催化剂孔道结构设计及孔内反应-扩散耦合模拟

采用硬球-拟颗粒(HS-PPM)耦合方法并结合简化的集总反应模型,模拟了碳四(C4)烯烃催化裂解过程中反应物和产物在孔道内的反应-扩散耦合过程,提出了一种多级孔道结构可控设计方法,实现了对孔隙率、孔径和孔体积占比等参数的独立量化调控,建立了包含三种不同孔径分布的催化剂孔道模型并定义了量化参数对反应-扩散耦合进行表征.结...     

Optimizing catalyst pore network structure in the presence of deactivation by coking

Designing the pore network structure is an effective approach to improve the performance of industrial catalyst particles, although it receives less attention than designing catalytic surfaces or active sites. This work presents a first example of the optimization of catalyst pore network structures in the presence of deactivation by coke formation, using a three-dimensional pore network model. Propane dehydrogenation in a Pt-Sn/Al₂O₃ catalyst particle is taken as the model reaction system. Catalyst particles with unimodal and bimodal pore-size distributions are investigated, both being commonly used in industry. The porosity, connectivity, pore size, and their spatial distributions are optimized under two separate assumptions: constant intrinsic activity per unit catalyst weight and constant intrinsic activity per unit internal surface area. The optimized catalyst shows up to 14-fold improvement in the time-averaged propene formation rate, when compared to a benchmark catalyst. This significant improvement is primarily because of reductions in diffusion resistance and pore blockage.     

Coupling non-isothermal trickle-bed reactor with catalyst pellet models to understand the reaction and diffusion in gas oil hydrodesulfurization

In this work, a trickle-bed reactor coupled with catalyst pellet model is employed to understand the effects of the temperature and catalyst pellet structures on the reaction–diffusion behaviors in gas oil hydrodesulfurization(HDS). The non-isothermal reactor model is determined to be reasonable due to non-negligible temperature variation caused by the reaction heat. The reaction rate along the reactor is mainly dominated by the temperature,and the sulfur concentration gradient in the catalyst pellet decreases gradually along the reactor, leading to the increased internal effectiveness factor. For the fixed catalyst bed volume, there exists a compromise between the catalyst reaction rate and effectiveness factor. Under commonly studied catalyst pellet size of 0.8–3 mm and porosity of 0.4–0.8, an optimization of the temperature and catalyst pellet structures is carried out, and the optimized outlet sulfur content decreases to 7.6 wppm better than the commercial level at 0.96 mm of the catalyst pellet size and 0.40 of the catalyst porosity.     

Optimization of catalyst pellet structures and operation conditions for CO methanation

A fundamental understanding of the effects of catalyst pellet structures and operation conditions on catalytic performance is crucial for the reactions limited by diffusion mass transfer. In this work, a numerical investigation has been carried out to understand the effect of catalyst pellet shapes (sphere, cylinder, trilobe and tetralobe) on the reaction-diffusion behaviors of CO methanation. The results reveal that the poly-lobe pellets with larger external specific surface area have shorter diffusion path, and thus result in higher effectiveness factors and CO conversion rates in comparison with the spherical and cylindrical pellets. The effects of operating conditions and pore structures on the trilobular catalyst pellet with high performance are further probed. Though lower temperature can contribute to larger effectiveness factors of pellets, it also brings about lower reaction rates, and pressure has little impact on the effectiveness factors of the pellets. The increase in porosity can reduce the pellet internal diffusion limitations effectively and there exists an optimal porosity for the methanation reaction. Finally, the height of the trilobular pellet is optimized under the given geometric volume, and the results demonstrate that the higher the trilobular catalyst, the better the reaction performance within the allowable mechanical strength range.     

