distillation material balance

[_Greg_]

1. The problem statement, all variables and given/known data

Question:

A mixture of 40/60 mole% pentane/heptane is to be separated to give a top product of 95 mole% pentane and a bottom product of 5 mole% pentane.

The column will operate at 1 bar witha reflux ratio of 0.5 kmol/kmol. It will have a total condenser and a partial reboiler. The feed is liquid at its bubble point.

Solution:

Taking a basis of 100 kmol feed:

Kmol-------Feed-----D---------B

pentane----40.0-----37.0----3.1
heptane----60.0-----1.9--- -58.0
total-------100------38.9----61.1

(D = distillate, B = bottoms product)


3. The attempt at a solution

I don't understand these numbers or how there derived??

I would have said, 95% of the 40 kmol pentane in the feed would be in the distillate, which would be 38 kmol.
And 5 % of the 40 kmol would be in the bottom, which is 2 kmol.

What am I doing wrong?

Thanks for looking :smile: