[0001] 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.
[0002] 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.
[0003] 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.
[0004] 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.
[0005] 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.
[0006] 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.
[0007] 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.
[0008] 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.
[0009] However, methane is considered to be one of the 'greenhouse gasses', such that it
is increasingly preferred not to vent it to atmosphere.
[0010] 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.
[0011] 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.
[0012] 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.
[0013] 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.
[0014] 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.
[0015] 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.
[0016] 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.
[0017] 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.
[0018] 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.
[0019] 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.
[0020] 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.
[0021] 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.
[0022] 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.
[0023] 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.
[0024] 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.
[0025] 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.
[0026] 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.
[0027] 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.
[0028] 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.
[0029] 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.
[0030] 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.
[0031] 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.
[0032] 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.
[0033] According to another embodiment of the present invention all the compressed BOG discharge
stream is cooled against the one or more first coolant streams.
[0034] 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.
[0035] 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
[0036] 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.
[0037] 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.
[0038] 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.
[0039] 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.
[0040] According to another embodiment of the present invention all the first cooled compressed
BOG stream is cooled against the second coolant stream.
[0041] According to another embodiment of the present invention all the second cooled compressed
BOG stream is cooled against the first expanded cooled BOG stream.
[0042] 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.
[0043] 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.
[0044] 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.
[0045] 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.
[0046] 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.
[0047] 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.
[0048] Optionally, the stages of compression are the compression stages of a multi-stage
compressor.
[0049] 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.
[0050] 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.
[0051] 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.
[0052] 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.
[0053] Optionally, the apparatus as defined herein is operable using the method as defined
herein.
[0054] Preferably, the second heat exchanger is an economiser.
[0055] 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.
[0056] 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.
[0057] 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.
[0058] 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.
[0059] 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.
[0060] 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.
[0061] 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.
[0062] 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.
[0063] 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.
[0064] 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.
[0065] 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.
[0066] 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.
[0067] 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.
[0068] 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.
[0069] 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.
[0070] 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.
[0071] 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).
[0072] 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.
[0073] 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.
[0074] 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.
[0075] 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.
[0076] 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.
[0077] 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.
[0078] 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.
[0079] 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.
[0080] 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.
[0081] 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.
[0082] Nevertheless, it is still preferred to maintain the use of this type of equipment
with its known CAPEX.
[0083] 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.
[0084] 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.
[0085] 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.
[0086] 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.
[0087] 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.
[0088] 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.
[0089] 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.
[0090] 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.
[0091] 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.
[0092] 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.
[0093] 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.
[0094] 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.
[0095] 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.
[0096] 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.
[0097] 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.
[0098] 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.
[0099] 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.
[0100] 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.
[0101] 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.
[0102] 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.
[0103] 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.
[0104] 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.
[0105] 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.
[0106] 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.
[0107] 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.
[0108] 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.
[0109] 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.
[0110] 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.
[0111] 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.
[0112] 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.
[0113] 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.
[0114] 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.
[0115] 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.
[0116] 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.
[0117] 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.
[0118] 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.
[0119] 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.
1. 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.
2. The method of claim 1 wherein the liquefied ethane cargo comprises >0.1 mol% methane
or >0.5 mol% methane.
3. The method according to any one of the preceding claims comprising three or four stages
of compression.
4. 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).
5. 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.
6. 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).
7. 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).
8. 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).
9. 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).
10. 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).
11. 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.
12. 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.
13. The method of any of the preceding claims wherein stages of compression (65, 75) are
the compression stages of a multi-stage compressor.
14. The method of any of the preceding claims wherein all of the second coolant stream
comprises the first expanded heated BOG stream (33b).
15. 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.
16. 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).
17. 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).
18. Apparatus as claimed in claim 17 which is operable using the method of any one of
claims 1 to 16.
19. Apparatus as claimed in claim 17 or claim 18 wherein the second heat exchanger (180)
is an economiser.
20. 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.
