Packed Columns vs Plate Columns

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 small-diameter columns (less than .!m" more choices in materials of construction for packings especially in corrosive service (e.g. plastic, ceramic, metal alloys" • •

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lower pressure drop (important in vacuum distillation"

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less li#uid entrainment $ low li#uid hold-up, especially suitable for thermally sensitive material

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foaming li#uids can be handled more readily (less agitation of li#uid by the vapour"

Conditions favouring plate columns: • variable li#uid and%or vapour loads • low li#uid rates $ large number of stages and%or diameter  •

high li#uid residence time

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dirty service (plate columns are easier to clean"

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 presence of thermal thermal or mechanical mechanical stress due to to large temperature temperature changes changes which might lead lead to cracked  packings exotherms re#uiring cooling coils inside column

&s a rule of thumb, plates are always used in columns of large diameters and towers that have more than ' to  stages. )acked 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. +oreover, 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. owever packed column are the best options  when* Costs* packed column tends are less expensive than plate column for small column diameter (.! m". Corrosive chemicals handling* cheap ceramic packing material, or other chemicals resistant packing element can be easily used. For foaming liquid* handling of foaming li#uid in packed column is more appropriate because of the relatively low degree of agitation by the gas. • •

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as absorption can be carried out in a packed column. /e will look at both column diameter and packed height in this 0ection.The 1igure below showed a typical gas-li#uid flow in a packed column. 2lick here for more information of packings.

3etermination of column diameter involves the analysis of pressure drop across the packed bed. &s for packed height, the design used in the early days was based on the 4T) method. This was largely replaced  by the +ethod of Transfer 5nits. Comparison between number of theoretical tra0s1 2!,P and ðod of ,ransfer %nits

The 6umber of Transfer 5nits (6T5" and eight of Transfer 5nits (T5" such as 6 7, 7 should not be confused with the number of theoretical trays (6", and the height e#uivalent to theoretical plate (4T)" respectively./hen the operating line and e#uilibrium line are straight and parallel* ',% 3 ' 8 and 2,% 3 2!,P 7therwise, the 6T5 can be greater than or less than 6 as shown in the 1igure below*

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

 where

is the Absorption Factor.