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.