GB2271993A - Separations using polyester or polyester copolymer membranes - Google Patents

Abstract

Mixtures comprising two or more components selected from alcohols/ethers/non-linear hydrocarbons are separated using polyester or polyester copolymer membranes under pervaporation conditions. An embodiment comprises the separation of an alcohol, preferably a C1-C4 alcohol, from a synthesis solution used for the production of ethers, such as methyl-tertiary butyl ether (MTBE) or tertiary-amyl methyl ether (TAME) used as octane enhancers in motor fuels.

Description

" 1 " 2271993 SEPARATIONS USING POLYESTER OR POLYESTER COPOLYMER MEMBRANES

The present invention relates to separations using polyester or polyester copolymer membranes; more particularly to the separation of alcohols or ethers from mixtures.

Environmental concerns have focused on reducing pollution associated with motor vehicles, especially gasoline burning internal combustion engine vehicles. While improvements in vehicle and engine design can account for part of the desired improvements in emission levels, much attention has been focused on improvements in the motor vehicle fuels, especially gasoline. Legislation is either being considered or being actively pursued on the state and federal level to mandate lower allowable aromatics and sulfur content in gasoline and reduce the final boiling point and alter the vapor pressure of gasolines. Much as when lead was phased out of gasoline it is anticipated that these changes in motor gasoline formulation will inevitably become world wide.

Lowering the emissions of fuels by removing lead octane additives and reducing aromatics contents however, have forced refmers to seek new, environmentally acceptable octane enhancers to make up the losses due to regulation.

Oxygen containing organic molecules such as ethers, in particular methyl tertiary butyl ether OJTBE) and tertiary amyl methyl ether (TAME) are very high octane components which are currently viewed as environmentally acceptable and the additives of choice to make up the octane shortfall. As a result, demand for these materials has increased as has their price.

MTBE and TAME and other ethers and mixtures thereof are produced by any of numerous processes which utilize methanol or other alcohols and hydrocarbons in the synthesis.

To be economically attractive these processes must convert the hydrocarbon, e.g., isobutylene or isoamylene to as high a degree as possible to ether, e.g., MTBE or TAME respectively. To accomplish this the alcohol, e.g., methanol is added in excess to shift the equilibrium as far as possible to product formation. Excess alcohol is also desirable in that an increase in catalyst life due to reduced polymerization is also achieved.

A problem is encountered however when one seeks to separate the unreacted alcohol from the ether and hydrocarbon for recycle and product purification. Alcohol forms an azeotrope with the etherlhydrocarbon mixture and cannot be readily separated by distillation.

Membrane separation processes are not affected by azeotrope formation. However, membrane processes typically do not achieve the high purities usually associated with distillation.

It is an object of the present invention to provide an improved membrane separation.

The present invention provides a method for separating a mixture containing at least two components selected from an alcohol, an ether and a hydrocarbon, which method comprises contacting the mixture with a polyester, or polyester copolymer, membrane under pervaporation conditions and recovering as the permeate (i) an alcohol, when present in the mixture or (ii) an ether, when an alcohol is absent from the mixture.

It has been discovered that dense film, non-porous polyester or polyester copolymer membranes can selectively separate and recover alcohols, especially C2-C1O' preferably Cl-C4 alcohols, from mixtures comprising alcohols with ethers, olefins, and non-linear hydrocarbons, preferably mixtures of alcohol and ether, mixtures of alcohols 10119303.RWP/mp i and non-linear hydrocarbons or mixtures of alcohol and olefins, or selectively recover ethers from ether/hydrocarbon mixtures to a high degree of purity at high flux under pervaporation conditions, preferably at atmospheric permeate pressures, at elevated temperatures.

Suitable feed mixtures for the process of the invention are solutions of alcohol, especially solutions of alcohols, ether, olefins, and non-linear hydrocarbons are typically the synthesis solutions encountered in the production of high value ethers such as methyl tertiary-butyl ether (MTBE) and tertiary-amyl methyl ether (TAME) used as octane enhancers in motor fuels. The alcohol is present as one of the starting materials in the ether synthesis but cannot readily be separated from the synthesis solution due to azeotrope formation.

The separation is preferably conducted at atmospheric permeate pressure in that this reduces or eliminates vacuum requirements and should simplify the design of the permeate section as well as the design of the separation elements. Operation at atmospheric pressure, however, requires the use of a membrane capable of operating at high temperature, temperatures above the feed boiling point.

