Vapor pressures of the three mixtures of light hydrocarbons, that is, isobutane-propane, n-butane-propane, and isobutane-n-butane-propane systems, were measured by a bubble point method in the temperature range from 273.15 to 323.15 K. The experimental apparatus used in this study is shown in Fig. 1. The equilibrium cell is a 18 mm o.d. × 200 mm long pyrex glass cell of about 20 cc coupled through an adapter to a pressure delivery system (Fig. 2). The vapor pressure was measured in accordance with the following procedure. (1) A sample mixture of the desired composition is charged into the cell. The cell is set into a thermostat controlled water bath. (2) Mercury is introduced into the cell by releasing valve 5 until the mixture is liquefied completely. Then the pressure is decreased by an increment of about 0.3 kPa until a bubble can be seen with use of a cathetometer. The vapor pressure was determined from the pressure at which the bubble first appeared in the liquid phase. The desired composition of the mixture was adjusted in the cell by using a balance with an accuracy of 0.001 g. The accuracy of the composition measurement was estimated to be within 0.0002 mole. The pressure of the cell was measured with a dead weight gage which was accurate to within ±0.1 kPa. The temperature of the water bath was measured with a mercury thermometer calibrated by a platinum resistance thermometer accurate to within ±0.01 K. The estimated precision of the vapor pressure determination was better than 2 kPa from the reproductivity of the data and the accuracies of the temperature, pressure, and composition measurements. The results of the isobutane-propane, n-butane-pro-pane, and isobutane-n-butane-propane systems are summarized in Tables 1 to 3, respectively. The vapor pressure vs. composition diagrams for the isobutane-propane and w-butane-propane systems are shown in Figs. 3 and 4, respectively. The vapor pressure vs. temperature diagram for the isobutane-w-butane-pro-pane system at four different compositions are shown in Fig. 5. It was found that the vapor pressures of these three systems show negative deviations from those obtained from an additive law and that the deviations become larger at higher temperature. The vapor pressure was calculated first by the Lee-Kesler's vapor pressure equation, Eq. (1), with use of the mixing rules of Eqs. (3)~(7). It was found that a binary interaction parameter, ktj, is needed for Eq. (8) to fit satisfactorily the experimental data of the binary systems. With use of the binary interaction parameters determined from the data of the binary systems, the vapor pressure for the isobutane-n-butane-propane system can be estimated with the maximum deviation of about 0.02 MPa. Next, four kinds of cubic equation of state were applied to calculate the vapor pressure. These equations are as follows; 1) Modified Redlich-Kwong equation; 2) Soave-Redlich-Kwong equation; 3) Peng-Robinson equation; 4) Patel-Teja equation. The parameters for a mixture were obtained from Eqs. (9)~(11). The maximum deviations between the experimental and calculated vapor pressures are summarized in Tables 4 to 6. It can be seen that the Modified Redlich-Kwong equation gives the best results and the Peng-Robinson and Patel-Teja equations give also good results, while the Soave-Redlich-Kwong equation gives slightly less accurate results.
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