Page 303 - Hydrocarbon Exploration and Production Second Edition

290 Oil and Gas Processing

liquid fuels from natural gas. Essentially, all the processes chemically bind short-
chain (gas) hydrocarbon molecules together to form long-chain (liquid) hydro-
carbons through catalytic reactions. The two main technologies for GTL to derive a
synthetic petroleum liquid are a direct conversion from gas, and an indirect
conversion via synthesised gas (syngas), for example using the Fischer–Tropsch
(F-T) synthesis with cobalt or iron catalysts, or from methanol using a zeolite
catalyst.
The direct method avoids the cost of syngas production, but is difficult to
control and requires a high activation energy level. The F-T syngas processes
convert the natural gas to hydrogen and carbon monoxide by either steam
reforming or partial oxidation, or a combination of these two processes, and the
subsequent conversion of syngas to liquid hydrocarbons requires an iron- or cobalt-
based catalyst (Figure 11.23).
The key parameters are pressure, temperature and type of catalyst, which
together determine the grade of synthetic crude produced. At lower temperatures
(e.g. 180–2501C and with a cobalt catalyst) the syncrude is predominantly diesel and
waxes which are almost free of sulphur and olefins. At higher temperatures around
3301C and with an iron catalyst mainly gasoline and olefins are produced, again
almost free of sulphur. The range and grade of liquid fuels produced can be further
optimised by careful selection of the catalysts.
One of the leading companies in this arena is South Africa’s SASOL, which
in partnership with Chevron and Qatar Petroleum has brought on stream a
GTL plant in Qatar in 2006 capable of producing 34,000 bpd of liquids from
a feed of 330 MMscf/d of gas. These plants are scalable, and firm plans are in
progress for expansion of the Qatar GTL facilities to produce 450,000 bpd of
liquids by 2015. The overall costs per barrel of produced liquids, including

natural gas synthesised
gas
natural
gas
reforming
Fischer-
oxygen
Tropsch
conversion
air
separation product
upgrading

GTL diesel
and
GTL naphtha

Figure 11.23 Schematic £owchart for Fischer^Tropsch GTL conversion process (source:
www.sasolchevron.com/technology.htm).