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.