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A Review of Clathrate Hydrate Based Desalination to Strengthen Energy-Water Nexus Ponnivalavan Babu, Abhishek Nambiar, Tianbiao He, Iftekhar A Karimi, Ju Dong Lee, Peter Englezos, and Praveen Linga ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01616 • Publication Date (Web): 25 May 2018 Downloaded from http://pubs.acs.org on May 27, 2018

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ACS Sustainable Chemistry & Engineering

A Review of Clathrate Hydrate Based Desalination to Strengthen Energy-Water Nexus Ponnivalavan Babu†,ᶵ, Abhishek Nambiar†,ᶵ, Tianbiao He†, Iftekhar A Karimi†, Ju Dong Lee‡, Peter Englezos§* and Praveen Linga†*, †

Department of Chemical and Biomolecular Engineering, National University of Singapore, Engineering Drive 4, Singapore 117585, Singapore ‡

Offshore Plant Resources R&D Center, Korea Institute of Industrial Technology, Busan Republic of Korea

§

Department of Chemical and Biological Engineering, University of British Columbia, 2360 East Mall, Vancouver V6T 1Z3, Canada

Abstract: Water resource management impacts almost all aspects of the economy, in particular health, food production and security, domestic water supply and sanitation, energy, industry and environmental sustainability. For last several decades, seawater has become an important source of fresh water as it is one of the most abundant resources on earth. Desalination is the process of removal of salts from seawater and is believed to be a core technology in alleviating this problem. Clathrate hydrate based desalination (HyDesal) is a potential technology for seawater desalination. Salts are excluded from hydrate formation, thereby resulting in solid hydrate and concentrated brine. After separation from brine, the solid hydrate crystals upon dissociation produce pure water. In this work, a detailed review of the literature (both patents and publications) so far on HyDesal is critically evaluated and, prospects and directions to commercialize the HyDesal process are presented. Further, innovation by coupling LNG cold energy with HyDesal can make it economically attractive and can strengthen the energy-water nexus. Keywords: gas hydrates; desalination; clathrate process; seawater, cold energy, LNG ᶵ Equal contribution from both authors *corresponding author(s): Praveen Linga (e-mail: [email protected]); Peter Englezos (email: [email protected])

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Table of Contents: Abstract ................................................................................................................................ 1 Introduction .......................................................................................................................... 3 Clathrate Hydrates ................................................................................................................ 5 The concept of clathrate hydrate based desalination (HyDesal) ............................................. 6 Literature Review................................................................................................................ 10 Clathrate hydrate based desalination (HyDesal) milestones ................................................. 15 Patent Review ..................................................................................................................... 16 Comparison of HyDesal with other technologies ................................................................. 28 Prospects and Challenges .................................................................................................... 31 Conclusion .......................................................................................................................... 33 Acknowledgment ................................................................................................................ 33 References .......................................................................................................................... 34 List of Figures ..................................................................................................................... 40 Synopsis.............................................................................................................................. 41 Concept Graphic ................................................................................................................. 41

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Introduction: Many countries suffer from a shortage of fresh water due to population rise and largescale expansion of industrial and agricultural activities. One in six human beings (about 1.2 billion) alive today lack adequate access to drinking water and a child dies every 8 seconds from drinking contaminated water.1 By 2025, 1.8 billion people will be living in countries or regions with absolute water scarcity, and two-thirds of the world’s population could be living under water stressed conditions1. Water resource management impacts almost all aspects of the economy, in particular health, food production and security, domestic water supply and sanitation, energy, industry and environmental sustainability. Since the future of water and energy resources are interrelated, there is a need for the development of innovative technologies to strengthen water-energy nexus.2-3 The freshwater resource is only around 2.5% of the total volume of water available on our planet. Of these freshwater resources, 70% is in the form of ice and snow cover in mountainous regions, the Antarctic and Arctic regions. The total usable freshwater supply for ecosystems and humans is less than 1% of all freshwater resources. For the last several decades, seawater has become an important source of fresh water because it is one of the most abundant resources (97.5%) on earth4. Desalination is the process of removal of salts from seawater or brackish water and is believed to be a core technology for alleviating freshwater scarcity. There have been significant advances in the last two decades in technologies to desalinate seawater and brackish water (available in saline aquifers and underground).4 The state of the art and challenges faced by seawater desalination has been reviewed in the literature.5 The traditional desalination plants based on the multi-stage flash (MSF) distillation and reverse osmosis (RO) processes have been evaluated to be reliable and established processes.6-7 MSF remains the primary technology for desalination in the Middle East due to easy availability of fossil fuel and poor feed water quality and contributes 50% of world’s desalination capacity. It has been estimated that about 203 million tons of oil per year is required to produce 22 million m3 of potable water/day.6 RO is the present state of the art technology for seawater desalination. RO process can treat the feed water within TDS (Total Dissolved Solids) range of 10,000 – 60,000 mg/L. The total water recovery of a typical seawater RO plant is less than 55%.4, 8 Total water recovery is the ratio of the volume of water produced to the volume of feed water. The total energy required for RO is 3 - 6 kWh/m3 of recovered potable water.9 The main drawback of these processes is that they are

