Liquid natural gas liquefier utilizing mechanical and liquid nitrogen refrigeration

Abstract

The present invention relates to a method and system for producing liquefied natural gas (LNG) from a stream of pressurized natural gas which involves a combination of mechanical refrigeration.

Claims

1. A natural gas liquefier system, comprising: a) a natural gas inlet in fluid communication to a source of natural gas; b) a liquid nitrogen inlet in fluid communication to a source of liquid nitrogen; c) at least one refrigerant inlet in fluid communication to a source of gaseous refrigerant fluid; d) at least one gaseous refrigerant outlet at a lower pressure than the refrigerant inlet in fluid communication to a device to receive the lower pressure refrigerant fluid; e) a liquefier in fluid communication to receive the natural gas, liquid nitrogen, inlet and outlet refrigerant flows which also includes at least one turbine; f) the at least one turbine which receives a flow of inlet refrigerant and discharges a flow of a reduce temperature refrigerant at a reduced pressure, wherein the inlet flow to the at least one turbine may or may not be pre-cooled within the liquefier module to a sub-ambient temperature; and g) said liquefier receiving the reduced temperature and pressure refrigerant fluid is then warmed where it is processed and discharged from the liquefier as the gaseous refrigerant outlet; and liquefied natural gas output coupled to the liquefier.

2. The method according to claim 1, where the refrigerant outlet fluid exiting the liquefier is compressed externally to the liquefier module and reintroduced to the liquefier as the refrigerant inlet fluid.

3. The method according to claim 1 where electrical or mechanic power is recovered from the at least one turbine.

4. The method according to claim 1 where the gaseous refrigerant fluid is composed on nitrogen.

5. The method according to claim 1 where a flow of vaporized liquid nitrogen leaves the liquefier as warmed gaseous nitrogen.

6. The method according to claim 4 where the warmed gaseous nitrogen is used to regenerate an adsorption based natural gas pre-purification scheme for removal of water and/or carbon-dioxide prior to the natural gas inlet.

7. The method according to claim 1 where the liquefier also includes a separator for removal of heavier hydrocarbons than methane from the natural gas inlet stream before the liquefied outlet natural gas natural leaves the liquefier.

8. The method according to claim 1 where the liquefier also includes the separator and a valve to remove lighter components than methane from a natural gas inlet stream before the liquefied natural gas leaves the liquefier.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above and other aspects, features, and advantages of the present invention will be better understood when taken in connection with the accompanying Figures in which:

(2) FIG. 1 is a schematic representation of a small LNG liquefier using a reverse Brayton expansion turbine for warm refrigeration and LIN vaporization for cold end refrigeration;

(3) FIG. 2 is a schematic representation of various heat exchanger configurations that apply to the hybrid liquefier embodiments, wherein:

(4) FIG. 2(a) is the heat exchanger (HX) configuration as shown in FIG. 1;

(5) FIG. 2(b) depicts dual pressure LIN boiling;

(6) FIG. 2(c) illustrates the cold end of the PHX;

(7) FIG. 2(d) depicts the pump utilized to increase the pressure of the LIN boiled in the HX;

(8) FIG. 2(e) illustrates a related pumped LIN process where LIN is boiled (or pseudo-boiled) and warmed;

(9) FIG. 2(f) illustrates an embodiment where ow pressure LIN is boiled in the cold end of the heat exchanger;

(10) FIG. 2(g) illustrates an embodiment where a portion of the NG feed is being split from the main cooled natural gas stream in the middle of the PHX;

(11) FIG. 2(h) depicts an embodiment where the PHX heat exchanger configuration where the multi-stream heat exchanger is generally oriented horizontally.

