The model of South et al. [South et al. (1995) Enzyme Microb Technol 17(9): 797-803] for simultan... more The model of South et al. [South et al. (1995) Enzyme Microb Technol 17(9): 797-803] for simultaneous saccharification of fermentation of cellulosic biomass is extended and modified to accommodate intermittent feeding of substrate and enzyme, cascade reactor configurations, and to be more computationally efficient. A dynamic enzyme adsorption model is found to be much more computationally efficient than the equilibrium model used previously, thus increasing the feasibility of incorporating the kinetic model in a computational fluid dynamic framework in the future. For continuous or discretely fed reactors, it is necessary to use particle conversion in conversion-dependent hydrolysis rate laws rather than reactor conversion. Whereas reactor conversion decreases due to both reaction and exit of particles from the reactor, particle conversion decreases due to reaction only. Using the modified models, it is predicted that cellulose conversion increases with decreasing feeding frequency (feedings per residence time, f). A computationally efficient strategy for modeling cascade reactors involving a modified rate constant is shown to give equivalent results relative to an exhaustive approach considering the distribution of particles in each successive fermenter.
The flow pattern, pressure drop and the mixing characteristics of Kenics™ static mixers are inves... more The flow pattern, pressure drop and the mixing characteristics of Kenics™ static mixers are investigated by means of computer simulations. The static mixer consists of a series of alternating left and right hand helical elements. The simulations gave new insights in the flow pattern in the helical mixing elements. The pressure drop predictions compare favorably with literature data. Mixing in the elements occurs through a combination of flow splitting and shearing at the junctions of successive elements and a stretching and folding mechanism within the elements. This makes the Kenics element an excellent radial mixing device.
Computational Fluid Dynamics (CFD) methods for turbulent flows have reached a stage where it is f... more Computational Fluid Dynamics (CFD) methods for turbulent flows have reached a stage where it is feasible to apply them to real problems such as mixer design in chemical process industries. However, there are many potential difficulties for CFD to predict turbulent mixing in complex geometries and in flows involving large-scale, time-dependent phenomena. Many closures (kappa-ε and full second-order Reynolds Stress models), while employed in commercial codes, have yet to be validated and improved for complex flows. ...
In this study, the application of large eddy simulation (LES) to the prediction of largescale cha... more In this study, the application of large eddy simulation (LES) to the prediction of largescale chaotic structures in stirred tanks is investigated. Flow regimes representing typical stirrer configurations were assessed: a single radial pumping impeller, and a single axial pumping pitched blade turbine. The turbulent flow field in each configuration was calculated using LES turbulence models. The impellers were modeled using the sliding mesh model. The predicted flow patterns compared well with digital particle image velocimetry data reported in the literature, and exhibited the long time scale instabilities seen in the experiments. The results of these studies open the way to a renewed interpretation of many previously unexplained hydrodynamic phenomena that are observed in stirred vessels.
The Eulerian-granular model as implemented in FLUENT 6 was validated based on experimental solids... more The Eulerian-granular model as implemented in FLUENT 6 was validated based on experimental solids concentration data for a number of challenging validation cases consisting of tall stirred vessels equipped with multiple impellers. It was concluded that good results could be obtained for a variety of systems. The use of a correction to the standard drag laws for particles settling in still fluids is necessary to obtain good results in turbulent stirred vessels. Good results were obtained using eddy-viscosity turbulence models for fully baffled vessels. For unbaffled vessels, the use of the multiphase Reynolds stress model was necessary in order to obtain a good comparison between the experimental data and the predictions.
An understanding of the parameters that govern the just-suspended impeller speed, N js , and the ... more An understanding of the parameters that govern the just-suspended impeller speed, N js , and the distribution of solids, is critical to the efficient operation of hydrometallurgical and other processes involving solid-liquid suspensions. In this paper, the distribution of solids in stirred tanks under a range of solids loadings (0.5 to 50 vol%) was predicted using CFD and validated against experimental data obtained from the literature. The multiphase flow is modeled using the Eulerian Granular Multiphase model. This paper will also review the established design parameter N js in the context of scale-up and compare it to the quality of solids dispersion as a means of assessing correct scale-up in suspension tank design. The results of this study will describe a straightforward procedure to obtaining comprehensive information about reactor behavior with complex CFD models. The performance of hydrofoil impellers and a 45° pitched-blade turbine at suspending solids under different agitation speeds was studied. Both single and dual impeller operation have been evaluated. The settled solids fraction for speeds below N js , and the cloud height for impeller speeds above N js were predicted. The CFD predictions are in good agreement with experimental literature data on velocity distribution and cloud height.
