U.S. patent application number 11/167591 was filed with the patent office on 2007-01-04 for compact reforming reactor.
Invention is credited to Michael Boe, John B. Hansen.
Application Number | 20070000173 11/167591 |
Document ID | / |
Family ID | 36915754 |
Filed Date | 2007-01-04 |
United States Patent
Application |
20070000173 |
Kind Code |
A1 |
Boe; Michael ; et
al. |
January 4, 2007 |
Compact reforming reactor
Abstract
Reforming reactor for the conversion of a process fluid into
hydrogen comprising: a reforming section and a boiler section which
are both contained within a common volume and a combustion section,
in which said reforming section contains one or more catalyst tubes
filled with reforming catalyst, said boiler section is provided
with one or more tubes carrying flue gas from the combustion
section and said combustion section is provided with at least one
burner, wherein the heat exchanging medium required for the
reforming of said process fluid in the one or more catalyst tubes
is a gas-liquid mixture that self-circulates and is encapsulated
inside said common volume containing said reforming and boiler
sections.
Inventors: |
Boe; Michael; (Klampenborg,
DK) ; Hansen; John B.; (Copenhagen, DK) |
Correspondence
Address: |
DICKSTEIN SHAPIRO LLP
1825 EYE STREET NW
Washington
DC
20006-5403
US
|
Family ID: |
36915754 |
Appl. No.: |
11/167591 |
Filed: |
June 28, 2005 |
Current U.S.
Class: |
48/197R ;
48/127.9 |
Current CPC
Class: |
C01B 2203/1223 20130101;
B01J 8/0492 20130101; C01B 2203/0822 20130101; Y02P 20/129
20151101; B01J 8/0242 20130101; B01J 2208/00504 20130101; C01B
2203/066 20130101; Y02P 20/10 20151101; B01J 8/065 20130101; B01J
8/0496 20130101; B01J 8/06 20130101; C01B 3/323 20130101; Y02P
20/128 20151101; B01J 2208/00106 20130101; B01J 8/0453 20130101;
B01J 8/048 20130101; B01J 8/067 20130101; C01B 2203/0405 20130101;
C01B 2203/044 20130101; C01B 2203/0233 20130101; B01J 8/0446
20130101; C01B 2203/043 20130101; C01B 2203/0811 20130101; B01J
8/0285 20130101; B01J 2208/00309 20130101; B01J 2208/0053 20130101;
B01J 8/0469 20130101; C01B 2203/0283 20130101; B01J 2208/00495
20130101 |
Class at
Publication: |
048/197.00R ;
048/127.9 |
International
Class: |
B01J 8/00 20060101
B01J008/00; C10J 3/46 20060101 C10J003/46 |
Claims
1. Reforming reactor for the conversion of a process fluid into
hydrogen comprising: a reforming section and a boiler section which
are both contained within a common volume and a combustion section,
in which said reforming section contains one or more catalyst tubes
filled with reforming catalyst, said boiler section is provided
with one or more tubes carrying flue gas from the combustion
section and said combustion section is provided with at least one
burner, wherein the heat exchanging medium required for the
reforming of said process fluid in the one or more catalyst tubes
is a gas-liquid mixture that self-circulates and is encapsulated
inside said common volume containing said reforming and boiler
sections.
2. Reactor according to claim 1, in which at least one process feed
tube carrying the process fluid to be converted extends inside said
common volume of the reactor.
3. Reactor according to claim 2, in which the at least one process
feed tube carrying the process fluid to be converted enters the
reactor through a conduct arranged in the outer wall of the reactor
and wherein said process fluid is preheated by indirect contact
with exiting converted gas from the reforming section of the
reactor.
4. Reactor according to claim 2, in which said at least one process
feed tube extends vertically into a transition compartment from
which at least one process tube carrying process gas to be
converted extends vertically inside the common volume of the
reactor and wherein the at least one process tube carrying the
process gas is formed as a coil.
5. Reactor according to claim 1 in which said common volume
containing said reforming section and boiler section are
substantially surrounded by an insulated housing, wherein said
insulated housing is encased by a first annular region carrying
flue gas and a second annular region carrying combustion air.
6. Reactor according to claim 1, wherein the process fluid entering
the reactor is a mixture of methanol and water and the gas-liquid
mixture is a saturated steam-water system circulating at a pressure
of 55 to 110 bar g and a temperature of 270.degree. C. to about
320.degree. C.
7. Reactor according to claim 1, wherein said combustion section is
provided with a single catalytic burner and wherein said catalytic
burner is provided as wire mesh layers arranged in series which are
coated with ceramic and impregnated with an oxidation catalyst,
whereby the heat generated in the combustion is transferred by a
convection mechanism to the self-circulating gas-liquid mixture via
the generated flue gas.
8. Reactor according to claim 1, further comprising a fixed bed of
catalyst arranged above said catalyst tubes, in which said fixed
bed covers substantially the whole horizontal cross section of the
reactor and wherein said fixed bed is adapted to receive the
process gas to be converted prior to the passage of said gas into
said catalyst tubes.
