Note: Descriptions are shown in the official language in which they were submitted.
CA 02413388 2002-12-20
WO 02/02220 PCT/US01/16513
IMPROVED SYSTEM FOR HYDROGEN GENERATION THROUGH STEAM
REFORMING OF HYDROCARBONS AND INTEGRATED CHEMICAL REACTOR FOR
HYDROGEN PRODUCTION FROM HYDROCARBONS
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to an integrated chemical reactor for the
production of
hydrogen from hydrocarbon fuels such as natural gas, propane, liquefied
petroleum gas,
alcohols, naphtha and other hydrocarbon fuels and having a unique unitized,
multifunctional
structure. The integrated reactor offers significant advantages such as lower
heat loss, lower
parts count, lower thermal mass, and greater safety than the many separate
components
employed in conventional systems to achieve the same end. The integrated
reactor is
especially well-suited to applications where less than 15,000 standard cubic
feet per hour of
hydrogen are required.
The present invention also relates to the generation of hydrogen for use in
industrial
applications, as a chemical feedstock, or as a fuel for stationary or mobile
power plants.
Discussion of the Back ound:
Hydrogen production from natural gas, propane, liquefied petroleum gas (LPG),
alcohols, naphtha and other hydrocarbon fuels is an important industrial
activity. Typical
industrial applications include feedstock for ammonia synthesis and other
chemical
processes, in the metals processing industry, for semiconductor manufacture
and in other
industrial applications, petroleum desulfurization, and hydrogen production
for the merchant
CA 02413388 2002-12-20
WO 02/02220 PCT/US01/16513
gas market. The demand for low-cost hydrogen at a smaller scale than produced
by
traditional industrial hydrogen generators has created a market for small-
scale hydrogen
production apparatus (< 15,000 standard cubic feet per hour (scfh)). This
demand has been
augmented by=the growing enthusiasm for hydrogen as a fuel for stationary and
mobile
powerplants, especially those employing electrochemical fuel cells, which
require hydrogen
as a fuel.
Hydrogen is typically produced from hydrocarbon fuels industrially via
chemical
reforming using combinations of steam reforming and partial oxidation. This is
typically
achieved at scales larger than one ton per day using well-known process and
catalyst designs.
For several reasons, it is difficult to adapt these. large-scale technologies
to economically
produce hydrogen at small scales. Typical industrial applications produce far
more than
15,000 standard cubic feet per hour(-1 ton per day), and often employ
catalytic steam
reforming of light hydrocarbons in radiantly-fired furnaces. Steam reforming
of
hydrocarbons is illustrated for the simple case of methane below.
CH4 + H20 -CO + 3Ha
The above reaction is highly endothermic, and the reacting fluid must have
energy
transferred to it for the reaction to proceed. Further, the extent of the
reaction is low at low
temperatures, such that greatly elevated temperatures, often as high as 800
C, are required by
conventional systems to convert an acceptable amount of hydrocarbon to
hydrogen and
carbon monoxide. The catalyst employed in industrial reactors is typically
composed of an
active nickel metal component supported on a ceramic support.
The radiantly-fired furnaces employed in large-scale industrial reactors have
many
disadvantages that make them unsuitable for small-scale systems. The most
important
disadvantage is the very high temperature of the radiant burners and the gas
contacting the
-2-
CA 02413388 2002-12-20
WO 02/02220 PCT/US01/16513
reactor surfaces, which are usually tubular in form. The temperature of the
radiant burners
often approaches or exceeds the melting temperature of the alloy from which
the tubes are
fabricated. Melting of the tubes is prevented by the rapid endothermic
catalytic reaction
inside the tubes. If, however, the catalyst fails due to carbon formation,
sulfur poisoning or
other causes, then the tubes form what is referred to in the literature as a
"hot spot," which
greatly accelerates the failure of the reactor tube in question. In large-
scale systems, careful
monitoring and control of the furnace and tube temperatures as well as
exceptionally rugged
construction of the tubes makes the risks of hot spots acceptable. For systems
producing
below 1 ton per day, however, the complexity and cost of such safety measures
can become
prohibitive. Nonetheless, small-scale steam reformers utilizing radiant heat
transfer are
known and described, for example, in U.S. 5,484,577 to Buswell, et al. The
extreme
measures necessary to control the temperature in arrays of reformer tubes are
likewise
documented in U.S. 5,470,360 to Sedercluist.
A means of transferring the necessary heat to the reacting gases without
radiant heat
transfer and its attendant risks, which is especially well-suited to small-
scale steam
reforming, is the use of compact heat exchange surfaces, such as arrays of
tubes or finned-
plates., The heat transfer mechanism in such devices is dominated by
convection and
conduction with minimal radiant transfer. An example of this approach is
described in U.S.
5,733,347 to Lesuir, wherein finned plates are employed to increase heat
transfer. Tubular
compact heat exchangers for steam reforming are sold by Haldor Topsoe, Inc. of
Houston,
TX.
Conventional hydrogen generation systems employing steam reforming of
hydrocarbon fuels typically include three main reaction steps for producing
hydrogen; steam
reforming, high-temperature water gas shift, and low temperature water gas
shift. The
-3-
CA 02413388 2002-12-20
WO 02/02220 PCT/US01/16513
important reactions for methane are as follows:
CH4 + HZO-> CO + 3H2 steam refonning
CO + H20 CO2 + H2 water gas shift
It is evident from the equation for steam reforming of hydrocarbon fuel that
the
principal products are hydrogen and carbon monoxide. The carbon monoxide may
be
converted into additional hydrogen via a catalytic reaction with steam (water
gas shift
reaction).
The water gas shift reaction is mildly exothermic and thus is
thermodynamically
favored at lower temperatures. However, the kinetics of the reaction are
superior at higher
temperatures. Thus, it is common practice to first cool the reformate product
from the steam
reformer in a heat exchanger to a temperature between 350 C and 500 C and
conduct the
reaction over a catalyst composed of finely divided oxides of iron and
chromium formed into
tablets. The resulting reformate gas is then cooled once again to a
temperature between
200 C and 250 C and reacted over a catalyst based upon mixed oxides of copper
and zinc.
An example of this approach is given in U.S. 5,360,679 to Buswell, et al. In
cases where an
exceptionally pure hydrogen product is required, the temperature of the low-
temperature shift
converter is controlled by including a heat exchanger in the reactor itself,
and an example of
this approach is given in U.S. 5,464,606 to Buswell, et al. In all cases, the
low temperature
shift converter is quite large because of the poor catalyst activity at low
temperatures.
In conventional systems, subsets of the process components are connected to
one
another via external plumbing; each component of the process being typically
referred to as a
"unit process," in the chemical engineering literature. This approach. is
preferred in large,
industrial units because standard hardware may be used. Owing to the large
size of industrial
-4-
CA 02413388 2002-12-20
WO 02/02220 PCT/US01/16513
units, the unit process approach also makes shipping of the components to the
site of the
installation feasible, as combinations of the components are sometimes too
large to be
transported by road or rail.
For systems producing less than 1 ton per day, however, the unit process
approach
has many disadvantages. The first disadvantage is the high proportion of the
total system
mass dedicated to the hardware and plumbing of the separate components. This
high mass
increases startup time, material cost, and system total mass, which is
undesirable for mobile
applications such as powerplants for vehicles.
Another disadvantage of the unit process approach in small systems is the
complexity
of the plumbing system to connect the components. - The complexity increases
the likelihood
of leaks in the final system, which presents a safety hazard, and also
significantly increases
the cost of the assembly process itself. Moreover, the requirement that each
component have
its own inlet and outlet provisions also adds considerable manufacturing cost
to the
components themselves.
A third disadvantage is the high surface area of the plumbing relative to the
unit
process hardware itself, which means that a disproportionately large amount of
heat is lost
through the connecting plumbing in small scale systems. This can drastically
reduce the
thermal efficiency of the system and adds cost and complexity associated with
adequately
insulating the plumbing system.
A fourth disadvantage to the unit process approach in small-scale systems is
that this
approach requires a large volume to package, as each component and its
associated plumbing
must be accessible for assembly and maintenance purposes. This is particularly
disadvantageous in space-sensitive applicatioiis such as building fuel cell
power stations, fuel
cell vehicle refueling stations, and fuel cell mobile powerplant hydrogen
generation.
-5-
CA 02413388 2002-12-20
WO 02/02220 PCT/US01/16513
Hydrogen is typically separated from the other reaction products using
pressure swing
adsorption (PSA) technology. The design of these PSA systems is largely
dictated by the
catalyst chemistry employed in the steam reformer and the low-temperature
water gas shift
reactor. These catalysts, typically based on nickel metal in the former and
copper in the latter
case, are extremely sensitive to poisoning and deactivation by sulfur or
molecular oxygen.
Thus, the incoming feed gas must be carefully treated to remove these
materials. Further, the
system must protect the catalysts against these agents during startup, shut-
down, and during
intervals when the system is shut down. Especially in the case of molecular
oxygen,
exposure of the active catalyst can lead to catalyst damage and even create a
safety hazard
through pyrophoric oxidation of the finely-divided base metal catalysts.
