How would one determine if a fluidised bed (with liquid flow) is determined to be a stable fluidised bed, which subsequently doesn't form a plug or bed at the top of a column (bounded by frits), when under pressure?
Use flow (which is a product of pressure).
If a single solid will be entrained at the flow rate then it will plug.
If the bed has a packing factor of 50% then flow velocity in the bed is twice pipe flow velocity. As the bed fluidizes the packing factor goes down as does flow velocity.
VelocityPacking = VelocityPipe / (1 - packing)
The smaller particles will be lost (plug) first.