Figure 6. Contour plots of water mole fraction, oxygen mole fraction,
and carbon dioxide mole fraction under nominal operating conditions for
the heat integrated reactor.
The Nusselt number profiles along the length of the heat integrated
reactor are presented in Figure 7 under nominal operating conditions for
thermochemically producing hydrogen from methanol by steam reforming.
Many chemical processes utilize catalysts to enhance chemical conversion
behavior. A catalyst promotes the rate of chemical conversion but does
not affect the energy transformations which occur during the reaction.
Often catalytic processes are conducted within tubes which are packed
with a suitable catalytic substance. The process gas flows within the
tube and contacts the catalytic packing where reaction proceeds. The
tube is placed within a hot environment such as a furnace such that the
energy for the process can be supplied through the tube wall via
conduction. The mechanism for heat transfer with this arrangement is
rather tortuous as heat must first be transferred through the outer
boundary layer of the tube, conducted through the often-heavy gauge wall
of the tube and then pass through the inner boundary layer into the
process gas. The process gas is raised in temperature and this energy
can be utilized by the process for chemical reaction. The process
engineer is often caused to compromise between the pressure drop within
the tube reactor with the overall heat transfer and catalytic
effectiveness. The inner heat transfer coefficient can be effectively
increased by raising the superficial velocity of the process gas. The
higher gas velocity therefore improves the thermal effectiveness of the
system. However, higher gas velocities increase the system’s pressure
drop and results in increased compressor sizes and associated operating
costs. A reactor must be of sufficient length to allow a reaction to
proceed to the required conversion [33, 34]. Utilizing high gas
velocities typically results in reactors with large length to width
ratios which again results in systems with high pressure drops [35,
36]. The smaller the characteristic dimension of the catalyst particle
the higher is the utilization of the catalyst [37, 38]. This is
sometimes expressed as a higher effectiveness factor [39, 40].
However, beds formed from small particles exhibit higher pressure drops
than similar beds formed from larger particle. So, an engineer designs a
system with expectable compromises between heat transfer, catalyst
utilization, system conversion, and pressure drop. Therefore, a reactor
for conducting catalytic processes which can promote overall heat
transfer and levels of conversion whilst minimizing pressure drop is
desired. The dividing walls must be of sufficient strength to maintain
the integrity of channels. The minimum wall thickness therefore depends
upon material of construction. The wall thickness is in the range of
about 0.5 millimeter to 5 millimeters and more particularly in the range
of about 0.5 millimeter to 2 millimeters. The wall will act as a thermal
barrier to heat transfer, however, as the wall is very thin its
resistance is small. The heat for the reaction is supplied directly
through the wall from the oxidation channels occurring on the opposing
side of the dividing wall. As the heat transfer characteristics are
highly independent of the bulk reactants velocity, a velocity can be
chosen to ensure that the reactants exiting the reactor has attained the
desired level of conversion or indeed reached any equilibrium. It is
interesting to note that in such an arrangement it is desirable to
operate the reactants in a co-current flow arrangement. This ensures
that the area with the greatest heat generation is adjacent to the area
with the greatest heat requirement. However, cases may exist where a
countercurrent flow arrangement is desirable. The system can be used to
for a number of reactions as a wide range of process conditions are
possible. A number of techniques are available in which to deposit an
active catalyst onto the wall of the monolith. One such technique is
that of the washcoat as is used in catalytic converters. Others include
the sol-gel technique, metal sputtering, or the grinding of commercial
catalyst pellets followed by attachment through the use of a cement or
sol-gel. Many of the coating techniques allow different thicknesses of
coating to be applied. It may also be possible to increase or decrease
the thickness of the coating along the channel length. This technique
can be used enhance the kinetics in the downstream sections of the
channel. The thickness of the catalyst coating depends upon the process
proceeding within the catalyst matrix. The products of some processes
are highly dependent upon the catalyst thickness. In this case, the
thickness should be no larger than the characteristic length beyond
which the product spectrum degrades. For some processes the catalyst
thickness has no effect on the product spectrum, an example of which is
the steam reforming of methane. In this case the catalyst thicknesses
can be of any dimension. However, excessively thick coatings are avoided
as the catalyst interior performs little reaction due to diffusion
limitations and acts as a thermal barrier. Many catalysts are prone to
deactivation due to diffusion of an impurity into the catalyst. In cases
where the catalyst is supported on a metallic surface, the source of the
impurity is often the metal surface itself. Metals have low diffusion
coefficients, however, as the catalyst is in intimate contact with the
support over extended periods and at elevated temperature, small amounts
of the metallic substrate will diffuse into the catalyst structure. A
common example of this effect is the poisoning of nickel-based steam
reforming catalysts with iron. It is possible to minimize this effect by
using a dense and nonporous barrier coating located between the metal
surfaces and the active catalyst. However, this problem can be
circumnavigated through the use of ceramic structures.