DE3854429T2 - Manufacture and use of composite membranes. - Google Patents

Description

Production and use of composite membranes Field of the invention

This invention relates to a novel process for the production of permeable membranes for the separation of fluids, such as gases. In particular, it relates to the production of permeable hollow fiber composite membranes with improved permeation and selectivity properties

Description of the state of the art

Permeable membranes capable of selectively permeating one component of a fluid mixture, liquid or gas, are considered in the art to be a convenient, potentially highly advantageous means of achieving desired fluid separations. In practical, commercial operations, permeable membranes must be capable of achieving an acceptable level of selectivity in separating the gases or liquids in a feed stream while at the same time achieving a desirably high productivity of fluid separation.

Various types of permeable membranes have been proposed in the art for carrying out a variety of gas separation processes. Such membranes can generally be classified as belonging to the (1) isotropic type, (2) asymmetric type, or (3) composite type. The so-called isotropic and asymmetric membranes consist essentially of a single permeable membrane material that can selectively separate desired components of a gas mixture. Isotropic membranes have the same density throughout their entire thickness. Such membranes generally have the disadvantage of low permeability, i.e. low permeate flux, due to the relatively large membrane thickness that is necessarily involved. Asymmetric membranes are characterized by the existence of two distinct morphological regions within the membrane structure. One such region has a thin, dense, semipermeable skin that can selectively permeate one component of a gas mixture. The other region has a less dense, porous, non-selective support region that serves to prevent collapse of the thin skin region of the membrane under pressure.

Composite membranes generally comprise a thin layer or coating of a suitable permeable membrane material overlying a porous substrate. The thin membrane separation layer, which determines the separation properties of the composite structure, is advantageously very thin so as to provide the desirable high permeability mentioned above. The substrate serves only to provide a support for the thin membrane layer disposed thereon.

As the advantages of permeable membranes have become increasingly recognized in the art, the performance requirements for such membranes have also increased. Therefore, the art is moving toward very thin membranes that have desirable permeability properties without sacrificing the separation or selectivity properties of the hollow fiber membrane or other permeable membrane structure. It is thus increasingly desirable to achieve more advantageous combinations of permeability and selectivity with respect to a variety of commercial gas separations. It is also desirable to advance and improve the methods used to deposit the thin membrane separation layer onto the porous substrate and to improve the uniformity of the thin membrane separation layer on the porous substrate.

As indicated above, isotropic membranes are not generally suitable for meeting those requirements. On the other hand, asymmetric membranes can be developed for such practical gas separation applications, but do not possess an inherent flexibility that allows them to be readily optimized for specific gas separation applications. Although the thin, dense, semipermeable layer of a specific asymmetric membrane material can be made thinner for increased permeability, the selectivity properties of said material, unless modified by special treatment techniques, may be no more than adequate with respect to the separation of the components of the gas being treated in the specific application.

The thin skin of such asymmetric membranes as described in US-A-3 133 132 is often found not to be perfect, but to have various defects. Such defects in the form of residual pores, pinholes and the like present relatively large openings through which the feed gas directed to a membrane made of such material will preferentially flow. As a result, a substantially reduced amount of gas separation will occur due to the interaction of the feed gas with the material of the permeation membrane itself, due to the presence of such defects in the membrane structure. In the case of asymmetric polysulfone hollow fibers, such defects result in the selectivity (as defined below) for air being about 1, as opposed to a Selectivity of about 6.0 for defect-free polysulfone. In a proposed solution to this problem, US-A-4,230,463 disclosed coating the asymmetric membrane with a coating material having a specified intrinsic separation factor less than that of the separation membrane material. The coating procedures used are described in Table XVI, columns 51 and 52 of US-A-4,230,463. They involve immersing the porous hollow fiber membrane in a liquid coating material, neat or diluted with a solvent, allowing the excess liquid coating material to drip off; in cases where a solvent was present, it was allowed to evaporate. Using this approach, silicone has been coated onto hollow polysulfone fibers with a selectivity of about 2 to increase their selectivity from the range of 1 to 1.5 indicated above to 2 to 6, with that selectivity usually approaching 6. The permeability (as defined below) of such silicone/polysulfone composites has generally been relatively low, i.e., about 0.2 or less, leading to the desire for thinner membranes, i.e., thinner, denser skins, particularly in light of the increasing demands in the art for high flux operation. However, thinner membranes lead to an increase in the number of deficiencies that must be corrected to achieve acceptable performance. While efforts to improve this approach continue, there remains a desire in the art for other approaches to provide a desirable combination of selectivity and permeability for practical, commercial processes. For such reasons, composite membranes using membrane materials specifically selected for a desired gas separation offer the greatest potential with respect to specific gas separations of commercial interest to achieve desirable combinations of selectivity and permeability. It will be appreciated that to achieve the performance requirements desired in the art, composite membranes must not only have very thin membrane layers, but must also have separating layer-substrate structures with optimum advantages for a desired gas separation process. One such application of significant commercial interest is in air separation, particularly where the membrane material selectively permeates oxygen for recovery as oxygen-enriched permeate gas, with a nitrogen-enriched stream withdrawn as non-permeate gas. There is therefore a real need and desire in the art to develop a composite membrane that is particularly suitable for air separation and other desirable gas separation operations, such as the recovery of hydrogen from ammonia purge gas and from refining gas streams, and carbon dioxide and methane separations in a variety of processes, such as tertiary oil production.

