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process is offered by acetylene:
9450 kJ/kg [47]. In these last two cases the
coking process is spontaneous due to the exothermicity of the process.
Therefore, when arcing is conducted with aromatic hydrocarbons like ben-
zene (or toluene) [44-46], because of their spontaneous tendency to carbon-
ization, the formation of PAH mixtures in relatively large quantity can be
observed together with pyrocarbon or coke. Instead, the amount of PAHs
and coke decreases significantly when n-hexane is arced and decreases
further when methanol is used in place of n-hexane for arcing. This is
because of the unfavorable thermodynamics in the carbonization process. As
shown above, the carbonization thermodynamics of alcohols are extremely
unfavorable because these molecules contain oxygen. Thus, in the case
of alcohols, PAH formation is extremely low to negligible and a similar
conclusion can be reached for pyrocarbon formation in these media.
8.3.5 F
ORMATION OF
C
ARBON
C
OKE OR
P
YROCARBON
:
AC
OMPARISON WITH A
S
ERIES OF
H
ALOGENATED
S
OLVENTS
[48]
In Section 8.3.4 we have shown that the formation of pyrocarbon during
arcing is very low in alcohols, increases in aliphatic solvents and reaches its
maximum in aromatic solvents. In this section we report the measured
amount of pyrocarbon formed in a series of selected solvents, including
halogenated solvents. Table 8.2 gives the amounts of pyrocarbon formed by
arcing with graphite electrodes and using 50ml of each selected solvent
under our standard arcing conditions at 10A for 6min. The anomaly is the
high formation rate of pyrocarbon when any halogenated solvents are arced.
TABLE 8.2
Coke Formation Rate in the Electric Carbon Arc
Solvent
Coke Formation Rate (mg/h at 10A ARC)
CCl
4
636
CHCl
3
735
CH
2
Cl
2
347
CH
2
I
2
319
CH
3
I
not determined
Methanol
7
Ethanol
10
n-propanol
14
n-heptane
43
n-octane
47
Decahydronaphthalene
62
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