Technical Field

The present invention relates to a clathrate hydrate desalination method and an annular bed reactor for use in the clathrate hydrate desalination method.

Background

Many countries suffer from the shortage of freshwater due to the increased population as well as the large expansion in industrial and agricultural activities. The fresh water resource is only around 2.5% of the total volume of water available in our planet. Of these fresh water resources, 70% is in the form of ice and paramount 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. Accordingly, seawater has become an important source of fresh water because it is one of the most abundant resources on earth.

Desalination is the process of removal of salts from seawater or brackish water and is believed to be a core technology in alleviating this problem. Multi-stage flash distillation and reverse osmosis are the traditional desalination technologies used for sea water desalination. The main drawback of these processes is that they are energy intensive. The energy cost of supplying the projected global water need with present technologies is excessive - especially in a carbon-constrained world.

Known clathrate hydrate based desalination processes have not been suitable for being scaled up at an industrial scale. One of the known methods pumps hydrate forming guest species directly in the ocean at depths of 1000 m. Subsequently, the formed crystals are separated and pure water is obtained by dissociating the crystals. However, the major challenge is separating the hydrates from the seawater and the stability of hydrate crystals. Another method proposed the use of halogenated hydrocarbons as guests for hydrate formation. However, these refrigerants are banned due to their environmental impact and cannot be used. The challenges which remain in commercialising and scaling up clathrate hydrate based desalination process relate to slow hydrate formation rates, difficulty in separating the hydrate crystals from the brine solution and energy required for the process. There is therefore a need for an improved clathrate hydrate based desalination process.

Summary of the invention

The present invention seeks to address these problems, and/or to provide an improved clathrate hydrate desalination method.

In general terms, the invention relates to a clathrate hydrate desalination method with enhanced rate of formation of hydrate, thereby improving the efficiency of obtaining desalinated water. Further, the method of the present invention minimises the energy consumption and therefore, would be a greener method compared to prior art methods. The method also enables easy separation of brine and hydrates. In this way, post- treatment of the desalinated water is minimised or even eliminated. In particular, the method of the present invention can be easily scaled up to an industrial scale.

According to a first aspect, the present invention provides a clathrate hydrate desalination method comprising: pumping feedwater into an annular bed reactor, the annular bed reactor comprising bed media;

adding a hydrate former to the annular bed reactor to enable formation of hydrates;

collecting the hydrates formed; and

dissociating the hydrates to obtain desalinated water and re-formed hydrate former.

The hydrate former may be any suitable hydrate former. For example, the hydrate former may be any suitable hydrate former which is able to draw water towards it and form hydrates. The hydrate former may be in the form of a gas or liquid. For example, the hydrate former may be, but not limited to, C ₃ H ₈ , C0 ₂ , H ₂ , CH ₄₎ C ₂ H _6> N _2> or a combination thereof.

According to a particular embodiment, the hydrate former may comprise a plurality of gases. In particular, the hydrate former may be a mixture of C ₃ H ₈ and C0 ₂ . The plurality of gases comprised in the hydrate former may be comprised in any suitable concentration. For example, the concentration of C ₃ H ₈ comprised in the hydrate former may be 2.5-100 vol%. In particular, the concentration of C ₃ H ₈ comprised in the hydrate former may be 5-95 vol%, 10-90 vol%, 15-85 vol%, 20-80 vol%, 25-75 vol%, 30-70 vol%, 35-65 vol%, 40-60 vol%, 45-55 vol%. Even more in particular, the concentration of C ₃ H ₈ comprised in the hydrate former may be 10 vol%. According to a particular embodiment, the bed media comprised in the annular bed reactor may be porous bed media. The bed media may be any suitable bed media. In particular, the bed media may have a suitable bed height. For example, the bed height of the bed media may be 0.9-5.0 cm. In particular, the bed height may be 1.0-4.5 cm, 1.2-4.0 cm, 1.5-3.5 cm, 1.8-3.0 cm, 1.9-2.5 cm, 2.0-2.2 cm. Even more in particular, the bed height may be 1.2-2.5 cm, or preferably about 1.5 cm.

The bed media may comprise particles of a suitable size. For example, the bed media may comprise particles having an average particle size of 0.1-1.4 mm. In particular, the bed media may comprise particles having an average particle size of 0.2-1.0 mm, 0.3- 0.9 mm, 0.4-0.8, 0.5-0.7, 0.6-0.65 mm. Even more in particular, the particles comprised in the bed media may have an average particle size of 0.21-0.29 mm.

The bed media may be comprised in a mesh. The mesh may be any suitable mesh. In particular, the mesh may be a mesh having a size which is smaller than the average particle size of the particles comprised in the bed media to prevent the particles comprised in the bed media from coming out of the mesh. According to a particular aspect, the formation of hydrates may be on a surface of the mesh opposite the surface in contact with the bed media.

The method may further comprise maintaining the annular bed reactor at a predetermined temperature and pre-determined pressure, wherein the pre-determined temperature and the pre-determined pressure may be a temperature and pressure, respectively, at which formation of hydrates are enabled. For example, the predetermined temperature may be -5-20°C. The pre-determined pressure may be 1-55 bar.

The method may further comprise dislodging the hydrates formed prior to the collecting. According to a particular aspect, the dissociating may comprise heating the hydrates to obtain the desalinated water. The heating may be by any suitable means and under suitable conditions.

