Water critical lines

Figure 1. Critical lines in water-gas shift binary pairs...

The study described above for the water-gas shift reaction employed computational methods that could be used for other synthesis gas operations. The critical point calculation procedure of Heidemann and Khalil (14) proved to be adaptable to the mixtures involved. In the case of one reaction, it was possible to find conditions under which a critical mixture was at chemical reaction equilibrium by using a one dimensional Newton-Raphson procedures along the critical line defined by varying reaction extents. In the case of more than one independent chemical reaction, a Newton-Raphson procedure in the several reaction extents would be a candidate as an approach to satisfying the several equilibrium constant equations, (25). 

For larger differences between the critical temperatures, one expects liquid-liquid miscibility gaps that are well-separated from the liquid-gas transition (type II). When further increasing the dissimilarity, these liquid-liquid immiscibilities are displaced to higher temperatures, and eventually the corresponding liquid-liquid-gas (I -L -G) three-phase line interferes with the L-G critical line (type III, IV, and V). Then, the L-G critical line starting from the critical point of the more volatile component (here water) is broken at a so-called upper critical end point (UCEP), where it meets the Li-L2-g three-phase line of the liquid-liquid equilibrium. 

For electrolyte solutions such as NaCl + water the critical temperatures of the pure components differ by about a factor of five. From the perspective of nonelectrolyte thermodynamics, the absence of a liquid-liquid immiscibility then comes as a great surprise. It is a major challenge for theory to explain why this salt, as well as similar salts such as KC1 or CaCl2, seems to show a continuous critical line. Perhaps there is a slight indication for a transition toward an interrupted critical curve in Marshall s study [151] of the critical line of NaCl + H20. Marshall observed a dip in the TC(XS) curve some K away from the critical point of pure water, which at first glance seems obscure. It was suggested [152] that the vicinity to an upper critical end point leaves its mark by this dip. 

The continuous critical line for systems such as NaCl + H20 offers a temperature window for studying the behavior of electrolyte solutions near their liquid-vapor transition. Pitzer [4,13,142,144] compiled much evidence that the nonclassical fluctuations in pure water are apparently suppressed when adding electrolytes. Thus, from the application s point of view, a classical EOS may be quite useful. The pressing question is to what degree these observations withstand more quantitative analysis. 

A key role in this debate was played by experiments by Bischoff and Rosenbauer [153], who reported accurate data on isothermal vapor-liquid coexistence curves as a function of pressure near the critical line of NaCl + H20. Far from the critical point of pure water, one expects the compositions... 

Mixtures of equisized charged spheres were also treated by the MSA. Such a system is then uniquely characterized by the ratio of the critical temperatures of the pure components. Harvey [235] found that a continuous critical curve from the dipolar solvent to the molten salt is maintained until the critical temperature of the ionic component exceeds that of the dipolar component by a factor of about 3.6. This ratio is much higher than theoretically predicted for nonionic model fluids. We recall that for NaCl the critical line is still continuous at a critical temperature ratio of about 5. Thus, the MSA of the charged-hard-sphere-dipolar-hard-sphere system captures, at least in part, some unusual features of real salt-water systems with regard to their critical curves. 

The present paper gives an overview of results on high-pressure phase equilibria in the ternary system carbon dioxide-water-1-propanol, which has been investigated at temperatures between 288 and 333 K and pressures up to 16 MPa. Furthermore, pressure-temperature data on critical lines, which bound the region where multiphase equilibria are oberserved were taken. This study continues the series of previous investigations on ternary systems with the polar solvents acetone [2], isopropanol [3] and propionic add [4], A classification of the different types of phase behaviour and thermodynamic methods to model the complex phase behaviour with cubic equations of state are discussed. 

The phase behaviour of systems with low molecular alcohols methanol and ethanol as well as of systems with acetone and propionic acid is relatively simple (pattern I). At lower pressures the single three-phase region is bound by a critical line (L3=L2)Vy at higher pressure the three-phase region is limited by either an upper critical line Lj(L2=V) or the binary three-phase line of the system carbon dioxide-water depending on temperature. 

Figure 5. Pressure-temperature diagram for carbon dioxide-water-1-propanol — critical lines calculated with Peng-Robinson EOS using the mixing rule of Panagiotopoulos-Reid, parameters fitted to ternary three-phase equilibria at temperatures between 303 and 333 K...

FIGURE 5.1. Liquid/vapour coexistence curves of SPC water (solid line) and OPLS methanol (dashed line) following hrom the RISM-KH theory. Molecular simulation results for water [45] and methanol [46] (open circles and squares, respectively) and critical point extrapolations (filled symbols). 

Figure 4. Boundary of limiting superheats of acetone and water solutions in acetone 1 - acetone, 2 - acetone + 10 % water, 3 - acetone + 30 % water. Solid line - line of liquid-acetone vapor phase equilibrium, C - critical point, dashed line - calculation by homogeneous nucleation theory for J = 10 s m (acetone). ...

