Lower explosive limit estimation

The possibilities offered by the lower explosive limit of estimating a flashpoint cc have already been discussed in detail. It would be possible, vice versa, to estimate a lower explosive limit given a flashpoint cc in which there is some confidence. To do so one simply needs to calculate, using equation (1) in paragraph 1.1.2, the vapour pressure of a substance at the temperature of the flashpoint, then for its equilibrium concentration C, in these conditions Ceq = LEL. But an error in has big consequences on Cgq and thus on LEL. This is the reason why this approach is less reliable than the other and can only be used when there Is certainty about the flashpoint. 

Another estimation method of mixture flashpointe was sugg ed by Gmehling (note p.63). The method uses the forecast technique of activity coefficients of iiquid mixtures called UNIFAC that would therefore enable calculation of the vapour pressure of the mixtures and, thanks to Le Chdtelier equation, calculate the temperature to which the mixture has to be heated so that its equilibrium concentration reaches the lower explosive limit. 

It is desired to operate the hood of Problem 3-29 so that the vapor concentration in the hood plenum is below the lower explosion limit of 12.5% by volume. Estimate the minimum control velocity required to achieve this objective. The amount of TCE evaporated within the hood is 5.3 lb per hour. The molecular weight of TCE is 131.4. The temperature is 70°F and the pressure is 1 atm. 

Evaluation of the above equations with normal alkanes disclosed favorable agreement of estimates and data. For lower explosive limit, very favorable agreement was experienced for small, intermediate and large size alkanes. For upper explosive limit, rough agreement was experienced for small and large size alkanes. More favorable agreement was experienced for intermediate size alkanes. 

Upon completion of data collection, estimation of values for the remaining compounds was performed. The estimates are primarily based on the methods of Shebeko (22), Gmehling and Rasmussen (23) and vapor pressure methods. The vapor pressure method is based on determining the temperature at which the vapor pressure will provide an equilibrium concentration that is equal to the lower explosive limit (LEL) concentration in air. The equations are briefly given below ... 

When a combustible substance is mixed with air, the mixture will explode only when it is neither too rich nor too lean. The lower explosion limit (LEL) is the minimum volume percent of the substance in air with flammability, which is separated from the upper explosion limit (UEL) by the explosive concentration range. The tabulations in handbooks are based on experimental data, and sometimes derived from estimation methods based on the elemental composition of the fuel as CmEtxOy. Figure 6.11 shows the LEL for the series of normal paraffins and of 1-alcohols versus the number of carbon atoms. There are two ways to plot the results, which show that, for paraffins, the volume percent shows a steeply declining trend, but the weight percent shows a mildly increasing trend. One may conclude that a smaller volume percent of higher paraffin... 

Property data from the literature (1-55,84-93) are given in Table 3-1. Critical constants have been determined experimentally for acetylene and methylacetylene (1-7). Critical pressure and volume are estimated for 1 butyne and 2 butyne (5). Additional property data such as acentric factor, enthalpy of formation, lower explosion limit in air and solubility in water are also available. The DIPPR (Design Institute for Physical Property Research) project (5) and recent data compilations by Yaws and co-workers (44-55) were consulted extensively in preparing the tabulation. 

One source gives estimated lower explosive limit of 1% by vol. Special first aid required in the event of poisoning antidotes must be available (with instructions). Flush contaminated clothing with water (fire hazard). 

Estimated lower explosive limit 1.2% vol. Transport Imarganey Card TEC(R)-30G15 ... 

Insufficient data on harmful atfacia to humans exercise great caution. Flash point (-18°C) estimated from vapor pressure curve and lower explosive limit. 

Experience tells that this estimate agrees fairly well with the measured values of the lower explosion limit for similar flammable gases. The upper limit shows larger deviations. The equations should be applied with care to safety technological questions, since the deviations may lie on both the safe and the unsafe side [4]. 

Nitrogen is the inerting medium used most frequently. When using nitrogen, the MOC can be estimated by multiplying the oxygen required for complete stoichiometric combustion by the lower explosive limit. An example of this is given for methanol ... 

Combustible Vapor Dispersion (CVD)— A mathematical estimation of the probability, location, and distance of a release of combustible vapors that will exist until dilution naturally reduces the concentration to below the lower explosive limit (LEL), or will no longer be considered ignitable (typically defined as 50% of the LEL). For basic studies, the normal expected wind direction is utilized (based on historical wind rose data). Real-time modeling is sometimes used during incident occurrence to depict area of vapor coverage on plant maps for visual understanding of the affected areas based on wind speeds and direction. 

Higher than normal flow rates, measured at 201b/min, have been used without incident. Still higher flow rates estimated as possibly 40 Ib/min (but not measured accurately) produced on a rare windless day a combustible gas indication of 34 of the lower explosive limit on the outside of the building 6 ft above ground level. 

The results for lower (LEL) and upper (UEL) explosive limits in air are presented in Table 3-1. The LEL and UEL values are the lower and upper concentrations (expressed as volume %) for flammability. The tabulation also provides the freezing and boiling point temperatures which are helpful in determining whether the substance is a gas, liquid or solid at ambient conditions. The tabulation is based on both experimental data and estimated values. 

Example 2 Estimate the lower (LEL) and upper (UEL) explosive limits in air for the gas mixture below ... 

Lower limits of explosibility of several organic dusts were determined and found to be in general agreement with estimated values [24], Probabalistic aspects of occurrence of dust explosions were studied experimentally with lycopodium powder in relation to dust and hot surface (or dust) temperature. The probability of explosion increases with dust concentration, then falls off, due to the ballast effect of unbumt dust. Probability also increases continuously with the temperature of the hot surface and/or dust particles. Application of the results to practical plant... 

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