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KARL KAMMERMEYER and DARRELL D. WYRlW State University of Iowa, Iowa City, Iowa
Effect of Adsorption in Barrier Separation A gas mixture composed of constituents having essentially the same molecular weight, can be separated in barrier flow strictly on the basis of adsorption behavior
SEPARATION
of gases or vapors by barriers is not yet widely used, except in the Atomic Energy Commission's gaseous diffusion plants. The effect of adsorption upon the separation of mixtures is still pretty much of an unknown quantity in flow through barriers. Recent publications (7, 3, 6-8, 70) have stressed the influence of adsorption upon the flow of gases and vapors in microporous media. A satisfactory theoretical interpretation of the relationship between adsorption and the magnitude of adsorbed flow has so far not been developed, even though a semiempirical relationship for single component systems has been proposed (7). The separation of gaseous mixtures by microporous barriers, therefore, should be affected by the differences in adsorbability of the components. T o investigate this effect, separation experiments were carried out with mixtures of propane and carbon dioxide in flow through porous glass ( 9 ) as the microporous barrier. Because these two components have essentially the same molecular weight, there would be almost no separation due to molecular flow, and passage of any mixtures of these gases through a barrier should give no change in composition unless adsorption had a controlling effect. The experimental separation results strikingly illustrate that appreciable separation does occur, and that the component which is enriched in passage through the barrier is propane. This means that propane must pass the barrier at a faster rate than carbon dioxide. Actually, this faster rate is caused by so-called adsorbed (or condensed) flow, which is greater for a more adsorbable material. Properties for the two components as reported in the literature are:
Propane, possessing the higher critical temperature, should be the more condensable component, and therefore should exhibit the greater amount of adsorbed flow. That this qualitative relationship holds for other gaseous components has been shown by Hagerbaumer and Kammermeyer ( 4 ) in the separation of an azeotropic mixture of benzene and methanol and a number of similar systems. More recently a quantitative relationship has been presented for the adsorbed flow of gases and vapors as a function of either boiling point or critical temperature (8). The adsorption of individual components from a mixture of vapors is not as yet amenable to theoretical interpretation. There is definite evidence that selectivity plays a major role, but it is not possible to predict readily which component is adsorbed preferentially. Preliminary results for the adsorption of propane and carbon dioxide on a porous glass barrier show that the individual components adsorb almost equally well, as pure components, at 25' C. and up to about 3-atm. absolute pressure. Beyond this pressure propane begins to adsorb more strongly than
carbon dioxide. The adsorption behavior of this particular system is now being studied in detail. I t is of considerable theoretical and practical interest that adsorbed flow behavior is controlling in microporous barrier separation. Actually a similar situation exists in flow of gases and vapors through plastic film-type barriers, where the diffusive solubility flow is controlling (7).
Equipment and Procedure The barrier was mounted in a pressure cell with provisions for feeding the gas mixture and taking off an unpermeated stream on the high pressure side. The permeated stream (enriched in propane) was taken off at the low pressure side. I n most respects the pressure cell resembled that used by Huckins ( 5 ) . Temperature of the constant temperature bath was within 2~0.1' C. over the range of -5' to 25' C. Gage pressure in the cell was maintained within 10.04 inch of mercury over the range of 10 to 40 inches. All gas flow rates (three streams) were measured within &l.O%. Operation was carried out at three feed
Physical Properties
Propane Molecular weight 44.094 Boiling point, O K, 231 Critical O
temp.,
368.8
Carbon Dioxide 44.010 194.7 (sublimes) 304.3
K.
Critical pressure, atm.
43
73.0
Present address, Archer-Daniels-Midland Co., Minneapolis 2, Minn.