9. Process for the production of hydrogen from a feed process fluid
in a reactor containing a combustion section, a boiler section and
a reforming section according to any preceding claim, the process
comprising: passing a preheated process gas through said reforming
section, heating the at least one catalyst tube in the reforming
section by indirect heat exchange with a gas-liquid mixture that
self-circulates and is encapsulated inside a common volume
containing said reforming section and said boiler section,
retrieving reformed process gas from said reforming section and
optionally cooling said reformed process gas by preheating of the
feed process fluid, introducing a fuel into the at least one burner
in the combustion section together with combustion air, in which
said combustion air is preheated by indirect heat exchange with
flue gas from the boiler section, retrieving flue gas from the
burner and passing said flue gas through a boiler section, and
heating said gas-liquid mixture that self-circulates and is
encapsulated inside a common volume in the reactor containing said
reforming section and said boiler section by indirect heat exchange
with the flue gas passing through said boiler section.
Description
FIELD OF THE INVENTION
[0001] This invention relates to an integrated and compact
reforming reactor for the production of hydrogen to be used in
industrial applications such as in the metallurgical industry,
chemical and pharmaceutical industry and fuel cell power plants. In
particular the invention relates to a compact reforming reactor for
the conversion of hydrocarbon feedstocks to hydrogen where the
reformed gas of the reactor is further enriched in hydrogen by
passage through a Pressure Adsorption Swing (PSA) unit, a Pd-alloy
membrane, water-gas shift unit or by Preferential Oxidation (PROX).
More particularly the invention relates to a compact reforming
reactor for the conversion of methanol to a hydrogen gas suitable
for use in fuel cell plants, especially where the reformed gas of
the reactor is further enriched in hydrogen by passage through a
PSA unit. The invention further involves a process for reforming
the hydrocarbon feedstock into a hydrogen gas using this
reactor.
BACKGROUND OF THE INVENTION
[0002] Fuel cell plants require often the supply of hydrogen as
fuel source and accordingly a reforming reactor is normally
integrated in fuel cell plants. The reforming reactor converts a
suitable hydrocarbon feedstock acting as energy carrier, such as
methane, liquid petroleum gas, gasoline, diesel or methanol, into a
hydrogen rich gas, which then may be passed through a
hydrogen-enrichment unit before entering a fuel cell assembly.
Compact fuel cell power plants may today provide about 20 kW of
power and even more, for instance up to 50 kW, thereby promoting a
wide range of applications. One such application is the use of
compact fuel cell plants in the automotive industry.
[0003] For widespread application, methanol is still regarded as
the best hydrocarbon feedstock for the production of hydrogen-rich
gas not only in connection with fuel cell plants but also for
application in small plants in other industrial fields. Roughly,
methanol is particularly suitable where the demand for hydrogen is
the range 50-500 Nm.sup.3/h, which is typical for small plants. For
a hydrogen demand of above 500 Nm.sup.3/h a hydrocarbon feedstock
such as natural gas is often more expedient. Below 50 Nm.sup.3/h
electrolysis or bottled hydrogen is normally more expedient.
[0004] Reactors for the reforming of fuel gases, particularly
methanol, and which are used in fuel cell plants are known in the
art. Dusterwald et al. disclose in Chem. Eng. Technol. 20 (1997)
617-623 a methanol steam reformer consisting of four reactor tubes
that are individually balanced. Each reactor tube consists of two
stainless tubes arranged concentrically with catalyst filling the
inner tube and in which the heat needed for the endothermic
reaction of a methanol-water mixture is provided by condensing
steam that flows in the gap between the tubes. It is also known
from U.S. Pat. No. 4,861,347 to oxidise a raw fuel such as methanol
in order to obtain an exothermic reaction, whereby the heat
generated by this reaction is used for the endothermic reforming
reaction of the hydrocarbon feedstock, which is normally a mixture
of methanol and water. The heat is transferred from the combustion
section of the reactor to its reforming section by means of heat
tubes through which a hot flue gas from the combustion section is
passed or as in JP-A-63248702 by means of heat pipes arranged in
the reactor. As a result, the heat generated in the combustion
system can be evenly distributed to the rest of the reactor,
whereby a uniform temperature distribution is obtained.
[0005] Often the heat transfer system in the reforming reactor is
not rapid enough to achieve the desired operating temperature after
changes in process conditions such as after sudden load changes or
during start-ups and shut-downs, especially when separate heat
pipes are provided in the reforming reactor. Normally a number of
more or less sequential steps are required for the start-up of the
reforming reactor resulting in a procedure that may be
significantly tedious and time-consuming.
