Reliquefaction of boil-off from liquefied natural gas

Abstract: The present invention relates to an improved process for the reliquefaction of boil-off gas containing up to 10% nitrogen resulting from the evaporation of liquefied natural gas (LNG) contained in a storage vessel. In the process, a closed-loop refrigeration cycle is utilized wherein an isenthalpically expanded stream (14) is warmed against an initially cooled boil-off stream (3). The boil-off LNG stream is initially cooled by indirect heat exchange with an isentropically expanded refrigerant stream (33).

Description: The present invention relates to a process for recovering liquefied natural gas (LNG) boil-off from a storage vessel. In ocean tankers carrying cargoes of liquid natural gas (LNG), as well as land based storage tanks, a portion of the liquid, normally amounting to approximately 0.1 to 0.25% per day in the case of LNG, is lost through evaporation as a result of heat leak through the insulation surrounding the LNG storage receptacle. Moreover, heat leakage into LNG storage containers on both land and sea causes some of the liquid phase to vaporize thereby increasing the container pressure. Shipboard LNG storage tank boil-off has typically been used as an auxiliary fuel source to power the ship's boilers and generators. However, recent LNG tanker designs have incorporated the use of diesel engines rather than steam driven engines thereby eliminating the need for supplemental energy supplied by LNG boil-off. Recently enacted legislation prohibiting tanker disposal of hydrocarbon-containing streams by venting or flaring within the vicinity of metropolitan areas coupled with an Increased desire to conserve energy costs have led to incorporation of reliquefiers into the design of new tankers for recovering LNG boil-off. Attempts have been made to recover nitrogen-containing natural gas boil-off vaporized from a storage tank. Typically, these systems employ a closed-loop refrigeration system wherein cycle gas is compressed, cooled and expanded to produce refrigeration prior to return to the compressor. The following patent is representative: US-A-3,874,185 discloses a reliquefaction process utilizing a closed-loop nitrogen refrigeration cycle wherein the lowest level or coldest level of refrigeration for condensation of LNG is provided by an isentropically expanded stream while the remaining refrigeration is provided by isenthalpic expansion of the residual second fraction of refrigerant. In one embodiment, the residual fraction of the isenthalpically expanded stream is subjected to a phase separation wherein liquid and vapor fractions are separated. During periods of low refrigeration requirements a portion of the liquid fraction is stored, and, during periods of higher refrigeration requirements, a portion of the stored liquid fraction is recycled into the refrigeration system. US-A-3,919,852 shows the use of an isentropically expanded nitrogen stream for cooling at higher temperature and pressure than the isenthalpically expanded stream and a dual expansion is realized by expansion valves. The present invention according to claim 1 provides a flexible and highly efficient process for reliquefaction of boil-off gas containing from about 5 to about 10% nitrogen. Prior art processes are typically unable to efficiently reliquefy boil-off where the nitrogen content varies over such a wide range. They are designed to operate optimally within a narrow concentration range. As the concentration of contaminants moves away from design criteria, the reliquefiers become less efficient. The present invention eliminates this deficiency. Several advantages are achieved by the present invention. They are: (a) an ability to obtain a closer match between the warming curve of the refrigerant cycle gases and the cooling curve of the LNG boil-off stream thereby reducing energy requirements to achieve liquefaction; and (b) an ability to obtain greater efficiency permitting reduction of the heat exchanger surface area required to achieve liquefaction. FIG. 1 is a process flow diagram illustrating the closed loop process referred to as an