A large number of semipermeable membranes of a variety of materials and shapes have been proposed in the art to effect such separations. So-called composite membranes have been proposed in which a thin layer of a suitable semipermeable membrane material is deposited on a relatively porous substrate. The separation layer is advantageously very thin to enable the desirably high flux levels described above. The substrate provides a support for the sensitive, very thin, permeable membrane layer or coating deposited thereon. Such composite membrane structures are described in US-A-4,155,793 and in US-A-3,616,607.

It is understood that membranes for such separation processes of both liquids and gases require membranes that have a high degree of perfection of the membrane or separation layer. In gas separation processes, evaporation, perstruction and the like, the best results would be achieved if the membrane layer were free of any residual pores or other defects. On the other hand, the membrane layer must be made as thin and uniform as possible in order to achieve desirably high permeation rates and thus a high overall productivity of the separation process. In such cases, it was found that morphological defects in the thin separation layer of the membrane assemblies in the form of residual pores, tiny holes and the like often occurred. Such defects can be introduced into the membrane system during the course of the various typical membrane manufacturing steps, such as spinning, casting, coating, curing and membrane module manufacturing processes.

To overcome the problem of voids, US-A-3,980,456 disclosed the application of a second, sealing coating over the very thin membrane to cover the voids caused by particulate contaminants. Composite structures so treated are complex in nature and, in addition, the use of a very thin membrane applied to a porous support substrate has not generally provided the desired selectivity of separation without unacceptably reducing the flux or permeability of the permeate gas.

US-A-4 214 020 discloses a process in which the outer surface of a hollow fiber membrane assembly is coated by immersing a bundle of hollow fibers in a coating solution and forcing the coating solution into the fiber from the outside to the inside of the hollow fibers using pressure. This process, which results in the formation of a continuous layer/coating on the outside of the hollow fibers, is particularly useful in the manufacture of highly selective gas separation membranes by coating asymmetric membranes having some residual porosity with coating solutions made of materials that are highly permeable to gases, as described in the above-mentioned US-A-4 230 463.

US-A-4 127 625 discloses a process for producing asymmetric hollow fibers by spinning a dope of a fiber-forming resin through the outer tube of a double-tube spinneret into an aqueous coagulation bath to form a hollow fiber and by simultaneously injecting an aqueous liquid through the inner tube of the spinneret to come into contact with the interior of the hollow fiber, followed by immersion in warm water and drying.

US-A-4 243 701 discloses a process for forming a thin film of a non-porous permeable membrane on the surface of a porous support by passing the porous support through a solution of a halogenated hydrocarbon solvent containing the permeable membrane-forming prepolymer and a cross-linking agent, removing the coated porous support from the solution, and then cross-linking the prepolymer at an elevated temperature to form the composite membrane.

US-A-4 467 001 discloses a method and apparatus for coating, drying and curing multiple webs. The liquid coating is applied to the web in a U-shaped tube and the drying and curing is carried out in a gas atmosphere.

EP-A-0 175 668 discloses a method for coating a porous substrate with a thin membrane separation layer. This is achieved by impregnating the porous substrate with a solvent to fill the pores, followed by immediately applying a polymer solution to form a thin membrane separation layer on the surface of the solvent-impregnated porous substrate. The solvent in the pores of the porous substrate prevents the polymer solution from penetrating into the pores of the substrate. The composite membrane is then dried by evaporation to remove both the impregnating solvent and the dissolving solvent.

In an article by J. Cabasso and AP Tamvakis, "Composite Hollow Fiber Membranes", J. App. Polym. Sci., 23, pp. 1509-1525, (1979), the preparation of composite hollow fibers is described, in which polysulfone fibers were coated with a dilute polyethyleneimine solution, dehydrated and dipped in m-toluene-2,4-diisocyanate, air dried for about 10 minutes and crosslinked at 100º to 110º C for 10 minutes. In addition, the coating of polysulfone hollow fibers with furan resins by polymerization of furfuryl alcohol (4 wt.% water solution) using sulphuric acid (2 wt.%) followed by curing at 150ºC.

From GB-A-1 473 857 a method for forming a hollow fiber composite membrane for fluid separation is known, in which a thin, substantially smooth membrane separation layer is arranged on a porous substrate, which is substantially free of voids, pinholes and other defects, in which method

(i) coating a preformed porous hollow fiber substrate with a membrane-forming composition comprising a solution containing a membrane-forming material and a solvent therefor by applying the composition to the porous hollow fiber substrate;

(ii) passing the porous hollow fiber substrate coated with the composition through a gas atmosphere to evaporate some, but not all, of the solvent(s) in the composition from the surface of the coated porous hollow fiber substrate;

(iii) the coated porous hollow fiber substrate thus partially dried is passed into a liquid coagulation bath which is a non-solvent for the membrane-forming material and a solvent for the solvent in the solution used in process step (i), whereby the membrane-forming material is coagulated on the surface of the porous hollow fiber substrate and the solvent for the membrane-forming material is dissolved in the liquid coagulation bath; and

(iv) the hollow fibre composite membrane is obtained.

In Example 1 of this patent, the solution used to form the preformed porous hollow fiber substrate has a viscosity of 25 Pa s (250 poise).