Several steps are necessary in conventional systems to prevent damage to the
refon-ning and Low Temperature Shift (LTS) catalysts.
(1) During operation, the incoming fuel must be tteated to remove both sulfur
and
molecular oxygen. Sulfur in particular is generally reduced below 1 part per
million, and
more preferably below 100 parts per billion. This is typically achieved
through a
combination of a partial oxidation to remove oxygen followed by a
hydrodesulfurization
(HDS) process. Such systems typically require recycle of high-temperature,
hydrogen-rich
product gas to the inlet through the use of a gas compressor or a fluid
ejector as exemplified
by U.S. 3,655,448 to Setzer, U.S. 4,976,747 to Szydlowski and Lesieur, and
U.S. 5,360,679
to Buswell, et al. Because accurate temperature control is required for the
HDS reaction,
several heat exchangers as well as active temperature control logic circuits
and flow control
valves are also required. Provision of these reactors, heat exchangers,
valves, as well as
sensors and controls adds significantly to the complexity of conventional
systems.
(2) Startup of conventional system requires bringing all of the components to
near
-6-
CA 02413388 2002-12-20
WO 02/02220 PCT/US01/16513
operating temperature, usually while blanketed in inert gas, then carefully
initiating the
reaction. Before the system is at operating conditions, full removal of sulfur
and molecular
oxygen is not guaranteed, so the process feed gas must be vented to the
atmosphere, wasting
fuel, generating air pollution, and creating a potential safety hazard while
further increasing
system complexity. Because the added components for fuel pretreatment add
significant
mass to the system, they also extend the warmup time required for hydrogen
production. In
situations with a variable hydrogen demand, this can create a need for
extensive onsite
hydrogen storage to supply the hydrogen demand while the system reaches
operating
conditions.
(3) During shutdown and periods'when the conventional system is not operating,
the
reaction system is typically purged with inert gases under pressure.
Alternatively,
substantially leak-tight valves must be supplied to prevent ingress of
atmospheric air to the
unit with the resultant catalyst deactivation/damage.
For large-scale applications the added cost/complexity of the conventional
systems
does not adversely affect the system economics. When this traditional approach
is applied to
small-scale systems, however, the relative cost of these added components
becomes
disproportionately large, and the resulting hydrogen cost is dominated by the
cost of the
system. Accordingly, it is not advantageous to simply scale down large scale
systems if a
small scale system is desired.
Conventional steam reformer systems for natural gas and other
light.hydrocarbons fall
into two broad classes. In the first, the reactors are operated at or near
ambient pressure at
low temperatures (typically less than 650 C). This is typical of conventional
systems
designed for small-scale applications producing impure hydrogen. For pure
hydrogen to be
produced, the reformer product must be compressed to high pressure for
subsequent cleanup
-7-
CA 02413388 2002-12-20
WO 02/02220 PCT/US01/16513
via PSA, metal separation membranes, or other conventional techniques. Because
steam
reforming creates additional moles of gas, the compression of the product gas
is very energy-
intensive and requires expensive and complicated compression and intercooling
equipment.
The second'class of reformers is typically used in large-scale applications
and is operated at
high pressures (often above 20 bar). Because of the thermodynamics of the
steam reforming
reaction, these high pressure reactors must be operated at much higher
temperatures, often
approaching 900 C, to attain adequate conversion of the hydrocarbon fuel to
hydrogen. The
higher temperatures and pressures require the use of more expensive materials
of
construction than are employed in the low-pressure systems, but this is more
than offset by
the reduction in reactor volume obtained due to enhanced chemical reaction
rates.
Unfortunately, in small-scale systems, the provision of compression and
pumping equipment
to deliver the reactants into a high-pressure (20 bar or higher) reactor can
undesirably
increase the cost of such a system.
Conventional pressurized steam reformer systems often are operated with very
high
temperatures in the combustion products used to heat the endothermic reaction
zone. This
high temperature allows a reduction in the amount of heat transfer area
required to complete
the reaction, and thus a reduction in reformer cost. Often, the mode of heat
transfer to the
wall of the tubes in the conventional reformers is a combination of radiation
and convection,
with the combustion carried out in a conventional premixed or diffusion-flame
burner. The
operation of the primary steam reformer with such high gas temperatures can
lead to
significant excursions in the reformer tube wall temperature due either to
poor control of the
distribution of the hot gases or to poisoning of the reforming catalyst. If
the catalyst for the
endothermic steam reforming reaction is locally-poisoned, the heat flux from
the combustion
products to the wall can form a local "hot spot." In either case, the increase
in the reformer
-8-
CA 02413388 2002-12-20
WO 02/02220 PCT/US01/16513
wall temperature can lead to premature reformer structural failure, presenting
both a safety
and an operational liability.
Conventional systems for hydrogen generation through steam reforming of
hydrocarbons have several inherent deficiencies which make them ill-suited to
economical
small-scale hydrogen production. The first is the requirement for strict
control of sulfur and
molecular oxygen concentrations in the steam reforming and LTS reactors. The
second
concerns the problems with operation in the ambient pressure regime where the
large volume
of reformate gas must subsequently be compressed prior to purification. The
third is
associated with operating the reactor in the high-pressure regime typical of
large-scale units
where appropriate compression and pumping equipment adds considerable cost at
small
scales. The final shortcoming is the risk of overheating the steam reforming
reactor structure
due to the very high gas temperatures employed in the combustors in
conventional systems
and their reliance on radiant heat transfer, especially in high-pressure
systems as employed in
large-scale applications.
It has been recognized previously that integrating the elements of the unit
process
more closely beneficially reduces heat losses and improves compactness. U.S.
5,516,344 to
Corrigan describes a steam reforming system wherein the unit process elements
are
integrated into a common mounting rack having a reduced requirement for
insulation and
having improved compactness. This approach, however, undesirably retains the
multiple
connections and extensive plumbing characteristic of the unit process
approach. Moreover,
because of its complicated packaging, the assembly of the Corrigan system
undesirably
presents a significant challenge.
Another attempt at improving compactness is described in U.S. 5,733,347 to
Lesieur,
wherein the primary reforming reactor and the catalytic burner are integrated
into a planar
-9-
CA 02413388 2002-12-20
WO 02/02220 PCT/US01/16513
reactor with. compact heat transfer surfaces. This reactor requires separate
heat exchangers to
cool the gas after the primary reformer, as well as separate reactors for the
water gas shift
reaction. These all require interconnections, as do the array of planar
reactors envisioned by
Lesieur. These connections once again present the same drawbacks found in unit
process
reactor systems.
SUMMARY OF THE INVENTION
Accordingly, one object of the present invention is to provide a reactor for
hydrogen
production that avoids the problems associated with conventional systems.
Another object of the present invention is to provide a reactor for hydrogen
production that is suitable for applications where less than 15,000 standard
cubic feet per
hour of hydrogen are required.
Another object of the present invention is to provide a reactor for hydrogen
production that is safer and more cost efficient than conventional systems.
Another object of the present invention is to provide a reactor for hydrogen
production that is less complex and is more space-sensitive than conventional
systems.
Another object of the present invention is to provide for the production of
hydrogen
from a hydrocarbon fuel such as natural gas, propane, naphtha, or other
hydrocarbons low in
sulfur content (< 100 ppm sulfur by mass).
Another object. of the present invention is to produce hydrogen which is
substantially
pure (>99.99%) by separating impurities using a pressure swing adsorption
(PSA) system.
Another object of the present invention is to provide for the elimination of
the
pretreatment of the fuel feed to the steam reformer for the removal of sulfur
and molecular
oxygen.
-10-
CA 02413388 2002-12-20
WO 02/02220 PCT/US01/16513
Another object of the present invention is the provide for the operation of
the system
in a mesobaric regime, between 4 and 18 atmospheres, where appropriate fluid
compression
devices of small capacity, low cost, high efficiency and high reliability are
readily available,
and the resultant thermal efficiency of the hydrogen production system is very
high.
Another object of the present invention is to provide for the feedback control
of the
delivery of fuel and/or air to a catalytic combustor in proportions such that
the peak
temperature of the gases entering the primary steam reformer does not exceed a
safe
maximum temperature determined by the metallurgy of the steam reformer.
Another object of the present invention is to provide for the operation of a
steam
reforming system without a low temperature water gas shift reactor.
Another object of the present invention is to provide for the operation of a
hydrogen
production system with feedback control of product carbon monoxide content.
Another object of the present invention is to provide a process having a
simplified
system construction, operation, and control resulting in low cost and
relatively fast start-up
and shut-down.
These and other objects have been achieved by the present invention, the first
embodiment of which provides a reactor, which includes:
a unitary shell assembly having an inlet and an outlet;
a flow path extending within the shell assembly from the inlet to the outlet,
the flow
path having a steam reformer section with a first catalyst and a water gas
shift reactor section
with a second catalyst, the steam reformer section being located upstream of
the water gas
shift reactor section;
a heating section within the shell assembly and configured to heat the steam
reformer
section; and
-11-
CA 02413388 2002-12-20
WO 02/02220 PCT/US01/16513
a cooling section within the shell assembly and configured to cool the water
gas shift
reactor section.
