Figure 7. Nusselt number profiles along the length of the heat
integrated reactor under nominal operating conditions for
thermochemically producing hydrogen from methanol by steam reforming.
The Sherwood number profiles along the length of the heat integrated
reactor are presented in Figure 8 under nominal operating conditions for
thermochemically producing hydrogen from methanol by steam reforming. 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. Another deficiency of traditional heat transfer equipment is
start-up time and thermal response to transients. As reactors are
traditionally large and heavy, they have significant thermal inertia.
Therefore, the system takes significant time to re-equilibrate from any
change in load or process operating conditions. Therefore, a reactor
with enhanced response characteristics particularly for rapid start up
is desired. The design consists of multiple packed tubes, of small
diameter, being placed in intimate contact with a heat generating flame.
The arrangement leads to improved heat transfer and therefore chemical
conversion. However, the packed tube results in a significant pressure
drop and the author states the process is still heat transfer limited.
Therefore, a reactor design which minimizes the process side pressure
drop and does not suffer from heat transfer limitation is required. It
is proposed that the arrangement can either be used as a heat exchanger,
where energy is transferred from one stream to another via conduction
through the wall or it is suitable as a chemical reactor where the
second set of channels allow the introduction of a heat transfer fluid
[41, 42]. It is noted that the reaction can be a catalytic process
and the catalytically active material can be coated onto the monolith
passage walls to minimize pressure drop [43, 44]. In this
arrangement the heat transfer from the process catalyst to the dividing
wall will be highly efficient, however, the uptake of the energy by the
heat transfer fluid will suffer from all of the limitations of
traditional heat transfer operations [45, 46]. In this case the
boundary layer will provide a significant resistance to heat transfer
and will severely limit the rate of the process [47, 48]. The high
velocities will reduce the characteristic thickness of the boundary
layer and ensure that a sufficient mass of heat transfer fluid is
available to absorb the heat of reaction without significantly changing
temperature. These requirements will lead to excessive pressure drop
through the coolant channels. Therefore, a reactor design which
minimizes the heat transfer fluid side pressure drop is required.
Combining endothermic and exothermic reactions on opposing sides of
dividing walls of adjacent channels can serve as an efficient method of
heat transfer. It is proposed that steam reforming of a hydrocarbon be
performed by one layer and the energy for this process be supplied by a
hydrocarbon oxidative process being promoted in the subsequent layer.
Various hybrids of this theme are proposed. However, as the heat is
supplied by an autothermal reaction, oxygen must be supplied along with
the fuel stream. As well as the oxygen, associated nitrogen is present.
This nitrogen acts to absorb process energy which lowers the thermal
efficiency of the process as well as diluting the desired product,
hydrogen. The presence of the nitrogen increases the load on downstream
partial oxidation units which act to oxidize carbon monoxide to carbon
dioxide. The nitrogen also reduces the streams suitability for use in
fuel cells. Therefore, a reactor which can supply sufficient energy to
an endothermic reaction without mixing the streams is needed. It should
be noted that an advantage 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, the gaseous components from a Fischer-Tropsch synthesis. It
should be further noted that 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. Suitable metals include copper, aluminum, stainless steel,
iron, titanium, and mixtures or alloys thereof.