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...

The storage of gases in porous adsorbents, such as activated carbon and carbon nanotubes, is examined here thermodynamically from a systems viewpoint, considering the entire adsorption-desorption cycle. The results provide concrete objective criteria to guide the search for the "Holy Grail" adsorbent, for which the adsorptive delivery is maximized. It is shown that, for ambient temperature storage of hydrogen and delivery between 30 and 1.5 bar pressure, for the optimum adsorbent the adsorption enthalpy change is 15.1 kJ/mol. For carbons, for which the average enthalpy change is typically 5.8 kJ/mol, an optimum operating temperature of about 115 K is predicted. For methane, an optimum enthalpy change of 18.8 kJ/mol is found, with the optimum temperature for carbons being 254 K. It is also demonstrated that for maximum delivery of the gas the optimum adsorbent must be homogeneous, and that introduction of heterogeneity, such as by ball milling, irradiation, and other means, can only provide small increases in physisorption-related delivery for hydrogen. For methane, heterogeneity is always detrimental, at any value of average adsorption enthalpy change. These results are confirmed with the help of experimental data from the literature, as well as extensive Monte Carlo simulations conducted here using slit pore models of activated carbons as well as atomistic models of carbon nanotubes. The simulations also demonstrate that carbon nanotubes offer little or no advantage over activated carbons in terms of enhanced delivery, when used as storage media for either hydrogen or methane.

We consider the thermodynamics of adsorption of gases in porous solids from both the perspective of absolute properties which appear naturally in theoretical studies and that of excess properties which are measured in experiments. Our thermodynamic description starts by treating the gas (or gas mixture) plus porous solid system as a mixture to which we can apply solution thermodynamics. We show that equations for the absolute thermodynamic properties for adsorption in rigid porous materials do not require an explicit reference to the pressure of the fluid confined in the porous material. We discuss how to relate absolute properties to excess properties by using an estimate of the helium void volume. We illustrate the thermodynamic formalism with calculations for a simple thermodynamic model in which the Langmuir equation is used to describe the absolute adsorption isotherm and the ideal gas equation of state is used for the bulk properties. The simplified model explains the apparently anomalous behavior of the thermodynamic functions for adsorption at high pressure up to 1000 bar.

Isosteric heats of adsorption and adsorption isotherms have been measured simultaneously in a calorimeter for a series of gases of increasing size and magnitude of quadrupole moment (Ar, O2, N2, CH4, C2H6, SF6, CO2) on adsorbents of varying pore structure and ion type (NaX, H-ZSM-5, Na-ZSM-5). Adsorption isotherms have been checked for reversibility by desorption experiments. The average experimental error in loading is ±0.6%; the average uncertainty in the isosteric heat of adsorption is ±0.5 kJ/mol. Heats of adsorption of nonpolar molecules (CH4, C2H6, SF6) increase in the order NaX, silicalite, H-ZSM-5, Na-ZSM-5 at low coverage. Heats of adsorption of nonpolar molecules are almost identical on silicalite, H-ZSM-5, and Na-ZSM-5 at high coverage. Heats of adsorption of the quadrupolar molecule CO2 increase in the order silicalite, H-ZSM-5, Na-ZSM-5, NaX. The electric field adjacent to Na+ sites is 6.2 V/nm, on the basis of the difference between the heat of adsorption of CO2 in Na-ZSM-5 and the heat of adsorption of CO2 in silicalite.

A Tian−Calvet type calorimeter is applied to the simultaneous determination of adsorption isotherms and heats of adsorption. This is the first of a series of studies of the effect of adsorbate size and polarity on the energetics of adsorption in zeolites. The adsorbate gases used in this study are quadrupolar (N2 and CO2) and nonpolar (Ar, O2, CH4, C2H6, and SF6). The heats of adsorption of both polar and nonpolar gases are either constant or increase with coverage, so silicalite may be classified as a relatively homogeneous adsorbent compared to X type zeolite. Reversibility was established by comparing adsorption and desorption isotherms. Reproducibility was studied by comparing runs for different samples of the same adsorbent. The average experimental error in loading is ±0.6%. The error in the isosteric heat of adsorption is ±2% for heats larger than 20 kJ/mol and ±5% for heats smaller than 20 kJ/mol.