U.S. patent application number 13/133448 was filed with the patent office on 2011-10-20 for co2 recovery and cold water production method.
This patent application is currently assigned to L'Air Liquide Societe Anonyme Pour L'Etude Et L'Exploitation Des Procedes Georges Claude. Invention is credited to Frederick Lockwood, Jean-Pierre Tranier, Claire Weber.
Application Number | 20110252827 13/133448 |
Document ID | / |
Family ID | 40943851 |
Filed Date | 2011-10-20 |
United States Patent
Application |
20110252827 |
Kind Code |
A1 |
Lockwood; Frederick ; et
al. |
October 20, 2011 |
CO2 Recovery And Cold Water Production Method
Abstract
The present invention relates to a method of capturing carbon
dioxide in a fluid comprising at least one compound more volatile
than carbon dioxide CO2, for example methane CH4, oxygen O2, argon
Ar, nitrogen N2, carbon monoxide CO, helium He and/or hydrogen
H2.
Inventors: |
Lockwood; Frederick; (Paris,
FR) ; Tranier; Jean-Pierre; (L'Hay-Les-Roses, FR)
; Weber; Claire; (Suresnes, FR) |
Assignee: |
L'Air Liquide Societe Anonyme Pour
L'Etude Et L'Exploitation Des Procedes Georges Claude
Paris
FR
|
Family ID: |
40943851 |
Appl. No.: |
13/133448 |
Filed: |
December 14, 2009 |
PCT Filed: |
December 14, 2009 |
PCT NO: |
PCT/FR2009/052509 |
371 Date: |
July 7, 2011 |
Current U.S.
Class: |
62/602 ;
62/617 |
Current CPC
Class: |
F25J 3/0266 20130101;
F25J 2290/42 20130101; B01D 2257/504 20130101; F25J 3/0223
20130101; F25J 2210/04 20130101; F25J 3/0257 20130101; F25J 3/067
20130101; F25J 2270/06 20130101; F25J 3/0655 20130101; F25J 2205/32
20130101; F23J 2900/15061 20130101; F25J 2270/14 20130101; F25J
2205/34 20130101; F25J 2230/30 20130101; F25J 2235/80 20130101;
F25J 3/04557 20130101; F25J 2220/80 20130101; Y02C 20/40 20200801;
F25J 2205/10 20130101; F25J 3/04539 20130101; F25J 3/04612
20130101; F25J 3/04533 20130101; F25J 2210/14 20130101; B01D 53/002
20130101; C10L 3/10 20130101; F25J 3/0295 20130101; F25J 3/0625
20130101; F25J 2230/32 20130101; F25J 2260/44 20130101; F25J
2245/42 20130101; F25J 2270/04 20130101; F25J 2205/04 20130101;
F25J 2270/88 20130101; F25J 3/0252 20130101; F25J 3/04836 20130101;
F25J 3/0219 20130101; F25J 3/0261 20130101; F25J 3/04545 20130101;
F25J 3/04563 20130101; F25J 2270/02 20130101; F25J 2270/80
20130101; F25J 2210/70 20130101; Y02C 10/12 20130101; F25J 2240/02
20130101; F25J 2270/58 20130101; F25J 2230/08 20130101; F25J 3/0695
20130101; F25J 2205/20 20130101 |
Class at
Publication: |
62/602 ;
62/617 |
International
Class: |
F25J 3/06 20060101
F25J003/06 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 19, 2008 |
FR |
0858866 |
Claims
1-7. (canceled)
8. A method for producing chilled water, at least one CO2-lean gas
and one or more CO2-rich primary fluids from a process fluid
containing CO2 and at least one compound more volatile than CO2 and
industrial water, comprising the following steps: a) separating
said process fluid into at least said CO2-lean gas and said
CO2-rich primary fluids; and b) cooling said industrial water by
exchange of heat with a non-zero fraction of said CO2-lean gas so
as to obtain said chilled water; wherein, in step b), said exchange
of heat is performed in a direct-contact tower.
9. The method of claim 8, further comprising c) exchanging heat
between a non-zero fraction of said process fluid and a non-zero
fraction of the chilled water produced in step b), thereby cooling
said process fluid prior to its use in step a).
10. The method of claim 9, wherein, in step c), said exchange of
heat is by direct contact.
11. The method of claim 10 further comprising a step c0) cooling
said process fluid, prior to being used in step c), by direct
contact with a stream of water at a temperature higher than the
wet-bulb temperature of the ambient air.
12. The method of claim 9, further comprising a step b1) cooling
the chilled water produced in step b) in a refrigerating unit prior
to its use in step c).
13. The method of claim 9, further comprising, a step c1) drying
said process fluid in an adsorption unit and/or compression unit,
subsequent to its use in step c) and prior to its use in step
a).
14. The method of claim 8, wherein step a) comprises of the
following sub-steps: a1) a first cooling) of said process fluid by
exchange of heat with no change in state; a2) a second cooling of
at least part of said process fluid cooled in step a) so as to
obtain at least one solid containing predominantly CO2 and at least
said CO2-lean gas; and a3) a step comprising liquefaction and/or
sublimation of at least part of said solid and making it possible
to obtain said one or more CO2-rich primary fluids.
Description
[0001] The present invention relates to a method of capturing
carbon dioxide in a fluid comprising at least one compound more
volatile than carbon dioxide CO2, for example methane CH4, oxygen
O2, argon Ar, nitrogen N2, carbon monoxide CO, helium He and/or
hydrogen H2.
[0002] The invention can be notably applied to units producing
electricity and/or steam from carbon fuels such as coal,
hydrocarbons (natural gas, fuel oil, petrochemical residue, etc),
household waste, biomass but can also be applied to gases from
refineries, chemical plants, steel-making plants or cement works,
to the treatment of natural gas as it leaves production wells. It
could also be applied to the flue gases from boilers used to heat
buildings or even to the exhaust gases from transport vehicles, and
more generally to any industrial process that generates
CO2-containing flue gases.
[0003] Carbon dioxide is a greenhouse gas. For environmental and/or
economic reasons, it is becoming increasingly desirable to reduce
or even eliminate discharges of CO2 into the atmosphere by
capturing it and then, for example, storing it in appropriate
geological layers or by realizing it as an asset in its own
right.
[0004] A certain number of techniques for capturing carbon dioxide,
for example methods based on scrubbing the fluids with solutions of
compounds that separate the CO2 by chemical reaction, for example
scrubbing using MEA, are known. These methods typically have the
following disadvantages:
[0005] high energy consumption (associated with the regeneration of
the compound used to react with the CO2),
[0006] degradation of the compound that reacts with the carbon
dioxide,
[0007] corrosion due to the compound reacting with the carbon
dioxide.
[0008] In the field of cryo-condensation, that is to say of cooling
until solid CO2 appears, mention may be made of document
FR-A-2820052 which discloses a method allowing CO2 to be extracted
by anti-sublimation, that is to say by solidification from a gas
without passing via the liquid state. The cold required is provided
by means of fractionated distillation of refrigerating fluids. This
method consumes a great deal of energy.
