Equilibrium pressure-composition diagram

Tie-lines giving the compositions of the liquid and gas in equilibrium are again horizontal. The bubble-point line gives the composition of the equilibrium liquid, and the dew-point line gives the composition of the equilibrium gas. The lengths of the tie-lines represent the quantities of gas and liquid at equilibrium in the same manner as for the pressure-composition diagram. 

Replot two isotherms of the data given in Figure 2-37 as pressure against composition in weight percent. Use temperatures of 75°F and 300°F. This is called a pressure-composition diagram. Label the bubble-point lines and dew-point lines. 2-23. Determine the compositions and quantities of gas and liquid when 10 lb moles of a mixture of 55 mole percent methane, 20 mole percent propane and 25 mole percent n-pentane is brought to equilibrium at 160°F and 1500 psia. 

Fig. 9-14. Pressure-composition diagram for liquid-solid equilibrium.

Figure 6A is a general representation of the pressure-composition diagrams for systems that display liquid -liquid phase separation. The binodal boundary represents the equilibrium demixing pressures for a monodisperse polymer system. Below the binodal there is another boundary known as the spinodal boundary. The binodal and the spinodal envelopes determine the metastable region (shaded area in between). They... 

Figure 8. Pressure-equilibrium phase composition diagram for isobutane-carbon dioxide system, calculations using Peng-Robin-son equation of state...

A common representation of the phase equilibrium is the phase envelope showing the dependence of the saturation pressure on temperature for a mixture of a given composition [6]. Such a representation may be applied to analysis of the capillary equihbrium in porous media, in order to evaluate deviations in the values of the dew-point pressures under the action of the capillary forces [40]. However, because temperature may be treated as an external parameter and is not modified by capillary forces, the pressure-composition diagram may be more informative for this case. Additionally, numerical calculations show that the compositional deviations under capillary equilibrium may be more significant than the deviations in the dewpoint pressures. A qualitative analysis of the P-z diagram for capillary equilibrium in binary mixtures was first carried out in Ref 22. We follow this analysis with some important modifications. 

The pressure-temperature-composition diagram presented by Morey is shown in Fig. 8. The vapor pressure of pure water (on the P-T projection) terminates at the critical point (647 K, 220 bar). The continuous curve represents saturated solutions of NaCl in water, i.e., there is a three-phase equilibrium of gas-solution-solid NaCl. The gas-phase pressure maximizes over 400 bar at around 950 K. Olander and Liander s data for a 25 wt. % NaCl solution are shown, and T-X and P X projections given. At the pressure maximum, the solution phase contains almost 80% NaCl. 

Shown is the free energy of mixing versus composition diagram of the A-B binary system at the temperature 7 and the pressure P. The diagram shows two terminal phases, a and P, and one intermediate phase y. If the overall composition of the system is given by the point X shown in the diagram, find the stable equilibrium phase(s) at 7 and P. 

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.

What determines whether nonstoichiometric behavior can readily be observed is largely the form of the dissociation pressure-composition isotherms in the equilibrium diagram. If the free energy rises too steeply with excess of either component, it may not be practicable to detect stoichiometric changes which, in principle, are taking place. The explicit relation between the stoichiometric defect (= x for the composition MOx+a.) and the free energy is complicated and awkward to work... 

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