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energy intensive. The energy cost of supplying the projected global water need with present technologies is high, especially in a carbon-constrained world. Since water and energy challenges are intimately linked, there is a need to develop innovative technologies that can strengthen the water-energy nexus and improve the efficiency of the existing processes to cater for the future demand of fresh water. Various processes have been proposed for desalinating seawater based on indirect and direct freezing (an alternative approach to MSF and RO processes). Although freeze distillation is a promising technology, it was proven to be too expensive for commercial use. In an indirect freezing approach, seawater is cooled by circulating cold fluid in the heat exchanger. The ice formed on the outer surface is removed, washed and melted to produce fresh water. In a direct freezing process, seawater is cooled by directly contacting it with a refrigerant. As refrigerant evaporates, water freezes into ice. A detailed review of freeze concentration process and its application for desalination is available in the literature.10 Clathrate hydrate based desalination (HyDesal) has been proposed for seawater desalination. HyDesal process effectively falls in the class of approaches based on freezing or freeze desalination. In this process, water molecules form cages around a guest gas/liquid component, thereby effectively separating themselves from brine solution even at temperatures higher than the normal freezing temperature of water. These hydrate crystals when melted are essentially fresh water and the guest component can be re-used for the desalination. One mole of hydrate consists of about 85% water and 15% guest gas, which signifies the high potential of producing relatively pure water from this process. The salt is just a thermodynamic inhibitor and is excluded from the hydrate cages.11 The advantage of HyDesal process is that it is less energy intensive since it operates at temperatures well above the normal freezing point of water.12 The use of clathrate hydrates as one of the methods to remove salts from seawater was reported way back in 194213 and a major research and development effort ensued in the 1960s and 1970s. The work was presented in a series of desalination symposia as well as in the refereed and patent literature. A summary of that early work was presented in a review of clathrate hydrates in 1993.14 It was reported that the speed of clathrate hydrate formation, crystal separation from the brine and recovery of the dissolved hydrate formers from the produced water were the major challenges preventing commercialization of the HyDesal process. The state of knowledge on clathrate hydrates has advanced considerably since then and it may be possible that the process operation challenges of separating crystals from concentrated brine and slow kinetics of the process may be overcome. With the exception of 4


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the review on clathrate hydrates in 1993 that also reviewed the clathrate hydrate based desalination, there is no other review of the work on this desalination method. Hence, the purpose of present study is to compile the state-of-the-art in the literature including patents and to identify the research and development challenges of the HyDesal process. Finally, the prospects to commercialize the HyDesal process are discussed. The paper is organized as follows. First, the subject of clathrate hydrates is presented with emphasis on topics relevant to desalination. The conceptual basis of the hydrate desalination method is presented next along with two metrics: water recovery percent and salt rejection percent. This is followed by a historical account of the work until the early 1990s. The work during the past 25 years is then reviewed, followed by a detailed account of the patents on the subject. Finally, the future challenges and prospects of HyDesal are discussed and a comparison with other technologies is presented.

Clathrate Hydrates: Clathrate hydrates are non-stoichiometric crystalline compounds formed when a guest molecule of suitable size and shape is incorporated into well-defined cages of the host lattice made up of hydrogen-bonded water molecules.15 Clathrates are not chemical compounds; the guest molecules inside the cage interact with the water molecules by van der Waals forces. The hydrates decompose readily upon thermal stimulation/ depressurization or both. In nature, natural gas hydrate exists in permafrost regions and marine sediments and it is considered as a huge potential energy resource for the future. The different types of gas hydrate deposits have been presented along with the potential of sustainably producing gas from these deposits in two recent reviews.16 17 Clathrate hydrates exist in three distinct polyhedral cavity structures, cubic structure I (sI), cubic structure II (sII) or hexagonal structure H (sH).15, 18 These structures are composed of cavities of different sizes that can accommodate one guest molecule per cavity in typical hydrates (except hydrogen molecules). Common hydrate formers include small hydrocarbons, fluorinated compounds, noble gases, carbon dioxide, small ether molecules, and hydrogen. Structure I hydrate consists of two different types of cavities. A unit cell of structure I hydrate consists of six 512 62 cavities, and two small 512 cavities created by 46 water molecules. Structure I is formed with molecules smaller than 6 Å such as methane, ethane, carbon dioxide and hydrogen sulfide. A unit cell of structure II hydrate consists of 16 small cavities (512) and 8 large cavities (512 64) made up of 136 water molecules. Propane, Iso-butane will 5



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form structure II. Nitrogen and small molecules (d Na+ > Na2+ + Na2+ > B3+ and 73 – 83% of each anion were removed in a single stage process without pre-treatment with CO2.51 The salt removal efficiency increased when the brine on the pellet surface was removed. Although the apparatus can remove salts effectively, it requires energy for gas/liquid mixing in the bubble generator and for the operation of dual cylinders with the piston to pelletize the hydrate slurry.

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Figure 4. Schematic of experimental apparatus for clathrate hydrate based desalination (adapted from Park et al.50) Enhanced hydrate formation kinetics in a fixed bed reactor with silica sand as porous media when the hydrate is formed from a gas mixture consisting propane as co-guest was reported recently.52 Kinetic experiments under the microscope showed that propane has the ability to draw dispersed water from silica sand towards the gas phase for hydrate growth. Based on this behavior, a conceptual clathrate hydrate based desalination process employing a fixed bed reactor and a simultaneous HyDesal-hydrate based gas separation hybrid process for CO2 capture, and seawater desalination process were presented and is shown in Figure 5. Although the conceptual process looks promising, it needs to be demonstrated using sea water. Further investigations need to be carried out to understand the effect of the sand bed surface area, salt rejection rate and efficiency of hydrate crystal separation form brine.

