In sparged vessels

Deshpande and Barigou [14] have tabulated a useful summary of the many published empirical studies of defoaming using various rotary devices. In Appendix 7.1, Table 7.A1, we produce an updated version of that table where later studies are included and an error in a citation is rectified. Whether performance has been studied in either bubble colunms or stirred vessels is indicated. Most of the reported studies are at laboratory scale, concerned with controlling the foam of dilute aqueous surfactant solutions in sparged vessels of <0.5 m diameter. Only two reported studies concern pilot scale with practical systems—Kraft (paper) mill effluent [15] and recombinant Bacillus fermentation [16]. 

The gas-liquid mixture can be mechanically agitated, as with an impeller, or, as in the simplest design, agitation can be accomplished by the gas itself in sparged vessels. The operation may be batch, semibatch with continuous flow of gas and a fixed quantity of liquid, or continuous with flow of both phases. 

In tray towers, entrainment of liquid in the gas is a form of back mixing, and there are back mbcing and axial mixing on the trays, which we have already considered. In sparged vessels, the liquid is essentially completely back-mixed to a uniform solute concentration. Both phases are largely completely mixed in mechanically agitated vessels. 

In an airlift fermenter, mixing is accomplished without any mechanical agitation. An airlift fermenter is used for tissue culture, because the tissues are shear sensitive and normal mixing is not possible. With the airlift, because the shear levels are significantly lower than in stirred vessels, it is suitable for tissue culture. The gas is sparged only up to the part of the vessel cross section called the riser. Gas is held up, fluid density decreases causing liquid in the riser to move upwards and the bubble-free liquid to circulate through the down-comer. The liquid circulates in airlift reactors as a result of the density difference between riser and down-comer. 

Quench pool/catch tank This type of system, as shown in Fig. 23-55, is used to condense, cool, react with, and/or collect a mixture of liquid and vapors discharging from a relief device by passing them through a pool of liquid in a vessel. Feed vapor and liquid (if present) are sparged into the pool of cool liquid, where the vapors are condensed and the liquid is cooled. If the feed materials are miscible with the pool liquid, they mix with and are diluted by the pool liquid if not, the condensate, feed liquid, and pool liquid separate into layers after the emergency relief event is over. The condensed vapors, feed liquid, and quench liquid are contained in the vessel until they are sent to final disposal. 

Hydrodynamics of slurry reactors include the minimum gas velocity or power input to just suspend the particles (or to fully homogeneously suspend the particles), bubble dynamics and the holdup fractions of gas, solids and liquid phases. A complicating problem is the large variety in reactor types (sec Fig. I) and the fact that most correlations are of an empirical nature. We will therefore focus on sparged slurry columns and slurries in stirred vessels. 

Stratco unit with the single mixer on one end is approximated by a single mixed tank, as shown in the upper part of the figure. However, the Kellogg cascade unit has a series of compartments with mixers and olefin is sparged into each compartment to keep the concentration low so that it reacts with the isobutane rather than polymerizing. The tank-in-series model may be used to model this type of unit and this is shown in the lower part of the figure. A mass balance can be made for a stirred tank reactor readily because the composition is the same everywhere in the vessel. 

The flow round the stirrer blades interacts with the stationary baffles and produces a complex, circulating turbulent flow. When gas is sparged in a tank it collects in low pressure zones behind the stirrer blades forming gas cavities, which considerably influence the flow and the turbulence in the vessel. 

Yagi II.. Yoshida F., Gas Absorption by Neivtonian and bfon-Neivtonian Fluids in Sparged Agitated Vessels, Ind. Eng. Chem. Process Des. Dev. 14 (1975) 4, p. 488-493... 

Chapman CM, Nienow AW, Middleton JC. (1980) Surface aeration in a smaU agitated and sparged vessel. Biotechnol. Bioeng., 22 981-994. 

Nikhade BP, Moulijn JA, Pangarkar VG. (2005) Critical impeller speed (N ) for solid suspension in sparged stirred vessels fitted with hehcal coils. Ind. Eng. Chem. Res., 44 4400-4405. 

All experiments were conducted with pasteurized 2% fat milk acquired from a local dairy. The milk was homogenized in two stages at 2500/500 psi. UHT processing was accomplished with a plate heat exchanger unit at 144.4 °C for 4 seconds. Flow rate was 2000 ml/min. Samples were collected in sterile stainless steel vessels. Aliquots were asepticaUy transferred to 250 ml amber bottles and sealed with teflon lined lids. For volatile ctmtmt analysis, 10 ml of UHT milk was transfored to a 100 ml sparging vessel and treated as described previously. 

Takahashi, K., McManamey, W.J., Nienow, A.W., Bubble size distributions in impeller region in a gas-sparged vessel agitated by a Rushton turbine, J. Chem. Eng. Jpn. 25(4) (1992), 427-432. 

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