milli-scale reaction system; metal foams; mass transfer; heat transfer; LIF; PIV; structured porous reactor; flow characterization; mixing in porous media; two phase flow; complex flow
Häfeli Richard, Rüegg Oliver, Altheimer Marco, Rudolf von Rohr Philipp (2016), Investigation of emulsification in static mixers by optical measurement techniques using refractive index matching, in Chemical Engineering Science
, 143, 86-98.
Häfeli Richard, Altheimer Marco, Butscher Denis, Rudolf von Rohr Philipp (2014), PIV study of flow through porous structure using refractive index matching, in Exp Fluids
, 55, 1717.
Häfeli Richard, Hutter Cédric, Damsohn Manuel, Prasser Horst-Michael, Rudolf von Rohr Philipp (2013), Dispersion in fully developed flow through regular porous structures: Experiments with wire-mesh sensors, in Chem Eng Proc
, 69, 104-111.
In the industry large scale equipment, predominantly dis- or semicontinuous batch processes, are still used due to the economy of scale. For chemical plants the investment costs are in general proportional to the capacity to the power of 0.5 to 0.7. The growing need of energy saving and sustainable production leads to a demand for small scale continuous devices as the recuperation of energy is more efficient and mass and heat transfer is strongly enhanced. In micro reaction systems the scale up is mostly realized by numbering up individual micro reactors, which is not an economical solution for large-scale production. In spite of the huge pressure drop and the predominant laminar flow within microsystems the small scales offer many advantages. The preferences of different scales lead to the conclusion that a favorable range for economical and sustainable large-scale production is found in between micro and macro. Our approach is to use high porous micro structured inserts for plug flow reactors in the millimeter range. The system offers the possibility of high throughput (liquid flow rate up to 2.5 l/min) at a comparable small pressure drop. A huge potential arises from open cell metal foams due to their high specific surface area combined with a high porosity which is typically larger than 85%. The induced turbulence in this micro-structured geometry leads to an enhanced heat and mass transfer which was observed in preliminary experiments at our laboratory. The study compared three metal foams with different pore sizes and showed an influence of the dimension and the shape of the ligaments. However, no measurements inside the foam have been possible to further investigate the influence of the pore geometry. To exploit such mini reaction systems using porous structures a thorough investigation of the transport mechanisms, i.e. their mixing and heat transfer characteristics with respect to their describing parameters (i.e.~pore size, porosity, ligament shape, etc.) is needed. The amount of turbulence or flow separation which is induced by these porous materials is an additional and open question. Therefore we propose to investigate the momentum and scalar transport by optical measurements inside milli-scale systems with different inner structures. To manufacture these geometries we apply Stereolithography respectively Selective Laser Sintering which allows to design highly reproducible milli-reactors with fully integrated static mixing elements. In a past study (SNF project 109560) we applied simultaneous PIV and LIF measurements to investigate scalar transport in complex flow situations. The obtained results revealed the suitability of these non--intrusive methods to fully characterize transport mechanisms and to relate them to identified turbulence structures. Thus we propose to apply these measurement techniques to the milli-scale system and to extend it to enable quantitative measurements inside the porous structure. In a first step we will address grid-structured mixing elements and thus the influence of the macroscopic geometry of the mixing elements on the induced turbulence and momentum exchange. We will perform PIV measurements between the different grid-planes and therefore inside the mixing element. In addition we will investigate the concentration field of a tracer dye injected in front of the mixing element by LIF. This is followed by the second step, where we increase the complexity of the mixing elements and focus on porous structures and the influence of their microscopic structure (pore size, pore geometry, ligament shape) on the transport properties. Therefore we propose simultaneous velocity and concentration measurements using PIV and LIF with different foam-like inserts. In a third step we propose to investigate the mixing performance of these inserts in a two-phase flow situation. Hereby we restrict ourselves in the beginning to liquid-liquid systems. Simultaneous velocity and concentration measurements will lead to a characterization of transport processes in porous structures when advancing to two-phase flows. The present project advances the existing knowledge in a defined step and addresses the fundamental turbulent transport phenomena in milli-scale reaction systems employing porous static mixing elements. The findings allow to describe the mechanisms of simultaneous momentum and mass transport in milli-scale systems relevant for industry and its correlation to turbulence production processes and turbulence structures inside the mixing elements.