CA2219244A1

CA2219244A1 - Method and apparatus for the manufacture of a hydrocarbon product as well as the product itself

Info

Publication number: CA2219244A1
Application number: CA 2219244
Authority: CA
Inventors: Otto Skovholt; Geir B. Lorentzen
Original assignee: Individual
Current assignee: Equinor ASA
Priority date: 1995-04-28
Filing date: 1996-04-26
Publication date: 1996-10-31
Also published as: NO951669L; WO1996034226A1; GB9722661D0; GB2315773A; AU5705296A; NO951669D0

Abstract

A method of producing a stable hydrocarbon product in the form of hydrate of at least one hydrate-forming hydrocarbon surrounded by or suspended in a hydrocarbon-containing liquid. The invention comprises the actual product, a method of manufacturing the product as well as a plant for manufacturing the product. The process comprises the following steps: a) hydrate-forming hydrocarbons and water are brought together in a hydrate-generating zone (1) under hydrate-forming process conditions. Heat energy being released during the hydrate formation is removed from the zone by direct cooling with a first cooling medium. Thereby a first intermediate product is formed, having an average temperature which is equal to or higher than the freezing point of water; b) any non-converted water being present is removed from the first intermediate product, whereby a second intermediate product is obtained, being substantially free of non-converted water c) the second intermediate product is cooled in a cooling zone (80) by direct cooling with a second, hydrocarbon-containing cooling medium, to a final temperature being in average lower than the freezing point of water. The end product comprises particles in a solid, hydrate-containing material, which is surrounded by or suspended in a liquid hydrocarbon carrier medium at a vapour pressure being lower than the final pressure at the final temperature.

Description

W 096~34226 PCTAN096/00098 Method and appara~us for the manu~acture of a hydrocarbon product as well as the product itself The present invention relates to a method and a plant for producing a hydrocarbon product in the form of a 5Us--pension comprising hydrate particles of at least one gas hydrate suspended in a hydrocarbon-based liquid. The hydro-carbon product itself also repr2sents a paF~ or this lnven-tion.
Suspensions including particles of gas hydrate sus-pended in a hydrocarbon-based liquid, are previously known per se, especially as a temporary intermediate product used in connection with treatment or transport of gas hydrate.
In this connection reference is made to US patent No.

2.363.529 which in particular relates to a suspension used in connection with a controlled fractioning o~ different hydrate-forming hydrocarbons from a fluid, and also to US
patent No. 2.356.407 which in particular relates to the use of a similar suspension for transporting gas hydrate from one place to another, for instance for storage. Finally US
patent No. 3.514.274 should be mentioned. This patent describes how natural gas may be transported as a hydrate in a slurry based on liquid propane. However, such a slurry will be unstable at atmospheric pressure unless the tempera-ture is below -42~C. In contrast to this the present inven-tion provides a hydrocarbon product being stable at atmos-pheric pressure even when the temperature is only a few degrees below zero.
According to the present invention a simplified and improved method for the generation of a hydrocarbon product is obtained, in which the gas hydrate particles are sur-rounded by or suspended in a hydrocarbon medium; and the invention also relates to a plant for producing a hydro-carbon product. Finally the present invention relates to a hydrocarbon product as mentioned above, with enhanced product qualities.
The gas being included in the gas hydrate when 3S liberated may be used in many different manners. The gas may be used in power production, for instance in power plants, or in central heating, or may be distributed to W 096/34226 PCT~096/00098 consumers through pipelines. The hydrocarbon component in the product may also be used as a raw material for the production of chemicals, and products such as synthetic gas, methanol, acetic acid etc. The heavier components in the product may be used as components of fuel or propellants, and as raw material within the petrochemical industry.
In this connection it can also be mentioned that it is previously known to produce gas hydrate for transporting and/or storing gas at advantageous pressure and temperature conditions, cf. Norwegian patent No. 175.656.
However, it has been found that great problems arise when gas hydrates are used in connection with transporting and storing gas using conventional technigues, for instance because the gas hydrate is cintered into a compact material during storage, which material is difficult to handle as it sticks to walls in the storage tanks and to the inner sur-faces of pipelines.
In addition, hydrate which is treated in a conventional manner has to be stored at high pressure or at very low temperatures. It would have been advantageous to reduce the storing pressure of the hydrocarbon product to 1 atmosphere.
This may be done when the present invention is used, as the gas hydrate included in the hydrocarbon product then obtains a stable and even temperature through the entire volume of the product, and this temperature may easily be maintained at a level ensuring a stable storage also at normal pressure.
The main object of the present invention is to provide a method and a plant for the efficient generation of large quantities of a new product including gas hydrate in large guantities, which requires an efficient heat transfer during the hydrate generation.
An additional object of the invention is to provide a new, preferably pumpable hydrocarbon product which is easy to handle, which in practical operation includes a hydro-carbon slurry or paste having a highest possible hydrate percentage, and in particular a product being stable at the prevailing pressure and temperature conditions in the -W 096/34226 PCTA~O~ C9 transport and storing areas, and accordingly a product which does not release gas which can lead to an undesirable increase of the pressure. Still a further object is to provide a hydrocarbon product which does not comprise any free water or ice or only contains insignificant amounts of ~ free water, i.e. water which is not converted into hydrate, as the occurrance of such free, non-converted water is deemed to represent one major reason why gas hydrate previously has been difficult to handle. In addition such free, non-converted water, will represent additional losses, as the water both represents an additional weight which again requires additional power during transport, and does not contribute to the transport of gas at all.
The expressions Uinsignificant'' or "unimportant" amounts of water or frozen water, indicate that the content of free, non-converted water should not be at such a level that the overall content of hydrate-forming, gaseous components in the product will be below an acceptable level. Economical considerations have shown that the conditions are acceptable when the relative volume of hydrate-forming gaseous com-ponents before the generation of hydrate as compared to the volume of solid gas hydrate and frozen water, after such generation, is 130 or higher. Accordingly the hydrocarbon product should include at least 130 Sm3 gas/m3 solid matter.
In particular it should be mentioned that the process condi-tions are set to obtain a final product where the solid, hydrate-containing material has a gas content corresponding to a density of at least 130 Sm3/m3, preferably above 150 Sm3/m3 solid material, when methane has been used as the hydrate-forming material.
Yet another object is to provide a method for gene-rating large amounts of a hydrocarbon product, continuously or in batches, by means of known and well-proven chemical engineering equipment.
Yet another object is to provide a new method for generating a new hydrocarbon product by a process including two-step direct cooling of the original materials and intermediate products with the aid of two identical or two W 096/34226 PCTA~03610~~9 different cooling agents.
Yet another object is to provide a new plant which either uses a common container for both generating and cooling, or which instead uses separate containers for carrying out each process step.
It is also an object to achieve a method which reduces the risk of ice and hydrate formation at undesired locations in the plant, for example in places where there is danger of clogging.
A further object of the present invention is to provide a suspension in which large amounts of gas hydrate occur in the form of particles being surrounded by, or suspended in, a carrying liquid, which makes possible an effective heat transfer between the gas hydrate included in the mass and the outside environment, and which thereby ensures an effective regulation and control of the overall temperature in the product.
Thus, both an energy carrying medium which can be stored and handled simply, substantially by means of traditional storage and transport equipment for liquids, pastes, dispersions and slurries, and at the same time a suspension with a very high energy content lying between the energy content of liquified gas (LNG) and that of compressed (CNG) gas, have been achieved without encountering corre-sponding problems of very high pressure and/or very lowtemperatures.
In order to fulfil the object of effective generation of large quantities of gas hydrate, direct contact is made, according to the present invention, between a first cooling medium which is added and the hydrate-forming hydrocarbons, where the latter usually exist in the form of gas. Large, direct contact surface between gas and cooling agent is the deciding factor. Such direct cooling has been shown empirically to be the cooling method which gives the highest rates of hydrate production and which therefore best lends itself to industrial applications.
Another advantage of the invention is that the process cannot only be carried out in a stationary plant on land,but W O 96134226 PCT~N096~'~0C9 can also be adapted for use on floating installations and ships at sea where there is a need for storing gas produced, either alone or associated with other petrochemical pro-~ ducts. Such compact plants can be built because the plant according to the present invention is relatively simple, andto a large extent is comprised of components which already are well tested and commercially available in the form of pumps, valves, cooling systems, tanks and so on.
The listed advantages and objectives can be achieved by using a method according to one, or several of the patent claims below, by implementing the method by means of equip-ment in accordance with the apparatus claims set out below, and by a product according to the product claims set out below.
A broad outline of the production of a hydrocarbon product according to the present invention is given in the following four steps.
- In step a) large quantities of hydrate are generated.
- In step b) redundent water is removed from the hydrate.
- In step c) the hydrate is cooled by the addition of a cold liquid containing hydrocarbon while it is ensured that the hydrate does not dissociate, and, - In step d) the end product is taken out of the process.
Any residual non-converted water will form a film around the individual hydrate particles, and hydrate products which contain large quantities of non-converted water will be difficult to handle if they are subjected to temperatures below the freezing point of water. Any re-dundant water can be removed from the hydrate by many means in order to form a "dry" hydrate, i.e. a hydrate where large quantities of non-converted water are no longer present, at least not in an amount sufficient to cause transport prob-lems. The three most important methods of removing non-converted water are:
- The hydrate can be treated mechanically, e.g. drained, compressed or compacted so that superfluous water is pressed out. Known treating units such as filters, centrifuges, or hydrocycolones can be of use. In any case this method will W 096/34226 PCT~0~6/00~$

