NO953797L - Process and plant for treating a brönnström produced from an offshore oil field - Google Patents

Description

Technical area.

This invention relates to a method and a plant for treating a well stream produced from an offshore oil field. The invention also relates to a method for converting a natural gas, in particular an associated natural gas, into a synthetic crude oil via a Fischer-Tropsch synthesis, in particular for execution offshore on a ship, a platform or other installation. The invention also relates to a facility for carrying out such a method, placed on easily replaceable frames, especially for installation on an FPSO ship (FPSO = "Floating Production, Storage and Offloading").

Background for the invention.

When producing crude oil from an offshore oil field, the well stream is separated into water, oil and gas. The natural gas that accompanies the produced crude oil in the well stream, and which is often referred to as "associated gas", must be handled in one way or another after separation. Handling is often done by burning the gas or re-injecting the gas in the oil field, but it can also be taken ashore for further processing. Burning has become an unacceptable method of getting rid of the gas, as such burning represents a waste of ever-dwindling hydrocarbon resources and is also a source of air pollution. Re-injection, which entails additional costs for the extraction of crude oil, will often be unacceptable both because of the costs and because of any adverse effects on the extraction of crude oil from the field. The third solution to the problem, transport of the gas away from the field, e.g. through a pipeline, for treatment in a land-based facility, will in some cases on remote fields be an expensive and unsuitable solution.

Conversion of a natural gas into synthesis gas (CO + H2) and conversion of this into synthetic crude oil by a Fischer-Tropsch synthesis is in itself a well-known process, which is described in an extensive literature, see e.g. G. A. Mills, "Status and future opportunities for conversion of synthesis gas to liquid fuels", Fuel Vol. 73 (8), pp. 1243-79 (1994). At the end of the 80s, the process became the subject of renewed interest, for the treatment of gas that was brought ashore from offshore petroleum fields, e.g. in South Africa and Malaysia. As far as the inventors behind the present invention have been able to bring in experience, facilities based on the Fischer-Tropsch technology have so far not been installed offshore, e.g. on platforms, jack-up rigs, FPSO units (FPSO = "Floating Production, Storage and Offloading"), including e.g. production ships and shuttle tankers; FSU units (FSU = "Floating Storage Unit"), semi-submerged platforms, etc.

In recent times, shuttle tankers have become known which are designed to connect to underwater loading buoys and which at the same time keep the ship anchored. Such underwater loading buoys form a gathering point for one or more flexible risers and control cables from e.g. a production system on the seabed. The buoys are adapted to be raised and fixed in a reception room in the relevant vessel, in order to establish a transport system for the petroleum products from the system on the seabed to e.g. cargo tanks in the vessel.

Based on this technique, vessels have recently been developed which, using simple means, can alternate between operating as: a) a shuttle tanker that connects to an underwater loading buoy, b) a storage ship that is permanently connected to a underwater loading buoy, and which at the same time has unloading equipment at the stern of the ship to unload oil, and c) a production vessel which is connected to an underwater loading buoy which includes a swivel device.

A vessel of this type, which is based on cooperation with a submerged, bottom-anchored loading buoy which may include a swivel unit with several pipe runs, is described in NO 940352. The vessel has near its front end a submerged recording room for recording the underwater buoy and a service shaft which extends between the recording room and the vessel's deck. The underwater buoy has an outer buoyancy part which is designed for introduction and releasable attachment in the submerged, downwardly open recording space in the vessel, and a rotatably stored, central part in the outer part which is anchored to the seabed and is connected by at least one transmission line which extends from a respective production well up to the buoy.

When a buoy of this type is fixed in the reception space of a vessel, the vessel is rigidly attached to the buoy's outer buoyancy part and is rotatable around the buoy's central part which is anchored to the seabed using a suitable anchoring system. The buoy itself thus constitutes a turning body ("turret") around which the vessel is allowed to turn under the influence of wind, waves and water currents.

This bending construction entails a number of significant advantages. The central part of the buoy has a small diameter and low mass, so that a correspondingly small diameter of the rotating body, i.e. the outer buoyancy part of the buoy, and thus low rotational mass and rotational resistance is achieved. Connection and disconnection between vessel and buoy can be carried out easily and quickly, even in bad weather with relatively high waves. Furthermore, the buoy can remain connected to the vessel in almost all kinds of weather, as a quick disconnection can be made if a weather restriction were to be exceeded.

In a vessel which is adapted for use together with the mentioned buoy structure, as mentioned, the reception room and the shaft arranged above this are suitably arranged in the bow section of the vessel. This enables a relatively simple and affordable conversion of existing vessels for adaptation to such a buoy loading system, for use e.g. as shuttle tankers. The combination of a submerged recording room and a shaft that extends between the recording room and the vessel's deck also enables a system that provides high operational safety and a low risk of polluting emissions.

For a more detailed description of the buoy construction mentioned above and of a vessel of the above-mentioned type, reference can be made to international patent applications No. PCT/N092/00054, PCT/N092/-00055 and PCT/NO92/00056.

An advantageous adaptation of such a buoy loading system for offshore oil and gas production on a production ship is described in NO 922043. In the embodiment described there, the system comprises a swivel unit which is designed to be lowered to or raised from an operating position at the lower end of the shaft and for in the operating position to be connected to a pipe system on the vessel. The swivel unit comprises inner and outer, mutually rotatable swivel parts. At the upper end of the buoy, a coupling unit is arranged in which the appropriate number of transmission lines are terminated, and this coupling unit is arranged for connection to, or release from, a corresponding coupling unit on the underside of the swivel unit.

In an advantageous embodiment of the system, the swivel unit is mounted on a lifting and lowering tool which is slidably mounted in a guide rail device which extends between the upper and lower ends of the shaft. Thereby, the swivel unit with its coupling unit can be easily placed in the correct position in a coupling compartment at the lower end of the shaft. As the most critical components, the swivel and coupling units will be easily accessible for maintenance or replacement. Connection to and disconnection from the buoy's transmission lines can be carried out as a one-step operation, with automatic shut-off valves on both sides of the coupling units. Vertical movement of the swivel unit when connecting and disconnecting is appropriately accommodated by flexible pipes that are mounted perpendicular to the axis of the swivel unit.

