Flash calculations

The BUBL P and DEW P calculations at T = 325,15 K of Example 3a and 3b show that for X = Z, Pbubi = 108,134 kPa, and for y = zi, Pdew = 87.939 kPa. Because P here lies between these values, the system is in two-phase equilibrium, and a flash calculation is appropriate. 

A trial calculation illustrates the nature of the solution. Vapor pressures are taken from Example 3a or 3fc a trial value of %i then allows calculation of Yi and Ys by Eqs. B) and (C) of Example 3. The values of Xi, K, and P that result are substituted into the summation equation. In the unlikely event that the sum is indeed unity, the chosen value of %i is correct. If not, then successive trials easily lead to this value. Note that the trivial solution giving E= 1 must be avoided. More elegant solution procedures can of course be employed. The answers are 

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When only the total system composition, pressure, and temperature (or enthalpy) are specified, the problem becomes a flash calculation. This type of problem requires simultaneous solution of the material balance as well as the phase-equilibrium relations. 

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

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... 

Isothermal Flash Calculations for Mixtures Containing Condensable and Noncondensable Components... 

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. 

Flash calculations for these mixtures usually require four to eight iterations. Cases 5 and 6 in Table 1 have feeds of this type, including noncondensable components in Case 6. Within the limits of the thermodynamic framework used here, no case has been encountered where FLASH has required more than 12 iterations for satisfactory convergence. 

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 calculation for a point on the flash curve that is intermediate between the bubble point and the dew point is referred to as an isothermal-flash calculation because To is specified. Except for an ideal binary mixture, procedures for calculating an isothermal flash are iterative. A popular method is the following due to Rachford and Rice [I. Pet. Technol, 4(10), sec. 1, p. 19, and sec. 2, p. 3 (October 1952)]. The component mole balance (FZi = Vy, + LXi), phase-distribution relation (K = yJXi), and total mole balance (F = V + L) can be combined to give... 

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. ... 

If a phase change occurs in the process stream for which heat duties are being calculated, it is best to perform a flash calculation and determine the heat loss or gain by the change in enthalpy. For a quick hand approximation it is possible to calculate sensible heat for both the gas and liquid phases of each component. The sum of all the latent and sen -i-ble heats is the approximate total heat duty. 

Choking, or expansion of gas from a high pressure to a lower pressure, is generally required for control of gas flow rates. Choking is achieved by the use of a choke or a control valve. The pressure drop causes a decrease in the gas temperature, thus hydrates can form at the choke or control valve. The best way to calculate the temperature drop is to use a simulation computer program. The program will perform a flash calculation, internally balancing enthalpy. It will calculate the temperature downstream of the choke, which assures that the enthalpy of the mixture of gas and liquid upstream of the choke equals the enthalpy of the new mixture of more gas and less liquid downstream of the choke. 

Step 7. Make a flash calculation with the feed composition and the K-values from Step 6. 

The calculation of y and P in Equation 14.16a is achieved by bubble point pressure-type calculations whereas that of x and y in Equation 14.16b is by isothermal-isobaric //cm-/(-type calculations. These calculations have to be performed during each iteration of the minimization procedure using the current estimates of the parameters. Given that both the bubble point and the flash calculations are iterative in nature the overall computational requirements are significant. Furthermore, convergence problems in the thermodynamic calculations could also be encountered when the parameter values are away from their optimal values. 

In an equilibrium flash process a feed stream is separated into liquid and vapour streams at equilibrium. The composition of the streams will depend on the quantity of the feed vaporised (flashed). The equations used for equilibrium flash calculations are developed below and a typical calculation is shown in Example 11.1. 

Flash calculations are often needed to determine the condition of the feed to a distillation column and, occasionally, to determine the flow of vapour from the reboiler, or condenser if a partial condenser is used. 

Efficient techniques for the solution of the trial and error calculations necessary in multicomponent flash calculations are given by several authors Hengstebeck (1976) and King (1980). 

The vapour and liquid compositions leaving the reboiler can be estimated using the same procedure as that for a flash calculation see Section 11.3.3. 

This route to K values is handy for flash calculations as... 

Many variations are possible around the basic flash calculation. Pressure and V/F can be specified and T calculated, and so on. Details can be found in King7. However, two special cases are of particular interest. If it is necessary to calculate the bubble point, then V/F = 0 in Equation 4.55, which simplifies to ... 

For flashing liquids composed of many miscible substances, the flash calculation is complicated considerably, because the more volatile components flash preferentially. Procedures are available to solve this problem.16... 

For multicomponent systems with boiling range greater than 80°C, a single adiabatic flash calculation to 80 to 90 percent of the inlet pressure P0 yields the two-phase specific volume oI at pressure P1 and co is calculated from (Nazario and Leung, Sizing Pressure Relief Valves in Flashing and Two-Phase Service An Alternative Procedure, J. Loss Prev. Process lnd. 5(5), pp. 263-269, 1992)... 

Adiabatic cracking reactor, 10 617-618 Adiabatic decomposition, of hydrogen peroxide, 14 61-62 Adiabatic dehydrogenation, 23 337 Adiabatic dehydrogenation unit, 23 339 Adiabatic evaporation, general separation heuristics for, 22 319 Adiabatic flame temperature, 12 322 Adiabatic flash calculation, 24 681 Adiabatic nitration process, 17 253—255 Adiabatic pressure-reducing valve,... 

The type of flash calculation--"dry", "wet" or "wet" plus methanol--has no practical effect on the predicted hydrocarbon liquid formation. 

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. 

The system is specified by the algebraic equations listed above. This is just a traditional steadystate equilibrium-flash calculation. 

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. 

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