Vortex formation, prevention

A vertical cylindrical, and mechanical agitated pressure vessel, equipped with baffles to prevent vortex formation is the most widely used fermenter configuration. The baffles are typically one-tenth of the fermenter diameter in widtli, and are welded to supports tliat extend from the sidewall. A small space between the sidewall and the baffle enables cleaning. Internal heat transfer tube bundles can also be used as baffles. The vessels must withstand a 45 psig internal pressure and full vacuum of -14.7 psig, and comply with the ASME code. 

Solid partieles in liquids generally tend to settle to the bottom of a vessel under gravity due to their exeess density. To maintain a suspension, some form of agitation is normally provided together with wall baffles to prevent vortex formation in the swirling flow (Figure 2.14). 

The vertical tubes serve as baffles to a certain extent, but not enough to prevent some vortex formation. The helical coll installations may have sidewall baffles (usually four Ao or Xj dla.), or baffles assembled with the coil itself (See Figures 5-23H, 5-231, 5-38 and 5-39.)... 

Use only stainless steel tanks minimize vortex formation to prevent aeration. Product attacks glass avoid contact with glass. 

Figure 7.2 Various methods to prevent vortex formation.

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. 

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. 

The operating mode of a stirred-tank reactor may be either continuous or batch. A STR consists of a vessel to contain the reactants, a heat exchanger, a mixer, and baffles to prevent vortex formation and to increase turbulence, enhancing mixing. 

For any given vessel, the values of the shape factors will be constant. Under this condition, % = TIN pDl will simply be a function of Re = pND Ip and a function of Fr = dXI8- The effect of the Froude number Fr is manifested in the rising and lowering of the water when the vortex is formed. Thus, if vortex formation is prevented, Fr will not affect the power number % and will only be a function of Re. 

Mixer tanks fFigure 13-311 usually have baffles to improve the mixing. If the tank is enclosed and is operated full of liquid, baffles are not required. If there is a liquid-vapor interface, baffles are required to prevent vortex formation. Baffles typically have a width in the range 1/12 to 1/10 times the tank diameter... 

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