2. Methods
Steam reforming is a process in which hydrogen is stripped from a
hydrocarbon fuel by thermal energy provided by a combustor. In alcohol
steam reforming, the feed stream contains steam and an alcohol or
alcohols. In the present design, methanol, ethanol, and propanol are
preferred with methanol being especially preferred. In an integrated
reactor, combustion or heat generation should occur in close proximity
to the endothermic reaction. Preferably, an exothermic reaction occurs
in microchannels that are interleaved with microchannels in which there
is an endothermic reaction. Co-flow of endothermic and exothermic
reaction streams is preferred; however, cross-flow or countercurrent
flow are also options. The heat of an exothermic reaction is conducted
from the exothermic reaction to the endothermic reaction catalyst, where
it drives the endothermic reaction. Preferably an exothermic reaction
channel and endothermic reaction channel in the integrated reactors is a
microchannel, that is, a channel having at least one dimension of 2
millimeters or less. The use of channels having a minimum dimension of
more than 2 mm may be less effective since heat and mass transfer
limitations may be magnified. An integrated combustor can use the high
surface area of reactor microchannels to remove heat as it is produced,
thus keeping microreactor components from exceeding material temperature
constraints while combusting with much less excess air or diluent than
would be necessary for an external combustor. The flow rate of reactants
will depend on the desired amount of hydrogen to be produced and on the
minimum or maximum capacity of the steam reformer. The rate of
combustion can be controlled to provide the desired amount of heat to a
steam reforming reaction in an adjacent reforming chamber. The steam
reforming reaction can be run over a broad pressure range from
sub-ambient to very high. The alcohol steam reforming reaction is
preferably carried out at temperatures of 200-400 °C, more preferably
220-300 °C, and in some cases 240-270 °C. In some preferred
configurations, the combustion temperature is approximately the same as
the average reformer temperature, that is, the average temperature of
the reforming catalyst.
The design provides a method of conducting an endothermic reaction in an
integrated combustion reaction, comprising: passing an endothermic
reaction composition into at least one endothermic reaction chamber,
passing a fuel and an oxidant into at least one exothermic reaction
chamber wherein the fuel and oxidant each have a contact time in the
combustion chamber of 50 milliseconds or less, wherein the exothermic
reaction chamber comprises at least one exothermic reaction chamber wall
that is adjacent at least one endothermic reaction chamber. The
endothermic reaction chamber comprises an endothermic reaction catalyst
in contact with at least the at least one endothermic reaction chamber
wall that is adjacent at least one exothermic reaction chamber, and
transferring heat from the at least one exothermic reaction chamber into
the at least one endothermic reaction chamber at a rate as based on the
internal surface area of the endothermic reaction chamber. The heat flux
can be measured based on either a single exothermic reaction chamber or
multiple chambers in a multi-chamber device. The reformate stream
usually comprises hydrogen, carbon dioxide, and carbon monoxide. Proton
exchange membrane fuel cells operate have a very low tolerance for
carbon monoxide. They can generally tolerate carbon dioxide and some
other gases such as nitrogen, but only up to a certain amount. Clean-up
of a reformate stream can be performed, for example by a multi-step
process consisting of water gas shift reactors, combined with selective
oxidation and carbon monoxide methanation, or by the use of a hydrogen
permeable membrane. Direct methanization of a reformate stream is
important without first passing the reformate through a
hydrogen-selective membrane, preferential oxidation, or water gas shift
reactor. This is highly desirable since hydrogen-selective membranes are
expensive, and additional process steps can be costly and result in
lowered yield. Eliminating the requirement of a preferential oxidation
also eliminates the need to add oxygen, including the need to vary
oxygen content to account for fluctuations in carbon monoxide
concentration. In preferred cases, the process adds heat to the steam
reforming step but does not have additional heat exchangers or heat
exchange steps for methanation or for other carbon monoxide-reducing
steps.
