Abstract:
Methods and apparatuses for reducing the effects of motion in a core-in-shell type heat exchanger are provided. One apparatus includes: (a) a heat exchanger, wherein the heat exchanger includes an internal volume defined within a shell and a plurality of spaced apart cores disposed within the internal volume of the shell, wherein the internal volume is flooded with a vaporizing fluid; and (b) a separation vessel connected to the heat exchanger, wherein the separation vessel is located at higher elevation than the heat exchanger, wherein the separation vessel is connected to the heat exchanger in such a manner so as to deliver a hot feed stream to heat exchanger and the receive a non-vaporizing stream from the heat exchanger.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority benefit under 35 U.S.C. Section 119(e) to U.S. Provisional Patent Ser. No. 61/578,144 filed on Dec. 20, 2011, the entire disclosure of which is incorporated herein by reference and is related to “Internal Baffle For Suppressing Slosh in a Core-in-Shell Heat Exchanger” filed on Dec. 18, 2012. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to a method and apparatus for reducing the effects of motion in a core-in-shell type heat exchanger. 
       BACKGROUND OF THE INVENTION 
       [0003]    Natural gas in its native form must be concentrated before it can be transported economically. The use of natural gas has increased significantly in the recent past due to its environmentally-friendly, clean burning characteristics. Burning natural gas produces less carbon dioxide than any other fossil fuel, which is important since carbon dioxide emissions have been recognized as a significant factor in causing the greenhouse effect. Liquefied natural gas (LNG) is likely to be used more and more in densely-populated urban areas with the increased concern over environmental issues. 
         [0004]    Abundant natural gas reserves are located all over the world. Many of these gas reserves are located offshore in places that are inaccessible by land and are considered to be stranded gas reserves based on application of existing technology. Existing technical reserves of gas are being replenished faster than oil reserves, making the use of LNG more important to meeting the demands of future energy consumption. In liquid form, LNG occupies 600 times less space than natural gas in its gaseous phase. Since many areas of the world cannot be reached by pipelines due to technical, economic, or political limits, locating the LNG processing plant offshore and utilizing a nautical vessels to directly transport the LNG offshore from the processing plant to the transportation vessel can reduce initial capital expenditure and release otherwise uneconomical offshore gas reserves. 
         [0005]    Floating liquefaction plants provide an off-shore alternative to on-shore liquefaction plants and alternative to costly subsea pipeline for stranded offshore reserves. A floating liquefaction plant can be moored off the coast, or close to or at a gas field. It also represents a moveable asset, which can be relocated to a new site when the gas field is nearing the end of its production life, or when required by economic, environmental or political conditions. 
         [0006]    One problem encountered in floating liquefaction vessels is the sloshing of vaporizing fluid inside heat exchangers. Sloshing in a heat exchanger may result in the production of forces that can affect stability and control of the heat exchanger. If the vaporizing fluid is allowed to slosh freely inside the shell of the heat exchanger, the moving fluid can have an adverse effect on the thermal function of the heat exchanger core. Furthermore, the cyclical nature of motion may result in cyclical behavior in heat transfer efficiency, and hence, process conditions in the LNG liquefaction plant may be impacted. These instabilities may result in poorer overall plant performance and may lead to narrower operating envelopes and limits to the available production capacity. 
         [0007]    Therefore, a need exists for a method and apparatus for reducing the effects of motion within a core-in-shell type heat exchanger. 
       SUMMARY OF THE INVENTION 
       [0008]    In an embodiment, a method for reducing the impact of motion in a heat exchanger includes: (a) flooding the heat exchanger with a vaporizing fluid, wherein the heat exchanger includes an internal volume defined within a shell and a plurality of spaced apart cores disposed within the internal volume of the shell; (b) introducing a hot process feed stream to an upper vessel, wherein the upper vessel is located above the heat exchanger, wherein the upper vessel is connected to the heat exchanger via a plurality of conductor pipes, wherein the conductor pipes include external pipes for connecting the upper vessel and the plurality of spaced apart cores, wherein the conductor pipes further include an internal diameter vapor riser, wherein the internal vapor riser is fixed to the top of the plurality of cores and floats inside the conductor pipe to allow for thermal expansion and contraction of the plurality of cores; (c) delivering the hot feed stream to a core within the internal volume of the shell via the conductor pipes, wherein the hot process feed stream undergoes indirect heat exchange with the vaporizing fluid thereby producing a warm liquid stream and a non-vaporized stream, wherein the non-vaporized stream includes a mixture of vapor and liquid; and (d) delivering the non-vaporized stream to the upper vessel via the conductor pipes to undergo disengagement. 
