We have performed the accurate calorimetric measurements of phase equilibria in the quinary mixtures of methane, propane, octane, nonane, and decane. Table 2 presents the composition of the measured hydrocarbon mixtures (see the mixtures No 1 and No 2). A method of the precision adiabatic calorimetry is used. The advanced adiabatic calorimetry gives the precision data of thermodynamic potentials and their temperature derivatives for the fluid systems. The phase transitions are localized by the finite discontinuity and singularities in temperature derivatives of the thermodynamic potentials. The phase diagrams for investigated mixtures are plotted based on the measured parameters of the phase transitions. A discussion on the uncertainty of the basic parameters of measurements one can find in [6]. A method of the precision adiabatic calorimetry is applied for the adiabatic calorimeter setup. A detailed description of the adiabatic calorimeter setup and experimental procedure one can find elsewhere [7, 8]. The heat capacity, internal energy, pressure, and temperature derivative of pressure at constant volume are measured in the range 170–250 K, up to 36 MPa.
Gas mixtures are prepared in the special high-pressure cylindrical sampler by weighing the pure components. The purity of the hydrocarbon components is between 0.9995–0.9999 mole fractions. Table 3 presents the summary of the gas parameters of the investigated mixtures.
The investigated systems presented as the quasi-binary mixtures of constant composition. The first quasi-component of the quasi-binary mixture is the binary mixture No 11. The second quasi-component of the quasi-binary mixture is the ternary mixture of octane, nonane, and decane. The measurements performed for the different values of xC8, xC9, and xC10 (see the mixtures No 1 and No 2 in Table 2). More information about the mixtures No 3–11 one can find in Refs. [2–6].
Figures 1, 2 and Table 4 present the phase diagrams for the quinary mixtures of methane, propane, octane, nonane, and decane. Using the procedure described in Ref. [2], these phase diagrams are plotted. The phase transitions, which occur for these mixtures, depicted in Table 5.
Figures 1a and 1b show the phase diagram for the quinary mixture of methane, propane, octane, nonane, and decane (1) presented as the combination of the simple hydrocarbon mixtures for the low concentration of octane (4), nonane (7), and decane (8). As well, the quinary mixture (1) is the combination of the quaternary hydrocarbon mixture for the low concentration of octane and nonane (3), and the simple hydrocarbon mixture for the low concentration of decane (8). By-turn, the quaternary hydrocarbon mixture for the low concentration of octane and nonane (3) is the combination of the simple hydrocarbon mixtures for the low concentration of octane (4) and nonane (7).
The phase diagram for the quinary mixture (1) includes the following parts:
1. The phase envelope for the macrophase composed of methane, propane, and partly dissolved octane, nonane, and decane, in the presence of the microphases formed by octane, nonane, and decane (I).
2. The phase envelope for the macrophase composed of methane, propane, octane and partly dissolved nonane and decane, in the presence of the microphases formed by nonane and decane (I¢).
3. The phase envelope for the macrophase, composed of methane, propane, octane, and nonane, and partly dissolved decane, in the presence of the microphase formed by decane (I²).
4. The typical phase envelope for the quinary mixture (II).
5. The lines of the microphase formation by octane, nonane, and decane (branches IIIC8, IIIC9, and IIIC10, (III¢C10)).
The inset in Fig. 1a shows the line of the microphase formation by nonane in the two-phase region of the macrophase, composed of methane, propane, octane, and partly dissolved nonane and decane, in the presence of the microphases formed by nonane and decane (LV(SC9+C10) + SC9+SC10). The inset in Fig. 1b shows the line of the microphase formation by decane in the three-phase region (L1 + L2 + V). The line AKB presents the P, T – projection of the immiscibility area (IV).
Figures 2a and 2b show the phase diagram for the quinary mixture of methane, propane, octane, nonane, and decane (2) presented as the combination of the simple hydrocarbon mixtures for the low concentration of octane (5), nonane (6), and decane (8). This phase diagram includes the following parts:
1. The phase envelope for the macrophase composed of methane, propane, and partly dissolved octane, nonane, and decane, in the presence of the microphases formed by octane, nonane, and decane (I).
2. The phase envelope for the macrophase, composed of methane, propane, octane, and nonane, and partly dissolved decane, in the presence of the microphase formed by decane (I²).
3. The typical phase envelope for the quinary mixture (II).
4. The lines of the microphase formation by octane, nonane, and decane (branches IIIC8C9, and IIIC10, (III¢C10)).
The inset in Fig. 2a shows the line of the microphase formation by decane in the three-phase region (L1 + L2 + V). The line AKB presents the P, T – projection of the immiscibility area (IV). The inset in Fig. 2b shows the lines of the microphase formation by octane and nonane in the two-phase region of the macrophase, composed of methane, propane, and partly dissolved octane, nonane, and decane, in the presence of the microphases formed by octane, nonane and decane (LV(S) + SC8+SC9+SC10).
