Chemistry Reference
In-Depth Information
Table 3.1 Comparisons of microwave and conventional heating technology.
Heating technology
Conventional
Microwave
Comments
Slow and
Superficial
Rapid and
Volumetric
Microwave irradiation is rapid and volumetric,
with the whole material heated simultaneously.
In contrast, conventional heating is slow and is
introduced into the sample from the surface.
This feature is very important for processing
of poor thermal conductors such as rubber
and wood.
12
d
n
9
r
3
n
g
|
8
Slow control
Instant control
Microwave heating can be controlled instantly
and the power applied can be accurately
regulated. This allows safe and precise control,
even when applying very rapid heating
rates.
8,18
Nonselective
Selective heating
As with radio-frequency heating the power will
selectively concentrate on the material that has
the highest dielectric loss factor.
18
-
Energy ecient
Calculations show that microwave-assisted
organic chemical reactions can be considerably
more energy ecient than those using
conventional heating.
19
-
Mobile
The advantage of microwave technology in terms
of mobility and waste reduction applications
have been highlighted by Ruan et al.
20
3
.
modification of each of the terms of the Arrhenius equation (pre-exponential
factor and activation energy) in the presence of a microwave field.
22,23
3.3 Specialised Microwaves Techniques
3.3.1 Microwave-Assisted Heterogeneous Gas-Phase Catalysis
The application of microwave dielectric heating in a range of environment-
related heterogeneous catalytic reaction systems has been reviewed by Zhang
and Hayward.
24
The reactions investigated include decomposition of hydrogen
sulfide, reduction of sulfur dioxide with methane, reformation of methane by
carbon dioxide, and hydrodesulfurisation of thiophene. For example, decom-
position of hydrogen sulfide into hydrogen and sulfur is commercially
important for the coal and petrochemical industry.
Since the early 1980s, research laboratories have shown an increasing interest
in microwave radiation as an energy source for the activation of oxidative
coupling of methane in gas-phase catalysis. For example, oxidative dehydration
of methane for synthesis of higher saturated and unsaturated hydrocarbons is a
deeply investigated research field with the goal to access natural-gas deposits as
a resource for the chemical industry in the context of progressively decreasing
oil reserves. Under classical conditions, selectivity of 80-85% to C
2
-hydro-
carbons at methane conversions of 10-15% are common.
25
It was found that
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