Jianan Zheng,Fanbao Cheng,Yuanping Li,Xin Lü,Mingjun Yang,*

1 Key Laboratory of Ocean Energy Utilization and Energy Conservation of Ministry of Education,Dalian University of Technology,Dalian 116024,China

2 Institute of Science and Technology,Shenzhen Branch,China National Offshore Oil Corporation,Guangzhou 510420,China

3 CNOOC Research Institute Co.Ltd.,Beijing 100027,China

Keywords:Gas hydrate Desalination Separation Phase equilibria Hydrocarbons Mixture

ABSTRACT The shortage of freshwater boosts the development of seawater desalination technology.As a novel method,the hydrate based desalination technology has been put forward for decades and achieved considerable development in the past years.This review focuses on the experimental progress at the aspects of the hydrate former choice,formation promotion and ion removal efficiency and conceptive innovation of hydrate separation and energy utilization.It should be noted that gaseous hydrate former with low formation pressure and insoluble liquid hydrate former are worthy for further study.Besides,the water migration caused by propane deserves to be investigated much more deeply for the potential value of wide application.Moreover,the utilization proposal of LNG cold energy brings more possibility of commercial application.In a word,the hydrate based desalination technology is hopefully an environment friendly,low-cost and widely used desalination technology in the near future.

1.Introduction

Fresh water is the basic desire for human to survive in a comfortable way.Along with the acceleration of urbanization process,the fresh water scarcity has become a pervasive problem troubling the life quality,especially in the developed inshore areas.Therefore,seawater desalination process has been conducted to mitigate the fresh water scarcity.The demand of seawater desalination constantly increases for agriculture sustaining,population supporting,and economic development [1].Till now,various seawater desalination technologies have been investigated and developed[2,3].

The traditional desalination technologies,widely used,include distillation and membrane methods,such as:multi-stage flash distillation(MSF),membrane distillation (MD),forward osmosis (FO),reverse osmosis(RO),and electro dialysis (ED)[4].The current desalination plants worldwide are basically using reverse osmosis method,while some old MSF plants are also operational.These commercial processes have about 90%of the total desalination capacity,yet there still are a lot of problems on energy consumption and cost for the reason that the desalination processes require enormous energy to realize the separation of salts from seawater[5,6].Therefore,there is still a need to update the existing technologies or develop innovative methods that can process seawater at a large scale.Meanwhile,minimizing the energy costs,maximizing the water recovery,and becoming more environment friendly are the basic requirements.In this view,more and more researchers have paid close attention to the hydrate based desalination(HBD)technology[7].

The HBD process is based on the formation and dissociation of clathrate hydrates.Clathrate hydrates are crystalline compounds made up of water and certain hydrate former molecules(e.g.,methane or carbon dioxide)by van der Waals forces [8,9].These former molecules are trapped in the cavities created by hydrogen-bonded water molecules.Besides,the formation and existence of hydrates need specific temperature and pressure (p-T)conditions.It is reported that gas hydrates are mostly non-stoichiometric and the occupancy of the hydrate cavities are about 70%[7],depending on the hydrate formers and p-T conditions.It should be noted that the hydrate crystals can exclude the dissolved ions from the aqueous phase when the hydrates form.If the hydrates(solid)are separated from the remaining brines(liquid),the fresh water can be obtained after hydrate dissociation.Above mentioned is the basic theory of hydrate based desalination process.

The concept of using clathrate hydrates is not brand new in seawater desalination field.Hydrate based desalination was proposed in the 1940s and then developed slowly[10,11].And then,researchers developed a few proposals on the extraction of fresh water from seawater via clathrate hydrates[12].During these years,the researches on the hydrate formers and their characteristics are the hotpot all the time.It has been found that the desalination cost of hydrate based technology may be competitive compared with the traditional desalination technologies.Yet,the difficulties in separating hydrate and brine,as well as hydrate former and dissociated water,are still the main technical obstacles.Besides,the microcosmic,mechanism and morphology investigation in hydrate based desalination technology gradually attract more and more researchers[13-15].

