Mixing and liquid-to-gas mass transfer under digester operating conditions

• Published in: Chemical engineering science 2017-10, Vol.170, p.606-627
• Main Authors: Trad, Zaineb, Vial, Christophe, Fontaine, Jean-Pierre, Larroche, Christian
• Format: Article
• Language: English
• Subjects: Anaerobic digester; Biomass suspension; CFD; Chemical and Process Engineering; Dual-impeller design; Engineering Sciences; Gas-liquid mass transfer; Stirred-tank bioreactor
• ISSN: 0009-2509; 1873-4405
• DOI: 10.1016/j.ces.2017.01.056

•A dual-impeller stirred tank devoted to dark fermentation was investigated.•Hydrodynamics, mixing and mass transfer were compared using nine mixer designs.•Local flow field, vortex formation, kLa and straw suspension were also studied.•Experimental data were successfully confronted to CFD simulations.•Finally, the best compromise for mixer design in the digester was defined. This work deals with the analysis and the optimization of a dual-impeller design in terms of mixing, hydrodynamics, mass transfer properties and power input in a mechanically stirred digester devoted to biohydrogen production through acidogenic fermentation of lignocellulosic waste. Various mixer designs involving Rushton turbines, an Elephant Ear impeller and a marine propeller, were compared. Experimental data were successfully confronted to CFD-based simulations used to reveal the respective roles of impeller type, geometry and clearance. The results showed that the flow pattern was strongly influenced by the off-bottom and inter-impeller clearances, and by the size and type of the lower impeller. Straw suspension was enhanced by a small disk turbine with a low off-bottom clearance and a large inter-impeller clearance that promoted an axial flow circulation together with a small mixing time due to the interaction with the larger turbine used as the upper impeller. Conversely, kLa evolution was weakly dependent on impeller design, position, and rotation speed until a deep vortex formed on the free surface, showing that power input was too weak to enhance liquid-to-gas mass transfer. Finally, the design including an Elephant Ear turbine as the upper impeller and a smaller Rushton turbine as the lower impeller was selected as the best compromise between distributive and dispersive mixing, while the objective of a power input lower than 10W/m3 was achieved.