OA11853A

OA11853A - Formation, processing, transportation and storage of hydrates.

Info

Publication number: OA11853A
Application number: OA1200100241A
Authority: OA
Inventors: Richard Allen Brown; David Alan Haynes; Gareth Richard Smith; Mark Raymond Taylor; Andrew Richard Williams
Original assignee: Bg Intellectual Pty Ltd
Priority date: 1999-03-24
Filing date: 2000-03-15
Publication date: 2006-03-01
Also published as: WO2000056684A1; JP2002540223A; EP1165470A1; GB9906731D0; GB2348437A; GB0006113D0; CA2368020A1; CN1344238A; MXPA01009597A; AU3180600A; ID30475A; TNSN00059A1; PL350569A1; TR200102798T2; AR022343A1

Abstract

The present invention relates to economically advantageous methods of forming, processing, transporting and storing fluids, especially natural gas in the form of solid crystalline gas hydrates. It has been found that fluid can be removed from a mixture of hydrate, liquid and optionally gas using a two stage fluid separation process in which a more efficient, but generally more expensive, fluid removing efficiency separation service is used for the second stage. A number of separation devices are also disclosed. A device for effectively and economically cooling substantially dry hydrate is also disclosed, as is a method for economically storing and transporting hydrate.

Description

118 5 3

FORMATION, PROCESSING, TRANSPORTATION AND

STORAGE OF HYDRATES

The présent invention relates to economically advantageousmethods of forming, processing, transporting and storingfluids, especially naturel gas in the font, of solidcrystalline gas hydrates.

Applications include the transport of gas from fields wherethere is no existing local market or gas transportinfrastructure and in. exporting associated gas from offshoreoilfields where there is no existing means of export ordisposai. The technology éliminâtes the environmentallyundesirable practice of flaring associated gas and gasreleased during well testing operations. A gas hydrate is an ice-like crystal structure comprisedmainly of water molécules during the formation of which gasmolécules are incorporated in molecular scale cavitieswithin the crystal structure. A unit volume of hydrate cancontain in excess of 150 volumes of gas when the gas is at20° C and atmospheric pressure.

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

PC 118 5 3

Hydrate formation is strongly influenced by température andpressure. For natural hydrocarbon, gases, hydrates will typically form at above 0° C (ice formation température) onlyat pressures above about 15 bar as shown in Figure 1.Hydrate formation in pipelines and equipment is thus acommonplace nuisance in offshore oil and gas fields, andexpensive coimtenneasures are used to prevent them. Basic;hydrate formation thermodynamics and properties are wellunderstood and published, see e.g. Sloan E.D. "ClathrateHydrates of Natural Gases" published by Marcel Dekker, New

York 1990.

The ability to couvert gas into a solid hydrate form ispotentially useful for several purposes including storage orlong distance transport because of the large araount of gasthat hydrate can contain in a unit volume. Several

processes hâve been proposed and patented for these purposesover many years, back to at least 1942, see for example US 5536893.

The detailed formation mechanisms of hydrates dépends onwhether the hydrate forming substance is, under thecontacting conditions, a gas, a liquid non-miscible withwater, or a miscible liquid. Most of the prior art patentsare aimed at manufacture of hydrates from gas ; in whichcase, production of hydrate occurs at the interfacialsurface between gas and water, and the proposed production 3 118 5 3 reactors are contacting devices which provide a largeinterface surface area to promote rapid formation. The engineering principles for several suitable contactor typesare well known; most prior art patents cover the use of a 5 single-stage spray (see for example US 2399723 and GB568290) or bubbling pool ("bubble column", "sparged column")reactor (see for example US 3975167 and US 3514274) . Thelatter type is frequently enhanced by the use of mechanicalagitators. 10 Recent prior art (such as WO 97/26494) has looked at optimumarrangements of processing plants for the manufacture of gashydrate.

Under the process conditions proposed by most prior art, theeffluent from the reactor vessel will comprise a mixture of 15 produced hydrate with a considérable amount of unreacted water in the form of a mixed slurry. This is a convenient form in which. to cohtinuously remove the hydrate product from the production reactor. However, the mixed slurrycontaining a considérable amount of unreacted water has alarge volume and mass and so processing, transport andstorage equipment must be correspondingly large to accommodate the slurry. Ail of the prior art, despite 50years of study and proposais, has failed to produce aneconomically advantageous system for any of the intended

r-t 118 5 3 applications. Το the best of our knowledge, no commercialuse has been achieved.

