(620bo) Driving Potentials for Mass Transfer, Reaction Kinetics and Thermodynamics Define Syngas Fermentation to Produce Ethanol

Several acetogenic bacteria can produce ethanol from CO, CO₂ and H₂ supplied from synthesis gas. The production reactions are defined by the Wood-Ljungdahl pathway, and cell mass, ethanol, acetic acid and other chemicals are built from these inorganic molecules. Fermentation of the sparingly soluble gas substrates CO and H₂ requires robust gas to liquid mass transfer to sustain production through the pathway reactions. Further, the dissolved concentrations of CO, CO₂ and H₂ determine the thermodynamic state of the syngas fermentation and the kinetics of production.

CO inhibits hydrogenase activity in bacteria, so that uptake of H₂ concurrent with CO consumption in active fermentation indicates very low dissolved CO concentration (c_CO) and CO is mass transfer limited. Under mass transfer limitation (c_CO ≈ 0) in a stirred tank fermenter (CSTR), the liquid film volumetric mass transfer coefficient for CO (k_L,COa) is calculated from the observed  inlet and effluent partial pressures of CO. Then k_L,CO2a and k_L,H2a derived from k_L,COa are used to calculate the dissolved concentrations c_CO2 and c_H2.

Observed conversion of CO and H₂ to acetic acid and ethanol confirms that each reaction in the biochemical pathway is thermodynamically favored and DG_r < 0. Concurrent uptake of CO and H₂ implies that the water gas shift reaction contained in the production pathway is in thermodynamic equilibrium, and the dissolved concentration c_CO calculated using c_CO2 and c_H2 confirms the assumed mass transfer limitation. The dissolved gas concentrations (c_CO, c_CO2 and c_H2) in the water gas shift reaction represent the electrochemical half-cells for CO and H₂ oxidation, and define the electrochemical potential inside the cells at intracellular pH. All electrochemical reactions within the active cells can be characterized at this same intracellular pH and potential by the boundary condition DG_r = 0 assuming equilibrium of the reaction.

Reduction of free acetic acid through acetaldehyde to ethanol establishes the intracellular ratio of ethanol to acetic acid c_EtOH/c_HAc as a thermodynamic parameter related to c_H2 and c_CO/c_CO2. The intracellular concentrations of ethanol and acetic acid cannot be measured directly. However, assuming that the acetate ion concentration is equal inside and outside the cell, as for assisted diffusion at equilibrium, intracellular c_EtOH/c_HAc can be estimated from c_EtOH and c_HAc measured in the fermentation broth. Intracellular pH and potential can then be determined from measurements of the macroscopic fermentation.

Fermentation in a CSTR with continuous gas and liquid flow established a near steady state condition, while analysis using the derived mathematical model established a dominant thermodynamic determinant of ethanol versus acetic acid production with culture growth supported by either product. The CSTR was operated using automated process control for over 4000 hours, achieving an ethanol concentration of about 25 g/L. The ratio c_EtOH/c_HAc+Ac was over 30, measured in the effluent fermentation broth representing moles of ethanol per mole of total acetic acid produced (free acetic acid plus acetate). Therefore, the ratio of c_EtOH/c_HAc for free acetic acid in the extracellular broth was over 60 mol/mol at pH of 4.76. However, the model analysis using these conditions estimated ethanol to free acetic acid, c_EtOH/c_HAc, over 300 mol/mol inside the cells. The developed model is a tool for controlling agitation speed and gas feed rate to achieve efficient operation of syngas fermentation reactors with sustained conversion of CO and H₂ to the desired product.