Model‐based optimization of hydrogen generation by methane steam reforming in autothermal packed‐bed membrane reformer

An autothermal membrane reformer comprising two separated compartments, a methane oxidation catalytic bed and a methane steam reforming bed, which hosts hydrogen separation membranes, is optimized for hydrogen production by steam reforming of methane to power a polymer electrolyte membrane fuel cell (PEMFC) stack. Capitalizing on recent experimental demonstrations of hydrogen production in such a reactor, we develop here an appropriate model, validate it with experimental data and then use it for the hydrogen generation optimization in terms of the reformer efficiency and power output. The optimized reformer, with adequate hydrogen separation area, optimized exothermic‐to‐endothermic feed ratio and reduced heat losses, is shown to be capable to fuel kW‐range PEMFC stacks, with a methane‐to‐hydrogen conversion efficiency of up to 0.8. This is expected to provide an overall methane‐to‐electric power efficiency of a combined reformer‐fuel cell unit of ～0.5. Recycling of steam reforming effluent to the oxidation bed for combustion of unreacted and unseparated compounds is expected to provide an additional efficiency gain. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

甲烷及水蒸气对生物质催化气化合成气和焦油的影响

以松木木屑为生物质原料,在两段式反应器上进行甲烷、水蒸气对生物质催化气化影响的实验研究,考察了甲烷与生物质之比α、水碳比S/C对气体产率、碳转化率、焦油产率、焦油组分和露点温度影响的变化规律。结果表明：α从0增加到0.4,合成气中H₂的产率增加了57.4%,甲烷的加入有利于生成富含氢气的合成气;α为0.2时碳转化率最高,为86.9%,焦油产率下降了30.5%,第二、五类焦油的产率达到最低,可见适量CH₄的添加能促进焦油的转化,特别是大分子焦油和酚类的反应。随着S/C的提高,H₂产率升高,CO产率降低;S/C从1增加到1.5,各类焦油的含量均有所降低,当S/C进一步增加到2时,第二、五类焦油含量却有所上升,说明水蒸气可以促进焦油向气体分子转化的反应,但过量的水蒸气抑制酚类和大分子焦油的分解。总之,甲烷和水蒸气的适量添加均可以提高合成气中H₂的含量,降低焦油产率,提高合成气的品质,有利于气化产物的进一步利用。     

CO2 methanation: Optimal start‐up control of a fixed‐bed reactor for power‐to‐gas applications

Utilizing volatile renewable energy sources (e.g., solar, wind) for chemical production systems requires a deeper understanding of their dynamic operation modes. Taking the example of a methanation reactor in the context of power‐to‐gas applications, a dynamic optimization approach is used to identify control trajectories for a time optimal reactor start‐up avoiding distinct hot spot formation. For the optimization, we develop a dynamic, two‐dimensional model of a fixed‐bed tube reactor for carbon dioxide methanation which is based on the reaction scheme of the underlying exothermic Sabatier reaction mechanism. While controlling dynamic hot spot formation inside the catalyst bed, we prove the applicability of our methodology and investigate the feasibility of dynamic carbon dioxide methanation. © 2016 American Institute of Chemical Engineers AIChE J, 63: 23–31, 2017     

Ni/CaO‐Al2O3 bifunctional catalysts for sorption‐enhanced steam methane reforming

Ni/CaO‐Al₂O₃ bifunctional catalysts with different CaO/Al₂O₃ mass ratios were prepared by a sol–gel method and applied to the sorption‐enhanced steam methane reforming (SESMR) process. The catalysts consisted mainly of Ni, CaO and Ca₅Al₆O₁₄. The catalyst structure depended strongly on the CaO/Al₂O₃ mass ratio, which in turn affected the CO₂ capture capacity and the catalytic performance. The catalyst with a CaO/Al₂O₃ mass ratio of 6 or 8 possessed the highest surface area, the smallest Ni particle size, and the most uniform distribution of Ni, CaO, and Ca₅Al₆O₁₄. During 50 consecutive SESMR cycles at a steam/methane molar ratio of 2, the thermodynamic equilibrium was achieved using the catalyst with a CaO/Al₂O₃ mass ratio of 6, and H₂ concentration profiles for all the 50 cycles almost overlapped, indicating excellent activity and stability of the catalyst. Moreover, a high CO₂ capture capacity of 0.44 was maintained after 50 carbonation–calcination cycles, being almost equal to its initial capacity (0.45 ). © 2014 American Institute of Chemical Engineers AIChE J, 60: 3547–3556, 2014     

Innovative steam methane reforming for coproducing CO-free hydrogen and syngas in proton conducting membrane reactor

