Applications include the transport of gas from fields where there is no existing local market or gas transport infrastructure and in exporting associated gas from offshore oilfields where there is no existing means of export or disposal. The technology eliminates the environmentally undesirable practice of flaring associated gas and gas released during well testing operations.

A gas hydrate is an ice-like crystal structure comprised mainly of water molecules during the formation of which gas molecules are incorporated in molecular scale cavities within the crystal structure. A unit volume of hydrate can contain in excess of 150 volumes of gas when the gas is at 20° C and atmospheric pressure.

Hydrates can be formed only by a limited range of compounds including methane, ethane, propane, butane, iso-butane, carbon dioxide, hydrogen sulphide, tetra-hydro furan and chlorofluorocarbons. The first six compounds named form the bulk of most natural gas fields.

Hydrate formation is strongly influenced by temperature and pressure. For natural hydrocarbon gases, hydrates will typically form at above 0° C (ice formation temperature) only at pressures above about 15 bar as shown in Figure 1.

Hydrate formation in pipelines and equipment is thus a commonplace nuisance in offshore oil and gas fields, and expensive countermeasures are used to prevent them. Basic hydrate formation thermodynamics and properties are well understood and published, see e. g. Sloan E. D."Clathrate Hydrates of Natural Gases"published by Marcel Dekker, New York 1990.

The ability to convert gas into a solid hydrate form is potentially useful for several purposes including storage or long distance transport because of the large amount of gas that hydrate can contain in a unit volume. Several processes have been proposed and patented for these purposes over many years, back to at least 1942, see for example US 5536893.

The detailed formation mechanisms of hydrates depends on whether the hydrate forming substance is, under the contacting conditions, a gas, a liquid non-miscible with water, or a miscible liquid. Most of the prior art patents are aimed at manufacture of hydrates from gas; in which case, production of hydrate occurs at the interfacial surface between gas and water, and the proposed production reactors are contacting devices which provide a large interface surface area to promote rapid formation. The engineering principles for several suitable contactor types are well known; most prior art patents cover the use of a single-stage spray (see for example US 2399723 and GB 568290) or bubbling pool ("bubble column","sparged column") reactor (see for example US 3975167 and US 3514274). The latter type is frequently enhanced by the use of mechanical agitators.

Recent prior art (such as WO 97/26494) has looked at optimum arrangements of processing plants for the manufacture of gas hydrate.

Under the process conditions proposed by most prior art, the effluent from the reactor vessel will comprise a mixture of produced hydrate with a considerable amount of unreacted water in the form of a mixed slurry. This is a convenient form in which to continuously remove the hydrate product from the production reactor. However, the mixed slurry containing a considerable amount of unreacted water has a large volume and mass and so processing, transport and storage equipment must be correspondingly large to accommodate the slurry. All of the prior art, despite 50 years of study and proposals, has failed to produce an economically advantageous system for any of the intended applications. To the best of our knowledge, no commercial use has been achieved.

The applicant has carried out a programme of experiments studying the formation of a range of gas hydrates and their processing and has subsequently investigated the storage properties of these hydrates. From this work an innovative combination of technologies has been devised which together comprise an economic means for the manufacture, processing, transport and storage of gas hydrates in many of the applications described above.

According to a first aspect of the present invention, there is provided an apparatus for removing fluid from a two phase mixture of hydrate and liquid at an elevated pressure or a three phase mixture of hydrate, liquid and gas at an elevated pressure, the apparatus comprising a first separation device of a first fluid removing efficiency for receiving an input mixture at elevated pressure of hydrate and liquid or hydrate, liquid and gas and for producing an intermediate mixture with a larger concentration of hydrate than the input mixture, and a second separation device having a second higher fluid removing efficiency than that of the first separation device, the second separation device comprising a centrifuge provided in a sealed pressure vessel for receiving the intermediate mixture from the first separation device at an elevated pressure and for producing an essentially dry hydrate or concentrated hydrate slurry output.

