Reactor comparison and scale-up for the microaerobic production of 2,3-butanediol by Enterobacter aerogenes at constant oxygen transfer rate

Stirred tank (STR), bubble column (BCR) and airlift (ALR) bioreactors of 0.05 and 1.5 m³ total volume were compared for the production of 2,3-butanediol using Enterobacter aerogenes under microaerobic conditions. Batch fermentations were carried out at constant oxygen transfer rate (OTR=35 mmol/lh). At 0.05 m³ scale, the STR reactor achieved much higher biomass and product concentrations than the BCR and ALR reactors. At 1.5 m³ scale, however, exactly the same biomass and product concentrations could be obtained in both STR and ALR reactors. The 1.5 m³ ALR reactor performed also much better than its counterpart at small scale, achieving a productivity 2.4-fold as high as that of the 0.05 m³ BCL and ALR reactors. No differences in performances were observed between BCR and ALR. As compared to STR the tower reactors have a 12 time higher energetic efficiency (referred to product formation) and thus should be the choice for large scale production of 2,3-butanediol.The criterion of constant OTR or constant k _L a is not applicable for the scale-up of this oxygen-sensitive culture due to strong influence of reactor hydrodynamics under microaerobic conditions. The effects of mixing and circulation time on growth and metabolism of E. aerogenes were quantitatively studied in scaled-down experiments with continuous culture. For a successful scale-up of this microaerobic culture it is necessary to have an homogeneous oxygen supply over the entire reactor volume. Under conditions of inhomogeneous oxygen supply an optimum liquid circulation time exists which gives a maximum production of 2,3-butanediol.List of Symbols BD 2,3-butanediol - mmol/l] saturation value of dissolved oxygen - D h^–1] dilution rate - D mm] reactor diameter - D _K mm] top section diameter - D _R mm] stirrer diameter - D _S mm] draft tube diameter - EtOH ethanol - E _P kg/kWh] energy efficiency refered to product formation - H mm] height of reactor - HAc acetate - H _L mm] height of liquid - k _L a h^–1] volumetric oxygen transfer coefficient - N rpm=min^–1] stirrer speed - OTR mmol/lh] oxygen transfer rate - OUR mmol/lh] oxygen uptake rate - p Pa] pressure - P kW] power input - P/V _L kW/m³] specific power input - mmHg] oxygen partial pressure (mmHg) or - mmol/l] dissolved oxygen (mmol/l) - mmol/gh] specific oxygen uptake rate - q _P mmol/gh] specific productivity - R Nm/kgK] gas constant, R = 287.06 - RQ respiration quotient - t _c s] liquid circulation time - T °C or K] temperature - TCA tricarboxylic acid - u _G cm/s] mean superficial gas velocity - v _G m/s] gas velocity at nozzels of gas distributor - V_G l/h] aeration rate at inlet - V m³ or l] total volume - V _L m³ or l] liquid volume - V _N l/mol] gas mole volume under normal conditions, V _N = 24.4116 - X g/l] biomass concentration - CO₂ mole fraction in the effluent gas - O₂ mole fraction in the effluent gas - inlet (above the gas distributor) - ratio of oxygen consumed through TCA cycle to the total oxygen uptake rate - g/l or kg/m³] density - %] degree homogeneity - outlet of fermenter or top of the dispersion phase Dedicated to the 65th birthday of Professor Fritz Wagner.We thank Dr. C. Posten and T. Gabel for support with the computer control system UBICON. T.-G. Byun gratefully acknowledges financial support by DAAD.