not remove all the water.
- Further amounts of hydrate-forming hydrocarbons can be added, as liquids or gases, and brought into contact with the non-converted water so that also the non-converted water is converted into hydrate. By ensuring the addition of an extra amount of hydrate-forming components at suitable pressure and temperature conditions, all the residual free water can be converted to hydrate(s) so that the finished hydrate will be completely dry.
- Excess amounts of water can also be removed by the addition of a water-absorbant medium, e.g. an alcohol or a ketone, e.g. acetone. Such media will however have a certain tendency also to dissolve hydrates and for that reason alone should only be used in special circumstances.
The expression "remove" therefore includes all these methods and combinations of them.
According to the present invention direct cooling is used, as already mentioned, where the product to be cooled and the cooling medium being used, come into direct contact with each other. This direct cooling can be conducted substantially in at least two steps, by the use of a first and a second cooling liquid, also denoted cooling agents.
The first cooling liquid is used during hydrate generation in step a) and has as its most important object to remove the amount of heat generated during the formation of hydrates, so that the temperature in the hydrate-generating zone is kept within the hydrate-generating limits at a given process pressure. The cooling liquid thus shall not only cool down "the gas" or the hydrocarbons which can form hydrates, but also the hydrate produced and the water present, to the extent being necessary. However, the cool-ing in the first step only goes down to a temperature which ensures that hydrates are formed in the desired amounts.
The first cooling liquid can be water, and in that case must be removed or converted to hydrate in step b), before the second cooling liquid, under process step c), brinqs the average temperature of the hydrocarbon product below the freezing point of water W O 96/34226 PCT~NO9''100 The second cooling liquid will perform several tasks, but first and most importantly it will cool the hydrates generated so that they are stable at the pressure of the ~ surroundings, e.g. at atmospheric pressure. The second cooling liquid cools the product down to temperatures of below 0~C but first after virtually all water is removed.
The non-converted water which is removed from the intermediate product in step b) can be, wholly or partially, cooled again and recirculated into the process, e.g. to step a) for producing further amounts of gas hydrate.
Any recirculation of the first and the second cooling liquids for maintaining the desired temperature of the product, takes place in separating the cooling agent from the hydrate to a greater or lesser extent, being cooled anew and recirculated alone. It is preferred that the recircu-lation flow which is cooled, should not contain hydrate particles, ice or water, as such components will have a tendency to precipitate as ice or hydrate on the cooling surfaces in the heat exchangers. The recirculated, newly cooled cooling liquid again cools the product by direct contact.
A crucial point concerning the present invention is that all the gas hydrate particles are in close contact with a liquid hydrocarbon. This fact ensures a stable temperature through the complete mass of hydrate and makes fast tempera-ture changes within the mass of hydrate possible when desired, as the mass nowhere will be thermally insulated from the temperature controlling medium, that is the liquid based on hydrocarbons, also referred to as the second cool-ing medium.
The suspension of hydrate particles in the firstcooling medium, possibly including a minor amount of free, non-converted water as it is found at the end of stage a, is referred to as the first intermediate product and has an average temperature just above the freezing point of water and a pressure equal to the hydrate-generating pressure.
The suspension of hydrate particles in the second cooling medium, with a ~;n; of free, non-converted water, W 096/34226 PCTANO9~ 9 as is found at the end of the process step b, is referred to as the second intermediate product. This has a temperature T4. The end product itself, however, shall be cooled down to a temperature being so low that the hydrate is stable at the prevailing storage pressure. The temperature in the end pro-duct may e.g. be reduced to -40~C and the pressure be reduced to approximately 1 atmosphere. A more detailed explanation is given below.
When the product has obtained stable temperature and pressure conditions, and superfluous cooling liquid has been removed, so that the product preferably has obtained a pumpable/transportable consistence, the desired end product has been obtained.
The end product may be handled by means of conventional transporting and storage equipment, developed for other products such as paste and slurries.
The conditions which have to be met to ensure that hydrate is generated are on the one hand, that pressure and temperature are within the hydrate-generating limits. In addition it is rather important that the hydrate-forming hydrocarbons and the water, possibly in frozen condition as snow or ice, are in close contact for long enough that the conversion into hydrate becomes as complete as possible.
When the hydrate generation takes place by spraying atomized water into the upper part of the hydrate-generating zone in the container 2 it is important that the container is high and that generated hydrate does not build up in too high piles in the container. This will ensure that the contact time between water and gas is sufficiently long-lasting so that large amounts of hydrate may be generated. In fig. 1, 2 and 3 it i5 assumed that the container 2 can have a very great height.
If instead a design solution is selected in which the gas is bubbled from the bottom of the container through water, it is important that the gas is well distributed through nozzles and the height up to the surface of the water is sufficient.
Another important condition is that the flow of W 096t34226 PCT~0~61C~~9~

