A great deal of work has been done in the measurement of adsorption isotherms of pure gases and vapors, but very little of the measurement of adsorption isotherms of mixed gases or vapors and much of this has been unintentional, being due to impurities in the adsorbate. Papers dealing with measurements of this type include those of Richardson and Woodhouse (2) and Bakr and King. (3) The method of Bakr and King has the objection that each experiment yields an isolated value. Richardson\u27s and Woodhouse’s method requires extensive gas analysis and yields an isotherm in which the composition of the adsorbate changes

Innes, W. B.

Rowley, H. H.

English

University of Northern Iowa

Proceedings of the Iowa Academy of Science Volume 48 Annual Issue Article 56 1941 Measurement of Adsorption Isotherms for Mixed Vapors H. H. Rowley State University of Iowa W. B. Innes State University of Iowa Copyright ©1941 Iowa Academy of Science, Inc. Follow this and additional works at: https://scholarworks.uni.edu/pias Recommended Citation Rowley, H. H. and Innes, W. B. (1941) "Measurement of Adsorption Isotherms for Mixed Vapors," Proceedings of the Iowa Academy of Science, 48(1), 253-257. Available at: https://scholarworks.uni.edu/pias/vol48/iss1/56 This Research is brought to you for free and open access by the Iowa Academy of Science at UNI ScholarWorks. It has been accepted for inclusion in Proceedings of the Iowa Academy of Science by an authorized editor of UNI ScholarWorks. For more information, please contact scholarworks@uni.edu. l\IEASCREl\IENT OF ADSORPTION ISOTHERl\IS FOH l\IIXED VAPORS H. H. ROWLEY AND \V. B. INNES A great deal of work has been done in the measurement of ad-sorption isotherms of pure gases and vapors, but very little of the measurement of adsorption isotherms of mixed gases or vapors and much of this has been unintentional, being due to impurities in the adsorbate. Papers dealing with measurements of this type in· elude those of Richardson and Woodhouse (2 ) and Bakr am! King.(~) The method of Bakr and King has the objection that each experiment yields an isolated value. Richardson's and \'lood-house's method requires extensive gas analysis and yields an iso-therm in which the composition of the adsorbate changes. Pressure data obtained where the composition of both the ad-sorbate and the vapor in equilibrium with it are unknown are dif-ficult to interpret and reproduce. Because of this we have at-tempted to develop a method of measurement so that the composi-tion of the adsorbed phase could be known and kept constant throughout the isotherm. Also we wished to know the composi-tion of the vapor phase in equilibrium with the adsorbate in order to obtain the partial pressures as well as the total pressures. A schematic cliagrarn of the apparatus de;-cloped for this pur-pose is given in figure I. The measurement of an isotherm is car-ried out in the following manner. A relati;-cly large amount of liquid mixture of known composition is introduced into the liquid reserrnir by means of tnbc (2). Considerable excess over the amount actually adsorbed is used so that the change in composi-tion of the liquid in the reservoir is negligible during an experi-ment. After sealing off tube (2) the dissolved air is removed by repeated freezing and evacuation until no bubbles are visible on thawing. Stopcock ( 3) is then closed and the tube containing the adsorbent ( 7) is sealed on to the apparatus. The adsorbent ( acti-vated cocoanut charcoal was used in this work) is then ontgassed by e\'acuation at the desired temperature (525°C.) until the l\Ic-Leod g-auge ( 8) g-fres a pressure of less than 2 X 10-4 mm. Hg. The vacuum system employed consists of a mercury condensation pump hacked up hy a Hyvac oil pump. The introduction of the adsorbate into the adsorbent is accom-plished by first opening stopcock (3) and allowing the liquid at 20°C. to equilibrate with its vapor phase in the vapor resen'oir 2.513 1Rowley and Innes: Measurement of Adsorption Isotherms for Mixed VaporsPublished by UNI ScholarWorks, 194125·b 3 1 liquid teservoir IOWA ACADE:\IY OF SCIENCE [VoL. XLVIII 2 S°C. Air Bath 110.por reservoir 7 adsorbent b 8 modified M' leod 9ou~e b Figure 1. Schematic diagram of apparatus. ( 5) at the air bath temperature (25°). (The temperature of the liquid reservoir ( 1) is maintained at 20° by immersion in a large vacuum bottle containing· water at this temperature. The compo-sition of the vapor corresponding to a known composition of liquid at 20° for the system employed is known from other work.( 1 ) ). The mercury in the vapor reservoir ( 5) is raised to a point corres-ponding to the amount of vapor which is to be introduced into the adsorbent ( 7). After equilibrium is established, the mercury in the cutoff ( 4) is raised and the levels of the mercury in ( 4), ( 5) and ( 6a) adjusted to fixed marks, thus confining the sample in a known volume. Since ( 6b) is connected to the vacuum system, the pressure of the sample in the vapor reservoir ( 5) can be deter-mined by measuring the difference in the mercury levels ( 6a) and ( 6b) by mans of a cathetometer. Since the volume of the vapor 2Proceedings of the Iowa Academy of Science, Vol. 48 [1941], No. 1, Art. 56https://scholarworks.uni.edu/pias/vol48/iss1/56l!HlJ AllSOllPTION ISOTHE!lMS reservoir is known from a pre\'ious calibration when the leYds arc at fixed marks and the pressure has been determined, the nnmbcr of moles of gas present in the reservoir can be calculated if the perfect gas law is assumecl. To completcl:v transfer this vapor to the adsorbent the mcrcnr;· in ( 6) is lowered uutil the mercury in (a) is below the connect-ing piece between (a) and ( c). (This type of nih·e proved n·r:· satisfactory and W!'.S capable of handling pressure differences as large as 12 cm. Hg.) _ \.11 the Yapor, except for a negligible amount in the connecting tnbf's, is then fort'cd ont of the vapor resen·oir hy displacing it with mercury and by immersing the charcoal tube in a carbon dioxide snow-acetone mixture. .