Abstract:
An apparatus for re-liquefying boil-off gas produced by liquid natural gas is described. The apparatus is made up of a collector, wherein the collector contains liquid natural gas, at least one first pump, wherein the first pump is in or adjacent to the liquid natural gas collector, a compressor, wherein the compressor communicates with the liquid natural gas (LNG) collector by a first conduit, at least one mixing device, wherein the mixing device communicates with the compressor by a second conduit, and wherein the mixing device communicates with the first pump by a third conduit, at least one separating device, wherein the separating device communicates with the mixing device by a fourth conduit, and wherein the separating device also communicates with the liquid natural gas (LNG) collector by a fifth conduit.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to liquid natural gas vaporization and more particularly to the recovery of boil-off gas vapors from n liquefied natural gas (LNG). 
     BACKGROUND OF THE INVENTION 
     Natural gas liquefaction by cryogenic cooling is practiced at remote natural gas rich locations to convert the natural gas to a transportable liquid for shipment to available markets. Before and after transport, the liquefied natural gas (LNG) is stored in a storage tank. Evaporation inside the tank causes the LNG to boil off natural gas vapors, which collect in the storage tank. The vapors generated are defined as boil-off vapors and are low-pressure natural gas. These boil-off vapors would eventually cause the tank pressure to rise above its design pressure if not appropriately addressed. 
     Various procedures exist in the art for dealing with these vapors including venting the vapors into the atmosphere, compressing the vapors to pipeline or process pressure and injecting them into the pipeline or process. These procedures, however, are not environmentally friendly and generally are not economically viable. To avoid these problems, boil-off vapors can be recondensed into the LNG that is being pumped to pipeline pressure (via booster pumps), which in turn is vaporized and injected into the pipeline. Not only is such procedure environmentally friendly and economically viable, but is also efficient because all of the natural gas, including its vapors, is utilized. 
     Natural gas recondenser systems in the industry typically require complicated internal components for the recondenser (such as pall rings, separator devices, deflector plates, internal piping, etc.). In addition, recondenser systems used in the industry typically require a liquid level to be maintained in the recondenser vessel, which also requires high-pressure make-up gas (pipeline gas or nitrogen) to be injected into the recondenser to force a liquid level in the recondenser. Some of these systems also contain separate heat exchangers, as well &amp;a complex controls, equipment, and valving. 
     It is desired therefore to have a natural gas recondenser system that recondenses boil-off vapors from a LNG tank via a simplified system with less equipment and less valving. It is also desired to halve a natural gas recondenser system that does not require a liquid level to be maintained in the vapor/liquid separator drum (recondenser) and hence does not require any high-pressure make-up gas. In addition, it is desired to have an efficient natural gas recondenser system that recycles excess vapor back to the LNG tank to have another chance to be recondensed. 
     SUMMARY OF THE INVENTION 
     One embodiment of the present invention is a process for re-liquefying boil-off gas produced by liquid natural gas. The process contains the steps of (a) containing liquid natural gas in a collector (LNG storage tank), wherein the liquid natural gas produces boil-off gas, (b) compressing and cooling the boil-off gas, to form a higher pressure vapor stream, (c) boosting the pressure of the liquid natural gas to form pressurized liquid natural gas, (d) combining this cooled vapor stream with the pressurized natural gas in a mixing device, to form a substantially liquid gas stream, (e) separating in a separating device any vapor, if vapor exists, from the substantially liquid gas stream to form a vaporless liquid gas stream, (f) returning any vapor from the separating device to the LNG storage tank, and (g) repeating steps (a) to (f). 
     Another embodiment of the present invention is an apparatus for re-liquefying boil-off gas produced by liquid natural gas. The apparatus is made up of a collector (LNG storage tank), wherein the LNG storage tank contains liquid natural gas, at least one first pump, wherein the first pump is in or adjacent to the LNG storage tank, a compressor, wherein the compressor communicates with the vapor space in the LNG storage tank by a first conduit, at least one mixing device, wherein the mixing device communicates with the compressor by a second conduit, and wherein the mixing device communicates with the first pump by a third conduit, at least one separating device, wherein the separating device communicates with the mixing device by a fourth conduit,and wherein the separating device also communicates with the vapor space in the LNG storage tank by a fifth conduit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematical flow diagram of one embodiment of the present invention. 
     FIG. 2 is a diagram of a liquid/vapor separator drum according to one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciate d that the present invention provides many applicable inventive concepts that may be err bodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention and do not limit the scope of the invention. 
