Phase split, vapor-liquid

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... 

When a mixture contains components with a broad range of volatilities, either a partial condensation from the vapor phase or a partial vaporization from the liquid phase followed by a simple phase split often can produce an effective separation. This is in essence a single-stage distillation process. However, by its very nature, a single-stage separation does not produce pure products hence further separation of both liquid and vapor streams is often required. 

Assume initially that a phase split can separate the reactor effluent into a vapor stream containing only hydrogen and methane and a liquid stream containing only benzene, toluene, and diphenyl and that the liquid separation system can produce essentially pure products. 

TABLE 4.3 Vapor-Liquid Phase Split Using the Soave-Redlich-Kwong Equation of State... 

As for LLE, an expression for G capable of representing liquid/liquid phase splitting is required as for X T.E, a vapor-phase equation of state For computing the is also needed. 

Table 13-1, based on the binary-system activity-coefficient-eqnation forms given in Table 13-3. Consistent Antoine vapor-pressure constants and liquid molar volumes are listed in Table 13-4. The Wilson equation is particularly useful for systems that are highly nonideal but do not undergo phase splitting, as exemplified by the ethanol-hexane system, whose activity coefficients are snown in Fig. 13-20. For systems such as this, in which activity coefficients in dilute regions may...

When a mixture contains components with large relative volatilities, either a partial condensation from the vapor phase or a partial vaporization from the liquid phase followed by a simple phase split can often produce an effective separation1. 

The liquid stream can readily be separated into relatively pure components by distillation, the benzene taken off as product, diphenyl as an unwanted byproduct and the toluene recycled. It is possible to recycle the diphenyl to improve selectivity, but it will be assumed that is not done here. The hydrogen feed contains methane as an impurity at a mole fraction of 0.05. The production rate of benzene required is 265 kmol-lr1. Assume initially that a phase split can separate the reactor effluent into a vapor stream containing only hydrogen and methane, and a liquid containing only benzene, toluene and diphenyl, and that it can be separated to produce essentially pure products. For a conversion in the reactor of 0.75,... 

When a mixture in a reactor effluent contains components with a wide range of volatilities, then a partial condensation from the vapor phase followed by a simple phase split can often produce a good separation. If the vapor from such a phase split is difficult to condense, then further separation needs to be carried out in a vapor separation process such as a membrane. The liquid from the phase split can be sent to a liquid separation unit such as distillation. 

The same reference (standard) state, f is chosen for the two phases, so that it cancels on both sides of equation 39. The products stffi and y" are referred to as activities. Because equation 39 holds for each component of a liquid—liquid system, it is possible to predict liquid—liquid phase splitting when the activity coefficients of the individual components in a multicomponent system are known. These values can come from vapor—liquid equilibrium experiments or from prediction methods developed for phase-equilibrium problems (4,5,10). Some binary systems can be modeled satisfactorily in this manner, but only rough estimations appear to be possible for multicomponent systems because activity coefficient models are not yet sufficiendy developed in this area. 

They split the liquid phase into two liquid phases using an approximate solution model, followed by rigorous solution, using k-values, for compositions of a vapor and two liquid phases at equilibrium. The FORTRAN program YFLASH (14) implements this algorithm. 

An example of heterogeneous-azeotrope formation is shown in Fig. 13-8 for the water-normal butanol system at 101.3 kPa. At liquid compositions between 0 and 3 mol % butanol and between 40 and 100 mol % butanol, the liquid phase is homogeneous. Phase splitting into two separate liquid phases (one with 3 mol % butanol and the other with 40 mol % butanol) occurs for any overall liquid composition between 3 and 40 mol % butanol. A minimum-boihng heterogeneous azeotrope occurs at 92°C (198°F) when the vapor composition and the overall composition of the two liquid phases are 25 mol % butanol. 

FIG. 13-8 Vapor-liquid equilibrium data for an n-butanol-water system at 101.3 kPa (1 atm) phase splitting and heterogeneous-azeotrope formation. 

A feed composition in the metastable set is stable to infinitesimal composition disturbances but is unstable to finite ones hence, for such a composition phase splitting can only occur by nucleation, and not simply by Brownian motion (which, at most, supposedly results in infinitesimal composition disturbances). Hence, a metastable composition may be observed as a one-phase system in the laboratory Superheated liquids and subcooled vapors are elementary one-component examples. In contrast with this, one-phase spinodal compositions, by virtue of being unstable to infinitesimal perturbations, will never be observed in the laboratory. 

We begin the analysis of phase equilibria (for those cases where phase splitting does in fact occur) with the simplest possible case, that of vapor-liquid equilibrium where both the liquid and the vapor phase are ideal, so that Raoult s law applies (Astarita, 1989 Bowman, 1949 Edminster, 1955). In this case the only parameter that completely characterizes every individual component is its vapor pressure b at the temperature considered hence, one begins by using b itself as the (dimensional) component label. Let X (b) and X (fc) be the mole fraction distributions in the liquid and vapor phases, respectively, and let p be the total pressure. Obviously, the zeroth moments of both distributions are unity. The continuous form of Raoult s law is ... 

The Wilson equation will not result in the prediction of liquid-liquid phase splitting and therefore can not be used in such applications or for vapor-liquid-liquid equilibria. The NRTL model has the advantage of having three adjustable parameters that allow it to be used for fitting the phase behavior of highly nonideal mixtures, though sometimes a is set to a fixed value (usually 0.2 for liquid-liquid equilibria and 0.3 for vapor-liquid equilibria). 

Flash2 - rigorous vapor-liquid split or vapor liquid liquid split FlashS - rigorous vapor-liquid-liquid split Decanter - separate two liquid phases Sep - use split fractions... 

In this method, a mixed A -value is defined as the ratio of the mole fraction of a component in the vapor to its mole fraction in the mixed liquid phase (Schuil and Bool, 1985). Applied to an equilibrium stage or a flash drum, the phase equilibrium is solved using the mixed /f-valucs instead of the usual vapor-liquid X-values to determine the flow rates and compositions of the vapor and the total liquid. The liquid phase split is then calculated on the basis of A -values for each liquid phase to determine the flow rates and compositions of the two liquid phases. An energy balance may also be included to determine the temperature or the heat transfer for the unit. 

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