1. Introduction
For
some reactions, for example certain single reactions that are chemically
irreversible or endothermic, maximizing the reaction temperature is
often desired because both kinetics and conversion increase with
increasing temperature. However, for many reactions, trade-offs exist
between kinetics, equilibrium, and reaction selectivity [1, 2]. For
example, reversible exothermic chemical reactions generally exhibit
improved reaction kinetics but lower equilibrium conversion with
increasing temperature [3, 4]. Lowering the reaction temperature
favors higher conversion but typically requires more catalyst and a
larger reactor [5, 6]. Accordingly, more efficient utilization of
catalyst and reactor resources for a desired conversion likely requires
a non-uniform temperature trajectory for the reactants as they progress
through the reaction process. For example, for a single reversible
exothermic reaction, such as the water-gas-shift reaction, a theoretical
optimum temperature trajectory would start at a high temperature to take
advantage of fast kinetics and proceed in monotonically decreasing
fashion to lower temperatures to improve conversion [7, 8]. More
complex optimum temperature trajectories are possible with reaction
sequences or competing reactions.
There are also reasons related to energy efficiency and energetic
efficiency to control the temperature trajectory of chemical reactions.
For both endothermic and exothermic chemical reactions, greater
thermodynamic reversibility, and therefore greater system efficiently
can theoretically be achieved with reaction temperature control [9,
10]. One conventional method for controlling the temperature
trajectory for exothermic reactants as they flow through a reactor
system is to employ a sequence of separate adiabatic reactors and heat
exchangers [11, 12]. In this approach, the outlet stream from one
adiabatic reactor is cooled in a heat exchanger prior to being fed to
the next successive reactor. However, within each reactor, the
temperature increases down the length due to the heat of reaction.
Consequently, a plot of the temperature through the series of reactors
is saw-toothed rather than monotonically decreasing. A sequence of two
water-gas-shift reactors with an intervening heat exchanger is the
typical approach for fuel processors being developed to produce hydrogen
from liquid fuels for fuel cell power applications [13, 14]. In this
application, the outlet from a fuel reformer is fed to a pair of shift
reactors in series [15, 16]. The reformate is first reacted at a
temperature of about 400 °C in a high temperature shift reactor, with
the outlet stream of the high temperature shift reactor cooled to a
temperature of around 250 °C prior to introduction in a second shift
reactor [17, 18]. Overall conversion of the carbon monoxide to
carbon dioxide is typically about 90 percent. Macroscale packed-bed
reactors have also been employed to improve the temperature trajectory
for reversible exothermic reactions [19, 20]. However, in this
reactor temperature differences between the hot and cold stream at a
given cross-section are on the order of 200 °C, implying large thermal
gradients across the bed and high heat transfer resistance.
Accordingly, there exists a need for improvements in the reactor design
to provide reactors with improved temperature control and that enable
better and more precise control of reaction temperatures [21, 22]. A
microreactor, or micro-structured reactor or microchannel reactor, is a
device in which chemical reactions are designed to take place in
confined spaces having very small lateral dimensions [23, 24].
Currently, there are major technological issues that prevent current
technology from meeting the needs of micro-structured reactors for
generating hydrogen, syngas, or performing specialty chemical synthesis
[25, 26]. In general, gas-phase reactions for generating hydrogen
and other specialty chemicals require microfabricated components that
can perform under harsh operating conditions such as high temperatures,
high temperature transients, or corrosive or erosive environments [27,
28]. In general, gas-phase reactions for generating hydrogen and other
specialty chemicals require microfabricated components that can perform
under harsh operating conditions such as high temperatures, high
temperature transients, or corrosive or erosive environments.
The present study aims to provide a unique microchannel fluid processing
system for performing chemical reactions with temperature control. The
present study relates to a unique method for performing reversible
endothermic, exothermic reactions, and competing reactions. The method
comprises flowing reactants through a reaction channel in thermal
contact with a heat exchange channel, and conducting heat in support of
the reaction between the reactants and fluid flowing through the heat
exchange channel to substantially raise or lower the temperature of the
reactants as they travel through the reaction channel. The heat exchange
channel may also be a reaction channel for another chemical reaction. It
is necessary to provide effective heat exchange in an endothermic
reactor to add heat of reaction and to increase reaction temperature and
to provide chemical reactor systems with high heat transfer power
densities and reduced temperature gradients across the catalyst. It is
also necessary to provide chemical reactor systems with reduced
temperature gradients across the catalyst and to manage the temperature
profile in a reversible exothermic reactor system to have a high initial
temperature with rapid kinetics promoting an initial rapid approach to
equilibrium and cooling of the reaction as it proceeds to increase
conversion. Particular emphasis is placed upon how to provide improved
conversion and selectivity in chemical reactions, provide chemical
reactor systems that are compact, and provide thermally efficient
chemical reactor systems.