By using flow-analysis and visualization techniques, engineers can identify potential trouble spo... more By using flow-analysis and visualization techniques, engineers can identify potential trouble spots in mixing systems. Increasingly, designers of various types of mixing equipment are turning to modern tools for flow analysis, such as computational fluid dynamics, Laser Doppler velocimetry and particle image velocimetry. Such quantitative design methods based on a fundamental understanding of microscopic as well macroscopic changes in the chemical and physical processes in the mixture are proving to be more effective than the traditional approaches.
The Canadian Journal of Chemical Engineering, 1997
A technique is proposed to predict the gassed power draw of mixed impeller systems as a weighted ... more A technique is proposed to predict the gassed power draw of mixed impeller systems as a weighted average of the gassed power draws of the individual impellers. The success of this method requires determining the gassed power draws of the individual impellers in the proper gassed environment. In the case of the upper impellers, this environment is provided by a rubber membrane sparger that distributes the gas evenly over the vessel cross section. Extensive comparisons of experimental gassed power draws of mixed impeller systems with model predictions verify the accuracy of the proposed technique. On propose une technique afin de predire la consommation de puissance en milieu aere de systemes de turbines multiples comme une moyenne ponderee de la consommation de puissance en milieu aere des turbines individuelles. Pour reussir, cette methode necessite qu'on determine les consommations de puissance en milieu aeree des turbines individuelles dans l'environnement d'aeration approprie. Dans le cas des turbines superieures, cet environnement est fourni par un aerateur a membrane de caoutchouc qui distribue le gaz de maniere egale au-dessus de la section transversale du reservoir. Des comparaisons extensives entre les consommations de puisance en milieu aere experimentales des systemes de turbines multiples et des predictions du modele demontrent la precision de la technique proposee.
The Canadian Journal of Chemical Engineering, 1998
The use of axial-flow impellers in the up-pumping mode has been increasing, but little performanc... more The use of axial-flow impellers in the up-pumping mode has been increasing, but little performance and design information is available. Up-pumping pitched-blade and high-efficiency impellers have been studied in solids suspension applications, and their performance has been compared to and contrasted with that of the more conventional down-pumping mode. Just-suspended speed data has been interpreted in terms of two literature correlations that can be used for design purposes. Just-suspended torque and power requirements are presented, as well as turbulent power number and flow pattern data. In general, the just-suspended torque and power requirements of up-pumping pitched-blade and high-efficiency impellers are substantially higher than those of the down-pumping mode. However, if a large impeller diameter is required to avoid critical speed limitations or to achieve sufficient power inputs at high solids loadings, then up-pumping impellers may be a viable option. L'utilisation de turbines a ecoulement axial en pompage ascendant s'est accrue, mais il existe peu d'informations sur leurs performances et leurs conceptions. Des turbines a pales inclinees et des turbines a grand rendement en pompage ascendant ont ete etudies dans des applications de suspensions de solides et leur performance a ete comparee et mise en perspective par rapport au mode de pompage descendant plus conventionnel. Des donnees de vitesse de mise en suspension critique ont ete interpretees relativement a deux correlations de la litterature scientifique, qui peuvent servir a des fins de conception. Le couple de mise en suspension critique et la consommation de puissance sont presentes, ainsi que le nombre de puissance turbulent et les donnees de profils d'ecoulement. En general, le couple de mise en suspension critique et la consommation de puissance des turbines a pales inclinees et des turbines a grand rendement en pompage ascendant sont substantiellement plus eleves que dans le cas du mode descendant. Cependant, si un gros diametre de turbine est necessaire pour eviter les limitations de vitesse critique ou pour obtenir des consommations de puissance suffisantes a des taux de solides eleves, alors les turbines en pompage ascendant peuvent ětre une option viable.
