A cheap handheld NIR spectrometric system for automatic determination of methane, ethane, and propane in natural gas and biogas

Abstract

In this study, a new NIR detection system using a cheap handheld NIR spectrometer, and a tungsten lamp coupled to a lab-made NIR flow cell was developed for the successful automatic quantification of methane, ethane, and propane in natural gas and biogas samples. To prepare standard gaseous mixtures for full-spectrum PLS multivariate calibration modeling and to record the NIR spectra of these mixtures and samples, this NIR detection system was connected to a flow automatic system and a gas chromatograph. The raw and second-derivative NIR spectra of the calibration set were then employed to construct PLS models using full cross-validation leave-one-out. All models presented high correlation coefficients (>0.9) and root mean square error (<5% mol mol−1) values. However, the RMSEP values using raw and preprocessed datasets were statistically similar according to the F-test at the 95% confidence level. The advantage of using raw data is the elimination of the preprocessing step for future prediction of samples to be analyzed. Once the NIR detection system has been previously calibrated via an automatic or batch process, it can be used to monitor light hydrocarbons or other gases inline (as in pipelines).