Separation of Gases by Plastic Membranes PERMEATION RATES AND EXTENT OF SEPARATION DAVID WILLIAM BRUBAKER1 AND KARL KAhIMERMEYER Chemical Engineering, State Uniz;ersity of Zowa, Iowa C i t y , Ioun
T
H E fact that nonporous plastic membranes show highcr permeabilities for some gases than for others has been knoirn since 1831 (I$), and has formed the basis of several old patents for the separation of air. However, until very recently little attention has been paid to the possible commercial application of this phenomenon. Although most of the work in this field has been done on natural rubber films, Weller and Steiner (16, 1 7 ) have reported data on other types of plastic membranes. The primary object of the present work was to obtain quantitative data on the rates of gas transmission through a number of polymeric films and to investigate the separation characteristics of these plastic membranes. I n addition, i t v a s desired to check existing relationships for predicting quantitatively the degree and the direction of separation from a knowledge of permeability data.
For pract>icaluse in effecting the separation of gases, films should possess the folloaing characteristics: High permeability, t o reduce area requirements High selectivity, to reduce both power requirements and the number of stages needed Chemical stability Physical stability The present theory for calculating the enrichment of a binary mixture was developed by Weller and Steiner ( 1 7 ) for a singlestage permeation. Their equations were derived for two specific cases: Case I, when perfect mixing of gases occurs on both sides of the film; and Case 11, when laminar flow (no mixing) occurs on either side of the membrane. I n Case I, the effective high pressure composition is assumed to be equal to the composition of the gas leaving the high pressure side of the membrane, and the low pressure composition is equal to the composition of the gas stream leaving the low pressure side. Based on these assumptions and Pick’s law of diffusion, the following equation may be obtained for a binar!. system ( 1 7 ) :
THEORY AND LITERATURE SURVEY
Since Mitchell’s (19) initial investigation on natural rubber in 1831, a great deal of work has been done on the theory undeylying the flow of gases and vapors through plastic membranes. In 1866 Graham (9) studied the permeability of gases through rubber and deduced that the permeation process was a sequence of solution, diffusion, and re-evaporation of the permeating gas, a viewpoint which is still held today ( 2 ) . The pressure, area of membrane, thickness of membrane, and temperature are the possible variables in the kinetics of permeation. The following equation for the flow of gases through the plastic mernbrane may be written: q = PAt(r - p)/d
XL = 1 - x,
(1)
x, =
=
DS
(2)
(4)
-b&d=
(5)
2a
+ + + + +
+
( R ’ T ’ W 2 ) ( X i ) 3- W [ T ’ ( @ R’BPc(r - p ) R’(T’nXf PAW - PcnX,A)](X;)* a[UrX,B (6) VPc(n - p ) T’nX/p TY(Pa R ’ ) ! ( X i ) - 82 = 0
+
(3)
m),
+
+
Equations for the other components are then:
Furthermore, it has been shown (1-3) that the effect of temperature on the diffusivity and the solubility may also be expressed as an exponential function in a manner similar to the permeability constant. Because organic membranes have the marked property of showing a much higher permeability toward some gases than toward others, they should be effective media for gas separation. This was demonstrated by Weller and Steiner (16, 1 7 ) in their investigation of films for use in the separation of air into nitrogen and oxygen and in the separation of helium from natural gas. 1
7 r x o
Huckins and Kainmermeyer (10) pointed out that by coinbining Equation 4 s i t h the over-all and component material balance equations, the following relation may be derived for a threc-component system in which the feed gas composition, the pressure, and the fraction permeated are known. This equation for X,” has been somewhat modified from their presentation in order to simplify its use.
over a moderate temperature range, and also that, the permeability constant, P , may be resolved into t,he product of the diffusivity constant, D, and the solubility coefficient, S, so that:
P
[x(1 - X , ) -- pP(lx,- ml
By combining this equation n.ith the over-all material balance and an individual component material balance, a quadratic equation is obtained vhich map be solved for the composition of the permeated gas stream. The final equation for a binary syptem is. therefore (IO),
The permeability constant, P , is then defined by this equation, and is equal to the rate of transfer of gas through a film of unit thickness, per unit, area, per unit partial pressure difference acros3 the membrane. Barrer (2) has pointed out that the effect of temperature on the permeability constant can be expressed by the relation:
P = Po exp. ( - E , / R T )
o1
x,c = 1 - (Xi 4- XgB)
(8)
X.! = ( X p - F X ; ) / V x,”= ( X ? - F X : ) / V xoc = 1 - ( X b x:)
(10)
+
(9) (11)
An expression for four components can be derived in a similar manner and then simplified to the following equation for Xi:
Present address. The Chsrnstrand Carp., Pensaoola, Fla.
