GB2100181A

GB2100181A - Anisotropic hollow fiber membranes

Info

Publication number: GB2100181A
Application number: GB8218393A
Authority: GB
Original assignee: Monsanto Co
Current assignee: Monsanto Co
Priority date: 1979-03-28
Filing date: 1980-03-27
Publication date: 1982-12-22
Also published as: BR8001873A; FI800977A; GB2047162A; ES489884A0; RO79464A; GR67255B; FR2452507A1; YU86080A; AU5689180A; AU532082B2; GB2047162B; DE3011917A1; NL8001845A; AR220972A1; NO800893L; IT8020971A0; PT71024A; IL59581A0; GB2100181B; DK131780A

Abstract

A dry, integral anisotropic polysulphone hollow fiber membrane for separating a gas of a gaseous mixture is described, comprising a homogeneously formed, thin, exterior, separating layer on an open cellular support, wherein a volume majority of the fiber wall consists of cells less than 2 microns major dimension and a substantial absence of macrovoids of major dimension greater than 3 microns and of length/width greater than 10. The membrane has a ratio of permeability of at least one of H2, He and NH3 to the permeability constant of the material of the membrane for the said gas of at least 5 x 10<4> reciprocal centimeters. The membrane has a permeability ratio (as defined in Claim 1) of at least 6 and a high collapse pressure.

Description

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GB 2 100 181 A 1

SPECIFICATION

Anisotropic hollow fiber membranes

This invention pertains to anisotropic membranes, especially hollow fiber membranes. Particularly attractive aspects of this invention include anisotropic polysulfone hollow fiber membranes suitable for 5 the separation of gases in which the material of the hollow fiber membrane effects separation by selective permeation.

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 which is desired, the total pressure losses caused by the apparatus for 10 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. 15 Particularly advantageous membranes are unitary anisotropic hollow fiber membranes which have a relatively thin layer (often referred to as separating 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 20 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, 25 anisotropic film membranes are prepared by casting a solution of the polymer 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 30 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 coagulation 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 35 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 40 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 45 attractive from the standpoint of convenience, in size and reduced complexity of design.

A new hollow fiber membrane has now been found that is suitable for the separation of gases.

The invention comprises 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 50 hollow fiber membrane consists of cells having a mean major dimension of less than 2 microns and said open, cellular support comprising a substantial absence of macrovoids having a major dimension greater than 3 microns and a ratio of maximum length to maximum width greater than 10, said hollow fiber membrane exhibiting a ratio of permeability of at least one of hydrogen, helium and ammonia to the permeability constant of the material of the hollow fiber membrane for said gas of at least 5 x 104 55 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 molecular weight gas divided by the square root of the molecular weight of said higher molecular weight gas of at least 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 60 and has a permeability constant in the polysulfone at least 10 times less than the permeability constant of said lower molecular weight gas in said material; and a collapse pressure of at least 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.

Hollow fiber membranes according to the invention can be made by the method described and

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GB 2 100 181 A2

claimed in British Patent Application No. 8010315 (2,047,162).

In an aspect of this invention the hollow fiber membranes 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.

5 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 10 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 to 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 15 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 20 about 40, say, greater than about 45 or 50, volume percent, and may range up to 65 or 70 or more volume percent.

Polysulfones useful for preparing the anisotropic membranes may be mixed with inorganics, e.g., fillers, reinforcements, and the like.

Generally, it is preferred that the polysulfone polymer be substantially non-crystalline, e.g., less 25 than about 25 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 preparation of anisotropic membranes.

The polysulfone polymeric material is preferably selected on the basis of its separation capabilities, 30 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 polysulfone polymer should have a molecular weight sufficient for 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 35 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 40 for forming the precursor. Therefore, it is often desired to utilize a polysulfone 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 a significant portion of lower molecular weight polymer molecules but provides a high solution viscosity due to the presence of substantially higher molecular weight polymer molecules. A convenient 45 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 polysulphone polymer should be sufficient to prevent damage to the 50 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 55 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 60 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 streams, e.g., gas streams, to be treated using membranes for separations contain one or more components 65 which can adversely affect the membrane material. By the use of a membrane material which exhibits

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GB 2 100 181 A 3

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 5 these reported permeations the mechanism of the separation apparently involves an interaction with 5 the material of the membrane.

