Van Konynenburg and Scott

Flalf a century later Van Konynenburg and Scott (1970, 1980) [3] used the van der Waals equation to derive detailed phase diagrams for two-component systems with various parameters. Unlike van Laar they did not restrict their treatment to the geometric mean for a g, and for the special case of b = hgg = h g (equalsized molecules), they defined two reduced variables. 

It is known that in five of the six principal types of binary fluid phase equilibrium diagrams, data other than VLE may also be available for a particular binary (van Konynenburg and Scott, 1980). Thus, the entire database may also contain VL2E, VL E, VL]L2E, and L,L2E data. In this section, a systematic approach to utilize the entire phase equilibrium database is presented. The material is based on the work of Englezos et al. (1990b 1998)... 

Data for the hydrogen sulfide-water and the methane-n-hexane binary systems were considered. The first is a type III system in the binary phase diagram classification scheme of van Konynenburg and Scott. Experimental data from Selleck et al. (1952) were used. Carroll and Mather (1989a b) presented a new interpretation of these data and also new three phase data. In this work, only those VLE data from Selleck et al. (1952) that are consistent with the new data were used. Data for the methane-n-hexane system are available from Poston and McKetta (1966) and Lin et al. (1977). This is a type V system. 

In this section we will discuss the phase behaviour of binary systems. In 2.2.2.1 the classification of fluid phase behaviour according to van Konynenburg and Scott [5] is discussed. The occurrence of solid phases introduces an extra complication in binary phase phase diagrams. This is discussed in 2.2.2.2. 

According to the classification of van Konynenburg and Scott [5] there are six basic types of fluid phase behaviour. The corresponding / -projections are shown in Figure 2.2-3. These / -projections can be complicated further by the occurrence of homo- and heteroazeotropes which will not be discussed here. 

The phase behaviour of many polymer-solvent systems is similar to type IV and type HI phase behaviour in the classification of van Konynenburg and Scott [5]. In the first case, the most important feature is the presence of an Upper Critical Solution Temperature (UCST) and a Lower Critical Solution Temperature (LCST). The UCST is the temperature at which two liquid phases become identical (critical) if the temperature is isobarically increased. The LCST is the temperature at which two liquid phases critically merge if the system temperature is isobarically reduced. At temperatures between the UCST and the LCST a single-phase region is found, while at temperatures lower than the UCST and higher than the LCST a liquid-liquid equilibrium occurs. Both the UCST and the LCST loci end in a critical endpoint, the point of intersection of the critical curve and the liquid liquid vapour (hhg) equilibrium line. In the two intersection points the two liquid phases become critical in the presence of a... 

Thermodynamic constraints to the SAS process can be summarized in the required miscibility between the liquid solvent and the supercritical antisolvent and the insolubility of the solute in the antisolvent and in the solvent-antisolvent mixture. Data are available for various binary mixtures liquid-supercritical fluid and can be described as type I using the classification of van-Konynenburg and Scott. If jet break-up is obtained and mass transfer is very fast, high-pressure VLEs of the ternary system liquid-I-solute-I-supercritical fluid can control the precipitation process. 

In the light of these considerations, a different approach based on ternary system thermodynamics could be considered. However, the phase behavior of temaiy systems could be very complex and there is a considerable lack of data on ternary systems containing a component of low volatility therefore, a possible compromise could be to consider that the solute addition can produce the shift of the mixture critical point (MCP) (i.e., the pressure at which the ternary mixture is supercritical) with respect to binary system VLEs and the modification of this kind of system that is formed according to the van-Konynenburg and Scott classification. ... 

The work of van Konynenburg and Scott stands out as a classic throughout all of thermodynamics. A basic understanding can be achieved by noting how phase behavior depends on the strength of the molecular... 

Type III behavior indicates the most extreme asymmetry between the components of a binary mixture. Nearly all H2 systems supply striking examples of type III behavior. CO2 mixtures with 2,5-hexanediol and 1-dodecanol are also classified as type III. The system CO2 -I- n-tridecane is peculiar because it was classified by van Konynenburg and Scott as type III, whereas Enick et al. have classified it as type IV, owing to experimental identification of a three-phase region. The system CO2 -I- n-tetradecane is a variation on type III, where the solute-rich locus terminates in a solid(wax)-liquid-liquid boundary. Several important systems fall into a similar category. For example, CO2 + naphthalene is commonly used as a model system for supercritical extraction. The naphthalene system differs from the n-tetradecane system in that the solute-rich locus terminates at a higher temperature... 

