Contacting trays

The separation operation called distillation utihzes vapor and hquid phases at essentially the same temperature and pressure for the coexisting zones. Various lands of devices such as r andom or sti uctui ed packings and plates or tr ays are used to bring the two phases into intimate contact. Trays are stacked one above the other and enclosed in a cyhndrical shell to form a column. Pacldngs are also generally contained in a cyhndrical shell between hold-down and support plates. A typical tray-type distillation column plus major external accessories is shown schematically in Fig. 13-1. 

The lighter (lower-boiling) components tend to concentrate in the vapor phase, while the heavier (higher-boihng) components tend toward the liquid phase. The result is a vapor phase that becomes richer in hght components as it passes up the column and a liquid phase that Becomes richer in heavy components as it cascades downward. The overall separation achieved between the distillate and the bottoms depends primarily on the r elative volatilities of the components, the number of contacting trays, and the ratio of the liquid-phase flow rate to the vapor-phase flow rate. 

General Listing and Comparison of the Major Contacting Trays ... 

Figure 10-173. Direct contact tray column for heat transfer. This could be a baffle tray, sieve type tray, bubble or other contact device, or open spray or random packed column. (Symbols only used by permission Smith, J. H. Hydrocarbon Processing, Jan. 1979, p. 147. Gulf Publishing Company. All rights reserved.)...

This overall flow pattern in a distillation column provides counter-current contacting of vapor and liquid streams on all the trays through the column. Vapor and liquid phases on a given tray approach thermal, pressure, and composition equilibria to an extent dependent upon the efficiency of the contacting tray. 

Contact Tray and Vacuum-Shelf Dryers Vacuum-shelf dryers are indirectly heated batch dryers consisting of a vacuum-tight chamber usually constructed of cast iron or steel plate, heated, supporting shelves within the chamber, a vacuum source, and usually a condenser. One or two doors are provided, depending on the size of the chamber. The doors are sealed with resilient gaskets of rubber or similar material. It is also possible, but much less common, to operate at atmospheric pressure without vacuum. 

The mass transfer efficiency of contacting trays is often expressed in several ways, but here only two efficiencies will be nsed point efficiency and tray efficiency. The former deals with the approach to equilibrium at some point on the tray and cannot be greater than 1.0 (100%). Clearly, the equilibrium can vary across the tray as liquid composition varies thus, there are a number of different values of point efficiency when the tray liquid is not completely mixed (the normal case). 

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