Effect of particle shape on fluid flow and heat transfer for methane steam reforming reactions in a packed bed

Numerical simulations of a cylindrical packed bed with tube to particle diameter ratio of 1.4, containing 10 particles, were performed to understand the effect of particle shape on pressure drop, heat transfer and reaction performance. Six particle shapes namely, cylinder as the reference, trilobe and daisy having external shaping, hollow cylinder, cylcut, and 7‐hole cylinder including internal voids were chosen. Methane steam reforming reactions were considered for the heat transfer and reaction performance evaluation. The present study showed that the external shaping of particles offered lower pressure drop, but lower values of effectiveness factor indicating strong diffusion limitations. The internally shaped particles offered increased surface area, led to higher effectiveness factor and allowed to overcome the diffusion limitations. The effective heat transfer and effectiveness factor of the trilobe‐shaped particle per unit pressure drop was found to be the best among the particle shapes considered in the present work. © 2016 American Institute of Chemical Engineers AIChE J, 63: 366–377, 2017     

Effect of restrictive diffusion on hydrodesulfurization of heavy residue oils over CoMo/Al2O3

The effect of the ratio of reactant molecular diameter to catalyst pore diameter (λ) on the restrictive diffusion under hydrodesulfurization (HDS) reactions of heavy residue oils over CoMo/Al₂O₃ catalysts was investigated. A series of Al₂O₃ with various pore structures were used as supports of CoMo sulfide. The HDS reaction was carried out in a semi-batch Carberry type reactor at 648 K and 10.3 MPa. Two empirical correlations for restrictive diffusion under HDS reaction conditions were developed depending on the value of λ. The results showed that the restrictive diffusion effect under HDS reaction conditions is severe for the lower values of λ. However, this effect seems not prominent for higher values of λ. The order of the magnitude of the effective diffusivity was in the range of 10^?6-10^?7 cm²/s for the sulfur-containing compounds of residue oil in CoMo/Al₂O₃ catalysts at 648 K.     

Alumina-silica binary mixed oxide used as support of catalysts for hydrotreating of Maya heavy crude

Alumina-silica binary mixed oxide support is used to prepare catalysts for hydrotreating of Maya heavy crude. Support is prepared by urea hydrolysis. Sequential incipient wetness and co-impregnation techniques are employed for preparation of catalysts. Ammonium heptamolybdenum is used as precursor of MoO₃. Catalysts are characterized by X-ray diffraction (XRD), temperature-programmed reduction (TPR) and the pore size distribution. Hydrodemetallation (HDM), hydrodesulfurization (HDS), hydrodenitrogenation (HDN), and asphaltene conversion (HDAsp) reactions are studied on these catalysts. One reference catalyst is also taken for comparison. Coke and metals depositions on spent catalysts are measured. The catalyst deactivation rate is also studied. The X-ray diffraction (XRD) results reveal that molybdenum atoms are well dispersed into CoMo catalyst, whereas MoO₃ crystalline phases are found in PCoMo and PNiMo catalysts. TPR reduction profiles are different for different catalysts. The laboratory made catalyst is reduced at one temperature, whereas the reference catalyst shows two reduction profiles. The reference catalyst shows the highest activities among four catalysts. The highest HDM and HDAsp activities of the reference catalyst may be due to its bigger pore diameter. The presence of TiO₂ in the reference catalyst enhances HDS and HDN activities. The CoMo catalyst shows higher activities than those of PCoMo and PNiMo catalysts. The presence of crystalline MoO₃ causes for lower activities of catalysts PCoMo and PNiMo.     

Maya crude hydrodemetallization and hydrodesulfurization catalysts: An effect of TiO2 incorporation in Al2O3

