U.S. patent application number 17/597181 was filed with the patent office on 2022-09-29 for process and plant for producing liquefied natural gas.
The applicant listed for this patent is Linde GmbH. Invention is credited to Heinz BAUER, Michael WARTER.
Application Number | 20220307765 17/597181 |
Document ID | / |
Family ID | 1000006432823 |
Filed Date | 2022-09-29 |
United States Patent
Application |
20220307765 |
Kind Code |
A1 |
BAUER; Heinz ; et
al. |
September 29, 2022 |
PROCESS AND PLANT FOR PRODUCING LIQUEFIED NATURAL GAS
Abstract
A process for producing liquefied natural gas, in which natural
gas feed having methane and higher hydrocarbons including benzene
is cooled down to a first temperature level in a first cooling step
using a first mixed coolant and then subjected to a countercurrent
absorption using an absorption liquid to form a methane-enriched
and benzene-depleted gas fraction, wherein a portion of the gas
fraction is cooled down to a second temperature level in a second
cooling step using a second mixed coolant and liquefied to give the
liquefied natural gas. In the plant proposed, the first and second
mixed coolants are low in propane or free of propane, and the
absorption liquid is formed from a further portion of the gas
fraction which is condensed above the countercurrent absorption and
returned to the countercurrent absorption without pumping. The
present invention likewise provides a corresponding plant.
Inventors: |
BAUER; Heinz; (Ebenhausen,
DE) ; WARTER; Michael; (Oberhaching, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Linde GmbH |
Pullach |
|
DE |
|
|
Family ID: |
1000006432823 |
Appl. No.: |
17/597181 |
Filed: |
July 10, 2020 |
PCT Filed: |
July 10, 2020 |
PCT NO: |
PCT/EP2020/025327 |
371 Date: |
December 28, 2021 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F25J 1/0022 20130101;
F25J 1/0214 20130101; F25J 1/0241 20130101; F25J 1/0092 20130101;
F25J 1/0057 20130101; F25J 2220/64 20130101; F25J 1/0295 20130101;
F25J 2210/60 20130101; F25J 2210/06 20130101; F25J 2205/50
20130101 |
International
Class: |
F25J 1/02 20060101
F25J001/02; F25J 1/00 20060101 F25J001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 2, 2019 |
EP |
19020457.8 |
Claims
1-14. (canceled)
15. A process for producing liquefied natural gas, in which natural
gas feed containing methane and higher hydrocarbons, including
benzene, is cooled down to a first temperature level in a first
cooling step using a first mixed refrigerant, and then subjected to
countercurrent absorption using an absorption liquid to form a
benzene-depleted gas fraction, wherein a portion of the gas
fraction is cooled down to a second temperature level in a second
cooling step using a second mixed refrigerant and liquefied to give
the liquefied natural gas, wherein the first and second mixed
refrigerants are low in propane or free of propane, and the
absorption liquid is formed from a further portion of the gas
fraction which is condensed above the countercurrent absorption and
returned to the countercurrent absorption without pumping.
16. The process according to claim 15, wherein a countercurrent
absorber is used in the countercurrent absorption, which is
operated with a head condenser arranged above an absorption region
of the countercurrent absorber, wherein the head condenser is used
for condensing the further portion of the gas fraction.
17. The process according to claim 16, wherein the head condenser
is integrated into the countercurrent absorber or is at least
partially arranged within the countercurrent absorber.
18. The process according to claim 15, wherein the first mixed
refrigerant comprises in total more than 90 mole percent,
preferably more than 95 mole percent ethane, isobutane and n-butane
and in total less than 10 mole percent, preferably less than 5 mole
percent nitrogen, methane, propane and hydrocarbons having five or
more carbon atoms.
19. The process according to claim 15, wherein the second mixed
refrigerant comprises in total more than 98 mole percent nitrogen,
methane and ethane and in total less than 2 mole percent propane
and heavier hydrocarbons.
20. The process according to claim 15, wherein a first heat
exchanger is used in the first cooling step, wherein the first
mixed refrigerant in gaseous form is subjected to, in particular,
single-stage compression in a first mixed refrigerant circuit,
condensed by cooling, subcooled, expanded, heated in the first heat
exchanger and, in particular, completely evaporated thereby, and
subsequently subjected to the compression again.
