FACTORS AFFECTING THE VALUES OF

VOLUMETRIC MASS TRANSFER COEFFICIENTS (kLa)

IN FERMENTATION SYSTEMS

FACTORS AFFECTING THE VALUES OF MASS TRANSFER COEFFICIENTS

IN FERMENTATION SYSTEMS

•Various operating parameters and physico-chemical properties of

fermentation broth affects mass transfer rate, which in turn show effect

on kLa.

Factors affecting the value of kLa are:

1. Bubble size

2.Gas Hold-up

3.Gas Velocity

4.Type of gas sparger

5.Type of agitation

6. Power input to agitator

7.Temperature

8.Pressure

9. Antifoaming agents

10. Presence of cells

11. Surface active solutes

1.Bubble size

•Bubble size has great effect on kLa as the gas is sparged discretely

into the fermentation broth in the form of small bubbles.

•Small bubbles have high interfacial area.

No.of bubbles Bubble volume

(mm3)

Bubble

Diameter (mm)

Surface Area

(mm2)

Increase in

area (%)

1 4.19 2 12.57 -

2 2X2.1 1.59 2X7.91 26.0

3 3X1.4 1.39 3X6.0 44.2

Effect of bubble size on interfacial area and various other parameters

Bubble

Dia

a kL Bubble

rise

velocity

Gas Hold-

up

Internal re-

circulation

Bubble

rigidity

Big Small High High Less Yes No

Small High Small Low High No Yes

2. Gas Hold-up

Gas Hold-up is the volume fraction of the gas held up in the total volume

comprising the liquid and the held-up gas together.

e= VG/(VG+VL) = VG/V

Higher values of e indicates higher amount of gas held up in the system.

Smaller bubbles (dB <<1mm) can be a nuisance in fermenters.

Very small bubbles will transfer the oxygen content in them to the liquid

broth quickly, and what is remaining is only nitrogen which does not

contribute any thing, except showing higher gas hold-up.

Though gas hold-up may not be directly proportional to the mass transfer

rates, kLa values decrease with decreasing bubble diameter less than 2

mm.

3. Gas Velocity

•The Superficial gas velocity (uG) is the linear velocity of gas obtained by

dividing the volumetric flow rate of the gas with the cross-sectional area of

vessel.

•Mass transfer rates increases with uG as:

•If gas flow rate is very high, it may not allow the oxygen to dissolve in the

liquid, and hence may escape in the outlet.

•In agitated vessels, the effect is compounded by the agitator speed also.

•For higher gas flow rates, the gas-liquid contact is poor and most of the gas

is unutilized, which results impeller flooding.

•Impeller flooding is the situation, where the gas is being supplied to the

system at a rate which the impeller is not able to disperse.

•To over come this, either gas flow rate should be reduced or the agitator

speed is to be increased.

20

70

3102 .

G

.

L uV

PXak

4. Type of Gas Sparger

The effect of spargers on mass transfer has not been studied

extensively.

Hassan and Robinson (1977) reported that sparger design did not

have much effect on gas-liquid dispersion in aerated aqueous phases.

5. Type of Agitator

Type of agitator and agitator design for effective mixing shows great

effect on mass transfer rates.

Types of agitators used for gas-liquid mass transfer are:

1. Propeller

2. Turbine - better gas-liquid contacting

3. Paddle

4. Vaned discs - better gas-liquid contacting

5. Anchor -highly viscous liquids and slurries

6. Helical Screws -highly viscous semi-solid masses.

Types of Agitators

Mixing

patterns

6. Power input to Agitators

The effect of power input on a as:

The interfacial area varies as : (P/V)0.4.

The power input is also related to agitator speed through the power

number (NP) as:

NP = P/(r n3 Di5) or P = NP r n3 Di

5

The dependence of power number on the agitator speed varies based on

the type of flow.

P a n2 for laminar regime

P a n3 for turbulent regime

50

60

20

40

441

.

t

.

L

.

L

.

u

uV

P

.a

r

7. Temperature

• The temperature has two effects on mass transfer:

1. It increases the diffusivity of the gas into the liquid.

2. Increases the value of kLa.

• However, increase in the temperature decreases the solubility of

gas and hence reduces C*O2,L.

• So, (C*O2,L-CO2,L) will reduce and there by it reduces mass

transfer.

• In the range of 10-400C, temperature rise increases the mass

transfer.

• But temperature > 400C, the mass transfer will decreases.

8. Pressure

Pressure affects the mass transfer by increasing the solubility of the gas

in the liquid phase, which is given by Henry’s law:

pO2,G = H X C*O2,L

The partial pressure and total pressure of the system are related by:

pO2,G = pT X yO2

Thus, a total pressure pT increases, pO2,G increases, and hence C*O2,L

increases, which in turn will increase driving force.

Generally, no high-pressure systems are used in fermentation processes.

9. Antifoaming agents

• Most of the fermentation broths contains proteins which causes foaming.

• Foaming is an unineviatble nuisance in fermentation broths, and should

be avoided.

• Otherwise,

• It may choke the pipelines, measuring instrument lines

• It may harbour unnecessary microorganisms to thrive and thrash the

fermentation subsequently.

• The choked pipe lines are difficult to be cleaned.

Foaming can be avoided by using- Mechanical foam breakers or

antifoaming agents

Use of Antifoaming agents reduces coalescence of smaller bubbles into

larger bubbles.

Thus they increase interfacial area and hence mass transfer.

10. Existence of cells

Oxygen transfer (or mass transfer) in fermentation broths is greatly

influenced by the presence of cells by two ways:

1.Physical Influence

2.Quantitative influence

In physical influence, cells interfere in the break-up and make-up of the

gas bubbles by influencing the surface properties.

As cells get absorbed on the gas-liquid interface, cells do not allow

smaller bubbles to coalesce into bigger bubbles, which increase the

interfacial area and in turn mass transfer.

In quantitative influence, the cells absorb oxygen during the process

which increases the driving force and hence oxygen transfer.

The influence of cells in enhancing the mass transfer depends on:

1. Type of cells 2. Morphology of cells 3. concentration of cells.

11. Surface active solutes

The surface active solutes, which are hydrophobic in nature

- alters the surface characteristics of the gas-liquid interface.

- do not allow the gas bubbles to coalesce.

This results in increased interfacial area.

The concentration of the solute could very low (0.05%), but its effect in

increasing the surface area could be large.

Since, the concentrations of solutes are very low, they do not affect the

interfacial tension to any appreciable or measurable extent.