Technical Field

The present invention relates to an apparatus for forming gas hydrates, and a method of forming gas hydrates.

Background

Natural gas (NG) is the cleanest burning fossil fuel and is used as a source of energy globally since its use results in reduction of carbon dioxide emissions as compared to using coal and oil. However, transportation of NG, which is usually through gas pipelines, poses a problem. Therefore, NG is usually stored and transported in a compressed gas form as compressed natural gas (CNG) or in a liquid form as liquefied natural gas (LNG).

LNG carriers used for transporting LNG and LNG storage are generally cryogenic ships and tanks which need to store LNG at a very low temperature of about -160°C. This results in high energy consumption and accordingly, high cost. CNG, on the other hand, has a very high pressure requirement of about 200 bars and above. This makes CNG impractical due to the high cost involved in designing high pressure and large volume storage tanks. Further, CNG is inherently explosive and flammable.

Another option of storing NG is in the form of solidified natural gas (SNG) in which NG is stored in the form of clathrate hydrates. SNG is a viable alternative for both transport and storage of NG and enables higher energy storage density at very mild conditions and is more eco-friendly compared to LNG and CNG. However, it has been a challenge to form hydrates on a large scale. This is due to a low rate of hydrate formation and low conversion of water to hydrates. Further, storing hydrates at temperatures of about - 20°C has also been a major hindrance for the large scale commercialization of the technology.

There is, therefore, a need for an improved apparatus and method for forming gas hydrates.

Summary of the invention

The present invention seeks to address these problems, and/or to provide an improved apparatus and method of forming gas hydrates. According to a first aspect, the present invention provides an apparatus for forming gas hydrates, the apparatus comprising:

a chamber comprising a hydrate formation zone and a hydrate pelletization zone, the hydrate formation zone and the hydrate pelletization zone being separated by a high pressure valve;

a gas inlet in fluid communication with the hydrate formation zone; a liquid inlet in fluid communication with the hydrate formation zone; a piston or an extruder movable through the hydrate formation zone and the hydrate pelletization zone and configured to transfer gas hydrates, formed in the hydrate formation zone, to the hydrate pelletization zone for forming gas hydrate pellets; and

a hydrate outlet for collecting formed gas hydrates, wherein the hydrate outlet is connected to the hydrate pelletization zone. According to a particular aspect, each of the hydrate formation zone and the hydrate pelletization zone may comprise a cooling system configured to circulate cooling liquid through the cooling system, thereby maintaining the hydrate formation zone and the hydrate pelletization zone at a first pre-determined temperature and a second pre determined temperature, respectively. The chamber may further comprise a pressure release valve configured to control the pressure in the chamber. The pressure release valve may be any suitable valve for the purposes of the present invention. According to another particular aspect, the high pressure valve may be any suitable high pressure valve. In particular, the high pressure valve may be, but not limited to: a gate valve, a ball valve, or a combination thereof.

The piston or the extruder may be configured to transfer the gas hydrates, formed in the hydrate formation zone, to the pellet die in the hydrate pelletization zone to form the gas hydrate pellets. According to a particular aspect, the hydrate pelletization zone may comprise a pellet die configured to form gas hydrate pellets. The apparatus may further comprise a liquid outlet in fluid communication with the hydrate pelletization zone for discharging residual liquid. According to a second aspect, the present invention provides a method of forming gas hydrates, the method comprising: feeding a hydrate forming liquid to a liquid inlet in fluid communication with a hydrate formation zone;

- injecting a gas into a gas inlet in fluid communication with the hydrate formation zone;

cooling the hydrate formation zone to a pre-determined temperature to enable formation of gas hydrates; and

pelletizing the gas hydrates in a hydrate pelletization zone.

The hydrate forming liquid may be any suitable liquid. According to a particular aspect, the hydrate forming liquid may comprise water. According to another particular aspect, the hydrate forming liquid may further comprise, but is not limited to: a thermodynamic promoter, a kinetic promoter, or a combination thereof. The thermodynamic promoter may be any suitable thermodynamic promoter. For example, the thermodynamic promoter may be, but is not limited to: si, sll-, sH-, or semiclathrate- forming compound. In particular, the thermodynamic promoter may be, but is not limited to: tetrahydrofuran, dioxolane, or a combination thereof.

The kinetic promoter may be any suitable kinetic promoter. For example, the kinetic promoter may be, but is not limited to: a surfactant or an amino acid. In particular, the kinetic promoter may be, but is not limited to: sodium dodecyl sulphate, tryptophan, or a combination thereof.

