Thermal Hazard Analysis And Micro-continuous Flow Improvement Of Key Synthesis Process Of Saccharin Sodium

The rapid development of the chemical industry reflects the increasing demand of human civilization for chemical products,but the contradiction between the increasing demand for fine chemicals and the frequent occurrence of production accidents is perplexing the development of fine chemical industry.The saccharin sodium manufacturing industry is the first batch of fine chemical industry in China,the synthetic route with phthalic anhydride as the raw material has been widely used in the sodium saccharin industry due to its many advantages.However,the synthetic route involves a variety of processes with strong exothermic characteristics and unstable substances,resulting in a great thermal runaway risk and potential safety hazards.The synthesis process of methyl anthranilate（MA）and MA diazonium salt,which have potential thermal runaway characteristics in the synthetic route of saccharin sodium,were selected as the research objects in this paper.Based on the process conditions set by the current industry production practice,reaction calorimeter,differential scanning calorimeter（DSC）and C80 microcalorimeter were employed to study the thermal hazard in the synthesis process.The results were as follows:（1）The intermediate rearrangement products which remain for a long time were the main hazard of the MA synthesis process.The DSC test results showed that rearrangement products exhibited obvious exothermic behavior at 40℃,with the heat release of 289.9 J/g and the T_D24of 6.5°C.Based on the thermal behavior data of MA synthesis process,the maximum temperature of synthetic reaction MTSR（57.3°C）was much higher than the T_D24of the rearrangement products.According to the scenario analysis method,the process thermal hazard level is determined as 5,which has a high risk of thermal runaway.（2）The synthesis process of MA diazonium salt had strong instantaneous exothermic property and the reaction system was highly sensitive to temperature.The results of reaction calorimetry experiment showed that when the reaction temperature was 30℃,the coupling side reaction occurred in the later stage of the reaction.C80microcalorimeter were employed to test the stability of MA diazonium salt,the results showed that the initial decomposition temperature of MA diazonium salt was 89.9℃,the heat release was 441.5 J/g and the T_D24was 48.7°C.Based on reaction process thermal behavior data determined that MA diazonium salt process thermal hazard level is 2,which has a low risk of thermal runaway.However,it is necessary to provide a strong cooling capacity to match its instantaneous strong heat release characteristic.Based on the results of thermal hazard analysis,micro continuous flow synthesis schemes(Coring AFR^TM-G1 microreaction system)were proposed to solve the safety problems and inherent defects of traditional semi-batch synthesis mode.Single factor experiments were designed for preliminary process development,and then Box-Behnken Design（BBD）center combination principle was used to construct the response surface model to optimize the process.The effects of interaction between different conditions were studied and compared with the laboratory scale semi-batch synthesis process.The results were as follows:（3）MA continuous synthesis scheme can separate the strong exothermic rearrangement reaction stage from the subsequent reaction,it effectively avoids the inherent disadvantages of long-term retention of rearranged products in MA semi-batch synthesis mode,which leads to the reduction of reaction selectivity and high risk of thermal runaway.The yield of MA could reach 86%under the optimized reaction conditions of n（phthalimide）∶n（Na Cl O）∶n（methanol）=1∶1.07∶4.6,reaction temperature 22℃,residence time 98 s.（4）The reaction requirements of MA diazonium salt synthesis process are highly consistent with the microchannel reactor,the continuous flow synthesis scheme of MA diazonium salt effectively solves the high temperature sensitivity and low temperature dependence of diazonium synthesis system.To a certain extent,it reduced the negative effect of the interaction among the reaction factors.The yield of MA diazonium salt could reach 92%under the optimized reaction conditions of n（MA）∶n（Na NO₂）∶n（HCl）=1∶1.15∶2.67,reaction temperature 35℃,residence time 45 s.