Benjamin Keith Harrison

When hot vapor condenses the nucleation particles initially formed coalesce rapidly and exothermically. Most of the heat of coalescence is released in less than a millisecond while droplets are less than 100 nm diameter. Consequently, the heat is released close to the source of the vapor and has no effect on dispersion. Coagulation rate is governed by an inverse square law with respect to number density; calculations showed that a vapor cloud initially within its flammable range should produce droplets smaller than 10 µm after a minute of coalescence. Such micronized droplets completely vaporize in flame preheat zones and behave as premixed vapors rather than as individual droplets; this characteristic is not due to the increased surface energy accompanying subdivision into micronized droplets but to short evaporation times in the thick preheat zones of limit flames. Droplet growth time is a nonlinear function of cloud mass concentration and if the latter exceeds about 10 times the minimum explosible concentration (MEC), droplets quickly grow to above 20–40 µm. These larger droplets tend to burn individually and the estimation of MEC becomes complicated, although rain‐out of large droplets might hasten dispersal to below the MEC.With the assumption that micronized crystalline organic materials behave similarly to liquid droplets, we derive “intrinsic” MEC values for both hydrocarbons and a series of CHO_x fuels having increasing O:C ratios. In each case, the MEC attains a constant “terminal value” as carbon number is increased. Terminal MEC values are 40 g m⁻³ for CH (hydrocarbons), 50 g m⁻³ for CHO (aldehydes, ethers, etc.), 60 g m⁻³ for CHOO (esters), 67 g m⁻³ (polyethylene glycols), and 103 g m⁻³ (carbohydrates). MEC values for CHO and CHOO decrease rapidly with increased carbon number since additional hydrocarbon units dilute the functional oxygen group. For polyethylene glycols and carbohydrates, the MEC is less dependent on carbon number since oxygen is contained in a repeating group. Although scission and/or decomposition replace simple evaporation as carbon number increases, flash pyrolysis data for polyethylene in air show that the products of decomposition can have little effect on MEC. Nevertheless, owing to experimental deficiencies published MEC values for polyethylene are as low as 10–30 g m⁻³. Of the many factors affecting MEC tests, we focus on the effects of gravity and flame radiation. Radiation not only affects MEC but also the burning rates of metal dusts such as titanium; such tests should be carried out on an adequately large scale. © 2017 American Institute of Chemical Engineers Process Process Saf Prog 37:4–17, 2018

The uses of the ASTM computer program CHETAH were described. The recently released version 10 is capable of accomplishing five different kinds of calculations related to chemical thermodynamics. The program enables the prediction of chemical reactive hazards, heats of reaction for combustion and other types of reactions, and a variety of flammability parameters. It can accomplish these calculations making use of its large internal databases of thermochemical properties or by making use of predictive methods for chemicals for which properties have yet to be determined.

This article discusses technical and practical aspects of the limiting oxygen concentration (LOC) including a detailed appraisal of possible sources of measurement error. LOC values obtained in the “classical” 5‐cm diameter, vertical tube apparatus have been described as nonconservative owing to flame quenching. However, much of the difference between classical LOC values and those obtained in closed vessels using ASTM E2079 is due to the change in the definition of “ignition.” Formerly defined as propagation of a visible flame through a vertical tube at least 1.5 m tall, which in a closed tube would typically generate an overpressure in excess of 100%, ignition is now defined as an overpressure that exceeds 7%. The small pressure rise criterion tends to underestimate the LOC in smaller test vessels, especially for tests made at elevated pressures and temperatures. More use should be made of ASTM E2079's flexibility regarding the use of larger pressure rise criteria. Having lost sight of the flame, it is important not to lose sight of the objective, namely to select test criteria that correctly predict the flammable limits pertaining to large volumes of gas mixture. Additional “reference quality” LOC data are needed both to improve the definition of “ignition” with respect to test vessel volume and to investigate the accuracy of LOC estimation techniques. Any large‐scale reference quality test program should investigate the effect of low levels of forced turbulence on measured LOC values.The NFPA “Explosion Protection Systems” Technical Committee recently supported the development of two Tentative Interim Amendments (TIAs) to NFPA 69 “Standard on Explosion Prevention Systems.” The first TIA resolves issues with the table of LOC values for gases, which had been modified and over‐corrected in the 2008–2014 editions. The second TIA introduces a simple method for estimating the LOCs of single gases and gas mixtures based on the equation LOC = LFL × S, where LFL is the lower flammable limit and S the molar stoichiometric ratio of oxygen to fuel. An Appendix describes the two TIA recommendations while a second Appendix describes how the estimating methods have been incorporated into ASTM's “CHETAH” Program. © 2016 American Institute of Chemical Engineers Process Saf Prog 35: 107–114, 2016

