CA1255061A - Separation membranes - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A process for the separation of water from water-miscible organic compounds, comprising contacting a mixture of water and an organic compound against one side of a membrane comprising a salt of an anionic cellulose derivative or blends thereof with a noncellu-losic polyanion, and withdrawing at the other side of said membrane a mixture having a higher concentration of water.

Description

~L~55436~

ANIONIC POLYSACCHARIDE SEPARATION MEMBRANES

This invention relates to sepaxation mèmbranes and to methods for removing water from organic compounds using separation me~branes.

The effecti.ve removal of water rom organic S fluids is important in pollution control and in numer-ous industries such as in distilleries, the preparation of the anhydrous chemicals and the like~ While such separation~ are comparatively simple when the or~anic compound is immiscible with water, many organic com-pounds are partially or completely soluble in water.Separation of such organic compounds from water is sometimes carried out by distilling the mixture but this process re~uires large amvunts of energy. Moreover, some organic li~uids which have boiling points close to that of water or which form azeotxopic mixtures with water cannot be readily separated using a distillation process.

It has been found that certain materials, when formed into thin membranes, possess the capacity to selectively permit water to pass there~hrough while 29,662-F -1-~ZS~6~

preventing the passage of organic compounds. Thus, Bi~ning et al . in U. S . Patent Nos . 2, 953, 502 and 3,035,060 teach the separation of ethanol rom water using cellulose acetate and hydrolyzed polyvinyl ac~tate membranes. Chiang et al. in U.S. Patent Nos . 3, 750, 735; 3,950,247; 4,035,291 and 4,067,805 describe the separation of formaldehyde from water - employing a variety of membranes.

Unfortunately, previously known separation membranes do not exhibit a selectivity as hiyh as desired for many applications; that is, the water which permeates therethrough contains substantial amounts of organic compounds. Thus, it would be desirable to de~elop a separation membrane which more efficiently separates water from organic compounds.

The present invention particularly resides in a water-selective permeation membrane comprising a salt of a polysaccharide or polysaccharide derivative which bears a plurality of anionic gxoups derived from a strong or weak acid, said anionic groups being present in an amount sufficient to allow the membrane to permeate water while substantially impeding the permeation of organic compou~ds therethrough.

In another aspect, this invention is a permeation membrane compxi~ing a blend of the afore-mentioned salt of a poly~accharide or polysaccharide derivative with a salt of a non-cellulosic pol~mer having a plurality of anionic groups. The membranes of this invention exhibit surprisingly good selectivity for water, i.e., when contacted on one side with a fluid mixture of an organic compound and water, they 29,662-F -2-~S5C~

allow wat~r to perm~ate there-through while substan-tially preventing the permeation of the organic materials therethrough.

In another aspect, the invention resides in a process for separating mixtures of water and an organic compound comprising (a) contacting one side of a membrane com-prising a salt of a polysaccharide or polysaccharide derivative having a plurality of anionic groups derived from a strong or weak acid with a fluid feed mixture containing water and an organic compound, and (b) withdrawing from the other side of said membrane a permeate in vapor form, said permeate containing a higher concentrate of water ~han said feed mixture.

According to the method, surprisingly efi-cient separations of water and organic compounds can be effected, with the permeate containing a higher concen-tration of water than permeates obtained using conven-tional separation membranes.

The polysaccharides or derivatives thereof suitably employed in this invention are those which contain a plurality of pendant anionic groups~ Said anionic groups are derived from strong or weak acids and lnclude -S03 , -OS03 , -COO , -As03 , -TeO~ , -P03-, -HP03 and the like, with sulfate, sulfonate and carboxylate groups being preferred. Exemplary polysac charide~ and derivatives thereo~ include, but are not limited to, alginic acid salts, xanthan gums and

2~,662-F -3-:~2~S~6~

dexivatives thereof, and salts of anionic cellulose derivatives such as carboxyalkyl cellulose, carboxy-alkylalkyl cellulose, sulfoalkyl cellulose, cellulose sulfate, cellulose phosphate, cellulose arsenate, cellulose phosphinate, cellulose tellurate and the like~ Also useful are salts of anionic derivatives of high molecular weight starches and gums such as tragacanth, karaya, guar and the like which by them-selves or in blends can be formed into ilms of suf-ficient strength to operate as membranes. Of these,the diverse cellulose derivatives and alginic acid salts are preferred. Especially preferred are salts of carboxylate, sulfate or sulfonate-containing cellulose derivatives. Most preferred are salts of carboxymethyl cellulose.

