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In-Depth Information
PBM
VBM
classic heating
microwave (multimode)
microwave (monomode)
ultrasound
1
10
100
1000
E
m
[kWh mol
-1
]
without cooling
with cooling
Figure 10.15 Comparison of the molar energy E
m
(Eq. 10.16) for the KMnO
4
-
mediated oxidation of p-toluidine (Scheme 10.3C) carried out with
different methods of energy entry.
62
The index i in Eq. (10.16) represents the number of milling beakers the
ball mill can be operated with, i.e. for MBMs i
ΒΌ
2 and for PBMs i varies
between 1 and 4 (Figure 10.2). The value E
m
is also defined as the energy
intensity of a desired reaction or process.
11,113,114
The energy e
ciency of
ball milling procedures expressed by E
m
has been proven for several
examples. Especially in comparison to microwave-assisted synthesis, the
application of ball mills is more sustainable, if similar reaction systems are
compared.
29,61,62
This fact is pointed out in Figure 10.15, which summarizes
E
m
for different reaction protocols regarding the examples discusses in
context with Figure 10.14.
62
The data are also interesting in that the product
yield for the different accomplishments differs by not as much as reported
for other reactions due to inecient mixing in the heterogeneous phase.
29,61
Sono- and mechanochemical processing result in comparable values
regarding the energy eciency, whereas the energy demand for microwave-
assisted synthesis and classical heating is approximately one magnitude
higher.
29,61,62
The conclusion can be drawn that the energy loss due to heat
dissipation (classical heating) and inecient energy transfer from electrical
to desired energy form (microwaves) is higher for these methods. Including
the energy demand for cooling of the liquid-phase reactions decreases the
energy eciency, which is manifested by an increase of E
m
in Figure 10.15.
62
10.6 Conclusion and Outlook
The concepts of sustainable chemistry and process intensification are the
main driving forces leading to application of ball mills as reactors in organic
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