U.S. patent application number 12/742752 was filed with the patent office on 2010-11-25 for method and apparatus for liquefying a hydrocarbon stream and floating vessel or offshore platform comprising the same.
Invention is credited to Michiel Gijsbert Van Aken, Jordan Adriaan Van Dam.
Application Number | 20100293996 12/742752 |
Document ID | / |
Family ID | 39521226 |
Filed Date | 2010-11-25 |
United States Patent
Application |
20100293996 |
Kind Code |
A1 |
Van Aken; Michiel Gijsbert ;
et al. |
November 25, 2010 |
METHOD AND APPARATUS FOR LIQUEFYING A HYDROCARBON STREAM AND
FLOATING VESSEL OR OFFSHORE PLATFORM COMPRISING THE SAME
Abstract
A method of liquefying a hydrocarbon stream, such as natural
gas, comprising at least the steps of : (a) partially liquefying a
hydrocarbon feed stream (10) to provide a partially liquefied
hydrocarbon stream (20); (b) passing the partially liquefied
hydrocarbon stream (20) through a first gas/liquid separator (B) to
provide a methane-enriched gaseous overhead stream (30a) and a
mixed C.sub.2.sup.+ enriched liquid bottom stream (30b); (c) adding
at least a part (30c) of the mixed C.sub.2.sup.+ enriched bottom
stream (30b) to a mixed refrigerant circuit (4) to change the
C.sub.2 component inventory of the mixed refrigerant (100) in the
mixed refrigerant circuit (4); and (d) liquefying the
methane-enriched gaseous overhead stream (30a, 30) against at least
a fraction (110, 125, 135) of the mixed refrigerant (100) in the
mixed refrigerant circuit (4) to provide a liquefied hydrocarbon
stream (50).
Inventors: |
Van Aken; Michiel Gijsbert;
(The Hague, NL) ; Van Dam; Jordan Adriaan; (The
Hague, NL) |
Correspondence
Address: |
SHELL OIL COMPANY
P O BOX 2463
HOUSTON
TX
772522463
US
|
Family ID: |
39521226 |
Appl. No.: |
12/742752 |
Filed: |
November 17, 2008 |
PCT Filed: |
November 17, 2008 |
PCT NO: |
PCT/EP2008/065687 |
371 Date: |
August 3, 2010 |
Current U.S.
Class: |
62/614 |
Current CPC
Class: |
F25J 2220/64 20130101;
F25J 1/0278 20130101; F25J 2220/62 20130101; F25J 1/0292 20130101;
F25J 1/0249 20130101; F25J 1/0279 20130101; F25J 1/0022 20130101;
F25J 1/0212 20130101; F25J 1/0247 20130101; F25J 1/025 20130101;
F25J 1/0055 20130101 |
Class at
Publication: |
62/614 |
International
Class: |
F25J 1/02 20060101
F25J001/02 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 16, 2007 |
EP |
07120866.4 |
Claims
1. A method of liquefying a hydrocarbon stream, comprising at least
the steps of: (c) circulating a mixed refrigerant through a mixed
refrigerant circuit: (a) partially liquefying a hydrocarbon feed
stream in a first cooling stage to provide a partially liquefied
hydrocarbon stream by passing the hydrocarbon feed stream against
at least a first fraction of the mixed refrigerant of the mixed
refrigerant circuit in at least one first heat exchanger and
reducing the temperature of the hydrocarbon feed stream from
0.degree. C. or higher to in the range -20.degree. C. to
-70.degree. C.; (b) passing the partially liquefied hydrocarbon
stream through a first gas/liquid separator to provide a
methane-enriched gaseous overhead stream and a mixed C.sub.2+
enriched liquid bottom stream; (d) adding, without fractionation,
at least a part of the mixed C.sub.2+ enriched bottom stream to the
mixed refrigerant circuit to change the C.sub.2+ component
inventory of the mixed refrigerant in the mixed refrigerant
circuit; and (e) liquefying the methane-enriched gaseous overhead
stream in a second cooling stage by passage through at least one
second heat exchanger and heat exchanging against at least a second
fraction of the mixed refrigerant circulating in the mixed
refrigerant circuit, to provide a liquefied hydrocarbon stream,
wherein in the temperature of the methane-enriched gaseous overhead
stream in the second cooling stage is reduced to below -100.degree.
C.; which method does not use fractionation to change the mixed
refrigerant inventory.
2. (canceled)
3. The method as claimed in claim 1, wherein the methane-enriched
gaseous overhead stream is partially liquefied during passage of
through the at least one second heat exchanger to form a further
partially_liquefied hydrocarbon stream, the method further
comprising passing the further partially-liquefied hydrocarbon
stream through a second gas/liquid separator to provide a second
methane-enriched gaseous overhead stream, and a second liquid
bottom stream; and using at least a part of the second liquid
bottom stream to change the component inventory of the mixed
refrigerant circuit.
4. The method as claimed in claim 1, further comprising the steps
of: (f) passing at least one fraction of the mixed refrigerant
through at least one heat exchanger to provide one or more cooled
mixed refrigerant streams; (g) passing at least one of the cooled
mixed refrigerant streams through at least one or more refrigerant
gas/liquid separator to provide one or more gaseous overhead
refrigerant streams and one or more liquid bottom refrigerant
streams; and (h) removing at least a part of at least one of the
gaseous overhead refrigerant streams and the liquid bottom
refrigerant streams to change the component inventory of the mixed
refrigerant circuit.
5. The method as claimed in claim 1, wherein at least one fraction
of the mixed refrigerant is removed from the mixed refrigerant
circuit to change the component inventory of the mixed
refrigerant.
6. The method as claimed in claim 5, wherein the component
inventory of at least one stream added to the mixed refrigerant
circuit is different from the component inventory of at least one
fraction of the mixed refrigerant being removed from the same mixed
refrigerant circuit.
7. (canceled)
8. The method as claimed in claim 1, wherein the first fraction of
the mixed refrigerant in the at least one first heat exchanger also
provides an at least partly evaporated mixed refrigerant stream;
the method further comprising: passing the at least partly
evaporated mixed refrigerant stream through a second refrigerant
gas/liquid separator to provide a gaseous overhead refrigerant
stream and a liquid bottom refrigerant stream; and removing at
least a part of the liquid bottom refrigerant stream from the mixed
refrigerant circuit to change the component inventory of the mixed
refrigerant.
9. The method as claimed in claim 1, further comprising: at least
partially evaporating at least one of the at least one fraction of
the mixed refrigerant in the first and second heat exchangers to
provide at least one of a liquid refrigerant bottom stream and at
least one gaseous refrigerant overhead stream from at least one of
the first and second heat exchangers; and removing at least a part
of at least one of the gaseous refrigerant overhead streams and the
liquid refrigerant bottom streams to change the component inventory
of the mixed refrigerant circuit.
10. The method as claimed in claim 1, wherein the mixed refrigerant
comprises at least two of the group comprising: nitrogen, methane,
ethane, ethylene, propane, propylene, butanes, pentanes.
11. The method as claimed in claim 1, wherein the hydrocarbon feed
stream comprises natural gas, and wherein the liquefied hydrocarbon
stream is LNG.
12. The method as claimed claim 1, further comprising: passing the
liquefied hydrocarbon stream through an end flash valve and an end
gas/liquid separator, to provide a gaseous overhead stream and a
liquid bottom stream; and combining the gaseous overhead stream
from the end gas/liquid separator and at least a part of the
gaseous overhead refrigerant stream from one of the refrigerant
gas/liquid separators, to provide a combined stream for compression
and use as a fuel gas.
13. The method as claimed in claim 1, wherein the mixed refrigerant
is initially formed of natural gas from which the C.sub.5+
components have been removed.
14. The method as claimed in claim 1, performed on a floating
vessel or off-shore platform.
15. (canceled)
16. The method as claimed in claim 1, wherein the mixed C.sub.2+
enriched liquid bottom stream is provided in step (b) without
fractionation of the partially liquefied hydrocarbon stream.
17. The method as claimed in claim 1, wherein the method comprises
a single mixed refrigerant circuit to provide cooling to the at
least one first heat exchanger and the at least one second heat
exchanger.
