(422d) Process Concept for Isolation of Low Molecular Weight Carboxylic Acids from Dilute Aqueous Feed

Market launch of biobased bulk products, such as low molecular weight carboxylic acids, alcohols and ketones, very often suffers from poor economics. Low concentration of target constituents is a major obstacle. Process intensification is a promising approach for addressing the economic feasibility of biobased bulk products.

Liquid-liquid extraction with chemical reaction in emulsified state offers excellent technological features for application in this challenge. This concept, successfully applied in multiphase homogeneous catalysis, was investigated for acetic acid isolation by esterification from dilute aqueous effluents with feed concentration of less than 60 g L^-1. Conversion rates of up to 55%, compared to conversion of just 4% esterification without catalysis were achieved. For further optimization of this process concept, investigating parameter interaction was necessary to maximize conversion. Design of experiments (DoE) was applied to evaluate relevant process parameters temperature, stoichiometric ratio and catalyst load in batch experiments. Evaluation of design of experiments revealed a possible mass transfer limitation for low phase ratio. Based on the DoE results a model was fitted with high reproducibility and agreement with experimental data. Accuracy of conversion prediction with the model was experimentally verified and confirmed. Downstream processing via transesterification by reactive distillation was applied to complete the technology concept.

n‑octanol, diluted in the solvent n‑undecane, was used as shuttle reactant. n-octanol was chosen because of its negligible water solubility. The anionic surfactant 4‑dodecylbenzenesulfonic acid catalyzed the esterification reaction. Emulsification during conversion provided large mass transfer area. Regeneration of the shuttle reactant n-octanol was performed by transesterification with short chain aliphatic alcohols (e.g. methanol) after emulsion splitting by centrifugation. The low molecular weight products methyl acetate/methanol can simply be separated from the reaction broth by single step distillation due to the huge boiling point difference. It has to be mentioned that the bottom stream of the distillation step may be recycled to the esterification without temperature adjustment. Reaction kinetics of esterification and transesterification for varying stoichiometric ratio and operation temperature level were subject of detailed analysis. For validation, the process concept was further tested for the low molecular weight carboxylic acids formic acid and propionic acid.