CA2039419A1

CA2039419A1 - Pervaporation process for separating a lower alcohol compound from a mixture of a lower alcohol compound and an ether compound

Info

Publication number: CA2039419A1
Application number: CA002039419A
Authority: CA
Inventors: Masayuki Nakatani; Makoto Matsuo; Kanji Nakagawa
Original assignee: Ube Industries Ltd
Current assignee: Ube Corp
Priority date: 1990-03-31
Filing date: 1991-03-28
Publication date: 1991-10-01
Also published as: JP2745767B2; GB2242429B; JPH03284334A; GB2242429A; GB9106535D0

Abstract

ABSTRACT OF THE DISCLOSURE
A lower alcohol compound is separated from a mixture of a lower alcohol compound and an ether compound by a pervaporation method comprising the steps of: bringing a feed comprising an ether compound and a lower alcohol compound into direct contact with a feed face of a specific asymmetric separating membrane comprising an aromatic imide polymer which has 70 to 100 molar% of recurring units selected from those of the formulae (I) and (II) (I) and

Description

- 1 - ;~039~19 PERVAPORATION PROCESS FOR SEPARATING A LOWER A~COHOL
COMPOUND FROM A MIXTURE OF A LOWER ALCOHOL
COMPOUND AND AN ETHER COMPOUND

BACKGROUND OF THE INVENTION
1) Field of the Invention The present invention relates to a pervapora-tion process for separating a lower alcohol compound from a mixture of a lower alcohol compound and an ether compound through an aromatic imide polymer asymmetric separating membrane.
More particularly, the present invention relates to a pervaporation process for separating a lower alcohol compound from a mixture of a lower alcohol compound and an ether compound through an aromatic imide polymer asymmetric separating membrane through which the liquid lower alcohol compound is selectively permeated and separated from the ether compound, at a high 5 selectivity and at a high permeation rate.

2) Description of the Related Arts It is known that a liquid mixture of two or more types of organic compounds can be separated into individual organic components by a distillation method, but in the distillation method, some of the organic compounds form an azeotropic mixture, or have boiling points close to each other, or are chemically modified at the distillation temperature, and therefore, the separation must be carried out by a complicated process, for example, a combination of the distillation procedure with ar. addition of a additional component or with a distillation-extraction procedure. It is very difficult to smoothly carry out this complicated separation procedure, and a large amount of energy is consumed thereby.
To avoid the above difficulty, a method of separating the liquid organic compound mixture by using - 2 - ~ ~39419 a semipermeable membrane has been attempted. In this method, wherein a semipermeable membrane is used to separate or concentrate an organic compound aqueous solution, a diluted aqueous solution of organic compounds is brought into contact with a face of a semipermeable membrane, to allow a specific liquid organic component to selectively permeate through the membrane due to a differential osmotic pressure. This method is referred to as a reverse osmosis method.
Usually, in the reverse osmosis method, a higher pressure than the osmosis pressure of the aqueous solution must be applied to the aqueous solution.
Therefore, the reverse osmosis method cannot be applied to a concentrated organic compound aqueous solution which exhibits a high osmotic pressure, and accordingly, the reverse osmosis method can be applied only to organic compound aqueous solutions having a limited concentration.
Recently, as a new type of separating method different from the conventional semipermeable membrane method, in which a separating membrane is used, a pervaporation method has been developed for a liquid organic compound mixture and is now under serious consideration in this field.
In the pervaporation method, an organic compound mixture in the state of a liquid is brought into direct contact with a feed side face of a separating membrane capable of selectively allowing a specific organic compound to permeate therethrough, and the opposite delivery side face of the membrane is exposed to a vacuum or a reduced pressure. The specific compound is allowed to selectively permeate through the membrane and is collected in the state of a vapor at the opposite delivery side of the membrane. This method is useful for selectively separating or concentrating an individual organic compound from a liquid organic compound mixture.

