Liquid phase splitting

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

For most LLE applications, the effect of pressure on the Yi < an be ignored, and thus Eq. (4-327) constitutes a set of N equations relating equilibrium compositions to each other and to temperature. For a given temperature, solution of these equations requires a single expression for the composition dependence of suitable for both liquid phases. Not all expressions for suffice, even in principle, because some cannot represent liquid/liquid phase splitting. The UNIQUAC equation is suitable, and therefore prediction is possible by the UNIFAC method. A special table of parameters for LLE calculations is given by Magnussen, et al. (Jnd E/ig Chem Process Des Dev, 20, pp. 331-339 [1981]). 

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. 

Once the best set of interaction parameters has been found, these parameters should be used with the EoS to perform the VLE calculations. The computed values should be plotted together with the data. A comparison of the data with the EoS based calculated phase behavior reveals whether correct or incorrect phase behavior (erroneous liquid phase splitting) is obtained. 

If the correct phase behavior i.e. absence of erroneous liquid phase splits is predicted by the EoS then the overall fit should be examined and it should be judged whether it is "excellent". If the fit is simply acceptable rather than "excellent", then the previously computed LS parameter estimates should suffice. This was found to be the case for the n-pentane-acetone and the methane-acetone systems presented later in this chapter. 

Therefore, although the stability function was found to be positive at all the experimental conditions it becomes negative at mole fractions between 0 and the first measured data point. Obviously, if there were additional data available in this region, the simplified constrained LS method that was followed above would have yielded interaction parameters that do not result in prediction of false liquid phase splitting. 

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. 

MESH) equations which are solved for the whole column, decanter included and taking into account the liquid-liquid phase split. Numerical treatment of the Differential Algebraic Equation (DAE) system and discrete events handling is performed with DISCo, a numerical package for hybrid systems with a DAE solver based on Gear s method. The column technical features and operating conditions are shown in Table 4. A sequence of two operational batch steps, namely... 

If the liquid mixture is extremely non-ideal, liquid phase splitting will occur. Here, we first consider the hypothetical ternary system. The physical properties are adopted from Ung and Doherty [17] and Qi et al. [10]. The catalyst is assumed to have equal activity in the two liquid phases. The corresponding PSPS is depicted in Fig. 4.5, together with the liquid-liquid envelope and the chemical equilibrium surface. The PSPS passes through the vertices of pure A, B, C, and the stoichiometric pole Jt. The shape of the PSPS is affected significantly by the liquid phase non-idealities. As a result, there are three binary nonreactive azeotropes located on... 

Liquid-liquid phase splits extn. - + Rapid assays. 

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). 

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. 

The liquid phase split is calculated by solving the following equation for a ... 

One of the more common cases of three-phase distillation is where the two liquid phases occur primarily in the condenser. Such occurrence is very common in crude oil and related distillation columns, where water separates in the condenser. When two liquid phases form in the column, one of them is usually taken out as a side draw at the tray where the liquid phase split occurs. This practice helps stabilize the column operation. 

The computations in three-phase distillation involve two sets of vapor-liquid equilibrium coefficients, or A -values, derived from the activity coefficients of each component in each liquid phase (Section 2.3.3). The calculations may be simplified in hydrocarbon systems if the second liquid phase is mostly water. In these situations it is possible to assume the aqueous phase to be pure water and account only for water dissolved in the organic phase. The description of rigorous solution methods for three-phase distillation is deferred to Chapter 13. The objective at this point is to consider tlie effect a liquid phase split can have on distillation. 

The two-parameter van Laar equation cannot represent maxima or minima in the activity coefficients, nor can it represent the mixed deviations from ideality exemplified by the benzene-hexaflnorobenzene system of Fig. 1.4-1. However, it is suparior to the Margules equation for some extremely nonideal systems such as alcohol-hydrocarbon mixtures, for example, the ethanol-n-haptane system of Fig. 1.4-1. For such mixtures the two-parameter Margules equation often incorrectly predicts liquid-liquid phase splitting. Higher order Margules equations can sometimes be u red for these systems, but at the expense of many additional parameters. 

Solutions containing carboxylic acids The Wilson model appears to best correlate data for mixtures containing carboxylic acTds, if the components are mutually soluble. (The Wilson model does not predict liquid-liquid phase splitting, as discussed in Sec. 11.2). Otherwise, the UNIQUAC, van Laar, or NRTL model should be used. 

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