WO2010010372A1

WO2010010372A1 - Clathrates for gas storage

Info

Publication number: WO2010010372A1
Application number: PCT/GB2009/050797
Authority: WO
Inventors: Andrew Cooper; David Adams; Christopher Bray; Weixing Wang
Original assignee: Ulive Enterprises Limited
Priority date: 2008-07-25
Filing date: 2009-07-07
Publication date: 2010-01-28
Also published as: EP2318486A1; GB0813650D0; CN102119207A; JP2011529036A; CA2731389A1; US20110185623A1

Abstract

This invention relates to a gas hydrate (often referred to as a clathrate) comprising a water- in-gas emulsion (egdrywater (DW)) and an enclathrated exogenous gas such as for example methane, natural gas, hydrogen orcarbon dioxide.

Description

Clathrates for Gas Storage

The present invention relates to a gas hydrate (often referred to as a clathrate) comprising a water-in-gas emulsion (eg dry water (DW)) and an enclathrated exogenous gas such as for example methane, natural gas, hydrogen or carbon dioxide.

Natural gas, methane and other gases are important sources of energy for (for example) vehicular systems. Cost-effective applications and uses of such gases rely to a large extent on effective, economical and user-friendly means of storage, transportation and release. The efficient capture of certain gases from waste streams (for example carbon dioxide) is also important.

The safe and economical storage and transportation of natural gas which is predominantly methane is an important societal challenge. The US Department of Energy has set a target storage capacity of 180 v/v CH₄ (STP). Technology based on the use of hydrogen (for example in fuel cells) is likely to be of increasing value in the future. Cost-effective hydrogen-based applications require effective means of storing hydrogen. At room temperature and pressure, hydrogen and methane are gases which are difficult to handle because of their large volume and reactivity. Liquefaction requires expensive and complex apparatus and cryogenic liquid materials are difficult and dangerous to handle. Known methods for producing hydrogen and methane from chemical compounds in situ are generally unsatisfactory for many industrial uses.

Gas hydrates comprise "host" assemblies of H₂O cages in which are entrapped "guest" gases. 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). However, the incorporation of hydrogen into a gas hydrate is a lengthy process, often taking days or weeks (see for example Strobel, T. A.; Taylor, C. J.; Hester, K. C; Dec, S. F.; Koh, C. A.; Miller, K. T.; Sloan, E. D. J. Phys. Chem. B 2006, 77(9, 17121). Furthermore the timescale for "freezing" the H₂-H₂O clathrate structure can be unpredictable. Enhancement of the kinetics of hydrogen enclathration together with good rechargeability is a major challenge to the development of gas hydrates as a commercially feasible material for hydrogen storage.

The use of methane gas hydrates (MGHs) has attracted recent attention (see Sloan, E. D.; Koh, C. A. Clathrate Hydrates of Natural Gases, 3^rd Edition; CRC Press: Boca Raton, 2007). However the very slow formation kinetics associated with clathrate formation is a major obstacle to the use of MGHs for practical gas storage.

Natural gas hydrate (NGH) is an important source of natural energy which also has potential for gas storage and transportation. For example, one volume of conventional methane gas hydrate (MGH) can yield approximately 180 v/v STP methane (Sloan, E. D. Nature, 2003, 426, 353-359). Liquefied natural gas has a much higher energy density but must be stored at very low temperatures (113 K). It has been suggested that it is economically feasible to transport natural gas in hydrated form (Sloan, E. D. Nature, 2003, 426, 353-359). There are numerous practical challenges such as very slow clathrate formation and the presence of trapped, unreacted interstitial water in the hydrate mass. NGH is typically synthesized by cooling a mixture of natural gas and water under pressure or by the reaction of natural gas with preformed ice.

In principle a wide range of gases may be stored within clathrates. The host and guest must be sufficiently compatible (for example in terms of the guest size) to form a stable clathrate structure, optionally in the presence of additional stabilizing agents. Known clathrates include those comprising (as guest) inter alia H₂, O₂, N₂, CH₄, CO₂, H₂S, Ar, Kr, Xe, He and Ne (see for example WO-A-2006/131738 and Lokshin, K. A. et al, Phys. Rev. Lett. 2004, 93, 125503) or a gas mixture (for example natural gas and air).

