Thermodynamics of mixed‐gas adsorption

Abstract

A simple technique is described for calculating the adsorption equilibria for components in a gaseous mixture, using only data for the pure-component adsorption equilibria a t the same temperature and on the same adsorbent. The proposed technique is based on the concept of an ideal adsorbed solution and, using classical surface thermodynamics, an expression analogous to Raoult's law is obtained. The essential idea of the calculation lies in the recognition that in an ideal solution the partial pressure of an adsorbed component is given by the product of its mole fraction in the adsorbed phase and the pressure which it would exert as a pure adsorbed Component a t the same temperature and spreading pressure as those of the mixture. Predicted isotherms give excellent agreement with experimental data for methaneethane and ethylene-carbon dioxide on activated carbon and for carbon monoxide-oxygen and propane-propylene on silica gel. The simDlicitv of the calculation, which requires no data for . . the mixture, makes it espec'ially useful for engineering applications.Adsorption equilibria are required in the design of heterogeneous chemical reactors and in certain types of separation equipment. In many cases the desired equilibria are for a mixed rather than for a pure gas, and it is therefore of considerable practical interest to develop a technique for estimating the adsorption equilibria of a gaseous mixture from the known adsorption isotherms of the pure components. Such a technique is described here. The principal idea on which the proposed technique is based is the proper definition of an ideal adsorbed solution in a manner analogous to that used for liquid-phase solutions. As shown towards the end of this work, the equations developed from the ideal-solution concept predict adsorption isotherms which are in excellent agreement with experimental adsorption data for gaseous mixtures.A complete review of the current status of mixed-gas adsorption is given in the excellent monograph by Young and Crowell (12). The usual procedure for the interpretation of experimental adsorption equilibria for a gaseous mixture is to compare the experimental data with the prediction of some theoretical model. Most models for physical adsorption contain two or three parameters, and it is usually assumed that the parameters for mixture adsorption can be written as some simple function of the purecomponent parameters and the composition of the adsorbed phase. Thus, a determination of the pure-component parameters from experimental data permits the prediction of mixture adsorption equilibria. Unfortunately, the above procedure has not been very successful; the predictions have not been in quantitative agreement with the experimental data ( 2 ) and often not even in qualitative agreement ( 1 ) .An alternative procedure for interpreting mixture data is the liquid entropy model of Arnold ( 1 ) . In this model, it was proposed that Raoult's law should be obeyed but only for adsorption sites having the same heat of adsorption, Using two addit...

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