Adsorption And Separation Of Alkanes In Zeolites By Molecular Simulations

Zeolite is microspore crystal material with framework. It was applied in adsorption, separation and catalyzes processing. There are many types of zeolites. Making scientific research on zeolite is a question for discussion for a long time. The research of adsorption of natural gas is of great importance because of its separation, purifying and storage. And the separation of petrochemical is implemented by zeolites. So it is urgent to find out the adsorption of alkane mixtures in zeolites distinctly. However, design of adsorption processes of pure and multi-component needs a great deal of adsorption and separation data. But experimental measurements and theoretical research of adsorption of mixtures are difficult so that experimental data are very scarce. It is time-consuming to measure adsorption isotherms by experimental, so it is of great importance to research that by molecular simulation techniques.In this paper, we employed GCMC combined with CBMC technique to simulate the adsorption and separation of alkanes and its mixtures in different zeolites. And gained the adsorption isotherms and adsorption phase diagrams of mixtures, and searched the location of alkane molecule, compared the selectivities of several zeolites. We got some results as follows:1. We can get the simulation results agreed with the experimental data by GCMC combined with CBMC technique with Lennard-Jones model. Molecular simulation technique, as a kind of "computer experiments", offers a method for the data taking for industry designing.2. For light alkane mixtures, the results show that the selectivities of the zeolites for longer alkanes for the methane-propane mixtures is in order of ISV>MFI>MEL> FER>TON>MOR (with mole fraction of 0.5 for methane in gas phase), and for the ethane-propane mixtures, the order is ISV>MOR>MFI>FER> MEL>TON (with mole fraction of 0.5 for ethane in gas phase). For ternary and quaternary mixtures the adsorption isotherms in MFI zeolite show that the longer chain component is preferentially adsorbed at low pressures. Its adsorption increases and then decreases as the pressure increases. But for the shorter chain component, it is adsorbed at higher pressures and its adsorption increases as the pressure increases. The adsorbed amount in mixtures is in order of ISV > MFI > MOR.3. The adsorption isotherms of binary mixtures of C4-C7 alkane isomers (with 0.5 mole fraction in gas phase) in the same zeolites show the same characteristic. The amounts of linear and branched alkanes are similar at low pressure. With pressure increasing the adsorption of the branched alkanes reaches a maximum and then decreases, and the adsorption of the liner alkanes increases with increasing pressure in MFI. But in ISV and MOR the amount of adsorption of the liner and branched alkanes all increases with pressure increasing. The amount of adsorption of the branched alkanes is higher than that of linear alkanes. The location of alkane isomers shows obvious orientation in MFI, but the location of alkane isomers is astatic in ISV and MOR.4. For the adsorption isotherm of C4-C6 isomers quaternary mixtures with 0.25-0.25-0.25-0.25 mole fraction in gas phase, the amount of adsorption of longer chain normal alkane increased with pressure increasing in MFI, but for other alkanes the increasing are not obviously. The 2-methylalkane locates at the intersection of channels, but the normal alkanes locate everywhere. For ISV the amount of adsorption of alkanes with more C atoms is more than that with less C atoms. At low pressure the amount of adsorption of normal alkanes is more than 2-methylalkane, but at higher pressure the amount of adsorption of 2-methylalkane is more than normal alkanes. Each kind of alkane locates everywhere in ISV. For MEL the adsorption of alkanes is similar to MFI.5. The modified Langmuir equation proposed by Vlugt et al. described the adsorption of alkanes in zeolites very well. By ideal adsorption solution theory (IAST) we calculated the phase diagram and adsorption isotherm with data of pure component. The calculation for adsorption of short chain alkanes mixtures in MFI and butane isomers mixtures in MOR are satisfied. But the result of calculation for butane isomers mixtures in MFI is not agreement with the result of simulation.