A significant advantage of this system is that it provides small system dimensions due to the use of the special buoy which itself constitutes a rotating body. This results in weight savings and a reduced scope of equipment, which results in significantly reduced costs.

Such a system would require minimal conversion of shuttle tankers that are adapted for the buoy loading system mentioned above, for transition to production ships. With such a production ship, seasonal operations will also be able to be carried out, in addition to continuous production from marginal fields, and also trial production. The ship can e.g. used for trial production in the summer half-year during a period with any surplus of shuttle tankers.

As the vessel's wheelhouse and its engine room are located in the vessel's bow and the service shaft up from the vessel's reception room is located just behind the wheelhouse, the service shaft will be in the lee of the wheelhouse. In addition to the safety this provides for crews who are to carry out tasks in the shaft, such an arrangement provides a large deck area from the rear part of the wheelhouse and backwards to the rear deck area. When the vessel is to be used as a production vessel, this area will be able to be used for the necessary process equipment and for equipment for well control.

As the vessel must be able to switch between different business areas, it is preferable that the entire process plant is divided into smaller, moveable modules.

A vessel as described above would be very suitable as a carrier of a plant for converting associated natural gas into, for example, synthetic crude oil and/or wax. Such an arrangement will also bring advantages in that the system with swivel unit will also be suitable for use in connection with water injection, operation of water treatment plants, well stimulation, etc., which gives great flexibility in the use of the vessel. The system will also be suitable for use in waters with drift ice and icebergs, as it enables quick disconnection if necessary, without risk of damage to the submerged buoy.

As mentioned at the outset, the inventors behind the present invention are not aware that plants based on the Fischer-Tropsch technology have been installed offshore. However, modular gas conversion plants have been described or

-systems for converting associated gas or remote gas into synthetic crude oil, for installation on ships, offshore platforms and other offshore installations, see Dr. David D.J. Antia and Dr. Duncan Seddon "Exploiting New Opportunities for Cost Reduction and Addition of Value through Conversion of Offshore Gas to Crude Oil", presented at SECONS 1994 (Strategy

and Economics in the North Sea), London, 28-29 November 1994.

The above-mentioned publication describes modular gas conversion plants or systems that can be added to new and existing production systems offshore. The modular plants are used for converting natural gas into synthetic crude oil, wax or methanol. The publication focuses in particular on facilities designed for fields that produce from 5 to 50 MMCF/D (0.14 - 1.42 Mill. m<3>/day) of associated gas. Two types of facilities are considered in the article: (a) facilities designed to extract value from the gas, before it is re-injected into the field, and (b) facilities designed for a complete treatment of the gas by converting it into easier-to-handle and valuable products, for thereby avoiding having to burn, re-inject or transport the gas

away.

In both types of plant, the process comprises two main steps: (1) the natural gas is converted into a synthesis gas consisting of a mixture of carbon monoxide, hydrogen and carbon dioxide in a unit where partial oxidation is carried out, and (2) the synthesis gas is converted in a Fischer-Tropsch reactor ( FT reactor) to a synthetic crude oil. The process is stated to be a flexible process that allows conversion during operation to other end products within the range from light condensates to microcrystalline wax.

The equipment for the two main stages of the process can be placed in separate ranune-mounted modules or module groups. The structure of the facility on a modular basis is said to provide flexibility, which can be seen, among other things, in that the facility's capacity can be upgraded or downgraded as needed, or that the facility can be operated with parallel currents that provide different products, e.g. synthetic crude oil, wax and methanol.

For producing a synthesis gas from the natural gas in the first main step of the process, several methods are mentioned in the publication by Antia et al. The most important of these are partial oxidation, steam reforming, autothermal catalytic reforming and combined reforming. Partial oxidation is preferred due to advantages which are stated to be linked to the process's effectiveness, costs, flexibility in the choice of product composition, plant size, product yield, logistics and economic operation.

For the production of synthetic crude oil and/or wax in the second main step of the process, the FT synthesis, a number of different types of reactor can be used, namely MTFB reactors, which are multi-tubular reactors with a stationary catalyst bed (MTFB "Multitubular Fixed Bed"), reactors with fluidized catalyst bed, reactors with an annular catalyst bed, slurry reactors and Linde's isothermal reactor. Of these, preferred according to Antia is al. The MTFB reactor, on the grounds that it is well-proven and cheap and is also flexible in that it can operate over a wide temperature range. Regarding the slurry reactor, it is said in the publication that this has been the subject of extensive research but has not succeeded commercially.

In the FT reactor, iron, cobalt or ruthenium catalysts are used. All these catalyst types are said to be able to give products that can vary in composition from light condensates to heavy paraffinic oils or microcrystalline or paraffinic waxes.

Although much of the foundation has thus been laid for an economically sound and environmentally friendly handling of associated natural gas by converting it into easier to handle and valuable products, there is a need for improved solutions to achieve safer and more profitable operation.

Not least is the above-mentioned MTFB reactor, which Antia et al. considered to be the preferred reactor for use in FT synthesis, beset with disadvantages due to heavy weight, expensive and complicated construction and a narrow operating temperature range. In order to keep the pressure drop through the catalyst layer in the MTFB reactor acceptable, large catalyst particles must also be used, which entails diffusion limitations. For this reason and due to difficulties with temperature regulation in the reactor, the turnover of synthesis gas per review lower than desired. Replacing the catalyst is also complicated with this type of reactor, and it is also not suitable for highly active catalysts.

Aim of the invention.

Against the above background, it is an aim of the invention to provide a method and a facility for on board a ship to treat a well flow produced on an offshore oil field, using a ship that cooperates with an underwater buoy which both the ship and the risers from the field are anchored to.

