Extraction equilibrium distribution coefficients

Barbari TA, King CJ. 1982. Equilibrium distribution coefficients for extraction of chlorinated hydrocarbons and aromatics from water into undecane. Environ Sci Technol 16(9) 624-627. 

The equilibrium distribution coefficient can be calculated by material balance, using the weight of the feed F, raffinate R, and extract E, plus the weight-fraction solute in the feed ay and raffinate x when the weight-fraction solute in the extraction solvent is zero [Eq. (15-8)]. 

The ideal solvent would be easily recovered from the extract. For example, if distillation is the method of recovery, the solvent-solute mixture should have a high relative volatility, low heat of vaporization of the solute, and a high equilibrium distribution coefficient. A high distribution coefficient will translate to a low solvent requirement and a low extract rate fed to the solvent recovery column. These factors will minimize the capital and operating costs associated with the distillation system. In addition to the recovery aspects, the solvent should have a high selectivity (ratio of distribution coefficients), be immiscible with the carrier, have a low viscosity, and have a high density difference (compared to the carrier) and a moderately low interfacial tension. 

With a large equilibrium distribution coefficient D (complete trapping) and a large phase ratio, Ee will increase linearly up to large values. In Eigure 12.4, which refers to an SLM flow system experiment, D in curve 1 is approximately 10,000 and the enrichment factor is linear at least up to at least 3000 times. In many cases, especially in flow systems, the extraction is not allowed to go to equilibrium, and an extraction efficiency E is defined as the fraction of the total amount of analyte that is transferred to the acceptor. Thus... 

King, C. J., Barbari, T. A., Joshi, D. K., Bell, N. E Senetar, J. J. Equilibrium Distribution Coefficients for Extraction of Chlorinated Organic Priority Pollutants from Water -/ EPA-600/2-84-0 a U. S. Dept, of Commerce, Nationd Technical Information Service Springfield, VA, 1984. 

Another point that should be observed in extraction calculations is the non-ideal nature of the system, which is responsible for the occurrence of two liquid phases in equilibrium. The liquid-liquid equilibrium distribution coefficients, or A -values, are highly composition-dependent and must be calculated by appropriate methods, namely those based on liquid activity coefficients. The NRTL and UNIQUAC liquid activity equations (Chapter 1) are among the more accurate ones for predicting liquid-liquid equilibria. The A -Value is defined as the ratio of the mole fraction of a component in one liquid phase to its mole fraction in the other, and is calculated as... 

When coextraction of water is an important consideration, the diluent can heve a large effect on the amonm of water coextracted and also can improve the separation of water in a subrequeni distillation step. It can be worthwhile to choose a diluanl with a lower equilibrium distribution coefficient for the solute, if the ratio of solute to water extracted is increased. 

FIGURE 15.2-2 Equilibrium distribution coefficients For solvent extraction of acetic acid from aqueous solution into solvent mixtures containing Alamine 336. ... 

FIGURE 15.2-5 Equilibrium distribution coefficients vs. temperature for extraction of phenol and catechol from dilute aqueous solution into a solvent mixture of 25% w/w TOPO in DIBK. Stoichiometric ratio 2.5 mol TOPO/mol solme.M... 

Table 15.2-1 Equilibrium Distribution Coefficients for Extraction of Phenol and Higher Phenols from Water with Various Solvents...

The maximum value of extract concentration in the supercritical solvent is given by the equilibrium solubility of extractable components. Due to different transport resistances and equilibrium distribution coefficients for different compounds, they may be successively extracted at different rates. The maxi-... 

Barbarl, T. A. and King, C. J. "Equilibrium Distribution Coefficients for Extraction of Chlorinated Hydrocarbons and Aromatics from Water into Undecane," Environ. Scl. Technol., Vol, 1 No. 9, 624-627, 1982. 

Various parameters such as adsorption and desorption isotherms, diffusion coefficients, liquid/gas, gas/solid and liquid/solid equilibrium distribution coefficients, as well as mass transfer coefficients and many other physicochemical property values have to be used in the models proposed for supercritical fluid extractions. These parameter values are either obtained from existing correlations, or from independent data sources using parameter estimation. However, in those cases where the above stated means are not sufficient to estimate the values of all parameters used in the model, the researcher(s) may be forced to use the model and the associated data to evaluate best fit or optimal values for the missing parameters. The fact is that, the number of reliable correlation s and methods for the SFE are still quite scarce. 

In heterogeneous extractions, K = M Km, where M is the solute equilibrium distribution coefficient for the external and internal solvem pair. Hence, for heterogeneous extractions M = M / and usually remains fairly constant. In all cases, m ... 

This may be illustrated by the following example. Suppose that 50 mL of water containing 0.1 g of iodine are shaken with 25 mL of carbon tetrachloride. The distribution coefficient of iodine between water and carbon tetrachloride at the ordinary laboratory temperature is 1 /85, i.e. at equilibrium the iodine concentration in the aqueous layer is 1 /85th of that in the carbon tetrachloride layer. The weight of iodine remaining in the aqueous layer after one extraction with 25 mL, and also after three extractions with 8.33 mL of the solvent, can be calculated by application of the above formula. In the first case, if x, g of iodine remains in the 50 mL of water, its concentration is x,/50 gmL 1 the concentration in the carbon tetrachloride layer will be (0.1 —x1)/25gmL 1. 

Equation (31) is true only when standard chemical potentials, i.e., chemical solvation energies, of cations and anions are identical in both phases. Indeed, this occurs when two solutions in the same solvent are separated by a membrane. Hence, the Donnan equilibrium expressed in the form of Eq. (32) can be considered as a particular case of the Nernst distribution equilibrium. The distribution coefficients or distribution constants of the ions, 5 (M+) and B X ), are related to the extraction constant the... 

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