[0009] Document FR-A-2894838 discloses the same type of method,
with some of the liquid CO2 produced recirculated. The cold may be
supplied by vaporizing LNG (liquefied natural gas). This synergy
reduces the specific energy consumption of the method, although
this remains high despite this, and requires an LNG terminal.
[0010] Document U.S. Pat. No. 3,614,872 describes a separation
method in which the adiabatic and isentropic expansion of the
carbon dioxide yields a refrigerating fluid.
[0011] It is one object of the present invention to provide an
improved method of capturing carbon dioxide from a fluid containing
CO2 and at least one compound more volatile than the latter.
The invention relates first of all to a method for producing
chilled water, at least one CO2-lean gas and one or more CO2-rich
primary fluids from a process fluid containing CO2 and at least one
compound more volatile than CO2 and industrial water, comprising
the following steps: [0012] a) separating said process fluid into
at least said CO2-lean gas and said CO2-rich primary fluids; and
[0013] b) cooling said industrial water by exchange of heat with a
non-zero fraction of said CO2-lean gas so as to obtain said chilled
water; characterized in that, in step b), said exchange of heat is
performed in a direct-contact tower.
[0014] The process fluid generally comes from a boiler or any plant
that produces flue gases. These flue gases may have undergone
various pre-treatments, notably with a view to removing NOx (oxides
of nitrogen), dust, SOx (oxides of sulfur) and/or water.
[0015] Prior to separation, the process fluid is either monophasic,
in gaseous or liquid form, or polyphasic. What is meant by
"gaseous" form is "essentially gaseous" form, that is to say that
it may notably contain dust, solid particles such as soot and/or
droplets of liquid.
[0016] The process fluid contains CO2 that is to be separated from
the other constituents of said fluid by cryo-condensation. These
other constituents comprise one or more compounds more volatile
than carbon dioxide in terms of condensation, for example methane
CH4, oxygen O2, argon Ar, nitrogen N2, carbon monoxide CO, helium
He and/or hydrogen H2. The process fluids generally comprise
predominantly nitrogen or predominantly CO or predominantly
hydrogen.
[0017] In step a) the process fluid is separated into at least one
CO2-lean gas and one or more CO2-rich primary fluids using methods
known to those skilled in the art. For example, this may be a
separation method using low-temperature solid cryo-condensation of
the CO2.
[0018] In step b) the CO2-lean gas produced in step a) is brought
into direct contact with one or more streams of industrial water.
This contact between a gas that is not saturated with water and
water causes some of said industrial water to vaporize. The heat of
vaporization of this water and any possible heating-up of the
CO2-lean gas serves to cool the non-vaporized water.
[0019] A direct-contact tower may simply consist of a spray system
so as to form water droplets which, under the action of gravity,
will drop down against the flow of the CO2-lean gas. In order to
improve the performance of the direct-contact tower, use may also
be made of gas-liquid contactors of the plates or packings (loose
or structured) type.
[0020] Depending on circumstances, the method according to the
invention may comprise one or more of the following features:
[0021] the method further comprises a step c) in which heat is
exchanged between a non-zero fraction of said process fluid and a
non-zero fraction of the chilled water produced in step b), so as
to cool said process fluid prior to its use in step a). [0022] in
step c), said exchange of heat is by direct contact in a similar
way to the direct contact performed in step b), that is to say in a
direct-contact tower, for example with spray and/or in packings
(loose or structured). [0023] the method further comprises a step
c0) in which said process fluid, prior to being used in step c), is
cooled by direct contact with a stream of water at a temperature
higher than the wet-bulb temperature of the ambient air. [0024] the
method further comprises a step c0) in which said process fluid,
prior to being used in step c), is cooled by direct contact with a
stream of water at a temperature higher than, but close to, the
wet-bulb temperature of the air. [0025] the method further comprise
a step b1) in which the chilled water produced in step b) is cooled
in a refrigerating unit prior to its use in step c). [0026] the
method further comprises, subsequent to its use in step c) and
prior to its use in step a), a step c1) of drying said process
fluid in an adsorption unit and/or compression unit. [0027] step a)
produces a CO2-lean gas that is not saturated with water. [0028]
Step a) is performed at least partially at low temperature, that is
to say at a temperature below 0.degree. C. [0029] step a) consists
of the following sub-steps: [0030] a1) a first cooling of said
process fluid by exchange of heat with no change in state; [0031]
a2) a second cooling of at least part of said process fluid cooled
in step a) so as to obtain at least one solid containing
predominantly CO2 and at least said CO2-lean gas; and [0032] a3) a
step comprising liquefaction and/or sublimation of at least part of
said solid (62) and making it possible to obtain said one or more
CO2-rich primary fluids (66, 68, 70).
[0033] In step a1) the process fluid is first of all cooled without
a change in state. This cooling may advantageously take place at
least in part by exchange of heat with CO2-rich fluids from the
separation process. In addition or as an alternative, it may
advantageously take place at least in part by exchange of heat with
the CO2-lean gas from the separation process. These cold fluids
from the separation process are heated up, while the process fluid
is cooled down. This makes it possible to reduce the amount of
energy required for the cooling operation.
[0034] Step a2) consists in solidifying the initially gaseous CO2
by raising the process fluid to a temperature below the triple
point for CO2 while the partial pressure of the CO2 in the process
fluid is below that of the triple point for CO2. For example, the
total pressure of the process fluid is close to atmospheric
pressure. This solidification operation is sometimes known as
"cryo-condensation" or "anti-sublimation" of the CO2 and, by
extension, of the process fluid.
[0035] Certain compounds more volatile than CO2 do not solidify and
remain in the gaseous state. Together with the non-solidified CO2
these will constitute said CO2-lean gas, that is to say will
constitute said gas that comprises less than 50% CO2 by volume and
preferably less than 10% CO2 by volume. According to one particular
embodiment, said CO2-lean gas contains less than 1% CO2 by volume.
According to another particular embodiment, it contains more than
2% thereof. According to another particular embodiment, it contains
more than 5% thereof. A solid comprising predominantly CO2, that is
to say containing at least 90% by volume if considered in the
gaseous state, preferably containing at least 95% by volume, and
more preferably still containing at least 99% CO2 by volume, is
formed.
[0036] This solid may comprise other compounds than CO2. Mention
may, for example, be made of other compounds which might also have
solidified, or alternatively of bubbles and/or drops of fluid
contained within said solid lump. This explains how the solid could
potentially consist of not only solid CO2. This "solid" may contain
non-solid parts such as fluid inclusions (drops, bubbles, etc).
[0037] This solid is then isolated from the compounds that have not
solidified after cryo-condensation and recovered. Next, in step
a3), it is returned to temperature and pressure conditions such
that it changes into a fluid, liquid and/or gaseous, state. At
least part of said solid may then liquefy. This then gives rise to
one or more CO2-rich primary fluids. These fluids are said to be
"primary" to distinguish them from treatment fluids which are said
to be "secondary". What is meant by "CO2-rich" is something
"comprising predominantly CO2" within the meaning defined
hereinabove.
