Continuous Production of Clathrate Hydrates From Aqueous and Hydrate-Forming Streams, Methods and Uses Thereof

Abstract

The present disclosure relates to a novel improved method for continuous crystallization of highly crystalline clathrate hydrates. The novel improved method utilizes a novel hydrator capable of overcoming heat and mass transfer limitations that usually constrain crystallization rate and thus reduces process productivity. The disclosed method and hydrator are for production of crystalline clathrates in general, CO.sub.2 capture, capture of other clathrate forming compounds, CO.sub.2 storage and transportation, storage and transportation of any clathrate forming compound in a solid lattice, gas separation or water desalination or purification purposes.

Claims

1. A method for producing clathrate hydrates in a continuous process the process comprising: lowering the temperature of an aqueous solution stream to a pre-defined temperature range; introducing the aqueous solution stream into a hydrator via a first inlet; introducing a stream of at least one hydrate-forming compound into the hydrator via a second inlet; contacting the hydrate-forming stream and the aqueous stream inside a mixing element of the hydrator to produce an aqueous slurry of clathrate hydrates, wherein the hydrator operates at a temperature in the range of from 0° C. to 5° C., at a pressure in the range of from 10 to 40 bar and at a-hydrate-forming stream rate ranging in the range of from 1 to 500 ton/h; and collecting the aqueous slurry of clathrate hydrates from an outlet of the hydrator; wherein the hydrator is an array of static mixing elements comprising one or more chambers.

2. The method claim 1, wherein the hydrate-forming stream is converted to the aqueous slurry of hydrates at a hydrate conversion rate of at least 80%.

3. The method claim 2, wherein the hydrate conversion rate is at least 90%.

4. The method claim 1, wherein the clathrate hydrates in the aqueous slurry of clathrate hydrates are crystalline compounds, the method further comprising concentrating the clathrate hydrate crystals.

5. The method claim 4 wherein the hydrator operates at a temperature in the range of from 2-5 ° C.

6. The method of claim 1 wherein the hydrator operates at a pressure in the range of from 20 to 40 bar.

7. The method claim 1 wherein the aqueous solution stream and the hydrate-forming stream are introduced into one or more static mixing elements of the hydrator through high energy jets that enhances the mixing intensity of the hydrate-forming stream and the aqueous stream inside the one or more chambers.

8. The method of claim 1 wherein the hydrate-forming stream is introduced into the hydrator at a hydrate-forming stream to water mass flow rate of at least 5%.

9. The method of claim 1 wherein the first and second inlets of the hydrator introduce the hydrate-forming stream and the aqueous stream in into a chamber of the hydrator with an energy characterized in a Reynolds number of least 100, and further wherein the contacting step inside the mixing element takes place over a time period times of less than or equal to 1 second.

10. The method claim 1 wherein the at least one hydrate-forming compound is a molecules with a molecular radius of less than 9 Å, wherein the at least one hydrate-forming compound is selected from a list comprising: carbon dioxide, carbon monoxide, hydrogen, nitrogen, C1-C5 hydrocarbons, hydrogen sulphide, or mixtures thereof.

11. The method of claim 10 wherein the C1-C5 hydrocarbons stream is methane, ethane, propane, or butane.

12. The method claim 1 wherein the array of static mixing elements comprising chambers, is a series of interconnected static mixing elements.

13. The method claim 1 wherein the hydrator comprises: a stack comprising a network mixer plate for performing hydration and a heat exchange plate, wherein the network mixer plate comprises the array of static mixing elements comprising one or more chambers, wherein each chamber is interconnected by at least two channels to at least two other chambers, for mixing and dividing one or more fluids sequentially through said chambers, wherein the heat exchange plate comprises a channel for the flow of a cooling liquid, wherein the channel of the heat exchanger plate and the chambers of the network mixer plate are lined up to transfer heat between said chambers and said channel, wherein each chamber of the network mixer plate is a spherical or cylindrical chamber, comprising at least two or three channels and at least two or three apertures for connection to said channels, wherein the temperature of the cooling liquid is in the range of from −5° C. to 0° C.