         [0009]    In another embodiment, a method for reducing the impact of motion in a heat exchanger includes: (a) flooding the heat exchanger with a vaporizing fluid, wherein the heat exchanger includes an internal volume defined within a shell and a plurality of spaced apart cores disposed within the internal volume of the shell; (b) introducing a hot process feed stream to a separation vessel, wherein the separation vessel is located above the heat exchanger, wherein the separation vessel is connected to the heat exchanger via a plurality of external pipes; (c) delivering the hot process feed stream to a core within the internal volume of the shell via the external pipes, wherein the hot process feed stream undergoes indirect heat exchange with the vaporizing fluid thereby producing a warm liquid stream and a non-vaporized stream, wherein the non-vaporized stream is a mixture of vapor and liquid; and (d) delivering the non-vaporized stream to the separation vessel to undergo separation. 
         [0010]    In yet another embodiment, a method for reducing the impact of motion in a heat exchanger includes: (a) flooding the heat exchanger with a vaporizing liquid, wherein the heat exchanger includes an internal volume defined within a shell and a plurality of spaced apart cores disposed within the internal volume of the shell; (b) introducing a hot process feed stream to a separation vessel, wherein the separation vessel is located at an elevation higher than the heat exchanger, wherein the separation vessel includes a liquid outlet and a vapor inlet; (c) delivering the hot feed stream to a core within the internal volume of the shell at or near the top of the core via the liquid outlet, wherein the hot feed stream undergoes indirect heat exchange with the vaporizing fluid thereby producing a warm liquid stream and a non-vaporized stream, wherein the non-vaporized stream is a mixture of vapor and liquid; and (d) delivering the non-vaporized stream to the separation vessel to undergo separation. 
         [0011]    In a further embodiment, an apparatus includes: (a) a heat exchanger, wherein the heat exchanger includes an internal volume defined within a shell and a plurality of spaced apart cores disposed within the internal volume of the shell, wherein the internal volume is flooded with a vaporizing fluid; and (b) a separation vessel connected to the heat exchanger, wherein the separation vessel is located at higher elevation than the heat exchanger, wherein the separation vessel is connected to the heat exchanger in such a manner so as to deliver a hot feed stream to heat exchanger and the receive a non-vaporizing stream from the heat exchanger. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The invention, together with further advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which: 
           [0013]      FIG. 1  is a schematic of a core-in-shell heat exchanger, according to one embodiment of the invention involving an external horizontal separator. 
           [0014]      FIG. 2  is a schematic of a core-in-shell heat exchanger, according to one embodiment of the invention involving a vertical separator. 
           [0015]      FIG. 3  is a schematic of a core-in-shell heat exchanger, according to one embodiment of the invention involving an external vertical separator. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0016]    Reference will now be made in detail to embodiments of the present invention, one or more examples of which are illustrated in the accompanying drawings. Each example is provided by way of explanation, not as a limitation. It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment can be used in another embodiment to yield a still further embodiment. Thus, it is intended that the present invention cover such modifications and variations that come within the scope of the appended claims and their equivalents. 
         [0017]    Referring to  FIGS. 1-3 , a heat exchanger  10  generally includes a shell  12  and a plurality of spaced apart cores, i.e., a first core  16 , a second core  18 , and a third core  20  within the shell  12 . The plurality of spaced apart cores within the heat exchanger  10  includes at least one core. For illustrative purposes, the heat exchanger is horizontally disposed; however, the heat exchanger may be positioned in any commercially operable manner, such as vertically, for example. The plurality of spaced apart cores within the shell  12  are completely submerged, i.e., flooded, in vaporizing fluid, i.e., a liquid refrigerant. 
         [0018]    A principle design of the core-in-shell heat exchanger provides cross exchange of a hot process feed stream against the colder vaporizing fluid. The vaporizing fluid resides in a pressure vessel where brazed aluminum compact exchanger cores are mounted and completely submerged in the vaporizing fluid which is at or near its boiling point. The liquid is drawn into the bottom face of the exchanger where it contacts the hotter surfaces within the core. The vaporizing fluid then transfers heat through the exchanger core channels. The majority of the heat transfer is from the latent heat of vaporization of the vaporizing fluid. The hot process feed stream is cooled or condensed as it passes through the opposite side of the channels in the exchanger cores. 
