Polyimide membranes for hyperfiltration recovery of aromatic solvents

Improved asymmetric hyperfiltration membranes and their method of preparation and use are disclosed. The membranes are fashioned from polyimides and conditioned with a lubricating oil. Permselective separation of aromatic hydrocarbons from non-aromatic hydrocarbons in a feed stream may be accomplished using the membranes under hyperfiltration conditions.

FIELD OF THE INVENTION
 The present invention relates to improved asymmetric membranes fashioned
 from a polyimide and the method of preparing the membranes. The invention
 also relates to the method of using the membranes for the separation of
 aromatic hydrocarbons from non-aromatic hydrocarbons under hyperfiltration
 conditions.
 Of particular interest, the membranes are useful for the recovery of
 aromatic hydrocarbons, i.e. toluene, having a high purity from process
 streams containing aromatic and non-aromatic hydrocarbons during a
 commercial process for the production of aromatic hydrocarbons.
 BACKGROUND OF THE INVENTION
 The separation of aromatics from non-aromatics is useful in upgrading
 aromatics containing streams in petroleum refineries, such streams
 including, naphtha streams, heavy catalytic naphtha streams, intermediate
 catalytic naphtha streams, light aromatic streams and reformate streams,
 and in chemical operations for the recovery of aromatics such as benzene,
 toluene, xylenes, naphthalene, etc.
 The use of membranes to separate aromatics from saturates has long been
 pursued by the scientific and industrial community. Methods of membrane
 separation include hyperfiltration (also known as reverse osmosis in
 aqueous separations), pervaporation and perstraction. Pervaporation 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. In perstraction, the permeate
 molecules in the feed diffuse into the membrane film, migrate through the
 film and reemerge on the permeate side under the influence of a
 concentration gradient. A sweep flow of liquid or gas is used on the
 permeate side of the membrane to maintain the concentration gradient
 driving force. In contrast, hyperfiltration does not require the use of
 external forces on the permeate side of the membrane, but drives the
 separation through application of a pressure gradient.
 Membrane separation of aromatics from saturates has been the subject of
 numerous patents.
 U.S. Pat. No. 3,370,102 describes a general process for separating a feed
 into a permeate stream and a retentate stream and utilizes a sweep liquid
 to remove the permeate from the face of the membrane to thereby maintain
 the concentration gradient driving force. The process can be used to
 separate a wide variety of mixtures including various petroleum fractions,
 naphthas, oils, hydrocarbon mixtures. Expressly recited is the separation
 of aromatics from kerosene.
 U.S. Pat. No. 2,958,656 teaches the separation of hydrocarbons by type,
 i.e., aromatic, unsaturated, saturated, by permeating a portion of the
 mixture through a non-porous cellulose ether membrane and removing
 permeate from the permeate side of the membrane using a sweep gas or
 liquid. Feeds include hydrocarbon mixtures, naphtha (including virgin
 naphtha, naphtha from thermal or catalytic cracking, etc.).
 U.S. Pat. No. 2,930,754 teaches a method for separating hydrocarbons e.g.,
 aromatic and/or olefins from gasoline boiling range mixtures, by the
 selective permeation of the aromatic through certain cellulose ester
 non-porous membranes. The permeated hydrocarbons are continuously removed
 from the permeate zone using a sweep gas or liquid.
 U.S. Pat. No. 4,115,465 teaches the use of polyurethane membranes to
 selectively separate aromatics from saturates via pervaporation.
 U.S. Pat. No. 4,929,358 teaches the use of polyurethane membranes for the
 separation of aromatics from non-aromatics. Permeation is conducted under
 pervaporation, perstraction, reverse osmosis, or dialysis conditions. None
 of the experimental results reported in this patent were obtained under
 reverse osmosis conditions.
 Polyimide membranes have been used for the separation of aromatics. U.S.
 Pat. No. 4,571,444 teaches the separation of alkylaromatics from aromatic
 solvents using a polyimide polymer membrane. The polyimide membrane of
 choice was an asymmetric polyimide polymer membrane prepared from a fully
 imidized, highly aromatic polyimide copolymer. Permeation was performed
 under reverse osmosis conditions.
 U.S. Pat. No. 4,532,029 discloses the use of an asymmetric polyimide
 membrane for the separation of aromatics from lower aromatic middle
 distillate feeds. Permeation of the feeds in the presence of a light polar
 solvent, e.g., acetonitrile, was required to obtain permeates having a
 high aromatic content, i.e., greater than 86%.
