Drawoff partial

Intermediate liquid outlets. Liquid may be withdrawn using a chimney tray or from a downcomer. A chimney tray is a flat, unperforated plate with vapor risers. It permits total withdrawal of liquid a downcomer drawoff permits only partial withdrawal because some weeping occurs through the tray. A downcomer drawoff may contain some entrained gas, which must be separated downstream or allowed for in downstream equipment design. 

Downcomer trapouts (Fig. 4.12) are mainly used for partial liquid drawoff from tray columns. They are preferred when the liquid withdrawn is only a small fraction of the column liquid, or when the drawoff feeds a once-through rehoiler (Sec. 4.8). They are sometimes also used for drawing most or all the column liquid at an intermediate point, hut are less suitable than chimney trays for this purpose. 

Chevron collectors are used in packed columns as liquid collectors for partial drawoff or for feeding to a redistributor which is not selfcollecting (e.g., a notched-trough redistributor). They are sometimes also used as total drawoffs, but are less suitable than chimney trays for this purpose. The chevron collector (Fig. 4.13) consists of evenly spaced chevron blades several inches high. Liquid collects at the bottom of the blades and runs into a draw pan. From there it can be taken out or fed to a redistributor. 

Figure 19.11 Side-stream stripper control, (a) Total drawoff (6) partial drawoff. (Concept based on N. Lieberman, Chemical Engineering, September 12,1977.)...

Partial drawoffs (commonly utilizing downcomer trapouts) are used when a side draw does not share a pumparound drawoff. Figure 19.116 shows the preferred controls (234). Operator action is required to ensiure that the correct distillate quantity is drawn and to prevent drying of trays below the drawoff. The dryout problem can often be mitigated by drawing the side product from the bottom seal pan (i.e., just above a chimney tray). In both arrangements (Fig. 19.11a and b), note the seal loop in the line from the main fractionator to the stripper. This loop prevents vapor backflow at low liquid rates (Sec. 5.1). 

No or low Delta P in a tower section may indicate missing trays or absence of liquid. High Delta P in a tower section may indicate that the drawoff is partially restricted or blocked, that may be due to high liquid rates in that section of the tower, flooding, or too much stripping steam. 

The concentration of the lightest component in the sidestream product is controlled by the location of the sidestream drawoff tray. As shown in Figure 7.6B, this easily can be implemented by conventional analog hardware. The fixed-gain relays are calibrated so that only two of the drawoff valves can be partially open simultaneously. Thus as the signal from the XSl composition controller increases, the valves for trays higher in the column open as valves for trays lower in the column close. 

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