Residence Time Distributions for Free Convection in Packed Beds

Experimental tracer pulse responses were recorded for a laboratory simulation of a fixed-bed reactor. The simulation consisted of a packed bed fitted with an electrical heater and supplied with a gas throughflow. Radically different responses were observed depending on whether the bed was undergoing free convection or not as inferred from the calculated Rayleigh number. For the free-convection cases, the response was characterized by a large early peak followed by a second smaller peak in contrast to the single broad peak characteristic of dispersed plug flow. Interpretation of the experimental results was obtained from numerical calculations of the Residence Time Distribution (RTD) and resulting plots of steady state stream function and concentration contours for different times after injection of the pulse. The successive peaks were seen to arise from the rotation of a high-concentration zone around a convection cell. The time between peaks is thus the mean circulation time of the convection cell. The method is rapid to use and allows immediate determination of the convecting state of a packed bed. It should be possible to use it for the verification of theoretical results, particularly for stability limits.