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Re: Definition of "Weathered" Ignitable Liquid Residue (ILR)
Posted by:
Gerald Hurst (IP Logged)
Date: July 11, 2007 10:31AM
You have hit on a complex problem.
The effects of evaporation on the composition of a liquid mixture depend on how the liquid is exposed to the atmosphere into which it evaporates, particularly if the evaporation of any given fraction occurs while the bulk of the liquid remains below its normal boiling point.
If you boil a liquid mixture, mixing occurs and the liquid components distill away at individual rates determined by their relative volatilities. Under these conditions, one can simply consult a distillation curve to determine the effect of "weathering" on composition at any given stage of the process.
Now imagine that you have a sample of mixed liquid in a tall narrow cylinder and you apply radiant or convective heat to the top surface. At first, the more volatile components will evaporate or distill from the surface layer in more or less normal fashion. Soon the top layer will consist primarily of a hot stratum of less volatile species exposed to much higher temperatures than are the inherently more volatile species below this layer. Under these conditions, the relative evaporation rates of the species will depend on both the temperature gradient and the rate at which volatile components can diffuse into the hot zone.
If a sample of gasoline is trapped in a porous medium, it will behave like the hypothetical liquid in the cylinder, with the added restriction that little or no convective mixing can occur. Thus, "weathering" (read change in composition) at fire scenes is not at all easy to quantify.