Effects of the feeding procedure on the thermal behaviors of autocatalytic esterifications in semibatch processes

Esterification reactions between anhydrides and alcohols catalyzed by sulfuric acid have broad applications in the food, cosmetic, and pharmaceutical industries. However, the exothermic behavior cannot be well explained, especially in semibatch reactors with different feeding procedures. In this study, a series of esterification processes in semibatch systems were conducted with a wide range of feeding rates with two different substrates. The structures of initial catalytic substrates were characterized using in situ Fourier Transform Infrared (FTIR) spectroscopy. A total of 11 reaction pathways, including those for the initial activation of substrates and the subsequent reactions of the feeding process, were calculated by the density functional theory (DFT) method. Three possible catalytic cycles of the esterification with specific acid catalysis, protonated intermediate, and protonated intermediate and H₂O, were established for the semibatch esterification reaction based on calculated results and experimental evidence. Thermal safety parameters such as reaction enthalpy (ΔH_r), adiabatic temperature rise (ΔT_r,ad), and maximum temperature of the synthetic reaction (MTSR) were determined based on Wilson equation used to calibrate mixing heat. The results reveal that mixing heat was 2.5 kJ/mol with same mole ratio of n-butanol (nB) and propionic anhydride (PA). The reactions started by PA are dominant, no matter whether the substrate is nB or PA. The initial reaction rate is restricted by the low concentration of catalyst and active reactant when nB is used as substrate, which lead to significant reactant accumulation with high MTSR of approximately 160 °C. Furthermore, feeding nB into PA allows a controllable increase in the ΔT_r,ad realized by changing the feeding rate and limiting the accumulation of reactant.