Patent Application
20220099365
The present invention relates to a method of cooling a boil-off gas (BOG) stream from a liquefied gas tank, such as a cargo tank, such as on a floating vessel, using a single mixed refrigerant (SMR), and apparatus therefor. It is particularly, but not exclusively, a method for cooling BOG from a floating LNG storage tank.
Traditionally, boil-off gas from liquefied natural gas (LNG) storage tanks on board ships carrying LNG as a cargo (typically LNG carriers) has been used in the ship engines to provide power to the ship. Any excess BOG is then considered ‘waste gas’, and is typically sent to a gas combustion unit (GCU), where it is disposed of by combustion.
However, ship engines have become increasingly more efficient, so that less of the BOG is required for the engines. This means a greater proportion of the BOG is sent to the GCU as waste gas. It is becoming economically attractive to reduce this loss of gas by re-liquefying it and returning it to the cargo tanks.
A conventional SMR cycle is shown in the accompanying
The heat exchanger 12 uses an external refrigerant (typically propane) supplied via pipeline 32, provided from a separate refrigerant cascade 13.
In the SMR recirculating system, the mixed refrigerant gas from refrigerant receiver 1 flows through a pipeline 22 to a compressor 2. The SMR gas is compressed into pipeline 24. The gas in pipeline 24 is sent into an aftercooler 6 which uses a readily available cooling medium (e.g. seawater, freshwater, engine room cooling water, air).
Downstream of the aftercooler 6, condensation of the refrigerant gas is performed using heat exchange against a cold external refrigerant (typically propane) i n condenser 11. The cold temperatures of this external refrigerant are created in the external refrigerant cascade 13. The refrigerant in pipeline 24 is at least partly condensed after passing through condenser 11, after which it enters a vapour-liquid separator 8 to provide vapour and liquid phases.
The refrigerant liquid in pipeline 29 has its pressure reduced by flash valve 9, leading to partial vaporisation and temperature reduction. The partially vaporised refrigerant liquid can then be sent into a multi-stream exchanger 7, where it is fully vaporised, thereby providing partial cooling to the hot streams in the exchanger 7. Meanwhile, the refrigerant vapour in pipeline 26 is sent directly into exchanger 7, where it is cooled substantially. It leaves the exchanger 7, fully or partly condensed, in pipeline 27, after which its pressure is reduced by a throttling valve 10 into pipeline 34 to its lowest temperature in the SMR recirculating system to achieve the required cooling in the exchanger 7. This provides the main cold stream for the exchanger 7.
The cold refrigerant in pipeline 34 is sent into exchanger 7, where it vaporises, cooling the hot streams. It merges with the depressurised liquid sent from valve 9, and the combined refrigerant stream leaves exchanger 7 as a vapour via pipeline 28, to re-enter refrigerant receiver 1.
Overall, the cooling duty for the re-liquefaction process in the conventional SMR cycle shown in
It is an object of the present invention to provide a simpler method, process and apparatus for cooling a BOG stream without an external refrigerant cascade, for applications requiring larger reliquefaction capacities, such as Q-flex or Q-max LNG carriers with cargo capacities typically greater than 200,000 m3.
Thus, according to the first aspect of the present invention, there is provided a method of cooling a boil-off gas (BOG) stream from a liquefied gas tank using a single mixed refrigerant (SMR) comprising at least the step of heat exchanging the BOG stream with the SMR in a liquefaction heat exchanger system to provide a cooled BOG stream,
(a) compressing the SMR using at least one centrifugal compressor to provide a post-compression SMR stream;
(b) passing the post-compression SMR stream into the liquefaction heat exchanger system to cool the post-compression SMR stream and provide a cooled first SMR vapour stream;
(c) withdrawing the cooled first SMR vapour stream from the liquefaction heat exchanger system;
(d) separating the cooled first SMR vapour stream to provide a liquid-phase SMR stream and a light SMR vapour stream;
(e) passing the light SMR vapour stream through the liquefaction heat exchanger system to provide a condensed SMR stream; and
(f) expanding the condensed SMR stream to provide an expanded lowest-temperature SMR stream to pass through the liquefaction heat exchanger system for heat exchange against the BOG stream.
The present invention utilises one or more centrifugal compressor package(s), which are suitable for applications requiring larger reliquefaction capacities than can be achieved in a single oil-injected screw compressor. Centrifugal compressors have found widespread use in land-based natural gas liquefaction plants, which typically have capacities much higher than that of off-shore reliquefaction. Unlike smaller oil-injected screw compressors, centrifugal compressors have the additional benefit of having effectively no oil carryover. The capacity of centrifugal compressors can provide a significantly higher flow capacity, which renders a larger system employing centrifugal compressors more economical overall.
