Your 100m3sparged mechanically agitated bioreactors are already operating close to oxygen limitation of your high density single-celled bacterial culture. Your process development department has now come up with scheme that uses even higher cell densities with even higher oxygen demands. What are you, the biochemical engineer, going to do about it?
Airlift bioreactors and mechanically stirred tanks are used in bio processing. Y. Chisti (1989).
There are useful because of less viscous fluids and gentle agitation and low cost oxygen transfer. In contrast, ordinary stirred fermenters have a larger range of use but they perform badly in highly viscous media, have a poorly defined mixing pattern relative to airlift reactors, and cannot be oxygenate at a high rate because of impeller flooding. A mechanically stirred hybrid airlift bioreactor with one or more failing pumping axial flow impellers located in the draft-tube (Figure. 1) and ventilation lock up to the annular zone, potentially overcome some of the limitations of both the normal stirred and the airlift tubes. This hybrid bioreactor has a highly directional flow pattern and it can deliver high rates of fluid rotation. A.W. Nienow (1998)
In viscous aerobic fermentations the performance of airlift reactors can be increase by installation of an axial flow impeller in the down comer to improve the distribution of the fluid. This approach has been shown with fermentations of the mycelia micro fungus. M. Moo-Young, Y. Chisti, D. Vlach (1993). It has also been found useful with less viscous yeast broths. In highly viscous, non-Newtonian, broths of Saccharopolyspora erythraea, the use of a marine blade located near the bottom of the draft-tube has been reported to increase the product of the antibiotic erythromycin by 45% in compare to the basic annulus-sparged airlift configuration. D.J. Pollard, A.P. Ison, P.A. Shamlou, M.D. Lilly (1998) For similar highly viscous broths of the micro fungus in annulus-sparged draft-tube reactors, Moo-Young had earlier established that the airlift configuration added feature with a low-shear axial flow impeller in the draft-tube was better than the pure airlift device. Also, the airlift-impeller hybrid reactors have been confirmed as more effective aerobic bioreactors than the ordinary, Rushton turbine-stirred fermenters. Whereas the stirred hybrid airlift reactor has been shown to perform well, little is known about such devices compared to the knowledge base for the design of normal stirred and airlift bioreactors. This work reports on hydrodynamic and mass transfer characterisation of a large impeller-assisted airlift bioreactor. Low-power hydrofoil impellers were used to increase fluid rotation in the reactor. Y. Chisti (1998)
2. Materials and methods
Great agitation can cause major problems like foaming, which can cause fermentation vessel to pollution. Antifoam cannot be always used to break the foam; it may stop the growth of the microorganism. A decent device is the ‘Funda-foam system', in which the foam is shattered by diffusive forces. The mineral solution held in the foam flows back into the bioreactor, and the air released from the foam leaves the vessel. Heat removal can be an issue in larger bioreactors, larger than 100m3, up to 12 baffles used, through coolant. C. Nicolella, M.C.M. van Loosdrecht, J.J. Heijnen (2000)
In system that are shear-sensitive and where are present, there are advantages in using an inclined bladed turbine. The number of agitator's increases on the pole, will be dependent on the height of liquid in the tube. For exact number of agitators on the pole, the height of liquid in the tube should be equal to the tank diameter, one agitator is required; if the height of liquid is two or three times of the tank diameter (H=2T or 3T), additional agitators should go up on the shaft, divided by a distance ɕ, then ɕ=T, where T represented of tank diameter. Installation of multi-sets of blades improves bonding and the quality of mass transfer. C. Nicolella, M.C.M. van Loosdrecht, J.J. Heijnen (2000)
Great turbulence is needed for good mixing: this is done by the swirl field which forms behind the blades. For all the gas to flow through this region it must enter the tube close and underneath the disc; therefore, it is advised that sparges should always be nearer, about a distance of D/2 below the agitator, where D is the blade diameter. Y. Chisti (1998)