Polyimide/aliphatic polyester copolymer membranes are examples of high temperature stable membranes which can be used in this separation. The membranes are thin and comprise polyimide and polyester segments, the polyimide being derived from a dianhydride having between 8 and 20 carbons and a diamine having between 2 and 30 carbons, and the polyester is a polyadipate, a polysuccinate, a polymalinate, a polyoxalate or a polyglutarate. Membranes of the type are described in U.S. Patent 4,944, 880 and U.S. Patent 4,990,275.

Polyurethane and polyurea-urethane membranes in so far as they also contain polyester moiety in their structure may also be employed.

10119303.RWP/mp Suitable aromatic polyurea-urethane membranes are characterized by possessing a urea index, defined as the percentage of the total of urea and urethane groups that are urea, of at least about 20% but less than 100%, an aromatic carbon content of at least about 15 mole %, a functional group density of at least 10 per 100 grams of polymer, and a C=O/NH ratio of less than about 8 and are described in U.S. Patent 4,914, 064. Thin film composites of polyurea-urethane film on support backing can be prepared as taught in U.S. Patent 4,837,054 and U.S. Patent 4,861, 628. U.S. Patent 4,879,044 describes anisotropic polyurea-urethane membranes.

Polyurethane and polyurea-urethane membranes described in U.S. Patent 4, 115,465 can also be used. If supported on thermally stable backings such as Teflon.

Halogenated polyurethane membranes described in U.S. Patent 5,028,685 and U.S. Patent 5,093,003 can also be employed in the separation.

Polyurethane-imide membranes, containing polyester moiety in their structure can also be used and are described in U.S. Patent 4,929,358.

Isocyanurate crosslinked polyurethane membranes containing polyester moiety in their structure are useable and are described in U.S. Patent 4, 929,357.

Other polyester containing membranes are described in U.S. Patent 4,976, 868 which describes polyester membranes per se, such as membranes made of polyethylene terephthalate/cyclohexane-dimethanoI terephthalate and U.S. Patents 4,946,594 and 4,997,906 which describe a crosslinked copolymer membrane derived from an aliphatic polyester, a dianhydride and a diisocyanate.

10119303.RWP/mp i New, multi-block copolymer membranes also suitable for use, when synthesized containing polyester moiety, are described in the following:

U.S. Patent 5,039,418, U.S. Patent 5,039,422, U.S. Patent 5tO39,417, U.S. Patent 5,221,481, U.S. Patent 5,096,592, U.S. Patent 5,049,281, and U.S. Patent 5,130,017.

Any of the membranes recited above, when prepared using a polyester component in the synthesis, are useful in the present separation of alcohol from mixtures of alcohol, ether and non-linear hydrocarbon or mixtures of alcohol/ether. The separation of ether from mixtures of ether and hydrocarbon can also be accomplished using the polyester membranes under pervaporation conditions.

The membranes can be used in any convenient form such as sheets, tubes or hollow fibers. Sheets can be used to fabricate spiral wound modules familiar to those skilled in the art.

An improved spiral wound element is disclosed in copending application USSN 921,872 filed July 29, 1992 wherein one or more layers of material are used as the feed spacer, said material having an open cross-sectional area of at least 30 to 70% and wherein at least three layers of material are used to produce the permeate spacer characterized in that the outer permeate spacer layers are support layers of a fine mesh material having an open cross-sectional area of about 10 to 50% and a coarse layer having an open cross-sectional area of about 50 to 90Y. is interposed between the aforesaid fine outer layers, wherein the fine layers are the layers in interface contact with the membrane layers enclosing the permeate spacer. While the permeate spacer comprises at least 3 layers, preferably 5 to 7 layers of alternating fine and coarse materials are used, fine layers always being the outer layers. In a further improvement an additibnal woven or non-woven chemically and thermally inert sheet may be interposed between the membrane and the multi-layer spacers, said sheet being for example a sheet of Nomex about I to 15 mils thick.

Alternatively, sheets can be used to fabricate a flat stack permeator comprising a multitude of membrane layers alternately separated by feedretentate spacers and permeate spacers. The layers are glued along their edges to define separate feed-retentate zones and permeate zones. This device is described and claimed in U.S. Patent 5,104,532.

Tubes can be used in the form of multi-leaf modules wherein each tube is flattened and placed in parallel with other flattened tubes. Internally each tube contains a spacer. Adjacent pairs of flattened tubes are separated by layers of spacer material. The flattened tubes with positioned spacer material is fitted into a pressure resistant housing equipped with fluid entrance and exit means. The ends of the tubes are clamped to create separate interior and exterior zones relative to the tubes in the housing. Apparatus of this type is described and claimed in U.S. Patent 4,761,229.

