Abstract:
A system and apparatus for condensing boil-off vapor from a Liquified Natural Gas (LNG) container are disclosed. A system for condensing vapor includes the steps of providing contact area within a condenser vessel, directing vapor to the condenser vessel, providing a condensing fluid and a pump fluid, directing the condensing fluid to the condenser vessel, varying the flow of the condensing fluid to control the pressure in the vessel, contacting the vapor with the condensing fluid to create a condensate, and combining the condensate and the pump fluid. An apparatus for condensing vapor includes a vessel, a liquified gas input to the vessel, control means on the liquified gas input for varying a first liquified gas stream to control the pressure in the vessel, a vapor input to the vessel, means for condensing vapor in the vessel, and means for combining condensed vapor with a second liquified gas stream.

Description:
RELATED APPLICATIONS 
     This application claims domestic priority from provisional application Serial No. 60/140,577, filed Jun. 23, 1999, the entire disclosure of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to storage and distribution systems for Liquified Natural Gas (LNG). More specifically, the present invention relates to an apparatus and method for condensing boil-off vapor received from an LNG storage tank and condensing the vapor into an output stream for routing into a distribution system. 
     BACKGROUND OF THE INVENTION 
     Imported Liquified Natural Gas (LNG) is stored at many locations throughout the world. The LNG is used when a local source of natural gas is not available or as a supplement to local sources. 
     Liquified Natural Gas (LNG) is typically stored at low pressure and in liquid form at cold temperatures at an import terminal. The LNG is usually pumped to a pressure that is slightly above the pressure of the natural gas distribution pipeline. The high pressure liquid is vaporized and sent to the distribution pipeline. The pumping operation typically involves a set of low pressure pumps located in a storage container connected in series to a set of high pressure pumps located outside the storage tank. 
     As is well known, heat input into the storage container generates boil-off vapor. Additional vapor generation may occur during filling of the storage container. Vapor may be obtained from an outside source such as a ship. Ideally, the boil-off vapor is included with the sendout to the distribution pipeline. 
     Compressors may be used to boost the vapor to the high operating pressure of the pipeline, which can be as high as 100 bar. Compressing the vapor to these high pressures requires considerable energy. A more energy efficient method for disposition of the vapor to the pipeline is desired. 
     A more energy efficient system utilizes the cold LNG sendout to condense vapor at a low interstage pressure. The vapor condensate combines with the liquid sendout flow and enters the high pressure pumps. The stream flows to the vaporizers from the high pressure pumps. Compressing the boil off vapor stream to the distribution pipeline pressures requires considerably more energy than boosting the boil off vapor condensate to the high pressure with a liquid pump. Several existing LNG import terminals have systems which condense boil off vapor at low pressure and pump the condensate with the liquid stream flowing to the vaporizer. However, the boil off vapor condensers at these prior art terminals lack the physical arrangement and control systems to obtain proper operation and high efficiency. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the invention, a system for condensing vapor includes the steps of providing contact area within a condenser vessel, directing vapor to the condenser vessel, providing a condensing fluid and a pump fluid, directing the condensing fluid to the condenser vessel, varying the flow of the condensing fluid to control the pressure in the vessel, contacting the vapor with the condensing fluid to create a condensate, and combining the condensate and the pump fluid. 
     In further accordance with a preferred embodiment, the condenser vessel is utilized as a pump suction vessel. The step of forming a surface layer of liquid that is substantially at its saturation temperature may be included. The condensate and the pump fluid may be mixed in the vessel, or the condensate and the pump fluid may be mixed outside the vessel. 
     The pressure in the vessel may be controlled by removing vapor during high pressure conditions in the vessel. Alternatively, the pressure in the vessel may be controlled by adding vapor during low pressure conditions in the vessel. Further, the pressure in the vessel may be controlled by removing vapor during high pressure conditions in the vessel and by adding vapor during low pressure conditions in the vessel. 
