Thermal runaway conditions of a partially diluted catalytic reactor

The generalized parametric sensitivity criterion was used to obtain runaway diagrams for one- and two-dimensional heterogeneous models and a one-dimensional pseudo-homogeneous model. The simulation work was based on the partial methanol oxidation to formaldehyde, which occurs in a fixed-bed reactor, with two distinct catalytic zones. At the inlet of the reactor, the catalyst was diluted with inert packing followed by a region with pure catalyst. This activity profile induces a reduction in the parametric sensitivity of the process to temperature runaway, when compared to a uniform activity bed, and the sensitivity functions reach a minimum when the inlet temperature is in the range of 530-540 K. The dilution of the catalytic bed leads to an enlargement of the stable region of operation, which allows a gain in the critical methanol concentration of 20% for a feed temperature of 530 K (industrial operating temperature), higher gains being possible (similar to 40%) for higher inlet temperatures. Moreover, depending on the particular set of the operating conditions, one or two hot spots can be developed, one in each zone of the bed, it being important to follow both because one or the other can determine the critical conditions of the system. The additional mass flux by intraparticle convection leads to an increase in the parametric sensitivity when compared with the case where diffusion is the only mechanism taken into account.