Summary of the invention

The present invention consists in a method as described in claim 1.

Detailed description of the invention

This invention provides a method for applying a substantially uniform coating of a thin membrane separation layer to a porous substrate. The use of membrane materials for selectively separating components present in a fluid mixture is known. They are used for liquid compositions, e.g. the separation of water from a saline solution, such as in the extraction of drinking water from oceans or brackish water sources, or for gaseous compositions, e.g. the selective separation of gases in air. The process of this invention can be used to prepare semipermeable membranes suitable for any separation process, but for convenience it will be described in terms of the preparation of materials used for the separation of gas components from one another through the use of semipermeable hollow fiber membrane composites.

There are numerous publications in the literature of methods for applying a thin membrane separation layer to a porous hollow fiber substrate. The most common of these publications is the method in which a solution of a membrane-forming material is applied to the surface of the porous hollow fiber substrate and then the treated composite is dried to remove solvent. Another procedure consists of immersing the porous substrate in a prepolymer solution, then dewatering and contacting the immersed composite with a cross-linking composition, and subjecting this final composite to an elevated temperature to cure the coating. Many modifications have been made to these basic procedures to solve some of the problems encountered, e.g., voids, holes, coating non-uniformity. Other procedures involve impregnating hollow fiber bundles using various techniques.

According to this invention, a method is provided for coating the surface of a porous substrate to form semipermeable composite membranes suitable for fluid separations. This method involves coating a surface of a porous substrate with a dilute solution of a solvent and the membrane-forming material, partially drying the coated porous substrate, and then coating it with a coagulation solution which is a non-solvent for the membrane-forming material and a solvent for the solvent of the solution, and recovering the composite membrane.

The composite membranes produced by the process of this invention generally have a more uniform coating of the thin membrane separating layer or membrane forming material on the porous substrate which is substantially free of voids and holes. Since the coating is more uniform, it is expected to be substantially smoother and thinner than coatings previously achieved by known processes. In the known evaporative coating techniques, it is expected that the surface of the coated material will be an undulating surface, as seen in an undulating meadow, having lower and higher areas; this being apparent from the hydrodynamics of the coating and from the non-uniform evaporation of solvent during the drying step or from the non-uniform application of coating material or crosslinking agent to the porous substrate. In the process of this invention, since the porous substrate coated with the membrane-forming material passes through the coagulation bath, the shear forces generated between the solvent-containing membrane-forming material on the porous substrate and the liquid coagulation bath tend to flatten the surface of the membrane-forming material and generally fill any voids and pinholes that may be present to provide a more uniformly applied composite that is free of those defects often found in many composites made by the known procedures mentioned. As a result, post-treatment is not required. It may also often be possible to obtain a thinner coating by the process of this invention.

The process of this invention also allows the use of a greater variety and number of solvents to prepare the membrane forming solutions. In known processes, these solvents are necessarily mild since they are removed by evaporation, either at ambient temperature or, as is most often the case, at elevated temperature. Whatever temperature condition should be used, it must be sufficiently low so that the porous substrate is not attacked. In the present process, temperature is not a significant factor since the solvent is removed in the coagulation bath and this is preferably done at ambient temperature. In general, therefore, solvents which could not be used in the known processes because their removal would have adversely affected the porous substrate can be used in the process according to this invention. Thus, an advantage of the process of this invention is that some porous substrates can now be coated which could not be coated previously because the substrate would have deteriorated during drying.

The invention will be described hereinafter with particular reference to polysulfone hollow fiber composite membranes for convenience of description. It is within the scope of the invention to use any composite membrane composition prepared as described herein in desired fluid separation operations.

The hollow fibers have continuous channels for fluid flow extending from the outer to the inner surfaces. Often the pores have an average cross-sectional diameter of less than about 2000 nm (20,000 Å), and in some hollow fibers the cross-sectional diameter is less than about 100 or 500 nm (1000 or 5000 Å). In particularly For advantageous hollow fibers, particularly for gas separations where the material of the hollow fiber provides a substantial portion of the separation, the average cross-sectional diameter of the pores is about 0.5 to 20 nm (5 to 200 Å). Often, the ratio of the total surface area to the total cross-sectional area of the pores of the hollow fiber is at least about 10:1. For membranes for gas separations, particularly for those membranes where a substantial portion of the separation is provided by the material of the hollow fiber, the ratio of the total surface area to the total cross-sectional area of the pores of the hollow fiber is at least about 10³:1, and some hollow fibers may have ratios of about 10³:1 to 10⁸:1.

Advantageously, the walls of the hollow fibers are sufficiently thick so that no special device is required for handling them. Often, the hollow fibers can have external diameters of about 20 to 1000 µm, for example about 50 to 1000 µm, e.g. about 250 to 1000 µm, and they have walls with a thickness of at least about 5 µm. The wall thickness of some hollow fibers can be up to about 200 or 300 µm. In order to provide desired flow rates through the hollow fibers, especially when using those hollow fibers whose wall thickness is at least about 50 µm, the hollow fibers can have a significant pore volume. Pores are areas within the walls of the hollow fibers which are free of the material of the hollow fibers. If pores are present, the density of the hollow fiber is therefore less than the density of the raw material of the hollow fiber. Often, when pores are desired, the pore volume of the hollow fibers is up to about 90, for example about 10 to 80 and sometimes about 20 or 30 to 70 percent, based on the surface volume, i.e., the volume contained within the overall dimensions of the hollow fiber. The density of the hollow fiber may be substantially the same throughout its thickness, i.e., isotropic, or the hollow fiber may be characterized by having at least one relatively dense region within its thickness which is in a blocking relationship with respect to fluid flow through the wall of the hollow fiber, i.e., the hollow fiber is anisotropic. Generally, a relatively dense region of anisotropic hollow fibers is located substantially on the outside of the hollow fiber, and preferably the coating is in contact with this relatively dense region.