 The majority of investigations for aromatic/non-aromatic separations have
 heretoafore involved pervaporation or perstraction separation techniques.
 This is probably due to reports of prior literature that very high
 operational pressures are required in hyperfiltration to reach a
 equivalent performance achievable by pervaporation and perstraction
 processes. Unfortunately, pervaporation and perstraction separation
 systems are higher cost than a hyperfiltration system due to expenses
 associated with vacuum, refrigeration and heat transfer systems.
 Consequently, it is an advantage of this invention to provide improved
 asymmetric polyimide membranes for the separation of aromatic hydrocarbons
 from non-aromatic hydrocarbons in a feed stream by hyperfiltration. It is
 also an advantage of this invention to provide a method of preparing the
 membrane by a phase inversion technique, which method permits variations
 in processing conditions to optimize the selective permeation of aromatic
 hydrocarbons through the membranes in the presence of non-aromatic
 hydrocarbons.
 Another advantage of the invention is to provide a membrane useful in a
 process of separating aromatic hydrocarbons as described in copending
 application Ser. No. 125,256, entitled "Recovery of Aromatic Hydrocarbons
 Using Lubricating Oil1-Conditioned Membranes", Mobil filed on even date
 herewith.
 Other facets and advantages of the present invention will be apparent from
 the ensuing description and the appended claims.
 SUMMARY OF THE INVENTION
 Improved asymmetric membranes which have high selectivity to permeate
 aromatic hydrocarbons in the presence of non-aromatic hydrocarbons under
 hyperfiltration conditions have been found. The membranes are prepared
 from a polyimide by a phase inversion technique and are thereafter treated
 with a lubricating oil to condition the membranes. Membranes in accordance
 with the invention exhibit over 30% rejection of the non-aromatic
 hydrocarbon materials at a commercially adequate flow rate in a
 temperature range of about -20 to 150.degree. C.
 Using the membranes of the invention, permselective separation of aromatic
 hydrocarbons from non-aromatic hydrocarbons in a feed stream may be
 accomplished by hyperfiltration with sufficient flux and selectivity to
 offer improved economics over pervaporation conditions. However, it is not
 intended to limit the use of the membranes to hyperfiltration mode of
 operation.
 A process for using the membrane of the present invention is disclosed in
 co-pending application Ser. No. 126,256, entitled "Recovery of Aromatic
 Hydrocarbons Using Lubricating Oil Conditioned Membranes", filed on even
 date herewith.
 DETAILED DESCRIPTION OF THE INVENTION
 The term "aromatic hydrocarbon" is used herein to designate a
 hydrocarbon-based organic compound containing one or more aromatic rings.
 An aromatic ring is typified by benzene having a single aromatic nucleus.
 Aromatic compounds having more than one aromatic ring include, for
 example, naphthalene, anthracene, etc. Preferred aromatic hydrocarbons
 useful in the present invention include those having 1 to 2 aromatic
 rings.
 The term "non-aromatic hydrocarbon" is used herein to designate a
 hydrocarbon-based organic compound having no aromatic nucleus.
 For purposes of this invention, the term "hydrocarbon-based organic
 compound" is used to mean an organic compound having a predominately
 hydrocarbon character. It is contemplated within the scope of this
 definition that a hydrocarbon compound may contain at least one
 non-hydrocarbon radical (e.g., sulfur or oxygen) provided that said
 non-hydrocarbon radicals do not alter the predominant hydrocarbon nature
 of the organic compound and/or do not react to alter the chemical nature
 of the polyimide of the membrane within the context of the present
 invention.
 Asymmetric membranes are defined for purposes of this invention as an
 entity composed of a dense ultra-thin top "skin" layer over a thicker
 porous substructure of the same or different material. Typically, the
 asymmetric membrane is supported on a suitable porous backing or support
 material.
 Polyimide membranes of the invention can be produced from a number of
 polyimide polymer sources. The identity of such polymers are presented in
 numerous patents. See, for example, U.S. Pat. No. 4,307,135, U.S. Pat. No.
 3,708,458, U.S. Pat. No. 3,789,079, U.S. Pat. No. 3,546,175, U.S. Pat. No.
 3,179,632, U.S. Pat. No. 3,179,633, U.S. Pat. No. 3,925,211, U.S. Pat. No.
