U.S. patent application number 15/999800 was filed with the patent office on 2019-05-16 for mehod for gradual sealing of a gas.
The applicant listed for this patent is LINDE AKTIENGESELLSCHAFT. Invention is credited to Thomas Bretschneider, Torben HOFEL, Martin Kamann, Eva-Maria Katzur, Jorg Matthes, Sean McCracken.
Application Number | 20190145703 15/999800 |
Document ID | / |
Family ID | 55456592 |
Filed Date | 2019-05-16 |
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United States Patent
Application |
20190145703 |
Kind Code |
A1 |
HOFEL; Torben ; et
al. |
May 16, 2019 |
MEHOD FOR GRADUAL SEALING OF A GAS
Abstract
A method is proposed for compressing a gas in stages in a
compressor arrangement (100, 200, 300, 400) having a plurality of
compression stages (I-VI) which are connected together sequentially
by a main line (1) and in which the gas, guided through the main
line (1), is respectively compressed from a suction-side pressure
level to a pressure-side pressure level and is heated by this
compression from a suction-side temperature level to a
pressure-side temperature level, wherein a feedback amount of the
gas, guided through the main line (1), is at least temporarily
removed from the main line (1) downstream of one of the compression
stages (V), is fed to an expansion process, and is fed back into
the main line (1) upstream of the same compression stage (V). It is
provided that the pressure-side pressure level of the compression
stage (V) downstream of which the feedback amount is removed from
the main line (1) is a supercritical pressure level, that the
feedback amount is expanded to a subcritical pressure level, that
the feedback amount is fed to the expansion process at the
pressure-side temperature level of the compression stage (V)
downstream of which it is removed from the main line (1), and that
the feedback amount is cooled only after being expanded and before
and/or after being fed back into the main line (1). The invention
also relates to a compressor arrangement (100, 200, 300, 400).
Inventors: |
HOFEL; Torben; (Munchen,
DE) ; McCracken; Sean; (Puchheim, DE) ;
Kamann; Martin; (Oberhaching, DE) ; Katzur;
Eva-Maria; (Unterschleissheim, DE) ; Matthes;
Jorg; (Geretsried, DE) ; Bretschneider; Thomas;
(Neuried, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
LINDE AKTIENGESELLSCHAFT |
MUNCHEN |
|
DE |
|
|
Family ID: |
55456592 |
Appl. No.: |
15/999800 |
Filed: |
February 20, 2017 |
PCT Filed: |
February 20, 2017 |
PCT NO: |
PCT/EP2017/053796 |
371 Date: |
August 20, 2018 |
Current U.S.
Class: |
62/612 |
Current CPC
Class: |
F25J 2245/02 20130101;
F25J 2230/04 20130101; F04D 29/582 20130101; F25J 2270/80 20130101;
F25J 2230/20 20130101; F25J 3/0219 20130101; F25J 2230/22 20130101;
F25J 2215/62 20130101; F25J 2230/60 20130101; F25J 2230/80
20130101; F04D 17/12 20130101; F25J 3/0238 20130101; F25J 2270/02
20130101; F25J 1/029 20130101; Y02C 20/40 20200801; F25J 3/0266
20130101; F25J 1/0282 20130101; F25J 2270/88 20130101 |
International
Class: |
F25J 1/02 20060101
F25J001/02 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 19, 2016 |
EP |
16156574.2 |
Claims
1. A method for compressing a gas in stages in a compressor
arrangement having compression stages (I-VI) which are connected
together sequentially by a main line and in which the gas, guided
through the main line, is respectively compressed from a
suction-side pressure level to a pressure-side pressure level and
is heated by this compression from a suction-side temperature level
to a pressure-side temperature level, a feedback amount of the gas,
guided through the main line, being at least temporarily removed
from the main line downstream of one of the compression stages (V),
being fed to an expansion process, and being fed back into the main
line upstream of the same compression stage (V), characterised in
that the pressure-side pressure level of the compression stage (V)
downstream of which the feedback amount is removed from the main
line is a supercritical pressure level, in that the feedback amount
is expanded to a subcritical pressure level, in that the feedback
amount is fed to the expansion process at the pressure-side
temperature level of the compression stage (V) downstream of which
it is removed from the main line, and in that the feedback amount
is cooled only after being expanded and before and/or after being
fed back into the main line.
2. The method according to claim 1, wherein a further feedback
amount of the gas, guided through the main line, is at least
temporarily removed from the main line downstream of a further
compression stage (VI), is fed to an expansion process, and is fed
back into the main line upstream of the same further compression
stage (VI), wherein the pressure-side pressure level of the further
compression stage (VI) downstream of which the further feedback
amount is removed from the main line is a supercritical pressure
level, wherein this further feedback amount is expanded to a
supercritical pressure level, and wherein the feedback amount is
cooled only after being expanded and before and/or after being fed
back into the main line.
