U.S. patent application number 10/534784 was filed with the patent office on 2006-05-18 for enrichment of oxygen for the production of hydrogen from hydrocarbons with co2 capture.
Invention is credited to Didier Grouset, Jean-Christopher Hoguet, Philippe Marty.
Application Number | 20060102493 10/534784 |
Document ID | / |
Family ID | 32116587 |
Filed Date | 2006-05-18 |
United States Patent
Application |
20060102493 |
Kind Code |
A1 |
Grouset; Didier ; et
al. |
May 18, 2006 |
Enrichment of oxygen for the production of hydrogen from
hydrocarbons with co2 capture
Abstract
The invention relates to a device which is used to produce
hydrogen from a hydrocarbon, obtaining high energy efficiency and
generating low levels of carbon dioxide and pollutants. The
inventive device comprises (a) a conversion reactor which is used
to convert the aforementioned hydrocarbons using water vapour.
According to the invention, pure or almost pure oxygen is fed into
the reactor in order to oxidize one part of the hydrocarbons and to
provide the heat necessary to convert virtually all of the other
part of the hydrocarbons into hydrogen, carbon monoxide and carbon
dioxide. The device also comprises: (b) means of pre-heating the
hydrocarbons, the oxygen flow and the water to be vaporized; (c) at
least one heat exchanger which is used to cool the conversion
product in order to recovery a fraction of the thermal energy of
said conversion product; and (d) hydrogen-enrichment equipment. The
above-mentioned reactor, the pre-heating means, the heat exchanger
and the enrichment equipment all operate at high pressures, e.g.,
above 30 bars.
Inventors: |
Grouset; Didier; (Albi,
FR) ; Marty; Philippe; (Albi, FR) ; Hoguet;
Jean-Christopher; (Rivieres, FR) |
Correspondence
Address: |
FULBRIGHT & JAWORSKI, LLP
666 FIFTH AVE
NEW YORK
NY
10103-3198
US
|
Family ID: |
32116587 |
Appl. No.: |
10/534784 |
Filed: |
October 29, 2003 |
PCT Filed: |
October 29, 2003 |
PCT NO: |
PCT/FR03/50109 |
371 Date: |
January 4, 2006 |
Current U.S.
Class: |
205/628 ;
205/637 |
Current CPC
Class: |
C01B 2210/0046 20130101;
C01B 2203/86 20130101; F28D 21/0001 20130101; B01J 2208/00309
20130101; C01B 2203/0822 20130101; C01B 2203/0244 20130101; C01B
2203/0811 20130101; Y02P 20/10 20151101; C01B 2203/148 20130101;
Y02P 30/30 20151101; C01B 2203/1294 20130101; C01B 3/501 20130101;
C01B 21/0405 20130101; C01B 2203/0233 20130101; B01J 8/0492
20130101; C01B 13/0229 20130101; Y02P 30/00 20151101; C01B 3/382
20130101; Y02P 20/124 20151101; C01B 2203/0475 20130101; Y02P
20/128 20151101; C01B 3/34 20130101 |
Class at
Publication: |
205/628 ;
205/637 |
International
Class: |
C25B 1/02 20060101
C25B001/02; C25C 1/02 20060101 C25C001/02 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 13, 2002 |
FR |
02/14187 |
Claims
1-21. (canceled)
22. A method for producing hydrogen from a hydrocarbon with high
energy efficiency while releasing very low or zero levels of carbon
dioxide and pollutants, said method comprising the steps of:
preheating reagents comprising hydrocarbons, a nearly pure flow of
oxygen and water to be vaporized; oxidizing a portion of said
hydrocarbons using the flow of nearly pure oxygen and converting
nearly all of the remaining portion of said hydrocarbons into
hydrogen, carbon monoxide and carbon dioxide by supplying heat and
water vapor at suitable temperature, thereby improving the hydrogen
production yield and forming a conversion product comprising a
mixture of said hydrogen, said carbon monoxide, said carbon
dioxide, and excess water vapor; cooling said conversion product to
recover a fraction of the thermal energy of said conversion product
which can be used to preheat said reagents and condensing at least
part of the water vapor contained in said conversion product;
extracting said hydrogen from said conversion product for
consumption or storage for later consumption; and wherein said
steps of said method being performed at suitably high pressures
above 30 bar to intensify the heat exchanges, promote the
liquefaction of said carbon dioxide and the condensation of the
water vapor by cooling, and/or improve the overall efficiency of
said method.
23. The method of claim 22, further comprising the step of
converting said carbon monoxide in said conversion product into
said carbon dioxide to form a final conversion product containing
only carbon dioxide and uncondensed water vapor.
24. The method of claim 23, further comprising the steps of
condensing said carbon dioxide and capturing said carbon dioxide in
a liquid form.
25. The method of claim 23, wherein the step of extracting hydrogen
extracts hydrogen from said conversion product using a membrane
that is selectively permeable to hydrogen; and further comprising
the step of lowering the partial pressure of said hydrogen
downstream from said permeable membrane by diluting the flow of
permeated hydrogen in a flow of extraction gas, thereby
facilitating the permeation of the hydrogen and recovery of pure
hydrogen through condensation of said extraction gas.
26. The method of claim 25, wherein the step of extracting is
performed simultaneously with the step of converting to lower the
partial pressure of said hydrogen during said step of converting,
thereby promoting the conversion of said carbon monoxide into said
carbon dioxide.
27. The method of claim 26, wherein the step of converting further
comprises the step of regulating the temperature by adjusting the
flow rate and/or the temperature of said extraction gas.
28. The method of claim 22, wherein said steps of preheating and
cooling are performed in a recovery exchanger so that said reagents
and said conversion product circulate continuously through said
recovery exchanger.
29. The method of claim 22, wherein said hydrogen extracted from
said conversion product feeds a fuel cell running with air; and
further comprising the step of reducing the pressure of said
conversion product and/or said hydrogen being fed to said fuel
cell.
30. The method of claim 22, further comprising the step of using a
hydrogen production method to generate a flow of said nearly pure
oxygen by electrolysis, thereby reducing the cost of producing said
nearly pure oxygen consumed in said method and increasing the
overall production of said hydrogen.
31. The method of claim 22, further comprising the step of using a
nitrogen production method to generate a flow of said nearly pure
oxygen, thereby reducing the cost of producing said nearly pure
oxygen consumed in said method.
32. Apparatus for producing hydrogen from a hydrocarbon with high
energy efficiency while releasing very low to zero levels of carbon
dioxide and pollutants, said apparatus comprising: a reactor for
converting hydrocarbons using water vapor at suitable temperature,
said conversion reactor being supplied with nearly pure oxygen to
oxidize a portion of said hydrocarbons and supplying heat to
convert nearly all of the remaining portion of said hydrocarbons
into hydrogen, carbon monoxide and carbon dioxide, thereby forming
a conversion product comprising a mixture of said hydrogen, said
carbon monoxide, said carbon dioxide and excess water vapor; a
heating device for preheating reagents comprising said
hydrocarbons, said nearly pure flow of oxygen, and water to be
vaporized; at least one cooling heat exchanger for cooling said
conversion product, for recycling a fraction of the thermal energy
of said conversion product to preheat said reagents, and for
condensing at least a part of the water vapor contained in said
conversion product; and a hydrogen recovery unit comprising an
extraction element for extracting said hydrogen from said
conversion product for consumption in a hydrogen-consuming device,
or storage in a hydrogen reservoir for later consumption; and
wherein said first conversion reactor, said heating device, said
heat exchanger, and said recovery unit operating at suitably high
pressures above 30 bar to intensify the heat exchanges, increase
the compactness of the method, promote the liquefaction of the
carbon dioxide by cooling, promote the condensation of the water
vapor by cooling, and/or improve the overall efficiency of the
apparatus.
