Arranging the Control Loops

Because few separations are truly binary, the ramifications that are encountered in column design and operation are extensive. A third component leaves with the distillate if it is the lightest component, or with the bottoms if it is the heaviest or it may be withdrawn as a side-stream if it boils at a temperature between those of the light and heavy 

As might be expected, each tower or combination of towers has its own particular requirements for control-loop arrangement,. Beyond this, even, there are various ways of controlling the same tower, some of which are more effective than others. Virtually any arrangement of loops will operate satisfactorily in the steady state—but in the steady state no control is really needed. It is the unsteady state that tests control performance. If feed rate or composition or enthalpy change, or steam pressure, or weather, what happens then  

With reflux on flow control, variations in vapor rate and reflux rate will both affect the flow of distillate. Earlier it was demonstrated that the distillate to feed ratio had the greatest effect on product quality. Therefore, it is imperative that distillate flow be controlled directly, instead of being subject to variations in vapor and reflux rates. If the flow of vapor leaving the top of the tower is the same as that generated 

The fractional change in D resulting from fractional variation in V and L 

FIG 11.8. The material balance at the feed tray is affected by feed enthalpy qp. 

---