Development of a multiparticle optical trajectography technique for hydrodynamic analysis of a stirred tank devoted to dark fermentation

Abstract

Dark fermentation (DF) is a biological process able to produce hydrogen gas from organic waste which can play a key role as a building block in biorefineries. But the optimization of hydrodynamic conditions of DF is still needed to enhance gas-liquid mass transfer, thereby reducing the self-inhibitory effect of soluble hydrogen. Mass transfer enhancement is constrained, as hydromechanical stress on microorganisms must be limited and the economic sustainability of the process maintained.Recent results showed that DF is enhanced in the transition region between laminar and turbulent regimes. To gain a better knowledge of 3D hydrodynamic features in this regime, a modified optical trajectography technique was developed and applied to a 2-L bioreactor equipped with a dual-impeller device. The proposed methodology aims at monitoring the trajectories of up to ten particles as tracers at the same time using three cameras but is also able to provide the real-time position of the particles in the 2D-images of each camera to minimize the post-treatment time.The methodology, including stereoscopic camera calibration, real-time and post- treatment to reconstruct the 3D trajectories, was applied, and validated against 2D-PIV and CFD data. A good agreement was achieved, but regions neighboring wall and impeller were difficult to capture due to the particle size. The results highlighted that operating with five particles was able to reduce by a factor of 3–4 the measurement time in comparison to optical trajectography with a single particle as a tracer, while a higher number of tracers increased the occurrence of artifacts.