CA2020696A1 - Isocyanurate crosslinked polyurethane membranes and their use for the separation of aromatics from non-aromatics - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

The present invention is directed to non-porous isocyanate-crosslinked polyurethane membranes.
These membranes are useful for the separation of aromatic hydrocarbons from non-aromatic hydrocarbons.
The separation can be performed using any commonly accepted membrane separation technique, e.g. reverse osmosis, dialysis, pervaporation or perstraction but is preferably performed under pervaporation or perstrac-tion conditions.

Description

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ISOCYANURATE CROSSLINKED POLYURETHANE MEMBRANES
AND THEIR USE FOR THE SEPARATION OF AROMATICS
FROM NQN-~ROMATICS
Description of the Inve~tion Isocyanate crosslinked polyurethane mem branes, which are non-porous, have been ~abricated.
Thesa membranes can be cast on a surfac:e which does not provide a backing layer, thereby produc:ing a symmetric membrane. Alternatively the membrane can be cast on a porous backing such as teflon, polypropylene etc. to produce an integral composite membrane. The membrane is particularly useful for separatin~ aromatics from non-aromatic~, e.g. aromatics from saturates~ especial-ly for upgrading naphtha streams. Such separations are preferably performed under pervaporation or parstrac-tion conditions.

Baçk~round ~f ths Invention The use of membranes to separate aromatics from saturates ha~ long been pursued by ~he sclenti~ic and industrial community and is the subject of numerous patents.

U.S. Patent 3,370,102 describes a genexal process for separating a ~eed 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 for~e. The process can be used to separate a wide variety of mixtures including various petroleum frac-tions, naphthas, oils, hydrocarbon mixtures. Expressly recited is the separation of aromatics from kerosene.

U.S. Patent 2,958,656 teaches the separation o~ hydrocarbons by type, i.e~ aromatic, unsaturated, saturated, by permeating a portion of the mixture ' ` ~

2~2~
- 2 ~

through a non-porous cellulose sther membrane and removing permeate from the per~eate side of the mem-brane using a sweep gas or liquid. Feeds include hydrocarbon ~ixtures, naphtha (including virgin naph-tha, naphtha from thermal or catalytic cracking, etc.).
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- U.S. Patent 2,930,754 teachQs a method for separating hydrocarbons e.g. aromatic and/or olefins ~rom gasoline boiling range ~ixtures, by the selective per~eation of the aromatic through certain c~llulose ester non-porous membranes. The permeated hydrocarbons are continuo~sly removed fro~ th~ p~r~eate zone using a ~weep gas or li~uid.

U.S. Paten~ 4,115,465 teaches the usa of po}yurethane membranes to select~vely separate aromat-ics fro~ saturate~ via pervaporation.

U.S. Patent 4,366,062 teaohe3 reverse osmosis using a composit~ isocyanurate membrane. The method selectively separates at least one water soluble material from an aqueous solution. The membrane comprises a ~icroporous substrate and a barrier layer about 0.01 to 0.1 micron thick. It is composed of a cro~linked polymeric material having isocyanurate structure and 3ub~tituents appended thereto selected fro~ hydrogen, glycidyl group~ and alk~l radical groups containing 2 to 5 carbon atoms which may also contain ~unctional hydroxyl groups or glycidyl groups. The cros~linked polymeric material has ester or ether linkages or combination thereo~ connecting the i~ocyanurat~ structures to each other. There are no urethane groups present.

U.S. Patent 4,5~7,949 teaches a method ~or making the reverse osmosis semipermeable membrane disclosed in U.S. 4,366,062.

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European Publication 0044B72 teaches selec-tively separating water soluble mat~rials from a solution under reverse osmosis conditions using a membrane having a porous support layer carrying a barrier layer of crosslinked isocyanurate polymer.

Japanese Publication 81/160960 teac~es a composite membrane for reverse osmosis made by applying a solution of a barrier layer-forming component to a substrate, then heating it.

Japanese Publication 78/121150 teaches an isocyanurate network terpolymer useful for the prodllc-tion of a selective permeation membrane. A polymer having hydroxyl groups and tert amine groups in the sid~ chain is reacted with cyanuric chloride and subject to terpolymerization by reacting the tert amine groups with produced hydrochloride to give a polymer-ized polyisocyanurate. A polymer made using glycidyl methacrylate-styrene copolymer, diethyl amine in benzene and methanol was produced having 2-hydroxy -~-diethylaminopropyl group. This polymer was crosslinked with cyanuric chloride and cast on a PTFE plate and kept 24 hours at 40 to give a 44~ membrane. This membrane was used to se~arate a mixture o~ cyclohexane and benzene under pervaporation conditions. A permeate gas which was 100% benzene was recovered at a rate of 0.0025 g/m2-hr.

