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Agitator / Impeller selection for stirred reactors (Part 2)

Agitator / Impeller selection for stirred reactors (Part 2)

16 Aug 2023

This blog post is the 2nd post in a two-part series on agitator selection for stirred reactors.

Very often during scale up parameters such as viscosity or solid fraction of a system are not accurately known. Even worse such parameters can often vary as the reaction proceeds e.g. some polymerizations can start relatively low viscosity but become extremely difficult to stir as the reaction proceeds. In such cases it is wise for the customer to size their agitator generously. Typically, the two limiting factors will be:

  • Shaft diameter: larger dia shafts allow more torque to be transmitted
  • Motor and magnetic coupling ratings

As we mentioned in the last blog post it is usually a good investment to oversize these components and also select a VFD in challenging lab systems. In particularly difficult situations we also advise customers to add a torque sensor to their system. This allows them to accurately measure the shaft torque and make sure that the system is not damaged by exceeding the design torque rating of any components. Such an interlock may also be provided which would alarm or shut off the system in case the actual torque approaches system limits.

Refer Fig. A for a photo of a typical torque sensor
Very often customers may not be aware of these possibilities. Another such option is the ability to interchange shafts. Often in labs one project may involve a low viscosity system where high rpm and a PBT is desirable but then the very next project is a high viscosity reaction which needs an anchor type impeller. Hence it is often quite economical to order multiple agitator options with the original order and the agitators can be quickly exchanged by the user themselves. Remembering to order such additional agitators with the original order can save weeks of waiting time later.

Refer Fig. C for the process to install a shaft
Finally, it is important to be always aware if a reaction is indeed mass transfer limited. If the rate limiting step is the reaction itself then changing agitators or rpm is unlikely to boost overall reaction rates.

Prediction of power draw by an impeller

This is more crucial during scale up since production planning requires an accurate estimate of power consumption at plant scale. Great resources are available for information on this. One useful reference is the book “Handbook of Industrial Mixing” by Paul and Kresta (Wiley-Interscience 2004). A figure reproduced from this reference is given below. By calculating the Renyolds number (Re) a figure like the one below allows an estimation of the Power number Np. Once Np is known the Power P can be estimated from the formula below. Note that at Re > 10,000 the flow within the agitated reactor is completely turbulent and in this regime the Power number is relatively constant. The equations below clearly show how the diameter of the impeller and the rpm have a huge effect on power consumption. As the rpm increases the power consumption shoots up and hence larger reactors typically will not be available with high rpm since the motor and shaft become very difficult to provide.

Again, talk to your vendor and they should have such correlations for their major impeller types which allows a customer some estimate of how much power their design will consume. Some other considerations often forgotten are to make sure that in case of flow reactors your inlet and outlet are not too close to each other lest fluids bypass the agitator and find their way to the outlet. For sensitive reactions another tip is to add some of the feed reagents very close to the agitator location often by using a dip tube. Dead zones are usually not a problem at lab scale but can be at larger pilot scale reactors. CFD (Computational Fluid Dynamics) simulations can provide useful insights about the presence of such dead zones with low mixing (e.g. behind baffles)

Scale up Considerations

Often the role of lab reactors during scale up is to allow the engineering team to accurately estimate power consumption, yields, selectivity and other performance parameters at production scale. In such cases it is crucial to use the exact same agitator design as is planned to be used at production plant. Ratios of agitator dia to vessel dia and height should also be maintained constant. Baffles (typically 8% or 10%) should also be chosen to closely mimic the final plant reactor. This is usually manufactured by a different vendor which makes it crucial to manage this part of design carefully. Incidentally one of the advantages of Amar is that we can manufacture reactors all the way up to 10,000 Litres and that allows increased flexibility with design under a single roof.

Things can get complicated when the lab reactor is manufactured from an exotic MOC like Hastealloy or Titanium however at plant scale a glass lined design is chosen. Glass coatings have severe limitations about what shapes they allow agitators to be made into. Often sharp corners are not compatible with glass coated agitator designs. In such cases it is better to first evaluate which impeller types are feasible at production scale and then order your scale up reactor accordingly. Lab reactors can usually be more flexible about agitator design and talk to our experts at Amar for various custom designs

Refer Fig. B for helical and other atypical designs for agitators

The important considerations during agitator scale up are “Geometric Similarity” which if ensured will make scale up more reliable. Of course, the difficult question is which kinematic parameters to keep constant. E.g. tip speed, rpm (usually not possible), power input per unit volume, blending times etc.

It is not possible to go into these details in this blog post but the interested reader should read the reference above (and many other excellent handbooks) or get in touch with Amar’s internal experts when planning to purchase your next lab reactor!

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