Patent Application
20110259045
The present invention is directed to a refrigerant compression circuit for use in the liquefaction of natural gas or other methane-rich gas streams.
The cryogenic liquefaction of natural gas is routinely practiced as a means of converting natural gas into a more convenient form for transportation and storage. Liquefaction of large volumes of gas using a refrigerant circuit is energy and capital intensive. Broadly speaking, a plant for liquefying natural gas comprises a main heat exchanger in which a hydrocarbon gas feed stream is liquefied by means of indirect heat exchange with evaporating refrigerant in one or more stages. The plant further comprises a refrigerant circuit in which evaporated refrigerant(s) are compressed, cooled and returned to the main heat exchanger. The refrigerant circuit typically includes a compressor train consisting of at least one compressor body driven by means of a mechanical driver (e.g., a gas turbine, a steam turbine, or an electric motor) that is connected to the shaft of the compressor body via a common shaft or via a gearbox. The configuration of compressors and mechanical drivers in a gas processing plant greatly influences the energy efficiency of the plant.
An important criterion for a refrigerant circuit to be cost effective is to make full use of the installed power of each mechanical driver. There exists a continuing need in the gas processing field to provide alternative plants and methods to improve the power balance between the refrigerant compressors mounted on each shaft and the mechanical drivers powering each shaft.
According to one aspect of the present invention there is provided a refrigerant compressor circuit for use in a liquefaction plant, the refrigerant compressor circuit comprising:
In one form, a first intercooling heat exchanger may be arranged between the first compression stage and the second compression stage for removing heat of compression from the mixed refrigerant gas.
Alternatively or additionally, the mass flow of refrigerant to the second compression stage may be directed to flow through a first segment and then a second segment within a single back-to-back compressor body.
In one form, the first driver has more power than the second driver.
Alternatively or additionally, the first compression stage may be provided with a single compressor body having:
In one form, the refrigerant circuit further comprises a first distribution means for splitting the mass flow of refrigerant gas to the first compression stage into the first stream and second stream such that the first stream fed to the first suction inlet and the second stream fed to the section suction inlet are symmetrical. In one form, the first distribution means may cause the mass flow of refrigerant to enter a branched tee such that half of the mass flow of refrigerant leaves the branched tee through one end of a straight run of pipe, whilst the other half of the mass flow of refrigerant leaves the branched tee in the opposite direction through the opposite end of the straight run of pipe.
In one form, the pre-cooling refrigerant compression stage comprises a single pre-cooling refrigerant compressor body with a plurality of suction inlets arranged to receive evaporated pre-cooling refrigerant at a corresponding plurality of different pressures.
Alternatively or additionally, the refrigerant circuit further comprises a third compression stage for compressing the mixed refrigerant gas from the third pressure to a fourth pressure. In this form, the second compression stage and the third compression stage may be co-axially mounted on a second shaft drivingly coupled to a second driver in the second compression string. Alternatively or additionally, the refrigerant circuit further comprises a second intercooling heat exchanger arranged between the second compression stage and the third compression stage for removing heat of compression from the refrigerant. In one form, the second and third compression stages may be combined within a single back to back compressor body.
According to a second aspect of the present invention, there is provided a plant for the production of a liquefied hydrocarbon product such as liquefied natural gas, the plant comprising:
According to a third aspect of the present invention, there is provided a method for cooling, preferably liquefying, a hydrocarbon stream, wherein the hydrocarbon stream to be cooled by indirect heat exchange with an evaporating refrigerant, and the evaporated refrigerant is cooled using a refrigerant circuit according to the first aspect of the present invention.
According to a fourth aspect of the present invention, there is provided a refrigerant circuit substantially as herein described with reference to and as illustrated in the accompanying drawings.
In order to facilitate a more detailed understanding of the nature of the invention embodiments will now be described in detail, by way of example only, with reference to the accompanying drawings, in which:
The present invention will now be described in greater detail with reference to the accompanying drawings wherein several preferred embodiments of the present invention are set forth. Those skilled in the art will recognized that the accompanying drawings are schematic representations only and therefore, many items of equipment that would be needed in a commercial plant for successful operation have been omitted for the sake of clarity. Such items might include, for example, compressor controls, flow, level, temperature and pressure controls, pumps, motors, filters, additional heat exchangers, and valves, etc. It will be readily appreciated that a person skilled in the art would be able to include such items in accordance with standard engineering practice.
The term “compressor” as used herein refers to a device used to increase the pressure of an incoming fluid by decreasing its volume.
The term “compressor body” as used herein refers to a casing which holds the pressure side of the fluid passing through a compressor. While the compressors bodies used for the LPMR stage may be centrifugal (radial) type or axial, it is preferable to use centrifugal (radial) compressor bodies for the MPMR and HPMR compression stages.
The term “compression string” is used to describe one or more compressor bodies mounted on a common shaft and driven by a common driver.
The term “driver” as used herein refers to a mechanical device such as a gas turbine, a steam turbine, an electric motor or a combination thereof which is used to cause rotation of a shaft upon which a compression string is mounted.