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Figure 5. Schematic block flow diagram of simultaneous HyDesal-hydrate based gas separation hybrid process for CO2 capture and seawater desalination process. Reprinted from Chemical Engineering Science, 117, P. Babu, R. Kumar, P. Linga, Unusual behavior of propane as a co-guest during hydrate formation in silica sand: potential application to seawater desalination and carbon dioxide capture, 342–351., Copyright (2014), with permission from Elsevier. Carbon dioxide hydrate formation and dissociation from 3 wt% NaCl solution dispersed in the interstitial pore space between the glass beads were investigated with three experimental procedures.53 Experiments with silica gel saturated with deionized water and 3 wt% NaCl solution were also conducted.54 It was reported that hydrate formation in a NaCl solution is faster than in deionized water. However, procedures to separate hydrate crystal from brine were not discussed. In order to increase the operating temperature, cyclopentane (CP) and cyclohexane (CH) was used as secondary hydrate former along with CO2.55 The hydrate formation rate of the double hydrate with CP and CH increased to 16 and 22 times higher than that of with pure CO2 hydrate. The salt removal efficiency also increased to 90%, while it was 70% for pure CO2 hydrate. Although the kinetics and salt removal efficiency improved, CP and CH are flammable and the process requires a secondary separation to remove CP and CH from the produced water after hydrate dissociation. The thermodynamics and kinetics of SF6 hydrate in NaCl solution for HyDesal application were investigated33. The hydrate phase equilibrium of SF6 hydrate with NaCl

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solution concentrations of 0, 2, 4 and 10 wt% was determined in the temperature range of 277 – 286 K and pressures below 1.4 MPa. As expected, experiments with 0, 2 and 4 wt% NaCl solution showed that addition of NaCl hinders the hydrate growth rate thereby resulting in low hydrate conversion. It is noted that the SF6 is the most potent greenhouse gas with a global warming potential of 23,900 times that of CO2.56 A system that combines clathrate hydrates and reverse osmosis system was also proposed.57 RO is the post-treatment process following the hydrate process. The RO concentrate should return to the feed stream of the hydrate process to increase the total water recovery. It was reported that salt rejection and energy consumption of the hydrate process are the key parameters for the economics of the hybrid system. An integrated thermodynamic approach to select suitable hydrate former prior to process design was proposed.58 Ethane and propane was found to be favourable hydrate former based on integrated thermodynamic approach.

Clathrate hydrate based desalination (HyDesal) milestones: The timeline progress of clathrate hydrate based desalination (HyDesal) process is presented in Figure 6. The various hydrate forming agents used over the period and reference to processes are made when applicable are presented along with the highlights.

Figure 6. Timeline of research progress of clathrate hydrate based desalination (HyDesal).

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Most of the literature studies focussed on the finding a good hydrate former to improve the kinetics of hydrate formation. The hydrate formers employed were methane, carbon dioxide, propane, chlorofluorocarbons, hydrofluorocarbons, SF6, cyclopentane, and cyclohexane. Despite all the effort there, a superior hydrate former has not been identified. Methane and CO2 are greenhouse gas and hence accidental release to the atmosphere may have an adverse environmental impact. Chlorofluorocarbons (CFC) usage has been restricted due to their contribution to ozone depletion. CFCs were replaced by hydrofluorocarbons (HFC). HFC’s are barely soluble in water and their hydrate formation kinetics are very slow.42 Cyclopentane and Cyclohexane are toxic and highly volatile. Moreover, when liquid hydrate formers are used, they need to be removed separately after decomposition of hydrate. To improve the kinetics of hydrate formation by increasing the interfacial gas-liquid contact area, several reactor configurations like stirring, bubbling column, spray column, fixed bed with silica sand, glass beads and variable volume reactor configurations were employed. Secondary treatment process after hydrate formation like washing, centrifuging etc., have been proposed to remove salt adsorbed to the surface of the hydrate crystals. These secondary treatment processes affect the process efficiency and the process economics. Although HyDesal process appears promising, so far, the HyDesal process has not yet been demonstrated successfully at the commercial scale.

Patent Review: Inventions have been made all over the world to desalinate seawater by employing clathrate hydrate formation process. Inventors have attempted to mitigate the challenges of slow kinetics, crystal separation from the brine and removal of salts occluded in the hydrate/brine slurry. A brief description of the key inventions made in clathrate hydrate based desalination (HyDesal) process is presented in Table 1.

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Table 1. Review of inventions. S. No. 1

Patent No.

Name of Inventor Wilm E. Donath

US2904511

2

US3027320 A

Ben B Buchanan

3

US3132096

Paul R. Walton

Key invention

Brief description

Comments

A conveyor was employed to separate the crystals from brine by transferring crystals from hydrate formation zone to 59 decomposition zone.

Hydrate formation was performed in an unstirred reactor configuration with separate hydrate formation and dissociation compartments. To remove salts occluded on hydrate, part of recovered water was used for washing step.

In the unstirred reactor, mass transfer limitation arises for gas diffusion into liquid due to the formation of thin film of hydrate at the gas-liquid interface which results in slow kinetics. Washing step reduces overall water recovery of the process. Additional energy input is required for the conveyor. Crystal contamination due to brine contact and uncontrolled hydrate decomposition as hydrate rise upwards could affect water recovery efficiency.

