Mass transfer modeling of gas–liquid separators during standby operation of alkaline water electrolyzers

Alkaline water electrolyzers integrated with renewable energy sources must frequently shift between standby and shutdown due to power intermittency. These transitions compromise hydrogen purity and system safety through gas crossover between the hydrogen and oxygen streams. To mitigate explosive hydrogen-oxygen mixture formation during prolonged standby, nitrogen purging of gas–liquid separators is commonly applied, although this practice can result in significant hydrogen losses in large-scale systems. Despite its importance, gas accumulation dynamics in gas–liquid separators during standby have not been systematically modeled to evaluate the necessity of nitrogen purging. This study develops a transient mass balance model to describe gas accumulation dynamics in gas–liquid separators during standby operation of alkaline water electrolyzers. The model is used to analyze hydrogen accumulation trends in the oxygen separator as a function of pressure (1 bar–50 bar), temperature (20°C–200°C), and KOH concentration (20 wt%–40 wt%). A dimensionless mixing constant is introduced to reduce the number of effective parameters by combining key variables into a single scalable term, simplifying the analysis and enabling generalized interpretation across different system sizes and flow conditions. The results show that higher pressures and potassium hydroxide concentrations slow impurity accumulation, while temperatures in the range 30°C–70°C are associated with more favorable accumulation behavior. Laboratory-scale experiments performed under controlled standby condition (1 bar, 60°C, 32.7 wt% KOH) at three flow rates validate model trends: the measured half–LEL times exceed the equilibrium-model predictions, confirming that the model yields conservative safety estimates. These findings provide mechanistic insight into gas–liquid mass transfer behavior during standby and identify operating regimes associated with slower impurity accumulation.