Surface Reaction Kinetics for Oxidation and Reforming of H2, CO, and CH4 over Nickel-based Catalysts

An experimental and kinetic modeling study of H2 and CO oxidation, as well as conversion of methane under oxidative and reforming conditions over nickel-based catalyst is presented. The numerical model is based on a newly developed surface reaction mechanism consisting of 52 elementary-steps reactions with 14 surface and 6 gas-phase species. The mechanism was evaluated against experimental data at varying operating conditions performed in this study and also taken from literature

Kinetics of the Direct DME Synthesis: State of the Art and Comprehensive Comparison of Semi-Mechanistic, Data-Based and Hybrid Modeling Approaches

Hybrid kinetic models represent a promising alternative to describe and evaluate the effect of multiple variables in the performance of complex chemical processes, since they combine system knowledge and extrapolability of the (semi-)mechanistic models in a wide range of reaction conditions with the adaptability and fast convergence of data-based approaches (e.g., artificial neural networks—ANNs). For the first time, a hybrid kinetic model for the direct DME synthesis was developed consisting of a reactor model, i.e., balance equations, and an ANN for the reaction kinetics. The accuracy, computational time, interpolation and extrapolation ability of the new hybrid model were compared to those of a lumped and a data-based model with the same validity range, using both simulations and experiments. The convergence of parameter estimation and simulations with the hybrid model is much faster than with the lumped model, and the predictions show a greater degree of accuracy within the models’ validity range. A satisfactory dimension and range extrapolation was reached when the extrapolated variable was included in the knowledge module of the model. This feature is particularly dependent on the network architecture and phenomena covered by the underlying model, and less on the experimental conditions evaluated during model development

Kinetics of the direct DME synthesis from CO2_{2} rich syngas under variation of the CZA-to-γ-Al2_{2}O3_{3} ratio of a mixed catalyst bed

The one-step synthesis of dimethyl ether over mechanical mixtures of Cu/ZnO/Al$_{2}$O$_{3}$ (CZA) and γ-Al$_{2}$O$_{3}$ was studied in a wide range of process conditions. Experiments were performed at an industrially relevant pressure of 50 bar varying the carbon oxide ratio in the feed (CO$_{2}$ in COx from 20 to 80%), temperature (503–533 K), space-time (240–400 kg$_{cat}$s m$_{gas}$$^{-3}$), and the CZA-to-γ-Al$_{2}$O$_{3}$ weight ratio (from 1 to 5). Factors favoring the DME production in the investigated range of conditions are an elevated temperature, a low CO$_{2}$ content in the feed, and a CZA-to-γ-Al$_{2}$O$_{3}$ weight ratio of 2. A lumped kinetic model was parameterized to fit the experimental data, resulting in one of the predictive models with the broadest range of validity in the open literature for the CZA/γ-Al$_{2}$O$_{3}$ system

Surface Reaction Kinetics of Steam- and CO₂-Reforming as Well as Oxidation of Methane over Nickel-Based Catalysts

An experimental and kinetic modeling study on the Ni-catalyzed conversion of methane under oxidative and reforming conditions is presented. The numerical model is based on a surface reaction mechanism consisting of 52 elementary-step like reactions with 14 surface and six gas-phase species. Reactions for the conversion of methane with oxygen, steam, and CO₂ as well as methanation, water-gas shift reaction and carbon formation via Boudouard reaction are included. The mechanism is implemented in a one-dimensional flow field description of a fixed bed reactor. The model is evaluated by comparison of numerical simulations with data derived from isothermal experiments in a flow reactor over a powdered nickel-based catalyst using varying inlet gas compositions and operating temperatures. Furthermore, the influence of hydrogen and water as co-feed on methane dry reforming with CO₂ is also investigated

Recent Progress in Direct DME Synthesis and Potential of Bifunctional Catalysts

PtX technologies are one major building block of the future energy system based on renewables sources. Dimethyl ether (DME) is an important PtX product that can be used as intermediate the production of CO$_{2}$-neutral base chemicals. New applications lead to an increase of the global production and the optimization of the process efficiency, especially when considering decentralized synthesis. This review article puts some spotlights on recent developments in methanol and the direct DME synthesis with a special focus on the modeling and bifunctional catalyst. This study is expected to provide a foundation for future works in the field of catalysis research based on catalysts design and kinetic modeling

Surface reaction kinetics of the methanol synthesis and the water gas shift reaction on Cu/ZnO/Al₂O₃

