ABSTRACT
This paper presents the general methodology to use spectroscopic measurements directly for on-line process monitoring and detection and diagnosis of disturbances. An application of the on-line monitoring of a chemical batch reaction using UV-visible spectroscopy is discussed in detail. Successful historical batches are used to build a statistical model of the batch reaction. The model uses external information such as the pure spectra of the compounds and their concentration profiles to improve the interpretability. Control charts are developed for on-line monitoring of new batches. It is shown that this model is capable of detecting erroneous batches. In combination with contribution plots, the actual cause of the disturbances can be diagnosed.
ABSTRACT
In process analytical applications it is not always possible to keep the measurement conditions constant. However, fluctuations in external variables such as temperature can have a strong influence on measurement results. For example, nonlinear temperature effects on near-infrared (NIR) spectra may lead to a strongly biased prediction result from multivariate calibration models such as PLS. A new method, called Continuous Piecewise Direct Standardization (CPDS) has been developed for the correction of such external influences. It represents a generalization of the discrete PDS calibration transfer method and is able to adjust for continuous nonlinear influences such as the temperature effects on spectra. It was applied to shortwave NIR spectra of ethanol/water/2-propanol mixtures measured at different temperatures in the range 30-70 degrees C. The method was able to remove, almost completely, the temperature effects on the spectra, and prediction of the mole fractions of the chemical components was close to the results obtained at constant temperature.