1. Ein Verfahren zum Kühlen eines Boil-Off-Gasstroms (01) aus einer Ladung verflüssigten
Ethans in einem schwimmenden Transportfahrzeug, wobei das Verfahren mindestens folgende
Schritte beinhaltet:
Verdichten eines Boil-Off-Gasstroms (01) aus der Ladung verflüssigten Ethans in zwei
oder mehr Stufen der Verdichtung, die mindestens eine erste Stufe (65) und eine letzte
Stufe (75) beinhalten, um einen verdichteten BOG-Auslassstrom (06) bereitzustellen,
wobei die erste Stufe (65) der Verdichtung einen Auslassdruck der ersten Stufe aufweist
und die letzte Stufe (75) der Verdichtung einen Saugdruck der letzten Stufe aufweist,
und einer oder mehrere optional gekühlte, verdichtete BOG-Zwischenströme (02, 03,
04) zwischen aufeinanderfolgenden Stufen der Verdichtung bereitgestellt werden;
Kühlen des verdichteten BOG-Auslassstroms (06) an einem oder mehreren ersten Kühlmediumströmen
(202, 302), um einen ersten gekühlten, verdichteten BOG-Strom (08) bereitzustellen;
Kühlen des ersten gekühlten, verdichteten BOG-Stroms (08) an mindestens einem zweiten
Kühlmediumstrom, um einen zweiten gekühlten, verdichteten BOG-Strom (34) bereitzustellen;
Kühlen des zweiten gekühlten, verdichteten BOG-Stroms (34) an einem dritten Kühlmediumstrom,
um einen dritten gekühlten, verdichteten BOG-Strom (35) bereitzustellen;
dadurch gekennzeichnet, dass das Verfahren ferner mindestens folgende Schritte beinhaltet:
Expandieren eines Teils des dritten gekühlten, verdichteten BOG-Stroms (35) auf einen
Druck zwischen demjenigen des Auslassdrucks der ersten Stufe und des Saugdrucks der
letzten Stufe, um einen ersten expandierten, gekühlten BOG-Strom (33a) bereitzustellen;
Verwenden des ersten expandierten, gekühlten BOG-Stroms (33a) als den dritten Kühlmediumstrom,
um einen ersten expandierten, erwärmten BOG-Strom (33b) bereitzustellen; und
Verwenden des ersten expandierten, erwärmten BOG-Stroms (33b) als den oder einen zweiten
Kühlmediumstrom.
2. Verfahren gemäß Anspruch 1, wobei die Ladung verflüssigten Ethans >0,1 mol-% Methan
oder >0,5 mol-% Methan beinhaltet.
3. Verfahren gemäß einem der vorangehenden Ansprüche, das drei oder vier Stufen der Verdichtung
beinhaltet.
4. Verfahren gemäß einem der vorangehenden Ansprüche, wobei das Kühlen des ersten gekühlten,
verdichteten BOG-Stroms (08) an dem zweiten Kühlmediumstrom in einem Economiser (180)
ausgeführt wird.
5. Verfahren gemäß einem der vorangehenden Ansprüche, wobei der gesamte erste gekühlte,
verdichtete BOG-Strom (08) an dem zweiten Kühlmediumstrom gekühlt wird.
6. Verfahren gemäß einem der vorangehenden Ansprüche, wobei der gesamte zweite gekühlte,
verdichtete BOG-Strom (34) an dem ersten expandierten, gekühlten BOG-Strom (33a) gekühlt
wird.
7. Verfahren gemäß einem der vorangehenden Ansprüche, das ferner folgende Schritte beinhaltet:
Bereitstellen eines gasförmigen Entlastungsstroms (51) aus dem ersten gekühlten, verdichteten
BOG-Strom (08);
Expandieren eines Teils des dritten gekühlten, verdichteten BOG-Stroms (35), um einen
vierten Kühlmediumstrom (33c) zu bilden; und
Kühlen des gasförmigen Entlastungsstroms (51) an dem vierten Kühlmediumstrom (33c),
um einen gekühlten Entlastungsstrom (53) und einen erwärmten vierten Kühlmediumstrom
(38) bereitzustellen.
