Figure 1. Temperature and oxygen mole fraction contour maps in the
micro-structured heat-exchanger reactor for hydrogen production by steam
methanol reforming.
The nitrogen and steam mole fraction contour maps in the
micro-structured heat-exchanger reactor are illustrated in Figure 2 for
hydrogen production by steam methanol reforming. The volume of a
reaction chamber, unless otherwise indicated, refers to the internal
volume where reaction substantially occurs but not adjacent material.
Where a catalyst is present, the volume includes at least the catalyst
volume and catalyst void fraction. Core volume of a heat exchanger
refers to the volume of the adjacent flow paths of the two fluids during
the portion that they are adjacent and subject to primary heat transfer
and including the volume of any intervening material, such as walls
between the adjacent flow paths. The initial composition is
representative of a reformate stream generated from steam reforming of
isooctane at a 1.5:1 steam to carbon ratio. While a reforming outlet
typically has at least these four compounds carbon monoxide, carbon
dioxide, steam, and hydrogen, the ratios of the components depend on the
type of reforming being performed, such as partial oxidation or steam
reforming as well as the operating conditions of the reformer. In
addition, additional material may be added to the reformate outlet prior
to performing a water gas shift reaction. Accordingly, the inlet stream
to a water gas shift reactor might have a carbon monoxide to carbon
dioxide molar ratio that ranges from about 2:1 to about 1:5. Typically,
the steam to gas ratio, defined as the moles of water divided by the
moles of the remaining gas, is between about 0.2 and 0.6. Once the
desired reactor temperatures are achieved, methanol and water injection
begin and eventually generates the desired heat transfer medium which
displaces the starter gas. In the meantime, the reformate exiting the
first reactor, although diluted with some starter gas, can still be used
in the fuel cell during system startup so long as the carbon monoxide
levels are low enough. Alternatively, the diluted reformate could be
diverted into the combustor for burning, or dumped to the atmosphere, if
desired, until acceptable carbon monoxide levels are achieved. Once the
desired operating temperatures are achieved, fresh methanol and water
are again injected into the recirculating gas, and normal fuel processor
operation resumes. Carbon monoxide output from the fuel processor is
controlled over the operating range of the processor by varying the
water-methanol ratio, the amount of air added to the reactor, and the
speed of the recirculating fan to respectively drive the reaction
equilibrium, oxidize the carbon monoxide and maintain the required heat
transfer within the processor. Preferably, the heat exchanger is a cross
flow exchanger adapted to flow the circulating heat transfer medium
through the cold side second conduits in a first direction and the
heating fluid through the hot side first conduits in a second direction
transverse the first direction. The heating fluid will comprise
combustion products exhausted from a combustor fueled by methanol and
unused hydrogen exiting the anode compartments of the fuel cell that is
fueled by the fuel processor. A variety of reactors can employ the steam
reforming catalysts. Examples include fuel cell reactors, steam
reformers, and conversion reactors. Instead of the fuel cell, the
hydrogen may be transported to one of a storage tank, a refueling
station, a hydrocracker, hydrotreater, or to additional hydrogen
purifiers. The hydrogen may be employed as a synthesis gas, as a
component in hydrogenation reactions, and the like. The alcohol
reforming reactor may also be configured by placing the reaction chamber
adjacent to a heat exchanger chamber that is comprised of an array of
microchannels or a single microchannel. The width of the reaction
chamber is dependent on the effective thermal conductivity of the
catalyst insert. The higher the effective thermal conductivity of the
catalyst insert, the wider the insert to enable rapid heat removal. In
another configuration, the reaction chamber may be connected to a fuel
tank such that alcohol from the tank can flow directly into the reaction
chamber. Although a fuel tank is commonly used, any alcohol fuel source
could be used. The liquid fuel stream may flow through a separate
vaporizer or be vaporized within a section of the steam-reforming
reactor. The alcohol is vaporized in a microchannel vaporizer and
preheated in a microchannel preheater. The reformation fuel channel is
disposed along the axis on a side of the reformation chamber opposite
the combustion chamber. The reformation products channel is disposed
outside the reformation fuel channel with respect to the axis and on the
side of the reformation chamber opposite the combustion chamber, and the
combustion exhaust channel is disposed outside the reformation fuel
channel with respect to the axis and on the side of the reformation
chamber opposite the combustion chamber.