Continuous multitubular millireactor with a Cu thin film for microwave-assisted fine-chemical synthesis

The productivity of microwave assisted continuous specialty chemical synthesis has been brought to a commercially interesting scale of 1 kg/day. To that end, a counter-current multi-tubular milli-reactor/heat exchanger (MTMR) assembly has been developed with the reactant flow through milli-tubular reactors while the coolant flows in the shell side. The efficiency of microwave absorption under continuous operation in a single mode microwave cavity has been improved with the deposition of a thin (350 ± 40 nm) Cu film on the inner walls of the reactor tubes. This design ultimately resulted in an enhanced product (1,3-diphenyl-2-propynyl piperidine) yield of a multi-component reaction (of piperidine, benzaldehyde, and phenylacetylene). The Cu film also improved the uniformity of microwave energy absorption in the reactor tubes along the radial direction. A near-isothermal operation was achieved by cooling with a counter-current flow of a microwave transparent coolant in the outer shell of the MTMR. A production rate of 213 ± 11 kgprod/(kgcat•hr) was achieved in a single microwave cavity at 373 ± 5 K and at a total reactant flow rate of 1.66•10-9 m3/s. The average production rate of 1,3-diphenyl-2-propynyl piperidine in the MTMR assembly (6 parallel tubes in a shell and tube reactor/heat exchanger) was 93 % of the production rate in a single tube due to a slightly uneven flow and temperature distribution. Kinetically determined mean Cu film temperature was 477 ± 10 K. Although the reactor tubes were placed at an equal distance from axial symmetry (6 parallel tubes in a hexagonal arrangement) a maximum temperature deviation of 8.0 ± 0.5 K was observed over the reactor tubes. The parallelization approach was demonstrated to be successful for scale up of continuously operated microwave reactors.