[0038] The inventors have demonstrated that it is particularly
advantageous to carry out the first and/or the second cooling of
the process fluid using one or more refrigerating cycles each
comprising at least one near-isentropic expansion of a gas. These
refrigerating cycles consist of several steps which cause a
so-called "working" fluid to pass via several physical states
characterized by given composition, temperature, pressure, etc
conditions. The presence, among the steps of the cycle, of at least
one near-isentropic expansion, that is to say of an expansion that
causes the entropy of the expanded fluid to increase by less than
25%, preferably less than 15% and more preferably still, less than
10% makes it possible to improve the energy consumption of the
separation process.
[0039] To provide another part of the cold required to carry out
the first and/or second coolings, recourse may be had to one or
more cycles comprising an expansion of a fluid that is not a
near-isentropic expansion, for example reverse-Rankine cycles.
These cycles are said to be reversed because they are used as
refrigerating cycles. Their benefit, as a supplement to the
refrigerating cycles employing near-isentropic expansion, is that
they do not require a large amount of working fluid. By contrast,
they are less energy-efficient.
[0040] According to one embodiment, some of the near-isentropic
expansions of the refrigerating cycle or cycles provide work. They
are, for example, carried out by introducing working fluid into a
turbine.
[0041] The working fluids may be of varying kinds. According to
various embodiments, these fluids may comprise nitrogen and/or
argon. They may also comprise all or part of the CO2-lean gas
obtained or of the process fluid. These fluids may be mixed with
other fluids or have undergone intermediate steps of compression,
expansion, etc.
[0042] When the working fluid of the refrigerating cycle comprises
all or part of the process fluid, the near-isentropic expansion or
expansions that do not provide external work may give rise to a
cooling of the working fluid such that solid CO2 appears. This may
constitute all or part of the second cooling of the process fluid.
According to one particular embodiment, these near-isentropic
expansions are carried out through a Venturi (a throat with Venturi
effect).
[0043] The abovementioned causing of the fluid to rotate can be
obtained by any conventional means, for example by suitably
oriented vanes. The increase in speed is achieved through a Venturi
effect. The temperature of the working fluid drops. Solid particles
of CO2 appear. The fluid has a rotational movement about an axis
substantially parallel to the direction of the flow, like a
corkscrew. This creates a centrifugal effect allowing these solid
particles to be recovered at the periphery of the flow.
[0044] According to a preferred embodiment, any work that might be
produced by the near-isentropic expansion or expansions serves in
part to compress the fluids in other steps of the method.
[0045] The invention also relates to the method applied to
industrial flue gases with a view to capturing CO2.
[0046] According to one particular embodiment, these flue gases
come from a plant producing energy (steam, electricity) and may
have undergone pretreatments.
[0047] Other specifics and advantages will become apparent from
reading the following description given with reference to the
figures in which:
[0048] FIG. 1 schematically depicts a plant employing a method
according to the invention, with a refrigerating cycle employing an
auxiliary fluid as working fluid,
[0049] FIG. 13 schematically depicts a plant employing a method
according to the invention with, on the one hand, a cycle for
producing energy using the cold of fusion of solid CO2 and, on the
other hand, additional purifications by distillation of the
compounds less volatile than CO2, then the compounds more volatile
than CO2,
[0050] FIGS. 2 to 7 schematically depict alternative forms of
refrigerating cycles that can be associated with the invention:
[0051] FIG. 2 schematically depicts an alternative form, with a
refrigerating cycle using the CO2-lean gas by way of working fluid
and comprising a near-isentropic expansion with the production of
work,
[0052] FIG. 3 schematically depicts an alternative form, with a
refrigerating cycle using the CO2-lean gas as working fluid and
comprising a near-isentropic expansion with the production of
work,
[0053] FIG. 4 schematically depicts an alternative form of the
method with a refrigerating cycle using the process fluid as
working fluid and comprising a near-isentropic expansion with the
production of work, during which there is no cryo-condensation of
CO2,
[0054] FIG. 5 schematically depicts an alternative form with a
refrigerating cycle using the process fluid as working fluid and
comprising a near-isentropic expansion with the production of work,
during which there is cryo-condensation of CO2,
[0055] FIG. 6 schematically depicts part of an alternative form
with a refrigerating cycle using the process fluid as working fluid
and comprising a near-isentropic expansion without the production
of work, during which there is cryo-condensation of CO2,
[0056] FIG. 7 schematically depicts an alternative form, in which
the second cooling also comprises liquefaction and further
comprising a refrigerating cycle using the process fluid as working
fluid and comprising near-isentropic expansions without the
production of work during which expansions there is
cryo-condensation of CO2,
[0057] FIG. 8 schematically depicts the use of a method according
to the invention in a plant for producing electricity on the basis
of coal with combustion in air,
[0058] FIG. 9 schematically depicts the use of a method according
to the invention in a plant for producing electricity on the basis
of coal with hybrid combustion or combustion in oxygen,
[0059] FIG. 10 schematically depicts a method of separating a gas
from a steel-making plant to obtain a CO2-lean gas 927 and a
CO2-rich primary fluid 73; the invention could in this case be
applied by bringing the dry CO2-lean gas 927 into direct contact
with the industrial water so as to obtain chilled water (not
depicted) for use in the exchangers 906 and/or 105.
[0060] FIG. 11 schematically depicts a method of separating a
synthesis gas from a synthesis gas production plant operating on
oxygen; the invention could in this case be applied by bringing the
dry CO2-lean gas 927 into direct contact with the industrial water
so as to obtain chilled water (not depicted) for use in an
exchanger of the unit 907 and/or in the exchanger 105.
[0061] FIG. 12 schematically depicts a method for separating a
synthesis gas from a plant for producing carbon monoxide from a
synthesis gas that comes from a steam reforming of a synthesis gas;
the invention could in this case be applied by bringing a CO2-lean
gas 927 and/or a dry gas 929 into direct contact with the
industrial water so as to obtain chilled water (not depicted) for
use in the exchangers 906 and/or 105.
[0062] FIGS. 14 and 15 depict a turbine for carrying out a
near-isentropic expansion of the process fluid with the production
of external work.
[0063] The plant illustrated in FIG. 1 implements the steps
described below.
[0064] The fluid 24 consisting of flue gases is compressed in a
compressor 101, notably to compensate for the pressure losses in
the various pieces of equipment in the plant. Let us note that this
compression may also be combined with the compression known as the
draft compression of the boiler that produces the flue gases. It
may also be carried out between other steps of the method, or
downstream of the CO2 separation method;
[0065] The compressed fluid 30 is injected into a filter 103 to
eliminate particles down to a level of concentration of below 1
mg/m.sup.3, preferably of below 100 .mu.g/m.sup.3.
[0066] Certain embodiments of the invention will be described in
greater detail hereinafter. The dust-free fluid 32 is cooled to a
temperature close to 0.degree. C., generally of between 0.degree.
C. and 10.degree. C., so as to condense the water vapor it
contains. This cooling is carried out in a tower 105, with water
injected at two levels, the cold water 36 and water 34 at a
temperature close to the wet-bulb temperature of the ambient air.
It is also possible to conceive of indirect contact. The tower 105
may or may not have packings.
[0067] The fluid 38 is sent to a unit that eliminates residual
water vapor 107, for example using one and/or another of the
following methods: [0068] Adsorption on fixed beds, fluidized beds
and/or rotary dryer, the adsorbent potentially being activated
alumina, silica gel or a molecular sieve (3A, 4A, 5A, 13X, . . . );
[0069] Condensation in a direct-contact or indirect-contact
exchanger.