33. The apparatus of claim 32, further comprising at least one
final conversion reactor operating with said hydrogen recovery unit
for converting said carbon monoxide in said conversion product into
carbon dioxide to form a final conversion product containing only
carbon dioxide and uncondensed water vapor.
34. The apparatus of claim 33 further comprising a condenser for
condensing said carbon dioxide and a container for storing said
carbon dioxide in liquid form.
35. The apparatus of claim 33, wherein said extraction element
comprises a membrane that is selectively permeable to hydrogen for
extracting hydrogen from said conversion product, wherein said
extraction element is operable to receive a feed of extraction gas
downstream from said permeable membrane, to lower the partial
pressure of the hydrogen downstream from said permeable membrane
and to dilute the flow of permeated hydrogen, thereby facilitating
the permeation of the hydrogen and recovery of pure hydrogen
through condensation of the extraction gas.
36. The apparatus of claim 33, wherein said extraction element
comprises a permeable membrane disposed inside said final
conversion reactor, for lowering the partial pressure of the
hydrogen during the conversion in said final conversion reactor,
thereby promoting the conversion of the carbon monoxide into carbon
dioxide.
37. The apparatus of claim 36, wherein said final conversion
reactor comprises a regulating device for regulating the
temperature in said final reactor by acting on the flow rate and/or
the input temperature of the extraction gas.
38. The apparatus of claim 35, wherein said selectively permeable
membrane is comprised of a plurality of tubes that descend into
said extraction element, wherein each tube has the shape of a glove
finger comprising an open end which opens to the outside of said
extraction element to introduce said extraction gas into said
tube.
39. The apparatus of claim 32, wherein said heating device and said
cooling heat exchanger are combined in a recovery exchanger so that
said reagents and said conversion product circulate continuously
through said recovery exchanger.
40. The apparatus of claim 32, wherein said hydrogen extracted from
said conversion product feeds a fuel cell running with air, and
further comprising an element for lowering the pressure of said
conversion product and/or said hydrogen produced to compress the
air required to run said fuel cell.
41. The apparatus of claim 32, further comprising a hydrogen
production unit for generating a flow of oxygen through an
electrolyzer.
42. The apparatus of claim 32, further comprising a nitrogen
production unit for generating a flow of oxygen to limit the cost
of producing the oxygen consumed in said apparatus.
Description
[0001] The present invention concerns a method and a device for
producing hydrogen from a hydrocarbon with high energy efficiency
while releasing low or zero levels of carbon dioxide and
pollutants.
[0002] In the sense of the present invention, the term hydrocarbon
generally designates any fossil or renewable fuel, including
substances that are oxygenated (alcohol, ester, etc.), gaseous,
liquid, or even in powdered solid form (handleable like a fluid),
provided that it forms only a small amount of inert solid residue,
i.e., an ash content of less than 1% by weight.
[0003] In essence, hydrogen as such does not exist in a natural
state and must be produced, for example for use in fuel cells,
either in a centralized way in order to be distributed to local
retailers and users, or in a decentralized way, locally, just
upstream from the fuel cell, for immediate consumption by the
latter.
[0004] Hydrogen can be produced from two separate sources: either
by so-called "downstream" means, i.e. by breaking down water
thermally at a very high temperature or electrically by
electrolysis, or by so-called "upstream" means, by converting a
hydrocarbon.
[0005] Since the hydrogen is intended to subsequently produce
electricity in a fuel cell, the use of "electrolysis" may seem
inappropriate, at least in terms of overall energy efficiency. But
if this electricity is from a renewable (wind, solar, geothermal)
or nuclear source, there is no production of CO.sub.2 or other
pollutants in this production-consumption chain. Whether for
stationary or mobile applications, the hydrogen in that case seems
to be an energy vector, making it possible, through the use of fuel
cells, to produce clean electricity in places that are totally or
periodically without access to nuclear or renewable energy.
[0006] In the "upstream" method, the conversion of a fossil or
renewable hydrocarbon generates hydrogen but also CO.sub.2, which
may limit the advantage of using fuel cells. However, this method
has the advantage of potentially high energy efficiency, thus
conserving fossil fuel resources or biomass products for energy
uses.
[0007] There is therefore an emerging need for technologies for the
centralized (large-scale) or decentralized (small-scale) production
of hydrogen with high energy efficiency and low generation of
CO.sub.2 or other pollutants.
[0008] There are three main families of known methods for producing
a hydrogen-rich mixture. These three methods are described
theoretically below:
[0009] 1. First family: Partial Oxidation combined with Water-Gas
Shift Conversion (POX+WGS).
[0010] The partial oxygen (POX) reaction corresponds to the
reaction of the fuel (C.sub.nH.sub.mO.sub.p) with oxygen. It
results in the formation of gaseous mixture of hydrogen, carbon
monoxide, and possibly nitrogen (if the oxygen is drawn from the
air): C.sub.nH.sub.mO.sub.p+[(n-p)/2]
(O.sub.2+.lamda.N.sub.2).fwdarw.nCO+(m/2)H.sub.2+[(n-p)/2].lamda.N.sub.2
[0011] .lamda. represents the N.sub.2/O.sub.2 molar ratio of the
oxidant mixture (standard air or oxygen-enriched air:
.lamda.<3.762). The POX reaction is exothermic; it does not
require an external supply of heat. Having extracted the hydrogen
from the fuel, it is possible to produce more of it using the
so-called water-gas shift or WGS reaction, in which the carbon
monoxide reacts with the water vapor to form carbon dioxide and
additional hydrogen through the following reaction:
CO+H.sub.2O.fwdarw.CO.sub.2+H.sub.2
[0012] Finally, the combination of the POX and WGS reactions is
written as follows:
C.sub.nH.sub.mO.sub.p+[(n-p)/2](O.sub.2+.lamda.N.sub.2)+nH.sub.2O.fwdarw.-
nCO.sub.2+(n+m/2) H.sub.2+[(n-p)/2].lamda.N.sub.2
[0013] POX, being endothermic, produces less hydrogen than the
vapor reforming (second family of methods) described below, and
moreover, has a tendency to produce solid carbon, which can foul or
clog the tubes and exchangers. For example, in the case of
gasoline, it is performed at around 1200.degree. C. without a
catalyst and at around 800.degree. C. with a catalyst. For diesel
fuel, it is conducted between 950.degree. C. and 1200.degree. C.
(Texaco-Shell.TM. burners).
[0014] 2. Second family: Complete Vapor Reforming (VRC)
[0015] It is also possible to produce hydrogen from the fuel using
the vapor reforming (VR) reaction, the principle of which is to
oxidize the carbon in the fuel by reducing the number of water
molecules in the gaseous phase.
C.sub.nH.sub.mO.sub.p+(n-p)H.sub.2O.fwdarw.nCO+(n+m)/2-p)H.sub.2
[0016] This reaction, automatically performed on a catalyst, is
very highly endothermic: this means that it requires an external
supply of heat, which makes the generating system complex. This
supply may be produced either through external combustion of a
fraction of fuel, or by recovering excess heat. The concatenation
of the VR and WGS reactions is known as Complete Vapor Reforming
(VRC) of the fuel, and is written as follows:
C.sub.nH.sub.mO.sub.p+(2n-p)H.sub.2O.fwdarw.nCO.sub.2+(2n+m)/2--
p)H.sub.2
[0017] Vapor reforming is a reaction that is well known in
petrochemistry, where the production of hydrogen from natural gas
is common. It requires a nickel-based catalyst, adapted to the
molecules to be reformed (methane and light hydrocarbons). It is
done at a temperature of 850 to 950.degree. C. at pressures of 15
to 25 bar and at H.sub.2O/F (fuel) ratios between 2 and 4. These
reactions, being endothermic, are conducted in large furnaces or
banks of parallel tubes filled with catalysts and heated externally
(mainly by radiation), which are passed through by the mixture to
be reformed. The energy required for the reforming reaction is
produced by oxidizing part of the fuel with air (producing CO.sub.2
and H.sub.2O) and is transmitted to the reagents to be reformed
through the walls of these tubes.
