Gassed power

Fig. 21. Gassed power curves for constantand JF where mixer speeds = and gas rates = < Q2 < Qs-...

When P/V is used as the eonstant eriterion, there is a deerease in power eonsumption under aerated eonditions. In the literature many relationships exist, but in praetiee the gassed power eonsumption is about 50% of the ungassed power eonsumption. The eorrelations for gassed power eonsumption (Pg) and the ungassed power eonsumption (Pj,) ean be expressed by [22]... 

The above correlation is valid for a bioreactor size of less than 3000 litres and a gassed power per unit volume of 0.5-10 kW. For non-coalescing (non-sticky) air-electrolyte dispersion, the exponent of the gassed power per unit volume in the correlation of mass transfer coefficient changes slightly. The empirical correlation with defined coefficients may come from the experimental data with a well-defined bioreactor with a working volume of less than 5000 litres and a gassed power per unit volume of 0.5-10 kW. The defined correlation is ... 

Using the plot of PgIP versus Na (Figure 6.7), the ratio of gassed power to ungassed power is defined. 

Dj = Dtank/3 = 0.8 m for three sets of impellers Impeller speed 150 rpm assume broth viscosity is 1 cp and the specific gravity of the broth is 1.2 aeration rate is 1 vvm given ratio of gassed power to un-gassed system is 0.4 specific oxygen uptake is 0.65 mmol 02/kg cell OTR = 6 X 10 3 kg/m3. 

In fact, the power number is a dimensionless number that is the ratio of ungassed power to gassed power in a normal bioreactor.3... 

Reading gassed power to ungassed power ratio from the plot for the ratio of PJP, defined in Figure 6.6, Chapter 6. 

The gassed power is always less than the ungassed power. 

Gassed power, Pg = 0.4 (15) 6hp=> for three sets of impellers using aeration... 

If you have access to the graph Pg/P that is good, read gassed power to ungassed power ratio off of the graph. 

Now we make use of Miller s correlation for gassed power calculations 5... 

Gas seal system, blast furnace, 14 507 Gassed power draw, during fermentation, 11 39 0... 

The gassed condition is important in mass transfer calculations. In general, the ungassed- and gassed-power are related by the equation PG = P. The value of P is calculated from the power correlation for Rushton turbines [16]. The correction factor, y, is calculated from the following equations ... 

Figure 10.11. Power consumption, (a) Ratio of power consumptions of aerated and unaerated liquids. Q is the volumetric rate of the gas (O) glycol ( X ) ethanol ( ) water. [After Calderbank, Trans. Inst. Chem. Eng. 36, 443 (1958)]. (b) Ratio of power consumptions of aerated and unaerated liquids at low values otQ/Nd3. Six-bladed disk turbine ( ) water ( ) methanol (10%) (A) ethylene glycol (8%) (A) glycerol (40%) Pg = gassed power input P = ungassed power input Q = gas flow rate IV = agitator speed d = agitator-impeller diameter. [Luong and Volesky, AIChE J. 25, 893 (1979)].

Gassed Impeller Power Bakker et al. (op. cit.) have given a gassed power correlation for the 6BD and CD-6 impellers. 

Gas holdup and volumetric gas-liquid mass-transfer coefficients are correlated with the gassed power input/volume and with the aeration rate (actual gas superficial velocity), e.g., the correlation of van t Riet [Ind. Eng. Chem. Proc. Des. Dev. 18 357 (1979)] for the volumetric mass-transfer coefficient of coalescing and noncoalescing systems ... 

FIGURE 12.7 Gassed power factor as a function of aeration number. 

Since impeller diameter is unknown, aeration number and, therefore, gassed power factor cannot be determined however, an estimate for power factor P/P0of 0.4 is usually a good initial estimate. By rearranging the expression for power number, that is, Np = P/(pN3D5), an expression for impeller diameter can be derived. The factor P/ Pf] must be introduced for the effect of gas and a conversion for units makes... 

Select standard size motor. A motor that is loaded to 85 percent by a 79.1-hp impeller will require a minimum size of (79.1 hp)/0.85 = 93.1 hp, which means a 100-hp (75-kW) motor. This motor and impeller assembly is correctly sized for conditions with the design gas flow. However, because of the gassed power factor, that is, P/P0 = 0.38, should the gas supply be lost for any reason, the impeller power would increase to 78/0.38 = 205 hp and seriously overload the motor. To avoid this problem, some method (typically electrical control) prevents motor operation without the gas supply. When the gas supply is off, the control either stops the agitator motor or, in the case of a two-speed motor, goes to a lower speed. 

For the low range of aeration numbers (Na), this corrrelation predicts ratios of Pg/Pu in the range of 0.3 to 0.68, which is lower than one normally expects. Thus, one is advised to use this correlation with a bit of caution. If the gassed power requirement seems somewhat low, a vendor should be consulted. 

To prevent the impeller from flooding, the gas flow rate can be decreased. However, this is undesirable because reduced gas flow rate leads to a lower mass-transfer rate. Therefore, it is decided to replace the flat-blade turbine with a concave-blade turbine that can handle larger gas flowrates. The gassed power draw of the flat-blade turbine is calculated to be 1,540 W. 

Impeller gassed power requirement, HP Impeller ungassed power requirement, HP Volumetric flow rate of the minor stream, ft /s Volumetric flow rate of major stream, ft /s Spacing between impellers on a shaft, ft (1) Heating or cooling time from Ti to Tp and (2) time from start of dissolving, s Vessel diameter, ft... 

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