Experimental And Numerical Studies On Thermal Decomposition And Vent Of Two Organic Peroxides

Organic peroxides are the organic compounds containing a peroxide functional group formed by replacing hydrogen atoms of hydrogen peroxide with an organic group such as alkyl,acyl,and aromatic.They are widely used as initiators and catalysts for synthetic resins,initiators for free radical polymerization and grafting reactions,crosslinkers and modifiers for rubber and plastics,curing agents for unsaturated polyesters.However,due to the weak peroxy bond,organic peroxides are sensitive to external excitation factors such as heat,impact and friction.They are easily decomposed,causing thermal runaway,fire and even explosion accidents,resulting in serious property losses and casualties.Therefore,it is necessary to study the thermal decomposition and vent process of organic peroxides,so as to contribute to the accurate assessment of their thermal hazards.In this paper,two typical organic peroxides,ditert-butyl peroxide(DTBP)and cumene hydroperoxide(CHP),were taken as the research objects.Firstly,thermodynamic parameters,such as the onset decomposition temperature,peak temperature,heat generation,were obtained through four groups of DSC dynamic experiments at different heating rates.The kinetic parameters were evaluated and verified by using Friedman isoconversional kinetics,formal model kinetics and reaction mechanism descriptive kinetics respectively,and the reaction rate equations of the two samples were obtained.It was shown that the results of three kinetic evaluation methods are in good agreement.The activation energy of DTBP ranges from 131 k J/mol to 143 k J/mol,which is a one-step N-order reaction.The activation energy of CHP decreases from 133 k J/mol to 46 k J/mol,which is a two-step autocatalytic reaction with strong autocatalytic characteristics.Then a self-designed Pressure Vessel Test(PVT)device,also named as decomposition intensity device of reactive substance,was used to test the thermal decomposition and vent processes of DTBP and CHP under 10 different experimental conditions,and the characteristic parameters such as maximum temperature,maximum pressure,reaction time and vent time were obtained.The influence of furnace temperature(200-300?)and sample mass(1-5g)on the characteristic parameters was analyzed qualitatively and quantitatively.Meanwhile,a high-speed photography was simultaneously applied to record the deformation and break of the rupture disc and the dispersion of residual materials in the vessel.It was shown that:(1)Under the current experimental conditions,the critical furnace temperature and critical sample mass for DTBP explosion are 260-280? and 3-4g,respectively.The higher the furnace temperature,the smaller the sample mass,the easier the DTBP vapor-air mixture system to explode.(2)With the increase of furnace temperature and sample mass,the thermal hazard of CHP increases.(3)The furnace temperature and sample mass affect the temperature rise,pressure rise,reaction time and whether explosion occurs,and different decomposition characteristics lead to different influence laws.However,there is no significant influence on the oscillation characteristics such as oscillation period and amplitude during the pressure drop process.Subsequently,based on the decomposition mechanism and reaction rate equation of the two samples,the mathematical models of their thermal decomposition and vent processes were established by combining the multi-phase conservation equations of mass,momentum,and energy,species transportation equation,turbulence equation and gas state equation.Then the three-dimensional numerical simulations of the decomposition and vent processes of the two samples in the PVT were carried out by using Computational Fluid Dynamics(CFD)method,and the simulation results were compared with those of the PVT experiments.In addition,numerical simulations of DTBP vent process under three different vent sizes(S,S/2,S/4)and three different vent pressures(1bar,3bar,5bar)were carried out to capture and compare the characteristics of pressure and airflow velocity.It was shown that:(1)the average heat production rate of CHP decomposition increased first and then decreased,and reached it's maximum at the reaction course of 0.8 approximately.The more samples,the longer the reaction time,the steeper the peak of average heat production rate,and the higher the peak value.(2)Negative pressure and low temperature were generated inside the vessel and near the vent during the DTBP vent process,and the the high-speed airflow area will continuously move upward.The amplitude of pressure oscillation at different locations along the vessel height is different,while the period is close to each other.(3)As the decrease of the vent size and the increase of the relief pressure,the longer the discharge time,the narrower the jet area in the external flow field,and the farther the high-speed airflow area is from the vent.Moreover,the pressure oscillation characteristics(amplitude and period)are more obviously affected by the vent size,but less affected by the relief pressure.Finally,a new method based on CFD numerical analysis for predicting thermal hazards of chemicals in industrial-scale reactors was presented in this paper.CFD method(three-dimensional calculation)and formal model method(one-dimensional calculation)were used to simulate and compare the reaction course of 2250 kg CHP in 4.3 m~3 vessel under fire condition.It was shown that the induction periods of thermal explosion obtained by CFD method and formal model method are 1003s and 828s,respectively.That is to say,the formal model calculation result was more conservative under current condition.Meanwhile,CFD simulation results showed that the hot spot will appear in the liquid near the wall.Therefore,thermal hazards of chemicals in industrial-scale can be evaluated more accurately by using the CFD method.In summary,this paper is of practical significance and application value for the thermal hazards assessment,inherently safer processes,emergency pressure relief system design,prevention and control of industrial disasters of organic peroxides in the production,storage,transportation and usage.It also provides useful reference for the application of the CFD method in the thermal hazards assessment of process,thermal decomposition and vent processes of hazardous chemicals.