Example of Multiphase Flash and Stability Analysis

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. 

Perform the, stability analysis on the overall mixture to examine the stability of the single-phase mixture. In case the original mixture is unstable, estimates of equilibrium ratios (if -values) are then on hand for the two-phase flash. 

Perform the two-phase flash using the if j-values from the stability test as the initial guess. 

Perform stability analysis on only one of the phases of the two-phase flash. If the selected phase is unstable, estimates of the equilibrium ratios for part of the three-phase flash are available. 

Perform the three-phase flash using the estimates of equilibrium ratios from steps 2 and 3. 

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