The injecting a gas may comprise pressurising the hydrate formation zone to a suitable pressure. In particular, the injecting may comprise pressuring the hydrate formation zone to a pressure of 30-95 bar.

The injecting may comprise injecting any suitable gas. For example, the injecting may comprise injecting a gas selected from, but not limited to: natural gas, biomethane, methane, ethane, propane, carbon dioxide, hydrogen, or a mixture thereof.

The pre-determined temperature may be any suitable temperature. For example, the temperature may be 1 -25°C. The method may further comprise: transferring the gas hydrates formed in the hydrate formation zone to the hydrate pelletization zone prior to the pelletizing.

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 process schematic of an integrated hydrate reactor apparatus in accordance with one embodiment of the present invention; Figure 2 shows a schematic of an integrated reactor with piston;

Figure 3 shows a schematic of an integrated reactor with an extruder;

Figure 4(A) shows the effect of temperature on methane uptake, Figure 4(B) shows the induction time and Figure 4(C) shows the time taken to achieve 90% completion of hydrate formation (tgo) during hydrate formation; Figure 5(A) shows the effect of pressure on methane uptake, Figure 5(B) shows the induction time and Figure 5(C) shows the tgo during hydrate formation;

Figure 6 shows the effect of volumetric scale-up on methane uptake, induction time and normalised methane uptake rate (NR _I ) during hydrate formation;

Figure 7 shows a comparison of gas uptake in mixed hydrates formed from pure water, salt water and seawater;

Figure 8 shows the effect of pressure on gas uptake using a ternary methane/ethane/propane gas mixture for mixed hydrate formation at 283.2 K;

Figure 9 shows a comparison of gas uptake using dioxolane and tetrahydrofuran during mixed methane hydrate formation at 7.2 MPa and 283.2 K; and Figure 10 shows stability test data of SNG mixed hydrate pellet at 1 atm and -2 ^e C over a period of 1 year. Detailed Description

As explained above, there is a need for an improved apparatus and method for forming gas hydrates. In general terms, the present invention provides a SNG forming system that is able to integrate typical steps such as hydrate formation, excess water removal and pelletization in a single system. The present invention also provides a method which only requires moderate operating pressures and higher temperatures as compared to conventional methods, while still achieving high hydrate conversion, making the system and method a lower cost method as well as more suited for large- scale natural gas hydrate production and storage.

According to a first aspect, the present invention provides an apparatus for forming gas hydrates, the apparatus comprising:

a chamber comprising a hydrate formation zone and a hydrate pelletization zone, the hydrate formation zone and the hydrate pelletization zone being separated by a high pressure valve;

a gas inlet in fluid communication with the hydrate formation zone; a liquid inlet in fluid communication with the hydrate formation zone; a piston or an extruder movable through the hydrate formation zone and the hydrate pelletization zone and configured to transfer gas hydrates, formed in the hydrate formation zone, to the hydrate pelletization zone for forming gas hydrate pellets; and

a hydrate outlet for collecting formed gas hydrates, wherein the hydrate outlet is connected to the hydrate pelletization zone.

A schematic representation of the apparatus of the present invention is shown in Figure 1 . As shown in Figure 1 , the apparatus 100 may comprise a chamber (not shown) which may comprise a hydrate formation zone 102 and a hydrate pelletization zone 104.

The apparatus 100 may also comprise a gas inlet 106 for feeding gas into the hydrate formation zone 102 and a liquid inlet 108 for feeding hydrate forming liquid into the hydrate formation zone 102.

The apparatus 100 also comprises a gas outlet 1 10 for discharging any excess gas fed into the hydrate formation zone 102. The gas outlet 1 10 may be in fluid communication with the gas inlet 106 such that any excess gas discharged from the hydrate formation zone 102 may be fed back into the hydrate formation zone 102 via gas inlet 106 at an appropriate time.

There may also be provided a high pressure valve 1 12 configured to adjust and maintain the pressure within the hydrate formation zone 102 and the hydrate pelletization zone 104. The high pressure valve 1 12 may be any suitable valve. For example, the high pressure valve 1 12 may be, but not limited to, gate valve, ball valve, or a combination thereof. In particular, the high pressure valve 1 12 may be a ball valve.

The apparatus 100 may further comprise a pressure release valve (not shown) configured to control the pressure in the hydrate formation zone 102 and the hydrate pelletization zone 104. The pressure release valve may be any suitable valve for the purposes of the present invention.

