Sieve trays phenomena

Figure 1332. Internals and mode of action of trays in tray towers, (a) Some kinds of bubblecaps (Glitsch). (b) Two kinds of valves for trays, (c) Vapor directing slot on a Linde sieve tray [Jones and Jones, Chem. Eng. Prog. 71, 66 (1975)]. (d) Vapor flow through a bubblecap. (e) Sieve tray phenomena and pressure relations hh is the head in the downcomer, h, is the equivalent head of clear liquid on the tray, hf is the visible height of froth on the tray, and h, is the pressure drop across the tray (Bolles, in Smith, 1963). (f) Assembly of and action of vapor and liquid on a bubblecap tray.

The relationship of vapor and liquid under spray was observed by Sakata and Yanagi (1979) for the sieve tray As the liquid rate reduces beyond a certain amount corresponding to weir loading of 2gpm/in. (gpm is gallons per minute), vapor rate must reduce to maintain the same entrainment rate. This reducing trend of both vapor and hquid rates under very small weir loading defines the spray phenomenon. This trend is different from the tray flood phenomenon under which vapor rate increases as liquid load reduces. 

Under certain conditions, sieve trays are salyect to lateral oscillations of the liquid, which may slosh from side to side or from the center to the sides and back. The latter Qrpe especially may seriously increase entrainment. The phenomenon is related to froth height relative to tower diameter 8, 9] and a change from frothing to bubbling [96] but is incompletely characterized. 

There are diree factors that limit the accuracy of the preceding analysis. The first of these relates to the phenomenon of inverse response discussed in Chapter 13. It is characteristic of valve tray columns and some sieve tray columns operating at low boilup rates. It exercises its most serious effect in those columns where base level is controlled via steam flow. If the level becomes too high, the level controller increases the steam flow. But this causes a momentary increase in base level due to the extra liquid coming down the column (also due to thermosyphon reboiler swell ). Without proper design the level controller can become very confused. This is discussed in detail in Chapter 16. 

Wang [24] simulated the Oldershaw sieve tray [24] with consideration of using two zones model for the liquid on the tray. The distillation is three-component non-ideal solution (ethanol, isopropanol, and water) for the purpose of investigating the bizarre phenomenon of multicomponent distillation. 

---