Patent Application
20180058752
The present invention relates to methods of removing refrigerant from a natural gas liquefaction system that uses a mixed refrigerant to liquefy and/or subcool natural gas, and to methods of altering the rate of production of liquefied or subcooled natural gas in which refrigerant is removed from the liquefaction system during shutdown or turn-down of production. The present invention also relates to natural gas liquefaction systems in which the above-mentioned methods can be carried out.
A number of liquefaction systems for liquefying, and optionally subcooling, natural gas are well known in the art. Typically, in such systems natural gas is liquefied, or liquefied and subcooled, by indirect heat exchange with one or more refrigerants. In many such systems a mixed refrigerant is used as the refrigerant or one of the refrigerants. Typically, the mixed refrigerant is circulated in a closed-loop refrigeration circuit, the closed-loop refrigeration circuit including a main heat exchanger through which natural gas is fed to be liquefied and/or subcooled by indirect heat exchange with the circulating mixed refrigerant. Examples of such refrigeration cycles include the single mixed refrigerant (SMR) cycle, propane-precooled mixed refrigerant (C3MR) cycle, dual mixed refrigerant (DMR) cycle and C3MR-Nitrogen hybrid (such as AP-X™) cycle.
During normal (steady state) operation of a such systems the mixed refrigerant circulates inside the closed-loop refrigeration circuit and is not intentionally removed from the circuit. Vaporized, warmed refrigerant exiting the main heat exchanger is typically compressed, cooled, at least partially condensed and then expanded (the closed-loop refrigeration circuit therefore typically including also one or more compressors, coolers and expansion devices) before being returned to the main heat exchanger as cold vaporized or vaporizing refrigerant to provide again cooling duty to the main heat exchanger. Minor amounts of mixed refrigerant may be lost over time, for example as a result of small leakages from the circuit, which may in turn require small amount of make-up refrigerant to be added, but in general no or minimal amounts of refrigerant are removed from or added to the circuit during normal operation.
However, under upset conditions, such as during shut down or turn down of the liquefaction system, mixed refrigerant may have to be removed from the closed-loop refrigeration circuit. During shut down, with the compressors, coolers and main heat exchanger out of operation, the temperature and hence the pressure of the mixed refrigerant inside the closed-loop refrigeration circuit will steadily rise over time as a result of ambient warming of the circuit, which in turn will necessitate removal of refrigerant from the circuit prior to the point at which the build of pressure is likely to lead to damage to the main heat exchanger or any other components of the circuit. During turn-down the inventory of the mixed refrigerant may need to be adjusted to properly match the reduced production rate (more specifically, the reduced amount of cooling duty required in the main heat exchanger) which again necessitates removal of some of the refrigerant from the closed-loop refrigeration circuit.
Refrigerant removed from the closed-loop refrigeration circuit may simply be vented or flared, but often the refrigerant is a valuable commodity, which makes this undesirable. In order to avoid this, another option that has been adopted in the art is to store the refrigerant removed from the closed-loop refrigeration circuit in a storage vessel so that it can be retained and subsequently returned to the closed-loop refrigeration circuit. However, this solution also involves operational difficulties. Mixed refrigerant removed from the the closed-loop refrigeration circuit typically will still need to be continuously cooled in order to for it to be stored in an at least partially condensed state, so as to avoid excessive storage pressures and/or volumes. Providing this cooling and condensing duty may involve, in turn, significant power consumption and associated operational costs.
For example, US 2012/167616 A1 discloses a method for operating a system for the liquefaction of gas, comprising a main heat exchanger and associated closed-loop refrigeration circuit. The system further comprises a refrigerant drum connected to the main heat exchanger or forming part of the refrigeration circuit in which refrigerant can be stored during shut down of the liquefaction system, so as to avoid having to vent evaporated refrigerant. The storage drum is provided with heat transfer means (such as for example a heat transfer coil through which a secondary refrigerant is passed) for cooling and liquefying refrigerant contained within the storage drum. The main heat exchanger may also be connected to a supply line through which liquid refrigerant may be injected directly into the main heat exchanger in order to cool down the refrigerant contained therein.
Similarly, IPCOM000215855D, a document on the ip.com database, discloses a method to prevent over-pressurization of a coil-wound heat exchanger during shut down. Vaporized mixed refrigerant is withdrawn from the shell side of the coil-wound heat exchanger and sent to a vessel having a heat transfer coil through which an LNG stream can be pumped, or into which LNG may be directly injected, in order to cool down and condense the mixed refrigerant, which is then returned to the shell side of the coil-wound heat exchanger. In an alternative arrangement, the cooling and condensing of the vaporized mixed refrigerant may take place in the shell side of the coil-wound heat exchanger, by placing the heat transfer coil inside the shell or injecting LNG directly into the shell. The LNG stream can be obtained from a storage tank or from any point in the cold end of the liquefaction unit.
