Selection, Scale-Up Operation, and Control of Bioreactors

Question 1

Factors for Consideration in Reactor Design

Answer 1

➢ Heat Removal
➢Foam Control
➢ Providing Oxygen
➢ Sterilization

Question 2

What factors limit the size of reactors?

Answer 2

Ability to provide oxygen and remove heat.

Question 3

Reactor types include:

Answer 3

➢ Stirred-Tank
➢ Bubble Column
➢ Airlift
➢ Propeller Loop
➢ Jet Loop

Question 4

Reactor types comparison: Agitated Tanks

Answer 4

➢ Good oxygen mass transfer
➢ High energy requirement for mixing
➢ Seal to maintain and keep sterile.

Question 5

Reactor types comparison: Bubble Column

Answer 5

➢ Low shear environment
➢ No seal needed
➢ Restricted to low viscosity
➢ Less mixing than agitated tank
➢ Bubble coalescence limits upper air flow rate.

Question 6

Reactor types comparison: Loop reactors

Answer 6

➢ Better mixing than bubble column with the same low shear and energy requirements and lack of seal.
➢Work with higher viscosity liquids
than bubble columns.
➢Still less mixing than agitated tanks

Question 7

It breaks bubbles into smaller ones to provide for better oxygen mass transfer

Answer 7

Impeller

Question 8

Comparison between bench-top tanks and commercial fermenters

Answer 8

Bench-top tanks are typically glass, commercial fermenters are typically stainless steel.

Question 9

Heat removal/addition is typically done by

Answer 9

coils along the wall, or a water jacket around the tank.

Question 10

It prevents foaming problems but can cause additional mass transfer resistance.

Answer 10

Antifoam

Question 11

What is “working volume”?

Answer 11

volume of liquid in tank; does not
include head space.

Question 12

Seals for _________________ must not allow contamination

Answer 12

agitator shaft

Question 13

These are used to augment mixing and gas dispersion.

Answer 13

Baffles

Question 14

Types of impellers

Answer 14

➢ Rushton Impellers
➢ Axial Flow Impeller

Question 15

Rushton impellers are:

Answer 15

➢ Disc with 6 to 8 blades.
➢ Pumps fluid in a radial direction.
➢ Compartmentalization with multiple
impellers on a shaft.

Question 16

Axial flow impellers are:

Answer 16

➢ Pumps liquid in a vertical direction.
➢ Lower energy for the same oxygen
mass transfer.
➢ Lower shear rates.

Question 17

Transfer rate at steady state is determined by the _____________

Answer 17

➢ slowest rate
➢ OTR is not the rate at which OUR = OTR you
provide air to the reactor. You will actually provide much more oxygen to the reactor than is transferred to the cells.

Question 18

Oxygen Profile 1-6

Answer 18

1. Bulk gas phase oxygen concentration.
2. Transfer across stagnant gas layer.
3. Partitioning into the liquid phase (C* at saturation).
4. Transfer across stagnant liquid layer.
5. Bulk liquid concentration ( ).
6. Transfer across stagnant liquid layer to cell.

Question 19

Experimental Determination of O2 Mass Transfer

Answer 19

➢ The previous correlation offers design estimates.
➢ Medium components, temperature, and pressure can
affect volumetric transfer coefficient and oxygen
solubility.
➢ Simple experiments can be done to measure volumetric
transfer coefficient.
➢ Unsteady state, steady state, dynamic and sulfite are
methods to measure volumetric transfer coefficient.

Question 20

Unsteady State Method

Answer 20

○ Fill the reactor with medium only – no cells.
○ Measure DO concentration in
the medium.
○ Remove oxygen from the
medium by sparging with N2.
○ Introduce air, and record the
increase in DO.

Question 21

Steady State Method

Answer 21

○ Requires an oxygen gas
analyzer for the effluent air.

○ Perform an O2 mass balance to
obtain OUR.

Question 22

Dynamic Method

Answer 22

○ Utilizes a fermentor with actively
growing cells.
○ Requires only a DO meter.
○ The air to the fermentor is shut
off, and the DO decreases due
to consumption by the
microorganisms. The air is then
turned on, and the DO increases.

Question 23

Scale -Up

Answer 23

○ Empirical
○ Make the controlling regime the same in the small scale as in the large scale

Question 24

Scale-Up Criterion

Answer 24

○ Power Input: Oxygen Transfer Rate
○ Liquid Circulation Rate
: Mixing Time
○ Tip Speed: Shear
○ Reynolds Number: Geometry

Question 25

COMMON ON-LINE INSTRUMENTATION

Answer 25

➢ pH ➢ Temperature ➢ Dissolved oxygen ➢ Foam ➢ Flow Rates ➢ Level ➢ Off-gas composition ○ Carbon dioxide ○ Oxygen gas ○ Volatile Organic Compounds

Question 26

_______________ control is generally not as sophisticated as chemical production process control due to a lack of on-line sensors.

Answer 26

Fermentation process

Question 27

Why is there a lack of on-line sensors?

Answer 27

Each probe into the fermentor increases the probability of contamination, difficult to sterilize some probes, probe fouling, probe placement (gradients within the fermentor).

Question 28

What does sterilization mean?

Answer 28

The absence of detectable, viable organisms. Sterilization is probabilistic: some portion of the population is more resistant to sterilizing agents than other portions.

Question 29

What does disinfection mean?

Answer 29

Reduction in the amount of detectable, viable organisms.

Question 30

Methods of Sterilization

Answer 30

➢ Filter ➢ Heating ➢ Radiation ➢ Chemical

Question 31

Filter is for:

Answer 31

➢ Heat-sensitive liquids and gases ➢ Most common for gases - ΔP important.

Question 32

Heating is:

Answer 32

➢ Most common for liquids and equipment. ➢ Typically steam at 121°C is used. ➢ Time and T are both important. ➢ Risk degrading medium components.

Question 33

Radiation is commonly used for:

Answer 33

Surfaces

Question 34

Chemicals are often used for:

Answer 34

Risk toxic residues

Question 35

Fluids and process equipment (filtration equipment, reactors, etc) can be sterilized by

Answer 35

➢ heat ➢ microfiltration ➢ radiation ➢ chemical agents ➢ UV light.

Question 36

Nature of the Problem: Chemostat

Answer 36

A faster growing contaminating organism can outgrow the desired organism and cause washout of the desired organism.

Question 37

Nature of the Problem: Batch

Answer 37

The product can be biologicaly contaminated (could be lethal) or the purity profile could be significantly affected.

Question 38

For continuous sterilization, there is:

Answer 38

High temperature, short exposure time.

Question 39

For batch sterilization, it cannot:

Answer 39

Cannot count the cooling and heating periods for sterilization.

Question 40

This is a common method for specific agents.

Answer 40

Thermal sterilization

Question 41

It is common for the insides of equipment that cannot be heat-sterilized or steam-sterilized.

Answer 41

Ethylene Oxide

Question 42

Media that cannot be heat-sterilized (heat-labile vitamins, proteins, sugars) must be filter-sterilized using filters with

Answer 42

narrow pore-size distributions.

Question 43

Commonly used to sterilize filtration equipment.

Answer 43

Weak (3%) sodium hypochlorite solution

Question 44

Sanitize means:

Answer 44

To clean with the purpose of removing possible biological and nonbiological health.

Question 45

Disinfect means:

Answer 45

To greatly reduce the number of living organisms.

Question 46

Sterilize means:

Answer 46

To eliminate al viable organisms present (often bioprocesses).