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 III (1975). 10 The suitability of a polymer for, e.g., a gas separation can readily be determined using, for instance, 10

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.

The anisotropic membranes of the invention are of polysulfone because of its strength, chemical 15 resistance and relatively good gas separation capabilities. Typical polysulfones are characterized by 15 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 20 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

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0 0 0 0

II II II I

-0—, —S—, —C—, —C—N—, —N—C—N—, —0—C—, etc.

25

H

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 bishphenyl moieties, bisphenyl methane and substituted 30 bisphenyl methane moieties having the nucleus

30

RH R

substituted and unsubstituted bisphenyl ethers of formula

R'

R-

R"

wherein X is oxygen or sulfur; and the like. In the depicted bisphenyl methane and bisphenyl ether

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GB 2 100 181 A 4

moieties, R3 to R12 represent substituents which may be the same or different and have the structure

X1

I

-fC4pZ X2

wherein X1 and X2 are the same or different and are hydrogen or halogen (e.g., fluorine, chlorine, and bromine); p is 0 or an integer, e.g., of 1 to about 6; and Z is hydrogen, halogen (e.g., fluorine, chlorine 5 and bromine),-fY4q R13 (in which q is 0 or 1, Y is—0—,—S—,—SS—, 5

0 0

II II

—OC—, or —C—,

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 10 or bicyclic with about 5 to 15 ring atoms, sulfato and sulfono, especially lower alkyl-containing or 10

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, 15 guanidyl, trialkylsilyl, trialkylstannyl, trialkylplumbyl, dialkylstibinyl, etc. Frequently, the substituents on 15 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 possess good thermal stability, are resistant to chemical attack, and have an excellent combination of toughness and flexibility.

20 Useful polysulfones are sold under trade names such as "P—1700", and "P—3500" by Union 20 Carbide, both commercial products are bisphenol methane-derived polysulfones (specially, 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, 25 bisphenol A-derived polysulfones sometimes contain a crystallized fraction which is believed to be an 25 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 solvents used in the method of British Patent Specification No. 30 8010315 (2047162) appear to provide solutions containing these polysulfone polymers which have a 30 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.

Preferred anisotropic hollow fiber membranes are of polysulfones containing bisphenyl methane 35 (including substituted bisphenyl methane) and bisphenylether (including substituted bisphenylether) 35 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 are particularly attractive for effecting gas separations.

40 A dry, integral anisotropic hollow fiber membrane of polysulfone in accordance with this invention 40 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 flow through 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 45 about 10, preferably greater than about 5. The preferred polysulfone hollow fiber membranes of this 45 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 50 strength. These properties depend on the chemical and physical nature of the material of the hollow 50

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GB 2 100 181 A 5

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 5 "submicroscopic" structures. 5

The combination of microscopic techniques (particularly scanning electron microscopy and transmission 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 membrane structures is scanning electron 10 microscopy. To assist in understanding the use of scanning electron microscopy in describing hollow 10 fiber membranes in accordance with this invention, references can be made to the provided figures.

Figures 1 and 2 are scanning electron microscopic photographs of hollow fiber membranes in which:

Figures 1 a to 1c depict a hollow fiber membrane prepared from a polymer solution containing 32 weight percent polysulfone (P—3500), about 7 weight percent formamide and about 61 weight 15 percent 1 -formylpiperidine. Figure 1 a shows the cross-section of the hollow fiber at a magnification of 15 about 300 times. Figure 1 b shows a segment of the cross-section of the hollow fiber at the exterior edge of the hollow 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.

