EP3183489B1

EP3183489B1 - Method of cooling boil off gas and an apparatus therefor

Info

Publication number: EP3183489B1
Application number: EP15756225.7A
Authority: EP
Inventors: Martin HALCROW
Original assignee: Babcock IP Management Number One Ltd
Current assignee: Babcock IP Management Number One Ltd
Priority date: 2014-08-21
Filing date: 2015-08-21
Publication date: 2019-12-04
Anticipated expiration: 2035-08-21
Also published as: CN107208841A; JP2017525910A; KR102379711B1; GB201414893D0; EP3183489A1; KR20170043637A; CN107208841B; WO2016027098A1; JP6553714B2

Description

The present invention relates to a method for the cooling, particularly the re-liquefaction, of a boil off gas (BOG) from a liquefied ethane cargo on a floating transportation vessel, and an apparatus therefor.
Floating transportation vessels, such as liquefied gas carriers and barges, are capable of transporting a variety of cargoes in the liquefied state. In the present context, a liquefied cargo is wholly or substantially ethane, generally being >90% ethane, or > 95%, or >96%, or >97% or >98% or > 99% ethane. Ethane is a useful product source for various industrial processes.
Ethane can be extracted from natural gas production, fracking, or produced in the refining of crude oil. As a consequence, ethane may be associated with a plurality of other components, in particular methane. It is often desirable to liquefy ethane in a liquefaction facility at or near its source, as it can be stored and transported over long distances (generally in excess of normal pipeline distances) more readily as a liquid than in gaseous form because it occupies a smaller volume and may not need to be stored at high pressures.
The long distance transportation of a liquefied ethane cargo having a boiling point of about -87 °C when measured at 1 atmosphere may be carried out in a suitable liquefied gas carrier, such as an ocean-going tanker having one or more storage tanks to hold the liquefied ethane cargo. These storage tanks may be insulated and/or pressurized tanks. During the loading of the tanks and the storage of liquefied ethane cargo, gas may be produced due to the evaporation of the cargo. This evaporated cargo gas is known as boil off gas (BOG). In order to prevent the build up of BOG in the tank (with consequent pressure build up problems), a system may be provided on the carrier to re-liquefy the BOG so that it can be returned to the storage tank in a condensed state. This can be achieved by the compression and cooling of the BOG against a cold source. Ethane has a critical temperature of 32.18 °C at a pressure of 47.7 barg, such that seawater at a similar temperature would be unsuitable as the primary cold source. In many systems, the compressed BOG is cooled and condensed against a secondary refrigerant.
Where the usual liquefied cargoes can be defined as 'pure', then there are known methods and apparatus for re-liquefying the BOG. However, liquefied ethane to be transported as a cargo in a floating transportation vessel can, and increasingly may, comprise concentrations of other components beyond a de minimus level. This is at least partly due to the increasing sourcing of 'non- pure' ethane from new sources or new industrial processes.
One of the possible other components is propane. However, because propane has a boiling point of about -40°C when measured at 1 atmosphere, the method and apparatus required to re-liquefy an ethane/propane BOG will inherently achieve re-liquefying of any propane portion of the BOG.
One other possible component is nitrogen. As its boiling point is about - 196°C when measured at 1 atmosphere, it is commonly not practical to attempt to re-liquefy any nitrogen in the BOG on a floating transportation vessel. Thus, nitrogen is generally considered to be at least the major component of those parts of the BOG defined as "in-condensable", i.e. it can never (practically) be condensed on a floating transportation vessel. However, nitrogen is a relatively 'safe' gas.
The major other possible component of concern in liquefied ethane cargoes is methane. Methane has a boiling point of about -162°C to -163°C when measured at 1 atmosphere. This boiling point is very significantly below the boiling point of ethane when measured at 1 atmosphere. As such, methane has hitherto typically been considered as a "non-condensable" component of liquefied cargoes, i.e. it can possibly be condensed (i.e. re-liquefied), but particularly special methods are required which may not be CAPEX and/or OPEX justifiable on a floating transportation vessel. Hence, the relative small amounts of methane in liquefied cargos such as those being wholly or substantially propane (i.e. LPG) or similar, have up to now been vented to atmosphere, as conventional LPG BOG re-liquefying methods and apparatus are not able to re-liquefy methane.
However, methane is considered to be one of the 'greenhouse gasses', such that it is increasingly preferred not to vent it to atmosphere.
Moreover, the types of ethane now expected to be increasingly transported as liquefied ethane is expected to have an increased amount of methane concentration in the cargo.
Furthermore, it is a particular disadvantage of methane that even a small concentration of methane in the liquefied cargo will lead to a disproportionate amount of methane in the BOG. For example, a concentration of only 0.5 mol% in the liquid phase could lead to the BOG of a liquefied ethane cargo being 25 mol% methane.
Thus, it may not be possible to re-liquefy all the components of the boil off gas from liquefied ethane cargoes, particularly those comprising lighter components, such as methane, present in concentrations above 0.1% mol%. Such non-condensable components may be returned to the liquefied ethane cargo storage tanks in the gaseous phase, but this will build up in the boil off gas in a closed system thereby increasing in concentration over time. Moreover, as the concentration of non-condensable components in the boil off gas increases, the volume of boil off gas which cannot be re-condensed will increase, reducing the effective capacity of the re-liquefaction system.
As mentioned above, the other alternative of venting of non-condensable components such as methane, which may be a greenhouse gas, is both environmentally and commercially undesirable.
US3919852 discloses a method of reliquefying boil-off gas of LNG, which requires full compression and full expansion of the boil-off gas to provide recovery of LNG back to its tank.
WO2012/143699A relates to a method and apparatus for re-liquefying a BOG stream from a liquefied cargo in a floating transportation vessel, said liquefied cargo having a boiling point of greater than -110°C at 1 atmosphere, wherein a cooled vent stream which may comprise non-condensed BOG components is heat exchanged with a portion of the compressed, cooled and then expanded BOG stream. This is particularly suitable for liquefied cargos having boiling points of greater than -110°C when measured at 1 atmosphere, but a need exists to provide an improved method of cooling, particularly re-liquefying as far as possible under reasonable OPEX and CAPEX, boil off gas from a liquefied ethane cargo, especially such cargoes comprising an increasing proportion of lighter components such as methane.
The present invention addresses these problems by triple cooling and using a compressed BOG stream. In this way, the triple cooled stream will condense previously non-condensed components, may be re-liquefied and subsequently returned to the liquefied ethane cargo tank in the liquid phase. The triple cooled compressed BOG stream provides a source of increased cooling duty compared to heat exchange media such as seawater, allowing the re-liquefaction of lighter components in the BOG stream.
Thus, for a given number of stages of compression, the method and apparatus disclosed herein allows liquefied ethane cargoes to be transported having an increased content of lighter components such as methane, without the need to add additional stages of compression or increase the venting of previously considered non-condensable components. Viewed in another way, the method and apparatus described herein allow the extension of a compression system having a given number of stages of compression to cargoes having components which could not normally be re-liquefied or condensed.
In a first aspect, the present invention provides a method of cooling a boil off gas stream from a liquefied ethane cargo in a floating transportation vessel, said method comprising at least the steps of:

