CN107208841B

CN107208841B - Method for cooling boil-off gas and device therefor

Info

Publication number: CN107208841B
Application number: CN201580050405.9A
Authority: CN
Inventors: 马丁·哈尔克罗
Original assignee: Babcock IP Management Number One Ltd
Current assignee: LGE IP Management Co Ltd
Priority date: 2014-08-21
Filing date: 2015-08-21
Publication date: 2020-06-16
Anticipated expiration: 2035-08-21
Also published as: WO2016027098A1; JP2017525910A; JP6553714B2; EP3183489B1; EP3183489A1; KR102379711B1; KR20170043637A; CN107208841A; GB201414893D0

Abstract

Method of cooling a boil-off gas stream from a liquefied ethane cargo in a floating transport vessel, the method comprising at least the steps of: compressing a boil off gas stream from the liquefied ethane cargo to provide a compressed BOG discharge stream in two or more compression stages comprising at least a first stage and a final stage, wherein the first stage of compression has a first stage discharge pressure and the final stage of compression has a final stage suction pressure, and one or more intermediate, optionally cooled, compressed BOG streams are provided between successive stages of compression; cooling the compressed BOG discharge stream against the 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 the pressure 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 a third coolant stream to provide a first expanded heated BOG stream; and using the first expanded heated BOG stream as the second coolant stream or a second coolant stream.

Description

Method for cooling boil-off gas and device therefor

The present invention relates to a method for cooling, in particular re-liquefying Boil Off Gas (BOG) from liquefied ethane cargo on floating transport vessels, and a device for use in the method.

Floating carriers, such as liquefied gas carriers and barges, are capable of transporting various cargo in a liquefied state. In this context, the liquefied cargo is entirely or substantially ethane, typically > 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, hydraulic fracturing (fracking), or produced in crude oil refining. Thus, ethane can be combined with a variety of other components, particularly methane. It is often desirable to liquefy ethane in a liquefaction facility at or near its source because as a liquid it can be stored and transported over long distances (typically beyond normal pipeline distances) more easily than in gaseous form because it occupies a smaller volume and may not require storage at high pressures.

The long distance transport of liquefied ethane cargo having a boiling point of about-87 ℃ when measured at 1 atmosphere can be carried out in a suitable liquefied gas vessel, 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 the liquefied ethane cargo, gases may be generated due to the vaporization of the cargo. This vaporized cargo gas is referred to as Boil Off Gas (BOG). To prevent BOG build-up (and consequent pressure build-up problems) in the tank, a system may be provided on board the vessel to re-liquefy the BOG so that it can be returned to the storage tank in a condensed state. This can be achieved by compressing and cooling the BOG against a cold source. Ethane has a critical temperature of 32.18 ℃ at a pressure of 47.7 bar (barg), so that seawater at similar temperatures would not be suitable as a primary heat sink. In many systems, compressed BOG is cooled and condensed against a secondary refrigerant.

In the case where a typical liquefied cargo may be defined as "pure", then there are known methods and apparatus for re-liquefying BOG. However, the liquefied ethane to be transported as cargo in floating transport vessels may and increasingly may include concentrations of other components that exceed a minimum level (de minimus level). This is due, at least in part, to the increasing supply of "non-pure" ethane from new sources or new industrial processes.

One of the possible other components is propane. However, since propane has a boiling point of about-40 ℃ when measured at 1 atmosphere, the method and apparatus required to reliquefy ethane/propane BOG will inherently achieve reliquefaction of any propane portion of the BOG.

Another possible component is nitrogen. Because its boiling point is about-196 ℃ when measured at 1 atmosphere, it is generally impractical to attempt to reliquefy any nitrogen in the BOG on a floating carrier. Thus, nitrogen is generally considered to be at least the major component of those portions of BOG defined as "non-condensable", i.e. it is not at all (indeed) condensable on floating transport vessels. However, nitrogen is a relatively "safe" gas.

A major other potentially important component in liquefied ethane cargo is methane. Methane has a boiling point of about-162 ℃ to-163 ℃ when measured at 1 atmosphere. This boiling point is very significantly lower than that of ethane when measured at 1 atmosphere. Likewise, methane has heretofore generally been considered to be the "non-condensable" component of liquefied cargo, i.e., it may be condensed (i.e., re-liquefied), but requires a particular special process that may not be CAPEX and/or OPEX justified on floating transport vessels. Thus, relatively small amounts of methane in liquefied cargo, such as those that are wholly or substantially propane (i.e., LPG) or the like, have up until now been vented to the atmosphere because conventional LPG BOG reliquefaction processes and devices are no longer capable of reliquefying methane.

However, methane is considered to be one of the "greenhouse gases", so that it is increasingly preferred not to discharge it into the atmosphere.

Furthermore, the type of ethane that is currently expected to be increasingly transported as liquefied ethane is expected to have increased concentrations of methane in the cargo.

Furthermore, a particular disadvantage of methane is that even small concentrations of methane in the liquefied cargo will result in a disproportionate amount of methane in the BOG. For example, a concentration of only 0.5 mol% in the liquid phase may result in a BOG of 25 mol% methane for the liquefied ethane cargo.

Thus, it may not be possible to re-liquefy all components of the boil-off gas from the liquefied ethane cargo, 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 tank in the gas phase, but this will accumulate in the boil-off gas in a closed system, thus increasing in concentration over time. Furthermore, as the concentration of non-condensable components in the boil-off gas increases, the volume of boil-off gas that cannot be recondensed will increase, thereby reducing the effective capacity of the reliquefaction system.