A second aspect of the present invention provides an annular bed reactor for use in a clathrate hydrate desalination method according to any preceding claim, the reactor comprising: bed media;

a first inlet for receiving feedwater;

a second inlet for receiving hydrate former;

a cooling jacket configured to circulate cooling liquid through the cooling jacket, thereby maintaining the annular bed reactor at a pre-determined temperature;

a first outlet for discharging brine; and

a second outlet for discharging formed hydrates.

The cooling liquid may be any suitable cooling liquid. For example, the cooling liquid may be a secondary coolant comprising an anti-freeze solution. In particular, the cooling liquid may be a coolant comprising an aqueous solution of glycol, methanol, or a combination thereof, which captures cold energy from liquefied natural gas (LNG).

According to a particular aspect, the bed media may be as described above. The bed media may be comprised within a mesh. The mesh may be as described above.

The annular bed reactor may further comprise a hydrate dislodging unit for dislodging hydrates formed on a surface of the mesh opposite the surface in contact with the bed media.

Brief Description of the Drawings

In order that the invention may be fully understood and readily put into practical effect there shall now be described by way of non-limitative example only exemplary embodiments, the description being with reference to the accompanying illustrative drawings. In the drawings:

Figure 1 shows a schematic representation of a clathrate hydrate desalination method; Figure 2 shows a schematic representation of an embodiment of the method of the present invention;

Figure 3 shows a representation of an embodiment of an annular bed reactor of the present invention;

Figure 4 shows a representation of a cylindrical core bed reactor according to one embodiment of the present invention;

Figure 5 shows a representation of an annular bed reactor according to one embodiment of the present invention;

Figure 6 shows a representation of an annular bed reactor according to one embodiment of the present invention;

Figure 7 shows a comparison of water recovery with different hydrate former compositions in a pure water system;

Figure 8 shows a comparison of water recovery with different hydrate former compositions in a 3 wt% NaCI system;

Figure 9 shows a comparison of water recovery with different concentrations of propane in the hydrate former composition in a 3 wt% NaCI system;

Figure 10 shows a comparison of water recovery with different hydrate former compositions in a 3 wt% NaCI system;

Figure 11 shows a comparison of water recovery with different concentrations of propane in the hydrate former composition in a 3 wt% NaCI system;

Figure 12 shows a comparison of water recovery with different bed heights for the bed media of a reactor in a 3 wt% NaCI system;

Figure 13 shows a comparison of water recovery with different particle sizes of the bed media of a reactor in a pure water system;

Figure 14 shows a comparison of water recovery with different particle sizes of the bed media of a reactor in a 3 wt% NaCI system; and Figure 15 shows a comparison of water recovery with different particle sizes of the bed media of a reactor in a synthetic seawater system.

Detailed Description

As explained above, there is a need for improved clathrate hydrate desalination method which may be easily scaled up.

The clathrate hydrate desalination method of the present invention minimises energy consumption and maximises water conversion and separation efficiency. In particular, the method of the present invention is carried out in an annular bed reactor. For the method of the present invention, the annular bed reactor provides effective cooling to enable formation of hydrates, as well as the ability to process more feedwater compared to use of flat bed reactors. Further, the annual bed reactor enables simpler and more efficient dislodgment and collection of formed hydrates.

In general terms, the clathrate hydrate desalination method of the present invention comprises a method, carried out in an annular bed reactor, in which water molecules form cages around a hydrate former, thereby effectively separating itself from the brine solution at temperatures higher than normal freezing temperature of water. The hydrates when melted are essentially fresh water and the hydrate former may be reused for the process again. The salt and impurities in the feedwater are a thermodynamic inhibitor and get rejected from the hydrates formed.

There is provided as a first aspect a clathrate hydrate desalination method comprising: pumping feedwater into an annular bed reactor, the annular bed reactor comprising bed media;

adding a hydrate former to the annular bed reactor to enable formation of hydrates;

collecting the hydrates formed; and

dissociating the hydrates to obtain desalinated water and re-formed hydrate former.

For the purposes of the present invention, feedwater refers to seawater, saline water or polluted water to be desalinated or treated. A schematic representation of the desalination method is shown in Figure 1. In particular, seawater may be pumped into a bed reactor to fill the interstitial pore space available between the bed media placed in the reactor. The reactor may then be cooled to a pre-determined temperature suitable for hydrate formation. When exposed to a suitable hydrate former, water will be drawn out from the interstitial pore space to form hydrates above the media. This may be referred to as the hydrate zone. The salts comprised in the saltwater may preferably get occluded in the media and in the left over brine solution below the retainer as shown in Figure 1. After the completion of the hydrate formation, the formed hydrates may be separated from the hydrate zone and dissociated to yield salt-free water.

According to a particular aspect, the hydrate former may be any suitable hydrate. For the purposes of the present invention, a hydrate former may comprise a molecule or a compound which may form hydrates. In particular, the hydrate former may be any suitable hydrate former which may be able to draw water towards it and form hydrates. The hydrate former may be in any suitable form. For example, the hydrate former may be in the form of a gas or liquid. In particular, the hydrate former may be in the form of a gas. The hydrate former may be, but not limited to, C C ₅ hydrocarbon, CC½, H ₂ , N ₂ , 0 ₂ , inert gases, or a combination thereof. For example, the C^Cs hydrocarbon may comprise, but is not limited to, C ₃ H ₈ , CH ₄ , C ₂ H ₆ , or a combination thereof. The inert gas may comprise, but is not limited to, Ar, Kr, Xe, or a combination thereof. Even more in particular, the hydrate former may comprise C ₃ H ₈ . The hydrate former may further comprise CH ₄ , C ₂ H ₆ , C0 ₂ , H ₂ , N ₂ , or a combination thereof.

