Investigation of entropy effects during sorption of mixtures of alkanes in MFI zeolite

doi:10.1016/S1385-8947(01)00253-4

Chemical Engineering Journal

Volume 88, Issues 1–3, 28 September 2002, Pages 81-94

https://doi.org/10.1016/S1385-8947(01)00253-4 Get rights and content

Abstract

We have carried out a comprehensive study of sorption of mixtures of alkanes, in the 1–7 C atom range, in MFI zeolite using configurational-bias Monte Carlo (CBMC) simulations. The isotherm characteristics of various binary, ternary and quaternary mixtures have been investigated. Our studies show that two types of entropy effects have a significant influence on mixture sorption:

1.
Size entropy effects arise due to differences in the saturation loading of the pure components. Size entropy effects favour the component with the smaller number of C atoms because the smaller molecule finds it easier to fill in the “gaps” within the zeolite matrix at high molecular loadings.
2.
Configurational entropy effects come into play for mixtures of alkanes that differ in the degree of branching. For a mixture of linear and mono-methyl alkanes with the same number of C atoms, configurational entropy effects favour the linear isomer because such molecules “pack” more efficiently within the MFI matrix. For a mixture of mono-methyl and di-methyl alkanes with the same number of C atoms, configurational entropy effects favour the single branched isomer. Configurational entropy effect comes into play when the loading exceeds four molecules per unit cell, when all the intersection sites are occupied, and results in the following hierarchy of sorption strengths: $linear alkanes ⪢ mono - methyl alkanes ⪢ di - methyl alkanes$ .

In all cases, the mixture isotherms can be predicted with good accuracy using the ideal adsorbed solution theory (IAST).

CBMC simulations of sorption of an 8-component mixture containing n-pentane (n-C₅), 2-methylbutane (2MB), n-hexane (n-C₆), 2-methylpentane (2MP), 2,2-dimethylbutane (22DMB), n-heptane (n-C₇), 2-methylhexane (2MH) and 2,2-dimethylpentane (22DMP) show that both size and configurational entropy effects contribute, leading to a sorption hierarchy depending on the degree of branching, $linear alkanes ⪢ mono - methyl alkanes ⪢ di - methyl alkanes$ . This result has considerable potential for commercial application in the petroleum industry in catalytic isomerization process where it is necessary to isolate the di-branched alkanes which are preferred ingredients in gasoline.

Introduction

The separation of mixtures of alkanes is an important activity in the petroleum and petrochemical industries. For example, the products from a catalytic isomerization reactor consist of a mixture of linear, mono-methyl and di-methyl alkanes. Of these, the di-branched molecules are the most desired ingredients in petrol because they have the highest octane number. It is therefore required to separate the di-methyl alkanes and recycle the linear and mono-methyl alkanes back to the isomerization reactor. In the detergent industry, the linear alkanes are the desired components and need to be separated from the alkane mixture.

Selective sorption on zeolites is often used for separation of alkane mixtures [1], [2], [3], [4], [5], [6], [7]. The choice of the zeolite depends on the specific separation task in hand. For example, small-pore Zeolite A are used for separation of linear alkanes using the molecular sieving principle; the branched molecules cannot enter the zeolite structure. Both linear and branched molecules are allowed inside the medium-pore MFI matrix and the sorption hierarchy in MFI will be dictated both by the alkane chain length and degree of branching. For the development of separation technologies we need to be able to calculate the mixture sorption characteristics for a wide range of operating conditions (pressures, temperatures, compositions). While there are several experimental studies on pure component isotherms [8], [9], [10], [11], [12], there is considerably less information on mixture isotherms. The lack of mixture sorption data is most probably due to the difficulty of experimentation.

In earlier publications [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], we had developed the configurational-bias Monte Carlo (CBMC) simulation technique to calculate the isotherms of pure components, and binary mixtures, of alkanes in MFI zeolite. The accuracy of the CBMC simulations have been demonstrated by extensive comparison with published experimental data. The objectives of the present study are threefold. First, we extend our previous studies by performing CBMC simulations of ternary and quaternary mixtures of alkanes in MFI in order to highlight subtle size and configurational entropy effects. Second, we examine the extent to which the mixture characteristics can be predicted from information on pure component sorption behaviour using the ideal adsorbed solution theory (IAST). Finally, we show how a proper understanding, and quantification, of entropy effects can afford new separation strategies with considerable industrial potential. We restrict our attention to alkanes in the 1–7 C atom range. We begin with a short summary of our CBMC simulation methodology.

Section snippets

CBMC simulation methodology

The linear and branched alkanes are described with a united-atom model, in which CH₃, CH₂, and CH groups are considered as single interaction centres. When these pseudo-atoms belong to different molecules or to the same molecule but separated by more than three bonds, the interaction is given by a Lennard–Jones potential. The Lennard–Jones parameters are chosen to reproduce the vapour–liquid curve of the phase diagram as shown in Siepmann et al. [23]. The bonded interactions include

Pure component isotherms

The sorption isotherms at 300 K for alkanes in MFI are shown in Fig. 1. n-Heptane shows a pronounced inflection at a loading of Θ=4. n-Hexane shows a slight inflection at this loading due to “commensurate freezing” effects [26]. All 2-methyl alkanes show inflection behaviour (see Fig. 1(c)); this is because these molecules prefer to locate at the intersections between straight and zig-zag channels, which offers more “leg-room” [20], [21]. At Θ=4, all intersections are fully occupied. To locate

Conclusions

We have examined the sorption characteristics of various mixtures of alkanes, in the 1–7 C atom range, at 300 K in MFI. The following major conclusions can be drawn:

1.
For binary mixtures of linear alkanes in the 1–4 C atom range, size entropy effects come into play at high mixture loadings and these counter chain length effects to reduce separation selectivities; see Fig. 3, Fig. 4.
2.
For binary mixtures of linear and mono-branched alkanes with the same number of carbon atoms, the sorption selectivity

Acknowledgements

All the authors of this study have received grants from the Netherlands Organisation for Scientific Research (NWO), through the Netherlands Research Council for Chemical Sciences (CW).

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Investigation of entropy effects during sorption of mixtures of alkanes in MFI zeolite

Abstract

Introduction

Section snippets

CBMC simulation methodology

Pure component isotherms

Conclusions

Acknowledgements

Micropor. Mesopor. Mat.

Chem. Eng. Sci.

Sep. Purif. Technol.

Chem. Eng. Sci.

J. Catal.

J. Catal.

Separation of branched hexane isomers using zeolite molecular sieves

AIChE J.

Adsorption equilibria of C5–C10 normal alkanes in silicalite crystals

J. Phys. Chem.

Adsorption equilibria of C1–C4 alkanes, CO2 and SF6 on silicalite

J. Phys. Chem.

Adsorption of branched and cyclic paraffins in silicalite. 1. Equilibrium

Ind. Eng. Chem. Res.

Adsorption of light alkanes in silicalite. 1. Reconciliation of experimental data and molecular simulations

Phys. Chem. Chem. Phys.

Adsorption equilibria of C₅–C₁₀ normal alkanes in silicalite crystals

Adsorption equilibria of C₁–C₄ alkanes, CO₂ and SF₆ on silicalite