The reagent stream (glowing beads) travels through a narrow coiled
tube reactor wrapped in a cooling jacket. Because the tube's surface
area-to-volume ratio is huge compared to a batch flask, heat generated
by the exothermic reaction is removed almost as fast as it forms —
unless flow rate and setpoint temperature outrun the jacket's cooling
capacity, in which case the hot-spot temperature climbs above the
setpoint and the coil glows red: a thermal runaway.
τ = V_reactor / flow_rate (residence time)
dT_hotspot/dt ∝ (heat generated) − (cooling capacity)
runaway risk ↑ as flow·ΔT_setpoint / cooling → large
- Flow rate — higher flow shortens residence time (less time to react) but raises the rate of heat generation per second.
- Reactor setpoint — the target operating temperature; higher setpoints speed the reaction exponentially (Arrhenius).
- System pressure — flow reactors run above the solvent's boiling point under pressure, keeping it liquid; the gauge shows this headspace pressure.
- Cooling capacity — how much heat the jacket can remove; the classic flow-chemistry safety lever for scale-up, since it scales with tube surface, not volume.
This is why flow chemistry is favoured for scale-up of hazardous,
highly exothermic reactions: a small-diameter coil never accumulates
the large thermal mass that can run away in a big batch reactor.