Patent Application
20210348838
The present invention relates to liquefaction, and more particularly, to a liquefier arrangement capable of producing liquid natural gas (LNG) and liquid nitrogen (LIN). Still more particularly, the present system and method relates to a liquefier arrangement configured for co-production of both LNG and LIN in using direct drive motor/generators for the warm and/or cold booster compressors and turbines.
There are various industrial gas business opportunities where the production of both liquid natural gas (LNG) and liquid nitrogen (LIN) is required. U.S. provisional patent application Ser. No. 62/976,049 filed Feb. 13, 2020, the disclosure of which is incorporated by reference herein, shows examples of liquefier arrangements capable of a liquefaction cycle that co-produces LNG and LIN. As disclosed in U.S. provisional patent application Ser. No. 62/976,049; liquefier arrangements capable of a liquefaction cycle that co-produce both LNG and LIN require a separate passage in a conventional nitrogen liquefier that is employed to cool and liquefy the natural gas. This modification typically requires changing the brazed aluminum heat exchanger (BAHX) arrangement to allocate one of the passages to cool the natural gas feed and then reallocate a portion of the high pressure gaseous nitrogen feed passages or layers. Since LNG is sufficiently subcooled at about 110 K it is withdrawn from the BAHX at a location corresponding to a temperature somewhat warmer than the cold end of the BAHX where the temperature is about 95 K to 100 K required to liquefy the nitrogen.
The natural gas feed is preferably pre-purified for removal of carbon dioxide and other contaminants as well as removal of minor amounts of moisture prior to entry in the cold box. Other potential contaminants may include H2S, mercaptans, mercury and mercury compounds which also must be removed or reduced to a satisfactory level. Usually, heavier hydrocarbons are sufficiently extracted in NGL facilities prior to supply. If this is not the case, a significant modification in the liquefier design would be required in order to capture and remove the heavier hydrocarbons at an intermediate temperature. Also, if the feed natural gas is at a low pressure, the liquefaction process may optionally require pre-compression of the natural gas feed, preferably to a pressure of about 450 psia to enable the use of a modified nitrogen liquefier design. If the pressure of the natural gas feed is below about 450 psia, the temperature difference in the natural gas condensing zone of the heat exchanger may exceed the allowable limits for many BAHX designs. Alternatively, if the feed natural gas is supplied at a lower pressure, the liquefier design would have to be changed so that at least the condensing portion of the heat exchanger is of a different design, for example, a stainless steel brazed heat exchanger or a stainless steel spiral wound heat exchanger. Thus, to avoid the much more expensive heat exchangers and to achieve improved efficiencies, natural gas pre-compression is preferred. needed when it is supplied at lower pressures. The further compressed natural gas feed would optionally be cooled in an aftercooler to remove the heat of compression.
During liquefaction of a high pressure natural gas feed pressures, the refrigeration demand of the warm turbine is greatly increased. This increased refrigeration demand is because natural gas liquefaction or pseudo-liquefaction is now taking place at a temperature above the exhaust temperature of the warm turbine. As a result, the warm turbine is larger and passes significantly more flow. The cold turbine refrigeration primarily is providing refrigeration for liquefaction or pseudo-liquefaction of the nitrogen while the warm turbine refrigeration primarily provides refrigeration for natural gas liquefaction or pseudo-liquefaction. This means that independent variation in the LNG demand and the LIN demand likely results in independent variation of the demand for refrigeration from each turbine and the optimal warm turbine to cold turbine flow ratio will vary significantly, depending on the output demand for LNG and LIN. The prior art liquefier arrangement capable of a liquefaction cycle that co-produces both LNG and LIN disclosed in U.S. provisional patent application Ser. No. 62/976,049 suffers from a disadvantage of not able to adjust the warm turbine to cold turbine flow ratio to achieve the optimal ratio when demand for LNG and LIN changes.
Varying demands of LNG and LIN in co-production natural gas liquefaction plants is common. For example, small peak shaver LNG plants are located strategically on natural gas pipelines and configured to store natural gas as LNG during the months when it is less expensive, and to return the natural gas to the pipeline when price and demand peaks, most often during cold winter weather and hot summer weather. These facilities produce LNG at maximum levels for part of the year and produce little or no LNG for the rest of the year. Co-production of LIN in such plants may be beneficial in strategic locations where demand for merchant LIN or back-up LIN is required. Of course, the potential for variation in merchant LIN demand and back-up LIN demand near a given LNG location can lead to wide changes in demand for LIN production.