14. The method claim 1 wherein a heat transfer coefficients of the hydrator is in the range of from 1000 to 8000 W.Math.m.sup.−2.Math.K.sup.−1.

15. The method of claim 1 wherein the aqueous slurry of clathrate hydrates is used for capturing gas, storing gas, transportation gas, gas separation, water desalination, or water purification.

16. A crystalline hydrate of CO.sub.2 produced by the method claim 1 wherein the crystalline CO.sub.2 hydrate has an XRD pattern comprising peaks at diffraction angles (2θ) of 18.2°±0.5%, 29.9°±0.5%, 30.8°±0.5% and 31.7°±0.5%; a melting point ranging from −50° C. to −40° C., or both an XRD pattern comprising peaks at diffraction angles (2θ) of 18.2°±0.5%, 29.9°±0.5%, 30.8°±0.5% and 31.7°±0.5% and a melting point ranging from −50° C. to −40° C.

17. The crystalline of CO.sub.2 hydrate claim 16, wherein the crystalline of CO.sub.2 hydrate does not contain of hexagonal ice and has an XRD pattern comprising the absence of peaks at diffraction angles (2θ) of 22.5±0.5%, 24.1±0.5% and 40±0.5%.

18. The method of claim 1, wherein the clathrate hydrates in the aqueous slurry of clathrate hydrates are crystalline compounds, the method further comprising gasifying the clathrate hydrate crystals.

19. The method of claim 10 wherein the aqueous solution stream contains salts, gases, organic promoters, or mixtures thereof.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0074] The following figures provide preferred embodiments for illustrating the description and should not be seen as limiting the scope of invention.

[0075] FIG. 1 shows a phase diagram of the operational conditions established for water and several hydrate-forming compounds such as: (1) methane CH.sub.4, (2) carbon dioxide CO.sub.2, (3) ethane C.sub.2H.sub.6 and (4) hydrogen sulphide H.sub.2S.

[0076] FIG. 2 shows the stacking of the plates of the (11) hydrator for continuous crystallization of clathrate hydrates comprising: (13) two heat exchange plates, (14) one structured network plate and (12) two closing plates.

[0077] FIG. 3 shows the mesa/micro structured network plate comprising a carved network of (21) cylindrical chambers, (22) prismatic channels, (23 and 24) inlet and outlet channels.

[0078] FIG. 4 is a process flow diagram for the method of sequestration and transport of CO.sub.2 using the hydrator.

[0079] FIG. 5 is a process flow diagram of the method of purification and separation of a mixture of gases using the hydrator.

[0080] FIG. 6 shows the X-ray diffraction patterns—XRD—of carbon dioxide hydrates obtained with the method disclosed.

[0081] FIG. 7 shows experimental evidence obtained from the example temperature and pressure plots.

DETAILED DESCRIPTION

[0082] The present disclosure relates to a novel improved method for continuous crystallization of highly crystalline clathrate hydrates. The novel improved method utilizes a novel hydrator capable of overcoming heat and mass transfer limitations that usually constrain crystallization rate and thus reduces process productivity.

[0083] As explained above, current methods for crystallization of hydrates either fail to do it continuously or present a series of problems regarding process stability due to inadequate heat and mass transfer rates for operating within the pressure and temperature conditions suited for crystallization to occur. Therefore, there is a need for an improved method.

[0084] The present disclosure relates to a novel improved method for efficient crystallization of clathrate hydrates in a continuous process. The novel improved method utilizes a novel hydrator capable of overcoming heat and mass transfer limitations that usually constrain crystallization rate and reduces process productivity.

[0085] In an embodiment, the method maximizes the rate of conversion of gas to hydrates through conditions that allows the method of the present disclosure to achieve close to the thermodynamic equilibrium conditions thus minimizing energy consumption.

