(517b) Design and Performance of Amine Based Solvents

Liquid-liquid extraction of biobased products such as acetic acid, hydroxypropionic acid etc. with tertiary amines is seemingly on a rise again. Reactive solvents based on tertiary amines have not established in acid extraction with distillative recovery due to strong chemical interaction between acid and amine. However, applications in liquid-liquid extraction with caustic recovery may gain attention in industrial applications. The design and performance of triisooctylamine based solvent mixtures in liquid-liquid extraction of the bulk chemical acetic acid was investigated.

Amine based solvents need admixture of modifiers and diluents to correct the liquid-liquid partition characteristic of aqueous acids with the pure extractant, and to correct the physical properties of the laden solvent by adjusting density, interfacial tension and viscosity, and to avoid third phase formation.

To gain data about the effect of solvent composition on liquid-liquid partition and mass transfer of acetic acid with tertiary amine based solvents liquid-liquid extraction of acetic acid was investigated with solvent mixtures comprising triisooctylamine (TiOA), dissolved in the industrial diluent Shellsol T (SST), and modified with either the Lewis basic modifier tributyl phosphate (TBP) or the Lewis acidic modifier isodecylalcohol (ISO). The modifiers were investigated for their potential of adjusting the association of acetic acid and triisooctylamine. Investigations considered the determination of liquid-liquid equilibrium data and the effect of solvent composition on mass transfer. The liquid-liquid equilibrium data were obtained from temperature controlled shaking funnel experiments. The mass transfer data were obtained from single droplet mass transfer experiments. Phase transfer of acetic acid starts with protonation of TiOA at the interface, followed by salt formation with the acetate anion and transfer of the triisooctylammonium acetate salt into the solvent phase. In principle the salt may stabilize the interface and retard mass transfer of the constituent acetic acid. On the other hand enhancement of phase transfer may be expected from neutralisation of acetic acid with triisooctylamine. To account for mass transfer phenomena, different solvent mixtures as well as different acid concentration in the aqueous phase and the effect of preloaded solvent on mass transfer were investigated. By preloading the solvent with the solute the experimental validation of extended droplet residence time was simulated. Due to salt formation enhancement of mass transfer might be expected. However high viscosity of the solvent and the low diffusion coefficient of the amine in the solvent exclude any enhancement. Data analysis of mass transfer experiments resulted in the conclusion that mass transfer of aqueous acetic acid in amine based solvent mixtures is simply controlled by diffusion of the constituent. Solvent viscosity, adjusted by mixing of appropriate amounts of solvent constituents controls mass transfer of acetic acid in the amine based solvent droplet. Whether stabilizing effects, provided by the “surfactant” TiOA and the reaction product triisooctylammonium acetate, or just viscosity or both effects lead to a significant difference between the dynamically obtained solvent load from single droplet mass transfer experiments and the liquid-liquid equilibrium partition, cannot be concluded from experiments.