Method for liquefaction of industrial gas by integration of methanol plant and air separation unit

Abstract

A method for the liquefaction of an industrial gas by integration of a methanol plant and an air separation unit (ASU) is provided. The method can include the steps of: (a) providing a pressurized natural gas stream, a pressurized purge gas stream composed predominately of hydrogen and originating from a methanol plant, and a pressurized air gas stream comprising an air gas from the ASU; (b) expanding three different pressurized gases to produce three cooled streams, wherein the three different pressurized gases consist of the pressurized natural gas stream, the pressurized purge gas stream, and the pressurized air gas stream; and (c) liquefying the industrial gas in a liquefaction unit against the three cooled streams to produce a liquefied industrial gas stream, wherein the industrial gas to be liquefied is selected from the group consisting of a first portion of the pressurized natural gas stream, a nitrogen gas stream, hydrogen and combinations thereof.

Claims

1. A method for the liquefaction of an industrial gas selected from the group consisting of natural gas, nitrogen, hydrogen, and combinations thereof, the method comprising the steps of: a) withdrawing a natural gas stream from a natural gas pipeline; b) removing carbon dioxide and water from the natural gas stream to produce a purified natural gas stream; c) expanding a first portion of the purified natural gas stream to form an expanded natural gas stream and warming the expanded natural gas stream in a first portion of a heat exchanger against the industrial gas to form a warmed natural gas stream; d) sending the warmed natural gas stream to a methanol production facility under conditions effective for producing a methanol stream, a purified hydrogen stream, and a purge gas rich in hydrogen; e) expanding the purge gas rich in hydrogen to form an expanded purge gas and warming the expanded purge gas in a second portion of the heat exchanger against the industrial gas to form a warmed purge gas stream; f) sending the warmed purge gas stream to the methanol production facility for use as fuel; and g) expanding a pressurized air gas stream from or derived from an air separation unit (ASU) to form an expanded air gas stream and warming the expanded air gas stream in a third portion of the heat exchanger against the industrial gas to form a warmed air gas stream, wherein the industrial gas is liquefied during step g), wherein the industrial gas comprises a second portion of the purified natural gas stream.

2. The method as claimed in claim 1, wherein the air gas from or derived from an ASU is a gas stream selected from the group consisting of compressed and purified air from a main air compressor, a nitrogen stream from a medium pressure column, and combinations thereof.

3. The method as claimed in claim 1, wherein the industrial gas further comprises trace components having a freezing point temperature warmer than that of methane, wherein the first portion of the heat exchanger and the second portion of the heat exchanger are kept at temperatures warmer than the freezing point temperature of the trace components, and wherein the trace components are removed from the industrial gas prior to the industrial gas being cooled in the third portion of the heat exchanger.

4. The method as claimed in claim 1, wherein the third portion of cooling is provided at colder temperatures than both the first portion of cooling and the second portion of cooling.

5. A method for the liquefaction of an industrial gas, the method comprising the steps of: (a) providing a pressurized natural gas stream, a pressurized purge gas stream originating from a methanol plant, and a pressurized air gas stream comprising an air gas from an air separation unit (ASU), wherein the pressurized purge gas stream is comprised predominately of hydrogen, wherein the pressurized air gas stream has an oxygen concentration at or below that of atmospheric air; (b) expanding a first portion of the pressurized natural gas stream to produce an expanded natural gas stream, expanding the pressurized purge gas stream to produce an expanded purge gas stream, and expanding the pressurized air gas stream to produce an expanded air gas stream; and (c) liquefying the industrial gas in a liquefaction unit against the expanded natural gas stream, the expanded purge gas stream, and the expanded air gas stream to produce a liquefied industrial gas stream, wherein the industrial gas to be liquefied is selected from the group consisting of a second portion of the pressurized natural gas stream, a nitrogen gas stream from a nitrogen pipeline, a nitrogen gas stream from the ASU, hydrogen and combinations thereof.

