Determination of API Gravity and Total and Basic Nitrogen Content by Mid- and Near-Infrared Spectroscopy in Crude Oil with Multivariate Regression and Variable Selection Tools

Abstract

American Petroleum Institute (API) gravity is an important parameter in the crude oil industry and the nitrogen compounds are related to the toxic effects of the oil in refineries and the environment. In this paper, 194 crude oil samples with API gravities ranging from 11.4 to 57.5 were used for the purpose of estimating the physicochemical properties: API gravity, total nitrogen content (TNC) and basic nitrogen content (BNC). Initially, infrared spectra in the mid and near regions (MIR and NIR) were collected, then full-spectral partial least squares (PLS) and the orthogonal projections to latent structures (OPLS) chemometric models were developed and validated, as well as models using interval PLS (iPLS), synergy interval PLS (siPLS) and competitive adaptive reweighted sampling PLS (CARSPLS) as variable selection tools. For API gravity and TNC, the best calibration technique is the NIR CARSPLS with a root mean square error of prediction (RMSEP) values of 0.9 and 0.0275 wt%, respectively. For BNC, the best technique is MIR siPLS with a prediction error of 0.0134 wt%. The results were validated based on the evaluation of the figures of merit, a statistical evaluation of the accuracy, characterization of the systematic error and measurement for errors in the residues. The results were satisfactory considering the high variability of the data and the diversity of the samples, demonstrating suitable applicability for practical analysis.

Keywords:

• API gravity
• basic nitrogen content
• crude oil
• mid-infrared (MIR) spectroscopy
• near-infrared (NIR) spectroscopy
• total nitrogen content

Acknowledgments

The authors would like to thank NCQP/LABPETRO/UFES and Petróleo Brasileiro S.A. (PETROBRAS) for providing the crude oil sample and analysis.

Conflicts of interest

The authors declare they have conflicts of interest or financial interest in this work.

ORCID

Karla Pereira Rainha https://orcid.org/0000-0001-6896-5659

Additional information

Funding

This work was supported by the FAPES (Process number: 33530.503.20537.12092017); and CNPq (Process number: 422515/2016-7). The authors thank Fundação de Amparo à Pesquisa e Inovação do Espírito Santo (FAPES – process number 33530.503.20537.12092017) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq – process number 422515/2016-7) for their support. This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES) – Finance Code 001.