21. The process according to claim 20, wherein a second heat
exchanger is used in the second cooling step, wherein the second
mixed refrigerant in gaseous from is subjected to an in particular
multi-stage compression in a second mixed refrigerant circuit,
condensed by cooling, subcooled, expanded, heated in the second
heat exchanger and, in particular, completely evaporated thereby,
and subsequently subjected to compression again.
22. The process according to claim 21, wherein the second mixed
refrigerant is used after heating in the second heat exchanger and
before compression during the condensation of the further portion
of the gas fraction from the countercurrent absorption and further
heated thereby.
23. The process according to claim 21, wherein the first heat
exchanger (E1) is used to cool down the first mixed refrigerant
(WMR) and/or the first (E1) and the second (E3) heat exchangers are
used to cool down the second mixed refrigerant (CMR).
24. The process according to claim 15, wherein in countercurrent
absorption, a rising gas phase is provided at least in part by
feeding in further natural gas feed which was not subjected to the
first cooling step and/or at least in part by evaporating a portion
of a sump liquid formed in the countercurrent absorption.
25. The process according to claim 15, wherein the natural gas feed
contains at least 80% methane and, in the methane-free remainder,
at least 50% ethane and propane.
26. A method according to claim 15, wherein the liquefied natural
gas contains at least 90% methane, wherein the methane content in
the liquefied natural gas is higher than in the natural gas
feed.
27. A plant configured to produce liquefied natural gas, having a
first heat exchanger configured to cool natural gas feed containing
methane and higher hydrocarbons, including benzene, to a first
temperature level in a first cooling step using a first mixed
refrigerant, a countercurrent absorber configured to subject the
natural gas feed to countercurrent absorption using an absorption
liquid after the first cooling step by forming a benzene-depleted
gas fraction, having a second heat exchanger configured to cool
down a portion of the gas fraction in a second cooling step to a
second temperature level using a second mixed refrigerant and is
liquefied to give the liquefied natural gas, wherein the plant is
configured to use low-propane or propane-free first and second
mixed refrigerants, and means are provided which are configured to
form the absorption liquid from a further portion of the gas
fraction, wherein they condense this above the countercurrent
absorption and return it to the countercurrent absorption without
pumping.
28. The plant according to claim 27, wherein the plant is
configured to carry out a process for producing liquefied natural
gas, in which natural gas feed containing methane and higher
hydrocarbons, including benzene, is cooled down to a first
temperature level in a first cooling step using a first mixed
refrigerant, and then subjected to countercurrent absorption using
an absorption liquid to form a benzene-depleted gas fraction,
wherein a portion of the gas fraction is cooled down to a second
temperature level in a second cooling step using a second mixed
refrigerant and liquefied to give the liquefied natural gas,
wherein the first and second mixed refrigerants are low in propane
or free of propane, and the absorption liquid is formed from a
further portion of the gas fraction which is condensed above the
countercurrent absorption and returned to the countercurrent
absorption without pumping.
Description
[0001] The invention relates to a process and to a plant for
producing benzene according to the preambles of the independent
claims.
PRIOR ART
[0002] For liquefaction and non-pressurized storage, natural gas
must be cooled down to low temperatures of approximately
-160.degree. C. In this state, the liquefied natural gas can be
economically transported by cargo ship or truck, since it has only
1/600th of the volume of the gaseous substance at atmospheric
pressure.
[0003] Natural gas generally contains a mixture of methane and
higher hydrocarbons, along with nitrogen, carbon dioxide, and
further undesirable constituents. Prior to liquefaction, these
components must be partially removed in order to avoid
solidification during liquefaction or in order to satisfy customer
requirements. The methods used for this purpose, such as
adsorption, absorption and cryogenic rectification, are generally
known.
[0004] For details of methods used in natural gas liquefaction,
reference is made to technical literature, such as the article
"Natural Gas" in Ullmann's Encyclopedia of Industrial Chemistry,
online publication Jul. 15, 2006, DOI:
10.1002/14356007.a17_073.pub2, in particular Section 3,
"Liquefaction."
