Packed-Bed Reactor Performance Enhancement by Membrane Distributed Feed: The Case of Methanol Oxidative Dehydrogenation
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Packed-Bed Reactor Performance Enhancement by Membrane Distributed Feed: The Case of Methanol Oxidative Dehydrogenation Victor Diakov and Arvind Varma* Department of Chemical Engineering, and Center for Molecularly Engineered Materials University of Notre Dame, Notre Dame, IN 46556 USA Abstract For methanol oxidative dehydrogenation to formaldehyde, the performance of the packed-bed membrane reactor (PBMR) is compared with that of the conventional fixed-bed reactor (FBR) over a wide range of operating conditions. The reaction was studied in three reactor configurations: the conventional FBR and the packed-bed membrane reactor, with either methanol (PBMR-M) or oxygen (PBMR-O) as the permeating component. The kinetics of methanol and formaldehyde partial oxidation reactions were determined and incorporated in a PBMR model. Both experimental data and model considerations demonstrate that the PBMR enhances reactant conversion and selectivity. Small oscillations in CO production were observed experimentally. Their amplitude was taken as a basis for comparison of packed-bed operation instability. The likely source of oscillatory behavior is the non-uniformity in reaction conditions along the reactor. It was found that membrane distributed feed, by providing a more uniform reactor operation, is an effective remedy from these instabilities. It is found, both by simulations and experimental observations, that relative reactor performance depends strongly on the operating conditions. Using formaldehyde yield as the basis for optimization, optimal reactor performances are determined to be in the order: PBMR-O > FBR > PBMR-M. Further PBMR productivity enhancement is possible by optimizing the membrane feed distribution pattern. 1. Introduction For more than a decade, inert inorganic membranes have been anticipated as a means to improve packed-bed reactor performance by distributing reactant feed to the catalyst bed [1,2]. Various publications have demonstrated the advantages of packed-bed membrane reactors (PBMR) over conventional fixed-bed reactors (FBR) [3]. A schematic diagram of the membrane used for distributed addition of a reactant is shown in Figure 1. The scope of the present work is to compare PBMR and FBR performances. For this, reliable reactor models are required. Recently, our efforts have been focused on the quantitative analysis and modeling of PBMR and FBR for the industrially important methanol oxidative dehydrogenation over Fe-Mo oxide catalyst [4,5] CH3OH + O2
CH2O + H2O + 1/2O2 2 CO + 2H2O governed by a consecutive reaction network. Our prior work [6] has dealt with ethylene epoxidation over Ag/α-Al2O3 catalyst, a parallel reaction network. For both reaction schemes, a plug-flow PBMR model, employing experimentally determined kinetics, was found to predict reactor performance accurately. In the present work, we use this model and the experimental data for the methanol partial oxidation network, to demonstrate the benefits of a PBMR as compared to a FBR. 1
* Author to whom correspondence should b
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