A method of maintaining the pressure in a liquefied gas storage tank

Abstract

A method of maintaining the pressure in a liquefied gas storage tank (10) during liquefied gas withdrawal, the storage tank having an inlet (22) and an outlet (20); comprising the steps of: (a) withdrawing liquefied gas (12) from the outlet of the storage tank to provide a withdrawn stream (30); (b) dividing the withdrawn stream (30) into a major stream (32) and a smaller return stream (33); (c) expanding the return stream (33) to provide an expanded return stream (34); (d) heat-exchanging the expanded return (34) stream of step (c) with the major stream of step (b) to provide a vaporised return stream (38); (e) compressing the warmer return stream (38) of step (d) to a pressure being wholly or substantially the same as the pressure inside the storage tank to provide a compressed return stream (42); and (f) passing the compressed return stream into the storage tank (10).

Description

[0001] A METHOD OF MAINTAINING THE PRESSURE IN A LIQUEFIED GAS STORAGE TANK

[0002] The present invention relates to a method of maintaining the pressure in a liquefied gas storage tank during liquefied gas withdrawal, particularly for a liquefied carbon dioxide storage tank, and a system therefor.

[0003] Background

[0004] Various gases such as carbon dioxide, hydrocarbons (for example LNG, LPG), oxygen, nitrogen etc., are most easily stored in liquefied form as a ’liquefied gas’ in specially designed tanks. This is especially for transportation, or at intertransportation stages, or to be ready for use.

[0005] A liquefied gas storage tank, whether onshore or offshore on a gas carrier or the like, can be said to have a lower portion of the liquid, and a portion of gas or vapour in the space above the liquid. The tank is generally kept at a temperature below the ambient temperature around the tank or carrier, etc.

[0006] A requirement for any storage tank is to discharge its stored liquid, typically using a pump or the like. The removal of the liquid from the tank will reduce the portion of the tank that contains liquid, and correspondingly increase the portion that is gas or vapour. This process will tend to reduce the pressure within the storage tank, which may be undesirable for various structural and / or operational requirements, especially for liquids being kept under non-ambient or non-atmospheric conditions such as liquefied gases.

[0007] A common method is to divert a portion of the discharged liquid to a heat exchange device (vaporiser) where an external source of energy, (such as steam, electricity, seawater, air), can be used to create a stream of vapour or gas which is then reintroduced to the storage tank.

[0008] But if the rate of liquid removal from the tank during discharge is large, then the required quantity of energy for this method can be very large, and this can be undesirable from a financial and / or environmental perspective, depending on the source of energy. For example, in regions where the ambient temperature is low (either year round or seasonally), it may not be possible to economically extract the energy required to vaporise the additional required gas from ambient sources such as seawater or air.

[0009] It is an object of the present invention to provide an improved method and system for providing additional gas for a storage tank when required.

[0010] Summary

[0011] According to one aspect of the present invention, there is provided a method of maintaining the pressure in a liquefied gas storage tank during liquefied gas withdrawal, the storage tank having an inlet and an outlet; comprising the steps of:

[0012] (a) withdrawing liquefied gas from the outlet of the storage tank to provide a withdrawn stream;

[0013] (b) dividing the withdrawn stream into a major stream and a smaller return stream;

[0014] (c) expanding the return stream to provide an expanded return stream;

[0015] (d) heat-exchanging the expanded return stream of step (c) with the major stream of step (b) to provide a vaporised return stream;

[0016] (e) compressing the warmer return stream of step (d) to a pressure being wholly or substantially the same as the pressure inside the storage tank to provide a compressed return stream; and

[0017] (f) passing the compressed return stream into the storage tank.

[0018] According to a second aspect of the present invention, there is provided a system for maintaining the pressure in a liquefied gas storage tank during liquefied gas withdrawal, comprising: a storage tank outlet to withdraw liquefied gas from the outlet of the storage tank to provide a withdrawn stream; a divider to divide the withdrawn stream into a major stream and a minor return stream; an expander to expand the return stream to provide an expanded return stream; a heat exchanger to heat-exchange the expanded return stream with the major fuel source stream to provide a warmer return stream; a compressor to compress the warmer return stream to a pressure being wholly or substantially the same as the pressure inside the storage tank to provide a compressed return stream; and a storage tank inlet to pass the compressed return stream into the storage tank.