Whilst improvements in the storage of gases in general would be desirable, there is a particular need to address technological and environmental aspects. The importance of gas storage is mentioned above and gas remediation solutions are required to trap undesirable gases. Climate change (in particular global warming) has prompted a search for better means of sequestration of (for example) CO₂. However the formation of clathrate structures is unpredictable and represents a significant barrier to industrial applicability. To date the feasibility of clathrates as gas storage means has been severely limited by poor kinetics and practical considerations. Common methods for increasing the kinetics of clathrate formation (eg high pressure, vigorous mechanical mixing, surfactants or micron- sized ground/sieved ice particles) are achievable in the laboratory but are neither cost effective nor practical in real gas storage applications.

The present invention is based on the recognition that preformed dry water powder exhibits desirable rates and levels of uptake of an exogenous gas by forming a gas hydrate.

Thus viewed from a first aspect the present invention provides a gas hydrate comprising: a water-in-gas emulsion of water droplets or particles 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 exogenous gas enclathration kinetics and recyclability. The exogenous gas is advantageously released rapidly from the gas which facilitates its ease of use at the point where it is required (for example in fuel cells).

Without wishing to be bound by theory, it is believed that the stabilization of the water-in- gas emulsion by the network of hydrophobic particles results in the formation of small domains. This compartmentalizes the water and the path length for diffusion of the enclathrated gas into (or out of) the water during clathrate formation (or dissociation) is reduced. The hydrophobic particles coat the water and divide up the host during clathrate formation but do not cover the water-gas interface sufficiently to prevent the ingress of gas.

A water-in-gas emulsion of water droplets is frequently referred to as dry water. It generally takes the form of a free-flowing powder. 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. The preparation of dry water is described (for example) in Binks, B. P.; Murakami, R. Nat. Materials 2006, 5, 865-869 and may be carried out straightforwardly in known apparatus. For example, hydrophobic particles and water may be aerated 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.

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.

Preferably the water-in-gas emulsion is a water-in-methane emulsion.

The exogenous gas may be methane. Typically CH₄ molecules are stored within H₂O cages formed within the matrix of the water-in-gas emulsion.

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

Preferably the water-in-gas emulsion comprises 5wt% or less of hydrophobic particles.

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, HDK13L, HDK15, HDK20, HDK30, HDK17, 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 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, 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.

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. The upper limit may be one of lOOμm, lOμm, or lμm. 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 average diameter of the primary droplets is less than about lOOμm, particularly preferably less than about 20μm.

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.

Preferably the size of the hydrophobic particles is less than 500 nm.

Viewed from a further aspect the present invention provides use of a water-in-gas emulsion (eg dry water) in the enclathration of an exogenous gas.

The use of a water-in-gas emulsion 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 hydrofluorocarbons from gas mixtures).

Dry water exhibits a rate of gas hydrate formation which allows an exogenous gas to be incorporated into clathrate cages more quickly than has previously been possible. This permits more straightforward and less expensive gas charging in a specialized gas charging plant or in situ. The use of dry water 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: exposing a water-in-gas emulsion 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.

The process may be carried out without forced agitation. Viewed from an even yet further aspect the present invention provides a method comprising the enclathration of a gas, and/or the dissociation of a gas from a gas hydrate, in the form of dry water.

Viewed from a still further aspect the present invention provides use of dry water in enhancing the enclathration of a gas, and/or the dissociation of a gas from a clathrate hydrate.

Viewed from an even still further aspect the present invention provides a composition in a form suitable for forming a gas hydrate comprising dry water and an isolated gas.

Viewed from a yet further aspect the present invention provides an apparatus comprising dry water and optionally a gas, wherein said apparatus is selected from one of the following devices or a component thereof: a fuel cell, an energy storage device, a gas storage device for example a modified gas tank, a gas separation device for example an in-line gas separation cartridge, a gas sequestration device for example an in-line gas sequestration cartridge, a gas transportation device for example a modified gas tank, and a vehicle for example an automobile.