Hydrate formers like hydrocarbon were pumped into salt water at suitable depths which favored hydrate formation.60

Hydrate formed at suitable depth in salt water rose upward due to buoyancy effect. At confined space in the top, hydrate decomposes to produce desalinated water. In one of the embodiment, washing water was introduced countercurrent to hydrate above hydrate formation zone. Continuous process for In a vertical reactor, hydrates Hydrate crystals are prone to clathrate hydrate based were produced at the bottom of contamination as crystals are desalination was developed a reactor by using clathrate in contact with brine. Very

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by dividing reactor into compartments for performing hydrate formation, hydrate washing step and hydrate dissociation using partition plates with risers.61

4

US3856492

Donald L. Klass

A heat exchange liquid which was immiscible in water was used to utilize the heat of hydration for hydrate decomposition and as a solvent for gas hydrate former.62

5

US5473904 A

Boyun Guo, Robert E. Bretz,

A mobile tank to produce, transport and dissociate hydrate was employed.

former like liquid propane. Hydrate rose upwards through brine due to buoyancy effect. It was made to pass through risers present in partition plates which aided in effective separation of hydrate crystals from saline solution. From dissociation compartment, part of fresh water produced was utilized for the washing step. To solve the problem of hydrate washing, heat exchanger (HE) liquid having a specific gravity lesser than that of brine or water was introduced. HE liquid also aided in the easy flow of hydrates out of the hydrate formation vessel to decomposition vessel since hydrate formed had a specific gravity lesser or equal to that of HE liquid. Heat exchange liquid and gas were recycled back to hydrate formation reactor. Local supercooling of gas was performed by releasing it from the nozzle. Hydrates were thus

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high energy input of about 25 MWh/m3 of produced water was required only for compressing propane which makes the process uneconomical.

High energy input was required for pumping heat exchange liquid, brine, and HE-hydrate slurry. Mass transfer limitation for gas to diffuse from HE liquid to HE–brine interface to form hydrates would be another challenge in this invention.

Since hydrate crystals were collected in the feed water tank, contamination of 18


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Robert L. Lee Local supercooling of gas by depressurization was performed before mixing gas with cooled water to form hydrate.63

formed by bubbling gas into pre-cooled water. Hydrates were blown from the pipeline into mobile tanks because of pressure differences between pipeline (hydrate formation region) and mobile tank. After the mobile tank was filled with hydrate, hydrate dissociation was performed in the mobile tank by isothermal depressurization.

hydrates would not be prevented. Plugging of flowlines could be another limitation of the process.

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6

US5553456 A WO1996036564 A1

Richard A. McCormack

Hydrate formers were pumped to suitable ocean depth through concentric pipes.64

7

US5873262 A US6158239 A WO1999000330 A1 EP1007476 A1 CN1261861 A

Michael D. Max, Robert E. Pellenbarg

Methane gas was injected into vertical columns at water depths with suitable pressure and temperature for hydrate formation.65

8

US6028234 A CA2252491 A1 EP0909265 A1 EP0909265 A4 WO1998027033 A1

Robert Frederick Heinemann, David DaThe Huang, Jinping Long, Roland Bernard Saeger

Hydrate formation and separation from the brine was carried out in a single step by depositing hydrate crystals on a moving surface like a conveyor belt, rotating drum or reciprocating surface 66 permeable to water.

The clathrate former was pumped to a suitable ocean depth at high pressure and low temperature through concentric and coaxial pipes. Hydrate slurry was produced in the annular region and retrieved at the surface. Hydrate being positively buoyant rise upwards through seawater. The heat of hydration gets dissipated to the surrounding ocean through columns. Atomized water was introduced into the reactor to increase the contact surface area of gasliquid. Reaction zone consisted of a moving surface onto which formed hydrate crystals gets deposited. Moving surface was permeable to water, thereby separating hydrate from feed or brine. A pair of rollers and grinders were used to transport washed hydrates from production to decomposition zone.

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The high cost of pumping hydrate former gas to suitable depths and environmental concerns of gas release into the atmosphere due to hydrate dissociation are challenging. Also, there was the possibility of some hydrates dissociating while rising upwards which would reduce water recovery efficiency of the process.

Additional energy input and maintenance of moving surfaces would be challenging.

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9

US6180843 B1 CA2306461 A1 EP1025070 A1 WO1999019282 A1 CN1281420 A

Robert Frederick Heinemann, David DaThe Huang, Jinping Long, Roland Bernard Saeger

Fluidized bed reactor was employed for hydrate formation wherein fluidized bed consisted of hydrate particles.67

10

US6531034 B1 US6562234 B2 US6565715 B1 US20030029713 A1

Michael D. Max

Hydrate crystal separation from brine took place due to buoyancy effect.68

11

US6890444 B1 EP1646585 A2 WO2005044733 A2 WO2005044733 A3

Michael D. Max

Hydrate forming gas was introduced upstream of the reactor into seawater under conditions not suitable for hydrate formation.69

Seawater was introduced from the top whereas gas was introduced from the bottom of the reactor. Hydrate particles which act as fluidized bed particles provide more surface area for gas-liquid contact. The flow of gas and water was obstructed by these fluidized hydrate particles, which resulted in increased residence time which aided in further hydrate formation. A single reactor was employed consisting of hydrate formation and decomposition region. Hydrate formed at the bottom of the reactor rose upward towards hydrate decomposition region due to its positive buoyancy. Hydrate forming gas was introduced upstream of the reactor into seawater under conditions not suitable for hydrate formation. To achieve better dissolution of gas into seawater, gas was also pumped

Additional energy input and gas were required for maintaining fluidized bed.

Hydrate crystals were in contact with brine during hydrate crystal separation step which would lead to salts getting occluded on to hydrate.

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12

US6969467 B1 US20050247640

Michael D. Max, Jens Korsgaard

A method to enhance natural hydrate circulation system in hydrate fractionation column has been presented.70

13

US7094341 B2 EP1501762 A2 US7008544 B2 US7013673 B2 US20030209492 A1 US20050194299 A1 US20050184010 A1 WO2004035167 A2 WO2004035167 A3

Michael D. Max

A continuous process for desalination was developed by employing a porous restraint.71

at different locations inside the reactor. In hydrate fractionation column part of product water is reintroduced into the column to effect the faster rise of hydrate from formation zone at the bottom to dissociation zone at the top of the column. A porous restraint was employed below which thick impermeable mat of hydrate formed. A thick mat of hydrate prevented the flow of saline water across restraint. The portion of hydrate adjacent to restraint is dissociated by lowering pressure on the collection side of the restraint.

