Desalination of Seawater through Gas Hydrates
Abstract
Water is an elixir of life and was basis of human civilization. Rapid growth in population promotes consumption of freshwater due to more industrialization and urbanization. Hence, the demand of freshwater is being increased for agriculture, industrial and domestic sectors day by day. Moreover, per capita water availability is also decreasing from 5117 m3/year in to 1371 m3/year in 2025 (MWR, GOI 2019). Shortage of water lies in more than 80 countries and almost 40% population of the world facing this problem (Sabil et.al., 2013). As about 97 % of water existed on the planet earth as seawater (Brown, 2017) and it may be good alternative for fulfill the demand of freshwater after desalination. Desalination is the process of the removal of salts from the seawater using economical processes to convert them to fresh water. It is estimated that about over 75 million people worldwide obtain freshwater by desalinating seawater or brackish water (Khawajia et al., 2010). Therefore, many countries in the world are investing heavily in the seawater desalination for production of freshwater. The five world leading countries by desalination capacity are Saudi Arabia (17.4%), USA (16.2%), United Arab Emirates (14.7%), Spain (6.4%), and Kuwait (5.8%) (Khawajia et al., 2008). Among the different methods of seawater desalination (reverse osmosis and multi-stage flash distillation etc.), gas hydrate-based desalination technology is a relatively new one that has created an interest among the researchers and institutions (Sangwai et.al, 2013). Gas hydrates are crystalline solids made of the water (host) and the gas molecules (guest) such as methane, carbon dioxide, nitrogen, etc., which are held within water cavities that are composed of hydrogen-bonded water molecules (Babu et.al 2018). Process in gas hydrates-based desalination technology is depend upon the phase change of liquid to solid thereby removing the solids from the liquid phase. Economically gas hydrates-based desalination technology as compared to the conventional technologies such as reverse osmosis (RO) and multistage flash (MSF) distillation looks a promising alternative for desalination of seawater (Park et al., 2011). As low temperature requirement is an important factor in gas hydrate formation process, implementation of gas hydrate desalination technology in the colder region would also enhance the economy of process by saving the energy cost for chilling the sea water. In future, the hydrate process can be made more economical by using some cheap and easily available hydrate formation promoter. Hence, the research in this direction is an ongoing process and may be very useful for fulfil the future demand of freshwater.
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Introduction
Many nations are experiencing freshwater crisis as a result of population increase and the tremendous development of industrial and agricultural operations. 1 in 6 of the people (approximately 1.2 billion) do not have enough access to safe drinking water, and the 1 out of 6 children dies for every 8 seconds after drinking polluted water. By 2025, 1.8 billion people will live in nations and territories with absolute water scarcity, and two-thirds of the world'spopulation may face water scarcity. The management of water resources affects nearly every aspect of the economy, especially health, food production and safety, domestic water and sanitation, energy, industry and environmental sustainability. The future of water and energy resources is inextricably linked, requiring the development of innovative technologies to strengthen the water-energy relationship. Freshwater resources represent the total amount of water available on earth. The total freshwater supply available for ecosystems and humans is less than 1% of total freshwater resources. In recent decades, seawater has emerged as an important source of freshwater, as it is one of the most abundant resources on earth (97.5%) and a core technology for alleviating freshwater scarcity. MSF remains the dominant desalination technology in the Middle East, accounting for 50% of global consumption, due to the ready availability of fossil fuels and poor feed water quality. RO is the current state of the art for seawater desalination The ROprocess can treat feedwater within the TDS (Total Dissolved Solids) range of 10,000 to 60,000 mg/L. Atypical seawater ROplant has a total water recovery of less than 55%. Total water recovery is the ratio of the amount of water produced to the amount of feed water The total energy required for RO is 3-6 kWh per m3 of recovered potable water. The biggest disadvantage of these methods is that they consume a lot of energy. The energy cost of meeting the expected global water demand with current technology is significant, particularly in a carbon-constrained society. Because water and energy. To meet the future demand for fresh water, there is a need to create novel technologies that may enhance the water-energy junction and increase the efficiency of existing thrust of freshwater. For seawater desalination, hydrate-based desalination has been suggested. The procedure essentially comes under the category of freezing or freeze desalination technologies. In the process water molecules create cages around a guest gas/liquid component in this mechanism, successfully isolating themselves from brine solution even at temperatures beyond the freezing point of water. When these hydrate crystals are melted, they become virtually fresh water, and the guest component may be reused in the desalination process. One mole of hydrate contains around 85% water and 15% guest gas, indicating that this technique has a great potential for creating reasonably pure water. The salt is only a thermodynamic inhibitor and is thus not allowed in the hydrate cages (Han et.al.,2017). The procedure has the benefit of using less energy because it runs at temperatures well above the freezing point of water.
Conclusion
One of the most promising solutions to help with a growing water issue is desalination. The most common desalination process in use today, reverse osmosis requires a lot of energy. Therefore, to increase the energy-water confluence, revolutionary energy-efficient technology must be developed. One such method is hydrate-based desalination. this paper gives an overview of some emphasised process and general information regarding hydrates and the function of desalination technology, the technology has been explored over the past 70 years, but due to delayed hydrate formation kinetics, challenges in hydrate crystal separation from brine without contamination, and expensive refrigeration costs, it was never commercialised. Using a more inventive reactor design, better hydrate-forming agents, and other technological advancements can be formalized for better mitigating the upcoming water scarcity issues in the world.