The exothermic reaction chamber has an internal dimension of less than
0.8 mm and a volumetric heat flux, based on reaction chamber volume of
greater than 8 Watts per cubic centimeter. Contact times in the
exothermic and endothermic reaction chambers are preferably less than
600 milliseconds. Area heat flux for the area of either reaction chamber
is preferably 0.8 Watts per square centimeter or more. The design
provides a method of steam reforming in an integrated
combustion-reforming reactor, comprising: step a) passing steam and
hydrocarbon into at least one endothermic reaction chamber wherein the
steam to carbon ratio is less than 2:1 with a pressure drop through the
endothermic reaction chamber of less than 6000 kPa, step b) passing a
fuel and an oxidant into at least one exothermic reaction chamber
wherein the fuel and oxidant each have a contact time in the combustion
chamber of 600 milliseconds or less, wherein the exothermic reaction
chamber comprises at least one exothermic reaction chamber wall that is
adjacent at least one endothermic reaction chamber, wherein the
endothermic reaction chamber comprises an endothermic reaction catalyst
in contact with at least the at least one endothermic reaction chamber
wall that is adjacent at least one exothermic reaction chamber, step c)
converting the steam and hydrocarbon to form carbon monoxide and
hydrogen such that the at least one endothermic reaction chamber has an
output demonstrating a conversion of at least 50 percent of the
hydrocarbon with a selectivity to carbon monoxide of at least 50
percent. A device is characterized by operation for 200 or 500 hours and
then cut open to reveal less than 0.08 gram of coke per each kilogram of
the fuel processed.
The endothermic reaction chamber and the combustion chamber are
separated by a thermally conductive wall. The endothermic reaction
composition endothermically reacts to form products. Where not otherwise
specified, the front of the combustion chamber is defined as where the
flow of fuel contacts a combustion catalyst and an oxidant, and the back
of the combustion chamber is defined as the last part of the reaction
chamber that contains a combustion catalyst and is in direct thermal
contact with an endothermic reaction chamber. The exhaust section is not
in direct thermal contact with the endothermic reaction chamber. The
design also provides a method of simultaneously conducting an
endothermic and an exothermic reaction in an integrated
combustion-reforming reactor, comprising: passing a mixture comprising
hydrogen and methanol through a microchannel in an integrated
combustion-reforming reactor; reacting the hydrogen and methanol with an
oxidant to form water, carbon dioxide and carbon monoxide and produce
heat, thus removing hydrogen and methanol from the mixture; wherein a
greater percentage of methanol is removed from the mixture than the
percentage of hydrogen removed from the mixture, as measured by
comparing the levels of hydrogen and methanol in the mixture before
passing through the microchannel with the levels of hydrogen and
methanol at any point after passing through the microchannel. This is an
extremely surprising result. The removing steps are by chemical
reactions, not separation techniques.
An integrated combustion-reforming reactor refers to an integrated
reactor that includes at least one combustion channel adjacent to at
least one endothermic steam reforming reaction channel. During
operation, a reactant enters a combustion or reaction chamber in a bulk
flow path flowing past and in contact with a porous material or porous
catalyst. In these cases, a portion of the reactant molecularly
transversely diffuses into the porous catalyst and reacts to form a
product or products, and then the products diffuse transversely into the
bulk flow path and out of the reactor. The term bulk flow region or bulk
flow path refers to open areas or open channels within the reaction
chamber. A reaction chamber with a bulk flow path or region will contain
a catalyst and there is a gap between the catalyst surface and a
reaction chamber wall or a second catalyst surface. A contiguous bulk
flow region allows rapid gas flow through the reaction chamber without
large pressure drops. In preferred cases, there is laminar flow in the
bulk flow region. Equilibrium conversion is defined in the classical
manner, where the maximum attainable conversion is a function of the
reactor temperature, pressure, and feed composition. For the case of
hydrocarbon steam reforming reactions, the equilibrium conversion
increases with increasing temperature and decreases with increasing
pressure. Fuel flow to combustor can be initiated at this point. Once
the fuel has begun reacting, the hydrogen flow is tapered off and the
fuel flow is increased. The excess air should not be too much, since the
extra air removes heat from the steam reformer. Air and methanol flows
are adjusted until the steam reformer is at the desired temperature. The
reformer fuel mixture flow is initiated at this point. Combustor flows
are adjusted as necessary to maintain desired temperatures. Preferred
forms of porous supports are foams and felts and these are preferably
made of a thermally stable and conductive material.
