Inline Sensors

Inline sensors are the process analytical technology (PAT) backbone of a flow chemistry platform — providing continuous, real-time measurement of critical process parameters directly in the flowing stream without requiring sample withdrawal, off-line analysis, or operator intervention. For continuous manufacturing to deliver consistent product quality, the reaction conditions must be monitored and controlled in real time: temperature measurement confirms the reaction zone is at the designed operating point; pressure measurement confirms system integrity and detects blockages or pump failures; and pH measurement confirms reagent stoichiometry, reaction completion, and workup efficiency in aqueous chemistry streams. Together, these three primary inline sensors constitute the minimum process monitoring suite for a safe, controlled continuous flow synthesis. Temperature measurement in flow chemistry uses miniaturised resistance temperature detectors (RTDs, typically PT-100 or PT-1000) or thermocouple probes (type K or type J) integrated directly into the flow channel at the reactor inlet, within the reaction zone, and at the reactor outlet. For microreactors and small-bore tubular reactors, the sensor probe diameter must not exceed approximately 1/3 of the channel diameter to avoid significant flow disturbance. RTDs offer higher accuracy (±0.1°C for PT-1000) and better long-term stability than thermocouples, at the cost of slightly slower response. Infrared thermometry through transparent tube walls (using fibre-optic IR sensors) allows non-contact temperature measurement without any wetted sensor components — useful for highly corrosive streams.

Pressure measurement uses piezoelectric or thin-film strain gauge pressure transmitters, installed at reactor inlet, outlet, and at defined intermediate points. Differential pressure monitoring across packed bed reactors detects catalyst bed compaction or fouling before it causes pump overpressure. For safety monitoring, high-pressure switches linked to the emergency shutdown (ESD) logic provide automatic system depressurisation if the operating pressure limit is exceeded. Inline pH measurement employs miniaturised combination pH electrodes (glass or ISFET) installed in flow-through cells with controlled flow velocity across the electrode surface to ensure representative measurement. ISFET (Ion-Sensitive Field-Effect Transistor) sensors are preferred for continuous flow applications as they have no fragile glass membrane, faster response, and longer calibration stability. For highly acidic (pH < 0) or highly alkaline (pH > 14) streams, and for streams incompatible with glass electrodes (HF, high-temperature organics), inline UV-Vis or Raman spectroscopy probes provide an alternative approach to reaction monitoring and endpoint detection.