The present invention relates to a method and a device for temporary storage of fluids, such as during transport of the fluids. In more detail, the invention relates to a method and a device for carrying out the method for alternating storage of two or more fluids in the same tanks, but where mixing of the fluids is avoided to the greatest extent possible. In particular, the present invention relates to a method and also a device for alternating storage, such as during transport, of natural gas and CO2, and also a vessel comprising the device for storage.
Technology for separation of CO2 from the flue gas from thermal power plants is being developed, where the separated CO2 is deposited, for example, by injection into an oil field or a gas field. It is often not possible or it is impracticable and costly to place a thermal power plant where fuel gas is available as fuel for the thermal power plant and at the same time there is a possibility for depositing CO2 nearby.
CO2 can be deposited in wells that are no longer in use, in aquifers which abandoned wells go through, or as a pressure support in producing wells. There may also be formations isolated from producing gas fields or oilfields near gas fields or oilfields that are suitable for safe deposition of CO2.
In instances where thermal power plants can not be built in direct connection to a gas field or an oilfield, gas as fuel for a thermal power plant must be transported from the field and to the thermal power plant, while CO2 which is separated from the flue gas must be transported to the deposition location.
Gas, such as natural gas as fuel for the thermal power plant, and also CO2 can be transported in pipelines, one for transport of the natural gas and one for return of CO2. However, it is costly to lay dedicated pipes to and from a thermal power plant. The flow to and from the field in such pipelines is small and for a power plant of 100 MW can constitute as little as 2 to 10% of the gas that is produced in a field. Such small pipelines over longer distances will often be unprofitable.
Pipelines from a field to a customer are normally pipes that transport gas and/or oil from production location to the customer. If, in addition to the pipe for transport of gas and/or oil, a pipe for return of CO2 is to be laid, the costs will be unacceptably high. Furthermore, planning, the decision process and the actual laying of such pipes take a long time.
An alternative can then be to transport fuel gas to a thermal power plant and return CO2 across ocean areas or along the coast in ships with separate tanks for CO2 and natural gas in pressurised and/or liquid form. The pressure in such tanks can be 200 to 300 barg, while it is required that gas is delivered to the thermal power plant at 20-40 barg. Corresponding pressures are also relevant for transport of CO2 and delivery of the same, respectively, for deposition at an oilfield/gas field. In other words, 10-15% of the gas will remain in the tanks after delivery of natural gas and CO2, respectively, to a gas driven power plant and deposition, respetively. If one should use the same tank for both gases, this will result in an unacceptable mixing of the gases. Firstly, an unacceptably large part of the costly natural gas would be returned to the field for deposition together with CO2 and secondly, an unacceptable amount of CO2 would be delivered together with the natural gas at the same time.
Thus, transport in tanks onboard ships will require that the gases/fluids are transported in separate tanks/containers, a solution which will be unacceptably costly and space demanding as the tanks for CO2 will stand empty during transport of natural gas and vice versa, so that a large part of the total transport capacity of the vessel will be unused at any time.
U.S. Pat. No. 5,203,828 describes use of a membrane in a tank for storage of different fluids, such as crude oil and water, where one fluid is stored on the one side of the membrane and the other on the other side of the membrane to avoid that the one fluid is contaminated by the other. However, this is a construction which will be subjected to wear and which is complicated to maintain.
An aim of the present invention is to provide a solution for temporary and alternating storage of different fluids and, in particular, for transport of natural gas and CO2 where the above mentioned disadvantages are overcome. This aim, and other aims, which a person skilled in the arts will understand by reading the enclosed description, are obtained by applying tanks connected in series as described below.
According to a first aspect, the present invention relates to a method for alternating storage of natural gas and CO2 in a tank installation, where the gases are stored in a plurality of tanks which are connected in series, where natural gas is supplied to and taken out of, respectively, a tank at one end of the tanks which are connected in series and where CO2 is supplied to and taken out of, respectively, a tank at the opposite end of the tanks that are connected in series.
According to one embodiment, the natural gas and CO2 have a pressure and temperature that lie above the cricondenbar of the actual gas. It is preferred that pressure and temperature are kept above the cricondenbar of the actual gas or gas mixture to avoid condensation of gas with the resulting problems of multiphase flow and collection of liquids in tanks and pipes. To ensure that the pressure in the tanks is above the cricondenbar of the gas, it is preferred that natural gas is stored at a pressure of from 120 to 300 barg, and CO2 is stored at a pressure of from 80 to 150 barg.
According to one embodiment, the tank installation is arranged onboard a vessel, where natural gas is supplied to the tank installation and where CO2 is removed from the tank installation when the vessel lies connected to a gas field, and is emptied of natural gas and supplied with CO2 when the vessel lies at a facility for use of the natural gas.
