262
The Chemistry and Technology of Petroleum
Specific heats are extremely important engineering quantities in refinery practice because they
are used in all calculations on heating and cooling petroleum products. Many measurements have
been made on various hydrocarbon materials, but the data for most purposes may be summarized
by the general equation
C
d
388
45t
=
+
1
0
0000
( .
.
)
where C is the specific heat at t°F of an oil whose specific gravity 60/60°F is d; thus, specific heat
increases with temperature and decreases with specific gravity.
10.4.6 lAtent HeAt
There are two properties that represent phase transformations: the latent heat of fusion and the latent
heat of vaporization. The latent heat of fusion, defined as the quantity of heat necessary to change
a unit weight of solid to a liquid without any temperature change, has received only intermittent
attention but, nevertheless, some general rules have been formulated. For hydrocarbons, latent heats
of fusion commence at approximately 15 cal/g for methane, rising to 40 cal/g for octane, then gradually approaching a limiting value of 55 cal/g. Branched paraffins usually have a lower latent heat of
fusion than the normal isomers; paraffin wax has a latent heat of fusion in the range 50–60 cal/g.
The latent heat of vaporization, defined as the amount of heat required to vaporize a unit weight
of a liquid at its atmospheric boiling point, is perhaps the most important property of the two and has
received considerably more attention because of its connection with equipment design. The latent
heat of vaporization at the atmospheric boiling point generally increases with increasing molecular
weight and, for the normal paraffins, generally decreases with increasing temperature and pressure.
10.4.7 entHAlPy or HeAt Content
Enthalpy is the heat energy necessary to bring a system from a reference state to a given state.
Enthalpy is a function only of the end states and is the integral of the specific heats with respect to
temperature between the limit states, plus any latent heats of transition that occur within the interval. The usual reference temperature is 0°C (32°F). Enthalpy data are easily obtained from specific
heat data by graphic integration, or, if the empirical equation given for specific heat is sufficiently
accurate, from the equation
H
d
388
225t 12 65
2
=
+
-
1
0
0000
( .
.
. )
Generally, only differences in enthalpy are required in engineering design; that is, the quantity of
heat necessary to heat (or cool) a unit amount of material from one temperature to another. Such
calculations are very simple since the quantities are arithmetically additive, and the enthalpy for
such a change of state is merely the difference between the enthalpies of the end states.
10.4.8 tHermAl ConduCtIvIty
The thermal conductivity K of hydrocarbons (in cgs units) is given by the equation
K
d
l
5 4 1
3
=
-
¥
-
0 28
0 000
0
.
(
.
)
where d is the specific gravity. The value for solid paraffin wax is about 0.00056, nearly independent
of temperature and wax type; the oil equation holds satisfactorily for waxes above the melting point.
The Chemistry and Technology of Petroleum
Specific heats are extremely important engineering quantities in refinery practice because they
are used in all calculations on heating and cooling petroleum products. Many measurements have
been made on various hydrocarbon materials, but the data for most purposes may be summarized
by the general equation
C
d
388
45t
=
+
1
0
0000
( .
.
)
where C is the specific heat at t°F of an oil whose specific gravity 60/60°F is d; thus, specific heat
increases with temperature and decreases with specific gravity.
10.4.6 lAtent HeAt
There are two properties that represent phase transformations: the latent heat of fusion and the latent
heat of vaporization. The latent heat of fusion, defined as the quantity of heat necessary to change
a unit weight of solid to a liquid without any temperature change, has received only intermittent
attention but, nevertheless, some general rules have been formulated. For hydrocarbons, latent heats
of fusion commence at approximately 15 cal/g for methane, rising to 40 cal/g for octane, then gradually approaching a limiting value of 55 cal/g. Branched paraffins usually have a lower latent heat of
fusion than the normal isomers; paraffin wax has a latent heat of fusion in the range 50–60 cal/g.
The latent heat of vaporization, defined as the amount of heat required to vaporize a unit weight
of a liquid at its atmospheric boiling point, is perhaps the most important property of the two and has
received considerably more attention because of its connection with equipment design. The latent
heat of vaporization at the atmospheric boiling point generally increases with increasing molecular
weight and, for the normal paraffins, generally decreases with increasing temperature and pressure.
10.4.7 entHAlPy or HeAt Content
Enthalpy is the heat energy necessary to bring a system from a reference state to a given state.
Enthalpy is a function only of the end states and is the integral of the specific heats with respect to
temperature between the limit states, plus any latent heats of transition that occur within the interval. The usual reference temperature is 0°C (32°F). Enthalpy data are easily obtained from specific
heat data by graphic integration, or, if the empirical equation given for specific heat is sufficiently
accurate, from the equation
H
d
388
225t 12 65
2
=
+
-
1
0
0000
( .
.
. )
Generally, only differences in enthalpy are required in engineering design; that is, the quantity of
heat necessary to heat (or cool) a unit amount of material from one temperature to another. Such
calculations are very simple since the quantities are arithmetically additive, and the enthalpy for
such a change of state is merely the difference between the enthalpies of the end states.
10.4.8 tHermAl ConduCtIvIty
The thermal conductivity K of hydrocarbons (in cgs units) is given by the equation
K
d
l
5 4 1
3
=
-
¥
-
0 28
0 000
0
.
(
.
)
where d is the specific gravity. The value for solid paraffin wax is about 0.00056, nearly independent
of temperature and wax type; the oil equation holds satisfactorily for waxes above the melting point.