Note: Descriptions are shown in the official language in which they were submitted.
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METHOD OF SEPARATING OLEFINS FROM MIXTURES WITH PARAFFINS
FIELD OF THE INVENTION
This invention relates to a method of separating or concentrating mixtures of
olefins and paraffins using a selectively perineable membrane. More
specifically, it
relates to a method of using certain polyimide membranes to selectively
separate olefinic
hydrocarbons from a gas or liquid mixture of olefinic and paraffinic
hydrocarbons such
as those generated by petroleum refining industries, petrochemical industries,
and the
like.
BACKGROUND OF THE INVENTION
Olefins, particularly ethylene and propylene, are important chemical
feedstocks.
Typically they are found in nature or are produced as primary products or
byproducts in
mixtures that contain saturated hydrocarbons and other components. Before the
raw
olefins can be used, they usually must be separated from these mixtures.
Currently, separation of olefin/paraffin mixtures is usually carried out by
distillation. However, the similar volatilities of the components make this
process costly
and complicated, requiring expensive distillation columns and energy-intensive
processing. Jarvelin reports that the fractional distillation of
propylene/propane mixtures
is the most energy-intensive distillation practiced in the United States
(Harri Jarvelin and
James R. Fair, Adsorptive separation of propylene/propane mixtures, Ind. Eng.
Chem.
Research 32 (1993) 2201-2207). More energy conserving separation processes are
needed.
Membranes have been considered for the separation of olefins from paraffins as
an alternative to distillation. However, the separation is difficult largely
because of the
similar molecular sizes of the components. Another difficulty is that the feed
stream
conditions are typically close to the gas/liquid phase boundary of the
mixture. Also, the
membrane must operate in a hydrocarbon environment under conditions of high
pressure
and temperature. Such harsh conditions tend to adversely affect the durability
and
stability of separation performance of many membrane materials. For example,
some
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CONFIRMATION COPY
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contaminants plasticize selectively permeable membrane materials and can cause
loss of
selectivity and/or permeation rate. A membrane with sufficiently high
olefin/paraffin
selectivity, and sufficient durability in long-term contact with hydrocarbon
streams under
high pressure and temperature is highly desired.
Membrane materials for separating olefinic hydrocarbons from a mixture of
olefinic and saturated hydrocarbons have been reported, but none can be easily
or
economically fabricated into membranes that offer the unique combination of
high
selectivity and durability under industrial process conditions.
For example, several inorganic and polymer/inorganic membrane materials with
good propylene/propane selectivity have been studied. See M. Teramoto, H.
Matsuyama, T. Yamashiro, Y. Katayama, Separation of ethylene from ethane by
supported liquid membranes containing silver nitrate as carrier, J. Chem Eng.
Japan 19
(1986) 1, and R.D. Hughes, J.A. Mahoney, E.F. Steigelmann, Olefin separation
by
facilitated transport, in: N.N. Li, J.M. Calo (eds.), Membrane Handbook, Van
Nostrand,
New York, 1992. Such materials are difficult to fabricate into practical
industrial
membranes. Liquid facilitated-transport membranes have been demonstrated to
have
attractive separation performance in the lab, but have been difficult to scale
up, and have
exhibited declining performance in environments typical of an industrial
propylene/propane stream.
Solid polymer-electrolyte facilitated-transport membranes appear more amenable
to fabrication into stable thin film membranes. See Ingo Pinnau and L.G. Toy,
Solid
polymer electrolyte composite membranes for olefin/paraffin separation, J.
Membrane
Science, 184 (2001) 39-48. Such a membrane is exemplified in U.S. Patent No.
5,670,051 (Pinnau et al, 1997) wherein a silver
tetrafluoroborate/poly(ethylene oxide)
membrane exhibited ethylene/ethane selectivity of greater than 1000. However,
these
membranes are severely limited by their poor chemical stability in the
olefin/paraffin
industrial environment.
Carbon hollow-fiber membranes have shown promise in laboratory tests
("Propylene/Propane Separation", Product Information from Carbon Membranes,
Ltd.,
Israel), but are vulnerable to degradation caused by condensable organics
present in
industrial streams. Moreover, carbon membranes are brittle and difficult to
form into
membrane modules of commercial relevance.
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Membranes based on rubbery polymers typically have olefin/paraffin selectivity
too low for an economically useful separation. For example, Tanaka et al.
report that the
single-gas propylene/propane selectivity is only 1.7 for a polybutadiene
membrane at
50 C (K. Tanaka, A. Taguchi, Jianquiang Hao, H. Kita, K. Okamoto, J. Membrane
Science 121 (1996) 197-207) and Ito reports a propylene/propane selectivity
only
slightly over 1.0 in silicone rubber at 40 C (Akira Ito and Sun-Tak Hwang, J.