6. The method as claimed in claim 5, wherein the air gas from an ASU is a gas stream selected from the group consisting of compressed and purified air from a main air compressor, a nitrogen stream originating from a medium pressure column of the ASU, and combinations thereof, wherein the air gas from the ASU is at a pressure between 4 and 40 bara.

7. The method as claimed in claim 5, wherein the pressurized natural gas stream comprises methane and trace components, wherein the trace components have freezing point temperatures warmer than that of methane.

8. The method as claimed in claim 7, wherein the trace components have a freezing point temperature warmer than about 140 C.

9. The method as claimed in claim 7, wherein in steps (b) and (c): the pressurized natural gas stream provides a first portion of cooling to the industrial gas, the pressurized purge gas stream provides a second portion of cooling to the industrial gas, and the pressurized air gas stream provides a third portion of cooling to the industrial gas, wherein the third portion of the cooling is at a temperature that is colder than the first portion of the cooling and the second portion of the cooling.

10. The method as claimed in claim 5, wherein the pressurized air gas stream further comprises nitrogen gas from a nitrogen pipeline.

11. The method as claimed in claim 5, wherein the air gas of the pressurized air gas stream is withdrawn at a location downstream a main air compressor (MAC) of the ASU at or slightly below an outlet pressure of the MAC.

12. The method as claimed in claim 11, wherein the flow of the air gas of the pressurized air gas stream withdrawn at a location downstream of the MAC and provided in step (a) has an increased flow during periods wherein there is no repressurization step taking place within a front end purification unit of the ASU.

13. The method as claimed in claim 11, wherein the flow of the air gas of the pressurized air gas stream withdrawn from the outlet of the MAC and provided in step (a) accounts for less than about 20% of the total volumetric flow of air coming from the outlet of the MAC.

14. The method as claimed in claim 11, wherein the flow of the air gas stream of the pressurized air gas stream withdrawn from the outlet of the MAC and provided in step (a) accounts for less than about 5% of the total volumetric flow of air coming from the outlet of the MAC.

15. The method as claimed in claim 5, wherein the three different expansions in step (b) are performed in at least three separate turbines.

16. A method for the liquefaction of an industrial gas selected from the group consisting of nitrogen, natural gas, hydrogen and combinations thereof, the method comprising the steps of: a) withdrawing a natural gas stream from a natural gas pipeline; b) sending a first portion of the natural gas stream to a methanol production facility under conditions effective for producing a methanol stream, a purified hydrogen stream, and a purge gas rich in hydrogen; c) expanding a second portion of the natural gas stream to form an expanded natural gas stream; d) expanding the purge gas rich in hydrogen from step b) to form an expanded hydrogen stream; e) expanding a pressurized air gas stream from or derived from an air separation unit (ASU) to form an expanded air gas stream, wherein the air gas from or derived from an ASU is a gas stream selected from the group consisting of compressed and purified air from a main air compressor (MAC), a nitrogen stream from a medium pressure column, and combinations thereof; f) cooling the industrial gas in a heat exchanger against the expanded natural gas stream and the expanded hydrogen stream; and g) further cooling and liquefying the industrial gas in the heat exchanger against the expanded air gas stream to produce a liquefied industrial gas.

17. The method as claimed in claim 16, wherein the third portion of cooling is provided at colder temperatures than both the first portion of cooling and the second portion of cooling.

18. The method as claimed in claim 16, wherein the third portion of the natural gas stream is cooled to a temperature warmer than about 140 C. in step f).

19. The method as claimed in claim 16, wherein the third portion of the natural gas stream is cooled to a temperature between 140 C. to 165 C. in step g).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, claims, and accompanying drawings. It is to be noted, however, that the drawings illustrate only several embodiments of the invention and are therefore not to be considered limiting of the invention's scope as it can admit to other equally effective embodiments.

(2) FIG. 1 shows an embodiment of the present invention.

(3) FIG. 2 shows a second embodiment of the present invention.

(4) FIG. 3 shows an embodiment of the air separation unit and nitrogen pipeline in accordance with an embodiment of the present invention.

