WO2019162250A1

WO2019162250A1 - Method and system for processing a gas-hydrate containing slurry

Info

Publication number: WO2019162250A1
Application number: PCT/EP2019/054042
Authority: WO
Inventors: Raghuraman PITCHUMANI; Pieter Hugo VAN RENSBERGEN
Original assignee: Shell Internationale Research Maatschappij B.V.; Shell Oil Company
Priority date: 2018-02-23
Filing date: 2019-02-19
Publication date: 2019-08-29
Also published as: JP2021514440A; JP2023182634A

Abstract

There is provided a method and system (42) for processing a slurry (12) which contains gas hydrate, the system comprising: a separator (422) configured to separate the slurry into a first stream (121) containing a higher level of the gas hydrate comparing to the slurry and a second stream (123) containing a lower level of the gas hydrate comparing to the slurry; a processing assembly (424) configured to process the first stream and the second stream separately, wherein the first stream is processed to recover gas hydrate from the first stream, or to recover gas from gas hydrate in the first stream, and wherein the second stream is processed to recover gas from gas hydrate in the second stream.

Description

METHOD AND SYSTEM FOR PROCESSING A GAS-HYDRATE CONTAINING

SLURRY

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a method and system for processing a gas-hydrate containing slurry, which may be obtained from water bottom.

BACKGROUND OF THE INVENTION

Methane (CH₄₎ hydrate sometimes forms from release of methane gas along oceanic geological faults. In cold climates and especially at deep sea levels or in deep lakes, at least a portion of the methane gas forms hydrate at or close to the seafloor on contact with cold water. Methane hydrate is considered as a promising alternative energy source. One liter of methane hydrate solid would contain approximately 168 liters of methane gas at Standard

Temperature and Pressure (STP) .

There have been attempts to develop systems and methods to harvest methane from these hydrate deposits. Underwater hydrate mining methods are known from US patent 6,209,965, US patent application US2003/0136585, international patent application WO98/44078, international patent application W02010/092145 Al, and Chinese patent application CN101182771. W02010/092145 Al further describes a method for processing hydrate-containing slurry obtained from water bottom, wherein the slurry is discharged into a slurry separation assembly and separated into a methane gas stream and a tailings stream. The tailings stream is pumped into a tailing returning conduit and returned to the water bottom for disposal .

Despite this and other attempts, it is still needed to develop an improved system and method to process gas-hydrate containing slurry obtained from water bottom. It is

preferred that the processing is done in a more energy efficient and/or economically viable manner.

SUMMARY OF THE INVENTION

To that end, in an aspect of the invention, there is provided a method for processing a slurry which contains gas hydrate, the method comprising the following steps:

- separating the slurry into a first stream containing a higher level of the gas hydrate comparing to the slurry and a second stream containing a lower level of the gas hydrate comparing to the slurry;

- processing the first stream and the second stream

separately, wherein the first stream is processed to recover gas hydrate from the first stream, or to recover gas from gas hydrate in the first stream, and wherein the second stream is processed to recover gas from gas hydrate in the second stream .

In another aspect of the invention, there is provided a system for processing a slurry which contains gas hydrate, the system comprising:

- a separator configured to separate the slurry into a first stream containing a higher level of the gas hydrate comparing to the slurry and a second stream containing a lower level of the gas hydrate comparing to the slurry;

- a processing assembly configured to process the first stream and the second stream separately, wherein the first stream is processed to recover gas hydrate from the first stream, or to recover gas from gas hydrate in the first stream, and wherein the second stream is processed to recover gas from gas hydrate in the second stream. These and other features, embodiments and advantages of the system and method according to the invention are

described in the accompanying claims, abstract and the following detailed description of non-limiting embodiments depicted in the accompanying drawings, in which description reference numerals are used which refer to corresponding reference numerals that are depicted in the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described herein below in more detail, and by way of example, with reference to the

accompanying drawings in which:

Figure 1 is a schematic view of an exemplary system for underwater mining, in which the system and method according to the present invention are applied.

Figures 2-5 are schematic block diagrams of a system for processing a gas-hydrate containing slurry obtained from water bottom.

Figure 6 is a schematic block diagram of a heating module applied in the system for processing a gas-hydrate containing slurry obtained from water bottom.

Figure 7 is a flow diagram of a method for processing a gas-hydrate containing slurry obtained from a water bottom deposit .

In the drawings, similar reference numerals refer to similar/equivalent components, steps or features.

