(735r) Characterization of a Mixed Flow Impeller in a Culture Medium

Mixing is a key unit operation that aims at the uniformity of a mixture of two or more phases. This process has applications in many industries such as chemical, petrochemical, and pharmaceutical [1].

Gas-liquid systems have been the subject of numerous studies in recent years [2, 3]. These systems have been characterized by high mass transfer, good liquid and gas mixing capacities, and longer residence times of gas in the liquid phase. These types of systems are commonly used in biological applications such as the culture of microorganisms [4,5]. In this sense, stirred tanks are used in such applications to obtain a complete homogenization of both liquid and gas phases.

To produce the required degree of uniformity inside the bioreactor the sparging and the agitation should be the best possible. The agitation inside of the bioreactor is carried out using impellers, which have been reported for different applications. These studies demonstrate that the traditional Rushton turbine impeller have some advantages over other impellers for gas-liquid mixing operations, although they exhibit drawbacks such as the formation of dead regions, poor pumping capacity, high strain rates, and high drag rate when using high viscosity fluids [6].

To overcome the deficiencies exhibited by the Rushton turbine, a hybrid impeller known as the axial radial impeller was designed and reported elsewhere [7], which is a combination of tilted blades and straight blades that creates a mixed flow. Posadas [8] studied the performance of the axial radial impeller in a gas-liquid stirred tank system using a model fluid. The results show that due to a combination of axial flow with radial flow achievement eliminates quasi-static flow zones, which resulted in better pumping, and better agitation intensity through the tank. These results show that axial-radial impellers may perform better than the traditional Rushton turbines in the fermentation processes.

In this work, the performance of a hybrid impeller called the axial-radial impeller (ARI) in a stirred tank with a culture of Azotobacter vinelandii was evaluated. Results are reported in terms of biomass growth, alginate production, and sucrose uptake. For comparison purposes, the biological performance of the axial-radial impeller was compared with the traditional Rushton turbine.

The bacterial strain A. vinelandii was grown in a modified Burk's medium which is shown in Figure 1a.

To avoid precipitation during autoclaving the solutions of and
were separated from the other medium components during sterilization. On the other hand, all components were sterilized at 121 °C for 35 minutes and were mixed in sterilized conditions.

The bioreactor consisted of a glass vessel of diameter T of 210 mm, and liquid height H of 202 mm, which provided an operating volume of 0.014 m³. The vessel is a cylinder with a plane bottom. Four baffles having a width J of 16 mm equally spaced to minimize vortexing in the free surface were collocated. Also, a ring sparger to supply air to the vessel at a distance of 25 mm measured from the bottom of the tank. Sets of three impellers with a diameter D_a of 70 mm coaxially placed along the shaft were used, namely: Rushton turbines (RTs), and axial radial impellers (ARIs). The first impeller on the shaft was collocated at a distance of 31 mm (C₁) measured from the sparger the second impeller at a distance of 70 mm (C₂) measured from the first impeller and the third impeller at a distance of 70 mm (C₃) measured from the second impeller, and finally distance between the third impeller and the free surface was of 31 mm (C₄).

Inside the vessel both the inoculum and culture medium were poured, which were operated under the conditions described in Figure 1b. The analytical determinations such as biomass growth, alginate production, and sucrose uptake were determined using the methodology described elsewhere [9].

The effect of each one of the two impellers on bacterial growth and alginate production is shown in Fig. 3a and b. As can be seen, the major biomass growth and alginate production were obtained with the axial radial impellers. The better performance of these impellers is due to that these impellers exhibit a major and better distribution of turbulent intensity maps as reported elsewhere [8], that is these impellers transfer more efficiently the energy from the engine to fluid, which is a better environment for the growth of microorganisms. On the other hand, as can be seen in Fig. 3c the sucrose consumption is very similar between both set impellers.

Finally, the results of the kinetics growth extracted from the graphs above show that the axial-radial impellers exhibited a superior performance with a cell yield coefficient, and product yield coefficient of , and , respectively. In contrast, the values obtained by the Rushton turbines were 0.123 and 0.173, respectively. Besides, the specific growth rate was calculated. The highest specific growth rate was with a specific alginate production rate of 7.0 g/alg/g biom/h, which was obtained with the axial-radial impellers.

In this study, the biological performance of a hybrid impeller was analyzed and compared to a Rushton turbine. The most relevant conclusions are:

The axial-radial impellers show better performance compared with the Rushton turbines. The axial-radial impellers exhibit the highest parameters compared with the one obtained with the Rushton turbines, which are listed following:

1. The specific growth rate was 195.3 % higher.
2. Its cell yield coefficient was 202 % higher.
3. Its product yield coefficient was 195.3 % higher.

The better performance of the axial-radial impellers is due to that these sets of impellers demand more power than the Rushton turbines, which is reflected in a better distribution of energy through the tank.

Figure 1. Kinetics of growth of A. vinelandii with ARIs and RTs: a) Components of modified Burk's medium; b) Bioreactor operating conditions; c) Biomass growth; d) Alginate production; e) Sucrose uptake

References:

[1] Paul, E. L., Atiemo-Obeng, V. A., & Kresta, S. M. (2004). Handbook of industrial mixing (pp. 1-143). NY: Wiley-Blackwell.

[2] Liu, Y., Guo, J., Li, W., Li, W., & Zhang, J. (2021). Investigation of gas-liquid mass transfer and power consumption characteristics in jet-flow high shear mixers. Chemical Engineering Journal, 411, 128580.

[3] Forte, G., Alberini, F., Simmons, M., & Stitt, H. E. (2021). Use of acoustic emission in combination with machine learning: monitoring of gas–liquid mixing in stirred tanks. Journal of Intelligent Manufacturing, 32(2), 633-647.

[4] Díaz-Barrera, A., Gutierrez, J., Martínez, F., & Altamirano, C. (2014). Production of alginate by Azotobacter vinelandii grown at two bioreactor scales under oxygen-limited conditions. Bioprocess and biosystems engineering, 37, 1133-1140.

[5] Medina, A., Castillo, T., Flores, C., Núñez, C., Galindo, E., & Peña, C. (2023). Production of alginates with high viscosifying power and molecular weight by using the AT9 strain of Azotobacter vinelandii in batch cultures under different oxygen transfer conditions. Journal of Chemical Technology & Biotechnology, 98(2), 537-543.

[6] Nienow, A. W. (1996). Gas-Liquid Mixing Studies-A Comparison of Rushton Turbines with Some Modern Impellers. Chemical Engineering Research & Design, 74(4), 417-423.

[7] Ascanio, G., Foucault, S., & Tanguy, P. A. (2003). Performance of a new mixed down pumping impeller. Chemical Engineering & Technology: Industrial Chemistry‐Plant Equipment‐Process Engineering‐Biotechnology, 26(8), 908-911.

[8] Posadas-Navarro, D., Palacios, C., Blancas-Cabrera, A., Trujillo-Roldán, M. A., Salinas-Vázquez, M., & Ascanio, G. (2022). Flow Patterns of Multiple Axial‐Radial Impellers for Potential Use in Aerated Stirred Tanks. Chemical Engineering & Technology, 45(5), 860-867.

[9] Peña, C., Campos, N., & Galindo, E. (1997). Changes in alginate molecular mass distributions, broth viscosity and morphology of Azotobacter vinelandii cultured in shake flasks. Applied Microbiology and Biotechnology, 48, 510-515.