Digital particle image velocimetry (DPIV) has been used to examine the flow field in a vessel agi... more Digital particle image velocimetry (DPIV) has been used to examine the flow field in a vessel agitated by an axial-flow impeller in turbulent operation. Both a pitched-blade turbine and a high-efficiency impeller were studied. Time series analysis indicates that the flow field is not steady; rather, it is subject to transients with frequencies well below the blade passage frequency (periods ranging from 40 to over 300 impeller revolutions have been observed). This result has important implications for computational modeling because current descriptions of agitated vessels are based upon time-averaged flow fields with superimposed turbulence. This modeling approach may not accurately capture the mixing associated with the low-frequency phenomena observed in this study.
The model of South et al. [South et al. (1995) Enzyme Microb Technol 17(9): 797–803] for simultan... more The model of South et al. [South et al. (1995) Enzyme Microb Technol 17(9): 797–803] for simultaneous saccharification of fermentation of cellulosic biomass is extended and modified to accommodate intermittent feeding of substrate and enzyme, cascade reactor configurations, and to be more computationally efficient. A dynamic enzyme adsorption model is found to be much more computationally efficient than the equilibrium model used previously, thus increasing the feasibility of incorporating the kinetic model in a computational fluid dynamic framework in the future. For continuous or discretely fed reactors, it is necessary to use particle conversion in conversion-dependent hydrolysis rate laws rather than reactor conversion. Whereas reactor conversion decreases due to both reaction and exit of particles from the reactor, particle conversion decreases due to reaction only. Using the modified models, it is predicted that cellulose conversion increases with decreasing feeding frequency ...
Computational Fluid Dynamics (CFD) methods for turbulent flows have reached a stage where it is f... more Computational Fluid Dynamics (CFD) methods for turbulent flows have reached a stage where it is feasible to apply them to real problems such as mixer design in chemical process industries. However, there are many potential difficulties for CFD to predict turbulent mixing in complex geometries and in flows involving large-scale, time-dependent phenomena.
... study of the gas dispersion process in those hydrodynamic regimes where the flow is stable. T... more ... study of the gas dispersion process in those hydrodynamic regimes where the flow is stable. The development of a predictive model for the gas-liquid flow in stirred vessels is considered. This model is incorporated in a code named GHOST (Gas Holdup Simulation Tool) which ...
The model of South et al. [South et al. (1995) Enzyme Microb Technol 17(9): 797-803] for simultan... more The model of South et al. [South et al. (1995) Enzyme Microb Technol 17(9): 797-803] for simultaneous saccharification of fermentation of cellulosic biomass is extended and modified to accommodate intermittent feeding of substrate and enzyme, cascade reactor configurations, and to be more computationally efficient. A dynamic enzyme adsorption model is found to be much more computationally efficient than the equilibrium model used previously, thus increasing the feasibility of incorporating the kinetic model in a computational fluid dynamic framework in the future. For continuous or discretely fed reactors, it is necessary to use particle conversion in conversion-dependent hydrolysis rate laws rather than reactor conversion. Whereas reactor conversion decreases due to both reaction and exit of particles from the reactor, particle conversion decreases due to reaction only. Using the modified models, it is predicted that cellulose conversion increases with decreasing feeding frequency (feedings per residence time, f). A computationally efficient strategy for modeling cascade reactors involving a modified rate constant is shown to give equivalent results relative to an exhaustive approach considering the distribution of particles in each successive fermenter.
The flow pattern, pressure drop and the mixing characteristics of Kenics™ static mixers are inves... more The flow pattern, pressure drop and the mixing characteristics of Kenics™ static mixers are investigated by means of computer simulations. The static mixer consists of a series of alternating left and right hand helical elements. The simulations gave new insights in the flow pattern in the helical mixing elements. The pressure drop predictions compare favorably with literature data. Mixing in the elements occurs through a combination of flow splitting and shearing at the junctions of successive elements and a stretching and folding mechanism within the elements. This makes the Kenics element an excellent radial mixing device.
Computational Fluid Dynamics (CFD) methods for turbulent flows have reached a stage where it is f... more Computational Fluid Dynamics (CFD) methods for turbulent flows have reached a stage where it is feasible to apply them to real problems such as mixer design in chemical process industries. However, there are many potential difficulties for CFD to predict turbulent mixing in complex geometries and in flows involving large-scale, time-dependent phenomena. Many closures (kappa-ε and full second-order Reynolds Stress models), while employed in commercial codes, have yet to be validated and improved for complex flows. ...
