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Rheology and Environmental Solutions: A Global Approach to Mitigating Green House Gas Emissions
This field report discusses the efforts by Professor Ian Frigaard and his team at the University of British Columbia, Canada, to understand and control gas bubble dynamics in Yield StressYield stress is defined as the stress below which no flow occurs; literally behaves like a weak solid at rest and a liquid when yielded.yield stress fluids, such as those found in oil sands tailings ponds, to mitigate greenhouse gas emissions.
Their research explores the rheological properties of model fluids like Carbopol gels and Laponite suspensions to better understand bubble entrapment and release mechanisms. Studies were made using the NETZSCH Kinexus rheometer. The findings have broad implications for reducing emissions in various industries, including mining, nuclear waste storage, and wastewater treatment.
„Understanding bubble dynamics in Yield StressYield stress is defined as the stress below which no flow occurs; literally behaves like a weak solid at rest and a liquid when yielded.yield stress fluids opens pathways to reduce emissions from oil sands tailings ponds. Rheology is a key method to understand the underlying mechanisms and thus to predict the behavior and conceive strategies to reduce emissions. These studies, made with the NETZSCH Kinexus rotational rheometer, have broad implications for various industries, including mining, nuclear waste storage, and wastewater treatment.“
Dr. Ian Frigaard is a professor in the Department of Mechanical Engineering at the University of British Columbia, Canada. He specializes in Non-NewtonianA non-Newtonian fluid is one that exhibits a viscosity that varies as a function of the applied shear rate or shear stress.non-Newtonian fluid mechanics, focusing on the industrial applications of visco-plastic fluids, particularly in the petroleum industry. His interdisciplinary research group combines mathematical, experimental, and computational approaches to address issues like well cementing and greenhouse gas emission control. Dr. Frigaard has also authored numerous scientific papers, contributing significantly to the understanding and advancement of fluid mechanics.
High Goals: Net-Zero Emissions by 2050
In June 2021, Canada took a significant step toward climate action by enacting the Canadian Net-Zero Emissions Accountability Act, which aims to reach net-zero emissions by 2050. This commitment underscores the urgency for all industries to examine their emissions footprint and minimize their environmental impacts. The oil sands industry is under the spotlight due to its major contributions to Canada's greenhouse gas emissions. Recent data indicates that in 2020, approximately seven megatons of methane and carbon dioxide were emitted from oil sands tailings ponds, where the by-products of the oil sands production process are stored.
Regions such as Canada, the United States, Brazil, Russia, and South Africa all face similar challenges with tailings ponds, particularly in their mining and oil extraction industries.
Professor Ian Frigaard and his team at the University of British Columbia's (UBC) Complex Fluids Group are tackling the issue from a fluid mechanics perspective. They aim to understand the mechanism of bubbles stability and migration in these systems, its link to the rheology of material and eventually engineer the system such that the gas bubbles release and entrapment can be controlled in an advantageous way. Their research holds significant potential not only for Canada but for all countries where industrial by-products must be stored safely and efficiently. From nuclear waste storage sites to gas emission from oil wells in the Middle East, Central Asia, and Latin America, and even sewage treatment facilities in Europe, understanding the dynamics of gas bubbles in viscoplastic fluids could greatly impact global efforts to reduce emissions and promote sustainable practices.
Understanding the Mechanism of Bubbles Stability and Migration by Means of Rheology
The tailings ponds consist of FFT (Fine Fluid Tailings) and MFT (Mature Fine Tailings) layers, comprising water, sand, anaerobic microorganisms, and naphtha. Microbial degradation of naphtha in these layers leads to the production of methane and carbon dioxide, contributing to GHG emissions.
The tailings materials exhibit characteristics of Yield StressYield stress is defined as the stress below which no flow occurs; literally behaves like a weak solid at rest and a liquid when yielded.yield stress fluids, behaving like a solid below a certain threshold StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.stress (Yield StressYield stress is defined as the stress below which no flow occurs; literally behaves like a weak solid at rest and a liquid when yielded.yield stress) and flowing like a liquid above this threshold, which enables them to retain gas bubbles.
The research conducted by the complex fluids group at UBC involves lab experiments, models, and computations to understand bubble entrapment and release, explore physical processes, and investigate how fluid rheology can potentially control GHG emissions from the ponds. The core of this fundamental research study is to determine the yielding limit for the static stability of bubbles in yield StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.stress fluids and establish its connection with the complex rheology of these materials, including yield StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.stress, elasticity, and thixotropic behavior. The rheological studies have been performed using NETZSCH Kinexus Pro + rheometer . They used Carbopol gels and Laponite as models for simple yield StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.stress fluids and thixotropic yield-StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.stress fluids, respectively.
