P
?
Silicone Rubber as a Selective Barrier Gas and VapQr Transfer High permeabilities and flow rates permit a tenfold reduction in surface a r e a requirements in gosews separation wih plostic barriers
WHEN sdicone rubber first became available, interest in its unusual properties did not immediately extend to applications in sheet or film form. To obtain samples of film, it was nenssary to enlist the cwperation of manufacturers active in the field. The first thin sheets of silicone rubber were obtained from the C h e m i d Department of the General Electric Co. These samples were unsupported films, compounded of polymethyMoxane and silica acrogel, essentially 10 mils (0.01 inch) thick. They had been prepared in the laboratory and their preparation was accomplished with some di5cdty. So far, these films have been the only ones available to investigate gas transmittance of silicone rubber without a supporting structure. Silicone rubber in sheet form, such as Silastic and Cohrlastic, is now available. These silicone rubber sheets are !up ported by glass fiber fabric. The thicknesw of the sheets are about the same as or greater than those of the unsupported ilmw-that is, about 10 to 20 mils. One film is available in a minimum thicknesa of about 4 mils. The presence of the supporting fabric makes the true permeability evaluation of Silicone rubber diflicult. None of the supported films have given permeabilities as high as those obtained with the original unsupported material. Nevertheless, permeability and gas separation data of the supported films are as valuable as those obtained for silicone rubber alone, beCause from a swngth standpoint the supported films are the only ones available which could be uscd in equipment. The determination of the permeabilities and the separation experiments were d e d out in equipment deswibcd previously (3,4). Permeability Data The original silicone films were procured as they might exbibit unusual scleftivity for specific gases. This was
true. The permeability toward carbon dioxide was exceptionally high compared wit& all other known plastic film materials. The selectivity of carbon dioxide pamcation in relation to other gases
such as hydrogen, oxygen, and nitrogen also was as high as that of natural rubber and polybutadiene-the two polymers known for preferential carbon dioxide permeation. Also, rates of permeation for hydrogen, oxygen, and nitrogen were much greater than any reported so far for other plastic film. Data on carbon dioxide, hydrogen, and helium permeabilities are shown in Figure 1, and oxygen and nitrogen permeabilities are plotted in Figure 2. The highest value of carbon dioxide permeability obtained on the unsupported films was: 315 X 10'
-_
. (rtd.cc.)(cm. thickness) (sec.)(sq.cm. area)(cm. H g p r e s dmp)
This value is an average of three determinations at 28.5' C. A more consistent average value would be about 270 X 10- at room temperature. Each point plotted in Figure 1 for the carbon dioxide film (GE) curve represents an average of 10 determinations. The points for Cohrlastic 2804 (4-mil thicknesa) are scattered somewhat and may be due to difficulties in obtaining a uniform thickness of the sheet in the low thickness range. A comparison with other plastic films is presented in Tables I and 11. Table I shows permeability data for a variety of plastic filmmaterials and Table I1 gives the selectivity ratios and gas flow rates of the unsupported silicone rubber samples. First, silicone rubber membranes are very much faster membranes than any other films which have been reported upon so far. Secondly, their selectivity for the gases tested is as g o d as that of the best ones known. As silicone films can withstand very much higher temperatures than the organic mataials, and the glass fibersilicone sheet combination is fairly rugged, silicone rubbers dearly constitute very promising barriers for gas and vapor separations. Another feahlre is that for Cohrlastic 3010 separations one can apparently use a material which possesses high flow rate and high sclectlvity. On the other hand, a similarly preferred material apparently is available for oxygen-nitrogea separations in the Cohrlastic 2804 and possibly the Silastic 50 materials. Thus, with the present available information, permeability and flow properties of silicone rubbers can be compounded to meet specific requirements.
+ ,.%.ma Figure 1. Silicone rubber fllms thawing permeabilities of carbon dioxide, hydrogen, and helium Birdwhistell and others (2) have presented some data on Silastic 50 membranes, which showed that permeabilities for hydrogen, nitrogen, and diborane (B&) were very high compared to some other plastic films. These findings partially confirm earlier results of the author. I
Barrier S e p a r i t i a n
8 ' '
A number of separation experiments have been carried out for a variety of gas mixtures. Data for the separation of a nitrogen-oxygen mixture and a carbon dioxideoxygen mixture are given in Tables I11 and IV. The data ..:.
'p$
>
T..C*lll
Figure 2. Silicone rubber film h a w ing permeabilities of oxygen and nitrogen VOL. 49. NO. 10
~
OCTOBER 1957
1685
Table 1.
Table 111.
Permeability Data of Plastic Film Materials P X 109 for given gas (room temperature)
Barrier, Cohrlastic silicone rubber N o 301 0
(std. cc.) (om.) in: (sec.) (sq. cm.) (crn. Hg Ap) Film Type 1. 2.
3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
Manufacturer
Silicone rubber Silicone rubber coated fabric Cohrlastic No. 3010 Cohrlastic No. 2804 Silicone rubber Silastic 50 Cellulose acetate
General Electric Co. Connecticut Hard Rubber Co.
HZ
25
Daw Corning Co. Celanese Corp. of America Du Pont Eastman Kodak Yisking Corp. Du Pont 'DowChem. Co. Dow Chem. Co. Goodyear Tire and Rubber Co. Bakelite Co.