[0086] In an embodiment, the method is carried out in a hydrator built by stacking at least two types of plates: meso/micro structured plates comprising a network of static mixing elements, where the crystallization occurs and where hydration heat is released; and heat exchange plates that absorb hydration heat from the structured network plates thus keeping its temperature within the range of operational temperatures. Due to the lack of any movable parts in the hydrator to promote mixing such as stirrers and other mechanical agitators, solely energy to the flow of the fluid streams is required in the hydrator.

[0087] In an embodiment, the hydrator comprises plates in which an aqueous stream and a hydrate-forming stream are fed into the hydrator through the meso/micro structured plates. The aqueous and hydrate-forming streams form hydrates of the hydrate-forming compounds in which a lattice of water molecules surrounds and traps the hydrate-forming molecules inside, forming polymorphic crystals of hydrates suspended in water in a slurry. Both the crystallization process and the guest hydrate-forming dissolution process (to achieve supersaturation) are exothermic, as such a cooling fluid must be fed to the heat exchange plates to absorb the heat released in the structured network plates.

[0088] In an embodiment, the method for the continuous crystallization of clathrate hydrates comprises the following steps: [0089] Decreasing of the temperature of an aqueous stream; [0090] Adding the aqueous stream into the hydrator; [0091] Adding a stream comprising at least one hydrate-forming compound into the hydrator to allow the hydrate-forming compound to come into contact with the aqueous stream, at pressure and temperature conditions within thermodynamic stability region; [0092] Collecting the aqueous hydrate slurry from the outlet of the hydrator.

[0093] In an embodiment, the aqueous stream may be fresh water or an aqueous solution. The aqueous solution can also contain salts, gases, or other compounds. The temperature of the aqueous stream is decreased by passing this stream through the hot side of a conventional heat exchanging device, such as a shell-and-tube heat exchanger or plates heat exchanger. A refrigerant enters the cold side of said heat exchanging device. The refrigerant can be any refrigerant capable of lowering the temperature of the aqueous stream down to the operational temperature. If operating in counter-current, the inlet temperature of the refrigerant must be lower than the outlet temperature of the aqueous stream, and the inlet temperature of the aqueous stream must be larger than the outlet temperature of the refrigerant. If operating in co-current, the inlet temperature of the refrigerant must be lower than the inlet temperature of the aqueous stream, and the outlet temperature of the aqueous stream must be larger than the outlet temperature of the refrigerant.

[0094] In an embodiment, the heat exchange operates counter-currently for a more efficient operation. The aqueous stream is then pumped into the hydrator at the operational pressure conditions, using a conventional centrifugal pump or positive displacement pumps such as diaphragm pump or screw pump. If pump dissipation results in the temperature of the aqueous stream increasing to a temperature that is outside the range of operational temperatures, then pumping should be done prior to cooling the aqueous stream.

[0095] In an embodiment, the hydrate-forming stream is a single-component gas, CO.sub.2, or a mixture of gases, such as CO.sub.2 and N.sub.2. Other suitable hydrate forming compounds or mixtures thereof include compounds with molecular radii less than 9 Å comprising C1-C5 hydrocarbons, carbon monoxide, hydrogen, hydrogen sulphide, amongst others.

[0096] In an embodiment, the hydrate-forming stream is introduced into the hydrator at a rate that allows supersaturation. Typically, for highly soluble gases such as carbon dioxide, the gas:water mass flow rate ratio is at least 5%. The maximum bound is limited by the desired amount of hydrates dispersed in the slurry.