compressing a boil off gas stream from said liquefied ethane cargo in two or more stages of compression comprising at least a first stage and a final stage to provide a compressed BOG discharge stream, wherein said first stage of compression has a first stage discharge pressure and said final stage of compression has a final stage suction pressure and one or more intermediate, optionally cooled, compressed BOG streams are provided between consecutive stages of compression;
cooling the compressed BOG discharge stream against one or more first coolant streams to provide a first cooled compressed BOG stream;
cooling the first cooled compressed BOG stream against at least one second coolant stream to provide a second cooled compressed BOG stream;
cooling the second cooled compressed BOG stream against a third coolant stream to provide a third cooled compressed BOG stream;
expanding a portion of the third cooled compressed BOG stream to a pressure between that of the first stage discharge pressure and the final stage suction pressure to provide a first expanded cooled BOG stream;
using the first expanded cooled BOG stream as the third coolant stream to provide a first expanded heated BOG stream; and
using the first expanded heated BOG stream as the or a second coolant stream.

That is, the first expanded cooled BOG stream is used as the third coolant stream in a heat exchange/exchanger against the second cooled compressed BOG stream, which heat exchange/exchanger provides a third cooled compressed BOG stream and a first expanded heated BOG stream as a heated third coolant stream, which can either indirectly, more preferably directly, be used as the primary or secondary second coolant stream.
That is, the first expanded heated BOG stream is used as the primary or secondary second coolant stream in heat exchange/exchanger against the first cooled compressed BOG stream, which heat exchange/exchanger provides a second cooled compressed BOG stream and a first expanded further heated BOG stream as a heated second coolant stream.
The terms "first", "second", "third", "fourth", etc. as used herein are intended to indicate a connection or relationship, which may or may not be a direct sequence except where explicitly stated. That is, there may be one or more other steps or processes or locations between a "second" and "third" feature. The terms are used to clarify a different nature or presence of an associated feature in or of a stream, and the present invention is not limited by these terms.
For the avoidance of doubt, the second coolant stream (i.e. first expanded heated BOG stream) is at a lower temperature than the first cooled compressed BOG stream; the third coolant stream (i.e. first expanded cooled BOG stream) is at a lower temperature than the second cooled compressed BOG stream; and the third coolant stream is at a lower temperature than the second coolant stream.
According to another embodiment, the method further comprises:
combining the first expanded heated BOG stream, as a heated second coolant stream, with an intermediate compressed BOG stream, such as a first or a second intermediate compressed BOG stream, preferably with a first intermediate compressed BOG stream.
According to another embodiment of the present invention, the step of cooling the compressed BOG discharge stream against one or more first coolant streams to provide a first cooled compressed BOG stream may comprise:
cooling the compressed BOG discharge stream against a first refrigerant stream as a first coolant stream to provide a first cooled compressed BOG stream.
That is, a first refrigerant stream is used as one of the one or more first coolant streams in a heat exchange/exchanger against the compressed BOG discharge stream, which heat exchange/exchanger provides a first cooled compressed BOG stream and a heated first refrigerant stream as a heated first coolant stream.
According to another embodiment of the present invention, the step of cooling the compressed BOG discharge stream against one or more first coolant streams to provide a first cooled compressed BOG stream may comprise:

pre-cooling the compressed BOG discharge stream against a pre-cooling coolant stream as a first coolant stream to provide a pre-cooled compressed BOG stream;
cooling the pre-cooled compressed BOG stream against a first refrigerant stream as a first coolant stream to provide a first cooled compressed BOG stream.

That is, a pre-cooling coolant stream is used as one of the one or more first coolant streams in a heat exchange/exchanger against the compressed BOG discharge stream, which heat exchange/exchanger provides a pre-cooled compressed BOG stream and a heated pre-cooling coolant stream as a heated first coolant stream.
That is, a first refrigerant stream is used as one of the one or more first coolant streams in a heat exchange/exchanger against the pre-cooled compressed BOG stream, which heat exchange/exchanger provides a first cooled compressed BOG stream and a heated first refrigerant stream as a heated first coolant stream.
According to another embodiment of the present invention, the pre-cooling coolant stream may be part of an open pre-cooling coolant system or a closed pre-cooling coolant system. The pre-cooling coolant stream may be selected from a water stream, an air stream or a pre-cooling refrigerant stream, with a water or air stream being preferred. Typically if an open pre-cooling coolant circuit is used, the pre-cooling coolant stream may be selected from a seawater stream and an ambient air stream. Typically, if a closed pre-cooling coolant circuit is used, the pre-cooling coolant stream may be selected from a pre-cooling refrigerant stream.
According to another embodiment of the present invention, the cooling of the pre-cooled compressed discharge stream against the pre-cooling coolant stream is carried out in a pre-cooling heat exchanger such as a shell and tube heat exchanger or a plate heat exchanger.
According to another embodiment of the present invention, the one or more first coolant streams comprise a first refrigerant stream, such as a first refrigerant comprising a single refrigerant or mixture of refrigerants. The first refrigerant should be capable of condensing ethane (i) at the discharge pressure of the compression system and the discharge temperature of the compression system or (ii) at the discharge pressure of the compression system and the temperature of the pre-cooled compressed BOG stream. The first refrigerant may comprise one or more organic compounds, ammonia, and particularly hydrocarbons and fluorinated hydrocarbons such as propane, propylene, difluoromethane and pentafluoromethane, including the fluorinated hydrocarbon mixture R-410A.
According to another embodiment of the present invention, the cooling of the compressed BOG discharge stream or the pre-cooled compressed discharge stream against the first refrigerant stream is carried out in a discharge heat exchanger such as a shell and tube heat exchanger, a plate heat exchanger or an economiser.
According to another embodiment of the present invention all the compressed BOG discharge stream is cooled against the one or more first coolant streams.
In one embodiment of the present invention, the liquefied ethane cargo comprises >0.1 mol% methane. Indeed, the liquefied ethane cargo may comprise > 0.4 mol% methane, including >0.5 mol%, 0.6 mol%, >0.7 mol%, >0.8 mol%, >0.9 mol% and >1.0 mol% methane. The present invention extends to a liquefied ethane cargo having 1-5 mol% methane, optionally >5 mol% methane.
The number of stages of compression is not a limiting factor of the present invention. Optionally, the method comprises three or four stages of compression
Optionally, it is desired to provide a fully condensed boil off gas as the first cooled compressed BOG stream, but the present invention extends to a method wherein the boil off gas is not fully condensed after cooling against the one or more first coolant streams.
The present invention overcomes the difficulty of using certain types of heat exchange, in particular certain types of heat exchanger, and more particularly conventional shell & coil economisers, where the temperature approach is limited by the composition of the fluid in the shell. Where the composition of the fluid in the shell may be a single component, i.e. a sufficiently 'pure' gas, its cooling against an expanded portion of the compressed BOG is well known and extensive. However, this cooling duty is reduced in a multi-component mixture, and is dramatically reduced in a multi-component mixture having a significant difference in boiling points , such as in particular ethane and methane. Thus, the present invention improves the coefficient of performance of the cooling cycle of a liquefied ethane cargo comprising a significant methane amount, i.e. the present invention improves the coefficient of performance of cargo currently considered de minimus (e.g. 0.1 mol% or less methane), and allows operation with cargoes comprising much higher methane contents (e.g. about or above 0.4 or 0.5 mol% methane.
The present invention also seeks to maintain the use of current onboard equipment and apparatus with its known OPEX and CAPEX, rather than seeking to introduce and work out how to use new equipment with new operating requirements.
Thus, according to another embodiment of the present invention, the cooling of the first cooled compressed BOG stream against the second coolant stream is carried out in an economiser.
According to another embodiment of the present invention all the first cooled compressed BOG stream is cooled against the second coolant stream.
According to another embodiment of the present invention all the second cooled compressed BOG stream is cooled against the first expanded cooled BOG stream.
In another embodiment of the present invention, the method further comprises the steps of:

providing a gaseous vent stream from the first cooled compressed BOG stream;
expanding a portion of the third cooled compressed BOG stream to form a fourth coolant stream:
cooling the gaseous vent stream against the fourth coolant stream to provide a cooled vent stream and a heated fourth coolant stream.

In this way, the present invention can further provide increased re-liquefying of previously considered 'non-condensables' or' non-condensing' components in the compressed BOG.
Preferably, the heated fourth coolant stream is or can be used as, a BOG recycle stream. Thus, the method may further comprise:
combining the heated fourth coolant stream with an intermediate compressed BOG stream, such as a first or second, preferably first intermediate compressed BOG stream.
Optionally, the method of the present invention comprising the further step of:
separating the cooled vent stream to provide a vent discharge stream and a cooled vent BOG return stream.
Optionally, the method of the present invention comprising the further steps of:

expanding the cooled vent BOG return stream to provide an expanded cooled vent BOG return stream;
passing the expanded cooled vent BOG return stream to a storage tank.

Optionally, the method comprises the further steps of:

expanding the cooled vent BOG return stream to provide an expanded cooled vent BOG return stream;
heat exchanging the expanded cooled vent BOG return stream against the vent discharge stream to provide a heat exchanged vent BOG return stream, a cooled vent discharge stream and a further vent discharge stream;
expanding the cooled vent discharge stream to provide an expanded cooled vent discharge stream;
passing the heat exchanged vent BOG return stream and the expanded cooled vent discharge stream to a storage tank.

Optionally, the stages of compression are the compression stages of a multi-stage compressor.
The first cooled compressed BOG stream is cooled against at least one second coolant stream to provide a second cooled compressed BOG stream. Optionally, the first cooled compressed BOG stream is wholly or substantially cooled against a second coolant stream only comprising the first expanded heated BOG stream. Preferably, all of the second coolant stream comprises the first expanded heated BOG stream. That is, first cooled compressed BOG stream may be cooled against one or more other second coolant streams, but these are secondary or minor compared to the cooling provided by the use of the first expanded heated BOG stream.
Optionally, the first expanded heated BOG stream used as the second coolant stream comprises both liquid and gas phases. That is, it does not need to be separated into separate gas and liquid phases prior to use as a second coolant stream.
Preferably, the liquid and gas phases of the first expanded heated BOG stream used as the second coolant stream are separated in the cooling of the first cooled compressed BOG stream. This is preferably by the apparatus allowing the first cooled compressed BOG stream to be cooled, preferably an economiser.
According to a second aspect of the present invention, there is provided an apparatus to cool a boil off gas stream from a liquefied ethane cargo in a floating transportation vessel comprising a plurality of components, said apparatus comprising at least:

a compression system to compress a boil off gas stream from a liquefied ethane cargo, said compression system comprising two or more stages of compression comprising at least a first stage and a final stage to provide a compressed BOG discharge stream, wherein intermediate, optionally cooled, compressed BOG streams are provided between consecutive stages of compression;
one or more first heat exchangers to cool the compressed BOG discharge stream to provide a first cooled compressed BOG stream;
one or more second heat exchangers to further cool the first cooled compressed BOG stream against a mixed phase coolant stream to be separated in the one or more second heat exchangers, t o provide a second cooled compressed BOG stream;
one or more third heat exchangers to further cool the second cooled compressed BOG stream to provide a third cooled compressed BOG stream and a pressure reduction device to expand a portion of the third cooled compressed BOG stream to a pressure between that of the first stage discharge pressure and the final stage suction pressure to provide a first expanded cooled BOG stream as the coolant in the one or more third heat exchangers and then the mixed phase coolant stream in the one or more second heat exchangers.

