Page 191 - Fluid mechanics, heat transfer, and mass transfer

MIXING EQUIPMENT 169

Without baffling, this swirling motion approximates
TABLE 6.5 Power Requirements for Baffled Vessels
3
Agitation Applications Power (kW/m ) solid body rotation in which little mixing actually
occurs.
Mild Blending, mixing 0.04–0.10
& Forexample,whilestirringacupofcoffeeorabowlof
Homogeneous reactions 0.01–0.03
soup, the majority of the mixing occurs when the
Medium Heat transfer 0.03–1.0
Liquid–liquid mixing 1.0–1.5 spoonisstoppedorthedirectionofstirringisreversed.
Vigorous Slurry suspension 1.5–2.0 & The primary purpose of baffling is to convert swirling
Gas absorption 1.5–2.0 motion into a preferred flow pattern to accomplish
Emulsions 1.5–2.0 process objectives.
Intense Fine slurry suspension >2.0
& The most common flow patterns are axial flow,
typically used for blending and solids suspension,
. What are the means normally used to increase mixing in and radial flow, used for dispersion.
a reactor? & The presence of baffles produces axial flow, in which
& By adding/improving baffles. the dischargeflow produced by the impeller impinges
on the base of the vessel, flows radially to the vessel
& Using a different mixer blade design.
wall, then up the wall, returning to the impeller from
& Installing higher rpm motor on the agitator.
above.
& Using multiple impellers.
& Baffling has some other effects as well, such as
& Pumped recirculation: addition and/or increasing the
suppressing vortex formation, increasing the power
quantity.
input, and improving mechanical stability.
& Usingin-linestaticmixersformixingenteringstreams.
. Givecommonly adopted geometries for an agitated tank
. How is a mixing operation scaled up? relative to its diameter (D).
& When changing the scale, that is, size of equipment, & Various geometries of an agitated tank relative to
with the goal of obtaining the same mixing perfor- diameter (D) of the vessel include the following:
mance at the new scale: ➢ Liquid level ¼ D.
(i) Geometric similarity should be preserved: di- ➢ Turbine impeller diameter ¼ D/3.
mensional ratios should be the same in the large
➢ Impeller level above bottom ¼ D/3.
tank as in the small one.
➢ Impeller blade width ¼ D/15.
(ii) Dynamic similarity should be preserved: Rey-
➢ Vertical baffle width ¼ D/10 (four baffles).
nolds numbers should be the same in the large
tank as in the small one. . What is standard baffling for an agitated vessel?
& It must be noted that the above analysis is correct for & Standard baffling consists of three or four flat vertical
Newtonian fluids only: it becomes more complex plates, each baffle occupying about 1/10th of tank
when the non-Newtonian behavior of materials is diameter and radially directed (i.e., normal to the
taken into account. vessel wall), spaced at 90 around the vessel periph-

ery and running the length of the straight side of the
vessel. They prevent tangential flow.
6.2 MIXING EQUIPMENT
& Flat plate baffles are common because of their ease of
manufacture and installation and the associated
6.2.1 Baffled Vessels
economy.
. What are the generally used dimensions for a mixing & Standard baffle width is 1/10th or 1/12th of the vessel
vessel? diameter.
& Generally circular with rounded bottoms to minimize
& Sometimes, baffles are flushed with the vessel wall
dead spots. and base, but, more often, gaps are left to permit the
& Depth to diameter ratio is normally 0.5–1.5 (1.0 is flowtocleanthebaffles.Recommendedgapsareequal
often recommended). to 1/72nd of the vessel diameter between the baffles
& If tall vessels are used, impellers should be installed and the vessel wall and 1/4th to one full baffle width
for each vessel diameter equal to height. between the bottom of the baffles and the vessel base.
. What is the purpose of using baffles in a mixer? . Give some rules of thumb for mixing.
& During agitation of a low-viscosity liquid, the rotat- & For fluid with viscosities less than 10,000 cP, baffles
ing impeller imparts tangential motion to the liquid. are highly recommended. There should be four