Flammability limits in flowing ethene-air-nitrogen mixtures: An experimental study

Abstract

A large pilot plant was constructed to study the upper flammability limit of ethene-air-nitrogen mixtures under conditions of flow. The gas mixtures flowed through an explosion tube with a length of 3.0 m and a diameter of 21 ram. An electrically heated wire was used as ignition source. Experiments were performed at pressures of 5 and 10 bar, with gas temperatures between 25 and 300 °C and with the wire fixed horizontally and vertically. Three different phenomena are observed: negligible reaction, local reaction, and explosion. The negligible reaction region occurs at power supply rates to the wire below a etitical value. Above this critical value either a local reaction or an explosion occurs. The critical oxygen concentration which separates the local reaction and explosion regimes depends on the experimental conditions: gas composition, pressure, temperature, wire size and orientation, and gas velocity. An increase in pressure increases the upper flammability limit. Also an increase in temperature causes an increase in the upper flammability limit and the results can be explained by assuming a constant flame temperature. Moreover, the upper flammability limit is influenced by the gas velocity. Under conditions of flow the explosion region becomes smaller, it shifts to higher oxygen concentrations. In practise this may mean that partial oxidaion reactions can safely be operated at higher oxygen concentrations, provided gas flow rates are kept high.

Influence of hydrodynamics on the upper explosion limit of ethene–air–nitrogen mixtures

“…1.5 MPa. Bolk et al 1996 did similar tests in a smaller 21-mm tube and found a flame temperature of around 1,500 K at a pressure of 0.5 MPa.…”

“…Some results have been pre-Ž . sented by Bolk et al 1996 , and others will be presented in a later study. The best fit for ␤ equals 1.53=10 y3 kgrm 3 , and this finally leads to Eq.…”

“…Ž . 1993 andBolk et al 1996 , who showed explosion experiments in quiescent mixtures, give worst-case results, because the experiments resulted in the lowest critical oxygen concentration at the explosion point.…”

“…Siccama and Westerterp 1993 performed experiments in a small-size tube of 21-mm diameter at low gas velocities, Ž . whereas Bolk et al 1996 used the same tube placed in a larger installation and higher gas flow rates. Explosion limits depended on the flow rate of the gas, an increase of which caused the explosion region to become narrower.…”

Effect of tube size and obstacles on explosion limits in flowing gases

“…wire. Westerterp 1993 andBolk et al 1996 showed that the upper explosion limit strongly depends on the flow rate of the gas; an increase in flow rate makes the explosion region smaller.…”