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 illustration of methane gas hydrate (MGH) forming within dry water (ie the enclathration of methane within a H₂O clathrate host where H₂O is in the form of dry water droplets);

Figure 2 shows a standard food blender which can be used to make dry water and free- flowing dry water powder photographed flowing through a funnel;

Figure 3 shows optical micrographs of three batches of dry water; Figure 4 is a schematic diagram of an experimental apparatus used to prepare and test the gas hydrate;

Figure 5 shows pressure versus temperature plots for control experiments which involved cooling and heating under CH₄ pressure;

Figure 6 shows pressure versus temperature plots for a control experiment in comparison with an experiment in accordance with the present invention;

Figure 7 shows kinetic plots (capacity versus time) for CH₄ enclathration in dry water in comparison to bulk water;

Figure 8 shows kinetic plots in a different format (pressure versus time) for the same experiments shown in Figure 7;

Figures 9 and 10 show plots relating to multiple re-use of the dry water system; and

Figure 11 shows pressure versus time plots for an experiment where the dry water was re- blended prior to being re-used.

Example 1 - Synthesis of dry water

Full details of the procedure for the preparation of dry water can be found elsewhere (Binks, B. P., Murakami, R. Nat. Mater. 2006, 5, 865-869). A sample of hydrophobic silica nanoparticles (H 18) was supplied by Wacker-Chemie. To prepare the dry water powder, deionized water (95 mL) was poured into a blender (Breville, Glass Jug Blender, BLl 8, 1.5 liter, fitted with lid) and Hl 8 (5 g) was added to the water. Mixing was carried out at three different speeds (speed 1: average 16,450 rpm; speed 2: average 17,500 rpm; speed 3: average 19,000 rpm) for 90 s. The material was produced as a free-flowing "dry" white powder which could be poured from one vessel into another (see Fig. 2). The morphology of the dry water was observed using an Olympus CX41RF Microscope. Photographs were taken with a C-5060 digital camera (Olympus). Figure 1 shows a schematic representation of dry water droplets coated with small hydrophobic particles to enable the enclathration of (for example) methane to form methane gas hydrate (MGH).

Example 2 - Apparatus for gas 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 communicates with a computer. Prior to experiments, the cell was slowly purged with methane (UHP 99.999%, BOC Gases, Manchester, 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) and pressure (P, psia) and time (t, min) were automatically interval-logged using MeterView 3.0 software (Cole-Parmer). Using this setup it was possible to obtain high resolution data (for example, 2 seconds between individual [T, P, i\ points, 120,000 data points in a 2000 min experiment). The apparatus is shown schematically in Fig. 4. Control experiments using glass beads showed that the system did not leak (ie no pressure drop occurred over 2000 min (Fig. 5)).

Example 3 (Comparative) - Control Experiments

Figure 5 shows Control Experiments: P-T plots for cooling and heating under CH₄ pressure (temperature ramp: 2.0 K/h): (a) and (b) 19.5 cm³ glass beads; (c) and (d) unblended mixture of water (19 g) and hydrophobic silica nanoparticles Hl 8 (Ig). The system approximated to ideal gas behavior for the glass bead control experiment and no leaks were detected. The behavior was similar for the unmixed water/silica control although a small pressure inflection (<0.1 MPa) was observed. This was most likely due to a very small degree of MGH formation at the gas-liquid interface.

Example 4 — Dry Water Samples

Figure 2 shows a typical sample of dry water formed by rapid mixing of hydrophobic silica (Hl 8), water and air in a conventional blender. The free-flowing dry water powder was prepared by aerating 5g of hydrophobic silica nanoparticles H18 and 95g of water at 19,000 rpm for 90 seconds. The powder is photographed flowing through a funnel.

The particle size can be altered by varying the speed at which mixing is carried out. Figure 3 shows optical micrographs showing three different batches of dry water prepared at different speeds (left to right [bottom to top]: 16450, 17500 and 19000 rpm). The scale bar in Figure 3 represents 50 μm in all cases.

