PART 1 - Transport Phenomena in Bioprocess System

Question 1

Mass can be transferred from one place to another under the influence of a__________________________ or____________________ in the system.

Answer 1

concentration difference
concentration gradient

Question 2

A simple example is when dye is dropped into a cup of water, mass-transfer processes are responsible for the movement of dye molecules through the water until _____________ is established and the concentration is __________.

Answer 2

equilibrium
uniform

Question 3

In biological multiphase systems, mass transfer takes place between ___________________, between a liquid and a solid phase, or between a gas and a liquid phase.

Answer 3

two liquid phases

Question 4

Most large-scale ____________________ (excluding ______________________ production) are aerobic and typically conducted in aerated gas-liquid bioreactors.

Answer 4

fermentation processes
bioethanol and lactic acid

Question 5

Before being utilized, oxygen must be moved via a sequence of processes from the _________ onto the _________ of cells. The solubility of oxygen within broth is typically very _______.

As a result, during aerobic cultures and fermentations, improving gas-liquid mass transfer is always given priority.

Answer 5

gas bulk
surfaces

low

Question 6

When sparingly soluble oxygen gas is transferred from a source, such as a rising air bubble, into a liquid phase that contains cells or liquid hydrocarbons used in hydrocarbon fermentation, it must pass through a ________________________________.

This refers to the relative magnitudes of which are determined by bubble (droplet) hydrodynamics, temperature, cellular activity and density, solution composition, interfacial phenomena, and other variables.

Answer 6

series of transport resistances

Question 7

Enumerate the transport resistances while explaining the transport process of oxygen from a gas bubble to inside a cell.

Answer 7

1. Diffusion from bulk gas-liquid interface.
2. Movement through the gas-liquid interface.
3. Diffusion of the solute through the relatively unmixed liquid region adjacent to the bubble
   into the well-mixed bulk liquid.
4. Transport of the solute through the bulk liquid to a second relatively unmixed liquid region
   surrounding the cells.
5. Transport through the second unmixed liquid region associated with the cells.
6. Diffusive transport into the cellular floc, mycelia, or soil particle.
7. Transport across the cell envelope and two intracellular reaction sites (e.g. the mitochondria)

Question 8

When the organisms take the form of individual cells, the _______ resistance disappears.

Answer 8

sixth

6. Diffusive transport into the cellular floc, mycelia, or soil particle.

Question 9

Microbial cells themselves have some tendency to _____________________. Thus, cells may preferentially gather at the vicinity of the gas-bubble-liquid interface.

Answer 9

adsorb at interfaces

Question 10

Adsorption of microbes/cells at interfaces!

Then, the diffusing solute oxygen passes through only _______________________________ before reaching the cell.

In this situation, the bulk dissolved O2 concentration does not represent the __________________ for the respiring microbes.

Answer 10

one unmixed liquid region and no bulk liquid

oxygen supply

Question 11

_____________________ on or near the hydrocarbon-emulsion interface has also been observed in the microbial utilization of other sparingly soluble substrates such as hydrocarbon droplets.

Answer 11

Cell adsorption

Question 12

In general, we can distinguish fluid motions induced by: (2)

Answer 12

• freely rising or falling bubbles or
• particles from fluid motion that occur as the result of applied forces other than the external gravity field (forced convection).

Question 13

Gas-liquid mixing and slowly stirred semi-batch system may have ________ contributions from naturally convected levels and mechanical stirring.

Answer 13

equal

Question 14

Gas-liquid contacting modes via free-falling, falling particles, fluids: (7)

Answer 14

1. Rising single bubble
2. Bubble swarms
3. Staged counter-current fermentor
4. Sparged airlift
5. Trickle filter counter current
6. Stratified, nearly horizontal flow
7. Naturally turbulent aeration

Question 15

Gas-liquid contacting modes via mechanically agitated: (5)

Answer 15

1. Semi-batch (batch liquid, continuous air
2. Continuous liquid and air
3. Multiple propellers (semi-batch or continuous)
4. Stirred tank with baffles (baffles are often used in designs 1 and 2 also)
5. Staged cross-current

Question 16

What do you mean by sparged?

Answer 16

moisten by sprinkling with water, especially in brewing.

Question 17

What do you mean by trickle?

Answer 17

flow in a small stream

Question 18

What do you mean by stratified?

Answer 18

placed close together in layers

Question 19

Mass transfer takes place by two basic processes: (2)

Answer 19

1. Convection
2. Diffusion

Question 20

_______________________ is the movement of component molecules in a mixture under the influence of a concentration difference in the system.

