Knowledge Management System Of Institute of process engineering,CAS
|关键词||合成气 输送床 甲烷化 催化剂 耐磨|
以煤为原料生产替代天然气可为我国边远地区低阶煤的高值利用开辟广阔的应用前景，同时对保障我国能源安全具有重要的战略意义。目前唯一商业化的甲烷化工艺——多段绝热固定床工艺存在流程复杂、控制难度大、催化剂易高温失活等问题。由于合成气甲烷化属于典型的快速表面反应过程，充分利用输送床高效传热、传质及处理量大的优点，本论文提出输送床-固定床两段甲烷化工艺。该工艺采用导热系数较高、热容量大的固体催化剂颗粒循环完成反应移热和反应温度的有效调控，实现合成气的高效转化，同时大大降低催化剂用量。输送床甲烷化技术需要突破的关键问题有两个：一是高效耐磨流化床甲烷化催化剂的开发，另一个是输送床甲烷化新工艺的研究。所以本论文的主要研究内容如下： （1）合成气两段甲烷化工艺的过程模拟。使用Aspen PlusTM软件对不同的两段甲烷化工艺进行过程模拟，以证明两段甲烷化工艺的可行性并确定最优的反应器组合方式。两段绝热固定床串联的甲烷化工艺需采用高的气体循环比来控制反应温度，因此能耗较高。流化床-固定床两段甲烷化工艺的CO转化率和CH4选择性都要高于两段固定床工艺，同时实验室研究也证实了流化床甲烷化在系统活性和降低催化剂表面积碳性能方面相对于固定床的技术优势。但低气速鼓泡流化床单位体积处理量小，大型化应用难。基于CO甲烷化快速的表面反应特性，“输送床+尾部净化固定床”甲烷化工艺采用导热系数较高的催化剂颗粒作为主要热载体，可以简化工艺，减小反应器尺寸，同时降低了催化剂用量。（2）添加不同粘结剂的Ni-Mg/Al2O3流化床甲烷化催化剂。采用共沉淀法制备Ni-Mg/Al2O3前驱体，然后添加不同的粘结剂制浆后喷雾造粒，形成具有一定粒度分布的球形颗粒。本论文所用粘结剂有铝溶胶(AS)、酸性硅溶胶(SS)、铝改性硅溶胶(AM) 和碱性硅溶胶(CC)，制备的催化剂分别命名为C-33AS、C-33SS、C-33AM和C-33CC，以C-33SS为例，其中33表示溶胶中氧化物占最终制得的催化剂中氧化物的质量百分含量。空气喷射磨损测试耐磨性能：C-33SS > C-33AM > C-33AS > C-33CC，硅溶胶的添加显著提高了催化剂的抗磨损能力，C-33SS样品的磨损指数为2.98%/h。催化剂颗粒结构分析表明颗粒内部大于20 nm的孔越多，催化剂耐磨性能越差。623-923 K下各催化剂活性(主要是CO转化率)：C-33AS > C-33SS > C-33AM ≈ C-33CC。铝溶胶制备的催化剂中金属Ni分散性好，可参与反应的表面活性位数量多，因此催化活性好，但磨损指数高达7.64%/h。C-33AS、C-33SS和C-33CC在900 K、2.5 MPa下20 h稳定性测试中均表现出较好的催化稳定性，而C-33AM催化剂由于在表面生成大量不活泼碳致使催化剂活性降低。（3）硅源对Ni-Mg/Al2O3催化剂性能的影响。本论文所用硅源有正硅酸四乙酯（C-10TEOS）、酸性硅溶胶（C-33SS、C-10SS）和硅酸钠（C-10NS）。磨损测试结果表明催化剂的抗磨损能力：C-10TEOS > C-33SS > C-10NS >> C-10SS。催化剂颗粒内部的多孔性以及骨架结构共同影响其耐磨性能。TEOS作为硅源时，其水解和金属盐沉淀同时进行，SiO2骨架网络均匀分散于前驱体颗粒中，提高了前驱体粒子强度，同时颗粒骨架连接致密，使得C-10TEOS催化剂耐磨性能最好，磨损指数为2.18%/h。同时常压下催化剂活性(主要是CO转化率)：C-10TEOS > C-10NS ≈ C-33SS。C-10TEOS催化剂具有较高催化活性和稳定性、较好的抗积碳和抗Ni烧结能力，比C-33SS催化剂表现出更好的性能。（4）其他制备因素对Ni-Mg/Al2O3催化剂性能的影响。考察TEOS添加量、NiO含量以及焙烧温度对催化剂性能的影响，确定最优的催化剂组成和制备工艺参数。不同TEOS添加量所制备催化剂的耐磨性能：C-10TEOS > C-15TEOS > C-20TEOS > C-5TEOS，催化活性随SiO2含量的增多而降低。催化剂颗粒抗磨损能力与其NiO含量呈非线性关系，当NiO含量为20 wt.%时，颗粒的抗磨损能力最强，且还原后催化剂颗粒耐磨性能变化不大；而催化活性则随催化剂内部NiO含量增加而提高，但当活性组分含量高于20 wt.%时，其活性增加幅度大幅减小。焙烧温度升高，催化剂颗粒的骨架强度及颗粒间的相互作用增强，相应提高了其抗磨损能力，但中等温度(873 K)焙烧的催化剂甲烷化活性最好。综合分析采用TEOS作硅源制备流化床甲烷化催化剂时，催化剂中SiO2含量优选为10 wt.%，NiO含量优选为20 wt.%，焙烧温度优选为873 K。（5）小型输送床甲烷化反应器内耐磨催化剂反应特性研究。提高反应器操作气速和返料阀的松动风速，反应器内颗粒循环量均增加，且操作气速对系统颗粒循环量的影响要大于松动风速的影响；但操作气速的提高使得气体在反应器内停留时间缩短，造成CO转化率降低，而松动风速的提高促进了CO的转化。当输送床内固体颗粒贮量大于100 g时，颗粒贮料量对颗粒循环量和催化活性的影响均很小。返料颗粒温度对系统催化活性影响很大，提高返料颗粒温度，催化活性显著提高。输送床反应器内高效的气固反应效率及传热效率使得在673 K、4.57 m/s（653 K）高操作气速下系统仍有86%的CO转化率，且输送床内床层温差只有不到10 K。输送床床层压降随颗粒返料温度的升高而增加，随操作气速的提高而增大。对输送床内反应进行热量衡算，发现常压输送床内反应产生的热量移出主要通过高热容的固体催化剂颗粒循环来实现。
Methanation of syngas from coal for the production of substitute natural gas (SNG) is considered to be an efficient way for the use of low-rank coal resource in remote areas. This is also a national strategic clean coal technology for securing the country’s supply of natural gas for civil utilization. As the only commercialized methanation process, the multi-stage adiabatic fixed bed process is complicated and relatively difficult to control, while its used catalyst is easy to be deactivated at high reaction temperatures. Taking advantage of the fast surface reaction characteristics of syngas methanation and the high heat and mass transfer efficiency as well as high superficial gas velocity in transport bed, an advanced methanation process was proposed in this study by combining a transport bed reactor and a tail-end clean-up fixed bed reactor. The use of catalyst particles which have high thermal diffusivity and high heat carrying capacity as the main heat carrier in the new process can realize the efficient removal of exothermic heat and the effective control of reaction temperature. Meanwhile, the new process can also achieve a sufficient conversion of syngas and require much less catalyst. Two big challenges for the transport bed methanation process are the development of highly attrition-resistant methanation catalyst and the operation of this new technology because they have not been studied before. This study covers the following five major research contents. 