Applied
Polymer Science, 38 (1989) 483-490).
Membranes based on glassy polymers have the potential for providing usefully
high olefin/paraffin selectivity because of the preferential diffusivity of
the olefin, which
has smaller molecular size than the paraffin.
Glassy polymers already used in gas separation have generally shown only
modest olefin/paraffin selectivity. For example, Ito has reported that films
of
polysulfone, ethyl cellulose, cellulose acetate and cellulose triacetate
exhibit
propylene/propane selectivity of 5 or less (Akira Ito and Sun-Tak Hwang,
Permeation of
propane and propylene through cellulosic polymer membranes, J. Applied Polymer
Science, 38 (1989) 483-490).
U.S. Patent No. 4,623,704 describes a process utilizing a cellulose triacetate
membrane for recovering ethylene from the reactor vent of a polyethylene
plant.
However, the vent stream that contained 96.5% ethylene is moderately upgraded
to only
97.9% in the permeate stream for recycle to the reactor.
Membrane films of poly(2,6-dimethyl-l,4-phenylene oxide) exhibited pure gas
propylene/propane selectivity of 9.1 (Ito and Hwang, Ibid.) Higher selectivity
has been
reported by Ilinitch et al. Q. Membrane Science 98 (1995) 287-290, J. Membrane
Science 82 (1993) 149-155, and J. Membrane Science 66 (1992) 1-8), but the
values at
higher pressure were uncertain and were accompanied by undesirable
plasticization of
the membrane by propylene.
Polyimide membranes have been studied extensively for the separation of gases
and to some degree for the separation of olefins from paraffins. Lee et al.
(Kwang-Rae
Lee and Sun-Tak Hwang, Separation of propylene and propane by polyimide hollow-
fiber membrane module, J. Membrane Science 73 (1992) 37-45) disclose a hollow
fiber
membrane of a polyimide that exhibited mixed-gas propylene/propane selectivity
in the
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range of 5-8 with low feed pressure (2-4 barg). The composition of the
polyimide was
not disclosed.
Krol et al. (J.J. Krol, M. Boerrigter, G.H. Koops, Polyimide hollow fiber gas
separation membranes: preparation and the suppression ofplasticization in
propane/propylene environments, J. Membrane Science. 184 (2001) 275-286)
report a
hollow fiber membrane of a polyimide composed of biphenyltetracarboxylic
dianhydride
and diaminophenylindane which exhibited a pure-gas propylene/propane
selectivity of
12; however, the membrane was undesirably plasticized by propylene at
propylene
pressure as low as 1 barg.
Polyimides based on 4,4'-(hexafluoroisopropylidene)diphthalic anhydride
(6FDA) and aromatic diamines have been found to provide a favorable
combination of
propylene permeability and propylene/propane selectivity. Permeation data for
dense-
film membranes of two different 6FDA-containing polyimides have been reported
to
have pure gas selectivity for propylene/propane in the range of 6-27. (C.
Staudt-Bickel et
al, Olefin/paraffin gas separations with 6FDA-based polyimide membranes, J.
Membrane Science 170 (2000) 205-214). Higher selectivity for similar 6FDA
polyimides has been reported in U.S. Patent No. 5,749,943 (Shimazu et al);
however, it
is anticipated that mixed-gas selectivity at high pressure will be much lower
due to
plasticization by the propylene-rich feed gas.
U.S. Patent Nos. 4,532,041; 4,571,444; 4,606,903; 4,836,927; 5,133,867;
6,180,008; and 6,187,987 disclose membranes based on a polyimide copolymer
derived
from the co-condensation of benzophenone 3,3',4,4'-tetracarboxylic acid
dianhydride
(BTDA) and a mixture of di(4-aminophenyl)methane and a mixture of toluene
diamines
useful for liquid separations.
U.S. Patent Nos. 5,605,627; 5,683,584; and 5,762,798 disclose asymmetric,
microporous membranes based on a polyimide copolymer derived from the co-
condensation of benzophenone-3,3',4,4'-tetracarboxylic acid dianhydride (BTDA)
and a
mixture of di(4-aminophenyl)methane and a mixture of toluene diamines useful
for
liquid filtration or dialysis membranes.
U.S. Patent No. 5,635,067 discloses a fluid separation membrane based on
blends
of phenylindane-containing polyimide polymers with polyimides derived from the
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condensation of benzophenone-3,3',4,4'-tetracarboxylic acid dianhydride (BTDA)
with
toluenediisocyanate (TDI) and 4,4'-methylene bisphenylisocyanate (MDI) and/or
polyimides derived from the condensation of BTDA and pyromellitic dianhydride
with
TDI and MDI.
