Quiz 3 Flashcards by tine cahayon

Mass can be transferred from one place to another under the influence of a

concentration gradient

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In biological multiphase systems, mass transfer takes place between

Two liquid phases, between a liquid and a solid phase, or between a gas and a liquid phase.

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most large-scale fermentation processes are

aerobic

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bioethanol and lactic acid production

anaerobic

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Before being utilized, _________ must be moved via a sequence of processes from the
gas bulk onto the surfaces of cells.

oxygen

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The solubility of oxygen within broth is typically _____

very low

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During aerobic cultures and fermentations, improving _________is always
given priority

gas-liquid mass transfer

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Example of transport resistances

Diffusion from bulk gas-liquid interface.
Movement through the gas-liquid interface.

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Microbial cells themselves have some tendency to adsorb at

interfaces

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__________concentration does not represent the oxygen supply for the
respiring microbes

the bulk dissolved O2

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Mass transfer takes place by two basic processes

convection and diffusion

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

molecular diffusion

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Diffusion of molecules occurs in the direction required to

eliminate the concentration gradient

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

Fick’s first law of diffusion

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Computing for diffusivity, When the solvent is water, Skelland (1974) recommends the use of the correlation
developed by

Othmer and Thakar

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The three most common theories for mass transfer mechanisms are the

two-film theory, the surface renewal theory, and the penetration theory

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

Two-Film Theory

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In two-film theory, Concentration gradients are assumed to exist only in the

films

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

inversely

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came up with the penetration theory in 1935

Higbie

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assumes that turbulent eddies travel from the bulk to the interface and at the interface, they remain for an exposure time equal to t0.

Penetration theory

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assumed by which the solute is assumed to penetrate a given eddy during its stay at the interface.

process of unsteady-state molecular diffusion

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Penetration theory predicts that kl (mass-transfer coefficient) and the _______________ are directly proportional, whereas kl varies inversely as the ________

square root of DAB (molecular diffusivity); exposure time t0.

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modified Higbie’s penetration theory for liquid phase mass transfer.

Danckwerts (1951)

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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 being 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

surface renewal theory

Similar to the penetration theory, the mass transfer coefficient is ___________ to the square root of diffusivity

directly proportional

at 1 atm of air and 25°C, the O2 collision rate at the surface is of the order of __________

10^24 molecules per square centimeter per second

At steady state, the oxygen transfer rate to the gas-liquid interface__________ its transfer rate through the liquid-side film

equals

Thus, essentially all the resistance to mass transfer lies on the

liquid-film side

for the gas-phase mass-transfer coefficient Kg is approximately equal to kg if

M (distribution coefficient) is very small

Head space gas, in calculating oxygen absorption rate, is

not included

It is important to recognize that 𝑄𝑜2 is defined

“at a point

The maximum possible mass-transfer rate is simply found by setting

𝑐𝑙 = 0

all oxygen entering the bulk solution is assumed to be ________ consumed

rapidly

f 𝑘𝑙𝑎𝑐𝑙 is much larger than 𝑥𝜇𝑚𝑎𝑥/𝑌𝑜2, 𝑌𝑜2 , the main resistance to increased oxygen consumption is ______________

microbial metabolism

If the main resistance to increased oxygen consumption is microbial metabolism, then the reaction appears to be

biochemically limited

the reverse inequality apparently leads to 𝑐𝑙 near zero, and the reactor seems to be in

the mass-transfer-limited mode

At steady state, the oxygen absorption and consumption rates must_________

balance

If the cell metabolic machinery is saturate with oxygen, then sufficient oxygen is available to accept immediately all electron pairs which pass through the respiratory chain, so that some other biochemical process within the cell is

rate- limiting

In the batch-system results of Figure 4, a maximum in specific O2 demand occurs in the

early exponential phase

affects oxygen demand in a major way

carbon nutrient

generally metabolized more rapidly than other carbohydrate substances

glucose

The component parts of oxygen utilization by the cell include

cell maintenance, respiratory oxidation for further growth, and oxidation of substrates into related metabolic end products

In metabolic stoichiometry, oxygen utilization for growth is typically coupled directly to

the amount of carbon-source substrate consumed

Oxygen may also be consumed as a reactant in

biotransformation

mass transfer between phases may often become the _________ of the overall process rate

limiting step

is used to improve mass transfer and also to insure homogeneous distribution of the nutrients in the liquid phase

Mixing ( by aeration and agitation )

oxygen transport from the air is necessary and obviously focused

aerobic systems

the transport of other gases such as methane or carbon dioxide usually takes place

anaerobic systems

one of the most important nutrients for aerobic cells that need oxygen dissolved in the broth for maintenance, growth and production of bioproducts

Oxygen

in aerated bioreactors the critical limiting factor to providing the optimal environment is the

oxygen transfer rate (OTR)

dependent on the vessel’s geometrical configuration and the physical properties of the fluids

mass-transfer coefficient 𝑘L

Three dimensionless numbers play a significant role in the mass transfer process

(1) Sherwood Number, (2) Schmidt Number, and (3) Grashof Number.

dimensionless number that represents the ratio of the convective mass transfer rate to the diffusive mass transfer rate

Sherwood number (Sh)

Characteristic length, in bioprocessing applications, will be equivalent to the

diameter of the air bubbles

A _____________ Sherwood number indicates that convection is the dominant mode of mass transfer

high

A ____ Sherwood number indicates that diffusion is the dominant mode

low

Describes the relationship between mass diffusivity over momentum diffusivity or vice versa

