Coupling Electrodialysis with bipolar membranes with renewable energies through advanced control strategies

The search for innovative and highly sustainable processes, which lead to the ecological transition, represents one of the most important recent guidelines of the European Union. Powering water treatment technologies with renewable energies, using the process buffering capacity as a way to indirectly store energy has been recently proposed as an effective strategy for smart energy use. With this respect, electrodialysis with bipolar membranes (EDBM) can be particularly suitable due to the high energy intensity, coupled with an extreme flexibility of the process. EDBM is an electromembrane process able to convert a concentrated brine into products with high added value such as acids and bases, simply applying an electric power. The objective of this work has been focused on the development of advanced control systems allowing EDBM to operate under transitory regimes following the highly variable trend of renewable energy availability, adapting the operating conditions and the process targets according to the variable power input offered during a working day. To do this, an EDBM stack at pilot scale (provided by FuMA-Tech GmbH) was operated and tested in Lampedusa in continuous mode (feed and bleed) under two different periods (summer and winter) at fixed target (0.5 of NaOH an HCl). In the hours of the day with available power higher than a minimum threshold (2-6.5 kW), the controller was able to keep the process target, fixed at 0.5M of sodium hydroxide and hydrochloric acid, varying the outlet flowrate of the product from 0.5 up to 3 l min-1. In these operating conditions, the controller was able to guarantee high product quality while still retaining high specific productivity (in the range of 0.1-0.2 kg h-1m-2) and low consumption (in the range of 1.3- 1.9 kWh kg-1 of NaOH and 1.8- 2.4 kWh kg-1 of HCl). In all cases, the parallel control logic allowed to tune the applied voltage in order to meet the electric power availability, while manipulating the outlet flowrate to meet the desired product specification. The obtained results have demonstrated for this first time at the pilot scale how EDBM is a particularly suitable technology to valorise, in industrial relevant scenarios, available saline streams and unstable energy sources, thus pushing towards the industrial implementation according to sustainable and circular values approaches.