733
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
134
+ +
Z R ’ 2 ” l r y 3 ( X t) ~ W*[I1 PDY f?’T’(TTPA - $11 + Z ( J 3 T’y + R’6)l:X;)’ 3IV[PnJI’ + R’T’W 5 Z1;3 f y 6) fJ p T ’ y + R‘s + dl17Pa]1Xi)2P2[Pol7 W ( J PA) f ( 3 f ”( S j ] ( S ; l j f $ 3 = 0 (12)
+
+
+
+
+
+
and in addition t,o Equations 7 , 9, and 10,
x:- _ _ xi= 3 -~6 r’rrx: X; = 1
xg
=
XD
=
-
(x;
+ x; t 1,”)
(3.”’ - FX, 1
-
(x: + x: + x::
Vol. 46, No. 4
I’REPAIIATION 01;XIATERIALS
The polymeric films which vere used are listed in Table I along with their compositions and the supplying company. These films included only commercial membranes. The gases were purchased from eommerciitl murces and were not purified before use. The purities quoted by the manufacturer in terms of percrntage were as follon-s:
(13)
(15, (I[i,
By analogous algebraic manipulations, equations cnn hr est ;rhlished for any number of components.
Approximately 100 99.8 99.5 99 e 99 0 99 95 99 90
Helium Hydrogen Carbon dioxide Oxygen Kitrogen Ammonia 8uliiv dioxide
(14)
All these gases iverc tic$giated 1):- the manufacturer t i s either dry or anhydrous. Therefore, no attempt was made to dry t h r gases further before use. EXI’ERI\IENTAL WORK AKVD DISCUSSION O F RESULTS
1. 600-Pound pressure Gas storage tank
gag?
2.
Flowrator Steel flange 15 inches in diameter and 6 s inch thick Rubber gasket 6 . Thermometer 7 . Gas-sample oollection bottle R . Liquid-displacement bottle 9. Plastic membrane 10. Filter paper or porous metal rnernhrane support 11. 100-Pound pressure gage, in 1-pound grnduationr 12. 50-Inch mercur? marnometer :3.
;:
I;
APPARATUS A S D PROCEDURE
The apparatus used t,o obtain gas permeability data v a s thc same as described earlier (6-7), and was based on the principle of measuring the rate of volume change of permeated gas under conditions of constmt pressure and temperature. In gas-separation experiment,s a gas-collection bulb l m s substituted for the glass capillary tubing and a gas-collection bulb and a Flowrator were placed on the upstream side of the film. This apparatus is shown in Figure 1. A s the rate of flow on the low pressure side was fixed by the actual permeability for a given upstream pressure, the fraction permeated, F,was varied by changing the rate of gas flow from the high pressure side. The composition of the gas mixtures was determined by analyzing 100-cc. samples in an Orsat apparatus. An aqueous solution of potassium hydroxide was used to remove the carbon dioxide or sulfur dioxide, a solution of pyrogallic acid and potassium hydroxide absorbed oxygen, and a dilute solution of sulfuric acid removed ammonia. Hydrogen was removed khy oxidizing it to water in a copper oxide furnace, and nitrogen and helium were determined by difference.
PERMEABILITY DATA. All permcability data are reported in terms of the, permeability constant, P, defined as the' numbw of standard cubic centimetcia (0” (;., 1 atmosphere pressure) of ga.passing through 1 sq. em. of film, 1 en). t,hick, per eecond, pcr centimeter 01 m e r c u r y p a r t i a l pressure differencc aero83 the film. The permeability data for the ~ w i ous films in the separation erperinicrita are shown in Figures 2, 3, 4, and 5. Different samples of the sane type of film! obtained from sheets processed :it different times, may vary widely in their permeability values (6), especiallj- with plasticized membranes. Separat,ion r(’sults may be expected to vary accordingly. Nost of the films investigated [wport,ed upon here and in previous publications (5,7)] either decomposed or wew chemically affected in an atmosphere of ammonia or sulfur dioside. Corisequently, data for ammonia m r e obtained only on polycthylene and oii Trit,hene (trifluoromonochloroethylcrx-) film. while sulfur dioside data vivcrc otitained only on polyethylene film. These data are shown in Figures 2 and 5~ In these cases, the characteristic straight line was encountered for the plot of the logarithm of the permcability constant versus the reciprocal of the absolute temperature. The permeability constants of the more readily condensable gases were found to be greater than thoso of thp “real” gases, uhich have a smaller molecular size. The relti-
USED TABLE I . LISTOF FILXS Film Sample Xo.