A significant shortcoming of published data for the separation of olefins from
paraffins using membranes is the absence of data under practical industrial
conditions:
e.g., high feed and permeate pressure and high temperature. These are
conditions under
which plasticization of the membrane material could become significant and
which could
result in substantial decline in membrane performance over extended periods of
time. In
spite of the considerable efforts to provide industrially viable membranes for
the
separation of olefins from paraffins, none has proven to meet the performance
criteria
required for industrial application.
SUMMARY OF THE INVENTION
The invention is directed to a membrane separation process for separating an
olefin
from a mixture of olefins and paraffins comprising:
(a) providing a two-sided, selectively permeable membrane comprising a
polymer or copolymer having repeating units of formula (I):
Ol 0
-R1-N)rR2,)fN-
0 O
(I)
in which R2 is a moiety of composition selected from the group of consisting
of
formula (A), formula (B), formula (C) and a mixture thereof,
Da -b - d- z O
(A) (B) (C)
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Z is a moiety of composition selected from the group consisting of formula
(L),
formula (M), formula (N) and a mixture thereof; and
0
0
0 -
II -s
,C~ i ~ II
(L) (M) (N)
Rl is a moiety of composition selected from the group consisting of formula
(Q),
formula (T), formula (S), and a mixture thereof,
O O
- O-C HZ O
CH3
CH3
(Q) (T) (S)
(b) contacting one side of the membrane with a feed mixture comprising an
olefin compound and a paraffin compound having a number of carbon atoms at
least
as great as the olefin compound,
(c) causing the feed mixture to selectively permeate through the membrane,
thereby forming on the second side of the membrane an olefin-enriched permeate
composition which has a concentration of the olefin compound greater than that
of
the feed mixture,
(d) removing from the second side of the membrane the olefin-enriched
permeate composition, and
(e) withdrawing from the one side of the membrane an olefin-depleted
composition.
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In accordance with another aspect, there is provided a membrane separation
process for separating an olefin from a mixture of olefins and paraffins
comprising
providing a two sided, selectively permeable membrane comprising a polymer or
copolymer having repeating units of formula (II)
0 0
fRI-N O O N
0 0 O
(II)
and in which moiety R, is of formula (Q) in 0-100% of the repeating units, of
formula
(T) in 0-100% of the repeating units, and of formula (S) in a complementary
amount
totaling 100% of the repeating units. In accordance with another aspect, the
moiety
R, is of formula (Q) in about 16% of the repeating units, of formula (T) in
about 64%
of the repeating units, and of formula (S) in about 20% of the repeating
units.
In accordance with another aspect, there is provided a membrane separation
process for separating an olefin from a mixture of olefins and paraffins
comprising
providing a two sided, selectively permeable membrane comprising a polymer or
copolymer having repeating units of formula (Ilia), formula (1I1b), and
mixtures
thereof
O O
4Ri _N a U gl N N - N
O O b a
if
0 O O
(IIIa) (IIIb)
in which moiety R, is of formula (Q) in about 1-99% of the repeating units,
and of
formula (T) in a complementary amount totaling 100% of the repeating units,
and in
which a is in the range of about 1-99% of a + b. In accordance with another
aspect,
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the moiety Ri is of formula (Q) in about 20% of the repeating units and of
formula (T)
in about 80% of the repeating units, and in which a is about 40% of a + b.
In accordance with another aspect, there is provided a membrane separation
process for separating an olefin from a mixture of olefins and paraffins
comprising
providing a two sided, selectively permeable membrane comprising a blend of
polymer or copolymer having repeating units of formula (IIIa), formula (111b),
and
mixtures thereof
O O
1-N O O N R1-N N
R
4 1 WI-Y
c lb a
O O
O 0
(IIIa) (IIIb)
with a second polymer having repeating units of formula (II)
O 0
tR,-N O N
C
11
0
O 0
(II)
and in which the second polymer moiety R1 is of formula (Q) in about 1-100% of
the
repeating units, and of formula (T) in 0-100% of the repeating units, and of
formula
(S) in a complementary amount totaling 100% of the repeating units. In
accordance
with another aspect, the second polymer constitutes about 10-90 wt% of the
blend of
polymer and the second polymer.
In accordance with another aspect, there is provided a membrane separation
process for separating an olefin from a mixture of olefins and paraffins
wherein the
feed mixture comprises ethylene and ethane. In accordance with another aspect,
the
6b
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feed mixture comprises propylene and propane. In accordance with another
aspect
the feed mixture comprises propylene and propane and is in the liquid state.