[0097] In an embodiment, the volume concentration of the hydrates in the slurry is not more than 40%. The gaseous stream may or may not be cooled down to the hydrator's operational temperature. If the hydrate-forming stream is at room temperature, the lower thermal inertia from the hydrate-forming stream enables the exchange of sensible heat with the aqueous stream without decreasing the temperature inside the hydrator at values that enable ice formation. The hydrate-forming stream must be introduced into the hydrator at a similar pressure than the aqueous stream. If the hydrate-forming stream is at a lower pressure, compression in a single stage or in multiple stages is required. If compression is required, the hydrate-forming stream is also cooled. If the hydrate-forming stream is available at temperatures larger than 25° C., the hydrate-forming stream is also cooled. Conventional heat exchangers, operating in co-current or counter-current, can be used for cooling the hydrate-forming stream. The same conditions for the refrigerant for cooling the aqueous stream is applied for the cooling of the gaseous stream.

[0098] In an embodiment, the hydrates are formed using the stacked micro/meso structured plates and heat exchange plates, and a closing mechanism to contain pressure and eliminate the possibility of inter-plate leakages. The hydrator may or may not comprise additional plates for fluids distribution in the networks structured plates and heat exchange plates, or for thermal insulation.

[0099] In an embodiment, as FIG. 2 illustrates, the hydrator comprises stacking of plates. The embodiment from FIG. 2 comprises a stacking of plates (11), delimited by closing plates (12) for compression, and two heat exchange plates (13) intercalated by one network plate (14). The closing plates are required to withstand the mechanical stresses due to the high pressure in the structured network plates. The top closing plate comprises inlets for the hydrate-forming and aqueous liquid streams (15), and one or more inlets for the refrigerant (16). The bottom closing plate comprises outlet for the flow of aqueous slurry (17) of hydrates, and one or more outlets for the flow of refrigerant (18). In FIG. 2, the refrigerant is flowing in co-current with the gaseous and aqueous streams. The structured network plates contain static mixing elements to promote mixing, to enhance heat and mass transfer rates, and to promote and accelerate hydrates crystallization.

[0100] In an embodiment, as illustrated in FIG. 3, the static mixing elements can be a network of cylindrical channels and prismatic chambers connected to each other in a bidimensional network, carved into the plates. The plates are made from a mechanically resistant material such as stainless steel or other metallic alloys. The network of chambers and channels are characterized by the number of rows n.sub.x in the direction of the flow, and number of columns n.sub.y in the direction normal to the flow. In the embodiment illustrated in FIG. 3, the number of rows n.sub.x is 25 and the number of columns n.sub.y is 8. The size of the network in both directions is easily adaptable by increasing or decreasing the number of chambers and channels, i.e., increasing or decreasing the number of rows and columns. This versatility allows a precise control of the contact time between guest hydrate-forming and liquid, as well as in terms of productivity, without affecting the mixing properties of the network. The chambers are characterized by a characteristic diameter Dc, and the channels are characterized by a width d, a length I and a depth ω. The depth of the channels and chambers is the same and is equal to ω.

[0101] In an embodiment, to maximize the mixing intensity in the chambers of the structured network, the ratio of the chamber diameter to the channel width is preferably 6 to 7. The ratio of the channel width to the channel depth should preferably be as high as possible, preferably 2 to 5. The mixing properties inside the chambers of the meso/micro structured network is due to the high energy jets from the two inlet channels of each chamber. The high energy jets impinge upon the chambers, promoting flow instability with periodic and chaotic characteristics that results in an intense mixing. However, there is a minimum amount of energy that the jets preferably possess in order to promote mixing. This energy is characterized by the channel's Reynolds number, defined as Re=pvd/μ, where p and μ are the fluids density and viscosity, respectively, ν is the velocity in the channel, and d is the channel's hydraulic diameter. For better results, the Reynolds number Re is larger than 100, preferably larger than 150. In addition to intense mixing, operating at these conditions of Reynolds number also have a positive impact on the increase of heat and interfacial mass transfer rates. In particular, instabilities caused by multiphase flow in such network results in interfacial shear stresses that lead to a significant increase of interfacial surface area. The network from FIG. 3 is built from repetition of cylindrical chambers (21), and prismatic channels (22). The aqueous stream and gaseous stream enter the network static mixer plates through inlet injection channels (23) that are connected to the mixing chambers of the first row of the network of said chambers and channels. The streams enter said network in different injection schemes, intercalated or not.