         [0019]    The thermal and hydraulic performance of the core-in-shell heat exchanger is dependent upon the liquid level in the exchanger. A driving force for circulation of the vaporizing fluid into the exchanger cores is the thermosiphon effect. The thermosiphon effect is a passive fluid transfer phenomenon resulting from natural convective thermal forces. As the vaporization of the fluid occurs, the vaporizing fluid is heated and the vaporizing fluid density decreases. As it naturally flows upward in the channels, fresh liquid is drawn in. This results in a natural circulation of the vaporizing fluid into the core channels induced by the thermal gradient inside the core. Not all vaporizing fluid in the channel is vaporized and a mixture of liquid and vapors typically are transported up through the exchanger core channels and expelled through the top of the core. Above the core, adequate space must be provided for the vapor and liquid to disengage so that only vapor leaves the overhead section of the shell side of the core. Liquid that separates in the upper section of the exchanger is then re-circulated to the bottom of the vessel where it is then vaporized in the core. The driving force for separation of the liquid and the gas in the upper section of the core-in-shell heat exchanger is gravity. 
         [0020]    The thermosiphon circulation effect in the core is enhanced or impaired by the external hydraulic pressure (level differences) between the effective liquid levels inside the core versus the liquid level outside the core. As the liquid level in the shell falls, the driving force for the transfer of the liquid into the exchanger core is decreased, and the effective heat transfer is reduced. When the liquid level falls below the core, the vaporizing fluid circulation stops due to the loss of the thermosiphon effect which results in the loss of heat transfer. If the heat exchanger is operated with a liquid level higher than the core, i.e., flooded, the heat transferred is impaired further as the vapor produced in the core has to overcome the additional head to escape from the core. 
         [0021]    To overcome the impact of a flooded heat exchanger and to ensure disengagement of the non-vaporized stream, a mixture of vapor and liquid, an upper vessel  30  is utilized, such as depicted in  FIG. 1 . The upper vessel  30  is located above the heat exchanger  10  and includes a plurality of vapor disengaging conductor pipes  32  connecting the upper vessel  30  to the shell  12  of the heat exchanger  10 . The conductor pipes  32  consist of an external pipe connecting the two vessels. Inside the conductor pipes resides an internal smaller diameter vapor riser. The vapor riser, not shown in  FIG. 1 , is fixed to the top of the core,  16 ,  18 ,  20  and floats inside the conductor pipe  32  so as to allow for thermal expansion and contraction of the exchanger cores  16 ,  18 ,  20 . The vapor riser terminates inside the upper vessel  30  above the normal liquid level of the upper vessel  30 . The vapor riser is fastened to the top of the cores  16 ,  18 ,  20  via a top manifold on the cores  16 ,  18 ,  20  so as to collect the non-vaporized stream exiting the internal cores  16 ,  18 , or  20 . The non-vaporized stream is introduced to the upper vessel  30 . The non-vaporized stream is then separated in the upper vessel  30  so that liquid flows down the annulus (between the outside of the vapor riser and the inside of the conductor pipe) of the conductor pipe  32  into the shell  12  of the heat exchanger  10  via the plurality of conductor pipes  32 . 
         [0022]    Upon entry into the heat exchanger shell  10  via a plurality of connecting pipes, the hot process feed streams are delivered to the plurality of spaced apart cores via externally connecting pipes in order to separately undergo indirect heat exchanger with the vaporizing fluid. The plurality of spaced apart cores each receives a separate hot process feed stream, allowing for simultaneous indirect heat transfer between the vaporizing fluid and the separate hot process feed streams. The vaporizing fluid and the hot process feed stream flow in a cross-current manner in alternating plate and fin channels inside each core. 
         [0023]    As previously discussed, only a portion of the vaporizing fluid is vaporized in the core resulting in a mixture of vapor and liquid, i.e., the non-vaporized stream, existing the top of the core, which must be disengaged in the upper vessel  30 . The vapor risers are designed such that the non-vaporized stream exiting the core remains a mixed phase stream and are routed inside the inner pipe connected via manifold from the top of the plurality of cores  16 ,  18 ,  20  of the heat exchanger  10  and terminates inside the upper vessel  30  above the normal liquid level in the upper vessel  30 . The vapor from non-vaporized stream disengages in the upper vessel  30  and is mixed with the vapor from the non-vaporized stream entering the upper chamber  30 . Liquid separates from the hot process feed stream and mixes with the re-circulating liquid from the vapor risers and flows in the outer annulus of the conductor pipes  32  to the bottom of the heat exchanger. In this embodiment, the upper vessel includes slosh suppressing baffles to minimize liquid surface movement. 
