Engineering Question #740

cheryl, a 21 year old female from singapore asks on April 5, 2002,

I read that to find the efficiency of a fractional distillation column, you can distill a mixture of 20% n-heptane (bp=98°C) and 80% methylcyclohexane (101°C). Then using fenske equation, you can calculate the number of plates in the column. I was wondering how the distillation concentration would vary with time? Since it is a batch distillation under total reflux, is it true to assume that concentration of n-heptane in the distillate will first increase as it is more volatile, then decrease as it becomes depleted? And how would you determine the efficiency since the number of plates would be changing as the distillate concentration changes as well? Do you take the highest or lower number of plates for efficiency or the number of plates when the distllate concentration becomes constant (if it ever does)?

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