Flammability Limits in Flowing Ethene-air-nitrogen Mixtures: an Experimental Study

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. INTRODUCTION Partial oxidation reactions are widely used in industry. A typical example is the reaction of ethene to ethene oxide. The stoichiometric mixture of this reaction is explosive and therefore the partial oxidation process must be operated at much leaner conditions. This is achieved by operating above the upper flammability limit with ethene in excess. For a safer and more economical process it is necessary to know the flammability limits precisely in order to operate ethene oxidation plants closer to their flammability limits. Flammability limits of a particular system of gases are affected by pressure, temperature, direction of flame propagation, ignition source and experimental set-up. Flammability limits are usually determined in closed vessels or explosion tubes, with in most cases a spark as the ignition source, see e.g. Lewis and Von Elbe (1961), Zabetakis (1965), and Lovachev et al. (1973). These results are considered to be also valid under industrial conditions. However, these laboratory experiments differ from industrial operating conditions, regarding the gas flow and the ignition source. Theretbre it is questionable whether the results of laboratory experiments can be used for the industrial situation. For partial oxidation processes the occurrence of sparks as an ignition source is unlikely, because water vapour produced in an oxidation reaction will lead to a good electrical conductivity and prevent accumulation of static electricity. It is more likely that a hot spot will act as an ignition source. Unfortunately different ignition sources lead to large differences in flammability limits, see e.g. Coward and Guest (1927), Cutler (1974) and Detz (1976). The gas flow affects the heat-transfer rate from the ignition source and therefore affects the ignition of a combustible gas mixture. In flowing systems it is more difficult to ignite an explosive mixture, because of the increased rate of heat removal caused by convection from the hot spot to the surrounding gas. Furthermore, flame propagation is more difficult through a fast flowing system because of the increased level of turbulence and therefore an increased heat transfer from the flame front to the unburnt gas, which may extinguish the flame. Lewis and Von Elbe (1961), Leuckel et al. (1989), Chippett (1993) and Watanabe et al. (1983) all come to the conclusion that the flammability limit is influenced by the degree of turbulence in the gas mixture. It has also been mentioned that flammability limits in reality differ from those found in laboratory experiments. For example the flammability limit of ammonia and air mixtures depends on the gas velocity, see Ullmann (1981 ). S iccama and Westerterp (1993) have also demonstrated that the explosion region of ethene-air-nitrogen mixtures, ignited with a hot surface, becomes smaller under conditions of flow. "Currently: ECN, P.O.Box 1, Petten, the Netherlands "'To whom correspondence should be addressed 2231