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
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
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-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
X ~ ~ X
~ ,~ Lno O 0~ X ~xl C ) ~i t`
~3 - ~
~l) ~ ~ l` au O
3 P- ~ O
~ ao dl U r ~ dl ~3 a) ~
~I z ~ ~C
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-