Various polysaccharides such as alginic acid and xanthan gums contain anion.ic groups and do not need chemical modification to place anionic groups thereon.
Other polysaccharides, notably cellulose, do not contain anionic groups and must be mod:ified to impart anionic groups thereto. Anionic groups are generally attached to polysaccharides by substitution of one or more of the hydroxyl groups on the anhydroglucose units of the polysaccharide molecule. Various methods for affixin~
anionic groups to polysaccharide molecules are known in the art and are described, for example, in Bogan et al., "Cellulose Derivatives, Esters" and Greminger, "Cellu-lose Derivatives, Ethers," both in Kirk-Othmer Encyclo-~edia of Chemical Technolo~y, 3d Ed., Vol. 5, John Wiley and Sons, New York (1979). Carboxyalkyl groups, for example, can be attached to cellulose by the reaction of cellulose with a haloalkylcarboxylate. The alkyl gxoup ca~ contain up to ive carbon atoms but because ., ~, 29,662-F -4-~2SS~6~

the alkyl group -tends to impart hydrophobic character~
istics to the molecule, it is preferred that the alkyl group be methyl or ethyl. Cellulose sulfate can be prepared by reacting cellulose with mixtures of sulfuric acid and aliphatic alcohols, followed by neutralization with sodium hydroxide, or alternatively by reacting a dimethylformamide-sulfur trioxide complex with cellu-lose using excess dimethylformamide as the solvent. It is noted that membranes prepared from cellulose sulfate are brittle when dry and are advantageously kept moist after their preparation and throughout the period of their use. Cellulose phosphate is advantageously prepared by reacting cellulose with phosphoric acid in molten urea, or with a mixture of phosphoric acid, phosphorus pentoxide and an alcohol diluent.

In addition to the methods described herein-before, the hydroxymethyl groups of cellulose and like polysaccharides can be converted directly to carboxylate groups by o~idation and hydrolysis according to well-~0 known processes.

When a cellulose derivative is employed inthe membra~, the amount o ani~nic substitution on a cellulose molecule is expressed as the average num~er o anionic groups per anhydroglucose unit of the mole-cule (degree of substitution (DS)~. Since there arethree hydroxyl groups per anhydroglucose unit of a cellulose molecule, the DS can range from O to 3. For the purpose of this invention, the anionic degree of substitution must be sufficiently high that the mate-rials prepared therefrom will allow water to permeatetherethrough while substantially impeding ~he permea-tion of organic compounds. Advantageously, the DS is , 29,662-F -5-~55~

in the range from 0.1 to 3.0, preferably rom 0.3 to 1.5. In addition to the anionic substituent, the cellulose derivative can also contain other substi-tution, i.e., methyl, ethyl, hydroxyalkyl and the like in an amount such that said substitution does not substantially increase the permeability of the cellu-lose to organic compounds.

The anionic polysaccharide or polysaccharide derivative is in the salt form, the counterion being any cation which forms an ionic bond with the anionic groups of the pol~mers. Said cations generally include alkali metals, alkaline earth metals, tr~nsition metals, as well as ammonium ions of the form, R4N , where each R is hydrogen or methyi. Recause of their relative ease in preparation and improved selectivity, the counterion is preferably an alkali metal. It has been found that the selectivity and the permeation rate, i.e., the rate at which water permeates the membrane, are dependent on the choice of the counterion. For alkali metals, the selectivity of the membrane generally decreases slightly as the counterion is changed from sodium to potassium to cesium while the permeation rate increases as the counterion is varied in the same sequence. Eowever, the selectivities of the membranes of the in~ention are superior to those of conventional separation membranes even when cesium is employed as the counterion.