[0070] The dried fluid 40 is then introduced into the exchanger 109
where the fluid is cooled down to a temperature close to, but in
all events higher than, the temperature at which CO2 solidifies.
This temperature can be determined by a person skilled in the art
aware of the pressure and composition of the process fluid 40. This
temperature is situated at around about -100.degree. C. if the CO2
content of the process fluid is of the order of 15% by volume and
for a pressure close to atmospheric pressure.
[0071] The fluid 42 which has undergone a first cooling 109 is then
introduced into a vessel 111 where it continues to be cooled down
to the temperature that provides the desired level of CO2 capture.
Cryo-condensation of at least part of the CO2 contained in the
fluid 42 occurs producing, on the one hand, a CO2-lean gas 44 and,
on the other hand, a solid 62 comprising predominantly CO2. The gas
44 leaves the vessel 111 at a temperature of the order of
-120.degree. C. This temperature is chosen as a function of the
target level of CO2 capture. At this temperature, the CO2 content
of the gas 44 is of the order of 1.5% by volume, namely a capture
level of 90% starting out from a process fluid containing 15% CO2.
There are various technologies that can be used for this vessel
111: [0072] Continuous solid cryo-condensation exchanger in which
solid CO2 is produced in the form of carbon dioxide snow, is
extracted, for example, using a screw and pressurized to introduce
it into a bath of liquid CO2 121 in which a pressure higher than
the triple point pressure for CO2 obtains. This pressurization can
also be carried out batchwise in a system of silos. Continuous
solid cryo-condensation may itself be performed in various ways:
[0073] Scraped surface exchanger, the scrapers for example being in
the form of screws to encourage extraction of the solid; [0074]
Fluidized bed exchanger so as to carry the carbon dioxide snow
along and clean out the tubes using particles for example of a
density greater than that of the carbon dioxide snow; [0075]
Exchanger in which solid is extracted by vibration, ultrasound, a
pneumatic or thermal effect (intermittent heating so as to cause
the carbon dioxide snow to fall); [0076] Accumulation on a smooth
surface with periodic "natural" fall into a tank; [0077] Batchwise
solid cryo-condensation: in this case, several exchangers in
parallel can be used alternately. They are then isolated,
pressurized to a pressure higher than the triple point pressure for
CO2, so as to liquefy the solid CO2 and possibly partially vaporize
it.
[0078] The fluid 46 is then heated up in the exchanger 109.
[0079] The invention in terms of step b) thereof will now be
described in greater detail: on leaving the exchanger 109, the
fluid 48 can also be used notably to regenerate the unit used for
eliminating residual vapor 107 and/or for producing cold water 36a
by evaporation in a direct-contact tower 115 into which a dry fluid
50 is introduced which then becomes saturated with water,
vaporizing some of it. The cold water could then potentially
undergo additional cooling in a refrigerator unit 119.
[0080] The solid 62 comprising predominantly CO2 is transferred to
a bath 121 of liquid CO2.
[0081] This bath 121 needs to be heated in order to remain liquid,
to compensate for the addition of cold from the solid 62 (latent
heat of fusion and sensible heat). This can be done in various
ways: [0082] by exchange of heat with a hotter fluid 72. The cold
energy from the fluid 74 can be used elsewhere in the method,
[0083] by direct exchange, for example by tapping a fluid 80 from
the bath 121, heating it in the exchanger 109, and reinjecting it
back into the bath 121.
[0084] Liquid 64 comprising predominantly CO2 is tapped from the
bath 121. This liquid is split into three streams. In the example,
the first is obtained by an expansion 65 to 5.5 bar absolute
producing a diphasic, gas-liquid, fluid 66. The second, 68, is
obtained by compression 67, for example to 10 bar. The third, 70,
is compressed for example to 55 bar. The 5.5 bar level provides
cold at a temperature close to the triple point temperature for
CO2. The 10 bar level allows the transfer of the latent heat of
vaporization of the fluid 68 at around -40.degree. C. Finally, at
55 bar, the fluid 70 does not vaporize during the exchange 109.
There is efficient use to be made of the cold energy contained in
the fluid 64 during the exchange 109 while at the same time
limiting the amount of energy required to produce a purified and
compressed stream 5 of CO2.
[0085] Part of the cold required for the first cooling 109 and for
the second cooling 111 is provided by a refrigerating cycle 200
employing a working fluid 51 which is argon. It comprises, in
succession: a compression 129, possibly two compressions 56 and 57,
a cooling by indirect exchange 109, a near-isentropic expansion 131
which gives rise to cooling, a heating-up in the vessel 111, and a
heating-up 109. During the cooling 109, part of the working fluid
is tapped off then undergoes near-isentropic expansion 130,
followed by indirect exchange 109 and finally compression 128
before reaching the compression stage 129. The near-isentropic
expansions 130 and 131 supply work part of which can be used for
the compressions 56 and 57.
[0086] This cycle 200 produces cold at between about -100 and
-120.degree. C. for the cryo-condensation 111 and between about
5.degree. C. and -100.degree. C. in order to offset the deficit of
cold during the exchange 109.
[0087] Another part of the cold needed for the first cooling 109 is
provided by an additional refrigerating cycle 181, 183, for example
of the reverse Rankine type.
[0088] Another part of the cold needed for the second cooling 111
is provided by an additional refrigerating cycle 191, 193, for
example of the reverse Rankine type.
[0089] Following the indirect exchange 109, the CO2-rich primary
fluids 66, 68, 70 are compressed in stages 141, 142, 143. For
example, the first stages compress gaseous streams. If need be, the
compressed CO2 75 is cooled by an indirect-contact exchanger to
convert it to liquid form. It is then mixed with the stream 73.
This liquid mixture is pumped to the transport pressure (fluid 5).
As the transport pressure is generally supercritical, the
supercritical fluids will, by extension, be considered to be liquid
at a temperature below that of the critical point for CO2.
[0090] FIGS. 2 to 7, which depict examples according to particular
embodiments of the invention, do not depict the steps which apply
to the process fluid 40 prior to its first cooling 109, nor do they
depict the compression of the CO2-rich primary fluids after the
exchange of heat 109. They depict only changes by comparison with
FIG. 1 relating essentially to the refrigerating cycles that
provide the cold for the exchanges 109 and 111.
[0091] FIG. 2 illustrates an alternative form of the
near-isentropic expansion with production of work, in which the
working fluid is the CO2-lean gas 44. The cryo-condensation method
is the same as in FIG. 1. Only the changes are detailed below.
[0092] The CO2-lean gas 44 is compressed, for example by a
multi-stage compressor 315. On leaving, the fluid 303 is cooled if
necessary to the inlet temperature for the exchanger 109 by the
exchanger 316. This may be a direct-contact or an indirect-contact
exchanger.
[0093] The compressed CO2-lean gas 304 is cooled in the exchanger
109 so that it can be expanded in the turbine 312 (near-isentropic
expansion) so as to provide some of the cold needed for the
exchange 111. The fluid 307 leaving the exchanger 111 is once again
expanded (near-isentropic expansion) to provide work and cold for
the exchanger 111 via the fluid 308. This loop in which the
CO2-lean gas is expanded can be repeated as many times as
necessary.