18. The method as claimed in claim 1, wherein the hydrocarbon feed
stream, essentially consists of natural gas, and wherein the
liquefied hydrocarbon stream is LNG.
Description
[0001] The present invention relates to a method and apparatus for
liquefying a hydrocarbon stream. In other aspects, the present
invention relates to a floating vessel or an offshore platform
comprising such an apparatus or on which such a method is
performed.
[0002] A commonly suggested hydrocarbon feed stream may comprise or
essentially consist of natural gas, but it could also be derived
from other sources.
[0003] Several methods of liquefying a natural gas stream thereby
obtaining liquefied natural gas (LNG) are known. It is desirable to
liquefy a natural gas stream for a number of reasons. As an
example, natural gas can be stored and transported over long
distances more readily as a liquid than in gaseous form because it
occupies a smaller volume and does not need to be stored at a high
pressure.
[0004] U.S. Pat. No. 3,274,787 describes a method wherein natural
gas is liquefied. The natural gas, which contains methane, C.sub.2
to C.sub.4 and some C.sub.5 and C.sub.6 hydrocarbons, arrives
through a conduit in a heat exchanger where it is cooled and
slightly condensed and then passes to a separator. The remaining
natural gas is further cooled, liquefied and sub-cooled. The
condensate enriched in heavier hydrocarbons passes through an
expansion valve to a rectification column, which is heated at the
bottom. The residual liquid separated in the sump of the
rectification column is introduced into a train of fractionation
columns each equipped with a condenser and a heater. Propanes and
butanes drawn from the tops of the fractionation columns are
combined with recycled fractions of cycling gas, which is used to
cool the natural gas, in order to compensate for inevitable butane
and propane losses in the recycling gas.
[0005] Thus, the process of U.S. Pat. No. 3,274,787 requires a
plurality of fractionation columns, in addition to a rectifier
column. These are expensive to build and operate, and also occupy
plot space.
[0006] U.S. Pat. No. 7,234,321 describes another method for
liquefying methane-rich gas. A stream of incoming feed gas (which
has been treated to remove components that would interfere with the
liquefaction, such as freezables) is pre-cooled and separated in a
condensate separator into a vapour stream and a liquid condensate
stream which consists mainly of propane, butane and C.sub.5.sup.+.
One of the purposes of this separation is to provide a source for
refrigerant fluid. The condensate liquid stream is flashed, heated
and fed into another separator to remove most of the methane
content from the liquid. A portion of the liquid, after further
heating, enters into yet another separator. The vapour stream from
this separator is admitted, when required, to the inventory of
refrigerant in a closed circuit refrigeration system. Liquid may be
removed from the refrigeration system in order to change the
composition of the refrigerant.
[0007] The cycled refrigerant in U.S. Pat. No. 7,234,321 is used to
cool the gas to a temperature of about -75 to -85 .degree. C.
before it flows to a liquefying expander wherein the gas is
liquefied.
[0008] The process of U.S. Pat. No. 7,234,321 uses only separators,
and no fractionation, to remove the natural gas liquids from the
natural gas. It is therefore a drawback in this process that is
will be difficult to get rid of all the methane and other light
components such as nitrogen in the natural gas liquid stream, and
in particular in the vapour part of the natural gas liquid stream
that is used to feed into the refrigerant inventory. These
components do not condensate and are therefore not effective in
removing heat from the natural gas. Moreover, these components are
hard to separate from the other components in the refrigerant at
the relative high temperatures (about -75 to -85.degree. C.) at
which the refrigerant cycle is operated.
[0009] The present invention provides a method of liquefying a
hydrocarbon stream, comprising at least the steps of: [0010] (a)
partially liquefying a hydrocarbon feed stream to provide a
partially liquefied hydrocarbon stream; [0011] (b) passing the
partially liquefied hydrocarbon stream through a first gas/liquid
separator to provide a methane-enriched gaseous overhead stream and
a mixed C.sub.2.sup.+ enriched liquid bottom stream; [0012] (c)
circulating a mixed refrigerant through a mixed refrigerant
circuit; [0013] (d) adding, without fractionation, at least a part
of the mixed C.sub.2.sup.+ enriched bottom stream to the mixed
refrigerant circuit to change the C.sub.2.sup.+ component inventory
of the mixed refrigerant in the mixed refrigerant circuit; and
[0014] (e) liquefying the methane-enriched gaseous overhead stream
by heat exchanging against at least a fraction of the mixed
refrigerant circulating in the mixed refrigerant circuit, to
provide a liquefied hydrocarbon stream.
[0015] The present invention also provides an apparatus for
liquefying a hydrocarbon stream, the apparatus at least
comprising:
[0016] one or more first heat exchangers for partially liquefying a
hydrocarbon feed stream to provide a partially liquefied
hydrocarbon stream;
[0017] a first gas/liquid separator (B) through which the partially
liquefied hydrocarbon stream can pass to provide a methane-enriched
gaseous overhead stream and a mixed C.sub.2.sup.+ enriched liquid
bottom stream;
[0018] a mixed refrigerant circuit comprising a mixed
refrigerant;
[0019] one or more lines to pass at least a part of the mixed
C.sub.2.sup.+ bottom stream into the mixed refrigerant circuit,
without a fractionator, to change the component inventory of mixed
refrigerant; and
[0020] one or more second heat exchangers to liquefy the
methane-enriched gaseous overhead stream using at least a fraction
of the mixed refrigerant to provide a liquefied hydrocarbon
stream.
[0021] The present invention further provides a floating vessel and
an off-shore platform, e.g. in the form of a caisson, having
apparatus and/or using a method as defined herein.
[0022] Embodiments of the present invention will now be described
by way of example only, and with reference to the accompanying
non-limiting drawings in which:
[0023] FIG. 1 is a diagrammatic scheme of a hydrocarbon liquefying
process showing an embodiment of the present invention;
[0024] FIG. 2 is a more detailed scheme of FIG. 1 showing various
embodiments of the present invention; and
[0025] FIG. 3 is a diagrammatic floating vessel showing another
embodiment of the present invention.
[0026] For the purpose of this description, a single reference
number will be assigned to a line as well as a stream carried in
that line. Same reference numbers refer to similar components.
[0027] The present disclosure provides an improved method of
liquefying a hydrocarbon stream, such as natural gas, which can be
self-sufficient in changing its mixed refrigerant inventory, in
particular in a space-limited location such as on a floating
vessel.
[0028] Moreover, the present disclose provides an improved method
of liquefying a hydrocarbon stream, such as natural gas, which does
not require fractionation to change the mixed refrigerant
inventory.
[0029] It is presently proposed to provide a refrigerant portion
from a hydrocarbon feed stream by partially liquefying the
hydrocarbon feed stream followed by simple separation and not
fractionation of the partially liquefied hydrocarbon stream in a
first gas/liquid separator to provide a mixed C.sub.2.sup.+
enriched stream. The mixed C.sub.2.sup.+ enriched stream can be
used to change the C.sub.2.sup.+ component inventory of the mixed
refrigerant in a mixed refrigerant circuit.
[0030] The term fractionation implies use of a fractionation
column, which is understood to be any type of distillation column
that has a source of heat in the lower part of the column (such as
a warm stream (e.g. a reboiler stream) or a heating coil) and/or a
drain of heat at the top (such as a condenser or a cold stream such
as a reflux stream). Separation, as opposed to fractionation,
merely separates vapour and liquid phases from a mixed phase
stream, but does not involve such source and/or drain of heat. As a
result, separation is not very selective in terms of separating
molecules, which thermodynamically inevitably causes a relatively
high amount of light molecules such as methane and nitrogen to be
present in the mixed C.sub.2.sup.+ enriched bottom stream which is
used to change the mixed refrigerant inventory.
[0031] However, in the present invention this is not a problem
because the mixed refrigerant is used to liquefy the
methane-enriched vapour stream. In order to reach temperatures low
enough to liquefy the methane-enriched vapour stream, a certain
inventory of light molecules in the mixed refrigerant is
effective.
