【作者】 唐盛伟；

【导师】 梁斌；

【作者基本信息】 四川大学 ， 化学工艺， 2005， 博士

【摘要】 苯甲酸和苯甲醛都是有机合成工业的重要中间体。甲苯液相空气氧化过程是环境友好的苯甲酸生产工艺,并副产一定量的苯甲醛。甲苯氧化反应是强放热自由基链式反应,容易产生爆炸,系统的安全性能是最重要的制约条件。已报道的研究很多,但要进行接近于实际操作条件的实验研究相当困难,已有的研究结果还不足以对反应过程进行准确的模拟和对工业现象进行合理的描述。因此,本文研究该反应过程的动力学规律和传质影响,研究催化剂的失活和流失规律,对该反应的工业实践和工程放大有着非常重要的意义。 与大多数文献以小型釜式反应器进行研究不同,本研究采用了模拟工业操作条件的鼓泡床反应器进行优化工艺条件研究。鼓泡反应器的直径为φ48mm,气体喷嘴为φ6mm的单孔鼓泡,以醋酸钴为催化剂。研究发现,现有工业反应过程的供氧量[0.37kg-Air/(kg-Toluene·h)]严重限制了工业反应器的生产能力,只有当供氧量达到0.62kg-Aid(kg-Toluene·h)时,这种限制才逐渐消除。催化剂初始加入量对反应活性影响不大,但对催化剂活性持续的时间呈正比例关系。实验结果表明,链式反应可以用0.1%(wt)的苯甲酸作为引发剂,但杂质苯或水浓度较高时(>2%),对反应有抑制作用。适宜的操作条件为165℃、1.0 MPa和空气鼓气量0.62 kg-Air/(kg-Toluene·h)。 对苯甲醛的浓度和选择性的研究表明,供氧充足的情况下,反应20min后苯甲醛浓度接近其最大值。在20min～100min内,苯甲醛的浓度和收率几乎不随反应时间变化。甲苯氧化反应存在一个诱导期,中间产物苯甲醛的浓度在诱导期内逐渐积累。当反应进入稳定的链反应阶段时,中间产物苯甲醛的浓度将基本保持不变。而任何影响苯甲醛继续氧化成苯甲酸的因素,如Co催化剂流更多还原

【Abstract】 Both benzoic acid and benzaldehyde are important intermediates in the organic synthesis industry. Liquid-phase oxidation of toluene by air is an environment benign technology for producing benzoic acid and benzaldehyde. The reaction is a strong exothermic radical reaction. The safety is crucial for the operation of the oxidation because of its explosive properties. Experimental research on the oxidation is difficult, and most of the works reported are significantly distinguished from the industrial operation condition. Data available are not sufficient to simulate the reaction system accurately and explain many industrial phenomena reasonably. In this work, reaction kinetics, influence of mass transfer and deactivation of catalyst were investigated. It will be useful for the commercial scaling up of this process and enrich the knowledge related to the oxidation of hydrocarbons.Different from most of published works conducted in small autoclave or stirred tank, investigation was conducted in a bubble column reactor under conditions near the industrial parameters. The reactor was a φ48mm column with a sparger of φ6mm single orifice. Cobaltous acetate was used as catalyst. Experimental results show that, an air throughput of 0.37kg-Air/(kg-Toluene·h) used in existing commercial device is not enough and restrains the production ability. Toluene conversion and/or benzoic acid selectivity increases along with air throughput until the throughput reached 0.62kg-Air/(kg-Toluene·h). The initial concentration of Co catalyst affects less on the reaction rate. However, the life of catalyst is proportional to the initial concentration of catalyst. As an initiator, 0.1% (mass fraction) of benzoic acid is high enough to initiate the reaction. Both benzene and water, impurities contained in toluene, significantly inhibits the reaction when their concentrations are above 2 %( mass fractions). Optimal operation conditions for the oxidation are 165℃, 1.0MPa and an air throughput of 0.62kg-Air/(kg-Toluene·h).Experimental research on the concentration and selectivity of benzaldehyde shows that, under a sufficiently aerated flow, benzaldehyde concentration reaches amaximum in 20 minutes. The concentration of benzaldehyde keeps stable in the next 80 minutes and then gradually drops. An induction period exists in liquid-phase oxidation of toluene. The concentration of immediate benzaldehyde gradually accumulates in this period. It keeps unchanged while the reaction enters the propagation period. Factors inhibiting the oxidation of benzaldehyde to benzoic acid, such as, the loss of Co catalyst, the water and benzene impurities would cause the decrease of the yield of benzoic acid and a relative rise of the selectivity of benzaldehyde.According to the analysis results of the oxidation products, a plausible mechanism and reaction network was proposed. Kinetic equations for toluene oxidation were constructed from the proposed mechanism. By data fitting with the experimental results under conditions simulating commercial process, a macrokinetics equation for 145°C-175°C was obtained in following form:dC *****dt "**Where, pre-exponential factor was 15.89 s"’-MPa"’. The apparent activation energy was estimated to be 41kJ/mol.The kinetics model of the oxidation of benzaldehyde was:Jf -18064UKj aldehyde _Using Film Theory, mass transfer parameters were calculated under reaction condition. Deducting the influence of mass transfer from previous models, an intrinsic reaction rate equation based on the liquid concentrations of toluene and dissolved oxygen was derived as following:dC \ 5735I1^L 5334l "iQJZHereafter, calculation under reaction condition shows that Hatta Number Ha<0.3 and the gas-liquid reaction effectiveness factor n. =0.606~0.728. The reaction is suggested to be in a reaction-controlling zone. The influences of mass transfer on the oxidation process were slight.The kinetic models and relative data obtained were used to simulate a commercial reactor. The simulation results agree well with the parameters obtained from the commercial device. Increasing air feeding, increasing gas and liquid feedings and using oxygen-enriched air were proposed to be potential measures to enhance the oxidation production. Amount them, using enriched air most significantly enhances the production. When the mole ratio of oxygen rises to 54% in the enriched air, the toluene conversion will reach 25.77% and 1.8 time of toluene feed will be reacted.In commercial plant, severe scaling often occurs, which results in the loss ofcatalyst. It not only lowering the reaction activity, but also bring about many operation problems. To make clear the mechanism of scaling, a set of analysis method for the chemical composition and physical properties was developed. The scaling sample collected from commercial device was analyzed by EDS (X-ray Energy dispersive spectrometry) , XRD, FT-IR, spectrophotometric analysis, Ion chromatographic analysis, TG and DSC. It is believed to be a kind of precipitate of CoC2O4-2H2O. Its composition was: Co: 27.3%(wt%), C2O42": 41.4%(wt%), CH3COO": 8.5%(wt%), residues: 4.1%(wt%), others: 2.7%(wt%).In the liquid oxidation product, oxalic acid, maleic acid and hydroquinone were checked out by HPLC. The results suggest that the oxalic acid is formed by the deep oxidation of toluene or its derivates. Scaling is due to the formation of oxalic acid.Analyzing the scaling phenomena under different operation parameters, we found the scaling was enhanced while the objective reaction to form benzoic acid was retarded. An optimal operation temperature range is 155-165°C, too high or too low a temperature will accelerate the scaling. In the range of <100 ppm, increasing Co catalyst content can ease the scaling in some extent. As the same reason, severe scaling phenomenon was observed without initiator benzoic acid feeding in the reaction system. According to the scaling mechanism and phenomena, methods to ease the scaling in industrial process include maintaining a high catalytic reaction activity, improving heat removal efficiency and avoiding deep-oxidation.更多还原