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Re-introduction of product water into column would reduce the overall efficiency of the process.

The major challenge would be to maintain steady state operation by maintaining a constant thickness of hydrate on restraint surface.

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15

16

US7485234 B2 EP2038223 A2 US7485222 B2 US20070284318 A1 US20080028781 A1 US20080053882 A1 US20080237137 A1 US20080264845 A1 WO2007145740 A2 WO2007145740 A3 US7560028 B1

Michael D. Max

Porous restraint panels in an enclosure were used for carrying out continuous clathrate hydrate based desalination process.72

Restraint panels with pores extending from one end of the restraint panel to another were used. The liquid hydrate forming material was employed. By allowing liquid hydrate to vaporize localized cooling was achieved.

Blake A. Simmons, Robert W. Bradshaw, Daniel E. Dedrick, David W. Anderson

Hydrochlorofluorocarbon, hydrofluorocarbon, chlorofluorocarbon compounds or their mixture were employed to prevent the dendritic growth of hydrate to minimize interstitial salt entrapment.73

US7569737 B2 US20070004945 A1

Tommy J. Phelps, Costas Tsouris, Anthony V. Palumbo,

To prevent washing step, ice formation over hydrate was carried out. As ice dissociates and flows it carries along with it salts occluded on hydrate.74

Hydrate formation was carried with seawater in a crystallizer with heat exchange liquid saturated with hydrate forming agent. Heat exchange liquid saturated with hydrate forming gas was used to capture and utilize the heat of hydration. After the formation step, hydrate and heat exchange liquid slurry were pumped to the separator. Hydrate forming gas and produced water were introduced in co-flow hydrate formation reactor. Following hydrate formation, rapid depressurization below hydrate

The need for pumping heat exchange liquid & hydrate slurry to the decomposition zone will increase the energy required for the process. HCFC, CFC & HFC are environmentally harmful.

Higher possibility of plugging of gas inlet nozzle.

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David E. Riestenberg, Scott D. McCallum

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stability zone was carried out to produce ice. Ice and hydrate rest on an elevated screen while the saline water gets drained from the bottom. As hydrate mass was warmed some of the ice melted and carried away some of the salts occluded on the surface.

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17

CN101289231 A CN101289231 B

Dongliang Li, Deqing Liang, Cuiping Tang

18

US8048309 B2 US20090028776 A1

John P. Osegovic, Michael D. Max, Shelli R. Tatro

19

CN 102351255 A

Dongliang Li, 3 stage hydrate formation Hydrate slurry from the top of Deqing Liang and dissociation process the 1st reactor was introduced to were employed.77 the bottom of next reactor. Concentrated saline water from the bottom of a reactor was reintroduced into a hydrate formation region of the previous reactor to improve the

Hydrate separation was performed on account buoyancy effect of 75 hydrate.

A vertical tubular reactor having upper and lower portions were employed for hydrate decomposition and formation respectively. Hydrate formation in the lower half of the reactor was carried out by stirring and crystal separation took place because of buoyancy effect. Pure water was recovered from the top of the reactor. A method to concentrate Sequestration of CO2 by brine solution by dissolving CO2 into seawater converting water into CO2 has been presented. To hydrate has been concentrate brine solution in which CO2 is dissolved, hydrate presented.76 based desalination has been proposed as a suitable medium.

As hydrate crystals rise from hydrate formation zone of the vertical reactor to decomposition zone through brine, salts may get occluded on to hydrate.

Potable water is a potential by-product since CO2 disposal is being performed by this process. Coupling HyDesal with CO2 sequestration could make the process economically feasible. 3 stage process could result in improved salt rejection. However, it could lead to increased energy requirement.

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US20120070344 A1 US7914749 US20070100178 WO2007002608 A2 WO2007002608 A3

Christopher Carstens, Wade Dickinson, Wayne Dickinson, Jon Myers

Modular gas hydrate system for desalination, natural gas storage, and thermal energy storage has been developed.78

21

US20140158635 A1 WO2013076737 A1 WO2013076737 A4 US9643860 B2

Amit Katyal

Hydrate forming gas was introduced into a salinewater in single tank reactor using microbubble generator system (microbubble nozzle and pump).79

efficiency of salt separation from seawater. Gas hydrate formation and dissociation modules have been incorporated in rack system to enable scalable and mobile hydrate based desalination process. The heat exchanger has been incorporated into the container for heating/cooling contents in the container. A single desalination tank consisting of hydrate formation and dissociation compartments was used. Microbubble enables further dissolution of gas in feed water compared to diffusion. Part of the fresh water produced was used for washing step to remove occluded salts. 2 reactors were operated in parallel so that when hydrate dissociates in one reactor, hydrate formation takes place in the other reactor.

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A modular and scalable solution for HyDesal process would be a promising technology for commercializing hydrate based applications.

Although hydrate formation is performed in a batch process, parallel reactors enable the continuous HyDesal process. However, washing step would reduce water recovery efficiency of the process.

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22

US20140223958 A1 WO2013049253 A1

Richard A. Sonic energy was used to Gas bubbles having a mean hydrate diameter between 10-3 and 10-2 McCormack, improve John A. nucleation.80 millimeter were introduced into Ripmeester salt water and sonic energy was supplied using an ultrasonic transducer to induce hydrate nucleation. Solid material particles like silica gel particles were introduced within the stream to increase gas-liquid contact surface area. A washing column was employed for removing the saline solution from the interstitial pore.

