Flash calculation three-phase

Table 3 shows results obtained from a five-component, isothermal flash calculation. In this system there are two condensable components (acetone and benzene) and three noncondensable components (hydrogen, carbon monoxide, and methane). Henry s constants for each of the noncondensables were obtained from Equations (18-22) the simplifying assumption for dilute solutions [Equation (17)] was also used for each of the noncondensables. Activity coefficients for both condensable components were calculated with the UNIQUAC equation. For that calculation, all liquid-phase composition variables are on a solute-free basis the only required binary parameters are those for the acetone-benzene system. While no experimental data are available for comparison, the calculated results are probably reliable because all simplifying assumptions are reasonable the... 

Figure 1 shows water content of lean, sweet natural gas. It can be used also for gases that have as much as 10% CO2 and/or HiS if the pressure is below 500psia. Above 500psia, acid gases must be accounted for by rigorous three-phase flash calculations or approximation methods. ... 

Three Phase Flash Calculation under High Pressure Using Continuous Thermodynamics Method+... 

Table 1 Three Phase Flash-Calculation Results for a Semicontinuous Mixture at 49 C and 164.8bar...

There are three basic phase equilibrium calculations (1) a flash calculation - phase split at specified conditions, (2) bubble point calculation, and (3) dew point calculation. For bubble and dew points, there are two types of calculations. First, the temperature is specified and the pressure is calculated. The alternative occurs when the pressure is specified and the temperature is calculated. 

Reliable and fast equilibrium calculations (or so-called flash calculations) are the mechanism by which thermodynamic properties are used in industry. This area has received much attention in the past. Algorithms include successive substitution with acceleration and stability analysis,Inside-Out and Interval methods, Homotopy continuation methods with application to three-phase systems, and systems with simultaneous physical and chemical equilibrium. An area of recent focus is the flash algorithm for mixtures containing polydisperse polymers. However, many challenging problems remain. 

Subroutine FLASH calculates the composition of the three phases given the distribution coefficients. Two equations must be solved for A and B. 

The predictions of three-phase equilibria considered so far were done as two separate two-phase calculations. Although applicable to the examples here, such a procedure cannot easily be followed in a three-phase flash calculation in which the temperature or pressure of a mixture of two or more components is changed so that three phases are formed. In this case the equilibrium relations and mass balance equations for all three phases must be solved simultaneously to find the compositions of the three coexisting phases. It is left to you (Problem 11.3-7) to develop the algorithm for such a calculation. 

These equations are restricted to two equilibrium phases. For a treatment of three-phase flash calculations, see E. J. Henley and E. M. Rosen, Material and Energy Balance Computations, J. Wiley Sons, Inc., 1968, Chapter 8. 

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,... 

With the reasonable assumption that the phases in a heterogeneous mixture are in phase (physical) equilibrium for a given reactor effluent composition at the temperature and pressure to which the effluent is brought, process simulators can readily estimate the amounts and compositions of the phases in equilibrium by an isothermal (two-phase)-flash calculation, provided that solids are not present. When the possibility of two liquid phases exists, it is necessary to employ a three-phase flash model, rather than the usual two-phase flash model. The three-phase model considers the possibility that a vapor phase may also be present, together with two liquid phases. 

In this case, the vapor is 94 mol% H2, for which a vapor separation section may not be needed. The organic-rich liquid phase (LI) is sent to a liquid separation section to recover a combined methanol and toluene stream for recycle to the reactor, ethylbenzene as a byproduct, and styrene as the main product. The water-rich liquid phase (L2) is sent to another liquid separation section to recover methanol for recycle to the reactor and water, which is sent to wastewater treatment to remove small quantities of soluble organic components. It is important to note that a two-phase flash calculation would produce erroneous results. If in doubt, perform a three-phase flash calculation, rather than a two-phase flash calculation. 

Chien, H. H.-Y., Formulations for Three-Phase Flash Calculations, AIChE Journal, 40 (6), 957 (1994). 

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. 

In Step 4, the ITj-values from steps 2 and 3 provide the initial estimates of the three-phase flash. From the three-phase flash, the compositions and the amounts of the three phase are calculated 0.093768,... 

Vapor-Liquid-Liquid Equilibrium. We have had limited experi-lence in rigorous three phase equilibrium calculations, vapor-liquid-liquid, primarily in single stage flash units. The implementation of such a three-phase equilibrium model in column calculation is scheduled in the future. Presently, a method also exists wherein complete immisclhility in the liquid phase can be specified between one component and all of the other components in the system e.g., between water and a set of hydrocarbons. The VLE ratios are normalized on an overall liquid basis so that the results can be used in conventional two-phase liquid-vapor equilibrium calculations. 

The different characterizations of the heavy ends, which make up less than 1% of the entire mixture, were considered. In Case 1 normal butane was selected to represent the heavy ends, in Case 2 normal pentane, and in Case 3 normal hexane. The phase envelopes for the three cases were predicted with the BWR—11 equation of state. The results are given in Figure 14. The bubble-point curves almost coincide, and the true critical points are not very sensitive to composition. However, as shown, the dew-point is greatly affected by the characterization of the heavy ends composition, and there is about a 70 F difference in the maximum dew-points of the three mixtures. Therefore, when working with a natural gas type system, if the breakdown of the C+ fraction into additional compounds is available, it should be used, especially in flash calculations, to get an accurate representation of the phase behavior. 

The programs used to make the above prediction have been successfully used for three phase flash, bubble point, and hydrocarbon and/or water dew point calculations. Generally the calculations converge rapidly and normally require only about 20 iterations. The programs predict hydrocarbon concentrations in the aqueous liquid phase which are several orders of magnitude lower than the reported experimental data. In order to obtain a good prediction of these very dilute concentrations a different and possibly temperature dependent may be required. 

Calculations for all three cases have been performed for the system described in Tables VII and VIII and Figure 6. In this case the raw feed gas was flashed at 66°C and 138 bars with sufficient water to assure that the gas leaving the separator was water saturated. Each of the calculational philosophies described above was used to predict the phase behavior of the systems at each pressure temperature point in the pipeline. The results of these calculations are summarized in Tables IX through XI and Figures 7 through 10. 

Besides providing feasible computer routes to handle the three aforementioned problems, special care must be taken to prevent the computer flash algorithm from oscillating back and forth in K constant (vapor-liquid equilibrium) calculations because the prescribed flash conditions of temperature and pressure do not fall in the two-phase region. It would be presumptuous to assume that all flash conditions are realistic engineers do, and will continue to, submit unrealistic temperature and pressures, in the range of subcooled liquids and superheated vapors. 

Solution The governing equations are (6.3.65), (6.3.55) and (6.3.54). First, one must make sure that the problem specifications are such that the flash drum conditions are in the two-phase region of vapor-liquid equilibrium. We calculate the value of /(lk, /Tiy) from equation (6.3.65) for two values of (lkft,/W((), namely 0 and 1. Consider (Tk, /1%) = 0 first. From Figure 4.1.6, we determine K, for the three species (see Table 6.3.2). 

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