[0094] After the exchanger 111, the CO2-lean gas 46 is heated up in
the exchanger 109. The outgoing fluid 48 is processed like the
fluid 48 in FIG. 1.
[0095] Some of the cold needed for the exchanger 111 may be
supplied by a refrigerating cycle 191, 193 of the Rankine type.
[0096] FIG. 3 illustrates another alternative form of the
near-isentropic expansion with the production of work.
[0097] The CO2-lean gas 44 gives up cold energy in the exchangers
111 and 109. It is then compressed by the multi-stage compressor
415. Next, it is cooled if necessary to the inlet temperature of
the exchanger 109 in the exchanger 416. This may be a
direct-contact or an indirect-contact exchanger.
[0098] The CO2-lean gas 404 is once again cooled in the exchanger
109 before it is being expanded by the turbine 412. This
near-isentropic turbine produces the cold required to compensate
for part of the deficit of cold energy in the exchanger 111.
[0099] Next, the fluid 407 is expanded again by the near-isentropic
turbine 414. The fluid 408 gives up its cold energy to compensate
for part of the deficit of cold energy in the exchanger 111. This
loop in which the CO2-lean gas is expanded can be repeated as many
times as necessary.
[0100] Following the exchanger 111, the CO2-lean gas 46 is heated
up in the exchanger 109. Finally, the outgoing fluid 48 is
processed as the fluid 48 in FIG. 1.
[0101] FIG. 4 illustrates another alternative form of the
near-isentropic expansion with production of work.
[0102] The process fluid 40 is compressed by the compressor 512
which may be a multi-stage compressor. The CO2-lean gas is expanded
in a near-isentropic turbine 514. The temperature of the fluid 503
must remain above the cryo-condensation temperature for CO2.
[0103] Part of the CO2 contained in the fluid 503 then condenses in
the vessel 111. The solid CO2 62 is tipped into the liquid bath 121
and the next steps are the same as those described in FIG. 1 (from
the bath 121 and stream 64 onwards). The CO2-lean gas 44 passes its
cold energy to the exchangers 111 and 109. The outgoing fluid 48 is
processed like the fluid 48 of FIG. 1.
[0104] FIG. 5 illustrates another alternative form of the
near-isentropic expansion with the production of work, in which the
working fluid is the process fluid.
[0105] A near-isentropic expansion with production of work is
carried out on the fluid 42 in the turbine 612 so as to cool the
fluid to a temperature below the cryo-condensation temperature for
CO2 and thus produce solid CO2 in the form of carbon dioxide snow
together with a CO2-lean gas 602.
[0106] This expansion turbine 612 needs to be designed with a great
deal of care. It has to be suited to the high flow rates such as
those of the flue gases 40 of an industrial plant, have very good
isentropic efficiency, and be resistant to potential additional
erosion due to the presence of solid CO2. To achieve this, carbon
dioxide snow is allowed to be present in the rotor part of the
turbine (the region contained between the leading edge 951 and the
trailing edge 954 in FIGS. 14 and 15) and is forbidden or minimized
in the stator part 960 upstream of the rotor part (the region
contained upstream of the trailing edge of the stator vanes 950) in
order notably not to cause erosion of the leading edge of the vanes
952 of the rotor part. Put differently, it is preferable for the
CO2 to be in the vapor or supersaturated vapor state in the stator
part or for it to have carbon dioxide snow nucleii that are small
enough (less than 10 .mu.m, preferably 1 .mu.m hydraulic diameter)
to avoid eroding the rotor part.
[0107] The turbine may be a radial turbine (centripetal or
centrifugal). It may be a supersonic shockwave turbine. It may be
axial.
[0108] The latter technology is the best suited to high flow rates,
but does require numerous successive stator and rotor stages. To
avoid erosion, it will be preferable for the carbon dioxide snow to
be separated out downstream of each rotor stage before the fluid
enters the next stator stage. The first two technologies have the
advantage of remaining effective for high expansion ratios (in
excess of 10) thus making it possible to avoid having to perform
numerous separation operations.
[0109] Moreover, other precautions have preferably to be taken in
order to create such a turbine: [0110] heterogeneous nucleation (on
the stator and rotor surfaces) needs to be minimized, for example
by heating some of these surfaces or by applying special coatings;
[0111] nucleation needs to be delayed by eliminating compounds less
volatile than CO2 (including solid particles) before they enter the
turbine, so that they do not form nucleii encouraging the
nucleation of solid CO2; [0112] the erosion resistance of the
surfaces needs to be increased by using stronger metals such as
titanium or by using special coatings or surface treatments; [0113]
in the case of centripetal radial turbines, it is preferable for a
sweeping gas to be passed across the back of the impeller 953. This
gas mixes with the expanded gas at the interface between the stator
part (vanes) and the rotor part (impeller) and thus avoids the
formation and build-up of solids behind the impeller.
[0114] This carbon dioxide snow is then separated from the CO2-lean
gas in a separator 612 to obtain a solid comprising predominantly
CO2 62 and a CO2-lean gas 44.
[0115] This separation may be performed downstream of the rotor
part by causing the fluid in the rotor part to rotate and by using
the centrifugal effect to separate a CO2-rich fraction at the
periphery from a CO2-lean fraction at the center. It may also be
advantageous to increase the speed and therefore achieve an
additional expansion of the fluid in a convergent nozzle 956 (a
turbine known as a Laval turbine). By reducing the pressure before
decelerating the gas the amount of solidified CO2 can be increased.
Most of the CO2-lean gas is recovered at the center of the flow 959
and most of the solid CO2 is recovered at the periphery 958, mixed
in with a fraction of the gas.
[0116] The benefit of a turbine for performing solid
cryo-condensation is that a great deal of solid CO2 can be
generated in a very small volume as compared with indirect-exchange
systems.
[0117] If necessary, an additional refrigerating cycle 191, 193 of
the Rankine type or which includes a near-isentropic expansion of a
working fluid with or without the production of work provides the
separator 612 with cold energy. The solid 62 comprising
predominantly CO2 is tipped into the liquid bath 121 and the next
steps are the same as those depicted in FIG. 1.
[0118] The CO2-lean gas 44 is heated up by exchange of heat with
the process fluid in the exchanger 109. The fluid 605 is then
compressed to a pressure higher than or equal to atmospheric
pressure. Finally, the outgoing fluid 48 is processed as in FIG.
1.
[0119] FIG. 6 illustrates one embodiment with near-isentropic
expansion without the production of work.
[0120] The process fluid 42 is still cooled to below the
cryo-condensation temperature for CO2 in the vessel 111 to produce
a cooled CO2-lean gas 701. It is also possible for this vessel to
be situated after the "expansion/Venturi" part 702 of the method,
and will now be described.
[0121] Some of the CO2 to be captured solidifies in the form of a
solid containing predominantly CO2 62 and is extracted from the
vessel 111. To improve CO2 capture, the fluid 701 is made to rotate
about an axis that is substantially parallel to the direction in
which it flows using a system of fixed vanes 717.