SMR is a term in the art used to refer to a range of refrigerants generally comprising a mixture of one or more hydrocarbons, in particular usually methane, ethane and propane, and possibly also at least butane, and nitrogen, optionally with one or more other possible refrigerants such as pentane. Various components and their ratios are known for forming a particular SMR, and are not further described herein.
Separating one or more of the streams as defined herein can be carried out in any suitable separator, many of which are known in the art, and which are generally intended to provide at least one gaseous stream, typically a lighter stream available at or near an upper part of the separator, and a heavier stream, typically comprising at least one liquid phase, typically available at a lower end of the separator.
Expansion of a stream is possible through one or more suitable expansion devices, generally including valves and the like.
The term “ambient cooling” as used herein relates to the use of an ambient cooling medium, usually provided at an ambient temperature. This includes seawater, freshwater, engine room cooling water, and air, and any combination thereof, which are typically easily available for use in providing ambient cooling to a stream.
Optionally, the cooled first SMR vapour stream and/or the light SMR vapour stream are cooled against the expanded lowest-temperature SMR stream.
All liquefied gas tanks create or release boil-off gas for known reasons, including tanks on liquefied gas carriers, barges and other vessels including transportation vessels. Liquefied gases can include those having normal boiling points (at 1 atm) below 0° C., typically at least below −40° C., such as various petroleum o r petrochemical gases, and including liquefied natural gas (LNG) having a normal boiling point below −160° C.
Whilst BOG from liquefied gas tanks may be more readily usable onshore, it is especially desired to seek re-liquefaction of BOG offshore. However, space is typically limited offshore, especially on floating vessels, and the ability to reduce the complexity of BOG re-liquefaction can often achieve a reduction in the required CAPEX and plot area required.
Optionally, the BOG is from a liquefied cargo tank in a floating vessel, optionally an LNG cargo tank.
It is possible that the compression of the SMR in step (a) comprises the use of more than one compressor, optionally in parallel or series or both, to provide the post-compression SMR stream. The invention is not limited by the method or type of compression of the SMR, other than the use of at least one centrifugal screw compressor.
The liquefaction heat exchanger system may be any form of one or more heat exchangers arranged in one or more units or stages, and able to allow heat exchange between two or more streams, and optionally having at least one stream running countercurrently to one or more other streams in a part or portion of the system, in particular between the BOG stream and one of the refrigerant streams.
Where the liquefaction heat exchanger system comprises more than one heat exchanger, the more than one heat exchangers may be in series or in parallel or a combination of in series and in parallel, and the more than one heat exchangers may be separate or conjoined or contiguous, optionally in a single cooled unit or box, and optionally in the form of one or more units or stages of providing the required heat exchange with the BOG stream to liquefy the BOG stream.
The liquefaction heat exchanger system may comprise any suitable arrangement of two-stream or multi-stream heat exchangers arranged into one or more connected sections, units or stages, optionally with one section, unit or stage being ‘warmer’ than another section, unit or stage, in the sense of the average temperature therein.
Many liquefaction heat exchangers are known in the art which are able to be part of or provide the liquefaction heat exchanger system, typically comprising plate-fin, shell & tube, plate & frame, shell & plate, coil wound, and printed circuit heat exchangers, or any combination thereof.
Optionally, the liquefaction heat exchanger system comprises a multi-unit liquefaction heat exchange comprising two multi-stream heat exchangers.
Alternatively, the liquefaction heat exchanger system comprises a multi-unit liquefaction heat exchange comprising one multi-stream heat exchanger and a plurality of two-stream heat exchangers.
Optionally, the liquefaction heat exchanger system in the present invention comprises one or more plate-fin heat exchangers.
Optionally, the liquefaction heat exchanger system in the present invention comprises a combination of one or more plate-fin heat exchangers and one or more two-stream plate-type (plate & frame or shell & plate) heat exchangers.
Heat exchangers generally have one or more entry points or ports for each stream, and one or more exit points or ports for said stream, with a temperature gradient or gradient pathway thereinbetween. Most streams passing through a heat exchanger pass typically through ‘all’ the heat exchanger, that is from an entry point or port at one end or side of the heat exchanger to an exit point or port, optionally at another end or side but not limited thereto, so as to achieve the maximum heat exchange possible between the entry and exit, i.e. the maximum temperature change or phase change possible along the temperature gradient pathway. Such streams have ‘fully’ or ‘wholly’ passed through the heat exchanger.
