The stability of free convection in a particle-fluid mixture with a free surface

Abstract

Recent MRI data have shown that the transition from trickle to pulsing flow in trickle-bed reactors occurs over a range of liquid velocities at constant gas velocity. The transition is initiated by isolated local pulsing events, which increase in number with increase in liquid velocity until a maximum number exists. Above this liquid velocity, which we have termed the transition point, the individual pulses merge until a single macro-scale pulse is formed and the whole bed demonstrates pulsing flow. In this paper we compare the characterisation of the transition obtained using conductance and pressure drop measurements with that obtained using MRI. Using the insights gained from the 3-D MRI measurements, recorded with a data acquisition time of 280 ms, it is shown that the conductance and pressure drop measurements are sensitive to different stages of the evolution of the hydrodynamic transition, a factor important when using these different measurements in the development and validation of numerical and theoretical models. Conductance measurements identify unambiguously only the onset of the single macro-scale pulse regime, consistent with a determination of the transition point made by visual observation. In contrast, pressure drop measurements are sensitive to both the onset of formation of local pulses and the liquid velocity at which the maximum number of liquid pulses occurs. We also show how a combination of conductance and pressure drop measurements can be used to fully characterise the transition, thereby enabling translation of the insights gained by MRI into a robust measurement strategy for use on larger-scale reactors. Data are reported for a cylindrical column of length 70 cm and inner diameter 43 mm, packed with cylindrical porous γ-Al2O3 packing elements of length and diameter 3 mm. The bed was operated under conditions of co-current downflow of air and water, at ambient temperature and a pressure of 2 barg. Gas and liquid superficial velocities were in the range 25–300 and 0.9–13.8mms-1, respectively.