Note: Descriptions are shown in the official language in which they were submitted.
~5~)9
Brief Description of _he Invention
High flux, thin film composite membranes
comprisillg a thin, dense, selective polymeric film
preferably of polyurethane, polyurea/urethane,
polyurethane/imide or polyurea/polyurethane copolymer
alloy on a microporous hydrophobic support backing are
prepared by wash coating an optimum wetting solution of
the polymer in solvent onto the hydrophobic support.
The viscosity of the wstting solution of polymer in
solvent should be between 5 and 100 cps, preferably
between 10 and 50 cps and more preferably between 20
and 40 cps. The optimum solution viscosity can be
obtained by adding viscosity modifiers, adjusting the
polymer concentration/solvent composition, by aging the
solution or a combination of these techniques.
Backqround of the Invention
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. See, for instance U.S. Patent 3,370,102, U.S.
Patent 2,958,656, U.S. Patent 2,930,754, and U.S.
Patent 4,115,465.
U.S. Patent 4,837,054 teaches thin film
composite membranes prepared by deposition from a
solution. A thin film of polyurea~urethane is coated
onto a microporous support substrate from a multi-
component solvent system. The solution of polyurea/-
urethane copolymer is prepared in a solution system
consisting of an aprotic solvent such as dimethylforma-
mide, a cyclic ether such as dioxane, a third component
comprising cellosolve acetate or methyl cellosolve and
- 2 - ~ ~45~3
a wettin~ agent such as crotyl alcohol. The polymer in
the solvent solution is depositecl as a thin film on a
support substrate such as polyet:hylene, polypropylene
or teflon after which excess polymer/solvent solution
is permitted to drain from the support. Thereafter the
solvents are permitted to evaporate leaving a thin
layer of active polyurea/urethane copolymer on the
support backing. The solvent system employed consti-
tutes a mixture of (a) an aprotic solvent such as DMF,
~b) a cyclic ether such as dioxane, (c) cellosolve
acetate or methyl cellosolve, and (d) a wetting agent
such as crotyl alcohol. These solvents are used in a
parts per hundred ratio of a/b/c/d in the range about
3-27/94-33/2-33/1-7. The polyurea/urethane copolymer
exists as a true, complete copolymer in the solvent
system and the polymer-solvent system exists as a true
solution. The polymer concentration in the solution
can range up to 40 parts polymer per 100 parts solvent,
preferably 0.5 to about 20 parts polymer, more prefer-
ably l to 10 parts polymer, most preferably 1 to 5
parts polymer per lO0 parts solvent solution.
Detailed Description of the Invention
Supported thin film composite membranes
comprising a thin active layer deposited onto a micro-
porous hydrophobic support are described. The thin
film composite membrane is prepared by depositing a
thin film of polymer from solution onto the microporous
hydrophobic support. The polymer solution is capable
of propexly wetting the hydrophobic support because the
solution includes a solvent of low surface tension and
a surfactant. The solvents must possess the optimum
wetting characteristics such that the solution wets the
surface but does not soak into the pores of the hydro-
phobic microporous support. Low flux membranes result
when the solution soaks into the pores. For example,
- 3 -
pure (1~0%) dimethylformamide will bead-up on teflon
and a non-continuous, defec~ive membrane layer will
result. The addition of a low surEace tension solvent
such as acetone, however, will allow the solvent
mixt~re to wet the teflon surface. A 60/40 DMF/acetone
mixture produces the optimum wetting characteristics
for a 5 wt% polymer solution since it coats the surface
but does not soak into the pores. A DMF/acetone ratio
greater than 10/90, however, will soak into the pores.
Although the actual ratio employed will
depend on the concentration of polymer present in the
solvent solution, the ratio will be within the ranges
10/90 to 90/10.
