Phase transition quadruple point

Each quadruple point occurs at the intersection of four three-phase lines (Figure 1.2). The lower quadruple point is marked by the transition of Lw to I, so that with decreasing temperature, Qi denotes where hydrate formation ceases from vapor and liquid water, and where hydrate formation occurs from vapor and ice. Early researchers took Q2 (approximately the point of intersection of line Lw-H-V with the vapor pressure of the hydrate guest) to represent an upper temperature limit for hydrate formation from that component. Since the vapor pressure at the critical temperature can be too low to allow such an intersection, some natural gas components such as methane and nitrogen have no upper quadruple point, Q2, and... 

A comprehension of Figure4.3 has value because a similar phase diagram could be drawn for a natural gas of fixed composition between the quadruple points (Qi and QaJ. The same phase transitions and boundaries would qualitatively occur, with the artificial constraint that all hydrocarbon phases be of the same composition as the original gas. A second useful outcome of binary phase diagrams like Figure 4.3 is the use of the lever rule (Koretsky, 2004, p. 367) at constant temperature to determine relative phase amounts note that the lever rule can be applied for quantitatively correct phase diagrams. 

Figure 5.17 shows a predicted pressure versus excess water composition plot for the ethane+propane+water system at 274 K. At 0.0 mol fraction ethane (propane+ water) sll form at approximately 2 bar, and at 1.0 mol fraction ethane (ethane + water) si form at approximately 5 bar. At the intermediate composition of 0.78 mole fraction ethane, a quadruple point (Aq-sI-sII-V) exists in which both incipient hydrate structures are in equilibrium with vapor and aqueous phase. This point will be referred to as the structural transition composition the composition at which the incipient hydrate formation structure changes from sll to si at a given temperature. 

On either side of a quadruple point the solution is supersaturated with respect to one of the solid phases, which would therefore change in contact with the solution into the hydrate of lower solubility. The quadruple point is therefore also called the transition point of the two hydrates. In Fig. 22 the solubility curves of the various hydrates of calcium chloride (a, 6, and c) are plotted for temperatures up to 260°. The formulae of the solid phases are written opposite the corresponding curves. 

Formal phase transition curves for different mechanisms of molecular interaction are shown in Fig. 2a. Since these mechanisms are independent, the curves after intersection continue to keep their course and divide the phase plane into four sections. The point D turns out to be not triple but quadruple. The area of existence of second intermediate state, second liquid L2, together with that of liquid, Li, appears on the phase plane. 

Simple thermodynamic reasoning implies that the quadruple point D cannot actually exist it will fall apart into two triple points Ds and Dg, connected by a line of the second-order phase transition Li L2 (Fig. 2b). 

---