Mass Transfer And Reaction Characteristics Of Nitrobenzene Nitrification Process In Microreactor

Dinitrobenzene is a key organic raw material in the production of printing and dyeing,pharmaceuticals,and pesticides.Nitration of nitrobenzene to produce dinitrobenzene in microreactor has many advantages,such as inherent safety,high heat and mass transfer efficiency,precise control over reaction temperature and time,etc.However,current research on nitration of nitrobenzene in microreactor only focuses on process optimization,and there is no related research on the reaction kinetics of nitration of nitrobenzene in microreactor or the liquid-liquid mass transfer process in two-phase nitration.This paper not only has important theoretical significance for advancing the mass transfer and reaction mechanism research of microscale nitration but also provides a basis and guidance for the industrial development of nitration of nitrobenzene in a microchannel reactor.The main contents are as follows:The equilibrium solubility of nitrobenzene in an acid phase under the influence of factors such as sulfuric acid mole fraction,the concentration of nitrobenzene in the organic phase,and water bath temperature was determined.Experimental results show that,with the increase of sulfuric acid mole fraction,the equilibrium solubility of nitrobenzene in the acid phase increases exponentially.When the sulfuric acid mole fraction increases to 55.0%,the equilibrium solubility of nitrobenzene can reach 2.76 mol/L.With the constant sulfuric acid mole fraction,as the concentration of nitrobenzene in the organic phase increases,the equilibrium solubility of nitrobenzene in the acid phase gradually increases and then sharply increases before finally trending towards a steady state.The effect of water bath temperature on the equilibrium solubility of nitrobenzene shows that at lower sulfuric acid mole fractions,the equilibrium solubility increases with the increase of water bath temperature.However,at higher sulfuric acid mole fractions,the sulfonation side reaction of nitrobenzene in sulfuric acid may increase,leading to a decrease in the measured equilibrium solubility with increasing water bath temperature.A homogeneous nitration reaction kinetics model of nitrobenzene was established and a microreactor nitration experimental setup was constructed to explore the effect of the water mole fraction,nitric acid mole fraction,and reaction temperature on the homogeneous nitration reaction of nitrobenzene in microreactor.Within the range of experimental conditions,the nitrobenzene conversion rate data under different residence times in the microreactor agrees well with the theoretical calculation values of the homogeneous nitration reaction kinetics model,indicating that homogeneous nitration of nitrobenzene in the microreactor follows the basic characteristics and rules of second-order reactions.Finally,the activation energy and pre-exponential factor of the nitration reaction of nitrobenzene under different water and nitric acid mole fractions were obtained based on the Arrhenius equation.The diffusion coefficient of the binary system of nitrobenzene-sulfuric acid was determined by a self-made metal diffusion cell apparatus,and the effect of different sulfuric acid mole fractions and water bath temperatures on the diffusion coefficient of the binary system of nitrobenzene-sulfuric acid was explored.A liquid-liquid mass transfer theoretical model based on Danckwerts surface renewal theory was established,and a microreaction apparatus was used to determine the two-phase nitration liquid-liquid mass transfer coefficient of nitrobenzene.The effect of water mole fraction,nitric acid mole fraction,acid phase volume fraction,and flow Reynolds number on the liquid-liquid mass transfer process of two-phase nitration of nitrobenzene in the microreactor was investigated.The experimental data of nitrobenzene two-phase nitration conversion rate at different residence times in the microreactor coincides well with the theoretical calculation values of the mass transfer model,indicating that the two-phase nitration process of nitrobenzene in the microreactor is in the liquid-liquid mass transfer control region under the range of experimental conditions.Finally,the nitration reaction process of nitrobenzene was studied in the microreactor,and the yield of dinitrobenzene was used as the evaluation index.Single-factor and orthogonal experiments were designed to explore the effects of sulfuric acid mole fraction,reaction temperature,n（HNO₃）:n（NB）ratio,nitrobenzene mole fraction,micro mixer inner diameter,and residence time on the yield of mixed dinitrobenzenes,respectively.The main and secondary factors for the nitration conditions of nitrobenzene in the microreactor were obtained as follows:the organic phase nitrobenzene mole fraction>temperature>n（HNO₃）:n（NB）>micro mixer inner diameter>residence time.Based on the results of the orthogonal experiment,the optimal nitration conditions for nitrobenzene in the microreactor were determined as follows:the organic phase nitrobenzene mole fraction was 35.0%,the reaction temperature was 85℃,the n（HNO₃）:n（NB）ratio was 1.3,the micro mixer inner diameter was 0.6 mm,and the residence time was 65.94 s.Under these conditions,the yield of mixed dinitrobenzenes obtained from the experiment was 95.92%,which was higher than all other results from the orthogonal experiments.