Vapour-liquid equilibrium measurements using a static total pressure apparatus.
A novel static total pressure apparatus was designed, built and commissioned for the measurement of VLE data at low to moderate pressures and temperatures. The apparatus of Fischer and Gmehling  was used as a basis for the current design. The continuous-dilution technique (Gibbs and Van Ness ) for sample introduction has been incorporated in our apparatus, so that the full composition range of a mixture can be covered in two runs. This procedure has the considerable advantage of speed. If the liquid is properly degassed, the main limitation of the method is the accuracy with which one can establish overall compositions from metered volumes. Accurate injection of the two components is accomplished with a patented dual-action piston-injector (Raal ). In the micromode the pump can accurately dispense submicrolitre volumes and the apparatus is thus particularly suited for VLE measurement in the very dilute region, and thus for determining limiting activity coefficients. r~ calculated using the method proposed by Maher and Smith  ranged from about 3.8 to 59. The estimated accuracy of the injected volumes is ± 0.002 cm3 ; this was obtained from calibration with distilled water. The estimated accuracies of the equilibrium temperature and pressure are ± 0.2 °C and ± 0.01 kPa respectively. The pure liquids were degassed for at least 8 hours according to the procedure proposed by Van Ness and Abbott . The static assembly and experimental procedure have been tested via pure component vapour pressure and binary vapour-liquid equilibrium measurements for a range of test systems (Water (1) + I-Propanol (2) at 313.17 K, Water (1) + 2-Butanol (2) at 323.18 K, n-Hexane (1) + 2-Butanol at 329.22 K). The test systems data compared well with literature data and a high degree of confidence was then placed on the equipment set-up and experimental procedure. New vapour-liquid equilibrium (VLE) data were measured for the following binary systems: • I-Propanol (1) + n-Dodecane at 342.83K and 352.68 K • 2-Butanol (1) + n-Dodecane at 342.83 K and 352.68 K • Water (1) + o-Cresol at 342.83 K The VLE measurements of the new systems were very challenging because of the large boiling point differences between the systems' constituents. An accurate new method for determining the net interior volume of the cell V cell was tested and P. -p J gave excellent linear plots of cumulative volume of injected liquid, vt against 1 Po 0 , with the sIot pe representing. Vcell The VLE data for all the systems measured were modeled using the combined (r - ¢ ) method. The Barker's method of data reduction was implemented to convert the number of moles of each component injected into the cell to mole fraction of the vapour and liquid phase (Uusi-Kyyny et al. ). Different Gibbs excess models namely NRTL, T-K Wilson and Van Laar together with the virial equation of state for vapour phase non-idealities were used. The T -K Wilson and NRTL gave the best fit.