As mentioned above, another alternative to the discharge of non-condensable components, such as methane (which may be a greenhouse gas), is both environmentally and commercially undesirable.

WO2012/143699a relates to a method and apparatus for reliquefying in a floating carrier a BOG stream from a liquefied cargo having a boiling point of greater than-110 ℃ at 1 atmosphere, wherein a cooled vent stream, which may include non-condensable BOG components, is heat exchanged with a portion of the compressed, cooled and then expanded BOG stream. This is particularly suitable for liquefied cargoes having a boiling point of greater than-110 ℃ when measured at 1 atmosphere, but there is a need to provide an improved method of cooling (in particular re-liquefying as much as possible at reasonable OPEX and CAPEX) boil-off gas from liquefied ethane cargoes (in particular such cargoes comprising an increased proportion of lighter components such as methane).

The present invention solves these problems by triple cooling and using a compressed BOG stream. In this manner, the triple cooling stream condenses previously uncondensed components, can be reliquefied and then returned to the liquefied ethane cargo tank in the liquid phase. The triple cooled compressed BOG stream provides a source of increased cooling duty (cooling duty) compared to heat exchange media such as seawater, allowing for re-liquefaction of lighter components in the BOG stream.

Thus, for a given number of compression stages, the methods and apparatus disclosed herein allow for a liquefied ethane cargo to be transported with an increased content of lighter components (e.g., methane) without the need to add additional compression stages or increase the venting of components previously considered non-condensable. Viewed in another way, the methods and apparatus described herein allow for expansion of a compression system having a given number of compression stages to a cargo having components that cannot generally be reliquefied 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 transport vessel, the method comprising at least the steps of:

compressing a boil off gas stream from the liquefied ethane cargo to provide a compressed BOG discharge stream in two or more compression stages comprising at least a first stage and a final stage, wherein the first stage of compression has a first stage discharge pressure and the final stage of compression has a final stage suction pressure, and one or more intermediate, optionally cooled, compressed BOG streams are provided between successive stages of compression;

cooling the compressed BOG discharge stream against the 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 the pressure 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 a third coolant stream to provide a first expanded heated BOG stream; and

using the first expanded heated BOG stream as the second coolant stream or a second coolant stream.

That is, the first expanded cooled BOG stream is used as the third coolant stream in a heat exchanger that relies on the second cooled compressed BOG stream to provide the third cooled compressed BOG stream and the first expanded heated BOG stream as the heated third coolant stream, which may be used indirectly, more preferably directly, 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 a heat exchange/exchanger against the first cooled compressed BOG stream, which provides the second cooled compressed BOG stream and the first expanded further heated BOG stream as the heated second coolant stream.

The terms "first," "second," "third," "fourth," and the like as used herein are intended to indicate a coupling or relationship, which may or may not be in direct sequence unless specifically stated otherwise. That is, there may be one or more other steps or processes or locations between the "second" and "third" features. These terms are used to clarify the existence of different properties or associated features in or of the stream, and the invention is not limited by these terms.

For the avoidance of doubt, the second coolant stream (i.e. the first expanded heated BOG stream) is at a lower temperature than the first cooled compressed BOG stream; the third coolant stream (i.e., the 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:

the first expanded heated BOG stream, which is the heated second coolant stream, is combined with an intermediate compressed BOG stream, such as the first or second intermediate compressed BOG stream, preferably with the first intermediate compressed BOG stream.

According to another embodiment of the invention, the step of cooling the compressed BOG discharge stream against the one or more first coolant streams to provide a first cooled compressed BOG stream may comprise:

the compressed BOG discharge stream is cooled against the first refrigerant stream as a first refrigerant stream to provide a first cooled compressed BOG stream.

That is, the first refrigerant stream is used as one of the one or more first refrigerant streams in a heat exchanger/exchanger relying on the compressed BOG discharge stream, which provides the first cooled compressed BOG stream and the heated first refrigerant stream as the heated first refrigerant stream.

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;

the pre-cooled compressed BOG stream is cooled against the first refrigerant stream as a first refrigerant stream to provide a first cooled compressed BOG stream.

That is, the pre-cooling coolant stream is used as one of the one or more first coolant streams in a heat exchanger/exchanger relying on the compressed BOG discharge stream, which provides the pre-cooled compressed BOG stream and the heated pre-cooling coolant stream as the heated first coolant stream.

That is, the first refrigerant stream is used as one of the one or more first refrigerant streams in a heat exchange/exchanger relying on the pre-cooled compressed BOG stream, which provides the first cooled compressed BOG stream and the heated first refrigerant stream as the heated first refrigerant stream.

According to another embodiment of the invention, the pre-cooling coolant flow 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, wherein a water stream or an air stream is preferred. Typically, if an open pre-cooling coolant circuit is used, the pre-cooling coolant flow may be selected from the group consisting of a sea water flow and an ambient air flow. Typically, if a closed pre-cooling coolant circuit is used, the pre-cooling coolant flow may be selected from the pre-cooling refrigerant flows.

According to another embodiment of the invention, the cooling of the pre-cooled compressed discharge stream by means of the pre-cooling coolant stream is performed 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 invention, the one or more first coolant streams comprise a first refrigerant stream, for example a first refrigerant comprising a single refrigerant or a 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 in particular hydrocarbons and fluorinated hydrocarbons, such as propane, propylene, difluoromethane and pentafluoromethane, including fluorinated hydrocarbon mixture R-410A.

According to another embodiment of the invention, the cooling of the compressed BOG discharge stream or the pre-cooled compressed discharge stream by means of the first refrigerant stream is performed in a discharge heat exchanger, such as a shell and tube heat exchanger, a plate heat exchanger or an economizer.

