Monte Carlo and energy minimization studies of binary xylene adsorption in AEL and AFI networks
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Monte Carlo and energy minimization studies of binary xylene adsorption in AEL and AFI networks S. Mardônio P. Lucena · Randall Q. Snurr · Célio L. Cavalcante Jr.
Received: 29 April 2007 / Revised: 13 July 2007 / Accepted: 23 July 2007 / Published online: 20 September 2007 © Springer Science+Business Media, LLC 2007
Abstract Adsorption of binary xylene mixtures in AEL and AFI networks was investigated using normal and biased GCMC simulations. Preferential o-xylene adsorption was evidenced in the simulations, as previously reported in single-component experimental data. In contrast to the FAU and MFI sieves, the AEL and AFI networks exhibit surprising azeotropic behavior. The selectivity switches from o-xylene to p-xylene at a gas phase mole fraction of ca. 0.5. Energy minimization was performed in the AlPO4 -11 molecular sieve to determine the energy differences between the adsorption sites. The minimization study showed that AlPO4 -11 has small adsorption energy differences between sites. The azeotropic behavior of the AEL and AFI networks can be explained using the two patch model proposed by Do and Do (Adsorption 5:319–329, 1999). Keywords Aluminophosphate · Xylenes · Simulation · Monte Carlo · Sorption · Binary
1 Introduction In the petrochemical industry, the separation of xylenes is the main application of adsorption separations. The current process of separating p-xylene from C8 aromatic mixtures is S.M.P. Lucena · C.L. Cavalcante Jr. () Universidade Federal do Ceará, Dept. Eng. Química, Grupo de Pesquisa em Separações por Adsorção—GPSA, Campus do Pici, Bl. 709, 60455-760 Fortaleza, CE, Brazil e-mail: [email protected] R.Q. Snurr Northwestern University, Department of Chemical & Biological Engineering, Evanston, IL 60208, USA
performed by taking advantage of the para-selective adsorption behavior of the FAU type zeolite (Buarque et al. 2005; Minceva and Rodrigues 2004). Ortho-selectivity in aluminophosphate molecular sieves was identified experimentally for xylene adsorption in AFI (AlPO4 -5) (Barthomeuf and De Mallmann 1990; Chiang et al. 1991) and AEL (AlPO4 -11) (Cavalcante et al. 2000). Those experimental studies were done with pure components. The only experimental work with multicomponent mixture was done by Rosenfeld and Barthomeuf (1983). The need to gain a detailed insight into the behavior of molecular sieve/sorbate systems on the molecular scale motivated us to perform molecular simulation studies of single components using the grand canonical Monte Carlo (GCMC) technique. We found for example that the selectivity in AFI and AET networks is due to the characteristic o-xylene face-to-face positioning, which allows more efficient packing within the pores (Lucena et al. 2006a, 2006b). On the other hand, AEL ortho-selectivity comes from the smaller length of the o-xylene molecules in the c-axis direction compared to p-xylene. We also found that channel modulation, creating wide and narrow regions along the aluminophosphate channels, is a decisive factor for the adsorption properties of xylenes. Con
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