WO1998019101A1

WO1998019101A1 - Method and means for preparing, storage and regasification of a hydrocarbon product, the product prepared thereby and applications thereof

Info

Publication number: WO1998019101A1
Application number: PCT/NO1997/000284
Authority: WO
Inventors: Geir B. Lorentzen; Tore A. Torp; Otto Skovholt; Ola Ruch; Erlend Straume; Morten AARVÅG
Original assignee: Den Norske Stats Oljeselskap A/S
Priority date: 1996-10-25
Filing date: 1997-10-27
Publication date: 1998-05-07
Also published as: AU4728797A; NO964544L; NO964544D0; NO311381B1

Abstract

A method of preparing a stable hydrocarbon product in the form of a hydrate or at least one hydrate-generating hydrocarbon surrounded by or suspended in a hydrocarbon containing liquid, the invention also comprising the product itself. The process comprises the following steps: a) hydrate-generating hydrocarbons and water are contacted in a hydrate-generating zone (1) under hydrate-generating process conditions. A first intermediate product is hereby generated, having a mean temperature which is equal to or above the freezing temperature of water; b) any non-converted water present is eliminated from the first intermediate product; c) the second intermediate product is cooled in a cooling zone (480) by direct cooling by a second, hydrocarbon containing cooling medium to an end temperature equal to or below formula (I) whose parameters are further explained in the specification. The end product comprises particles of solid, hydrate containing material, surrounded by or suspended in a liquid hydrocarbon carrier medium at a vapour pressure which is below the end pressure at the end temperature.

Description

Method and means for preparing, storage and regasification of a hydrocarbon product, the product prepared thereby and applications thereof.

The present invention relates to a method for preparing a hydrocarbon product, comprising hydrates of hydrate-generating hydrocarbons surrounded by or suspended in a hydrocarbon containing liquid and which is stable at a storage pressure equal to or close to ambient atmospheric pressure; wherein a hydrocarbon material comprising hydrate-generating hydrocarbons and water are contacted in a hydrate-generating zone under hydrat-generating process conditions to generate a substantially water- and ice-free hydrate mass, which in a cooling zone is cooled to a mean end and storage temperatur, which is lower than the freezing temperature of water, thereby generating a hydrocarbon product, said hydrocarbon comprising liquid being supplied to the hydrate-generating zone as a part of the hydrocarbon material or is supplied during the preparation or cooling of the hydrate mass, a method of storage and transportation of a hydrocarbon product, containing hydrates of hydrate-generating hydrocarbons surrounded by or suspended in a hydrocarbon containing liquid, the use of this product as a medium for storage and transportation of natural gas, for storage and transportation of volatile components (VOC), which are released during loading, unloading and transportation of processed crude oil for storage and transportation of normally gaseous or volatile components occuring with or being released from crude oil during the production and processing of crude oil and natural gas, and as fuel or engine fuel for the generation of heat or energy or as a medium for storage and transportation of normally gaseous or volatile hydrocarbons intended for such purposes.

The expression hydrate-generating conditions imply that the heat of hydration must be removed during the hydrate-generation, further that the process must be controlled to obtain a substantial water- and ice-free hydrate mass and low temperatures leading to ice formation should be avoided. Water possibly eliminated occurring in the hydrate mass can be eliminated by filtration or similar.. Suspensions comprising particles of gas hydrate suspended in a hydrocarbon based on liquid is previously known, particularly as a temporary intermediate product by the treatment or transportation of gas hydrate.

It shall be referred to US patent patent number 2.363.529 in this connection which specificly discloses to a suspension used in combination with controlled fractionation of different, hydrate-generating hydrocarbons from a fluid; and to US patent number 2.356.407 which particularly discloses the use of related suspensions for the transportation of gas hydrate from one site to another, e.g. for storage purposes. US patent number 3.514.274 can at last be mentioned also disclosing the transportation of natural gas as a hydrate in a "slurry" with liquid propane. However, such a slurry will not be stable at atmospheric pressure unless the temperature is below - 42 °C. This implies that if such a composition is to be stored and transported at atmospheric pressure, this must take place at a temperature equal to or below the boiling temperature of propane (- 43°C) . On the contrary the present invention generates a hydrocarbon product which is stable at a atmospheric pressure although the temperature partly can be substantially above the boiling temperature of propane at atmospheric pressure.

Jon Steinar Gudmundson alleges in US patent number 5.536.893 that agglomerated particles of hydrocarbon hydrate are stable at atmospheric pressure or at a low gauge pressure at temperatures below 0°C, preferably at -10°C to -15°C. This condition is previously described as meta-stable, since the agglomerated hydrate particles in this case is clearly off the range where hydrate constitue the thermodynamically most stable phase and since the hydrate material still seems comparatively stable. However, the basis of the gas hydrate observations in meta-stable condition are performed on "dry" and likely compressed or agglomerated gas hydrate , i.e gas hydrate not being in contact with liquid hydrocarbons. A possible explanation of the meta-stable condition which is observed for the "dried" gas hydrate is that the particles or the lumps of more or less compressed gas hydrate during meta-stable conditions - temperatures at some few degrees below the freezing temperature of water and down to -10°C to -15°C, and a ambient pressure below than sufficiently thermodynamically stable gas hydrate, e.g. about atmospheric pressure, at a gas hydrate generated from a methane rich natural gas - are surrounded by an ice- layer enveloping the real gas hydrate material. This generation of ice-layer may possible result in to the accumulation of an internal pressure in those particles or lumps of gas hydrate, so that the gas hydrate material within this ice-shell still has at a pressure keeping the gas hydrate material within the thermodynamically stability range.

On the other hand, when the gas hydrate is in contact with liquid hydrocarbons, the metastable condtion is less pronounced. Experiments performed with gas hydrates generated from hydrocarbon gases having different compositions existing as a slurry in hydrocarbon containing liquids, indicates that the material is clearly more unstable with respect to the release of gas components of the hydrate containing material (composition of gas hydrate + hydrocarbon liquid) . In some cases it is observed that such materials can loose more than 50 % of their original content of gaseous components within a few days at storage under pressure and temperaure conditions which until recently have been regarded representing the meta-stable range.

The present invention accordingly provides a simplified and improved method of preparing a hydrocarbon product, where gas hydrate particles are surrounded by or suspended in a hydrocarbon medium, which hydrocarbon product has improved product qualities.

The gas of the gas hydrate can be used for many purposes. It can be used in preparing energy, either for power generation in power plants, for heating operations centrally or for distribution to consumers in a pipeline net. The hydrocarbon components of the product can also be used as feed stock in preparing chemical products and such products as synthesis gas, methanol, acetic acid, etc. The heavier components of the product are useful as components in fuel or as raw materials in a wide range of petrochemical processes.

In this relation referring to the prior art it is known to produce gas hydrate for the transportation and/or storage of gas at advantageous pressure and temperature conditons, see NO patent number 175.656.

Technically, however, it has been evident that large problems connected rise in the tranportation and storage of gas in the form of gas hydrate when using the prior art technique, as the gas hydrate during storage sinter to a hard and processable mass which in addition easily adhere to both walls in the storage containers and conveyor's inside..

In addition, when handling the hydrate according to prior art, it had to be stored either at high pressures or at very low temperatures. It would be an advantageous to reduce the pressure at which the hydrocarbon is stored, all the way down to atmospheric pressure. This can be achieved by the present invention wherein the gas hydrate of the hydrocarbon product has a stable and constant temperature throughout the product and the temperature is low enough enabling the hydrate to be stable also at atmospheric pressure.

The main object of the present invention is to provide an efficient method of manufacturing for generating large amounts of a new product comprising gas hydrates in large amounts in a storage stable condition, assuming an efficiently thermal transmission during the generation of hydrate. Another object is to provide a new, easy to handle and preferably pumpable hydrocarbon product, i.e. a slurry or pastous hydrocarbon product with a highest possibe content of hydrate, and especially a product which is stable at those pressures and temperatures which prevail in the transportation and storage areas, and in this way not releasing gas which result in undesirably pressure accumulation. A further object is to provide a hydrocarbon product not containing any, or only insignificant amounts of free water or ice, that is water not being converted in to hydrate, as occurrences of such free, non-converted water is expected to be a reason why gas hydrates previously have been difficult to handle. Free, unconverted water will in addition represent loss, because the water constitute an unnecessarily weight requiring extra energy for the transportation, and the water does not contribute to the transportation of further amounts of gas. The statement insignificant or minor amounts of water, or frozen water, is to be understood that the content of free, non-converted water is not so high that the content of hydrate-generating gas components in the product become unacceptably low. Economic estimates have indicated that satisfactory conditions normally will prevail when the volume ratio between the hydrate-generating gas components prior to the hydrate-generation and solid gas hydrate + frozen water after the hydrate-generation, is greater or equal to 130. This indicates that the end product shall contain at least 130 Sm³ gas pr. m³ solid mass. It shall particularly be mentioned that the process conditions are adjusted in such a way that an end product is obtained wherein the solid hydrate containing material contain an amount of gas which corresponds to a packing density of at least 130 Sm³/m³, preferably of more than 150 Sm³/m³ solid substance, using methane as hydrate- generating hydrocarbon.

A further object is to provide a method of preparing continuous or batchwise generation of large amounts of a hydrocarbon product using known and established chemical engineered facilities.

Another object of this invention is to provide a new method of preparing by generating a new hydrocarbon product by a two-stage direct cooling of the starting materials and intermediate products by means of two identical or two different cooling media. A plant which use a common container for generating and cooling is required for this purpose, or which utlilize of separate containers to accomplish one or more process stages.

It is also an aim to reduce the risk of generating ice and hydrate at undesirable sites in the plant, e.g. in sites wherein it is a risk of clogging. Further objects of the present invention is to provide a suspension wherein large quantities of gas hydrate exist in the form of particles surrounded by or suspended in a carrier liquid, said liquid enabling an efficient thermal transmission between the gas hydrate in the mass and the exterior surroundings, thereby ensuring an effective operation and control of the temperature in the product. Thus both an energy carrier medium is obtained which simply can be stored and handled by an assistance of traditional storage and transportation equipment for liquids, pastas, dispersions and semi-solid masses, and at the same time a suspension having a large energy content in the range between the energy content of liquified gases (LNG) and in compressed (CNG) gases is achieved, without getting corresponding problems with high pressures and /or very low temperatures. To fulfil the object of effective generation of large amounts of gas hydrate, the present invention can provide for direct contact between a first cooling medium which is supplied and the hydrate-generating hydrocarbons, the last mentioned generally being a gas. A large, direct contact surface between gas and cooling medium is required. It has been experimentally demonstrated that such direct cooling is the cooling method which results in the highest production rate of hydrate, and thus most suitable for industrial uses.

Another advantage of the present invention is that the process can not only be realised in a stationary plant on ground, but it can also be adapted for use at floating installations and crafts offshore requiring to take care of the gas produced, either alone or associated with other products of petroleum. Such compact plants can be realised as the plant according to present invention is relative simple and to a high degree has components which are already thoroughly tested and commercially available such as pumps, valves, cooling systems, containers etc. The mentioned advantages and objects are achieved by using a method according to one or several of the claims set forth below by means which are described in more detail in the following, and which produces the desired product. Roughly, a hydrocarbon product according to the present invention can be obtained by a process having the following four steps: In step a large amounts of hydrate is generated.

In step b excessive water from the hydrate is eliminated. In step c the hydrate is cooled by addition of a cold hydrocarbon liquid while ensuring that the hydrate does not dissociate, and in step d is removed from the end product of the process. Possibe non-converted water will deposit as a film around the individual hydrate particles, and hydrate products containing great amounts of unconverted water will become unhandy on exposive to temperatures below the freezing temperature of water. A possible surplus of water can be removed from the hydrate in many ways to generate a «dry» hydrate, being a hydrate where a large amount of unconverted water is no longer present, at least not in a detrimental degree. The three most main methods of eliminating non-converted water is: The hydrate can be treated mechanically, e.g. drainage, compressing or compacting in such a way that the water is squeezed out. Known treatment devices such as filters, centrifuges or hydrocyclones, can be useful. This method can still nevertheless not eliminate all the water. Further amounts of hydrate-generating hydrocarbons can be added, in liquid or gas form , which are contacted with the non-converted water, in a manner resulting in the said water being converted to the hydrate. By supplying an excess of hydrate-generating components at suitable pressure/temperature-conditions, all remaining free water can be converted to hydrate so that the final hydrate will be completely dry.

Excess of water can further be eliminated by the addition of a water absorbing medium, e.g. an alcohol or a ketone, e.g. acetone. However, such media also have a tendency to dissolve hydrate, and one for this reason only used in particular cases. The term «eiiminate» therefore comprises all of these methods and combinations thereof.