1 2
3
4
Trade
Kame
Composition Viequeen Po!yethylene Trithene B Tnfluoromonochloroethylene (plasticized) Bakelite T‘B Cop,olyiner of poly1925 vinvl chloride with polyvinyl acetate (oiasticized) . Kodapak I1 Celluloee acetateregular butyrate
Tiiickness. Inch Manufactureis 0.0015 Visking Gorp. 0 0033 Vislcing Corp. 0 001
Bakelite CO., Division of Union Carbide and C a r bon Cora.
0 001
Tennessee Eastman
Corp.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Ay,ril 1954
135
time were expended in an attempt to obtain reproducible results. ( r = A 990 a t m . abs. p = 0 991 atin. abu : T = 306’ K.) Because of the relatively sudden change in behavior, i t was thought that the anomalous results obtained on the 0 265 0 181 0 160 0.199 0 200 0 178 0,250 0.326 0.11G 0 027 0 400 cellulose a c e t a t e - b u t y r a t e 0 262 0 272 0 130 0,297 0.116 0.114 0 265 0 450 0.181 0.182 0 015 0.296 0.113 0 261 0 450 0 161 0.182 0.270 0.0195 0 265 0.116 0.181 might have been due t o a first0.223 0 422 0.170 0.265 0 216 0.253 0.116 0,083 0.181 0.183 0.235 order change in the film at a 0.238 0 410 0 180 0.210 0.313 0 265 0 198 0.118 0.074 0.181 0.180 0.185 0 178 0 385 0 185 0.222 0.116 0.065 0.181 0.176 0.420 0 265 temperature of about 11” C. 0 188 0.180 0 265 0 170 0 380 0.212 0.116 0.060 0.181 0.170 0 460 However, neither dilatometric measurements nor x-ray difTABLE 111. SEPAR.4TION DATAFOR AMMONIA-HYDROGEN POLYETHYLENE MEMBR.4iT.E fraction patterns showed any .vp x 108, such discontinuity as would P T, ,-Etd. Cc.-Cm. Mole Fraction .4tln. .Stin. occur with a transitional struc.4bs. Abs. K. Sec.,-Sq. Cm. F XFz X7a x;z (X,”Z)’ tural change. It is possible 4.30 0.976 304 0 509 0.069 0 300 0,289 0.470 0.466 0 459 0 0644 0 300 0 287 4.30 0.972 304 0.440 0.460 that the self-plasticizing char0.0734 0 509 0 300 4.30 0.452 0,290 0,972 304 0,466 4.30 0 508 0 300 0.0715 0.290 0.972 304 0.460 0.460 acteristics of cellulose acetate0.300 0.0740 0,225 4 33 0 976 0 543 295 0.442 0,450 0.0715 0.263 0 512 0 300 0.440 0.452 4.33 0.976 302 butyrate overshadowthe effects 0.515 1.11 0 985 306 0 861 0.300 0.0745 0,283 0.511 of such a transitional change, 7.11 0.252 0.246 0 818 0.461 0.460 0,985 306 0 300 0 264 0.485 306 0.300 7.11 0 839 0.1632 0,486 0 985 thereby making its detection 0.300 7.11 0.472 0 773 0.400 0.985 306 0 207 0.404 difficult. The much less intense break in the nitrogen curve for Trithene does not tive importance of the solubility coefficient, 8, and the diffuappear to have resulted from a transitional change, Dilatosion constant, D, as defined by Barrer (2) (see Equation 3), metric measurements bear out this conclusion. The existence of these abnormalities with nitrogen for both films, and a160 for account for the fact that the permeability constants for ammonia, sulfur dioxide, and carbon dioxide are greater than those for the oxygen with cellulose acetate-butyrate, must be considered of significance, even though i t is impossible a t present to postulate smaller hydrogen, helium, nitrogen, and oxygen molecules. a satisfactory explanation. It is difficult to explain why the nitrogen permeability data for both cellylose acetate-butyrate (Figure 3) and Trithene (Figure 5) SEPARATION DATA. When the permeability data of tthe films films do not follow the usual exponential function of temperature. presented in two previous publications (6, 7 ) are considered, i t is noted that only a limited number of the films possess the necessary Ilowever, because the permeability constant is a complex term, this deviation may be due to the dependence of either the difcharacteristics of good separation membranes. Therefore, the number of films investigated with regard to gas separation was fusivity or the solubility on the temperature. Van Amerongen (1) found that both the activation energy of diffusion and the restricted t o polyethylene, cellulose acetate-butyrate, a copolymer of polyvinyl chloride with polyvinyl acetate, and Trithene. heat of solution in conjunction with the solubility are functions of temperature for rubber membranes. Satisfactory data on The separation data are presented in Tables 11 through V and oxygen permeability could not be obtained for the cellulose are also shown in Figures 6 through 12. I n each of these figures acetate-butyrate film at temperatures below 11’ C. because the the permeated gas composition was plotted against the fraction results were inconsistent, even though considerable effort and SEP4RATION DATA FOR CARBON DIOXIDE-HYDROGEN-~XYGEN-XITROGEN POLYETHYLENC LfEivfBRANE
TABLE 11.