The anionic polysaccharide or polysaccharide derivative is advantageously converted to salt form by contacting said derivative with a dilute solution of the hydroxide of the desired counterion. Generally, 29,662-F -6-the salt can be formed in this manner at ambient condi-tions using relatively dilute, i.e., 0.02 to 1 molar solu-tions of the desired hydroxide. When the cationic specie~ form an insoluble hydroxide, a solution of a soluble ~alt of said cation is contacted with the anionic polysaccharide in order to convert said anionic polysaccharide to the desired salt form through an ion exchange proces~.

In a preferred embodiment of this invention, the anionic polysaccharide or polysaccharide derivative is blended with a salt of a polyanion which is not a polysaccharide having a plurality of groups derived from strong or weak acids such as are descri~ed herein-before. In general, the polyanion is chosen such that it forms solutions which are sufficiently compatible with solutions of the anionic polysaccharide or poly-saccharide derivative such that blends can be produced therefrom. The polyanion is employed in the salt form, with the counterions being those described hereinbefore.
The polyanion can be a homopolymer containing repeating anionic units such as polyacrylic acid or poly(sodium vinylsulfonate), or may be a copolymer having repeating anionic units and repeating nonionic units such as a styrene/sodium vinylsulfonate copolymer or sodium acrylate/alkyl acrylate copolymers. The polyanion has a molecular weight sufficiently high that ilms prepared therefrom do not rapidly dissolve or become distoxted in the presence of the water/organic mixture to be contacted therewith. Preferably, the polyanion is a homopolymer of an ethylenically unsaturated sulfonate or carboxylate with sodium polyacrylate, sodium poly-(vinyl sulfonate) and sodium poly(styrene sulfonate) being preferred.

,: .
29,662-F -7-8 ~;~S5(:~6~

The polyanion is employed in amounts suffici~nt to increase the charge density on the membrane but in amounts less than that which causes substantial incompatibility with the anionic polysaccharide derivative in the preparation of the membrane. Generally, such substantial incompatibility is evidenced by the separation of a solution containing these components into distinct phases. Said phase separation makes it difficult to prepare a film which is a blend of` the polyanion and the polysaccharide. In general, the polyanion will comprise up to about 70 weight percent, preferably less than 50 weight percent, more preferably less than 30 weight percent of the membrane.
The membranes of this invention are advantageously formed into films or hollow fibers.
Film~ may be formed by casting a solution of the selected materials onto a suitable surface and removing the solvent therefrom. Said films may be, for example, flat, concavs, or convex. Preferably, the membrane is cast from an aqueous solution. The solvent is generally rsmoved by evaporation at ambient ~ondition~
or at elevated temperatures, low pressures, or by other suitable techniQues. Membrane~ which are blends of an anionic poly~accharide or polysaccharide derivative and a polyanion are generally formed in the manner described hereinbefore by casting a film from the

3 solution containing both materials. Solutions containing both the anionic polysaccharide derivative and the polyanion are advantageously prepared by mixing solution~ of the anionic polysaccharide derivative with a solution of the polyanion or by mixing finely divided portions of each material and dis~olving the mixture into a suitable ~olvent.

29,662-F -8-~r~ `
,. ,~

~25S~

The anlonic polysaccharide derivative and the polyanion described hereinbefore are generally soluble in water and the use thereof is generally restricted to feed mixtures having relati~ely low concentrations of water, i.e., less than 50 weight percent water. Accord-ingly, it is highly preferred to crosslink the membranes in order to render them insoluble in water. Cross-linking of polysaccharides is known in the art and can be accomplished, for example, by reacting said polysac-charide with glyoxal or epihalohydrin ammonium hydroxide.When a blend of a polysaccharide and a polyanion is employed, crosslinks may be formed between the polysac-charide and the polyanion, solely between the polysac-charide, or solely between the polyanion.