A higher fluid removing efficiency relates to the ability of a device to produce a greater concentration of solids for the same input mixture. An elevated pressure is taken to mean a pressure greater than atmospheric. Since hydrate manufacturing plants such as that disclosed in WO 97/26494 generally operate at high pressure, the hydrate slurry produced from the plant will be at an elevated pressure.

The provision of a higher fluid removing efficiency centrifuge provided in a sealed pressure vessel enables the centrifuge to operate at an elevated pressure. By maintaining the elevated pressure through the first and the second separation devices, the hydrate is maintained in a stable condition without having to be excessively cooled which can be very expensive.

The applicant has found that removing a proportion of the liquid and, if applicable, gas, from a two phase mixture of hydrate and liquid or a three phase mixture of hydrate, liquid and gas before supplying it to a generally more expensive but higher liquid removing efficiency second separation device including a centrifuge significantly reduces the number and capacity of higher efficiency separating devices required whilst producing a greater quantity of the same quality output of essentially solid hydrate or concentrated slurry. This significantly reduces costs and increases production levels making the use of hydrates more commercially attractive.

The provision of an apparatus having two separation devices of differing liquid removing efficiencies enables the production of essentially liquid free hydrate at reasonable cost. If one were to use one or more lower liquid removal efficiency separation devices the final product would still contain an excessive quantity of liquid. If one were to use a series of higher liquid removal efficiency separation devices then the apparatus would be prohibitively expensive.

The applicant has found that an essentially solid or concentrated slurry final product form is especially useful for applications requiring handling of stable hydrate product at pressures substantially lower than the pressure required for hydrate production.

A device according to a second aspect of the present invention and which may be used as at least part of the first separation device of the first aspect of the present invention for separating gas from a three phase mixture of hydrate, liquid and gas comprises a vessel with an inlet for receiving a three phase mixture of hydrate, liquid and gas; the vessel having an internal surface against which the mixture is arranged to be directed with sufficient force such that the impact of the mixture against the surface disengages gas from the mixture; and the vessel having a chamber to collect mixture remaining after it has been directed against the internal surface, the chamber having an outlet and means to direct hydrate floating on liquid in the chamber to the outlet when in use.

The means to direct hydrate floating on liquid in the chamber to the outlet is preferably an upper boundary of the chamber, at least a portion of which is inclined to the horizontal when in use with the outlet located at an upper portion of the chamber defined below the inclined portion of the upper boundary.

A device for separating gas, liquid and solid hydrate according to a third aspect of the present invention and as may be used as the first separation device of the first aspect of the present invention comprises a vessel for receiving an input mixture of gas, liquid and hydrate; a straining means mounted within the vessel; and means to direct an input mixture of gas, liquid and hydrate against the straining means such that gas is evolved to be collected or removed from the vessel, liquid passes through the straining means to be collected or removed from the vessel, and hydrate is collected by the straining means.

The straining means may be for example a perforated screen or a woven mesh.

An essentially solid or concentrated slurry final product form which may be produced according to the first aspect of the present invention is preferably cooled before being stored or transported to enable it to remain stable for longer periods of time.

The cooling of a solid or concentrated slurry is difficult and expensive because of the poor heat transfer characteristics of such systems and the need to avoid the freezing of solids to the surfaces of a cooling device.

The inventors have solved this problem according to a further aspect of the present invention with a substantially dry hydrate cooling apparatus comprising a container for receiving essentially solid or concentrated slurry hydrate; a gas distribution device arranged to be supplied with fluidising gas when in use, the gas distribution device being arranged to be positioned in the container to pass fluidising gas through essentially solid or concentrated slurry hydrate in the container when in use to fluidise the hydrate; and means to provide cooling of the fluidised hydrate in the container.

The means to provide cooling to the fluidised hydrate is preferably the distribution device which is arranged to supply cooled fluidising gas. Alternatively or additionally the means to provide cooling to the hydrate may be means to supply a stream of cooled fluid through the fluidised hydrate to provide the cooling. This stream of cooled fluid may be passed through the fluidised hydrate in one or more conduits.