Furthermore, it is an aim of the invention to provide a method for converting a natural gas, especially an associated natural gas, into a synthetic crude oil and/or wax, which method is suitable for execution in locations with limited space, e.g. offshore on a ship, platform or other installation.

It is a further aim of the invention to provide a simple, compact and reliable plant for converting a natural gas into a synthetic crude oil and/or wax. In particular, it is an aim to provide such a plant for the conversion of an associated natural gas, located on frames that can be attached easily replaceably to a ship, an offshore platform or other offshore installation, in particular an FPSO ship (FPSO » "Floating Production , Storage and Offloading").

A further aim is to provide a plant of the specified type which is easily adjustable for the purpose of manufacturing products with different specifications, and which is also easily adjustable with respect to its capacity.

Summary of the Invention.

According to a first aspect of the invention, there is now provided a method for treating on board a ship a well stream produced in an offshore oil field, using a ship cooperating with an underwater buoy to which both the ship and the risers from the field are anchored. , as a swivel unit is arranged in the ship, on the upper side of the buoy. The procedure is characterized by the fact that the well flow is first led to a processing plant which is placed on easily replaceable frames attached to the ship's deck on each side of a pipe passage centrally located in the ship's longitudinal direction, that water, oil and gas are separated from each other in this processing plant, that separated, stabilized oil is stored in at least some of the ship's tanks, and that the separated gas is taken to a facility for converting the gas into synthetic crude oil and/or wax, which is then stored in tanks in the ship, with the synthetic crude oil possibly being mixed with the stabilized oil.

According to a second aspect of the invention is provided

a facility for treating a well stream produced from an offshore oil field, which facility is arranged for installation on board a ship and includes a processing facility where oil, gas and water are separated from each other. The facility is characterized by the fact that it also includes a facility for converting the separated gas into synthetic crude oil and/or wax, and that this conversion facility includes at least a synthesis gas unit and a Fischer-Tropsch unit, and that the total facility (processing facility and conversion facilities) are placed on frames that can be attached easily replaceably to the ship's deck.

According to a further aspect of the invention, a method is provided, in particular for execution offshore on a ship, a platform or other installation, for the conversion of a natural gas, in particular an associated natural gas, into a synthetic crude oil and/or wax in two stages, where (1) the natural gas is converted into a synthesis gas consisting of a mixture of carbon monoxide, hydrogen and carbon dioxide in a synthesis gas unit, and (2) the synthesis gas is converted into a synthetic crude oil and/or wax in a Fischer-Tropsch synthesis. The method is characterized by the fact that the synthesis gas from step (1) for carrying out the Fischer-Tropsch synthesis is introduced into a slurry consisting of liquid products, finely divided catalyst particles and synthesis gas in a reaction zone in a slurry bubble column reactor

(SBCR reactor) where an internal separation of the liquid products from the rest of the slurry is carried out.

A preferred embodiment of this approach is that the synthesis gas from step (1), after cooling and separation of water, is introduced into the bottom of the reaction zone in the slurry bubble column reactor, the reaction zone being arranged both to accommodate the slurry consisting of liquid products, finely divided catalyst particles and added synthesis gas and to accommodate a volume of gas above the slurry phase, that liquid product is separated from the rest of the slurry by means of a filtration section including a chamber and a filter element which together define a filtrate zone with an outlet for the product filtrate, which filter element is arranged so that it is in contact with the slurry in the reaction zone, that fluid communication is created between the filtrate zone and the part of the reaction zone that contains the gas volume above the slurry phase, and that a mean pressure difference is created across the filter element.

According to yet another aspect of the invention, there is provided a plant for converting natural gas, especially an associated natural gas, into a synthetic crude oil and/or wax in two stages, in which (1) the natural gas is converted into a synthesis gas consisting of a mixture of carbon monoxide, hydrogen and carbon dioxide in a synthesis gas unit, and (2) the synthesis gas from this unit is converted to a synthetic crude oil and/or wax in a Fischer-Tropsch unit. The plant is characterized by the fact that the Fischer-Tropsch unit comprises one or more slurry bubble column reactors (SBCR reactors) comprising a reactor zone arranged to contain a slurry consisting of liquid products, finely divided catalyst particles and synthesis gas, and that the reactors e) are arranged for internal separation of liquid products from the rest of the slurry.

i A preferred embodiment of this plant is characterized in that each reactor comprises: a container defining a reaction zone arranged to contain both the slurry phase and a volume of gas above the slurry phase; devices for introducing the synthesis gas into the slurry phase in the lower part of the container; a filter

annular section arranged to separate liquid products from the slurry phase, including a chamber which at least partially surrounds the container, and a filter element which, together with the chamber, delimits a filtrate zone with an outlet for the product filtrate, the filter element being arranged so that it is in contact with the slurry in the slurry zone; devices for creating fluid communication between the filtrate zone and the part of the reaction zone which, during operation, will contain the gas volume above the slurry phase; and means for creating a mean pressure difference across the filter element.

In preferred embodiments, the plant is placed on frames that can be attached easily replaceably to a ship, an offshore platform or other offshore installation, in particular an FPSO ship (FPSO = "Floating Production, Storage and Offloading").

Brief description of the drawings.

Fig. 1 is a simplified flow diagram for an embodiment of the method according to the invention. Fig. 2 is a schematic section through a slurry bubble column reactor for use in the FT synthesis unit in the method according to the invention, Fig. 3 is a perspective sketch, partly in section, of a cargo and production ship with a bow loading system for loading hydrocarbons, with space for the installation of a facility for carrying out the method according to the invention. Fig 4 is a perspective sketch of a production ship with a modular plant according to the invention mounted on board.

Detailed description of the invention.

