Baffles agitated vessels

A. Solid particles suspended in agitated vessel containing vertical baffles, continuous phase coefficient -2 + 0.6Wi f,.Wi D Replace Osi p with Vj = terminal velocity. Calculate Stokes law terminal velocity [S] Use log mean concentration difference. Modified Frossling equation K, -< T.d,P. [97] [146] p.220... 

The values of eonstant C and the exponents a, b, and e depend on the type of agitator, whether baffles are used and their type, and whether the transfer is via the vessel wall or to eoils. Baffles are normally used in most applieations, and the values of a, b and e in the literamre are 2/3, 1/3, and 0.14 respeetively. Tables 7-14 and 7-15 give typieal eorrelations for various agitator types. 

Surface area for heating or cooling agitated vessels can be provided by eidier external jacketing or internal coils (or tubular baffles). Jacketing is usually preferred because of ... 

In a mixed agitated vessel with high agitation rate, at the centre of the vessel a vortex often forms. To prevent a central vortex in tanks less than 3 m in diameter, four baffles each with a baffle width of 15-20 cm are necessary. A basic assumption is to select a ratio of liquid height to tank diameter from 2 1 to 6 1. 

A reaction is to be carried out in an agitated vessel. Pilot scale tests have been carried out under fully turbulent conditions in a tank 0.6 m in diameter, fitted with baffles and provided with a flat-bladed turbine, and it has been found that satisfactory mixing is obtained at a rotor speed of 4 Hz when the power consumption is 0.15 kW and the Reynolds number 160,000. What should be the rotor speed in order to achieve the same degree of mixing if the linear scale of the equipment if increased by a factor of 6 and what will be the Reynolds number and the power consumption ... 

Mixing times in mechanically agitated vessels typically range from a few seconds in laboratory glassware to a few minutes in large industrial reactors. The classic correlation by Norwood and Metzner for turbine impellers in baffled vessels can be used for order of magnitude estimates of... 

The factors that can affect the rate of heat transfer within a reactor are the speed and type of agitation, the type of heat transfer surface (coil or jacket), the nature of the reaction fluids (Newtonian or non-Newtonian), and the geometry of the vessel. Baffles are essential in agitated batch or semi-batch reactors to increase turbulence which affects the heat transfer rate as well as the reaction rates. For Reynolds numbers less than 1000, the presence of baffles may increase the heat transfer rate up to 35% [180]. 

Detailed investigations of dispersions in agitated and baffled vessels have been made by Vermeulen, Williams, and Langlois (V3), Rodger, Trice, and... 

The crystallizer was an agitated vessel with an Inside diameter of 9.0 cm and a volume of about 1 1. It was equlppet with four vertical baffles, a water jacket to keep the solution temperature constant, and a nozzle through which nitrogen gas was Introduced In several experiments to suspend a speed crystal more effectively In the solution. Agitation was accomplished with a 5.0 cm stainless steel marine propeller having three blades driven by a variable speed motor. 

The dimensionless group in the left-hand side of Eq. (9.53) is known as power number NP, which is the ratio of drag force on impeller to inertial force. The first term of the right-hand side of Eq. (9.53) is the impeller Reynolds number NRe. which is the ratio of inertial force to viscous force, and the second term is the Froude number NFr which takes into account gravity forces. The gravity force affects the power consumption due to the formation of the vortex in an agitating vessel. The vortex formation can be prevented by installing baffles. 

Figure 7-16. Mixing times in agitated vessels. Dashed lines represent unbaffled tanks solid lines represents a baffled tank. (Source McCabe, W. L, et at., Unit Operations of Chemical Engineering, 4th ed., McGraw-Hill Book Company, New York, 1985.)...

In recent years attempts have been made to improve the gas-liquid mass transfer by changing the design of the mechanically agitated vessel. Mann et al. (1989) evaluated the use of horizontal baffles mounted near the gas-liquid surface. Horizontal baffles prevent vortex formation, generate less shear than standard baffles, increase gas holdup, and improve gas-liquid mass transfer. The latter two results are due to the rotational flow below the baffles, which causes gas bubbles to move upward in a spiral trajectory and induces surface aeration. For a 12-inch i.d. and 18-inch-tall stirred vessel, they showed kLat to be improved by a factor of 1.6 to 2.3 with 30 to 50% lower agitation power compared to the standard vessel. 

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. 

Fig. 21. Various types of agitated vessels with H/d, = 1 (baffles drawn with dotted lines indicate that the stirrer can be used in baffled or unbaffled vessels).

Another important design parameter for miscible liquids is the power consumption, which can be obtained from Fig. 24. For viscous liquids, flow is in most part laminar. When the agitated vessels contain baffles, turbulence is achieved at a lower Reynolds number. Once the flow becomes turbulent, the power number attains a constant value. 

A. Solid particles suspended in agitated vessel containing vertical baffles, continuous phase coefficient A = 2 + 0.6N tNS Replace vz [p with uT = terminal velocity. Calculate Stokes law terminal velocity c d lp,-pjg K 18 ic and correct 1 10 100 1,000 10,000 100,000 [S] Use log mean concentration difference. Modified Frossling equation = Vn "P ° Re (Reynolds number based on Stokes law.) V -vTdrP° A Re,r — (terminal velocity Reynolds number.) kl almost independent of dp. Harriott suggests different correction procedures. Range ki/k is 1.5 to 8.0. [74] [ 138] p. 220-222 [110] [74]... 

Treybal (T5) has reported that the volume-fraction of dispersed phase actually present in an agitated vessel may be considerably less than the volume-fraction in the feed, especially in an upflow system when the dispersed phase is the lighter one. For baffled vessels and flat-blade turbines, with cocurrent upward flow, the volume-fraction of dispersed phase in the vessel is about 20% of the volume-fraction in the feed at energy inputs less than 100 ft.-lb./ft. of feed. This ratio rises rapidly with increased power above this level and is between 80% and 100% at energy inputs above 400 ft.-lb./ft. of feed. This effect is still incompletely understood. 

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