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If the pressure is lowered in a distillation column, then the equilibrium curve will move further away from the diagonal (bulge out more) in an equilibrium diagram. This leads to a decrease in ideal step stages required to reach the separation requirement.

My question is why does the ideal step stages decrease? Is it only because of how the stages are drawn; meaning if the equilibrium curve is more bulged less horizontal and vertical stages are required. Or has it to do with the driving force increasing as pressure is lowered?

All help is appreciated. Thank you!

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In an x-y diagram, the "bulging out" is demonstrating the increase in relative volatility of the components; the number of stages is a strong function of relative volatility, so an increase in relative volatility leads to less stages.

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