Control of boil-up rate

We have seen that the theoretical plate number is atrongly dependent on load in some columns. In addition, if a column is operated near to flooding, the control of the bod-up rate should be close enough not to allow flooding to take place. Two methods of control are available  

that of keeping the heat input constant, especially in simple distillation and 

control of the flask heater by the pressure drop in the column in countercurrent distillation. 

The method of supplying the electric flask heater with a constant current can be unreliable since it involves frequent checking of the load, especially if there are large differences in the heats of evaporation of the components being distdled. As the pressure drop in a column depends on the vapour velocity (section 4.11), it provides a convenient means for controlling the heater. The method also has the advantage of not being seriously affected by mains fluctuations or by variations in the gas pressure [13]. 

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