There is still a long way to solve many complex and fundamental issues,before the hydrate desalination technology becomes commercial reality.In general,researchers prefer to investigate the scientific problems concerning hydrate nucleation,agglomeration and growth,ion removal and recycling hydrate formers.This review analyzes and discusses the development of hydrate based desalination technology and concludes the progress on hydrate formers,formation enhancement and novel proposals based on quantities of up to date literatures.Meanwhile,we put forward several conception and suggestion for other researchers'consideration.

2.Feasibility and Efficiency

Hydrates are one kind of crystals,made up of hydrogen-bonded water molecules and guest(hydrate former)molecules inside the hydrogenbonded cages.When meeting specific temperature and pressure,hydrates can appear in the liquid seawater that contains former in the form of solid phase,as shown in Fig.1.The hydrate crystals accompany the exclusion of dissolved ions and salts during formation process[11].Among the seawater desalination methods,the hydrate based method is more similar with the freezing method.Hydrate based method needs certain high pressure and low temperature to provide driving force,and freezing method requires much lower temperature.So,it is hard to judge them from an energy point of view.However,it is reported that if the freezing rate is not quite slow,the salts may be trapped in ice and are hard to be separated[16].Thus,the freezing method still needs further mechanical purification,like washing,centrifuging and so on[17].On account of this,researchers are willing to attempt hydrate based method for the essential difference between ice and hydrate.

Fig.1.Proposal sketch of hydrate based desalination and gas separation.

Recently,quite a few researchers worldwide have carried out the removal efficiency tests of hydrate based desalination.In 2012,Cha and Seol[18]measured the removal efficiency of high salinity(9 wt%)water using CO2and insoluble hydrocarbon to form double hydrates in a stirring reactor.They achieved higher than 90%removal efficiency of salts (Na+,Mg2+,K+,and Ca2+).In the same year,Yu et al.[19]studied desalination effects of CO2hydrate with different concentrations sodium chloride solution progressively.They analyzed the relationship between the initial salinity and remnant salinity and suggested the remnant salinity should be controlled below 4 wt%.In 2013,Liu et al.[20]adopted multi-grade hydrate based desalination using CO2with CH3CCl2F(R141b)additive.They found that the addition of R141b (1:70 volume ratio with seawater)improved the removal efficiency by three times,and the removal rates were able to reach up to 98.4%with the grade increasing.Later in 2014,Kang et al.[4]tested the removal characteristics of dissolved ions by squeezing gas hydrate pellets.They found no obvious difference (less than 1%)of removal efficiency between cation and anion using CO2as hydrate former.Moreover,the removal efficiency of each ion using methane is lower than using CO2for the difference of relative hydraulic pressure.Recently in 2017,Kang et al.[21]used the same apparatus to produce CO2hydrate pellets from salt water and confirmed the removal efficiency of dissolved ions higher than 75%by ICP-AES(inductively coupled plasma atomic emission spectroscopy).Therefore,there is a great possibility of hydrate based desalination as a seawater treatment method.

Laboratorial experiments have proved the feasibility of hydrate based desalination,but there are still two problems to be solved by way of innovation in the near future.One is the hydrate former choice and operation condition,which has great effects on the production cost.This needs systematic investigation on various types of formers,considering the phase state,solubility,safety,price,recycling and so on.Another is the further separation and purification,which decides the removal efficiency of fresh water.In the following two sections,the development and prospect of the two aspects will be discussed in detail respectively.

3.Hydrate Former Investigation

As mentioned in the previous sections,the hydrate former is the key factor affecting the fundamental characteristics.Hydrates formed by different formers are different on the aspects of the induction time,formation p-T condition,density,crystal structure and so on.The prior studies are mostly on the single hydrate former,and the latter studies are mainly about the mixed formers.Due to the advantage of gaseous hydrate formers on easy separation,gaseous materials are greatly likely to be basic hydrate formers with necessary support of liquid or soluble solid co-formers.A summary on different types of hydrate formers has been listed in Table 1.This section will discuss the various formers according to the phase state and solubility for the convenience of further separation.