8. Verfahren gemäß Anspruch 7, das ferner folgende Schritte beinhaltet:
Expandieren des gekühlten Entlastungsstroms (53), um einen expandierten weiteren gekühlten
Entlastungsstrom (63) bereitzustellen; und
Leiten des expandierten weiteren gekühlten Entlastungsstroms (63) zu einem Speicherbehälter
(50).
9. Verfahren gemäß Anspruch 7, das ferner folgende Schritte beinhaltet:
Trennen des weiteren gekühlten Entlastungsstroms (53), um einen Entlastungsauslassstrom
(55) und einen gekühlten Entlastungs-BOG-Rückstrom (57) bereitzustellen;
Expandieren des gekühlten Entlastungs-BOG-Rückstroms (57), um einen expandierten,
gekühlten Entlastungs-BOG-Rückstrom (59) bereitzustellen; und
Leiten des expandierten, gekühlten Entlastungs-BOG-Rückstroms (59) zu einem Speicherbehälter
(50).
10. Verfahren gemäß einem der vorangehenden Ansprüche, wobei der Schritt des Kühlens des
verdichteten BOG-Auslassstroms (06) an einem oder mehreren ersten Kühlmediumströmen
(202, 302), um einen ersten gekühlten, verdichteten BOG-Strom (08) bereitzustellen,
Folgendes beinhaltet:
Vorkühlen des verdichteten BOG-Auslassstroms (06) an einem Vorkühl-Kühlmediumstrom
(202) als einen ersten Kühlmediumstrom, um einen vorgekühlten, verdichteten BOG-Strom
(07) bereitzustellen; und
Kühlen des vorgekühlten, verdichteten BOG-Stroms (07) an einem ersten Kältemittelstrom
(302) als einen ersten Kühlmediumstrom, um den ersten gekühlten, verdichteten BOG-Strom
(08) bereitzustellen.
11. Verfahren gemäß Anspruch 10, wobei es sich bei dem Vorkühl-Kühlmediumstrom (202) um
einen oder mehrere von der aus einem Meerwasserstrom, einem Luftstrom, insbesondere
einem Umgebungsluftstrom, und/oder einem Kältemittelstrom ausgewählten Gruppe handelt.
12. Verfahren gemäß Anspruch 10 oder Anspruch 11, wobei es sich bei dem ersten Kältemittelstrom
(302) um eines oder mehrere von der aus Propan und Propylen ausgewählten Gruppe handelt.
13. Verfahren gemäß einem der vorangehenden Ansprüche, wobei es sich bei Stufen der Verdichtung
(65, 75) um die Verdichtungsstufen eines Mehrstufenverdichters handelt.
14. Verfahren gemäß einem der vorangehenden Ansprüche, wobei der gesamte zweite Kühlmediumstrom
den ersten expandierten, erwärmten BOG-Strom (33b) beinhaltet.
15. Verfahren gemäß einem der vorangehenden Ansprüche, wobei der als der zweite Kühlmediumstrom
verwendete erste expandierte, erwärmte BOG-Strom (33b) sowohl eine Flüssig- als auch
eine Gasphase beinhaltet.
16. Verfahren gemäß Anspruch 15, wobei die Flüssig- und die Gasphase des als der zweite
Kühlmediumstrom verwendeten ersten expandierten, erwärmten BOG-Stroms (33b) bei dem
Kühlen des ersten gekühlten, verdichteten BOG-Stroms (08) getrennt werden.
