Transient Effects during Dynamic Operation of a Wall‐Cooled Fixed‐Bed Reactor for CO2 Methanation

The power‐to‐gas process is an option to transform fluctuating renewable electric energy into methane via water electrolysis and subsequent conversion of H₂ by methanation with CO₂. The dynamic behavior of the methanation reactor may then be a critical aspect. The kinetics of CO₂ methanation on a Ni‐catalyst were determined under isothermal and stationary conditions. Transient isothermal kinetic experiments showed a fast response of the rate on step changes of the concentrations of H₂, CO₂; in case of H₂O, the response was delayed. Non‐isothermal experiments were conducted in a wall‐cooled fixed‐bed reactor. Temperature profiles were measured and the effect of a changing volumetric flow was studied. The experimental data were compared with simulations by a transient reactor model.     

Applying Reaction Kinetics to Pseudohomogeneous Methanation Modeling in Fixed-Bed Reactors

Reactor simulations can reduce the effort when designing fixed-bed reactors for methanation processes. Several microkinetic models were developed under a variety of operating conditions. However, most production-scale fixed-bed methanation processes exceed the temperature range in which these kinetic models were obtained. In addition, heat and mass transport limitations strongly influence the reaction kinetics. In this work, microkinetic rate equations for CO and CO₂ methanation were analyzed with respect to their suitability for high-temperature, pseudohomogeneous reactor modeling. The best-suited kinetic model was fitted to the operating conditions and validated by means of CFD simulations. It is shown that the simulations match the experimental data for various operating conditions.     

Global Reaction Kinetics of CO and CO2 Methanation for Dynamic Process Modeling

For the design and optimization of methfanation processes detailed modeling and simulation work is advisable. However, only a few kinetics published in literature rely on wide temperature and pressure ranges, which are prevalent at modern methanation applications with dynamic operation. Especially the simulation‐based design of methanation processes with commercial catalysts is difficult due to legal restrictions regarding the publication of kinetic data of those catalysts. In this work, rate equations for the dynamic modeling and simulation of methanation processes operating with commercial Ni/Al₂O₃ catalysts are selected, adapted, and tested in a dynamic reactor model. The results suggest that the catalyst's nickel content is an indicator for the choice of a rate equation. Testing of the equations in a reactor model meets published data for CO and CO₂ methanation and own measurements.     

Raney Ni catalysts derived from different alloy precursors Part II. CO and CO2 methanation activity

Catalytic activity, in conjunction with reaction mechanism, was studied in the methanation of CO and CO₂ on three Raney Ni catalysts derived from different Ni-Al alloys using different leaching conditions. Main products were CH₄ and CO₂ in CO methanation, and CH₄ and CO in CO₂ methanation. Any other hydrocarbon products were not observed. Over all catalysts, CO methanation showed lower selectivity to methane and higher activation energy than CO₂ methanation. The catalyst derived from alloy having higher Ni content using more severe leaching conditions, namely higher reaction temperature and longer extraction time, showed higher specific activity and higher selectivity to methane both in CO and CO₂ methanation. In CO and CO₂ methanation on Raney Ni catalyst, catalytic activity was seen to have close relation with the activity to dissociate CO This paper was presented at the 2004 Korea/Japan/Taiwan Chemical Engineering Conference held at Busan, Korea between November 3 and 4,2004.     

Dynamic modeling and simulations of the behavior of a fixed‐bed reactor‐exchanger used for CO2 methanation

A multidimensional heterogeneous and dynamic model of a fixed‐bed heat exchanger reactor used for CO₂ methanation has been developed in this work that is based on mass, energy and momentum balances in the gas phase and mass and energy balances for the catalyst phase. The dynamic behavior of this reactor is simulated for transient variations in inlet gas temperature, cooling temperature, gas inlet flow rate, and outlet pressure. Simulation results showed that wrong‐way behaviors can occur for any abrupt temperature changes. Conversely, temperature ramp changes enable to attenuate and even fade the wrong‐way behavior. Traveling hot spots appear only when the change of an operating condition shifts the reactor from an ignited steady state to a non‐ignited one. Inlet gas flow rate variations reveal overshoots and undershoots of the reactor maximum temperature. © 2017 American Institute of Chemical Engineers AIChE J, 64: 468–480, 2018     

Mixed oxygen ionic-carbonate ionic conductor membrane reactor for coupling CO2 capture with in situ methanation

CO₂ methanation is one of the vital reactions to utilize CO₂ and realize power to gas process. To decrease the CO₂ capture cost and alleviate the hot spots during the strong exothermic methanation reaction, here, we report a coupling of CO₂ capture process with in situ CO₂ methanation process through a ceramic-molten carbonate (MC) dual phase membrane reactor over the Ni-based catalyst. The performance of the membrane reactor was systematically investigated and compared with the traditional fixed-bed reactor. The results show that the performance of the membrane reactor is higher than that of the fixed-bed reactor, since the produced steam through the methanation process can be partially removed through the dual-phase membrane, which promotes the reaction shift to right side. A stability test shows no obvious degradation within 32 h. These results indicate that the membrane reactor is promising for coupling CO₂ capture with in situ methanation process.     

