【作者】 米冠杰；

【导师】 陈标华； 李建伟；

【作者基本信息】 北京化工大学 ， 化学工程与技术， 2010， 博士

【摘要】 Fe-ZSM-5分子筛对N2O氧化苯制苯酚反应具有良好的初始活性和选择性,但存在快速失活问题,制约了该工艺路线的工业化进程。目前的研究主要集中在催化剂制备和性能改进两个方面,并试图解决催化剂快速失活问题,但效果并不显著,因此亟需新的解决办法。本文基于Fe-ZSM-5分子筛催化剂,立足于循环流化床反应技术应用,对N2O氧化苯制苯酚的反应过程特性进行系统的实验和理论研究,包括工艺条件、反应网络结构和催化剂的定态/非定态性能特性等,以期为该工艺过程的循环流化床反应技术的开发提供理论指导,为其失活问题的解决提供思路。文中,首先采用等温固定床反应装置(尺寸为Φ21×3mm,L450mm),以单因素实验法系统考察了温度(623-773K)、空间速度(2500-10000h-1)和反应器入口苯：N2O摩尔比(2-12)对Fe-ZSM-5分子筛上N2O氧化苯制苯酚反应过程及产物分布规律的影响,结果表明：N2O转化率随反应温度的升高而增加,苯酚选择性随温度升高逐步降低,苯酚收率和时空产率随温度的升高呈现先增大后减小的趋势；高空速有利于苯酚选择性提高,但会导致N2O转化率降低,适宜的空速将有利于提高苯酚时空产率；提高苯：N2O摩尔比有利于N20转化率和苯酚选择性的提高,但过高的苯：N2O摩尔比对提高苯酚时空产率不利。最后采用L16(45)正交实验设计综合考察了以上因素对反应过程的影响,最终确定了相对适宜的反应条件为：温度673K,空速6000h"1,苯：N20摩尔比5：1。以等温固定床反应器和热重分析相结合的方式研究了Fe-ZSM-5分子筛上N2O氧化苯制苯酚的积炭行为,考察了反应时间、温度、原料配比和空速对积炭量的影响。依据实验观察现象和积炭物组分分布的气相色谱-质谱分析结果,提出了Fe-ZSM-5分子筛上N2O氧化苯制苯酚的积炭机理。结果表明,Fe-ZSM-5分子筛上的积炭物种主要来源于产物苯酚的深度氧化或缩聚,属连串失活。并基于所提出的积炭机理和实验数据,建立了Fe-ZSM-5分子筛催化剂的积炭动力学模型,统计分析和残差分布检验结果表明,所建模型与实验结果良好相容,是适宜和可信的。根据固定床反应装置研究结果,利用变空速实验和积炭机理研究所获得的信息,对该反应体系的热力学特性和反应网络结构进行了较为系统的研究。确定了该体系的主要反应和主要副反应,提出了能合理解释实验观察现象的动力学网络结构。所进行的热力学分析结果表明,当反应温度在500K以上时,生成苯酚主反应的绝热温升趋近于零,但实际过程中的反应器有明显温升,因此氧化过程反应热主要是由副反应造成的。在500K-900K温度范围内,各反应的平衡常数值极大,因而可以不考虑逆反应对反应转化率的影响。依据热力学分析结果和所提出的动力学网络结构,分别构建了描述该反应体系动力学特性的的幂函数型和机理型动力学模型。采用等温积分反应器(尺寸为Φ6×2mm,L300mm),在消除内、外扩散影响的实验条件下,对Fe-ZSM-5分子筛上N2O氧化苯制苯酚的本征动力学进行了实验研究。以单纯形法对两模型参数进行了搜索、选优,并经统计检验表明,所建立的两种本征动力学模型均能较好地吻合实验数据,是适宜和可靠的。但相对而言,机理模型的预测结果与实验数据更加吻合,因此将其作为实验所用催化剂的本征动力学模型将更为可靠。由前述研究结果可见,Fe-ZSM-5分子筛催化剂具有良好的初始活性和选择性,但存在快速失活问题,因此在对其进行反应过程研究时需考虑其性能随时间的变化规律,为此建立了动态在线质谱分析实验装置和分析方法,通过程序升温动态实验,研究了Fe-ZSM-5分子筛上N2O氧化苯制苯酚的反应机理和动态特性,提出了与实验结果相容的基元反应序列结构。即,N2O首先在催化剂表面上进行弱化学吸附,达到一定温度才会发生分解反应,在两种不同活性位上形成表面氧、氮物种,此分解步骤是整个基元反应过程的速率控制步骤,且表面氧物种的活性明显高于表面氮物种,吸附态氧物种则氧化苯生成苯酚。此外,N2O在与催化剂表面相互作用时还会伴随产生相对较强吸附的NO物种。基于以上基元反应序列结构,建立了固定床积分反应器的一维非均相瞬态动力学模型,并依据瞬态实验数据对模型参数进行了估值及数理检验,结果表明所建瞬态动力学模型能够合理地描述N20催化氧化苯反应的瞬态行为,其结果为深入认识基元过程的相对快慢和速率控制步骤及进一步采取措施优化反应效率提供了重要的理论参考。在催化剂积炭机理及定态/非定态反应动力学研究的基础上,考虑到流化床反应器易于实现反应和再生循环操作,可以有效解决催化剂失活问题,因此,在固定流化床反应装置上,真实考察了N2O氧化苯制苯酚周期操作工艺条件的影响规律及Fe-ZSM-5分子筛的催化性能,结果表明：周期操作可以显著提高催化剂的反应性能,且该催化剂经反复再生后,活性水平基本可以恢复到新鲜催化剂的水平。在反应工艺条件为反应温度703K、原料苯/N2O配比9：1、操作气速0.050m·s-1；再生工艺条件为再生温度703K、N2O浓度12.5%、再生操作气速0.059m·s-1；周期操作工艺条件为操作周期6min,循环裂度0.4的条件下,苯酚收率比定态操作提高了10.46%,其结果为循环流化床反应器的开发提供了更为直接的基础数据。最后,在反应网络、瞬态动力学和固定流化床实验研究的基础上,采用一系列合理假设,建立了固定流化床反应器的一维、非均相、瞬态数学模型,并用固定床定态实验数据对催化剂利用效率和总传热系数进行了校正。采用Crank-Nicolson差分法和二阶改进的Rosenbrock法求解所建立的数学模型,并用MATLAB语言编写了相应的模拟程序。通过模拟研究,分析了操作周期和循环裂度对反应性能的影响,模拟结果与实验现象基本一致：适当缩短操作周期和增大循环裂度均能显著提高反应的苯酚收率,这表明,采用循环流化床反应器在经济和技术上将更加有利。更多还原

【Abstract】 Fe-ZSM-5 zeolite, as a catalyst, was widely used in the oxidation of benzene with N2O to phenol with good initial activity and selectivity. However, the rapid deactivation of the catalyst leads that the industrialization of the process route was restricted. While most researches were focused in the preparation and modification of the catalyst, it failed to solve the problem of high deactivation rate. Based on the use of Fe-ZSM-5 zeolite in circulating fluidized bed, a series of experimental and theoretical studies were carried out on the characteristic of the oxidation of benzene to phenol with N2O, including reaction conditions, reaction network and steady or unsteady state performance of the catalyst in the reaction. The purpose of this paper was to provide the theoretical instructions for the development of the circulating fluidized bed process and a new methods for the solution of the catalyst deactivation.At first, based on an isothermal fixed bed reactor (SizeΦ21×3mm, L450mm), the effects of temperature (623-773K), space velocity (2500～10000h-1) and benzene/N2O molar ratio (2-12) at the entrance of the reactor on the reaction process and the distribution of products during the oxidation of benzene with N2O to phenol over Fe-ZSM-5 zeolite were investigated by the single-factor test method, respectively. The results revealed that, with the increase of temperature, the conversion of N2O increased and the selectivity of phenol decreased, the productivity and yield of phenol first increased and then decreased; With