Detailed Description of the Invention The present invention is directed to a non-porous isocyanurate crosslinked polyurethane membrane and its use for the s~paration of aromatic hydrocarbons from non-aromatic hydrocarbons e.g.
aromatics from saturates as, for example, in upgrading aromatics containing streams in petroleum refineries, 2 ~ % ~

and chemical plants, such streams including by way of exa~pla and not limitation naphtha ~treams, heavy cat naphtha stream3, intermediate cat naphtha streams, light aromatic streams boiling in the Cs-300-F range, LCCO boiling in the 400-650-F range BTX containing streams, etc.

The isocyanurate crosslinXed polyurethane m~mbran~ is produced employing standar~ membrane ca~ting procedures. A prepolymer o~ polyurethane is prepared by reacting dihydroxy or polyhydroxy compounds (Q . g., polyethers or polyester ) of about 250 to 5000 ~olecular weight, or mixtures of di~ferent molecular weight polymers o~ th~ same type with aliphatic, alkylaromatic or aromatic diisocyanates or polyisocya-nate~.

Mixtures of polyesters and polyethers can al50 be used.

This isocyanate end-capped polyurethane prepolymer i~ trimarized using a standard trimerization catalyst such as N,N',Nn-tris(di~ethylaminopropyl)-s-hexahydrotriazine, Sodium ethoxid~, Potassium octoate, N-Hydroxypropyl-trimethyla~monium-2-ethylhexanote, Pota~ium 2-~thylhexanoate, Trialkyl phosphines, 2,4,6-Tris(di~ethyla~inomethyl)phenol and ~ixtures th~r~o~. U~ing these catalyst yields a mixture which ~lo~ly thickens due to crosslinking accounted for by the ~or~ation of isocyanurate crosslinked rings.
Before this ~ixture beco~es too thick, it i5 deposited as a thin film on an appropriate substrate and permit-ted to ~ully gel, after which the membrane coat is treated to complete the formation o~ isocyanurate crosslinked polyurethane. This final treat can consti-tute no mor~ than waiting a ~ufficiently long time to be certain that trimerization is complete. More likely thi~ final treat will involv9 ~arious degrees of drying followed, pre~erably, by heating to complete the tri~erization to the isocyanurate crosslinksd polyure-thane.

As previously stated, thQ me~branes are produced by standard casting technique~e from a polymer made from dihydroxy or polyhydro~y compounds, such as polyethers or polyester oP 250 to 5000 molecular waigh~, end capped with aliphatic, allkylaro~atic or aro~atic diisocyanates or polyisocyarlates to ~or~l a polyurethane prepoly~er which i~ thsn trimerized through the free iso~yanate group using 2 catalyst to produce the isocyanurate cro slinked polyurethane. Th~
end capped polyurethane prepolymer is produced using a 1:2 mole ratio of diol wi~h diisocyanate.

The polyester polyol components are prepared from aliphatic or aromatic dicarboxylic aci~s and aliphatic or aromatic dialcohols. ~liphatic dicarboxy-lic acids re~er to those ~aterials having the general formula HOOC~COOH where R contain~ 2 to 10 carbons (and may be either a straight or branched chain configura-tion). Aromatic dicarboxylic acids refer to those matQrial~ having the general structure ~OOCRCOOH where R i~:
R' R' R''' R'~

(C)n I R'~' II

wherein R', R'' and R''' may be the same or different and are selected from the group consisting of H and ~2~

Cl-Cs carbon~ or C~Hs and combinations thereof, and n is O to 4. It is to bQ understood thai: in the above ~ormula each R' or R' ' may itself represent a mixture of H, Cl-cs or C6Hs-Dialcohol~ hav~ the general structure HOROHwhere R may be R' ~R' --(I)n ~O~

R' III IV

wher~ n i~ 1 to 10, preferably 4 to 6, and R' is H, C
to Cs or C6Hs or R' R' ~ ' R' ' ~ I ~
----W ^ _ ( C) n--R' ' ' ~I

wher~ R', R'', R''' and n are defined, in ~he same ~anner a~ for the aromatic dicarboxyl ic acids . An exampl~ o~ a use~ul dialcohol is bisphenol ~.