The material used for the hollow fiber may be a solid natural or synthetic substance. The selection of material for the hollow fiber may be based on the thermal resistance, solvent resistance and/or mechanical strength of the hollow fiber, as well as other factors dictated by an intended separation process in which the hollow fiber is used and the operating conditions to which it will be exposed. For example, the selection of material for the hollow fiber may be based on whether the material the hollow fiber is intended to substantially effect fluid separation or not, in which case the material of the hollow fiber has useful separation properties, or whether the coating is intended to effect fluid separation. The hollow fibers may be flexible or substantially rigid. The material used for the hollow fibers may be inorganic, e.g. to make hollow glass fibers, ceramic fibers, cermet fibers, or fibers of sintered metal or the like. In the case of polymers, both addition and condensation polymers are included, which can be prepared in any suitable manner to form porous hollow fibers. Generally organic, or organic polymers blended with inorganic materials (e.g. fillers), are used to make the hollow fibers. Typical polymers may be substituted or unsubstituted polymers, and may be selected from the following: polysulfones; Poly(styrenes) including styrene-containing copolymers such as acrylonitrile-styrene copolymers, styrene-butadiene copolymers and styrene-vinylbenzyl halide copolymers; polycarbonates; cellulosic polymers such as cellulose acetate; cellulose acetate butyrate, cellulose propionate, ethylcellulose, methylcellulose, nitrocellulose, etc.; polyamides and polyimides including arylpolyamides and arylpolyimides; polyethers; poly(arylene oxides) such as poly(phenylene oxide) and poly(xylylene oxide); poly(esteramide diisocyanate); polyurethanes; polyesters (including polyarylates) such as poly(ethylene terephthalate), poly(alkyl methacrylates), poly(alkyl acrylates), poly(phenylene terephthalate), etc.; polysulfides; Polymers of monomers having alpha-olefinic unsaturation other than those mentioned above, such as poly(ethylene), poly(propylene), poly(butene-1), poly(4-methylpentene-1), polyvinyls, e.g. poly(vinyl chloride), poly(vinyl fluoride), poly(vinylidene chloride), poly(vinylidene fluoride), poly(vinyl alcohol), poly(vinyl esters), such as poly(vinyl acetate), poly(vinyl propionate), poly(vinyl pyridines), poly(vinyl pyrrolidones), poly(vinyl ethers), poly(vinyl ketones), poly(vinyl aldehydes), such as poly(vinyl formal), poly(vinyl butyral), poly(vinylamides), poly(vinylamines), poly(vinyl urethanes), poly(vinyl ureas), poly(vinyl phosphates), and poly(vinyl sulfates), polyallyls, poly(benzobenzimidazole), polyhydrazides, polyoxadiazoles, polytriazoles, poly(benzimidazole), polycarbodiimides; polyphosphazines, etc. and copolymers including block terpolymers containing repeating units of the above, such as partial polymers of acrylonitrile, vinyl bromide sodium salt of para-sulfophenylmethallyl ethers, and graft polymers and blends containing any of the foregoing. Typical substituents forming substituted polymers include halogens such as fluorine, chlorine and bromine; hydroxyl groups; lower alkyl groups; lower alkoxy groups; monocyclic aryl; lower acyl groups and the like.

The thin membrane separation layer or membrane-forming material deposited on the porous substrate is generally applied as a solution in a suitable solvent. Those skilled in the art will appreciate that many such materials are available, and that some exhibit selectivities for some fluid mixtures and not for others. This has no implications for this invention since the process of this invention is directed to making composite membranes. US-A-4,214,020 contains a fairly detailed disclosure of membrane-forming organic synthetic substances which can be used in this invention to the extent that a suitable solution can be made.