_ 3 _ ~0~941~

Many proposed pervaporation methods have been reported.
For example, Japanese Unexamined Patent Publication No. 52-11188~ discloses a separation of a benzene-cyclohexane mixture solution or benzene-hexane mixture solution by using an ionomer type polymer; and Japanese Unexamined Patent Publication No. 59-30441 discloses a pervaporation separation of the above-mentioned mixture solu~ion by using a polyamide membrane. Also, Japanese Unexamined Patent Publication No. 2-35921 discloses a pervaporation separation of an organic compound aqueous solution through an aromatic imide polymer membrane.
Nevertheless, the conventional separating membranes for the pervaporation method are disadvanta-geous in that;
(1) the permeation rate of the meI~brane for individual organic compounds to be separated or concen-trated is unsatisfactorily low;
(2) the selectivity of the membrane for separating the individual organic compounds is unsatisfactory;

(3) the heat resistance and solvent resistance of the membrane are unsatisfactory; and (4) the membrane has a low durability, and thus cannot be employed continuously over a long time;
e.g., when separating various individual organic compounds from a mixture.
Accordingly, the conventional separating membrane for the pervaporation separation method is practically useful only for limited aqueous solutions or mixtures of specific organic compounds, and only data on the separating properties of the above-mentioned specific compounds is disclosed. Accordingly, the conventional pervaporation separation method cannot be industrially utilized, and therefore, substantially no example of industrial utilization of the conventional - 4 - 2039~19 pervaporation separation has been reported.
Particularly, a separation of lower alcohol compound from a liquid mixture of the organic ether compound with a lower alcohol compound can be effected only by an improved distillation method. This method, however, is disadvantageous in that it involves a large consumption of energy, and therefore, it is not useful as an industrial separation process.
Vnder the above-mentioned circumstances, there is an urgent need for the provision of an improved pervaporation process for separating a lower alcohol compound from a mixture of a lower alcohol compound with an ether compound through a separating membrane, with a small energy consumption and at a high efficiency.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a pervaporation method of separating a lower alcohol compound from a mixture of a lower alcohol compound with an ether compound through a separating membrane, at a high selectivity and a high permeation rate.
Another object of the present invention is to provide a pervaporation method of industrially separating a lower alcohol compound from a mixture o~ a lower alcohol compound with an ether compound through a specific aromatic imide polymer asymmetric separating membrane at a high efficiency.
The above-mentioned objects can be attained by the pervaporation method of the present invention for separating a lower alcohol compound from a mixture of a lower alcohol compound with an ether compound, which method comprises the steps of bringing a feed comprising a liquid aliphatic ether compound having 2 to 8 carbon atoms and a liquid aliphatic alcohol compound having l to 4 carbon atoms into direct contact with a feed face of an asymmetric separating membrane comprising a heat resistant aromatic imide polymer; exposing a delivery face opposite to the feed face of the separating ~39~19 membrane to an atmosphere under a reduced pressure to cause the lower aliphatic alcohol compound to selectively penetra~e and permeate through the separating membrane and then to be vaporized in the delivery face side of the asymmetric separating membrane, and collecting the lower alcohol compound at the delivery face side of the separating membrane.
DESCRIPTION OF THE PREFERRED EMBODIME~TS
In the first step of the pervaporation method of the present invention for separating a lower alcohol compound from a mixture of a lower alcohol compound with an ether compound, a feed comprising a liquid aliphatic ether compound having 2 to 8 carbon atoms and a liquid lower aliphatic alcohol compound having l to 4 carbon atoms is brought into direct contact with a feed face of an asymmetric separating membrane comprising a heat resistant aromatic imide polymer.
The aromatic imide polymer usable for forming the asymmetric separating membrane preferably contains 70 to l00 molar%, more preferably 80 to l00 molar%, still more preferably 90 to l00 molar%, of at least one type of recurring units selected from those of the formulae (I) and(II~:

~ 6C ~ ~ >N-R ~ (I) and ~ ,C ~ X ~ (II) wherein R represents a divalent aromatic group having at least a two benzene ring structure, and X represents a member selected from the group consisting of -S-, -~2-' -CO-~ -O-~ -C(CH3)2-~ -CH2- and -C(CF3)2-- The benzene 2039~9 rin~ structure represented by R in the formulae (I) and (II) is derived from an aromatic diamine having at least a two benzene ring structure, of the formula:
H2N R-NH2.
The above-mentioned aromatic imide polymer can be prepared by dissolving a mixture of:
(A) an aromatic tetracarboxylic acid component comprising:
(a) preferably 70 to l00 molar%, more preferably 80 to l00 molar%, still. more preferably 90 to l00 molar%, of at least one principal member selected from aromatic tetracarboxylic acids of the formulae (III) and (IV):
HOOC ~ f~ COOH (III) HOOC COOH
and HOOC ~ X ~ COOH (IV) HOOC COOH
wherein X is as defined above, and dianhydrides, esters and salts of the above-mentioned principal acids, and (b) preferably 0 to 30 molar~, more pref-erably 0 to 20 molar%, still more preferably 0 to l0 molar%, of at least one additional member selected from arornatic tetracarboxylic acids and dianhydrides, esters and salts of those acids which are different from the above-mentioned compound for the principal member;
with (B) an aromatic diamine component comprising:
(c) preferably 70 to l00 molar%, more preferably 80 to l00 molar%, still more preferably 90 to l00 molar% of at least one principal member selected from aromatic diamine having at least two, preferably 2 to 4, benzene ring structures, and (d) preferably 0 to 30 molar%, more pref-erably 0 to 20 molar%, still more preferably 0 to lO molar%, of at least one additional member selected from aromatic diamines different from the above-mentioned compounds for the principal member, in an organic solvent comprising at least one phenol compound, and then by subjecting the solution to a polymeriza-tion-imidization procedure a~ a high temperature of 150C to 250C or at a low temperature of about 10C to 100C in the presence of a imidizing agent, for example, acetic anhydride or pyridine.
The phenol compound usable for the organic solvent is preferably selected from phenol, 2-chloro-phenol, 4-chlorophenol, 4-bromophenol, and cresol. The solvent may contain N,N-dimethyl acetamide or dimethyl-sulfoxide.
The aromatic tetracarboxylic acid compound of the formula (III) for the principal acid member (a) is preferably selected from the group consisting of 2,3,3',4~-biphenyltetracarboxylic acid, 3,3~,4,4~-biphenyltetracarboxylic acid, and dianhydrides, lower alkyl esters, preferably having l to 3 carbon atoms and salts of the above-mentioned acids.
Also, the aromatic tetracarboxylic acid compound of the formula (IV) for the principal acid member (a) is preferably selected from the group consisting of diphenylether tetracarboxylic acids, for example, 3,3',4,4'-diphenylether tetracarboxylic acid;
benzophenone tetracarboxylic acids, for example, 3,3~,4,4'-benzophenone tetracarboxylic acid; diphenyl-sulfone tetracarboxylic acids, for example, 3,3',4,4'-diphenylsulfone tetracarboxylic acid; and 2,2-diphenyl-propane- or hexafluoropropane-tetracarboxylic acids, for example, 2,2-bis(3,4-carboxyphenyl) propane or 2,2-bis(3,4-carboxyphenyl) hexafluoropropane, and dian-hydrides, lower alkyl esters, preferably having l to 3 carbon atoms, and salts of the above-mentioned acids.
Among the above-mentioned aromatic tetra-carboxylic acid compounds usable for the principal - 8 - ~039419 member (a), 3,3',4,4~-biphenyl tetracarboxylic dianhydride and 3,3',4,4~-diphenylether tetracarboxylic dianhydride exhibit an excellent polyimide-forming property and greatly contribute to the obtaining of an aromatic imide polymer with a superior film-forming property and the forming of an asymmetric separating membrane having a high separating property for the aliphatic ether-alcohol mixtures, and to a satisfactory durability in practical use and a high mechanical strength and heat resistance.
The aromatic tetracarboxylic acid compound for the additional member (b) is preferably selected from the group consisting of pyromellitic acid, and dianhydride, lower alkyl esters, preferably having l to 3 carbon atoms, and salts of the above-mentioned acld.
In the aromatic tetracarboxylic acid component (A) when the content of the principal aromatic tetra-carboxylic acid component (a) is less than 70 molar%, or the content of the additional aromatic tetracarboxylic acid compound (b) is more than 30 molar%, the resultant aromatic imide polymer sometimes exhibits a poor solubility in phenolic solvents, and thus it becomes difficult to produce an asymmetric separating membrane having a uniform quality or a satisfactory pervapora-tion-separation property for organic compounds.
The aromatic diamine compound having two or more benzene ring structures and usable for the principal member (c) of the aromatic diamine component (B) are preferably selected from the group consisting of diaminodiphenylethers, diaminodiphenylthioethers, diaminodiphenylsulfons, diaminodiphenylmethanes, diaminodiphenylpropanes, diaminodibenzothiophenes, diaminodiphenylenesulfon!" diaminothioxanthones, and diaminothioxanthenes which have -two benzene ring structures; bis(aminophenoxy) benzenes and di(amino-phenyl) benzenes, which have three benzene ring structures; and di[(aminophenoxy)phenyl]alkanes, di[(aminophenoxy)phenyl]sulfons and di(aminophenoxy)bi-phenyls, which have four benzene ring structures.