According to a still further aspect of the present invention there is provided a method for at least one of storage and transport of gas in the form of stable hydrate which is preferably used with hydrates prepared using one or more of the above aspects of the invention.

The invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 shows a typical natural gas hydrate equilibrium curve showing the pressure and temperature conditions required for formation of stable hydrate, with stable hydrate existing above the curve; Figure 2 diagrammatically shows an apparatus according to the first aspect of the present invention for the production of an essentially solid or concentrated slurry of hydrate; Figure 3 diagrammatically shows a sequence of steps in a process for forming hydrate incorporating the method according to the first aspect of the present invention; Figures 4 to 8 show preferred devices for performing various steps in the process shown in Figure 3; and Figure 9 is a diagram showing the temperature of various regions of a mass of hydrate stored for 5 days in a ship's hold at ambient temperature and pressure.

Figure 2 diagrammatically shows a system according to the first aspect of the present invention for the two stage removal of fluid from a two phase mixture of hydrate and liquid or a three phase mixture of hydrate, liquid and gas.

The mixture 1 is supplied to a first stage 3 of a fluid removal system 2. The first stage 3 may be any suitable separating device such as a hydrocyclone which is well known in the art or a device to mechanically remove hydrate floating on a liquid which may be liquid separated from the slurry as described later. The output 4 from the first stage 3 is fed to the second stage 5 which is a more efficient separating device than the first stage, in this case a centrifuge in a pressure vessel which produces a substantially dry hydrate product 6.

Figure 3 is an outline of a hydrate formation process which has been tested using a pilot plant and laboratory experiments, the process incorporating the fluid removal system shown in Figure 2. A process reactor 10, for example as shown in our earlier international patent application published as W097/26494, produces a hydrate/gas/liquid mixture 11. The mixture 11 is passed to device 12 described below which is arranged to separate the majority of the gas phase from the mixture 11. A separated substantially liquid and solid free gas stream 13 can be utilised by for example being returned to the process reactor 10 for the formation of further hydrate or by being delivered to a device for power generation or it may be burned. A substantially gas free liquid and solid slurry stream 14 is passed to a first separating device 15 forming the first stage 3 of the water removal system 2, an example of which is described later, which is arranged to produce a liquid stream 16 containing a low level of solids and a slurry stream 17 with a higher solid hydrate concentration than input stream 14. Stream 16 is passed back to the process reactor 10 to be used in the further production of hydrate. Alternatively gas separator 12 and first separating device 15 may be combined into a single device 30 described later.

Stream 17 is passed via an optional cooling device 18 to the second stage of the water removal system 2, comprising the more efficient separating device 5 than that of the first stage 3. The separator of the second stage is a centrifuge in a pressure vessel to enable it to separate fluid at high pressure so that the hydrate may be maintained in a stable condition without having to be excessively cooled. The inventors of the present invention have found that a continuous screening centrifuge 5 produces a 95% to 99.5% liquid free stream 19 in the form of a granular, flowable solid and a liquid stream 20 containing extremely low levels of solids which may be returned to the process reactor 10. The centrifuge has been found by the inventors to be particularly suited to large scale applications.

Stream 19 may optionally be passed into device 21 where it is cooled either by direct contact with a gas stream 22 supplied in this case at high pressure and low temperature or indirectly by passing an additional stream of cooling medium 23 through conduits passing through the body and walls of the device. This latter option increases the process complexity but means that a smaller high pressure gas stream 22 is needed only to aid motion of the solids termed'fluidisation'and improve heat transfer to the solids. The gas stream 22 may be either of hydrate forming or non-hydrate forming gases-in the former case an advantage is gained in that any moisture entering device 21 in stream 19 may be converted into additional hydrate.

The fluidising/cooling gas 22 and cooling medium 23 exit device 21 separately (streams 24 and 25). The dry cold solids stream exits device 21 and may be depressurised by device 26 to atmospheric conditions and loaded into a transportation or storage device 27.