[0019] Brief description of the drawings

[0020] Figure 1 is a diagrammatic plan of a method and system according to one embodiment of the present invention;

[0021] Figure 2 is a diagrammatic view of a vessel using the embodiment of Figure 1 ;

[0022] Figure 3 is a schematic plan with example parameters at defined positions around an embodiment of the system of the present invention;

[0023] Figures 4a and 4b are diagrammatic views of variations of Figure 1.

[0024] Detailed description of the invention

[0025] Various gases such as carbon dioxide, hydrocarbons (such as for example LNG and LPG), oxygen, nitrogen etc., are most easily stored in liquefied form as a ’liquefied gas’ in specially designed tanks. This is especially for transportation, or at intertransportation stages, or to be ready for use.

[0026] Such storage tanks are usually designed to store the liquefied gas in the most efficient manner with appropriate insulation, etc. In order to store it, the right tank is needed, sometimes using cryogenic.. technology, to guarantee its optimal and safe storage until is needed.

[0027] There are a number of standards for the design and constructions of liquefied gas storage tanks, including for example BS EN 14620 and API 625. The kinds of steel that are needed are determined by the type of liquefied gas that will be kept, as well as the temperature and type of tank that will be used. Liquefied gas storage tanks can have a volume anywhere from a few hundred square meters, to 1 ,000 m3, to 30,000 m3, or larger. For example, some liquefied gas carriers range in capacity from small tankers of between 500 and 6,000 m3, for example for the shipment of propane, butane and the chemical gases at ambient temperature, up to the fully insulated or refrigerated seagoing tankers of over 100,000 m3capacity for the transport of LNG and LPG from production to users.

[0028] Liquefied gas storage tanks include being onshore, whether underground or aboveground.

[0029] A liquefied gas storage tank, whether onshore or offshore on a gas carrier or the like, can be said to have a lower portion of the liquid, and a portion of gas or vapour in the space above the liquid which the ‘fills’ the tank to its capacity. The tank is generally kept at a temperature below the ambient temperature around the tank or carrier.

[0030] During storage, the tank design is to minimise any vaporisation of the liquid content. For example, low-pressure tanks (less than 10 kilopascals), and ship or carrier cargo tanks, especially for liquefied gas transportation, all generally having double walls for enhanced protection, and a vacuum that provides thermal insulation and thus maintains the cold temperatures needed. But the discharge of liquefied gas is required at some stage. This is generally driven by an external pump at or near a tank outlet.

[0031] The removal of the liquid from the tank will reduce the portion (in terms of any of the proportion, volume, amount and / or size) of the tank that contains liquid, and correspondingly increase the portion (in the same terms) that is gas or vapour. This process will tend to reduce the pressure within the storage tank, which may be undesirable for various well known structural or operational requirements, such as avoiding a vacuum, avoiding a distinct pressure differential, tank construction, etc.

[0032] Known methods to maintain the pressure inside the tank at a constant pressure include the additional of inert gases such as nitrogen from an external source, adding energy to the liquid remaining in the tank to cause vaporisation, and commonly diverting a portion of the discharged liquid to a heat exchange device (vaporiser) where an external source of energy, (such as steam, electricity, seawater, air), can be used to create a stream of vapour or gas which is then reintroduced to the storage tank. All of these methods are designed to maintain the capacity of the storage tank during a discharge of the liquid portion.