The applicant has carried out a programme of experimentsstudying the formation of a range of gas hydrates and theirProcessing and has subsequéntly investigated the storageproperties of these hydrates. From this work an innovativecombination of technologies has been devised which togethercomprise an économie means for the manufacture, Processing,transport and storage of gas hydrates in many of theapplications described above.

According to a first aspect of the présent invention, thereis provided an apparatus for removing fluid from a two phasemixture of hydrate and liquid at an elevated pressure or athree phase mixture of hydrate, liquid and gas at anelevated pressure, the apparatus comprising a first séparation device of a first fluid removingefficiency for receiving an input mixture at elevatedpressure of hydrate and liquid or hydrate, liquid and gasand for producing an intermediate mixture with a largerconcentration of hydrate than the input mixture, and . a second séparation device having a second higher fluidremoving efficiency than that of the first séparation 5 118 5 3 · -·:η cicvice coi.ipi'a ceuLmuge provided in a sealed pressure vess.el for receiving theintermediate mixture from the first séparation device at anelevated pressure and for producing an essentially dryhydrate or concentrated hydrate slurry output. A higher fluid removing efficiency relates to the ability ofa device to produce a greater concentration· of solids forthe same input mixture. An elevated pressure is taken tomean a pressure greater than atmospneric. Since hydratemanufacturing plants such as that disclosed in WO 97/26494generally operate at high pressure, the hydrate slurryproduced from the plant will be at an elevated pressure.The provision of a higher fluid removing efficiencycentrifuge provided in a sealed pressure vessel enables thecentrifuge to operate at an elevated pressure. By maintaining the elevated pressure through the first and thesecond séparation devices, the hydrate is maintained in astable condition wi'thout having to be excessively cooledwhich can be very expensive.

The applicant has found that removing a proportion of theliquid and, if applicable, gas, from a two phase mixture ofhydrate and liquid or a three phase mixture of hydrate,liquid and gas before supplying it to a generally moreexpensive but higher liquid removing efficiency secondséparation device including a centrifuge significantly 118 5 3 ec reduces the number and capacity of higher efficiencyseparating devices required whilst producing a greaterquantity of the same quality output of essentially solidhydrate or concentrated slurry. This significantly reducescosts and incr.eases production levels making the use ofhydrates more commercially attractive.

The provision of an apparatus having two séparation devicesof differing liquid removing efficiencies enables theproduction of essentially liquid free hydrate at reasonablecost. If one were to use one or more lower liquid removalefficiency séparation devices the final product would stillcontain an excessive quantity . of liquid. If one were touse a sériés of higher liquid removal efficiency séparationdevices then the apparatus would be prohibitively expensive.

The applicant has found that an essentially solid orconcentrated slurry final product form is especially usefulfor applications requiring handling of stable hydrateproduct at pressures substantially lower than the pressurerequired for hydrate production. A device according to a second aspect of the présentinvention and which may be used as at least part of thefirst séparation device of the first aspect of the présentinvention for separating gas from a three phase mixture ofhydrate, liquid and gas comprises 7 118 5 3 a vessel with an ' inlet for receiving a three phasemixture of hydrate, liquid and gas; the vessel having an internai surface against which themixture is arranged to be directed with sufficient force 5 such that the impact of the mixture against the surface disengages gas from the mixture; and the vessel having a chamber to collect mixtureremaining after it has been directed against the internaisurface, the chamber having an outlet and tneans to direct ® hydrate floating on liquid in the chamber to the outlet when in use.

The means to direct hydrate floating on liquid in thechamber to the outlet is preferably an upper boundary of thechamber., at least a portion of which is inclined to the 5 horizontal when in use with the outlet located at an upper portion of the chamber defined below the inclined portion ofthe upper boundary. A device for separating gas, liquid and solid hydrateaccording to a third aspect of the présent invention and asmay be used as the first séparation device of the firstaspect of the présent invention comprises 8 118 5 3 a vessel for receiving an input mixture of gas, liquidand hydrate; a straining means mounted within the vessel; and means to direct an input mixture of gas, liquid andhydrate against the straining means such that gas is evolvedto be collected or removed from the vessel·, liquid passesthrough the straining means to be collected or removed fromthe vessel, and hydrate is collected by the straining means.The straining means may be for example a perforated screenor a woven mesh.

An essentially solid or concentrated slurry final productform which may be produced according to the first aspect ofthe présent invention is preferably cooled before beingstored or. transported to enable it to remain stable forlonger periods of time.

The cooling of. a solid or concentrated slurry is difficultand expensive because of the poor heat transfercharacteristics of such Systems and the need to avoid thefreezing of solids to the surfaces of a cooling device.

