U.S. patent application number 12/665329 was filed with the patent office on 2010-06-03 for method and system for producing lng.
This patent application is currently assigned to KANFA ARAGON AS. Invention is credited to Inge Sverre Lund Nilsen.
Application Number | 20100132405 12/665329 |
Document ID | / |
Family ID | 40304530 |
Filed Date | 2010-06-03 |
United States Patent
Application |
20100132405 |
Kind Code |
A1 |
Nilsen; Inge Sverre Lund |
June 3, 2010 |
Method and system for producing LNG
Abstract
A method is described for production of LNG from an incoming
feed gas (1) on an onshore or offshore installation, and it is
characterised by the following steps: 1) the feed gas is led
through a fractionation column (150) where it is cooled a separated
in an overhead fraction with a reduced content of pentane (C5) and
heavier components, and a bottom fraction enriched with heavier
hydrocarbons, 2) the overhead fraction from the fractionation
column is fed to a heat exchanger system (110) and is subjected to
a partial condensation to form a two-phase fluid, and the two-phase
fluid is separated in a suitable separator (160) a liquid (5) rich
in LPG and pentane (C3-C5) which is re-circulated as cold reflux to
the fractionation column (150), while the gas (6) containing lower
amounts of C5 hydrocarbons and hydrocarbons heavier than C5 is
exported for further processing in the heat exchanger system (110)
for liquefaction to LNG with a maximum content of ethane and LPG 3)
the cooling circuit for liquefaction of gas in the heat exchanger
system comprises an open or closed gas expansion process with at
least one gas expansion step. A system for carrying out the method
is also described.
Inventors: |
Nilsen; Inge Sverre Lund;
(Bergen, NO) |
Correspondence
Address: |
CHRISTIAN D. ABEL
ONSAGERS AS, POSTBOKS 6963 ST. OLAVS PLASS
OSLO
N-0130
NO
|
Assignee: |
KANFA ARAGON AS
Bergen
NO
|
Family ID: |
40304530 |
Appl. No.: |
12/665329 |
Filed: |
June 20, 2008 |
PCT Filed: |
June 20, 2008 |
PCT NO: |
PCT/NO08/00229 |
371 Date: |
February 15, 2010 |
Current U.S.
Class: |
62/611 ;
62/618 |
Current CPC
Class: |
F25J 1/0082 20130101;
F25J 1/0205 20130101; F25J 1/0241 20130101; F25J 1/0052 20130101;
F25J 1/0278 20130101; F25J 1/0216 20130101; F25J 1/0037 20130101;
F25J 2220/64 20130101; F25J 1/0201 20130101; F25J 1/0202 20130101;
F25J 1/005 20130101; F25J 1/0072 20130101; F25J 1/0215 20130101;
F25J 1/0294 20130101; F25J 2210/06 20130101; F25J 1/0212 20130101;
F25J 1/0092 20130101; F25J 1/0022 20130101; F25J 1/0097 20130101;
F25J 2270/16 20130101; F25J 1/0057 20130101; F25J 2270/90 20130101;
F25J 1/0288 20130101; F25J 1/0232 20130101; F25J 1/0204 20130101;
F25J 1/0238 20130101 |
Class at
Publication: |
62/611 ;
62/618 |
International
Class: |
F25J 1/02 20060101
F25J001/02; F25J 3/02 20060101 F25J003/02 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 22, 2007 |
NO |
2007 3245 |
Claims
1. Method for production of LNG from an incoming feed gas (1),
characterised by the following steps: a) the feed gas is led
through a fractionation column (150) where it is cooled and
separated into an overhead fraction (2), with a reduced content of
the hydrocarbons that have a molecular weight higher than pentanes
(C5) and enriched with most of the butane (C4) and hydrocarbons
with a lower normal boiling point than butane, and a bottom
fraction (3) wherein hydrocarbons having a molecular weight from
hexanes (C6) and higher of the feed gas are conserved. b) the
overhead fraction of the fractionation column is led into a heat
exchanger system (110) and is subjected to a partial condensation
to form a two-phase fluid, and the two-phase fluid is separated in
a suitable separator (160) into a fluid (5) rich in LPG and pentane
(C3-C5) which is re-circulated as cold reflux to the fractionation
column (150), while the gas (6), enriched with most of the butane
(C4) and hydrocarbons with a lower normal boiling point than
butane, is reintroduced into the heat exchanger system (110) for
liquefaction to LNG consisting essentially of methane, ethane,
propane and butane, and wherein c) the cooling circuit for
liquefaction of gas in the heat exchanger system comprises an open
or closed gas expansion process with at least one gas expansion
step.
2. Method according to claim 1, characterised in that the
fractionation column (150) and the separator (160) are operated at
pressures and temperatures which lead to the complete system (the
fractionating column 150 and reflux separator 160) generating a
component split/separation point in the normal boiling point range
(NBP) between -12.degree. C. and 60.degree. C.
