Sulfur and Total Carboxylic Acid Number Determination in Vacuum Gas Oil by Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy

Abstract

Sulfur removal is one of the key functions of vacuum gas oil (VGO) hydrotreating reactors. Knowing feed and product properties real-time or near-real-time improves reactor operations. The VGO section of crude distillation unit is also prone to severe high-temperature sulfidic and naphthenic acid corrosion. In this article, we evaluate a single-reflectance attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy as a possible quick and cost-effective methodology to determine total carboxylic acid number (TCAN) and total sulfur content of VGO. The study shows that single-reflectance diamond ATR crystal methodology has the right signal-to-noise ratio to accurately predict TCAN and total sulfur within the primary method’s repeatability. Statistical models have been developed using 64 sample sets of vacuum gas oil and out of which 10 samples were used for the cross-validation of the model. The range of TCAN in VGO samples used in this study was between 0.37 and 13.8 mg KOH/g, and sulfur content was between 0.8 to 5.4% by mass. Models have been evaluated by determining the correlation coefficient (R 2), linearity curves obtained by plotting measured versus predicted values, and the errors associated with the prediction and cross-validation. The models showed a correlation coefficient of 0.9991 for TCAN and 0.9974 for total sulfur between reference and the measured values for calibration set of samples. A root-mean-square error of calibration (RMSEC) and prediction (RMSEP) for TCAN were found to be 0.0903 and 0.0885 mg KOH/g. Similarly, RMSEC and RMSEP values for sulfur content were 0.0829 and 0.107% by mass, respectively. The proposed methodology for the prediction of total sulfur and TCAN is fast, efficient, and cost-effective and has several advantages over the standard methods.

Multivariate Calibration of Sulfur in Sour Crude Oils via Near-Infrared Spectra

“…To the best of our knowledge there are no NIR-based studies specifically dedicated to quantifying sulfur in crude oils, let alone on this large number of samples or during such a lengthy period of time. NIR and FT-MIR spectroscopies were used for determining sulfur content in Brazilian petroleum fractions, while FT-IR spectroscopy is used for determining sulfur and total carboxylic acid number in vacuum gas oils and for sulfur speciation . The first of these articles also addresses a variety of chemometric details, primarily with regards to selecting wavenumbers of interest for multivariate calibration.…”