Steam methane reforming (SMR) is a commercial process to produce syngas. Normally, the as-produced syngas is characterized by a H₂/CO ratio of 3. However, such H₂/CO ratio is unsuitable for Fischer–Tropsch synthesis. The hydrogen obtained by subsequent upgrading of syngas usually contains residual CO, which readily deactivates Pt electrocatalysts in fuel cells. Here we report an innovative route by coupling SMR with H₂ removal in a proton conducting membrane reactor to coproduce syngas with a preferable H₂/CO ratio of 2 and CO-free H₂ on opposite sides of the membrane, which can be directly used for Fischer–Tropsch synthesis and fuel cells, respectively. Notably, H₂ is in-situ extracted by the membrane that only allows the permeation of H₂ as protons through the oxide lattice with infinite selectivity, and thus the obtained H₂ is CO-free. This work could provide an alternative option in one-step conversion of methane into two inherently separated valuable chemicals.     

Effects of cationic polymer on start‐up and granulation in upflow anaerobic sludge blanket reactors

The upflow anaerobic sludge blanket (UASB) has been used successfully to treat a variety of industrial wastewaters. It offers a high degree of organics removal, low sludge production and low energy consumption, along with energy production in the form of biogas. However, two major drawbacks are its long start‐up period and deficiency of active biogranules for proper functioning of the process. In this study, the influence of a coagulant polymer on start‐up, sludge granulation and the associated reactor performance was evaluated in four laboratory‐scale UASB reactors. A control reactor (R1) was operated without added polymer, while the other three reactors, designated R2, R3 and R4, were operated with polymer concentrations of 5 mg dm^?3, 10 mg dm^?3 and 20 mg dm^?3, respectively. Adding the polymer at a concentration of 20 mg dm^?3 markedly reduced the start‐up time. The time required to reach stable treatment at an organic loading rate (OLR) of 4.8 g COD dm^?3 d^?1 was reduced by more than 36% (R4) as compared with both R1 and R3, and by 46% as compared with R2. R4 was able to handle an OLR of 16 g COD dm^?3 d^?1 after 93 days of operation, while R1, R2 and R3 achieved the same loading rate only after 116, 116 and 109 days respectively. Compared with the control reactor, the start‐up time of R4 was shortened by about 20% at this OLR. Granule characterization indicated that the granules developed in R4 with 20 mg dm^?3 polymer exhibited the best settleability and methanogenic activity at all OLRs. The organic loading capacities of the reactors were also increased by the addition of polymer. The maximum organic loading of the control reactor (R1) without added polymer was 19.2 g COD dm^?3 d^?1, while the three polymer‐assisted reactors attained a marked increase in organic loading of 25.6 g COD dm^?3 d^?1. Adding the cationic polymer could result in shortening of start‐up time and enhancement of granulation, which may in turn lead to improvement in the efficiency of organics removal and loading capacity of the UASB system. Copyright © 2004 Society of Chemical Industry     

Theoretical and experimental analysis of methane steam reforming in a membrane reactor

Thermal effects on methane steam reforming process were analyzed, in a Pd-Ag (23wt%) membrane reactor as a function of several parameters, such as temperature, reactant and sweep-gas flow rate, and reactant molar ratio. Heat transfer from the oven was very important for the outlet methane conversion, which also depends on the temperature profile along the reactor. In particular, when the reactant flow rate was high the conversion degree decreased because the energy supplied was not sufficient to maintain the temperature in the reactor. A non-isothermal mathematical model was presented which reproduced the experimental data.     

Stable equilibrium shift of methane steam reforming in membrane reactors with hydrogen‐selective silica membranes

Equilibrium shifts of methane steam reforming in membrane reactors consisting of either tetramethoxysilane‐derived amorphous hydrogen‐selective silica membrane and rhodium catalysts, or hexamethyldisiloxane‐derived membrane and nickel catalysts is experimentally demonstrated. The hexamethyldisiloxane‐derived silica membrane showed stable permeance as high as 8 × 10^?8 mol m^?2 s^?1 Pa^?1 of H₂ after exposure to 76 kPa of vapor pressure at 773 K for 60 h, which was a much better performance than that from the tetramethoxysilane‐derived silica membrane. Furthermore, the better silica membrane also maintained selectivity of H₂/N₂ as high as 10³ under the above hydrothermal conditions. The degree of the equilibrium shifts under various feedrate and pressure conditions coincided with the order of H₂ permeance. In addition, the equilibrium shift of methane steam reforming was stable for 30 h with an S/C ratio of 2.5 at 773 K using a membrane reactor integrated with hexamethyldisiloxane‐derived membrane and nickel catalyst. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

On optimal sensing and actuation design for an industrial scale steam methane reformer furnace

