Monitoring gradient formation in a jet aerated bioreactor.

Jet aerated loop reactors (JLRs) provide high mass transfer coefficients (kLa) and can be used for the intensification of mass transfer limited reactions. The jet loop reactor achieves higher kLa values than a stirred tank reactor (STR). The improvement relies on significantly higher local power inputs (∼104) than those obtainable with the STR. Operation at high local turnover rates requires efficient macromixing, otherwise reactor inhomogeneities might occur. If sufficient homogenization is not achieved, the selectivity of the reaction and the respective yields are decreased. Therefore, the balance between mixing and mass transfer in jet loop reactors is a critical design aspect. Monitoring the dissolved oxygen levels during the turnover of a steady sodium sulfite feed implied the abundance of gradients in the JLR. Prolonged mixing times at identical power input and aeration rates (∼100%) were identified for the JLR in comparison to the STR. The insertion of a draft tube to the JLR led to a more homogenous dissolved oxygen distribution, but unfortunately a reduction of mixing time was not achieved. In case of increased medium viscosities as they may arise in high cell density cultivations, no gradient formation was detected. However, differences in medium viscosity significantly altered the mass transfer and mixing performance of the JLR.

Jet aeration as alternative to overcome mass transfer limitation of stirred bioreactors.

In industrial biotechnology increasing reactor volumes have the potential to reduce production costs. Whenever the achievable space time yield is determined by the mass transfer performance of the reactor, energy efficiency plays an important role to meet the requirements regarding low investment and operating costs. Based on theoretical calculations, compared to bubble column, airlift reactor, and aerated stirred tank, the jet loop reactor shows the potential for an enhanced energetic efficiency at high mass transfer rates. Interestingly, its technical application in standard biotechnological production processes has not yet been realized. Compared to a stirred tank reactor powered by Rushton turbines, maximum oxygen transfer rates about 200% higher were achieved in a jet loop reactor at identical power input in a fed batch fermentation process. Moreover, a model-based analysis of yield coefficients and growth kinetics showed that E. coli can be cultivated in jet loop reactors without significant differences in biomass growth. Based on an aerobic fermentation process, the assessment of energetic oxygen transfer efficiency [kgO2 kW-1 h-1] for a jet loop reactor yielded an improvement of almost 100%. The jet loop reactor could be operated at mass transfer rates 67% higher compared to a stirred tank. Thus, an increase of 40% in maximum space time yield [kg m-3 h-1] could be observed.

Impact of nozzle operation on mass transfer in jet aerated loop reactors. Characterization and comparison to an aerated stirred tank reactor.

The impact of mass transfer on productivity can become a crucial aspect in the fermentative production of bulk chemicals. For highly aerobic bioprocesses the oxygen transfer rate (OTR) and productivity are coupled. The achievable space time yields can often be correlated to the mass transfer performance of the respective bioreactor. The oxygen mass transfer capability of a jet aerated loop reactor is discussed in terms of the volumetric oxygen mass transfer coefficient kLa [h-1] and the energetic oxygen transfer efficiency E [kgO2 kW-1 h-1]. The jet aerated loop reactor (JLR) is compared to the frequently deployed aerated stirred tank reactor. In jet aerated reactors high local power densities in the mixing zone allow higher mass transfer rates, compared to aerated stirred tank reactors. When both reactors are operated at identical volumetric power input and aeration rates, local kLa values up to 1.5 times higher are possible with the JLR. High dispersion efficiencies in the JLR can be maintained even if the nozzle is supplied with pressurized gas. For increased oxygen demands (above 120 mmol L-1 h-1) improved energetic oxygen transfer efficiencies of up to 100 % were found for a JLR compared to an aerated stirred tank reactor operating with Rushton turbines.