According to a particular embodiment, the hydrate former may comprise a plurality of gases. In particular, the hydrate former may be a mixture of C ₃ H ₈ and C0 ₂ . The plurality of gases comprised in the hydrate former may have any suitable concentration. For example, the concentration of C ₃ H ₈ comprised in the hydrate former may be 2.5-100 vol%. In particular, the concentration of C ₃ H ₈ comprised in the hydrate former may be 5-95 vol%, 10-90 vol%, 15-85 vol%, 20-80 vol%, 25-75 vol%, 30-70 vol%, 35-65 vol%, 40-60 vol%, 45-55 vol%, based on the total volume of the hydrate former. Even more in particular, the concentration of C ₃ H ₈ comprised in the hydrate former may be about 10 vol%. The bed media comprised in the annular bed reactor may be any suitable bed media for the purposes of the present invention. According to a particular embodiment, the bed media may be a porous bed media. The bed media may comprise any suitable media. For example, the media may comprise, but is not limited to, silica sand particles, silica gel particles, glass beads, metal particles or a combination thereof. In particular, the bed media may in the form of silica sand layers.

Further, the bed media may have a suitable bed height. For example, the bed height of the bed media may be 0.9-5.0 cm. In particular, the bed height may be 1.0-4.5 cm, 1.2- 4.0 cm, 1.5-3.5 cm, 1.8-3.0 cm, 1.9-2.5 cm, 2.0-2.2 cm. Even more in particular, the bed height may be 1.2-2.5 cm, or preferably about 1.5 cm.

The bed media may comprise particles of a suitable size. For example, the bed media may comprise particles having an average particle size of 0.1-1.4 mm. In particular, the bed media may comprise particles having an average particle size of 0.2-1.0 mm, 0.3- 0.9 mm, 0.4-0.8, 0.5-0.7, 0.6-0.65 mm. Even more in particular, the particles comprised in the bed media may have an average particle size of 0.21-0.29 mm. For the purposes of the present invention, the average particle size of a material may be defined by its largest dimension. Any standard method suitable for determining the particle size may be used.

The annular bed reactor may comprise a mesh. In particular, the bed media may be comprised within the mesh. The mesh may be any suitable mesh and may have a suitable size. In particular, the mesh may be a mesh having a size which is smaller than the average particle size of the particles comprised in the bed media to prevent the particles comprised in the bed media from coming out of the mesh.

According to a particular aspect, the formation of hydrates following the adding of a hydrate former to the annular bed reactor in the method of the present invention may be on a surface of the mesh opposite the surface in contact with the bed media. In particular, the formation of the hydrates may be on the surface of the mesh and facing a wall of the annular bed reactor.

The method may further comprise maintaining the annular bed reactor at a predetermined temperature and a pre-determined pressure, wherein the pre-determined temperature and the pre-determined pressure may be a temperature and pressure, respectively, at which formation of hydrates is enabled. For the purposes of the present invention, hydrates refer to ice-like, solid crystalline compounds formed of water molecules and gas molecules, in which each gas molecule is included inside a cage formed of hydrogen-bonded water molecules within a three-dimensional lattice structure. The pre-determined temperature may be -5-20°C. The pre-determined pressure may be 1-55 bar. The pre-determined pressure is much lower compared to the pressure required for other desalination processes, such as reverse osmosis.

Accordingly, it may be necessary to cool the annular bed reactor during the method of the present invention in order to maintain the pre-determined temperature. Any suitable cooling method may be used for the purposes of the present invention. For example, the cooling may be by, but not limited to, utilising liquefied natural gas (LNG) cold energy during re-gasification or a refrigeration system. The refrigeration system may be an ammonia refrigeration system or any other suitable chiller system. In particular, the cooling may be by a secondary coolant comprising an anti-freeze solution. In particular, the cooling liquid may be a coolant which may transfer cold energy from LNG or a refrigeration system to the annular bed reactor. Even more in particular, the cooling may comprise using a coolant comprising an aqueous solution of glycol, methanol, or a combination thereof, which captures cold energy from LNG. The advantage of utilising LNG cold energy during re-gasification is that the energy consumption of the clathrate hydrate desalination process is reduced since this cold energy is available as waste energy from the LNG re-gasification process.

The method may further comprise dislodging the hydrates formed prior to the collecting. In particular, any suitable dislodging method may be used to dislodge the hydrates formed. For example, the dislodging may comprise using a hydrate dislodging unit. The hydrate dislodging unit may comprise, but is not limited to, a scrapper or plunger. Even more in particular, the method may comprise dislodging the hydrates formed from the surface of the mesh opposite the surface in contact with the bed media. According to a particular aspect, the collecting may comprise collecting the dislodged hydrates.