Nitrogen liquefiers are typically capable of efficient turndown over a very broad range. Turndown to about 20% of capacity is achievable at reasonably good efficiency. Turndown is accomplished naturally by keeping the turbine nozzles unchanged. As the liquefier is turned down, the feed nitrogen flow is reduced and the pressure levels within the liquefier fall commensurately. As a result, the volumetric flows through the turbines, their respective boosters, and the recycle compressor remains unchanged at their design rates. The pressure ratios across the machines also remain unchanged. So, while the machines become more unloaded, they each continue to operate essentially at their ideal design point. This means that the aerodynamic efficiencies of the rotating machines remain unchanged. The feed gas compressor is an exception to this, as it must be turned down with guide vanes or a suction throttle valve due to its lower flow and discharge pressure, with a constant supply pressure. The power demand of the recycle compressor is much larger than that of the feed gas compressor, though. So, it doesn't have a very large effect. Other than this, the only penalties for turndown are those associated with the mechanical and motor losses of the rotating machinery (which increase as a proportion of the total power consumption at turndown), and a significant thermodynamic penalty for the lower pressure liquefaction of nitrogen. This thermodynamic penalty occurs because at lower pressures, and particularly below its critical point pressure, the liquefaction of nitrogen results in a more thermodynamically irreversible temperature profile. The larger temperature invariant zones at lower nitrogen liquefaction pressures result in both tight pinch deltaT (ΔT) values and large deltaT (ΔT) values.
What is needed, therefore is a liquefier arrangement capable of co-production of LNG and LIN that is capable of operating efficiently in normal or full capacity production modes as well as in selected turn-down operating modes, including operating modes targeting low or zero LNG production capacity
The present invention may be characterized as a liquefaction system configured to co-produce liquid nitrogen and liquid natural gas, the liquefaction system comprises: (a) a natural gas feed stream; (b) a gaseous nitrogen feed stream; (c) a multi-pass brazed aluminum heat exchanger; (d) a recycle compressor configured to compress the gaseous nitrogen feed stream and a gaseous nitrogen recycle stream to produce an effluent stream; (e) a cold recycle circuit, (f) a warm recycle circuit; and (g) a subcooler configured to subcool a second portion of the primary nitrogen liquefaction stream to produce a subcooled liquid nitrogen stream. The multi-pass brazed aluminum heat exchanger also includes a fourth heat exchange passage and a fifth heat exchange passage configured to liquefy the natural gas feed stream in the fifth heat exchange passage against a first portion of the subcooled liquid nitrogen stream in the fourth heat exchange passage and the gaseous nitrogen recycle stream, wherein the liquid nitrogen product stream is a second portion of the subcooled liquid nitrogen stream and the liquid natural gas stream is the liquefied natural gas exiting a cold end of the fifth heat exchange passage.
The warm recycle circuit comprises a warm booster compressor and a booster loaded warm turbine operatively coupled to a direct drive motor/generator arrangement and configured to compress a second portion of the effluent stream in the warm booster compressor to form warm nitrogen recycle stream, cool the further compressed warm nitrogen recycle stream in a third heat exchange passage in the multi-pass brazed aluminum heat exchanger; expand the cooled stream in the booster loaded warm turbine to produce a warm turbine exhaust; warm the warm turbine exhaust in the second heat exchange passage to form part of the gaseous nitrogen recycle stream.
The cold recycle circuit preferably comprises a cold booster compressor and a booster loaded cold turbine and configured to further compress a first portion of the effluent stream to form a primary nitrogen liquefaction stream; cool the primary nitrogen liquefaction stream in a first heat exchange passage of the multi-pass brazed aluminum heat exchanger; expand a first portion of the cooled primary nitrogen liquefaction stream extracted at a cold intermediate location of the first heat exchange passage in the booster loaded cold turbine to produce a cold turbine exhaust; warm the cold turbine exhaust in a second heat exchange passage of the multi-pass brazed aluminum heat exchanger; and recycle the warmed stream exiting the second heat exchange passage as the gaseous nitrogen recycle stream. The cold booster compressor and the cold booster loaded turbine are operatively coupled to a second direct drive motor/generator arrangement.