Example 5 - Storage of CHU in dry water

Figure 6 shows the cooling/heating curves for the CH₄-dry water system with and without mixing. In this Figure are shown P-T plots for CH₄ and dry water during cooling and heating (temperature ramp: 2 K/h): (A) unblended mixture of water (19 g) and hydrophobic silica nanoparticles Hl 8 (1 g); (B) 20 g dry water powder (portion of a sample produced from 95 g of water + 5 g of hydrophobic silica nanoparticles Hl 8) formed by mixing at 19,000 rpm for 90 seconds.

In the unmixed system (curve A; bulk water plus Hl 8 silica), the P-T relationship for CH₄ 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 (temperature ramp = 2.0 K/h, Figure 5). By contrast, MGH formation and subsequent dissociation occurred when particulate dry water was employed (curve B), as shown respectively by the dramatic pressure drop upon cooling and the rapid pressure rise upon heating.

The CH₄ uptake is very high in this dry water system (175 v/v). MGH formation occurred when cooling to 279.0 K with an associated exotherm. During warming, MGH dissociation commenced at around 277.5 K and was completed at about 290.5 K. The formation of dry water-MGH can be attributed to the highly dispersed water phase (Fig. 2) which has a large surface area/volume ratio compared to bulk water.

Figure 7 shows kinetic plots for CH₄ enclathration in dry water at 273.2 K (starting pressure = 8.6 MPa, see Fig. 8). No mixing was applied in these gas uptake experiments. In all cases, the dry water powders were formed by blending and then poured into an unmixed pressure vessel.

In the bulk water system with silica, only a very small pressure drop was observed even after 2000 min (see also Fig. 8) and the gas capacity was very small (less than 3 v/v) over this time. These very slow kinetics could be due to the formation of a MGH "skin" at the gas-water interface. By contrast, much faster methane enclathration was observed for the dry water systems. A total capacity of 175v/v was reached and the time to reach 90% of this capacity (tc_>o) was 160min.

Figure 8 shows P-t kinetic plots for dry water-MGH formation at 273.2 K: (a) Control experiment, 19.5 cm glass beads; no pressure change is observed after initial thermal equilibration upon adding CH₄ gas which occurred in < 3 min; (b) Control experiment, 19.0 g water and 1 g silica (Hl 8), no mixing; very little pressure change is observed; (c) 19.0 g water and 1 g silica (Hl 8), dry water powder formed by blending at 16,450 rpm for 90s; (d) 19.0 g water and 1 g silica (H18), dry water powder formed by blending at 17,500 rpm for 90s; (e) 19.0 g water and 1 g silica (Hl 8), dry water powder formed by blending at 19,000 rpm for 90s.

The methane uptakes calculated from pressure changes were consistent with values obtained by decomposing the dry water-MGH and measuring the volume of methane that was released.

The following table shows the amount of CH₄ enclathrated in dry water-MGH, as measured by volumetric release experiments (Circone, S., Kirby S.H., and Stern L.A. J. Phys. Chem. B 2005, 109, 9468-9475; Stern L.A., Circone, S., and Kirby S.H. J. Phys. Chem. B 2001, 105, 1756-1762).

These measurements confirm the capacities calculated from P-T plots (Fig. 8). The MGH was stabilized at 120 K prior to venting the excess CH₄. Very similar release volumes (±5%) were also measured when the MGH was stabilized at 263 K followed by rapid venting / valve closure prior to MGH decomposition and gas release.

Vent temperature Volume of methane released

Samples (K) (liters, 293.2 K, l atm)

19.0 g water and 1 g silica

293.2 2.2

(Hl 8), 16,450 rpm for 90s

293 2 19.O g water and 1 g silica 3 Q

(H 18), 17,500 rpm for 90s

293 2 19.O g water and 1 g silica 4 3 (Hl 8), 19,000 rpm for 90s.

Calculation of Capacity

The free space volume of the vessel was obtained by subtracting the sum volume of methane gas 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 out of the water and the guest gas respectively (neglecting any dissolution of the guest 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.

The density of the Hl 8 was 2.2 g/ cm³. The density of water DH_2O was 1.0 g/ cm³. The density of methane in GAS PHASE D_g and the density of methane at STP Dem, _STP were from GASPAK v3.41 software according to P_g & T_g. M_CH₄ & M _H2o, were molecular weight of methane and water respectively. The mass of sample m_s° was 20.0 g, the initial volume of sample V₅ ⁰ was 19.5 cm³. The initial volume of methane in GAS PHASE V_g° was 48.5 cm³.

