Screening of zeolite adsorbents for separation of hexane isomers: A molecular simulation study

doi:10.1016/j.seppur.2006.12.011

Separation and Purification Technology

Volume 55, Issue 2, 15 June 2007, Pages 246-255

https://doi.org/10.1016/j.seppur.2006.12.011 Get rights and content

Abstract

Recent developments in Monte Carlo (MC) simulation techniques have enabled the accurate calculation of sorption isotherms of a variety of molecules in zeolites. Using the example of separation of isomers of hexane we demonstrate the power of MC simulations for screening of zeolite adsorbents for maximum separation selectivity. The simulations underline the exploitation of entropy effects during mixture sorption

Introduction

Zeolites are crystalline nanoporous materials that are widely used in the chemical industry as catalysts and adsorbents [1]. Zeolite crystals are incorporated into binders and used in the form of pellets in fixed or (simulated) moving bed reactors. Alternatively, zeolite crystals are coated on to a porous membrane support and used in (catalytic) membrane reactors and separation devices. More than 180 zeolite structures are known [2]; Fig. 1 shows some common topologies. AFI, MOR and LTL, consist essentially of uni-dimensional channels; MOR also has side pockets. MFI, ISV and BEA have intersecting channel structures. FAU, LTA, CHA and DDR consist of cages that connect to one another through windows; these windows may be large (FAU), or narrow (LTA, CHA, DDR). For any given separation or reaction duty, there is an optimum zeolite structure offering the maximum selectivity or reactivity; arriving at this optimum zeolite by experiments is often a demanding and time consuming exercise.

Recent development in Monte Carlo (MC) simulation techniques, coupled with the availability of high performance computing facilities, have enabled the determination of the adsorption characteristics of a variety of molecules in different zeolites with a reasonable degree of accuracy and reliability [3], [4], [5], [6], [7]. The major objective of this paper is to illustrate the potential of MC simulations in screening zeolites by considering an example of current interest, i.e. the separation of hexane isomers. We demonstrate that molecular simulations can provide insights into adsorption mechanisms, siting and orientation of molecules that are not available from experiments alone. Furthermore it is demonstrated how entropy effects during mixture sorption can be exploited to obtain maximum sorption selectivities. Such insights allow the development of more fundamental models for process design.

Section snippets

Separation of hexane isomers

Isomerization of alkanes, for the purposes of octane improvement, is a process of increasing importance in the petroleum industry [8], [9], [10]. Fig. 2a shows an example of a process for isomerization of n-hexane (nC6). The product from the isomerization reactor, that commonly uses MOR as catalyst, consists of an equilibrium distribution of unreacted nC6, along with its mono-branched isomers 2-methylpentane (2MP), 3-methylpentane (3MP) and di-branched isomers 2,2-dimethylbutane (22DMB) and

Conclusions

In this paper we have attempted to demonstrate the potential uses of MC simulations in the screening of zeolite adsorbents for separation of hexane isomers. MC simulations can be invaluable adjuncts to experiments, and allow isotherm data to be produced up to saturation conditions often difficult to achieve in experiments, especially for small molecules. In the case of mixtures, experimental data are very scarce [28] and MC simulations are invaluable in unraveling entropy effects arising out of

Acknowledgements

R.K. and J.M.vB. acknowledge the grant of a TOP subsidy from the Netherlands Foundation for Fundamental Research (NWO-CW) for intensification of reactors and NWO/NCF for provision of high performance computing resources. The CBMC simulations were carried out with the BIGMAC program developed by Dr. T.J.H. Vlugt.

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Citation Excerpt :
An interesting question can be raised with regards to whether one can use molecular simulation in order to screen for zeolites in a particular separation process. This screening approach was used for the separation of linear and branched hydrocarbons [402,403], of mixtures of linear alkanes, and of methane and carbon dioxide [404]. For example, certain molecular simulations predicted that, for the separations of linear and branched hexane isomers, MFI was a very good candidate [402,403].
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Screening of zeolite adsorbents for separation of hexane isomers: A molecular simulation study

Abstract

Introduction

Section snippets

Separation of hexane isomers

Conclusions

Acknowledgements

Chem. Eng. Sci.

Chem. Phys. Lett.

Chem. Phys. Lett.

Microporous Mesoporous Mater.

Sep. Purif. Technol.

J. Catal.