The aromatic diamine compound having two benzene ring structures is preferably selected from 4,4~-diaminodiphenylether, 3,4'-diaminodiphenylether, 4,4'-diaminodipheny].methane, 3,4'-diaminodiphenyl-methane, 3,3'-dimethyl-4,4~-diaminodiphenylmethane, 2,2-bis(4-aminophenyl)propane, 2,2-bis(3-amino-phenyl)propane, 3,4'-diamino-(2,2-di-phenylpropane), 4,4'-diaminodiphenylsulfone, 3,3'-diaminodiphenylsul-fone, o- and m-dianisidine, and diamine compounds of the formulae (V) and (VI):
Rl ~ 2 '1 (V' H2N Sn NH2 and H~N S~r ~ ~H2 (VI) wherein R1 , R2 r R3 and R4 respectively and indepen-dently from each other represent a member selected from a hydrogen atom and methyl and ethyl radicals and n represents zero or 2.
The diamine compounds of the formula (V) include diaminobenzothiophene compounds, for example, 3,7-diaminodibenzothiophene, 2,8-dimethyl-3,7-diamino-dibenzothiophene, 2,6-dimethyl-3,7-diaminodibenzo-thiophene, 2,8-diethyl-3,7-diaminodibenzothiophene, 2,6-diethyl-3,7-diaminodibenzothiophene, and 4,6-diethyl-3,7-diaminodibenzothiophene, and diaminodibenzothiophene-5,5-dioxide(diaminodiphenylene-sulfon) compounds, for example, 3,7-diaminodibenzo-thiophene-5,5-dioxide, 2,8-dimethyl-3,7-diaminodibenzothiophene-5,5-dioxide, 2,6-dimethyl-3,7-diamino-dibenzothiophene-5,5-dioxide, 4,6-dimethyl-- 1 o Z0394~9 3,7-diaminodibenzothiophene-5,5-dioxide, 2,8-diethyl-3,7-diaminodibenzothiophene-5,5-dioxide, 2,6-diethyl-3,7-diaminodibenzothiophene-5,5-dioxide, and 4,6-diethyi-3,7-diaminodibenzothiophene-5,5-dioxide.
The diamine compounds of the formula (VI) include diaminothioxanthene compounds, for example, 3,7-diaminothioxanthene, 2,8-dimethyl-3,7-diaminothio-xanthene, 2,6-dimethyl-3,7-diaminothioxanthene, and 4,6-dimethyl-3,7-diaminothioxanthene; and diaminothio-xanthene-5,5-dioxide compounds, for example, 3,7-di-aminothioxanthene-5,5-dioxide, 2,8-dimethyl-3,7-diamino-thioxanthene-5,5-dioxide, 2,6-dimethyl-3,7-diaminothio-xanthene-5,5-dioxide, and 4,6-dimethyl-3,7-diaminothio-xanthene-5,5-dioxide.
The aromatic diamine compounds having three benzene structures include bis(aminophenoxy)benzene compounds, for example, l,4-bis(4-aminophenoxy)benzene, l,4-bis(3-aminophenoxy)benzene, and l,3-bis(4-amino-phenoxy)benzene.
The aromatic diamine compounds usable for the additional member of the aromatic diamine component include phenylene diamine compounds, for example, m-phenylene diamine p-phenylene diamine, which are used preferably in a content of lO molar% or less, and diaminobenzoic acid compounds and alkylphenyl diamine compounds which are used preferably in a content of 30 molar% or less.
The above-mentioned type of aromatic imide polymer is soluble in the afore-mentioned organic solvent, and thus can be converted to an asymmetric separating membrane.
The asymmetric separating membrane is preferably in the form of a hollow filament or a film.
The asymmetric membrane usable for the method of the present invention can be prepared by dissolving the solvent soluble polymerization product of the aromatic tetracarboxylic acid component (A) with the - 1 1 - z~3~3~19 aromatic diamine component (B), namely, a solvent soluble specific aromatic imide polymer, in a solvent comprising at least one phenolic compound to provide a dope solution; shaping the dope solution into a hollow filament-formed stream or a film-formed layer; bringing the shaped dope solution into contact witn a coagulating bath to provide a solidified membrane; washing the solidi~ied membrane with an organic solvent not capable of dissolving the solidified membrane; drying the washed membrane; and aging the dried membrane.
In the asymmetric membrane-preparing process, the aromatic imide polymer is dissolved in the above-mentioned solvent to provide a dope solution.
Preferably, in the dope solution, the aromatic imide polymer is in a concentration of 5 to 30% by weight, more preferably lO to 25% by weight.
The dope solution is shaped in a hollow filamentary stream thereof by extruding through a spinning nozzle for hollow filaments or in a flat filmy Z layer by extruding through a slit for film, or by spreading on a film-forming surface, for example, a horizontal surface of a film-forming plate or a peri-pheral surface of a rotating film-forming drum. The resultant shaped dope solution is brought into contact with a -oagulating liquid to provide a solidified membrane. The coagulating liquid is compatible with the solvent in the dope solution but cannot dissolve therein the aromatic imide polymer in the dope solution. The coagulated aromatic imide polymer asymmetric membrane is washed with an organic solvent not capable of dissolving the solidified membrane. The washing organic solvent comprises at least one member selected from, for example, lower aliphatic alcohols, for example, methyl alcohol, ethyl alcohol, propyl alcohols and butyl alcohols and aliphatic and cycloaliphatic hydrocarbons, for example, n-hexane, n-heptane n-octane and cyclohexane.

