Equipment and process for liquefaction of LNG boiloff gas

[0001] The present Divisional application claims priority from U.S. application Ser. No. 11/474,787, filed Jun. 26, 2006 now U.S. Pat. No. 7,581,411, which claims priority from U.S. Provisional Patent Application Ser. No. 60/798,696 filed May 8, 2006.

[0002] The present invention is directed to the reliquefaction of boiloff vapors from liquefied natural gas (LNG) storage tanks. Such storage tanks are used on large ocean-going vessels for transport of LNG, and are in widespread use on land in many applications.

[0003] This invention is particularly applicable to shipboard re-liquefaction of boil-off natural gas from LNG carriers, where simplicity, weight, energy consumption, cost, and maintenance must strike an economic balance.

[0004] Such systems have typically incorporated a refrigeration cycle, composed of a working fluid such as nitrogen gas in mufti-stage compression and one or two turboexpanders which may drive compressors; and the boiloff gas is typically compressed in two stages. Such prior art is shown in existing patents: WO 98/43029 A1 (Oct. 1, 1998), WO 2005/057761 A1 (May 26, 2005), WO 2005/071333 A1 Aug. 4, 2005, each issued to Rummelhoff; and U.S. Pat. No. 6,449,983 B2 (Sep. 17, 2002) and U.S. Pat. No. 6,530,241 B2 (Mar. 11, 2003), each issued to Pozivil; and has also been prominently displayed in publications and web sites. The designs in the prior art include turboexpansion of the refrigerant gas through wide pressure and temperature ranges, considered essential for process efficiency under the selected overall plant design, leading to compression of the refrigerant gas in multistage compressors of increased weight and complexity. None of these patents (and other published material) has openly considered the viability of a single stage of refrigerant compression, though shipboard liquefaction of boiloff gas has been a topic of serious investigation. Hence, the advantages of single-stage compression of a refrigerant gas in a main compressor have not been obvious to practitioners with skill in the specific technology.

[0005] Since these installations are considered primarily (but not exclusively) aboard ship, size and weight, and the number of pieces of equipment, especially machinery, take on great importance. Additionally, requirements for unbroken on-stream time may necessitate full duplication of all rotating equipment, effectively doubling the savings which accrue from a reduction in component machinery and complexity.

[0006] In view of the compound requirements for achieving efficient reliquefaction and reducing the number of components, including their weights and complexity, it would be advantageous to develop a process which achieves both ends.

[0007] It has been determined that under certain design configurations, a refrigeration cycle requiring a main single-stage compressor for the refrigerant, can have high thermodynamic efficiency (low specific power); and have the aforementioned benefits of reductions in component rotating equipment.

[0008] The current invention breaks the state-of-the-art barrier to an efficient refrigeration cycle based on a low compression ratio for the refrigerant gas, and enables employment of a single-stage main compressor for the refrigerant gas. The current system offers attractive alternatives to other proposed and constructed systems.

[0009] This invention achieves the objectives of net capital cost and overall weight reduction by reducing the compression of nitrogen in a main compressor to one centrifugal stage, saving a large investment over a main compressor of multiple stages and its coolers. Further compression may take place in compressors which are shaft-connected to turboexpanders.

[0010] Another aspect of this invention is that the refrigeration cycle is so designed as to efficiently achieve boiloff gas condensation while utilizing only one turboexpander, while maintaining a low compression ratio on the single-stage refrigerant compressor.

[0011] This invention relates to a process and equipment configuration to liquefy natural gas boiloff, wherein gas machinery for the refrigeration cycle is composed of a single-stage main compressor and one or two turboexpanders, which may drive compressors.

[0012] Additional improvements may include, all or individually, a single-stage boiloff gas compressor; an inserted heat exchanger to enable compression of the boiloff gas from an ambient temperature condition; and throttling a small refrigerant sidestream at low temperature in order cover the complete cooling range, while maintaining a low compression ratio on the single-stage main cycle compressor without an increase in energy consumption. This is especially effective when the condensed boiloff gas is brought to a subcooled condition.

[0013] The object of this invention is to provide equipment and process for reliquefaction of LNG boiloff gas which is thermodynamically efficient, in an installation which has a lower capital cost, smaller size (volume, footprint), lower weight, and less need for maintenance than systems utilizing the prior art.