     The present invention includes an apparatus and process for re-liquefying boil-off gas generated from liquid natural gas (LNG). The present invention includes an apparatus and process that eliminates the need for a liquid level to be maintained in the recondenser vessel and similarly, eliminates the need for make-up gas to be injected into the recondenser to force a liquid level in the recondenser. In addition, the present invention provides a simplified vapor/liquid recondenser or separator, and provides a mechanism where excess boil-off vapors and non-condensed vapors may be routed back through the system (recycled back to the LNG storage tank) for another chance to be absorbed in an LNG stream. 
     Generally, the present invention may be useful for reliquification of boil-off gas vapors from liquid natural gas stored in a saturated liquid state at cryogenic temperatures (generally about −160° C.) and at pressures sufficient for the liquid natural gas to be at or below its bubble point. As used herein the term “bubble point” generally is the temperature and pressure at which a liquid begins to convert to a gas. For example, if a certain volume of liquid natural gas is held at constant pressure, but its temperature is increased, the temperature at which bubbles of gas begin to form in the liquid natural gas is the bubble point. Similarly, if a certain volume of liquid fluid natural gas is held at constant temperature but the pressure is reduced, the pressure at which gas begins to form defines the bubble point. At the bubble point, the liquid natural gas is a saturated liquid. If liquid natural gas is further cooled, this subcooling reduces the amount of boil-off vapors during its storage, transportation and handling. Liquid that does not boil is said to be subcooled. 
     Nevertheless, heat leakage into the LNG storage tank may cause the LNG to boil off natural gas vapors, which are collected in tile LNG storage tank. These vapors generally may be handled by either venting the vapors into the atmosphere, compressing the vapors to pipeline or process pressure and injecting them into the pipeline or process, recondensing the vapors and placing them back into the LNG storage tank, or recondensing the vapors into the LNG stream that is flowing to booster pumps, which in turn is pumped to pipeline pressure, vaporized and injected into the pipeline. The present invention addresses this last point. 
     The present invention and its advantages may be described with reference to the flow diagram illustrated in FIG.  1 . The overall system is generally indicated at  100 . The system contains a collector or liquefied natural gas (LNG) collector (for example an LNG storage tank  105 ), a vapor compressor  115 , a static mixer  120 , a separator  125 , a secondary pump  130 , restrictors (as for example restrictor  135 ), and interconnecting conduits to be described. Liquefied natural gas (LNG) may be stored in an LNG storage tank  105 . LNG in collector or LNG storage tank  105  is generally stored at near atmospheric pressure in a saturated liquid state (boiling) at cryogenic temperatures (about −160° C.). In one embodiment of the present invention, the LNG may be stored at pressures in the range of from about 0 psig to about 250 psig and at temperatures in the range of from about −160° C. to about −130° C. LNG storage tank  105  may be any suitable receptacle for storing LNG or another saturated liquid. 
     Generally, in order to transport the sub-cooled (low pressure) LNG, the LNG must eventually be pumped to pipeline pressure. In one embodiment of the present invention, the LNG is pumped from LNG storage tank  105  via an “in tank” or primary pump  110 . Generally, the primary pump  110  may be any device sufficient to increase the pressure of LNG contained in LNG storage tank  105 . Primary pump  110  may be located within LNG storage tank  105  or may be located immediately outside LNG storage tank  105 . In one embodiment of the present invention primary pump  110  increases the pressure of the LNG to a pressure in the range of from about 3 to about 10 barg (150 psig) in order to provide the required Net Positive Suction Head (NPSH) for subsequent pumps, such as secondary booster pump  130 , which further increases the LNG pressure up to pipeline pressure. 
     The LNG, after being pumped via primary pump  110 , passes through conduit  155  and into static mixer  120 . Static mixers are common tools employed in various process facilities and are commercially available from Chemineer, Inc., Kam Controls, Inc., Lightnin Co., as well as others. Generally, static mixer  120  serves to provide intimate contact between the LNG and the boil-off gas vapors (described below). Because the LNG is sub-cooled and is at a sufficient pressure, the LNG has the capacity to absorb a portion (or all) of the natural gas vapors completely back into the liquid state. Any static mixer may be employed in the present invention. Useful examples of static mixers include, but are not limited to, straight pieces of pipe (approximately 20 feet) with internal stationary fan blades welded inside. Generally, the material may be for example stainless steel type  304 L or type  316 L material suitable for cryogenic temperatures. The ends generally may be raised face weld neck (FRWN)  150 # class flanges. In one embodiment of the present invention, the orientation of static mixer  120  is vertical with the LNG and vapor entering from the top. Orientation of the static mixer can be at any angle. 