3. Method according to claims 1-2, characterised in that the light
key component for the separation is butane (C4) with a normal
boiling point between -12.degree. C. and 0.degree. C., and the
heavy key component is a C6 component with a boiling point between
50.degree. C. and 70.degree. C., whereby the overhead gas stream
(6) of the system will contain the most of comprising a
considerably reduced content of n-butane and hydrocarbons with a
lower normal boiling point than n-butane, and the reject stream (3)
of the system comprises most of C6 and components with a normal
boiling point higher than C6.
4. Method according to claims 1-3, characterised in that the
fractionation column (150) and the separator (160) are operated so
that pentane (C5, NBP=28-36.degree. C.) is a transitional component
that is distributed both in the overhead gas stream (6) of the
system and the reject stream (3) of the system.
5. Method according to one of the preceding claims, characterised
in that the temperature of the feed gas is reduced through the
fractionation column (150) so that the temperature of the gas when
it is fed into the heat exchanger system (110) is lower than the
temperature of the cooling gas stream at the hot end of the heat
exchanger system (hot pinch point temperature).
6. Method according to one of the preceding claims, characterised
in that a reboiler (135) is connected to the fractionation column
(150) to reduce the steam pressure of the bottom product.
7. Method according to one of the preceding claims, characterised
in that the heat exchanger for liquefaction (LNG production)
comprises one or more multi-stream heat exchangers.
8. Method according to one of the preceding claims, characterised
in that it is carried out with a closed gas expansion process with
at least one nitrogen expander.
9. Method according to one of the preceding claims, characterised
in that it is carried out with a closed hybrid cooling process with
methane/nitrogen as a cooling agent, where the cooling agent is
used both in the gas phase and in the liquid phase, and where the
cooling circuit has at least one gas expander and at least one
appliance for expansion of a liquid cooling agent.
10. Method according to one of the preceding claims, characterised
in that it is carried out with an open gas expansion process with
at least one gas expander, in which a suitable gas at a higher
pressure is used as cooling gas, and where the expanded gas at a
lower pressure is not recompressed for recycling but is used for
another purpose.
11. System for carrying out the method according to claims 1-10
comprising a fractionation column (150) for feeding in a feed gas,
a heat exchanger system (110) for cooling down and partially
condensing the overhead gas stream of the fractionation column, a
separator (160) to separate the two-phase stream from the heat
exchanger system, set up to recycle fluid from the separator to the
fractionation column and import this fluid to the upper part of the
column as a reflux, and appliance to lead the gas from the
separator back to the heat exchanger system for further cooling
down and liquefaction to LNG, characterised in that the cooling
system which is used for cooling down, condensing and liquefying of
gas in the heat exchanger system comprises an open or closed gas
expansion process with at least one gas expansion step.
12. System according to claim 11, characterised in that the system
is designed and configured to separate the feed gas such that the
overhead gas stream (6) of the system will be enriched with most of
the butane (C4) and hydrocarbons with a lower normal boiling point
than butane, and the bottom product in the fractionation column
will be enriched with most of the C6 and components with a normal
boiling point higher than C6.
13. An optimized gas liquefaction system of the gas expansion type
for the production of LNG from an incoming natural gas stream,
characterised in that the system comprises: a) an open or closed
gas liquefaction circuit comprising a gaseous refrigerant, at least
one gas expander for cooling the refrigerant by gas expansion, and
one or more heat exchangers for heat exchange between the natural
gas, LNG and refrigerant streams, b) a fractionation column
arranged for receiving the incoming natural gas prior to
introduction of the natural gas into the liquefaction circuit, said
fractionation column being further arranged to cool and separate
the incoming natural gas into an overhead gaseous fraction, with a
reduced content of the hydrocarbons that have a molecular weight
higher than pentanes (C5) and enriched with most of the butane (C4)
and hydrocarbons with a lower normal boiling point than butane, and
a bottom liquid fraction wherein hydrocarbons having a molecular
weight from hexanes (C6) and heavier are conserved, and arranged to
lead the overhead gaseous fraction to at least one of the heat
exchangers of the liquefaction circuit, whereby the overhead feed
gas fraction is cooled to a two-phase fluid, and c) a separator
arranged for receiving the two-phase fluid and separating the
two-phase fluid into a gas component wherein hydrocarbons having a
molecular weight from butanes and lighter are conserved, and a
liquid component rich in C3-C5 hydrocarbons, and further arranged
to return the liquid component back to the fractionation column as
a cold reflux liquid and to return the gas component to the
liquefaction circuit for further cooling and condensation to
LNG.
14. (canceled)
15. (canceled)
16. A system according to any of claim 13, wherein the liquefaction
circuit comprises the gaseous refrigerant at an inlet pressure of
3-10 MPa being fed to the heat exchanger or system of heat
exchangers and cooled to a temperature between 0 and -120 deg C.,
and further wherein the cooled gaseous refrigerant is expanded to a
pressure between 5% and 40% of the inlet pressure, and then being
led back to the heat exchanger or system of heat exchangers to
provide cooling.
17. A system according to any of claim 13 or 16, wherein the
liquefaction circuit comprises two expansion stages, wherein the
gaseous refrigerant at an inlet pressure of 3-10 MPa is split in
two parts either before or after pre-cooling, and where the parts
are pre-cooled to different temperatures before expansion to
essentially the same lower pressures and led back to the heat
exchanger or system of heat exchangers to provide cooling.