Optionally, the apparatus as defined herein is operable using the method as defined herein.
Preferably, the second heat exchanger is an economiser.
According to a further aspect of the present invention, there is provided a floating transportation vessel for a liquefied ethane cargo having the apparatus as defined herein or operating the method as defined herein.
The present invention is applicable to any floating transportation vessel for a liquefied ethane cargo. The present invention may be utilized in floating transportation vessels where the liquefied ethane cargo storage tanks are fully refrigerated to maintain the cargo in liquid phase at approximately atmospheric pressure by lowering the temperature, as well as in those vessels in which the cargo in the storage tanks is maintained in the liquid phase by a combination of reduced temperature and increased pressure versus ambient.
In order to obtain the benefits of the method and apparatus disclosed herein, the use of economizers is not required. However, in certain embodiments, heat exchangers such as economizers can be placed between consecutive stages of compression, such as between the first and second stages, to cool the intermediate compressed BOG streams. Where three or more stages of compression are present, heat exchangers, such as economizers or intercoolers, such as seawater intercoolers, to allow the cooling of an intermediate compressed BOG streams may be provided between the second and final stages of compression.
For instance, an intercooler can be situated between the second and third stages of compression. Alternatively, an economizer can be situated between the second and third, as well as between the first and second stages of compression. In an economizer, an expanded, optionally further cooled, portion of the cooled compressed BOG stream can be heat exchanged with an intermediate compressed BOG stream. In a further embodiment, an expanded, optionally further cooled, portion of the cooled compressed BOG stream can be heat exchanged with an optionally further cooled portion of the cooled compressed discharge stream. This leads to further improvements in the coefficient of performance and increased cooling, particularly re-liquefaction, capacity.
It will be apparent that the method and apparatus disclosed herein can be applied to an existing floating transportation vessel as a retro-fit, by maintaining the number of stages of compression present and adding the necessary piping, valves and controls to carry out the cooling of a second cooled compressed BOG stream against an expanded portion of the third cooled BOG stream.
As used herein, the term "multiple stages of compression" defines two or more stages of compression in series in a compression system. Each stage of compression may be achieved by one or more compressors. The one or more compressors of each compression stage may be independent from those of the other stages of compression, such that they are driven separately. Alternatively, two or more of the stages of compression may utilize compressors which are linked, typically powered by a single driver and drive shaft, with optional gearing. Such linked compression stages may be part of a multi-stage compressor.
The method and apparatus disclosed herein requires at least two stages of compression. After the first stage of compression, each subsequent stage provides an increased pressure compared to the pressure at the discharge of a previous stage. The term "consecutive stages" refers to pairs of adjacent stages of compression i.e. a stage (n) and the next (n+1) stage where 'n' is a whole number greater than 0. Consequently, consecutive stages are, for instance, first and second stages or second and third stages or third and fourth stages. Intermediate compressed streams (and cooled intermediate compressed streams) refer to those streams connecting consecutive stages of compression. The terms "next stage of compression" or "subsequent stage of compression" used in relation to the cooled intermediate compressed stream refer to the numerically higher number (and higher pressure stage) of the two consecutive stages defining the intermediate stream.
The heat exchange steps may be indirect, where the two or more streams involved in the heat exchange are separated and not in direct contact. Alternatively, the heat exchange may be direct, in which case the two or more streams involved in the heat exchange can be mixed, thereby producing a combined stream.
According to a further aspect of the present invention, there is provided a method integratively designing apparatus to cool a boil off gas stream from a liquefied ethane cargo in a floating transportation vessel comprising a plurality of components, comprising the steps of:

selecting a compression system to compress a boil off gas stream from a liquefied ethane cargo, said compression system comprising two or more stages of compression comprising at least a first stage and a final stage to provide a compressed BOG discharge stream, wherein intermediate, optionally cooled, compressed BOG streams are provided between consecutive stages of compression,
selecting one or more first heat exchangers to cool the compressed BOG discharge stream to provide a first cooled compressed BOG stream;
selecting one or more second heat exchangers to further cool the first cooled compressed BOG stream against a mixed phase coolant stream to be separated in the one or more second heat exchangers, to provide a second cooled compressed BOG stream; and
selecting one or more third heat exchangers to further cool the second cooled compressed BOG stream to provide a third cooled compressed BOG stream.

Optionally, the method further comprises the steps of:

running a process simulation for said apparatus;
determining the effectiveness of the method;
altering a process variable in said process simulation; and repeating the process simulation.

According to a further aspect of the present invention, there is provided a method of designing process for the cooling a boil off gas stream from a liquefied ethane cargo in a floating transportation vessel, said method comprising at least the steps of:

designing a compression system to compress a boil off gas stream from a liquefied ethane cargo, said compression system comprising two or more stages of compression comprising at least a first stage and a final stage to provide a compressed BOG discharge stream, wherein intermediate, optionally cooled, compressed BOG streams (are provided between consecutive stages of compression,
designing one or more first heat exchangers (to cool the compressed BOG discharge stream to provide a first cooled compressed BOG stream;
designing one or more second heat exchangers to further cool the first cooled compressed BOG stream against a mixed phase coolant stream to be separated in the one or more second heat exchangers to provide a second cooled compressed BOG stream; and
designing one or more third heat exchangers to further cool the second cooled compressed BOG stream to provide a third cooled compressed BOG stream.

Optionally the method further comprising the steps of:

running a process simulation for said process;
determining the effectiveness of the method;
altering a process variable in said process simulation; and
repeating the process simulation.

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.
Certain features of the present invention and its method of design may be described in terms of a set of numerical upper limits and a set of numerical lower limits. It should be appreciated that any ranges formed by any combination of such limits are contemplated to fall within the scope of the invention. Further the overall design is contemplated to include the selection of additional structures for use with the combination herein specifically defined. The various structures operational parameters may be selected for a limited or fixed basis or selected for flexible or multiple operational use within the vessel. Hence, it is intended that the method of design covers alternatives, modifications, and equivalents with respect to the overall design of the vessel and any off-vessel that are included within the spirit and scope of the invention.
Embodiments of the invention will now be described by way of example only, and with reference to the accompanying non-limiting drawings in which:

Figure 1 shows a schematic diagram of one possible known system of re-liquefying boil off gas from a cargo tank in a carrier;
Figure 2 shows a schematic diagram of a system of cooling, particularly re-liquefying, boil off gas from a liquefied ethane cargo in a floating transportation vessel according to one embodiment of the invention;
Figures 3a and b are economiser temperature profiles of temperature against heat flow for a pure component BOG cooling system (3a) and a wide boiling multicomponent mixture cooling system (3b); and
Figure 4 shows a schematic diagram of a system for cooling, particularly re-liquefying, boil off gas from a liquefied ethane cargo in a floating transportation vessel according to another embodiment of the invention.