3.1.Gaseous formers

Gases are easily separated from water and should be the optimal choice as the formers of hydrate based desalination,if the cost forpressurization is ignored.As mentioned above,the formation of gas hydrate needs a certain pressure as the driving force.Thus,not all gases that can form hydrates will be chosen as the formers.The common evaluation basis of gaseous formers is the hydrate phase equilibrium condition,the fundamental p-T parameters to know when the gas hydrates is stabilized.Hence,the hydrate equilibrium conditions should be measured systematically as the fundamental data for the application of HBD process in advance.

Table 1 A summary on different types of hydrate formers potential for gas hydrate based desalination

The potential formers may be CO2and propane for their moderate formation p-T conditions.CO2hydrate has been widely investigated for decades [25].The formation characteristics of CO2hydrate have already been investigated adequately [22,26-28].The researches on CO2hydrate phase equilibrium in salt solutions of different concentrations have also been conducted[29-32].In order to make the formation condition clear,phase equilibrium experiments of hydrates are usually conducted to determine the formation p-T conditions,for the design of operation conditions.Table 2 lists several experimental result of hydrate phase equilibrium using CO2former and CO2-containing mixture formers.It should be noted that single CO2hydrate is bound to be used with other formers,otherwise the p-T conditions are hard to be accepted.

Table 2 Several phase equilibrium experiments of CO2-containing hydrate systems

Compared with CO2hydrate,propane hydrate is quite easy to form at several atmospheric pressures [41].However,propane is rarely used as hydrate former alone for its flammability.Thus,propane is usually appeared as co-former of other gas like CO2[14,42].Yang et al.[23]employed the mixture of CO2and propane as hydrate formers and experimentally confirmed the thermodynamic improvement of propane by several degrees at the same gas pressure.Besides,propane is also mixed into gas mixture to assistant gas to form hydrates for CO2capture,natural gas storage as well as hydrogen storage[43-49].

In terms of hydrate density,the density of propane hydrate is just lower than water,while CO2hydrate's density is slightly larger than water [50].Besides the phase equilibrium condition,the density of double hydrates like CO2-C3H8hydrates should also be investigated for the possibility of developing efficient separation based on the density difference.So,if gas leak-free during the whole process,xenon may be an optional for the density of xenon hydrate is near double of water.Nevertheless,propane or propane containing are thought as the most potential gaseous hydrate formers.

3.2.Liquid formers

Fig.2.Desalination process using CO2with cyclopentane[18].

Some researchers realized the high energy cost of gas supply and put forward liquid hydrate based desalination method.Common liquid formers include tetrahydrofuran(THF),hydrocarbon,hydrochlorofluorocarbon(HCFC)and so on[51,52].Due to the intermiscibility of THF and water,it is rarely used for desalination because the separation of THF and water will be a trouble.Thus,the insoluble hydrocarbon and HCFC are preferred(see Fig.2).Yu et al.[53]employed HCFC-R141b to investigate the hydrate formation process and the salinity change in the residual solution.Song et al.[54]tested the removal characteristics of separating heavy metal ions from aqueous solution by forming HCFC-R141b hydrate at atmospheric pressure.Their final removal efficiencies of Cr3+,Cu2+,Ni2+and Zn2+are 88.01%-90.82%.Recently,cyclopentane (CP)has attracted much attention for hydrate based desalination[55-58].Besides,CP and its derivatives are also chosen as co-former of gas hydrate[13,18,38,59].

It should be noted that the liquid hydrates at atmospheric pressure are easy to be centrifugally separated and wished.The advantage of liquid formers is the giant decrease of pressurization cost,but the enormous volume of liquid formers is a new question for large scale production.As for the gaseous and liquid formers,both of them still need to be further developed.

3.3.Functional additives

Fig.3.Common enhancement methods for hydrate formation.

