desalination of seawater using gas hydrate ...

Proceedings of HYDRO 2013 INTERNATIONAL 4-6 Dec 2013, IIT Madras, Chennai, INDIA

DESALINATION OF SEAWATER USING GAS HYDRATE TECHNOLOGY – CURRENT STATUS AND FUTURE DIRECTION Jitendra S. Sangwai1,*, Rachit S. Patel2, Prathyusha Mekala3, Deepjyoti Mech3, Marc Busch5 1 Assistant Professor, Department of Ocean Engineering, Indian Institute of Technology Madras, Chennai 600 036, India, Email: [email protected]. *Corresponding Author. 2 Project Officer, Department of Ocean Engineering, Indian Institute of Technology Madras, Chennai 600 036, India, Email: [email protected] 3 Research scholar, Department of Ocean Engineering, Indian Institute of Technology Madras, Chennai 600 036, India, Email: [email protected] 4 Research scholar, Department of Ocean Engineering, Indian Institute of Technology Madras, Chennai 600 036, India, Email: [email protected] 5 Exchange Student, Department of Ocean Engineering, Indian Institute of Technology Madras, Chennai 600 036, India, & Depart of Mechanical Engineering, RWTH Aachen Germany, Email: [email protected]

Abstract: The water forms an integral part of mankind and hence should require prime attention. Due to industrialization and increased population, the shortage of water in several developing countries is observed. Seawater forms a huge source of potable water provided the economical desalination technology is in place. The available desalination technology, though mature, require development to make them more economical. Gas hydrates may come at help to make the process more economical. 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. The gas hydrate as a technology has been successful for several potential applications in various engineering fields, such as, gas separation, carbon dioxide sequestration, gas storage and transportation, energy source, refrigeration and not the least, in the desalination of salt water. The current work focuses on the use of hydrate for desalination of salt water. The desalination process is based on the phase change of liquid to solid thereby removing the solids from the liquid phase. We present a principle behind the use of hydrate technology for desalination of salt water, the science and engineering aspects of the process and future directions. Keywords: Clathrate; Desalination; Economics; Hydrate; Multi-flash Distillation; Reverse Osmosis; seawater. INTRODUCTION World population has increased drastically resulting in more demands for potable water. Past few decades saw the ground water level depleted drastically and available fresh water deposits accounting less than 0.5 % of the earth’s total water supply, forcing us to find alternative water reserves. The oceans represent the earth’s major water reservoir. Seawater shows potential alternative for a source of potable water provided suitable water purification technologies can be incorporated economically to make them feasible. Desalination is the process of the removal of salts from the seawater using economical processes to convert them to fresh water. Now a days, several countries depend on desalination processes, particularly Middle East countries, where desalination is vital for human beings. It is estimated that about over 75 million people worldwide obtain fresh water by desalinating seawater or brackish water (Khawajia et al., 2008). 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). Two of the most commercially important desalination processes the multi-

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XVIII Conference on Hydraulics, Water Resources, Coastal and Environmental Engineering

