Patent Application
20090139263
This invention relates to processes for the separation of nitrogen from a liquid natural gas stream comprising nitrogen, methane, and possibly heavier hydrocarbons.
Crude natural gas is often liquefied to enable storage of larger quantities in the form of liquid natural gas (LNG). Because natural gas may be contaminated with nitrogen, nitrogen is advantageously removed from LNG to produce a nitrogen-diminished LNG product that will meet desired product specifications. Several methods of effectuating nitrogen removal from LNG have been disclosed in the prior art.
One simple method for separating nitrogen from a LNG stream is to isentropically expand the crude LNG stream in a turbine and then inject the stream into a flash separator. The liquid product removed from the flash separator will contain less nitrogen than the crude LNG stream, whereas the vapor product will contain a higher proportion of nitrogen.
A different method is disclosed in U.S. Pat. No. 5,421,165 (“the '165 patent”). A process is disclosed wherein crude LNG is isentropically expanded in a turbine and cooled in a reboiler heat exchanger. The cooled and expanded LNG stream is then passed through a valve, where it undergoes static decompression, prior to its injection into a denitrogenation column. Within the column, nitrogen is stripped from the falling liquid by the rising vapor, so that the vapor stream exiting the top of the column is enriched with nitrogen. A liquid LNG stream is withdrawn from the bottom of the column as a nitrogen-diminished product. Within the column, at a level below the level of injection of the LNG feed stream, a liquid stream is withdrawn and passed through the heat exchanger to cool the feed and then reinjected into the column at a level below that at which it had been withdrawn, to provide boilup to the column. In effect, the passage of the withdrawn stream through the heat exchanger provides an additional equilibrium stage of separation.
A similar method for separating nitrogen from an LNG stream replaces the turbine driven dynamic decompression with a valve for static decompression, such that the expansion takes place isenthalpically rather than isentropically. The use of the isentropic expansion in the process of the '165 patent allegedly permits greater methane recovery.
Another method for removing nitrogen from an LNG stream is described in U.S. Pat. No. 5,041,149 (“the '149 patent”). This patent discloses a method of removing nitrogen from a crude natural gas stream by first cooling the stream and then passing it through a phase separator, to produce a liquid stream and a vapor stream. The liquid stream is further cooled and injected into a denitrogenation column. The vapor stream is condensed and cooled further to produce a second liquid stream, prior to injection into the denitrogenation column at a higher level than that of the first liquid stream. Nitrogen-enriched vapor is removed from the top of the column and used to cool the incoming second liquid stream. The sump of the column is divided by a baffle, one side of which is filled with liquid from the lowest tray of the column. This bottoms liquid is withdrawn and at least partially vaporized in the heat exchanger, while condensing the vapor stream from the phase separator, and returned to the column as a reflux stream to provide boilup. The liquid remaining in the reflux stream falls to the other side of the baffle in the sump. This liquid reflux is then removed as a nitrogen-diminished product stream, pumped to a higher pressure, warmed and vaporized, and then dynamically expanded to reduce the temperature and pressure of the vapor product. Similar to the reboiler heat exchange of the '165 patent, the reflux of the bottoms liquid serves as an additional equilibrium stage of separation.
A disadvantage of these prior art nitrogen separation methods is that they each require that the entire liquid flow off of one tray be recycled through the reboiler. Another disadvantage is that they each are completely dependent upon liquid head in the column to drive the heat exchangers. These characteristics limit the flexibility of these methods, as the entire process must be designed to accommodate this large amount of flow. A further disadvantage associated with the prior art is that the processes tend to require a large area for the placement of equipment.
Accordingly, it is an object of the present invention to provide a process which allows for greater flexibility in the design of the equipment necessary for nitrogen rejection from an LNG stream. This greater flexibility allows for the design of relatively inexpensive process equipment, thus lowering the capital costs associated with the process. It is another objective of the present invention to provide a nitrogen separation process which can be less costly and can save valuable space through the elimination of certain equipment required of the prior art processes.
The present invention provides an improved process for the denitrogenation of an LNG stream contaminated by nitrogen. This process allows for economic benefits by permitting a greater flexibility in the process design and eliminating the requirement of some equipment.