The crosslinking agent is employed in an amount ~ufficient to render the membrane essentially insoluble in water. The crosslinking agent advanta-geously comprises from 1 to 30 weight percent of the membrane. The crosslinked membranes of this in~ention can be effectively employed using feed compositions containing even very high, i.e., 90 weight percent or more, concentrations of water~

In the preparation of crosslinked mem~ranes, the crosslinking agent is advantageously added to a solution of the polysaccharide, and ~he membrane formed into the desired shape. The membrane is cured after the removal of the solvent therefrom to crosslink the membrane. The particular means employed for curing the membrane will depend on a variety of factors includin~
the particular polymers and crosslinkers employed.
Generally, known procedures for curing crosslinked polymers, such as heating, irradiation and ~he like, 29,662-E' ~9-~ 24~

are advantageously employed to crosslink the membranes of the invention.

The membrane has a minimum thickness such that it is essentially continuous, i.e., there are essentially no pinholes or other leakage passages therein. However, the rate at which water permeates the membranes of this invention is inversely proportional to the thickness of the membrane. Accordingly, it is preferred to prepare a membrane as thin as possible in order to maximize the permeation rate while ensuring the integrity of the membrane~ The thickness of the membrane is advantageously in the range from about 0.1 to 250 microns, preferably from about 10 to about 50 microns. Mechanical strength can be imparted to ~he membrane by affixing the membrane to a porous supporting material. Particularly thin membranes can be formed by casting the membrane directly onto the porous support-ing material.

Separation of water from organic compounds is ~0 effected with the membranes of this invention using general procedures described in U.S. Patent Nos. 3,950,247 and 4,035,291 to Chiang e~ al. I~ general, the separation proce~s comprises contacting one side of the membranes of this invention with the fluid mixture containing an or~anic compound and water and withdrawing from the other side of the membrane a mixture containing a subætan'ially higher concentration of water. The feed mixture can be a mixture of gaseous and liquid components.
The permeate side of the membrane is maintained at a pressure less than the vapor pressure of water and is advantageously as low as about 0.1 mm of mercury.
Superatmospheric pressure may also be e~erted on the 29,662-F ~10-feed side of the membrane. The temperature at which the separations are conducted affects both the selec-tivity and the permeation rate. As the temperature increases, the permeation rate rapidly inc~eases, while selectivity decreases slightlyO The increase in rate, however, may be compensated for by the increase in energy needed to maintain the system at an elevated temperature. In general, the temperature is sufficiently high that the water has a substantial vapor pressure at the pressures at which the separation is effected, and is sufficiently low that the membrane remains stable.
Advantageously, the temperature is from -10C to g5C.

The membranes of this invention are most useful in separating water from organic compounds which are miscible with water. Exemplary water-miscible compounds include, but are not limited to, aliphatic alcohols such as methanol, ethanol, propanol, hexanol and the like; ketones such as ethyl methyl ketone, acetone, diethyl ketone and the likei aldehydes ~uch as formaldehyde, acet~ldehyde and the like~ alkyl esters of organic acids such as ethyl acetate, methylpropio-nate and the like; p-dioxane, alkyl and cycloalkyl amines and other water-miscible organic compounds which do not chemically react with or dissolve the membranes of this invention. In addition, the organic compound may be one in which water has a limited solubility, such as the chlorinated alkanes like chloroform and carbon tetrachloride~ Preferably, the organic compound is an aliphatic alcohol, a ketona, or an aldehyde, with lower alcohols, especially ethanol, being preferred.

The ability of a membrane to selectively permeate one component of a multi-component mixture is 29, 662-F

S~3~

expressed as the separation factor ~ which is defined as wt % A/wt % B in permeate ~A/B w t ~ A/wt % B in feed wherein A and B represent the components to be separated.
For the purposes of this invention, A will represent water.

