Trapout pans

Figure 4.7 Reboiler trapout-pan arrangements, (a) Single-pass trays (once-through reboiler) (6) two-peiss trays, single reboiler (once-through reboiler) (c) two-pass trays, two reboilers (once-through reboiler) (d) single-pass trays (recirculating reboiler).

The above benefits are most thoroughly realized with the baffle ar-rtmgement shown in Fig. 4.66 or with the once-through arrangements in Fig. 4.7a to c. With the baffle arrangements in Fig. 4.6a, c, and d, with packed columns, and with the trapout pan arrangement in Fig. 4.7d, benefits 1 and 3 above are only partially realized, because some... 

When the bottom sump does not supply liquid to a reboiler (e.g., when a reboiler trapout pan is used, or when the column has a bottom feed and no reboiler), the design in Fig. 4.86 can be used with thermally unstable materials. This arrangement eliminates the need for a surge drum and self-venting lines, and immediately quenches liquid reaching the column base. 

In tray columns, liquid to once-through reboilers (Fig. 4.7a to c), as well as liquid to many recirculating reboilers (Fig. 4.7cf), is normally withdrawn from the bottom tray via a trapout pan. In packed columns, a chimney tray is usually preferred for this purpose. This trapout pan can often be troublesome. Successful operation requires close attention to the following guidelines ... 

The wall of the trapout pan should be 4 to 8 in higher than the wall of the seal pan (237). This prevents the liquid overflowing the seal... 

Sufficient vertical height must be provided from the top of the trapout pan overflow weir to the bottom tray. The overflow pan liquid is usually less aerated and therefore denser than the downcomer fluid. This will cause greater downcomer backup. 

If the column liquid flows to more than one reboiler using separate trapout pans (Fig. 4.7c), a liquid balance line is needed to ensure that liquid is equally distributed to each reboiler. 

The trapout pan is most suitable for once-through reboilers (Fig. 4.7o to c). With a recirculating reboiler, the trapout pan can still be used, but its arrangement is more complex. Here the reboiler return nozzle needs to be located above the trapout pan, with liquid returning from the reboiler routed into the trapout pan and away from the seal pan. This is usually accomplished by a baffle (Fig. 4.7d) and/or by using a wide pan. Failure to properly route the reboiler return liquid into the trapout pan may starve the reboiler of liquid, and/or lower reboiler AT, and/or reduce the reboiler mass transfer efficiency. 

Figure 4.12 Downcomer trapout pan arrangements, (a) No positive seal b, c) with positive seal (c ) with pumparound, no positive seal (e) with pumparound and positive seal.

In services where leakage is to be minimized and pressure surges tend to occur, it is often recommended to specify heavier gage for chimney trays and trapout pans (232). 

Installing a valved dump line connecting the column bottom outlet line with the reboiler inlet line (Fig. 15.4a). This technique is only needed when the column reboiler sump is separated from the column bottom sump by a baffle or when the reboiler liquid comes from a trapout pan. The valve remains shut during normal operation, but is opened during startup to lower the level and inspire thermosiphon action during startup. One case where this technique was successfully used has been described (237) the author has had several similar experiences. 

Change In tube vapor volume with change in heat input. Stepping up heat input generates more vapor and expands the volume of fluids in the tubes. Fluid swell following an abrupt increase in heat input may displace liquid back into the column base. If the liquid to the reboiler comes from a trapout pan, this can flood the bottom tray. This problem is most pronounced at low heat loads (67, 68). 

A once-thn>i) thermosiphon r iler was starved of liquid because the overflow weir of the trapout pan feeding the rdboiler was level with a seal pan weir. Liquid bypassed the tr xnit pan. Problem was solved by raising tnqxmt pan weirly 6in. 

Trapout pan weir should be hi er than seal pan weir when feeding a once-throu thermosiphon reboiler. 

If the tower is not equipped with a reboiler start-up line, try increasing the flow of liquid to the trapout pan. This is done by increasing the tower feed or the reflux rate. If the trapout pan is leaking, the increased liquid flow to the pan will cause the AT between the inlet and outlet to decrease. 

Trim gas oil (TGO) is a black oil stream withdrawn immediately above the vacuum-tower flash zone. It consists of 20%-50% resid and 50%-80% gas oil. This relatively light material may cause the bottoms pump to cavitate when it overflows its trapout pan at a nonuniform rate. Reduce the TGO pan level to see if this helps the NPSH problem on the bottoms pump. 

Figure 20-3 illustrates a typical water trapout tray scheme. The trapout pan itself is too small to make a decent separation between water and hydrocarbon. A mixed phase is drawn off and flows to a small drum (boot) located at grade. In the boot, a good separation is made between the two... 

Leaking amine water cooler, 103 Leaking drawoff trays, 23-25 welded trapout pans, 23-24 Leaking feed-effluent exchanger (glycol), 444-445... 

Reactor vapor, 184, 186 Reboiler circulation, 272-273 Reboiler corrosion, 109 Reboiler leaks, 92 Reboiler problems (distillation lowers), 271-275, 280, 388 Irapoul pans, 271-272 plugged reboilers, 272-273 trapout pan repair, 273 steam-side problems, 273 blown condensate seal, 273-275 distillation, 388... 

A leaking total trapout pan drops clean gas-oil product into black oil. The lost gas-oil will eventually be boiled out of the black oil again. 

The temperature difference obtained should be compared to the design AT (temperature difference). If the measured AT is much greater (more than 50%) than the design AT, circulation through the reboiler is insufficient. The two usual causes of decreased reboiler circulation are plugged bundles and leaking trapout pans. 

The process engineer can perform a series of experiments to determine whether the reboiler bundle needs to be cleaned to remove plugging or whether the distillation tower must be opened to repair a malfunctioning trapout pan. Following are details in diagnosing and correcting insufficient reboiler circulation. 

As a first step in testing for leaking trapout pans, consult Figure 12-9. If the tower has a reboiler start-up line, open it. Increase the tower bottom... 

The exact cause of malfunctioning trapout pans can easily be determined once the tower is out of service. Hose water into the center downcomer, disconnect the tower from the reboiler inlet, and observe the flow of water. 

A leak in the LVGO pan will have the same effect as letting the pan overflow, i.e., draw temperatures and hence crude preheat will decline. Tight trapout pans in vacuum towers almost always save energy. 

A large hole in the trapout pan will also reduce the capacity to remove water. That is why such pans should be checked when a tower is opened for inspection. 

Plugged circulation line Hole in trapout pan Water drains plugs Circulation rate too high Using two drawoffs at the same time... 

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