(12) FIG. 3a is a schematic representation of a small LNG liquefier depicting three separate liquefier deployment phases, wherein:

(13) FIG. 3(b)illustrates Phase 1: LIN only mode (no reverse Brayton refrigeration) for production of relatively low amounts of LNG;

(14) FIG. 3(c): illustrates Phase 2: addition of reverse Brayton refrigeration equipment to the Phase 1 equipment to boost LNG production and reduce specific LIN use;

(15) FIG. 3(d) illustrates Phase 3: upgrade Brayton refrigeration equipment and pre-purifier to further boost capacity and/or reduce LIN use to make final liquefier competitive with pure mechanically refrigerated LNG liquefiers; and

(16) FIG. 4 is a schematic representation of various heat exchanger configurations as they apply to the phased capital investment concept; where:

(17) FIGS. 4(a) depicts portion of boiled GAN being re-distributed to turbine air layers on the warm end of the PHX;

(18) FIG. 4(b) depicts LIN being boiled and warmed to fully take advantage of the entire turbine pass;

(19) FIG. 4(c) illustrates an embodiment where LIN is being boiled in the turbine air passes on the cold end of the heat exchanger; and

(20) FIG. 4(d) illustrates two separate phases as shown in FIGS. 4(a) and (c), respectively.

DETAILED DESCRIPTION

(21) With reference to FIG. 1, a pressurized natural gas feed 1, is routed to the hybrid liquefaction process. Natural gas feed could be supplied from a pressurized source and/or compressed before being fed to this process. Natural gas could be sub or supercritical. Natural gas feed 1, is supplied to operation unit 2 such as a liquid separator, and vapor is fed to a step or series of steps for water, acid gas, CO.sub.2 removal. In this exemplary embodiment, unit operation 5 is shown as a regenerable adsorption based unit for water and CO.sub.2 removal from the feed natural gas stream. CO.sub.2 is typically removed to a level of 50 ppm or less in the case of low pressure LNG product, and routed to operation unit 7. Thus unit 7 is a non-regenerable adsorption based unit, for example for removal of mercury and/or other species that could interfere with the downstream liquefaction process. It is understood that there are many configurations of natural gas pre-purification that can result in a stream suitable for natural gas liquefaction in terms of feed levels of moisture, CO.sub.2, heavy hydrocarbons, NGL's, sulfur species, mercaptans, mercury, etc. These approaches include but are not limited to adsorption, absorption (pressure or temperature swing), amine systems, and membranes.

(22) Clean pressurized natural gas stream 8 enters the primary LNG heat exchanger (PHX) 10, where it is cooled and liquefied. Heat exchanger 10 can be a single multi-stream heat exchanger, but the heat exchanger could be split up into multiple heat exchangers for example to accommodate heat exchanger limitation (maximum temperature differentials, block size, etc.). Natural gas feed is cooled to an intermediate temperature and taken as stream 11, where if necessary NGL's can be rejected. In this embodiment, NGL rejection is shown taking place in a single separator 12, but it is understood that the NGL and/or ethane rejection can be achieved using one or more separators, reboiled or refluxed columns, etc., in order to achieve final LNG product specifications or to ensure certain natural gas components do not freeze in the heat exchanger. Furthermore, it is understood that stream 14 can be further warmed in the PHX to recover refrigeration from this stream. Stream 13 is further cooled in the PHX to form a cooled and pressurized LNG stream (which may or may not be supercritical). The LNG stream is flashed across a valve 16 or expanded in a dense phase expander to a lower pressure which would typically be a pressure suitable for LNG storage. Depending on stream 15 temperatures and natural gas composition flashing the LNG across valve 16 which is routed to separator 18, where vapor stream 20 is taken and warmed in the PHX, while LNG product stream 19 is directed to storage. Separator 18 could also be exchanged for a reboiled and/or refluxed column for removal of N.sub.2 and/or ethane from LNG. Stream 20 which is typically enriched in nitrogen, is warmed and then flared or used as regeneration energy or used in a natural gas driver or natural gas engine to supply all or part of the site liquefier power 21. Warmed stream 21 can also be sent to a recirculating methane rich circuit that generates warm end liquefier refrigeration through the reverse Brayton process.

