In microbial syngas fermentation, the gaseous substrates, such as CO and H2, require trans­port from gas phase to the cell surface (Vega et al., 1990). In that case, the gaseous substrate is first absorbed at the gas-liquid interface and then diffused through the culture media to the cells. Microbes consume the diffused substrates as their carbon and energy sources and pro­duce the metabolites such as biofuel and other byproducts.

There are several intermediate steps involved in transporting substrate gases into the microbial cells. These steps include the diffusion through the bulk gas to the gas-liquid interface, moving across the gas-liquid interface, transport into the bulk liquid surrounding the microbial cells, and the diffusive transport through the liquid-solid boundary. Out of these, the gas-liquid inter­face mass transfer is the major resistance for gaseous substrate diffusion (Klasson et al., 1992).

Poor solubility of a gaseous substrate in the culture media results in low substrate uptake by microbes and, thus, leads to low productivity. The volumetric mass transfer coefficient (kLa) is often used to quantify the solubility of a gas in the liquid phase. Klasson et al. (1992) proposed the following equation (Equation (10)) to calculate the mass transfer coeffi­cient (kL) in the liquid phase.

where N(?(mol) is the molar substrate transferred from the gas phase, VL (L) is the volume of the reactor, PG and Pi (atm) are the partial pressures of the gaseous substrate in gas and the liquid phase, H (Latm/mol) is Henry’s law constant, and a (m2/L) is the gas-liquid interfacial area for unit volume.

The difference in the partial pressures of the gaseous substrate (PG — P| ) is the driving force for mass transfer and thus controls the solubility of the substrate. High-pressure opera­tion improves the solubility of the gas in aqueous phase. However, at higher concentrations of gaseous substrates, especially CO, anaerobic microorganisms are inhibited. Therefore, the determination of a correlation between the substrate diffusion and the specific substrate uptake rate (qs) (h-1) is important in order to evaluate the process kinetics (Equation (11)).

where qm (h-1), W (atm) and Kp(atm) are empirical constants. Furthermore, Qs (mol/L h), the substrate uptake rate, can be written as Qs = qsX, where X (mol/L) is the microbial cell concentration. By comparing Equations (10) and (11), it is evident that the difference between the partial pressures of the substrate gases and the cell concentration of the reactor is directly proportional (Vega et al., 1990).

Different approaches such as high gas and liquid flow rates, large specific gas-liquid interfacial areas, increased pressure, different reactor configurations (Munasinghe and Khanal, 2010 (b)), innovative impeller designs, modified fluid flow patterns, varying mixing times and speeds, and the use of microbubble dispersers have been examined to enhance gas solubility in the liquid phase. Many of these approaches increase the agitator’s power input to volume ratio which facilitates bubble breakup, and increases the interfacial surface area available for mass transfer. This approach, however, is not economically attrac­tive for commercial syngas fermentation due to high energy costs. Additionally, higher power inputs can also damage the sensitive microorganisms in the culture media. In order to achieve energy efficient mass transfer, alternative bioreactor configurations such as trick­ling beds and airlift reactors were examined for syngas fermentation (Bredwell et al., 1999; Munasinghe and Khanal, 2010 (b)).

Yang et al. (2001) reported kLa values of 54 and 119 h-1 for H2 and CO gases, respectively, in a slurry bubble column operated at temperature of 20 °C and pressure of 10 bar. In a separate study, Bredwell et al. (1999) reported a maximum kLa of 190 and 75 h-1 for H2 gas in a stirred — tank reactor at a mixing speed of 300 rpm with and without microbubble sparging, respec­tively. The authors used a mixed culture of sulfate-reducing bacteria (SRB) in their study. Munasinghe and Khanal (2010 (b)) reported kLa values for CO, ranging from 0.4 to 91 L/h for eight different reactor configurations including a submerged composite hollow fiber membrane (CHFM) reactor. Some of the reported values of kLa for different reactor configurations under various hydrodynamic conditions are shown in Table 2.