The separation factor ~ is dependent on the type and concentrations of the components in the feed mixture as well as the relative concentrations thareof in the feed. Accordingly, it is also advantageous to express the efficiency of the separation membrane in terms of the composition of ~he permeate. The separa-tion membranes of this invention will generally haveseparatio~ factors for water/ethanol mixtures o~ at least 50, pre~erably at least 100, more preferably at least 500 and often will have separation factors of 2500 or more. The permeates obtained with the use of the separation membranes of this in~ention to separate ethanol/water mixtures will generally contain at least 90 weight percen~, preferably at least 98 weight per-cent, more preferably at least 99.5 weight percent water.

The separation membranes of this invention are especially useful in the pr~paration o anhydrous organic compounds, particularly when said compound forms an azeotropic mixture with water. In such systems, the membranes of this invention presant an economical alt~rnative to azeotropic distillation. The membranes of this invention can also be used in conjunction with 29,662-F -12-~5~

distillation processes to efect rapid, e~ficient removal of water from organic compounds.

The following examples are intended to illus-trate the invention but not to limit the scope thereof.
All parts and percentages are by weight unless otherwise indicated.

Membrane Sample No, 1 is prepared from an aqueous solution containing 4.25 percent sodium carboxy-methylcellulose. The carboxymethylcellulose has acarboxymethyl degree of substitution of about 0.9. The membranes are prepared by casting an excess of the solution onto a glass plate and allowing the water to evaporate, thereby yielding a film having a thickness of about 19.8 microns (O.78-mil).

The following apparatus is used to evaluate membrane Sample No. 1 and the samples in all subsequent examples. The membrane is placed into a Gelman in-line stainless steel filter holder which is modified so that a 14.19 cm2 section of the mem~rane is open to the feed solution. The membrane is supported with cellulosic filter paper and a porous metal disk~ The permeate side of the ilter holder is connected to a vacu~m pump with two cold traps placed in line to collect the permeate by condensation. The membrane and holder are then immersed in a closed flask containing the mixture to be separated. The flask is equipped with a thermo~
couple or thermometer for measuring temperature a~d a re~lux condenser to prevent feed loss due to evaporation.

2~662-F -13--~2S~

Separation is efected by pulling a vacuum of about 0.1 mm/EIg on the permeate side of the membrane and collecting the permeate in the cold traps. The temperature of the feed solution is as indicated in the S individual examples. The permeation rate is calculated by periodically weighing the collected permeate. The permeate composition is determined by gas chromatography analysis using a Hewlett Packard 5840A gas chromatoyraph e~uipped with a thermal conductivity detector. The column is a 1.83 met x O.32 cm (6 ft x 1/8 inch inside diameter) in Poropak QS column.

Sample No. 1 is evaluated according to the foregoing procedure using various ethanol/water mixtures as the feed composition. Each separation is effected at 25C ~mtil a steady state condition is obtained, i.e., until the permeation rate and permeate content are nearly constant over time. Once a steady state is reached, the content of the permeate and permeation rate are determined. The respective concentrations of water in the feeds, concentrations of water in the permeates, separation factoxs a~d permeation rates are as reported in Table I following.

T~BLE I

Separa- Permeation 25 % H O % ~ O tion Rate in ~eedn Pe ~eate Factor ~q-mil/m2~hr) 5.68 99.36 `2578 12.2 9.93 99.25 1200 37.8 18.19 99.49 ~77 126.5 20.10 9g.62 1042 128 29,662-F -14-~5~
-lS-It is seen from the foregoing Table I that the separation me~branes made ~rom sodium carboxymethyl-cellulose exhibit excellent selectivity for water/ethanol mixtures as expressed in terms of the separation factor or as expressed as the composition of the permeate.

A 4.25 percent solids solution containing 77 weight percent of the sodium carboxymethylcellulose having a degree of substitution of 0.85 and 23 weight percent sodium polyacrylate (based on the total solids weight) is prepared by mixing separate solutions of the sodium carboxymethylcellulose and the sodium polyacry-late. Membrane Sample No. 2 with an area of 14.1g cm2 and a thickness of 15.2 microns ~0.6 mil) is prepared as described in Example I. This membrane is used to separate several ethanol/water mixtures at 25C with ~he results given in Table II following.

