Factors Affecting Kla

FACTORS AFFECTING THE VALUES OF

VOLUMETRIC MASS TRANSFER COEFFICIENTS (k

a)

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 k_La.

Factors affecting the value of k_La 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

1.Bubble size

•Bubble size has great effect on k_La 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 k_L 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= V_G/(V_G+V_L) = V_G/V

Higher values of e indicates higher amount of gas held up in the system. Smaller bubbles (d_B <<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 (u_G) 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 u_G 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. 2 0 7 0 3 10 2 _G. . L u V P X a k        

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.

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 (N_P) as:

N_P = P/(r n3 _D

i5) or P = NP r n3 Di5

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}

5 0 6 0 2 0 4 0 44 1 . t . L . L . u u V 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 k_La.

• 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.

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:

p_O2,G = H X C* O2,L

The partial pressure and total pressure of the system are related by: p_O2,G = p_T X y_O2

Thus, a total pressure p_T increases, p_O2,G increases, and hence C* O2,L

increases, which in turn will increase driving force.

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.

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.