Design and Optimization of a two-stage bioreactor with cellular recycling for intensive ethanol production

This paper focuses on ethanol production optimization in the steady-state when the bioprocess involves a two-stage bioreactor with a cell recycling loop to reach high biomass concentrations. This bioprocess is described by a nonlinear model; several nonlinear optimizations for different configurations of the bioprocess are made with a Sequential Quadratic Programming (SQP) algorithm. Biomass residence times in the two bioreactors (TB1 and TB2) are the main parameters of the optimization. These parameters are connected to the volume ratio between the two bioreactors (V1/V2) and the substrate feed concentration (Sf1 and Sf2). The optimal configuration has been found for a volume ratio in the range 3.5 to 4.5 with the volume of the second bioreactor remaining constant. This configuration allows ethanol production higher than 2 kg/h, with a cellular viability greater than 80 %, a null glucose residual concentration and an ethanol titer of 11.6° GL.