A compressed liquid mixture of 42 mol percent n-hexane (1) and 58 mol percent ethane (2) is to be fed to a flash tank where the pressure is reduced and a liquid and vapor stream separated. The flashing operation will be carried out at 32°C. The liquid product should have a composition of 81.7 mol percent n-hexane to meet the requirement of the succeeding unit.

At what pressure should the tank be operated? What will be the composition of the vapor stream? Assume that the vapor is an ideal gas.

Data:                                                        n-hexane                     ethane

      Molar liquid volume (cm3/mol)          132.0                          45.7

      Solubility parameter (J/cm3)1/2            14.9                         19.4

      Vapor pressure (bar)                              0.284                       46.195