With reference to the attached fig. 1, a preferred embodiment of a method and a plant according to the invention for converting a natural gas, especially an associated natural gas, into a synthetic crude oil and/or wax in two stages shall first be described in the main features. An associated gas with a pressure of approx. 40 bar is heated to approx. 400 °C and introduced into an absorption tower 1, where sulphur, which will only be present in the form of H2S, is absorbed in a layer of ZnO particles. The desulphurised gas from the absorption tower is mixed with steam, and the mixture is heated to approx. 500 °C and introduced into an autothermal reformer 2. Oxygen extracted from air is mixed with water vapor and introduced at approx. 300 °C in the autothermal reformer. A recirculation gas from the FT synthesis, heated to approx. 300 "C. The autothermal reformer comprises a burner where the reactants are mixed, a combustion zone where hydrocarbons are burned with oxygen to CO and H20, a thermal zone and a subsequent catalyst-filled zone where the remaining hydrocarbons and water are converted to CO and H2, and where the equilibrium between CO and H20 on the one hand and between C02 and H2 on the other hand sets in (the water shift reaction). The ratio between the reactants and the reaction conditions in general is set so that the molar ratio between H2 and CO in the outlet from the autothermal reformer 2 is in the range of 1.6: 1 to 2.0:1.

The synthesis gas taken from the autothermal reformer is quenched to approx. 300 °C through direct injection of water into the gas. The synthesis gas is further cooled in a heat exchanger, and water is separated (not shown). The synthesis gas is then introduced at approx. 200 °C in the FT synthesis unit, which in the plant shown in the figure consists of two reactors 3. The components of the synthesis gas react with each other to form hydrocarbons and water in an exothermic process. The FT reactors are of the slurry bubble column reactor ("Slurry Bubble Column Reactor" = SBCR), and a Co-based catalyst is used on an aluminum oxide carrier. The term "slurry" here denotes a three-phase mixture of solid catalyst particles, liquid hydrocarbons consisting of products from the FT synthesis, and gas consisting of unreacted reactants and gaseous hydrocarbons formed by the FT synthesis.

The excess heat is removed by heat exchange with water through heat exchanger tubes placed inside the SBCR reactors 3. The hydrocarbons will be present both in gas phase and in liquid phase under the reaction conditions, which include a temperature of approx. 230 °C. Unreacted synthesis gas and gaseous hydrocarbon product are taken out at the top of the reactors 3. In the upper part of the reactor, a filter system is arranged which separates the catalyst from the liquid products.

The gaseous products from the reactors 3 are cooled (not shown) and introduced into a separation unit 4 where water is separated,

and where, in addition, a liquid stream consisting of synthetic crude oil is separated as the desired product. Part of the separated water is recycled to the reformer's 2 inlets. Part of the uncondensed gas obtained from the separation unit 4 is recycled to the reformer 2 inlet, while the rest of this gas can be used as fuel gas for heating the feed to the autothermal reformer and/or used for energy production in electric generators or for the production of fresh water from sea water. It is also possible to use parts of this condensed gas for injection purposes.

The two SBCR reactors 3 shown in the figure are connected in series, but they can also be connected in parallel, which is indicated by dotted lines in the figure. When connecting the reactors in series, it may be relevant to remove reaction water and liquid hydrocarbons (C5+) from the product stream after the first reactor in order to improve the efficiency in the second reactor. Each of the reactors 3 can also be run separately, when desired.

Any catalyst suitable for use in a Fischer-Tropsch synthesis for the production of synthetic crude oil and/or wax can be used in the SBCR reactors in the plant according to the invention, e.g. one of the known iron, cobalt, nickel or ruthenium catalysts for this application. A preferred catalyst is a cobalt-rhenium catalyst on an alumina support. The catalyst can optionally also be activated with metal from the group of rare earth metals. For example, a cobalt-rhenium catalyst can be used containing 20% by weight Co and 1% by weight Re of y_a12°3- Such a catalyst is described in US patent no. 4,801,573 and can be produced by impregnating y- Al2O3 with an aqueous dissolution of Co(N03)2-6H20 and HRe04 according to the pore filling method ("incipient wetness method").

With preferred FT catalysts, more than 85% turnover of CO per throughput (up to 98% is possible), a C5+ selectivity > 88% and a polymerization probability a according to the Anderson-Schultz-Flory distribution of 0.9-0.95.

A combination of the preferred catalysts and the described SBCR reactors gives high C54 selectivity, high turnover of CO per review, stable activity and regenerability of the FT catalyst.

In an autothermal reforming process, such as that used in the above-described plant according to the invention, partial oxidation and adiabatic steam reforming are combined. The product gas is in chemical equilibrium at the reactor's outlet temperature, which is determined by the inlet temperature and the adiabatic temperature rise. The process is carried out in a reactor with a stationary catalyst bed. Autothermal reforming requires less equipment than conventional steam reforming and is also a flexible process capable of producing synthesis gas of different composition, depending on the setting of the operating conditions.

For the production of the synthesis gas from the natural gas supplied to the plant, however, it will also be possible to use other versions of the reforming process. Among these can be mentioned steam reforming; combined reforming, consisting in a steam reforming with subsequent autothermal reforming; combined reformation with pre-reformation; partial oxidation; and gas-heated reforming, consisting of an autothermal reforming and a subsequent steam reforming. Other possibilities will e.g. be a combined autothermal reformer or reformer in a Kellogg reformer heat exchange system.

The Slurry Bubble Column Reactor (SBCR)

Among the three-phase system reactors in use in land-based Fischer-Tropsch plants, mechanically stirred slurry reactors and circulating and slurry bubble column reactors should be mentioned. In all of these, small catalyst particles are used that are dispersed in the liquid, and in most cases the liquid will therefore have to be separated from the slurry to remove liquid products or for catalyst regeneration purposes.

The operation of slurry bubble column reactors is simple, as mechanically moving parts are avoided. Therefore, and due to low diffusion resistance and efficient heat transfer, these reactors are attractive for many industrial processes. However, solid-liquid separation is usually performed outside the reactor in cumbersome filtration and sedimentation systems. The catalyst slurry must be recycled to the reactor, occasionally using a slurry pump. Thus, serious problems can arise during continuous operation of slurry bubble column reactors.