Some streams may only pass through a partial portion or amount of a heat exchanger, generally by either having an entry point or port at an intermediate temperature or location along the maximum possible temperature gradient pathway, or by having an exit point or port at an intermediate temperature along the temperature gradient pathway, or both. Such streams have passed through only part of the heat exchanger.
In the present invention, the liquefaction heat exchange can be provided in a single stage or in a multi-stage arrangement, optionally in line with the number of liquefaction heat exchangers in the liquefaction heat exchanger system, but not limited thereto where more than one heat exchange stage can be provided with a single liquefaction heat exchanger.
Optionally, the liquefaction heat exchanger system is a single liquefaction heat exchanger. In one further option, the method comprises passing the light SMR vapour stream partly through the single liquefaction heat exchanger prior to step (f), i.e. passing the light SMR vapour stream into the single liquefaction heat exchanger at an intermediate temperature along the heat exchange.
In another further option, the method comprises passing the light SMR vapour stream fully through the single liquefaction heat exchanger prior to step (f).
Optionally, where the liquefaction heat exchanger system is a single liquefaction heat exchanger, withdrawal of the cooled first SMR vapour stream from the liquefaction heat exchanger system in step (c) can occur at an intermediate temperature along the heat exchange occurring in the heat exchanger, optionally at a temperature that is similar to the entry for the light SMR vapour stream into the liquefaction heat exchanger system to provide a condensed SMR stream.
Thus, optionally, step (c) of the present invention comprises withdrawing the cooled first SMR vapour stream from the liquefaction heat exchanger system prior to the coolest part of the liquefaction heat exchanger system, i.e. achieving partial passageway through the liquefaction heat exchanger system.
The light SMR vapour stream may be passed (back) into the liquefaction heat exchanger system at a temperature that is higher than, lower than, the same as, or similar to, the temperature of the withdrawn cooled first SMR vapour stream of step (c).
Optionally, the light SMR vapour stream passes into the liquefaction heat exchanger system at a temperature that is similar to the temperature of the withdrawn cooled first SMR vapour stream of step (c).
Alternatively, the liquefaction heat exchanger system may be a multi-unit liquefaction heat exchange or exchanger comprising two, optionally more than two, units, and the expanded lowest-temperature SMR stream passes through each unit.
Where the liquefaction heat exchange is provided by more than one liquefaction heat exchanger units and/or stages, optionally the cooled first SMR vapour stream passes into a first unit and/or stage, and the light SMR vapour stream passes into a second unit and/or stage. Alternatively optionally, the cooled first SMR vapour stream passes into a first heat exchange unit, and the light SMR vapour stream passes into both a first heat exchange unit and a second heat exchanger unit.
Where the liquefaction heat exchange is provided by more than one liquefaction heat exchanger units and/or stages, also optionally the first or warmer stage comprises either a multi-stream heat exchanger such as a plate-fin heat exchanger, or a series of distinct heat exchangers, optionally in series, in parallel, or both, at least one of which is able to cool the post-compression SMR stream and provide a cooled first SMR vapour stream prior to separating the cooled first SMR vapour stream to provide a liquid-phase SMR stream and a light SMR vapour stream.
Optionally, the method of the present invention further comprises the steps of expanding the liquid-phase SMR stream of step (d), and passing the expanded liquid-phase SMR stream into the liquefaction heat exchanger system.
Optionally, the method of the present invention further comprises the step of combining the expanded liquid-phase SMR stream with the expanded lowest-temperature SMR stream in the liquefaction heat exchanger system, further optionally, between two stages or units of a multi-stage or multi-unit liquefaction heat exchanger system.
Optionally, the method of the present invention alternatively further comprises the step of combining the expanded liquid-phase SMR stream with the expanded lowest-temperature SMR stream after the liquefaction heat exchanger system.
The method of the present invention provides a post-liquefaction heat exchange SMR stream, or a post-cooling vapour SMR stream, for recirculation or reuse as part of the SMR recirculating system. This post stream is optionally the expanded liquid-phase SMR stream combined with the expanded lowest-temperature SMR stream, being combined either within or after the liquefaction heat exchanger system.
Thus, optionally, the method of the present invention further comprises recycling the expanded lowest-temperature SMR stream after the liquefaction heat exchanger for providing the SMR, typically with the additional expanded liquid-phase SMR stream.