DETAILED DESCRIPTION OF THE DEPICTED EMBODIMENTS

The invention may be embodied or carried out in a system for processing a slurry which contains gas hydrate. The slurry may be transported under a first pressure to the system, while at least a part of the system is operated under a second pressure being lower than the first pressure and higher than atmospheric pressure. The first pressure may be selected to inhibit dissociation of the gas hydrate, and the second pressure may be selected to allow dissociation of the gas hydrate.

The system may comprise a separator to separate the slurry into a first stream containing a higher level of the gas hydrate compared to the slurry and a second stream containing a lower level of the gas hydrate compared to the slurry. The separator may be configured to separate the slurry into the first stream and the second stream based on at least one of the following properties of solid matters in the slurry: mass, size, density, wettability or magnetic susceptibility .

The system may comprise a processing assembly to process the first stream and the second stream separately. The first stream is processed to recover gas hydrate from the first stream, or to recover gas from gas hydrate in the first stream. The second stream is processed to recover gas from gas hydrate in the second stream. The gas recovered from gas hydrate in the second stream may be used to power the system.

The processing assembly may comprise at least one heating module configured to provide heat to at least one of the first stream and the second stream. The heating module may comprise:

a first unit configured to subject an inflow to a first heating medium;

a second unit configured to separate an outflow of the first unit into a first flow and a second flow;

a third unit in connection with the second unit and is configured to subject only the first flow to a second heating medium; a fourth unit configured to join the heated first flow together with the second flow to obtain an outflow of the heating module.

The processing assembly may further comprise at least one dewatering unit configured to dewater the first stream and/or the second stream. The dewatering unit is preferably located before the heating module of the respective first and or second stream. Dewatering of the first stream and/or the second stream preferably is done before feeding the

respective stream to the respective heating.

Processing the first stream may comprise grinding the first stream to reduce size of solid matters therein prior to feeding the first stream into the dewatering unit or the heating module. The system may thus further comprise a grinder configured to reduce size of solid matters in the first stream.

A pre-processing assembly may be provided upstream of the separator. The pre-processing assembly may be configured to subject the slurry to pre-processing before the slurry is sent to the separator. Pre-processing may include at least one of the following: grinding; pre-heating; and/or

dewatering .

Figure 1 illustrates an exemplary system for underwater mining. In specific, the system 1 is designed to excavate, lift and process gas hydrate from a water bottom deposit 11. The system and method for processing gas-hydrate containing slurry obtained from water bottom according to the present invention can be used in this system to recover valuable gas such as methane, the system and method are equally applicable to hydrates of other gases.

As shown in Figure 1, a seabed excavator 10 excavates hydrate from hydrate deposit 11 buried at water bottom 19 and passes a slurry 12 which contains excavated hydrate in solid state, particulate sediment and seawater through a flexible hose 13 to a pumping station 14 set at a certain depth above the water bottom 19. The pumping station 14 raises the pressure of the slurry 12 and causes it to move upwards in a substantially turbulent flow regime through a slurry riser conduit 15. In practice, a plurality of pumps can be distributed along the conduit 15 to maintain the pressure in the entire conduit, if needed.

The slurry riser conduit 15 terminates at a platform 2 floating at the water surface 21, where the slurry 12 enters a system 22 and gets processed. In specific, in system 22, methane hydrate in slurry 12 may dissociate into water and methane gas. The methane gas may be collected from the top of the system 22. The collected methane gas may be further dried and pressurized or otherwise processed as required by a downstream system such as a system for compressed natural gas (CNG) , a system for liquefied natural gas (LNG) , or a pipeline export system. A tailings stream 23 comprising residual water and sediment is drawn from for instance a bottom of the system 22 and enters a tailings return conduit 16 extending from the platform 2 back down to an area 18 of the water bottom 19 suitable for tailings disposal.

The applicant found that it's important to keep the gas hydrate in its (hydrate) stability zone (field) in conduit 15. Preferably, slurry 12 is transported under a first pressure (Pi) to the system 22, Pi being selected to inhibit dissociation of the hydrate at the temperature in the conduit 15, so that hydrate in slurry 12 remains within its stability zone as long as the slurry 12 is still in the conduit 15.