According to the present invention can as mentioned, direct cooling be used, that is direct contact between the product which is to be cooled and the cooling medium. The direct cooling can practically be effected in at least two steps by the application of a first and a second cooling liquid, also named cooling media. The first cooling liquid is applied in the hydrate generation of the step a and its main purpose is to eliminate the amount of heat which is generated during the hydrate formation to keep the temperature of the hydrate generating zone is kept within the hydrate-generating range at a given operating pressure. The cooling liquid shall in this manner not only cool «the gas» or the hydrate generating hydrocarbons, but also the hydrate obtained and the water present to the required extent. The cooling in the first step is only effected down to a temperature which ensures that the hydrate is generated in the desired amounts. The first cooling liquid may be water, and must in that case be eliminated or converted to a hydrate in step b, prior to the second cooling liquid, during the process step c, reduces the mean temperature of the hydrocarbon product to a temperature T₀ + AT

wherein

\ + 40Y_r

0,61 - 0,15-7_Cι + 0,15-r_Cj + 0,15-F^ + 0,1 -Y_Nj 0,55 -Y_C{ + l,5-Y_C2 + l,5-Y_Cι + 0,*-Y_Nι + l,l-r_co, 0,55.Y_Ci + l,5-Y_C2 + l,5-Y_Cy + 0,8,7^ + l,l-r_COι 1,2 + 95 -Y_c

0,64 + 0,32 -Y_c + 0,42 • Y_N ^■

In the above mentioned P is the total pressure, Y_t is the mole fraction of the individual gas components, A, is gas-specific constants, n, is gas compositions- dependent exponents. The two latter groups are written in the vector form, meaning that letters having an index are mutually connected to numbers or terms at the same line for the groups written inside the brackets and having = between them.

ΔT = a numerical value defining the error margin in the expression for T_end = T₀ and ranging from +1 to -15°C.

ΔT is experimentally assayed. s The equation above is considered to be valid in the temperature range of -

10 to -50°C.lt was previously expected that the cooling of a hydrocarbon hydrate product according to US patent number 5.536.893 after the generation of the hydrate was sufficiently low for further storage and transportation of the product. However, further studies carried out by the present inventors have established o that at the temperatures -10°C to -15°C, which are indicated as the preferred and optimal ones in this patent document, considerable decomposition of the hydrocarbon hydrate takes place during the further storage and tranportation, resulting in part of the original gas which hydrate was generated being dissolved in the other cooling liquid, while the hydrate water will freeze into ice again at such 5 temperatures and may deposit as an ice-film on the hydrocarbon hydrate product. It has now been found that if storage of hydrocarbon products which comprise hydrate-generating gas components from hydrate-generating gas components surrounded by or suspended as particles in a hydrocarbon containing liquid are stored within selected pressure- and temperature ranges, such products may still be stored and transported under pressure- and temperature conditions which are considered as interesting by the industry. Generally spoken the chosen temperature range is lower than what previously was considered as the most preferably range for meta stable storage of "dry" hydrate at further identical pressure conditions. For many current of hydrocarbon gas compositions the temperature range for stable products is thus below -20°C at atmospheric pressure. Below the upper temperature limit of stable product experiments indicate a completely stable product, i.e. that the product during storage at such temperatures do not release detectable amounts of gas even after storage for a long time. It is further observed that the transition between a more or less completely stable condition, as described above, and a more unstable condition at temperatures above the discovered temperature limit, is remarkably defined. Thus an example of the product will during slowly heating from a temperature well below the mentioned temperature limit to a temperature above this limit not release observable amounts of gas before the temperature limit is reached, while the product sample immediately after will passing of the limit release easily detectable amounts of gas.

Further it has been discovered that the present hydrocarbon products possess other rheology properties when the products exists at temperatures below the above mentioned temperature limits than the properties of the same product at temperatures above these limits, particularly at temperatures considered as preferably for storage of "dry" gas hydrate material within the meta stable temperature range (i.e. -10 to -15°C). In this manner products which are kept outside the stable temperature area have a pronounced tendency of sintering at straining pressure. A sample of the material will for example be converted to a solid mass after exposure to a pressure of 0.1 bar in a press after being stored for a few days. The sample can be divided into smaller arts when exposed to hammer strokes of a defined impact. The hydrocarbon material that is kept below the upper limit of the stable range will have a comparatively lower sintering tendency. A sample which has been exposed to the same experiment as mentioned above will though result in a compact mass of the particles of the gas hydrate surrounded by hydrocarbon liquid, but the sample will rapidly disintegrate to a granular mass of gas hydrate particles in the hydrocarbon liquid after a considerably milder mechanical load.

The previously unknown properties render hydrocarbon products of this kind considerable industrially interesting and the discovery of these properties constitutes the basis of the present invention.

Without being limited to a definite theory, it is possible to explain the difference in properties of "dried" hydrate and of the instant hydrocarbon products which has been mentioned as meta-stable condition, that is above and outside the recently discovered stable range, as follows: When "dry" hydrate is stored under meta-stable conditions, the generation of the protected ice-layer possibly occurs of the gas molecules in the last surface layer of gas hydrate particles escaping from the gas hydrate structure and leaving a layer of the gas hydrate structure which is poor in or free of gas molecules. Such a lattice structure of water molecules (lacking or with a low content of gas molecules) is thermodynamically less stable than a normal ice structure, and the water molecules in the lattice structure will successively reorganise to the lattice structure of normal ice. The process will proceed at diminishing rate as long as gas molecules can escape during formation of a increasing thicker ice shell which envelop the hydrate material. The same physical conditions for such a process are presumably not available for gas hydrate which is kept in contact with a liquefied phase of hydrocarbon, under generally similar conditions. When the gas hydrate is outside the area where the hydrate structure thermodynamically is the most stable lattice structure and the temperature at the same time is below the freezing temperature of water, ice and free dissolved gas constitute the stable phases. Thus one can imagine that the changed physical conditions which the presence of a liquefied hydrocarbon phase involve, resulting in the mechanism for the generation of the ice structure (outside the established stable range) being different from "dry" hydrate, for example by both water and gas molecules escaping from the surface of the gas hydrate particles, and by the water molecules either in the hydrocarbon phase or at the surface of the hydrocarbon particles combining and generating ice as separate ice crystals. Thus, a protective ice layer is not created at the surface of the gas hydrate particles. New surface layers of gas hydrate materials are instead constantly exposed to the influence of the hydrocarbon phase, and the gas hydrate disintegrates faster in the presence of the hydrocarbon phase, than when this not being present. As indicated above, for a person skilled in the art, it will become evident when the temperature is sufficiently low enough at a certain pressure, the gas hydrate structure will constitute the thermodynamically most stable phase. It is therefore relatively simple to explain that the hydrocarbon product is stable when the product is kept at temperatures below the upper limit for the established stable range.

However, when the upper temperature limit of the stable range is approached from below, it is however more uncertain whether the thermodynamic conditions alone, which decide the stability of the gas hydrate, or whether the observed stability is caused by phenomena of kinetic kind. For example it is possible that the upper temperature limit indicates a change reciprocally in the relation between parameters such as solubility, rate of diffusion and surface tension of the different phases in the system, and that this change causes a transfer from one mechanism to another thereby generating a hydrate in a stable, but not necessarily a thermodynamically stable form. With respect to a preferable method which can be used for preparing of the hydrocarbon product, the second cooling liquid can perform several tasks, but primarily cool the generated hydrate in such a manner that it becomes stable at ambient pressure, i.e. at atmospheric pressure. The second cooling liquid cools according to the present invention the product to a temperature below

wherein the definitions are as defined above, but only after almost all the water is eliminated.

The water which has not been involved in the hydrate generation with hydrocarbon gas at step a as described above, is removed from the intermediate product at step b, may, completely or partly, again be cooled and recirculated in the process, for example to the step a for preparing additional amounts of gas hydrate.

Optional recycling of the first or second cooling liquid for the maintenance of the desired temperature in the product is effected by more or less separating the cooling agent from the hydrate, is cooled again and recycled separately. It is preferred that the recycling stream which is cooled, does not contain particles of hydrate, ice or water, as such components have a tendency to be deposited as ice or hydrate on the cooling surfaces in heat exchangers. The recycled, recooled cooling liquid coolsl the product again by direct contact therewith.

Essentiall to the present invention is that all the gas hydrate particles are in intimate contact with a liquefied hydrocarbon. This ensure a stable temperature throughout the entire hydrate mass and enables effekting rapid, desired temperature adjustments in the hydrate mass which will not anywhere will be thermally isolated from the temperature controlled medium which constitute the hydrocarbonous liquid, also named the second cooling medium.

The suspension of hydrate particles in the first cooling medium, possibly with some free non-converted water, as at the end of stage a, constitute the first intermediate product and has a mean temperature just above the freezing temperature of water and a pressure similar to the hydrate generating pressure. The suspension of hydrate particles in the second cooling medium, having as far as possible reduced contents of free unconverted water as at the end of step b, is named the second intermediate product. This has a temperature T₄. However, the end product itself is to be brought to such a low temperature that the hydrate is stable at the revailing pressure. The temperature of the end product can for example be as low as -40°C and the pressure can be as low as approximately 1 atmosphere. See also the following description. When the product has achieved stable temperature- and pressure conditions, and excessive amounts of the cooling liquid is eliminated, the product having preferably reached a pumpable/transportable consistence, the desired end product is formed. The end product can be handled by conventional transport and storage equipment developed for other types of paste and slurry products.

The conditions that have to be satisfied generate a hydrate, are of course primarily that the pressure and temperature are within the hydrate-generating range. In addition it is very important that the hydrate-generating hydrocarbons and the water, possibly in frozen condition as snow or ice, are allowed the sufficient contact time for the conversion to hydrate to take place as completely as possible. When the hydrate generation take place by the atomised water being sprayed at the top of the hydrate-generating zone in the container 2, is it important that the container is tall and that generated hydrate is not allowed to build up too highly in the container. This ensure that the contact time between water and gas is sufficient long for generating large amounts of the hydrate. In the figures 2, 3 and 4 it is indicated that the container 2 may be very high.

If an apparatus where the gas is bubbled in from the bottom through the water, is selected instead, it is atomised gas via nozzles and a large height up to the water surfaces correspondingly important

A further important condition is obviously that the streams of mass out and in from the hydrate-generating zone are sufficiently large.

The temperatures which are mentioned in this application, have the following mutual relations: T, is the temperature of the first cooling medium when this is brought to the hydrate-generating zone. T, must necessarily be substantially below the equilibrium temperature for generating/decomposing gas hydrate at the current operating pressure. T₂ is the temperature of the first cooling medium when this leaves the hydrate-generating zone. This temperature is close to the equilibrium temperature of the hydrate.

T₃ is the temperature of the second cooling medium normally when fed to the cooling zone. T₄ is the temperature of the end product, also termed the storage temperature T_storage elsewhere in the specification.

T₅ is the temperature of the second intermediate product.

T₆ is the temperature of the hydrate mass after cooling hydrate mass at step c to a temperature below the freezing temperature of water, where the cooling medium, which contain destabilising amounts of volatile components, can be substituted by a medium having a substantially lower content of such components, and the most important relative conditions can be expressed as follow: T, « T₂and 0°C < T₂

T₃ < T₄ « 0°C og

T₄ < T₆ < 0°C < T₅

Some other detail conditions which may be of interest, are mentioned below. It must be noticed that the first cooling liquid can be contacted at a low temperature for generating relatively small amounts of ice intermittently. However, it is important that the first cooling medium is not contacted in at such large amounts or at such low temperature that great amounts of ice are formed. The invention also relates to the processes where some ice is found in the hydrate- generating zone, but where such ice is later melted by heat exchange with the further amounts of gas and liquid in the hydrate-generating zone.

The temperature in the first cooling medium can be below 0°C, especially if the cooling medium is a hydrocarbon, on contacting to the hydrate-generating zone. This appear from the conditions above.

T₄ < T₆ < 0°C <τ₅

T₅ is the temperature in the hydrate mass after generation and after removing of non-converted water at step b, that is previous to cooling at step c. Therefore must 0°C < T₅ . T₆ is the temperature to which the hydrate mass must be cooled to replace the hydrocarbon medium containing destabilising amounts of volatile components by a medium having a low content of such components, dissociate to that non-generated gas hydrate to a higher degree due to the lack of stabilising concentrations of hydrate-generating components. After the temperature in the hydrate mass is brought below the temperature T₆, the gas in the gas hydrate will for practical purposes be irreversibly bound in the gas hydrate structure. This also appear from the conditions above.

The water which is to be converted to hydrate, can already at the beginning be in the forms of snow and ice. The requirement for cooling of the first cooling medium remains reduced. It shall be stressed that the first cooling medium can be contacted at such a temperature and in such an amount that some ice is generated or maintained, but not at such amounts that ice is transferred to the next step in the process.

The second cooling liquid which is used at the step c, is constituted of a hydrocarbon containing liquid, and the temperature thereof must be sufficiently low at the outlet of the cooling zone to obtain a composition of gas hydrate and hydrocarbon liquid which has a temperature resulting in a stable composition at ambient pressure, which normally is about 1 bar. However, it is important that the first cooling liquid, especially if this contains water, must not have a temperature below its own freezing temperature. The first cooling medium can, however, readily contain hydrate-generating hydrocarbons, which also may be other hydrocarbons than what is found in the gaseous in the hydrate-generating zone. Concerning the composition of the second cooling medium, the following shall be noted:

On the one hand, to stabilize/not dissociate the second intermediate product, still having a temperature above 0°C, the total partial pressure of hydrate- generating hydrocarbons should not be reduced substantially below the limit value for the hydrate generation at the current temperature.

