We wish to distill a mixture of ethane, propane, n-butane. The column has a partial reboiler and a partial condenser and operates at 400 kPa. The feed flow rate is 200 kmol/h. The feed is a saturated liquid and is 22 mol% ethane, 47 mol% propane, and 31 mol% n-butane. We wish to recover 97% of the ethane in the distillate and 99% of the propane in the bottoms. The reflux is a saturated liquid, and the external reflux ratio L0/D = 3.0. Find the optimum feed stage and the total number of equilibrium contacts required. Assume constant molal overflow, and use the DePriester charts for K values.