A differential vapor pressure (DYP) cell is disposed in a divided wall
column that receives a feed comprising a first, second, and third
component. A separation section on the feed side of the divided wall
column separates the feed in a vapor comprising the first and second
component, and a liquid comprising the second and third component. The
DYP cell is disposed in the divided wall column at a level below the
point where the feed enters the column, and the DYP cell measures the
concentration of the first component.