Mathematical modeling of regeneration of coked Cr-Mg catalyst in fixed bed reactors

Abstract

On the base of two-dimensional (bed length and pellet radius coordinates) two-temperature (gas and catalyst phases) mathematical model the coked CrF3/MgF2 catalyst regeneration process in the adiabatic reactor of gas-phase perchloroethylene hydrofluorination to pentafluoroethane (Freon 125) is studied. The parameters of the mathematical model such as coke burning rate constant, temperature of the coke burning beginning T* (ignition point), heat of the reaction and activation energy were determined from experiments in lab-scale reactor and from the thermal analysis data. The influence of process parameters, e.g. oxygen and coke concentrations, initial gas and catalyst temperatures and pellet size was investigated. The simulation was carried out under conditions of low oxygen concentration (oxygen shortage) and showed that if the inlet gas temperature Tgo is lower than initial catalyst temperature Tp0 (Tgo<Tp0, Tpo≥T*), the coke burning occurs only near the outer surface of catalyst pellet, because the bed is “cooled” by the inlet gas flow. This leads to the dramatic rise of temperature in a narrow “peak burning wave” and to an unexpected bed overheating that exceeds the adiabatic heating. This effect was not discussed in publications but was detected in industrial reactor during the coked chromium–magnesium catalyst regeneration. This result is very significant for industrial applications since it helps to avoid the catalyst overheating and the following catalyst activity decreasing during the regeneration.