(24f) Effect of Particle Swelling on the Effective Reaction Rate in a Fixed Bed Reactor

Fixed bed catalytic reactors (FBRs) are
the most commonly used reactors in industrial practice. A commonly used type of
FBR is a tubular reactor packed with spherical catalyst particles, and often
small tube diameter-to-particle diameter ratios are realized. This makes an
analytical description of the phenomena in these FBRs difficult, and
computational or experimental techniques must be used to access reactor
performance. In this talk we will focus on a combined computational (using
Direct Numerical Simulations, DNS) and experimental investigation of a
heterogeneous reaction relevant for fine chemicals production.

As a model reaction, an exothermic
esterification to produce acetylsalicylic acid was investigated. This reaction
takes place on the surface of an organic catalyst (Amberlite IR120). It was
found that due to the organic nature of the catalyst particles, solvent can
diffuse into the solid phase which causes particle swelling. The swelling leads
to a significant bed compaction which (i) on the one hand influences the
transport processes, and (ii) on the other hand complicates the computing of
characteristic geometric bed properties such as the particle volume fraction or
the surface area. The particle bed geometry was simulated applying a Discrete
Elements Method (DEM). By Introducing a Monte-Carlo integration method it was
possible to determine the particle volume fraction and the (for chemical
reaction available) particle surface area. Investigations on a single sphere
were used to quantify effects due to heat transfer outside and inside the
catalyst particles, and its influence on the reaction rate. In subsequent DNS
studies the reactive flow inside a short section of the particle bed was
investigated and process relevant quantities (i.e., pressure drop and
conversion) were computed for different reactor operation conditions. Extreme
spatial grid refinement near the particle surface was employed to resolve
concentration gradients in this high Schmidt-number flow (Figure 1). An
analytical model was calibrated by use of the DNS data which makes it possible
to precisely predict the overall reactor performance of packed bed reactors.

Acknowledgement

The authors acknowledge funding through
the ?NAWI Graz? project by providing access to dcluster.tugraz.at. SR
acknowledges funding through the NanoSim project (http://www.sintef.no/projectweb/nanosim).

Figure 1: Concentration distribution on
the surface of a reacting particle bed at high Schmidt numbers (the velocity
profile is illustrated at the outlet of the domain).