[0005] In particular, mixed refrigerants consisting of various
hydrocarbon components and nitrogen are used in natural gas
condensing processes. For example, methods in which two mixed
refrigerant circuits are used (dual mixed refrigerant (DMR) are
known. In this way, natural gas, for example, which, in addition to
methane, contains higher hydrocarbons, such as ethane, propane,
butane, etc., but has already been freed of acid gases and dried
beforehand, can be subjected to separation of the higher
hydrocarbons and subsequent liquefaction. The separation of the
higher hydrocarbons is accompanied by a separation of benzene,
which is undesirable in the remaining liquefied natural gas.
Benzene is used as a key or marker component in corresponding
methods and can also be used as an indicator component for the
separation.
[0006] Methods known from the prior art for natural gas
liquefaction using corresponding mixed refrigerant circuits are
often proven to be in need of improvement in practice for the
reasons explained below.
[0007] The object of the present invention is, therefore, to
improve natural gas liquefaction using two mixed refrigerant
circuits.
DISCLOSURE OF THE INVENTION
[0008] Against this background, the present invention proposes a
process for producing liquefied natural gas and a corresponding
plant according to the preambles of the respective independent
claims. Each of the embodiments are the subject matter of the
dependent claims and of the description below.
[0009] Prior to explaining the features and advantages of the
present invention, some of the principles of the present invention
are explained in greater detail and terms used below are
defined.
[0010] The present application uses the terms "pressure level" and
"temperature level" to characterize pressures and temperatures,
which is supposed to mean that corresponding pressures and
temperatures in a corresponding plant do not have to be used in the
form of exact pressure or temperature values. However, such
pressures and temperatures typically fall within certain ranges
that are, for example, .+-.10% about an average. In this case,
corresponding pressure levels and temperature levels can be in
disjointed ranges or in ranges which overlap one another. In
particular, pressure levels, for example, include unavoidable or
expected pressure losses. The same applies to temperature levels.
The pressure levels indicated here in bar are absolute
pressures.
[0011] Where "expansion machines" are referred to here, they are
typically understood to mean known turboexpanders, which have
radial impellers arranged on a shaft. A corresponding expansion
machine can, for example, be mechanically braked or coupled to a
device, such as a compressor or a generator. Expansion of a mixed
refrigerant within the scope of the present invention is typically
carried out using an expansion valve and not using an expansion
machine.
[0012] A "heat exchanger" for use in the context of the present
invention can be designed in any manner constituting usual practice
in the field. It serves for the indirect transfer of heat between
at least two fluid flows guided, for example, in countercurrent,
here in particular a comparatively warm natural gas feed flow or a
gaseous fraction formed therefrom and one or more cold mixed
refrigerant flows. A corresponding heat exchanger can be formed
from one or more heat exchanger sections connected in parallel
and/or in series, e.g., from one or more wound heat exchangers or
corresponding sections. In addition to wound heat exchangers of the
type already mentioned, other types of heat exchangers may also be
used within the scope of the present invention.
[0013] The relative spatial terms "upper," "lower," "over,"
"under," "above," "below," "adjacent to," "next to," "vertical,"
"horizontal," etc. here refer to the reciprocal arrangement of
components during normal operation. An arrangement of two
components "one above the other" is understood here to mean that
the upper end of the lower of the two components is located at a
lower geodetic height than the lower end of the upper of the two
components or at the same geodetic height thereas, and the vertical
projections of the two components overlap. In particular, the two
components are arranged exactly one above the other, that is to say
the central axes of the two components run on the same vertical
straight line. However, the axes of the two components need not lie
exactly vertically one above the other, they may also be offset
from one another.
[0014] In the context of the present invention, a countercurrent
absorber is used. As regards the design and configuration of
corresponding apparatuses, reference is made to relevant textbooks
(see, for example, K. Sattler: Thermische Trennverfahren.