material out from and into the hydrate-generating zone is sufficiently large.
The inter-relations of the temperatures referred to in this application are as follows:
T1 is the temperature of the first cooling medium when it enters the hydrate-generating zone. T, has to be so much below the equilibrium temperature for generation/dis-sociation of gas hydrate at the prevailing working pressure, that hydrate will be generated.
T2 is the temperature of the first cooling medium when it leaves the hydrate-generating zone. This temperature is rather close to the equilibrium temperature of hydrate.
T3 is the temperature of the second cooling medium when it is supplied to the cooling zone.
T4 is the temperature of the end product.
T5 is the temperature of the second intermediate product.
T6 is the temperature within the hydrate when the hydrate product in step c is cooled down to a temperature below the freezing point of water, at which the cooling medium comprising destabilizing amounts of volatile components may be replaced by a medium having a substantially lower content of such components, and the most important relative conditions can be expressed as follows:
T, << T og oCc ~ T~
T3 < T4 << oCC og T4 < T~ < oCC s TL.
Below some further details are mentioned, which can be of great importance.
It should be noted that the first cooling liquid may be supplied at so low temperature that small amounts of ice are generated locally for a short time. However, it is important that the first cooling medium is not supplied in so large ~ amounts or at so low temperatures that large quantities of ice are generated. Accordingly this invention also covers methods according to which some ice is generated in the hydrate-generating zone, but is later melted by mutual heat transfer between the remaining quantities of gas and liquids W O 96/34226 PCTA~0~6~C~9 in the hydrate-generating zone.
The temperature of the first cooling medium may be below 0~C, in particular when the cooling medium is a hydrocarbon, when it is supplied to the hydrate-generating zone. This fact is also understood from the conditions below:
T4 < T6 ~ 0~C ~ Ts T~ is the temperature in the hydrate after it is generated and after removal of non-converted water in step b, i.e. before the cooling process in step c. Accordingly T~
has to be above or equal to 0~C. T6 is the temperature which the hydrate at least must have to ensure that the hydro-carbon medium which incorporates the stabilizing amounts of volatile components, can be replaced by a medium having a low content of such components so that the generated hydrate shall not dissociate due to the absence of stabilizing consentrations of hydrate-forming components. When the temperature in the gas hydrate has come below the tempera-ture T6, the gas within the gas hydrate for all practial purposes will be irreversibly included in the gas hydrate structure. This is also according to the conditions stated above.
The water which is to be converted into hydrate may already at the start of the process be introduced as snow or ice. Then the requirement for cooling the first cooling medium is reduced. It should therefore be pointed out that the first cooling medium can be supplied at such a tempera-ture and in such an amount that a small amount of ice is generated or is maintained, but not in such amounts that ice is carried over to the next step in the process.
The second cooling liquid which is used in step c, consists of a hydrocarbon-based liquid, and the temperature in this liquid has to be sufficiently low that at the output from the cooling zone there is obtained a mixture of gas hydrate and hydrocarbon liquid having a temperature which leads to a stable mixture at the surrounding pressure which normally will be approximately 1 bar. However, it is impor-tant that the first cooling liquid, in particular when this W 096/34226 PCTANO~ 9 liquid includes water, must not have a temperature below its own freezing point. The first cooling medium can, however, also include hydrate-forming hydrocarbons which may be other hydrocarbons than those which exist as gas in the hydrate-generating zone.
When the composition of the second cooling medium is considered, the following should also be noted:
On the one hand the total partial pressure of the hydrate-forming hydrocarbons should not be reduced substan-tially below the limit for hydrate generation at the presenttemperature. This is to ensure that the second intermediate product which still has a temperature above O'C is maintained stable or does not dissociate.
If the second cooling medium does not include any hydrate-forming components, the supply of this cooling medium will reduce the partial pressure from those hydro-carbons and accordingly the hydrate will become unstable and will dissociate. Therefore the cooling zone, at least at the beginning of step c, should be supplied with sufficient amounts of hydrate-forming hydrocarbons to ensure that the hydrate is kept stable until the temperature has reached T=T~, i.e. the temperature of the end product.
If insufficient amounts of hydrate-generating components are present during the cooling in step c, there is a risk that a portion of hydrate will dissociate before the temperature T=T4has been reached.
On the other hand it is required that the content of destabilizing components should be removed or reduced. These components may be methane, ethane, propane or other volatile components. These components should be removed from the hydrocarbon medium to obtain a stable end product. Accord-~ ingly a hydrocarbon medium included in the end product will not at the temperature T=T include the stabilizing compo-- nents in amounts leading to a collective partial pressure from these components over the hydrocarbon medium, exceeding the pressure in the environments (normally approximately 1 bar) at temperature T=T .
The hydrocarbon medium in the end product should not CA 022l9244 l997-l0-27 W 096/34226 PCTANO96/OC~93 12 include the stabilizing amounts of light hydrocarbons such as methane and ethane. The partial pressure for each of the destabilizing components can, at least as a first approxi-mation, be calculated from Henrys law: Pi = H . C , in which:
P~ x;mum acceptable partial pressure of component i, Hi = Henrys constant found experimentally, and ci = the consentration (measured in mol/volume unit) of said component.
The sum of the partial pressure of such volatile components, ~Pi must be below the prevailing pressure in the surround-ings. If the sum of the partial pressures exceeds this limit, the end product becomes unstable as it releases volatile gases such as methane, ethane and to a certain degree also propane when the end product is exposed to the prevailing pressure (- 1 bar) of the environments, even when the final temperature T=T4 (<<O~C) has been reached.
The method and the apparatus according to the invention can comprise a mechanical treatment, e.g. implemented by means of at least one mixer. The purpose of this solution is to prevent generation of agglomerates and large quantities of hydrate and thus contribute to an increased transport of hydrate-forming components to the interfaces between hydrate and non-converted water in the compound, and also to an equalization of the temperature in the hydrocarbon product.
Even if the hydrate product may be exposed to mechani-cal treatment to produce a suspension of hydrate particles in a cooling liquid, such a mechanical treatment will not always be required. Depending on the composition of said liquids, and the pressure and temperature conditions, the hydrate can in many circumstances disintegrate naturally and thereby generate small separated particles so that a sus-pension is generated as soon as a hydrate is brought together with the hydrocarbon containing liquid.
In other words the object is to obtain a product which represents a mixture of - a dry hydrate produced from hydrate-forming hydro-carbons, and W 096/34226 PCTANO~J~9 - a hydrocarbon-containing liquid, as none o~ these components includes essential amounts of a free, i.e. non-converted water, so that the mixture may be - exposed to temperatures below the freezing point of water without any risk of ice generation. Accordingly, the temperature of said mixture can be controlled within wide limits, and the mixture will be stable at a pressure of down to 1 atmosphere. Especially this last condition makes the product unique and well adapted to transport and storing.
Besides, it is very advantageous that the hydrocarbon liquid has good thermal contact with all the particles within the suspension and accordingly acts as an efficient temperature stabilizing and controlling medium for these particles.
Again it is assumed that irreversible generation of large amounts of ice takes place.
As soon as free water no longer exists, at least not in considerable quantities, after the generation of hydrate, e.g. as it is removed according to one of the methods explained above, the second cooling medium, e.g. the liquid hydrocarbon, preferably consisting of the so-called condensate fraction of crude oil anyhow can be supplied relatively soon after the generation of hydrate, and then at a temperature which may be substantially below the freezing point of water, as the risk of ice generation resulting in clogging, is then strongly reduced.
When the cooling in step c is considered, the following essential features should be mentioned:
- The cooling is undertaken in the presence of necessary amounts of hydrate-generating components until the tempera-ture T has reached a value T.; below O C.- The cooling is preferably undertaken in the absence of destabilizing hydrocarbon components when T is below T~.
- The destabilizing components may be removed in several different manners:
i) A medium comprising destabilizing components may be replaced by a cold hydrocarbon medium which does not include destabilizing components exceeding a limit value determined from the stability requirements for W O 96/34226 PCT~NO~ D~

the end product, either when the temperature T has reached the value T6, or when the compound of hydrate has reached the temperature T=T4, or ii) step c is undertaken using a different cooling medium and in the presence of necessary amounts of hydrate-forming components until the temperature T=T~, whereupon the content of destabilizing components is removed from the second intermediate product as the product, (gas hydrate included in a hydrocarbon medium) is exposed to a sufficiently low pressure so that the destabilizing components are released as gas from the hydrocarbon medium, until an end product satisfying the stability requirements has been generated. Gas being released may be compressed anew and recirculated to the hydrate-generating step a. Pressure relief and removal of residual portions of volatile components in the end product can be undertaken while the product is still in the cooling zone. Remaining portions o~ volatile components released as gas upon the pressure relief process, can also be removed after the end product has been transferred to a storage tank. This storage tank then has to be equipped with a gas outlet at its upper point and also must be connected to equipment necessary for the further handling of released gas, e.g. pipe-lines and compressors adapted for recirculation of the gas to step a.
The method in its most simple embodiment, can be performed in one single production line in which each operation is undertaken consecutively and continuously. Then the generation of hydrate takes place first, whereupon any water present is removed from the hydrate and the dry hydrate is cooled by means of a suitable cooling liquid.
However, such a simple production line with only one course, requires a batchwise treatment of gas, both at the input and at the output. A preferred method therefore is to use a process including at least two parallel production lines, each adapted for performing at least some of the above-mentioned production steps. By an arrangement according to CA 022l9244 l997-l0-27 W O 96/34226 PCTANO~6/n~9 which the various production lines all the time are at different steps of the process, so that the production lines start the production of hydrate at different instants of - time, the complete plant which accordingly comprises two or several parallel production lines running in different "phases", will when considered as a unit obtain a relatively steady or uniform flow of gas at the input and will also provide a relatively steady flow of the end product from the output, and this will in many cases be an essential require-ment for commercial plants intended for gas management.
Even if a hydrocarbon based liquid is preferred as the cooling medium, and even if it is deemed to be preferable that the same cooling liquid is used in all parts of the plant where cooling is required, the invention also covers methods and plants using different cooling agents at differ-ent places in the process. Accordingly a cooling medium may for example be the same in all the process steps mentioned above, or a different cooling medium may be used of differ-ent places, and of these some may comprise or be consisted of water.
It should also be emphasized that a stable end product requires conditions which ensure that the hydrocarbon pro-duct is stable for all practical purposes. The term stable here also covers the conditions which in the literature often is referred to as "meta-stable".
The method described above will also more or less include a description of the plant itself, and this plant may, in its simplest embodiment comprise only one single container or reactor provided with inlets for gas, water and at least one cooling medium, and also provided with outlets for generated hydrate and excess water and/or cooling liquid. Such a container may, if required, also be provided with inlets and outlets adapted for the circulation of at least portions of the cooling liquid which then may be cooled down in an external heat exchanger arrangement, and then lead back to the container for repeated cooling of the product. In a similar manner the container may, if required, be provided with a recirculating loop which leads at least CA 022l9244 l997-l0-27 W 096/34226 PCT~NOg~'03~9 16 some of the excess water back for repeated cooling in a second, external heat exchanger before the water is returned to the container.
The different components described above must of course be interconnected by means of the necessary communication connections, and must also be provided with the necessary valves, detectors and control accesories. In a different embo~;~e,Pt the plant can finally comprise two or several containers, as the product is transferred to new containers as the production steps are completed. The plant therefore preferably may include a separate storage container for hydrate. This storage container can preferably be heat insulated and can also be connected to an external heat exchanger via a circulation loop through which at least a portion of a liquid fraction of the hydrocarbon product can circulate. Details of such a plant is described below in several different embodiments and with reference to the drawings.
To give a still clearer understanding of the present invention reference is made to a detailed description of the method and the plant according to the present invention, and to the accompanying drawings in which:
Figure 1 illustrates one simple embodiment of the plant according to the present invention, in which water which is to be converted into hydrate, may circulate several times through the generator, while being cooled in between. The hydrate-generating zone and the cooling zone is in this embodiment built in one common container. This will give a clear presentation of the main principle.
Figure 2 illustrates a somewhat different embodiment of the plant, also according to the present invention, but here the water which is to be converted passes through the process only once (the once through principle).
Figure 3 illustrates a different embodiment in which ~he cooling zone is represented by a separate unit, -CA 022l9244 l997-l0-27 W 096134226 PCT~NO~ 9 Figure 4 illustrates a flow chart for an industrial plant and here some calculated values and capacities for the plant moduls are given, and also some parallel process paths are assumed in some of the process stages.
It should be noted that some of the details of the implementation has been omittet in the drawings, which mainly comprise the principles of the present invention. It should also be mentioned that same reference numbers are used in all of the drawings when considered appropriate, while the different Figures and parts of Figures are not necessarely shown in the same scale.
A first explanation of the principle is given with reference to Figure 1 which illustrates one of the simplest possible implementations of the present invention. The Figure shows the main features of a plant adapted to perform the method according to the invention.
The first embodiment of the method according to the invention is undertaken in a plant comprising a pressure tank or container 2 which in step a acts as the hydrate-generating zone 1 and which in step c acts as the cooling zone 80, and its associated cooling circuits for water and/or for the first and the second cooling medium. These components represent the main components of the system. As shown in Figure 1 the container or reactor 2 is connected to a storage unit 50 for storing the end product.
In the following a first embodiment will be explained, in which water, possibly seawater, is used as the first cooling medium. A different modification of this first embodiment, in which a hydrocarbon liquid is used as the first cooling medium, will be explained later on.
The container or the reactor 2 is made of a suitable material, e.g. stainless steel, and the construction is such that the container will endure a selected internal working pressure, with sufficient margins.
Hydrate-forming hydrocarbons, e.g. a natural gas comprising 90% methane, 4% ethane, 2% propane and a minor residual portion comprising heavier hydrocarbons and other W 096/34226 PCTANO~GJ~OO9~