17. Eine Vorrichtung zum Kühlen eines Boil-Off-Gasstroms (01) aus einer Ladung verflüssigten
Ethans in einem eine Vielzahl von Komponenten beinhaltenden schwimmenden Transportfahrzeug,
wobei die Vorrichtung mindestens Folgendes beinhaltet:
ein Verdichtungssystem (60) zum Verdichten eines Boil-Off-Gasstroms (01) aus einer
Ladung verflüssigten Ethans, wobei das Verdichtungssystem zwei oder mehr Stufen der
Verdichtung beinhaltet, die mindestens eine erste Stufe (65) und eine letzte Stufe
(75) beinhalten, um einen verdichteten BOG-Auslassstrom (06) bereitzustellen, wobei
optional gekühlte, verdichtete BOG-Zwischenströme (02, 03, 04) zwischen aufeinanderfolgenden
Stufen der Verdichtung bereitgestellt sind,
einen oder mehrere erste Wärmetauscher (200, 300) zum Kühlen des verdichteten BOG-Auslassstroms
(06), um einen ersten gekühlten, verdichteten BOG-Strom (08) bereitzustellen;
dadurch gekennzeichnet, dass die Vorrichtung ferner Folgendes beinhaltet:
einen oder mehr zweite Wärmetauscher (180) zum weiteren Kühlen des ersten gekühlten,
verdichteten BOG-Stroms (08) an einem in dem einen oder den mehreren zweiten Wärmetauschern
zu trennenden Mischphasen-Kühlmediumstrom (33b), um einen zweiten gekühlten, verdichteten
BOG-Strom (34) bereitzustellen; und
einen oder mehrere dritte Wärmetauscher (195) zum weiteren Kühlen des zweiten gekühlten,
verdichteten BOG-Stroms (34), um einen dritten gekühlten, verdichteten BOG-Strom (35)
bereitzustellen, und eine Druckminderungseinrichtung (80) zum Expandieren eines Teils
des dritten gekühlten, verdichteten BOG-Stroms (35) auf einen Druck zwischen demjenigen
des Auslassdrucks der ersten Stufe und des Saugdrucks der letzten Stufe, um einen
ersten expandierten, gekühlten BOG-Strom (33a) als das Kühlmedium in dem einen oder
den mehreren Wärmetauschern (195) und dann den Mischphasen-Kühlmediumstrom (33b) in
dem einen oder den mehreren zweiten Wärmetauschern (180) bereitzustellen.
18. Vorrichtung gemäß Anspruch 17, die unter Verwendung des Verfahrens gemäß einem der
Ansprüche 1 bis 16 betreibbar ist.
19. Vorrichtung gemäß Anspruch 17 oder Anspruch 18, wobei es sich bei dem zweiten Wärmetauscher
(180) um einen Economiser handelt.
20. Ein schwimmendes Transportfahrzeug für ein verflüssigtes Ethan, das die Vorrichtung
gemäß einem der Ansprüche 17 bis 19 aufweist oder das Verfahren gemäß einem der Ansprüche
1 bis 16 verwendet.
1. Un procédé de refroidissement d'un flux de gaz d'évaporation (01) d'une cargaison
d'éthane liquéfié dans un navire d'acheminement flottant, ledit procédé comprenant
au moins les étapes consistant à :
comprimer un flux de gaz d'évaporation (01) de ladite cargaison d'éthane liquéfié
en deux étages de compression ou plus comprenant au moins un premier étage (65) et
un dernier étage (75) pour fournir un flux de décharge de BOG comprimé (06), dans
lequel ledit premier étage (65) de compression a une pression de décharge de premier
étage et ledit dernier étage (75) de compression a une pression d'aspiration de dernier
étage, et un ou plusieurs flux de BOG comprimés intermédiaires, facultativement refroidis,
(02, 03, 04) sont fournis entre des étages de compression consécutifs ;
refroidir le flux de décharge de BOG comprimé (06) contre un ou plusieurs premiers
flux d'agent refroidisseur (202, 302) pour fournir un premier flux de BOG comprimé
refroidi (08) ;
refroidir le premier flux de BOG comprimé refroidi (08) contre au moins un deuxième
flux d'agent refroidisseur pour fournir un deuxième flux de BOG comprimé refroidi
(34) ;
refroidir le deuxième flux de BOG comprimé refroidi (34) contre un troisième flux
d'agent refroidisseur pour fournir un troisième flux de BOG comprimé refroidi (35)
;
caractérisé en ce que le procédé comprend en outre au moins les étapes consistant à :
dilater une portion du troisième flux de BOG comprimé refroidi (35) jusqu'à une pression
comprise entre celle de la pression de décharge de premier étage et celle de la pression
d'aspiration de dernier étage pour fournir un premier flux de BOG refroidi dilaté
(33a) ;
utiliser le premier flux de BOG refroidi dilaté (33a) comme troisième flux d'agent
refroidisseur pour fournir un premier flux de BOG chauffé dilaté (33b) ; et
utiliser le premier flux de BOG chauffé dilaté (33b) comme le ou un deuxième flux
d'agent refroidisseur.