1. Process simulation of two-stage syngas methanation processes for SNG. Process simulation using Aspen PlusTM was conducted to identify the technical feasibility and optimal reactor combination for a simple two-stage methanation process producing SNG. With two adiabatic fixed bed reactors in series the reaction temperature control requires high product gas recycle ratio which is much energy-consuming. An isothermal fluidized bed combining a fixed bed was shown to be an efficient one-pass two-stage methanation technology in which the CO conversion and the product gas quality was higher than that from the two-stage fixed bed process. Laboratory tests showed also the technical superiority of syngas methanation in fluidized bed than in fixed bed over the same catalyst in terms of the realized activity and carbon-deposition resistance. Nonetheless, the low operating velocity of the bubbling fluidized bed makes it difficult to be scaled up for the high-capacity system producing SNG. Taking advantage of the fast surface reaction characteristics of syngas methanation and high heat carrying capacity of solid catalyst particles, the methanation process based on a transport bed combing a tail-end clean-up fixed bed can not only simplifies the methanation process but also reduces the reactor size and catalyst amount required. 2. Attrition-resistant Ni-Mg/Al2O3 catalysts with different binders for fluidized bed syngas methanation. Spray granulation of catalyst precursor prepared by co-precipitation using different binders was employed to make attrition-resistant Ni-Mg/Al2O3 catalysts with suitable particle size distribution for fluidized bed methanation. In this study, the tested binders included alumina sol (AS), acidic silica sol (SS), alumina-modified silica sol (AM) and alkaline silica sol (CC). The obtained catalysts are denoted respectively as C-33AS, C-33SS, C-33AM and C-33CC, where 33 means the weight percentage of binder. By air-jet attrition test it was found that the attrition strength of the resulting catalysts followed an order of C-33SS > C-33AM > C-33AS > C-33CC. The silica binders obviously improved the attrition strength of the prepared catalysts and the attrition index of C-33SS was 2.98%/h. Characterization shows that the higher volume of pores above 20 nm, the less attrition resistance of the catalyst was. Syngas methanation over the catalysts in a fluidized bed clarified an activity order of C-33AS > C-33SS > C-33AM ≈ C-33CC at 623-923 K. The AS binder enabled highly dispersed metallic Ni and more surface active sites for methanation reactions, thus C-33AS catalyst showed the better catalytic performance but with a large attrition index of 7.64%/h. Continuous methanation tests for 20 h at 900 K and 2.5 MPa verified the stability of the catalysts using binders AS, SS and CC. Analyzing the spent catalysts via TPO demonstrated a high amount of inactive carbon on C-33AM to cause its deactivation in the 20-h test.3. Ni-Mg/Al2O3 catalysts with different silica sources for fluidized bed syngas methanation. In this study, the tested silica sources included acidic silica sol (C-33SS, C-10SS), sodium silicate (C-10NS) and tetraethyl orthosilicate (C-10TEOS). Air-jet attrition tests showed that the attrition strength of the resulting catalysts followed an order of C-10TEOS > C-33SS > C-10NS >> C-10SS. Characterizations showed that the porosity and skeletal structure have strong correlation with the catalyst attrition strength. Simultaneous hydrolysis of TEOS and