Cellulose acetate Cellulose acetate Polyethylene Polyethylene Ethylcellulose Polystyrene Pliofilm (rubber hydrochloride) Copolymer of poly(viny1) chloride) and poly(vinyl acetate) Natural rubber Polybutadiene
N2
... ...
0.7-0.9 0.8-1.1 0.86 1.90 3.20 9.10 0.16-0.23 1.0
.
10 16
...
I
.
270anb
21 48
130Cvd -2005
26
104f 0.7-2.1
...
.*.
0.8-1.5
COa
0 2
*..
65
Separation of NitrogenOxygen Mixture
...
...
0.12
0.35
0.84 0.78
2.65 2.40
0.007
0.3
1.7
0.81 0.645
2.3 1.9
13.10 13.88
... ...
...
...
...
0.6-0.8 0.9-1.4 1.40 1.20 4.40 3.70 0.06-9.15
ExDerimental unsuuuorted silicone rubber film. Data obtained AugkHt 1951. I n a coated fabric thickness determination of coating is not possible, only total thickness of coated fabric. This may account for difference in P values between 1 and 2. Hence, silicone rubber only, 1, is the more correct value. d Data obtained May 1952. e D a t a obtained November 1954. f Data obtained August 1954. a Reference ( I ) . 6
Pressure Drop Acioss Barrier Fraction Per- Lb 1 Sq. Mole yo 0 2 in nieated, F Inch 61 Permeated Ga1.000 0.436 0.346 0.162 0.064
61
20.2 (feed) 25.9 27.0 29.1 30.4
1.000 0.475 0.300 0.157 0.0343
61.6
28.2 (feed) 34.7 37.6 39.4 41.0
1.000 0.499 0.309 0.106 0.0313
61.5
48.5 (feed) 54.9 88.5 61.6 62.4
1.000 0.707 0.460 0.227 0.052
100
20.2 (feed) 23.4 26.7 29.0 31.5
c
Table Membrane Material
II.
COn Flow P c o ~ I P H ~ P C O I I P O ~ Rate"
...
4.15
1 . O-1. 5 1.63
4.0
Ethylcellulose
1.37
1.65
1.6
Polystyrene
0.4
1.54
1.4
Natural rubber Polybutadiene
5.2
... ...
1.7
... ...
0 2
Rateb
2.1 3.0
100 228 124
...
100
6.2 4.2 4.0
48 74 39 0.4 0.5
...
5.6
0.6
5.7
4.9
7.2
5.1
Flow
P02IPN,
...
2.95 2.5-2, 7d 3.15 2.4-3. 3d 3.1 3.0d 4.3 1.5d 2.8 2 . 1-2.6d 2.96
...
...
1.7
12.6 11.5 1.4 11.0 9.1
Based on silicone 1, Table I. Based on silicone 2, Table I. Average of No. 4,5, 6, Table I. d Reference (6). e Average of No. 7 and 8, Table I. a
b
c
when plotted appear as smooth curves, and are represented very well by theoretical curves computed from the Weller-Steiner equation for complete mixing of the gas on the high pressure side of a barrier ( 5 ) . Silicone resin and rubber barriers may be applied to gas-gas, gas-vapor, and vapor-vapor systems. The selectivities of the barriers investigated have been
1686
yo COZin EnFraction of riched FracGas Mixture tion (FracPassing tion Perthrough meated Ohrough Membrane yGof Feed Membrane)
Ratio of Permeability Values
General Electric samples Cohrlastic No. 3010 No. 2804 Silastic 50 Cellulose acetateC Polyethylene"
Poly(viny1 chloride)-poly(vinyl acetate)
Table IV. Separation of Carbon Dioxide-Oxygen Mixture
very good, and the rates of permeation are of a magnitude that a one hundredfold reduction in operating areas can be expected compared with other films available to date. Literature Cited
(1) Amerongen, G. J. van, J . Polymer Sci. 5 , 307 (1950).
( 2 ) Birdwhistell,
INDUSTRIAL A N D ENGINEERING CHEMISTRY
R. K., Quill, L. L.,
37.5 61.8 61.9 76.1 80.7
Film 1 74.2 63.5 63.3 60.4 59.1
% Enrichment over Feed Mixture 21.0 10.3 10.1 7.2 5.9
Film 2 74.3 21.1 35.0 66.1 12.9 63.1 66.0 12.8 66.9 82.7 58.4 5.2 Barriers Molded Silicone Rubber Sheets (General Electric C o . ) Permeability ratios = Pco,/Pon ~ . ~. - = Film 1 = 5.1 Film 2 = 6.8 Operating conditions, room temperature Pressure drop over barrier = 50 lb./sq. inch Feed to barrier = 53.2% COz 46.870, 0 2 Values for concentration changes, when plotted against fraction permeated for both films, give a rather straight line. At 50% of gas mixture passing through the membrane (a practical operating value) change in concentration is 16.5% of COr-an excellent separation for one step.
Johnson, R . E., Chem. Eng. News 34, 4690 (1956). ( 3 ) Brubaker, D. W.: Kammermeyer, Karl, IND. END. CHEhI. 44, 1465-7 119521. (4) Zbihl, 451-1148-52 (1953). ( 5 ) Weller, Sol, Steiner, Waldo, J . Ap&l. Phys. 21, 279--83 (1950). RECEIVED for review February 1 3 1957 ACCEPTEDMay 3, 1957 Investigation carried o u t in cooperation with Selas Corp. of America.