The present invention may also be characterized as a method for liquefaction to co-produce liquid nitrogen and liquid natural gas, the method comprising the steps of: (i) receiving a gaseous nitrogen feed stream; (ii) compressing the gaseous nitrogen feed stream and one or more gaseous nitrogen recycle streams in a recycle compressor to produce a gaseous nitrogen effluent stream; (iii) further compressing a first portion of the effluent stream in a cold booster compressor to form a part of a primary nitrogen liquefaction stream and further compressing a second portion of the effluent stream in a warm booster compressor to form a warm nitrogen recycle stream, wherein the warm booster compressor is coupled to a direct drive motor/generator arrangement; (iv) cooling the primary nitrogen liquefaction stream in a first heat exchange passage in a multi-pass brazed aluminum heat exchanger; (v) expanding a first portion of the cooled primary nitrogen liquefaction stream extracted at a primary intermediate location of the first heat exchange passage in a cold booster loaded turbine to produce a cold turbine exhaust; (vi) warming the cold turbine exhaust in a second heat exchange passage in the multi-pass brazed aluminum heat exchanger to form a gaseous nitrogen recycle stream; (vii) cooling the warm nitrogen recycle stream in a third heat exchange passage in the multi-pass brazed aluminum heat exchanger; (viii) expanding the cooled stream exiting the third heat exchange passage in a warm booster loaded turbine to produce a warm turbine exhaust wherein the warm booster loaded turbine is also operatively coupled to the direct drive motor/generator arrangement; (ix) warming the warm turbine exhaust in the second heat exchange passage to form part of the gaseous nitrogen recycle stream; (x) subcooling a second portion of the primary nitrogen liquefaction stream to produce a subcooled liquid nitrogen stream; (xi) liquefying a natural gas feed stream in a fifth heat exchange passage of the multi-pass brazed aluminum heat exchanger against a first portion of the subcooled liquid nitrogen stream in a fourth heat exchange passage of the multi-pass brazed aluminum heat exchanger and the gaseous nitrogen recycle stream to produce the liquid natural gas; and (xii) taking a second portion of the subcooled liquid nitrogen stream as the liquid nitrogen.
While the present invention concludes with claims distinctly pointing out the subject matter that Applicants regard as their invention, it is believed that the invention will be better understood when taken in connection with the accompanying drawings in which:
Turning now to the drawings, there are shown three different embodiments of the present system and method for the liquefaction of both LNG and LIN where the warm and/or cold booster compressors and associated turbines are driven by one or more direct drive motor/generators.
The warm booster discharge stream 67 and the cold booster discharge stream 25 are subsequently cooled in aftercoolers 66A. 66B to remove the heat of compression generated from the warm booster compressor 65 and the cold booster compressor 30, respectively. The cooled, cold booster discharge stream is the primary nitrogen liquefaction stream 26 that is directed to a first heat exchange passage 51 in a brazed aluminum heat exchanger (BAHX) 50 for cooling to temperatures suitable for nitrogen liquefaction. A first portion 27 of the primary nitrogen liquefaction stream in the first passage 51 of the BAHX 50 is extracted at an intermediate location of the first heat exchange passage 51 and directed to the cold turbine 28 where the first extracted portion 27 is expanded to produce a cold turbine exhaust 29. The cold turbine exhaust 29 is then directed to the cold end of a second heat exchange passage 52 in the BAHX 50. The cold turbine exhaust 29 is warmed in the BAHX 50 and the warmed exhaust 15 is recycled to the compressed nitrogen feed stream 19.
A second portion 31 of the primary nitrogen liquefaction stream continues through the BAHX 50 to produce a liquid nitrogen stream 32. The liquid nitrogen stream 32 is optionally diverted to a generator loaded liquid turbine 33 where it is expanded to produce a liquid turbine exhaust stream 34. The liquid turbine exhaust stream 34 is directed to subcooler 35 configured to produce a subcooled liquid nitrogen stream 36. The use of the generator loaded liquid turbine 33 shown in in the drawings is optional. Use of the liquid turbine likely depends on the power savings that the liquid turbine provides relative to the cost of electricity at a given installation site. In lieu of using the generator loaded liquid turbine 33, the liquid nitrogen stream 32 may proceed directly to subcooler 35 via control valve 37.
A first portion 38 of the subcooled liquid nitrogen stream is routed back via valve 39 through another passage of the subcooler 35 and then to a fourth heat exchange passage 53 of BAHX 50 to provide the requisite cooling for the nitrogen and natural gas streams. The resulting recycle stream exiting the warm end of the fourth heat exchange passage 53 is recycled as stream 13 to the gaseous nitrogen feed stream 12. A second portion of the subcooled liquid nitrogen stream is the liquid nitrogen product stream 40 preferably directed to a liquid nitrogen product storage tank 42.
The cooled, warm booster discharge stream 69 is directed to a third heat exchange passage 55 in the BAHX 50 where it is partially cooled. The partially cooled refrigerant stream 68 is extracted from the third heat exchange passage 55 of BAHX 50 at an intermediate location and directed to the warm turbine 70 where it is expanded. The exhaust stream 72 from the warm turbine 70 is returned to an intermediate location of the second heat exchange passage 52 in the BAHX 50 where it is warmed with the warmed exhaust stream 15 being recycled to the compressed nitrogen feed stream 19.