Preferably, at least a part of the system 22 is operated under a second pressure (P₂₎ which is selected to allow dissociation of the gas hydrate. For instance, the slurry 12 may experience a pressure drop from Pi to P₂ when it leaves conduit 15 and enters the system 22. Gas hydrate in the slurry then gets out of its stability zone and starts to dissociate. The second pressure P₂ is preferably selected to be higher than an atmospheric pressure. In an example, P₂ is about 10 bar. By setting P₂ higher than the atmospheric pressure, energy required to re-pressurize the gas produced from the system 22 is reduced. This is particularly useful if the recovered gas needs to be pressurized to e.g., form CNG or LNG for storage and transportation.

The system for processing a slurry which contains gas hydrate according to the invention will be further elaborated with reference to Figures 2-5.

Figure 2 is schematic block diagram of a system 22 for processing a gas-hydrate containing slurry obtained from water bottom. This system 22 may be applied in the system 1 illustrated in Figure 1 for processing slurry 12. This system 22 mainly comprises a separator 222 and a processing assembly 224. In an example, slurry 12 arriving at the system 12 may contain the following: seawater, hydrate in solid state, sediment in solid state. As will be introduced later below, the system 22 may include a pre-processing assembly (not shown) set upstream of the separator, so that the slurry 12 may be pre-processed before being sent to the separator .

In the example shown in Figure 2, the separator 222 receives and separates the slurry 12 into two streams: a first stream 121 containing a higher level of the gas hydrate comparing to the slurry 12, and a second stream 123

containing a lower level of the gas hydrate comparing to the slurry 12.

In an example, this is embodied by that a weight ratio of the gas hydrate in the first stream 121 is higher than that in slurry 12, and a weight ratio of the gas hydrate in the second stream 123 is lower than that in slurry 12.

The system 22 may benefit from the separation for at least the following reasons:

1) Applicant found that the size of hydrate shall be as small as possible to allow fast dissociation, especially when the goal is to recover gas instead of solid gas hydrate. To that end, it is preferred (but optional) to grind the gas hydrate to reduce the size of hydrate. However, the slurry also contains rocks and sediments lumps, which the operators do not want to spend energy to grind. Separating the slurry into the two streams may avoid the need to grind rocks or sediment for energy saving purpose .

2) Applicant also found that heating (and even

repetitive heating) may be needed to fully dissociate hydrate contained in the slurry. Separating the slurry into a first stream rich in hydrate but poor in sediment and a second stream poor in hydrate but rich in sediment would allow the operator to only apply additional heating to the first stream containing more hydrate. This is more energy efficient because otherwise, in absence of the separation, the system may have to heat a considerable amount of sediment and rocks because the hydrate is mixed therein.

Applicant found that solid state pure hydrate, solid state pure sediment and a mixture of the two are usually different from each other in terms of at least one of the following properties: mass, size, density, wettability and magnetic susceptibility. Therefore, the separator 222 may separate the slurry 12 into the first stream 121 and the second stream 123 according to at least one of these

properties. For instance, the separator 222 may perform centrifugal separation which makes use of the differentiating densities of e.g., sediment and gas hydrate. The first stream 121 and the second stream 123 are passed to the processing assembly 224. The processing assembly 224 processes the first stream 121 and the second stream 123 separately. In specific, the first stream 121 is processed to recover gas from gas hydrate in the first stream 121 or to recover gas hydrate from the first stream. The second stream 123 is processed to recover gas from gas hydrate in the second stream 123. In various examples, the processing assembly 224 may process the two streams

differently, as described below.

In an example, the separator 222 and the processing assembly are operated under a second pressure P₂ suitable for the dissociation of the gas hydrate. Part of the gas hydrate dissociates in the separator 222, and the rest dissociates in the processing assembly 224. To enhance the recovery rate, a residual flow obtained by processing the first stream 121 may not directly form a part of tailings flow 125, instead, the residual flow may be sent back in the processing assembly 224 for another round of treatment, to ensure as much gas hydrate as possible dissociates.

Gas released from the dissociation is collected for further processing such as drying and re-pressurization.

Without loss of generality, gas recovered from processing the second stream 123 may be used to supply power to the system 22, or even the entire platform 1 shown in Figure 1. The pressure under which elements of the system 22 is operated may be selected by taking into account the following: i) to maximize the driving force to enhance hydrate dissociation kinetics (relative to the hydrate stability curve); ii) to satisfy the highest possible gas export pressure

requirements .