[0018] For other hydrocarbons, the conditions and catalysts are
different. For hydrocarbons that are heavier than methane, the
temperatures are lower than for methane (850.degree. C.). Methanol
is easier to reform; temperatures of 250.degree. C. are sufficient,
and the catalyst is Cu/Zn/Al-based. Reforming gasoline requires a
temperature higher than 800.degree. C. Hydrocarbons that contain
sulfur require pre-desulphurization, as the catalyst would be
poisoned by the sulfur. The reforming is therefore done under
temperature and pressure conditions that are adapted to the fuel
and that can be calculated using the laws of thermodynamics
involving chemical equilibrium. It is always a slow reaction, which
is why the reforming is necessarily catalytic.
[0019] 3. Third family: Autothermal Vapor Reforming (VRA)
[0020] In this method, a fraction of the fuel, which we'll call v,
is burned in the reformer in order to supply exactly enough energy
to produce the endothermic complete vapor reforming reaction of the
remaining fraction (1-v) of the fuel. The Autothermal Vapor
Reforming reaction is thus theoretically a thermal. It is usually
produced catalytically, at temperature levels between that of the
POX-based method and that of the VRC method, for example on nickel
at 25 bar and at 950.degree. C. It is written in the following way:
C.sub.nH.sub.mO.sub.p+v(n+m/4-p/2)(O.sub.2+.lamda.N.sub.2)+[(1-v)(2n-p)-v-
(m/2)]H.sub.2O.fwdarw.nCO.sub.2+(1-v)(2n+m/2-p)H.sub.2+v.lamda.(n+m/4-p/2)-
N.sub.2
[0021] The fraction v that should be burned depends solely on the
atomic composition of the fuel and its heat-generating power, as
well as that of the hydrogen. VRA is a combination of reforming and
partial oxidation (with water and air injection). This technology
has been adapted for small-scale facilities both in EPYX.TM.
technology and in single-reactor HOTSPOT.TM. technology, initially
developed for methanol.
[0022] In all reforming techniques, there is an energy need that is
satisfied by oxidizing part of the fuel with atmospheric air. This
oxidation takes place outside the hydrogen producing reactor in the
case of vapor reforming, or inside the hydrogen producing reactor
in the case of partial oxidation and autothermal vapor reforming.
It consumes oxygen and produces CO.sub.2.
[0023] The air is compressed before being introduced into the
reforming process. In the case of a reforming process in connection
with a fuel cell, air is also compressed in order to be introduced
into the fuel cell. The air compressors represent auxiliary
equipment that consumes a significant part of the electric power
produced by the fuel cell. To limit this consumption, the tendency
is to use low levels of pressurization relative to the atmospheric
pressure, both for the fuel cell and for the reforming process when
it is performed in direct connection with a fuel cell.
[0024] The invention concerns a method for producing hydrogen from
a hydrocarbon with high energy efficiency while releasing very low
or zero levels of carbon dioxide and pollutants.
[0025] The method comprises a step (a) for using a flow of (pure or
nearly pure) oxygen to (i) oxidize a portion of the hydrocarbons
and (ii) supply the heat required to convert, using water vapor, at
suitable temperatures, nearly all of the other portion of the
hydrocarbons into hydrogen, carbon monoxide and carbon dioxide.
Suitable temperatures means temperatures like those used in the
techniques described above.
[0026] The method also comprises a step (b) for preheating the
hydrocarbons, the flow of oxygen and the water to be vaporized. The
hydrocarbons, the flow of oxygen, and the water to be vaporized are
hereinafter referred to as the reagents.
[0027] The result of this combination of technical characteristics
is that, nitrogen being absent from the reagents, there is no
generation of nitrous oxide and no need for energy to preheat it.
The hydrogen production yield is thus distinctly improved.
[0028] The mixture formed by the hydrogen, the carbon monoxide, the
carbon dioxide and the excess water vapor is hereinafter referred
to as the products of the conversion. Nitrogen being absent from
the reagents, it does not dilute the conversion products; the
subsequent steps (b) through (f) of the method are facilitated, and
overall efficiency is increased.
[0029] The method also comprises steps (c) for cooling (at least
one) of the conversion products in order to recover a fraction of
the thermal energy of the conversion products for the purpose of
preheating the reagents and condensing at least part of the water
vapor contained in the conversion products.
[0030] The method also comprises the following steps:
[0031] a step (d) for recovering the hydrogen by extracting the
hydrogen from the conversion products, either in order to consume
it or with a view to storing it for later consumption.
[0032] Steps (a) through (d) are performed at suitably high
pressures, above 30 bar, in order to: [0033] intensify the heat
exchanges, and/or [0034] increase the compactness of the method,
and/or [0035] promote the liquefaction of the carbon dioxide by
cooling, and/or [0036] promote the condensation of the water vapor
by cooling, and/or [0037] improve the overall efficiency.
[0038] Preferably, the method according to the invention also
comprises:
[0039] steps (e) for the final conversion of the carbon monoxide
into carbon dioxide; if necessary, these steps are performed during
the step for recovering the hydrogen.
[0040] The result of this combination of technical characteristics
is that, at the end of steps (a) through (e), the residual flow no
longer contains, apart from the water vapor that has not yet
condensed, anything other than carbon dioxide.
[0041] Preferably, the method according to the invention is
performed at sufficient pressure to implement:
[0042] a step for condensing (f) the carbon dioxide contained in
the conversion products and/or the final conversion products,
[0043] a step for capturing the carbon dioxide in liquid form.
[0044] Preferably, the method according to the invention uses a
membrane that is selectively permeable to hydrogen to extract the
hydrogen from the conversion products. In the case of this variant
of embodiment, the method also comprises a step for lowering the
partial pressure of the hydrogen downstream from the membrane by
diluting the flow of permeated hydrogen in a flow of extraction
gas, particularly a condensable gas. The result of this combination
of technical characteristics is that the permeation of the hydrogen
is facilitated.
[0045] Preferably, in the case of this variant of embodiment of the
invention, the extraction of hydrogen by means of a permeable
membrane is performed at the same time as the step for the final
conversion of the carbon monoxide into carbon dioxide. The result
of this combination of technical characteristics is that the
partial pressure of the hydrogen during the final conversion step
is lowered, which promotes the conversion of the carbon monoxide
into carbon dioxide.
[0046] Preferably, in the case of this variant of embodiment of the
invention, the method also comprises a step for regulating the
temperature of the final conversion by adjusting the flow rate
and/or the temperature of the flow of extraction gas.
[0047] Preferably, according to the invention, the method is such
that the preheating and cooling steps are combined in a recovery
exchanger so that the reagents and the conversion products
circulate continuously through the recovery exchanger.
[0048] Preferably, in the case where the method is more
specifically intended to produce hydrogen for the purpose of
feeding a fuel cell running with air, the method according to the
invention also comprises a step for lowering the pressure of the
conversion products and/or the final conversion products and/or the
hydrogen produced while compressing the air required to run the
fuel cell.
[0049] Preferably, the method according to the invention can also
be combined with a hydrogen production method that generates a flow
of oxygen, particularly by electrolysis. The result of this
combination of technical characteristics is that it is thus
possible:
[0050] to limit the cost of producing the oxygen consumed in the
method according to the invention, and
[0051] to increase the overall quantity of hydrogen produced.