The purified, natural gas feed stream 82 is received from a source of natural gas (not shown) and is optionally compressed in natural gas compressor 84 and optionally cooled in aftercooler 85. The conditioned natural gas feed 86 is then directed to a fifth heat exchange passage 54 in BAHX 50 where it is cooled to temperatures suitable for liquefaction of natural gas. The LNG stream 44 existing fifth heat exchange passage 54 in BAHX 50 is sent to LNG storage tank 45.
As seen in
Advantageously, this liquefier arrangement 100 allows each direct drive motor/generator 99A, 99B to augment or reduce the power transmitted by the coupled turbine to the associated booster compressor. To increase the refrigeration production of the warm turbine 70 the interposed direct drive motor/generator 99A would add power by increasing its shaft speed. This increase in shaft speed would increase the outlet pressure of the warm booster compressor 65 and correspondingly increase the pressure ratio and refrigeration production of the warm turbine 70. Similarly, increasing the shaft power to the direct drive motor/generator 99B would increase the outlet pressure of the cold booster compressor 65 and correspondingly increase the pressure ratio and refrigeration production of the cold turbine 70. Note that the turbine boosters are in a “bootstrapped” configuration such that power addition or power reduction by each direct drive motor/generator has a multiplicative effect and provides a greater change than that due to the power change only. In this manner, coupling a direct drive motor/generator to each booster compressor and turbine pair, allows the addition or reduction of energy to the liquefier to more efficiently follow desired changes in LNG production and/or LIN production.
Similar to conventional nitrogen liquefiers, the load on both the warm turbine and the cold turbine in the present embodiment can be turned down or turned up by adjusting the nitrogen feed flow which, in turn, effects the overall circuit pressure levels, and particularly the recycle compressor discharge pressure which in many ways governs the overall capacity of LIN and LNG production from the liquefier arrangement. However, the present embodiments can also be loaded or unloaded with its interposed direct drive motor/generator, which enables more efficient loading of the turbines following of independent capacity changes of LIN or LNG.
For example, if it were desired to operate the present embodiments of the liquefier so that LIN production is at or near 40% of full capacity and LNG is at or near 60% of full capacity, the liquefier arrangement could be adjusted so that the recycle compressor discharge pressure is set at a level required for about 50% of full capacity of both products. The direct drive motor/generator on the cold turbine would extract power (i.e. operate as a generator) so that the cold turbine refrigeration production is further reduced to better match the load needed for the desired LIN production of at or near 40% of full capacity. The direct drive motor/generator on the warm turbine would add power (i.e. operate as a motor) so that the refrigeration production of the warm turbine is comparatively increased to better match the load needed for the desired LNG production of at or near 60% of full capacity. In this manner, the direct drive motor/generators on both the cold turbine booster and the warm turbine booster helps increase the rangeability of the liquefier.
As many of the components and streams in the embodiment of
Turning now to
In the embodiment illustrated in
However, during turn-down operation, including low or zero LNG production operation, there is a bypass circuit around the warm booster compressor 65 that is employed. In such low or zero LNG production modes, valve 92 is closed and valves 94 and 96 are open while the check valve 93 essentially isolates the warm booster circuit. In this operating mode, the first portion 23 of the cooled compressed nitrogen feed stream is diverted around the warm booster compressor 65 via valve 96 directly to the third heat exchange passage 55 in the BAHX 50 and to the warm turbine 70. In other words, the warm turbine is fed directly from the recycle compressor 20. As a result, the pressure ratio of the warm turbine 70 is greatly decreased and its refrigeration production is diminished. Also, in this low or zero LNG production operating mode, the generator 97 is engaged electrically and it essentially fixes the rotating speed of the warm turbine 70 and warm booster compressor 65. The energy that still goes to the warm booster compressor 65 simply recirculates flow in its circuit via valve 94. Ideally, the generator 97 extracts most of the load from the warm turbine 70 so that little of the energy is wasted by the warm booster compressor 65.
While the present invention has been described with reference to several preferred embodiments, it is understood that numerous additions, changes and omissions can be made without departing from the spirit and scope of the present system and method for natural gas and nitrogen liquefaction as set forth in the appended claims.
This application claims the benefit of and priority to U.S. provisional patent application Ser. No. 63/020,042 filed May 5, 2020 the disclosure of which is incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 63020042 | May 2020 | US |