The inventors hâve solved this problem according to afurther aspect of the présent invention with a substantiallydry hydrate cooling apparatus comprising 9 118 5 3 a container for receiving essentially solid orconcentrated slurry hydrate; a gas distribution device arranged to be supplied withfluidising gas when in use, the gas distribution device 5 being arranged to . be positioned in the container to pass fluidising gas through essentially solid or concentratedslurry hydrate in the container when in use to fluidisé thehydrate ; and means to provide cooling of the fluidised hydrate in 10 the container.

The means to provide cooling to the fluidised hydrate ispreferably the distribution device which is arranged tosupply cooled fluidising gas. Altematively or additionally the means to provide cooling to the hydrate may 15 be means to supply a stream of cooled fluid through the fluidised hydrate to provide the cooling. This stream ofcooled fluid may be passed through the fluidised hydrate inone or more conduits.

According to a still further aspect of the présent invention 20 there is provided a method for at least one of storage and transport of gas in the form of stable hydrate which ispreferably used with hydrates prepared using one or more ofthe above aspects of the invention. 10 118 5 3

The invention will now be described by way of example withrsfersnce to the accompanying drawings in which:

Figure 1 shows a typical raturai gaa hydrateequilibrium cuve showing the pressure and températureconditions required for formation of stable hydrate, withstable hydrate existing above the curve;

Figure 2 diagrammatically shows an apparatus accordingto the first aspect of the présent invention for theproduction of an essentially solid or concentrated slurry ofhydrate ;

Figure 3 diagrammatically shows a sequence of steps ina process for forming hydrate incorporating the methodaccording to the first aspect of the présent invention;

Figures 4 to 8 show preferred devices for performingvarious steps in the process shown in Figure 3; and

Figure 9 is a diagram showing the température ofvarious régions of a mass of hydrate stored for 5 days in aship's hold at ambient température and pressure.

Figure 2 diagrammatically shows a System according tothe first aspect of the présent invention for the two stageremoval of fluid from a two phase mixture of hydrate and 11 118 5 3 liquid or a three phase mixture of hydrate, liquid and gas.The mixture 1 is suppiied . to a first stage 3 of a fluidremoval System 2. The first stage 3 may be any suitableseparating device such as a hydrocyclone which is well known 5 in the art or a device to mechanically remove hydrate floating on a liquid which may be liquid separated from theslurry as described later. The output 4 from the firststage 3 is féd to the second stage 5 which is a moreefficient separating device than the first stage,, in this ® case a centrifuge in a pressure vessel which produces a substantially dry hydrate product δ.

Figure 3 is an outline of a hydrate formation processwhich has been tested using a pilot plant and laboratoryexperiments, the process incorporating the fluid removal 5 System shown in Figure 2. A process reactor 10, for example as shown in our earlier international patentapplication published as WO97/26494, produces ahydrate/gas/liquid mixture 11. The mixture 11 is passed todevice 12 described below which is arranged to separate the ® tnajority of the gas phase from the mixture il. A separated substantially liquid and solid free gas stream 13 can beutilised by for example being returned to the processreactor 10 for the formation of further hydrate or by beingdelivered to a device for power génération or it may be '5 burned. A substantially gas free liquid and solid slurry stream 14 is passed to a first separating device 15 forming 2 118 5 3 the first stage 3 of the water removal system 2, an exampleof which is described later, which is arranged to produce aliquid stream 16 containing a low level of solids and aslurry stream 17 with a higher solid hydrate concentrationthan input stream 14. Stream 16 is passed back to theprocess reactor 10 to be used in the further production ofhydrate. Alternatively gas separator 12 and firstseparating device 15 may be combined into a single device 30described later.

Stream 17 is passed via an optional cooling device 18 tothe second stage of the water removal system 2, comprisingthe more efficient separating device 5 than that of thefirst stage 3. The separator of the second stage is acentrifuge in a pressure vessel to enable it to separatefluid at high pressure so that the hydrate may be maintainedin a stable condition withôut having to be excessivelycooled. The inventors of the présent invention hâve foundthat a continuous screening centrifuge 5 produces a 95% to99.5% liquid free stream 19 in the form of a granular,flowable solid and a liquid stream 20 containing extremelylow levels of solids which may be returned to the processreactor 10.· The centrifuge has been found by the inventorsto be particularly suited to large scale applications.

Stream 19 may optionally be passed into device 21 wnere itis cooled either by direct contact with a gas stream 22 118 5 3 supplied in this case at high pressure and low températureor indirectly by passing an additional stream of eoolingmedium 23 through conduits passing through the body andwalls of the device. This latter option increases the 5 process complexity but means that a smaller high pressure gas stream 22 is needed only to aid motion of the solidstermed 'fluidisation' and improve heat transfer to thesolids. The gas stream 22 may be either of hydrate formingor non-hydrate forming gases - in the former case an 1® 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 eooling medium 23 exitdevice 21 separately (streams 24 and 25) . The dry coldsolids stream exits device 21 and may be depressurised by 15 device 26 to atmospheric conditions and loaded into a transportation or storage device 27.