US 2014/075986 A1 describes a method of using the main heat exchanger and closed-loop refrigeration circuit of a liquefaction facility for separating ethane from natural gas during start up of facility, instead of for producing LNG, so as to speed up the production of ethane that is to be used as part of the mixed refrigerant during subsequent normal operation of the liquefaction facility.
US 2011/0036121 A1 describes a method of removing natural gas contaminants that have leaked into a circulating nitrogen refrigerant that is being used in the reverse Brayton cycle for liquefying natural gas. A portion of the nitrogen refrigerant is withdrawn from the cycle, liquefied in the cold end of the main heat exchanger and introduced into the top of a distillation column as reflux. The purified nitrogen vapor withdrawn from the top of the distillation column to returned to the cycle. The liquid withdrawn from the bottom of the distillation column, comprising the natural gas contaminants, may be added to the LNG stream produced by the liquefaction system.
US 2008/0115530 A1 describes a method of removing contaminants from a refrigerant stream employed in a closed-loop refrigeration cycle of an LNG facility. The refrigerant stream may be a methane refrigerant or an ethane refrigerant employed in a cascade cycle, with the contaminant comprising a heavier refrigerant (e.g. ethane or propane, respectively) that has leaked into the refrigerant from a separate closed-loop circuit of the cascade cycle. The system employs a distillation column to remove the contaminants. The contaminated refrigerant is introduced into the distillation column at an intermediate location. A vapor stream of contaminant-depleted refrigerant is withdrawn from the top of the column and returned to its closed-loop refrigeration circuit. A contaminant-enriched liquid is withdrawn from the bottom of the column and discarded.
According to a first aspect of the present invention, there is provided a method of removing refrigerant from a natural gas liquefaction system that uses a mixed refrigerant to liquefy and/or subcool natural gas, the mixed refrigerant comprising a mixture of methane and one or more heavier components, and the liquefaction system comprising a closed-loop refrigeration circuit in which the mixed refrigerant is circulated when the liquefaction system is in use, the closed-loop refrigeration circuit including a main heat exchanger through which natural gas is fed to be liquefied and/or subcooled by indirect heat exchange with the circulating mixed refrigerant, the method comprising:
(a) withdrawing vaporized mixed refrigerant from the closed-loop refrigeration circuit;
(b) introducing the vaporized mixed refrigerant into a distillation column and providing reflux to the distillation column so as to separate the vaporized mixed refrigerant into an overhead vapor enriched in methane and a bottoms liquid enriched in heavier components;
(c) withdrawing overhead vapor from the distillation column to form a methane enriched stream that is removed from the liquefaction system; and
(d) reintroducing bottoms liquid from the distillation column into the closed-loop refrigeration circuit, and/or storing bottoms liquid such that it can subsequently be reintroduced into the closed-loop refrigeration circuit.
According to a second aspect of the present invention, there is provided a method of altering the rate of production of liquefied or subcooled natural gas in a natural gas liquefaction system that uses a mixed refrigerant to liquefy and/or subcool the natural gas, the liquefaction system comprising a closed-loop refrigeration circuit in which the mixed refrigerant is circulated, the mixed refrigerant comprising a mixture of methane and one or more heavier components, and the closed-loop refrigeration circuit including a main heat exchanger through which natural gas is fed to be liquefied and/or subcooled by indirect heat exchange with the circulating mixed refrigerant, the method comprising:
a first period of time during which natural gas is fed through the main heat exchanger at a first feed rate and mixed refrigerant is circulated in the closed-loop refrigeration circuit at a first circulation rate so as to produce liquefied or subcooled natural gas at a first production rate;
a second period of time during which the production of liquefied or subcooled natural gas is stopped, or the rate of production of liquefied or subcooled natural gas is reduced to a second production rate, by stopping the feed of natural gas through the main heat exchanger or reducing the feed rate thereof to a second feed rate, stopping the circulation of the mixed refrigerant in the closed-loop refrigeration circuit or reducing the circulation rate thereof to a second circulation rate, and removing refrigerant from the liquefaction system, wherein the method of removing refrigerant from the liquefaction system comprises:
According to a third aspect of the present invention, there is provided a natural gas liquefaction system that uses a mixed refrigerant, comprising a mixture of methane and one or more heavier components, to liquefy and/or subcool natural gas, the liquefaction system comprising:
a closed-loop refrigeration circuit for containing and circulating a mixed refrigerant when the liquefaction system is in use, the closed-loop refrigeration circuit including a main heat exchanger through which natural gas can be fed to be liquefied and/or subcooled by indirect heat exchange with the circulating mixed refrigerant;
a distillation column for receiving vaporized mixed refrigerant from the closed-loop refrigeration circuit and operable to separate the vaporized mixed refrigerant into an overhead vapor enriched in methane and a bottoms liquid enriched in heavier components of the mixed refrigerant;
means for providing reflux to the distillation column;
conduits for transferring vaporized mixed refrigerant from the closed-loop refrigeration circuit to the distillation column, for withdrawing from the distillation column and removing from the liquefaction system a methane enriched stream formed from the overhead vapor, and for reintroducing bottoms liquid from the distillation column into the closed-loop refrigeration circuit.
