Sieve trays operating ranges

Valve trays. These may be regarded as a cross between a bubble-cap and a sieve tray. The construction is similar to that of cap types, although there are no risers and no slots. It may be noted that with most types of valve tray the opening may be varied by the vapour flow, so that the trays can operate over a wide range of flowrates. Because of their flexibility and price, valve trays are tending to replace bubble-cap trays. Figure 11.53 shows a typical tray. 

Bubble-cap trays may be operated over a far wider range of vapor flows, without loss of tray efficiency. It is the author s experience that bubble-cap trays fractionate better in commercial service than do perforated (valve, or sieve) trays. Why, then, are bubble-cap trays rarely used in a modern distillation ... 

A sieve tray-type absorption column is proposed. The design specifies a column 1.8 m diameter, approximately 32 m high, and containing 59 sieve plates. Weak-acid condensate is added to tray 13 and make-up water is added at tray 59 (the top tray). Crossflow-type trays are employed from plates 1 to 13, however, a decreased liquid loading demands reverse flow-type trays for plates 14 to 59. The operating pressure is approximately 950 kPa and the operating temperature range is from 8°C to 65°C. 

FIGURE 8.10 Approximate operating range of sieve trays. Note 1 psia = 6.895 kPa. 

These contacting devices can extend the operating range of a crossflow sieve tray, as indicated by Figure 12.26, which shows that the sieve tray under observation loses efficiency at an F-factor of about 1.0 in/s(kg/m )o, whereas the addition of valves permits operation down to an F-factor of... 

The suitable control range of differential pressure controllers may be narrow, especially with valve trays (Fig. 19.126). In the valvethrottling range of a valve tray, and below the weep point of a sieve tray, pressure drop may be insensitive to vapor loads. Differential pressure control may therefore be difficult to apply imder turned-down conditions. Packed towers are not prone to this limitation (44), because pressure drop tends to be sensitive to vapor load over the entire operating range (Fig. 14.3). 

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