According to a second aspect, the present invention relates to a combined installation for alternating storage of natural gas and CO2, where the installation comprises a plurality of tanks that are connected in series with the help of connecting pipes and where a CO2 line is arranged for supply of CO2 to and removal of CO2 from, respectively, the tank installation which is connected to a first tank in the series of tanks, and a natural gas line for removal of natural gas and supply of natural gas, respectively, to a last tank in the series of tanks.
According to one embodiment, the CO2 line has an outlet near the bottom of the first tank and that the natural gas line has an outlet near the top of the last tank. CO2 is heavier than natural gas. As CO2 is filled from the bottom of the first tank, the least possible mixing of the gases will be ensured in this tank, as CO2 will lie predominately at the bottom and rise upwards as the tank is filled, while the natural gas will lie uppermost in the tank and be pushed up and out of the tank.
According to a second embodiment, the connecting pipes have a first opening near the top of the tank that streamwise lies nearest the first tank and a second opening near the bottom of the next tank in the series of tanks. CO2 or gas mixtures with a high concentration of CO2 will be heavier than natural gas. It is therefore appropriate, from the same consideration as in the paragraph above, always to fill the heaviest gas from the bottom of any tank in the series.
It is appropriate that the installation encompasses from 5 to 200 tanks in series.
According to a special embodiment, the installation encompasses from 20 to 50 tanks in series.
According to a third aspect, the present invention relates to a vessel for alternating transport of natural gas and CO2, where the vessel comprises a tank installation encompassing a plurality of tanks which are connected together in series with the help of connecting pipes and where a CO2 line is arranged for supply of CO2 to and removal of CO2 from, respectively, the tank installation which is connected to a first tank in the series of tanks, and a natural gas line for removal of natural gas and supply of natural gas, respectively, to a last tank in the series of tanks.
a shows a longitudinal section through a third preferred tank;
b shows a transverse section of the tank according to
The present invention relates to a combined storage facility for hydrocarbon gas, such as natural gas, and CO2, where the storage facility is used, for one period of time, for storage of CO2 and, for another period of time, is used for storage of natural gas. Such a combined storage facility is especially appropriate for transport, for example, onboard a vessel, where natural gas is brought from a gas field to a land-based installation and CO2 for re-injection is transported from land to the gas field.
CO2 gas has a greater density that natural gas, which is mainly comprised of methane. The connection pipes 4, 4′, etc. run therefore from the top of the tank that is nearest the supply of CO2 to the bottom of the next tank. In this way, gas is taken out near the top of the tank that is nearest the CO2 supply and is supplied near the bottom of the next tank at filling of CO2. At the same time as CO2 is being loaded in this way, natural gas is taken out through the natural gas pipe 3.
At filling of natural gas, the natural gas is supplied through the natural gas pipe which has its opening near the top of the last tank 1n′. Then the gas flows from tank to tank via the connection pipes 4, opposite to the flow direction of the gas during loading of CO2.
The present installation is thus emptied of natural gas at the same time as it is being filled with CO2 and vice versa. By adapting the speed of emptying and filling, respectively, one can prevent undesirable vortex formation and mixing of gases in each tank. The fact that the connection pipes carry the gas stream from the top of one tank to the bottom of the next, or vice versa at reversed direction of flow, has the effect that the density of the gases helps to obtain an approximately plug flow through the present installation.
If it can be tolerated for the intended purpose, the natural gas which is taken out of the natural gas pipe 3 can be supplied directly to the intended purpose. If the intended purpose for the natural gas is use in a gas driven power plant, it can be appropriate to ensure that the gas taken out at the end, which contains some CO2, is mixed with pure natural gas before use. Alternatively, the gas can be cleaned as described in
At the start of the unloading of natural gas, the gas in gas line 3 will be relatively clean with small amounts of CO2, and separation is unnecessary. Therefore, the gas can be taken out via a circulation pipe 10 to a natural gas outlet 12. When the content of CO2 in the stream taken out in gas line 3 rises above an predetermined level, which one does not wish to exceed, the circulation pipe 10 is closed and the gas from pipe 3 is led through a separation unit 11 for separation of natural gas and CO2. CO2 from the separation unit 11 is fed via a return pipe 13 and is pumped, with the help of a pump 14, back to the CO2 pipe 2 and is led into the storage installation. Cleaned natural gas is then taken out through the natural gas outlet 12.
In
a and 5b show a longitudinal section and a transverse section, respectively, of an alternative tank where the inside of the tank is divided in two by a partition that runs axially in the tank. CO2 and natural gas, respectively, or a mixture of the two, are led into and out of the tank as shown in
It is important that the supply or the connection pipe 4 which comes in from the side that is nearest the CO2 pipe, runs out into the bottom of the tank and that the supply or the connection pipe 4′ that lies nearest the natural gas pipe, runs out at the top of the tank in all the tanks. In this way one can use the fact that CO2 has a greater density than natural gas and remains lying in the bottom of the tank and only to a small extent mixes with the natural gas which may be present in the tank. As the natural gas is always taken out from the top of the tank and CO2 is always taken out from the bottom, one uses the effect which is provided by this density difference.
