Metal-foam-structured Ni-based Catalysts:Wet Chemical Etching Preparation, Syngas Methanation Performance And Structure-dependent Heat/Mass Transfer

With the depletion of petroleum reserves, combined with ever-increasing demand for clean and reliable energy supply, it is exigent to develop renewable and alternative energy sources. In recent years, production of substitute natural gas (SNG) via methanation of syngas as a promising way towards the clean utilization of coal and energy utilization of biomass, has been attracting more and more attentions.On the one hand, syngas methanation is a heat transfer limited process because of the strong exothermicity of this process, and the adiabatic temperature rise can reach as high as ～620℃. Due to the poor mass/heat transfer of traditional packed beds, undesired hotspots and even temperature runaway often arise in the reactor beds, which make the temperature-control difficult. To move the reaction heat away from the reactors, such designed SNG plants have to be operated with a series of reactors and/or product gas recycling, which will result in high energy consumption. On the other hand, as one type of structured supports, metal foams with inherent outstanding features such as low pressure drop, desired mechanical robustness and high heat/mass transfer have gained ever-growing interests in the heterogeneous catalysis. In spite of these marked promising features, what to be most note-worthy is that the practical applications of these metal-foams in catalysis are still severely restricted by their low surface areas and unsatisfied catalytic functionalization. For example, the coating method which is widely used in catalytic functionalization of metal foams is still faced with many problems such as low coating amount, poor adherence to foam-surface, binder harmful contamination and high cost. Thus, the development of an energy-efficient and non-cycle SNG process with the combination of excellent catalytic performance and enhanced mass/heat transfer is still a great challenge.To solve the above problems, this paper proposed a "Top-Down" reverse design strategy for in-situ catalytic functionalization of monolithic metal foam struts in "non-coating" manner. We originally developed a new technology to prepare metal-foam-structured Ni-based catalysts via a modified wet chemical etching method. And we screened out the best monolithic catalyst Ni-CeO2-Al2O3/Ni-foam. Then the feasibility of feed gas, sulfur-resistant and stability tests were conducted. We also discussed the permeability, mass/heat transfer of metal-foam-structured Ni-based catalysts and analyzed the process intensification efficiency and structure-dependent relationship of mass-heat transfer. Finally, the intrinsic kinetics of methanation over Ni-CeO2-Al2O3/Ni-foam catalyst was investigated. The main results are shown below.· Preparation of metal-foam-structured Ni-base catalysts via the modified wet chemical etching method and their catalytic performance during syngas methanationMetal foams with enhanced mass/heat transfer and optimized fluid structure were employed as supports. Based on the inherently active chemical-property of the metal foam struts, we proposed and successfully demonstrated the preparation of self-supported Ni-based catalysts by using a modified wet chemical etching method. The chemical etching solution consisting of 1.0 mmol L-1 sodium dodecyl sulphate (SDS, C12H2.5-OSO3Na),0.2 mol L-1 aceticacid (CH3COOH) and 0.3 mol L-1 aluminium nitrate (Al(NO3)3) was employed for the Ni- and CuNi-alloy foams, while such solution with nickel nitrate (Ni(NO3)2) adding to a concentration of 0.2 mol L-1 was utilized for the Cu-foam. The as-prepared structured Ni-Al2O3/Ni-foam exhibited excellent catalytic performance as well as high heat conductivity. The characterization and evaluation results indicated that wet chemical etching method can effectively and efficiently created a highly active oxide catalytic layer of 1-3 μm on the Ni-foam struts. But the modified wet chemical etching method was not suitable for Cu-foam and CuNi-alloy-foam due to the lack of enough highly active sites.We carefully investigated the effects of cell density, additives, preparation conditions (the temperature of etching solution, calcination temperature and time, reduction temperatue and time) and reaction conditions (reaction temperature and pressure, gas hourly space velocity) on the catalytic performance of Ni-foam-structured Ni-based catalysts and screened out the best monolithic catalyst Ni-CeO2-Al2O3/Ni-foam. The effects of feed gas composition and sulfur content were also investigated, and the results indicated that the Ni-foam-structured Ni-based catalyst showed good catalytic performance in various types of feed gas including coke oven gas and biomass syngas and showed a certain level of sulfur resistance. A 1500 h long-term test of CO methanation and a 1200 h long-term test of CO2 methanation were carried out over 10 mL Ni-CeO2-Al2O3/Ni-foam. In both tests the Ni-CeO2-Al2O3/Ni-foam catalyst showed a combination of excellent catalytic activity and good heat conductivity with good resistance of carbon deposition and sintering, which opened a new opportunity for non-recycle and high throughput SNG plant design by taking the advantage of significantly enhanced heat/mass transfer.● Process intensification efficiency of the Ni-foam-structured Ni-based catalyst for syngas methanationThe permeability of Ni-foam-structured Ni-based catalyst was investigated by computational fluid dynamics (CFD) software Fluent at pore scale. Firstly, by comparing the results of calculation models and that of experiment, we found that the accuracy of centric ball cube model was higher. We subsequently investigated the effects of porosity and cell density on the permeability of metal foam. The results showed that the pressure drop decreased quickly with increasing the porosity but the reduction percentage of pressure drop decreased. The pressure drop increased with increasing cell density (PPI). We obtained the correlation of f= 0.163+18.8/Re after fitting the data of fanning friction factor and Reynolds number (Re).The mass transfer of Ni-foam-structured Ni-based catalyst was investigated with CO methanation as model reaction. The results indicated that mass transfer coefficient increased with increasing the velocity of fluid and the correlation of mass transfer was Shs= 0.013Res0.44Sc1/3. When Re (reactor diameter as characteristic length) was less than 19.5 in methanation, Ni-foam-structured Ni-based catalyst can meet the requirements of low pressure drop and enhancement of mass transfer at the same time.The heat transfer of Ni-foam-structured Ni-based catalyst was investigated by computational fluid dynamics (CFD) software Fluent. Due to the excellent heat transfer of Ni-foam-structured Ni-based catalyst, the released reaction heat of methanation can be transferred quickly. Thus the temperature rise of reactor bed packed with Ni-foam-structured Ni-based catalyst was much lower than the traditional randomly packed bed with catalyst pellets, and the distribution of reaction rate of Ni-foam-structured Ni-based catalyst bed was relatively uniform, which reduced the risks of temperature run-away of reactors, deactivation of catalysts caused by sintering and/or carbon deposit and improved the utilization efficiency of catalyst. The experimental results indicated that the simulation model was rational and reliable.· Kinetics and modeling study of methanationUnder the conditions of 240-330℃,0.1-2.0 MPa and GHSV of 5000-15000 h-1, we investigated the intrinsic kinetics of methanation over Ni-foam-structured Ni-based catalyst in an isothermal integral reactor. The intrinsic kinetic model of Langmuir-Hinshelwood type was derived and the parameters were fitted by Marquardt and Quasi-Newton methods. The statistical test and residual analysis indicated the model was rational and reliable.