     In accordance with another aspect of the invention, a system for condensing vapor includes the steps of providing contact area within a condenser vessel, directing vapor to the condenser vessel, providing a condensing fluid and a pump fluid, varying the flow of the pump fluid to control a liquid level in the vessel, directing the condensing fluid to the condenser vessel, varying the flow of the condensing fluid to control the pressure in the vessel, contacting the vapor with the condensing fluid to create a condensate, and combining the condensate and the pump fluid. 
     In accordance with a further aspect of the invention, a system for condensing vapor includes the steps of condensing vapor in a condenser vessel to create a condensate, mixing the condensate with a pump fluid to create a combined stream, measuring a temperature difference between the condensate and the combined stream, and utilizing the temperature difference to provide control logic for a vapor compressor. 
     In accordance with yet another aspect of the invention, a vapor condenser includes a vessel, a liquified gas input to the vessel, control means on the liquified gas input for varying a first liquified gas stream to control the pressure in the vessel, a vapor input to the vessel, means for condensing vapor in the vessel, and means for combining condensed vapor with a second liquified gas stream. 
     In accordance with a still further aspect of the invention, a vapor condenser includes a vessel, a liquified gas input to the vessel, control means on the liquified gas input for varying a first liquified gas stream to control the pressure in the vessel, a vapor input to the vessel, means for condensing vapor in the vessel, a second liquified gas input to the vessel, second control means on the second liquified gas input for varying a second liquified gas stream to control liquid level in the vessel, and means for combining condensed vapor with the second liquified gas stream. 
     In accordance with another aspect of the invention, a vapor condenser includes a vessel, means for condensing vapor in the vessel, control means for varying the flow of a first stream of liquified gas to control pressure in the vessel, means for combining the condensed vapor with a second stream of liquified gas to create a combined stream, means for measuring a temperature difference between the condensate and the combined stream, and means for using the temperature difference to control a vapor compressor. 
     Other features and advantages are inherent in the apparatus claimed and disclosed or will become apparent to those skilled in the art from the following detailed description and its accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic illustration of a Liquified Natural Gas storage container operatively connected to a boil-off vapor condenser assembled in accordance with the teachings of the present invention; and 
     FIG. 2 is a schematic illustration of a boil-off vapor condenser assembled in accordance with the teachings of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The embodiments described herein are not intended to be exhaustive or to limit the scope of the invention to the precise forms disclosed. Rather, the following embodiments have been chosen and described in order to best explain the principles of the invention and to enable others skilled in the art to follow the teachings thereof. 
     Referring now to the drawings, FIG. 1 illustrates a boil-off vapor condenser constructed in accordance with the teachings of the present invention and which is generally referred to by the reference numeral  10 . The Vapor condenser  10  is shown operatively connected to various components of a Liquified Natural Gas (LNG) storage and distribution system  12 . The LNG storage and distribution system  12  includes a storage tank  14  having one or more internal or external pumps  16 , a low stage compressor  18  having an optional cooler  19 , a high stage compressor  20  having an optional cooler  21 , one or more high pressure pumps  22 , and a vaporizer  24 . It will be understood that vapor from the vaporizer is routed to a distribution pipeline (not shown) in a well known manner. 
     A pipeline  26  connects the pumps  16  to the vapor condenser  10  for routing a stream of LNG to the vapor condenser  10 . A pipeline  28  connects a vapor vent  30  in the storage tank to the low stage compressor and then to the vapor condenser  10  as will be discussed in greater detail below. As would be known, the vapor from the storage tank  14  is typically at a low first pressure, and must first be boosted to an interstage second pressure by the low stage compressor  18 . Preferably, a “T”  32  is provided in the pipeline  28  which enables a portion or all of the vapor to be diverted from the pipeline  28  into a pipeline  34 , thereby bypassing the condenser  10 . The diverted portion of the vapor may be boosted to a high pressure by the high stage compressor  20  for routing directly to the distribution pipeline (not shown). An output pipeline  36  connects the Vapor condenser  10  to the high pressure pumps  22  for routing the LNG from the Vapor condenser  10  to the vaporizer  24 , from where the LNG is converted to vapor and routed to the distribution pipeline (not shown) in a known manner. 