p
ior
%
6
$
Q
$
2
I
I
32
33
I
I
35
36
1
z3 2 v, g 0
10
c
B + J
I5
30 4 0
03 2 0 2
3 ;O O I O I3 0
I
31
I
32
I
33
“.pe Figure 2.
I
3.4
I
35
I
3.6
ax
30
31
34 lQ?..Q
T-*K
Gas Permeability through Polyethylene Film Sample 1
010
37
Figure 3.
T-OK
Gas Permeability through Cellulose Acetate-Butyrate Film Sample 4
37
INDUSTRIAL AND ENGINEERING CHEMISTRY
136
Vol. 46,No. 4
sample of cellulose acetatebutyrate are presented. It is CELLULOSE-;\CET.~TEBUTYRATE >IE.IIBR.%XE apparent that for this film,as 7 = 303' l'i 'I p = 0 905 a t i n . ab3.: ' (6 = 6 00 atiii. ab..: well as the following films, s p x 106, t'here is agreement between t h t BIole Std. Cc.-Cm. ~ _ Fraction _ esperirnental arid the calcuSec.-Pq. Cm. F xyi s,"t sy> x,H.2 lated values. Since ammonia 0.110 0 27,; 0.16.i 0.102 0 267 0.167 0.480 0.114 0.293 0.377 0.041 0 110 0.27.5 0.16: 0 097 0.280 0.478 0 120 0 068 0 260 0.168 0.367 and sulfur dioside react with 0.110 0 27.5 0,lAZ 0 093 0 280 0.478 0 120 0.079 0 2.58 0 169 0 364 this type of membrane, sepmi0,110 0.273 0 175 0 125 0.348 0.120 0.16d 0.088 0.248 0.170 0 270 0.110 0 272 0.325 0.190 0.165 0.078 0,230 0 465 0.136 0.230 0 172 tione involving these gases s e r e 0.27: 0.162 0.1:,0 0.250 0 110 0,070 0.213 0,175 0.230 0.460 0.305 0.16.i 0.110 0 273 0.430 O.16d 0.0:4 0.176 0 174 0.193 0.260 0,398 not practical, 0.110 0.275 0 239 0.513 0.163 0.046 0.!32 0.172 0.173 0.403 0.174 The results on a sample of Trithene membrane are also i u TABLE T;. SEPARATION DATAFOR CARBON DIOXIDE-HYDROGEX-OXI-GES-XITROGEX good agreement with the thcoTRITHESE ~ I E M B R A X E reticd values. Pertinent data ( 7 i = 6 7 : atin. ab,.: p = 0 . 9 8 atin. ah?.; T = 301" IC.) are shown in Figures 11 and 12 and in Table V. With the quatcrriar). system (Figure 1I ) , 0.20: 0.218 0.025 0.200 0.210 0 233 0.480 0.170 0.201 0.103 0.0808 there is an inversion of hydro0 203 0,218 0.222 0 26'2 0.450 0.180 0.089 0.200 0.200 0 17.5 0.0769 0 218 0.200 0 20.i 0.0743 0.132 0.196 0.164 0.222 0 263 0.438 0.188 m d carbon dioside sepagen 0.203 0.218 0.223 0 266 0.415 0.195 0.188 0.200 0.191 0.121 0.0799 0.206 0.218 ration as compared to that 0.230 0.200 0.141 0.228 0 269 0.1% 0.400 0.200 0.0686 0 202 0.218 0 177 0 123 0.221 0.267 0.371 0.215 0.312 0.200 0.0641 obtainrd on the other mem0.200 0.205 0.218 0.162 0 330 0.223 0.102 0.211 0 262 0.0379 0 428 0.20: 0.218 0.313 0.230 0.496 0.200 0.206 0 280 0.133 0.000 0.0546 branes. This change may be predicted from the permwbility data. Separations wcrc made on a system contiiinirig permeated, F . As F approaches 0, the greatest enrichment in animoriia with this membrane, and again good agreement \vas the permeated gas st,ream O C C U ~ Aand a s F approaches 1, t,he found between experimental and calculated