Chemistry Reference
In-Depth Information
Although these consequences are not in dispute,
there is some dispute over the significance of both
rising CO
2
levels and the level of contribution made
by the burning of fossil fuels [40]. Governments of
most countries now accept that reduction of both the
use of fossil fuels and CO
2
emissions will be of envi-
ronmental benefit and agreements and legislation
are being put in place to meet these objectives
[41].
Energy requirements of chemical reactions fre-
quently are overlooked at the R&D stage and, for all
but the largest commodity processes, were not con-
sidered seriously at the production stage either, at
least until the oil crisis in the 1970s. As energy has
become more expensive and legislative drivers have
encouraged greater energy efficiency and conserva-
tion, we have seen significant changes in process
design. Many of these changes have focused on engi-
neering aspects, such as using hot process streams
from one part of a process to heat up incoming raw
materials, whereas a combination of process re-
engineering and catalysis has led to energy savings
in many large-scale chemical processes. Even so,
energy conservation is one of the most ignored of the
12 Principles of Green Chemistry, especially by
chemists!
The production of sulfuric acid has gone through
similar historical improvements [43]. The main
energy savings have been made in the production of
sulfur dioxide, which is the initial step in the process.
Originally this was produced by roasting the ore
(pyrites) in multiple hearth furnaces and later rotary
kilns, the energy produced being lost to the sur-
roundings. The development of fluidised bed tech-
nology enabled more than 50% of the excess energy
to be recovered and used to raise steam. Many
modern plants use sulfur (recovered from oil and
gas) as the feedstock and this produces much cleaner
SO
2
, eliminating the requirement for a cleaning step
and saving further energy.
When considering the eco-efficiency and compet-
itiveness of competing processes it is vital that the
energy requirements of the process are considered.
Unfortunately this detailed information is not readily
available for most small- to medium-scale processes.
As a striking example of how the energy require-
ments for producing a given chemical can vary from
process to process, let us consider titanium dioxide
production. The annual production of TiO
2
is
approximately 4.5 million tonnes, made via two
competing processes—the sulfate process and the
chloride process.
The sulfate process essentially involves three
stages:
5.1 Some energy efficiency improvements
(1)
Dissolution of the ore (ilmenite) in sulfuric acid
and removal of iron impurities
(2)
Formation of hydrated TiO
2
by treatment of the
sulfate with base
(3)
Dehydration in a calciner
Ammonia has been synthesised chemically for
almost 100 years. The original electric ark process
operated at temperatures of over 3000°C and was
highly inefficient. The Haber process was a huge leap
for energy efficiency, brought about by the use of
a reduced magnetite catalyst [42]. Although the
underlying principles of the Haber process have
changed little, the energy consumption of the
process is now less than 40% of the original process
[43].
Initially the energy utilisation of the process was
less than 20%, however with the replacement of coal
by oil, and later gas, as the preferred feedstock the
energy efficiency rose to around 60%. Optimisation
of turbine equipment, the steam distribution net-
works and the design of radical flow converters
with small-sized catalyst particles have made sig-
nificant contributions to energy efficiency improve-
ments without significant changes being made to the
chemistry.
All three stages are energy intensive [44].
By contrast, the chloride process can, for simplic-
ity, be broken down into two steps:
(1)
Chlorination of the ore with Cl
2
and purification
of TiCl
4
by distillation
(2)
Oxidation by burning
Overall, the chloride process is much less energy
intensive (by a factor of around 5 [44]), one of the
main reasons being the avoidance of large amounts
of water that need to be removed by energy-
intensive evaporation.
With such a huge energy differential it could be
assumed that the chloride process should have shut-
down economics, however around 40% of TiO
2
is
