Continuous extractive fermentation using hollow-fiber membrane extractors for enhanced propionate production

A membrane based extractive fermentation process with continuous substrate feeding and continuous cell free product removal was developed. The research includes three parts: equilibrium studies of amine-based carboxylic acid extraction, mass transfer studies of amine-based extraction in a hollow fiber system, and an evaluation of an extractive fermentation process for enhanced propionic acid production.; First, the thermodynamic aspect of amine-based solvent extraction was studied. Amine based extraction was considered as a molecular form reaction in organic phase combined with an ionic form reaction in aqueous phase. The distribution coefficient (K{dollar}rmsb{lcub}d{rcub}){dollar} was studied as the function of amine content in the organic phase and the initial acid concentration in the aqueous phase. Anion interference in the amine-based extraction was attributed to the competition for amine between inorganic acid and organic acid. Computer simulation based on this mechanism was consistent with the experimental observation.; Then, the mass transfer of amine-based extraction in a hollow fiber membrane extraction system with extraction followed by solvent regeneration by base stripping was studied. A reaction model instead of a conventional extraction model was developed. The model assumed that the reaction was instantaneous and happened only at the phase interface. The computer simulation was consistent with the experimental results and the model parameters were determined by fitting the predicted results to the experimental data. This model, which includes all the operation variables and equipment conditions, may serve as a basis for further scale-up.; The fundamentals of an organic acid extractive fermentation were studied further. Two critical problems: solvent toxicity and different pH requirement for extraction and fermentation in developing such a process, were resolved. The acid salt concentration can be used to raise pH in the fermentor without losing the extraction ability of the solvent for the fermentation product (acid). Solvent toxicity can also be avoided without reducing extraction power by using low amine content stoichiometrically equal to the acid concentration in the fermentor. The operation mode of continuous fed batch with no pH control is considered critical for organic acid extractive fermentation. A mathematical model was developed for an extractive fermentation process. The effects of operation variables such as amine content, circulation flow rate, and salt concentration were studied by computer simulation. An experimental demonstration of such a process was then run for 1.5 months. The propionate productivity of the extractive fermentation was 1.2 g/L/h, 300% higher than that from the batch fermentation. The propionate yield was increased from 45% g/g in the conventional batch to 58% g/g in the extractive fermentation. The ratio of propionic acid to acetic acid was 6:1, compared with 3:1 in the batch. The final propionate concentration obtained in the extractive fermentation was 7.5% (w/v) compared to only 3.0% (w/v) in the conventional batch fermentation.