In another example, the separator 222 may not need to operate under the second pressure P₂, instead, the pressure in the separator 222 could be set high enough so that gas hydrate would not dissociate in the separator 222 and no gas is released. The gas hydrate leaving the separator 222 may be subject to a pressure drop (for instance down to P₂₎ to stimulate the dissociation of hydrate in the first stream and/or the second stream in a downstream unit/module, for instance a grinder as described later below or the processing assembly .

In another example, the processing assembly 224 is not configured to recover gas from gas hydrate in the first stream 121 by heating or other means. Instead, the

processing assembly 224 is configured to process the first stream 121 to obtain relatively pure gas hydrate, which is preferably dry. To that end, a dewatering module (as described later below) can be included in the processing assembly 224 to treat the first stream 121 to remove seawater therefrom. The first stream 121 may be sent back to the separator 222 before or after processing in the processing assembly in order to take out more sediment from the first stream 121. An outflow 127 obtained by processing the first stream 121 may therefore contain dry gas hydrate suitable for storage and/or transportation. Pressure and temperature in the separator 222 and elements/modules in the processing assembly 224 assigned to process the first stream 121 shall be carefully controlled to keep gas hydrate in the first stream in its stability zone.

The processing assembly 224 produces a tailings stream 125 as a result of processing the first stream 121 and the second stream 123. The tailings stream 125 mainly contains water and sediment and is returned to a suitable place at the water bottom 19.

As briefly mentioned, there are mainly two different ways to process the first stream, depending on the product that is expected from the processing of the first stream 121. In a first example, the first stream 121 is not subject to heating or a significant pressure drop which both promote dissociation of gas hydrate. Instead, in the processing assembly 224, the first stream is dewatered (or dried) to take out liquid water, and may be repeatedly fed to the separator 222 to take out sediment therefrom, in order to obtain relatively pure dry hydrate which is then exported as a product of the system 22. To that end, it may be important to ensure the separator and other elements such as the dewatering module assigned for the first stream 121 is operated under a pressure which is suitable to keep the gas hydrate in the first stream from dissociation. The

temperature in the separator and the dewatering module assigned to the first stream 121 shall also be kept low enough to not promote dissociation. To produce gas hydrate instead of gas from the first stream, a grinding process as mentioned with reference to Figure 3 later can be used, although it is optional.

Gas recovered from processing the second stream 123 may be used to power the system 22 or even the entire platform 2.

Figure 3 is a schematic block diagram of a system 32 for processing a slurry which contains gas hydrate. Comparing system 32 to system 22, the main difference is that system 32 further comprises a grinder 326 arranged downstream the separator 322 and upstream the processing assembly 324. The grinder 326 is configured to receive and grind the first stream 121, so as to reduce size of solid matters especially hydrate cuttings in the first stream 121. By reducing size of solid matters in the first stream 121 especially hydrate therein, hydrate in the first stream is provided with a greater superficial area which may facilitate its

dissociation. Other suitable means instead of grinder can also be used to reduce size of hydrate cuttings in the first stream 121.

The grinder 326 may be operated under a pressure which promotes dissociation of the gas hydrate, gas may be

partially recovered from the first stream 121 in the grinder 326 and collected.

An outflow 327 from the grinder 326, which is formed by reducing size of solid matters in the first stream 121, is provided to the processing assembly 324 where the outflow 327 and the second stream 123 are processed separately for gas recovery .

It should be noted that the grinder 326 is optional.

The example in Figure 3 has a variation in which the treatment of the first stream 121 is performed to recover gas hydrate instead of gas. A similar example has been

elaborated with reference to Figure 2.

Figure 4 is a schematic block diagram of a system 42 for processing a slurry 12 which contains gas hydrate. The system 42 is a further development based on system 32 shown in Figure 3, and gives more specifics about the processing assembly .

As similarly described with reference to Figures 2-3, in system 42, a separator 422 is set to separate the intake slurry 12 into a first stream 121 having a higher hydrate level comparing to slurry 12 and a second stream having a lower hydrate level comparing to slurry 12. The first stream 121 is then passed through a grinder 426 where size of solid matters in the first stream 121 gets reduced.

The processing assembly 424 comprises a heating module 4242a and a reactor vessel 4244a (or a reactor pipe) arranged in serial order to process the outflow 427 of the grinder 426. The processing assembly 424 further comprises a heating module 4242b and a reactor vessel 4244b (or reactor pipe) arranged to receive and process the second stream 123. In this way, the first stream 427 after grinding and the second stream 123 are processed separately, and differently if needed .

