M. Gödde, H. K. Cammenga, E. Brandes
Effect of Pressure on Autoignition Temperatures of Organic Compounds

Normally, the autoignition temperatures (AIT) of organic compounds determined by a standard test method (DIN 51795, IEC 79-4, ASTM E 659-78) are available only for atmospheric pressure. The present paper describes an experimental method suited to determine the autoignition temperatures of liquid organic substances at pressures of up to 25 bar. In contrast to earlier investigations carried out at various pressures, the AIT was measured with the fuel-air-ratio varied.

Apparatus and Procedure

The apparatus used to determine the effect of pressure on the autoignition temperature consists of a cylindrical stainless-steel vessel of half litre volume, which can be uniformly electrically heated. The temperature inside the vessel is measured with a thermocouple, the pressure is measured with a piezoresistive pressure transducer. Because of the high heat capacity of the vessel it is not feasible to determine the AIT by varying the temperature at a given pressure. Instead, the lowest autoignition pressure at several fixed temperatures is determined in order to produce an AIT-pressure-curve.

The liquid substance is metered into the vessel by means of an HPLC pump at a pre-determined air-pressure and temperature. An ignition is evidenced by a sharp rise of pressure and temperature inside the vessel within 10 minutes after introduction of the organic liquid. With the pre-selected air pressure and sample amount varied, the minimum autoignition pressure is determined at several temperatures. In addition, the AIT is determined in the open vessel at atmospheric pressure and compared with the AIT measured by the IEC 79-4 standard test method.

Pressure - AIT curves of acetone, butanone, 2-pentanone and 2-hexanone were examined.

Results

The AITs determined by the standard test method and those measured with the open vessel are in a good agreement. Experiments with the closed vessel show that a small rise of the initial pressure lowers the AIT distinctly. When starting at 1 bar, especially at higher temperatures the rise of the system pressure by evaporation of the liquid sample is sufficient to reach the minimum ignition pressure. With decreasing temperatures the minimum ignition pressure increases. For instance, the AIT of acetone drops from 535 °C at atmospheric pressure to 240 °C at 17 bar. The lowest values are found in mixtures very rich in fuel. For the ketones investigated, the AITs decrease at a given pressure with increasing chain length. Within the examined pressure range the pressure - AIT curve of acetone shows a lobe due to a change of the oxidation mechanism. This effect cannot be observed with the other substances because this phenomenon occurs at pressures below 1 bar in these cases.

The decrease of the AIT of liquids with higher pressures is limited mainly by the vapour pressure of the combustible.

The apparatus presented has proved to be useful to extend the measurements of the AIT to pressures higher than atmospheric. As result of the variation of mixture composition, minimum values are obtained by this procedure.