Mixed refrigerants are a valuable commodity in a natural gas liquefaction plant. Typically, they can be extracted and manufactured from the natural gas feed itself, using a natural gas liquids (NGL) recovery system either in integration with the liquefaction or prior to liquefaction. However, while components of the mixed refrigerant such as methane can easily be obtained in this way, some other components are far more time consuming and difficult to isolate (such as for example ethane/ethylene and higher hydrocarbons that are present only in small amounts in the natural gas) or may not be possible to obtain in this way at all (for example HFCs, which are not present in the natural gas at all). In practice, therefore, the heavier components of the mixed refrigerant may have to be imported into the facility, at significant expense. Consequently, the loss of such refrigerants has a significant financial impact.
Equally, however, under upset conditions, such as during shut down or turn down of the liquefaction system, refrigerant may have to be removed from the closed-loop refrigeration circuit for reasons discussed above. Mixed refrigerant removed from the closed-loop refrigeration circuit may simply be vented or flared, but then this refrigerant, and in particular the heavier components thereof, has been lost. Alternatively, the removed mixed refrigerant may be stored in an at least partially condensed state, but then the cooling duty required for this is likely to involve significant power consumption and associated operational costs, as also discussed above.
The methods and systems in accordance with the first, second, and third aspects of present invention, as described above, address these problems by separating the vaporized mixed refrigerant initially removed from the closed-loop refrigerant circuit in a distillation column into a methane enriched fraction (that collects as overhead vapor in the distillation column) and a heavier component enriched fraction (that collects as bottoms liquid in the distillation column), allowing a methane enriched stream to be rejected from the liquefaction system and a stream enriched in the heavier components to be returned to the closed-loop refrigeration circuit and/or stored for subsequent reintroduction into the closed-loop refrigeration circuit.
In this way, the heavier components of the mixed refrigerant (such as for example ethane/ethylene and higher hydrocarbons) can largely be retained, thereby avoiding the difficulties and/or costs of having to replace these components in the mixed refrigerant once the reasons for having to remove the refrigerant have passed and normal operation of the liquefaction system can be restored. At the same time, by removing a methane enriched stream, formed from the overhead vapor, from the distillation column and from the liquefaction system (either by simply flaring this stream or by putting it to some other use), the difficulties and costs associated with storing the methane until normal operations are restored are also avoided. As noted above, since methane is present as the main component of the natural gas that is available on site, replacing the methane in the refrigerant is a relatively easy and quick process. Likewise, where nitrogen is also present in the mixed refrigerant, and thus also removed as part of the methane enriched stream, this is usually also relatively easy to replace, since natural gas liquefaction systems typically require nitrogen for inerting purposes and so often have nitrogen generation facilities on site. Furthermore, as methane, nitrogen (if present) and any other light components present in the mixed refrigerant will have higher vapor pressures than the heavier components of the mixed refrigerant, they inherently require colder storage temperatures (or higher storage pressures), which also makes the rejection rather than storage of these components beneficial.
The articles “a” and “an”, as used herein and unless otherwise indicated, mean one or more when applied to any feature in embodiments of the present invention described in the specification and claims. The use of “a” and “an” does not limit the meaning to a single feature unless such a limit is specifically stated. The article “the” preceding singular or plural nouns or noun phrases denotes a particular specified feature or particular specified features and may have a singular or plural connotation depending upon the context in which it is used.
As used herein, the term “natural gas” encompasses also synthetic and substitute natural gases. The major component of natural gas is methane (which typically comprises at least 85 mole %, more often at least 90 mole %, and on average about 95 mole % of the feed stream). Other typically components of the natural gas include nitrogen, one or more other hydrocarbons, and/or other components such as helium, hydrogen, carbon dioxide and/or other acid gases, and mercury. However, prior to being subjected to liquefaction, components such as moisture, acid gases, mercury and natural gas liquids (NGL) a removed from the feed, down to the levels necessary to avoid freezing or other operational problems in the heat exchanger in which liquefaction takes place.