Z~)39~ 9 Then, the washed membrane is dried and aged or heat treated at a temperature of 150C to 400C, preferably 160C to 350C for 1 second to 20 hours.
When the aging or heat treatment is thoroughly carried out at a high temperature of 250C or more, the aromatic imide polymer in the asymmetric separating membrane is partially cross-linked, and thus the resultant aged membrane becomes insoluble in or resistant to swelling in the organic sol~ent, and exhibits an enhanced chemical resistance and durability in practical use.
The method of producing a polymeric asymmetric membrane is disclosed, for example, in Japanese Unexamined Patent Publication Nos. 56-21602 and The asymmetric separating membrane usable for the present invention is composed of a dense layer having a thickness of about 0.001 to 5 ~m and a porous layer continuously incorporated with the dense layer and having a thickness of about 10 to 2000 ~Im.
Generally, when the aromatic imide polymer asymmetric membrane is used for the pervaporation method of the present invention to selectively separate a specific alcohol compound x having a highest permeation rate; from a liquid organic compound mixture containing the compound x and the ether compound y, the asymmetric membrane preferably allows the specific alcohol compound x to permeate therethrough at a permeation rate Q of about 0.1 kg/m2.hr or more, more preferably 0.2 to 7 kg/m2.hr, and exhibits a separating coefficient ~ for the specific compounds x and y, of 20 or more, more preferably from 30 to 10,000.
The permeation rate Q of a specific compound fraction through a separating membrane is defined by the equation:
Q = A/B
wherein A represents an amount (in kg) of the specific l3 - 2~39~19 compound fraction permeated through the membrane per hour and B represents a permeation area in m2 of the membrane through which the specific organic compound fraction permeates.
The separation coefficient ~ of a separating membrane for the specific compounds x and y is defined by the equation:
C2/Cl wherein Cl represents a proportion in weight of the specific compound x to the weight of organic compound y in the organic liquid mixture to be fed and separated and C2 represents a proportion in weight of the specific compound x permeated through the membrane to the weight of the compound y permeated through the membrane.
In the pervaporation method of the present invention a mixture of two or more types of organic compound in the state of a liquid is brought into contact with one surface of the aromatic imide polymer separating membrane; the opposite face of the aromatic imide polymer sepaxating, membrane is exposed to an a~mosphere under a reduced pressure, to cause a fraction consisting of at least one type of the organic compound in the liquid mixture to selectively penetrate into and permeate through the separating membrane while leaving the other fraction consisting of at least one type of the organic compound in the feed face side of the separating membrane; the permeated fraction is collected in the state of a vapor in the delivery face side of the separating membrane; and the left fraction is collected in the state of a liquid in the feed face side of the separating membrane.
Practically, the pervaporation separating process of the present invention is carried out in the following steps.
(a) A liquid mixture comprising an aliphatic ether compound having 2 to 8 carbon atoms and an aliphatic alcohol compound having l to 4 carbon atoms i8 - l4 -- ~0394~9 fed to a feed side of a separating membrane module containing a number of aromatic imide polymer asymmPtric membranes (in the form of a hollow filament or a flat film) so that the fed liquid mixture comes into direct contact with one face of each membrane.
(b) A delivery side opposite to the feed side of the separating membrane module is exposed to a reduced pressure by connecting the delivery side to a pressure-reducing or vacuum pump placed outside of the separating membrane module, if necessary while flowing a carrier gas (sweeping gas), for example, helium, nitrogen, and argon gases and air, through the delivery side, to selectively allow the alcohol compound to penetrate into and permeate through the separating membranes and to be withdrawn as a vapor at the delivery side of the module.
(c) Finally, the remaining non-permeated portion of the liquid mixture is recovered from the feed side of the module, and the permeated portion in the state of a vapor is collected from the delivery side of the module, and if necessary, condensed by cooling.
The non-permeated portion of the liquid mixture comprises the ether compound in an increased concentration and the permeated portion of the liquid mi~ture comprises the alcohol compound in an increased conceniration.
Usually, the liquid organic compound mixture is fed into the separating membrane module preferably at a temperature of from about 0C to 120C, more pref-erably from 20C to lO0C.
In the method of the present invention, the feed side of the separating membrane module is under a gauge pressure of from 0 to 60 kg/cm2G, preferably from 0 to 30 kg/cm2G.
Also, the pressure at the delivery side of the separating membrane module is lower than the atmospheric pressure, and is preferably about 200 Torr or less, more ;~)39~