[0122] The fluid 703 is expanded as it leaves the vanes and cools,
to below the cryo-condensation temperature for CO2, without
producing work. The expansion may take place through the Venturi
effect by passing the fluid through a restriction 718. Solid
particles comprising predominantly CO2 form and are recovered at
the periphery of the flow thanks to the centrifugal effect caused
by the rotation of the fluid.
[0123] A mixture 705 of solid comprising predominantly CO2 and gas
is recovered. The outgoing non-condensables 44, 46 give up their
cold energy in the exchangers 111 and 109.
[0124] The stream 705 is made up predominantly of solid, although
it may be necessary to separate the residual gas from the solid in
a separator 731. The non-condensable part then gives up its cold
energy in the exchangers 111 and 109.
[0125] The solid comprising predominantly CO2 62 is tipped into the
liquid bath 121 and undergoes the same steps as those described in
FIG. 1.
[0126] The streams 48 are used to cool the water, in the same way
as the stream 50 in FIG. 1.
[0127] FIG. 7 illustrates another embodiment with near-isentropic
expansion without the production of work.
[0128] The process fluid 40 is under pressure, for example as much
as 60 bar (compression performed by the compressor 101 or by an
additional compressor). It may potentially be more concentrated in
CO2 than in the other examples, typically containing between 50 and
90% by volume.
[0129] The exchange 809 comprises the same features as the exchange
109 in FIG. 1. The exchanger 811 cools the process fluid 42 to a
temperature below the liquefaction temperature of CO2. From this
there emerges a cooled process fluid 801 which is sent to a
separator 812.
[0130] A CO2-rich liquid 816 is extracted by the separator 812. The
residual fluid 802 is made to rotate about an axis substantially
parallel to the direction in which it flows by a system of fixed
vanes 817. It is expanded as it leaves 803 the vanes having been
rotated and cooled to below the cryo-condensation temperature for
CO2 without producing work. The expansion may take place through a
Venturi effect by passing the fluid through a restriction 818.
[0131] Solid particles comprising predominantly CO2 form and are
recovered at the periphery of the flow thanks to the centrifugal
effect caused by the rotating of the fluid. The stream 805 is made
up predominantly of solid, although it may be necessary to separate
the residual gas from the solid in a separator 841. The
non-condensables 44 give up their cold energy in the exchangers 811
and 809.
[0132] In order to improve the level of CO2 capture, a second (or
even a third or more) step in which the fluid 806 undergoes a
near-isentropic expansion with Venturi effect may be added. This
step is identical to the previous one: [0133] the fluid 806 is made
to rotate about an axis substantially parallel to the direction in
which it flows using a system of fixed vanes 807; [0134] after it
has been made to rotate, the fluid leaving the vanes 808 is
expanded to cool it to below the cryo-condensation temperature for
CO2 without the production of work. The expansion may take place
through a Venturi effect by passing the fluid through a restriction
822.
[0135] The solid 62 comprising predominantly CO2 recovered at the
outlet from the separators 841 and possibly 851 is tipped into the
liquid bath 121 and processed as in FIG. 1. Streams 48 are used to
cool the water, in the same way as the stream 50 in FIG. 1.
[0136] FIG. 8 depicts a plant for producing the electricity from
coal, employing various units 4, 5, 6 and 7 for purifying the flue
gases 19.
[0137] A primary airflow 15 passes through the unit 3 in which the
coal 15 is pulverized and carried along toward the burners of the
boiler 1. A secondary airflow 16 is applied directly to the burners
in order to provide additional oxygen needed for near-complete
combustion of the coal. Feed water 17 is sent to the boiler 1 to
produce steam 18 which is expanded in a turbine 8.
[0138] The flue gases 19 resulting from the combustion, comprising
nitrogen, CO2, water vapor and other impurities, undergo various
treatments to remove some of said impurities. The unit 4 removes
the NOx for example by catalysis in the presence of ammonia. The
unit 5 removes dust, for example using an electrostatic filter, and
the unit 6 is a desulfurization system for removing the SO2 and/or
SO3. The units 4 and 6 may be superfluous depending on the
composition of the product required. The purified flow 24 from the
unit 6 (or 5 if 6 is not present) is then sent to a low-temperature
cryo-condensation purification unit 7 to produce a relatively pure
flow 26 of CO2 and a nitrogen-enriched residual flow 25. This unit
7 is also known as a CO2 capture unit and implements the method
that forms the subject of the invention, as illustrated, for
example in FIGS. 1 to 7.
[0139] FIG. 9 depicts a plant for producing electricity from coal,
implementing various units 5 and 7 for purifying the flue gases
19.
[0140] A primary airflow 15 passes through the unit 3 where the
coal 15 is pulverized and carried along toward the burners of the
boiler 1. A secondary flow of oxidant 16 is supplied directly to
the burners in order to provide the additional oxygen needed for
near-complete combustion of the coal. This secondary oxidant is the
result of the mixing of flue gases 94 recirculated using a blower
91 with oxygen 90 produced by a unit 10 for separating air gases.
Feed water 17 is sent to the boiler 1 to produce steam 18 which is
expanded in a turbine 8.
[0141] The flue gases 19 from the combustion of the coal,
comprising nitrogen, CO2, water vapor and other impurities, undergo
various treatments to remove some of said impurities. The unit 5
(ESP) removes the dust, for example using an electrostatic filter.
The dust-free flow 24 from the unit 5 is sent to a low-temperature
cryo-condensation purification unit 7 to produce a relatively pure
flow 26 of CO2 and a nitrogen-enriched residual flow 25. This unit
7 is also known as a CO2 capture unit and implements the method
that forms the subject of the invention, as illustrated, for
example, in FIGS. 1 to 7.
In this case, the presence of a unit for separating the air gases
is used to provide cold at low level for the solid
cryo-condensation of CO2 in the unit 7 and to carry out
cryo-condensation, preferably by direct exchange with the process
gas. The fluid 93 may be in liquid, gaseous or diphasic form and
consists of a mixture of cooled air gases. For example, this may be
cold gaseous nitrogen or air (at between -56.degree. C. and
-196.degree. C.), or alternatively liquid nitrogen or air. It is
intended to be introduced into the vessel referenced 111 in FIGS. 1
to 4 and in FIG. 6, referenced 612 in FIG. 5, 731 in FIGS. 6, and
841, 851 in FIG. 7.
[0142] The unit 7 may also produce a fluid 92 which will be used in
the unit for separating air gases. This may, for example, be a
fraction of the lean gas leaving the vessel 111 in FIGS. 1 to 4 and
6, 612 in FIG. 5, 731 in FIGS. 7 and 841, 851 in FIG. 8. This lean
gas in some way restores cold to the unit 10 at a temperature level
higher than that afforded from the unit 10 by the fluid 93. It is
advantageous for the flow rate of this injection of fluid 93 to be
varied over time. For example, liquid nitrogen may be produced and
stored by night, when energy is available and inexpensive and the
liquid nitrogen may then be injected by day in order to reduce the
energy consumption. The time at which the cold is produced by the
unit 10 (for example liquid nitrogen) is separated from the time at
which it is used in the unit 7. In such a circumstance, the
near-isentropic expansion of a gas can be carried out in the unit
10 rather than in the unit 7.