Another embodiment of the present invention provides a reactor for the
production of
hydrogen from at least one selected from the group including natural gas,
propane, liquefied
petroleum gas, alcohols, naphtha, hydrocarbon fuels and mixtures thereof, the
reactor
including:
a unitary shell assembly having an inlet and an outlet;
a flow path extending within the shell assembly from the inlet to the outlet,
the flow
path including a convectively-heated catalytic steam reformer and a
convectively-cooled
water gas shift reactor.
Another embodiment of the present invention provides a method for producing
hydrogen, which includes:
feeding at least one fuel selected from the group including natural gas,
propane,
liquefied petroleum gas, alcohols, naphtha, hydrocarbon fuels and mixtures
thereof, into a
reactor which includes a unitary shell assembly having an inlet and an outlet,
and a flow path
extending within the shell assembly from the inlet to the outlet, the flow
path including a
convectively-heated catalytic steam reformer and a convectively-cooled water
gas shift
reactor, whereby hydrogen is produced.
Another embodiment of thepresent invention provides a method for producing
hydrogen from at least one fuel selected from the group including hydrocarbon
fuel, natural
gas, propane, naphtha, hydrocarbons with < 100 ppm sulfur by mass, and
mixtures thereof,
which includes:
producing hydrogen by steam reforming the fuel; and
substantially purifying said hydrogen with a pressure swing adsorption (PSA)
system;
-12-
CA 02413388 2008-05-26
wherein prior to the producing, no pretreatment of the fuel to remove at least
one impurity
selected from the group including sulfur and molecular oxygen and mixtures
thereof is
carried out.
Another embodiment of the present invention provides a simplified hydrogen
production process, which includes the catalytic steam reforming and
subsequent high
temperature water gas shift of low-sulfur (< 100ppm by mass) hydrocarbon fuels
followed by
hydrogen purification through the pressure swing adsorption (PSA).
Another embodiment of the present invention provides an improved system for
hydrogen generation through mesobaric (4-18 bar) steam reforming of natural
gas, propane,
naphtha and other low-sulfur hydrocarbon feedstocks.
According to a further broad aspect of the present invention, there is
provided a
reactor comprising; a unitary shell assembly having an inlet and an outlet; a
flow path
extending within said shell assembly from said inlet to said outlet, said flow
path having a
steam reformer section with a first catalyst and a water gas shift reactor
section with a second
catalyst, said steam reformer section being located upstream of said water gas
shift reactor
section; a heating section within said shell assembly and configured to heat
said steam
reformer section; and a cooling section within said shell assembly and
configured to cool said
water gas shift reactor section; wherein said first catalyst comprises a steam
reforming
catalyst; and wherein said second catalyst comprises a water-gas shift
catalyst.
According to a further broad aspect of the present invention, there is
provided a
reactor for the production of hydrogen from at least one selected from the
group consisting of
natural gas, propane, liquefied petroleum gas, alcohols, naphtha, hydrocarbon
fuels and
mixtures thereof, said reactor comprising: a unitary shell assembly having an
inlet and an
outlet; a flow path extending within said shell assembly from said inlet to
said outlet, said
flow path including a convectively-heated catalytic steam reformer and a
convectively-cooled
water gas shift reactor; wherein the steam reformer comprises a steam
reforming catalyst;
wherein the water-gas shift reactor comprises a water-gas shift catalyst; and
wherein the
steam reformer is upstream of the water-gas shift reactor.
According to a still further broad aspect of the present invention, there is
provided a
method for producing hydrogen, comprising the step of: feeding at least one
fuel selected
from the group consisting of natural gas, propane, liquefied petroleum gas,
alcohols, naphtha,
13-
CA 02413388 2008-05-26
hydrocarbon fuels and mixtures thereof, into a reactor comprising a unitary
shell assembly
having an inlet and an outlet, and a flow path extending within the shell
assembly from the
inlet to the outlet, the flow path comprising a convectively-heated catalytic
steam reformer
and a convectively-cooled water gas shift reactor, whereby hydrogen is
produced; wherein
the steam reformer comprises a steam refonning catalyst; wherein the water-gas
shift reactor
comprises a water-gas shift catalyst; and wherein the steam reformer is
upstream of the
water-gas shift reactor.
According to a still further broad aspect of the present invention, there is
provided a
method for producing hydrogen from a feedstock comprising water and at least
one fuel
selected from the group consisting of hydrocarbon fuel, natural gas, propane,
naphtha,
hydrocarbons with <100 ppm sulfur by mass, and mixtures thereof, comprising:
in a reactor,
producing hydrogen by steam reforming said fuel; and purifying said hydrogen
with a
pressure swing adsorption (PSA) system; wherein prior to said producing, no
pretreatment of
said fuel to remove at least one impurity selected from the group consisting
of sulfur and
molecular oxygen and mixtures thereof is carried out; wherein a molar ratio of
water
molecules to carbon in said feedstock ranges from 3:1 to 8:1; and wherein said
reactor
comprises: a unitary shell assembly having an inlet and an outlet; a flow path
extending
within said shell assembly from said inlet to said outlet, said flow path
having a steam
reformer section with a first catalyst and a water gas shift reactor section
with a second
catalyst, said steam reformer section being located upstream of said water gas
shift reactor
section; a heating section within said shell assembly and configured to heat
said steam
reformer section; and a cooling section within said shell assembly and
configured to cool said
water gas shift reactor section; wherein said first catalyst comprises a steam
reforming
catalyst; and wherein said second catalyst comprises a water-gas shift
catalyst.
BRIEF DESCRIPTION OF THE FIGURES
Various other objects, features and attendant advantages of the present
invention will
be more fully appreciated as the same becomes better understood from the
following detailed
description when considered in connection with the accompanying drawings in
which like
reference characters designate like or corresponding parts throughout the
several views and
wherein:
- 13a -
CA 02413388 2008-05-26
Figures la and lb are schematics of two preferred embodiments of the reactor
flow
geometry on both the tube and shell sides. Figure lb differs from Figure la in
that it has an
integral catalytic burner. Figure 2 shows a preferred embodiment of the
reactor of the present invention without
an internal catalytic burner and without extended surfaces on the tubes in the
tubular array,
which is provided with baffles to create a multi-pass cross-flow geometry in
the shell side
fluid pathway.
13b -
CA 02413388 2002-12-20
WO 02/02220 PCT/US01/16513
Figure 3 shows a preferred embodimerit of the reactor of the present invention
with
plate fin heat exchange surfaces attached to the tubes on the shell side and
an adiabatic water
gas shift reactor zone placed after the convectively cooled water gas shift
reactor zone. This
figure also illustrates the preferred combination of extended tube surfaces
and baffles.
Figure 4 shows a preferred embodiment of the reactor of the present invention
with
baffles, and with shell side extended surface comprising loose packing
material. This figure
also shows one manifestation of a catalytic burner included within the reactor
shell. Figure 4
also 'depicts an outer housing and insulation system.
Figure 5 is a schematic of the hydrogen production system of a preferred
embodiment
of the present invention.
Figure 6 is a logic diagram for a preferred combustor outlet temperature
control
apparatus of the present invention.
Figure 7 is a logic diagram for a preferred gas purity control apparatus of
the present
invention.
Figure 8 illustrates a computer system upon which a preferred embodiment of
the
present invention may be implemented.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Various other objects, features and attendant advantages of the present
invention will
be more fully appreciated as the same becomes better understood from the
following detailed
description of the preferred embodiments of the invention.
Preferably, according to one embodiment of the present invention, an integral
reactor
for the production of hydrogen from natural gas, propane, liquefied petroleum
gas, alcohols,
naphtha and other hydrocarbon fuels and mixtures thereof is provided where
several
-14-
CA 02413388 2002-12-20
WO 02/02220 PCT/US01/16513
components of the process system are combined into a single mechanical
structure. These
components will preferably include a convectively-heated catalytic steam
reformer, a cooler
for the reformate product from the steam reformer and a convectively-cooled
water gas shift
reactor. The reactor niay additionally and optionally include a preheat
section to heat the inlet
feeds. The packing of this preheat section may additionally and optionally
serve as a sulfur
absorbent bed. The reactor may additionally and optionally include an
adiabatic water gas
shift reactor appended to the exit of the convectively cooled water gas shift
reactor.
. Preferably, the reactor of present invention includes a tubular array
wherein the fuel
and water to be reformed flow through the tubes, and the cooling and heating
fluids flow
outside the tubes, with a single reforming side inlet tube header and a single
reforming side
outlet tube header. The interior of these tubes is preferably provided with a
catalyst in the
form of a coating, a monolith, or as a loose packing of pellets, extrudates or
the like.
Preferably, the-reactor also. includes a shell assembly, with a means of
thermal expansion
relief, one or more inlets for a cooling medium for the water gas shift
reactor, and one or
more outlets for the hot combustion product. The reactor shell assembly may
additionally
and optionally have one or more outlets for heated coolant for the water gas
shift reactor and
one or more inlets for the hot combustion product to heat the steam reforming
reactor.
Preferably, the reactor tube array surface area may be enhanced on the shell
side of
the tubes for the purposes of aiding heat transfer between the shell side
fluid and the tube
walls. The surface augmentation may be accomplished through the use of twisted
tubes,
finned tubes, rifled tubes, plate fins, by means of a loose packing material,
or by other means
apparent to one skilled.in the art.