Floating re-liquefaction systems draw the vapor, also known as boil off gas, from one or more storage tanks and pass the boil off gas to a compressor in which it is compressed such that the compressed vapor can be cooled and condensed against one or more coolants as the heat sink/ refrigerant. For instance, seawater may be used to pre-cool, typically de-superheat, the compressed vapour in an open cycle pre-cooling circuit. The pre-cooled compressed vapour can then be further cooled against a refrigerant in a closed cycle refrigerant circuit.
Those lighter components of the compressed vapor which cannot be condensed against the refrigerant are usually vented to the atmosphere or recycled to the storage tanks in vapor form. Typically, the liquefied cargo is kept in the storage tank under one or both of reduced temperature (versus ambient) and increased pressure (versus atmospheric).
Figure 1 shows a schematic diagram of a known system for re-liquefying boil off gas from an ethane cargo. Currently, ethane cargo tends to be transported in a repurposed ethylene carrier vessel. Liquefied ethane cargo is stored in a tank 50a which may be insulated and/or pressurized in order to maintain the ethane in a liquefied state. Vaporization of the ethane in the tank, for instance due to imperfect thermal insulation, will result in the formation of ethane gas in the overhead space of the tank 50a, and such gas is commonly termed boil off gas (BOG). In order to prevent the build-up of this gas, it is removed from the tank 50a as a boil off gas stream 01a. All the components are compressed, and as many of the components as possible of the removed boil off gas are normally cooled to condense them before it is returned to the tank 50a.
The boil off gas stream 01a can be passed to a compression system 60, such as the two stage compressor shown in Figure 1 which comprises a first compression stage 65 and a second compression stage 75. The two - stage compression system 60 produces a compressed BOG discharge stream 06a which can be passed to a pre-cooling heat exchanger 100, in which the compressed BOG discharge stream 06a is cooled against a seawater stream 102. The pre-cooling heat exchanger 100 produces a pre-cooled compressed BOG stream 07a and a warmed seawater stream 104. The pre-cooling heat exchanger 100 can de-superheat the compressed BOG discharge stream 06a.
The pre-cooled compressed BOG stream 07a can be passed to a refrigerant heat exchanger 250, in which the pre-cooled compressed BOG stream 07a is cooled against a refrigerant stream 252. The refrigerant should be capable of condensing ethane at the discharge pressure of the compression system 60. The refrigerant may be propane or propylene. The refrigerant stream 252 can be part of a refrigerant circuit (not shown) comprising the refrigerant heat exchanger 250, a refrigerant compressor and a refrigerant cooler. The refrigerant circuit may be a closed refrigerant system. Such refrigerant circuits, also called refrigerant packs, are well known.
The refrigerant heat exchanger 250 produces a cooled compressed BOG stream 08a and a heated refrigerant stream 254. The cooled compressed BOG stream 08a is an at least partially condensed stream comprising those components of the boil off gas capable, at the discharge pressure of the second stage of compression 75, of 're-liquefaction', i.e. condensation, against the refrigerant.
The 'non-condensed' components which are incapable of re-liquefaction against the refrigerant in this system, and which may comprise both non-condensable' components and 'in-condensable' components as discussed herein, may be removed from the refrigerant heat exchanger 250, or an associated accumulator (not shown) located downstream of the refrigerant heat exchanger 250 as a vent stream 49, which is a vapor stream. The vent stream 49 is typically vented to the atmosphere, after expansion to atmospheric pressure.
The cooled compressed BOG stream 08a can be passed to a further heat exchanger 80, to provide a cooled return fluid stream 18, which is typically a fully condensed stream.
The cooled return fluid stream 18 may then be passed to a return pressure reduction device 22, such as an expander or Joule-Thomson valve, to provide an expanded cooled return fluid stream 24. Typically, the return pressure reduction device 22 will reduce the pressure of the cooled return fluid stream 18 from at or near the pressure of the compressed BOG discharge stream 06a to a pressure close to that of the liquid ethane and BOG in the tank 50a, such as a pressure just above that of the BOG in the tank which is sufficient to ensure an adequate flow of the expanded cooled return fluid stream 24 to the tank 50a. The pressure of the expanded cooled return fluid stream 24 is below that of the discharge pressure of the first stage 65 of compression.
Returning to compression system 60, the first stage 65 of compression provides a first intermediate compressed BOG stream 02a, which is passed to further heat exchanger 80. The first intermediate compressed BOG stream 02a can be heat exchanged against an expanded portion 8b of the cooled compressed BOG stream 08a in the further heat exchanger 80 to provide a cooled first intermediate compressed BOG stream 03a, which can then be passed to the suction of the second stage 75 of compression. The second stage 75 compresses the cooled first intermediate compressed BOG stream 03a to provide the compressed BOG discharge stream 06a.
Turning to Figure 3a, the graph shows a typical temperature profile for the cooling of a 'pure' substance in a conventional shell & coil economiser, with the 'xxxx' line representing the shell side, and the 'oooo' line representing the tube or coil temperature. It can be seen that the shell side temperature is 'flat', so that there is no change in the shell side temperature with increased heat flow. This represents cooling a 'pure' substance such as pure ethane.
However, Figure 3b shows the temperature profile in the same economiser (and using the same line formats) for a multi-component mixture having 'wide boiling points', such as the difference in the boiling points of ethane and methane. Figure 3b shows that it is difficult to achieve a constant temperature for the tube side There is decreased efficiency is clear across all heat flows, so that for multi-component mixtures, the cooling efficiency is dictated by the heavier components, reducing the potential cooling that can be achieved in this type of equipment.
Nevertheless, it is still preferred to maintain the use of this type of equipment with its known CAPEX.
The method and apparatus disclosed herein seeks to provide an improved method and apparatus of re-liquefying BOG. An embodiment of the method and apparatus according to the present invention is disclosed in Figure 2. Where appropriate, identical stream and component names, and the same reference numerals as those in Figure 1 have been used for corresponding streams and components in the remaining Figures.
Figure 2 shows a liquefied ethane cargo storage tank 50 in a floating transportation vessel, such as an ethane carrier. The liquefied ethane cargo may comprise ethane and methane. In order to cool, particularly re-liquefy, evaporated cargo from the storage tank 50, a boil off gas stream 01, comprising evaporated cargo, is passed to a compression system 60 having two or more stages of compression. The boil off gas stream 01 may have a pressure (the "BOG pressure") in the range of from above 0 to 500 kPa gauge. The compression system 60 may be a multi-stage compressor comprising two or more stages. By "multi-stage compressor" it is meant that each compression stage in the compressor is driven by the same drive shaft. Alternatively, the compression system 60 may comprise independently driven compressors for each of the stages of compression. When the compression system 60 is a multi-stage compressor, it is typically a reciprocating compressor.