Rheological Behavior of model fluids
The representative rheological curves for Carbopol gels are shown in the following figure. The rheology of Carbopol was measured through a shear-rate controlled ramp-up and ramp-down test using a roughened parallel-plate geometry. Above the yielding point, no thixotropic behavior was observed. Below the yielding point, the elastic response of the gel caused a deviation between the ramp-up and ramp-down flow curves. The inset of this figure shows the Elastic modulusThe complex modulus (elastic component), storage modulus, or G’, is the “real” part of the samples the overall complex modulus. This elastic component indicates the solid like, or in phase, response of the sample being measurement. elastic modulus (G') and Viscous modulusThe complex modulus (viscous component), loss modulus, or G’’, is the “imaginary” part of the samples the overall complex modulus. This viscous component indicates the liquid like, or out of phase, response of the sample being measurement. viscous modulus (G'') as functions of StrainStrain describes a deformation of a material, which is loaded mechanically by an external force or stress. Rubber compounds show creep properties, if a static load is applied.strain amplitude, obtained from an amplitude sweep test at a frequency of 2 rad/s. For StrainStrain describes a deformation of a material, which is loaded mechanically by an external force or stress. Rubber compounds show creep properties, if a static load is applied.strain amplitudes below approximately 0.1%, both moduli remain constant, indicating linear behavior.
These findings demonstrate that Carbopol, at concentrations below 2%, behaves as a simple elasto-viscoplastic fluid without discernible thixotropic behavior.
Carbopol 0.15% (Simple yield StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.stress fluid) [3]
Laponite has been confirmed as a model fluid exhibiting thixotropic behavior through a series of rheological tests. The following figure presents the flow curve for a 1% Laponite sample being at rest for 10 minutes following a pre-shearing. Up on the rest period, the sample was subjected to a StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.stress-controlled ramp-up (circles) and ramp-down (downward-pointing triangles) using a roughened geometry. The thixotropic behaviour of the material manifests itself in the discernible discrepancy between ramp-up and ramp-down curves. They also measured the Elastic modulusThe complex modulus (elastic component), storage modulus, or G’, is the “real” part of the samples the overall complex modulus. This elastic component indicates the solid like, or in phase, response of the sample being measurement. elastic modulus (squares) and Viscous modulusThe complex modulus (viscous component), loss modulus, or G’’, is the “imaginary” part of the samples the overall complex modulus. This viscous component indicates the liquid like, or out of phase, response of the sample being measurement. viscous modulus (plus signs) versus StrainStrain describes a deformation of a material, which is loaded mechanically by an external force or stress. Rubber compounds show creep properties, if a static load is applied.strain, through a dynamic StrainStrain describes a deformation of a material, which is loaded mechanically by an external force or stress. Rubber compounds show creep properties, if a static load is applied.strain amplitude sweep at a frequency of 2 Hz. The results which are shown in the inset of the following figure confirmed the linear viscoelastic behavior of the material at strains below 1%.
Laponite 1% (thixotropic yield StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.stress fluid): Flow curves [4]
Representative rheology curves for a Laponite suspension (Laponite 1%):
This figure displays the ramp-up and ramp-down flow curves obtained from a shear rate-controlled test. There is a discernible hysteresis in the flow curves which marks the thixotropic behaviour of the material. The dynamic behaviour of the material measured using an amplitude sweep test is presented in the inset of this figure. Results show that Laponite is a suitable model for a time-dependent viscoplastic fluid.
Laponite 1% (thixotropic yield StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.stress fluid) : static and dynamic yield stresses [4]
Single shear rate tests for a Laponite suspension (Laponite 1%): The test was performed for a sample at various ageing times including 10 min (red), 2 h (blue), and 2 days (black). The material was imposed to a constant low shear rate of 0.001/s after a resting period following a 100/s pre-shear for 2 min. The results show the growth of static yield StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.stress (marked by filled circles) with the ageing time.