[0014] Reliquefaction systems for liquefaction of LNG boiloff gas can be composed of a circulating working fluid, such as nitrogen in a closed cycle, which includes compression and machine expansion; as well as compression of the LNG boiloff gas. Such systems are machinery-intensive, i.e. the machinery size, weight, cost, and potential maintenance constitute major factors in the practicality and economy of the installation. This invention directly addresses machinery-intensive systems by means of a reduction in machinery components, i.e. stages of compression, while maintaining, and even improving, the energy requirements for reliquefaction.

[0015] The signal feature of the invention incorporates a single-stage main compressor for the circulating refrigerant fluid (nitrogen). Since each stage of compression in a main compressor requires an aftercooler (intercooler, if followed by another stage of compression), a reduction in stages of compression also reduces the heat exchanger requirements for cooling the compressed gas. Of course, savings are multiplied, if an installation must have a spare compressor.

[0016] Additionally, features can be incorporated in the invention which improve the thermodynamic efficiency (reduction in power consumption) of the reliquefaction process. These features include:

• • [0017] 1. The cold boiloff gas emerging from the storage tank is warmed to approximately ambient temperature before it is compressed. Compression of cold gas has a thermodynamic penalty and leads to higher energy consumption.
  • [0018] 2. A small refrigerant stream is liquefied, reduced in pressure, and introduced into the cold end of the main heat exchanger in order to achieve final cooling or subcooling of the reliquefied boiloff gas, as a means of reducing the overall compression ratio required for compression of the refrigerant.

[0019] The invention allows choices for employment of one or two stages of boiloff gas compression; one or two refrigerant turboexpanders; how the turboexpander(s) is loaded, i.e. by compressors, electric generators, mechanical load, and/or dissipative brakes; whether a combination of compressors is in series or parallel; if there are two turboexpanders, whether they operate in series or in parallel; and whether a turboexpander-driven compressor operates over the same pressure range as the main compressor, or a different pressure range.

[0027] The drawings show the arrangement of equipment for effecting this process and its modifications.

[0028] (FIGS. 1 & 2) A refrigerant cycle gas 14, such as nitrogen, is compressed in a single-stage compressor 2. Through an arrangement of heat exchangers 6 and one turboexpander 8, refrigeration is delivered to the compressed natural gas boiloff from the cargo of a liquefied natural gas carrier ship, or other liquefied natural gas storage container.

[0029] The compressed nitrogen 3 is cooled in an aftercooler 4 against cooling water or ambient air, and is partially cooled in a heat exchanger 6 against low-pressure returning streams. A first part of the partially-cooled compressed nitrogen 7 is withdrawn from the heat exchanger and is work-expanded in a turboexpander 8. The exhaust stream 9 from the turboexpander re-enters the heat exchanger 6 and flows countercurrent to the feed streams and exits as stream 14 which returns to the suction side to the aforementioned single-stage nitrogen compressor.

[0030] The second divided stream 10 is further cooled in the heat exchanger 6. It is removed and passed through a throttle valve 11 and stream 12 exits the throttle valve at the same or nearly the same pressure as the turboexpander exhaust pressure of the first divided stream. The valve-throttled stream 12 also re-enters the heat exchanger 6 and flows countercurrent to the feed streams. Stream 12 may be combined with stream 9 at junction point 13 and also returns to the suction side to the aforementioned single-stage nitrogen compressor. Power recovery from the turboexpander 8 may be by mechanical shaft connection to the single-stage nitrogen compressor or by means of an electric generator. In some cases, power recovery may not be practiced.

[0031] In FIG. 1, natural gas boiloff 21 is warmed in a heat exchanger 22 and then compressed in either a single stage compressor, or in two stages with intercooling. The compressed boiloff gas 25 is cooled in an aftercooler 26 against cooling water or ambient air, and the cooled, compressed boiloff gas 27 is then cooled in the above-mentioned heat exchanger 22 by refrigeration derived from warming the aforementioned natural gas boiloff. The cooled, compressed boiloff natural gas 28 undergoes further cooling in heat exchange against the refrigerant in heat exchanger 6. This stream 28 is further de-superheated and then partially or fully condensed. The condensate may be further subcooled. The condensate 29 is returned to the cargo tank of the vessel. The condensate 29 may be flashed to lower pressure with recycle or venting of vapor prior return of the liquid to the cargo tank of the vessel.