Hydrotreating of Maya heavy crude oil over high specific surface area CoMo/TiO₂–Al₂O₃ oxide supported catalysts was studied in an integral reactor close to industrial practice. Activity studies were carried out with Maya crude hydrodesulfurization (HDS), hydrodemetallization (HDM), hydrodenitrogenation (HDN), and hydrodeasphaltenization (HDAs) reactions. The effect of support composition, the method of TiO₂ incorporation, and the catalyst deactivation are examined. Supported catalysts are characterized by BET specific surface area (SSA), pore volume (PV), pore size distribution (PSD), and atomic absorption. It has been found that sulfided catalysts showed a wide range of activity variation with TiO₂ incorporation into the alumina, which confirmed that molybdenum sulfided active phases strongly depend on the nature of support. The pore diameter and nature of the active site for HDS, HDM, HDN, and HDAs account for the influence of the large reactant molecules restricted diffusion into the pore, and/or the decrease in the number of active sites due to the MoS₂ phases buried with time-on-stream. The textural properties and hysteresis loop area of supported and spent catalysts indicated that catalysts were deactivated at the pore mouth due to the metal and carbon depositions. The atomic absorption results agreed well regarding the textural properties of spent catalysts. Thus, incorporation of TiO₂ with γ-Al₂O₃ alters the nature of active metal interaction with support, which may facilitate the dispersion of active phases on the support surface. Therefore, the TiO₂ counterpart plays a promoting role to HDS activity due to the favorable morphology of MoS₂ phases and metal support interaction.     

Second row transition metal sulfides for the hydrotreatment of coal-derived naphtha I. Catalyst preparation, characterization and comparison of rate of simultaneous removal of total sulfur, nitrogen and oxygen

Naphtha derived from an Illinois No. 6 coal contains appreciable quantities of sulfur-, nitrogen- and oxygen-containing compounds. The hydrotreatment of this naphtha has been evaluated over unsupported transition metal sulfide catalysts of the second row in the Periodic Table. The catalysts were prepared by a room temperature precipitation reaction. Surface areas, crystalline phase and particle size distributions were determined by Brunauer-Emmet-Teller (BET), X-ray diffraction (XRD) and transmission electron microscopy (TEM), respectively. A comparison of average particle sizes calculated from these three techniques has enabled the understanding of the morphology of the transition metal sulfides. The catalysts exhibit a so-called volcano plot for the HDS of dibenzothiophene. Similar so-called volcano plots are also exhibited for the simultaneous hydrodesulfurization (HDS), hydrodenitrogenation (HDN) and the hydrodeoxygenation (HDO) of the coal-derived naphtha containing a mixture of heteroatoms. The order of reactivity of the transition metal catalysts is the same for all three of the processes. Ruthenium sulfide (RuS₂) is the most active catalyst for HDS, HDN and HDO of the coal-derived naphtha.     

微波辅助制备Ni-W-P/γ-Al2O3煤焦油加氢催化剂

采用微波辅助浸渍法、微波管式焙烧制备了Ni-W-P/γ-Al₂O₃催化剂,并以中低温煤焦油轻油为原料,在固定床反应器装置上评价了催化剂的加氢活性。通过N₂吸附-脱附、GC-MS等方法对催化剂的物化性能及加氢产物油进行表征,并根据FHH模型,计算出催化剂的表面分形维数。结果表明,添加助剂P可调节催化剂的微观孔结构,改变催化剂的酸性分布与强度,并有助于加氢饱和反应的进行;当助剂P含量为0.9%时,催化剂的加氢脱硫、脱氮活性最高,加氢饱和性能最好;焙烧温度直接影响催化剂物性参数,当温度为500 ℃时,加氢活性最高、加氢产物品质最佳;微波焙烧相比常规制备方法,可增加晶粒烧结程度,形成更多三维孔隙结构,为加氢反应提供更大的表面和空间,且增加中等强度酸的酸量,更有助于表面活性组分的分散及硫化性的增强。     

分形多孔介质的孔隙特性对气体扩散的影响

采用随机行走方法建立了分形多孔介质生成模型,生成的颗粒在形貌上与真实多孔颗粒接近,且能够反映其固有分形特征。在此模型基础上,根据经典分子动理论建立扩散控制方程,对气体在多孔介质中的扩散进行数值模拟。分析了比表面积、平均孔径、孔隙率等孔隙特性参数对扩散的影响,获得了分形多孔介质中气体扩散系数与平均孔径的函数关系。结果表明,扩散系数随平均孔径的增大以幂函数形式增大,相应的指数表征扩散系数对平均孔径的敏感度,其值随孔隙率的增大呈线性减小。     

Platinum embedded within carbon nanospheres for shape selective liquid phase hydrogenation