3. The method according to claim 1, wherein an additional feedback
amount of the gas, guided through the main line, is at least
temporarily removed from the main line downstream of an additional
compression stage (III, IV), is fed to an expansion process, and is
fed back into the main line upstream of the same additional
compression stage (III, IV), wherein the pressure-side pressure
level of the additional compression stage (III, IV) downstream of
which the further feedback amount is removed from the main line is
a subcritical pressure level, wherein this additional feedback
amount is expanded to a subcritical pressure level, and wherein the
feedback amount is cooled only after being expanded and before
and/or after being fed back into the main line.
4. The method according to claim 1, wherein a further heat
exchanger is used in a return line used to return the feedback
amount and/or in the main line.
5. The method according to claim 1, wherein the feedback amount is
fed back into the main line upstream of one or more compression
stages (I-1V) which are arranged upstream of the compression stage
(V) downstream of which the feedback amount is removed from the
main line.
6. The method according to claim 1, wherein the feedback amount is
controlled based on an attainable or attained suction-side or
pressure-side pressure level of one of the compression stages
(I-VI).
7. The method according to claim 1, wherein the gas is cooled
between the compression stages (I-VI) using cooling water which is
maintained within a predetermined temperature range.
8. The method according to claim 1, wherein the gas is ethylene or
an ethylene-rich gas, which is provided in particular using a steam
cracking method, or wherein the gas is ethane or an ethane-rich
gas, or wherein the gas is carbon dioxide or a carbon dioxide-rich
gas.
9. The method according to claim 1, wherein a plurality of the
compression stages (1-VI) are driven by one or more common shafts,
by which the respective compression stages (I-VI) are mechanically
coupled.
10. The method according to claim 9, wherein a plurality of the
compression stages (I-VI) are respectively driven by a plurality of
common shafts.
11. The method according to claim 10, wherein the plurality of
common shafts are mechanically coupled by a transmission.
12. A plant which is configured to compress a gas in stages and
which comprises a compressor arrangement with compression stages
(I-VI) which are connected together sequentially by a main line and
in which the gas, guided through the main line, can be respectively
compressed from a suction-side pressure level to a pressure-side
pressure level and can be heated by this compression from a
suction-side temperature level to a pressure-side temperature
level, means being provided which are configured to at least
temporarily remove a feedback amount of the gas, guided through the
main line, from the main line downstream of one of the compression
stages (V), to feed it to an expansion process and to feed it back
into the main line upstream of the same compression stage (V),
characterised in that the plant is configured to be operated such
that the pressure-side pressure level of the compression stage (V)
downstream of which the feedback amount is removed from the main
line is a supercritical pressure level and in that the feedback
amount is expanded to a subcritical pressure level, in that means
are provided which are configured to feed the feedback amount to
the expansion process at the pressure-side temperature level of the
compression stage (V) downstream of which the feedback amount is
removed from the main line, and in that means are provided which
are configured to cool the feedback amount only after it has been
expanded and before and/or after it is fed back into the main
line.
13. The plant according to claim 12 which is configured to
implement a method for compressing a gas in stages in a compressor
arrangement having compression stages (I-VI) which are connected
together sequentially by a main line and in which the gas, guided
through the main line, is respectively compressed from a
suction-side pressure level to a pressure-side pressure level and
is heated by this compression from a suction-side temperature level
to a pressure-side temperature level, a feedback amount of the gas,
guided through the main line, being at least temporarily removed
from the main line downstream of one of the compression stages (V),
being fed to an expansion process, and being fed back into the main
line upstream of the same compression stage (V), characterised in
that the pressure-side pressure level of the compression stage (V)
downstream of which the feedback amount is removed from the main
line is a supercritical pressure level, in that the feedback amount
is expanded to a subcritical pressure level, in that the feedback
amount is fed to the expansion process at the pressure-side
temperature level of the compression stage (V) downstream of which
it is removed from the main line, and in that the feedback amount
is cooled only after being expanded and before and/or after being
fed back into the main line.
Description
[0001] The invention relates to a method for compressing a gas in
stages in a compressor arrangement having a plurality of
compression stages which are connected together sequentially by a
main line, and to a corresponding compressor arrangement according
to the preamble of the independent claims.
PRIOR ART
[0002] Methods and devices for steam cracking hydrocarbons are
known and are described for example in the article "Ethylene" in
Ullmann's Encyclopaedia of Industrial Chemistry, online since 15
Apr. 2009, DOI 10.1002/14356007.a10_045.pub3.
[0003] In steam cracking, gas mixtures are obtained which, after
separating water and oily constituents, if present (so-called
pyrolysis oil), still substantially contain hydrogen, methane and
hydrocarbons having two and more carbon atoms. Gas mixtures of this
type can be separated in separating sequences, as are basically
known to a person skilled in the art and are also described in the
mentioned article. The conventional target product in steam
cracking, namely ethylene, is also separated from the other
components in appropriate separating sequences. In this respect,
ethylene is typically drawn off from the top of a so-called C2
splitter.