Gas separating device 12 may be of the type as illustrated in Figure 4. An input stream 11 is arranged to enter a pressure vessel 40 which is able to withstand the elevated pressure at which the input stream 11 arrives from the hydrate production plant. The input stream 11 enters pressure vessel 40 via inlet 41 and in this example is directed downwards by a suitably shaped portion 42 of the inlet. The gas present in the input mixture 11 is disengaged from the mixture by impacting the mixture against a suitable surface 43, in this case part of an insert 44.

The generated gas exits the vessel as a gas stream 13 via outlet 45. The surface 43 directs the remaining liquid and solids from the mixture into a downcomer 46 which is sized (by known methods) to ensure that particles of hydrate are entrained in the downward flow. The insert 44 is shaped to provide a space or chamber 47 at the base of the vessel 40 with an upper boundary 48 which is inclined or sloping relative to the horizontal when in use. An offtake 49 for a substantially gas free liquid and solid slurry stream 14 is located at the upper part of this chamber 47 and the inclined or sloping upper boundary 48 is arranged to direct hydrate floating on liquid in the chamber 47 to the offtake 49. This design avoids accumulation of hydrate within the device and subsequent blockage.

In operation the level of liquids collected in or above the downcomer is maintained to enable hydrate to be entrained in the downward flow into chamber 47. The flow of liquid into the downcomer 46 has been found to generally produce a vortex which entrains hydrate in the downward flow.

Maintaining the level of liquid in or above the downcomer when in use produces a seal to prevent the passage of separated gas into chamber 47 or out of offtake 49 so that a substantially gas free liquid and solid slurry stream 14 is produced. Maintenance of the liquid/hydrate slurry surface level in or above the downcomer may be achieved using a first level sensor 401 positioned at the minimum liquid level in the downcomer 46, a second level sensor 402 positioned at the maximum liquid level in or above the downcomer 46, a valve 403 connected to the offtake 49 and a control means 404 connecting them together via control lines 405. When the liquid level in the downcomer falls to the minimum level, the first level sensor 401 is activated and the control means 404 closes valve 403 so that the liquid level in the downcomer 46 rises due to the continued addition of the stream from input 11 to ensure that hydrate continues to be caught in the downward flow of liquid in the downcomer 46. Conversely when the maximum liquid level sensor 402 is activated the control means 404 opens the valve 403 so that water and hydrate may be passed out of offtake 49 to lower the liquid level.

The chamber 47 may be provided with a water outlet 404 at its lower portion to withdraw water from the chamber 47 and thus increase the concentration of hydrate from offtake 49.

By positioning the water outlet 404 at the lower portion of chamber 47, hydrate floating on water in the chamber is unlikely to be withdrawn through water outlet 404, especially if the liquid control system described in the previous paragraph is used. However, a filter 405 may be provided at the outlet to prevent hydrate passing into the water outlet 404.

Either of two preferred devices can perform the function of separating device 15. One is a hydrocyclone-a device familiar to those skilled in the art of solid-liquid separation but normally used for the separation of solids from liquid of lower density than in the present invention.

Studies performed by the applicant have identified an alternative device 15 suitable for this application which involves separating solids from a liquid of higher density.

This device is illustrated in Figure 5. Stream 14 enters a vessel 50 of the device 15 via an inlet 51 and is directed upwards by a suitable portion 52 of the inlet.

Liquid stream 16 is removed from the base of the vessel through outlet 53. The diameter of the vessel 50 is such that hydrate particles are not drawn down by the flow of liquid in stream 16-instead the hydrate collects to form a floating mass 54 in the upper part of the vessel 50. The mass of hydrate 54 floats on the liquid contained in the vessel. The section of the mass of hydrate 54 which floats above the surface of the liquid becomes drained of liquid by gravity. A scraping device 55 positioned at the top of the vessel 50 scrapes hydrate off the top of the floating mass 54 to an outlet 56 to form stream 17.

Device 30 which may be used as an alternative to the combination of devices 12 and 15 is illustrated in Figure 6.