 4,113,628, U.S. Pat. No. 3,816,303, U.S. Pat. No. 4,240,914, U.S. Pat. No.
 3,822,202, U.S. Pat. No. 3,853,754 and British Patent No. 1,434,629.
 A preferred polyimide polymer useful to prepare the membranes of the
 invention is available as Matrimid 5218 from Ciba Geigy. The structure of
 the polyimide, Matrimid, is shown below. The polyimide is also known as
 the polymer with 1 (or 3)-(4-aminophenyl)-2,3-dihydro-1,3,3 (or
 1,1,3)-trimethyl-1H-inden-5-amine and
 5,5'-carbonylbis-1,3-isobenzofurandione (CAS Number 62929-02-6). A common
 name for Matrimid is the polymer with diaminophenylindane and benzophenone
 tetracarboxylic dianhydride.
 ##STR1##
 Most preferably, the membranes of the invention are prepared from a
 polyimide polymer described in U.S. Pat. No. 3,708,458, assigned to
 Upjohn. The polymer, available from HP Polymers, Inc, Lewisville, Tex. as
 Lenzing P84, is a copolymer derived from the co-condensation of
 benzophenone 3,3',4,4'-tetracarboxylic acid dianhydride (BTDA) and a
 mixture of di(4-aminophenyl) methane and toluene diamine or the
 corresponding diisocyanates, 4,4'-methylenebis(phenyl isocyanate) and
 toluene diisocyanate.
 The obtained copolyimide has imide linkages which may be represented by the
 structural formulae:
 ##STR2##
 wherein the copolymer comprises from about 10 to 90% I and 90 to 10% II,
 preferably about 20% I and about 80% II.
 Another polyimide useful to prepare a membrane in accordance with the
 invention is a polymer, available from HP Polymers, Inc., Lewisville, Tex.
 as Lenzing P84 HT. The polymer is the co-condensation of
 1H,3H-Benzo[1,2-c:4,5-c']difuran-1,3,5,7-tetrone with
 5,5'-carbonyl[bis1,3-isobenzofurandione], 1,3-diisocyanato-2-methylbenzene
 and 2,4-diisocyanato-1-methylbenzene. The structure of the polyimide is
 shown below.
 ##STR3##
 Membranes in accordance with the invention can be made by dissolving the
 desired polyimide polymer in a solvent to give a viscous, polymer dope
 solution, spreading the solution upon a porous support to form a film,
 partially evaporating the solvent, and quenching the film in water. This
 precipitates the polymer and forms an asymmetric membrane by the phase
 inversion process.
 The polyimide polymer dope solution is prepared by dissolving the polyimide
 polymer in one or a mixture of the following water-miscible solvents:
 N-methyl-2-pyrrolidone, hereinafter referred to as NMP, tetrahydrofuran,
 hereinafter referred to as THF, N,N-dimethylformamide, hereinafter
 referred to as DMF, dioxane, .gamma.-butyrolactone, water, alcohols,
 ketones, and formamide.
 The weight percent of the polyimide polymer in solution may range from 12%
 to 30% in the broadest sense, although a 18% to 28% range is preferable
 and a 20% to 26% range will produce the best results.
 Additives such as viscosity enhancers may be present in amounts up to 10%
 by weight of the said polyimide polymer dope solution and these include
 polyvinyl pyrrolidones, polyethylene glycols and urethanes. Additionally
 additives such as void suppressors may be used in amounts up to 5% of the
 weight of said polyimide polymer dope solution, and in this case maleic
 acid produces the desired results.
 Once the polyimide polymer is dissolved in the solvent system described, it
 is cast onto a suitable porous support or substrate. The support can take
 the form of an inert porous material which does not hinder the passage of
 permeate through the membrane and does not react with the membrane
 material, the casting solution, the gelation bath solvent, or the aromatic
 materials being separated. Typical of such inert supports are metal mesh,
 sintered metal, porous ceramic, sintered glass, paper, porous nondissolved
 plastic and woven or non-woven material. Preferably, the support material
 is a non-woven polyester, polyethylene, or polypropylene material.
 Following the casting operation, a portion of the solvent may be evaporated
 under conditions sufficient to produce a dense, ultra-thin, top "skin"
 layer on the polyimide membrane. Typical evaporation conditions adequate
 for this purpose include air blown over the membrane surface at 15.degree.
 to 25.degree. C. for a duration of less than 30 seconds.