Figure 7. Spinodal of superheated liquid water solid line - by the empirical equation of state 1 - by Filrth equation , 2 -by Gimpan equation , - saturation line, C - critical point.

Fkj. 10-8. Critical mass and volume of spheres and circular cylinders containing solutions of and surrounded by water. Straight lines show mass of U ... 

Figure 7.2. Photoswitch of the solubility of chains, (a) Schematic drawing of the phototriggered coiiapse and aggregation of azobenzene-containing polymers in poor soivent conditions or ciose to iow critical solubility temperature (LCST). (b) Typical variation of the radius of the chains as a function of solvent parameter, or temperature in the case of chains having a LCST in water. Bold line parent chain with no azobenzene dashed and dot-dashed lines azo-modified chains, respectively, exposed to UV and dark-adapted.

Two main differences are found as one replaces hexanol with pentanol the first is the existence of a critical line and the second is the possibility of going continuously from the water -f surfactant vertex to the oil vertex when X is larger than 3. In the hexanol system. 

Type I mixtures have continuous gas-liquid critical line and exhibit eomplete miseibil-ity of the liquids at all temperatures. Mixtures of substances with eomparable eritieal properties or substances belonging to a homologous series form Type I unless the size difference between components is large. The critical locus could be convex upward with a maximum or concave down with a minimum. Examples of Type I mixtures are Water -l-1-propanol, methane -i- n-butane, benzene -I- toluene, and carbon dioxide -I- n-butane. 

Type II have systems have liquid-liquid immiscibility at lower temperatures while locus of liquid-liquid critical point (UCST) is distinct from gas liquid critical line. Examples include water -l- phenol, water -l- tetralin, water -l- decalin, carbon dioxide -l- n-oetane, and carbon dioxide -I- n-decane. 

Many aqueous binaries me Type-Ill. In Fig. 9, we show the high-pressure critical lines for several of these, such as air constituents and n-alkanes, emanating from the steam critical point in the P-T representation. There are also some Type-I systems indicated, such as water with NH3 or NaCl. The cross-hatches indicate the respective 2-phase region. 

Figure 9. Schematic plot of critical lines of some Type-III, and also of some Type-1 aqueous systems near the water critical point. The cross-hatches indicate on which side of the critical line the system exists in two phases. (Reprinted from 12] Fig. 11, copyright 1994, with kind permission from Kluwer Academic Publishers)...

In Fig. 11, we draw schematically the case of fluid-solid phase behavior for the Type-I fluid mixture water-NaCl. For critical temperatures this far apart, the three-phase line Sb-L-V from the low-temperature quadruple point (where four three-phase lines meet) to the solutes triple point develops a high maximum that reaches above water s critical pressure and temperature. If a salt solution is heated at a pressure above the critical pressure of water, the vapor-liquid critical line is crossed first, and a two-phase L-V region entered. At high enough temperature the three-phase line Sb-L-V may be crossed, and solid salt will form. Thus supercritical water, fully miscible with air constituents and hydrocarbons, becomes a poor solvent for salts. 

Fig. 53. Sodium cholate-water binary phase diagram. Expressed as wt %. Vertical axis, temperature degrees C horizontal axis, percentage of water. The line at about 5% water represents a monohydrate of sodium cholate. This monohydrate is in equilibrium with a liquid phase. The dotted line marked Te represents the solubility of sodium cholate in water. The solubility increases slightly with increasing temperature. The liquid phase in the dilute region is made up of small micelles. CMC, critical micellar concentration (this line is very approximate see Section VIII. B) (42).

In the next section, we review a number of lattice gas models for which the addition of directional interactions not only allows for polyamorphism and two liquid phases but also introduces the possibility of a richer phase diagram, in which a critical line following the liquid-liquid first-order phase transition substitutes the critical point. Even though not explored in the literature, this picture is not inconsistent with known experimental results for water and other tetrahedral liquids [38]. 

As for real water, it is not clear if the orientation imposed by hydrogen bonding is so relevant as to actually lead to a critical line instead of a critical point at the end of the hypothetic two liquid phase coexistence. However, this picture can not be excluded. The peaks in the specific heat observed in the confined water system could be an indication of criticality, indication that would only be confirmed if experiments in bulk water would be possible [38]. 

The question of how the solvent water would behave around and above its critical point was first addressed by the Dutch chemist Bakhuis Roozeboom and his school, who were experts at measuring and classifying the phase separation of binary and ternary mixtures, including sohd phases. By 1904, Bakhuis Roozeboom had explored the case of the liquid-vapor-solid curve intersecting the critical line of a binary mixture in two critical endpoints and predicted that this would also happen in aqueous solutions of poorly soluble salts, as his successors indeed confirmed in 1910. His experiments and classification scheme pertain to a multitude of both non-aqueous and aqueous binary and ternary systems. 

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