Criteria for Locating the Control Temperature

Three major criteria are used for determining the location of the temperature control point (301, 332, 400) sensitivity, correlation with product(s), and dynamic response. Sensitivity is the most important dsmamic response is least important. 

One case has been reported (237) where the control temperature was located at the reboiler outlet of a column producing a pure bottom product. At that point, the control temperature was insensitive to composition. This, together with the operator s natural reaction, promoted flooding. The problem was solved by switching the controller from automatic to manual. 

In services where the difference between column top and column bottom temperature is small, it may be difficult to find any point where temperature is sufficiently sensitive to composition. Sometimes the 

If an intermediate component is present, the sensitivity of tray temperature to its concentration must also be taken into account (287, 

The sensitivity of the control temperature to key components concentration must also exceed its sensitivity to any expected pressure variations. This is most important under vacuum. 

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