\fter completion of this operation the mercury in (6) is raised and the carbon dioxide snow-acetone mixture is rcmoYcd from tl1c charcoal and the sys-tem allowed to equilibrate at bath temperature (2!5°). The pres-sure in the adsorption ch am bcr is then read by measnring the dif-ference between len:ls ( 6b) and ( (ic), the levels (Ge) all(l ( 8) being at fixed marks so that the volume of the dead space is known. The amount of dead space is such that the vapor in tl1is n1lume is small compared with the amount of material adsorbed on 2 grams of charcoal, varying from 0 to ;3 per cent as the equil-ibrinm pressure increases. Hence, except for the last part of the isotherm, it is justifiable to say that the composition of the adsor-bate is the same as the introduced vapor. Even at higher pressures of the isotherm this would be a good approximation. By measure-ment of the composition of the ,·apor in the dead space, knowing the volume of the dead space and the composition and amount of vapor introdnccd, the exact composition of the adsorbate can be calculaled if desired. To !lderminc the composition of the Yapor in lhe dead space the modified }fcLcod gauge (8) is employed. The rnlume in the gauge is a known function of the height of the mercury in (Sa) and by measuring the heights of the mercury levels in (Sa) and ( 8b) (making necessary capillary corrections) the pressure of the vapor in the gauge !'an he determined. By making a series of measurements and plotting pv-zi a break is obtained at the con· densation pressure. Since the cornlensation pressure may alrea(ly be known as a function of composition of the vapor or can be in-dependently determined, the composition of the Yapor can be found if the condensation pressure is known. If the composition of the vapor and the total pressure are known, the partial pressures can be calculated if all vapors are assumed to be perfect. 3Rowley and Innes: Measurement of Adsorption Isotherms for Mixed VaporsPublished by UNI ScholarWorks, 1941IOWA ACADEMY OF SCIENCE [VoL. XLVIII To obtain further points on the adsorption isotherm, the process decribed can be repeated until condensation of the vapor phase in the dead space occurs without compression. The condensation pressure tells the composition of the liquid and vapor phases that would be in equilibrium with the adsorbate of known composition. Therefore, the selective adsorption, which is a measure of the dif-ference in composition between the adsorbate and the liquid phase and which can be defined as n (na - Na), can be determined. This definition can be shown( 4 ) to be related to the definition of selection adsorption, (N 6 Na), used by Bartell and coworkers (5 ) by the following relationship. n (Ila -Na) = ( N - n) 6 Na n total moles present in the adsorbed phase. Ila mole fraction of the first component in the surface phase. Na mole fraction of the first component in the liquid phase. moles of adsorbate plus moles of liquid phase. change of moles fraction of the first component in the liquid phase due to selective adsorption. For measurements of very low pressures the McLeod gauge can be employed. Another alternative for the low pressure range is also possible. If adsorption equilibria is established with the con-nection between the charcoal tube and reservoir open, a fifteen fold magnification in pressure can be obtained by raising the mer-cury in ( 5) and ( 6) and reading the difference between levels (Ga) and (6b). To illustrate the results obtained in a particular case the iso-therm for a mixture containing a mole fraction of 59% carbon tetrachloride and 41 % methanol on activated charcoal is plotted in figure 2 together with the isotherms obtained for the pure com-ponents on the same material. Partial pressures were not deter-mined for this run. The mole composition of the liquid phase which would be in equilibrium with this adsorbate as determined from the condensation pressure is 59% carbon tetrachloride. Hence, the selective adsorption per gram of adsorbent from a liquid phase having this equilibrium composition is: n (na - Na) = n (0.59 - 0.59) = 0 This mole fraction of adsorbate happens to correspond with zero selective adsorption, but this was not found to be true for other adsorbate compositions. For example for na = 0.24, Na was found to be 0.07. 4Proceedings of the Iowa Academy of Science, Vol. 48 [1941], No. 1, Art. 56https://scholarworks.uni.edu/pias/vol48/iss1/561941J ADSOR PTIO~ ISOTHEJUIS 257 300 CH10H 250 .1... m 1.00 S9'1. CG! CCI 100 50 5 10 15 20 p in c.m, Hj Figure 2. Adsorption isotherms for carbon tetrachloride, methanol, and a mixture containing a mole fraction of 59% carbon tetrachloride and 41 % methanol. The amount adsorbed, x/m is expressed in cc. of vapor at n.t.p. adsorbed per gram of charcoal (corrected for ash content and loss of weight on outgassing). Data obtained by measurements of this nature can be used for some interesting thermodynamic calculations including the evalua-tion of the reversible work of wetting both for the pure liquids and their mixtures and for checking the Gibb's adsorption equa-tion, as we hope to show in future work. DIVISION OF PHYSICAL CnEMISTRY, STATE UNIVERSITY OF IowA, IowA CITY, IowA. REFERENCES I. Nils Fontell, Soc. Sci. Fennica, vol. 8 (1936). 2. L. B. Richardson and J. C. Woodhouse, J.A.C.S., 45, 2647 (1923). 3. A. M. Bakr and J. E. King, J.A.C.S., 119, 545 (1921). 4. W. B. Innes, S.U.I. Master's Thesis (1939). 5. F. E. Bartell, E. Miller and E. Almy, J.A.C.S., 55, 466 (1933). 5Rowley and Innes: Measurement of Adsorption Isotherms for Mixed VaporsP blished by UNI ScholarWorks, 1941

Measurement of Adsorption Isotherms for Mixed Vapors

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Measurement of Adsorption Isotherms for Mixed Vapors

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