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Paul Kubera, Lightnin Americas
Efficient agitators disperse gas, keep solids suspended and circulate tank contents in multiphase systems
With the drive for increasingly reliable, predictable and productive process operations, it's critical to match process equipment to the application. The range of possible mixing applications provides process designers with plenty of challenge. Currently available hardware, however, provides ample opportunity to meet -- and often exceed -- expectations.
- Simultaneous solids suspension and gas dispersion/mass transfer (for example, in bacterial oxidation of minerals).
- Simultaneous gas dispersion and liquid circulation for heat transfer (e.g., hydrogenation).
- Applications requiring low values of impeller shear rate (e.g.,with fermentations involving shear-sensitive cultures).
Figure 3. Does Double Duty
A down-pumping impeller is designed for both solid suspension and gas dispension.
For gas rates below 5 percent, the impeller power draw under gassed conditions is slightly greater than when not gassed. As the gas rate exceeds 5 percent, power draw drops below that of the ungassed condition.
For gas rate between 7.5 percent and 21 percent, the high-efficiency impeller system can be used if the sparger introduces gas below the impeller, just outside its swept diameter. This sparge system uses simple downward-pointing pipes, rather than the conventional perforated ring, which makes it ideal if solids are present.
At the high end of the gas range, the power draw under gassed may drop to 70 percent of that for ungassed operation. Using a two-speed motor avoids under-utilizing the mixer's drive system because it takes advantage of the mixer's power response.
Improved efficiency designs
A number of innovations have improved the flow efficiency and power draw characteristics of radial flow impellers. A ring sparger encircling an efficient pump turbine-like impeller, for example, eliminates the large decrease in power draw that normally occurs with sparging.
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Many processes involve simple liquid or liquid-solid mixing. However, gas-liquid or gas-liquid-solid applications are more complex from both the process and mechanical design perspectives. Although a variety of impellers is available for mixing in multi-phase systems, no single approach is ideal for every application.
Flat-bladed turbine
The flat-bladed Rushton turbine (Fig. 1) is the historic standard for gas-liquid applications, and is widely used throughout the process industries. Given its simple construction, it can be "tuned" easily to an application by changing the blade count or by adjusting the turbine's diameter by changing the blade's radial position. While it features high pumping capacity, its radial discharge is not efficient for solids suspension. Similarly, it's capable of dissipating a large amount of energy, although this is done through its high shear rate.
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Figure 1. Flat-bladed Old Faithful
The flat-bladed impeller, the historic standard for gas-liquid applications, is not efficient for solids suspension.
The shape of the Rushton turbine blade and the vortex that forms behind it lead to a marked decrease in power draw when gas is introduced. Two-speed motors can optimize the drive system for both gassed and ungassed operation.
The variety of specialized gas handling impellers is better suited for dispersing gas than the Rushton turbine. However, applications requiring high-shear or high-energy dissipation rates remain the primary use for flat-blade turbines.
High-efficiency impellers
Axial flow impellers, typified by high-efficiency fluidfoil (Fig. 2) and marine propellers, are well suited for flow-controlled processes. These impellers develop the greatest pumping or liquid circulation rate for a given power input. They're ideal for simple blending, suspending solids and transferring heat. They also can be effective for dispersing small volumes of gas introduced beneath the impeller, as long as the gas rate is no more than 2 percent of the impeller's primary pumping capacity.
Figure 2. Fluidfoil Floods Frequently
High-efficiency fluidfoil impellers develop the greatest pumping rate for a given power output, but are prone to gas flooding.
A conventional, down-flow, high-efficiency fluidfoil impeller is the best choice for suspension of a solid-liquid slurry when there are small amounts of impurities to be oxidized using sparged air. Low gas rates don't affect the impeller's power draw markedly.
As the ratio of gas volume to impeller pumping capacity exceeds 2 percent, conventional fluidfoil impellers don't have sufficient blade area to disperse the gas effectively. Much of the gas simply passes through the impeller, a condition known as flooding. The impeller's power draw also fluctuates and drops during the flooded condition.
Gas-dispersing impellers
Introducing higher gas volumes beneath a down-pumping axial flow impeller has the same deleterious effect as increasing the system's overall resistance. Gas rates between 2.5 percent and 7.5 percent require high-efficiency impellers that feature a greater number of wider blades to disperse the gas effectively and maintain adequate liquid circulation. Impellers of this type (Fig. 3) have been used for:
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