The dissociating may comprise heating the hydrates to obtain the desalinated water. The heating may be by any suitable means and under suitable conditions. In particular, the heating may comprise increasing to a suitable temperature. The temperature may be a hydrate dissociation temperature of the hydrate former. The increase in temperature to the hydrate dissociation temperature may result in the dissociation of the formed hydrates and release of the hydrate former and desalinated water. Once the hydrate former and the desalinated water have been formed, the hydrate former may be transferred back to a storage tank comprising the hydrate former.

The method of the present invention may further comprise washing the hydrates formed prior to the dissociating step. In this way, any impurities or salts on the surface of the hydrates formed may be washed away, thereby preventing any mixing of the impurities or salts with the desalinated water when the hydrates are subsequently dissociated.

A schematic flow diagram showing the method of the present invention according to a particular embodiment is as shown in Figure 2. In particular, method 200 involves utilisation of LNG cold energy obtained from the re-gasification of LNG to natural gas (NG). The NG may be supplied to a power plant. The LNG may be at any suitable temperature. For example, the LNG may be at about -162°C. The LNG cold energy may be fed to the annular bed reactor. The LNG cold energy may optionally be fed to a tank comprising the feedwater and a tank comprising the hydrate former in order to maintain the feedwater and hydrate former at a suitable temperature. The LNG cold energy fed to the annular bed reactor may be for cooling the annular bed reactor, particularly to enable the formation of hydrates during the clathrate hydrate desalination process.

Feedwater, comprising seawater, is fed into the annular bed reactor from the feedwater tank. Likewise, hydrate former is fed into the annular bed reactor from the tank comprising the hydrate former. Since the annular bed reactor is maintained at a suitable temperature at which hydrate formation is enabled by the LNG cold energy, hydrates may be formed. As explained above, the formation of the hydrates will cause the salts and other impurities to remain in the bed media. The resultant brine solution is then discharged, leaving behind the formed hydrates.

In order to obtain the desalinated water, the formed hydrates are dissociated. The dissociation comprises heating and may therefore utilise waste heat. Following the dissociation, the obtained desalinated water may optionally undergo further post- treatment in order to obtain clean water. The post-treatment may comprise, but is not limited to, chlorination, remineralisation, pH adjustment, or a combination thereof.

A second aspect of the present invention provides an annular bed reactor for use in the clathrate hydrate desalination method described above. The annular bed reactor comprises: bed media;

a first inlet for receiving feedwater;

a second inlet for receiving hydrate former;

a cooling jacket configured to circulate cooling liquid through the cooling jacket, thereby maintaining the annular bed reactor at a pre-determined temperature;

a first outlet for discharging brine; and

a second outlet for discharging formed hydrates.

An example of an annular bed reactor according to the present invention is as shown in Figure 3. There is provided an annular bed reactor 300 comprising bed media 302, a feedwater inlet 304, a hydrate former inlet 306, a cooling jacket 308 comprising an inlet 308a for receiving a coolant and an outlet 308b for discharging the coolant, an outlet 310 for discharging brine and an outlet 312 for discharging formed hydrates.

According to a particular aspect, the bed media 302 may be any suitable bed media. For example, the bed media 302 may be as described above. The bed media may be comprised within a mesh (not shown). The mesh may be as described above.

The feedwater inlet 304 may be in fluid communication with a feedwater tank. The feedwater inlet 304 may supply feedwater into the annular bed reactor 300. According to a particular aspect, the feedwater inlet 304 may be in fluid communication with a perforated tube to evenly disperse the feedwater into the bed media 302.

The hydrate former inlet 306 may be in fluid communication with a hydrate former tank. The hydrate former may be supplied into the annular bed reactor 300 via inlet 306.

The cooling jacket 308 cools the annular bed reactor 300 by circulating a coolant through the cooling jacket 308. In particular, the coolant enters the cooling jacket 308 via inlet 308a and exits the cooling jacket through outlet 308b. The coolant may be any suitable coolant for the purposes of the present invention. For example, the coolant may be a secondary coolant comprising an anti-freeze solution. In particular, the cooling liquid may be a coolant comprising an aqueous solution of glycol, methanol, or a combination thereof, which captures cold energy from liquefied natural gas (LNG) or a refrigeration system. The refrigeration system may be an ammonia-based system or any other chiller system.

Following each cycle of the clathrate hydrate desalination method, brine is discharged from the outlet 310, while the hydrates formed are collected via outlet 312. The outlet 312 may be in the form of a hydrate collection zone.

According to a particular aspect, the annular bed reactor 300 may further comprise a hydrate dislodging unit (not shown) for dislodging hydrates formed on a surface of the mesh opposite the surface in contact with the bed media. Even more in particular, the hydrate dislodging unit may be configured to dislodge the formed hydrates from the surface of the mesh to the hydrate collection zone. The hydrate dislodging unit may be of any suitable form. For example, the hydrate dislodging unit may be in the form of a scrapper, a plunger, or the like.

Particular embodiments of annular bed reactors according to the present invention will now be described.

Figure 4 shows a cylindrical core bed reactor 400. Reactor 400 comprises a bed media 402 comprising a sand bed, a feedwater inlet 404 for allowing feedwater to enter the reactor 400, a hydrate former inlet 406 to allow hydrate former to enter the reactor 400, and a cooling jacket 408 configured to have a coolant flow through the cooling jacket 408 thereby maintaining the temperature of the reactor 400. The feedwater inlet 404 is in fluid connection with a perforated tube 405 to disperse the feedwater. In particular, the sand bed 402 may be a silica sand bed.