Gas separating device 12 may be of the type as illustratedin Figure 4. An input stream il is arranged to enfer apressure vessel 40 which is able to withstand the elevated 2® 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 isdirected downwards by a suitably shaped portion 42 of theinlet. The gas présent in the input mixture 11 is disengaged from the mixture by impacting the mixture against 14 118 5 3 a suitable surface 43, in this case part of an insert 44.The generated gas exits the vessel as a gas stream 13 viaoutlet 45. The surface 43 directs the remaining liquid andsolids from the mixture into a downcomer 46 which is sized(by known methods) to ensure that particles of hydrate areentrained in the downward flow. The insert 44 is shaped toprovide a space or chamber 47 at the base of the vessel 40with an upper boundary 48 which is inclined or slopingrelative to the horizontal when in use. An offtake 49 fora substantially gas free liquid and solid slurry stream 14is located at the upper. part of this chamber 47 and theinclined or sloping upper boundary 48 is arranged to directhydrate floating on liquid in the chamber 47 ta the offtake 49. This design avoids accumulation of hydrate within thedevice and subséquent blockage.

In operation the level of liquide collected in or above thedowncomer is maintained to enable hydrate to be entrained inthe downward flow into chamber 47. The flow of liquid intothe downcomer 46 has been found to generally produce avortex which entrains hydrate in the downward flow.Maintaining the level of liquid in or above the downcomerwhen in use produces a seal to prevent the passage ofseparated gas into chamber 47 or out of offtake 49 so that asubstantially gas free liquid and solia slurry stream 14 isproduced. Maintenance of the liquid/hydrate slurry surfacelevel in or above the downcomer may be achieved using afirst level sensor 401 positioned at the minimum liquid ' PC'f 118 5 3 level in the downcomer 46, a second level sensor 402positioned at the maximum liquid level in or above thedowncomer 46, a valve 403 connected to the offtake 49 and acontrol means 404 connecting them together via control lines405. When the liquid level in the downcomer falls to theminimum level, the first level sensor 401 is activated andthe control means 404 closes valve 403 so that the liquidlevel in the downcomer 46 rises due to· the continuedaddition of the stream from input 11 to ensure that hydratecontinues to be caught in the downward flow of liquid in thedowncomer 46. Conversely when the maximum liquid level sensor 402 is activated the control means 404 opens thevalve 403 so that water and hydrate may be passed out ofofftake 49 to lower the liquid level.

The chamber 47 may be provided with a water outlet 404 atits lower portion to withdraw water from the chamber 47 andthus increase the concentration of hydrate from offtake 49.By positioning the water outlet 404 at the lower portion ofchamber 47, hydrate floating on water in the chamber isunlikely to be withdrawn through water outlet 404,especially if the liquid control system described in theprevious paragraph is used. However, a filter 405 may beprovided at the outlet to prevent hydrate passing into thewater outlet 404. 16 118 5 3

WG

Either of two preferred dèvices can perform the function ofseparating device 15. One is a hydrocyclone - a devicefamilial* to those skilled in the art of solid-liquidséparation but normally used for the séparation of solidsfrom liquid of lower density than in the présent invention.

Studies performed by the applicant hâve identified analternative device 15 suitable for this application whichinvolves separating solids from a liquid of higher density.This device is illustrated in Figure.5. Stream 14 enters a 10 vessel 50 of the device 15 via an inlet 51 and is directedupwards by a suitable portion 52 of the inlet.

Liquid stream 16 is removed from the base of the vesselthrough outlet 53. The diameter of the vessel 50 is suchthat hydrate particles are not drawn down by the flow of IJ liquid in stream 16 - instead the hydrate collects to form afloating mass 54 in the upper part of the vessel 50. Themass of hydrate 54 floats on the liquid contained in thevessel. The section of the mass of hydrate 54 which floatsabove the surface of the liquid becomes drained of liquid by 20 gravity. A scraping device 55 positioned at the top of thevessel 50 scrapes hydrate off the top of the floating mass54 to an outlet 56 to form stream 17.

