Adsorption And Diffusion On Zeolites Used In Olefins Catalytic Cracking Technology: Experiment And Molecular Simulation

With the C₄ olefin-rich hydrocarbons as the feed to produce ethylene andpropylene on special shape-selectivity and acidic ZSM-5 zeolite catalysts, theolefins catalytic cracking (OCC) technology developed by Shanghai ResearchInstitute of Petrochemical Technology of SINOPEC is a novel technology, andthis technology is of great importance in the utilization of C₄ olefin-richhydrocarbons and increase of benefit for petrochemical industry. Therefore, itis necessary to investigate adsorption and diffusion of C₄ hydrocarbons onZSM-5 zeolites with different Si/Al ratios, used in the catalytic crackingprocess.In this work, aimed at the OCC technology, the combine method "experiment measurement,theory model and molecular simulation" was used to study the adsorption and diffusion of C₄ hydrocarbons (n-butane and butene-1) on special shape-selectivity and acidic ZSM-5 zeolites with different Si/Al ratios used as catalysts in OCC technology, which were prepared by Shanghai Research Institute of Petrochemical Technology. The adsorption and diffusion model of C₄ hydrocarbons on ZSM-5 zeolite catalysts were proposed here. The main contents and finding are summarized as follows:1．First, five ZSM-5 zeolite samples are characterized by XRD, SEM and the nitrogen adsorption isotherm at 77 K. It is found from the XRD results that the positions of the characteristic peaks for five zeolite samples are the same with those of the standard ZSM-5 zeolite samples, indicating that the samples are all ZSM-5 zeolites. The SEM photograph shows that the ZSM-5 zeolite particulates have the regular crystal morphology and a narrow crystal size, which about is 0．1～0．3μm. As the addition of a large amount of binder, the BET surface area of five samples are all lower than that of the literature values and some macropores are formed in the ZSM-5 zeolite sample crystal particulates. Furthermore, based on the catalytic cracking conditions recommended by Shanghai Research Institute of Petrochemical Technology, the adsorption equilibrium and kinetic data of C₄ hydrocarbons on ZSM-5 zeolite with different Si/Al ratios and modified components are measured by the intelligent gravimetric analyzer (IGA-003, Hiden) at different temperatures and pressures.2．The double Langmuir (DL) model is used to correlate the equilibrium data of pure C₄ hydrocarbons on ZSM-5 zeolite samples, and the heat of adsorption, entropy change of adsorption and Henry coefficient are calculated and the affection of Si/Al ratio to adsorption thermodynamic character are discussed. Then, by combining the ideal adsorbed solution theory (IAST) with the double Langmuir (DL) model, the IAST-DL model which is capable to model the butene-1(1) /n-butane(2) mixture adsorption on different ZSM-5 zeolite samples is proposed and is used to predict the selectivities and adsorption of butene-1(1) /n-butane(2) mixture on different ZSM-5 zeolite samples covering a wide range of compositions, temperatures and pressures. The correlation between the selectivity and Si/Al ratio of sample is proposed here.3．The united-atoms (UA) representation is employed, and the interaction parameters between UA model are obtained by fitting the NVT and GCMC, which are combined with CBMC (Configurational-bias Monte Carlo) technique, simulation results with the experimental data (heat of adsorption, Henry coefficient and adsorption isotherm). The force field parameters used for industrial catalysts are proposed here. Then, the model and force field parameters are used to simulate the mixture adsorption isotherm and selectivity. The simulated results are good agreement with the experiment data and the calculated results of IAST-DL model. The adsorption localization of C₄ hydrocarbons on ZSM-5 zeolite samples are simulated, and the simulated results show that the addition of a large amount of binder has a distinctly affection for the adsorption localization of C₄ hydrocarbons on ZSM-5 zeolites.4．The adsorption kinetic data of C₄ hydrocarbons on ZSM-5 zeolites are correlated in terms of the Fick diffusion model, and the diffusion coefficients of n-butane and butene-1 on ZSM-5 zeolites with different Si/Al ratios at 275, 288 and 300 K are obtained. In addition, the affection of temperature, pressure and Si/Al ratio to diffusion coefficients are discussed.