WO2011021019A1

WO2011021019A1 - Gas hydrate

Info

Publication number: WO2011021019A1
Application number: PCT/GB2010/051234
Authority: WO
Inventors: Andrew Cooper; David Adams; Benjamin Carter; Weixing Wang
Original assignee: Ulive Enterprises Ltd
Current assignee: Ulive Enterprises Ltd
Priority date: 2009-08-21
Filing date: 2010-07-27
Publication date: 2011-02-24
Anticipated expiration: 2012-02-21
Also published as: GB0914625D0

Abstract

The present invention relates to a gas hydrate (often referred to as a clathrate) comprising a water-in-gas emulsion containing a gelling agent which is stabilised by a network of hydrophobic particles and an enclathrated exogenous gas such as for example methane, natural gas, hydrogen, krypton or carbon dioxide.

Description

Gas Hydrate

Gas hydrates are non-stoichiometric, crystalline inclusion compounds which comprise a "host" assembly of water cages in which is entrapped a "guest" gas. The hydrogen-bonded water lattice traps gas molecules within polyhedral cavities. There is currently significant interest in the use of gas hydrates as an effective, economical and user-friendly means of storage, transportation and release of a range of gases. For example, one volume of methane gas hydrate (MGH) can yield approximately 180 v/v STP methane leading to the proposal that it may be economically feasible to transport natural gas in a hydrated form to provide an important source of natural energy (see Sloan, E. D. Nature, 2003, 426, 353-359). Similarly gas hydrates have the potential to provide a safe and environmentally friendly means for hydrogen storage (see for example Struzhkin, V. V.; Militzer, B.; Mao, W. L.; Mao, H. K.; Hemley, R. J. Chem. Rev. 2007, 707, 4133).

The use of clathrates as gas storage means has so far been severely limited by practical considerations such as trapped, unreacted interstitial water in the hydrate mass, thermal stability and the slow rates of formation. Common methods for increasing the kinetics of clathrate formation are generally achievable in the laboratory but are generally neither cost effective nor practical in real gas storage applications. For example, the rate of nucleation and growth can be increased by operating at either higher pressure or lower temperatures but such conditions may be undesirable for many applications. Alternatively the kinetics of clathrate formation may be increased by increasing the interfacial contact between liquid water and the gas by (for example) vigorous mixing, grinding of ice particles, surfactants and supports such as silica or a high surface area emulsion-templated polymer to generate a thin, supported water layer in contact with the gas. Vigorous mixing increases gas- liquid contact but requires substantial and sustained energy input. Promoters such as tetrahydrofuran, cyclopentane or an alkyl ammonium salt are generally effective. However THF and cyclopentane are volatile and highly flammable and THF is toxic. Furthermore the promoter occupies some of the cavities in the clathrate and the volume available for the trapping of gas is reduced. Grinding and sieving of ice to form high surface area produces small crystals but is laborious. The particles also have a tendency to sinter with the resulting loss of the advantages of the particulate nature. A high surface area support such as a polyHIPE may be used to aid mass transport.

Dry water (DW) is a water-in-air emulsion consisting of water droplets surrounded by a network of hydrophobic fumed silica nanoparticles which prevent droplet coalescence (see Binks, B. P.; Murakami, R. Nature Mater. 2006, 5, 865-869). It generally takes the form of a free-flowing powder which can be readily handled under ambient conditions and which is prepared by mixing water, hydrophobic silica particles and air at high speed. The network of hydrophobic particles prevents coalescence by effectively coating the water droplets thereby reducing the energy required to keep the water in droplet form in a gaseous medium relative to the energy required to form bulk water. Dry Water exhibits high rates of methane uptake to form dry water-methane gas hydrate DW-MGH (see Wang, W. X.; Bray, C. L.; Adams, D. J.; Cooper, A. I. J. Am. Chem. Soc. 2008, 130, 11608-11609).

The present invention is based on the recognition that the stability of dry water to coalescence can be increased significantly by the inclusion of a gelling agent. In particular, the present invention relates to a gas hydrate containing a gelling agent which imparts a level of cyclability previously unseen in gas hydrates without the need for repeated stirring or a porous support.

Thus viewed from a first aspect the present invention provides a gas hydrate comprising: a water-in-gas emulsion containing a gelling agent which is stabilised by a network of hydrophobic particles; and an exogenous gas enclathrated in the water-in-gas emulsion.

