Abstract

Near-infrared (NIR) spectroscopy has been successfully used for the monitoring of important com ponents in the <i>p</i>-diethylbenzene (PDEB) separation process. The process is composed of mostly diethylbenzene isomers (<i>ortho</i>, <i>meta</i>, and <i>para</i>) and extractant (<i>p</i>-xylene), as well as other C₉-C₁₁ aromatic hydrocarbons. Therefore, the major concern in using NIR spectroscopy in this process was the spectral resolution of NIR spectra among diethylbenzene isomers, since the molecular structures of each isomer were very similar. NIR spectral features of <i>o</i>-diethylbenzene (ODEB), <i>m</i>-diethylbenzene (MDEB), and PDEB showed considerable spectral differences in the 2100-2220 nm range. These combination bands originated from the combination of the = C-H stretch at 3100-3000 cm^-1 and C = C ring stretch at 1600-1450 cm^-1. Characteristic C = C ring stretches of each isomers in the IR region result in selective and identifiable spectral features in the NIR region. Partial least-squares (PLS) regression was used to build each calibration model for ODEB, MDEB, PDEB, and <i>p</i>-xylene (PX). PLS calibration results of the four components showed excellent correlation with gas chromatography data. The combination region (2100-2500 nm) provided the important isomeric spectral information for PLS calibration since the absorption bands in this region were the most sensitive and selective.

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