Another preferred embodiment of the invention provides that the fluids flowing
outside the tubes in the shell side may be forced to flow across the tubular
array, substantially
-15-
CA 02413388 2002-12-20
WO 02/02220 PCT/US01/16513
normal to the axis of the tubes, by baffles. These baffles may be employed
with or without
the surface area enhancements which are another embodiment of the present
invention.
Another preferred embodiment of the present invention provides that a
catalytic
burner may be incorporated in the shell side of the reactor assembly. This
catalytic burner
may be provided with one or more inlets for fuel delivery. This'buriner may
also''be provided
with a means of mixing the fuel and heated air. This burner may also be
provided with a
means of preheat and/or ignition. This burner may also be provided with one or
more
temperature sensors.
Preferably, the steam reforming catalyst is resistant to poisoning by sulfur
and
molecular oxygen.
Preferably, the water gas shift catalyst is resistant to poisoning by sulfur.
Preferably, the present invention is carried out without pretreatment of the
fuel feed to
the steam reformer for the removal of sulfur and molecular oxygen.
Pretreatments which are
preferably excluded from the present invention include any or all of partial
oxidation,
hydrodesulfurization, adsorption, or absorption. Other such pretreatment
methods known to
one of ordinary skill in the art are preferably excluded as well.
The system according to the invention preferably employs improved steam
reforming
catalyst and a simplified water gas shift system in order to eliminate the
requirement for
oxygen and sulfur removal upstream of the reforming system required in the
conventional
systems.
Preferably, the reactor includes an outer housing and insulation assembly.
Referring to Figure 1 a, one embodiment of the overall flow geometry of the
reactor of
the present invention is provided with an inlet on the tube side for entry of
vaporized, mixed
water and fuel, which flow through a first region packed with steam reforming
catalyst,
-16-
CA 02413388 2002-12-20
WO 02/02220 PCT/US01/16513
where catalytic steam reforming takes place, and a second region packed with
water gas shift
catalyst, where the water gas shift reaction takes place, after which the
reformed gases exit
the reactor. A second fluid stream enters the shell side near the outlet of
the tube side, and
flows generally in counterflow to the reformate flowing through the tube side.
This second
fluid stream is lower in temperature than the exiting reformate, and it
removes heat from the
water gas shift portion of the tube side of the reactor. In the embodiment of
Figure 1 a, the
heated air then exits the shell side through an outlet port and is conveyed to
an external
catalytic combustor, where the heated air is mixed with one or more fuel
streams and
combusted over a catalyst or in a conventional burner. The hot combustion
product is then
returned to the shell side of the reactor, where the bot combustion product
convectively heats
the lower temperature reformate in the steam reformer section of the tube
side.
Another preferred embodiment of the overall flow geometry of the present
invention
is shown in Figure lb, which differs from that of Figure 1 a in that the
catalytic combustor is
located within the shell side of the reactor. In the embodiment of Figure lb,
the fuel for the
combustor is introduced into the shell-side fluid flow, and the fuel-air
mixture is combusted
on a catalyst, which is located inside the reactor shell and intimately in
contact with the
reactor tube walls.
Referring to Figure 2, the preferred reactor of the present invention has an
inlet for
mixed, pre-vaporized fuel and steam 1, which communicates with a plenum 2,
which
distributes the mixture to the array of reactor tubes 3. These reactor tubes
are mounted to the
inlet tube header 4 by welding, brazing, swaging or other processes capable of
creating a
leak-tight joint in the materials of construction. Most preferably, the
reactor tubes 3 are
joined to the inlet header 4 by brazing or welding. The reactor tubes are
provided, as is
illustrated in the cut-away view of Figure 2, with a charge of steam reforming
catalyst
-17-
CA 02413388 2002-12-20
WO 02/02220 PCT/US01/16513
material 5. This catalyst material 5 may be a loose packing as illustrated, or
may be a
catalytic coating, or may be a section of monolithically-supported catalyst.
Such coated,
packed bed, or monolithic catalyst systems are well known to those skilled in
the art. The
reactor tubes are also provided with a water gas shift catalyst 50, which is
located
downstream from the steam reforming catalyst, 5. The tubes 3 are further
joined to an outlet
tube header 6 by processes similar to those for attaching the tubes to the
inlet header 4. The
outlet tube header 6 communicates with an outlet plenum 7, which delivers the
reformate
product to an outlet port 8. The reactor tubes 3 pass through holes in one or
more baffles 9,
which share the same geometrical pattern of holes as the inlet and outlet
headers 4 and 6.
The spacing between these baffles is governed by the allowable pressure drop
and required
heat transfer rate on the shell side of the reactor. The baffle spacing may be
different in
various portions of the reactor. The baffles 9 shown in Figure 2 are chorded
to allow fluid to
flow.around the end of the baffle and along the tube axis through a percentage
of the cross-
sectional area of the shell. The baffles are chorded between 50% and 10%; more
preferably
they are chorded between 40% and 15%, most preferably they are chorded between
30% and
20%. The direction of the chorded side alternates by 180 degrees such that
fluid is forced to
flow substantially perpendicular to the long axis of the tubes 3. Alternative
baffle designs are
apparent to one skilled in the art and are included within the scope of the
present invention.
Preferred examples of alternative baffle designs include baffles chorded in
more than one
location, circular baffles of alternating ring and circle shapes, and wedge-
shaped baffles.
The baffles fit with a close tolerance to allow a sliding fit to the shell
assembly 10.
The shell assembly is secured to either one or both of the inlet and outlet
headers by welding,
brazing, swaging, or other methods which are apparent to one skilled in the
art. The close
tolerance fit between the baffles and the shell is chosen such that the
baffles will not bind
-18-
CA 02413388 2002-12-20
WO 02/02220 PCT/US01/16513
against the shell wall during assembly and operation while still minimizing
leakage between
the baffles and the shell wall. If the shell 10 is rigidly secured to both
headers it is especially
preferable to provide a means for relative thermal expansion and contraction
between the
reactor tubes and the shell to occur without undue restraint. In Figure 2
thermal expansion is
provided for by a corrugated tube or bellows 11. If the shell is fixed to only
one header,
relative expansion may be provided for with a sliding fit and seal system
between the shell
bore and the outer surface of the other reactor header to minimize leakage
while allowing
free thermal expansion of the tubular array. Other means of providing free
thermal
expansion of the tube array will be apparent to one skilled in the art, and
are included within
the scope of the present invention.
The reactor of Figure 2 employs the overall flow geometry of Figure 1 a, and
is thus
provided in the shell-side of the water gas shift section with a cold air
inlet 12 as well as a hot
air outlet 13. Most of the shell-side air is prevented from bypassing the hot
air outlet 13 by
an unchorded baffle 14, which fits snugly against the shell assembly 10 inner
wall. The
reactor is further provided in the shell side of the steam reforming section
with a hot
combustion product inlet 15 and a cooled combustion product outlet 16. The
inlets and
outlets are depicted as single tube sections in Figure 2, but it must be
understood that other
inlet and outlet types are possible, including ring manifolds and multiple
tube fittings. Such
alternative embodiments may be advantageously employed to reduce thermal
stresses in the
tubes, to modify heat transfer characteristics, or for other purposes apparent
to one skilled in
the art.
The reactor of Figure 2 is provided with an external burner assembly 18. In
the
embodiment of Figure 2 this burner assembly is a catalytic burner with
catalyst zone 22, and
it will be understood that alternative burner designs are known in the art
which employ
-19-
CA 02413388 2002-12-20
WO 02/02220 PCT/US01/16513
premixed or diffusion burning or combinations thereof. Other preferable types
of burners are
also apparent to one skilled in the art, and it is intended that the choice of
external burner
shall not limit the reactor of the present invention. The external burner
assembly 18 is
provided with at least one fuel injection port 19. The air inlet may
additionally be provided
with at least one air preheater element 23. This preheater element may
alternatively be
replaced or augmented with a pilot light, a spark ignitor, or an electrically
heated catalyst.
These and other modifications to the burner assembly are apparent to one
skilled in the art.
The tubes 3 are preferably filled with at least two catalyst systems. In the
steam
reforming zone, a catalyst 5 active for steam reforming is used, while in the
water gas shift
zone a catalyst 50 active for water gas shift but substantially inactive for
methanation is
employed. These catalyst systems may be in the form of surface coatings, a
packed bed of
loose particles, or as a monolithically-supported catalyst of the shape of the
inside of the
tubes. Most preferably the catalyst is either coated or is in the form of a
packed bed. In
Figure 2, the catalyst is a packed bed of loose particles retained between the
inlet and outlet
headers by catalyst support screens 17. Prior to the steam reforming zone, a
zone of
chemically inert packing may be provided as a heat transfer media only. In
this
configuration, the reactants may be preheated in order to bring their
temperature to a level
where the catalytic steam reforming reaction occurs at a meaningful rate. A
preferred
embodiment of the present invention replaces the inert packing in this preheat
zone with a
sulfur absorbent such as zinc oxide. This sulfur absorbent can serve as a
guardbed to protect
the steam reforming catalyst from poisoning by sulfur.
