Mathematical modeling for investigating the effect of operating parameters and radial diffusion on oxidative dehydrogenation of normal butane in the fixed bed reactor and the inert membrane reactor

Abstract

The oxidative dehydrogenation of n-butane in a ceramic membrane reactor was studied. Mathematical models were developed to investigate reactor performance at various operating conditions. Kinetic data of V/MgO catalyst with 24wt% V2O5 and permeation data of gases through a porous gamma-alumina membrane (membralox) with 4 nm pore size were used in the models. The non-isothermal condition and radial dispersion of both mass and heat transfer were included in the models. Because the oxidative dehydrogenation of n-butane is a highly exothermic reaction, hot spot is a major problem found in conventional fixed-bed reactors. From this study it was found that the selectivity to C4 hydrocarbon increased with the increase of operating temperature and the hot spot problem and the effect of radial dispersion were pronounced particularly near the entrance of the reactor. The use of the ceramic membrane to control the distribution of oxygen feed to the reaction side could significantly reduce the hot spot temperature the results also showed that there were optimum feed ratios of air/n-butane for the fixed-bed rector and membrane reactor, however, the hot spot temperature was not sensitive to the feed ratio for the membrane reactor. The membrane reactor outperformed the fixed-bed reactor in term of yield C4 at high values of the ratio. In addition there is an optimum membrane reactor size. At the optimum reator size, when the reactor size increased, the conversion of n-butane and selectivity to C4 decreased due to the effect of radial dispersion and when the reactor size decreased, the extent of reaction decreased due to the smaller amount of catalyst. As a result, the yield to C4 hydrocarbon decreased. It was found that the increase of wall temperature increased the yield but the radial dispersion effect was more pronounced. Finally the feed air temperature was found to be able to control the temperature profile along the reactor length.