In a report recently issued by the United States Department of Energy, the question of separation of catalyst and wax in Fischer-Tropsch slurry reactor systems has been addressed. The report concludes with the following: "Internal filters immersed in the reactor slurry, which are used in some units on a laboratory scale or semi-industrial scale, do not work satisfactorily due to problems during operation. A reactor in which a section of the wall serves as a filter can be applicable in a semi-industrial scale plant, but is not applicable as a commercial reactor. Internal filters are subject to the risk of clogging, which can cause a premature shutdown of operations, and in commercial plants, no chances can be taken."

In another place in the report, it is stated that an internal filter in the reaction slurry has been used in a research project. Although initially it was possible to maintain a flow of filtrate using a pressure differential, the filter soon became clogged and it was concluded that continuous operation would not be practical and that for commercial scale operation it would be necessary to carry out the solid/liquid separation outside the reactor.

More recent development of the slurry bubble column reactor, which the patent applicant has undertaken, and which is described, among other things, in a. in international patent application PCT/N094/00023 (corresponding to Norwegian patent application no. 952956), has however shown that despite the above-mentioned teaching it is possible to provide a continuous reaction system for a Fischer-Tropsch synthesis , in which system it is not necessary to perform the solid/liquid separation in an external filter unit, and where a sufficiently high filtrate flow rate is achieved for commercial operation.

A slurry bubble column reactor for such a continuous reaction system for a Fischer-Tropsch synthesis, which is suitable for use in the plant according to the invention, is a reactor where a liquid product is separated from a slurry phase containing finely divided catalyst in a liquid medium, and which comprises: a container which defines a reaction zone arranged to accommodate the slurry phase and a volume of gas above the slurry phase; devices for introducing the synthesis gas into the slurry phase in the lower part of the container; a filtration section arranged to separate the liquid product from the slurry phase, including a chamber which at least partially surrounds the container, and a filter element which, together with the chamber, delimits a filtrate zone with an outlet for the product filtrate, the filter element being arranged so that it is in contact with the slurry in the slurry zone; devices for creating fluid communication between the filtrate zone and the part of the reaction zone which, during operation, will contain the gas volume above the slurry phase; and means for creating a mean pressure difference across the filter element.

It has been shown that the communication between the filtrate zone and the reaction zone, which is achieved with the above-described design of the reactor, prevents the accumulation of solids on the filter element. The mode of operation is considered to be as follows: The turbulent movement of the slurry, caused by gas bubbles passing up through the slurry, causes fluctuations or oscillations in the pressure in the filter element. The fluid communication between the reaction zone and the filtrate zone facilitates or enhances these pressure fluctuations or oscillations. Such a system is therefore relatively simple but still effective. The separation step, which is usually considered to be particularly problematic, is carried out without unnecessary complications, and under well-chosen operating conditions

geyser, the filter element is self-cleaning.

Important advantages achieved with such an SBCR reactor compared to an MTFB reactor are, among other things, the following: - Improved heat regulation of the exothermic FT reaction can be achieved by using efficient heat exchangers integrated in the reactor. The improved heat regulation enables high productivity for both catalyst and reactor. - The reactor is compact and simple, with few parts inside the reactor. The installation costs for the reactor are 50-70% lower than for reactors with stationary catalysts (MTFB reactors). - An internal separation of the catalyst from the FT product is carried out, which eliminates the need for equipment for external separation of the catalyst. - Because the catalyst is suspended in a slurry in the reactor, continuous replacement of the catalyst during operation is enabled. - Because the reactor contains a slurry of small catalyst particles, it is well suited for the use of highly active catalysts.

No less important is the great flexibility shown by the SBCR reactor in terms of operating temperature, product composition, production quantities and operating situations.

The linear gas velocity, catalyst concentration and temperature can be varied in an SBCR reactor, without this causing any major operational problems. Both the quantity produced and the turnover per throughput will thus be able to be varied. In an MTFB reactor, on the other hand, it is necessary to maintain very high linear gas velocities in order to achieve a favorable heat transfer between the stationary catalyst layer and the reactor wall. The turnover rate and the quantity produced cannot therefore be varied to a very large extent.

The product composition (wax/liquid ratio) in an SBCR reactor can be varied within wide limits by changing the reactor temperature. This cannot be done in an MTFB reactor, because the reaction rate (for a given catalyst) is determined by the heat balance. The MTFB reactor will therefore primarily only be able to be used in the lower temperature range, e.g. in the range 180-220 °C, i.e. in an area that gives a high wax/liquid ratio. Admittedly, also for the SBCR reactor, the reaction rate per unit effective reactor volume determined by the heat balance, but in this case a constant heat production can be maintained through a simultaneous change in catalyst concentration.

An SBCR reactor will be more robust than an MTFB reactor in unforeseen operating situations, such as e.g. a total stoppage in the natural gas supply to the plant's synthesis gas unit. For an SBCR reactor, this will not cause any major problems, because the liquid phase that is present in an SBCR reactor has a very high heat capacity and will therefore effectively dampen temperature variations, also in the event of other possible operational disturbances. An MTFB reactor, on the other hand, will have to be flushed with inert gas to avoid damage to the catalyst as a result of over-temperature. When restarting the plant, the catalyst in an SBCR reactor will simply be resuspended when the synthesis gas supply is restarted, while an MTFB reactor will require a more elaborate start-up procedure to avoid uncontrolled temperature increases.

Fig. 2 schematically shows a suitable embodiment of a slurry bubble column reactor, indicated at 11, which includes a reactor container 12 and a filtration section 13. The reactor container 12 includes a generally tubular section 14 and above this an inverted cone-shaped part 15. The tubular section 14 delimits the slurry zone 20, which will accommodate a slurry of finely divided catalyst in a liquid medium of e.g. product hydrocarbon. The cone-shaped part 15 serves as an expansion chamber to prevent the slurry from foaming over and defines a gas space 16 above the reaction zone. The cone-shaped part 15 may contain further devices (not shown) to break up the foam or reduce the formation of foam.