Expander J-T process. FIG. 2 Is a process flow diagram of a closed loop process referred to as the Dual J-T process in accordance with the present invention. FIG. 3 is a process flow diagram of a prior art closed loop process for recovering boil-off gas. The improvement in this process for reliquefying boil-off gases resulting from the evaporization of liquefied natural gas contained in a storage vessel is achieved through the modification of a closed-loop refrigeration system. Conventionally, the closed loop refrigeration systems use nitrogen as a refrigerant or working fluid, and in the conventional process, the nitrogen is compressed through a series of multi-stage compressors, usually in combination with aftercoolers, to a preselected pressure. This compressed nitrogen stream is split with one fraction being isenthalpically expanded and the other being isentropically expanded. Typically, the work from the isentropic expansion is used to drive the final stage of compression. Refrigeration is achieved through such isenthalpic and isentropic expansion and that refrigeration is used to reliquefy the boil-off gas. The objective is to match the cooling curves with the warming curves and avoid significant separations between such curves. Separations are evidence of lost refrigeration value. To facilitate an understanding of one embodiment of the invention, reference is made to Figure 1. In accordance with the embodiment referred to as the Expander-JT process as shown in Figure 1, natural gas (methane) to be reliquefied is withdrawn from a storage tank (not shown) via conduit 1 and compressed in a boil-off compressor 100 to a pressure sufficient for processing during reliquefaction. Refrigeration requirements for reliquefying the LNG boil-off are provided through a closed-loop refrigeration system using nitrogen as the working fluid or cycle gas. In this refrigeration system, nitrogen is compressed from ambient pressure through a series of multi-stage compressors having aftercoolers 102 to a sufficient pressure, e.g., 41,37 to 61,65 bar (600-900 psia). Thermodynamic efficiency is enhanced by using large pressure differences in the nitrogen cycle. The exhaust from the final compressor is split into a first stream 10 and a second stream 30. These streams are cooled in heat exchangers 104 and 106. The first stream 10 is passed through heat exchanger 104, line 11 through exchanger 106 reduced in temperature and then via line 13 isenthalpically expanded through Joule-Thompson (JT) Valve 108 to a pressure from about 13,79 to 22,06 bar (200-320 psia) and a temperature from about -151 to -165°C (-240°F to -265°F). Both liquid and gas fractions are formed. The effluent from Joule-Thompson Valve 108 then is warmed in indirect heat exchange in heat exchangers 110, 106, and 104 via lines 14, 18, and 19 prior to return to an intermediate section of the multi-stage compressor system 102 via lines 20 and 21 or 20 and 22. The remaining refrigeration is obtained wherein second stream 30 is also cooled in heat exchanger 104 then line 31 in heat exchanger 106 to a temperature of -62.2 to -84.4°C (-80 to -120°F) and then via line 32 isentropically expanded in expander 112. The pressure after expansion is from about 4,826 to 8,22 bar (70-120 psia) and the temperature is from about -157 to -173°C (-250°F to -280°F). In contrast to prior art processes, the isentropically expanded fluid is withdrawn from expander 112 through line 33 and passed through exchangers 106 and 104 which are operated at temperatures higher than the final temperature of the condensed boil-off gas. The warmed working fluid is then returned or recycled via lines 36 and 37 to compressor system 102. In prior art processes, isentropically expanded fluid withdrawn via line 33 was used to provide the "coldest" level of refrigeration for the LNG, whereas in the Expander JT process, the isenthalpically expanded stream via line 14 is used to provide its coldest level of refrigeration and thus, refrigerate the boil-off to its coldest level. Reliquefaction of the boil-off gas is achieved by cooling against the isenthalpically expanded stream and the