Transport Phenomena in Bioprocess Engineering
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Questions and Answers

What is the primary focus of studying transport phenomena in bioprocess systems?

  • Understanding chemical reaction rates only
  • Studying how substances move during biochemical processes (correct)
  • Analyzing nutrient efficiency in plants
  • Evaluating equipment performance in isolation

Which of the following is a factor influencing oxygen transfer efficiency in bioreactor systems?

  • Temperature gradients within the solution (correct)
  • The concentration of carbon dioxide only
  • The viscosity of the growth medium (correct)
  • Size of the bioreactor tank

What does the volumetric mass transfer coefficient (KLa) indicate?

  • Momentum transfer between fluids
  • The rate of heat loss in bioreactors
  • The effectiveness of oxygen transfer (correct)
  • Efficiency of solute concentration

Which method is typically used to determine oxygen transfer rates in bioreactors?

<p>Applying the mass balance approach (C)</p> Signup and view all the answers

Which phenomenon can dominate the overall rate of solute processing in a reaction volume?

<p>Physical-transport phenomena (C)</p> Signup and view all the answers

How can foam formation impact oxygen transfer in bioprocesses?

<p>It can reduce oxygen transfer efficiency (C)</p> Signup and view all the answers

What is momentum transport primarily related to in bioprocess engineering?

<p>Fluid dynamics and circulation (C)</p> Signup and view all the answers

What is one of the key roles of transport phenomena in biochemical engineering?

<p>To design efficient bioprocesses (A)</p> Signup and view all the answers

What was the percentage decrease in kL when 10 ppm of sodium lauryl sulfate (SLS) was added?

<p>56 percent (B)</p> Signup and view all the answers

What is the primary characteristic of passive transport?

<p>It occurs naturally from high to low concentration. (C)</p> Signup and view all the answers

At what concentration of surfactant did the lowest kLa' value occur?

<p>10 ppm (A)</p> Signup and view all the answers

What happened to the value of kLa' with increasing concentrations of surfactant?

<p>It plateaued after a certain point (A)</p> Signup and view all the answers

Which type of transport requires energy to move substances across membranes?

<p>Active transport (A)</p> Signup and view all the answers

How much did kLa' increase with the addition of 4.0 ppm of sodium dodecyl sulfate?

<p>400 percent (C)</p> Signup and view all the answers

Which resistance is encountered first during the transport of a gas bubble into a cell?

<p>Diffusion from bulk gas to gas-liquid interface (C)</p> Signup and view all the answers

What effect does cellular activity have on gas transport efficiency?

<p>It decreases the transport resistances. (C)</p> Signup and view all the answers

What is the approximate reduction in kL typically observed with the addition of surfactants?

<p>60 percent (D)</p> Signup and view all the answers

What effect do electrolytes have on bubble size in mechanical force-generated bubbles?

<p>They decrease bubble size (A)</p> Signup and view all the answers

Which of the following is NOT a factor affecting the efficiency of gas transfer?

<p>Concentration of the gas in the atmosphere (C)</p> Signup and view all the answers

What happens to the sixth resistance in gas transport if microbial cells are individual rather than aggregated?

<p>It does not occur during transfer. (B)</p> Signup and view all the answers

What effect does electrostatic potential at the surface of aqueous electrolyte solutions have?

<p>It decreases the rate of bubble coalescence (D)</p> Signup and view all the answers

What is recommended when using correlations for kL values obtained with surfactants?

<p>To substitute them with experimental values whenever feasible (D)</p> Signup and view all the answers

What role do microbial cells have at the gas-liquid interface during gas transport?

<p>They create a barrier preventing diffusion. (A)</p> Signup and view all the answers

What is the main process involved in the transport of solute across the cell envelope?

<p>Active transport (D)</p> Signup and view all the answers

What assumption does the penetration theory make about turbulent eddies?

<p>They remain at the interface for a specific time, t0. (C)</p> Signup and view all the answers

How is the mass transfer coefficient (kL) related to molecular diffusivity (DAB) in the penetration theory?

<p>It is directly proportional to the square root of molecular diffusivity. (D)</p> Signup and view all the answers

What modification did Danckwerts make to Higbie's penetration theory?