(5) FIG. 4 shows an embodiment of a methanol production facility in accordance with an embodiment of the present invention.

(6) FIG. 5 shows an embodiment of an integrated methanol production facility with an ASU and liquefier in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION

(7) While the invention will be described in connection with several embodiments, it will be understood that it is not intended to limit the invention to those embodiments. On the contrary, it is intended to cover all the alternatives, modifications and equivalence as may be included within the spirit and scope of the invention defined by the appended claims.

(8) In one embodiment, it is proposed to utilize the potential high pressure energy of the two streams in the above-described methanol process: 1) high pressure natural gas letdown to fuel and 2) high pressure hydrogen rich purge gas letdown as fuel. At least a portion of these streams can be diverted to an exchanger where they can be cooled down, then expanded in a turbine to extract energy and produce a colder process stream, which is then re-warmed in the exchanger to cool the turbine inlet streams, as well as the fluid to be liquefied (e.g., natural gas for LNG or nitrogen for LIN). For a chosen turbine discharge temperature, the turbine inlet temperature can be a result of the available pressure ratio across the turbine. Therefore, in certain embodiments, the two diverted streams from the methanol process are not cooled down prior to expansion.

(9) Because the natural gas stream and purge gas streams can contain some trace components such as, but not limited to ethane, propane, and butane+, there is a low temperature limit for the turbine discharge to prevent liquid formation, which creates process complexities at low levels and turbine damage at high levels. In certain embodiments, this temperature limit can be in the range of 100 C. depending on composition and pressure. For purposes herein, about 100 C. includes 100 C.+/30 C. Those of ordinary skill in the art will recognize that this lower level temperature limit for the natural gas stream is preferably selected to prevent adverse process conditions (e.g., excess liquid production) coming out of the turbine discharge. In one embodiment, the molar fraction of liquid at the turbine discharge is less than 20%, preferably less than 15%, more preferably less than 10%. In one embodiment, no liquid production is preferred in the discharge of the turbines.

(10) The liquefaction temperature of low pressure natural gas is approximately 160 C.; therefore, an additional refrigerant is included in embodiments of the present invention in order to liquefy the natural gas at temperatures below the lower limit of the natural gas stream. In one embodiment, the additional refrigerant provides refrigeration in the temperature range from about 100 C. to 160 C. In one embodiment, this additional refrigeration can be provided by expansion of nitrogen and/or air from the ASU. As such, the additional refrigeration can be provided by a stream originating from an ASU and/or from a gas stream derived from an ASU (e.g., nitrogen from a pipeline being a gas stream derived from an ASU).

(11) Oxygen and nitrogen are separated from atmospheric air by cryogenic distillation. The required separation energy is provided by a main air compressor (MAC). Air at approximately 6 bara from the MAC is purified to remove H.sub.2O and CO.sub.2 for cryogenic processing in the medium pressure (MP) column (sometimes also referred to as higher pressure (HP) column). The air flows upward in the MP column as it is enriched in nitrogen and is then condensed by heat exchange against vaporizing liquid oxygen in the LP column. At least a portion of this condensed nitrogen provides the reflux needed for the medium pressure (MP) distillation column. During operation of a typical double column, a portion of the pure nitrogen stream can be withdrawn from the MP column and sent to the top of the lower pressure (LP) column to provide reflux for the LP column. These nitrogen rich reflux streams for the MP and LP columns are used to separate the oxygen from nitrogen or wash down the oxygen. This reflux is often in excess of what is required for efficient distillation. For example, a portion of the nitrogen at the top of the MP column can be removed as product without significantly reducing the distillation recoveries (>99% O2 recovery can still be achieved).

(12) This product nitrogen, which has been removed from the MP column, is often valorized by injecting at an intermediate stage of a nitrogen product compressor in order to reduce the nitrogen compression energy compared to a nitrogen compressor with only a low pressure feed.

(13) For air separation plants where this potential for medium pressure N.sub.2 is not valorized as described above, there is an opportunity to utilize this medium pressure nitrogen compression energy directly in an external liquefier.

