Decarbonisation Technology August 2025 Issue

101

100

Process stage

Figure 2 Normal (left) and vacuum (right) pressure profile across AEL process stages

• Start-up and shutdown transients: Pressure dips may occur before gas production stabilises or after it stops while flow continues. • Membrane dynamics: Pressure imbalance across membranes can draw gases through, causing localised vacuum and risk of cross- contamination. • Flow path resistance or blockage: Blockages or high resistance downstream can create suction upstream, reducing pressure unintentionally. • Cooling and condensation effects: Rapid gas cooling causes vapour condensation and volume shrinkage, lowering pressure if not compensated. This matters because most sensors are calibrated for near-atmospheric pressure. A vacuum can skew thermal conductivity readings, reduce accuracy, and allow ambient air ingress, compromising gas purity. Flow velocity characteristics Gas velocities in AEL systems range from 3-10 m/s in product lines to 15 m/s in off-gas lines (see Figure 3 ). These variations stem from several factors: • Changing gas production rates: Higher electrolysis rates increase H₂/O₂ volumes and flow speeds. • Pipe diameter and flow path design: Narrow pipes, valves, and bends raise velocity, whereas wider or vented lines slow it down. • System stage and equipment: Gas flow accelerates at the electrodes, slows after the separator, and stabilises after drying.

cooler and drier components lead to lower RH (see Figure 1 ). At full operation, heat from reactions and water vapour push RH toward saturation. Moisture carryover from the separator or incomplete drying can further raise RH downstream. Pressure profile AEL systems typically operate between 0.8-40 bara. Minor pressure drops occur after the separator and dryer. Stable pressure is essential for accurate thermal conductivity measurements. Pressure rises gradually during start-up as gas production increases (see Figure 2 ). Fluctuations may result from flow restrictions, separator back pressure, compression demands, or mismatched H₂ production and withdrawal rates, potentially causing transient drops or vacuum if extraction exceeds generation. Vacuum and pressure drop conditions Transient vacuum conditions can occur due to mismatches in gas production and extraction rates (see Figure 2). These must be monitored to avoid membrane stress and gas cross- contamination. In AEL systems, a vacuum (low absolute pressure) can occur temporarily, which may affect membrane integrity and measurement accuracy. Vacuums can arise for several reasons: • Differences between gas production and gas extraction rates: If H₂ or O₂ is withdrawn faster than they are generated, pressure can drop below ambient, forming a partial vacuum.