However, using high gas flow rates decreases the gas conversion efficiency. The increase in agitation speed has been widely used to increase the kLa in STRs. The hydrodynamic shear generated by the impeller reduces the bubble size and increases the interfacial area for mass transfer. However, the use of high agitation speed increases the power requirement for large reactors. reported a dual impeller scheme with axial flow impeller at the top and lower concave impeller that resulted in a similar kLa and less power requirement compared to Rushton impellers. used a microsparger that was shown to be energy efficient and increased the kLa by six fold compared to conventional gas sparging. In the case of an air lift reactor, the use of a 20 μm bulb diffuser was reported to provide higher mass transfer coefficient (91 h-1) than air lift reactor configurations with column diffusers, gas spargers with mechanical mixing. Also, it was claimed that due to the simple reactor configuration and low energy requirements, the scale up of air lift reactors with a 20 μm bulb diffuser will be easy and cheap compared to a conventional STR.