The hydrate number n was 5.89 (CH₄ ^• n H₂O) and the density of methane gas hydrate was: D_h (g/ cm³) =0.92645 - 0.239*10^"3-r(°C) - 3.73* 10^'7T(⁰C)²

( n =5.89) [Waite, W. F., Stern, L. A., Kirby, S. H., Winters, W. J. And Mason, D. H. Geophys. J Int 2007, 169 (2), 161-114]

GAS PHASE(CH₄) GAS PHASE(CH₄)

V_g (volume)

Va⁰ (initial volume) P_g (pressure) P_g (initial pressure) m_g (mass) m_g° (initial mass) D_g (density) D_g° (initial density) T_g (temperature) T_g (initial temperature)

\ ΔV

HYDRATE

DRY WATER

V_s (volume )

V_s° , m_s° m_s (mass) (initial volume&mass)

V₀ = V_g° + V_s ⁰ = V₈ + V_s = 68.0 cm 3 : volume of vessel

^' Δm = m_g° - m_g = m_g° -D_g* V_g = m_g° -Dg*(V_g ⁰ - ΔV) : consumed methane Wh = (( M_CH4+n* M mo) / Mem)* Δm : mass of hydrate ΔV = (W_h / D_h) - ((n* M H2o) / D_H2O*M_CH4)* Δm : volume change of sample β = ( M_CH4+n* M H2θ) / (D_h*McH4) - (n* M _H2o) / (DH20* M_CH4) : defined constant

so: r Δm _ = (/_nD_g0^υ - D_g)*V_g ^υ / (1 - β*D_g) ΔV = β*(D_g° - D_g)*V_g° / (l - β*D_g)

So:

Capacity (v(STP)/v(hydrate at 273.2K)) = ( Δm/ D_CH4, STP) / ( ΔV + V_s°) The dry water-MGH formation rate and saturation methane capacity was related to the size of the water droplets. The higher the mixing speed, the lower the average particle size (Fig. 1). The dry water prepared at the highest mixing speed (19,000 rpm) exhibited a saturation CH₄ uptake of 175 v(STP)/v at 273.2 K after 1500 min (t₉₀, the time to achieve 90% of this capacity, was about 160 min). This CH₄ capacity is close to the maximum capacity for si or sll MGH assuming single occupancy of all cages (~180 v/v at STP) (Sloan, E. D.; Koh, C. A. Clathrate Hydrates of Natural Gases, 3^rd Edition; CRC Press: Boca Raton, 2007; Khokar, A.A.; Gudmundsson, J.S.; Sloan, E.D. Fluid Phase Equil 1998, 150-151, 383-392; Sun, Z.G.; Wang, R.; Ma, R.; Guo, K.; Fan, S. Energy Cons. Man. 2003, 44, 2733-2742). The effective storage pressure (ie the CH₄ pressure at saturation uptake (Fig. 8)) was 2.73 MPa in this case. A short induction time was observed (typically 5- 10 min) prior to dry water-MGH formation.

The present invention can be contrasted with prior art methods which attempt to increase MGH formation rates by stirring the mixture vigorously (Sloan, E. D.; Koh, C. A. Clathrate Hydrates of Natural Gases, 3^rd Edition; CRC Press: Boca Raton, 2007; Koh, C. A. Chem. Soc. Rev. 2002, 31, 157-167; Sloan, E. D. Nature, 2003, 426, 353-359, and references therein; Susilo, R. Methane Storage and Transport via Structure H Clathrate Hydrate, Ph.D. thesis, 2008, The University of British Columbia). However, the energy required to stir the thickening slurry is significant. Similarly, crushed and sieved ice particles can be employed but this is quite laborious and the material must be handled without melting. In the system according to the invention, the gas-water interfacial surface area is increased by forming a dispersed water phase at ambient temperature prior to enclathration. The weight "penalty" is low because only 5 wt % silica is added with respect to water.