[0143] This scheme may prove well suited to instances where
existing plants are being modified, where replacing the primary air
sent to the coal pulverizers with a mixture of recirculated flue
gases plus oxygen could prove complicated, partly because of the
increase in water content, the flue gases containing far more water
than damp air, and partly for safety reasons, although that should
not be overestimated.
[0144] Moreover, it may prove advantageous to combine the units 7
and 10 into a single unit, notably by carrying out one (or more)
exchange(s) of heat between fluids of the 2 units.
[0145] FIG. 10 schematically depicts the use of a method according
to the invention in a steel-making plant. A unit 10 for separating
the air gases supplies oxygen 90 to a blast furnace 900 into which
iron ore 901 and carbon products 902 (coal and coke) are also
introduced. The blast furnace in that instance operates in the
presence of little nitrogen.
[0146] The blast furnace gases 903 made up for example of 47% CO,
36% CO2, 8% N2 and 9% other compounds such as H2 and H2O can be
split into two. Most 905 goes to the CO2 capture unit with another
proportion 904 used to reduce the nitrogen concentration in the
loop. The fluid 905 is cooled beforehand in a direct-contact
exchanger 906, has its dust removed in the filter 103, and is then
compressed by a compressor 901, is cooled in an exchanger 105 and
dried in a drier 107 before entering the low-temperature exchanger
109 where it will be cooled and then partially liquefied to a
temperature close to the triple point for CO2 without the formation
of solid. The diphasic gas-liquid fluid 912 obtained is separated
into a gaseous fraction 502 and a liquid fraction 920 in the
separator 928. The gaseous fraction 502 is then cooled by
near-isentropic expansion, for example in a turbine 514, so as to
obtain a diphasic gas-solid fluid 503. This is separated in the
vessel 111 into a gaseous fraction 44 and a CO2-rich solid fraction
62. The solid fraction 62 is compressed, for example by an endless
screw and mixed with the liquid 920 in the bath 121, which is
heated by gas 72 produced by vaporizing liquid 74 in the exchanger
109. The liquid CO2 64 is compressed by a pump 69 to obtain a
pressurized liquid 70 and is heated up in the exchanger 109 without
undergoing vaporization or pseudo-vaporization if the pressure is
above the supercritical pressure. The lean gas is successively
heated up by a compressor 315 and by the exchanger 109.
[0147] The invention may also be adapted to types of blast furnace
operating on enriched air, for example by adding a CO/N2 separation
using cryogenic distillation, cooling the gas 44 to the required
temperature.
[0148] FIG. 11 schematically depicts the use of a method according
to the invention in a plant for producing synthesis gas from an
oxygen process (partial oxidation, gasification, auto-thermal
reformer, etc.). A unit 10 for separating air gases supplies oxygen
90 to a reactor 900 into which a carbon product 902 (coal, natural
gas, biomass, household waste, etc.) is introduced.
[0149] The synthesis gases 903 chiefly comprise the compounds CO,
CO2, H2 and H2O. The CO can be converted (in a so-called shift
reaction) into CO2 and H2 in the presence of water vapor:
CO+H2O<->CO2+H2 in the unit 907. This unit may also include
one or more exchangers for cooling the gas prior to compression.
The fluid 905 may possibly have its dust removed in a filter 103,
then be compressed by a compressor 101, cooled in an exchanger 105
and dried in a dryer 107 before entering the low-temperature
exchanger 109 where it may be partially liquefied at a temperature
close to that of the triple point for CO2. This diphasic gas-liquid
fluid 912 is separated into a gaseous fraction 502 and a liquid
fraction 920 in the separator 928. The gaseous fraction 502 is then
cooled by near-isentropic expansion, for example in a turbine 514,
to obtain a diphasic gas-solid stream 503. This is separated into a
gaseous fraction 44 and a CO2-rich solid fraction 62 in the vessel
111. The solid fraction 62 is mixed with the liquid 920 in the bath
121, which is heated with gas 74 produced by the vaporizing of the
liquid 72 in the exchanger 109. The liquid CO2 64 is compressed by
a pump and heated up in the exchanger 109 without vaporizing, or
pseudo-vaporizing if the pressure is above the supercritical
pressure. The lean gas 44 is successively heated up via a
compressor 924 and the exchanger 109. This lean gas essentially
consisting of hydrogen may be sent to a gas turbine to be combusted
without the emission of CO2. The unit 10 may supply hot nitrogen
90a which is introduced downstream of the dryers 910, and/or cold
nitrogen 90b, introduced directly into the vessel 111 to increase
the amount of CO2 captured. In the first instance, the expansion in
the turbine 514 of the hot nitrogen present in the stream 502
provides additional cold energy for solid cryo-condensation of CO2
in the turbine 514; in the second instance, the cold nitrogen 90b,
by heating up upon contact with the fluid 503, leads to solid
cryo-condensation of the CO2. The other benefit of hot nitrogen 90a
is that it increases the molecular weight of the gas 502, something
that may prove advantageous in reducing the cost of the expansion
514 and/or of the compression 924. What actually happens is that
when these gases are very rich in hydrogen, it is not easy for
these gases to be compressed/expanded using the technologies best
suited to high flow rates, namely technologies of the axial, radial
or supersonic shockwave type. It then becomes necessary to use
technologies of the positive-displacement type, for example using
pistons or screws, which are very expensive to implement.
[0150] FIG. 12 schematically depicts the use of a method according
to the invention in a plant producing synthesis gas from steam
reforming. A carbon product 902 (natural gas, methanol, naphtha,
etc.) is introduced into a reactor 900.
[0151] The synthesis gases 903 produced in the reactor 900 chiefly
comprise the compounds CO, CO2, H2 and H2O. The fluid 905 may
potentially be compressed by a compressor 101, cooled in an
exchanger 105 and dried in a dryer 107 before entering a
low-temperature exchanger 109 where it may be partially liquefied
at a temperature close to that of the triple point for CO2. The
diphasic gas-liquid fluid 912 obtained is separated into a gaseous
fraction 502 and a liquid fraction 920 in the separator 928. The
gaseous fraction 502 is then cooled by a near-isentropic expansion,
for example in a turbine 514, so as to obtain a gas-solid diphasic
mixture 503. This is separated into a gaseous fraction 44 and a
CO2-rich solid fraction 62 in the vessel 111. The solid fraction 62
is mixed with the liquid 920 in the bath 121, which is heated with
gas 74 produced by the vaporizing of the liquid 72 in the exchanger
109. The liquid CO2 64 is compressed by a pump and heated up in the
exchanger 109 without vaporizing or pseudo-vaporizing if the
pressure is above the supercritical pressure. The lean gas 44 can
then be purified in terms of CO2 at a low temperature, for example
by adsorption using a molecular sieve 13X before being introduced
into a cryogenic unit 924 for the production of CO. This unit
operates, for example, by methane scrubbing or partial condensation
of the CO. This unit 924 produces a hydrogen-enriched gas 929 and a
CO-enriched gas 925. One or more fluids of this unit may be
compressed at low temperature, then reintroduced into the heat
exchanger 926.