18. A system according to any of claim 13 or 16-17, wherein cooling
in the fractionation column is essentially provided by the reflux
liquid from the separator.
19. A system according to any of claim 13 or 16-18, wherein a
reboiler (135) is connected to the fractionation column (150) to
reduce the vapour pressure of the bottom product.
20. A system according to any of claim 13 or 16-19, wherein the
overhead gas fraction is cooled into the two-phase fluid by being
in heat-exchanging, counter-current relationship with the gaseous
refrigerant in one or more heat exchangers of the liquefaction
circuit.
21. A system according to any of claim 13 or 16-20, wherein the
liquefaction circuit comprises one or more multi-stream heat
exchangers configured in series or parallel, or both.
22. A system according to any of claim 13-22, wherein the gas
expander essentially isentropically cools the refrigerant.
23. A system according any of claim 13 or 16-23, wherein the
liquefaction circuit comprises a closed gas expansion process with
two or more gas expansion stages for essentially isentropically
cooling the refrigerant by gas expansion, and where the refrigerant
inlet temperature for the second gas expander stage is lower than
the refrigerant inlet temperature for the first gas expander
stage.
24. A system according to either any of claim 13 or 16-24, wherein
the gaseous refrigerant comprises nitrogen gas, or a mixture of
nitrogen and hydrocarbons.
Description
[0001] The present invention relates to a method for optimal
production of LNG on a fixed or floating offshore installation, as
can be seen in the preamble of the independent claim 1.
[0002] The invention also relates to a system for implementing the
method comprising a fractionation column for feeding feed gas, a
heat exchanger system for cooling down and partially condensing the
overhead gas stream from the fractionation column, a separator for
separation of the two-phase stream from the heat exchanger system,
a device for return of liquid from the separator to the
fractionation column and feeding this liquid to the upper part of
the column as reflux, and a device for routing the gas from the
separator back to the heat exchanger system for further cooling and
liquefaction to LNG.
[0003] The invention aims to use a closed gas expansion process to
liquefy the natural gas, and in that the gas is first fed through a
fractionation column where the gas is cooled and separated into an
overhead fraction with reduced content of pentane (C5) and heavier
components, and a bottom fraction enriched with the heavier
hydrocarbons, furthermore, in that the fractionation column reflux
is generated as an integrated part of the system for liquefaction
in that the overhead gas is partially condensed. By carrying out
the liquefaction in accordance with the invention, production of
liquefied gas with maximum content of ethane and LPG (liquid
petroleum gas) is achieved at the same time as the efficiency of
the gas expansion process is increased and the by-production of
unstable/volatile liquid with a high content of ethane, LPG and
pentane is minimised.
[0004] In particular, the invention comprises a method and a system
for liquefaction of natural gas or other hydrocarbon gas from a gas
field or from a gas/oil field, where it is appropriate to liquefy
the gas to facilitate transportation of the gas from the source to
the market. This is particularly relevant for offshore oil/gas
fields.
[0005] In this context, natural gas means a mixture of hydrocarbons
where an essential part consists of methane. Natural gas is
normally liquefied by considerably cooling down the gas such that
it condenses and becomes a liquid. With LPG is meant liquid
petroleum gas that encompasses propane and butanes (C4, C4
components).
[0006] The aim of the invention is to render liquefaction of gas
energy efficient at the same time as the process is kept simple so
that the equipment can be used offshore, and then especially on
floating installations. By-production of condensate during the
liquefaction is minimised and the efficiency is maximised (the need
for fuel gas is minimised).
[0007] The method according to the invention is characterised by
the following steps: [0008] 1) that the feed gas is led through a
fractionation column (150) where it is cooled and separated into an
overhead fraction with reduced content of pentane (C5) and heavier
components, and a bottom fraction enriched with heavier
hydrocarbons, [0009] 2) that the overhead fraction from the
fractionation column is fed into a heat exchanger system (110) and
is subjected to partial condensation to form a two-phase fluid, and
the two-phase fluid is separated in a suitable separator (160) to a
liquid (5) rich in LPG and pentane (C3-C5) which is re-circulated
as cold reflux to the fractionation column (150), while the gas (6)
containing lower amounts of C5 hydrocarbon and hydrocarbons heavier
than C5, is routed for further treatment in the heat exchanger
system(110) for liquefaction to LNG with maximum content of ethane
and LPG, and [0010] 3) that the cooling circuit for liquefaction of
gas in the heat exchanger system comprises an open or closed gas
expansion process with at least one gas expansion step.
[0011] The preferred embodiments of the method are defined in the
dependent claims 2-10.
[0012] The system according to the invention is characterised in
that the cooling system which is used for cooling, condensing and
liquefaction of the gas in the heat exchanger system comprises an
open or closed gas expansion process with at least one gas
expansion step. The system is preferably designed and configured to
separate the feed gas so that the overhead gas stream of the system
will be enriched with the majority of the butane (C4) and
hydrocarbons with a lower normal boiling point than butane, and the
bottom product of the fractionation column will be enriched with
most of C6 and components with a normal boiling point higher than
C6.