results. Unfortunately, expwiments with sulfur dioside did not yield permeated gas stream composition beromes t,hat of the feed gas. Separation data with a sample of polyethylene film are presivtrrit results. The calculations and data included in this paper are based on a sented in Tables I1 and I11 and in Figures 6 through 9. The solid lines represent the theoretical values as calculated from single stage of separation. In order to obtain a greater separation Equations 5, 6, and 12. The data agree well with the theoretical a multistage recycle system would be necessary. This subject, values. The rate from the lox pressure side of the membrane which is outpide the scope of this paper, is discussed hp Benedirt, was fixed by the actual permeahilities. In order to obtain high (,i) and ('ohen (8). F values it is necessary that the rates of flow on the downstream The temperature dependence of thc gas pprmcability of side be a t least t,no or three times the rate obtained on the upplastic membranes has been discussed. However, the slopes of the linea, [ts shown in Figures 2 through 5 , are usually not the stream side of the membrane. A s the rate of flox- obtained on the downstream side was small, it was not pract,ical t o investigate same for the different gases and thus, in the majority of casw, increasing or decreasing the t,emperature will result in an irivalues of F greater t'han about 0.50. Good agreement \va8 obtained between experiment,al and theoretical values for the crease or decrease in the rate of permeabilities. As pointed out bp Weller and Steiner (17),if this ratio is decreased with an irinormal gases and also for mixturm containing ammonia and sulfur dioxide. In Table IT' and Figure 10 the results of separation with a
TABLE IF'.
SEP.~RATION D a ~ aFOR C ~ R BDIOXIDE-IIYDROCES~OXY~ OS
(9011-
"2 Oo531
h
32
33
34
'';'
__
j
35
36
37
T-DK
Figure 4. Gas Permeability through Copolymer Poly>inyl Chloride-Polyiinyl Acetate Film Sample 3
Figure 5 .
Gas Permeabilitj through Plasticized Trithene
Film Sample 2
April 1954
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
E 0.40 E 0.70
0
3i u)
E 0.35
u)
a
0
a
0 c
0
; i0.60
0.30
3
0)
.I-
5
0
$ .-
0.25
E 2
050 =
:: 0 2 0
-
-a2
C
L IA 0)
2
COIMMERCIA L APPLICATIONS
An evaluation of the commercial applications of plastic membrane separation requires a complete knowledge of the economics involved. However, even though such data are not available, the following discussion gives
.I-
2 a20
015
results might follow the conditions set forth under laminar flow and thus the second case proposed by Weller and Steiner would apply. Weller and Steiner (16)found that the second case applied in their experimental work for the separation of gases by means of plastic membranes.
040
.5 0.33 .-O
.c
.991Atmr.