Grundlagen, Auslegung, Apparate. Weinheim: Wiley-VCH, 3rd Edition
2001). A liquid fraction ("sump liquid") and a gaseous fraction
("head gas") can typically be removed from a lower region ("sump")
or from an upper region ("head"). Countercurrent absorbers are also
generally known from the field of separation technology. They are
used for absorption in the phase countercurrent and are therefore
also referred to as countercurrent columns. During absorption in
the countercurrent, the releasing gas phase flows upwards through
an absorption column. The receiving solution phase, provided from
above and withdrawn at the bottom, flows towards the gas phase. The
gas phase is "washed" with the solution phase. Built-in components
that ensure a gradual (trays, spray zones, rotating plates, etc.)
or continuous (random filling of filling material, packings, etc.)
phase contact are typically provided in a corresponding absorption
column. A liquid stream, also referred to as "absorption liquid,"
is fed into an upper region of a countercurrent absorber, whereby
components are washed out of a gaseous stream that is fed in more
deeply.
[0015] Where a "feed of natural gas" is referred to below, this is
to be understood to mean natural gas that has been subjected, in
particular, to acid gas removal and optional further conditioning.
In particular, heavy hydrocarbons, such as butanes and/or pentanes,
along with hydrocarbons having six or more carbon atoms, may
already have been separated from corresponding feed of natural gas.
The feed of natural gas is, in particular, anhydrous and has a
content of, for example, more than 85% methane and contains, in
particular, ethane and propane in the remainder. Nitrogen, helium
and other light components may also still be contained.
[0016] Where "liquefied natural gas" is referred to below, it is
understood to mean a cryogenic liquid at the atmospheric boiling
point of methane or below, especially at -160 to -164.degree. C.,
which comprises more than 85% methane, especially more than 90%
methane, and the methane content of which is in any case higher
than that of the natural gas used. The liquefied natural gas is, in
particular, significantly lower in benzene than the feed of natural
gas and comprises benzene only at a maximum content given
below.
FEATURES AND ADVANTAGES OF THE INVENTION
[0017] A method for producing liquefied natural gas using two mixed
refrigerants is disclosed, for example, in U.S. Pat. No. 6,119,479.
In this process, the higher hydrocarbons contained in the natural
gas feed can be separated from it this in a countercurrent absorber
as needed.
[0018] For this purpose, the natural gas feed can be cooled down in
a first colling step, depending on the composition, to a
temperature in the range of -20.degree. C. to -70.degree. C. and
then fed into the countercurrent absorber. The countercurrent
absorber can have a sump heater. Sump liquid separated in the
countercurrent absorber contains at least a portion of the higher
hydrocarbons from the natural gas feed. The sump liquid can be
returned to the countercurrent absorber in part as absorption
liquid and, if necessary, also partially supplied to a head gas of
the countercurrent absorber after its removal from the
countercurrent absorber. In this way, the head gas of the
countercurrent absorber is depleted of at least a portion of the
higher hydrocarbons and is subsequently subjected to a second
cooling step, which initiates the liquefaction. Benzene is also
used as the key component here, which benzene may be contained in
the head gas of the countercurrent absorber, and thus in the
natural gas to be liquefied, in particular at less than 1 ppm on a
molar basis. The contents of other higher hydrocarbons result
therefrom; however, these are typically less critical. Benzene is,
in particular, to be regarded as critical in natural gas
liquefaction because it can solidify at the low temperatures
used.
[0019] Mixed refrigerants are used in corresponding refrigerant
circuits both in the first and in the second cooling step of the
method just explained. In particular, a first mixed refrigerant
(warm mixed refrigerant, WMR) is subjected to compression in
gaseous form in the order indicated below, condensed by cooling,
subcooled, expanded, heated in the first heat exchanger, in
particular completely evaporated thereby, and subsequently
subjected to compression again. The first mixed refrigerant can be
subcooled in particular in the first heat exchanger, the previous
cooling takes place in a further heat exchanger. Furthermore, a
second mixed refrigerant (cold mixed refrigerant, CMR) can be
subjected to compression in gaseous form, condensed subcooled by
cooling, relaxed, heated in the second heat exchanger, in
particular completely evaporated thereby, and subsequently
subjected to compression again. The subcooling of the second mixed
refrigerant can take place in particular in the second heat
exchanger, the previous cooling in the first and the second heat
exchangers.
