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Questions and Answers
What is the effect of high local pH on animal cell growth during culture?
What is the effect of high local pH on animal cell growth during culture?
How do pseudoplastic fluids behave under shear in stirred vessels?
How do pseudoplastic fluids behave under shear in stirred vessels?
What is a more economical solution for improving mixing instead of increasing fluid velocity?
What is a more economical solution for improving mixing instead of increasing fluid velocity?
What happens to the Reynolds number (Re)i as viscosity increases during mixing?
What happens to the Reynolds number (Re)i as viscosity increases during mixing?
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What is the primary characteristic of pseudoplastic fluids?
What is the primary characteristic of pseudoplastic fluids?
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Which factor primarily determines the intensity of turbulence in a mixing vessel?
Which factor primarily determines the intensity of turbulence in a mixing vessel?
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What is a potential drawback of increasing impeller speed for better mixing?
What is a potential drawback of increasing impeller speed for better mixing?
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What is a critical consequence of non-turbulent conditions during agitation?
What is a critical consequence of non-turbulent conditions during agitation?
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Which statement accurately relates to the effects of methanol on cell growth?
Which statement accurately relates to the effects of methanol on cell growth?
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What is the primary benefit of using multiple injection points in bioprocessing?
What is the primary benefit of using multiple injection points in bioprocessing?
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Study Notes
Mixing Mechanisms
- Effective mixing requires fluid circulation to sweep the entire vessel efficiently.
- Impeller speed must ensure the fluid can reach remote areas, promoting turbulence for optimal mixing.
- Mixing involves three physical processes: distribution, dispersion, and turbulence.
Distribution
- Distribution is the bulk transport of materials throughout the vessel via circulation currents.
- This process can be slow, especially in large tanks where circulation paths are extensive.
- High impeller speed can enhance distribution by introducing turbulence into the flow.
Dispersion
- Turbulent flow results in the formation of eddies, which range in size and are essential for material transfer.
- Larger eddies break into smaller ones, facilitating rapid material transfer and leading to heat dissipation.
- The size of the smallest eddies restricts the degree of homogeneity achievable during mixing.
Mixing Time
- Mixing time (tm) quantifies the time required to reach a specific level of homogeneity from a segregated state.
- Factors affecting tm include tank size, impeller type, fluid viscosity, and speed.
- Mixing time can be measured by injecting tracers (e.g., acids, salts) and monitoring concentration changes.
Mixing Time Measurement
- Tracers are tracked using pH probes or conductivity cells for concentration analysis.
- Mixing time is defined as the interval where the tracer concentration stabilizes within 10% of the total concentration difference (Cf− Ci).
- For stirred tanks, mixing time (tm) is approximately four times the circulation time (tc).
Power Requirements for Mixing
- Impeller driving power is influenced by the fluid's resistance during rotation.
- Power consumption varies with tank size: 10 kW m−3 for small vessels and 1-2 kW m−3 for larger ones.
- Electrical power usage considers energy loss due to friction in motors and gearbox, necessitating higher input than the mixing power.
Improving Mixing
- Installing baffles enhances turbulence and overall mixing efficiency.
- Optimal impeller positioning is critical; it should be located below the vessel's geometric center for effective circulation.
- Multiple impellers can be used for improved mixing but require additional power input.
Fermenter Design and Challenges
- Tall cylindrical bioreactors enhance contact time between air bubbles and liquid due to increased hydrostatic pressure.
- Continuous or fed-batch systems may develop localized concentrations if bulk distribution is slow.
- Multiple injection points assist in distributing added material, which is cost-effective compared to increasing fluid velocity.
Rheological Properties
- Turbulent conditions, necessary for effective mixing, are gauged by the impeller's Reynolds number.
- Increasing fluid viscosity directly impacts Reynolds number and can hinder mixing efficiency.
- Pseudoplastic fluids exhibit shear-thinning behavior, affecting viscosity based on shear rate, resulting in variable flow patterns.
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Description
This quiz explores the essential principles of fluid mixing, focusing on the role of impellers, turbulence, and the methods of distribution and dispersion. Understanding these concepts is crucial for effective mixing in various applications. Test your knowledge on how these factors influence the mixing process.
