Activity, coefficient rules

Concentrated, Binary Mixtures of Nonelectrolytes Several correlations that predict the composition dependence of Dab. re summarized in Table 5-19. Most are based on known values of D°g and Dba- In fact, a rule of thumb states that, for many binary systems, D°g and Dba bound the Dab vs. Xa cuiwe. CuUinan s equation predicts dif-fusivities even in hen of values at infinite dilution, but requires accurate density, viscosity, and activity coefficient data. 

Thermodynamic models are widely used for the calculation of equilibrium and thermophysical properties of fluid mixtures. Two types of such models will be examined cubic equations of state and activity coefficient models. In this chapter cubic equations of state models are used. Volumetric equations of state (EoS) are employed for the calculation of fluid phase equilibrium and thermophysical properties required in the design of processes involving non-ideal fluid mixtures in the oil and gas and chemical industries. It is well known that the introduction of empirical parameters in equation of state mixing rules enhances the ability of a given EoS as a tool for process design although the number of interaction parameters should be as small as possible. In general, the phase equilibrium calculations with an EoS are very sensitive to the values of the binary interaction parameters. 

A number of authors have suggested various mixing rules, according to which the quantity a could be calculated for a measured electrolyte in a mixture, starting from the known individual parameters of the single electrolytes and the known composition of the solution. However, none of the proposed mixing relationships has found broad application. Thus, the question about the dependence of the mean activity coefficients of the individual electrolytes on the relative contents of the various electrolytic components was solved in a different way. 

These relationships are termed the Harned rules and have been verified experimentally up to high overall molality values (e.g. for a mixture of HC1 and KC1 up to 2 mol-kg-1). If this linear relationship between the logarithm of the activity coefficient of one electrolyte and the molality of the second electrolyte in a mixture with constant overall molality is not fulfilled, then a further term is added, including the square of the appropriate molality ... 

If the molecular species of the solute present in solution is the same as those present in the crystals (as would be the case for nonelectrolytes), then to a first approximation, the solubility of each enantiomer in a conglomerate is unaffected by the presence of the other enantiomer. If the solutions are not dilute, however, the presence of one enantiomer will influence the activity coefficient of the other and thereby affect its solubility to some extent. Thus, the solubility of a racemic conglomerate is equal to twice that of the individual enantiomer. This relation is known as Meyerhoffer s double solubility rule [147]. If the solubilities are expressed as mole fractions, then the solubility curves are straight lines, parallel to sides SD and SL of the triangle in Fig. 24. 

Activity coefficients in concentrated solutions are often described using Harned s rule (l ). This rule states that for a ternary solution at constant total molality the logarithm of the activity coefficient of each electrolyte is proportional to the molality of the other electrolyte. The expressions for the activity coefficients are written ... 

For dilute solutions, Equations 4 and 5 reduce to the Bronsted-Guggenheim equations, and the parameters a23 and cu2 can be expressed in terms of the interaction parameters of tne Bronsted-Guggenheim theory. For concentrated solutions, Harned s rule is a simple empirical extension of the Brb nsted-Guggenheim theory. Thus, 1t 1s surprising how well the rule describes activity coefficients 1n highly concentrated solutions. 

The success of Harned s rule for ternary solutions is largely fortuitous, and the rule has no theoretical basis to expect that it would be useful for solutions containing more than two electrolytes. Furthermore, for high concentrations of several electrolytes, activity coefficients such as Y3(g are hypothetical. There are, unfortunately, few experimental data available to test Harned s rule for concentrated solutions of three or more electrolytes. 

The Bronsted-Guggenheim equations provide a highly satisfactory description of the activity coefficients in dilute solutions however, their empirical extension to concentrated solutions (Harned s rule) introduces several serious problems. 

Equations 11 and 12 were fit to the experimental activity coefficients of HC1 and NaCl as described by Harned s rule. 

For the ternary solution, the Gibbs-Duhem equation can be easily integrated to calculate the activity coefficient of water when the expressions for the activity coefficients of the electrolytes are written at constant molality. For Harned s rule, integration of the Gibbs-Duhem equation gives the activity of water as ... 

The Gibbs phase rule is the basis for organizing the models. In general, the number of independent variables (degrees of freedom) is equal to the number of variables minus the number of independent relationships. For each unique phase equilibria, we may write one independent relationship. In addition to this (with no other special stipulations), we may write one additional independent relationship to maintain electroneutrality. Table I summarizes the chemical constituents considered as variables in this study and by means of chemical reactions depicts independent relationships. (Throughout the paper, activity coefficients are calculated by the Debye-Hiickel relationship). Since there are no data available on pressure dependence, pressure is considered a constant at 1 atm. Sulfate and chloride are not considered variables because little specific data concerning their equilibria are available. Sulfate may be involved in a redox reaction with iron sulfides (e.g., hydrotroilite), and/or it may be in equilibrium with barite (BaS04) or some solid solution combinations. Chloride may reach no simple chemical equilibrium with respect to a phase. Therefore, these two ions are considered only to the... 

The approximation that the MgCl2 mean ionic activity coefficient remains constant is introduced into the linkage relation by first expanding Eq. (21.26) by the standard chain rule ... 

The properties of the stationary phase manifest themselves in the activity coefficient in eqn.(3.6). A very simple expression for the activity coefficient can be obtained from the concept of solubility parameters (see section 2.3.1). This expression can be seen as a special form of Hildebrand s regular mixing rule, and it reads [303]. 

These equations allow calculation of activity coefficients based on Henry s law from activity coefficients based on the Lewis/Randall rule. In the limit as Xi - 0,... 

In order to correlate the results obtained, a modified SRK equation of state with Huron-Vidal mixing rules was used. Details about the model are reported in the paper by Soave et al. [16]. This approach is particularly adequated when experimental values of the critical temperature and pressure are not available as it was the case for limonene and linalool. Note that the flexibility of the thermodynamic model to reproduce high-pressure vapor-liquid equilibrium data is ensured by the use of the Huron-Vidal mixing rules and a NRTL activity coefficient model at infinite pressures. Calculation results are reported as continuous curves in figure 2 for the C02-linalool system and in figure 3 for C02-limonene. Note that the same parameters values were used to correlated the data of C02-limonene at 45, 50 e 60 °C. 

As far as mixed strong electrolyte solutions are concerned, Harned s rule [38] holds. At constant ionic strength, the activity coefficient of one electrolyte (A) in the mixture is a function of the fractional ionic strength (y I) of the other electrolyte (B) ... 

At the other end of the spectrum, Inverse sequences are observed for TiOj, a-FegOg and y-AljOg. Now the sites are the relatively small sRO groups which have a relatively strong electric field in their neighbourhood, and therefore prefer the smaller Ions. Thus, phenomenologically speaking, for site-adsorption the "like seeks like" rule seems to apply. This rule is also observed for ionic interactions in electrolytes, as expressed in the activity coefficients (sec. 1.5.4). 

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