Schmidt number (Sc)

this dimensionless number is most applicable to cases that simultaneously involve mass and momentum diffusion

Schmidt number (Sc)

often used to design and optimize processes that involve mass transfer, such as aeration, fermentation, and filtration

Schmidt number (Sc)

used to determine the rate of oxygen transfer from the air to a culture of cells in a fermenter

Schmidt number (Sc)

A _______ Schmidt number indicates that momentum transfer is more important than mass transfer, while a _______ Schmidt number indicates that mass transfer is more important than momentum transfer

High, low

This number is mostly associated with the cases involving free (natural) convectio

Grashof Number (Gr)

used to characterize the flow of fluids in bioreactors and other vessels

Grashof Number (Gr)

A ________ Grashof number indicates that the flow is laminar, while a ______Grashof number indicates that the flow is turbulent

low , high

generally more effective at mixing fluids, which can be important for bioprocesses that require good mixing, such as fermentation

Turbulent flow

measure of how quickly an organism consumes oxygen

Oxygen-Absorption Rate (OAR)

Oxygen-Absorption Rate (OAR) is also known as

Respiration Rate or Metabolic Rate

OAR is typically measured in units of

milliliters of oxygen per minute per gram of body weight (ml O2/min/g BW)

As oxygen is categorized as a sparingly soluble gas, the _______ is equal to e liquid-side individual mass transfer coefficient

overall mass transfer coefficient KL

oxygen-transfer rate can be maximized by ___________ and air flow rate

minimizing the power consumption

The oxygen absorption can be enhanced by increasing

interfacial area a, mass-transfer coefficient 𝑘𝐿 , and the concentration difference

𝑘𝐿𝑎 is not a property of the ________, but a parameter of the _________

system; model adopted

came up with the dynamic gassing out technique in 1979

Van’t Riet

__________ monitors the change in the oxygen concentration, while an oxygen-rich liquid is deoxygenated by passing ________ through it

Dynamic gassing out technique, nitrogen

usually used to measure the DO concentration

polarographic electrode

It works by inducing an electrochemical reaction with the dissolved oxygen, and the resulting electrical signal is proportional to the DO concentration

polarographic sensor

In direct measurement technique, _______________ used to estimate the oxygen concentration in the gas stream entering and leaving the fermenter

a gaseous oxygen analyzer

generally used to measure and evaluate the oxygen concentration of the liquid in a fermenter

oxygen sensor

largely depends on the size of the fermenter.

Difference in concentration term

used the dynamic technique to estimate the 𝑘𝐿𝑎 value for the oxygen transfer during an actual fermentation process with real culture medium and microorganisms

Taguchi and Humphrey

implemented in a microorganism-rich aerated batch fermenter over which the oxygen material balance is applied

Dynamic technique

Despite a steady dissolved oxygen level in the fermenter, the oxygen concentration will be _________ if the air supply is turned off

decreased sharply

based on the oxidation of sodium sulfite to sodium sulfate in the presence of catalyst (Cu++ or Co++)

Sodium sulfite oxidation method (Cooper et al., 1944)

it is possible to determine the oxygen transfer rate by using an

Oxygen consuming chemical reaction

Traditional sulfite method is based on the _______ by oxygen

oxidation of sulfite to sulfate

the rate of sulfite consumption is determined solely by the rate at which

oxygen is transferred from the gas phase

The dissolved oxygen concentration, cO, is virtually ________ due to the very rapid reaction with sulfite

zero

__________ is determined by measuring the concentration of sulfite in samples taken at a set of time values after the start of the experiment

The reaction rate

can be determined by adding an excess amount of iodine, and thereafter back-titrate with thiosulfate, using starch as an indicator

sulfite concentration in the samples

may enhance the oxygen absorption, since the rapid chemical reaction may occur not only in the bulk liquid, but also in the liquid film

sulfite oxidation

The significant coalescence-reducing effects of sulfite on the bulk liquid results in a_________specific interfacial area.

higher

Both the enhancement of mass transfer due to reaction in the film, and the _________, may lead to an overestimation of 𝑘𝐿a

coalescence-reducing effect of sulfite

This method due to Hickman (1988) is, like the sulfite method, a chemical method

Hydrogen peroxide method

the transfer of oxygen from the liquid to the gas phase is measured

Hydrogen peroxide method

generated by the enzyme-catalyzed decomposition of hydrogen peroxide

oxygen

At steady state, the rate of decomposition of hydrogen peroxide equals____________ from the liquid phase to the gas phase

half the rate of oxygen transfer

calculated from the gas phase partial pressure of oxygen.

interfacial concentration of oxygen

Hickman used the assumption of _______ of both the liquid and the gas phase

complete back-mixing

The risk of enhancing the mass transfer by reaction in the liquid film is _______ in h2o2 method

smaller

the effect of the small added amount of catalase on the coalescence of the medium is _________ in h2o2 method

negligible

The method has been applied with good results for 𝑘𝐿𝑎 measurements in large industrial reactors, also with viscous, non-Newtonian media

H2O2 method

in fermentation processes is a common problem

Foam formation

Foaming is controlled routinely in fermenters using a

foam sensor and pump for automatic addition of antifoam agent

Therefore, antifoam addition will be triggered more often if the working volume of the vessel is

increased