Device 30 which may be used as an alternative to thecombination of devices 12 and 15 is illustrated in Figure 6. 17 110 5 3 wc

It comprises a straining means, in this case a perfcratedscreen 6 0 mounted within a pressure vessel 61 to withstandthe pressure of the input stream 11 from the hydrateproduction plant. Stream : 11 enters the vessel through inlet 62 and is direct ed downwards by a suitabledistribution device 63 which may be a suitably directedportion of the inlet 62. The input stream 11 is directedagainst a surface 60 with sufficient force to- generate gas. A gas stream 13 is generated by the impact of the inputhydrate/liquid/gas mixture onto the screen 60 and thegenerated gas stream 13 exits from the top of the vessel 61through a gas outlet 64. Liquid and hydrate which impingeagainst the screen 60 travel down it under the influence ofgravity 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 isgenerally required, to maintain the hydrate in a stablecondition, increases the amount of liquid which passesthrough the perforations in screen 60 thus providing betterséparation. The concentrated slurry (stream 17) is drawnfrom the vessel through outlet 65. The liquid 66 thatpasses through the screen 60 accumulâtes in the base of thevessel 61 and is drawn out of the vessel, as stream 16,through outlet 67. Laboratory tests at process conditionshâve found that such a device can concentrate a stream 118 5 3 containing less than 5% by volume hydrate to one containingmore than 30% by volume of hydrate.

The second more efficient stage 5 of the two stage waterrénovai apparatus is in this example a centrifuge 71 5 provided in a pressure vessel 72. The centrifuge 71 comprises a ring 73 of gauze or mesh acting as a screeningsurface. The centrifuge is mounted on an axis 74 supportedby the pressure vessel 72 and is arranged to be rotatable onthe axis at a suitable rate. If desired an arrangement of 10 plates and blades (not shown) may be provided inside thecentrifuge to assist in the séparation. An intermediatestream from the first stage 3 of the two stage water removalSystem 2 is delivered to the centrifuge 73 via inlet 75.The rotation of the centrifuge forces water through the 15 screening surface to be collected at the bottom of thepressure vessel 72 whilst hydrate collects on the inside ofthe screening surface. Below the centrifuge 71 is mounteda duct 76 to receive hydrate collected on the inside of thecentrifuge 71 and pass it out of the pressure vessel 72 via 2® hydrate outlet 77, Water collected at the bottom of thepressure vessel 72 is collected via liquid outlet 78. Thecentrifuge 73 thus produces a continuous flow of hydrate.Laboratory work with a small pressurised centrifuge hasshown that a full size centrifuge can produce a hydrate 25 product which contains less than 2% by volume of water. 118 5 5 19

Device 21 is a fluidised bed in the présent example The applicant has found that the fluidisation of hydrate and ice particles is feasible at low températures and high pressures. In laboratory studies beds of hydrate and ice 5 particles could be fluidised smoothly at a température of -10° C and below provided high pressure was maintained.Experiments were performed at -10° C to · -70° C and atpressures of 3.5 to 28 bar. By comparison of the heattransfer rates from device 21 with those seen in 1® conventional cooling devices for substantially solid streams we hâve found this to be the most économie method ofadequately cooling the hydrate product to a températuresuitable for transport. Figure 8 shows an arrangement ofsuch a bed. The bed 80 is contained within a pressure 15 vessel 81 suitable for the pressures and températures necessary for the process. The pressure vessel 81 is shaped with the bed 80 arranged to be located in a lowerportion 82 of the vessel 81. The upper portion 83 of thevessel 81 is arranged to direct any fluidised particles 2® leaving the lower portion 82 back to the lower portion. In the example illustrated in Figure 8, this is achieved by theprovision of an inclined lip 84 around the upper peripheryof the lower portion 82 to direct any particles leaving thebed 80 back to the lowerportion of the vessel 81. This 25 structure contains the bed 80 and avoids the carriage of smaller solid particles out of the top of the bed 80 20 118 5 3 (alternatively shaped internais may be used to provide thedesired bed geometry in a pressure vessel of moreconventional shape), Solids from separator 5 are added tothe bed 80 via inlet '85 so that they fall down into the bed.Fluidising gas 22 is introuced through inlet 86 and thencevia a distribution System 87 to the majority of the base ofthe bed. The fluidising gas 22 is preferably a hydrateforming gas so that any moisture entering the fluidised bedfrom stream 19 is converted into hydrate to maintain thehydrate virtually dry. The fluidising gas 22 may also provide cooling to the hydrate in the bed 80. If desired acooling medium 23 such as evaporative réfrigérant from oneor more external sources may be passed through the bed 8 0from the distribution system 87 with the fluidising gas. Thefluidising gas 22 exits the bed 80 and leaves the vessel 81via outlet 88 after passing, optionally, through aconventional cyclone device 89 to remove small entrainedparticles of ice and hydrate. As illustrated in Figure 8, cooling medium may be passed through the fluidised bed inconduits 90 made of good heat conducting material,preferably métal such as Steel. By passing the coolingmedium through the bed in conduits, liquid coolant may beused which can absorb far more heat than gaseous coolantproducing a better cooling effect. As further solids areadded to the bed 90 the level of the bed rises and solidsoverflow out of the bed 90 via chute 91 and outlet 92maintaining the level of the bed 90 substantially constant. 21 T--' 118 5 3

