CA1280041C

CA1280041C - Pervaporation separation of ethanol-water mixtures using polyacrylic acid composite membranes

Info

Publication number: CA1280041C
Application number: CA000507673A
Authority: CA
Inventors: Hermann H. Neidlinger
Original assignee: US Department of Energy
Current assignee: US Department of Energy
Priority date: 1985-05-07
Filing date: 1986-04-25
Publication date: 1991-02-12
Anticipated expiration: 2008-02-12
Also published as: GB2174619B; GB8609815D0; JPS61281138A; IT1191792B; FR2581556A1; GB2174619A; IT8620334A1; IT8620334A0; DE3615325A1

Abstract

ABSTRACT
Synthetic, organic, polymeric membranes were prepared from polyacrylic acid salts for use with pervaporation apparatus in the separation of ethanol-water mixtures. The polymeric bacterial was prepared in dilute aqueous solution and coated onto a polysulfone support film, from which excess polymeric material was subsequently removed. Cross-links were then generated by limited exposure to toluene-2,4-diisocyanate solution, after which the prepared membrane was heat-cured. The resulting membrane structures showed high selectivity in permeating water over a wide range of feed concentrations.

Description

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BAC~GROUND OF T~E INVENTION
1. FIELD OF THE INVENTION
The invention generally relates to liquid purification or separation. More specifically, the invention relates to membrane materials for separation of ethanol and water mixtures. The invention also relates generally to coating processes in which a permselective product is produced, specifically a thin, dense coating on a microporous substrate.

2. DESCRIPTION OF TH~ PRIOR ART
Ethanol is commonly produced by fermentation processes, wherein the ethanol product is found in a water ~o()~

mixtore. The production of fuel-grade ethanol requires that the fermentation product be dried beyond the azeotrope. The usual drying process of distillation requires a significant amount of energy. Therefore, it is desirable to separate ethanol from fermentation beers by a more econonical method, such as by nxmbrane separation. In addition, significant preferential passage of ethanol at feed concentrations corresponding to fermentstion beers can be a significant result because it may pennit a fermentation process to operate at a low ethanol concentration while yielding a pervaporate sufficiently enriched for further processing by distillation or other ~eans.
Selective membranes have been used in reverse osmosis processes, such as in the desalination of seawater and the separation of azeotropic mixtures of aronatic and aliphatic hydrocarbons or close boiling isomers. A principal disadvantage of reverse osmosis is that a high pressure is needed in excess of the prevailing osmotic pressure to drive the permeate through the membrane. Pervaporation avoids the limitation of osmotic pressure imposed on reverse osmosis processes by m~intaining the permeate below its saturated vapor pressure. The heat of vaporization must be supplied to the permeating fraction in pervaporation, whereas during reverse osmosis there is no phase change and the heat of vaporization is not required. Thus, membranes used in the pervaporation 0 () L1 ~

