| THEORY & FORMULAE |
Packed columns are most frequently used in chemical engineering to remove contaminants from a gas stream (absorption), and for removing volatile components from a liquid stream by contacting it with an inert gas (stripping). They are also used in distillation applications where the separation is particularly difficult due to close boiling components.
Selecting a type and size of is usually the first step in packed column design, and the choice is between random and structured packing. Generally, the column diameter to packing size ratio should be greater than 30 for Raschig rings, 15 for ceramic saddles, and 10 for rings or plastic saddles.
The second step is the determination of column diameter. Most methods for determining the size of randomly packed towers are derived from the Sherwood's generalized correlation. This is usually given in graphic form where line on the graph is marked with an acceptable pressure drop in inches of water per foot of packing. Guidelines which largely designed around "flooding pressure drops" documented in literature, are as follows: Moderate to high pressure distillation = 0.4 to 0.75 in water/ft packing; Vacuum Distillation = 0.1 to 0.2 in water/ft packing; Absorbers and Strippers = 0.2 to 0.6 in water/ft packing.
Flooding is caused when the energy potential of the vapor is greater than that of the liquid, so that the liquid travels up and the column floods. Packed towers are typically operated at a gas velocity which corresponds to about 50 - 80% of flooding conditions.
Sherwood's correlation relates the following three dimensionless groups:
where
L = liquid rate head
G = gas rate
F = packing factor
ρL = liquid density
ρG = gas density
μ = liquid viscosity
gc = gravitational constant
where
ΔP = pressure drop
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