Inherent Safety Analysis Of The Preparation Process Of Methyl Anthranilate

Methyl anthranilate（MA）is an essential industrial chemical intermediate,with stable properties and wide application,mainly prepared by"phthalic anhydride method"in China.The semi-batch synthesis process of phthalic anhydride method involves Hofmann rearrangement reaction,which has the characteristics of large heat release and complex reaction mechanism,and the high activity and corrosiveness of the halogen reaction reagents,resulting in a large risk of thermal runaway and safety hazards in the reaction process.However,there are relatively few studies on the risk of thermal runaway during the MA reaction process,and the study of the reaction mechanism is not deep enough,so its process safety needs to be improved urgently.Therefore,studying the thermal hazards of MA synthesis process and optimizing the semi-batch process will be of great significance to promote safe production and advance the process of intrinsic safety.Accordingly,this paper aims to improve the inherent safety of the MA synthesis process.Firstly,the kinetics of the thermal decomposition of the intermediate in the Hoffmann rearrangement reaction and the exothermic characteristics of the reaction process under different process conditions were investigated by calorimetric experiments,and the thermal hazard levels under different process conditions were evaluated.Subsequently,the application of a tubular reactor,based on an inherent safety strategy of process intensification,enabled the transformation of the MA semi-batch production process into continuous process.The thermal decomposition behavior and mechanism of the intermediate during MA synthesis were investigated by differential scanning calorimetry（DSC）.The thermal behavior of the intermediate was simulated under isothermal and adiabatic environments,and its thermal hazard was evaluated in combination with the thermal hazard parameter TMR_ad（time to maximum reaction rate in adiabatic condition）.The results showed that the intermediate is an extremely unstable substance with an onset decomposition temperature of 10.75℃ and an average heat release of 1088.05±58.10 J/g.The activation energies of the intermediate calculated by the Friedman and Ozawa methods followed the same trend with values ranging from 35 k J/mol to 60 k J/mol and 50 k J/mol to 63k J/mol,respectively.At an operating temperature of 0℃,the intermediate will be completely decomposed in 12 days under isothermal conditions,compared to 1.6 days under ideal adiabatic conditions.When TMR_ad is 24 h,the surrounding temperature is5.7℃,and that of 8 h,the corresponding temperature is 19.4℃.The DSC curves at different heating rates all showed two exothermic peaks below 130℃,reflecting the complexity of the decomposition reaction.The decomposition mechanism was analyzed by Malke method,and the results showed that the heating rate influenced the decomposition of the intermediate and the decomposition mechanism function does not follow any of the standard kinetic models.A thermal hazard assessment of the Hoffmann rearrangement reaction during the semi-batch synthesis of MA was performed using reaction calorimeter（RC1）.The heat release of the reaction was researched at different reaction temperatures and different dosing times.The parameters such as adiabatic temperature rise(ΔT_ad)and maximum temperature of synthesis reaction（MTSR）were calculated to assess the thermal hazard.The results showed that with the increase of reaction temperature,the exothermic rate of reaction increased,and the reaction time and heat release decreased;with the increase of feeding time,the exothermic rate of reaction decreased,but the reaction time and heat release increased.The risk level of the process was assessed as 5 by the failure scenario analysis,so it was necessary to improve the original process to reduce the risk of uncontrolled reaction.The results of the assessment of the hazard level of the MA semi-batch synthesis process show that the traditional semi-batch process has a high hazard level and that it is necessary to optimize the operating conditions to minimize the accumulation of materials or to change to continuous operation to improve the level of inherent safety.On this basis,a continuous tubular reactor system was designed and constructed,and single-factor experiments were first conducted for the initial process development.The response surface methodology（RSM）was further used to optimize the process parameters and analyze the effect of the interaction of the factors on the MA yield,as well as to compare and analyze the results with those of the semi-batch process optimization.The optimal process conditions were obtained as follows:reaction temperature of 21℃,reaction time of 52 s,n（phthalimide）:n（sodium hypochlorite）:n（methanol）=1:1.13:3.84.The yield can reach 93%.The continuous synthesis process enables Hoffmann rearrangement reaction and hydrolysis reaction to be carried out continuously,allowing the reaction to be carried out at a higher temperature,reducing the reaction residence time and material usage,effectively solving the inherent drawbacks of long residence time of the intermediate,poor selectivity of the reaction and high risk of thermal runaway in the semi-batch synthesis process of MA,and the inherent safety of the process is improved.