Mechanical mixing is foundational to the efficient and effective running of bioindustrial processes, ensuring the maximisation of yields and productivity. Whether focusing on enhancing product formation or minimising energy consumption, ensuring the right configuration of mechanical mixing is mission–critical. Two examples of stirred bioreactors are shown in Figure 1.
Figure 1. Examples of CFD domains for bioreactors with different types of mechanical mixers. (a) anaerobic digester with horizontal propeller and (b) fermenter with vertical dual flat blade turbine.
According to various industries, mechanical mixing is critical for:
– Bio process performance
- Efficient mechanical mixing ensures optimal contact between microorganisms and substrate without excesive stress by shear, promoting their growth and/or biochemical activity.
- Rapid mixing is fundamental to processes such as disinfection and flocculation in wastewater treatment plants. However, the right trade-off with energy consumption minimisation must be found for sustainability and profitability purposes.
– Temperature control
- Mechanical mixing helps to mantain homogeneous temperature profiles in bioreactors, ensuring consistent and favourable conditions for microorganisms to thrive.
– Reduced maintenance costs
- Proper mechanical mixing helps to minimise the precipitation of solids, reducing the mainteneance costs associated with plant power down for bioreactors cleaning.
– Process stability
- By maintaining stable mixing, key parameters such as pH levels can be better controlled, optimising conditions for stable microorganism’s activity.
Computational Fluid Dynamics (CFD) allows us to simulate gas and liquid behaviour within complex stirred systems such as bioreactors, so that we can analyse the fluid behaviour, flow patterns and the distribution of nutrients and microorganisms. Figure 2 illustrates a few examples. The retrieved information serves as an input to decision-making to reduce CAPEX and OPEX spending. It also provides crucial data that can support industrial plants in achieving their environmental sustainability goals.
Figure 2. Examples of CFD simulations of stirred bioreactors. (a) Prediction of surface vortex formation; (b) characterisation of flow patterns; and (c) profile of species concentration over time and space.
CFD also allows us to predict, characterise and measure how the mechanical mixing is operating within these systems, in order to continually pursue a more effective and more efficient process. Reaching a (pseudo) steady state with minimal dead zones is a desired condition in most bioprocesses. Implementing a virtual sensor (also known as soft sensor) within the simulation allows us to monitor the evolution of a particular variable, such as velocity, and precisely determine when the system is well-mixed and homogeneous. Figure 3 illustrates the use of CFD for mixing quality monitoring.
Figure 3. Use of CFD simulations as soft sensors in stirred bioreactors. The diagrams show virtual monitoring of (a) dead zones based on the volumetric distribution of the velocity magnitude, and (b) steady state based on the stability of velocity at points far away from the impeller.
In summary, mechanical mixing is a core tool in the management of bioprocesses, enabling industrial plants to rapidly enhance yields and reduce costs. And CFD modelling enables us to optimise it, reducing costs and risks.
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