Computergestützte Analyse des reaktiven Strömungsmischens in T-Mikroreaktoren

Due to the large area-to-volume ratio micro-reactors exhibit an effective mass and heat transfer. They also show immediately responses to changes in operating conditions. Therefore micro-reactors are suitable for the analysis of fast exothermic chemical reactions. The multiscale analysis of laminar mixed reaction systems requires the full resolution of all relevant time scales and length scales. In a T-shaped micro-reactor the largest length scales are given by the dimensions of the reactor itself whereby the smallest lengths scales are equal to the interatomic distances. The related time scales are the residence time and the reaction time. Using the finite volume method the full resolution of all length scales is obtained by a combination of simulations on a 3-D and a 2-D grid: the transport equations for mass, momentum and species concentration are solved on a 3D grid as long as the flow of the aqueous solvent shows no distinct direction but areas, where backflow occurs. Once no further backflow emerges, the reorientation of the flow towards the outlet of the straight micro channel allows the handling of the axial flow direction as a pseudo-time variable, so that the evolution of the concentration profile can be computed on successive 2-dimensional cross sections, thereby following the main axial flow direction. The conversion of the axial flow direction into a pseudo-time leads to the parabolised version of the species equation. This equation is derived from the stationary species equation under the neglect of the diffusion in the mean flow direction. The model is complemented by homogeneous Neumann boundary conditions concerning the species distribution. Investigating the species distribution of the fluorescent complex [CaFluo4] which is formed in a reaction of second order between the Ca(II) and the Fluo4 ions the described numerical approach is validated by experimental data. Beside the quasi-irreversible formation of [CaFluo4] further reaction systems of second order are investigated. Thereby numerical simulations of very fast i. e. quasi-instantaneous chemical reactions lead to the problem of stiff differential equation systems. To avoid such numerically unmanageable equation systems an approach described by Toor is developed further, whereby the impossibility of a local coexistence of both educts is exploited. The developed methods to handle fast and quasi-instantaneous reaction systems facilitate the detailed analysis of the influence of an increased flow rate on the yield and selectivity of reactions of second order. The numerical simulations reveal the enhancement of the yield and selectivity due to an increased but still stationary and laminar flow rate. However, the analysis of the conversion also shows that it is not possible to ameliorate continuously the conversion by the acceleration of the flow. Due to the accompanying reduction of the residence time a maximum conversion can not be exceeded without leaving the range of a stationary laminar flow. An increase of the conversion and the selectivity is also achievable by the miniaturisation of the micro-reactor itself under constant flow conditions. Downsizing the dimensions of the micro-reactor by a factor of b << 1 a reaction of second order is accelerated relatively to the transport processes due to the different scaling behaviour of transport processes and chemical reactions. An increase of the space-time-yield by a factor b multiplied by b is found. Varying the rate constant of a reaction of second order it is revealed that even the changeover to a micro-reactor and the acceleration of the flow rate lead to a limitation of the conversion. In case of a reaction system with side reactions the noticed residence time dependent decrease of selectivity can be lead back to an inhomogenous mixing which causes local excess of those species, which react in the undesirable side reactions. In summary, the full resolution of all relevant time scales and length scales as well as the variation of boundary conditions and species characteristics allows a detailed analysis of the interactions of transport processes and fast chemical reactions. The influences on different yield and selectivity of a reaction system can be revealed. This enables an explicit comprehension of the reactive mixing and opens the access to further optimizations.

Grantor dc:publisher

Publikationsserver der RWTH Aachen University

Year dc:date

2010

Author dc:creator

• Lojewski, Alexander

Contributors dc:contributor

• Bothe, Dieter

• info:eu-repo/classification/ddc/500
• Mikroreaktor
• Numerische Strömungssimulation
• Chemische Reaktion
• Bimolekulare Reaktion
• Mischen
• Konvektion
• Diffusion
• Transportprozess
• Inkompressibles Fluid
• Newtonsche Flüssigkeit
• Finite-Volumen-Methode
• Naturwissenschaften
• Flüssig-flüssig-Strömungsmischen
• parabolisierte Speziesgleichung
• reactive mixing
• parabolised species equation

Statement dc:rights

• info:eu-repo/semantics/openAccess

Language dc:language

ger

Identifier

https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-124558%22

OAI identifier oai:identifier

oai:publications.rwth-aachen.de:63100