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.