BACKGROUND
[0013] Liquefaction of natural gas can be carried out with the use
of a gas expansion process, where a cooling medium goes through a
processing circuit based on compression, cooling, expansion and
thereafter heat exchange with the fluid that is to be cooled down.
For example, for liquefaction of natural gas, one can use a
compressed cooling medium in gas phase, normally nitrogen or
methane, which is pre-cooled and thereafter expanded across an
expansion valve or a turboexpander. The gas expansion generates
very cold gas, or a mixture of gas and liquid, which is then used
for liquefaction of natural gas and to pre-cool the compressed
cooling medium gas. The gas expansion processes are relatively
simple and therefore very well suited to offshore installation.
However, the processes have a somewhat lower efficiency than the
more advanced processes, such as, for example, mixed refrigerant
cycle processes, and thus require much compression equipment and
much energy.
[0014] In order to produce LNG it is normally required that the gas
has a relatively high content of methane. However, most of the feed
gases will also contain some heavier hydrocarbons such as ethane,
propane, butane, pentane, etc. Some requirements with respect to
the content of heavier hydrocarbons in the liquid gas are normally
present:
[0015] The specific energy content per cubic meter of liquefied gas
must normally not exceed given sales specifications.
[0016] The content of pentane (C5) and upwards, and also aromatic
compounds of the liquid gas, must be kept below defined limits to
avoid freeze out in the cooling process.
[0017] The simplest way to limit the content of heavier
hydrocarbons in the liquid gas is to partially condense the gas and
then separate the condensed liquid from the gas, which is further
cooled for liquefaction. The separation is normally carried out as
an integrated part of the cool down process, typically at a
temperature between 0.degree. C. and -60.degree. C. Separated
condensate can be heated up again as a part of the cooling process
to utilise the refrigeration potential.
[0018] In large land based LNG installations (so called "base load"
installations) most of the propane and heavier hydrocarbons are
normally removed and in many cases also a considerable part of
ethane, before or as a part of, the liquefaction. This is done to
meet the sale specifications and to be able to produce and sell the
valuable ethane, LPG and condensate/naphtha. Comprehensive
processes are normally used with low temperature fractionation
columns both as a part of the cool down process and as separate
units outside the cooling system.
[0019] However, for offshore LNG production it is undesirable to
handle products other than the liquid natural gas. Where oil or
condensate is also produced one can however permit separation of
condensate for stabilisation and export together with another oil
and/or condensate. However, stabilised condensate will, in the
main, consist of C6+ with a relatively low content of pentane and
lighter components. Hydrocarbons lighter than C6 can generally not
be stored or transported safely without being cooled down or being
under pressure. Some separated hydrocarbons or condensate can be
used as fuel, but beyond that it is desirable to retain these
components in the LNG product. Due to smaller LNG volumes and the
possibility for later blending into large LNG volumes, it can be
appropriate offshore to produce a liquid natural gas with a
considerably higher, and preferably a maximum, content of heavier
hydrocarbons.
[0020] The present invention represents a considerable optimisation
for application offshore, and especially on a floating unit, in
that a relatively simple and robust gas expansion process is used
for liquefaction of natural gas, and in that the energy efficiency
of this process is increased at the same time as the amount of
liquid gas is maximised by maximising the content of ethane and
LPG, at the same time as the amount of hydrocarbons heavier than
methane which is separated out as bi-products in the liquefaction
process is minimised.
[0021] An installation which comprises the system according to the
invention can thereby simply be adapted and be installed, for
example, on board floating offshore installations where space is
often a limiting factor.
[0022] References to Known Technology and Other Publications, and
Comparisons with the Present Invention:
[0023] Initially reference is made to EP-1.715.267 which describes
a method which includes natural gas being cooled and being led
through a fractionation column where it is separated into an
overhead fraction and a bottom fraction. The bottom fraction is
enriched with heavier hydrocarbons and is exported out of the
system. The overhead fraction is cooled and forms a two-phase fluid
which is separated in a separator. The liquid phase is
re-circulated to the fractionation column whilst the gas phase is
fed further to a heat exchanger system. Cooling of the overhead
fraction is carried out with a free standing cooler. The EP patent
consequently describes a classical and well-known distillation
process.
[0024] Furthermore, the set-up is standard practice in so-called
"base load" LNG installations, where both cooler 5 and cooler 11
(ref. figures in the EP patent) are parts of the pre-cooling
installation of the plant, which is normally carried out as a
multistep propane cooling installation. However, the set up in the
EP patent does not integrate a fractionation column and a
downstream LNG condensation process as one aims with the present
invention. Integration is here meant that two systems are tightly
connected together and function as one system and that material
streams and/or energy streams are flowing both ways between the
systems.
[0025] The refrigeration work which according to EP-1.715.267 cools
the overhead fraction and generates so-called reflux to the
fractionation column, comes according to the description not from
the same cooling circuit that carries out further cooling and
condensation of the natural gas, but apparently from an external
cooling process.
[0026] International patent application WO-2005/071333 describes a
well-known double gas expansion which is used to liquefy boil off
gas from storage tanks for LNG. In practice, such boil off gases
contain only methane and nitrogen.