The reactor 400 also has outlets 4 0a, 410b, 410c to discharge brine following a cycle of desalination, and an outlet 418 for discharging desalinated water. The outlets 410a, 410b and 410c are in fluid connection with a perforated tube 411 which collects brine from the reactor 400 and distributes the brine to the outlets 410a, 410b and 410c. There is also provided a hydrate collection zone 412 for collecting formed hydrates 420 following desalination. The hydrate collection zone 412 may comprise a heating jacket 416 around the zone to heat the hydrate collection zone 412. The reactor 400 also comprises a scrapper 414 for dislodging formed hydrates 420.

The sand bed 402 is comprised within a cylindrical mesh 403. The mesh 403 is placed at the centre of the reactor 400. In particular, the diameter of the cylindrical mesh 403 may be varied depending on the amount of feedwater to be treated. The size of the mesh 403 is smaller than the sand particles comprised within the mesh 403 to prevent the sand from coming out of the mesh 403.

In use, feedwater enters the reactor 400 at the top of the mesh 403 via inlet 404 and a perforated tube 405 disperses the feedwater into the sand bed 402. The reactor 400 may be cooled by circulating a coolant through the cooling jacket 408. The reactor 400 may be pressurized with hydrate former entering the reactor 400 via the hydrate former inlet 406, wherein the hydrate former comprises C ₃ H ₈ . In particular, the C ₃ H ₈ present in the hydrate former draws water dispersed within the sand bed towards a gas phase 422 to form hydrates 420. At a suitable temperature and pressure, hydrates may be formed between walls of the reactor 400 and the cylindrical mesh 403. The formed hydrates 420 may stick to the surface of the cylindrical mesh 403 opposite the surface of the mesh 403 in contact with the sand bed 402. When the scrapper 414 is rotated, the scrapper 414 scrapes/removes the hydrates 420 attached to the surface of the cylindrical mesh 403 at regular intervals of time. Alternatively, a plunger may be used instead of the scrapper 414 to remove the hydrates 420 from the outer surface of the cylindrical mesh 403. The scrapped hydrates 420 are collected at the bottom of the reactor 400 within the hydrate collection zone 412, where the hydrates 420 may be dissociated by thermal stimulation by way of the heating jacket 416 and depressurization to recover desalinated water. Desalinated water may be removed from the bottom of the reactor 400 via outlet 418. Thereafter, the brine left in the sand bed 402 may be removed by washing the sand bed 402 with feedwater. Outlet 410a is used to remove washed brine to prevent mixing of the desalinated water with brine. In the event the brine overflows out from the cylindrical mesh 403 during the washing step, the brine may be discharged from the outlets 410b and 410c, thereby avoiding contamination with the formed hydrates 420. Figure 5 shows another example of the reactor of the present invention. Figure 5 shows an annular bed reactor 500. Reactor 500 comprises bed media 502 comprising a sand bed, feedwater inlets 504a and 504b for allowing feedwater to enter the reactor 500 and a cooling jacket 508 configured to have a coolant flow through the cooling jacket 508 by entering the cooling jacket 508 via coolant inlet 508a and out of the cooling jacket 508 via coolant outlet 508b, thereby maintaining the temperature of the reactor 500. The sand bed 502 may be a silica sand bed.

The reactor 500 also has outlets 510a and 510b to discharge brine following a cycle of desalination, and an outlet 518 for discharging desalinated water. There is also provided a hydrate collection zone 512 in the form of a dissociation chamber for collecting formed hydrates following desalination. The hydrate collection zone 512 may comprise a heating jacket 516 around the zone to heat the hydrate collection zone 512. The reactor 500 may also comprise a scrapper 514 for dislodging formed hydrates.

The sand bed 502 is comprised within a cylindrical mesh 503. The mesh 503 is placed at the centre of the reactor 500. In particular, the diameter of the cylindrical mesh 503 may be varied depending on the amount of feedwater to be treated. The size of the mesh 503 is smaller than the sand particles comprised within the mesh to prevent the sand from coming out of the mesh 503.

In use, feedwater enters the reactor 500 at the top of the mesh 503 and is dispersed into the sand bed 502 via inlets 504a and 504b. The reactor 500 is pressurized with hydrate former consisting of C ₃ H ₈ as one of its constituents. In particular, C ₃ H ₈ present in the hydrate former draws water dispersed within the sand bed towards a gas phase 522 to form hydrates. Contents of the reactor 500 may be cooled by circulating a coolant through the cooling jacket 508. At a suitable temperature and pressure, hydrates may be formed and grow towards the gas phase 522 on the outer surface of the mesh 503, i.e. the surface of the mesh 503 opposite the surface of the mesh 503 in contact with the sand bed 502.

When the scrapper 514 is rotated, the scrapper 514 scrapes/removes the hydrates attached to the surface of the cylindrical mesh 503 at regular intervals of time. The scrapped hydrates are collected at the bottom of the reactor 500. Alternatively, a plunger may be used instead of the scrapper 514 to remove the hydrates from the outer surface of the cylindrical mesh 503. When the hydrates have been collected, valve 524 is opened to collect the hydrates in the hydrate collection zone 512 which may be maintained at the same pressure as the reactor 500. Once all the hydrates are collected in the hydrate collection zone 512, the valve 524 is closed. The hydrates may be dissociated by thermal stimulation and depressurization by way of the heating jacket 516 to recover desalinated water. Desalinated water may be removed from the hydrate collection zone 512 via outlet 518 by opening valve 526. The hydrate former may be recycled.

