Q$2.(10$ Marks$)[$CO$1,$ CO$2,$ CO$3,$ CO$4]$

In a peanut processing plant $10$ ton $\frac{?}{h}$ of miscella $(15wt\%$ peanut oil in hexane$)$ leaves at $35C.$ As a

process engineer you are asked to propose a processing scheme which can separate the hexane from the

oil so that the final oil contains less than $0\mathrm{.}01\%$ hexane and such that the temperature never exceeds

$80C.$ Begin with what seems to be the most economical process 'distillation', start answering the

following questions in sequence and come out with your proposal for processing scheme:

a$)$ What makes you so confident that distillation is plausible?

b$)$ The vapor pressure of hexane in a solution containing $0\mathrm{.}01\%$ hexane in peanut oil at $80C$ is only

$0\mathrm{.}6mmHg.$ Normally, even vacuum distillation towers do not operate at pressure less than $100mm$

$Hg.$ How will you operate the distillation column at this lower pressure?

c$)$ How are you going to choose the tower pressure? Justify.

d$)$ For the sake of argument, fix the still pressure at $200mmHg$ and assume that the still can be

designed. Will you face any other problem?

e$)$ Indeed this one of the plausible alternatives. Vapor liquid equilibrium calculations reveal that a

flash of the hexane$-$peanut oil solution at $75C$ and $1$atm will remove $85\%$ of the hexane, and at

$75C$ and $300mmHg98\%$ of the hexane will be removed. Can you suggest some modification in

the existing alternative to reduce the load on the distillation. Draw the flowsheet.

Additional Information

Mol. wt$.$ of: Peanut Oil $=885\mathrm{.}02,$ Hexane $=86\mathrm{.}18,$ Vapor pressure of peanut oil is $0mmHg$ at $20C,$

Dew point of hexane is $87C$ at $151mmHg$ of pressure. Cooling water is available at $25C.$