20 Figure 2 depicts a cross-section of a hollow fiber membrane prepared from a polymer solution 20 containing 36 weight percent polysulfone (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 25 hollow fiber in hexane and immediately placing the hollow fiber in liquid nitrogen such that the hollow 25 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 30 coating. 30

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 underlying 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 35 hollow fiber wall. The predominant structure of the cells in the middle region of the cross-section of the 35 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., less than 0.1 micron in major dimension. This open, cellular structure enables gases readily to pass through the hollow fiber 40 wall with minimal resistance. Advantageously, the exterior skin (and interior skin, if any) provides the 40 major portion of resistance to gas 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 45 individual cells are considered to be defects in the cell wall, and therefore, the cell dimensions do not 45 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 50 analyzers to inspect and analyze the photograph. Suitable image analyzers include image analyzers 50 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 55 cell is by dividing the cross-sectional cell area by the square of perimeter of the cell at that cross- 55

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 60 the use of an image analyzer to inspect photographs. Conveniently, the configuration ratio can be ' 60 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 5 microns). In many desirable 65 hollow fiber membranes in accordance with this invention the mean configuration ratio is at least about 65

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GB 2 100 181 A 6

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 5 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, 10 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.

15 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 20 exists. As stated earlier, the size range of cells in the wail 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 25 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 to 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 30 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 35 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 to 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 40 fiber wail.

Scanning electron microscopy may sometimes be useful in examining the porosity of the internal (bore side) skin of the hollow fiber membrane. 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 45 (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 microscopy, 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 50 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 55 given thickness (2) is 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 is 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 60 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 many techniques available for determining permeabilities and permeability 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) 65 are disclosed by Hwang, et al., Techniques of Chemistry, Volume VII, Membranes in Separations, John

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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 5 barrier layer, and the range of sizes of the pores in the barrier layer, is by determining the ability of the 5 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 10 (i.e., the intrinsic permeability constant) of the polysulfone at least about 10 times less than the intrinsic 10 permeability constant of the lower molecular weight gas in the polysulfone. The analysis can be conveniently conducted at ambient temperatures (e.g., room temperature of about 25°C.) at a pressure of about 7.8 atmospheres 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 15 membranes of this invention exhibit for at least one pair of gases a permeability ratio of (i) the 15

permeability of the lower molecular weight gas (P/I)L, divided by the permeability of the higher molecular weight gas (P/I)H, to (ii) the square root of the molecular weight of the lower molecular weight gas \/MW„ divided by the square root of the molecular weight of the higher molecular weight gas,

\/MWH, of at least about 6, frequently at least about 7.5. The theoretical maximum permeability ratio is 20 the ratio of the intrinsic separation factor for the gas pair to the quotient of the square root of the 20

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 25 material. Such a material may be referred to as being continuous or non-porous. The intrinsic separation 25 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 30 variations in membrane preparation, and the like. Consequently, the determined intrinsic separation 30 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 35 determining the permeabilities of a pair of low molecular weight gases having approximately the same 35 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 40 relationship can be expressed as the quotient of the difference between the permeability of the more 40 readily permeated gas, (P/I)F, and the permeability of the less readily permeated gas, (P/I)s, divided by • (P/I)s times the permeability constant of the less readily permeated gas, Ps< divided by the permeability constant of the more readily passed gas PF:

(P/l)F - (P/X)s

- <pWs .

— —

p„

_s

_ F _

45 In many instances, this relationship is at least about 0.001, say, at least about 0.01, preferably, at least 45 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 fiber membrane is the ratio of the permeability of a gas to the permeability constant of the polysulfone for the gas:

(P/l)

50 50

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 5 x 104, say, at least about 1 x 105, preferably at least about 2 x 105, up to about 1 x 106, say, about 1 x 10s to 0.6 x 106, reciprocal centimeters. This relationship is indicative of a low resistance to gas flow through the hollow fiber 55 membrane structure and thus the high permeabilities of the desired permeating gas that can be 55

achieved.

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GB 2 100 181 A 8

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 0.5 x 10-6, or even less than about 0.2 x 10~6, say, less than about 5 0.1 x 10-6, and sometimes less than 0.01 x 10~6 cm3 (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 temperature 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 10 about 100 angstroms.