process must meet more stringent membrane performance. To minimize energy input, membranes that pass water selectively would be of importance for solutions concentrated in ethanol, while membranes that pass ethanol selectively could remove ethanol directly from a fermentation bath. In either of these concentration regimes, osmotic pressures would hinder the competitlve use of reverse osmosis.
The membrane separation of ethanol from water is difficult, and those membranes used for the separation of ethanol from either simple aqueous mixtures or from fermentation beers using reverse osmosis or pervaporation have been successful usually only in achieving a permeate that is enriched in water. A small number of exceptions to this result have been noted in published literature, as follcw. It is reported in Heisler, E.G., A.S. Hunter, J.
Siciliano, R.H. Treadway, Science, Vol. 124, p. 77, 1956, that adding benzoic acid to the feed yielded a slight enrichment of ethanol in the permeate when used with a cellophane membrane. Eustache, H., and G. Histi, J. Membr.
Sci., Vol. 8, p. 105, 1981, report the use of pervaporation with a membrane of polydimethylsiloxane to yield a permeate enriched in ethanol. However, the latter measurements used feeds of only very low ethanol concentrations (ca. 0.1-1.0%).
Finally, Hoover, K.C., and S. T. Hw2ng, J. Membr. Sci., Vol.
10, p. 253, 1982, report the use of a silicone rubber ~0()41 membrane in a pervaporation column with good separation factors at low ethanol concentrations; however, there was essentially no separation at very high ethanol concentrations. Thus, the prior art has not produced a membrane that is well suited to the separation of ethanol from water over a wide range of concentrations.
A primary problem encountered in membrane technology used to separate ethanol from water mixtures remains the creation of a mem~rane material that optimizes the properties that permit high separation efficiency and permeability. Some of the factors that influence the permeation process in polymers include chemical composition, membrane homogeneity, and the imposed driving forces causing permeation. It remains unpredictable as to what membrane composition will best perform in these areas, as the mechanism or mechanisms of membrane separation remain somewhat controversial, although the general sorption-diffusion theory is supported by a growing body of evidence.
The efficiency of liquid permeation separations through polymer films depends primarily on whether there is an interaction, chemical or physical, between the solvent, solute, and polymer. The extent of the liquid-polymer interaction determines how swollen the polymer becomes.
These interactions arise in general from polar-, steric-, nonpolar-, or ionic-character of each of the above three 004~

components in the membrane system. The overall result of their interactions determines whether solvent, solute, or neither is preferentially sorbed at the membrane-solution interface.
Further, it has been observed that the permselectivity of a polymeric material increases as the general level of flux rate decreases. This aspect of transport behavior must be overcome for economic separation processes by appropriate changes in membrane geometry and by adjusting polymer composition, structure, and morphology to enhance transport behavior Gf the chosen penetrant. Both the diffusion coefficient and solubility coefficient of a penetrant are quite sensitive to minor variations in polymer composition and structure, which provides a possibility to experimentally derive useful permselective membrane materials.
Changes in membrane geometry are of great importance, as flux is inversely dependent on film thickress, while permeability constants are independent of thicXness.
Consequently, a very thin film can be highly permselective with excellent overall fluxes of the desired penetrant species. However, the presence and damaging effects of pinholes or other defects increase with decreasing membrane thickness. In order to develop optimum thin film materials, it is therefore essential that the dependence of permeability on factors that control transport processes be understood.

1~0041 The above noted factors, among others, demonstrate the difficulty faced in the development of a ~embrane having the combination of high selectivity and concurrent high flux of the permeating species. To achieve the foregoing and other objects and in accordance with the purpose of the present invention, as embodied and broadly described herein, the membrane and rnethod of rnanufacture of this invention may conprise the following.

SINUY~RY CF DNVENrICN
Against the described background, it is therefore a general object of the invention to provide a permselective membrane for water/ethanol mixtures.
Another general object of the invention i3 to provide a ~mbrane adapted to separate water-ethanol mixtures with a conbination of high selectivity and concurrent high flux of the permeating species.
A ~re specific object is to provide a synthetic, organic, polymeric menbrane that permeates water over a wide range of feed compositions in order to obtain a more highly concentrated water-ethanol solution.
Another specific object is to provide a process for making a In3nbrane capable of efficiently permeating at least a portion of the water fran a feed solution in pervaporation process.

Additional objects, advantages and novel features of the invention shall be set forth in part in the descrip-tion that follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by the practice of the invention. The object and the advantage of the invention may be realized and attained by means of the instrumentalities and in combinations parti-cularly pointed out in the appended claims.
In a process for preparing a composite membrane capable of selectively permeatiny water from a water-ethanol mixture by pervaporation, an aqueous solution is prepared of a synthetic, organic, polymeric substance comprising a salt of polyacrylic acid, wherein the solution contains no more than about 2% of polyacrylic acid, and wherein the sodium salt of polyacrylic acid has approximately 10~ residual acid groups and a molecular weight of about 6000. Then a micro-porous support member i.s coated with the prepared aqueous solution for a predetermined time sufficient to deposit a uniform coati.ng of the polymeric substance on the support member to provide high flux of water therethrough. There-after, the surface of the polymeric substance is cross-linked by treating the surface with a cross-linking agent for a predetermined time to produce interfacial addition reactions, after which the membrane is heat-cured to provide high selectivity to water permeation.

