Volatility vapor-liquid equilibrium

Figure 8-2 illustrates a typical normal volatility vapor-liquid equilibrium curve for a particular component of interest in a distillation separation, usually for the more volatile of the binary mixture, or the one where separation is important in a multicomponent mixture. 

Most batch distillations/separations are assumed to follow the constant relative volatility vapor-liquid equilibrium curve of... 

Then calculate tray by tray from xr up Nr trays (using component balances and constant relative volatility vapor-liquid equilibrium relationships) to obtain the vapor composition on the top tray yvr- Compare this value with that obtained from a component balance around the reactor, Eq. (5.43). 

As pointed out in the previous chapter, the separation of a homogeneous fluid mixture requires the creation of another phase or the addition of a mass separation agent. Consider a homogeneous liquid mixture. If this liquid mixture is partially vaporized, then another phase is created, and the vapor becomes richer in the more volatile components (i.e. those with the lower boiling points) than the liquid phase. The liquid becomes richer in the less-volatile components (i.e. those with the higher boiling points). If the system is allowed to come to equilibrium conditions, then the distribution of the components between the vapor and liquid phases is dictated by vapor-liquid equilibrium considerations (see Chapter 4). All components can appear in both phases. 

On the other hand, rather than partially vaporize a liquid, the starting point could have been a homogeneous mixture of components in the vapor phase and the vapor partially condensed. There would still have been a separation, as the liquid that was formed would be richer in the less-volatile components, while the vapor would have become depleted in the less-volatile components. Again, the distribution of components between the vapor and liquid is dictated by vapor-liquid equilibrium considerations if the system is allowed to come to equilibrium. 

A portion of volatile LNAPL in the subsurface vaporizes into air-filled pore spaces until the vapor-liquid equilibrium concentrations are established. If the soil zone is naturally permeable from the fluid surface to the soil surface, a concentration gradient is established. Eventually, most of the volatile mass can be transferred from the LNAPL to the atmosphere, minus that portion retained by sorption on the soil, or biologically degraded. 

Figure 1. Vapor-liquid equilibrium in an aqueous system of volatile weak...

An application to one binary mixture of a volatile electrolyte and water will illustrate the choice of parameters H and K, an approach is proposed to represent the vapor-liquid equilibrium in the whole range of concentration. Ternary mixtures with one acid and one base lead to the formation of salts and high ionic strengths can be reached. There, it was found useful to take into account... 

Acetic Acid-Water Mixture. CRUZ (4) chose this example to illustrate his method of representation of vapor-liquid equilibria of volatile weak electrolyte and to show how to obtain simply from experimental vapor-liquid equilibrium data the significant parameters. ... 

This effect, in and of itself, tends to increase the yield of tar (and therefore of total volatiles), for the reason discussed earlier. However, increasing the ambient pressure also shifts the vapor-liquid equilibrium of the tar species to smaller tar species (with higher vapor pressures) and thus tends to diminish the overall release of tar. Wire-mesh experiments with well-controlled particle heating rates show a significant reduction in the yield of tar and total volatiles as the pressure is increased. The rate of devolatilization, however, is nearly insensitive to pressure, as would be expected for unimolecular reaction processes. 

Example 2.7. To show what form the energy equation takes for a two-phase system, consider the CSTR process shown in Fig. 2.6. Both a liquid product stream f and a vapor product stream F (volumetric flow) are withdrawn from the vessel. The pressure in the reactor is P. Vapor and liquid volumes are and V. The density and temperature of the vapor phase are and L. The mole fraction of A in the vapor is y. If the phases are in thermal equilibrium, the vapor and liquid temperatures are equal (T = T ). If the phases are in phase equilihrium, the liquid and vapor compositions are related by Raoult s law, a relative volatility relationship or some other vapor-liquid equilibrium relationship (see Sec. 2.2.6). The enthalpy of the vapor phase H (Btu/lb or cal/g) is a function of composition y, temperature T , and pressure P. Neglecting kinetic-energy and potential-energy terms and the work term,... 

The effect of salts on the vapor-liquid equilibrium of solvent mixtures has been of considerable interest in recent years. Introduction of a salt into a binary solvent mixture results in a change in the relative volatility of the solvents. This effect can be used to an advantage where the separation of the solvents is of interest. Furter and co-workers have demonstrated the potential importance of salts as separating agents in extractive distillation (J, 2, 3). 

