Patent Publication Number: US-8117852-B2

Title: LNG vapor handling configurations and methods

Description:
This application claims priority to our copending U.S. provisional patent application with the Ser. No. 60/792,196, which was filed Apr. 13, 2006. 
    
    
     FIELD OF THE INVENTION 
     The field of the invention is LNG vapor handling, and especially as it relates to vapor handling during LNG storage, ship unloading, and transfer operation. 
     BACKGROUND OF THE INVENTION 
     Despite its apparent simplicity, LNG ship unloading poses various significant challenges in several economic and technical aspects. For example, when LNG is unloaded from an LNG ship to a storage tank, LNG vapors are generated in the storage tank due to, among other factors, volumetric displacement, heat gain during LNG transfer and pumping, boil-off in the storage tank, and flashing (due to the pressure differential between the ship and the storage tank). In most cases, these vapors need to be recovered to avoid flaring and pressure buildup in the storage tank system. 
     Moreover, LNG unloading docks and LNG storage tanks are often separated by relatively large distances (e.g., as much as 3 to 5 miles), which frequently causes significant problems to maintain LNG in the transfer line at cryogenic temperatures (i.e., −255° F. and lower). Worse yet, additional heat is introduced into the LNG by the transfer pumps as the ship unloading pumping horsepower is relatively high to overcome pressure losses due to the long distance between the ship and the storage tanks. As a consequence, large amounts of LNG vapor are formed that must be further processed. 
     Furthermore, the LNG storage and unloading system must also be maintained at a stable pressure. To that end, a portion of the vapor coming from the storage tank is typically compressed by a vapor return compressor and returned to the ship to make up for the displaced volume. In such configurations, a dedicated vapor return line is required which adds significant cost to the LNG receiving terminal. The excess vapor from the storage tanks is compressed to a sufficiently high pressure by a boil-off gas compressor for condensation in a vapor condenser that utilizes the refrigeration content from the LNG sendout from the storage tank. As relatively large volumes of vapor are handled by such compressors, currently known compression and vapor absorption systems require significant energy and operator attention, particularly during transition from normal holding operation to ship unloading operation. During normal holding operation, the LNG transfer line generally remains stagnant, which leads to an increase in temperature and thermal stress on the transfer line. Alternatively, vapor control can be implemented using a reciprocating pump in which the flow rate and vapor pressure control the proportion of cryogenic liquid and vapor supplied to the pump as described in U.S. Pat. No. 6,640,556 to Ursan et al. However, such configurations are often impractical and fail to eliminate the need for vapor recompression in LNG receiving terminals. 
     Alternatively, or additionally, a turboexpander-driven compressor may be employed as described in U.S. Pat. No. 6,460,350 to Johnson et al. Here the energy requirement for vapor recompression is typically provided by expansion of a compressed gas from another source. However, where compressed gas is not available from another process, such configurations are typically not implemented. In still other known systems, methane product vapor is compressed and condensed against an incoming LNG stream as described in published U.S. Pat. App. No. 2003/0158458. While such systems increase the energy efficiency as compared to other systems, various disadvantages nevertheless remain. For example, vapor handling in such systems requires costly vapor compression and is typically limited to plants in which production of a methane rich stream is desired. 
     In yet another system, as described in U.S. Pat. No. 6,745,576, mixers, collectors, pumps, and compressors are used for re-liquefying boil-off gas in an LNG stream. In this system, the atmospheric boil-off vapor is compressed to a higher pressure using a vapor compressor such that the boil-off vapor can be condensed. While such a system typically provides improvements on control and mixing devices in a vapor condensation system, it nevertheless inherits most of the disadvantages of known configurations as shown in Prior Art  FIG. 1 . 
     Thus, most of the currently known processes and configurations for LNG ship unloading and regasification require vapor compression and absorption that are typically energy inefficient. Therefore, there is still a need for improved configurations and methods for vapor handling in LNG unloading and regasification terminals. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to configurations and methods of LNG transfer from an LNG source to an LNG storage tank, where refrigeration content of compressed, condensed, and expanded boil-off from the LNG storage tank is employed to subcool the LNG stream in a position intermediate the LNG source and the LNG storage tank. Such configurations and methods advantageously reduce boil-off volume in the storage tank, and further eliminate the need for a vapor return line and compressor between the LNG source and the LNG storage tank, especially where the LNG source is an LNG carrier. 