The temperature of the hydrate formation zone 102 and the hydrate pelletization zone 104 may be maintained by means of a cooling system 1 14. According to a particular aspect, each of the hydrate formation zone 102 and the hydrate pelletization zone 104 may comprise a cooling system 1 14 configured to circulate cooling liquid through the cooling system 1 14, thereby maintaining the hydrate formation zone 102 and the hydrate pelletization zone 104 at a first pre-determined temperature and a second pre determined temperature, respectively. The first pre-determined temperature and the second pre-determined temperature may be any suitable temperature. The first pre-determined temperature and the second pre determined temperature may be the same or different from each other. According to a particular aspect, the first pre-determined temperature may be 1 -25°C, 5-20°C, 7-18°C, 10-15°C. Even more in particular, the first pre-determined temperature may be 1 -10°C. According to another particular aspect, the second pre-determined temperature may be -25-5°C, -20-3°C, -18-2°C, -15-1 °C, -10-0°C, -8-1 °C, -5-2°C. Even more in particular, the second pre-determined temperature may be -2°C.

The cooling liquid may be any suitable cooling liquid. For example, the cooling liquid may be, but not limited to, refrigerated cooling fluids from an external circulating bath such as R717, R404A, R22, or a combination thereof. The apparatus may further comprise a temperature controller (not shown). The temperature controller may be configured to adjust the amount of cooling liquid circulating through the cooling system 1 14 to maintain the hydrate formation zone 102 and the hydrate pelletization zone 104 at the first pre-determined temperature and the second pre-determined temperature, respectively. In particular, the temperature controller may comprise at least one thermocouple port to measure the temperature at each of the hydrate formation zone 102 and the hydrate pelletization zone 104.

Apparatus 100 may also comprise a piston or extruder 120 configured to move hydrates formed in the hydrate formation zone 102 to the hydrate pelletization zone 104 and eventually out of the apparatus via hydrate outlet 1 16. According to a particular aspect, the hydrate pelletization zone 104 may comprise a pellet die configured to form gas hydrate pellets. The gas hydrate pellets formed may be of any suitable shape and size, as desired.

The piston or extruder 120 may be configured to move by any suitable means. For example, the piston or extruder 120 may be operated by a suitable drive system. In particular, the drive system may be an automatic drive. Further, the piston or extruder 120 may comprise high pressure seals which may be able to withstand hydrate formation pressure.

The piston or extruder 120 may be configured to transfer all the gas hydrates formed in the hydrate formation zone 102 to the hydrate pelletization zone 104. According to a particular aspect, the piston or the extruder 120 may have a diameter which may be in close tolerance to an inner diameter of the chamber comprising the hydrate formation zone 102 and the hydrate pelletization zone 104 such that maximal hydrate transfer may be achieved. There is also provided a hydrate outlet 1 16 for discharging the gas hydrate pellets from the hydrate pelletization zone 104. The hydrate outlet 1 16 may be connected to the hydrate pelletization zone 104 for collecting the formed gas hydrate pellets.

The apparatus may further comprise a liquid outlet 1 18 in fluid communication with the hydrate pelletization zone 104 for discharging residual hydrate forming liquid. The residual hydrate forming liquid may be discharged during the pelletizing of the formed gas hydrates in the hydrate pelletization zone 104. The residual hydrate forming liquid may be fed back into the apparatus 100 for subsequent use. Accordingly, the liquid outlet 1 18 may be in fluid communication with the liquid inlet 108.

According to a particular aspect, the chamber may comprise a stand for fixing the chamber such that the inclination of the chamber may be changed. The apparatus 100 may be provided as a single unit or as multiple units. In particular, when the apparatus 100 is provided as multiple replicating units, the apparatus 100 may be used for continuous gas hydrate pellet formation. This may result in continuous gas consumption at a pre-determined rate. For example, the multiple units may be connected to a common gas feed system, thereby requiring constant gas consumption. When the apparatus 100 is provided as multiple units, each of the two or more units of apparatus 100 may be arranged in parallel formation so that when one unit of apparatus 100 is in gas hydrate pelletization cycle, the other unit may undergo the gas hydrate formation cycle.