(23) Refrigeration in this cycle is supplied by liquid nitrogen (LIN) stream 31, which is supplied from storage. The LIN is supplied to the PHX and boiled and/or warmed in PHX 10. LIN could be boiled and/or warmed in the PHX in a sub or supercritical state. Typically, LIN is boiled above a certain pressure (3.5 bara) to avoid the possibility of freezing LNG on the cold end of the PHX. Advantages of boiling LIN at a high pressure (possibly requiring a LIN pump between the storage tank and PHX) allow for a reduction in the stream-to-stream maximum temperature delta on the cold end of the PHX. Limiting the maximum temperature delta in the cold end of the HPX can allow for a single brazed aluminum heat exchanger to be used for the entire PHX. Otherwise PHX 10 could need to be split between 2 heat exchangers, typically a brazed aluminum HX on the warm end and another HX that can mechanically tolerate large temperature differentials on the cold end. Also it is understood that LIN can be boiled at multiple pressures.

(24) Boiled LIN emerges from the warm end of the PHX as gaseous nitrogen (GAN) stream 34. This GAN can be used for adsorbent bed regeneration stream 35, and/or for other purposes (stream 41) such as cold-box purging, instrument air, LIN tank pressure building, and makeup for nitrogen circuit compressor and turbine seal leakage.

(25) The warm end refrigeration needed to liquefy the natural gas feed is generated through the reverse Brayton process where the working fluid is typically nitrogen but could also be derived from the natural gas feed (such as supplied by flash gas stream 21) or other fluids which can also be employed. Since the preferred recirculating fluid is nitrogen for small LNG liquefiers the remaining embodiments are described with the use of nitrogen in the recirculating circuit.

(26) Pressurized nitrogen stream 56 is fed to the PHX and cooled and withdrawn from the PHX as stream 57. This stream is work expanded to a lower pressure in a turbine 58 to produce a low pressure N.sub.2 stream 59. The turbine work can be dissipated in an oil brake system, used to drive a compressor such as one stage of N.sub.2 compression, or used to drive a generator. This turbine is preferably a radial inflow turbine since high isentropic efficiencies are achievable with this type of turbine, but many other types of turbines or expanders could be used (e.g., scroll expanders).

(27) The cold low pressure nitrogen stream 59 is then warmed and removed from the PHX as stream 52. Stream 52 is typically combined with makeup nitrogen 51 that is needed to replenish compressor and turbine and piping seal losses. The combined stream is subsequently compressed in one or more stages of compression, 53. This compressor could be composed of multiple stages or compressors with each stage or compressor possibly being of a different type (centrifugal, dry or oil-flooded screw, reciprocating, axial, etc.) with intercooling and/or aftercooling within or between compression stages. The pressure ratio across compressor 53 is typically between 3 and 8. The final compressed N.sub.2 can be aftercooled and optionally split where a major portion of N.sub.2 returns to the PHX as stream 56 and a minor portion 61 is employed for LIN tank pressure building, instrument air, adsorbent bed repressurization, etc.

(28) As shown in FIG. 2, several exemplary embodiments are illustrated where the potential PHX and process variants as they apply to the configuration of the main process heat exchanger 10. These exemplary embodiments could be expanded upon and/or combined together with the particular heat exchanger design. FIG. 2(a) is the heat exchanger (HX) configuration as shown in FIG. 1. FIG. 2(b) depicts dual pressure LIN boiling, for example, in order to reduce exchanger maximum temperature difference in the cold end of the HX, or this configuration could also be advantageous if the N.sub.2 recycle compressor suction pressure is above that of the low pressure boiled GAN fluid 34. In this way stream 134 could be used as the makeup source for the recirculating N.sub.2 fluid.