If the second cooling liquid does not contain any hydrate-generating components, delivery of this cooling medium will reduce the partial pressure of these hydrocarbons, thus making the hydrate unstable and dissociated. The cooling zone should therefore, at least at the beginning of step c, be supplied sufficient amounts of hydrate-generating hydrocarbons to ensure that the hydrate remains stable until the temperature has reached T = T₄ which is the temperature in the end-product.

If not sufficient amounts of hydrate-generating components are present during cooling at step c, it is a risk that a part of the hydrate dissociates before the temperature T = T₄ is reached.

On the other hand, it is necessary to eliminate or reduce the content of destabilising components, such as methane, ethane, propane and other volatile components, from the hydrocarbon medium, to stabilize the end-product, meaning that the hydrocarbon medium in the end-product (at temperature T = T₄) does not contain destabilising components in amounts resulting in a total partial pressure of these components above the hydrocarbon medium exceeding the ambient pressure (which normally would be about 1 bar) at T = T₄ .

The hydrocarbon medium in the end-product should not contain destabilising amounts of light hydrocarbons such as methane and ethane. The partial pressure of each of the destabilising components can, at least as a first approach, be calculated from Henry's law:

P, = H, c,

wherein: p, = maximum permitted partial pressure of component i,

H, = Henry's constant which is defined experimentally, and c, = the concentration (in mole per volume unit) of the same component.

The total amount of the partial pressures of volatile components, ∑p, , must be less than prevailing ambient pressure. If the total of the partial pressures exceeds this limit, this will result in an unstable end-product, as it releases volatile gases as methane, ethane and to a certain degree propane when the end-product is exposed to prevailing ambient pressure (~1 bar), even when the end- temperature T = T₄ (« 0°C) is reached.

The process according to the invention may comprise a mechanical processing by an apparatus, for example as at least an agitator. The purpose is to prevent the formation of agglomerates and large accumulations of hydrate, and to contribute to increased transport of hydrate-generating components forward to the boundary surface of non-converted water in the hydrate mass, as well as an equalisation of the temperature in the hydrocarbon product. Although the hydrate product may be exposed to a mechanical processing to generate a suspension of hydrate particles in a cooling liquid, such processing will not always be necessary. Depending on the liquid composition, pressure and temperature condition, the hydrate may frequently disintegrate by itself generating separate particles thereby generating a suspension as soon as the hydrate is contacted with the hydrocarbon containing liquid.

The object of the invention is in other words, is to generate a product being a composition of

- a «dry» hydrate prepared of hydrate-generating hydrocarbons, and

- a hydrocarbon containing liquid; as none of these components contains considerable amounts of free, that is unconverted, water, so the composition can be exposed to temperatures below the freezing temperature of water without any risk of agglomerating ice formation. This means that the temperature of the composition can be regulated considerably adjusted, the composition remaining stable at a pressure down to one atmosphere. Particularly the last condition renders the product unique and suitable for storage and transportation. Further it is beneficial that the hydrocarbon containing liquid is in good heat-conducting connection with all of the particles in the suspension acting as an effective temperature stabilising and adjusting medium too these. The provision is that this does not irreversibly result in the formation of larger amounts of ice. As soon as water in free form no longer exists after the generation of the hydrate, at least not in substantial amounts, i.e. because being eliminated in one of the methods mentioned above, the second cooling medium may, i.e. in the form of a hydrocarbon liquid, preferably as the so- called condensate fraction in crude oil, still be supplied fairly soon after hydrate- generation, and at a temperature which may be substantially lower than the freezing temperature of water, thereby greatly reducing the risk of ice formation and the subsequent clogging. Concerning the cooling at stage c, the following important aspects shall be emphasized:

. The cooling is carried out in presence of the required amounts of hydrate-generating components until the temperature has reached a value T, being below a value T₆, below 0°C.

. The cooling is preferably effected in the absence of destabilising hydrocarbon components when T is below T₆ . Destabilising components may be removed in different ways:

A) An medium containing destabilising components may be replaced by a cold hydrocarbon medium not containing destabilising components beyond a limit value determined from the required stability of the end product, or after the hydrate mass reaching the temperature T=T₄, or

B) stage c is effected with another cooling medium in the presence of the necessary amounts of hydrate-generating components until T=T₄, whereupon the content of destabilising components is eliminated from the second intermediate product exposing the product (gas hydrate in hydrocarbon medium) to a sufficiently low pressure releasing the destabilising components as gas from the hydrocarbon medium to generate the end product satisfying the requirement of stability. Released gas may be recompressed and recirculated to the hydrate- generating stage a. Depressurizing and elimination of remnants of volatile components in the end product can be accomplished while the product is still in the cooling zone. Remnants of volatile components, released as gas after depressurizing, can also be eliminated after transferring the end product to a storage container. The storage container must in that case be provided with a gas outlet at the vertex of the container and be connected to equipment for further handling of released gas, e.g. pipes and compressors for recycling of gas to the stage a.

The method may, in the most simple embodiment, be accomplished in one simple of production line performing each operation in subsequent order. The preparstion of the hydrate will thereby first be accomplished, whereupon possibly obliging water is eliminated from the hydrate prior to cooling the dry hydrate with a suitable cooling liquid. Such a simple, one-track production line will however require a batchwise treatment of the gas, at both the inlet and outlet. A preferred method is therefore to accomplish the process by means of at least two parallel production lines each of which being arranged to accomplish at least some of the production stages mentioned above. By providing that the individual production lines at any time are at different stages of the process cycle, that is starting the production lines with the preparation of hydrate at different, displaced points of time, it may be obtained that the whole plant, including two or more parallel production lines working in different «phases», together achieves quite a steady receipt of gas and also generates quite a uniform production stream at the outlet, which in a lot of instances are strictly required in commercial plants handling gas.

Although it is preferably to use a cooling medium in the form of a hydrocarbon containing liquid, and although it is presumed an advantage to use the same composition of cooling liquid everywhere in the plant where cooling is required, the invention still also comprise methods and plants using different cooling mediums at different cites in the process. Thus the cooling medium may either being the same at all process stages as mentioned above, or a different cooling medium, some may also comprise or consist of water, be used.

It must be stressed that «stable» end product require conditions being such that the hydrocarbon product for all practical purposes behaves stable.

The method discussed above also indirectly discloses a plant for the preparation of the stable hydrocarbon product. The production plant in the most simple embodiment is a production plant for the hydrocarbon product consisting of a simple container or reactor with an outlet of gas, water and cooling liquid, together with outlet of final generated hydrate and outlet of excess water and/or cooling liquid. Such a container may, if necessary, also have an inlet and outlet to circulate at least portions of the cooling liquid, which in that case can be cooled in an external heat exchanger arrangement prior to recirculating of the cooling liquid to the container to regenerate direct cooling of the product. The container may also if required be provided with a return loop to circulate at least the surplus water to another cooling in a second, external heat exchanger prior to returning to the container.

The different components described must of course be connected by the necessary joints and be equipped with the necessary valves, detectors and regulating means. Finally a production plant preparing the hydrocarbon product may in another embodiment comprise two or several containers, as the product is conveyed to new containers as one or several production stages are accomplished. The plant may preferably comprise a separate storage container for the hydrate. This storage container is preferably heat-insulated and may also be connected to an external heat exchanger via a circulating loop through which at least a liquefied fraction of the hydrocarbon product may circulate. Details concerning the plant are described in the following through different embodiments and with references to the figures.

As mentioned in the introduction the present invention also relates to a method of storage and degassing a hydrocarbon product and particularly a hydrocarbon product containing a large amount of gas hydrate, comprising at least a storage container wherein the hydrocarbon product, or at least the hydrate existing therein, is kept prior to degasification.

A plant has previously been suggested for the transportation and use of gas wherein the transportation and partly the use of the gas takes place as the gas with obliging water is transferred into a gas hydrate.

In practice it has been shown difficult both to transport hydrate, which easily clog and adhere to the inside of the transportation plant, and to utilise the gas of the gas hydrate at a suitable rate; a.o. because it is difficult to achive a constant and monitored heating of the gas hydrate which itself is a poor heat conductor, and because large sections of the plant at some times will be exposed to high pressures with the risks this involved.

As examples of previously known plants for the transportation and storage of gas hydrate, it is referred to the Norwegian patents Nr. 149.976; 175.656 and US patent Nr. 3.888.434. The present invention aims at storing and thereupon degassing all the different hydrocarbon products comprising or exclusively comprising of a gas hydrate, in a effective, economical and as far as possible safe manner.

The present invention particularly, but not exclusively, aims at providing a plant and a method of using gas hydrate being in a different form than previously used. The form of the gas hydrate which is going to be used according to the present invention, is included in a suspension partly comprising gas hydrate in the form of particles, partly a carrier liquid not containing water, but preferably being of hydrocarbons, and particular hydrocarbons which are not hydrate-generating. Such suspension are further described in the Norwegian patent applications Nr. 95.1669 and 95.1670.

It is also preferred that the hydrocarbons of in re possess such properties as being in the liquid form and that they all being compatible and in the liquid form at the operating temperatures or temperature ranges of the present plant. The greatest advantage of such a suspension comprising particles of gas hydrate in a carrier agent as a hydrocarbon based liquid, is that the carrier medium both keep the hydrate particles separated preventing sintering and the formation of solid masses or deposits, and that the carrier medium is suitable monotoring temperature of large amounts of the hydrate mass at any time at the desired rate, e.g. releasing gas or stabilising the hydrate mass.

The present invention primarily relates to a method wherein a hydrocarbon product may be only a gas hydrate or be a suspension of a carrier liquid and said gas hydrate, possibly being stored for a certain time and then decompose to release gas for further transportation or use, and according to the present 5 invention the hydrocarbon product is stored in one or more storage containers at a temperature sufficiently low to keep the hydrate in a stable hydrate condition at the storage pressure, possibly being close to the normal atmospheric pressure. The storage container(s) may be built very simple without reinforcing structures and thick walls. These storage containers are used in the plant with at least a o decomposition container having a smaller volume compared to the storage container(s), and the decomposition container(s) are dimensioned for a pressure corresponding to degassing pressure of the released gas as the hydrate dissociates, practically meaning e.g. a pressure of about 50 - 60 bar.

When the hydrocarbon product can be stored above -40°C, a propane cooling cycle can be used for cooling the different process streams by indirect cooling,. Shell and tube heat exchangers can for instance be used. Further it is not require use special steel in the equipment directly in contact with the product, as the instance when the temperature is below about -40°C.

In this manner the division of the terminal plant into achieves two pressure zones, one storage zone and one dissociating zone, particularly two advantages: 1 - The large gas volume which is stored in the storage zone may be kept at a low pressure close to normal atmospheric pressure. This involve possible damages to the storage part of the plant only result in small leakages and not to big bursts or blow-outs. 2 - Previous to application of the gas the hydrocarbon product is transferred with the hydrate to the decomposition zone and into at least one decomposition container, preferably having a far smaller size. This size is in practice decided only by the desired consumption of gas per time unit and the temperature at which the decomposition will occur. The calculations of this volume are easily accomplished by a person skilled in the art and will not be further discussed here. It is supposed to be an advantage when the hydrocarbon product is in the form of a suspension comprising relatively small particles of gas hydrate suspended in a carrier liquid preferably being a hydrocarbon liquid or a composition of hydrocarbon liquids, preferable of a mainly non-hydrate- generating nature. The amount of hydrocarbon liquid during transportation and storage can be reduced so far that the main amount of the suspension, e.g. about 70 %, just are hydrate particles containing and preferably being saturated with gas, whereas the smallest part of the suspension, about 30 percent by volume, is a carrier agent or carrier liquid as a preferably non-hydrate-generating hydrocarbon liquid. One the most important objects of the carrier agent is to provide a buyoant force to the gas hydrate particles. This is of significance when the gas hydrate is loaded into loading containers, e.g. in a transportation craft. A hydrocarbon medium ensures that the gas hydrate mass gets a buyoant force which prevents or at least to a substantial degree reduces the tendency of sintering of the hydrate mass into a solid mass in the bottom of the loading container.

Such a hydrocarbon based hydrate suspension provides for good temperature control possibilities of the gas hydrate, and further can be transported through pipes and be pumped by known pumps developed for dispersions, pasta and other masses being more or less a composition of solid-matters and liquids. This involves in practice that the construction of a plant according to can use previously known and established single components developed for temperature regulation, transportation and pumping of other kinds of suspensions and masses. These single components are for this reason not defined in detail.

The invention is however not limited to the use in relation to such suspensions, but can be used for all gas hydrate containing materials, or for purely gas hydrate. One purpose of the present invention is thus to develop a method of degassing of the hydrate in a hydrocarbon product, particular a hydrocarbon product having a high content of gas hydrate, but where the plant is not encumbered with the above mentioned risks and disadvantages. This is achieved by modelling the terminal plant and using a method in accordance with the specifications in the following drafted claims.

To give a better understanding of the present invention, it is referred to the following detailed specifications of the methods according to the present invention, and to the accompanying figures where:

The general aspects of the present invention is described in more detail in the following (fig.1).