An embodiment of the apparatus of the present invention is shown in Figure 2. Figure 2 shows an apparatus for forming gas hydrates, wherein the apparatus comprises a hydrate formation zone and a hydrate pelletization zone in a horizontal chamber, wherein the hydrate formation zone and the hydrate pelletization zone are separated by a ball valve. The hydrate formation zone may be designed for higher pressure rating to form gas hydrates while the hydrate pelletization zone may be operated at atmospheric pressure. The hydrate formation zone and the hydrate pelletization zone may be independently cooled to hydrate formation temperature and hydrate storage temperature, respectively. In particular, there is provided two separate refrigerated circulating baths. The hydrate formation temperature may be about 10°C while the hydrate storage temperature may be about -2°C. In use, the ball valve (rated for high pressure) may be initially closed during the hydrate formation cycle. Once the formation is complete, the pressure may be lowered to atmospheric pressure. The ball valve may be opened following which a piston operated by an electric hydraulic pump may transfer the formed gas hydrates into the hydrate pelletization zone. The piston may be equipped with high pressure seals to withstand hydrate formation pressure and shall be suited to traverse along the entire chamber including the hydrate formation zone and the hydrate pelletization zone. The piston retrieving all the formed gas hydrates from the hydrate formation zone may compress the gas hydrates into gas hydrate pellets by pounding on to a die in the hydrate pelletization zone.

There is also provided a drain port to collect the unconverted hydrate forming liquid that may be recycled for successive hydrate formation cycles. The pellet die/piston may be fabricated to produce single or multiple pellets.

Another embodiment of the apparatus of the present invention is as shown in Figure 3. As shown in Figure 3, there is provided a hydrate formation zone and a hydrate pelletization zone connected through a hopper. Like the apparatus as shown in Figure 2, the apparatus of Figure 3 also comprises a gas inlet port for receiving gas in the hydrate formation zone and a solution inlet port for feeding hydrate forming liquid into the hydrate formation zone. The hydrate pelletization zone may comprise a solution outlet port from which any unconverted hydrate forming liquid may be collected and optionally recycled back to the solution inlet port for successive hydrate formation cycles. There is also provided a cooling system for cooling the hydrate formation zone and the hydrate pelletization zone to suitable hydrate formation temperature and hydrate storage temperature, respectively.

The apparatus also comprises a screw conveyor in a horizontal cylindrical reactor comprising the hydrate formation zone. During hydrate formation, a high pressure knife gate valve may remain closed and the screw conveyor may be equipped with high- pressure seals rated to withstand the gas hydrate formation. After the completion of the gas hydrate formation, gas in the hydrate formation zone may be vented following which the knife gate valve will be opened. The screw conveyor may be operated at optimal speed to extrude the formed gas hydrates in the reactor to the hopper. The hopper may collect the formed gas hydrates into the hydrate pelletization zone which may be cooled to a storage temperature of about -2°C. A piston with a suitable drive arrangement may be operated to compress the formed gas hydrate into gas hydrate pellets at atmospheric pressure. The gas hydrate pellets may be collected and stored in a storage tank. Therefore, it can be seen that the apparatus of the present invention provides a hydrate formation zone and a hydrate pelletization zone integrated into a single apparatus unit for the formation of gas hydrates. Such an apparatus results in minimal energy requirements and reduced capital/operating costs as compared to other hydrate formation apparatus known in the art.

According to a second aspect, the present invention provides a method of forming gas hydrates, the method comprising: - feeding a hydrate forming liquid to a liquid inlet in fluid communication with a hydrate formation zone;

injecting a gas into a gas inlet in fluid communication with the hydrate formation zone;

cooling the hydrate formation zone to a pre-determined temperature to enable formation of gas hydrates; and

pelletizing the gas hydrates in a hydrate pelletization zone.

The method will be described in relation to use of apparatus 100 described above.

The method of the present invention may comprise feeding a hydrate forming liquid into liquid inlet 108. The hydrate forming liquid is fed into the hydrate formation zone 102. The hydrate forming liquid may be any suitable liquid. According to a particular aspect, the hydrate forming liquid may comprise water. According to another particular aspect, the hydrate forming liquid may further comprise, but is not limited to: a thermodynamic promoter, a kinetic promoter, or a combination thereof. The thermodynamic promoter may be any suitable thermodynamic promoter. For example, the thermodynamic promoter may be, but is not limited to: si, sll-, sH-, or semiclathrate- forming compound. In particular, the thermodynamic promoter may be, but is not limited to: tetrahydrofuran (THF), dioxolane, or a combination thereof. Even more in particular, the thermodynamic promoter may be TFIF. The kinetic promoter may be any suitable kinetic promoter. For example, the kinetic promoter may be, but is not limited to: a surfactant or an amino acid. In particular, the kinetic promoter may be, but is not limited to: sodium dodecyl sulphate, tryptophan (TRP), or a combination thereof. Even more in particular, the kinetic promoter may be TRP. Following the addition of the hydrate forming liquid into the hydrate formation zone 102, pressure of the hydrate formation zone 102 may be increased to a pre-determined pressure. The pre-determined pressure may be any suitable pressure. For example, the pre-determined pressure may be 30-95 bar. The pre-determined pressure may be achieved by injecting a gas into the hydrate formation zone 102 via the gas inlet 106. During the injecting, the high pressure valve 1 12 may be closed.