Typical of suitable membrane-forming materials are polymers, which may be substituted or unsubstituted. The materials include synthetic rubbers; natural rubbers; liquids with relatively high molecular weight and/or high boiling points; organic prepolymers; poly(siloxanes) (silicone polymers); polysilazanes; polyurethanes; poly(epichlorohydrin); polyamines; polyimines; polyamides; acrylonitrile-containing copolymers, such as poly(α-chloroacrylonitrile) copolymers; polyesters (including polyactams and polyarylates), e.g. poly(alkyl acrylates) and poly(alkyl methacrylates) in which the alkyl groups have, for example, 1 to about 8 carbon atoms, polysebacates, polysuccinates and alkyd resins; cellulosic polymers; polysulfones, particularly aliphatic-containing polysulfones; Poly(alkylene glycols), such as poly(ethylene glycol), poly(propylene glycol), etc.; Poly(alkylene), polysulfates; polypyrrolidones; Polymers made from monomers having α-olefinic unsaturation, such as poly(olefins), e.g. poly(ethylene), poly(propylene), poly(butadiene), poly(2,3-dichlorobutadiene), poly(isoprene), poly(chloroprene), poly(styrene) including poly(styrene) copolymers, e.g. styrene-butadiene copolymer, polyvinyls such as poly(vinyl alcohols), poly(vinyl aldehydes) (e.g. poly(vinyl formal) and poly(vinyl butyral)), poly(vinyl ketones) (e.g. poly(methyl vinyl ketone)), poly(vinyl esters) (e.g. poly(vinyl benzoate)), poly(vinyl halides) (e.g. poly(vinyl bromide)), poly(vinylidene halides), poly(vinylidene carbonate), poly(N-vinylmaleimide), etc., Poly(1,5-cyclooctadiene), poly(methylisopropenyl ketone); fluorinated ethylene copolymers, poly(arylene oxides), e.g. poly(xylylene oxide); polycarbonates, polyphosphates, e.g. poly(ethylene methyl phosphate) and the like, and any copolymers, including block copolymers containing repeating units of the above, and graft polymers and blends containing any of the foregoing components, and monomers of the foregoing polymers.

Additional useful materials include poly(siloxanes). Typical poly(siloxanes) may have aliphatic or aromatic groups, and often have repeat units containing from 1 to about 20 carbon atoms. The molecular weight of the poly(siloxanes) can vary widely, but is generally at least about 1000. Often the poly(siloxanes) have a molecular weight of about 1000 to 300,000 when applied to the hollow fiber membrane. Common aliphatic and aromatic poly(siloxanes) include the poly(monosubstituted and disubstituted siloxanes), in which, for example, the substituents are lower aliphatic, e.g., lower alkyl including cycloalkyl, particularly methyl, ethyl and propyl; lower alkoxy; aryl including mono- or bicyclic aryl including phenylene, 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 with, for example, halogens, e.g., fluorine, chlorine and bromine, hydroxyl groups, lower alkyl groups, lower alkoxy groups, lower acyl groups, and the like. Some poly(siloxanes) do not wet a hollow fiber, e.g., a polysulfone hollow fiber, sufficiently to provide contact to the desired extent. However, achieving contact can be facilitated by dissolving or dispersing the poly(siloxane) in a solvent for the poly(siloxane) which does not substantially affect the polysulfone. Suitable solvents typically include liquid alkanes, e.g. pentane, cyclohexane, etc.; aliphatic alcohols, e.g. methanol; some halogenated alkanes and halogenated dialkyl ethers; and dialkyl ethers and the like, and mixtures thereof.

As previously indicated, the membrane-forming material is applied to the surface of the porous substrate from a solution, the membrane-forming solution. This is generally accomplished by passing the porous substrate surface through or immersing it in the solution. It will be understood that the porous substrate may be either dry or pre-wetted with a liquid compatible with the solvent used to prepare the membrane-forming solution. Any suitable solvent or mixture of solvents may be used for the membrane-forming material, and those skilled in the art will know which solvent is suitable for a particular membrane-forming material. The solvent is one which dissolves the membrane-forming material, and it may be inorganic or organic and form a homogeneous or uniform solution. The solvent used to prepare the membrane-forming material solution may also be a mixture of one or more solvents which dissolve the membrane-forming material and one or more non-solvents for the membrane-forming material. Such mixtures are generally preferred. The important consideration whether a single solvent or a mixture of solvent and non-solvent is used is that the membrane forming solution solution is preferably a substantially homogeneous or uniform solution. The solution should have a sufficiently low viscosity so that it can be easily applied as a smooth coating to the porous substrate and wets the porous substrate surface. The viscosity at the application temperature can vary from less than about 5 10⁻² Pa s (50 centipoise), and is generally between about 5 10⁻² Pa s to about 1 10⁻² to 2 10⁻² Pa s (about 0.05 to about 10 to 20 centipoise). The concentration of membrane-forming material in the membrane-forming solution may be between about 0.25 and about 10 percent by weight of the solution.

Illustrative examples of the materials that can be used to prepare the solution are the liquid aliphatic and aromatic hydrocarbons such as benzene, toluene, pentane, hexane, 2-ethylhexane, cyclopentane, methylcyclopentane, cyclohexane, etc.; alkanols, e.g. methanol, ethanol, the propanols, the butanols, cyclopentanol, cyclohexanol, benzyl alcohol, etc.; ketones, e.g. acetone, dimethyl ketone, diethyl ketone, methyl ethyl ketone, etc.; acids, e.g. acetic acid, propionic acid, hydrochloric acid, etc.; water; esters, e.g. ethyl acetate, etc.; halogenated alkanes and dialkyl ethers, dialkyl ethers; and the like, mixtures of which can also be used. In some cases, one of the materials in the mixture may be the solvent for the membrane-forming material, while the other of the materials may be the non-solvent; in other cases, both components may be solvents for the membrane-forming material.

A preliminary laboratory test to determine the suitability of a particular material as a solvent for the membrane forming solution for use in applying the coating to the porous substrate can be carried out by applying the material to be tested alone to the porous substrate and passing the treated substrate through the coagulation bath. A suitable material would not significantly affect the substrate, but an unsuitable one would. Using this test, materials can be selected which can be used in the process of the invention but which would not be suitable in known processes in which heating was necessary to remove the solvent material from the surface of the porous substrate, since heating quite often had a detrimental effect, particularly when the temperature required was too high. It is also possible to choose non-volatile solvents, e.g. strong mineral acids or high-boiling materials, if these are the only solvents suitable for dissolving the membrane-forming polymer material, since they can be easily removed by means of the coagulation bath.

