Rheology, the study of the flow and deformation of materials, plays a critical role in the biogas industry as the anaerobic digestion (AD) process involves the mixing of complex fluids with non-Newtonian behaviour. This blog will explore why an understanding of rheology is so crucial, but first, who remembers the fun of playing with cornstarch and water? (See Figure 1).
Figure 1. Oobleck, cornstarch and water solution, reacting as solid.
Oobleck and Non-Newtonian Fluids
Oobleck, the mixture of cornstarch and water, has long delighted children with its seemingly fickle tendencies: at rest it flows like a liquid, but under sudden pressure or force, it solidifies. This is because Oobleck is a non-Newtonian fluid.
Fluids can be categorised into Newtonian and non-Newtonian. For Newtonian fluids, including oil and water, the ratio of shear stress to shear rate is independent of the shear stress. On the other hand, for non-Newtonian fluids, this ratio depends on the applied stress. Non-Newtonian fluids can be further categorised, depending on how this ratio reacts to changes in stress (see Figure 2). The ratio of shear stress to shear rate is known as viscosity or apparent viscosity depending on whether we are referring to Newtonian or non-Newtonian fluids, respectively, and represents the resistance of a fluid to flow.
Figure 2. Classification of non-Newtonian fluids
Let’s say we have a stirred tank filled with a fluid at a constant temperature. If that fluid is water, its viscosity will be the same at every point of the system, no matter how close or far away from the stirrer and regardless of the stirring velocity. However, if that fluid is non-Newtonian, the apparent viscosity will have a spatial profile. This is because its value at each point will depend on the shear experienced by the fluid, which will vary depending on the distance to the stirrer and external walls (see Figure 3). It will also depend on the mixing velocity. The apparent viscosity may even show time variations.
Figure 3. (a) Illustration of shear stress distribution in a single-impeller tank. (b) Apparent viscosity contour for a multiple-impeller tank filled with a non-Newtonian fluid.
As a result, understanding the rheology of fluids is fundamental to optimising processes. For the biogas industry, it directly impacts mixing effectiveness in the digester and, therefore, energy consumption and biogas production.
Non-Newtonian Fluids in AD Systems
Non-Newtonian fluids in AD systems include substrates such as animal manures, organic slurries and sludge. The rheological properties of these different fluids depend on the total solids content and other biochemical compounds. Within the digester, these fluids are subjected to different factors, such as temperature, pH and stress, for example from pneumatic and mechanical mixing. The interaction of all these factors will determine the apparent viscosity of the fluids and ultimately the AD system’s efficiency.
Now, let’s consider three different substrates in AD systems: food waste, agricultural residues, and sewage sludge (see Figure 4). Each substrate exhibits unique rheological properties:
- Food Waste: Often behaves as a shear-thinning fluid. Its viscosity decreases with increased agitation, making it easier to mix at higher shear rates. Understanding this can help in designing mixers that operate efficiently at specific speeds, reducing energy consumption.
- Agricultural Residues: These may form fibrous slurries that exhibit yield stress behaviour, requiring a certain amount of force to initiate flow. Proper rheological analysis ensures that mixers can overcome this initial resistance, preventing clogging and ensuring uniform mixing.
- Sewage Sludge: This can be a complex non-Newtonian fluid with thixotropic behaviour, meaning its viscosity decreases over time under constant shear. This property can be exploited by continuously operating mixing systems to maintain low viscosity, enhancing flow and digestion rates.
Figure 4. Examples of AD substrates. (a) food waste, (b) agricultural residues, and (c) sewage sludge.
Hence, leaders can maximise energy efficiency and mixing effectiveness by understanding how the fluid rheology will affect the process under different operating conditions. This is where we often support leaders, using our virtual prototypes and piloting services to test mixing systems and then recommend the ideal operating conditions for each unique digester.
Optimising Biogas Production Through Virtual Prototyping and Piloting
By accurately modelling and predicting flow behaviour, virtual prototypes can help optimise system design and operation of anaerobic digesters. Factoring in substrate rheology is key to boosting the productivity and sustainability of biogas production in three main ways:
1. Energy Efficiency
Virtual prototypes allow for estimations of the energy consumption of a mixing system and this information is used to support the selection of the right mixing system and operating conditions for each digester (see Figure 5).
Figure 5. Virtual prototype of the MDA©, an anaerobic digester developed by ProCycla, for assessing the mixing efficiency of different pneumatic systems. Rheological parameters were used as a function of total solids content.
2. Mixing Efficiency
Virtual prototypes can be used to identify and mitigate issues such as stratification, sedimentation, bypass and the formation of dead zones, all of which can hinder the digestion process (see Figure 6).
Figure 6. Virtual prototype of the MDA©, an anaerobic digester developed by ProCycla, for assessing the effect of stirring velocity on mechanical mixing energy consumption. Rheological parameters were used as a function of total solids content.
3. Biogas Production
Virtual prototypes that integrate both fluid dynamics and kinetic modelling enable a comprehensive view of how the mixing heterogeneities, resulting from the rheological complexities of the fluid, can affect AD stability and biogas yield (see Figure 7).
Figure 7. CFD-based compartmental modelling approach used by Modela to integrate the fluid dynamics and kinetics of AD processes. Rheological parameters were used as a function of total solids content.
Conclusion
Productivity in the biogas industry. By recognising and leveraging the unique flow behaviours of non-Newtonian fluids within anaerobic digesters, industry leaders can design more efficient systems that maximise biogas production while minimising energy consumption, thereby enhancing the productivity and sustainability of the system.