The viscosity of the wetting solution of
polymer in solvent should be between 5 and 100 cps at
20C, preferably between 10 and 50 cps at 20C and more
preferably between 20 and 40 cps at 20C. The optimum
solution viscosity can be obtained, for example, by
either adding viscosity modifiers (such as Monsanto's
Modaflow), adjusting the polymer concentration/solvent
composition, aging the solution or a combination of
these techniques. The polymer solution should be aged
for at least 1 day prior to being coated on the hydro-
phobic support. Preferably the polymer solution is
aged 3 days and more preferably about 7 days prior to
being coated onto hydrophobic supports, such as teflon
or polypropylene. No special atmosphere need be used,
previded the polymer solution is not exposed to an
atmosphere with which it chemically reacts. Aging
temperature is not critical except that aging at
slightly elevated temperature will result in a decrease
in aging time needed for the solution to achieve a
viscosity in the desired previously recited ranges.
_ 4 _ ~ ~5~
The polymPr solution used is preferably a
solution of polyurea/urethane copolymer, polyurea/-
polyurethane copolymer alloy, polyurethane-imide or
polyurethane in solvent.
The polymer concentration in the polymer
solution can range from 0.1 to 10.0 wt~. Thinner
active layers are obtained when a lower concentration
solution is used. Thin active layers in the range 0.1
to 10 micron, preferably 0.5 to 5 microns in thickness
can be obtained.
Thin film composite membrane made by deposit-
ing a thin active layer of polyurea/urethane,
polyurethane/imide or polyurethane from a polymer
solvent solution system onto a microporous support
substrate are useful for separating aromatic hydrocar-
bons from saturated hydrocarbons and are of particular
utility in the chemicals industry for recovering/-
concentrating aromatics such as benzene, toluene,
xylenes, etc. from chemicals streams and in the petro-
leum industry for recovering aromatics from saturates
in heavy feed streams such as naphtha, catalytic
naphtha, heavy cat naphtha, light gas oils, light cat
gas oils, reformates etc.
Examples of polyurea/urethane copolymers
which can be used to produce the thin active layer of
the thin film composite membrane herein described and
which are effective when in the form of membranes in
performing the separating are described in U.S. Patent
No. 4,914,0~4 and in its continuation-in-part applica-
tion USSN 336,172 filed April 11, 1989 in the name of
Robert C. Schucker.
The polyurea/urethane membrane described in
U.S. Patent 4,914,064 which is effective in separating
- 5 ~
aromatics from saturates is distinguished by being
aromatic in nature and possessing other certain and
specific characteristics.
The aromatic polyure~/urethane polymer used
to produce the thick dense film membrane of that
invention is characterized by possessing a urea ind~x
of at least about 20% but less than 100%, an aromatic
carbon content of at least about 15 mole percent, a
functional group density of at least about 10 per 1,000
grams of polymer and C=O/NH ratio of less than about 8.
In that disclosure the dense, thick film
aromatic polyurea/urethane layer is produced using an
aromatic polyurea/urethane copolymer which is itself
prepared by reacting dihydroxy or polyhydroxy compounds
(e~g., polyethers or preferably polyesters of about 250
to 5000 molecular weight, or mixtures of different
molecular weight polvmers of the same type, i.e. about
30:70/70:30 mixtures of an about 500 molecular wt.
component (polyester or polyether) and an about 2000
molecular wt. component (polyester or polyether) with
aliphatic, alkylaromatic or aromatic diisocyanates or
polyisocyanates and low molecular weight chain extend-
ers, such as diamines, polyamines or amino alcohols.
The choice of the molecular weight of the polyether or
polyaster component is a matter of compromise. Poly-
ether or polyester components of 500 molecular weight
give membranes of highest selectivity, but lower flux.
Polyesters or polyethers of higher molecular weight
(e.g. 2000) give membranes of lower selectivity but
higher flux. Thus, the choice of the single molecular
weight or blend is matter of choice and compromise
between selectivity and flux. The ratio of these
components used in producing the polyurea/urethane
copolymer is governed by the aforementioned character-
istics possessed by the membranes useful for aromatic
~ 3 9
-- 6
from saturate separa-tion. ~he copolymer produced
possesses a urea index of at least about 20% but less
than 100%, preferably at least about 30~ but less than
1~0%, most preferably at least about 40% but less than
100%. By urea index is meant the percentage of urea
groups relative to the total urea plus urethane yroups
in the polymer. The copolymer also contains at least
about 15 mole percent, and preferably at least about 20
mole percent aromatic carbon, expressed as a percent of
the total carbon in the polymer. The copolymer also
possesses a particular density of functional groups (DF
ratio) defined as the total of C-O+NH per 1000 grams of
polymer, the density of functional group being at least
about 10, preferably at least about 12 or greater.
