Figure 6. Hydrogen molar fraction contour plots in the combined parallel
plate heat exchanger-reactor for hydrogen production by steam-methanol
reforming.
The Sherwood number profiles in the exothermic process are presented in
Figure 7 along the length of the combined parallel plate heat
exchanger-reactor. The combined parallel plate heat exchanger-reactor
system can be used to for a number of reactions as a wide range of
process conditions are possible. It is inevitable that the catalyst
coating will eventually deactivate to the point where economics drive
for its replacement. It is sometimes possible to extend a catalyst life
and reclaim some activity by techniques such as hydrogen treatment or
methods to remove carbon buildup. These techniques can be readily
applied to the combined parallel plate heat exchanger-reactor. The
combined parallel plate heat exchanger-reactor also allows the body to
be removed and monolith replacement performed. This is simply achieved
by removing the relevant inlet manifold and outlet manifold and removing
any monolith supports or containment structure. A new monolith can be
inserted and reverse procedure applied. If the endothermic catalyst
requires high temperature hydrogen activation, the heat can be supplied
via the exothermic channels. The spent monolith can be recycled after
recovery of any of the precious metal components of the catalyst. 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 methanol. 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. An advantage of the
arrangement is the low thermal inertia of the system. This allows the
reactor to operate with inherently fast thermal response and is
particularly advantageous during startup. The low thermal inertia will
minimize startup time to the order of minutes from the order of hours,
which is typical for large packed tube technology. With suitable
ancillary equipment, the system can be operated with a level of control
and operating flexibility not encountered in traditional steam
reformers. An advantage of the reactor system is the ability to use low
calorific fuel for the exothermic reaction. Such fuel is not ideally
suited to homogeneous combustion and results in a highly unstable flame.
Heterogeneous combustion aids in spreading the heat generation along the
length of the channel and helps prevent hotspot formation. The use of
low caloric value gas allows the use of certain waste streams as the
fuel to supply the heat. Examples of such streams include the off-gas
stream from a fuel cell, and the stream remaining after hydrogen removal
from a membrane gas shift reactor. The heat generation rate per unit
area is approximately matched to the heat requirement in the adjacent
channel. This can be achieved by controlling the catalyst thickness in
each channel. A trial-and-error process may be required to obtain the
optimum catalyst thicknesses for some processes. If the processes are
not thermally matched, the overall efficiency of the reactor will be
reduced. When using large channels, it is possible that the reaction
will become diffusion limited, such that the rate of reaction is
dictated by the rate at which unreacted molecules can diffuse from the
center of the channel into the catalyst matrix. In this case, it is
possible to add flow disturbance elements in the channel or emanating
from the wall. These elements will produce a degree of convective mixing
by forming local flow disturbances in an otherwise laminar environment.
If a heat transfer fluid is used to remove the heat of reaction from a
reaction occurring in any adjacent channel, then these flow disturbance
elements would provide a useful and low pressure drop method of
enhancing thermal performance.