Flash calculation multiphase

The calculations described in the earlier section gave the incipient point only. At the incipient point, the amount of hydrate formed is infinitesimally small. At conditions that depart from the incipient pressure and temperature, it is possible to estimate the phase fractions (or amounts) of hydrates that have formed. The pioneering work in this field was that of Bishnoi et al., in which solid phases of structure I and II gas hydrates were included in multiphase flash calculations. The multiphase equilibrium calculations are based on a Gibbs free energy minimization methodology... 

Bishnoi, P.R. Gupta, A.K. Englezos, P. Kalogerakis, N. Multiphase equilibrium flash calculations for systems containing gas hydrates. Fluid Phase Equilibria 1989, 53, 97-104. 

The methods presented in previous sections can be combined to attack multiphase equilibrium problems. To illustrate, we combine the gamma-phi method wi the gamma-gamma method to solve three-phase, vapor-liquid-liquid problems. We again choose to pose these problems as analogies to isothermal flash calculations, as in 11.1.5. Then such problems are well-posed when we have specified values for T independent properties, where T is given by (9.1.12) with S = 0,... 

Coupled phase-reaction equilibrium problems not only raise no new thermodynamic issues, but they also raise few new computational issues. By building on the phase and reaction-equilibrium algorithms presented earlier in this chapter, we can devise an elementary algorithm. Reaction-equilibrium problems typically start with known values for T, P, and initial mole numbers N° in a phase-equilibrium context, these variables identify an T problem, such as an isothermal flash calculation. Therefore we can combine the Rachford-Rice method with the reaction-equilibrium calculation given in 11.2 an example is provided in Figure 11.8 for a vapor-liquid situation. This is a traditional way for attacking multiphase-multireaction problems [21, 22] ... 

Example of multiphase flash and stability analysis. We will, in detail, discuss the stability analysis of a three-component system of Ci/CO /nCif at T = 294.0K and P — 67 bar with — 0.05. 2 co.> = 0.90, and = 0.05. At fixed temperature and pressure, from the phase rule F — c - -2 — p, there can be a maximum of three phases when the interface between the phases is flat. The first question is what types of phases may exist—gas, liquid, or solid. As we will see in Chapter 5, a solid phase does not exist for the above system. Therefore one might expect (1) a single gas phase or a single liquid phase, (2) gas and liquid phases, (3) liquid and liquid phases, or (4) gas-liquid-liquid phase separation. The difficulty in liquid-liquid (L-L) and vapor-liquid-liquid (V-Lr-L) and higher-phase equilibria (for more than three components) is how many phases should be considered for flash calculations. One approach is to determine whether one, two, or more phases are to be considered without prior knowledge of the true number of phases. In certain cases, as we will see in the next chapter, it is possible from thermodynamic stability analysis to determine the true number of phases a priori without performing a flash. However, in general, we do not know the true number of phases. One may, therefore, follow a sequential approaches outlined next for the Ci/C02/nCiQ example. 

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