[0006] In the particular field of fuel cells, the advent of fuel
cells with increased power, for instance of up to 20 kW or even
more, for instance up to 50 kW has resulted in a need for providing
a plurality of catalyst tubes in a single reforming reactor. This
in turn imposes more demands in reactor design in terms of i.a.
compactness, better temperature distribution and thermal
efficiency. In particular, the provision of a uniform temperature
distribution by which all catalyst tubes inside the reactor are
heated to the same temperature becomes more difficult to achieve
when the heating required in reforming has to be provided by means
of a single burner in the reactor.
[0007] In addition, the catalyst within the catalyst tubes may
often be not evenly distributed so that the catalyst may for
instance be better packed in some tubes than others. This may
create undesired variation in temperature conditions across the
catalyst tubes.
SUMMARY OF THE INVENTION
[0008] It is therefore an object of the invention to provide a
reforming reactor with improved temperature distribution across all
catalyst tubes.
[0009] It is also an object of the invention to provide a reforming
reactor which is compact and free for mechanical means for
circulating a heat exchanging medium from the high temperature
section of the reactor to the reforming section of the reactor.
[0010] It is a further object of the invention to provide a
reforming reactor which is compact whilst at the same time is able
to rapidly and simply achieve or maintain its operating temperature
after a change in process conditions, such as a change in
hydrocarbon feed flow or temperature or a change in burner
conditions or during a start-up operation.
[0011] It is another object of the invention to provide a reforming
reactor which is less sensitive to divergent catalyst packing
across the catalyst tubes.
[0012] It is another object of the invention to provide a reforming
reactor which is simple in its construction, inexpensive and with
lower heat loss than in conventional reforming reactors.
[0013] It is yet another object of the invention to provide a
reforming reactor which is compact and suitable for use in fuel
cell plants, particularly for fuel cell plants capable of producing
up to 20 kW of power or even more for instance up to 50 kW.
[0014] These and other objects are achieved by the reactor and
process of the invention.
[0015] In a first aspect of the invention we provide a reforming
reactor for the conversion of a process fluid into hydrogen
comprising: a reforming section and a boiler section which are both
contained within a common volume and a combustion section, in which
said reforming section contains one or more catalyst tubes filled
with reforming catalyst, said boiler section is provided with one
or more tubes carrying flue gas from the combustion section and
said combustion section is provided with at least one burner,
wherein the heat exchanging medium required for the reforming of
said process fluid in the one or more catalyst tubes is a
gas-liquid mixture that self-circulates and is encapsulated inside
said common volume containing said reforming and boiler
sections.
[0016] Accordingly, in the invention a gas-liquid mixture
circulating outside the catalyst tubes in the reforming section and
outside the tubes carrying the flue gas in the boiler section
provides for a large heat sink that enables the accumulation and
supply of heat for the reforming reaction so that all metal parts
within the reactor, particularly the catalyst tubes, maintain or
rapidly reach the same temperature, and a robust operation of the
reactor is obtained as it becomes i.a. less sensitive to temporary
changes in process conditions, such as changes in burner duty.
[0017] By the term "self-circulates" it is meant that the
gas-liquid mixture acting as heat exchanging medium moves
internally in the reactor without the need of any mechanical means.
The gas flows to surfaces or catalyst tube walls, where
condensation takes place in a movement driven by the slightly lower
pressure created by the volume reduction of the gas as it
transforms into liquid. Liquid flows then to the boiler section
driven by gravity forces.
[0018] In the reactor of the invention at least one process feed
tube carrying the process fluid to be converted, such as a liquid
mixture of methanol and water, may extend inside said common volume
of the reactor. Accordingly, the at least one process feed tube may
extend into any location inside said common volume containing the
reforming and boiler section, for example the at least one process
feed tube may extend from a region at the top of the reactor and
above the reforming section into this reforming section or even
further into the boiler section arranged below. The at least one
process feed tube carrying the process fluid to be converted is
introduced to the reactor through a conduct in the outer wall of
the reactor and may then extend into the reactor from said conduct
arranged in the outer wall. Preferably said process feed tube
extends substantially co-axially of the reactor wall inside said
common volume from the reforming section of the reactor to the
boiler section of the reactor. This enables the provision of a
compact reactor as the at least one process feed tube, for example
a single substantially straight tube or a tube bundle, is
advantageously integrated within the reactor whereby the preheating
or evaporation of the process fluid can advantageously be effected
as the gas in the self-circulating gas-liquid mixture outside the
tube condenses. Hence it is possible to integrate the required
evaporation stage inside the reactor thus avoiding the inexpedient
provision of separate evaporation means outside the reactor.
[0019] By the term "extends substantially co-axially" it is meant
that a portion of the process feed tube, particularly the inlet
portion cooperating with the conduct in the outer wall of the
reactor, may extend into the center of the reactor in a direction
which is perpendicular to the reactor length axis, thereafter
bending 90.degree. and consequently extending vertically into the
reforming section or boiler section below.
[0020] The at least one process feed tube may extend vertically
into a transition compartment from which at least one process tube
carrying process gas to be converted extends vertically upwards
inside the common volume of the reactor and wherein the at least
one process tube carrying the process gas is formed as a coil.