3D CFD Simulation of Reaction Cells,Cooling Cells,and Manifolds of a Flatbed Reactor for CO2 Methanation

The methanation of CO₂ has attracted great interest in recent years as a technology to generate renewable synthetic natural gas and to recycle CO₂ from different sectors. The actual development state of a flatbed reactor for the methanation of pure stoichiometric feed gas is presented. Additionally, computational fluid dynamics (CFD)-based design strategies are introduced which can be applied for the development and optimization of different processing units. The results of the reactor development demonstrate a good heat exchange and flow distribution in the reactor.     

Kinetics of two-step methanol synthesis in the slurry phase

The carbonylation of methanol using potassium methoxide catalyst and hydrogenolysis of methyl formate using a copper-chromite catalyst (39% Cu; 37% Cr and 3% Mn) were studied in the temperature ranges of 60–110°C and 100–140°C and pressure ranges of 25–65 and 30–60 bar respectively in a mechanically agitated reactor. Kinetic rate expressions are presented for both reactions. The carbonylation reaction was found to be rapid and limited by equilibrium at the conditions studied. The apparent activation energy for the carbonylation was found to be 67.7 ± 1.5 kJ/mol. CO₂ reacts with the potassium methoxide catalyst and stops the reaction. The hydrogenolysis reaction was found to be slow at the studied conditions with an apparent activation energy of 69.8 ± 2.0 kJ/mol. CO inhibited the hydrogenolysis reaction over the copper-chromite catalyst used. CO₂ poisoned the copper-chromite catalyst. A Langmuir-Hinshelwood type rate model was used to fit the experimental data. A brief discussion of the feasibility of the two-step methanol synthesis in a single stage reactor is given. The data would be useful for evaluating the possibility of synthesizing methanol from H₂ and CO using these reactions either in two separate reactors or concurrently in one reactor.     

In situ perpetual regeneration of a Ni-faujasite methanation catalyst

A prolonged lifetime of a Ni-faujasite methanation catalyst and a stable rate of methanation can be achieved by operating the fluidized catalyst bed under unsteady state conditions. A higher rate of methanation is accompanied by a shift in selectivity compared to a steady state operation due to an enhanced disproportionation of CO.     

OPTIMUM REACTOR DESIGN IN METHANATION PROCESSES WITH NONUNIFORM CATALYSTS

A generic methodology is developed to design a heterogeneous catalytic reactor for methanation processes. For the optimization of a heterogeneous catalytic reactor, nonuniform catalyst pellets such as a layered catalyst are considered with respect to reaction type, reactor performance, and component distribution inside the catalyst. Heterogeneous uniform and nonuniform catalyst models were developed to analyze the effect of mass and heat transfer between both bulk phase and catalyst surface and inside a catalyst pellet. Then, concentration profiles of hydrogen and carbon monoxide in the catalyst pellet and along the reactor axis were obtained by analyzing simulation results. It was shown that the application of different types of nonuniform catalyst pellets at a certain number of separate zones within a reactor could produce higher catalyst performance than a reactor with uniform catalyst. Furthermore, it proved a significant decrease of catalyst deactivation behavior such as coking and sintering. Layered catalysts were optimized to maximize an overall reactor performance over the catalyst lifetime, achieving capital cost reduction characterized by reactor size, catalyst amount, and degree of catalyst deactivation. Last, temperature control throughout the reactor operating periods was strategically planned for a reactor operation with distribution of nonuniform catalyst pellets. This methodology can also be usefully applied to the design of heterogeneous catalytic reactors for other processes such as hydro-treating process and cracking process.     

Synthetic natural gas from CO hydrogenation over silicon carbide supported nickel catalysts

Silicon carbide supported nickel catalysts for CO methanation were prepared by impregnation method. The activity of the catalysts was tested in a fixed-bed reactor with a stream of H₂/CO = 3 without diluent gas. The results show that 15 wt.% Ni/SiC catalyst calcined at 550 °C exhibits excellent catalytic activity. As compared with 15 wt.% Ni/TiO₂ catalyst, the Ni/SiC catalyst shows higher activity and stability in the methanation reaction. The characterization results from X-ray diffraction and transmission electron microscopy suggest that no obvious catalyst sintering has occurred in the Ni/SiC catalyst due to the excellent thermal stability and high heat conductivity of SiC.     