[0004] Subject to the requirements of downstream processes or
consumers, ethylene can be released to the plant limit under
different conditions. Typically, ethylene is released under
elevated pressure in the form of gas. Provided that the ethylene is
not already present under the desired conditions, compression is
carried out in single-stage or multi-stage turbocompressors. Since
ethylene is also used as a refrigerant in the mentioned separating
sequences, it is also compressed for this purpose. Ethylene is
typically compressed for the mentioned purposes in a common
turbocompressor having a plurality of compression stages and
intermediate cooling, which is thus also used as the product
compressor and as the refrigerant compressor. The ethylene is
removed from this turbocompressor for use as refrigerant and as
product at different pressure levels corresponding to different
compression stages. A corresponding turbocompressor is shown
schematically in the accompanying FIG. 1 and is described in more
detail below. However, in principle it is also possible to use a
plurality of separate, in particular multi-stage, turbocompressors
for refrigerant and product compression.
[0005] Although the present invention is mainly described with
reference to ethylene and to ethylene-rich gases, it is equally
suitable for the compression of gases which behave similarly
thermodynamically, such as ethane and carbon dioxide. The
description with regard to ethylene is therefore used only as an
example.
[0006] In particular cases, the release of ethylene at a
supercritical pressure level is required. For this purpose, the
ethylene can either be liquefied and then conveyed at a
supercritical pressure level by a pump, or it is brought to the
corresponding pressure level in a multi-stage compressor of the
described type. The latter case is shown schematically in the
accompanying FIG. 2 and is also described in more detail below.
Here, the product is compressed in stages IV to VI and the
refrigerant is compressed in stages I to III. Compression without
liquefaction is the more favourable alternative in terms of
energy.
[0007] However, problems can arise in multi-stage compression at
supercritical pressure levels in conventional multi-stage
turbocompressors, as described below. The present invention is
intended to overcome these problems.
DISCLOSURE OF THE INVENTION
[0008] In view of the above, the invention proposes a method for
compressing a gas in stages in a compressor arrangement having a
plurality of compression stages which are connected together
sequentially by a main line, and a corresponding compressor
arrangement having the features of the independent claims.
Embodiments are the subject of the dependent claims and of the
following description.
[0009] To characterise pressures and temperatures, the present
application uses the terms "pressure level" and "temperature
level", which are intended to signify that corresponding pressures
and temperatures in a corresponding plant do not have to be used as
exact pressure and temperature values in order to realise the
inventive concept. However, such pressures and temperatures are
typically within particular ranges which lie, for example .+-.1%,
5%, 10%, 20% or even 50% around an average. In this respect,
corresponding pressure levels and temperature levels can lie within
disjoint ranges or within overlapping ranges. In particular, for
example pressure levels include pressure losses which are
unavoidable or which are to be expected. The same applies
accordingly to temperature levels. Pressure levels which are stated
here in bar are absolute pressures.
Advantages of the invention
[0010] Before the ethylene reaches the supercritical pressure level
in a multi-stage turbocompressor, it is possible for liquefaction
to occur at relatively moderate temperatures at a high, but still
subcritical pressure level. The critical temperature of ethylene is
approximately 8.degree. C. This temperature is low enough for
liquefaction to be ruled out by an intermediate cooling with
cooling water downstream of a compression stage. Cooling water is
typically at a temperature of at least 10.degree. C. Howevor,
typically provided in multi-stage turbocompressors are return
lines, or so-called kickbacks, which, under partial load or other
intermittently occurring operating states, expand ethylene to a
lower pressure level from the pressure side of a compression stage
and feed it back on the suction side to the same compression stage
or to a compression stage which is arranged upstream thereof. This
is also shown in FIG. 2 and is described in more detail below.
[0011] If the ethylene is cooled too much before a corresponding
recirculation, an undesirable partial liquefaction can result
during expansion. If the compressed ethylene is at approximately 70
bar for example after stage V of the turbocompressor according to
FIG. 2, and if it is cooled to 20.degree. C. at this pressure level
and then expanded isenthalpically to approximately 40 bar by a
throttle to feed it back to stage V, a direct transition into the
two-phase region results in the enthalpy diagram. For details,
reference is made to enthalpy diagrams for ethylene published in
the relevant specialist literature. However, a corresponding
partial liquefaction is a disadvantage, because the multi-stage
turbocompressors which are used are not configured for conveying
liquid phases.
[0012] To overcome this disadvantage, the present invention
proposes a method for compressing a gas in stages in a compressor
arrangement having a plurality of compression stages which are
connected together sequentially by a main line. The compression
stages can be configured in particular as turbocompression stages,
as previously described. In particular, the compression stages can
be partly or entirely driven by common mechanical devices, for
example common shafts, and in this way they are coupled together
mechanically.