In accordance with another aspect, there is provided a membrane separation
process for separating an olefin from a mixture of olefins and paraffins
wherein the
steps of (a) providing a two sided, selectively permeable membrane as
described
above; (b) contacting one side of the membrane with a feed mixture; (c)
causing the
feed mixture to selectively permeate through the membrane, thereby forming on
the
second side of the membrane an olefin-enriched permeate composition which has
a
concentration of the olefin compound greater than that of the feed mixture;
and (d)
removing from the second side of the membrane the olefin-enriched permeate
composition, are executed continuously for a duration after an initial time at
which
the feed mixture first contacts the membrane, and in which the membrane
exhibits a
permeance for the olefin compound and the permeance at 72 hours of executing
steps
(a)-(d) continuously is at least 60% of the permeance at the initial time.
In accordance with another aspect, there is provided a membrane separation
process for separating an olefin from a mixture of olefins and paraffins in
which the
membrane provides a selectivity of olefin compound relative to the paraffin
compound of at least 10.
In accordance with another aspect, there is provided a membrane separation
process for separating an olefin from a mixture of olefins and paraffins
wherein the
feed mixture is in the liquid state and in which the membrane provides a
permeance
of the olefin compound of at least about 0.4 GPU.
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DETAILED DESCRIPTION OF THE INVENTION
This invention is directed to a method of selectively separating olefinic
hydrocarbons from paraffinic hydrocarbons using a membrane containing certain
polyimide polymers, copolymers and blends thereof. The polymers which form
these
polyimides have repeating units as shown in the following formula (I):
IOJ O
-Rl -N)rR2-_yN-
O O
(I)
in which R2 is a moiety of composition selected from the group of consisting
of
formula (A), formula (B), formula (C) and a mixture thereof,
)a -b-Z-d-
(A) (B) (C)
Z is a moiety of composition selected from the group consisting of formula
(L),
formula (M), formula (N) and a mixture thereof; and
0
0 -Is
o~ -
/c~ ~ II
(L) (M) (N)
Rl is a moiety of composition selected from the group consisting of (formula
(Q),
formula (T), formula (S), and a mixture thereof,
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CH2 CH3
CH3
(Q) (T) (S)
In a preferred embodiment the polyimide that forms the selective layer of the
membrane has repeating units as shown in the following formula (II):
0 0
fR1NN
C
O O
(II)
In this embodiment, moiety Rl is of formula (Q) in 0-100% of the repeating
units, of
formula (T) in 0-100% of the repeating units, and of formula (S) in a
complementary
amount totaling 100% of the repeating units. A polymer of this structure is
available
from HP Polymer GmbH under the tradename P84 and is much preferred for use in
the
present invention. P84 is believed to have repeating units according to
formula (II) in
which R1 is formula (Q) in about 16 % of the repeating units, formula (T) in
about 64 %
of the repeating units and formula (S) in about 20 % of the repeating units.
P84 is
believed to be derived from the condensation reaction of benzophenone
tetracarboxylic
dianhydride (BTDA, 100 mole %) with a mixture of 2,4-toluene diisocyanate (2,4-
TDI,
64 mole %), 2,6-toluene diisocyanate (2,6-TDI, 16 mole %) and 4,4'-methylene-
bis(phenylisocyanate) (MDI, 20 mole %).
In another preferred embodiment, the polyimide that forms the selective layer
has
repeating units of compositions selected from among those shown in the
following
formulas (Illa and IIIb):
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O O
R1-N O N R1-N O N
C lb a
O p O O. 0
(IIIa) (IIIb)
The repeating units can be exclusively of formula (IIIa) or formula (IIIb).
Preferably, the
repeating units are a mixture of formulas (IIIa) and (IIIb). In these
embodiments, moiety
Rl is a composition of formula (Q) in about 1-99 % of the repeating units, and
of formula
(T) in a complementary amount totaling 100 % of the repeating units, and a is
in the
range of about 1- 99 % of the total of a and b.
A preferred polymer of this structure is available from HP Polymer GmbH under
the tradename P84-HT325. P84-HT325 is believed to have repeating units
according to
formulas (IIIa and IIIb) in which the moiety Rl is a composition of formula
(Q) in about
20 % of the repeating units and of formula (T) in about 80 % of the repeating
units, and
in which a is about 40 % of the total of a and b. P84-HT325 is believed to be
derived
from the condensation reaction of benzophenone tetracarboxylic dianhydride
(BTDA, 60
mole %) and pyromellitic dianhydride (PMDA, 40 mole %) with 2,4-toluene
diisocyanate (2,4-TDI, 80 mole %) and 2,6-toluene diisocyanate (2,6-TDI, 20
mole %).
In yet another preferred embodiment, the selectively permeable portion of the
membrane can be formed of a material comprising a blend of the above mentioned
polymers. For example, it is contemplated that a membrane can be formed from a
blend
comprising a first polymer having repeating units of formula (IIIa), formula
(111b) as
defined above, or a mixture of formulas (IIIa) and (IIIb) and a second polymer
having
repeating units of formula (II) as defined above. Greater preference is given
to a
membrane of a blend consisting essentially of the first and second polymers.
