Flat disc turbine

Figure 4.1. Stirred vessels with baffles, and common impeller types flat disc turbine, propeller, and pitched blade impeller.

We find then that the coefficient P is between 2 and 4 for propellers and pitched blade impellers, and between 1 and 2 for flat disc turbines. That would mean that the latter are the most power-effective mixers. 

In those gas/liquid processes where the gas flow is not sufficient for obtaining an effective contact between the phases, a stirrer can be used for disperging the gas. This applies particularly to small and medium sized reactors. A turbine impeller combined wiA baffles is quite effective (see figure 4.1). The gas can best be introduced via a perforated ring that is placed underneath the impeller. Another well known impeller type has a hollow shaft and hollow blades, through which the gas is sucked from the gas space and introduced into the liquid at the blade tips. When a common flat disc turbine is used, the gas phase is dispersed into small bubbles that circulate with the liquid in upward and downward directions (compare section 4.2.1). Particularly in the zones with lower flow rates, the bubbles will coalesce at a certain rate. A fraction of the larger bubbles will return with the circulating liquid flow to the impeller zone, where they are redispersed, but the... 

The most common impellers are the marine impeller or disc flat-blade turbine the flowpatterns which typically result are illustrated in Fig. 15-22. 

The power for agitation of two-phase mixtures in vessels such as these is given by the cuiwes in Fig. 15-23. At low levels of power input, the dispersed phase holdup in the vessel ((j)/ ) can be less than the value in the feed (( )df) it will approach the value in the feed as the agitation is increased. Treybal Mass Transfer Operations, 3d ed., McGraw-HiU, New York, 1980) gives the following correlations for estimation of the dispersed phase holdup based on power and physical properties for disc flat-blade turbines ... 

FK . 15-22 Uqiiid agitation by a disc flat blade turbine in the presence of a gas-liquid interface a) without wall baffles, (h) with wall baffles, and (c) in full vessels without a gas-bqiiid interface (continuous flow) and without baffles. [Couitesy Treyhal, Mass Transfer Operations, 3rd ed., p. 148, McGraw-Hill, NY,... 

Radial-flow impellers include the flat-blade disc turbine, Fig. 18-4, which is labeled an RlOO. This generates a radial flow pattern at all Reynolds numbers. Figure 18-17 is the diagram of Reynolds num-ber/power number curve, which allows one to calculate the power knowing the speed and diameter of the impeller. The impeller shown in Fig. 18-4 typically gives high shear rates and relatively low pumping capacity. 

Flat-blade disc turbine (ftushton turbine)... 

Flat blade disc turbine, baffled or unbaffled vessel, transfer to vessel wall, Re < 400 ... 

The reactor described in Example 12.12 is fitted with a flat blade disc turbine agitator 0.6 m diameter, running at 120 rpm. The vessel is baffled and is constructed of stainless steel plate 10 mm thick. 

Disc-type turbines In this type of turbines, blades can be straight or curved. In the related literature, the term straight is some times replaced by the term flat. The most popular turbine of this type is the straight-blade disc turbine, which is better known as the Rushton turbine (Figure 3.18). The same turbine is also called flat-blade turbine, vaned disc or simply flat-blade turbine. ... 

For Rushton turbines (flat-blade disc turbines), the following equation can be used (Nouri and Hockey, 1998) ... 

Flat blade disc turbine with six blades 0.99 1.4 2.18... 

The impellers used in this study are shown in Figure 1. They are both of the radially discharging type which is characterized by a strong radial jet of fluid that moves out to the vessel walls while entraining fluid from above and below. Near the wall it splits into two circulation zones. The flat blade turbine (FBT) is commonly used in industry and consists of a disc with several paddles fixed normal to the disc which serve to generate the radial flow. The novel impeller design... 

Problem A recently built reactor, equipped with a flat-blade disc turbine, does not satisfy the process requirements. By examining the hquid surface, the operator suspects that the gas is rising straight through the impeller to the hquid surface. In other words, the impeller is flooded. The mass-transfer rate is usually lower when the impeller is flooded than when it is able to completely disperse the gas. Pertinent information about the reactor is T = 2m D = 0.8m N = 1.13rps Qg = 0.12m /s /i = 1 cP p = 1,000 kg/m Na = 0.21 Npr = 0.1. 

The most comprehensive correlation for heat transfer to vertical baffle-type coils is for a disc flat-blade turbine over the Reynolds number range 10 to (2)(10 ) ... 

Three types of impellers are commonly used in the low viscosity region, propellers, Fig. 1 turbines. Fig. 2 and axial flow turbines. Fig. 3. Impellers used on small portable mixers shown in Fig. 4, are oflen inclined at an angle as well as being off-center to give a good top-to-bottom flow pattern in the system. Fig. 5. Large top-entering drives usually use either the axial flow turbine or the radial flow flat blade turbine. For aerobic fermentation, the radial flow disc turbine is most common and is illustrated in Fig. 6. 

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