Phase equilibria lower critical solution temperature

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... 

Phase equilibrium resulting in a UCST is the most common type of binary (liquid + liquid) equilibrium, but other types are also observed. For example, Figure 14.5 shows the (liquid + liquid) phase diagram for (xiH20 + jc2(C3H7)2NH. 7 A lower critical solution temperature (LCST) occurs in this system/ That is, at temperatures below the LCST, the liquids are totally miscible, but with heating, the mixture separates into two phases. 

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],...

For the hydrocarbon--CO2 systems studied here, at pressures above the critical pressure (7.383 MPa) and above the critical temperature (304.21 K) of C02 the isobaric x,T coexistence plots of liquid and vapor phases form simple closed loops. The minimum occurs at the lower consolute point or the Lower Critical Solution Temperature (LCST). Since pressure is usually uniform in the vicinity of a heat transfer surface, such diagrams serve to display the equilibrium states possible in a heat transfer experiment. 

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. ... 

Liquid-liquid Equilibrium Lines of the Amin -Water Systems Saturated with NaCl. The liquid-liquid equilibrium lines of the binary DiPA-H20 and DMiPA-HiO systems are reported in the literature (2) (Figure 5). The lower critical solution temperatures (LCST) of the DiPA-HiO and the DMiPA-HiO systems were estimated to be 27 and 65 respectively. Below these temperatures there is one single liquid phase present regardless of the concentration of the amine. 

Certain principles mnst be obeyed for experiments where liquid-liquid equilibrium is observed in polymer-solvent (or snpercritical flnid) systems. To understand the results of LLE experiments in polymer solutions, one has to take into acconnt the strong influence of polymer distribution functions on LLE, because fractionation occnrs dnring demixing. Fractionation takes place with respect to molar mass distribution as well as to chemical distribution if copolymers are involved. Fractionation during dentixing 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 mnlticomponent system. Clond-point cnrves are measnred instead of binodals and per each individnal feed concentration of the mixtnre, two parts of a coexistence cnrve occnr below (for upper critical solution temperatnre, UCST, behavior) or above the clond-point cnrve (for lower critical solution temperature, LCST, behavior), i.e., produce an infinite nnmber of coexistence data. 

Fig. 1.4.1(a) Binary phase diagram of a binary amorphous polymer/solvent system undergoing phase separation above the lower critical solution temperature (LCST). 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... 

FIGURE 3.15 Liquid-liquid equilibrium in the system HDPE-butyl acetate. The system displays both upper and lower critical solution temperature behaviors. The experimental data for molecular weights 13,600 and 64,000. Lines are simplified PC-SAFT correlations with kjj = 0.0156 for both molecular weights. (From Fluid Phase Equilib., 222-223, von Solms, N., Kouskoumvekaki, I.A.. Lindvig, T., Michelsen, M.L., and Kontogeorgis, G.M., A novel approach to liquid-liquid equilibrium in polymer systems with application to simplified PC-SAFT, 87-93, Copyright 2004, with permission from Elsevier.)... 

Madbouly Wolf, 2002, Equilibrium phase behavior of polyethylene oxide and of its mixtures with tetrahydronaphthalene or/and p ly (ethylene oxide-block-dimethylsiloxane), /, Chem. Phys., Vol. 117, No. 15, PP. 7357-7363 Maderek et al. 1983, High-temperature demixing of poly(decyl methacrylate) solutions in isooctane and its pressure-dependence, Makromol. Chem., Vol. 184, No. 6, PP. 1303-1309 Lower critical solution temperatures of poly(decyl methacrylate) in hydrocarbons, Eur. Polym.., Vol. 19, No. 10, PP. 963-965 Patterson Robard, 1978, Thermodynamics of polymer compatibility. Macromolecules, Vol. 11, No. 4, 690-695... 

VSH Vshivkov, S.A., Tager, A.A., Lantseva, N.V., and Loginova, L., Phase equilibrium and stracture of solutions of oligomeric poly(oxypropylene)diols with upper and lower critical solution temperatures (Russ.), Protsessy Studneobras. Polimern. Sistem., (2), 3, 1977. 

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. 

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 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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