Heat exchange, pumparound

The eadiest use of heat-exchange network synthesis was in the analysis of cmde distillation (qv) units (1). The cmde stream entering a distillation unit is a convenient single stream to heat while the various side draws from the column are candidate streams to be cooled in a network. So-called pumparounds present additional opportunities for heating the cmde. The successful synthesis of cmde distillation units was accompHshed long before the development of modem network-synthesis techniques. However, the techniques now available ensure rapid and accurate development of good cmde unit heat-exchange networks. 

Internal reflux is induced by means of externally cooled liquid pumparounds. A pumparound simply removes hot liquid from the tower, pumps it through a heat exchanger and then introduces this cooled liquid into the tower a few trays above. Use of pumparounds allows a better distribution of tower loadings than if all the heat were removed from the VPS using an overhead condenser. Four to six trays between sidestreams and two pumparounds are normally specified for a lube VPS. The three liquid sidestream products to be used as lube plant feed stocks are steam stripped to remove lighter boiling components which condense with tire sidestreams. 

All reactor modes sometimes can be operated advantageously with recycle of part of the product or intermediate streams. When the recycle is heated or cooled appropriately it can serve to moderate undesirable temperature travel. This function is well served with pumparound from a stirred tank through an external heat exchanger. Recycle streams also can be processed for changes in composition before return. 

A second order reaction is conducted in a CSTR that is provided with a pumparound heat exchanger as sketched. Heat transfer rate in the exchanger is Q - 15000(AT) lm Btu/hr, The reactor is to be kept at 200 F. Other temperatures are shown on the sketch. Feed rate is V = 100 cfh, inlet concentration is 0.5 lbmol/cuft Also AHr= -50000, p = 50, Cp = 0.8. 

I have not shown the flow of liquid on the cold side of the pumparound heat exchanger in Fig. 12.1, but we can calculate its flow. Let s assume that the specific heat of the cold-side liquid is 0.5 Btu/L(lb)(°F)]. Then the cold-side (or shell-side) flow is... 

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. 

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

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. 

The stage for each feed, product, heat exchanger, and pumparound... 

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. 

Pumps (both regular and spare) must be started up to flush out suction and discharge lines. Heat exchangers, side product lines, blowdown and emergency lines, and any connections used intermittently should be flushed and drained during this step. Circulation should be established and maintained in pumparound lines (296). Instruments should be flushed and be fully operational before this step... 

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