[0020] The first and second heat exchangers are in particular used
in a known manner per se as coiled heat exchangers (coil wound heat
exchanger, CWHE), wherein the heating of the mixed refrigerant
takes place, after its expansion, in particular on the shell side,
i.e., in a jacket space surrounding the heat exchanger tubes or,
into which the mixed refrigerant is expanded. The media to be
cooled down are guided tube-side, i.e., through the correspondingly
provided heat exchanger tubes. The heat exchanger tubes are
provided in bundles in corresponding heat exchangers, so that the
term "tube-side" or "bundle-side" is used here for a corresponding
flow guide.
[0021] Processes and plants of a similar type are also disclosed,
for example, in U.S. Pat. No. 6,370,910 A and AU 2005224308 B2.
[0022] Processes for natural gas liquefaction must be able to be
flexibly adapted to different plant capacities and operating
conditions. The processes as explained, which use two mixed
refrigerant circuits, are preferably used, where large ambient
temperature fluctuations result in significantly different
refrigerant condensation conditions. These can be taken into
account more efficiently if a mixture comprising refrigerant
components is used instead of a single pure component, such as
propane.
[0023] In addition, corresponding processes do not contain large
inventories of liquid hydrocarbons having a higher molecular weight
than air, which would present a significant safety risk.
Corresponding hydrocarbons can accumulate in more deeply located
regions and possibly lead to explosions. Propane in this sense is
considered to be the most dangerous refrigerant due to a
combination of high volatility and high molecular weight. Processes
using two mixed refrigerant circuits and a correspondingly reduced
propane fraction therein are, therefore, a preferred solution for
plant layouts having limited installation space, for example
modularized plants and/or floating plants, in which the base area
is limited.
[0024] A compact plant layout (e.g., mandatory for offshore
installations) can be achieved by minimizing the number of plant
components and by reducing the space between the plants, which can
be determined based on safety aspects. The plant components known
to be hazardous include pumps for liquid hydrocarbons (risk of
leakage and liquid discharge) and all types of devices that contain
significant amounts of liquid propane.
[0025] The present invention solves the problems explained by
dispensing with hydrocarbon pumps and largely dispensing with
propane as a refrigerant component in corresponding mixed
refrigerants. These advantages are achieved by the measures
according to the invention proposed below and corresponding
advantageous embodiments.
[0026] In the process proposed according to the invention for
producing liquefied natural gas, a natural gas feed of the type
explained above, which contains methane and higher hydrocarbons,
including benzene, is cooled down as a whole in a first cooling
step using a first ("warm") mixed refrigerant cooled down to a
first temperature level, in particular from -20.degree. C. to
-70.degree. C., and then subjected to countercurrent absorption
using an absorption liquid to form a benzene-depleted gas fraction.
The benzene-depleted gas fraction has, in particular, a content of
less than 1 ppm on a molar basis of benzene, wherein the content of
benzene in the natural gas feed is significantly higher, for
example 5 to 500 ppm. Particularly in comparison to the natural gas
feed, the gas fraction formed is enriched in methane and depleted
of the higher hydrocarbons.
[0027] Known means can, in principle, be used for countercurrent
absorption. The gas fraction can also be (essentially) free of
hydrocarbons having five and optionally more carbon atoms, so that
depletion (essentially) to zero can take place. However, higher
hydrocarbons may also still be contained, and a sump liquid formed
during countercurrent absorption can also have certain proportions
of methane. The degree of separation or accumulation and depletion
achieved in countercurrent absorption depends upon the subsequent
use of corresponding fractions and the respective tolerable
contents of the specified components.
[0028] In the context of the present invention, a portion of the
gas fraction from countercurrent absorption, which is
correspondingly depleted (or essentially free) of benzene (and
other higher hydrocarbons), is cooled down in a second cooling step
using a second ("cold") mixed refrigerant cooled down to a second
temperature level of, in particular, -145.degree. C. to
-165.degree. C. and liquefied to give liquefied natural gas.
Liquefied natural gas formed in this way can be subjected to any
further processing or conditioning (expansion, subcooling,
etc.).
