(633b) Small-Scale Multiple Effect Distillation (MED) for Brackish Groundwater Desalination

Multiple effect distillation (MED) is the oldest thermal desalination process and has a typical plant capacity of 600 to 300,000 m³/day. Because MED units do not use membranes, the capital cost, system complexity, and maintenance costs are usually lower than those for reverse osmosis (RO) systems. The purpose of this study was to engineer a small unit that could be operated in the field using renewable energy-sourced heat, namely, heat from the pyrolysis of waste biomass.

A small-scale MED unit was designed and fabricated at NMSU to produce approximately 0.5 m³/day of very pure water (<10 ppm total dissolved solids (TDS)) from brackish groundwater. The unit is being used to study scaling behavior as a function of water chemistry and identify optimum operating conditions for each water. The fabricated MED unit consists of two falling film evaporators with horizontal smooth copper tubes in a parallel feed arrangement; the flow rates are approximately 60 kg/hr. brackish feed water and 20 kg/hr. of distilled water based on brine concentration factor of 1.5. The feedwater is sourced from the Brackish Groundwater National Desalination Research Facility (BGNDRF) in Alamogordo, NM and has a TDS concentration of 1,000-5,000 ppm. The MED operates under partial vacuum (0.2 bar absolute) and temperatures of 60-70°C. The MED performance was improved by adding a heat exchanger, water heater, and pump to bring the feedwater temperature closer to its boiling point prior to be sprayed into the effects. The aluminum parts in the MED effects were anodized to prevent corrosion. Remote thermocouples were used to monitor the temperature at the surface of copper tubes to study the effects of dry spots. The temperatures and pressures selected in this design are meant to be compatible with local brackish groundwater chemistry in terms of salt solubility to limit scaling on the heat exchanger surfaces.