Two-staged multi-effect distillation for energy efficient brine concentration

Multi-effect distillation (with or without a membrane, MEMD/MED) can achieve a high product recovery in a single pass of feed and is therefore the preferred technology for brine concentration towards zero or minimal liquid discharge. The specific thermal energy consumption of MED/MEMD systems decreases with increasing number of effects (N) since condensing steam is used internally to drive further evaporation multiple times. However, increasing N is limited by the boiling point elevation of the saline brine. This study proposes multi-staging of a parallel-feed MEMD system, with initial treatment in a first stage with high N followed by further brine concentration in a second stage with low N, to save approximately 35 % energy and 17 % on the specific product water cost compared to conventional single-stage operation. A flexible design of industrial forward-feed MED is also proposed such that it can be operated in either 4-effect or 8-effect mode by controlling valves. Such a design enables reducing the energy consumption by up to 40 % when the effluent treatment load is low, through temporally multi-staged operation, as opposed to a conventional 4-effect system which operate for fewer hours when the treatment load reduces. Multi-staging of MED offers an additional degree of freedom to improve the energy efficiency of the energy-intensive brine concentration processes.