[0032] Alternatively (FIG. 2), the cold natural gas boiloff 23 enters the boiloff gas compressor 24 at the temperature it leaves the cargo tank piping, and the stream 25 which exits a one- or two-stage boiloff gas compressor directly enters the heat exchanger 6 for further cooling. Compressed boiloff natural gas undergoes further cooling in heat exchanger 6 against the refrigerant, where the boiloff gas is further de-superheated and then partially or fully condensed. The condensate may be further subcooled prior to cargo tank return. The condensate 29 may be flashed to lower pressure with recycle or venting of vapor prior return of the liquid to the cargo tank of the vessel.

[0033] FIGS. 3 and 4 show arrangements similar to FIGS. 1 and 2, but incorporating two turboexpanders in the refrigeration circuit. The turboexpanders operate over different temperature ranges, which may partially overlap. These systems consume less energy than single turboexpander systems, at the cost of an additional machine and related complexity.

[0034] FIGS. 5 and 6 show arrangements similar to FIG. 1 and FIG. 3, respectively, with the exception that the turboexpanders drive compressors. The refrigeration cycle then includes the effects of further compression by these means. The processes represented in FIGS. 2 and 4 could also be modified to include turboexpander-driven compressors as part of the process cycle.

[0035] There are a large number of combinations of how turboexpander-driven compressors are employed in a refrigeration cycle. The common element in each of the figures is the single-stage centrifugal main refrigeration compressor.

[0036] kgmoles/hr=kilogram moles per hour (flow)

[0037] ° C.=degrees Celsius (temperature)

[0038] bar=bar (absolute pressure)

[0039] composition %=molar percentages

[0040] FIG. 5 shows a process for the reliquefaction of boiloff gas 21 evolved from the cargo tanks of an ocean-going LNG transport vessel, where the boiloff gas evolution rate is 395.9 kgmoles/hr, reaching the deck at a temperature of −130° C. and a pressure of 1.060 bar. The boiloff gas composition is 91.46% methane; 8.53% nitrogen; and 0.01% ethane. The boiloff gas is warmed in heat exchanger 22 and stream 23 exits at 41° C. and 1.03 bar. Stream 23 enters boiloff gas compressor 24 and is compressed to 2.3 bar and 122° C. Stream 25 is cooled in aftercooler 26 to 43° C. and 2.2 bar. Typically, cooling water is the cooling medium in indirect heat transfer with the boiloff gas for this aftercooler and other aftercoolers in the process. The cooled, compressed gas 27 enters heat exchanger 22 in indirect heat transfer with stream 21, and exits as stream 28 at −126.7° C. and 2.17 bar. Stream 27 enters heat exchanger 6 for further cooling, condensation, and subcooling. Stream 29 exits heat exchanger 6 at −169.2° C. and 2.02 bar. It then can be re-injected into the storage tank.

[0041] The refrigeration cycle working fluid in this case is nitrogen. A nitrogen stream 3 at 8.73 bar and 43.12° C. is compressed in a single-stage compressor 2 to 16.64 bar and 123.1° C. at a flow rate of 6875 kgmoles/hr. This stream is cooled in aftercooler 4 to 43° C. and 16.50 bar. Stream 41 is further compressed in turboexpander-driven compressor 81 to 18.99 bar and 59.53° C. Stream 42 cooled in aftercooler 82 to 43.0° C. and 18.89 bar, and stream 5 enters heat exchanger 6, where it is cooled to −142.0° C. A division of nitrogen flow occurs here. Stream 7 is routed to turboexpander 8 at a flow of 6825 kgmoles/hr. The balance of the flow of 50 kgmoles/hr remains in heat exchanger 6 and is cooled to −163.0° C. and 18.49 bar and exits as stream 10.

[0042] Stream 10 is valve-throttled to 9.00 bar which produces a two-phase mixture 12 at a temperature of −171.0° C., which enters the cold end of heat exchanger 6 and is vaporized and warmed as it further removes heat from the boiloff gas stream.

[0043] Stream 7 undergoes a work-producing turboexpansion which is utilized to drive compressor 81. The discharged stream 9 is at −167.7° C. and 8.99 bar. This stream enters heat exchanger 6 at a point where the returning cold stream is at that temperature. The returning streams may be combined as they are warmed to 42.19° C. and 8.73 bar leaving the heat exchanger as stream 14, transferring their refrigerative value to the incoming streams.

[0044] Stream 14 enters the suction side of the single-stage compressor 2 as part of the closed refrigeration cycle.

[0045] While particular embodiments of this invention have been described, it will be understood, of course, that the invention is not limited thereto, since many obvious modifications can be made; and it is intended to include with this invention any such modifications as will fall within the scope of the invention as defined by the appended claims.