(14) As described above, the pressurized air from the MAC of the ASU can be available at approximately the operating pressures of the MP column (e.g., 4 to 7 bara) and at relatively low cost due to the typical operating range of the ASU and worst case equipment design basis. Moreover, with integration of the methanol plant, additional savings can be achieved by driving the MAC and BAC using steam turbines with available steam from the methanol plant. In certain embodiments, the MAC and purification unit operate at higher pressures (10 to 40 bara) such that refrigeration for the ASU is provided by pressure letdown through a turbine to the MP column operating at 4-7 bara.

(15) In another embodiment, a portion of the high pressure air that is not sent to the ASU cold box can be mixed with available medium pressure nitrogen (MPN.sub.2) from the MP column if the MPN.sub.2 is not being valorized in the nitrogen compressor. This can result in a low-cost, lean synthetic medium pressure air (or impure N.sub.2) stream in the range of 4-7 bara to be available for expansion refrigeration in the liquefier. In one embodiment, the air injection may be limited by the combustibility limit of O.sub.2 in methane (approx 12%) or O.sub.2 in H.sub.2 (approx 6%) or by design margin to these limits if there is potential leak.

(16) Now turning to FIG. 1. Natural gas 2 is purified of carbon dioxide and water to form stream 12 before being compressed in compressor 20 to form pressurized natural gas 22. Pressurized natural gas 22 is then cooled by aftercooler 25 to remove heat of compression from compressor 20.

(17) For the warm end refrigeration (i.e., cold temperatures that are still warm enough to prevent freezing of trace components in the natural gas and high pressure purge gas, which in some embodiments is considered to be temperatures warmer than about 100 C. to 140 C.), pressurized natural gas 22 is then cooled within heat exchanger 50, wherein a first portion of the pressurized natural gas 24 is withdrawn at a first intermediate point of the heat exchanger where it is expanded in turbine 30 to low pressure to form first expanded portion 32, before being warmed in heat exchanger 50 and subsequently sent to the methanol plant to be used as fuel (see lines 32 and 308 of FIG. 5). Alternatively, after exiting aftercooler 25, the pressurized natural gas may be sent directly to the inlet of turbine 30 via line 27 rather than cooling the gas in exchanger 50 for the purpose of limiting the temperature at the discharge of the turbine. Alternatively, cooler 25 may be reduced or removed to further warm stream 32.

(18) First expanded portion 32 provides a first portion of the refrigeration used to cool and eventually liquefy the industrial gas, which in the embodiment shown is natural gas. The turbine 30 can drive a generator or booster to pre-boost the total natural gas feed as shown, or only the natural gas to be liquefied, or only the natural gas to be expanded, or to post-boost the natural gas which was expanded.

(19) In the embodiment shown in FIG. 1, purge gas 48 can be expanded in first turbine 61. Following expansion, the expanded hydrogen rich purge gas 63 is warmed before being boosted in first booster 75 and second booster 71, which can be driven by turbines 65 and 61, respectively. The compressed purge stream is then partially re-cooled in the main exchanger, expanded again in a second set of turbines 65 and re-warmed in the main exchanger, thereby providing additional refrigeration to the industrial gas. The expanded purge gas 62 is then sent to the methanol plant for use as fuel. This arrangement provides increased refrigeration production by utilizing the higher pressure ratios of the purge gas letdown while at least partially offset by additional cycle complexity and capital cost.

(20) In the embodiment shown in FIG. 2, purge gas 48, which is a high pressure hydrogen rich stream (see 312 of FIG. 3) received from the methanol plant, can be partially cooled (e.g., the purge gas is removed from an intermediate section of the heat exchanger), expanded in second turbine 60 (or set of turbines arranged in series or parallel), which is linked to a generator G, to form expanded purge gas 62 before being warmed in the main exchanger, and sent back to the methanol plant as low pressure fuel (see 316 of FIG. 3). Expanded purge gas 62 provides a second portion of the refrigeration used to cool and eventually liquefy the natural gas.

