Impellers, agitation kinds

Equipment suitable for reactions between hquids is represented in Fig. 23-37. Almost invariably, one of the phases is aqueous with reactants distributed between phases for instance, NaOH in water at the start and an ester in the organic phase. Such reac tions can be carried out in any kind of equipment that is suitable for physical extraction, including mixer-settlers and towers of various kinds-, empty or packed, still or agitated, either phase dispersed, provided that adequate heat transfer can be incorporated. Mechanically agitated tanks are favored because the interfacial area can be made large, as much as 100 times that of spray towers, for instance. Power requirements for L/L mixing are normally about 5 hp/1,000 gal and tip speeds of turbine-type impellers are 4.6 to 6.1 i7i/s (15 to 20 ft/s). 

Open-type turbines This kind of turbines is also called full-blade turbines (Figure 3.17). The blades are vertical (parallel to the axis of the drive shaft) and could be straight or curved. They are cost-effective impellers for operations veiy near the floor of a tank for agitating the... 

Air is most commonly dispersed with mechanical agitation. Figure 19.8 illustrates a popular kind of flotation cell in which the gas is dispersed and the pulp is circulated with impellers. Such vessels have capacities of 300-400 cuft. Usually several are connected in series as in Figure 19.8(b). The froth is removed from... 

SECONDARY NUCLEATION. The formation of nuclei attributable to the influence of the existing macroscopic crystals in the magma is called secondary nucleationP Two kinds are known, one attributable to fluid shear and the other to collisions between existing crystals with each other or with the walls of the crystallizer and rotary impellers or agitator blades. 

---