A new numerical method for axial dispersion characterization in microreactors

Abstract

new numerical method for axial dispersion characterization in microreactors.(2017) Chemical Engineering Science, vol. 168. pp. 178-188. ISSN 0009-2509 Open Archive TOULOUSE Archive Ouverte (OATAO) OATAO is an open access repository that collects the work of Toulouse researchers and makes it freely available over the web where possible. h i g h l i g h t sAn innovative numerical method for axial dispersion characterization is proposed. The method is validated thanks to a RTD experiment in a microchannel. The method is applied to rectangular millimetric wavy channels. The impact of the channel geometry on the axial dispersion coefficient is studied. Keywords:Axial dispersion Numerical method CFD simulations Microreactor Wavy channel a s t r a c t Axial dispersion is a key phenomenon in reactor engineering that can affect yield and selectivity when reactions are carried out. Therefore its characterization is necessary for an adequate modelling of the reactor. The development of compact reactors to fit with process intensification expectations requires the use of characterization methods adapted to small-scale devices. An original method not-frequently used up to now for the estimation of axial dispersion coefficients is presented and applied to millimetric wavy channels. It is based on CFD simulations to calculate velocity and concentration fields from which axial dispersion coefficient can be estimated. This method is used to predict the impact of the wavy channel geometry and of the fluid velocity on axial dispersion in laminar flow regime. The investigated geometrical parameters are the hydraulic diameter (2-4 mm), the cross-sectional aspect ratio defined as the ratio between the channel width and its depth (0.25-1) and the internal curvature radius of the bends (2-3.4 mm). The range of Reynolds number considered is Re = 70-1 600. Axial dispersion coefficient increases with velocity, values range from 2.8 Á 10 À4 to 3.2 Á 10 À3 m 2 Ás À1. It appears that axial dispersion varies slightly in function of the channel hydraulic diameter. Square wavy channels generate less axial dispersion than rectangular wavy ones. Finally, axial dispersion coefficient increases with the internal curvature radius which shows the positive impact of sharp bends to reduce axial dispersion effect.

Mixing Performance in a Distributed-Feed Plate-Type Reactor with Multinozzle Injection for Fine Chemical Production Scale

“…It can thus be concluded that bend-induced vortices in combination with modulated flow velocities along the flow path are well able to generate a very small axial dispersion with the unit cell smallest hydraulic diameters between 5.5 and 9.3 mm for the given flow rate scale. The obtained axial dispersion coefficients could with the stated simplifications consistently be described by the correlation obtained by Moreau et al 65 which may open the way to estimate axial dispersion for a specific reactor configuration employed in a given process and may help to design additional plates. Proper numerical investigations could help to further verify the assumptions made, for example, for the curvature radii.…”

“…The results are compared in Figure 5b with experimentally observed D ax . The correlation derived by Moreau et al 65 is seemingly able to describe the experimental results obtained in this study despite the somewhat larger Re numbers based on the smallest cross section of the respective unit cell employed here. It is noted that considering center streamlines in the images of the unit cells, the above employed curvature radii were quite plausible but may be associated with uncertainties in the order of 30%.…”

“…It was shown by Moreau et al 65 that the axial dispersion coefficient for microreactors with sinusoidal channels varies only slightly with d h for a constant flow velocity, but changes with aspect ratio. The authors found that the intensity of axial dispersion can be correlated to Re by the Dean number…”

Application of transfer function for quick estimation of gas flow parameters—A useful model‐based approach to enhancing measurements

“…Moreau et al 19 presented an original experimental method not‐frequently used up to now to estimate axial dispersion coefficients for a liquid flow. The method was validated thanks to an experiment in a millimetric channel.…”

“…The presented method is similar to the method reported by Moreau et al 19 However, in contrast to the example discussed there, we consider gas flow for two practical importance cases: empty and packed columns. In both cases, the method is helpful and predicts the coefficient values with good precision.…”