The objective of this simulation is to observe thermal runaway in a tubular reactor and identify the critical parameters that represent the crossover from a thermally well-behaved reactor to one that exhibits thermal runaway. Consider a chemical reactant that is converted irreversibly and exothermically to products in a tubular catalytic reactor. The reactive mixture in the inner pipe flows from left to right and is cooled using a concentric double-pipe heat exchanger. The cooling fluid in the annular region can flow either concurrently or countercurrently with respect to the reactive fluid.