As used herein, the term “mixed refrigerant” refers, unless otherwise indicated, to a composition comprising methane and one or more heavier components. It may also further comprise one or more additional light components. The term “heavier component” refers to components of the mixed refrigerant that have a lower volatility (i.e. higher boiling point) than methane. The term “light component” refers to components having the same or a higher volatility (i.e. the same or a lower boiling point) than methane. Typical heavier components include heavier hydrocarbons, such as but not limited to ethane/ethylene, propane, butanes and pentanes. Additional or alternative heavier components may include hydrofluorocarbons (HFCs). Nitrogen is often also present in the mixed refrigerant, and constitutes an exemplary additional light component. When present, nitrogen is separated by the distillation column with the methane, such that both the overhead vapor from the distillation column and methane enriched stream that is removed from the liquefaction system are also enriched in nitrogen. In a variant, the methods and systems of the present invention could also be applied to methods and systems where the mixed refrigerant does not contain methane but contains instead nitrogen and one or more heavier components (such as for example an N2/HFC mixture), the overhead from the distillation column being enriched in nitrogen and a nitrogen enriched stream being removed from the liquefaction system. However, this is not preferred.
The liquefaction system in the methods and systems in accordance with the present invention can employ any suitable refrigerant cycle for liquefying, and optionally subcooling, natural gas, such as but not limited to the single mixed refrigerant (SMR) cycle, propane-precooled mixed refrigerant (C3MR) cycle, dual mixed refrigerant (DMR) cycle and C3MR-Nitrogen hybrid (such as AP-X™) cycle. The closed-loop refrigeration circuit, in which the mixed refrigerant is circulated, can be used to both liquefy and subcool the natural gas, or alternatively it can be used just to liquefy the natural gas, or to subcool natural gas that has already been liquefied by another part of the liquefaction system. In systems where more than one mixed refrigerant-containing closed-loop circuit is present, the methods of removing refrigerant in accordance with the present invention can be used in connection with the mixed refrigerant present in just one of the closed-loop circuits, or can be used in connection with the mixed refrigerants present in more than one, or all, of the closed-loop circuits.
As used herein, the term “main heat exchanger” refers to the part of the closed-loop refrigeration circuit through which natural gas is passed to be liquefied and/or subcooled by indirect heat exchange with the circulating mixed refrigerant. The main heat exchanger may be composed of one or more cooling sections arranged in series and/or in parallel. Each such section may constitute a separate unit having its own housing, but equally sections may be combined into a single unit sharing a common housing. The main heat exchanger may be of any suitable type, such as but not limited to a heat exchanger of the shell and tube, coil-wound, or plate and fin type, though it is preferred that the heat exchanger is a coil-wound heat exchanger. In such exchangers, each cooling section will typically comprise its own tube bundle (where the exchanger is of the shell and tube or coil-wound type) or plate and fin bundle (where the unit is of the plate and fin type). As used herein, the “warm end” and “cold end” of the main heat exchanger are relative terms, referring to the ends of the main heat exchanger that are of the highest and lowest temperature (respectively), and are not intended to imply any particular temperature ranges, unless otherwise indicated. The phrase “an intermediate location” of the main heat exchanger refers to a location between the warm and cold ends, typically between two cooling sections that are in series.
The vaporized mixed refrigerant that is withdrawn from the closed-loop refrigerant circuit is preferably withdrawn from a cold end of and/or from an intermediate location of the main heat exchanger. Where the main heat exchanger is a coil-wound heat exchanger, the vaporized mixed refrigerant is preferably withdrawn from the shell-side of the coil-wound heat exchanger.
As used herein, the term “distillation column” refers to a column (or set of columns) containing one or more separation stages composed of devices, such as packing or a tray, that increase contact and thus enhance mass transfer between the upward rising vapor and downward flowing liquid flowing inside the column. In this way, the concentration of methane and any other light components (such as nitrogen when present) is increased in the rising vapor that collects as overhead vapor at the top of the column, and the concentration of heavier components is increased in the bottoms liquid that collects at the bottom of the column. The “top” of the distillation column refers to the part of the column at or above the top most separation stage. The “bottom” of the column refers to the part of the column at or below the bottom most separation stage.
The vaporized mixed refrigerant withdrawn from the closed-loop refrigeration circuit is preferably introduced into the bottom of the distillation column. Reflux to the distillation column, i.e. downward flowing liquid inside that distillation column, can be generated by any suitable means. For example, reflux may be provided a reflux stream of condensate obtained by condensing at least a portion of the overhead vapor in an overhead condenser by indirect heat exchange with a coolant. Alternatively or additionally, reflux may be provided by a reflux stream of liquid that is introduced into the top of the distillation column. The coolant and/or the reflux stream of liquid can, for example, comprise a stream of liquefied natural gas taken from liquefied natural gas that is being or has been produced by the liquefaction system.