[0052] Preferably, the method according to the invention can also
be combined with a nitrogen production method that generates a flow
of oxygen. The result of this combination of technical
characteristics is that it thus possible to limit the cost of
producing the oxygen consumed in the method according to the
invention.
[0053] The invention concerns a device for producing hydrogen from
a hydrocarbon with high energy efficiency while releasing very low
or zero levels of carbon dioxide and pollutants. The device
comprises a reactor for converting (a) the hydrocarbons using water
vapor. The conversion reactor is supplied with pure or nearly pure
oxygen in order to (i) oxidize a portion of the hydrocarbons and
(ii) supply the heat required to convert into hydrogen, carbon
monoxide and carbon dioxide, at suitable temperatures, nearly all
of the other portion of the hydrocarbons. The mixture formed by the
hydrogen, the carbon monoxide, the carbon dioxide and the excess
water vapor is hereinafter referred to as the products of the
conversion.
[0054] The device also comprises means for preheating (b) the
hydrocarbons, the flow of oxygen and the water to be vaporized. The
hydrocarbons, the flow of oxygen and the water to be vaporized are
hereinafter referred to as the reagents.
[0055] The device also comprises:
[0056] at least one heat exchanger (c) for (i) cooling the
conversion products, for (ii) recovering a fraction of the thermal
energy from the conversion products in order to preheat the
reagents, and for (iii) condensing at least a part of the water
vapor contained in the conversion products.
[0057] a hydrogen recovery unit (d).
[0058] The hydrogen recovery unit comprises an extraction element
for extracting the hydrogen from the conversion products in order
to consume it in a hydrogen-consuming device (for example in a fuel
cell) or store it in a reservoir for later consumption.
[0059] The conversion reactor, the preheating means, the heat
exchanger, and the recovery unit operate at suitably high
pressures, above 30 bar, in order to: [0060] intensify the heat
exchanges, and/or [0061] increase the compactness of the method,
and/or [0062] promote the liquefaction of the carbon dioxide by
cooling, and/or [0063] promote the condensation of the water vapor
by cooling, and/or [0064] improve the overall efficiency.
[0065] Preferably, the device according to the invention also
comprises:
[0066] at least one reactor for the final conversion (e) of the
carbon monoxide into carbon dioxide, if necessary combined with the
hydrogen recovery unit.
[0067] The result of this combination of technical characteristics
is that the residual flow that leaves the device according to the
invention no longer contains, apart from the water vapor not yet
condensed, anything other than carbon dioxide.
[0068] Preferably, according to the invention, the pressure inside
the device is sufficient to implement:
[0069] a condenser (f) for condensing the carbon dioxide contained
in the conversion products and/or the final conversion
products,
[0070] a container for storing the carbon dioxide in liquid
form.
[0071] Preferably, according to the invention, the extraction
element includes a membrane that is selectively permeable to
hydrogen for extracting the hydrogen from the conversion products.
The extraction element also includes a feed of extraction gas,
particularly an easily condensable gas, located downstream from the
membrane, which lowers the partial pressure of the hydrogen
downstream from the membrane by diluting the flow of permeated
hydrogen. The result of this combination of technical
characteristics is that the permeation of the hydrogen is
facilitated.
[0072] Preferably, in the case of this variant of embodiment of the
invention, the extraction element with a permeable membrane is
disposed in the final conversion reactor. The result of this
combination of technical characteristics is that the partial
pressure of the hydrogen during the final conversion is lowered,
which promotes the conversion of the carbon monoxide into carbon
dioxide.
[0073] Preferably, in the case of this variant of embodiment of the
invention, the device also comprises means for regulating the
temperature of the final conversion by acting on the flow rate
and/or the input temperature of the extraction gas.
[0074] Preferably, in the case of this variant of embodiment of the
invention, the device is such that the permeable membrane is
composed of a plurality of tubes that descend into the extraction
element. Each tube is shaped like a glove finger whose open end
opens to the outside of the extraction element. The open end makes
it possible to introduce the extraction gas into the tube.
[0075] Preferably, the device according to the invention is such
that the preheating means and the cooling heat exchanger are
combined in a recovery exchanger so that the reagents and the
conversion products circulate continuously through the recovery
exchanger.
[0076] Preferably, in the case where the device is more
specifically intended to produce hydrogen for the purpose of
supplying a fuel cell running with air, the device according to the
invention also comprises an element for reducing the pressure of
the conversion products and/or the final conversion products and/or
the hydrogen produced, making it possible to compress the air
required to run the fuel cell.
[0077] Preferably, the device according to the invention can also
be combined with a hydrogen production unit that generates a flow
of oxygen, particularly by means of an electrolyzer. The result of
this combination of technical characteristics is that it is thus
possible:
[0078] to limit the cost of producing the oxygen consumed in the
method according to the invention, and
[0079] to increase the overall quantity of hydrogen produced.
[0080] Preferably, the device according to the invention can also
be combined with a nitrogen production unit that generates a flow
of oxygen. The result of this combination of technical
characteristics is that it is thus possible to limit the cost of
producing the oxygen consumed in the method according to the
invention.
[0081] Other characteristics and advantages of the invention will
become apparent through the reading of the description of variants
of embodiment of the invention given as an illustrative and
non-limiting example, and of
[0082] FIG. 1, which illustrates the variation in the fraction (fa)
of hydrocarbon oxidized with pure oxygen as a function of the
reagent preheating temperature in the case of diesel fuel,
[0083] FIG. 2, which illustrates the variation in the fraction (fa)
of hydrocarbon oxidized with air as a function of the reagent
preheating temperature in the case of diesel fuel,
[0084] FIG. 3, which illustrates, in block diagram form, a variant
of embodiment of a unit for producing pure hydrogen stored under
pressure,
[0085] FIG. 4, which illustrates, in block diagram form, another
variant of embodiment of a unit for producing pure hydrogen,
intended to be used immediately in a low-temperature, low-pressure
PEMFC-type fuel cell,
[0086] FIG. 5, which illustrates, in block diagram form, another
variant of embodiment of a unit for producing a mixture of hydrogen
and carbon dioxide, intended to be used immediately in a
low-temperature, medium-pressure PEMFC-type fuel cell,
[0087] FIG. 6, which illustrates in block diagram form, another
variant of embodiment of a unit for producing a mixture of hydrogen
and carbon dioxide, intended to be used immediately in a
high-temperature, medium-pressure SOFC-type fuel cell,
[0088] FIG. 7, which illustrates a variant of embodiment of a means
for preheating the reagents and a heat exchanger for cooling the
associated products, constituting a regeneration system, the
regeneration system being combined with a conversion reactor,
[0089] FIG. 8, which illustrates another variant of embodiment of a
means for preheating the reagents and a heat exchanger for cooling
the associated products, constituting a recovery exchanger, the
recovery exchanger being combined with a conversion reactor,
[0090] FIG. 9, which illustrates in a graph the increase in the
efficiency of the hydrogen permeation as a function of the ratio
between the molar flow rate of the extraction gas downstream from
the membrane and the molar flow rate of the hydrogen to be
extracted upstream from the membrane,
[0091] FIGS. 10a and 10b, which illustrate a reactor for converting
CO into CO.sub.2 equipped with a hydrogen-permeable membrane,
supplied with extraction water vapor on the downstream end
[0092] FIG. 11, which illustrates a reactor for converting CO into
CO.sub.2, equipped with a series of closed-end tubes that descend
into the core of the reactor, each of which supports a
hydrogen-permeable membrane.
