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Limiting Oxygen Concentration Recent Results and their Presentation in Chemsafe

By Connie Lawrence,2014-05-08 22:50
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Limiting Oxygen Concentration Recent Results and their Presentation in Chemsafe

W. O. Möller, M. Molnarne, R. Sturm

    Limiting Oxygen Concentration: Recent Results and their Presentation in Chemsafe

    Summary

    Substances like alcohols, ethers and esters have recently gained in importance as solvents for paints and varnishes, for surface degreasing and cleaning. Furthermore, fluorinated hydrocarbons are used as refrigerants, replacing chlorofluorocarbons. Because reliable data were not available for most of these substances, measurements of explosion limits and limiting oxygen concentration (MOC) have been carried out by PTB (flammable liquids) and BAM (flammable gases).

    The poster presents the results of the most recent measurements of MOC for ethanol, i-propanol, ethyl acetate, propyl formate, hexane and for gases as e.g. difluoroethane and methane. Explosion limits and limiting oxygen concentrations have been measured in mixtures with air under ambient conditions and at elevated temperatures according to the German standard DIN 51649.

    Introduction

    It is often not possible to avoid the presence of flammable gases and vapours inside plants. In these cases, purging with an inert gas like nitrogen is a suitable way of preventing an explosive mixture. Inerting requires the knowledge of the limiting oxygen concentration with respect to the inert gas or the explosion limits of the ternary system consisting of combustible, air and inert gas as a function of the mixture composition. The European Standard EN 1127-1 defines the limiting oxygen concentration as the maximum oxygen concentration (MOC) in a mixture

    of combustible, air and inert gas, in which an explosion will not occur, determined under specified test conditions.[1]

    The maximum oxygen concentration is directly available from measurement or can be derived graphically (cf. Figure 1): In a triangle diagram showing the experimentally determined explosion area (hatched), a tangent to the boundary curve is drawn parallel to the inert gas axis. The intersection with the air axis will yield the maximum air concentration value not yet leading to ignition. This air concentration value has to be converted into oxygen concentration in order to get the required MOC value.

Figure 1: Explosion area of a ternary system: combustible, inert gas, air

    Determination of MOC according to DIN 51649

    MOC values reported in literature are mostly determined at room temperature and atmospheric pressure [2]. However, for many processes, these values are needed for elevated temperature and pressure.

    In the past, mainly hydrocarbons were used as solvents for technical purposes. The data of these substances are sufficiently well known. But recently other substances like alcohols, ethers and esters have gained in importance as solvents for paints and varnishes, for surface degreasing and cleaning. Furthermore, fluorinated hydrocarbons are used as refrigerants, replacing chlorofluorocarbons. Because no reliable data were available for most of these substances, measurements have been carried out by PTB (flammable liquids) and BAM (flammable gases).

    Table I presents the results of these measurements for the liquids ethanol, i-propanol, ethyl acetate, propyl formate, hexane and for the gases difluoroethane and methane. Explosion limits and limiting oxygen concentrations have been measured in mixtures with air under ambient conditions and at elevated temperatures according to the German standard DIN 51649 [6].

    Table I: Lower explosion limits (LEL), upper explosion limits (UEL) and MOC (in % vol.) compared with data reported by Zabetakis [2] (values in brackets).

    Data on methane were taken from CHEMSAFE [4] and Rennhack et al.[5]

     ethanol i-propanol ethyl propyl hexane difluoro-ethane methane

    acetate formate

    LEL 20?C 2.0 2.0 2.3 4.0 3.1 1.0 4.4

    (3.3) (1.2) (5.0)

     100?C 2.9 1.9 1.8 1.8 0.85 3.95

    UEL 20?C 18.5 16.3

    (15.0)

     100?C 27.7 13.4 12.8 7.8 17.3 8.9

    (18.8)

    (7.5)

    MOC 20?C 8.7 9.8 9.8 9.8 8.6 9.3 10.7

    (10.7) (11.9) (12)

     100?C 8.1 8.1 9.1 9.2 8.9 9.9

    For a couple of substances literature data are available [2]. Comparison with these data shows that the explosion range measured in the standardized apparatus is significantly wider. The data reported by Zabetakis had been measured in an explosion chamber which was longer and had a smaller diameter [3].

    Graphical representation of data using CHEMSAFE

    These results are available in the latest update of the CHEMSAFE database [4]. The newly developed graphical user interface of the in-house version of CHEMSAFE (Figure 2) allows a graphical representation of the data as triangular diagrams showing the explosion limits as a function of the concentration of combustible, air and inert gas.