59th Annual Report on Research 2014

In the second year of the grant, we investigated new reactor configurations that improve the robustness of autothermal microchannel reactors. These systems consist of millimeter high channels separated by thin, catalyst coated plates. A set of endothermic reactions is carried out concurrently with one or more exothermic reactions, with the latter supplying heat to the former. Ideally, the heat generated at any longitudinal point is absorbed at the same location by the endothermic reactions. However, in practice, the rates of the exothermic and endothermic reactions can differ significantly, leading to local or overall mismatches in the rates of heat generation and consumption. Synchronizing heat generation and consumption remains an open problem, both at steady state and in transient situations (e.g., in the presence of disturbances, such as fluctuations in flow rate or composition at the channel inlets).

In the first year, we perfected a new concept for optimizing the steady-state longitudinal temperature profile of microchannel reactors. Our approach consisted of a segmented catalyst geometry, alternating catalytically active and inactive (blank) sections in the exothermic channels. In the second year, we concentrated on improving reactor dynamics. To this end, we introduced a temperature control concept based on change materials (PCMs), specifically, using a thin layer of PCM embedded in the reactor structure (Figure 1).

Figure 1: Novel reactor structure with embedded phase change material layer and segmented catalyst configuration in the combustion channel (reproduced from Pattison and Baldea, 2014, Proceedings of FOCAPD, Cle Elum, WA, 399-404).

If the rate of heat generation (temporarily) exceeds the rate of heat consumption at any location in a conventional reactor, the local temperature rises, potentially threatening the structural integrity and safety of the system. The PCM layer prevents the advent of such “hotspots.” Its operation is based on the thermodynamics of phase transformations: melting and solidification occur with latent heat exchange, and the temperature of the PCM remains constant during the transformation. Thus, should such temporary disparity in the rates of heat generation and consumption arise, in the proposed PCM-enhanced reactor configuration the local temperature would rise only until the melting point is reached. At this time, the PCM layer would begin melting, and the local temperature is maintained at the melting point for the entire melting cycle.

We determined the optimal thickness of the PCM layer in conjunction with the optimal number, length and longitudinal position of the catalytic and blank segments by solving an optimization problem of the form:

where  is the PCM layer thickness, i, L_combi and z_combi represent, respectively, the number, length and axial position of the catalytic segments in the exothermic channels, T_sp(z) is the value of the optimal wall target temperature at z, T^s(z) is the temperature of the wall plate at z. The constraints impose, respectively, lower bounds on the conversion in the exothermic and endothermic channels, a maximum on the possible temperature of the wall plates, a minimum length for each catalyst segment and, finally, ensure that the equations of the mathematical model of the reactor are satisfied. The problem was solved using the strategy developed in Year 1 of the grant (Pattison et al., 2014, Ind. Eng. Chem. Res., 53, 5028-5037).

To validate the theoretical concepts, a prototype autothermal system for carrying out the (endothermic) methane steam reforming reactions, supported by the (exothermic) catalytic combustion of methane was considered. The system was described using a detailed two-dimensional (2D) reactive flow model comprising a catalytic wall plate and the adjacent exothermic and endothermic half-channels, choosing a PCM with properties similar to sterling silver (melting point 893 °C) to best enforce the temperature safety limits. The optimization calculations resulted in a four-segment catalyst configuration for the combustion channels and a PCM thickness of 0.56 mm. The results are illustrated in Figure 2. The PCM-enhanced reactor has a more uniform temperature distribution compared to other configurations. This can be attributed to the thermal conductivity of the PCM, which is higher than that of the wall plates, and is an additional benefit of the proposed PCM-enhanced structure.

Figure 2: Steady-state temperature profiles (a reactor with continuous combustion catalyst structure is considered as the base case) (Pattison and Baldea, 2014, FOCAPD).

We tested the dynamic performance of the PCM-enhanced reactor by considering a significant reduction in the flow rate to the reforming channel. Simulation results (Figure 3) emphasize the benefit of the PCM, whose presence delays the advent of a temperature hotspot. Evidently, the temperature control effect ceases when the melting process is complete, and we designed nonlinear supervisory feedback controller (Pattison and Baldea, 2013, AIChE J. 59, 2051-2061) to supplement the operation of the PCM. The performance of this two-tiered control structure is shown in Figure 4.

Figure 3: Temperature response of the reactor to a 47.5% reduction of the flow rate to the reforming channel, imposed at t=100s (Pattison and Baldea, 2014, FOCAPD).

Figure 4: Temperature response of the reactor under feedback control to a 47.5% reduction of the flow rate to the reforming channel, followed by an increase to 75% of the nominal value (Pattison and Baldea, 2014, FOCAPD).

Ongoing work focuses on integrating steam-methane reforming autothermal reactors in larger process flowsheets, with the goal of devising gas-to-liquids conversion processes that are economically viable at a small scale.

Our work has been reported in four peer-reviewed publications (with one more in preparation) and six conference presentations. ACS funding has substantially supported the formation of graduate students and enabled them to present their work at major conferences. The financial support provided by this grant has been essential to the PI, allowing him to continue building his research group and to establish new, fundamental modeling and optimization concepts that will be used to expand the frontiers of process intensification research in the future. The grant has also fostered exciting research interactions, and the results of this work have been presented in several seminars that the PI gave at leading academic institutions and industry R&D centers.