Use of propylene oxide versus nitrogen as an ethylene oxide diluent



Decomposition flame propagation limits of ethylene oxide (EO) diluted either with nitrogen or propylene oxide were measured under initial conditions enveloping those commonly used in simple stirred ethoxylation reactors. Tests conducted in a 600 liter vessel at 6.5 bar (95 psia) and 165°C verified that “energetic ignition” decreases the EO propagation limit relative to the predictions of “weak ignition” models. The limit in nitrogen found using “energetic ignition” (1 gm gunpowder squib or EO decomposition flame fired into the test mixture) was about 38 mol% EO while the limit found using hot coil igniters was about 46% EO, similar to the predictions of “weak ignition” models. The rates of pressure rise due to decomposition of near-limit mixtures were highly sensitive to low levels of turbulence but much less than values previously reported for mixtures containing 90% EO.

Propylene oxide (PO) has been reported to be a more effective EO diluent than nitrogen. However, tests using “energetic ignition” in the large vessel revealed a potentially hazardous feature of PO dilution. Although the EO decomposition limit was raised from 38% in nitrogen to about 48% in PO, the latter started to decompose at greater EO concentrations, becoming a hazardous reactant rather than a diluent. Decomposition of near-limit EO + PO mixtures generates much greater pressures than can be achieved by decomposing near-limit EO + nitrogen mixtures. At the test conditions used in this study, decomposition of 55% EO in nitrogen produces a maximum pressure of about 34.5 bar (500 psia) while 55% EO in PO produces about 51.7 bar (750 psia). The PO decomposition phenomenon has not been previously reported. A plausible explanation involves the fate of ethylene formed during PO decomposition. In diluent mode, PO decomposes endothermically producing ethylene, CO, and hydrogen. The large heat losses and short duration of decomposition events in small test vessels allows ethylene to persist as a stable product and PO therefore appears to be an effective diluent. However, in large test vessels, the greater temperature and pressure transients developed during EO decomposition cause ethylene to decompose and the net decomposition reaction of PO becomes exothermic.