I
J. N. REEDS’ and KARL KAMMERMEYER State University of Iowa, Iowa City, Iowa
Adsorption of Mixed Vapors The existence of adsorption azeotropes was established in vapor adsorption studies on the benzene-methanol system. This fact has implications in all separation processes using adsorption methods
WHEN
azeotropes in the vapor state are separated by diffusion through a microporous barrier (5),the component which diffuses most rapidly does not always have the lower molecular weight. This seemingly anomalous behavior results from adsorption (8-70). However. the rate a t which a component diffuses does not always depend on the degree of its adsorption on the barrier. Selective adsorption occurs (2, 72. 7 5 ) and the adsorption isotherm (or isobar) of a vapor mixture cannot be predicted from the isotherms of the individual pure components ( 3 ) . Therefore, to establish if adsorption is selective and to what extent. the system methanolbenzene \cas investigated.
Experimental Materials. The adsorbent was porous Vycor glass, which had been used as the barrier in the separation experiments with azeotropes (5). Its physical properties have been reported (7, 5, 76). The actual sample of adsorbent consisted of three blocks of porous glass, approximately 0.7 X 1.2 X 2.0 cm. weighing a total of 9.1470 grams (Figure 1). To remove impurities adsorbed from the atmosphere. the blocks were normalized by heating in an electric furnace a t 500’ C. for 50 hours. Readsorption of components of the air during cooling probably had a negligible effect due to the much greater adsorption of the vapors used in the investigation. The adsorbate was a mixture of methanol and benzene, and a number of different vapor mixture compositions were investigated. This system is nonideal, and the variation of its azeotropic composition a t various temperatures and pressures is shown in Table I. hfallinckrodt analytical reagent grade materials were used without further purification. The benzene was stored over sodium wire. Gas chromatographic analyses showed no traces of impurities. Apparatus. T h e adsorption studies
Present address, National Science Foundation, Washington 2 5 , D. C.
weye carried out by generating vapor mixtures in an adsorbate boiler and mrasuring adsorption with a spring balance of the McBain type (74). Details of the apparatus are shown in Figure 2. The portion of the equipment located below the jacketed helix case was immersed in a constant-temperature bath. The adsorbent was suspended on a glass hook which passed through the large-bore stopcock; G. When the stopcock was closed: the hook would break and the adsorbent \vas isolated in the adsorbent chamber, K . A laboratorybuilt canned rotor pump was used in some runs in vessel R for vapor circulation. Vibrations from the pump caused loosening of joints and the pump had to be removed. Vapor sampling was done with a 10ml. sample bulb fitted with a 10 ’30 borosilicate glass taper male joint a t one end, and with a glass stopcock a t each end. Liquid samples of adsorbate were obtained by desorption under vacuum from chamber K through a cold trap immersed in a dry ice-acetone bath to freeze out the desorbed vapors. Procedure. Before making adsorption measurements, the adsorbent chamber with the adsorbent in place was evacuated for 4 hours. T h e adsorbate boiler was then charged with about 110 ml. of benzene-methanol mixture of an
approximate composition to yield a vapor composition M ithin a desired range. The amount of liquid in the boiler was sufficient so that the change in composition of the liquid, due to preferential adsorption from the vapor and from repeated vapor sampling during a run. was rather small. The temperature and pressure in the adsorption apparatus were adjusted to the required level (Table I ) , and vaporization was started. T h e room temperature was kept slightly higher than that of the equipment to avoid condensation. The increase in weight of the adsorbent was followed by a cathetometer focused on a reference point of the hrlix arrangement. No correction was made for buoyancy in view of the rather low density of the vapor a t the prevailing subatmospheric pressures. The end of adsorption-that is, equilibrium--was
Table 1. Azeotropic Composition and Pressure of Methanol-Benzene Liquid Mixtures a t Several Temperatures This system i s nonideol
Tot a1
Mole Fraction of Methanol
Temp., O
C. 15 25 26 35 42
Mm. Hg Abs. Licht Measured Calcd. and Horsley Rao Denzler
Reeds
...
