Air spargers

The KEN-FLOTE column (11) is one of several column flotation processes based on a countercurrent principle. The feed slurry containing reagents is iatroduced iato the column just below the froth zone. Air is iujected at the bottom of the column via an air sparger. Wash water is sprayed within the froth zone to reject the entrained impurities from the froth. Test results on this column iadicate that a 6% ash product coal having a combustible-recovery of 75—80% can be obtained. A 70—80% pyrite reduction is also claimed. Figure 2 shows the operation of such a column. 

Leaf Tests A bomb filter is used for small-scale leaf tests to simulate the performance of pressure-leaf (leaf-in-shell) filters. The equipment used is a small [50.8- by 50.8-mm (2- by 2-in)] leaf, covered with appropriate filter medium, suspended in a cell large enough to contain sufficient shiny to form the desired cake (Fig. 18-108). The shiny may be agitated gently, for example, by an air sparger. 

Mechanical agitation is needed to break up the gas bubbles but must avoid rupturing the cells. The disk turbine with radial action is most suitable. It can tolerate a superficial gas velocity up to 120 m/h. (394 ft/h) without flooding, whereas the propeller is limited to about 20 i7i/h (66 ft/h). When flooding occurs, the impeller is working in a gas phase and cannot assist the transfer of gas to the liquid phase. Power input by agitation and air sparger is 1 to 4 W/L (97 to 387 Btu/[fF-h]) of liquid. 

Fig.2 and Fig.3 show the typical liquid velocity and gas hold up distribution in the ALR. From the figures, one notices that the cyclohexane circulates in the ALR under the density difference between the riser and the downcomer. An apparent large vortex appears near the air sparger when the circulating liquid flows fi om the downcomer to the riser at the bottom. In the riser, liquid velocity near the draft-tube is much larger than that near the reactor wall, the latter moved somewhat downward. The gas holdup is nonuniform in the reactor, most gas exists in the riser while only a little appears in the dowmcomer. 

In Figure 11.2 a schematic view of a stirred vessel is given. The vessel is cyhndrical with a height (m) and a diameter T (m). Usually is equal to or greater than 2 T. It is equipped with a stirrer in the lower compartment. TTiis stirrer is mounted near the bottom, usually at a distance equal to the stirrer diameter. At a lower position the stirrer and bottom interact, leading to a decrease in power consumption. At a higher position hquid circulation problems can occur because, at increased gas flow rate in case of aeration, the bubbles will not be recirculated in the lower compartment. Sometimes the upper compartment (s) are also equipped with a stirrer. The vessel is equipped with baffles to prevent rotation of the contents as a whole. For aeration an air sparger is mounted below the stirrer. For mass transfer... 

FIGURE 9.3 Aeration units (a) turbine aerator with an air sparger (b) porous ceramic diffuser and (c) surface aerator. 

In the pharmaceutical industry, the reactors are closed to maintain aseptic conditions, and agitation and aeration are provided as needed. The classic example is the Monod chemostat, often found in laboratories. A representative chemostat, fermentation vessel, or bioreactor, is shown in Fig. 2 with an air sparger, agitator, and feed or exit streams. The continuous flow system is usually replaced by a batch process for production in the pharmaceutical and biotechnology industries. The general relationships for a constant volume system are developed in terms of cell concentration (A), often the component of interest, and substrate concentration (5), such as a sugar or oxygen. 

Figure 3 gives the results for the steady state removal of the three compounds using bubble fractionation on the small scale 5-cm diameter bubble column. The aqueous influent feed rate was 6 mL.min. The total aqueous height was 90 cm and the influent was fed at a height of 50 cm above the air sparger. The bottom effluent was maintained at 3 mL.min and hence the overflow rate was 3 mL.min ... 

During the course of the many laboratory-scale experiments conducted, it was found that the addition of surface agents (i.e., anionic, cationic and nonionic) did not improve the crystalline habit of the resultant product. An attempt to use an air sparger to simultaneously agitate the mother liquor and speed evaporation of the ammonia resulted in a rapid clogging of the sparger by precipitated silver azide. 

Sterilized and introduced to a bioreactor or fermenter, is typically equipped with agitators, baffles, air spargers, and sensing devices for the control of the operating conditions. A pure strain of microorganisms is introduced into the vessel. The number of cells multiplies exponentially after a certain period of lag time and reaches a maximum cell concentration as the medium is depleted. The fermentation is then stopped and the contents are pumped out for the product recovery and purification. This process is operated either by batch or continuous mode. 

Air sparger pressure drop Oxidation tank agitator Hp Solid sulfite oxidation ... 

An alternative method of treating wastewaters is to add powdered carbon to a tank of solution using mechanical stirrers or air spargers to keep the particles suspended. With the fine particles, the adsorption is much more rapid than with granular carbon, but large equipment is needed to remove the spent carbon by sedimentation or filtration. The treatment with powdered carbon can be done... 

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