Passive Mixer cum Reactor using Threaded Inserts: Investigations of flow, mixing and heat transfer characteristics

Significant efforts have been and are being spent on developing intensified tubular reactors for continuous manufacturing of fine and specialty chemicals. In this work, we have proposed a new design of a passive mixer cum reactor for process intensification and development of continuous processes. The mixer/reactor consists of threaded inserts with cone-shaped ends, placed concentrically in the tube such that fluid flows through the annular region between the inserts and the tube. The proposed design is easy to fabricate and maintain, and overcomes the limitations of scale up/scale down compared to most of the commercial passive mixers. The spliting and recombination of flow around inserts, the swirling effect generated by threads, the change in the swirl direction due to a change in the direction of the screw threads, and the pinching effect/expansion at the cone-cone shaped ends realize desired enhancements in mixing and heat transfer. A detailed computational study has been carried out on the mixer cum reactor to characterize flow, mixing, and heat transfer at different operating conditions using a verified and validated CFD model. Various designs and configurations of threaded inserts were considered: 5-channel, 7-channel, 9-channel, smooth surface (no threading), and smooth surface-extended rear end inserts. The flow, mixing, and heat transfer were characterized over the Reynolds number range of 100 to 1600. The structure of the generated swirling flow, effect of pinching/expansion, direction reversal of flow, tracer fraction, temperature, and pathlines were investigated systematically to gain new insights. Threaded inserts could achieve excellent mixing (>99% of mixing intensity) and heat transfer (7 times with smooth inserts and 20 times without inserts). The presented results will provide a sound basis for selecting appropriate threaded inserts for intensifying mixing and heat transfer in tubular reactors. The work also provides a useful starting point for further work on multiphase flows in a tubular reactor with threaded inserts.