Hydrate-generating hydrocarbons and water are brought together in a hydrate generating zone 101 through a pipe 102, respectively 103. The hydrate- generating zone 101 is pressurised and the temperature is regulated such as hydrate-generation pressure and temperature conditions for creation of hydrates of the hydrate-generation hydrocarbons are reached in the zone. The flow which contains the hydrate-generating hydrocarbons can in addition to hydrocarbons also contain other components, among them components as nitrogen and carbondioxide which also at mentioned conditions can generate hydrates.

During the generation of hydrates of the hydrate-generating component which is supplied the zone 101 will heat be released. This hydrate-generating heat is eliminated by heat exchanging directly or indirectly against a cooler cooling medium, which is supplied from a first cooling aggregate 106 through a pipe 107. Heated cooling medium is lead in return to the cooling aggregate through the pipe 108.

Possibly unreacted water is eliminated from the hydrate generating zone 101 through a pipe 105. Preferably the temperature in the hydrate generating zone should be kept at a level which prevent generation of more an essential amount of frozen water as ice or snow.

After possibly non-converted water is eliminated from the hydrate mass, is the hydrate mass lead through a pipe 111 to a cooling zone 112, where the hydrate mass is cooled down to a desired storage temperature T|_ager below the temperature limit T(0). The cooling take place by direct or indirect heat exchanging against a colder cooling medium, which is supplied from another cooling aggregate 113 through a pipe 110 and is possibly recycled to the cooling aggregate through a pipe 109. Hydrocarbon liquid, which constitute a part of the hydrocarbon product according to the invention, can be supplied in the hydrate generating zone 101 via a separate pipe 130 or to the cooling zone 112 via a pipe 114, or hydrocarbon liquid can be supplied as a part of the cooling medium, which is used for cooling during hydrate generation in the hydrate generation zone 101 or for cooling of the hydrate mass in the cooling zone 112. If one or both of the zones is cooled by direct heat exchanging. In the last-mentioned case must necessary amount hydrocarbon liquid be supplemented for replacement of hydrocarbon liquid which is being used for formation of the end product, i.e. the product constituted of hydrates surrounded by or suspended in the hydrocarbon liquid. Preferably constitutes the hydrocarbon liquid of hydrocarbons with four or more hydrocarbons in the molecule. Further cooling of the hydrocarbon product to a storage temperature T_storage which is above about -40°C. After generation and cooling of the hydrocarbon product to a desired storage temperature T_storage is the product transferred from the cooling zone via pipes 1 15, 1 16 to a storage arrangement 1 17 for the hydrocarbonproduct, where the product is being stored at low pressure, preferably close to the ambient atmospheric pressure, at cooled condition. The storage arrangement can constitute of a suitable dimensioned container, which is preferably heat insulated. The storage temperature in the storage arrangement is maintained by a third cooling aggregate 118, which is connected to the storage arrangement via the pipes 119 and 120, particular if it is actual to store the product for a certain time. When it is desired to use the hydrocarbon product, the product is lead out of the storage arrangement 1 17 via a pipe 121.

The gas hydrate mass in the product can decompose at heating to a temperature whereby the gas hydrate becomes unstable.

The hydrocarbon product according to the present invention can be used for several objects. Generally the product can be transferred from the storage arrangement via the pipes 121 , 122 to a conversion plant 123, where the product is used as such directly as combustibles or fuel for production of mechanical or electric power which is being exported from the conversion plant via suitable arrangement, indicated at the figure by the arrow 124, or the product can by supply of necessary heat energy via suitable arrangement indicated by the arrow 125 and at the figure, decompose into its own individual components, i.e. components which was earlier connected in the hydrate mass or, possibly to a minor degree, dissolved in the hydrocarbon liquid. The hydrocarbon liquid as such and water, eventually ice, from decomposed hydrate mass. The last mentioned components are taken out of the conversion unit 123 partly as gaseous substances via a outlet 126 and partly as liquefied substances via a outlet 127. A system for preparing, storage and transportation of the hydrate can comprise one or several storage arrangement 1 15. The connection line between the production plant comprising the hydration zone 101 and cooling zone 1 12; and storage arrangement 117 can in this manner be permanent or non-continuous. Likewise the connection line between the storage arrangement 1 17 and conversion plant can be permanent or non-permanent. This is indicated in the figure by the dotted lines between pipe pieces 115 and 116, respectively 121 and 122.

For example, a production plant comprising the hydration zone 101 and cooling zone 112 can at at production platform or a production ship permanently s be connected to an intermediate storage 117(1). If it is required the hydrocarbonproduct can be transferred from a intermediate storage to a storage arrangement 117 (2) in the form of one or several load containers on a transportation crew via a provisional connection line, and at arrival to the loading place the hydrate product can be transferred via a provisional connection line from o the storage arrangement at the crue to a storage arrangement on shore, e.g. near by a conversion plant 123 as in instance will be connected to the last mentioned storage arrangement via a permanent connection line.

The method according to the present invention for preparation of a hydrocarbon product can be used for catching of hydrate-generating gases, e.g. s volatile gases, so called VOC-gases, which is released during loading, transportation and unloading of crued oil and the product according the invention can be used for storage of such material. Except from the recently invented temperature conditions for cooling and storage of the hydrocarbon product, the technical methods for such catching of VOC-components and for storage and use 0 of the corresponding hydrocarbon products are described in the Norwegian patent applications Nos. 96.1666 and 96.1667.

Likewise the present technology can be used for stabilisation of relatively untreated and unstabilised crued oil eventual in combination with so called associated natural gas. Except from the mentioned recently discovered 5 temperature conditions of such matter described in the Norwegian patent applications No. 96.4489.

The invented methods and products can also be used for transportation of natural gas, particularly from distant gas field to new or established consumption areas for natural gas. Except from the mentioned recently invented temperature 0 conditions of such methods described in the Norwegian patent applications Nos. 95.1669 and 95.1670. It can also be referred to the Norwegian patent application No. 95.5364 which describes a terminal plant and a method for storage and decomposition of the gas hydrate material.

Depending of situation and practical conditions either to or three of the cooling aggregates 106, 113 and 118 can be constituted of integrated cooling systems, i.e. that the cooling aggregates was completely or partly utilized common facilities and resources or are built as one unit, which cover the complete cooling demand for the different units. Such an integration of the cooling aggregates are indicated with the dotted lines 128 and 129 at the figure 1.

Fig. 2 indicate a simple embodiment of a plant according to the present invention, where water which shall be into converted to hydrate can pass at several occasions through the generator, with intermediate cooling. The hydrate- generating zone and the cooling zone consists in this embodiment of one and the same container. The fundamental principle is evident.

Fig. 3 indicates a somewhat different embodiment of the plant according to the present invention, wherein the water is converted, only passing once in the process (the "once through" principle).

Fig.4 indicates a further embodiment wherein the cooling zone constitutes of a separate unit,

Fig. 5 illustrates a flow-diagram of an industrial plant, wherein some of the calculated values and capacities are indicated, and where some parallel process- routes are indicated at the different stages of the process.

It shall be noted that part of the practical and constructional details are omitted in the figures, which primarily indicates the fundamental relations according to the present invention. In addition it shall be mentioned that the same reference numbers are used in all of the figures as long as it is suitable for the purpose and the different figures and parts of these are not necessarily indicated in the same scale.

To give a first discussion of the principle of the present invention, fig. 2 indicates one of the most simple manners to realise the present invention. The figure roughly shows the construction of a plant to accomplish the method . The first embodiment of the invented method is performed in a plant comprising a pressurised container 202, which at stage a works as the hydrate- generating zone 201 and as the cooling zone 280 at stage c and appending cooling loops for water and/or the first and second cooling medium, as major

5 components. As shown in fig.2 the container 202 is connected to a storage unit 250 for the storage of the end product.

In the following a first embodiment shall be explained with water, possibly sea water, is used as the first cooling medium. A variant of the first embodiment, wherein hydrocarbon liquid is used as the first cooling medium, will be explained lo later on.

The container or reactor 202 ismanufactured from a suitable material, e.g. stainless steel, and is constituted in such a way that the container will resist a selected internal operating pressure with sufficient margins.

Hydrate-generating hydrocarbons , e.g. a natural gas containing 90 % i5 methane, 4% ethane, 2% propane and a residue comprising heavier hydrocarbons and other gaseous components (N₂, CO_2> and similar), is supplied through a pipe 207 to the upper , gas-filled part 211 of the container 202. Apart from the gas being supplied through the pipe 207 having to possess a pressure in accordance with the selected operating pressure, no fixed conditions apply to the

20 properties of the gas to require particular pre-treatment processes.

Water is supplied to the gas volume 211 in the upper part of the reactor 202 through a pipe 205 and is sprayed into the gas volume through at least one nozzle 206. The water is taken from an available source, e.g. a cold freshwater source (not shown), and must when fed to the reactor 202 through a nozzle 206 have a

25 temperature T=T, which is below the equilibrium temperature for the generation/decomposition of gas hydrate at the current operating pressure. The relation between the temperature of the hydrate-equilibrium and required gas pressure will be known to a person skilled in the art from the literature, ref. e.g. Sloan, E.D.Jr., "Clathrate hydrates of natural gases", Marcel Dekker, Inc., New

30 York 1990. Notice is also drawn to the conditions in the introduction.

When the operating pressure is set to 60 bar,l a temperature T=T, at plus 10-12°C will be sufficiently low for the generation of hydrate in the reactor container 202. However, is it evident that the temperature T, beneficially can be considerably lower, e.g. close to 0°C. If the first cooling liquid is water, this temperature should as mentioned not be below the freezing temperature of water. Provided that the temperature in the gas phase 211 in the upper part of the reactor container 202 is kept at least 2-3°C below the temperature of the hydrate equilibrium at the current operating pressure with sufficient supply of amount cold water as cooling medium, gas hydrate is generated as a slurry of gas hydrate particles in water. This material will immediately after generation possess a consistence and an appearance as slush and will contain large amounts of unconverted water.

Generated gas hydrate and unconverted water will be collected in the bottom part of the reactor container 202. Gas hydrate is similar to ice somewhat lighter than water, and the slurry of gas hydrate and water will to a certain degree separate in an upper fraction containing essential all the gas hydrate as a aqueous slurry of gas hydrate, and a bottom fraction consisting of unconverted water and residues of gas hydrate particles. The interface between those two fractions can however be diffuse or non-existing if the liquefied phase includes relatively large amounts of gas hydrate particles and if there is a lot of motion and turbulence in the material. During the hydrate-generation unconverted water is drained off at a temperature T=T₂ ( which is a sligthly higher than the generating temperature T=T,) as of the bottom of the reactor container 202 through the pipe 213. If required water can also drained from the system through a pipe 219 connected to the pipe 213. Water be recycled to the hydrate-generating zone is passed through a pump 214 and a heat exchanger 217 and returned to the water inlet 205 via the intermediate pipes 216 and 218.

The heat exchanger 217 can be cooled by a suitable external cooling medium. If large amounts of water at low temperature, e.g. 5 °C or below, are available, is used as a cooling medium. Frequently it will however be more relevant to use cooling mediums such as propane, ammonia or other mediums for the cooling of recycled water, as such media having a normal boiling temperature substantially below 0 °C contribute to larger temperature differences and thereby more compact heat exchangers 217.

The water used during the preparation of the gas hydrate in the hydrate- generation, must to the required degree be substituted by the addition of further s amounts of water.

After the desired amounts of gas hydrate have been generated in the reactor container 202, the process stage a is fulfilled and the water supply is thereby stopped, e.g. by a valve which is not shown, and unconverted water is separated at stage b from the hydrate mass e.g. by drainage. If necessary a filter o (not shown) may be installed above the outlet at the bottom of the reactor to avoid loss of gas hydrate.

Significant amounts of water will still after such a simple drainage will still be connected to the hydrate mass as a water film outside the hydrate particles and between the hydrate particles because of capillary effects. These residues of s water can as mentioned in the general part of the specification be eliminated in several known ways. E.g. further amounts of hydrate-generating gas and cooling amounts of hydrocarbon may be passed through the hydrate mass thereby converting the residual amounts of water to gas hydrate. This take place at process stage b, but still in the same container. 0 After most of the free, non-converted water is removed, the hydrate mass is supplied, which is still present in the reactor container 202, during process stage c, a second hydrocarbonous cooling medium through an inlet 225. There may be expressed as if the product is now in a cooling zone 280, although the product has not left the container 202. As described elsewhere, however, the cooling zone 280 5 can possibly be in another container. The second cooling medium is supplied to the reactor container 202 during the process stage c in such amounts and at such a temperature that the composition of the hydrate and the hydrocarbon obtain the intended end temperature T=T₄ , whereby the gas hydrate is stable or meta stable at atmospheric pressure, that is in general when T=T₄ = -10°C or below. During o process stage d the stabilising end product is transferred to a storage container 251. Simple estimates with basis in specific heating capacities of the hydrocarbon medium and the gas hydrate, will provide indications of the required amounts of cold hydrocarbon cooling medium at a given amount of gas hydrate mass, at a given starting temperature T=T₅ in the gas hydrate mass, the temperature T₃ in the supplied, second cooling medium and the end-temperature T=T₄.

