Experimental Investigation of the Oxidative Coupling of Methane in a Porous Membrane Reactor: Relevance of Back-Permeation

Synthesis and Characterization of Silica-Tantala Microporous Membranes for Gas Separations Fabricated Using Chemical Vapor Deposition

Composite membranes consisting of microporous tantalum-doped silica layers supported on mesoporous alumina substrates were fabricated using chemical vapor deposition (CVD) in both thermal decomposition and counter-flow oxidative deposition modes. Tetraethyl orthosilicate (TEOS) was used as the silica precursor and tantalum (V) ethoxide (TaEO) as the tantalum source. Amounts of TaEO from 0 mol% to 40 mol% were used in the CVD gas mixture and high H₂ permeances above 10^-7 mol m^-2 s^-1 Pa^-1 were obtained for all conditions. Close examination was made of the H₂/CH₄ and O₂/CH₄ selectivities due to the potential use of these membranes in methane reforming or partial oxidation of methane applications. Increasing deposition temperature correlated with increasing H₂/CH₄ selectivity at the expense of O₂/CH₄ selectivity, suggesting a need to optimize membrane synthesis for a specific selectivity. Measured at 400 °C, the highest H₂/CH₄ selectivity of 530 resulted from thermal CVD at 650 °C, whereas the highest O₂/CH₄ selectivity of 6 resulted from thermal CVD at 600 °C. The analysis of the membranes attempted by elemental analysis, X-ray photoelectron spectroscopy, and X-ray absorption near-edge spectroscopy revealed that Ta was undetectable because of instrumental limitations. However, the physical properties of the membranes indicated that the Ta must have been present at least at dopant levels. It was found that the pore size of the resultant membranes increased from 0.35 nm for pure Si to 0.37 nm for a membrane prepared with 40 mol% Ta. Similarly, an increase in Ta in the feed resulted in an increase in O₂/CH₄ selectivity at the expense of H₂/CH₄ selectivity. Additionally, it resulted in a decrease in hydrothermal stability, with the membranes prepared with higher Ta suffering greater permeance and selectivity declines during 96 h of exposure to 16 mol% H₂O in Ar at 650 °C.

Reactors for Process Intensification: Recent Advances and Key Applications

Coupling industrial development and environmental protection is one of the most important challenges for the coming years [...].

Thermochemical Performance Analysis of the Steam Reforming of Methane in a Fixed Bed Membrane Reformer: A Modelling and Simulation Study

Pd-based membrane reformers have been substantially studied in the past as a promising reformer to produce high-purity H₂ from thermochemical conversion of methane (CH₄). A variety of research approaches have been taken in the experimental and theoretical fields. The main objective of this work is a theoretical modelling to describe the process variables of the Steam Reforming of Methane (SRM) method on the Pd-based membrane reformer. These process variables describe the specific aims of each equation of the mathematical model characterizing the performance from reformer. The simulated results of the mole fractions of components (MFCs) at the outlet of the Fixed Bed Reformer (FBR) and Packed-Bed Membrane Reformer (PBMR) have been validated. When the H₂O/CH₄ ratio decreases in PBMR, the Endothermic Reaction Temperature (ERT) is notably increased (998.32 K) at the outlet of the PBMR’s reaction zone. On the other hand, when the H₂O/CH₄ ratio increases in PBMR, the ERT is remarkably decreased (827.83 K) at the outlet of the PBMR’s reaction zone. An increase of the spatial velocity (S_sp) indicates a reduction in the residence time of reactant molecules inside PBMR and, thus, a decrease of the ERT and conversion of CH₄. In contrast, a reduction of the S_sp shows an increase of the residence time of reactant molecules within PBMR and, therefore, a rise of the ERT and conversion of CH₄. An increase of the H₂O/CH₄ ratio raises the conversion rate (CR) of CH₄ due to the reduction of the coke content on the catalyst particles. Conversely, a reduction of the H₂O/CH₄ ratio decreases the CR of CH₄ owing to the increase of the coke content on the catalyst particles. Contrary to the CR of CH₄, the consumption-based yield (CBY) of H₂ sharply decreases with the increase of the H₂O/CH₄ ratio. An increase of the ERT raises the thermochemical energy storage efficiency (η_tese) from 68.96% (ERT = 1023 K), 63.21% (ERT = 973 K), and 48.12% (ERT = 723 K). The chemical energy, sensible heat, and heat loss reached values of 384.96 W, 151.68 W, and 249.73 W at 973 K. The selectivity of H₂ presents higher amounts in the gaseous mixture that varies from 60.98 to 73.18 while CH₄ showed lower values ranging from 1.41 to 2.06. Our work is limited to the SRM method. In terms of future uses of this method, new works can be undertaken using novel materials (open-cell foams) and the physical-mathematical model (two-dimensional and three-dimensional) to evaluate the concentration polarization inside membrane reactors.

Analysis and Model-Based Description of the Total Process of Periodic Deactivation and Regeneration of a VOx Catalyst for Selective Dehydrogenation of Propane

This study intends to provide insights into various aspects related to the reaction kinetics of the VOx catalyzed propane dehydrogenation including main and side reactions and, in particular, catalyst deactivation and regeneration, which can be hardly found in combination in current literature. To kinetically describe the complex reaction network, a reduced model was fitted to lab scale experiments performed in a fixed bed reactor. Additionally, thermogravimetric analysis (TGA) was applied to investigate the coking behavior of the catalyst under defined conditions considering propane and propene as precursors for coke formation. Propene was identified to be the main coke precursor, which agrees with results of experiments using a segmented fixed bed reactor (FBR). A mechanistic multilayer-monolayer coke growth model was developed to mathematically describe the catalyst coking. Samples from long-term deactivation experiments in an FBR were used for regeneration experiments with oxygen to gasify the coke deposits in a TGA. A power law approach was able to describe the regeneration behavior well. Finally, the results of periodic experiments consisting of several deactivation and regeneration cycles verified the long-term stability of the catalyst and confirmed the validity of the derived and parametrized kinetic models for deactivation and regeneration, which will allow model-based process development and optimization.