NH4HCO3–water solutions regeneration in RED closed loop applications

Reverse Electrodialysis (RED) in closed loop arrangement (Reverse Electrodialysis Heat Engine - REDHE) is a promising technology to convert low-grade waste heat into electricity. RED is a membrane process exploiting the salinity gradient between a concentrated and a diluted solution to generate electrical current. Due to the transfer phenomena occurring in the RED unit, the two exiting solutions are partially mixed. Thermal regeneration processes can be used to restore the initial conditions of the two solutions, thus closing the loop. In this regard, ammonium hydrogen carbonate (NH4HCO3) salt solutions are suitable for such applications, being able to decompose at temperatures above 40-45 °C into a gaseous phase containing NH3 and CO2. Low temperature waste heat (T<80°C) can be used in a suitable thermal regeneration unit to recover these gases contained in the dilute solution. The decomposed gases, once stripped, can be finally reabsorbed in the concentrate one, thereby restoring the initial concentrations. The present work investigates two kinds of regeneration units fed by NH4HCO3-water solutions. More precisely (i) an air stripping column and (ii) a distillation column were designed, built and analysed both via experiments and process modelling. The effect of several operative conditions on the removal efficiency of the two systems was investigated. In particular, for the case of the stripping column the influence of flowrate (air and inlet solution) and regeneration temperature was analysed, while, for the case of the distillation column, different pressures and reboiler duties were tested. Effect of feed solution concentrations was studied in both cases. Finally, two models were developed on Aspen Plus® software, one for each kind of regeneration unit. Model results were compared with the experimental ones and a good agreement was found, thus validating the two models proposed.