Cost and Energy Needs of RO-ED-Crystallizer Systems for Zero Brine Discharge Seawater Desalination

Cost and energy needs of RO-ED-crystallizer systems for zero brine discharge seawater desalination The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. Citation Nayar, Kishor G. et al. “Cost and Energy Needs of RO-ED- Crystallizer Systems for Zero Brine Discharge Seawater Desalination.” Desalination 457 (May 2019): 115–132 © 2019 Elsevier B.V. As Published https://doi.org/10.1016/j.desal.2019.01.015 Publisher Elsevier Version Author's final manuscript Citable link http://hdl.handle.net/1721.1/120779 Terms of Use Creative Commons Attribution-Noncommercial-Share Alike Detailed Terms http://creativecommons.org/licenses/by-nc-sa/4.0/ Cost and energy needs of RO-ED-crystallizer systems for zero brine discharge seawater desalination Kishor G. Nayara, Jenifer Fernandesb, Ronan K. McGoverna, Bader S. Al-Anzib, John H. Lienhard Va,∗ aDepartment of Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue,Cambridge, MA 02139, United States bEnvironmental Technology and Management Department, College of Life Sciences, Kuwait University, Kuwait Abstract A zero brine discharge seawater desalination concept integrating reverse osmosis (RO), electrodialysis (ED) and crystallizer into a single system (REC) is presented. Analytical models were used to optimize parameters and minimize water production costs. Parameters varied were: the ratio of seawater to RO brine in the ED diluate channel, ED current density, ED diluate outlet salinity, electricity and salt prices, and RO recovery by adding high pressure RO (HPRO). Using only RO brine instead of only seawater in the ED diluate channel reduced water production costs by 87% from 27 to 3.5 $/m3 while increasing salt production costs 26% from 135 to 170 $/tonne-salt. The former was best for brine minimization, and the latter for salt production. Optimizing ED current density reduced REC costs by another 14% to 3.0 $/m3 while increasing specific 3 energy consumption 26% to 12.7 kWhe/m , corresponding to a Second Law efficiency of 18%. Adding an HPRO stage was uneconomical as it increased specific costs 21%. A salt price of 104.5 $/tonne-salt will justify the cost of adding an ED-crystallizer. REC systems may be economically feasible in parts of the Middle-East. Producing other products such as Mg(OH)2 or Br2 may further improve economics. Keywords: reverse osmosis, electrodialysis, hybrid, salt production, brine concentration, seawater 1. Introduction Climate change and rapid population growth is causing rising water scarcity across the globe [1, 2]. Seawater desalination is increasingly being deployed to meet water demand around the world. As of 2017, around 55 million m3 of freshwater was produced by seawater desalination plants around the world [3] along with around 75 million m3 of saltier brine as a byproduct. Currently, seawater desalination brine is discharged back into the sea using outfalls. There is now a growing concern around the environmental impact of desalination plants especially the impact on marine life close to the plant intakes and outfalls [4, 5]. Countries especially in the Middle East in particular are considering more stringent regulations of the ∗Corresponding author: [email protected] Email addresses: [email protected] (Kishor G. Nayar), [email protected] (Jenifer Fernandes), [email protected] (Ronan K. McGovern), [email protected] (Bader S. Al-Anzi), [email protected] (John H. Lienhard V ) Preprint submitted to Desalination January 14, 2019 K.G. Nayar, J. Fernandes, R.K. McGovern, B.S. Al-Anzi, J.H. Lienhard, “Cost and energy needs of RO-ED crystallizer systems for zero brine discharge seawater desalination,” Desalination, online 8 February 2019, 457:115-132, 1 May 2019. discharge of seawater desalination brine including potential zero discharge regulations. Policy makers and the industry have significant interest in understanding the technical and economic implications of zero brine discharge regulations. To achieve zero brine discharge seawater desalination, three steps are necessary: desalination, brine concentration and crystallization of salts. Seawater RO has been shown to be the most energy efficient [6{ 12] and lowest cost [9, 13] method to desalinate seawater and produce freshwater. For the crystallization step, the state-of-the-art technology currently used are multi-effect evaporators [14]. In a previous work, some of the authors of this paper had shown that state-of-the-art crystallizers are more efficient than state-of-the-art brine concentrators and suggested that research work should focus on improving the brine concentration step [14]. For seawater brine concentration, two proven technologies that can be used are mechanical vapor compressors (MVC) systems [10, 15{18] and electrodialysis (ED) systems [19{26]. Chung et al. [14] had concluded that the potential for further efficiency improvements in MVC systems was limited. ED has been used for a wide range of applications: brackish water desalination [24, 27, 28], industrial water treatment [24, 29], whey demineralization [30], in-home