As mentioned above,single former or double formers are still hard to satisfy the HBD requirement.As shown in Fig.3,functional additives are as one kind of chemical method to promote the formation of hydrates.As a result,additives are usually used to promote the formation of hydrates[60-63].Surfactant as kinetics additives are often used in the formation promotion studies,like sodium dodecyl sulfonate (SDS)[64],sodium dodecyl benzene sulfate(SDBS)[65],alkyl polyglycosides(APG)and linear alkyl sodium,and so on.They can reduce the surface tension of aqueous solution and result in solubilization,wetting,emulsifying,foaming and other effects to change the interface status of the system and the mechanism of hydrate formation,so as to either can accelerate the rate of hydrate formation or can increase the gas density of hydrates.But they have negligible solubility in water and are not particularly suitable for hydrate based desalination.In addition,some large molecules including tetrahydrofuran(THF)[61,66],tetra-butyl ammonium bromide (TBAB)[67-71]and cyclopentane (CP)[59,72]are served as additives as well.As co-formers of slight amount,the functional additives can greatly reduce the hydrate formation pressure and promote hydrate formation rapidly by participating in the construction of the crystal structure of the hydrate[73,74].

Due to the limited promoting effect of a single former,it is better to combine the advantages of several different formers or use composite additives in hydrate formation to receive the desired results.The so called C3H8,CP or R141b may be a consideration for co-formers of CO2hydrate.As a result,the characteristics of polysystem containing former molecules and hydrates will be investigated thoroughly and completely in the period ahead.

4.Formation Enhancement

After the suitable formers are chosen,the formation enhancement,especially the formation rate and induction time,should also be taken into consideration seriously.The induction time is related with the elapsed time for hydrate nucleation[75-77].It is an energy consumption process for hydrate formation,thus the time that hydrate formation need is as short as possible.That's to say,it is essential to moderate the formation pressure of hydrates and improve the formation rate at the same time,from the perspective of physical and chemical techniques(see Fig.3).The chemical enhancement has been discussed in above section,and this section is focused on three kinds of physical methods.

4.1.Mechanical

The formation of water and gaseous former should be assisted by mechanical methods to ensure the hydrate formation amount and rate.Mechanical enhancements,shown in Fig.4,include stirring[78],bubbling[79]and spraying[80].Magnetic stirring is one kind of conventional method to increase the contact surface of gas and water[81].In the opinion of some researchers,stirring is particularly suitable for the immiscible liquids[82].However,it is not optimal for desalination for the following reasons:The stirring rod may be the original occurrence of hydrate and bring separation trouble.And if the hydrates form massively in short time,it is quite difficult to rapidly separate the hydrates from the brine[83].

The bubble method is by a bubble plate creating gas bubbles that move to the top from the bottom.Gas hydrates could form from the gas-liquid boundary around the bubbles and grow into hydrate particles in the bottom side of the bubbles gradually [79].The spraying method is to spray liquid water downward through a gas phase to increase the contact of gas and water[80].Bubbling and spraying may be more suitable as the enhancing formation methods of hydrates because they have little influence on the further separation of hydrates.

4.2.Porous media

Porous media are often used as a carrier improve the formation kinetics in hydrate researches[75,84].The porous media can enlarge the contact surface and are helpful for the hydrate nucleation[85,86].Lots of studies concerning CO2hydrate are in porous media and it is reported a good CO2hydrate stability in porous media[84,87,88].Besides,the presence of porous media with specific wettability may be a carrier to make the immiscible water and hydrocarbon co-attached on the media surface,increasing the contact between them as well as gas[38].The trouble of using porous media to enhance hydrate formation is mainly the hydrate separation from the porous media.In the recent years,Linga's lab has found the water migration through the porous media and hydrate towards gas phase region,with propane or cyclopentane as co-formers [13,14,59].In other words,the hydrates firstly form at the surface of wetted porous media and gas,and then will grow gradually out of the porous media.Based on this,they designed a hydrate based desalination apparatus[89].As shown in Fig.5,it is an annular bed reactor filled with bed media,the seawater and propane are fed into the annular bed reactor to enable the hydrates form.The hydrates will grow out of the bed media and the cylindrical mesh.And then,the hydrates that have stick to the cylindrical mesh can be scraped off by a rotating scrapper and collected at the reactor bottom to be dissociated to recover desalinated water.

