Adsorption of fuel odorants provides clue to fading. (General Developments).

Sulfur compounds, specifically thiols and alkylsulfides, are added to fuel gas to give the fuel distinctive odors that alert consumers to leaks. Over time, however, odorant concentration in fuel gases may decrease below olfactory thresholds. This phenomenon, referred to as odorant fading, occurs through various mechanisms. When natural gas and liquefied petroleum gas leaks occur in distribution lines, adsorption and absorption processes between the odorants and surrounding soil media often result in diminished odorant concentration which cannot be directly compensated for by utilities. In addition, the magnitude of the interactions between the fuel gas odorants and soil media is not known. Recent research conducted by NIST furnished some answers by determining the magnitude of these sorption interactions.

The magnitude of these sorption processes was determined by measuring the heats of adsorption and interaction of the odorants on clay and organo-clay substrates. These substrates served as soil surrogates. The measurements were performed using a wall-coated, open-tubular (WCOT) column gas chromatographic technique developed by NIST researchers in Boulder. The researchers created clay stationary phases using the synthetic clay Laponite-RD. Subsequent coatings with an organic such as octadecane create an organo-clay stationary phase.

While the results to date include substrates representative of only two types of soil media, valuable insight into the magnitude of the odorant fading in soils is possible. Experimental results show that, as a class, the sulfide odorants have larger adsorption enthalpies on clay and organo-clay surfaces than the thiol odorants. As a result, the sulfides are more likely to be sequestered on soil surfaces. The difference in enthalpy values between the fuel gases and their respective odorants is valuable information from a utility operation standpoint.

CONTACT: Thomas J. Bruno, [phone]; [e-mail].