Proceedings of HYDRO 2013 INTERNATIONAL 4-6 Dec 2013, IIT Madras, Chennai, INDIA stage flash (MSF) distillation and reverse osmosis (RO) processes. Some estimate shows that MSF and RO accounts of the order of 40 % and 50 % of the world’s total desalination capacity. MSF technology is observed to be of an industrial scale with a capacity of over 5000 m3/day. RO technology is more concentrated for small scale unit production. However, the RO technology gained significance in desalination due to its lower cost and simplicity. The desalination process using MSF distillation technique involves flash evaporation of water at reduced pressure and higher temperature conditions. Regenerative heating helps to achieve the economic criterion where the seawater flashing in each flash drum exchange heat to the incoming stream of seawater. This helps in increase in the temperature by utilizing the heat of the stream at maximum. The typical MSF plant consists of heat input, heat recovery, and heat rejection sections. The MSF process may involve 19 to 24 stages depending upon the volume of sea water to be treated and operates at about 90-120 oC, however, higher operating temperature attract formation and deposition scale/corrosion in the process equipments challenging the safe and economical operation. On the other side, the RO process uses a semi-permeable membrane to separate salts from water. In this method, external pressure is applied to overcome osmotic pressure, which is driven by chemical potential. In this method the solute (salts, ions) is retained on the pressurized side of the membrane and the pure water is allowed to pass to the other side in reverse direction of the natural flow pattern across the membrane. In this process, no heating or phase separation is required. The major energy required for desalting is for pressurizing the seawater feed. The proper choice of membrane is the key for a better solute retention and having pure water as a product. The typical RO plant consists of feed water pre-treatment, high pressure pumping, membrane separation, and permeates post-treatment. Although these methods found to be more reliable, these need constant improvements in research and development to make the water purification technologies more economically and approachable (Kalogirou, 2005; Park et al., 2011). Gas hydrate offers one of the promising economical alternatives for desalination of seawater (Park et al., 2011). GAS HYDRATE AND DESALINATION Gas hydrates (clathreate) are crystalline ice-like 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 hydrogenbonded water molecules as shown in Figure1 (Sloan 1998). It is observed that 1m3 hydrate, if dissociated, can produce up to 164m3 of gas and 0.8 m3 of pure water at standard temperature and pressure. Looking at the amount of pure water produced per m3 of hydrate pellet, the process of using hydrate for desalination seems promising. Gas hydrate typically forms at low temperature, T (typically < 20°C), and high pressure, P (typically > 30 bar), conditions. The hydrate formation (P, T) conditions required for propane and CO2 hydrate is lower than most of the commonly used guest molecules to form hydrates. Thus, these hydrate show potential to be used for desalination technologies. Gas hydrates are non-stoichiometric compounds and on a mole basis methane gas hydrate consists of 85.69 (± 0.14) % water and 14.31 (± 0.14) % methane. Due to the presence of such a large amount of water in the hydrates, the physical (i.e., density, refractive index) and thermal (specific heat) properties are similar to ice with some exceptions. Gas hydrate shows different structures such as, structure I (sI), structure II (sII) and structure H (sH), depending upon the type of guest molecules. The gas hydrate as a technology has been successful for several potential applications in various engineering fields, such as, gas separation, carbon dioxide sequestration, gas storage and transportation, energy source, refrigeration and not the least, in the desalination of salt water. Hydrate as a technology for desalination was developed way back in the 1940s and gained attention in the 1970s followed by the development of desalinating process by Sweet Water Development Co. and Koppers and Company (Parker, 1942; Knox et al., 1961). Some researchers in the early 70s had investigated the kinetics and separation of minerals using hydrate technology; this was followed by development of pilot scale plant for desalination (Barduhn et al., 1962; Barduhn, 1967; 1968). Concentrated brines such as seawater which primarily contains NaCl are known to be very good inhibitors for gas hydrate formation. However, the salinity is zero in hydrate once formed and this can be utilized to separate salts from water. The main obstacle for early industrialization of hydrate based technology was


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Proceedings of HYDRO 2013 INTERNATIONAL 4-6 Dec 2013, IIT Madras, Chennai, INDIA the separation of hydrate phase out of the concentrated brine liquid phase resulting in uneconomical operation (Rautenbach and Seide, 1978; Park et al., 2011). Recent studies showed that the hydrate have potential to give economical desalination process with the rate of $0.46–0.52/m3 of saline water (McCormack and Andersen, 1995). Ngan and Englezos (1996) investigated the recovery of water from effluents and 2.5 wt.% NaCl solutions using hydrate of propane in a moderately operated vessel in which hydrate nucleation, growth, separation, and melting occur (Ngan and Englezos, 1996). The average reduction in the salt content of the recovered water from the NaCl solutions was found to be 31%.