Heating module 4242a is configured in such a way that it provides heat to stream 427 and hence promotes dissociation of gas hydrate therein. Gas released from the dissociation is collected from a gas outlet (not illustrated) of that heating module 4242a, and an outflow 429a from the heating module 4242a is provided to the reactor vessel 4244a set downstream the heating module 4242a. The outflow 429a typically contains residual water generated from dissociation happened in the heating module 4242a, seawater and sediment originally carried in the first stream 427 after grinding. Those skilled in the art would appreciate that the heating module 4242a and the reactor vessel 4244a may be omitted and replaced by a dewatering module if it is expected to process the first stream to recover gas hydrate instead of gas .

Similar examples have been described with reference to

Figures 2 and 3.

Heating module 4242b is configured to provide heat to the second stream 123 to promote dissociation of gas hydrate therein. Gas released from the dissociation is collected from a gas outlet (not shown) of heating module 4242b, and an outflow 429b from the heating module 4242b is fed to a reactor vessel 4244b. The outflow 429b from the heating module 4242b typically contains residual water generated from dissociation happened in heating module 4242b, seawater and sediment originally carried in the second stream 123.

The reactor vessels 4244a and 4244b can be each formed by a large tank. At each large tank, the inflow (429a or 429b) enters from the top and leaves from the bottom. In each of reactor vessels 4244a and 4244b, gas hydrate remaining in the inflow may have sufficient time to

dissociate to achieve a high recovery rate, i.e., to get as much hydrate as possible to dissociate in the system 42.

Since the first stream is rich in gas hydrate, to ensure a high dissociation ratio, the flow 425a may be sent back to the heating module 4242a for another round of heating. In this way, additional heating is provided to the first stream which contains a higher level of gas hydrate than the incoming slurry, to enhance the gas recovery rate .

Reactor vessels 4244a and 4244b generate tailings streams 425a and 425b respectively, which can be pumped in a suitable manner back to the water bottom 19.

As mentioned, each reactor vessel may be replaced by a reactor pipe in which the inflow flows and the remaining gas hydrate in the inflow can dissociate.

The reactor vessel 4244b can be optional, in which case the outflow of the heating module 4242b may be taken as a tailings stream and returned to the water bottom 19.

Figure 5 illustrates a system 52 for processing a slurry containing gas hydrate. System 52 may be considered as a further development based on system 42 shown in Figure 4. Comparing to system 42, system 52 further comprises a dewatering function, as further described below.

Separator 522, grinder 526, heating modules 5242a and 5242b, reactor vessels 5244a and 5244b, are similar to those described with reference to Figures 2-4.

In system 52, preferably, dewatering is done before the first stream and the second stream are heated by respective heating modules. It is assumed that in this dewatering installation 50% of the water is removed from each stream, it is also assumed that no sediment is removed. If that would happen, it only would have positive effects. The flow rate of each flow may decrease because of the dewatering. The effect of adding the dewatering modules 5246a and 5246b to dewater the first stream 527 after grinding and the second stream 123 is that less water needs to be heated.

Furthermore, the flow velocities decrease, if the same equipment is used, increasing the amount of heat that is transferred with seawater. The result is that the total power that is supplied heating modules may be decreased by about 35%, according to estimation.

To ensure a high recovery ratio of gas from gas hydrate in the first stream, the outflow 525a of the reactor vessel 5244a may be sent back to the dewatering module 5246a or the heating module 5242a, to further dewater, heat residual hydrate left therein. Sending the flow 525a back to the dewatering module 5246a may be preferable because this will remove residual water from flow 525a which does not need to be heated.

According to a variation of the example shown in Figure 5, the dewatering module 5246b and/or the reactor vessel 5244b can be optional. In other words, the second stream 123 which contains a lower level of hydrate comparing to the slurry 12 may be directly sent to the heating module 5252b.

Tailings flows 525a and 525b are formed as a result of treating the first and second streams by the processing assembly 524.

The example in Figure 5 has a variation in which the heating module 5242a and the reactor vessel 5244a are omitted, so that dry gas hydrate is recovered by treating the first stream. Similar examples have been mentioned with reference to Figures 2-4.

Examples shown in Figures 2-5 can all be modified by adding an optional pre-processing assembly (not shown) upstream the separator. The pre-processing may include at least one of the following: pre-heating; dewatering;

grinding .

Figure 6 illustrates an exemplary block diagram of a heating module 6 according to an embodiment of the invention.