According to one embodiment of the invented process, a crude LNG stream comprising between about 1% and 10% nitrogen, and the remainder methane and heavier hydrocarbons, is expanded in a means for expansion, and cooled in a thermosyphon reboiler. The resultant crude LNG stream is introduced into a nitrogen rejection column, wherein the nitrogen content of the LNG is reduced as the liquid flows down the column. A nitrogen-enriched vapor stream is withdrawn from the top of the column, and a first nitrogen-diminished liquid stream is withdrawn from the bottom of the column. This nitrogen-diminished liquid stream may be recovered as a LNG product.
A second nitrogen-diminished liquid stream is also withdrawn from the bottom of the column. This second stream is passed through the thermosyphon reboiler, thus cooling the crude LNG stream, and at least partially vaporizing the second stream. The partially vaporized second stream is reinjected into the column at a level above the level of withdrawal of the nitrogen-diminished bottoms LNG stream and below the level of introduction of the crude LNG feed stream to provide column boilup.
In an alternative embodiment of the invented process, the first and second nitrogen-diminished liquid streams are withdrawn from the column together, through the same conduit, and are separated after withdrawal.
In another alternative embodiment of the invented process, the thermosyphon reboiler is placed within the sump of the column so that only one nitrogen-diminished liquid stream is withdrawn from the column.
As will become apparent, several variations of these processes are within the scope of the invention. For example, in one embodiment, the initial crude LNG stream is expanded in a dense fluid expander, which may be placed either upstream or downstream of the thermosyphon reboiler. A valve may also be placed immediately upstream of the nitrogen rejection column, such that the crude LNG stream is throttled through the valve prior to injection into the column.
The present invention achieves flexibility of design and process economic advantages in an LNG denitrogenation operation by using, in part, a thermosyphon reboiler, the flow through which is driven by the density difference between the input and output streams in conjunction with the liquid head of the column, rather than solely by the liquid head of the column, thus permitting a greater flexibility in the overall process design. The present invention also permits variability in the amount of fluid flow through the reboiler which further increases process flexibility. Additionally, the processes according to the present invention permit the elimination of some equipment, including collection trays, nozzles, and large reboilers, that would otherwise be required of prior art processes, and can therefore achieve the additional advantages of saving both cost and space.
As will be clarified in the following description, achieving this flexibility, while allowing for the removal of process equipment and the maintenance of output levels and energy requirements, involves the introduction of a small thermodynamic inefficiency. However, the flexibility and cost and space savings afforded by the present invention more than compensate for this thermodynamic inefficiency, especially given the ease and low expense with which it may be remedied.
The term “nitrogen-enriched stream” is used herein to mean a stream containing a higher concentration of nitrogen when compared with an initial feed stream.
The term “nitrogen-diminished stream” is used herein to mean a stream containing a lower concentration of nitrogen when compared with an initial feed stream.
The term “below” is used herein to mean at a position of lesser height, i.e., closer to the ground.
The term “above” is used herein to mean at a position of greater height, i.e., farther from the ground.
The term “boilup” is used herein to mean vapor which rises up the column.
A preferred embodiment of the invention will now be described in detail with reference to
As set forth in
A first nitrogen-diminished liquid stream 110 is withdrawn from the bottom of column 150 and may be recovered as a product stream. By “nitrogen-diminished,” it is herein understood to mean containing a lower concentration of nitrogen than that of high-pressure LNG stream 100. A second nitrogen-diminished liquid stream, reboiler stream 112 is also withdrawn from the bottom of column 150. The flow rate of reboiler stream 112 is typically between about 15% and about 100% of the flow rate of liquid stream 110. Reboiler stream 112 is at least partially vaporized in thermosyphon reboiler 106 to produce partially vaporized reboiler stream 114, which is then injected into the bottom of column 150, below the lowest tray in the case of a tray column, or below the packing material in the case of a packed column, to provide boilup. In design stage, the flow rate of reboiler stream 112 may be adjusted as necessary to provide different recirculation rates (i.e., the ratio of the outlet liquid flow to vapor flow).
In an alternative embodiment, the means for expanding the LNG stream 102 may be placed downstream of thermosyphon reboiler 106. In this manner, high-pressure stream 100 is cooled in thermosyphon reboiler 106 prior to undergoing expansion in the means for expanding the LNG stream 102.
In another alternative embodiment, nitrogen-diminished liquid stream 110 and reboiler stream 112 may be withdrawn from the bottom of column 150 as a single stream through a single conduit. According to this embodiment, nitrogen-diminished liquid stream 110 would then be separated from the combined stream and optionally recovered as a product stream. The remaining stream would be reboiler stream 112, and would proceed through the thermosyphon reboiler as before.