The gas hydrate of the invention exhibits advantageous enclathration kinetics and recyclability. The gas hydrate may represent a potential platform for recyclable gas storage or gas separation on a practicable timescale in a static, unmixed system.

Preferably the water-in-gas emulsion comprises droplets or particles.

Preferably the gelling agent is present in the water phase as an aqueous suspension, dispersion, colloid or solution. Typically the gelling agent is present in an amount in water of 30wt% or less, preferably 20wt% or less, particularly preferably in the range 1 to 20wt%, more preferably 10 to 20wt%.

Preferably the gelling agent is a hydrocolloid gelling agent.

Preferably the gelling agent gelates at low concentration in water. Preferably the gelling agent gelates at an aqueous concentration of 20wt% or less, particularly preferably 10wt% or less.

The gelling agent may be natural or synthetic. The gelling agent may be protein-based or polysaccharide-based. Typically the gelling agent is natural gum, starch, pectin, agar, agarose or gelatin, preferably natural gum (eg gellan gum), agarose or gelatin.

Preferably the gelling agent is agarose. Preferably the gelling agent is gelatin.

Preferably the gelling agent is a gum, particularly preferably gellan gum. The addition of gellan gum typically allows effective re-use of the gas hydrate over at least eight clathration cycles without the need for reblending.

Preferably the water-in-gas emulsion containing gelling agent is obtainable by agitating an aqueous solution of the gelling agent in the presence of hydrophobic particles. Particularly preferably the aqueous solution of the gelling agent is at an elevated temperature. Typically the concentration of the aqueous solution of the gelling agent is 10wt% or less.

Preferably the water-in-gas emulsion containing gelling agent is obtainable by gelating an aqueous solution of the gelling agent to form a gel and agitating the gel in the presence of hydrophobic particles. The aqueous solution of the gelling agent is typically prepared at an elevated temperature and is gelated by cooling to ambient temperature. Typically the concentration of the aqueous solution of the gelling agent is 10wt% or more.

Typically the exogenous gas predominantly occupies cavities in the caged framework structure of the water-in-gas emulsion. The gas hydrate may be a clathrate. A proportion of the exogenous gas may form part of the caged framework structure. The gas hydrate may be a semi-clathrate.

Preferably the water-in-gas emulsion is a water-in-air emulsion. Preferably the gas of the water-in-gas emulsion is the same as the exogenous gas. This conveniently promotes optimum enclathration or release of the exogenous gas and avoids its contamination. For example, the water-in-gas emulsion may be a water-in-methane emulsion.

Preferably the water-in-gas emulsion comprises 95wt% or more of water.

Preferably the water-in-gas emulsion is non-agglomerative. Preferably the water-in- gas emulsion is non-coalescent.

The hydrophobic particles may be selected from the group consisting of modified silica particles, polymer particles, hydrophobically modified inorganic particles and polymer latex particles.

The hydrophobic particles may be particles of a fluoropolymer such as Zonyl® MP 1400 (Dupont).

The hydrophobic particles may be hydrophobic silica particles (preferably hydrophobic fumed silica particles). The hydrophobic silica particles may be surface modified (eg surface modified by siloxy groups such as dimethylsiloxy, polydimethylsiloxy or trimethylsiloxy groups). The hydrophobic silica particles may be alkylated or fluorinated silica.

Hydrophobic silica particles are available commercially from Wacker Chemie AG. The hydrophobic silica particles may be one or more of the group consisting of HDKl 8. HDKl 3L, HDKl 5, HDK20, HDK30, HDKl 7, HDK2000, HDK20RM and HDK30RM from Wacker Chemie AG. Preferred is HDKl 8.

Preferably the hydrophobic silica particles have a residual silanol content of 66wt% or less, particularly preferably 50wt% or less, more preferably 25wt% or less.

Enclathration of an exogenous gas may be carried out for example by adding the exogenous gas to a conventional containment vessel such as a pressure reactor in the absence of mechanical agitation and proceeding at high pressure and low temperature. Enclathration may be monitored for example by observing the pressure drop or temperature rise in the vessel as a function of time using conventional apparatus.