A method for evaluating the size and openness of the cellular support is by subjecting the hollow fiber membrane 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 15 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., 20 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 fibrillar structure instead of a cellular structure. Fibrillar structures can sometimes be relatively weak in comparison to cellular structures. Commonly, 25 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 30 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 35 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 40 foam and the uniformity of the hollow fiber configuration (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 (t/D). This approximation relationship can be expressed in terms of the tensile strength of the material of the 45 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 (-)3,

D

50 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 t

1O(0)(Tg)( )3

D

wherein tj> is the volume fraction of material of the hollow fiber membrane in the wall of the hollow fiber 55 membrane. The volume fraction of the material of the hollow fiber membrane ((4) is therefore 1 minus the quantity of void volume (volume percent)/100.

Anisotropic membranes in accordance with this invention can be prepared to minimize the separating, or barrier, layer thickness. Frequently, the tendency to produce pores in the membranes is

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GB 2 100 181 A 9

increased when the barrier layer thickness is decreased. In accordance with the teachings of Henis, et al., Belgian Patent No. 860,811, corresponding to British Application No. 47,269/77 (1,590,813),

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 5 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 10 permeates the material of the membrane) to the permeability constant of the material of the membrane for the gas is at least about 1 x 10s, preferably, at least about 2 x 105, up to, say, 1 x 106, 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 to 2 to about 10, 15 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 20 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 5 x 104, preferably, 25 at least about 1 x 10s, 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 contact with the 30 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 35 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 40 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 of 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 45 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; acrylonitrile-containing copolymers such as poly(a-chloroacrylonitrile) copolymers; polyesters (including 50 polylactams and polyarylates), e.g., poly(alkyl 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 ar-olefinic unsaturation such as poly(olefins), e.g., 55 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 polyvinyl alcohols), polyvinyl aldehydes) (e.g., polyvinyl formal) and poly(vinyl butyral)), polyvinyl ketones) (e.g., poly(methylvinylketone)), polyvinyl esters) (e.g., polyvinyl benzoate)), polyvinyl halides) (e.g., polyvinyl bromide)), polyvinyl halides), poly(vinylidene carbonate), 60 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(ethylenemethylphosphate): and the like, and any interpolymers including block interpoiymers 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.

65 Particularly useful materials for coatings comprise poly(siloxanes). Typical poly(siloxanes) can

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GB 2 100 181 A 10

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 weight of about 1,000 to 300,000 or more when applied to the membrane. Common aliphatic and aromatic poly(siloxanes) include the 5 poly(monosubstituted and disubstituted siloxanes), e.g., wherein the substituents are lower aliphatic, for instance, lower alkyl, 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 10 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. Cross-linking may occur prior to application of the poly(siloxane) to 15 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(hydro-genmethylsiloxane), poly(phenylmethylsiloxane), poly(trifluoropropylmethylsiloxane), copolymer of a-methylstyrene and dimethylsiloxane, and post-cured poly(dimethylsiloxane)-containing silicone rubber 20 having a molecular weight of about 1,000 to 50,000 or more prior to cross-linking. Some poly(siloxanes) do 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, 25 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.

30 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 35 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 40 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 coating membrane.

45 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, 50 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 50°C., say, about 10° to 35°C., are usually employed. Treatment of the membrane with a densifying agent prior to coating to increase selectivity 55 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 60 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., 65 methane; carbon dioxide from at least one of carbon monoxide and hydrocarbon of 1 to about 5 carbon

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GB 2 100 181 A 11

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 hydrocarbon of 1 to about 5 carbon atoms. It is emphasized that membranes in accordance with this invention may find beneficial 5 application in the separation operations using other gas mixtures. 5

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 weight, 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 10 atmosphere at the bore side of the hollow fiber membrane unless otherwise stated. 10

EXAMPLE 1

Thirty two parts by weight of polysulforie (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 15 atmosphere. Prior to being charged to the dope mixer, the polysulfone polymer is dried at about 125°C. 15 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 20 heated dope mixer the polymer solution is maintained at about 80°C. to 100°C. for a time sufficient to 20 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 25 polymer solution in the cup and the polymer solution flows over the edge of the cup and down the 25

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 30 degassing may also be achieved using substantially atmospheric pressure in the deaerator. Adequate 30 deaeration is generally achieved in less than about 7 hours residence time in the deaerator.

