Odorant Monitoring in Natural Gas Pipelines Using Ultraviolet–Visible Spectroscopy

The remote, timely and in-field detection of sulfured additives in natural gas pipelines is a challenge for environmental, commercial and safety reasons. Moreover, the constant control of the level of odorants in a pipeline is required by law to prevent explosions and accidents. Currently, the detection of the most common odorants (THT = thiophane, and TBM = Tertiary Butyl Mercaptan) added to natural gas streams in pipelines is made in situ by using portable Gas Cromatography (CG) apparatuses. In this study, we report the analysis of the UV-spectra obtained by a customized UV spectrophotometer, named Spectra, for the in-field detection of THT and TBM. Spectra was conceived to accomplish the remote analysis of odorants in pipelines of natural gas stream through the adoption of technical solutions aimed to adapt a basic bench UV-spectrophotometer to the in-field analysis of gases. The remotely controlled system acquires spectra continuously, performing the quantitative determination of odorants and catching systemic or accidental variations of the gaseous mixture in different sites of the pipeline. The analysis of the experimental spectra was carried out also through theoretical quantum mechanical approach aimed to detect and to correctly assign the nature of the intrinsic electronic transitions of the two odorants, THT and TBM, that cause the UV-absorptions. So far, these theoretical aspects have never been studied before. The absorption maxima of THT and TBM spectra were computationally simulated through the usage of selected molecular models with satisfactory results. The good matches between the experimental and theoretical datasets corroborate the reliability of the collected data. During the tests, also unexpected pollutants and accidental malfunctions have been detected and also identified by Spectra, making this instrument suitable for many purposes.