In a preferred embodiment, the steam reforming catalyst is capable of
operation in the
presence of less than 100 ppm of sulfur by mass in the fuel feed and is
insensitive to the
presence of molecular oxygen in the fuel feed. More preferably, the catalyst
is capable of
-20-
CA 02413388 2008-05-26
being shut down from operation and restarted without the use of reducing or
inert gas. Most
preferably, the catalyst active metal is chosen from one or more of those in
group VIIIB of
the periodic table. Examples of the preferred metals are ruthenium, rhodium,
iridium,
platinum and palladium. These metals are preferably supported on a ceramic
support of high
surface area. Preferred examples of supports are oxides of aluminum, zirconium
and
magnesium, as well as mixed oxide spinels such as calcium aluminate, nickel
aluminate or
magnesium aluminate. Other ceramic supports will be apparent to one skilled in
the art and
are included in the scope of the present invention.
In a preferred embodiment, the water gas shift catalyst is capable of
operation in the
presence of less than 100 ppm of sulfur by mass in the fuel feed and can be
started in the
presence of partially reacted mixtures of fuel and water, i.e. reformate from
the steam
reforming reactor. The catalyst also preferably does not require inert gas for
shutdown. An
example of a preferred catalyst is a finely divided mixture of oxides of iron
and chromium,
marketed as high temperature water gas shift, or "ferrochrome" catalyst. A
second example of
a preferred catalyst includes platinum supported on aluminum oxide, with or
without
promotion by oxides of cerium or other metal oxides.
Referring to Figure 3, a preferred embodiment of the reactor of the present
invention
is depicted which employs both baffles 9 as in Figure 2 as well as extended
heat exchange
surfaces on the outer walls of the reactor tubes 3. In this case, a plurality
of closely-spaced
plate fins 20 are provided. These fins may be bonded to the reactor tube by
brazing, or more
preferably by hydraulically expanding the tubes 3 into close contact with the
plate fins 20.
The plate fins, like the baffles 9, also have a pattern of holes which is
identical to that in the
inlet and outlet headers.
Figure 3 also shows an adiabatic water gas shift reactor 21 appended to the
outlet tube
-21-
CA 02413388 2002-12-20
WO 02/02220 PCT/US01/16513
header 6. This reactor increases the volume of catalyst accommodated without
increasing the
usage of the expensive metal alloy reactor tubing. The additional catalyst
volume can be
used to better approach the equilibrium conversion of the water gas shift
reaction at the outlet
temperature conditions. This is typically desired when the outlet temperature
of the
reformate from the water gas shift reactor is below 400 C.
Figure 4 shows a preferred embodiment of the reactor of the present invention
wherein the overall flow geometry is that of Figure lb. In this embodiment the
shell side is
packed with a loose packing material to provide extended surface area for the
tubes 3. It
should be understood that other types of extended surfaces are possible, such
as finned tubes,
rifled tubes, twisted tubes, and combinations thereof. All of these eliminate
the possibility of
using baffles in conjunction with the extended surface area, whereas plate
fins and loose
packing do not. It should be noted that in the embodiment of Figure 4 baffles
are not
employed, and the overall flow is substantially parallel to the axis of the
tubes 3. The
embodiment of Figure 4 also includes a catalytic burner integrated within the
reactor shell.
The catalytic burning is accomplished by a zone of packing 29 which is
catalyzed with an
appropriate combustion catalyst, such as mixtures of palladium and platinum
supported on a
ceramic support. The size of this catalyzed zone is chosen to meet the
requirements of the
specific application, and may fill the entire reactor shell above the
unchorded baffle 14.
Alternatively, the unchorded baffle 14 may be omitted, and the heated air from
the water gas
shift zone may proceed directly to the steam reforming zone where catalytic
combustion will
occur.
Near the shell side inlet 15 to the catalytic combustion zone there is a fuel
distribution
assembly 24, which allows fuel to be introduced into the catalytic combustion
zone.
Alternatively, and preferably, the combustion zone may be provided.with more
than one fuel
-22-
CA 02413388 2002-12-20
WO 02/02220 PCT/US01/16513
distribution assembly 24, which may be employed to control the temperature
profile in the
combustion zone. It should be understood that any number of configurations for
this fuel
distribution assembly are possible and may be employed in the reactor of the
present
invention. The one or more fuel distribution assemblies 24 are provided with a
fuel feed
controller 25, and the reactor is provided with at least one temperature
sensor 26 to be used in
control of the combustion temperature. The temperature sensor 26 is
illustrated as located at
the shell of the reactor, but may be alternatively located in a thermowell
located in a reaction
tube or at other locations within the reactor. The intimate contact
between,:the catalyst and
the reactor tubes allows very good heat transfer, but makes temperature
control more difficult
as well. Another preferred embodiment of the present invention replaces the
loose, catalyzed
packing with a catalyzed monolith provided with a pattern of holes which are
larger than the
reactor tube outer diameters. This type of monolithic combustion catalyst,
because it is
aligned by the shell assembly, and does not contact the reactor tubes, will
pose less of a
danger to overheating the reactor tubes and causing the formation of hot
spots. Even if the
monolithic combustion support did contact the tube walls locally, hot spotting
would be less
likely as combustion is distributed throughout the monolith volume, rather
than being
localized only at the tube wall.
Figure 4 also depicts an outer housing 27 that can be constructed to extend
over the
entire outer surface of the insulation system 28 or a portion thereof, and
which is only
depicted over a small segment of the insulation system 28. The insulation
system may be
constructed from one or more layers of insulation materials, and may be either
rigid or
flexible. The precise amount and type of insulation employed is dependent upon
a variety of
factors such as allowable heat loss, maximum surface temperature of the outer
housing 27,
and the mode of structural support for the reactor. These variables do not
materially affect
-23-
CA 02413388 2002-12-20
WO 02/02220 PCT/US01/16513
the performance advantages of the reactor of the present invention, and any
number of
possible insulation configurations are considered within the scope of the
present invention.
The outer housing 27 also does not materially affect the operation of the
reactor and is
designed instead based upon factors ~uch as structural requirements,
environmental
conditions, and aesthetics. Thus, any number of configurations for the outer
housing are
considered within the scope of the present invention.
Several surprising and unexpected advantages of the reactor of the present
invention
are apparent when it is compared to the conventional systems.
The first advantage is the great simplification in the construction of the
reactor system
afforded by combining the steam reforming and water gas shift reactors and
their associated
heat transfer functionalities into a single mechanical device. This eliminates
the requirement
for separate inlet and outlet zones, fittings, and interconnecting plumbing.
This advantage is
even further evidenced in systems incorporating the feed preheat function
and/or the internal
catalytic burner. 'In small hydrogen generation applications (< 15,000 scf/hr
or 1 ton per
day), this reduction in physical components and interconnects can greatly
reduce the cost of
the completed system.
A second advantage is the great reduction in heat loss achieved by the reactor
of the
present invention when compared to the unit process approach of the
conventional systems.
In part because the number of fittings and interconnecting plumbing is
decreased in the novel
reactor of the present invention, the amount of heat transfer surface with the
ambient
environment is greatly diminished. Consequently, the heat lost to the ambient
environment is
desirably proportionally reduced. This heat loss can otherwise undesirably
form a large
energy requirement in conventional small-scale, hydrogen-generating reactors.
The
reduction in heat loss achieved by the present invention leads to a higher
energy efficiency of
-24-
CA 02413388 2002-12-20
WO 02/02220 PCT/US01/16513
the reactor and a faster warmup time. Additionally, the low heat loss of the
reactor of the
present invention allows it to be maintained in a hot condition for extended
periods of time
without generating hydrogen and without consuming much fuel, which desirably
makes "hot
standby" of the novel reactor more practical than in conventional reactors.
A third advantage of the reactor of the present invention is its ability to
start up from a
cold condition more rapidly than conventional reactors. This is believed to be
due to both the
lower structural weight of the reactor and its lower heat loss when compared
to conventional
discrete reactors and heat exchangers. The rapid warmup capability allows the
reactor of the
present invention to be operated intermittently without excessive penalties in
warmup time or
warmup fuel usage.
Preferably, the present invention is carried out without pretreatment of the
fuel feed to
the steam reformer for the removal of sulfur and molecular oxygen.
Pretreatments which are
preferably excluded from the present invention include any or all of partial
oxidation,
hydrodesulfurization, adsorption, or absorption. Other such pretreatment
methods known to
one of ordinary skill in the art are preferably excluded as well.
Low-pressure water and hydrocarbon fuel are admitted to separate or combined
fluid
compression devices; and they are subsequently heated to their vaporization
points and
admitted to a primary steam reforming reactor. This steam reforming reactor is
provided -
with a catalyst which is resistant to poisoning by both sulfur and molecular
oxygen, and is
preferably based upon catalytically-active group VIIIB metals such as
ruthenium, rhodium,
iridium, platinum, palladium or combinations thereof supported on a ceramic
support of high
surface area. In this primary steam reforming reactor, the vaporized fuel and
steam are
further heated by a separate stream of hot combustion product which is
separated from the
reactants by the walls of the reactor, which also form heat exchange surfaces.