The diffusive forces will carry the gas into the system, which assure enough turbulence is made. For this, a power input greater than 100W/m3 is needed from the agitator. Y. Chisti (1998)
3. Dissolved oxygen, measurement and mixing
In biochemical engineering process, measurement of dissolved oxygen (DO) is vital. The production of SCP (Production of single-cell protein) may reach a steady-state condition by keeping the DO level constant, while the accessible protein is gradually produced. The concentration of protein is proportional to oxygen uptake rate. Control of DO would guide us to achieve steady SCP production. Eviation of DO may affect retention time and other process variables such as substrate and product concentration, retention time, dilution rate and aeration rate. C. Nicolella, M.C.M. van Loosdrecht, J.J. Heijnen (2000)
Microbial cells in aerobic condition take up oxygen from the gas and the liquid phases. The rate of oxygen transfer from the gas phase to liquid phase is essential. At high cell densities, the cell growth is limited by the availability of oxygen in the medium. The growth of aerobic bacteria in the fermented is then controlled by the availability of oxygen, substrate, energy sources and enzymes. Air has to be supplied for aerobic process in order to increase the cell growth. Oxygen restriction may reduce the cell growth rate. The supplied oxygen from the gas phase has to fill into the microorganism. The oxygen first must travel through the gas-liquid interface, then the volume of liquid and finally into the microbial cell. D.J. Pollard, A.P. Ison, P.A. Shamlou, M.D. Lilly (1998)
4. Concluding remarks
The principal conclusions are as follows:
1. Use of low-power axial flow impellers in the down comer of an airlift bioreactor can substantially increase the rate of liquid distribution, mixing and gas-liquid mass transfer relative to operation without the agitator; however, the performance complement at the cost of a disproportionate increase in the power consumption.
2. Increasing concentration of the relatively light fibrous solids greatly reduces the volumetric gas-liquid mass transfer coefficient.
3. Surface aeration contributes but little to the total gas-liquid mass transfer in large bioreactors.
4. In mechanically agitated draft-tube reactors, air sparging of the riser zone may or may not improve the mixing performance, depending on the powerful mechanical agitation. At sufficiently high aeration rates, whether mechanical agitation is used or not has little bearing on the mixing characteristics of the reactor.
Finally, mechanically stirred hybrid airlift reactors are well-suited for use with shear sensitive fermentations that require good oxygen transfer and volume mixing than can be provided by an ordinary airlift reactor. Y. Chisti (1998)
Y. Chisti, Airlift Bioreactors,(1989) P 12.
A.W. Nienow, Hydrodynamics of stirred bioreactors, (1998) P 32.
Y. Chisti, U.J. Jauregui-Haza / Biochemical Engineering Journal 10 (2002) 143
M. Moo-Young, Y. Chisti, Considerations for designing bioreactors for shear-sensitive culture, Biotechnology 6 (11) (1988) P 1296.
Y. Chisti, Shear sensitivity, in: M.C. Flickinger, Encyclopaedia of Bioprocess Technology: Fermentation, Biocatalysts, and Bio separation, Vol. 5, Wiley, New York, 1999, P 2406.
D.J. Pollard, A.P. Ison, P.A. Shamlou, M.D. Lilly, Reactor heterogeneity with Saccharopolyspora erythraea airlift fermentations, Biotechnol. Bioeng. 58 (1998) P 463.
Y. Chisti, Pneumatically agitated bioreactors in industrial and environmental bio processing: hydrodynamics, hydraulics and transport phenomena, 51 (1998) P 112.
C. Nicolella, M.C.M. van Loosdrecht, J.J. Heijnen, Wastewater treatment with particulate bio film reactors, J. Biotechnol. 80 (2000) P 30 -33.
Y. Chisti, Pneumatically agitated bioreactors in industrial and environmental bio processing: hydrodynamics, hydraulics and transport phenomena, 51 (1998) P 100.
D.J. Pollard, A.P. Ison, P.A. Shamlou, M.D. Lilly, Reactor heterogeneity with Saccharopolyspora erythraea airlift fermentations,Biotechnol. Bioeng. 58 (1998) 452-463.
Y. Chisti, Pneumatically agitated bioreactors in industrial and environmental bioprocessing: hydrodynamics, hydraulics and transport phenomena, 51 (1998) P 115.