gaseous components (N2, C02, etc.), are supplied through a pipeline 7 to the upper part 11 of the container 2 filled with gas. Apart from the requirement that the gas supplied through the pipeline 7 must have a pressure according to the selected working pressure, no specific conditions have to be met when the qualities of the gas are considered, and accordingly no specific retreatment is required.
Water is supplied and enters the volume of gas 11 in the upper part of the reactor 2 through a pipeline 5 and is flooded into the gas volume through at least one nozzle 6.
The water comes from any available source, e.g. a cold source of fresh water (not shown on the Figure), and must, when it enters the reactor 2 through the nozzle 6, have a temperature T=T1 below the equilibrium temperature for generation/dissociation of gas hydrate at the prevailing pressure. The relation between the equilibrium temperature of hydrate and the required gas pressure is known for a person skilled in the art from literature such as Slaan, E.D. Jr., "Clathrate hydrates of natural gases", Marcel Dekker, Inc., New York 1990. Ref. is also made to the conditions mentioned in the first part of this specifi-cation.
If the working pressure is selected to 60 bar, a temperature T=Tl of +10 - +12'C will be sufficiently low for generation of hydrate in the reactor container 2. However, it is obvious that the temeprature T; can preferably be much lower, e.g. down towards 0CC. If the first cooling liguid is water, this temperature should preferably not be below the freezing point of water.
If the temperature in the gaseous phase 11 in the upper part of the reactor container Z is maintained at at least 2-3'C below the equilibrium temperature at the prevailing work-ing pressure by supplying a sufficient amount of cold water as the cooling medium, gas hydrate will be generated and take the form of a slurry comprising particles of gas hydrate in water. Just after generation this material will have a consistancy and a look similar to wet snow and it will contain a larqe percentage of non-converted water W 096/34226 PCT~N096/00098 Generated gas hydrate and non-converted water will gather in the lower part of the reactor container 2. Gas hydrate is, just as ice, lighter than water, and the slurry comprising gas hydrate and water will to a certain degree separate into one upper fraction containing substantially all of the gas hydrate as the water containing gas hydrate slurry, and a lower fraction comprising non-converted water and residual amounts of gas hydrate particles. However, the distinction between the two fractions may be unclear or non-existant if the floating phase comprises relatively largeamounts of gas hydrate particles and the material is moving or turbulent.
During the generation of hydrate non-converted water having a temperature T=T2 (which is a little above the generating temperature T=T) is discharged from the lower part of the reactor container 2 through the pipeline 13.
When required, water can also be discharged from the system via a pipeline 19 connected to the pipeline 13. Water to be returned to the hydrate-generating zone is applied through a pump 14 and a heat exchanger 17 returning to the water inlet 5 between the pipelines 16 and 18.
The heat exchanger 17 can be cooled by a suitable, external cooling medium. If large amounts of seawater having a low temperature, e.g. 5 C or below, are available, it is viable as a cooling medium. In many cases it would, however, be more proper to use a cooling medium such as propane, ammonia or similar media for cooling the recirculated water, since such media having a normal boiling point substantially below O'C give higher temperature differences and accordingly also more compact heat exchangers 17.
The water used for production of gas hydrate has to be replaced by supplying more water.
When the desired amount of gas hydrate has been gene-rated in the reactor container 2, the process step a is finished and accordingly the water supply is stopped, e.g.
by closing a valve (not shown), and non-converted water is separated from the hydrate in step b, e.g. by draining. If required a filter (not shown) can be installed above the W 096t34226 PCTA~O~G

discharge pipe at the bottom of the reactor to avoid loss of gas hydrate.
Considerable amounts of water will still be bound to the hydrate a~ter such a simple draining process, mainly as a film of water on the outer surface of the hydrate particles and in between the particles due to capillary attraction. These remaining amounts of water may be removed, as mentioned in the general part of the specification, in different ways previously known per se. Additional amounts of hydrate-forming gases and a cooling hydrocarbon agent may for example be supplied to flow through the hydrate com-pound, and thus cause a conversion of remaining water into gas hydrate. This takes place in process step b, but in this embodiment in the same container.
When the substantial amount of free, non-converted water has been removed, a second cooling medium containing hydrocarbon is supplied to the hydrate compound, which is still kept in the reactor container 2 during process c. This second cooling medium containing hydrocarbon is supplied through an inlet 25 for the cooling medium. Accordingly the product may now be considered as being in the cooling zone 80, even if the product itself has not been taken out of the container 2. As described in other places in the descrip-tion, the cooling zone 80 may possibly be arranged within a different container. The second cooling medium which is supplied to the reactor container 2 during the process step c, is supplied in such an amount and at such a temperature that the mixture of gas hydrate and hydrocarbon obtains the assumed final temperature T=T , at which the gas hydrate is stable or meta-stable at atmospheric pressure, i.e. general-ly at temperature T=T~=- 10 C or below. In the process stage d the stable end product is transferred to a storage tank 51.
A quite simple estimate based on the specific heat capacities of hydrocarbon and gas hydrate, will lead to indications about required amounts of hydrocarbon cooling medium with a certain amount of gas hydrate, a selected output temperature T=T of the gas hydrate, and the temperature T~ in the second cooling medium supplied as well W 096/34226 PCT~No~ Dn3S

as the final ~r~ature T=T4.
The second hydrocarbon cooling medium is preferably a mixture of light, liquid hydrocarbons, and in particular a so-called condensate fraction. This medium should preferably not contain components which may participate as wax or solids or possibly higly viscous materials on cool surfaces within the plant. At the same time the hydrocarbon liquid used as the second cooling medium as explained above, con-tains a minimum of hydrate-forming components.
Heated cooling medium, i.e. the second cooling medium after passing through the gas hydrate in order to to cool it, is discharged from the container at the temperatures T=T5, and is recirculated through a second cooling circuit which can comprise for example a pump 21, a heat exchanger 24 and the required circulation pipelines 20,23 and 25. The heat exchanger 24 is fed by a suitable cooling medium such as amonia, propane, mixtures of light hydrocarbons or freon.
Supplemental amounts of the second cooling medium containing hydrocarbon adapted to replace the amount of hydrocarbon liquid included in the end product, may be supplied via a pipeline 22 connected to the cooling circuit.
When the desired final temperature T~ has been reached in the gas hydrate within the container 2, the end product which can comprise gas hydrate particles within a hydro-carbon liquid, is discharged through the pipe 8 and thevalve 9, and transferred to a storing tank 51. Theoretically the end product may be stored in the same container 2, but a specific storage container 51 is preferred so that the generator 2 again is free for further production. To reduce the flow of heat into the storage tank 51, this tank may be insulated thermally by a suitable material 57. The tempera-ture in the stored gas hydrate may be controlled by draining and circulation of the hydrocarbon liquid through a separate - cooling circuit (not shown) connected to the container 51 via pipes 52 and 53. The storage tank 51 is provided with an outlet 64 adapted for the transfer of the hydrocarbon product or the end product (gas hydrate in a hydrocarbon liquid) to further transport, storage or processing units.