the increase of space velocity, the conversion of N2O decreased and the selectivity of phenol increased. A appropriate space velocity was beneficial to increase the productivity of phenol. With the increase of benzene/N2O molar ratio, the conversion of N2O and the selectivity of phenol both increased. But overhigh benzene/N2O molar ratio would result in a reduction in the productivity of phenol. Finally, based on the results of the single-factor experiments above, the global influence of temperature, space velocity and benzene/N2O molar ratio on the reaction process was investigated by means of the orthogonal test of Li6(45). The optimal reaction conditions were determined as follows:temperature of 673K, space velocity of 6000 h-1 and benzene/N2O molar ratio of 5:1.The coking behavior of the Fe-ZSM-5 zeolite catalyst during the oxidation of benzene with N2O to phenol was studied in an isothermal fixed bed reactor using thermogravimetric analyzer. The effect of reaction time, temperature, molar ratio of reactants, and space velocity on coking was investigated. Based on the informations obtained from experiments and the gas chromatography-mass spectrometry analysis of the coke on the catalyst, a mechanism for coke formation was suggested. The results showed that the formed coke mainly came from the further oxidation or polymerization of phenol as product, revealing that sequential deactivation occurred in the reaction process. Based on the suggested mechanism and the experimental data, a kinetic model for coke formation that describes coke deposition over the Fe-ZSM-5 zeolite catalyst was developed. A statistical test and a residual distribution analysis showed that the established kinetic model was in good agreement with the experimental data and reliable.Based on the experimental results of the isothermal fixed bed reactor and the various space velocities, as well as the mechanism of coke formation, the thermodynamic properties and the reaction network of the reaction system were studied systematically. The main reaction and main side reactions were determined and the kinetic reaction network for the oxidation of benzene with N2O to phenol was proposed, which can reasonably explain the experimental phenomena. The results of the thermodynamic analysis showed that when the temperature was above 500 K, the adiabatic temperature rise of the main reaction, which phenol was produced in, tended to zero. However, there was a remarkable temperature rise in the reactor actually. Accordingly, the reaction heat in the oxidation process mainly came from side reactions. In the temperature range of 500 K to 900 K, the reaction equilibrium constants were extremely high so that the effect of counterreactions on reaction conversion was not considered.Based on the thermmdynamic analysis and the kinetic reaction network proposed, the power function type and the mechanism type kinetic models, which described the kinetic characteristic of the reaction system, were established. The intrinsic kinetic experiments of the oxidation of benzene with N2O to phenol over Fe-ZSM-5 zeolite were carried out in an isothermal integral reactor (sizeΦ6×2 mm, L300 mm) on condition that internal and external diffusions were excluded. The kinetic parameters were estimated by means of Simplex optimal method. Statistical test showed that the two kinetic models established were in good agreement with the experimental data and reliable. By comparison between the two models, the mechanmism type kinetic model showed better agreement with the experimental data than the power function type. Therefore, the mechanism type kinetic model, which was used for describing the catalyst kinetics, was more reliable.It can be seen from the above results that Fe-ZSM-5 zeolite catalyst possessed favorable initial activity and selectivity in the oxidation of benzene with N2O to phenol, whereas deactivated rapidly. Accordingly, the rule of the catalyst performance changing with time should be considered when the reaction process was investigated. The experimental apparatus