Preferably, a single process tube descends from the conduct in the
outer wall where the hydrocarbon feed for example a liquid
hydrocarbon feed enters the reactor to the transition compartment.
The transition compartment is arranged as a box having inlet
openings adapted to accommodate the at least one process tube
carrying a process fluid present in substantially liquid form and
outlet openings adapted to accommodate the at least one process
tube carrying a process fluid present in substantially gas form.
These tubes extend vertically upwards and are formed as a coil or
spiral. This ensures a better heat transfer for the preheating of
the process gas prior to reforming and provides at the same time a
compact reactor design as the same heat transfer area as for
instance a straight tube can be accommodated in a lower height.
Furthermore, the use of a coil or spiral imparts a centrifugal
effect on the two-phase flow (gas-liquid) thereby enabling backflow
of any liquid not yet evaporated and facilitating the upward flow
of process gas.
[0021] Preferably the at least one process tube extends from a
transition compartment in the boiler section of the reactor to the
reforming section in order to ensure that the process gas is heated
to the proper reaction temperature in the reforming section.
[0022] In the invention it is also possible to extend the at least
one process feed tube into a transition compartment located in the
combustion section for example in a flue gas region just underneath
the boiler section.
[0023] In this specification the term "hydrocarbon feedstock" is
used interchangeably with the term "process fluid" or "feed process
fluid". Normally, the feed inlet to the reactor, for example a
mixture of methanol and water is present in liquid form whereas
when entering into the reforming section it is present in gas form.
When entering the reactor, the hydrocarbon feed is also referred as
process fluid and after evaporation in the process tube the
resulting fluid is also referred as process gas. The term "process
feed tube" as used herein refers to the at least one tube carrying
the process fluid and which enters the transition compartment. The
tubes protruding from the transition compartment and carrying the
evaporating gas that is directed to the reforming section are
referred simply as "process tubes".
[0024] In another embodiment of the invention the at least one
process feed tube carrying the process fluid to be converted enters
the reactor through a conduct arranged in the outer wall of the
reactor and said process fluid is preheated by indirect contact
(i.e. across a heat transfer surface) with exiting converted gas
from the reforming section of the reactor, in which said exiting
converted gas preferably passes in the annular region of said
conduct. Normally the PSA unit downstream requires a relatively
cold stream of hydrogen-rich gas and accordingly cooling means such
as an air cooler downstream the reactor is used. Hence, this
embodiment enables the reformed gas from the reactor (hydrogen-rich
gas) to be cooled from normally about 280.degree. C., which is
typical for the reforming of methanol to about 150.degree. C.,
thereby reducing the effect required in the air cooler downstream
and accordingly also reducing its size. The portion of the at least
one process tube carrying the process fluid which is in contact
with the exiting converted gas from the reforming section may
advantageously be formed as a coil to ensure an even more compact
reactor design without too noticeable protruding parts. Said
conduct is preferably located in the upper portion of the reactor,
e.g. near its top. In an alternative embodiment, an outlet tube
carries the exiting converted gas and runs parallel with the
process feed tube inside said conduct.
[0025] In the combustion section arranged preferably in the lower
portion of the reactor and below the boiler section, a suitable
fuel, such as methanol is injected through a fuel inlet and is
subjected to a reaction with preheated combustion air in the at
least one burner. Hot flue gases are produced by the exothermic
oxidation of methanol and are then passed to the boiler section.
The tubes carrying said flue gases may extend vertically from the
combustion section into the boiler section and their outlets may
then protrude from said boiler section towards an annular section
of the reactor.
[0026] The boiler section is contained within a compartment or
common volume in which a gas-liquid system, preferably a saturated
gas-liquid mixture, such as a saturated water-steam mixture,
self-circulates. The compartment contains one or more tubes through
which hot flue gas from a combustion section arranged below passes.
The hot flue gas supplies heat to the gas-liquid mixture thereby
evaporating part of the liquid and promoting its circulation
upwards internally in the reactor. Part of the heat in the
gas-liquid mixture is also delivered to the at least one process
tube carrying the gas or liquid or gas-liquid mixture to be
converted, e.g. methanol-water. The process tubes extend away from
the boiler section and upwardly through the middle portion of the
reactor and further up to the reforming section inside which one or
more vertical catalyst tubes are disposed. The reforming section is
also contained within the same compartment or common volume as the
boiler section, but is preferably arranged separately in the upper
portion of the reactor. Hence, said boiler and reforming section
are both contained within a common volume. The term catalyst tube
means that these tubes are filled with solid catalyst particles
suitable for the reforming of a given hydrocarbon feedstock, such
as a mixture of methanol and water.
