Environmental Engineering Reference
In-Depth Information
[11]. The different reforming processes are classified based on the source of
heat and the type of reactants. In general, the glycerol reforming is endother-
mic and can be represented as follows:
C H O
+
x
H O
+
y
O
→
a
CO
+
b
CO
+
c
H O
+
d
H
+
e
CH
.
(2.9)
3
8
3
2
2
2
2
2
4
When
x
> 0 and
y
= 0, the process is steam reforming that, as a vapor phase
catalytic reaction, occurs at high temperature (e.g., 800°C) and low pressure
(e.g., 0.1 MPa). The reaction can be represented as:
C H O
+
3
H O
→
3
CO
+
7
H
∆
H
=
128
kJ mol
⋅
-
.
1
(2.10)
3
8
3
2
2
2
For every mole of glycerol, 7 mol of H
2
is produced based on stoichiom-
etry, which is highly favorable. Major issues include byproduct formation
(e.g., CO), catalysts deactivation, and high energy consumption. To lower
the energy requirement, the reforming can be carried out in the aqueous
phase at lower temperature (e.g., 225°C) and high pressure (e.g., 2 MPa).
One added advantage of this process is the lower CO concentration in the
reformate since the water gas shift reaction is favored at lower temperature.
However, limitations of this process include the high pressure requirement
and low H
2
selectivity due to the possible formation of alkanes.
When
x
= 0 and
y
> 0 and oxygen is used to provide the heat required
internally, catalytic partial oxidation takes places and can be represented as:
C H O
+
3 2
/ O
→
3
CO
+
4
H
∆
H
=−
603
kJ mol
⋅
−
1
.
(2.11)
3
8
3
2
2
2
This overall process is exothermic and produces 4 mol of H
2
for every
mole of glycerol. The conversion efficiency, as well as reaction temperature
and pressure required, depend strongly on the catalysts used, for example,
Ni, Co, Pt, Ru, and Rh. A more detailed discussion of some examples of
catalysts and related reactions mechanisms will be given later.
Similarly, in autothermal reforming (
x
> 0 and
y
> 0), oxygen, water
steam, and glycerol react in the presence of a catalyst. The reaction can be
given as:
-
.
(2.12)
1
C H O
+
3 2
/ H O
+
O
→
3
CO
+
11 2
/ H
∆
H
=−
240
kJ mol
⋅
3
4
3
8
3
2
2
2
2
For every mole of glycerol, 5.5 mol of H
2
is produced. It should be pointed
out that the heat of reactions for processes represented by Equation 2.10,
Equation 2.11, and Equation 2.12 vary substantially in different reports [12].
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