According to another embodiment of the invention, all of the compressed BOG discharge stream is cooled against one or more first coolant streams.

In one embodiment of the invention, the liquefied ethane cargo comprises >0.1 mol% methane. In practice, the liquefied ethane cargo may comprise >0.4 mol% methane, comprising >0.5 mol%, 0.6 mol%, >0.7 mol%, >0.8 mol%, >0.9 mol% and >1.0 mol% methane. The invention extends to a liquefied ethane cargo having 1-5 mol% methane, optionally >5 mol% methane.

The number of compression stages is not a limiting factor of the invention. Optionally, the process comprises three or four compression stages.

Optionally, it is desirable to provide the fully condensed boil-off gas as the first cooled compressed BOG stream, but the invention extends to a process in which the boil-off gas is not fully condensed after cooling against the one or more first coolant streams.

The present invention overcomes the difficulties of using certain types of heat exchange, particularly certain types of heat exchangers, and more particularly conventional shell & coil economiser (shell & coil economiser), in which the temperature method is limited by the composition of the fluid in the shell. While the composition of the fluid in the shell may be a single component, i.e., a sufficiently "pure" gas, it is well known and widespread to rely on cooling of the expanded portion of the compressed BOG. However, this cooling load is reduced in multicomponent mixtures and drastically reduced in multicomponent mixtures with significant boiling point differences (such as ethane and methane in particular). The present invention thus improves the coefficient of performance of the cooling cycle of a liquefied ethane cargo containing significant amounts of methane, i.e. it improves the coefficient of performance of the cargo currently considered to be minimal (e.g. 0.1 mol% or less methane) and allows operation with cargo containing much higher methane content (e.g. about or above 0.4 mol% or 0.5 mol% methane).

The present invention also seeks to maintain the use of current onboard equipment and devices under its known OPEX and CAPEX, rather than attempting to introduce and tailor how to use new equipment with new operational requirements.

Thus, according to another embodiment of the invention, the cooling of the first cooled compressed BOG stream by means of the second coolant stream is performed in an economizer.

According to another embodiment of the invention, all of the first cooled compressed BOG stream is cooled against the second coolant stream.

According to another embodiment of the invention, all of the second cooled compressed BOG stream is cooled against the third coolant stream.

In another embodiment of the present invention, the method further comprises the steps of:

providing a gaseous exhaust stream from the first cooled compressed BOG stream;

expanding a portion of the third cooled compressed BOG stream to form a fourth coolant stream;

the gaseous exhaust stream is cooled against the fourth coolant stream to provide a cooled exhaust stream and a heated fourth coolant stream.

In this manner, the present invention may also provide for increased reliquefaction of components previously considered "non-condensable" or "uncondensed" in the compressed BOG.

Preferably, the heated fourth coolant stream is a BOG recycle stream or may be used as a BOG recycle stream. Accordingly, the method may further comprise:

the heated fourth coolant stream is combined with an intermediate compressed BOG stream, such as the first or second, preferably the first intermediate compressed BOG stream.

Optionally, the process of the invention comprises the further steps of:

the cooled vent stream is separated to provide a vent discharge stream (vent discharge stream) and a cooled vent BOG return stream.

Optionally, the process of the invention comprises the further steps of:

expanding the cooled vent BOG return stream to provide an expanded cooled vent BOG return stream;

the expanded cooled vented BOG return stream is passed to a storage tank.

Optionally, the method comprises the further steps of:

heat exchanging the expanded cooled vent BOG return stream against the vent stream to provide a heat exchanged vent BOG return stream, a cooled vent stream, and an additional vent stream;

expanding the cooled discharge stream to provide an expanded cooled discharge stream;

the heat exchanged exhaust BOG return stream and the expanded cooled exhaust vent stream are passed to a storage tank.

Optionally, the compression stage is a compression stage 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 fully or substantially cooled against a second coolant stream comprising only the first expanded heated BOG stream. Preferably, all of the second coolant stream comprises the first expanded heated BOG stream. That is, the first cooled compressed BOG stream may be cooled against one or more other second coolant streams, but these are secondary or less than the cooling provided by using the first expanded heated BOG stream.

Optionally, the first expanded heated BOG stream used as the second coolant stream comprises both a liquid phase and a gas phase. That is, it is not necessary to separate it into separate gas and liquid phases before use as the second coolant stream.

Preferably, the liquid and vapor 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 done by means, preferably an economizer, that allows for a first cooled compressed BOG stream to be cooled.

According to a second aspect of the present invention, there is provided an apparatus for cooling a boil-off gas stream from a liquefied ethane cargo in a floating transport vessel, the boil-off gas stream comprising a plurality of components, the apparatus comprising at least:

a compression system to compress a boil off gas stream from a liquefied ethane cargo, the compression system comprising two or more compression stages, the compression stages comprising at least a first stage and a final stage to provide a compressed BOG discharge stream, wherein an intermediate, optionally cooled, compressed BOG stream is provided between successive 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 to 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.

Optionally, the device as defined herein is operable using the method as defined herein.

Preferably, the second heat exchanger is an economizer.

According to a further aspect of the present invention there is provided a floating carrier vessel for liquefying ethane cargo having an apparatus or operating method as defined herein.

The invention is applicable to any floating carrier vessel used for liquefying ethane cargo. The invention may be used in a floating transport vessel in which a liquefied ethane cargo storage tank is fully refrigerated at about atmospheric pressure by lowering the temperature to maintain the cargo in the liquid phase; and in those vessels in which the cargo in the tank is maintained in the liquid phase by a combination of reduced temperature and increased pressure (relative to ambient temperature and pressure).

