ISOSTERIC HEATS OF MULTICOMPONENT ADSORPTION - THERMODYNAMICS AND COMPUTER-SIMULATIONS

Heats of adsorption were obtained by molecular simulations for the binary mixtures C2H4-CO2, CO2-CH4, and i-C4H10-C2H4 adsorbed on zeolite 13-X. These simulations provide the first comprehensive sets of data on adsorption isotherms and isosteric heats of single gases and their mixtures. The simulations agree very well with experimental isotherms for the amount adsorbed, but individual heats of adsorption have not been measured calorimetrically and the calculation of mixture heats from adsorption isotherms using the methods of thermodynamics is impractical. Heats of adsorption from gas mixtures are predicted from single-gas heats of adsorption by use of two thermodynamic models based on an ideal adsorbed solution (IAS). The IAS heats are in excellent agreement with results from molecular simulations at low surface coverage. At high surface coverage, the predictions agree with the simulations for binary mixtures that form ideal solutions (C2H4-CO2). However for a highly nonideal, azeotropic mixture such as i-C4H10-C2H4 in zeolite X, IAS predictions of isosteric heats are poor. For adsorption on energetically heterogeneous surfaces, the excess enthalpy is exothermic and individual heats of adsorption are always greater than those predicted for an ideal solution.