As used herein, reference to the overhead vapor, or the stream removed from the liquefaction system, being “enriched” in a component (such as being enriched in methane, nitrogen and/or another light component) means that said overhead vapor or stream has a higher concentration (mole %) of said component than the vaporized mixed refrigerant that is withdrawn from the closed-loop refrigeration circuit and introduced into the distillation column. Similarly, reference to the bottoms liquid being “enriched” in a heavier component means that said bottoms liquid has a higher concentration (mole %) of said component than the vaporized mixed refrigerant that is withdrawn from the closed-loop refrigeration circuit and introduced into the distillation column.
The methane enriched stream that is removed from the liquefaction system can be disposed of or put to any suitable purpose. It may, for example, be flared, used as fuel (for example in order to generate power, electricity, or useful heat), added to a natural gas feed that is to be liquefied by the liquefaction system, and or exported (for example via a pipe-line) to an off-site location.
Where some or all of the bottoms liquid from the distillation column is stored prior to being reintroduced into the closed-loop refrigeration circuit, bottoms liquid can be stored in the bottom of the distillation column and/or can be withdrawn from the distillation column and stored in a separate storage vessel. In preferred embodiments, all of the bottoms liquid that is produced by the distillation column is reintroduced into the closed-loop refrigeration circuit (either directly and/or after temporary storage).
The method of removing refrigerant according to the first aspect of the present invention is preferably carried out in response to a shutdown of or turn-down in the rate of natural gas liquefaction and/or subcooling by the liquefaction system. Alternatively, the method could be carried out in response to other occurrences or upset situations, such as for example where a leak is detected or discovered in the main heat exchanger.
In the method of altering production rate according to the second aspect of the present invention, the first period of time may, for example, represent normal operation of the system, with the first production rate corresponding to the normal rate of production of liquefied or subcooled natural gas, and the second period of time representing a period of turn-down or shutdown when the rate of production of liquefied or subcooled natural gas is reduced (to the second, or turn-down, production rate) or is stopped altogether.
The method of altering production rate according to the second aspect of the present invention may further comprise a further, or third, period of time after the second period of time, during which the rate of production of liquefied or subcooled natural gas is increased to a third production rate, by increasing the feed of natural gas through the main heat exchanger to a third feed rate, adding refrigerant to the liquefaction system, and increasing the circulation of the mixed refrigerant to a third circulation rate. The step of adding refrigerant to the liquefaction system may comprise introducing methane into the closed-loop refrigeration circuit. Some or all of this methane may be obtained from the natural gas supply that provides natural gas for liquefaction in the liquefaction system. If bottoms liquid has not already been reintroduced into the closed-loop refrigeration circuit in step (d) of the second time period (or if some bottoms liquid has been stored, and heavier components still need to be reintroduced into the closed-loop refrigeration circuit) then the step of adding refrigerant to the liquefaction system may also comprise reintroducing stored bottoms liquid into the closed-loop refrigeration circuit. The third production rate of liquefied or subcooled natural gas, third feed rate of natural gas and third circulation rate of mixed refrigerant are preferably the same as or less than the first production rate, first feed rate and first circulation rate, respectively. In particular, the third production rate, third feed rate and third circulation rate may be the same as the first production rate, first feed rate and first circulation rate, respectively, with the third period of time representing the restoration of the liquefaction system to normal operation.
The natural gas liquefaction system in accordance with the third aspect of the present invention is, in particular, suitable for carrying out methods in accordance with the first and/or second aspects of the invention.
Preferred aspects of the present invention include the following aspects, numbered #1 to #29:
(a) withdrawing vaporized mixed refrigerant from the closed-loop refrigeration circuit;
(b) introducing the vaporized mixed refrigerant into a distillation column and providing reflux to the distillation column so as to separate the vaporized mixed refrigerant into an overhead vapor enriched in methane and a bottoms liquid enriched in heavier components;
(c) withdrawing overhead vapor from the distillation column to form a methane enriched stream that is removed from the liquefaction system; and
(d) reintroducing bottoms liquid from the distillation column into the closed-loop refrigeration circuit, and/or storing bottoms liquid such that it can subsequently be reintroduced into the closed-loop refrigeration circuit.