737
OJO
an idea of the possible uses and indicates the amount of separation which may be obX ;a tained. ooo x 0.10 NITROGEN-HYDROGES-AXMONIA SYSTEM. F - Fraction Perrneoted F - Fraction Permeated The system nitrogen-hydrogen-ammonia is of Figure 7. Polyethylene MemFigure 6. Polyethylene RIembrane brane for separation of Carbon commercial importance in ammorlia synthesis. for Separation of ~ I e l i u m and Dioxide, Hydrogen, oxygen, and illthough separation into its components b y Oxygen Nitrogen means of membrane separation is not, economical a t this tinie, it demonstrates the overall picture as i t would affect other commercial applications of plastic membranes and, therefore, merits considercrease in temperature, a compromise has to be established ation. between the use of higher temperatures to obtain higher permeaExample 1. A. Single-stage enrichment for a mixture of bilities and lower temperatures t o obt2diii higher se1ectivit)ies. hydrogen-nitrogen-ammonia by means of a polyethylene memHowever, when the selectivity increases with an increase in hrline 0.0015 inch thick. The permeability constanh a t 50" C. temperature, the only restrictions are the physical characteristics of the film. are: EFFECT OF PRESSURE ox SEPARATIOS. T h e effect of pressure p S N O 11.0 x 10-9 std. cc.-cm./sec.-sq. cm.-cm. mercury e n separation is shown in Table I11 for the system ammoniaP H ~= 3.5 x 10-9 std. cc.-cm./sec.-sq. cm.-em. mercury hydrogerl Jvith a polyethylerle film. Separation increased with PXa = 0.72 X std. c~.-cm./sec.-sq.cm.-cm. mercury a n increase in the pressure differential across the membrane, as The total pressure difference across the membrane may predicted by Equation 4. If the downstream pressure is zero, arbitrarily be taken as 100 pounds per square inch (6.8 atmosthen the pressure terms in Equation 4 cancel out, 5 0 t h a t pressure pheres) and the low pressure as 14.69 pounds per square inch will have no effect) on separat'ion. Hov-ever, if the low pressure absolute (1 atmosphere). Perfect mixing of t,he gas on bot,h sides has a finite value greater than zcro, then the total pressure terms of tile membrane is assumed. The initial concentrations of the will not cancel. If the high pressure is increased while the lower components are taken as: pressure remains constant a t some value greater than zero, the X,"%= 0.456 mole fract'ion degree of separation will also be increased, However, if the low XFz = 0.262 mole fraction Xy2 = 0.282 mole fraction pressure also increases, the result may be an increase, a decrease, or no change in the degree of separation. The limit to which the F factor of o.5 is used. hi^ value of the fraction permeated pressure differential may be raised is determined b y the physical has been recommended bs Keith ( I 2 ) for the most economical strength of the plastic membrane. membrane separation. The resulting permeated and nonNo attempt was made to predict the separations of gases with plast'ic membranes on the basis of gas molecular weight, since the permeability constants are not a simple function of the molecular weight or of any other physical property of the gas (6). 6 0.60 E Where three or more components are considered, the permeated gas composition of the C5 0.50 one or two intermediate components goes through a maximum value as the value of F is ~o varied from 0 to 1. This result is analogous to E the behavior of intermediate components in 0,30 multicomponent distillation and is predicted ,I
1
0
. I -
by the separation equations. Even though the flow of gas on either side of the membrane would ordinarily be expected t o be laminar, there appears t o be complete mixing on both sides of the membrane. This conclusion is based upon the fact that the separations obtained agree with the values predicted by Equations 5, 6, and 12. However, such a behavior may be characteristic only of the particular in this If another type of cell arrangement v'ere used, the
d
.-E ~0.20
5
E9 olo 2, 0.0
* 7 1 0 9 Atms.
0.2 0.4 0.6 0.8 1.0 F - Fraction Perrneoted Figure 8. Polyethylene Membrane €or Separation of Ammonia, Hydrogen, and Nitrogen
x"
0
o
0.2
F
-
0.4 0.6 o,e Fraction Permeoted
1.0
Figure 9. Polyethylene Membrane for Separation o€ Sulfur Dioxide, Oxygen, and Nitrogen
138
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 46, No. 4
-is many menibranes such as poiyctjliylcrir, Tiithene, alid Teflon ( t e t r a f l i i o i o c t l i ~ l e i i ~ ~ have great chemical stability, they could p i ' o r t l very useful in separation processes, especially if their permeability ra.tes could be incre through compounding and/or film-processing techniques. PLMTIC~IEMIBRANEG F O E SEI'$ X.\'PI ~ ( i i = 307OC VAPORS. While the Pepsration of vapors, or of gas-vapor mixtures, xva? not considered here, t-he possibility of using plastic membraiics for such separations should be given &ous thought,. Simril and Hershberger ( 1 4 ) have stated that the permeability rates of v a p o i ~are from 1000 to 100,000 times greater than gae permeability rates. Consequently, a much giwiier separation could be obtained per c ~ l la i i t i a o 0.2 0.4 0.6 0.a IO F - Fraction Permeated sizable reduction in the coat of comprei;sow a s F - Fraction Permeated Figure l l . Trithene Film for !Tell as pon.er requirenients would be posiil)le. Figure 10. Cellulose -4cetateSeparation of Carbon Dioxide, Also, the number of capcades required n-oultl he Butyrate Membrane for SepaHydrogen, Oxygen, and Nitrogen reduced. This iiiet,hod of separation ma)prov[' .