Hydrodynamic and Mass Transfer Correlation in a Microbubble Aerated Stirred Tank Reactor

Abstract

Many chemical and biocatalytic reactions are consuming gaseous species like oxygen, provided by the mass transfer across interfaces of multiphase contact apparatuses. For biocatalytic reactions a macroscopic aeration can lead to reduced enzyme activity by foaming and shear forces and for fast chemical reactions in multiphase ows, the mass transfer limitation is often the bottleneck for a process optimization. The present study investigates the use of bubbles with diameters less than 100 µm for aeration of a 3 L lab scale stirred tank reactor. For demineralized water and a solution of glucose and bovine serum albumin (BSA) as biocatalytic model protein solution, two di erent membrane spargers with a mean pore size of 1 µm and 2 µm are investigated. Determining the in uence of the energy input on the hydrodynamics of the system, endoscopic measurements of bubble size distributions are carried out. The mass transfer performance of the two spargers is analyzed by measurements of the oxygen k l a value for varying gas ow rates. As a result microbubble aeration shows a signi cant higher mass transfer performance compared to an open tube aeration saving 60% of the gaseous phase by reaching the same k l a values. Besides high speci c interfacial areas and long residance times, the Laplace pressure inside the bubble is identi ed as an enhancing force for mass transfer at microscale.

Comparative investigation of fine bubble and macrobubble aeration on gas utility and biotransformation productivity

“…Consequently, the shrinking behavior as well as the bubble formation is depending on the surface tension. In a previous study, we reported a decrease of 15.5% in surface tensions of a buffer solution containing 67.8 mg/L BSA or GOx, compared with pure water (Matthes et al, 2020). This change applies to the underlying system.…”

“…As reported in the literature, especially the pore size distribution is expected to affect the oxygen transfer rate through the generated bubbles size distribution (Kazakis, Mouza, & Paras, 2008b; Kulkarni & Joshi, 2005; Tsuge and Hibino, 1983). In previous investigations using the same microbubble aerators, no significant difference in the BSD between the SPG and sintered frit for gassing rates up to 0.067 vvm, generated in BSA containing sodium acetate buffer, is determined (Matthes et al, 2020). The 5 mm (single hole) open pipe macrobubble aerator and the 6 × 0.5 mm (multiple holes) open pipe are included in this comparison.…”

“…This effect is also expected for the application of GOx. The presence of glucose shows no significant influence on BSD (Matthes et al, 2020). To gain a deeper understanding of the aerator's specific mass transfer performance, the investigated range of aeration rate is expanded up to 16.667 vvm (Figure 3) from the previously studied lower aeration rates of up to 0.067 vvm (Matthes et al, 2020).…”

“…The presence of glucose shows no significant influence on BSD (Matthes et al, 2020). To gain a deeper understanding of the aerator's specific mass transfer performance, the investigated range of aeration rate is expanded up to 16.667 vvm (Figure 3) from the previously studied lower aeration rates of up to 0.067 vvm (Matthes et al, 2020). Enabling the comparison of the aerators by mass transfer phenomena in the underlying study, the GOx is substituted by BSA to ensure the steady‐state condition and avoiding an overestimation of the k L a through the consuming GOx reaction (Figure 3).…”

“…In consequence, the formation of the resulting bubble size is expected to be highly influenced due to the process of secondary bubble formation and the disintegration of unstable bubbles (Durst & Beer, 1969). In contrast to macrobubbles, stirrer introduced shear forces affect the gas dispersion of microbubbles less, due to their small size (Matthes et al, 2020). In the experimental setup, mixing is provided by a pitch blade turbine with Ne = 1.37 and constant power input of 0.12 W/L.…”