Hydrodynamics and mass transfer performance of microbubble flow in the bubble column with a contraction section

Abstract

Bubble columns have many applications in the biochemical, petrochemical, and metallurgical industries. The gas-liquid system composed of microbubbles is characterized by significant phase interfacial area, high mass transfer efficiency, fast dissolution rate, strong adsorption capacity, and long self-sustaining time. This paper investigated the performance of pressurized dissolved gas microbubble generation (PDG) using a Venturi tube as the releaser using a plug-in online imaging instrument. The hydrodynamic and mass transfer characteristics of the microbubble flow generated by PDG in a bubbling column with a contraction section (BCS) were then investigated. One of the primary studies was the effect of the contraction section on the flow performance of the bubbles, the mass transfer performance, and the aeration efficiency of the PDG versus the throat inlet Venturi bubble generator (VBG). The results show that the initial bubble Sauter mean diameter is essentially constant with dissolved gas pressure, and the bubble size distribution(BSD) is similar. The bubble stream passing through the contraction section exhibits two different BSD at the center and side walls of the BCS outlet section. Microbubbles in the inlet section in the absence of external forces almost do not occur in bubble coalescence. The flow of bubbles in the contraction section is subject to a slight coalescence effect. The diameter of bubbles in the contraction section is mainly larger than 20 μm. In the contraction section, coalescence occurs mainly at the side wall. The way of occurrence is adsorption coalescence and collision coalescence. The bubbles move in a gradually confined space with a high degree of interference with each other's bubbles, and the liquid linear velocity acts as an ‘external force’ that benefits the flow of the bubbles out of the confined space. The standard aeration efficiency of PDG is 3–11 times higher than that of the VBG. At 0.3 MPa, the highest standard aeration efficiency for the PDG method was between 0.18 and 0.64 kgO2/kW∙h.