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According to another aspect of the invention, a permeation apparatus is provided, haviny a microporous sup-port member, a coating on the support member of a salt of acrylic acid with approximately 10% residual acid groups remaining and having a Molecular weight of about 6000, and a partially cross-linked surface network on the coating to provide high selectivity to water permeation. The apparatus forms a water permeable pervaporation separation membrane for separating water from ethanol-water mixtures.

'`` 1;~00~1 DESCRIPTION OE THE PREF'ERRED EMBODIMENTS
Synthetic organic polymeric membranes were developed that separate ethanol-water mixtures over a wide range of ethanol-water feed compositions. The membranes are characterized by the presence of a polymer group consisting of acrylic acid. In each case, the process of testing the membranes involved contacting a liquid feed mixture of ethanol and water against one side of a membrane and withdrawing at the second side a vapor phase mixture having a higher concentration of ethanol or water than was present in the feed mixture.
Membrane performance was measured and calculated to determine relevant parameters relating to performance with ethanol (e) or water (w). The diffusion flow or flux, Jw~ f substance _ through a film is defined as the amount passing during a unit time through a surface of unit area normal to the direction of flow.
A separation factor SFaw for substance _ in a system of two penetrants in a pervaporation process is defined as the ratio of the permeability constants of each penetrant in the membrane when the downstream pressure is close to zero, according to the equation:

SFaw = PW/ Pe where P is the permeability constant for the respective 1;~800~

substance w or e and is defined by the product of the solubility coefficient and the diffusion coefficient for the respective substance.
An alternative separation factor SFbW is defined as:
SFbW = PW/fw where p is the weight fraction of the substance w in the downstream phase (permeate) and fw is the weight fraction of the substance w in the upstream phase (feed). For a w selective membrane, SFaw will be greater than SFbW.
The efficiency or productivity factor of a pervaporator equipped with a given w-selective mernbrane can be derived to be proportional to the product:
(SFbw - I)J
where J is the permeate flux.
Efficient and selective poly~eric membranes were prepsred fr3n acrylic acid by dissolving the polymeric substance in water to a predeterrnined concentration, dip-coating a microporous support in the aqueous solution, applying a cross-linking agent to the treated support for a time sufficient to achieve a predetermined degree of cross-linking in the n~nbrane surface, and heat curing.
The m~nbrane support menber was chosen for its ability to carry the pol~neric merr~branes without interfering with or contributing to the separation. A polysulfone film 1~0(~1 - lo -was selected as the preferred support film, as an uncoated polysulfone film does not exhibit any selectivity in a water-ethanol system and, due to its microporosity, has a large flux of about S0 Llm2h.
The polymeric substances were dissolved in aqueous solution in order to obtain a desired fi~n thinness and uniformity by the dip-coating process. It has been found that concentrations of less than about 2% by weight are suitable, with the preferred concentration being generally less than about 1% for polyacrylic acid in order to produce a membrane that hss the desired thinness and uniformity. After the coating is applied to the support, a limited degree of cross-linking is desired to establish a cross-linked polymeric coating over the surface of the supported msmbrane structure. A suitable cross-linking agent such as toluene-2,4-diisocyanate in a hexane solution may be used to achieve the limited cross-linking by an interfacial addition reaction.
Exposure for approximately one minute is adequate to achieve the desired cross-linked surface on the polymeric film.
The composite ~mbrane is cured after cross-linking by drying in an oven at a temperature frcm 100C to 150C. The curing process also removes residual water and hexane fron the polymer and its supporting structure.
The membrane may be a simple disk or sheet of the 1~80()~1.

menbrane substance. H~vever, other forms of membrane may also be employed, such as hollow tubes and fibers. ~arious other shapes and sizes are readily adaptable to comnercial installations.
Synthetic organic polymeric membranes characterized by the presence of acrylic acid groups were produced and evaluated.
The membranes were prepared in different variations, as illustrated in the following examples:

