Ratio control is a feed-forward control scheme that measures and maintains a constant ratio between two disturbances (loads), usually flow rates. It is commonly used to control the ratio of reactants entering a reactor, the reflux ratio in distillation columns, and the fuel/air ratio in burners. Ratio control allows linking two streams to produce and maintain a defined ratio simply without needing a complex model. However, it assumes the correct ratio is maintained by controlling only one measured flow rate.

1. Ratio Control
ADNAN AKHTAR
2017-MS-CH-05

2. Ratio Control Scheme
Ratio Control scheme is a special type of feed-forward
control scheme where two disturbances (loads) are
measured and held in a constant ratio to each other.
Mostly used to control the Flow rates of two streams.
Wild stream
Controllable stream

3. Configuration of Ratio Control

4. Configuration of Ratio Control

5. Applications of Ratio Control
Hold Constant Reflux ratio in a distillation column
Control the ratio of two reactants entering a reactor at a
desired value
Keep the ratio of Fuel/Air in a burner at it’s optimum value

6. Fuel/Air Ratio

7. Advantages
Allows user to link two streams to produce and maintain a defined
ratio between the streams.
Ratio control scheme is very simple to use.
It does not require a very complex model.

8. Disadvantages
Often one of the flow rates is not measured directly and the
controller assumes that the flows have the correct ratio through the
control of only the measured flow rate.
Requires a ratio relationship between variables that needs to be
maintained.
Not as useful for variables other than flow rates.