The embodiment of Figure 2 shows a compression system 60 having a first stage 65 and a second stage 70 and a third and final stage 75, although the method and apparatus described herein is also applicable to compressors having two stages or more than three stages. The first stage 65 and final stage 75 of compression provide low and high pressure streams respectively at their discharge.
The compression system 60 compresses the boil off gas stream 01 to provide a compressed BOG discharge stream 06. The compressed BOG discharge stream 06 may have a pressure (the "final stage pressure") in the range of from 1.5 to 3.2 MPa or above, eg. up to 6 MPa.
The compressed BOG discharge stream 06 is cooled in one or more first heat exchangers 200, 300 against one or more first coolant streams 202, 302 to provide first cooled compressed BOG stream 08. In the embodiment of Figure 2, the compressed BOG discharge stream 06 can be passed to a pre-cooling heat exchanger 200 as one of the one or more first heat exchangers. The compressed BOG discharge stream 06 is pre-cooled against a pre-cooling coolant stream as one of the one of more first coolant streams. The pre-cooling coolant stream 202 may be an air or a water stream, such as an ambient air or seawater stream. The pre-cooling heat exchanger 200 may be a shell and tube heat exchanger or a plate heat exchanger. The pre-cooling heat exchanger may de-superheat the compressed BOG discharge stream 06. The pre-cooling heat exchanger 200 provides a pre-cooled compressed BOG stream 07 and heated pre-cooling coolant stream 204. Typically, the seawater used as the pre-cooling coolant would have a temperature of +36 °C or below, more typically +32 °C or below.
The pre-cooling heat exchange/exchanger 200 is optional in the method and apparatus disclosed herein. It is advantageous because it reduces the cooling duty of the subsequent cooling steps. However, is it not an essential aspect, such that in an alternative embodiment, the compressed BOG discharge stream 06 can be passed directly to the discharge heat exchanger 300 via line 06', such that the equipment shown by numeral 210 may be omitted. In such circumstances, the cooling capacity of the discharge heat exchanger 300 would have to be increased to compensate for the absence of pre-cooling.
The pre-cooled compressed BOG stream 07 can then be passed to a discharge heat exchanger 300 as another of the one or more first heat exchangers. The discharge heat exchanger 300 cools the pre-cooled compressed BOG stream 07 against a first refrigerant stream 302 as another of the one or more first coolant streams. The discharge heat exchanger 300 provides a first cooled compressed BOG stream 08 and a heated first refrigerant stream 304.
The first refrigerant stream 302, discharge heat exchanger 300 and heated first refrigerant stream 304 may be part of a first refrigerant system (not shown). Such a first refrigerant system may further comprise a first refrigerant compressor to compress the heated first refrigerant stream 304 to provide a compressed first refrigerant stream, a first refrigerant cooler to cool the first refrigerant to provide a cooled compressed first refrigerant stream and a first refrigerant expansion device to expand the cooled compressed first refrigerant stream to provide the first refrigerant stream 302. The first refrigerant system may be a closed system. The first refrigerant may comprise one or more organic compounds, particularly hydrocarbons and fluorinated hydrocarbons such as propane, propylene, difluoromethane and pentafluoromethane, including the fluorinated hydrocarbon mixture R-410A, as well as one or more inorganic compounds such as ammonia.
The first cooled compressed BOG stream 08 may be a partially condensed, compressed BOG stream, comprising those components of the boil off gas which can be condensed against the first refrigerant at the discharge pressure of the final stage of compression. Any non-condensed components can be removed either from the discharge heat exchanger 300 as a vent stream (not shown) or from a discharge receiver (not shown) which functions as a gas/liquid separator located downstream of the discharge heat exchanger 300. Discharge heat exchangers suitable for the separation of gaseous and liquid components are shell and tube heat exchangers in which the cooled compressed BOG is located in the shell-side.
Any discharge receiver may be an accumulator and can operate to maintain a liquid seal in the discharge heat exchanger 300 and/ or maintain the discharge pressure at the final stage 75 of compression.
The discharge heat exchanger 300 may be of a type which could not adequately separate vapor and condensed phases into separate streams, such as a plate and fin type heat exchanger. In such a situation, the discharge receiver will be located downstream of the discharge heat exchanger 300 to separate the non-condensed components as a vent stream.
The first cooled compressed BOG stream 08 is then second cooled. This can be achieved by passing the first cooled compressed BOG stream 08 to a second heat exchanger 180. The second heat exchanger 180 may be of any type, and an intermediate stage, particularly first stage, economizer for cooling the intermediate BOG streams 02 or 04 as well as the first cooled compressed stream 08 is shown in Figure 2.
The cooling of the first cooled compressed BOG stream 08 is against a second coolant stream to provide a second cooled compressed BOG stream 34. Optionally, a portion of the first cooled compressed BOG stream 08 can be used elsewhere prior to passage into the second heat exchanger (180), but in the present invention, it is preferred that wholly or substantially all of the first cooled compressed BOG stream 08 passes into the first heat exchanger 180.
The action of the second coolant, described hereinafter, is to provide a second cooled compressed BOG stream 34. Again, a portion of this stream 34 could be used elsewhere, but preferably wholly or substantially all of the second cool compressed BOG stream 34 passes into a third heat exchanger 195 to further cool the second cooled compressed BOG stream 34 and to provide a third cooled compressed BOG stream 35.
The third heat exchanger 195 may be of any type, such as an economiser, but is preferably a countercurrent heat exchanger such as a plate and fin heat exchanger known in the art.
In the present invention, a portion of the third cooled compressed BOG stream 35 is expanded to a pressure between that of the first stage discharge pressure and the final stage suction pressure to provide a first expanded cooled BOG stream 33a. This action can be carried out through a pressure reduction device 80 such as a Joule-Thomson valve or expander in a manner known in the art.
The first expanded cooled BOG stream 33a is used as the third coolant in the third heat exchanger 195, which heat exchange provides the third cooled compressed BOG stream 35, and a first expanded heated BOG stream 33b as heated third coolant stream 33b, which can either indirectly, or more preferably directly, be used as the second coolant stream 33b. The first expanded heated BOG stream/second coolant stream 33b is not separated (to separate gas/liquid phases) prior to use as the second coolant stream 33b, to fully utilise all of the remaining cooling effect of the first expanded heated BOG stream after use in the third heat exchanger 195.
The first expanded heated BOG stream/second coolant stream 33b is passed into the second heat exchanger 180, such that the heat exchange with the first cooled compressed BOG stream 08 provides the second cooled compressed BOG stream 34 and a heated second coolant in the second heat exchanger 180. The heated second coolant may comprise vapour and liquid components, which are conveniently separated in the second heat exchanger 180, and which is discussed hereinafter. The heated second coolant stream, which is a first expanded further heated BOG stream, may be passed to an intermediate compressed BOG stream of the appropriate pressure. In the embodiment of Figure 2, the heated second coolant stream is combined with the first intermediate compressed BOG stream 02.
