Bioreactors power requirements

Slurry bioreactors offer the most aggressive approach to maximizing contact between the contaminated soil and the degrading organisms. Slurry bioreactors are usually the most expensive bioremediation option because of the large power requirements, but under some conditions this cost is offset by the rapid biodegradation that can occur. 

Since the biomass production is an aerobic process and air should be supplied to cultures, efforts are being made to develop bioreactors having higher oxygen transfer rates and lower power requirements. 

Until recently most industrial scale, and even bench scale, bioreactors of this type were agitated by a set of Rushton turbines having about one-third the diameter of the bioreactor (43) (Fig. 3). In tliis system, the air enters into the lower agitator and is dispersed from the back of the impeller blades by gas-filled or ventilated cavities (44). The presence of these cavities causes the power drawn by the agitator, ie, the power required to drive it through the broth, to fall and tliis has important consequences for the performance of the bioreactor with respect to aeration (35). k a has been related to the power per unit volume, P/T, in W/m3 and to the superficial air velocity,, in m/s (20), where vs is the air flow rate per cross-sectional area of bioreactor. Tliis relationship in water is... 

Mechanical agitators bioreactor cost of mixer = (maximum impeller shear rate)" The power required being about 20 to 40% less than would be required to mix liquid. 

The working reactor volume is 20 ml Therefore, = 225/20,000 1.1 x 10" W/kg. This value is sufficiently low for use in animal cell bioreactors. The low power requirement can be attributed to the relatively very low wm used. The low vvm in turn is a result of the much lower oxygen utilization rate (lower peak OUR/higher doubling times as compared to bacteria/yeast in Table 7B.6 in such systems). 

Looking at larger industrial bioreactors, the mixing times should be kept short and may require additional feed or air/gas injection points on the lateral side of the reactor instead of increasing the power intake. Considering the power requirements of 2-10 kW m (Table 1.7), a 250 m bioreactor would need a 0.5-2.5 MW motor installed to keep the medium fully dispersed and turbulent. 

Power used for wastewater treatment. The power required at different WWTPs varies widely, but general trends can be developed based on the type of treatment process. TFs are reported to use 430 kW per mVs, with AS using 2.4 to 5.9 times as much power (1020-2550 kW per m /s) (Table 9.1). These estimates for AS are a little higher than Shizas and Bagley (2004) reported for the North Toronto Wastewater Treatment (NT) plant of 680 kW per m /s (30 kW/mgd). The wastewater at this plant contained 431 mg-COD/L and 1930 mg-TS/L, and contained in the influent a total of 2616 kW. Thus, the energy content of the wastewater was 9.3 times that required to treat the wastewater (283 kW). A membrane bioreactor (MBR) system requires 8520 kW per m /s for treatment, and thus could not be self-sustaining based on the energy content of the wastewater. 

Biodiesel to Fuel a Large Power Plant. Researchers at ASU s Center for Bioenergy and Photosynthesis have calculated that a 25 x 25 km field of bioreactors using cyanobacteria to fix carbon could uptake all of the carbon dioxide produced by a 1.6 GW power plant and subsequently provide the biomass as lipid to fuel the power plant. The parameters necessary to achieve this goal are a seven percent power conversion efficiency for photosynthesis, 40 percent conversion efficiency of biomass to fuel, 50 percent conversion efficiency of fuel to electricity, and 80 percent conversion efficiency of land area covered by the bioreactors. This system would then be carbon neutral in operation and produce about 1.6 GW of electrical power. The key to making this feasible is to achieve a seven percent power conversion efficiency for cyanobacteria. Moore noted that the area required to produce a specified amount of energy scales directly with the energy conversion efficiency of the system or device. 

Mechanically stirred hybrid airlift reactors (see Fig. 6) are well suited for use with shear sensitive fermentations that require better oxygen transfer and mixing than is provided by a conventional airlift reactor. Use of a low-power axial flow impeller in the downcomer of an airlift bioreactor can substantially enhance liquid circulation rates, mixing, and gas-liquid mass transfer relative to operation without the agitator. This enhancement increases power consumption disproportionately and also adds other disadvantages of a mechanical agitation system. 

The area ratio effects on the liquid-phase mass transfer coefficient are more difficult to predict. Area ratio effects are usually studied by keeping the bioreactor volume equal, which requires the effective bioreactor height to be adjusted. As the height is increased, the interfacial solute gas concentration increases as well, which decreases the gas solubility and, in turn, the liquid-phase mass transfer coefficient. In addition, an increase in the area ratio decreases the liquid circulation rate, which increases gas holdup, but may decrease surface renewal. The greater height also raises the pressure drop and power consumption, which increases surface renewal and the liquid-phase mass transfer coefficient. The extent of these effects is dependent on the operational scale and power level, and it is hard to predict which will dominate. 

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