[0032] Moreover, even if the concentration of light molecules in
the mixed refrigerant happens to become too high, it is relatively
easy to separate and remove these molecules from the mixed
refrigerant because of the low temperatures needed anyway to
liquefy the methane-enriched vapour stream.
[0033] In the present specification, the term "liquefy", as opposed
to "partially liquefy", implies full liquefaction.
[0034] An advantage of the present invention is that a
fractionation column is not required to further fractionate the
C.sub.2.sup.+ enriched stream before using it to change the mixed
refrigerant inventory. This saves space. It also saves the need for
further processing units and equipment, including separate storage
facilities, for providing every separate hydrocarbon which it
usually desired to have available for changing the C.sub.2.sup.+
component inventory of a mixed refrigerant in a mixed refrigerant
circuit. This is especially propane and butane storage tanks, which
require higher safety requirements, especially in space-limited
environments.
[0035] The present invention may provide a source of one or more of
the components of the mixed refrigerant from one or more of the
gas/liquid separators described herein, so that change of the
component inventory of the mixed refrigerant can be provided for
from the method of liquefying without separate supply of one of the
more of the components. Separate supply is conventionally provided
in the art by the location of a number of storage tanks nearby to
the hydrocarbon liquefying process, each storage tank storing one
separated component such as ethane, propane, etc, which can supply
its component to the mixed refrigerant on demand. It is an
advantage of the present invention that separate nearby storage of
the components of the mixed refrigerant can be avoided.
[0036] Each gas/liquid separator throughout the present disclosure
may be provided in the form of a tank, optionally provided with
internals as known in the art including a schoepentoeter and/or a
mist mat and/or a swirl deck.
[0037] FIG. 1 shows a general scheme 1 for a hydrocarbon liquefying
process, generally involving cooling and liquefying a hydrocarbon
feed stream 10, such as natural gas, by heat exchanging against a
mixed refrigerant stream being circulated in a mixed refrigerant
circuit 4.
[0038] The hydrocarbon feed stream 10 may be any suitable gas
stream to be cooled and liquefied, but is usually a
methane-comprising gas stream such as a natural gas stream obtained
from natural gas or petroleum reservoirs. As an alternative the
hydrocarbon feed stream may also be obtained from another source,
also including a synthetic source such as a Fischer-Tropsch
process.
[0039] Usually a natural gas stream is comprised substantially of
methane. Preferably the hydrocarbon feed stream comprises at least
50 mol % methane, more preferably at least 80 mol % methane.
[0040] Depending on the source, natural gas may contain varying
amounts of hydrocarbons heavier than methane such as in particular
ethane, propane and the butanes, and possibly lesser amounts of
pentanes and aromatic hydrocarbons. The composition varies
depending upon the type and location of the gas.
[0041] Conventionally, the hydrocarbons heavier than butanes are
removed as far as efficiently possible from the hydrocarbon feed
stream prior to any significant cooling for several reasons, such
as having different freezing (particularly those higher than
liquefaction temperature of methane) temperatures that may cause
them to block parts of a methane liquefaction plant. Ethanes,
propanes, and butanes are typically removed to meet a desired
specification of the liquefied hydrocarbon product such as LNG.
[0042] It is presently intended that the hydrocarbon feed stream is
maintained with at least some of the hydrocarbons heavier than
methane. At least some of the hydrocarbons heavier than methane may
still be removed if desired, for example where relatively few
heavier hydrocarbons are expected to be required as described
hereinafter. Preferably, there is less or even no heavy hydrocarbon
removal from the hydrocarbon feed stream during start-up of the
liquefaction process, so that there are more heavier hydrocarbons
available for addition to the mixed refrigerant circuit.
[0043] Another advantage of the present invention is that it is not
limited to only refilling the refrigerant with a single component
fraction, such as an ethane-rich fraction as is the case in U.S.
Pat. No. 5,588,306.
[0044] The hydrocarbon feed stream may also contain
non-hydrocarbons such as H.sub.2O, N.sub.2, CO.sub.2, Hg, H.sub.2S
and other sulphur compounds, and the like. If desired, the
hydrocarbon feed stream comprising the natural gas may be
pre-treated before cooling and liquefying to remove such
components. This pre-treatment may comprise reduction and/or
removal of undesired components such as CO.sub.2 and H.sub.2S or
other steps such as early cooling, pre-pressurizing or the like. As
these steps are well known to the person skilled in the art, their
mechanisms are not further discussed here.
[0045] To provide a partially liquefied hydrocarbon stream, the
hydrocarbon feed stream 10 as hereinbefore described could be
cooled prior to feeding it into a first gas/liquid separator (B).
Such initial cooling could be provided by a number of methods known
in the art. One example is by passing the hydrocarbon feed stream
10 against a refrigerant (101,160), such as at least a first
fraction of the mixed refrigerant of the mixed refrigerant circuit,
in one or more first heat exchangers 12, to provide a partially
liquefied hydrocarbon stream 20. The partially liquefied
hydrocarbon stream 20 is preferably at a temperature below
0.degree. C.
[0046] Preferably, any such first heat exchangers 12 producing a
partially liquefied hydrocarbon stream could form part of a first
cooling stage, and one or more second heat exchangers 22 used to
liquefy any part of the hydrocarbon stream could form part of one
or more second or third cooling stages.
[0047] In this way, the present invention may involve two or more
cooling stages, each stage having one or more steps, parts etc. For
example, each cooling stage may comprise one to five heat
exchangers. The or a part of a hydrocarbon stream and/or the mixed
refrigerant may not pass through all, and/or all the same, the heat
exchangers of a cooling stage.
[0048] In one embodiment of the present invention, the hydrocarbon
liquefying process comprises two or three cooling stages. A first
cooling stage is preferably intended to reduce the temperature of a
hydrocarbon feed stream to below 0.degree. C., usually in the range
-20.degree. C. to -70.degree. C. Such a first cooling stage is
sometimes also termed a `pre-cooling` stage.
[0049] The second cooling stage is preferably separate from the
first cooling stage. That is, the second cooling stage comprises
one or more separate heat exchangers 22. Such a second cooling
stage is sometimes also termed a `main cooling` stage.
[0050] The second cooling stage is preferably intended to reduce
the temperature of a hydrocarbon stream 30, usually at least a part
(30a) of the partially liquefied hydrocarbon stream 20 cooled and
partially liquefied by the first cooling stage, to below
-100.degree. C.
[0051] Heat exchangers for use as the one or more first or the one
or more second heat exchangers are well known in the art. At least
one of the second heat exchangers is preferably a spool-wound
cryogenic heat exchanger known in the art. Optionally, a heat
exchanger could comprise one or more cooling sections within its
shell, and each cooling section could be considered as a cooling
stage or as a separate `heat exchanger` to the other cooling
locations.
[0052] Further, the person skilled in the art will readily
understand that after liquefaction, the liquefied hydrocarbon
stream 50 may be further processed, if desired. As an example, the
obtained LNG may be depressurized by means of a Joule-Thomson valve
51 and/or by means of a cryogenic turbo-expander.
[0053] In yet another embodiment of the present invention, the
method further comprises the steps of: [0054] (f) passing one or
more fractions of the mixed refrigerant through one or more heat
exchangers to provide one or more cooled mixed refrigerant streams;
[0055] (g) passing at least one of the cooled mixed refrigerant
streams through one or more refrigerant gas/liquid separators to
provide one or more gaseous overhead refrigerant streams and one or
more liquid bottom refrigerant streams; and [0056] (h) removing at
least a part of at least one of the gaseous overhead refrigerant
streams and the liquid bottom refrigerant streams to change the
component inventory of the mixed refrigerant circuit.
[0057] The mixed refrigerant of the mixed refrigerant circuit may
be formed from a mixture of two or more components selected from
the group comprising: nitrogen, methane, ethane, ethylene, propane,
propylene, butanes, pentanes, etc. The present invention may
involve the use of one or more other refrigerants, in separate or
overlapping refrigerant circuits or other cooling circuits.