In the present invention, it is intended that the post-compression SMR stream of step (a) does not undergo any external refrigerant cooling prior to step (d), such that an external refrigerant cascade is not required. The SMR liquefaction heat exchanger system itself wholly or substantially provides the refrigerant cooling required to condense the light SMR vapour stream prior to its expansion back into the liquefaction heat exchanger system.
Optionally, the BOG stream also does not undergo any external refrigerant cooling prior to passing through the liquefaction heat exchanger.
In this way, the expanded lowest-temperature SMR stream provides the cooling of the post-compression SMR stream, and preferably, the expanded lowest-temperature SMR stream provides all the sub-ambient refrigerant cooling duty for cooling the BOG stream and in the SMR recirculating system.
According to another aspect of the present invention, there is provided an SMR recirculating system for use with a method of cooling a boil-off gas (BOG) stream from a liquefied gas tank using a single mixed refrigerant (SMR) comprising at least the step of heat exchanging the BOG stream with the SMR in a liquefaction heat exchanger system to provide a cooled BOG stream,
(a) compressing the SMR using at least one centrifugal compressor to provide a post-compression SMR stream;
(b) passing the post-compression SMR stream into the liquefaction heat exchanger system to cool the post-compression SMR stream and provide a cooled first SMR vapour stream;
(c) withdrawing the cooled first SMR vapour stream from the liquefaction heat exchanger system;
(d) separating the cooled first SMR vapour stream to provide a liquid-phase SMR stream and a light SMR vapour stream;
(e) passing the light SMR vapour stream through the liquefaction heat exchanger system to provide a condensed SMR stream; and
(f) expanding the condensed SMR stream to provide an expanded lowest-temperature SMR stream to pass through the liquefaction heat exchanger system for heat exchange against the BOG stream.
Optionally, the SMR recirculating system is for use in cooling BOG from a liquefied cargo tank in a floating vessel, optionally an LNG cargo tank.
Optionally, the SMR recirculating system is for use with a liquefaction heat exchanger system as defined herein.
Optionally, the SMR recirculating system further comprises one or more further steps as herein described in relation to the method of cooling a BOG stream.
It is intended that the SMR recirculating system of the present invention is able to provide all the sub-ambient refrigerant cooling duty for cooling a boil-off gas stream from a liquefied gas tank and in the SMR recirculating system.
According to another aspect of the present invention, there is provided an apparatus for cooling a boil-off gas (BOG) stream from a liquefied gas tank comprising a single mixed refrigerant (SMR) recirculating system as defined herein and a liquefaction heat exchanger for heat exchange against the BOG stream.
According to a further aspect of the invention, there is provided a method of integratively designing a vessel having a method of cooling a boil-off gas (BOG) stream from a liquefied gas tank using a single mixed refrigerant (SMR) comprising at least the step of heat exchanging the BOG stream with the SMR in a liquefaction heat exchanger system to provide a cooled BOG stream, wherein the SMR is provided in an SMR recirculating system comprising at least the steps of:
(a) compressing the SMR using at least one centrifugal compressor to provide a post-compression SMR stream;
(b) passing the post-compression SMR stream into the liquefaction heat exchanger system to cool the post-compression SMR stream and provide a cooled first SMR vapour stream;
(c) withdrawing the cooled first SMR vapour stream from the liquefaction heat exchanger system;
(d) separating the cooled first SMR vapour stream to provide a liquid-phase SMR stream and a light SMR vapour stream;
(e) passing the light SMR vapour stream through the liquefaction heat exchanger system to provide a condensed SMR stream; and
(f) expanding the condensed SMR stream to provide an expanded lowest-temperature SMR stream to pass through the liquefaction heat exchanger system for heat exchange against the BOG stream.
According to a further aspect of the invention, there is provided a method of integratively designing an SMR recirculating system for use with a method of cooling a boil-off gas (BOG) stream from a liquefied gas tank comprising the same or similar steps as described herein.
According to a still further aspect of the invention, there is provided a method of designing a process for cooling a boil-off gas (BOG) stream from a liquefied gas tank using a single mixed refrigerant (SMR) comprising the same or similar steps as described herein.
According to a still further aspect of the invention, there is provided a method of designing an SMR recirculating system for use with a method of cooling a boil-off gas (BOG) stream from a liquefied gas tank comprising the same or similar steps as described herein.
