Pumparounds heat duty, increase

Increasing pumparound heat duty will unload the overhead condenser. This will cool off the reflux drum. A colder reflux drum will absorb more gas into the distillate product. Less gas will be vented from the reflux drum, and this is often desirable. 

Figure 3-6 illustrates how one combination tower was retrofitted to increase the pumparound draw temperature by 45°F. The heavy coker gasoil feed to the steam stripper was modified to be withdrawn from the tower in two streams. The 3,100 B/D of relatively light, cool gas oil was trapped out and drawn off the tower before it could fall into the pumparound section. Computer calculations predicted that segregating this lighter gas-oil cut from the pumparound section would increase both the pumparound draw and return temperatures, with only a 3% reduction in pumparound heat duty. 

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. 

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... 

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. 

Reducing the pumparound heat-removal duty increases the vapor flow from tray 8 in the column shown in Fig. 17.5. The extra pounds of vapor flow up the tower and raise the tower-top temperature. The reflux control valve opens to cool the tower-top temperature back to its temperature set point. Then the liquid flow rates from trays 1, 2, and 3 onto tray 4 all increase. If the diesel draw-off rate is maintained constant, the liquid overflow rate onto trays 5, 6, and 7 will increase. This liquid flow is called the internal reflux. Trays 5, 6, and 7 are the... 

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. 

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