Crude tower, reflux rate

The effect of increasing the reflux rate on a crude tower, even with a constant condenser duty, will be to increase the condenser pressure drop. This happens because the volume and the weight of the tower overhead vapors will increase as the tower-top temperature drops. At the Calumet Refinery in Shreveport, Louisiana, the crude tower reflux rate was increased by 25 percent and the tower-top temperature dropped by 50 F-60 F. The overhead condenser AP increased from about 12 to 24 psi. The tower pressure was kept constant, but the reflux drum pressure declined from 17 to 5 psig. 

Refinety Crude tower Column pressure and product gas rate sharply fluctuated during low-rate operation in this and other luiits. The condenser was a partial condenser located at ground level, with an elevated reflux drum. Problem was caused by slug flow in the riser from the condenser to the drum. Size risers to avoid sltig flow even during low-rate operations. 

So far, all our examples have dealt with tworeflux rate is large compared to the overhead product rate, and many of our towers do run with a lot of reflux. But we can all think of distillation columns where the top reflux rate is small compared to the overhead product, and the overhead product itself consists of a dozen widely different chemical compounds. Figure 7.4 represents such a column. It is called a crude preflash tower. 

To test alternative control strategies, Prett and Morari (1986) provide a linearized model, referred to as the Shell process, of a distillation tower to separate crude oil into fractions in a refinery. Part of the model describes the dynamics of the two top compositions as a function of the manipulated variables (the two top draw rates) and two key disturbances (the heat removal loads in pump-around streams used to remove heat and create intermediate reflux). For this example, it is sufficient to examine die matrices specific to the nominal model ... 

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