10 Determination of KLA

Chemical Engineering Laboratory I

BKF2741 BIOREACTOR DETERMINATION OF KLa

OBJECTIVE To investigate the effect of agitation speed and air flow rate on volumetric oxygen transfer rate (KLa) in stirred tank bioreactor. EQUIPMENT / APPARATUS / MATERIAL 1. 2 L Fermenter

2. Dissolve oxygen probe

3. Ruler

4. Caliper

5. 2 L Distilled water

6. N2 and O2 supply

7. 2 g NaCl

8. 5 ml cooking oil (antifoam)

INTRODUCTION The main parameter of interest for design of an aerobic bioreactor are K L and a. It is extremely difficult to determine KL and a separately and thus, most researcher concentrate on the determination of KLa, termed as volumetric oxygen transfer coefficient. KLa can be determined in various systems of the gas-liquid dispersion. KLa is influence not only by the operating variable of the stirred tank bioreactor but also the geometry and design of impeller. Understanding of the system and its effect on oxygen transfer and mixing may help to efficiently use the bioreactor for fermentation process. Estimation of the KLa of a fermentation system by using dynamic gassing-out techniques depends upon monitoring the increase in dissolved oxygen concentration of a solution during aeration and agitation. The oxygen concentration of the solution is lowered by gassing the liquid out with nitrogen gas. Aeration is then initiated at a constant air flow rate and the increase in dissolved oxygen tension (DOT) is monitored using dissolved oxygen probe. The profile of DOT during deaeration and aeration is shown in Figure 1. Increased in DOT during aeration can be expressed by Eq. 1

dC L

dt

 K L a(C E  C L )  Qd

(1)

where CL is dissolved oxygen conc., CE is saturated dissolve oxygen conc. and Q d is spesific respiration rate. Mass balance for the system:

Chemical Engineering Laboratory I Rate of change in O2 conc. = Rate of O2 in – Rate of O2 out – Rate of usage Qd

Air Off

CL (% saturation)

100

BKF2741

Air On

0 0

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10

Time (min) Figure 1 DOT profile If microorganism is not present in the solution, Qd = 0, Eq 1 becomes

dC L

dt  K L a(C E  C L )

(2)

Can be written as,

dC L

Plot of

dC L

dt   K L a.C L  K L a.C E

dt

against different value of CL will give a slope as –KLa. However determining

dC L

dt

values may be a problem. There are two methods in determination of K La. First, the difference method and second, integral method. (A) Difference method Obtained CL value at constant time interval aeration. 2

Chemical Engineering Laboratory I Time (min) CL

Assume that Therefore,

dC L

C L

dt

t



≈

C L

t0 CL0

BKF2741 t1 CL1

t2 CL2

t3 CL3

t4 CL4

t5 CL5

dt

 C L1  C L 0  ,  C L 2  C L1   t 2  t1   t1  t 0 

Corresponding to CL value will be CL(average)

C L ( average ) 

 C L 0  C L1  , (C L1  C L 2 )  2  2

Eq. 2 can be expressed as,

C L Plot of

C L

t  K L a (C E  C L (average) ) t

versus C L (average ) will produce a straight line where the slope is equal to -KLa.

(B) Integral method

dC L

dt  K L a(C E  C L )

dC L

(C E  C L )  K L a dt

Integrate CL=C0, when t=0 CL=CL, when t=t ln(C E  C L )  ln(C E  C L )   K L a.t ln(C E  C L )   K L a.t  ln(C E  C L )

Plot of ln(CE-CL) versus t will produce a straight line where the slope is equal to -KLa

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Chemical Engineering Laboratory I

BKF2741

PROCEDURE A) Geometry of Stirred Tank Reactor 2 L stirred tank bioreactor was filled with 2 L distilled water. Measure the geometrical ratios of the bioreactor using a caliper and ruler. Fill in the table below: Dimension of components Operating volume Liquid height (L) Tank or vessel diameter (D) Ratio 0f L/D Impeller diameter (P) Baffle width (B)

Liter/mm

Dimension of components Ratio of P/D Ratio of B/D Impeller height (I) Ratio of I/D Number of impeller

Liter/mm

B) Determination of KLa at different air flow rate 1. Set the agitation of bioreactor to 300 rpm. 2. Oxygen concentration of the solution is lowered by gassing the liquid out with N 2 at 9 L/min until saturate and stops the flow. 3. Supply O2 to the bioreactor at flow rate of 1.0 L/min until saturate, then stops the flow. 4. Obtain CL value at constant time interval during aeration. 5. Repeat step 2-4 for different oxygen flow rate (1.5 and 2 L/min). 6. Determine the KLa of stirred tank reactor at different air flow rate C) Determination of KLa at different agitation speed 1. Set the agitation of bioreactor to 200 rpm. 2. Oxygen concentration of the solution is lowered by gassing the liquid out with N 2 at 9 L/min until saturate and stops the flow. 3. Supply O2 to the bioreactor at flow rate of 1 L/min until saturate, then stops the flow. 4. Obtain CL value at constant time interval during aeration. 5. Repeat step 1-4 for different agitation speed (400 and 600 rpm). 4

Chemical Engineering Laboratory I 6. Determine the KLa of stirred tank reactor at different agitation speed.

BKF2741

RESULTS Variable

Time (min)

2

4

6

8

10

12

14

1.0 L/min 1.5 L/min 2.0 L/min CL 200 rpm

(%

400 rpm

saturation)

600 rpm DISCUSSION 1. Plot a graph to show the effect of air flow rate and agitation speed on KLa. 2. Discuss the possible cause of error in determination of KLa by using this dynamic gassing out technique.

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