The diisoryanate~ are preferably aromatic diisocyanates having the general structure:

2 0 ~
_ 7 ~

R~ R''' R'' OCI~{~(C) n ~Co R' ' VI

wherein R' and R'' are thQ sam~ or dli~erent and are selected ~rom the group consisting o~ H, Cl-Cs and C~Hs ~nd mix~ure# thereo~ and n ranges fro~ O to 4.

Examples of th~ polyether polyols use~ul in the present invention as polymer precursors are poly-ethylene glycol, ~PEG), polypropylene glycol (PPG), polytramethylene glycol, PEG/PPG random copolymers, etc. having molecular weight ranginq ~rom about 250 to 4000. ~liphatic diisocyanates which may be ut$1ized are exe~plified by hexamethylene diisocyanate (~DI), 1,6-diisocyanato-2,2,4,4-tetramethylhexane ~TMDI), 1,4-cyclohexanyl diisocyana~e (CHDI~, isophorone diisocyanat~ (IPDI)~ while useful alkylaromatic diisocyanates are exe~plified by toluene diisocyanate (TDI3 and bitolylene diisocyanate (TODI). Aromatic ocyanate~ are exemplified by 4,4'-diisocyanato diphenyl~ethan~ (MDI). Polyisocyana~es are exemplified by poly~eric ~DI fP~DI) and carbodiimide modified M~I.

Trimerization ca~alysts are exempli~ied by N,N',N~-tris(dimethylaminopropyl~-s-hexahydro~riazine, Sodium e~hoxide~ Potassium ootoate, N-Hy~roxypropyl-tri~ethyla~monium-2-ethylhexanote, Potassium 2-ethyl-hexanoate, Trialkyl phosphines, and mixtures thereof.

The above are presented solely by way of exa~ple. Those skilled in the art, with the present ~ 0 2 0 bS ~ ~

teaching before them, can select from the innumerable materials available the various starting materials which upon combination as d~scribed herein will produce a polyisocyanate crosslinked polyurethane which can thsn be cast into the me~branes useful ~or the separa-tion of aromati c3 fro~ saturates.

The m2mbranes ara produced by preparing the polyisocyanurate crosslinXed polyurethane in an appro-priat~ solvent, such as Dimethyl~or~amide (DM~), N-~ethyl pyrrolidone (N~P), 2-Etho~ethyl acetat~
(cQllosolve acetate), Di~athyl acetamlde (DMAC), Dimethyl sulfoxide (DMSO) and ~ixtures thereo~, to produce a pourable, spreadabl~ solution. To this end, once the components are mixed, the mixture should be poured or spread be~ore the isocyanurate polymex gels to too high a viscosity. Thus, the isocyanurate crosslinking polyurethane mixture can be poured or spread almost immediately upon tha acldition of the trimerizing catalyst, if the surface on which it is pourad or spread i3 not porous. A thin film of this ~iXtUrQ iS loft on the 3ur~ace (glass, metal, non-porous ~abric etc.) and per~itted to trimerize over time until tha reaction to the isocyanurate-crosslinked polyurethan~ ~s completed. Alternatively if a porous support ~B used, in order to prevent thP casting solution ~ro~ simply soaking ~hrough the sur~ace or b2co~ing ~mbédded in the pores o~ the fabric or back-ing, the c2~ting mixture is le~t to ~and so as ~o gel to ~o~e extent prior to being poured or cast onto the ~upport.

In either case, after the casting solu~ion ha-c been spread and has gelled it can be left ~o complet~ its trimerization simply by standing. Alter-natively tAe gelled film can be dri~d in air or inert gas stream at 25-lOO-C to induce completion of ~ c3 _ 9 ~

~ros~llnking. Prefarably ~ollowing drying the film may bo heated if necessary at 50-lOO~C in air or other gas or vacuum to complete trimerization to the polyisocya-nurate.

The ca ting solution of end capped poly-urethane and catalyRt in solvent has a concentration o~
1 to 50 wt% polymerization component in solvent, pre~erably 2 to 25 total wt% component~s in the solvent.