FRACTiOH PERMEATED ( Figure 1.
e)
Changes in pressure drop at constant temperature VOL. 50, NO. 9
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-- W E L L E R - S T E I N E R CURVE -EXPERIMENTAL CURVE 0.4 0.6 0.8 FRACTION PERMEATED ( 0 1
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Changes in temperature at constant pressure
compositions-0.25, 0.50, and 0.75 mole fraction of propane. The extent of molecular or Knudsen flow of the components through the porous glass barrier was computed from helium measurements (8). Some earlier data on the separation of propane and carbon dioxide had been obtained by Brubaker ( Z ) , who had not succeeded in obtaining a sufficient number of runs with satisfactory material balances. I n the present work particular attention was paid to the necessity of checking a11 streams to obtain good material balances. Results
A total of 157 experimental separation runs were made. The maximum deviations of material balances were +4.97 and -3.7%, and 85% of all runs agreed within f2% of the material balance. Separation and, therefore, enrichment varies with the fraction 0 of the feed mixture which permeates the barrier. I t is greatest when only an infinitesimal amount permeates, a t e = 0, and nil when all of the mixture passes through the barrier, at 0 = 1.0. Representative sets of data are shown in Figures 1 and 2. Figure 1 gives separations obtained a t various pressure drops over the barrier a t a constant temperature, and Figure 2 presents data a t different temperatures for a fixed pressure drop. Separation data are often conveniently correlated by means of the Weller and Steiner equations (77, 72), which are reasonably valid if the amount of adsorbed flow is rather small. However,
0.501 0
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Figure 3. When adsorbed flow controls separafion, the equations do not express data well
when the adsorbed flow actually controls the separation, the equations do not express the experimental data to a satisfactory degree. This is illustrated by Figure 3, where experimental data for one set of conditions are shown with curves computed from the Weller and Steiner equations. The PT and PS curves were computed by using total observed permeability ratios (PT) and “adsorbed flow” permeability ratios (PS), respectively. The adsorbed flow values were obtained by calculating molecular flow pefmeabilities from helium measurements and subtracting these values from the total observed permeabilities. The PT and PS numbers represent the ratios of the permeabilities of propane and carbon dioxide a t the appropriate pressure and temperature. Sufficient data were obtained to show that the Weller and Steiner equations would fit the data much more closely when the adsorbed flow component was very great; then the PS curve can be used with good approximation. Summary
Experiments have shown that components of essentially equal molecular weight can be separated on the basis of differences in adsorption on a microporous barrier. Mixtures of propane and carbon dioxide gave appreciable enrichment in propane, the more condensable component, entirely on the basis of adsorbed flow. Therefore, vapor mixtures can effectively be separated with barriers by utilizing differences in the adsorbed flow of the components by
1 3 10 INDUSTRIAL AND ENGINEERING CHEMISTRY
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proper adjustment of operating conditions. Studies are under way to establish the degree of adsorption of propane and carbon dioxide on the porous glass barrier under the conditions of the separation experiments. Acknowledgment
The support of the studies by the United States Atomic Energy Commission is gratefully acknowledged. Literature Cited
(1) Barrer, R. M., Strachan, E., Proc Roy.Soc. (London) 231, 52-74 (1955). (2) Brubaker, D. W., Kammermeyer, K., Proceedings, 1953 Conference on Nuclear Engineering, Univ. of California, Berkeley, F-9 to 28 (Sept. 9-11, 1953). (3) Carman, P. C., “Flow of Gases through Porous Media,” Academic Press, New York, 1956. (4) Hagerbaumer, D. H., Kammermeyer, K., Chem. Eng. Progr. Symj. Series, No. 10,50,25-44 (1954). (5) Huckins, H. E., Kammermeyer, K., Chem. E n g . Progr. 49, 180-4 (1953). ( 6 ) Kammermeyer, K., A.1.Ch.E. Symposium, September 1957, Baltimore. (7) Kammermeyer, K., IND.ENG. CHEw 50, 697-702 (1958). (8) Kammermeyer, K., Rutz, L. O., A.1.Ch.E. Symposium, September 1957, Baltimore. (9) Nordberg, M. E., J . A m . Ceram. SOL 27,299-305 (1944). (10) Russell. J. L.. Ph.D. thesis. Massachusetts Institute‘of Technolor;. 1955. (11) Weller, S., Steiner, W. “A; Chem. Eng. Progr. 46, 585-90 (1950). (12) Weller, S., Steiner, W. A,, J . Appl. Phys. 21,279-83 (1950). RECEIVED for review January 27, 1958 ACCEPTED March 31, 1958