Finally, to insure that the functional groups are not
mostly carbonyl, the C-O~N~ ratio is less than about 8
and preferably less than absut 5Ø This insures that
there is sufficient hydrogen bonding within the polymer
to result in strong polymer chain interactions and high
selectivity. This polyurea/urethane copolymer formula-
tion can be used in producing the polymer solutions
described in the present invention employed in making
the thin film composite membrane described herein.
Other polyurethane and polyurea/urethane
polymers described in the literature, such as those
described in U.S. 4,115,465 (which can be characterized
as aliphatic polyurethanes or polyurea/urethanes) can
also be employed in the present solution casting
procedure to produce TFC membranes of the present
invention.
The thin film composite membranes made by the
process of the present invention are especially well
suited for separating aromatics from saturates in heavy
feeds, such as heavy cat naphtha, wherein the constitu-
ents making up the feed include, in some cases, highly
3~3
- 7
complex, multi-ring, heavily substituted aromatic
species.
As previously stated, t:he thin film composite
membranes are produced from a polyurea/urethane
copolymer made from dihydroxy or polyhydroxy compounds,
such as polyethers or polyester of 500 to 5000 molecu-
lar weight, reacted with aliphatic, alkylaromatic or
aromatic diisocyanates or polyisocyanates and low
molecular weight chain extenders, such as diamines,
polyamines or amino alcohols.
The polyester components are prepared from
aliphatic or aromatic dicarboxylic acids and aliphatic
or aromatic dialcohols. Aliphatic dicarboxylic acids
refer to those materials having the general formula
HOOCRCOOH where R contains 2 to 10 carbons (and may be
either a straight, branched chain or cyclo configura-
tion). Aromatic dicarboxylic acids refer to those
materials having the general structure HOOCRCOOH where
R is:
R' R' R''' R''
(I)n
1 R''' II
wherein R', R'' and R''' may be the same or different
and are selected from the group consisting of H and
C1-Cs carhons or C6Hs and combinations thereof, and n
is 0 to 4. It is to be understood that in the above
formula each R' or R'' may itself represent a mixture
of H, Cl-Cs ~r C6H5
Dialcohols have the general structure HOROH
where R may be
~ 'J~
R' R'
,1
R'
III IV
where n is 1 to 10, preferably 4 to 6, and R' is H, C
to Cs or C6Hs or
R' R''' R''
)n ~ \
R'''
V
where R', R'', R''' and n are defined in the same
manner as for the aromatic dicarboxylic acids. An
example of a useful dialcohol is bisphenol A.
The diisocyanates can be aromatic diisocya-
nates having the general structure:
R' R''' R''
OCN ~ (C)n~ C - NCO
,,,
VI
wherein R', R'' and R''' are the same or different and
are selected from the group consisting of H, C1-Cs and
C6Hs and mixtures thereof and n ranges from O to 4.
Aliphatic, cycloaliphatic, aromatic, and araliphatic
diisocyanates or polyisocyanates can also be used, thus
resulting in the production of aromatic or aliphatic
2~ S~l~ r3
g
polyurethanes, polyurea~ureth2rles or polyurethane
imid~s.
Diamine chain extenders have the general
formula H2NRNH2 where R includes aliphatic and aromatic
moieties such as
R'
I
-(C)n
VII
where n is 1 to 10 and R' may be the same or different
and are selected from the group consisting of H, C1-Cs
carbons and C6Hs and mixtures thereof.
Also included are diamine chain extenders of
the formula:
R' R''' ~''
H2N ~ O ` ~C)n ~ NH2
R'''
VIII
where R', R'' and R''' are the same or different and
are selected from the group consisting of H or Cl or a
Cl to Cs or C6Hs and mixtures thereof and n ranges from
O to 4.
