If I set the time step value to a large number like 2 seconds or 10 seconds, an error occurs. Is there a way to avoid this, or does it always have to be set as a decimal?
You have set the minimum time step to 1 second (DT_MIN). This is likely way too large to obtain convergence for a fluidized bed. MFiX will try to adapt the time step between DT_MIN and DT_MAX to get the fastest run. When convergence does not occur, the time step decreases down to at most DT_MIN. Once you reach DT_MIN, the simulation is considered failed and will stop. Here you should use a much smaller DT_MIN to get a chance to converge.