isentropically expanded stream in heat exchanger 106 and 110. As a first step, the boil-off gas is initially compressed from ambient to about 2,068 bar (30 psia) in compressor 100. Then it is cooled in heat exchanger 106 against both the isentropically expanded and isenthalpically expanded working fluid to form a partially condensed boil-off stream. It is then cooled to its ultimate liquefaction temperature e.g., -153 to -161°C (-244°F to -258°F) in heat exchanger 110. Refrigeration for heat exchanger 110, to provide final condensation of the partially condensed stream however, is supplied by the isenthalpically expanded first stream. The reliquefied boil-off gas from heat exchanger 110 is withdrawn through line 4 and then pressurized by pumping through pump 114 and returned to the storage vessel. In another embodiment of the invention, referred to as the Dual JT or Joule-Thompson process, more efficient refrigeration can be achieved than through the particular embodiment referred' to as the Expander JT process just described particularly those LNG streams containing higher concentrations of nitrogen, e.g., from about 5-10% by volume. To facilitate an understanding of the Dual JT process, reference is made to Figure 2. To some extent, the embodiment is essentially the same as that of the Expander JT system except that the first stream is cooled and isenthalpically expanded to an intermediate pressure to form a subcooled liquid. A minor portion of the resulting liquid undergoes a second isenthalpic expansion, and provides the lowest level of refrigeration. Thus, a major portion of the liquid produced in the first isenthalpic expansion provides the bulk of the refrigeration in parallel with the isentropically expanded stream. For expediency, the numbering system used in Figure 1 has been used in Figure 2 and the components function and operate in essentially the same manner as in the embodiment described in Figure 1. In the reliquefaction process, the first stream 10 via line 11 is cooled in heat exchangers 104 and 106 and further via line 12 cooled in heat exchanger 110. The cooled first stream at a temperature from about -168 to -174°C (-270°F to -282°F) is withdrawn through line 213 and expanded in JT valve 215 under conditions sufficient to generate a subcooled liquid e.g., to a pressure from about 8,96 to 17,93 bar (130 to 260 psia). Separator 217 is provided after the first isenthalpic expansion to permit storage of liquid for subsequent use in the event of flowrate or composition change and to permit the separation of vapor, if generated by the expansion, from the liquid. The vapor space in separator 217 communicates through dashed line 219 with conduit line 18 exiting heat exchanger 110 for permitting vapor flow from conduit line 18 to separator 217 or vice versa. The liquid fraction is withdrawn from separator 217 and split into two portions. One portion i.e., the major portion is removed via line 14 and warmed against boil-off gas and against the first stream prior to its first isenthalpic expansion via lines 18, 19 and 20 prior to return to compressor system 102. The balance or minor portion of stream 221 is expanded through Joule-Thompson valve 223 to a pressure from about 2,41 to 3,45 bar (35 to 50 psia) and conducted via line 114 through heat exchanger 116. In heat exchanger 116, the boil-off gas is condensed and cooled to its lowest temperature level e.g., -179 to -184°C (-290°F to -300°F) against the expanded refrigerant. The isenthalpically expanded minor portion is then conveyed via lines 118, 119 and 120 through heat exchangers 106 and 104 to compressor system 102. The isentropic expansion of second stream 30 is conducted in essentially the same manner as was done in the Expander JT Fig. 1 process. However, some process modifications should be made because of the increased nitrogen content and greater refrigeration requirements. Second stream 30 is cooled to a temperature from about -62.2 to -84.4°C (-80 to 120°F) and then conveyed via line 32 to expander 112. It is then expanded to a pressure of about 4,14 to 6,89 bar (60 to 100 psia) which is intermediate the pressure