co-precipitation made the C-10TEOS have dense and continuous skeletal structure to cause the high strength of its precursor, which thus improved the attrition resistance of the sprayed catalyst to have an attrition index of only 2.18%/h. Atmospheric syngas methanation over the catalysts at an SV of 600 NL·g-1·h-1 in a fixed bed reactor clarified an activity order of C-10TEOS > C-10NS ≈ C-33SS. The C-10TEOS catalyst exhibited high activity and stability under the tested harsh conditions and also good resistance to carbon formation and Ni sintering. Therefore, C-10TEOS catalyst showed the better performance than C-33SS catalyst.4. Influence of other preparation parameters on performance of Ni-Mg/Al2O3 catalyst. Attrition-resistant Ni-Mg/Al2O3 catalysts for fluidized bed syngas methanation were prepared with different amounts of TEOS (C-5TEOS, C-10TEOS, C-15TEOS and C-20TEOS) or NiO (C-10Ni, C-15Ni, C-20Ni and C-25Ni), and then calcined at different temperatures (CC-773, CC-873, CC-973 and CC-1073). This study is expected to optimize the parameters for preparing attrition-resistant methanation catalyst without much sacrifice of catalytic activity. For catalysts prepared with different amounts of TEOS, air-jet attrition tests showed that attrition strength of these catalysts followed an order of C-10TEOS > C-15TEOS > C-20TEOS > C-5TEOS, but their catalytic performance decreased with increasing the SiO2 content in the catalyst. There was a non-linear relationship between the NiO content and particle strength resisting attrition. The catalyst with 20 wt.% NiO had the highest attrition resistance and there is little change on catalyst attrition strength before and after reduction. The CO conversion increased greatly with increasing the NiO content until it was over 20 wt.%. Raising the calcination temperature increased the interaction between primary particles and thus raised the attrition resistance of the resulting catalyst. However, only those catalysts with moderate metal-support interaction, such as calcined at 873 K, showed high and stable activity of methanation. In conclusion, the catalyst granulated with 10 wt.% SiO2, 20 wt.% NiO and calcined at 873 K showed good activity and stability and high attrition resistance (2.18%/h) for fast fluidized bed methanation.5. Reaction characteristics of attrition-resistant catalyst in a laboratory transport bed reactor. The particle circulation rate increased with increasing the aeration gas velocity and superficial gas velocity. Varying superficial gas velocity had the larger effect on particle circulation rate than changing aeration gas velocity. Raising aeration gas velocity increased CO conversion, but the gas residence time reduced with increasing the superficial gas velocity to lower the CO conversion. The catalyst inventory in the reactor had little influence either on particle circulation rate or on catalytic performance, provided the inventory is above 100 g. The realized catalytic performance greatly varied with temperature of recycled particles, and CO conversion obviously increased with increasing the temperature of recycled particles. Due to the high heat transfer and reaction efficiency, the transport bed can be operated at high superficial gas velocity, such as 4.6 m/s (653 K), and the realized CO conversion can achieve 86%, with a bed temperature gradient less than 10 K. The pressure drop in the reactor increased with increasing the superficial gas velocity and the temperature of recycled particles. A heat balance calculation showed that the exothermic heat of the reactions in the transport bed under atmospheric pressure was mainly carried by the catalyst particles.
|崔佃淼. 合成气输送床甲烷化催化剂与工艺研究[D]. 北京. 中国科学院研究生院,2016.|