Figure 4. Effect of wall thermal conductivity on the average Nusselt
number in the endothermic process of the combined parallel plate heat
exchanger-reactor.
The temperature contour plots in the combined parallel plate heat
exchanger-reactor are illustrated in Figure 5 for hydrogen production by
steam-methanol reforming. 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. Utilizing high gas velocities typically results in
reactors with large length to width ratios which again results in
systems with high pressure drops. The smaller the characteristic
dimension of the catalyst particle the higher is the utilization of the
catalyst. This is sometimes expressed as a higher effectiveness factor.
However, beds formed from small particles exhibit higher pressure drops
than similar beds formed from larger particle. Consequently, 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 interior walls of the channels of either or both groups
of channels may be coated with a catalytic material converting that
group of channels into a catalytic reactor. Consequently, this reactor
group is in intimate contact with the separately manifolded channels of
the other group because of the common wall design and as a result,
highly exothermic or endothermic reactions may be carried out under much
greater temperature control than was possible in fixed bed or fluidized
bed or countercurrent flow reactors. This greater temperature control
can result in much greater product selectivity and yield for a wide
variety of chemical reactions. Now heat can be removed or added to the
reactor as required the temperature being regulated merely by careful
control of the flow of reactants through the reactor channels and
control of coolant or heat source flow through the coolant channels. The
temperature control possible with this system is extremely efficient
because all the catalytic material in the reactor channels, where heat
is either absorbed or liberated due to chemical reactions taking place,
is on the surface of the walls that can transmit heat through the walls
directly to the thermal control channels. Most of the heat liberated or
absorbed in the chemical reaction is conducted through the walls which
support the catalyst and thereby eliminating the necessity for heat
conduction through the gas phase which is a much less efficient way to
cool a catalytic particle. In addition, this configuration allows not
only most of the heat to be conducted through solid walls but also
allows all the walls on which the catalysts reside to have all common
walls with the coolant channels. Consequently, one has an extremely
uniform and precise method of controlling the catalyst temperature
because no two catalytic surfaces share a common wall, Also, it is
possible to fabricate a structure with very thin walls and this too
contributes to greater efficiency and ease of temperature control. In
traditional systems, the catalyst is on particles randomly packed into a
reactor tube. Exothermic reactions occurring generate heat at the
catalytic particles even in the center of the tube. This heat must
migrate through the gas phase and through the particles in the reaction
tube to the wall of the tube and only then pass through the wall to be
dissipated in a coolant. This passage of heat through the reactor tube
gas and particle phase before finally contacting the wall is responsible
for the difficulty encountered in controlling the temperature in the
reactors traditionally used.