In such
preferred composition, the second polymer should constitute about 10-90 wt. %
of the
total of the first polymer and the second polymer.
The polyimides should be of suitable molecular weight to be film forming and
pliable so as to be capable of being formed into continuous films or
membranes. The
polyimides of this invention preferably have a weight average molecular weight
within
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the range of about 20,000 to about 400,000 and more preferably about 50,000 to
about
300,000. The polymer can be formed into films or membranes by any of the
diverse
techniques known in the art. The polymers are usually glassy and rigid, and
therefore,
may be used to form a single-layer membrane of an unsupported film or fiber of
the
polymer. Such single-layer films are normally too thick to yield commercially
acceptable transmembrane flux of the preferentially permeable component of the
feed
mixture. To be more economically practical, the separation membrane can
comprise a
very thin selective layer that forms part of a thicker structure. This
structure may be, for
example, an asymmetric membrane, which comprises a thin, dense skin of
selectively
permeable polymer and a thicker micro-porous support layer which is adjacent
to and
integrated with the skin. Such membranes are described, for example, in U.S.
Pat. No.
5,015,270 to Ekiner.
In a preferred embodiment, the membrane can be a composite membrane, that is,
a membrane having multiple layers of typically different compositions. Modern
composite membranes typically comprise a porous and non-selective support
layer. It
primarily provides mechanical strength to the composite. A selective layer of
another
material that is selectively permeable, is placed coextensively on the support
layer. The
selective layer is primarily responsible for the separation properties.
Typically, the
support layer of such a composite membrane is made by solution-casting a film
or
spinning a hollow fiber. Then the selective layer is usually solution coated
on the
support in a separate step. Alternatively, hollow-fiber composite membranes
can be
made by co-extrusion of both the support material and the separating layer
simultaneously as described in U. S. Patent No. 5,085,676 to Ekiner.
The membranes of the invention may be housed in any convenient type of
separation unit. For example, flat-sheet membranes can be stacked in plate-and-
frame
modules or wound in spiral-wound modules. Hollow-fiber membranes are typically
potted with a thermoset resin in cylindrical housings. The final membrane
separation
unit can comprise one or more membrane modules. These can be housed
individually in
pressure vessels or multiple modules can be mounted together in a common
housing of
appropriate diameter and length.
In operation, a mixture of one or more olefin compounds and one or more
paraffin compounds is contacted with one side of the membrane. Under a
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driving force for penneation, such as imposing a pressure difference between
the feed
and permeate sides of the membrane, the olefin compounds pass to the permeate
side at
higher rate than the paraffin compounds of the same number of carbon atoms.
That is, a
three carbon olefin permeates faster than a three carbon paraffin. This
produces an
olefin-enriched stream which is withdrawn from the permeate side of the
membrane.
The olefin-depleted residue, occasionally referred to as the "retentate", is
withdrawn
from the feed side.
The novel process can operate under a wide range of conditions and is thus
adapted to accept a feed stream supplied from diverse sources. If the feed
stream is a gas
that exists already at a sufficiently high, above-atmospheric pressure and a
pressure
gradient is maintained across the membrane, the driving force for separation
can be
adequate without raising feed stream pressure farther. Otherwise, the feed
stream can be
compressed to a higher pressure and/or a vacuum can be drawn on the permeate
side of
the membrane to provide adequate driving force. Preferably the driving force
for
separation should be a pressure gradient across the membrane of about 0.7 to
about 11.2
MPa (100 - 1600 psi).
The novel process can accept a feed stream in either the gaseous state or the
liquid state. The state of matter will depend on the composition and on the
pressure and
temperature of the olefin/paraffin feed stream. When the feed stream is in the
liquid
state, the separation can be carried out by the pervaporation mechanism.
Basically, in
pervaporation, components of the liquid feed mixture in contact with the
membrane
permeate and evaporate through the membrane, thereby separating the component
in the
vapor phase.
This invention is particularly useful for separating propylene from
propylene/propane mixtures. Such mixtures are produced as effluent streams of
olefin
manufacturing operations, and in various process streams of petrochemical
plants, for
example. Thus in a preferred embodiment, the process involves passing a stream
comprising propylene and propane in contact with the feed side of a membrane
that is
selectively permeable with respect to propylene and propane. The propylene is
concentrated in the permeate stream and the retentate stream is thus
correspondingly
depleted of propylene. The membranes of this invention exhibit unexpectedly
high
propylene/propane selectivity which distinguishes them from prior art
membranes.
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Furthermore, the membranes of this invention exhibit stable performance over
long
periods of time under conditions where membranes of the prior art degrade
significantly
in performance.
