What are different methods to control distillation?
Answers
Hey mate here we go
In a distillation column it is usually necessary to regulate the pressure in some way. Below there are five different methods described for doing this.
*Vent to Atmosphere
*Cooling Water
*Flooded Condenser - 1
*Flooded Condenser - 2
*Partial Condenser
One thing to note is that in none of them is a valve simply placed on the vapour line. This would lead to the use of a large expensive control valve. Instead the pressure is controlled indirectly involving the use of the condenser and/or reflux drum.
*Vent to Atmosphere:-
In this case the cooling water flow stays constant and the reflux drum is vented to atmosphere. Thus the reflux drum and hence the top of the column are at atmospheric pressure. The advantage of this scheme is that it requires one less control valve. The disadvantage is that the tops have to be subcooled so that a minimal amount of vapour is lost through the vent. Hence more energy is required from the reboiler when the reflux is added to the top of the column.
*Cooling Water:-
In this case if the cooling water flow is increased then more vapour is condensed and the vapour pressure is reduced (and vice versa).
*Flooded Condenser - 1:-
Again in this setup, as with the first example, there is no valve on the cooling water. Instead the valve is in the liquid line between the condenser and reflux drum.
If this valve is closed then the condensed vapour i.e. liquid will build up and flood the condenser. This has the effect of reducing the heat exchange area, thus reducing the amount of vapour being condensed and hence increasing the pressure.
The valve can then be opened, the liquid level will fall, increasing the heat exchange area and hence decreasing the pressure.
*Flooded Condenser - 2:-
The first thing to notice about this setup is that the reflux drum and condenser are at the same level. The second important point is that the vapour line, on which there is the control valve, is very small in comparison with the overhead line. If the valve is opened there is a small escape of gas into the reflux drum. This pushes the liquid level down in the drum and up in the condenser, flooding it and reducing the heat exchange area as in the last example.
Therefore to increase the pressure the valve is opened and to decrease the pressure the valve is closed.
*Partial Condenser:-
The above scheme is used if the overhead product is required as a vapour.
I hope this will helps you
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