V.3 : Ratio Control

A ratio controller is a special type of feedforward controller where disturbances are measured and their ratio is held at a desired set point by controlling one of the streams. The other uncontrolled stream is called wild stream. Fig V.11 shows the schematic of a ratio controller. The ratio of flow rates of two streams are being held at a desired ratio by controlling the flow rate of one stream. The flow rates are measured through flow transmitters (FTs).

Fig V.11: The schematic of a ratio control structure

The chemical process industries have various applications for ratio controllers. Following are a few such examples:

•  Reflux ratio and reboiler feed ratio in a distillation column

•  Maintaining the stoichiometric ratio of reactants in a reactor

•  Keeping air/fuel ratio in a combustion process