A three-site mean-field extended microkinetic model was developed based on ab initio DFT calculations from the literature, in order to simulate the conversion of syngas (H2/CO/CO2) to methanol on Cu (211) and Cu/Zn (211). The reaction network consists of 25 reversible reactions, including CO and CO2 hydrogenation to methanol and the water-gas shift reaction. Catalyst structural changes are also considered in the model. Experiments were performed in a plug flow reactor on Cu/ZnO/Al2O3 at various gas hourly space velocities (24–40 L h−1 gcat−1), temperatures (210–260 °C), pressures (40–60 bar), hydrogen feed concentrations (35–60% v/v), CO feed concentrations (3–30% v/v), and CO2 feed concentrations (0–20% v/v). These experiments, together with experimental data from the literature, were used for a broad validation of the model (a total of 690 points), which adequately reproduced the measurements. A degree of rate control analysis showed that the hydrogenation of formic acid is the major rate controlling step, and formate is the most sensitive surface species. The developed model contributes to the understanding of the reaction kinetics, and should be applicable for industrial processes (e.g. scale-up and optimization)

COx_{x} Fixation to Elementary Building Blocks: Anaerobic Syngas Fermentation vs. Chemical Catalysis

Heterogeneous catalysis and anaerobic syngas fermentation represent two different approaches for the conversion of synthesis gas into chemicals and fuels. This review provides a unique comparison of different reaction paths for the fixation of CO$_{2}$, CO and H$_{2}$ into elementary building blocks such as methanol, acetic acid and ethanol. Operating conditions, reactor engineering, influence of gas impurities, yields, conversion efficiencies as well as downstream product recovery are compared. It was found that mass-specific productivity ranges in the same order of magnitude for both technologies, while space-time yield of heterogeneous catalysis is up to three orders of magnitude higher

A Detailed Process and Techno-Economic Analysis of Methanol Synthesis from H₂ and CO₂ with Intermediate Condensation Steps

In order to increase the typically low equilibrium CO₂ conversion to methanol using commercially proven technology, the addition of two intermediate condensation units between reaction steps is evaluated in this work. Detailed process simulations with heat integration and techno-economic analyses of methanol synthesis from green H₂ and captured CO₂ are presented here, comparing the proposed process with condensation steps with the conventional approach. In the new process, a CO₂ single-pass conversion of 53.9% was achieved, which is significantly higher than the conversion of the conventional process (28.5%) and its equilibrium conversion (30.4%). Consequently, the total recycle stream flow was halved, which reduced reactant losses in the purge stream and the compression work of the recycle streams, lowering operating costs by 4.8% (61.2 M€·a$^−$¹). In spite of the additional number of heat exchangers and flash drums related to the intermediate condensation units, the fixed investment costs of the improved process decreased by 22.7% (94.5 M€). This was a consequence of the increased reaction rates and lower recycle flows, reducing the required size of the main equipment. Therefore, intermediate condensation steps are beneficial for methanol synthesis from H₂/CO₂, significantly boosting CO₂ single-pass conversion, which consequently reduces both the investment and operating costs

Direct DME synthesis on CZZ/H-FER from variable CO2_{2}/CO syngas feeds

Catalyst systems for the conversion of synthesis gas, which are tolerant to fluctuating CO/CO$_{2}$ gas compositions, have great potential for process-technical applications, related to the expected changes in the supply of synthesis gas. Copper-based catalysts usually used in the synthesis of methanol play an important role in this context. We investigated the productivity characteristics for their application in direct dimethyl ether (DME) synthesis as a function of the CO$_{2}$/CO$_{x}$ ratio over the complete range from 0 to 1. For this purpose, we compared an industrial Cu/ZnO/Al$_{2}$O$_{3}$ methanol catalyst with a self-developed Cu/ZnO/ZrO$_{2}$ catalyst prepared by a continuous coprecipitation approach. For DME synthesis, catalysts were combined with two commercial dehydration catalysts, H-FER 20 and γ-Al$_{2}$O$_{3}$, respectively. Using a standard testing procedure, we determined the productivity characteristics in a temperature range between 483 K and 523 K in a fixed bed reactor. The combination of Cu/ZnO/ZrO$_{2}$ and H-FER 20 provided the highest DME productivity with up to 1017 g$_{DME}$ (kg$_{Cu}$ h)$^{-1}$ at 523 K, 50 bar and 36 000 ml$_{N}$ (g h)$^{-1}$ and achieved DME productivities higher than 689 g$_{DME}$ (kg$_{Cu}$ h)$^{-1}$ at all investigated CO$_{2}$/CO$_{x}$ ratios under the mentioned conditions. With the use of Cu/ZnO/ZrO$_{2}$//H-FER 20 a promising operating range between CO$_{2}$/CO$_{x}$ 0.47 and 0.8 was found where CO as well as CO$_{2}$ can be converted with high DME selectivity. First results on the long-term stability of the system Cu/ZnO/ZrO$_{2}$//H-FER 20 showed an overall reduction of 27.0% over 545 h time on stream and 14.6% between 200 h and 545 h under variable feed conditions with a consistently high DME selectivity