(31e) Equilibrium Study on Reactive Extraction of Lactic Acid with Tri-N-Butyl Phosphate in N-Hexane

Extraction of carboxylic acids from dilute aqueous solution using traditional solvents such as ketones, alcohols, ethers, and ester is inefficient because the distribution coefficient is rather low. Reactive extraction which exploits reversible chemical complexation is an effective separation process for extraction of carboxylic acids from aqueous streams such as fermentation broths and wastewaters. In the extraction process, selection of the solvent is an important aspect to be considered. Considering its solubility in water, cost and availability, tri-n-butyl phosphate (TBP) seems to be an attractive solvent for the extraction of lactic acid from aqueous solution.

The purpose of this experiment is to study the equilibrium of the reactive extraction of lactic acid in aqueous solution with TBP in n-hexane. The parameters studied in this experiment were initial concentration of lactic acid in the aqueous phase, TBP concentration in n-hexane phase, and the extraction temperature. The experiments at ambient temperature were carried out using a separatory funnel, while the experiments at other than ambient temperature were carried out using erlenmeyer flask and water bath shaker to adjust the temperature. In this experiment, the initial concentration of lactic acid was varied from 0.1 to 0.5 gmol/dm³. The range of initial TBP concentrations in n-hexane was 0.1 to 1.0 gmol/dm³ and the extraction temperature range was 283 to 313 K.

The experimental results showed that the higher the initial concentration of lactic acid in aqueous solution the higher the distribution coefficient for fixed TBP concentration and extraction temperature. For fixed initial concentration of lactic acid in aqueous solution and extraction temperature, the distribution coefficient of lactic acid was increased by increasing TBP concentration. The overall equilibrium constants (K_pq) for the experiments using TBP concentration ranging from 0.1 to 1.0 gmol/dm³ at the extraction temperature of 293 K were calculated to be 0.0668 to 0.5144. K_pq for the experiments at the temperature ranging from 283 to 313 K at the initial temperature of lactic acid of 0.2 gmol/dm³ were found to be 0.0122 to 0.8856. The K_pq as a function of temperature (T) in K can be expressed as ln K_pq = 10,596/T-38.08 with sum of square of error of 0.14.

Keywords: Reactive Extraction, Equilibrium, Lactic Acid, Tri-n-Butyl Phosphate,n-Hexane.