Page 28 - Chemical Technology

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Chemical Technology • January 2013

Figure 9: Telltales on BT-6 impeller blade

solids handling

Under turbulent flow conditions, it is usually very

beneficial, both from an erosion standpoint as well as a

process efficiency standpoint, to select a high efficiency

style impeller. In transitional flow there are still benefits

to be achieved by the use of a high efficiency impeller,

but not as profound as in turbulent flow. In laminar flow,

there is no advantage in using a high-efficiency impeller

either for erosion or solids suspension efficiency.

Horsepower and speed selection

As discussed earlier, erosion is very dependent on veloc-

ity, and typically in erosion-corrosion environments, the

velocity exponent for the volumetric rate or weight rate of

erosion is generally observed to be 2,5 to 4,0. In design-

ing an agitator for the suspension of solids, the designer

has a choice of selecting a number of power and speed

combinations. Because solid suspension impeller ef-

ficiencies change with impeller style, impeller to tank

diameter and off-bottom clearance to tank diameter

19

, a

number of possible horsepower and speed choices can

meet process objectives.

The selection of a specific agitator design in the end

should come down to economics. There are capital costs

and operating costs. Capital costs are largely associated

with the general size of the machine. The torque can gen-

erally best characterize the capital cost. Operating costs

include the energy costs to operate the machine plus

any maintenance costs. Maintenance costs include the

costs of oil changes, new bearings, new gears, new seal

components, and in the case of erosion applications, the

replacement of in-tank wear components, typically impel-

ler blades. A close examination of the various horsepower

and speed options should be examined closely in order to

make an economic selection.

Example

For the sake of demonstration let us assume that we

need to design an agitator for suspending a 10% solu-

tion of sand in water. The sand will be assumed to have a

weight mean particle size of 360 μm. The tank is 3,66 m

diameter with a 2:1 elliptical dished bottom, and the water-

sand slurry will have a liquid volume such that the depth

of liquid in the tank is 3,66 m. The sand will be assumed

to have a specific gravity of 2,4 and the water a specific

gravity of 1,0. The viscosity of water will be assumed to be

1 mPa-s. The process solution requires that the solids be

suspended to the 'just suspended' condition such that no

ent and a gas must be dispersed they are often used. As

with axial flow impellers, impeller efficiency changes with

design. The Rushton or D-6 impeller, introduced in the

late 1940s, creates a pair of vortices behind each blade

in turbulent flow. These vortices, as we’ve observed with

the pitched-blade impeller, can cause severe erosion

from the backside of the blade. The Figure 7 photo of the

intertwined telltales on the backside of the blade demon-

strates the size and nature of these vortices.

Figure 6: Chemineer Maxflo W impeller

Figure 7: Telltales on D-6 impeller blade

Figure 8: Telltales on CD-6 impeller blade

There exist today highly efficient radial flow impellers.

The Chemineer CD-6 impeller was introduced into the

marketplace in 1988, and the even more efficient Chem-

ineer BT-6 was introduced in 1998. Both of these impel-

lers exhibit very little tendency for vortex formation on the

back-side of the blade. This is demonstrated in telltale

photos in Figure 8 and Figure 9. In each of these photos

the blades are rotating into the plane of the paper.