Calculation Procedure

1. Determine the relevant vapor-pressure data. Design calculations involving vapor-liquid equilibrium (VLE), such as distillation, absorption, or stripping, are usually based on vapor-liquid equilibrium ratios, or K values. For the ith species, K _i is defined as K _i= y _i /x _i, where yi is the mole fraction of that species in the vapor phase and xi is its mole fraction in the liquid phase. Sometimes the design calculations are based on relative volatility ? _i,j, which equals K _i /K _j , the subscripts i and j referring to two different species. In general, K values depend on temperature and pressure and the compositions of both phases.

When a system obeys Raoult s law and Dalton s law, it is known as an ideal system (see Related Calculations for guidelines). Then , and , where is the vapor pressure of the (pure) ith component at the system temperature and P is the total pressure.

One way to obtain the necessary vapor-pressure data for benzene and toluene is to employ the Antoine equation:

(See also Section 1.) When P is in millimeters of mercury and t is temperature in degrees Celsius, the following...