A gas inlet 17 and a gas distributor 18 are arranged at the bottom of the container 12 through which gas can be introduced into the slurry zone. A gas outlet 19 from the gas space 16 is arranged on top of the container 12. A series of heat transfer tubes 21 are arranged inside the reactor container and extend between a common inlet 22 and a common outlet 23 for a heat exchange medium. The apparatus 11 will be regulated by means of a large number of transducers, control valves, pumps, etc., among which one (a pressure or temperature sensor) is indicated by reference numeral 24 as an example.

The filtering section 13 comprises an annular chamber 25 which surrounds the container 12 just below the cone-shaped part 15. Inside the chamber 25, part of the container wall is made of sintered metal and thus forms a filter element 26. Non-porous parts 27 of the container wall extend into the chamber 25 at the top and bottom of the chamber. The chamber 25 and the container wall effectively define a filtrate zone 28 and, above this, a gas space 29.

An outlet from the filtrate zone 28 serves as a constant-level device for the filtrate. A tube 31 extends upwardly from an outlet opening 32 near the bottom of the chamber 25. A horizontal connecting section 33 determines the filtrate level 34 in the filtrate zone 28 and extends downwardly to an outlet valve 35. The valve 35 is opened to discharge the collected liquid product into the downward direction of the tube share. Of course, the downward-facing part can be replaced with a collection tank for the liquid product. The outlet pipe 31 is filled with liquid product between the opening 32 and the horizontal section.

A connecting pipe 38 connects the two gas chambers 16 and 29 to each other. The tube 38 has a valve 39. The connecting tube 30 is also connected to the tube 31 and thus provides fluid communication between the gas chambers 16 and 29 and the outlet tube 31. The chamber 25 also has an inlet 36 near the top, with a valve 37.

In operation, gaseous reactants are introduced into the slurry of catalyst and liquid product via the gas distributor 18, the catalyst particles being held in suspension. The correct temperature for the reaction is maintained using the various sensors, e.g. 24, and the heat transfer system 21, 22, 23. Liquid product is filtered through the filter element 26 and flows into the filtrate zone 28. This filtration is promoted by a pressure difference across the filter element, caused by a hydrostatic pressure resulting from the level difference between the slurry and the filtrate. The filtrate level 34 is kept constant as a result of the vertical position of the horizontal section 33 of the outlet pipe 31.

The turbulent movement of the slurry helps to prevent the build-up of some filter cake and tends to prevent the filter element 26 from being clogged with catalyst particles by causing fluctuations or oscillations in the pressure above the filter element 26, where the valve 39 is open.

Gaseous products and any unreacted reactant gases are taken out via the outlet 19. Accumulation of gas above the filtrate in the room 29 is avoided as a result of the presence of the connecting pipe 38.

The filtrate section 13 can be flushed, either with a suitable gas such as e.g. synthesis gas, or a suitable liquid, such as e.g. purified product, by the valve 37 being opened and the valves 35 and 39 being opened. This forces the flushing fluid back through the filter element 26.

During normal operation, part of the catalyst can be removed and replaced either with a new catalyst or with a regenerated catalyst. For the sake of overview, fig. 3 no devices are shown for this purpose, and it will be understood that any standard system for making such a replacement will be able to be used.

Preferably, the chamber surrounds the reactor container around the entire circumference, at least in part of the extent of the reactor container. The filter element can, as shown in fig. 3 is made up of part of the wall of the reactor container, as this is made up of a filter material. In an alternative embodiment, the filter element is arranged on the outside of the container, and the container is then discontinuous in the area of the filter element. In another alternative, the filter element is arranged inside the container, the chamber being made up of part of the container wall. Preferably, the fluid communication takes place between the gas volume above the slurry phase and a gas volume above the filtrate.

The communication between the space above the slurry in the reaction zone and the space above the filtrate in the filtrate zone prevents the build-up of pressure differentials beyond that which corresponds to the hydrostatic pressure. The communication can suitably take place via a pipe which extends between the top of the reaction zone and the top of the filtrate zone and which is open to both. Preferably, the pipe connecting the two gas volumes is arranged to facilitate the escape of any gas that may have accumulated in the upper part of the filtrate zone.

Preferably, the amplitude or size of the fluctuations or oscillations in the pressure difference across the filter element will be approximately the same or greater than the mean value of the static pressure difference. Preferably, the average pressure difference across the filter element should be kept at a relatively low level, in typical cases lower than 6 mbar (600 Pa). If the average pressure difference is lower than a critical value (e.g. 6 mbar), the filter will be self-cleaning.

The pressure fluctuation value can be of the same order of magnitude as the pressure difference, e.g. from 10 to 200% of the pressure difference. The current value of the pressure difference can be from 1 to 100 mBar, preferably from 2 to 50 mBar.

The device for introducing gaseous reactants or components can comprise any suitable device, such as e.g. a bell plate, a set of nozzles, a frit plate, etc., preferably located at the bottom of the reaction vessel.

For a more detailed description of the SBCR reactor, reference is made to international patent application PCT/N094/00023 (corresponding to Norwegian patent application no. 952956), which is incorporated herein in its entirety by reference. Reference is also made to international patent applications no. PCT/NO93/00030 (corresponding to Norwegian patent application no. 943121) and PCT/NO93/00031 (corresponding to Norwegian patent application no. 943122) as well as to Norwegian patent application no. 943084.

Installation of the facility on an FPSO unit.

The method according to the invention is particularly applicable for implementation in a facility installed on board a so-called FPSO unit (FPSO = "Floating Production, Storage and Offloading"), which could be a ship designed and equipped for loading/unloading hydrocarbons by offshore oil and gas production wells, storage of such hydrocarbons, as well as production, primarily conversion and processing of hydrocarbons produced from the wells.

