Vapor flash calculation

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

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... 

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. 

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. )... 

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

The API-RP-520 [10] recommends calculating the amount of vapor flashed and the amount of residual liquid (unfiashed) and then sizing valve orifices for each condition. Select a valve (s) area that has a total area at least equal to the sum of these two areas. Before settling for this approach, this author recommends examining... 

Both "wet" flash calculations predict a higher concentration of water in the vapor phase than the graphical correlations. The presence of methanol reduces the predicted water content of the vapor phase. 

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. 

FLLO = old value of assumed total vapor flow in iterative flash calculation... 

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. 

The dew point and bubble point calculations do not present peculiar problems, but the flash calculation does. Let X fx) be the mole fraction distribution in the feed to a flash, and let a be the vapor phase fraction in the flashed system. The mass balance is ... 

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