TABLE II
r Separa- Permeation 20 % H O % H O tion Rate in ~eed in Pe~meate Factor (g-mil~ hr~

4.4 9~.6 3200 7.2 16O1 99.8 2600 103 19.7 99.7 1355 161 24.1 99.3 447 308 At all feed compositions, the permeate is essentially free of ethanol when a sodium carboxymethyl-cellulose/sodium polyacrylate membrane is employed to separate e~hanol and water mixtures.

29,~62-F -15-3~

Example 3 Membrane Sample No. 3 comprising 78.5 percent sodium carboxymethylcellulose having a degree of substi-tution of 0.9 and 21.5 weight percent polysodiumvinyl sulfonate is prepared according to the.methods described in Example 1. The membrane is 12.7 microns ~0.5 mil) thick and is evaluated with various ethanol/water mixtures at 25C with the results as given in Tahle III
following.

TABLE III

Separa- Permeation % H O % H O tion Rate in ~eed in Pe ~eate Factor (g-mil/m2-hr)

5.6 99.3 2391 4.6 1514.5 9~.9 5~91 56 19.1 99.9 4231 114 This membrane exhibits very high separation actors at al.1 feed compositions evaluated, with the permeate in each instance comprising almost entirely water..

Example 4 An aqueous solution o~ the sodium salt of cellulose sulfate having a sulfate degree of substi-tution of 2.5 is prepared.

A 1.5-mil membrane is prepared in the manner described in Example 1. The membrane is evaluated for 96.25 hours at 255C with results as reported in Table IV.

29,662-F -16-.~255~6~L

T~BLE IV

Separa- Permeation Time % H 0 % H~0 tion Rate (hr~ in F~ed in Pe~meate Factor (~-mil~m2_hr) 50.75 20.20 9~.68 ~9S 440.9

6.79 19.79 g9.64 1122 413.2 23.63 18.57 99.68 1366 372.8 96.2~ 14.57 99.85 3gO3 261.6 As can be seen from Table IV, excellent s~parations are obtained using the cellulose sulfate membrane.

Examy~le 5 A 19 micron (0.75 mil) thick film of alginic acid, sodium salt, prapared according to the general procedures described in Ex~mple 1, is used to separate an ethanol/water mixture. After 47 hours of operation, the average per~eation rate is 163 g-mil/m2 hr. The ~eed mi~ture comprises 19.2 percent by weight water and 80.8 percent by weight ethanol. The permeate coI~tains 99.5 percent water. The separation Xactor is 837.

To demonstrate the efect of the counterion on selectivity and permeation rate, a membrane is prepared from 80 percent sodium carbox~methylcellulose having a degree of substitution of 0.9 and 20 percent sodium polyacrylate. This membrane is converted to the hydrogen form by soaking the mem~rane in a 0.4 M ~Cl solution in 90 percent ethanol and 10 percent water.

`~:.,' 29,662-F -17-tj~6~ `

Conversion to acid form is confirmed from the IR spec-trum. The membrane is th~n soaked in a ~resh 90 percent ethanol, 10 percent water solution and evaluated for the separation of ethanol/water solution as described in Ex~mple 1. The feed composition initially contains lO.l percent of wat~r. After 52 hours of operation, the permeate contains 69.8 percent of water yielding a separation factor of 21. The permeation rate is 93 g-mil/m2-hr (2.36 g-mm/m2-hr).

The membrane is then converted to potassium form by soaking in a 0.5 M potassium hydroxide solution and 90 perce~t ethanol, 10 percent water for 3.75 hours~
The membrane is then soaked in fresh 90 percent ethanol, 10 percent water solution for 16 hours and dried. The conversion to potassium form is con~irmed by IR spectrum.
The membrane is then evaluated using an ethanol/water feed containing 20 percent water. When the water content of the feed is reduced to 19.2 percent, the separation factor is 697. When the water content of the feed is reduced to 13.9 percent, the separation ~actor is 51~8. At feed water content of 10.2 percent, the separation factor is 8795. In all cases, the permeate contains over 99 percent water. In addition to the greatly improved separatlon factor, the permea-tion rate increases when the membrane is converted topotassium form from about 93 g-mil/m2-hr (2.36 ~-mm/m2-hr) to as much as 595 g-mil/m2-hr (14.8 g-mm/m2-hr).

