Although bacterial ABE (Acetone, Butanol and Ethanol) fermentations have been known for more than 150 years, widespread large-scale ABE fermentations remain absent in today's economy. Main hurdles to overcome include low productivities, strain degeneration and inefficient product recovery techniques. This work proposes and investigates concepts aimed at improving the overall efficiency of the ABE fermentation process using a partially degenerated strain of Clostridium acetobutylicum (ATCC 824). In particular, three interrelated concepts were studied in detail. First, strain degeneration, a process where-by the original solvent-producing strain mutates or is outgrown by a degenerated version incapable of solventogenesis, was investigated. Here, two viable alternatives to circumvent the problem of strain degeneration of solvent producing strains of ATCC 824 are being proposed. They consist of (i) focusing on the precursor of solvents, namely organic acids and (ii) applying a technique of systematic pH control that can trigger the degenerated culture into solventogenesis. Second, a reactor system consisting of a plug flow reactor (PFR) and two continuously stirred tank reactors (CSTR) was studied. This setup was capable of avoiding strain degeneration while maintaining high final titers and volumetric productivities. The reactor system was continuously operated for 40 days while the effects of dilution rate and substrate composition were investigated. The highest combined product titers for both ABE and organic acids could be achieved when the ABE fermentation was followed by an organic acid fermentation. Here, the final combined product concentration reached 27.7 g/l. The reactor system was modeled using a set of differential equations reflecting the biochemical and physiological behavior of the culture. Finally, product removal was studied using adsorption onto activated carbon and liquid-liquid extraction. ItCould be shown that both techniques are capable of recovering equi-molar ratios of organic acids and alcohols into a micro-aqueous system. Subsequent esterification was investigated using a lipase as biocatalyst. Reaction mechanisms and kinetic models for the enzymatic conversion are being proposed.
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