Fluid Mixing Principles and Techniques

Questions and Answers

What is the effect of high local pH on animal cell growth during culture?

It increases biomass yield.

It severely affects cell growth. (correct)

It enhances cell proliferation.

It has no significant effect.

How do pseudoplastic fluids behave under shear in stirred vessels?

Their viscosity increases with shear rate.

They exhibit constant viscosity at all shear rates.

Their viscosity is unaffected by shear conditions.

They have low viscosity in high-shear zones and high viscosity elsewhere. (correct)

What is a more economical solution for improving mixing instead of increasing fluid velocity?

Increasing the number of injection points. (correct)

Implementing a two-stage mixing process.

Reducing the viscosity of the feed material.

Using more power input to the impeller.

What happens to the Reynolds number (Re)i as viscosity increases during mixing?

It decreases in direct proportion. Signup and view all the answers

What is the primary characteristic of pseudoplastic fluids?

Their apparent viscosity decreases with increasing shear. Signup and view all the answers

Which factor primarily determines the intensity of turbulence in a mixing vessel?

The impeller’s Reynolds number. Signup and view all the answers

What is a potential drawback of increasing impeller speed for better mixing?

It significantly increases power consumption. Signup and view all the answers

What is a critical consequence of non-turbulent conditions during agitation?

Increased mixing time and poor mixing. Signup and view all the answers

Which statement accurately relates to the effects of methanol on cell growth?

Slow mixing of methanol contributes to decreased biomass yields. Signup and view all the answers

What is the primary benefit of using multiple injection points in bioprocessing?

It improves the homogeneity of the mixture. Signup and view all the answers

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.

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.