[0033] But if the rate of liquid discharge or removal from the tank is large, then the required quantity of energy can be very large to maintain the internal tank pressure, and this can be undesirable from a financial and / or environmental perspective, depending on the source of energy. For example, in regions where the ambient temperature is low (either year round or seasonally), it may not be possible to economically extract the energy required to vaporise the additional gas from ambient sources such as seawater or air. Thus, additional energy sources are needed, increasing cost

[0034] Thus, the present invention provides a method of maintaining the pressure in a liquefied gas storage tank during liquefied gas withdrawal, the storage tank having an inlet and an outlet; comprising the steps of:

[0035] (a) withdrawing liquefied gas from the outlet of the storage tank to provide a withdrawn stream;

[0036] (b) dividing the withdrawn stream into a major stream and a smaller return stream;

[0037] (c) expanding the return stream to provide an expanded return stream;

[0038] (d) heat-exchanging the expanded return stream of step (c) with the major stream of step (b) to provide a vaporised return stream;

[0039] (e) compressing the warmer return stream of step (d) to a pressure being wholly or substantially the same as the pressure inside the storage tank to provide a compressed return stream; and

[0040] (f) passing the compressed return stream into the storage tank.

[0041] The storage tank may be of any suitable shape and design, and the present invention is not limited by the nature of the storage tank. Many liquefied gas storage tanks are typically designed for a specific liquefied gas.

[0042] The storage tank may have one or more inlets and one or more outlets, with such inlets and outlets optionally serving different purposes. The storage tank has at least one discharge outlet for expected discharge of the tank content at a suitable dock, handling port of the like. Step (a) of the present invention comprises withdrawing liquefied gas in the liquefied gas storage tank from an outlet of the storage tank to provide a withdrawn stream. The withdrawing of the liquefied gas from the storage tank may be carried out by any suitable means, including the use of one or more pumps, vacuum, gravity or other forces, to create a discharge of the tank content in a manner known in the art.

[0043] According to one embodiment of the present invention, withdrawing liquefied gas from the outlet of the storage tank is provided by a pump at or near the outlet, or otherwise in line with the outlet of the storage tank.

[0044] Step (b) of the present invention comprises dividing the withdrawn stream into a major stream, such as >50% or >60% or >70% or>80% or >90%, and a smaller return stream. Various stream valves, splitters, dividers, etc. are known in the art, able to divide a liquefied gas stream into two or more liquefied stream, particularly into desired proportions, such as >90%;<10%, 90;10, 80:20, 70:30, 60:40, or thereinbetween, such as in the range of 90% to >50% versus 10% to <50%.

[0045] Step (c) of the present invention comprises expanding the smaller return stream to provide an expanded return stream. Various valves, devices and expanders are known for expanding a liquefied gas stream, and are not further described herein.

[0046] Step (d) of the present invention comprises heat exchanging the expanded return stream of step (c) with the major stream of step (b) to provide a vaporised return stream. In this way, the present invention uses the inherent energy content of the liquefied gas that is being provided as the major stream from the withdrawn stream, as the heat energy required to achieve full vaporisation of the smaller return stream.

[0047] The heat exchanging of step (d) may be provided at or near the storage tank.

[0048] The step (d) of the present invention also achieves a change in the energy content of the major stream after the heat exchanging of step (d). Typically, the major stream has become cooler. Cooling of the major stream provides the additional benefit of offsetting undesirable energy inputs into the major stream, such as heat transfer from the ambient surroundings, and through the action of withdrawal in step (a) and / or the dividing in step (b). In this way, the temperature of the major stream after step (d) can be controlled to a more desired temperature, in particular reducing or avoiding any undesired temperature rise after step (a).

[0049] Step (e) of the present invention comprises compressing the warmer compressed stream step (d) to a pressure being wholly or substantially the same as the pressure inside the storage tank, to provide a compressed return stream. The compressing of step (e) is typically provided by one or more suitable compressors. Such compressors are well known in the art, and generally comprise a suitable inlet and a suitable outlet, through which a gaseous stream can be compressed. In the present invention, such a compressor is typically found at or near liquefied gas storage tanks. Typically, such compressors are useable for one or more other functions in relation to the liquefied gas, including compression of boil-off gas (BOG), etc.