[0058] A mixed refrigerant or a mixed refrigerant stream as
referred to herein comprises at least 5 mol % of two different
components. A common composition for a mixed refrigerant can
be:
TABLE-US-00001 Nitrogen 0-10 mol % Methane (C.sub.1) 30-70 mol %
Ethane (C.sub.2) 30-70 mol % Propane (C.sub.3) 0-30 mol % Butanes
(C.sub.4) 0-15 mol %
[0059] The total composition comprises 100 mol %.
[0060] It is known that the composition and inventory of a mixed
refrigerant may be varied by a liquefaction plant user depending
upon the geographical location of the liquefaction plant, one
factor being the expected daily or weekly average ambient
temperature, and another factor being seasonal adjustment to take
account of variation in ambient temperature over a year. A
refrigerant stream may also need replenishment due to leakage.
[0061] The present invention may also provide a process for
reducing one or more of, optionally each of, the above components
from a mixed refrigerant circuit via one or more of the gas/liquid
separators and/or one or more of the heat exchangers involved, so
that change of the component inventory of the mixed refrigerant in
the mixed refrigerant circuit can also be provided thereby. Removal
of a component also changes the composition and/or inventory of the
mixed refrigerant. In its simplest form, one or more fractions of
the mixed refrigerant can be removed from the mixed refrigerant
circuit to change the component inventory of the mixed
refrigerant.
[0062] For example, a fraction of the mixed refrigerant in a heat
exchanger, such as the first 12 and second heat exchanger(s) 22, is
at least partly evaporated in cooling and/or liquefying the
hydrocarbon stream, to provide a bottom refrigerant stream
(respectively 180a 180b) and a gaseous refrigerant overhead stream
(respectively 190a 190b) from the heat exchanger (respectively 12,
22a), and at least a part of at least one of the gaseous
refrigerant overhead stream and/or bottom refrigerant stream could
also be removed from the mixed refrigerant to change the component
inventory of the remaining mixed refrigerant.
[0063] The term "inventory" as used herein relates to both the
amount (in mass or volume) of all the components, and to the
percentage or proportion (as a ratio or %) of each component, in
the mixed refrigerant circuit. Thus, the addition of a stream may
change the amount of all the components, but not necessarily the
proportions of the components, in a mixed refrigerant circuit.
Addition of a stream having one or more specific components, or at
least a majority of such component(s), can change both the amount
and the proportions of the component(s) of the mixed refrigerant
circuit. Similarly, removal of a phase of a stream from a separator
through which at least a part of the mixed refrigerant passes, may
change both the total amount of components of the mixed refrigerant
circuit, and change the component proportions, as usually a
separator provides a component(s)-specific stream.
[0064] In this way, the present invention can also provide a
combination of a method of increasing and a method of reducing the
inventory of one or more mixed refrigerant components in a mixed
refrigerant circuit from only the mixed refrigerant circuit and/or
a hydrocarbon stream being liquefied thereby.
[0065] The mixed refrigerant may initially be formed of a
hydrocarbon feed stream. During start-up of the method of
liquefying, the mixed refrigerant may for instance initially be a
natural gas stream from which only the C.sub.5.sup.+ components
have been removed. Then, as the method of liquefying proceeds
through its start-up procedure, the component inventory of the
mixed refrigerant can be changed using the method of the present
invention, to achieve an inventory of the mixed refrigerant as
desired, such as that described hereinbefore. In this way, the
present invention provides a self-sufficient method of changing the
mixed-refrigerant to that desired, even from a start-up
procedure.
[0066] The present invention may provide a nominal capacity (or
name plate) of a liquefied hydrocarbon stream in the range of 1 to
8 millions (metric) tonnes per annum (MTPA). The term "nominal
capacity" is defined at the daily production capacity of a plant
multiplied by the number of days per years the plant is intended to
be in operation. For instance, some LNG plants are intended to be
operational for an average of 345 days per year. Preferably the
nominal capacity of the hydrocarbon cooling process of the present
invention is in the range of 2 to 3 MTPA.
[0067] Referring to the drawings, FIG. 1 shows a general scheme 1
for a hydrocarbon liquefying process. As part of a hydrocarbon
liquefying process, and prior to any major cooling, an initial
hydrocarbon stream containing natural gas is conventionally
pre-treated to reduce and/or remove heavier hydrocarbons therefrom.
A common form of such separation is termed `natural gas liquids`
(NGL) extraction, in which proportions of C.sub.2.sup.+
hydrocarbons are removed from the gas stream--to provide a
methane-enriched stream which is subsequently cooled--and
fractionated to provide one or more single or multi-component
streams for the C.sub.2.sup.+ components, such as NGL and LPG
product streams.
[0068] In the present invention, at least some removal of heavier
hydrocarbons may still be desired where less C.sub.2.sup.+
components are subsequently required to change the component
inventory of the mixed refrigerant. However, it is a further
benefit of the present invention that less or no prior separation
of heavier hydrocarbons is required from the initial hydrocarbon
stream as discussed hereinafter. The initial hydrocarbon stream may
nevertheless have initially been cleared from C.sub.5.sup.+
components prior to its use. Such C.sub.5.sup.+ removal can be
carried out using a separator such as a scrub column or an NGL
extraction column, of significantly reduced size and scale compared
to columns required for C.sub.2.sup.+ separation and fractionation,
such that there is further CAPEX, OPEX and beneficial space-saving
in this way.
[0069] In a hydrocarbon liquefying start-up procedure, it is
possible for the initial hydrocarbon stream to be initially used as
a or the refrigerant stream, whose component inventory can then be
changed in a manner as discussed hereinafter. In this way, separate
provision and/or storage of each of the mixed refrigerant
components in advance of the start-up is not required, leading to
further space saving.
[0070] After any pre-treatment and C.sub.5.sup.+-removal,
processes, steps or stages, optionally carried out remotely such as
described in for instance WO 2008/006788, the initial hydrocarbon
stream is provided as a hydrocarbon feed stream 10 as shown in FIG.
1.
[0071] The hydrocarbon feed stream 10 passes through one or more
heat exchangers 12 which can also define a first cooling stage.
Preferably, the first cooling stage cools the hydrocarbon feed
stream 10 to below 0.degree. C., such as between -20.degree. C. and
-70.degree. C., preferably either between -20.degree. C. and
-45.degree. C., or between -40.degree. C. and -70.degree. C., to
provide a partially liquefied hydrocarbon stream 20. Cooling in the
one or more first heat exchangers 12 can be provided by an incoming
first refrigerant stream 101, which is at least partially
evaporated and discharged from the heat exchanger(s) 12 as an
outgoing first refrigerant stream 160 in a manner known in the
art.
[0072] The partially liquefied hydrocarbon stream 20 then passes
into a first gas/liquid separator "B" to provide a methane-enriched
gaseous overhead stream 30a and a mixed C.sub.2.sup.+ enriched
liquid bottom stream 30b. In its simplest form, the first
gas/liquid separator B may consist of a pressure vessel, whose use
and installation minimises CAPEX and OPEX for the process, because
no fractionation of the partially liquefied hydrocarbon stream 20
is required to provide the mixed C.sub.2.sup.+ enriched liquid
bottom stream 30b.
[0073] The mixed C.sub.2.sup.+ enriched liquid bottom stream 30b
comprises at least 5 mol % of two different C.sub.2.sup.+
components, such as ethane, propane and butane.
[0074] The C.sub.2.sup.+ enriched liquid bottom stream 30b can be
divided by a divider 24 into a separated portion 30c, useable to
change the C.sub.2.sup.+ component inventory of the mixed
refrigerant 100 in a mixed refrigerant circuit 4 as hereinafter
described, and a continuing stream 30d. The division of the
C.sub.2.sup.+ enriched liquid bottom stream 30b may be between
0-100%, depending upon various factors including the amount of the
part stream 30c required to change the component inventory of the
mixed refrigerant 100. In some circumstances, such as during
start-up, a high percentage, including but not limited to 100%, of
the C.sub.2.sup.+ enriched liquid bottom stream 30b may be required
to change the C.sub.2.sup.+ component inventory of the mixed
refrigerant 100. Under other circumstances, such as during a
settled and continuing cooling period, little or 0% of the
C.sub.2.sup.+ enriched liquid bottom stream 30b may be required for
the part stream 30c.
