The Pumparound

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. 

Let s calculate the heat removed in the pumparound circuit shown in Fig. 12.1. Assume that the specific heat of the pumparound liquid is 0.7 Btu/[(lb)(°F)]  

What I have just illustrated is the most powerful tool in my bag of tricks—calculating a flow from a heat balance. 

The decrease in the heat duty of the pumparound heat exchanger would equal the increase in the heat duty of the overhead condenser. Thus, we say that the heat balance of the tower is preserved. Some of the heat that was being recovered to the cold fluid, shown in Fig. 12.2, is now lost to cooling water, in the overhead condenser. This shows the most important function of pumparounds recovering heat to a process stream that would otherwise be lost to the cooling tower. 

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It would be possible to remove all of the heat by pumping cold reflux from the distillate drum to the top of the tower and thus eliminate the cost of the pumparound circuit. Where more than one sidestream is withdrawn, however, it is usually economical to withdraw part of the heat in a pumparoimd reflux system farther down the tower. The following economic factors affect the choice ... 

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. 

Pumparound Flow Failure - The relief requirement is the vaporization rate caused by an amount of heat equal to that removed in the pumparound circuit. The latent heat of vaporization would correspond to the temperature and pressure at PR valve relieving conditions. "Pinchout" of steam heaters may be considered. 

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. 

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. 

Let s say that the cooling-water outlet temperature from the condenser was 140°F. This is bad. The calcium carbonates in the cooling water will begin to deposit, as water-hardness deposits, inside the tubes. It is best to keep the cooling-water outlet temperature below 125°F, to retard such deposits. Increasing the pumparound heat removal will lower the cooling-water outlet temperature. 

Another purpose of the pumparound is to suppress top-tray flooding. If tray 1 in Fig. 12.2 floods, the operator would observe the following ... 

Finally, is is best remembered that, as we said earlier in this chapter, heat recovered in the pumparound heat exchanger is often a valuable... 

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... 

But what will happen to the flow of the vapor leaving tray 3, 4, or 5 I ask this question assuming that the pumparound heat duty is fixed. Will the pounds per hour of vapor flowing through trays 3, 4, or 5 ... 

If an increase in the tower-top reflux rate causes the top of the tower to flood, how should the operator respond She should then increase the pumparound flow to reduce the pounds of vapor flow to tray 5, in Fig. 12.4. But suppose this causes the pumparound trays 6, 7, and 8 to flood, because of the extra liquid flow She should increase the cold liquid flow through the pumparound heat exchanger. If this cannot be done, either, then the tower pressure can be increased. This will increase the density of the flowing vapors and shrink the volume of the vapors which the trays must handle. 

We could reduce the amount of diesel product from the tower. That could wash the heavier gas oil out of the diesel. But it would also increase the amount of diesel in the gas oil. Increasing the heat removed in the pumparound would have a similar effect less gas oil in diesel, but more diesel in gas oil. 

How about decreasing the heat removed in the pumparound This would seem to allow the lighter diesel to more easily vaporize out of the gas-oil product. But will this action result in increasing the contamination of diesel, with heavier gas oil This answer is no—but why not ... 

Reducing the pumparound heat-removal duty increases the vapor flow from tray 8 in the column shown in Fig. 12.5. The extra pounds of... 

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. 

From an operating standpoint, we can see when this flooding starts. As we decrease the pumparound duty, the temperature difference between the diesel- and gas-oil product draws should increase. When these two temperatures start to come together, we may assume that we have exceeded the incipient flood point, and that trays 5, 6, and 7 are beginning to flood. 

Pervaporation can also operate in batch mode, and this is done typically when testing membranes for small plants and for some larger multipurpose plants. Batch pervaporation systems are robust, well proven, and flexible in operation. The pumparound rate on batch systems is normally set high to give a low permeate quantity per pass. Pervaporative cooling effects are small, and such systems can be built with a single preheater and unheated modules (Fig. 3). 

The trays that carry the pumparound liquid are the heaviest loaded in the tower. Sometimes extra vertical spacing is provided at these trays, or high-capacity trays or packing may be used. 

Distillation columns and other multistage columns generally require heat addition and/or removal, which normally take place at the reboiler and/or condenser. For a variety of reasons, it may be desirable in certain types of columns to add or remove heat at intermediate trays aside from the condenser and reboiler. Examples of such columns include demethanizers that utilize a side reboiler alongside the bottom reboiler, multi-product columns where intermediate condensers are associated with some of the side products, and absorber intercoolers used for partial removal of the heat of absorption. The method most commonly employed for exchanging heat between a column tray and a heat source or sink is the pumparound, where a fluid is drawn from the tray, sent to a heat exchanger, then pumped back to the column. The following sections pertain to the various applications of side heaters and coolers and the different types of pumparounds. 

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. 

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