The spatial temperature distribution in the highly energy‐intensive furnace unit in a steam reforming‐based hydrogen manufacturing plant determines the energy efficiency of the plant. While the fuel distribution among the burners can be manipulated to control the furnace temperature distribution, adequate temperature measurements is a prerequisite. Typical furnaces have hundreds of tubes and burners, and economic considerations dictate that the number of temperature sensors and flow actuators required for automatic temperature optimization be minimized. In this article, we investigate several formulations for the design of the optimal sensor and actuation configurations for an industrial furnace. We initially formulate the optimal sensor placement problem as a bi‐level optimization problem, and exploit the problem structure to obtain an equivalent mixed‐integer linear program formulation. We then provide an extension to the combined sensor and actuator placement. We demonstrate the efficacy of our approach through simulation case studies based on industrial data. © 2016 American Institute of Chemical Engineers AIChE J, 62: 3225–3237, 2016     

Simultaneous oxidative conversion and CO2 or steam reforming of methane to syngas over CoO–NiO–MgO catalyst

CO₂ reforming, oxidative conversion and simultaneous oxidative conversion and CO₂ or steam reforming of methane to syngas (CO and H₂) over NiO–CoO–MgO (Co: Ni: Mg=0·5: 0·5:1·0) solid solution at 700–850°C and high space velocity (5·1×10⁵ cm³ g⁻¹ h⁻¹ for oxidative conversion and 4·5×10⁴ cm³ g⁻¹ h⁻¹ for oxy-steam or oxy-CO₂ reforming) for different CH₄/O₂ (1·8–8·0) and CH₄/CO₂ or H₂O (1·5–8·4) ratios have been thoroughly investigated. Because of the replacement of 50 mol% of the NiO by CoO in NiO–MgO (Ni/Mg=1·0), the performance of the catalyst in the methane to syngas conversion process is improved; the carbon formation on the catalyst is drastically reduced. The CoO–NiO–MgO catalyst shows high methane conversion activity (methane conversion >80%) and high selectivity for both CO and H₂ in the oxy-CO₂ reforming and oxy-steam reforming processes at ⩾800°C. The oxy-steam or CO₂ reforming process involves the coupling of the exothermic oxidative conversion and endothermic CO₂ or steam reforming reactions, making these processes highly energy efficient and also safe to operate. These processes can be made thermoneutral or mildly exothermic or mildly endothermic by manipulating the process conditions (viz. temperature and/or CH₄/O₂ ratio in the feed). © 1998 Society of Chemistry Industry     

Particle‐resolved simulations of methane steam reforming in multilayered packed beds

Particle‐resolved CFD simulations of multilayered packed beds containing 30 particles of different particle shapes (trilobe, daisy, hollow cylinder, cylcut, and 7‐hole cylinder) with a tube to particle diameter ratio of 5, were performed to understand the effect of particle shape on pressure drop (ΔP), dispersion, CH₄ conversion and effectiveness factors for methane steam reforming reactions. The effect of different boundary conditions and particle modeling approaches were analyzed in detail. The empirical correlations (Ergun and Zhavoronkov et al.) over‐predicted the ΔP and a modified correlation was developed to predict ΔP for the particles with different shapes. Overall, the externally shaped particles (trilobe and daisy) offered lower ΔP and higher dispersion because of the lower surface area and higher back flow regions, whereas the internally shaped particles (cylcut, hollow, and 7‐hole cylinder) offered higher CH₄ conversion and effectiveness factors because of the better access for the reactants. The cylcut‐shape offered the highest CH₄ conversion/ΔP. © 2018 American Institute of Chemical Engineers AIChE J, 64: 4162–4176, 2018     

Operation of an industrial steam reformer under severe condition: A simulation study

A rigorous two‐dimensional model is developed for simulating the operation of a less‐investigated type steam reformer having a considerably lower operating Reynolds number, higher tube diameter, and non‐availability of extra steam in the feed compared with conventional steam reformers. Simulation results show that reasonable predictions can only be achieved when certain correlations for wall to fluid heat transfer equations are applied. In all cases, strong radial temperature gradients inside the reformer tubes have been found. Furthermore, the results show how a certain catalyst loading profile will affect the operation of the reformer.     

An integrated process of oxidative coupling of methane and pyrolysis of naphtha in a scaled‐up unit

A heat‐effective ‘integrated’ process of C₂H₄ production, incorporating exothermic oxidative coupling of methane (OCM) carried out in the catalytic section of a flow tubular reactor, and endothermic pyrolysis of naphtha carried out in the postcatalytic section of the same reactor, studied earlier in a small silica reactor, was examined now in a scaled‐up unit with a stainless‐steel (1H18N9T) reactor (volume 400 cm³, Li/MgO catalyst bed 165 cm³). It was demonstrated that depending on the operating conditions, such an integrated process could be realized over a wide range of the relative contribution of the two component processes, leading always to an increase in the C₂H₄ yield, as compared with OCM or pyrolysis alone. A high degree of additivity of the yields of all products was observed in all cases, independently of the relative contribution of OCM and pyrolysis. Such results indicated that in the scaled‐up unit with a stainless‐steel reactor, the interactions between the component processes and products were only negligible under experimental conditions. The overall balance of CH₄, being consumed in OCM and formed in pyrolysis, was negative, equal to zero, or positive, depending on the relative contribution of the component processes. The integrated process could be based, therefore, either on CH₄ and naphtha as raw materials or exclusively on naphtha, with the recirculation of the excess of CH₄ to the OCM section. Copyright © 2004 Society of Chemical Industry     