 The dense ultra-thin top "skin" layer of the asymmetric polyimide membranes
 of the invention is characterized by pore sizes below 50 .ANG. in
 diameter, is highly resistant to the greater than 500 psi operating
 pressures and has high operating efficiency and stability in the presence
 of solvent streams having a high aromatic content.
 The coagulating or quenching medium may consist of water, alcohol, ketones
 or mixtures thereof, as well as additives such as surfactants, e.g.,
 Triton X-100.RTM. available from Aldrich Chemical Company, Milwaukee, Wis.
 (octylphenoxy-polyethoxyethanol). The conditions for effecting coagulation
 are conventional.
 The asymmetric polyimide membranes of the present invention can be washed
 and dried according to the following techniques. Typically a water-soluble
 organic compound such as low molecular weight alcohols and ketones
 including but not limited to methanol, ethanol, isopropanol, acetone,
 methylethyl ketone or mixtures thereof and blends with water can be used
 for removing the residual casting solvent (e.g., NMP) from the membrane.
 Alternatively the membrane may be washed with water. Removal of the
 residual casting solvent may require successive wash blends in a
 sequential solvent exchange process. Both membrane efficiency and flow
 rate can be enhanced by the proper solvent exchange process.
 The membrane is then conditioned by contacting the membrane with a
 conditioning agent dissolved in a solvent to impregnate the membrane. The
 conditioning agent is a lubricating oil. Lubricating oils include, for
 example, synthetic oils (e.g., polyolefinic oils, silicone oils,
 polyalphaolefinic oils, polyisobutylene oils, synthetic wax isomerate
 oils, ester oils and alkyl aromatic oils) and mineral oils, including
 solvent refined oils and hydroprocessed mineral oils and petroleum wax
 isomerate oils. The lubricating oil may be a light neutral oil having a
 boiling temperature of 400-450.degree. C. to a heavy lubricating oil
 having a boiling temperature from 450-500.degree. C. It is also within the
 scope of the invention to use other natural lubricating oils such as, for
 example, vegetable fats and oils, however, such fats and oils may be less
 desirable to avoid introducing unwanted contaminants into the process
 streams. Suitable solvents for dissolving the conditioning agent includes
 alcohols, ketones, aromatics, or hydrocarbons, or mixtures thereof.
 The use of a conditioning agent in accordance with the invention allows the
 membrane to maintain a high flux while exhibiting a high selectivity to
 permeate aromatics in the presence of non-aromatics. The conditioning
 agent also allows the membrane to be wetted with hydrocarbon solvents, to
 maintain a suitable pore structure in a dry state for permeation of
 aromatics, and to produce a flat sheet membrane with improved flexibility
 and handling characteristics.
 Following treatment with the conditioning agent, the membrane is typically
 dried in air at ambient conditions to remove residual solvent. Preferably
 the membrane is dried in a forced air drying oven designed to capture
 solvent emissions.
 Heat treatment can also be used to increase membrane rejection of
 non-aromatic hydrocarbons. After the conditioning step, the membrane may
 be heated to about 150.degree. C. to about 320.degree. C., preferably
 about 200.degree. C. for about 1 minute to 2 hours. At about 200.degree.
 C., the heating time is typically about 5 minutes. It is preferred that
 the membrane be air dried before heating.
 Once the membranes are formed they may be processed into spiral wound
 modules, into hollow fiber configurations, into flat sheet or into plate
 and frame configurations.
 In the practice of a preferred embodiment of the present invention, a feed
 stream containing the aromatic hydrocarbons and non-aromatic hydrocarbons
 to be separated will be contacted with the dense active layer side of the
 polyimide membrane under pressure and at a temperature sufficient to
 effect the desired separation. Such contacting will typically be at about
 -20.degree. C. to about 150.degree. C., preferably about 20.degree. C. to
 about 80.degree. C. The pressure employed will be at least greater than
 that sufficient to overcome the osmotic pressure difference between the
 feed stream and the permeate stream. Preferably there will be at least a
 net driving force of about 100 to 1000 psi across the membrane, more
 preferably a net driving force of about 400 to 1000 psi, most preferably
 about 600 to 800 psi. Preferably, no additional heating or cooling of the
 stream is made to minimize energy requirements.