[0050] Step (f) of the present invention comprises passing the compressed return stream of step (e) into the storage tank. The passing of the compressed return stream may be carried out through a suitable inlet, and may be provided by a suitable compressor .

[0051] In this way, the vaporisation energy required to vaporise a suitable return stream is inherently supplied from the major product stream, and so does not require any external energy input. Thus, the provision of a suitable amount or volume of balance gas (to maintain or balance the pressure in the storage tank during the liquefied gas discharge) only requires the power input or consumption of the compressor. This is typically less than 20% or even 10% of the vaporisation energy needed, providing a significant reduction in the overall energy input requirement.

[0052] In one embodiment of the present invention, the liquefied gas storage tank is a pressurised tank. Pressurised tanks for the storage of liquefied gases are well known in the art, typically having strengthened walls, such as those discussed herein above. In one embodiment of the present invention, the liquefied gas storage tank is a pressurised tank at a pressure above 5 barg. Optionally, the pressure in the liquefied gas storage tank is in the range 8 to 16 barg.

[0053] The present invention is useable with a range of liquefied gases, such as those described herein. In one embodiment of the present invention, the liquefied gas has a boiling point below -50°C at 1 atmosphere. Such gases include carbon dioxide, hydrocarbons (such as for example LNG and LPG), oxygen and nitrogen.

[0054] Furthermore, a liquefied gas in the gas storage tank may not be a ‘pure’ gas, in the sense of being 100%, but may include impurities, and / or a mixture of gases. Thus, the present invention is not limited by the requirement that the gas is a pure or single liquefied gas, but may be considered to be of a sufficient proportion or standard to be defined as a ‘liquefied gas’. For example, liquefied carbon dioxide storage may realistically only comprise 90% or higher, such as 95% or higher, carbon dioxide.

[0055] One or more impurities may not be volatile, or at least not volatile at the pressures and temperatures used in the present invention. Such non-volatile impurities may accumulate in any heat-exchanger used for step (c). Accumulation of material in a heat exchanger is generally undesirable, as this could lead to reduced efficiency of the heat exchanger, and even corrosion.

[0056] Thus, an embodiment of the present invention includes the further steps of increasing the pressure the compressed return stream or a portion thereof, and using the increased pressure stream to collect and return any non-volatile impurities in the expanded return stream to the storage tank.

[0057] Optionally, the compressed return stream is pressurized and divided into a first tankreturn stream and a second tank-return stream, and the increased pressure of the second return tank stream is used.

[0058] In one embodiment, the second return tank stream passes onto a unit such as an ejector or pressure vessel able to either cause the withdrawal of any non-volatile impurities away from the expanded return stream and / or to help drive any nonvolatile impurities from the expanded return stream back towards the storage tank.

[0059] As such, the term “wholly or substantially” is used herein to define a liquefied gas that can be labelled as a single substance or product, but which may include one or more impurities or one or more mixtures of gases. In another embodiment of the present invention, the liquefied gas is wholly or substantially carbon dioxide. Liquefied carbon dioxide is an established commercial product, used in a number of industries including for example freezing and chilling of food products, carbonation of beverages, water treatment, low temper testing of electronic components and as a chemical reactant or in a number of well known chemical reactions. Moreover, it is increasingly desired to achieve ‘carbon capture’ in relation to benefitting the environment. There is increasing technology for ‘capturing’ gaseous carbon dioxide, either from the atmosphere or from various reactions or processes, which captured carbon dioxide is then desired to be stored at and / or transported to a suitable location. The present invention assists the overall input required for carbon capture, by minimising and reducing the energy required when withdrawing liquefied carbon dioxide from carbon dioxide storage tanks.

[0060] According to a further embodiment of the present invention, the method further comprises having a compressor near a storage tank gas inlet, and step (e) is carried out in the compressor, and step (f) is carried out through the inlet.

[0061] Optionally, the method of the present invention further includes the step of using a pump to at least pump withdrawal of the liquefied gas from the outlet of the storage tank.