[0075] The methane-enriched gaseous overhead stream 30a and that
proportion of the C.sub.2.sup.+ enriched liquid bottom stream 30b
provided as the continuing stream 30d can be combined to provide a
combined cooled hydrocarbon stream 30 for subsequent processing as
described hereinafter.
[0076] The component inventory of the part stream 30c will vary
depending upon the nature of the hydrocarbon feed stream 10, the
degree of cooling provided by the one or more first heat exchangers
12, and/or the pressure and/or temperature of the first gas/liquid
separator B. Typically, the heavier hydrocarbon proportions of the
part stream 30c will be greater than the proportions of the same
components of the partially liquefied hydrocarbon stream 20 prior
to separation in the first gas/liquid separator B.
[0077] The part stream 30c can be added into the mixed refrigerant
100 at any suitable location around the mixed refrigerant circuit
4, and where its component proportions are different to that of the
mixed refrigerant 100 at that point of the mixed refrigerant
circuit 4, then the component proportions of the mixed refrigerant
thereafter in the mixed refrigerant circuit 4 will change.
Typically, the part stream 30c provides a source of one or more of
ethane, propane and one or more of the butanes.
[0078] Where the pressure of the part stream 30c is different to
that of the mixed refrigerant 100 at the intended point of their
combination, one or more pressure-reducing valves may be used. By
way of example only, the part stream 30c could be at a pressure
above 50 bar, whilst the mixed refrigerant 100 may be in the range
1-10 bar. Pressure-reducing valves can include "knock-out" drums
known in the art, which drums are also able to take off a usually
heavier hydrocarbon liquid stream. In normal operations, it is not
intended to alter the component proportions of the portion stream
30c prior to its addition to the mixed refrigerant 100.
[0079] The combined cooled hydrocarbon stream 30 in FIG. 1 passes
to one or more second heat exchangers 22, preferably a main
cryogenic heat exchanger. After passage through the second heat
exchanger 22, there is provided a liquefied hydrocarbon stream
50.
[0080] Cooling in the one or more second heat exchangers 22 is
provided by an incoming second refrigerant stream 110 comprising at
least a fraction of the mixed refrigerant of the mixed refrigerant
circuit 4. The second refrigerant stream 110 is at least partly
evaporated through the one or more second heat exchangers 22 to
provide an outgoing second refrigerant stream 140 in a manner known
in the art.
[0081] In various embodiments of the present invention, the method
of liquefying a hydrocarbon stream comprises one refrigerant
circuit comprising one mixed refrigerant, preferably being
evaporated at different temperature and pressure levels. The
inventory of the mixed refrigerant may change around the circuit
according to additions or removals of stream parts as herein
described as part of the present invention. An example of such a
so-called "single mixed refrigerant" process is shown in FIG.
2.
[0082] FIG. 2 is a more detailed scheme 2 of a hydrocarbon
liquefying process incorporating an embodiment of the general
scheme 1 shown in FIG. 1. As with FIG. 1, there is provided a
hydrocarbon feed stream 10 which may have been pre-treated to
reduce and/or remove at least some of the non-hydrocarbons in an
initial feed stream such as natural gas, and optionally some of the
hydrocarbons heavier than methane, particularly those heavier than
butane, as discussed hereinabove.
[0083] The hydrocarbon feed stream 10 passes through a first heat
exchanger 12, which may comprise one or heat exchangers in series,
parallel, or both, in a manner known in the art.
[0084] The cooling in the first heat exchanger 12 is provided by a
first fraction 154 of the mixed refrigerant 100. In the scheme 2
shown in FIG. 2, the first fraction 154 is shown passing into the
top or upper part of the first heat exchanger 12. The provision of
the first fraction 154 is described herein below. After providing
its cooling in the first heat exchanger 12, the first fraction 154
becomes an at least partly evaporated, usually fully evaporated,
mixed refrigerant stream 160.
[0085] The cooled hydrocarbon stream 20 then passes into the first
gas/liquid separator B to provide a methane-enriched gaseous
overhead stream 30a and a mixed C.sub.2.sup.+ enriched liquid
bottom stream 30b. The C.sub.2.sup.+ enriched liquid bottom stream
30b can be divided by a divider 24 into a separated part 30c,
useable to change the C.sub.2.sup.+ component inventory of the
mixed refrigerant 100 as hereinbefore described, and a continuing
stream 30d which can be combined with the methane-enriched gaseous
overhead stream 30a to provide a combined cooled hydrocarbon stream
30 for subsequent liquefying.
[0086] The part stream 30c can be added into the mixed refrigerant
100 at any suitable location around the mixed refrigerant circuit
4. Typically, the part stream 30c provides a source of one or more
of ethane, propane and one or more of the butanes.
[0087] The second heat exchanger 22 has two sections, a first or
lower section, and a second or upper section. These are shown in
FIG. 2 as a first second heat exchanger 22a and a second second
heat exchanger 22b. The arrangement of two or more heat exchangers
as sections in a, for example cryogenic, heat exchanger are known
in the art, and are not further discussed herein.
[0088] In another embodiment of the present invention, a second
partly-liquefied hydrocarbon stream 40 formed after further cooling
of the methane-enriched gaseous overhead stream in the one or more
of the second heat exchangers 22, could be passed through a second
gas/liquid separator (F) to provide a second methane-enriched--and
optionally nitrogen-enriched--gaseous overhead stream 40a, and a
second liquid bottom stream 40b. At least a part (40c) of the
second liquid bottom stream 40b, could be used to change the
component inventory of the mixed refrigerant circuit 4. The
overhead and bottom streams from such a second gas/liquid separator
F could provide different sources of one or more of the components
required in the mixed refrigerant circuit 4.
[0089] Optionally, after some cooling of the combined cooled
hydrocarbon stream 30, a further partially-liquefied hydrocarbon
stream 40 outflows the second heat exchanger 22 employing a bundle
break below the top of the first section, and passes into a second
gas/liquid separator F to provide a second methane-enriched, and
optionally nitrogen-enriched, gaseous overhead stream 40a, and a
second liquid bottom stream 40b. The temperature of the further
partly liquefied hydrocarbon stream 40 may be such that little or
no gaseous overhead stream 40a is produced.
[0090] The second liquid bottom stream 40b from the second
gas/liquid separator F can optionally be divided (in a similar
manner described hereinabove in relation to the liquid bottom
stream 30b from the first gas/liquid separator B) into a second
continuing stream 40d and a second separated stream 40c. The second
continuing stream 40d can return as a return stream into the second
heat exchanger 22 for further cooling in the second second heat
exchanger 22b, whilst the second separated stream 40c can be used
to change the component inventory of the mixed refrigerant 100.
[0091] The component proportions of the second separated stream 40c
will again vary depending upon a number of factors, but will
usually predominantly be methane. Typically, the component
proportions of the second separated stream 40c are different to the
component proportions of the mixed refrigerant 100, such that the
addition of the second separated stream 40c to the mixed
refrigerant 100 will change the overall component proportions of
the subsequent mixed refrigerant 100.
[0092] After passage through the second heat exchangers 22a, 22b,
there is provided a liquefied hydrocarbon stream 50.
[0093] As in FIG. 1, the obtained liquefied hydrocarbon stream 50
may be depressurized, e.g. by means of a Joule-Thomson valve
51.
[0094] The liquefied hydrocarbon stream may further be passed
through an end gas/liquid separator J such as an end-flash vessel
to provide a gaseous overhead stream 60 and a liquid bottom stream
70, optionally for storage in a storage tank as the liquefied
product such as LNG. Where a part, preferably the coldest part, of
the refrigerant stream passes through a suitable gas/liquid
separator, at least a part of the gaseous overhead stream from this
gas/liquid separator could be combined with the gaseous overhead
stream from the end gas/liquid separator, and optionally boil-off
gas from the storage tank, to provide a combined stream for
compression and use as fuel gas.