Hollow fibers can be employed in bundled arrays potted at either end to form tube sheets and fitted into a pressure vessel thereby isolating the insides of the tubes from the outsides of the tubes. Apparatus of this type are known in the art. A modification of the standard design involves dividing the hollow fiber bundle into separate zones by use of baffles which redirects fluid flow on the outside of the bundle and prevents fluid channelling and polarization on the outside of the hollow fibers. This modification is disclosed and claimed in U.S. Patent 5,108,605.

Preferably the direction of flow in a hollow fiber element will be counter-current rather than co-current or even transverse. Such countercurrent flow can be achieved by wrapping the hollow fiber bundle in a spiral wrap of flow-impeding material. This spiral wrap extends from a central mandrel at the center of the bundle and spirals outward to the outer periphery of the bundle. The spiral wrap contains holes along the top and bottom ends whereby fluid entering the bundle for flow outside the hollow fibers at one end is partitioned by passage through the holes and forced to flow parallel to the hollow fibers down the channel created by the spiral wrap. This flow - 7 lirection is counter-current to the direction of flow inside the hollow fibers. At the bottom of the channels the fluid re-emerges from the hollow fiber bundle through the holes at the opposite end of the spiral wrap and is directed out of the module. This devise is disclosed and claimed in copending application USSN 802,158 filed December 4, 1991( u. sp. 5" 2-34 5-cit).

Separation is preferably performed under pervaporation conditions, most preferably pervaporation operation at atmospheric permeate pressure at elevated temperature.

The pervaporation process is run generally at high temperature, e.g., greater than 100% preferably greater than 160C and more preferably at even higher temperature, and generally relies on vacuum on the permeate side to evaporate the permeate from the surface of the membrane and maintain the concentration gradient driving force which drives the separation process. The maximum temperatures employed in pervaporation will be those necessary to vaporize the components in the feed which one desires to selectively permeate through the membrane while still being below the temperature at which the membrane is physically damaged. While a vacuum may be pulled on the permeate side, operation at atmospheric pressure on the permeate side is also possible and economically preferable. In pervaporation it is important that the permeate evaporate from the downstream side (permeate side) of the membrane. This can be accomplished by either decreasing the permeate pressure (i.e., pulling a vacuum) if the permeate boiling point is higher than the membrane operating temperature or by increasing the membrane operating temperature above the boiling point of the permeate in which case the permeate side of the membrane can be at atmospheric pressure. The second option is possible when one uses a membrane capable of functioning at high temperatures. In some cases if the membrane operating temperature is greater than the boiling point of the permeate, the permeate side pressure can be greater than 1 atmosphere.

As previously stated, the preferred membranes employed in the present invention are generally described as polyimide/aliphatic polyester copolymer membranes and are described and claimed in U.S. Patent 4,944,880 and U.S. Patent 4,990,275.

The polyimide/aliphatic polyester copolymer membranes are made from copolymers comprising polyimide segments and oligomeric aliphatic polyester segments, the polyimide being derived from a dianhydride having between 8 and 20 carbons and a diamine having between 2 and 30 carbons and the oligomeric aliphatic polyester being a polyadipate, a polysuccinate, a polymalonate, a polyoxalate or a polyglutarate. Alternately, an activated anhydride acid chloride such as trimellitic anhydride acid chloride may be used instead of a dianhydride.

The diamines which can be used include phenylene diamine, methylene dianiline (MDA), methylene di-o-chloroaniline (MOCA), methylene bis(dichloroaniline)(tetrachloro MDA), methylene dicyclohexylamine (H12MDA), methylene dichlorocyclohexylamine (H12-MOCA), methylene bis(dichlorocyclohexylamine)(tetrachloro H12-MA), 4, 41(hexafluoroisopropylidene)-bisaniline (6F diamine), 3,3'-diaminophenyl sulfone (3,3' DAPSON), 4,4'-diaminophenyl sulfone (4,4' DAPSON). 4,4'dimethyl-3,31-diaminophenyl sulfone (4,4'-dimethyl-3,3' DAPSON), 2,4diamino cumene, methyl bis(di-o-toluidine), oxydianiline (ODA), bisaniline A, bisaniline M, bisaniline P, thiodianiline, 2,2-bis[4(4- aminophenoxy) phenyl] propane (BAPP), bis[4-(4-aminophenoxy) phenyl] sulfone (BAPS), 4,4'-bis(4-aminophenoxy)bipheny1 (BAPB), 1,4-bis(4- aminophenoxy)benzene (TPE-Q), and 1,3-bis(4-aminophenoxy)benzene (TPE-R).