[0013] As mentioned, the gas used in the present invention can be,
for example, an ethylene-rich gas. An ethylene-rich gas of this
type can also be pure or substantially pure ethylene, i.e. it can
contain at least 90%, 95% or 99% ethylene. Since in the context of
the present invention the ethylene-rich gas can be removed in
particular from the top of a known C2 splitter (see the specialist
literature mentioned at the beginning), it has in particular the
usual ethylene content in this connection. For simplification
purposes, a corresponding ethylene-rich gas will also be referred
to in the following as "ethylene". However, the present invention
is also suitable, for example, for the compression of ethane or
carbon dioxide, which have comparable thermodynamic characteristic
quantities, or for corresponding ethane-rich and carbon
dioxide-rich gases.
[0014] In the compression stages, the gas which is guided through
the main line is respectively compressed from a suction-side
pressure level to a pressure-side pressure level and is heated by
this compression from a suction-side temperature level to a
pressure-side temperature level.
[0015] Here, the term "suction-side pressure level" is understood
as meaning the pressure level at which the gas is fed to the
compression stage. This suction-side pressure level is also
commonly known as "suction pressure". The "pressure-side pressure
level" is the pressure level to which the compression stage
compresses the gas.
[0016] Here, the term "suction-side temperature level" is
understood as meaning the temperature level at which a
corresponding gas is fed to the compression stage. This temperature
level is no longer actively influenced before the gas is fed into
the compressor, in particular the gas is no longer actively heated
or cooled from a suction-side temperature level. Accordingly, a
"pressure-side temperature level" denotes the temperature level
directly downstream of a corresponding compression stage, thus the
pressure-side temperature level is the pressure level at which a
corresponding gas leaves the compression stage. Therefore,
downstream of the compression stage, the temperature level is no
longer actively influenced to reach the pressure-side temperature
level, in particular there is no heating or active cooling in a
cooler. If an intermediate cooler is used downstream of the
compression stage, the "pressure-side temperature level" is present
up to the entry of the gas into the intermediate cooler.
[0017] Within the context of the present invention, according to
conventional kickback lines, a feedback amount of the gas guided
through the main line is at least temporarily removed from the main
line downstream of one of the compression stages, is fed to an
expansion process and after expansion is fed back into the main
line upstream of the same compression stage.
[0018] The expression feedback "upstream of the same compression
stage" can mean, as also explained below, feedback directly
upstream of the compression stage downstream of which the feedback
amount was removed; however, feedback can also take place upstream
of one or more further compression stages which are arranged
upstream of the compression stage downstream of which the feedback
amount was removed from the main line.
[0019] As already stated, if appropriate kickbacks are used in the
form of the described feedback amount, the previously described
problems of partial liquefaction can occur when an arrangement is
used which has been previously described and is shown in FIG. 2. In
such an arrangement, an appropriate feedback amount is removed from
the main line downstream of an aftercooler so that it is already in
a cooled state at the removal point. The feedback amount is further
cooled by the expansion process, which is necessary for feedback.
This is particularly critical in the case of fluctuating or
generally low cooling water temperatures, because (partial)
liquefaction can result here. As mentioned, this can take place for
example in the case of ethylene or ethylene-rich gases even at
relatively moderate temperatures, namely for example if the
feedback amount is at a pressure of approximately 70 bar, is cooled
to 20.degree. C. at this pressure level and is then expanded
isenthalpically to approximately 40 bar by a throttle.
[0020] A corresponding liquid phase formation cannot exclusively
occur when the pressure-side pressure level of the compression
stage downstream of which the feedback amount is removed from the
main line is above a supercritical pressure level and this feedback
amount is expanded to a subcritical pressure level, but during
normal operation of a corresponding plant it is a disadvantage
particularly during a "transcritical" expansion of this type.
Therefore, the present invention focuses on corresponding cases of
transcritical compression and expansion.
[0021] The invention therefore provides that the pressure-side
pressure level of the compression stage downstream of which the
feedback amount is removed from the main line is a supercritical
pressure level, that the feedback amount is expanded to a
subcritical pressure level and that the feedback amount is fed to
the expansion process at the pressure-side temperature level of the
compression stage downstream of which it is removed from the main
line. To obtain the advantages according to the invention, the
feedback amount is cooled only after being expanded and before
and/or after being fed back into the main line.
[0022] In principle, corresponding problems can also arise if the
feedback amount is expanded during normal operation from a
supercritical to a supercritical pressure level (i.e. for example
in stage VI of the arrangement shown in FIG. 2 or of a
corresponding feedback amount). For example, during the start-up or
during a malfunction of a corresponding plant, the pressure level
of the feedback can temporarily lie below the critical pressure
level or can fall to a corresponding value. During start-up or
during disruptions, considerable fluctuations in the pressure
levels can potentially be recorded until a corresponding plant has
(again) reached a steady state.
