Packed Columns Vs Plate Columns 4w475e

Packed Columns vs. Plate Columns The difference in cost between plate and packed columns is not too great, although packing’s 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 favouring 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. Packed columns offers narrower operating range than cross-current columns:  For too low liquid flow rate: the wetting of the packing material may not be adeguate.  For too high liquid flow rate: the treatment in plate columns is often more economic. Moreover,  If solid particles are present in the liquid: the cleaning of a packing section is much too complicated and expensive compared with the plate discharge.  Working under stressed conditions of temperature variations and pressure: the packing elements are easily breakable. However packed column are the best options when:  Costs: packed column tends are less expensive than plate column for small column diameter (<0.6 m).  Corrosive chemicals handling: cheap ceramic packing material, or other chemicals resistant packing element can be easily used.  For foaming liquid: handling of foaming liquid in packed column is more appropriate because of the relatively low degree of agitation by the gas.

PACKED COLUMN FOR GAS ABSORPTION (DILUTE SYSTEMS) Gas absorption can be carried out in a packed column. We will look at both column diameter and packed height in this Section.The Figure below showed a typical gas-liquid flow in a packed column. Click here for more information of packings.

Determination of column diameter involves the analysis of pressure drop across the packed bed. As for packed height, the design used in the early days was based on the HETP method. This was largely replaced by the Method of Transfer Units. Comparison between number of theoretical trays, HETP and Method of Transfer Units The Number of Transfer Units (NTU) and Height of Transfer Units (HTU) such as NOG, HOG should not be confused with the number of theoretical trays (N), and the height equivalent to theoretical plate (HETP) respectively.When the operating line and equilibrium line are straight and parallel: NTU = N ; and HTU = HETP Otherwise, the NTU can be greater than or less than N as shown in the Figure below:

When the operating line is straight but not parallel, we have the following relationships:

where

is the Absorption Factor.