The use of an economizer is not required in order to obtain the benefits of the methods and apparatus disclosed herein. However, in certain embodiments, a heat exchanger, such as an economizer, may be disposed between successive stages of compression, such as between the first and second stages, to cool the intermediate compressed BOG stream. Where three or more compression stages are present, a heat exchanger, such as an economizer or intercooler (e.g., a seawater intercooler), that allows for cooling of the intermediate compressed BOG stream may be provided between the second and final compression stages.

For example, an intercooler may be located between the second and third compression stages. Alternatively, the economizer may be located between the second and third compression stages, and between the first and second compression stages. In the economizer, the expanded, optionally further cooled, portion of the cooled compressed BOG stream can be heat exchanged with an intermediate compressed BOG stream. In further embodiments, the expanded, optionally further cooled, portion of the cooled compressed BOG stream may be heat exchanged with the optionally further cooled portion of the cooled compressed vent stream. This leads to a further improvement of the coefficient of performance and an increased cooling (in particular re-liquefaction) capacity.

It is apparent that the methods and apparatus disclosed herein can be applied as a retro-fit (retro-fit) to existing floating transport vessels by maintaining the number of compression stages present and adding the necessary piping, valves and controls to effect cooling of the second cooled compressed BOG stream against the expanded portion of the third cooled BOG stream.

As used herein, the term "multiple compression stages" defines two or more compression stages in series (in series) in a compression system. Each compression stage may be achieved by one or more compressors. One or more compressors of each compression stage may be independent of the compressors of the other compression stages such that they are driven separately. Alternatively, two or more compression stages may utilize connected compressors, typically powered by a single drive and drive shaft and optional gearing. Such connected compression stages may be part of a multi-stage compressor.

The methods and apparatus disclosed herein require at least two compression stages. After the first compression stage, each subsequent stage provides an increased pressure compared to the pressure at the discharge of the previous stage. The term "successive stages" refers to adjacent pairs of compression stages, i.e., stage (n) and the next (n +1) stage, where "n" is an integer greater than 0. Thus, the successive stages are, for example, the first and second stages, or the second and third stages, or the third and fourth stages. Intermediate compressed streams (and cooled intermediate compressed streams) refer to those streams that couple successive stages of compression. The term "next compression stage" or "subsequent compression stage" as used in relation to the cooled intermediate compressed stream refers to the numerically higher number (and higher pressure stage) defining the two successive stages of the intermediate stream.

The heat exchange step may be indirect, in which the two or more streams participating in the heat exchange are separate and not in direct contact. Alternatively, the heat exchange may be direct, in which case two or more streams participating in the heat exchange may be mixed, thereby producing a combined stream.

According to a further aspect of the invention, there is provided a method of integrated design (integrated design) plant for cooling a boil-off gas stream from a liquefied ethane cargo in a floating transport vessel, the boil-off gas stream comprising a plurality of components, the method comprising the steps of:

selecting a compression system to compress a boil off gas stream from the liquefied ethane cargo, the compression system comprising two or more compression stages, the compression stages comprising at least a first stage and a final stage to provide a compressed BOG discharge stream, wherein an intermediate, optionally cooled, compressed BOG stream is provided between successive 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 in dependence on the mixed phase coolant stream to be separated in the one or more second heat exchangers to provide a second cooled compressed BOG stream; and

one or more third heat exchangers are selected 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 the device;

determining the effectiveness of the method;

changing a process variable in the process simulation; and

and repeating the process simulation.

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

the compression system is designed to compress a boil-off gas stream from the liquefied ethane cargo, the compression system comprising two or more compression stages, the compression stages comprising at least a first stage and a final stage, to provide a compressed BOG discharge stream, wherein an intermediate, optionally cooled, compressed BOG stream is provided between successive 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

one or more third heat exchangers are designed 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 the process;

determining the effectiveness of the method;

changing a process variable in the process simulation; and

and repeating the process simulation.

The design method as discussed herein may incorporate computer-aided processes for incorporating associated operational equipment (operational equipment) and controllers into the overall ship structure, and may incorporate associated cost, operational capability parameters into methodology and design. The methods described herein may be encoded on media suitable for reading and processing on a computer. For example, code to perform the methods described herein may be encoded on magnetic or optical media, which may be read by and copied to a personal or mainframe computer. The method may then be performed by a design engineer using such a personal or mainframe computer.

Certain features of the invention and methods of designing the same may be described in terms of a set of numerical upper limits and a set of numerical lower limits. It should be understood that any range formed by any combination of such limitations is contemplated to fall within the scope of the present invention. In addition, it is contemplated that the overall design includes a selection of additional structures for use with the combinations specifically defined herein. Within the vessel, various structural operating parameters may be selected for a defined or fixed basis, or for flexible or multiple operating uses. Accordingly, it is intended that the design methodology encompass alternatives, modifications, and equivalents as to the overall design of the marine vessel and any overboard (off-vessel) that are included within the spirit and scope of the present invention.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying non-limiting drawings, in which:

fig. 1 shows a schematic view of a possible known system for reliquefying boil-off gas from cargo tanks in a ship;

figure 2 shows a schematic diagram of a system for cooling (particularly re-liquefying) boil-off gas from a liquefied ethane cargo in a floating transport vessel, according to an embodiment of the invention;

FIGS. 3a and 3b are the economizer temperature curves of temperature versus heat flow for a pure component BOG cooling system (3a) and a wide boiling point multi-component mixture cooling system (3 b); and is

Fig. 4 shows a schematic diagram of a system for cooling (in particular re-liquefying) boil-off gas from a liquefied ethane cargo in a floating transport vessel, according to another embodiment of the invention.

