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Residence Time Distribution in a Packed Bed Microreactor

Franz Trachsel and Philipp Rudolf von Rohr. Institute of Process Engineering, ETH Zurich, Sonneggstrasse 3, Zurich, 8092, Switzerland

Microreactors allow precise process control resulting in increased constant product quality due to their small characteristic length scales. Heat and mass transfer is enhanced and the reduced reactor size is favorable to high pressure and hazardous reactions. To compare the performance of microreactors with their macroscale counterparts in heterogeneously catalyzed reaction the use of the same type of catalyst in both cases is essential. A packed bed microreactor allows the use of commercially available catalyst, the reduction of hot spots during reaction and a reduction in diffusion times on the catalyst surface.

The characterization of residence time distribution in microreactors is of great importance for describing the entire reaction process in continuously driven reactors. This work describes the residence time distribution in liquid/liquid and gas/liquid flow in transparent microreactors.

Single channel microreactors with hydraulic diameters of 370 – 520 μm are made of polydimethylsiloxane (PDMS) and silicon/glass using standard soft lithography and photolithographic, dry etching and anodic bonding techniques respectively (Fig. 1). The packed bed consists of a 30 mm long channel filled with catalyst particles with mean diameters of 100 - 120 μm.

Fluorescence microscopy is used to observe the broadening of a sample pulse. Rhodamine B tracer is injected with a HPLC valve and an “on-chip” method [1] into the reactor and recorded with a CCD camera. The actual residence time distribution of the microreactor is calculated by the deconvolution of the temporal concentration distributions at input and output locations using fast fourier transformations (FFT). The porosity of the packed bed is calculated from pressure drop measurements and visualized with laser scanning microscopy (LSM).

Results of residence time distribution show that the measured residence time distribution in a packed bed microreactor is only restricted comparable to conventional macroscopic packed bed reactors. This is due to a relatively large boundary layer with high porosity at the channel walls compared to the overall reaction volume. A decreased dispersion hence a narrow residence time distribution is realized by increasing the gas flow rate. Such segmented gas/liquid flow has a positive impact on the product quality and the catalyst activation during the reaction process.

Fig. 1. Photograph of a silicon/glass packed bed microreactor. A static micro mixer provides a steady segmented gas liquid flow through the reaction channel. The catalyst inlet serves to insert and remove catalyst. This inlet is closed during experiments. The packed bed has a length of about 30 mm which correspond to a catalyst weight of 2-3 mg. Micromachined filter elements at the common outlet prevent catalyst particles from being removed.

[1] Trachsel, F., Günther, A., Khan, S.A., Jensen, K.F. 2005. Measurement of Residence Time Distribution in Microfluidic Systems. Chemical Engineering Science 60 (21), 5729-5737.