The injecting may comprise injecting any suitable gas. For example, the injecting may comprise injecting a gas selected from, but not limited to: natural gas, biomethane, methane, ethane, propane, carbon dioxide, hydrogen, or a mixture thereof. According to a particular aspect, the gas may be but not limited to, natural gas, biomethane, a mixture of methane, ethane and propane, a mixture of methane and carbon dioxide, or a mixture of hydrogen and carbon dioxide.

Following the injecting, the gas inlet 106 and the liquid inlet 108 may be closed. The method may then comprise cooling the hydrate formation zone 102 to a pre-determined temperature to enable formation of gas hydrates. In particular, the cooling may comprise maintaining the temperature of the hydrate formation zone 102 at a constant pre-determined temperature.

The pre-determined temperature may be any suitable temperature. For example, the temperature may be 1 -25°C, 5-20°C, 7-18°C, 10-15°C. Even more in particular, the temperature may be 1 -10°C.

According to a particular aspect, the cooling may be by any suitable means. For example, the cooling may be by using a cooling system 1 14. In particular, the cooling system 1 14 may comprise circulating a cooling liquid through a cooling jacket around the hydrate formation zone 102.

Upon formation of the gas hydrates in the hydrate formation zone 102, the method may further comprise releasing the pressure inside the hydrate formation zone 102. The releasing may comprise releasing the pressure inside the hydrate formation zone 102 to a pre-determined pressure which may be lower than the pressure at which gas hydrate formation occurs. For example, the pre-determined pressure may be about atmospheric pressure.

The releasing may be by any suitable manner. For example, the releasing may comprise opening a pressure release valve provided in the hydrate formation zone 102. In particular, the releasing may comprise releasing the unused gas inside the hydrate formation zone 102. The method may further comprise opening the high pressure valve 1 12.

The method may further comprise: transferring the gas hydrates formed in the hydrate formation zone to the hydrate pelletization zone prior to the pelletizing. The transferring may comprise moving the piston or extruder 120 to transfer the formed gas hydrates from the hydrate formation zone 102 to the hydrate pelletization zone 104. The transferring may be at a pre-determined speed. In particular, the moving of the piston or extruder 120 may increase the compactness of the formed gas hydrates during the transferring. Once the formed gas hydrates are transferred to the hydrate pelletization zone 104, the formed gas hydrates are subjected to pelletizing in the hydrate pelletization zone 104. The pelletizing may be by any suitable method. According to a particular aspect, the pelletizing may be by compacting the formed gas hydrates using the piston or extruder 120 against a pellet die within the hydrate pelletization zone 104. In particular, the rate of the piston or extruder 120 and the pressure of compacting may be selected according to the desired level of compactness desired for the hydrate gas pellets.

The pelletizing may be performed at any suitable pressure and temperature. For example, the pelletizing may be at atmospheric pressure.

The method may further comprise cooling the hydrate pelletization zone 104 to a pre- determined temperature to enable storage of the gas hydrates. For example, the cooling may be by any suitable means. In particular, the cooling may comprise maintaining the temperature of the hydrate pelletization zone 104 at a constant pre determined temperature. The pre-determined temperature may be any suitable temperature. For example, the temperature may be -25-5°C, -20-3°C, -18-2°C, -15-1 °C, -10-0°C, -8-1 °C, -5-2°C. Even more in particular, the pre-determined temperature may be -2°C.

According to a particular aspect, the cooling may be by any suitable means. For example, the cooling may be by using a cooling system 1 14. In particular, the cooling system 1 14 may comprise circulating a cooling liquid through a cooling jacket around the hydrate pelletization zone 104.

The method may further comprise collecting residual hydrate forming liquid discharged from the liquid outlet 1 18 which is in fluid communication with the hydrate pelletization zone 104. The residual hydrate forming liquid may be fed back to the liquid inlet 108 for the next cycle of forming gas hydrates.

The method may comprise collecting the formed gas hydrate pellets from the hydrate outlet 1 16. The collecting may be by removing the formed gas hydrate pellets through a flange opening in the hydrate pelletization zone 104. 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.