The designing methods as discussed herein may incorporate computer aided processes for incorporating the relevant operational equipment and controls into the overall vessel construction and may incorporate relevant cost, capacity of operation parameters into the methodology and design. The methods described herein may be encoded onto media that is suitable for being read and processed on a computer. For example, code to carry out the methods described herein may be encoded onto a magnetic or optical media which can be read by and copied to a personal or mainframe computer. The methods may then be carried out by a design engineer using such a personal or mainframe computer.
Embodiments and an example of the present invention will now be described by way of example only and with reference to the accompanying schematic drawings in which:
Where relevant, the same reference numerals are used in different Figures to represent the same or similar feature.
In more detail,
The liquefaction heat exchanger system 40 may comprise any form or arrangement of one or more heat exchangers able to allow heat exchange between two or more streams, optionally between multiple streams, and optionally having at least one stream running countercurrently to one or more other streams in a part or portion of the system, in particular between the BOG stream and one of the refrigerant. Any arrangement of more than one heat exchanger may be in series or in parallel or a combination of in series and in parallel, and the heat exchangers may be separate or conjoined or contiguous, optionally in a single cooled unit or box, and optionally in the form of one or more stages of providing the required heat exchange with the BOG stream to liquefy the BOG stream.
Liquefaction heat exchanger systems comprising more than one heat exchanger generally have one section, unit or stage being ‘warmer’ than another section, unit or stage, in the sense of the average temperature therein.
Some variants of suitable liquefaction heat exchanger systems are discussed and shown hereinafter. The skilled person can recognise other variants, and the invention is not limited thereby.
In the general liquefaction heat exchanger system 40 shown in
In the SMR system 200, an initial stream of SMR refrigerant gas 74 from a refrigerant receiver 51 is sent to a centrifugal compressor 52. Centrifugal compressors are well known in the art, and not further described herein. Centrifugal compressors are well proven in industry and are cost effective, especially for larger scale or larger volume compression.
In
The post-compression SMR stream 79 is cooled in a second ambient heat exchanger 56 using a readily available cooling medium (e.g. seawater, freshwater, engine room cooling water, air) to provide a cooler first vapour stream 80. Depending on the composition and pressure of the refrigerant, as well as on the temperature achieved in the second ambient heat exchanger 56, some condensation of the SMR may start to occur.
The cooler first vapour stream 80 passes into the liquefaction heat exchanger system 40, where the refrigerant is cooled and at least partially condensed. The cooled first SMR vapour stream 81 is withdrawn from an intermediate temperature along the liquefaction heat exchanger system 40, and enters a vapour-liquid separator 58. In the separator 58, a liquid-phase SMR stream 82 can be drained via pipeline 82.
Thereafter, the pressure of the liquid-phase SMR stream 82 can be reduced by a flash valve 59, resulting in some vaporisation and an associated reduction in temperature. The expanded, or at least partly vaporised, liquid-phase SMR stream 83 can be sent into the heat exchanger system 40, where it provides some cooling to warmer streams, while itself being vaporised.
In the separator 58, a light SMR vapour stream 84 is also sent into the heat exchanger system 40. In
The expanded lowest-temperature SMR stream 86 is sent back into heat exchanger system 40, where it vaporises as it heats up, and in doing so, cools the warmer streams in the heat exchanger system 40 to provide the majority of the cooling duty. The SMR refrigerant stream 86 can merge with the expanded liquid-phase SMR stream 83 to form a single stream which leaves the heat exchanger system 40 as a post-cooling vapour stream 89, to be returned to refrigerant receiver 51.
In this way, the requirement in prior art arrangement in
The first and second heat exchange units 64 and 62 may be contiguous or separate.
In
Meanwhile, the cooled and compressed BOG stream 71 passes into a third two-stream heat exchanger 98 to provide a cooler BOG stream 72 to pass into the second cooler unit 62.
The second unit 62 in
Where the liquefaction heat exchanger system comprises multiple heat exchanger units, the present invention is not limited by the relative positioning of the first and second units, which may be contiguous or separate.
It is possible that the composition and/or ratio of components in the SMR can be varied to achieve best effect for each arrangement of the present invention. It is also possible that the SMR composition is different in each of the examples shown in
The present invention is a modification of a typical single mixed refrigerant (SMR) cycle for LNG re-liquefaction in particular, that allows the use of a centrifugal compressor in the mixed refrigerant system, without the requirement of an external refrigerant cascade. In comparison with the typical arrangement, the present innovation allows for reduced complexity, fewer pieces of equipment, reduced capital cost, and is suitable for applications requiring larger reliquefaction capacities.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1901942.3 | Feb 2019 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/GB2020/050263 | 2/5/2020 | WO | 00 |
