Importance of Simulating Stirred Mixers

Many production facilities use stirred reactors in their processes. To maximize fluid contact and reactions it is important to ensure that any solids within the fluid are properly distributed. This can be true whether the solid is a reactant, catalyst, microbial growth media or even microbial nutrients.

“However, simply spinning the impeller at high speeds to ensure a well-mixed vessel isn’t always the best plan of attack,” said Luke Munholand Technical Services Engineer at ANSYS Inc. “Blindly increasing impeller velocity is power intensive and will create high shear stresses. This stress can benefit gas transfer, but can also be very damaging to particles like those in mammalian cell cultures.  In addition, locally high shear rates may cause non-Newtonian fluids to mix poorly."

He added, “Disposable reactor liners and impellers often used in the bio pharma industry can become damaged by the stresses of higher mixing rates.”

Therefore, using CFD simulations to ensure optimal mixing while limiting power consumption, damage, and shear does have its advantages.

“Simulation modeling and methodology is getting advanced while HPC computing cost is getting lower. This makes for a great environment for researchers and designers to carry out detailed parametric study and design optimization using complex multiphase models in CFD simulations. This may not have been possible 10 years ago,” expressed Shitalkumar Joshi, Sr. Manager at ANSYS

Simple Modeling & Pre/Post Processing for Stirred Reactors

“ANSYS has a lot of experience in simulation of single phase and multiphase reactors,” said Joshi. “Our turbulence models are suitable for different situations like baffled, un-baffled, even single use bio-reactors. In fact, we offer a template which can be used to get the simulation running quickly and accurately.”

The mixing template allows users to build their tank’s geometry and mesh in a matter of minutes with little training or knowledge of simulation.

Users are asked a series of questions such as the type of tank used, as well as the type and number of impellers, baffles, probes and spargers. Once these questions are answered, the mesh and geometry is created automatically. The template even sets up the flow, mixing, and residence time within the reactor.

The mesh requirements for the simulation is typically light. As a result, the automatic tetrahedral mesh used by the template will be the best option in many situations. However, “It is important to have at least 2-3 cells in across the thickness of impeller blades,” explained Joshi.

Standard automatic tetrahedral meshes tent to work in these simulations. Just ensure at least 5 cells in slow moving areas.

More advanced users can use the template as a starting point for their more complicated designs and simulations. Users will be able to modify the geometry, mesh, and simulation parameters to match their own designs and customizations.

“As the template is based on ANSYS Workbench, users will also be able to perform parametric studies, optimizations and Design of Experiments (DoE) of their simulations.,” said Joshi.

Additionally, the ANSYS workbench stirred tank template can be used to automate the post processing and visualizations of the results.

“The workbench template not only allows you to perform parameter based optimizations, it can create comprehensive mixing report. This is a big advantage for those that need to quickly complete a report for upper management,” explained Joshi.

Turbulent Dispersion Forces’ Effect of Modeling Cloud Height

Maximizing contact between the solid and liquid phases facilitates mass transfer and reaction. Therefore, assessing the ability of the process to suspend is vital.

Eulerian models are generally used to accurately model solids suspension. It takes a few hours of simulation on 8 cores to run a steady state simulation.

“Eulerian-Granular will be more specific to those that want to detail the movement of suspended particles,” explained Munholand. “It will include drag functions and turbulent dispersion terms that predict particles very nicely.”

Joshi further explained that interactive forces like drag and turbulence interactions are important to properly simulate the particle movements, diameters, and density in the model. He said, “Drag is the most important directional force but turbulent dispersion forces are the second most important force associated with solids dispersion in stirred reactors. Unfortunately, this force isn’t well studied.”

To model the turbulent dispersion, Joshi suggested users choose the Simonin model. “It is typically useful with small particle sizes. Using this dispersion force model, analysts can better predict the cloud height within the reactors.”

Joshi and Munholand explained that ANSYS’s CFD models are tried and true by the time they make it into the software. The simulation techniques are based on academically peer reviewed models. Munholand said, “these models will yield accurate results and the template will ensure you get those results quickly. This will ensure there will be no surprises when your test the mixer in real life.”

To learn more about optimizing the solid distribution in a stirred tank follow this link.

To learn more about the simulation mixing tool follow this link.

ANSYS has sponsored this post. They have no editorial input to this post - all opinions are mine. Shawn Wasserman.