[0029] In the context of the present invention, the first and
second mixed refrigerants are low in propane (having a content of
less than 5 mole percent propane) or (essentially) propane-free,
and the absorption liquid for the countercurrent absorption is
formed from a further portion of the gas fraction from the
countercurrent absorption, which (geodetically) is condensed above
the countercurrent absorption and returned to the countercurrent
absorption without pumping. For the term "above," reference is made
to the above definitions.
[0030] By means of the proposed measures, the present invention
reduces or eliminates the use of appreciable amounts of
propane-containing media. As mentioned, propane is considered a
dangerous refrigerant due to the combination of high volatility and
high molecular weight. A corresponding refrigerant must necessarily
be conveyed by means of machines, with which there is an increased
probability of propane leakage. This is no longer the case within
the scope of the present invention, which means that it is also
suitable and advantageous, in particular, for plant layouts having
limited installation space, for example modularized plants and/or
floating plants with which the base area is limited and additional
installation space requiring safety equipment can only be installed
with difficulty.
[0031] Since the absorption liquid for countercurrent absorption is
formed from the further portion of the gas fraction from the
countercurrent absorption, condensed above the countercurrent
absorption and returned to the countercurrent absorption without
pumping, this (possibly propane-containing) medium does not require
the detrimental use of pumps having the problems explained.
[0032] The invention thus provides a solution in which the use of
significant amounts of propane-containing media is essentially
dispensed with, either by using refrigerant mixtures that
previously contained propane in a low-propane or propane-free
manner or by conveying a propane-containing head gas from the
countercurrent absorption without pumping. Surprisingly, it has
been found out that the process proposed in the context of the
present invention has the same or a higher thermodynamic efficiency
in comparison to known methods. Within the scope of the present
invention, the investment costs can be reduced without increasing
the operating costs.
[0033] In the process proposed according to the invention, a
countercurrent absorber is advantageously used in countercurrent
absorption, which countercurrent absorber is operated with a head
condenser arranged above an absorption region of the countercurrent
absorber, wherein the head condenser is used to condense the
further portion of the gas fraction. An "absorption region" is to
be understood here as meaning the region having built-in components
as explained above.
[0034] The head condenser can be integrated into the countercurrent
absorber or at least partially arranged within the countercurrent
absorber. An integrated head condenser comprises a heat exchange
structure in a common column jacket, in which material exchange
structures of the type explained above are also arranged, wherein
the heat exchange structure, for example, a cooling coil or the
like is separated from a region containing the material exchange
structures, in particular by a liquid accumulation bottom or a
liquid-tight bottom.
[0035] The latter allows a controlled return of condensate to the
region having the material exchange structures. In contrast, a head
condenser arranged outside is not arranged in a common column
jacket having the material exchange structures.
[0036] In the process proposed according to the invention, the
first refrigerant mixture advantageously comprises in total more
than 90 mole percent ethane, isobutane and n-butane and in total
less than 10, preferably less than 5 mole percent nitrogen,
methane, propane and hydrocarbons having five or more carbon atoms.
Compared to known processes, the small amount of propane proves
unproblematic. By contrast, the second mixed refrigerant
advantageously has more than 98 mole percent nitrogen, methane and
ethane in total and less than 2 mole percent propane and higher
hydrocarbons in total.
[0037] In the context of the present invention, a first heat
exchanger is advantageously used in the first cooling step, wherein
the first mixed refrigerant in gaseous form is subjected to, in
particular, single-stage compression in a first mixed refrigerant
circuit, condensed by cooling, subcooled, relaxed, heated in the
first heat exchanger, in particular completely evaporated thereby,
and subsequently subjected to compression again. The subcooling of
the first mixed refrigerant can take place, in particular, in the
first heat exchanger, the previous cooling in a further heat
exchanger. In contrast to processes not according to the invention,
the compression of the first mixed refrigerant thus takes place, in
particular, in a single stage and without intermediate cooling,
which would constitute a risk of partial condensation and a need to
convey the condensate to the high-pressure side of the compressor.
This disadvantage is remedied here.
