| THEORY & FORMULAE |
The packed bed represents a workhorse configuration for a wide variety of mass transfer operations in the chemical process industry, such as distillation, absorption and liquid/liquid extraction. The packed bed configuration also facilitates the intimate mixing of fluids with mismatched densities, largely due to increased surface area for contact.
Flow through a packed bed can be regarded as fluid flow past some number of submerged objects. When there is no flow through the packed bed, the net gravitational force (including bouyancy) acts downward. When flow begins upward, friction forces act upward and counterbalance the net gravitational force. For a high enough fluid velocity, the friction force is large enough to lift the particles. This represents the onset of fluidization. The frictional force can be expressed in terms of a friction factor. This leads to equations describing the the flow of a fluid past a collection of particles. The Ergun correlation is one such equation:
where
ΔP = pressure drop
L = unit depth of packed bed
gc = dimensional constant
μ = fluid viscosity
V = superficial fluid velocity
G = superficial mass velocity
D = effective particle diameter
ε = void fraction of bed
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