Example 6 - Reuse of dry water systems

The dry water system can be reused after MGH dissociation (Figs. 9 to 10). Re-blending the dry water results in regeneration of the original enclathration kinetics (Fig. 11). By contrast, the kinetic advantages of using crushed ice particles are entirely lost after one cycle if the water is allowed to melt. Figure 9 shows a P- T plot illustrating partial recyclability of dry water-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 10 shows a P-t kinetic plot illustrating partial recyclability of dry water-MGH system (19.0 g water and 1 g H18, formed at 19,000 rpm for 90s) over three cooling / heating cycles under CH₄ pressure.

Fig. 11 shows a P-t kinetic plot illustrating recovery of fast CH₄ uptake kinetics after reblending the destabilized dry water sample for 90 s.

The CH₄ storage capacities for these dry water-MGHs come very close to US Department of Energy targets (Buchell, T.; Rogers, M. SAE Tech. Pap. Ser. 2000, 2000-01-2205) and compare favorably with high surface area physisorptive materials such as metal organic frameworks (MOFs) (see Eddaoudi, Kim, J.; Rosi, N.; Vodak, D.; Wachter, J.; O'Keeffe, M.; Yaghi, O. M. Science 2002, 295, 469-472), activated carbons (see Lozano-Castello, D.; Cazorla-Amoros, D.; Linares-Solano, A. Energy Fuels 2002, 16, 1321-1328) and microporous organic polymers (see Wood, C. D.; Tan, B.; Trewin, A.; Su, F.; Rosseinsky, M. J.; Bradshaw, D.; Sun, Y.; Zhou, L.; Cooper, A. I. Adv. Mater. 2008, 20, 1916-1921). MGHs are in principle stable for significant periods at atmospheric pressure with moderate cooling (Stern L.A., Circone, S., Kirby S.H. J. Phys. Chem. B 2001, 105, 1756-1762). This is very difficult to achieve with physisorptive materials.

Example 7 - Storage of carbon dioxide in dry water

The procedure for preparing a CO₂-dry water system is the same as that described for the CH₄-dry water system in Example 5 but with a starting pressure of 3.3MPa. The starting pressure is lower than for methane in order to keep the carbon dioxide in gaseous form. The time required to reach 90% capacity was (tgo) was 200 min. An uptake of 150v/v was achieved.

Example 8 - Storage of Krypton in dry water Experiments have shown that enclathration of krypton in dry water is possible.

Conclusion

Dry water gas hydrates represent a viable platform for recyclable gas storage on a practicable timescale in a static, unmixed system.

Claims

1. A gas hydrate comprising: a water-in- gas emulsion of water droplets or particles 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 water-in-gas emulsion is dry water.

3. A gas hydrate as claimed in claim 1 or 2 wherein the water-in-gas emulsion is a water-in-air emulsion.

4. A gas hydrate as claimed in any preceding claim wherein the water-in-gas emulsion comprises 5wt% or less of hydrophobic particles.

5. A gas hydrate as claimed in any preceding claim wherein the hydrophobic particles are hydrophobic fumed silica particles.

6. A gas hydrate as claimed in any preceding claim wherein the exogenous gas is hydrogen, carbon dioxide or a saturated or unsaturated hydrocarbon.

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

8. A gas hydrate as claimed in claim 1 wherein the exogenous gas is CH₄, CO₂, O₂, H₂, N₂, H₂S, Ar, Kr, Xe, He, Ne or a mixture thereof.

9. A gas hydrate as claimed in claim 1 wherein the exogenous gas is CH₄.

10. A gas hydrate as claimed in claim 1 wherein the exogenous gas is CO₂.

11. A gas hydrate as claimed in claim 1 wherein the exogenous gas is H₂.

12. A gas hydrate as claimed in any preceding claim wherein the average diameter of the droplets is less than about lOOμm.

13. A gas hydrate as claimed in any preceding claim wherein the size of the hydrophobic particles is less than 500nm.

14. Use of a water-in-gas emulsion in the enclathration of an exogenous gas.

15. A process for preparing a gas hydrate as defined in any of claims 1 to 13 comprising: exposing a water-in-gas emulsion to an exogenous gas in a contained environment at a temperature less than ambient temperature and an elevated pressure.