It comprises a straining means, in this case a perforated screen 60 mounted within a pressure vessel 61 to withstand the pressure of the input stream 11 from the hydrate production plant. Stream 11 enters the vessel through inlet 62 and is directed downwards by a suitable distribution device 63 which may be a suitably directed portion of the inlet 62. The input stream 11 is directed against a surface 60 with sufficient force to generate gas.

A gas stream 13 is generated by the impact of the input hydrate/liquid/gas mixture onto the screen 60 and the generated gas stream 13 exits from the top of the vessel 61 through a gas outlet 64. Liquid and hydrate which impinge against the screen 60 travel down it under the influence of gravity in the direction illustrated. During this passage, liquid passes through the perforations in the screen 60.

Operation of device 30 at an elevated pressure, as is generally required to maintain the hydrate in a stable condition, increases the amount of liquid which passes through the perforations in screen 60 thus providing better separation. The concentrated slurry (stream 17) is drawn from the vessel through outlet 65. The liquid 66 that passes through the screen 60 accumulates in the base of the vessel 61 and is drawn out of the vessel, as stream 16, through outlet 67. Laboratory tests at process conditions have found that such a device can concentrate a stream containing less than 5% by volume hydrate to one containing more than 30% by volume of hydrate.

The second more efficient stage 5 of the two stage water removal apparatus is in this example a centrifuge 71 provided in a pressure vessel 72. The centrifuge 71 comprises a ring 73 of gauze or mesh acting as a screening surface. The centrifuge is mounted on an axis 74 supported by the pressure vessel 72 and is arranged to be rotatable on the axis at a suitable rate. If desired an arrangement of plates and blades (not shown) may be provided inside the centrifuge to assist in the separation. An intermediate stream from the first stage 3 of the two stage water removal system 2 is delivered to the centrifuge 73 via inlet 75.

The rotation of the centrifuge forces water through the screening surface to be collected at the bottom of the pressure vessel 72 whilst hydrate collects on the inside of the screening surface. Below the centrifuge 71 is mounted a duct 76 to receive hydrate collected on the inside of the centrifuge 71 and pass it out of the pressure vessel 72 via hydrate outlet 77. Water collected at the bottom of the pressure vessel 72 is collected via liquid outlet 78. The centrifuge 73 thus produces a continuous flow of hydrate.

Laboratory work with a small pressurised centrifuge has shown that a full size centrifuge can produce a hydrate product which contains less than 2k by volume of water.

Device 21 is a fluidised bed in the present example. The applicant has found that the fluidisation of hydrate and ice particles is feasible at low temperatures and high pressures. In laboratory studies beds of hydrate and ice particles could be fluidised smoothly at a temperature of -10° C and below provided high pressure was maintained.

Experiments were performed at-10° C to-70° C and at pressures of 3.5 to 28 bar. By comparison of the heat transfer rates from device 21 with those seen in conventional cooling devices for substantially solid streams we have found this to be the most economic method of adequately cooling the hydrate product to a temperature suitable for transport. Figure 8 shows an arrangement of such a bed. The bed 80 is contained within a pressure vessel 81 suitable for the pressures and temperatures necessary for the process. The pressure vessel 81 is shaped with the bed 80 arranged to be located in a lower portion 82 of the vessel 81. The upper portion 83 of the vessel 81 is arranged to direct any fluidised particles leaving the lower portion 82 back to the lower portion. In the example illustrated in Figure 8, this is achieved by the provision of an inclined lip 84 around the upper periphery of the lower portion 82 to direct any particles leaving the bed 80 back to the lower portion of the vessel 81. This structure contains the bed 80 and avoids the carriage of smaller solid particles out of the top of the bed 80 (alternatively shaped internals may be used to provide the desired bed geometry in a pressure vessel of more conventional shape). Solids from separator 5 are added to the bed 80 via inlet'85 so that they fall down into the bed.