Reactant shape selective catalysis occurs when substrates of different sizes and shapes are consumed at different rates over catalysts that combine molecular sieving transport processes with reaction. By contrast the same substrates react at nearly equivalent rates over catalysts that have large, open pores that do not induce any form of molecular sieving. Here we describe the design and synthesis of reactant shape selective catalysts for liquid phase hydrogenation reactions. Using an emulsion polymerization of furfuryl alcohol, we have made catalysts that consist of microporous carbon nanospheres within which are embedded platinum nanoparticles. The porosity of the carbon spheres was found to be a key parameter affecting catalyst activity and selectivity; porosity was varied by adding pore forming agents, such as polyethylene glycol with different molecular weights, during synthesis, or by mild oxidation of the as-synthesized catalyst using carbon dioxide. In addition to increasing porosity to reduce mass transfer limitations, a synthesis of smaller carbon spheres (<200 nm) was devised to reduce the micropore diffusion length. Decreasing the particle size of the catalyst by adjusting the surfactant composition during polymerization, improved the effectiveness factor by approximately one order of magnitude making it as active as a comparable standard metal catalyst.     

Effect of temperature on activity and selectivity of carbon supported Mo sulfide in simultaneous hydrodenitrogenation of pyridine and hydrodesulfurization of thiophene

Activity and selectivity of carbon supported Mo catalyst was tested in parallel hydrodenitrogenation (HDN) of pyridine and hydrodesulfurization (HDS) of thiophene in the temperature range 260–350 °C at 2 MPa of hydrogen pressure and compared with that of commercial NiMo-alumina catalyst Shell 324. The main advantages of carbon supported Mo sulfide over commercial NiMo catalyst can be summarized as follows: the markedly higher HDN and better HDS activities normalized to moles of active metals, the lower content of piperidine in the reaction products and the distinctly better selectivity towards HDN reaction.     

Computational analysis of the reacting flow in a microstructured reformer using a multiscale approach

A multiscale methodology is presented to analyze the transport and reaction processes in the catalyst coating of a microstructured reformer and to elucidate the effect of catalyst morphology on transport limitations and the reformer performance. This analysis includes three‐dimensional simulations of methane steam reforming at both reactor level (macroscale) and catalyst microstructure level (microscale). Hypothetical catalyst microstructures are generated using an in‐house particle packing code. Based on the generated structures, the effective transport properties of the porous catalyst and the average reaction rates in the microstructure are determined to be applied in the pseudohomogeneous model used in the macroscale simulation. Parametric study is done to demonstrate the significant effect of the catalyst intraparticle and interparticle porosity as well as the particle size on the reaction effectiveness factor and methane conversion. This study shows that an optimal catalyst coating has a decreasing porosity along the reformer length based on the difference in the degree of diffusion limitation. © 2014 American Institute of Chemical Engineers AIChE J, 60: 2263–2274, 2014     

Mass transfer advantage of hierarchical structured cobalt-based catalyst pellet for Fischer–Tropsch synthesis

The low effectiveness factor of catalyst pellet caused by high internal diffusion limitation is a common issue in fixed-bed reactor. Nevertheless, hierarchical structured catalyst provides a promising solution for the contradiction between reaction activity and diffusion efficiency in large catalyst pellets. Herein, we studied the effect of pore structure parameters of the meso-macroporous catalyst on Fischer–Tropsch synthesis performances through experiment and pellet scale reaction–diffusion simulation. The pellet simulation firstly elucidated the reason for the significant improvement on activity and product selectivity for the meso-macroporous catalyst observed in our experiment. Further optimization via pellet simulation indicated the critical influences of wax filling degree and that the perfect matching between reaction and mass transfer rates by increasing macropore size and adjusting porosity within pellet enables the C₅₊ space–time yield to the maximum. This work could provide a theoretical guideline for the engineering design of the hierarchical structured catalyst pellet.     