The fundamental steps of the separation process include
contacting one side of the membrane with a feed mixture comprising an olefin
compound
and a paraffin compound having a number of carbon atoms at least as great as
the olefin
compound,
causing the feed mixture to selectively permeate through the membrane, thereby
forming
on the second side of the membrane an olefin-enriched permeate composition
which has a concentration of the olefin compound greater than that of the feed
mixture,
removing from the second side of the membrane the olefin-enriched permeate
composition, and
withdrawing from the one side of the membrane an olefin-depleted composition
which
has a concentration of the olefin compound less than that of the feed mixture.
This invention is now illustrated by examples of certain representative
embodiments thereof, wherein all parts, proportions and percentages are by
weight
unless otherwise indicated. All units of weight and measure not originally
obtained in SI
units have been converted to SI units.
EXAMPLES
Example 1: Propylene/Propane Gas Separation with P84 Membrane
Asymmetric hollow-fiber membrane of P84 was spun from a solution of 32 %
P84, 9.6 % tetramethylenesulfone and 1.6 % acetic anhydride in N-
methylpyrrolidinone
(NMP) with methods and equipment as described in US Patent Nos. 5,034,024 and
5,015,270. The nascent filament was extruded at a rate of 180 cm3/hr through a
spinneret with fiber channel dimensions of outer diameter 559 gm and inner
diameter
equal to 254 gm at 75 C. A fluid containing 85 % NMP in water was injected
into the
bore of the fiber at a rate of 33 cm3/hr. The nascent fiber traveled through
an air gap of 5
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cm at room temperature into a water coagulant bath at 24 C and the fiber was
wound up
at a rate of 52 m/min.
The water-wet fiber was washed with running water at 50 C to remove residual
solvent for about 12 hours and then sequentially exchanged with methanol and
hexane as
taught in U.S. Patent Nos. 4,080,744 and 4,120,098, followed by vacuum drying
at room
temperature for 30 minutes. After that the fibers were dried at 100 C for one
hour.
Samples of fiber were formed into four test membrane modules of 52 fibers
each. The
fiber in the modules was treated to seal defects in the separating layer with
a method
similar to the method described in U.S Patent No. 4,230,463. The fiber was
thus
contacted with a solution of 2% wt. 1-2577 Low-VOC Conformal Coating (Dow
Coming Corporation) in 2,2,4-trimethylpentane for 30 minutes and then dried.
The modules were measured in permeation of a feed of mixed propylene/propane
(50:50 mole %). The feed mixture was provided in the vapor state by
controlling the
feed pressure at 2.8 MPa (400 psig) and the feed temperature at 90 C. The feed
mixture
was supplied to contact the outside of the fibers and the permeate stream was
collected at
atmospheric pressure. The permeate flowrate was measured by volumetric
displacement
with bubble flowmeters. The feed flowrate was maintained at greater than
twenty times
the permeate flowrate. This rate was high enough that the composition on the
feed side
remained roughly constant while the feed mixture permeated the membrane. This
was
done to simplify calculation of the membrane permeation performance. The
composition
of the permeate stream was measured by gas chromatography with a flame
ionization
detector. The average permeate composition was 92.2 % propylene and 7.8 %
propane.
The performance of the membrane was expressed in terms of propylene
permeance and propylene/propane selectivity. The permeance is the flowrate of
propylene across the membrane normalized by the membrane surface area and the
propylene partial pressure difference across the membrane. It is reported in
gas
permeation units ("GPU"). One GPU equals 10-6 cm3(at standard temperature and
pressure "STP")/(sec = cm2 = cmHg). The propylene/propane selectivity is the
ratio of the
permeance of propylene divided by the permeance of propane. The performance of
the
four modules is shown in Table 1.
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Table I
Propylene Permeance (1) Propylene/Propane
GPU selectivity (1)
1.3 12.0
0.97 12.5
1.4 12.9
1.3 13.1
(1) measured after 24 hours
Example 2: Propylene/Propane Gas Separation with P84 Non-posttreated Membrane
A sample of the fiber from Example 1 was processed and formed into a test
module as in Example 1 except that the fiber was not treated to seal defects
in the
separating layer. The propylene permeance was 1.7 GPU and the
propylene/propane
selectivity was 7.5. Although the selectivity was lower than the selectivity
of the treated
fiber of Example 1, it was high enough to suggest that the P84 fiber with
acceptable
performance characteristics can be produced as an asymmetric membrane without
the
sealing posttreatment.
Example 3: Propylene/Propane Gas Separation with P84 Membrane
Asymmetric hollow-fiber membrane of P84 was prepared as in Example 1 with
the following two changes: (a) the water-bath temperature was lowered to 8 C
and (b)
the spinneret temperature was increased to 87 C. The fiber was washed, dried
and built
into test modules and tested in permeation of a 50:50 mole % mixed
propylene/propane
feed mixture as in Example 1. The propylene permeance was 0.61 GPU and the
propylene/propane selectivity was 15.
