Novel Design of an Integrated Pulp Mill Biorefinery for the ...

invention of Fischer-Tropsch in the early 1930’s[80]. The reactions that are supposed to be
occurring in the Fischer-Tropsch reactor are as follows:
nCO + 2nH2 → CnH2n + nH2O (2.4.4)
CO + 3H2 → CH4 + H2O ∆H298º=-247kJ / mol
CO + H2O → CO2 + H2 ∆H298º=-41kJ / mol
2CO → C + CO2 ∆H298º=-172kJ / mol
methanation
(2.4.5)
(water gas shift) (2.4.6)
(Boudouard reaction) (2.4.7)
H2 + CO → C + H2O (coke deposition) (2.4.8)
Reaction 2.4.4 is the production of heavy hydrocarbons of varying chain length. This is
the reaction that dominates when cobalt catalyst is used. Reaction 2.4.5 creates methane gas,
which is sent with the offgas to turbines for power creation. Reaction 2.4.6 demonstrates the
water gas shift, which creates the H2 to make up for an inadequate syngas ratio. The water gas
shift reaction only occurs for the iron catalyst, requiring a lower temperature reaction. The
Boudouard reaction creates carbon and CO2 , both undesirable products, that required cleaning.
To avoid the creation of undesirable products, product selectivity must be utilized in the Fischer-
Tropsch process. Product selectivity is accomplished through the use of temperature, pressure
and catalyst. Generally, creation of long chain heavy hydrocarbons is accomplished by utilizing
high pressures and low temperatures [4].
2.4.4. Fischer-Tropsch Product Selection
The equation that is accepted to predict the chain growth within Fischer-Tropsch
synthesis is the Anderson-Schulz-Flory (ASF) distribution [4, 72, 75, 80, 81].
xCn = α n−1 (1−α ) (2.4.9)
Where XCn is the molar yield in carbon number α is the chain growth probability factor,
N is the length of the hydrocarbon. The “1- α” part of the equation describes the chance that
chain growth will terminate [75]. To maximize wax production and thus to maximize FTD
production, α should be as high as possible [4, 79]. A plot of the chain growth is given in Figure
9.
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