Key findings:
In summary, this research has uncovered two distinct mechanisms governing bubble release from yield stress fluids. In a homogeneous gel with non-thixotropic behavior, a quasi-uniform bubble cloud forms, and the overall rheological characteristics of the material, coupled with the proximity of the bubbles, dictate their release and entrapment within the system. At a pretty high gas concentration, this might lead to a bubble cloud burst upon static instability onset. However, when time-dependent rheology (ThixotropyFor most liquids, shear thinning is reversible and the liquids will at some point in time gain their original viscosity when a shearing force is removed.thixotropy) comes into play, the physical picture of the problem becomes more intricate.
The non-uniform structure of the materials coming from their shear history dependent rheology leads to the formation of damaged layers within which the material structure is weaker. The presence of these damaged layers within the material significantly influences bubble release and entrapment, preventing gas accumulation. In this case, the polydisperse bubble suspensions emerge and the bubble release occurs gradually through damaged layers rather than suddenly.
Mechanism of bubble release [4]
Normalized standard deviation of intensity (I) in sequential images of bubbles, captured right after the onset of instability for (a) a simple yield stress fluid and (b) a thixotropic yield stress fluid. Both gels have high initial gas content. The white spots in the images indicate areas where bubbles are moving within the gel, while the dark spots represent areas where the bubbles are stagnant. The network-like structure in panel (b) suggests that bubbles follow re-used pathways.
As larger bubbles escape to the surface, local shearing weakens the gel due to the material's shear history-dependent rheology, forming invisible conduits with less resistance. Bubbles then migrate towards these conduits, creating lateral weakened layers and eventually invisible networks of damaged layers connected to vertical conduits.
These networks allow smaller bubbles to be gradually released, preventing bubble accumulation, thus acting as safety valves in the system.
Broader Applications:
While this research is primarily motivated by the issue of GHG emissions from oil sands tailings, the findings have far-reaching implications. Understanding how gas is trapped and emitted in viscoplastic fluids has applications in several other fields: For example, nuclear waste storage can lead to “bubble and sludge” problems, wastewater treatment (sewage) involves Non-NewtonianA non-Newtonian fluid is one that exhibits a viscosity that varies as a function of the applied shear rate or shear stress.non-Newtonian suspensions and gas bubbles, and oil and gas wells experience gas kicks during construction, where bubble propagation through yield stress fluids is common. Other applications include foaming of concrete for construction and chocolate for taste enhancement.
In summary, understanding bubble dynamics in yield stress fluids offers a pathway to reduce emissions from oil sands tailings and opens doors to innovations across various industries. Rheology is a key method to understand the underlying mechanisms and thus to predict the behavior and reduce the emissions.
Dr. Frigaard's interdisciplinary research team, focusing on viscoplastic fluids and the application of non-Newtonian fluid properties in industrial processes:
A few papers showcasing their findings are listed below:
The following papers explained the theoretical models developed for the stability of bubbles in yield stress fluids. The yielding limit for the bubbles and the effects of the bubbles’ shapes and bubbles’ interactions on it are studied in these theoretical works.
[1] Pourzahedi, A., Chaparian, E., Roustaei, A., & Frigaard, I. A. (2022). Flow onset for a single bubble in a yield-stress fluid. Journal of Fluid Mechanics, 933, A21.
[2] Chaparian, E., & Frigaard, I. A. (2021). Clouds of bubbles in a viscoplastic fluid. Journal of Fluid Mechanics, 927, R3.
The following papers studied the bubbles growth and stability in yield stress material using an experimental approach. The role of complex rheology of the material including its elasticity and ThixotropyFor most liquids, shear thinning is reversible and the liquids will at some point in time gain their original viscosity when a shearing force is removed.thixotropy are explained in the following papers. Also, different scenarios for bubble clouds instability and its link to the rheology and structure of the material is explained here.
[3] Daneshi, M., & Frigaard, I. A. (2023). Growth and stability of bubbles in a yield stress fluid. Journal of Fluid Mechanics, 957, A16.
[4] Daneshi, M., & Frigaard, I. A. (2024). Growth and static stability of bubble clouds in yield stress fluids. Journal of Non-NewtonianA non-Newtonian fluid is one that exhibits a viscosity that varies as a function of the applied shear rate or shear stress.Non-Newtonian Fluid Mechanics, 327, 105217.
The effect of non-uniform rheology of the material on the bubble stability and migration is highlighted in the following work. Numerical simulations combined with experiments are used to investigate this problem.
[5] Zare, M., Daneshi, M., & Frigaard, I. A. (2021). Effects of non-uniform rheology on the motion of bubbles in a yield-stress fluid. Journal of Fluid Mechanics, 919, A25.