Vapour-liquid equilibria prediction

Fig. 5. Vapour-liquid equilibrium predictions using the HCToolkit. This figure shows the experimental data from (Keshtkar et al. 1997), an accurate binary calculation (UNIQUAC), the phase envelope (in bold) and vapour phase isofractions (shaded).

Perry et al. (1997) give a useful summary of solubility data. Liquid-liquid equilibrium compositions can be predicted from vapour-liquid equilibrium data, but the predictions are seldom accurate enough for use in the design of liquid-liquid extraction processes. 

Before beginning the series of runs to determine the relief size, the physical property and kinetic data need to be correlated in the form required, by the code. In some cases, the code may already have the components required on a database. In all other cases, physical property data must be found, estimated or measured and correlated in the appropriate form. Some codes have a front-end program for curve fitting of data. For tempered systems, the vapour/ liquid equilibrium models are of critical importance since errors will cause the code to open the relief system at the wrong temperature and reaction rate. It is therefore worthwhile to spend time to ensure reasonable behaviour of the vapour pressure predictions. Check that all correlations behave sensibly over the entire temperature range of relevance for relief sizing. A good test for the physical property and kinetic data supplied to the code is to first model the (unrelieved) adiabatic calorimetric test which was used to obtain the kinetic data.. . ... 

At high pressures, especially approaching the critical pressure, the accuracy of the vapour-liquid equilibrium data are questionable. Direct measurement is not easy prediction with equations of state is risky [1],... 

For the analytical equations, there are two methods to compute the vapour-liquid equilibrium for systems. The equation of state method (also known as the direct or phi-phi method) uses an equation of state to describe both the liquid and vapour phase properties, whereas the activity coefficient method (also known as the gamma-phi approach) describes the liquid phase via an activity coefficient model and the vapour phase via an equation of state. Recently, there have also been modified equation of state methods that have an activity coefficient model built into the mixing mles. These methods can be both correlative and predictive. The predictive methods rely on the use of group contribution methods for the activity coefficient models such as UNIFAC and ASOG. Recently, there have also been attempts to develop group contribution methods for the equation of state method, e.g. PRSK. " For a detailed history on the development of equations of state and their applications, as well as activity coefficient models, refer to Wei and Sadus, Sandler and Walas. ... 

The augmented and fixed dipole moments of the models could give good estimates for the vapour-liquid equilibrium only as a result of fortuitous cancellation of forces. In Table 2 we present the critical values of the state variables for the different models. None of the models predicts all the three state variables with acceptable accuracy. Both the pressures and in most cases the temperatures are smaller than the experimental values. For further details see Refs.4, 9,14. 

The implementation of the EOS components allows very tight control over the accuracy of calculations. As proof of concept, two examples of calculations are included. The first is a prediction of vapour-liquid equilibrium in a binary CO2-cyclopentane system. This system is somewhat... 

He found experimentally that conversion was enhahced by an increase in feed rate of the acid or the concentration of the catalyst but decreased by an increase in reflux ratio. He attributed differences between predicted and experimental values to inadequacy of the predicted vapour-liquid equilibrium data. The same system was studied experimentally by Corrigan and Ferris (17) in an Oldershaw sieve-tray column. Batch and continuous experiments... 

Romero, C. Villares, A. Haro, M. Giner, B. Lafuente, C. Experimental and predicted vapour-liquid equilibrium of 1,4-dioxane with cycloalkanes and benzene Fluid Phase Equilib. 2005,238,1-6... 

Binary variables are used to represent the occurrence of molecular structural groups (e.g. -CH3, -CHO, -OH. ..) found in the group contribution correlations. This allows molecules to be generated according to a set of structural and chemical feasibility constraints. In addition, a variety of pure component physical and environmental property prediction equations, non-ideal multi-component vapour-liquid equilibrium equations (UNIFAC), process operational constraints and an aggregated process model form part of the overall procedure. Finally, the solvent identification task is solved as a mixed integer non-linear programming (MINLP) problem (Buxton et ai, 1999). 