Example 4: Durability of P84 Membrane in Propylene/Propane Gas Separation with
P84
Membrane
Asymmetric hollow-fiber membrane of P84 similar to the fiber of Example 3 was
tested for duration of 4 days at 90 C with a 50:50 mole % feed mixture of
propylene/propane at 2.8 MPa (400 psig). The test was designed to simulate
commercial
operating conditions. Results are shown in Table II. No decline in selectivity
was
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observed. A slight decline was observed in propylene permeance, which
stabilized after
the second day.
Table II
Feed Propylene/
Pressure Propane Propylene permeance
Time MPa (psig) Selectivity GPU
4 hours 1.7 (250) 13 0.76
1 day 1.7 (250) 13 0.96
2 days 1.7 (250) 13 0.73
3 days 2.8 (400) 12 0.61
4 days 2.8 (400) 14 0.61
Example 5: Propylene/Propane Liquid Feed Separation with P84 Membrane
One of the modules of Example 1 was tested using a 50:50 mole % feed mixture
of propylene/propane. Feed pressure and temperature were controlled at 2.8 MPa
(400
psig) and 50 C, respectively, to place the feed mixture in the liquid state.
The permeate
was withdrawn at atmospheric pressure, therefore the permeate was in the vapor
phase.
For this type of separation the concentration difference across the membrane
is usually
considered to be the driving force for separation instead of the partial
pressure difference
as used in gas or vapor permeation. For comparison of the results of this
Example with
permeation under vapor state feed conditions, the simplifying mathematical
treatment
described in J.G. Wijmans and R.W. Baker, A simple predictive treatment of the
permeation process in pervaporation, J. Membrane Science 79 (1993) 101-113)
was
applied. Such analysis assumes that the liquid feed evaporates to produce a
saturated
vapor phase on the feed side of the membrane and then permeates through the
membrane
driven by a partial pressure gradient. This analysis provides a mathematical
model that
includes terms for feed-side and permeate-side vapor pressures and permeance
and
selectivity comparable to those used in the separation of gaseous state feed
mixtures.
The model also contains a term related to the liquid-vapor equilibrium. With
the feed
mixture of 50:50 mole % propylene/propane in the liquid state, the membrane
produced
a permeate stream of 93% propylene. By application of the model, it was
determined
that the propylene permeance was 0.46 GPU and the propylene/propane
selectivity was
16. In separate testing with feed mixture of the same composition in the vapor
state at
2.8 MPa (400 psig) and 90 C, the propylene permeance was 0.95 GPU and the
propylene/propane selectivity was 13. This shows that the membrane of P84 can
be
useful for separation service for liquid propylene/propane.
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Example 6: Propylene/Propane Gas Separation with a Membrane of P84 blended
with
P84-HT325
Asymmetric hollow-fiber membrane of a 1:1 blend of P84 and P84-HT325 was
spun from a solution of 16 % P84, 16 % P84-HT325, 9.6 % tetramethylene sulfone
and
1.6 % acetic anhydride in NMP by the process described in Example 1. The
spinning
conditions and equipment were similar except that the spinneret temperature
was 85 C,
the bath temperature was 8 C and the air gap was 10 cm. The fiber was formed
into a
module which was tested for permeation of a propylene/propane (50:50 mole %)
feed
mixture as in Example 1. The permeation performance was 1.9 GPU propylene
permeance and 11.9 propylene/propane selectivity.
Example 7: Propylene/Propane Liquid Feed Separation with a Membrane of P84
blended with P84-HT325
The module of 1:1 blend of P84 and P84-HT325 of Example 6 was tested with
50:50 mole % feed mixture of propylene/propane. The feed mixture was
maintained in
the liquid state by applying the conditions described in Example 5, i.e., the
feed pressure
was 2.8 MPa (400 psig) and the temperature was 50 C. The permeate was
withdrawn as
a vapor at atmospheric pressure.
The membrane produced a permeate with 93.6 % propylene; the propylene
permeance was 0.6 GPU and the propylene/propane selectivity was 15.5. This
shows
that the membrane of 1:1 blend of P84 and P84-HT325 can provide useful
separation
with liquid propylene/propane feed.
Example 8: Propylene/Propane Liquid Feed Separation with a Membrane of P84
blended with P84-HT325
The test in Example 7 (i.e., with membrane of 1:1 blend of P84 and P84-HT325)
was continued for a duration of 100 hours, to assess membrane performance
stability
under simulated commercial conditions. Results are shown in Table III. No
significant
decline was observed.