         [0024]    Alternatively, a separation vessel  40  can be utilized to overcome the effects of a flooded heat exchanger to ensure disengagement of the non-vaporized stream, as depicted in  FIG. 2 . The separation vessel  40  is vertically disposed above the heat exchanger  10  and includes a plurality of conductor pipes  42  connected to the heat exchanger  10 . The non-vaporized stream is introduced to the separation vessel  40 . The vapor and liquid in non-vaporized stream is separated in the separation vessel  40  and the liquid flows through the external separation vessel to the heat exchanger  10  via the plurality of conductor pipes  42  through the bottom of the separation vessel  40 . The vapor risers within the conductor pipes are designed such that the non-vaporized liquid vapor mixture exiting the core remains a mixed two phase stream and are routed inside the inner pipe connected via a manifold from the top of the plurality of cores  16 ,  18 ,  20  and terminates inside the separation vessel  40  above the normal liquid level in the horizontal return header  43  connected to the vapor space of the separation vessel  40 . The vapor from the plurality of cores  16 ,  18 ,  20  disengages in the separation vessel  40  and is mixed with the vapor from the non-vaporized stream entering the separation vessel  40  and exits the top of the separation vessel  40 . Re-circulating liquid is returned to the bottom of the shell  10  via the annulus of the conductor pipes or via the bottom of the separation vessel  40 . 
         [0025]    Upon entry into the shell  12  of the heat exchanger  10  via a plurality of connecting pipes, the hot process feed streams are delivered to the plurality of spaced apart cores via externally connecting pipes in order to separately undergo indirect heat exchanger with the vaporizing fluid. The plurality of spaced apart cores each receives a separate hot process feed stream, allowing for simultaneous indirect heat transfer between the vaporizing fluid and the hot process feed streams. The vaporizing fluid and the hot process feed stream flow in a cross-current manner in alternating plate and fin channels inside each core. 
         [0026]    The resulting non-vaporized stream is disengaged in the plurality of external pipes  42 . The liquid stream is re-circulated back into the heat exchanger  10  via the external side of the annulus pipe inside the conductor pipe  42 . The vapor stream flows through the external pipes  42  into the separation vessel  40 . A liquid level is maintained in the separation vessel via an external liquid level control valve. Since the separation vessel  40  is vertical, motion suppression may not be required. However, in an embodiment, slosh suppressing baffles are utilized in the separation vessel. 
         [0027]    Referring to  FIG. 3 , a separation vessel  50  located at an elevation higher to or at the same elevation as than the heat exchanger  10  can be utilized to overcome the effects of motion and is an alternative method to flood completely flooding the heat exchanger  10 . In  FIG. 3 , the separation vessel  50  is located to the side of the heat exchanger  10  and includes a feed liquid outlet pipe  52  and a plurality of vapor disengaging conductor inlet pipes  54 . A hot process feed stream, i.e., a mixed phase feed stream, is introduced to the separation vessel  50 . The vaporizing fluid is delivered to the heat exchangers  10  from the separation vessel  50  through one or more free drains via the feed outlet pipe  52 . Upon entry into the heat exchanger shell  10  via a plurality of connecting pipes, the hot process feed streams are delivered to the plurality of spaced apart cores via externally connecting pipes in order to separately undergo indirect heat exchanger with the vaporizing fluid. The plurality of spaced apart cores each receives a separate feed stream, allowing for simultaneous indirect heat transfer between the vaporizing fluid and the separate feed streams. The vaporizing fluid and the feed stream flow in a cross-current manner in alternating plate and fin channels inside each core. 
         [0028]    The resulting non-vaporized stream is disengaged in the plurality of vapor inlet pipes  54 . The liquid stream is re-circulated back into the heat exchanger  10 . The vapor stream flows through the vapor disengaging conductor inlet pipes into the separation vessel  50 . A liquid level is maintained in the separation vessel  50  via an external liquid level control valve. Since the separation vessel  50  is vertical, motion suppression may not be required. However, in an embodiment, slosh suppressing baffles are utilized in the separation vessel  50 . 
         [0029]    In order to minimize the size of the vapor disengaging space required in the external or integral separator device, internal devices like vane packs, demister pads, or cyclonic devices could be utilized to reduce the diameter required in the separator and thus reduce the overall size required to remove the liquid droplets from the vapor stream. 
         [0030]    In closing, it should be noted that the discussion of any reference is not an admission that it is prior art to the present invention, especially any reference that may have a publication date after the priority date of this application. At the same time, each and every claim below is hereby incorporated into this detailed description or specification as additional embodiments of the present invention. 
         [0031]    Although the systems and processes described herein have been described in detail, it should be understood that various changes, substitutions, and alterations can be made without departing from the spirit and scope of the invention as defined by the following claims. Those skilled in the art may be able to study the preferred embodiments and identify other ways to practice the invention that are not exactly as described herein. It is the intent of the inventors that variations and equivalents of the invention are within the scope of the claims while the description, abstract and drawings are not to be used to limit the scope of the invention. The invention is specifically intended to be as broad as the claims below and their equivalents.