(265e) Reactive Extraction of Pyruvic Acid Using TOA in Octanol From Single and Mixed Acid Solutions

Interest in pyruvic acid has been increasing, due to an increase in its potential areas of use as well as its significant role in metabolic reactions (Xu, 2008). Additionally high production cost of conventional processes has caused researchers to look for alternative methods of production. Thus in recent decades, biocatalytic reactions have replaced conventional synthesis. Production methods involving fermentation , resting cells and enzymes are becoming increasingly important (Li, 2001).

Reactive extraction has been shown to be a promising technique with its high distribution coefficients compared to other recovery methods. Reactive extraction consists of both forward and backward extraction steps. Some carboxylic acids, i.e. lactic, citric and acetic acids were successfully separated by reactive extraction (Wasewar, 2002). As described by King and his co-workers (1990), the nature of the solute, concentrations of the solute and the extractant, and the type of diluent in the organic phase are important parameters affecting the separation. A well-planned kinetic investigation is required to complete the design of a separation system which is to be coupled with the production medium

In the present study, equilibrium and kinetic data on the reactive extraction of pyruvic acid were obtained. While distribution coefficients, loading ratios and equilibrium constants were obtained through the equilibrium studies, reaction rates and orders were calculated from the kinetic data. In addition to studies of reactive extraction with the single acid, pyruvic acid was also separated from mixed acid solutions comprising three carboxylic acids

As expected, it was observed that octanol compared to oleyl alcohol yielded much higher distribution coefficients in pure solvent extraction of pyruvic acid because of having higher polarity. When tertiary amines dissolved in diluents were used as the organic phase, the trend was not changed. On the other hand, oleyl alcohol had the disadvantage of forming an additional third emulsion phase in between the two phases (Gurkan et al., 2006). It was observed that the distribution coefficients with TOA and Alamine 336 were not significantly different (Marti and Gurkan, 2006). The results showed that because the extractant in the organic phase was the limiting reagent for reversible complexation of carboxylic acid in forward extraction, the distribution coefficients were low at higher aqueous phase concentrations, while high at higher organic phase and lower aqueous phase concentrations.

It was seen that as the initial pH value increased, the distribution coefficient and the loading ratio decreased. The most significant reason for this is that tertiary amine reacts with the undissociated portion of the acid while forming an acid-amine complex in the organic phase. The results showed that at a pH value of two units higher than the pKa value of the acid, there was practically no extraction of acetic, lactic and pyruvic acids. Comparison of the distribution coefficient of the acids at the same pH value showed that, the higher the pKa value of the acid was, the higher was the distribution coefficient . This sequence of distribution coefficients led to a selective recovery scheme of carboxylic acids from the production medium by the adjustment of the pH.

To optimize the process conditions for the recovery of pyruvic acid, a mixed acid solution that had an equivalent carboxylic acid composition of a biological production medium comprised of 0.75 M pyruvic, 0.08 M acetic, and 0.05 M lactic acid was utilized. This composition was chosen in accordance with earlier published data. The results showed that due to its low concentration in solution and also due to possessing the highest pKa value among the acids, acetic acid was preferentially transferred at low extractant concentrations. An increase of extractant concentration brought in an increase in the distribution coefficient of pyruvic acid (Marti and Gurkan 2008). Lactic acid has a higher pKa value than pyruvic acid but because of the presence of the hydroxyl group on lactic acid which enhances its hydrophilic character, preventing its extraction.

To evaluate the kinetic data, a comprehensive study based on the theory of extraction accompanied with chemical reaction, as proposed by Doraiswamy and Sharma (1984) is being carried out. Preliminary experiments in a stirred cell has shown that the reaction between pyruvic acid and TOA in octanol falls in Regime 3, that is extraction accompanied by a fast chemical reaction in the diffusion film. The reaction was found to be first order with respect to trioctylamine and pyruvic acid and second order rate constant was calculated as 0.94 L mol-1 s-1.

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