[0152] In this case, solid cryo-condensation replaces elimination
of CO2 by absorption with amines (MDEA or MEA). If there is a
desire to produce pure hydrogen, then it is possible to add an H2
PSA into this scheme either upstream of this solid
cryo-condensation purification, that is to say on the outlet side
of the reformer 900 after the cooling of the synthesis gas, or on
the H2-rich gas 929.
[0153] It might be supposed that these solid cryo-condensation
methods are deficient in cold. In actual fact, this is not the case
at all. On the contrary, these solid cryo-condensation methods with
near-isentropic expansion of the process gas produce excessive
amounts of cold, especially if the method also provides external
work. The problem is then that the CO2-rich fluids and the CO2-lean
gas exit at low temperature, which represents an appreciable energy
loss. In order to minimize the energy consumption of this method,
one or more of the following operations may be carried out: [0154]
internally: [0155] cold compression of one of the fluids of the
cryo-condensation method: [0156] process gas cooled to low
temperature prior to compression; [0157] CO2-lean gas that is
compressed at low temperature (cf. FIG. 2). It can then either be
expanded again or compressed under vacuum to return it to
atmospheric pressure or it can be expanded after it has been heated
in the hot part of the method that produced the process gas; [0158]
indirect solid cryo-condensation in an exchanger; [0159]
externally: [0160] cold compression of any fluid of the plant;
[0161] production of liquid nitrogen and/or liquid air; [0162]
transcritical Rankine cycle on the CO2
[0163] FIG. 13 schematically depicts an alternative form of the
invention combined with a method implementing a transcritical
Rankine cycle on the CO2. It also includes the features of a method
in which a liquid cryo-condensation and then a solid
cryo-condensation are performed in succession and in which the
purity of the CO2 produced is improved using two distillation
columns, one of them to eliminate the compounds less volatile than
CO2 (NO2 or N204, SO2, etc.) and another to eliminate the compounds
that are more volatile.
[0164] The fluid 24 consists of flue gases and may be at a
temperature of the order of 150.degree. C. and is injected into a
filter 103 to remove the particles down to a concentration level of
below 1 mg/m.sup.3, preferably below 100 .mu.g/m.sup.3.
[0165] Certain features of the invention will now be described in
greater detail: the dust-free fluid 30 is cooled to a temperature
of close to 0.degree. C., generally of between 0.degree. C. and
10.degree. C., so as to condense the water vapor it contains. This
cooling is carried out in a tower 105b, with water injected at two
levels, cold water 36b and water 34b at a temperature close to the
wet bulb temperature of the ambient air. It is also possible to
conceive of indirect contact. The tower 105 may or may not have
packings. This tower may also serve as a scrubbing tower for the
SO2.
[0166] On leaving this first tower, the fluid that may have been
desaturated, is compressed to a pressure of between 5 and 50 bar
abs in the compressor 101. The fluid 32 is cooled to a temperature
close to 0.degree. C. and generally of between 0.degree. C. and
10.degree. C. so as to condense the water vapor it contains. This
cooling is carried out in a tower 105 with water injected at two
levels, cold water 36 and water 34 at a temperature close to the
wet bulb temperature of the ambient air. It is also possible to
conceive of indirect contact. The tower 105 may or may not have
packings.
[0167] The fluid 38 is sent to a unit 107 that eliminates the
residual water vapor, for example using one and/or another of the
following methods:
[0168] Adsorption on fixed beds, fluidized beds and/or rotary
dryer, it being possible for the adsorbent to be activated alumina,
silica gel or a molecular sieve (3A, 4A, 5A, 13X, etc.);
[0169] Condensation in a direct-contact or indirect-contact
exchanger.
[0170] The process fluid 40 is cooled then brought into contact in
a distillation column 79 with pure CO2, so as to recover the
compounds less volatile than CO2 in the form of a liquid containing
CO2 and, for example, NO2 (or its dimer N2O4). This liquid can be
pumped and vaporized in the unit 78, then sent either to a
combustion chamber to reduce the NO2 or to the unit for purifying
the stream 30 by low-pressure scrubbing of the SO2, where it acts
as a reagent, either directly in the form of NO2 or in the form of
nitric acid having been reacted with water.
[0171] The process fluid 74a is then cooled and partially condensed
into liquid form and sent to the separator 76. The liquid fraction
76a is sent to the bath 121. The gaseous fraction 76b is sent to an
expansion turbine so as there to produce a gas-solid diphasic
stream 42 which is then sent to the vessel 111 where it is
separated into a CO2-lean gas 44 and solid CO2 62. An auxiliary
fluid 93, for example from an air gas separation unit, may
potentially supply additional cold for solid cryo-condensation.
When it does, it may be advantageous to tap from the CO2-lean gas
44 a fluid 92 which returns to the unit that supplied the fluid 93.
The solid 62 is compressed for example by an endless screw and
injected into the bath 121 of liquid CO2, from which a liquid 64 is
tapped. This liquid may potentially be pumped and introduced into a
distillation column 75 where its compounds more volatile than CO2
are eliminated. The pure liquid 68 is heated up without vaporizing
or pseudo-vaporizing if it is supercritical. It may once again be
pumped to obtain the fluid 5 ready for transport. A part of the
fluid 5 may be tapped off to be vaporized or pseudo-vaporized in a
unit 72. This unit 72 is, for example, any arbitrary source of heat
of the plant that produces the process fluid. This part of the
fluid 80 is then expanded in a turbine 73 used to produce
electricity or mechanical power and is then cooled in the exchanger
109 and condensed by direct exchange in the bath 121, at the same
time melting the solid CO2.
[0172] The invention, in respect of step b) thereof, will now be
described in greater detail: on leaving the exchanger 109, the
fluid 48 can still notably be used to regenerate the unit that
eliminates residual vapor 107 and/or for producing cold water 36a
by evaporation in a direct-contact tower 115 into which a dry fluid
50 is introduced and becomes saturated with water, vaporizing part
of it. Potentially, the cold water may undergo additional cooling
in a refrigerating unit 119. Thereafter, this cold water can be
used in one and/or other of the towers 105 and 105b to cool the
process gas before and/or after compression.
[0173] FIGS. 14 and 15 depict a turbine for carrying out
near-isentropic expansion of the process fluid with the production
of external work in accordance with the invention. The upstream
stator part 960 begins with the volute (not depicted) followed by
vanes 950 which may be fixed or variable. Next comes the rotor part
960 which, for example, comprises blades 952 with a leading edge
951 where the rotor part 960 begins and a trailing edge 954 where
it ends.
[0174] Downstream of the rotor part, if centrifugal force is not to
be used on the solid parts, the rotor part may consist of a simple
deceleration cone.
If the downstream stator part 961 is to be used to achieve a first
separation, then the fact that the fluid has been made to rotate in
the rotor part and the centrifugal effect can be used to separate a
CO2-rich fraction at the periphery from a CO2-lean fraction at the
center. It may also be advantageous to increase the speed and
therefore perform an additional expansion of the fluid in a
convergent nozzle 956 (a so-called "Laval" turbine). By reducing
the pressure before decelerating the gas, the amount of solidified
CO2 can be increased. Most of the CO2-lean gas is recovered at the
center of the flow 959 and most of the solid CO2 is recovered at
the periphery 958, mixed in with a fraction of gas.
* * * * *