between the first and second isenthalpic expansion of the first stream. The isentropically expanded stream is conveyed via line 33 to heat exchanger 110 then via lines 34 and 36 through heat exchangers 106 and 104 and then via line 37 to compressor system 102. Again, the coldest level of refrigeration for the boil-off is supplied through the isenthalpic expansion of the working fluid in contrast to systems which have used isentropically expanded working fluids as the coldest level of refrigeration. Liquefaction of boil-off is achieved in the following manner: The boil-off gas is removed from the storage vessel via line 1 and compressed in boil-off gas compressor 100 and then passed via lines 2, 3 and 4 through heat exchangers 106, 110, 116 for liquefaction. On exiting heat exchanger 116, the liquefied LNG 15 removed via line 5 and pressurized in pump 225 where it is transferred via line 6 to the storage vessel. To summarize when the boil-off gas nitrogen content is from 5-10%, the pressure required for the isenthalpically expanded stream to totally liquefy the boil-off gas decreases. The Dual JT process to effect reliquefaction of the boil-off stream uses two levels of refrigeration. The bulk of the refrigeration is supplied by a higher pressure isenthalpically expanded stream in parallel with an isentropically expanded stream and the final cooling is provided by a minor stream which undergoes a second isenthalpic expansion to the required lower pressure. Through this two stage isenthalpic expansion enhanced process efficiency is achieved when higher nitrogen concentrations e.g., 5-10% by volume are present in the feed. The following examples are provided to illustrate various embodiments of the invention and are not intended to restrict the scope thereof. Example 1 Expander JT Process A recovery system for LNG boil-off was carried out in accordance with the process scheme as set forth in Figure 1. Nitrogen concentrations varied from 0% to about 10% by volume of the boil-off gas. Table 1 provides stream properties and rates in kg (lb) moles/hr corresponding to the numbers designated in Figure 1 for a boil-off gas containing 0% LNG. Table 2 provides field properties corresponding to numbers designated in Figure 1 or for a boil-off gas containing approximately 10% nitrogen by volume. Table 3 provides stream properties corresponding to a prior art process scheme described in U.S. Patent 3,874,185 where the nitrogen concentration in the boil-off gas is 0%. Table 4 provides stream properties for liquefaction of a boil-off gas containing 10% nitrogen. Fig. 1 - EXPANDER JT - 0% N₂ Stream No. N₂ kg (lb) Moles/hr CH₄ Moles/hr T °C (°F) Press. bar (Psia) Phase 1 -- 327 -102 (-151) 1,03 (14.9) VAP 2 -- 327 -47 (-54) 2,07 (30) VAP 3 -- -- -153 (-243) 1,93 (28) VAP 4 -- 327 -154 (-244) 1,86 (27) LIQ 10 909 -- 35 (95) 54,88 (796) VAP 13 909 -- -153 (-243) 54,33 (788) VAP 14 909 -- -156 (-248) 21,72 (315) LIQ 18 909 -- -157 (-249) 21,58 (313) VAP 20 909 -- 31 (87) 21,17 (307) VAP 21 909 -- 31 (87) 21,17 (307) VAP 30 1879 -- 35 (95) 55,16 (800) VAP 31 1879 -- -48 (-54) 54,88 (796) VAP 32 1879 -- -75 (-105) 54,61 (792) VAP 33 1879 -- -160 (-256) 66,19 (96) VAP 36 1879 -- -160 (-256) 63,43 (92) VAP 37 1879 -- -61 (-77) 62,06 (90) VAP Fig. 1 - EXPANDER JT - 10% N₂ Stream No. N₂ kg (lb) Moles/hr CH₄ Moles/hr T °C (°F) Press. bar (Psia) Phase 1 32 289 -130 (-202) 1,07 (15.5) VAP 2 32 289 -87 (-125) 2,07 (30) VAP 3 32 289 -155 (-246) 1,93 (28) VAP 4 32 289 -182 (-296) 1,86 (27) LIQ 10 736 -- 37 (99) 55,16 (800) VAP 13 736 -- -155 (-246) 54,33 (788) LIQ 14 736 -- -184 (-300) 3,10 (45) VAP 18 736 -- -157 (-250) 2,96 (43) VAP 20 736 -- 35 (95) 2,55 (37) VAP 22 736 -- 35 (95) 2,55 (37) VAP 30 1746 -- 37 (99) 55,16 (800) VAP 32 1746 -- -80 (-112) 54.61 (792) VAP 33 1746 -- -162 (-260) 66,19 (96) VAP 36 1746 -- -99 (-147) 63,43 (92) VAP 37 1746 -- 35 (95) 62,06 (90) VAP PRIOR ART - FIGURE 3 - U.S. PATENT 3,874,185 - 0% N₂ Stream No. N₂ kg (lb) Moles/hr CH₄ Moles/hr T °C (°F) Press. bar (Psia) Phase or Dew Point °C 1 - 292 -94 (-138) 1,03 (14.9) VAP 2 292 -39 (-38) 2,07 (30) VAP 3 292 -152 (-243) 1,93 (28) V+L 4 292 -171 (-276) 1,86 (27) LIQ 45 2368 - 35 (95) 45,02 (653) VAP 46 2368 - -101 (-150) 44,61 (647) VAP 47 2368 - -172 (-278) 62,81 (91.1) VAP 48 2368 - -154 (-245) 60,74 (88.1) VAP 60 2368 - 32 (90) 58,61 (85) VAP 52 415 - 35 (95) 45,02 (653) VAP 54 415 - -153 (-243) 44,20 (641) LIQ 55 415 - -155 (-247) 23,99 (348) LIQ 56 415 - -87 (126) 23,65 (343) VAP 58 415 - 32 (90) 23,24 (337) VAP PRIOR ART - FIGURE 3 - U.S. PATENT 3,874,185 - 10% N₂ Stream No. N₂ kg (lb) Moles/hr CH₄ Moles/hr T °C (°F) Press. bar (Psia) Phase 1 32 289 -130 (-202) 1,07 (15.5) VAP 2 32 289 -87 (-125) 2,07 (30) VAP 3 32 289 -162 (-260) 1,93 (28) V+L 4 32 289 -182 (-296) 1,86 (27) LIQ 5 32 289 -181 (-295) 41,37 (60) LIQ 45 2056 -- 37 (99) 45,02 (653) VAP 46 2056 -- -1,09 (-164) 33,10 (480) VAP 47 2056 -- -178 (-298) 3,31 (48) VAP 48 2056 -- -163 (-263) 3,10 (45) VAP 60 2056 -- 34 (94) 2,99 (42) VAP 52 391 -- 37 (99) 45,02 (653) VAP 54 391 -- -162 (-260) 44,20 (641) VAP 55 391 -- -163 (-263) 13,93 (202) V+L 56 391 -- -101 (150) 13,58 (197) VAP 58 391 -- 33 (94) 13,17 (191) VAP Calculations were made determining the heat exchanger requirements expressed as U times A where U is the heat transfer coefficient and A is the area of heat exchanger surface for the processes set forth in Tables 1-4. Compressor power requirements are also given. These values are set forth in Table 5. Process Boil-off N₂% Heat Exchanger (UA) kJ/H F °C (BTU/H F °F) Power Watt (HP) Table 1 0 1663,132 (779,715) 2023.9 (2,713) Table 2 10 1510,975 (708,380) 2603.5 (3,490) Table 3 0 1700,246 (797,115) 2090.3 (2,802) Table 4 10 1497,579 (702,100) 2648.3 (3,550) From these results, it can be seen the Expander JT system (Table I) is superior to the Table 3 prior art system at a 0% N₂ level in the feed. At the 10% N₂ level, the process is comparable. Example 2 The procedure of Example 1 was repeated except that the process scheme of Figure 2 was utilized for the 10% nitrogen case. As noted from Fig. 2, the Expander JT process of Fig. 1 is modified slightly to handle the additional load required by the higher nitrogen content in the feed. A minor fraction of the liquid from isenthalpic expansion undergoes a second isenthalpic expansion to supply the coldest level refrigeration for condensing the boil-off gas. Table 6 presents stream properties for a Dual-JT process scheme using boil-off gas containing 10% nitrogen. FIG. 2 - DUAL JT - 10% N₂ Stream No. N₂ kg (lb) Moles/hr CH₄ Moles/hr T °C (°F) Press. bar (Psia) Phase 1 32 289 -130 (-202) 1,07 (15.5) VAP 2 32 289 -87 (-125) 2,07 (30) VAP 3 32 289 -155 (-246) 1,93 (28) VAP 4 32 289 -172 (-278) 1,86 (27) LIQ 5 32 289 -182 (-296) 1,72 (25) LIQ 6 32 289 -181 (-295) 41,37 (60) LIQ 10 502 -- 37 (99) 48,27 (700) VAP 12 502 -- -155 (-246) 47,44 (688) LIQ 213 502 -- -172 (278) 47,23 (685) LIQ 14 464 -- -171 (-276) 16,20 (235) LIQ 114 38 -- -184 (-300) 3,03 (44) V+L 118 38 -- -174 (-282) 2,99 (42) VAP 120 38 -- 34 (94) 2,48 (36) VAP 18 464 -- -157 (-250) 16,00 (232) VAP 20 464 -- 34 (94) 15,58 (226) VAP 30 2118 -- 34 (94) 48,33 (701) VAP 32 2118 -- -97 (-144) 47,71 (692) VAP 33 2118 -- -174 (-282) 5,79 (84) VAP 34 2118 -- -120 (-184) 5,65 (82) VAP 37 2118 -- 34 (94) 6,07 (78) VAP The UA and horsepower requirements for this embodiment are shown in Table 7 along with values reproduced from Table 5. Process Boil-off N₂% Heat Exchanger (UA) kJ/H F °C Power Watt (HP) (BTU/H F °F) Table 1 0 1663,132 (779,715) 2021.7 (2,710) Table 2 10 1510,975 (708,380) 2603.5 (3,490) Table 3 0 1700,246 (797,115) 2088.8 (2,800) Table 4 10 1497,579 (702,100) 2648.3 (3,550) Table 6 10 1513,747 (709,680) 2193.2 (2,940) From the above Table 7 and Table 5 in Example 1, the Expander-JT is the most efficient process when the nitrogen content in the boil-off gas is essentially in the 0-5% by volume range while the Dual JT process is the most efficient where the nitrogen content is approximately 5-10% by volume in the boil-off gas. The processes described in U.S. 3,874,185 are less efficient than the Expander-JT where the nitrogen content is from about 0-5% nitrogen and the Dual JT process is more effective when the nitrogen content is approximately 5-10%.