In general the backing or support ~an be glass, metal, woven fabric, or ~on-woven abric. Woven fabric backing includes woven fiber gl3ss, nylon~
polye~ter ~tc. Non-woven backing~ include non-woven porou~ polypropylene or teflon. Tha backing is one which is not attacked by ths solvant used to procluce the casting solution. It is also one which will stand up to the environment to which the active membrane layer will be exposed. That environment includes the materials in the ~ixtureR to be separated as well as the te~peratures used in the separation~ Clearly, separations practiced at ~levated temperatures require the US8 of a high temp~rature backing, e.g. sintered metal or tePlon rather than polypropylene~

The m~nbrane active layer ( i . e . membrane less any backing or ~upport which may be used) may be cast in any thicknes8, me~brane~ ranging in thickness of ~ro~ about 0.1 to about 50 ~icrons being preferred.

Alternatively a very thin layer of the casting solution (gelled to a manageable viscosity) can be deposited into a highly permeable, non-porous, non-selective polyurethane layer producing a composite membrane comprising a thin, dense, selective layer of isocyanurate-crosslinked polyurethane which would otherwise be mechanically unmanayeable due to their ~1~2~$

thlnness. Due to the chemical similarity between the polyursthane support layer and the isocyanurate-cros~linked polyurethane activa, selective layer, the two layers interact through hydrogen bonding to produce a very strong adhe.~ion.

If one were to use this technique to produce sheet material, the thick, permeable polyurethane layer can be depo~ited on a ~uitable backing material such as porous fiber glass, polyethylene, pol~propylene, nylon, ~eflon, etc. after which ths thin, danse ~elective polyurea/urethane layer would be deposited onto the polyurethane layer.
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In producing hollow fibers or tubes using this composite membrane technique, first a tube or hollow fiber o~ suitable support material, such as nylon, teflon or permeable polyurethane is produced aftar which a thin dense layer of the selective poly-lsocyanurate crosslinked polyuret~ane material is deposited on either ~he outer or inner surface of the tube or fiber support. It is also possible to deposit a layer o~ the aforesaid porous polyurethane on ~he hollow ~iber and then put ~own a thin, dense film of polyisocyanurate crosslinked polyurathane thereon.

Th~ permeable polyurethane layer can be prepared ~ro~ polyether glycols such as polypropylene glycol or polybutylene glycol plus aliphatic. and/or aromatic diisocyanates (pre~erably aliphatic diisocya-nates) using polyols (diols or triols) preferably aliphatic diols as chain extenders. These permeable polyurethane sublayers will possess characteristics well outside the minimums recited for the polyurea/-urethane me~branes taught herein. Polyurethane mem-brane materials which satisfy the above requirement of permeablllty ~re th~ polyur2thane m~mbranes described in U.S. Patent 4,115,465.

The membraneg arQ use~ul for the separation of aromatics fro~ saturate~ in petroleum and chemical streams, and hav~ been ~ound to b~ particularly useful for the separation o~ larg~ substituted arQ~atiCs from saturates as are encountered in heavy cat naphtha ~trea~s. Other strea~3 which are al~o ~uitabl~ feed stream~ for arosatic~ ~ro~ ~turate~ separation are int~rmediate cat naphtha streams, (200-320-F) light aromatics content streams boiling in the Cs-300-F
range, light catalytic cycle oil boiling in tha 400-650 F range as wQll a~ ~tream~ in chemical plant~ which contain recoverable quantitie~ of benzene, toluene, xylene (BTX~ or other aromatic~ in co~bination with saturates. The separation techniques whlch may suc-cessfully 2mploy the membranes o~ ~he present inven~ion include perstraction and pervaporation.
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Perstraction involves the selective di~solu-tion of particular componen~s contained in a mixture into the membrane, the di~fu~ion o~ those components through the membrane and the removal of the diffused co~ponents ~rom the downstream side of ~he membrane by u~e o~ a liquid sweep ~trea~. In the perstractive separ~tion of aromatics fro~ saturates in petroleu~ or chemical stream~ (particularly heavy cat naph~ha ~tr~a~) tha aro~atic molecule~ present in the feed-strea~ dissolve into the ~embrana fil~ due to similari-tie~ between the membrane solubility parameter and those of th~ aromatic species in the feed. The aroma-tics then permeate (diffuse) through the membrane and are swept away by a sweep liquid which is low in aromatics content. This keeps ~he concentration of aromatics at the permeate side of the membrane film low and maintains the concentration gradient which is , ~ 0 2 ~

respon~ble for ths permeation of the aro~atics through tha ~e~brane.

