Intensification of gas–liquid mass transfer using a rotating bed of porous packings for application to an E. coli batch fermentation process

In the present study, the intensification of oxygen transfer in an E. coli fermentation system has been investigated by employing various types of highly porous packing elements as agitators in batch fermentors. Three types of porous materials namely Declon© mesh, compact fibre mesh and knitted stainless steel wire mesh were first characterised in terms of their mass transfer performance and their power consumption in air–water and air/50% water–50% glycerol (v/v) mixture representing Newtonian liquid viscosities of 0.7 and 4.7 mNs/m2, respectively, at 35 °C. Comparisons between the porous impellers and a conventional double Rushton turbine impeller indicate the transfer of oxygen into the liquid medium is approximately doubled with the knitted wire mesh design for the range of agitator speeds tested at 1 vvm aeration rate. For example, a volumetric mass transfer coefficient, KLa, of 0.048 s−1 is achieved at a power input of 1000 W/m3 using 40 cm length of knitted wire mesh attached to the shaft in air/water system at 1 vvm of aeration rate compared to KLa of 0.021 s−1 obtained with the double Rushton turbine at the same power input. The degree of mass transfer enhancement is even greater with the knitted wire mesh in the more viscous liquid system compared to the Rushton turbine at similar power inputs. The potential for achieving the principle of process intensification is demonstrated by the knitted wire mesh where mass transfer performance is improved without increased power consumption through a more efficient rotating packed bed design.Preliminary tests to evaluate the application of the porous impeller designs in real fermentation systems show that the knitted wire mesh impeller resulted in higher overall E. coli cell growth rate than the Rushton turbine. This may be attributed to its higher oxygen transfer capability than the Rushton turbine. More tests are currently underway to study these effects in more detail.