Examples

Method Flydrate formation experiments performed were of batch type and in an unstirred reactor configuration. A hydrate forming liquid comprising 5.6 mol% tetrahydrofuran (THF) (promoter) solution was prepared. The hydrate forming liquid solution was transferred to the reactor that was cooled by external refrigerated circulator to temperatures of 10-20°C. After reaching the required experimental temperature, air present in the reactor was purged and the reactor was pressurized with methane gas to a pressure of 3-7.2 MPa.

Gas hydrates started forming in the reactor after a specific time interval referred to as ‘induction time’. Since the start of hydrate formation, progressive decrease in reactor pressure was observed due to the enclathration of gas molecules in hydrate cages. The pressure drop observed, was related to the number of moles of gas consumed during the gas hydrate formation. The time dependent pressure and temperature allowed estimation of the gas uptake for gas hydrate formation and the normalized rate of methane uptake.

Results & Analysis (i) Effect of temperature on methane/THF hydrate formation kinetics

Experimental pressure was kept constant at 7.2 MPa and experiments were performed at three temperatures of 283.2 K, 288.2 K and 298.2 K (i.e. 10, 15 and 20°C). Figure 4(A) presents plots of gas uptake profiles at each of these conditions and Figure 4(B) presents the variation of induction (nucleation) time, while Figure 4(C) provides the time taken to reach 90% completion for experimental trials performed at the three temperatures.

From the Figure 4(B), it can be seen that time taken for nucleation is lower at lower temperatures (i.e. 283.2 K and 288.2 K) but substantially higher at 293.2 K. Lower driving force at 293.2 K resulted in longer nucleation times. As expected, higher the driving force, the faster the completion of hydrate formation. On average, it took about

46.2 minutes, 1 12.4 minutes and 402.0 minutes for 90% completion of hydrate formation at 283.2 K, 288.2 K, and 293.2 K, respectively as shown in Figure 4(C). It was also found that hydrate formation occurred in multiple stages for experiments at

288.2 K (two stages with the latter stage exhibiting the higher rate of hydrate formation) and 293.2 K (three stages with the third stage exhibiting the highest rate of hydrate formation).

To improve the kinetics of hydrate formation at 293.2 K, sodium dodecyl sulphate (SDS) surfactant (i.e. a kinetic promoter) was added to the THF solution. SDS concentration was varied between 5-2500 ppm and experiments were performed at 293.2 K and at 7.2 MPa.

It was found that addition of 100 ppm SDS resulted in drastic improvement in the rate of hydrate formation resulting in 90% completion of hydrate formation in just 55.6 minutes at 293.2 K, which was similar to that observed at 283.2 K without any kinetic promoter both at the same experimental pressure of 7.2 MPa (see Figure 4(A)). However, the induction time for THF solution with SDS was longer with an average of 223.9 minutes as shown in Figure 4(B) than for experiments conducted without SDS. From these results, it can be seen that presence of small concentrations of a kinetic promoter drastically improved the kinetics of methane hydrate formation at higher temperatures. This is a significant finding due to which there will be a substantial reduction in cooling cost of hydrate formation at 293.2 K in comparison to at 274.2 K or lower temperatures typically used for si pure methane hydrate formation.

(ii) Effect of pressure on methane/TFIF hydrate formation kinetics

To investigate the effect of experimental pressure on the formation kinetics of mixed methane/TFIF hydrates, experiments were performed at different starting pressures of 3.0, 5.0 and 7.2 MPa and at a constant temperature of 283.2 K. The driving force (in terms of pressure) at each of these starting pressures was 6.7 MPa, 4.5 MPa and 2.5 MPa, respectively.

Figure 5(A) presents plots of the gas uptake, Figure 5(B) presents variation of induction (nucleation) time and Figure 5(C) presents the time taken to reach 90% completion for experimental trials performed at the three pressures. The gas uptake was found to lower progressively with the reduction of driving force from 7.2 MPa to 3.0 MPa. However, despite 60% drop in experimental pressure (from 7.2 MPa to 3.0 MPa), only about a 20% drop in total methane uptake was observed (see Figure 5(A)), thus showing substantial methane uptake even at reduced experimental pressures. From Figure 5(B), it can be concluded that induction time increased with the decrease in pressure due to the corresponding reduction in the pressure driving force. Due to the single stage growth observed for 5 and 7.2 MPa, time take for 90% completion of hydrate formation for these two experimental pressures was similar between 40-42 minutes, whereas the lower 3.0 MPa experiments resulted in about an average of about 80 minutes for 90% completion of hydrate formation due to a lower driving force (Figure 5(C)). Substantial methane gas uptake was observed even at lower pressure of 3.0 MPa, thereby resulting in a considerable reduction in gas compression cost when scaling the SNG technology for commercial deployment.

