A computer implemented method simulates the behavior of a gas-solid mixture in a fluidized bed having a particulate comprising Geldart group A particles using a Eulerian model comprising, for each time step (a current time step) and for each cell: obtaining an initial value of an agglomerate diameter corresponding to the cell and corresponding to the beginning of the current time step; performing a force-balance procedure for the cell to determine a first agglomerate diameter corresponding to the cell and corresponding to the current time step, wherein performing the force-balance procedure comprises determining a plurality of forces for the cell based at least in part on the initial value of the agglomerate diameter; updating one or more drag relationships for the current time step based at least in part on the first agglomerate diameter; and performing a computational fluid dynamic (CFD) solve procedure to determine, for the current time, the any one or more of: one or more of: a gas velocity {right arrow over (V)}g for the cell, a particle velocity {right arrow over (V)}s for the cell, a gas pressure for the cell, a particulate pressure for the cell and a solid fraction εs for the cell. Performing the CFD solve procedure is based at least in part on the updated drag relationships.