Fluid Mixing in Bioreactors

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Questions and Answers

Which of the following impellers are commonly used for viscous mixing?

Helical impellers

Gate-anchors

Paddle-anchors

All of the above (correct)

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.

fermenters Signup and view all the answers

What is necessary for the application of helical ribbon and anchor impellers in fermenters?

Low oxygen demand Signup and view all the answers

What does fluid mixing mean?

A physical process that aims at reducing non-uniformities in fluids by eliminating gradients of concentration, temperature gradients, and other properties. Signup and view all the answers

Which of the following components is NOT part of a stirred tank?

Condenser Signup and view all the answers

What is a common use of fluid mixing in the processing industry?

Dissolving solids in liquids Signup and view all the answers

Distribution is often the slowest step in the mixing process.

True Signup and view all the answers

What type of flow is characterized by Re < 10?

Laminar Flow Signup and view all the answers

What is mixing time?

The time required to achieve a given degree of homogeneity starting from a completely segregated state. Signup and view all the answers

What does the equation tm = 4tc represent?

It represents the relationship between mixing time (tm) and circulation time (tc) in stirred tanks. Signup and view all the answers

In which scenario is multiple impellers advantageous?

In tall fermenters Signup and view all the answers

In non-Newtonian fluids, the apparent viscosity increases with increasing shear rate.

False Signup and view all the answers

What affects the power requirements for mixing?

Power requirements depend on the resistance offered by the fluid to the rotation of the impeller. Signup and view all the answers

In stirred tanks, the presence of ______ can be correlated with the impeller Reynolds number.

turbulence Signup and view all the answers

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.