[0023] An advantageous embodiment of the method according to the
invention therefore provides that a further feedback amount of the
gas, guided through the main line, is at least temporarily removed
from the main line downstream of a further compression stage, is
fed to an expansion process, and is fed back into the main line
upstream of the same further compression stage, that the
pressure-side pressure level of the further compression stage
downstream of which the further feedback amount is removed from the
main line is a supercritical pressure level, that this further
feedback amount is expanded to a supercritical pressure level, and
that the feedback amount is cooled only after being expanded and
before and/or after being fed back into the main line. In this way,
liquefaction is also avoided in this case.
[0024] In particular cases, liquefaction can also occur when a
pressure-side pressure level is below the supercritical pressure
level, namely when the feedback amount before expansion is at a
subcritical pressure level and simultaneously at a temperature
level at which the two-phase region can be achieved by simple
expansion. An example of this is a pressure level of approximately
48 bar and a temperature level of approximately 10.degree. C. In
the pressure enthalpy diagram, a point defined by a corresponding
pressure level and temperature level is located above the two phase
line.
[0025] To avoid liquefaction in this case as well, a further
embodiment of the present invention provides that an additional
feedback amount of the gas, guided through the main line, is at
least temporarily removed from the main line downstream of an
additional compression stage, is fed to an expansion process, and
is fed back into the main line upstream of the same additional
compression stage, that the pressure-side pressure level of the
additional compression stage downstream of which the further
feedback amount is removed from the main line is a subcritical
pressure level, that this additional feedback amount is expanded to
a subcritical pressure level, and that the feedback amount is
cooled only after being expanded and before and/or after being fed
back into the main line.
[0026] The present invention solves the problem of liquefaction in
the mentioned cases in that the feedback amount (the following
explanations also relate to a plurality of feedback amounts) is fed
to the expansion process at the pressure-side temperature level of
the compression stage downstream of which it is removed from the
main line. In other words, within the context of the present
invention, a corresponding feedback amount is not cooled downstream
of the relevant compression stage at which the feedback amount is
formed, before the expansion thereof. This is an essential
difference over the prior art. In the case of an isenthalpic
expansion starting from a corresponding pressure level, but from a
higher temperature than previously mentioned (because aftercooling
has still not taken place), the two-phase region of a corresponding
enthalpy diagram is not attained and therefore the feedback amount
remains fully in the gaseous state. As already stated, the
expansion preferably takes place isenthalpically, i.e. a throttle
valve is preferably used for the expansion.
[0027] As already mentioned, in the context of the present
invention, cooling does not take place before the expansion of the
feedback amount after it has been removed from the main line.
However, within the context of the present invention, the feedback
amount is cooled following expansion and before and/or after being
fed back into the main line, for which purpose a separate heat
exchanger can be provided in a return line used for returning the
feedback amount and/or in the main line downstream of the infeed
point of the feedback amount. A separate heat exchanger of this
type can be advantageous, because in this way the cooling of a
corresponding feedback amount can be adapted individually to the
respectively required conditions. However, it is also possible to
carry out a cooling process in the main line without a separate
heat exchanger. In this case, the feedback amount is fed into the
main line without cooling or after (partial) cooling in a separate
heat exchanger in the return line and is cooled there by the heat
exchanger, which is also used for cooling the remaining gas which
has not been fed back and is present in the main line. In this way,
a corresponding plant can be set up in a relatively simple and
cost-effective manner.
[0028] As already mentioned, within the context of the present
invention the feedback amount does not necessarily have to be fed
back into the main line directly upstream of the compression stage
downstream of which it was removed from the main line. Instead, the
feedback amount can be advantageously fed back into the main line
upstream of one or more compression stages which are arranged
upstream of the compression stage downstream of which the feedback
amount is removed from the main line. In this way, the suction-side
pressure levels of a plurality of upstream compression stages can
be influenced in a particularly advantageous manner using a
feedback amount.
[0029] Within the context of the present invention, the feedback
amount is advantageously controlled based on an attainable or
attained suction-side or pressure-side pressure level of one of the
compression stages. In particular, in a corresponding method, the
product pressure can be fixed by a controlled valve which is
arranged downstream of the last compression stage. This valve fixes
the product pressure, i.e. the pressure-side pressure level of the
last compression stage. A pressure level of this type can be for
example approximately 125.6 bar. In this case, the pressure-side
temperature level, downstream of an aftercooling downstream of the
last compression stage, can be for example approximately 40.degree.
C.
[0030] The suction-side pressure level of an upstream compression
stage which is charged with ethylene from a high pressure ethylene
refrigerant circuit and which compresses the gas in the main line
to a pressure level of, for example, approximately 22.5 bar can be
adjusted by the rotational speed of this compression stage. The
suction-side pressure level of the compression stages arranged
upstream thereof is also fixed thereby in a corresponding
multi-stage turbocompressor. If a corresponding suction-side
pressure level is not attained, for example during partial load, a
control can be carried out by opening appropriate kickbacks, i.e.
by providing or increasing an appropriate feedback amount.
