Liqiud Mass Transfer And Mixing Characteristics Of A Three-Phase Continutious Loop Reactor

In view of the unbalanced energy structure, relatively abundant in coal but lack of oil, the direct coal liquefaction technology has particularly strategic and realistic significance for the energy security of China. Multiphase loop reactors, in which the liquid is circulated mainly by the difference of the density between the riser and downcomer but not driven by the expensive and complex pump, present a lot of advantages for the coal direct liquefaction technology. As a first step to industrialize this focusing coal liquefaction technology, it is essential to study the multiphase transport characteristics of this new type reactor.In this paper, mass transfer and mixing characteristics of a three-phase continuous loop reactor with a concentric drag tube were measured by the dynamic dissolved oxygen and saturated KCl solution pulse trace methods, with glass beads as solid phase, air as gas phase and tap water as liquid phase, respectively. Effects of different operation conditions, including superficial gas velocity, superficial slurry feeding velocity, inlet solid loading, solid particle diameter, liquid surface intension and gas aeration modes, on the gas-liquid phase mass transfer and mixing characteristics have been invesgated in terms of the volumetric liquid mass coefficient and the axial dispersion coefficient. Empirical correlations for evaluating the transport coefficients and liquid circulation velocity in the downcomer are presented. Furthermore, mass transfer and liquid mixing models were developed for studying the local mass transfer and mixing characteristics within the three phase bed of the loop reactor. With simplified assumptions, the models were solved analytically. Mass transfer coefficients derived from the mass transfer coefficient model suggest that flows in the riser and down comer are essensially a plug flow, but the whole reactor could be considerd as a continuous stirred tank in the mass transfer coefficient investigation. An analysis of the results calculated by the axial dispersion coefficient model shows that the axial dispersion coefficient values in the riser calculated by model is a little higher than that in downcomer when the superficial gas velocity is low. Furthermore, our axial dispersion coefficient results suggest that the liquid mixing for the whole bed could be better considered as a continuous stirred tank.