An Extensive Analysis on the Start‐up of a Simple Distillation Column with Multiple Steady States

The impact of start‐up procedures on the behaviour of simple distillation columns with multiple steady states is analyzed. A well‐known system with hysteresis is studied in this paper and transient responses for a binary distillation column yielding different steady states during the start‐up operation are shown. Several dynamic simulation results showing an interesting behaviour are presented. Through the examination of the profile evolutions corresponding to given start‐up policies, it can be seen how the column arrives to different steady states. It is shown that it is possible to identify a set of critical values for the start‐up supervision. Also, guidelines of general validity are achieved with the aim of finding the appropriate start‐up policy to obtain the desired solution.     

Computational study of staged membrane reactor configurations for methane steam reforming. II. Effect of number of stages and catalyst amount

The present work complements part I of this article and completes a computational analysis of the performances of staged membrane reactors for methane steam reforming. The influence of the number of stages and catalyst amount is investigated by comparing the methane conversion and hydrogen recovery yield achieved by an equisized‐staged reactor to those of an equivalent conventional membrane reactor for different furnace temperatures and flow configurations (co‐ and counter‐current). The most relevant result is that the proposed configuration with a sufficiently high number of stages and a significantly smaller catalyst amount (up to 70% lower) can achieve performances very close to the ones of the conventional unit in all the operating conditions considered. This is equivalent to say that the staged configuration can compensate and in fact substitute a significant part of the catalyst mass of a conventional membrane reactor. To help the interpretation of these results, stage‐by‐stage temperature and flux profiles are examined in detail. Then, the quantification of the performance losses with respect to the conventional reactor is carried out by evaluating the catalyst amount possibly saved and furnace temperature reduction. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

The start‐up of anaerobic sequencing batch reactors at 20 °C and 25 °C for the treatment of slaughterhouse wastewater

The objective of this research was to evaluate the feasibility, the stability and the efficiency of a start‐up at 20 °C and 25 °C of anaerobic sequencing batch reactors (ASBRs) treating slaughterhouse wastewater. Influent chemical oxygen demand (COD) and suspended solids concentrations averaged 7500 and 1700 mg dm^?3, respectively. Reactor start‐up was completed in 168 and 136 days at 20 °C, and 25 °C, respectively. The start‐up process was stable at both temperatures, except for a short period at 20 °C, when effluent volatile fatty acid (VFA) concentrations increased from an average of 40 to 400 mg dm^?3. Effluent quality varied throughout start‐up, but in the last 25 days of the experiment, as the ASBRs were operated under organic loading rates of 2.25 ± 0.21 and 2.86 ± 0.24 kg m^?3 d^?1 at 20 °C and 25 °C, respectively, total COD was reduced by 90.3% ± 1.3%. Methanogenesis was not a limiting factor during start‐up. At 20 °C, the limiting factor was the acidification of the soluble organics and, to a smaller extent, the reduction of propionic, isobutyric and isovaleric acids into lower VFAs. At 25 °C, the limiting factor was the hydrolysis of particulate organics. To minimize biomass loss during the start‐up period, the organic loading rate should be increased only when 75 –80% of the COD fed has been transformed into methane within the design hydraulic retention time. © 2001 Society of Chemical Industry     

催化剂颗粒形状对甲烷水蒸气重整反应的影响及工业反应器模拟

甲烷水蒸气重整工艺是现阶段最主要的工业制氢技术,催化剂颗粒形状和反应器操作条件是影响重整反应器性能和产物组成的重要因素。首先从颗粒尺度研究催化剂形状对甲烷水蒸气重整反应的影响,在不同的反应温度和压力下,计算并比较了球形、柱形和环形催化剂的效率因子,其大小顺序为:柱形 < 球形 < 环形。其次,将反应器床层的质量、热量和动量传递与环形催化剂颗粒的扩散-反应方程相结合,建立了用于描述甲烷水蒸气重整工业反应器的一维轴向数学模型。计算并分析了反应器进口温度和压力对反应器床层的温度和压力分布、催化剂效率因子以及甲烷转化率和各组分浓度分布的影响,确定了适宜的工业反应器进口温度和压力,分别为773 K和3 MPa。