Examples of the polyether polyols useful in
the present invention as polymer precursors are poly-
ethylene glycols, (PEG), polypropylene glycol (PPG),
- 10 ~
polytramethylene glycol, PEG/P]?G random copolymers,
etc. having molecular weiyh~ ranging from about 250 to
4000.
Aliphatic diisocyanates which may be utilized
are exemplified by hexamethylene di.isocyanate (HDI),
1,6-diisocyanato-2,2,4,4-tetramethylhexane (TMDI),
1,4-cyclohexanyl diisocyanate (CHDI), isophorone
diisocyanate (IPDI), while uceful alkylaromatic
diisocyanates are exemplified by toluene diisocyanate
(TDI) and bitolylene diisocyanate (TODI). Aromatic
diisocyanates are exemplified by 4,4'-diisocyanato
diphenylmethane (MDI). Polyisocyanates are exemplified
by polymeric MDI (PMDI) and carbodiimide modified MDI.
Useful polyamines are exemplified by polyethylene-
imines, 2,2',2'' triaminotriethylamine, 4,4'-diamino
3,3'dichloro-diphenylmathane (MOCA). Useful amino
alcohols are exemplified by 6-aminohexanol, 4-amino-
phenol, 4-amino-4'-hydroxyl-diphenylmethane.
~ he above are presented solely by way of
example. Those skilled in the art, with the present
teaching before them, can select from the innumerable
materials available the various starting materials
which upon combination as described herein will produce
a polyurea/urethane copolymer having the desired
characteristics which can then be cast into the mem-
branes useful for the separation of aromatics from
saturates.
Polyurethanes are prepared using the reac-
tants recited above, omitting the polyamine or amino
alcohol chain extenders.
Polyurethane imides are produced by endcap-
ping a polyol selected from those recited above with a
polyisocyanate also selected from those recited above
while aliphatlc and cycloaliphatic di- an~ polyisocya-
nates can also be used as can be mixtures of aliphatic,
cycloaliphatic, aralkyl and aromatic polyisocyanates
followed by chain extending by reaction with a poly-
anhydride which produces the imide ~irectly or with di
or poly car~oxylic acids which produce amic acid groups
which can be condensed/cyclized to the imide~ Poly-
urethane imides are the subject of U.S. Patent No.
4,929,358.
The polvmer, preferably polyurea/urethane
copolymer, polyurethane, or polyurethane/imide is
prepared in a suitable dissolving solvent. The sol-
vent(s) chosen must not only be capable of dissolving
the polymer, preferably polyurea/urethane copolymer,
polyurethane or polyurethane/imide but must also be
capable Gf wetting the hydrophobic support upon which
the polymer solution is to be coated. The solvent(s)
must possess the optimum wetting characteristics such
that the solution wets the surface but does not soak
into the pores of the hydrophobic microporous support.
For example, with a solvent mixture of dimethylforma-
mide (high solvency) and acetone (low surface tension)
complete soak through can be obtained at high acetone
concentrations while non-wetting conditions result at
high dimethylformamide concentrations. Thus, the
optimum dimethylformamide/acetone solvent ratios lies
betwaen 10/90 and 90/10. The surface tension at 20C
of the low surface tension solvent should be less than
35 dyne/cm, preferably less than 30 dyne/cm and more
preferably less than 25 dyne/cm. The surface tension
of acetone at 20C is 23 dyne/cm. Other examples of
low surface tension solvents are toluene, heptane and
hexane.
Solvents with high solvency (good solvents)
are characterized by a high polar solubility parameter.
~ 5~
- 12 -
The polar solubility parameter at 25~C of a goodsolvent should be greater than 3 (cal/cc)l/2, prefer-
ably greater than 5 (cal/cc)1/2 and more preferably
greater than 7 (cal/cc)l/2. Di.methylformamide has a
polar solubility parameter of 8.07 (cal/cc)1/2. Other
examples of good solvents are climethylsulphoxide and
dimethylacetamide.
The solvent used in the membrane preparation
process will comprise a mixture of high solvency
solvent and low surface tension solvent used in a ratio
of about 10/90 to 90~10, preferably about 20/80 to
80/20, most preferably about 40/60 to 60/40.
