Abstract:

Mixsim, one of computational fluid dynamics (CFD) methods, was used to simulate a 3 L mechanically stirred bioreactor to study the effect of agitation speed on agitation current field. Along with this, effect of agitation speed on formation of mycelial pellets of Rhizopus oryzae and lactic acid production in the real bioreactor. Results showed that higher agitation speeds corresponding to higher shear stress rates resulted in the formation of more than 98% of mycelial pellets of 0.1 – 0.3 mm in diameter, with lower lactic acid production and biomass. In contrast, when agitation speed was controlled at lower levels and shear stress rate was also lower, the formation of mycelial pellets was inhibited and higher lactic acid production was achieved. However, the large amount of dispersed mycelia exhibited a negative impact on latter fermentation. A maximum lactic acid production of 106.7 g/L fermentation broth was achieved at the agitation of 400 r/min. Meanwhile, more than 80% of Rhizopus oryzae mycelia were formed into pellets of 1.0－1.5 mm in diameter. The biggest shear stress rate simulated by CFD was 1.58 m/s.

Key words: CFD simulation, bioreactor, lactic acid, Rhizopus oryzae