For many novel or unusual chemicals, there may be only one pressure/temperature point that is available for estimating flammability and/or toxicity hazards. This may be the atmospheric-pressure boiling point, the vapor pressure at room temperature, or the vapor-pressure/temperature point that corresponds to both the flash point and the lower flammable limit (LFL). It is shown here how an equation for the vapor-pressure/temperature relationship above and below that point can be developed through use of a second “infinite point.” Thus, the liquid temperature that corresponds to a given toxic concentration can be estimated, and the flash point and LFL can also be estimated with only one set of T and P.
An infinite point for hydrocarbons was first noted in 1923, and this concept was “rediscovered” and extended to a wide variety of other chemicals in 1949. This article summarizes the previous work and provides a rearrangement of the Antoine equation so that an infinite point can be easily used to estimate vapor pressure (and thereby concentration) at any given temperature.
The primary purpose of this article is to provide guidance concerning the use of an infinite point to aid in establishing hazardous concentrations of flammable and/or toxic vapors, particularly in poorly ventilated enclosures, where a range of vapor-pressure data is not readily available. © 2011 American Institute of Chemical Engineers Process Saf Prog, 2011