Thereafter, the reactor 500 is depressurised and the sand bed 502 is washed to remove the brine via outlets 510a and 510b.

Figure 6 shows another example of a reactor according to the present invention. Figure 6 shows an annular bed reactor 600. Reactor 600 comprises bed media 602 comprising a sand bed, feedwater inlets 604a and 604b for allowing feedwater to enter the reactor 600, a hydrate former inlet 606 to allow hydrate former to enter the reactor 600, and a cooling jacket 608 configured to have a coolant flow through the cooling jacket 608 thereby maintaining the temperature of the reactor 600. The coolant enters the cooling jacket 608 through inlet 608a and exits the cooling jacket via outlet 608b. In particular, the sand bed 602 may be a silica sand bed.

The reactor 600 also has outlets 610a, 610b, 610c and 610d to discharge brine following a cycle of desalination, and an outlet 618 for discharging desalinated water. There is also provided a hydrate collection zone 612 for collecting formed hydrates following desalination. The hydrate collection zone 612 may comprise a heating jacket 616 around the zone to heat the hydrate collection zone 612. The reactor 600 also comprises a scrapper 614 for dislodging formed hydrates.

The sand bed 602 is comprised within a cylindrical mesh 603. The mesh 603 is placed at the centre of the reactor 600. In particular, the diameter of the cylindrical mesh 603 may be varied depending on the amount of feedwater to be treated. The size of the mesh 603 is smaller than the sand particles comprised within the mesh to prevent the sand from coming out of the mesh 603.

In use, feedwater enters the reactor 600 and into the sand bed 602 via inlets 604a and 604b. The reactor 600 may be cooled by circulating a coolant through the cooling jacket 608. Hydrate former is drawn from a reservoir comprising hydrate former and supplied to the reactor 600 via inlet 606 and valve 628. The hydrate former may comprise C ₃ H ₈ . In particular, the C ₃ H ₈ present in the hydrate former draws water dispersed within the sand bed towards a gas phase to form hydrates.

The contents of the reactor 600 may be cooled using a circulating coolant through a cooling jacket 608. At a suitable temperature and pressure, hydrates may form and grow towards the gas phase on the outer surface of the mesh 603 opposite the surface in contact with the sand bed 602. The formed hydrates may be attached to the surface of the cylindrical mesh 603 opposite the surface of the mesh 603 in contact with the sand bed 602.

When the scrapper 614 rotates, the scrapper 614 scrapes/removes the hydrates attached to the surface of the cylindrical mesh 603 at regular intervals of time. The scrapped hydrates may be collected at the bottom of the reactor 600. When the hydrates have been collected, valve 624 is opened to collect the hydrates in the hydrate collection zone 612 which may be maintained at the same pressure as the reactor 600. Once all the hydrates are collected in the hydrate collection zone 612, the valve 624 is closed. The hydrates may be dissociated by thermal stimulation and depressurization by way of the heating jacket 616 to recover desalinated water. Desalinated water may be removed from the hydrate collection zone 612 via outlet 618 by opening valve 626. The hydrate former may be recycled.

Thereafter, the reactor 600 is depressurised. Prior to depressurization, the valve 628 is closed. The presence of the hollow cylindrical scrapper 614 will limit the gas volume inside the reactor and thereby reduce the loss due to depressurization. The sand bed 602 is then washed to remove brine via outlets 610a and 610b. In the event the brine overflows out from the cylindrical mesh 603 during the washing step, the brine may be collected from the outlets 610c and 61 Od, thereby avoiding contamination with the formed hydrates.

It can be seen from the above that the present invention provides a simple and effective clathrate hydrate desalination method. In particular, the method is a green method and also cost-effective since energy consumption is low, particularly when cold energy from LNG re-gasification plants is utilised. Further, the method of the present invention is able to achieve a much higher water recovery at a lower cost compared to other methods such as traditional desalination and reverse osmosis. In particular, the hydrate formation rate is also high, therefore making the method a commercially viable one.

Whilst the foregoing description has described exemplary embodiments, it will be understood by those skilled in the technology concerned that many variations may be made without departing from the present invention.

Having now generally described the invention, the same will be more readily understood through reference to the following examples which are provided by way of illustration, and are not intended to be limiting.

EXAMPLE

Example 1A - Effect of hydrate former composition

The effect of hydrate former composition was investigated by employing a flat bed reactor configuration. A gas/gas mixture comprising C ₃ H ₈ was employed to as a hydrate former. Porous bed media employed was added to the flat bed reactor, in which the porous bed media comprised silica sand. Different gas mixtures comprising C ₃ H ₈ were tested as follows:

(a) 10 vol% C ₃ H ₈ and 90 vol% N ₂ ;

(b) 2.5 vol% C ₃ H ₈ , 38.1 vol% C0 ₂ and 59.4 vol% H ₂ ;

(c) 2.0% vol% C ₃ H ₈ , 93.0 vol % CH ₄ , 5.0 vol% C ₂ H ₆ ; and

(d) 10 vol % C ₃ H ₈ , 90 vol % C0 ₂ .