water treatment [31{33] and for the concentration of seawater for salt production [21{24, 26, 34]. Given the 50 years of industrial operational experience using ED for concentrating seawater [34], and given limited efficiency improvements possible in MVC [14], we focus the present paper on the potential ED has for cost reductions and efficiency improvements. Several studies in the literature have proposed hybridizing RO with other desalination technologies for various reasons such as reducing water costs, energy consumption, membrane scaling or to better manage brine discharge [35{44]. Hybridizing RO with ED has been discussed previously for brackish water desalination [35, 38, 45{47], co-production of salt and water from seawater [36, 37, 48{52] and for industrial water treatment [41]. In 1996, Hayashi et al. [36] patented a concept of using RO, ED, an evaporator and relevant pre-treatment systems for producing both salt and water from seawater. In 2001, Ohya et al. [37] proposed a high-level concept using multi-ion adsorption columns, nanofiltration (NF), multi-stage RO, ED and an evaporator for producing water along with several salts and minerals. While potential revenue from all seawater derived products from a 1 million m3 seawater desalination plant was discussed, the authors did not discuss the specific energy consumption and the costs of the systems. However, Ohya et al. [37] identified where research and development efforts should focus to realize the concept. In 2006, Davis [53] evaluated the economics of using RO, ED and evaporator systems for producing water, salt, bromine and magnesium hydroxide. Davis used literature data, industry sources, experimental setups and an analytical model to report the final capital and operating costs and product revenues for the concept. However, process model equations were not disclosed and the model could not be directly reproduced or verified. However, it was clear from Davis's report that production of magnesium hydroxide and bromine were key to making the process profitable, with both products increasing profit margins 2.5 times than when 2 just water and sodium chloride were produced [53]. In 2012, Casas et al. [49] piloted a 500 L/hr ED plant to concentrate seawater RO (SWRO) brine to produce sodium chloride rich brine for the chlor-alkali industry and reported energy consumption, ED current densities and sodium chloride concentrations. Despite the RO brine being super-saturated in carbonate and sulfate salts, the presence of anti-scalants from the RO process and maintaining a pH of 5.5 inhibited scaling on the ED membranes. Jiang et al. [51] also carried out experiments on an ED stack treating SWRO brines comparing the performance of different ED membranes. Jiang et al. also reported on the water transport across the membranes under various conditions. Reig et al. [50] further established technical feasibility of using ED to concentrate SWRO brine through a pilot system, concentrating SWRO brine from 70 g/L of NaCl to 245 g/L of NaCl with the specific energy consumption varying between 120-190 kWh/tonne- NaCl as the current density was varied from 350-500 A/m2. Reig et al. also determined that the specific energy consumption for producing 185 g/L of NaCl brine was lowest at a current density of 350 A/m2. In 2015, Tanaka et al. [52] reported results from a computer simulation of ED treating desalination brine. The simulation was based on prior experimental investigations, and reported results for the specific energy consumption, ED current densities, voltages and the effect of linear flow velocity on performance. In 2017, Thiel et al. [54] reviewed several methods to concentrate seawater RO brine and to produce sodium hydroxide from brine. The energetic advantages of using ED and partial desalination over complete desalination to concentrate brine was highlighted. A framework was also presented for calculating the thermodynamic least work of separation for sodium hydroxide production. In 2018, Du et al. [55], presented a process of using NF, ED, MVC, chemical softening, ion-exchange and an electrolyzer to produce sodium hydroxide from desalination brine. Du et al. had modeled the sub-systems using Aspen with the ED model being a black-box adaptation of a simple ED model developed by Nayar et al. [56] and reported energy consumption and operating costs. While there have been several experimental studies and a few analytical model studies conducted on the RO-ED-crystallizer concept for producing salt and water from seawater, to the best of our knowledge, no study has reported the specific cost of an RO-ED-crystallizer system designed for zero brine discharge seawater desalination and evaluated the economics along with the energy needs of such a system. Furthermore, no study has looked at how the flow configuration and coupling between the RO and ED systems changes the costs. Inclusion of novel high pressure RO (HPRO) systems into the RO-ED-crystallizer concept have also not been considered in the literature on zero brine discharge desalination.

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