Residence time distribution of solids in a fluidised bed

TL;DR: In this article, the effect of gas flow rate, solids flow rate and dilution on the first, second and third moments and on F-curves of a continuous fluid-ised bed for uniform size and for binary solid mixture at different dilutions was investigated.

Abstract: The present work involves an experimental investigation on residence time distribution of solids in a continuous fluid­ ised bed for uniform size and for binary solid mixture at different dilutions. The column was tested for improvement in per­ formance with increase in dilution. The effect of gas flow rate, solids flow rate and dilution on the first, second and third moments and on F-curves have been determined. Fractional tank extension model fits the experimental data. High values of N as dilution increases, indicates reduced solids mixing, thus reducing spread of residence times, giving rise to more uni­ form quality of product. The work finds importance in that similar improved performance of the column using internals in­ side the bed can be obtained by :'~ i ng a mixture of solids rather than uniform size. Fluidised beds find wide applications due to uniform temperatu~e throughout th e bed because of solids mixing. But this rapi d solids mixing in the bed leads to non-uniform residence times of solids in the reac­ tor. For continuous treatment of solids, this gives non-uniform product and lower conversions, espe­ cially at higher conversion level s. Despite its serious drawbacks, fluidis ed beds have been widely used by maki;1g modifications inside the bed. One such modi­ fication is to section the bed horizontalli or verti­ ca lll, known as a multistage fluidised bed or using a spiral internal to make it a spiral fluidised bedS . These modifications have been made to reduce the gross irregularities and instability of the bed induced by the gas flow . The present work is aimed at reducing the spread of residence times in a single stage flat fluidised bed, which is achieved by using a binary solid mixture. The uniform sized particles give rise to more non­ uniform re sidence times for solids. Hence a binary mixture has been used, where the fine size formed the major pO/iion and th e coarser size formed the minor portion. In other words, the fine sized solids are di­ luted with the coarser sized particles and the percent­ age of dilution (same as the percentage of coarse size) is varied. Experiments were carried out to test the improved performance of the bed with increase in dilution. Similar observations of betterment in fluidi­ sation phenomena were observed by Chen and Pei 2