In this study, the application of large eddy simulation (LES) to the prediction of largescale cha... more In this study, the application of large eddy simulation (LES) to the prediction of largescale chaotic structures in stirred tanks is investigated. Flow regimes representing typical stirrer configurations were assessed: a single radial pumping impeller, and a single axial pumping pitched blade turbine. The turbulent flow field in each configuration was calculated using LES turbulence models. The impellers were modeled using the sliding mesh model. The predicted flow patterns compared well with digital particle image velocimetry data reported in the literature, and exhibited the long time scale instabilities seen in the experiments. The results of these studies open the way to a renewed interpretation of many previously unexplained hydrodynamic phenomena that are observed in stirred vessels.
The Eulerian-granular model as implemented in FLUENT 6 was validated based on experimental solids... more The Eulerian-granular model as implemented in FLUENT 6 was validated based on experimental solids concentration data for a number of challenging validation cases consisting of tall stirred vessels equipped with multiple impellers. It was concluded that good results could be obtained for a variety of systems. The use of a correction to the standard drag laws for particles settling in still fluids is necessary to obtain good results in turbulent stirred vessels. Good results were obtained using eddy-viscosity turbulence models for fully baffled vessels. For unbaffled vessels, the use of the multiphase Reynolds stress model was necessary in order to obtain a good comparison between the experimental data and the predictions.
An understanding of the parameters that govern the just-suspended impeller speed, N js , and the ... more An understanding of the parameters that govern the just-suspended impeller speed, N js , and the distribution of solids, is critical to the efficient operation of hydrometallurgical and other processes involving solid-liquid suspensions. In this paper, the distribution of solids in stirred tanks under a range of solids loadings (0.5 to 50 vol%) was predicted using CFD and validated against experimental data obtained from the literature. The multiphase flow is modeled using the Eulerian Granular Multiphase model. This paper will also review the established design parameter N js in the context of scale-up and compare it to the quality of solids dispersion as a means of assessing correct scale-up in suspension tank design. The results of this study will describe a straightforward procedure to obtaining comprehensive information about reactor behavior with complex CFD models. The performance of hydrofoil impellers and a 45° pitched-blade turbine at suspending solids under different agitation speeds was studied. Both single and dual impeller operation have been evaluated. The settled solids fraction for speeds below N js , and the cloud height for impeller speeds above N js were predicted. The CFD predictions are in good agreement with experimental literature data on velocity distribution and cloud height.
By using flow-analysis and visualization techniques, engineers can identify potential trouble spo... more By using flow-analysis and visualization techniques, engineers can identify potential trouble spots in mixing systems. Increasingly, designers of various types of mixing equipment are turning to modern tools for flow analysis, such as computational fluid dynamics, Laser Doppler velocimetry and particle image velocimetry. Such quantitative design methods based on a fundamental understanding of microscopic as well macroscopic changes in the chemical and physical processes in the mixture are proving to be more effective than the traditional approaches.
The Canadian Journal of Chemical Engineering, 1997
A technique is proposed to predict the gassed power draw of mixed impeller systems as a weighted ... more A technique is proposed to predict the gassed power draw of mixed impeller systems as a weighted average of the gassed power draws of the individual impellers. The success of this method requires determining the gassed power draws of the individual impellers in the proper gassed environment. In the case of the upper impellers, this environment is provided by a rubber membrane sparger that distributes the gas evenly over the vessel cross section. Extensive comparisons of experimental gassed power draws of mixed impeller systems with model predictions verify the accuracy of the proposed technique. On propose une technique afin de predire la consommation de puissance en milieu aere de systemes de turbines multiples comme une moyenne ponderee de la consommation de puissance en milieu aere des turbines individuelles. Pour reussir, cette methode necessite qu'on determine les consommations de puissance en milieu aeree des turbines individuelles dans l'environnement d'aeration approprie. Dans le cas des turbines superieures, cet environnement est fourni par un aerateur a membrane de caoutchouc qui distribue le gaz de maniere egale au-dessus de la section transversale du reservoir. Des comparaisons extensives entre les consommations de puisance en milieu aere experimentales des systemes de turbines multiples et des predictions du modele demontrent la precision de la technique proposee.