     In one aspect of the inventive subject matter, a system for transfer of LNG from an LNG carrier to an LNG storage tank comprises an exchanger (preferably located at the unloading dock) that is configured to subcool the unloaded LNG using refrigeration content of a portion of the LNG from the LNG storage tank. In such configurations, it is typically preferred that a separator is configured to receive and separate depressurized heated LNG into a vapor phase and a liquid phase. A return line may then be configured to feed the vapor phase to the LNG carrier, and a pump may be configured to pump the liquid phase to the LNG storage tank. Typically, a compressor is configured to receive boil-off from the LNG storage tank. 
     In further contemplated aspects, a bypass provides at least a portion of the sendout LNG liquid to mix with the compressed boil-off from the LNG storage tank, and a condenser or absorber is configured as a contacting device for the compressed boil-off vapor and is still further configured to receive sendout LNG from the LNG storage tank to thereby form the condensed boil-off from the LNG storage tank. 
     In another aspect of the inventive subject matter, an LNG unloading plant includes an LNG source that is configured to provide an LNG stream and that is fluidly coupled to an LNG storage tank configured to provide a liquid LNG and an LNG vapor. A compressor and a condenser/absorber are fluidly coupled to the LNG storage tank and configured to receive the LNG boil-off vapor and to produce a pressurized send-out LNG. Contemplated plants further include a pressure reduction device that reduces pressure of the pressurized LNG sendout liquid and a heat exchanger that subcools the unloaded LNG stream using the depressurized LNG sendout liquid from the condenser or absorber. 
     Most typically, the pressure reduction device is configured to cool via reduction of pressure the saturated LNG liquid to a temperature that is lower than the temperature of the LNG source (e.g., at least 1 to 3° F.). A separator downstream of the heat exchanger receives the depressurized heated saturated LNG liquid and provides a vapor and a liquid, wherein most preferably a vapor return line delivers the vapor from the separator to the LNG source, and wherein a pump pumps the depressurized liquid to the LNG storage tank. 
     Consequently a method of transferring an LNG stream from an LNG source (e.g., an LNG carrier) includes a step of forming a pressurized saturated LNG liquid from a vapor of an LNG storage tank, and another step of cooling the unloaded LNG stream (e.g., 1° F. or lower) using a heat exchanger that receives refrigeration content from the depressurized sendout LNG liquid. Most typically, the depressurized sendout LNG liquid is heated in the heat exchanger and separated into a vapor portion and a liquid portion, wherein the liquid portion is fed to the LNG storage tank, and/or wherein the vapor portion is fed to the LNG source. In such methods, the LNG storage tank provides a boil-off that is compressed, and the compressed boil-off is preferably mixed with sendout liquid LNG, and wherein the mixture is condensed in a condenser or absorber to thereby form the pressurized saturated LNG liquid. 
     Various objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Prior Art  FIG. 1  is an exemplary schematic of a known LNG unloading station. 
         FIG. 2  is an exemplary schematic of an LNG unloading station according to the inventive subject matter. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention is directed to various configurations and methods for an LNG receiving terminal in which sendout LNG liquid from a storage tank is employed as refrigerant to subcool LNG that is being unloaded. Using such configurations, it should be noted that vapor generation from the tank is reduced to a significant degree and that the vapor return compressor and the return line to the LNG carriers of heretofore known configurations can be eliminated. It should still further be appreciated that the circulation line and pump system for the sendout LNG liquid can be advantageously used during normal holding operation, which will maintain the LNG transfer line at cryogenic temperature. 
