Scaling study of miniaturised continuous stirred tank reactors via residence time distribution analysis and application in the production of iron oxide nanoparticles

Magnetically agitated miniaturised continuous stirred tank reactors (mCSTRs) are an attractive platform for the intensification of chemical reactions involving solids by combining active stirring and intensified heat and mass transfer due to their small dimensions. This work investigated the operation of mCSTRs at flowrates up to 60 ml/min (space time of 3 s per tank) as a means of increasing the throughput of fast reactions. Investigation of the residence time distribution under varying operational (flowrate, stirrer rotational speed) and reactor geometrical (stirred volume, stir bar size) parameters, showed deviation from the ideal CSTR behaviour at increasing flowrates, which could be mitigated by keeping the stir bar length close to the tank diameter, increasing stirrer rotational speed, and using larger tank sizes. Assembling mCSTRs into cascades did not amplify non-ideal behaviour and allowed narrowing the residence time distribution at high throughput. Various configurations of mCSTR cascades were evaluated for the synthesis of iron oxide nanoparticles (IONPs) via iron chloride co-precipitation with NaOH, demonstrating the importance of residence time distribution (RTD) control when increasing the throughput of nanoparticle production. Using a 5 × 3 ml mCSTR cascade for the core formation followed by a 5 × 3 ml mCSTR cascade for deagglomeration/stabilisation, the IONP flow synthesis was scaled successfully, producing high quality nanoparticles (7.3 $\pm$ 2 nm) at 60.5 ml/min (l/h scale).