Heat and mass transport in a nonlinear fixed-bed catalytic reactor: Hot spots and thermal runaway

Transient heat and mass transport in a wall-cooled tubular catalytic bed reactor is numerically investigated. A two-dimensional pseudo-heterogeneous model, accounting for transport in the solid and fluid phases, with axial and radial dispersions is used to describe transport in the reactor. The effects of inlet process conditions, viz., temperature and concentration, are investigated and their impact on the development of thermal runaway and hot spots in the reactor is analyzed. Under typical process conditions the calculation results show the development of a hot spot downstream the reactor inlet. At reduced feed temperature thermal runaway develops for an inlet concentration of 0.505 mol/m3. A criterion for thermal runaway limit has been developed whereby runaway can be detected at a point in time during the process when the time derivative of temperature increases monotonously with time throughout the bed. Under low feed concentration and temperature a simpler pseudo-homogeneous model can be used to describe the reactor.