Mixtures quadruple point

Applying the phase rule, it is found that, in the two component system NaP03 + H20, a maximum of four phases is possible at the quadruple points 199,302). In addition to, at the most, two crystalline substances and water vapor, an amorphous glass-like phase is always present. This phase consists of mixtures of polyphosphates, the chain length of which rises with increasing temperature. Only Na2H2P207, Maddrell s salt (h) and trimetaphosphate occur as stable solid phases in addition to NaH2P04. 

The two calculation methods in Section 4.2 enable prediction of the three-phase (Lw-H-V) gas mixture region extending between the two quadruple points Qi and Q2 in Figure 4.1. Section 4.3 provides a method to use the techniques of Section 4.2 to locate both quadruple points on a pressure-temperature plot. Section 4.3 also discusses equilibrium of three condensed phases [aqueous liquid-hydrate-hydrocarbon liquid (Lw-H-Lhc)] Determination of equilibrium from condensed phases provides an answer to the question, Given a liquid... 

In the last common condition, for four phases in equilibrium (such as I-Lw-H-V) at the lower quadruple point, the number of intensive variables must equal the number of components minus two. For a mixture of methane and water (or for a gas mixture in large excess so that the composition does not change) no intensive variables are required—that is, the lower quadruple point is fixed at a unique pressure, temperature, as well as the composition of all the phases. 

In Figure 4.2c for natural gases without a liquid hydrocarbon (or when liquid hydrocarbons exist below 273 K), the lower portion of the pressure-temperature phase diagram is very similar to that shown in Figure 4.2a. Two changes are (1) the Lw-H-V line would be for a fixed composition mixture of hydrocarbons rather than for pure methane (predictions methods for mixtures are given in Section 4.2 and in Chapter 5) and (2) quadruple point Qi would be at the intersection of the Lw-H-V line and 273 K, at a pressure lower than that for methane. The other three-phase lines of Figure 4.2a (for I-Lw-H and I-H-V) have almost the same slope at Qj. Otherwise, the same points in Section 4.1.1 apply. 

When a liquid hydrocarbon mixture is present, the Lw-V-Lhc line in Figure 4.2b broadens to become an area, such as that labeled CFK in Figure 4.2c. This area is caused by the fact that a single hydrocarbon is no longer present, so a combination of hydrocarbon (and water) vapor pressures creates a broader phase equilibrium envelope. Consequently, the upper quadruple point (Q2) evolves into a line (KC) for the multicomponent hydrocarbon system. 

Both the gas gravity method and the Kysi-value method enable the estimation of three-phase (Lw-H-V) equilibrium between quadruple points Qi and Q2 for mixtures as well as for simple natural gas hydrate formers such as those in Table 4.2. 

Chapter 14 describes the phase behavior of binary mixtures. It begins with a discussion of (vapor -l- liquid) phase equilibria, followed by a description of (liquid + liquid) phase equilibria. (Fluid + fluid) phase equilibria extends this description into the supercritical region, where the five fundamental types of (fluid + fluid) phase diagrams are described. Examples of (solid + liquid) phase diagrams are presented that demonstrate the wide variety of systems that are observed. Of interest is the combination of (liquid + liquid) and (solid 4- liquid) equilibria into a single phase diagram, where a quadruple point is described. 

Concentration-Temperature Diagram.—In this diagram the temperatures are taken as the abscissae, and the composition of the solution, expressed in atoms of chlorine to one atom of iodine, is represented by the ordinates. In the diagram, A represents the melting-point of pure iodine, 114°. If chlorine is added to the system, a solution of chlorine in liquid iodine is obtained, and the temperature at which solid iodine is in equilibrium with the liquid solution will be all the lower the greater the concentration of the chlorine. We therefore obtain the curve ABF, which represents the composition of the solution with which solid iodine is in equilibrium at different temperatures. This curve can be followed down to 0°, but at temperatures below 7 9 (B) it represents metastable equilibria. At B iodine monochloride can be formed, and if present the system becomes invariant B is therefore a quadruple point at which the four phases, iodine, iodine monochloride, solution, and vapour, can co-exist. Continued withdrawal of heat at this point will therefore lead to the complete solidification of the solution to a mixture or conglomerate of iodine and iodine monochloride, while the temperature remains constant during the process. B is the eutectic point for iodine and iodine monochloride. 

The following symbols are used T = temperature p = (total) pressure X = mole fraction (of component II if not indicated) c = concentration M = moldm" L = liquid phase, G = gaseous phase, S = solid phase CP I, CP II = critical point of the pme component I or II CP = critical point of a mixture UCST (LCST = upper (lower) critical solution temperature A, B, C, D, E, K = critical end point Qx, Qa = quadruple point. 

Effects of presence of hydrate. Chlorine hydrate forms whenever operating conditions are suitable. The quadruple point referred to in Section 9.1.3.5C is that for hydrate, gas, and two liquid phases. At low temperature the water phase also freezes, and the four possible phases are gas, wet liquid chlorine, ice, and hydrate. This mixture has a quadruple point at — 3 C and 244torr. 

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. 

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