Drawoff total

Bottom tray downcomer total drawoff other ... 

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 Liquid may be withdrawn using a chimney tray or from a downcomer.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. 

Column pressure at the reflux drum is established so as to condense totally the overhead vapor or some fraction thereof. Flash-zone pressure is approximately 69 kPa (10 psia) higher. Crude oil feed temperature at flash-zone pressure must be sufficient to vaporize the total distillates plus the overflash, which is necessary to provide reflux between the lowest sidestream-product drawoff tray and the flash zone. Calculations are made by using the crude oil EFV curve corrected for pressure. For the example being considered, percent vaporized at the flash zone must be 53.1 percent of the feed. 

Other methods. A cycling procedure can also be used for the column operation. The unit operates at total reflux until a steady state is establisned. The distillate is then taken as total drawoff for a short time, after which the column is returned to total reflux operation. This cycle is repeated throughout the course of distillation. Another possibility is to optimize the reflux ratio to achieve the desired separation in a minimum time. More complex operations may involve withdrawal of sidestreams, provision for intercondensers, addition of feeds to trays, and periodic feed additions to the pot. 

The column is initially solved at total reflux, constant overhead vapor rate, and steady-state conditions. At time t = 0 transient conditions develop as distillate drawoff begins. Numerical integration of the differential equations is performed for finite amounts of distillate draw-off, using initial values of the variables from the steady-state solution. 

When all liquid in a column section is withdrawn ("total drawoff ). 

Chimney trays equipped with rectangular risers that stretch from one end of the tray to another often use open-top gutters ("rainwater conduits ) rather than hats (see Fig. 8.66 later). The gutters can be V-shaped or U-shaped, are mounted above the risers, and slope toward the short edges of the rectangles. Compared to hats, these reduce riser pressure drop and eliminate the downward velocity component of vapor leaving the chimneys, but at the expense of allowing some liquid to rain into the risers. This makes them less suitable for total drawoffs. 

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. 

Particular care should be taken to see that the nuts and bolts at the drawoff trays (particularly for total drawoffs) are properly tightened. At least one experience (85) has been reported where poor column performance resulted from failure to tighten the drawoff box nuts and bolts. 

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

Lube oil was drawn as a side cut from a trapout tn (total drawoff). When all tower bafQe trays were r laced by valve tntys, lube oil rate declined 1 12% due to leakage at the trapout tray. To restore the original draw rate, the traqxnit tray was replaced first by a bubble-cap tray, then by a valve tray with venturi openings finally the outlet nozzles were 63q>anded. Each of these steps progressively further lowered the lube oil rate. Installation of a seal-welded chimn trapout tray solved the problem and achieved a lube oil rate 19% above original. 

A leakproof chimney tray should be used for total drawoffs. 

For surge vessels, allow 2 min liquid residence time for drawoff, use 15 min for reflux use 5 min provided this allows sufficient time for controllers to function. Total volume = 1.3 X holdup if the holdup volume is > 3 m . 

For total reflux a separate switch may be provided that actuates air-operated, three-way valves in the air lines to the drawoff valves. The startup/shutdown switch then controls only the feed and steam valves. This design permits the operator to put the column into total reflux, when desired, or to take it out of total reflux during startup upon ascertaining that it is proper and safe to do so. 

Low base temperature or low top temperature holds all drawoff valves closed (Figure 9.21). Usually a gain 6 override is adequate, but this really depends on column composition dynamics. The function here is to hold the column on total reflux until overhead and base compositions are nearly correa. On-stream analyzers would permit a better job here. 

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