Note: Descriptions are shown in the official language in which they were submitted.
3~
IMPROVE~ PROCES~ FOR SEP~RATING
CO~ FROM OTHFR G~SES
This is an i~proved process for separating carbon
dio~id~ from oth~r gases usi~g a gas permeable me~brane.
In p~rticular, this p:rocess utiliæes a dry, asymmetric
cellulose ester membrane.
Numerous references exist which describe various
processes ~or separating gases by means of semi-permeable
me~branes. U.S.P. 3,415,038 describes a method for
separatillg a first gas from a gaseous mixture using a
thin, dry, asymmetric cellulose acetate membrane.
U.S.PO's 3,84 ,515, 4,080,744, 4,080,743 and 4,127,625
describe other techni~ues for drying a water-wet membrane
to be used in gas separation. U~S.P. 4,130,403 teaches
the separation of C02 from a natural gas stream by
means of a dry cellulose ester membrane.
The subject inventio~ is an improved process for
separatins carbon dio~ide from a fluid containing other
gases or liquids by selectively permeatlng the carbon
dioxide through a thin, dry, asymmetric cellulose ester
memb.rane at a temperature of less tharl about 10C.
Surprisingly, it has been found that the separation
28,416-F
-2-~
i3~
factor of carbon diox.ide xelative to o~her principal
components pres~nt in the feed st.ream is fre~uently at
least 10 percent greater at temperatures less -than
about 10C. than at temperatuxes of 25C. or higher.
Principal components are those making up at leas-t 10
volume percerlt of the feed stream when said components
are .in ~le gas p.hase. With cextain pxeferred principal
com~onents, such as metha~e, th~ improvement in the
separation facto~ at these lower temperatures may be 2
percent or more of the separaklon fackox obtained at
25~C. at otherwise equivale~t operating conditions.
The present in~ention reside in a process for
sepa.rating carbon dioxide rom a 1uid containing
carbon dioxide a~d other gases or liquids by selectively
penmeating ~he carbon dioxide ~hrough a thin, dry,
as~mmetric cellulose ester membrane, the improvement
wherein khe separation is conducted at a temperature no
greater than about 10C~ and the separation factor of
carbon dio~ide relative to at least one other principal
gas in the feed is at least 10 percent greater than
that exhibited at 25Co undar otherwise equivalent
operating conditions,
The thl~, dry asymmetric cellulose ester membranes
used in this i~ention axe described in detail in the
prior art~ Such membra~es are desc.ribed in U.S.P.'s
3,415,038, 3,842,515, 4,080,743, 4,127,625 and 4,130,403.
Preferred as cellulose esters are cellulose acetate,
cellulose diacetate, cellulose triacetate, cellulose
propionate, cellulose butyrate, cellulose cyanoethylate,
cellulose methacrylate and mixtures thereof. Mixed
esters of cellulose, such as cellulose acetate butyrate,
28, 416-F -2-
mixed cellulose acetates and cellulose acetate meth~
acrylate, are also operable. Commercial cellulose
triacetate, containiny from 42.7 to 44 weight percent
acetate, .is the material of choice for the membranes
used in the subject method.
Inasmuch as the permeation flu~ of gas or liquid
is generally invexsely related to the membrane thic~ness,
it is desixable that the discriminating layer o~ the
membrane be as thin as possible while maintalning
ade~uate me~brane s-trength and good rejection. Typically,
the asymmetxic cellulose ester membrane will have a
dense discriminating layer less than on~ micron thick
and a much thicker, relatively porous suppor-ting sublayer.
The membrane may also be cast on dissimilar porous
supporting materials to provide additional strength.
For example, micropo.rous polysulfone materials can be
used as a second supporting layer. Of course, this
dissimilar porous support can comprise a second dis
criminating layer, but generally a second discriminating
layer is nei-ther necessary or desirable.
The term "membrane" as u~ed herein is intended to
encompass a wide variety of possible configurations
known in the prior art. For e~ample, the membrane may
be used as a flat film, tubular film or hollow fiber.
The hollow fiber form is generally preferred and can be
readily prepared by ~echniques known in ~he art.
Asymmetric cellulose ester films suitable for
membrane use are commercially available. However,
these cellulose esters as manufactured are water wet.