<p>He modified it for liquid phase mass transfer. (D)</p> Signup and view all the answers

What is the relationship between exposure time (t0) and the mass transfer coefficient in penetration theory?

<p>It is inversely proportional. (A)</p> Signup and view all the answers

According to penetration theory, what allows the solute to penetrate the eddy at the interface?

<p>Unsteady-state molecular diffusion. (B)</p> Signup and view all the answers

What is a key element of the surface renewal theory?

<p>It focuses on surface renewal events in mass transfer. (D)</p> Signup and view all the answers

Which equation represents the relationship used in the penetration theory?

<p>$k_L = 2 \frac{D}{\pi t_0}$ (D)</p> Signup and view all the answers

What does the constant D represent in the equation related to mass transfer coefficient?

<p>The diffusivity of the solute. (C)</p> Signup and view all the answers

What is one advantage of the dynamic pressure method in kLa measurement?

<p>It eliminates the need for gas composition adjustments. (C)</p> Signup and view all the answers

Why is the sulfite oxidation method generally discouraged for measuring kLa?

<p>Results vary unpredictably with operating conditions. (A)</p> Signup and view all the answers

What complicates measurements in the dynamic pressure method?

<p>Changes in bubble size after a pressure shift. (A)</p> Signup and view all the answers

What is a key aspect of modifying methods used to estimate kLa?

<p>Integrating mass transfer processes and liquid film resistance. (A)</p> Signup and view all the answers

How does bubble size influence kLa measurements in salt solutions?

<p>It affects the coalescence properties of bubbles. (C)</p> Signup and view all the answers

What technique is often compared to the sulfite oxidation method for measuring kLa?

<p>Dynamic pressure method. (B)</p> Signup and view all the answers

What impact does the oxygen probe's pressure tolerance have on kLa measurement?

<p>Allows for accurate readings during pressure fluctuations. (C)</p> Signup and view all the answers

What is a common issue when applying the simple dynamic method for kLa measurement?

<p>Complications with gas hold-up. (D)</p> Signup and view all the answers

What is the relationship between the diffusivity of A in B and the correlation suggested by Othmer and Thakar for water?

<p>It requires specific SI units for accuracy. (A)</p> Signup and view all the answers

What does the variable $D°$ represent in the provided equations?

<p>Diffusivity of A in B in a very dilute solution. (B)</p> Signup and view all the answers

How is the association factor ($\xi$) for the solvent water described in the content?

<p>2.26 for water. (D)</p> Signup and view all the answers

Which of these parameters is NOT part of the equations provided?

<p>Density of the solvent. (D)</p> Signup and view all the answers

What is the experimental value given for the diffusivity of oxygen in water at 25°C?

<p>$2.5 imes 10^{-9} m^2/s$ (B)</p> Signup and view all the answers

Which correlation is suggested for use with water as the solvent?

<p>Othmer and Thakar correlation. (B)</p> Signup and view all the answers

Why should the equations for $D°$ not be considered for concentrated solutions?

<p>They are derived from experimental values in dilute conditions. (D)</p> Signup and view all the answers

What unit is specified for the viscosity ($oldsymbol{ u}$) in the equations?

<p>kg/m·s (B)</p> Signup and view all the answers

In the context of the equations, what does the term $V$ represent?

<p>Solute molecular volume at the normal boiling point. (D)</p> Signup and view all the answers

What is a common misconception regarding the application of the discussed equations?

<p>All solvents exhibit the same association factors. (D)</p> Signup and view all the answers

Flashcards

Transport Phenomena in Bioprocess

The study of how substances like fluids, heat, or particles move during chemical or biochemical processes.

Gas-Liquid Mass Transfer

The movement of gases between a gas phase and a liquid phase.

Oxygen Transfer Rate

The rate at which oxygen moves from a gas phase to a liquid phase, crucial in bioreactors.

Volumetric Mass Transfer Coefficient (KLa)

A measure of how effectively oxygen transfers in a bioreactor.

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Momentum Transport

The movement of momentum in fluids. Fluid dynamics.

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Mass Transport

The movement of chemical species, such as in oxygen transport to microbes.

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Bioreactor Systems

Systems that support biological reactions, often involving oxygen supply.

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Foam Formation

The creation of foam in bioreactors, negatively affecting oxygen transfer.