preferably lO0 Torr or less. If necessary, a sweeping gas is made to flow through the delivery side of the module, to promote the permeation of the specific organic compound.
In the method of the present invention, the feed liquid comprises preferably at least 70% by weight or more, more preferably 80% by weight or more, of a mixture of a liquid aliphatic ether compound having 2 to 8 carbon atoms and a liquid aliphatic alcohol compound having 1 to 4 carbon atoms.
The aliphatic alcohol compound is preferably selected from methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol and butyl alcohol, more preferably from methyl alcohol and ethyl alcohol.
The aliphatic ether compound is selected from preferably dimethylether, diethylether, di-n-propyl-ether, di-n-butylether, methyl-tert-butylether, ethyl-tert-butyl-ether, methyl-tert-amylether.
The liquid feed preferably contain 30% by weight or less, more preferably 20% by weight or less, of at least one organic compound different from the aliphatic ether and alcohol compounds, for example, lower alkane compounds such as ethane, propane and butane, and lower alkane compounds such as propylene and isobut-^ne.
In the li~uid feed, the aliphatic ether compound and the aliphatic alcohol compound are not limited to a specific mixing ratio in weight and can be mixed in any proportions.
There is no restriction on the structure, type, and size of the separating membrane module to be subjected to the pervaporation method of the present invention, but preferably the separating membrane module is a plate and frame type module, spiral type module or hollow filament type module.
EXAMPLES
The present invention will be further illustrated - 16 - 2 O 39 ~1 9 by way of specific examples, which are merely represen-tative and do not restrict the scope of the present invention in any way.
In the examples, the permeating rate Q and separating coefficient ~ were determined in the following manner.
When a liquid organic compound mixture was subjected to a pervaporation me~hod, a fraction was permeated through a separating membrane, liquefied by cooling, and then collected, and the weight of the liquefied fraction was measured. Then an internal standard liquid was added to the liquefied fraction, and the whole subjected to TCD-gas chromatography to determine the proportions in weight of organic compounds in the fraction to the total weight of the fraction.
The permeating rate Q and the separating coeffi-cient ~ were determined in accordance with the equatio..s:
Q = A/B
~ C2/C
as defined above.
In the examples, the compounds are represented by the following abbreviations.
(A) Aromatic tetracarboxylic dianhydride s-BPDA: 3,3',4,4'-biphenyltetracarboxylic dianhydride a-BPDA: 2,3,3',4'-biphenyltetracarboxylic dianhydride ETDA: 3,3',4,4'-diphenylether tetracarboxylic dianhydride DSDA: 3,3',4,4'-diphenylsulfone tetracarboxylic dianhydride 6-FDA: 2,2-bis(3,4-carboxyphenyl)hexa-fluoropropane dianhydride PMDA: Pyromellitic dianhydride BTDA: 3,3',4,4~-benzophenonetetra-carboxylic dianhydride - l7 - Z039419 (B) Aromatic diamine TSN: Isomeric mixture of 2,8-dimethyl-3,7-diaminodibenzothiophene-5,5-dioxide, 2,6-dimethyl-3,7-diamino-dibenzothiophene-5,5-dioxide and 4,6-dimethyl-3,7-diaminodibenzo-thiophene-5,5-dioxide DADE: 4,4'-diaminodiphenylether DADM: 4,4'-diaminodiphenylmethane DABA: 3,5-diaminobenzoic acid TPEQ: l,4-bis(4-aminophenoxy)benzene BAPB: 4,4'-di(4-aminophenoxy)biphenyl DMMB: 4,6-dimethyl-m-phenylenediamine Referential Examples l to 13 In each of Referential Examples l to 13, an aromatic imide polymer solution was prepared by polym-erizing and imidizing the aromatic tetracarhoxylic acid component and the aromatic diamine component having the composi~ions as shown in Table l, in substantially equal molar amounts, in the organic polar solvent consisting of p-chlorophenol at a polymerization temperature of 180C cr the polymerization time as shown in Table l.
The resultant aromatic imide polymer solution had the polymer concentration and the solution viscosity, which is a rotation viscosity (poise) at a temperature of 100~, as shown in Table l.
The aromatic imide polymer solution was extruded, as a spinning dope solution, through a hollow filament-spinning nozzles, the resultant hollow filamentary streams of the dope solution were travelled through atmospheric air and then introduced into a coagulating liquid consisting of an aqueous solution of 60% by volume of ethyl alcohol at a temperature of 0C, the resultant solidified hollow filaments were withdrawn from the coagulating liquid at a take-up speed of lO m/min in accordance with a semi dry-wet membrane-forming method, and the hollow filaments were washed - 18 - ~039~19 with ethyl. alcohol and then with an aliphatic hydro-carbon, and dried and aged under the conditions as indicated in Table 1, for 30 minutes.
The resultant hollow filaments had an asymmetric layered structure and the dimensions (outside diameter and membrane thickness of the hollow filament) as shown in Table 1.
The type of resultant hollow filament will be represented hereinafter by the number of the Referential Example in which the hollow filament was prepared.

