Note: Descriptions are shown in the official language in which they were submitted.
~2~
-- 1 `
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.).
:
- 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.
2`~
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 ~ % ~
- 4 -
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~$
- 10 -
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.
..
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.
;
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 ~
- 12 -
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~
- 13 -
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.