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Passive Transport

Movement of substances from high to low concentration, naturally occurring, and requiring no energy.

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Active Transport

Movement of substances against a concentration gradient (low to high), requiring energy.

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Gas Transfer Resistances

Obstacles in the path of gas movement from a gas bubble to a cell, including diffusion, interfacial movement, and liquid transport.

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Diffusion (gas to liquid interface)

Process where gas molecules move from higher to lower concentration across the interface between a gas bubble and a liquid.

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Transport through bulk liquid

Movement of oxygen in the large volume of the liquid, sometimes unmixed, towards the cell.

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Cellular Transport Resistances

Obstacles encountered by oxygen reaching the target cell, including layers, walls, and internal structures within the cell.

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Oxygen Transport

The process by which oxygen moves from a gas bubble or droplet to an organism's cells, requiring overcoming multiple transport resistances.

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Interfacial Phenomena

Events at the boundary between the gas bubble and liquid that affect oxygen transport, such as adsorption of microbes.

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Diffusivity (D°)

The rate at which a substance (solute) diffuses through another substance (solvent), measured in m²/s.

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Othmer-Thakar Correlation

A formula used to estimate diffusivity of gases in liquids, especially when the solvent is water.

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Wilke-Chang Correlation

A formula used to estimate diffusivity of gases in liquids that is more general than Othmer-Thakar.

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What is the SI unit for molecular weight (M)?

The SI unit for molecular weight is kg/kmol.

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What is the SI unit for viscosity (μ)?

The SI unit for viscosity is kg/(m⋅s).

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Units of Molecular Volume (V)

The SI unit for molecular volume is m³/kmol, representing the volume occupied by one kilomole of the substance at its normal boiling point.

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Association Factor (𝜉)

A factor that accounts for the association of solvent molecules, influencing diffusivity.

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Why does water have a high association factor?

Water molecules are highly associated due to strong hydrogen bonding, which makes them stick together more strongly.

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What are the units for diffusivity, molecular weight (M), and viscosity (μ)?

Diffusivity (D°) - m²/s, Molecular Weight (M) - kg/kmol, Viscosity (μ) - kg/(m⋅s)

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How is the Association Factor relevant to diffusivity?

A higher association factor indicates stronger interactions between solvent molecules. This can lead to a slower diffusion rate of the solute (gas) through the solvent, reducing diffusivity.

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Two-Film Theory

A model explaining mass transfer across a gas-liquid interface, where the transfer is limited by two film resistances: one in the gas phase and one in the liquid phase.

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Penetration Theory

A mass transfer theory suggesting that turbulent eddies in fluids bring fresh gas to the interface, where it dissolves for a short time before being mixed back into the bulk.

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Mass Transfer Coefficient (kL)

A measure of how effectively a substance transfers from one phase to another. It reflects the rate of transfer.

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Surface Renewal Theory

A modification of the penetration theory, taking into account the continuous renewal of the liquid surface exposed to the gas.

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What is the relationship between mass transfer coefficient (kL) and molecular diffusivity (DAB) in penetration theory?

In the penetration theory, the mass transfer coefficient (kL) is directly proportional to the square root of the molecular diffusivity (DAB). This means that the faster the molecules can diffuse, the faster the transfer.

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How does the exposure time (t0) affect the mass transfer coefficient (kL) in penetration theory?

In the penetration theory, the mass transfer coefficient (kL) is inversely proportional to the exposure time (t0). This means that the longer the exposure time, the slower the transfer and the lower the coefficient.

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What is the key difference between the two-film theory and penetration theory?

The two-film theory assumes steady-state diffusion, while the penetration theory assumes unsteady-state diffusion. This means that the two-film theory assumes a constant concentration gradient, while the penetration theory assumes a changing concentration gradient.

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How does the surface renewal theory relate to the penetration theory?

The surface renewal theory is a modification of the penetration theory developed by Danckwerts for liquid phase mass transfer. It takes into account the continuous renewal of the liquid surface exposed to gas.

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kLa

The volumetric mass transfer coefficient, representing how effectively oxygen moves from the gas phase to the liquid phase in a bioreactor.

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Modified kLa Estimation Methods

Techniques like normalizing dissolved oxygen data or using the dynamic pressure method to account for factors like electrode response, liquid film resistance, and gas-phase dynamics.

