Stirred Tank Reactors (CSTR)

The Continuous Stirred Tank Reactor (CSTR) is the flow chemistry adaptation of the conventional batch stirred-tank reactor — configured for continuous reagent input and continuous product withdrawal, with active mechanical agitation maintaining well-mixed conditions and a defined mean residence time in the reactor volume. In a CSTR, reagent feed streams enter the vessel at the top or side, are mixed by an impeller or magnetic stirring mechanism, react at steady-state conditions, and the product stream is continuously withdrawn — typically through an overflow weir or bottom discharge valve maintaining constant liquid level and hence constant residence time. The residence time distribution (RTD) of an ideal CSTR follows an exponential decay profile — meaning a fraction of the entering reagent exits immediately while some material remains for extended periods. For reactions where narrow residence time distribution is important (e.g., polymerisations or reactions sensitive to over-processing), multiple CSTRs in series are used, with the RTD of n CSTRs in series approaching plug-flow behaviour as n increases. In flow chemistry practice, 2–5 CSTRs in series are commonly used to achieve the combination of back-mixing (beneficial for highly exothermic reactions where temperature spikes must be damped) and plug-flow character.

For laboratory and kilo-lab scale, CSTR units are available in glass-jacketed formats (10 mL to 5 L) with precision overhead stirring, PT-100 temperature sensing, and level control. At pilot and production scale, CSTR units in SS 316L or Hastelloy C-276 with external jacket heating/cooling, multiple reagent inlet nozzles, internal baffles, and control valve-based level and feed management are used. Magnetic coupling agitators are available for completely sealed designs required for highly toxic or air-sensitive chemistries. Slurry handling — critical for reactions producing solid products (e.g., salt formation, crystallisation) — is addressed through bottom-discharge valve designs, slurry pumps, and smooth internal geometry to prevent fouling.