(139e) The Influence of Meso-Scale Flow Structure on Transport Phenomena in a Circulating Fluidized Bed

A circulating fluidized bed (CFB) has been applied widely in the industrial process due to homogeneous mass and heat transfer. However, due to lacking of clear image of understanding the influence of meso-scale flow structure, up to now, directly scaling up this process based on the theoretical model is still a challenge for chemical engineering. In the present study, the heterogeneous flow structure in circulating fluidized bed is firstly divided into three homogeneous theoretical structures, for example, the dispersed phase, the cluster phase and the inter-phase phase. And thus, in these homogeneous structures, momentum transfer, mass transfer and heat transfer coefficient can be evaluated by the equations from the experimental or theoretical results based on homogeneous assumptions.

Firstly， the parameters characterizing heterogeneous flow structure for the dispersed phase, the cluster phase and the inter-phase phase should be solved based on the models, for example, EMMS and some semi-empirical equations from fitting experimental data in literatures. And thus, the momentum transfer coefficient between gas and solid for TFM model can be evaluated by solving the force balance equations among three homogeneous phases. Mass and heat transfer coefficient between gas and solid can be obtained by respectively solving the mass and energy conservation equations in the dispersed phase and the cluster phase and then summing up it. As a result, the relationship between meso-scale flow structure and transport coefficient in CFB can be obtained.

Two flow model (TFM) has been widely used to simulate the fluidized bed. However, due to the limitation of computer’s ability, coarse calculating grid has to be used to simulate the industrial fluidized bed process. And thus, there are still heterogeneous flow structures in each calculating grid with employing it to simulate CFB. Therefore, in order to obtain the reasonably predicted results through TFM theory, which should be improved by using transport coefficients considering the influence of meso-scale flow structure in each calculating grid.

In our study, TFM model improved by considering the relationship between heterogeneous flow structure and transport coefficient is used to simulate circulating fluidized bed. Momentum and heat transfer for TFM has been verified by comparing for the axial or radial distribution of viodage and temperature between theoretical results and experimental data in literature. Regarding to mass transfer, the relative experiments were performed in a circulating fluidized bed with ID 50mm and Length 4m by using the catalytic oxidization of carbon monoxide as a model reaction. The experimental results are analyzed by the Muti Scale Mass Transfer (MSMT). And there are good agreements between experimental data and simulating results for the axial distribution of carbon monoxide.

Keywords: Circulating fluidized bed, Transport coefficient, Meso-scale, Cluster phase.