Ratio control

In chemical processes it is often necessary to regulate the ratio of the flowrates of two streams, for example:

• In a distillation column, one important manipulation could be the reflux ratio
• In absorption columns the liquid to gas ratio is important.
• In a reactor with multiple feeds it is often necessary to keep them in some ratio to own another.

Ratio control is really very simple. One of the streams in the problem is designated the 'wild stream'. This wild stream is not manipulated by the ratio controller, but the other streams are adjusted to keep in ratio with the wild stream. The ratio controller itself is just a simple multiplier: it multiplies the measurement signal from the wild stream by a gain and then sends this signal either directly to a valve on a manipulated stream, or, more usually, to a secondary flow controller setpoint. The gain used as a multiplier is often the output of another controller (e.g. a distillate composition controller on a distillation column may use the reflux ratio as a manipulation).