Additional energy would be required for employing ultrasonic actuator. Washing step would reduce overall water recovery efficiency of the process.

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The inventions made in relation to the HyDesal process have mainly focused on solving the critical problem of efficient hydrate crystal separation from the leftover brine. Since hydrate nucleation is a stochastic event, attempts were also made to reduce induction time by introducing microbubbles,

ultrasonic energy,

localized supercooling by

depressurizing liquid propane, etc. Although several attempts have been made to commercialize the HyDesal process, these efforts have not met with success because of additional technical challenges. Design criteria for HyDesal plant could not be met because of formation of small amounts of hydrate crystals, which made the process both costly and difficult for crystal separation from brine. Hydrate formation is an energy-intensive process because it requires low temperature and high-pressure conditions along with further energy input to separate crystals from brine. Secondary treatments like washing step needs to be introduced to remove salts occluded on the hydrates. This step reduces the overall water recovery of the process. Certain hydrate formers like CFC are toxic and pose risks to the environment. Hence, there is a need to address the challenges of energy efficiency and cost, crystal/brine separation and of course carry out the process in a manner that fully addresses potential environmental concerns.

Comparison of HyDesal with other technologies: Table 2 summarizes the advantages and limitations of the RO and MSF desalination technologies together with the HyDesal process. It is noted that the freeze desalination method is a variation of the HyDesal process. Reverse osmosis is the most widely used technology for seawater desalination. RO is a membrane separation process where seawater is pressurized above the osmotic pressure on one side of the membrane and water is recovered on the other side of the membrane. It operates at a pressure of 50 – 80 bar which accounts for most of the energy consumed. The operating pressure increases with increase in the concentration of salt in the brine which results in low water recovery.7 Typical water recovery in RO is less than 55%.4 The membranes used in RO are sensitive to impurities, pH, algae, bacteria and other foulants81-82. Therefore, the feed water needs to be pre-treated otherwise will result in fouling of the membrane.7 Simulation of the RO process with a plant capacity of 3185000 m3/day was performed in IMS DESIGN HYDRANAUTICS version 1.216.73 tool, developed by Nitto group. The plant's total capacity was divided into 17 trains which were operated in one-stage two pass process with considering pressure exchanger as energy recovery device. Each train consisted of 221 pressure vessels in the first pass, 95 pressure vessels in the second pass and each 28


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pressure vessel consisted of 6 SWRO membrane modules to treat the seawater with TDS concentration of 35000 mg/l (3.346 %w). In this simulation study, the first pass recovery percentage was 45% and the second pass was 85% to satisfy the design constraints. The study concluded that specific energy consumption of RO process was 3.55 kWh/m3 of water produced with the cost of production at 1.01 SGD/m3. Multi-stage distillation (MSF) is another popular technique for seawater desalination.83 In recent years, renewable energies such as geothermal, wind and solar energy have been proposed to power existing desalination plants to reduce the total energy usage and reliance on fossil fuel during the production of potable water.

Figure 7. Utilizing LNG cold energy for clathrate hydrate based desalination process (ColdEn-HyDesal) to strengthen the energy-water nexus (adapted from He et al.84 ). Recently, a model of the traditional HyDesal process was built and simulated in Aspen HYSYS.84 The study reported that utilizing LNG waste cold energy for HyDesal process (ColdEn-HyDesal) could reduce the specific energy consumption to 0.60 – 0.84 kWh/ m3, which makes the ColdEn-HyDesal process very attractive to the industry to strengthen the energy-water nexus further84. The schematic of the ColdEn-HyDesal process is shown in Figure 7. It can be seen that LNG replaces the external refrigeration cycle in the

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traditional HyDesal process to cool the hydrate former, seawater and coolant. The hydrate former and seawater are firstly cooled to the hydrate formation temperature by two substreams of LNG from the LNG regasification terminal, and then enter the hydrate formation reactor to form hydrates. Then the hydrate is separated from the hydrate formation reactor and goes into the dissociation reactor to dissociate into potable water and hydrate former by heat stimulation. Since the hydrate formation is exothermic, the heat generated in the hydrate formation reactor is removed by the coolant. One part of the LNG, as the heat sink, is utilized to cool the high-temperature coolant and absorb the reaction heat.

Table 2: Advantages/Disadvantages of Other Technologies Process

Advantage/ Limitations

Water Cost ($/ton Recovery (%) of water)

85-86

Up to 20% − High energy cost (Temperatures of 80 – 95 °C) − Low water recovery − Applicable for high concentration of TDS 4 Membrane − Requires pretreatment Up to 55% (RO) − Pressure of 50 – 80 bars − Very sensitive to impurities − Energy-intensive − Replacement of membrane frequently − Low water recovery HyDesal NA* − Low maintenance − High Water recovery − Applicable for a high concentration of TDS Up to 20% Freeze − Energy-intensive desalination − Low total water recovery * Experimental data not available in the literature Distillation (MSF)

Specific Energy Consumption (kWh/m3 of water)

0.56 1.7587-88

13.5 – 25.589

0.45 – 0.6639, 87

1.85 – 36.390

0.46 – 0.529, 38-39

0.60 – 65.2984, 91-93

0.3492

11.992

The two important factors for commercialization of any desalination technology are water recovery and energy consumption. As can be seen in Table 2, for HyDesal, the cost of producing one ton of potable water is comparable to the more mature RO and is significantly 30


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lower than MSF. Figure 8 shows the 2×2 matrix of these technologies as a function of energy consumption and water recovery. Considering that the HyDesal process operates at a temperature above the freezing point of water, it would be advantageous to use the HyDesal process as long as the compression penalty is not prohibitive when LNG cold energy is used.