[0093] We will now describe FIG. 1. This figure illustrates the
variation in the fraction (fa) of hydrocarbon oxidized with pure
oxygen as a function of the reagent preheating temperature in the
case of diesel fuel. As this graph shows, and as the description
below explains, the fraction (fa) of hydrocarbon oxidized depends
on the desired conversion temperature. The curves shown
respectively correspond to the following conversion temperatures
(Tconv): 1000.degree. C., 1200.degree. C., 1400.degree. C. They
have been plotted for water factor (fe) values equal to 1.5 and a
pressure of 5 bar. In the sense of the present invention, the term
water factor (fe) means the ratio between the flow of water
actually made available by injection into the conversion reactor
and the stoichiometric flow of water required for a complete
conversion of the fraction of hydrocarbon to be converted:
C.sub.nH.sub.mO.sub.p+fa(n+m/4-p/2)(O.sub.2+.lamda.N.sub.2+[fe(1-fa)(2n-p-
)-fa(m/2)]H.sub.2O.fwdarw.nCO.sub.2+(1-fa)(2n+m/2-p)H.sub.2+fa.lamda.(n+m/-
4-p/2)N.sub.2+(fe-1)(1-fa)(2n-p)H.sub.2O
[0094] The water factor has an influence on the formation of soot,
carbon particles or polyaromatic hydrocarbons during the
conversion. FIG. 1 shows that fa diminishes when the preheating
temperature of the reagents increases. In fact, the amount of
energy supplied with preheated reagents makes it possible to reduce
the fraction of fuel to be burned in order to reach the desired
temperature level and to promote the endothermic conversion
reactions.
[0095] Such figures can be established for each fuel or mixture of
fuels and are not specific to diesel fuel. Nor are they specific to
the reforming method used (vapor reforming or partial oxidation,
catalytic or non-catalytic, etc.).
[0096] Any conversion of a hydrocarbon into hydrogen using water
vapor, possibly in the presence of oxygen, requires obtaining a
sufficient temperature level in the conversion reactor. The
temperature level to be applied depends both on the hydrocarbon to
be converted and on whether or not a catalyst is present. For
example, in the case of a catalytic vapor reforming type reaction,
a temperature of 200 to 250.degree. C. is sufficient in the case of
methanol. If on the other hand the fuel is methane, temperatures of
800 to 950.degree. are necessary. The partial oxidation of gasoline
can be conducted at 800.degree. C. in the presence of a catalyst
and at 1200.degree. C. in the absence of a catalyst. Non-catalytic
conversion using water vapor requires 1200.degree. C. for any fuel
and 1400.degree. C. to obtain in less than one second a complete
conversion, i.e., one that releases products that are free of even
light hydrocarbons such as methane, ethane or ethylene.
[0097] It order to reach these temperature levels at the end of
conversion, it is wise to preheat the reagents. A portion of this
preheating heat can be recovered by cooling the products that leave
the conversion reactor, and transferred to the reagents through
external exchange. On the other hand, it is technologically easier,
and also faster, to supply heat to gasses at a high temperature
(higher than 800.degree. C., for example) by oxidizing a fraction
of the fuel, either with air, with oxygen, or with an oxygen-rich
flow; the other, non-oxidized fraction of this fuel being converted
into a mixture of hydrogen and carbon monoxide and dioxide.
[0098] For an adequate conversion temperature, a given water factor
and a chosen reagent preheating temperature, the fraction fa and
the flow of oxygen can be determined, as shown in FIG. 1. Thus, the
three reagent flows to be placed in contact inside the conversion
reactor are identified.
[0099] The introduction into the conversion reactor of the
preheated reagents, and in particular pure oxygen, generates highly
active oxidation zones in contact with the hydrocarbon, which can
lead to very high temperatures, for example higher than
2500.degree. C. In the present invention, one must therefore be
careful:
[0100] (i) first, to simultaneously introduce the hydrocarbon and
the water vapor required for the conversion, so that the water
vapor absorbs part of the energy given off by the oxidation of the
hydrocarbon,
[0101] (ii) second, to organize the gradual injection of the
reagents into the reactor and their mixture inside this reactor so
as to gradually release the oxidation energy, as well as the energy
required for the conversion, and thus establish a satisfactory
temperature profile inside the reactor, and
[0102] (iii) and possibly, to provide thermal protection for the
walls, for example using a parietal film of hydrocarbons and/or
water vapor that is relatively cool compared to the reaction
mixture.
[0103] The combination of these technical characteristics, and in
particular the proper use of a flow of nearly pure oxygen, makes it
possible to obtain a nearly complete conversion of the hydrocarbon
into hydrogen and CO/CO.sub.2: light hydrocarbons such as methane,
ethylene, and ethane, as well as polyaromatic hydrocarbons, are
present only in trace amounts. Thus, the products of the conversion
contain only H.sub.2, CO, CO.sub.2 and H.sub.2O and are not diluted
in nitrogen.
[0104] The hydrogen conversion reaction being endothermic, no
matter how high the preheating temperature, it will be necessary in
all cases to oxidize a fraction of the hydrocarbon in order to
compensate for the heat of the conversion reaction. This minimum
fraction to be burned can be determined as a function of the fuel's
enthalpy of formation and its composition. This particular value of
fa is marked v. This value is characteristic of the fuel. It is
equal to 0.2565 in the case of diesel fuel.
[0105] Knowing the value of fa and the value of v relative to the
fuel makes it possible to directly determine the value of the
hydrogen production yield, give or take a few losses, in the
subsequent steps of the method. The yield .eta. is equal to:
.eta.=(1-fa)/(1-v)
[0106] In the case of a non-catalytic conversion of diesel fuel
with oxygen at 1400.degree. C., with a preheating of the reagents
at 700.degree. C., the yield is equal to:
.eta.=(1-0.374)/(1-0.2656)=0.852.
[0107] The flows of oxygen and diesel fuel to be used are therefore
in a ratio of 1.27. A higher preheating temperature, with
exchangers made of ceramic material, produces better yields. Yields
of 80 to 90% are therefore attainable with the technology according
to the invention for oxygen and diesel fuel consumption in a ratio
of 1.15 to 1.40 and hydrogen production yields of 0.283 to 0.253 kg
H.sub.2 per kg of diesel fuel.
[0108] If air were used instead of oxygen, the value of v would be
unchanged, but higher values of fa would be required to reach the
same conversion temperatures. In fact, it is necessary to heat all
of the nitrogen that is injected into the conversion reactor at the
same time as the oxygen. FIG. 2 illustrates, in the case of diesel
fuel, the variation in the fraction (fa) of hydrocarbon oxidized
with air as a function of the reagent preheating temperature. To
facilitate the comparison of FIGS. 1 and 2, the desired conversion
temperatures are the same (1000, 1200 or 1400.degree. C.) as are
the water factor fe=1.5 and the pressure P=5 bar.
[0109] Thus, for diesel fuel, for a conversion temperature equal to
1400.degree. C. and a preheating at 700.degree. C., a value of fa
equal to 0.444 is necessary. From this, it may be deduced that the
conversion yield with air is: .eta.=(1-fa)/(1-v); or
.eta.=(1-0.444)/(1-0.2656)=0.757
[0110] This yield is clearly more advantageous than with pure
oxygen (0.852).
[0111] Moreover, with air, the oxidation of the fraction of the
fuel and the conversion of the remaining fraction are less sudden
than with oxygen. The maximum temperatures reached are lower, and
there may remain larger contents of light hydrocarbons such as
methane, ethylene and ethane as well as polyaromatic hydrocarbons.
These contents are on the order of a few per thousand to a few
percent, depending on the family of conversion methods used and the
temperature applied.
[0112] We will now demonstrate that the use of pure or nearly pure
oxygen makes it possible to operate under pressure, which has
several advantages. We will also demonstrate that it is possible to
use pure or nearly pure oxygen under pressure without thereby
reducing the yield.