The floating reliquefaction system draws vapor, also referred to as boil-off gas, from one or more storage tanks and delivers the boil-off gas to a compressor where it is compressed so that the compressed vapor can be cooled and condensed against one or more coolants as a heat sink/refrigerant. For example, seawater may be used to pre-cool (typically desuperheat) the compressed vapor in an open-cycle pre-cooling circuit. The pre-cooled compressed vapor may then be further cooled against refrigerant in a closed-cycle refrigerant circuit.

Those lighter components of the compressed vapor that cannot rely on refrigerant condensation are typically vented to the atmosphere or recycled to the storage tank in vapor form. Typically, the liquefied cargo is maintained in the tank at one or both of a reduced temperature (relative to ambient) and an increased pressure (relative to atmospheric).

Fig. 1 shows a schematic diagram of a known system for reliquefying boil-off gas from an ethane cargo. Currently, ethane cargo tends to be transported in converted ethylene carriers. The liquefied ethane cargo is stored in a tank 50a, which tank 50a may be insulated and/or pressurized to maintain the ethane in a liquefied state. Vaporization of ethane in the tank, for example due to imperfect thermal insulation, will result in the formation of ethane gas in the head space of tank 50a, and this gas is commonly referred to as Boil Off Gas (BOG). To prevent accumulation of this gas, the gas is removed from the tank 50a as a boil-off gas stream 01 a. All components are compressed and typically as much of the components of the boil-off gas removed are cooled to condense them before they are returned to the drum 50 a.

The boil-off gas stream 01a may be delivered to a compression system 60, such as the two-stage compressor shown in fig. 1 comprising 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 may be passed to a pre-cooling heat exchanger 100, where 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 may desuperheat the compressed BOG discharge stream 06 a.

The pre-cooled compressed BOG stream 07a may be passed to a refrigerant heat exchanger 250 where 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 may be part of a refrigerant circuit (not shown) including a refrigerant heat exchanger 250, a refrigerant compressor, and a refrigerant cooler. The refrigerant circuit may be a closed refrigerant system. Such a refrigerant circuit, also referred to as a refrigerant pack, is 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 that includes those components of the boil-off gas that can be "reliquefied" (i.e., condensed) against the refrigerant at the discharge pressure of the second compression stage 75.

The "uncondensed" components that are not capable of being re-liquefied against refrigerant in the system, and which may include "uncondensable" components and "uncondensable" components as discussed herein, may be removed as an effluent stream 49 from the refrigerant heat exchanger 250 or an associated accumulator (not shown) located downstream of the refrigerant heat exchanger 250, the effluent stream 49 being a vapor stream. After expansion to atmospheric pressure, the discharge stream 49 is typically vented to atmosphere.

The cooled compressed BOG stream 08a may be passed to an additional 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 a joule-thomson valve, to provide an expanded cooled return fluid stream 24. Typically, the return depressurization device 22 reduces the pressure of the cooled return fluid stream 18 from the pressure of the compressed BOG vent stream 06a or near the pressure of the compressed BOG vent stream 06a to a pressure near the pressure of the liquid ethane and BOG in the tank 50a, e.g., a pressure just above the pressure of the BOG in the tank, which is sufficient to ensure proper flow of the expanded cooled return fluid stream 24 to the tank 50 a. The pressure of the expanded cooled return fluid stream 24 is lower than the discharge pressure of the first compression stage 65.

Returning to the compression system 60, the first compression stage 65 provides a first intermediate compressed BOG stream 02a, which is passed to a further heat exchanger 80. The first intermediate compressed BOG stream 02a may be heat exchanged against the expanded portion 8b of the cooled compressed BOG stream 08a in a further heat exchanger 80 to provide a cooled first intermediate compressed BOG stream 03a, which may then be passed to the suction of the second compression stage 75. The second stage 75 compresses the cooled first intermediate compressed BOG stream 03a to provide a compressed BOG discharge stream 06 a.

Turning to fig. 3a, this figure shows a typical temperature profile for cooling of "pure" material in a conventional shell-coil economizer, where the 'xxxx' line represents the shell side and the 'ooo' line represents the tube or coil temperature. It can be seen that the shell side temperature is "flat" such that there is no change in shell side temperature with increasing heat flow. This represents cooling of a "pure" material such as pure ethane.

However, fig. 3b shows the temperature profile in the same economiser (and using the same line format) for a multi-component mixture with a "broad boiling point" (e.g. the difference in boiling points of ethane and methane). Fig. 3b shows that it is difficult to achieve a constant temperature for the tube side. It is evident that there is a reduced efficiency on all heat flows, so that for multi-component mixtures the cooling efficiency is determined by the heavier component, which reduces the potential cooling that can be achieved in this type of equipment.

However, it is still preferred to use this type of equipment maintained under its known CAPEX.

The methods and apparatus disclosed herein seek to provide improved methods and apparatus for reliquefying BOG. An embodiment of the method and apparatus according to the present invention is disclosed in fig. 2. Where appropriate, the same stream and component names and the same reference numerals as those in fig. 1 have been used for the corresponding streams and components in the remaining figures.

Fig. 2 shows a liquefied ethane cargo tank 50 in a floating carrier, such as an ethane carrier. The liquefied ethane cargo may include ethane and methane. For cooling (in particular re-liquefying) the vaporized cargo from the storage tank 50, the boil-off gas stream 01 comprising the vaporized cargo is conveyed to a compression system 60 having two or more compression stages. Boil-off gas stream 01 can have a pressure in the range from above 0kPa to 500kPa gauge ("BOG pressure"). 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 include an independently driven compressor for each compression stage. When the compression system 60 is a multi-stage compressor, it is typically a reciprocating compressor.