 The membranes of the invention are preferably used in accordance with the
 invention as described in co-pending application Ser. No. 126,256 entitled
 "Recovery of Aromatic Hydrocarbons Using Lubricating Oil Conditioned
 Membranes", filed on even date herewith. In accordance with the co-pending
 application, the invention membranes are contacted with a feed stream
 having at least 10 wt % of aromatic hydrocarbons. In a preferred
 embodiment, the membranes are contacted with a feed stream containing an
 aromatic hydrocarbon content of above 50 wt %, most preferably 70 wt % or
 higher. In one embodiment the feed stream contains an aromatic hydrocarbon
 content of 80 wt % or higher, preferably 90 wt % or higher.
 The membranes of the invention can be used to upgrade aromatics containing
 streams in petroleum refineries, such streams including, naphtha streams,
 heavy catalytic naphtha streams, intermediate catalytic naphtha streams,
 light aromatic streams and reformate streams. The membranes are also
 useful in commercial chemical operations for the recovery of aromatics
 such as, for example, benzene, toluene, xylene, and alkyl naphthalene. Of
 particularly interest, the membranes of invention can have application at
 various points in a commercial aromatics, i.e. toluene, production unit to
 upgrade the aromatics content of process streams. It is within the scope
 of the invention to use the invention membranes alone or in combination
 with other adsorption, distillation, extraction or reforming processes.
 When used in a hybrid process with other separation technologies, the
 invention membrane is not required to make 100% separation of aromatics
 from non-aromatics, but can instead be used to perform partial separation
 of aromatics to complement overall separation processes. The membrane of
 the invention thus offers excellent efficiencies in bulk separation
 processes which when coupled with other more selective unit operations can
 offer enhanced performance.

The Examples below are for illustrative purposes only, and do not limit the
 invention, or the claims which follow them.
 EXAMPLES
 In the cases shown here, the asymmetric polyimide membranes preferentially
 permeate aromatic over non-aromatic hydrocarbons. The membranes were
 tested on a small bench unit with four (4) test cells in series under
 reverse osmosis conditions The feed solution was continuously flushed over
 the membrane surfaces, and the permeate and retentate streams were
 combined and recycled. A typical feed solution consists of a high toluene
 concentration (80-100%), along with lesser amounts of other aromatic
 compounds such as benzene and p-xylene and non-aromatic hydrocarbons
 including branched and unbranched C6 to C9 isomers. The feed was
 pressurized, heated to operating temperatures, and pumped over the surface
 of the membrane. If desired, permeate lines could also be pressurized.
 Permeate samples were generally collected after overnight operation
 (18+hours).
 Membrane coupons were small disks with 14.2 cm.sup.2 surface area. Flows
 were determined in ml/min and then converted to gallons per square foot
 per day (GFD). Sample sizes were kept at less than 1% by weight of
 material, so that retentate and feed compositions were essentially equal.
 GC analysis was used to identify concentrations of aromatic and
 non-aromatic compounds. All concentrations were expressed as weight
 percent. Rejection was calculated from the sum of non-aromatic compounds
 in both the permeate and retentate streams with the formula Rejection
 (%)=(1-% per/% ret) 100%. In some cases, specific rejections for a given
 hydrocarbon were calculated.
 Example 1
 A viscous solution containing 22% Lenzing P84 polyimide (HP Polymers, Inc.,
 Lewisville, Tex.), 67% dioxane, and 11% dimethylformamide (DMF) was
 prepared and filtered through a 10 micron filter. This solution was cast
 at 10 ft/min onto a moving web of nonwoven polyester fabric (Hollytex 3329
 from Ahlstrom Filtration, Mt. Holly Springs, Pa.) using a knife blade set
 at a gap of 7 mil above the fabric. After about 15 seconds with an air
 flow of 1 SCFM the coated fabric was quenched in water at 22.degree. C. to
 form the membrane structure. The membrane was washed with water to remove
 residual solvents. then solvent exchanged by immersion into methyl ethyl
 ketone (MEK) for 3 hours, followed by immersion in a solution of 20% light
 neutral lube oil/40% MEK/40% toluene for 3 hours. The membrane was then
 air dried.
 The membranes were tested as flat sheet coupons at 50.degree. C. and 800
 psi with various circulating solutions consisting of a high toluene
 concentration and additional aromatic and non-aromatic C6 to C9
 hydrocarbons typically found in a toluene process stream in refinery
 operations. Total aromatic content in one of the feeds was 99.37 wt %.