基于Aspen Plus的焦炉煤气甲烷化工艺模拟及分析

焦炉煤气作为优质的二次能源,利用焦炉煤气甲烷化合成天然气(SNG)是焦炉煤气资源化利用的最佳方式。借助Aspen Plus软件,采用BWRS状态方程,设定主要工艺参数,对绝热式三段固定床焦炉煤气甲烷化工艺进行模拟计算分析,通过调节循环率和水蒸汽添加量控制反应器出口温度,模拟结果与实际试验数据较吻合,证明模拟可靠。考察了循环率、分流率、原料气组成、进口气压力和空速对反应器出口温度和组成的影响,结果表明循环率和分流率对反应器出口温度和转化率影响明显。     

Entwicklung des Katalysatorbetts in einem Flachbettreaktor zur CO2-Methanisierung

This work presents a development step for a novel flatbed reactor for CO₂ methanation as well as a CFD-based method that can be used for the development and optimization of reactors. The development step comprises the construction of the catalyst bed in the flat bed reactor into which a pure stoichiometric reactive gas is fed. The results show that a high CO₂ conversion (92.9 %) and a small rise in temperature in the catalyst bed (12.4 °C) can be achieved with an arrangement of the catalyst bed. The heat flow reaches 1924 W m⁻² in the hot spot zone.     

Ni/Al2O3催化剂对高纯HCl中微量CO2甲烷化反应的催化性能

用浸渍法制备了以Al₂O₃为载体、Ni为活性组分的Ni/Al₂O₃二氧化碳甲烷化催化剂,在等温固定床反应器中研究了在Ni/Al₂O₃催化剂作用下,高纯氯化氢中微量CO₂甲烷化反应效果,并考察了温度、压力、氯化氢体积空速以及H₂/CO₂摩尔比对CO₂转化率的影响,同时研究了催化剂活性、稳定性及其再生性能。结果表明,在温度为250℃、压力为4.0 MPa、氯化氢空速为100 h^-1、H₂/CO₂摩尔比为500:1条件下,CO₂甲烷化反应效果最好,其转化率可达到90%左右,对于高纯氯化氢中微量CO₂的脱除起到很好的效果;催化剂在温度高于300℃时,反应不久后会迅速失活;催化剂再生性能只能部分恢复到新鲜水平。     

Synthesis of Nanocrystalline CeO2 with High Surface Area by the Taguchi Method and its Application in Methanation

Mesoporous nanocrystalline cerium(IV) oxide (CeO₂) with high surface area was synthesized by precipitation using a cationic surfactant and employed as support for a nickel catalyst in CO methanation. The preparation factors of CeO₂ were optimized by the Taguchi method to achieve a sample with high surface area. The obtained results reveal that the sample prepared under optimized conditions has a mesoporous structure with high surface area and crystallite size. The addition of a surfactant significantly influences the structural properties of CeO₂ and improves the specific surface area. The optimized sample was employed as support for a nickel catalyst in CO methanation reaction. The prepared catalyst possessed a high activity compared to a commercial methanation catalyst.     

Pure Hydrogen and Propylene Coproduction in Catalytic Membrane Reactor-Assisted Propane Dehydrogenation

Propane dehydrogenation on a commercial Pt-Sn/Al₂O₃ catalyst in a Pd-Ag membrane reactor is considered. A mathematical model is developed to evaluate the performance of the catalytic membrane reactor for the process of propane dehydrogenation. Design and operating conditions are systematically evaluated for key performance metrics such as propane conversion, propylene selectivity, hydrogen selectivity, and hydrogen recovery under different operating conditions. The results confirm that the high performance of the membrane reactor is related to the continuous removal of hydrogen from the reaction zone to shift the reaction equilibrium towards the formation of more propylene and hydrogen.     

Studies of the fischer-tropsch synthesis on a cobalt catalyst II. Kinetics of carbon monoxide conversion to methane and to higher hydrocarbons

Six Langmuir-Hinshelwood-Hougen-Watson models have been derived for the kinetics of conversion of carbon monoxide to hydrocarbons in the Fischer-Tropsch synthesis. The models were fitted to experimental data obtained in an internal recycle reactor over a wide range of operating conditions. Two models, one based on the hydrogenation of surface carbon and the other on a hydrogen-assisted dissociation of carbon monoxide as rate limiting steps were both able to provide a satisfactory fit to the experimental rate data. A general model was also developed for the rate of methanation in the presence of higher hydrocarbons. The same two rate limiting assumptions as those used in formulating the rate of total CO conversion are used in these models. The two models were fitted to experimental data for methane formation. It was the model assuming CH formation as rate limiting that showed the best fit for both CO conversion for CH₄ formation.     