The dianhydride is preferably an aromatic dianhydride and is most preferably selected from the group consisting of pyromellitic dianhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, 4,41(hexafluoroisopropylidene)-bis(phthalic anhydride), 4,C-oxydiphthalic anhydride, diphenylsulfone-3,3',4,4'-tetracarboxylic dianhydride, and 3, 31,4,4'-biphenyl-tetracarboxylic dianhydride.

Examples of preferred polyesters include polyethyleneadipate and polyethylenesuccinate.

t 1 The polyesters used generally have molecular weights in the range of 500 to 5000, preferably 1000 to 2000.

In general the membrane may be synthesized as follows. One mole of a polyester, e.g., polyadipate, polysuccinate, polyoxalate, polyglutarate or polymalonate, preferably polyethyleneadipate or polyethylenesuccinate, is reacted with two moles of a dianhydride, e.g., pyromellitic dianhydride, with stirring for about 5 hours at about 170C to make a prepolymer in the end-capping step. One mole of this prepolymer is then reacted with one mole of a diamine, e.g., methylene di-o-chloroaniline (MOCA) to make a copolymer. Finally, heating of the copolymer at 260-300C for about 10 minutes leads to the copolymer containing polyester segments and polyimide segments. The heating step converts the polyamic acid to the corresponding polyimide via imide ring closure with removal of water.

In the synthesis an aprotic solvent such as dimethylformamide (DMF) is used in the chain-extension step. DMF is a preferred solvent but other aprotic solvents are suitable and may be used. A concentrated solution of the polyamic acid/polyester copolymer in the solvent is obtained. This solution is used to cast the membrane. The solution is -spread on a glass plate or a high temperature porous support backing, the layer thickness being adjusted by means of a casting knife. The membrane is first dried at room temperature to remove most of the solvent, then at 120C overnight. If the membrane is cast on a glass plate it is removed from the casting plate by soaking in water. If cast on a porous support backing it is left as is. Finally, heating the membrane at 260-300C for about 10 minutes results in the formation of the polyimide. Obviously, heating at 2603000C requires that if a backing is used the backing be thermally stable, such as teflon, fiber glass, sintered metal or ceramic or high temperature polymer backing.

The present invention is demonstrated by the following non-limiting examples.

- 10 Examifl e Membrane I --PEI One point zero nine (1.09) grams (0.005 mole) of pulverized pyromellitic dianhydride (PMDA) was placed into a reactor. Five (5.0) grams (0.0025 mole) of predried 2000 MW polyethylene adipate diol (PEA) was added to the reactor. The PEA was dried at 60C and a vacuum of approximately 20" Hg. The prepolymer mixture was heated to 140C and stirred vigorously for approximately 1 hour to complete the end-capping of PEA with PMDA. The viscosity of the prepolymer increased during the endcapping reaction ultimately reaching the consistency of molasses.

The prepolymer temperature was reduced to 70C and then diluted with 40 grams of dimethylformamide (DMF). Zero point six seven (0.67) grams (0. 0025 mole) of 4,4'-methylene bis(o-chloroaniline) (MOCA) was added to 5.2 grams of W. 'The solution viscosity increased as the chain extension progressed. The solution was stirred and the viscosity was allowed to build up until the vortex created by the stirrer was reduced to approximately 50% of its original height. DMF was added.incrementally to maintain the vortex level until 73.2 grams of DMF had been added. Thirty minutes was taken to complete the solvent addition. The solution was stirred at 70C for 2 hours then cooled to room temperature.

The polymer solution prepared above was cast on 0.211 pore teflon and allowed to dry overnight in N2 at room temperature. The membrane was further dried at 120C for approximately another 18 hours. The membrane was then placed into a curing oven. The oven was heated to 2600C (approximately 40 min) and then held at 260% for 5 min and finally allowed to cool down close to room temperature (approximately 4 hours).

Membrane 2 - PES To 2.18 g (0.01 mole) of pulverized pyromellitic dianhydride (PMDA) heated (about 1OWC) "under N2 in a reactor was added 10 g i (0.005) mole of polyethylene succinate diol with a molecular weight of about 2000 (PES) with stirring. The temperature was approximately 150C, and the stirring continued for about 5 hours to complete the end-capping step. The temperature is presented as an approximation due to a measurement error encountered during this step in this example. To the reactor content was added 41.3 g of DMF, and the temperature was dropped to about 80C with stirring for about 0.5 hour. To this reactor content was added 1.34 g (0.005 mole) MOCA in 10 g DMF solution dropwise. The solution was stirred at 80C for 2.5 hours. Then, a viscous solution resulted, which indicated the chainextension reaction. The solution was then cooled to room temperature. The resulting solution containing the copolymer with the polyamic acid hard segment and the polyethylene succinate soft segment had suitable consistency for solution casting in the preparation of membranes.