We note that in each of the described embodiments, valve 109 is optional, and, in the alternative, cooled LNG stream 108 can be directly injected into nitrogen rejection column 150. Where valve 109 is not present, the means for expanding the LNG stream 102 is preferably a two-phase dense fluid expander.
A particularly preferred embodiment is provided wherein a crude LNG stream 100 is substantially isentropically expanded in a dense fluid expander 102 and cooled in a thermosyphon reboiler 106. This cooled, expanded LNG stream 108 is substantially isenthalpically expanded through valve 109 and injected into a nitrogen rejection column 150. Within the column, rising vapor strips the nitrogen from the falling liquid, and a nitrogen-enriched stream 130 is withdrawn from the top of the column. A nitrogen-diminished liquid stream 110 is withdrawn from the bottom of the column and may be recovered as a product stream. Reboiler stream 112 is also withdrawn from the bottom of column 150. Reboiler stream 112 is at least partially vaporized in thermosyphon reboiler 106 to produce partially vaporized reboiler stream 114, which is then injected into the bottom of column 150, below the lowest tray in the case of a tray column, or below the packing material in the case of a packed column, to provide boilup. The recirculation rate of the reboiler 106 is preferably at least about 4.
The liquid portion of the partially vaporized reboiler stream 114 mixes with the liquid from the lowest column stage upon reinjection into column 150 such that the nitrogen-diminished liquid stream 110 is not exclusively the liquid from the bottom stage of the rejection column 150, or from the thermosyphon reboiler 106, but rather a mixture of both. There is a thermodynamic loss associated with the mixing of the liquid streams to provide the withdrawn nitrogen-diminished stream 110. However, this can easily and cheaply be compensated for by the addition of a stage or stages to the nitrogen rejection column 150.
The recirculation rate for the thermosyphon reboiler may be any desired rate determined by the geometry of the heat exchanger, and therefore, the ratio of the flow rate of the reboiler stream 112 to the flow rate of the nitrogen-diminished liquid stream 110 can be flexibly defined, and may be easily optimized for the particular process.
An alternative embodiment is illustrated in
A nitrogen-enriched vapor stream 130 is withdrawn from the top of column 150. A nitrogen-diminished liquid stream 110 is withdraw from the bottom of the column and may be recovered as product. As in the embodiments described above, in the design stage the recirculation rate may be adjusted to any value to provide the desired amount of boilup.
The present invention provides a significant improvement in the adaptability and flexibility of a LNG denitrogenation process through the implementation of a hydraulically different process from those of the prior art. By permitting a thermosyphon reboiler 106 to assist in driving the flow of the reboiler streams 112/114 rather than relying exclusively on the column head, and by allowing variability in the selection of the design recirculation rate, greater flexibility of design is permitted. This flexibility can lead to a smaller capital expense at the remediable cost of a minor thermodynamic loss. For example, by altering the recirculation rate (and, consequently, the flow through the reboiler), the reboiler and piping requirements associated with the process can be adjusted to minimize capital expenditures. The use of a thermosyphon rebolier for the denitrogenation of an LNG stream can also lead to improvements in the controllability of the overall process.
In both the internal and external thermosyphon processes of the present invention, there is no need for a liquid collection tray below the distillation section of the column, which would be required were all of the liquid coming off of a tray to pass through a reboiler. Additionally, when the internal thermosyphon process of the present invention is used, the nozzles required for withdrawal of the recycle stream are also eliminated. Moreover, by placing the thermosyphon reboiler inside of the column, valuable space can be saved because there is no longer a need to dedicate space outside of the column to the reboiler and associated piping. When the external thermosyphon reboiler is used, space may still be saved, due to the simplified piping, smaller required heat transfer surface area, and small footprint associated with thermosyphon reboilers as compared to other reboiler types.
