Heat Management in Microreactors for Fast Exothermic Organic SynthesesFirst Design Principles

Microreactors permit continuous operation of various organic syntheses. Due to their excellent heat transfer characteristics, many authors assume isothermicity and ignore the issue of heat management. This assumption can lead to severe misinterpretations of experimental results if a hot spot in a microchannel remains undetected. A simple methodology is presented that permits a first assessment of heat management in microreactors for fast exothermic reactions. Several typical reaction classes in organic synthesis are subdivided into three categories according to their heat production potential (HPP, [kW/L]). Category 1 reactions (HPP > 100 kW/L) are unsuitable for industrial-scale continuous syntheses in single channel arrangements. Category 2 reactions (HPP > 10 kW/L) are critical, and careful selection of channel diameters is necessary. Category 3 reactions (HPP < 10 kW/L) are suitable for continuous synthesis up to the millimeter scale. Very fast reactions with half-lives of less than 1 s are often category 1. To properly remove heat without generating significant hot spots, channel diameters must be smaller than 500 μm. A short-cut approach is proposed for the simple assessment of maximum channel diameters permitting near-isothermal operation.