     Most preferably, LNG is provided from an LNG carrier vessel or other remote source using conventional LNG transfer lines and one or more pumps to a conventional LNG storage tank that is fluidly coupled to a boil-off compressor and vapor condenser or absorber. The vapor condenser or absorber produces saturated liquid at high pressure, providing at least a portion preferably to an LNG unloading dock. There, the saturated LNG liquid is let down in pressure, heat exchanged with the unloaded LNG from the carrier vessel or other remote source to thereby chill the unloaded LNG. Vapor evolved from the saturated LNG liquid after passing through the heat exchanger is advantageously returned to the ship to maintain the pressure in the transport vessel, while the flashed liquid is pumped to the LNG transfer line to the storage tank. Thus, it should be recognized that the unloaded LNG is subcooled, which eliminates or at least substantially reduces vapor flashing to the storage tank. Consequently, vapor evolution from the storage tank is reduced, which in turn reduces the duty on the vapor recompression and condenser system. Moreover, due to the reduced vapor generation from the storage tank, the vapor return compressor system and the relatively long vapor return line common to most known configurations can be eliminated. 
     To illustrate the advantages over previously known configurations and methods, a typical prior art LNG unloading terminal is shown in Prior Art  FIG. 1 . Here, LNG at about −255° F. to −260° F. is unloaded from an LNG carrier ship  50  via unloading arm  51  and transfer line  1  into storage tank  54 , typically at a flow rate of 40,000 GPM to 60,000 GPM. The unloading operation typically lasts for about 12 to 16 hours, and during this period an averaged rate of 40 MMscfd of vapor is generated from the storage tank as a result from the heat gain during the transfer operation (e.g., by the ship pumps, heat gain from the surroundings), the displacement vapor from the storage tanks, and the liquid flashing due to the pressure differential between the carrier and the storage tank. The LNG carrier ship typically operates at a pressure slightly less than that of the storage tank (e.g., LNG ship at 16.2 psia to 16.7 psia, storage tank at 16.5 psia to 17.2 psia). The vapor stream  2  from the storage tank is split into two portions, stream  20  and stream  4 . Stream  20 , typically at an average flow rate of 20 MMscfd, is returned to the LNG ship via a vapor return compressor  64  that discharges to vapor line  3  to the LNG ship via vapor return arm  52  for replenishing the displaced volume from the unloading process. The power consumption by compressor  64  is typically 500 HP to 1,500 HP, predominantly depending on the tank boil off flow rate and compressor discharge pressure, which in turn depends on the vapor return line size and distance between the storage tank  54  and the LNG carrier  50 . It should be appreciated that the vapor return compressor and the vapor return line substantially contribute to the capital and operating cost of such ship unloading systems. 
     Stream  4 , typically at an average flow rate of 20 MMscfd, is compressed by compressor  55  to about 80 psig to 115 psig and fed as stream  5  to the vapor absorber  58 . Here vapor is de-superheated, condensed, and absorbed by a portion of the sendout LNG which is delivered via valve  56  and stream  6 . The power consumption by compressor  55  is typically 1,000 HP to 3,000 HP, depending on the vapor flow rate and compressor discharge pressure. LNG from the storage tank  54  is pumped by the in-tank primary pumps  53  to about 115 to 150 psia at a typical sendout rate of 250 MMscfd to 1,200 MMscfd. Stream  6 , a subcooled liquid at −255° F. to −260° F., is routed to the absorber  58  to mix with the compressor discharge stream  5  using a heat transfer contacting device such as trays and packing. The operating pressures of the vapor absorber and the compressor are determined by the LNG sendout flow rate. A higher LNG sendout rate with higher refrigeration content would lower the absorber pressure, and hence require a smaller compressor. However, the absorber design is also designed to operate under the normal holding operation when the vapor rate is lower, and the liquid rate may be reduced to a minimal. 
     The flow rate of stream  6  and the bypass stream  8  are controlled using the respective control valves  56  and  57  as needed for controlling the vapor condensation process. The vapor condenser produces a bottom saturated liquid stream  7  typically at about −200° F. to −220° F., which is then mixed with stream  8  forming streaming  10 . Stream  10  is pumped by high pressure pump  59  to typically 1000 psig to 1500 psig forming stream  11 , which is heated in LNG vaporizers  60  forming stream  9  at about 40° F. to 60° F. to meet pipeline specifications. The LNG vaporizers are typically open rack type exchangers using seawater, fuel-fired vaporizers, or vaporizers using a heat transfer fluid. 
     Therefore, it should be appreciated that prior art configurations and methods require substantial energy for compression of the vapors coming off the storage tank for both vapor condensation and return to the LNG source (typically LNG carrier). Moreover, and especially in relatively long distance between the carrier and the tank, the handling of vapor evolution from the tank is very costly. 