[0095] Cooling of the combined cooled hydrocarbon stream 30 in the
second heat exchangers 22a, 22b is provided by two fractions 125,
135 of the mixed refrigerant 100, which fractions enter the second
heat exchanger 22 at different locations, so as to provide the
different heat-exchanging sections within the second heat exchanger
22 in a manner known in the art. At or near the base of the first
second heat exchanger 22a, the mixed refrigerant after its use can
be collected as an at least partly, usually fully, evaporated mixed
refrigerant stream 140, which passes through a refrigerant
gas/liquid separator H to provide a compressor feed stream 141.
[0096] The refrigerant gas/liquid separator H may be a knock-out
drum. Any liquid bottom refrigerant stream 143 provided by the
refrigerant gas/liquid separator H could also be removed from the
mixed refrigerant 100 so as to change the component proportions of
the mixed refrigerant 100.
[0097] Line 140 also shows an example of one location where an
additive stream such as stream 30c can be added to the mixed
refrigerant circuit 4 comprising the mixed refrigerant 100 through
line 200.
[0098] For example, an at least partly evaporated mixed refrigerant
stream 160 from a first heat exchanger(s) 12 could pass through a
second refrigerant gas/liquid separator (I) to provide a gaseous
refrigerant stream 161 and a liquid bottom refrigerant stream 162.
By removing at least a part of the liquid bottom refrigerant stream
162 from the mixed refrigerant stream 160 the component inventory
of the remaining mixed refrigerant 100 in the mixed refrigerant
circuit 4 could be changed.
[0099] The compressor feed stream 141 is compressed by a first
compressor 32 to provide a first compressed stream 142, cooled by a
first cooler 26, and then combined with the gaseous stream 161 from
the refrigerant gas/liquid separator I to provide a combined
compressor feed stream 170 for a second compressor 34. The second
compressor 34 provides a compressed refrigerant stream 171, which
passes through a second cooler 28 to provide a mixed refrigerant
stream 101 ready for reuse. Line 202 shows a second example of a
suitable location in the mixed refrigerant circuit 4 where an
additive stream 202 can be added to the mixed refrigerant 100.
[0100] The second compressor 34 may optionally be provided with an
intercooler 27 between successive stages. If such intercooler 27 is
provided, an intercooler refrigerant gas/liquid separator K may be
provided to clear any condensed fraction 203 of the refrigerant
before supplying the remaining vaporous refrigerant to the next
compression stage in the second compressor 34. The liquid bottom
refrigerant stream in line 203, or part thereof, may be pumped to
higher pressure using one or more liquid pumps, and then reinserted
into the refrigerant circuit in line 171, preferably downstream of
second cooler 28.
[0101] The mixed refrigerant stream 101 can pass through a first
refrigerant gas/liquid separator E to provide a gaseous overhead
refrigerant stream 102 and a liquid bottom refrigerant stream
103.
[0102] The gaseous overhead refrigerant stream 102 and liquid
bottom refrigerant stream 103 pass through the first heat exchanger
12. Where there are more than two first heat exchangers 12, such as
3, 4 or 5 heat exchangers, one or more of the hydrocarbon stream
10, the gaseous overhead refrigerant stream 102 and the liquid
bottom refrigerant stream 103, may not pass through all of the
first heat exchangers 12, but may be selected to pass through
certain of the first heat exchangers 12 to provide a particular
arrangement of cooling to the three streams in a manner known in
the art.
[0103] The liquid bottom refrigerant stream 103 is cooled by its
passage through the first heat exchanger 12 to provide a cooled
stream 150, which can be expanded through a valve and passed into
another refrigerant gas/liquid separator G, which provides a
gaseous overhead refrigerant stream 151 and a liquid bottom
refrigerant stream 152. The gaseous overhead refrigerant stream 151
can be divided in a manner similar to that described hereinabove
for the liquid bottom stream 30b, to provide a fraction refrigerant
stream 153 and a continuing refrigerant stream 153a, which
continuing refrigerant stream 153a passes back into the first heat
exchanger 12 as stream 154 to provide cooling as described
hereinabove. The fraction refrigerant stream 153 will typically be
methane-enriched and usually also ethane-enriched, and provides
another source of components which could be added to or removed
from the mixed refrigerant 100. Its removal will typically increase
the proportion of the heavier hydrocarbons in the mixed refrigerant
100. Its addition to the mixed refrigerant 100 may be at a time
when a higher proportion of lighter hydrocarbons are required at
another part of the mixed refrigerant circuit 4.
[0104] Similarly, the removal of at least a part 152a of the liquid
bottom refrigerant stream 152, which typically has a greater
proportion of heavier hydrocarbons than the gaseous overhead
refrigerant stream 151, increases the proportion of lighter
hydrocarbons in the mixed refrigerant 100. Alternatively, it may be
used as a source of heavier hydrocarbons at another time or in
another part of the mixed refrigerant circuit 4. The remaining part
of the liquid bottom refrigerant stream 152 can be combined with
the continuing part 153a of the gaseous overhead refrigerant stream
153 for passing back as stream 154 into the first heat exchanger 12
as the cooling medium therein.
[0105] The gaseous overhead refrigerant stream 102 from the
refrigerant gas/liquid separator E is also cooled by the first heat
exchanger 12 to provide a cooled refrigerant stream 110, which then
passes through another refrigerant gas/liquid separator D to
provide a gaseous overhead refrigerant stream 111 and a liquid
bottom refrigerant stream 112. Typically, the liquid bottom
refrigerant stream 112 has a greater proportion of heavier
hydrocarbons, and is also typically termed a heavy mixed
refrigerant stream (HMR). By means of a suitable divider 25, the
liquid bottom refrigerant stream 112 can be arranged to provide
between 0-100% of one, two or three further streams, these being a
first cooling stage return stream 114 (to pass to the refrigerant
gas/liquid separator G), a second cooling stage cooling stream 113
for use in the first second heat exchanger 22a, and a part stream
115 for removal from the mixed refrigerant circuit 4. Similar to
separated refrigerant streams 152 and 162, the removal of part
stream 115 changes the component inventory of the mixed refrigerant
100, (and the subsequent re-introduction of this part stream 115
into the mixed refrigerant circuit 4 at another location and/or
time would also change the component inventory of the mixed
refrigerant 100 at that location and/or time).
[0106] The gaseous overhead refrigerant stream 111 from the
refrigerant gas/liquid separator D, commonly also termed a light
mixed refrigerant stream (LMR), passes through the second heat
exchanger 22 to provide a cooled refrigerant stream 120, which
passes through a valve and into another refrigerant gas/liquid
separator A. From the separator A, there is a gaseous overhead
refrigerant stream 121 and a liquid bottom refrigerant stream
122.
[0107] Typically, the cooled refrigerant stream 120 is the coldest
refrigerant stream in the refrigerant circuit 4, such that the
gaseous overhead refrigerant stream 121 is typically enriched with
nitrogen. Thus, separation of a part stream 123 from the gaseous
overhead refrigerant stream 121 provides a source of a
nitrogen-enriched stream and/or a source for removal of nitrogen
from the mixed refrigerant 100. The remaining part stream 124 of
the gaseous overhead refrigerant stream 121 can be combined with
the liquid bottom refrigerant stream 122 to provide a fraction 125
of the mixed refrigerant 100 for cooling in the second second heat
exchanger 22b in a manner known in the art.
[0108] Similarly, the cooling stream 113, being at least a part of
the liquid bottom refrigerant stream 112 from the refrigerant
gas/liquid separator D, is also cooled by passage through the first
second heat exchanger 22a to provide a cooled refrigerant stream
130, which is expanded by passage through a valve and passed into
another refrigerant gas/liquid separator C, which provides a
gaseous overhead refrigerant stream 131 and a liquid bottom
refrigerant stream 132. A separated fraction 133 of the gaseous
overhead refrigerant stream 131 is methane-enriched, and can also
be nitrogen-enriched, whose removal from the mixed refrigerant
circuit 4 changes the inventory of the mixed refrigerant 100. The
remaining fraction 134 of the gaseous overhead refrigerant stream
131 can be combined with the liquid bottom refrigerant stream 132
to provide a fraction 135 for re-entry into the first second heat
exchanger 22a and cooling in a common or separate section of the
second heat exchanger 22 in a manner known in the art.
