Packed Columns Vs tray column

Packed Columns vs. Plate Columns 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 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

CHOICE BETWEEN PLATE AND PACKED COLUMN

The choice between use of tray column or a packed column for a given mass transfer operation should, theoretically, be based on a detail cost analysis for the two types of contactors. However, the decision can be made on the basis of a qualitative analysis of relative advantages and disadvantages, eliminating the need for a detailed cost comparison. Which are: 1. Because of liquid dispersion difficulties in packed columns, the design of tray column is considerably more reliable. 2. Tray columns can be designed to handle wide ranges liquid rates without flooding. 3. If the operation involves liquids that contain dispersed solids, use of a tray column is preferred because the plates are more accessible for cleaning.

4. For non-foaming systems the plate column is preferred. 5. If periodic cleaning is required, man holes

will be provided for cleaning. In packed

columns packing must be removed before cleaning. 6. For large column heights, weight of the packed column is more than plate column. 7. Design information for plate column is more readily available and more reliable than that for packed column. 8. Inter stage cooling can be provided to remove heat of reaction or solution in plate column. 9. When temperature change is involved, packing may be damaged. 10. Random-packed columns generally are not designed with diameters larger than 1.5 m, and diameters of commercial tray column are seldom less than 0.67m. As my system is non foaming and diameter calculated is larger than 1.5m so I am going to use tray column. Also as average temperature calculated for my distillation column is higher that is approximately equal to 98oc. So I prefer Tray column. PLATE CONTACTORS: Cross flow plate are the most commonly used plate contactor in distillation. In which liquid flows downward and vapours flow upward. The liquid move from plate to plate via down comer. A certain level of liquid is maintained on the plates by weir. Other types of plate are used which have no down comer (non-cross flow) the liquid showering down the column through large opening in the plates (called shower plates). Used when low pressure drop is required. Three basic types of cross flow trays used are

(1)

Sieve Plate (Perforated Plate)

(2) (3)

Bubble Cap Plates Valve plates (floating cap plates) I prefer sieve plate because: (1) Their fundamentals are well established, entailing low risk. (2) The trays are low in cost relative to many other types of trays.

(3) They can easily handle wide variations in flow rates. (4) They are lighter in weight. It is easier and cheaper to install. (5) Pressure drop is low as compared to bubble cap trays. (6) Peak efficiency is generally high. (7) Maintenance cost is reduced due to the ease of cleaning.