The exogenous gas enclathrated in the water-in-air emulsion is typically released by heating. Typically the exogenous gas is a non-ambient gas (ie a gas other than air). Preferably the exogenous gas is hydrogen, carbon dioxide or a saturated or unsaturated hydrocarbon (eg a Ci_-4 hydrocarbon). The exogenous gas may be for example methane, ethane, ethene, propane, propene or butane.

The exogenous gas may be CH₄, CO₂, O₂, H₂, N₂, H₂S, Ar, Kr, Xe, He, Ne or a mixture thereof. The exogenous gas may be mixed with air.

Preferred is an exogenous gas selected from the group consisting of methane, hydrogen, krypton, carbon dioxide, a hydrocarbon and natural gas.

Preferably the exogenous gas is hydrogen. Preferably the exogenous gas is carbon dioxide. Preferably the exogenous gas is methane. Preferably the exogenous gas is krypton. Preferably the exogenous gas is natural gas.

The gas hydrate may further comprise a stabilizer or promoter. The stabiliser or promoter may serve to lower the gas clathration pressure. The stabilizer or promoter may be enclathrated or may form part of the caged framework structure.

Preferably the average size of the primary droplets is less than about lmm, preferably less than lOOμm. The droplets may form agglomerates which are larger than the primary droplet. The diameter of the agglomerates may be (for example) up to lmm.

Preferably the amount of hydrophobic particles relative to the amount of water and any stabiliser is no more than 20wt%, more preferably no more than 15wt%, more preferably no more than 10wt%, even more preferably no more than 5wt%, most preferably no more than lwt%.

Typically the hydrophobic particles have an upper size limit of 500 nm, 100 nm or 50 nm. For example, the size of the hydrophobic particles may be approximately 10 nm to 20 nm. Viewed from a further aspect the present invention provides use of a water-in-gas emulsion containing a gelling agent which is stabilised by a network of hydrophobic particles in the enclathration of an exogenous gas.

The use of a water-in-gas emulsion containing a gelling agent which is stabilised by a network of hydrophobic particles according to the present invention may be deployed in gas storage, gas transportation/distribution, fuel use, gas sequestration, waste gas trapping and the separation of one or more gases from a mixture (for example by preferential enclathration of methane over hydrogen, ethane or propane, carbon dioxide over methane or nitrogen or hydro fluorocarbons from gas mixtures).

The use of a water-in-gas emulsion containing a gelling agent which is stabilised by a network of hydrophobic particles allows gas uptake to occur rapidly and reproducibly without agitation or mechanical mixing.

Viewed from a yet further aspect the present invention provides a process for preparing a gas hydrate as hereinbefore defined comprising:

(a) exposing a water-in-gas emulsion containing a gelling agent which is stabilised by a network of hydrophobic particles to an exogenous gas in a contained environment at a temperature less than ambient temperature and an elevated pressure.

The pressure may be in the range 1 to 12MPa, preferably 6 to 10MPa. The temperature may be 293K or less, preferably 273K or less. Step (a) may be carried out without forced agitation. Preferably step (a) is preceded by:

(aθ) agitating an aqueous solution of the gelling agent in the presence of hydrophobic particles. Particularly preferably the aqueous solution of the gelling agent is at an elevated temperature. Typically the concentration of the aqueous solution of the gelling agent in step (aθ) is 10wt% or less.

Alternatively preferably step (a) is preceded by:

(aOO) gelating an aqueous solution of the gelling agent to form a gel; and

(aθl) agitating the gel in the presence of hydrophobic particles. The aqueous solution of the gelling agent is typically prepared at an elevated temperature and is gelated by cooling to ambient temperature. Typically the concentration of the aqueous solution of the gelling agent in step (aOO) is 10wt% or more.

Steps (aθ) and (aθl) may be carried out by mixing or blending. For example, step (aθ) and step (aθl) may be carried out by mixing at a speed of 19000rpm or more.

Steps (aθ) and (aθl) may be carried out straightforwardly in known apparatus. For example, the solution of the gelling agent may be agitated by stirring at high speed {eg in a domestic food blender). For industrial applications, a rotastater, a blender, a folder or a sonicator may be used.

The aqueous solution of the gelling agent may be at a concentration of 30wt% or less, preferably 20wt% or less, particularly preferably in the range 1 to 20wt%, more preferably 10 to 20wt%.