The resulting solution was centrifuged for about 5 minutes. Following centrifugation, a membrane was knife-cast onto a microporous teflon support (about 50 micron thickness, 0.2 micron pores and 807. porosity). DMF was allowed to evaporate from the membrane in a nitrogen box in a hood at ambient conditions over a period of about 17 hours. The membrane was then dried in an oven at 120C for about 16 hours. Finally, the membrane was heated to 300C, maintained at this temperature for 1.5 hours and then cooled to room temperature in the curing step. The resulting membrane synthesized from PES/PMDA/MOCA at a molar ratio of 1/2/1 contained 29 wt9.' polyimide hard segment and 71 wt% polyethylene succinate soft segment.

Both of these membranes were tested in a pervaporation operation using a 10% MeOH in MTBE mixture.

Table 1 summaries the performance of PEI and PES membranes operated at 100C and 660 mbar vacuum. The PEI membrane had an excellent flux (213 kg/m2-d) with an extremely high permeability (4700 kg-A/m2-d). The high permeability identifies the potential flux capability of this membrane. The 7.7 selectivity was extremely good in combination with the high flux. The selectivity of PES membrane 12 - was substantially higher (24) despite operating at a relatively poor vacuum of 660 mbars.

Table 2 shows that the PES flux increases as expected with an increase in temperature. Table 2 also compares the high temperature (ACC) performance of PES operated at vacuums of 660 mbar and atmospheric pressure. Even at atmospheric permeate pressures the PES membrane is still able to achieve an excellent overall performance; a flux of 203 kg/m2-d and a selectivity of 20.

ll Table 1 Me0H/MTH Separation Usinq Polyimide/Polvester Membranes, (Feed: 10% MeOH in MTBE, Pervap @ 110 C/660 mbar) PEI PES Membrane Prep Example 1 Example 2 Flux 213 51 (kg/m2-d) Permeabi 11 ty 4700 460 (kg-jj/m2-d) Permeate [MeOH] 44 71 (wt%) Selectivity 7.7 24 (wt fr) Selectivity =([MeOH]/[MTBEI)Perm/([MeON]/[MTBE])Feed TABLE 2 Me0H/KBE SeRaration Using a PES Membrane (PES Example 2 Membrane, Feed: 10% MeOH in MTBE, Pervaporation) Vacuum 660 660 1000 (mbar) Temperature 110 140 140 (deg C) Flux 51 200 203 (kg/m2-d) Permeability 460 1800 1825 (kg-11/m2-d) Perm [MeOH] 71 69 68 MY.) Selectivity 24 21 20 (wt fr) Selectivity -([MeOH]/[MTBE])Perm/([MeOH]/[MTBEI)Feed

Claims (12)

1. CLAIMS:

   A method for separating a mixture containing at least two components selected from an alcohol, an ether and a hydrocarbon, which method comprises contacting the mixture with a polyester, or polyester copolymer, membrane under pervaporation conditions and recovering as the permeate (i) an alcohol, when present in the mixture or (h) an ether, when an alcohol is absent from the mixture.
2. 2. The method of claim 1, wherein the mixture comprises at least one alcohol and at least one ether.
3. 3. The method of claim 1, wherein the mixture comprises at least one alcohol and at least one olefin.
4. 4. The method of claim 1, wherein the mixture comprises at least one alcohol and at least one non-linear hydrocarbon.
5. 5. C10 alcohol(s).

   The method of any preceding claim, wherein the alcohol(s) islare Cl-
6. 6. The method of claim 5, wherein the alcohol(s) islare C l-C4 alcohol(s).
7. 7. The method of claim 6, wherein the alcohol is methanol.
8. 8. The method of claim 1, wherein the mixture comprises at least one ether and at least one hydrocarbon.
9. 9. The method of claim 1 or claim 2 or claim 8, wherein the ether is methyl tertiary butyl ether andlor tertiary amyl methyl ether.

   10119303.RVYT/mp 1 I-
10. 10. The method of any preceding claim, wherein the membrane is a dense film, non-porous membrane.
11. 11. The method of claim 10, wherein the dense film non-porous polyester copolymer membrane is a polyimide/aliphatic polyester copolymer membrane.
12. 12. The method claimed in claim 1 and substantially as herein described with reference to the Example.

    10119303.RWP/mp