[0113] Reforming units on petrochemical sites commonly operate at
high pressures of several tens of bar. On the other hand, for a
small-scale unit that feeds, for example, a low-pressure fuel cell,
using a partial oxidation or autothermal vapor reforming unit at
high pressure is detrimental to the overall efficiency of the
system since it is necessary to compress the air to be injected
into the conversion reactor, which is energy-expensive. It is
therefore preferable to operate at a pressure close to the
atmospheric pressure.
[0114] Conversely, in the case of the present invention, with a
supply of oxygen rather than air, the cost of compressing the
oxidant becomes negligible: either the oxygen is supplied in
gaseous form in bottles or reservoirs already compressed to 200 bar
or more, or the oxygen is supplied in liquid form under a few bar
of pressure, but the compression energy of the liquid is
negligible. It is therefore possible and advantageous to perform
the hydrogen production steps between 30 and 60 bar; this technical
characteristic provides several advantages:
[0115] (i) The high pressures of the gasses lead to higher gas
densities, higher heat exchange coefficients through the walls, and
often also faster chemical kinetics, making it possible to
considerably reduce the size of the equipment of the method.
[0116] (ii) The high pressure of the products makes it possible to
use a hydrogen-permeable membrane, a membrane that normally
requires typical partial pressure difference of 15 bar (in reality
from a few bar to 40 or 50 bar), in order to efficiently extract
hydrogen and separate CO.sub.2 and H.sub.2, as explained below.
[0117] (iii) The high pressure of the products makes it possible,
after cooling, to easily condense the water contained in the
products and to recycle it for use in the conversion reactor.
[0118] (iv) The high pressure of the products makes it possible in
certain cases, as explained below, to condense the carbon
dioxide.
[0119] (v) With a method of production between 30 and 60 bar, the
pressure of the products can also be skillfully used, as explained
below, to reduce the use of energy-expensive auxiliary equipment
and thus increase the overall efficiency of the installation.
[0120] A hydrogen production unit according to the invention can be
embodied in various ways. Four variants of embodiment are shown as
examples in FIGS. 3 through 6.
[0121] We will now describe FIG. 3, which illustrates, in block
diagram form, a variant of embodiment of a unit for producing pure
hydrogen stored under pressure.
[0122] The production unit, also called the device 1, is composed
of the following elements: [0123] a hydrocarbon reservoir: 2 [0124]
a reservoir for oxygen under pressure or in liquid form: 3 [0125] a
CO+H.sub.2 conversion reactor at 60 bar: 4 [0126] a means for
preheating the reagents: 5 [0127] a first heat exchanger for
cooling the products: 6a [0128] a reactor for the final conversion
of CO into CO.sub.2: 11 [0129] a hydrogen-permeable membrane at 60
bar/20 bar: 7 [0130] a hydrogen compression element: 8 [0131] an
H.sub.2 storage reservoir: 10 [0132] a carbon dioxide condenser at
60 bar: 14 [0133] a water condenser at 60 bar: 13 [0134] a second
cooling exchanger at 60 bar: 6b [0135] a post-combustion of the
final products at 60 bar: 12 [0136] a storage container for the
CO.sub.2 at 60 bar: 16 [0137] a water reservoir at 60 bar: 15
[0138] The production unit 1 is used to produce pure hydrogen. The
latter is stored under high pressure (200 to 350 bar or more) for
later use. The pressure in the conversion reactors 4 of this unit
is on the order of 50 to 60 bar. Downstream from the membrane 7,
the pressure of the flow of hydrogen extracted is still significant
(20 to 30 bar); the compression effort required to reach the
storage pressure is thus considerably reduced.
[0139] We will now describe FIG. 4, which illustrates, in block
diagram form, another variant of embodiment of a unit for producing
pure hydrogen, intended to be used immediately in a
low-temperature, low-pressure PEMFC-type fuel cell.
[0140] The production unit, also called the device 1, is composed
of the following elements: [0141] a hydrocarbon reservoir: 2 [0142]
a reservoir for oxygen under pressure or in liquid form: 3 [0143] a
CO+H.sub.2 conversion reactor at 60 bar: 4 [0144] a means for
preheating the reagents: 5 [0145] a heat exchanger for cooling the
products: 6 [0146] a reactor for the final conversion of CO into
CO.sub.2: 11 [0147] a hydrogen-permeable membrane at 60 bar/20 bar:
7 [0148] a carbon dioxide condenser at 60 bar: 14 [0149] a water
condenser at 60 bar: 13 [0150] a cooling exchanger at 60 bar: 6
[0151] a post-combustion of the final products at 60 bar: 12 [0152]
a storage container for the CO.sub.2 at 60 bar: 16 [0153] a water
reservoir at 60 bar: 15 [0154] an air compression element at 1
bar/2 bar: 19 [0155] an element for reducing the pressure of the
hydrogen from 20 bar/2 bar: 18 [0156] a PEFC fuel cell at 2 bar and
80.degree. C.: 17
[0157] The production unit 1 produces pure hydrogen, which is
immediately put to use in another system, for example a PEMFC
(Proton Exchange Membrane Fuel Cell) type fuel cell 17, running at
a relatively low temperature (60 to 120.degree. C.) and low
pressure (between 1 and 5 bar). The production unit is identical to
the one in FIG. 3 until just downstream from the membrane 7, where
the pressure of the flow of hydrogen extracted is still significant
(20 to 30 bar) and its temperature is high (350.degree. C.). The
release of pressure from the hydrogen downstream from the membrane
7 by means of a turbo compressor 18, 19 supplies the energy for
compressing the air that feeds the cell 17, which normally requires
a piece of auxiliary equipment that is costly in terms of the
overall efficiency of the method.
[0158] We will now describe FIG. 5, which represents, in block
diagram form, another variant of embodiment of a unit for producing
a mixture of hydrogen and carbon dioxide, intended to be used
immediately in a low-temperature, medium-pressure PEMFC-type fuel
cell. The production unit, also called the device 2, is composed of
the following elements: [0159] a hydrocarbon reservoir: 2 [0160] a
reservoir for oxygen under pressure or in liquid form: 3 [0161] a
CO+H.sub.2 conversion reactor at 60 bar: 4 [0162] a means for
preheating the reagents: 5 [0163] a first heat exchanger for
cooling the products: 6a [0164] a reactor for the final conversion
of CO into CO.sub.2: 11 [0165] a water condenser at 7 bar: 13
[0166] a second heat exchanger for cooling the products: 6b [0167]
a post-combustion of the final products at 7 bar: 12 [0168] a water
reservoir at 7 bar: 16 [0169] an air compression element at 1 bar/7
bar: 19 [0170] an element for reducing the pressure of
H.sub.2/CO.sub.2 from 60 bar/7 bar: 18 [0171] a CO.sub.2 storage
container at 7 bar: 15
[0172] The production unit 1 produces hydrogen for immediate use in
a mixture with CO.sub.2 in a fuel cell 17 at a relatively low
temperature and medium pressure. In the case of this variant of
embodiment, the production unit 1 does not include a hydrogen
permeation membrane 7, but includes an additional cooling 6b of the
products during the final conversion of the CO into CO.sub.2. The
hydrogen production unit 1 operates at a high level of pressure (30
to 60 bar). The energy recovered during the release of pressure 18,
19 from the H.sub.2/CO.sub.2 mixture makes it possible to compress
the air admitted into the fuel cell 17. The recoverable energy is
substantial since the mass and volume rate of the H.sub.2+CO.sub.2
mixture whose pressure is to be reduced is higher than in the case
of the production unit represented in FIG. 4. It is possible to
have the cell 17 run at a higher pressure (5 or 7 absolute bar
rather than 1 bar), which promotes the recycling of the water
leaving the cell 17 in order to feed the conversion reactor 4, and
which also promotes the compactness of the equipment.