Figure 1: Schematic of gas hydrate structure (Sloan, 1998) The incipient equilibrium hydrate formation conditions for the main constituents of natural gases and for several mixtures have been determined in the presence of electrolytes. Recent studies suggest novel hydrate-based desalination unit with a potential to easily extract dehydrated high-density gas hydrates from a reactor containing hydrate slurries and separate various mineral ions (Park et al., 2011). This study gives proof that the hydrate method may address the separation difficulty between hydrate crystals and concentrated brine solutions, thus can be applied for effective desalination processes (Park et al., 2011). They also showed the effect of ionic radius and ionic charge present on the cation in the form of mineral to removal efficiency of salt from seawater using hydrate. Sarshar and Sharafi (2011) carried out the experimental study for seawater desalination and CO2 capture simultaneously using the gas hydrate technology. They have evaluated process condition for CO2 hydrate formation in Persian Gulf water with 6-52% saline removal efficiency. Colten et al. (1972) have investigated economic losses due to hydrolysis of hydrating agent used for desalination of seawater in hydrate technology. The authors have investigated results for hydrolysis rates of several halogenated hydrocarbons which have been considered for use as hydrating agents in hydrate desalination. Younos and Tulou (2005) presented a review study of different well known desalination technology such as thermal (distillation), membrane (RO) and chemical (ion exchange) and also given a brief introduction about several other desalination technologies which are under research and development, which includes hydrate as a potential future desalination technology. Sabil et al. (2010) presented a kinetic study of formation for single carbon dioxide and mixed carbon dioxide and tetrahydrofuran hydrates in water and sodium chloride aqueous solution. The presence of tetrahydrofuran in the hydrate system significantly reduces consumption of CO2 in hydrate form and presence of sodium chloride also reduces the CO2 uptake slightly, potentially implicating desalination process using CO2 hydrate technology. Bardhn and Lee (1978) have done a thermodynamic study of F-22 (CHCIF) hydrate system in aqueous sodium chloride solution. They presented thermodynamic information such as, hydrate decomposition conditions and the invariant points including the eutectic point for checking the performance of F-22 as an agent for use in the hydration process for desalination of seawater. Corak et al. (2011) gave the thermodynamic and kinetic data for cyclopentane hydrate in brine. They also observed the effect of sub-cooling and amount of cyclopentane present on hydrate formation. The higher degree of sub-cooling favors the desalination process by hydrate method. Aliev et al. (2011) did mathematical modeling for Freon R-142B–water hydrate system. They have taken column type reactor with a sieve plate for gas–liquid process for hydrate formation

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Proceedings of HYDRO 2013 INTERNATIONAL 4-6 Dec 2013, IIT Madras, Chennai, INDIA in the Freon–water system. They carried out an optimization study for a determination optimum design condition for desalination processes by hydrate technology. Though the hydrate technology for desalination was appeared way back in the 1940s, the attention and importance of hydrate process for the desalination is gaining since recent past. As not much information is available in open literature on hydrate technology for desalination, this calls and provides scope for efforts in this direction. HYDRATE DESALINATION PROCESS In the absence of much scientific information on hydrate technology for desalination process, we propose, based on the literature survey carried so far, a process for desalination using hydrates as shown in Figure 2. The desalination process using gas hydrate technology is based on the phase change process, wherein, the liquid water is transformed into solid hydrate by separating the dissolved solids from the liquid phase. The desalination process using hydrate technology need hydrate former such as gas to be mixed with the salted water, which is then followed by separation using hydrate formation. Process flow sheet as shown in Figure 2 describes the procedure of desalination of seawater through gas hydrate technology. Before entering the seawater into the reactor, the heat content of water is removed through the heat exchanger.

Figure 2: A typical process flow sheet proposed for desalination via hydrate The reactor is connected with guest gas chamber from which gas is being supplied to the reactor. However, a makeup gas cylinder is provided for overcoming the gas losses during process flow. The seawater and the guest gas which is present inside the reactor are thoroughly mixed with the help of a stirrer at a desired RPM for thorough mixing to form gas hydrate by making a proper interface bonding between the water and gas molecules at a required pressure and temperature conditions. A low temperature and high pressure conditions are maintained in the reactor for given gas hydrate system. After the formation of hydrate slurry, it is then transferred to the crystallizer via a hydrate slurry pump. This hydrate slurry is converted into a crystalline solid structure along with the concentrated brine. This concentrated brine is drained out from the crystallizer. Further the crystalline solid structure of gas hydrate transferred to a decomposer where again the gas hydrate decomposes into gas and water by the addition of heat. The gas then flows into the gas storage tank through the upper portion of the decomposer and the remaining water is collected at the bottom, which can be used in the industrial and domestic areas for daily routines. ECONOMICS Economic study is a very important factor for checking the feasibility of the process. The operation cost of desalination of seawater by hydrate technology depends on many factors such as sea water temperature, salt


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Proceedings of HYDRO 2013 INTERNATIONAL 4-6 Dec 2013, IIT Madras, Chennai, INDIA content, mobility of salt and yield (Javanmardi and Moshfeghian, 2003). The removal efficiency of desalination plant is also depends on ionic radius and ionic charge of the cation present in the form of mineral (Park et al., 2011). In the Figure 3, it can be observed that, as we go for bigger cation size and low ionic charge, the efficiency of removal of salts gets increase using hydrates but reverse is for the ionic charge.