The embodiment of fig. 2 shows a compression system 60 having a first stage 65 and a second stage 70 and a third and final stage 75, but the methods and apparatus described herein may also be applicable to compressors having two or more than three stages. The first and

final stages

65 and 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 ("final stage pressure") in the range of from 1.5MPa to 3.2MPa or higher, for example up to 6 MPa.

The compressed BOG discharge stream 06 is cooled in the one or more

first heat exchangers

200, 300 against the one or more first coolant streams 202, 302 to provide a first cooled compressed BOG stream 08. In the embodiment of fig. 2, the compressed BOG discharge stream 06 may be passed to a pre-cooling heat exchanger 200, which is one of the one or more first heat exchangers. The compressed BOG discharge stream 06 is pre-cooled against a pre-cooling coolant stream that is one of the one or more first coolant streams. The pre-cooling coolant flow 202 may be an air flow or a water flow, such as an ambient air flow or a seawater flow. 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 desuperheat the compressed BOG discharge stream 06. The pre-cooling heat exchanger 200 provides a pre-cooled compressed BOG stream 07 and a heated pre-cooling coolant stream 204. Typically, the seawater used as the pre-cooling coolant will have a temperature of +36 ℃ or less, more typically +32 ℃ or less.

Pre-cooling heat exchange/exchanger 200 is optional in the methods and apparatus disclosed herein. This is advantageous because it reduces the cooling load of the subsequent cooling step. However, this is not a necessary aspect, such that in an alternative embodiment, the compressed BOG discharge stream 06 may be passed directly to the discharge heat exchanger 300 via line 06', such that the equipment shown by reference 210 may be omitted. In this case, the cooling capacity of the discharge heat exchanger 300 would have to be increased to compensate for the lack of pre-cooling.

The pre-cooled compressed BOG stream 07 may then be passed to a discharge heat exchanger 300, which is 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 refrigerant 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, the discharge heat exchanger 300, and the heated first refrigerant stream 304 may be part of a first refrigerant system (not shown). Such a first refrigerant system may further include a first refrigerant compressor for compressing the heated first refrigerant stream 304 to provide a compressed first refrigerant stream; a first refrigerant cooler for cooling a first refrigerant to provide a cooled compressed first refrigerant stream; and a first refrigerant expansion device for expanding the cooled compressed first refrigerant stream to provide a first refrigerant stream 302. The first refrigerant system may be a closed system. The first refrigerant may comprise one or more organic compounds, in particular hydrocarbons and fluorinated hydrocarbons, such as propane, propene, difluoromethane and pentafluoromethane, including the fluorinated hydrocarbon mixture R-410A, and one or more inorganic compounds, such as ammonia.

The first cooled compressed BOG stream 08 may be a partially condensed, compressed BOG stream, including those components of the boil-off gas that may be condensed against the first refrigerant at the discharge pressure of the final stage of compression. Any uncondensed components can be removed as a vent stream (not shown) from the vent heat exchanger 300 or from a vent receiver (not shown) that functions as a gas/liquid separator located downstream of the vent heat exchanger 300. A discharge heat exchanger suitable for the separation of gas and liquid components is a shell and tube heat exchanger in which the cooled compressed BOG is located on the shell side.

Any discharge receiver may be an accumulator and may operate to maintain a liquid seal in the discharge heat exchanger 300 and/or to maintain discharge pressure at the final stage of compression 75.

The discharge heat exchanger 300 may be of a type that does not adequately separate the vapor and condensed phases into separate streams, such as plate and fin heat exchangers. In this case, the discharge receiver would be located downstream of the discharge heat exchanger 300 to separate the uncondensed components as a discharge stream.

The first cooled compressed BOG stream 08 is then cooled a second time. This may be accomplished 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 is used to cool the

intermediate BOG stream

02 or 04 and the intermediate stage of the first cooled compressed stream 08, particularly the first stage economizer is shown in fig. 2.

The cooling of the first cooled compressed BOG stream 08 relies on the second coolant stream to provide a second cooled compressed BOG stream 34. Optionally, a portion of the first cooled compressed BOG stream 08 may be used elsewhere before passing into the second heat exchanger (180), but in the present invention, it is preferred that all or substantially all of the first cooled compressed BOG stream 08 enters the first heat exchanger 180.

The function of the second coolant described below is to provide a second cooled compressed BOG stream 34. Again, a portion of this stream 34 may be used elsewhere, but preferably all or substantially all of the second cooled compressed BOG stream 34 enters the 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 economizer, but is preferably a counter-flow heat exchanger, such as plate and fin heat exchangers known in the art.

In the present invention, a portion of the third cooled compressed BOG stream 35 is expanded to a pressure between the first stage discharge pressure and the final stage suction pressure to provide a first expanded cooled BOG stream 33 a. This action may be performed by 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 provides a third cooled compressed BOG stream 35, and a first expanded heated BOG stream 33b as a heated third coolant stream 33b, which heated third coolant stream 33b may be used indirectly or more preferably directly as the second coolant stream 33 b. The first expanded heated BOG stream/second coolant stream 33b is not separated (to separate the vapor/liquid phases) prior to use as the second coolant stream 33b to take full advantage of any 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 heat exchange with the first cooled compressed BOG stream 08 provides the second cooled compressed BOG stream 34 and the heated second coolant in the second heat exchanger 180. The heated second coolant may include vapor and liquid components that are conveniently separated in the second heat exchanger 180 and will be discussed below. The heated second coolant stream, which is the first expanded further heated BOG stream, may be passed to an intermediate compressed BOG stream of suitable pressure. In the embodiment of fig. 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 that is not used to provide the first expanded cooled BOG stream 33a may be returned to the storage tank 50 as a return stream via a pressure reduction device 82 as an expanded cooled BOG return stream 36 in a manner known in the art.

