Effects of fin configuration on heat transfer rate in packed bed reactors for improvement of their thermal characteristics

Packed bed reactors are utilized for catalysts, chemical heat pumps, etc. Because the effective thermal conductivities of the packed beds of particles are generally low (≈10^-1 W/mK), this matter often results in low performance and degradation of catalyst. Many heat transfer tubes with fins and/or much filler with high thermal conductivities are inserted in the packed bed reactors to improve heat transfer rate. In return to this, the volume of reactive particles packed into the reactors, or stored energy, decreases. In this study, the effect of fin configurations on the heat transfer rate in the reactors is numerically investigated. Three configurations of fins are studied. (1) "Sheet type" is a longitudinal fin attached on the heat transfer tubes. It is placed to connect between heat transfer tubes. (2) "Straight type" is several longitudinal fins in the half length of the tube pitch attached on the tube with radial structure. (3) "Spiral type" is many narrow rectangular fins attached on the tube with spiral structure. To discuss the effect of fin configuration on the heat transfer generally, the heat conduction equation in the packed bed around the tube is converted to the dimensionless form. The transient temperature responses in the packed bed and fins at a uniform temperature are calculated when the temperature of the tube surface is stepwise changes. In another analytical system, a homogeneous body around the tube is assumed. To evaluate the thermal performance of the fin, apparent thermal conductivity is defined as the thermal conductivity which gives the same thermal response as that calculated in the heterogeneous system. As a result, the spiral type rather than the straight and sheet types effectively increases apparent thermal conductivity. The apparent thermal conductivity of the spiral type is two-three times larger than the straight type, and ten times as large as the sheet type. This result indicates dispersion of fins in packed bed is essential to improve the thermal response in the reactors.