Effect of process conditions on fluidization

Abstract

The influence of process conditions such as temperature and the presence of fines on the fluidization behaviour of gas-fluidized beds is of major importance in industrial fluid bed processes, which are often operated at temperatures well above ambient and where it is common practice to add fine particles to improve the reactor performance. Several works have demonstrated that process conditions can influence the role of the interparticle forces in the fluidization behaviour of powders. In particular, the beneficial effect on fluidization of adding fines to the bulk of the material is well known. The objective of this paper is to review experimental and theoretical studies of gas-solids fluidized beds operated at high temperature and the effect of fines and fines distribution within the bed. The survey begins with a review of the effect of temperature on fundamental fluidization parameters such as minimum fluidization, bed expansion and deaeration, and the role of hydrodynamic and interparticle forces at ambient conditions and high temperature is discussed. The effect of temperature and fines and fines size distribution on the dynamics of gas-fluidized beds is considered next, highlighting areas of current gaps in knowledge. Given the complexity of the phenomena involved, a direct quantification of the particle-particle interactions in fluidized beds and of their changes under process conditions is very difficult. The review concludes by touching upon powder rheology as a methodology to evaluate indirectly the effects of the IPFs on fluidization. This leads to a review of the work done at UCL on linking rheological measurements to fluidization tests in the attempt to quantify the effect of process conditions, i.e. high temperature and the effect of fines on fluidization.