(614e) Techno-Economic Assessment of Membrane Distillation for Shale Gas Produced Water Treatment

Unconventional shale gas is a promising energy resource with major economic benefits but is accompanied by a host of environmental challenges. In particular, the rapidly developing shale gas industry faces a critical challenge of managing vast quantities of high salinity wastewater generated in the process of hydraulic fracturing. Membrane distillation (MD) is an emerging desalination technology for treating high salinity wastewaters including the produced water from shale gas activities. MD has lower operating temperature (30-90⁰C) and pressure compared to conventional desalination technologies. Moreover, MD can treat wastewaters with up to 350,000 mg/Liter total dissolved solids (TDS) with up to 99% salt rejection where most pure thermal processes or pressure driven membrane processes are not applicable. While MD offers several advantages, techno-economic assessment (TEA) is necessary for evaluating the economic feasibility of MD for shale gas produced water treatment. To date, there has been little emphasis on evaluating the performance of MD technology for treating produced water and to best of our knowledge TEA of MD technology for produced water is not available in the existing literature.

We present a detailed TEA for a hypothetical MD plant for treating shale gas produced water based on a combination of experimental results, ASPEN Plus process model, and best available engineering knowledge. The analysis is carried out for a hypothetical 0.5 million gallons per day (MGD) MD plant that concentrates produced water from 10% (100,000 mg/Liter) TDS to 30% salinity. Preliminary results reveal that thermal energy cost for MD operation contributes the most to total cost of treating produced water in a MD plant. We also perform sensitivity analysis to identify input parameters that have the highest impact on total cost of purified water using MD technology. The results of sensitivity analysis reveal that plant capacity and feed TDS level have a significant impact on the cost of treating produced water. Subsequently, we evaluate the economic potential of MD for treating shale gas produced water under the scenario of utilizing waste heat for meeting the thermal energy requirements of the MD process. Specifically, we focus on utilizing available waste heat from existing natural gas (NG) compressor stations (CS) to meet the heat requirements of the MD process. Our prior work has developed quantitative estimates of waste heat from existing NG CS in the U.S. This information is coupled with experimental results on MD performance to propose a systems-level integration of MD with available waste heat at NG CS. Subsequently, we quantify the amount of produced water that can be treated using MD driven by available waste heat using ASPEN Plus simulation and experimentally measured flux rates. The results of this study reveal that utilizing waste heat as the energy source for MD reduces the total water cost significantly. A comparison of MD with existing desalination technologies including reverse osmosis (RO), multi-stage flash (MSF), and multi-effect distillation (MED) will be discussed. We also compare our findings with the business-as-usual scenario where produced water is transported and disposed in class II injection wells to highlight the potential and limitations of the MD technology for produced water treatment.