0.525
... ... ...
(6)
(17)
(IS) 79.45 129.25
0.578
0.5139 0.5347 0.5368 0.5556 0.5702
... ... 0.557 .. .
...
203.73
...
Figure 1. The adsorbent specimens consist of three porous glass blocks VOL. 5 1 , NO. 5
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707
A.
1-cm. diameter helix, about 200 turns of No. 28 stainless steel wire B . Double-walled glass case for helix, 25-mm. tubing inside and 40-mm. tubing outside. C . Wire extension hook, No. 28 stainless steel wire, length about 25 cm. D. T 29/42 taper joint E . Glass suspension hook, diameter about 1 mm. and length about 25 cm. F. 29/42 taper joint G. 1 0-mm. bare stopcock H. 4-mm. bore three-way stopcock 1. T 1 0 1 3 0 female joint for cold trap and vacuum pump connection 1. 5-mm. tubing for cold trap and vacuum pump connection K. Double-walled adsorbent chamber, 25-mm. tubing inside and 35-mm. tubing outside 1. 29/42 taper joint M. Outlet for constant temperature water jacket N. T 29/42 taper joint 0.Inlet far constant temperature water jacket P. 18/9 b a II a n d soc k e t joints Q. 6-mm. bore stopcock R. 25-mm. diameter case for "canned rotor" circulating Pump S. T 24/40 taper joint for adsorbate boiler 150-ml. adsorbate boiler U. Three blocks of Vycor porous glass adsorbent about 0.7 X 1.2 X 2.0 cm. All eauipment is made of . . borosilicate glass, except as stated
-
Figure 2.
Adsorption apparatus
attained when no further elongation of the spring was observed. During adsorption, six periodic vapor samples of about 10 ml. were taken from the vapor space of the adsorbent chamber for analysis. When adsorption was complete, the adsorbent and the adsorbent chamber were sealed from the rest of the equipment by closing all stopcocks leading to the adsorbent chamber. The chamber was evacuated by a vacuum pump through a cold trap to collect the adsorbate removed during the desorption process. When desorption was complete, the liquid sample collected in the cold trap was allowed to warm rapidly to room temperature, and then the refractive index was measured with a Bausch and Lomb precision refractometer. Composition of the liquid sample was determined from a calibration curve of refractive index as a function of the composition, Figure 3. The extent of adsorption and desorption could usually be observed visually, because the optical characteristics of the sample of adsorbent changed markedly during both processes.
Adsorption. Before any adsorption was started, the adsorbent had a translucent appearance, which permitted printing to be read through it. As adsorption proceeded, the adsorbent
708
-
r.
exhibited a white and very cloudy appearance. Light would scarcely pass through the sample, and even then in a highly diffused state. When the adsorption was almost complete, the adsorbent became very transparent, and vision through the sample was far better than that even for the nonadsorhed state.
Desorption. Almost immediately upon the starting of desorption by applying a vacuum, the adsorbent became very cloudy again. As desorption progressed, the original translucent appearance returned at the edges of the sample, leaving only the center of the samples of the adsorbent with a cloudy appearance. When desorption neared completion, the translucent portion approached the center of the adsorbent, and the cloudy portion eventually disappeared completely when desorption was finished. The vapor samples were taken to provide some vapor circulation and also to establish the vapor composition around the adsorbent during adsorption. The samples were analyzed by planimetric graphical integration of the chromatograms obtained with a Beckman 17300 gas chromatograph containing a silicone grease column. When adsorbent was released from the spring at the completion of adsorption,
INDUSTRIAL A N D ENGINEERING CHEMISTRY
the wire helix rebounded against the top of the case. This caused a permanent change in the characteristics of the spring and necessitated recalibration after each run. All calibrations were carried out at the appropriate temperatures.