4.3.Electric field

According to the previous studies,the salts prevented hydrate formation by changing the phase equilibrium condition and delaying the hydrate nucleation[4,90,91].The salt ions in the solution are ionized dispersed among water molecules by coulombic force[92].It is reported that the hydrate formation process will be inhibited by salt ions,and the inhibition of the anion on gas hydrate formation is stronger than that of the cation[91,93,94].That's to say,the containing-seawater system will inhibit the formation of hydrates by itself.Given this,applying a weak electric field through the reactor during hydrate formation is a good idea to solve this problem[95].The existence of electric field can lead the ions rearrange orderly,so that the inhibition of salt ions will be whittled.

Fig.4.Sketch of three mechanical enhancement for hydrate formation.

Fig.5.A novel apparatus for desalination via clathrate hydrate[89].

5.Novel Apparatus and Proposal

5.1.Pelleting

Generally,the new formed gas hydrates in aqueous solution will appear as slurry mixtures of hydrate crystals and brine solutions.It is still very difficult to separate gas hydrate particles from slurries,because salt ions may be included into hydrate particles and adsorbed onto the surface of hydrates.Therefore,the form of hydrate pellets is proposed to complete the separation process of gas hydrate and brine liquid phase by Park and Lee[90,96].They developed a pelleting equipment with squeezing operation in a continuous process by a dual cylinder unit.The hydrates are produced in the seawater and extracted to hydrate pellets from the reactor containing hydrate slurries.However,the pelleting method is not suitable for desalination well to some extent,accounting for the conflict between squeezing and separation.Thus,the pelleting method has been successfully applied on energy storage,like solidified natural gas[97].

5.2.Integrated proposal using LNG cold energy

Due to the high energy cost of gas supply,CO2is not the optimal former choice for hydrate based desalination.Recently,the utilization of LNG(liquefied natural gas)became a focus for the researches,requiring cold energy[98].In general,the storage temperature of LNG terminals is at the level of 111 K,yet the utilization of LNG cold energy was barely satisfactory.The giant cold energy released by LNG vaporization happens to be suitable for the cold energy demand of gas hydrate based desalination process.

Song et al.[99]designed an integrated flue gas hydrate based desalination and CO2capture system using LNG cold energy.As shown in Fig.6,the hydrate formers are supplied by the flue gas and the cold energy is provided by LNG vaporization.The final emission of LNG is just CO2mixed in the flue gas,and the hot flue gas can be used for HBD process in return.Besides,most LNG terminals are distributed in the coastal cities where they may have enough seawater but lacking freshwater.Therefore,it is very likely for the coastal areas with LNG terminals to conduct the gas hydrate based desalination application utilizing the cold energy of LNG vaporization in the near future.

6.Conclusion and Forecast

Based on the research progress in recent years,the development and application of hydrate based technology on seawater desalination have been summarized in this review.After a systematic review of literatures,it is found that quantity experiments have been conducted on the hydrate former choice,formation promotion and ion removal efficiency.It should be noted that gaseous hydrate former with low formation pressure and insoluble liquid hydrate former are worthy for further study.Besides,the water migration caused by propane should be studied much more deeply for the potential value of wide application.Moreover,the utilization proposal of LNG cold energy brings great hope for the commercial HBD technology.

However,few novel apparatuses and innovative techniques have been put forward in these years.The following research attention should focus on the high-efficiency separation and ion removal efficiency,making hydrate based technology more convincing.After that,the energy analysis and cost estimate are just meaningful.In a word,the HBD technology is bound to be the most potential with great advantages from the perspective of the principle and process.And we expect an environment friendly and low-cost desalination technology using clathrate hydrates to become commercial soon.

Fig.6.The scheme of a flue gas hydrate based desalination system utilizing LNG cold energy[99].