Behaviour of Nafion® 350 membrane in sodium sulfate electrochemical splitting:: continuous process modelling and pilot scale tests

A two-compartment membrane electrolysis cell is used to split sodium sulfate into sulfuric acid and sodium hydroxide. The cell is equipped with a Nafion(R) 350 cation-exchange membrane. Due to the dissociation of the strong acid, free hydrogen ions migrate through the membrane together with sodium ions. This transfer decreases current efficiency. The transport properties of Nafion(R) 350 membrane are studied in a laboratory cell. Current efficiency varies either with sulfuric acid to total sulfate concentration ratio in the anolyte or with sodium hydroxide concentration depending on the membrane state. Water transport through the membrane is due to electroosmosis. Hydrogen and sodium ions carry three to four molecules of water per ion. Modelling of a continuous feed and bleed process in the steady-state is performed using material balance and transport data obtained in a laboratory scale. Tests in a pilot plant (scaling factor 13) were undertaken. The model predictions agree well with experimental results. As a consequence, the model may be used for industrial purposes. Due to current efficiency decrease when salt conversion increases, the use of a cascade of cells in series is advantageous compared to a single stage.