Claims: A process for liquifying boil-off gas containing from about 5 to 10 % nitrogen by volume resulting from the evaporation of liquified natural gas contained in a storage vessel, the boil-off gas being cooled and liquified in a closed-loop refrigeration system and then returned to said storage vessel wherein said closed-loop refrigeration system comprises the steps: compressing nitrogen as a working fluid in a compressor system (102) to form a compressed working fluid; splitting said compressed working fluid into a first (10) and a second (30) stream; expanding said first stream (10) to produce a cooled first stream, then warming against recycle compressed working fluid and boil-off gas; expanding the second stream (30) to form a cooled expanded stream (33) which is then warmed against working fluid prior to return to the compressor system (102); which process comprises the further steps: (a) effecting isenthalpic expansion of said first stream (10) and effecting isentropic expansion of said second stream (30), the isenthalpic expansion of the first stream (10) being effected under conditions such that at least a liquid fraction (221) is generated at a pressure higher than the isentropically expanded stream (33); (b) warming the liquid fraction (221) against the partially condensed boil-off gas and against the compressed working fluid prior to returning said fraction to the compressor system (102); (c) separating any vapor fraction (219), if generated, from the liquid fraction (221), (d) warming the vapor fraction (219), if generated, against boil-off gas and recycle compressed working fluid; characterised by (e) splitting the liquid fraction (221) formed in step (a) into a first major portion (14) and a second minor portion (114); (f) warming the first major portion (14) of the liquid fraction (221) against boil-off gas in parallel with said expanded second stream (33); and (g) isenthalpically expanding the second minor portion (114) to produce a second cooled liquid fraction and a second vapor fraction and then warming the second cooled liquid fraction and said second vapor fraction against the partially condensed boil-off gas for effecting final condensation of boil-off gas in a further heat exchange step separate from step (f). The process of claim 1, wherein the nitrogen working fluid is compressed to a pressure from about 41,37 to 61,65 bar (600 to 900 psia). The process of claim 2, wherein the first stream (10) is cooled to a temperature from about -168 to -174°C (-270 to -282°F) prior to the first isenthalpic expansion. The process of claim 3, wherein the first stream (10) is expanded to a pressure from 8,96 to 17,93 bar (130 to 260 psia) in the first isenthalpic expansion. The process of claim 4, wherein the second stream (30) is cooled to a temperature of from about -62 to -84°C (-80 to -120°F) prior to the isentropic expansion. The process of claim 5, wherein the second stream (30) is expanded to a pressure fom about 4,14 to 6,89 bar (60 to 100 psia). The process of claim 6, wherein the pressure is reduced to about 2,43 to 3,45 bar (35 to 50 psia) by the second isenthalpic expansion of the first stream (10).

Inventors: COOK PHILIP JOSEPH, COOK, PHILIP JOSEPH
Applicants: AIR PROD & CHEM, AIR PRODUCTS AND CHEMICALS, INC.
Application year: 1989
Main classification: F25J1
IPC codes: F25J1