GB2047162A

GB2047162A - Anisotropic membranes

Info

Publication number: GB2047162A
Application number: GB8010315A
Authority: GB
Original assignee: Monsanto Co
Current assignee: Monsanto Co
Priority date: 1979-03-28
Filing date: 1980-03-27
Publication date: 1980-11-26
Also published as: IT8020971A0; GR67255B; NO800893L; GB2100181B; BR8001873A; AU532082B2; DE3011917A1; IT1131098B; CA1141510A; IL59581A0; FI800977A; FR2452507A1; AR220972A1; GB2047162B; ES489884A0; SE8002388L; GB2100181A; NL8001845A; PT71024A; YU86080A

Abstract

Anisotropic membranes are prepared from a solution of a membrane- forming polymer in a liquid carrier. The polymer solution is provided in the form of a precursor and is then coagulated in a liquid coagulant comprising water to form the anisotropic membrane. The liquid carrier used in the methods comprises an N-acylated heterocyclic solvent which can be represented by the structure: <IMAGE> wherein X is -CH2-, -N(R')- or -O-; R is hydrogen, methyl or ethyl; and R' is hydrogen or methyl. The methods are particularly useful for preparing anisotropic hollow fiber membranes. In forming anisotropic hollow fiber membranes, the solution of hollow fiber- forming polymer in the liquid carrier is extruded through an annular spinnerette to form a hollow fiber precursor, and the hollow fiber precursor is coagulated in a liquid coagulant comprising water to form the hollow fiber membrane. The methods are attractive for producing polysulfone hollow fiber membranes which have a homogeneously-formed, thin selective skin on an open cellular support and which exhibit high resistance to collapse. The membranes are especially useful for the separation of gases.

Description

SPECIFIATION Methods for preparing anisotropic membranes and membranes produced therefrom This invention pertains to methods for preparing anisotropic membranes, especially hollow fiber membranes. Particularly attractive aspects of this invention include methods for preparing polysulfone hollow fiber membranes suitable for the separation of gases in which the material of the hollow fiber membrane effects separation by selective permeation and anisotropic polysulfone hollow fiber membranes.

The viability of the use of membranes for fluid separations as compared to other separation procedures such as absorption, adsorption, and liquefaction often depends on the cost of the apparatus and its operation including energy consumption, degree of selectivity of separation which is desired, the total pressure losses caused by the apparatus for conducting the separation procedure which can be tolerated, the useful life of such apparatus, and the size and ease of use of such apparatus. Thus, membranes are sought which provide desired selectivities of separation, fluxes and strength. Moreover, in order to be commercially attractive on an economic basis, the membranes are preferably capable of being manufactured in large quantities while achieving a reliable product quality and being readily and relatively inexpensively assembled in a permeator.Particularly advantageous membranes are unitary anisotropic hollow fiber membranes which have a relatively thin layer (often referred to as separation layer, barrier layer, or active layer) integral with a porous structure which provides support to the separating layer and offers little, if any, resistance to the passage of fluids. In order to prepare these integral anisotropic membranes, a unitary membrane structure must be formed which possesses diametrically opposed structures. The separating layer must be formed such that it is thin and possesses few, if any, pores or other defects. On the other hand, the conditions which make the integral anisotropic membrane must also provide a support structure which is highly open such that it offers little resistance to fluid flow.

Membranes have been prepared in film and in hollow fiber form. Numerous proposals have been made pertaining to the preparation of integral anisotropic membranes in film form. In general, anisotropic film membranes are prepared by casting a solution of the poymer to form the membrane in a solvent onto a surface, e.g., a polished glass surface. The polymer may be allowed to coagulate, at least partially, in air or a gaseous or vaporous environment and then it is usually immersed into a liquid coagulant. Considerable flexibility exists in preparing anisotropic film membranes. For instance, since the polymer solution is placed on a support, the membrane precursor structure need not be self supporting at least until after coagulation is completed.Similarly, since one surface of the cast membrane is in contact with the support, each side of the membrane may be subjected to different coagulant conditions thereby permitting substantially different structures to be achieved at each surface of the membrane. Accordingly, membranes having a relatively thin layer having an essential absence of pores may be achieved at one surface of the film membrane, while the remainder of the membrane may be relatively porous. Moreover, since the film membrane precursor is supported, the coagulation conditions including coagulation times, can be widely varied to achieve the desired film membrane structure.

In some instances, however, film membranes may not be as attractive as other gas separation apparatus due to the need for film membranes to be supported to withstand operating conditions and the overall complexity of apparatus containing film membranes. Membranes in the configuration of hollow fibers may overcome some of the deficiencies of film membranes for many separation operations. The hollow fibers are generally self-supporting even under operating conditions, and can provide a greater amount of membrane surface area per unit volume of separation apparatus than that which may be provided by film membranes.

Thus, separation apparatus containing hollow fibers may be attractive from the standpoint of convenience, in size and reduced complexity of design.

Many different considerations are involved in making a film membrane than are involved in making a hollow fiber membrane. For instance, no solid support, or interface, can be provided in a process for spinning a hollow fiber membrane. Moreover, in spinning procedures, the polymer solution must be of sufficient viscosity to provide a self-supporting extrudate prior to and during coagulation, and the coagulation must be quickly effected after extrusion such that the hollow fiber membrane is not adversely affected.

Processes for the formation of integral anisotropic membranes must not only meet the criteria for forming integral anisotropic hollow fiber membranes but also must be compatible with hollow fiber spinning capabilities. Hence, many constraints are placed upon the techniques available to produce integral anisotropic hollow fiber membranes. Commonly, in hollow fiber membrane spinning procedures, a solution of the polymer to form the hollow fiber membrane in a solvent is extruded through a spinnerette suitable for forming a hollow fiber structure, and a gas or liquid is maintained within the bore of the hollow fiber extrudate such that the hollow fiber configuration can be maintained. The hollow fiber extrudate must quickly be coagulated, e.g., by contract with the non-solvent for the polymer, such that the hollow fiber configuration can be maintained.The hollow fiber spinning process contains many variables which may affect the structure, or morphology, of the hollow fiber membrane such as the conditions of the polymer solution when extruded from the spinnerette, the nature of the fluid maintained in the bore of the hollow fiber membrane extrudate, the environment to which the exterior of the hollow fiber extrudate is subjected, the rapidity of coagulation of the polymer in the hollow fiber extrudate, and the like. Since the hollow fiber forming process is one of coagulation of polymer from a polymer solution, the nature of the solvent for the polymer may be highly influential in deterining the morphology of the hollbwfiber membrane and its separation properties.

The solvent must possess numerous properties in order to be suitable for forming anisotropic membranes (especially anisotropic hollow fiber membranes). For example, the solvent (or a liquid carrier containing solvent) must be capable of dissolving the polymer for forming the hollow fiber membrane but yet permit the polymer to readily coagulate to form the anisotropic structure. Furthermore, when hollow fiber membranes are desired, the solvent (or a liquid carrier containing solvent) should enable polymer solution to be prepared having suitable viscosities for hollow fiber membrane formation. and advantageously, these viscosities can be obtained without using excessively high concentrations of polymer in the solution.Since advantageous hollow fiber membranes are often obtained when the extrudate incurs a significant temperature drop upon exiting the spinnerette, the liquid carrier should enable suitable viscosities for hollow fiber formation to be provided at elevated temperatures which elevated temperatures facilitate achieving the significant temperature drop. The solvent or any other components of the liquid carrier should not be subject to degadation at such elevated temperatures. The solvent should be miscible with non-solvent used to assist in coagulating the polymer and should be capable of being removed, e.g., by washing, from the coagulated structure such that the membrane is not unduly plasticized, and thereby weakened, by the solvent.

Moreover, the solvent (or a liquid carrier containing solent) should not exhibit excessive heats of dilution in the non-solvent used to assist in coagulating the polymer.

In order for a procedure to be attractive for the production of commercial quantities of membranes, it is also desired that the procedure be safe and economical. Thus, the solvent should not be unduly toxic, and advantageously, the solvent exhibits a very low vapor pressure to minimize risk of inhalation and/or air pollution. Moreover, a solvent having a very low vapor pressure may also minimize the risk of explosion and fire. Furthermore, waste materials from the spinning process should be able to be economically and safely discarded or recycled.

Since the solvent is only one component used in the spinning procedure, other components such as fluid within the bore of the hollow fiber extrudate, non-solvent to assist in effecting coagulation, washing fluids to remove solvent from hollow fiber membranes, and the like should also be economical and safe. Heretofore proposals have been made to use, e.g, gasoline, kerosene or other hydrocarbonaceous materials in the spinning procedure either as coagulants or to assist in drying such as disclosed by Arasaka, et al., in U. S.

Patent No. 4,127,625. Such materials clearly pose toxicity and fire risks as well as disposal problems.

Moreover, in the quantities required to effect, e.g., coagulation, washing, etc., the expense of the hydrocarbonaceous materials could be a factor in the economics of the spinning process. Accordingly, it is desired to use highly safe, readily available materials, such as water, wherever possible in the spinning process, especially as non-solvent to assist in effecting coagulation and in washing to remove solvent from the hollow fiber membrane. The ability to use water, of course, will depend to a large extent upon the properties of the solvent with respect to water, i.e., solubility in water, heat of dilution in water, stability in water, and the like.

By this invention methods are provided for preparing anisotropic membranes, which methods utilize certain advantageous solvents. The methods of this invention can provide anisotropic membranes characterized by having a very thin separating layer having relatively few pores on an underlying region having an open, cellular structure which offers little resistance to fluid flow. These membranes may be particularly attractive for gas separations. Moreover, polymers which may be utilized in accordance with this invention to provide membranes include polysulfones which exhibit desirable strength and chemical resistance as well as desirable permeation properties in terms of permeate flux and selectivity.

Advantageously, the methods of this invention can utilize water as a non-solvent to assist in effecting the coagulation of the polymer and to wash solvent from the membranes. Moreover, the solvents have low vapor pressures, do not pose an undue risk of explosion or fire, and may be economically and safely discarded or recycled.

In accordance with this invention, an anisotropic membrane is prepared from a solution of membraneforming polymer in a liquid carrier comprising an N-acylated heterocyclic solvent which can be represented by the structure:

wherein X is -CH2-, -N(R')- or -0-; R is hydrogen, methyl, or ethyl; and R' is hydrogen or methyl. The polymer solution is provided in the form of a precursor and is then coagulated in a liquid coagulant comprising water to form the anisotropic membrane. The anisotropic membrane may be a film or, preferably, a hollow fiber. Typical N-acylated heterocyclic solvents include 1-formylpiperidine, 1acetylpiperidine, l4ormylmorpholine, and 1-acetylmorpholine. A preferred N-acylated heterocyclic solvent comprises 1-formylpiperidine.Reilly Tar & Chemical Corporation, in its brochure entitled "1 Formylpiperidine", reports that 1 -formylpiperidine has a boiling point of about 222 C, a vapor pressure of 0.111 millimeters of mercury at 25 C, a flash point of about 1 02 C, and and LD50 (oral-rats) of about 0.88 grams per kilogram of body weight.

In the methods of the aspect of this invention pertaining to making hollow fiber membranes the polymer solution is at a sufficient temperature to substantially maintain the fiber-forming polymer in solution and to provide the polymer solution at a fiber4orming viscosity prior to the extrusion of the hollow fiber precursor.

The polymer solution is extruded through an annular spinnerette to form a hollow fiber precursor. A fluid is injected into the bore of the hollow fiber precursor as it is being extruded from the spinnerette et a rate sufficient to maintain the bore of the hollow fiber precursor open. The injection fluid is preferably highly miscible with the liquid carrier and often, therefore, comprises water. The hollow fiber precursor is then contacted with the liquid coagulant which is a non-solvent for the polymer in the polymer solution. The liquid coagulant is preferably highly miscible with the liquid carrier and the injection fluid. Usually the temperature of the liquid coagulant is sufficiently low that the polymer solution at that temperature is extremely viscous and may even be a gel.The contact of the hollow fiber precursor with the liquid coagulant is for sufficient duration to substantially completely coagulate the polymer in the hollow fiber precursor under the conditions of the liquid coagulant and thereby provide a hollow fiber. The hollow fiber is then washed, i.e., contacted, with a non-solvent comprising water for the polymer which is miscible with the liquid carrier to reduce the content of the liquid carrier in the hollow fiber to less than about 5 weight percent based on the weight of the polymer in the hollow fiber. The washed hollow fiber may then be dried at a temperature at which the selectiviy or flux exhibited by the hollow fiber is not unduly adversely affected.

In an aspect of this invention the hollow fiber mebranes preferably have a relatively circular cross-section with circular, concentric bores. In many instances, the hollow fiber membrane can be circular and concentric within the tolerances of the machining required to make suitable spinnerettes. The outside diameter of the hollow fiber membrane may vary widely, e.g., from about 100 or 150 to 1000 or more microns. Frequently the outside diameter of the hollow fiber membrane is about 200 or 300 to 800 microns.

The ratio of thickness of the wall to outside diameter of the hollow fiber membrane may also vary widely depending upon the required collapse pressure which must be exhibited for a given separation operation.

Since the cellular support is open, an increase in wall thickness may not result in an undue reduction in flux through the membrane wall. Typical ratios of thickness to outside diameter are about 0.1 or 0.15 to about 0.45, most often about 0.15 to 0.4.

The void volume of the hollow fiber membranes, i.e., regions within the wall of the hollow fiber membrane vacant of the material of the hollow fiber membrane is substantial. The void volume can be determined by a density comparison with a volume of the bulk polymer (or other material of the hollow fiber) which would correspond to a hollow fiber of the same superficial gross physical dimensions and configurations as the wall of the hollow fiber membrane. Since the cellular support of the hollow fibers is open, relatively low void volumes for anisotropic hollow fiber membranes can be achieved without unduly low permeate flux occurring. Often, the void volume is at least about 30 to 35, e.g., at least about 40, say, greater than about 45 or 50, volume percent, and may range up to 65 or 70 or more volume percent.

The liquid carrier may consist essentially of the solvent or the solvent may be admixed with one or more other components to provide the liquid carrier. The other components may be solid or liquid at room temperature but are dissolved when in the liquid carrier. In many instances, especially when the precursor is exposed to the atmosphere prior to coagulation of the polymer, it is preferred that the liquid carrier (including any diluent) be relatively non-volatile, e.g., the liquid carrier advantageously has an essential absence of a component (solvent or diluent) having a vapor pressure above about 0.8, say, above 0.6, atmosphere at the temperature of the polymer solution at extrusion.If, however, a more volatile component in the liquid carrier is used, and the precursor is exposed to an atmosphere prior to contacting the liquid coagulant, the atmosphere to which the precursor is exposed, may beneficially contain substantial amounts of such a more volatile component to retard the loss of the component from the precursor. Gaseous components are generally avoided but may be of use especially when the precursor is immediately contacted with the liquid coagulant. The components are preferably compatible with the solvent and the liquid coagulant.Often the other components are highly soluble in the N-acylated heterocyclic solvent, e.g., at least about 50, preferably at least about 100, parts by weight can be dissolved in 100 parts by weight of the N-acylated heterocyclic solvent at room temperature (about 25 C), and the other components are preferably miscible with the N-acylated heterocyclic solvent and the liquid coagulant in all proportions. Components other than the N-acylated heterocyclic solvents which may be useful in the liquid carrier include solvents, such as dimethylacetamide, dimethylformamide, N-methylpyrrolidone, dimethylsulfoxide, etc.; viscosity modifiers, such as isopropyl amine; surfactants; plasticizers; and the like. A particularly useful component for the liquid carrier is a diluent which may be a solvent or non-solvent for the polymer. Preferably, the N-acylated heterocyclic solvent comprises at least about 50, say, at least about 60, weight percent of the liquid carrier.

The use of diluents can provide particularly attractive liquid carriers such that certain solvent activities with the individual polymer molecules are exhibited such that frequently, the polymer can be coagulated or gelled from the polymer solution with little, if any, change in energy. The behavior of polymer molecules in solution is theorized to be influenced by not only the interaction among the polymer molecules, but also by the effect of the solvent (and liquid carrier) on the polymer molecule.