(21) Remaining with FIG. 2, the refrigeration can be supplemented by a supplemental high pressure nitrogen 91 sourced from a high pressure nitrogen source (e.g., a pipeline), which is preferably at a pressure of 15-100 bara. The supplemental high pressure nitrogen 91 is cooled in the main exchanger 50 and expanded in expander 140 to a pressure sufficient to mix with the ASU MAC air discharge pressure and the medium pressure nitrogen draw pressure (e.g., pressure of stream 236). In one embodiment, this high pressure turbine 140 drives a generator; however, those of ordinary skill in the art will recognized that it could also provide pre- or post-boost to the nitrogen stream.

(22) Now returning to FIG. 1, in an optional embodiment, if second portion of the pressurized natural gas 26 (stream to be liquefied as product LNG) contains heavy hydrocarbons such as butane and heavier, it can be withdrawn at a second intermediate point of the heat exchanger and introduced to a liquid/gas separator or distillation column to remove heavy hydrocarbons 42, leaving a top gas 44 that is more concentrated in methane. Top gas 44 is reintroduced into the intermediate and/or cold end of heat exchanger 50, wherein it is further cooled and liquefied to form liquefied natural gas (LNG) 46.

(23) Cold end refrigeration (i.e., temperatures colder than what can be achieved from the purge gas and natural gas expansion or colder than about 100 C. to 140 C.) can be provided by a boosted air stream 84, a nitrogen stream 236, and/or a nitrogen-air mixture 86. In the embodiment shown in FIG. 1, nitrogen-air mixture 86 is used. In the embodiment shown, nitrogen-air mixture 86 is at a medium pressure (e.g., approximately 4-7 bara) before being boosted by booster 110, cooled in aftercooler 115 to form high pressure air mixture, wherein it is partially cooled in heat exchanger 50, expanded in expander 120 to approximately the range of 1.1 to 2.0 bara to form expanded air mixture, and then warmed in heat exchanger 50. Expanded air mixture 122 provides a third portion of the refrigeration used to cool and eventually liquefy the natural gas. Following heat transfer, expanded air mixture 122 can be vented to the atmosphere, used as a dry gas to an evaporative cooling system, or recompressed.

(24) In another embodiment, which is shown in FIG. 2, there may be a requirement for utility nitrogen at a pressure of 5 to 10 bara at the facility, which is typically supplied by a high pressure N.sub.2 pipeline. In this embodiment, the discharge pressure of expander 140 may be adjusted slightly per the customer's demand pressure, such that a first portion 97 can be removed as product with the remainder 95 available for mixing with air 84 from MAC 210.

(25) In another embodiment, heat exchanger 50 may be split into parallel trains such that there is little to no risk of cold refrigerant leaking and being in contact with the hydrogen rich purge gas, or natural gas streams. In this embodiment, the oxygen content in the cold refrigerant can be increased to that of the combustibility limits.

(26) FIG. 3 provides a schematic representation of an air separation unit in accordance with an embodiment of the invention. Air is compressed in main air compressor (MAC) 210, which can be driven by a motor (not shown) or by steam turbine 215 if excess steam is available from a nearby source, such as the methanol unit, to produce compressed air 212. Compressed air 212 is then purified in purification unit 220 to remove components that will freeze at cryogenic temperatures (e.g., carbon dioxide and water). From there, compressed air 212 can be split into first portion 222, second portion 224, and third portion 84, wherein the second portion is cooled in ASU heat exchanger 225 and then introduced to the double distillation column for rectification therein. In certain embodiments, first portion 222 can be further compressed in booster air compressor (BAC) 80 before being cooled in the ASU heat exchanger 225 and then introduced into medium pressure column 230. Third portion of the compressed air stream 84 is sent to the liquefier as described in FIG. 1 or FIG. 2, either alone or combined with nitrogen stream 236 from medium pressure nitrogen stream 236 (e.g., the combination of air 84 and nitrogen 236 forms nitrogen-air mixture 86).