_ _ _ _ , W 096/34226 PCT~NO9~'~~D3~

Prior to the transfer of the product ~rom the reactor container 2, superfluous amounts of hydrocarbon liquid may be drained from the gas hydrate.
The end product will, as previously explained, contain particles of gas hydrate suspended in or surrounded by a liquid containing hydrocarbon at the temperature T4. The dimensions and the shape of the particles will vary, and will be a result of process conditions and possible post treatment of the gas hydrate compound. The size of the particles may vary from fractions of 1 mm up to several centimeters, all within the scope of the present invention.
A stirring apparatus 31,32, respectively 55,56 may be installed in the hydrate-generating zone 1/the cooling zone 80 and/or in the storage zone 50. Such stirring units may be required to obtain a sufficiently fine grain in the materi-ale and effective heat exchange between the components at different stages of the process. The stirring of the product in the storage container may also reduce the tendency for sintering in the end product.
As an alternative to supplying the gas through the pipeline 7 to the upper part of the reactor 2, the gas may be supplied to the lower part of the container through a pipeline 61. Using such a method for supplying the gas, the gas may be bubbled through a mixture of solids and liquids in the lower part of the reactor 2. Such a solution willcontribute to a high concentration of hydrate-generating components from the gas in the liquid phase, and therefore also contribute to a powerful generation in the liquid phase during step a, and possibly step b of the process. Non-converted gas or gas which to a large extend has been poorwhere hydrate-forming components are considered, may, when such an embodiment is used, be discharged from the plant as a flow of gas through an outlet 62 close to the top of the reactor container 2. Supplies of gas from both upper and lower parts of the container 2 may also be combined.
A further modification of the embodiment described above is that water is replaced, completely or partly by a hydrocarbon medium already as a first cooling medium. This W O 96/34226 PCTAN09~J'~C9 may take place as the cooling circuit for hydrocarbon liquids is connected to the reactor container 2 which, according to the Figure 1, comprises the circulation pump 21 and the heat exchanger 24, which is constructed in such a 5 manner that the cooling requirements of step a is covered by circulation of a hydrocarbon medium instead of water. If a substantial part of the hydrate generation is to take place within the gas-filled volume 11 in the reactor container 2, it is necessary that the cooling medium containing hydro-10 carbon is supplied at least partly to this volume of gas, preferably as droplets (sprayed or flooded in) through an alternative supply line 25' (indicated by means of a dotted line in Figure 1).
In Figure 2 a different embodiment is shown, substan-15 tially distinguished from the embodiment in Figure 1 as non-converted water is not recirculated during the generating step a, but only flows through the plant one single time (in principle once). Gas is supplied as previously via the pipeline 7. Cooled water, preferably cold seawater, is 20 supplied to the reactor container 2 through the pipeline 5 and the nozzles 6, both as a raw material for generating hydrate and as the first cooling medium. During steps a and b non-converted water will gather at the bottom of the reactor container 2. This amount of non-converted water is 25 just discharged through the pipeline 19. Diluted gas which may be found in the discharged water, may if required be removed by means of a hydrocyclone 41 or a similar liquid/-gas separator. In many embodiments it will however, be possible to reduce the pressure so much that the remaining 30 amounts of diluted gas may be removed from the water and handled in a suitable manner without any use of other G equipment than a simple gas/liquid separator.
The supplied amount of cooled water through the pipe 5 - and the nozzles 6 may be controlled, e.g. by means of 35 valves, so that all the heat which is generated in the process is removed from the reactor container 2 as heated, non-converted water through the outlet 19. In this manner the need for further cooling is reduced or avoided. A better cooling is in other words obtained simply by increasing the input of cooling water from the pipe 5.
The reactor or the hydrate generator 2 will be exposed during the working process to a medium high pressure (50-80 bar a). Even if substantially larger amounts of water have to be pumped through the reactor against this pressure, a corresponding increase of the pumping power is not re~uired.
In a rather simple way a pressure sluice assembly may be arranged in which the discharged flow of liquid at a high pressure is sluiced against an inwardly directed flow of liquid at low pressures. Theoretically, only the water which has been used for generating gas hydrate within the reactor will require external pumping power.
The central outlet 43 from the hydrocyclone 41 will contain hydrocarbons as gas or liquid, and these hydrocar-bons may again be put under high pressures whereupon they may flow back into the process loop, or they may be used as power source for motors in pumps, compressors and similar e~uipment in a plant, e.g. by using suitable engines.
The hydrate compound may preferably be cooled down to a temperature at least 15CC, typically 20-30CC, below the prevailing temperature in steps a and b of the process.
Accordingly the pressure requirements for the storage container 51 is much lower than the pressure requirements for the generator 2 which has to endure a pressure of at least 60 bar. Accordingly it may be preferred to let step c and step d be effected in a different container than the reactor which has been used in step a and step b of the process.
A process plant using separate cooling in a specific container 81 is shown in Figure 3, in which the reference number 80 still refers to the cooling zone where the step c is undertaken. The cooling container 81 is itself preferably insulated by a layer of heat insulating material 82. In Figure 3 it is also assumed that, during draining of liquids in process step b it may be the case that the liquid discharged from the bottom of the reactor 2 through the pipe 75 will contain a mixture of a liquid medium containing W 096/34226 PCT~NO~ c~9s hydrocarbon and water. This mixture may be separated in a specific separator 78. When step b is finished in the reactor container 2, the hydrate compound is transferred to the container 81. The fluid communication through a tube 8 5 which connects the gas volumes 11 and 86, found in upper parts of the containers 2 and 81 respectively, will give the required pressure compensation which allows easy passage of compound into and out of the container 81, as soon as the valve 9 is opened. The hydrate compound in the container 81 is cooled down, in a similar manner as described in step a, by direct cooling using recirculation of a second cooling medium containing hydrocarbon through a cooling loop this time comprising a heat exchanger 87.
When the hydrate compound is cooled to the desired temperature which is preferably below -10~C, the compound is transferred to the storage tank Sl, of which a small part is seen in the lower part of the Figure.
In connection with the embodiments shown above as a plant adapted for carrying out the method according to the present invention, the following possibilities for modifi-cations should be mentioned:
When water has been drained off in step b, the hydrate compound which still may contain small amounts of free water, can be exposed to an additional, hydrate-generating step in which the free water is brought into contact with hydrate-forming gas components such as-methane, ethane and propane. This may take place for example as such components can be supplied through a pipe 61 (Figure 3) near the bottom of the reactor 82. In this manner an additional drying of the hydrate compound can be obtained, and the result is a hydrate compound comprising only gas hydrate without free waters or with ~uite insignificant amounts of free water.
Larger amounts of free water in the hydrate compound, would - as already mentioned, lead to problems when the hydrate is cooled down in step c in the process, as the free water would then freeze to ice and form bridges of ice on and between the surface of each gas hydrate particle. However, as mentioned above, very small amounts of free water may be W 096/34226 PCT~NO