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Z039~19 Examples 1 to 21 In each of Examples 1 to 21, 4 hollow filaments of the type shown in Table 2 and having a length of 7.5 cm were arranged in parallel to each other to form a hollow filament bundle, and at one end of the bundle, the ends of the hollow filaments were sealed with an epoxy resin to provide a hollow filament bundle element.
The hollow filament bundle element was placed in a container having an inlet for feeding a liquid feed, an outlet for recovering a non-permeated fraction, and an outlet for collecting a permeated fraction, to provide a separating membrane module.
A liquid feed comprising methyl-tert-butylether in an amount as shown in Table 2 and the balance consisting of methyl alcohol was fed into a feed face side of the separating membrane module so that the feed came into contact with the outside peripheral surfaces of the hollow filaments. The hollow spaces of the hollow filaments were connected to a pressure-reducing or vacuum apparatus, and the pressure in the hollow spaces was reduced to 3 Torr or less, to cause methyl alcohol fraction to be selectively permeated through the hollow filaments. The permeated fraction in the state of a vapor was cooled, and the resultant liquefied methyl alcohol was collected.
The permeating rate Q of the permeated fraction through the hollow filaments and the separating coeffi-cient ~ of the hollow filaments for methyl alcohol and methyl-tert-butylether are shown in Table 2.
Example 22 and 23 In Examples 22 and 23, the asymmetric separating hollow filaments made in Referential Examples 4(2) and 5(1) were converted to hollow filament bundle elements in the same manner as in Examples 8 and 9, respectively.
Each of the above-mentioned hollow filament bundle elements was immersed in a treating liquid consisting of 1 part by weight of methyl alcohol and 9 parts by weight 2039~'19 of methyl-tert-butylether at a temperature of 80C for 20 hours. The treated hollow filament bundle element was placed, without drying, in a container having an inlet for feeding a liquid feed, an outlet for dis-charging a non-permeated fraction of the feed liquid and an outlet for discharging a permeated fraction of the feed liquid, to provide a separating ~embrane module.
A liquid feed having the composition as shown in Table 2 was fed into the separating membrane module in the same manner as in Example 1.
The results are shown in Table 2.
Examples 24 and 25 In each of Examples 24 and 25, the same procedures as in Example 1 were carried out with the following exceptions.
In Examples 24 and 25, the same hollow filament bundle elements as in Examples 22 and 23 were dried at a temperature of 30C for 20 hours, and then converted to separating hollow filament modules in the same manner as in Example, respectively.
A liquid feed having the composition as indicated in Table 2 were fed into the separating module to separate the methyl alcohol and methyl-tert-butylether from each other. The results are shown in Table 2.