(29) FIG. 2(c) illustrates the cold end of the PHX split 110, split off from warm end of the heat exchanger 10. This could be advantageous because it could allow a relatively inexpensive, compact and efficient brazed aluminum heat exchanger (BAHX) to be used for the warm multi-stream heat exchange while a separate heat exchanger can be used on the cold end of the process where the temperature differential is higher. The cold end heat exchanger could also be a BAHX or it could be a coil-wound heat exchanger, brazed stainless steel heat exchanger, shell and tube heat exchanger (with 2 or more streams), etc.

(30) In the embodiment of FIG. 2(d) pump 130 is utilized to increase the pressure of the LIN boiled in the HX. A LIN pump allows for the LIN storage tank to remain at a low pressure (reduced pressure builder penalty) but can allow for reduced temperature differentials within the PHX 10, or the pump can be used to slightly warm up the temperature of a potentially cold LIN storage tank such that LNG is not frozen at the cold end of the PHX (or a combination of the factors described above).

(31) The embodiment of FIG. 2(e) illustrates a related pumped LIN process where LIN is boiled (or pseudo-boiled) and warmed, before it is removed from the PHX as stream 201 which joins the cooled recirculating high pressure N.sub.2 flow 57, to be expanded in the turbine 58. In this way extra refrigeration can be extracted from high pressure stream and the PHX can be simplified with less different types of passages. Further, the addition of stream 201 to the recirculating N.sub.2 circuit serves as the N.sub.2 circuit makeup. Stream 34b is the low pressure N2 to be used for pre-purifier regeneration, coldbox purge, etc.

(32) With reference to FIG. 2(f) low pressure LIN is boiled in the cold end of the heat exchanger and this stream 210, is then introduced in the turbine discharge 59, before the combined cold GAN is returned to the PHX. This configuration also simplifies the heat exchanger and recirculating GAN makeup. In this embodiment, stream 34c is the low pressure N.sub.2 to be used for pre-purifier regeneration, coldbox purge, etc.

(33) In the embodiment of FIG. 2(g) a portion of the NG feed is being split from the main cooled natural gas stream in the middle of the PHX. This portion of NG is then reduced in pressure and returned to the heat exchanger to be warmed and used for fuel in NG engine drives and/or NG genset and/or in NG fired regen heater. Throttling the NG at a warmer temperature like this serves to take advantage of the large JT effect of isentropically expanding warmer natural gas.

(34) With respect to the embedment of FIG. 2(h) a PHX heat exchanger configuration where the multi-stream heat exchanger is generally oriented horizontally for much of the sensible heat exchange with a vertical section to the right where LIN is boiling and LNG is condensing or pseudo condensing is provided. In this embodiment, it could be possible to configure the entire heat exchange process in to one PHX and furthermore the cold-box height can be reduced to reduce field erection costs and enable the employment of equipment that is either portable or more easily re-locatable. In the exemplary embodiment of FIG. 2(h) the turbine discharges into the horizontal section but it could discharge either into the horizontal section or in to the vertical section depending on natural gas pressure and location where NG condensation or pseudo-condensation will start. Additionally, it is understood that the LIN boiling section could also be split off into a separate heat exchanger combining the concepts of FIGS. 2(c) and (h) as the LIN boiling heat exchanger is generally small. The turbine discharge could be routed into the bottom of the vertical section of heat exchanger 10b as shown (e.g., in an additional parallel vertical pass where stream 33 is shown entering heat exchanger 10b).

(35) FIG. 3(b) shows a configuration which is very similar in performance to the process shown in FIG. 1. However, the PHX 10 as shown in FIG. 1 is split into two sections, namely 10c and 120. Splitting the heat exchange in this way results in no or limited process efficiency penalty but allows for some advantages such as potential for deferring capital as the liquefier is upgraded and reducing the size of the heat exchanger 10c which has many streams. In heat exchanger 120 high pressure recirculating N.sub.2 is cooled before being expanded in the turbine against warming low pressure recirculating N.sub.2. The portion of total system duty and UA required to cool and warm recirculating N.sub.2 in heat exchanger 120 is about 50-75% of total duty and 75 to 85% of total UA. This heat exchange can be achieved very efficiently and cost-effectively in a 2 stream BAHX (as well as in other types of heat exchanger).