Preferably the solution uses not only a
wetting solvent but also have a wetting surfactant
present such as crotyl alcohol or zonyl FSN, a Dupont
fluoro-surfactant. Preferably, less than 5% wetting
surfactant should be used.
In order to insure that the membrane coating
layer is thin and in the range of 0.1 to 10~, prefer-
ably 0.5 to 5~, the polymer concentration in solution
should be in the range of about 10 wt% polymer, and
less, preferably lower concentrations are used, 0.5 to
8 wt%, more preferably 0.5 to 5 wt%.
The viscosity of the wetting solution of
polymer in solvent should be between 5 and 100 cps,
preferably between 10 and 50 cps and more preferably
between 20 and 40 cps. The optimum solution viscosity
can be obtained, for example, by either adding viscosi-
ty modifiers, adjusting the polymer concentration/-
solvent composition, by aging the solution or by a
combination of these techniques. Aginy the polymer
solution unexpectedly produces composite membranes of
higher selectivity as compared to thin film composite
~ ~t~
- 13 -
membrane made using a polymer solution of the same
composition but which was not aged and which did not
possess a viscosity in the desired range. The polymer
solution is aged for at least 1 day, preferably at
least 3 days, more preferably at least 7 days. Aging
is accomplished by permitting the polymer solution to
stand at room temperature in a non-reacting atmosphere.
Using temperatures higher than room temperature will
reduce the aging time to achieve an equiYalent viscosi-
ty.
The support which is coated with this aged
polymer solution is a hydrophobic, microporous support
such as teflon or polypropylene.
Following deposition of a layer of the
polymer solution on the hydrophobic microporous sup-
port, the excess solution is poured off and the solvent
portion of what remains is permitted to evaporate.
Solvent evaporation can be performed by simply permit-
ting the solvent to dispel into the atmosphere or
solvent evaporation can be augmented by the addition of
heat and/or the application of a vacuum.
The thin film composite membranes are useful
for the separation o~ aromatics from saturates in
petroleum and chemical streams, and have been found to
be particularly useful for the separation of larg~
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,
(200-320F) light aromatics content streams boiling in
the Cs-300F range, light catalytic cycle oil boiling
in the 400-650F range as well as streams in chemical
plants which contain recoverably quantities of benzene,
toluene, xylene (BTX) or other aromatics in combination
r~
with saturates. The separation techniques whi~h may
successfully employ the membr2lnes of the present
invention include perstraction and pervaporation.
Perstraction involves t:he selective dissolu-
tion 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
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 molPcules present in the feed-
stream dissolve into the membrane film due to similari-
ties between the membrane solubility parameter and
those of the aromatic species in the feed. The aromat-
ics then permeate (diffuse) through the membrane and
are swept away by a sweep liquid which is low in
aromztics 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 gradi-
ent. 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, C3 to C6 saturated hydrocarbons and lube
basestockS (C15-C20)o
The perstraction process is run at any
convenient temperature, preferably as low as possible.
- 15 -
The choice of pressure is not critical since
the perstraction process is not dependent on pressure,
but on the ability of the aromatic compon~nts in the
feed to dissolve into and migrate through the membrane
under a concentration driving force. Consequently, any
convenient pressure m~y 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 C3 or C4 sweep liquids are used at 25C 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
re-emerge on the permeate side under the influence of a
concentration gradient~ Pervaporative separation of
aromatics from saturates can be performed at a tempera-
ture of about 25C for the separation of benzene from
hexane but for separation of heavier aromatic/saturate
mixtures, such as heavy cat naphtha, higher tempera-
tures of at least 80C and higher, preferably at least
100C and higher, more preferably 120C and higher
should be used. The maximum upper temperature limit is
that temperature at which the membrane is physically
damaged or delaminates. 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
~'3
- 16 -
should be below the de~ point of the pPrmeate at agiven vacuum level.
The membrane itself m~,y be in any convenient
form utiliy~ing any convenient ~odule 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 vacuumj in t.he interior space of the tube or fiber,
the complimentary environment obviously being main-
tained on the other side.
The present invention will be better under-
stood by reference to the following Examples which are
offered by way of illustration and not limitation.