We consider these two types simultaneously because they share their distinctive feature. That feature is an interruption in the critical locus where two liquid phases appear over a short range of compositions before the critical locus reappears as a liquid-liquid critical point. Unlike low-temperature LL behavior, varying the pressure has a strong impact on type IV or V liquid-liquid-equilibria, (LLE) making it appear or entirely disappear over a remarkably narrow range of pressures. Systems that exhibit type IV behavior include methane -I- 1-hexene and benzene -I- polyisobutylene, the only polymer solution mentioned by van Konynenburg and Scott. Peters has also speculated that methane and ethane mixed with alkylbenzenes will form type II-rV solutions, in contrast to the I, III, V solutions of the n-alkanes.f ... 

As an example, some members of the CO2 + alkane family are presented in Fig. 5. The figure demonstrates that the system octane -f CO2 exhibits liquid-liquid immiscibility and belongs to type (3 in Fig. 4a. For CO2 + hexadecane and CO2 + squalane, continuous transitions between Ig and 11 critical states on the critical line are found according to type / or 7 in Fig. 4b. As it has been shown elsewhere [29,30], the system CO2 + tridecane corresponds to a transition type between both (type IV of the classification of van Konynenburg and Scott [28]). 

In Fig. 7, the critical phase behavior of binary aqueous solutions of several selected hydrocarbons and additionally fluorobenzene is shown, most of them having been measured in our laboratory [3]. The dashed curve is the vapor pressure curve of pure water, and the solid lines are parts of the branches of the binary critical p T) curves that start from the critical endpoint llg (systems 9 and 10) or from the critical point of pure water CP(H20). Whereas naphthalene H- water (system 9) and biphenyl -f water (system 10) show class-II behavior, all other systems belong to class III according to the classification of van Konynenburg and Scott, and thus exhibit gas-gas equilibria of the second kind. The consequence is that naphthalene and biphenyl are completely miscible with water already at quite low pressures near the vapor pressure curve of pure water. This behavior is of interest for measurements in mixed solvents and for separations. 

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. 

In this section the first five types of fluid phase behavior according to the classification of Van Konynenburg and Scott [5,6] will be introduced. This classification for binary systems consists of six types of fluid phase behavior, of which originally the first five could be derived from the van der Waals equation of state [7]. Section 2.1 contains a description of the types I to V of fluid phase behavior. In addition, some possible transitions between the types II, III and IV are presented. In Section 2.2 the occurrence of type-I and -V fluid ph e behavior is discuss briefly. 

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.

All types of fluid phase behavior presented can be distinguished by the number and nature of the various CEP s occurring. These CEP s are used to identify which type of fluid phase behavior a certain mixture belongs to. In this work, the binary classification of Van Konynenburg and Scott [5,6] is applied to ternary mixtures of constant ratio of the two solutes, considering the CEP s occurring for these mixtures. 

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... 

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... 

In conclusion, both argumentations point into the same direction. Theoretically, i.e., according to fluid phase theory, the types II and IV of fluid phase behavior seem to be more likely than types I and V for molecules with simple interaction forces. As has become apparent in this chapter, type-I and type-V fluid phase behavior are very much related to the types II and IV, respectively. Because of solidification it becomes easy to erroneously confuse a type-II with a type-IV system and a type-I with a type-V system, respectively. Many examples for this are given above. In addition, already Van Konynenburg and Scott [6] pointed out that of their example systems given for the types I or V some might, after further (theoretical) examination, have to be considered as to show type-II or -IV fluid phase behavior instead. 

These diagrams can take various forms depending on the properties of the solute and solvent molecules. A very thorough study of the possibilities has been made by van Konynenburg and Scott [15] who identified six principal types of fluid/fluid equilibrium behaviour in binary systems. For a complete discussion, reference should be made to the above study and also to the works of Rowlinson [16] and other authors [17-19]. In the discussion below a simplified classification is used to describe typical forms of fluid/fluid phase behaviour in the near-critical region. (Some readers may find it helpful to read... 

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