a first period of time during which natural gas is fed through the main heat exchanger at a first feed rate and mixed refrigerant is circulated in the closed-loop refrigeration circuit at a first circulation rate so as to produce liquefied or subcooled natural gas at a first production rate;
a second period of time during which the production of liquefied or subcooled natural gas is stopped, or the rate of production of liquefied or subcooled natural gas is reduced to a second production rate, by stopping the feed of natural gas through the main heat exchanger or reducing the feed rate thereof to a second feed rate, stopping the circulation of the mixed refrigerant in the closed-loop refrigeration circuit or reducing the circulation rate thereof to a second circulation rate, and removing refrigerant from the liquefaction system, wherein the method of removing refrigerant from the liquefaction system comprises:
a closed-loop refrigeration circuit for containing and circulating a mixed refrigerant when the liquefaction system is in use, the closed-loop refrigeration circuit including a main heat exchanger through which natural gas can be fed to be liquefied and/or subcooled by indirect heat exchange with the circulating mixed refrigerant;
a distillation column for receiving vaporized mixed refrigerant from the closed-loop refrigeration circuit and operable to separate the vaporized mixed refrigerant into an overhead vapor enriched in methane and a bottoms liquid enriched in heavier components of the mixed refrigerant;
means for providing reflux to the distillation column;
conduits for transferring vaporized mixed refrigerant from the closed-loop refrigeration circuit to the distillation column, for withdrawing from the distillation column and removing from the liquefaction system a methane enriched stream formed from the overhead vapor, and for reintroducing bottoms liquid from the distillation column into the closed-loop refrigeration circuit.
Solely by way of example, certain preferred embodiment of the invention will now be described with reference to
In the embodiments illustrated in
Also, in the embodiments illustrated in
The mixed-refrigerant used in these embodiments comprises methane and one or more heavier components. Preferably, the heavier components comprise one or more heavier hydrocarbons, and nitrogen is also present as an additional light component. In particular, a mixed refrigerant comprising a mixture of nitrogen, methane, ethane/ethylene, propane, butanes and pentanes is generally preferred.
Referring to
The natural gas liquefaction system comprises a closed loop refrigeration circuit that, in this instance, comprises main heat exchanger 10, refrigerant compressors 30 and 32, refrigerant coolers 31 and 33, phase separator 34, and expansion devices 36 and 37. The main heat exchanger 10 is, as noted above, a coil-wound heat exchanger that comprises three helically wound tube bundles 11, 12, 13, housed in a single pressurized shell (typically made of aluminium or stainless steel). Each tube bundle may consist of several thousand tubes, wrapped in a helical fashion around a central mandrel, and connected to tube-sheets located above and below the bundle.
Natural gas feed stream 101, which in this embodiment has already been pre-cooled in a pre-cooling section (not shown) of the liquefaction system that uses propane or mixed refrigerant in a different closed-loop circuit to pre-cool the natural gas, enters at the warm end of the coil-wound heat exchanger 10 and is liquefied and subcooled as it flows through the warm 11, middle 12 and cold 13 tubes bundles, before exiting the cold end of the coil-wound heat exchanger as subcooled, liquefied natural gas (LNG) stream 102. The natural gas feed stream 101 will also have been pre-treated as and if necessary to remove any moisture, acid gases, mercury and natural gas liquids (NGLs) down to the levels necessary to avoid freezing or other operational problems in the coil-wound heat exchanger 10. The subcooled, liquefied natural gas (LNG) stream 102 exiting the coil-wound heat exchanger may be sent directly to a pipeline for delivery off-site (not shown), and/or may be sent to an LNG storage tank 14 from which LNG 103 can be withdrawn as and when required.
The natural gas is cooled, liquefied and subcooled in the coil-wound heat exchanger by indirect heat exchange with cold vaporized or vaporizing mixed refrigerant flowing through the shell-side of the coil-wound heat exchanger, from the cold end to the warm end, over the outside of the tubes. Typically there is, located at the top of each bundle within the shell, a distributor assembly that distributes the shell-side refrigerant across the top of the bundle.
Warmed, vaporized mixed refrigerant 309 exiting the warm end of the coil-wound heat exchanger is compressed in refrigerant compressors 30 and 32 and cooled in inter- and after-coolers 31 and 33 (typically against water or another ambient temperate cooling medium) to form a stream of compressed, partially condensed mixed refrigerant 312. This is then separated in phase separator 34 into a liquid stream of mixed refrigerant 301 and a vapor stream of mixed refrigerant 302. In the illustrated embodiment, the refrigerant compressors 30 and 32 are driven by a common motor 35.
The liquid stream of mixed refrigerant 301 is passed through the warm 11 and middle 12 tube bundles of the coil wound heat exchanger, separately from the natural gas feed stream 101, so as to also be cooled therein, and is then expanded in expansion device 36 to form a stream of cold refrigerant 307, typically a temperature of about −60 to −120 C, that is re-introduced into shell-side of the coil-wound heat exchanger 10, at an intermediate location between the cold 13 and middle 12 tube bundles, to provide part of the aforementioned cold vaporized or vaporizing mixed refrigerant flowing through the shell-side of the coil-wound heat exchanger.
