Bridging the gap: A nested-pipe reactor for slow reactions in continuous flow chemical synthesis

A multi-purpose production facility for continuous flow operation of medium-fast to slow chemical reactions was realised. The reactor concept followed the typical three-stage sequence for numerous organic reactions: non-reactive mixing at low temperature, main conversion with selectivity control at medium temperature, and completion of the reaction at high temperature. For the first two stages, a static mixer for immediate mixing and a micro-structured heat exchanger for efficient heat transfer were utilised, respectively. A lack of equipment availability was experienced for the third stage, requiring reaction times in the hours range at well-defined flow conditions. In order to bridge this gap between (micro-)structured low-volume devices and traditional macro-scale equipment for long reaction times, a novel nested-pipe reactor design based on annular slit geometries was realised. These reactor modules provide reaction volumes between 0.25 and 8.0 L, depending on the width of the slit and the number of reactor modules connected in series. The entire unit was constructed as a compact, mobile, self-consistent device with a high degree of automation. As a representative process example, the solvent-free synthesis of the ionic liquid [EMIM]EtSO4 from N-methylimidazole and diethylsulfate was selected, since this reaction is characterised by high initial rates of reaction and heat release, and requires long reaction times in order to achieve high conversion.A characterisation of the residence time behaviour of the annular slit reactors was performed by means of fibre optical mid-infrared spectroscopy. Tracer experiments in step-change mode showed a well predictable response behaviour with a pronounced laminar contribution in the studied flow parameter range.