[0031] As also explained in the following with reference to the
accompanying figures, in particular operating states, for example
during partial load or at varying cooling water flow temperatures,
the entry conditions of the individual compression stages can vary
to different extents. In addition, the thermodynamic
characteristics of the fluid under elevated pressure can have a
disproportionate effect. Consequently, downstream compression
stages in a corresponding multi-stage turbocompressor increasingly
generate too much pressure, possibly during partial load or if the
cooling water is too cold. To solve this problem as well, a control
can be carried out by adjusting appropriate feedback amounts.
[0032] The present invention is particularly advantageous in cases
of fluctuating cooling water temperatures, because within the
context of the present invention the temperature of the feedback
amount can no longer fall below the cooling water temperature,
because this is firstly expanded and is only then cooled. However,
in the case of cooling (not according to the invention) of the
feedback amount before expansion, the temperature level will fall
below the cooling water temperature, because further cooling takes
place starting from the temperature reached by cooling. This is a
particular disadvantage in cases in which, as explained, for
example during partial load, the suction-side pressure level of a
compression stage is set by the opening the provision or increase
of the feedback amount. The pressure-side temperature level of such
a compression stage becomes increasingly cold by increasing the
correspondingly cold feedback amount. This produces disadvantages
in terms of control in this compression stage and in the downstream
compression stages.
[0033] As an alternative or in addition to the described measures,
to overcome this problem, it can be provided to cool the gas
between the compression stages using cooling water which is
maintained within a predetermined temperature range. The rotational
speed of compression stages can also be controlled separately in
the high pressure range in this case.
[0034] As already mentioned, within the context of the present
invention it is possible for a plurality of the compression stages
to be driven by one or more common shafts, to which the respective
compression stages are mechanically coupled. Appropriate shafts
allow a plurality of compression stages to be driven jointly, so
that only one drive unit has to be provided. The use of a plurality
of shafts is advantageous if particular compression stages are to
be controlled separately, particularly in the previously mentioned
cases.
[0035] However, within the context of the present invention, it is
also possible for a plurality of the compression stages to be
respectively driven by a plurality of common shafts, thereby
simplifying a corresponding control. In such cases, a plurality of
common shafts can be mechanically coupled together by a
transmission, so that for example a particular transmission ratio,
which can also be adjustable by an adjustable transmission, can be
achieved.
[0036] The present invention also relates to a plant which is
configured for compressing a gas in stages and to a compressor
arrangement which comprises a plurality of compression stages which
are connected together sequentially by a main line and in which the
gas, guided through the main line, can be respectively compressed
from a suction-side pressure level to a pressure-side pressure
level and can be heated by this compression from a suction-side
temperature level to a pressure-side temperature level, means being
provided which are configured to at least temporarily remove a
feedback amount of the gas, guided through the main line, from the
main line downstream of one of the compression stages, to feed it
to an expansion process and to feed it back into the main line
upstream of the same compression stage.
[0037] According to the invention, the plant is configured to be
operated such that the pressure-side pressure level of the
compression stage downstream of which the feedback amount is
removed from the main line is a supercritical pressure level and
the feedback amount is expanded to a subcritical pressure level.
Means are provided which are configured to feed the feedback amount
to the expansion process at the pressure-side temperature level of
the compression stage downstream of which the feedback amount is
removed from the main line. Furthermore, means are provided which
are configured to cool the feedback amount only after it has been
expanded and before and/or after it is fed back into the main
line.
[0038] A plant of this type benefits from the previously explained
features and advantages. It is advantageously configured to
implement a method which has been previously described. Therefore,
reference is explicitly made to the corresponding features and
advantages.
[0039] In the following, the invention will be described in more
detail with reference to the accompanying drawings, which show
embodiments of the invention compared to embodiments which are not
according to the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0040] FIG. 1 shows a multi-stage compressor arrangement according
to an embodiment which is not according to the invention.
[0041] FIG. 2 shows a multi-stage compressor arrangement according
to an embodiment which is not according to the invention.
[0042] FIG. 3 shows a multi-stage compressor arrangement according
to a particularly preferred embodiment of the invention.
[0043] FIG. 4 shows a multi-stage compressor arrangement according
to a particularly preferred embodiment of the invention.
[0044] FIG. 5 shows a multi-stage compressor arrangement according
to a particularly preferred embodiment of the invention.
DETAILED DESCRIPTION OF THE DRAWINGS
[0045] In the following figures, mutually corresponding elements
have been provided with identical reference signs. For the sake of
clarity, they are not described in every figure, unless
corresponding elements perform a different function and/or are
configured in a different manner.