In accordance with the theory described by Flory, Principles of Polymer Chemistry (1953), herein incorporated by reference, at Chapter XIV, pages 595 to 639, in a given solvent-polymer system at a given temperature, the molecular dimension of the polymer may be unperturbed by intramolecular interactions.

As the temperature increases from this given temperature (theta temperature), the polymer swells in the solution. The greater the "swelling" of the polymer molecule at a reference temperature, the better the solvent is for the polymer at the reference temperature. If the temperature is decreased from the theta temperature, a polymer molecule of infinite molecular weight would gel or precipitate, i.e., the intramolecular forces of the polymer molecule are greater than the forces between the solvent and polymer molecule.

A meaningful evaluation of a liquid carrier polymer system can be made by viscosity measurements of a very dilute solution of the polymer in the liquid carrier to determine the intrinsic viscosity (the capacity of a polymer to enhance the viscosity of a solution) (e.g., at approximately 250C for the sake of consistency although other temperatures so long as consistent, can be used) and the intrinsic viscosity of the polymer at the theta temperature (intrinsic theta viscosity). The ratio of the intrinsic viscosity to the intrinsic theta viscosity (hereafter "intrinsic viscosities ratio") is indicative of the effect of the solvent (liquid carrier) on the polymer molecules.The intrinsic viscosity for a given liquid carrier polymer system is obtained by determining the viscosity of polymer solution (n) at several various dilute polymer concentrations. The viscosity of the liquid carrier (n0) is also determined at the same temperature. The ratio of n o is the relative viscosity and a plot of the natural logarithm of the relative viscosity divided by the polymer concentration versus the polymer concentration when extrapolated to zero polymer concentration provides the intrinsic viscosity. Conveniently, the dilute polymer solutions employed are selected to provide relative viscosities of from about 1.1 to 2.5.In order to determine the intrinsic viscosity of the polymer at the theta temperature, several techniques may be employed such as disclosed by Flory, supra, at pages 612 to 622. For instance, the intrinsic viscosity of the polymer in a theta solvent at the theta temperature can be determined by viscosity measurements as set forth above. Theta solvents and theta temperatures for many polymers can be found in the literature, e.g., see for instance Brandrup, et al., Polymer Han dbook, Second Edition (1975), pages lV-1 57 to IV-173, herein incorporated by reference.If necessary, theta solvents and temperatures can be determined by any of the techniques set forth by Brandrup, et al., at pages lV-1 57 to IV-159. The temperature of the polymer solution during the viscosity determination must be precisely maintained as close as possible to the theta temperature since the intrinsic viscosity of many polymer solutions changes rapidly at temperatures close to the theta temperature.Alternatively the intrinsic theta viscosity can be estimated by the use of a light scattering technique to determine the second virial coefficients of the polymer in a solvent at different temperatures.A suitable procedure is disclosed by Kaye in "Low Angle Laser Light Scattering", Analytical Chemistry, 45 (1973) pages 221 to 225, and involves the determination of the amount of light scattering of a laser beam passed through the polymer solution at several temperatures. See for instance, Stacy, Light Scattering in Physical Chemistry (1956)pages117 et seq., for description of the relationship between light scattering and the second virial coefficient. Care should be taken to remove any solids from the polymer solution, e.g., by filtration, which can affect the light scattering data.The intrinsic viscosity of the polymer is obtained at each of the temperatures at which the second virial coefficient is determines, and a plot of the intrinsic viscosities and second virial coefficients can be prepared and extrapolated to the point at which the second virial coefficient is zero. The intrinsic viscosity at this point is the intrinsic theta viscosity. The method for determining the intrinsic theta viscosity is an approximation; however, such an approximation may still be useful in providing a suitable polymer-liquid carrier system for use in accordance with this invention to make a hollow fiber membrane from a polymer. Generally suitable polymers in the liquid carriers in accordance with this invention will provide dilute solutions exhibiting an "intrinsic viscosities ratio" of about 1.05 to 1.7, say, about 1.05 to 1.5, and most frequently about 1.1 to 1.5.

The second virial coefficient of the liquid carrier/polymer system may similarly be useful in determining suitable liquid carrier/polymer systems. The second viral coefficient exhibited by many useful liquid carrier/polymer systems often is below about 15, e.g., below about 12, say below about 10, mol-cm3/gm2 (times 104) (at 250C). The second virial coefficientforthe liquid carrier/polymer systems is frequently above about 0.5, say, above 1, and sometimes above 3, for instance, between about 3 and 8, mol-cm3/gm2 (times 104) (at 25").

Another useful method for determining liquid carrier/polymer systems which may be advantageous for providing hollow fiber membranes in accordance with the methods of this invention is the interaction parameter described by Blanks, et al., in "Solubility of Styrene-Acrylonitrile Copolymers," Structure Solubility Relationships in Polymers, edited by Harris, et al., (1977) pages 111 to 122, herein incorporated by reference.The interation parameter (A12) can be defined as follows

where V, is the molar volume of the solvent in cubic centimeters per mole; R isthe gas constant (about 12 cal/mol); T is temperature, K; dp and 6d,s are the (London) dispersive force component of the solubility parameter, (cal/cc)i, of the polymer and solvent respectively, bp,p and bp are the polar bonding terms of the solubility parameter, (cal/cc);, of the polymer and solvent respectively; and 5h.0 and bh are the hydrogen bonding terms of the solubility parameter, (cal/cc)1, of the polymer and solvent respectively.The total solubility parameter (6T) can be defined as 6T2 = bd2 + tj.p2 + oh2 Solubility parameters are conveniently employed to characterize liquid carriers and solutes, and values for the solubility parameters and components can often be found in the literature, for instance, see Shaw "Studies of Polymer-Polymer Solubility Using a Two-Dimensional Solubility Parameter Approach", J. of Applied Polymer Science, 18, pp. 449to472 (1974), and Brandrup, et al., The Polymer Handbook, Second Edition, pages lV-337 to 1\1-359 (who also describe experimental techniques for obtaining solubility parameters and components). The components).The components of the solubility parameter which are used to calculate the interaction parameterfora liquid carrier comprising morethan one component can be estimated on a volume fraction average basis This estimation is an approximation since in many instances the volume of the components of liquid carrier are not additive.Frequently the interaction parameter at 200 to 300C of the polymer and liquid carrier is less than about 0.5, say, less than about 0.3. The interaction parameter generally is at least about 0.01, preferably, at least about 0.02, e.g., at least about 0.05.

Also, the liquid carrier advantageously has a solubility parameter approximately equal to (i.e., within 5 or 10 percent of) or above the solubility parameter of the polymer. In many instances, at least one, and sometimes both, the polar bonding term and hydrogen bonding term of the solubility parameter are greater (i.e., no more than) 0.5, say, greater than 0.3 (cal/cc)i, below the corresponding term for the polymer.

Advantageous membranes can be made from polymer solutions in which the addition of liquid coagulant, even in very small amounts, does not substantially improve the ability of the solvent to dissolve the polymer.

Often, the polar bonding term and hydrogen bonding term of the solubility parameter of the liquid carrier are each greater than the corresponding terms of the solubility parameter of the polymer.

Often, the surface tension of the liquid carrier is within about 10, say, about 5 or 7, dynes per centimeter, of the surface tension of the polymer at room temperature (250cm. Generally, if the liquid carrier/polymer system, when in a dilute polymer solution, exhibits a suitable "intrinsic viscosities ratio", second virial coefficient, and interaction parameter, the liquid carrier also exhibits a surface tension approximating the surface tension of the polymer. Frequently, particularly useful liquid carriers exhibit a surface tension at room temperature (25 C) less than about 3 or 5 dynes per centimeter above the surface tension of the polymer.Surface tensions for polymers can be experimentally determined by any conventional procedure, and one such procedure is described by Tanny in J. of Applied Polymer Science, Volume 18, pages 2153 to 2154(1974).

Often liquid carriers used in the method of this invention contain a diluent which is a non-solvent for the polymer. Non-solvents are generally characterized by exhibiting little capability of dissolving polymer, e.g., the polymer solubility is less than about 10, say, less than 2, often less than about 0.5, grams per 100 milliliters of non-solvent. The non-solvent preferably exhibits little, if any, swelling action on the polymer.

The non-solvent, if added in a sufficient amount, is usually capable of resulting in a phase separation of the polymer solution. Preferably, the non-solvent is not added in an amount such that the polymer solution is unduly unstable at the processing conditions prior to forming the precursor. Frequently, the amount of non-solvent in the liquid carrier is at least about 2, e.g., at least about 5, weight parts per 100 weight parts of the liquid carrier.

Suitable non-solvents can include the liquid coagulant or one or more components of the liquid coagulant.

Some desirable solvents can maintain the polymer in solution in the presence of at least about 5, say, at least about 10, weight percent of the liquid coagulant based on the weight of the solvent (e.-g., at 25 C). Preferably, however, the addition of relatively small quantities of liquid coagulant to a solution of the polymer in the liquid carrier will result in phase separation or gelling of the polymer solution.

Typical diluents, including non-solvents,include formamide, acetamide, ethylene glycol, water, trimethylamine, triethylamine, isopropylamine, isopropanol, methanol, nitromethane, 2-pyrrolidone, acetic acid, formic acid, aqueous ammonia, methylethylketone, acetone, glycerol and the like. Low molecular weight inorganic salts such as lithium chloride, lithium bromide, zinc chloride, magnesium perchlorate, lithium nitrate, and the like may also be useful in the liquid carrier.

A sufficient amount of polymer is contained in the polymer solution to enable the precursor to be formed.

Hence, when preparing a hollow fiber membrane, the polymer concentration is sufficient that the polymer solution is at a fiber-forming viscosity at the temperature of the polymer solution when formed (i.e., extruded) into a hollow fiber precursor. Unduly low viscosities can result in the breakage of the hollow fiber precursor and the inability to maintain the desired hollow fiber configuration. High viscosities are desirable, but excessively high viscosities may be undesirable due to the pressures required to extrude the polymer solution. Frequently the viscosity of the polymer solution being extruded is at least about 5000, often at least about 10,000, centipoises and may be as high as 500,000 or 1,000,000 centipoises at the temperature of the extrusion. Many attractive hollow fiber membranes are prepared utilizing polymer solution viscosities at the temperature of extrusion in the range of about 10,000 to 500,000 centipoises. In some instances, sufficient polymer may be contained in the polymer solution that at the temperature of the liquid coagulant, the polymer solution becomes a substantially non-flowing structure, for instance, becomes extremely viscous or undergoes a physical change, e.g., to form a gel, ti.e., an elastic structure in which at least some of the polymer is not soluble in the liquid carrier and liquid carrier is entrapped in the interstices) orto result in phase separation.

The polymer concentration of the polymer solution is conveniently sufficiently high in order to ensure that the membrane contains sufficient polymer to provide the desired high strength to the membrane. If the polymer concentration of the polymer solution is too low, large voids, including large cells and macrovoids, may occur in the membrane wall, thereby resulting in a low strength wall structure. At higher polymer concentrations the resulting hollow fiber wall is generally more dense and macrovoids are generally more infrequent, hence the hollow fiber membrane can exhibit greater strength. Macrovoids are large voids having a major dimension greaterthan about 3 microns. Preferably, a sufficiently high polymer concentration is employed such that under the coagulation conditions, few, if any, macrovoids are formed.

Since this invention advantageously can provide an open, cellular structure in the membrane wall, the increase in polymer density can be achieved with little, if any, reduction in flux through the matrix of the membrane wall. The maximum polymer concentrations which can be used.in a polymer solution is generally determined on a practical basis by the capability of conventional apparatus to form membranes from highly viscous polyer solutions. The maximum, preferred concentrations of polymer in the polymer solution will also depend on the nature of the polymer and of the liquid carrier. For instance, with lower molecular weight polymers, higher polymer concentrations can be more desirably utilized than with higher molecular weight polymers. Frequently, the polymer concentration is at least about 25 weight percent of the polymer solution.

Polymer concentrations as high as 45 or 50 weight percent may be useful in some situations. Polymer concentrations of about 28 or 30 to 38 or 40 weight percent are most often desired.

Frequently, the viscosities of the polymer solutions used in accordance with this invention are of relatively high activation energies. An activation energy is a relationship between temperature and viscosity, and this relationship has been characterized as: r = A.eE!RT where n is the viscosity of the polymer solution, E is the activation energy, R is the gas constant (approximately 2 cal/ K, mole) and T is the temperature ( K). See Kunst, et al., Fifth International Symposium on Fresh Water from the Sea, Vol. 4 (1976), herein incorporated by reference.The slope of a plot of the logarithm of the viscosity (poise) to the reciprocal absolute temperature ( K-1), is generally liner, and from that slope, the activation energy (cal/mole) can be approximated. Often, the activation energy is at least about 8 iccal/mole, say, about 8.5 to 15, e.g., about 9 to 12, most frequently about 9.5 to 12, kcal/mole.

Usually, with desirable liquid carriers containing a non-solvent as a diluent, the activation energy increases with increased concentration of the non-solvent.

The polymer solution can be prepared in any convenient manner, for instance, the liquid carrier can be added to the polymer, the polymer can be added to the liquid carrier, or the polymer and the liquid carrier can be simultaneously combined. The liquid carrier, of course, if comprising more than one component, can be admixed with the polymer on a component-by-component basis. For instance, if the liquid carrier comprises a solvent and a non-solvent, the polymer may advantageously be admixed with the solvent prior to the addition of the non-solvent. When the non-solvent is added subsequent to dissolving the polymer in the solvent, generally the non-solvent is added slowly such that the localized zones of increased non-solvent concentrations are minimized to avoid coagutation, or precipitation, of the polymer at such localized zones.

Elevated temperatures may be utilized to facilitate the mixing of the polymer and liquid carrier. The temperature however, should not be so high as to deleteriously affect any of the components of the solution being formed. The time required to effect mixing to provide a polymersoltion can vary widely depending upon the rate of solution of the components, the temperature, the efficiency of the mixing apparatus, the viscosity of the polymer solution being prepared, and the like. Desirably, the mixing of the polymer solution should continue until a substantially uniform composition exists throughout the polymer solution. Any suitable mixing equipment may find application in preparing polymer solutions suitable for use in accordance with this invention.Preferably, the mixing equipment does not induce undue air entrapment in the polymer solution in order to facilitate any subsequent degassing of the polymer solution. Often components of the polymer solution have an appreciable volatility under the conditions of mixing. These volatile components may present undue risk in terms of fire and health hazards, and the loss of volatilized components will also alter the composition of the polymer solution. Accordingly, in most instances the mixing of the polymer and liquid carrier is conducted in an essentially sealed container. An inert atmosphere may be provided in such sealed containers.It is possible that components for the polymer solution may be adversely affected when exposed to atmospheric conditions (e.g., ambient air).The total pressure of the atmosphere in the mixing container is preferably relatively low in order to minimize any dissolving or entrapment of the inert atmosphere in the polymer solution during formation of the polymer solution. In most instances, pressures below about 2 or 3 atmospheres absolute are used.

Frequently the polymer solution contains entrapped or dissolved gases, which gases mayresult in the formation of anomalies, e.g., macrovoids, in the membranes. Thus, it is generally desirable to subject the polymer solution to degassing operations. Preferably, the polymers solution is substantiallyafree of entrapped or dissolved gases prior to extruding the hollow fiber precursor. Any suitable degassing apparatus and conditions may be useful in effecting the desired degassing. For example, adequate degassing may be achieved by holding the polymer solution in a closed vessel for a time sufficient to allow the entrapped or dissolved gases to escape the polymer solution. Conventional degassing equipment such as "J" tubes, centrifugal degassers, ultrasonic degassers, and the like, may find application in degassing the polymer solution.Since many of the polymer solutions useful in accordance with this invention are of relatively high viscosity, the polymer solution in the degasser may be at an elevated temperature in order to reduce its viscosity and facilitate release of the entrapped or dissolved gases. The elevated temperatures to which the polymer solution may be subjected and the times for which the polymer solution is at such elevated temperatures, should not be so excessive as to result in deleterious effects to the polymer solution or its components. The degassing may be conducted at relatively low absolute pressures (i.e., vacuum) or at such higher pressures as to prevent e.g., undue volatilization of any of the components of the polymer solution. The pressure should not be so high as to result in an undue dissolving or redissolving of gases from the atmosphere in the degasser.In general, the degassing is preferably conducted at an absolute pressure below about 2 atmospheres, and most often the degassing is conducted at a subatmospheric pressure. The degassing may be conducted in a sealed vessel. An inert atmosphere may be used during the degassing operation.