The sweep liquid i8 low in aromatics contsnt so as not to itself decrease the concentratlon gradi-an~. ~he sweep liquid i8 pr~f~rably a ~aturated hydroc~rbon liquid with a boiling point much lower or much higher than that of the permeated aromatics. This i~ to facilitate separa~ion, a~ by simple distillation.
Suitable sweep liquid~, therefore, would include/ ~or exaMple~ C3 to C6 saturated hydrocarbons and lube basestoCkS (C15-C2~)-The perstraction process is run at any convenient temperature, preferably as low as possible.

I'he choice of preæsura is no~ critical since the perstraction process is not dependent on pressure, but on the ability o~ the aromatic components ln the fee~ to dissolve into an migrate ~hrough ths membrane under a conc2ntra~ion driving force. Consequently, any convenient pre~sure ~ay be employed, the lower the better to avoid un~esirabl¢ compaction, i~ the ~embrane is supported on a porous backinq, or rup~ure of the ~embran~ it i~ not.

IP C3 or C4 sweep liquids are used at 25-C or abov~ in liquid C~ate, the pressure mus~ be increased to keep them in the liqui~ pha~e.

Pervaporation, by co~parison, is run at generally higher ~e~peratures than pers~raction and relies on vacuum on the permeate side to evaporate the permeat2 from the surface of the membrane and maintain the concentration gradient driving force which drives the separation process. As in perstraction, the aromatic molecules present in the feed dissolve into 2~2~

th~ ~embrane ~ilm, migr~ through said film and re-~erqe on the per~ea~e sidQ under the influence of a concentration gradient. Pervaporative separation of aromatic~ ~rom ~aturates can bQ perfo~ed at a tempera-ture of about 25'C for the separation of ~en2ene from hexane but for saparation of heavier aromatic/saturate mixtures, ~uch as heavy cat naphtha, higher te~pera-tures of at least 80C and high~r, pre~erably at least lOO-C and higher, more praferably 1;20-C and higher should be used, the maximu~ upper l;imik being that temperatUrQ at which the membrane i~ phy~ically dam-aged. Vacuum on the order oP 1-50 ~m Hg is pulled on the permaate side. The vacuum strea~ containing the perme~te is coolsd to condense ou~ the highly aromatic per~eate. Condensation tempera~ure should be be.low the dew point o~ the permeat~ at a given vacuum level.

The membrane itsel~ may be in any convenient form utili~ing any convenient modulQ design. Thus, sheet~ o~ me~brane ~aterial may be use~ in spiral wound or plate and ~rame permeation cell modules. Tubes and hollow fibar~ Or membranes may be used in bundled co~figurations with either the feed or the sweep liquid (or vacuum) in the intarnal space of the tub~ or fib~r, the oth~r material obviously being on the other side.

Most conveniently, the me~brane is used in a hollo~ fiber configuration with the feed introduced on the exterior side of the fiber, the sweep li~uid or vacuu~ being on the inside or outside o~ the hollow ~iber to sweep away the permeated highly aromatic specie~, thereby maintaining the desired concentration gradient. The sweep liquid, along with the aromatics contained therein, is passed to separation means, typically distilla~ion means, however, if a sweep liquid of low enough ~olecular weight is used, such as liquefied propane or butane, the sweep li~uid can be 2 0 2 ~ 3 parmitted tG si~ply evaporate, the liquid aromatics b~ing recovered and the gasaous propane or butane (for ex~mplQ) b~ing recovered and reliquefied by application of pressure or lowerlng of te~perature.

The present invention will be better under-~tood by re~erenca to th~ ~ollowing Examples which are o~ered by way of illustration and not li~itation.

T~enty-~ive point two nin2 gra~s ~approxi-~at~ly 0.05 mole) polyethylsn~ adipate (500 ~W) and 25.0 grams (0.10 mol~) methylene diisocyanate were placed in a wida ~outh round bottom flas~ ~quipped with a ~echanical stirrer. The mixture was stirred and heated at 95-C for 2 hours to for~ an isocyanate-capped urethane prepolymQr. OnQ gra~ o~ this prepolymer was di~solved in 17 gram~ o~ DMF to which wa~ added 2 grams o~ DMF containing 0.01 gram 2~4,6-Tris(dimethylamino-methyl)phenol (DABC0 TMR-30) and 0.002 gram Potassium 2-ethylhexanoatQ (D~BCO ~-15) catalysts. Ths solution was stirred until it began to thicken due ~o crosslink-ing reaction~ (approxi~ately 60 minutes); and then a s~ll a~ount of it was poured onto a pieca of porous T~lon mQm~ran~ (DSI K-100, 0.02 ~4 pore size~. ~he cro~ nking continued until th~ layer on the Teflon had co~pl~t~iy gelled up. The coated Teflon was air dri~d for 30 minutes, then placed in~o an oven at 160C
oYernight under a constant nitrogen purge to complete thQ formation of th~ polyisocyanurate. It was tested for per~tractive ~eparation of sa~urates ~rom aromatics in a ~mall laboratory ~embrane testing unit at 863C
using n-heptane a~ t~e sweep liquid and a feed consist-ing of 14.6 wt~ p-xylene~ 28.1 wt% mesitylene, 13.0 wt%
l-decene and 44 . 3 wt% n-decaneO The run was subse-quently repeated at 113-C using n-hexadecane as the ~02~

