Influence of cooling design on fixed-bed reactors dynamics

Abstract

Tubular fixed-bed reactors of the heat-exchanger type are selected commonly in industrial plants to carry out highly exothermic reactions. Due to the important heat effects involved, these units usually exhibit the well-known problems of a pronounced maximum in the axial temperature profile (hot spot), combined with high parametric sensitivity. The authors analyzed the response of axial temperature and concentration profiles to changes in the reactant's inlet conditions for the cocurrent design. Nevertheless, more critical disturbances are changes at the inlet coolant temperature which is often used as an indirect manipulated variable for control purposes. Therefore, the influence of this variable on the reactor dynamics will be studied in this paper. Recent steady-state simulation results have shown that the behavior of this type of reactors depends strongly on the mutual direction of the reacting fluid and coolant streams. As optimal operation conditions cannot be defined only on the basis of steady-state analysis, this work complements the above-mentioned works by means of a similar development, performed under nonsteady-state conditions. The underlying idea is that to be confirmed as the best choice, the optimal design found from steady-state analysis should demonstrate also to exhibit an acceptable dynamic performance.