Bioreactor System

ERT 314

Sidang 1 2010/2011

Chapter 2:Types of Bioreactors

Week 3

Mass Transfer

Mass transfer occurs in mixtures containing local

concentration variation

Mass is transferred from one location to another

under the influence of a concentration difference or

concentration gradient in the system

For example, in bioprocessing, the supply of oxygen

in bioreactors for aerobic culture

Concentration of oxygen at the surface of air bubbles

is high compared with the rest of the fluid, therefore

this concentration difference promotes oxygen

transfer from bubbles into the medium

Molecular Diffusion

Molecular diffusion is the movement of component

molecules in a mixture under the influence of a

concentration difference in the system

It occurs in the direction required to destroy the

concentration difference

If the gradient is maintained by constantly supplying

material to the region of high concentration and

removing it from region of low concentration,

diffusion will be continue

Diffusion Theory

Diffusion Theory (cont’d)

In the previous figure, concentration CA varies from

CA1 to CA2 is a function of distance y

Molecules A will diffuse away from the region of high

concentration until eventually the whole system

acquires uniform composition

Diffusion Theory (cont’d)

According to Fick’s Law of diffusion, which states

that mass flux is proportional to the concentration

gradient:

dy

dCD

a

NJ A

ABA

A

where:

JA = mass flux of component A

NA = rate of mass transfer of component A

a = is are across which mass transfer occurs

DAB = binary diffusion coefficient or diffusion of component A

CA – concentration of component A

y = distance

DCA/dy = concentration gradient

Role of Diffusion in Bioprocessing Fluid mixing is carried out in most industrial processes where mass

transfer takes place

Areas of bioprocessing in which diffusion plays major role aredescribe below:

Scale of mixing – Turbulence in fluids produces bulk mixing on ascale equal to the smallest eddy size. Within the smallest eddies,flow is largely streamline so that further mixing must occur bydiffusion of fluid components.

Solid phase reaction – in biological systems, reaction aresometimes mediated by catalyst in solid form. When cells or enzymemolecules are clumped together into a solid particle, substrates mustbe transported into the solid before reaction can take place.

Mass transfer across a phase boundary – mass transfer betweenphases occurs often in bioprocessing. Oxygen transfer from gasbubbles to fermentation broth, penicillin recoveryfrom aqueous toorganic fluid, and glucose transfer from liquid medium into mouldpellets are typical examples.

Gas-Liquid Mass Transfer

It is important in bioprocessing because of the requirement for oxygen in aerobic fermentation.

From the figure beside, assume A is transferred from gas phase into the liquid.

The concentration of A in the liquid bulk is CAL; at the interface is CALi

whereas in gas CAG in bulk and CAGi at the interface.

Concentration gradients for gas-

liquid mass transfer

Gas-Liquid Mass Transfer (cont’d)

The rate of mass transfer of A through the gas boundary layer is:

AGiAGGAG CCakN

The rate of mass transfer of A through the liquid boundary layer is:

ALALiLAL CCakN

For some gasses equilibrium concentration in the gas phase is a linear function of liquid concentration, where m is distribution factor:

m

CC AGi

ALi

Gas-Liquid Mass Transfer (cont’d)The equilibrium relationship between gas phase and liquid phase at steady state are:

ALAG

LG

A

ALAG

LG

A

Cm

C

akamkN

mCCak

m

akN

11

1

The overall gas-phase mass transfer coefficient KG:

aK

m

akaK LGG

11

The overall liquid-phase mass transfer coefficient KL:

aKamkaK LGL

111

Gas-Liquid Mass Transfer (cont’d)The rate of mass transfer in gas-liquid systems can therefore be expressed in:

AL

AGLA C

m

CaKN

The equation is usually expressed in equilibrium, where mCAL is equal to C*AG, and (CAG/m) is equal to C*AL. Therefore the equation become:

ALALLA

AGAGGA

CCaKN

CCaKN

*

*

Gas-Liquid Mass Transfer (cont’d)When solute A is very soluble in liquid (e.g ammonia to water), the liquid side resistance is small compared with that posed by the gas interfacial film. Therefore, KGa is approximately equal to kGa due liquid side resistance is small and kLa is large:

*

AGAGGA CCakN

Conversely, if solute A is poorly soluble in liquid (e.g

oxygen in aqueous solution), the liquid phase mass

transfer ressistance dominates and kGa >>kLa,

therefore KLa approximately equal to kLa:

ALALLA CCaKN *

Oxygen Transfer in Cell Cultures

Cell in aerobic culture take up oxygen from the

liquid.

Therefore, the mass transfer of oxygen from gas to

liquid is importance, especially at high densities

when cell growth is likely to be limited by availablity

of oxygen in the medium.

The difference (C*AL-CAL) between the maximum

possible and actual oxygen concentration represent

concentration-difference driving force for mass

transfer.

Oxygen Transfer in Cell Cultures (cont’d)

where:

NA = rate of oxygen per unit volume (gmol m-3 s-1)

kL = liquid phase mass transfer coefficient (ms-1)

a= gas-liquid interfacial area per unit volume fluid (m2 m-3)

CAL = the oxygen concentration in broth (gmol m-3)

C*AL = the oxygen concentration in broth in equilibrium with

gas phase (gmol m-3)

ALALLA CCaKN *

Oxygen Transfer in Cell Cultures (cont’d)

The solubility of oxygen in aqueous solution at

ambient temperature and pressure is about 10 ppm

This amount of oxygen is quickly consumed in

aerobic cultures and must be constantly changed by

sparging

Low solubility of oxygen will show the difference

(C*AL-CAL) is very small

Factors Affecting Cellular Oxygen

The rate at which oxygen is consumed by cells in

bioreactors determines the rate which it must be

transferred from gas to liquid

Many factors influence oxygen demand, the most

important of these are:

Cell species

Culture growth phase

Nature of the carbon source in the medium

The concentration of cell increases during course of

batch culture and the total rate of oxygen uptake is

proportional to the no. of cell present

Factors Affecting Cellular Oxygen (cont’d)

The rate of oxygen consumption per cell also known

as oxygen uptake rate (OUR):

xqQ OO where:

Qo = oxygen uptake rate per volume broth (gL-1 s-1)

qo = specific oxygen transfer rate (gg-1 s-1)

Factors Affecting Cellular Oxygen (cont’d)

The demand of an organism for oxygen depends

primarily on the biochemical nature of the cell and its

nutritional environment.