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Dynamic Pressure Method

A technique to estimate kLa by temporarily increasing pressure in the fermenter, avoiding issues with gas composition changes.

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Sulfite Oxidation Method

A method to measure kLa by observing the oxidation of sodium sulfite to sulfate, but often less accurate and not recommended due to limitations.

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Gas Hold-up

The volume fraction of gas bubbles present in a liquid.

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Bubble Size

The diameter of individual gas bubbles in a liquid.

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Coalescence

The merging of smaller bubbles into larger ones.

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Pressure Shift

A change in pressure applied to a fermenter, often used in the dynamic pressure method.

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Effect of Surfactants on kL

Adding surfactants like sodium lauryl sulfate (SLS) to water initially decreases kL, but at higher concentrations, it plateaus.

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Surface Area per Volume (a')

A measure of how much surface area is available for gas transfer in a liquid volume.

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kLa' vs. a' and kL

While a' (surface area) can increase with surfactants, the product kLa' (overall oxygen transfer rate) might not increase proportionally due to a decrease in kL (oxygen transfer rate).

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Electrolyte Effect

Bubbles in electrolyte solutions are smaller than in pure water.

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Why Electrolyte Bubbles are Smaller

Electrolytes create an electrostatic potential at the surface of bubbles, reducing bubble coalescence (merging).

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kLa' Data and Clean Water

Most kLa' data is obtained using clean water. Electrolytes can significantly affect bubble size and kLa'.

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Importance of Experimental Data

It's important to rely on experimental kLa' values obtained in the specific bioreactor system and conditions rather than just using correlations.

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Study Notes

Introduction

  • Biological systems grow increasingly complex as they scale from molecular to fluid volumes.
  • Transport phenomena can be a major factor affecting overall reaction rates in large biological systems.

Learning Objectives

  • Understanding transport phenomena in bioprocess systems
  • Identifying methods to determine oxygen transfer rates in bioreactors
  • Understanding the volumetric mass transfer coefficient (KLa) and its effect on oxygen transfer efficiency
  • Recognizing factors influencing oxygen transfer efficiency and their effect on cell culture performance
  • Evaluating the impact of foam formation and anti-foam agents on oxygen transfer efficiency

Gas-Liquid Mass Transfer in Cellular Systems

  • Transport phenomena involves the movement of substances like fluids, heat, or particles during chemical and biochemical processes.

  • Principles of transport are fundamental to the design and operation of bioprocesses.

  • Momentum transport (fluid dynamics): important processes like blood circulation and mixing in bioreactors

  • Mass transport: important for movement of chemical species, like oxygen transport to aerobic microorganisms

  • Energy (heat) transport: critical for processes like reactor sterilization and temperature control.

Determination of Oxygen Transfer Rates

  • Aerobic cultures require a continuous oxygen supply for cell growth and metabolic activity.
  • Insufficient oxygen can damage or kill cells.
  • Engineers utilize aeration to maintain optimal oxygen levels.

Determination of KLa values

  • The volumetric mass transfer coefficient (kLa) is a crucial parameter in bioreactors.
  • It characterizes the efficiency of oxygen transfer from the gas phase to the liquid phase.

Factors Affecting KLa values in bioreactors

  • Degree of Agitation: Increased power input generally means smaller bubbles, enhancing oxygen transfer but exceeding optimal levels may be detrimental to microorganisms.

  • Medium Culture Rheology: The viscosity of the medium impacts mass transfer rates; non-Newtonian fluids exhibit various viscosities in different conditions.

Effects of Foam and Anti-foam on Oxygen Transfer

  • Foaming can affect the gas-liquid interface's effective area for oxygen transfer, affecting oxygen uptake rates
  • Factors like surface-active substances, hydrodynamic conditions, and cell/product concentration can affect foam generation and stability.
  • Anti-foam agents help control foam and thus maintaining steady oxygen supply to the culture.

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Transport Phenomena in Bioprocess System PDF

Description

This quiz focuses on the fundamental concepts of transport phenomena within bioprocess systems. Questions explore topics such as oxygen transfer efficiency, volumetric mass transfer coefficient (KLa), and the effects of surfactants on bioreactor performance. Dive deep into the principles that govern the movement of substances in biochemical engineering.

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