The present invention will now be described further by way of non-limiting example with reference to the following drawings in which:

Figure 1 is a schematic diagram of the experimental setup;

Figure 2 illustrates free-flowing dry water powder photographed flowing through a funnel;

Figure 3 shows methane uptake kinetics in DW at different formation temperatures (initial pressure 10.0 MPa). A formation temperature of 273 K was found to be optimal under these conditions;

Figure 4 shows P-T plots for cooling and heating under CH₄ pressure (temperature ramp: 2.0 K/h) for dry water prepared using different ratios of hydrophobic silica to water. For each ratio, three different initial pressures are shown. The si MGH phase boundary curve was generated using CSMPlug software (Sloan, E. D.; Koh, C. A. Clathrate Hydrates of Natural Gases; 3rd ed.; CRC Press: Boca Raton, 2008);

Figure 5 illustrates methane uptake kinetics in DW at 273.2 K with different ratios of silica to water (initial pressure 8.5 MPa);

Figure 6 shows optical microscope images at 1 Ox magnification for (a) dry water, (b) a water- in-air emulsion prepared with 1 wt% gellan gum and (c) a water-in-air emulsion prepared with 3 wt% gellan gum; Figure 7 shows P-T plots for a water-in- air emulsion prepared with 1 wt% gellan gum over multiple cycles (initial pressure 8.6 MPa);

Figure 8 shows P-T plots for a water-in-air emulsion prepared with 3 wt% gellan gum over multiple cycles (initial pressure 8.8 MPa);

Figure 9 illustrates (a) free-flowing powder prepared from blending gellan gum gel ( 10 wt%) with silica (5 wt%), (b) free-flowing powder prepared from blending gellan gum gel (20 wt%) with silica (10 wt%), (c) optical microscope image at 10Ox magnification of the powder shown in (a) and (d) optical microscope image at 10Ox magnification of the powder shown in (b);

Figure 10 illustrates (a) methane uptake kinetics in water, DW and "dry gels" prepared from 10 and 20 wt% gellan gum gel at 273.2 K with different ratios of silica to water (initial pressure 8.5 MPa), (b) recyclability of "dry gel" for methane storage (1O g dry gel prepared from 10 wt% gellan gum gel with 5 wt% silica) and (c) recyclability of "dry gel" for methane storage (1O g dry gel prepared from 20 wt% gellan gum gel with 10 wt% silica);

Figure 11 shows P-T plots for CO₂ and DW during cooling and heating (temperature ramp: 2.5 K/h): (A) unblended mixture of water (9.5 g) and hydrophobic silica nanoparticles Hl 8 (0.5 g) and (B) 10 g DW powder (95 g of water and 5 g of hydrophobic silica nanoparticles Hl 8) formed by mixing at 19,000 rpm for 90 seconds;

Figure 12 illustrates P-T dependence for H₂O-CO₂ system during cooling and heating (temperature ramp: 2.5 K/h);

Figure 13 illustrates CO₂ uptake kinetics in DW at 273.2 K (initial pressure 3.3 MPa);

Figure 14 shows P-T plots for Kr and DW during cooling and heating (temperature ramp: 2.5 K/h): (A) unblended mixture of water (9.5 g) and hydrophobic silica nanoparticles Hl 8 (0.5 g) and (B) 10 g DW powder (95 g of water and 5 g of hydrophobic silica nanoparticles Hl 8) formed by mixing at 19,000 rpm for 90 seconds;

Figure 15 illustrates krypton uptake kinetics in DW at 273.2 K (initial pressure 4.5 MPa);

Figure 16 is P-T plots showing partial recyclability of a DW-MGH system (19.O g water and 1 g Hl 8 formed at 19,000 rpm for 90s) over three cooling/heating cycles under

CH4 pressure (temperature ramp: 2.0 K/h); Figure 17 illustrates P-T plots for a water-in-air emulsion prepared with 10 wt% agarose over multiple cycles;

Figure 18 illustrates P-T plots for a water-in-air emulsion prepared with 10 wt% gelatin over multiple cycles; and

Figure 19 illustrates P-T plots for a water-in-air emulsion prepared with 5wt% gellan gum over multiple cycles for CO2.

EXAMPLE Materials and Methods

Hydrophobic silica nanoparticles (Hl 8) were supplied by Wacker-Chemie. Glass beads (diameter 3mm) were purchased from Sigma-Aldrich. Gellan gum was purchased from Apollo Scientific. The morphology of the various dry water samples was observed using an Olympus CX41RF Microscope fitted with a Linkam FDCS 196 variable temperature stage. Photographs were taken with an Olympus C-5060 digital camera.

