Measuring residence time distributions of wood chips in a screw conveyor reactor

Abstract

In rotating screw conveyors both the average and the distribution of the residence time influence the extent and the uniformity of the transformation. Experimenters have applied two distinct experimental approaches to obtain the residence time distribution of granular solids in longitudinal reactors: 1) measuring the mass flow rate of product at the exit from the reactor in response to a step change (either positive or negative) in the mass flow rate of feedstock into the reactor or 2) measuring the appearance of a tracer in the flow exiting the reactor in response to either a pulse or a step change addition of tracer in the inlet. We found that all three methods reveal residence time distributions that are approximately normal (i.e., symmetrical and bell-shaped), but the distribution estimated from the pulse input of tracer exhibited a long trailing tail that was not detectable in either the positive or negative step changes. Second, we demonstrated that a normal probability plot proved valuable in displaying and analyzing the residence time distribution obtained by the pulse addition of tracer. Finally, we observed that all three methods yielded mean residence times that consistently differed from the nominal values. The positive step change averaged 8% shorter, the pulse addition of tracer averaged 7% longer, and the negative step change averaged 60% longer.