A new and rapid method for the evaluation of the liquid-solid contact resulting from liquid injection into a fluidized bed

摘要

Liquid feeds are injected into fluidized bed reactors such as fluid cokers, fluid catalytic crackers and gas-phase polymerization reactors. In these industrial processes, it is of crucial importance to optimize the contact between the injected liquid and the bed solids to minimize agglomeration, to ensure good reactor operability, and to allow reactions to proceed under minimum heat and mass transfer limitations in order to maximize the yields of valuable products. It has been shown [P. House, M. Saberian, C. Briens, F. Berruti, E. Chan, Injection of a liquid spray into a fluidized bed: particle-liquid mixing and impact on fluid coker yields. Industrial & Engineering Chemistry Research 43 (2004) 5663-5669., S. Bruhns, J. Werther, An investigation of the mechanism of liquid injection into fluidized beds. AIChE Journal 51 (2005) 766-775] that the nozzle technology and the operating conditions have a significant effect on the quality of the liquid-solid interaction resulting from the injection of gas-atomized liquid feed. The goal of this study was to develop a rapid and reliable experimental technique to assess the liquid-solid contact efficiency resulting from the injection of a liquid feed into a fluidized bed. Air-fluidized silica sand particles were first charged by triboelectrification as a result of their random collisions with the inner walls of the fluidized bed. Immediately after the injection of water through an aerated nozzle, the fluidization air was stopped and the wetted bed solids were allowed to settle. While the bed was defluidized, the triboelectric charges accumulated on the particles migrated to a grounded electrode through the low-resistance paths offered by the conductive liquid. A stronger electric current flowing through the electrode indicated that the liquid was more evenly distributed on the solid particles. The intensity of the current flowing through the electrode was, therefore, used to define a spray nozzle performance index. This technique was used to examine the effect of increasing the nozzle aeration, and, specifically, the gas-to-liquid mass ratio (G/L) through the nozzle on the liquid-solid contact efficiency. The results showed that changing the nozzle geometry can change how the contact efficiency between atomized liquid and fluidized solids varies with (G/L), especially at relatively high G/L ratios. A model of the time-evolution of the electric current generated during defluidization of the bed solids is also presented.