[0109] From the above, it can be seen that it is an advantage of
the present invention to use one or more of a number of possible
overhead streams and bottom streams. In this way, there can be
greater flexibility of the composition proportions and/or inventory
of the mixed refrigerant 100, especially all the different
components of a mixed refrigerant 100, at any suitable location
around the mixed refrigerant stream 4.
[0110] It is a further advantage of the present invention that by
using one or more of a number of possible overhead streams and
bottom streams from one or more of the hydrocarbon streams and the
mixed refrigerant streams at points around the cooling process or
mixed refrigerant circuit, separate storage of the components of
the mixed refrigerant can be avoided, such that there is some space
saving that allows a process or a plant using the present invention
to be arranged or designed with more space-efficiency, or otherwise
to locate a process or plant using the present invention in a
space-limited location not previously possible, such as on a
floating vessel.
[0111] The scheme 2 shows use of a single mixed refrigerant circuit
4, although the present invention is not limited thereto. The use
of single mixed refrigerant circuits to provide cooling to two or
more different sets of heat exchangers is known in the art, and an
example is shown in WO 96/33379 A1 incorporated herein by way of
reference. Alternatively, the scheme 2 may involve one or more
other, further or separate refrigerant circuits to provide other,
further or separate cooling to one or more of the streams. The
present invention is not limited by the nature of arrangement of
the refrigerant circuit or circuits.
[0112] The scheme 2 shown in FIG. 2 provides the ability to select
one or more of the overhead gaseous streams and liquid bottom
streams, and/or parts thereof, both from the refrigerant gas/liquid
separators and the other gas/liquid separators, so as to refine the
nature of the mixed refrigerant 100 at one or more suitable
locations around the refrigerant circuit 4 to best match the
requirements of the hydrocarbon cooling process. These requirements
can change over time and/or location.
[0113] For example, the ambient temperature around a hydrocarbon
liquefaction process or plant may require refinement of the
particular proportions of the components of the mixed refrigerant
to affect greater and/or lower degrees of cooling in the first heat
exchanger(s) and/or in the second heat exchanger(s). In another
example, the hydrocarbon cooling process can be started up using
only a part of a hydrocarbon feed stream 10 in the mixed
refrigerant circuit 4, with heavier hydrocarbons being provided
into the mixed refrigerant 100 by one or more of the heavier
hydrocarbon part streams, such as the part stream 30c from the
gas/liquid separator B, so as to enhance the heavier hydrocarbon
proportions of the mixed refrigerant 100 during the start-up
procedure.
[0114] In the scheme 2, it is also possible to remove, such as
vent, at least a fraction of the mixed refrigerant 100 as a gaseous
overhead refrigerant stream or as a liquid bottom refrigerant
stream from a heat exchanger itself. FIG. 2 shows two gaseous
overhead vent streams 190a, 190b and two liquid vent bottom
refrigerant streams 180a, 180b from the first and second heat
exchangers 12, 22 respectively. Removal of one of the vent streams
180a, 180b, 190a, 190b can also be used to change the component
inventory of the mixed refrigerant 100.
[0115] As the present invention can be self-sufficient in supplying
the components to, and changing the composition of, the mixed
refrigerant without the need of a fractionation column, it is
particularly suitable where there is limitation of the space
available for a hydrocarbon cooling process, either as a
stand-alone process, or as part of a larger process or plant such
as including one or more pre-treatment processes, post-liquefaction
processes, and/or storage of a liquefied hydrocarbon stream
including the requirement for one or more storage tanks.
[0116] Thus, the present invention is particularly suitable for
location on a floating vessel or an off-shore platform, e.g. in the
form of a caisson. A floating vessel may be any movable or moored
vessel, generally at least having a hull, and usually being in the
form of a ship such as a `tanker`. Examples of such a floating
vessel provided with a hydrocarbon liquefaction apparatus are shown
and described in for instance WO 2007/064209 and WO
2008/006842.
[0117] Such floating vessels can be of any dimensions, but are
preferably elongate. Whilst the dimensions of a floating vessel are
not limited at sea, building and maintenance facilities for
floating vessels may limit such dimensions. Thus, in one embodiment
of the present invention, the floating vessel or off-shore platform
is less than 600 m long such as 500 m, and a beam of less than 100
m, such as 80 m, so as to be able to be accommodated in existing
ship-building and maintenance facilities.
[0118] An off-shore platform may also be movable, but is generally
more-permanently locatable than a floating vessel. An off-shore
platform may also float, and may also have any suitable
dimensions.
[0119] In another embodiment of the present invention, the method
is for liquefying natural gas to provide liquefied natural gas.
[0120] Preferably, the liquefied hydrocarbon stream provided by the
present invention is stored in one or more storage tanks, which
storage tanks maybe also located on any floating vessel or
off-shore platforms.
[0121] FIG. 3 shows a floating vessel 52 such as a tanker,
including a hydrocarbon liquefying process based on scheme 2 shown
in FIG. 2. In FIG. 3, a hydrocarbon feed stream 10 passes through a
first cooling stage 12a having one or more first heat exchanger(s)
12 such as shown in FIG. 2, to provide a partially liquefied
hydrocarbon stream 20/30, which passes through a second cooling
stage 22c involving one or more second heat exchangers to provide a
liquefied hydrocarbon stream 50. The liquefied hydrocarbon stream
50 passes via a cryogenic turbo-expander 53 into an end gas/liquid
separator J, to provide an overhead gaseous stream 60, and a liquid
bottom stream 70 which passes into a storage tank 42 such as an LNG
storage tank 42.
[0122] FIG. 3 also shows a cooled refrigerant stream 120 similar to
that in FIG. 2, from the second cooling stage 22c, which passes
into the refrigerant gas/liquid separator A also shown in FIG. 2.
The gas/liquid separator A provides an gaseous overhead refrigerant
stream 121, a fraction of which passes through a valve to provide a
gaseous stream 123 which can be combined with the gaseous overhead
stream 60 from the end gas/liquid separator J to provide a combined
stream 80. To the combined stream 80 could be added any boil-off
gas from the storage tank 42 passing along line 75 to create a
further combined stream 80a which can be compressed and
subsequently used as a fuel stream 90. The arrangement shown in
FIG. 3 provides a method of using any fraction of the gaseous
overhead refrigerant stream 121 from the usually coldest
refrigerant gas/liquid separator A without requiring additional
CAPEX.
[0123] Table I gives an overview of estimated compositions and
phases of some of the gaseous overhead streams and liquid bottom
streams from the gas/liquid separators of an example process of
FIG. 2. An assumption has been made that liquefaction is performed
just below the cricondenbar, and that the temperature of the
hydrocarbon stream at the bundle break in the second heat exchanger
22 supplying line 40 is -45.degree. C. such that the hydrocarbon
stream is in the two-phase regime.
TABLE-US-00002 TABLE 1 Separator Stream Phase N.sub.2 C.sub.1
C.sub.2 C.sub.3 iC.sub.4 nC.sub.4 A 121/123 V 81.22% 18.77% 0.01%
0.00% 0.00% 0.00% 122 L 10.69% 68.35% 18.32% 2.51% 0.08% 0.05% B
30a V 1.00% 90.99% 4.99% 1.99% 0.49% 0.49% 30b/30c L 0.28% 60.01%
11.96% 12.23% 5.99% 7.82% C 131/133 V 33.20% 66.13% 0.67% 0.01%
0.00% 0.00% 132 L 0.73% 27.91% 45.96% 22.22% 1.71% 1.47% D 111 V
16.85% 64.01% 16.73% 2.30% 0.07% 0.04% 112/115 L 2.80% 30.38%
43.06% 20.79% 1.60% 1.38% E 102 V 5.74% 37.41% 37.54% 16.93% 1.28%
1.10% 103 L 1.12% 13.90% 37.08% 37.65% 5.00% 5.26% F 40a V 1.04%
92.07% 4.63% 1.60% 0.33% 0.31% 40b L 1.00% 90.95% 5.00% 2.00% 0.50%
0.50% G 151/153 V 8.37% 63.16% 24.37% 3.88% 0.14% 0.09% 152 L 0.29%
10.08% 41.65% 38.32% 4.75% 4.90% H 141 V 6.94% 40.30% 35.30% 15.34%
1.15% 0.99% I 161 V 1.61% 18.71% 38.84% 32.72% 4.00% 4.12% J 60 V
11.70% 88.29% 0.01% 0.00% 0.00% 0.00% 70 L 0.44% 91.09% 5.26% 2.10%
0.53% 0.53%
[0124] From Table 1, the different component proportions of various
streams or part streams shown in the example scheme 2 of FIG. 2 can
be seen. For example, the part stream 123 from the gaseous overhead
refrigerant stream 121 from the coldest refrigerant gas/liquid
separator A has a high nitrogen proportion, and the removal of this
part stream 123 from the mixed refrigerant 100 will decrease the
nitrogen inventory of the overall mixed refrigerant 100. Stream
190b would also be a good place to remove light components from the
mixed refrigerant. Heavy components may be removed from the mixed
refrigerant inventory by removing at least a part of either one of
the liquid vent bottom refrigerant streams 180a, 180b from the
first and second heat exchangers 12, 22 respectively, or by
removing a portion of the inter-cooled bottom refrigerant stream
203 from separator K if such is provided. These streams, and in
particular stream 203 are expected to have the highest molecular
weight in the refrigerant circuit.