     Referring now to FIG. 2, the Vapor condenser  10  includes a vessel  38  having a generally dome shaped top  40 , a generally dome shaped bottom  42 , and a sidewall  44 , all of which enclose a chamber  46 . The chamber  46  includes a top portion  48 , a central portion  50 , and a bottom portion  52 . A plurality of random packing elements  54  are disposed within the chamber  46 , with the random packing elements  54  preferably being disposed toward the top portion  48  of the chamber  46 . The packing elements  54  together define an enhanced surface area  56 . Random packing elements  54  can be  2 ″ Pall rings. The heat and mass transfer area for vapor condensing can be provided by alternate surface area arrangements including structured packing, tray columns or spray elements. 
     A diaphragm plate  58  is disposed within the vessel  38 , and generally separates the bottom portion  52  and the central portion  50  of the chamber  46 . The diaphragm plate  58  includes an upper collection surface  60  and a central drain aperture  62 . An input  64  is provided which routes a first stream, condensing fluid  66  of cold LNG into the top portion  48  of the chamber  46 . Preferably, the input  64  is positioned such that the first stream  66  enters the chamber  46  above the packing elements  54 . Still preferably, a liquid distributor  68  is provided which helps to disperse the first stream  66  over the packing elements  54 . 
     Another input  70  is provided which routes a second stream, pump fluid  72  of cold LNG into the bottom portion  52  of the chamber  46 . It will be noted that in the embodiment shown the input  70  is positioned immediately above the diaphragm plate  58 , with the input  70  including an internal conduit  74  which extends through the drain aperture  62  such that the second stream  72  enters the bottom portion  52  of the chamber  46 . Alternatively, the input  70  may be positioned to directly enter the bottom portion  52  of the chamber  46  or would join the condensate stream remote from the vessel. A vapor input  76  is provided which is in flow communication with the pipeline  28  for routing boil-off vapor (indicated as  78 ) into the chamber  46 . Preferably, the boil-off vapor  78  will enter the central portion  50  of the chamber  46 . An output  80  is provided in the bottom portion  52 . The output  80  is in flow communication with the pipeline  36  for routing an output stream  82  to the high pressure pumps  22  (shown in FIG.  1 ). Preferably, a “T”  83  is disposed in the pipeline  26  for dividing the stream of LNG flowing through the pipeline  26  into the first stream  66  and the second pump fluid stream  72 . 
     Referring still to FIG. 2, the bottom portion  52  of the chamber  46  includes a mixing chamber  84 . The mixing chamber  84  is bounded primarily by the diaphragm plate  58  and the bottom  42  of the vessel  38 . Depending on the location of the diaphragm plate  58 , a portion of the sidewall  44  may also bound a portion of the mixing chamber  84 . Preferably, the mixing chamber  84  will have disposed therein a vortex breaker  86 . The vortex breaker  86  preferably includes an upper plate  88  which sits atop a generally cylindrical tube  90  which defines therewithin a subchamber  92 . The subchamber  92  generally surrounds the output  80 . A plurality of apertures  94  are provided for providing flow communication between the mixing chamber  84  and the subchamber  92  so that fluid within the mixing chamber may drain or otherwise be pumped or drawn through the output  80  via the apertures  94 . 
     An input  96  may be provided which is in flow communication with the high pressure pumps  22  via a pipeline (not shown). The input  96  preferably enters the chamber  46  adjacent to a passage or downcomer  98  between the central portion  50  and the bottom portion  52  of the chamber  46 . The downcomer  98  includes an upper end  102  disposed above the diaphragm plate  58  and includes a lower end  104  disposed below the diaphragm plate  58 . A splash plate  100  is preferably provided. The vessel  38  will preferably also include pressure vent  106  and a pump vent  108 . Vessel  38  is also used as a suction pot for the high pressure pumps. The pump vent  108  is connected to the high pressure pumps to vent vapor from the pumps, generally prior to starting the pumps. 
