Agitator, baffle paddle

Cooper et al. (C9) were the first to determine mass-transfer coefficients by measuring the oxidation rate of sodium sulfite in an aqueous solution catalyzed by cupric ions. Their data were taken for a vaned-disk agitator with 16 blades and for a flat paddle. The ratio of agitator-to-tank diameter was 0.4, and the ratio of paddle to tank diameter was 0.25. The tank was equipped with four baffles, with baffle-width to tank diameter ratio of 0.1. 

Kneule and Weinspach (1967) also measured the suspension characteristics of numerous stirrer types and agitated vessels. They found the optimum stirrer diameter d, and distance from the bottom H, to be given by dT/d, = 3.0-3.5 and Hj/d, = 0.3-0.5. The optimum shapes for the vessel bottom are hemispherical and elliptical a flat vessel bottom is unsuitable for particle suspension. For a vessel with an elliptical bottom, baffles, and a propeller stirrer installed at HJd = 0.2-0.8 pumping the liquid toward the floor, the constant b in Eq. (3.22) has the value b = 3.06. For a turbine stirrer with six paddles and Hj/d, = 0.3, the value is b = 1.21. In order to keep the particles in the same material system in suspension, the propeller stirrer must therefore operate at a rotational speed (3.06/1.21)1/2 = 1.59 times higher than a turbine stirrer of the same size. 

This work employed rather small vessels (6-8 in.), usually without baffles, and turbine, paddle, and propeller agitators. The observed mixing times ranged from 5 to 30 seconds. For Reynolds numbers (D2Np/fi) of about 10s—104, the following proportionality was found. 

Air oxidation of sodium sulfite solutions, containing a trace of cupric ion as catalyst, was first studied by Cooper et al. (C7). These workers found the reaction rate to be essentially independent of sulfite and sulfate concentrations and therefore especially convenient for their study. Most of their runs were made using a vaned-disk agitator (a flat disk with sixteen radial vanes on its lower face) in baffled vessels 6-17 in. in diameter. Tank-to-impeller diameter ratio was 2.5, and the impeller was 0.3 tank diameter off the bottom, in every case. Impeller speeds were 60-900 r.p.m. A few measurements were made with a simple flat paddle, with a total length one-fourth the vessel diameter, in 9.5- and 96-in. diameter tanks. Air was introduced from a single open-end pipe just below the center of the agitator. Superficial air velocities were 0.3-11 ft./min. 

Two studies have been concerned with measurement of the interfacial area obtained by agitation of liquid-liquid systems. Each of these investigations relied on the use of a photoelectric probe which measured the light transmission of the two-phase dispersion. Vermeulen and co-workers (V2) made measurements in two geometrically similar, baffled vessels of 10- and 20-in. diameter. They used a very simple four-blade paddle-like stirrer, with a tank-to-impeller diameter ratio of about 1.5, and a 0.25 blade-width/impeller-diameter ratio. The impeller was located midway between the top and bottom of the vessel, which had a cover and was run full. Impeller speeds varied from about 100 to 400 r.p.m. A wide variety of liquids was employed. Volume fractions of dispersed phase varied from 10% to 40%. The mean droplet diameters observed ranged from 0.003 to 0.1 cm. The results were correlated with a mean deviation of about 20% by an empirical equation relating the specific interfacial area near the impeller to several system and operating variables as follows ... 

Oyama and Endoh (012) studied the solution of sugar in water in 6.7- and 10.8-in. baffled vessels using paddles and flat-blade turbines. They report a mass-transfer coefficient which was proportional to the cube root of the particle diameter and to the cube root of the impeller power consumption per unit mass of agitated liquid. 

The paddle agitator (without baffles) consists merely of arms placed on a vertical shaft inserted through the top of a tank. It is rotated by any suitable motive power (this type is very seldom used without baffles but in order to bring out the faults of this class better we will discuss it briefly). 

The only result of such an apparatus is to set the entire solution to rotating with the paddles. Fairly heavy particles will settle out from the solution very readily and after a very short time the entire system will come to equilibrium, the solution and solids and paddles all revolving at approximately the same speed and the agitation ceases, becoming only that due to retardation on sides and bottom of tank. The natural improvement is to place along the sides of the tank and between the paddles some stationary horizontal baffles. 

Some tanks are designed with horizontal baffling to create a number of layered zones and so to improve the approximation to plug flow. These may use low-speed flat-paddle agitators with diameters close to that of the tank. Liederbach [40] recommends flow into the top of a tank and out the bottom, and Sutter [53] provides sketches indicating the same approach. 

Pa s (3000 cp) turbines can be used below about 100 Pa-s (100000 cp) modified paddles such as anchor agitators can be used above 50 Pa-s to about 500 Pa-s (500000 cp) helical and ribbon-type agitators are often used above this range to about 1000 Pa-s and have been used up to 25000 Pa-s. For viscosities greater than about 2.5 to 5 Pa-s (5000 cp) and above, baffles are not needed since little swirling is present above these viscosities. 

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