[0062] The present invention also provides a system for maintaining the pressure in a liquefied gas storage tank during liquefied gas withdrawal, comprising: a storage tank outlet to withdraw liquefied gas from the outlet of the storage tank to provide a withdrawn stream ; a divider to divide the withdrawn stream into a major stream and a minor return stream; an expander to expand the return stream to provide an expanded return stream; a heat exchanger to heat-exchange the expanded return stream with the major fuel source stream to provide a warmer return stream; a compressor to compress the warmer return stream to a pressure being wholly or substantially the same as the pressure inside the storage tank to provide a compressed return stream; and a storage tank inlet to pass the compressed return stream into the storage tank.

[0063] Optionally, the system further comprises a pump to at least pump withdrawal of the liquefied gas from the outlet of the liquefied gas storage tank.

[0064] Optionally, the system further comprises wherein the liquefied gas storage tank is a pressurised tank at a pressure above 5 barg. Optionally, the pressure in the liquefied gas storage tank is in the range 8 to 16 barg.

[0065] Optionally, the system further comprises is for maintaining the pressure in a liquefied gas storage tank for storing a liquefied gas having a boiling point below -50°C at 1 atmosphere. Such gases include, but are not limited to carbon dioxide, hydrocarbons (such as for example LNG and LPG), oxygen and nitrogen.

[0066] Optionally, the liquefied gas in the storage tank is wholly or substantially carbon dioxide.

[0067] Optionally, the system further includes a pressuriser to increase the pressure the compressed return stream or a portion thereof, and a unit to use the increased pressure stream to collect and return any non-volatile impurities in the expanded return stream to the storage tank.

[0068] Optionally, the compressed return stream of the system is pressurized and divided by a divider into a first tank-return stream and a second tank-return stream, and the increased pressure of the second return tank stream is used to collect and return any non-volatile impurities in the expanded return stream to the storage tank. In one embodiment, the second return tank stream passes onto a unit such as an ejector or pressure vessel able to either cause the withdrawal of any non-volatile impurities away from the expanded return stream and / or to help drive any non-volatile impurities from the expanded return stream back towards the storage tank.

[0069] As described hereinabove, the storage tank of the system of the present invention may be of any suitable shape and design, and the present invention is not limited by the nature of the storage tank. Many liquefied gas storage tanks are typically designed for a specific liquefied gas.

[0070] Such storage tanks could be fitted with vapour transfer devices, such as compressors, to remove vapour from the tank during filling, (i.e. in the converse case where liquid is introduced into the tank), since otherwise the pressure rise within the tank may be undesirable.

[0071] Referring to the Figures, Figure 1 shows in a diagrammatic form, an example of a method of maintaining the pressure in a liquefied gas storage tank 10 according to one example of the present invention.

[0072] The liquefied gas storage tank 10 may be for the storage of any suitable liquefied gas 12, for example being carbon dioxide, but not limited thereto. Liquid carbon dioxide storage is a common action or requirement for many industries and chemical processes, and increasingly for transportation thereof after ‘carbon capture’.

[0073] In the liquefied gas storage tank 10, there is a liquefied gas 12 comprising a portion of the storage tank 10 volume, and a portion of the storage tank 10 in the space above the liquid, typically being gas or vapour of the liquefied gas.

[0074] During withdrawing of the liquefied gas 12 from an outlet 20 of the storage tank 10 to provide a withdrawn stream 30, it is commonly desired to achieve, to maintain, or as required achieve a minimum change, in the pressure within the storage tank 10. A change in pressure in the storage tank 10 will occur due to the withdrawal of the portion of the liquefied gas 12, and the present invention provides a more efficient method of maintaining the pressure within the liquefied gas storage tank 10 during the liquefied gas 12 withdrawal.