     Preferably, a control system is provided for controlling the vapor condenser  10 . The control system includes pressure control valve  114 , liquid level control valve  116 , gas vent control valve  132 , and make up gas control valve  118 . In the embodiment shown, the valve  114  is disposed downstream of the “T”  83 . A liquid level transmitter  120  is disposed to measure the level of liquid (indicated as  122 ) within the chamber  46 , with the desired liquid level preferably being at a level within the central portion  50  as shown in FIG.  2 . The liquid level transmitter  120  is operatively connected to a level controller  124 , which controls the valve  116 . The control valve  116  controls the second stream  70  entering the vessel  38  to maintain the desired liquid level. 
     The control system also includes a pressure transmitter  126  which measures the pressure within the vessel  38 . The pressure transmitter  126  is operatively connected to a pressure controller  128 , which controls the control valve  114 . The control valve  114  controls the first stream  66  entering the vessel  38 . The pressure transmitter  126  is also connected to a second pressure controller  130 , which is operatively connected to a valve  132  disposed at the pressure vent  106 . The control system preferably also includes a pressure controller  134  for controlling the valve  118  connected to a source of make-up gas (not shown) which may be routed into the vapor pipeline  28 . 
     Preferably, the control system will also include a pair of temperature elements  138 ,  140 , which may be thermocouples or resistance temperature detectors, or any other suitable temperature element. The temperature element  138  is disposed to measure the condensate temperature within the vessel  38 , preferably within the central portion  50 , while the temperature element  140  is disposed to measure the temperature within the output stream  82 . A temperature difference controller  142  is operatively connected to both of the temperature elements  138 ,  140 . 
     In operation, boil-off vapor is routed to the vapor condenser  10  via the pipeline  28  and enters the central portion  50  of the vessel  38  via the input  76 . The LNG stream is conveyed from the storage tank  14  in cold, liquid form through the pipeline  26  to the vapor condenser  10 . At the “T”  83 , which is disposed in the pipeline  26 , the stream of LNG is split into a first stream  66  which enters the vessel  38  via the input  64  and a second stream  72  which enters the vessel  38  via the input  70 . The first stream  66  enters the top portion  48  of the chamber  46  at a point above the packing elements  54 . The first stream  66  comes into contact with liquid distributor  68 , which helps to disperse the first stream  66  over the packing elements  54 , such that the first stream  66  spreads out over the enhanced surface area  56  of the packing elements  54 . 
     At the same time, the boil-off vapor  78  enters the chamber  46  via the input  76  from the pipe line  28 . Once the vapor  78  has entered the central portion  50  of the chamber  46 , it will rise in the direction indicated by the reference arrow “A”. The rising vapor  78  contacts the dispersed first stream  66  which is spread out over the enhanced surface area  56  of the packing elements  54 . It will also be appreciated that the first stream  66  is very cold, being at cryogenic temperatures, while the boil-off vapor  78  is relatively warm. The boil-off vapor  78  thus comes into direct contact with the dispersed first stream  66 , with the cold first stream  66  causing the vapor  78  to condense thus forming a condensate which drains downwardly in the direction indicated by the reference arrow “B”. Condensate may be defined as a fluid containing condensing liquid and condensed vapor as major components. The condensate gathers on the upper collection surface  60  of the diaphragm plate  58 . The diaphragm plate  58  provides a separation such that the upper vessel can operate at a pressure close to the saturation pressure of the condensate liquid. The pool of condensate liquid  60  helps to maintain a stable operating pressure in the vessel. Baffles or other means can be used as alternatives to plate  58  to help maintain conditions approaching saturation in the upper portion of the vessel. It will be noted in the embodiment shown that the first stream and the condensate flow generally in a direction that is counter to the upward, rising direction of the vapor  78 . 
