1.1. The products produced in this process

Fischer-Tropsch synthesis

Ultra clean liquid transportation fuels and various other chemicals can
be produced from synthesis gas using the well known and established catalytic
chemical reaction process called Fischer-Tropsch synthesis (FTS). It is
invented by two German researchers Franz Fischer and
Hans Tropsch at Kaiser-Wilhelm institute for coal research
in Germany in 1923 1–3.
FTS is a catalytic reaction, which involves the production of long chain
hydrocarbons (wax) from synthesis gas obtained from alternative energy sources
such as natural gas, biomass and coal via gasification and steam reforming as
shown in Figure 1.1 4–6. The ideal FTS reaction of
producing hydrocarbons of various molecular weights is according to Equation 1 7,8.

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——– (1)

Where ‘n’ is a positive
integer. When n=1, the reaction represents the formation of methane, which is an
undesirable byproduct. Apart from the major hydrogenation reaction, there are
other side reactions taking place in this process, of which the unwanted water-gas-shift
(WGS) reaction is the most predominant as shown in Equation 2 7,9,10. Also, small quantities of oxygenated compounds (alcohols,
carboxylic acids, ketones etc.) also produced in FTS 11. Based on the source of the synthesis gas, the process is often referred
to as gas-to-liquids (GTL), coal-to-liquids (CTL) and biomass to liquids (BTL) 12.

Figure 1.1: Schematic illustration of Fischer-Tropsch synthesis.

FTS process rapidly gained academic
and industrial importance and eventually is used to create approximately 2% of the total
world’s fuel 13. The products produced in this process became major alternative
transportation fuels due to their high quality and compatibility with existing
petroleum infrastructure and vehicles 14. Besides the environmental concerns, abundant reserves
of natural gas in many parts of the world ignited the global interest in FTS
and new plants based on FTS technology are commissioned periodically 4. Shell International B.V is one of the largest companies working on FTS
process. Shell’s Pearl GTL plant in Qatar is one of the largest industrial
applications of the FTS process producing 140,000 barrels of high quality liquid
products per day as shown in Figure 1.2. It is one of the world’s largest,
most complex and challenging energy projects ever commissioned. The products
produced in this GTL plant are virtually sulphur free and have no contaminants
such as heavy metals or aromatics leading to colourless, odourless and
clean-burning products 15. Along with the clean
fuels, Pearl’s GTL plant also produces 280,000 barrels of water per day. This
water is used for cooling by evaporation, steam generation and watering for
trees around the plant 16. Another Shell’s GTL plant
in Bintulu, Malaysia producing 14,700 barrels per day is shown in Figure 1.3 17. Shell International B.V invested more than
one billion dollars and filed over 3,500 patents covering all the stages of GTL
process 15.

1.2. Reaction kinetics and product

FTS has been recognized as a very complex polymerization reaction with a
large number of species involved. The general proposed mechanism of FTS
consists of chain initiation, chain propagation and chain termination steps. Various
reaction mechanisms, like carbide 18, enolic 18 and CO insertion 19–21 have been proposed to explain the FTS reaction.

reaction products produced in FTS form a complex multi-component mixture with
substantial variation in carbon number and product type. However, the major products
are linear paraffins and ?-olefins. The distribution of products
obtained in FTS can be understood by the frequently used Anderson-Schultz-Flory
(ASF) kinetic model given by Equation 3 22–24.


Where, Wn is the mass fraction of
the carbon atoms with a chain containing n carbon atoms. The chain growth probability
factor (?) is given by Equation 4, where Rp and Rt represent
the rate of chain propagation and the rate of chain termination, respectively:

————————– (4)

value of ? depends upon process conditions and type of the catalyst 25–28 and generally, the range
of ? value for FTS catalysts is 0.7-0.95 29. Significant deviations
from the ASF distribution are reported in the literature. These deviations are
predominantly caused by secondary reactions of readsorbed ?–olefins on the
active sites of the catalyst surface and reinsertion of primary hydrocarbon
products into the chain growth process 30,31. A graphical
representation of the ASF distribution of FTS products is illustrated in Figure