The present invention relates to the cooling and liquefaction of gases, and more particularly to the liquefaction of natural gas.
The demands for natural gas have increased in recent years. The transport of natural gas is through pipelines or through the transportation on ships. Many areas where natural gas is located are remote in the sense that there are no convenient pipelines to readily transfer the natural gas to. Therefore natural gas is frequently transported by ship. The transport of natural gas on ships requires a means to reduce the volume and one method of reducing the volume is to liquefy the natural gas. The process of liquefaction requires cooling the gas to very low temperatures. There are several known methods of liquefying natural gas as can be found in U.S. Pat. No. 6,367,286; U.S. Pat. No. 6,564,578; U.S. Pat. No. 6,742,358; U.S. Pat. No. 6,763,680; and U.S. Pat. No. 6,886,362.
One of the methods is a cascade method using a shell and tube heat exchanger. The apparatus, the shell and tube heat exchanger, is very large and very expensive, and presents problems of economics and feasibility for remote and smaller natural gas fields. It would be desirable to have a device for liquefying natural gas that is compact and relatively inexpensive to ship and use in remote locations, especially for natural gas fields found under the ocean floor, where collection and liquefaction of the natural gas can be performed on board a floating platform using a compact unit.
The invention is a block heat exchanger comprising a plurality of plates that have been stacked and bonded together into a single block. Within the plates open channels have been formed for carrying fluids. The channels form conduits when the plates are stacked and bonded together, and the open channels are covered by a side of a neighboring plate that is in sealing contact, forming a lightweight and compact heat exchanger.
In another embodiment, the heat exchanger comprises plates having channels defined therein, and with the channels inlets and outlets disposed upon an edge of a plate. The plates when stacked form a block having covered channels, or conduits, traversing through the block for carrying fluids. An individual channel in this embodiment does not cross between plates, but is disposed within a single plate. The plates have a channel side and a non-channel side, and are stacked such that a channel side of one plate is in sealing contact with the non-channel side of a neighboring plate.
Additional objects, embodiments and details of this invention can be obtained from the following detailed description of the invention.
The use of liquefied natural gas (LNG) is increasing, as fuel and a means of transporting natural gas from remote sites having natural gas, without a nearby gas pipeline, to more distant areas where the natural gas is consumed. Natural gas is typically recovered from gas wells that have been drilled and is in the gas phase at high pressure. The present invention is directed to a heat exchanger for cooling the natural gas at the gas wells. By providing an inexpensive heat exchanger for cooling and liquefying natural gas in remote locations, natural gas can be recovered on site and transported as LNG, rather than requiring a natural gas pipeline, or transporting the gas at very high pressures.
The basic invention comprises a novel design using the bonding of plates together to form a single unit. Each of the plates has channels formed in the plates, by etching, milling, or methods known in the art. When the plates are bonded together, the channels are covered and form conduits through which fluids can flow. The bonding method will depend on the materials of construction, such as with aluminum plates, bonding involves brazing the aluminum plates together. With steel, diffusion bonding can be performed to bond the steel plates together.
The most common commercial design of a heat exchanger for the cooling of natural gas is a spiral wound heat exchanger where the coolant cascades within a shell over spiral wound tubes carrying the gas to be cooled. Benefits of the present design over the spiral wound design include lower cost, lower weight, and a more compact structure as well as improved heat transfer characteristics.
An apparatus for heat exchange between fluids is fabricated from a plurality of first plates having channels defined therein for carrying a fluid to be cooled. Each channel has an inlet and an outlet, and each plate has channeling ports passing through the plates. The plates each have an upper and lower face, with the channels defined in the upper face. The apparatus further includes a plurality of second plates having channels defined therein for carrying a coolant. Each channel has an inlet and an outlet, and each plate has channeling ports passing through the plates. The second plates each have an upper and lower face, with the channels defined in the upper face. The plates are stacked in an alternating manner—first plate, second plate, first plate, second plate, etc.—wherein a first plate upper face is in sealing contact with a second plate lower face, and a second plate upper face is in sealing contact with a first plate lower face. When the plates are stacked, the channels become covered conduits.
Another method of fabricating the apparatus does not require ports for fluids to pass from channels in one plate to channels in another plate, but the plates are fabricated to have the entire channel defined within a plate, and the inlets and outlets to the channels are disposed along an edge of the plate. The plates have a channel side, or first side, and a non-channel side or second side. The plates would consist of coolant plates for carrying coolant, and cooling plates for carrying fluids to be cooled. The plates are stacked in an alternating sequence to provide the maximum thermal contact between the plates. The plates are stacked such that the first side, or channel side, of one plate is in sealing contact with the second side, or non-channel side, of a second plate, where the channels become covered conduits with the inlets and outlets to the channels disposed along edges of the plates.
The invention is further illustrated by the following descriptions of specific embodiments.