[0027] Patent publications US2006/0260355 A1 and U.S. Pat. No.
6,662,589 describe systems which apparently are similar to the
present invention, but which in reality are considerably different
from the present invention. The systems in the referred
publications comprise processes for simultaneous liquefaction of
natural gas and recovery/separation of components heavier than
methane, i.e. ethane and heavier components, where ethane, LPG and
heavier components are fractionated into sales products and where
the liquid gas has a considerably reduced content of ethane and
heavier components. This is achieved by leading the feed gas to a
fractionation column where it is contacted with an ethane rich
reflux such that the fractionation column separates the feed into
an overhead gas fraction with a considerably reduced content of
components heavier than methane and a liquid stream from the bottom
considerably enriched with components heavier than methane. The
ethane rich reflux is generated in that the gas from the
fractionation column is partially condensed, and in addition by
cooling down and condensing a stream rich in ethane which is
re-circulated from a fractionation train for fractionation of the
bottom fraction from the fractionation column.
[0028] Patent publications U.S. Pat. No. 6,401,486, U.S. Pat. No.
6,742,358 and WO02006/115597 A2 describe systems for simultaneous
liquefaction of natural gas and recovery/separation of components
heavier than methane, i.e. ethane and heavier components. The
processes themselves are also considerably different from and more
complex than the present invention in that the feed gas is first
cooled down in, amongst others, the heat exchanger(s) for
liquefaction of gas, and also by heat exchange with a flash
expanded separated liquid and with fluid from the bottom of the
column. Furthermore, the whole or part of the feed gas stream is
expanded through a turboexpander or a Joule-Thompson expansion
valve before it is led to the fractionation column.
[0029] The patent publications US 2006/0260355 A1, U.S. Pat. No.
6,662,589, U.S. Pat. No. 6,401,486 and also U.S. Pat. No. 6,742,358
consequently relate to processes to minimise the content of ethane,
LPG and also the heavier hydrocarbons in the liquid gas, whilst the
present invention comprises a system and a method to maximise the
content of methane, ethane and LPG in the liquid gas. None of the
US patent application 2006/0260355 A1, U.S. Pat. No. 6,662,589,
U.S. Pat. No. 6,401,486 or U.S. Pat. No. 6,742,358 describe the
increase in energy efficiency which can be achieved for a gas
expansion process with the integrated separation column that
receives a reflux rich in C3-C5 from the liquefaction heat
exchanger(s) for production of LNG.
[0030] A process is described in DE patent 10205366 for
simultaneous liquefaction of natural gas and recovery/separation of
components heavier than ethane, and where separated LPG and heavier
components are fractionated to sales products. This is achieved by
first partially cooling down the feed gas in the condensation
installation for liquefaction of natural gas and then by leading
the cooled down feed gas to a fractionation column where it comes
into contact with a reflux rich in ethane so that the fractionation
column separates the feed into an overheard gas fraction with a
considerably reduced content of components heavier than ethane, and
a liquid stream from the bottom considerably enriched with
components heavier than ethane. The reflux rich in ethane is
generated in that the gas from the fractionation column is
partially condensed and thereafter brought into contact with a
C4/C5 stream in a second fractionation column, and where the C4/C5
fraction is re-circulated from a fractionation train for
fractionation of the bottom product from the first fractionation
column. DE patent 10.205.366 comprises, in other words, a process
to minimise the content of LPG of the liquid gas, and also the
heavier hydrocarbons, while the present invention comprises a
system and a method to maximise the content of LPG in the liquid
gas. The publication DE 10.205.366 does not describe an increase in
energy efficiency which can be achieved in a gas expansion process
with the integrated separation column which receives a reflux rich
in C3-C5 from the liquefaction heat exchanger(s) for production of
LNG.
[0031] In U.S. Pat. No. 4,690,702 an LNG process is described where
the feed gas is firstly pre-cooled in the cooling installation for
LNG production, thereafter to be fed to a first fractionation
column where it is brought into contact with a cooled ethane rich
reflux that is re-circulated from a second fractionation column for
fractionation of the bottom stream from the first column. The
publication does not comprise a system where a reflux rich in C3-C5
for a fractionation column is achieved by partially condensing the
overhead gas product from the fractionation column as an integrated
part of an LNG process.
[0032] U.S. Pat. No. 7,010,937 shows a system for simultaneous
liquefaction of natural gas and recovery / separation of components
heavier than methane. According to this publication the gas feed is
pre-cooled and partially condensed so that a liquid stream can be
separated in a separator and where this liquid stream is
fractionated in a first fractionation column to generate an
overhead gas which is cooled down to produce a reflux for a second
fractionation column. The gas flow from the separator is expanded
across a gas expander and fed to the second fractionation column.
Therefore this US patent has little in common with the present
invention as it is defined in the subsequent claims.
DESCRIPTION OF THE INVENTION
[0033] The invention will now be described in more detail with
reference to the enclosed figures in which:
[0034] FIG. 1 shows a principal embodiment with main components and
main functionality.
