Experimental And Kinetic Study Of The Nitration Of 3-[2-Chloro-4-（Trifluoromethyl） Phenoxy] Benzoic Acid In Microreactors

As an important chemical process,aromatic nitration still faces unmet technical challenges in relation to being heterogeneous and highly exothermic.The safety and production efficiency of the heterogeneous nitration process in traditional batch are limited.The development in microreactor technology has demonstrated its unique advantages in the research and enhancement of heterogeneous nitrification processes,owing to its large specific surface area,excellent mass and heat transfer rate,intrinsic process safety and accurate control over reaction parameters.In this study,a continuous flow process was developed for the mixed acid nitration of aromatic compounds based on a droplet microreactor.The chemical kinetics of the reaction were studied in depth,and a pseudo-homogeneous kinetic model of the reaction was established.In this work,the nitration reaction of 3-[2-chloro-4-（trifluoromethyl）phenoxy]benzoic acid with mixed acid was investigated in different microreactors.The effects of process parameters,including the temperature,the residence time,the M-ratio,N/S,the flow rate,tube diameter and the acetic anhydride content in organic phase,on the reaction process were performed.The results indicated that the optimized reaction temperature,M-ratio and N/S should be set to 308 K,1.6 and 0.57,respectively.Under optimal parameters,the conversion of 83.03%and the selectivity of 79.52%were obtained in a microreactor with4.0 m tube length and 0.5 mm diameter,and the reaction time could be brought down to5.5 min.In these conditions,the effect of total flow rate on conversion and selectivity can be ignored.The addition of acetic anhydride content can enhance the dispersion effect of etherate in organic phase and absorb the produced water which inhibits the decrease of sulfuric acid strength and promotes the generation of NO₂⁺as well as the etherate conversion.However,the excessive acetic anhydride will reduce the product selectivity,and the optimum acetic anhydride content was 3.04 wt%.The results from three reactor channel geometries（i.d.0.5,1.0 and 1.59 mm）were indicative that,under the given experimental conditions,with a slight decrease in reactant conversion（from 80%to 70%）it was able to give a fourfold increase in total volumetric flow rate when the reactor channel diameter was doubled（from 0.5 mm to 1.0 mm）.As the reaction carried out for a period time,it is difficult for this reaction to achieve complete conversion quickly with the apparent reaction rate decreased.In order to achieve high conversion and improve the feasibility of the reaction,the experiments were carried out by expanding the tube diameter,secondary circulation and heating scheme,and multiple rounds of circulation.The scheme of expanding the tube diameter in stages and heating up enhanced the reaction rate,however,more than 1.7 h was required to complete the reaction.By adopting the secondary circulation and heating scheme,the mass transfer and heat transfer performance of the process are improved by recirculating the two-phase reaction after the reaction,and the conversion rate of the reaction is>95.8%,and the selectivity is>74.0%.Compared with the traditional process,the secondary circulation and heating scheme shortens the reaction time from 10 h to 11 min,the production efficiency and safety was improved,and it is beneficial to reduce the production costs.Based on the assumption of pseudo-homogeneous model and the M_c function of sulfuric acid acidity,the apparent kinetics(k_app)and the intrinsic kinetics（k~0）of the nitration reaction were determined through the experimental data.According to the designed kinetics experiments,the linear relations between the rate constant and the residence time are obtained and the reaction rate is in compliance with the second-order assumption.The A and E_a of the reaction were determined as 3.75×10~4 m~3·（mol·s）^-1 and 37.9 k J·mol^-1,respectively.The calculated results based on the kinetics model are in good agreement with the experimental results.