To render these films suitable for the separa-tion of
28,416-F -3
non~aqueolls fluids the film must be carefully dried so
as to avoid significant disruption of the membrane
structure~ A number o methods for drying cellulose
ester membranes are taught in the prior art. One
S preferxed techni~le for drying a water~wet me~brane is
to first anneal the fiber in 80C. water ~or abou-t 1.5
rni.nutes. The water is then extracted from the fiber
with isopropanol and the isopropanol displaced with
he~ane, heptane or isooctane in the manner taugh-t in
U.S~P. 3,842,515. ~ particularly preferred techni~ue
for dr~ing water-wet hollow fiber membxane bundles is
~o intxoduce a 50:50 volume per ent mi~tu.re of
isopropanol and isooc-tane down the bore of each fiber
while an ine.rt gas stream is passed over the hollow
fiber's out.er surfaceO When the fiber is essentially
free o~ water, the introduction o the isopropanol/-
isooctane mix-ture is terminated and the liquid remaining
in the bore pervaporated through the membrane. Generally,
such mem~ranes will exhibit a CO2 flux of at least
about lX10 6 cm3/(sec cm~ cm of Hg) at 10C. and
4.53 kg/cm2 (50 psig) wi~h a pure C02 feed.
As some shrinkage occurs in drying the hollow
fiber, if -the fiber is assembled in a bundle prior to
drying, the construction of the bundle should allow
tolerance for some shrinkage. For example, if a per-
forated core is employed, it should be designed so that
some reduction in length will take place as the fibers
shrink. Also, the epoxy resin tubesheet should be
cured with an agent which promotes good adhesion with
3 0 the hollow fibers, such as the aliphatic amine marketed
by Paci~ic Anchor Chemical under the trade name SURWETR~
The fluid in the feed stream contains carbon
dioxide along with other gases or liquids. Pre:~erably,
28, 416--F -4-
the predominant components present in addition to
carbon dio~ide are lower aliphatic hydrocarbons, such
as methane, ethane, ethylene, propylene, butane and
propane, nitrogen and other components present in a
natural gas stxeam ta~en directly from a yas well.
P.re.ferably, such feed streams will contain up to about
90 volume percent CO2 with a remaining amount of lower
allphatic hydrocarbons and optionally a small amount
(less than about 10) of H2S. For the instant separation
10 pxocess to work most e:Eficlently, the feed gas preferably
contains at least about 10 volume percent CO2.
The feed stream is advantageously substantially
free of water to avoid wetting the dry membrane which
adversely a:Efects membrane proper~ies. However, some
15 water can be tolera~ed al-though the long term performance
of the membrane may suffer. Of course, the feed stream
can be dried by a number of techniques well known in
the art.
Depending on the pressure and temperature of the
feed s~ream, it can be present as either a gas or
liquid. In fact, as C02 is removed from a light hydro~
carbon stream, some condensation o~ the gas to li~uid
may occur. Ad~an-tageously, the liquid present in the ..
feed stream is not vaporized while in contact with the
membrane inasmuch as such contart can adversely affect
~he long~term membrane performance.
The pressure of the fluid feed stream can vary
over a wide range. A pressure of from about 2.8 to
50 k~/cm2 (25 to 700 psig) is operable, with a pressure
from 5.6 to ~6 kg/cm2 (50 to 500 psig) being preferred.
A11 other operating parameters being egual, the carbon
28,416 F ~5~
~6--
dioxide flux of the membrane generally increases with
incxeased pressure in the feed stream.
The d.iffe.rentlal pressure acro~s the membrane can
also operably va.ry over a considerable range. The
pressure on ~le feed side of the membrane should be at
least 0O7 kg/cm2 (10 psi) greater than that on the
permea-te side of the membrane. A pressure differential
of at least about 7 kg/cm2 (100 psi) is preferred.
The tem~eratu.re at which ~he gas separation is
conducted should be no greater than about 10Co Although
the carbon dio~ide flu~ is greater at higher temperatures,
the car~on d:ioxide/methane separation factor decreases
significantly. Generally, the greatest separation
actor of carbon dioxide relative to methane is effected
at an operating temperature of no greater than about
about 10C. ak a pxessure of at least about 15 kg/cm2
(200 psig). Lower temperatures are operable, so long
as the carbon dioxide present in both the feed and
permeate remains in the gas or liquid phase. The
preferred temperature range for this separation is
about 5 to about 15C.
Cooling of the feed stream and membrane from
ambient temperatures to ~he temperature desired for
separation can be accomplished by conventional refrigera~
tion techniques or any o~her convenient means. In one
preferred embodiment, the feed stream is cooled by
exp~nsion of a high pressure feed gas to lowex the
pressure of the gas to the pressure employed in separa-
tion.
28,416-F ~6
Seve.ral of the instant membrane separation units
can be operated in parallel to increase the overall
capacity of the separa-tion device. ~lternatively,
several membranes can be employed i.n series -to improve
rejection.
