Sparger Design

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. 

For example, at a power consumption of 0.3 kW/m in an ELARL, Joshi et al. (1990) observed a local maximum when = lOm this led to the conclusion that 

Approaches used to develop gas-liquid mass dansfer correlations in bubble columns have been ported over to airlift bioreactors, and, unfortunately, they have brought some issues along with them. Airlift bioreactor correlations are highly empirical, and a unifying development method does not exist. Some correlations attempt to be very specific and use multiple inputs, which are hard to quantify in an industrial setting, or use inputs that are not independent of each other. The suggestion is similar as with the bubble column experimental units can use these more complicated correlations, but pilot or indusdial scale units will have to depend on empirical and design specific correlations. 

Further problems are presented by the downcomer and riser. Conditions in the two different sections can be very different, either by design or operation. Gas 

Air-(water, 1 wt% aqueous solutions of methanol, ethanol, w-propanol, isopropanol, and w-butanol) 

Mersmann (1978) has suggested the following criteria for uniform gassing of all the orifices  

Thorat et al. (2001) have given Equation 10.22 for the critical Froude number for bubble columns when the dispersion height is much larger than the maximum encountered in sieve trays  

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The sparger design can be based on all vapor flow, but the heat balance must include the liquid. 

The mixture acts hke a liquid and the vapor condensation is dependent on jet mixing. This will require a different type of sparger design. 

Can handle high liquid loading with special sparger design. 

Alternative sparger designs are shown in AIChE-CCPS (1998). 

Birch, D J., Ahmed, N., Solids suspension in aerated agitated vessels Role of sparger design, 9th European Conference on Mixing Mixing 97, Vol. 11, 177 (1997)... 

Agitation speed requirements (MSSR) or sparger design (both MSSR and BSCR) ... 

Gas sparger design Number of holes in sparger ring Minimum speeds of suspension, re--suspension and flooding... 

In the practice of the sparger design, it is very important to maintain equal flows through each orifice. For a perforated-pipe sparger the following three factors have to be calculated [27] ... 

The ammonia sparger design provides a smooth reaction and good mixing of the reactants. 

Some qualitative observations can be made. Increase of the superficial gas velocity increases the holdup of gas, the interfacial area, and the overall mass-transfer coefficient. The ratio of height to diameter is not important in the range of 4 to 10. Increase of viscosity and decrease of surface tension increase the interfacial area. Electrolyte solutions have smaller bubbles, higher gas holdup, and higher interfacial area. Sparger design is unimportant for superficial gas velocities > 5 to 10 cm/s (0.16 to 0.32 ft/s). Gas conversion falls off at higher superficial velocities, so values under 10 cm/s (0.32 ft/s) are advisable. 

Therefore, a more elaborate model taking into account the presence of the lateral walls and the sparger design will now be presented. 

The most important parameter governing stability is the dispersion coefficient of the dispersed phase such as bubbles, drops, and particles. The published information is not sufficient. A comprehensive research program is needed for the measurement of dispersion in all multiphase reactors over a wide range of terminal velocities, column diameters, column heights, sparger designs, phase velocities, and continuous-phase physical properties. 

An ore-containing slurry is to be processed in a froth flotation tank at a rate of 300 lons/h. The slurry consists of 20.0 wt% solids (the ore, SG = 1.2) and the remainder an aqueous solution with a density close to that of water. Air is sparged (blown through a nozzle designed to produce small bubbles) into the slurry at a rate of 40.0 ft (STP)/1000 gal of slurry. The entry point of the air is 10 ft below the slurry surface. The tank contents are at 75 F and the barometric pressure is 28.3 in. Hg. The sparger design is such that the average bubble diameter on entry is 2.0 mm. 

Ranade, V.V. and Tayaliya, Y. (2001), Modeling of fluid mechanics and mixing in shallow bubble column reactors influence of sparger design, Chem. Eng. Sci., 56, 1667-1675. 

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