Heats of Adsorption and Relative Adsorbability bf Some Gaseous J
Hydrocarbons SILICA GEL AND CARBON BLACK ADSORBENTS 'WALTER R . SMITH
RALPH A. BEEBE
Godfrey L. Cabot, Inc., Boston, Mass.
Amherst College, Amherst, Mass.
I t is demonstrated that 1-butene is selectively adsorbed by silica gel from a mixture of 1-butene and butane in the vapor state. The heats of adsorption for 1-butene and butane on silica gel have been determined calorimetrically and it is found that the heat of 1-butene exceeds that of butane by nearly 3 kg.-cal. per mole in the first adsorbed layer. Calorimetric measurements reveal little difference in the heats of adsorption of 1-butene and butane on
carbon black and of propene and butane on silica gel especially in the monolayer. In a limited number of experiments, the authors have failed to obtain any evidence of separation of 1-butene from butane over carbon black or of propene from butane over silica gel. Thus in the systems studied a correlation appears to exist between selective adsorption from a hydrocarbon pair and the relative values of the heats of adsorption of the two components.
A
ing bottles and a device, L, for maintaining constant pressure as the mixture was assed from one reservoir t o the other through the adsorption ce6, C, containing the silica gel sample. All oonnections between the adsorption cell and the reservoir were 2mm. capillary. The volume of the cell was approximately 7 ml. The adsorption cell was immersed in a manually controlled thermostat which permitted a constancy of about 0.5" C. during the run. The apparatus was connected to the pumps as indicated, and the silica gel evacuated at 180' C. prior to each run. Synthetic mixtures of the hydrocarbons were prepared by measuring the appropriate volumes in B and thoroughly mixed by passing the sample between reservoir R and the buret. The entire sample was finally transferred t o R and assed alowly through the silica gel by proper manipulation o r t h e mercury leveling devices a t R and R'. Each pass requjred about 4 minutes. Six to eight passes were made and the gas mixture was analyzed; a second set of passes was made and the mixture once again analyzed. I n all instances, the analyses showed that equilibrium was reached well within the first set of passes. -4t the conclusion of each run the major portion of the unadsorbed vapor mixture was isolated in reservoir R, the pressure in cell C and the "dead space" being carefully adjusted to 760 mm. The unrtdsorbed mixture was then transferred to B, the volume was measured, and the composition was determined by analysis. The adsorbed vapor was then desorbed from the gel by heating C to 100' C. By using R as a Toepler pump the desorbed vapor was collected in B for measurement and analysis. In this manner less than 1 mm. of gas was left
WMBER of investigators (1,5, 6,8-10)have reported the successful use of adsorption columns for the separation of
hydrocarbons from mixtures either in the liquid or the vapor phase. For example, Mair (9) has reported the small scale separation on a quantitative basis of such liquid mixtures as 2,2,4-trirnethylpentane, 2,4,4-trimethyl-l-pentenel and toluene using silica adsorbent in a column. In separations of this sort, advantage is taken of the differences in adsorbability of the various hydrocarbons on the adsorbent surface. These differences must depend t o a considerable degree on differences in the energies of binding, and, therefore, might be expected t o manifest themselves in differences in the heats of adsorption. A calorimetric method has been developed in the Amherst laboratory for the direct measurement of heats of adsorption of hydrocarbon vapors of low molecular weight ( 2 , s ) . This method has now been applied to the determination of the heats of adsorption of butane and of I-butene on a silica gel sample, and the authors have tested relative adsorbabilities of these two hydrocarbons on silica gel. In addition, the heat of adsorption of propene has been measured and relative adsorbabilities of propene and butane have been tested on silica gel. I n an investigation previously reported, it was shown that unlike the silica adsorbent, a carbon black surface exhibits almost identical binding energies for 1-butene and butane within the monolayer. I n a limited number of experiments no evidence has been obtained for vapor phase separation of I-butene from butane over a carbon black surface. In the systems considered, the authors appear to find evidence of a correlation between the relative values of the heats of adsorption of the components of a mixture and the relative adsorbabilities of these components. EXPERIMENTAL
APPARATUS.The calorimeter employed in the present measurements has been described ( 3 ) . The apparatus for the measurement and admission of the hydrocarbon vapors, out of contact n-ith stopcock grease, was an improved modification of that described by Wendell ( 1 2 ) . The improvement devised by Wendell consisted in the substitution of sintered-glass disk mercury cutoffs for the part of the train leading from the hydrocarbon supply to the measuring buret. No change was made, however, in the adjustable capillary syringe cutoff between the buret and the adsorption chamber. I n Figure I is illustrated the Pyrex apparatus used for testing the vapor phase separation of the hydrocarbon mixtures in contact with the adsorbent surfaces; this consisted of a 100-ml. gas buret, B , with pressure compensator for measuring gas volumes, and two 200-ml. reservoirs, R a n d R', equipped with mercury level-
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Figure 1. Apparatus for Testing Vapor Phase Separation
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol, 41, No. 7
One silica sample, used in thc hcat measurements only, was supplied by Total Davisoii Chemical Corporation and KO.of Volume T'olume Volume Residual Relative Fraction Diff. Heat designated as No. 22-08-X1926; this Admitted, Adsorbed, Adsorbed, Pressure, Pressure, of Monoof Adsorption, Increwas a 200-mesh gel of 724 square hIl. 311, M1. p/po layer, v/v, b Kg,-Cal,/Mole ment 4Im. meters snecific surface area. and was 4.50 4.50 1.9 0.002 0,049 4.56 10.90 identical* with the material' used by 0.007 10.17 5.1 5.67 0.111 10.09 5.78 Mair (9, 10). (This figure, 724, is 7.05 17.22 10.1 0.188 0.013 9.60 7.21 25.50 8.28 0,279 0.024 18.2 8.55 9.28 4 based on the weight of the silica 33.26 7.76 0,364 0.035 27.2 8.06 9.32 5 samples afrer drying in vacuo a t 0.052 42.15 0.461 40.0 8 89 9.31 9.25 6 200" C. During this drying process 30.31 54.4 8.16 0.070 0,550 8.64 8.77 7 14.28 8 5 . 1 0.110 64.59 0.706 15.30 . . . 8 the loss of weight due to removal 122.9 0.159 78.36 13.77 15.02 0.856 ... 9 of volatile material was 2.0% of the 88.05 153.7 0.198 9.69 8.55 0,963 10.70 10 dry weight.) The other silica used a Adsorbent, 1.578 g. of silica gel 22-08-Xl926 a t 0' c.; run 73; P O = 775.4 mm.; e m = 57.9 ml./g.: both in heat measurements and in galvanometer on 1/fi sensitivity. vapor phase separations was Davison b Based 011 value of u , the total volume adsorbed. Xo. 659528-2000 of 28- to 200-mesh and 534 square meters specific surface area. All surface areas were deTABLE 11. ADSORPTION DATAFOR BUTENE^ termined by nitrogen adsorption by Total the method of Brunauer, EmmetJ, Residual Relative Fraction Diff. Heat Volume Tolume Volume N ~of. of Monoof Adsorption Admitted, Adsorbed, Adsorbed, Pressure, Pressure, Inoreand Teller ( 4 ) assuming 16.2 sq. A. Ml. MI. p/po layer. v/vmb I