(27) The double distillation column shown is a typical double distillation column in an air separation unit comprising lower pressure column 240, shared condenser/reboiler 241, and higher pressure column 230 (sometimes also referred to herein as medium pressure column). Bottoms liquid 232 rich in oxygen is expanded across a valve before being introduced into lower pressure column 240 for further separation. Nitrogen stream 234 is also introduced into lower pressure column 240 as reflux. Liquid oxygen stream 244 is produced at a bottom section of lower pressure column 240 as product or vaporized in ASU heat exchanger 225 for gaseous oxygen production (not shown). Low pressure nitrogen stream 242 is produced at the top of low pressure column 240, and medium pressure nitrogen stream 236 is produced at a top portion of higher pressure column 230.

(28) In one embodiment, low pressure nitrogen stream 242 can be further compressed by compressor 260 to form high pressure nitrogen 262, which can then be introduced to nitrogen pipeline 100.

(29) FIG. 4 provides a schematic overview of a methanol plant 301. Natural gas is withdrawn from natural gas pipeline 300, with a first portion of the natural gas 302 being sent to a hydro-desulfurization (HDS) unit to remove sulfur to form a desulfurized natural gas 304. This stream is then sent to a steam methane reformer (SMR) under conditions effective for producing syngas 306, which is then pressurized in compressor 310 before being sent to the methanol production facility (MEOH) under conditions effective for producing methanol and a purge gas 311. A first portion of the purge gas 314 is then sent to a pressure swing adsorber (PSA) in order to recover purified hydrogen. In a typical methanol plant, second portion of the purge gas 312, which is at an increased pressure of only nominally less than that of the discharge pressure of compressor 310, is typically expanded across a valve to very low pressure (e.g., about atmospheric pressure) and then sent to the SMR for use as purge gas fuel 316. Similarly, it is typical to supplement this fuel by use of a second portion of the natural gas 1 for use as fuel to the SMR after expansion in a valve via line 308.

(30) FIG. 5 provides a schematic overview of an integrated methanol plant, liquefier and ASU in accordance with an embodiment of the present invention. In embodiments of the present invention, instead of wasting the expansion energy of natural gas 1 and second portion of the purge gas 312 by expansion across a valve, natural gas 2 and purge gas 48 are sent to the liquefier, as described in FIG. 1 or FIG. 2, in order to provide a portion of the refrigeration used to cool and liquefy the natural gas.

(31) Integration of the methanol plant, liquefier, and ASU provides significant energy savings compared to a stand-alone natural gas liquefier. In one embodiment, all of the refrigeration used for liquefaction of the gas stream is provided by the cooling energy provided from the expansion of the natural gas, purge gas and air gas from the ASU (or nitrogen pipeline), thereby providing liquefaction of the natural gas and/or nitrogen gas with minimal or no additional compression costs. Alternatively, for increased liquefaction, this liquefaction energy can be supplemented by one or more additional liquefaction energy sources such as a cycle compressor driven by electric, gas turbine, or steam turbine drive.

(32) In the embodiment shown in FIG. 1, 274 mtd of LNG can be produced using between 92 and 163 kW/mt depending on if free steam is available to drive the main air compressor. In the embodiment of FIG. 2, approximately 428 mtd LNG can be produced for about 146 kW/mt if free steam is available to drive the air compressor and 223 kW/mt if free steam is not available. In comparison, a stand-alone LNG plant would produce that amount of LNG for about 660 kW/mt. Clearly, even without free steam, embodiments of the invention provide a significant operational savings.

(33) As used herein, purge gas stream is process gas to be withdrawn from the pressurized synthesis process to remove impurities and inerts from the catalytic process. The purge gas from methanol plants typically contains between 50-80% hydrogen.

(34) While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations that fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, language referring to order, such as first and second, should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps or devices can be combined into a single step/device.

(35) The singular forms a, an, and the include plural referents, unless the context clearly dictates otherwise. The terms about/approximately a particular value include that particular value plus or minus 10%, unless the context clearly dictates otherwise.

(36) Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.

(37) Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and/or to the other particular value, along with all combinations within said range.