~ ration of Carbon Dioxide, € 1 ~ u~efulfor gas-vapor sytenis and for VRPOI' drogen, Oxygen, and Nitrogen mixtures. Anothei,possible applicittion of the wp;ri,;i I ion c.!laiacteristics of plastic membranes is in the removai of a ga..cous permeated gas streams a i t h their compositions are shon n in reaction product' evolved during a chemical reaction. Thc reFigure 13 moval of such a gas would result in an increase of the yield oi' the Per pound-mole of pe~meatedgas It nouid be necessary to derired product).when the prcsciice of t,he gas wpprecses the rcaecompress 2 moles of feed gas and t o decompress 1 mole of untion. The reactmion vesi;el could be opei,ated under pressure, n.iwi permeated gas. The use of poaer recovery units would greatly a fluitable support for the film is used, as the membrane assembly reduce the cost of individual compressions Becauqe of the can work D-ith a reasonable partial picsrure differential. CIIrelatively low absolute permeability rates of the films, large areas fortunately, the maximum tcniperature to which ple are needed for gas production in commercial amounts. Thip branes may be heated i R rather Ion. about 60" C. for most plastics separation would require 3.65 X lo4 square feet of area per mole ( 1 1 ) . Exceptions t o t>hisarc special products such as 'I'eflon, (359 standard cubic feet) of gar pelnleated per minute. Trithene, and silicone compounds. While the use of the plastic B. Where high purity cuts are to be obtained, a niultistage membranes would, therefore, be mbject to serious limitat,ioiis, it recycle system would be required. An example is the production is possible t o resort to such membrane materiala a,s ceramics, of anhydrous ammonia, for B hich the final conrentration must be. porous glasp (IO), and thin metallic films. X v H 3 = 0.9995 mole fraction ~ tpresent, , the same principle is being applied in essence, where XHaiXN2= 0.0005 mole fraction Preliminary calculations indicate that an ammonia plant capable of producing 150 tons per day of anhydrous ammonia would require 35 cascades in the ammonia recovery system. As indicated previously, this particular separation would not be 2 Moles economical a t this time. The cost of the compressors alone ~ ~ o u l d be very greatly in excess of the cost of the absorption towers n hich are currently being used to accomplish ammonia recovery XFe = 0,2621 A similar analysis of a system containing sulfur dioxide, oxygen, X y t =0,282/ and nitrogen indicates that its cost also could not compete with E the cost of the current method E for obtaining pure sulfur dioxide k . 060 m Although a t present even the "fastest" films have a relatively 050 low gas transmission rate, memB brane. may be designed which 1s ill 040 permit a much greater f l o ~rate. This increase mould be possible n" E ! 030 I Mole through proper selective com0 pounding of the plastics. To c 0 X t H 3 = 0.270 date, most of the work on plastics e 020 LL ha8 been directed toward reducing X,Hg =0.268 the permeability rate rather than g 0.10 TI 7 . 0 0 A t m s . i n c r e a s i n g it. -4s considerable p =0995Alms. X i z = 0.462 a progress has been made toward X reducing the permeability rates J of plastics, i t would appear that F - Fraction Permeated Figure 13. Material Flow Diagram for "faster" films could also be proSingle-Stage Separation of AmmoniaFigure 12. Trithene Film for SepaHydrogen-Nitrogen System duced if research were directed ration of Ammonia, Hydrogen, along this line. and Nitrogen PolSethylene film v)
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April 1954
INDUSTRIAL AND ENGINEERING CHEMISTRY
plastic membranes are used as covers for farm silos. Here the use of a plastic which possesses low permeability for oxygen and a relatively high permeability for carbon dioxide makes possible the safe storage of silage wit,h a minimum of degradation. The application of plastics in the packaging industry depends largely upon differential gas permeability. For example, packaging materials for fresh fruit? must have low moisture permeability and high carbon dioxide, oxygen, and nitrogen permeability. It is apparent, therefore, that the commercial use of the differential gas permeability of plastic membranes is currently being employed advantageously. With the development of membranes which possess high selectivity and also high permeability, i t is possible that' the use of plastic membranes may hecome competitive with conventional techniques, a t least for specialty applications. CONCLUSIONS
Data on permeability of four plastic membranes to helium, hydrogen, oxygen, nitrogen, and carbon dioxide, as well as ammonia and sulfur dioxide, have been obtained. The plastic films included in this study were polyethylene, trifluoromonochloroethylene, a copolymer of polyvinyl chloride with polyvinyl acetate, and cellulose acetate-butyrate. Most of the gas permeabilities increased exponentially with an increase in the temperature. However, two cases were observed in which this relationship did not exist-that is, the slope of the line resulting from a plot of the logarithm of the permeability constant versus the reciprocal of the absolute temperature did not