These heated
-25-
CA 02413388 2002-12-20
WO 02/02220 PCT/US01/16513
gases are then encouraged to react by the aforementioned catalyst to form a
hydrogen-rich
product gas with a composition near its equilibrium value at the reactor
outlet conditions.
This hydrogen-rich gas is then cooled and passed over a second catalyst which
is also sulfur
resistant, and is active for the water gas shift reaction while being
substantially-inactive for
the reverse of the steam reforming reaction, the methanation reaction. An
example of such a
catalyst is a finely divided mixture of oxides of iron and chromium, which is
well-known in
the art as "high temperature" water gas shift or "ferrochrome" catalyst. The
hydrogen rich
gas stream has much of its carbon monoxide converted to carbon dioxide and
hydrogen in the
water gas shift reactor, and exits with a carbon monoxide concentration
between 0.3% and
4%, at a temperature above 200 C.
In conventional systems, a further low-temperature water gas shift reactor is
provided,
whereas in the system of the present invention no such reactor is provided, as
an active,
sulfur-tolerant catalyst operable at such low temperatures is not easily made.
The product
gas mixture is then further cooled either by heat exchange with ambient air or
cool water or
by quenching with cool water in an evaporative cooler. Condensed water is then
removed
from the gas via a separator, and the thus partially-dried gas mixture is
admitted to the PSA
purification system. In the PSA system, impurities are adsorbed from the gas
while the
product hydrogen is delivered at a high purity and at an elevated pressure
(slightly below the
steam reformer pressure). The impurities are then purged with a small portion
of the
hydrogen product at low pressure and are delivered as a fuel to a catalytic
combustor, which
is provided with an exit temperature sensor and a means of controlling the
rate of admission
of air. The rate of air admission is thus controlled such that the exit
temperature from the
combustor is below the maximum allowable temperature of the reformer
metallurgy. This
hot combustion product is then ducted to a heat transfer interface in the
steam reformer to
-26-
CA 02413388 2002-12-20
WO 02/02220 PCT/US01/16513
provide heat for the endothermic reaction therein to proceed. The combustion
product, at a
reduced temperature, may then be used to heat and vaporize the pressurized
water and, if
desired, fuel streams.
As described below, the rate of air admission is controlled by a feedback loop
based
upon the outlet temperature of the catalytic combustor. The calorific value of
the low-
pressure mixed fuel gas expelled from the PSA system is determined by the
degree of
hydrogen purity required. A feedback control system based upon a product
carbon monoxide
sensor based on either infrared or electrochemical principles will be used to
set the rate at
which the PSA system purges itself of contaminants. When high purity is
desired, a high
purge rate is employed and the calorific value of the low-pressure gases is
high. When less
stringent purity is required, the rate of purging may be lower, and the
calorific value of the
purged gases may be correspondingly lower. Indeed, the purge rate may be
reduced to a
point where the calorific value of the purge gas is too low to sustain the
reactor temperature,
at which point unreacted hydrocarbon fuel may be provided from a valve to make
up the
deficit. Whereas the carbon monoxide concentration is of special significance
for fuel cell
applications, in other applications it is understood that another impurity may
be more critical,
and feedback based upon concentrations of that species may accordingly be
employed.
Referring to Figure 5, the hydrogen production system of the present invention
can
process hydrocarbon fuels such as natural gas, town gas, refinery off-gas,
propane, liquefied
petroleum gas, naphtha, alcohols or any other hydrocarbon fuel with a sulfur
content less
than 100 parts per million (ppm) by mass. Natural gas or liquified petroleum
gas are
preferred. More preferably, the sulfur content is less than 75 ppm, most
preferably, the sulfur
content of the fuel is less than 50 ppm. The second feed to the system is
water, which is
subsequently chemically reacted with the fuel to yield hydrogen. This water
feed must be
-27-
CA 02413388 2002-12-20
WO 02/02220 PCT/US01/16513
conditioned to remove particles, organics, and ionized species. This may be
achieved using
methods apparent to one skilled in the art. The molar ratio of the water to
the fuel is such
that the ratio of water molecules to carbon molecules is between 2.5:1 and
8:1. More
preferably, the ratio is between 3:1 and 5:1.
The water feed to the system is pressurized using an appropriate pump 66 to a
pressure greater than the operating pressure of the system, which is
preferably 4 atm to 18
atm. The pressurized water is then admitted to a heat exchanger 84 where it is
heated by a
second fluid, which is the cooled combustion product exhausted from the steam
reforming
reactor hot side 60. It must be understood that this heat exchanger may
include more than
one individual unit, and that alternative strategies may be employed to heat
the feed water
such as by removing heat from the hot hydrogen-containing gas exiting the
water gas shift
reactor 62, or from other high temperature streams in the system. Irrespective
of the exact
arrangement of the heat exchange means, sufficient energy is transferred to
the water to cause
it to vaporize and allow it to be mixed with the fuel at 56.
The fuel is pressurized using compressor 54. This device may be a pump if the
fuel is
a liquid, and may also be replaced and/or augmented by a steam ejector
employing pressure
energy stored in the vaporized water to pressurize the fuel. The fuel is mixed
with the
vaporized water at 56. The resulting pressure of the mixed fuel and water
preferably exceeds
that of the steam reforming reactor 58, which is between 4 atm and 18 atm.
This requires that
sufficient energy be imparted to one or more of the fluids to maintain the
resulting mixture in
the vapor phase at the steam reforming reactor inlet. This may require the
addition of an
evaporator for a liquid fuel, or may be achieved through superheat of the
vaporized water.
The steam reforming reactor includes a high pressure, cold side 58 wherein is
disposed a quantity of catalytically active material as well as a lower
pressure, hot side 60.
-28-
CA 02413388 2008-05-26
The mixed, vaporized fuel and water enter the cold side 58 and are heated by
the hot
combustion product which flows through the hot side 60. These fluids are
prevented from
mixing by the shared structure of the reactor, which fonns a heat exchange
surface, or a
plurality of heat exchange surfaces. The pressure of the fluid in the cold
side of the reactor is
between 4 atm and18 atm. More preferably, the pressure is between 5 atm and 15
atm. Most
preferably, the pressure is between 10 atm and 15 atm. The catalyst disposed
in the cold side
of the reactor is resistant to both the adsorption of sulfur compounds and
oxidation by both
steam and molecular oxygen. The catalyst preferably includes an active metal
or mixture
thereof supported upon a ceramic support material of high surface area.
Preferably the
catalyst active metal or metals is selected from the group VIIIB metals of the
periodic table.
Most preferably the catalyst active metal includes one or more of the
following group VIIIB
metals singly or in combination; ruthenium, iridium, rhodium, platinum and
palladium. The
temperature of the reacting mixture is increased in the steam reformer. The
exit temperature
of the heated refonnate, or hydrogen rich mixture, depends upon the fuel,
pressure, steam to
carbon ratio and metallurgy of the reactor. The exit temperature from the cold
side 58 is
preferably between 500 C and 900 C. More preferably, the temperature is
between 600 C
and 800 C. Most preferably, the temperature is between 700 C and 800 C.
This heated reformate gas passes from the cold side 58 of the steam reformer
to the
hot side 62 of the water gas shift reactor, part or all of which is cooled by
cooler, lower
pressure air flowing through the cold side of the water gas shift reactor 64.
Like the steam
reformer, the water gas shift reactor is thus provided with one or more heat
transfer surfaces
for transferring heat between these two fluids. Alternatively, the hot gases
may be partially or
completely cooled to the water gas shift reactor temperature before being
admitted to its
-29-
CA 02413388 2002-12-20
WO 02/02220 PCT/US01/16513
hot side 62. A catalyst active for water gas shift and inactive for
methanation is disposed
within the hot side of the water gas shift reactor 62. This catalyst must also
be resistant to
poisoning by sulfur compounds. An example of a commercially-available catalyst
is a finely
divided mixture of oxides of iron and chromium which is fonned into pellets or
tablets. The
gas exiting the water gas shift reactor hot side 62 is prefeiably greater than
200 C in
temperature. More preferably, the gas is greater than 250 C and less than 400
C in
temperature. Most preferably the gas is greater than 275 C and less than 350
C.
The hot, hydrogen rich reformate is then passed through a cooler 68. This is
depicted
in Figure 5 as being cooled by cool external air from a fan. Alternatively,
the cooling may be
accomplished via a series of heat exchangers including heating the water feed
to the system
and cooling with air. Altematively, the reformate may be cooled by heat
exchange with cool
water. Alternatively, the reformate may be cooled through the use of an
evaporative chiller
using directly injected water. These embodiments may also be combined in a
variety of
configurations apparent to one skilled in the art, which do not in any way
limit the scope of
the present invention. The reformate exits this cooler 68 at a temperature
below 100 C.
More preferably, the temperature is between 80 C and 25 C. Most preferably
the
temperature is between 60 C and 30 C. Because the reformate gas is
pressurized, the
cooling will cause some portion of the water vapor to condense. This condensed
water
vapor, and any condensed fuel residuals, is then removed in a condensate
separator 74.