[0046] FIG. 1 shows a multi-stage compressor arrangement according
to an embodiment which is not according to the invention and is
designated overall by reference sign 500. The compressor
arrangement 500 is configured to provide ethylene at a pressure
level of approximately 40 bar, i.e. at a subcritical pressure
level. As mentioned, the invention is also suitable for compressing
other gases such as methane and carbon dioxide. The compressor
arrangement 500 comprises a plurality of compression stages which
are designated here by Roman numerals I to IV. The compression
stages I to IV are connected together by a main line 1. The
compression stages I to IV are arranged on a common shaft 8 in the
compressor arrangement 500. Ethylene is fed to compression stages
I, II and III from ethylene refrigerant circuits at different
pressure and temperature levels via corresponding lines 2 to 4.
Line 2 conveys ethylene out of a low-pressure refrigerant circuit
at approximately 1.05 bar, line 3 conveys ethylene out of a
medium-pressure refrigerant circuit at approximately 3 bar and line
4 conveys ethylene out of a high-pressure refrigerant circuit at
approximately 8.1 bar.
[0047] In the first compression stage I, the ethylene is compressed
from the mentioned 1.05 bar, the suction-side pressure level of the
first compression stage I, to a pressure-side pressure level of
approximately 3 bar, which is at the same time the suction-side
pressure level of the second compression stage II. Compression
stage II compresses the ethylene in the main line 1 to a
pressure-side pressure level of approximately 8.1 bar, which is at
the same time the suction-side pressure level of the third
compression stage III. In compression stage III, the ethylene is
compressed to a pressure-side pressure level of approximately 22.5
bar, which is at the same time the suction-side pressure level of
the fourth compression stage IV. In compression stage IV, the
ethylene is compressed to a pressure-side pressure level of
approximately 40 bar, at which it can be released as product via a
line 5. Via the line 4, further ethylene is fed in, for example
from the top of a C2 splitter. Since the compressor arrangement 500
is configured as a combined refrigerant and product compressor, an
intermediate extraction line 6 is provided for extracting
refrigerant and optionally a return flow to the C2 splitter.
[0048] To dissipate the compression heat due to the compression in
compression stages II to IV, respective aftercoolers IIa to IVa are
provided in which the ethylene is respectively cooled to
approximately 40.degree. C. Since on the suction side of the third
compression stage III cold ethylene is also fed in from the
high-pressure refrigerant circuit, upon entry into the third
compression stage III a mixed temperature of approximately
18.degree. C. is produced. The entry temperature of the ethylene
out of the low-pressure refrigerant circuit into the first
compression stage I is approximately -57.degree. C. and the entry
temperature of the ethylene out of the medium-pressure refrigerant
circuit into the second compression stage II is approximately
14.degree. C.
[0049] Via a plurality of return lines 7, feedback amounts can be
respectively removed from the main line 1 downstream of compression
stages II to IV and can be fed back into the main line 1 upstream
of these compression stages. In this respect, the feedback amounts
are expanded via valves which are not denoted separately. In a
multi-stage compressor arrangement 500 as shown in FIG. 1, the
problem of the initially mentioned liquefying effects typically
arises to a lesser extent, because a supercritical pressure level
is not reached here.
[0050] FIG. 2 shows a compressor arrangement 600 according to a
further embodiment which is not according to the invention. The
compression stages I to IV and the interconnection thereof has
already been described. In the compressor arrangement 600, the
compression stages I to IV are arranged on a common shaft 8.
[0051] In the compressor arrangement 600 according to FIG. 2, two
further compression stages V and VI are provided. These are
arranged on a common shaft 9 in the compressor arrangement 600 and
further compress the ethylene, released via the line 5 as product
in the compressor arrangement 500 according to FIG. 1 to a
supercritical pressure level. The ethylene, compressed to
approximately 40.2 bar and cooled to a temperature level of
approximately 40.degree. C., is fed to the fifth compression stage
V in the compressor arrangement 600. Thus, the suction-side
pressure level of this compression stage V is approximately 40.2
bar. In compression stage V, the ethylene is compressed to a
pressure-side pressure level of approximately 70.4 bar from this
suction-side pressure level. In so doing it heats up, and is cooled
in an aftercooler Va to approximately 40.degree. C. Thereafter, the
ethylene is fed to a compression stage VI in which it is compressed
to a pressure-side pressure level of approximately 125.6 bar. After
cooling in an aftercooler VIa to approximately 40.degree. C., the
ethylene is released as product at a temperature level of
approximately 40.degree. C. and at the mentioned pressure level via
a line 5.
[0052] Also provided downstream of compression stages V and VI are
return lines 7, by which feedback amounts can be respectively
removed from the main line 1 and can be fed back into the main line
upstream of the respective compression stages. However, as
mentioned, disadvantageous liquefying effects possibly occur during
a compression, particularly in compression stage V, during a
feedback and an expansion.