[0075] Figure 1 shows a pump 46 located in line with the stream of the withdrawal stream 30 from the outlet 20 of the storage tank 10, which pump can provide the withdrawn stream 12 to a suitable divider 24. Dividers are well known in the art, and the divider shown in Figure 1 provides a major stream 32 and a smaller return stream 33. The return stream 3 undergoes expanding through a suitable expander 26, such as an expansion valve, to provide an expanded return stream 34. In the method of the present invention, there is heat exchange between the expanded return stream 34 and the major stream 32 in a suitable heat exchanger 28. Heat exchangers are well known in the art, generally comprising one or more parallel or cross flows, designed to achieve heat exchange between the incoming streams.

[0076] The expanded return stream 34 takes energy from the major stream 32 so as to become a vaporised return stream 38.

[0077] Following the withdrawal of energy from the major stream 32, there is typically a cooler major stream 36. The cooler nature of the cooler return stream 36 can be beneficial, where such stream has become undesirably warmer, such as following the action of the withdrawal, pumping and dividing of the withdrawn stream 30.

[0078] The vaporised return stream 38 is then compressed by a compressor 40.

[0079] Various compressors are known in the art, and may comprise one or more stages, one or more trains and the like to achieve a compressed return stream 42. In industries and processes and transportation of liquefied gases, compressors are typically already in location at or near liquefied gas storage tanks, such that the present invention can take advantage of the citing of an existing compressor, for use in the present invention.

[0080] The compressor 40 increases the pressure of the vaporised return stream 38 to a raised pressure, usually of at least several bar.

[0081] The present invention is achieved by passing the compressed return stream 42 into the storage tank 10 through the inlet 22. A suitable controller or controlling means can be used to control the amount or proportion of the compressed return stream 42 back into the storage tank 10 to achieve maintaining the pressure in the liquefied gas storage tank 10 following or during the liquefied gas withdrawal described herein above.

[0082] In this way, the present invention is able to compress a returning gas stream before it is returned back into a liquefied gas storage tank for maintaining the pressure therein, using an existing compressor. Thus, the only power requirement to achieve the method of the present invention is for the compressor. This can be contrasted with conventional method of requiring external energy for the production of the vaporised return stream.

[0083] Figure 2 shows a vessel 50, the vessel being a refrigerated seagoing tanker having one or more, typically at least two, storage tanks. The vessel is on the sea 52, and docked by or near a dock 54 having an onshore facility 56 able to use or convey or store a liquefied gas.

[0084] Using the method and system shown and described in the embodiment of Figure 1 above, Figure 2 shows a storage tank 10 having a liquefied gas 12 therein that is being discharged to the onshore facility 56.

[0085] As described above in relation to Figure 1, the liquefied gas 12 is discharged from the storage tank 10 through an outlet to form a withdrawal stream 30, which is pumped by an in-line pump 46, and then divided by a divider 24 into a major stream 32 and a smaller return stream 33. The smaller return stream 33 is expanded by a suitable expansion valve to pass into a suitable heat exchanger 28 to heat exchange with the major stream 32.

[0086] In this way, the expanded return stream takes energy from the major stream 32 so as to become a vaporised return stream 38. The vaporised return stream 38 passes through a suitable compressor 40 prior to entry through a suitable inlet back into the storage tank 10 to maintain the pressure in the storage tank 10 during the discharge of the liquefied gas 12.

[0087] Figure 3 is a schematic plan with example parameters at defined positions around an embodiment of the system of the present invention.

[0088] Figure 3 shows a schematic provision of a liquefied gas 101 stream discharged from a storage tank (not shown) and passing through a pump P-100. The parameters and measurements of energy, actual volume flow, feed pressure, product pressure, product temperature and mass flow, of the stream 101 at the pump P-100 are set out in Figure 3. From pump P-100, the liquefied gas passes into a suitable divider 120 to produce a major stream 122 and a smaller return stream 124. The return stream 124 passes through a suitable expansion valve 126 to produce an expanded return stream 128 that passes into a heat exchanger E-102. The duty of the heat exchanger E-102 is set out in Figure 3 as 751.1 kW.

[0089] The major stream 122 also passes through the heat exchanger E-102 to heat exchange with the expanded return stream 128, to produce a cooler ongoing stream 110. Figure 3 sets out the parameters of temperature, pressure, mass flow and actual volume flow for the cooler ongoing stream 110.