     Boil-off gas vapors present in LNG storage tank  105  are passed through conduit  150  and into vapor compressor  115  where the vapors are compressed. Vapor compressor  115  may be any compressor known in the art. Particularly useful vapor compressors include, but are not limited to, boil-of gas compressors. After the vapors are compressed, the vapors pass through restrictor  135  (control valve) and then are injected into static mixer  120 . The internal blades of static mixer  120  make tiny bubbles out of the incoming boil-off vapor (which increases its surface area) and then mixes them in the main LNG, flowing from primary pump  110  in conduit  155 . The main LNG flow absorbs most or all the tiny bubbles of natural gas and then flows into conduit  160 . The restrictor  140  can be in the open or closed position, or in a partially open or partially closed position. In the most preferred embodiment, the restrictor or valve is in the partially open/partially closed position. 
     In one embodiment of the present invention, restrictor  135  is a pinch or control valve. Generally, restrictor  135  remains in an open position, thus allowing vapors that have been collected in LNG storage tank  105  to pass through restrictor  135  and into static mixer  120 . As will be more fully described below, restrictor  135  may progressively move to a closed position, depending on the level contained in separator  125 , also described below. Boil-off gas vapors present in LNG storage tank  105  are passed through conduit  150  and into vapor compressor  115  where the vapors are compressed. Vapor compressor  115  may be any compressor known in the art. Particularly useful vapor compressors include, but are not limited to, boil-off gas compressors. After the vapors are compressed, the vapors pass through restrictor  135  and then are injected into static mixer  120 . The internal blades of static mixer  120  make tiny bubbles out of the incoming boil-off vapor (which increases its surface area) an then mixes them in the main LNG, flowing from primary pump  110  in conduit  155 . The main LNG flow absorbs the tiny bubbles of natural gas. 
     Boil-off gas vapors injected into static mixer  120  may be absorbed by the LNG flowing from primary pump  110  in conduit  155  and preferably are converted back to a liquid state. Occasionally, all of the boil-off gas vapor are not absorbed into the LNG. This may be due to a number of factors such as too much vapor flow, too low LNG flow, or non-condensable vapors present in the boil-off gas vapors. Because subsequent pumps (such as secondary booster pump  130 ) may be designed to operate with zero vapor (bubbles) in the LNG, all vapors generally must be removed before the LNG reaches secondary booster pump  130 . 
     To remove these non-condensed vapor before they reach secondary booster pump  130 , the LNG must be directed through a vapor/liquid separator drum (“separator”)  125 . After the LNG and boil-off gas vapors are mixed in static mixer  120 , the LNG stream and optionally a LNG/LNG vapor stream is passed through conduit  160 , and into separator  125 . 
     Separator  125  may be any device that is capable of containing a liquid and providing means in which vapors, if present, may be separated from the liquid. Generally, separating devices useful in the present invention include drum shaped containers, although any shaped container may be used. After the LNG stream, and optionally a LNG/LNG vapor stream, enters separator  125 , the liquid fills separator  125  from the bottom to the top. Generally, separator  125  provides a forum where vapors are allowed to separate from the liquid and float to the top of separator  125 . These LNG vapors in separator  125  are vented back to storage tank  105  where they again enter the cycle and may be recondensed in the LNG stream. According to one embodiment of the present invention, these vapors pass from separator  125  through conduit  165  passing through restrictor  140  and then back into LNG storage tank  105 . At this point, the vapors are allowed to proceed through the recondensing process again. In one embodiment of the present invention, restrictor  140  is a control valve. 
     Generally, restrictor  140  remains in the closed position when separator  125  contains mostly liquid. If separator  125  contains vapor, restrictor  140  opens to allow the non-condensed vapors to be directed back to LNG storage tank  105  via conduit  165  to be recycled back through static mixer  120 . 
     After the LNG or LNG/LNG vapor passes through separator  125 , the LNG portion passes from the bottom of separator  125  directly to secondary booster pump  130  (without entrapped vapors). From secondary booster pump  130 , the LNG stream passes through restrictor  146  and flows through vaporizer  170 . One function of vaporizer  170  is to vaporize and heat the LNG to a temperature compatible with requirements of receiver  175 . From vaporizer  170 , the LNG is conveyed to receiver  175 . Examples of useful receivers include, but are not limited to, a pipeline, the inlet of a gas turbine, the inlet of a chemical process, a compressor inlet, a burner inlet and the like. In one embodiment of the present invention, the receiver is a pipeline. 