Distillation is a widely used unit operation in pharmaceutical manufacture to remove components from a system to an acceptable level. The process engineer is able to assist in the selection of the optimum solvent system, providing vapour-liquid equilibrium information and predictions of the efficiency of the separation. A recent example highlighted the effectiveness of CAPE tools in the design and prediction of distillation performance. 

E. W. Lemmon, A Generalized Model for the Prediction of the Thermodynamic Properties of Mixtures Including Vapour-Liquid Equilibrium, Ph.D. Dissertation, University of Idaho, Moscow, 1996. 

Figure 8.4 Pressure as a function of methanol mole fraction x(CH30H) for the methanol + hexane vapour-liquid equilibrium at temperatures of (333, 343 and 348) K. , measurements —, results predicted from the PC-...

The prediction of volatilities and melting points of pure ionic liquids was possible by associating first principles calculations and calorimetric measurements, as presented, for example, by Verevkin and co-workers. They showed that the knowledge of gas-phase thermodynamic properties is important for the modelling of vapour-liquid equilibrium on pure ionic fluids [97]. 

In order to improve volume predictions (Peneloux et al., 1982) proposed a consistent correction for cubic EoS which does not change the vapour-liquid equilibrium conditions. Their method consists in translating the EoS by replacing v hy v+c and by b+c. The... 

Bandres, L Giner, I. Pera, G. Giner, B. Lafuente, C. Vapour-liquid equilibrium of cyclic ethers with 1-chlorohexane Experimental results and UNIFAC predictions. Fluid Phase Equilib. 2007, 257, 70-77. 

Arce, A. Martinez-Ageitos, J. Rodil, E. Soto, A. Measurement and prediction of isobaric vapour-liquid equilibrium data of flie system ethanol + meflianol + 2-methoxy-2-methylpropane. Fluid Phase Equilib. 1998, 146, 139-153. 

B. J. Lee and W. C. Edmister A generalized method for predicting vapour-liquid equilibrium. 

The UNIQUAC equation developed by Abrams and Prausnitz is usually preferred to the NRTL equation in the computer aided design of separation processes. It is suitable for miscible and immiscible systems, and so can be used for vapour-liquid and liquid-liquid systems. As with the Wilson and NRTL equations, the equilibrium compositions for a multicomponent mixture can be predicted from experimental data for the binary pairs that comprise the mixture. Also, in the absence of experimental data for the binary pairs, the coefficients for use in the UNIQUAC equation can be predicted by a group contribution method UNIFAC, described below. 

For the synthesis of heterogeneous batch distillation the liquid-liquid envelope at the decanter temperature is considered in addition to the residue curve map. Therefore, the binary interaction parameters used in predicting liquid-liquid equilibrium are estimated from binary heterogeneous azeotrope or liquid-liquid equilibrium data [8,10], Table 3 shows the calculated purity of original components in each phase split at 25 °C for all heterogeneous azeotropes reported in Table 1. The thermodynamic models and binary coefficients used in the calculation of the liquid-liquid-vapour equilibrium, liquid-liquid equilibrium at 25 °C and the separatrices are reported in Table 2. 

Furter [91] has analyzed the state of the art from the point of view of employing the salt effect in industrial processes, especially in extractive distillation. In addition, he ha.s made up a list of references covering the years 1966 to 1977 [91 a]. Schubert et al. [92] investigated the effect of some metal chlorides and other salts on the isothermal = 60°C) phase equilibrium behaviour of the systems n-propanol-water, n-butanol-water and methanol-water. Using CH30H/H20/NaBr as an example, the method of predicting salt effects for vapour-liquid equilibria as developed by Schuberth has been extended to uusaturated solutions [92a]. 

Muir, R.F. and Howatt, C.S. (1982) Predicting solid-liquid equilibrium data from vapour-liquid data. Chemical Engineering, 22 Feb, 89-92. 

In an attempt to improve the behaviour of the cubic equation of state, the more elaborate Huron-Vidal mixing rules were used with the Peng-Robinson equation of state.As shown in Figure 4.5 for the (vapour + liquid + liquid) (VLLE) equilibrium of (water + hex-1-ene) the Huron-Vidal mixing rules improved significantly the predicted solubility of hex-1-ene over the standard van der Waals mixing rules however, there was also a significant decrease in the ability of the model to correlate of solubility of water. 

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