The degassed polymer solution can then be used for forming the membrane precursor. In transporting the polymer solution, it is preferred that the piping and pumping apparatus be designed such that gas (e.g., air) does not leak into the transport system and enter the polymer solution. The polymer solution may contain solid impurities, e.g., dust, polymeric oligomers, and the like, which may adversely affect the membrane.

Often, therefore, the polymer solution is filtered prior to entering the spinnerette. Preferably, the filtering is sufficient to remove substantially all solid particles greater than about, say, 50 micron in major dimension, and often substantially all particles greater than about 0.5 micron in greatest dimension and removed by filtering. Elevated temperatures may be used to facilitate any transport and filtering of the polymer solution.

Such elevated temperatures however should not be so high as to result in any deleterious effects to the polymer solution or its components.

Frequently, water comprises a major amount of the liquid coagulant sufficient to provide a suitable strength non-solvent to effect the polymer coagulation. For instance, the liquid coagulant may comprise at least about 50, say, at least about 75, and most often at least about 85, weight percent water. Advantageously the liquid coagulant does not cause any appreciable swelling of the polymer. Since water often comprises a major amount of the liquid coagulant, the polymer preferably exhibits little, if any, swelling in water.

Desirably, the composition of the liquid coagulant is such that the heat of dilution of the liquid carrier in the liquid coagulant is greater than about -3.5, say, greater than about -3, e.g., greater than about -2.5, and frequently, greater than about -2 kilocalories per mole. The absolute value of the heat of dilution of the squid carrier in the liquid coagulant is often less than about 3 kilocalories per mole. In many instances, the heat of dilution is within the range of about -0.5 to -2 kilocalories per mole. The heat of dilution can be determined using any conventional procedure and generally is determined at about 25 C.

The liquid coagulant, particularly when making hollow fiber membranes on a continuous basis, will contain liquid carrier which is removed from the precursor. The liquid carrier may detract from the non-solvent strength of the liquid coagulant. The liquid coagulant is therefore desirably replaced with fresh, or recycled, liquid coagulant containing little, if any, liquid carrier. Frequently, the concentration of the liquid carrier in the liquid coagulant is less than about 10, preferably less than about 5, say, less than about 2, weight percent.

The liquid coagulant may contain additional components such as surfactants, materials which increase or decrease the non-solvent strength of the liquid coagulant with respect to the polymer, materials which enhance the solubility of components of the liquid carrier in the liquid coagulant, materials which reduce the heat of dilution of the liquid carrier in the liquid coagulant, freezing point depressors, and the like. Useful additional components for the liquid coagulants which may find application in making the membranes include low molecular weight alcohols such as methanol, isopropanol, etc., salts such as sodium chloride, sodium nitrate, lithium chloride, etc., organic acids such as formic acid, acetic acid, etc., and the like.

The liquid coagulant is desirably maintained at a temperature at which the polymer solution is substantially non4lowing. While suitable temperatures are generally low, some polymer solutions may be substantially non-flowing at room temperature or above. Hence, liquid coagulant temperatures of up to about 900C or higher may find application.Often, however, the liquid coagulant temperature is below about 35 C, and highly desirable membranes are frequently produced using liquid coagulant temperatures below about 20 C, say, below about 1 0 C. Most often, for the sake of convenience, its temperaure is above about 0 C, e.g., about 00to 10"C. However, with suitable refrigeration equipment and the presence of freezing point depressors in the liquid coagulant, temperatures of -150C and below may be achieved.

The residence time of the precursor in the liquid coagulant should be sufficient that the amount of coagulation at the temperature of the liquid coagulant provides adequate strength to the membrane for further processing. Frequently, it is desired that coagulation of the polymer in the precursor occur within a few seconds such that the equipment sizes for the coagulation step are not unduly large. Since the exterior of, e.g., the hollow fiber precursor directly contacts the liquid coagulant, it is generally almost instantaneously coagulated. In some instances, essentially complete coagulation of the polymer in the precursor under the conditions of the liquid coagulant may almost instantaneously occur. More frequently, the precursor exhibits an observable transition in the liquid coagulant from a clear or translucent structure to an opaque structure.

This transistion may be gradual, and sometimes the progress of the transition can be observed. The time for this transition under the conditions of the liquid coagulant can vary widely, but in many instances is at least about 0.001, for example, about 0.01 to 1, say, about 0.02 to 0.5,seconds.

Generally, even after coagulation of thp nrl Irenr the membrane still contains substantial amounts of liquid carrier which can adversely affect its strength properties and may even enable coagulated polymer to be redissolved. Most conveniently, therefore, the membrane is subjected to at least one washing step to further remove liquid carrier. Often, the washing of the membrane is initiated under substantially the same conditions (e.g., temperature) that the coagulation occurred. The temperatures employed for washing are often based on the strength of the liquid carrier-containing membrane, the useful range of temperatures to which the washing liquid can be subjected, and convenience of obtaining the washing liquid at such temperatures.In general, temperatures should be avoided during wasing which could result in undesirable annealing of the membrane surface. Often the temperature of washing ranges from about 0 to 50 C preferably about 0 to 35 C. Conveniently, especially if the liquid carrier and liquid coagulant are miscible with water, water is employed as the washing fluid. The washing fluid may contain additives to, e.g., enhance removal of the liquid carrier. For instance, if water is the washing fluid, then a water miscible organic material (e.g., methanol, isopropanol, etc.) may assist in facilitating the removal of the liquid carrier.

Often the liquid carrier content of the membrane can readily be reduced to say less than about 20 weight percent of liquid carrier (e.g., by passing the hollow fiber through a washing liquid for 2to 5 minutes).

However, the reduction of the liquid carrier content of the membrane to desirably low levels, e.g., less than about 5, and somtimes less than about 2, weight percent liquid carrier, may require relatively long periods of additional washing. This additional washing is often at least about 3 hours, and may range up to 20 or more days. The additional washing may be conducted by continuously passing washing liquid having little, if any, liquid carrier over the membrane (rinsing) or by soaking, or storage, in washing liquid. Generally, a combination of rinsing and storage is employed to remove the liquid carrier from the membrane. When the membranes are stored in washing liquid, a periodic replacement of washing liquid may be advantageous if the concentration of liquid carrier in the washing liquid becomes undesirably high.Although the membranes can be stored in the washing liquid for periods longer than 20 days, usually little additional amounts of liquid carrier are removed from the membranes after such long storage periods. Most frequently, the washing of the membranes continues until the liquid carrier content of the membrane is less than about 4 or 5 weight percent of the membrane.

The membranes, after washing, are dried to remove the washing liquid. The presence of liquid in the anisotropic membranes can significantly reduce the flux of moieties, e.g., gases, permeating the membrane and therefore is generally not desired. Suitable drying can be accomplished by exposure to a gaseous atmosphere. Air is usually a suitable gaseous atmosphere. Useful drying conditions can vary widely. For instance, suitable drying conditions may include temperatures ranging from -15 C or below to 90 C or more and relative humidities ranging from about 0 to 95, say, about 5 to 60 percent. Frequently, the temperature ranges from about 0 to 80 C with absolute humidities less than about 20 or 30, say, about 5 to 15, grams per cubic meter.In many instances, drying membranes under ambient laboratory conditions, e.g., about 20 to 25 C and 40 to 60 percent relative humidity, is acceptable.

Materials useful for preparing anisotropic membranes may be organic polymers, or organic polymers mixed with inorganics, e.g., fillers, reinforcements, and the like. Thus, the method of this invention may find advantageous applicability in the preparation of metalic hollow fiber membranes in accordance with the teachings of Dobo, et al., United States Patent No.4,175,153, herein incorporated by reference. Suitable polymers are soluble in the N-acylated heterocyclic solvents employed in accordance with the method of this invention. Desirably, at least about 50, preferably at least about 100, parts by weight of polymer can be dissolved in 100 parts by weight of the N-acylated heterocyclic solvent (at 25 C), and most often, the N-acylated heterocyclic solent and polymer are miscible in all proportions.However, even though the N-acylated heterocyclic solvent is a good solvent for the polymer and is miscible with the liquid coagulant, the polymer may not be acceptable to produce membranes if the solvent is not capable of being readily removed from the membrane after coagulation. If the polymer retains the solvent the polymeric structure of the membrane may be so weakened as to result in undue compaction of the anisotropic structure thereby resulting in an undesirable reduction of the permeability of the membrane to permeating moieties, e.g., gases. Moreover, if undue amounts of solvent are retained even after drying, the resulting membrane may not have sufficient structural strength to withstand separation conditions.

Frequently, the "intrinsic viscosities ratio" for suitable polymers in dilute solution in the N-acylated heterocyclic solvent is at least about 1.05, preferably at least about 1.3. Often the "intrinsic viscosities ratio" is up to about 2, say, about 1.3 to 1.75, or even 1.5 to 1.75.

Frequently, the second virial coefficient for polymer solutions containing a suitable polymer in the N-acylated heterocyclic solvent is below about 20, say, below about 15, mol-cm3/g2 (times 104) (at 25 C). In many instances, the second virial coefficient is at least about 0.5, say, at least about 1, e.g., at least about 3, mol-cm3/g2 (times 104) (at 25 C). Often the interaction parameter for suitable polymers with the N-acylated heterocyclic solyent at about 20 to 30 C is less than about 0.5, say, less than about 0.3, and most frequently at least about 0.005, say, at least about 0.01, and sometimes at least about 0.02.

Generally, it is preferred that the polymer be substantially non-crystalline, e.g., less than about 2F weight percent, and frequently less than about 1 or 2 weight percent, crystalline, since crystalline polymers usually exhibit low permeability coefficients. However, polymers containing substantial crystallinity may sometimes be desired. Also, the polymer is often non-ionic to facilitate the prepartion of anisotropic membranes.

The polymeric material is preferably selected on the basis of its separation capabilities, i.e., its ability to provide the desired selectivity of separation at a suitable permeate flux strength and chemical resistance, especially with respect to the components of the fluid streams intended to be treated using the membrane.

The polymer should have a molecular weight sufficient for, e.g., hollow fiber formation. In general, for a given type of polymer, the longer the polymeric chain, the greater the tensile strength exhibited by the polymer and the higher the glass transition temperature of the polymer. These increases in physical properties with increasing molecular weight can often be accomplished with little, if any change in the separation capabilities of the polymer. Thus, by providing polymers of high molecular weight, the anisotropic membrane can be made stronger and tolerate to a greater extent the components of feed streams which can deleteriously affect the polymer. However, increased polymer molecular weights generally result in an increase in viscosity of the polymer solutions for forming the precursor.Therefore, it is often desired to utilize a polymer having a narrow distribution of molecular weights, such that the polymer exhibits high strength properties without an undue increase in viscosity. For instance, with a broad molecular weight distribution, the polymer can contain asignificantportion of lower molecular weight polymer molecules but provide a high solution viscosity due to the present of substantially high molecular weight polymer molecules. A convenient manner for characterizing the molecular weight distribution of a polymer is by the ratio of the weight average molecular weight of the polymer to the number average molecular weight of the polymer. The higher ratios indicate that the molecular weight distribution is wider.

Frequently, this ratio is less than about 3.

The strength of the polymer should be sufficient to prevent damage to the separating layer during expected handling and transporting operations in making the membrane, fabricating a permeator containing the membrane, and installing and using the permeator. Also, the polymer must exhibit sufficient strength such that the membrane can withstand gas separation operating conditions. The strength of the polymer can be expressed in terms of, e.g., tensile strength, Young's modulus, etc. While these mechanical properties may each affect the overall strength of the membrane, often an indication whether the polymer has adequate strength can be obtained by a consideration of the tensile strength of the polymer.Generally, suitable polymers for membranes exhibit a tensile strength of at least about 350, say, at least about 400, preferably at least about 500, say, at least about 600 kilograms per square centimeter. The tensile strength may be as high as achievable, however, few polymers which are advantageous as membrane materials exhibit tensile strengths above about 2000 kilograms per square centimeter. The strength of a polymer can be adversely affected by materials which, for instance, dissolve, plasticize or swell the polymer. Often many suitable polymers for membrane separations will be dissolved, plasticized, swollen or otherwise adversely affected by one or more materials. In some instances, industrial streams, e.g., gas stream, to be treated using membranes for separations contain one or more components which can adversely affect the membrane material.By the use of a membrane material which exhibits high strength, a reduction in strength of the membrane due to the presence of such deleterious components in the feed mixtures may be tolerated since the weaker membrane may still withstand the separation operating conditions.

The selective permeation of many fluids, including gases, in many polymers have been reported. In these reported permeations the mechanism of the separation apparently involves an interaction with the material of the membrane. hence, widely differing separation capabilities can be achieved by the use of different polymers. For instance, polyacrylonitrile permits nitrogen to permeate over 10 times as fast as methane, whereas in a polysulfone membrane, methane may permeate slightly faster than nitrogen. The literature is replete with information regarding the permeation properties of polymers.For instance, see Hwang, et al., "Gas Transfer Coefficients In Membranes", Separation Science, Volume 9, pages 461 to 478 (1974) and Brandrup, et al., Polymer Handbook, Second Edition, Section 111(1975). The suitability of a polymer for, e.g., a gas separation can readily be determined using, for instance, conventional techniques for determining permeability constants, permeabilities and separation factors such as disclosed by Hwang, et al., in Techniques of Chemistry, Volume VII, Membranes in Separations(1975) at Chapter 12, pages 296 to 322.

Typical polymers which may be suitable for making integral, anisotropic membranes include substituted and unsubstituted polymers selected from polysulfones; poly (styrenes), including styrene-containing copolymers such as acrylonitrile-styrene copolymers and styrene-vinylbenzyl-halide copolymers; polycarbonates; cellulosic polymers, such as cellulose acetate-butyrate, cellulose propionate; ethyl cellulose, methyl cellulose, nitrocellulose, etc.; polyamides and polyimides, including aryl polyamides and arylpolyimides; polyethers; polyacetal; polylarylene oxides) such as poly(phenylene oxide) and poly(xylylene oxide); poly(esteramide-diisocyanate); polyurethanes; polyester (including polyarylates), such as poly (ethylene terephthalate), poly(alkyl methacrylates), poly(alkyl acrylates), poly(phenylene terephthalate), etc.; polysulfides; polymers from monomers having alphaolefinic unsaturation other than mentioned above such as polyvinyls, e.g., poly(vinyl chloride), poly(vinyl fluoride), poly(vinylidene chloride), poly(vinylidene fluoride). poly(vinyl alcohol), poly(vinyl esters) such as poly(vinyl acetate) and poly(vinyl propionate), poly(vinyl pyridines), poly(vinyl pyrrolidones), polyvinyl ethers), poly(vinyl ketones), poly(vinyl aldehydes), such as poly(vinyl formal) and poly(vinyl butyral), poly(vinyl amines), poly(vinyl phosphates), and poly(vinyl sulfates); polyallyls; poly(benzobenzimidazole); polyhydrazides; polyoxadiazoles; polytriazoles; poly(benzimidazole); polycarbodiimides; polyphosphazines; etc., and interpolymers including blockinterpolymers containing repeating units from the above such as terpolymers of acrylonitrile-vinyl bromide-sodium sa It of para-sulfophenylmethallyl ethers; and grafts and blends containing any of the foregoing. Typical substituents providing substituted polymers include halogens such as fluorine, chlorine and bromine; hydroxyl groups; lower alkyl groups; lower alkoxy groups; monocyclic aryl; lower acyl groups and the like.

One class of polymers which may be attractive for many gas separations are copolymers of styrene and acrylonitrile or terpolymers containing styrene and acrylonitrile. Frequently, the styrene is up to about 60 or 70, say, about 10 to 50, mole percent of the total monomer in the polymer. Advantageously, the acrylonitirile monomer comprises at least about 20, e.g.,about 20 to 90, often about 30 to 80 mole percent of the polymer.