The partially dried reformate is then admitted to the Pressure Swing
Adsorption
(PSA) system 72. PSA systems are known to those skilled in the art. The PSA
system 72
removes impurities from the reformate; thus delivering a substantially pure
hydrogen product
at a pressuie slightly lower than the reactor pressure due to pressure drop.
The contaminant
species are purged from the PSA system 72 using some of the pure hydrogen
product. This
-30-
CA 02413388 2002-12-20
WO 02/02220 PCT/US01/16513
purge gas is rejected at lower pressure than the hydrogen is delivered as
product. It is also
possible to provide a vacuum pump to reduce the pressure at which the low-
pressure exhaust
is rejected to thus improve the performance of the PSA system 72. The average
hydrogen
purity may be controlled by varying the rate with which the beds in the PSA
system 72 are
purged. This rate of purging may be controlled via a feedback loop of the
present invention
which is described herein. The PSA product outlet may optionally be provided
with a gas
composition sensor 70 for use in the control of the system.
The low-pressure purged gases from the PSA system 72 are fed to the catalytic
combustor 78, where they are mixed with the process air which is compressed by
the feed
compressor 76, and heated by the reformate in the cold side of the water gas
shift reactor 64.
The catalytic combustor is provided with an inlet end and an outlet end, with
a means of
preheat or ignition, a charge of combustion catalyst, and an outlet
temperature sensor. The
flowrate of air delivered by the feed compressor 76 is regulated such that the
temperature of
the combusted mixture does not exceed the maximum temperature allowed by the
metallurgy
.of the steam reformer. The strategy for this control is disclosed later in
this document. The
system is also provided with an auxiliary fuel metering valve 82, which may
deliver low-
pressure fuel as shown, or may be required to deliver pressurized fuel, to
match the pressure
utilized in the combustion loop. This valve may be used to deliver fuel during
system
startup, and to augment the low-pressure reject fuel gas from the PSA system
72 if it is
insufficient to supply the steam reformer heat requirements.
The hot combustion product is delivered to the hot side of the steam reformer
60,
where it is cooled in exchanging heat with the reformate. It then flows
through the water
preheater 84 to transfer heat for the purpose of vaporizing the reactants.
After leaving the
water preheater 84, the combustion product is sufficiently cooled to be
exhausted to the
-31-
CA 02413388 2002-12-20
WO 02/02220 PCT/US01/16513
atmosphere. This exhaust may be unrestricted, flow through a back-pressure
regulator, or
flow through a gas turbine or other work recovering device. Such modifications
are included
within the scope of the present invention.
Referring to Figure 6, the preferred embodiment of the temperature control
scheme
for the reactor-combustor system is shown. The control scheme employs a
minimum of two
temperature sensors shown in Figure 5, the first temperature sensor 80 in the
hotter outlet
stream of the catalytic combustor and the second temperature sensor 52 in the
outlet stream
of the colder, steam reforming side of the steam reformer. The temperatures at
these two
points are preferably measured at repeated intervals; and'their values are
compared to target
values.
If the combustor outlet temperature measured by the sensor 80 is above the
preset
value, which is dependent upon the fuel, the steam to carbon ratio, the
reactor pressure, and
the reactor design, then the temperature of the heated reformate measured by
sensor 52 is
checked. If this temperature is below the minimum value consistent with proper
performance, then the flowrate of air to the combustor must be increased and
the cycle
repeated. This change in the airflow may be affected by a variation in the
compressor or
blower speed, or by the application of a throttling valve. The air to fuel
stoichiometry will
always be fuel lean in the reformer system of the present invention in order
to control the
peak temperature to a safe level. If the reformate temperature is above the
minimum
temperature, the flowrate of the auxiliary fuel must be checked. If this
flowrate is zero, then
the air flowrate must be increased and the cycle restarted. If the flowrate of
auxiliary fuel is
not zero, then the flowrate should be decreased and the cycle restarted.
If the combustor outlet temperature does not exceed the maximum temperature,
then
the reformate temperature must be checked. If the reformate temperature is
above the
-32-
CA 02413388 2002-12-20
WO 02/02220 PCT/US01/16513
minimum value, then all is well and no changes are required. The control
system will then
continue to cycle until something disturbs the steady-state condition. If,
however, the
reformate temperature is below the minimum value, the flowrate of auxiliary
fuel must be
increased and the cycle repeated.
Other control strategies which achieve the twin aims of maintaining a maximum
temperature in the combustion product and a minimum temperature in the
reformate will be
apparent to one skilled in the art. Modifications to the control strategy of
Figure 6 designed
to improve the response of the system or to reduce oscillations about the
steady state
condition may also be envisioned. These alternative and modified control
strategies are
encompassed within the scope of the present invention.
Figure 7 presents a preferred example of a feedback control strategy for the
PSA
subsystem based upon the signal from a carbon monoxide sensor. If the carbon
monoxide
sensor detects a concentration above the maximum value, the purge rate for the
PSA system
is increased and the control cycle is repeated. If the carbon monoxide
concentration is below
the maximum value, and above -the minimum value, then no action is taken and
the control
cycle repeats. If the value is below the minimum value then the purge rate is
decreased and
the control cycle repeats. The minimum contaminant concentration is determined
by
minimum allowable system efficiency, as running at arbitrarily high purge
rates will greatly
reduce hydrogen recovery and thus system thermodynamic efficiency. As noted
previously,
the example of carbon monoxide, though particularly suitable for fuel cell
applications, is not
limiting. Feedback control based upon the exit concentrations of other gases
may also be
employed, and is within the scope of the present invention.
The improved hydrogen generation system of the present invention has many
advantages compared to conventional systems, especially for applications
requiring less than
-33-
CA 02413388 2002-12-20
WO 02/02220 PCT/US01/16513
one ton per day of hydrogen. Preferably, the present invention is used in a
reactor system
producing less than 1 ton per day of hydrogen, more preferably less than 7/8
ton per day, and
most preferably less than 3/4 ton per day.
The improved system of the present invention eliminates partial oxidation of
the fuel,
sulfur removal (via hydrodesulfurization or other processes), and low
temperature water gas
shift. These simplifications reduce system cost relative to conventional
systems by
eliminating components. It also improves safety and durability by reducing the
number of
interconnections which may develop leaks in service.
The improved system of the present invention is capable of quicker and simpler
startup from a cold or idle condition. This is due to several factors,
including the reduced
mass of the present system due to the elimination of many components as well
as the fact that
the rugged catalysts employed in the system of the present invention are
insensitive to fuel
impurities which require bypassing the feed in conventional systems until full
operating
temperature is attained. The startup is further simplified as the rugged
catalysts do not
require inert purging during startup. The rugged catalysts of the present
invention also do not
require special precautions on shutdown such as inert purging. This simplifies
the design of
the system further, thus reducing cost and improving safety.
The improved system of the present invention preferably operates in a pressure
regime where suitable pressurization equipment is commercially-available and
very
inexpensive. Conventional systems operate either at low pressure in the steam
reformer, with
subsequent compression of the reformate product at high cost and complexity,
or at very high
pressures where small-scale compression equipment is not readily available.
The improved system of the present invention preferably employs active control
of
the reactor peak temperal.ure. This temperature is limited to a value
consistent with extended
-34-
CA 02413388 2002-12-20
WO 02/02220 PCT/US01/16513
operation of the reformer. In conventional systems, the peak gas'temperatures
were often
above an acceptable service temperature of the reactor structure, and if any
upset in the
endothermic catalytic reaction took place the structure might be badly
overheated.
Any embodiment of the hydrogen production system of the present invention may
be
implemented on a computer system. Figure 8 illustrates a preferred computer
system 801
upon which an embodiment of the present invention may be implemented. The
computer
system 801 includes a bus 802 or other communication mechanism for
communicating .
information, and a processor 803 coupled with the bus 802 for processing the
information.
The computer system 801 also includes a main memory 804, such as a random
access
memory (RAM) or other dynamic storage device (e.g., dynamic RAM (DRAM), static
RAM
(SRAM), and synchronous DRAM (SDRAM)), coupled to the bus 802 for storing
information and instructions to be executed by processor 803. In addition, the
main memory
804 may be used for storing temporary variables or other intermediate
information during.. the.
execution of instructions by the processor 803. The computer system 801
further includes a
read only memory (ROM) 805 or other static storage device (e.g., programmable
ROM
(PROM), erasable PROM (EPROM), and electrically erasable PROM (EEPROM))
coupled
to the bus 802 for storing static information and instructions for the
processor 803.
The computer system 801 also includes a disk controller 806 coupled to the bus
802
to control one or more storage devices for storing information and
instructions, such as. a
magnetic hard disk 807, and a removable media drive 808 (e.g., floppy disk
drive, read-only
compact disc drive, read/write compact disc drive, compact disc jukebox, tape
drive, and
removable magneto-optical drive). The storage devices may be added to the
computer
system 801 using an appropriate device interface (e.g., small computer system
interface
(SCSI), integrated device electronics (IDE), enhanced-IDE (E-IDE), direct
memory access
-35-
CA 02413388 2002-12-20
WO 02/02220 PCT/US01/16513
(DMA), or ultra-DMA).
