HOW TO DESIGN AND VERIFY A REASONABLE STIRRING SYSTEM FOR BIOREACTORS CFD SIMULATION STUDY OF STIRRED BIOREACTOR

1. Research purpose

Through the research and analysis of the fermentation process characteristics of high-aerobic fermentation products, combined with the company's experience in bioreactor system design, a bioreactor system that can meet the needs of the fermentation process of this product was proposed. It is planned to use computational fluid dynamics (CFD) method to verify the entire reactor design from the perspectives of flow field, mixing and mass transfer.

2. Scheme design and model parameters

There are three types of simulated production-scale fermentation tanks, with nominal volumes of 100L, 1000L and 10000L. The 100L and 1000L fermentation tanks use a combination of straight blade disc turbine + inclined blade propeller + straight blade disc turbine stirring impeller, and the heat exchange method is jacketed. The 10000L mixing system uses a half-tube disc turbine stirring impeller with strong gas dispersion ability in the bottom layer, a tilted blade disc turbine stirring impeller in the second layer, and a four-wide blade axial flow type stirring impeller with good mixing performance in the upper layer. The heat exchange method It is a built-in tube. In order to meet the high oxygen consumption demand, the stirring motor powers used in the three fermentation tanks are 1.5kW, 5.5kW and 22kW respectively, and the maximum stirring speeds are 600rpm, 500rpm and 200rpm respectively. The main design scheme and model parameters of the reactor are shown in Table 1.

Table 1. Reactor design scheme and model parameters

3. Simulation results

3.1. Velocity field comparison

Since this design scheme uses a mixing combination of radial flow blades + axial flow blades, it takes into account both gas dispersion performance (i.e. mass transfer performance) and axial mixing performance. Judging from the velocity vector diagram (Fig. 1) and velocity cloud diagram (Fig. 2) obtained from the simulation, the flow pattern control capabilities of the bottom blades of each scheme are strong, forming a stable radial flow, the gas dispersion effect is good, and no gas dispersion occurs. Angry. Because the airflow is controlled, the upper two layers of blades also form a stable flow pattern. The flow pattern in the entire reactor is relatively stable, has strong axial circulation, and has good mass transfer and mixing performance.

3.2. Energy dissipation rate

For the stirring-controlled fluid movement process, the energy dissipation rate in the reactor is mainly concentrated in the fluid discharge area of the paddle. Judging from the energy dissipation rate distribution in the reactor (see Figure 3), the energy dissipation rate is mainly concentrated in the fluid discharge area of the blade. The axial flow stirring blade forms a larger energy dissipation rate in the axial direction, while The radial flow type stirring paddle forms a larger energy dissipation rate in the radial direction. It can be seen that the flow and energy transfer in the reactor are mainly controlled by the paddles.

3.3 Gas holdup

From the perspective of gas holdup distribution (see Figure 4), under this design scheme, the bottom gas flow is completely controlled, the gas is well dispersed, and there is almost no oxygen supply dead zone at the bottom of the reactor. The gas distribution in the entire reactor is very uniform, and the gas content rate is high. The three reactors have good gas dispersion and oxygen mass transfer capabilities.

3.5 Bubble size distribution

The simulation process uses the PBM model to analyze the bubble aggregation and breakup process in the reactor. The bubble size distribution is shown in Figure 5. Due to the high energy dissipation rate in the blade discharge area, the diameter of the bubbles in this area is small. Generally speaking, the bubble diameter in the upper part of the reactor is larger than that in the lower part, mainly because the bubbles coalesce during the rising process.

3.6 Engineering parameters

Table 2 compares and analyzes various engineering parameters in the reactor. Under the conditions of this design scheme, since there is no gas flooding in the bottom layer, the difference in gas content in each area is small and the gas is evenly dispersed. The effective powers of the three fermentation tank stirring paddles are 0.286, 3.843 and 15.840kW respectively, and the power per unit volume reaches more than 2kW/m3.

The total gas content of the three fermentation tanks is about 30%, and the gas content is relatively high. The average bubble sizes are 4.21, 4.31 and 4.23mm respectively. The specific mass transfer area a between gas and liquid can be calculated based on the gas holdup and the average diameter of the bubbles. The calculation method is shown in Formula 1.

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（1）

In the formula, α is the gas holdup rate, and d is the bubble diameter. From this, the gas-liquid ratio mass transfer area is 224m-1. Through the energy dissipation rate TED obtained by simulation, the mass transfer coefficient kL in the reactor can be calculated according to the penetration theory. Further, kLa can be calculated based on a and kL, and then the maximum oxygen mass transfer capacity OTRmax can be calculated according to Formula 2.

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? （2）

In the formula, c* is the saturated oxygen concentration under working conditions (directly related to temperature and tank pressure). It can be seen from the calculation results that the highest OTR of the three fermentation tanks is above 200 mmol O2/L/h, and the oxygen supply effect is good, which can meet the oxygen demand of the high-aerobic fermentation process.

4. Conclusion

This study systematically analyzed the flow field characteristics and engineering parameters of three fermentation tanks, verified the design plan based on this, and reached the following conclusions:

1) The three designed fermentation tanks have good gas dispersion and stable flow patterns under the maximum working conditions.

2) The axial flow and radial flow in the three reactors are well combined, and the mixing effect is good.

3) The stirring power of the stirring system after ventilation reaches more than 2kW/m3, and the stirring power does not exceed the rated power of the motor.

4) Under simulation conditions, the maximum oxygen supply capacity of the three reactors reaches 200 mmol/L/h, which can meet the high oxygen consumption demand.

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