Inverse Response

The phenomenon of inverse response was apparendy first noted by Rijnsdorp.4 As mentioned in Chapter 13, it occurs in some columns when an increase in vapor flow causes a momentary increase in liquid flow down the column. This is due to a decrease in foam density on the trays and a consequent momentary overflow into the downcomer.

As may be seen from Figure 18.2—18.4 and 18.6, an increase of liquid flow onto a tray increases low boiler concentration. Therefore, although in the long run a vapor flow increase will decrease low boiler concentration, in the short run it increases it. This momentary change of concentration in the wrong direction gives rise to the term "inverse response."

The significance of inverse response in the design of feedforward compensators and feedback controllers for composition is discussed later. It is the subject of a paper by Luyben.5

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