Pumparounds fractionation

To obtain a low flash zone pressure, the number of plates in the upper section of the vacuum pipe still is reduced to the minimum necessary to provide adequate heat transfer for condensing the distillate with the pumparound streams. A section of plates is included just above the flash zone. Here the vapors rising from the flash zone are contacted with reflux from the product drawoff plate. This part of the tower, called the wash section, serves to remove droplets of pitch entrained in the flash zone and also provides a moderate amount of fractionation. The flash zone operates at an absolute pressure of 60-90 mm Hg. 

Steam stripping is not adequate for the bottoms purity required. More positive stripping is obtained by charging the tower bottom liquid to a heating unit known as a reboiler. In a typical reboiler, 50% of the feed is vaporized and returned to the tower below the bottom plate. A fractionating tower equipped with a steam heated reboiler is shown in Figure 4. The reboiler may also be heated by a hot oil stream, such as a pumparound reflux stream from the primary fractionator of a cracking unit, or by a fired furnace. 

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). 

Heavy cycle oil, heavy naphtha, and other circulating side pumparound reflux streams are used to remove heat from the fractionator. They supply reboil heat to the gas plant and generate steam. The amount of heat removed at any pumparound point is set to distribute vapor and liquid loads evenly throughout the column and to provide the necessary internal reflux. 

One method of maximizing the LCO end point is to control the main fractionator bottoms temperature independent of the bottoms pumparound. Bottoms quench ( pool quench ) involves taking a slipstream from the slurry pumparound directly back to the bottom of the tower, thereby bypassing the wash section (see Figure 9-9). This controls the bottoms temperature independent of the pumparound system. Slurry is kept below coking temperature, usually about 690°F, while increasing the main column flash zone temperature. This will maximize the LCO endpoint and still protect the tower. 

If flooding occurs in the main fractionator, increasing the bottoms pumparound rate reduces vapor loading, but can have a negative affect on fractionation. 

Removing more heat from the pumparound returns, either by generating steam or adding coolers. This can decouple the fractionator from the reboilers in the gas concentration unit. 

Often, we remove heat from a tower, at an intermediate point, by use of apumparound or circulating reflux. Figure 7.11 is a sketch of such a pumparound. In many towers, the liquid flows in the pumparound section are greater than in the other sections, which are used for fractionation. That is why we are often short of capacity and initiate flooding in the pumparound or heat-removal section of a column. 

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. 

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. 

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... 

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. 

The hot oil to the kettle boiler is a circulating pumparound stream, from a fluid catalytic cracker fractionator, slurry-oil circuit. There is a fundamental difference between this sort of boiler and the utility plant boilers discussed previously. In the kettle boiler, the heating medium is inside, rather than outside, the tubes. To obtain the full capacity of... 

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... 

The separation brought about in a given column section depends on the number of stages and the UV ratio in that section. The UV ratio is a function of the feeds and product rates, heater and cooler duties, and pumparounds associated with the section. The fractionation may be of a distillative or an absorptive or stripping nature (Chapters 7 and 8), although one effect or the other may be predominant in different situations. The various features of complex distillation are described in this chapter. 

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. 

For example, let us assume that the overhead stream from a main fractionator is condensed, enters the overhead accumulator (inventory), and then feeds a gas plant downstream. Assume also that the main fractionator has pumparounds lower down the tower that are used to supply reboiler heat to the same gas plant. This is an example of a heat-integrated plant. 

Liquid circulation lines These are required when liquid circulation is performed at startup (Sec. 11.10). Often, a jumpover from the column bottom to the reflux line is needed, but other lines may also be required. In a refinery crude fractionator, it has been recommended (237) to install the jumpover line to the return line of the uppermost pumparound and to size it for 20 percent of the net distillate product rate. 

Direct-contact condensers (Fig. 15.14f). These are used for minimizing pressure drop in vacuum condensation. To accomplish this, the direct-contact zone contains low-pressure-drop internals such as packings, or is a spray chamber. Another common application is intermediate heat removal ("pumparounds ) in refinery fractionators. Here the main purpose is to maximize heat recovery at the highest possible temperature levels. A third common application is intermediate heat removal from absorbers or reactive distillation columns in which an exothermic reaction takes places. In all these applications, condensation... 

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). 

---