Hydrodynamic and mass transfer correlation in a microbubble aerated stirred tank reactor

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 flows, 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 different membrane spargers with a mean pore size of 1 µm and 2 µm are investigated. Determining the influence 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_la value for varying gas flow rates. As a result microbubble aeration shows a significant higher mass transfer performance compared to an open tube aeration saving 60% of the gaseous phase by reaching the same k_la values. Besides high specific interfacial areas and long residance times, the Laplace pressure inside the bubble is identified as an enhancing force for mass transfer at microscale.