Discussion of Results The primary objective was to establish the relationship between the compositions of adsorbed and vapor phases. As the adsorbent samples were surrounded by a vapor atmosphere at the end of adsorption, it was necessary to correct the composition of the desorbed sample for the amounts of benzene and methanol present in the vapor space of the adsorbent chamber. This was done by using the analyses of the vapor sample and calculating, by the gas laws, the amounts of constituents from the known free volume of the chamber (197.7 ml.). A considerable number of runs were made, but many of the results were in error due to difficulties with the chromatograph in analysis and air leakage into the equipment (detected chromatographically). T h e results which are considered reliable are summarized in Table I1 for temperatures of lj', 25', and 35' C. Figure 4 shows the data for 25' C. together with a vapor-liquid equilibrium curve and a computed adsorption equiTable II. Readings Which Are Out of Line Are Shown to Give an Idea of Actual Data Obtained G. Adsorbate/ Mole Fraction G. G. of Methanol Adsorbate Adsorbent Adsorbate Vapor At 15' C. 1.5477 1.5337 1.5302 1.5288 1.5213 1.5106 1.4691 1.4562
0.1692 0.1677 0.1673 0.1671 0.1663 0.1651 0.1606 0.1592
1.7763 1.6827 1.6821 1.5720 1.7079 1.7084 1.5614 1.5934 0.6721 1.4863 1.3308 1.3007
0.1942 0.1840 0.1839 0.1718 0.1867 0.1868 0.1707 0.1742 0.0735 0.1625 0.1455 0.1422
1.6574
0.1812
1.6495 1.6019 1.6449 1.9591 1.5490 1.6142 1.6231 1.7846 1.8151 1.6575 1.5568
0.1803 0.1751 0.1798 0.2142 0.1693 0.1765 0.1774 0.1951 0.1984 0.1812 0.1702
0.0000 0.4977 0.5524 0.6340 0.7206 0.7934 0.9651 1 .OOOO
0.0000 0.7592 0.7028 0.7282 0.7521 0.7642 0.8262 1.0000
At 25' C. 0.0000 0.3171 0.3890 0.4070 0.5320 0.6327 0.6414 0.6429 0.7441 0.7721 0.9661 1 .OOOO
0.0000 0.4702 0.5302 0.5405 0.6221 0.6752 0.6615 0.6656 0.6903 0.7380 0.7592 1.0000
At 35' C.
...
...
0.0000 0.0146 0.2781 0.2915 0.3206 0.3998 0.4927 0.5878 0.5893 0.7085 0.7482 0.9386 1.0000
0.0000 0.0682 0.4555 0.4833 0.4528 0.5242 0.4439 0.6318 0.5719 0.6802 0.6883 0.7451 1.0000
VAPOR ADSORPTION 1.50 I
\
I
I
\.
Refractive Index E 1.38
(t = 250
c., a= 5893
A.)
versus Mole Fraction Methanol in a
1.34
Methanol -Benzene Solution \
I
I
0.2 Mole Fraction Methanol Figure 3. Composition of liquid benzene-methanol mixtures can b e determined from refractive index curve
librium curve. This latter curve represents an attempt to use a van Laar type of correlation ( 7 7 ) for predicting the complete curve from the azeotropic composition (4)and vapor pressure data (7). The most significant finding is the existence of an adsorption azeotrope in analogy to vapor-liquid azeotropes. T h e shift of the azeotropic composition with temperature is summarized in Table 111. Perhaps the greatest source of error in the data could come from an appreciable change in liquid composition in the adsorbate boiler. This possible error can be estimated. As a n extreme case, each time a vapor sample was taken, a 100-ml. volume was removed from the system at the prevailing pressure to provide flushing and the 10-ml. vapor sample. For six samples, this amounts to 600-ml. vapor, or about 0.4 gram of liquid. The amount of liquid removed as adsorbate during a 25” C. run would be perhaps 1.7 grams. For a n initial boiler content of 110 ml. (-90 grams), total liquid change would If all be around 2.1/90 or 2.3%. of this change were to occur in one component, then the maximum change of the vapor composition during adsorption could have been 7y0 in the adsorption azeotrope composition (Figure 4), and a Table Ill.
Temp., O
c.