The second hydrocarbon medium is preferably a composition of light, liquefied hydrocarbons, particularly a so-called condensate fraction. The medium should preferably not contain components which can be precipitated as a wax or solid or viscous materials at the cooling surfaces in the plant. Simultaneous the hydrocarbon liquid which is used as the second medium, as thourougiy discussed in the general part of the specification, should contain the least possible of amounts of hydrate-generating components.

Heated cooling medium, that is the second cooling medium which has been flowing through the gas hydrate mass and thus been contributing to the cooling of the gas hydrate, is drawn from the container at the temperature T=T₅, recycled through a secong cooling loop, which e.g. comprises a pump 221 , a heat exchanger 224 and the required circulation pipes 220, 223 and 225. The heat exchanger 224 is feed with a suitable cooling medium such as ammonia, propane, compositions of light hydrocarbons or freon. The feeding of the supplemental amounts of the second hydrocarbonous cooling medium as a replacement for the amount of hydrocarbon liquid which gets included in the end-product, can be made through a pipe 222 connected with the cooling circuit.

After the intended end-temperature T₄ is reached in the gas hydrate mass in the container 202, the end product, being as gas hydrate particles in hydrocarbon liquid is tapped, through the pipe 208 and the valve 209 preferably to a storage container 251. The end-product can theoretically be stored in the same container 202, but a separate storage container 251 is preferred to release the generation container 202 for new production. To reduce the heat flow into the storage container 251 , the container may be heat insulated with a suitable material 257. The temperature of the stored gas hydrate mass can be adjusted at the tapping and circulation of the hydrocarbon liquid through a separate cooling loop (not shown) connected to the container 251 via the pipes 252 and 253. The storage container 251 is equipped with a outlet 264 for transferring of the hydrocarbon product or the end-product (gas hydrate mass in hydrocarbon liquid) to other transportation-, storage- or processing units. Prior to the transfer of the product from the reactor container 202, can redundant amounts of hydrocarbon liquid may be drained from the gas hydrate mass.

The end-product will as earlier mentioned be constituted by particles of gas hydrate surrounded by or suspended in hydrocarbon containing liquid at the temperature T₄. The size and shape of the particles will vary and will be set from process conditions and any post-treatment of the gas hydrate mass. Particle sizes from fractions of a millimetre to several centimetres are within the scope of the invention.

The agitation means 231 , 232 respectively 255, 256 may be installed in the hydrate-generating zone of the cooling zone 280 and / or in the storage zone 250. Such agitation means may be desirable to obtain sufficient pulverising of the material and good thermal exchange between the components at the different stages in the process. Agitation in the storage phase can further reduce of the sintering of the end product. Instead of feeding the gas through the pipe 207 to the upper part of the reactor container 202, alternatively the gas may be fad at the bottom of the container through the pipe 261. In such a way of supplying the gas it may be bubbled through a composition of solid and liquefied material at the bottom part of the reactor 202. This will contribute to keeping the liquefied phases at a high concentration of the hydrate-generating components of the gas phase and thereby contribute to strong hydrate-generation in the liquid phase during stage a and perhaps stage b of the process. Non-converted gas or gas of which is depleted with respect to the hydrate-generating components, can by this embodiment of the plant be taken out as a gas flow through an outlet 262 at the top of the reactor container 202. Feed of gas both at the top and the bottom in the container 202 can also be combined. A further variant of the above mentioned embodiment comprises water totally or partly replaced by a hydrocarbon medium already as the first cooling medium. This can take place by dimensioning the cooling circle of hydrocarbon liquid connected to the reactor container 202 and which according to fig. 2 consist of the circulating pump 221 and the heat exchanger 224, to cover the cooling requirement at stage a by circulation of a hydrocarbon medium instead of water. If a substantial part of the hydrate-generation shall take place in the gas-filled volume 211 in the reactor container 202, it is necessary that the hydrocarbon containing cooling medium at least partly is supplied to this gas volume, preferably o as drops ( shower or spray) , through an alternative feeding line 225' (indicated with a dash line in fig. 2).

In fig. 3 is another embodiment shown which is mainly distinguished from the one shown in fig.2 by non-converted water not being recycled during the generating stage a, but is just passed through the plant once (once through ). The s gas is supplied as previously via pipe 307. Cold water, preferably cold sea water, is supplied to the reactor container 302 through the pipes 305 and the nozzles 306, both as starting material for the hydrate-generation and as the first cooling medium. During the stage a and b will non-converted water now be accumulated at the bottom of the reactor container 302. The non-converted water is tapped out o through the pipe 319. Dissolved gas which may appear in the tapped water, can if necessary be eliminated by means of a hydrocyclone 341 or a similar liquid/gas separator. However, in many embodiments it is possible to reduce the pressure sufficiently to eliminate residues of dissolved gas from the water which can be taken care of in a suitable manner without using other equipment than a simple 5 gas/liquid separator.

The amount of cold water supplied through the pipe 305 and the nozzles 306 can be monitored, e.g. by valve gear, to release the whole amount of heat released by the hydrate generation through remitasion from the reactor container 302, as heated, non-converted water through the outlet 319. Thereby the 0 requirements for further cooling is reduces or disappear. An increase of the cooling effect thus occur by simply increasing the cooling water feed via the pipe 305. The reactor or the hydrate generator 302 shall during operating be under an high pressure average (50-80 bar a). Although substantially larger amounts of water must be pumped through the reactor against this pressure, this does not require a corresponding increased pumping power requirement. It can in a simple manner be arranged a pressure-lock-arrangement wherein outgoing liquid flow at high pressure is locked against the inlet liquid flow at low pressure. Preferably only water should be used during the generation of gas hydrate in the reactor which will require the use of external pump power.

The central outlet 343 of the hydrocyclone 341 will contain hydrocarbons in gas or liquid form which can be recompressed, whereup they are recircled to the process loop or used as a fuel for driving gear for pumps, compressors and similar in the plant, e.g. by using suitable combustion engines.

The hydrate mass can suitably be cooled to a temperature of at least 15°C, typically 20-30°C below the temperature when carrying out the stages a and b. This means that the hydrate generator 302 must resist a pressure of at least 60 bar, and that the pressure requirements of the storage container 351 are much lower. Therefore it can be preferable to perform the stages c and d in another container than the reactor container which has been used at the stages a and b. A process plant having separate cooling in a separate container 481 is shown in fig. 4 where the reference number 480 consistantly assign the cooling zone for the performance of step c. The cooling container itself 481 is preferably surrounded by a layer of heat insulating material 482. Fig. 4 in addition indicates that during the drainage of liquid at stage b it may happen that the liquid tapped off at the bottom of the reactor container 402 through the pipe 475, will consist of a composition of hydrocarbon containing liquid medium and water. This composition can be separated in a separator 478. After the accomplishing of stage b in the reactor container 402, the hydrate mass is transferred to the container 481. The fluid-connection through a pipe 485 connecting the gas volumes 411 and 486, respectively in the upper parts of the containers 402 and 481 , will ensure pressure- compensation, and thereby to a unobstructed transfer of mass in and out of the container 481 , when the valve 409 is open. The hydrate mass in the container 481 is cooled, such as described at stage a, bydirect cooling recycling a hydrocarbon containing second cooling medium through a loop which comprise herein a heat exchanger 487.

After the cooling of the hydrate mass to the desired temperature, which preferably is below -10°C, the mass is transferred to the storage container 451 , a part of which is indicated at the bottom of the figure.

In relation to the examples of embodiment of a plant described above for accomplishing a method according to the present invention, the following modifications shall be mentioned: After drainage of water at stage b the hydrate mass, which still may contain small amounts of free water may, be exposed to an futher, hydrate-generating stage wherein the free water is contacted with the hydrate-generating gas components such as methane, ethane and propane. This may e.g. take place by feeding such gas components through a pipe 461 (fig.4) at the bottom of the reactor container 482. In this manner additional drying of the hydrate mass will be obtained resulting in a hydrate mass only containing gas hydrate without or having insignificant amounts of free water. Large amounts of free water in the hydrate mass will as mentioned previously, involve problems when cooling the hydrate mass at stage c in the process, the free water will freeze into ice and form bridges of ice between and at the surface of the gas hydrate particles. As also previously mentioned, small amounts of free water can be tolerated.

At some embodiments it is supposed to be an advantage when the second cooling medium do not containing hydrate-generating components or at least that some of these components are absent at this stage in the process, since such components may result in reduced stability of the end-product. In such instances it is recommended that the content of volatile components in the hydrocarbon medium are kept at a level which involve that the vapour pressure of the hydrocarbon medium at the storage temperature is below the ambient pressure. This can e.g. be achieved, at least in the end phase of stage c, by using a hydrocarbons medium which substantially only contain hydrocarbon which comprising at least five carbon atoms, as the second cooling medium.

The hydrate mass obtained after the stages b, c or d, may be subjected to a drainage or compressing stage wherein redundant humidity is eliminated or squeeze out. Preferably is the end product suspension having about 80 percent by volume of hydrate and about 20 percent by volume of hydrocarbonous liquid, roughly speaking identical with the second cooling liquid, but possibly having small amounts of free water in the frozen form and the residues of the first cooling liquid if this had a composition different from the second cooling liquid.

A more detailed description of a current plant for accomplishment of the hydrate process, which is explained according to the principle drawings above, is given in the following example which is referring to fig. 5. The capacity of the plant is also indicated for a lot of the current parameters. It is supposed to use three parallel reactor containers, at fig. 5 denoted as

502 A, B, C; and where only 502A is shown in detail, which in 502B and 502C are only the connection points shown in the total plant. All of the reactor containers 502A, 502B and 502C will during operation be at different stages of the production process, such as transferring the produced hydrate in sequences to the cooling container 581 , which can be common to all the reactor containers. How many reactor containers 502 which can be connected to a common cooling container, depends i.a. of how much time the different process stages use. The figure shows the situation at the end of the process stage a, and it is referred to text at the figures to get an understanding of the positions of the different valves and fluid flows which are operated at this stage.

Following is a short description of the reactor container 502A with the connected plant forward to where the end product is transferred to the storage container 551 (at the bottom, to the right in fig.5).

The hydrate-generating process is based on the use of sea water both as hydrate water and cooling water in the reactor after the "one through " principle which, as the name indicate, use one simple through-put of the water which shall be included in the hydrate. This imply that sea water feed flows via the pump 100 and the water inlet 505, through the hydrate-generating reactor 502Adivided in two parts, and is directly discharged into the sea ( after a simple treatment in a hydrocyclone plant 541), The sea water feed at 8 °C is pumped into the reactor system by means of a sea water pump 100. The reactor 502A operates at a pressure of 60 bar a. In the reactor chamber is sea water spread in a regular manner beyond the total volume by means of nozzles 506 installed in the ceiling and/or at the walls of the cylinder. The hydrate-generation takes place when sea water is contacted with the natural gas feed which has arrived through the pipe 507. At the bottom of the reactor 502 is the temperature 13 °C (equilibrium temperature ). The amounts of natural gas supplied to the reactor system can be e.g. 700 000 Sm³/d (standard cubic meter per. 24 hour). The reactor container 502A is a "semi-batch"-unit where the generation of hydrate product takes place continuously, while the tapping of the product takes place in portions at which the hydrate product once are in a while emptified into a collecting container 502', placed under the reactor 502A.

As previously mentioned consists the reactor system of three parallel reactors, 502A/B/C, which may have their own, or as shown on figure 5, a common collecting container, 581. The units are controlled sequentially, that is that they are operated in cycles where each cycle consist of three sequences or intervals. In the first interval is the reactor 502A drained off for the hydrate product and sea water by opening the valve between the reactor and the collecting container 502A' and closing the outlet line for sea water at the bottom of the reactor. When the reactor 502A is drained, is one valve between the reactor 502A and the collecting container 502A' closed. After this is as much as possible of the sea water squeezed, which have been following the hydrate mass, out from the collecting container 502A', e.g. by means of supplied gas under pressure . The "dry" hydrate mass is assumed to contain a degree of packing at 130 Sm³ ga/sm³ hydrate.

When the sea water have been squeezed out, is the second interval starting where the hydrate product in the reactor container 502A' is being flushed with condensate from the cooling container 581 , by the means of a condensate pump 501. A slurry-product from the hydrate og condensate which is easier to handle are obtained. In the third and last interval is this hydrate slurry drifted out of a collecting container 502A' and to the cooling container 581 , where the hydrate- slurry is cooled to -20°C. The driving forces at this operation is the large pressure difference between the collecting container 502A' (60 bar) and the cooling container 581 (15bar). Each interval is put to 4 minutes, the total cycle time being 12 minutes. The three reactor units A,B,C are sequential monitored by a controlling system which is not shown at the figure, in such a manner that they at any time are operating at different intervals. In this way can adjacent, common process equipment as the cooling tank 581 , condensate pump 501 operates continuously towards that reactor 502A,b or c which is connected at any time. During each interval is it necessary with pressure equalising between the reactor 502A and the collecting container 551. This is done by the mean of a open pressure equalisation (not shown) between those to containers. The cooling of the hydrate product takes place secondary in the collecting container 581 where the hydrate slurry is cooled during the flushing in of cold condensate (-20°C). Since the hydrate slurry from the collecting container 502A' is partly cooled , may the cooling container 551 be operated at 15 bar without getting any problems concerning dissociation of the hydrate product. The total cooling is operated by a cold condensate cycle 587 connected to the cooling container, where filtered condensate from the cooling container at -20 °C, is cooled to -30 C in a circulation cooler 587 for the condensate, and is returned to the cooling container 581. In the circulation cooler 587 is the condensate cooled by evaporation of propane by a cooling circuit compressor and a propane condenser 579 (sea water based). The cooled slurry product from the cooling container 581 is feed to a hydrate/condensate-separator 511 , where the product is separated as a "hydrate- pasta" (20 percent by volume of condensate + 80 percent by volume hydrate) and is stored at atmospheric pressure. Separated condensate is returned to the cooling container. Make-up condensate is added the cooling container 581 to cover the demand for condensate which follows the hydrate product ("pasta"- product).

