Quantitative In-Line Analysis in Supercritical CO2 Using Fibre-Optic NIR Spectroscopy and Multivariate Calibration: A Potential Method for Monitoring Continuous Flow Processes

Abstract

A set-up for fibre-optical in-line near infrared spectroscopic monitoring of hydrocarbons in supercritical CO2 has been developed, which can be operated under flow-through conditions. It consists of a high-pressure optical cell, which is adapted to an FT-NIR spectrometer via a fibre-optic interface. NIR spectra of squalane in supercritical CO2 have been measured for varying concentration, temperature and pressure conditions. A partial least squares (PLS) regression model was developed with a calibration set of 36 samples spanning the 0–32 × 10−3 g cm−3 squalane concentration range and with CO2 densities between 0.62–0.86 g cm−3. Based on the corresponding absorbance data in the 4900–4200 cm−1 spectral range, squalane and CO2 were modelled with standard deviations of 0.24–10−3 and 3.8 × 10−3 g cm−3, respectively. Comparable standard deviations were obtained by validating the model with independent test samples. The developed calibration models allow for the in-line quantification of both constituents under varying temperature and pressure conditions from a single spectral measurement. The results obtained encourage the use of NIR spectroscopy/PLS calibration for fast in-line analysis of continuous flow extraction processes for the deoiling of metal parts based on supercritical CO2.