Flash calculation, vapor-liquid

Illustrates use of subroutine FLASH for vapor-liquid equilibrium separation calculations for up to 10 components and of subroutine PARIN for parameter loading. 

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. 

This problem is similar in some ways to a vapor-liquid flash and here is referred to as a liquid-liquid flash calculation. 

In modern separation design, a significant part of many phase-equilibrium calculations is the mathematical representation of pure-component and mixture enthalpies. Enthalpy estimates are important not only for determination of heat loads, but also for adiabatic flash and distillation computations. Further, mixture enthalpy data, when available, are useful for extending vapor-liquid equilibria to higher (or lower) temperatures, through the Gibbs-Helmholtz equation. ... 

The same fundamental development as presented here for vapor-liquid flash calculations can be applied to liquid-liquid equilibrium separations. In this case, the feed splits into an extract at rate E and a raffinate at rate R, which are in equilibrium with each other. The compositions of these phases are... 

It is important to stress that unnecessary thermodynamic function evaluations must be avoided in equilibrium separation calculations. Thus, for example, in an adiabatic vapor-liquid flash, no attempt should be made iteratively to correct compositions (and K s) at current estimates of T and a before proceeding with the Newton-Raphson iteration. Similarly, in liquid-liquid separations, iterations on phase compositions at the current estimate of phase ratio (a)r or at some estimate of the conjugate phase composition, are almost always counterproductive. Each thermodynamic function evaluation (set of K ) should be used to improve estimates of all variables in the system. 

Both vapor-liquid flash calculations are implemented by the FORTRAN IV subroutine FLASH, which is described and listed in Appendix F. This subroutine can accept vapor and liquid feed streams simultaneously. It provides for input of estimates of vaporization, vapor and liquid compositions, and, for the adiabatic calculation, temperature, but makes its own initial estimates as specified above in the absence (0 values) of the external estimates. No cases have been encountered in which convergence is not achieved from internal initial estimates. 

Examples of Vapor-Liquid Separation Calculations Conducted with Subroutine FLASH... 

Liquid-liquid equilibrium separation calculations are superficially similar to isothermal vapor-liquid flash calculations. They also use the objective function. Equation (7-13), in a step-limited Newton-Raphson iteration for a, which is here E/F. However, because of the very strong dependence of equilibrium ratios on phase compositions, a computation as described for isothermal flash processes can converge very slowly, especially near the plait point. (Sometimes 50 or more iterations are required. )... 

Examples of main programs calling subroutines FLASH and ELIPS for vapor-liquid and liquid-liquid separation calculations, respectively, are described in this Appendix. These are intended only to illustrate the use of the subroutines and to provide a means of quickly evaluating their performance on systems of interest. It is expected that most users will write their own main prograns utilizing FLASH and ELIPS, and the other subroutines presented in this monograph,to suit the requirements of their separation calculations. 

Some organic compounds can be in solution with water and the mixture may still be a flammable mixture. The vapors above these mixtures such as ethanol, methanol, or acetone can form flammable mixtures with air. Bodurtha [39] and Albaugh and Pratt [47] discuss the use of Raoult s law (activity coefficients) in evaluating the effects. Figures 7-52A and B illustrate the vapor-liquid data for ethyl alcohol and the flash point of various concentrations, the shaded area of flammability limits, and the UEL. Note that some of the plots are calculated and bear experimental data verification. 

Dewpoint calculations must be made when we know the composition of the vapor yj and P (or T) and want to find the liquid composition Xj and T (or P). Flash calculations must be made when we know neither Xj nor yj and must combine phase equilibrium relationships, component balance equations, and an energy balance to solve for all the unknowns. 

An equilibrium-flash calculation (using the same equations as in case A above) is made at each point in time to find the vapor and liquid flow rates and properties immediately after the pressure letdown valve (the variables with the primes F , F l, y], x j,.. . shown in Fig. 3.8). These two streams are then fed into the vapor and liquid phases. The equations describing the two phases will be similar to Eqs. (3.40) to (3.42) and (3.44) to (3.46) with the addition of (1) a multi-component vapor-liquid equilibrium equation to calculate Pi and (2) NC — 1 component continuity equations for each phase. Controller equations relating 1 to Fi and P to F complete the model. 

