Patent Application
20230076753
Natural gas can be processed into liquified natural gas (“LNG) via a variety of processes that decrease the temperature of the natural gas to produce a liquified natural gas. Some processes utilize a cold box. Refrigerants required by LNG processes can be stored inside tanks that are located within the cold box.
The features and advantages of certain embodiments will be more readily appreciated when considered in conjunction with the accompanying figures. The figures are not to be construed as limiting any of the preferred embodiments.
Natural gas, composed primarily of methane, can be used in a variety of applications, such as in homes and buildings as a source of heat. However, for storage and transportation purposes, methane in a gas state occupies a large volume and can require very large storage or transportation tanks. Accordingly, natural gas can be processed into a liquid state called liquified natural gas (“LNG”). LNG occupies about 1/600th the volume of natural gas in the vapor state, thereby allowing more methane to be stored and transported in a tank of a given volume. LNG is also used as a fuel alternative to gasoline for vehicles. If LNG is to be used as an energy source, for example in homes or buildings, the LNG can be converted back into a vapor state.
There are a variety of ways to process methane gas into a liquid state. All processes involve cooling methane gas to the temperature at which the gas becomes a liquid—typically around −160° C. (−256° F.). The LNG process typically uses hydrocarbon refrigerants (e.g., propane and ethane or ethylene) in one or more vapor compression cycles. Although the refrigerant cycles are closed processes, small amounts of refrigerant vent or leak; for example, through compressor seals or depressurization after shutdown and before restart. Therefore, a processing facility will commonly have refrigerants stored in pressurized containers at the facility to replace these losses.
There are several hazards and disadvantages for refrigerant storage. For example, risk analysis reveals that refrigerant storage is among the most hazardous areas at LNG facilities because the refrigerants are highly volatile, combustible, and are stored under pressure. Propane has a molecular weight heavier than air, while ethane and ethylene have approximately the same molecular weight as air but have a higher density than air when released from storage. The combination of high vaporization rates, high combustibility, and high density can cause a very large, low, pooling cloud of vapor from a leak that can creep over the ground and can find ignition sources. If the leaked vapor finds an ignition source, then a flame can rapidly propagate back to the source of the leak. Once the flame impinges on the pressurized storage container, it will cause the stored refrigerant to heat up and boil. If the storage container mechanically fails and explodes due to the heat and pressure, a resulting boiling liquid expanding vapor explosion (“BLEVE ”) event can destroy large areas of the processing facility and can even cause mass casualties. Some of the worst hydrocarbon explosion events, with hundreds or thousands of casualties, have been caused by BLEVEs from stored ethane and propane; for example, in 1984 in Mexico City, Mexico, and in 1978 in Tarragona, Spain.
Previous efforts to reduce the risk of hazards at LNG facilities to an acceptable level include placing the refrigerant storage containers within a dedicated plot area that is physically distant from other areas of the facility where personnel are continually present or that contain other flammable materials. Some safety measures that have been incorporated into the design of the facilities and the refrigerant storage plot area can include pressure relief, emergency isolation and depressurization, spacing requirements, storage vessel orientation, fire and gas detection equipment, fire monitors, water deluge/curtains, fire protection insulation, firewalls, and spill-impoundment areas.
Accordingly, the requirements for refrigerant storage can result in a complex and expensive processing facility. By way of example, the refrigerant storage area typically requires as much plot area as possible in order to provide adequate spacing and set off distances from other areas of the processing facility. This space is often difficult to come by in facilities with limited available area, particularly in offshore facilities. The refrigerant storage area is also frequently the primary determinant for the overall maximum water demand for fire eradication at facilities. Despite these efforts to improve the safety of refrigerant storage areas, the residual risk profile of possible refrigerant leaks often dominates the overall risk to the facility and workers. Therefore, there is a long-felt need and an ongoing industry concern for improved LNG processing facility systems that reduce the risk of fires and explosions and decrease the acreage needed.
It has been discovered that LNG processes that utilize cold boxes for the refrigeration processes can house the refrigerant storage tanks within the cold box. There are several advantages of the various embodiments disclosed. The primary advantage is that the refrigerants may be stored at temperatures lower than ambient temperature and at pressures very close to ambient pressure. Because the refrigerant vapor pressure is much lower than ambient pressure at the temperatures inside the cold box, the refrigerant storage tanks can use a lower design pressure and can cost less compared with traditional refrigerant storage tanks at ambient temperature. Ethane or ethylene are typically stored in refrigerant storage tanks at LNG facilities at elevated pressure and at temperatures slightly below ambient temperature. A typical temperature, for example, is in the range of −20° C. to −40° C. (−4° F. to −40° F.), with a resulting pressure generated from the refrigerant vapor on the order of 10 to 20 bar. Because the stored liquid is slightly cryogenic, any heat leak into the storage tanks will increase the pressure within the tanks and requires intermittent venting to prevent pressure accumulation and eventual overpressure. By storing the refrigerants inside the cold box at temperatures below the normal ambient pressure boiling point (e.g., about −89° C. (−128.2° F.) for ethane) eliminates the potential for pressure accumulation and the loss of stored refrigerant from venting.
