Energy consumption in desalinating produced water from shale oil and gas extraction

On-site treatment and reuse is an increasingly preferred option for produced water management in unconventional oil and gas extraction. This paper analyzes and compares the energetics of several desalination technologies at the high salinities and diverse compositions commonly encountered in produced water from shale formations to guide technology selection and to inform further system development. Produced water properties are modeled using Pitzer's equations, and emphasis is placed on how these properties drive differences in system thermodynamics at salinities significantly above the oceanic range. Models of mechanical vapor compression, multi-effect distillation, forward osmosis, humidification–dehumidification, membrane distillation, and a hypothetical high pressure reverse osmosis system show that for a fixed brine salinity, evaporative system energetics tend to be less sensitive to changes in feed salinity. Consequently, second law efficiencies of evaporative systems tend to be higher when treating typical produced waters to near-saturation than when treating seawater. In addition, if realized for high-salinity produced waters, reverse osmosis has the potential to achieve very high efficiencies. The results suggest a different energetic paradigm in comparing membrane and evaporative systems for high salinity wastewater treatment than has been commonly accepted for lower salinity water. © 2014 Elsevier B.V. All rights reserved.

The effects of increased salinity on desalination system performance have been investigated in the context of produced water. Based on produced water composition, we first conclude that: • Whereas the rule of thumb for the least work required to desalinate seawater at 50% recovery is about 1 kWh/m 3, the minimum work required to desalinate produced water depends on the salinity but can be up to 9 kWh/m3 —nearly an order of magnitude higher. Efficiency models were developed for MVC, MED, FO, HDH, PGMD, and a hypothetical RO system. Considering a fixed brine salinity of 26%, we can draw the following conclusions about the performance of each system: • For single effect MVC, the energy consumption ranges from about 23–42 kWh/m3, depending mainly on the system size and compressor efficiency. The primary effect of salinity on energy consumption is the increase in BPE within the evaporator condenser, requiring a higher • • • • • discharge pressure in the compressor. A two-effect MVC system can require as little as 20 kWh/m3 to treat produced water at mediumlarge scale. For MED, the GOR ranges from about 3 to 7, depending on the number of effects and the feed salinity. At higher feed salinities (lower recovery), MED behaves more like multistage flash, where large BPEs in the feed heaters penalize heat recovery and diminish GOR. For FO with thermal regeneration, an exergetic input of 25–150 kWh/ m3 is required, depending on the size of the FO unit and the efficiency of the draw regeneration process. Most of the exergy destruction occurs in the thermal draw regeneration step, so future efforts should focus on improving regeneration efficiency. For zero-extraction HDH, the GOR ranges from 1 to 3, depending on the system size and brine salinity. With brine recirculation, the GOR of HDH is generally independent of feed salinity. Alternate configurations with extraction and injection may improve performance. For PGMD, the GOR ranges from about 1 to 2, depending on system size and salinity. Like HDH, because it is inherently low recovery in a single pass, brine recirculation is required to obtain high RR. Alternate configurations, like multi-stage MD, can be used to effect better heat recovery and improve GOR. For a hypothetical two-stage RO system, energy inputs range from about 4–16 kWh/m3, depending on the salinity and system size. Because the feed must be pressurized above the osmotic pressure of the brine, the system tends to perform best at moderate recovery ratios, where the imbalance in the RO unit is not too high and the amount of product per unit feed is not too low.