To more particularly demonstrate some of the important differences between the process of the present invention and the prior art, process simulations of the entire natural gas liquefaction process were run, using an ASPEN process simulator, comparing two embodiments of the invention (“current process”) with the process disclosed in the '165 patent. The comparison basis is an equal LNG production and a satisfied fuel balance (the amount of LNG product flash required to drive a gas turbine driving the process). The respective reference numerals used in this example refer to
A. Recirculation rate: 2.9
With reference to
B. Recirculation rate: 26.0
With reference to
With reference to FIG. 1 of the '165 patent, following expansion in turbine 21, semidecompressed LNG stream 22, at a flow rate of 125,451 lbmol/hr, a pressure of 71.76 psia, a temperature of −242.9° F., and containing 2.96% N2, 95.47% methane, 1.10% C2 hydrocarbons, and 0.47% heavier hydrocarbons, is cooled in indirect heat exchanger 2 to a temperature of −252.6° F. and a pressure of 64.50 psia. This cooled, expanded stream is throttled through valve 3 and introduced into denitrogenation column 5 comprising 6 trays, at a pressure of 18 psia. An overhead vapor stream 10 is withdrawn from the top of the column 5 at a flow rate of 8,122 lbmol/hr, and contains 31.17% N2, 68.83% methane, and trace amounts of heavier hydrocarbons, at a pressure of 18 psia and a temperature of −261.9° F. Bottoms stream 11 is withdrawn from the column 5 at a flowrate of 117,329 lbmol/hr, a pressure of 19.45 psia, a temperature of −256.8° F., and contains 1.01% N2, 97.32% methane, 1.17% C2 hydrocarbons, and 0.50% heavier hydrocarbons. First LNG fraction 6 is withdrawn from the lowest tray of the column at a flow rate of 121,047 lbmol/hr, a temperature of −259.7° F., a pressure of 19.74 psia, and contains 1.56% N2, 96.81% methane, 1.14% C2 hydrocarbons, and 0.49% heavier hydrocarbons. This first LNG fraction 6 is passed through indirect heat exchanger 2 to produce stream 7, which is at a temperature of −256.8° F., a pressure of 19.45 psia, and has a vapor fraction of 3.1%. Stream 7 is returned to column 5 under the lowest tray to provide boilup. This liquefaction process also requires approximately 229 MW of power.
Table 1 sets forth data of corresponding streams of the current process with a recirculation rate of 2.9 and the prior art process in order to more clearly illustrate the comparison. We first note that the respective feed streams, 104 and 22, and the respective product streams, 110 and 11, and 130 and 10, are substantially identical with respect to all relevant properties. This equivalency of feed streams and product streams enables a valid comparison of the two processes.
As demonstrated in Table 1, a significant difference between the two processes is that the reboiler stream of the current process 112 is at a flow rate of 17,704 lbmol/hr, which is only 14.6% of the flow rate of the reboiler stream 6 of the '165 patent process, 121,047 lbmol/hr. This difference is attributable to the fact that, while the '165 patent process requires that the entire liquid flow off of a column tray be recycled through the reboiler heat exchanger, the current process may be designed to function with various recirculation rates, permitting optimization of the amount of flow necessary to achieve the desired separation, and therefore only recycles the amount of bottoms liquid necessary to produce the required product. Another noteworthy difference between these processes is that, while the total fluid flow through the reboiler is substantially less for the current process than for the process of the '165 patent, because the same amount of heat is transferred in each reboiler, a greater percentage of the reboiler stream is vaporized in the current process, 25.3% versus 3.1%. The amount of vapor actually returned to the column for boilup is therefore greater for the current process (4479 lbmol/hr), than for the '165 patent process (3752 lbmol/hr).
Table 2 sets forth data of corresponding streams of the current process with a recirculation rate of 26.0 and the prior art process in order to demonstrate the flexibility of the current process. We first note that the respective feed streams, 104 and 22, and the respective product streams, 110 and 11, and 130 and 10, are, again, substantially identical with respect to all relevant properties. This equivalency of feed streams and product streams enables a valid comparison of the two processes.
As demonstrated in Table 2, altering the recirculation rate allows for great variation in the amount of fluid recycled through the thermosyphon reboiler 106, while maintaining equivalent recovery. The total fluid flow through the reboiler (streams 112 and 6) is much closer in this comparison (117,353 lbmol/hr and 121,047 lbmol/hr), as is the percentage of fluid vaporized in the rebolier (3.7% and 3.1%). Thus, the current process has the flexibility to achieve equivalent separation not only when implementing the preferred lower recycle flow rate, but also under flow rate conditions similar to the prior art.
Although the invention has been described in detail with reference to certain embodiments, those skilled in the art will recognize that there are other embodiments within the scope of the claims.