Another significant advantage is that by providing a storage tank for storing refrigerants at near-ambient pressure eliminates the possibility of a BLEVE. For a BLEVE to occur, high-pressure refrigerant storage tanks require significant internal heat and pressure buildup prior to rupturing of the tank, and a low-pressure tank designed for ambient pressure simply cannot accumulate enough energy for the most destructive forms of explosions. Additionally, because the storage tanks are located within the cold box, the tanks are protected from external forces that can cause failure. By way of example, the cold box and insulation within the cold box provides significant protection against fire impingement as well as mechanical impingement from external projectiles, thereby reducing the risks associated with the stored flammable refrigerants. Moreover, any small vapor leaks from the stored refrigerant can be contained within the cold box and can be detected by a standard cold box inert purge, vent, and detection system, effectively providing secondary containment of modest leaks. This can eliminate a major source of process risk to tanks that are located outside of a cold box.
Another advantage is a cost savings due to a higher density of stored refrigerant. The density of refrigerants at the cold box cryogenic temperatures is significantly higher compared to the density of refrigerants stored at traditional temperatures. For example, propane stored at −160° C. (113 K) has a density 40% higher (or more dense) than propane stored at 15° C. (288 K). Reducing the required storage volume of the storage tanks for the same quantity of refrigerant by 40%, for example, can provide significant cost savings. Another significant cost savings is eliminating a large, separate refrigerant storage area from the LNG processing facility. This can significantly reduce the total acreage needed for the LNG processing facility.
A system for processing liquified natural gas can include: a natural gas feed; a cold box; one or more natural gas cooling components; and a storage tank configured to store a refrigerant, wherein the one or more natural gas cooling components and the storage tank are located within the cold box.
The LNG processing system includes a natural gas feed. The natural gas can be methane. The natural gas can also include ethane or other compounds. The natural gas in the natural gas feed is at near-ambient temperature (24° C. (75° F.)) and at an elevated pressure generally in the range of 30 to 90 bar. The natural gas in the natural gas feed can be cooled to a temperature less than 24° C. (75° F.) for example. The natural gas can be cooled using one or more refrigeration cycles (discussed in more detail below).
The LNG processing system also includes a cold box. The LNG processing system can use any type of process that processes LNG using a cold box. The cold box can be an enclosure that is airtight or mostly airtight. Externally, the cold box can be a cube, or rectangular cuboid, with a leak-tight carbon-steel enclosing frame, filled with the required processing equipment, piping, and loose-fill insulation, including but not limited to expanded perlite. The cold box can be maintained slightly above atmospheric pressure (i.e., >1 atmosphere), for example, in the range of 1.01 atm-1.10 atm. The cold box can be constructed in a remote fabrication yard. The cold box can be fitted with cryogenic flanged terminations to enable simple on-site hookup to facility process pipework without intensive field construction.
The cold box can have dimensions that are selected based on the specific type of LNG processing. The cold box can be made from a variety of materials known to those skilled in the art. The cold box can further include a loose-filled insulating material that insulates the components inside the cold box against increases in temperature. The cold box can be purged with a constant, low pressure, dry, inert gas (for example, nitrogen gas) to prevent air and water vapor ingress. The cold box can further include an inert gas inlet and a vent.
Turning to the figures, the cold box can be a methane cold box 130 as shown, for example, in
As shown in
As discussed above, the natural gas in the natural gas feed can be cooled prior to entering the methane cold box. One non-limiting example of a process that utilizes a cold box is called a pure component cascade process.
As can be seen, the LNG processing 100 system can include a propane refrigerant loop and propane cooling equipment. The refrigerant, propane, can flow through a closed-loop vapor compression refrigeration cycle. In this cycle, propane vapor is compressed to high pressure and condensed against a cooling medium, e.g., air or water. The high-pressure liquid propane is expanded to lower pressure, and the resulting cold, low-pressure liquid propane provides refrigeration in one or more heat exchangers to the desired high temperature process stream(s)—for example, as shown in
The system also includes a storage tank configured to store a refrigerant, wherein the storage tank is housed within the cold box. According to a first example, the storage tank can be housed within a methane cold box.
If more than one storage tank is included, then the first storage tank 201 and the second storage tank 202 (and any other storage tanks) can be oriented on top of each other or side by side. A fluid communicator connector 203 can connect the first and second storage tanks 201/202 such that fluid communication and pressure communication occurs between the tanks. The fluid communicator connector 203 can be used when the same type of refrigerant (e.g., propane) is stored within both the first and second storage tanks 201/202.