Other monomers which may be employed with styrene and acrylonitrile to provide, e.g., terpolymers, include olefinic monomers such as butene, butadiene, methylacrylate, methlmethacrylate, maleic anhydride, and the like. Other monomers which may be employed include vinyl chloride. The copolymers or terpolymers of styrene and acrylonitrile often have a weight average molecular weight of at least about 25,000 or 50,000, say, about 75,000 to 500,000 or more.

One of the preferred polymers for anisotropic membranes is polysulfone due to its strength, chemical resistance and relatively good gas separation capabilities. Typical polysulfones are characterized by having a polymeric backbone comprised of the repeating structural unit:

where R1 and R2 can be the same or different and are aliphatic or aromatic hydrocarbyl-containing moieties, say, of 1 to about 40 carbon atoms, wherein the sulfur in the sulfonyl group is bonded to aliphatic or aromatic carbon atoms, and the polysulfone has an average molecular weight suitable for film or fiber formation, often at least about 8000 or 10,000. When the polysulfone is not cross-linked, the molecular weight of the polysulfone is generally less than about 500,000 and is frequently less than about 100,000.The repeating units may be bonded, i.e., R1 and R2 may be bonded, by carbon-to-carbon bonds or through various linking groups such as

Particularly advantageous polysulfones are those in which at least one or R1 and R2 comprises an aromatic hydrocarbyl-containing moiety and the sulfonyl moiety is bonded to at least one aromatic carbon atom.

Common aromatic hydrocarbyl-containing moieties comprise phenylene and substituted phenylene moieties, bisphenyl and substituted bisphenyl moieties, bisphenyl methane and substituted bisphenyl methane moieties having the nucleus

substituted and unsubstituted bisphenyl ethers of formula

wherein X is oxygen or sulfur; and the like.In the depicted bisphenyl methane and bisphenyl ether moieties, R3 to R12 represent substituents which may be the same or different and have the structure

where X1 and X2 are the same or different and are hydrogen or halogen (e.g., fluorine, chlorine, and bromine); p is O or an integer, e.g., of 1 to about 6; and Z is hydrogen, halogen (e.g., fluorine, chlorine and bromine),

qis0orl,Yis -0-, -S-, and R13 is hydrogen, substituted or unsubstituted or unsubstituted alkyl, say, of 1 to about 8 carbon atoms, or substituted or unsubstituted aryl, say, monocyclic or bicyclic of about 6 to 15 carbon atoms), heterocyclic with the heteroatom being at least one of nitrogen, oxygen and sulfur and being monocyclic or bicyclic with about 5 to 15 ring atoms, sulfato and sulfono, especially lower alkyl-containing or monocyclic or bicyclic aryl-containing sulfato or sulfono, phosphorous-containing moieties such as phosphino and phosphato and phosphono, especially lower alkyl-containing or monocyclic or bicyclic aryl-containing phosphato or phosphono, amine including primary, secondary, tertiary and quaternary amines often containing lower alkyl or monocyclic or bicyclic aryl moieties, isothioureyl, thioureyl, guanidyl, trialkylsilyl, trialkylstannyl, trialkylplumbyl, dialkylstibinyl, etc. Frequently, the substituents on the phenylene groups of the bisphenyl methane and bisphenyl ether moieties are provided as at least one of-R3 to R6, and R7 to R10 are hydrogen.

The polysulfones having aromatic hydrocarbyl-containing moieties in general posses good thermal stability, are resistant to chemical attack, and have an excellent combination of toughness and flexibility.

Useful polysulfones are sold under trade names such as "P-1700", and "P-3500" by Union Carbide, both commercial products are bisphenol methane-derived polysulfones (specifically, bisphenol A-derived) having a linear chain of the general formula

where n, representing the degree of polymerization, is about 50 to 80. These commercially-available, bisphenol A-derived polysulfones sometimes contain a crystallized fraction which is believed to be an oligomer. With solvents for polysulfone such as dimethyl formamide and dimethyl acetamide, this crystallized fraction may be undissolved and may even increase in crystal size and fraction. This crystallized fraction may increase the difficulty in obtaining desirable anisotropic hollow fiber membranes.The N-acylated heterocyclic solvent used in the method of this invention appear to provide solutions containing these polysulfone polymers which have a very small, if any, fraction of these crystals. Moreover, the polymer solutions exhibit good stability against undue crystal formation; hence, the polymer solution can even be stored prior to use. This stability provides adequate polymer solution life to effect adequate degassification.

With respect to polysulfone polymer, e.g., P-3500, Table A provides a list of solvents and diluents which may be attractive for making membranes, especially hollow fiber membranes, in accordance with the methods of this invention.

TABLE A General liquid carrier Preferred liquid carrier compositions, parts by compositions, parts by weight weight 1-Formyl- 1-Formyl- Diluent piperidine Diluent piperidine Diluent None 100 0 100 0 Formamide 85-98 2-15 87-95 5-13 Ethylene glycol 65-98 2-35 75-95 \ 5-25 Water 90-98 2-10 95-98 2-5 The methods of this invention are particularly advantageous for preparing hollow fiber membranes. With respect of this aspect of the invention, the polymer solution is extruded through a spinnerette in order to form a hollow fiber precursor. While any suitable hollow fiber spinnerette may find application in producing the hollow fibers of this invention, it is preferred that the spinnerette be of the tube-in-orifice type.

Tube-in-orifice type spinnerettes are characterized by having a continuous annular ring surrounding an interior injection tube. Most desirably, the injection tube is concentrically positioned within the orifice such that substantially the same amount of polymer solution is extruded at all points of the annular ring and the resulting hollow fiber precursor has an essentially uniform wall thickness. In mosttube-in-orifice type spinnerettes the polymer solution enters a cavity behind annular ring to assist in the distribution of polymer solution around the annular ring. Sometimes, at least about 4, preferably at least about 5, polymer solution ports are provided, and they may desirably be equidistantly spaced.

In spinning some polymer solutions it may be desired to maintain the polymer solution at an elevated temperature in order to provide desirable spinning conditions. In such instances it is desirable to provide a heating means for maintaining the spinnerette at the desired extrusion temperature. Suitable heating means include electrical or fluid heating jackets, embedded electrical heating coils, and the like. Since the residence time of the polymer solution in the spinnerette is often relatively brief, frequently the polymer solution is heated to at or near the desired extrusion temperature prior to entering the spinnerette.Often the temperature of the polymer solution in the spinnerette is at least about 200C but preferably is not at such high temperatures that during the residence time of the polymer solution at such temperatures, the polymer solution is unduly adversely affected. Frequently, the polymer solution in the spinnerette is at a temperature in the range of about 200 to 90 C, say, about 25 to 800C.

The size of the spinnerette will vary with the desired inside and outside diameters of the sought hollow fiber membrane. One class of spinnerettes may have orifice diameters (outside diameters) of about 300 to 1000 microns and an inside orifice diameter (often the injection tube outside diameter) of about 100 to 500 microns with an injection capillary within the injection tube. The diameter of the injection capillary may vary within the limits established by the injection tube. The projection of the injection tube usually to the plane of the spinerette orifice. Most often, spinnerettes are described in terms of a configuration of the entrance to the annular ring and the ratio of length to width of the annular ring portion of the spinnerette.The entrance configuration to the annular ring porion of the spinnerette may be abrupt, e.g., at an angle greater than about 800 to the direction of flow of polymer solution in the annular ring portion of the spinnerette, or may be sloped into the annular portion of the spinnerette, e.g., at an angle below about 80 C, say, about 750 to 30% from the direction of polymer solution flow through the annular spinnerette. The ratio of the length to which the annular ring portion extends from the face of the spinnerette to the width of the annular ring at spinnerette face may vary within wide ranges; however, the length of the annular ring portion of the spinnerette sould not be so excessive that undue pressure drop is incurred while passing the polymer solution through the spinnerette.Frequently, these ratios vary between about 0.7 to 2.5, say about 1 to 2.

The dimensions of-the hollow fiber precursor will depend on, for instance, the dimensions of the spinnerette, the rate of flow of the polymersolution through the spinnerette, the jet stretch on the hollow fiber precursor, and the rate and pressure of the fluid being injected into the bore of the hollow fiber precursor. The rate of flow of the polymer solution through the spinnerette may vary widely depending upon the take-up speed of the hollow fiber. The rate of flow, however, should not be so great as to cause fracturing of the hollow fiber precursor.Frequently, since the size of the hollow fiber may vary widely, e.g. from about 100 or 150 microns to 1000 or more microns in outside diameter and the length of hollow fiber produced per unit time (spinning speed or take-up speed) may also vary widely, e.g., from about 5 to 100 meters per minute (although higher spinning speeds can be employed providing the fiber is not unduly stretched and sufficient resience time is provided in subsequent processing steps), the spinning operations is often described in terms of jet stretch.Spinning (jet) stretch is herein defined as the ratio of (i) the cubic centimeters per minute of polymer solution extruded through the spinnerett & ivided by the cross-sectional area of the annular extrusion zone in square centimeters to (ii) the length in centimeters of said hollow fiber precursor extruded per minute (i.e., take-up speed). The pinning stretch is often about 0.6 to 2, say about 0.7 to 1.8, preferably about 0.75 to 1.5.

An injection fluid is introduced into the bore of a hollow fiber precursor in an amount sufficient to maintain the bore of the hollow fiber precursor open. The injection fluid may be gaseous, or, preferably, liquid at the conditions of the spinning of the hollow fiber. The injection fluid should be miscible (and is preferably miscible in all proportions) with the liquid coagulant and with the liquid carrier. Often the injection fluid is substantially a non-solvent for the polymer. In many instances the injection fluid need not be a strong non-solvent for the polymer. The injection fluid may contain one or more components, and frequently comprises at least one component of the liquid coagulant in order to enhance miscibility of the injection fluid with the liquid coagulant. Thus, frequently, the injection fluid comprises water.Often, the injection fluid also contains a solvent or weak solvent for the polymer. The temperature of the injection fluid generally approximates The temperature of the polymer solution being extruded and the temperature of the spinnerette because of heat transfer through the wall of the injection capillary positioned within the spinnerette.

However, with suitable insulation or other provisions to minimize heat transfer with the injection fluid in the spinnerette, or the use of an injection fluid which is chilled prior to passing to the spinnerette, the injection fluid may be significantly cooler than the temperature of the polymer solution being extruded. The rate and pressure of the injection fluid being introduced into the bore of the hollow' fiber precursor should be insufficient to result in any deleterious effects to the separation capability or structure of the hollow fiber membrane. Frequently, the rate and pressure of injection fluid is such that the ratio of the inside diameter of the hollow fiber to the inside orifice diameter of the spinnerette is less than about 2.5, and preferably this ratio is about 0.5 to 1.5 or 2.

The spinnerette may be positioned below the level of the liquid coagulant (wet jet) or, preferably, above the liquid coagulant (dry jet). When the dry jet spinning technique is employed, it is preferred that the components of the liquid carrier be substantially non-volatile. The distance above the liquid coagulant at which the spinnerette is positioned (gap) may vary considerably without noticeable effect on the properties of the hollow fiber membrane. Frequently, the spinnerette is positioned within about 20 or 30 centimeters.

Desirable hollow fibers may be produced when the face of the spinnerette is within about 0.5 or less centimeters of the liquid coagulant. For sake of convenience, the spinnerette is often positioned within about 0.5 to 15 centimeters of the liquid coagulant. The dry jet spinning technique is also preferred as a matter of convenience when the polymer solution must be extruded at elevated temperatures and the coagulation liquid is maintained at a temperature substantially below, e.g., at least 25 C, say, at least 300C, below the temperature of the polymer solution at extrusion.

The methods of this invention are particularly advantageous for producing anisotropic hollow fiber membranes of polysulfone, especially polysulfones containing bisphenyl methane (including substituted bisphenyl methlane) and bisphenylether (including substituted bisphenylether) moieties. The polysulfone hollow fiber membranes of this invention exhibit several significant structural characteristics which enable desirable strength properties to be combined with advantageous fluid separation properties. The polysulfone membranes of this invention can be particularly attractive for effecting gas separations.

A dry, integral anisotropic hollow fiber membrane of polysulfone in accordance with the invention has a wall having a thin exterior skin on an open, cellular support. The exterior skin has few, if any, pores (i.e., continuous channels for fluid flowthrough the exterior skin). The cells in the cellular support of the hollow fiber wall are preferably relatively small in all dimensions. Desirably, the cellular support has a substantial absence of macrovoids having a maximum length to maximum width ratio greater than about 10, preferably greater than about 5. The preferred polysulfone hollow fiber membranes of this invention have a substantial absence of macrovoids.

A hollow fiber membrane for gas separations is often evaluated in terms of its gas separation characteristics, i.e., its selectivity for permeating.one gas as compared to at least one other gas of the gaseous mixture and flux of the selectively permeated gas through the membrane wall, and also its strength.

These properties depend on the chemical and-physical nature of the material of the hollow fiber as well as the structure of the hollow fiber. Since morphological structures which play important roles in determining the gas separation characteristics of the hollow fiber membrane may be on the order of tens of angstroms or less in dimension, such structures can not be visually perceived even using the best optical microscopic techniques available. Such small structures are thus termed "submicroscopic" structures.

The combination of microscopic techniques (particularly scanning electron microscopy and transission electron microscopy) and observable performances in characterizing hollow fiber membrane structures on a gross basis and a submicroscopic basis can be highly useful. A particularly useful microscopic technique for characterizing hollow fiber membane structures is scanning electron microscopy. To assist in understanding the use of scanning electron microscopy in describing hollow fiber membranes in accordance with this invention, reference can be made to the provided figures.Figures 1 and 2 are scanning electron microscopic photographs of hollow fiber membranes in which: Figures la to 1c depict a hollow fiber membrane prepared from a polymer solution containing 32 weight percent polysulfone (P-3500), about 7 weight percentformamide and about 61 weight percent 1formylpiperidine. Figure 1 a shows the cross-section of the hollow fiber at a magnification of about 300 times.

Figure 1 b shows a segment of the cross-section of the hollow fiber at the exterior edge of the holow fiber and is at a magnification of about 20,000 times. Figure 1 c shows the structure of the cross-section of the hollow fiber in an area approximately in the middle of the wall of the fiber and is at a magnification of about 20,000 times.

Figure 2 depicts a cross-section of a hollow fiber membrane prepared from a polymer solution containing 36 weight percent polysufone (P-3500) and 64 weight percent 1 -formylpiperidine and is at a magnification of about 300 times.

Samples of hollow fiber membranes (such as depicted in the figures) for examination by scanning electron microscopy can conveniently be prepared by thoroughly drying the hollow fiber, immersing the hollow fiber in hexane and immediately placing the hollow fiber in liquid nitrogen such that the hollow fiber can be fractured. Upon obtaining a clean fracture of the hollow fiber, the specimen can be mounted and then sputter coated with a gold and palladium mixture (using, for instance, a Hummer II sputter coater). The coating procedure usually results in a coating of about 50 to 75 angstroms being placed on the hollow fiber sample.

Accordingly, the dimensions of very small features may be obscured by the coating.

As can be seen especially from Figure 1, the structure of the hollow fiber membranes is characterized by having a relatively dense structure adjacent the exterior surface with an underying open cellular support. An increase in the size of the cells in the hollow fiber wall may be seen as the distance from the exterior surface increases to a generally maximum size in a middle region within the hollow fiber wall. The predominant structure of the cells in the middle region of the cross-section of the hollow fiber wall may be observed to be intercommunicating cells (cells with open passage for gas flow into adjacent cells), i.e., an open, cellular structure. This intercommunication between cells, however, may not be capable of being visually perceived if the cells are very small, e.g., le,ss than 0.1 micron in major dimension.This open, cellular structure enables fluids to readily pass through the hollow fiber wall with minimal resistance. Advantageously, the exterior skin (and interior skin, if any) provides the major portion of resistance to fluid flow through the membrane wall.

The dimensions of the cells, especially larger cells which often occur in the middle region of the hollow fiber wall, can frequently be estimated using, e.g., scanning electron microscopic techniques. In estimating the approximate size and configuration of the cells the observable passages between individual cells are considered to be defects in the cell wall, and therefore, the cell dimensions do not continue through these passages. Another feature of the cells which may have importance in evaluating the strength of the membrane is the thickness of the material of the cell wall defining the larger cells in the hollow fiber membrane wall. These estimations can be conducted by visually inspecting photographs of the cell structure or by using available computer scanning techniques such as image analyzers to inspect and analyze the photograph.Suitable image analyzers include image analyzers useful in analysis of conventional textile fibers such as the Quantimet 720-2 image analyzer available from Quantimet, Munsey, New York.

