A Comparative Study of ATR-FTIR and FT-NIR Spectroscopy for In-Situ Concentration Monitoring during Batch Cooling Crystallization Processes

Crystallization is a widely used separation and purification process in the chemical industry. The final product properties such as the crystal size distribution and the morphology are largely defined during this process. In order to achieve proper control over the crystallization process, accurate in situ measurements are necessary. This paper presents the assessment of in situ infrared spectroscopic techniques for solute concentration measurements. It presents a comparative study of attenuated total reflectance (ATR), Fourier transform infrared (FTIR), and Fourier transform (FT) near infrared (NIR) spectroscopy for batch cooling crystallization of four different crystallization systems. These two techniques were compared based on the partial least square models developed by extensive calibration in the laboratory, the independent data sets generated while determining saturation concentrations in the presence of crystals and monitoring the batch cooling crystallization processes. The ATR-FTIR spectroscopy was found to perform better statistically and also in the presence of crystals. The FT-NIR spectroscopy was less reliable as the probe head was more susceptible to scaling. The average relative error in predicting the solubilities in the presence of crystals was sensitive to the crystallizing system and was on the order to 2% for ATR-FTIR while it was much higher for FT-NIR.