Figure 8. Potential of HyDesal process based on performance in comparison with Multi-stage distillation (MSF), Reverse Osmosis (RO).

Prospects and Challenges: The HyDesal process was first proposed in the US more than 50 years ago, as a potential technology for desalination. However, its application on an industrial/commercial scale has been unsuccessful due to its high energy consumption for achieving the required low-temperature operating conditions, slow kinetics of hydrate formation and difficulty of hydrate crystal separation from brine. The first major challenge of cold energy requirement (low-temperature requirement for hydrate formation) can be alleviated by utilizing LNG cold energy to replace the external refrigeration cycle in HyDesal process. LNG is regasified in LNG regasification terminals by

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employing seawater as the heat source. During the regasification process, the cold energy stored in LNG is wasted and can be utilized for HyDesal process. According to Organisation for Economic Co-operation and Development, Asia accounts for more than half of the total LNG imports in the World. Globally, LNG regasification capacity has nearly doubled from 2004 to 2014. China has built 17 LNG regasification terminals over the past decade, will have 9 more coming new terminals by 2018 while Singapore is working on plans to build a second regasification terminal. It is expected that India will likely develop many such regasification terminals along its coast. In medium-term, inter-regional gas trade is set to rise by 40%. LNG will contribute to 65% of this increase94. Hence, developing sustainable technology based on LNG cold energy utilization has great market potential. In Singapore, a potential market for LNG cold energy utilization with HyDesal process is the industrial water (IW) market. In Jurong island which is Singapore’s industrial hub, IW is used for non-potable use like industrial cooling. Since SLNG (Singapore Liquified Natural Gas) regasification terminal is also located at Jurong island, there is a potential to produce and consume desalinated IW in Jurong island by using its existing IW delivery network. Recently, Wang and Chung proposed a hybrid process of freeze desalination (FD) and membrane distillation (MD) utilizing the cold energy from the LNG regasification unit.95 The HyDesal process is advantageous compared to the traditional freeze distillation (FD) method as the water recovery in an FD is typically low, about 19% in a two-hour operation.95 It is also plausible to develop hybrid approaches by coupling the HyDesal with MD to utilize the potential advantages of both these independent approaches. The utilization of LNG cold energy dramatically reduces the total energy consumption of the HyDesal process by up to 90% lower.84 The limitations of slow kinetics for HyDesal can be overcome through the judicious choice of favorable guest gas/liquid and improved reactor designs. In the literature, mostly CO2, methane, propane, cyclopentane, and refrigerants like CFC were employed. Faster kinetics with suitable promoters could enable combining HyDesal process with gas separation process at industrial scale to increase its commercial viability and improve its overall efficiency. Limitation of effective crystal separation needs to be efficiently mitigated to minimize crystal contamination by leftover brine. One such novel approach is employing a fixed bed reactor as proposed by Babu et al.52 Propane as co-guest hydrate former has the ability to

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draw dispersed water from porous media to form hydrates outside the porous bed. This effectively facilitates a natural separation of hydrate crystals from leftover brine. Such innovative reactor designs can efficiently remove hydrate crystals from the hydrate formation zone.96 A gas mixture with propane as co-guest also resulted in faster hydrate formation kinetics. With further research and development to identify better hydrate forming agents and reactor designs can make HyDesal process a more attractive technology for seawater desalination and brackish water treatment. Apart from seawater and brackish water desalination, there are high salinity solutions which requires treatment. One such example is produced water from shale gas fields, which have up to 25% salinity. Conventional desalination technologies have limited efficiencies to treat such high salinity. Hence, there is a high potential for the application of HyDesal process to treat produced water.55, 97-98

Conclusion: Desalination is one of the most promising technologies to mitigate an emerging water crisis. Although reverse osmosis is the most widely employed desalination technology in the world, it is energy intensive. Hence, there is a need to develop innovative energy-efficient technologies to strengthen the energy-water nexus. Clathrate hydrate based desalination (HyDesal) is one such technology. In this work, we have comprehensively reviewed the stateof-the-art in the literature including patents and highlighted key technical contributions. Although HyDesal technology has been studied for past 70 years, it never commercialized due to slow kinetics of hydrate formation, difficulty in hydrate crystal separation from brine without contamination and high refrigeration cost. With further research and development to identify better hydrate forming agent, innovative reactor design and utilizing waste LNG cold energy, HyDesal can be a sustainable solution for desalination.

Acknowledgment: The work was funded in part under the Energy Innovation Research Programme (EIRP, Award No. NRF2014EWTEIRP003-006), administrated by the Energy Market Authority (EMA). The EIRP is a competitive grant call initiative driven by the Energy Innovation Programme Office, and funded by the National Research Foundation (NRF). The authors would like to acknowledge funding support from our Industrial collaborator, Shell Energy Singapore. This study also has been conducted with the support of the Korea Institute of Industrial Technology (EO160061). 33



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List of Figures: Figure 1: A simple block flow diagram illustrating the concept of clathrate hydrate based desalination (HyDesal). Figure 2: Hydrate phase equilibrium of CO2 in presence of pure water and various concentrations of NaCl solutions. Figure 3: Effect of water recovery on salt concentration in brine. Figure 4: Schematic of experimental apparatus for clathrate hydrate based desalination (adapted from Park et al. 50) Figure 5: Schematic block flow diagram of simultaneous HyDesal-hydrate based gas separation hybrid process for CO2 capture and seawater desalination process. Reprinted from Chemical Engineering Science, 117, P. Babu, R. Kumar, P. Linga, Unusual behavior of propane as a co-guest during hydrate formation in silica sand: potential application to seawater desalination and carbon dioxide capture, 342–351., Copyright (2014), with permission from Elsevier. Figure 6: Timeline of research progress of clathrate hydrate based desalination (HyDesal). Figure 7: Utilizing LNG cold energy for clathrate hydrate based desalination process (ColdEn-HyDesal) to strengthen the energy-water nexus. Figure 8: Potential of HyDesal process based on performance in comparison with Multi-stage distillation (MSF), Reverse Osmosis (RO).