In this way one can obtain a better separation and less mixing of the gases than what seems theoretically possible from the above considerations. The stream can thus be very close to plug flow and the number of tanks where there actually will be a mixing of the gases can be reduced to a few tanks.
If there is more natural gas than CO2 on a volume basis, transport can be carried out at a higher pressure for natural gas. On the other hand if there is more CO2 than natural gas on a volume basis, transport can be carried out at a higher pressure for CO2 than for natural gas.
With supply of natural gas fuel to a thermal power plant and return of 90%, or more, of produced CO2, there typically will be more natural gas on a volume basis. It will be possible to carry out transport at 200 to 250 barg for natural gas at a temperature from 10-25° C. Depending on the composition of the natural gas, return transport of CO2 will be at 100 to 150 barg at a temperature from 35-60° C.
Typical pressure and temperature in the present storage device will, for natural gas, be 200 barg at 10° C. and for CO2 will be 120 barg and 38° C. If one should use the same pressure for both gases, the temperature of the natural gas ought to be 45-50° C. colder than the temperature of CO2 so that one can transport the same amount of gas both ways. At the same temperature of the gases, the pressure of the natural gas must be about 150 bar higher than the pressure of CO2 to transport the same amount of gas both ways. If the amount of gas is larger one way than the other way, pressure and temperature can be adjusted accordingly.
Typically, the tanks will work above the cricondenbar for the gas mixtures that might occur, i.e. in an area where liquid does not occur (the same is normal at transport in pipelines). The cricondenbar varies with the gas composition, but lies typically from somewhat below to somewhat above 100 barg.
The FIGS. 6 to 9 illustrate that the performance of such a system, i.e. the approach to ideal plug flow, improves with the number of tanks connected in series.
The calculations on which the FIGS. 6 to 9 are based assume that the tanks are ideally mixed vessels, where the gases in the tank are completely mixed at any time. However, CO2 has a tendency to go to the bottom of the tank during filling and emptying while natural gas will lie at the top of a tank where both gases are present. Mixing of the gases will be slow and determined by the flow pattern in the tank and diffusion phenomena. This slow mixing of the gases, together with the gas containing most CO2 being loaded and emptied from the tanks through an opening near, or in the bottom of the tank, will result in the flow of gas through the tank being nearer plug flow than what appears to be the case in the FIGS. 6 to 9. To reduce even further the mixing of the gases in tanks where both gases are present, the openings of the CO2 pipe, natural gas pipe and also connecting pipes can be formed so that vortexing in the tanks is reduced as much as possible during emptying and filling. This is illustrated in
Even if the
With reference to
The flexible pipes 106, 110 run up from the anchorage buoy and up to a swivel 112 on the deck of the ship. From the swivel 112, the incoming natural gas is led in a pipe 110 that comes from the gas well 101, in a natural gas pipe 108 to a storage unit 113. The storage unit 113 is a storage unit as described above, comprising a series of tanks connected in series, but is represented by one tank in the figure for simplicity. It can be appropriate that a sand trap 117, with associated sand storage facility 119, is arranged between the swivel 119 and the storage unit 113 for removal of sand that follows the incoming gas. Furthermore, a pump 116 must be arranged between the gas well 101 and the storage unit 113 to pump the well stream into the storage unit.
Natural gas is supplied as described above with reference to
CO2 is taken out from the storage installation 113 in a CO2 line 114 and is pumped further down into the injection well with the help of a pump 115.
After loading of the well stream and emptying of CO2, the vessel goes to an installation ashore.
The natural gas can be led directly to a pre-treatment unit 126 and to the storage unit 123. The natural gas, whether it comes directly from the ship or has been temporarily stored in the storage unit 123, will normally contain a certain fraction of condensable components. These components are condensed in the pre-treatment unit 126. From the pre-treatment unit, the condensate is led, via a pipe 133, to a storage unit 130. The condensate can be exported from the installation from the storage unit 130.
The remaining gas from the pre-treatment unit 126 is led to a gas-driven power plant 131 via a fuel pipe 127.
The gas-driven power plant comprises a separation unit for CO2, and separated CO2 is led, via a CO2 pipe 124, to the storage unit 123 or can be sent directly onboard the vessel if this is connected to the installation.
The land-based installation shown is only an example and other types of land-based installations, where CO2 is generated from natural gas, of course can be used.
The present invention makes utilisation of smaller oilfields and gas fields possible. These fields are not developed today as it is too costly to build processing installations on the fields, or to build pipelines. The processing of the gas and use of this can be placed ashore and one can utilise such fields without placing processing equipment on the field or laying pipelines. In the present description it must be understood that a gas-driven power plant and other processing installations, respectively, must not necessarily lie ashore, but can also lie on an installation at sea.
Number | Date | Country | Kind |
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20035622 | Dec 2003 | NO | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/NO04/00390 | 12/16/2004 | WO | 9/1/2006 |