Effect of pore size on bitumen hydrocracking over Al2O3-AlPO4 supported Ni-Mo catalysts

A study of co-precipitated aluminum oxide-aluminum phosphate (AAP) materials as supports of Ni-Mo heavy oil upgrading catalysts has been completed. Results of both short duration (8 h) and longer duration (up to 200 h) experiments at conditions relevant to the commercial H-Oil process are reported and compared with a commercial NiMo/Al₂O₃ catalyst. The initial activity of the Ni-Mo/AAP catalysts correlates with the catalyst average pore diameter which is determined by the P content of the AAP support. An optimum pore diameter of about 20 run exists for HDM whereas for HDS a pore diameter < 10 nm is desirable. After 100 h operation the HDM conversion of the best Ni-Mo/AAP catalyst was approximately 10 percentage points greater than for the commercial catalyst. The HDS and CCR conversions were comparable over the two catalysts. The difference in performance between the catalysts is attributed primarily to the smaller pore size of the Al₂O₃ support compared to the AAP support. The amount of coke deposited on the Ni-Mo/AAP catalyst was less than that on the commercial catalyst, presumably due to differences in pitch conversion levels.     

水滑石法制备渣油加氢催化剂

用离子交换法合成钼酸盐阴离子柱撑镍铝水滑石,并进一步合成了渣油加氢催化剂（MoO₃-NiO/Al₂O₃）。通过XPS检测发现MoO₃以单分子层存在于催化剂孔道中,其饱和分散容量为4.69μmol/m²。通过XRD分析显示中间产物水滑石结构晶形完好,且置换后晶粒尺寸有所增大。采用低温氮气吸附法研究了不同阶段产物的比表面积、孔容和孔体积的变化规律。采用固定床反应器以沙轻常压混合渣油为原料进行催化剂评价,发现水滑石法制备的催化剂较浸渍法制备的催化剂在金属脱除率、脱硫率、脱氮率和脱残炭率等方面初活性有明显提高,但活性衰减较快,寿命相对较短。     

基于体视学的SAP混凝土气孔结构分析与研究

为了研究混凝土在添加高吸水性树脂后其内部气孔结构的变化规律,以及内部气孔结构的变化对混凝土试样抗压强度的影响,基于定量体视学的电子显微图像分析方法,对添加SAP混凝土的内部气孔结构特征展开了相关试验研究.采用干拌的方法拌和SAP混凝土,对不同配合比和不同SAP掺量的混凝土进行分批试验,测定出各组试样的气孔结构参数和抗压强度等,以深层次的剖析SAP对混凝土强度和内部气孔结构的影响.研究结果表明:混凝土内部气孔的平均孔径与间距系数随着SAP含量和水灰比的增加而增大,而混凝土抗压强度则呈现出相反的变化趋势.在配合比相同的情况下,小粒径SAP颗粒对混凝土气孔率的提高效果更为明显,而大粒径SAP颗粒对混凝土气孔平均孔径的提高效果更为明显;水灰比对气孔平均间距系数影响显著,而SAP粒径对混凝土内部气孔平均间距系数的影响不明显.     

中油型加氢裂化催化剂工艺条件的影响

以NNY分子筛和Hβ分子筛为酸性组分,以γ-Al₂O₃为载体原料、Ni-W为金属组分、P为改性剂,采用较合适的配比利用挤条成型法和等体积饱和浸渍法制备较优的中油型加氢裂化催化剂,并针对此催化剂,在恒压15 MPa条件下,反应温度、空速和氢油体积比的变化对加氢裂化过程中馏分油转化率、产品分布、中油选择性和HDS、HDN效果的影响进行探究。结果表明,随着反应温度升高,转化率增大,产品分布向轻组分偏移,脱硫率和脱氮率增加,但中油选择性降低;随着空速增大,转化率、脱硫率和脱氮率均降低,中油选择性增大;随着氢油体积比增大,转化率、脱硫率和脱氮率先增大后趋于稳定,产品分布和中油选择性基本不变。在反应压力15 MPa、反应温度380 ℃、空速0.7 h^-1和氢油体积比1 500∶1条件下,转化率84.6%,中油选择性91.3%,生成油硫含量9.28 μg·g^-1,氮含量1.46 μg·g^-1。