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| Time line |
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| 1999: Imported SAS reactors soon create new production records |
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| 1981 |
Sasol begins designing a radically different
kind of Fischer-Tropsch reactor known
as Sasol Advanced Synthol (SAS) using a
100 barrels-a-day pilot plant |
| 1995 |
The first SAS reactor comes on stream at Secunda |
| 1996 |
Sasol decides to mothball its Synthol reactors |
| 1999 |
Imported SAS reactors travel by road to Secunda where they soon create new production records |
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| By the late-1960s Sasol’s research engineers and plant operators had,
as we’ve seen, overcome the main problems presented by the Synthol
reactors. Nevertheless, they remained tricky as well as expensive to
operate and maintain. |
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| That seemed unavoidable, given the reactor’s basic design
concept of having the catalyst, entrained in rapidly moving
gas, flow through large-diameter pipes from hopper to cooling
section, through cyclones and back to hopper. |
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| That approach meant, among other things, that only the
catalyst that, at any moment, was in the gas between the
bottom of the hopper and the cooling section was taking
part in the conversion of gas into hydrocarbon molecules -
and that amounted to no more than one-third of all the
catalyst in the reactor. Driving the gas with sufficient pressure
to entrain the catalyst also consumed a lot of electricity. And
having many tons of abrasive catalyst swirling around inside
the pipes meant the latter had to be lined with expensive
ceramic material. |
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| Why, asked Sasol’s young research scientists in the late-1960s,
had Kellogg come up with the idea of the Circulating Fluidised
Bed? What was wrong with feeding gas upwards through an
essentially static bed of catalyst? It wouldn’t have to be under
great pressure, and each particle of catalyst would move only
slightly, floating in the gas that would thus make contact with
each particle’s entire surface area, which is where chemical
reaction takes place. |
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| It wasn’t a new idea. Indeed, it already had a name, Fixed
Fluidised Bed (FFB), to distinguish it from other kinds of
reactors. "Fixed" meant it didn’t swirl around, as in the Synthol
reactor. "Fluidised" meant its particles could move around,
rather than being jammed immovably into tubes, as in the Arge
reactor. "Bed" meant that, as in a coal gasifier, the solid
material lay metres-thick at the bottom of a reaction vessel
rather than in, say, a hopper. |
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| It not only had a name, it also had a history - but, alas, not an
encouraging one. During the 1940s, when there was worldwide
concern that crude oil reserves would soon be used up, a group
of Americans, headed by "Dobie" Keith of Hydrocarbon
Research Inc, a company engaged in developing new refining
processes, had a plant built in Brownsville, Texas, to convert
natural gas into petrol using an FFB reactor. It ran into technical
problems, however, and when the price of natural gas started
to rise steeply and fears of oil shortages disappeared with the
discovery in the late-1940s and early-1950s of huge reserves in
the Middle East, the project was abandoned. |
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| The nature of its technical problems was never revealed, but
others have reckoned they stemmed from the industry’s paucity
of knowledge at that time about catalysts and how they perform, let alone how to get them to produce only what you
want. The Brownsville operators, they say, watched helplessly
as the gas formed wax and carbon on the catalyst particles. The
wax caused particles of catalyst to stick together, forming balls
too heavy to float in the gas. Carbon also affected the bed’s
fluidisation properties because it is less dense (ie, it’s lighter)
than the catalyst, which means you need less pressure of gas to
keep a cubic centimetre of carbon-coated catalyst afloat than
you do for uncoated catalyst. In short, the FFB reactor
produced petrol, but it was seriously unstable. |
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| New high-temperature technology increases output hugely |
| Barrels per day |
Barrels per day |
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| Circulating Fluidised Bed reactor
(relative size) |
Sasol Advanced Synthol (SAS) reactor
(relative size) |
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