Trapout trays

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. 

The operating superintendent of a medium-sized crude unit obliterated all of the above difficulties. He tore out the valve trapout tray and drawoff pan. A new, all-welded pan was installed. Several large chimneys allowed for vapor passage. The pan was seal-welded to the tray ring and the problem was corrected. (Note, in large-diameter towers, a seal weld can fail due to thermal expansion.)... 

Leaking trapout trays above the flash zone in crude units—or above the hot vapor inlet in coker and FCCU main fractionators—cause clean gas-oil products to be reprocessed. The barrels of leakage do not vary with throughput. This source of energy waste is discussed in detail in Chapter I. 

If both calculation and observation confirm that water is settling out inside a tower, the operability of the existing trapout tray should be checked. If no trapout tray exists, one should be installed at the earliest opportunity. 

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... 

If substantial reflux is provided, one can be sure that the amine is well stripped before it is drawn off the reboiler trapout tray. The regenerator reflux rale (Ib/hr water) should be 10%-30% of the reboiler steam rate. To double-check stripping efficiency, pull samples of lean amine and reboiler feed. Both should have the same H2S concentration. Remember, H2S must be stripped out of the regenerator trays, not in the regenerator reboiler. This is important enough to repeat Keep a decent reflux rate in the regenerator to prevent reboiler tube corrosion. 

A gas plant reboiled absorber is the most common example of a tower that requires a water trapout tray (Fig. 20-4). Water accumulates in such absorbers because they are not refluxed. In general, towers that run at low reflux ratios are most subject to internal water accumulation. 

Sometimes, a distillation column will begin losing efficiency due to water accumulation on the trays or perhaps the bottoms product has turned cloudy because of moisture. If there is no trapout tray, try cooling off the tower s feed. This will reduce the moisture content of the hydrocarbon. 

Arrangements such as bottom sumps, chimney trays, side-draw drums, and surge drums are usually designed for avoiding the presence of vapor in the liquid outlet. On the other hand, downcomer trapouts are usually designed to allow for the presence of vapor in the outlet liquid. 

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).

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 ... 

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. 

Chimney trays (Fig. 4.10a, 6) are used for withdrawing intermediate liquid streams from the column in a packed tower, they are also used as liquid collectors or vapor distributors. Alternative devices used for liquid withdrawal are downcomer trapouts in tray columns, chevron collectors, and some redistributors in packed columns. Compared to these alternative devices, chimney trays have the following advantages ... 

In tray columns, chimney trays do not suffer from unsealing problems and are generally less troublesome than downcomer trapouts. 

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. 

Downcomer trapouts seldom provide sufficient residence time for vapor disentrainment, and the venting process must be completed downstream of the column outlet. The vent-retum nozzles must always be located above the tray s liquid level. Downstream piping must be designed for self-venting flow (Fig. 4.5). 

For similar reasons, in partial trapout services, it is desirable to use a leak-resistant tjqie of tray above the downcomer trapout. 

Downcomer trapouts may be troublesome with foaming systems. Chimney trays are often preferred in such services. 

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). 

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). 

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). 

A valve tray used in trapout service excessive leaked. The leakage was eliminated seal-welding trqy sections, and by welding a strip onto the periphery of the tray, a few inches from the siqpport ring. 

A leaking valve tray in trapout service was replaced by an aU-welded chimney trEQT, which was seal-welded to the tray ring. Leakage was eliminated. 

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

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