Synthesis of Dry Water

Dry water was prepared by rapid mixing of hydrophobic silica (Hl 8), water and air in a conventional blender as detailed below. The particle size was altered by varying the speed at which mixing was carried out.

Deionized water (95 mL) was poured into a blender (Breville Glass Jug Blender, BLl 8, 1.5 L, total air volume with the lid fitted was estimated to be 1.7 L) and Hl 8 (5 g) was added. Mixing was carried out at the highest speed setting (average speed of 19,000 rpm) for 90s in three 30s bursts to minimize particle dissociation due to heat generated while mixing. The volume ratio of air to water/silica was the same in each preparation. The material was produced as a free-flowing dry white powder which could easily be poured from one vessel into another without any residue (see Figure 2) and which was stable in a sealed polypropylene bottle under ambient conditions for one month.

A water-in-air emulsion containing gellan gum was prepared in the same fashion using a solution at the appropriate concentration prepared at a blending temperature in the range 333-343 K to ensure complete dissolution. The blender was pre-warmed with hot water before addition of the gellan gum solution. Alternatively a pre-formed gel was directly blended with silica. For example, a pre-formed, cooled 10 wt% gellan gum solution was prepared and then blended at room temperature for 3 minutes at 19,000 rpm. For a 20 wt% gellan gum solution, blending was carried out at 19,000 rpm for 90 seconds and then at 37,000 rpm for 30 seconds.

Clathrate Hydrate Formation

To carry out gas uptake kinetic experiments, 20.0 g of dry water (or control samples consisting of either glass beads (19.5 cm³) or unmixed water and silica (19 cm³ + 0.5 cm³)) was loaded into a 68.0 cm³ high pressure stainless steel cell (New Ways of Analytics, Lδrrach, Germany). The temperature of the coolant in the circulator bath was controlled by a programmable thermal circulator (HAAKE Phoenix II P2, Thermo Electron Corporation). The temperature of the compositions in the high pressure cell was measured using a Type K Thermocouple (Cole-Parmer, -250-400 ⁰C). The gas pressure was monitored using a High- Accuracy Gauge Pressure Transmitter (Cole-Parmer, 0-3000 psia). Both thermocouple and transmitter were connected to a Digital Universal Input Panel Meter (Cole-Parmer) which communicated with a computer. Prior to experiments, the cell was slowly purged with gas (Methane (UHP 99.999%); CO₂ (liquid withdrawal): BOC Gases, Manchester, UK; Krypton (compressed): Air Liquide, Birmingham, UK) three times at atmospheric pressure to remove any air and then pressurized to the desired pressure at the designated temperature. The temperature (T, K), pressure (P, Psi) and time (t, min) were automatically interval-logged using MeterView 3.0 software (Cole-Parmer). Using this set-up it was possible to obtain high resolution data (for example, 2 seconds between individual [T, P, t] points, 120,000 data points in a 2000 min experiment). The apparatus is shown schematically in Figure 1.

Pressure-Temperature (P-T) Dependence Measurements

Studies of the onset and extent of clathration were typically carried out by pressurizing a 20 g sample before ramping slowly (2 K/h) from 293 K to 273 K and back. The temperature and pressure were logged at 10 minute intervals. At the onset of clathration, an exotherm was observed due to the heat of crystallization. This was accompanied by a drop in system pressure (AP) as the gaseous guest was incorporated into the solid hydrate. Conversely dissociation was observed as a sudden rise in pressure upon release of enclathrated gas with the P-T curve departing from an ideal gas-type relationship.

The recyclability of samples was examined by back-to-back repeats of the ramping cycle. A drop in system capacity was evidenced by a reduction in AP as less gas was incorporated in the clathrate. Capacity was measured directly by conducting only the cooling ramp of the cycle before quickly venting the vessel of non-enclathrated gas and re-sealing. After warming back to 293 K, the volume of gas released from the sample was measured via volumetric displacement of water.

Kinetic Measurements

The rate of gas uptake at a constant temperature was studied by observing gas pressure change as a function of time. Typically a 20 g sample was cooled to and held at 273 K. The vessel was then pressurized with gas (as required). Temperature and pressure were logged at 10 minute intervals over a period of hours or days as appropriate.