(iii) Multi-scale validation of rapid methane hydrate formation Figure 6 presents the methane uptake achieved at the end of 60 minutes (1 hour) of hydrate formation, normalized methane uptake rate (NR15) and induction time for experiments conducted using three different hydrate forming liquid solution volumes of 2 ml (small scale), 53 ml (medium scale) and 220 ml (large scale) represented as a volumetric scale up factor of 1 , 26.5 and 1 10, respectively. All experiments were conducted at 283.2 K and 7.2 MPa. Each point in the Figure 6 is the average of three or more experiments. It can be seen that the methane uptake rate for the small scale experiments was significantly faster 0.22±0.02 kmol/m ³ /min compared to the medium (0.065±0.01 kmol/m ³ /min) and large scale experiments (0.075±0.01 kmol/m ³ /min).

It is also seen from Figure 6 that the methane uptake capacity is slightly lower at the medium and large scale, signifying the presence of slightly more unconverted hydrate forming liquid. This drop in rate and methane capacity may be attributed to the scale factor and can be seen that for the medium and large scale the values are in the same range (both rate and capacity). In contrast, the nucleation of hydrates is much faster in the medium and large scale reactors resulting in less induction times (about 2-3 minutes) as compared to the small scale reactor, where it took about 120 minutes for nucleation.

(iv) Effect of salt on methane/THF hydrate formation kinetics

Kinetics of mixed methane/THF hydrate formation in the presence of 3.0 wt % NaCI was investigated to assess the possibility of using seawater for methane storage via clathrate hydrates. The use of saline water (i.e. about 3.0 wt% NaCI) for methane storage via clathrate hydrates was demonstrated in a simple quiescent reactor configuration. About 90 v/v methane storage capacity in hydrates was observed when saline water was used and the reaction kinetics was extremely fast resulting in tgo of 13±1 minute. Further, a few preliminary trials of mixed methane hydrate formation with seawater from Singapore having 2.72 wt % salinity were performed at 7.2 MPa and 283.2 K. Methane uptake data of seawater experiments were very similar to saline water (3 wt% NaCI) experiments with tgo of 14.8±0.2 minutes. Thus, there is no significant impact of other salts being present in the seawater on the mixed hydrate formation kinetics. The volume of gas stored per unit volume of hydrate calculated using pure water, salt water and seawater in presence of 5.6 mol% THF is provided in Figure 7. Though the gas content in hydrates in presence of salt water or seawater is slightly lower than fresh water, the formation kinetics is comparable between all experiments. The use of seawater will improve the economic feasibility of SNG formation process as it eliminates the need of pure water, which is scarce in nature and expensive to produce.

(v) Effect of higher hydrocarbons on mixed hydrate formation kinetics

Investigation on the kinetics of mixed hydrate formation using a ternary C1 (93%)- C2(5%)-C3(2%) {C1 - methane, C2-ethane and C3-propane} gas mixture representing the natural gas was performed. The main objective was to examine the effect of higher hydrocarbons (ethane and propane) in influencing the mixed hydrate formation kinetics with the additional presence of thermodynamic promoter, tetrahydrofuran (TFIF). The advantage of employing the natural gas mixture is that the equilibrium conditions of hydrate formation may be milder than that envisaged for pure methane gas, thus resulting in more moderate temperature and pressure conditions of hydrate formation.

Three experimental pressures of 3, 4 and 5 MPa were chosen to determine the effect of varying pressure driving force on mixed C1/C2/C3/THF hydrate formation at 283.2 K. Gas uptake profiles at all three experimental pressures were plotted in Figure 8 along with gas uptake profile recorded for mixed gas hydrate formation employing pure methane at 5 MPa and 283.2 K for comparison. At 3 MPa, the rate of gas uptake was sluggish for about 160 minutes after induction time, subsequently increasing to reach a final gas uptake of 3.44 kmol/m ³ water, with a time taken to 90% completion of 224.67 minutes. While the gas uptake potential was promising, the rate of mixed C1/C2/C3/THF hydrate formation at 3.0 MPa start pressure was too slow. This was due to the lower pressure driving force available that might be insufficient to promote rapid formation of si I mixed hydrate crystals. At subsequent higher experimental pressures of 4 MPa and 5 MPa, higher gas uptakes of about 4.2 and 4.0 kmol/m ³ water respectively were observed. Moreover, the rate of gas uptake for the mixed C1/C2/C3/THF hydrates at both experimental pressures were very similar, except for a varied higher gas uptake rate between 90 to 1 10 minutes.

