Three-fluid phase behavior

As an example for complicated fluid phase behavior in multi-component systems, in the group of Maurer [1,2,3] fluid four-phase behavior liquid-liquid-liquid-gas in ternary and quaternary aqueous systems is examined Thus, already for ternary systems complications can occur. Also Patton et al. [4] encountered unexpected phase behavior in a ternary system. For the system CO2 + 1-decanol + tetradecane they found a so-called two-phase hole ig enclosed in the three-phase surface ttg. 

Based on s discovery, a systematic and extensive experimental investigation of related ternary systems containing near-critical CO2 as the solvent and two heavier solutes has been carried out. The temperatures, pressures and compositions examined are within the range of conditions at which processes in super- and near-critical fluid technology applications take place. In ternary systems of the nature CO2 + 1-alkanol + alkane critical endpoint data were determined experimentally to characterize the three-phase behavior tig. To explain the observed fluid phase behavior, the binary classification of Van Konynenburg and Scott [5,6] was adapted to ternary systems, see section 2. 

Figure 1 shows the schematic p,T-projections for different types of fluid phase behavior. The vapor pressure curves of the two pure components, marked g(A) and g(BX with their critical points are shown. As can be seen in these figures, the diagrams differ with respect to the number and the nature of the critical en( )oints (CEP s) occurring and also with respect to the critical lines and three-phase loci gy which are connected directly to these CEP s. A CEP is a critical phase in equilibrium with an additional phase. 

Figure L Schematic p,T-projections of types of binaiy fluid phase behavior according to the classification of Van Konynenburg and Scott [5,6] —, vapor pressure curve of a pure component - -, critical line —three-phase line ffg , critical point of a pure component o, UCEP =g+ , LCEP f =r+g o, UCEP f-r+g X, DCEP a, TCP (a) Type-III fluid phase behavior (b) DCEP, transition between type-HI and type-IV fluid phase behavior (c) Type-fV fluid phase l havior (d) TCP, transition between type-IV and type-II fluid phase behavior (e) Type-II fluid phase behavior (f) Type-I fluid phase behavior (g) Type-V fluid phase behavior.

Figure la is a representation of type-III fluid phase behavior. Towards higher temperature and pressure, the three-phase line g is terminated by an upper critical endpoint of the nature =g " (UCEP which can be seen as the characteristic feature for this type of fluid phase behavior. A critical line connects this CEP with the critical point of pure component A. Another critical line emerges fi om the critical point of component B and tends towards higher pressures. Very often this critical line may show a minimum and a maximum in pressure and/or temperature (although temperature maxima are quite rare) and gradually chants nature into = ... 

If, on the other hand, the gap between the two three-phase loci widens, the temperature range of the higher-temperature three-phase locus will decrease. When this higher-temperature three-phase line just vanishes, two CEP s of different nature (the LCEP f= f+g and the UCEP t=g ) merge, see Figure Id. The resulting point is called a tricritical point (TCP), since at this point three phases become identical simultaneously (g=t= ). This state can be seen as a transition state between type-IV and type-II fluid phase behavior. 

Type-II fluid phase behavior is represented schematically in Figure le. The observed three-phase locus g is terminated by a UCEP from where a critical... 

Towards higher N, the region of type-IV fluid phase behavior ends with the occurrence of a DCEP, and type-III fluid phase behavior can be found. With the appearance of the DCEP (transition point between type-III and type-IV fluid phase behavior), the gap between the higher- and the lower-temperature three-phase loci has vanished. For constant N > Ndcep one continuous three-phase locus has formed, which is terminated towards higher temperature by an UCEP i.e., type-III fluid phase... 

Various mixtures first classified as belonging to type-V fluid phase behavior were found later to show type-IV instead. Rowlinson and Freeman [27] found some CO2 + hydrocarbon polymer mixtures to show type-V fluid phase behavior, i.e., they did not find any lower-temperature three-phase locus. However, using the van der Waals equation of state. Van Konynenburg and Scott [6] classified these systems as type-IV fluid phase behavior. Also the systems methane + pentane and methane + hexane, being members of the homologous series methane + alkane, were classified by Van Konynenburg and Scott [6] as a type-II and a type-IV system, respectively, although the UCEP s are situated below the solid phase boundary. Also Davenport and... 

Rowlinson [28] report these systems as just indicated and Dickinson et al. [29] found strong evidence for a metastable UCEP t=i"- rg near 150 K in the shape of the melting curve. In addition, the system CO2 + decanoic acid has to be considered as an example for type-IV fluid phase behavior [30], although no lower-temperature three-phase line iig can be found. A critical line interrupted by crystallization, indicated a meta-or instable UCEP t=r g. CO2 + undecanoic acid belongs to type-III fluid phase behavior [30]. 

For example, the binary system methane + ethane belongs to type-II fluid phase behavior according to the van der Waals equation of state. For this equation of state. Van Konynenburg and Scott [6] introduced the three parameters... 