The configuration of the cells, especially in the larger cells which often exist in the middle region of the hollow fiber wall, can vary widely. A convenient manner for characterizing the configuration of the cell is by dividing the cross-sectional cell area by the square of perimeter of the cell at that cross-section, to provide a configuration ratio. Thus the configuration ratio of a perfect circle would be about 0.8, a square would be about 0.06, and a rod, 0.01 or less. When the configuration ratio is in the range of 0.03 to 0.04, the cross-section of the cells are often roughly polygonal in appearance. Such analyses can be conducted by visually inspecting photographs of the cell structure of the hollow fiber wall or by the use of an image analyzer to inspect photographs.Conveniently, the configuration ratio can be determined by analyzing a random segment of the cross-section of the hollow fiber wall, which segment contains cells which are sufficiently large to be inspected, and which segment has an absence of macrovoids. The area of a segment examined in order to be representative is generally at least about 25 square microns (e.g., a segment having a minimum dimension of at least about 5 microns). In many desirable hollow fiber membranes in accordance with this invention the mean configuration ratio is at least about 0.025, and in many instances is at least about 0.03 or even at least about 0.035. Preferably, segments of the cross-section of the hollow fibers which contain the largest cells (excluding macrovoids) will have a mean configuration ratio of at least about 0.03, e.g., at least about 0.035..

When the cell sizes in a segment of a cross-section of the hollow fiber wall are greater than about 0.1, say, greater than 0.2, micron, analysis of photographs of cell structures as, e.g., perceived through scanning electron microscopy, may be also useful in determining the ratio of mean wall thickness to mean cell cross-sectional area. In general, the greater the ratio of thickness of the polymer supporting the cell to the cross-sectional area of the cell, the stronger the hollow fiber. Frequently, this ratio, especially for the largest cells in the hollow fiber membrane cross-section, will be at least about 0.05, say, at least about 1, and sometimes in the range of about 2 to 20, reciprocal centimeters.In analyzing photographs of smaller cells and smaller wall thicknesses of scanning electron microscope images, the thickness of the reflecting coating may become significant and should be taken into account. Often, at least a majority of the cells capable of being observed in such photographs will also have observable passages communicating with adjacent cells.

Preferably a volume (as determined by cross-sectional area) majority of wall of the hollow fiber consists of cells having a mean major dimension less than about 2 microns. Frequently, at least about 75, say, at least about 90, volume percent of the wall consists of cells having a major dimension less than about 2 microns, preferably less than 1.5, say, less than 1, micron. In some advantageous hollow fiber membranes a substantial absence of cells having major dimensions greater than 2 or 1.5 microns exists. As statedearlier, the size range of the cells in the wall of the hollow fiber membranes usually varies substantially from a size too small to be resolved through scanning electron microscopy to cells having major dimensions of 2 microns or greater.In some hollow fibers in accordance with this invention, the predominant major dimensions of the larger cells in the hollow fiber membrane wall are less than about 0.75 microns, and in some instances essentially all of the cells in the zone of the largest cells in the hollow fiber membrane wall have major dimensions of about 0.1 to 0.7 microns. Often; this zone of the largest cells is at least about 30, say, at least about 40, e.g., up to about 50 or 75, volume percent of the wall of the hollow fiber. Often, desirable hollow fiber membranes have mean major dimensions of cells which are relatively uniform regardless of the segment (say, of at least about 25 square microns in area with a minimum dimension of at least about 5 microns) observed in at least a volume majority (say, at least about 75 volume percent) of the hollow fiber wall.

In some instances hollow fiber membranes can exhibit adequate resistance to collapse even though macrovoids may be present. Macrovoids, when they appear in the hollow fiber, can vary widely in shape and position within the hollow fiber wall. Macrovoids which more closely approach the shape of a circle can generally be tolerated to a greater extent without an undue reduction in resistance to collapse than long, finger-like, macrovoids. Desirably, if macrovoids occur, they occur substantially only in the thickness of the hollow fiber membrane from the interior surface to about 0.5 or 0.75 the distance of the exterior wall.

Preferably, the hollow fiber membrane has a substantial absence of macrovoids having a maximum length to maximum width ratio greater than about 10, say, about 5. The major dimension of the macrovoids is preferably less than about 0.4, say, less than 0.3, times the thickness of the hollow fiber wall.

Scanning electron microscopy may sometimes be useful in examining the porosity of the internal (bore side) skin of the hollow fiber mernbrane. Preferably, the internal skin is highly porous to enable permeating gases to pass into the bore of the hollow fiber with little resistance. A useful microscopic technique for estimating extremely small structural features, e.g., the porosity of the external surface (and possibly the internal surface) of the hollow fiber membranes is the surface replicate technique. In this technique, a metal coating, e.g., a platinum coating, is applied to the surface, and then the polymeric hollow fiber is dissolved from the metal coating.The metal coating is then analyzed using transmission electron miroscopy, e.g., at magnifications of 50,000 times or greater, in order to determine the surface characteristics of the hollow fiber which have been replicated by the metal coating. In general, the most preferred hollow fiber membranes of this invention provide smooth surface replicates of the external surface having a substantial absence of anomalies, or irregularities, (which could represent pores) greater than about 100, often greater than about 75 or even 50, angstroms.

A particularly valuable analytical tool for kinetic evaluations of hollow fiber membranes is the permeability of gases through the membrane. The permeability of a given gas through a membrane of a given thickness (")its the volume of gas, standard temperature and pressure (STP), which passes through the membrane per unit area of membrane per unit of time per unit of partial pressure differential. One method for reporting permeabilities in cubic centimeters (STP) per square centimeter of membrane area per second for a partial pressure differential of 1 centimeter of mercury across the membrane (cm3/cm2-sec-cmHg). Unless otherwise noted, all permeabilities are reported herein at standard temperatures and pressures and are measured using pure gases.The permeabilities are reported in gas permeation units (GPU) which is cm3(STP)/cm2-sec-cmHg x 106; thus, 1 GPU is 1 x 10-6cc(STP)/cm2-sec-cmHg. Several of the manytechniques available for determining permeabilities and permability constants (e.g., the volume of gas (STP) passing through a given thickness of the material of the membrane per unit area per unit time per unit pressure differential across the thickness) are disclosed by Hwang, et al., Techniques of Chemistry, Volume Vll, Membranes in Separations, John Wiley & Sons, 1975 (herein incorporated by reference) at Chapter 12, pages 296 to 322. The permeability of a given gas reflects permeation of that gas through the membrane regardless of the mechanism by which the gas passes through the membrane.

One useful kinetic relationship which is indicative of the freedom from pores in the separating,-or barrier layer, and the range of sizes of the pores in the barrier layer, is by determining the ability of the membrane to separate different molecular weight gases, e.g., low molecular weight gases, of substantially different molecular weights. Suitable gas pairs for this analysis often include one of molecular hydrogen and helium and one of molecular nitrogen, carbon monoxide or carbon dioxide. Preferably, the higher molecular weight gas of the gas pair selected will have a permeability constant (i.e., the intrinsic permeability constant) of the polysulfone at least about 10 times less than the intrinsic permeability constant of the lower molecular weight gas in the polysulfone.The analysis can be conveniently conducted at ambient temeratures (e.g., room temperature of about 25 C) at a pressure of about 7.8 atmosperes absolute on the exterior side of the hollow fiber membrane and about 1 atmosphere absolute on the bore side of the hollow fiber membrane.The polysulfone hollow fiber membranes of this invention exhibit for at least one pair of gases a permeability ratio of (i) the permeability of the lower molecular weight gas (P/f )v, divided by the permeability of the higher molecular weight gas (P/f )H, to (ii) the square root of the molecular weight of the lower molecular weight gas, v/ WL, divided by the square root of the molecular weight of the higher molecular weight gas, \/-EWH, of at least about 6, frequently at least about 7.5.The theoretical maximum permeability ratio is the ratio of the intrinsic separation factor for the gas pair to the quotient of the square root of the molecular weight of the lower molecular weight gas divided by the square root of the molecular weight of the higher molecular weight gas.

An intrinsic separation factor as referred to herein is the separation factor for a material which has no channels for gas flow across the material, and is the highest achievable separation factor for the material.

Such a material may be referred to as being continuous or non-porous. The intrinsic separation factor of a material can be approximated by measuring the separation factor of a compact membrane of the material.

However, several difficulties may exist in the determination of an intrinsic separation factor including imperfections introduced in the preparation of the compact membrane such as the presence of pores, the presence of fine particles in the compact membrane, undefined molecular order due to variations in membrane preparation, and the like. Consequently, the determined intrinsic separation factor can be different than the intrinsic separation factor. Accordingly, a "determined intrinsic separation factor" as employed herein refers to the separation factor of a dry compact membrane of the material.

Another useful method to characterize hollow fiber membranes on a kinetic basis is by determining the permeabilities of a pair of low molecular weight gases having approximately the same molecular weights but one has a permeability constant in the polysulfone at least about 5, preferably, at least about 10, times greater than that of the other gas. Typical gas pairs which have approximately the same molecular weight but substantially differ in permeability constants for many polysulfones suitable for hollow fiber membranes include ammonia and methane; carbon dioxide and propane; etc. This relationship can be expressed as the quotient of the difference between the permeability of the more readily permeated gas, (P/")F, and the permeability of the less readily permeated gas, (P/e)s, divided by (P)s times the permeability constant of the less readily permeated gas, Ps, divided by the permeability constant of the more readily passed gas PF:

In many instances, this relationship is at least about 0.001, say, at least about 0.01, preferably, at least about 0.02. This relationship is indicative of the material of the hollow fiber membrane effecting at least a portion of the separation.

Another useful kinetic analysis of a hollow fibre membrane is the ratio of the permeability of a gas to the permeability constant of the polysulfone for the gas: (P/) p The gas employed to determine this relationship preferably readily permeated the membrane, e.g., hydrogen, helium, ammonia, or the like. Often this ratio is at least about 5x104, say at least about 1 x105, preferably at least about 2x 106, up to about 1 x106, say about 1 xl x105to t6 0.6x 106, reciprocal centimeters. This relationship is indicative of a low resistance to gas flow through the hollow fiber membrane structure and thus the high permeabilities of the desired permeating gas that can be achieved.

The structure of the hollow fiber membranes of this invention can also be observed through other kinetic evaluations. For instance, dried hollow fiber membranes exhibit very little liquid water permeability due to the existence of few, if any, pores in the separating or barrier; layer. The water permeability is often less than about 0.5x10-6, or even less than about0.2x106, say, less than about 0.1 x106, and sometimes less than 0.01 x 1 0-6cm3 (liquid)/cm2-sec-cmHg, even after soaking in room temperature (25 C) water for 4 days.The water permeability determination can conveniently be at room temperture with a pressure drop of 3.4 atmospheres from the feed side to permeate side of the membrane at an absolute pressure of about 4.4 atmospheres on the feed side. Frequently, the maximum pore size is less than about 250 angstroms, and often is less than about 150, e.g., less than about 100 angstroms.

A method for evaluating the size and openness of the cellular support is by subjecting the hollow fibermembrane to the conventional Brunauer, Emmett and Teller (BET) adsorption analysis for determining surface areas. In a BET analysis, a substantially monolayer of a gas, e.g., nitrogen, is sought to be adsorbed on the available internal surface of the hollow fiber and the amount of adsorbed gas is determined and is believed to be proportional to the available surface area. In the BET analysis technique described herein, the analysis is substantially conducted such that the internal surface area of cells which are not open does not contribute significantly to determination of the available surface area. Smaller cells provide more surface area per gram of polymer than larger cells.Consequently, a BET analysis can provide an indication of the openness of cells which are too small to be resolved by, e.g., scanning electron microscopy. Many hollow fiber membranes in accordance with this invention exhibit surface areas as determined by the BET method of at least about 18 or 20, preferably at least about 22, square meters per gram. At very high BET determined surface areas, e.g., above 50 or 70, square meters per gram, the hollow fiber is often observed to have a fibrillarstructure instead of a cellular structure. Fibrillarstructurescan sometimes be relatively weak in comparison to cellular structures. Commonly, the hollow fiber membrane internal surface area as determined by a BET analysis is in the range of about 18 or 22 to 30 square meters per gram.Hollow fibers which have a predominantly closed-cell structure often exhibit BET determined surface areas of less than about 18 or 20 square meters per gram. Another useful method for evaluating surface areas determined by BET analysis is in terms of surface area per unit volume of hollow fiber wall. Frequently, desirable hollow fiber membranes of the invention exhibit surface areas as determined by a BET analysis of about 10 to 30, say, about 10 to 20, square meters per cubic centimeter of hollow fiber wall volume.

The structures of the polysulfone hollow fiber membranes of this invention enable the hollow.fibers to withstand large pressure differentials from the exterior to the bore of the hollow fiber membrane, i.e., the hollow fiber membranes exhibit high collapse pressures. Thus, the hollow fiber membrane can have a relatively low ratio of thickness (wall) to outside diameter while maintaining a desired structural strength and thereby provide an advantageously large bore diameter to minimize the pressure drop to gases passing within the bore.The collapse strength from external pressure of a tube having a foamed wall structure depends upon the strength of the material of the tube, the wall thickness of the tube, the nature of the foamed wall structure, the outside diameter of the tube, the density of the foam and the uniformity of the hollow fiber confiuration (i.e., whether the outside and inside configurations are concentric and circular). In general, the collapse pressure of the hollow fiber membranes of this invention exhibits an approximation relationship to the strength of the material of the hollow fiber and the wall thickness divided by the outside diameter of the hollow fiber (tit). This approximation relationship can be expressed in terms of the tensile strength of the material of the hollow fiber and the cube of the ratio of the wall thickness to the outside diameter.Similar relationships can exist with respect to other strength properties of the material of the hollow fiber such as modulus of elasticity and the like. Frequently, the hollow fiber membranes of this invention exhibit a collapse pressure (kilograms/square centimeters) greater than about 4 Ts (tit)3, where Ts is the tensile strength at yield (or at break if appropriate) in kilograms per square centimeter. More advantageous hollow fiber membranes in accordance with this invention also exhibit a collapse pressure greater than about 1 O(Q)(Ts)(VD)3 where Q is the volume fraction of material of the hollow fiber membrane in the wall of the hollow fiber membrane. The volume fraction of the material of the hollow fiber membrane () is therefore 1 minus the quantity of void volume (volume percent)/100.

Anisotropic membranes can be prepared in accordance with the method of this invention to minimize the separating, or barrier, layer thickness. Frequently, the tendency to produce pores in the membranes is increased when the barrier layer is decreased. In accordance with the teachings of Henis, e al., Belgian Patent No.860,811, corresponding to British Patent Application No.47,269/77, herein incorporated by reference, a coating may be provided in occluding contact with a gas separation membrane containing pores to enhance the selectivity of separation exhibited by the gas separation membrane wherein the coating material does not significantly effect the separation. Usually, suitable membranes for coating in accordance with the invention of Henis, et al., exhibit a permeability ratio as defined above of at least about 6.5, frequently, at least about 7.5.In many instances, the uncoated membranes which are most advantageously coated exhibit a permeability ratio of less than about 40, e.g., less than about 20 or 25. The ratio of the permeability of a gas (preferably a gas which readily permeates the material of the membrane) to the permeability constant of the - material of the membrane for the gas is at least about lx 1 x105, preferably, at least about 2x105, up to, say, 1 x108, reciprocal centimeters. For instance, advantageous polysulfone (e.g., P-3500) hollow fiber membranes (and many other hollow fiber membranes) for coating exhibit a hydrogen flux of about 100 to 1200 GPU or more and exhibit ratios of hydrogen permeability to methane permeability of about 1.5 or 2 to about 10, say, about 2 to 7.