Modeling and optimization of a large-scale slurry bubble column reactor for producing 10,000 bbl/day of Fischer-Tropsch liquid hydrocarbons

A user-friendly simulator based on a comprehensive computer model for slurry bubble column reactors (SBCRs) for Fischer-Tropsch (F-T) synthesis, taking into account the hydrodynamics, kinetics, heat transfer, and mass transfer was developed. The hydrodynamic and mass transfer data obtained in our laboratories under typical F-T conditions along with those available in the literature were correlated using Back Propagation Neural Network and empirical correlations with high confidence levels. The data used covered wide ranges of reactor geometry, gas distributor, and operating conditions. All reactor partial differential equations, equation parameters and boundary conditions were simultaneously solved numerically.The simulator was systematically used to predict the effects of reactor geometry (inside diameter and height) as well as superficial gas velocity and catalyst concentration on the performance of a large-scale SBCR provided with cooling pipes and operating under F-T conditions with cobalt-supported catalyst and H₂/CO = 2. The performance of the SBCR was expressed in terms of CO conversion, liquid hydrocarbon yield, catalyst productivity, and space time yield. The simulator was also used to optimize the reactor geometry and operating conditions in order to produce 10,000 barrels/day (bbl/day) of liquid hydrocarbons.     

Transient behaviour of an adiabatic fixed-bed methanator—II: Methanation of mixtures of carbon monoxide and carbon dioxide

The transient behaviour of an adiabatic fixed-bed methanator has been studied using the hydrogenation of mixtures of CO and CO₂ at concentrations up to 2·7 vol.% carbon oxide in hydrogen as the test reactions. Responses to disturbances in feed conditions were studied by measuring the axial temperature profile as a function of time. The results show that the dynamic behaviour of the reactor is complicated by the inhibition by CO of the methanation of CO₂.The agreement between theory and experiment was again quite satisfactory: the quasi-homogeneous plug flow model which applied to experiments using binary mixtures of hydrogen and a carbon oxide applies to the data obtained with mixtures of CO, CO₂ and hydrogen, provided that the successive hydrogenation of CO and CO₂ is taken into account. However, it is improbable that the quasi-homogeneous model can be applied to industrial methanation, when the higher temperatures and consequent faster rates of methanation are likely to cause heat and mass transfer limitations. Nevertheless, there is no doubt that response times of but a few seconds must be expected in industrial methanation.     

A novel reverse flow reactor coupling endothermic and exothermic reactions. Part I: comparison of reactor configurations for irreversible endothermic reactions

A new reactor concept is studied for highly endothermic heterogeneously catalysed gas phase reactions at high temperatures with rapid but reversible catalyst deactivation. The reactor concept aims to achieve an indirect coupling of energy necessary for endothermic reactions and energy released by exothermic reactions, without mixing of the endothermic and exothermic reactants, in closed-loop reverse flow operation. Periodic gas flow reversal incorporates regenerative heat exchange inside the reactor. The reactor concept is studied for the coupling between the non-oxidative propane dehydrogenation and methane combustion over a monolithic catalyst.Two different reactor configurations are considered: the sequential reactor configuration, where the endothermic and exothermic reactants are fed sequentially to the same catalyst bed acting as an energy repository and the simultaneous reactor configuration, where the endothermic and exothermic reactants are fed continuously to two different compartments directly exchanging energy. The dynamic reactor behaviour is studied by detailed simulation for both reactor configurations. Energy constraints, relating the endothermic and exothermic operating conditions, to achieve a cyclic steady state are discussed. Furthermore, it is indicated how the operating conditions should be matched in order to control the maximum temperature. Also, it is shown that for a single first order exothermic reaction the maximum dimensionless temperature in reverse flow reactors depends on a single dimensionless number. Finally, both reactor configurations are compared based on their operating conditions. It is shown that only in the sequential reactor configuration the endothermic inlet concentration can be optimised independently of the gas velocities at high throughput and maximum reaction coupling energy efficiency, by the choice of a proper switching scheme with inherently zero differential creep velocity and using the ratio of the cycle times.In this first part, both the propane dehydrogenation and the methane combustion have been considered as first order irreversible reactions. However, the propane dehydrogenation is an equilibrium reaction and the low exit temperatures resulting from the reverse flow concept entail considerable propane conversion losses. How this ‘back-conversion’ can be counteracted is discussed in part II Chemical Engineering Science, 57, (2002), 855-872.