Calculation of Capacity

The free space volume of the vessel was obtained by subtracting the sum volume of methane clathrate hydrate, unreacted water and Hl 8. Taking into account non-ideality factors, GASPAK v3.41 software (Horizon Technologies, USA) was employed to calculate the methane enclathration capacity according to the pressure and the temperature. It was assumed that the liquid and gas phases inside the vessel were formed exclusively from water and the gas respectively (neglecting any dissolution of the gas into the liquid phase and any mixing of the water vapor in the gas phase). The temperature inside the vessel was assumed to be uniform throughout the operation.

Effect of Temperature on the Kinetics of MGH Formation

The rate of formation was found to be fast over a range of formation temperatures (Figure 3). The time required to reach 90% of total methane uptake (t₉₀) was measured as 160 min at 273 K. However, the final capacity of the system was strongly affected by the temperature of MGH formation with the highest capacity being obtained between 273 and 277 K. A formation temperature of 273 K was found to be optimal with the gas uptake capacity and rate falling off both above and below this temperature. Given that the dry water sample was cooled prior to pressurizing with methane, gas uptake occurs across a gas/solid rather than a gas/liquid interface at temperatures below 273 K.

Ratio of Silica to Water

Figure 4 shows the cooling/heating curves for CH₄-DW using DW prepared at three different ratios of water to silica. The droplet size in particle-stabilized systems is known to be controlled by the process of limited coalescence. Because the solid silica particles are totally and irreversibly adsorbed at the interface (ie there are no free particles in the continuous phase), the inverse average droplet radius varies linearly with the amount of particles. It was anticipated that the droplet size in the DW systems would be reduced at lower ratios of water to silica. However this was difficult to measure directly and quantitatively without perturbing the system. At higher particle concentrations, the density of dry water is known to be lower. Imaging the DW particles by optical microscopy was carried out for samples produced at various blending speeds. However water tended to be lost during imaging and the droplet size was too small for size quantification. Scanning electron microscopy (SEM) in vacuo was similarly not possible. High-speed freeze-drying also yielded no useful results since the silica shell (which is not covalently bonded) did not maintain its integrity in the absence of water. Estimates of DW particle size were therefore made using a temperature-regulated optical microscope stage set to ~278 K. Nevertheless it proved difficult to image sufficient particles to provide a reliable quantitative size distribution before significant water loss occurred.

In an unmixed bulk control system, the P-T relationship for CH₄ and water approximated to the ideal gas law during a continuous cooling/heating cycle. There was no evidence for substantial MGH formation or dissociation under these conditions. By contrast, MGH formation and subsequent dissociation occurred in all cases for particulate DW as shown by the dramatic pressure drop upon cooling and the rapid pressure rise upon heating respectively (see Figure 4). In all cases, the CH₄ uptake was high (approximately 175 v/v). MGH formation occurred when cooling to 270.0 K at a rate of 2.0 K/h with an associated exotherm at approximately 279 K attributable to the heat of crystallization for the clathrate. MGH dissociation occurred on warming. The dissociation temperature was strongly dependent on the pressure of the system with a higher pressure resulting in a higher dissociation temperature. This dissociation temperature closely followed the phase boundary curve for si MGH. As can be seen in Figure 4, varying the amount of silica used to prepare the dry water had no discernable effect on the formation or dissociation temperatures of the MGH at different pressures. As such, there is no advantage in this respect in increasing the silica loading above 10 wt % and indeed this introduces an additional gravimetric penalty.

The kinetics of methane uptake were significantly greater for DW than for bulk water (Figure 5). At 273 K, a very low uptake was observed for bulk water over 1000 minutes. With DW, a rapid uptake of methane was observed. In keeping with the data presented in Figure 4, no significant kinetic differences were observed between systems in which DW was prepared using different ratios of silica to water. The lack of variation in uptake rate between samples prepared at different ratios of silica to water implies that particle size does not vary significantly as a function of silica content during mixing. Blending speed appears to have a greater impact on droplet size.

