Question

From a control process, what is the effect of an overshoot in products composition in a distillation column?

can it be concluded that it is a consequence of an overshoot in temperature inside the column?

Answer 1

Overshoot is a measure of how much our "controlled variable (to which we want to control)" exceeds its steady value for a step change in "manipulated variable".And we are concerned about 'Overshoot' so that we can choose our control parameters to stay within that limit.

The effect of an overshoot in product compositions is that we will not get desired purity of the product as per the market requirement which will be a huge loss to the industry if this occurs in terms of economy as well as product purity.Normally for getting desired specification of product,we do sampling using "Gas Chromatography" which requires huge investment and maintainence cost.Also it shows larger time lag.

So for getting desired composition,we measure the temperature of the product stream at every instant and hence manipulate the temperature of reflux which is sent to the column and so the final product composition is decided by the temperature of the column.Composition is measured intermitently to verify whether the process under this temperature control is going in the right direction or not.So we can say the overshoot in product composition may be the possible consequence of overshoot in the temperature inside the column.

But it is not only the temperature as the composition for a ideal soln is function of temperature and pressure while for non-ideal solution,it is also the function of activity coefficient which is the property of component itself.So the overshoot in the product composition may also occur because of overshoot in the pressure of the column which is directly affected by the flow rate of Vapour inside the column.

(Po is used for the saturation pressure.sorry i forgot to write Psat somewhere,i have used Po). Hope it is clear.If any doubt,please ask.