Estimation of lower flammability limits in high-pressure systems. Application to the direct synthesis of hydrogen peroxide using supercritical and near-critical CO2 and air as diluents

A method for the prediction of the LFL at high pressures, where data are scarce, based on the calculation of adiabatic flame temperatures for the mixtures H2 + O2 in CO2 and N2, between 1.0 and 300 bar and 288–348 K is presented. A group contribution equation of state (GC-EoS) has been selected to predict thermodynamic properties of the mixture, i.e. residual enthalpy, heat capacity and others, as well as phase equilibrium data, giving deviations lower than 10% at high pressures. The use of CO2 as a diluent increases the operational margin from 4.5 mol% H2 at 1 bar up to ca. 7.0–9.0 mol% H2 at 200 bar due to the increase in the heat capacity. On the other hand, the use of nitrogen or air as a diluent only increases the margin from 5.2 mol% H2 at 1 bar up to ca. 6.0 mol% H2 at 200 bar.