Draw pan

The author recommends that for adequate vapor disengagement, liquid residence time should be at least 1 minute, based on the above definition. The author is familiar with experiences where sump residence times of the order of 20 to 40 s were insufficient to adequately disentrain vapor from liquid in sumps and draw pans of columns handling nonfoaming liquids. A greater residence time may be required... 

Vapor disentrainment is not required in a sump or draw pan used exclusively to feed a thermosiphon reboiler, because the reboiler can handle some vapor bubbles in its liquid supply. It is important to size the reboiler sump for self-venting flow otherwise vapor pockets may choke the reboiler sump. Figure 4.5 can be applied to determine the maximum allowable velocity through the reboiler sump. The pipe diameter on the y axis of the diagram is considered... 

The downcomer in Fig. 4.12a is likely to lose its seal whenever its liquid height drops below tray level. When the seal is lost, vapor fium the tray ascends the downcomer, which may cause flooding, cycling, and/or poor separation. Downcomer unsealing by this mechanism is most likely to occur when the liquid drawn constitutes a laige portion of the downcomer liquid flow, when the quantity drawn tends to fluctuate, and/or when excessive leakage takes place due to tray weeping or draw pan leaks. 

Whenever possible, downcomer trapouts and draw pans should be positioned at the column shell in order to minimize internal piping. Internal flanges should be avoided, as these may leak and deprive the drawoff of liquid. One troublesome experience with such leakage has been reported (237). 

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 5.6 Misleading liquid level measurement in a draw pan. (Courtesy of Doug S. Bouck (Sohio), private communication.)...

Internal demisters are expensive, have a high plugging tendency, and their sections may disintegrate. The author is familieir with one incident where chunks of wire mesh finm an internal demister tore loose and found their way to a draw pan above and to the bottom pump. For these reasons, they should only be installed in clean services experiencing entrainment problems that cannot be eliminated by alternate means. It is a good practice to install an internal demister only in the problem regions rather than above every tray in a column. In one case (278), it was foimd that demisters installed in the nonproblem regions did little to improve performance. 

Weep holes usually range in size from Vi to % in (48, 73, 86, 237, 248, 257, 371, 409). Small holes are preferred in clean, noncorrosive services but should be avoided in fouling or corrosive applications. The recommended hole area is 4 in per 100 ft of tray area (48, 257, 371, 409). This hole area will drain a column containing 50 bubble-cap (or other leak-tight) trays with 4-in weirs in about 8 hours (48, 371). Alternatively, the required weep-hole area can be derived from the required drainage time (usually a few hours) by using Bolles (48, 371) or Lockhart s (248) procedure. Excessive hole area will cause leakage, and should be avoided, especially in draw pans one case in which this was troublesome was reported (231). 

Insufficient circulation is usually caused by plugging, a leaking reboiler preferential baffle or draw pan, or by insufficient liquid head (alternatively, excessive pressure drop in the reboiler loop). Leakage across the preferential baffle is implied when the bottom sump level influences reboiler heat transfer rate despite the presence of a baffle. 

A portion of the valves on three pumparound trays was removed and their opening blanked. Ettds of distribution pans and draw pans were seal-welded. Leakage was reduced, sqrara-tion and heat recovery woe improved. 

Refinery Vaporchctkingafalonglinefromcol-umn to rdboiler caused premature tower flooding. To solve the problem, the draw pan was converted to a degassing pan and the line was sloped and vented. As for 703. 

Coluinn consisted of two secticms, separated an upward bulging internal head, which served as a draw pan. The liquid outlet was 3 in above the lowest po water accumulated below that. When hot oil later filled the pan, a pressure surge occurred and damaged trt ... 

Pressure surges occurred upon feed introduction and caused tray damage in several cases. The startup procedure did not use oil circulation to flush out water. The surges resulted from pockets of water remaining in draw pans and pumparound circuits. 

When the new trays were delivered, the debutanizer was taken offline, steamed out, and opened. When I entered the tower, I discovered that each and every tray in the column was totally and completely intact and clean. Everything was just as 1 had last seen it six months ago. Actually, the only thing that had changed was me. 1 had become smarter in the last six months. For example, 1 now noticed that the sides of the draw pan (see Fig. 49.3) were missing. True, they had been missing last year too, but I had not noticed their absence then. 

This explained why I had been forced to open valve A (the start-up line) to get flow to the reboiler. That is, the liquid from the bottom tray seal pan was largely overflowing the draw pan and bypassing the reboiler. Hence, the only way I could get enough liquid into the reboiler was to gravitate it out of the bottom of the tower through valve A. 

As the amount of liquid overflowing the draw pan increases, and the amount of liquid flowing to the reboiler decreases, the tower bottoms temperature will start to approach the tray temperature. For example, with valve A wide open, I had observed the following temperatures ... 

My boss generously gave me the opportunity to explain all this to the plant manager. 1 explained that we had extended the height of the sides of the draw pan by 18 inches. The debutanizer was now back on-line. Gasoline was on specification with 1 percent butane. 

For years thereafter, the plant manager consistently referred to my anti-inflationary foresight as gross stupidity. His narrow-minded, hostile, and totally unfair comments being based on my supposed negligence in not observing that the sides of the draw pan were missing before the initial start-up in 1969. 

Sun W, Cai Q, Li P, Deng X, Wei Y, Xu M, Yang X (2010) Post-draw PAN-PMMA nanofiber reinforced and toughened bis-GMA dental restorative composite. Dent Mater 26 873-880... 

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