In some circumstances, e.g. a large installation, a bed maybe subdivided by a sériés of substantially vertically orientated baffles 93 only one of which is shown in Figure 8, over which solids will flow from entry région(s) 93a of a f irst température to exit régions 93b of lower température.

Depressuring device 26 shown diagrammatically in Figure 3could be any of a range of known technologies for reducingthe pressure of a solid stream. The applicant uses a lookhopper System where batches of solids are introduced intopressurised vessels, the vessels are then isolated by raeansof valves and the vessels then depressurised with theexhaust gas optionally being initially routed to previouslydepressurised vessels to save on the costs of recompression.

Any means of transporting or storing a chilled bulk solidmass of hydrate may be used as convenient. Examples mightbe a container, the hold of a ship or a railway wagon. Thetransporting or storing means is preferably insulated.

It has been found from économie studies that the gas contentof any hydrate used for transportation or storage shouldpreferably be of the order of 150 to 200 volumes of gas (forgas at atmospheric pressure and température) per volume ofhydrate. If such a hydrate gas content is not achievedthen such large ships or large numbers of small ships or w< 22 118 5 3 containers will be required as to mdke the use of hydratesuneconomic when compared with other known alternatives forgas storage or transportation.

The applicant has surprisingly found that according to a5 still further aspect of the présent invention , storage ortransport of hydrate with at least the majority of thehydrate remaining in a stable State for at. least 24 hourscan be performed by the provision of the stable hydrate in a mass without the need for external cooling. 10 Figure 9 shows the température profile of just such a mass of hydrate in a ship1 s hold with an initial storagetempérature of -50°C after 5 days hâve elapsed. The ambient température at the top of the hold is 20°C and theambient température at the bottom of the hold is 15°C. As 15 can be seen only the edges of the original mass of hydratefall below the stable température of approximately -37°C atatmospheric pressure and are converted into water (ice) andnatural gas. The vast majority, in this case 95%, of thehydrate remains as stable hydrate with only 5% being 20 converted into natural gas and water in the f orm of ice.

Although insulation of the mass of hydrate is not necessaryits use is preferred in some circumstances as it willenhance the length of time that the hydrate remains stable.Insulation may be provided in whatever transport or storage

PO' 118 5 3 device is being used such as the hold of a ship, containeror railway wagon.

Since only the edges of a mass of hydrate are decomposedinto ice and gas within a normal transport or storage period 5 of a few days, as the size of the mass of hydrate is increased the proportion of hydrate that remains stable overthe same period is increased. A preferred mass of hydratefor use in the présent invention has a minimum dimension of2m in any direction or a more preferred dimension of at 1® least 10m in any direction. However, this of course dépends upon the expected duration of the transportation orstorage.

The hydrate used for the above method of storage andtransport is preferably substantially pure to provide a 15 commercially viable volume of gas in a suitably small volume of hydrate.

The hydrate used for the above method of storage andtransport is preferably substantially dry or a concentratedslurry to reduce the proportion of non-gas carrying material 2Θ to be stored or transported making the method of storage or transport of the présent invention even more economicallyattractive. 24 118 5 3

Vv

Many modifications may be madeabove without départing from thedefined in the following daims,first séparation device may beapparatus for removing fluidoptionally gas mixture. to the examples describedscope of the invention asFor example any suitableused in the two stagecm a hydrate, liquid and