Exan~le 1. Synthetic organic polymeric membranes characterized by the presence of acrylic acid groups were prepared from the sodium salt of polyacrylic acid. Polyacrylic acid was treated with sodiun hydroxide to partially neutralize the acrylic acid groups, with l~K residual acid groups remaining. The resulting campound, having a molecular weight of approximately 6000, was dissolved in water to produce an aqueous 0.68%
solution. A polysulfone support membrane was dip-coated in the solution by soaking for 10 minutes to form ~ membrane of substantially uniform thickness. Thereafter, the membrane ~vas drained for one minute and then dipped into a 0.5% TDI solution in hexane for one minute to generate cross-links through an interfacial film reaction. The prepared membrane was then heat-cured in a convection oven at 150C for one hour.
N~3nbrane perform~nce was evaluated in a pervaporation apparatus that consists of a constant temperature bath and pump 1~30()~

that circulates the feed through a radial-flow cell at a rate of about 1.4 L/min and with bath temperatures controlled to 0.1C.
The membrane is mounted on a porous plate of stainless steel embedded in the membrane cell. A downstream compartment consists of two parallel pumping stations that allow alternate sampling from cold traps. Five centimeter dianeter pumping lines connect to the lower surface of the nYnbrane to ensure that pressures downstream are well bel~w the saturated vapor pressures even for membranes passing up to 170 L/~ h. A thermocouple gauge located irrmediately downstream frcm the membrane was used as a serniquantitive rnonitor of the perrneate pressure. Resulting pervaporation data are presented below in Table 1, Table 2, and Table 3 for, respectively, 23C, 33C, and 43C bath temperatures.

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()()41 Analysis of the resulting data demonstrates that this membrane has a high water selectivity that increases with increasing feed concentration of ethanol. Membrane flux varies with temperature, increasing with the feed temperature. The permeability of water is decreased when ethanol is present.
The PAA membrane has a high efficiency over the entire concentration range. The productivity parameter (SFbW - l)J generally increases with increasing ethanol feed concentration up to about 90% ethanol. Above this concentration, membrane efficiency seems to level off or decrease.
Example 2. Synthetic polymeric membranes were prepared according to the techniques of Example 1, with the addition of an immersion in a 9.2~ by weight calcium chloride solution for 93 hours. The membranes were evaluated in a pervaporation apparatus and with procedures as described in Example 1. Resulting pervaporation data are presented below in Table 4.

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The data show an increase in water selectivity and a decrease in flux in comparison to the membranes discussed in Example 1. The high water selectivity is demonstrated, for example, by the permeate from a 9.6% by weight water feed composition, which becomes enriched to 92~ water.
The foregoing is considered as illustrative only of the principles of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described, and accordingly all suitable modifications and equivalents may be regarded as falling within the scope of the invention as defined by the claims that follow.

Claims

1. A process for preparation of a composite membrane capable of selectively permeating water from an ethanol-water mixture by pervaporation, comprising:
preparing an aqueous solution of a synthetic, organic, polymeric substance comprising a salt of polyacrylic acid said solution containing no more than about 2% by weight of said polyacrylic acid; wherein said sodium salt of polyacrylic acid has approximately 10% residual acid groups and molecular weight of about 6000;
coating a microporous support member with said prepared aqueous solution for a predetermined time sufficient to deposit a thin uniform coating of the polymeric substance substantially over a surface defined by the support member to provide high flux of water therethrough;
limited cross-linking the surface of said polymeric substance coating by treating the surface with a cross-linking agent to produce interfacial addition reactions for a predetermined time; and heat curing said cross-linked membrane to provide high selectivity to water permeation.

2. The process of claim 1, wherein said coating step is by soaking said microporous support member in said aqueous solution for approximately ten minutes.

3. The process of claim 2, further comprising, after soaking the support member, draining the support member for approximately one minute before applying said cross-linking agent.

