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Example 4: Continuous Distillation

A distillation column operating at 1 atm is to be designed for separating an ethanol-water mixture. The feed is 20 mole% ethanol and the feed flow rate is 1000 kg-mole/hr of saturated liquid. A distillate composition of 80 mole% ethanol and a bottoms composition of not more than 2 mole% ethanol are desired. The reflux ratio is 5/3.

Determine:
(a) the total number of equilibrium stages required
(b) the optimum feed plate location
(c) the distillate and bottoms flow rates in kg-mole/hr


Equilibrium data for ethanol-water system at 1 atm pressure are in Table E-4 given below:

VLE for Ethanol-Water

Analyse the graphical solution. What can you conclude about the extent of separation in the rectifying section and stripping section respectively?

Obtain the mole fractions in liquid and vapour for each tray and plot a concentration profile for your design (vertical-axis: number of trays, top-down, horizontal-axis: mole fractions x and y).

Solution to Example 4

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