Beyond Stack Metrics: Process-Economic Design of Sodium Sulfate Electro–Electrodialysis

Abstract

Na2SO4 is generated in large quantities as a low-value industrial byproduct, and under zero-liquid-discharge constraints, its recovery by crystallization can be costly. Electro-electrodialysis (EED) can convert concentrated Na2SO4 into valuable NaOH and H2SO4, but which reactor design is economically most favorable, and at which stack conversion it should be operated, remain unclear. Here, three EED architectures are compared in gap and zero-gap configurations using an experimentally parametrized electrochemical model coupled with process-level techno-economic analysis. Across reactor architectures, plant scales, and electricity and steam price scenarios, net cost is minimized at an intermediate stack conversion of about 60–70%, reflecting a trade-off between stack-side penalties at high conversion and downstream separation demand at low conversion. CEM–CEM architectures outperform the alternatives across a broad range of economic conditions. The results, therefore, identify both the economically relevant conversion window for stack assessment and the reactor architectures most favorable for circular Na2SO4 valorization.