Mass transfer in external‐loop airlift bioreactors using static mixers

The influence of static mixers on the overall gas‐liquid volumetric mass transfer coefficient ( K L a L ) was examined in an external‐loop type airlift bioreactor (approximately 15 L volume, 1.8 m static liquid height, A r /A d = 0.444). The study was conducted with aqueous salt solution (0.15 kmol ṁ m −3 NaCl) and with pseudoplastic solutions of carboxymethyl cellulose (0.2 − 0.6 wt./vol. % (g/100 mL) CMC). Over a broad range of power law parameters K (10 −3 − 10 Pa ṁ s n ) and n (0.5 − 1.0), the presence of static mixers in the riser was found to enhance the K L a L relative to mixer‐free mode of operation. The extent of increase in K L a L depended on the fluid “thickness”, K : the higher the K , the greater the K L a L intensification due to static mixers. For otherwise identical conditions, the presence of static mixers improved K L a L by 30‐500%, depending on the fluid. The boost in K L a L was associated with increased gas holdup and gas‐liquid interfacial area, which arose due to bubble breakup accomplished by the static mixing elements. Potential advantages of static mixers in upgrading the performance of oxygen‐limited fermentations were pointed out.