Crosslinked copolymers of aliphatic polyester diols and dianhydrides

A crosslinked copolymer composition is derived from an aliphatic polyester, a dianhydride and a diicoyanate. The copolymer membrane has high thermal stability and good aromatic/saturate selectivity and permeability.

BACKGROUND 
The present invention relates to a composition of matter for the separation 
of aromatics from saturates. 
The use of membranes to separate aromatics from saturates has long been 
pursued by the scientific and industrial community and is 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, e.g., 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 non-porous cellulose 
ester 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. 
Compared to distillation, membrane permeation can lead to considerable 
energy savings. A membrane can separate a mixture of aromatics and 
saturates, e.g., a heavy cat naphtha, into a high-octane, mainly aromatic 
permeate and a high-cetane, mainly saturated retentate. Both permeate and 
retentate are more valuable than the starting heavy cat naphtha. 
SUMMARY OF THE INVENTION 
The present invention is a composition of matter and its use in a process 
for separating aromatics from feeds which are mixtures of aromatics and 
non-aromatics. The composition is formed into a membrane which includes a 
crosslinked copolymer composition wherein the copolymer is derived from an 
aliphatic polyester diol and a dianhydride. The aliphatic polyester may be 
a polyadipate, a polysuccinate, a polymalonate, a polyoxalate or a 
polyglutarate. Crosslinking can be accomplished by a variety of methods. 
Preferably, the copolymer is crosslinked by reaction with diisocyanates. 
In a preferred embodiment, polyester is a polyethylene adipate or a 
polyethylene succinate, the dianhydride has between 8 and 20 carbons, and 
the diisocyanate has between 4 and 30 carbons. 
In a preferred embodiment, the dianhydride is selected from the group 
consisting of pyromellitic dianhydride, 3,3',4,4'-benzophenone 
tetracarboxylic dianhydride, 4,4'-(hexafluoroisopropylidene)-bis-(phthalic 
anhydride), 4,4'-oxydiphthalic anhydride, 
diphenylsulfone-3,3',4,4'-tetracarboxylic dianhydride, and 
3,3',4,4'-biphenyltetra-carboxylic dianhydride. 
In a preferred embodiment, the diisocyanate is selected from the group 
consisting of 2,4-toluene diisocyanate (TDI), methylene diphenylisocyanate 
(MDI), methylene dichlorophenylisocyanate (dichloro-MDI), methylene 
dicyclohexylisocyanate (H12-MDI), methylene dichlorocyclohexylisocyanate 
(dichloro-H12-MDI), methylene bis(dichlorophenylisocyanate) 
(tetrachloro-MDI), and methylene bis(dichlorocyclohexylisocyanate) 
(tetrachloro-H.sub.12 -MDI). 
The present invention also includes a method for separating aromatics from 
feeds which are mixtures of aromatics and non-aromatics which method 
comprises selectively permeating the aromatic hydrocarbon through a thin 
membrane including a crosslinked copolymer composition wherein the 
copolymer is derived from an aliphatic polyester diol, a dianhydride, and 
a diisocyante crosslinking reagent.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The present invention is new copolymers for membranes to separate aromatics 
from feed streams of aromatics and non-aromatics. 
The new copolymers contain aliphatic polyester soft segments and hard 
segments derived from the dianhydride and the diisocyanate crosslinking 
reagent. FIG. 1 shows the synthesis and composition of the copolymer 
containing polyethylene adipate soft segment and crosslinked hard segment. 
In the synthesis, one mole of polyethylene adipate diol with a molecular 
weight of 2000 reacts with one mole of pyromellitic dianhydride (PMDA) to 
make a copolymer. The copolymer is then dissolved in dimethyl formamide 
(DMF) and 2,4-toluene diisocyanate (TDI) (at 0.5 mole to each mole of 
PMDA) is added to the solution. The new copolymer membrane can be prepared 
by casting the solution on a glass plate, or a porous support, adjusting 
the thickness by means of a casting knife, and drying the membrane first 
at room temperature to remove most of the solvent, then at 160.degree. C. 
overnight to complete the crosslinking of polymer chains with TDI. The 
membrane is then removed from the glass plate via soaking in water. 
Finally, this membrane is then dried at 120.degree. C. overnight. 