The term “stage” as used herein means a compressor or compressor segment having one or more impellers wherein the mass flow of the fluid being compressed in the stage is constant through the stage. For mixed refrigerant compression, each stage is optionally defined by intercooling between them.
The term “intercooling” is used to refer to a process by which heat of compression is removed from a fluid between stages.
The term “back-to-back compressor” refers to a compressor body having two compression sections within a single casing, each stage having one inlet and one outlet.
As used herein, the terms “upstream” and “downstream” shall be used to describe the relative positions of various components of a natural gas liquefaction plant along the flow path of natural gas through the plant.
Preferred embodiments of the present invention are ideally suited to LNG trains with a capacity in the range of 4.5-6.5 million tonnes per annum (“mtpa”), but can be modified to suit processing plants of other capacities.
Numerous systems exist in the prior art for the liquefaction of a hydrocarbon feed stream by heat exchange with one or more refrigerants such as propane, propylene, ethane, ethylene, methane, nitrogen or combinations of the preceding refrigerants (so-called “mixed refrigerant” systems). For example, U.S. Pat. No. 4,698,080 discloses a liquefaction plant of the so-called cascade type having three refrigeration circuits operating with different refrigerants, propane, ethylene and methane. An alternative to the cascade-type liquefaction plant is a so-called propane-pre-cooled multi-component or “mixed refrigerant” (MR) liquefaction plant. Examples of liquefaction processes using mixed refrigerants are given in U.S. Pat. No. 5,832,745, U.S. Pat. No. 6,389,844, U.S. Pat. No. 6,370,910 and U.S. Pat. No. 7,219,512 (the contents of which are hereby specifically incorporated by reference). As methods and systems for liquefying a hydrocarbon stream are well known in the art they do not form a portion of the present invention and thus the operating conditions of the refrigeration side and the compositions of the refrigerants are not discussed in detail here.
The hydrocarbon stream to be liquefied may be any suitable hydrocarbon-containing stream, such as a natural gas stream obtained from natural gas or petroleum reservoirs or natural gas from a synthetic source such as a Fischer-Tropsch process. Whilst the composition of this gas stream may vary significantly, the hydrocarbon stream is comprised substantially of methane (e.g. >60 mol % methane). Depending on the source, the hydrocarbon stream may contain varying amounts of hydrocarbons heavier than methane such as ethane, propane, butane and pentane as well as some aromatic hydrocarbons. The hydrocarbon stream may also contain undesirable non-hydrocarbon components such as H2O, mercury, CO2, H2S, mercaptans, and other sulphur compounds. Various pre-treatment steps provide a means for removing undesirable components from the natural gas feed stream prior to liquefaction. As these pre-treatment steps are well known to the person skilled in the art, they do not form a portion of the present invention and are not further discussed here.
During normal operation a pre-treated hydrocarbon feed stream is pre-cooled using one or more pre-cooling heat exchangers before being supplied to a main cryogenic heat exchanger system (MHE) comprising one or more main heat exchangers. In the main heat exchanger system, the pre-cooled hydrocarbon feed stream is subjected to further cooling, and liquefied by means of indirect heat exchange with a refrigerant, in this example, an evaporating mixed refrigerant. When the hydrocarbon feed stream is natural gas, liquefied natural gas is removed from the discharge end of the main heat exchanger system. Specific examples of an indirect heat exchanger for use as one of the main heat exchangers include a spiral wound heat exchanger, a shell-and-tube heat exchanger, and a brazed aluminium plate-fin heat exchanger. Evaporated mixed refrigerant is removed as a gas from the main heat exchanger system and is passed to a refrigerant compressor circuit for compressing the evaporated refrigerant gas so that it can be re-used in the main heat exchanger system. The refrigerant compressor circuit consists of at least two, and preferably three, mixed refrigerant compression stages. It is contemplated that more than three stages of compression may be found to be desirable for a particular application. Downstream of the last compression stage, the mixed refrigerant is supplied to one or more heat exchangers where the mixed refrigerant is progressively cooled and at least partially liquefied, before being recycled to the main heat exchanger system.
Reference is now made to
Therefore, using the refrigerant compression circuit of the present invention, the power absorbed by the compressor bodies on each compressor string is matched closely to the available driver power on that string. Where two drivers are being used, the absorbed power of the compressor bodies mounted upon each shaft should be in same proportion as the available power from each driver. In cases where the available power is equal the target is to have a 50%-50% split between the compressor bodies on each shaft. This principle is independent of any capacity limitations.
The pre-cooling refrigerant compression stage (18) may compress pre-cooling refrigerant evaporated by the cooling of the mixed refrigerant. Alternatively or additionally, it may be used to compress pre-cooling refrigerant evaporated in one or more pre-cooling heat exchangers used for the purpose of pre-cooling the hydrocarbon feed stream before it enters the main heat exchanger system, or for other purposes such as the fractionation of NGLs removed from the natural gas. Thus, the refrigerant being compressed using the pre-cooling refrigerant compressor body could be a substantially pure refrigerant such as propane or ammonia, or alternatively a separate mixed refrigerant with a different composition to the mixed refrigerant evaporated in the main heat exchanger system.
