The flow of gases through packed-bed columns at elevated pressures was investigated by displacement experiments with a stepwise change in the tracer concentration. The experiments with different tracers, flow rates, pressures, particle sizes, tube diameters, and flow directions were used to illustrate the effect of buoyancy forces on the apparent fluid mixing in packed beds. The experimental results exhibit a strong influence of the density differences between the tracer and carrier gas on the flow behavior at higher pressures. Axial dispersion significantly increases due to hydrodynamical instabilities when the density increases with the height, and is reduced in the presence of stable density gradients. The effect of density differences on the fluid flow becomes more pronounced with increasing tube diameters. It can be argued that in pressure equipment the usual correlations for mass and heat transfer may not be valid any more and that plug flow in such equipment is hardly possible to achieve.
Benneker, A. H., Benneker, A. H., Kronberg, A. E., & Westerterp, K. R. (1998). Influence of buoyancy forces on the flow of gases through packed beds at elevated pressures. AIChE journal, 44(2), 263-271. https://doi.org/10.1002/aic.690440205