Treatment of wet flue gas desulfurization wastewater from fluid catalytic cracking units using bipolar membrane electrodialysis

Wet flue gas desulfurization (WFGD) is widely used for flue-gas control in fluid catalytic cracking (FCC) units, generating large volumes of high-salinity wastewater containing organic constituents. To address its high salinity and the need for valorization, bipolar membrane electrodialysis (BMED) was investigated in this study. First, the diffusion and electromigration experiments were conducted to systematically confirm the charge properties and migration of organics in the wastewater. Second, a three-compartment BMED system was constructed, in which a bipolar membrane (BPM), an anion-exchange membrane (AEM), and a cation-exchange membrane (CEM) were arranged to form acid, feed, and base compartments for converting Na₂SO₄ into H₂SO₄ and NaOH. The effects of key process factors, including current density, initial feed pH, and the volume ratio among the feed, acid, and base compartments, were then evaluated. Under a current density of 50 mA cm −2 , an initial pH of 7, and a volume ratio of 1:1:1, sodium sulfate in FCC–WFGD wastewater was converted into approximately 0.20 mol L −1 sulfuric acid and 0.44 mol L −1 sodium hydroxide within 100 min, while the SO 4 2− concentration in the feed effluent decreased to below 600 mg L −1 . The effluent from the BMED process showed potential for reuse as make-up water in the plant's circulating cooling-water system. Overall, this study demonstrates a potentially feasible pathway for source reduction and resource recovery of FCC–WFGD wastewater, providing practical implications for salt valorization and circular chemical utilization in petroleum refining.