Question

Ethanol boils at 78.4°C, water boils at 100°C, but it forms a positive azeotrope that boils at 78.2°C, which is lower than for either of its constituents. If you start from a mash that has 10% alcohol content, how many distillation steps do you require to get to 40% alcohol (typical for Vodka)? (use the figure below to sketch the steps). How many steps would be required to make Whiskey with 60% alcohol? Why can you not make a solution of more than 95% alcohol by fractional distillation? (note that the figure is not completely to scale, but let's just use it to get approximate answers and you can sketch the process in this one). boiling point boiling point 100°C vapour composition liquid composition 78.5°C 78.2°C 0% ethanol 100% water % by mass 100% ethanol 0% water 95.6% ethanol

Answer 1

It an example of low-boiling azeotrope. Phase diagrams showing a minimum indicate that
mixture is destabilized relative to the ideal solution, interactions being unfavourable.

• From 10 % alcohol content to 40 % (Vodka) : to get this we will require atleast 2 -distillation steps.We start with a mixture of composition 10%, and follow the changes in the composition of the vapour, The mixture boils at 10% to give a vapour of composition ~20% (A2), This vapour condenses to a liquid of the same composition (now marked A3'). That liquid is boiled and reaches equilibrium with its vapour at (A3), which condenses to give a liquid of the same composition (~ 40 %)
• Mixture at A4 (~40 % ) if distilled once more reaches equilibrium with its vapour at A4', which condenses to give a liquid of the same composition (~ 60 %). So, for whiskey we will require atleast 3 -distillation steps.

When the azeotropic composition of of this mixture is reached, distillation cannot separate water and Ethanol,  because the condensate has the same composition as the azeotropic mixture. Fot this mixture it is 95.2 % Ethanol, thus by fractional distillation we cannot make solution beyond 95% ethanol. Boils will remain unchanged when ethanol content is 95.2 % by mass and the temperature is 78.2°C.

boiling point bailing point 100C A2 0% vapour composition A3 A4' NA0% liquid composition 78.5°C 60 % 78.2°C 0% ethanol 100% water 100% ethanol 0% water %by mass 95.6% ethanol