2. Le procédé de la revendication 1 dans lequel la cargaison d'éthane liquéfié comprend
> 0,1 % molaire de méthane ou > 0,5 % molaire de méthane.
3. Le procédé selon n'importe laquelle des revendications précédentes comprenant trois
ou quatre étages de compression.
4. Le procédé selon n'importe laquelle des revendications précédentes dans lequel le
refroidissement du premier flux de BOG comprimé refroidi (08) contre le deuxième flux
d'agent refroidisseur est effectué dans un économiseur (180).
5. Le procédé selon n'importe laquelle des revendications précédentes dans lequel l'intégralité
du premier flux de BOG comprimé refroidi (08) est refroidie contre le deuxième flux
d'agent refroidisseur.
6. Le procédé selon n'importe laquelle des revendications précédentes dans lequel l'intégralité
du deuxième flux de BOG comprimé refroidi (34) est refroidie contre le premier flux
de BOG refroidi dilaté (33a).
7. Le procédé selon n'importe laquelle des revendications précédentes comprenant en outre
les étapes consistant à :
fournir un flux d'évacuation gazeuse (51) depuis le premier flux de BOG comprimé refroidi
(08) ;
dilater une portion du troisième flux de BOG comprimé refroidi (35) pour former un
quatrième flux d'agent refroidisseur (33c) ; et
refroidir le flux d'évacuation gazeuse (51) contre le quatrième flux d'agent refroidisseur
(33c) pour fournir un flux d'évacuation refroidi (53) et un quatrième flux d'agent
refroidisseur chauffé (38).
8. Le procédé selon la revendication 7 comprenant les étapes supplémentaires consistant
à :
dilater le flux d'évacuation refroidi (53) pour fournir un flux d'évacuation refroidi
davantage dilaté (63) ; et
faire passer le flux d'évacuation refroidi davantage dilaté (63) vers une cuve de
stockage (50).
9. Le procédé selon la revendication 7 comprenant l'étape supplémentaire consistant à
:
séparer le flux d'évacuation refroidi davantage (53) pour fournir un flux de décharge
d'évacuation (55) et un flux en retour de BOG d'évacuation refroidi (57) ;
dilater le flux en retour de BOG d'évacuation refroidi (57) pour fournir un flux en
retour de BOG d'évacuation refroidi dilaté (59) ; et
faire passer le flux en retour de BOG d'évacuation refroidi dilaté (59) vers une cuve
de stockage (50).
10. Le procédé de n'importe lesquelles des revendications précédentes dans lequel l'étape
consistant à refroidir le flux de décharge de BOG comprimé (06) contre un ou plusieurs
premiers flux d'agent refroidisseur (202, 302) pour fournir un premier flux de BOG
comprimé refroidi (08) comprend :
pré-refroidir le flux de décharge de BOG comprimé (06) contre un flux d'agent refroidisseur
de pré-refroidissement (202) comme premier flux d'agent refroidisseur pour fournir
un flux de BOG comprimé pré-refroidi (07) ; et
refroidir le flux de BOG comprimé pré-refroidi (07) contre un premier flux réfrigérant
(302) comme premier flux d'agent refroidisseur pour fournir le premier flux de BOG
comprimé refroidi (08).
