Selectivity in complex reactions in fluidised-bed reactors: effect of temperature

Abstract

It is well known that the performance of a fixed-bed reactor with an optimum temperature profile is better than that of an isothermal fixed-bed reactor. In the case of a fluidised-bed reactor, the vigorous mixing of the solids ensures isothermal operation, thus precluding the possibility of a temperature profile along the reactor length. However, one still has a choice of using two (or three) fluidised-bed reactors in series, each operating isothermally at a different temperature. It is of interest to know how much improvement in performance can be obtained in this manner. An upper limit to the possible improvement can be obtained by considering the limiting case of a large number of fluidised-bed reactors operating in series. Mathematically this is equivalent to a fluidised-bed reactor with a temperature profile. In the present work optimum temperature profiles are derived for the case of a consecutive and parallel reaction carried out in a fluidised bed. The simple Kunii-Levenspiel model is assumed to describe the bed behaviour. An interesting feature of the analysis is the fact that the temperature derivative (dT/dl) can have either positive or negative value depending on the parametric values chosen, indicating that the operating temperature profile could be either a rising or a decreasing profile for the same reaction. This is in contrast to observations in the case of fixed-bed reactors. Finally, a numerical example is presented for the purpose of illustration.