15 25 35
Adsorption Azeotrope Shift with Temperature
Mole Fraction. Methanol in Constant boiling
Adsorption
liquid
azeotrope
0.5139 0.5347 0.5556
0.764 0.680 0.664
0.4
I
0.6
1.0
0.8
Adsorbate Mole Fraction Methanol liiiililllllllll Liquid Mole Fraction Methanol Figure 4. Adsorption vapor-adsorbate equilibrium for benzene-methanol vapors on ‘porous glass shows that some changes occur in vapor composition during adsorption
similar shift in the other points of the curve. Actual refractive index measurements in a number of runs indicated changes in M e O H composition of 1.3 to 1.6 wt. % in the liquid. T h e appearance of the data plots is such that a fair accuracy is indicated. Conclusions
At the three temperatures investigated-that is 15”, 25’, and 35” C.-benzene was preferentially adsorbed from the azeotropic vapor-liquid equilibrium mixtures. This could definitely account for the preferential flow of benzene from an azeotropic vapor through a porous glass barrier as found by Hagerbaumer and Kammermeyer ( 5 ) , because all of their experimental conditions were such that flow studies were made with benzenerich compositions. However, at very high concentrations of methanol in the binary mixture of vapor, on the methanol-rich side of the azeotropic mixture, methanol, rather than benzene, was preferentially absorbed. Thus in the intermediate region, a composition must exist which would absorb unchanged in composition. The equilibrium between the composition of the vapor and the composition of the adsorbate bears a graphic similarity to common isothermal vapor-liquid equilibria for minimum boiling mixtures of binary constituents (see Figure 4). Therefore. this cross-over composition is called an adsorption azeotrope. T h e curve for vapor-adsorbate equilibrium is, however, shifted toward them ethanol-rich side of the vapor-liquid equilibrium sys-
tem. This shift is more pronounced a t lower temperatures or lower pressures. literature Cited
(1) Amberg, C . H., McIntosh, R., Can. J . Chem. 30, 1012 (1952). (2) Bakr, A. M., King, J. E., J. Chem. SOC. 119, 454 (1921). (31 . , Burkhardt. R. W.. M.S. thesis, State University df Iowa,‘1955. (4) Carlson, H. C., Colburn, A. P., IND. ENG.CHEM.34, 581 (1942). (5) Hagerbaumer, D. H., Kammermeyer, K., Chem. Eng. Progr. Symposium Ser. 50,NO. 10, 25-44 (1954). ( 6 ) Horsley, L. H., Anal. Chem. 19, 603 (1947). (7) Jordan, T. E., “Vapor Pressure of Organic Compounds,” Interscience, New York. 1954. (8) Kammermeyer, K., IND.ENG. CHYM. 50, 697-702 (1958). (9) Kammermeyer, K., Brubaker, D. il’., Chem. Eng. Progr. 50, 560-4 (1954). (10) Kammermeyer, K., Wyrick, D. D., IND.ENG.CHEM.50, 1309-10 (1958). (11) Laar. J. J. van, 2. .phvsik. _ Chem. 72, 723 (19iO). (12) Lewis, W. K., Gilliland, E. R.; Chertow, B., Cadogan, W. P., IND. ENG. CHEM.42, 1319 (1950). (13) Licht, W., Jr., Denzler, C. G., Chem. Eng. Progr. 44, 627 (1948). (14) hkBain, J. W., Bakr, A. M., J. A m . Chem. SOC.48, 609 (1926). (15) Manegold, E., “Kapillarsystemc,” Chemie und Technik, Heidelberg, 1955. (16) Nordberg, M. E., J . A m . Ceram. Soc. 27, (10) 299-305 (1944). (17) Rao, V. N. K., Sarma, K . J. K., Swami, D. R., Rao, M. N., J . Sci. Ind. Research (India) 16B, 4 (1957). RECEIVED for review August 27, 1958 .~CCEPTED January 7, 1959 Work done under financial support of the United States Atomic Energy CoinI
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mission. VOL. 51, NO. 5
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