Temperature Control of Microchannel Reactors Using Bimetallic Thermally-Actuated Valves
Richard Pattison, Akash Gupta, Melissa Donahue, and Michael Baldea
McKetta Department of Chemical Engineering
The University of Texas at Austin, 1 University Station C0400, Austin, TX 78712
Microchannel reactors coupling exothermic and endothermic reactions are one of the most prominent results of the process intensification paradigm. The high surface-area-to-volume characteristics of CPRs result in a process with minimal transport limitations and consequently, a size and cost that are an order of magnitude less than conventional reactors of the same capacity .
While CPRs bring significant potential economic benefits, they also pose significant control and operational challenges. Synchronizing heat generation and consumption in the reactors is particularly challenging, and if not properly addressed, could lead to the formation of temperature hot spots that may damage the catalyst coating of the plates or the reactor structure itself [2-4]. Furthermore, in practical scenarios, the reactors are subject to disturbances in the inlet conditions and potentially unequal distribution of flow to the channels, which may further contribute to the formation of hot spots. The implementation of measurements (temperature, composition, and/or flow rate) at the channel level is practically challenging. As a consequence, thus far, the majority of feedback control systems for CPRs rely on measuring the properties (typically temperature) of the bulk outlet streams, and on boundary actuation, in the sense that the total (rather than channel-wise) flow rate of fuel is adjusted in response to changes in reactor outputs.
In this work, we present a novel approach for microchannel reactor control, that provides actuation and temperature control at the individual channel level. Specifically, we introduce a new class of thermally-actuated valves constructed from bimetallic strips . Bimetallic strips consist of two different metal strips that are rigidly attached. The difference in thermal expansion properties of the two metals causes the strips to deflect upon changes in temperature, transforming temperature variations into mechanical displacement. Thus, affixing bimetallic strips to either side of the combustion channels in microchannel reactors is equivalent to implementing a thermally-actuated valve as seen in Figure 1.
Figure 1: Response of a thermally-actuated valve to temperature changes. Tss represents the design temperature (adapted from ).
In this system, the flow rate through the channel is dictated by the pressure drop across the valve. If the operation deviates from the nominal conditions, temperature changes in the reactor will result in a deflection of the strips and a change in the valve position, and consequently an adjustment in the fuel flow rate. For example, in the presence of a disturbance that reduces the amount of heat consumed in the reforming channels, the temperature in the reactor will rise causing the strips to deflect toward the channel center. Consequently, the fuel flow rate will be restricted, and, in turn, the heat generated in the combustion channel will drop, returning reactor temperature towards the nominal value.
Using a detailed model of a steam methane reforming CPR as a case study, we illustrate the considerations involved in the design of thermally-actuated control valves, and demonstrate via dynamic simulations their effectiveness for temperature control when the reactor is subject to flow maldistribution and operational disturbances.
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 Pattison, R.C., Donahue, M.M., Gupta, A., Baldea, M. Localized Temperature Control in Microchannel Reactors Using Bimetallic Thermally-Actuated Valves, Ind. Eng. Chem. Res., in press