Whilst dry water dramatically promotes the rate at which MGH is formed, a key practical question is the re-use and recyclability of the system. Figure 16 shows that dry water is not that stable over three re-uses. It would be advantageous (for example) to form a powder by blending which could be used for multiple gas storage and discharge cycles without remixing. The DW system can be reused after MGH dissociation but the storage capacity and kinetics degrade significantly after a few cycles. This stems from partial agglomeration of the water droplets which are destabilized by the freezing and warming process. Reblending the DW results in the original enclathration kinetics being regenerated. In principle, increased silica content might give rise to greater stability and recyclability but this was not found to be the case. In all cases, partial agglomeration of water droplets occurred upon warming. The volume of separated bulk water thus evolved was 1.8 ± 0.3 mL for all ratios of silica to water. Direct recycling of the system (without re-blending) led to some drop-off in gas uptake capacity.

Gellan gum: "Dry gel "formation

A water-in-air emulsion ("dry gel") was prepared using a solution of hydrocolloid gelling agent. A hot aqueous solution of 1.0 wt% gellan gum was blended with silica to yield particles which were imaged by optical microscopy (see Figure 6b). Optical measurements taken from a cold microscope stage (278 K) indicated an average primary particle size calculated from at least 130 droplets of 79 ± 23 μm for the 1.0 wt% solution blended at 19,000 rpm (compared with 52 ± 14 μm for dry water). P-T experiments with methane revealed that the "dry gel" did not lead to significant improvements in recyclability or in the gas uptake capacity relative to dry water (Figure 7). A gradual loss of capacity was observed over repeat cycles.

Increasing the concentration of gellan gum to 3 wt % resulted in the formation of a water-in-air emulsion with an average primary droplet size calculated from 130 droplets of 163 ± 46 μm (see Figure 5c). The more viscous solution also yielded some larger secondary "dry gel" particles (of the order of millimeters). The water-in-air emulsion containing 3 wt% gellan gum performed better in terms of methane capacity and cyclability than the water-in- air emulsion containing 1 wt% gellan gum. Over five successive cycles, only a small reduction in ΔP was observed (1.23 MPa). This suggests that dry gel maintains its integrity despite multiple cycles of freezing and thawing. In addition, the dry gel particles of the water- in-air emulsion containing 3 wt% gellan gum remained discrete following clathration (see Figure 8) and released a volume of methane equivalent to a capacity of 144 v/v. These findings show that a gelling agent can impart a level of cyclability previously unseen in clathrates without the need for repeated stirring or a porous support.

In a variation on these experiments, a gellan gum gel was pre-prepared by allowing a hot solution of the gellan gum to cool and gelate. The pre-prepared gellan gum gel was cut up and blended in the presence of silica. The pre-prepared gel blended in this way generated a free-flowing powder with a visual appearance and flow properties which were similar to dry water (see Figures 9a and b). A first water-in-air emulsion was prepared at a concentration of 10 wt% gellan gum with 5 wt% silica and a second water-in-air emulsion was prepared at a concentration of 20 wt% gellan gum with 10 wt% silica. It should be noted that an attempt to prepare water-in-air emulsions with similar concentrations of gellan gum in hot solution failed due to the high viscosity.

The water-in-air emulsion prepared from 10 wt% gellan gum appeared qualitatively similar to dry water and consisted of well-defined droplets with an average diameter calculated from 151 droplets of 64 ± 15 μm (see Figure 9c). The water-in-air emulsion prepared from 20 wt% gellan gum had a similar morphology but also contained elongated, string-like structures (see Figure 9d). As such, the average droplet size was not calculated. The methane clathration properties of both emulsions were examined and gas uptake kinetics for both were found to be similar to dry water (see Figure 10a). The final gas capacities (156 v/v for the water- in-air emulsion prepared from 10 wt% gellan gum and 130 v/v for the water-in-air emulsion prepared from 20 wt% gellan gum) were found to be lower than that observed for dry water. This decrease in storage capacity was offset by a remarkable improvement in recyclability over many cycles (Figures 1 Ob and c) without the need for any re-blending between runs. In particular, the water-in- air emulsion prepared from 20 wt% gellan gum was found to be perfectly recyclable over eight heating/cooling cycles. This is a major improvement over dry water. The "dry gel" method therefore results in a water-in-air emulsion which combines the benefits of fast kinetics, easy handling, good gas storage capacity and excellent recyclability.

Recyclability can be achieved for CO₂ clathration using gellan gum. This is illustrated in the P-T plots for a water-in-air emulsion prepared with 5wt% gellan gum over multiple cycles for CO₂ (see Figure 19).

