Dipl.-Ing. Annika Rosseburg,
Jürgen Fitschen, M.Sc.

Motivation & Objectives

• Aerated stirred tanks are widely used in the field of bio fermentations.
• Oxygen mass transfer is of great importance for many industrial processes.
• Many ways of improving the mass transfer are known
• enhancement of specific surface area a (by smaller bubbles or higher gas flow rates)
• enhancement of the transfer rate kL (by lager shearing of the bubbles)

But the relation between those factors are very complex and thus
hard to estimate.

Figure 1: complexity of the influencing factors on mass transfer rate

Objective:
Further investigation on the influence of the bubble shearing are going to be done. The main objective is placed on the influence of shear stress induced by the stirrer blades on the concentration boundary layer which seems to be of strong importance for mass transfer performance.

Experimental Set Up & Procedure

Figure 2: Experimental set up with two locations for aeration

To investigate the impact of the stirrer blades two identical sparger with different location of aeration are going to be used (Figure 2)

• Small scale bio reactor
• D = 0.11 m
• H = 0.25 m
• Rushton turbine
• wg0<0,5cm/s

Calculation of energy input by measuring the momentum.

Results

1. Comparison of both positions:

Bubble diameter d₃₂ and spec. surface area a are similar (measured with endoscopic method [2])

Figure 3a: Bubbles position A; Figure 3b: Bubbles position B

2. Determination of mass transfer

Calculation of mass transfer kLa by dynamic method:

Figure 4: Oxygen increase mea-sured with PreSens fibreoptical sensor

3. Determination of local energy dissipation:

Via PIV the flow field will be measured to identify the local shear stress on the bubbles within the liquid phase (Figure 5).

Figure 5: velocity distribution in stirrer tank obtained with PIV [3]

Outlook

Bubble-blade punching leads to an increase of mass transfer rate (Figure 6)

Figure 6: Comparison of O2 mass transfer rates for two aeration positions

Position B:
Increased kL value by 10 and 50 %.

Further experiments will be done with high speed imaging and Laser Induced Fluorescence to get a deeper insight into the mass transfer enhancement by local stress due to stirrer blade-bubble punching.

References

[1] F. Garcia-Ochoa, E. Gomez, Biotechnol. Adv. 27 (2009) 153.
[2] M. Schlüter, CIT 82 (2011) 992.
[3] S. Richter, IMS Technical University Hamburg, (2011)