Evaluation of percrystallization coupled with electrodialysis for zero liquid discharge in the pulping industry

Abstract

We evaluated percrystallization at laboratory scale to determine its suitability as core technology for achieving Zero Liquid Discharge (ZLD) in a Kraft effluent desalination process. Compared with conventional evaporation/crystallization techniques, percrystallization allows to operate at room temperature and with barely pressurized fluids, using relatively unexpensive membranes and vacuum to allow evaporation of aqueous brine solutions. For further comprehension of the technology before experimentation, a computational fluid dynamics model was developed, showing how temperature affects the performance of percrystallization in terms of transmembrane flux. Additionally, we performed experiments with single and double salt solutions (NaCl and NaCl/Na₂SO₄) and concentrated industrial effluent from a Kraft pulp mill (brine from the effluent desalination with electrodialysis). Percrystallization of the concentrated industrial effluent was successfully achieved at laboratory scale, showing no signs of fouling on the membrane surface. However, high energy consumptions (above 3000 kWh/ton of evaporated water) were measured. Theoretical power consumptions of an optimized industrial percrystallization system were therefore computed. Percrystallization showed a more efficient performance compared with similar membrane systems, such as vacuum membrane distillation, but higher energy consumptions than conventional ZLD technologies (mechanical vapor compression), having an estimated energy consumption of around 110-150 kWh/ton of removed water, depending on the feed fluid temperature. Nevertheless, percrystallization could be suitable for ZLD applications where low-cost heating (e.g., solar) is available, since the vacuum energy demand is only 32-140 kWh/ton. Alternatively, it could be applied to low scale processes where the temperature of the solution must remain low (e.g., less than 40 °C).

Keywords: Electrodialysis concentrate; Kraft pulping effluent ZLD; Percrystallization; Vacuum process; ZLD; Zero liquid discharge.