(36) In the embodiment of FIG. 3(a) a LIN to LNG process where the main PHX 10c is configured to add the reverse Brayton refrigeration at a later time (Phase 1) is provided. In this embodiment, there is relatively little penalty to design heat exchanger 10c because heat exchanger 120 has been separated from the main PHX. The initial process operated in FIG. 3(a) could then be upgraded to what is shown in FIG. 3(b) (Phase 2) which could cut the specific LIN use (LIN required per gallon of LNG produced) by 70% to 80% or more and would also allow the process to produce 3 to 4× the LNG produced by the FIG. 3(a) process embodiment. It is understood that along with the upgrade in going from 3a to 3b as shown in FIG. 3 it is likely that the pre-purification system, LNG storage system and LNG off-loading systems may also need to be upgraded. In addition, splitting the heat exchange liquefaction process as shown in FIG. 3 could be advantageous even if there is no need or desire to ever operate in a LIN only mode as shown in FIG. 3(b).

(37) In the embodiments of FIGS. 3(c) and 3(d) a further upgrade to the system shown in FIG. 3(b) is provided where the reverse Brayton refrigeration system is further upgraded to reduce LIN and/or to boost LNG production capacity. The embodiment of FIG. 3(c) illustrates a second upgrade (Phase 3) where a second expansion turbine is added and FIG. 3(d) illustrates similar second upgrade (alternate Phase 3) where the recycle compressor is upgraded, 53b, for a higher pressure ratio which would result in a lower turbine discharge pressure such that the turbine discharge would optimally be fed to a lower location in the main PHX, 10c. Along with the upgrades shown in FIG. 3(c) and FIG. 3(d) other equipment may be included such as inter/aftercooler upgrades, turbine upgrades, valve/control upgrades, pre-purifier upgrades (more beds, different adsorbent, higher regen temperature, etc.) to accommodate the lower available GAN regen flow (or the pre-purification system could be replaced with a system not requiring GAN for regen).

(38) The embodiments of FIG. 4 shows heat exchanger configurations that apply to Phases 1 (LIN only operation) and Phases 2 (LIN+reverse Brayton operation) as described above. FIGS. 4(a), 4(b) and 4(c) show heat exchanger configurations that allow for enhanced use of the turbine discharge heat exchanger passes in the main heat exchanger 10c, when in LIN only mode of operation. The total heat exchanger volume associated with the passes used to warm turbine discharge would be about ⅓.sup.rd (or more) of the total heat exchanger volume so it is advantageous to utilize this heat exchanger volume if possible to improve cycle efficiency and/or to reduce heat exchanger size. FIG. 4a shows a portion of boiled GAN being re-distributed to turbine air layers on the warm end of the PHX, stream 452. FIG. 4(b) depicts LIN being boiled and warmed to fully take advantage of the entire turbine pass to fully take advantage of the entire turbine pass via streams 433, 434, 435, 436. When the turbine streams were added in Phase 2 some piping changes would be needed to again free up the turbine passes in the middle of HX 10c for warming turbine discharge. FIG. 4(c) illustrates an embodiment where LIN is being boiled in the turbine air passes on the cold end of the heat exchanger and GAN being re-distributed and warmed in the turbine air passes on the warm end of the HX. In this embodiment, the turbine air passes in the middle of the heat exchanger are reserved for turbine air to be added at a later dated.

(39) FIG. 4(d) depicts Phase 2 configuration corresponding to Phase 1 operation as shown in FIG. 4(a). FIG. 4(e) illustrates the Phase 2 configuration corresponding to Phase 1 operation as shown in FIG. 4(c).

(40) Although various embodiments have been shown and described, the present disclosure is not so limited and will be understood to include all such modifications and variations as would be apparent to one skilled in the art.