'' 15 --8we~p. Rasults fro~ both run8 are shown in Table balow.

Table 1 Perstraction o~ ~odal Fead ~with a Isocyanurat~ Crosslinked Polyurethana Membrane Te~peratur~ (-C) 86 113 Flux (kg/~2/d~U.1010.416 S~1QCtiVitY
(V8 n-decana~ to p-~ylene 16.75 15.83 mesitylene 7.~6 7.18 As can b~ seen from the data~ the selectivity o~ these membranes to aromatics is quite good. In addition, b~cause this i5 a crosslinked pol~mer, the sel~ctiv~ty doe~ not show much decline with elevated temperatur~ whil~ th~ flux increased fourfold. The relatively low ab~olute flux rates can be increased by ~aking t~e membrane3 thinn~r.

Claims (17)

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

1. A non-porous isocyanurate crosslinked polyurethane membrane.

Z. The membrane of claim 1 wherein the isocyanurate crosslinked polyurethane is supported on a backing.

3. The membrane of claim 2 wherein the backing is selected from teflon, polypropylene.

4. The membrane of claim 1 or 2 wherein the polyurethane is prepared by reacting a dihydroxy or poly hydroxy compound of about 250 to 5000 molecular weight and mixtures thereof with aliphatic, alkylaro-matic or aromatic diisocyanates or polyisocyanates.

5. A method for producing a non-porous isocyanurate crosslinked polyurethane membrane compris-ing the steps of preparing 2 prepolymer of polyurethane by reacting dihydroxy or polyhydroxy compounds with a di or poly isocyanate, trimerizing this prepolymer using a trimerization catalyst to produce a casting solution mixture which slowly thickens due to cross-linking caused by the formation of isocyanurate cross-linked rings, depositing this casting solution on a substrate to produce a thin film, permitting the thin film to fully gel into the desired isocyanurate cross-linked polyurethane membrane.

6. The method of claim 5 wherein the thin layer of casting solution is dried to produce the desired isocyanurate crosslinked polyurethane membrane.

7. The method of claim 5 wherein the layer of casting solution is heated to complete trimerization to produce the desired isocyanurate crosslinked poly-urethane membrane.

8. The method of claim 5, 6 or 7 wherein the dihydroxy or polyhydroxy compound and the di or poly isocyanate compound is used in a mole ratio of 1:2.

9. The method of claim 5, 6 or 7 wherein the casting solution is prepared by mixing the components in the presence of a solvent.

10. The method of claim 9 wherein the solvent is selected from dimethylformamide (DMF), N-methyl-pyrrolidone (NMP), 2-ethoxyethyl acetate (cellosolve acetate), dimethylacetamide (DMAC), dimethyl sulfoxide (DMSO) and mixtures thereof.

11. The method of claim 9 wherein the casting solution has a concentration of 1 to 50 wt%
polymerization component in solvent.

12. The method of claim 5, 6 or 7 wherein the casting solution is spread on a substrate selected from non-woven polypropylene or teflon.

13. The method of claim 5, 6 or 7 wherein the casting solution is spread on a substrate compris-ing highly permeable, non-porous, non-selective poly-urethane layer.

14. The method of claim 5, 6 or 7 wherein the casting solution is spread on either the inside or outside surface of a hollow fiber of suitable support material.

15. A method for separating aromatic hydro-carbons from mixtures of same with non-aromatic hydrocarbons comprising the step of contacting the mixture with one side of a non-porous isocyanurate crosslinked polyurethane membrane under conditions such that the aromatic hydrocarbon selectively permeates through the membrane.

16. The method of claim 15 wherein the permeation is conducted under pervaporation or per-straction conditions.

17. The method of claim 15 wherein the permeation is conducted under pervaporation conditions.