Continuous Multitubular Millireactor with a Cu Thin Film for Microwave-Assisted Fine-Chemical Synthesis

Abstract

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 multitubular millireactor/heat exchanger (MTMR) assembly has been developed with the reactant flow through millitubular 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 multicomponent 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 333 ± 11 kgprod/(kgcat·h) 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 (six 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 (six 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.