At least a portion of the cooling of the mixed refrigerant upstream of the main heat exchanger system may be via indirect heat exchange with a pre-cooling refrigerant in one or more pre-cooling heat exchangers. Pre-cooling refrigerant streams evaporated by various heat exchangers at similar pressures are combined and collected using gas liquid/separators. The pre-cooling refrigerant compression stage (18) is used to compress the combined vapour flows at a plurality of different pressures. In the embodiment illustrated in
In the embodiment illustrated in
The embodiment illustrated in
In the embodiments illustrated in
In the embodiment illustrated in
It is to be understood that it is possible to use only two stages of mixed refrigerant compression as illustrated in
The embodiment illustrated in
In the embodiment illustrated in
A first distribution means (74) is used for splitting the mass flow of refrigerant gas to the first compression stage (12) into the first stream (54) and second stream (60), such that the first stream (54) fed to the first suction inlet (52) and the second stream (60) fed to the section suction inlet (58) are as “symmetrical” as possible (ie the resistance to the mass flow of refrigerant through each of the first and second segments is as even as possible). One way in which symmetrical flow is achieved is to use a “branched tee” (i.e. a terminating pipe which intersects a straight run of pipe at a perpendicular angle). The flow of refrigerant enters the branched tee through one end of the terminating pipe. Half of the mass flow of refrigerant leaves the branched tee through one end of the straight run of pipe, whilst the other half of the mass flow of refrigerant leaves the branched tee in the opposite direction through the opposite distal end of the straight run of pipe. Both halves of the flow therefore perform a 90 degree angle turn. This allows the pipework to be symmetrical and therefore the mass flow rate of the first stream (54) of refrigerant fed to the first suction inlet (52) is thus substantially equal to the mass flow rate of the second stream (60) of refrigerant fed to the second suction inlet (58).
When the first compression stage (12) is provided with split flow to the first and second suction inlets (52 and 58, respectively), the maximum allowable volumetric flow rate for the combined suction flow (74) is increased. However, with this arrangement, the effective number of impellers installed in each of the first and second segments (56 and 62, respectively) of the first compression stage (12) is reduced in comparison to the number of impellers that could otherwise be installed in a single compressor body if the mass flow of refrigerant to the first compression stage was not split. Once the effective number of impellers is reduced for the first compression stage (12), the power requirement for the first compression stage (12) is reduced, thereby allowing the first compression stage (12) to be mounted co-axially on the same shaft as the compressor body (32) of the pre-cooling compression stage (18).
As the pressure increases through the compression stages, the density increases and the volumetric flow decreases. By virtue of the compression ratio, the second and third compression stages have lower actual volumetric flow rates than the first (low pressure) compression stage. By splitting the mass flow of refrigerant gas to the first compression stage (12) evenly across the first and second suction inlets (52 and 58, respectively), the volumetric flow in each of the first and second segments (56 and 62, respectively) of the first compressor body (50) is halved. As a result the actual volumetric flow to each compressor segment (56, 62, 68 and 72) is inherently more even. This has the potential to allow the volumetric flow rates to be better matched to the preferred rotational speeds of the compressor segments, thereby allowing higher efficiency.
By way of example, the suction volumetric flow of a first compression stage might typically be about ten times the volumetric flow at the suction inlet of a third compression stage. By splitting the mass flow of refrigerant to the first compression stage across the first and second suction inlets, each segment in the first compressor body of the first compression stage would only require a suction volumetric flow five times the volumetric flow of the third compression stage. By way of further example, for an LNG train producing about 6 mtpa, the suction flow inlet size for a single compressor body used for the first compression stage would need to be about 300,000 m3/h, which is greater than the largest commercially available compressor on the market at this time. Using the process of the present invention as illustrated in
Splitting refrigerant flow in this way has the result that the actual volumetric flow to each of the first, second and (optional) third compression stages is more consistent, allowing better matching with the ideal rotational speed when mounted on the same shaft or a separate shafts driven by similar drivers. Also, using this arrangement in the absence of a restriction on the suction volumetric flow to the first compression stage, the LNG train size can be increased or the refrigerant circuit can be operated at a lower pressure, thereby possibly allowing greater efficiency.
In the embodiment illustrated in
In the embodiment illustrated in
In the embodiment illustrated in
It will be apparent to persons skilled in the relevant art that numerous variations and modifications can be made without departing from the basic inventive concepts. For example, the pre-cooling refrigerant compression stage (18) may be split between two compressor bodies depending on the performance limits of each of the pre-cooling refrigerant compressors when operated in the same compression string as the first compression stage. All such modifications and variations are considered to be within the scope of the present invention, the nature of which is to be determined from the foregoing description and the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2008905954 | Nov 2008 | AU | national |
This application is a continuation of PCT/AU2009/001477, filed on Nov. 13, 2009, which claims priority from Australian Patent Application No. AU 2008905954, filed on Nov. 17, 2008, the disclosures of which are incorporated herein by reference in their entireties.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/AU2009/001477 | Nov 2009 | US |
| Child | 13105168 | US |