[0173] We will now describe FIG. 6, which illustrates, in block
diagram form, another variant of embodiment of a unit for producing
a mixture of hydrogen and carbon dioxide, intended to be used
immediately in a high-temperature, medium-pressure SOFC-type fuel
cell.
[0174] The production unit, also called the device 1, is composed
of the following elements: [0175] a hydrocarbon reservoir: 2 [0176]
a reservoir for oxygen under pressure or in liquid form: 3 [0177] a
CO+H.sub.2 conversion reactor at 60 bar: 4 [0178] a means for
preheating the reagents: 5 [0179] a first heat exchanger for
cooling the products: 6a [0180] a water condenser at 7 bar: 13
[0181] a second cooling exchanger at 7 bar: 6b [0182] a
post-combustion of the final products, CO/H.sub.2 at 7 bar: 12
[0183] a water reservoir at 7 bar: 15 [0184] an air compression
element at 1 bar/7 bar: 19 [0185] an element for reducing the
pressure of H.sub.2/CO.sub.2 from 60 bar/7 bar: 18 [0186] an SOFC
fuel cell at 7 bar and 800.degree. C.: 20 [0187] a CO.sub.2 storage
container at 7 bar: 16
[0188] The production unit 1 produces hydrogen in a mixture with CO
and CO.sub.2 for use in an SOFC (Solid Oxide Fuel Cell) type fuel
cell running at a high temperature (600 to 900.degree. C.) and
relatively medium pressure (between 1 and 7 bar). In the case of
this other variant of embodiment, the production unit 1 does not
include a hydrogen-permeable membrane 8 either; nor does it include
the final conversion 11 of the CO into CO.sub.2 since the CO can be
used by the SOFC. The hydrogen production unit 1 operates at a high
level of pressure (30 to 60 bar). The energy recovered during the
release of pressure 18 from the H.sub.2/CO/CO.sub.2 mixture makes
it possible to compress the air 19 admitted into the fuel cell 17.
It is possible to have the SOFC run at a medium pressure (5 or 7
absolute bar instead of 1), which promotes the recycling of the
water leaving the cell in order to feed the conversion reactor 4,
as well as the compactness of the equipment.
[0189] The means for preheating the reagents 5 and the heat
exchanger for cooling 6 the products in the case of the variants of
embodiment illustrated in FIGS. 3 through 6 may advantageously be
combined so that the energy recovered from the products is
transferred to the reagents. Both capabilities of the combination,
regeneration or recovery, can be implemented in the variants of
embodiment described above.
[0190] We will now describe FIG. 7, which illustrates a variant of
embodiment of a means for preheating the reagents 5 and a heat
exchanger for cooling the associated products 6, constituting a
regenerative system. In the case of the variant of embodiment
represented in FIG. 7, the reagent preheating means previously
referenced 5 is referenced 22, and the cooling heat exchanger
previously referenced 6 is referenced 23.
[0191] In the case of regenerative exchangers, the heat is stored
in the elements made of ceramic material placed in the reagent
preheating means 22 and in the cooling heat exchanger 23. The
reagent preheating means 22 and the cooling heat exchanger 23 are
disposed on either side of the conversion reactor 4.
[0192] The flows are periodically alternated. The cold reagents
enter the reagent preheating means 22, wherein the ceramic elements
are hot, heat up on contact with it and cool it, while the hot
products enter the cooling heat exchanger 23, which is relatively
cold, cool off in contact with the ceramic elements and reheat
them. After a certain amount of time (on the order of one minute to
30 minutes depending on the size of the installation), the flows
are reversed by means of valves 21, and the roles of the reagent
preheating means 22 and the cooling heat exchanger 23 are reversed.
The reagents flow into the cooling heat exchanger 23, which has
become hot enough to serve as the reagent preheating means 22, then
pass through the conversion reactor 4 in the opposite direction.
The conversion products leave the conversion reactor 4 in the
direction of the reagent preheating means 22. The latter has become
cold enough to serve as the cooling heat exchanger.
[0193] The ceramic elements have the advantage of being able to be
used at a very high temperature.
[0194] We will now describe FIG. 8, which represents a variant of
embodiment of a means for preheating the reagents 5 and a heat
exchanger for cooling the associated products 6, constituting a
recovery system 24.
[0195] In the case of the variant of embodiment represented in FIG.
8, the means for preheating the reagents 5 and the heat exchanger
for cooling the products 6 form two sides of the same piece of
equipment, and the heat is transferred from one to the other
through the impermeable surface that separates them. This
configuration has the advantage of continuous operation and does
not require a system of flow-reversing and control valves. The
thermal inertia is also much lower.
[0196] The hydrocarbons, the oxygen and the water or vapor enter
the recovery system 24, where they are heated, cooling the hot
products of the conversion. They are then injected into the
conversion reactor 4 on the opposite side of the recovery system 24
through feed circuits 25. The hot conversion products then enter
the recovery system 24.
[0197] In the case of the four variants of embodiment represented
in reference to FIGS. 3 through 6, after the extraction of the
hydrogen or its use by the fuel cell, the gas may still contain a
small amount of residual hydrogen. The same is true of the CO after
its conversion into CO.sub.2 or its use by the SOFC-type fuel cell.
The gas is then subjected to a post-combustion 12 of these
residues, which transforms them into H.sub.2O and CO.sub.2.
[0198] The gas, under high pressure (50 or 60 bar in the case of
the variants of embodiment in FIGS. 3 and 4) or medium pressure (5
to 7 bar in the case of the variants of embodiment in FIGS. 5 and
6), no longer contains anything other than water vapor and carbon
dioxide (with small traces of CO, H.sub.2 if the post-combustion is
incomplete, and nitrogen if the oxygen used is not pure). Cooling
it to a temperature on the order of 40.degree. C. will result in
the condensation 13 of nearly all of the water, which can be
recycled back to the beginning of the hydrocarbon conversion
process via a reservoir of water under pressure 15.
[0199] The residual gas is then nothing but nearly pure CO.sub.2
(with traces of CO, H.sub.2, N.sub.2, H.sub.2O). At a pressure of
60 bar, the CO.sub.2 can be easily condensed by cooling in contact
with a wall at ambient temperature. The effective rate of
condensation of the CO.sub.2 will depend on the temperature of the
cold wall and the level of residual impurities in the gas: for
example, at 60 bar and with a wall at 110.degree. C., a 92% to
99.2% condensation of the CO.sub.2 will be obtained if the traces
of CO, H.sub.2 and N.sub.2 represent 2% to 0.2%, respectively, in
the products leaving the post-combustion chamber. The CO.sub.2 can
then be stored in dense liquid form. Pressures as low as 30 bar are
acceptable for condensing the CO.sub.2; in that case, it is
necessary to use a refrigerant at negative temperatures such as
-20.degree. C. in order to obtain substantial levels of CO.sub.2
condensation, commensurate with the level of residual impurities in
the gas.
[0200] In the case of the variants represented in FIGS. 5 and 6,
the flow of CO.sub.2 generated can be stored at a pressure of 7
bar, or possibly recompressed in order to be condensed.
[0201] In all cases, it is possible not to discharge the CO.sub.2
generated by the hydrocarbon conversion into the atmosphere. It may
be re-used in a CO.sub.2-consuming process or stored in underground
layers. Finally, if the hydrogen is from a fossil hydrocarbon
source, its production will not have generated any additional
greenhouse effect. If the hydrogen is from a renewable biomass
source, its production is accompanied by a carbon sink.