The Canadian Journal of Chemical Engineering, 1998
The use of axial-flow impellers in the up-pumping mode has been increasing, but little performanc... more The use of axial-flow impellers in the up-pumping mode has been increasing, but little performance and design information is available. Up-pumping pitched-blade and high-efficiency impellers have been studied in solids suspension applications, and their performance has been compared to and contrasted with that of the more conventional down-pumping mode. Just-suspended speed data has been interpreted in terms of two literature correlations that can be used for design purposes. Just-suspended torque and power requirements are presented, as well as turbulent power number and flow pattern data. In general, the just-suspended torque and power requirements of up-pumping pitched-blade and high-efficiency impellers are substantially higher than those of the down-pumping mode. However, if a large impeller diameter is required to avoid critical speed limitations or to achieve sufficient power inputs at high solids loadings, then up-pumping impellers may be a viable option. L'utilisation de turbines a ecoulement axial en pompage ascendant s'est accrue, mais il existe peu d'informations sur leurs performances et leurs conceptions. Des turbines a pales inclinees et des turbines a grand rendement en pompage ascendant ont ete etudies dans des applications de suspensions de solides et leur performance a ete comparee et mise en perspective par rapport au mode de pompage descendant plus conventionnel. Des donnees de vitesse de mise en suspension critique ont ete interpretees relativement a deux correlations de la litterature scientifique, qui peuvent servir a des fins de conception. Le couple de mise en suspension critique et la consommation de puissance sont presentes, ainsi que le nombre de puissance turbulent et les donnees de profils d'ecoulement. En general, le couple de mise en suspension critique et la consommation de puissance des turbines a pales inclinees et des turbines a grand rendement en pompage ascendant sont substantiellement plus eleves que dans le cas du mode descendant. Cependant, si un gros diametre de turbine est necessaire pour eviter les limitations de vitesse critique ou pour obtenir des consommations de puissance suffisantes a des taux de solides eleves, alors les turbines en pompage ascendant peuvent ětre une option viable.
Digital particle image velocimetry (DPIV) has been used to examine the flow field in a vessel agi... more Digital particle image velocimetry (DPIV) has been used to examine the flow field in a vessel agitated by an axial-flow impeller in turbulent operation. Both a pitched-blade turbine and a high-efficiency impeller were studied. Time series analysis indicates that the flow field is not steady; rather, it is subject to transients with frequencies well below the blade passage frequency (periods ranging from 40 to over 300 impeller revolutions have been observed). This result has important implications for computational modeling because current descriptions of agitated vessels are based upon time-averaged flow fields with superimposed turbulence. This modeling approach may not accurately capture the mixing associated with the low-frequency phenomena observed in this study.
The model of South et al. [South et al. (1995) Enzyme Microb Technol 17(9): 797–803] for simultan... more The model of South et al. [South et al. (1995) Enzyme Microb Technol 17(9): 797–803] for simultaneous saccharification of fermentation of cellulosic biomass is extended and modified to accommodate intermittent feeding of substrate and enzyme, cascade reactor configurations, and to be more computationally efficient. A dynamic enzyme adsorption model is found to be much more computationally efficient than the equilibrium model used previously, thus increasing the feasibility of incorporating the kinetic model in a computational fluid dynamic framework in the future. For continuous or discretely fed reactors, it is necessary to use particle conversion in conversion-dependent hydrolysis rate laws rather than reactor conversion. Whereas reactor conversion decreases due to both reaction and exit of particles from the reactor, particle conversion decreases due to reaction only. Using the modified models, it is predicted that cellulose conversion increases with decreasing feeding frequency ...
Computational Fluid Dynamics (CFD) methods for turbulent flows have reached a stage where it is f... more Computational Fluid Dynamics (CFD) methods for turbulent flows have reached a stage where it is feasible to apply them to real problems such as mixer design in chemical process industries. However, there are many potential difficulties for CFD to predict turbulent mixing in complex geometries and in flows involving large-scale, time-dependent phenomena.
... study of the gas dispersion process in those hydrodynamic regimes where the flow is stable. T... more ... study of the gas dispersion process in those hydrodynamic regimes where the flow is stable. The development of a predictive model for the gas-liquid flow in stirred vessels is considered. This model is incorporated in a code named GHOST (Gas Holdup Simulation Tool) which ...
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