To eliminate oxygen limitation and allow cell

metabolism to function at its fastest, the dissolved

oxygen concentration at every point in the bioreactor

must be above Ccrit.

The value of Ccrit depends on the organism, but

under average operation it usually falls between 5%-

10% of air saturation.

Oxygen Transfer from Gas Bubble to Cell

Oxygen Transfer from Gas Bubble to Cell

(cont’d)

In aerobic fermentation, oxygen molecules must overcome a series of transport resistance before being utilized by the cell

Eight mass transfer steps involved in transport of oxygen from the interior of gas bubbles to the site of intracellular reaction

1. transfer from the interior of the bubble to the gas-liquid interface

2. movement across the gas-liquid interface

3. diffusion through the relatively stagnant liquid film surrounding the bubbles

4. transport through the bulk liquid

5. diffusion through the relatively stagnant liquid film surrounding the cells

6. movement across the liquid-cell interface

7. if the cells in a floc, clump or solid particle, diffusion through the solid to the individual cell

8. transport through the cytoplasm to the site of reaction

Oxygen Transfer from Gas Bubble to Cell

(cont’d)

For most bioreactors, the analysis of mass transfer is as below1. Transfer through the bulk gas phase in the bubble is relative fast

2. The gas-liquid interface itself contributes negligible resistance

3. The liquid film around the bubbles is the major resistance to oxygen transfer

4. In a well-mixed bioreactor, concentration gradients in the bulk liquid are minimized and mass transfer resistance in this region is small

5. Because the single cells are much smaller than gas bubbles, the liquid film surrounding each cell is much thinner than around the bubbles and its effect on mass transfer can generally neglected

6. Resistance at the cell liquid interface is generally neglected

7. When cells are in clumps, intraparticle resistance is likely to be significant as oxygen has to diffuse through the solid pellet to reach the interior cells. The magnitude of resistance depends on the size of the clumps

8. Intracellular oxygen transfer resistance is negligible because of small distance involved

Oxygen Transfer from Gas Bubble to Cell

(cont’d)

When cell are dispersed in the liquid and the bulk fermentation broth is well mixed, the major resistance to oxygen transfer is the liquid film surrounding the gas bubbles

Therefore, transport through this film becomes rate limiting step and controls the overall mass transfer rate

At steady state, no accumulation of oxygen at any location in the bioreactor, therefore rate of oxygen transfer from bubbles must be equal to rate of oxygen consumption by cells:

xqCCaK OALALL *

Oxygen Transfer from Gas Bubble to Cell

(cont’d)

If kLa is small, the ability of the bioreactor to deliver

oxygen is limited

The maximum cell concentration can be supported

by mass transfer function of the bioreactor is:

O

ALL

q

Cakx

*

max

If xmax is lower than the cell concentration required

in fermentation process, kLa must be improved

Another important parameter is the minimum kLa

required to maintain CAL>Ccrit in bioreactor

critAL

O

critLCC

xqak

*

Oxygen Transfer in Bioreactor

Rate of oxygen transfer in fermentation broth is influences by several physical and chemical factors that change the value of kL of value of a or driving force (C*AL-CAL)

kL in fermentation liquid is usually in the range of 3-4 x 10-4 ms-1 for bubbles greater than 2-3 mm diameter, but it can be reduced to 1x10-4 ms-1 if smaller bubbles produced

If substantial improvement in mass transfer rate is required, it is more productive to increase the interfacial area, a

In production scale bioreactor, value of kLa is typically in the range of 0.02s-1 to 0.25s-1

Bubbles

Bubble behavior strongly affects the value of kLa,

which some may affect mainly on kL, whereas some

changes interfacial area, a

Bubble in lab-scale bioreactor frequently subjected

to severe distortion as they interact with turbulent

liquid current, whereas bubble in industrial stirred

tanks spend large proportion of their time floating

free and impeded through the liquid after initial

dispersion at impeller

The most important property of air bubbles in

bioreactor is their size

Bubbles (cont’d)

Advantages of small bubbles such as:

Can produce high level of gas dispersion by providing

more interfacial area, a

Since it has slow rise velocities, they can stay in the liquid

longer, allowing more time for oxygen to dissolve

Therefore, small bubble create high gas hold-up

defined as fraction of fluid volume in the reactor

occupied by gas:

GL

G

VV

V

where

ξ=gas hold upVG = volume of gas bubblesVL= volume of liquid

Bubbles (cont’d)

Disadvantages of small bubbles

Bubbles <<1mm can become nuisance in bioreactor where

oxygen concentration equilibrates with that in the medium

within seconds, so that the gas hold-up no longer reflects the

capacity of the system for mass transfer

Small bubble in non-Newtonian broth (viscous) will remain

lodged for long periods due to its velocity is reduced through

time

Bubble size also affect the value of kL

Bubble with diameter less than 2-3 mm, the surface tension

effects dominate the behaviour of the bubble surface

For bubble with diameter >3 mm, it can develop internal and

relatively mobile surface, depending on liquid properties