L Earlier Observations on Unexpected Fluid Phase Behavior Patton et al. [4] found unexpected fluid multiphase behavior for the system CO2 + 1-decanol + tetradecane. The two binary border systems CO2 + 1-decanol and CO2 + tetradecane both show type-III fluid phase behavior, see [41] and [10,43], respectively, in the classification of Scott and Van Konynenburg [14,30] (section 2), with its characteristic UCEP For the ternary system, the three-phase surface Ug is... 

It is believed that the selected systems are representative for systems to be met in supercritical fluid applications. Also it became apparent from this work that in the region of interest for supercritical fluid applications in very narrow concentration windows the nature of the fluid phase behavior sometimes may change several times or, in other words, the number of coexisting phases may change several times from two into three and vice versa. For obvious reasons this makes the design of processes with near-critical carbon dioxide extra complicated since it may be expected that the phenomena discussed in this work are very general and not only limited to the solutes investigated in this study. 

The complete form of the liquid-liquid immiscibility region can be achieved only if there is no interference of immisci-bihty region and crystallization surface, and this region does not touch the crystallization surfaces but exists only in solid unsaturated solutions (the main types of fluid phase behavior). Types lb, Ic and Id are the versions of complete phase diagrams with three different types of immiscibility region in their complete form. 

Fluid-fluid phase separations have been observed in many binary mixtures at high pressures, including a large number of systems in which helium is one of the components (Rowlinson and Swinton, 1982). Fluid-fluid phase separation may actually be the rule rather than the exception in mixtures of unlike molecules at high pressures. Fig. 6.4 shows the three-dimensional phase behavior of a binary mixture in schematic form. This diagram includes the vapor pressure curves and liquid-vapor critical points of the less volatile component (1) and the more volatile component (2) in their respective constant-x planes. The critical lines are interrupted one branch remains open up to very high temperatures and pressures. Systems that can be represented by a diagram such as Fig. 6.4, those for which the critical lines always have positive slope in the p — T projection, have been called fluid-fluid mixtures of the first kind. A second class of system, in which the critical line first drops to temperatures below T (l) and then increases, exhibit fluid-fluid equilibrium of the second kind. There is, however, no fundamental distinction between these two classes of fluid mixtures. 

Of course, LC is not often carried out with neat mobile-phase fluids. As we blend solvents we must pay attention to the phase behavior of the mixtures we produce. This adds complexity to the picture, but the same basic concepts still hold we need to define the region in the phase diagram where we have continuous behavior and only one fluid state. For a two-component mixture, the complete phase diagram requires three dimensions, as shown in Figure 7.2. This figure represents a Type I mixture, meaning the two components are miscible as liquids. There are numerous other mixture types (21), many with miscibility gaps between the components, but for our purposes the Type I mixture is Sufficient. 

Figure 7.2 A three-dimensional phase diagram for a Type I binary mixture (here, CO2 and methanol). The shaded volume is the two-phase liquid-vapor region. This is shown ti uncated at 25 °C for illustration purposes. The volume surrounding the two-phase region is the continuum of fluid behavior.

In order to increase the sustainability of chemical processes, environmentally friendly solvents such as supercritical fluids (SCFs) are widely investigated. Han and coworkers studied the ethenolysis of ethyl oleate in SC C02 in relation with the phase behavior of the reaction mixture [62]. They carried out the ethenolysis reaction at 35°C in the absence of C02 and in the presence of C02 at three different pressures (50, 82, and 120 bar). The reaction in the absence of C02 reached equilibrium in 1 h at 80% conversion. The reaction rate in the presence of 50 bar of C02 was higher than without C02 and, at 82 bar, again increased with respect to 50 bar. However, when the pressure was increased to 120 bar, the reaction rate decreased. This effect was explained according to the variations on the phase behavior with the pressure an increase in the C02 pressure carried an increase of solubility of reactants, products, and C02, which produced a decrease of the viscosity of the reaction mixture. This positive effect was enhanced at 82 bar and was accompanied by an increase of selective solubility of the products in the vapor phase that further increased both reaction rate and conversion. The decrease of efficiency at 120 bar was related to an increase of the solubility of the reactants in the C02 phase. 

The potential of supercritical extraction, a separation process in which a gas above its critical temperature is used as a solvent, has been widely recognized in the recent years. The first proposed applications have involved mainly compounds of low volatility, and processes that utilize supercritical fluids for the separation of solids from natural matrices (such as caffeine from coffee beans) are already in industrial operation. The use of supercritical fluids for separation of liquid mixtures, although of wider applicability, has been less well studied as the minimum number of components for any such separation is three (the solvent, and a binary mixture of components to be separated). The experimental study of phase equilibrium in ternary mixtures at high pressures is complicated and theoretical methods to correlate the observed phase behavior are lacking. 

The surfactant AOT forms reverse micelles in non-polar fluids without addition of a cosurfactant, and thus it is possible to study simple, water/AOT/oil, three component systems. To determine micelle structure and behavior in water/AOT/oil systems, investigators have studied a wide range of properties including conductivity (15), light (JL ), and neutron (12) scattering, as well as solution phase behavior (1 ). From information of this type one can begin to build both microscopic models and thermodynamic... 

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