(555a) Temperature Control of Microchannel Reactors Using Bimetallic Thermally-Actuated Valves

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

email: mbaldea@che.utexas.edu

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 [1].

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 [5]. 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 [5]).

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.

References

[1] Zanfir, M.;
Gavriilidis, A. Catalytic combustion assisted methane steam reforming in a
catalytic plat reactor. Chem. Eng. Sci. 2003, 58,
3947-3960.

[2] Baldea, M.;
Daoutidis, P. Dynamics and Control of Autothermal Reactors for the Production
of Hydrogen. Chem. Eng. Sci. 2007, 62, 3218-3230.

[3] Zanfir, M.; Gavriilidis, A. Influence of flow
arrangement in catalytic plate reactors for methane steam reforming. Chem.
Eng. Res. & Des. 2004, 82, 252-258.

[4] Vaccaro, S.; Malangone, L.; Ciambelli, P.
Modelling of a catalytic micro-reactor coupling endothermic methane reforming
and combustion. Int. J. of Chem. Reactor Eng. 2010, 8.

[5] 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