After the porous substrate has been coated with the solution of the membrane-forming material, as previously indicated, the coated porous substrate is passed through a coagulation bath. The coagulation bath contains a liquid material which is a non-solvent for the polymeric membrane-forming material, but which is a solvent for the solvent used to prepare the solution of the polymeric membrane-forming material. In other words, the solvent of the coating material will be soluble in the coagulation bath, but the polymeric membrane-forming material will be substantially insoluble in the coagulation bath. As is known to any ordinarily educated scientist, this coagulation phenomenon is well known to those skilled in the art. Therefore, once one knows what solvent was used to dissolve the membrane-forming material, the ordinarily educated scientist can readily select an appropriate coagulation solvent for the coagulation bath. In general, when a mixture of solvent and non-solvent is used to prepare the solution of membrane-forming material, the coagulation bath can be the same non-solvent. Consequently, the specific composition of the coagulation bath will vary with the particular composition applied to the porous substrate, a fact which is obvious to those familiar with this coagulation phenomenon.

The following description sets forth one mode for carrying out the method of this invention. It will be apparent that this is not the only mode of operation and should not be construed as such.

The process begins by passing the porous hollow fibers, either wet or predried, at a temperature of from about 0°C or less to about 70°C or more, preferably at ambient temperature, through a solution of the membrane-forming material prepared as previously described. Any temperature may be used provided it does not have a detrimental effect on the porous substrate or coating. The fibers exit downwardly from the bottom of the vessel containing the solution of the membrane-forming material through a suitable nozzle arrangement as separate fibers, that is, they exit as individual fibers and do not contact each other. Such nozzle arrangements are known in the art and are commercially available or already in existence. The separated fibers exiting the nozzle continue downwardly and are subjected to a drying step, preferably by passing them through air at ambient conditions for a period of from about 1 to about 80 seconds, preferably from about 10 to about 20 seconds. Any other gaseous atmosphere may be used provided it does not have a detrimental effect on the product. The extent of this drying period will vary depending on the particular system used to coat the porous substrate and the temperature at which drying occurs, since temperatures below or above ambient temperature may be used if desired. It is important that some evaporation or drying of the solvent occurs and it is equally important that complete drying does not occur. A critical feature is that some residual amount of solvent is retained by the membrane forming solution on the coated porous substrate, generally about 10 to 90 percent by weight, and most frequently about 30 to 70 weight percent when it enters the coagulation bath. It is equally important that the downwardly moving coated fibers do not come into contact with each other or with any solid surface after they exit the nozzle assembly after being passed through the coagulation bath during the drying period, otherwise defects may form on the coated porous substrate. Following the brief drying step, the separated fibers enter the coagulation bath. This, again, may be at a temperature below or above ambient conditions, e.g., about 0°C or less to about 75°C or more, but ambient temperature is preferred. This bath is of sufficient size so that coagulation of the membrane-forming material on the surface of the porous substrate is completed before the coated porous substrate comes into contact with a solid surface. Simultaneous with this coagulation, the solvent present in the membrane-forming solution is dissolved in the coagulation bath. After coagulation has taken place, the separated fibers are guided around a pulley arranged at the bottom of the coagulation bath in the opposite direction to their original direction, they rise and leave the bath. They then exit via the pulley and the fiber separation assembly and the hollow fiber composite membrane is collected on a winding reel either wet or after a drying step.

In order to achieve maximum benefit during fluid separation using the composite membranes prepared by the process of this invention, it is usually preferred to use the highly selective coating materials for coating the porous substrate. These have high selectivity values and can be found in the Polymer Handbook published by John Wiley & Sons, N.Y., 1975. However, if high flux is desired, a material with a lower selectivity can be chosen. These aspects of membrane technology are well known to those skilled in the art.

The thin membrane applied to the surface of the porous substrate by the method of the invention has a thickness of about 50 to about 70 nm (about 500 to about 7000 Å), preferably about 75 to about 200 nm (about 750 to about 2000 Å).

Among the advantages that can be achieved by the process of the invention are the ability to produce permeable membranes that are thinner than those previously achievable; membranes with a more uniform surface; membranes that are substantially free of voids and pinholes; the ability to coat polymers that would be damaged by the solvent used to prepare the membrane-forming solution and the ability to use higher boiling solvents for the membrane-forming composition. As a result, the composite membranes have generally have improved values for selectivity, flux and permeation rates.

It is understood that as used herein, the selectivity or separation factor of a membrane or membrane module assembly represents the ratio of the permeation rate of the more easily permeable to the less easily permeable component of a mixture to be separated, with permeability being expressed in cm³(STP)/cm² day kPa (ft³(STP)/ft² day psi).

The following examples serve to illustrate the invention in more detail.

example 1

A spun porous polysulfone hollow fiber was dried at about 115 ºC and then passed through a hot air oven at about 182 ºC for a residence time of 10 seconds. This procedure resulted in the formation of an asymmetric substrate.