tolerated.
In connection with certain embodiments it is deemed preferable that the second cooling medium should not include hydrate-forming components or that such components are not present in this step of the process, as such components could lead to reduced stability in the end product. In such cases it is recommended that the content of volatile components in the hydrocarbon medium be kept at a level which gives a partial pressure from the hydrocarbon, at the storage temperature below the pressure in the environment.
This again may be obtained if the second cooling medium, at least quite at the end of step c, is a hydrocarbon medium which is comprised substantially of hydrocarbons with at least five carbon atoms.
The hydrate compound obtained after the steps b, c or d, may be exposed to a draining or compressing stage in which superfluous humidity is reduced, removed or squeezed out. A proper composition of the end product is a suspension having approximately 80 volume % hydrate and approximately 20 volume % liquid hydrocarbon, mainly identical with the second cooling liquid but possibly including small fractions of free water in a frozen condition, and residual amounts of the first cooling liquid if this liquid had a different composition from the second cooling liquid.
A somewhat more detailed description of a realistic plant for carrying out the hydrate-generating process which is explained in connection with the principle drawings above, is also given in the following example referring to Figure 4. Here too, the capacity of the plant is vaguely assumed and the values of some of the parameters are given as calculated examples.
Three parallel reactors are now used, in the Figure 4 referred to as 2A,B,C; and of which only 2A is shown in detail, while the containers 2B and 2C are referred to only as their connections are shown. The reactor containers 2A, 2B and 2C will, when working, be at different stages in the production process, so that the reactors transfer the manufactured hydrate in sequence to the cooling tank 81, W O 96t34226 PCT~N09'!~009 which may be common for all the reactors. How many reactors 2 which may be connected to a common cooling tank, depends among other things on how long the different steps of the - process need. The figure shows the situation at the end of 5 the process stage a, and the text is referred to on the r figure ~or understanding the positions of di~ferent valves, and which fluid flows are affected at this stage.
Here follows a short description of the reactor container 2A and its associated units in the plant, to the 10 point where the finished end product is transferred to the storage tank 51 (lower right on Figure 4).
The hydrate-generating process is based on the use of seawater both as hydrate-generating water and as cooling water in the reactor arranged according to the "once through"
15 principle which, as the name assumes, only uses one single through-flow of the water which is to be included in the hydrate. According to this the seawater which is supplied, flows through the pump 100 and the water inlet 5, through the divided hydrating reactor 2A and is then lead directly out into the sea (after simple treatment in a hydrocyclone plant 41). The incomming seawater is at a temperature of 8''C
pumped into the reactor system by means of the seawater pump 100. The reactor 2A operates at 60 bar a. Within the reactor the seawater is evenly distributed within the total volume by means of nozzles 6 installed in the ceiling and/or in the cylinder wall. The generation of hydrate takes place as the seawater contacts the natural gas which has entered the system via the pipe 7. In the bottom of the reactor 2 the temperature is about 13'C (the equilibrium temperature~. The natural gas which is entering the reactor system may e.g.
amount to 700 OOO Smj/d (standard cubic meter each day). The reactor 2A itself is a so-called "semi-batch' unit in which the generation of the hydrate takes place continueously, while the output of the product takes place in batches as the hydrate product at intervals is discharged into a gathering tank 2A, arranged below the reactor 2A.
As mentioned above the reactor system consists of three parallel reactors, 2A/BtC, which may have separate or (as W 096/34226 PCTA~O~GI'~

shown on Figure 4) one common collector tank 81. The units are sequencially controlled, so that they operate in cycles where each cycle consists of three sequences or intervals.
In the first interval the reactor 2A is discharged of the hydrate product and seawater as the valve is open between the reactor and the collector tank Z'. At the same time the discharge line for seawater close to the bottom of the reactor is closed. When the reactor 2A is emptied, a valve between the reactor 2A and the collector tank 2A is closed.
Then as much as possible of the seawater which has been included in the hydrate compound, is squeezed out from the collector tank 2A, e.g. by means of gas supplied under pressure. The hydrate compound which in this manner has been "dried" is supposed to have a compacting density of approxi-mately 130 Sm3 gas/m3 hydrate.
When the seawater has been squeezed out of the product,the second interval starts, in which the hydrate product in the reactor 2A is flooded with condensate delivered from a cooling tank 81 by means of condensate pump 101. In this manner a slurry product is obtained, containing hydrate and condensate, and this slurry product is more easy to handle.
In the third and last interval this hydrate slurry is transferred from the collector tank 2A to the cooling tank 81, in which the hydrate slurry is cooled down to -20'C. The power behind this operation is the great differensial pressure between the collector tank 2A (60 bar) and the cooling tank 81 (15 bar).
Each interval is calculated to 4 minutes and accordingly one cycle of the process is 12 minutes. The sequential operation of the three reactor units A, B, C is controlled by a control system not shown, in such a manner that each unit at a given time works in different intervals.
In that manner the adjacent common process equipment such as the cooling tank 81, the condensate pumpe 101 etc. work 3S continuously against that of the reactors 2A, B or C which at each time is connected. During the different intervals it is necessary to compensate the pressure between the reactor 2A and the collecting tank 51. This is achieved by means of W 096/34226 PCTnN096/00098 an open pressure balancing connection (not shown) between the two tanks). The cooling down of the hydrate product takes place secondarily within the collector tank 81 in which the hydrate slurry is cooled down during flooding by S means of cooled condensate (-20CC). As said hydrate slurry from the collector tank 2A is partly cooled down, the cooling tank 51 may operate at 15 bar without any problems in connection with dissociation of the hydrate product. The complete cooling down process is obtained by means of a cold condensate circuit 87 connected to the cooling tank, and in this the filtrated condensate is delivered from the cooling tank at -20~C, is then cooled further down to -30~C in a circulation cooling loop 87 for condensate, and then returned to the cooling tank 81. In the circulating cooler 87 the condensate is cooled down by evaporation of propane by means of a cooling circuit compressor and a propane condensator 79 (based on seawater).
The cooled slurry product from the cooling tank 81 is supplied to a hydrate/condensate separator 111, in which the product is separated as a hydrate paste (20 volume % conden-sate and 80 volume % hydrate) and stored at atmospheric pressure. Separated condensate is returned to the cooling tank. Make-up condensate is added to the cooling tank 81 to cover the requirement for condensate included in the hydrate product (the paste).
Excess seawater from the reactor units 2A, 2B, 2C is treated first in a plant comprising flash tanks and a battery 41 of hydrocyclones, which respectively degases and separates oil/condensate droplets from the seawater which afterwards is let out into the ocean.
Below follows a list showing capacity, power ~ requirements, pressure and temperature at some important places in the plant:
- seawater inlet (at 5) 3495 m /h 35 seawater pump (100) 9015 kW differensial pressure 65 bar, gas inlet tat 7) 700.000 Sm3/d hydrate reactor (2A) 60 bar, 13 c CA 022l9244 l997-l0-27 W 096/34226 PCTA~09~'C0~9 outlet of seawater (from 2A) 1098 m3/h slurry valve (from 2A) 673 m~/h, oGC, 15 bar cooling tank ( 81) 15 bar circulating pump (for 87) 585 kW
condensate cooler (87) 11465 kW, -20GC to -30CC
Besides, the text on Figure 4 states how the different sequences are mainly controlled.
The method and the apparatus can be modified in many ways within the scope of the claims below.
10 Below some specific conditions which may be of some importance when the present invention is to be implemented, are mentioned. To obtain a stable end product such as this term is defined above, the hydrocarbon carrier in the end product should have a low content of volatile hydrocarbon components. This may be obtained in two different ways:
1) By replacing one hydrocarbon medium (used as the second cooling medium) which comprises many volatile components, with a cold hydrocarbon medium having a low content of such components.
2) After reduction of the pressure, i.e. when the pressure is released and has come down to the pressure in the environment, the volatile components which are released from the hydrocarbon medium (the second cooling medium) are released as gas, if the hydrocarbon containing medium at the end of step c still incorporates significant amounts of volatile components.
The stabilizing may of course also include a combi-nation of these techniques.
When step c is finished, the product will still be under high pressure (approximately the same pressure as in step a) within the cooling zone 80. Normally the end product will therefore be exposed to pressure in the environment when removed from the cooling zone.
Releasing the pressure may take place while the hydrate product is still in the cooling zone 80, or at the same time as the hydrate product is taken out of the cooling zone.
Remaining amounts of volatile (destabilizing) components in the hydrocarbon medium will in both cases be released as CA 022l9244 l997-l0-27 W 096/34226 PCTA~O9~C~9 gas. Such gas may be taken away, eventually to be recom-pressed and returned to the earlier stages of the process.
If pressure release takes place at the same time as the hydrate product is removed from the cooling zone 80, the final stabilizing (removal of remaining amounts of volatile components) of the product may take place in the storage tank 51, so that the end product will not be ready until after such stabilizing in the storage tank.
The end product itself may have a paste or slurry consistence, and the size of the hydrate particles may vary within wide limits so that the hydrate may comprise rather big lumps with a dimension going up to several centimeters within a liquid hydrocarbon medium. Of course it may also be preferable with hydrate particles of varying size in one end lS product, as the small hydrate particles will fill up spaces between larger hydrate particles without any substantial reduction of the gas content.
Of course the storage tanks have to be dimensioned to endure a certain excess pressure. If the pressure in the environments is 1 bar, this does not mean that the final pressure also has to be 1 bar. With an excess pressure of 0,5 bar the final pressure in the end product may for example be approximately 1,5 bar.
When it is stated that the second cooling medium shall have a partial pressure which at the final temperature is below the final pressure, it is accepted that the cooling medium may contain a certain amount of volatile hydrocarbons such as isobuthane and propane, and this does not jeopardize the stability requirements. However, it is an assumption that the sum of the partial pressures of the single com-ponents in the cooling medium mixture is below the final pressure as stated in connection with Henrys formula in the specification.
If the method used leads to such a condition that the water added to the hydrate-generating zone is so strongly cooled that it comprises or consists of ice or snow, the hydrate conversion and the temperature control taken place in process step a has to go on until all ice or snow has W 096/34226 PCTA~096/00098 32 been converted into hydrate and meltet water.
It is also preferred that the process conditions during step a) are set so that an end product is obtained in which the solid hydrate-containing material has a gas content corresponding to a density of at least 130 Sm~/m3, preferably above 150 Sm3/m3 solids, when methane is used as the hydrate-generating hydrocarbon.
It should also be emphasized that the hydrate-generating pressure and temperature conditions in process step c has to be maintained until the hydrate compound has reached a temperature at which the tendency to dissociate in the generated hydrate will be negligible for practical purposes. If cooling takes place at a high speed, this temperature will be reached just after the moment at which the water freezes.
Finally it should be emphasized that the final pressure or the storage pressure is normally preselected according to design requirements for containers and piping. The final pressure is a nominal pressure which mainly is dependent on the construction of the plant.
The method, the plant and the product according to the present invention may be used in different industrial connections. Thus the invention may be used for conversion of natural gas into a hydrocarbon product which may be stored and transported at technically speaking simple conditions. The method may therefore be used in connection with production and transport of natural gas from primary gas fields, especially from remote gas fields to a terminal plant in connection with the market or the location of the users. Gas from so-called associated gas fields, that is oil fields which in addition to the hydrocarbon liquid contain larger or smaller amounts of gas components, may also be converted into hydrocarbon products according to the present invention, and the conversion of such gas may accordingly lead to a profitable oil and gas production from such oil and gas fields.
Further, the invention may be used where there is a need for ta~ing care of and storing volatile hydrocarbon W 096/34226 PCT~N036/C~~9 compounds during a shorter or longer period. Such needs may also rise where superfluous gas is developed in connection with oil production or oil raffing, in connection with loading and unloading as well as transporting crude oil, where gathering of volatile connections (VOC) from the crude oil, and in connection with loading refined products as petrol, diesel etc.
The product according to the invention may be used for several purposes, e.g. as a medium for storage and transport of natural gas, as a fuel for motors or in heating plants, or again as a source of natural gas components and light hydrocarbon liquids which may be further treated in petro-chemical plants. In particular it may be of interest to use a product as a propellant for vessels, e.g. as an environ-mental friendly propellant for ferries.
The plant according to the invention may be installedon vessels or on offshore platforms, or may be built as stationary plants on land.
There follows below a short explanation of Figure 4, in particular so as to indicate capicities in a practical embodiment of such a hydrate plant.
Figure 4 shows the actual generating and cooling part of a hydrate-generating plant according to the present invention, the plant being based on the use of seawater both for the hydrate generation and for cooling. Presumably it is possible to obtain a packing degree of about 130 Sm3/m'.
The elements of the figure will be described broadly from the inlet at the upper, lefthand part of Figure 4, to the outlet at the lower, righthand part of Figure 4.
At the lefthand side of the figure there is shown a hydrate reactor 2A and a collecting/flushing tank 2A
connected thereto. It is to be understood that the plant can comprise several hydrate reactors connected in parallel, as indicated in the figure by arrows leading respectively to and from such parallel reactors, as denoted 2B and 2C.
The pump P-100 takes in seawater to all the generators 2A, 2B, 2C through the supply pipes 5 in an amount of approximately 3.500 m /h. The seawater is considered to have a temperature o~ about 8~C and the pump P-100 will increase the pressure by about 65 bar. The power consumption of this pump will then be about 9 MW.
Gas is introduced into the reactors through supply conduits 7 ~rom the oil and gas processing in an amount of approximately 700.000 Sm3/day. Superfluous seawater is removed from the reactor via the treatment plant 41 for discharged water, and it is estimated that the total output of seawater at 42 is about 3.300 m3/h at a temperature of about 13~ C. Moreover it is seen from the le~thand side of figure 4 how the reactor 2A, 2A can be subdivided into an upper reactor operating under a pressure of about 60 bar and where the temperature at the bottom of the reactor is about 13~ C, and a lower collecting/flushing tank where the pressure can also be about 60 bar. At the bottom of flush-ing tank 2A there is indicated a sieve located for retain-ing hydrates being thereby collected above this sieve.
Through pipe connections and control valves hydrate slurry is conveyed from all the reactors in the plant according to a controlled se~uence, to the cooling tank 81 which operates at a pressure of about 15 bar. The cooling in the cooling tank is effected by means of the circulation cooler 87 which can for example be cooled by means of a propane cooling circuit 79, which in turn is cooled with seawater being supplied by means of the pump P-104, which has an estimated power consumption of somewhat less than 400 KW. The actual propane circuit 79 will have a power con-sumption of somewhat less than S MW and the propane cooling circuit 79 is considered to cool the condensates in the circulation cooler 87 down to about -30' C. The condensate is circulated by means of the pump 102 which has an esti-mated power consumption somewhat less than 600 KW when handling about 3.000 m condensate per hour. The slurry product containing hydrate is delivered at the bottom of the cooling tank 81 and is conveyed to the hydrate/condensate separator 111 which returns condensate to the cooling tank and discharges the finished hydrate product, which may well be in paste form, further for storage in tank 51 being W 096/34226 PCT~NO95'dG098 located outside this figure of the drawings.
Moreover the figure contains the most important valves and pumps and connecting pipes being necessary for imple-- menting the generating and cooling part of the plant. The figure illustrates in particular how several reactor tanks can cooperate with a single cooling tank.
-

