On-line monitoring and quantification of a process reaction by near-infrared spectroscopy. Catalysed esterification of butan-1-ol by acetic acid

The acid-catalysed esterification of butan-1-ol by acetic acid was monitored and quantified using near-infrared spectroscopy (NIRS). Partial least-squares regression (PLSR) was used to construct calibration models based on synthetic mixtures of the esterification components (viz. butan-1-ol, acetic acid, n-butyl acetate and water) with a view to determining all of them simultaneously. The constructed models were used to monitor a set of esterification reactions based on a factorial design using initial composition and temperature as factors; the results thus obtained were consistent with those provided by the GC reference method, even for reactions performed at higher temperatures than those used for calibration, with relative standard error of prediction (RSEP) values less than 3.5% in all cases. The kinetics of the reaction were calculated and it was shown that the reaction follows theoretical second-order kinetics, which is expected for this esterification. The proposed method allows one to determine quantitatively rate constants, end-points, yields and equilibrium constants, and also the influence of initial water on the performance of the reaction. A statistical study of these variables, based on the characteristics of the used experimental design, proved that the studied factors and their interaction were significant, and that higher temperatures and an excess of acetic acid were the best conditions. The proposed NIR method is thus an effective alternative to traditional methods and is suitable for the on-line quantitative control of batch processes.