In one embodiment, the apparatus, as shown in
Upon stacking the plates, first exterior plate 10, interior second plate 20, interior third plate 30, etc., and finally exterior plate 40, a block is formed when the plates are diffusion bonded together. Within the block, there is defined a first set of contiguous conduits comprising the channels 22 defined in the second plates 20 and in fluid communication with one another through the channeling ports 34 defined in the third plates 30. Additionally there is a second set of contiguous conduits comprising the channels 32 defined in the third plates 30 and in fluid communication with one another through the channeling ports 24 defined in the second plates 20.
The first set of contiguous conduits provide at least one fluid conduit for the transport of a fluid to be cooled. The second set of contiguous conduits provide fluid conduits for a coolant. In the embodiment as shown in
In an alternative embodiment, a fluid to be cooled can be directed through multiple channels through a bifurcation defined in a plate. As shown in
Multiple channels 22 can also be combined into single broad channels as shown in
The design can include intermediate drawoff ports for drawing off the natural gas and passing the natural gas through an adsorbent unit for removing water, carbon dioxide, and other undesired components in the natural gas to create a dry, enriched natural gas stream. With the use of an intermediate drawoff for passing the natural gas through an adsorbent unit, the design would include intermediate inlet ports for entering the dried natural gas stream into the heat exchanger.
A second embodiment is shown in
A fluid to be cooled enters through an inlet port 12, traverses along channels 22, through connecting ports 34, and exits through outlet port 44. A coolant enters through inlet ports 42, traverses along channels 32, through connecting ports 24, and exit outlet ports 14, or an intermediate outlet port 36. Optionally, a coolant can enter through a single port 42, traverse through one set of channels 32, and connecting ports 24, exiting one outlet port 14, whereby the coolant is passed through an expander (not shown), further cooling the coolant. The expanded coolant is directed back to the heat exchanger through a second coolant inlet port 42, traverses through a second set of channels 32, and connecting ports 24, and exiting a second outlet port 14. Another option, is to pass the expanded coolant in a reverse direction, entering through a port 14 or 36 and exiting at port 42.
A third embodiment of the heat exchanger is shown in
A second coolant stream is injected into a third coolant conduit 144 and travels in a con-current direction relative to the fluid to be cooled. The second coolant stream is withdrawn from an outlet 146 where the second coolant is passed to a second expander 150, wherein the second coolant stream is expanded and cooled. The cooled second coolant stream reenters the heat exchanger at an inlet port 152 and traverses along a fourth coolant conduit 154 in a counter-current direction relative to the fluid being cooled, and exiting the conduit 154 at outlet port 156.
A final plate 170 is added to the stack of plates forming the heat exchanger to enclose the channels 110 in the last plate 100 of the interior stack of plates 100. The final plate 170 can include a port 172 for the outlet of the cooled fluid. Additional cooling can be provided by cooling the coolant streams before directing the coolant streams to the respective expanders 130, 150.
The expanders 130, 150 can comprise a Joule-Thomson valve, a turbine expander, or other device for expanding the coolant and dropping the temperature of the coolant.
A fourth embodiment of the heat exchanger is shown in
When stacking the plates 200, 220, the inlets and outlets of the various channels are in fluid communication with a manifold for collecting or distributing like streams to respective like outlets or inlets. A benefit of the fourth embodiment, is that alignment of ports 120 as in the first through third embodiments is not necessary, as the conduits formed from the channels are completely defined within a single plate. This can reduce fabrication costs by removing the need for precision alignment of ports in the plates.
In one embodiment, the apparatus can include a restriction device 216 disposed within a channel 210, as shown in
The plates that are bonded together can, also, each have a single channel etched, milled, or otherwise created in an individual plate. As shown in
An intermediate temperature stream enters an intermediate manifold 342 where the intermediate temperature stream is distributed to the inlets 344 of the intermediate plates 340. The stream exiting the intermediate plates 340 is collected into an intermediate manifold 346. The intermediate stream is a pre-refrigerant stream, and can be natural gas that has been pre-cooled and recycled.
A cold stream comprising a refrigerant, enters a cold manifold 302 where the refrigerant is distributed to the inlets 304 of the cold plates 300. The refrigerant passes along the cold plate channels 310 and is collected in the cold outlet manifold 306.
In another embodiment as shown in
The design of the present invention allows for variations such that refrigerant after cooling the hot natural gas can be expanded to and recycled to provide further cooling as shown in
The use of the diffusion bonded heat exchanger of the present invention provides for optimization of natural gas liquefaction, by taking advantage of the synergies presented with this compact heat exchanger. In
The refrigerant is used to cool itself, by expansion and passing the expanded refrigerant back through the heat exchanger 400. This provides a temperature difference that is a driving force for cooling and allows for interesting optimization. The effect of refrigerant flow rate for this system is shown in
The efficiency of the heat exchanger is affected by the composition of the refrigerant. The refrigerant composition is selected to heat flow over a broad range of temperatures, and providing continuous boiling of the refrigerant over the temperature range of interest as shown in
While the invention has been described with what are presently considered the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but it is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims.
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