A membrane-forming solution of 1.5 weight percent cellulose acetate in a solvent mixture by volume of 50/50 acetic acid and isopropanol was prepared and filtered into a vessel having a multi-hole nozzle at the bottom thereof. The porous polysulfone hollow fibers prepared above were coated as previously described. They were passed through the cellulose acetate solution at a rate of 1.5 m (5 feet) per minute to coat the fibers at room temperature and exited the nozzle orifices. The solution-coated composite hollow fibers exited through their individual holes or orifices in the nozzle and moved downward in air for 14 seconds to evaporate solvent at ambient temperature without contacting each other or a solid surface. They were then immersed in a 10 L container of isopropanol at ambient temperature, which served as the coagulation bath. Isopropanol is a nonsolvent for the cellulose acetate and a solvent for the acetic acid/isopropanol solvent mixture. After coagulation, the cellulose acetate-coated porous polysulfone hollow fiber composite membranes were recovered as previously described.

The hollow fiber composite membranes were formed into 16-strand fluid separation modules with a length of 203 mm (8 inches) by conventional means for experimental purposes.

The porous polysulfone hollow fiber composite membrane modules were tested for their gas separation properties with a mixed gas feed of 30.5 vol.% hydrogen and 69.5 vol.% nitrogen at 13.8 bar (200 psi) and 25 ºC. The average selectivity for hydrogen separation between hydrogen and nitrogen was 59; the mean permeation rate (P/t) of hydrogen was 4.07 cm³(STP)/cm² day kPa (0.92 ft³(STP)/ft² day psi).

The same polysulfone hollow fibers were coated with the membrane-forming cellulose acetate solution at different coating speeds to vary the air evaporation time. Fluid separation modules were prepared and tested as described above. The results are summarized in Table I. Table I Run Coating Speed Air Evaporation Selectivity (Average)

As can be seen, the best results in terms of selectivity between hydrogen and nitrogen and in terms of permeation rate (P/t) are obtained with an air drying or air evaporation time of 14 seconds. For shorter air drying times, the selectivity is lower, but the permeation rate is higher.

Comparison test A

For comparison purposes, a module was prepared using the porous asymmetric polysulfone hollow fibers of Example 1 without the application of a cellulose acetate coating and tested for hydrogen separation properties as described in Example 1. The average selectivity between hydrogen and nitrogen was 3.3 and the average permeation rate was 446 cm³(STP)/cm² day kPa (10 0.92 ft³(STP)/ft² day psi). The separation was carried out at 1.7 bar (25 psi) and 26ºC.

Comparison test B

For comparison purposes, using the porous polysulfone hollow fibers of Example 1 before subjecting them to the heating step and without applying a cellulose acetate coating A module was fabricated and tested for hydrogen separation properties as described in Comparative Experiment A. The average selectivity between hydrogen and nitrogen was 1.3 and the average permeation rate was 619 cm³(STP)/cm² day kPa (140 ft³(STP)/ft² day psi).

A comparison of the permeation rate of Comparative Experiment A with that of Example 1 shows that in the Comparative Experiment the permeation rate or flux is substantially higher. However the selectivity is considerably lower, namely 3.3 in Comparative Experiment A and about 44 to 59 in Example 1. Thus, when the task is to separate one gas from another, in this case hydrogen from nitrogen, the membranes produced by the process of the invention are substantially superior; a high flux or permeation rate did not result in a satisfactory separation.

Example 2

A membrane forming solution of 1.0 weight percent cellulose acetate in a solvent mixture by volume of 50/50 acetic acid and isopropanol was prepared. The porous polysulfone hollow fibers described in Example 1 were coated with this solution following the procedure described in Example 1. The resulting cellulose acetate coated porous polysulfone hollow fiber composite membranes were tested for their hydrogen-nitrogen gas separation properties using the procedures described in Example 1. Table II summarizes the data and results. Table II Run Coating Speed Air Evaporation Selectivity (Average)

It can be observed that the selectivity decreases with decreasing air evaporation time, with an air evaporation time of 14 seconds showing the best selectivity when membranes produced by the process according to the invention are used with this system.

Example 3

A membrane forming solution of 0.75 weight percent cellulose acetate in a solvent mixture by volume of 50/50 acetic acid and isopropanol was prepared. The porous polysulfone hollow fibers described in Example 1 were coated with this solution following the procedure described in Example 1. The resulting cellulose acetate-coated porous polysulfone hollow fiber composite membranes were used to construct fluid separation modules and tested for their gas separation properties between hydrogen and nitrogen following the procedures described in Example 1. Table III summarizes the data and results. Table III Run Coating Speed Air Evaporation Selectivity (Average)

It should be noted that the selectivity decreases with decreasing air evaporation time, with an air evaporation time of 14 seconds showing the best selectivity when membranes produced by the process according to the invention are used with this system.

It is also noted that the concentration of cellulose acetate in the membrane-forming solution has little or no effect on the selectivity or permeation rate of the porous polysulfone hollow fiber membranes produced by the process of the invention. composite membranes and that in all cases the selectivity was better than that achieved with Comparative Example A or Comparative Example B.

