The scale of demand for synfuel was recognised only recently. The total global capacity of synthetic fuel, built up over several decades, remained an unremarkable 200,000 bpd until 2005. However, major GTL/CTL projects currently under construction will add 450,000bpd by 2011. A further 1 million bpd of projects are under consideration once the projects currently under construction are operating.
The F-T process uses catalytic reactions to synthesise complex hydrocarbons from simpler organic chemicals. During this process, syngas is converted into a waxy syncrude (a synthetic form of crude oil) in the presence of catalysts. This is then converted by hydrocracking, chemical work-up and refining to produce diesel fuel, gasoline, naphtha, LPG and jet fuel, as well as methane and ethane. Fuels produced by F-T synthesis are of high quality owing to their very low aromatic hydrocarbon content and zero sulphur content.
Different companies and technology suppliers use different reactor systems (fixed-bed, slurry-phase or fluidised-bed reactors, for example) and catalysts (either iron or cobalt based), depending on the suitability for their process. The technologies used are broadly classified into two types:
The high-temperature process takes place at 300-330° C and produces mainly chemicals and gasoline. The products are virtually free of sulphur but contain aromatic hydrocarbons. Sasol is currently using this process at its Secunda plant in South Africa.
The low temperature process takes place at 200-230° C and produces extremely clean fuel, which is virtually free of both sulphur and aromatic hydrocarbons. Sasol has applied this technology at its Sasolburg plant and at Oryx GTL, Pearl GTL and Escravos GTL, also employ this technology.
Recently, some ?technology-orientated players, including Rentech and Syntroleum in USA and Oxford Catalysts in the UK, are actively developing F-T with a view to licensing the technology at a smaller scale.
F-T synthesis is extremely exothermic. One of the greatest challenges of the process is the optimisation of the energy integration between syngas generation and F-T conversion. Oryx GTL will be able not only to generate electricity to meet its captive needs but also to supply surplus to the grid.
Catalysts
The F-T process uses two major types of catalysts – iron and cobalt. The first are much cheaper, their disposal entails fewer toxicity problems, they are more robust with respect to the quality of the syngas feed or a sulphur ‘slip’, and will be more likely to maintain conversion efficiencies in a CTL application than a cobalt catalyst. However, they have a much shorter lifespan and produce products with heavier molecular weights as well as more olefinic products than the cobalt catalyst. Although cobalt catalysts are far more expensive, the replacement cost is relatively minor compared with the potential costs of plant downtime and consequently are now gaining prominence in most GTL facilities.
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