P–x data for binary systems using a novel static total pressure apparatus

A new static isothermal vapor–liquid equilibrium (VLE) apparatus, designed for operation at pressures up to 2000 kPa incorporates novel dual-mode piston pumps which, in the micro-mode, can dispense microlitre volumes repeatably. Pure component vapor pressure and VLE measurements for two test systems (water + 1-propanol at 313.17 K and n-hexane + 2-butanol at 329.22 K) showed excellent agreement with literature data. New VLE data are presented for n-dodecane + 1-propanol at 342.8 and 352.7 K, and n-dodecane + 2-butanol at 342.7 and 352.7 K. An accurate new method for finding the cell net interior volume from cumulative injected liquid volumes and corresponding pressures is presented. For systems of high relative volatility the liquid equilibrium composition (xi) may differ significantly from the charge composition (zi) in the region dilute in the more volatile component. Exact equations for the fractional difference (zi − xi)/xi conveniently show the influence of relative volatility, system pressure, vapor space, and vapor non-ideality, and are illustrated for the n-dodecane + 1-propanol system. Equations showing the dependence of the limiting activity coefficient on the virial coefficient values (B11, B12, and B22) used in their computation indicate that the influence may be considerable for derivatives numerically larger than about 4 × 10−5 mol cm−3.