Relative Merits of Plates, and Characteristics of a Tower Packing

The selection of a particular kind of contractor plate or packed tower for a given mass transfer operation generally rests on cost and profit considerations but in many cases, the choice can be made based on a quantitative analysis of the relative merits and demerits of the plate or packed towers. For selecting the right type of contractor for a given mass transfer duty following merits and demerits should be considered. 

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Relative Merits of Plates

1. Plate columns operate over a wide range of liquid flow rates without flooding. 

2. Plate columns by repeated mixing and separation provide more positive contact between fluid phases whereas packed columns may lead to back mixing Or bypassing. 

3. Because of difficulties arising in the dispersion of liquid in the packed tower, a plate tower is more reliable and needs fewer safety factors at the low liquid-to-gas mass velocity ratios. 

4. Side streams are very easily taken out from plate towers. 

5. For plate towers design information is generally more readily available and it is more reliable than it is for packed towers. 

6. Whenever a liquid mixture containing dispersed solids is to be handled plate towers should be preferred as cleaning of the plate is very easy. 

7. Whenever intermediate cooling arrangements are required to remove the heat of a solution or reaction plate towers are preferred. Cooling coils are easily incorporated in plat towers. 

8. For a given duty the total weight of dry plate towers is normally less than the weight of a packed tower but the weight of both towers is approximately the same if the liquid holdup in the plate tower during operation is taken into account. 

9. For a column diameter of more than 1200 mm, packed towers are seldom designed and for a column diameter of less than 600 mm, packing is cheaper than plates. 

10. For a liquid having the tendency to foam, packed towers are usually preferred. 

11. High values of liquid gas ratio are best handled in packed towers. 

12. The liquid hold-up is considerably low in packed towers so they are used in cases where the liquid deterioration might occur at high temperature (for handling heat-sensitive liquids). 

13. The pressure drop through a packed tower is usually low which makes the packed tower particularly desirable for vacuum distillation operations. 

14. Packed towers may be more economical than plate towers when highly corrosive fluids are to be handled because of corrosion-resistant ceramic/plastic packing. 

15. Plate towers are often preferred when large temperature changes are involved as in distillation thermal expansion or contraction of the equipment component may crush the packing.

Packed columns and plate columns are the commonly used equipment for gas-liquid operations such as distillation and gas absorption. They are also used for liquid extraction. 

Comparison of Packed Column and Plate

Packed columns are differential contactors wherein mass transfer occurs throughout the length of the contactor and equilibrium is not reached at any point between the phases in contact whereas plate columns are stagewise contactors wherein mass transfer occurs intermittently and equilibrium is established between the phases at several discrete stages. In packed columns, packing is used as gas-liquid contacting devices whereas in a plate column plates are used as gas-liquid-containing devices. The design of a packed column mainly involves the calculation of the height of the transfer unit and the number of transfer units for a given separation whereas the design of a plate column requires the calculation of the number of theoretical stages required to effect a given separation construction wise packed columns are simple in construction whereas plate columns are complex in construction. 

Ranching rings, pall rings, Berl saddles, and into saddles are the various types of packings used in packed columns whereas bubble cap plates sieve plates, and valve plates are the various types of plates used in plate columns. The selection of a particular type of equipment for gas-liquid operation in a particular situation will depend upon many factors such as efficiency, capacity, pressure drop, corrosion, hold-up, cost, etc. 

Characteristics of a Tower Packing

1. It should provide a large interfacial area for phase contact. i.e. it should have a large wetted surface area per unit volume of packed space for high efficiencies. 

2. It should provide a large void volume or empty space in the packed bed so that reasonable throughputs of the phases are handled without excessive pressure drop. 

3. It should have high corrosion resistance. 

4. It should possess good wetting characteristics. 

5. It should be relatively cheap/inexpensive.

6. It should have a low bulk density so that the weight of the entire packed bed is low which thereby reduces serious support problems. 

7. It should possess enough structural strength. 

8. It should be chemically inert to the fluids handled in the tower.

Take these Notes is, Orginal Sources: Unit Operations-II, KA Gavhane