An Investigation On The Liquid Phase Oxidation Of Toluene By Air

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.