and the analysis method of dynamic online mass spectrograph were established. The reaction mechanism and the dynamic characteristics of the oxidation of benzene with N2O to phenol over Fe-ZSM-5 zeolite were studied by the temperature programmed dynamic experiments. The sequence of the elementary reactions, which showed well agreement with the experimental data, was proposed. The first step corresponded to a week chemical adsorption of N2O on the catalyst surface followed by the dissociation of N2O on the condition that the temperature was above 634 K. The surface atomic oxygen and nitrogen adsorbed upon two different active sites. The dissociation of N2O was the most crucial step, dominating the overall reaction process. Moreover, the surface atomic oxygen had a remarkable higher activity than the surface atomic nitrogen. Benzene was oxidated by the chemisorbed atomic oxygen to phenol. Furthermore, the formation of the strongly adsorbed NO on the Fe-ZSM-5 by temperature programmed desorption of N2O during the reaction process was discovered. A one-dimensional heterogeneous transient kinetic model for the integral fixed bed reactor based on the elementary reaction steps above was developed. The corresponding parameters of this model were evaluated and statistically tested using the transient experimental data. The results showed that the constructed transient kinetic model can reasonably describe the transient reaction behavior for the oxidation of benzene with N2O to phenol. The results also provided important theoretical references for not only understanding the relative rates and the rate controlling step of elementary reactions thoroughly, but also taking further measures to optimize reaction efficiency.On the basis of the mechanism of coke formation on the catalyst, the steady/unsteady state reaction kinetics, and the solution of the catalyst deactivation by fuildized bed reactor due to realizing easily the periodic operation of reaction and regeneration, the rule of the effect of periodic operation conditions and the catalytic performance of Fe-ZSM-5 zeolite during the oxidation of benzene with N2O to phenol were investigated in a fixed fluidized bed equipment in practice. The results showed that the periodic operation can improve evidently the reaction performance of the catalyst, and that after the deactivation, the catalyst was regenerated again and again, and the activity of the deactivation catalyst can be basically recovered to the level of fresh catalyst. When reaction conditions were reaction temperature of 703 K, benzene/N2O molar ratio of 9:1 and operational gas velocity of 0.050 m·s-1, and regeneration conditions were regeneration temperature of 703 K, N2O concentration of 12.5% and regeneration gas velocity of 0.059 m·s-1, and periodic operation conditions were cycling period of 6 minutes and cycling split of 0.4, the yield of phenol increased by 10.46% than that operated under steady state. The results provided directly basic data for the development of circulating fluidized bed reactor.Finally, based on reaction network, transient kinetics and the experiments in the fixed fluidized bed, a one-dimensional heterogeneous transient mathematical model for the fixed fluidized bed reactor was proposed on some acceptable assumptions. The utilization efficiency of the catalyst and the total heat transfer coefficient were corrected using steady state experimental data in the fixed bed. A Crank-Nicolson finite difference algorithm and an improved second order Rosenbrock method were used to solve the mathematical model established. The simulation program was programmed in MATLAB software. The effects of operation period and cycling split on the reaction performance were analyzed by simulation study. The simulation results were in general agreement with the experimental phenomena. Shortening operation period and augmenting cycling split both increased the yield of phenol remarkably. It suggested that circulating fluidized bed reactor was more promising than fixed bed reactor in economy and technology更多还原