139
have a constant value. This anomalous behavior was attributed to the complexity of the permeability constant. A nonexponential variation of either the diffusion constant or the solubility coefficient with the temperature could be responsible for this behavior. Separation data obtained with mixtures of gases or mixtures of gases and such vapors as carbon dioxide, sulfur dioxide, or ammonia agreed remarkably well with the calculated values based upon experimental permeability data. For purposes of calculation, Equations 5, 6, and 12, which were derived on the basis of complete mixing on each side of the membrane, may be used with considerable confidence for binary and multicomponent systems over a rather wide range of pressures and temperatures. These results are in contradiction with those reported by Weller and Steiner (16), who found laminar flow conditions to exist. These equations represent a theoretical expression based upon simplifying assumptions. On the basis of the experiments, the plastic membranes were shown to be a n efficient nonporous medium for the separation of gaeeq. This method of gas or vapor separation is not of economical importance in its present state of development, a t least, afar as the more important large scale commercial operations ai P concerned. However, i t may find limited applications in specific instances where conventional methods are inadequate. The development of better plastic membranes, which have a greatei selectivity and a greater permeability, would, of course, bring thr use of such membranes for separations into a more favorable economic position.
(Separation of Gases by Plastic Membranes)
NEW DIFFUSION CELL DESIGN JAMES 0. OSBURN AND KARL BAMMERMEl*ER Chemical Engineering, State University of Iowa, Iowa C i t y , Iowa
E
QUIPhlENT design for plaPtic film cells is still in the development stage. At present, the only equipment for wing plastic films is that described by Steiner and Weller ( 1 5 ) . They introduced the idea of supporting of the plastic film by a porous material, such as paper, to help the film withstand the necessary high pressure differenlial across it. Their equipment contains a number of alternate layers of paper and film clamped together to form a unit. In the apparatus described in this paper, porous supporting material is used, but the design is otherwise entirely different from that described by Weller and Steiner. The equipment is easy to construct and to make leak-free, and its pel formance in separating gases by diffusion is satisfactory. EQUIPMENT
The apparatus consists of three main parts: an inner conduit, a film of flat plastic tubing wrapped around the conduit, and an outer chamher which encloses the other parts. Details are shown in Figure 1 and Figure 2 is an exploded view of the assembly. Figures 3 and 4 are photographs of the apparatus. Gas enters the outer chamber under pressure, diffuses through the film, and flows through the porous material inside the plastic tubing and through holes into the conduit, leaving through the
outlet. The outer chamber ia a piece of 2-inch standard steel pipe, 5.25 cm. in internal diameter and 30.5 em. long. It is closed at one end by a reducing coupIing which internslly supports the inner conduit. At the other end, the outer chamber is connected by a
reducing coupling and a l/r-inch pipe to a pressure gage and a supply of compressed gas. The inner conduit is a 1-inch pipe 2.7 em. in internal diameter and 35.5 cm. long, capped a t one end and at the other screwed inside a 2-inch t o 1-inch reducing coupling. Loose inside the inner pipe is a steel rod 2.1 cm. in outside diameter and 28 em. long, which reduces the volume of gas in this pipe. I n a line parallel to the longitudinal axis of the pipe, a number of holeq 0.25 cm. in diameter are drilled 1 or 2 inches apart. The surface around the hole is highly polished t o allow the film to adhere tightly. The film, 8 inches wide and 100 em. long, with a 1.5-mil (0.038-mm.) wall thickness, is made of flat polyethylene plastic tubing, heat-sealed at both ends. Inside is a layer of porous paper 18 cm. wide, 100 cm. long, and about 11 mils (0.28 mm.) thick when compressed. It conducts the gas from the inner surface of the film, to which i t has diffused, t o holes near one end of the tubing. The tubing around the holes is sealed to the inner conduit, with pressure-sensitive tape, so that only gas which has diffused through the film can flow into the conduit. As the film is wound around the conduit, a layer of porous paper 0.10 mm. thick is wound with it, so that all of the surface of the tubing is exposed t o the pressure which exists in the outer chamber. As diffusion takes place through film on both sides of the paper, the surface area available for diffusion is 2 X 18 X 100, or 3600 eq. cm. Diffusion does not take place through the tubing where there is no paper to carry away the diffused gases. The diameter of the entire inner assembly is 4.6 cm.
