Ternary systems, phase-equilibrium behavior

Experimental results are presented for high pressure phase equilibria in the binary systems carbon dioxide - acetone and carbon dioxide - ethanol and the ternary system carbon dioxide - acetone - water at 313 and 333 K and pressures between 20 and 150 bar. A high pressure optical cell with external recirculation and sampling of all phases was used for the experimental measurements. The ternary system exhibits an extensive three-phase equilibrium region with an upper and lower critical solution pressure at both temperatures. A modified cubic equation of a state with a non-quadratic mixing rule was successfully used to model the experimental data. The phase equilibrium behavior of the system is favorable for extraction of acetone from dilute aqueous solutions using supercritical carbon dioxide. 

The system carbon dioxide - acetone - water was investigated at 313 and 333 K. The system demonstrates several of the general characteristics of phase equilibrium behavior for ternary aqueous systems with a supercritical fluid. These include an extensive LLV region that appears at relatively low pressures. Carbon dioxide exhibits a high selectivity for acetone over water and can be used to extract acetone from dilute aqueous solutions. 

Becke, H., Quitzch, G. (1977) The phase equilibrium behavior of ternary systems of the C4-alcohol-hydrocarbon type. Chem. Tech. (Leipzig) 29, 49-51. 

Figure 11.2-11/, for the liquid-liquid phase equilibrium behavior of liquid carbon dioxide with pairs of other liquids, has been included to illustrate the variety of types of ternary system phase diagrams the chemist and engineer may encounter. Complete discussions of these different types of phase diagrams are given in numerous places (including A. W. Francis, Liquid-Liquid Equilibriums, John V Tley Sons, New York, 1963). 

Residue curve maps (RCMs) have been long used as a tool for analyzing a given ternary system s phase equilibrium behavior. These maps, originally pioneered by Schreinemakers in 1902 [1], enable design engineers to quickly scan possible separation trains or sequences, and also to identify areas of difficult separation due to azeotropes. 

In addition to the experimental data, the partitioning behavior of MMA between water and CO2 has been modeled. The Peng-Robinson equation of state combined with various mixing rules as described in Section 14.4.1 has been assessed on the ability to correlate phase equilibrium data from literature of the binary subsystems CO2-H2O, MMA-CO2 and MMA-H2O. Subsequently, the model has been used to predict the phase equilibrium behavior of the ternary system CO2-H2O-MMA. Partition coefficients were calculated at four different temperatures at pressures ranging from 5 to 10 MPa. In order to provide a means for comparison, the experimentally determined partition coefficients obtained in the high-pressure extraction unit were used to evaluate the results of the predictive model for phase equilibrium behavior. 

With the help of the binary parameters kn or g -model parameters now the phase equilibrium behavior, densities, enthalpies, Joule-Thomson coefficients, and so on, for binary, ternary and multicomponent systems can be calculated. For the calculation of the VLE behavior the procedure is demonstrated in the following example for the binary system nitrogen-methane using classical mixing rules. The same procedure can be applied to calculate the VLE behavior of multicomponent systems and with g -mixing rules as well. 

Figure 11.14 Phase equilibrium behavior of the ternary system acetone-chloroform-methanol at atmospheric pressure calculated using modified UNIFAC. (a) Tx-behavior (b) lines of constant separation factors (ofi2 = 1, 0fi3 = 1, of23 = l)l...

For the design study of a particular separation system, we typically start by using the Aspen built-in parameters of a suitable physical property model. The phase equilibrium behavior predicted by the Aspen built-in parameters should be compared with experimental data for validation purpose. It is obvious that an inaccurate description of the phase equilibrium behavior of a separation system will give flowsheet results that do not match the results of the true system. The worst case may be a failure of the separation task in the proposed design flowsheet. Thus, the validation stage is important before doing any design study. The experimental data that can typically be found in hterature include the Txy and yx data, binary and ternary LLE data, VLLE data, and azeotropic information. 

Note that selecting a proper physical property method is extremely important for obtaining reliable simulation results of a process flowsheet. Aspen Physical Property System gives the recommended classes of property methods for different applications. The phase equilibrium behavior predicted by the selected physical property method should be compared to experimental data for validation purpose. The experimental data that can typically be found in literamre includes the Txy and yx data binary and ternary LLE data VLLE data and azeotropic information. For details of how to validate the prediction of the selected physical property method, please refer to Chapter 2. 

