Questions and Answers
Which of the following impellers are commonly used for viscous mixing?
Helical agitators are used to increase shear damage in viscous cell suspensions.
False
What is one advantage of large-diameter impellers operating at slow speed?
Superior bulk mixing
Mixing effectiveness and adequate mass transfer require a compromise in the design of _____ for viscous cultures.
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What is necessary for the application of helical ribbon and anchor impellers in fermenters?
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What does fluid mixing mean?
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Which of the following components is NOT part of a stirred tank?
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What is a common use of fluid mixing in the processing industry?
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Distribution is often the slowest step in the mixing process.
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What type of flow is characterized by Re < 10?
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What is mixing time?
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What does the equation tm = 4tc represent?
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In which scenario is multiple impellers advantageous?
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In non-Newtonian fluids, the apparent viscosity increases with increasing shear rate.
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What affects the power requirements for mixing?
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In stirred tanks, the presence of ______ can be correlated with the impeller Reynolds number.
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Study Notes
Fluid Mixing
- Fluid mixing is a physical process central to bioreactors, aimed at reducing non-uniformities like concentration and temperature gradients.
Significance of Fluid Mixing
- Essential in chemical processing for various operations such as blending miscible liquids, dissolving solids, dispersing gas in liquids, suspending solids, and agitation for heat transfer.
Stirred Tank Components
- Motor: Powers the agitator.
- Agitator: The primary mixing device.
- Baffle: Enhances mixing efficiency.
- Cooling Jacket: Maintains temperature.
Agitator Types
- Different impeller designs include:
- Anchor
- Propeller
- Disc-turbine
- Paddle
- Gate anchor
- Helical and ribbon
Flow Patterns in Agitated Tanks
- Flow patterns depend on:
- Fluid properties
- Tank geometry
- Baffle types
- Agitator types
Mechanisms of Mixing
- Effective mixing requires fluid circulation to sweep across the entire vessel, creating turbulence vital for material transfer.
Distribution
- Involves the transportation of materials via bulk currents, often the slowest step in mixing.
Dispersion
- Relies on turbulent fluid kinetic energy, breaking down bulk flow into smaller eddies for rapid material transfer.
Mixing Effectiveness: Mixing Time
- Defined as the time needed to achieve homogeneity, influenced by tank size, impeller type, fluid viscosity, and stirring speed.
- Mixing time can be measured using tracers or by temperature response.
Power Requirements for Mixing
- Power depends on fluid resistance; typically ranges from 10 kW/m³ for small vessels to 1-2 kW/m³ for larger ones.
- Impeller Reynolds number indicates turbulence, categorized as laminar (Re < 10), transitional (10 < Re < 10000), or turbulent (Re > 10000).
Scale-up of Mixing Systems
- Scale-up ensures conditions in large vessels mimic those in small prototypes, but maintaining consistent mixing times is often technically challenging.
- Power per volume requirement increases significantly with tank size.
Improving Mixing in Fermenters
- Strategies include:
- Installing baffles to increase turbulence.
- Positioning the impeller below the tank’s geometric center.
- Using multiple impellers for enhanced circulation.
Effect of Rheological Properties
- Turbulent mixing is affected by fluid viscosity; higher viscosity leads to longer mixing times.
- Pseudoplastic fluids exhibit shear-thinning behavior, reducing viscosity near the impeller, improving mixing efficiency.
Impeller Design Considerations
- Large-diameter, low-speed impellers recommended for viscous fluids.
- Helical ribbon and paddle anchors help reduce shear damage and enhance mixing.
- Compromises are necessary between mixing effectiveness and mass transfer for viscous cultures.
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Description
This quiz explores the fundamental concepts of fluid mixing in bioreactors, including the significance of mixing and the various components of stirred tanks. It covers agitator types, flow patterns, and the mechanisms of effective mixing essential in chemical processing. Test your understanding of how these elements work together to enhance mixing efficiency.
