7.4 Ideal Distillation The most common separation method in the process industry is distillation. This process separates a liquid mixture in its two constituents based on their difference in volatility (boiling point). The main piece of equipment is a vertical column and two heat exchangers. The bottom exchanger vaporises liquid. The vapour rises through the column and is condensed at the top. So we have a circulating flow with vapour upwards and liquid downwards. The liquid concentrates the heavy or high boiling component in the bottom; the vapour strips out the light or volatile component and transports it to the top. A mixture is fed somewhere near the middle of the column, and products are drawn off at the top and bottom. The details of a distillation column are complicated, but you can (as always) get hold of the main characteristics by considering an ideal process. You only need an enthalpy and an entropy balance.

We consider a feed consisting of pentane and hexane. The feed is at its boiling point and the two products are withdrawn as liquids at their boiling points. You are to calculate the heat flows in and out of the column. Data required are:

pentane feed = n1 0.6mol/s hexane feed = n2 0.4mol/s feed temperature Tfeed = 273 + 50K top temperature Ttop = 273 + 36K bottom temperature Tbottom = 273 + 69K pressure p = 0.1 MPa heat capacity pentane Cp(l) = 160 J/molK heat capacity hexane Cp(l) = 200J/molK