[0090] The cooler return stream 105 exiting the heat exchanger E-102 has the parameters set out in Figure 3 for temperature, pressure, mass flow and actual volume flow.

[0091] The cooler stream 102 passes through a suitable compressor K-100, which requires a power input of 67.85 kW as set out in Figure 3, to produce a compressed return stream 106 having the temperature, pressure and actual volume flow set out in Figure 3.

[0092] The plan sets out how in Figure 3, this example of the present invention utilises energy in the major stream 122 to vaporise the smaller return stream 128 to a pressure lower than that of the storage tank. The product pressure at pump P-100 is 22 bar, and the pressure of the expanded and cooler return stream 105 is 7.8 bar. The cooler return stream 105 passes through the compressor K-100 to increase the pressure of the compressed stream 106 to 13 bar.

[0093] In the example shown in Figure 3, the vaporisation energy of 751.1 kW involved in the heat exchanger E-102 to vaporise a suitable return stream is inherently supplied from the major product stream 122, and so does not require any external energy input. Thus, the provision of a suitable amount or volume of balance gas (to maintain or balance the pressure in the storage tank during the liquefied gas discharge) only requires the power input or consumption of 67.85 kW by the compressor K-100. This is a reduction in energy input of approximately 90%. As mentioned above, the removal of heat energy from the major stream 122 has the additional benefit of cooling the major stream, off-setting undesirable energy inputs such as heat transfer from the surroundings, and pumping energy, both of which may cause an undesirable temperature increase in the major stream being discharged. The example in Figure 3 confirms a product temperature of -31.31°C at pump P-100, and a lower temperature of the post-heat exchanger major stream 110 of -35.59 °C.

[0094] Figures 4a and 4b show variants of the example shown in Figure 1, further comprising the steps of increasing the pressure of the compressed return stream or a portion thereof, and using the increased pressure stream to collect and return any non-volatile impurities in the expanded return stream to the storage tank. As described above, the liquefied gas 12 in the storage tank 10 may comprise one or more impurities, which impurities may not be volatile at the pressures and temperatures used in the present invention. Such non-volatile impurities can accumulate in any heat-exchanger 28, and accumulation of material in the heat exchanger 28 is undesirable.

[0095] In Figure 4a, the initial compressed return stream 59 directly provided by the compressor 40 can be increased in pressure when desired to at least above the pressure in the storage tank 10 by using a suitable flow restrictor 64. Upon operation of the restrictor 64, the compressed return stream 59 is divided by a divider 58 into a first tank return stream 42 (in the manner of the compressed return stream 42 described above), and a second tank return stream 60. The second tank return stream 60 passes into an ejector 68 operating in a manner known in the art, the ejector 68 is able to withdraw from the heat exchanger 28 any non-volatile impurities collecting in the heat exchanger 28. Figure 4a shows a combined postejector stream 62 for return to the storage tank 10.

[0096] In Figure 4b, the initial compressed return stream 59 directly provided by the compressor 40 is similarly increased in pressure when desired to a pressure above the pressure in the storage tank 10 by using a suitable restrictor 64. With operation of the restrictor 64, the compressed return stream 59 can be divided when required (described below) by a divider 58, into a first tank return stream 42 (in the manner of the compressed return stream 42 described above), and a second tank return stream 60.

[0097] Meanwhile, a drainage line 61 from the heat exchanger 28 provides a passage for any non-volatile impurities collecting in the heat exchanger 28 to a collecting vessel 72. A drainage valve 74 in the drainage line 61 allows flow along the drainage line 61when required.

[0098] Once the collecting vessel 72 is appropriately filled, the drainage valve 74 can be shut, and a vessel entry valve 70 opened, to allow the second tank return stream 60 when created to pass into the collection vessel 72. A vessel exit valve 76 can also be opened. Because of the higher pressure of the second tank return stream 60, the impurities collected in the collection vessel 72 are now driven out of the collection vessel to form a post vessel stream 62 for return to the storage tank 10.