[0125] The part stream 30c from the first gas/liquid separator B
has relatively high C.sub.3 and C.sub.4 proportions, whose addition
to the mixed refrigerant stream will generally increase its
inventory of C.sub.3 and the C.sub.4s.
[0126] By use of one or more of the streams shown in Table 1 and
other streams mentioned above, either by its addition to or removal
from the mixed refrigerant 100, and the proportion of such addition
and/or removal, allows refinement of the component inventory of the
mixed refrigerant 100 to suit specific requirements or changing
conditions. In this way, the hydrocarbon liquefying process can be
self-sufficient in refining the component proportions of the mixed
refrigerant, reducing and/or removing the need for separate
sources, in particular separate storage, of each component of the
mixed refrigerant. For example, separate storage tanks for propane
and butane are not required, allowing the hydrocarbon liquefying
process to be located in a smaller area, increasing safety by the
reduction and/or removal of high-risk gas storages.
[0127] The person skilled in the art will understand that the
present invention can be carried out in many various ways without
departing from the scope of the appended claims.
[0128] Applicants also propose that the methods and apparatuses
described above, wherein the refrigerant portion is provided by
partially liquefying the hydrocarbon feed stream and separating the
mixed C.sub.2.sup.+, represent merely one way of obtaining a
refrigerant portion to change the mixed refrigerant inventory.
Moreover, applicants propose that the methane-enriched gaseous
overhead stream is merely one example of the stream that can be
liquefied by the at least the fraction of the mixed refrigerant
circulating in the mixed refrigerant circuit.
[0129] Defined more broadly, the present invention provides a
method of liquefying a hydrocarbon stream, comprising at least the
steps of: [0130] (A) providing a hydrocarbon feed stream; [0131]
(B) drawing from the hydrocarbon feed stream a refrigerant portion;
[0132] (C) circulating a mixed refrigerant through a mixed
refrigerant circuit; [0133] (D) adding, without fractionation, at
least a part of the refrigerant portion to the mixed refrigerant
circuit to change the inventory of the mixed refrigerant in the
mixed refrigerant circuit; and [0134] (E) cooling and liquefying a
part of the hydrocarbon feed stream not comprised in the
refrigerant portion of step (B) by heat exchanging against at least
a fraction of the mixed refrigerant circulating in the mixed
refrigerant circuit, to provide a liquefied hydrocarbon stream.
[0135] In the embodiments discussed above with reference to FIGS. 1
and 2, the refrigerant portion of step (B) is provided in the form
of the mixed C.sub.2.sup.+ enriched stream 30c, while the part of
the hydrocarbon feed stream to be cooled and liquefied in step (E)
that does not form part of the refrigerant portion is provided in
the form of the methane-enriched stream 30a and/or stream 30.
[0136] Other ways of providing the refrigerant portion, include
drawing (e.g. by means of a divider) a portion of the feed stream
upstream of any liquefaction, such as e.g. from line 10 upstream of
the one or more first heat exchangers, to provide a stream which
may be used to form the refrigerant portion of step (B). Of course,
the not-liquefied portion of the feed stream may be partially
liquefied after having been drawn from the feed stream, to provide
a C.sub.2.sup.+ enriched liquid bottom stream that can be used to
form the refrigerant portion. This has as advantage that excess
methane and other light components are removed before adding to the
mixed refrigerant inventory in the refrigerant circuit.
[0137] However, the refrigerant portion may alternatively be formed
without forming the mixed C.sub.2.sup.+ enriched liquid bottom
stream. This has an advantage that the mixed C.sub.2.sup.+ enriched
stream does not have to be formed and that therefore no gas/liquid
separator needs be supplied for that purpose. Clearly, the
molecular weight of the refrigerant portion used to change the
refrigerant inventory would be then be lower than that of the mixed
C.sub.2.sup.+ enriched liquid bottom stream from the separator. In
embodiments that use the composition of the feed gas in line 10 to
add to the refrigerant inventory, without first partly liquefying
and gas/liquid separating the feed gas, this may therefore result
in adding more of the light components to the refrigerant circuit
than needed to obtain a target composition. Like in any other
embodiment described herein, such light components may subsequently
be removed via e.g. line 190b or 123, to achieve that the remaining
refrigerant inventory meets the target composition. This may
require first liquefying the part of the hydrocarbon feed stream
that is not comprised in the refrigerant portion using a
sub-optimal mixed refrigerant inventory that has a composition that
is off the target composition. The final target refrigerant
composition may then be obtained via an iterative process over time
of feeding streams to the refrigerant circuit and/or removing
selected parts of the refrigerant from the refrigerant circuit.
[0138] Part of the hydrocarbon feed gas that is not used to form
the refrigerant portion may be subjected to the cooling and
liquefying of step (E). This may be done simultaneously to steps
(B) and (D), such as in the embodiments explained above with
reference to FIGS. 1 and 2, but it may also be done later in
time.
[0139] In the latter case, the removal of the refrigerant portion
in step (B), and the steps (C) and (D) are performed earlier in
time than step (E). This would be an attractive option for instance
during starting up of the process and making up the refrigerant
inventory for the first time. Also in such embodiments, steps (f),
(g), and (h) as defined hereinabove or any other way to remove one
or more fractions of the mixed refrigerant from the mixed
refrigerant circuit described herein may be applied to change the
component inventory of the mixed refrigerant to change the
component inventory.
[0140] In any of the embodiments, the circulating of the mixed
refrigerant through the mixed refrigerant circuit in step (c) may
comprise compressing a vaporous fraction of the mixed refrigerant
though at least to stages of compression, and subsequently passing
the compressed mixed refrigerant though one or more coolers. This
may be taken advantage of when removing heavy components from the
mixed refrigerant in the mixed refrigerant stream as follows:
[0141] drawing an inter-stage compressed mixed refrigerant stream
from a first one of the at least two stages of compression; [0142]
intercooling the inter-stage compressed mixed refrigerant stream;
[0143] passing the intercooled mixed refrigerant through one or
more intercooler refrigerant gas/liquid separator to provide one or
more gaseous overhead refrigerant streams and one or more liquid
bottom refrigerant streams; [0144] feeding the one or more gaseous
overhead refrigerant streams to a second one of the at least two
stages of compression; and [0145] removing at least a part of the
liquid bottom refrigerant stream to change the component inventory
of the mixed refrigerant circuit.
[0146] At any time that no full liquefaction of any part of the
hydrocarbon feed gas is pursued, as e.g. during starting up of the
process, it is an option to use any of the refrigerant gas/liquid
separators provided in the refrigerant circuit, such as refrigerant
gas/liquid separator D, as the first gas/liquid separator to
provide the methane-enriched gaseous overhead stream and a mixed
C.sub.2.sup.+ bottom stream. The refrigerant gas/liquid separator
so used should be bypassed with a refrigerant bypass line in order
to allow the refrigerant to be circulated. The refrigerant
inventory can then be changed using the principles set forth herein
based on these streams.
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