In many instances the selectivity of separation for at least one pair of gases after coating the membrane is at least about 40 percent, say, at least about 50 percent, of that exhibited by an essentially non-porous membrane of the same material, i.e., a non-porous membrane through which gases pass essentially only by interaction with the material of the membrane. The selectivity of separation of the coated membrane may be at least about 35, e.g., at least about 50 or 100, percent greater than the selectivity of separation for said pair of gases which could be provided by the material of the coating when in the form of an essentially non-porous membrane.Moreover, after coating, the flux which can be obtained using the coated membrane is not unduly reduced, e.g., the ratio of permeability of a gas to the permeability constant of the material of the membrane is preferably at least about 5x104, preferably, at least about 1 x105, reciprocal centimeters.

Particularly advantageous materials for coatings to enhance the selectivity of separation of the membranes do not unduly reduce the permeation rate (flux) of the gas or gases desired to be permeated through the wall of membrane. Often, the materials for the coatings have relatively high permeability constants. The material of the coating should be capable of providing occluding cotact with the exterior surface of the membrane.

For instance,when applied it should sufficiently wet and adhere to the hollow fiber to enable occluding contact to occur. The wetting properties of the material of the coating can be easily determined by contacting the material of the coating, either alone or in a solvent, with the material of the membrane. Moreover, based on estimates of the average pore diameter in the separating thickness of the membrane, materials for the coating of appropriate molecular size can be chosen. If the molecular size of the material of the coating is too large to be accommodated by the pores, the material may not be useful to provide occluding contact. If, on the other hand, the molecular size of the material for the coating is too small, it may be drawn through the pores of the membrane during coating and/or separation operations.When the pores are in a wide variety of sizes, it may be desirable to employ a polymerizable material for the coating material which is polymerized after application to the membrane, or to employ two or more coating materials of different molecular sizes, e.g., by applying the materials of the coating in order of their increasing molecular sizes.

The materials for the coating may be natural or synthetic substances, and are often polymers, and advantageously exhibit the appropriate properties to provide occluding contact with the membrane.

Synthetic substances include both addition and condensation polymers. Typical of the useful materials which can comprise the coating are polymers which can be substituted or unsubstituted, and which are solid or liquid under gas separation conditions, and include synthetic rubbers; natural rubbers; relatively high molecular weight and/or high boiling liquids; organic prepolymers; poly(siloxanes) (silicone polymers); polysilazanes; polyurethanes; poly(epichlorhydrin); polyamines; polyimines; polyamides; acrylonitrilecontaining copolymers such as poly(a-chloroacrylonitrile) copolymers; polyesters (including polylactams and polyarylates), e.g., poly(alyl acrylates) and poly(alkyl methacrylates) wherein the alkyl groups have, say, 1 to about 8 carbons, polysebacates, polysuccinates, and alkyd resins; terpinoid resins; linseed oil; cellulosic polymers; polysulfones, especially aliphatic-containing polysulfones; poly(alkylene glycols) such as poly(ethylene glycol), poly(propylene glycol) etc.; poly(alkylene) polysulfates; polypyrrolidones; polymers from monomers having a-olefinic unsaturation such as poly(olefins), e.g., poly(ethylene), poly(propylene), poly(butadiene), poly(2,3-dichlorobutadiene), poly(isoprene), poly(chloroprene), poly(styrene) including poly(styrene) copolymers, e.g., styrene-butadiene copolymer, polyvinyls such as poly(vinyl alcohols), poly(vinyl aldehydes) (e.g., poly(vinyl formal) and poly(vinyl butyral)), poly(vinyl ketones) e.g., poly(methylvinylketone)), poly(vinyl esters) (e.g., poly(vinyl benzoate)), poly(vinyl halides) (e.g., poly(vinyl bromide)), poly(vinyl halides), poly(vinylidene carbonate), poly(N-vinylmaleimide), etc., poly(1,5,-cyclooctadiene), poly(methylisopropenylketone), fluorinated ethylene copolymer; poly(arylene oxides), e.g., poly(xylylene oxide); polycarbonates; polyphosphates, e.g., poly'(etylenemethylphosphate); and the like, and any interpolymers including block interpolymers containing repeating units from the above, and grafts and blends containing any of the foregoing. The polymers may or may not be polymerized after application to the membrane.

Particularly useful materials for coatings comprise poly(siloxanes). Typical poly(siloxanes) can comprise aliphatic or aromatic moieties and often have repeating units containing 1 to about 20 carbon atoms. The molecular weight of the poly(siloxanes) may vary widely, but is generally at least about 1000. Often, the poly(siloxanes have a molecular weightof about 1,000 to 300,000 or more when applied to the membrane.

Common aliphatic and aromatic poly(siloxanes) include the poly(monosubstituted and disubstituted siloxanes), e.g., wherein the substituents are lower aliphatic, for instance, lower alky, including cycloalkyl, especially methyl, ethyl, and propyl, lower alkoxy; aryl including mono or bicyclic aryl including bisphenylene, naphthalene, etc.; lower mono and bicyclic aryloxy; acyl including lower aliphatic and lower aromatic acyl; and the like. The aliphatic and aromatic substituents may be substituted, e.g., with halogens, e.g., fluorine, chlorine and bromine, hydroxyl groups, lower alkyl groups, lower alkoxy groups, lower acyl groups, glycidyl groups, amino groups, vinyl groups, and the like.The poly(siloxane) may be cross-linked in the presence of a cross-linking agent to provide a silicone rubber, and the poly(siloxane) may be a copolymer with a cross-linkable comonomer such as a-methylstyrene to assist in the cross-linking. Typical catalysts to promote cross-linking include the organic and inorganic peroxides. Crnss-Iinking may occur prior to application of the poly(siloxane) to the membrane, but preferably the poly(siloxane) is cross-linked after being applied to the membrane. Frequently, the poly(siloxane) has a molecular weight of about 1,000 to 100,000 or more prior to cross-linking.Particularly advantageous poly(siloxanes) comprise poly(dimethylsiloxane), poly(hydrogenmethylsiloxane), poly(phenylmethylsiloxane), poly(trifluoropropylmethylsiloxane), copolymer of a-methylstyrene and dimethylsiloxane, and post-cured poly(dimethylsiloxane)-containing silicone rubber having a molecular weight of about 1,000 to 50,000 or more prior to cross-linking. Some poly(siloxanes) to not sufficiently wet the material of the hollow fiber to provide as much occluding contact as is desired. However, dissolving or dispersing the poly(siloxane) in a solvent which does not substantially affect the material of the membrane can facilitate obtaining occluding contact.Suitable solvents include normally liquid alkanes, e.g., pentane, isopentane, cyclohexane, etc.; aliphatic alcohols, e.g., methanol, some halogenated alkanes and dialkyl ethers; dialkyl ethers; and the like; and mixtures thereof.

The coating may be in the form of an essentially non-interrupted membrane, i.e., an essentially non-porous membrane, in contact with the anisotropic membrane, or the coating may be discontinuous, or interrupted. Preferably, the coating is not so thick as to adversely affect the performance of the membrane, e.g., by causing an undue decrease in flux or by causing such a resistance to gas flow that the separation factor of the coated membrane is essentially that of the coating. Often the coating may have an average thickness of up to about 50 microns. When the coating is interrupted, of course, there may be areas having no coating material. The coating may often have an average thickness ranging from about 0.0001 to 50 microns. In some instances, the average thickness of the coating is less than about 1 micron, and may even be less than about 5000 angstroms.The coating may comprise one layer or at least two separate layers which may or may not be of the same materials. The coating may be applied in any suitable manner, e.g., by a coating operation such as spraying, brushing, immersion in an essentially liquid substance comprising the material of the coating or the like. The material of the coating is preferably contained in an essentially liquid substance when applied and may be in a solution using a solvent for the material of the coating which is substantially a non-solvent for the material of the membrane. Conveniently, the coating may be applied after the assembly of the membranes for later installation in a permeator, or even after installation in the permeator, to minimize the handling of the coated membrane.

It is frequently possible to treat the exterior surface of a membrane with a densifying agent, e.g., a plasticizer, swelling agent, non-solvent, or a solvent (preferably a weak solvent) for the material of the anisotropic membrane to densify the exterior surface of the membrane. The selection of a suitable densifying agent will depend upon the polymer of the membrane. Densifying agents which may find application include liquids and gases such as ammonia, hydrogen sulfide, acetone, methyl ethyl ketone, methanol, isopentane, cyclohexane, hexane, isopropanol, dilute mixtures of solvents for the polymer (e.g., toluene, benzene, methylene chloride) in non-solvents for the polymer, and the like. The treatment may be conducted before or after the membrane has been dried, and may be conducted over a wide range of temperatures.Temperatures of about 0 to 500C, say, about 10' to 35 C, are usually employed. Treatment of the membrane with a densifying agent prior to coating to increase selectivity may be advantageous in some instances.

The membranes of this invention are widely applicable in gas separation operations, particularly the separation of low molecular weight gases, by interaction of the permeating gases with the material of the membrane. Gaseous mixtures which may be employed in gas separation operations are comprised of gaseous substances, or substances that are normally liquid or solid but are vapors at the temperatures under which the separation is conducted.Typical gas separation operations which may be desired include separations of, for example, oxygen from nitrogen; hydrogen from at least one of carbon monoxide, carbon dioxide, helium, nitrogen, oxygen, argon, hydrogen sulfide, nitrous oxide, ammonia, and hydrocarbon of 1 to about 5 carbon atoms, especially methane, ethane, and ethylene; ammonia from at least one of hydrogen, nitrogen, argon, and hydrocarbon of 1 to about 5 carbon atoms, e.g., methane; carbon dioxide from at least one of carbon monoxide and hydrocarbon of 1 to about 5 carbon atoms, e.g., methane; helium from hydrocarbon of 1 to about 5 carbon atoms, e.g., methane; hydrogen sulfide from hydrocarbon of 1 to about 5 carbon atoms, for instance, methane, ethane, or ethylene; and carbon monoxide from at least one of hydrogen, helium, nitrogen, and hyrdocarbon of 1 to about 5 carbon atoms. It is emphasized that membranes in accordance with this invention may find beneficial application in the separation operations using other gas mixtures.

The following examples are provided to assist in the understanding of the invention and are not in limitation of the invention. All parts and percentages of gases are by volume and all parts and percentages of liquids are by wight, unless otherwise noted. All gas permeabilities are determined using substantially pure gases with a shell side pressure of about 70 atmospheres absolute and about 1 atmosphere at the bore side of the hollow fiber membrane unless otherwise stated.

Example 1 Thirty two parts by weight of polysulfone (P-3500, obtainable from Union Carbide Corporation) and 68 parts by weight of a liquid carrier consisting of 90 parts by weight of 1-formylpiperidine and 10 parts by weight of formamide (reagent grade) are charged to a heated dope mixer having a dry nitrogen atmosphere.

Prior to being charged to the dope mixer, the polysulfone polymer is dried about 125"C for five to seven days under vacuum (e.g., at an absolute pressure of less than about 10 or 20 millimeters of mercury). The formylpiperidine has a moisture content of about 0.45 grams per 100 milliliters and a formamide, about 0.15 grams per 100 milliliters. The exposure of formamide to air during the charging of the heated dope mixer is minimized to avoid degradation of the formamide. In the heated dope mixer the polymer solution is maintained at about 80 C to 1000C for a time sufficient to completely dissolve the polysulfone (residence times of about 8 hours are generally employed).The polymer solution is transferred to a deaerator having a dry nitrogen atmosphere which is heated to about 80 C. The deaerator contains a conical-shaped partition with a cup at the apex of the partition. The polymer solution when fed to the holding tank is introduced into the cup at below the level of polymer solution in the cup and the polymer solution flows over the edge of the cup and down the conical-shaped partition in the form of a thin film. The polymer solution passes from the lower edge of the conical-shaped partition onto a wire mesh which leads into the polymer solution in the deaerator. A recycle stream of the polymer solution in the deaerator is maintained during deaeration. The deaerator is maintained at an absolute pressure of about 250 to 350 millimeters of mercury.Satisfactory degassing may also be achieved using substantially atmospheric pressure in the deaerator. Adequate deaeration is generally achieved in less than about 7 hours residence time in the deaerator.

The deaerated polymer sqlution is pumped through a Zenith-type H pump at a rate of 8.6 grams per minute to a tube-in-orifice-type spinnerette having an orifice diameter of 533 microns, an injection tube outside diameter of 203 microns and an injection capillary diameter of 127 microns. The spinnerette has five equidistant polymer solution entrance ports positioned behind the annular extrusion zone and is maintained at a temperature of approximately 50 to 58 C by the use of an external, electrical heating jacket; and the temperature of the polymer solution approximates this temperaure. Deionized water at about ambient temperature (20 to 25 C) is fed to the spinnerette at a rate of about 1.2 milliliters per minute.While in the spinnerette, the deionized water is warmed by heat transfer. The spinnerette is positioned about 10.2 centimeters above the coagulation bath. The hollow fiber precursor is extruded at a rate of about 42.7 meters per minute.

The hollow fiber precursor passes downwardly from the spinnerette into an elongated coagulation bath. The coagulation bath contains substantially tap water and sufficient fresh tap water is added and coagulation bath liquid (liquid coagulant) purged to maintain the concentration of 1-formylpiperidine in the coagulation bath less than about one weight percent. The liquid coagulant is maintained at a temperature of about 2 to 4 C. The hollow fiber precursor passes vertically downward into the liquid coagulant for a distance of about 7.8 centimeters, passes around a roller to a slightly upwardly slanted path through the liquid coagulant and then exits from the coagulation bath. The distance of immersion in the coagulation bath is about 1.4 meters.

The hollow fiber from the coagulation bath is then washed with tap water in three sequential godet baths.

In the first godet bath, the hollow fiber is immersed for a distance of about 7.3 meters. The first godet bath is maintained at a temperature of about 2 to 4 C and the concentration of 1 -formylpiperidine in the bath is maintained below about three weight percent by the addition of fresh water and purging of the washing medium. The second and third godet baths are at ambient temperature (e.g., about 15 to 25 C depending on the temperature of the tap water and the laboratory room temperature} and fresh water is continuously added to the godet baths. The wet hollow fiber has an outside diameter of about 440 microns and an inside diameter of about 155 microns.

The hollow fiber, while being maintained wet with water, is wound on a bobbin using a Leesona winder with as little tension as possible. The bobbin contains about 3600 meters of hollow fiber, and the thickness of hollow fiber wound on the bobbin is less than about 2 centimeters. The bobbin is placed in a vessel containing tap water, and tap water is added and purged from the vessel for about 24 hours at a rate of about 4 to 5 liters per hour per bobbin at tap water temperatures (10 to 20 C). The bobbin is then stored for seven days in the vessel containing tap water at about ambient temperature (20 to 25 C). After storage, the hollow fiber, while being maintained wet, is wound on a skeiner to form hands of hollow fiber about 3 meters in length. The winding of the hollow fiber on the skeiner is conducted using the minimum tension required to effect the winding. The hanks of hollow fiber are hung vertically with the bores of the hollow fibers open at the bottom and are allowed to dry for at least about three days at ambient laboratory conditions (about 20t25 C and 50 percent relative humidity). The dried hollow fiber has an outside diameter of about 390 microns and an inside diameter of about 140 microns and appears, under scanning electron microscopy, to resemble the hollow fiber depicted in Figure 1.

Test loops of ten hollow fiber each of about 15 centimeters in length are prepared. At one end, the test loop is embedded in a tube sheet through which the bores of the hollow fibers communicate. The other end is plugged. Th-e bores ofthe hollow fibers in each test loop are subjectedto a vacuum (about 50 to 100 millimeters of mercury absolute pressure) for about 10 minutes. Each of the test loops is immersed in a coating solution of about 2 weight percent post-curable polydimethylsiloxane (Sylgard 184 obtainable from Dow Corning Corp.) in isopentane while maintaining the vacuum for about 10 minutes and then -removed from the coating solution.The vacuum is contained for about 10 minutes after the test loop is removed from the coating solution, and the test loops are dried at ambient laboratory conditions (20 to 25 C, about 40 to 60percent relative humidity). The permeabilities of the test loops to hydrogen and methane are determined using essentially pure gases at ambient laboratory temperature (about 20 to 25 C) with the exterior, feed surface side (shell side) ofthe hollow fibers being at a pressure of about 70 atmospheres absolute and the bore side of the hollow fiber being at about 1 atmosphere absolute.The determined permeabilities generally vary somewhat from test loop to test loop, and some variations may be due to leaks in the tube sheet, damage to the hollow fiber due to handling, or the like. The average hydrogen permeability often ranges from about 75-to 150 GPU (1 x106 cc(STP)/cm2-sec-cmHg) and the average methane permeability often ranges from about 1 to 2 GPU. The ratio ofthe permeabilities (separation factor) for hydrogen over methane is often about 50 to 60. The failure pressure of the hollow fiber is determined to be over 110 kilograms per square centimeter (1600 pounds per square inch).

