Chemistry Reference
In-Depth Information
still produced via the sulfate route, although this is
diminishing. There are two main reasons for this:
the sulfate process can use lower grade and there-
fore less-expensive ores; and it produces anatase pig-
ments as well as rutile, which is the sole product of
the chloride process.
end-of-pipe technologies should not be overlooked.
For example, photocatalysis plays an important role
in the purification and treatment of wastewater [48],
whereas the use of electrochemical techniques for
the recovery of heavy metals from electroplating
processes is becoming widespread [49].
6 Reduction of Risk and Hazard
5.2 Alternative energy sources
The energy required to bring about chemical reac-
tions is supplied largely by external thermal sources
of heat, such as steam, hot oil and electrical heating
elements. When designing a process for energy effi-
ciency these conventional energy sources, which do
not target the energy, may not be the most efficient
and alternatives should be considered. There is
currently growing interest in alternative sources of
energy that can target specific molecules or bonds,
giving both energy savings and improved selectivity.
Such alternative energy sources include microwaves
and photochemical, ultrasonic and electrochemical
sources, some of which are discussed in detail in
other chapters of this topic. Industrial manufac-
turing processes using electrochemistry [45] (the
obvious example being chlorine/sodium hydroxide
manufacture) and, to a somewhat lesser extent, pho-
tochemistry [46] (e.g. the synthesis of vitamin D
3
, as
discussed in most photochemistry textbooks) have
been used for many years, with a great deal of
success for niche products. Others, such as the use
of microwave reactors, are still confined largely
to the R&D laboratory. One fairly rare example of
microwave energy being used for chemical produc-
tion is in the vulcanisation of rubber [47], where
heat-up rates can be up to 100 times faster than
when conventional heating methods are used. As
well as saving energy, process productivity is greatly
improved and the rubber obtained is less contami-
nated than that produced using a liquid curing
medium.
Although strictly speaking outside the scope of
green chemistry, the importance of photochemical
and electrochemical techniques to remediation and
6.1 Inherently safe design
December 1984 saw the worlds' worst chemical dis-
aster, with over 3000 people killed and 50 000 people
injured. The name Bhopal became synonymous with
all that was bad about the chemical industry [50]
and the repercussions from the tragedy transformed
industries' views on risk and hazard forever. The
product made at Bhopal was the insecticide carbaryl
(Fig. 2.15); although the chemistry involved was
fairly simple, it involved the use of two highly
hazardous chemicals, phosgene and methyliso-
cyanate (MIC).
The immediate cause of the accident was the
ingress (during routine maintenance) of a large
quantity of water into a storage tank containing up
to 60 tonnes of intermediate MIC. This caused a large
increase in temperature and pressure, eventually
causing the storage tank to explode and a toxic gas
cloud containing MIC and its hydrolysis products,
including hydrogen cyanide, to be released over the
nearby town.
It is very easy to blame people, procedures and
equipment failure in cases such as this but statisti-
cally these will always occur because people are
subject to human error, procedures can always be
improved with hindsight and even the most well-
engineered equipment will fail eventually. It is much
more beneficial to identify the real root cause of the
problem and to eliminate it. In this case the root
cause was the large-scale storage of toxic MIC, so the
question is: could carbaryl have been manufactured
efficiently without MIC being stored? The answer
is yes, but that is irrelevant; what is relevant is that
Fig. 2.15
The Bhopal route to carbaryl.
