Environmental Engineering Reference
In-Depth Information
CH
4
þ
1
=
2
O
2
!
CO
þ
2H
2
DH
¼
35
:
6 kJ/mol
ð
2
:
9
Þ
or for higher hydrocarbons:
C
n
H
m
þ
n
=
2
O
2
!
nCO
þ
m
=
2
H
2
ð
2
:
10
Þ
The theoretical H
2
to CO ratio results lower than that of SR (about 2/3), as the
main oxidant is O
2
instead H
2
O. However, a small amount of water is often added
to the reactor feed, to better control reaction temperature and coke formation [
16
].
The reactions (
2.9
)or(
2.10
) are not the exclusive routes of the process as other
stoichiometric equations are thermodynamically compatible with the mixture
composition fed to the reactor. Equations
2.1
-
2.8
involved in hydrocarbon SR
might occur also in partial oxidation, i.e. they are possible reaction pathways in
addition to (
2.9
)or(
2.10
). On the other hand, it is necessary to consider that
further equations related to several oxidation reactions could occur during fuel
conversion:
CH
4
þ
2O
2
!
CO
2
þ
2H
2
O
DH
¼
801
:
6 kJ/mol
ð
2
:
11
Þ
CO
þ
1
=
2
O
2
!
CO
2
DH
¼
282
:
7 kJ/mol
ð
2
:
12
Þ
H
2
þ
1
=
2
O
2
!
H
2
O
DH
¼
241
:
6 kJ/mol
ð
2
:
13
Þ
POX involves the combustion of hydrocarbon feedstock in a flame with less
than stoichiometric oxygen required by complete combustion with production of
carbon dioxide (CO
2
) and water (H
2
O), according to Eqs.
2.11
-
2.13
, which in turn
react with the unreacted hydrocarbon (Eqs.
2.1
and
2.4
in
Sect. 2.1.1.1
), to produce
carbon monoxide and hydrogen. Usually a slight excess (20-30%) of oxygen with
respect to the stoichiometric value required by equations (
2.9
)or(
2.10
) is fed to
the system. The most recognized reaction mechanism hypothesis is that the highly
exothermic total oxidation reaction consumes essentially all the available oxygen,
and the large amount of thermal power produced by the combustion is exploited by
endothermic reforming reactions. However, the POX process remains globally
exothermic.
A non-catalytic partial oxidation process based on the above reactions has been
largely used for the past five decades for a wide variety of feedstocks, in particular
heavy fractions of refinery, such as naphtha, vacuum fuel oil, asphalt residual fuel
oil, or even whole crude oil. The absence of catalysts implies that the operation of
the production unit is simpler (decreased desulfurization requirement) but the
working temperatures results higher than 1200C. The high values of this
parameter permit satisfactory yield to H
2
and CO to be obtained without using a
selective catalyst.
A catalytic partial oxidation (CPO) reaction permits operation temperature to be
lowered and meets the requirements of recently proposed decentralized applica-
tions based on small-scale reformer plants [
17
], better than the SR or the non-
catalytic partial oxidation process. This evaluation is based on the dependence of
costs associated with both SR and CPO manufacture and management plants by
