Dispersion aeration rate

Impeller Reynolds number and equations for mixing power for particle suspensions are in Sec. 5. Dispersion of gasses into liquids is in Sec. 14. Usually, an increase in mechanical agitation is more effective than is an increase in aeration rate for improving mass transfer. 

M.E. Sensei, K.J. Myers, and J. B. Fasano, Gas dispersion at high aeration rates in low to moderately viscous systems, AIChE Symposium Series, No. 293, 89, 76-84(1993). 

Ar = aeration rate Fg = volumetric gas flow rate Vr = fermenter reaction volume Vs = superficial gas velocity Fg = volumetric gas flow rate A = fermenter cross section T = gas hold up Vr = fermenter reaction volume Fg = volumetric gas flow rate e = fractional gas hold up Vg = dispersed gas volume ... 

Gas dispersion in bioreactors is a process that has a complex dependence on various factors. Firstly, the gas flow rate has an important bearing on the hydrodynamics of two-/three-phase reactors. A measure of aeration rate that has been conventionally used to represent the air flow rate in stirred bioreactors is vvm. Chapman et al. (1983b) have shown that the use of vvm eliminates the scale-up effect since vvm is the reciprocal of the residence time of the gas phase Schluter and... 

The analysis above shows that higher specific power input is necessary both to prevent impeller flooding and to achieve complete dispersion at the large scale. Also, from the flooding correlation, for constant aeration rate and a fixed vessel size,... 

Until recently most industrial scale, and even bench scale, bioreactors of this type were agitated by a set of Rushton turbines having about one-thind the diameter of the bioreactor (43) (Fig. 3). In this system, the air enters into the lower agitator and is dispersed from the back of the impeller blades by gas-fiUed or ventilated cavities (44). The presence of these cavities causes the power drawn by the agitator, ie, the power requited to drive it through the broth, to fall and this 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/ U, in W/m and to the superficial air velocity, in m/s (20), where is the air flow rate per cross-sectional area of bioreactor. This relationship in water is... 

Process performance is affected by temperature. The reaction rate decreases with temperature over a range of 4—31°C. As the temperature decreases, dispersed effluent suspended sohds increase. In one chemical plant in West Virginia, the average effluent suspended sohds was 42 mg/L during the summer and 105 mg/L during the winter. Temperatures above 37°C may result in a dispersed floe and poor settling sludge. It is therefore necessary to maintain aeration basin temperature below 37°C to achieve optimal effluent quahty. 

One of the functions of the impeller in an aerated stirred tank is to disperse gas into the hquid as bubbles. For a given stirrer speed there is a maximum gas flow rate, above which the gas is poorly dispersed. Likewise, for a given gas flow rate there is a minimum stirrer speed, below which the stirrer cannot disperse gas. 

The flow rate required for the transition from regimes a to b or b to c is correlated to the Froude number and other system parameters, as shown in Table VI. The flowrate for the a-b transition depends on the direction in which JV is changed, as well as on the reactor scale (Zlokarnik and Judat, 1967). Van t Riet and Smith (1975) showed that the dispersion does not occur if Fr < 0.1. The aeration number for the b-c transition is mainly a function of the Froude number. 

Determine impeller size for required power input. The impeller required for the process must draw 7.91 hp per 1000 gal or 79.1 hp for the 10,000-gal batch. This power level must be achieved at the gas flow rate required by the process, which is, Qa = 1507 ft3/min. The power required to operate an agitator impeller for gas dispersion can be much less than the power required for a liquid without gas. The ratio of power with gas to power without P/Po is shown in Fig. 12.7 and is a function of the dimensionless aeration number NAe =Qa/ND3. 

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