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Synopsis: A comprehensive review of clathrate hydrate based desalination process, which can utilize LNG cold energy to sustainably address energy-water nexus is presented.

Concept Graphic:

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Biography of Authors Dr. Ponnivalavan Babu

Dr. Ponnivalavan Babu currently serves as a Postdoctoral Research Fellow in the Department of Chemical and Biomolecular Engineering at the National University of Singapore (NUS). He received his PhD in Chemical Engineering from the National University of Singapore in 2014. His Ph.D. research involved identification of promoter, and enhancing the kinetics and maximizing the CO2 capture capacity from fuel gas mixture via gas hydrates. He has received the “Outstanding Ph.D. Thesis” award from the 9th International Conference on Gas Hydrates and Outstanding Young Researcher award from AIChE Singapore Local Section in 2017. His research interests are in the areas of clathrate (gas) hydrates, carbon dioxide capture, LNG cold energy utilization processes, seawater desalination and energy storage and recovery.

Mr. Abhishek Nambiar

Mr. Abhishek Nambiar is a Research Engineer in the Department of Chemical and Biomolecular Engineering at National University of Singapore. He received his MSc in Chemical Engineering at National University of Singapore in 2015. His current research

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interests include clathrate (gas) hydrate, clathrate hydrate based desalination and LNG cryogenic energy utilization.

Dr. Tianbiao He

Dr. Tianbiao He currently serves as a Postdoctoral Research Fellow in the Department of Chemical and Biomolecular Engineering at the National University of Singapore (NUS). Prior to joining NUS, Tianbiao received his PhD in Refrigeration and Cryogenics Engineering from the Institute of Refrigeration and Cryogenics at Shanghai Jiao Tong University (SJTU) doing research on small-scale moveable LNG technology. His research interests now include gas hydrate based desalination technologies, LNG cold energy utilization processes, natural gas liquefaction technologies, and process modelling, optimization and integration in chemical and cryogenics engineering.

Dr Iftekhar A Karimi

Iftekhar A Karimi is a Professor in the Department of Chemical & Biomolecular Engineering at the National University of Singapore. He has published extensively on process modeling, 43



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design, simulation, integration, operations, and optimization; and led several industrycollaborative research and consulting projects. His current research interests include energy systems, energy efficiency, process integration and intensification, natural gas supply chain, planning and scheduling, and surrogate modeling. He received DuPont's Best Manufacturing Technology award in 1994 and 2002 Best Paper Award of Computers and Chemical Engineering.

Dr Ju Dong Lee

Dr. Ju Dong Lee is an executive director in Offshore Plant Resources R&D Center at Korea Institute of Industrial Technology (KITECH). He received his Ph.D. in the Department of environmental engineering from Kyungpook National University, Korea, in 2001. In 20022006, as a postdoctoral research fellow at the University of British Columbia, he studied fundamental researches of gas hydrates. Then he joined KITECH in 2006 and conducted researches to apply the basic investigations of gas hydrates to industry. His recent research has focused on the development of hydrate-based water treatments, multi-phase flow loops and in-situ Raman spectroscopy to monitor the gas hydrates in micro and macro-level. He has published 47 scientific papers and has registered about 30 patents related to gas hydrates.

Dr Peter Englezos

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Dr. Peter Englezos is Professor and Head of the Department of Chemical & Biological Engineering at the University of British Columbia (UBC). He is an internationally recognized expert on clathrate or gas hydrates, carbon capture and storage and chemical thermodynamics. He has published extensively and is among the most cited researchers in the field of gas hydrates. He taught courses on thermodynamics, conceptual process design and optimization. He has served on several University committees at various levels and Nationally and Internationally including tenure and promotion, review of academic programs, expert panels, review of research proposals, editorial board member and others. Dr. Englezos received several honours/awards including UBC Izaak Walton Killam Memorial Faculty Research Fellowship, Fellow of the Tokyo Electric Power Company Chair at Keio University and Fellow of the Canadian Academy of Engineering.

Dr Praveen Linga:

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Professor Praveen Linga holds bachelor’s, master’s and PhD degrees in Chemical Engineering from the University of Madras, Indian Institute of Technology (IIT) Kharagpur and the Univeristy of British Columbia, respectively. His research interests are in the areas of clathrate (gas) hydrates, storage and transport of fuels, carbon dioxide capture, storage & utilization (CCS & U), seawater desalination and recovery of energy. His research group at NUS particularly focuses on enhancing the kinetics of hydrate formation for several applications of interest by developing novel reactor designs, experimental methods and techniques. Up to date, he has published more than 85 research articles and delivered about 50 keynote/invited talks and seminars. He has won numerous awards including the 2017 NUS Young Researcher Award, 2017 NUS Engineering Young Researcher Award, 2017 Energies Young Investigator Award and the 2017 Donald W. Davidson Award for outstanding contributions to gas hydrate research.

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A Review of Clathrate Hydrate Based Desalination ... - ACS Publications

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