Britton discovered that with increased “net heat of oxidation” (ΔH_ox), the maximum flame temperatures of CH and CHO fuels in air increase linearly while flame temperatures at the lower flammable limit (LFL) decrease linearly. Maximum flame temperature is a major factor determining the combustion rate of optimum fuel‐air mixtures and relationships were found between ΔH_ox and “optimized” flammability parameters such as minimum ignition energy. The LFL is the fuel concentration needed to attain the lower limit flame temperature; since less fuel is needed to attain a smaller flame temperature, the LFL varies inversely with ΔH_ox. Simple expressions derived between ΔH_ox and parameters commonly used in process safety were previously published in this journal. The commercially available computer program “CHETAH™” now solves these expressions and outputs the flammability parameters plus the internally generated thermodynamic data used in the solutions. This article updates the original expressions together with new findings and explanatory material. © 2014 American Institute of Chemical Engineers Process Saf Prog 33: 314–328, 2014

This study introduces a new method for characterizing mineral-associated microbial diversity in sedimentary environments, a habitat that has been intrinsically challenging to study in regard to microbe-mineral interactions. Mineral components were enriched from bulk environmental samples by magnetic susceptibility or density separation techniques and used in subsequent molecular and microscopic analyses. Testing and optimization of the method was performed on geochemically-distinct sediment horizons from Eel River Basin methane seeps and pyrite and sphalerite-rich hydrothermal vent samples from the Lau Basin. Initial results show reproducible variations in microbial diversity between mineral fractions from marine sedimentary environments enriched in authigenic pyrite and/or transition metal-bearing clay minerals. Specifically, different archaeal clades associated with the anaerobic oxidation of methane and putative sulfate-reducing deltaproteobbacteria show preferential colonization patterns, suggesting potential ecophysiological differences between closely-related taxa. These results indicate that mineral colonization may influence the extent and distribution of microbial diversity throughout unconsolidated sediments of the marine subsurface. The combination of mineral separation and molecular analyses introduced here provide a new approach for revealing previously concealed patterns of mineral-associated microbial diversity across a wide range of environments, from hard rock habitats to fine-grained lithologies.

Accurate flammability and ignitability data for chemicals form the cornerstone of procedures used to assess the hazards associated with commercial chemical production and use. Since 1967 the ASTM E27 Committee on the Hazard Potential of Chemicals has issued numerous, widely used consensus standards dealing with diverse testing and predictive procedures used to obtain relevant chemical hazard properties. The decision to issue a standard rests solely with the membership, which consists of representatives from industry, testing laboratories, consulting firms, government, academia, and instrument suppliers. Consequently, the procedures are automatically relevant, timely, and widely applicable. The purpose of this paper is to highlight some of the widely used standards, complemented with hypothetical but relevant examples describing the testing strategy, interpretation, and application of the results. A further goal of this paper is to encourage participation in the consensus standards development process.The paper is published in two parts. The first part (in the preceding issue of Process Safety Progress) dealt with the E27 standards pertaining to thermodynamics, thermal stability, and chemical compatibility. The second part, published here, focuses on the flammability, ignitability, and explosibility of fuel and air mixtures. © 2005 American Institute of Chemical Engineers Process Saf Prog, 2005

Accurate reactive chemicals data form the cornerstone of procedures used to assess the hazards associated with commercial chemical production and use. Since 1967, the ASTM E27 Committee on the Hazard Potential of Chemicals has issued numerous, widely used consensus standards dealing with diverse testing and predictive procedures used to obtain relevant chemical hazard properties. The decision to issue a standard rests solely with the membership, which consists of representatives from industry, government, consulting firms, and instrument suppliers. Consequently, the procedures are automatically relevant, timely, and widely applicable. The purpose of this paper is to highlight some of the widely used standards, complemented with hypothetical but relevant examples describing the testing strategy, interpretation, and application of the results. A further goal of this paper is to encourage participation in the standards development process.The paper is published in two parts. The first part deals with the E27 standards pertaining to thermodynamics, thermal stability, and chemical compatibility. The second part, to be published in the next issue of this journal, focuses on the flammability, ignitability, and explosibility of fuel/air mixtures. © 2004 American Institute of Chemical Engineers Process Saf Prog, 2004