Baffled tanks

H. Liquid drops in baffled tank with flat six-blade turbine 2.621 X iQ- [E] Use arithmetic couceutratiou difference. Studied for five systems. [154] p. 437... 

Baffled Tanks For vigorous agitation of thin suspensions, the tank is provided with baffles which are flat vertical strips set radially along the tank wall, as illustrated in Figs. 18-15 and 18-16. Four baffles are almost always adequate. A common baffle width is one-tenth to one-twelfth of the tank diameter (radial dimension). For agitating slurries, the baffles often are located one-half of their width from the vessel wall to minimize accumulation of solids on or behind them. 

FIG. 18-15 Typical flow pattern in a baffled tank with a propeller or an axial-flow turbine positioned on center. 

FIG. 18-16 Typical flow patter n in a baffled tank witb a turbine positioned on center. 

Figure 11. Row patterns in a baffled tank with centrally mounted impeller.

Figure 16. Draft lubes in baffled tanks (A) turbine (B) propeller.

Sinee the reaetors were a series of baffled tanks, Case 1 is most likely. Case 2 ean be evaluated quantitatively by using the kineties applieable to the reaetion and making suitable ehanges to allow for the reeyeling of ethylene glyeol. Case 3 is extremely unlikely beeause the impurities formed are water-soluble and should, therefore, be removed by the water purged from the system. 

Rushton et al. [10] performed extensive measurements of the power requirements for geometrically similar systems and found that for baffled tanks, the Froude number plays no part in determining the power requirements, as vortices do not form in such systems. For unbaffled systems, the Froude number plays a part above N g of about... 

If off-eentered and inelined propellers without baffles or sideentering propellers without baffles are used, no vortex forms and the O versus N[ g eurve for the eorresponding baffled tank ean be used to estimate the power requirements. 

Prochazka and Landau [19] developed a mixing time conelation for a single Rushton turbine impeller in a baffled tank in the standard configuration for > 10" ... 

Intensities of agitation with impeller m baffled tanks are measured by power input, hp/1,000 gal, and impeller tip speeds ... 

For Nr, > 1000, the properly baffled tank is turbulent throughout. Nq and P, are independent of Nr,. If the tank is not baffled, a forced vortex dominates the flow in the vessel. 

A related but somewhat more recent work by Oldshue presents heat transfer to and from helical coils in a baffled tank, using standard baffling of T/12 located either inside the coil diameter or outside ... 

Froude number, 304 Heal transfer, 312 In baffled tanks, 301 Performance relationships, mixing variables, 306... 

Mild agitation is obtained by circulating the liquid with an impeller at superficial velocities of 0.1-0.2 ft/sec, and intense agitation at 0.7-1. Oft/sec. Intensities of agitation with impellers in baffled tanks are measured by power input, HP/1000 gal, and impeller tip speeds ... 

The following convenient expression may be used to approximately evaluate the mixing times in baffled tanks ... 

Small blade high speed agitators are used to mix low to medium viscosity liquids. Two of the most common types are the six-blade flat blade turbine and the marine type propeller shown in Figures 5.1 and 5.2 respectively. Flat blade turbines used to mix liquids in baffled tanks produce radial flow patterns primarily perpendicular to the vessel wall as shown in Figure 5.3. In contrast marine type propellers used to mix liquids in baffled tanks produce axial flow patterns primarily parallel to the vessel wall as shown in Figure 5.4. Marine type propellers and flat blade turbines are suitable to mix liquids with dynamic viscosities up to 10 and 50 Pa s, respectively. 

Calculation of power for a turbine agitator in a baffled tank 179... 

As discussed in Volume 1, Chapter 7, the two key parameters determining power are the Reynolds number for the agitator and the power number. The Reynolds number should exceed 104 for optimum agitation. This gives a power number of about 6 for a fully baffled tank. It is important to note that the power number and the tip speed cannot both be kept constant in scale-up. 

Figure 3.21 Typical flow pattern in a baffled tank.

The values of VP are approximate. At low Reynolds numbers, about 300, the power number curves for baffled and unbaffled tanks are identical (McCabe et al., 1993). For higher NAV, the power number for unbaffled tanks is lower than the values for baffled tanks. 

Figure 3.24 Dimensionless power number in stirred tanks for a typical configuration (Perry and Green, 1999). A typical configuration consists of a baffled tank with DT/B values of 10 - 12 and a six-blade impeller.

Harriot (1962) measured the mass transfer coefficients in baffled tanks, using six-blade turbines and several liquids such as water and glycerine. According to that study,... 

The coefficients in unbaffled tanks increased with only the 0.3 power of the stirrer speed. At the speed needed for complete suspension in a baffled tank, the coefficients are about the same with or without baffles. At higher speeds, the more uniform dispersion of the particles and the greater velocity fluctuations make the coefficients larger with baffles present. 

Both large-scale motion (mass flow) and small-scale motion (turbulence) are usually required to bring about effective mixing (R5). Different ratios of mass flow to turbulence can be obtained for a given impeller type for the same power input large ratios for large values of d/T and slow speed, small ratios for small values of d/T and high speed. The requirements peculiar to batch liquid extraction have not been established, but for other services d/T = 0.2 to 0.5 is usually recommended for baffled tanks. 

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