     In one embodiment of the present invention, separator  125  may be divided into multiple levels to provide a convenient way to measure the amount of liquid contained in separator  125 . Although an infinite number of levels may be designated, separator  125  generally has from between 2 to 4 levels. In one embodiment of the present invention, separator  125  contains four levels. In addition, at each level, separator  125  may contain means for monitoring or sensing the level of liquid contained in separator  125 . Useful means for monitoring the level of liquid contained in separator  125  include, but are not limited to, level indicating controllers, level transmitters and the like. Level transmitters/controllers are common tools employed in all types of process facilities. A level transmitter is a device that is contained in a vessel and sends a signal, the strength of which is proportional to the level it senses. A level controller receives this signal and then opens or closes a valve per programmed instructions based on the level signal received. Level transmitters/controllers are known in the art (for example, those commercially, available from American Magnetics, Inc.) In FIG. 1, a level indicating controller (“LIC”) is indicated at  240 . The level transmitter plugs into a panel on or near separator  125 . LIC  240  generally contains a “split range” controller, which means that more than one valve may be controlled based on the level signal received. 
     FIG. 2 is a diagram illustrating various level measurements in separator  125  according to one embodiment of the present invention. As shown in FIG. 2, separator  125  contains 4 levels: LEVEL  1  (L 1 ) shown at line  200  between level  1   a  at line  205  and level  1   b  at line  210 , LEVEL  2  (L 2 ) shown at line  215  between level  2   a  at line  220 (and level  2   b  at line  225 , LEVEL  3  (L 3 ) shown at line  230 , and LEVEL  4  (L 4 ) shown at line  235 . 
     In the present invention, separator  125  may operate completely full of LNG or may operate partially full. If separator  125  is partially full of LNG, and that level is anywhere above level  1   a  (at line  205 ) restrictor  140  remains in its normally closed position. As the level of liquid LNG drops below level  1   a  (at line  205 ), a level controller signals restrictor  140  to open slightly to allow non-condensed vapors back to LNG storage tank  105 . As the liquid level drops to level  1   b  (line  210 ), restrictor  140  is allowed to progressively open to its maximum opening. 
     Similarly, if more vapors enter separator,  125  causing the level to drop to level  2   a  (line  220 ), a level controller signals restrictor  135  to slightly close. As described above, restrictor  135 , which may for example be a valve, is generally located in conduit  150  between vapor compressor  115  and static mixer  120 . This action causes fewer vapors to be injected into static mixer  120 . As the level drops below level  2   a  (line  220 ), a level controller signals restrictor  135  to progressively close and may be fully closed, for example, at level  2   b  (line  225 ), or whatever level or position the operator chooses. This action completely stops additional vapors from entering static mixer  120 . In a zero vapor flow case, the compressor will automatically unload itself if it is a reciprocating compressor or go on automatic bypass if it is a centrifugal or rotating compressor. 
     Level  3  (L 3 ) (at line  230 ) may be described as a position below L 2  and L 1  where, should the LNG drop to this level, a level controller may cause an alarm to sound thus alerting the operator that low levels exist in separator  125 . In another embodiment of the present invention, no alarm is activated at L 3 . In one embodiment of the present invention, level  4  (L 4 ) (at line  235 ) is a position below L 3 , L 2  aid L 1 , where, should the LNG drop to this level or below, a level controller causes a shutdown of secondary booster pump  130  so as to protect the pump from damage due to cavitation. According to one embodiment of the present invention, separator  125  contains no liquid. 
     In another embodiment of the present invention, should non-condensable vapors accumulate in LNG storage tank  105  causing high-pressures, a tank pressure controller may signal restrictor  145  to open. This action would allow the non-condensable rich vapor to vent in the atmosphere or other safe location. 
     It should be understood that the conditions of temperature and pressure set forth herein are dependent on efficient system components, which are assumed. In addition, these conditions may change depending on the temperature and pressure of the LNG, the nature of receiver  175 , efficiency of the equipment (for example, vapor compressor  115 , static mixer  120  and separator  125 ), and other factors. FIG. 1 also shows element  185 , which can be a flare connected to conduit  180 , which communicates with the tank or collector  105 . In an alternative embodiment, the flare  185  could be a vent  185  also in communication with the collector  105 , which could be a tank.