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

Abstract

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 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.