Claims

25 118 5 3 CLAIMS

1. An apparatus for removing fluid from a . two phasemixture of hydrate and liguid at an ' elevated pressureor a three phase mixture of hydrate, liquid and gas atan elevated pressure, the apparatus comprising 5 a first séparation device of -a first fluid removing efficiency for receiving an input mixture ofhydrate and liquid or hydrate, liquid and gas and forproducing an intermediate mixture with a largerconcentration of hydrate than the input mixture; and 1© a second séparation device having a second higher fluid removing efficiency than that of the firstséparation device, the second séparation devicecomprising a centrifuge provided in a sealed pressurevessel for receiving the intermediate mixture from the 15 first séparation device at an elevated pressure and for producing essentially solid hydrate or concentratedhydrate slurry output.
2. An apparatus according to claim 1, wherein the firstséparation device comprises a vessel to receive an 2© input mixture of hydrate and liquid, means to remove 26 118 5 3 hydrate floating on top of the mixture and. means toremove iiquid from a lower portion of the vessel.
3. An apparatus according to claim 2, wherein the means toremove hydrate floating on top of the Iiquid is a 5 scraper arranged to direct removed hydrate to an outlet.
4. An apparatus according to claim 1, wherein the firstséparation device is a hydrocyclone. '
5. An apparatus according to claim 1, wherein the first 10 séparation device includes: a vessel with an inlet for receiving a three phase’mixture of hydrate, Iiquid and gas; the vessel having an internai surface againstwhich the mixture is arranged to be directed such that 15 the impact of the mixture against the surface disengages gas from the mixture; and the vessel having a chamber to collect mixtureremaining after it has been directed against theinternai surface, the chamber having an outlet and 20 means to direct hydrate floating on Iiquid in the chamber to the outlet when in use. 118 5 3
6. An apparatus according to claim 5, wherein the means todirect solid floating on liquid in the chamber to theoutlet is an upper boundary of the chamber, at least aportion of which is inclined to the horizontal when inuse with the outlet located at an upper portion of thechamber defined below the inclined portion of the upperboundary.
7. An apparatus according to claim 5 or claim 6, whereinthe internai surface of the vessel against which themixture is arranged to be directed is a surface abovethe chamber when in use and a downcomer is provided todirect mixture remaining after it has been directedagainst the surface to the chamber.
8. An apparatus according to claim· 7, wherein the internaisurface of the vessel is shaped to direct mixtureremaining after it has been directed against theinternai surface to the downcomer under gravity when inuse.
9. An apparatus according to claim 1, wherein the firstséparation device comprises : a vessel for receiving an input mixture of gas,liquid and solid hydrate; 28 118 5 3 a straining means mounted within the vessel; and means to direct an input mixture of gas, liquidand hydrate against the straining means such that gasis evolved to be collected or removed from an upper 5 portion of the vessel, liquid passes through thestraining.means to be collected or removed from a lowerportion of the vessel, and hydrate is collected by thestraining. means.
10. An apparatus according to claim 9, wherein the 10 straining means is a perforated screen arranged such that hydrate collected by the screen travels down thescreen to be collected or removed from the vessel.
11. An apparatus according to claim 11, wherein the screenis curved and the means to direct the mixture against 15 the screen is arranged to direct the mixture downagainst the screen such that hydrate collected by thescreen slides off the screen with a component of itsmotion in an arc.
12. An apparatus according to any of the preceding daims . 20 including a hydrate cooling. apparatus for cooling an essentially dry hydrate or côncentrated slurry outputproduced by the second séparation device, the hydratecooling device comprising: 29 118 5 3 container for receiving essentially solid orconcentrated slurry hydrate; a gas distribution device arranged to be suppliedwith fluidising gas when in use, the gas distribution 5 device being arranged to be positioned in the container to pass fluidising gas through essentially dry hydrateor concentrated slurry hydrate in the container when inuse to fluidisé the hydrate; and means to provide the passage of a cooling medium 10 through fluidised hydrate in the container, when in use.
13. An apparatus according to claim 12, wherein the meansto provide the passage of a cooling medium throughfluidised hydrate in the container is the gasdistribution device which is arranged to supply cooled 15 fluidising gas.
14. An apparatus according to claim 12 or claim 13, whereinthe means to provide the passage of a cooling mediumthrough fluidised hydrate is a means to supply a streamof cooling fluid, separate from the fluidising gas, 20 through the fluidised hydrate. 30 118 5 3
15 . An apparatus according to any of daims 12 to 14,wherein the gas distribution device is arranged to besupplied with hydrate forming fluidising gas.

