Explosion regions and limiting oxygen concentrations of methyl propionate, methyl acetate, dimethyl carbonate with air and inert gas mixtures

Highlights

• The explosion regions of three ester–air–inert (N₂, CO₂, He) gas mixtures are determined experimentally.

• The temperature dependence of the explosion region can be predicted quantitatively by the extended CAFTP method.

• The CAFTP method is improved to model explosion regions when He is the inert and the respective Lewis numbers are missing.

Abstract

New data about explosion regions with special focus on limiting oxygen concentrations for methyl propionate, methyl acetate, dimethyl carbonate with air in the presence of nitrogen, helium and carbon dioxide were determined at ambient initial pressure and 423 K. The measurements were executed according to EN 1839 method T. The changes of the explosion regions with temperature and type of inert gas were also modeled mathematically using an extended calculated adiabatic flame temperature profile (CAFTP) method. The shift of the explosion region boundaries with temperature when switching from nitrogen to carbon dioxide were reproduced well. For a switch to helium a good agreement could be reached only if the very high thermal conductivity of helium had been considered properly by using the Lewis number. This requires the knowledge of the respective Lennard-Jones parameters. The LOC of helium-containing mixtures can, however, be calculated with acceptable accuracy even if the Lennard-Jones parameters of the flammable substance are not known exactly by using reasonable estimations.

Introduction

When handling mixtures of flammable substances with air, it is often desired to keep the fuel concentration outside the explosion range. To do so, knowledge of the lower and upper explosion limits (LEL, UEL) of the substances in question is required. They are the limiting concentrations of a flammable substance in air that cannot support flame propagation and lead to an explosion because the mixture is either too lean or too rich of the flammable substance. For their determination several standardized methods have been developed (DIN EN 1839, 2017 (method T and B); ASTM E681-09, 2015; ASTM E2079-07, 2013).

If concentrations of the flammable substance within the explosion range of the mixture cannot be avoided, the addition of an inert gas is a way to prevent an explosion. Addition of an inert gas leads to narrowing of the explosion region and finally to the coalescence of the LEL and UEL in one point. The oxygen concentration at this point is called limiting oxygen concentration (LOC) when nitrogen is the inert gas. It is called maximum oxygen concentration in a mixture of flammable substance, air and an inert gas other than nitrogen. Additionally, explosion regions can be characterized by the following terms (Molnárné et al., 2008):

• MXC – maximum permissible amount of substance fraction of flammable gas (in a vapor/inert mixture; in mol %)

• ICR – minimum inert gas/flammable gas ratio

• MAI – minimum required amount of inert gas (in an inert/air mixture; in mol %)

• IAR – minimum inert gas/air ratio.

An exact definition of these quantities is given by Molnárné et al. (2008).

As the experimental determination of the explosion region of a vapor/air/inert gas mixture may be time-consuming, numerous numerical methods have been developed for prediction of explosion regions. They comprise semiempirical methods (Wang et al. 2010; Mendiburu et al., 2015), or calculation based on modified LeChatelier equations (Kondo et al., 2006a; 2006b; Ma 2011). Other predictions are based on thermal models (Ma et al., 2013; Chen et al., 2009; Liaw et al., 2012, 2016). Bertolino et al. (2019) predicted the flammability region using a freely propagating flame method, stressing the importance of detailed reaction kinetics and soot formation (which increases radiation losses) especially at the UEL.

The calculated adiabatic flame temperature (CAFT) method is useful for predicting the flammability envelope of the gaseous and vapor mixtures, see by Mashuga and Crowl (1999); Razus et al. (2004); and Brooks and Crowl (2007), however, as already observed by these authors, when used in its pure form (one temperature for the whole range of explosion limits), its predictive power is limited. Therefore, in the present work, the method is used in a modified version (calculated adiabatic flame temperature profile CAFTP) proposed by Askar et al. (2008, 2019). This modification takes into account amongst others the fact that the flame temperature depends on the amount of inert added and the special inerting properties of He by introducing the Lewis number of the mixtures. Using this modified method, the explosion regions of propane, alcohols (1-propanol, 2-propanol, 1-butanol) acetone, and methyl acetate/inert gas/air mixtures, have been successfully calculated by Abdelkhalik et al. (2016, 2019) and for ethylene oxide and difluormethane by Askar et al. (2019). With respect to He as inert gas often information on Lewis numbers is missing. The aim of this investigations was to find solutions for this problem and to check the parameters quantifying the inerting power of the inert gases for elevated temperatures.

The flammable liquids investigated -methyl acetate, methyl propionate and dimethyl carbonate-have been chosen due to systematic considerations concerning the ratio of C to O in the molecule in extension to the investigations reported by Abdelkhalik et al. (2016, 2019) and because of their wide use in industrial processes. A major use of methyl acetate and methyl propionate is as solvents or as raw material for the production of paints, varnishes and other chemicals. Dimethyl carbonate is considered in addition an option for meeting the oxygenate specifications on gasoline, as a means of converting natural gas to a liquid transportation fuel (Pacheco and Marshall, 1997), and can be utilized in an unmodified conventional diesel engine (Durbin et al., 2017; Rounce et al., 2010). It presents interest as a fuel oxygenate additive (Durbin et al., 2017; Pacheco and Marshall, 1997; Rounce et al., 2010).

The inert gases investigated were nitrogen, carbon dioxide both widely used as a safety measure in explosion safety, and helium due to its pronounced inerting properties.

In this work, the measurements were executed according to EN 1839 method T (DIN EN 1839, 2017) at ambient initial pressure and 423 K. This temperature was chosen to extend the investigations by Abdelkhalik et al. (2019) to higher temperatures and to demonstrate that such temperature-related widening of the explosion regions can also be modeled by CAFTP.