| m-Dinitrobenzene is an important basic chemical which is widely used in pesticide,photosensitive material,printing and dyeing industries.In industry,m-dinitrobenzene is generally produced by nitration of benzene or nitrobenzene with mixed acid as a nitrating agent.Nitration reaction is a non-homogeneous,rapid and highly exothermic reaction,while the traditional batch reactor has a limited production efficiency and poor safety.As a new type of continuous flow reactor,the microreactor has the advantages of intrinsic safety,high efficiency of heat and mass transfer,thereby has high controllability for nitration reaction,which brings new opportunities for the production of m-dinitrobenzene.In this work,the transfer and reaction characteristics were systematically investigated in a continuous flow microreactor for the mixed-acid nitration of nitrobenzene.Firstly,the effects of flow rate and temperature on the sulfuric acid/nitrobenzene two-phase flow pattern were investigated under the temperatures of 25℃～65℃and the total flow rate of 150μL/min～5500μL/min.When the total flow rate is less than 1200μL/min,it is easier to form slug flow or droplet flow.With the increase of flow rate,it gradually generates irregular droplet flow or slug-droplet flow.Based on the formation mechanism of droplets,a correlation for droplet size was proposed,with the prediction error less than 10%.The interphase mass transfer behavior of nitrobenzene was then investigated under slug flow pattern.It is found that the mass transfer reached equilibrium within 2 s,verifying the excellent mass transfer performance of the microreactor.Secondly,the reaction characteristics of nitrobenzene nitration in microreactors were evaluated under different operating conditions,i.e.,by changing the residence time,reaction temperature,sulfuric acid strength,molar ratio of H2SO4to HNO3,molar ratio of nitrobenzene to HNO3and total volume flow rate.The conversion of nitric acid increased with increasing reaction temperature,residence time,concentration and amount of sulfuric acid.The selectivity of m-dinitrobenzene maintained around 88%,but decreased to 85%when the temperature increased to 65℃.The flow rates of nitrobenzene and mixed acid also influence the reaction,ascribed to the variation of the interfacial area.Since the measured reaction rate decreased with increasing the phase ratio of nitrobenzene to mixed acid,the reaction was verified to take place not only in the bulk aqueous phase but also at the phase interface.When the total flow rate exceeded 1000μL/min,the effect of mass transfer and interfacial reaction can be neglected.The optimized reaction conditions were obtained by balancing conversion and selectivity:temperature of 65℃,residence time of10 min,sulfuric acid strength of 98%,molar ratio of nitrobenzene to nitric acid of 4.3,molar ratio of sulfuric acid to nitric acid of 2.Under these conditions,the nitric acid conversion was 95.08%,with the selectivity and yield of m-dinitrobenzene 85.76%and81.54%,respectively.Compared with the industrial production（typically taking tens of minutes or even several hours）,microreactor exhibits advantages for process efficiency improvement.Finally,kinetic experiments were then carried out at a total volume flow rate of 1500μL/min.The pseudo-homogeneous phase model was used to describe the reaction rate of heterogeneous nitration in microreactor.From the reaction kinetic model based on nitric acid,it is found that the observed reaction rate constant is dependent on not only temperature,but also sulfuric acid strength.By decoupling the sulfuric acid concentration from the observed reaction rate constant based on nitric acid,the real reaction rate constant based on nitronium ion the real nitrating species,was calculated.Accordingly,the pre-exponential factor and activation energy were obtained,2455 m3/（mol·s）and 71.23k J/mol,respectively.By comparing the predicted with experimental results,the developed kinetic model well describes the experimental data. |
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