Meshing industrial scale mixing tank
 

I am trying to model an industrial scale mixing tank (500k-1M gal). The tank is baffled with a pitched blade impeller. I am using MRF and 5 Eulerian phases to simulate solids behavior. The solution is steady state.

I have modeled this correctly for a 1/2 gal lab vessel, and am now seeking to do scale up. My original mesh contained 509k cells. When I try scaling in Fluent, the system diverges immediately. Now I am trying to refine the mesh in Icem to model the larger tank.

My question is this: is there any rule of thumb for how high the mesh count should be? No one in my group has modeled anything this large before.

Thanks