11. Le procédé de la revendication 10 dans lequel le flux d'agent refroidisseur de pré-refroidissement
(202) est un ou plusieurs flux du groupe sélectionnés parmi : un flux d'eau de mer,
un flux d'air, plus particulièrement un flux d'air ambiant et/ou un flux de réfrigérant.
12. Le procédé de n'importe lesquelles de la revendication 10 ou de la revendication 11
dans lequel le premier flux de réfrigérant (302) est un ou plusieurs éléments du groupe
sélectionnés parmi le propane et le propylène.
13. Le procédé de n'importe lesquelles des revendications précédentes dans lequel des
étages de compression (65, 75) sont les étages de compression d'un compresseur multi-étage.
14. Le procédé de n'importe lesquelles des revendications précédentes dans lequel l'intégralité
du deuxième flux d'agent refroidisseur comprend le premier flux de BOG chauffé dilaté
(33b).
15. Le procédé de n'importe lesquelles des revendications précédentes dans lequel le premier
flux de BOG chauffé dilaté (33b) utilisé comme deuxième flux d'agent refroidisseur
comprend à la fois des phases liquides et gazeuses.
16. Le procédé de la revendication 15 dans lequel les phases liquides et gazeuses du premier
flux de BOG chauffé dilaté (33b) utilisé comme deuxième flux d'agent refroidisseur
sont séparées dans le refroidissement du premier flux de BOG comprimé refroidi (08).
17. Un appareil pour refroidir un flux de gaz d'évaporation (01) d'une cargaison d'éthane
liquéfié dans un navire d'acheminement flottant comprenant une pluralité de composants,
ledit appareil comprenant au moins :
un système de compression (60) pour comprimer un flux de gaz d'évaporation (01) d'une
cargaison d'éthane liquéfié, ledit système de compression comprenant deux étages de
compression ou plus comprenant au moins un premier étage (65) et un dernier étage
(75) pour fournir un flux de décharge de BOG comprimé (06), dans lequel des flux de
BOG comprimés intermédiaires, facultativement refroidis, (02, 03, 04) sont fournis
entre des étages de compression consécutifs,
un ou plusieurs premiers échangeurs de chaleur (200, 300) pour refroidir le flux de
décharge de BOG comprimé (06) pour fournir un premier flux de BOG comprimé refroidi
(08) ;
caractérisé en ce que l'appareil comprend en outre :
un ou plusieurs deuxièmes échangeurs de chaleur (180) pour refroidir davantage le
premier flux de BOG comprimé refroidi (08) contre un flux d'agent refroidisseur de
phase mélangée (33b) à séparer dans ces un ou plusieurs deuxièmes échangeurs de chaleur,
pour fournir un deuxième flux de BOG comprimé refroidi (34) ; et
un ou plusieurs troisièmes échangeurs de chaleur (195) pour refroidir davantage le
deuxième flux de BOG comprimé refroidi (34) pour fournir un troisième flux de BOG
comprimé refroidi (35) et un dispositif de réduction de pression (80) pour dilater
une portion du troisième flux de BOG comprimé refroidi (35) jusqu'à une pression comprise
entre celle de la pression de décharge de premier étage et celle de la pression d'aspiration
de dernier étage pour fournir un premier flux de BOG refroidi dilaté (33a) comme agent
refroidisseur dans ces un ou plusieurs troisièmes échangeurs de chaleur (195) et ensuite
le flux d'agent refroidisseur de phase mélangée (33b) dans ces un ou plusieurs deuxièmes
échangeurs de chaleur (180).
18. Appareil tel que revendiqué dans la revendication 17 qui peut fonctionner en utilisant
le procédé de n'importe laquelle des revendications 1 à 16.
19. Appareil tel que revendiqué dans la revendication 17 ou la revendication 18 dans lequel
le deuxième échangeur de chaleur (180) est un économiseur.
20. Un navire d'acheminement flottant pour un éthane liquéfié, ayant l'appareil tel que
défini dans n'importe laquelle des revendications 17 à 19 ou utilisant le procédé
tel que défini dans n'importe laquelle des revendications 1 à 16.