Fluidising gas 22 is introuced through inlet 86 and thence via a distribution system 87 to the majority of the base of the bed. The fluidising gas 22 is preferably a hydrate forming gas so that any moisture entering the fluidised bed from stream 19 is converted into hydrate to maintain the hydrate virtually dry. The fluidising gas 22 may also provide cooling to the hydrate in the bed 80. If desired a cooling medium 23 such as evaporative refrigerant from one or more external sources may be passed through the bed 80 from the distribution system 87 with the fluidising gas. The fluidising gas 22 exits the bed 80 and leaves the vessel 81 via outlet 88 after passing, optionally, through a conventional cyclone device 89 to remove small entrained particles of ice and hydrate. As illustrated in Figure 8, cooling medium may be passed through the fluidised bed in conduits 90 made of good heat conducting material, preferably metal such as steel. By passing the cooling medium through the bed in conduits, liquid coolant may be used which can absorb far more heat than gaseous coolant producing a better cooling effect. As further solids are added to the bed 90 the level of the bed rises and solids overflow out of the bed 90 via chute 91 and outlet 92 maintaining the level of the bed 90 substantially constant.

In some circumstances, e. g. a large installation, a bed may be subdivided by a series of substantially vertically orientated baffles 93 only one of which is shown in Figure 8, over which solids will flow from entry region (s) 93a of a first temperature to exit regions 93b of lower temperature.

Depressuring device 26 shown diagrammatically in Figure 3 could be any of a range of known technologies for reducing the pressure of a solid stream. The applicant uses a lock hopper system where batches of solids are introduced into pressurised vessels, the vessels are then isolated by means of valves and the vessels then depressurised with the exhaust gas optionally being initially routed to previously depressurised vessels to save on the costs of recompression.

Any means of transporting or storing a chilled bulk solid mass of hydrate may be used as convenient. Examples might be a container, the hold of a ship or a railway wagon. The transporting or storing means is preferably insulated.

It has been found from economic studies that the gas content of any hydrate used for transportation or storage should preferably be of the order of 150 to 200 volumes of gas (for gas at atmospheric pressure and temperature) per volume of hydrate. If such a hydrate gas content is not achieved then such large ships or large numbers of small ships or containers will be required as to make the use of hydrates uneconomic when compared with other known alternatives for gas storage or transportation.

The applicant has surprisingly found that according to a still further aspect of the present invention storage or transport of hydrate with at least the majority of the hydrate remaining in a stable state for at. least 24 hours can be performed by the provision of the stable hydrate in a mass without the need for external cooling.

Figure 9 shows the temperature profile of just such a mass of hydrate in a ship's hold with an initial storage temperature of-50°C after 5 days have elapsed. The ambient temperature at the top of the hold is 20°C and the ambient temperature at the bottom of the hold is 15°C. As can be seen only the edges of the original mass of hydrate fall below the stable temperature of approximately-37°C at atmospheric pressure and are converted into water (ice) and natural gas. The vast majority, in this case 95%, of the hydrate remains as stable hydrate with only 5% being converted into natural gas and water in the form of ice.

Although insulation of the mass of hydrate is not necessary its use is preferred in some circumstances as it will enhance the length of time that the hydrate remains stable.

Insulation may be provided in whatever transport or storage device is being used such as the hold of a ship, container or railway wagon.

Since only the edges of a mass of hydrate are decomposed into ice and gas within a normal transport or storage period of a few days, as the size of the mass of hydrate is increased the proportion of hydrate that remains stable over the same period is increased. A preferred mass of hydrate for use in the present invention has a minimum dimension of 2m in any direction or a more preferred dimension of at least 10m in any direction. However, this of course depends upon the expected duration of the transportation or storage.

The hydrate used for the above method of storage and transport is preferably substantially pure to provide a commercially viable volume of gas in a suitably small volume of hydrate.

The hydrate used for the above method of storage and transport is preferably substantially dry or a concentrated slurry to reduce the proportion of non-gas carrying material to be stored or transported making the method of storage or transport of the present invention even more economically attractive. Many modifications may be made to the examples described above without departing from the scope of the invention as defined in the following claims. For example any suitable first separation device may be used in the two stage apparatus for removing fluid from a hydrate, liquid and optionally gas mixture.