Thermal Hazard Analysis And Continuous Flow Improvement Of Key Synthesis Process Of Pyrazolones

The diazotization reaction in the production process of pyrazolone is not only violently exothermic in the reaction process,but also the synthesized diazonium salt is extremely unstable,which is listed as one of the typical 18 dangerous processes.The previous research on diazotization reaction mostly focused on finding suitable acid as catalyst to improve the safety of the reaction process.However,due to the problem of reaction amplification and high cost,it is difficult to realize industrialization.The research on diazonium salts is mostly the test and analysis of solid pure products,which cannot accurately analyze the thermal hazard of diazonium salts in the production process.In this paper,three different concentrations of aniline diazonium salt solutions were analyzed by differential scanning calorimetry（DSC）,and the basic data of the decomposition of diazonium salts at different concentrations were obtained.The diazonium salt solution has a sharp exothermic peak in the range of 0～200℃,and the higher the concentration of the diazonium salt solution,the more unstable it is.At the heating rate of 2 K/min,the high concentration of diazonium salt solution decomposed at 41℃.The average specific heat release values of the three different concentrations of diazonium salt solutions in descending order of concentration were 177.46±2.10 J/g,215±4.3 J/g and 367.02±22.47 J/g,respectively.The magnitude of the heat release of the diazonium salt solution is not a simple linear relationship with the concentration.First,the kinetic parameters of the reaction progress,reaction rate and activation energy of the diazonium salt were obtained by using the Advanced Kinetic Thermal analysis Software（AKTS）to perform kinetic analysis on the test data of diazonium salts with different concentrations.Then,the thermal behavior of diazonium salts was predicted and compared under isothermal simulation and adiabatic conditions,as well as TMR_ad curves of different concentrations of diazonium salts,and it was found that the diazonium salt solution of T_D24 was only 6.2°C under normal process conditions.Finally,the decomposition process of diazonium salt was studied by Málek method,and it was found that the decomposition reaction was complex and did not follow a single mechanism.The complexity of the decomposition mechanism cannot be judged only by the change of activation energy.The effects of feeding time,reaction temperature and stirring speed on the exothermic characteristics of the diazotization reaction were systematically studied by reaction calorimeter,and a series of thermochemical parameters of the diazotization reaction were obtained.The evaluations results of the severity of the runaway diazotization reaction under the experimental conditions are all"critical",and if the runaway occurs,it will cause short-term damage to the plant.When the feeding time is too short or the reaction temperature is too high,the possibility of runaway reaction is"occasionally",and the other process conditions are"rarely".The risk matrix assessment showed that the risk of reaction under the conditions of experiments 1 and 4 was grade II,and the risk of the remaining experimental conditions was grade I.Finally,the risk rating of the diazotization process is carried out using the runaway scenario analysis method;the risk level of the diazotization process under the semi-batch normal process conditions is level 2.If a reliable feeding and cooling system is adopted to control the material not to be in a state of heat accumulation for a long time,the possibility of accidents is low.If the feeding time is too short（Experiment 1）and the reaction temperature is too high（Experiment 4）,the danger level of the reaction will be increased from level 2 to level 5;therefore,the feeding time must be extended and the reaction temperature must be lowered to ensure safe production.Due to the inherent defects of the semi-batch process,it was improved to a tubular continuous reaction,and the single factors of the semi-batch and continuous reaction time,reaction temperature and material ratio were systematically investigated.Combined with Response Surface Analysis（RSM）,the factors were optimized and analyzed,and the response surface model was constructed using the Box-Behnken Design（BBD）central combination principle.Design Expert 12.0 software was used to make 3D surface and contour plots of the interaction between factors,and the interaction between semi-interval and continuous factors was compared and analyzed.Finally,the optimal reaction conditions for semi-batch and continuous processes were obtained through optimization analysis,and the prediction results were verified.