15. An apparatus for producing an essentially solid hydrateor concentrated hydrate slurry substantially ashereinbefore described with reference to theaccompanying drawings.
17. A method of storing or transporting hydrate produced bythe apparatus according to any of the preceding daimscomprising providing the hydrate in a stable form.
18. A method according to daim 17, wherein the hydrate isessentially dry hydrate or concentrated slurry hydrate.
19. A method according to daim 17 or daim 18, wherein thehydrate is provided in an insulated container.
20. A method according to any of daims 17 to 19, whereinthe hydrate is provided in a bulk mass having a minimumdimension of 2 métrés in any direction.
21. A method of storing or transporting hydratesubstantially as hereinbefore described with referenceto the accompanying drawings. 31 118 5 3
22. A device for separating gas from a three phase mixtureof hydrate, liquid and gas, the dévies comprising a vessel with an inlet for receiving a three phasemixture of hydrate, liquid and gas; the vessel havingan internai surface against which the mixture isarranged to be directed such that the impact of themixture against the surface disengages gas from themixture ; and the vessel having a chamber to collect mixtureremaining after it, has been directed against theinternai surface, the chamber having an outlet andraeans to direct hydrate floating on liquid in thechamber to the outlet when in use.
23. A device according to claim 23, wherein the means todirect hydrate floating on liquid in the chamber tothe outlet is an upper boundary of the chamber, atleast a portion of which is inclined to the horizontalwhen in use with the outlet located at an upper portionof the chamber defined below the inclined portion ofthe upper boundary.
24. A device according to claim 22 or claim 23, wherein adowneomer is provided to direct mixture remaining after 32 118 5 3 it has been directed against the surface to thechamber.
25. A device according to claim 24, wherein the level ofmixture remaining after it has been directed againstthe surface is maintained in or above the downcomer.
26. A device according to claim 24 or claim 25, wherein theinternai surface of the vessel is shaped to directmixture remaining after . it has been directed· againstthe internai surface to the downcomer under gravitywhen in use.
27. A device according to claim 26, wherein the internaisurface of the vessel présents a substantially conicalor frusto-conical surface with the conical or frusto-conical axis arranged substantially vertically when inuse and the narrower part of the cône or frusto-conearranged below the wider part. 2Θ. A device according to any of daims 22 to 27, whereinthe vessel has an outlet for gas disengaged from themixture to exit the vessel.
29. A device for separating gas from a three phase mixtureof hydrate, liquid and gas substantially as 118 5 3 33 hereinbefore described wifch reference to figure 4 ofthe accompanying drawings.
30. An apparatus for separating gas, liquid and hydratecomprising: a vessel for receiving an input mixture of gas,liquid and hydrate; straining means mounted within the vessel; and means to direct an input mixture of gas, liquid andhydrate against the straining means such that gas isevolved to be collected or removed from the vessel,liquid passes through the straining means to becollected or removed from the vessel, and hydrate iscollected by the straining means.
31. An apparatus according- to claim 30, wherein thestraining means is a perforated screen arranged suchthat hydrate collected by the screen travels down thescreen to be collected or removed from the vessel.
32. An apparatus according to claim 31, wherein the screenis curved and the means to direct the mixture againstthe screen is arranged to direct the mixture downagainst the screen such that hydrate collected by the 34 118 5 3 screen slides off the scrsen with a component of itsmotion in an arc.
33. An apparatus according to any of daims 30 - 32, wherein the interior of the vessel is maintained at an 5 elevated pressure.
34. An apparatus for separating gas, liguid and solidhydrate substantially as hereinbefore described withreference to figure 6 of the accompanying drawings.
35. A hydrate cooling apparatus comprising: 10 a container for receiving essentially dry or concentrated slurry hydrate; a gas distribution device arranged to be suppliedwith fluidising gas when in use, the gas distributiondevice being arranged to be positioned in the'container 15 to pass fluidising gas through essentially dry or concentrated slurry hydrate in the container when inuse to fluidisé the hydrate; and means to provide the passage of a cooling mediumthrough fluidised hydrate in the container when in use. 35 118 5 3
36. An apparatus according to claim 35, wherein the meansto provide the passage of a cooling medium throughfluidised hydrate in the container is the gasdistribution device which is arranged to supply cooled 5 fluidising gas.
37. An apparatus according to claim 35 or claim 36,wherein the means to provide the passage of a coolingmedium through fluidised hydrate is a means to supply astream of cooling fluid, separate from the fluidising 10 gas, through the fluidised hydrate.
38. An apparatus according to claim 37, wherein the meansto supply a stream of cooling fluuid, separate from thefluidising gas, comprises one or more conduits arrangedto convey cooling fluid therethrough to pass through 15 the fluidising bed when in use.
39. An apparatus according to claim 38 wherein the one ormore conduits are arranged to convey a substantiallyliquid stream of cooling fluid. An apparatus according to any of daims 35 to 39,wherein the fluidised bed is provided with one or more,baffles arranged substantially vertically when in useto divide the bed into. a number of régions and hydrateis arranged to be received in a first région and 40 20 118 5 3 41. 42 . ovêrflow into subséquent régions . as more hydrate isadded. An apparatus according to any of claims 3 5 to 4Q,wherein the gas distribution device is arranged to besupplied with hydrate forming gas. A hydrate cooling apparatus substantially ashereinbefore described with reference to figure 8 ofthe accompanying drawings.