The vapor stream of mixed refrigerant 302 is passed through the warm 11, middle 12 and cold 13 tube bundles of the coil wound heat exchanger, separately from the natural gas feed stream 101, so as to also be cooled and at least partially condensed therein, and is then expanded in expansion device 37 to form a stream of cold refrigerant 308, typically at a temperature of about −120 to −150 C, that is re-introduced into shell-side of the coil-wound heat exchanger 10 at the cold end of the coil-wound heat exchanger, to provide the remainder of the aforementioned cold vaporized or vaporizing mixed refrigerant flowing through the shell-side of the coil-wound heat exchanger.
As will be recognized, the terms ‘warm’ and ‘cold’ in above context refer only to the relative temperatures of the streams or parts in question and, unless otherwise indicated, do not imply any particular temperature ranges. In the embodiment illustrated in
Referring to
Where the liquefaction system is operating under turn-down conditions then natural gas feed stream 101 is still being passed through the coil-wound heat exchanger 10 to produce subcooled LNG stream 102, but the feed rate of the natural gas (i.e. flow rate the natural gas feed stream 101) and the production rate of LNG (i.e. the flow rate of subcooled, LNG stream 102) is reduced as compared to the feed and production rates in
A stream of vaporized mixed refrigerant 201 is withdrawn from the closed-loop refrigeration circuit by being withdrawn from the shell-side of the coil-wound heat exchanger 10 at the cold-end thereof, and is introduced into the bottom of a distillation column 20 containing multiple separation stages, composed for example of packing or trays, that serve to separate the vaporized mixed refrigerant into an overhead vapor that accumulates at the top of the distillation column and a bottoms liquid that accumulates at the bottom of the distillation column. The overhead vapor is enriched, relative to the mixed-refrigerant that is fed into the column, in methane and any other light components of the mixed refrigerant. For example, when nitrogen is present in the mixed refrigerant, the overhead vapor is also enriched in nitrogen. The bottoms liquid is enriched, relative to the mixed refrigerant that is fed into the column, in components of the mixed refrigerant that are heavier than methane. Exemplary heavier components include, as previously noted, ethane/ethylene, propane, butanes and pentanes, for example. The operating pressure of the distillation column is typically less than 150 psig (less than 100 atm).
Reflux to the distillation column is generated in this embodiment by cooling and condensing at least a portion of the overhead vapor in an overhead condenser 22 by indirect heat exchange with a coolant 207. The overhead condenser 22 may be integrated with or part of the top or the distillation column 20, or it may (as illustrated in
Overhead vapor 202 from the distillation column 20 passes through the condenser 22 and is, in this embodiment, partially condensed to form a mixed phase stream 203. The mixed phase stream 203 is then separated, in phase separator 21, into a liquid condensate that is returned to the top of the distillation column as reflux stream 210, and a remaining, methane enriched, vapor portion that is removed from the liquefaction system as methane-enriched stream 204. In an alternative embodiment (not shown), the overhead vapor 202 could be fully condensed in the overhead condenser, and the condensed overhead then divided into two streams, one of which is returned to the top of the distillation column as reflux stream 210 and the other of which forms the (in this case liquid) methane-enriched stream 204 withdrawn from the liquefaction system.
This would allow phase separator 21 to be dispensed with, but would also require increased cooling duty for the overhead condenser, and so is not generally preferred.
The methane-enriched stream 204 withdrawn from the liquefaction system is preferably largely free of heavier components. For example, where the heavier components comprise ethane and higher hydrocarbons, it typically contains less than about 1% of these components. Where nitrogen is also present in the mixed refrigerant, stream 204 is enriched in both methane and nitrogen. The nitrogen to methane ratio in the stream will depend on their ratio in the vaporized mixed refrigerant withdrawn from the closed-loop refrigeration circuit, but will typically range from about 5-40 mole % N2. The methane enriched stream 204 may be disposed of by being sent to and flared in a flare stack (not shown) or other suitable device for flaring the stream, but preferably it is used as a fuel, sent to an external pipeline or external natural gas use, or is added to the natural gas feed stream 101 so as to provide additional feed for generating additional subcooled LNG. If the methane enriched stream 204 is used as fuel it may, for example, be combusted in a gas-turbine (not shown) or other form of combustion device in order to generate power for onsite use (such as by the motor 35 driving refrigerant condensers 30 and 32), to generate electricity for export, and/or to provide process heating in the plant such as in the acid gas removal unit.