As illustrated, a first unit 601 receives an inflow 611 of the heating module 6, which may be the slurry 12, the first stream or the second stream as shown in any of Figures 2-5. The first unit 601 also receives a first heating medium 616 introduced to heat the inflow 611 at the first unit 601. The first heating medium 616 is for instance seawater which is warmer than the cold slurry. In an example, the first unit 601 is implemented by a shell and tube heat exchanger. Methane may be partly recovered from the heating process in the first unit 601, as indicated in Figure 6.

An outflow 612 from the first unit 601 is guided to a second unit 602, which in an example can be any means that divides the outflow 612 into two flows 613 and 615, for example a hydro-cyclone or a Y-joint. Flow 613 may be a small portion (e.g., 20%, 10% or less) of the outflow 612.

Flow 613 is fed to a third unit 603, where it's further heated by e.g., a second heating medium 617. Part of the gas in the gas-hydrate contained in flow 613 is recovered at this third unit 603. In an example, the third unit 603 is implemented by a spiral heat exchanger, and the second heating medium 617 is heating water. The heating water could be heated by waste heat and/or added heat, or by burning gas recovered previously e.g., by treating a seconds stream.

After processing the flow 613 in the third unit 603, an outflow 614 is generated and fed to a fourth unit 604 which may be another Y-joint where the outflow 614 and the main flow 615 join again. The joined flow 616 will be provided to e.g., a reactor vessel as previously described for further processing . In an alternative example, the second unit 602 may be implemented by a hydro-cyclone, where smaller hydrate particles (e.g., fine particles) are separated from larger hydrate particles (e.g., courser particles) . The flow 613 carrying the smaller hydrate particles is fed to the third unit 603 which may be implemented by a plate heat exchanger or a number of parallel plate heat exchanger, and heated by heating water from waste heat or added heat. The flow 615 carrying the larger hydrate particles is optionally subject to a further heating in a shell and tube heat exchanger by seawater. Those two flows are joined together at the fourth unit 604 after passing the heat exchangers.

In the reactor vessel, the hydrates have the time to dissociate. When all hydrates are dissociated and the gas is tapped off, water and soil is returned to the seabed.

In another alternative example, the first unit 601 may be implemented by a number of spiral heat exchangers, placed in parallel.

With reference to Figure 6, a heating module 6 which separates the inflow into two flows 613 & 615 and treat the two flows has been described. Alternatively, the heating module may be implemented by a preheater and a main heater placed in serial order. For instance, a shell and tube heat exchanger may be used as the preheater for pre-heating the inflow with seawater. The main heating is done by e.g., injecting steam. After steam injection, the outflow of the heating module is formed and will be fed to a reactor vessel where all gas hydrates dissociate.

Working conditions of these and other modules, units, components of the system can be well controlled by using a mathematical model that describes the kinetics of hydrate dissociation . Certain examples of the system according to the

invention have been described with reference to the drawings according to which the building blocks (unit, modules) are arranged in a certain order. Those skilled in the art would appreciate that the invention is not limited to these examples. For instance, at least a part of those building blocks shall be able to change order and/or be put in loops or parallel.

According to variations of the arrangement shown in the drawings, the slurry may be pre-processed before the

separation. For instance, a pre-processing assembly for pre heating the slurry may be set upstream the separator to pre heat the slurry with warm seawater. A heating module

illustrated in Figure 6 may be used for this purpose.

In another variation, a dewatering module may be set upstream the separator to dewater the slurry before the slurry is fed into the separator. This dewatering module does not have to replace the at least one dewatering module in the processing assembly, in an example, dewatering is performed before the separation and in the processing assembly .

In another variation, the grinder may be set upstream the separator. This may result in extra energy consumption by grinding non-hydrate ingredients of the slurry, at least a part of the processing assembly may still benefit from being set downstream the separator.

Figure 7 illustrates an exemplary flow chart of a method for processing gas-hydrate containing slurry obtained from water bottom according to a preferred embodiment of the invention. Various steps of this method mirrors the

functions of units, modules, assemblies in systems 22, 34, 42 and 52 described with reference to Figures 2-5. Therefore, the method can be explained already by referring to the description provided in connection with Figures 2-5.