Figure 3: Removal efficiency of each dissolved mineral with its ionic radius and ionic charge (Park et al. 2011) The typical comparison of operating conditions for the well-known desalination process such as RO and MSF technologies with gas hydrate technology are shown in Table 1. In the case of MSF, the operating temperature range is very high as compared to that of hydrates and RO. At the same time pressure is below 1bar for MSF and relatively higher for others. The capital investment for hydrate is higher but shows low operating cost against MSF and RO processes. The total production cost per ton of desalted water is lower for hydrates. Table 1: Comparison of MSF, RO and gas hydrate technology Parameter Physio-chemical principal Temperature (oC) Pressure (Bar) Capacity (ton/day) Capital Investment (M$)* Operating cost ($/ton)* Total Product Cost ($/ton)* Maintenance

MSF Flash evaporation 90-120 Below 1 1000 2.93 2 3.26 Corrosion issue

RO Solute diffusion 20-35 55-70 1000 2.3 3.23 4.47 Membrane replacement

Hydrate Phase change process Vicinity of 0 4.5-6.5 (Propane hydrate) 1000 5.46 1.2 2.76 No maintenance

*(Javanmardi and Moshfeghian, 2003)

Figure 4: Energy consumption for distillation, membrane and hydrate (Clathrate Method) (Lee JD, 2011)

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Proceedings of HYDRO 2013 INTERNATIONAL 4-6 Dec 2013, IIT Madras, Chennai, INDIA Figure 4 shows the energy consumption (kJ/kg) with respect to salt content in water. It is observed that clathrate hydrate for desalination are more economical than the two conventional methods. By comparing these three cases though the capital investment for hydrate is more, the operating cost is less. The operating cost of hydrate technology can be further reduced by using suitable nontoxic promoters. The overall total cost using hydrate technology is competitive with respect to other conventional technologies. The hydrate technology is expected to gain importance for desalination process in near future leading to a further decrease in the capital investment for this process over a period of time. CONCLUSION AND FUTURE DIRECTION Seawater desalination by different major technologies is discussed in this work. After seeing the economic aspect of desalination plant based on hydrate technology, it can be concluded that desalination by hydrate route looks a promising alternative compared to the conventional technologies such as reverse osmosis (RO) and multi stage flash (MSF) distillation. 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, research in this direction is an ongoing process. REFERENCES 1.

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Corak, D., Barth, T., Høiland, S., Skodvin, T., Larsen, R. and Skjetne, T. 2011. Effect of subcooling and amount of hydrate former on formation of cyclopentane hydrates in brine. Desalination, 278(1), 268-274.

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Proceedings of HYDRO 2013 INTERNATIONAL 4-6 Dec 2013, IIT Madras, Chennai, INDIA 14. Park, K., Hong, S.Y., Lee, J.W., Kang, K.C., Lee, Y.C., Ha, M.G. and Lee, J.D. 2011. A new apparatus for seawater desalination by gas hydrate process and removal characteristics of dissolved minerals (Na+, Mg2+, Ca2+, K+, B3+). Desalination, 274, 91–96. 15. Parker, A. 1942. Potable water from seawater. Nature, 149,184-186. 16. Rautenbach, R. Seide, A. 1978. Technical problems and economics of hydrate-processes. Proceeding 6th International Symposia Fresh Water from Sea, 4, 43-51. 17. Sabil, K.M. Duarte, A.R.C. Zevenbergen. J. Ahmad, M.M. Yusup, S. Omar. A.A. Peters, C.J. 2010. Kinetic of formation for single carbon dioxide and mixed carbon dioxide and tetrahydrofuran hydrates in water and sodium chloride aqueous solution. International Journal of Greenhouse Gas Control, 4, 798–805. 18. Sarshar, M. Sharafi, A.H. 2010. Simultaneous water desalination and CO2 capturing by hydrate formation. 1st International Conference on Water and Wastewater Treatment, April Isfahan, Iran, 21–22. 19. Sloan, E.D. Jr. 1998. Clathrate Hydrates of Natural Gases, Second Edition, Marcel Dekker, NY. 20. Younos, T. Tulou, K.E. 2005. Overview of Desalination Techniques. Journal of Contemporary Water Research & Education, 132, 3-10.

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