[0027] Prior to reforming, the process gas to be reformed leaves
the process tubes at a suitable position in the reformer section,
preferably above the one or more catalyst tubes. The one or more
catalyst tubes are normally arranged as a plurality of
circumferentially and radially spaced catalyst tubes. Often the
number of catalyst tubes is over 5 or 20, more often over 50 and
even above 100 or 200 depending on the hydrogen capacity of the
reactor. The process gas to be reformed enters the catalyst tubes
and flows downwards through the catalyst particles so as to be
gradually converted along its passage through the catalyst tubes.
The heat required for the reforming reaction is provided by the
gas-liquid mixture which self-circulates outside said catalyst
tubes. As the gas-liquid mixture delivers heat to the catalyst
tubes, the gas condenses and via gravity is forced to flow
downwards to the boiler section. The gas-liquid mixture acting as
heat exchanging medium moves therefore inside the reactor in a
self-circulating manner in a region which is encapsulated inside
said common volume containing the boiler section and the reforming
section. This enables the continuous circulation of the gas-liquid
mixture through said boiler section and said reforming section
inside the reactor.
[0028] It would therefore be understood that the gas-liquid mixture
self-circulates outside the at least one process feed tube, outside
the at least one process tube carrying the process gas to be
converted, outside the tubes carrying the flue gas, and outside the
one or more catalyst tubes in a hermetically sealed compartment.
The gas or liquid in the mixture, for instance steam when the
mixture is a saturated water-steam mixture, is not utilised for
other purposes other than as heat transfer medium as described
above.
[0029] Preferably at least said reforming and boiler sections are
arranged co-axially in the reactor so as to be able to fit into an
outer substantially cylindrical housing. Accordingly, in one
embodiment said combustion, reforming and boiler sections are
arranged co-axially in the reactor. In another embodiment the
reforming and boiler section may be arranged co-axially in the
reactor, while the combustion section may be arranged normal to
said boiler section so as to form an L-shaped reactor. This enables
a lower length in the reactor and may facilitate its transport
under circumstances where reactor length is a limiting factor.
[0030] Said reforming section is preferably arranged in series with
respect to the boiler section in which the at least one process
tube carrying the process gas and optionally the at least one
process feed tube carrying the process fluid inlet are disposed
co-axially. The boiler section is preferably arranged in series
with respect to a combustion section, which apart from the one or
more burners may also comprise a fuel inlet for the introduction of
a suitable fuel, preferably methanol, and optionally a co-axially
arranged fuel inlet for the introduction of another fuel, which is
preferably off-gas from the PSA unit or any other off-gas from a
hydrogen enrichment step. Typically during normal operation of the
reactor, the off-gas from the PSA serves as main fuel, whereas
methanol serves as supporting fuel, whereas upon a start-up it is
methanol that serves as the main fuel. The use of off-gas from the
PSA unit and optionally the anode off-gas from the fuel cell
enables better overall thermal efficiency in for instance a fuel
cell plant comprising said reactor and said accompanying PSA
unit.
[0031] The combustion section of the reactor is also provided with
at least one burner. Because of the requirement of reactor
compactness the number of burners is kept at a minimum. Preferably
a single burner is provided; more preferably a single catalytic
burner is provided. The catalytic burner may be a ceramic hollow
cylinder with oxidation catalyst on its outer surface to which fuel
gas premixed with air is supplied internally. The catalytic burner
is preferably a burner arranged in a flow channel and provided as
wire mesh layers arranged in series which are coated with ceramic
and impregnated with an oxidation catalyst. The heat generated in
the combustion is transferred by a convection mechanism to the
self-circulating gas-liquid system via the generated flue gas.
Accordingly, in another embodiment of the invention, in the reactor
said combustion section is provided with a single catalytic burner
and wherein said catalytic burner is provided as wire mesh layers
arranged in series which are coated with ceramic and impregnated
with an oxidation catalyst, whereby the heat generated in the
combustion is transferred by a convection mechanism to the
self-circulating gas-liquid mixture via the generated flue gas.
This enables a better transfer of heat than in for instance systems
in which heat transfer occurs by a radiation mechanism, while at
the same time enables a compact reactor design since only a single
burner is used.
[0032] In another embodiment of the invention said reforming
section and boiler section are substantially surrounded by an
insulated housing, wherein said insulated housing is encased by a
first annular region carrying flue gas and a second annular region
carrying combustion air. This enables a low heat loss to the
surroundings since the hotter parts within the main body of the
reactor containing the reforming section, combustion section and
the common volume carrying the gas-liquid system serving as heat
exchanging medium is encased by first an insulated housing, then a
sleeve through which flue gas is passed and finally a second
(outer) annular region carrying combustion air to be used in the
burner. This may also enable that combustion gas and any other
suitable fuel gas, such as off-gas from a hydrogen-purification
unit downstream, be preheated by indirect heat exchange with the
flue gas, which preferably runs counter-currently on its way out of
the reactor. In a preferred embodiment, the flue gas enters into
said first annular region directly from the boiler section via an
annular region outside said boiler section. This annular region is
fed with flue gas by means of tubes carrying this gas that protrude
from the boiler section. The flue gas may also enter into said
first annular region directly from the combustion section of the
reactor, whereby a higher temperature in the flue gas may be
effected.
