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Alternative TitleThermodynamic Calculation and Nickel Catalysts Development for Methanation of Syngas
Thesis Advisor苏发兵
Degree Grantor中国科学院研究生院
Degree Discipline化学工艺
Keyword甲烷化   合成气   热力学   镍催化剂   六铝酸盐
Abstract合成气甲烷化过程(CO+3H2→CH4+H2O)作为“煤制天然气”路线的重要步骤之一,工艺和催化剂是其核心。本课题针对合成气甲烷化过程强放热,存在多个副反应,催化剂易烧结、积碳失活等核心科学问题,开展如下研究: (1)甲烷化反应工艺条件方面,采用Gibbs自由能最小化法系统地计算了反应温度、压力、氢碳比、原料气中加入水蒸汽、产品CH4循环率、少量烃类、O2等因素对CO转化率、CH4选择性和得率、积碳量的定量影响,从而确定了热力学上优选的甲烷化反应工艺条件,即较低的温度(低于450 oC),较高的压力(例如2.0-3.0 MPa),较高的氢碳比(H2/CO摩尔比不低于3.0)有利于提高CH4得率,同时减少积碳。加入少量水蒸汽,对CH4得率影响较小,但可以显著减少积碳;另外,反应过程中应尽可能减少产品CH4的循环率,同时脱除烃类和O2等杂质,以利于甲烷化反应。实验结果和文献中报导结果与理论计算的结果对比表明,两者基本一致,证明了该方法的可靠性。 (2)以典型的Ni/Al2O3为模型催化剂,系统地研究了Al2O3载体的性质(比表面积、孔结构、晶型、表面酸碱性)、金属-载体相互作用、Ni颗粒尺寸(5-35 nm)等因素对甲烷化催化剂的活性、选择性、稳定性、抗积碳性能的影响,确定了适用于合成气甲烷化催化剂的载体性质(即经过高温处理的,表面无明显酸性的,无复杂孔结构的稳定载体)以及Ni颗粒尺寸范围(即约为10-20 nm)。Ni颗粒尺寸过小将导致催化剂积碳量略微增加,Ni颗粒过大会导致催化剂的活性有所降低;这为优化甲烷化催化剂的性能提供一定参考,但是该类催化剂的稳定性需要大幅提升。 (3)基于上面研究结果,采用高温热稳定的六铝酸盐复合氧化物(BaO?6Al2O3)材料作为载体,负载Ni(NiO: 2-10 wt%)设计开发了低Ni含量的较高活性、较高稳定性、抗积碳的甲烷化催化剂,与典型的Ni/Al2O3催化剂相比,在相同Ni含量下,性能得到明显提升。接着采用炭黑模板法制备出高比表面积(> 100 m2?g-1)的六铝酸盐作为载体,并提高Ni的负载量(NiO: 10-40 wt%)以进一步提升了该类催化剂的催化性能,使催化剂具备较好的低温活性(300 oC下CO转化率大于50 %)和高温热稳定性。 (4)对改进后的六铝酸盐型Ni催化剂进行了实验室千克级放大制备和成型工艺研究。结果表明,实验室单次制备1500 g催化剂的活性与单次制备50 g的催化剂的活性基本一致。成型过程中加入一定量(2-10 wt%)的石墨,采用压片机成型得到的催化剂可以获得较高的强度,径向强度大于200 N/cm。催化性能测试表明放大制备并成型后催化剂的强度及催化性能接近商业催化剂水平,并在实验室规模200 h寿命测试中稳定运行,CH4得率稳定在94 %左右,稳定性略优于商业催化剂。
Other AbstractMethanation of syngas (CO+3H2→CH4+H2O) is an important step in "Coal to SNG" process, and methanation catalyst together with technology are the key points. Aiming at the core scientific issues of the strongly exothermic synthesis gas methanation process such as, exsisting several side reactions, easily coking, sintering, and deactivating of the catalysts, we carried out systematic research in the following aspects: (1) For the methanation reaction process, a comprehensive thermodynamic analysis of reactions occurring in the methanation of carbon oxides (CO and CO2) is conducted using the Gibbs free energy minimization method. The effects of temperature, pressure, ratio of H2/CO (and H2/CO2), and the addition of other compounds (H2O, O2, CH4, and C2H4) in the feed gas (syngas) on the conversion of CO and CO2, CH4 selectivity and yield, as well as carbon deposition, were carefully investigated. The thermodynamically preferred reaction conditions of syngas methanation were determined: relatively low temperature (< 450 oC), relatively high pressure (such as 2.0-3.0 MPa), high H2/C ratio (usually more than 3.0) is favorable for methanation, gets high CH4 yield, and reduces carbon deposition. Additionally, decreasing the recycle ritio of CH4 and remove the hydrocarbon and O2 impurity are also favorable for methanation process. The experimental results and those in references were compared with the calculated ones, and they are consistant, demonstrating the reliability of thermodynamic analysis results. (2) Typical Ni/Al2O3 catalysts were used as a model system to study the effects of the properties of Al2O3 support (specific surface area, pore structure, crystal phase, surface acidity), metal-support interaction, Ni particle size (about 5-35 nm), etc. on the activity of methanation catalyst, stability, resistant carbon performance, and we determined the preferred nature (no surface acidity and complex pore structure after calcined at high temperature and stable) of the support for methanation catalyst and Ni particle size range (about 10-20 nm). Too small Ni particles will lead a little more carbon deposition while too large ones cause some decreasing of the activity. However, the stability of these catalysts need to be inhanced dramatically. (3) Based on the above results, we used hexaaluminate composite oxide (BaO?6Al2O3) material with high thermal stability as support to design and develop relatively active, stable, and anti-coking Ni methanation catalyst (NiO: 2-10 wt%). Compared with typical Ni/Al2O3 catalysts, the performance was enhanced. Based on this, we prepared hexaaluminate with high surface area (> 100 m2?g-1) using carbon black as a hard template and used high Ni content (NiO: 10-40 wt%) to further enhance the catalytic performance of the catalyst. The improved catalyst has relatively high activity at low temperature (CO conversion > 50 % at 300 oC) and relatively high stability at high temperature. (4) The improved Hexaaluminate supported Ni catalysts were scaled up (kg class) in laboratory and the shaping process was also investigated. The performance of the catalyst produced at 1500 g per time is similar to that of the one produced at 50 g per time. The results showed that adding graphite (2-10 wt%) and using compression shaping method could obtain catalyst with good mechanical strength. The shaped catalysts showed similar strength and activity with the commercial ones, and is very stable during 200 h life test in laboratory scale. CH4 yield could stabilze at 94 % and the perfommance was a litte better than that of commercial catalysts.
Document Type学位论文
Recommended Citation
GB/T 7714
高加俭. 合成气甲烷化过程的热力学计算及其镍基催化剂开发[D]. 中国科学院研究生院,2014.
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