(93c) Heat Management in Chemical Reactors-Solved and Unsolved Problems

The management of heat generated in chemical reactors is essential for the safe operation of any chemical reactor. Lilenroth pointed out already in 1922 that the heat generation in a chemical reactor may lead to multiple steady states. Significant advances have been made in gaining an understanding of the impact of start up policy or transient perturbations on the dynamic behavior and its stability and sensitivity. Some dynamic features of chemical reactors, such as the wrong-way behavior, are counterintuitive, and reaction engineers know now how to circumvent the problems that they may lead to. The interaction between the temperature change and the fluid properties can have a major impact on the behavior of some reactors. For example, it may have a major impact on the behavior of some polymerization reactors, lead to thermoflow multiplicity in multi-tube reactors and cause self-ignition of coal piles. Most existing safe heat management rules and guide lines are concerned with continuous-flow reactors. Missing are similar rules for the optimal safe feed of reactants to batch reactors in which highly exothermic reactions among several reactants occur. .

Still missing is an understanding of how to circumvent the formation of local hot zones in a reactor, in which the overall heat management predicts safe operation. The local overheating may lead to major operation problems in industrial reactors. Among the reported problems that will be discussed are the formation of small hot regions in packed bed reactors, the formation of polymer sheets during polyolefin polymerization and i destruction due to melting of diesel particulate filters.

In some applications periodic reactor operation may enhance the energy utilization of the process. For example, a reverse flow operation enables circumventing the need to add fuel during the catalytic combustion of feeds containing a low concentration of pollutants.. A remaining unsolved challenging problem is how to use that reactor to conduct an endothermic reaction using the heat generated by an exothermic one.