Equilibrium studies on lactic acid extraction with N,N-didodecylpyridin-4-amine (DDAP) extractant

The extraction of aqueous lactic acid (LA) by the extractant N,N-didodecylpyridin-4-amine (DDAP) diluted in 1-octanol was studied at three temperatures (298 K, 310 K and 328 K) in a batch system for an initial concentration of 0.013 M (0.12 wt%) and a range of Vorg/Vaq ratios (0.02–1). A reactive extraction model based on mass action law was applied to describe the experimental data. The parameters of this model, including aqueous phase dissociation, and a heterogeneous complexation with stoichiometry coefficient and complexation constant were fitted to the experimental data. The aqueous phase dissociation constants were fitted for the three temperatures, from which the enthalpy and entropy of dissociation were found using a Van׳t Hoff-plot. The found values are ΔHd=−2.12×104J mol−1 and ΔSd=1.44×102 J mol−1 K−1. The equilibrium constants for 298 K, 310 K and 328 K were determined to be 570.42 L/mol, 349.77 L/mol and 175.46 L/mol, respectively. The stoichiometry coefficient was found independent on temperature, having a value of 1.2. Using a Van׳t Hoff plot, the thermodynamic constants for the complexation were estimated at ΔHc=−3.19×104 J mol−1 and ΔSc=−0.54×102 J mol−1 K−1. The developed model was used for short-cut calculations on multistage extraction – back extraction operations. By applying back-extraction at higher temperature, at atmospheric pressure and T=373 K with wash/extract ratio (W/E)=5.4, the initial 0.013 mol/L could be concentrated to between 0.041 mol/L and 0.078 mol/L lactic acid (concentration factor ranging from 3.15 to 6.06). Pressurized recovery at 413 K allows for concentrations up to 0.112–0.21 M (concentration factor ranging from 8.66 to 16.66). Even though such results are clearly outperforming the previously reported values, additional polishing techniques are required (i.e. evaporation) to obtain pure product of high quality.