The predictions of the flash calculation show only the total amounts of vapor and liquid resulting. If a side vapor or side liquid flow has been specified from the stage it must be subtracted from the total phase, the remainder constituting the flow to the adjoining stage. This is done be-between statements 106 and 111. If it is found, for example, the predicted amount of liquid is less than the set amount of side liquid flow, all of the predicted liquid phase will be used as side flow and the flow of liquid to the stage below will be set at zero. The vapor is treated in the same way, but in either case the program notes that the problem has been altered. 

Subroutine isotfl calculates in similar fashion, except that the smaller phase is changed by a fraction of itself, the fraction being 10% at the start and diminishing by a factor of 10 each time the calculation crosses the correct solution. A flash calculation at a set temperature can result in all vapor or all liquid, if the temperature is above the dew point or below the bubble point of the feed this possibility is provided for by setting the small phase to zero if it falls below one millionth of the feed. 

The proper design of distillation and absorption columns depends on knowledge of vapor—liquid equilibrium, as do flash calculations used to determine the physical state of streams at given conditions of temperature, pressure, and composition. Detailed treatments of vapor—liquid equilibria are available (6,7). 

Since K-vslue is defined as the ratio of mol fraction of a component in the vapor to mol fraction of that component in the equilibrium liquid, when K-values arc applied in a conventional equilibrium flash calculation the condition which will give the greatest proportion of a component in the liquid phase is the condition where its K-value is lowest. 

Calculations of gas-liquid equilibria using either Equation 12-17 or Equation 12-18 often are called flash vaporization calculations. Usually the term is reduced to flash calculations. 

Adiabatic flash calculation Liquid and vapor enthalpies off charts in the API data book are fitted with linear equations... 

Flash Calculations. The ability to carry out vapor-liquid equilibrium calculations under various specifications (constant temperature, pressure constant enthalpy, pressure etc.) has long been recognized as one of the most important capabilities of a simulation system. Boston and Britt ( 6) reformulated the independent variables in the basic flash equations to make them weakly coupled. The authors claim their method works well for both wide and narrow boiling mixtures, and this has a distinct advantage over traditional algorithms ( 7). 

One further vapor/liquid equilibrium problem is the flash calculation. origin of the name is in the change that occurs when a liquid under press passes through a valve to a pressure low enough that some of the liquid vapori or flashes, producing a two-phase stream of vapor and liquid in equilibri We consider here only the P.f -flash, which refers to any calculation of quantities and compositions of the vapor and liquid phases making up a two-ph system in equilibrium at known P, T, and overall composition. 

Table 12.2 shows the results of a P, T-flash calculation for the system n-hexane( 1)/ethanol(2)/methylcyclopentane(3)/benzene(4). This is the same system for which the results of a BUBL T calculation were presented in Table 12.1, and the same correlations and parameter values have been used here. The given P and T are l(atm) and 334.15 K. The given overall mole fractions for the system Zi are listed in the table along with the calculated values of the liquid-phase and vapor-phase mole fractions and the K-values. The molar fraction of the system that is vapor is here found to be V = 0.8166. 

Flash calculations can also be made for light hydrocarbons with the Figs. 14.2 and 14.3. The procedure here is exactly as described in conn with Raoult s law in Sec. 10.5. We recall that the problem is to calculate fi system of given overall composition z at given T and P the fraction o system that is vapor V and the compositions of the vapor phase yf and liquid phase xj. The equation to be satisfied is... 

In most industrial processes coexisting phases are vapor and liquid, although liquid/liquid, vapor/solid, and liquid/solid systems are also encountered. In this chapter we present a general qualitative discussion of vapor/liquid phase behavior (Sec. 12.3) and describe the calculation of temperatures, pressures, and phase compositions for systems in vapor/liquid equilibrium (VLE) at low to moderate pressures (Sec. 12.4).t Comprehensive expositions are given of dew-point, bubble-point, and P, T-flash calculations. 

Thermodynamic calculations are used to evaluate vapor-liquid equilibrium constants, enthalpy values, dew points, bubble points, and flashes. Established techniques simulate the heat exchangers and distillation columns, and handle convergence and optimization. 

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