Experimental determination of the lower flammability limit and limiting oxygen concentration of propanal/air mixtures under elevated temperatures and pressures

Abstract

Aside from being a critical intermediate species in the combustion of hydrocarbons, propanal, as an isomer of acetone and propylene oxide, is also an important chemical raw material widely used in propionic acid production under elevated temperatures and pressures. Unfortunately, propanal reacts readily with air or oxygen, resulting in severe flame or explosion accidents under those special conditions. To reduce the risk, the concentration of propanal and oxygen needs to be controlled strictly. Therefore, the lower flammability limit (LFL) and the limiting oxygen concentration (LOC) of propanal at elevated temperatures and pressures need to be known. To this end, experiments were carried out in a reinforced 20 L spherical explosive vessel at the temperatures of 75 ℃ and 95 ℃ and pressures varying from 0.5 MPa to 2.2 MPa. The influences of temperature and pressure on the LFL and LOC were analyzed. The results show that the oxidation of propanal proceeds slowly during the air injection at 95 ℃. It is worth noting that air can be substituted by a mixture of 79% N2 and 21% O2. During the gas distribution, mixing N2 with propanal first can effectively reduce the oxidation reaction, and the resultant LFLs of propanal/N2/O2 are slightly lower than that of propanal/air. In the presence of the throttling effect, gaseous propanal and its oxides will condense into droplets near the exhaust port. Then the droplets absorb heat and vaporize into gases again under elevated temperatures, increasing the concentration of combustible components in the confined space. Thus, there is still an explosion risk in the vessel after the unignited low concentration propanal has been rapidly discharged from elevated pressures to the atmospheric pressure. Also, propanal has a lower LOC than conventional hydrocarbon gases (e.g., methane and propane), and the LOC decreases with increasing initial pressure. Specifically, at 95 ℃, the LOCs of the propanal/N2/O2 mixtures were 4.3% and 3.3% for 1.0 and 2.2 MPa, respectively. In addition, the empirical formula in the existing literature is not suitable for estimating the LOC of propanal.