The portion of the third cooled compressed BOG stream 35 which is not used to provide the first expanded cooled BOG stream 33a can be returned as a return stream to the cargo tank 50 via a pressure reduction device 82 as expanded cooled BOG return stream 36 in a manner known in the art.
It is a particular feature of the present invention that no CAPEX change is required in the nature of the first heat exchangers 200, 300 and second heat exchanger 180, such that the operator can continue to use a 'conventional' shell and tube economiser as the second heat exchanger 180, and that the present invention can be achieved simply with the addition of the third heat exchanger 195. This allows the overall BOG re-liquefying system to be controlled by existing level controllers in at least the second heat exchanger 180, avoiding potential issues with temperature control that might arise with the use of different BOG compositions and different inter stage pressures.
Indeed, an improvement of 10-15% in the refrigeration capacity of a BOG re-liquefying method and apparatus for a liquefied cargo is possible for ethane cargoes containing methane (in the liquid phase) above a de minimus level, and even above 0.4 or 0.5 mol% methane. Such methane-containing liquefied ethane cargos may be increasingly common where new or other sources of ethane are being provided, but the desire to purify the ethane (by reducing or eliminating any methane content) prior to transportation is not cost effective, or in some cases, not locally possible. Figure 4 shows a further embodiment of the method and apparatus of the present invention. In common with Figure 2, Figure 4 shows a liquefied ethane cargo storage tank 50 from which a boil off gas stream 01, comprising evaporated cargo, is passed to a compression system 60, having three stages of compression being a first stage 65, a second and intermediate stage 70 and a third and final stage 75. The first stage 65 provides a first intermediate compressed BOG stream 02 which passes into the second heat exchanger 180 to provide a cooled first intermediate BOG stream 03 which passes into the intermediate compression stage 70, to provide a second intermediate compressed BOG stream 04 which passes into the suction of the final stage 75 of compression.
The compression system 60 provides a compressed BOG discharge stream 06 which can be passed into a pre-cooling heat exchanger 200 as one of the one or more first heat exchangers to be cooled against one first coolant being seawater in a seawater stream 202 in a manner previously described, to provide a pre-cooled compressed BOG stream 07.
The pre-cooled compressed BOG stream 07 can then be passed to a discharge heat exchanger 300 as another of the one or more first heat exchangers in a manner previously described. The discharge heat exchanger 300 provides a first cooled compressed BOG stream 08 and a heated first refrigerant stream 304.
The first cooled compressed BOG stream 08 can be provided either directly, or optionally after passage through a discharge receiver 305 as shown in Figure 4.
Where the cooled compressed BOG stream 08 is not fully condensed, there is a gaseous vent stream 51 also provided, either from the discharge heat exchanger 300 as stream 51a, and/or from the discharge receiver 305 as stream 51b. Whilst Figure 4 shows the two streams 51a, 51b as separate, such streams may be provided separately or combined or without any distinction, depending upon the nature and construction of the discharge heat exchanger 300 and the discharge receiver 305. The provision of these stream or streams is known in the art.
The gaseous vent stream 51 may comprise both 'non-condensable' components and 'in-condensable' components. The in-condensable components are generally considered to be components which cannot practically ever by compressed and condensed within the confines and operating parameters of a particular floating transportation vessel BOG cooling system, and primarily relate to nitrogen.
Conventionally, it is considered that the major non-condensable component is methane, whose boiling point at 1 atmosphere is significantly below the boiling point of ethane, and whose condensing was therefore considered generally not practical again within the confines and operating parameters of a floating transportation vessel.
In WO2012/143699A , there is shown a method and apparatus for increasing the amount or proportion of condensing of the gaseous vent stream in order to increase the recovery thereof.
In the present invention, the method and apparatus may further comprise, as shown by way of example in Figure 4, the steps of expanding a portion of the third cooled compressed BOG stream 35 to form a fourth coolant stream 33c, generally by passage of a portion of the third cooled compressed BOG stream 35 through a pressure reduction valve 87 in an amount which allows that portion of the third cooled compressed BOG stream 35 to act as a fourth coolant 33c in a fourth heat exchanger 197, such as a vent heat exchanger.
The fourth heat exchanger 197 may be of any type, but is preferably a countercurrent heat exchanger such as a plate and fin arrangement. As shown in Figure 4, the gaseous vent stream 51 can be cooled against the fourth coolant stream 33c to provide a cooled vent stream 53 and a heated fourth coolant stream 38.
Optionally, the heated fourth coolant stream 38 is a BOG recycle stream which can pass into the second heat exchanger 180 such that vapour therefrom can be used as part of the cooled first intermediate BOG stream 03.
The cooling of the gaseous vent stream 51 in the vent heat exchanger 197 can condense a portion of the components of the boil off gas which could not be condensed in the discharge heat exchanger 300 against the first refrigerant such as propane or propylene. The cooled vent stream 53 is typically an at least partly condensed stream.
In one embodiment, the cooled vent stream 53 can be passed to a vent stream pressure reduction device 61 (dashed line), such as a Joule-Thomson valve or expander, where its pressure is reduced to provide an expanded further cooled vent stream 63 (dashed line). The expanded further cooled vent stream 63 may have a pressure at or slightly above the pressure of the liquefied ethane cargo storage tank 50, so that it can be returned to the tank, for instance by addition to expanded cooled BOG return stream 36 to provide combined expanded cooled BOG return stream 11.
In another embodiment shown in Figure 4, the cooled vent stream 53 can be passed to a vent stream separator 150, such as a gas/ liquid separator. The vent stream separator 150 provides a vent discharge stream 55 being wholly or substantially the in-condensable components, which is typically a vapour stream, and a cooled vent BOG return stream 57, which is typically a condensed stream, comprising those components of the boil off gas which were condensed in the fourth heat exchanger 197. The pressure of the vent discharge stream 55 may be reduced, for instance to a pressure appropriate for return to the storage tank 50, for storage elsewhere or for venting.
The cooled vent BOG return stream 57 may be passed through a vent return stream pressure reduction device 58, such as a Joule-Thomson valve or expander, to provide an expanded cooled vent BOG return stream 59. The expanded cooled vent BOG return stream 59 can be passed to the storage tank 50, for instance by addition to the expanded cooled BOG return stream 36.
That portion of the third cooled compressed BOG stream 35 that is not passed to the pressure reduction devices 80 and 87 to provide the third and fourth coolant streams 33a, 33c, provides a BOG return stream 10, which may be expanded by a pressure reduction valve 82 to at or near the pressure of the storage tank 50 as expanded cooled BOG return stream 36. This can then be returned to the storage tank 50.
The person skilled in the art will understand that the invention can be carried out in many various ways without departing from the scope of the appended claims. For instance, the invention encompasses the combination of one or more of the optional or preferred features disclosed herein.