The deaerated polymer solution 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 35 spinnerette has five equidistant polymer solution entrance ports positioned behind the annular extrusion 35 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 temperature. 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 40 spinnerette is positioned about 10.2 centimeters above the coagulation bath. The hollow fiber precursor 40 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 45 the coagulation bath less than about one weight percent. The liquid coagulant is maintained at a 45

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.

50 The hollow fiber from the coagulation bath is then washed with tap water in three sequential 50

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 55 temperature (e.g., about 1 5 to 25°C. depending on the temperature of the tap water and the laboratory 55 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 1 55 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, artd 60 the thickness of hollow fiber wound on the bobbin is less than about 2 centimeters. The bobbin is placed 60 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 sevel 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 would on a skeiner to form hanks

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12

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 20°—25°C. and 50 percent relative humidity). The dried 5 hollow fiber has an outside diameter of about 390 microns and an inside diameter of about 140 5

microns and appears, under scanning electron miscoscopy, 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 10 other end is plugged. The bores of the hollow fibers in each test loop are subjected to a vacuum (about 10 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 15 after the test loop is removed from the coating solution, and the test loops are dried at ambient 15

laboratory conditions (20° to 25°C., about 40 to 60 percent 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) of the hollow fibers being at a pressure of about 70 atmospheres absolute and the bore side of the hollow fiber 20 being at about 1 atmosphere absolute. The determined permeabilities generally vary somewhat from 20 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 x 106 cc(STP)/cm2-sec-cmHg) and the average methane permeability often ranges from about 1 to 2 GPU. The ratio of the permeabilities (separation factor) for hydrogen over methane is often about 25 50 to 60. The failure pressure of the hollow fiber is determined to be over 110 kilograms per square 25 centimeter (1600 pounds per square inch).

EXAMPLE 2

A degassed polymer 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 30 described in Example 1 into a hollow fiber membrane in accordance with the procedure set forth in 30

Table I. The polymer solution (dope) temperature for runs 885—1, 533—1—7, and 537—1 is not directly 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 1-formylpiperidine in the coagulation bath is maintained 35 below about one percent by weight through purging. The hollow fiber is immersed in the coagulation 35 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

40

30.5

56.9

40

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 45 running tap water for about 16 to 24 hours, then stored in a bucket filled with water for seven days. The 45 hollow fiber is then dried under ambient laboratory conditions (22° to 25°C. 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

Run No.

1 "formylpiperidine Concentration in Liquid Carrier, Wt. %

Fiber* Dimensions OD(p)/ID(p)

Jet Size, OD(p), ID(p) Capillary(p)

Jet Stretch

Dope Rate cc/min.

Spinning Speed, m/min.

Dope Temp, at Jet,

°C.

Distance of Jet From Coagulation Bath, cm.

D) CO

o o

d)

o

0 +-»

CO

T—

CD "o O

CD ■o c <N

•4—'

<D

■o

O

(3 -o po

885-1

92.5

416/134

457/127/76

0.96

6

36.6

About 60°

12.7

3

4

RT

533-1-7

92.5

410/136

457/127/76

0.96

6

38.1

About; 60°

12.7

3

4

RT

537-1

92.5

416/136

457/127/76

0.96

6

38.1

About 60°

12.7

3

RT

893-4-6

92.5

425/150

533/203/127

1.15

7.1

42.7

53°

10.2

2

1-30

1.5-20

RT

895-1

90

456/150

533/203/102

0.98

8.3

42.7

53°

10.2

1

RT

895-2

90

448/148

533/203/102

0.98

8.3

42.7

55°

10.2

1

RT

894-1

87

456/152

533/203/102

0.98

8.3

42.7

52°

10.2

3

RT

894-2

87

448/149

533/203/102

0.98

8.3

42.7

54°

10.2

3

RT

204-3

90

479/217

457/127/76

0.77

6

30.5

55°

5.1

1

11

204-6

90

445/205

457/127/76

0.77

6

30.5

65°

5.1

1

11

893-2

92.5

406/146

635/178/102

1.75

7.1

42.7

64°

10.2

2

3

RT

*Before Drying.

