A solution of methanol and water at 101 kPa containing 50 mol% methanol is continuously rectified in a seven-theoreticalplate, perforated-tray column, equipped with a total condenser and a partial reboiler heated by steam. During normal operation, 100 kmoVh of feed is introduced on the third plate from the bottom. The overhead product contains 90 mol% methanol, and the bottoms product contains 5 mol% methanol. One mole of liquid reflux is returned to the column for each mole of overhead product. Recently it has been impossible to maintain the product purity in spite of an increase in the reflux ratio. The following test data were obtained:

What is the most probable cause of this poor performance? What further tests would you make to establish definitely the reason for the trouble? Could some 90% product be obtained by further increasing the reflux ratio, while keeping the vapor rate constant? Vapor-liquid equilibrium data at 1 atm in mole-fraction methanolare

Stream kmol/h mol% alcohol 100 Feed 51 Waste 62 12 Product 53 80 Reflux 94