In a basis embodiment of the method, which mirrors the system 22 shown in Figure 2, the method comprises the 5 following steps:

(i) separating the slurry into a first stream containing a higher level of the gas hydrate comparing to the slurry and a second stream containing a lower level of the gas hydrate comparing to the slurry; and

(iij.0 processing the first stream and the second stream

separately, wherein the first stream is processed to recover gas hydrate from the first stream, or to recover gas from gas hydrate in the first stream, and wherein the second stream is processed to recover gas from gas hydrate in the second 15 stream.

In the example shown in Figure 7, in step 701 (mirroring the function of a separator as described) , the intake slurry raised from water bottom is separated by a separator (322,

422, 522) into a first stream containing a higher level of 20 the gas hydrate comparing to the slurry and a second stream containing a lower level of the gas hydrate comparing to the slurry .

The slurry is transported under a first pressure to the separator, the separator may be operated under a second 25 pressure, the first pressure is selected to prevent

dissociation of the gas hydrate. At least a part of the steps of the method may be performed under a second pressure P₂ is selected to be lower than the first pressure and higher than an atmospheric pressure to allow the gas hydrate to 30 dissociate. In other words, at least a part of the system is operated under a second pressure which is selected to be lower than the first pressure and higher than atmospheric pressure to allow the gas hydrate to dissociate. Alternatively, the pressure drop may happen after the separation, as previously mentioned.

In step 702, the first stream is subject to grinding so that size of solid matters especially solid-state gas hydrate in the first stream is reduced.

In step 703, the first stream (after grinding) and the second stream are dewatered respectively.

In step 704, the dewatered streams are heated, using respective heating modules as previously described.

In step 705, the heated streams are fed to respective reactor vessels, where the remaining gas-hydrate further dissociate until gas has been sufficiently recovered from the slurry .

As previously mentioned, the second stream may or may not need to be subject to dewatering and a reactor vessel.

In other words, the second stream may be directly sent for heating for gas recovery and an outflow generated after heating can be taken as a tailings stream and sent for disposal .

An embodiment of step 704 may include the following processes, mirroring the heating module 6 described with reference to Figure 6.

subjecting an inflow of the heating module to a first heating medium to obtain a pre-heated flow;

- separating the pre-heated flow into a first flow and a second flow;

subjecting only the first flow to a second heating medium; and

joining the heated first flow together with the second flow to obtain an outflow of the heating module.

The method may further comprise a step (not shown) of subjecting the slurry to at least one of the following treatments before the slurry is sent to the separator: grinding;

pre-heating;

dewatering .

This optional step mirrors the pre-processing assembly described with reference to Figures 2-5.

In a variation of the method shown in Figure 7, the first steam is not heated nor subject to a reactor vessel. Instead, the first stream is mainly dewatered to remove water therefrom. The firs stream after dewatering may be sent back to the separator to enhance the separation and take out more sediment from the first stream. Examples of such variation have already been elaborated with reference to Figures 2-5.

While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the invention. The embodiments described herein may be embodied in a variety of other forms. Furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the scope of the invention. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope of the invention.

Claims

C L A I M S

1. A method for processing a slurry which contains gas hydrate, the method comprising:

separating the slurry into a first stream containing a higher level of the gas hydrate comparing to the slurry and a second stream containing a lower level of the gas hydrate comparing to the slurry;

processing the first stream and the second stream separately,

wherein the first stream is processed to recover gas hydrate from the first stream, or to recover gas from gas hydrate in the first stream,

and wherein the second stream is processed to recover gas from gas hydrate in the second stream.

2. The method of claim 1, wherein the slurry is transported under a first pressure to the system, the first pressure is selected to prevent dissociation of the gas hydrate, the method further comprising:

operating at least a part of the system under a second pressure which is selected to be lower than the first pressure and higher than atmospheric pressure to allow the gas hydrate to dissociate.

3. The method of any one of the preceding claims, further comprising :

heating at least one of the first stream and the second stream by at least one heating module, the heating step comprising :

subjecting an inflow of the heating module to a first heating medium to obtain a pre-heated flow; separating the pre-heated flow into a first flow and a second flow;

subjecting only the first flow to a second heating medium; and

4. The method of any one of the preceding claims, further comprising :

subjecting the slurry to at least one of the following pre-treatments before the slurry is sent to the separator: grinding;

pre-heating;

dewatering .

5. A system for processing a slurry which contains gas hydrate, the system comprising:

a separator configured to separate the slurry into a first stream containing a higher level of the gas hydrate comparing to the slurry and a second stream containing a lower level of the gas hydrate comparing to the slurry; a processing assembly configured to process the first stream and the second stream separately,