The shift of the lines of the microphase formation by octane and nonane for the quinary mixture No 1 compared to the ternary mixtures No 4 and No 7 takes place (see Fig. 1b). As well, the shift of the line of the nonane microphase formation for the quinary mixture No 1 compared to the quaternary mixture No 3 occurs. The line of the microphase formation by octane for the quinary mixture No 1 coincides with the line of the microphase formation by octane for the quaternary mixture No 3. An additional shift of the line of the microphase formation by nonane for the quinary mixture No 1 compared to the quaternary mixture No 3 is a result of the decane addition into the quaternary mixture No 3. The shift of the line of the microphase formation by decane for the quinary mixture No 1 compared to the ternary mixture No 8 takes place due to the octane and nonane addition into the ternary mixture No 8. Octane and nonane, dissolved in the macrophase for the region of microphase formation by decane, transform the macrophase composition and ipso facto transform the microphase formation by decane.
The shift of the lines of the microphase formation by octane and nonane for the quinary mixture No 2 compared to the ternary mixtures No 5 and No 6 has been shown in Fig. 2b. Due to the octane concentration in the mixture No 2 twice as high as the nonane concentration, the superposition of octane phase transition on nonane phase transition takes place. We believe that formation of microphase by octane and nonane realized simultaneously. IIIC8C9 are the lines of the double microphase formation by octane and nonane. Nevertheless, octane and nonane form partly isolated microphases IIIC8+C9 for the low density of the quinary mixture (see the inset in Fig. 2b). Like the quinary mixture No 1, the shift of the line of the microphase formation by decane for the quinary mixture No 2 compared to the ternary mixture No 8 takes place due to the addition of octane and nonane into the ternary mixture No 8.
Thus, the presented above shifts are the result of the quinary mixtures transformation due to the addition of the heavy components into the ternary (the simple mixtures) and quaternary mixtures. As well, the shift of the lines of the microphase formation by decane for the quaternary mixtures No 9 and No 10 compared to the ternary mixture No 8 presented in [4] takes place due to the addition of octane into the ternary mixture No 8 (see Fig. 4, in [4]). The volume of the added octane predetermines the shift of the line of the microphase formation by decane. The higher the octane concentration the larger is the shift of the line of the microphase formation by decane. Due to the complete dissolution of octane in the macrophase for the region of microphase formation by decane, octane transforms the macrophase composition and ipso facto transforms the microphase formation by decane.
The heat capacity at constant volume for the mixtures No 1 and No 2 has been presented in Fig. 3a, 3b, and Table 6. Two isochores, ρ = 307.217 kg⋅m− 3 and ρ = 295.516 kg⋅m− 3, have been shown. The singularity C8 corresponds to the octane microphase formation (the branch IIIC8). As well, the singularity C9 corresponds to the nonane microphase formation (the branch IIIC9); and the finite discontinuity C10 corresponds to the decane microphase formation (the branch III′C10). The inset in Fig. 3b shows the heat capacity at constant volume for the partly isolated microphases IIIC8+C9 for the low density of the quinary mixture No 2. The finite discontinuity and singularity of heat capacity of the fluid systems have been described in Refs. [11–13]. Besides, Figs. 3a and 3b show the finite discontinuities in the heat capacity at constant volume, which correspond to the phase envelope for macrophase composed of methane, propane, octane, nonane, and partly dissolved decane in the presence of the microphase formed by decane (L(SC10) + SC10–LV(SC10) + SC10) and the phase envelope for the quinary mixture (L2–L2V).
The comparison of experimental phase diagrams and phase diagrams calculated with the ThermoFast software from the University of Western Australia performed for the quinary mixtures No 1 and No 2. The calculation data, performed for the investigated quinary systems, are similar to the calculation data performed for quaternary mixtures of methane, propane, octane, and decane presented in detail in Ref. [4]. Calculations predict the shift of the line of solid appearance for the quinary mixtures No 1 and No 2 to the lower temperature due to the octane and nonane dissolving. Unfortunately, calculations do not predict the formation of microphases by octane and nonane. Besides, the three-phase L1L2V area is not predicted.
As noted above, the investigated quinary hydrocarbon systems are presented as the combination of the simple hydrocarbon mixtures. In general, the simple hydrocarbon mixture consists of C1 − 3 components forming the macrophase and one of C4+ components forming a microphase. The investigated pure macrophase is a binary mixture with the constant methane/propane ratio having a single critical point Tc=211.60 K, Pc=6.40 MPa (see Ref. [3]). For the variable composition of the macrophase the critical point transforms into the line.
Our investigations show that like the classification scheme of van Konynenburg and Scott for the binary mixtures with the (P, T) projections for type V mixtures, the simple hydrocarbon mixture has been characterized by the absence of the continuous gas-liquid critical line, due to the presence of liquid-liquid immiscibility [3, 14, 15]. The critical line of the simple hydrocarbon mixture is divided into two parts: first, a part, which is vapor-liquid in nature, starts at the critical point of the pure macrophase (the heavy hydrocarbon concentration is zero) and ends in an upper critical end point. Further, the three-phase L1L2V line is spreading (see, for example, [3]). The three-phase line ends at lower temperatures and pressures at the lower critical end point that connects, through the second part of the critical line, with the gas-liquid critical point of the heavy component (one of C4+ components forming a microphase). Parameters of the heavy component critical points presented in Table 1. The second part of the critical line is liquid – liquid in nature at low temperature and changes its character into vapor – liquid at high temperature.