Claims

A method of cooling a boil off gas stream (01) from a liquefied ethane cargo in a floating transportation vessel, said method comprising at least the steps of:
compressing a boil off gas stream (01) from said liquefied ethane cargo in two or more stages of compression comprising at least a first stage (65) and a final stage (75) to provide a compressed BOG discharge stream (06), wherein said first stage (65) of compression has a first stage discharge pressure and said final stage (75) of compression has a final stage suction pressure, and one or more intermediate, optionally cooled, compressed BOG streams (02, 03, 04) are provided between consecutive stages of compression;

cooling the compressed BOG discharge stream (06) against one or more first coolant streams (202, 302) to provide a first cooled compressed BOG stream (08);

cooling the first cooled compressed BOG stream (08) against at least one second coolant stream to provide a second cooled compressed BOG stream (34); cooling the second cooled compressed BOG stream (34) against a third coolant stream to provide a third cooled compressed BOG stream (35);

characterised in that the method further comprises at least the steps of;

expanding a portion of the third cooled compressed BOG stream (35) to a pressure between that of the first stage discharge pressure and the final stage suction pressure to provide a first expanded cooled BOG stream (33a);

using the first expanded cooled BOG stream (33a) as the third coolant stream to provide a first expanded heated BOG stream (33b); and

using the first expanded heated BOG stream (33b) as the or a second coolant stream.
The method of claim 1 wherein the liquefied ethane cargo comprises >0.1 mol% methane or >0.5 mol% methane.
The method according to any one of the preceding claims comprising three or four stages of compression.
The method according to any one of the preceding claims wherein the cooling of the first cooled compressed BOG stream (08) against the second coolant stream is carried out in an economiser (180).
The method according to any one of the preceding claims wherein all the first cooled compressed BOG stream (08) is cooled against the second coolant stream.
The method according to any one of the preceding claims wherein all the second cooled compressed BOG stream (34) is cooled against the first expanded cooled BOG stream (33a).
The method according to any one of the preceding claims further comprising the steps of:
providing a gaseous vent stream (51) from the first cooled compressed BOG stream (08);

expanding a portion of the third cooled compressed BOG stream (35) to form a fourth coolant stream (33c); and

cooling the gaseous vent stream (51) against the fourth coolant stream (33c) to provide a cooled vent stream (53) and a heated fourth coolant stream (38).
The method according to claim 7 comprising the further steps of:
expanding the cooled vent stream (53) to provide an expanded further cooled vent stream (63); and

passing the expanded further cooled vent stream (63) to a storage tank (50).
The method according to claim 7 comprising the further step of:
separating the further cooled vent stream (53) to provide a vent discharge stream (55) and a cooled vent BOG return stream (57).

expanding the cooled vent BOG return stream (57) to provide an expanded cooled vent BOG return stream (59); and

passing the expanded cooled vent BOG return stream (59) to a storage tank (50).
The method of any of the preceding claims wherein the step of cooling the compressed BOG discharge stream (06) against one or more first coolant streams (202, 302) to provide a first cooled compressed BOG stream (08) comprises:
pre-cooling the compressed BOG discharge stream (06) against a pre-cooling coolant stream (202) as a first coolant stream to provide a pre-cooled compressed BOG stream (07); and

cooling the pre-cooled compressed BOG stream (07) against a first refrigerant stream (302) as a first coolant stream to provide the first cooled compressed BOG stream (08).
The method of claim 10 wherein the pre-cooling coolant stream (202) is one or more of the group selected from: seawater stream, an air stream, more particularly an ambient air stream and/or a refrigerant stream.
The method of any of claim 10 or claim 11 wherein the first refrigerant stream (302) is one or more of the group selected from propane and propylene.
The method of any of the preceding claims wherein stages of compression (65, 75) are the compression stages of a multi-stage compressor.
The method of any of the preceding claims wherein all of the second coolant stream comprises the first expanded heated BOG stream (33b).
The method of any of the preceding claims wherein the first expanded heated BOG stream (33b) used as the second coolant stream comprises both liquid and gas phases.
The method of claim 15 wherein the liquid and gas phases of the first expanded heated BOG stream (33b) used as the second coolant stream are separated in the cooling of the first cooled compressed BOG stream (08).
An apparatus to cool a boil off gas stream (01) from a liquefied ethane cargo in a floating transportation vessel comprising a plurality of components, said apparatus comprising at least:
a compression system (60) to compress a boil off gas stream (01) from a liquefied ethane cargo, said compression system comprising two or more stages of compression comprising at least a first stage (65) and a final stage (75) to provide a compressed BOG discharge stream (06), wherein intermediate, optionally cooled, compressed BOG streams (02, 03, 04) are provided between consecutive stages of compression,

one or more first heat exchangers (200, 300) to cool the compressed BOG discharge stream (06) to provide a first cooled compressed BOG stream (08);

characterised in that the apparatus further comprises;

one or more second heat exchangers (180) to further cool the first cooled compressed BOG stream (08) against a mixed phase coolant stream (33b) to be separated in the one or more second heat exchangers, to provide a second cooled compressed BOG stream (34); and

one or more third heat exchangers (195) to further cool the second cooled compressed BOG stream (34) to provide a third cooled compressed BOG stream (35) and a pressure reduction device (80) to expand a portion of the third cooled compressed BOG stream (35) to a pressure between that of the first stage discharge pressure and the final stage suction pressure to provide a first expanded cooled BOG stream (33a) as the coolant in the one or more third heat exchangers (195) and then the mixed phase coolant stream (33b) in the one or more second heat exchangers (180).
Apparatus as claimed in claim 17 which is operable using the method of any one of claims 1 to 16.
Apparatus as claimed in claim 17 or claim 18 wherein the second heat exchanger (180) is an economiser.
A floating transportation vessel for a liquefied ethane, having the apparatus as defined in any one of claims 17 to 19 or using the method as defined in any one of claims 1 to 16.