The difference in cost between plate and packed columns is not too great, although packings are generally more expensive than plates.
In addition, the difference in column height is not usually significant if the flow rates are such that efficiencies are near maximum.
Conditions favouring packed columns:
Small-diameter columns (less than 0.6m)
More choices in materials of construction for packings especially in corrosive service (e.g. plastic, ceramic, metal alloys)
Lower pressure drop (important in vacuum distillation)
Less liquid entrainment · low liquid hold-up, especially suitable for thermally sensitive material
Foaming liquids can be handled more readily (less agitation of liquid by the vapour)
Conditions favoring plate columns:
Variable liquid and/or vapour loads
Low liquid rates · large number of stages and/or diameter
High liquid residence time
Dirty service (plate columns are easier to clean)
Presence of thermal or mechanical stress due to large temperature changes which might lead to cracked packings
Exotherms requiring cooling coils inside column
As a rule of thumb, plates are always used in columns of large diameters and towers that have more than 20 to 30 stages.
Attributes | Packed Tower | Tray Tower |
Gas Pr Drop | Smaller | |
Liquid hold up | Smaller | |
Liquid / gas ratio | High values can be best handled | Low values can be best handled |
Liquid cooling | Cooling coils more readily built | |
Side Stream | More readily removed | |
Foaming system | Operate with less bubbling of gas & hence more suitable. | |
Corrosion | For difficult corrosion, less costly | |
Solid present | Not satisfactory | Not satisfactory |
Cleaning | Frequent, Cleaning difficult | Easier |
Temp. Fluctuation | Fragile packing tend to crush |