[0102] In an embodiment, as illustrated in FIG. 3, the network size is n.sub.x=25 and n.sub.y=8. The number of inlet channels is the same as the number of network columns n.sub.y. Said inlet channels may be connected in the same direction or perpendicular to the direction of the flow. In FIG. 3, said inlet channels are oriented in the same direction as the flow. Moreover, aqueous and hydrate-forming streams can be premixed prior to entering the network static mixer plates, being its flow equally or not divided through the number of rows of the network of mixing chambers and channels. The aqueous slurry stream exits the network plates through similar injection channels (24) through n.sub.y channels.

[0103] In an embodiment, the heat exchange plates are stacked adjacent to the structured network plates. Several combinations of structured network plates and heat exchange plates can be used. For better results, one structured network plate is surrounded by one heat exchange plate on each side to maximize heat transfer rates. Heat exchange plates contain static elements to improve fluid flow distribution and to maximize heat transfer capacity.

[0104] In an embodiment, heat exchange plates comprise channels and/or vertically mounted rods to promote flow distribution and to distribute compressive stresses, caused by high pressure conditions in the structured network plates, along the whole stacking of the hydrator. A cooling fluid flows inside the heat exchange plates to absorb the heat released by the adjacent structured network plates during the crystallization of the hydrates. To maximize the heat transfer performance, temperature difference between the network plates is as large as possible. For better results, and to avoid freezing of the fluid inside the network plates, a temperature difference ranging from 2° C. to 5° C. is preferable. The cooling fluid enters the heat exchange plates at temperatures ranging from −5° C. to 0° C. The pressure of the cooling fluid is atmospheric plus the energy required for the cooling fluid to flow inside, i.e., pressure to overcome the pressure drop inside the heat exchange plates. The cooling fluid can be any refrigerant capable of flowing at temperatures ranging from −5° C. to 0° C., to operate with the temperature difference mentioned above. In particular, ethanol or glycol aqueous solutions, or a combination thereof, can be used. The refrigerant flow rate is set so that the temperature difference between the bulk of the heat exchange plates and the bulk of the structured network plates is constant. For better results, the flow rate is 5 to 20 times larger than the water flow rate in the structured network plates. Refrigerant and heat exchange geometry are chosen to obtain heat transfer coefficients ranging from 1000 to 8000 W.Math.m.sup.−2 k.sup.−1, preferably in the range of 4000 to 8000 W.Math.m.sup.−2 k.sup.−1.

[0105] In an embodiment, distribution or collecting plates may or may not be installed in the hydrator to distribute both aqueous and gaseous stream into multiple inlets to feed the inlet channels and the network, or to collect the produced slurry of hydrates into a single connection. These are installed at the top and beginning of the stacking.

[0106] In an embodiment, the hydrator also comprises a closing system to contain the pressure inside the structured network plates. The advantage of using a stacking of plates is that the inner pressures are distributed along the stack, hence pressure containment is only required at the top and bottom of the stacking.

[0107] The use of a stacking of meso/micro structured network plates and heat exchange plates makes it easier to scale up the process. The coupling of plates can be in parallel or series. The number of plates in the stack can be increased or decreased.

[0108] In an embodiment, a stream of aqueous slurry of polymorphic and nanocrystalline hydrates is collected from the outlet of the hydrator. The temperatures and pressure conditions of this stream ranges from 0° C. to 5° C. and 10 to 30 bar respectively depending on the guest hydrate-forming and aqueous stream composition introduced at the inlet of the hydrator. Depending on the final application, the stream of aqueous slurry of hydrates obtained can be further processed or purified. Further processing of the aqueous slurry includes multi-step crystallization of hydrates, gas-liquid-solid separation, solid concentration, stream recycling or dissociation of the hydrates.