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TABLE II

Run No.

Permeability, GPU**

Separation Factor

Approximate Collapse Pressure, kg/cm2

Hydrogen

Methane

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 -5, formylpiperidine, the spinnerette has an outside diameter of 533 microns, an inner diameter of 203 5

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 10 coagulation bath is at a temperature of about 10 to 2°C.; the first godet bath, about 1 ° to 2°C„ and the 10 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 15 hollow fiber is about 170 kilograms per square centimeter. The hollow fiber is formed into test 15

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 20 collapse pressure of about 100 to 150 kilograms per square centimeter and, when coated and in test 20 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 25 weight of 1 -formylpiperidine and 13 parts by weight of formamide is spun at a temperature of about 25 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 centimeters 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 30 3°C., and the second and third godet baths are at about room temperature. The residence time in each 30 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 35 are determined for the hollow fiber stored one day using a shell side pressure of about 7.8 atmospheres 35 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 III.

15

GB 2 100 181 A 15

TABLE III

Days in Storage Bucket: 1 2 24

Uncoated H2 457 — —

Permeability, GPU

Uncoated CH4 Permeability, GPU

Coated H2 Permeability, GPU

Coated CH4 Permeability, GPU

Collapse Pressure, kg/cm2

167 — —

93 102 114

2.8 3.3 1.9

140 140 170

1-formylpiperidine 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 5 chimney is positioned between the spinnerette and the liquid level in the coagulation bath such that a 5 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 10 methane permeabilities determined after 7 days storage in water. The results are as follows: 10

Chimney No Chimney

H2 Permeability, GPU 129 123

CH4 Permeability, GPU 1.5 1.3

Outside Diameter, microns 370 364

15 Inside Diameter, microns 171 165 15

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.

20 through a spinnerette having an outside diameter of 635 microns, an inside diameter of 228 microns, 20 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 residence times of the 25 hollow fiber in each of the three godet baths is about 56.9 seconds. The coagulation bath contains 7.5 25 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 30 determined at a shell pressure of about 7.8 atmospheres absolute and a bore pressure of about 1 30

atmosphere absolute). The results are as follows:

Uncoated Coated

H2 Permeability, GPU 409 136

CH4 Permeability, GPU 107 2.1

35 EXAMPLE 7 35

The procedure of Example 6 is substantially repeated except that the spinnerette has an outside diameter of 483 microns; an inside diameter of 1 52 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 in Bucket

Stored in Water

Stored in Water: Isopropanol 5:5 by Weight

Collapse Pressure, kg/cm2

7

175

-

Collapse Pressure, kg/cm2

28

175

-

Weight Percent 1-formylpiperidine in Fiber

7

2.45

0.85

Weight Percent 1-formylpiperidine in Fiber

28

1.93

0.35

Outside Diameter, Microns

-

439

427

Inside Diameter, Microns

-

131

125

Permeabilities, GPU

h2

Coated Uncoated

7

205 759

243 2330

ch4

7

5.7 262

9.27 1060

h2

28

•140 854

108

ch,

28

3.1 321

24

17

GB 2 100 181 A 17

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 1 5 parts by weight of ethylene glycol and is spun at about 70°C.

5 through a spinnerette having an outside diameter of 635 microns, an inside diameter of 229 microns, 5 and an injection capillary diameter of 1 52 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 10 each of the three godet baths is about 40.6 seconds. The outside diameter of the hollow fiber is about 10 456 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 15 contains 32 weight percent polysulfone P—-3500 and 68 weight percent liquid carrier having 90 parts 1 5 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 20 the coagulation bath. The coagulation bath and first godet bath are at about 5°C. and the second and 20 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 25 determined using a gas mixture of about 25 mole percent hydrogen and 75 mole percent carbon 25

monoxide using a shell side pressure of about 1 50 centimeters of mercury absolute and laboratory vacuum 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 30 prepared 32 weight percent polysulfone P—3500 and 68 weight percent liquid carrier having 80 parts 30 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 35 minute. The spinnerette is about 20.3 centimeters from the liquid level in the coagulation bath. The 35 coagulation bath and first godet bath are at about 0°C. 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 40 uncoated hollow fiber is about 225 GPU, and after coating, about 82 GPU, and the methane 40

permeability of the uncoated hollow fiber is about 50 GPU, and after coating, about 1.6 GPU.

