A quaternary light hydrocarbon mixture of ethane, propane, n-butane, and n-penta
ID: 502454 • Letter: A
Question
A quaternary light hydrocarbon mixture of ethane, propane, n-butane, and n-pentane is separated in a distillation column operating at 5 atm. The column has a total condenser and a partial reboiler. The feed flow rate is 1000 kmol/hr. The feed is a saturated liquid. Feed composition is 8.0 mol % ethane, 33.0 mol % propane, 49 mol % n-butane and 10.0 mol % n-pentane. A 98 % recovery of propane is desired in the distillate. A 99.2 % recovery of n-butane is desired in the bottoms. Assume relative volatility is constant at value calculated at the bubble point temperature of the feed (bubble point of feed is 12 degree C). Use DePriester chart given in Wankat (pp. 31-32; Third Edition) to find value of K (distribution or equilibrium constant) and find relative volatility (alpha) of each component with respect to suitable reference component. Answer the following questions; a) What is the heavy and light key for this design problem? b) Derive the Fenske equation and clearly state your assumptions (refer to your notes) c) What are the K values for individual components under given feed conditions? d) Find N_min using the Fenske equation derived above e) Find (L/D)_min using the two Underwood equations given in your class notes Use L/D = 1.2 (L/D)_min and estimate N_actual and N_Feed (based on the optimum feed stage) actual using the Gilliland correlation.Explanation / Answer
a) The heavy key is the component with higher boiling point and the light key is the component with lower boiling point.
the boiling point of methane is -161.5 oC
ethane is -89 oC
n-butane is -1 oC
n-pentane is 36.1 oC
So, the light key is methane
and heavy key is n-pentane