[0109] The disclosed method is used in several applications. The following examples are offered by way of illustration purposes only and not as a limitation.

EXAMPLE 1

Continuous CO.SUB.2 .Hydrates Production for Transportation

[0110] In an embodiment, after the hydrates are collected at the outlet of the hydrator, they can be transported into a storage site and deposited far from the atmosphere, in high pressure and low temperature environments such as the ocean seabed. FIG. 4 shows schematically the method of the present disclosed method for the continuous crystallization of an aqueous slurry of CO.sub.2 hydrates, including the transport of the CO2 hydrates through a pipeline to a storage site. In this process, the pure CO.sub.2 stream (31) is pressurized in a multi-stage compressor (32) and then cooled in a shell-and-tube heat exchanger (33). The water stream (34) is pressurized using a centrifugal pump (35) and cooled using a second shell-and-tube heat exchanger (36). Both gaseous and aqueous streams are pressurized to a pressure ranging from 20 to 40 bar and cooled to a temperate ranging from 2° C. to 5° C. The gas-liquid mass flow rate ratio ranges from 10% to 15%. In this example, an aqueous solution of 30% in volume of ethylene glycol is used as refrigerant, entering the heat exchangers at a temperature ranging from −10° C. to −5° C. The streams are introduced into the hydrator (11) disclosed above. An aqueous slurry of CO.sub.2 hydrates at a mass concentration ranging from 20% to −40% is obtained at the outlet of the hydrator. The heat released during the crystallization of hydrates and the heat from CO.sub.2 dissolution in water are absorbed by the refrigerant flowing in the heat exchange plates of the hydrator. The refrigerant is an aqueous solution of 30% in volume of ethylene glycol, entering the hydrator at a temperature ranging from −5° C. to 0° C. The hydrates slurry is then pressurized using a pump (37) and hydraulically transported through a pipeline (38) to a storage site where it is deposited (39). The pressure at which the hydrates are transported depends on the distance to the storage location. The pressure typically ranges from 60 to 70 bar for a storage site 100 kilometers from the production site. Alternatively, the use of pumps in several booster stations for long-distance transportation may be required.

EXAMPLE 2

Continuous CO.SUB.2 .Capture and Separation From Flue Gas

[0111] In an embodiment of the disclosed method of producing clathrate hydrates is for the capture of CO.sub.2. CO.sub.2 capture can be used for power plants or any other industrial processes where large amounts of CO.sub.2 are produced. The clathrate hydrates can be used to separate CO.sub.2 from the other gases.

[0112] Example 2 focuses on the process of capturing CO.sub.2 from flue gas (a mixture of CO.sub.2 and N.sub.2). Both N.sub.2 and CO.sub.2 can produce hydrates, but they require different conditions. CO.sub.2 forms hydrates at lower pressures than N.sub.2, as such the clathrate hydrates produced by the disclosed method are CO.sub.2-rich, resulting in a N.sub.2-rich and cleaner gaseous stream output. The method is similar to that shown above, consisting of allowing the CO.sub.2/N.sub.2 mixture to come into contact with water under pressure and temperature conditions that lead to the formation of hydrates. The method further comprises separation steps after the clathrate hydrates production process.

