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Propionic acid extraction from the aqueous phase is important in view of its number of uses. The recovery technique should provide high selectivity and yield. A number of techniques like distillation, membrane, dialysis, electrodialysis, and reactive extraction are available. The profound success of reactive extraction has been noted in the recovery of carboxylic acids from dilute solutions. In the present paper, equilibrium with propionic acid using tri-n-butyl phosphate, an organophosphorous compound, in eight different diluents (1-dodecanol, benzene, toluene, heptane, hexane, butyl acetate, petroleum ether, and paraffin liquid) has been studied. Equilibrium parameters such as distribution coefficients, loading ratio, degree of extraction, and equilibrium complexation constants have been presented. The data obtained are useful in understanding the equilibrium characteristics and efficient design of the recovery process of propionic acid by reactive extraction.

Reactive extraction of propionic acid from aqueous solution containing 0.05 to 0.4 kmol/m3 acid using tri-n-butylphosphate (TBP) (organophosphorous compound), tri-n-octylamine (TOA) (tertiary amine), and Aliquat 336 (quaternary amine) as extractants in 1-octanol was studied. The comparison among the different categories of extractants was made. The order of extraction power was found to be TOA > Aliquat 336 > TBP with K E values of 25.67, 3.58, and 2.36 m3/kmol, respectively. The highest value of equilibrium complexation constant (25.67 m3/kmol) and distribution coefficient (14.09) of TOA suggested it to be the best extractant among the three. The greatest support to highest recovery was also due to the choice of diluent for the system. 1-Octanol was an active diluent which provided good solvation of the acid-extractant complexes. In all the extractions (1:1) acid-extractant complexes were formed with loading ratios less than 0.5, except in Aliquat where values higher than 0.5 were also obtained. This led to the definition of an another equilibrium constant (K E(2:1)) as (2:1) acid−Aliquat complexes were formed, with the value of K E(2:1) = 26.54 (m3/kmol)2.

Reactive extractions of acrylic, propionic, and butyric acids using Aliquat 336 in oleyl alcohol were carried out to study the effects of temperature (305-333 K). The effects of temperature on the partition (P) and dimerization (D) coefficients were evaluated, and it was found that P decreases with increasing temperature, whereas the effects of temperature on D vary. Chemical extractions using Aliquat 336 in oleyl alcohol at temperatures ranging from 305 to 333 K show an increase in K E(1:1) values with temperature up to 313 K for acrylic and propionic acids but a decrease with increasing temperature for butyric acid over the range studied. Differences in the hydrophobicity and octanol-water partition coefficient are suggested as the reasons for the differences in extraction by the respective acids using Aliquat 336 in oleyl alcohol. The enthalpy (∆H) and entropy (∆S) of reaction were evaluated at different temperatures, and their difference was assessed in terms of different parameters.

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