Adiabatic flash vaporization

The calculation of single-stage equilibrium separations in multicomponent systems is implemented by a series of FORTRAN IV subroutines described in Chapter 7. These treat bubble and dewpoint calculations, isothermal and adiabatic equilibrium flash vaporizations, and liquid-liquid equilibrium "flash" separations. The treatment of multistage separation operations, which involves many additional considerations, is not considered in this monograph. 

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. ... 

FLASH determines the equilibrium vapor and liquid compositions resultinq from either an isothermal or adiabatic equilibrium flash vaporization for a mixture of N components (N 20). The subroutine allows for presence of separate vapor and liquid feed streams for adaption to countercurrent staged processes. 

The temperature and composition of each feed stream and the stream ratios are specified along with a common feed pressure (significant only for the vapor stream) and the flash pressure. For an isothermal flash the flash temperature is also specified. Resulting vapor and liquid compositions, phase ratios, vaporization equilibrium ratios, and, for an adiabatic flash, flash temperature are returned. 

A step-limited Newton-Raphson iteration, applied to the Rachford-Rice objective function, is used to solve for A, the vapor to feed mole ratio, for an isothermal flash. For an adiabatic flash, an enthalpy balance is included in a two-dimensional Newton-Raphson iteration to yield both A and T. Details are given in Chapter 7. 

Adiabatic flash of liquid phase 40 wt% vaporized 90% of propylene vaporized ... 

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

The adiabatic flash pressure Pf, maintained slightly below the triple-point pressure, causes liquid to spontaneously vaporize and solidify. The ratio of solid to vapor is determined by the heats of fusion and vaporization for carbon dioxide about 1.7 moles of solid are formed for each mole vaporized. The solid, more dense than the liquid, falls through a liquid head and forms a loosely packed crystal bed at the bottom. The liquid head is about 10-12 feet, and increases the hydrostatic pressure on the solid to melter pressure Pm. The crystal bed depth is about two... 

A specific feature of the Carnit process is the self-regulating combination of the adiabatic flash. This occurs upon pressure reduction with the first isobaric concentration step where the flashed vapors supply the heat of evaporation. Efficient pH control achieves a liquid effluent with less than 50 ppm of AN227. 

The feed to a stage is split into the two phases of an adiabatic flash as viF for the vapor portion and for the liquid portion. The coefficients of the component vapor rates in Eq. (4.29) form the elements of the tridiagonal matrix... 

From the American Petroleum Institute s (API) Technical Data Book—Petroleum Refining, specific heats, specific gravities, latent heats of vaporization, and percent vaporization can be obtained, for a given oil, as a function of flash-zone temperature (percent vaporization and flash-zone temperature are functionally related because the flash vaporization takes place adiabatically). This suggests a trial-and-error procedure Assume a flash-zone temperature and the associated percent vaporization then make an energy balance to check the assumptions. Finally, complete the material balance. 

The dependent variables are the pressure or temperature, the vapor fraction, and the vapor and liquid compositions. In a truly adiabatic process Q = 0 (or h2 + H2 = the term adiabatic flash is generally applied to a process where the heat duty is specified. The problem is to determine a temperature (or pressure if the temperature is specified) at which the total enthalpy of the products satisfies the heat duty specification. Once and are known, the problem is handled as an isothermal flash. Again, in this case, the solution could result in a single phase or a mixed phase, and any set of temperature (or pressure) and heat duty specification is feasible. 

At the thermal expansion valve (stage 4), adiabatic flash evaporation of part of the liquid occurs. Autorefrigeration lowers the temperature of the liquid and vapor refrigerant mixture so that it is colder than the temperature of the enclosed space to be refrigerated. 

If a liqnid is released while its temperatnre is above the atmospheric-pressnre boiling point, some of the liqnid will adiabatically and instantaneously flash to vapor that is, the energy that is released as the liqnid temperatnre drops to the boiling point is ntihzed in vaporizing some of the liqnid. The fraction of liqnid that is flash-vaporized often can be obtained from a thermodynamic diagram (snch as a pressnre-enthalpy plot [8, 13]) for the material of interest, where values of x (if shown) indicate the weight-fractions of vapor ( quality ) in the two-phase mixture. 

This is a one-component adiabatic flash process. I will assume that only vapor + liquid are present, and then show that this is indeed the case. 

Equations 10.1-7 and 10.1-8, together with the equilibrium relations, can be used to solve problems involving partial vaporization and condensation processes at constant temperature. For partial vaporization and condensation processes that occur adiabatically, the final temperature of the vapor-liquid mixture is also unknown and must be found as part of the solution. This is done by including the energy balance among the equations to be solved. Since the isothermal partial vaporization or isothermal flash calculation is already tedious (see Illustration 10.1-4), the.adiabatic partial vaporization (or adiabatic flash) problem will not be considered here. ... 

When the pressure of a liquid stream of known composition, flow rate, and temperature (or enthalpy) is reduced adiabatically across a valve as in Fig. l. a, an adiabatic flash calculation can be made to determine the resulting temperature, compositions, and flow rates of the equilibrium liquid and vapor streams for a specified flash drum pressure. In this case, the procedure of Fig. 7.4o is applied in an iterative manner, as in Fig. 7.8, by choosing the flash temperature Tv as the iteration or tear variable whose value is guessed. Then X, y, and L are determined as for an isothermal flash. The guessed value of Tv (equal to TJ is checked by an enthalpy balance obtained by combining (7-15) for Q = 0 with (7-14) to give... 

The second iteration starts with initial estimates for almost all streams. An exception is stream L4, for which no estimate is needed. Initial estimates of streams V3 and Lr are used in an adiabatic flash calculation for stage 4 to determine an initial estimate for stream L4. Subsequently, flash calculations are performed in order for stages 3, 2, and 1, and then back up the column for stages 2, 3, and 4 followed by the total condenser and reflux divider. At the conclusion of the second iteration, generally all internal vapor and liquid flow rates are increased over values generated during the first iteration. 

Figure 7.1. Continuous single-stage equilibrium separation, (a) Flash vaporization. (Adiabatic flash with valve isothermal flash without valve when Tv is specified. (6) Partial condensation. (Analogous to isothermal flash when Ty is specified.

When cascade sections are coupled, a feed stage is employed and an adiabatic flash is carried out on the combination of feed, vapor rising from the cascade below, and liquid falling from the cascade above. The absorption factor for the feed stage is related to the flashed streams leaving by... 

The reactor effluent pressure is reduced and adiabatically flashed to recover acetic acid as vapor. The liquid phase contains the homogeneous catalyst which is pumped back to the reactor. The flashed vapor enters the light ends column where low molecular weight hydrocarbons are removed and a heavy fraction including water and hydrogen iodide is condensed and recycled to the reactor flash tank. The acetic acid product is removed from the column as a liquid side draw and further purified in downstream distillation columns. 

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