[0113] In an embodiment, FIG. 5 provides a schematic diagram of the hydrate-based CO.sub.2 capture process for separating CO.sub.2 from N.sub.2. In the disclosed method, a stream composed of a mixture of CO.sub.2 and N.sub.2 (41), is pressurized to pressures between 5 and 40 bar in a multi-stage compressor (42), and is cooled in a shell-and-tube heat exchanger (43) down to temperatures in the range of 2° C. to 5° C. The concentration of CO.sub.2 at the gaseous stream may be from 5% to 90% vol. The feed aqueous stream (44) is pressurized using a centrifugal pump (45), after it is mixed with an aqueous stream from two recycled streams. Tetrahydrofuran at concentration of about 1% mol is present in the feed aqueous stream to lower the equilibrium pressure for the gaseous mixture. The said stream is cooled using a shell-and-tube heat exchanger (46) to the same temperature and pressure as the stream (41). The gas-liquid mass flow rate ratio ranges from 5% to 20%. In this example, an aqueous solution of 30% in volume of ethylene glycol is used as refrigerant, entering the heat exchangers at a temperature ranging from −10° C. to −5° C. The streams are put in contact with each other inside the hydrator (11) described. The heat released during the crystallization of hydrates and during gas dissolution (mainly CO.sub.2 gas dissolution) in water, is absorbed by the refrigerant flowing in the heat exchange plates of the hydrator. The refrigerant is also an aqueous solution of 30% in volume of ethylene glycol, entering the hydrator at a temperature ranging from −5° C. to 0° C. The N.sub.2-rich vapour obtained at the outlet of the hydrator is further separated from the aqueous slurry in a flash drum separation unit (47). The gaseous phase exits from the top of the flash unit and passes through a turbine (48) which reduces the stream (49) pressure to atmospheric pressure thus enabling energy recovery. The gaseous phase is essentially composed of N2. The output stream (49) is cleaner, containing lower CO.sub.2 concentration than the feed stream (41). CO.sub.2 concentration in the output stream (49) ranges from 0.1% to 5% vol. The aqueous slurry exits at the bottom of the flash unit (47) and undergoes a solid-liquid separation in a hydrocyclone (51) which concentrates the slurry to a concentration ranging from 50% to 80% in volume. The concentrated solids are then sent to a substitution reactor (52). The liquid stream that exits the hydrocyclone is pressurized using a centrifugal pump (50) to counterbalance the pressure losses during the process and bring the pressure to the same level as the pressure of the water feed stream (44), thus enabling the recycling of both water and the promoter tetrahydrofuran. A stream of pure CO.sub.2 is fed to the substitution reactor (52) to promote the substitution of the existing N.sub.2 in hydrates formed by CO.sub.2. This CO.sub.2 stream comes from a recycle from a later stage of the process. The reactor (52) operates at temperature and pressure conditions similar to the hydrator (11), and can be a stirred tank or a hydrator built from the stacking of structure network plates and heat exchanger plates. N.sub.2 exits from the top of the substitution reactor and is mixed with the N.sub.2 stream from the flash unit (47) and goes into the turbine (48). The hydrate stream that exits the reactor (52) is heated in a shell-and-tube heat exchanger (54), for temperatures larger than the equilibrium temperature of the formation of CO.sub.2 hydrates at the operating pressure (FIG. 1—curve 2), ranging from 10° C. to 15° C. In an alternative embodiment, the heating is done in the dissociation reactor 55. In a further alternative embodiment, the dissociation may be accomplished by pressure reduction. The hydrate dissociation occurs in the unit (55), which can be a stirred tank or a hydrator built from the stacking of structure network plates and heat exchanger plates. An aqueous stream composed essentially of water and tetrahydrofuran exits from the bottom of unit (55). This aqueous stream is recycled, passing through a pump to offset the pressure losses and then mixed with the aqueous stream (44) to be re-introduced into the hydrator (11). This dissociation step is only required for recycling tetrahydrofuran that was not consumed during the process. From the top of unit (55) exits a pure stream of CO.sub.2 which is split into two streams. One of these streams is pressurized and then fed to the substitution reactor (52) as stated above. The remaining goes into a compressor and is allowed to come into contact with a cooled and pressurized aqueous stream. The hydrate formation process, now without N.sub.2, is repeated according to the method disclosed, and under the same conditions as in Example 1.

