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ublic concern about global warming has led to increased attention on the monitoring and control of almost all greenhouse gases (GHGs). Recognizing that the global warming potential of methane (the second important GHG after carbon dioxide (CO2)) is 21 times higher than that |
of carbon dioxide, scientists have considered the catalytic combustion of the more harmful methane (CH4) into CO2.
Catalytic combustion is chosen because it can occur at lower temperatures and produces fewer harmful by-products. The combustion unit will also be smaller and can be located in or combustion units will not be allowed. Also, methane combustion is an exothermic reaction and hence, the heat produced from combustion can be used for other purposes. Our work is concerned with lean methane emissions, i.e. methane emitted mainly through leaks in natural gas transmission facilities such as pipelines, compressor stations, upstream oil and gas production facilities.
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Figure 1: Schematic of a Reverse Flow Reactor. |
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Figure 2: Typical results showing the effectiveness of RMPC. |
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Through a grant from the Academic Research Fund, we investigated the issues in the operation of a reverse flow reactor to combust methane. Figure 1 illustrates the concept of a Reverse Flow Reactor (RFR). It consists of a packed bed reactor in which the flow direction is reversed periodically. During start-up, the reactor section is preheated (using an external heat source) to the ignition temperature. The control valves A and D are open (valves B and C are closed) for the forward flow (Figure 1a) and the control valves B and C are open (valves A and D are closed) in order for the flow to be in the opposite direction, i.e. reverse flow (Figure 1b). When the flow is in a particular direction (either forward flow or reverse flow), the heat released by the reaction is trapped in the inert monolith section which is located next to the reactor section. This trapped heat is used to heat up the feed when the flow direction is reversed. Thus, a sustained autothermal
operation is possible.
The system was modeled in COMSOL. The importance of various phenomena in the process, the sensitivity of various physical and operating parameters on the reactor sustainability, and derivations of useful analytical expressions useful for model based control were obtained from scaling. Based on the knowledge obtained from this theoretical analysis, a reduced order model was deduced and used to implement advanced control strategies for the RFR. Considering the periodic nature of the system, a novel control strategy that combines basic concepts of Iterative Learning Control, Repetitive Control and Model Predictive Control (called Repetitive Model Predictive Control, RMPC) was proposed and verified via simulations. This control algorithm controls the maximum temperature and the exit concentration for both lean and rich feed conditions (see Figure 2). Finally, it has been shown that the RFR is efficient for lean feed conditions while an alternate configuration called the multiport switching reactor
(MPSR) is appropriate for rich feed conditions. Based on the results obtained, a new reactor configuration which can be operated more efficiently over a broad range of operating conditions has been proposed. This funded project led to three tier 1 publications and one tier 2 publication.
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Dr
Lakshminarayanan is an Assistant Professor in the Department of Chemical and Biomolecular Engineering, NUS since July 2001. Prior to that, he was a Senior Principal Consultant at Mitsubishi Chemical Corporation, Japan. His research interests span Informatics, Advanced Process Control and Optimization. He has authored about 53 journal articles, made about 90 conference presentations and presented several invited and keynote talks. Besides research, he is keenly interested in student centered pedagogy and actively participates in outreach efforts.
Email: chels@nus.edu.sg |
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