Pumparounds trays, fractionation

Since we do not rely on pumparounds to fractionate—but just to remove heat—good vapor-liquid distribution is not critical. A bed of 4 or 5 ft of structured packing is often, then, an excellent selection for the pumparound section of a tower. The capacity of such a bed potentially has a 30 to 40 percent advantage over trays. 

Let s refer back to Fig. 12.1. Note that the vapor temperature leaving tray 9 is 450°F. The temperature of the liquid leaving tray 10 is 500°F. This sort of temperature difference shows that fractionation is taking place across the pumparound trays. The temperature difference between... 

Higher pumparound rates will reduce fractionation. This is shown in Figure 1-3. In this sketch, trays 3,4, and 5 provide the fractionation between furnace oil and FCCU feed. Trays 6 and 7 are the pumparound trays and, hence, contribute little toward separating the two products. 

The bottom section of the main column provides a heat transfer zone. Shed decks, disk/doughnut trays, and grid packing are among some of the contacting devices used to promote vapor/liquid contact. The overhead reactor vapor is desuperheated and cooled by a pumparound stream. The cooled pumparound also serves as a scrubbing medium to wash down catalyst fines entrained in the vapors. Pool quench can be used to maintain the fractionator bottoms temperature below coking temperature, usually at about 700°F (370°C). 

The process design engineer typically assumes that a pumparound is simply a way to extract heat from a tower. The engineer does not expect the trays used to exchange heat between the hot vapor and the cold liquid to also aid in fractionation. In practice, this is not what happens. 

Again, this improvement in the degree of fractionation developed by trays 5, 6, and 7 is a result of reducing the amount of heat duty removed by the pumparound flowing across trays 8, 9, and 10. 

Reducing the pumparound duty increases the tray loadings on trays 1 through 7. But in so doing, the trays operate closer to their incipient flood point. This is fine. The incipient flood point corresponds to the optimum tray performance. But if we cross over the incipient flood point, and trays 5, 6, and 7 actually start to flood, their fractionation efficiency will be adversely affected. Then, as we decrease the pumparound heat-removal duty, the mutual contamination of diesel and gas oil will increase. 

Crude oil fractionators are an example of a more elaborate system. They make several products as side streams and usually have some pumparound reflux in addition to top reflux which serve to optimize the diameter of the tower. Figure 3.13 is of such a tower operating under vacuum in order to keep the temperature below cracking conditions. The side streams, particularly those drawn off atmospheric towers, often are steam stripped in external towers hooked up to the main tower in order to remove lighter components. These strippers each have four or five trays, operate... 

Either the entire amount of the liquid on a tray or part of it may be taken out and returned to the tray directly below it. The combined liquid flow (the pumparound and the liquid flowing directly to the tray below) would be the same as the liquid flow down to the tray below had there been no pumparound. From the standpoint of equilibrium stages, the column would perform exactly as though the pumparound did not exist although the tray efficiency may be affected due to the different flow pattern. If the liquid is returned several trays below the draw tray, the trays between the draw tray and the return tray are bypassed by the pumparound liquid. The amount of fractionation in that column section is therefore reduced. This pumparound thus tends to lower the overall number of effective trays in the column. 

In another type of pumparound, the liquid drawn from one tray is returned a number of trays above the draw tray. This process also tends to lessen the effective number of trays and fractionation in that column section because of back-mixing higher-boiling liquid from the lower tray with lower-boiling liquid in the upper tray. 

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). 

Pump-Around Many fractionation towers have pump-arounds to remove excess heat in the key sections of the tower. The effect of increasing pump-around rate is reduced internal reflux rate in the trays above the pump-around, but increased internal reflux rate below the pump-around. Thus, change in pumparound duty affects fractionation. On the other hand, pump-around rates and return temperature have effects on heat recovery via the heat exchanger network. It is not straightforward in optimizing pump-around duties and temperamres since the effects on both fractionation and heat recovery can only be assessed in a simulation model. An APC application incorporated with process simulation should be able to handle this optimization. 

Under extreme circumstances, the trays below the furnace oil draw may have no liquid at all on them. Dry trays do not fractionate. When this happens, the furnace oil end point will skyrocket. Reducing the pumparound duty will correct this problem. 

Note that as the pumparound duty is decreased, the vapor and liquid loads on the trays above the pumparound return tray will increase. This should ordinarily enhance fractionation. However, the reduction in pumparound duty could cause trays 3, 4, and 5 shown in Figure 1—3 to flood. This would reduce the separation efficiency between FCCU feed and furnace oil. 

The question then is As an operator reduces pumparound duty, how can he tell if fractionation between two adjacent products is getting better (due to increased internal reflux on the trays) or worse (due to tray deck flooding and entrainment) The simple answer is to observe the difference in draw temperatures (AT), between the adjacent cuts. 

Table 8-4 summarizes the product distillations for the FCCU fractionator. These distillations are consistent with the vapor and liquid flows presented in Table 8-1. The degree of separation between the tower overhead and LCO products is represented by the ASTM 5% to 95% gap of —2°F. Based on the Packie method, calculations indicate that there are 10 effective trays above the LCO draw tray. This is equivalent to roughly six theoretical separation stages. Since each pumparound section is usually represented as a single stage, we can see by referring to Figure 8-2 that the naphtha wash section is equivalent to four theoretical separation stages.

For example, in an FCCU s main fractionator, large amounts of heat must be removed to keep the fractionator in heat balance. This is done by circulating a hot-oil stream (called a pumparound) drawn from an intermediate tray through a series of heat exchangers (see Fig. 18-8). 

In an olefins plant, the gas stream at a pressure of 140 to 180 psia is washed with a caustic soda solution to remove all acidic components present. Typically, the inlet gas contains 300 to 1,000 ppm of H2S plus CO2. The exit gas specifications will be 1 to 5 ppm by volume total acid gases in order to prevent freeze-up in the cold section fractionators. This caustic scrubber usually has been constructed with 30 to 50 total trays. The top three to five trays serve as a water-wash section to prevent caustic carry-over in the exit gas stream. The remaining trays constitute two or three pumparound sections. Fresh caustic solution is fed to the upper section along with the pumparound return liquid. Excess partially spent solution overflows a trap tray at the bottom of the section and goes to the next lower section where it is mixed with pumparound return liquid from the bottom of that section. The liquid effluent from the bottom of the column is a salt solution from which 65% to 75% of the sodium hydroxide has been consumed. The pressure drop through such a trayed column normally is 5 to 8 psi. 

In a typical design, this tower will contain at least four open-type trays in the lower section and a minimum of seven fractionating trays in the upper section. The function of the lower section is to cool the incoming gas by sensible heat transfer with a pumparound of cooled quench oil (bottoms). This quench oil will leave the column at 350° to 400°F, and the pumparound will be cooled to between 270° and 330°F before being returned to irrigate the lower section trays. At least 90% of the components in the cracked gas feed that are heavier than C-lOs will be condensed by these angle trays, baffle trays, or splash decks. A small slip-stream of bottoms is sent to a stripper to remove the C-8 and lighter components, because the stripped bottoms have only fuel value. 

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