Question

A feed containing 30 mol.% n‐pentane and 70 mol.% n‐heptane is distilled in a continuous counter‐ current tower to produce an overhead product that contains 98 mol.% n‐pentane. 96 % of the n‐ pentane that enters to column must be recovered in the overhead product.  

a) Determine the flows of both products (per 100 mol feed) and the bottoms composition.

b) Determine the minimum number of stages required to perform this separation.

c) Calculate the minimum reflux ratio if the feed enters the column as a saturated liquid at its boiling point.

d) Determine the number of theoretical stages needed if an actual reflux ratio of 0.9105 is used. State the location of the feed tray and the temperature of the vapour and liquid streams exiting this stage.

e) At the conditions in part d), determine the liquid and vapour rates (per 100 mol of feed) in the rectifying and stripping sections of the column.

f) Determine the number of actual stages required at a reflux ratio of 0.9105 if the Murphree stage efficiency is 70 %.

Answer 1

With the help of material balance on distillation column, we will be able to find the flowrates of product streams and their composition.

Minimum no. of trays (i.e. at total reflux condition) is find out using Fenske equation or by Mccabe thiele diagram using slope of operating lines at 45° i.e. at diagonal line.

Minimum reflux ratio is found using the slope of operating line equation.Slope is to be find in between The point at which feed line cuts the equilibrium curve and the point of distillate composition as shown below.

Theoretical number of trays at given reflux ratio is to be found using McCabe thiele diagram as shown below.

The graphs are not based on actual scale, so actual answers may vary. You can follow the technique and procedure.

Fed tray is the tray at which feed point lies when we draw triangles between equilibrium and operating lines.