Experiments were carried out with both pure water and 3 wt% NaCI solution in a reactor having 1.5 cm sand bed height.

The reactor comprised a crystallizer having a volume of 980 mL and an inner diameter of 10.2 cm. The crystallizer comprised a cooling jacket. There was also provided a pressure transmitter to measure the pressure of the crystallizer. A thermocouple was placed at height of 1.2 cm from the bottom of the bed within the reactor to measure the temperature of the silica sand bed in the crystallizer. An external refrigerator was used to maintain the temperature of the setup. A data acquisition device coupled with a computer was used to record the data. All experiments were conducted under batch conditions.

Once a bed height of 1.5 cm was prepared within the crystallizer, a desired quantity of water or 3 wt% NaCI solution was dispersed into the interstitial pore spaces of the silica sand particles comprised in the sand bed. All the experiments were conducted with 100% saturation in silica sand. The crystallizer was closed and the thermocouples were connected. The crystallizer was cooled to experimental temperature, which varied depending on the hydrate former used. The crystallizer was then flushed three times with the hydrate former and then pressurized to the experimental pressure. The data was recorded for every 20 seconds, with time zero corresponding to the time at which the pressure and temperature of the crystallizer reached the experimental conditions. The crystallizer pressure dropped as a result of gas dissolution and hydrate formation. The experiment was terminated after 1 hour from hydrate nucleation. The crystallizer was depressurized and the hydrates formed were collected. The hydrates were then decomposed to yield the recovered water.

Water recovery is a common measure to describe the efficiency of desalination technologies. It may be defined as the ratio of quantity of potable water produced to the amount of seawater fed to the process. For a typical one stage clathrate hydrate desalination process, the water recovery may be as follows:

Volume of water recovered after hydrate decomposition _ _~—

Water recovery % = -— ^■ ■ xlOO

Volume of feed solution

Figure 7 shows the comparison of water recovery with the different gas mixtures tested in a pure water system. In particular, in 1 hour of hydrate formation, water recovery of 44.5% (±7.8) was achieved for 10% propane in nitrogen gas mixture (mixture (a)). For 10% propane in carbon dioxide gas mixture (mixture (d)), water recovery of 76.5 % (±1.5) was achieved in 1 hour. For d-yCsHe/CsHs (93.0% / 5.0% / 2.0%) gas mixture (mixture (c)), water recovery of 47.2% (±11.9) was achieved in 1 hour. For CO2/H2/C3H8 (38.1/59.4/2.5%) gas mixture (mixture (b)), water recovery of 55.1 % (±1.1 ) was achieved. Figure 8 shows the comparison of water recovery with different gas mixtures in 3 wt% NaCI solution. The gas mixtures tested were mixtures (a), (b) and (d). The water recovery for each of the tested mixtures (a), (b) and (d) were 5.3% (±0.5), 25.9% (±1.8) and 42.4% (±7.6), respectively. Generally, the water recovery of water from salt solution was lower compared to water recovery from a pure water system. This is because the presence of salt in the water which hinders the kinetics of hydrate formation.

It can be seen that the choice of composition of the hydrate former affects the kinetics of hydrate formation.

Example 1 B - Effect of hydrate former concentration

As seen from Example 1A, a combination of propane and carbon dioxide as the hydrate former yielded good water recovery results. The effect of the concentration of propane in the hydrate former composition was then investigated.

With the same reactor configuration as Example 1A, a hydrate former composition comprising 2.5-20% C ₃ H ₈ in C0 ₂ gas mixture at constant temperature of 274.2K and a driving force ΔΡ of 1.97 MPa with 3 wt% NaCI solution was tested. The driving force is defined as the difference between the experimental pressure and the hydrate equilibrium pressure at a specific temperature.

It was found that a hydrate former composition comprising 2.5 vol% C ₃ H ₈ in C0 ₂ had no effect and hydrates were not formed even after 3 days. Figure 9 shows the comparison of water recovery from different propane concentrations in C0 ₂ gas mixture. As can be seen, 10% C ₃ H ₈ in C0 ₂ yielded almost double water recovery as compared to 5% or 20% C ₃ H ₈ in C0 ₂ gas mixture.

Example 1C - Effect of hydrate former composition

As can be seen from Example 1A, gas mixture (d) yielded a higher water recovery compared to the other tested gas mixtures. Since Example A was conducted on flat bed reactors, tests were conducted to see whether the same results would be obtained in an annular bed reactor. Accordingly, a gas mixture comprising 10% C3H8 in argon in both annular and flat bed reactor configurations were tested. For the annular bed reactor, the reactor comprised a crystalliser having a volume of 980 ml_ and an inner diameter of 10.2 cm, which was surrounded by a cooling jacket. A cylindrical mesh of outer diameter 7.2 cm was placed at the centre of the reactor. Silica sand was added within the annular region between the cylindrical mesh and inner wall of the reactor to prepare an annular bed having a bed height of 1.5 cm. The interstitial pore spaces within silica sand bed was filled with the desired quantity of feedwater. All experiments were conducted under batch conditions with 100% saturated silica sand. For 10% C ₃ H ₈ in argon system, water recovery of 1.15% (±0.1) and 4.36% (±0.6) was achieved for flatbed and annular bed reactor, respectively, in 1 hour.