[0202] FIGS. 3 and 4 show two variants of embodiment comprising two
successive steps. The purpose of one step is to convert CO in to
CO.sub.2 using the gas's catalytic reaction to water:
CO+H.sub.2O.fwdarw.CO.sub.2+H.sub.2. The purpose of the other step
is to extract the hydrogen formed by means of a membrane 7. The use
of oxygen in place of air promotes the extraction by the membrane 7
since the partial pressure of the hydrogen, which is not diluted in
nitrogen, is higher.
[0203] Moreover, it is advantageous to be able to combine several
functions in the same piece of equipment. The use of a vector gas
or an easily condensable extraction gas, for example water vapor or
ammonia, downstream from the permeation membrane 7, makes it
possible to lower the partial pressure of the hydrogen downstream
and thus to extract more of the hydrogen in the reformate. FIG. 9
shows the increase in the efficiency of the hydrogen extraction as
a function of the ratio between the molar flow rate of the hydrogen
to be extracted and the molar flow rate of the flow of extraction
gas. In the case illustrated in FIG. 9, the total pressure upstream
from the membrane 7 is 45 bar, the molar fraction of the hydrogen
upstream is 50.9%, and the total pressure downstream from the
membrane 7 is 5 bar. Extraction efficiencies of 90 to 100% may be
obtained, even with back pressures of 5 bar downstream from the
membrane 7. After the extraction of the hydrogen, the flow of
extraction gas may be condensed by cooling so as to be recycled to
the extraction membrane, leaving behind a flow of pure hydrogen to
be used or stored.
[0204] Any gas that is inert with respect to hydrogen and the
membrane, such as nitrogen, argon, water vapor, ammonia, etc., may
be used to lower the partial pressure of the hydrogen downstream
from the membrane and thus extract the hydrogen more easily.
However, it is advantageous to use an easily condensable extraction
gas such as water vapor or ammonia; a step for cooling and
condensing the vector gas/hydrogen mixture will make it possible to
separate them and to recover pure hydrogen.
[0205] We will now describe, in reference to FIGS. 10a and 10b, two
variants of embodiment according to the invention of a reactor for
the final conversion of CO into CO.sub.2 11, comprising a
hydrogen-permeable membrane 7 that makes it possible to extract the
hydrogen. In the case of low-capacity, small scale installations,
it is possible to organize the permeation chamber 27 so that it is
concentric to the conversion reactor. In the case of the variant of
embodiment represented in FIG. 10a, the hydrogen is extracted at
the center of the final conversion reactor 11.
[0206] The membrane tube 26 is placed on the axis of the chamber 27
and is fed with extraction water vapor. The conversion catalyst is
placed in the ring-shaped chamber around the membrane tube 26 and
is passed through by the gasses to be converted, generally in the
opposite direction from the extraction water vapor. In the case of
the variant of embodiment represented in FIG. 10b, the hydrogen is
extracted at the periphery of the final conversion reactor 11. The
water vapor for extracting the hydrogen circulates at the periphery
of the final conversion reactor 11. The conversion catalyst is
placed in the center.
[0207] We will now describe, in reference to FIG. 11, another
variant of embodiment according to the invention of a reactor for
the final conversion of CO into CO.sub.2 11, equipped with a series
of closed-end tubes that descend into the core of the reactor, each
of which supports a hydrogen-permeable membrane that makes it
possible to extract hydrogen.
[0208] In the case of a high-capacity installation, the membrane
surface to be installed would result in an excessive diameter and
length if the configuration represented in FIG. 10a or 10b were
retained. Likewise, the quantity of catalyst required would result
in a ring that is too thick. For this reason, the compositions and
temperatures in each section would not be homogeneous. It is
preferable to divide up the catalyst thickness using a number of
membrane tubes 26 shaped like the fingers of a glove. The tubes 26,
of small diameter and length, descend from the external wall right
into the core of the conversion reactor 11.
[0209] Reactors like those described in reference to FIGS. 10a and
10b make it possible not to separate the steps for the final
CO/CO.sub.2 conversion and for the extraction of the hydrogen. They
are performed in the same chamber. It is thus possible to reduce
the partial pressure of the hydrogen during the final CO/CO.sub.2
conversion and thereby shift the equilibrium toward the formation
of CO.sub.2 and H.sub.2O; the conversion reaction is accelerated. A
smaller quantity of catalyst or a smaller size chamber may be used
to achieve equivalent performance. This configuration is possible
because the conversion of the CO into CO.sub.2 and the extraction
through a hydrogen-permeable membrane are done at the same
temperature level: on the order of 250 to 400.degree. C. The
reaction of the gas to water is exothermic, and heat must be
extracted in order to maintain the gas within the optimal operating
temperature range of the catalyst. In the case where the membrane
is located inside the final conversion reactor, the flow of
extraction water vapor can advantageously be used to cool the
CO/CO.sub.2 conversion chamber. Likewise, when the installation is
cold and must be reheated in order for the catalyst and the
permeable membrane to reach their best operating temperature range,
the flow of extraction water vapor may be used to supply heat to
this conversion reactor. The tube or tubes that support the
permeation membrane 26 and are passed through by the extraction
water vapor can advantageously serve as heat exchangers, thus
avoiding the use of specific equipment for this heat exchange
function. The temperature of the conversion chamber can thus be
regulated by varying the flow rate and the temperature of the flow
of extraction water vapor.
[0210] Nitrogen may be produced by distilling air under cryogenic
conditions. The production of one kg of nitrogen is accompanied by
the production of 0.30 kg of oxygen. This oxygen, in liquid form,
may be used onsite to produce hydrogen using the method according
to the invention described in reference to FIGS. 3 through 6. It
may also be transported in order to be used at another site using
the method according to the invention described in reference to
FIGS. 3 through 6.
[0211] With the oxygen produced, for an 80 to 90% energy efficiency
of the method for producing hydrogen, the consumption of diesel
fuel is respectively equal to 0.21 kg/kg of nitrogen and 0.26 kg/kg
of nitrogen for a quantity of captured CO.sub.2 respectively equal
to 0.67 kg per kg of nitrogen and 0.82 kg per kg of nitrogen. The
quantity of hydrogen generated is respectively equal to 0.054 kg of
H.sub.2 per kg of nitrogen produced and 0.073 kg of H.sub.2 per kg
of nitrogen produced, representing a chemical energy of 7.7 to 10
MJ and an electrical energy of 1.1 to 1.45 kWh after use in a fuel
cell.
[0212] Hydrogen can also be produced by water electrolysis. The
production of one kg of electrolytic hydrogen is accompanied by the
production of 8 kg of oxygen. The electrolyzers operate under
medium pressure, from a few bar to several tens of bar. The oxygen
produced may be put to use according to any of the variants of
embodiment represented in FIGS. 3 through 6. The variant of
embodiment represented in FIG. 3, however, has the advantage of
using the oxygen onsite to produce hydrogen. The method according
to the invention makes it possible to obtain a flow of chemical
hydrogen in addition to the electrolytic hydrogen, while
contributing to CO.sub.2 capture and to the amortization of all the
utilities for conditioning the hydrogen produced.
[0213] The leverage is considerable, since with 8 kg of oxygen
produced, for an 80 to 90% energy efficiency of the chemical method
for producing hydrogen, the consumption of diesel fuel is
respectively equal to 5.75 to 6.9 kg/kg of electrolytic hydrogen
for a quantity of captured CO.sub.2 respectively equal to 19.1 kg
per kg of electrolytic hydrogen and 21.8 kg per kg of electrolytic
hydrogen. The quantity of hydrogen generated is respectively equal
to 1.45 kg of chemical hydrogen per kg of electrolytic hydrogen and
1.96 kg of chemical hydrogen per kg of electrolytic hydrogen.
[0214] Combining the two methods results in an increase in the
quantity of hydrogen produced by a factor of nearly 3, which
largely compensates for the efficiency loss of electrolysis.
* * * * *