4. The process of claim 1, wherein said cross-linking agent is a solution of toluene-2,4-diisocyanate in hexane.

5. The process of claim 1, wherein said cross-linking agent is approximately a 0.5% by weight solution of toluene-2,4-diisocyanate in hexane, and said predetermined time of exposure is approximately one minute.

6. The process of claim 1, wherein said heat curing step is conducted at approximately 150°C. for about one hour.

7. The process of claim 1, wherein said microporous support member comprises a polysulfone film.

8. The process of claim l, wherein said microporous support member comprises a polysulfone film, and wherein said cross-linking agent is a solution of toluene-2,4-diisocyanate in hexane.

9. A permeation apparatus, comprising:
a microporous support member;

a thin coating substantially covering a surface defined by said support member to provide high water flux there-through, said coating comprising a salt of acrylic acid with approximately 10% residual acid groups remaining and having a molecular weight of about 6000;
a partially cross-linked surface network on said coating to provide high selectivity to water permeation; and said permeation apparatus forming a water permeable pervaporation separation membrane operative to separate water from ethanol-water mixtures.

10. The permeation apparatus of claim 9, wherein said support member comprises a polysulfone film.

11. The permeation apparatus of claim 9, wherein said apparatus is the product of the process comprising:
soaking said microporous support in an aqueous solution of the acrylic acid coating material containing no greater than about 2% by weight of acrylic acid for a time sufficient to deposit a uniform coating layer on the support;
removing drainable solution from the coated support;
and exposing the coated support to a cross-linking agent for a time sufficient to form a partially cross-linked surface network of the coating material.

12. The permeation apparatus of claim 9, characterized by said apparatus being the product of the process wherein said cross-linking agent is 0.5% by weight toluene-2,4-diisocyanate applied to the surface of said coating for about 1 minute.

13. The permeation apparatus of claim 9, characterized by said apparatus being the product of the process further comprising, after forming said cross-linked surface network, applying heat in a quantity and for a time sufficient to cure the network.

14. The permeation apparatus of claim 9, characterized by said coating having the resultant composition of one prepared by the process of a polysulfone support member being soaked for approximately 10 minutes in an aqueous 0.67% by weight solution of a sodium salt of polyacrylic acid; the coated member thereafter having cross-links generated by exposure for about 1 minute to 0.5% by weight toluene-2,4-diisocyanate in hexane; and followed by heat curing at about 150°C. for approximately l hour.

15. The apparatus of claim 14, wherein before said cross-links are generated, said support member is drained for about 1 minute.

16. The method of separating water from an ethanol-water mixture through a membrane film, comprising:

providing a polymeric membrane film of polyacrylic acid groups supported on a microporous member and having a par-tially cross-linked and heat-cured surface, said polymeric film being formed by soaking said microporous support in an aqueous solution of a coating material containing no greater than about 2% by weight acrylic acid groups for a time suf-ficient to deposit a uniform thin coating layer on the sup-port to provide high water flux and selectivity, wherein the coating material is characterized by the presence of poly-acrylic said in which the acrylic acid groups are partially neutralized such that about 10% residual acid groups remain and the resulting compound has a molecular weight of approx-imately 6000, and then partially cross-linking the surface of said thin polymeric layer followed by heat curing the cross-linking network;
providing an ethanol-water feed solution on a first side of said membrane film;
applying a pressure differential between first and second sides of the membrane film sufficient to include a high flux diffusion flow of a substantially water permeate from the feed solution through the membrane; and recovering residual ethanol product solution from the first side of the membrane film.

17. The method of claim 16, wherein said permeate com-prises a relatively decreased weight fraction of ethanol as compared to the feed solution at substantially any water-ethanol concentration.

18. The method of claim 16, wherein said cross-linking agent comprises 0.5% by weight toluene-2,4-diisocyanate applied to the surface of said coating for about 1 minute.

19. The method of claim 16, wherein said polymeric membrane film is formed by the process further comprising treating the heat-cured membrane with calcium chloride to increase water selectivity in the permeate.

20. The method of claim 19, wherein the membrane is immersed in approximately a 9.2% solution by weight calcium chloride for a time sufficient to increase water selectively in the permeate.