One particular feature of the present invention is that no CAPEX changes are required based on the nature of the first 200, 300 and second 180 heat exchangers so that the operator can continue to use a "conventional" shell and tube economizer as the second heat exchanger 180 and the invention can be implemented simply by adding the third heat exchanger 195. This allows the entire BOG reliquefaction system to be controlled by existing level controllers in at least the second heat exchanger 180, avoiding potential problems with temperature control that may arise from using different BOG compositions and different pressures between stages.

Indeed, for ethane cargoes containing above a minimum level of methane (in the liquid phase), and even above 0.4 mol% or 0.5 mol% methane, improvements of 10% -15% in the refrigeration capacity of BOG reliquefaction processes and apparatus for liquefying the cargo are possible. Such liquefied ethane cargoes containing methane may become increasingly popular when new or other sources of ethane are provided, but the need to purify the ethane prior to transportation (by reducing or eliminating any methane content) 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. As with fig. 2, fig. 4 shows a liquefied ethane cargo tank 50, from which tank 50a boil-off gas stream 01 comprising a vaporised cargo is passed to a compression system 60, which compression system 60 has three stages of compression, namely 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, the BOG stream 02 entering the second heat exchanger 180 to provide a cooled first intermediate BOG stream 03, the BOG stream 03 entering the intermediate compression stage 70 to provide a second intermediate compressed BOG stream 04, the BOG stream 04 entering the suction of the final stage of compression 75.

The compression system 60 provides a compressed BOG discharge stream 06 which discharge stream 06 may be passed to a pre-cooling heat exchanger 200 as one of one or more first heat exchangers, cooled against a first coolant as seawater in seawater stream 202 in the manner previously described to provide a pre-cooled compressed BOG stream 07.

The pre-cooled compressed BOG stream 07 may then be passed to a discharge heat exchanger 300, which is another of the one or more first heat exchangers, in the 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 may be provided directly, or optionally after passing through the discharge receiver 305, as shown in fig. 4.

In the event that the cooled compressed BOG stream 08 is not fully condensed, a gaseous vent stream 51 is also provided from the vent heat exchanger 300 as stream 51a and/or from the vent receiver 305 as stream 51 b. Although fig. 4 shows the two

streams

51a, 51b as separate, these streams may be provided separately or in combination or without any distinction, depending on the nature and structure of the discharge heat exchanger 300 and the discharge receiver 305. The provision of these streams or multiple streams is known in the art.

The gaseous effluent stream 51 may include "non-condensable" components and "non-condensable" components. Non-condensing components are generally considered to be components that can never be compressed and condensed in practice within the limits and operating parameters of a particular floating carrier BOG cooling system, and are primarily related to nitrogen.

It is generally believed that the major non-condensable component is methane, which has a boiling point significantly lower than that of ethane at 1 atmosphere, and therefore its condensation is generally considered impractical within the limitations and operating parameters of a floating carrier.

In WO2012/143699a, a method and apparatus for increasing the amount or proportion of condensation of a gaseous effluent stream to increase its recovery rate is shown.

In the present invention, as shown by way of example in fig. 4, the method and apparatus may further include the step of expanding a portion of the third cooled compressed BOG stream 35 to form a fourth coolant stream 33c, typically by passing the portion of the third cooled compressed BOG stream 35 through a pressure reducing valve 87 in an amount that allows the portion of the third cooled compressed BOG stream 35 to act as the fourth coolant 33c in a fourth heat exchanger 197 (e.g., a discharge heat exchanger).

The fourth heat exchanger 197 may be of any type, but is preferably a counter-flow heat exchanger, such as a plate and fin arrangement. As shown in fig. 4, the gaseous exhaust stream 51 may be cooled against the fourth coolant stream 33c to provide a cooled exhaust stream 53 and a heated fourth coolant stream 38.

Optionally, the heated fourth coolant stream 38 is a BOG recycle stream, which may enter the second heat exchanger 180 such that steam therefrom may be used as part of the cooled first intermediate BOG stream 03.

Cooling of the gaseous discharge stream 51 in the discharge heat exchanger 197 may condense a portion of the components of the boil-off gas that cannot be condensed against the first refrigerant, e.g., propane or propylene, in the discharge heat exchanger 300. The cooled exhaust stream 53 is typically an at least partially condensed stream.

In one embodiment, the cooled discharge stream 53 may be passed to a discharge 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 discharge stream 63 (dashed line). The expanded further cooled discharge stream 63 may have a pressure at or slightly above the pressure of the liquefied ethane cargo storage tank 50 so that it may be returned to the tank, for example by adding to the expanded cooled BOG return stream 36 to provide the combined expanded cooled BOG return stream 11.

In another embodiment shown in fig. 4, the cooled effluent stream 53 may be passed to an effluent stream separator 150, such as a gas/liquid separator. The vent stream separator 150 provides a vent stream 55, which is typically a vapor stream, of all or substantially non-condensable components; and a cooled vent BOG return stream 57, which is typically a condensed stream, including those components of the boil-off gas that are condensed in the fourth heat exchanger 197. The pressure of the discharge stream 55 may be reduced, for example, to a pressure suitable for return to the storage tank 50 for storage elsewhere or for discharge.

