| Abstrakt: |
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 CHOx 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 [ABSTRACT FROM AUTHOR] |
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