Due to the higher rate for mixed natural gas hydrate formation, the time taken for 90% completion was about 120 and 1 10 minutes for 4 MPa and 5 MPa experiments respectively. Induction time of less than 3 minutes was observed for these experiments in comparison to about 20 minutes for 3 MPa experiment. Kinetics of mixed hydrate formation for the natural gas (in presence of ethane and propane) was lowered as the pure methane resulted in comparable gas uptake but took longer time for completion. Thus, it showcased the need for the addition of suitable kinetic promoters like surfactants or amino acids for improving the kinetics under the experimental conditions for the natural gas system. In the presence of 200 ppm tryptophan (TRP), at 5 MPa and 283.2 K, the kinetics of hydrate formation had improved considerably, proceeding to completion in about 80 minutes in comparison to about 1 10 minutes for experiments without the tryptophan promoter.

Gas composition of mixed hydrates analyzed after dissociation showed methane composition of 93.1 ±0.1 mol % which was similar as compared to that of the feed gas composition considered. Thus, there was no significant change in the gas composition during the mixed hydrate formation, highlighting the advantage of employing SNG technology for storing natural gas.

(vi) Alternate thermodynamic promoters for mixed hydrate formation

Investigation of the kinetics of methane hydrate formation in the presence of other thermodynamic promoters such as dioxolane and tetrahydropyran were performed. Dioxolane is obtained when one CH group of tetrahydrofuran gets interchanged with an oxygen. Like tetrahydrofuran, dioxolane (1 ,3 dioxolane) is also miscible with water having higher boiling point of 75°C as compared to THF which has a boiling point of 66°C. The higher boiling point results in lower volatility and lower loss of the promoter into the gas phase, thereby favouring multiple cycles of mixed hydrate formation.

Figure 9 presents the comparison between dioxolane and tetrahydrofuran with stoichiometric concentrations at 7.2 MPa and 283.2 K of gas uptake during mixed hydrate formation. It can be observed that the hydrate formation kinetics is sluggish in presence of dioxolane compared to tetrahydrofuran. Further, the gas uptake achieved in presence of dioxolane was about 10% higher than that with tetrahydrofuran under same experimental conditions. Also, with the addition of low concentration of a suitable kinetic promoter (such as 300 ppm tryptophan), the kinetics observed with dioxolane was much faster than the kinetics of tetrahydrofuran along with a higher gas uptake. This showed that alternate thermodynamic promoters may be used for mixed methane (natural gas) hydrate formation in SNG technology.

Another thermodynamic promoter, tetrahydropyran (THP), was also investigated, which was obtained from tetrahydrofuran with an addition of one CH group. Although the boiling point of tetrahydropyran is 88°C, which is much higher than tetrahydrofuran and dioxolane, the solubility in water is very low, thus rendering immiscible solution (presence of a layer of THP above the water). The gas uptake achieved was much lower and the kinetics of hydrate formation is quite sluggish. Thus, it may be concluded that a thermodynamic promoter which is soluble in water and having similar chemical structures would aid in mixed methane hydrate formation with enhanced kinetics as compared to those which are insoluble in water.

(vii) Stability analysis of stored gas hydrate pellets

Mixed CH ₄ -THF hydrates were formed at 7.2 MPa and 283.2 K in a high-pressure reactor using 5.56 mol % tetrahydrofuran. Formed gas hydrates were recovered from the reactor at a low temperature (268 K and atmospheric pressure) after completion of gas hydrate formation. The mixed methane-THF hydrates were known to be stable at temperatures lower than 277.7 K at atmospheric pressure. The recovered gas hydrates were taken in a pre-cooled pelletizer die and pressed through a manual hydraulic press till a pressure of about 10 bar. Gas hydrate pellets were produced which had a diameter of about 4 cm and a 1.1 cm thickness. The mixed gas hydrate pellets were stored at about 1 atm and -2°C in a stainless steel container connected to a pressure gauge and a thermocouple for continuous monitoring in a simple conventional laboratory freezer. As can be seen from Figure 10, stability of the si I mixed (CH ₄ -THF) hydrates was demonstrated for a period of twelve months. The exemplary embodiments above are only examples, and are not intended to limit the scope, applicability, operation or configuration of the invention in any way.