The cooled discharge BOG return stream 57 may be passed through a discharge return stream pressure reduction device 58 (e.g., a joule-thomson valve or expander) to provide an expanded cooled discharge BOG return stream 59. The expanded cooled vented BOG return stream 59 may be passed to the storage tank 50, for example by addition to the expanded cooled BOG return stream 36.

The portion of the third cooled compressed BOG stream 35 that is not passed to the

pressure reducing devices

80 and 87 to provide the third and

fourth coolant streams

33a, 33c provides a BOG return stream 10, and the BOG return stream 10 may be expanded by a pressure reducing valve 82 to a pressure at or near the pressure of the storage tank 50 as an expanded cooled BOG return stream 36. Which can then be returned to the reservoir 50.

It will be appreciated by a person skilled in the art that the invention can be implemented in many different ways without departing from the scope of the appended claims. For example, the present invention encompasses combinations of one or more of the optional or preferred features disclosed herein.

Claims

1. A method of cooling a boil off gas stream (01) from a liquefied ethane cargo in a floating transport vessel, the method comprising at least the steps of:

compressing a boil off gas stream (01) from the liquefied ethane cargo in two or more compression stages comprising at least a first stage (65) and a final stage (75) to provide a compressed BOG discharge stream (06), wherein the first stage (65) of compression has a first stage discharge pressure and the 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 successive 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);

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

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

using the first expanded heated BOG stream (33b) as part of the second coolant stream or the entire second coolant stream; and

returning a portion of the third cooled compressed BOG stream (35) not used to provide the first expanded cooled BOG stream (33a) to the storage tank (50) via a pressure reduction device (82) as an expanded cooled BOG return stream (36).

2. The method of claim 1, wherein the liquefied ethane cargo comprises >0.1 mol% methane.

3. The method of claim 2, wherein the liquefied ethane cargo comprises >0.5 mol% methane.

4. A method according to any preceding claim, comprising three or four compression stages.

5. The method according to claim 1, wherein the cooling of the first cooled compressed BOG stream (08) against the second coolant stream is performed in an economizer (180).

6. The method according to either one of claims 1 and 5, wherein all of the first cooled compressed BOG stream (08) is cooled against the second coolant stream.

7. The method according to either one of claims 1 and 5, wherein all of the second cooled compressed BOG stream (34) is cooled against the third coolant stream.

8. The method of claim 1, 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 (33 c); and

cooling the gaseous exhaust stream (51) against the fourth coolant stream (33c) to provide a cooled exhaust stream (53) and a heated fourth coolant stream (38).

9. The method of claim 5, further comprising the steps of:

10. The method of claim 6, further comprising the steps of:

11. The method of claim 7, further comprising the steps of:

12. The method of any of claims 8-11, including using the heated fourth coolant stream (38) as a BOG recycle stream.

13. The method according to claim 8, comprising the further step of:

expanding the cooled discharge stream (53) to provide an expanded further cooled discharge stream (63); and

passing the expanded further cooled discharge stream (63) to a storage tank (50).

14. The method according to claim 8, comprising the further step of:

separating the cooled vent stream (53) to provide a vent stream (55) and a cooled vent BOG return stream (57).

15. The method according to claim 14, comprising the further step of:

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).

16. The method of claim 1, 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 refrigerant stream to provide the first cooled compressed BOG stream (08).

17. The method of claim 16, wherein the pre-cooling coolant stream (202) is one or more selected from the group of: a flow of seawater, a flow of air, and/or a flow of refrigerant.

18. The method of claim 16, wherein the first refrigerant stream (302) is one or more selected from the group of propane and propylene.

19. The method of claim 1, wherein the compression stage (65, 75) is a compression stage of a multi-stage compressor.

20. The method of claim 1 wherein the first expanded heated BOG stream (33b) used as the second coolant stream comprises both a liquid phase and a gas phase.

21. The process of claim 20 wherein the liquid and vapor 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).

22. A method according to claim 6 wherein all of the second cooled compressed BOG stream (34) is cooled against the third coolant stream.

23. The method of claim 22, further comprising the steps of:

24. The method of claim 23 including using the heated fourth coolant stream (38) as a BOG recycle stream.

25. The method of claim 16, wherein the pre-cooling coolant stream (202) is one or more selected from the group of: a flow of seawater, a flow of ambient air, and/or a flow of refrigerant.

26. An apparatus for cooling a boil-off gas stream (01) from a liquefied ethane cargo in a floating transport vessel, the boil-off gas stream (01) comprising a plurality of components, the apparatus comprising at least:

a compression system (60) to compress a boil off gas stream (01) from a liquefied ethane cargo, the compression system comprising two or more compression stages, the compression stages comprising at least a first stage (65) and a final stage (75) to provide a compressed BOG discharge stream (06), wherein an intermediate, optionally cooled, compressed BOG stream (02, 03, 04) is provided between successive 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);

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);

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);

a pressure reducing device (80) that expands 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 a third coolant in one or more third heat exchangers (195); and

a pressure reduction device (82) that expands a portion of the third cooled compressed BOG stream (35) that is not used to provide the first expanded cooled BOG stream (33a) to return to the storage tank (50).

27. The apparatus of claim 26, operable using the method of any one of claims 1 to 25.

28. The apparatus of claim 26 or claim 27, wherein the second heat exchanger (180) is an economizer.

29. A floating carrier vessel for liquefied ethane cargo in a floating carrier vessel, the floating carrier vessel having an apparatus as defined in any one of claims 26 to 28 or using a method as defined in any one of claims 1 to 25.