Handbook of Solvents - George Wypych - ChemTech - Ventech!

54 George Wypych
Heat of combustion, kJ mol -1
6000
5000
4000
3000
2000
1000
n-hydrocarbons
alcohols
0
1 2 3 4 5 6 7 8
Number of carbon atoms
Figure 2.3.17. Heat of combustion vs. number of carbon
atoms in molecule.
The effect of the air to solvent ratio on
autoignition temperature is similar to that
on ignition energy (see Figure 2.3.14). As
the oxygen concentration increases within
the range of the flammability limits, the
autoignition temperature increases. The
autoignition temperature increases when
the size of combustion chamber decreases.
Rapid heating reduces the autoignition
temperature and catalytic substance may
drastically reduce it.
2.3.10 HEAT OF COMBUSTION
(CALORIFIC VALUE)
Heat of combustion, also known as calorific
value, is the quantity of energy per
mole released during combustion. It coincides
with the heat of reaction. Solvents
have higher heats of combustion than typi-
cal fuels such as natural gas, propane or butane. They can be very good source of energy in
plants which process solutions. In addition to supplying energy, the combustion of solvents
can be developed to be one of the cleanest method of processing from solutions. Two
approaches are commonly used: solvent vapors are directed to a combustion chamber or
spent solvents are burned in furnaces. The heat of combustion of a liquid solvent is less than
1% lower than the heat of combustion of a vapor.
Figure 2.3.17 shows the relationship between the heat of combustion and the number
number of carbon atoms in the molecule. The heat of combustion increases as molecular
weight increases and decreases when functional groups are present.
2.3.11 HEAT OF FUSION
Heat of fusion is the amount of heat to melt the frozen solvent. It can be used to determine
the freezing point depression of solute.
2.3.12 ELECTRIC CONDUCTIVITY
Electric conductivity is the reciprocal of specific resistance. The units typically used are either
ohm -1 m -1 or, because the conductivities of solvents are very small picosiemens per meter
which is equivalent to10 -12 ohm -1 m -1 . The electric conductivity of solvents is very low
(typically between 10 -3 -10 -9 ohm -1 m -1 ). The presence of acids, bases, salts, and dissolved
carbon dioxide might contribute to increased conductivity. Free ions are solely responsible
for the electric conductivity of solution. This can be conveniently determined by measuring
the conductivity of the solvent or the conductivity of the water extract of solvent impurities.
The electronic industry and aviation industry have the major interest in these determinations.
2.3.13 DIELECTRIC CONSTANT (RELATIVE PERMITTIVITY)
The dielectric constant (or relative permittivity) of a solvent reflects its molecular symmetry.
The value of the dielectric constant is established from a measurement relative to vacuum.
The effect is produced by the orientation of dipoles along an externally applied