Chemistry Reference
In-Depth Information
Fig. 2.12
Routes to 6-aminopenicillanic
acid.
continue to use resources at the current rate we will
face a significant shortage (combined with a very
high price) some time in the second half of this
century [27]. The use of non-renewable resources
for chemicals manufacture must be put into per-
spective: approximately 90% of crude oil currently
is used to provide energy via burning of oil, gasoline
and diesel, with only 8% of crude being converted
into chemicals. The two main arguments for reduc-
ing our dependency on fossils and increasing our use
of renewable feedstocks are:
Table 2.2
Some disadvantages of vegetable-oil-based diesel
High viscosity
Lower volatility
Reactivity of unsaturated chains, leading to gum formation
Increased coking
There is nothing particularly new about using
vegetable-based diesel oils [28] but during the last
60 years or so the advent of relatively inexpensive
and technically superior petroleum-based diesel has
prevented their widespread use. In recent years there
has been considerable renewed interest in the com-
petitive production of biodiesel to overcome many
of the environmental issues associated with the
petroleum-based material [29]. Some of the disad-
vantages of vegetable-oil-based diesel are shown in
Table 2.2.
These disadvantages generally preclude the use of
unmodified vegetable oils, although there are many
examples of 20-50% blends with conventional diesel
being used for prolonged periods [30]. Trans-
esterification has been the major technique
employed to overcome these technical problems
(especially high viscosity), although at added cost.
Typically the anhydrous oil (triglycerides) is heated
with methanol and a basic catalyst to give a mixture
of methyl esters and glycerol, which is recovered as
a valuable co-product. Although sodium hydroxide
and sodium methoxide are widely used as catalysts,
a 'green' process involving a reusable immobilised
lipase catalyst and supercritical carbon dioxide has
been demonstrated [31].
The main obstacle to widespread use of biodiesel
is the cost, of which up to 75% can be the raw
(1)
To conserve valuable supplies of fossil fuels
for future generations (a core principle of
sustainability).
(2)
To reduce global emissions of greenhouse gases,
especially carbon dioxide (renewable resources
being CO
2
-neutral overall).
Reduction in the use of fossil fuels for chemicals
manufacture will have some benefit on conserving
resources and reducing CO
2
emissions, but these will
be small compared to what can be achieved by using
renewable resources for energy production. Chemi-
cals manufacture from renewable resources, there-
fore, ideally should provide additional benefits such
as reduced hazard, more efficient process, reduced
cost, reduced pollution, meeting market needs, etc.
Additionally, it is important to look at the whole
process, including growing, transport, etc., to ensure
that the total energy consumed (or total CO
2
emis-
sion) is lower when employing the renewable
resource. Chemistry does have a vital role to play in
reducing the requirement for fossil fuels, e.g. more
efficient combustion processes, the development of
energy-efficient solar and fuel cells and the produc-
tion of biodiesel.