Example 2 A degassedpolymer solution containing 32 weight percent polysulfone P-3500 and a liquid carrier consisting of 1 -formylpiperidine.and formamide is spun through a spinnerette (jet) of the type described in Example 1 into a hollow fiber membrane in accordance with the procedure set forth in Table I. The polymer solution (dope) temperature for runs 885-1, 533-1-7, and and 537-1 is not direct!y measured and may be 60 to 70 C. Deionized water is employed as the injection fluid and is provided at a rate sufficient to maintain the inside diameter of the hollow fiber.The coagulation bath comprises water, and the concentration of lformylpiperidine in the coagulation bath is maintained below about one percent by weight through purging.

The hollowfiber is immersed in the coagulation bath for a distance of about 1 meter. The residence time of the hollow fiber in each of the three wash baths (first, second and third godet baths) is as follows Approximate spinning speed, Approximate residence time meters per minute in each wash bath, seconds 30.5 56.9 36.6 47.4 38.1 45.5 42.7 40.6 The hollow fiber is wound on a bobbin while being maintained wet, and the bobbin is washed in cold running tap water for about 16 to 24 hours, then stored in a bucket filled with water for seven days.The hollow fiber is then dried under ambient laboratory conditions (22 to 26C and about 40 to 60 percent relative humidity), and formed into test loops, coated and tested as described in connection with Example 1 except that a 1 weight percent Sylgard 184 solution is employed and twenty hollow fibers are used in the test loop. The observed performance of the hollow fiber is provided in Table II.

TABLE I Temperature, C 1-formylpiperidine Distance of concentration in Fiber* Jet size, Dope Spinning Dope temp. jet from liquid carrier, dimensions OD( ),ID Jet rate speed, at let, coagulation Run no. wt. OD( )/ID( ) capillary( ) stretch cc/min. cc/min C bath, cm 885-1 92.5 416/134 457/127/76 0.96 6 36.6 About 60 12.7 3 4 RT RT 533-1-7 92.5 410/136 457/127/76 0.96 6 38.1 About 60 12.7 3 4 RT RT 637-1 92.5 416/136 457/127/76 0.96 6 38.1 About 60 12.7 3 3 RT RT 893-4-6 92.5 425/150 533/203/127 1.15 7.1 42.7 53 10.2 2 1- 1.5- RT 3 20 895-1 90 456/150 533/203/102 0.98 8.3 42/7 53 10.2 1 1 RT RT 895-2 90 448/148 533/203/102 0.98 8.3 42.7 55 10.2 1 1 RT RT 894-1 87 456/152 533/203/102 0.98 8.3 42.7 52 10.2 3 3 RT RT 894-2 87 448/149 633/203/102 0.98 8.3 42.7 54 10.2 3 3 RT RT 204-3 90 479/217 457/127/76 0.7 6 30.5 55 5.1 1 1 11 11 204-6 90 445/217 457/127/76 0.7 6 30.5 65 5.1 1 1 11 11 893-2 92.5 406/146 635/178/102 1.75 7.1 42.7 64 10.2 2 3 RT RT *Before drying TABLE II Permeability, GPU** Approximate Separation collapse pressurs, Run no. Hydrogen Methane factor kg/cm 885-1 124 1.8 70 140+ 533-1-7 187 4.2 45 125 537-1 157 2.7 58 125 893-4-6 132 2.4 56 125 895-1 98 0.97 101 155 895-2 117 1.53 77 155 894-1 97 1.2 83 110 894-2 130 1.7 78 155 204-3 89 1.2 74 70-110 204-6 142 2.3 63 70-110 893-2 131 8.8 15 125 **Based on fiber dimension before drying.

Example 3 The procedure of Example 2 is substantially repeated except as follows. The degassed polymer solution consists essentially of 35 weight percent polysulfone P-3500 and 64 weight percent 1-formylpiperidine, the spinnerette has an outside diameter of 533 microns, an inner diameter of 203 microns and an injection capillary diameter of 127 microns. The spinnerette is positioned about 10.2 centimeters above the liquid level in the coagulation bath, the polymer solution feed rate to the spinnerette is about 11.8 cubic centimeters per minute' and the spinning speed is about 42.7 meters per minute.The temperature of the polymer solution extruded from the spinnerette is about 54 C. The coagulation bath is at a temperature of about 1 to 2 ; the first godet bath, about 1" to 2 C, and the second and third godet baths, about 20 C. The residence time of the hollow fiber in each of the godet baths is about 40.6 seconds. The hollow fiber has an outside diameter of 445 microns and an inside diameter of 166 microns prior to drying. A scanning electron microscope photograph of a dried hollow fiber in accordance with this Example is depicted in Figure 2. The collapse pressure exhibited by the hollow fiber is about 170 kilograms per square centimeter.The hollow fiber is formed into test loops, coated (except using a 2 weight percent Sylgard 184 solution) and tested as described in connection with Example 1. The hydrogen and methane permeabilities of the uncoated hollow fiber are about 39 GPU and 2.1 GPU. A similarly prepared hollow fiber except that the temperature of the coagulation bath is about 5"C, has an inside diameter (wet) of about 190 microns, and exhibits a collapse pressure of about 100 to 150 kilograms per square centimeter and, when coated and in test loops, a hydrogen permeability of about 100 GPU and a methane permeability of about 3.3 GPU.

Example 4 The procedure of Example 2 is substantially repeated except as follows. A degassed polymer solution of 36 weight percent polysulfone P-3500 and 64 weight percent of a mixture of 87 parts by weight of 1 -formylpiperidine and 13 parts by weight of formamide is spun at a temperature of about 52 C through a spinnerette having an outside diameter of 457 microns, an inside diameter of 127 microns and an injection capillary diameter of 76 microns at a rate of 5 cubic centimeteres per minute. The spinnerette is positioned 10.2 centimeters from the liquid level in the coagulation bath. The spinning speed is about 30.5 meters per minute. The coagulation bath and first godet baths are at about 3 C, and the second and third godet baths are at about room temperature.The residence time in each of the three godet baths is about 56.9 seconds. The hollow fiber is stored in water in a storage bucket for 1,2 and 24 days instead of 7 days, and samples of the hollow fibers are analyzed for hydrogen and methane permeabilities, collapse pressures, and solvent content after each of the storage periods. The hydrogen and methane permeabilities of a test loop containing the sample of the uncoated hollow fiber are determined for the hollow fiber stored one day using a shell side pressure of about 7.8 atmospheres absolute and a bore pressure of about 4.4. atmospheres absolute and for the coated hollow fibers stored for 1,2 and 24 days using a shell side pressure of about 70 atmospheres absolute and a bore pressure of about 1 atmosphere absolute. The hollow fiber, when dry, has an outside diameter of 456 microns and an inside diameter of 188 microns. The results are provided in Table Ill.

TABLE Ill Days in storage bucket: 1 2 24 Uncoated H2 457 permeability, GPU Uncoated CH4 167 - permeability, GPU Coated H2 93 102 114 permeability, GPU Coated CH4 2.8 3.3 1.9 permeability, GPU Collapse pressure, 140 140 170 kg/cm2 l4ormylpiperidine in 2.25 1.46 0.95 fiber, weight % Example 5 The procedure of Example 4 is substantially repeated except that the spinning rate is about 45.7 meters per minute and the residence time in each of the godet baths is about 38 seconds. In one run, a chimney is positioned between the spinnerette and the liquid level in the coagulation bath such that a saturated atmosphere exists within the chimney. The spinnerette is positioned 10.2 centimeters from the liquid level in the coagulation bath.Another run is conducted which is substantially identical except that no chimney is employed and the spinnerette is positioned 5.1 centimeters above the liquid level in the coagulation bath.

Hollow fiber samples are formed into test loops, coated and hydrogen and methane permeabilities determined after 7 days storage in water. The results are as follows: Chimney No chimney H2 permeability, GPU 129 123 CH4 permeability, GPU 1.5 1.3 Outside diameter, microns 370 364 Inside diameter, microns 171 165 Example 6 The procedure of Example 2 is substantially repeated except as follows. A degassed polymer solution consisting of 38 weight percent polysulfone P-3500 and 62 weight percent liquid carrier of 87 parts by weight 1 -formylpiperidine and 13 parts by weight formamide is spun at about 70 C through a spinnerette having an outside diameter of 635 microns, an inside diameter of 228 microns, and an injection capillary diameter of 152 microns at a rate of 6 cubic centimeters per minute.The spinnerette is positioned about 2.5 centimeters above the liquid level in the coagulation bath, and the spinning rate is about 30.5 meters per minute. The coagulation bath and first godet bath are at about 3 C and the seconds and third godet baths are at about room temperature. The resistance times of the hollow fiber in each of the three godet baths is about 56.9 seconds. The coagulation bath contains 7.5 weight percent acetic acid. The dried hollow fibers have an outside diameter of 444 microns and an inside diameter of 171 microns. The hollow fiber exhibits a collapse pressure of about 180 kilograms per square centimeter.Samples of the hollow fiber are fabricated into test loops and hydrogen and methane permeabilities are determined both before and after coating. (The uncoated permeabilities are determined at a shell pressure of about 7.8 atmospheres absolute and a bore pressure of about 1 atmosphere absolute). The results are as follows: Uncoated Coated H2 Permeability, GPU 409 136 CH4 Permeability, GPU 107 2.1 Example 7 The procedure of Example 6 is substantially repeated except that the spinnerette has an outside diameter of 483 microns; an inside diameter of 152 microns, and an injection capillary diameter of 76 microns, and the hollow fibers are stored in a bucket under different conditions as set forth in Table IV.

TABLE IV Days stored Stored in water : iso in bucket Stored in water propanol 5:5 by weight Collapse pressure, kg/cm 7 175 Collapse pressure, kg/cm 28 175 Weight percent 1-formiylpiperidine in fiber 7 2.45 0.85 Weight percent 1-formiylpiperidine in fiber 28 1.93 0.35 Outside diameter, microns - 439 427 Inside diameter, microns - 131 125 Permeabilities, GPU Permeabilities, GPU Coated Uncoated Coated Uncoated H2 7 205 759 243 2330 CH4 7 5.7 262 9.27 1060 H2 28 140 854 108 CH4 28 3.1 321 24 - Example 8 The procedure of Example 2 is substantially repeated except as follows.The polymer solution contains 32 weight percent polysulfone P-3500 and 68 weight of a liquid carrier of 85 parts by weight of 1 -formylpiperidine and 15 parts by weight of ethylene glycol and is spun at about 70 C through a spinnerette having an outside diameter of 635 microns, an inside diameter of 229 microns, and an injection capillary diameter of 152 microns at a rate of 7.2 cubic centimeters per minute. The spinning rate is about 42.7 meters per minute. The spinnerette is about 10.2 centimeters from the liquid level in the coagulation bath. The coagulation bath and first godet baths are at about 3 C, and the second and third godet baths are at about room temperature. The residence time of the hollow fiber in each of the three godet baths is about 40.6 seconds.The outside diameter of the hollow fiber is about 456 microns and the inside diameter is about 205 microns. The hydrogen permeability of the coated hollow fiber is about 64 GPU and the carbon monoxide permeability is about 0.65 GPU.

Example 9 The procedure of Example 2 is substantially repeated except as follows. The polymer solution contains 32 weight percent polysulfone P-3500 and 68 weight percent liquid carrier having 90 parts by weight 1-formylpiperidine, 5 parts by weight ethylene glycol, and 5 parts by weight water. The polymer solution is spun at 65-70 C through a spinnerette having an outside diameter of 508 microns, an inside diameter of 152 microns, and an injection capillary diameter of 102 microns. The spinning rate is 43.5 meters per minute. The spinnerette is about 17 centimeters from the liquid level in the coagulation bath. The coagulation bath and first godet bath are at about 5 C and the second and third godet baths are at about room temperature. The residence time of the hollow fiber in each of the three godet baths is about 38 seconds.The outside diameter of the hollow fiber is about 450 microns and the inside diameter is about 236 microns. The hydrogen permeability of the coated hollow fiber is about 100 GPU, and the carbon monoxide permeability is about 2.3 GPU. The permeabilities are determined using a gas mixture of about 25 mole percent hydrogen and 75 mole percent carbon monoxide using a shell side pressure of about 150 centimeters of mercury absolute and laboratoryvacum on the bore side of the hollow fibers.

Example 10 The procedure of Example 2 is substantially repeated except as follows. The polymer solution is prepared 32 weight percent polysulfone P-3500 and 68 weight percent liquid carrier having 80 parts by weight 1 -formylpiperidine,10 parts by weight ethylene glycol, and 10 parts by weight acetone. The concentration of the acetone is not determined after degassing. The polymer solution is spun at about 80 C through a spinnerette having an outside diameter of 635 microns, an inside diameter of 228 microns, and an injection capillary diameter of 152 microns. The spinning rate is 26.2 meters per minute. The spinnerette is about 20.3 centimeters from the liquid level in the coagulation bath.The coagulation bath and first godet bath are at about 00C and the second and third godet baths are at about 14 C. The residence time of the hollow fiber in each of the three godet baths is about 66 seconds. The bore of the hollow fiber is off-center and the outside diameter of the hollow fiber (wet) is about 342 microns and the inside diameter is about 165 microns. The hydrogen permeability of the uncoated hollow fiber is about 225 GPU, and after coating, about 82 GPU, and the methane permeability of the uncoated hollow fiber is about 50 GPU, and after coating about 1.6 GPU.

Example 1 1 The procedure of Example 2 is substantially repeated except as follows. The polymer solution contains 40 weight percent polysulfone P-1700 (a lower molecular weight polysulfone but otherwise similar to P-3500 available from Union Carbide Company) and 60 weight percent of a liquid carrier having 87 parts by weight l4ormylpiperidine and 13 parts by weight formamide. The polymer solution is spun at about 70"C through a spinnerette having an outside diameter of 635 microns, an inside diameter of 229 microns, and an injection capillary diameter of 152 microns at 9 cubic centimeters per minute. The spinnerette is about 7.6 centimeters from the liquid level in the coagulation bath. The spinning rate is about 30.5 meters per minute.The coagulation bath and first godet bath are at about 4"C, and the second and third godet baths are at about room temperature. The residence time of the hollow fiber in each of the three godet baths is about 56.9 seconds. The outside diameter of the hollow fiber is about 439 microns and the inside diameter is about 154 microns. The collapse pressure exhibited by the hollow fiber is about 175 kilograms per square centimeter.

Samples of the hollow fibers are stored in water for 7 or 22 or 23 days, dried and assembled into test loops.

The test loops are treated in several manners. Some test loops prior to coating, are flushed with isopentane.

In this procedure, the bores of the hollow fibers are subjected to a vacuum for about 10 minutes, immersed in isopentane at room temperature (about 22" to 25'C) for 10 minutes while maintaining the vacuum on the bore side of the hollow fibers, and then removed from the isopentane and the vacuum is maintained for an additional 10 minutes. Also, test loops (coated and uncoated) are suspended in a closed container of gaseous ammonia at room temperature and at a pressure of about 18 kilograms per square centimeter. Both coated and uncoated hollow fibers are subjected to this ammonia treatment. The permeabilities of the hollow fiber for hydrogen and methane are determined using different shell side pressures. The bore side pressure is about 1 atmosphere absolute. The results are provided in Table V.

TABLE V Coated after isopsentane Isopentane Coated after flush permeability, GPU Uncoated flush Coated isopentane slush at 68 atm.

Ammonia Permeability, Permeability, Permeability, Permeability, 1 day 3 day treatment, Days stored GPU at GPU at GPU at GPU at ammonia ammonia days in bucket 7.8 atm.* 7.8 atm. 7.8 atm. 68 atm. 7.8 atm. 68 atm. treatment treatment H2 0 7 626 270 - - 178 - - CH4 0 7 213 23 - - 3.5 - - H2 0 22 380 193 193 143 161 140 - CH4 0 22 94 19 4.0 2.4 2.2 2.3 - H2 0 23 518 246 - - 131 127 68 62 CH4 0 23 143 40 - - 2.6 1.9 1.0 1.3 H2 3 23 39 34 30 29 - - - CH4 3 23 0.18 1.9 0.26 0.2 - - - *All pressures are absolute pressures at sell side.

