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Fluids/Solids Handling

Optimize Mixing by Using the Proper Baffles
Kevin J. Myers, University of Dayton Mark F. Reeder and Julian B. Fasano, Chemineer, Inc.

It is common knowledge that baffles promote better flow in an agitated vessel, but how to apply them and what kind to use take some ingenuity.

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gitated vessels are used throughout the chemical engineering industries (CEI) for diverseapplications including storing, blending and reacting materials. Agitator design requires specification of the motor, drive and impeller system that will satisfy both process (1, 2, 3) and mechanical (4) requirements. In addition, most agitated vessels are baffled, and the design of the baffle system must also economically satisfy process objectives.

Why use baffling? During agitation of alow-viscosity liquid, the rotating impeller imparts tangential motion to the liquid. Without baffling, this swirling motion approximates solid-body rotation in which little mixing actually occurs. Think about stirring a cup of coffee or a bowl of soup: The majority of the mixing occurs when the spoon is stopped or the direction of stirring is reversed. The primary purpose of baffling is to convert swirlingmotion into a preferred flow pattern to accomplish process objectives. The most common flow patterns are axial flow, typically used for blending and solids suspension, and radial flow, used for dispersion. However, baffling also has some other effects, such as suppressing vortex formation, increasing the power input and improving mechanical stability. A common agitation objective is suspendingsettling solids in a low-viscosity liquid, and Figure 1 il-

lustrates the effect of baffling on this. In the unbaffled vessel on the left, the swirling flow field is ineffective at dispersing the solids that are grouped in a rotating pile below the pitched-blade impeller. Also, a large surface vortex is visible at the top of the shaft. In the vessel on the right, the baffles are visible on the leftand right sides of the vessel and as a thin gray strip that bisects the impeller and shaft. The presence of baffles produces axial flow, in which the discharge flow produced by the impeller impinges on the base of the vessel, flows radially to the vessel wall, then up the wall, returning to the impeller from above. This flow pattern can be inferred from the solids that are distributed ratheruniformly throughout the liquid. All parameters (impeller, speed, solids, etc.) are the same in the two vessels in

s Figure 1. Agitation in an unbaffled vessel (left) leads to swirling flow
with surface vortex formation and poor solids distribution, while standard baffling (right) with a pitched-blade turbine promotes axial flow that results in good solids distribution.

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www.cepmagazine.orgFebruary 2002

CEP

Figure 1. The only difference is the presence of baffles. However, note that baffles do lead to a difference in power input. This point will be discussed later.

Why consider baffling alternatives? As evidenced by Figure 1, in most applications, the key to successful agitation is providing the proper flow field to achieve process objectives. In some instances, particularly the mostchallenging ones, selection of the proper baffling system is critical to providing the optimal flow field. Standard baffling Many agitated vessels, including the one on the right in Figure 1, use standard baffling, which consists of four flat vertical plates, radially-directed (i.e., normal to the vessel wall), spaced at 90 deg. around the vessel periphery, and running the length of the vessel’sstraight side. Standard baffle width is 1/10 or 1/12 of the vessel dia. (T/10 or T/12) (5). Sometimes, baffles are flush with the vessel wall and base, but, more often, gaps are left to permit the flow to clean the baffles. Recommended gaps are equal to 1/72 of the vessel dia. (T/72) between the baffles and the vessel wall, and 1/4 to one full baffle width between the bottom of the baffles and...