Factors Affecting Tray Efficiency

The actual number of trays needed for a particular separation duty depends on the efficiency of the plate, and the packings if they are used. Thus, any factors that cause a decrease in tray efficiency will also change the performance of the colunm. Tray efficiencies are affected by such factors as fouling, wear and tear and corrosion, and the rates at which these occur depends on the properties of the liquids being processed. Thus the proper materials of construction must be selected for tray construction. 

Factors Affecting Tray Efficiency Below is a summaiy based on the industry s experience. A detailed discussion of the fundamentals is found in Lockett s book (Distillation Tray Fundamentals, Cambridge University Press, Cambridge, England, 1986). A detailed discussion of the reported experience, and the basis of statements made in this section are in Kister s book (Distillation Design, McGraw-Hill, New York, 1992). 

Prior to discussing scaleup, the factors that affect tray efficiency need to be addressed. These factors are addressed in Secs. 7.3.1 to 7.3.3. Considerations relevant to the effect of flow regime were previously discussed in Secs. 6.4.4 and 6.4.5. 

Figure T.10 Some factors affecting sieve tray efficiency. FRI data, total reflux, DT = 4 It, S = 24 in, hu, = 2 in, dH = 0.5 in. Both parts show a small efficiency rise with pressure. Both parts show little effect of vapor and liquid loads above about 40 percent of flood, (a) Showing efficiency reduction when fractional hole area is increased from 8 to 14 per-cent of the bubbling area (6) emphasizing little effect of vapor and liquid loads, and an efficiency increase with pressure. Af 0.14 (Both parts repeated with permission from T. Yanagi and If. Sakata, lad. Eng. Chan. Proc. Use. Dev. 21, p. 712, copyright 19S2, American Chemical Society.)...

Errors in relative volatility are the most underrated factor that affects both tray and packing efficiency. The effects are direct when VLE errors affect separation stage requirement at a constant reflux ratio, and indirect when VLE errors affect the reflux ratio requirement (which in turn affects the stage requirement). Since higher relative volatility lowers both stage and reflux requirements (and vice versa), the direct and indirect effects complement each other and do not counteract each other. The discussion below applies to hoth tray and packed towers. 

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