The new membranes can be used for the separation of aromatics from 
saturates. In our separation experiments, the membranes are employed to 
separate a mixture containing toluene and isooctane in a pervaporation 
apparatus. The initial mixture contains about equal weights of the two 
hydrocarbons. The pervaporation apparatus is a cell, separated into two 
compartments by a porous metal plate, on which the membrane is supported. 
During a pervaporation experiment the toluene-isooctane mixture is 
circulated through the upper compartment at the desired temperature. The 
lower compartment is kept at reduced pressure. The permeate is collected 
in a trap cooled with dry ice-acetone or isopropanol and periodically 
analyzed by gas chromatography. 
The membranes are useful for the separation of aromatics from saturates in 
petroleum and chemical streams, and have been found to be particularly 
useful for the separation of large substituted aromatics from saturates as 
are encountered in heavy cat naphtha streams. Other streams which are also 
suitable feed streams for aromatics from saturates separation are 
intermediate cat naphtha streams boiling at 93-160.degree. C., light 
aromatics content streams boiling in the C.sub.5 -150.degree. C. range, 
light catalytic cycle oil boiling in the 200-345.degree. C. range as well 
as streams in chemical plants which contain recoverable quantities of 
benzene, toluene, xylenes (BTX) or other aromatics in combination with 
saturates. The separation techniques which may successfully employ the 
membranes of the present invention include perstraction and pervaporation. 
Perstraction involves the selective dissolution of particular components 
contained in a mixture into the membrane, the diffusion of those 
components through the membrane and the removal of the diffused components 
from the downstream side of the membrane by the use of a liquid sweep 
stream. In the perstractive separation of aromatics from saturates in 
petroleum or chemical streams (particularly heavy cat naphtha streams) the 
aromatic molecules present in the feedstream dissolve into the membrane 
film due to similarities between the membrane solubility parameter and 
those of the aromatic species in the feed. The aromatics then permeate 
(diffuse) through the membrane and are swept away by a sweep liquid which 
is low in aromatics content. This keeps the concentration of aromatics at 
the permeate side of the membrane film low and maintains the concentration 
gradient which is responsible for the permeation of the aromatics through 
the membrane. 
The sweep liquid is low in aromatics content so as not to itself decrease 
the concentration gradient. The sweep liquid is preferably a saturated 
hydrocarbon liquid with a boiling point much lower or much higher than 
that of the permeated aromatics. This is to facilitate separation, as by 
simple distillation. Suitable sweep liquids, therefore, would include, for 
example, C.sub.3 to C.sub.6 saturated hydrocarbons and lube basestocks 
(C.sub.15 -C.sub.20). 
The perstraction process is run at any convenient temperature, preferably 
as low as possible. 
The choice of pressure is not critical since the perstraction process is 
not dependent on pressure, but on the ability of the aromatic components 
in the feed to dissolve into and migrate through the membrane under a 
concentration driving force. Consequently, any convenient pressure may be 
employed, the lower the better to avoid undesirable compaction, if the 
membrane is supported on a porous backing, or rupture of the membrane, if 
it is not. 
If C.sub.3 or C.sub.4 sweep liquids are used at 25.degree. C. or above in 
liquid state, the pressure must be increased to keep them in the liquid 
phase. 
Pervaporation, by comparison, is run at generally higher temperatures than 
perstraction and 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. As in 
perstraction, the aromatic molecules present in the feed dissolve into the 
membrane film, migrate through said film and emerge on the permeate side 
under the influence of a concentration gradient. Pervaporative separation 
of aromatics from saturates can be performed at a temperature of about 
25.degree. C. for the separation of benzene from hexane but for separation 
of heavier aromatic/saturate mixtures, such as heavy cat naphtha, higher 
temperatures of at least 80.degree. C. and higher, preferably at least 
100.degree. C. and higher, more preferably 120.degree. C. and higher 
should be used. Temperatures of about 170.degree. C. have been 
successfully used with membranes of the present invention, the maximum 
upper limit being that temperature at which the membrane is physically 
damaged. Vacuum on the order of 1-50 mm Hg is pulled on the permeate side. 
The vacuum stream containing the permeate is cooled to condense out the 
highly aromatic permeate. Condensation temperature should be below the dew 
point of the permeate at a given vacuum level. 
The membrane itself may be in any convenient form utilizing any convenient 
module design. Thus, sheets of membrane material may be used in spiral 
wound or plate and frame permeation cell modules. Tubes and hollow fibers 
of membranes may be used in bundled configurations with either the feed or 
the sweep liquid (or vacuum) in the internal space of the tube or fiber, 
the other material obviously being on the other side. 