[0099] The present invention confirms a method of maintaining the pressure in a liquefied gas storage tank during liquefied gas withdrawal, with an exemplified 90% reduction in energy input required to achieve such maintenance. The present invention therefore provides a much more efficient method, typically using existing apparatus near to liquefied storage tanks, such as a compressor.

Claims

CLAIMS1. A method of maintaining the pressure in a liquefied gas storage tank (10) during liquefied gas withdrawal, the storage tank having an inlet (22) and an outlet (20); comprising the steps of:(a) withdrawing liquefied gas (12) from the outlet of the storage tank to provide a withdrawn stream (30);(b) dividing the withdrawn stream (30) into a major stream (32) and a smaller return stream (33);(c) expanding the return stream (33) to provide an expanded return stream (34);(d) heat-exchanging the expanded return (34) stream of step (c) with the major stream of step (b) to provide a vaporised return stream (38);(e) compressing the warmer return stream (38) of step (d) to a pressure being wholly or substantially the same as the pressure inside the storage tank to provide a compressed return stream (42); and(f) passing the compressed return stream into the storage tank (10).

2. A method as claimed in claim 1 wherein the liquefied gas storage tank (10) is a pressurised tank at a pressure above 5 barg.

3. A method as claimed in claim 2 wherein the pressure in the liquefied gas storage tank (10) is in the range 8 to 16 barg.

4. A method as claimed in any preceding claim wherein the liquefied gas (12) has a boiling point below -50°C at 1 atmosphere.

5. A method as claimed in any one of the preceding claims wherein the liquefied gas (12) is wholly or substantially carbon dioxide.

6. A method as claimed in any one of the preceding claims having a compressor (40) near the inlet (22), and step (e) is carried out in the compressor, and step (f) is carried out through the inlet.

7. A method as claimed in any one of the preceding claims including the step of using a pump (46) to at least pump withdrawal of the liquefied gas from the outlet of the storage tank.

8. A method as claimed in any one of the preceding claims including the further steps of increasing the pressure the compressed return stream or a portion thereof, and using the increased pressure stream to collect and return any non-volatile impurities in the expanded return stream to the storage tank.

9. A system for maintaining the pressure in a liquefied gas storage tank (10) during liquefied gas withdrawal, comprising: a storage tank outlet (20) to withdraw liquefied gas (12) from the outlet (20) of the storage tank to provide a withdrawn stream (30); a divider (24) to divide the withdrawn stream into a major stream (32) and a minor return stream (33); an expander (26) to expand the return stream to provide an expanded return stream (34); a heat exchanger (28) to heat-exchange the expanded return stream with the major fuel source stream (32) to provide a warmer return stream (38); a compressor (40) to compress the warmer return stream to a pressure being wholly or substantially the same as the pressure inside the storage tank (10) to provide a compressed return stream (42); and a storage tank inlet (22) to pass the compressed return stream into the storage tank (10).

10. A system as claimed in claim 9 further comprising a pump (46) to at least pump withdrawal of the liquefied gas (12) from the outlet (20) of the liquefied gas storage tank (10).

11. A system as claimed in claim 9 or claim 10 wherein the liquefied gas storage tank (10) is a pressurised tank at a pressure above 5 barg.

12. A system as claimed in any one of claims 9 to 11 wherein the pressure in the liquefied gas storage tank (10) is in the range 8 to 16 barg.

13. A system as claimed in any one of claims 9 to 12 wherein the liquefied gas (12) has a boiling point below -50°C at 1 atmosphere.

14. A system as claimed in any one of claims 9 to 13 wherein the liquefied gas(12) is wholly or substantially carbon dioxide.

15. A system as claimed in any one of claims 9 to 14 wherein claims including a pressuriser to increase the pressure the compressed return stream or a portion thereof, and a unit to use the increased pressure stream to collect and return any non-volatile impurities in the expanded return stream to the storage tank.

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