Estudo cinético da decomposição térmica de coque sobre zeólita beta modificada com Zn e La

Abstract

Environmental regulations have intensified measures to reduce the sulfur content of gasoline, as sulfur oxides are generated after combustion, aggravating environmental and human health problems. The reduction of this gasoline contaminant can be done directly in the FCC by employing additives that perform in situ desulfurization. The FCC process is severely impacted by the deactivation of catalysts and additives, due to coke formation. This problem is circumvented with the cyclic regeneration of materials, through the combustion of coke, which requires kinetic data of its thermal decomposition to determine the parameters of temperature and residence time in the regenerator. Given the above, this work evaluated the activation energy required for the thermoxidation of coke, deposited on zeolite Beta modified with La and Zn, which are promising materials for the reduction of sulfur in gasoline range, and performed a comparison of the data with two commercial additives. The samples were deactivated, in an accelerated manner, with coke deposition by catalytic cracking of cyclohexane containing 2% sulfur, at a temperature of 500 °C and a space velocity of 0.02 s-1 . Modifications of the Beta zeolite were carried out by incorporating 2% zinc and 2% lanthanum via wet impregnation. The synthesized materials were characterized with XRF, XRD, N2 physisorption and FTIR techniques, which indicated that there were no significant structural changes in the Beta zeolite after the modifications. The coke deactivated samples were analyzed by thermogravimetry, which showed that the reference zeolite Beta and the one modified with lanthanum obtained the highest and lowest deposited coke content, respectively. The Ozawa-Flynn-Wall method was applied for the kinetic study, which uses the data from the thermogravimetric analyses. It was found that the lowest activation energy for coke thermoxidation was found in the zeolite modified with lanthanum, while the unmodified Beta zeolite presented the highest value. The Raman spectroscopy analyses showed a direct relationship of the degree of graphitization with the activation energy of coke combustion, and the zeolite modified with lanthanum showed the lowest activation energy for combustion, indicating that the coke is more easily consumed during the regeneration of the catalyst in the FCC.