Claims

C L A I M S

1. A method for manufacturing a hydrocarbon product in the form of a hydrate of at least one hydrate-forming hydrocarbon enclosed in or suspended in a liquid containing hydrocarbon, c h a r a c t e r i z e d in that the product is manufactured by a process including the following steps:
a) bringing together hydrate-forming hydrocarbons and water in a hydrate-generating zone (1) under hydrate-generating process conditions, as heat energy produced during hydrate generation is removed from said zone by direct cooling with a first cooling medium, which, when the cooling medium is water, is obtained in a manner known per se, to provide a first intermediate product which includes a suspension of hydrate in a liquid carrier containing residues of the first cooling medium and possibly non-converted water, and having an average temperature at or above the freezing point of water, b) removing any non-converted water from said first intermediate product in a way known per se, so that a second intermediate product being substantially free of non-converted water is obtained, c) cooling of said second intermediate product in a cooling zone (80) by direct cooling with a second cooling medium containing hydrocarbon, to an average final temperature below the freezing point of water, which temperature is so low that the hydrate is stable at an final pressure substantially equal to the surrounding pressure, whereupon d) the compound of material containing hydrate in surrounding hydrocarbons are relieved of pressure and removed from the cooling zone; so that a stable end-product is obtained comprising particles of a solid, hydrate-containing material surrounded by or suspended in a liquid hydrocarbon carrier medium having a vapour pressure which at the final temperature is below the final pressure.

2. A method according to claim 1, c h a r a c t e r i z e d in that the final pressure is approximately 1 bar.

3. A method according to claim 1 or 2, c h a r a c t e r i z e d in that the process conditions in step a) are set so that a first intermediate product is generated free of frozen water.

4. A method according to claim 1, 2 or 3, c h a r a c t e r i z e d in that steps a) and b) are carried out in such a manner that the solid, hydrate-containing material in the final product obtains a gas content corresponding to a packing density of at least 130 Sm3/m3, preferably above 150 Sm3 gas/m3 solids.

5. A method according to one of the claims 1-4, c h a r a c t e r i z e d in that the first cooling medium is water, possibly seawater, being supplied to the hydrate-generating zone at a temperature below 8°C, preferably approximately 4°C.

6. A method according to claim 1, 2, 4 or 5, c h a r a c t e r i z e d in that the first cooling medium is a liquid of liquid hydrocarbons, which is supplied to the hydrate-generating zone (1) at a temperature below 0°C.

7. A method according to claim 1, 2, 4, 5 or 6, c h a r a c t e r i z e d in that the water which is supplied to the hydrate-generating zone, completely or in part consists of water in a frozen state, such as snow or ice.

8. A method according to one of the claims 1-7, c h a r a c t e r i z e d in that the second cooling medium mainly contains hydrocarbons having at least 5 carbon atoms per molecule.