The Bottoms liquid 221/222 from the distillation column 20 is reintroduced into the closed-loop refrigeration circuit and/or is stored so that it can be subsequently reintroduced into the closed-loop refrigeration circuit. The bottoms liquid is, as noted above, enriched in the heavier components, and preferably consists mainly of these heavier components. Preferably it contains less than 10 mole % methane and any other light components (for example, less than 10 mole % CH4+N2). It may be reintroduced into the closed-loop refrigeration circuit at any suitable location. For example, the bottoms liquid 221 may be reintroduced into the same location of the coil-wound heat exchanger from which the vaporized mixed refrigerant was withdrawn (using, for example, the same conduit), or it may, as shown in
As discussed above, by reintroducing (or storing and then reintroducing) the bottoms liquid back into the closed-loop refrigeration circuit, the heavier components of the mixed refrigerant (such as for example ethane/ethylene and higher hydrocarbons) can be retained, thereby avoiding the need to replace these components in the mixed refrigerant once normal operation of the liquefaction system is restored, which can be a costly, difficult and time consuming operation. At the same time, by removing a methane enriched stream, formed from the overhead vapor, from the distillation column and from the liquefaction system (either by simply flaring this stream or by putting it to some other use), the difficulties associated with storing the methane and any other additional light components of the mixed refrigerant (such as for example nitrogen) are avoided.
The coolant used in the overhead condenser can come from any suitable source. For example, if available on-site, a liquefied nitrogen (LIN) stream could be used. However, in a preferred embodiment, as shown in
LNG tank and the vapor flared or used as fuel or refrigerant make-up or for some other use as described previously for the overhead vapor.
Control of the flow of the various streams depicted in
As described above, in the embodiment shown in
Referring to
Again, in the embodiment shown in
In another embodiment, the embodiments shown in
In the embodiments shown in
For example, referring to
Referring to
During restoration of normal operation the feed rate of natural gas (i.e. flow rate the natural gas feed stream 101) through the coil-wound heat exchanger 10 and the resulting production rate of LNG (i.e. the flow rate of subcooled, LNG stream 102) is increased until the normal production rate is again reached. Likewise, the circulation rate of the mixed-refrigerant in the closed-loop refrigeration circuit (i.e. the flow rate of the mixed-refrigerant around the circuit and, in particular, through main heat exchanger 10) is increased so as to provide the increased cooling duty that this increase in the LNG production rate requires. In order to provide this increase in the circulation rate of the mixed-refrigerant it is, in turn, necessary to add refrigerant back into the closed-loop refrigeration circuit to provide make-up for the refrigerant previously removed when the liquefaction system was operating under turn-down or shutdown conditions.
In the embodiments shown in
The reintroduction of refrigerant back into the closed-loop refrigeration circuit also typically will require the addition of methane and any other light components, such as for example nitrogen, that are designed to be present in the mixed refrigerant and that have been removed from the liquefaction system during the period of turn-down or shutdown operation as part of methane enriched stream 204. It may be preferable that methane and any other light refrigerants are introduced into the closed-loop refrigeration system prior to the reintroduction of the bottoms liquid 401 back into the closed-loop refrigeration system from recovery drum 24. The make-up methane (and any other light components) may be obtained from any suitable source, and may also be introduced into the closed-loop refrigerant at any suitable location.
In particular, as natural gas is mainly methane (typically about 95 mole %) the natural gas supply that provides natural gas feed stream 101 provides a convenient and easy source of make-up methane for the closed-loop refrigeration circuit. As described above, the natural gas feed, prior to being introduced into the coil-wound heat exchanger for liquefaction, is typically scrubbed to remove NGLs. These natural gas liquids are typically processed in a NGL fractionation system (not shown) that includes a series of distillation columns, including a demethanizer column or a scrub column that produces a methane rich overhead. This methane rich overhead may, for example, be used as a make-up methane 402 that can, for example, be added to the closed-loop refrigeration circuit downstream of the coil-wound heat exchanger 10 and upstream of the first refrigerant compressor 30.
In order to illustrate the operation of the invention, the process of removing refrigerant from a natural gas liquefaction system as described and depicted in
The basis of this example is a 5 million metric tons per annum (mtpa) LNG facility using a C3MR cycle which produces about 78,000 lbmoles/h (35380 kgmoles/h) of LNG. The example is a shutdown where the exchanger has been sitting for several hours until the pressure builds to 100 psi (6.8 atm) due to heatleak of about ˜130 k btu/hr (38 kW). The simulation represents the initial operation of the distillation column 20. The conditions of the streams are listed in table below. For this example the distillation column is 0.66 ft (20 cm) in diameter, 15 ft (4.57 m) long and contains packing in the form of 1″ (2.5 cm) Pall rings. These results show that the distillation column is efficient in separating the light components (methane and nitrogen) from the heavier components (ethane/ethylene, propane and butanes) of the mixed refrigerant, and is thereby effective in retaining and recovering said valuable heavier components during an extended shutdown.
It will be appreciated that the invention is not restricted to the details described above with reference to the preferred embodiments but that numerous modifications and variations can be made without departing from the spirit or scope of the invention as defined in the following claims.
The present application is a continuation application and claims the benefit of priority of U.S. application Ser. No. 14/511,774, filed Oct. 10, 2014.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 14511774 | Oct 2014 | US |
| Child | 15671622 | US |