[0038] Furthermore, in the method according to the invention, a
second heat exchanger is advantageously used in the second cooling
step, wherein the second mixed refrigerant in a second mixed
refrigerant circuit is subjected to, in particular multi-stage,
compression, condensed by cooling, subcooled, relaxed, heated in
the second heat exchanger, in particular completely evaporated
thereby, and subsequently subjected to compression again. The
subcooling of the second mixed refrigerant can take place, in
particular, in the second heat exchanger, the previous cooling in
the first and the second heat exchangers.
[0039] As mentioned, the first and second heat exchangers may be
designed as wound heat exchangers and, in particular, with one or
two (serial) bundles in a common jacket in each case.
[0040] In the context of the present invention, a header for the
second mixed refrigerant, which receives said refrigerant after it
has condensed, can be designed in particular for a pressure that is
2 to 10 bar above a suction pressure of a compressor or of a first
of several compressors used in compressing the second mixed
refrigerant.
[0041] In particular, a series of three compressors can be used for
compressing the first and the second mixed refrigerant, a first of
which compresses the first mixed refrigerant and the further two
compress the second mixed refrigerant. Such compressors can be
designed for (almost) identical shaft powers, i.e., 331/3.+-.3% of
the total power consumption.
[0042] The second mixed refrigerant is advantageously used after
heating and evaporation in the second heat exchanger and before
compression during the condensation of the further portion of the
gas fraction from the countercurrent absorption and is further
heated thereby. In this way, a particularly advantageous
utilization of this second mixed refrigerant results.
[0043] In the context of the present invention, the first (but not
the second) heat exchanger is advantageously used for cooling the
first mixed refrigerant and/or the second (and additionally the
first) heat exchanger is used for cooling the second mixed
refrigerant. Further cooling after compression or after compression
steps can take place in a known manner, for example using air or
water coolers.
[0044] In the context of the present invention, in an alternative,
a rising gas phase is formed in countercurrent absorption at least
in part by feeding in further natural gas feed, which was subjected
to the first cooling step. In this way, there is no need for a
reboiler, but a higher separation performance is caused in
countercurrent absorption. However, the rising gas phase can also
be provided at least in part by evaporation of a portion of a sump
liquid formed in the countercurrent absorption.
[0045] In the context of the present invention, working liquid
expanders can be used instead of expansion valves. This reduces
energy consumption.
[0046] The present invention is suitable for typical natural gases
so that the natural gas feed can contain, in particular, at least
80% methane and, in the methane-free remainder, at least 50% ethane
and propane. The liquefied natural gas advantageously contains at
least 90% methane, wherein a methane content in the liquefied
natural gas is higher than in the natural gas feed.
[0047] The present invention further extends to a plant for
producing benzene, reference being made to the corresponding
independent claim with regard to its specific features. For further
features and embodiments of such a plant and of preferred
embodiments, reference is expressly made to the above explanations
regarding the method according to the invention and its respective
advantageous embodiments. Advantageously, such an arrangement is
designed for carrying out a process as previously explained in
different embodiments.
[0048] The invention is described in more detail hereafter with
reference to the accompanying drawings, which illustrate a natural
gas liquefaction plant according to an embodiment of the present
invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0049] FIG. 1 illustrates a plant according to an embodiment of the
present invention in the form of a simplified process flow
diagram.
[0050] FIG. 2 illustrates a plant according to a further embodiment
of the invention in the form of a simplified process flow
diagram.
DETAILED DESCRIPTION OF THE DRAWINGS
[0051] In FIG. 1, a plant according to a particularly preferred
embodiment of the present invention is shown in the form of a
greatly simplified schematic process flow diagram and is designated
as a whole by 100.
[0052] The plant 100 illustrated in FIG. 1 is supplied with natural
gas feed NG, which is first divided into two partial streams. A
first partial stream is cooled down in a first heat exchanger E1,
which can in particular be designed as a wound heat exchanger, in a
first cooling step to a first temperature level of, for example,
-20.degree. C. to -70.degree. C. and then fed approximately
centrally into a countercurrent absorber T1.
