Non-intrusive method for measuring residence time distribution in microreactors
Special Symposium - EPIC-1: European Process Intensification Conference - 1
EPIC-1: Poster Session (EPIC - Poster) - P1
Keywords: RTD, microreactors, optical injection, non-intrusive
S. Lohse*,**,(1st author), P.S. Dittrich**, D. Janasek**, J. Franzke** and D.W. Agar*
* University of Dortmund, Dept. of Biochemical and Chemical Engineering, 44221 Dortmund, Germany
** ISAS – Institute for Analytical Sciences, 44139 Dortmund, Germany
Miniaturisation is a powerful technique for enhancing the yield and selectivity of chemical syntheses [1]. In addition to the advantages of increased heat and mass transfer, miniaturised systems offer the realisation of chemical processes with a virtually identical processing history for each converted mole-cule. A crucial requirement enabling one to exploit the full potential of process intensification using miniaturisation is the availability of accurate methods for the characterisation of microreactors. The main objective of this work is to develop and evaluate an appropriate experimental method for the purposes of microreactor design and performance assessment. The residence time distribution of chemical reactors is a crucial ‘parameter’ for describing their true behaviour.
Conventionally, injective methods are widely used for the determination of residence time distribution (RTD) in chemical reactors: an easily detectable tracer is introduced into a steady-state flow in the reactor inlet and the distortion of the signal determined in the reactor outlet. The residence time distri-bution is then calculated by deconvolution of the inlet and outlet signals. Previous research [2, 3] has shown that the main challenge in measuring microreactor RTD is to obtain a well-defined signal at the reactor inlet, which is independent on the actual flow rate, without disturbing the flow.
To avoid the shortcomings of conventional methods in microreactors a novel technique for RTD de-termination has been investigated. The underlying idea is the continuous introduction of a photo-activated fluorescent dye dissolved in the aqueous solution used in the experiment. This procedure has already been used for visualisation of velocity fields in microchannels [4]. By exposing a defined sec-tion of the inlet channel to a pulse of UV-light, a certain amount of tracer is activated or ‘de-caged’ and thus becomes fluorescent. This kind of optical ‘injection’ is non-intrusive and disturbs the flow in no way. Due to the possibility of locating the ‘injection’ site directly at any position within a transpar-ent device, artefacts from peripheral equipment can be eliminated. The method generates almost ideal input signals, which simplifies the numerical treatment of experimental data necessary. A comparison demonstrating the superiority of this new approach over various traditional injection methods has been presented recently [5].
To obtain the RTD, the activated fluorophores are detected at a given cross-section in the outlet of the structure being studied by using an EMCCD-Camera mounted on an inverse research microscope (Olympus). For complex structures, this kind of detection corresponds to a closed/closed system, i.e. the measured data yield the RTD itself directly. For simple straight or meandering channels (open/open boundaries) the velocity profile of the fluid in the detection sections has to be considered. This is feasible by virtue of the spatially resolved measurement provided by the EMCCD. Initial ex-periments to show the feasibility of this procedure for determining the RTD of complex microreactor structures has been carried out. With the experimental data obtained using this technique one can re-liably validate mathematical models describing mass transfer behaviour in (micro)structures with ex-tremely unfavourable aspect ratios.
Acknowledgements: to DFG for financial support of the project AG26/9-1
[1] V. Hessel, S. Hardt and H. Löwe, Chemical Micro Process Engineering, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 2004
[2] M. Günther, S. Schneider, J. Wagner, R. Gorges et al., Chem. Eng. J., 101, 373-378 (2004)
[3] F. Trachsel, A. Gunther, S. Khan and K. F. Jensen, Chem. Eng. Sci., 60, 5729 – 5737 (2005)
[4] P. H. Paul, M. G. Garguilo and D. J. Rakestraw, Anal. Chem., 70, 2459-2467 (1998)
[5] S. Lohse, I. Gerlach, D. Janasek, P.S. Dittrich and D.W. Agar, IMRET 9 (2006)
Presented Wednesday 19, 13:30 to 14:40, in session EPIC-1 Poster Session (EPIC - Poster) - P1.