Peters and Luthy (1993, 1994) performed a detailed analysis of the equilibrium behavior of solvent coal tar water mixtures in work that was complementary to column studies performed by Roy, et. al. (1995). Peters and Luthy successfully modeled ternary phase diagrams of coal tar/n-butylamine/water systems. In addition, Peters and Luthy identified n-butylamine as the leading solvent for coal tar extraction. Pennell and Abriola (1993) report the solubilization of residual dodecane in Ottawa sand using a nonionic surfactant, polyoxyethylene sorbitan monooleate, which achieved a 5 order of magnitude increase over the aqueous solubility, but is still 7 times less than the equilibrium batch solubility with the same surfactant system. 

A correlation of the detergency performance and the equilibrium phase behavior of such ternary systems is expected, based on the results presented by Miller et al. (3,6). The phase behavior of surfactant - oil - water (brine) systems, particularly with regard to the formation of so-called "middle" or "microemulsion" phases, has been shown by Kahlweit et al. (7,8) to be understandable in teims of the... 

Ternary Systems. As one of a series of model systems, we studied the carbon dioxide - acetone - water ternary system at 313 and 333 K. The most interesting feature of the system behavior is an extensive three-phase region at both temperatures. The three-phase region is first observed at a pressure of less than 30 bar at 313 K and approximately 35 bar at 333 K, extending up to approximately the critical pressure of the binary carbon dioxide - acetone system. Table I summarizes our experimental results for the composition of the three phases at equilibrium as a function of pressure and temperature. 

The ternary-phase diagrams presented here illustrate only a small sample of the variety of equilibrium behaviors observed in solid-liquid equilibria in three-component systems. This subject, along with kinetic considerations make up much of the subject matter of a variety of fields, including metallurgy and... 

Fig. 4.5. Potential singular point surface (dashed-dotted curve) for a ternary system with phase splitting behavior and the single reaction A + B C. RA = reactive azeotrope solid curve = chemical equilibrium surface.

To illustrate the system behavior, the ternary mixture 1 = iso-propanol, 2 = water, and 3 = air is considered here. In order to obtain an algebraic solution, both the dif-fusivities of iso-propanol in air and iso-propanol in water vapor were assumed to be approximately the same, which is not far from reality. The liquid phase mass transfer resistance was negligibly small, as will be shown below. The phase equilibrium constants K/,c and Kjrs were calculated with activity coefficients from van Laar s equation. Water vapor diffuses 2.7-fold faster in the inert gas air than iso-propanol. The ratio of the respective mass transfer coefficients kj3 equals the ratio of the respective diffusivities to the power of 2/3rd according to standard convective mass transfer equations Sh =J Re, Sc). 

The phase behavior observed in the quaternary systems A and B is also evidenced in ternary systems. Figure 4 shows the phase diagrams for systems made of AOT-water and two different oils. The phase diagram with decane was established by Assih (14) and that with isooctane has been established in our laboratory. At 25°C the isooctane system does not present a critical point and the inverse micellar phase is bounded by a two-phase domain where the inverse micellar phase is in equilibrium with a liquid crystalline phase, as for system B or system A when the W/S ratio is below 1.1. In the case of decane, a critical point has been evidenced by light scattering (15). Assih and al. have observed around the critical point a two-phase region where two microemulsions are in equilibrium. A three-phase equilibrium connects the liquid crystalline phase and this last region. 

In conclusion, the same phase behavior is evidenced when we change the alcohol or the W/S ratio in a quaternary system, and the oil in a ternary system. This behavior can be characterized by two types of phase diagrams. In the first type no critical point occurs. In this case, the inverse micellar phase is bounded by a two-phase region where it is in equilibrium with a liquid crystalline phase. 

For slags exhibiting a transition in behavior, the transition temperature could usually be associated with a ternary eutectic temperature in the phase equilibrium diagram for the most closely related ternary system. 

Using one of these activity coefficient equations it is possible to calculate liquid-liquid equihbrium (LLE) behavior of multicomponent hquid systems. Consider, for example, the ternary system of Figure 1. A system of overall composition A splits into two liquid phases B and C. The calculation of compositions of B and C is analogous to the flash ciculation of vapor-liquid equilibrium problems. By using the UNIQUAC equations to obtain the partition coefficients, Kj, this problem can be solved for any composition A of the overall system. The calculations are lengthy but computer programs for this purpose (2) have been published. In this paper simpler approximate methods for phase equilibrium problems of environmental interest is sought. For the moment it is sufficient to note that the activity coefficients provide the means of complete liquid-liquid equihbrium computations. 

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