EXAMPLE 11

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 45 similar to P—3500 available from Union Carbide Company) and 60 weight percent of a liquid carrier 45 having 87 parts by weight 1 -formylpiperidine 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 50 spinning rate is about 30.5 meters per minute. The coagulation bath and first godet bath are at about 50 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 1 54 microns. The collapse pressure exhibited by the hollow fiber is about 175 kilograms per square centimeter. Samples of the hollow fibers 55 are stored in water for 7 or 22 or 23 days, dried and assembled into test loops. The test loops are 55

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 60 for an additional 10 minutes. Also, test loops (coated and uncoated) are suspended in a closed container 60 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 Isopentane

Isopentane

Coated After

Flush Permeability, GPU

Uncoated

Flush

Coated

Isopentane Flush at 68 atm.

Ammonia

Permeability,

Permability,

Permeability,

1 Day

3 Day

Treatment,

Days Stored

GPU at

Ammonia

Days

in Bucket

7.8 atm?

7.8 atm.

68 atm.

7.8 atm.

68 atm.

T reatment

Treatment h2

0

7

&

626

270

-

178

-

ch4

0

7

213

23

-

3.5

-

h2

0

22

380

193

143

161

140

-

ch4

V

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

-

* Al I pressures are absolute pressures at shell side.

19

GB 2 100 181 A 19

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 1 -5 formylpiperidine. The polymer solution is spun at about 70° to 80°C. through a spinnerette having an 5 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 10 of the three godet baths is about 86 seconds. The outside diameter of the hollow fiber is about 685 10 microns, and the inside diameter is about 380 microns. The permeabilities are determined using the procedure described in Example 9. The coated hydrogen permeability 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 15 75 square meters per gram using a B.E.T. analysis. 15

EXAMPLE 13

The procedure of Example 2 is substantially repeated except as follows. The degassed polymer solution contained 32 weight percent polyfphenylene ether) sulfone available from ICI, Ltd., Great Britain and 68 weight percent of a liquid carrier consisting of 90 weight percent 1 -formylpiperidine and 20 10 weight percent formamide. The spinnerette dimensions are outside diameter, 533 microns; inside 20 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 25 40° to 55°C. and the dimension of the hollow fiber membranes before drying are outside diameter 25 about 435 microns and inside diameter about 1 50 microns — however, one sample has an oval bore configuration. The observed performances of the hollow fiber membranes are provided in Table VI.

TABLE VI

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

*AII permeabilities determined with a bore side pressure at about one atmosphere absolute. The shell side pressures in atmospheres absolute are provided parenthetically.

EXAMPLE 14

30 The procedure of Example 2 is substantially repeated except as follows. The degassed polymer 30 solution contains 32 weight percent polysulfone (P—3500) and 68 weight percent of a liquid carrier consisting of about 90 weight percent 1 -acetylpiperidine 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

35 the liquid coagulant and is at a temperature of about 57°C. The take-up rate is about 42.4 meters per 35 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

Water Injection

Permeabilities, GPU**

Dope Rate

Rate

Pressure

Fiber Dimensions*

Coated (68)

Run No.

cc/min.

mmHg

OD^/IDfo)

h2

X o

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.

21

GB 2 100 181 A 21

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 5 conditions and then fabricated into a test loop. The hydrogen and methane permeabilities of coated and 5 uncoated hollow fiber are determined. The results are provided in Table VIII.

TABLE VIII

Drying Temperature °C

Relative

Humidity

%

Absolute Humidity g/m3

Permeabilities, GPU*

Uncoated

Coated h2

ch4

h2

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)

25

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)

* Al I permeabilities determined with a bore side pressure of about one atmosphere absolute. The shell side pressures in atmospheres absolute are provided parenthetically.