     In contrast, contemplated configurations and methods alleviate the above problems by subcooling the LNG flow between the LNG carrier and the LNG storage tank using refrigeration content of expanded sendout LNG liquid and/or compressed storage tank vapor condensate. Thus, preferred configurations include an LNG source that is configured to provide an LNG stream and that is fluidly coupled to an LNG storage tank that is configured to provide a liquid LNG and an LNG vapor. A compressor and a condenser or absorber are fluidly coupled to the LNG storage tank and configured to receive the LNG vapor and to thus provide a pressurized saturated LNG liquid. A pressure reduction device (e.g., JT valve, expansion turbine, etc.) is configured to reduce pressure of at least a portion of the pressurized sendout LNG liquid, and a heat exchanger employs the refrigeration content of the expanded sendout LNG to subcool the unloaded LNG stream to a temperature that is lower than the temperature of the LNG source. 
     Most preferably, a separator is fluidly coupled to and located downstream of the heat exchanger and configured to receive the depressurized heated saturated LNG liquid. The separator provides a vapor and a liquid, wherein a return arm is configured to deliver the vapor to the LNG source. The depressurized liquid is fed to the LNG storage tank using a pump. 
     One exemplary configuration according to the inventive subject matter is depicted in  FIG. 2  in which an LNG ship unloading system is coupled to an LNG circulation system. In such circulation system, a portion of the sendout LNG and the saturated liquid from the vapor condenser is provided to the LNG docking area, letdown in pressure to thereby chill the unloaded LNG. Flashed vapor is used to supply vapor to the ship, which eliminates the need for a vapor return compressor and the long vapor return line. Flashed liquid is returned to the storage tank. Among other advantages, it should be recognized that contemplated configurations and methods reduce vapor loads on the vapor recompression and condensation system, and also substantially decrease the capital and energy requirements. 
     Here, LNG from ship  50  is unloaded via liquid unloading arm  51  and is cooled in a heat exchanger  61  using a portion of the saturated liquid (stream  13 ) from the bottom of the vapor condenser  58  or sendout LNG stream  8  via a bypass (e.g., when valve  56  is closed; not shown in  FIG. 2 ). Stream  13 , at a pressure between about 80 psig to 115 psig and at a temperature of about −220° F. to −250° F., is provided at a rate of about 600 to 1200 gpm via a circulation line to the LNG ship unloading area. Stream  13  is letdown in pressure to about 1 to 2 psig in a letdown valve  64  forming a chilled stream  21  at −257° F. to −259° F. This chilled liquid is then used to cool the unloaded LNG from LNG unloading arm  51 , from −254° F. to about −255° F. It should be appreciated that even a slight reduction in the unloaded LNG temperature (typically 1° to 2° F. or lower) will significantly reduce the vapor load when LNG is unloaded to the storage tank  54 , mainly due to the large unloading flow rate of 40,000 gpm to 60,000 gpm. The two phase stream  14  leaving the heat exchanger  61  is separated in separator  62 . The separated vapor stream  17  is returned to the LNG ship via the vapor return arm  52  to maintain the ship pressure. The flashed liquid  15  is pumped by a pump forming stream  16 , which is preferably combined with the unloaded LNG in LNG transfer line  1  and returned to the storage tank  54 . It should be appreciated that using such circulation, the vapor return compressor  64  and vapor return line  3  of the plant of Prior Art  FIG. 1  are no longer needed. Additionally, as heat exchanger  61  subcools the unloaded LNG, vapor generation from the LNG in storage tank  54  is reduced, which in turn reduces the vapor loads on the boil-off gas compressor  55  to a significant degree. 