The computer system 801 may also include special purpose logic devices (e.g.,
application specific integrated circuits (ASICs)) or configurable logic
devices (e.g., simple
programmable logic devices (SPLDs), complex programmable logic devices
(CPLDs), and
field programmable gate arrays (FPGAs)).
The computer system 801 may also include a display controller 809 coupled to
the
bus 802 to control a display 810, such as a cathode ray tube (CRT), for
displaying
information to a computer user. The computer system includes input devices,
such as a
keyboard 811 and a pointing device 812, for interacting with a computer user
and providing
information to the processor 803. The pointing device '812, for example, may
be a mouse, a
trackball, or a pointing stick for communicating direction information and
command
selections to the processor 803 and for controlling cursor movement on the
display 810. In
addition, a printer may .provide printed listings of the data
structures/information shown in
Figures 3 and 4, or any other data stored and/or generated by the computer
system 801.
The computer system 801 performs a portion or all of the processing steps of
the
invention in response to the processor 803 executing one or more sequences of
one or more
instructions contained in a memory, such as the main memory 804. Such
instructions may be
read into the main memory 804 from another computer readable medium, such as a
hard disk
807 or a removable media drive 808. One or more processors in a multi-
processing
arrangement may also be employed to execute the sequences of instructions
contained in
main memory 804. In alternative embodiments, hard-wired circuitry may be used
in place of
or in combination with software instructions. Thus;embodiments are not limited
to any
specific combination of hardware circuitry and software.
As stated above, the computer system 801 includes at least one computer
readable
-36-
CA 02413388 2002-12-20
WO 02/02220 PCT/US01/16513
medium or memory for holding instructions programmed according to the
teachings of the
invention and for containing data structures, tables, records, or other data
described herein.
Examples of computer readable media are compact discs, hard disks, floppy
disks, tape,
magneto-optical disks, PROMs (EPROM, EEPROM, flash EPROM), DRAM, SRAM,
SDRAM, or any other magnetic medium, compact discs (e.g., CD-ROM), or any
other
optical medium, punch cards, paper tape, or other physical medium with
patterns of holes; a
carrier wave (described below), or any other medium from which a computer can
read.
Stored on any one or on a combination of computer readable media, the present
invention includes-software for controlling the computer system 801,4or
driving a device or
devices for implementing the invention, and for enabling the computer system
801 to interact
with a human user (e.g., print production personnel). Such software may
include, but is not
limited to, device drivers, operating systems, development tools, and
applications software.
Such computer readable media further includes the computer program product of
the present
invention for performing all or a portion (if processing is distributed) of
the. processing
performed in implementing the invention. .
The computer code devices of the present invention may be any interpretable or
executable code mechanism, including but not limited to scripts, interpretable
programs,
dynamic link libraries (DLLs), Java classes, and complete executable programs.
Moreover,
parts of the processing of the present invention may be distributed for better
performance,
reliability, and/or cost. =
The term "computer readable medium" as used herein refers to any medium that
participates in providing instructions to. the processor 803 for execution. A
computer
readable medium may take many forms, including but not limited to, non-
volatile media,
volatile media, and transmission media. Non-volatile media includes, for
example, optical,
-37-
CA 02413388 2002-12-20
WO 02/02220 PCT/US01/16513
magnetic disks, and magneto-optical disks, such as the hard disk 807 or the
removable media
drive 808. Volatile media includes dynamic memory, such as the main memory
804.
Transmission media includes coaxial cables, copper wire and fiber optics,
including the wires
that make up the bus 802. Transmission media may also take the form of
acoustic or light
waves, such as those generated during radio wave and infrared data
communications.
Various forms of computer readable media may be involved in carrying out one
or
more sequences of one or more instructions to processor 803 for execution. For
example, the
instructions may initially be carried on a magnetic disk of a remote computer.
The remote
computer can load the instructions for implementing all or a portion of the
present invention
remotely into a dynamic memory and send the instructions over a telephone line
using a
modem. A modem local to the computer system 801 may receive the data on the
telephone
line and use an infrared transmitter to convert the data to an infrared
signal. An infrared
detector coupled to the bus 802 can receive the data carried in the infrared
signal and place
the data on the bus 802. The bus 802 canries the data to the main memory 804,
from which
the processor 803 retrieves and executes the instructions. The instructions
received by the
main memory 804 may optionally be stored on storage device 807 or 808 either
before or
after execution by processor 803.
The computer system 801 also includes a communication interface 813 coupled to
the
bus 802. The communication interface 813 provides a two-way data communication
coupling to a network link 814 that is connected to, for example, a local area
network (LAN)
815, or to another communications network 816 such as the Internet. For
example, the
communication interface 813 may be a network interface card to attach to any
packet
switched LAN. As another example, the communication interface 813 may be an
asymmetrical digital subscriber line (ADSL) card, an integrated services
digital network
-38-
CA 02413388 2002-12-20
WO 02/02220 PCT/US01/16513
(ISDN) card or a modem to provide a data communication connection to a
corresponding
type of communications line. Wireless links may also be implemented. In any
such
implementation, the communication interface 813 sends and receives electrical,
electromagnetic or optical signals that carry digital data streams
representing various types of
information.
The network link 814 typically provides data communication through one or more
networks to other data devices. For example, the network link 814 may provide
a connection
to a another computer through a local network 815 (e.g., a LAN) or through
equipment
operated by a service provider, which provides.communication services through
a
conimunications network 816. In preferred embodiments, the local network 814
and the
communications network 816 preferably use electrical, electromagnetic, or
optical signals,
that carry digital data streams. The signals through the various networks and
the signals on
the network link 814 and through the communication interface 813, which carry
the digital
data to and from the computer system 801, are exemplary forms of carrier waves
transporting
the information. The computer system 801 can transmit and receive data,
including program
code, through the network(s) 815 and 816, the network link 814 and the
communication
interface 813.
The mechanisms and processes set forth in the present description may be
implemented using a conventional general purpose microprocessor programmed
according to
the teachings in the present specification, as will be appreciated to those
skilled in the
relevant art(s). Appropriate software coding can readily be prepared by
skilled programmers
based on the teachings of the present disclosure, as will also be apparent to
those skilled in
the relevant art(s).
The present invention thus also includes a computer-based product which may be
-39-
CA 02413388 2002-12-20
WO 02/02220 PCT/US01/16513
hosted on a storage medium and include instructions which can be used to
program a
computer to perform a process in accordance with the present invention. This
storage
medium can include, but is not limited to, any type of disk including floppy
disks, optical
disks, CD-ROMs, magneto-optical disks, ROMs, RAMs, EPROMs, EEPROMs, flash
memory, magnetic or optical cards, or any type of media suitable for storing
electronic
instructions.
Preferred embodiments of the invention are listed below:
A. A system for the production of hydrogen from hydrocarbon fuel such as
natural
gas, propane, naphtha, and other hydrocarbons low in sulfur content (< 100 ppm
sulfur by
mass); wherein the product hydrogen is made substantially pure (>99.99%) by
separating
impurities using a pressure swing adsorption (PSA) system, wherein no
pretreatment of the
fuel feed to the steam reformer by the removal of sulfur and molecular oxygen
is carried out.
B. The hydrogen production system of A wherein the steam reforming catalyst is
insensitive to sulfur and molecular oxygen, preferably the catalyst active
metal comprises one
or more of the following group VIIIB metals singly or in combination;
ruthenium, iridium,
rhodium, platinum and palladium supported upon a high area ceramic support.
C. The hydrogen production system of A wherein the operation of the system is
in a
mesobaric regime, between 4 and 18 atmospheres, more preferably the pressure
is between 5
atm and 15 atm, most preferably the pressure is between 10 atm and 15 atm.
D. The hydrogen production system of A wherein feedback control is employed
for
the delivery of fuel and or air to a catalytic combustor in proportion such
that the peak
temperature of the gases entering the primary steam reformer does not exceed a
safe
maximum temperature determined by the metallurgy of the steam reformer.
E. The hydrogen production system of A where no low temperature water gas
shift
-40-
CA 02413388 2008-05-26
reactor is employed, wherein the exit temperature of the high temperature
water gas shift
reactor employed is preferably above 200 C, more preferably the temperature is
greater than
250 C and less than 400 C, most preferably the temperature is greater than 275
C and less
than 350 C.
F. The hydrogen production system of A wherein feedback control of product
carbon
monoxide, or other impurity, concentration is employed.
G. The hydrocarbon production system of A wherein the pretreatment includes at
least one selected from the group including partial oxidation,
hydrodesulfurization,
adsorption, and absorption.
I. A hydrogen production method, which includes the catalytic steam reforming
and
subsequent high temperature water gas shift of low-sulfur (< 100ppm by mass)
hydrocarbon
fuels to produce hydrogen followed by hydrogen purification with pressure
swing adsorption
(PSA).
Having now fully described the invention, it will be apparent to one of
ordinary skill
in the art that many changes and modifications may be made thereto, without
departing from
the spirit or scope of the invention as set forth herein.
It is therefore to be understood that within the scope of the appended claims,
the
invention may be practiced otherwise than as specifically described herein.
The entire contents of each of the above-mentioned patents, references and
published
applications is hereby incorporated by reference, the same as if set forth at
length.
-41 -