[0033] By the term "substantially surrounded by an insulated
housing" as used herein is meant that some portions of the reactor
may not be insulated. For instance it is possible that part of the
reforming section does not require insulation. It is also possible
that a small portion of the reforming or boiler section is not
surrounded by said insulated housing. For instance, the insulated
housing may not cover the lower portion of the boiler section
closest to the combustion section.
[0034] The reactor may be adapted to cooperate with a Pressure
Swing Adsorption unit (PSA), which is the preferred
hydrogen-purification unit for the further treatment of the
reformed process gas leaving the reactor. As mentioned above, the
off-gas from the PSA unit may be utilised in the reactor as fuel.
Hence, in yet another embodiment of the invention an inlet is
adapted to said second annular region carrying combustion air for
the passage of PSA off-gas. This enables the preheating of said
off-gas prior to introduction into the at least one burner in the
combustion section.
[0035] Instead of a PSA-unit a Pd-alloy membrane may also be used
to enrich the reformed process gas. Normally a higher degree of
purity may be obtained by using Pd-alloy membranes which may be
incorporated into the reactor. Accordingly, in the invention it is
also possible that a hydrogen purification unit, such as a Pd-alloy
membrane is integrated within the reactor. However, a PSA
purification unit is still preferred as it is less sensitive and
more inexpensive than Pd-alloy membranes. Normally a Pd-alloy
membrane requires also a relatively high temperature in the
reformed gas for instance about 350.degree. C. Hence, in methanol
reforming the reformed gas leaving the reactor at about 300.degree.
C. will require heating in order to conform to the requirements of
a Pd-alloy membrane. Other hydrogen enrichment units such as
conventional water-gas shift step, e.g. low shift and the selective
oxidation of carbon monoxide in what is also referred as
Preferential Oxidation (PROX) of carbon monoxide, may
advantageously be used particularly in connection with fuel cells.
The water-gas shift and PROX steps enable the removal of carbon
monoxide from the reformed hydrogen-rich gas. This results in an
increase in the efficiency of electrochemical reactions in proton
exchange membrane (PEM) fuel cells, since carbon monoxide adsorbed
in the Pt anode of the PEM fuel cell inhibits the dissociation of
hydrogen to protons and electrons and consequently strongly reduces
the power output or performance of the PEM fuel cell.
[0036] The second annular region of the reactor carrying the
combustion air is preferably connected to the combustion section.
Accordingly, said second annular region may preferably extend into
the combustion section in order to ensure that the preheated
combustion air enters into the burner together with the inlet fuel,
which preferably is methanol and the other fuel, which preferably
is off-gas from the PSA unit. It would be understood that instead
of air, any other suitable oxidant, such as oxygen enriched air,
may be used.
[0037] The gas-liquid mixture is preferably a saturated steam-water
system that self-circulates at a pressure of about 55 to 110 bar g,
preferably 65 to 110 bar g and a temperature of 270.degree. C. to
about 320.degree. C., preferably 280.degree. C. to about
320.degree. C. Most preferably the saturated steam-water system
self-circulates at a pressure of 65 bar g and a temperature of
280.degree. C. It would be understood that the temperature is
determined by the saturated steam pressure in the circulating
system, in this case 280.degree. C. where the pressure of the
saturated steam-water system is 65 bar g. Accordingly, the
saturated steam-water system may also self-circulate at a pressure
of 110 bar g and a temperature of about 320.degree. C., or at a
pressure of 55 bar g with a temperature of 270.degree. C. The
saturated steam-water system enables the provision of a
self-circulating system in which the temperature required in the
reforming section for the conversion of methanol to hydrogen, for
example 280.degree. C., is easily achieved. The above pressures and
temperatures are particularly suitable when the process gas to be
reformed comprises methanol, for example a mixture of methanol and
water, since the reforming of methanol normally occurs in the
temperature range of 250-350.degree. C. Accordingly, in another
embodiment of the invention the process fluid entering the reactor
is a mixture of methanol and water and the gas-liquid mixture is a
saturated steam-water system circulating at a pressure of 55 to 110
bar g and a temperature of 270.degree. C. to about 320.degree. C.
(more specifically 318.degree. C.). The high heat capacity of the
saturated steam-water system enables therefore the provision of a
large heat sink in the reactor. Heat is accumulated and ready to be
used when the circumstances, e.g. changes in reactor operation or
burner duty, so require it. Heat is distributed throughout the
reactor by the self-circulating steam-water system, in which water
is vaporized by heat exchange with hot flue gas from the catalytic
burner, while steam condenses where heat is consumed.