The methanol conversion is calculated by using a carbon balance on the
system. Reaction chamber volume is the internal volume of a reaction
chamber either exothermic or endothermic. This volume includes the
volume of the catalyst, the open flow volume if present and metal
support ribs or fins if present within the reaction chamber volume. This
volume does not include the reaction chamber walls. The reaction chamber
volume must contain a catalyst somewhere within its cross-section and
must be directly adjacent another reaction chamber for heat transport.
This volume is used for calculations of endothermic reaction chamber
volumetric heat flux, area heat flux, and endothermic reaction contact
time. The reactor core volume is defined as the reaction chamber volume
and all combustion chamber volume and the metal webs that separate the
two chambers. The combustion chamber volume is defined as the chamber
volume in which the exothermic heat generating reaction occurs and is
adjacent to the reaction chamber volume. Perimeter metal is not included
in reactor core volume. The reactor core volume does not include any
preheat exchanger zone volume that may or may not be attached to the
reactor core volume. The preheat exchanger zone may be attached to the
reactor but does not contain an endothermic reaction catalyst along any
plane that bisects the device orthogonal to the direction of flow. The
use of electric heating for system start-up is eliminated by following
the subsequent procedure. Hydrogen and air are fed to combustor to
initiate combustion and heat the vaporizers. Once the vaporizers are
heated to approximately 80 °C, methanol is fed to the vaporizer. The
hydrogen is slowly tapered off as the methanol feed is increased until
only methanol and air are being fed to the combustor and the device is
completely self-sustaining. The methanol-air mixture is adjusted until
the steam reformer reaches the desired temperatures depending on the
conditions being tested. The methanol-water solution feed is then
initiated.
Endothermic reaction chamber heat flux is defined as the endothermic
reaction heat duty divided by the reaction chamber volume. Reactor core
volume heat flux is defined as the endothermic reaction heat duty
divided by the reactor core volume. Heat exchanger flux is defined as
the total heat transferred to the cold streams divided by the heat
exchanger core volume. Heat exchanger core volume is defined as the
total heat exchanger volume inclusive of microchannels, ribs between
microchannels, and the walls separating microchannels for all fluid
streams transferring heat. The heat exchanger volume is inclusive of the
heat exchanger zone. The heat exchanger core volume does not include the
perimeter metal or manifolds or headers. The heat exchanger core volume
does not include the endothermic reaction chamber nor any volume that
could be included within any plane that bisects the endothermic reaction
chamber orthogonal to the direction of flow. Average area heat flux is
defined as the endothermic reaction heat duty divided by the area of the
endothermic reaction chamber heat transfer surface. The endothermic heat
transfer surface is defined by a planar area, which may be intermittent
in the case of ribs or other structures in the endothermic reaction
chamber, above which there is area for flow of reactants and below which
there is a wall that separates the endothermic reaction chamber and the
exothermic reaction chamber. This area is the path for heat transfer
from the exothermic reaction chamber to the endothermic reaction
chamber. The apparent equilibrium conversion temperature is the apparent
temperature based on methane conversion or, more generally, hydrocarbon
conversion or the temperature required to produce an equilibrium methane
conversion equal to the measured methane conversion at the measured
average process pressure. Average process pressure is assumed to be the
average of the measured inlet and outlet pressures.