Numerical Simulation And Temperature Runaway Study Of Reactor For The Preparation Of Phthalic Anhydride From O-xylene

Phthalic anhydride is an important organic chemical raw material,which is widely used in the production of plasticizers,unsaturated resins,polyesters and dyes.The industrial production of phthalic anhydride is a strong exothermic reaction.Because of the sensitive characteristics of the reactor parameters,the high temperature and high pressure may lead to the explosive damage of the reactor and result in serious environmental pollution.Taking the reactor for preparation of phthalic anhydride from o-xylene as example,the distribution of physical field in the reactor,the influencing factors and the distribution characteristics of physical field at critical temperature was simulated by the numerical analysis.The concentration gradient and temperature gradient of key components change greatly in the 1/3 section of reactor,which is the main reaction zone for phthalic anhydride production from o-xylene,while 2/3 section is mainly used to improve the yield of phthalic anhydride.In view of the physical field distribution characteristics,it is more reasonable to evaluate the design length of reaction tube by simulated instantaneous selectivity.The yield and hot spot distribution of phthalic anhydride were obtained by changing feed temperature,space velocity and cooling molten salt temperature.The shows that feed temperature and space velocity are non-sensitive operation parameters,cooling molten salt temperature is sensitive operation parameters.In the operating process,temperature fluctuation range of cooling molten salt should be strictly controlled to prevent reactor temperature runaway.On the basis of the above simulation,taking the hot spot temperature as reference,the variation of the temperature field under the critical temperature runaway was simulated by changing the temperature of the cooling molten salt.The difference of the radial temperature is large in critical temperature runway.The maximum radial temperature difference is consistent with the change trend of the heat transfer temperature difference along the axial direction.The hot spot is the most sensitive point in the reactor,and heat transfer balance at the hot spot is broke easily by the axial temperature increase.Temperature runaway mechanism of the reactor is not only caused by the increase of heat generation rate,but also caused by the change of heat generation mode.In the temperature range of non-sensitive region,partial oxidation is the dominant mode of heat generation.When the temperature reaches a certain node limit,the deep oxidation reactions are stimulated,and then heat generation of deep oxidation reactions is induced.The heat production capacity of deep oxidation reaction is stronger than that of partial oxidation reaction.Due to the heat accumulation,the heat production rate shows the characteristic of exponential explosion,which results in the temperature runway.At the same time,the peroxide products in the reactor increased significantly.