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I have a line in my text that states the following:

"For systems with low boiling points (e.g. Ethane-Ethylene, propane-propylene etc.), high pressures are used so the vapour leaving the column at the top is cooled by water."

I don't understand the utility of high pressure over here. Isn't usage of water in the condenser already the case?

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In ambient pressure, ethane boiling point is -89 °C; ethylene boiling point is -103.7 °C. Water, to act as efficient coolant needs to be liquid, so above 0 °C. You're not going to condense ethane vapor at ambient pressure with coolant at 89 degrees above its boiling point!

Your two options are to either use a much colder - and more expensive and difficult to handle coolant (say, liquid nitrogen) or increase the pressure of the distillate so that it condenses at temperature of the cheap, ubiquitous coolant like water.

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