EXAMPLE 3

Experimental Continuous CO.SUB.2 .Hydrates Production

[0114] In an embodiment, we show the continuous CO.sub.2 hydrates production. Proof of concept of the continuous production of pure CO.sub.2 hydrate crystals was made using the method disclosed in the present disclosure. Gaseous pure CO.sub.2 from pressurized cylinder and tap water are fed to a hydrator comprising three structured network plates and 4 heat exchange plates. Two closing plates and two additional distribution plates are also included in the hydrator. Prior to their entry in the hydrator, aqueous stream is pressurized using a pump and cooled to about 3-4° C. using a heat exchanger. Inlet flow rates were set to 1 kg.Math.h.sup.−1 and 10 kg.Math.h.sup.−1, for the gaseous and aqueous stream respectively, resulting in a gas-liquid mass flow rate ratio of 10%, and a Reynolds number of approximately 200. An aqueous ethanol mixture is flowing in counter-current in the heat exchange plates, entering in the hydrator at a temperature of about 0° C. The temperature at the outlet of the structured network plates stabilized at 5° C. and the pressure is set to 30 bar using a back-pressure valve downstream the hydrator, well within the region of stability of CO.sub.2 hydrates (FIG. 1). The contact time in the hydrator of the present example is 1-3 seconds. Immediately after contacting gaseous and aqueous stream in the hydrator, and aqueous slurry of hydrates exit said hydrator.

[0115] It is evident from the above description and exemplification that the method of the present invention substantially reduces process times when compared to state-of-the art methods, using smaller hydrator. The present invention provides greater process control of gaseous and aqueous temperature, pressure and heat transfer rates. Greater control results in larger productivities and greater gas conversion. The resultant hydrates have no ice in its structure, maximizing the amount of gas enclathred in the lattice cages.

[0116] The CO.sub.2 hydrates resultant from the application of the present invention were isolated using a filtration device, washed with pure CO.sub.2 and characterized through X-Ray diffraction technique, using a Rigaku Smartlab. The diffractograms (FIG. 6) were obtained with Cu-Ka source (wavelength of 1.5406 Å), at a temperature of 263 K, and diffraction angles 15°≤2θ≤40° . The pattern from the real sample (61) can be compared with the simulated pattern (63). The sample contains only cubic structure I material in their structure, and no presence of ice is detected. The pattern (62) represents the difference between the experimental and the simulated data.

[0117] In an embodiment, experimental pressure and temperature data are presented in FIG. 7 to demonstrate the process variables stability using the method of the present invention. The pressure at the inlet of the hydrator of the aqueous (71) and gaseous (72) streams have similar behaviour, which is a characteristic of the use of network plates. The pressure at the outlet of the hydrator (73) is lower and is set through the back-pressure regulator to about 30 bar. Temperature at the inlet of the hydrator of the aqueous (74) and gaseous (75) streams are also shown. Temperature of the gas (75) is greater than temperature of water (74). Gaseous stream may or may not be cooled prior to enter the hydrator. In this specific example, gaseous stream was not cooled. The aqueous stream needs to be cooled to temperatures ranging from 2° C. to 5 ° C. The outlet stream temperature (76) is slightly larger, but not large enough to inhibit the production of hydrates. Furthermore, the temperature increase at the outlet is a prove that hydrates are being formed in a continuous way.

[0118] In an embodiment, FIG. 6 shows that crystalline CO.sub.2 hydrate form has any one or more of the following: an XRD pattern comprising peaks at diffraction angles (2θ) of 18.2°±0.5%, 29.9°±0.5%, 30.8°±0.5% and 31.7°±0.5%. Additionally, the absence of these ice peaks in the XRD pattern of FIG. 6 clearly demonstrates that the hydrates obtained by the method of the present disclosure are absence of hexagonal ice.

[0119] The above described embodiments are combinable.

[0120] The term “comprising” whenever used in this document is intended to indicate the presence of stated features, integers, steps, components, but not to preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.