Phase equilibria upper critical solution temperature

For example, 0 describes the temperature dependence of composition near the upper critical solution temperature for binary (liquid + liquid) equilibrium, of the susceptibility in some magnetic phase transitions, and of the order parameter in (order + disorder) phase transitions. 

Using the estimated interaction parameters phase equilibrium computations were performed. It was found that the EoS is able to represent the VL2E behavior of the methane-n-hexane system in the temperature range of 198.05 to 444.25 K reasonably well. Typical results together with the experimental data at 273.16 and 444.25 K are shown in Figures 14.14 and 14.15 respectively. However, the EoS was found to be unable to correlate the entire phase behavior in the temperature range of 195.91 K (Upper Critical Solution Temperature) and 182.46K (Lower Critical Solution Temperature). 

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... 

Figure 14.10 The five types of (fluid + fluid) phase diagrams according to the Scott and van Konynenburg classification. The circles represent the critical points of pure components, while the triangles represent an upper critical solution temperature (u) or a lower critical solution temperature (1). The solid lines represent the (vapor + liquid) equilibrium lines for the pure substances. The dashed lines represent different types of critical loci. (l) [Ar + CH4], (2) [C02 + N20], (3) [C3H8 + H2S],...

Figure 13.15 is drawn for a single constant pressure equilibrium phase compositions, and hence the locations of the lines, change with pressure, but the general nature of the diagram is the same over a range of pressures. For the majority of systems the species become more soluble in one another as the temperature increases, as indicated by lines CG and DH of Fig. 13.15. If this diagram is drawn for successively higher pressures, the corresponding three-phase equilibrium temperatures increase, and lines CG and DH extend further and further until they meet at the liquid/liquid critical point Af, as shown by Fig. 13.16. The temperature at which this occurs is known as the upper critical solution temperature, and at this temperature the two liquid phases become identical and merge into a single phase.

Generally, liquid-liquid phase equilibrium (or phase separation) occurs only over certain temperature ranges, bounded above by the upper consolute or upper critical solution temperature, and bounded below by the lower consolute or lower critical solution temperature. These critical solution temperatures are indicated on the liquid-liquid phase diagrams given here. All partially miscible mixtures should exhibit either one or both consolute temperatures however, the lower consolute temperature may be obscured by the freezing of the mixture, and the upper consolute temperature will not be observed if it is above the bubble point temperature of the mixture, as vaporization will have instead occurred. ... 

In a polynary system at two-phase equilibrium, there is a curve of coexistence of phases on the 7 vs ip diagram for each polymer concentration, and it shows a break at high temperatures (if the system has the upper critical. solution temperature UCS I ) and at all the concentrations except one ip = fic-... 

Fig. 1.4.1(b) Binary phase diagram of a binary amorphous polymer/solvent system undergoing phase separation below the upper critical solution temperature (UCST). The temperature of interest, T, intersects with the binodal curve at the composition (wt fraction) of a and p phases at equilibrium, c" and c. On the other hand, T intersects with the spinodal at compositions (wt fraction) c and c . Outside the binodal curve is the single-phase region, while inside it is the two-phase region... 

Liquid—Liquid Phase Separation, in contrast to solid-liquid phase separation, lowering temperature can induce liquid-liquid phase separation of a polymer solution with an upper critical solution temperature and when the crystallization temperature of the solvent is sufficiently lower than the phase separation temperature. In an equilibrium phase diagram of a polymer solution, the spin-odal curve divides the liquid-liquid phase separation region into two regions a thermodynamically metastable region (between the binodal and spinodal) and a thermodynamically unstable region (enclosed by the spinodal) (Fig. 11). Above the... 

Pig. 11. Schematic equilibrium phase diagram for a polymer solution with an upper critical solution temperature. 

Certain principles must be obeyed for experiments where liquid-hquid equilibrium is observed in polymer-solvent (or supercritical fluid) systems. To understand the results of LLE experiments in polymer solutions, one has to take into account the strong influence of polymer distribution functions on LLE, because fractionation occurs during demixing. Fractionation takes place with respect to molar mass distribution as well as to chemical distribution if copolymers are involved. Fractionation during demixing leads to some effects by which the LLE phase behavior differs from that of an ordinary, strictly binary mixture, because a common polymer solution is a multicomponent system. Cloud-point curves are meastrred instead of binodals and per each individual feed concentration of the mixture, two parts of a coexistence curve occur below (for upper critical solution temperature, UCST, behavior) or above the cloud-point curve (for lower critical solution temperature, LCST, behavior), i.e., produce an infrnite number of coexistence data. 

Figure 7.1 shows schematically a phase diagram for a typical polymer-solvent system, plotting temperature vs. the fraction polymer in the system. At low temperatures, a two-phase system is formed. The dotted tie lines connect the compositions of phases in equilibrium, a solvent-rich (dilute-solution) phase on the left and a polymer-rich (swollen-polymer or gel) phase on the right- As the temperature is raised, the compositions of the phases become more nearly alike, until at the upper critical solution temperature (UCST) they are identical Above the UCST, the system forms homogeneous (single-phase) solutions across the entire composition range. The location of the phase botmdary depends on the... 

For the two-component, two-phase liquid system, the question arises as to how much of each of the pure liquid components dissolves in the other at equilibrium. Indeed, some pairs of liquids are so soluble in each other that they become completely miscible with each other when mixed at any proportions. Such pairs, for example, are water and 1-propanol or benzene and carbon tetrachloride. Other pairs of liquids are practically insoluble in each other, as, for example, water and carbon tetrachloride. Finally, there are pairs of liquids that are completely miscible at certain temperatures, but not at others. For example, water and triethylamine are miscible below 18°C, but not above. Such pairs of liquids are said to have a critical solution temperature, For some pairs of liquids, there is a lower (LOST), as in the water-tiiethylamine pair, but the more common behavior is for pairs of liquids to have an upper (UCST), (Fig. 2.2) and some may even have a closed mutual solubility loop [3]. Such instances are rare in solvent extraction practice, but have been exploited in some systems, where separations have been affected by changes in the temperature. 

CRITICAL COMPOSITION. Systems consisting of two liquid layers that are formed by the equilibrium between two partly miscible liquids, frequently have a constitute temperature or a critical solution temperature, beyund which Ihe two liquids are miscible in all proportions. At this temperature, ihe phase boundary disappears, and the two liquid layers merge into one. The composition of the mixture al that point is called the critical composition. There is, in some cases, a lower consolutc temperature as well as an upper consolule temperature. 

The physical picture that underlies this behavior, as pointed out first by Elgin and Weinstock (1), is the salting out effect by a supercritical fluid on an aqueous solution of an organic compound. As pressure is increased, the tendency of the supercritical fluid to solubilize in the organic liquid results in a phase split in the aqueous phase at a lower critical solution pressure (which varies with temperature). As pressure is further increased, the second liquid phase and the supercritical phase become more and more similar to each other and merge at an upper critical solution pressure. Above this pressure only two phases can coexist at equilibrium. This pattern of behavior was also observed by Elgin and Weinstock for the system ethylene - acetone - water at 288 K. In addition, the same type of... 

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