[0038] On the process fluid side, the pressure is kept at a lower
level, normally in the range of 3 to 30 bar g, such as 20 to 30 bar
g. For instance the pressure of the process fluid entering the
reactor, here a liquid mixture of methanol and water, may be about
22 bar g and its temperature in the range 0.degree. C. to
50.degree. C., while in the reformed gas leaving the reactor the
pressure may be slightly lower, for example 20 bar g and the
temperature in the range 120.degree. C. to 270.degree. C. The
hydrogen production from the reactor (exiting reformed gas) is
normally in the range 10-5000 Nm.sup.3/h, often 15-1000 Nm.sup.3/h,
preferably 25-1000 Nm.sup.3/h, more preferably 25-500 Nm.sup.3/h.
Normally the composition of said reformed gas is about 65% vol.
H.sub.2, 11% vol. H.sub.2O, 2.1% vol. CO, 23% vol.CO.sub.2 and 1.4%
vol. methanol. The methanol conversion in the reactor is normally
above 90%, often above 95%, for example 97 to 99%. For a reactor
having a hydrogen capacity (production) of 600 Nm.sup.3/h the
number of catalyst tubes is normally in the range 110-120. The
catalyst tubes are normally 2.5 to 3.0 m long and with internal
diameter of 20 mm. The temperature in the reactor across the
catalyst tubes in the reforming section is kept at a uniform level,
for instance at 280.degree. C., and this level is determined by the
saturated steam pressure in the circulating system, in this case 65
bar g. For higher temperature applications, the self-circulating
system may comprise sodium or potassium instead of a water-steam
mixture.
[0039] The reactor may further comprise a fixed bed of catalyst
arranged above said catalyst tubes, in which said fixed bed covers
substantially the whole horizontal cross section of the reactor and
wherein said fixed bed is adapted to receive the process gas to be
converted prior to the passage of said gas into said catalyst
tubes. The fixed bed of catalyst may surround the one or more
process tubes carrying the process gas to be converted.
Accordingly, the fixed bed is arranged upstream the one or more
catalyst tubes of the reforming section. The one or more process
tubes carrying the process gas extends through the fixed bed and
may protrude slightly away from the bed. The process tubes may thus
be provided with an outlet opening right above the fixed bed to
allow the passage of process gas through said bed and subsequently
through the catalyst beds inside the one or more catalyst tubes.
The fixed bed of catalyst covering substantially whole horizontal
cross section of the reactor serves as a poison guard catalyst
layer and enables often that the process gas flows into the
catalyst tubes downstream evenly and consequently better
temperature distribution across the horizontal cross section of the
reactor is achieved.
[0040] It would be understood that the integrated and compact
reactor according to the invention integrates in a single unit a
number of process units or steps which may otherwise require
stand-alone operation outside the reactor, such as heaters for the
preheating and evaporation of the hydrocarbon feedstock, preheating
of combustion air and optionally preheating off-gas from a PSA
unit, as well as catalytic burners and the common volume
encapsulating said gas-liquid mixture (gas-liquid system) serving
as heat exchanging medium. The reactor does not require the use of
moving parts such as valves and pumps, for instance it is not
necessary to have a pump to provide for the internal circulation of
the gas-liquid mixture serving as heat exchanging medium inside the
reactor.
[0041] In a second aspect the invention encompasses also a process
for the production of hydrogen. Accordingly, we provide a process
for the production of hydrogen from a feed process fluid in a
reactor containing a combustion section, a boiler section and a
reforming section as described herein, the process comprising:
[0042] optionally preheating a feed process fluid by indirect heat
exchange with exiting reformed process gas from said reforming
section, [0043] optionally further heating and evaporating said
feed process fluid in the reactor to form a preheated process gas
by indirect heat exchange with a gas-liquid mixture that
self-circulates and is encapsulated inside a common volume
containing said reforming section and said boiler section, [0044]
passing a preheated process gas through said reforming section,
[0045] heating the at least one catalyst tube in the reforming
section by indirect heat exchange with a gas-liquid mixture that
self-circulates and is encapsulated inside a common volume
containing said reforming section and said boiler section, [0046]
retrieving reformed process gas from said reforming section and
optionally cooling said reformed process gas by preheating of the
feed process fluid, [0047] introducing a fuel into the at least one
burner in the combustion section together with combustion air, in
which said combustion air is preheated by indirect heat exchange
with flue gas from the boiler section, [0048] retrieving flue gas
from the burner and passing said flue gas through a boiler section,
and [0049] heating said gas-liquid mixture that self-circulates and
is encapsulated inside a common volume in the reactor containing
said reforming section and said boiler section by indirect heat
exchange with the flue gas passing through said boiler section.
[0050] The process enables the production of reformed process gas
which is rich in hydrogen and which is particularly suitable for
use in PSA-units. Alternatively, where a Pd-alloy membrane or
similar is used as hydrogen-purification unit instead of a PSA,
further heating of the reformed process gas may advantageously be
effected by means of indirect heat exchange with flue gas. The
hydrogen-purification unit may thus be a membrane which may also be
integrated within the reactor.