Subsequently, the water recovery in an annular bed reactor using 10% C ₃ H ₈ in argon and 10% C ₃ H ₈ in C0 ₂ as the hydrate former was tested. Figure 10 shows the comparison of the water recovery of 10% C ₃ H ₈ in argon and 10% C ₃ H ₈ in C0 ₂ from feedwater of 3 wt% NaCI solution. It can be seen that a hydrate former comprising 10% C ₃ H ₈ in C0 ₂ yielded higher water recovery compared to other gas mixtures in annular bed reactor.

Example 1 D - Effect of hydrate former concentration

As seen from Example 1C, a combination of propane and carbon dioxide as the hydrate former yielded good water recovery results. The effect of the concentration of propane in the hydrate former composition was then investigated.

With the annular reactor configuration, a hydrate former composition comprising 10% and 20% C ₃ H ₈ in C0 ₂ gas mixture at constant temperature of 274.2K and a driving force ΔΡ of 1.97 MPa with 3 wt% NaCI solution was tested. Figure 11 shows the comparison of water recovery from different propane concentrations in C0 ₂ gas mixture. 10% C ₃ H ₈ in C0 ₂ yielded better water recovery than 20% C ₃ H ₈ in C0 ₂ gas mixture. The water recovery achieved in 1 hour was 29.5% (±0.04) and 18.45% (±2.2) using hydrate former comprising 10% and 20% C ₃ H ₈ in C0 ₂ gas mixture, respectively.

Example 2 - Effect of height of bed media In a fixed bed reactor, the contact area of exposure is important in addition to the tuning of the hydrate former composition in order to be able to improve the ability of the hydrate former to draw the water outside the porous bed media. Accordingly, the height of the bed media was optimised to achieve maximum water recovery and hydrate formation.

Experiments with different annular bed height of 1.0, 1.5, 1.9, 2.55 cm were carried out at 2.6 MPa, 274.2 K, using silica sand (Sigma Aldrich) having particle size range of 0.21 -0.297 mm and 3 wt% NaCI solution as feedwater to optimize the bed height. The hydrate former used was a gas mixture comprising 10 vol% C ₃ H ₈ and 90 vol% C0 ₂ . Figure 12 shows the effect of bed height on water recovery. Annular bed height of about 1.9 cm was found to yield higher water recovery. Water recovery achieved in 1 hour was 19.2% (±0.1 ), 29.5% (±0.1 ), 32.0% (±1.1 ) and 21.8% (±3.3) for annular bed thickness of 1 , 1.5, 1.9 and 2.55 cm respectively.

Example 3A - Effect of particle size of bed media

Particle size of the bed media may also affect the kinetics of the hydrate formation and water recovery. Accordingly, four different types of sand commonly used as bed media with different particle sizes were investigated for hydrate formation kinetics and water recovery using two different hydrate former compositions as shown in Table 1.

With the different sands indicated in Table 1 , experiments were carried out to investigate the hydrate formation kinetics and water recovery with a bed height of 1.5 cm, at a pressure of 5.0 MPa and temperature of 275.7 K with 10% C ₃ H ₈ in N ₂ with pure water as the feedwater. Figure 13 shows the effect of different particle size on water recovery with 10% C ₃ H ₈ in N ₂ . As can be seen, Sigma Aldrich (SA) sand having particle size of 0.21-0.297 mm performed better when compared to the other sand particles. The water recovery achieved in 1 hour with Toyura, Sigma Aldrich, coarse and granules was 9.1 % (±2.9), 44.5% (±7.8), 1.7% (±0.6) and 4.2% (±1.6), respectively. Similarly, with these four different sand particles, experiments were carried out at a pressure of 2.6 MPa and temperature of 274.2 K using 10% C ₃ H ₈ in C0 ₂ as the hydrate former and with 3 wt% NaCI solution as the feedwater. A bed height of 1.5 cm was employed. The effect of particle size on water recovery with 10% C ₃ H ₈ in C0 ₂ is shown in Figure 14. As can be seen, the effect of particle size on water recovery is minimal with 10% propane in carbon dioxide gas mixture. Water recovery achieved in 1 hour was 45.2% (±2.9), 42.4% (±7.5), 39.5% (±0.9) and 49.5% (±1.9) with Toyura, Sigma Aldrich, coarse and granules.

Based on the experiments with the two different gas mixtures as the hydrate formers in pure water and 3 wt% NaCI solution, it can be seen that Sigma Aldrich sand may be suitable as bed media compared to the other particle sizes. Moreover, granules may not retain feedwater between their interstitial pore spaces, therefore making them unsuitable for the purposes of the present invention.

Example 3B - Effect of particle size of bed media

In view of the results obtained in Example 3A, experiments were conducted using Toyura and Sigma Aldrich sand particles as the bed media in an annular reactor configuration. Experiments with coarse and granule sand particles were not conducted since they were unable to hold water within the interstitial pore space in the annular configuration. The hydrate former used was a gas mixture comprising 10 vol% C ₃ H ₈ and 90 vol% C0 ₂ . Effect of particle size on water recovery is shown in Figure 15. Water recovery achieved in 1 hour was 25.13% (±1.0) and 31.96% (±1.1 ) with Toyura and Sigma Aldrich sand, respectively.