The system can include a refrigerant fill line 220. The refrigerant fill line 220 can be used to replace the refrigerant to a desired volume. If the storage tanks store different types of refrigerants, then each type of refrigerant storage tank can have its own refrigerant fill line 220. The system can also include an inert gas inlet 240. An inert gas (for example, nitrogen gas) can be supplied to the first and second storage tanks 201/202 via the inert gas inlet 240 to pressurize the first and second storage tanks 201/202 to a desired pressure, which can eliminate the possibility of operating the storage tanks in vacuum conditions and can provide a driving force for draining the refrigerant from the storage tanks. According to any of the embodiments, the first storage tank 201 and any other storage tanks are designed to withstand a desired internal pressure; for example, in excess of the inert gas supply pressure. The desired pressure can be, for example, 10 Bar (1 megapascal) or other pressure as required by the system. As with the refrigerant fill line 220, more than one inert gas inlet 240 may be needed if a different type of refrigerant is stored within the storage tanks.
The system can also include a refrigerant drain line 230. The refrigerant drain line 230 can be connected to a series of storage tanks that store the same type of refrigerant, or there can be a refrigerant drain line 230 for each storage tank or series of storage tanks for each type of refrigerant. The refrigerant being stored in the storage tank(s) can flow out of the storage tank via the refrigerant drain line 230. The first storage tank 201, the second storage tank 202, or any of the storage tanks can be slightly sloped towards the refrigerant drain line 230 to allow complete drainage of the refrigerant from the tank(s).
The storage tank can also be housed within an ethylene cold box that is shown in
In a pure component cascade process as discussed above with reference to
According to any of the embodiments, the first storage tank 201 and any other storage tanks are thermally coupled to each other and at least one of the one or more natural gas cooling components. According to any of the embodiments, the storage tank is thermally coupled to the coldest gas cooling component (for example, the third flash vessel 114 shown in
The dimensions of the first storage tank 201 and any other storage tanks can be selected such that a desired mass of refrigerant is stored within the storage tank. The desired mass can be 1 to 2 times the mass needed to fill the corresponding refrigerant cycle. By way of example, if a propane refrigerant cycle utilizes 500 tonnes of propane, then the first storage tank 201 for propane can be in the range of 500 to 1,000 tonnes. In this manner, if all of the propane in the refrigerant cycle needs to be purged, then there is sufficient propane stored in the first storage tank 201 to fill the refrigerant cycle. The volume of the storage tank can be calculated based on the required mass of refrigerant divided by the density at the refrigerant storage temperature. The storage tank can have a small amount of volume reserved for vapor space above the stored liquid refrigerant. For example, the storage tank can include 95% of the volume reserved for liquid refrigerant and 5% for vapor space. Other percentages can be selected based on the specifics of the LNG processing 100 system.
The LNG processing 100 system can include a variety of other components. The other components can include but are not limited to: instrumentation for monitoring temperatures, pressure, leaks, and flow rates; control systems, including flow control valves and flow meters; inert gas controllers for the cold box and storage tank, including inlets and vents; and safety systems, including relief valves. Another component that can be included in the system is a pre-cooler for the refrigerant fill line 220. In this manner, the refrigerant can flow into the storage tank at a temperature less than the temperature outside the cold box and reduce or eliminate an increase in the internal temperature of the cold box. The refrigerant in the fill line can be pre-cooled using a small slip stream of the produced LNG. Another component that can be included in the system is a warmer for the refrigerant in the refrigerant drain line 230. The warmer can be a mechanical heater or an external heat exchanger; for example, a slip stream of natural gas from the natural gas feed that increases the temperature of the refrigerant in the drain line prior to flowing into the refrigerant loops.
Therefore, the present invention is well adapted to attain the ends and advantages mentioned as well as those that are inherent therein. The particular embodiments disclosed above are illustrative only, as the present invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is, therefore, evident that the particular illustrative embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the present invention.
As used herein, the words “comprise,” “have,” “include,” and all grammatical variations thereof are each intended to have an open, non-limiting meaning that does not exclude additional elements or steps. While compositions, systems, and methods are described in terms of “comprising,” “containing,” or “including” various components or steps, the compositions, systems, and methods also can “consist essentially of” or “consist of” the various components and steps. It should also be understood that, as used herein, “first,” “second,” and “third,” are assigned arbitrarily and are merely intended to differentiate between two or more storage tanks, fill lines, drain lines, etc., as the case may be, and do not indicate any sequence. Furthermore, it is to be understood that the mere use of the word “first” does not require that there be any “second,” and the mere use of the word “second” does not require that there be any “third,” etc.
Whenever a numerical range with a lower limit and an upper limit is disclosed, any number and any included range falling within the range is specifically disclosed. In particular, every range of values (of the form, “from about a to about b,” or, equivalently, “from approximately a to b,” or, equivalently, “from approximately a-b”) disclosed herein is to be understood to set forth every number and range encompassed within the broader range of values. Also, the terms in the claims have their plain, ordinary meaning unless otherwise explicitly and clearly defined by the patentee. Moreover, the indefinite articles “a” or “an,” as used in the claims, are defined herein to mean one or more than one of the elements that it introduces. If there is any conflict in the usages of a word or term in this specification and one or more patent(s) or other documents that may be incorporated herein by reference, the definitions that are consistent with this specification should be adopted.
