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  • Photothermal Analyzer Detects Fuel Adulteration

Photonics Spectra
Mar 2004
Daniel S. Burgess

The adulteration of gasoline with cheap industrial solvents or with aromatic or light hydrocarbons is a common practice in many developing countries as retailers attempt to stretch their stocks. In some cases, the practice has little or no effect beyond reduced tax revenues. Higher concentrations of adulterants, however, can increase tailpipe emissions, damage engines and fuel systems, produce stalling or vapor lock, and even present immediate health risks.

Now a team of researchers at Universidade Estadual do Norte Fluminense in Campos dos Goytacazes and at Instituto Nacional de Pesquisas Espaciais in São José dos Campos, both in Brazil, has devised a photothermal technique that can detect the presence of adulterants in gasoline. In tests with 210 samples purchased from gas stations in the Rio de Janeiro area, the method proved to be simple, fast and accurate, suggesting that it may be suitable for use in a portable device for field analysis.

The incentive to doctor retail fuels can be powerful in developing countries. A 2002 study by the Centre for Science and Environment in New Delhi estimated that a gas station that sells 10,000 liters of gasoline per day could increase its profits by more than $550 per day by cutting the fuel with 15 percent naphtha. The center's inspection of 70 randomly collected samples revealed that 26 percent had been adulterated.

A 2001 policy note from the World Bank in Washington suggests that the lack of a monitoring regime or an effective fuel tax system in many nations further promotes cheating. But the will to enforce compliance and the policies to express that will are hollow without an effective means of detecting noncompliance.

Fuel analysis

The group in Brazil believes that it has found that means. Its photothermal gas analyzer employs a 30-mW 665-nm AlGaInP laser diode to heat a 15-µm-thick piece of aluminum. The laser light, which is electronically modulated at 10 Hz, heats the foil, causing it to launch thermal waves through a cell containing a mixture of air and vapor from the liquid hydrocarbon that is under test. The thermal waves are detected at the other end of the 2-mm-long cell using a thin-film pyroelectric sensor, and the amplitude and phase of the signal from the sensor are converted into thermal diffusivity values for the sample that decay over time.

Gasoline typically is doctored with lighter substances. According to the kinetic theory of gases, the thermal diffusivity of a gas is inversely proportional to the square root of the effective mass of the gas molecules. Such an adulterant, therefore, decreases the effective mass of the fuel vapor in the air/vapor mixture in the cell and increases the thermal diffusivity.

Comparing the results of tests with the photothermal gas analyzer against those performed with a densimeter, distillers and an IR spectrometer, the researchers found that the new method displayed an uncertainty of 5.7 percent, incorrectly identifying 12 of the 210 samples. Crucially, the analyzer requires only on the order of 15 minutes to perform a measurement, and its simplicity makes it promising for field analysis.

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