9. A method according to one of the claims 1-8, c h a r a c t e r i z e d in that the cooling of the second intermediate product in step c) is carried out in the presence of hydrate-generating hydrocarbons until the temperature in the hydrate compound has reached a temperature below the freezing point of water, preferably a temperature at which the hydrate mainly is stable.

10. A method according to one of the claims 1-9, c h a r a c t e r i z e d in that a liquid hydrocarbon medium which after the cooling process of the second intermediate product in presence of hydrate-generating hydrocarbons in step c), comprises destabilizing amounts of volatile components, is replaced by a liquid hydrocarbon medium which has a lower content of such components.

11. A method according to one of the claims 1-10, c h a r a c t e r i z e d in that destabilizing amounts of volatile components from the liquid hydrocarbon carrier medium after the cooling of the second intermediate product in the presence of hydrate-forming hydrocarbons in step c), are removed by pressure relief and degasing of the mass of hydrate-containing materials in the surrounding hydrocarbon carrier medium.

12. A method according to one of the claims 1-11, c h a r a c t e r i z e d in that step a) and step c) are carried out in separate pressure containers.

13. A method according to one of the claims 1-12, c h a r a c t e r i z e d in that the steps a), b) and/or c) are carried out consecutively in one common pressure container.

14. A method according to one of the claims 1-13, c h a r a c t e r i z e d in that step b) is carried out completely or partly by having additional amounts of hydrate-forming hydrocarbons added to the first intermediate product, so that possible remaining amounts of non-converted water in the first intermediate product are converted into hydrate.

15. A method according to one of the claims 1-14, c h a r a c t e r i z e d in that the process is carried out in a process plant comprising two or more parallel production lines for carrying out the steps a), b), c) and d), preferably in such a manner that the different phases of the process (which comprises one or more of the steps a), b), c) and d)) are carried out in a cyclic; such manner that the complete plant obtains an even consumption of gas.

16. A method according to one of the claims 1-15, c h a r a c t e r i z e d in that the first cooling medium comprises at least one hydrate-forming hydrocarbon which in the hydrate-generating zone is completely or partly converted into hydrate.

17. A method according to one of the claims 1-16, c h a r a c t e r i z e d in that the portion of non-hydrate-forming hydrocarbons in the liquid hydrocarbons which are used as the first and/or the second cooling medium, is comprised substantially by a C5- to C10-petroleum fraction, preferably a condensate fraction.

18. A method according to one of the claims 1-17, c h a r a c t e r i z e d in that the hydrate-forming hydrocarbons and finely dispersed water are brought together in a gas-filled volume of the hydrate-generating zone and that the first cooling medium completely or partly is supplied in droplets to the gas-filled volume of the generating zone (1).

19. A method according to one of the claims 1-18, c h a r a c t e r i z e d in that a portion of the first cooling medium is added to that part of the hydrate-generating zone (1) in which liquid and hydrate are collected.

20. A method according to one of the claims 1-19, c h a r a c t e r i z e d in that at least one of the intermediate products and/or the end product is exposed to shear forces or to a mechanical treatment, e.g. by stirring, thus reducing the hydrate particle size and providing for fine distribution of hydrate particles in the surrounding, liquid medium.

21. A method according to one of the claims 1-20, c h a r a c t e r i z e d in that the second cooling medium is supplied to the cooling zone (2) at a temperature lower than -20°C, preferably approximately -40°C.

22. A method according to one of the claims 1-21, c h a r a c t e r i z e d in that the final temperature is lower than -7°C, and preferably between -10°C and -35°C.

23. A plant for producing a hydrocarbon product in the form of a hydrate and at least one hydrate-forming hydrocarbon suspended in a hydrocarbon-containing liquid, c h a r a c t e r i z e d in that the plant comprises:
- a hydrate-generating zone (1) which in a way known per se is provided with inlets (7,61) for hydrate-forming hydrocarbons, means (5,6) for supplying water and a first cooling device (20,24,25) adapted for direct cooling of the contents in the zone (1) with a first cooling medium, down to a first temperature (T=T1) which in average is equal to or higher than the freezing point of water, - means as known per se for removing non-converted water from a first intermediate product being produced according to step a) in claim 1, in the hydrate-generating zone (1), and - a cooling zone (80) being equipped with a second cooling device (24,87) adapted for direct cooling of a second intermediate product being obtained according to step b) in claim 1, with a second cooling medium, down to a final temperature (T=T4) which in average is lower than the freezing point of water and so low that the hydrate is stable at a final pressure being substantially equal to the ambient pressure, and an outlet (8,27) for an end product obtained.

24. A plant according to claim 23, c h a r a c t e r i z e d in that the first and/or the second cooling device comprises a recirculating cooling circuit adapted for repeated cooling of separated portions of the first or the second cooling medium, and pipe connections adapted to feed back recirculating cooling medium for re-use in direct cooling.

25. A plant according to claims 23 or 24, c h a r a c t e r i z e d in that the hydrate-generating hydrocarbons are introduced through gas inlets at the top (7) and/or at the bottom (61) of the hydrate-genrating zone (1).

26. A plant according to one of the claims 23-25, c h a r a c t e r i z e d in that the hydrate-generating zone (1) and the cooling zone (80) are formed by separate pressure containers (2,81) with a suitable pipe connection (8) between them.

27. A plant according to one of the claims 23-26, c h a r a c t e r i z e d in that the hydrate-generating zone (1) and the cooling zone (80) are formed by one and the same pressure container (2 in Fig. 1 and Fig. 2) and that the pressure container (2) is provided with means (19,41;75,78) for removing non-converted water from the first intermediate product.

28. A plant according to one of the claims 23-27, c h a r a c t e r i z e d in that the plant comprises two or more production lines (A,B,C), each comprising a separate hydrate-generating zone (1A,1B,1C), said production lines (A,B,C) being preferably controlled by an associated control system so that there is obtained an even output of gas from the plant in relation to the individual production line (e.g. A) which operates batchwise.

29. A plant according to one of the claims 23-28, c h a r a c t e r i z e d in that the means (6) for supplying finely distributed water are adapted to supply the water in finely distributed form to a gas-filled volume (11) of the hydrate-generating zone (1), and that said means (5,6) for supplying the first cooling medium are adapted to supply said medium completely or partially in drop form to the gas-filled volume (11) of the zone (1).

30. A plant according to one of the claims 23-29, c h a r a c t e r i z e d in that the hydrate-generating zone (1) and the cooling zone (80) are provided with means (31,32;55,56) subjecting the intermediate products and/or the end product to shear forces or mechanical treatment, e.g. stirring, resulting in a reduction of the hydrate particle size and a fine distribution of the hydrate particles in the surrounding, liquid medium.

31. A hydrocarbon product in the form of a hydrate of at least one hydrate-forming hydrocarbon suspended in a hydrocarbon-containing liquid, and manufactured according to a method as claimed in any of the claims 1-22;
c h a r a c t e r i z e d in that the product is provided as a suspension of solid, hydrate-containing material in a liquid hydrocarbon carrier medium at a storage temperature (T=T4) being lower than the freezing point of water and so low that the hydrate is stable at a storage pressure being substantially equal to the ambient pressure, the liquid hydrocarbon carrier medium having a vapour pressure being lower than the storage pressure at the storage temperature, and that the hydrocarbon carrier medium mainly contains hydrocarbons with at least 5 carbon atoms per molecule, preferably a C5- to C10-petroleum fraction, in particular a condensate fraction.

32. A product according to claim 31, c h a r a c t e r i z e d in that the complete volume of the solid hydrate-containing material is substantially higher than the complete volume of the hydrocarbon carrier medium, preferably equal to or larger than 70 volume-% of the total volume.

33. A product according to claim 31 or 32, c h a r a c t e r i z e d in that the storage temperature is lower than -7°C, and preferably between -10°C and -35°C,and that the hydrocarbon carrier medium has a vapour pressure lower than about 1 bar at the storage temperature.

34. A product according to one of the claims 31-33, c h a r a c t e r i z e d in that the solid, hydrate-containing material in the product has a gas content corresponding to a packing density of at least 130 Sm3/m3, preferably more than 150 Sm3 gas/m3 solid substance.

35. A product according to one of the claims 31-34, c h a r a c t e r i z e d in that it is able to be produced by a method according to claim 1.

36. A product according to one of the claims 31-36, c h a r a c t e r i z e d in that the solid, hydrate-containing material in the product is in the form of finely distributed hydrate particles.

37. A method according to one of the claims 1-22, c h a r a c t e r i z e d in that the process conditions for step a) are so set that there is obtained an end product in which the solid, hydrate-containing material has a gas content corresponding to a packing density of at least 130 Sm3/m3, preferably more than 150 Sm3/m3 solid substance, when methane is used as a hydrate-forming hydrocarbon.