[0053] Furthermore, the second partial flow of the natural gas feed
NG, which is expanded via a valve V6, is fed into a lower region of
the countercurrent absorber T1, where it rises essentially in
gaseous form. Gas is withdrawn from an upper region of the
countercurrent absorber T1 and is cooled down in a head condenser
E2, which can be designed, for example, as a plate heat exchanger,
and fed into a head space of the countercurrent absorber T1. Liquid
precipitating here is returned as a return flow to the
countercurrent absorber T1 and washes out heavier components from
the natural gas feed, which pass into a sump liquid of the
countercurrent absorber T1.
[0054] The sump liquid of the countercurrent absorber T1 can be
expanded via a valve V5 and discharged from the plant 100 as a
heavy fraction HHC (heavy hydrocarbon). Head gas of the
countercurrent absorber T1, i.e., a methane-rich gas fraction, is,
in contrast, cooled down to a liquefaction temperature in a second
heat exchanger E3, which can also be designed as a wound heat
exchanger, and, after expansion, discharged via a valve V4 as
liquefied natural gas LNG from the plant 100.
[0055] The plant 100 comprises two mixed refrigerant circuits. In a
first mixed refrigerant circuit, a first ("warm") mixed refrigerant
WMR is subjected to single-stage compression in gaseous form in a
compressor C1 and subsequently cooled down in an air cooler and/or
water cooler E4 and thereby condensed. Condensate can be obtained
in a separation vessel D1. This is first further cooled down in the
first heat exchanger E1 on the bundle side, then expanded via a
valve V1 and fed into the jacket space of the first heat exchanger
E1, where it is heated, completely evaporated and subsequently
subjected to compression again.
[0056] In contrast to processes not according to the invention, the
compression of the first mixed refrigerant takes place here, in
particular, in the single-stage compressor C1 without intermediate
cooling, which would constitute a risk of partial condensation and
a need to convey the condensate to the high-pressure side of the
compressor. This disadvantage is remedied here.
[0057] Furthermore, in the plant 100, a second mixed refrigerant
CMR is subjected to a gradual compression in compressors LP C2 and
HP C2 in gaseous form and subsequently cooled down in each case,
for example in air coolers and/or water coolers E5 and E6. Further
cooling takes place on the bundle side in the first heat exchanger
E1 and then in the second heat exchanger E3. After subsequent
expansion in a valve V2, feeding into a buffer vessel D2 takes
place. Condensate withdrawn therefrom is expanded via a valve V3
and fed jacket-side into the second heat exchanger E2, where it is
heated and completely evaporated. The gaseous second mixed
refrigerant CMR is used as refrigerant in the aforementioned head
condenser E2 before it is again subjected to compression.
[0058] A return pump can be dispensed with by installing the head
condenser E2, which is operated using tactile heat of the second
mixed refrigerant, which leaves the second heat exchanger E3 as a
vapor above the countercurrent absorber T1. In contrast, the return
flow formed from the gas from the countercurrent absorber T1 is
returned to the countercurrent absorber T1 purely by the effect of
gravity.
[0059] In FIG. 2, a plant according to a further embodiment of the
present invention is shown in the form of a greatly simplified
schematic process flow diagram and is designated as a whole by
200.
[0060] A first difference from the embodiment of the plant 100
according to FIG. 1 here is that the countercurrent absorber T1 is
not supplied with a partial flow of the natural gas feed, but
instead a reboiler E7 is provided which evaporates a portion of the
sump liquid of the countercurrent absorber T1 and thus forms a
portion of the rising gas phase in the countercurrent absorber
T1.
[0061] A further difference from the embodiment of the plant 100
according to FIG. 1 is, furthermore, that the head condenser E3 in
the form of corresponding heat exchanger structures is displaced
into the head space of the countercurrent absorber T1, whereby
corresponding installation space is potentially saved.
[0062] Finally, as illustrated here, an expansion of the liquid
natural gas LNG leaving the second heat exchanger E3 is provided
via an expansion machine
[0063] X1 and a corresponding expansion of the cooled second mixed
refrigerant CMR in an expansion machine X2. Analogously, the valve
V1 can also be replaced by a expansion machine X3 (not shown).
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