The carbon dioxide~enriched permeate has a variety
o:E uses. The ca.rbon dioxi~e can be lnjected into
oil~bearing formations to enhance oil recovery ln
accordance wi~h known m~thodsO The carbon dioxide can
also be used as an inerting gas. If H2S is present in
the gas feed, it will generally permea-te along with the
CO20 It is desirable to selec-tively eliminate the H~S
from the penmeat@ by ~nown methods, if -the gas is to be
used for inextingO The removal of carbon dioxide may
al~o enhance the commercial value of the other gases
present in the feed.
The following examples are presented to illustrate
-the in~entionO
Example 1
A hollow fiber of cellulose triacetate having an
inside diameter of 90 microns and an outside diameter
of 250 microns is spun dry, ~uenched in water for about
3 seconds at 4C. and then drawn through water at 18C.
for 45 seconds to remove plasticizer in a conventional
spinning process. This fiber is spun from a solution
of 40 weigh~ percent cellulose triacetate and 60 weight
percent o a mix-ture of 78 weight percent tetramethylene
sulfone and 22 percent of a polyethylene glycol having
a molecular weight of about 400. The fiber is then
annealed fox 1.5 minutes in water at 30C. The resulting
fiber contains 65 weight percent water.
28,416-F ~7
3~
The water=wet fibers are immersed at ambient
tempera-tures first in isopropanol and then isooctane to
remove the water in accordance with methods known in
the art. ~he residual liguid is removed by drying the
5 f].ber in a rapidly moving air stream.
Twenty~:four of the dry hollow fibers are assembled
in a staxldaxd loop cell for testing. These fibers are
about 30 centimeters in len~h. Pure methane and
ca.rbon dloxide gases are intxoduced into the loop cell
external to the fibers at pxessures of 4.5; 9.0 and
13 4 kg/cm2 (50, 100 and 192 psig) at temperatures of
25, 10, 2 and -10C. The flux o carbon dio~ide
and me~hane pexmeating through the fiber are recorded
fox each set of temperatures and pressures. The carbon
dioxide flux in units of cm3/(sec cm2cm of Hg) and the
calculated ca.rbon dioxide/methane separation factor
(~ CO2/CH4) for each set of conditions are tabulated in
Table I. The maximum sepaxation factor observed for
mixed gases containing both methane and carhon dioxide
would be expected to be much lower, but the same trend
and temperature dependence has been observed in tests
with mi~ed gases.
28,416-F -8-
N --
\''1 ~ d~ 3tj3~
~C ~ N
Cl O ~ O t` ~
n v . " ~1 o
~ X n ~ d;
a) ~ o o o
1~ ~ ~1 1~ r~ r I
Pl ~ ~ ! X X
O O N
~q ',:~
O 0
00 ~ ~ O t~ ~
o~ ~ ~ Cl) d'
O
~6 L~ Lr~ Ln LO '~
o o o o
P~
~ ~ O t~ ~r~
h VN ~ N ~ ~ rl
~ ~ L
~ ~D ~ C5~ 0
n v ~ ~ n ~
s~ ~ ~
~ Gl ,,
rn n n n n
~ . ~o 1~ 10 10
P~ ~ ~I ~I ~
~ o~ ~ X z
~c~ n o o
o ~J r-l N ~1
28, 416-F -g-
- 10
Ex~mple 2
The separation properties of dry hollow fibers
p.repared in accordance with Example 1 are tested in a
stan(laxd 1QP cell at two diffexent ~emperatures . The
prepared feed gas consists of 54.92 vo:Lume percent C02
and a remaining amount of C~I4. T.he feed gas is intro~
duced external to the fibers and 10ws countercurrent
to the permeate. The pressure of the feed gas was 18.5
kg/cm2 (250 psig)0 The other opera-ting conditions are
tabulated in Table IIo
28,416-F ~10
h
O
r ' ~
h O ¦ ~I N
V~
O O
X ~ ~ X
~Y,
t`
$
~ ,~ Lno
O 0~ X ~xl
C ) ~i t`
H
~3 - ~
~l)
~ ~ l` au
O
3 P- ~ O
~ ao dl
U r ~ dl ~3
a) ~
~I z
~ ~C
K
d~ d'
E-i N
~8, 416-F 11
~:1.2--
i3~
As can be seen fxom the da ta in Table I I, the
separatlon factox is sigIlificaIltly higher at 4 . 8C .
than at 2405C. This difference can be very important
in the commercial separatioxl of C0~ f:rom CEI4.
28, 416 F ~12-