A review is presented of techniques for the correlation and prediction of vapor-liquid equilibrium data in systems consisting of two volatile components and a salt dissolved in the liquid phase, and for the testing of such data for thermodynamic consistency. The complex interactions comprising salt effect in systems which in effect consist of a concentrated electrolyte in a mixed solvent composed of two liquid components, one or both of which may be polar, are discussed. The difficulties inherent in their characterization and quantitative treatment are described. Attempts to correlate, predict, and test data for thermodynamic consistency in such systems are reviewed under the following headings correlation at fixed liquid composition, extension to entire liquid composition range, prediction from pure-component properties, use of correlations based on the Gibbs-Duhem equation, and the recent special binary approach. 

The use of a dissolved salt in place of a liquid component as the separating agent in extractive distillation has strong advantages in certain systems with respect to both increased separation efficiency and reduced energy requirements. A principal reason why such a technique has not undergone more intensive development or seen more than specialized industrial use is that the solution thermodynamics of salt effect in vapor-liquid equilibrium are complex, and are still not well understood. However, even small amounts of certain salts present in the liquid phase of certain systems can exert profound effects on equilibrium vapor composition, hence on relative volatility, and on azeotropic behavior. Also extractive and azeotropic distillation is not the only important application for the effects of salts on vapor-liquid equilibrium while used as examples, other potential applications of equal importance exist as well. 

The original equation for salt effect in vapor-liquid equilibrium, proposed by Furter (7) and employed subsequently by Johnson and Furter (8), described the effect of salt concentration on equilibrium vapor composition under the condition of a fixed ratio of the two volatile components in the liquid phase. The equation, derived from the difference in effects of the salt on the chemical potentials of the two volatile components, with simplifying approximations reduces to the form... 

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. 

Separation processes which involve non-volatile salts arise in two situations. First, as an alternative to extractive or azeotropic distillation, salts may be added to a system to alter the vapor-liquid equilibrium behavior. Second, there are cases where a salt is generated in the process before final product purification. For example, product streams from processes involving esterification, etherification, or neutralization contain salts and are often fed to separation units such as distillation or stripping columns. 

The establishment of the method of prediction has been attempted by the reverse calculation of the preferential solvation number from measured values, using Equations 4 and 7 which are based on the assumption that the salt effect in the vapor-liquid equilibrium is caused by the preferential solvation formed between a volatile component and a salt. The observed values were selected from Ciparis s data book (4), Hashitani s data (5-8), and the author s data (9-15). S was calculated by Equation 7 when the relative volatility as in the vapor-liquid equilibrium with salt is increased with respect to the relative volatility a in the vapor-liquid equilibrium with salt, but by Equation 4 when as is decreased. The results are shown in Figures 5-12. From these figures, it will be seen that the following three relations exist ... 

The effect of the magnesium nitrate on the vapor-liquid equilibrium of nitric acid and water can be seen in Figure 3. As the concentration of magnesium nitrate in the liquid increases, the volatility of nitric acid also increases. 

Measurements of binary vapor-liquid equilibria can be expressed in terms of activity coefficients, and then correlated by the Wilson or other suitable equation. Data on all possible pairs of components can be combined to represent the vapor-liquid behavior of the complete mixture. For exploratory purposes, several rapid experimental techniques are applicable. For example, differential ebulliometry can obtain data for several systems in one laboratory day, from which infinite dilution activity coefficients can be calculated and then used to evaluate the parameters of correlating equations. Chromatography also is a well-developed rapid technique for vapor-liquid equilibrium measurement of extractive distillation systems. The low-boiling solvent is deposited on an inert carrier to serve as the adsorbent. The mathematics is known from which the relative volatility of a pair of substances can be calculated from the effluent trace of the elutriated stream. Some of the literature of these two techniques is cited by Walas (1985, pp. 216-217). 

The vapor-liquid equilibrium (VLE) is assume to be described by a constant relative volatility between components A and B of a= 1.5. On each tray in the column, the compositions (mole fraction A) of the liquid x and vapor v are related as follows ... 

Assuming constant relative volatilities ay of the components, the vapor-liquid equilibrium is given by ... 

Further, an ideal vapor liquid equilibrium is assumed with constant relative volatilities according to... 

Assuming the relative volatility of a benzene-toluene mixture is 2.90, the vapor-liquid equilibrium compositions can be calculated as shown in Table 5.1. The resultant curve is plotted in Fig. 5.1a. 

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