Agarose: " Dry gel " formation

In a similar manner, agarose (10wt%) was used as a gelling agent with silica (10wt%) for methane clathration and re-use. The results are given in Figure 17.

Gelatin: "Dry gel" formation

In a similar manner, gelatin (10wt%) was used as a gelling agent with silica (10wt%) for methane clathration and re-use. The results are given in Figure 18.

Clathrate Formation in Dry Water with CO₂ and Kr

The P-T plot for CO₂ in an unmixed system containing water and hydrophobic silica approximated to the ideal gas law during a continuous cooling/heating cycle (ie very little CO₂ was observed). In the case of DW, a rapid pressure drop was observed on cooling with an associated exotherm which can be ascribed to CO₂ clathration (see Figure 1 1). As for MGH, the formation and dissociation temperatures are related to the pressure of the system (see Figure 12). There is a maximum pressure under which the clathrate can be formed which is defined by the gas-liquid phase boundary for CO₂ as shown in Figure 12.

The rate of formation of CO₂ gas hydrate was significantly enhanced in dry water compared with unmixed systems (see Figure 13). The clathration rate for the unmixed system was however higher than that observed for MGH probably because of the greater solubility of CO₂ in water compared to methane. The maximum CO₂ capacity for the system was found to be -150 v/v. Based on structural data for si CO₂ hydrate, the expected maximum should be comparable with that of si MGH (180 v/v). This reduced figure for CO₂ hydrate capacity might be a result of incomplete occupancy of the si CO₂ hydrate cages or the co- crystallisation of sll CO₂ hydrate alongside the expected si phase

Cooling DW under a pressure of krypton resulted in the characteristic exotherm associated with clathration (see Figure 14). On re-warming, dissociation of the krypton clathrate was observed. An increase in the rate of clathration was also observed as compared to the unmixed control system (see Figure 15). It is therefore clear that DW can be used to increase the kinetics of formation of both si and sll clathrates.

Conclusion

Dry water can be used to increase the kinetics of formation of a range of gas hydrates including CH₄, CO₂ and Kr. The incorporation of a hydrocolloid gelling agent dramatically increases recyclability.

Claims

1. A gas hydrate comprising: a water-in-gas emulsion containing a gelling agent which is stabilised by a network of hydrophobic particles; and an exogenous gas enclathrated in the water-in-gas emulsion.

2. A gas hydrate as claimed in claim 1 wherein the gelling agent is a hydrocolloid gelling agent.

3. A gas hydrate as claimed in claim 1 or 2 wherein the gelling agent gelates at an aqueous concentration of 20wt% or less.

4. A gas hydrate as claimed in any preceding claim wherein the gelling agent is protein- based or polysaccharide-based.

5. A gas hydrate as claimed in any preceding claim wherein the gelling agent is natural gum, agarose or gelatin.

6. A gas hydrate as claimed in any preceding claim wherein the water-in-gas emulsion is a water-in-air emulsion.

7. A gas hydrate as claimed in any preceding claim wherein the exogenous gas is selected from the group consisting of methane, hydrogen, krypton, carbon dioxide, a hydrocarbon and natural gas.

8. A gas hydrate as claimed in any preceding claim wherein the water-in-gas emulsion is obtainable by agitating an aqueous solution of the gelling agent in the presence of hydrophobic particles.

9. A gas hydrate as claimed in claim 8 wherein the aqueous solution of the gelling agent is at an elevated temperature.

10. A gas hydrate as claimed in any of claims 1 to 7 wherein the water-in-gas emulsion is obtainable by gelating an aqueous solution of the gelling agent to form a gel and agitating the gel in the presence of hydrophobic particles.

11. Use of a water-in-gas emulsion containing a gelling agent which is stabilised by a network of hydrophobic particles in the enclathration of an exogenous gas.

12. A process for preparing a gas hydrate as defined in any of claims 1 to 10 comprising:

13. A process as claimed in claim 12 wherein step (a) is preceded by:

(aθ) agitating an aqueous solution of the gelling agent in the presence of hydrophobic particles.

14. A process as claimed in claim 12 wherein step (a) is preceded by:

(aOO) gelating an aqueous solution of the gelling agent to form a gel; and (aθl) agitating the gel in the presence of hydrophobic particles.