Excess of sea water from the reactors 502A,502B, 502C are first treated in a treatment plant consisting of flush containers and hydrocyclone-batteries 541 , which respectively degas and removes oil/condensate droplets from sea water before this is exhausted into the sea.

Below follows an arrangement showing capacities, power requirement, pressure and temperature at some important places in the plant: Seawater inlet (505) 3495 m³/t

Sea water pump (500)

9015 kW pressure difference65 bar gas inlet (at 507)

700.0 00 Sm³/d hydrate reactor (502A)

60 bar, 13 °C outlet for sea water (from 502A) 1098 m³/t slurryvalve (from 502') 673 m³/t, 0°C,15 bar cooling container (581)

15 bar circulation pump (for 587) 585 kW condensate cooler (587) 11465 kW, -20 °C to -30° C

It appear from the text at fig. 5 how the different sequence in coarse features are monitored.

The method described above can be changed in a lot of way within the frame of the claims.

In the following is some conditions summarised which can be important by the exercise of the present invention.

To obtain a stable end product, as such this expression is defined, must the hydrocarbon medium in the end product have a low content of volatile hydrocarbon components. This can be obtained in to different ways: 1) To replace a the hydrocarbon medium (which is used as the second cooling medium), which contain a lot of volatile components with a cold hydrocarbon medium, which possess a low content of such components. 2) After the pressure equalising, that is after the pressure is close to the ambient pressure by eliminating those volatile components, which is released from the hydrocarbon medium (the second medium) as gas, if the hydrocarbon containing medium at the end of stage c still contain considerable amounts of volatile components. The stabilisation can of course also be a combination of these drafts. After finishing stage c, the product will still exist at high pressure (about similar to the pressure at stage a) in the cooling zone (580). Normally will the end pressure therefor be below the ambient pressure after being taken out of the cooling zone. The depressurizing may occur while the hydrate product still is in the cooling zone 580, or simultaneous as the hydrate product is taken out of the cooling zone. Remaining amounts of volatile (destabilishing) components in the hydrocarbon medium will in both instances be released as gas. Released gas is taken away, if possible for decompressing and in return to earlier stages in the process.

If the depressurizing take place at the same time as the hydrate product is taken out of the cooling zone (580), will the definitive stabilisation (elimination of remaining amounts of volatile components) of the product happen in the storage container 551 , so that the end product first will be available after such a stabilisation in the storage container.

Concerning the end product itself, can this be in pasta- or slurry-form, and the size can vary within large areas so that the hydrate will be as large pieces or clogs with proportions up to several centimetre's in a liquefied hydrocarbon medium. Of course can it be advantageous with hydrate particles of strongly varying dimension in the same end product, as small hydrate particles will fill up the spaces between larger particles without any worth mentioning reduction of the gas content.

The storage containers must obviously be dimensioned to resist a certain excess pressure. If the ambient pressure is 1 bar, this do not involve that the end- pressure necessarily also shall be 1 bar. With a excess pressure of 0,5 bar will e.g. the end pressure in the end product be about 1 ,5 bar.

Given that the second cooling medium shall have a vapour pressure at the end temperature is below the end pressure, is it allowed that the cooling medium can contain a certain amounts of volatile hydrocarbon such as iso-butane and propane, without effecting the stability demand. The assumption is however that the total partial pressure of the individual components in the cooling composition is below the end pressure as given concerning Henry's law in the specification. If the used method is such as that water leading to the hydrate-generating zone is so strongly cooled that is contains ice or snow, must the hydrate convertion and the temperature monitoring which take place in the process stage a continue until all the ice and snow is generated in to hydrate and melting water. It is also advantageous that the process conditions for stage a) is adjusted thereby obtaining a end product where the solid, hydrate containing material possess a gas content which correspond a degree of packing at least 130 Sm³/m³, preferable at more than 150 Sm³/m³ solid matter, when methane is used as hydrate- generating hydrocarbon. It must also be defined precisely that the hydrate-generating pressure- and temperature conditions at process stage C must be maintained until the hydrate mass has reached a temperature where tendency for decomposition of generated hydrate can be ignored for practical reasons. If the cooling take place rapidly, will this temperature be reached immediately after the freezing temperature of water is passed.

At last it shall be defined that end pressure or storage pressure normally is determined in advance from construction demands of the containers and compounds. The end pressure is a nominal pressure which is decided from the construction of the plant. The terminal plant which is used for storage and degassing of the stored product are further on described at the following figures where; fig. 6 shows a quite simple plant for storage and decomposition of large amounts of hydrate in the same container, fig. 7 shows a terminal plant according to the present invention. Fig. 8 shows a detail of a plant according to the present invention.

To get a better overview is the figures only showing the components which are necessary for the understanding of the present invention. It shall also be mentioned that the same reference numerals are used at all the immediate figures mentioned above as long as this has been found suitable, and that the different figures for part of these are not shown in the same scale, but are only provided to give a principle explanation and understanding of the invention. As far known has large terminal plants for degassing of hydrate never been built. But generally it can be said that decomposition or dehydratisation of the gas hydrate shall take place in the same container, as the storage container, as previously has been considered as the most natural method if it is desirable to use the principle commercial, had this combined storage- and decomposition container T as indicated in fig. 6, to be constructed to resist the maximum pressure of degassing, e.g. until 100 bar, and all of the cargo of gas hydrate had to be pumped during loading in the combined pressure- and storage container, e.g. from a transportation craft 602 which lie at quay, against this high pressure, as will demand pumps 603 with a large capacity and a lot of excess power. The transportation would then have been through the pipe 604. The previously most known or suggested techniques are indicated in fig. 6, where it is also indicated that the hydrate can be sprinkled of relatively hot water which is being supplied to the container through nozzles in the upper part of the container T. This is however not a part of the present invention. This previously known solution is leading to the disadvantages mentioned above, so that the hydrate has to be pumped into the container against a nominal pressure of degassing, and that the container has to be constructed resisting this high pressure and with those security problems and costs this necessarily may bring. The hydrate can be as a suspension such as previously explained, or in another form. In the following it is supposed that the hydrate is a suspension. Another natural way of accomplishing the decomposition would have been to decompose the gas hydrate in a storage container which is kept about atmospheric pressure (not shown). For this reason the gas had to be in the most object comprimated to a suitable degassing pressure before use, and this would have demanded supplying of power in one or another form. The total power consumption for the process would with both these methods get large. There are several important pressures, that is:

- Storage pressure, which can be low, readily near by 1 bar.

- The operating pressure, which is the pressure the gas shall be used at and which can be completely in the direction of 100 bar, but preferably about 60 bar. - The degassing pressure, which is the power in the zone where the hydrate is heated until decomposition occur.

- Equilibrium pressure, which depends of the temperature and of the composition of the hydrates. The equilibrium pressure is the pressure where the currently gas hydrates dissossates at the current temperature. Equilibrium pressure is also depending of the decomposition temperature and the relation between these is often represented by a equilibrium curve for generation/decomposition of the gas hydrate in the form of a pressure/temperature diagram. (Analogue with a pressure-depending equilibrium curve for phase transfer from the liquid phase to solid phase for freezing liquids. At fig. 7 it is shown a principle drawing of a terminal plant according to the present invention.

The terminal plant shown at fig. 7 is built up in the following way;

The core in the plant are the storage container 701 which is arranged to be filled with the hydrocarbon-containing product preferably in the form as a gas hydrate-containing suspension via the pump 714. The storage container 701 does not have to be dimensioned for especially high pressure. If it is suitable, serveral storage containers can be present.

Another important component in the plant is a substantial miner pressure container or decomposition container 740. The decomposition container has to be dimensioned to stand against the degassing pressure of the gas as this is released from the hydrate suspension. Since the decomposition container 740 is substantially minor than the storage container 701 , will be costs by dimensioning the decomposition container to the degassing pressure become substantially minor than the costs would have been if the storage container 701 should have been dimensioned for the degassing pressure. If it is suitable, there can also be several of such decomposition containers.

If several storage containers 701 and/or several decomposition containers 740, the plant must of course contain additional connection pipes, pumps and valves. It is provided that the shown valves and pumps to the degree of demand are provided with the necessary regulating cycles 780 which is not shown in detail. At fig. 7 it is in addition shown a storage container 701 which can be provided with a temperature regulating loop 706 comprising a pump 711 and a cooling cycle 712 connected with a liquefied connection 713. It shall be noted that all liquefied connections at the figures are shown in a simplified form as simple, solid drawn lines.

The decomposition container 740 is in a similar way provided with a temperature regulating cycle 741 which, in the shown performance, comprising a heat exchanger 743 connected with the content in the decomposition container together with a heat exchanger 744 placed outside the decomposition container. The heating of the content in the decomposition tank can however also occur via a direct injection of a warm liquid, e.g. water.

This performance of the terminal plant as shown at fig. 7, comprises in addition the following relatively minor components: a pump 714 arranged for pumping in a hydrate mass to the storage container 701 ; a transportation 730, which in a simple performance of the invention constitutes of a pump, which is being able to pressurise and transport the material in the storage container to the decomposition container, but preferably it will also contain a pressure equalising arrangement such as a pressure lock, primarily arranged for the leading of hydrate mass from the storage container 701 to the decomposition container 740; to the compression units 708, 709 connected with a connection 710 which lead access gas out from the storage container 701 ; in a gas conditioning unit 750 arranged for after treatment of gas which is being released from the gas hydrate prior to sending this gas out, e.g. to consumption via the gas tapping 751 ; a separation containe 761 for treatment of liquid or the suspension which emptyfyes out from the decomposition container 740; also a hydrocyclon 762 for after treatment of liquid which date from the separation container 761. When the hydrocarbon material is a suspension of hydrate particles in a carrier liquied, the tank 761 is a separator for separation of water and hydrocarbon liquids (a carrier medium for gas hydrate). The upper outlet 763 is for the hydrocarbon medium, the outlet 764 at the bottom is for water.

The separation container 761 and the hydrocyclon 762 with the outlets 766 and 767, are included in a liquid treatment unit 760. Inside the storage container 701 it is preferably arranged a paddle mechanism 715 driven by a motor 716, it is not shown in the figure how this motor is supplied with energy. At last it is shown at the figure a gas outlet from the storage container 701 to a burner flame at 707, contemporary as the necessary liquid, and gas-communicating connections, such as 717 and 719 which respectively lead from bottom and top of the storage container 701 to the transportation application 730, and 718 which lead from the transportation application 730 to the decomposition container 740, which is indicated with solid drawn lines as already mentioned.

It is supposed for simplicity that the hydrocarbon material is a suspension as previously mentioned. As long as the suspension from the gas hydrate in a carrier liquid, e.g. in a condensate, is kept cool at a temperature from -25 to -35°C, will the gas hydrate suspension be stable, or at least meta-stable, all the way to 1 bar. Instead of pumping the gas hydrate in a combined high pressure storage and decomposition container T as shown in fig. 6, it is suggested at the present invention that the hydrocarbon product containing or constituting of gas hydrate, is loaded from transportation containers e.g. on board the transportation crafts 602 and in a storage container 701 which only being used for storage and which only is dimensioned to bear a low storage pressure. The gas hydrate in the storage container is kept, by means of a cooling cycle 706 and the heat exchanger 712, at or under a temperature

in which equation the earlier defined definitions are applied, and where the gas hydrate is stable, which is indicated in fig. 6. The storage container 701 should be surrounded by heat insulating material 705 and if necessary also be provided with a cooling cycle 706 for a suitable cooling medium as condensate or similar. The container 701 is only required to be dimensioned for bearing the hydrostatic pressure of the gas hydrate suspension with a full container at storage temperature and a certain gas gauge pressure at e.g. 0,5 bar. The storage container 701 can preferably have a volume at 20,000-25,000 m³, while de decomposition container 740 can be much minor and e.g. with advantage have a volume of only a few ten or by hundreds m³.

The simplest method for converting gas hydrate into gas at a degassing pressure at e.g. 60 bar, is considered as follows: The gas hydrate-suspension is pumped by the means of a transportation application 730, from the storage container 701 to a minor decomposition container 740 which is equipped with a heat exchanger system 741. At fig. 7 it is indicated in direct heating, but direct heating can be similar useful. As a heat exchanger medium one can use available water sources with suitable temperatures, provided that the temperature is a bit above the hydration temperature at the degassing pressure. The cooling water from a thermal power station will obviously be a current source.

