4. Conclusions
In this work, a continuous-flow microreactor system was proposed for the rapid and accurate measurement of CB nitration kinetics at high mixed acid concentrations. The influence of mass transfer between the two phases on the kinetic measurements was eliminated by dissolving CB in concentrated sulfuric acid, which allowed for a homogeneous reaction environment. The effects of temperature, residence time and sulfuric acid concentration on conversion and selectivity were systematically investigated, and it took only a few seconds to obtain a single data point. The results demonstrated that at the same residence time, the conversion of CB and the selectivity of o -NCB increased slightly with increasing reaction temperature (from 0 to 15 °C), while the selectivity ofp -NCB slightly decreased. The selectivity of m -NCB ranged from 1‰ to 3‰. Due to the excellent heat transfer rate and accurate temperature control realized in the microreactor system, the selectivity of each nitration product did not change much with changes in the residence time. Similar to the nitration of other aromatic compounds, the nitration of CB is a second-order reaction, with both CB and nitric acid exhibiting first-order kinetics. Based on conversion data obtained at different times, a kinetic model was established to obtain the observed reaction rate constants. The results showed that the nitration rate was highly related to temperature and the sulfuric acid concentration. As the temperature increased, k obsincreased; at the same temperature, as the mass fraction of sulfuric acid increased from 85% to 95%, k obs increased by more than 1 order of magnitude. To explain the relationship between the reaction rate and sulfuric acid concentration, a polynomial equation describing M c as a function of sulfuric acid concentration was fitted. The essence of the homogeneous nitration of aromatics in mixed acids involves the attack of on the aromatics. By summarizing data reported in the literature, a prediction model for concentration as a function of sulfuric acid concentration and temperature was established. Based on the above analysis, the reaction rate parameters based on were obtained. With increasing temperature, the value of k * increased. n is a specific thermodynamic parameter that is characteristic of different aromatic compounds and was approximately 0.71 in this work.E a for CB nitration was 30.11 ± 1.68 kJ/mol, andE a for o -NCB and p -NCB was 35.12 ± 1.79 kJ/mol and 29.52 ± 1.02 kJ/mol, respectively, indicating thato -NCB was more sensitive to the reaction temperature. Finally, at a sulfuric acid mass fraction of 93%, the kinetic model was used to predict the reaction performance. The experimental results were in good agreement with the values predicted by the model, indicating that the model is of crucial theoretical significance and practical value for optimizing the reactor design and understanding nitration processes in-depth. We envisage that the kinetics-based study of other fast and highly exothermic reactions is possible with this continuous-flow microreactor system. In our future work, more kinetics studies on nitration will be carried out.