Adsorption equilibria of binary ethylmercaptan/hydrocarbon mixtures on a NaX zeolite
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Adsorption equilibria of binary ethylmercaptan/hydrocarbon mixtures on a NaX zeolite J.P. Bellat · F. Benoit · G. Weber · C. Paulin · P. Mougin · M. Thomas
Received: 11 April 2007 / Revised: 22 April 2008 / Accepted: 2 June 2008 / Published online: 20 June 2008 © Springer Science+Business Media, LLC 2008
Abstract We present here coadsorption data of ethylmercaptan with n-heptane or toluene on NaX at 298 K in the aim to use this adsorbent for removing mercaptans from natural gas. Results show that NaX has an adsorption affinity for sulphur compound strong enough to perform a deep desulphurization. NaX adsorbs preferentially ethylmercaptan over a large domain of pore filling. A displacement of n-heptane by ethylmercaptan is even observed. However, toluene becomes preferentially adsorbed at high filling. The dependence of filling and composition on selectivities and failed prediction of coadsorption equilibria by the IAS Theory indicate that the adsorbed mixture behaves as a non-ideal solution. Calorimetric measurements of coadsorption heats show that the coadsorption process is not governed by enthalpic effects but by entropic effects consecutive to steric hindrance of molecules in confined spaces. Keywords Coadsorption · Ethylmercaptan · n-heptane · Toluene · NaX · Calorimetry 1 Introduction Natural gases contain some impurities as water, carbon dioxide, hydrogen sulphide and light mercaptans (mainly J.P. Bellat () · F. Benoit · G. Weber · C. Paulin Institut Carnot de Bourgogne, UMR 5209 CNRS et Université de Bourgogne, BP 47870, 21078 Dijon Cedex, France e-mail: [email protected] P. Mougin IFP, 1 and 4 avenue du Bois-Préau, 92852 Rueil-Malmaison Cedex, France M. Thomas IFP-Lyon, BP 3, 69390 Vernaison, France
methyl- and ethylmercaptans), which must be removed before industrial use. Indeed, the environmental legislation will impose in a near future a drastic reduction of sulphur emission in the atmosphere (less than 20 molar ppm). On the other hand, natural gas used in fuel cell must be highly purified because sulphur acts as a fearful poison for catalysts that are used for conversion of methane into hydrogen. In the conventional process, the natural gas is firstly cleaned of the acid gases carbon dioxide and hydrogen sulfide in an amine-washing unit then dried in a glycol unit. However, these treatments do not eliminate completely sulphur compounds because light mercaptans are not acidic enough to be removed by this way. An additional step in the treatment process is required to reduce to a lower level the sulphur content in the gas. Gas purification by selective adsorption on zeolites is an alternative way that is in progress. Although papers on desulphurization of liquid fuels by adsorption on zeolites are more and more numerous these last six years (Yang et al. 2001; Velu et al. 2003; Hernandez-Maldonado and Yang 2003, Hernandez-Maldonado et al. 2004), fundamental studies on the adsorption of mercaptans in these nanoporous materials are rather scarce. As shown in previous papers (Weber et al. 2
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