Transverse concentration and temperature nonuniformities in adiabatic packed-bed catalytic reactors

Several catalytic reactions such as partial oxidations and hydrogenations a re carried out in packed-bed reactors with diameter-to-length (aspect) rati os of order unity and a reactor-to-particle diameter ratios of 20 or more. In the design and operation of these reactors one objective is to prevent t he side reactions that may occur due to localized hot spots or temperature variations. In such reactors, the transport coefficients in the flow and tr ansverse directions can be quite different. In the present study, we report a new type of instability, the transverse instability, which leads to non- uniform temperature and concentration profiles in the lateral direction (ac ross the diameter). This instability can occur even when the physical prope rty variations are negligible and velocity is constant and unidirectional. We consider a reaction of the type A + nu B --> P with a Langmuir-Hinshelwo od-type kinetic expression and use a pseudohomogeneous model. For a fixed n umber of catalyst particles in the direction of flow (L/d(p)), the boundary that determines the onset of transverse concentration and temperature nonu niformities is presented in the parameter space defined by the number of ca talyst particles in the transverse direction (R/d(p)) and the residence tim e (Da). Our results indicate that transverse nonuniformities are likely to occur for typical values of the reaction parameters when the reactor to par ticle diameter exceeds about 5. For aspect ratios of order unity, this numb er is nearly independent of the number of particles in the flow direction. (C) 1999 Elsevier Science Ltd. All rights reserved.