 The membrane exhibited good rejection (54%) of non-aromatics and a flux of
 22.9 GFD. The aromatic content in the permeate was 99.71 wt %.
 Example 2
 A viscous solution containing 24% Lenzing P84 polyimide (HP Polymers, Inc.,
 Lewisville, Tex.), 56% dioxane, and 20% dimethylformamide (DMF) was
 prepared. This solution was cast at 4 ft/min onto a moving web of nonwoven
 polyester fabric (Hollytex 3329 from Ahlstrom Filtration, Mt. Holly
 Springs, Pa.) using a knife blade set at a gap of 7 mil above the fabric.
 After about 3 seconds with an air flow of 20 SCFH the coated fabric was
 quenched in water at 20.degree. C. to form the membrane structure. The
 membrane was washed with water to remove residual solvents, then solvent
 exchanged by immersion into methyl ethyl ketone (MEK) for 1 hour, followed
 by immersion in a second solvent exchange bath of light neutral lube oil
 in 50/50 MEK/toluene for 1 hour. The membrane was then air dried.
 A series of Lenzing P84 membranes with differing oil content were prepared
 by changing levels of oil in the 2nd exchange bath from 0 to 60% oil,
 while maintaining a 50/50 ratio of MEK/toluene.
 A feed solution consisting of 88 wt % toluene with six compounds (n-decane
 (C10), 1-methylnaphthalene (C11), n-hexadecane (C 16), 1-phenylundecane
 (C17), pristane (C19), and n-docosane (C22) each at 2% levels was
 prepared. Coupons of each membrane were tested at 600 psi and 50.degree.
 C. Results are recorded in Table 1 below.
 TABLE 1
 Oil Content in 2nd Solvent Exchange
 Bath (%) Flux (GFD) C10 Rejection (%)
 0 2.4 65
 20 25.3 45
 33 26.3 43
 50 27.7 42
 60 27.4 42
 The membrane having no lube oil present as a conditioning agent had an
 unacceptably low flux. The membranes conditioned with lube oil exhibited
 over 40% rejection of non-aromatic with a significant increase in flux
 (GFD) over the unconditioned membrane.
 Example 3
 A viscous solution containing 26% Matrimid 5218 polyimide (Ciba Geigy,
 Hawthorne, N.Y.), 15% acetone, and 59% dimethylformamide (DMF) was
 prepared and filtered through a 10 micron filter. This solution was cast
 at 10 ft/min onto a moving web of nonwoven polyester fabric (Hollytex 3329
 from Ahlstrom Filtration, Mt. Holly Springs, Pa.) using a knife blade set
 at a gap of 8 mil above the fabric. After about 15 seconds with an air
 flow of 1 SCFM the coated fabric was quenched in water at 22.degree. C. to
 form the membrane structure. The membrane was washed with water to remove
 residual solvents, then solvent exchanged by immersion into methyl ethyl
 ketone (MEK) for 3 hours, followed by immersion in a solution of 20% light
 neutral lube oil/40% MEK/40% toluene for 3 hours. The membrane was then
 air dried.
 A feed solution consisting of 94 wt % toluene with three non-aromatic
 compounds (n-decane, n-hexadecane, and n-docosane) each at 2% levels was
 prepared. Coupons of each membrane were tested at 600 psi and 50.degree.
 C.
 The membrane demonstrated 25% rejection of n-decane, 51% rejection of
 n-hexadecane and 68% rejection of n-docosane with a flux of 18.0 GFD.
 Example 4
 A Lenzing P84 membrane was prepared as in Example 2 and with the second
 solvent exchange bath containing 33% oil. An annealed membrane was
 prepared by clipping a sample to a glass plate, and heating in an oven at
 220.degree. C. for set times. The samples were tested with a toluene
 solution under pressure as in Example 2. Results are recorded in Table 2
 below.
 TABLE 2
 Flux and Rejection for Lenzing P84 Membrane at 600 psi and 50.degree. C.
 Anneal Time Flux Rejection (%)
 (minutes) (GFD) C10 C11 C16 C17 C19 C22
 0 29.7 45 0 75 70 83 95
 2 19.3 54 0 74 63 81 95
 4 17.4 59 16 88 84 97 100
 6 9.4 66 27 83 75 88 100
 8 3.9 75 34 91 100 100 100
 Rejection of the non-aromatic components by the membrane increased with
 heat treatment of the membrane.
