Date Published: October 9, 2018
Publisher: Public Library of Science
Author(s): Emad Ali, Jamel Orfi, Abdullah Najib, Jehad Saleh, Nicolas Roche.
Desalination of geothermal brackish water by membrane distillation (MD) provides a low recovery rate, but integrating MD with reverse osmosis (RO) can maximize the production rate. In this study, different design configurations of a hybrid system involving brine recycling and cascading are studied via simulations, and the performance improvement due to the process integration is substantiated via the increased recovery rate and reduced specific energy consumption. Brine recycling is also found to improve the recovery rate considerably to 40% at an energy cost of 0.9 $/m3. However, this achievement is only valid when the final brine is recycled to the RO feed: when the final brine is recycled to the MD feed, the overall performance degrades because the recycled brine cools the feed and causes a serious reduction in the driving force and the consequent production rate. Configuring the hybrid system in multiple stages connected in series increases the recovery rate to 90% and reduces the specific energy consumption to 0.9 MJ/kg. Although the specific energy cost increases dramatically because external inter-stage heating is implemented, using a free energy source (such as a geothermal or waste-energy source) for inter-stage heating could provide the optimum configuration.
Geothermal energy is derived from hot water or steam drawn from sub-soil and is mainly used for generating electricity, producing heat, and cooling. Lund et al.  reviewed direct global applications of geothermal energy worldwide and determined that it was used for bathing and swimming, heating spaces and districts, and acted as a ground source heat pump. Specific data and information on the use of geothermal energy in 82 countries up to year 2015 were gathered, presented, and discussed. The report showed that the amount of thermal power installed globally for direct utilization at the end of 2014 was approximately 71,000 MW, and the electrical capacity of the European Union reached 993.6 MW in 2015 (with 915.5 MW in Italy) . However, geothermal energy use is limited in Saudi Arabia, where there is an installed capacity of only 40 MW for bathing and swimming and 4 MW for animal farming, providing a total direct use application of 152.89 TJ/year .
In the authors’ previous study [25,26], extensive experiments were conducted on a single pilot-scale MD to analyze the process performance and develop a rigorous model; this previously validated model of the MD unit will be used here. However, previous experimental and theoretical studies performed on a single MD by the authors [25,26] revealed that the performance of this desalination system was low and needed improvement. For example, it was found that the recovery ratio and production rates are too low: the recovery ratio does not exceed a maximum value of 5%. Similar conclusions are supported by other studies in the literature. This present work proposes and assesses methods that can be used to enhance the desalination performance; for example, by integrating MD with RO and brine recycling. The feed water is considered to be brackish water with fixed properties, and the presented results therefore relate to brackish water that has the same baseline feed conditions: a hot flow rate of Qho = 300 L/h and hot water feed temperature and salinity of Tho = 70°C and Cso = 1.414 kg/m3, respectively, (the latter two conditions are representative of certain wells in the Riyadh region).
Fig 10 depicts the simulation results for Option 1 at a range of recycling ratios for both fixed and varying makeups, and the results show that the recycling ratio has an obvious effect on the salinity and temperature of the MD feed. Typically, for a large-size MD, as used in this study, the brine leaves the MD at a low temperature; therefore, recycling the brine reduces the feed temperature substantially, which degrades the driving force at the membrane interface and hence results in reduced water production. Consequently, the recovery ratio diminishes as the recycle ratio increases, which then decreases the performance ratio and increases the specific energy demand. The situation deteriorates further when a varying fresh feed is adopted. With a decrease in the makeup flow rate, less energy is supplied to the system; this leads to a sharp drop in the feed temperature, which then causes a considerable degradation of mass production. Although water production is not shown in the figure, its effect is reflected in the specific energy and performance ratio, which grow exponentially at higher recycling rates. Although energy for pumping is constant for the fixed feed flow and corresponds to a varying makeup, the associated water production decreases rapidly, which leads to a sharp increase in specific energy cost. Interestingly, variations in the recovery ratio with changes in the recycling ratio are almost the same for both varying and fixed makeups, and this occurs because water production and makeup ratios remain almost the same due to their relative change rates. It is of note that pure water is obtained, which means that there is 100% salt rejection. Nevertheless, the performance obtained using Option 1 is not promising.
Geothermal brackish water is usually distilled using RO units after the treated feedstock has been cooled. However, there is much potential in using the thermal energy of geothermal water and employing MD, which provides the added advantages of producing high-purity water and being insensitive to feed salinity. In this study, different design configurations of MD/RO hybrid systems were investigated, which included differences in brine recycling and the cascading structure. The results substantiated the superiority of the integrated system compared to the conventional one; the recovery ratios ranged from 30% to 40% and the energy costs per m3 ranged from 0.4 to 0.9 $ when using variations in the RO operating pressure between 6 and 40 bar, respectively. It was found that brine recycling also improved the recovery rate and performance ratio only when brine was reused around the RO unit. When the rejected brine was fed back to the MD feed, the performance deteriorated because cold recycled brine quenched the MD feed and led to a reduced driving force inside the MD membrane. It was also revealed that using a multi-stage MD/RO interplay system connected in series enhanced the performance. In fact, a 90% recovery ratio and 0.9 MJ/kg performance ratio were obtained when 8 stages were employed. Although the production cost increased to 9 $/m3 because inter-stage heating was involved, if waste heat was used for inter-stage heating, the specific energy cost would be considerably reduced.