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;~03~19 Table 2 clearly shows that the aliphatic ether compounds having 2 to 8 carbon atoms and the aliphatic alcohol compounds having 1 to 4 carbon atoms can be separated from each other at a high separation coeffi-S cient by the pervaporation separation method of thepresent invention in which a specific aromatic imide polymer asymmetric separating membrane is employed.
Accordingly, the aliphatic ether compounds and the aliphatic alcohol compounds can be industrially isolated and recovered from mixtures thereof under a stable condition over a long period.

Claims

1. A pervaporation method of separating a lower alcohol compound from a mixture of a lower alcohol compound and an ether compound, comprising the steps of:
bringing a feed comprising a liquid aliphatic ether compound having 2 to 8 carbon atoms and a liquid aliphatic alcohol compound having 1 to 4 carbon atoms into direct contact with a feed face of an asymmetric separating membrane comprising a heat resistant aromatic imide polymer;
exposing a delivery face opposite to the feed face of the separating membrane to an atmosphere under a reduced pressure to cause a lower aliphatic alcohol compound to selectively penetrate into and permeate through the separating membrane and then to be vaporized in the delivery face side of the separating membrane; and collecting the vaporized lower alcohol compound at the delivery face side of the separating membrane.

2. The method as claimed in claim 1, wherein the aliphatic ether compound is selected from the group consisting of dimethylether, diethylether, di-n-propyl-ether, di-n-butylether, methyl-tert-butylether, ethyl-tert-butylether and methyl-tert-amylether.

3. The method as claimed in claim 1, wherein the aliphatic lower alcohol compound is selected from the group consisting of methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol and butyl alcohol.

4. The method as claimed in claim 1, wherein the asymmetric separating membrane has a dense layer having a thickness of about 0.001 to about 5 µm and a porous layer having a thickness of about 10 to about 2000 µm.

5. The method as claimed in claim 1, wherein the asymmetric separating membrane allows the lower aliphatic alcohol compound to permeate through the membrane at a permeation rate of 0.1 kg/m2?hr or more.

6. The method as claimed in claim 1, wherein the feed is brought into contact with the feed face of the asymmetric separating membrane at a temperature of 0°C
to 120°C.

7. The method as claimed in claim 1, wherein the feed is brought into contact with the feed face of the asymmetric separating membrane under a pressure higher than that at the delivery face side of the asymmetric separating membrane.

8. The method as claimed in claim 7, wherein the feed face side of the asymmetric separating membrane is under a gauge pressure of 0 to 60 kg/cm2.

9. The method as claimed in claim 1, wherein the delivery face side of the asymmetric separating membrane is under a reduced pressure of 200 Torr or more.

10. The method as claimed in claim 1, wherein the aromatic imide polymer has 70 to 100 molar% of at least one type of recurring units selected from those of the formulae (I) and (II):
(I) and (II) wherein R represent 2 divalent aromatic group having at least two benzene ring structures, and X represents a member selected from the group consisting of -S-, -SO2-, -CO-, -O-, -C(CH3)2-, -CH2- and -C(CF3)-.

11. The method as claimed in claim 1 or 10, wherein the aromatic imide polymer is a polymerization-imidization product of:
A) an aromatic tetra carboxylic acid component comprising:
(a) 70 to 100 molar% of at least one principal member selected from the group consisting of biphenyl tetracarboxylic acids, diphenylether tetracarboxylic acids, and dianhydrides, esters and salts of the above-mentioned acids, and (b) 0 to 30 molar% of at least one additional member selected from the group consisting of aromatic tetracarboxylic acids, and dianhydrides, esters and salts of those acids which are different from the above-mentioned compounds for the principal member; with B) an aromatic diamine component comprising:
(c) 70 to 100 molar% of at least one principal member selected from aromatic diamines having at least two benzene ring structures, and (d) 0 to 30 molar% of at least one aromatic diamine other than the principal member.

12. The method as claimed in claim 1, wherein the asymmetric separating membrane is in the form of a film or sheet.

13. The method as claimed in claim 1, wherein the asymmetric separating membrane is in the form of a hollow filament.