In a number of patent documents and applications, the patent applicant has shown and described a type of ship of the type mentioned above, which is here referred to as an MST ship (MST = "Multipurpose Shuttle Tanker"). Ships of this type are particularly suitable as carriers of a facility for carrying out the method according to the invention and will make it possible to utilize to the greatest possible extent the flexibility and possibility of integration that is implicit in such a facility.

A vessel as mentioned above is schematically shown in side view in fig. 3. At the forward end of the vessel, there is a submerged, downwardly open recording room 2 for recording an underwater buoy 3, and a service shaft 4 extends between the recording room 2 and the vessel's deck 5. The arrangement is designed so that in the recording room it can be pulled up and fixed a submerged buoy for loading/unloading of hydrocarbons, as shown and described in more detail in the Norwegian patent applications no. 923814, 923815 and 923816, and further so that a buoy which is designed to cooperate with a swivel unit which is arranged at the lower end of the shaft, for use of the vessel as a production ship, as shown and described in more detail in the Norwegian patent applications no. 922043, 922043 and 922045. Reference is made here to the aforementioned applications for a more detailed description of the designs in question.

As can be seen, the vessel's wheelhouse 6 is located close to the vessel's bow 7, and furthermore the engine room 8 with its diesel-electric main machinery is located below the wheelhouse. The service shaft 4, which extends between the buoy 3 and the vessel's deck 5, is located just behind the wheelhouse, so that crew going down the shaft will be in the lee behind the wheelhouse.

A loading manifold/swivel 9 is shown to be arranged above the buoy for connection to the buoy 3, and further a connection pipe with an oil line valve 10. Monitoring devices 11 are also shown, e.g. TV cameras, a hatch 12 for shutting off the shaft 4 above the recording room, and a guide device 13 for use in connection with pulling up the buoy. A towing winch 14, a storage unit 15 and a service crane 16 are also arranged on the deck for use in connection with, among other things, maintenance. Two bow thrusters 17 are arranged in the vessel's bow.

Process equipment for processing oil is placed on frames on the deck between the front and rear of the vessel. The well stream produced on the field, which is brought up to the ship via the risers from the field and underwater buoy 3, is here separated into water, oil and gas. This equipment is shown in the form of a number of movable modules 26. Between the front and the rear part, the vessel contains a number of cargo spaces or tanks 28. In the rear area, a flame boom 27 is also shown arranged.

Fig. 4 is a perspective view of a production ship fitted with a plant according to the invention for converting associated natural gas into a synthetic crude oil and/or wax. The facility is mounted behind a wheelhouse 30 in the ship's bow and behind a possible reception room (not shown) for reception of an underwater loading buoy. The reference numbers 1, 2 and 3 refer to the same process apparatus as the corresponding reference numbers in fig. 1, namely an absorption unit 1 where sulfur is removed from the natural gas, a synthesis gas reactor 2, consisting of an autothermal reformer, and two slurry bubble column reactors 3 for carrying out the Fischer-Tropsch synthesis. A plant for recovering hydrogen from part of the synthesis gas is shown at 31, and an oxygen plant for extracting oxygen for supply to the autothermal reactor is shown at 32. The process equipment 1, 2, 3, 31 and 32 as well as other equipment which is directly connected to the facility (indicated in the figure without reference numbers) are mounted on standardized, replaceable frame structures 33 attached to the ship's deck. These frame structures can be easily removed, so that the ship's deck is freed up for other uses.

It is an important aspect of a preferred embodiment of the facility according to the invention that the facility is fully adapted to and integrated into the technology underlying the MST ship which in a preferred embodiment will be the facility's main base. This means, among other things, that the facility's design and construction will be adapted to the production ship's framework dimensions for module assembly, that it is adapted to the infrastructure of the production ship, including e.g. a central pipeline, and that it is adapted to the various auxiliary systems that provide cooling water, steam, oxygen, etc. Furthermore, the plant should initially be best adapted to the oil production in any given case, especially the amount of associated gas that is produced and the scope of gas injection. Advantages achieved by integrating the facility with the auxiliary systems on board a production ship include that unconverted gas from the plant can be used for the production of electric power in an electric generator, or for the production of fresh water from seawater. Furthermore, it is an advantage that the relatively large amount of water that is separated from the product from the FT reactors, and which contains acid (e.g. acetic acid) and alcohol (e.g. methanol), can be used for injection purposes on the field. A further advantage is the easy access to seawater for cooling.

The plant according to the invention will suitably have a production capacity in the range of 420-21,000 bbl C5t/day (53.5-2675 tonnes C5+/day), corresponding to a supply of natural gas of 0.1-5.0 Mill. Sm<3>/day, preferably a production capacity in the range of 2100-8400 bbl C5+/day (267.5-1070 tonnes C5t/day), corresponding to 0.5-2.0 Mill. Sm<3>natural gas per day and night. In a particularly current size of the facility, this will have a production capacity of approx. 4200 bbl C5+/day (approx. 535 tonnes C5h/day), corresponding to approx. 1.0 Mill. Sm<3>natural gas per day and night.

The synthetic crude oil and/or wax obtained as a product from the plant can be mixed with the crude oil produced from the wells) and thus shipped together with this. Alternatively, the product from the plant can be taken out to separate product tanks for separate unloading from the production ship and marketing/refining. This can in many cases be profitable, because the product obtained from the plant's synthesis gas will usually be superior to conventional crude oil in terms of quality and properties, i.a. it contains practically no sulphur, and it can e.g. be suitable as starting material for the production of diesel fuels with a high cetane number and various high-grade lubricating oil components.

A further advantage of the frame-mounted plant is that it can be mounted on suitable facilities on land to produce synthetic crude oil during the periods when it is not in use on the production ship.

Similar facilities, but which are not adapted for an MST ship, will be able to be used on dedicated ships, on permanent installations offshore or in places on land where, e.g. remote gas may be relevant as feed for the plant.

A simulated implementation of the Fischer-Tropsch part of the method according to the invention, in a plant as shown in fig. 2 and described generally above.