Example 7 Membrane Nos. VIIA-VIIF, having thicknesses as noted in Table V, are prepared from a 4.25 percent solids a~ueous solution containing 80 percent sodium-29,662-F -18-~2S~13~L

carboxymethylcellulose and 20 percen-t sodium polyacry-late, said percentages being based on the weight of the solids. The membrane is used to separate, at 25C, mixtures containing 11 weight percent water and 89 weight percent of the organic compounds noted in Table V
following. The permeate composition, selectivity factor ~, and permeation rates for each separation are as reported in Table V ollowing.

2~, 662-F -19-5~
~20'-~:: N
rl ~ O Lr) ' ~ O ~ ~ ' r~l Ul L~ N 11~~i N
~rl ~--I N N dl d h t~
P~

o rl S-l O O O O O O
O O O O O ~ O
ffl ~ r~ O O O ~ U) )-I O
a~ (d ~ O O o U') ~ O O~`
a~ a~ CO N

_ ~ I~ ~
~ ~ ~ ~ In a~
a~

.,, a O ~ O ~
O ~ ~ O ~ O
~ Q~
E~11~ ~~:: o ~ o -~ ~ o r~:l ~ a ~
h O .C Q. r4 ~ ,q o :1 .
O U ~ I I I I O O
aJ ~ ~ _l ,i ~ ~

tn r~ ~ I N O h aU rl ~ o N N C0 ~ O
~1 _ ,,,, O o a~
C~
.,, .
o a ~ ~ ,1 E~ ~ ,~ ~7 J~
a) h O
Z;
1~3 ~ H H H H H
~Z) t-~ H ~-1 H H H ~1 ., ,;
29, 662-F -20-.

~$~

As can be seen from the foregoing table, the membranes of this invention can be used to perform very efficient separations of water from a variety of organic compouncls.

29,662~F -21-.

Claims (10)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS
FOLLOWS:

1. A water-selective permeation membrane comprising a substantially non-porous anionic separation layer consisting essentially of a polysaccharide composition or blend, said polysaccharide being selected from the anionic groups consisting of (1) an alkali metal carboxyalkyl cellulose, (2) an alkali metal sulfoalkyl cellulose, (3) a C1-C4 alkyl or hydroxyalkyl derivative of (1) or (2), wherein the separation layer contains a plurality of pendant anionic groups in salt form and the anionic groups are present in an amount sufficient to allow water to permeate the separation layer while substantially impeding the permeation of organic compounds therethrough.

2. The membrane of Claim 1 wherein the polysaccharide is an alkali metal carboxyalkyl cellulose.

3. The membrane of Claim 2 wherein said polysaccharide derivative is a cellulose derivative containing from 0.1 to 3.0 anionic groups per anhydroglucose unit of the cellulose molecule.

4. The membrane of Claim 3 wherein the cellulose derivative is carboxymethyl cellulose.

5. The membrane of Claim 1 further comprising a salt of a polymer which is not a polysaccharide having a plurality of anionic groups in an amount sufficient to increase the charge density on said membrane.

6. The membrane of Claim 5 wherein the polymer which is not a polysaccharide is a polymer of acrylic acid, vinylsulfonic acid or styrene sulfonic acid.

7. The membrane of Claim 1 wherein the alkali metal is cesium.

8. The membrane of Claim 5 wherein the polymer which is not a polysaccharide comprises from 1 to 70 weight percent of said membrane.

9. The membrane of Claim 1 or 6 which is crosslinked in an amount sufficient to be insoluble in water.

10. The membrane of Claim 1 wherein the separation layer is supported on a porous substrate.