Vapor-liquid equilibrium presenting experimental data

Several activity coefficient models are available for industrial use. They are presented extensively in the thermodynamics literature (Prausnitz et al., 1986). Here we will give the equations for the activity coefficients of each component in a binary mixture. These equations can be used to regress binary parameters from binary experimental vapor-liquid equilibrium data. 

A procedure is presented for correlating the effect of non-volatile salts on the vapor-liquid equilibrium properties of binary solvents. The procedure is based on estimating the influence of salt concentration on the infinite dilution activity coefficients of both components in a pseudo-binary solution. The procedure is tested on experimental data for five different salts in methanol-water solutions. With this technique and Wilson parameters determined from the infinite dilution activity coefficients, precise estimates of bubble point temperatures and vapor phase compositions may be obtained over a range of salt and solvent compositions. 

All of the necessary experimental data [Vf, H2,i, 7 2,3, and E (Margules parameter)] were taken from the original publications (indicated as footnotes to Table 1) or calculated using the data from Gmehling s vapor-liquid equilibrium data compilation. Figure 1 and Table 1 show that the present eq 25 is in much better agreement with experiment than Krichevsky s eq 1 and equations A2-3—5 from Appendix 2, which involve the Margules expression for the activity coefficient. The new eq 25 provides predictions that are comparable to those of an empirical correlation for aqueous mixtures of solvents, which involves three adjustable parameters. 

With all these choices, and limited knowledge of your system, you will likely want to use the recommended options and make predictions of vapor-liquid equilibrium using Aspen Plus in order to compare those predictions with experimental data. Chapter 3 presented an example of such a comparison for the ethanol-water system. 

At the lowest pressure in the figure, P = 0.133 bar, the vapor-liquid equilibrium curve intersects the liquid-liquid equilibrium curve. Consequently, at this pressure, depending on the temperature and composition, we may have only a liquid, two liquids, two liquids and a vapor, a vapor and a liquid, or only a vapor in equilibrium. The equilibrium state that does exist can be found by first determining whether the composition of the liquid is such that one or two liquid phases exist at the temperature chosen. Next, the bubble point temperature of the one or either of the two liquids present is determined (for example, from experimental data or from known vapor pressures and an activity coefficient model calculation). If the liquid-phase bubble point temperature is higher than the temperature of interest, then only a liquid or two liquids are present. If the bubble point temperature is lower, then depending on the composition, either a vapor, or. a vapor and a liquid are present. However, if the temperature of interest is equal to the bubble point temperature and the composition is in the range in which two liquids are present, then a vapor and two coexisting liquids will be in equilibrium. 

It is usually desirable to present the experimental vapor-liquid equilibrium data graphically. A number of methods of presentation have been developed, but the most important are the temperature-composition and the vapor-liquid composition diagrams. 

Evaluate the performance of the UNIQUAC model in the prediction of the vapor-liquid equilibrium behavior for the system carbon tetrachloride(l)-benzene(2)-i-propyl alcohol(3) at 1.013 bar by comparing your results to the experimental data presented in Table 13.E.12 (Hala et al, 1968, pp.493-494).The required UNIQUAC binary parameters are included in the same Table and were obtained by regressing the corresponding VLE data at 343.15 K for the 1-2 and 1-3 binaries (Hirata et al, 1975) and at 760 mm Hg for the 2-3 one (see Example 13.10). 

Vapor pressure is the most important of the basic thermodynamic properties affec ting liquids and vapors. The vapor pressure is the pressure exerted by a pure component at equilibrium at any temperature when both liquid and vapor phases exist and thus extends from a minimum at the triple point temperature to a maximum at the critical temperature, the critical pressure. This section briefly reviews methods for both correlating vapor pressure data and for predicting vapor pressure of pure compounds. Except at very high total pressures (above about 10 MPa), there is no effect of total pressure on vapor pressure. If such an effect is present, a correction, the Poynting correction, can be applied. The pressure exerted above a solid-vapor mixture may also be called vapor pressure but is normallv only available as experimental data for common compounds that sublime. 

In principle, it would be desirable to have information on vapor-liquid equilibria of all binary systems in the temperature range in which the RD is carried out, which is about 100-150 °C in the case studied here. Furthermore, it would be desirable to have at least some data points for ternary systems (all of which are reactive) and for the quaternary system to be able to check the predictive power of the phase equilibrium model. That ideal situation is almost never encountered in reality. In many cases, even reliable experimental data on the binary systems is missing. In the present study, no data was available for the binary systems acetic acid + hexyl acetate and 1-hexanol - - hexyl acetate. Estimations of missing data using group contribution methods such as UNIFAC are possible, but their quality is often hard to assess. 

The equilibrium vapor pressure of CO2 over a solution containing the equivalent of 20% potassium carbonate as a function of conversion to bicarbonate, based on the data of Tosh et al. (1959), is presented in Figure 5-9. These authors, who investigated vapor/liquid equilibria for 20, 30, and 40% equivalent potassium carbonate solutions, found that the CO2 equilibrium vapor pressure remains practically the same for the range of 20 to 30%. This in effect confirms the observation of Buck and Leitch (1958) for commercial operations. The experimental CO2 vapor pressure data were used by Tosh el al. (1959) as the basis for calcu-... 

Two generally accepted models for the vapor phase were discussed in Chapter 3 and one particular model for the liquid phase (UNIQUAC) was discussed in Chapter 4. Unfortunately, these, and all other presently available models, are only approximate when used to calculate equilibrium properties of dense fluid mixtures. Therefore, any such model must contain a number of adjustable parameters, which can only be obtained from experimental measurements. The predictions of the model may be sensitive to the values selected for model parameters, and the data available may contain significant measurement errors. Thus, it is of major importance that serious consideration be given to the proper treatment of experimental measurements for mixtures to obtain the most appropriate values for parameters in models such as UNIQUAC. 

The hydrate and phenol clathrate equilibrium data of the water-carbon dioxide, phenol-carbon dioxide, and water-phenol-carbon dioxide systems are presented in Table 1 and depicted in Figure 2. In order to establish the validity of the experimental apparatus and procedure the hydrate dissociation pressures of carbon dioxide measured in this work were compared with the data available in the literature (Deaton and Frost [7], Adisasmito et al. [8]) and found that both were in good agreement. For the phenol-carbon dioxide clathrate equilibrium results, as seen in Figure 2, the dramatic increase of the dissociation pressures in the vicinity of 319.0 K was observed. It was also found in the previous study (Kang et al. [9]) that the experimental phenol-rich liquid-phenol clathrate-vapor (Lp-C-V) equilibrium line of the binary phenol-carbon dioxide system could be well extended to the phenol clathrate-solid phenol-vapor (C-Sp-V) equilibrium line (Nikitin and Kovalskaya [10]). It is thus interesting to note that a quadruple point at which four individual phases of phenol-rich liquid, phenol clathrate, solid... 

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