[0053] In the compressor arrangement 600 according to FIG. 2, the
shafts 8 and 9 can be connected together by a transmission, as also
shown in the following FIGS. 3 and 4. Thus, the rotational speed of
compression stages I to VI can no longer be controlled
independently of the other compression stages. If the suction-side
pressure level of compression stage V is now reduced, for example
because a smaller amount of ethylene is fed in via line 4, this can
only be counteracted by opening the return line 7 downstream of the
aftercooler Va. This is not a problem provided that it is ensured
by the cooling water temperature in the aftercooler Va that the
feedback amount, guided in the return line 7 downstream of the
aftercooler Va, is at a sufficiently high temperature, for example
approximately 40.degree. C. However, in an extreme case, with
colder cooling water, the feedback amount, guided in the return
line 7 downstream of the aftercooler Va, can fall to a value of for
example 20.degree. C. This temperature is further reduced due to
the expansion in the expansion valve. The suction-side temperature
level of compression stage V thereby also falls, and thus also the
suction-side pressure level. Here again, this can only be
counteracted by returning a greater feedback amount which in turn,
however, causes the suction-side temperature level of compression
stage V to fall further. Ultimately, a very large amount of
ethylene is circulated without any benefit. This also affects the
downstream compressor stages.
[0054] FIG. 3 schematically shows a compressor arrangement
according to an embodiment of the invention which is designated
overall by reference sign 100. The compressor arrangement 100 is
largely the same as the compressor arrangement 600 according to
FIG. 2. However, whereas in the compressor arrangement 600
according to FIG. 2 the return line 7 is arranged downstream of the
aftercooler Va, a corresponding return line, designated here by
reference sign 10 for the purposes of clarity, according to the
embodiment of the compressor arrangement 100 according to the
invention which is shown in FIG. 3, branches off from the main line
1 upstream of this aftercooler and directly downstream of
compression stage 5.
[0055] This measure can ensure that a feedback amount which is
guided through the return line 10 and is branched off from the main
line 1 is expanded in an expansion device 11, for example an
expansion valve, from a higher temperature level than in the
compressor arrangement 600 according to FIG. 2. In this way, no
liquefying effects can occur during expansion in the expansion
device 11.
[0056] Provided downstream of the expansion device 11 in the return
line 10 is a separate cooler 12 which can cool the expanded
feedback amount in the return line 10. After cooling, the feedback
amount is fed back into the main line 1 out of the return line
10.
[0057] The embodiment of the compressor arrangement 100 according
to the invention which is shown in FIG. 3 also differs from the
compressor arrangement 600 according to FIG. 2 in that the common
shaft 8 interconnects compression stages I to IV and the common
shaft 9 interconnects compression stages V and VI. The shafts 8 and
9 are connected together by a transmission 13. A common drive 14,
for example a steam turbine, can thus drive the shaft 8 and the
shaft 9. The speed of the transmission 13 can be configured to be
variable or fixed.
[0058] The disadvantages in terms of control, described with regard
to the compressor arrangement 600 according to FIG. 2, are overcome
by the embodiment of the compressor arrangement 100 according to
the invention which is shown in FIG. 3. Even when the temperature
of the cooling water in the aftercooler Va is reduced, it is
ensured that the feedback amount, guided in the return line 7
downstream of the aftercooler Va, is not cooled to the great extent
mentioned with regard to the compressor arrangement 600 according
to FIG. 2. The maximum cooling is restricted by the heat exchanger
12 because subsequently no further expansion takes place. This can
prevent an excessive drop in the suction-side temperature level of
compression stage V.
[0059] FIG. 4 shows a compressor arrangement according to a further
embodiment of the invention which is designated overall by
reference sign 200. The compressor arrangement 200 according to
FIG. 4 is largely the same as the compressor arrangement 100
according to FIG. 3, although here as well, the return line is
configured downstream of compression stage VI, just as the return
line downstream of compression stage V. For the sake of clarity,
the same reference signs are used and reference is made to the
above descriptions. As stated above, in the case of compression
stage VI as well, undesirable liquefaction is thus avoided which
could occur during abnormal operating states, such as start-up or
malfunction.
[0060] FIG. 5 shows a compressor arrangement according to a further
embodiment of the invention which is designated overall by
reference sign 300. Like FIG. 4, here the return line 10 branches
off directly downstream of compression stage VI and delivers a
feedback amount of ethylene to an expansion process in an expansion
valve 11. However, here, the ethylene is fed back into the main
line 1 directly downstream of the expansion in the expansion device
17, more specifically not directly upstream of compression stage
VI, but upstream of compression stage V. A further aftercooler 15
is provided downstream of the ethylene feed-in point of the return
flow from the return line 16.
[0061] The shafts 8 and 9 of the compressor arrangement 300 are
configured separately from one another, separate drives 14 being
respectively provided.
* * * * *