Fischer-Tropsch synthesis Building Complex Hydrocarbons from simple ones

Fischer-Tropsch synthesis: " CO reacts with two moles of H2... Fischer-Tropsch synthesis

In the (exothermic) Fischer-Tropsch (FT) one mole of CO reacts with two moles of H2 to afford a hydrocarbon chain extension (-CH2-). The oxygen from the CO is released as product water:

CO + 2H2 → - CH2 - + H2O
ΔH = -165 kJ/mol

The reaction implies a H2/CO ratio of at least 2 for the synthesis of the hydrocarbons. When the ratio is lower it can be adjusted in the reactor with the catalytic Water-Gas Shift (WGS) reaction:

CO + H2O → CO2 + H2
ΔH = -42 kJ/mol

When catalysts are used with WGS activity the water produced in the reaction can react with CO to form additional H2. In this case a minimal H2/CO ratio of 0.7 is required and the oxygen from the CO is released as CO2:

2CO + H2 → - CH2 - + CO2
ΔH = -204 kJ/mol

The reaction affords mainly aliphatic straight-chain hydrocarbons (CxHy). Besides these straight-chain hydrocarbons also branched hydrocarbons, unsaturated hydrocarbons (olefins), and primary alcohols are formed in minor quantities. The kind of liquid obtained is determined by the process parameters (temperature, pressure), the kind of reactor, and the catalyst used. Typical operation conditions for the FT synthesis are a temperature range of 200-350°C and pressures of 15-40 bar, depending on the process.

Products
The subsequent FT chain-growth process is comparable with a polymerisation process resulting in a distribution of chain-lengths of the products. In general the product range includes the light hydrocarbons methane (CH4) and ethane (C2), LPG (C3-C4), gasoline (C5-C12), diesel (C13-C22), and light and waxes (C23-C32 and >C33, respectively). The distribution of the products depends on the catalyst and the process operation conditions (temperature, pressure, and residence time). The (theoretical!) chain length distribution can be described by means of the Anderson-Schulz-Flory (ASF) equation, which is represented as:

log Wn

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n
n • log α + log (1 - α)2

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α


where Wn is the mass fraction of a product consisting of n carbon atoms and the chain growth probability factor (α). A plot of the relative mass fractions of products formed as function of the ASF chain growth factor α is given in Figure below. As can be seen from this plot, higher values of α give higher molecular weight products.

With respect to the production of green diesel, process conditions can be selected to produce maximum amounts of products in the diesel-range. However, an even higher yield of diesel can be achieved when the FT synthesis is optimised towards production of wax (i.e. high α). Subsequently, the wax can be selectively cracked to yield predominantly diesel.

Catalysts
Several types of catalysts can be used for the Fischer-Tropsch synthesis - the most important are based on iron (Fe) or cobalt (Co). Cobalt catalysts have the advantage of a higher conversion rate and a longer life (over five years). The Co catalysts are in general more reactive for hydrogenation and produce therefore less unsaturated hydrocarbons and alcohols compared to iron catalysts. Iron catalysts have a higher tolerance for sulphur, are cheaper, and produce more olefin products and alcohols. The lifetime of the Fe catalysts is short and in commercial installations generally limited to eight weeks.

Reactors
ECN Biomass has two operational reactors available for Fischer-Tropsch synthesis. The “FITOR” bench-scale slurry bubble column reactor and the “POTTOR” micro-flow fixed-bed reactor. Both reactors were used in successful integrated BG-FT test runs.
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