When the membrane is used in a hollow fiber configuration with the feed 
introduced on the exterior side of the fiber, the sweep liquid flows on 
the inside of the hollow fiber to sweep away the permeated highly aromatic 
species, thereby maintaining the desired concentration gradient. The sweep 
liquid, along with the aromatics contained therein, is passed to 
separation means, typically distillation means, however, if a sweep liquid 
of low enough molecular weight is used, such as liquefied propane or 
butane, the sweep liquid can be permitted to simply evaporate, the liquid 
aromatics being recovered and the gaseous propane or butane (for example) 
being recovered and reliquefied by application of pressure or lowering of 
temperature. 
The new copolymer composition of the present invention comprises the soft 
segment of an aliphatic polyester and the hard segment derived from a 
dianhydride and a diisocyanate. The aliphatic polyester may be a 
polyadipate, a polysuccinate, a polymalonate, a polyoxalate or a 
polyglutarate. 
In a preferred embodiment, the aliphatic polyester is a polyethylene 
adipate or a polyethylene succinate, the dianhydride has between 8 and 20 
carbons, and the diisocyanate has between 4 and 30 carbons. 
In a preferred embodiment, the dianhydride is selected from the group 
consisting of pyromellitic dianhydride, 3,3',4,4'-benzophenone 
tetracarboxylic dianhydride, 4,4'-(hexafluoroisopropylidene)-bis-(phthalic 
anhydride), 4,4'-oxydiphthalic anhydride, 
diphenylsulfone-3,3',4,4'-tetracarboxylic dianhydride, and 
3,3',4,4'-biphenyltetra-carboxylic dianhydride. 
In a preferred embodiment, the diisocyanate is selected from the group 
consisting of 2,4-toluene diisocyanate (TDI), methylene diphenylisocyanate 
(MDI), methylene dichlorophenylisocyanate (dichloro-MDI), methylene 
dicyclohexylisocyanate (H.sub.12 -MDI), methylene 
dichlorocyclohexylisocyanate (dichloro-H.sub.12 -MDI), methylene 
bis(dichlorophenylisocyanate) (tetrachloro-MDI), and methylene 
bis(dichlorocyclohexylisocyanate) (tetrachloro-H.sub.12 -MDI). 
It has been observed that the new membranes from the new copolymer 
composition of the present invention can separate toluene from isooctane, 
showing good selectivity and permeability. The membrane has a high thermal 
stability of about 170.degree. C. in pervaporation separation of the 
toluene/isooctane mixture. The present invention will be better understood 
by reference to the following examples which are offered by way of 
illustration and not limitation. 
EXAMPLE 1 
Synthesis of Toluene Diisocyanate Cross-linked Polyethylene 
Adipate-Pyromellitic Dianhydride Copolymer Membrane 
To 10 g (0.005 mole) of polyethylene adipate diol with a molecular weight 
of 2000 at about 80.degree. C. under N.sub.2 in a reactor was added 1.09 g 
(0.005 mole) of pyromellitic dianhydride (PMDA) with stirring. The 
temperature increased to about 100.degree. C., and the stirring continued 
for about 6 hours at this temperature for polymerization. To the reactor 
content was added about 20 g of DMF with stirring. Then, the reactor 
content was cooled to room temperature overnight. Finally, about 0.44 g 
(0.0025 mole) of TDI was added to the reactor content to give the 
resulting solution with 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 glass plate with a knife 
gap setting of 14 mils. DMF was allowed to evaporate from the membrane in 
a hood at ambient conditions over a period of about 17 hours. The membrane 
was then dried in an oven at 160.degree. C. overnight to complete the 
cross-linking of polymer chains with TDI. The membrane was then removed 
from the glass plate by soaking it in a water bath. Finally, the membrane 
was dried at 120.degree. C. overnight. The resulting membrane had a 
thickness of about 95 microns. 
EXAMPLE 2 
Pervaporation Results 
The resulting membrane described in Example 1 was evaluated for 
aromatic/saturate separation with the feed mixture of 50 wt% toluene and 
50 wt% isooctane in the pervaporation apparatus described above. FIG. 2 
shows the toluene/isooctane selectivity and permeability for the copolymer 
membrane as a function of temperature. As shown in this figure, this 
copolymer membrane had good selectivity and permeability. This figure also 
shows that this copolymer membrane had a good thermal stability of about 
170.degree. C.