     The vapor stream  2  from storage tank  54 , typically at a flow rate of 10 to 20 MMscfd is routed to the compressor  55  as stream  4  and compressed to about 80 psig to 115 psig and fed as stream  5  to the vapor absorber  58 . As in known configurations, the compressed vapor is de-superheated, condensed, and absorbed by a portion of the sendout LNG which is delivered via valve  56  and stream  6 . The flow rate of stream  6  and the bypass stream  8  are controlled using the respective control valves  56  and  57  as appropriate for controlling the vapor condensation process. The vapor condenser produces a bottom saturated liquid stream  7  typically at about −200° F. to −250° F. One portion of stream  7 , stream  12 , is then mixed with stream  8  forming stream  10 . Stream  10  is pumped by high pressure pump  59  to typically 1000 psig to 1500 psig forming stream  11 , which is heated in LNG vaporizers  60  forming stream  9  at about 40° F. to 60° F. to meet pipeline specifications. The LNG vaporizers are typically open rack type exchangers using seawater, fuel-fired vaporizers, or vaporizers using a heat transfer fluid. The other portion of stream  7 , stream  13 , is the fed to the pressure reduction device  64  as described above. Further configurations, methods, and contemplations are presented in our copending International patent application with the publication number WO 2005/045337, which is incorporated by reference herein. 
     Therefore, a system for transfer of LNG from an LNG carrier to an LNG storage tank will comprise an exchanger that is configured to receive and subcool unloaded LNG from the carrier using refrigeration content of sendout LNG and condensed and expanded boil-off from the LNG storage tank. Most preferably, contemplated configurations also include a separator that receives and separates the two-phase LNG downstream of the exchanger into a vapor phase and a liquid phase. The vapor from the separator may then be routed via a return arm to the LNG carrier. However, in alternative embodiments, the vapor may also be condensed or used as refrigerant in other processes. The liquid from the separator is preferably pumped to the LNG storage tank as a separate stream, or as a combined stream with the LNG that is being unloaded from the carrier. Alternatively, the liquid may also be stored separately or otherwise utilized (e.g., as refrigerant in a thermally coupled process). Similar to known configurations, contemplated unloading terminals will preferably include a compressor receives and compresses the boil-off from the LNG storage tank. Typically, the pressure is selected such that the vapor can be condensed in an absorber or other contact device via combination with an LNG stream, for example, from the carrier, but more preferably from a position downstream of the LNG storage tank). Therefore, in preferred configurations, a bypass is configured to provide LNG liquid to the compressed boil-off from the LNG storage tank for condensation of the boil-off vapor. In such configurations, it is preferred to include a condenser or absorber that receives the compressed boil-off from the LNG storage tank and that further receives liquid from the LNG storage tank to thereby form condensed boil-off from the LNG storage tank. Such combination of compressed vapors and LNG may be done upstream of or within the condenser or absorber. 
     Consequently, it should be appreciated that a method of transferring an LNG stream from an LNG source includes a step of forming a pressurized saturated LNG liquid from a vapor of an LNG storage tank, and a further step of cooling the LNG stream using a heat exchanger that receives refrigeration content from the depressurized sendout LNG liquid. Most preferably, the depressurized sendout LNG liquid is heated in the heat exchanger against the LNG that is being unloaded, and separated into a vapor portion and a liquid portion. The liquid portion is preferably fed to the LNG storage tank, while the vapor portion is preferably fed to the LNG source (e.g., LNG carrier). It should be noted that in such methods the liquid stream from the LNG source is subcooled at least 1° F., and more typically between 1.1° F. and 5.0° F. 
     The LNG storage tank provides a boil-off that is compressed using a conventional compressor (which may be energetically coupled with an expander where appropriate) and the compressed boil-off vapor is then mixed with sendout LNG upstream of or within an absorber, condenser, or other contact device. Thus, it should be appreciated that a pressurized sendout LNG liquid is formed, wherein one portion is combined with LNG leaving the storage tank, while another portion is used as refrigerant after expansion (which may be a JT valve or expansion turbine). 
     Thus, specific embodiments and applications of LNG vapor handling configurations and methods have been disclosed. It should be apparent, however, to those skilled in the art that many more modifications besides those already described are possible without departing from the inventive concepts herein. The inventive subject matter, therefore, is not to be restricted except in the spirit of the present disclosure. Moreover, in interpreting the specification and contemplated claims, all terms should be interpreted in the broadest possible manner consistent with the context. In particular, the terms “comprises” and “comprising” should be interpreted as referring to elements, components, or steps in a non-exclusive manner, indicating that the referenced elements, components, or steps may be present, or utilized, or combined with other elements, components, or steps that are not expressly referenced. Furthermore, where a definition or use of a term in a reference, which is incorporated by reference herein is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.