[0051] The fuel introduced into the at least one burner in the
combustion section together with combustion air may be a
hydrocarbon fuel, such as methanol, but is often only off-gas from
a PSA-unit downstream used as hydrogen-enrichment unit.
[0052] The above process may further comprise the steps of: [0053]
passing the cooled reformed process gas through an air cooler,
[0054] subsequently passing said cooled reformed process gas
through a hydrogen-purification unit to form a hydrogen-enriched
gas, and [0055] introducing off-gas from said hydrogen-purification
unit into the at least one burner of the reactor.
[0056] Where the hydrogen-purification unit is a PSA-unit, this
unit and the air cooler are preferably located outside the reactor.
The off-gas from the PSA unit may then be introduced into the at
least one burner, as described above. The hydrogen-enriched gas
from the hydrogen-purification unit may then be used for any
suitable industrial application, such as in the metallurgical
industry, electronics, chemical and pharmaceutical industry or as
hydrogen source in fuel cell plants.
BRIEF DESCRIPTION OF THE DRAWINGS
[0057] The invention is further illustrated by the accompanying
drawing in which the sole FIGURE shows a schematic of the reactor
according to one embodiment of the invention for production of
25-1000 Nm.sup.3/h of hydrogen for use with a PSA-unit.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0058] In FIG. 1 a cylindrical integrated reactor 1 with capacity
of 80 Nm.sup.3/h of hydrogen contains a combustion section 2,
boiler section 3 and reforming section 4. The cylindrical reactor 1
has a total weight of 300 kg and is about 1.6 m high, with a
diameter (except for the combustion section) of about 0.4 m. The
total volume of the reactor is about 0.275 m.sup.3 while the total
catalyst volume is 0.020 m.sup.3.
[0059] The reforming section 4 encompasses also a fixed bed of
reforming catalyst 5 arranged above the region of the reforming
section in which catalyst tubes are disposed. These sections are
arranged co-axially in the reactor so as to be able to fit into an
outer substantially cylindrical housing.
[0060] A mixture of methanol and water is introduced to reactor 1
through a conduct 6 in the outer wall of the reactor. Through the
conduct 6 runs a process feed tube 7 carrying the process fluid
(methanol and water mixture). The process tube extends vertically
downward to boiler section 3.
[0061] The boiler section is arranged in a compartment or common
volume 8 inside which a saturated water-steam mixture 9
self-circulates here illustrated by the hatched region. The
saturated water-steam mixture moves therefore inside the reactor in
a self-circulating manner in a region which is encapsulated inside
said common volume 8 containing the boiler section and the
reforming section. The compartment or common volume 8 contains one
or more tubes 10 through which hot flue gas 11 from the combustion
section 2 arranged below passes. In combustion section 2 arranged
in the lower portion of the reactor below the boiler section 3, a
suitable fuel such as methanol is injected through fuel inlet 12
which is adapted as a spray nozzle. Methanol is then subjected to a
reaction with preheated combustion air entering via inlet 13 in a
single catalytic burner 14 comprising wire meshes impregnated with
oxidation catalyst and which is disposed in a flow channel
co-axially of the cylindrical reactor 1. Hot flue gases 11 are
produced and are then passed to boiler section 3. The tubes 10
carrying said flue gases extend vertically from combustion section
2 into boiler section 3 and their outlets 15 protrude towards an
annular section 16 of the reactor.
[0062] In boiler section 3 within common volume 8 part of the heat
in the saturated water-steam mixture 9 is delivered to a system of
process tubes 17. The process tubes 17, here formed as a coil or
spiral extend away from a transition compartment 18 in boiler
section 3 and upwardly through the middle portion of the reactor
and further up to the reforming section 4. The reforming section 4
inside which one or more vertical catalyst tubes 19 are disposed is
arranged in the compartment or common volume 8 in the upper portion
of the reactor. The process gas to be reformed travelling inside
process tubes 17 leaves above the fixed bed of catalyst 5 passes
through this bed and enters the catalyst tubes 19. The reformed gas
leaves the reforming section through outlet pipe 20 at the bottom
of the catalyst tubes 19 and is used to preheat the hydrocarbon
feed being transported inside process feed tube 7 in conduct 6 at
the outer wall of the reactor.
[0063] The reforming section 4, 5 and boiler section 3 are
surrounded by an insulated housing 21. This insulated casing 21 is
encased by a first annular region 22 carrying flue gas and a second
annular region 23 carrying combustion air which enters via inlet
13. The combustion air is preheated by indirect heat exchange with
the flue gas 11 running counter-currently in annular section 22
towards the flue gas exit 24. The combustion section 2 is also
surrounded by a separate insulated housing 25. Off-gas from a
PSA-unit downstream is also used as fuel and enters via inlet 26 to
the burner 14. The flue gas 11 enters into said first annular
region 22 directly from the boiler section via an annular region 27
outside said boiler section. The second annular region 23 carrying
the combustion air is connected to the combustion section 2 via
narrow passageway 28.
* * * * *