The dimensions at the decomposition container 740 will mainly be decided by the claims to gas delivery capacity and the temperature at the heat exchanging medium. The decomposition container 740 is however supposed, as earlier mentioned, to be substantially minor than the storage container 701.

Heat which is applied to the decomposition container 740 via the heat exchanger system 741 cause a decomposition of the gas hydrate suspension.

Released gas leaves the decomposition container through the pipe 742 and is possibly transportet through a gas conditioning plant 750 previous to leading the gas to a pip line net or directly to a consumer, e.g. a gas power plant, via the outlet 751.

Liquid which can comprise water and condensate or light oil, generated at gas hydrate decomposition in a decomposition container 740 is lead to a liquid treatment plant 760 which e.g. comprises a separation container 761 and a hydrocyclon 762. When the decomposition of the gas hydrate mass is accomplished by indirect heat exchange such as indicated in fig. 7, avoiding to have to clean the heat exchanger medium. This will possibly be necessary if the decomposition is executed by direct heat exchanging, i.e. by supply of a heat exchanger medium which is in direct physical contact with the gas hydrate mass. When the hydrate mass is going to be dissociated in a dissociating container 740, contains liquefied hydrocarbons as carrier medium, the liquefied hydrocarbons are separated after the decomposition of the gas hydrate in the decomposition container 761 , as the light (upper) liquid phase in the container (at 763), while the heavier (bottom) water phase is taken out of the container through the bottom outlet 764 and is treated further on in a hydrocyclon 762 where a purified water phase is tapped through the point outlet 767 and residuum of the liquefied hydrocarbons is tapped off in the top outlet 766.

The liquid will at the output of the decomposition container 740 possess a pressure equal to the degassing pressure for the gas which also correspond to the equilibrium pressure at the degassing pressure. The liquid can thus be used as replacement medium in a lock for inlocking of gas hydrate suspension from the storage container 701 to pressure or dissociation container 740, such as mentioned below, particularly concerning the figure 8.

The temperature in the outgoing liquid from the decomposition container 740 nearly corresponding to the equilibrium temperature for the hydrate generation/dissociation at the equilibrium pressure. Depending of the temperature can the liquid out of the container 740 also be used to different cooling objects, e.g. for cooling of the external coating 705 in the storage container 701 in the area where such cooling will be desirable. Other possibilities are that the liquid can be used as heat drain in a possibly cooling cycle 706 for condensate or light oil in connection to the storage container 701. Cooling of air for management of turbins in a heat power plant is another arrangement area and the cooling medium in the heat power plant a further application area.

Gas being at low pressure in the upper part of the storage container 701 , should, of security reasons, be directed to a torch via the outlet 707 for burning off as already mentioned. Small amounts of gas which possibly will be released in the storage container 701 , can also be compressed, e.g. into two compressing stages 708 and 709 for management of a gas turbin or similar (in the connection 710), and be used as a grant to the main gas flow out from the plant at the main outlet 751. All the units in the plant with exception of the storage container 701 can be small and relatively cheap. Some of the indicated units necessarily in every executions, this concerns e.g. the compressors 708, 709 for compression of gas from the storage container 701.

An example of an arrangement which can enter into a transportation arrangement 730, as e.g. comprising both a pump and lock arrangement, which is shown in fig. 7.

The two main units in the transportation arrangement according to the performance in fig. 8, is the lock chamber 870 and the decomposition container 840. Both of these are dimensioned to bear the pressure of the gas after and under degassing. Also in fig. 8 is fluid leading pipe connections in a simple way indicated as solid drawn lines with arrows which assign the flow direction for the fluid which is being transported via the pipe. These pipe connections, of which some are inlet and/or outlet from the current containers, are given the reference numerals 817- 823. It is used square symbols for pumps and round symbols for walls connected in the transportation lines as shown. The figures comprise in this manner the pumps 873, 874, 875, 876 and valves V_{1 (} V₂, V₃, V₄, V₅, V₆, V₇ and V₈. An inlet 847 is also shown for dispersion of water in the decomposition container 840, when direct heating of a hydrate is used.

This arrangement are meant for combined inlocking of hydrate gas suspensjon and varm decomposition liquid to the pressure or decomposition container 840. It can be used direct heating of the gas hydrate mass in the decomposition container, as shown in fig. 8, and/or in direct heating, as shown in fig. 7.

Decomposition of hydrate by supplying of heat, e.g. at influshing of water at 5 a temperature above the decomposition temperature at a given gas pressure in the container 840, can take place in a continuous without large surge in the container 840. This is one of the advantages of the invention.

In fig. 8 it is indicated a locking container 870 and a decomposition container 840 of approximately the same size. However, in practice it will be o expected to be advantages to use lock container 870 which is considerably minor than a decomposition container 840. As an example can a lock container 870 container volume which is less than 20 percent by volume to the decomposition container 840.

A person skilled in the art will be able to calculate the relative dimensions of the different parts of the plant, such as the relative sizes of the locking container 870 and the decomposition container 840. Likewise a person skilled in the art can calculate the operating parameters for the operating cycle to the lock container 870, so that the pressure variations as a consequence of opening and locking of the valve V₃ shall be in given maximums limited when such values as volume and temperature/ pressure are decided. The operating method for the lock-arrangement appears besides from the following operating explanation; output position: the valves V, and V₂ are open, the other are closed. The hydrate mass is pumped into the lock chamber 870 from the storage container 701 through the connection 817 by the means of the pumps 873. V₂ being open, ensures the connection 819 with the gas volume in the storage container 701 in such a way that the transferring of hydrate mass can be done without construction of gas pressure at compressing in the lock chamber 870.

When the lock chamber 870 is filled up, are V., and V₂ closed. V₄ keeping closed while V₃ is opened. This lead to pressurisation of the hydrate mass in the lock chamber 870 to a existing pressure in pressure or decomposition container 840. After the pressure equalisation the V₄ is opened and the hydrate mass is pumped by the pump 874 to the decomposition container 840. The connection 818 thereby draining the lock chamber 870 of hydrate.

When the lock chamber 870 is empty, V₄ is closed and V₆ is opened. The liquid components (water and liquid medium generated at decomposition of gas hydrate) is pumped by the pump 875 into the lock chamber 870 via the connection 871 from the decomposition container 840. Filling of the lock chamber 870 with these liquid components dispel gas from the lock chamber 870 back to the decomposition container 840 through the connection 820 and the open valve V₃. When the lock chamber 870 is filled up with liquid components from the decomposition container 840, are V₃ and V₆ closed. V₂ and V₅ are opened. The locked chamber 870 is thereby drained of liquid components through the connection 822 and the valve V₅.

The cycle is repeated after the lock chamber 870 is drained and the valve

V₅ is closed. For further to explain the principle of the manner can the following conditions particularly be mentioned.

The pressure in the lock container 870 changes as the pressure, during filling of hydrate, is similar to the storage pressure in the storage container 801 , while the pressure in the lock chamber 870 at emptifying of decomposition liquid and by inserting decomposition liquid, is similar to the degassing pressure in the decomposition container 840.

Further is the lock chamber 870 operating as a batch and working as a pressure lock, in that way so that the pump never need to work against a large counter pressure. If the lock container 870 is small compared to the decomposition container 840, will the batchwise drift of the lock container 870 not have perceptible consequences for the gassing pressure in the container 840.

Another performance can be to have several decomposition container 840 in parallel, and/or several lock container 870 in parallel. Such parallel containers

(840 and/or 870) can if possibly be regulated in such a way as being in different working phases, by the means of controlling the necessary valves and pumps.

Such modifications will be understood by a person skilled in the art, and will not be shown or explained in more details.

The low pressure zone 701 can be a storage container, as described earlier, eventually can the low pressure zone constituted of a loading rule at the transportation craft 602 for the gas hydrate. Further on can the high pressure zone

840 be constituted of a degassing container, as described earlier. If possible the high pressure zone 840 can be constituted of a cavity/one cavern fitted in a mountain formation. Such a cavity/cavern in the mountain can possess any practical dimensions. Because of security, the process below will be described thorough as it is supposed to be separated into the following stages:

Step I: Filling of the lock chamber When hydrate being pumped into the lock chamber 870, the valves V., and V₂ are open. The hydrate mass is being pumped in through the connection 817 while the valve \Λ, is open, the pressure is kept similar to the storage pressure because of the pressure equalisation through the connection 819 and the opened valve V₂ between the storage container 701 and lock container 870. Step I: Filling of lock chamber

When the hydrate is being pumped into the lock chamber 870 the valves V₁ and V₂ are open. The hydrate mass is pumped in through the pipes 817 while the valve NT, is open, the pressure is kept equal to the storage pressure for the reason of pressure equalising through the connection 819 and the open valve V₂ between the storage container 701 and lock container 870. Step II: Pressure equalisation

After the valves V., and V₂ have been closed, is the pressure in the lock chamber 870 increased as the lock chamber and the decomposition container 840 are connected with each other through the connection 820 because the valve V₃ is open without temporary transfer of hydrate, and without transmitting the pressure back to the storage container 701. Step HI: Transferring of hydrate Primarily when the pressure is equalised, is the hydrate transferred from the lock container 870 to the decomposition container 840 through the connection 819 while the valve V₄ is open. Step IV: Decomposition

Primarily when the decomposition container 840 is filled with hydrate in predetermined degree (at batchwise operation), is the temperature raised, e.g. by flushing of temporated water; if possible via the lock chamber 870, from the connection 823, through the open valves V₇ and V₈ driven by the pump 876. During this process is the valve V₃ still open to give pressure equalising. But it must be mentioned that dissociating of the gas hydrate may also take place continuously, e.g. by several parallel lock chambers 870 which work in different phases, one by one filling the one and similar decomposition container 840. Step V: Emptyfying Dissociating gas is further directed out of the outlet 842 in the decomposition container 840. Liquied from the decomposition is directed out of the decomposition container 840 through the valve V₆ to the lock chamber 870 and out through valve V₅. Step VI: Filling of lock container with liquid

After the hydrate mass being transferred from the lock container 870 to the decomposition container 840, the lock container 870 will be filled with gas because the valve V₃ is open. The gas volume in the lock container 870 must be replaced by a uncompressed medium avoiding a loss of gas from the high pressure zone. This is achieved by leading liquid from the decomposition container 840 is lead back to the lock container 870 so that the gas in the lock container is displaced and laid back to the decomposition container through the connection 820 and the open valve V₃. It is preferably that all gas in the lock container should be replaced with liquid from the decomposition container. Liquid and gas communication between the lock container 870 and the decomposition container 840 is closed by the valves V₃. V₄ and V₆ is being closed. Afterward the valve V₃ is opened. As through the connection 719 provide the gas communication between the lock container 870 and the storage container 701. The opening of the valve V₃ take place without any large pressure search in the plant because the lock container 870 at this time will be filled up with a incompressible fluid. That is liquid from the decomposition container 840.

IT should also be mentioned that feeding of a heat medium to the decomposition container 840 can take place in two ways. If available heat medium exists at low pressure, e.g. at about 1 bar, the pressure search of the heat medium to the degassing pressure in the decomposition container 840 can be done through the lock container 870. This is shown at fig. 8 with the possible connection from the inlet 823 to the decomposition container 840 when the valve V₇ is open via the outlet 848 form the lock container 870 to the inlet 847 in the decomposition container 840 when the valve V₈ is open and the pump 876 is working.

The heat medium can of course also be pressurised by the aid of a particular pump (not shown) and be lead directly into the decomposition container 840 from random source. If the heat medium already exists at high pressure, it is not necessary with such arrangement, as the heat medium can be lead directly into the decomposition container 840 from such a high pressure source.

The following conditions must particularly be mentioned to define that the invention must not be understood too narrowly.

The invention is not limited to storage containers 701 which only can stand an atmospheric pressure. The higher pressure the storage containers can stand, less cooling is necessary. Which pressure the storage container(s) shall be dimensioned to, is defined from economic and practical conditions in the individual plant.

The invention does not always demand a cooling plant 806 for the storage container 701. If the storage time is short, the cooling of the gas hydrate can be omitted.

A lot of modifications can be done inside the frame of the present invention. Particularly it should be mentioned that lock containers 870 and/or several decomposition containers such as 840 connected in parallel can be used, but regulated to be filled/emptyfied different times to give a constant gas-flow to the consumer; the separate pressure or lock container can also have different sizes, e.g. adjusted to varying gas demand; the different parts of the system can be connected and regulated by known regulation principles, which among other things comprise the detectors x for pressure, temperature and flow and corresponding return connection circles to regulate valves, pumps and something of the sort by the means of actuators y. Such a regulation system with the regulation central 880 is indicated by dotted lines at fig. 8, while the detectors and actuators only are indicated at some places to avoid the figure to be overloading and to indicate that the type of detectors, actuators and the replacing of these not are critical, but can easy be realised in a practical way by a person skilled in the WS-field. As an example of detectors it can be mentioned pressure, temperature, level and flow detectors. And the temperature regulation can as mentioned take place directly and/or indirectly by recirculation or by one occasion supply of the temperature regulation medium from a suitable source. In this way the medium used for the decomposition of the gas hydrate can being directly supplied to the decomposition container(s) 840.

