Tray Capacity Enhancement

Hole Sizes Small holes slightly enhance tray capacity when limited by entrainment flood. Reducing sieve hole diameters from 13 to 5 mm ( to in) at a fixed hole area typically enhances capacity by 3 to 8 percent, more at low liquid loads. Small holes are effective for reducing entrainment and enhancing capacity in the spray regime (Ql < 20 m3/hm of weir). Hole diameter has only a small effect on pressure drop, tray efficiency, and turndown. 

High-capacity trays evolved from conventional trays by including one or more capacity enhancement features such as those discussed below. These features enhance not only the capacity but usually also the complexity and cost. These features have varying impact on the efficiency turndown, plugging resistance, pressure drop, and reliability of the trays. 

Antijump baffles (Fig. 6.16) are often installed in the center and off-center downcomers of multipass trays in order to avoid a phenomenon similar to liquid throw (Sec. 6.16). Here the concern is that liquid jumping across the center downcomer, from one side to another, may cause excessive localized liquid buildup near the tray outlet, which may lead to premature flooding. Simulator tests (144) showed that antijump baffles can enhance tray capacity by as much as 25 percent in some instances. 

FIGURE 12-7 Methods to enhance tray capacity without sacrificing fractionation efficiency... 

Fractional hole area (sieve trays). Eight to 10% is generally considered optimum. Higher area may enhance capacity at the expense of more weeping at low gas flow rates. 

Systems restricted by downcomer choke With such systems, increasing tray spacing does little to enhance capacity and there is an incentive to minimize tray spacing. Unfortunately, downcomer choke usually coincides with a high foaming tendency, and guideline 6 above overrules. 

The following example is designed with the purpose of enhancing the understanding of column tray design, operation window and the capacity limits. 

Thus, the 50 IMTP packing can revamp the trayed column to obtain a 55% increase in capacity at the same separation efficiency. The total pressure drop through the packed column is only 42 mm Hg, even at a 55% higher feed rate. This is only about one-half the pressure drop through the 26 valve trays at the present lower flow rates. Further, the reduced pressure at the bottom of the column enhances the stripping action and lowers the bottom column temperature. 

---