Example 4

A membrane forming solution of 1.5 weight percent brominated polyphenylene oxide in carbon tetrachloride was prepared and filtered and placed in a vessel having a multi-hole nozzle at the bottom thereof. Following the procedure described in Example 1, the polysulfone hollow fibers described in Example 1 were coated with this solution using an air evaporation time of 14 seconds at ambient temperature. The resulting porous polysulfone-hollow fiber composite membranes coated with the brominated polyphenylene oxide were used to construct fluid separation modules and tested for their gas separation properties between oxygen and nitrogen using air as the feed at 25°C and 6.9 bar (100 psi). The mean selectivity for oxygen separation between oxygen and nitrogen was 4.2 and the mean permeation rate was 0.75 cm³(STP)/cm² day kPa (0.17 ft³(STP)/ft² day psi).

Example 5

A membrane-forming solution of 1.5 weight percent tetramethylbisphenol-A polysulfone polymer in a solvent mixture by volume of 50/50 1,1,1-trichloroethane and carbon tetrachloride was prepared and filtered. Following the procedure described in Example 1, the polysulfone hollow fibers described in Example 1 were coated with this solution, air dried at ambient temperature for 14 seconds, and immersed in a 10-L container of isopropanol used as a coagulation bath. Isopropanol is a nonsolvent for the tetramethylbisphenol-A polysulfone polymer and a solvent for the 1,1,1-trichloroethane/carbon tetrachloride solvent mixture. The porous polysulfone hollow fiber composite membranes coated with tetramethylbisphenol-A polysulfone polymer were used to construct fluid separation modules and tested for their gas separation properties between hydrogen and nitrogen using air as the feed at 25 ºC and 6.89 bar (100 psi). The average selectivity for oxygen separation between oxygen and nitrogen was 3.9 and the average permeation rate was 0.49 cm³(STP)/cm² day kPa (0.11 ft³(STP)/ft² day psi).

Claims (16)

1. A method of forming a hollow fiber composite membrane for fluid separation comprising: disposing on a porous substrate a thin, substantially smooth membrane separation layer that is substantially free of voids, pinholes and other defects, comprising: (i) coating a preformed porous hollow fiber substrate with a membrane-forming composition comprising a solution containing a membrane-forming material and a solvent therefor by applying the composition to the porous hollow fiber substrate; (ii) passing the porous hollow fiber substrate coated with the composition through a gas atmosphere to evaporate some, but not all, of the solvent(s) in the composition from the surface of the coated porous hollow fiber substrate; (iii) the coated porous hollow fiber substrate thus partially dried is passed into a liquid coagulation bath which is a non-solvent for the membrane-forming material and a solvent for the solvent in the solution used in the process step (i), whereby the membrane-forming material is coagulated on the surface of the porous hollow fiber substrate and the solvent for the membrane-forming material is dissolved in the liquid coagulation bath; and (iv) the hollow fibre composite membrane is obtained, characterized in that - in step (i) a uniform solution is used which has a viscosity of less than 5 10-2 Pa s (50 centipoise) at the application temperature, the concentration of the membrane-forming material in said solution being from about 0.25 to about 10% by weight of the solution; - contact of the coated surface of the hollow fiber substrate with a solid surface is avoided from the time of application of the solution of the membrane-forming composition to the surface of the porous hollow fiber substrate until the coagulation of the membrane-forming material on the surface of the porous hollow fiber substrate; and - the coated porous hollow fiber substrate is passed through the coagulation bath in such a way that shear forces are generated between the solvent-containing membrane-forming material on the porous substrate and the liquid coagulation bath, which shear forces tend to level the surface of the membrane-forming material and substantially fill any voids and pinholes that may be present there. 2. A method for forming a hollow fiber composite membrane according to claim 1, wherein the solvent solution used in step (i) comprises one or more solvents for the membrane-forming material. 3. A method for forming a hollow fiber composite membrane according to claim 1, wherein the solvent solution used in step (i) comprises one or more solvents for the membrane-forming material and one or more nonsolvents for the membrane-forming material. 4. A method of forming a hollow fiber composite membrane according to claim 1, wherein the temperature of the membrane-forming composition is between about 0°C and about 70°C. 5. A method for forming a hollow fiber composite membrane according to claim 1, wherein the temperature of the membrane-forming composition corresponds to the ambient temperature. 6. A method of forming a hollow fiber composite membrane according to claim 1, wherein the drying step (ii) has a duration of about 1 to 80 seconds. 7. A method of forming a hollow fiber composite membrane according to claim 1, wherein the drying step (ii) has a duration of about 10 to 20 seconds. 8. A method of forming a hollow fiber composite membrane according to claim 1, wherein the temperature of the liquid coagulation bath is about 0°C to about 75°C. 9. A method for forming a hollow fiber composite membrane according to claim 1, wherein the temperature of the liquid coagulation bath corresponds to the ambient temperature. 10. The method of claim 1, wherein the membrane-forming material is cellulose acetate. 11. The method of claim 1, wherein the membrane-forming material is brominated polyphenylene oxide. 12. The method of claim 1, wherein the membrane forming material is tetramethylbisphenol A polysulfone polymer. 13. The method of claim 1, wherein the surface of the preformed porous hollow fiber substrate has been pre-wetted prior to contacting the membrane-forming composition. 14. The method of claim 1, wherein the porous hollow fiber substrate is made of polysulfone. 15. Use of a selectively permeable hollow fiber composite membrane obtained by the process according to one of the preceding claims for separating at least one fluid in a fluid mixture from at least one other fluid in the fluid mixture. 16. Use according to claim 15, wherein the fluids are gases.