Execution example

Using a mathematical simulation model for slurry bubble column reactors (SBCR reactors), developed by the patent applicant and based on reaction kinetic data for the catalyst described below and recognized correlations for mass transfer and hydrodynamics in slurry bubble columns, data was obtained for the operation of the Fischer-Tropsch part of a plant according to the invention as shown in fig. 2, comprising two series-connected SBCR reactors, with condensation of water and C5+ between the reactors, for the production of liquid hydrocarbons (C5+) from a synthesis gas.

In the reactors, a cobalt-rhenium catalyst containing 20% by weight of Co and 1% by weight of Re on y~Al2O3 was used. The catalyst is described in US patent document no. 4,801,573 and had been produced by impregnation of y-Al^a me(^ an aqueous solution of Co(N03)2-6H20 and HReO4 using the pore-filling method ("incipient wetness method").

A synthesis gas of composition as indicated in Table 1 below was introduced into the first of the two series-connected reactors (reactor 1) in an amount of approx. 153,000 Sm<3>/h. The added amount of synthesis gas corresponds to approx. 1 Mill. Sm<3>/day natural gas supplied to the plant's reformer part + recycled synthesis gas from SBCR reactor 2. The composition of the synthesis gas is typical of a synthesis gas for a remote gas.

The operating conditions and the most important data for the two reactors 1 and 2 are given in table 2 below.

The various mass flows to and from reactors 1 and 2 are given in table 3.

The total conversion rate for CO in the two reactors turned out to be 89% per throughput.

Claims (8)

1. Procedure for on board a ship to treat a well stream produced on an offshore oil field, using a ship cooperating with an underwater buoy to which both the ship and the risers from the field are anchored, in the ship, on the upper side of the buoy , a swivel unit is arranged, CHARACTERIZED in that the well flow is first led to a processing plant which is placed on easily replaceable frames attached to the ship's deck on each side of a centrally located pipe passage in the ship's longitudinal direction, that water, oil and gas are separated from each other in this processing facility, that separated, stabilized oil is stored in at least some of the ship's tanks, and that the separated gas is taken to a facility for converting the gas into synthetic crude oil and/or wax, which is then stored in tanks in the ship, as the synthetic crude oil may be mixed with the stabilized oil. 2. Facility for treating a well stream produced from an offshore oil field, which facility is arranged for installation on board a ship and includes a processing facility where oil, gas and water are separated, CHARACTERIZED in that it also includes a facility for conversion of the separated gas into synthetic crude oil and/or wax, and that this conversion plant comprises at least a synthesis gas unit and a Fischer-Tropsch unit, and that the total plant (processing plant and conversion plant) is placed on frames that can be attached easily replaceably to ship's deck. 3. Method, especially for carrying out offshore on a ship, platform or other installation, for converting a natural gas, especially an associated natural gas, into a synthetic crude oil and/or wax in two stages, where (1) the natural gas is converted into a synthesis gas consisting of a mixture of carbon monoxide, hydrogen, and carbon dioxide in a synthesis gas unit, and (2) the synthesis gas is converted to a synthetic crude oil and/or wax in a Fischer-Tropsch synthesis, CHARACTERIZED by synthesizing the synthesis gas from step (1) to perform Fischer - The Tropsch synthesis is introduced into a slurry consisting of liquid products, finely divided catalyst particles and synthesis gas in a reaction zone in a slurry bubble column reactor (SBCR reactor) where an internal separation of the liquid products from the rest of the slurry is carried out. 4. Method according to claim 3, CHARACTERIZED in that the synthesis gas from step (1), after cooling and separation of water, is introduced into the bottom of the reaction zone in the slurry bubble column reactor, the reaction zone being arranged both to accommodate the slurry consisting of liquid products, finely divided catalyst particles and added synthesis gas and to accommodate a volume of gas above the slurry phase, that liquid product is separated from the rest of the slurry by means of a filtration section including a chamber and a filter element which together define a filtrate zone with an outlet for the product filtrate, which filter element is arranged so that it is in contact with the slurry in the reaction zone, that fluid communication is created between the filtrate zone and the part of the reaction zone that contains the gas volume above the slurry phase, and that a mean pressure difference is created across the filter element. 5. Plant for the conversion of natural gas, especially an associated natural gas, into a synthetic crude oil and/or wax in two stages, in which (1) the natural gas is converted into a synthesis gas consisting of a mixture of carbon monoxide, hydrogen and carbon dioxide in a synthesis gas unit, and (2) the synthesis gas from this unit is converted to a synthetic crude oil and/or wax in a Fischer-Tropsch unit, CHARACTERIZED in that the Fischer-Tropsch unit comprises one or more slurry bubble column reactors (SBCR reactors) comprising a reactor zone arranged to contain a slurry consisting of liquid products, finely divided catalyst particles and synthesis gas, and that the reactors) are arranged for internal separation of liquid products from the rest of the slurry. 6. Plant according to claim 5, CHARACTERIZED in that each slurry bubble column reactor comprises: a container defining a reaction zone arranged to accommodate both the slurry phase and a volume of gas above the slurry phase; devices for introducing the synthesis gas into the slurry phase in the lower part of the container; a filtration section arranged to separate liquid products from the slurry phase, including a chamber which at least partially surrounds the container, and a filter element which, together with the chamber, delimits a filtrate zone with an outlet for the product filtrate, the filter element being arranged so that it is in contact with the slurry in the slurry zone; devices for creating fluid communication between the filtrate zone and the part of the reaction zone which, during operation, will contain the gas volume above the slurry phase; and means for creating a mean pressure difference across the filter element. 7. Installation according to claim 6, CHARACTERIZED in that it is placed on frames that can be attached easily replaceably to a ship, an offshore platform or other offshore installation. 8. Installation according to claim 7, CHARACTERIZED in that it is adapted for installation on an FPSO ship (FPSO = "Floating Production, Storage and Offloading").