In Chile, commercial liquefied petroleum gas (LPG) is customarily odorized with dimethyl sulfide (DMS) and tert-butyl mercaptan (TBM) at a rate of about 10 grams of odorant per 1 cubic meter of LPG. The current control of the odorization practice involves the human nose to detect the odor in some cases, and the determination of total sulfur concentration in others. None of those methods provide the speciation of the sulfur species in LPG.
On the other hand, both DMS and TBM are higher boiling than propane or butane. Therefore, the odorant concentration emanating from the odorized liquid is not uniform in the vapors released. Furthermore, the gas will also be much leaner than the odorized liquid. For those reasons, the Chilean industry recommends analyzing the odorant directly from the liquid phase.
To address the problem of odorant quantification at the industry this research aims to develop a reliable and objective measure for odorized LPG that allow not only verify the odorization itself but also quantify the species incorporated with the fuel, and other sulfur compounds present naturally in the LPG. In this regard, an Automated Gas Chromatograph (Auto-GC) with an electrochemical detector was coupled with an LPG valve, which continuously takes samples of 1 µL of liquid propane. The extracted sample is then vaporized and injected into the Auto-GC to be analyzed.
The methodology is capable of distinguishing and quantifying distinct odorants near real-time (within 8 minutes), as well as other sulfur impurities present in the LPG such as tetrahydrothiophene (THT) and methyl mercaptan (MeSH). Typical retention times for TBM and DMS corresponded to 282 s and 418 s respectively. More importantly, the proposed method provides meaningful information to verify the optimum odorization, i.e., the minimum value to alert in case of leak and maximum level that assures efficient combustion.