[0035] FIG. 2 shows the invention with an alternative
embodiment.
[0036] FIG. 3 shows the invention with an alternative embodiment
that includes further stabilisation of the heavier hydrocarbons
that are separated out (condensate).
[0037] FIG. 4 shows the invention in detail carried out by using a
double gas expansion process.
[0038] FIG. 5 shows the invention carried out by using a hybrid
cooling circuit with a gas expansion loop and a liquid expansion
loop.
[0039] FIG. 6 shows an example of a hot temperature curve and a
cold temperature curve (composite curve) for a conventional
nitrogen expansion cycle.
[0040] FIG. 7 shows an example of a hot temperature curve and a
cold temperature curve (composite curve) for a nitrogen expansion
cycle obtained by using the present invention.
[0041] FIG. 8 shows a comparison of the curves shown in the FIGS. 6
and 7.
[0042] With reference to FIG. 1 the system for optimised
liquefaction of gas comprises, as a minimum, the following
principle components: [0043] an incoming gas stream 1 which shall
be cooled down and liquefied, [0044] a fractionation column 150 in
which the incoming gas is cooled and is separated into an overhead
fraction 2 with a reduced content of pentane and heavier
components, [0045] a bottom fraction 3 enriched with the heavier
hydrocarbon components, [0046] a system of heat exchangers 110, in
which the incoming gas is cooled down and partially condensed for
separation of heavier hydrocarbons, and further cooling and
liquefaction, [0047] a product stream 11 comprising cooled
liquefied gas, [0048] a product stream 3, which mainly comprises
pentane and heavier hydrocarbons, and [0049] a cooling system for
cooling and liquefaction of the gas comprising a gas phase cooling
medium stream 20, at least one cycle compressor 100, at least one
aftercooler 130, at least one gas expander 120.
[0050] Incoming and cleaned feed-gas 1, for example, a methane rich
hydrocarbon gas, is first fed to a fractionation column 150, where
the gas is cooled down in contact with a colder reflux fluid.
During the cooling and counter current contact with the colder
fluid, the feed gas is separated into an overhead fraction 2 with a
reduced content of the hydrocarbons that have a molecular weight
higher than pentane (C5), and a bottom fraction 3 enriched with C6
and hydrocarbons that have a higher molecular weight than C6. The
overhead fraction 2 from the fractionation column is then led to
the heat exchanger system 110, where the gas is cooled down and
partially condensed so that the resulting two-phase fluid 4 can be
separated in a suitable separator 160. A liquid 5 rich in LPG and
pentane (C3-C5), which is separated in the separator 160, is
re-circulated as cold reflux to the fractionation column 150.
(Note: Her er det feil i originalteksten (150/160) men det bo r
vaere opplagt for behandlende instans at dette er skrivefeil og at
meningen ikke forandres) As this fluid is generated by condensation
by cooling, the reflux liquid 5 will have a lower temperature than
the feed gas 1. The gas 6 from the separator 160 has now further
reduced its content of C5 hydrocarbons and hydrocarbons higher than
C5. This gas is then led back to the heat exchanger system 110 for
further cooling, condensation and sub cooling. The liquefied gas 11
is alternatively led through a control valve 140 that controls the
operating pressure and flow through the system.
[0051] In a preferred embodiment the gas feed stream 1 is
pre-cooled by a suitable external cooling medium such as available
air, water, seawater or a separate suitable refrigeration
system/pre-cooling system. For the latter external cooling method,
a separate closed mechanical refrigeration system with propane,
ammonia or other appropriate refrigerant is often used.
[0052] In a preferred embodiment the fractionation column 150 and
the separator 160 are operated at pressures and temperatures such
that the complete system (the fractionation column 150 and reflux
separator 160) generate a component split/separation point in the
normal boiling point area (NBP) between -120.degree. C. and 60 C.
This can, for example, correspond to the light key component for
the separation being butane (C4) with a normal boiling point
between -12.degree. C. and 0.degree. C., and the heavy key
component being a C6 component with a boiling point between
50.degree. C. and 70.degree. C. The overhead gas stream 6 of the
system will then be enriched with most of the butane (C4) and
hydrocarbons with a lower normal boiling point than butane. The
bottom product 3 from the fractionation column will be enriched
with most of C6 and components with a normal boiling point higher
than C6, while pentane (C5, NBP=28-36.degree. C.) is a transition
component which is distributed in the gas product of the system and
the bottom product from the fractionation column.
[0053] Cooling and condensation of the feed gas in the heat
exchanger system 110 is provided by a closed or open gas expansion
process. The cooling process starts in that a cooling medium 21
comprising a gas or a mixture of gases (such as pure nitrogen,
methane, a hydrocarbon mixture, or a mixture of nitrogen and
hydrocarbons), at a higher pressure, preferably between 3 and 10
MPa, is fed to the heat exchanger system 110 and cooled to a
temperature between 0.degree. C. and -120.degree. C., but such that
the cooling medium stream is mainly a gas at the prevailing
pressure and temperature 31. The pre-cooled cooling medium 31 is
then led into a gas expander 121 where the gas is expanded to a
lower pressure between 5%-40% of the inlet pressure, but preferably
to between 10% and 30% of the inlet pressure, and such that the
cooling agent mainly is in the gas phase. The gas expander is
normally an expansion turbine, also called turboexpander, but other
types of expansion equipment for gas can be used, such as a valve.