     As an alternative to the counter-current flow shown, the vessel  38  and the various inputs may be arranged such that the vapor  78  and the first stream  66  and the condensate flow in a parallel (i.e., downward) direction. 
     The second stream  72  enters the bottom portion  52  of the chamber  46 . The condensate liquid above the diaphragm plate  58  drains through the drain aperture  62  into the mixing chamber  84  defined in the bottom portion  52  of the chamber  46 . The condensate mixes with the second stream  72 . The liquid entering the mixing chamber  84  eventually drains through the apertures  94  into the subchamber  92 . The liquid then leaves the vessel  38  as an output stream through the output  80  into the pipeline  36 . The apertures  94  serve to enhance the mixing process. 
     It will appreciated that after the boil-off vapor  78  and the first stream  66  form the condensate, that the condensate will be warmer than the LNG initially entering as the first stream. The condensate is at or near its saturation temperature at the vessel operating pressure. Accordingly, when the condensate is mixed with the colder second stream  72  within the mixing chamber  84 , the output stream  82  will be subcooled. The output stream  82  is then routed through the pipeline  36  to the high pressure pumps  22 . 
     The control system will preferably maintain the pressure within the vapor condenser  10  at a predetermined range. In the event an upset causes the pressure within the vessel  38  to rise above the desired pressure range, the pressure controller  130  will open the valve  132 , such that vapor is vented through vent  106 . An upset condition may occur when sufficient LNG is not available to condense the incoming vapor flow. On the other hand, in the event the pressure within the vessel  38  drops below the desired pressure range, pressure controller  134  will open the valve  118 , which allows a quantity of makeup gas to enter the vessel  38  thereby raising the pressure. 
     Ideally, however, the pressure can be controlled without venting and without using makeup gas. Normally, the vessel pressure would be controlled by throttling the valve  114  using the controller  128 . A portion of the low pressure pump flow in line  26  flows through valve  114  and is used to condense the vapor and maintain the vessel pressure at its operating condition. The flow not required for condensing is directed through valve  116  to chamber  84 . Further, it will be noted that the central portion  50  of the chamber  46  encloses a relatively large volume, which gives the vessel  38  a liquid level capacitance. 
     The temperature difference between the condensate temperature provided by the temperature element  138  and the combined stream temperature provided by the temperature element  140  gives an indication of the amount of subcooling of the combined stream  36 . The amount of subcooling can be utilized to provide logic for control of system compressors. A highly subcooled stream could indicate that additional vapor from the low stage compressor could be condensed and/or the high stage compressor capacity could be reduced. 
     The vessel pressure can also provide logic for compressor control. An increase in vessel pressure can initiate an increase in the high stage compressor capacity. The vapor flow entering the condenser vessel will decrease. The LNG flow required for condensing the vapor will be decreased. 
     A boil off vapor condenser assembled in accordance with the above-described teachings of the invention will meet at least some of the following objectives: 
     1) Condense the maximum quantity of boil off vapor for the complete range of liquid flows and will reduce power consumption; 
     2) Provide controlled LNG sub-cooling for effective high pressure pump operation, and will maintain net positive suction head for the high pressure pumps to prevent cavitation and failure; 
     3) Accommodate the turn-up and turn-down of step changes in the liquid flow rates; 
     4) Accommodate the turn-up and turn-down of step changes in the vapor flow rates; 
     5) Maintain constant condenser vessel pressure without repeated vapor venting or vapor make up, will stabilize vessel pressure, reduce vapor loss, increase efficiency, and stabilize pumping system flow; and 
     6) Provide logic for compressor control and interstage pump flow, and be adjustable for operating energy savings. 
     Numerous modifications and alternative embodiments of the invention will be apparent to those skilled in the art in view of the foregoing description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the best mode of carrying out the invention. The details of the structure may be varied substantially without departing from the spirit of the invention, and the exclusive use of all modifications which come within the scope of the appended claims is reserved.