The boiling point of a pure compound at a given pressure
is the temperature at which vapor and liquid exist together
at equilibrium. If the pressure is 1 atm, the boiling point is
called the normal boiling point. However, usually the term
boiling point, Tb, is used instead of normal boiling point and
for other pressures the term saturation temperature is used.
In some cases, especially for heavy hydrocarbons in which
thermal cracking may occur at high temperatures, boiling
points at pressures other than atmospheric is specified. Boiling
points of heavy hydrocarbons are usually measured at 1,
10, or 50 mm Hg. The conversion of boiling point from low
pressure to normal boiling point requires a vapor pressure relation
and methods for its calculation for petroleum fractions
are discussed in Chapter 3. The boiling point, when available,
is one of the most important characterization parameters for
hydrocarbons and is frequently used in property estimation
methods.