 Example 5
 An annealed Lenzing P84 membrane was prepared as in Example 4 by clipping a
 sample to a glass plate, and heating in an oven at 180.degree. C. for 5
 minutes. Coupons of this membrane were tested with a toluene stream
 obtained from a refinery. The non-aromatics in this sample were isomers of
 C7 and C8 including methyl heptanes, ethyl hexanes, dimethyl hexanes,
 methyl ethyl pentanes, trimethyl pentanes, methyl ethyl cyclopentanes,
 trimethyl cyclopentanes, and dimethyl cyclohexanes. Results are recorded
 in Table 3 below.
 TABLE 3
 Rejection and Flux for Lenzing P84 Membrane at 800 psi and 58.degree. C.
 Non-
 Ben- Aromatics
 zene Toluene p-Xylene % Non- Flux Rejection
 (%) (%) (%) Aromatics (GFD) (%)
 Feed 0.10 94.89 0.28 4.73
 Permeate 0.10 97.58 0.28 2.04 34.7 57
 As shown in Table 3, the membrane exhibited favorable rejection of
 non-aromatics at favorable process conditions and permeate flow rates.
 Example 6
 A viscous solution containing 24% Lenzing P84 HT polyimide (HP Polymers,
 Inc., Lewisville, Tex.), 38% dioxane, and 38% dimethylformamide (DMF) was
 prepared. This solution was cast at 4 ft/min onto a moving web of nonwoven
 polyester fabric (Hollytex 3329) using a knife blade set at a gap of 7 mil
 above the fabric. After about 3 seconds with an air flow of 10 SCFH the
 coated fabric was quenched in water at 21.degree. C. to form the membrane
 structure. The membrane was washed with water to remove residual solvents,
 then solvent exchanged by immersion into MEK for 1 hour, followed by
 immersion in a solution of 33% light neutral lube oil/33% MEK/33% toluene
 for 1 hour. The membrane was then air dried.
 Coupons were tested with a 230-270.degree. C. distillation cut of light
 cycle oil containing a high percentage of aromatic compounds. The
 percentage of 1-ring, 2-ring, and 3-ring aromatics and non-aromatic
 compounds were determined with supercritical fluid chromatography methods.
 Results from coupon tests at 1000 psi and 57.degree. C. are reported in
 Table 4.
 TABLE 4
 Rejection and Flux for Lenzing P84 HT Membrane
 Non- 1-ring 2-ring Non-
 aroma- aroma- aroma- 3-ring aromatics
 Flux tics tics tics aromatics rejection
 (GFD) (%) (%) (%) (%) (%)
 Feed 16.2 10.3 73.5 0.0
 Permeate 1.1 8.5 8.1 83.4 0.0 47
 Example 7
 A viscous solution containing 22% Lenzing P84 polyimide, 11% acetone, and
 67% N-methyl-2-pyrrolidone (NMP) was prepared. This solution was cast at 4
 ft/min onto a moving web of non-woven polyester fabric (Hollytex 3329)
 using a knife blade set at a gap of 7 mil above the fabric. After about 3
 seconds the coated fabric was quenched in water at 22.degree. C. to form
 the membrane structure. The membrane was washed with water to remove
 residual solvents, then solvent exchanged by immersion into MEK for 1
 hour, followed by immersion in a solution of 40% light neutral lube
 oil/30% MEK/30% toluene for 1 hour. The membrane was then air dried.
 A feed solution was obtained from a refinery and consisted of three
 components, 11% p-xylene, 15% 1-methylnaphthalene, and 74% of a light
 neutral lube distillate. Coupons were tested at 600 psi and 107.degree. C.
 The lube distillate component of the feed was fractionated by refining
 processes into an aromatic-rich extract oil fraction and a
 non-aromatic-rich raffinate oil fraction. Analysis of the feed and
 permeate solutions and the refinery extract and raffinate fractions were
 performed with GC and UV/visible spectrometry. By monitoring the
 absorption at 350 nm in methylene chloride, an estimate of the percentage
 of aromatic-rich extract oil was determined. Results are recorded in Table
 5 below.
 TABLE 5
 Fractionation of Lube Distillate Oil with Lenzing P84 Membrane
 1-methyl- lube est. % est. %
 p- naphtha- distil- extract oil raffinate
 Flux xylene lene late in lube oil in lube
 (GFD) (%) (%) (%) distillate distillate
 Feed 11.0 14.8 74.2 43.7 56.3
 Permeate 4.1 12.9 17.4 69.7 49.8 50.2
 as shown in table 5, the membrane showed good rejection of non-aromatics as
 indicated by the increased aromatic content in the permeate.