Pumparounds return tray

The purpose of the pumparound is to cool and partially condense the upflowing vapors. The vapors to pumparound tray 10 are at 600°F. The vapors from the pumparound return tray 9 are at 450°F. There are two pumparound trays (9 and 10) in the column. This is the minimum number used. A typical number of pumparound trays is two to five. 

Pumparound return tray 6 ES2, equilibrium stage 1, duty Q2... 

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. 

Figure 17.1 shows an alternate method, called circulating reflux or pumparound, to remove heat from a tower. Hot liquid, at 500°F, is drawn from tray 10, which is called the pumparound draw tray. The liquid pumparound is cooled to 400°F. The cooled liquid is returned to the tower at a higher elevation onto tray 9. It appears from Fig. 17.1 that the cold 400°F pumparound return liquid is entering the downcomer from tray 8. This is often good design practice. Tray 9 is called the pumparound return tray. 

Pumparound return tray The tray below the pumparound return nozzle. 

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 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. 

Feed enters T-2 at tray 5. There is a pump-through reboiler. Another pump withdraws material from the bottom and sends it to tower T-3. Liquid is pumped from tray 18 through a cooler and returned in part to the top tray 20 for temperature and reflux control. A portion of this pumparound is withdrawn after cooling as unsaps product. Steam leaves the top of the tower and is condensed in the barometric. 

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. 

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. 

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. 

Assume that the returning pumparound liquid is 150 degrees F cooler than the tray to which it is being returned. This will provide an adequate temperature difference within the tower for the required heat transfer. Since this also sets the temperature difference for the pumparound liquid, calculate the circulation rate of this stream. 

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