With advancements in horizontal drilling and hydraulic fracturing methods, unconventional shale gas extraction is rapidly developing. However, shale gas production is accompanied with a host of environmental challenges. One such critical challenge is the sustainable management of large quantities of high salinity wastewater generated in the process of hydraulic fracturing. While recycling may be a short-term strategy, long term solutions are needed for economical and environmentally conscious management of high salinity wastewaters. Desalination is one possible technological solution to treat the produced water and reduce the brine volume for final disposal while simultaneously producing fresh water for other beneficial uses. Membrane distillation (MD) system is a vapor pressure driven emerging membrane technology capable of handling hypersaline wastewater. Since MD can be operated at low temperature (30-900
C), it has the potential to be integrated with low-grade waste heat sources. Another advantage of MD is the low capital investment of the process. However, low energy efficiency of MD results in high operating costs when waste heat is not available. Mechanical vapor recompression (MVR) is another desalination technology capable of treating hypersaline produced water. Unlike MD, MVR system is a well-established work driven desalination process, in which the heat recovery and energy efficiency is relatively high by utilizing the vapor compression cycle. In contrast with MD, MVR system requires considerable amount of high quality energy sources. Considering the competitive desalination efficiency of these two technologies, there is a need for a systematic way to comparison their energetic and economic performance. To date, very limited work exists on techno-economic optimization and evaluation of MD and MVR technologies for produced water treatment.
We present a mixed-integer non-linear programming (MINLP) formulation of a direct contact membrane (DCMD) process design for produced water treatment while minimizing the total treatment cost. The proposed model simultaneously optimizes the operating conditions (i.e. the temperature, pressures, and flowrate of all streams) and DCMD plant configuration (number of membrane stages, number of modules in each stage, and membranes geometry). Also, the optimization model, takes into account the possibility to improve system performance by including brine recirculation and heat integration. The results of the proposed optimization model are compared with our previous work on MVR optimization for application in produced water treatment. The optimization model is tested for a hypothetical desalination plant with 0.5 million gallons per day (MGD) capacity, in which produced water with 10% salinity is concentrated to 30% salinity of the exiting brine. The results are also validated through ASPEN simulations. Sensitivity analysis is also conducted to analyze the effect of input parameters on the system performance and economics. Tradeoffs between MD and MVR technologies for treatment of shale gas produced water will be discussed.