23

GB 2 100 181 A 23

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 5 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.

Claims

10 CLAIMS

1. 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 2 microns and said open,

15 cellular support comprising a substantial absence of macrovoids having a major dimension greater than 3 microns and a ratio of maximum length to maximum width greater than 10, said hollow fiber membrane exhibiting a ratio of permeability of at least one of hydrogen, helium and ammonia to the permeability constant of the material of the hollow fiber membrane for said gas of at least 5 x 104 reciprocal centimeters; a permeability ratio of (i) the permeability of a lower molecular weight gas

20 divided by the permeability of a higher molecular weight gas to (ii) the square root of the molecular weight of said lower molecular weight gas divided by the square root of the molecular weight of said higher molecular weight gas of at least 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 10 times less than the

25 permeability constant of said lower molecular weight gas in said material; and a collapse pressure of at least 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.

2. A polysulfone hollow fiber membrane of Claim 1, having a substantial absence of macrovoids.

3. A hollow fiber membrane of either Claim 1 or Claim 2, in which at least 90 volume percent of

30 the wall of the hollow fiber membrane consists of cells having a major dimension less than 2 microns.

4. A hollow fiber membrane of any of Claims 1 to 3, in which at least 90 volume percent of the wall of the hollow fiber membrane consists of cells having a major dimension less than 1 micron.

5. A hollow fiber membrane of any of Claims 1 to 4, in which the mean configuration ratio of - segments of the wall of hollow fiber membrane containing the cells having the largest mean major

35. dimension and having an absence of macrovoids is at least 0.03.

6. A hollow fiber membrane of any of Claims 1 to 5, 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 1 reciprocal centimeter.

7. A hollow fiber membrane of any of Claims 1 to 6, in which the void volume of the wall of the

40 hollow fiber membrane is at least 40 and up to 70 volume percent.

8. A hollow fiber membrane of any of Claims 1 to 7, in which the void volume of the wall of the hollow fiber membrane is greater than 45 and up to 65 volume percent.

9. A hollow fiber membrane of any of Claims 1 to 8, in which the collapse pressure of the hollow fiber membrane is greater than 10 {</>)(Ts)(t/D)3 wherein <j> is the volume fraction of the material of the

45 hollow fiber membrane in the wall of the hollow fiber membrane.

10. A hollow fiber membrane of any of Claims 1 to 9, 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

50 readily permeated gas of at least 0.01, wherein the more readily permeated gas has a permeability constant at least 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.

11. A hollow fiber membrane of any of Claims 1 to 10, in which the thickness of the external skin

55 is less than 1000 angstroms.

12. A hollow fiber membrane of any of Claims 1 to 11, in which the thickness of the external skin is less than 500 angstroms.

13. A hollow fiber membrane of any of Claims 1 to 12, in which the permeability ratio is at least

7.5.

60

14. A hollow fiber membrane of any of Claims 1 to 13, exhibiting a maximum pore size of less than 150 angstroms.

15. A hollow fiber membrane of any of Claims 1 to 14, in which the polysulfone comprises polysulfone having the repeating structural unit:

5

10

15

20

25

30

35

40

45

50

55

60

24

GB 2 100 181 A

24

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

16. A hollow fiber membrane of any of Claims 1 to 15, in which at least one of R1 and R2 comprises an aromatic hydrocarbyl-containing moiety.

17. A hollow fiber membrane of any of Claims 1 to 16, in which the polysulfone comprises a polysulfone having a structure represented by

~ CH3 ?

_Jn where n is 50 to 80.

10

18. A hollow fiber membrane of any of Claims 1 to 17, in which the material of the hollow fiber membrane exhibits a tensile strength of at least 350 kilograms per square centimeter.

19. A hollow fiber membrane according to Claim 1, substantially as described in any of the Examples.

10

Printed for Her Majesty's Stationeiy Office by the Courier Press, Leamington Spa, 1982. Published by the Patent Office, 25 Southampton Buildings, London, WC2A 1AY, from which copies may be obtained