Fluidisation considerations in packed beds

Pressure drop increases with velocity in a packed bed according to the Ergun equation.

$$\frac{\partial P}{\partial x } = 150(\frac{\mu u}{d_p^2}\frac{(1-\epsilon)^2}{\epsilon ^3})+1.75(\frac{\rho u^2}{d_p}\frac{1-\epsilon}{\epsilon^3})$$

If the pressure gradient exceeds a critical value:

$$\frac{\partial P}{\partial x} >= (\rho_s - \rho_f)g$$

Then the porous media will be fluidised. In certain applications, that may be desirable, but in PSA and TSA systems, fluidisation is bad news.

Instantaneous fluidisation is known as “bumping the bed” resulting in mixing of media that will affect the cyclic steady state of the system as the concentration and temperature profile will be perturbed. It is particularly bad news in multi-layered systems where mixing of layers can cause permanent operational problems requiring replacement of the adsorbents. Fluidisation can also cause dusting, which in turn increases pressure drop and could potentially be entrained into the gas stream causing downstream issues.

Fluidisation is typically only considered on the feed step of a PSA or TSA cycle, however, it can happen on any step - for instance, fast (de)pressurisation. This can be analysed through dynamic 1D simulations of the system, and there are various methods for preventing it from occurring.

For instance, flow direction can be changed to prevent fluidisation by choosing downwards flow on the step where the maximum approach to fluidisation occurs. Typically, 1.5-2 times the minimum approach to fluidisation can be endured in down flow where the limiting factor becomes the mechanical strength of the distribution screen. Screen design must then account for the dynamic load, rather than the static load. Dynamic load includes the weight of the media + the pressure drop * area.