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Table III
Time Propylene/Propane Propylene
Hours Selectivity Permeance
GPU
24 15.5 0.56 GPU
60 15.9 0.59 GPU
84 15.6 0.67 GPU
110 15.8 0.67 GPU
Example 9: Propylene/Propane Gas Separation with P84 Dense Film Membrane
A thin dense film of P84 polymer was cast from a solution comprising 20 % P84
in NMP. The film was dried at 200 C in a vacuum oven for four days. A sample
of the
polymer film was tested in a modified 47-mm ultrafiltration style permeation
cell
(Millipore), using a feed mixture of 50:50 mole % propylene/propane at 2.8 MPa
(400
prig) pressure and 90 C. temperature. The permeate pressure was 2-5 mm Hg. The
feed
flowrate was high enough to ensure low conversion of the feed into permeate so
that the
composition on the feed side was constant. The compositions of the feed and
permeate
streams were measured by gas chromatography with a flame ionization detector.
The
permeate flowrate was determined from the increase in pressure over time in
the fixed-
volume permeate chamber of the permeation cell.
The permeation performance of the polymer is characterized by the two
parameters: propylene permeability and propylene/propane permselectivity. The
permeability is the flowrate of propylene across the film normalized by the
film surface
area and film thickness and by the propylene partial pressure difference
across the film.
Units of permeability are Barrers. One Barrer equals 10-10 cm3(STP) = cm/(sec
= cm2 =
cm Hg). The propylene/propane permselectivity is the ratio of the propylene
and
propane permeabilities. The propylene permeability of the P84 film at 90 C and
2.8
MPa (400 psig) was 0.24 Barrers; and the propylene/propane permselectivity was
15.5. .
The permselectivity was in good agreement with the selectivity measured with
hollow-
fiber membranes of P84 polymer.
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Example 10: Propylene/Propane Separation with a Membrane of TDI +
BTDA:BPDA(1:1)
A dense film of a copolymer of toluenediisocyanate (TDI, a mixture of 20% 2,6-
toluenediisocyanate and 80% 2,4-toluenediisocyanate) and a 1:1 mixture of
benzophenone-3,3',4,4'-tetracarboxylic acid dianhydride (BTDA) with 3,3',4,4'-
biphenyl
tetracarboxylic dianhydride (BPDA) was tested in permeation with 50:50 mole %
mixed
propylene/propane feed at 2.8 MPa (400 psig) and 90 C as in Example 9. The
propylene
permeability of the film was 0.48 Barrers and the propylene/propane
permselectivity was
over 16.
Comparative Example 1: Polypropylene/propane separation with a traditional
composition fiber membrane
Samples of composite hollow-fiber membrane of Matrimid 5218 a copolymer of
5,x-
amino-(4-aminophenyl)-1,1,3 trimethyl indane and 3,3',4,4'-benzophenone
tetracarboxylicdianhydride (Vantico, Inc.) were tested in permeation over a 72-
hour
period with a feed mixture of 50:50 mole % propylene/propane at 1.7 MPa (250
psig)
and 90 C as in Example 1. The purpose of the test was to determine the
membrane
performance stability under simulated commercial conditions. This membrane,
described in US Patent 5,468,430 is a commercial gas-separation membrane
produced by
MEDAL, LP. Results of the test are shown in Table IV.
Table IV
Time Propylene/ Propane Propylene permeance
hours Selectivity GPU
2 5.5 9.0
24 7.0 4.8
48 7.1 4.0
72 7.2 3.8
As apparent from these results, the membrane exhibited low selectivity and
lost greater
than 50% of its initial permeance during the test, unlike the membranes of
this invention.
Comparative Example 2: Propylene/Propane Separation with a Polyaramid Membrane
Samples of asymmetric hollow-fiber membrane made from a blend of two
aromatic polyamides were tested in permeation of a feed mixture of 50:50 mole
%
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propylene/propane at 2.8 MPa (400 psig) and 90 C. as in Example 1. This
membrane is
described in US Patent No. 5,085,774 (Example 15). The fiber was spun at a
draw ratio
of 7.3. It is an established gas-separation membrane applied in the separation
of
hydrogen from mixtures with hydrocarbons or carbon monoxide. It exhibited a
propylene permeance of 0.23 GPU and a propylene/propane selectivity of 9.5.
This
performance was less than that of the novel membranes having composition of
formula
(I). This result was unexpected because the membrane of aromatic polyamide has
very
high selectivity in separations of other mixtures, for example a selectivity
of higher than
200 for H2/CH4 at 90 C.
Although specific forms of the invention have been selected for illustration
in the
preceding description which is drawn in specific terms for the purpose of
describing
these forms of the invention fully and amply for one of average skill in the
pertinent art,
it should be understood that various substitutions and modifications which
bring about
substantially equivalent or superior results and/or performance are deemed to
be within
the scope and spirit of the following claims.
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