EXAMPLE 1
A solution containing a polyurea-urethane
polymer is prepared as follows. Four point five six
~4.56) grams ~0.00228 moles) of polyethylene adipate
(MW = 2000), 2.66 grams (0.00532 moles) of 500 MW
polyethylene adipate and 3.81 grams (0.0152 moles) of
4,4'diphenylmethane diisocyanate are added to a 250 ml
flask equipped with a stirrer and drying tube. The
temperature is raised to 90C and held for 2 hours with
stirring to produce an isocyanate-end-capped prepoly-
mer. Twenty grams of dimethylformamide is added to
this prepolymer and the mixture is stirred until clear.
One point five grams (0.0076 moles~ of 4,4' diamino-
diphenylmethane is dissolved in ten grams of dimethyl-
formamide and then added as chain extender to the
prepolymer solution. This mixture was then allowed to
react at room temperature (approx. 22C) for 20 min-
utes. The viscosity of the solution was approximately
100 cps. Films were cast on glass using a 5 mil
- 17 -
casting knife and then dried in an oven at 90C for 2
hours. This technique produced a 20 micron dense film
as a comparison for the thin film composites (TFC).
XAMPLE 2
The polymer solution was made according to
Example 1 and then diluted to 5 wt% such th~t the
solution contained a 60/40 wt% blend of dimethylforma-
mide/acetone. The solution was allowed to stand for 7
days at room temperature. The viscosity of the aged
solution was 35 cps. After this period of time one wt~
Zonyl FSN (Dupont) fluorosurfactant was added to the
aged solution. (Note: the fluorosurfactant could also
be added prior to aging). A microporous teflon mem-
brane (K-150 from Desalination Systems Inc.) with
nominal 0.1 micron pores was wash-coated with the
polymer solution. The coating was dried with a hot air
gun immediately after the wash-coating was complete.
This technique produced composite membranes with the
polyurea/urethane dense layer varying between 3 to 4
microns in thickness. Thinner coatings could be
obtained by lowering the polymer concentration in the
solution while thicker coatings are attained at higher
polymer concentrations.
3~3
- 18 -
TABLE 1
(HCN Feed: 52 vol% arom, pervap @140C/5-10 mbar,
< 5 LV% yields)
Membrane Example 1 Example 2
Type Dense FilmTFC
Coating Thicknesst~) 21 3
Permeate Quality
RI @20C 1.50041.5000
Arom, vol% (1) 85.1 85.7
Permeate Flux, kg/m2-d 40.9 270.0
~1~ Aromatics concentration based on an RI correlation
developed for the specific HCN feed using an FIA
analysis (Fluorescent Indicating Analysis, ASTM
Dl319) (Arom = 807.99 x RI - 1126.24).
The data in Table 1 show more than a six-fold
increase in the flux performance of the thin film
composites compared to the dense films. Clearly, this
represents a significant improvement.
3~ 3
~ 19 -
_XAMPLE 3
A thin film composite was prepared from the
same solution as in Example ~ ~x~ept that the solution
was only allowed to age for 3 days. The solution had a
viscosity of only 3 cps.
TABLE 2
(HCN Feed: 52 vol% arom, pervap Ql40C 15-10 mbar,
< 5 LV% yields)
Run M-229R M-228
Membrane Example 2 Example 3
Type TFC TFC
Solution Age, days 7 3
Coating Thickness(~) 3
Permeate Quality
RI Q20C 1.5000 1.4713
Arom, vol~ (1) 85.7 52.0
Permeate Flux, kg/m2-d 270.0 >2000
(1) Aromatics concentration based on an RI correlation
developed for the specific HCN feed using an FIA
analysis (Arom = 807.99 x RI - 1126.24).
The data in Table 2 clearly shows that aging
the solution improves the membrane performance.
EXAMPLE 4
A thin film composite was prepared as in
Fxample 2 except that a nylon microporous support (0.1
microns) was washed-coated. (same aged solution)
- 20 -
TABLE 3
(HCN Feed: 52 vol~ arom, pervap ~140C/5-10 mbar,
< 5 LV% yielcls)
Run M-229R M-228
Membrane Exa~ple 2 Example 4
Type TFC T~C
Support 0.1~ Teflono.l~ Nylon
Permeate Quality
RI Q20C 1.5000 1.4953
Arom, vol~ (1) 85.7 81.8
Permeate Flux, kg/m2-d 270.0 87.0
(1) Aromatics concentration based on an RI correlation
developed for the specific HCN feed using an FIA
analysis (Arom = 807.99 x RI - 1126.24~.
