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| Olefins |
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Another task looms for Sasol as South Africa’s
first naphtha cracker is built in 1969 |
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| When you use the Fischer-Tropsch process to get hydrogen and
carbon monoxide to form hydrocarbon molecules - or when you
crack oil into its various fractions - you end up with molecules
containing varying numbers of carbon atoms. Those that are
saturated and contain three carbon atoms form propane, while
four carbon atoms gets you butane. Dehydrogenate them to
unsaturated molecules - also known as olefins - and you have, respectively, propylene and butylene. All three olefins - C2H4,
C3H6 and C4H8 - get to be polymerised: the first two to produce
plastics, C4H8 to produce synthetic rubber
(and all of them to produce motor fuel). |
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| That would not have been possible
without the understanding gained by
research scientists during the 1920s
and 1930s, both of how to get
molecules to form long chains, and of the different ways in which those chains can relate to each other
and thereby produce products with markedly different
characteristics. That knowledge was put to use during World
War Two almost entirely to produce synthetic rubber and
plastics for such things as aircraft windows and raincoats. Once
the war was over in 1945, production facilities in Europe and
the US were redirected to satisfying pent-up consumer demand
for cars, refrigerators, washing machines, radios and toys. |
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| There were no such facilities, however, in South Africa. In
1945 it was still largely a mining and agricultural
country; there was little secondary industry outside of
engineering shops serving the mining industry.When
the National Party came to power in 1948, it set about
trying to encourage the broadening of local industry,
offering tariff protection, investment incentives and
so on. But it was not until the mid-1960s that it had
in place the beginnings of a programme for
encouraging the local assembly of cars. Not until
then, either, that it realised that plastics had great
prospects and South Africa - like Ben in The Graduate - should
get into them. |
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| But where to start? Well, why not with ethylene? It was the
basic building block for the biggest-selling plastic of them all,
polyethylene (a name often shortened to polythene).
Moreover, it was - together with benzene, which was readily
available from South Africa’s oil refineries - a basic component
of styrene, which was used to make synthetic rubber as well as
the plastic, polystyrene. Ethylene was being produced in Sasol’s
Synthol reactors, but not enough to meet the needs of a new
plastics industry. So why not obtain it by cracking naphtha? |
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| Naphtha |
| A fraction of crude oil, naphtha was almost being given away
on world markets at the time because its straight molecular
chains gave motor fuels produced from it a low octane rating.
Using heat, pressure and steam, the molecules in naphtha -
ranging from C6 to C9 - could be broken down and reformed.
That would produce shorter chains, notably ethylene, and
branched-chain or ring-shaped molecules containing six
carbon atoms (benzene). Both types of molecule did wonders
for petrol’s octane rating. |
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| Why, then, was naphtha being sold so cheaply? Because a
naphtha-cracking plant was costly to build and to run because
of its high power consumption. Why make that investment if
naphtha’s products could be obtained more cheaply from
distilling crude oil? |
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| For all that, the South African government decided to invest in
naphtha cracking, and almost inevitably handed the task to
Sasol. Other private-sector companies would establish plants
alongside the naphtha cracker in Sasolburg to use its products.
African Explosives and Chemical Industries (AECI), for
example, would produce polyethylene since one of its two
major shareholders was Britain’s Imperial Chemical Industries
(ICI), which had pioneered the polymerising of ethylene back
in the 1930s. Butadiene - derived from butane - and styrene
would go the Synthetic Rubber Company, controlled by
Afrikaner business. |
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| Sasol’s first naphtha cracker was commissioned in 1966. It
produced 80 000 tons a year of ethylene, which soon proved
insufficient to meet AECI’s needs. In 1969 a second naphtha
cracker was installed, with a design capacity of 200 000 tons of ethylene a year. At that time naphtha crackers didn’t yield
more than about 10 per cent of what was put into them as
ethylene, which meant Sasol’s two crackers were treating
2,8 million tons of naphtha a year. They were also producing
about 40 000 tons of butadiene a year.Most of the rest of the
naphtha ended up as high-octane fuel components. |
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When Sasol Two came on stream, far more ethylene came out
of it than was produced by the two naphtha crackers at Sasolburg; a great deal more, too, than the market then
required. AECI, together with another chemicals company,
Sentrachem, had decided in the mid-1970s to provide some of
its own plastics feedstock by making acetylene (more about
that below). By the early 1980s, too, the local synthetic rubber
industry was also facing extinction due to a surge - reflecting
that of crude oil and naphtha - in the price of its feedstocks
from Sasol, even as the market was being flooded with natural
rubber from the Far East. With demand from the two companies
for ethylene being met by Secunda through a pipeline that had
been laid in 1980 to connect the two towns, Sasol decided its
two naphtha crackers were no longer economic and in 1983
closed them down. Years later, when demand for ethylene
exceeded Secunda’s supply, ethane was sent through that same
pipe to Sasolburg. There, in 1988, one of the naphtha crackers was modified and recommissioned to crack ethane to ethylene.
By 1995 Sasol was producing 360 000 tons of ethylene a year.
Today the figure stands at
420 000 tons, the increase achieved
by some additions to installed capacity, as well as by plant
debottlenecking. |
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