Answer 20

Molecular diffusion

Question 21

Diffusion of molecules occurs in the direction required to ____________ the concentration gradient.

Answer 21

eliminate

• to be in equilibrium

Question 22

If the gradient is maintained by constantly supplying material to the region of high concentration and removing it from the region of low concentration, diffusion will be _______________. This situation is often exploited in mass-transfer operations and bioreaction systems.

Answer 22

continuous

Question 23

It states that the molar flux of component A relative to the average molal velocity of all
constituent JA is proportional to the concentration gradient dCA/dz as expressed in:

Answer 23

Fick’s First Law of Diffusion

Question 24

What is D_AB

Answer 24

Diffusivity of Component A through B

Question 25

This is a measure of diffusive mobility

Answer 25

Diffusivity

Question 26

The molar flux relative to stationary coordinate NA is equal to:

Answer 26

NA = Ca/C (Na-Nb)…..

Question 27

The kinetic theory of liquids is much less advanced than that of gases. Therefore, the correlation for diffusivities in liquids is _________________ as that for gases

Answer 27

not as reliable

Question 28

This correlation is the most widely used for dilute solutions of nonelectrolytes.

Answer 28

Wilke-Chang correlation (Wilke and Chang, 1955)

Question 29

When the solvent is ________, ________ (1974) recommends the use of the correlation developed by ___________________

Answer 29

water

Skelland
Othmer and Thakar

Question 30

This theory states that two fictitious films are present on either side of the interface that cause the overall resistance to mass transfer and that takes place due to molecular diffusion.

Answer 30

Two-Film Theory

Question 31

Two-Film Theory

______________________ are assumed to exist only in the films.

Outside the film, the phases are assumed to be at _______________________ and are referred to as gas bulk and liquid bulk.

Answer 31

Concentration gradients
uniform concentrations

Question 32

Mathematically, the two-film theory states that kl (mass-transfer coefficient) varies _________ as the film thickness δ, whereas kl and DAB (diffusivity) are ________ proportional to each other.

Answer 32

inversely
directly

Question 33

The _____________________ assumes that turbulent eddies travel from the bulk to the interface and at the interface, they remain for an exposure time equal to ____.

Answer 33

penetration theory

t₀

Question 34

___________ came up with the penetration theory in 1935

Answer 34

Higbie

Question 35

In addition, the process of unsteady-state molecular diffusion is assumed in this theory by which the solute is assumed to:

Answer 35

Penetration theory

penetrate a given eddy during its stay at the interface

Question 36

This theory assumes that there is a region in which steady-state molecular transfer is controlling

Answer 36

film

Question 36

This theory assumes that the interface is continuously replaced by eddies and that unsteady state molecular transfer into the eddies controls the transfer in this region.

Answer 36

Penetration theory

Question 36

In contrast to the two-film theory, the penetration theory predicts that kl (mass-transfer coefficient) and the square root of DAB (molecular diffusivity) are ________ proportional, whereas kl varies __________ as the exposure
time t0.

Answer 36

directly
inversely

Question 36

In 1951, ___________ modified Higbie’s penetration theory for liquid phase mass transfer.
The modified theory is widely known as the _______________________.

Answer 36

Danckwerts
Surface renewal theory

Question 37

What does the surface renewal theory state?

Answer 37

The surface renewal theory
states that a portion of the mass-transfer surface is replaced with a new surface by the motion of
eddies near the surface assuming that the liquid elements at the interface are randomly
swapped by fresh elements from bulk and each of the liquid elements at the surface has the same probability of being substituted by a fresh element.

Question 38

Similar to the penetration theory, the mass transfer coefficient in surface renewal theory is ________ proportional to the square root of diffusivity.

Answer 38

directly

Question 39

For sparingly soluble species such as oxygen or hydrocarbons in water, the two equilibrated interfacial concentrations c_gi and c_li on the gas and liquid sides, respectively, may typically be related through a _________________________ such as ________ law.

Answer 39

linear partition-law relationship
Henry’s la

Question 40

The oxygen-transfer rate per unit of reactor volume Q_O2 is given by:

Answer 40

(flux)(interfacial area)/reactor liquid volume

Question 41

The maximum possible utilization is:

Answer 41

xmyumax/Yo2

Question 42

Oxygen may also be consumed as a reactant in __________________________.

Answer 42

biotransformation