Example 12 The procedure of Example 2 is substantially repeated except as follows. The polymer solution contains 34 weight percent polysulfone P-3500 and 66 weight percent of a liquid carrier comprising a mixture of 42.4 grams of substantially anhydrous lithium chloride per 1000 milliliters of solution with l4ormylpiperidine.

The polymer solution is spun at about 70 to 80 C through a spinnerette having an outside diameter of 635 microns, an inside diameter of 228 microns, and an injection capillary diameter of 152 microns. The spinnerette is 3.5 centimeters above the liquid level in the coagulation bath. The spinning rate is about 20 meters per minute. The coagulation bath is at 5 C, the first godet bath is at 5 C, and the second and third godet baths are at about room temperature. The residence time in each of the three godet baths is about 86 seconds. The outside diameter of the hollow fiber is about 685 microns, and the inside diameter is about 380 microns. The permeabilities are determined using the procedure described in Example 9.The coated hydrogen permeaility is about 80 GPU; and the coated carbon monoxide permeability is about 2.9 GPU. The hollow fiber has a fine, nodular structure as observed from scanning electron microscope photographs and exhibits an internal surface area of about 75 square meters per gram using a B.E.T. analysis.

Example 13 The procedure of Example 2 is substantially repeated except as follows. The degassed polymer solution contained 32 weight percent poly(phenylene ether) sulfone available from ICI, Ltd., Great Britain and 68 weight percent of a liquid carrier consisting of 90 weight percent 1 -formylpiperidine and 10 weight percent formamide. The spinnerette dimensions are outside diameter, 533 microns; inside diameter, 203 microns, and the injection capillary diameter, 134 microns. The polymer solution rate through the spinnerette is about 7.1 cubic centimeters per minute and the take-up rate is about 42.4 meters per minute.The coagulation and first wash baths are at about 1" to 2 C and the second and third wash baths are at about 19 C. The approximate polymer solution temperature is varied from about 40 to 55 C and the dimension of the hollow fiber membranes before drying are outside diameter about 435 microns and inside diameter about 150 microns; however, one sample has an oval bore configuration. The observed performances of the hollow fiber membranes are provided in Table Vl.

TABLE Vl Approximate Permeabilities, GPU* Run no. dope temperature 'C Uncoated (7.8) Coated (68) H2 CH4 H2 CH4 1 55 325 120 41 1.3 2 50 400 140 47 1.0 3 40 170 50 35 0.3 *All permeabilities determined with a bore side pressure at about one atmosphere absolute. The shell side pressures in atmospheres absolute are provided parenthetically.

Example 14 The procedure of Example 2 is substantially repeated except as follows. The degassed polymer solution contains 32 weight percent polysulfone (P-3500) and 68 weight percent of a liquid carrier consisting of about 90 weight percent 1 -acetyl piperidine and 10 weight percent formamide. The spinnerette dimensions and outside diameter, 584 microns; inside diameter, 178 microns; and injection capillary diameter, 101 microns.

The spinnerette is positioned about 5 centimeters above the level of the liquid coagulant and is at a temperature of about 57 C. The take-up rate is about 42.4 meters per minute. The coagulation and first wash baths are at about 2 C and the second and third wash baths are at about 16 C. Further details are provided in Table VII.

TABLE VII Permeabilities, GPU** Water injection Coated (68) Dope rate Rate Pressure Fiber dimensions* Run no. cc/min. cc/min. mmHg OD(/lD(u) H2 CH4 1 8.4 0.75 65 455/150 70 0.9 2 9.9 0.75 50 500/150 73 0.7 3 8.4 0.6 35 465/130 53 1.4 * Before drying ** All permeabilities determined with a bore side pressure of about one atmosphere absolute. The shell side pressures in atmospheres absolute are provided parenthetically.

Example 15 A hollow fiber is prepared in accordance with the procedure substantially set forth in Run No. 533-1 of Example 2 except that the hollow fiber is stored in water for over seven days. Samples of the hollow fiber are dried in a temperature and humidity controlled atmosphere (air) under different conditions and then fabricated into a test loop. The hydrogen and methane permeabilities of coated and uncoated hollow fiber are determined. The results-are provided in Table VIII.

TABLE VIII Permeabilities, GPU* Drying Relative Absolute Uncoated Coated temperature humidity Humidity C % g/m H2 CH4 H3 CH 10 50 4.7 739(7.8) 205(34) 4.7(68) 15 77 9.8 - 730(7.8) 126(68) 2.8(68) 22 50 9.8 - 795(7.8) 163(34) 4.1(68) 26 50 11.5 1128(7.8) 413(7.8) 160(34) 4.0(68) 35 25 9.8 - 771(7.8) 158(34) 3.6(68) 40 50 26 1343(7.8) 496(7.8) 138(68) 2.7(68) 60 50 65 603(7.8) 193(7.8) 94(68) 1.1(68) 80 50 147 252(7.8) 62(7.8) 68(68) 0.6(68) *All permeabilities determined with a bore side pressure of about one atmosphere absolute. The shell side pressures in atmospheres absolute are provided parethetically.

Example 16 The procedure of Example 1 is substantially repeated except as follows. The polysulfone is dried for about - 2 days at atmospheric pressure under a forced, dry air atmosphere. the deaeration of the polymer solution is conducted at atmospheric pressure under an atmosphere of dry air. The polymer solution is extruded through the spinnerette at about 8.1 cubic centimeters per minute and the take-up speed is about 42.4 meters per minute. The outside diameter of the wet hollow fiber is about 450 microns and the inside diameter is about 155 microns. When the hanks of hollow fibers are hung to dry, the hollow fibers remain looped, i.e., are not open at the bottom. The dried hollow fiber membranes have an outside diameter of about 400 microns and an inside diameter of about 140 microns.

Example 17 The procedure of Example 12 is substantially repeated except as follows. The polymer solution contains 34 weight percent of a suspension polymerized copolymer of 33 weight percent acrylonitrile and 67 weight percent styrene in 66 weight percent of a liquid carrier containing 75 weigt percent l4ormylpiperidine and 25 weight percent formamide. The spinnerette is about 0.6 centimeter above the level of the liquid coagulant, and the take-up rate is about 13 to 14 meters per minute. The coagulation bath and first wash bath are at about 2 to 3 C and the second and third wash baths are at about 17 C. The hollow fibers are dried and dipped in methanol or pentane and then exposed to achieve vapor at low partial pressure.The hollow fiber membranes are then coated with a solution of about 3 weight percent sylgard in pentane. The coated hydrogen permeability is about 20 to 40 GPU, and the separation factor for hydrogen over carbon monoxide is about 20 to 40.

Example 18 The procedure of Example 12 is substantially repeated except as follows. The polymer solution contains 30 weight percent of a methyl brominated poly(phenylene oxide) (about 45 mole percent brominated) in 70 weight percent 1 -formylpiperidine. The spinnerette is about 2.6 centimeters above the level of the liquid coagulant, and the take-up rate is about 13 to 14 meters per minute. The coagulation and first wash baths are at about 46 to 48 C and the second and third wash baths are at about 14 C. When coated, the hollow fiber membranes exhibit a hydrogen permeability of about 90 to 100 GPU and a separation factor for hydrogen over carbon monoxide of about 13.

Claims

1. In a method for making an anisotropic membrane in which a polymer solution of a membrane-forming polymer in a liquid carrier containing solvent for the membrane-forming polymer is provided in the form of a precursor and then is coagulated in a liquid coagulant comprised of water, the improvement wherein the liquid carrier comprises N-acylated heterocyclic solvent having the structural formula

wherein X is -CH2-, -N(R')-, or -0-; R is hydrogen, methyl or ethyl; and R' is hydrogen or methyl.

2. A method of claim 1 in which an anisotropic hollow fiber membrane is made, a said method comprising: a. extruding said polymer solution through an annular spinnerette to form a hollow fiber precursor and said polymer solution during extrusion being at a temperature sufficient to substantially maintain said polymer in solution; b. injecting a fluid into the bore of said hollow fiber percursor as it is being extruded from said spinnerette, said injection fluid being miscible with said liquid carrier and being injected at a rate sufficient to maintain the bore of said hollow fiber precursor open;; c. contacting the exterior of said hollow fiber precursor with said liquid coagulant, said liquid coagulant being essentially comprised of water and containing less than about 5 weight percent of said liquid carrier, said liquid coagulant being miscible with said liquid carrier and injection fluid, and said contact being sufficient to coagulate polymer in said hollow fiber precursor at the conditions of the liquid coagulant to provide a hollow fiber; and d. washing said hollow fiber with non-solvent for said polymer to reduce the content of said liquid carrier in said hollow fiber to less than about 5 weight percent based on the weight of the polymer in said hollow fiber.

3. The method of claim 1 or claim 2 wherein the N-acylated heterocyclic solvent comprises at least one of 1 -formylpiperidine, 1 -acetylpiperidine, 1 -formylmorpholine, and 1 -acetylmorpholine.

4. The method of any of claims to 3 wherein-with respect to said polymer, the liquid carrier and polymer system has a second virial coefficient of up to about 15x10-4 mol-cm3/g2 at 25 C.

5. The method of any of claims 1 to 4 wherein the liquid carrier has a heat of dilution at 25 C in the liquid coagulant of greater than about -3.5 kilocalories per mole.

6. The method of any claims 1 to 5 wherein the said polymer comprises polysulfone.

7. The method of claim 6 in which the polysulfone comprises polysulfone having the repeating structural unit

where R' and R2 are the same or different and are aliphatic or aromatic hydrocarbyl-containing moieties of 1 to about 40 carbon atoms and the sulfonyl group is bonded to an aliphatic or aromatic carbon atom.

8. The method of claim 7 in which at least one of R' and R2 comprises an aromatic hydrocarbyicontaining moiety.

9. The method of claim 7 wherein the polysulfone has a structure represented by

wherein n is about 50 to 80.

10. The method of any claims 1 to 9 wherein the liquid carrier comprises non-solvent.

11. The method of any of claims 1 to 10 wherein the washed hollow fiber is dried at a temperature which does not unduly adversely affect the selectivity or flux exhibited by the hollow fiber membrane.

12. The method of any of claims 1 to 11 wherein the liquid coagulant comprises at least about 75 weight percent water.

13. The method of any of claims 1 to 12 wherein the temperature of the liquid coagulant is at least about -15'Cto below about 20 C.

14. The method of claim 13 wherein the temperature of the liquid coagulant is about 0 to 10 C.

15. The method of any of claims 1 to 14 wherein the annular spinnerette is positioned above the surface of the liquid coagulant.

16. The method of any of claims 1 to 15 wherein the polymer solution contains at least about 25 weight percent.

17. The method of any of claims 1 to 16 wherein the polymer solution contains about 28 to 40 weight percent polymer.

18. The method of any of claims 1 to 17 wherein the polymer solution has a viscosity at the temperature of extrusion of about 10,000 to 500,000 centipoises.

19. The method of any of claims 1 to 18 wherein the liquid carrier comprises non-solvent and the non-solvent comprises at least one of formamide, ethylene glycol, and water.

20. An anisotropic membrane made by the method of any of claims 1 to 19.

21. A dry, integral anisotropic hollow fiber membrane for the separation of at least one gas of a gaseous mixture comprising a homogeneously-formed, thin, exterior, separating layer on an open, cellular support and comprising polysulfone wherein a volume majority of the wall of said hollow fiber membrane consists of cells having a mean major dimension of less than about 2 microns and said open, cellular support comprising a substantial absence of macrovoids having a major dimension greater than about 3 microns and a ratio of maxmum length to maximum width greater than about 10, said hollow fiber membrane exhibiting a ratio of permeability of at least on of hydrogen, helium and ammonia to the permeability constant of the material of the hollow fiber membrane for said gas of at last about 5x 1 04 reciprocal centimeters; a permeability ratio of (i) the permeability of a lower molecular weight gas divided by the permeability of a higher molecular weight gas to (ii) the square root of the molecular weight of said lower moleularweight gas divided by the square root of the molecular weight of said higher molecular weight gas of at least about 6, wherein said lower molecular weight gas is one of hydrogen and helium and said high molecular weight gas is one of nitrogen, carbon monoxide and carbon dioxide and has a permeability constant in the polysulfone at least about 10 times less than the permeabiliy constant of said lower molecular weight gas in said material; and a collapse pressure of at least about 4(Ts)(t/D)3 wherein Ts is the tensile strength of the polysulfone, t is the wall thickness and D is the outside diameter of the hollow fiber membrane.

22. The polysulfone hollow fiber membrane of claim 21 having a substantial absence of macrovoids.

23. The hollow fiber membrane of claim 21 or 22 in which at least about 90 volume percent of the wall of the hollow fiber membrane consists of cells having a major dimension less than about 2 microns.

24. The hollow fiber membrane of any of claims 21 to 23 in which at least about 90 volume percent of the wall of the hollow fiber membrane consists of cells having a major dimension less than about 1 micron.

25. The hollow fiber membrane of any of claims 21 to 24 in which the mean configuration ratio of segments of the wall of hollow fiber membrane containing the cells having the largest mean major dimension and having an absence of macrovoids is at least about 0.03.

26. The hollow fiber membrane of any of claims 21 to 25 in which the wall of the hollow fiber membrane contains cells having a major dimension greater than 0.2 micron and the ratio of mean wall thickness to mean cell cross-sectional area of the largest cells is at least about 1 reciprocal centimeter.

27. The hollow fiber membrane of any of claims 21 to 26 in which the void volume of the wall of the hollow fiber membrane is at least about 40 and up to about 70 volume percent.

28. The hollow fiber membrane of any of claims 21 to 27 in which the void volume of the wall of the hollow fiber membrane is greater than about 45 and up to about 65 volume percent.

29. The hollow fiber membrane of any of claims 21 to 28 in which the collapse pressure of the hollow fiber membrane is greaterthan about 10 (4))(T5)(t/D)3wherein Q is the volume fraction of the material of the hollow fiber membrane in the wall of the hollow fiber membrane.

30. The hollow fiber membrane of any of claims 21 to 29 which exhibits a relationship of (i) the quotient of the difference between the permeabilities of a more readily permeated gas and a less readily permeated gas divided by the permeability of the less readily permeated gas times (ii) the ratio of the permeability constant of the less readily permeated gas to the permeability constant of the more readily permeated gas of at least about 0.01, wherein the more readily permeated gas has a permeability constant at least about 5 times greater than the permeability constant of the less readily permeated gas and the more readily permeated gas and less readily permeated gas have approximately the same molecular weight.

31. The hollow fiber membrane of any of claims 21 to 30 in which the external skin is less than about 1000 angstroms.

32. The hollow fiber membrane of any of claims 21 to 31 in which the external skin is less than about 500 angstroms.

33. The hollow fiber membrane of any of claims 21 to 32 in which the permeability ratio is at least about 7.5.

34. The hollow fiber membrane of any of claims 21 to 33 exhibiting a maximum pore size of less than about 150 angstroms.

35. The hollow fiber membrane of any of claims 21 to 34 in which the polysulfone comprises polysulfone having the repeating structural unit:

where R1 and R2 are the same or different and are aliphatic or aromatic hydrocarbyl-containing moieties of 1 to about 40 carbon atoms and the sulfonyl group is bonded to an aliphatic or aromatic carbon atom.

36. The hollow fiber membrane of any of claims 21 to 35 in which a least one of R1 and R2 comprises an aromatic hydrocarbyl-containing moiety.

37. The hollow fiber membrane of any of claims 21 to 36 in which the polysulfone comprises a polysulfone having a structure represented by

where n is about 50 to 80.

38. The hollow fiber membrane of any of claims 21 to 37 in which the material of the hollow fiber membrane exhibits a tensile strength of at least about 350 kilograms per square centimeter.

39. A method according to claim 1, substantially as described in any of the Examples.

40. A hollow fiber membrane according to claim 21, substantially as described in any of the Examples.