Determination of the Dispersion Characteristics of Miniaturized Coiled Reactors with Fiber-Optic Fourier Transform Mid-infrared Spectroscopy

A miniaturized modularized tubular reactor setup designed for flexible small-scale experimentation under continuous flow conditions was investigated for its residence time behavior. Studies focused on the question of whether the coil shape of the tubular reactor segments beneficially affects the dispersion behavior of the system by inducing secondary flows in the dispersed plug flow pattern. In the studies, fiber-coupled sensors for the attenuated total reflectance technique were applied to measure concentration profiles of tracer substances by Fourier transform mid-infrared spectroscopy. Sensor flow cells with specifically structured flow geometries were designed in order to reduce the impact on the flow characteristics, and in this context they have proven to be superior to nonstructured flow cells. Quantitative evaluation of the measured spectra yielded good-quality time profiles of the tracer concentration that were subjected to a mathematical approximation with the well-established dispersion model by Taylor and Aris. In accordance with model predictions from literature, the axial dispersion coefficient is drastically reduced in coiled capillaries as compared to the coefficient in the Taylor−Aris case for straight tubes. Depending on the influence of the tube curvature and the flow properties, expressed by the product of the square Dean number and the Schmidt number, the reduction of the axial dispersion coefficient can be by up to a factor of 4. For chemical processes with critical requirements regarding selectivity, this can be of great advantage since the detrimental impact of axial back-mixing is effectively reduced.