The flow of pre-cooled cooling agent is expanded in the gas
expander 121 at a high isentropic efficiency, such that the
temperature drops considerably. In certain embodiments of the
invention, some liquid can be separated out in this expansion, but
this is not a requirement for the process. The cold stream of
cooling agent 32 is then led back to the heat exchangers 110 where
it is used for cooling and alternatively condensing of the other
incoming warm cooling medium streams and the gas that shall be
cooled, condensed and sub cooled.
[0054] After the cold cooling medium stream 32 has been heated in
the heat exchanger system 110, the cooling medium will exist as the
gas stream 51, which in a closed loop embodiment is recompressed in
an appropriate way for recycle, and is cooled with an external
cooling medium, such as air, water, seawater or an appropriate
refrigeration unit.
[0055] Alternatively, the cooling system in an open embodiment will
use a cooling medium 21 consisting of a gas or a mixture of gases
at a higher pressure received from an appropriate source, for
example, from the feed gas that is to be treated and cooled down.
Furthermore, the open embodiment comprises that the low pressure
cooling medium stream 51 is used for other purposes or, in an
appropriate way, is recompressed to be mixed with the feed gas that
is to be treated and cooled down.
[0056] In a preferred embodiment, the returning cooling medium
stream 51 is led from the heat exchanger 110 to a separate
compressor 101 driven by the expansion turbine 121. In this way,
the expansion work is utilised, and the energy efficiency of the
process is improved. After the compressor 101, the cooling agent is
cooled further in a heat exchanger 131, before the stream is
further compressed in the cycle compressors 100. The cycle
compressors 100 can be one or more units, alternatively one or more
stages per unit. The cycle compressor can also be equipped with
inter cooling 132 between the compressor stages. The compressed
cooling medium 20 is then cooled by heat exchange in an aftercooler
130 with the help of an appropriate external cooling medium, such
as air, water, seawater or a suitable separate refrigeration cycle,
to be re-used as a compressed cooling medium 21 in a closed
loop.
[0057] In a preferred embodiment, the system of heat exchangers 110
is one heat exchanger which comprises many different "warm" and
"cold" streams in the same unit (a so-called multi-stream heat
exchanger).
[0058] FIG. 2 shows an alternative embodiment where several
multi-stream heat exchangers are connected together in such a way
that the necessary heat transfer between the cold and warm streams
can be accomplished. FIG. 2 shows a heat exchanger system 110
comprising several heat exchangers in series. However, the
invention is not related to a specific type of heat exchanger or
number of exchangers, but can be carried out in several different
types of heat exchanger systems that can handle the necessary
number of hot and cold process streams.
[0059] FIG. 3 shows an alternative embodiment where the
fractionation column 150 is equipped with a reboiler 135 to further
improve the separation (a sharper split between light and heavy
components), and also to reduce the volatility of the bottom
fraction in the column. This can be used to directly produce
condensate which is stable at ambient temperature and atmospheric
pressure.
[0060] FIG. 4 shows in details the invention applied in a more
advanced embodiment where a double gas expansion process is used.
In this embodiment, the compressed cooling medium stream 21 is
first cooled down to an intermediate temperature. At this
temperature, the cooling agent stream is divided into two parts,
where one part 31 is taken out of the heat exchanger and is
expanded in the gas expander 121 to a low pressure gas stream 32.
The other part 41 is pre-cooled further to be expanded in the gas
expander 122 to a pressure essentially equal to the pressure in
stream 32. The expanded cold cooling agent streams 32, 42 are
returned to different inlet locations on the heat exchanger system
110 and are combined to one stream in this exchanger. Heated
cooling agent 51 is then returned to recompression. In an
alternative embodiment to the system in FIG. 3, the compressed
cooling agent stream 20 in the double gas expansion circuit can be
split into two streams before the heat exchanger 110 to be cooled
down to different temperatures in separate flow channels in the
heat exchanger 110.
[0061] The same applies for the heating of the returned cold
cooling agent streams 32, 42. The embodiment is otherwise in
accordance with FIG. 3.
[0062] FIG. 5 shows in detail the invention carried out with the
use of a hybrid cooling loop where one cooling medium is used both
in a pure gas phase and in a pure liquid phase. In this embodiment
a closed cooling loop provides the cooling of the feed gas in the
heat exchanger system 110. The Said cooling cycle starts by methane
or a mixture of methane and nitrogen, where methane makes up at
least 50% of the volume, being compressed and aftercooled to a
compressed cooling medium stream 21, and where this cooling medium
stream is pre-cooled, and at least a part 31 of the cooling medium
stream is used in the gas phase in that it is expanded across a gas
expander 121 and that at least a part 41 of the cooling agent
stream is condensed to liquid and is expanded across a valve or
liquid expander 141.
