9 July 1999 Ž . Chemical Physics Letters 307 1999 413–418 www.elsevier.nlrlocatercplett Comparative study of mean-field theory and Monte Carlo simulation of supercritical methane adsorption in zeolites Zhimei Du a , Lawrence J. Dunne a,b, ) , Martin F. Chaplin a , George Manos a,c a School of Applied Science, South Bank UniÕersity, London SE1 0AA, UK b School of Chemistry, Physics and EnÕironmental Science, UniÕersity of Sussex, Falmer, Brighton BN1 9QJ, UK c Department of Chemical Engineering, UniÕersity College London, Torrington Place, London WC1E 7JE, UK Received 6 April 1999; in final form 11 May 1999 Abstract A comparison of mean-field theory and Monte Carlo simulations, supplemented by molecular modelling, of adsorption of methane in the zeolite silicalite at supercritical conditions is presented. It is found that for the ‘hot’ supercritical temperature regime, adsorption isotherms predicted by mean-field theory are virtually indistinguishable from those of Monte Carlo simulations thereby providing a straightforward route for the prediction of supercritical adsorption isotherms for this system. q 1999 Elsevier Science B.V. All rights reserved. 1. Introduction An improved understanding of the adsorption of alkanes in zeolites is important for the optimisation of adsorptive separation processes particularly those involving natural gas. Experimental measurements of adsorption isotherms are expensive and time inten- sive and, as a consequence, experimental data are often lacking. Furthermore, accurate theoretical pre- diction of alkane adsorption in zeolites has a number of challenging features. Molecular simulation tech- w x niques, like Monte Carlo 4,8,11,12 seem to offer an attractive method for the computation of adsorption isotherms for short chain hydrocarbons but those of larger molecules are much more difficult and expen- ) Corresponding author. Fax: q44 171 815 7999; e-mail: dunnel@vax.sbu.ac.uk sive to simulate. In this situation there is a need to circumvent these problems and find reliable models for description of the adsorption of longer chain alkanes. A central part of the development of practi- cally useful models of adsorption of large molecules is the testing of such models against accurate theory, such as the Monte Carlo method. In this Letter, we follow the above strategy and present the outcome of a mean-field theory descrip- tion, supplemented by molecular modelling, of the adsorption of methane in silicalite under supercritical conditions and compare the results with Monte Carlo simulations which are known to describe experimen- tal adsorption isotherms well. These calculations treat methane molecules as effectively spherical and inter- acting with a Lennard–Jones potential which we discuss below. Furthermore, the zeolite is regarded as being a rigid incompressible structure and unaf- fected by gas adsorption. 0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. Ž . PII: S0009-2614 99 00534-5