Mass transport effects during measurements of gas-solid reaction kinetics in a fluidised bed

Artikel i vetenskaplig tidskrift, 2007

Design, modelling and scaling of many gas-solid reaction processes in a fluidised bed (FB) require reliable intrinsic kinetic data obtained under conditions similar to those in industrial-scale units. The determination of such data in a laboratory-scale FB seems to be the best route. Despite this, owing to practical difficulties, few experimental reactivity data are available from FB. One of the drawbacks of FB reactors is the plausible interference of physical processes on kinetic measurements. In this work a simple methodology is developed for the assessment of fluid-dynamic and mass transport effects during kinetic experiments in FB (FBKE). A modelling approach is proposed, which combines a kinetic particle model with a simple two-phase flow model. The parameters resulting from the model are expressed in terms of three observable quantities, making it possible to evaluate the transport effects in a straightforward way from gas concentration measurements. Charts for direct evaluation of transport effects in FBKE are included. The analysis presented aims at facilitating the selection of optimum operating conditions for FB tests to determine gas-solid kinetics. Moreover, it may support dimensioning of new rigs designed for this purpose. This study primarily aims at the assessment of transport effects in batch-operated laboratory FBKE, where isothermal conditions are assumed and only one reaction occurs: the reaction between CO2 and char is taken as an example. The methodology developed can be applied to other isothermal systems to estimate the influence of diffusional effects on the observed reaction rate, i.e., char oxidation and other catalytic or non-catalytic systems.