The data in Table 3 clearly show that using
high surface tension hydrophilic supports such as nylon
produces low flux membranes. Polymer solution soaked
into the nylon.
EXAMPLE 5
A solution was prepared as in Example 2
except that the polymer solvent was 100% dimethylforma-
mide (DMF) instead of a 60/40 blend of DMF/acetone.
This solution did not coat the 0.1 micron teflon
support despite using 1 wt% Zonyl FSN (Dupont) fluoro-
surfactant, and a defective membrane layer was ob-
tained. As a result of the poor coatability of the
solution, these membranes showed high flux and no
aromatics/saturates separation.
5~
- 21 -
EXAMPLE Ç
A solution containing a polyurea-urethane
polymer is prepared as follows. Ten point six (10.6)
grams (0.00532 moles) of polyethylene adipate (MW
2000), 2.66 grams (0.00532 moles) of 500 MW polyethy-
lene adipate and 5.33 grams (0.02128 moles~ of 4,4-
diphenylmethane diisocyanate are added to a 250 ml
flask eyuipped with a stirrer and drying tube. The
temperature is raised to 90C and held for 2 hours with
stirring to produce an isocyanate-end-capped prepoly-
mer. Twenty grams of dimethylformamide is added to
this prepolymer and the mixture is stirred until clear.
Four point two grams ~0.02128 moles) of 4,4'diamino-
diphenylmethane is dissolved in two grams of dimethyl-
formamide and then added as a chain extender to the
prepolymer solution. This mixture is then allowed to
react at room temperature (approx. 20C) for 20 min-
utes. The polymer solution is then diluted to 10 wt%
with dimethylformamide. The viscosity of the solution
is approximately 75 cps. One wt% Zonyl FSN fluorosur-
factant (Dupont) is added and then the solution is wash
coated onto a 0.1 micron porous teflon membrane sample
(~-150 from Desalination Systems Inc.). Despite
possessing a viscosity in the desired range, this
solution does not wet the teflon very well and as a
result large gaps exist in the polymer coating after
drying.
EXAMPLE 7
A polymer solution is prepared as in Example
1 and then diluted to 10 wt% in a 50/50 dimethylforma-
mide/acetone solvent mixture. The viscosity of the
solution was approximately 75 cps. This solution was
used to wash coat 0.1 micron porous teflon. Initially
the solution wetted the teflon but as the acetone
~ 2~ -
evaporated the remaining solution beaded-up on the
surface forming a nsn-continuous polymer layer.
EX~MPLE 8
One wt~ Zonyl FSN fluorosurfactant (Dupont)
was added to the solution prepared as in Example 7.
This solution was used to coat 0.1 micron porous
teflon. The solution wetted the teflon and form~d a
continuous dense layer after drying. Viscosity was
already about 75 cps so aging was not needed. Coating
thickness after drying was 7~.
As shown in Table 4 a continuous dense layer
is required to achieve separation (i.e., Example 8
membrane). Table 4 shows that a continuous dense layer
can be formed with a polymer solution in a dimethylfor-
mamide/acetone solvent mixture and Zonyl FSN fluorosur-
factant. These examples, in combination with Example 3
demonstrate that the polymer solution must possess a
viscosity in the recited range and, further, the need
for a low surface tension solvent such as acetone and a
fluorosurfactant such as Zonyl FSN to obtain a thin,
continuous dense separation barrier.
- 23 -
TABLE 4
Effect of Solvent ~ixture ancl Fluorosurfactant
on Wettinq
5Model feed; 50% aromatics, p~rstraction Q~0C,
<5 LV~ yields)
Membrane 6 7
SolventDMF DMF/acetone DMF/acetone
Zonyl FSN, wt% 1 o
Coating non- non-
Integrity continuous continuous continuous
Permeate
Aromat.ics 50 50 86