If the gas hydrate is being stored as solid gas hydrate (as powder, particles or more or less solid mass) in the container 701 , that is without liquefied carrier medium, the terminal plant must also comprise transportation means, as for instance feed screws, for transferring of the mass from the storage container 701 to the transportation arrangement 730 and from the transportation arrangement 730 to the decomposition container 740. The transferring from the container 730 to the container 740 can in these occasions eventually be done easier by supply of a liquefied medium, which can be detect water from the decomposition by hydrate in the container 740, to the transportation arrangement 730 after a pressure increase happened (at least in such a way that the supply of water do not cause any substantial decomposition of the gas hydrate in parts of the plant where decomposition is not desirable, e.g. in the container 701.

Stability experiment

The effect achieved through the present invention regarding the stability of the gas hydrates was proved through experiment which is described in the following. The experiment apparatus which was used is shown at the texted figures 9 and 10, which is self-explanatory concerning the experiment specification.

Pressure and temperature inside a container with gas hydrate read off at different test point, and these pressure and temperature values indicated the limit for stable hydrates. The hydrate was stable if the measured temperature at a given pressure was lower than the estimated stability temperature, or if the measured pressure at a given temperature was higher than the estimated stability pressure.

Measuring of the stability of the gas hydrates in a pressure container Hydrate as in advance was cooled to -20°C was placed in the pressure container. The top flange to the container was installed with O-rotary tightening between the flank and the rest of the container. The container was placed in a freezer with temperature regulation in the range of -10 to -55°C. The hydrate container was firstly cooled to a temperature which was considerably below the estimated equilibrium temperature of the hydrate at atmospheric pressure, and was depressurized. The temperature of the freezer was then raised with 1°C per 24 hours. When the pressure had raised above 1 atm., the stability temperature at atmospheric pressure was reached.

To verify the stability temperature, the container was depressurised and closed again. Then the pressure was measured, and if the same pressure value were reached again, real equilibrium temperature was reached.

Measuring of stabilities of the gas hydrates in a glass container at atmospheric pressure.

Hydrate as in advance was cooled to -20°C and placed in a glass container which afterwards was connection to a gas experiment bag through a male-frosted/tube adaptery and a tube. The experiment tube collected gas which was sublimed from the hydrate when it was stored and stable. The container was placed in a freezer with temperature regulation in the range of -10 to -55°C. The hydrate container was first cooled below a temperature which was below the estimated equilibrium temperature of the hydrate at atmospheric pressure. The temperature of the freezer was then raised with 1°C per 24 hours. When the gas experiment bag was filled up with gas, the stability temperature at atmospheric pressure was reached. The results appear from the following tables.

TABELL 1

^* Hydrate produced from gas having a composition corresponding to the gasphase after the condensation out of the heavy components at 2°C and 15 bar.

In most cases, i.e. for the majority of gas compositions that are examined, the transition between stable and less stable product quite marked. The product has also better rheology properties in the stable condition. Among other things are the tendency of sintering during compressive load substantially reduced, and clogs of compressed product are easy to break during mechanical influence, e.g. agitating, to a granulated slurry.

Claims

CLAIMS 1. A method of preparing a hydrocarbon product comprising hydrates of hydrate-generating hydrocarbons surrounded by or suspended in a hydrocarbon- containing liquid, which is stable at a storage pressure equal to or close to ambient atmospheric pressure; wherein a hydrocarbon material comprising hydrate-generating hydrocarbons and water are contacted in a hydrate-generating zone under hydrate-generating process conditions generating substantially a water- and ice-free hydrate mass, which in a cooling zone is cooled to a mean end- and storage temperature, which is lower than the freezing temperature of water, thereby generating the hydrocarbon product, said hydrocarbon comprising liquid being supplied to the hydrate-generating zone as a part of the hydrocarbon material or is supplied during the preparing or cooling of the hydrate mass, characterized in that the hydrate mass is cooled to a mean end- and storage temperature T_storage which is equal to or lower than the a temperature value T₀, which is the temperature at which the product leaves a relative unstable condition to a substantially stable condition according to the equation:

T₀ = T(P,Y) + AT₀

wherein l + 4or_C;

0,61 - 0,15- Y_Cι + 0,15 -Y_Cj + 0,15 • F_Cj + 0,1 -Y_Nι 0,55 -Y_Cι + l,5-r_Cι + l,5-r_C4 + 0,8-7^ + u-r_co. 0,55 -Y_Cι + l,5-Y_Cι + l,5-r_Cj + 0,8,F„₂ + l,l-Y_cc,₂ 1,2 + 95-F_Cj 0,64 + 0,32 -F_C| + 0,42 • Y_N≥ ^■

ΔT₀ = a numerical value, which defines the error margin in the expression for T₀ being in the range from +1 to -15°C, and P is the total pressure, Y, Is the mole fraction of the individual gas components, A, are gas-spesific constants, n, are gas composition dependent exponents.

2. The method according to claim ^ c h a r a c t e r i z e d in that the hydrate mass is cooled to a mean end- and storage temperature T_storage, which is equal to or below a temperature T₀, being the temperature whereby the product is leaving a relatively unstable condition to a condition wherein more than 95 percent by volume of the hydrate-generating hydrocarbons are present in the product after storage for three days.

3. The method of preparing a hydrocarbon product according to claim 1 and 2, said hydrocarbon material comprising natural gas having a substantial content of methane, particularly having a content of methane exceeding 80 percent by volume of the gas, wherein the hydrate mass is cooled to a mean end and storage temperature T_storage, which is equal to or below a temperature T₀ , which is equal to -30°C or lower at a storage pressure of approximately 1 bar (normal atmospheric pressure).

4. The method of preparing a hydrocarbon product according to claim 1 and 2, where the hydrocarbon material comprise composition of hydrate-generating components having a total content of ethane and propane of maximum 35 percent by volume, wherein the hydrate mass is cooled to a mean end- and storage temperature T_storage, which is equal to or lower than a temperature value T₀ , which is equal to -20°C or lower at a storage pressure of approximately 1 bar (normal atmospheric pressure).

5. The method according to the claims 1 to 4, wherein a hydrocarbon containing cooling medium is used in the preparation or cooling of the hydrate mass, the hydrocarbon containing cooling medium being in direct contact with the remaining phases in the hydrate-generating zone, respectively the cooling zone.

6. The method according to the claims 1 to 5, wherein hydrocarbons or compositions thereof are used as said hydrocarbon containing cooling medium the hydrocarbons of which having at least four carbon atoms.

7. The method according to the claims 1 to 6, wherein the proportion of the non-hydrate-generating hydrocarbons in said hydrocarbon contenting cooling medium in substantially comprised by a C₅ - C₁₀ - petroleum fraction, preferably a condensate fraction.

8. The method according to the claims 1 to 7, wherein a liquid comprising components of said hydrocarbon containing cooling medium is used as a hydrocarbon containing liquid. 0

9. The method according to the claims 1 to 8, wherein a hydrocarbon containing medium is used for cooling said hydrate mass , which at storage temperature has a vapour pressure equal to or below the storage pressure.

s 10. The method according the claims 1 to 9, wherein destabilising quantities of volantile components, being dissolved in the hydrocarbon containing liquid after cooling of the hydrate mass, are eliminated from the product by depressurizing and degassing, prior to storage and transportation of the product.

o 11. The method according to the claims 1 to 10, wherein the hydrate mass is cooled to a temperature of 1 to 10°C below the temperature T₀.

12. The method of storage and tranportation of a hydrocarbon product, which contains hydrates of hydrate-generating hydrocarbons surrounded by or 5 suspended in a hydrocarbon containing liquid, at a storage pressure equal to or closely to ambient atmospheric pressure, wherein the product is stored or kept cooled at a mean storage temperatur T_storage which is equal to or below a temperature T₀, which is the temperature at which the product is transferred from a relative unstable condition to a substantially stable condition according to the 0 equation:

wherein

1 + 40F_r

0,61 - 0,15 • F_C| + 0,15 • F_Cj + 0,15 • Y_Ct + 0,1 • F„₂ 0,55 -F_C| + 1,5-F_C2 + 1,5-F_C + 0,8-F„₂ + l,l-F_COι 0,55 -F_C| + 1,5-F_C2 + 1,5-F_C) + 0,8,F_Wj + l,l-7_CO| 1 ,2 + 95 • F_r

0,64 + 0,32 -Y_c + 0,42 • Y_N ^■

where the meaning of P, Y\, and n, are as defined in claim 1 , and ΔT = a numerical value, which defines the error margin in the expression for T_enύ being in the range of +1 to-15°C.

13. The method according to claim 12, wherein the product is stored or is kept cooled at a mean end- and storage temperature T_storage, which is equal to or below a temperature T₀, which is the temperature where the product is passing from a relative unstable condition to a condition wherein more than 95 percent by volume of the hydrate-generating hydrocarbons are present in the product after three days storage.

14. The method of preparing a hydrocarbon product according to the claims 11 and 13, wherein the hydrocarbon material comprises nature gas having a substantial content of methane, particularly having a content of methane which exceeds 80 percent by volume of the gas, wherein said product is stored or is kept cooled at a mean storage temperatur T_storage which is equal to or below a temperature T₀ , which is equal to -30°C or lower at a storage pressure of approximate 1 bar (normal atmospheric pressure).

15. The method of preparing a hydrocarbon product according to claim 12 and 13, said hydrocarbon material comprising a composition of hydrate-generating components having a total of ethane and propane until 35 percent by volume, whrein said product is stored or is kept cooled at a mean end and storage temperature T_storage, which is equal to or lower than a temperature value T₀ , which is equal to -20°C or lower at a storage pressure of approximately 1 bar (normal atmospheric pressure).

16. The method according to the claims 12 to 15, wherein the storage temperature above -40°C is used.

17. The method according to claim 11 -16, wherein said product is stored or maintained refrigerated at a storage temperature T_storage which is from 1 to 10°C lower than the temperature T₀.

18. A hydrocarbon product containing a hydrate of at least one hydrate- generating hydrocarbon surrounded by or suspended in a hydrocarbon containing liquid, wherein the product being at a storage temperatur T_storage which is equal to or below a temperature value T₀, which is the temperature wherein said product is transferred from a relative unstable condition to a substantially stable condition according to the equation:

T₀ = T(P,Y) = 19 ln( ) + + ΔΓ

wherein

1 + 40F_C2

0,61 - 0,15- _Cι + 0,15-F_Cj + 0,15 -F^ + 0,1 -Y_Nι

0,55-F_c_ + l,5.y_Cι + 1,5-F_C4 + 0,%-Y_Nι + \,\-Y_COi

0,55 -Y_C + 1,5-F_C2 + l,5-F_Cj + 0,8,7„₂ + \,\-Y_COl

1,2 + 95-F_r2

0,64 + 0,32 -Y_c + 0,42 ■ Y_N ■

wherein the meanings of P, Yi'Ai og n, are defined in claim 1 , and ΔT = a numerical value, which defines the error margin in the expression of T₀ and which is in the range from +1 to -15°C.

19. The product according to claim 18, wherein said product being at a storage temperature T_storage, which is equal to or lower than a temperature value T₀, being the temperature at which the product is transferred from a relative unstable condition to a condition where more than 95 percent by volume of the hydrate- generating hydrocarbons are present in the product after storage for three days.

20. The product according to the claimslδ and 19; prepared from a hydrocarbon material which comprises natural gas having a substantial content of methane, particularly having a content of methane exceeding 80 percent by volume of the gas, wherein said product being at a storage temperature T_storage, which is equal to or below a temperature T₀ equal to -30°C or lower at a storage pressure of approximately 1 bar (normal atmospheric pressure).

21. The product according to the claims 18 - 20, prepared from a hydrocarbon material which comprises a composition of hydrate-generating compounds having a total of ethane and propane of maximum 35 percent by volume, wherein said product being at a storage temperature T_storage, which is equal to or below a temperatureT₀ equal to

-20°C or lower by a storage pressure of approximately 1 bar (normal atmospheric pressure).

22. The product according to the claims16 - 21 , wherein said product being at a storage temperature T_storage, which is from 1 to 10°C below the temperature T₀.

23. The product according to the claims 16 - 22, wherein said product being at a storage temperature T_storage above -40°C.

24. The product according to the claims 18 - 23, wherein said hydrocarbon containing liquid has a vapour pressure at storage temperature which is equal to or below the storage pressure.

25. The product according to the claims 18 - 24, wherein said product can be prepared in accordance with the method of preparing defined in the claims 1 - 10.

26. The use of a product according to the claims 18 - 25 as a medium for the storage and transportation of natural gas.

27. The use of a product according to the claims 18 - 25 as a medium for the storage and transportation of volatile components (VOC), which are released during loading, unloading and transportation of processed crude oil.

28. The use of a product according to the claims 18 - 25 as a medium for the storage and transportation of normal gaseous or volatile components being combined with, or which are released from crude oil during production and processing of crude oil and natural gas.

29. The use of a product according to the claims 18 - 25 as a fuel or engine fuel for the generation of heat or energy or as a storage and transportation medium for normal gaseous or volatile hydrocarbons intended for such purposes.