An investigation of the influence of fines distribution and high temperature on the fluidization behaviour of gas fluidized beds linked with rheological studies
The influence of process conditions, such as temperature and 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 (IPFs) 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. However, the influence on fluidization of the different IPFs introduced when adding "big" fines (e.g. 26-45 microm) or "small" fines (e.g. 0-25 microm) has not been evaluated yet. Given the complexity of the phenomena involved, a direct quantification of the particle-particle interactions in fluidized beds and of their changes at process conditions is very difficult. Within this framework, powder rheology represents an appealing tool to evaluate indirectly the effects of the IPFs on fluidization. The present work investigated the effects of adding different fines cuts on the fluidization and the rheological behaviour of an alumina powder. To this end, a two fold approach was followed and a link between the two aspects of the work was sought. On the one hand, the fluidization behaviour of the alumina was studied at process temperatures ranging from ambient to 400 °C and by adding to the material previously deprived of fines two fine sub-cuts of nominal size 0-25 and 26-45 am respectively. Experiments were carried out changing the total fines content from 22% to 30% by weight.