Question

Ethanol-water pressure swing distillation is to be designed with the use of FUG method. first column is at 1 bar and second is at 7 bar.

Column 1 results: minimium number of theoretical stages = 7, Rmin = 0.99 ,

Column 2 results: minimuim number of theoretical stages = 40 Rmin =8.4. How come the second distillation column is so costly due large amount of stages needed . This may be due to the relative volatility being close to 1 in the second column.

Is Pressure swing distillation a wise choice for the separation of water and ethanol

Answer 1

Distillation process is the method of separation of binary components based on the difference of volatility. for he azeotropic mixtures the conventional distillation fails, in that case the the methods which are used are:

1. azeotropic distillation

2. membrane distillation

3. azeotropic distillation

4. pressure swing distillation

pressure swing distillation is a method of distillation in which the relative volatility of the mixture is altered by changing the pressure. advantages of pressure swing distillation is this process we don not require to add any third solvent like in azeotropic distillation method. and this has attracted the special attention.

in the problem we are given that ethanol and water is being separated using the pressure swing distillation method. the operating pressure is 1 bar for the fist column and 7 bar in the second column.

now for the first column is designed with 7 stages and the second column is designed with 40 stages. as the number of the stages increase the cost of the production of the column also increases.

this increases in the stages could be incorporated because of two factors:

1. the mixture of water and ethanol is an azeotropic mixture, which means that relative volatility is very clos to 1. so they will boil at the same boiling point.for these kind of the mixtures the driving force is very less and hence as the driving force is less the number of stages increases.

2. as the mixture is an azeotropic mixture the best method for the component separation is pressure swing distillation method. Now , in the first column maximum amount of ethanol gets separated from water and hence very less amount is present in the bottom product. and then the bottom product is sent to the next column, so because of very less amount of ethanol is separated from the mixture we more number of stages because the driving force of concentration difference is very less. that's why the number of stages increases in the second stage.

these could be the reasons for increase in the number of stages for the second column.

the decision of pressure swing distillation is a wise choice for the separation of ethanol and water mixture, because the mixture is an azeotropic mixture and thus by varying the pressure could lead to change in the volatility of the mixture and thus the components will boil at different temperature. hence the separation will become more easier.