[0063] It is emphasised that the embodiment of the invention is not
limited to the cooling processes described above only, but can be
used with any gas expansion cooling process for liquefaction of
natural gas or other hydrocarbon gas, where the cooling is mainly
achieved by using one or more expanding gas streams.
[0064] By carrying out the liquefaction of the natural gas in
accordance with the invention, a product of liquefied gas is
produced which has a maximum content of methane, ethane and LPG,
however, at the same time does not contain more than the permitted
level of pentane and heavier hydrocarbons with a normal boiling
point above 50-60.degree. C. At the same time, the by-production of
volatile hydrocarbons with considerable content of ethane, propane
and butane is minimised or eliminated, where such will be difficult
to handle on an offshore installation for LNG production. At the
same time more liquid natural gas will also be produced with lower
energy consumption than for similar cooling cycles configured
without the fractionation column which receives cold and LPG-rich
reflux from the cooling down process. (Note: Her er det skrivefeil
i originaltekst)
[0065] The reason for the energy consumption for the gas expansion
processes for liquefaction of the natural gas is being reduced with
the use of the invention compared to a similar cooling process
without the integrated separation column has several aspects:
[0066] The heavier hydrocarbons which are essential to separate out
to prevent freezing during the liquefaction will be condensed and
be separated at considerably higher temperatures than in
conventional methods, in that much of the condensing takes place in
the fractionation column. This reduces the energy loss in the
cooling process in that cooling load is moved to a higher
temperature range.
[0067] The heat exchanger system 100 of the cooling process
receives the gas which is to be liquefied as stream 2 (the overhead
gas stream in the fractionation column), which has a reduced
temperature with respect to the actual gas feed stream 1. A gas
expansion process is characterised in that the warm and cold
cooling curves are dominated by the large amount of gas which is
used as cooling medium. These gas streams form linear cooling
curves. The reduced feed temperature into the heat exchanger
results in a "break point" on the warm cooling curve (the sum of
the streams which are being cooled), so that it is possible to
obtain a general reduction of the distance between the warm and
cold cooling curves. This provides a better temperature adaption,
reduced energy loss and thus a reduced energy consumption to drive
the cooling process.
[0068] Preliminary analyses and comparisons show that necessary
compressor work per kg liquid natural gas which is produced can be
reduced by 5-15% for a gas expansion cycle carried out in
accordance with the invention compared to conventional methods.
[0069] FIG. 6 shows warm and cold cooling curves (warm and cold
composite curves, i.e. the sum of all warm streams that are to be
cooled down and the sum of all cold streams that are to be heated
up, respectively) for the heat exchanger system 110 carried out in
accordance with the present invention, and with a double nitrogen
expansion process as cooling system. FIG. 7 shows corresponding
warm and cold cooling curves for a corresponding cooling process
with the same feed, but carried out in a conventional way without
the fractionation column. The curves appear to look alike, but by
considering FIG. 8, which shows a section and both the systems in
(Note: skrivefeil i originaltekst) the same curve, the "break
point" and the better adaption can clearly be seen.
Example
[0070] The example below shows natural gas with 90.4% methane by
volume which is to be liquefied, where the invention is used to
maximise the amount of liquid gas and at the same time minimise the
by-production of unstable hydrocarbon liquid with a high content of
ethane, propane and butane. The stream data refer to FIG. 1, 2, 3,
4 or 5.
TABLE-US-00001 Stream No. 1 2 3 4 5 6 11 Gas fraction 1.00 1.00
0.00 0.95 0.00 1.00 0.00 Temperature 40.0 19.2 35.9 -20.0 -20.0
-20.0 -155.0 (.degree. C.) Pressure 2740 2738 2745 2725 2730 2723
2655 (kPa abs) Mole flow 4232 4422 44 4422 235 4185 4185 (kmol/h)
Mass flow 78980 87539 3410 87539 11969 75541 75541 (kg/h) Mole
fraction (%) Nitrogen 0.51% 0.49% 0.02% 0.49% 0.03% 0.52% 0.52%
Methane 90.4% 87.4% 11.8% 87.4% 19.5% 91.3% 91.3% Ethane 4.38%
4.53% 2.58% 4.53% 6.84% 4.40% 4.40% Propane 2.29% 2.95% 4.17% 2.95%
15.04% 2.27% 2.27% i-Butane 0.68% 1.25% 2.80% 1.25% 11.92% 0.65%
0.65% n-Butane 0.66% 1.52% 3.79% 1.52% 17.30% 0.62% 0.62% i-Pentane
0.17% 0.70% 2.52% 0.70% 10.57% 0.14% 0.14% n-Pentane 0.17% 0.79%
3.61% 0.79% 12.49% 0.12% 0.12% n-Hexane 0.44% 0.32% 43.62% 0.32%
6.25% 0.02% 0.02% n-Heptane 0.19% 0.00% 18.29% 0.00% 0.02% 0.00%
0.00% n-Octane 0.055% 0.000% 5.187% 0.000% 0.000% 0.000% 0.000%
n-Nonane 0.014% 0.000% 1.339% 0.000% 0.000% 0.000% 0.000% n-Decane+
0.002% 0.000% 0.214% 0.000% 0.000% 0.000% 0.000%
* * * * *