Relative Merits Of Plate And Packed Towers
The choice between use of a plate tower or a packed tower for a given mass-transfer operation should, theoretically, be based on a detailed cost analysis for the two types of contactors. Thus, the optimum economic design for each type would be developed in detail, and the final choice would be based on a consideration of costs and profits at the optimum conditions. In many cases, however, the decision can be made on the basis of a qualitative analysis of the relative advantages and disadvantages, and the need for a detailed cost comparison is eliminated. The following general advantages and disadvantages of plate and packed towers should be considered when a choice must be made between the two types of contactors:
1. Stage efficiencies for packed towers must be based on experimental tests with each type of packing. The efficiency varies, not only with the type and size of packing, but also with the fluid rates, the fluid properties, the column diameter, the operating pressure, and, in general, the extent of liquid dispersion over the available packing surface.
2. Because of liquid-dispersion difficulties in packed towers, the design of plate towers is considerably more reliable and requires less safety factor when the ratio of liquid mass velocity to gas mass velocity is low.
3. Plate towers can be designed to handle wide ranges of liquid rates without flooding.
4. If the operation involves liquids that contain dispersed solids, use of a plai.. tower is preferred because the plates are more accessible for cleaning.
5. Plate towers are preferred if interstage cooling is required to remove heats of reaction or solution, because cooling coils can be installed on the plates or the liquid-delivery line from plate to plate can be passed through an external cooler.
6. The total weight of a dry plate tower is usually less than that of a packed tower designed for the same duty. However, if liquid holdup during operation is taken into account, both types of towers have about the same weight.
7. When large temperature changes are involved, as in distillation operations, plate towers are often preferred because thermal expansion or contraction of the equipment components may crush the packing.
8. Design information for plate towers is generally more readily available and more reliable than that for packed towers.
9. Random-packed towers are seldom designed with diameters larger than 4 ft, and diameters of commercial plate towers are seldom less than 2 ft.
10. Packed towers prove to be cheaper and easier to construct than plate towers, if highly corrosive fluids must be handled.
11. Packed towers are usually preferred if the liquids have a large tendency to foam.
12. The amount of liquid holdup is considerably less in packed towers.
13. The pressure drop through packed towers may be less than the pressure drop through plate towers designed for the same duty. This advantage, plus the fact that the packing serves to lessen the possibility of tower-wall collapse, makes packed towers particularly desirable for vacuum operation.
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