Page 229 - Synthetic Fuels Handbook

FUELS FROM SYNTHESIS GAS 215

Or by the gasification of any carbonaceous source, such as biomass:
C + H O → H + CO
2 2
The energy needed for this endothermic reaction is usually provided by (exothermic) com-
bustion with air or oxygen:
2C + O → 2CO
2
The detailed behavior of these other reactions (Table 7.2) is not known with any
degree of certainty and still remains somewhat speculative. The reactions are highly
exothermic, and to avoid an increase in temperature, which results in lighter hydrocar-
bons, it is important to have sufficient cooling, to secure stable reaction conditions. The
total heat of reaction amounts to approximately 25 percent of the heat of combustion of
the synthesis gas, and lays thereby a theoretical limit on the maximal efficiency of the
Fischer-Tropsch process.

TABLE 7.2 Fischer-Tropsch Reactions
Reaction enthalpy:
Reaction Δ H [kJ/mol]
300 K
→ −CH − + H O −165.0
CO + 2H 2 2 2
2CO + H → −CH − + CO −204.7
2 2 2
CO + H O → H + CO 2 −39.8
2
2
3CO + H → −CH − + 2CO −244.5
2 2 2 2
CO + 3H → −CH − + 2H O −125.2
2
2
2
2
The reaction is dependent on a catalyst, mostly an iron or cobalt catalyst where the reac-
tion takes place (van Berge, 1995). There is either a low-temperature Fischer-Tropsch (LTFT)
or high-temperature Fischer-Tropsch (HTFT) (with temperatures ranging between 200 and
240°C for LTFT and 300 and 350°C for HTFT). LTFT uses an iron catalyst, and HTFT either
an iron or a cobalt catalyst. The different catalysts include also nickel-based and ruthenium-
based catalysts, which also have enough activity for commercial use in the process. But the
availability of ruthenium is limited and the nickel based catalyst has high activity but produces
too much methane, and additionally the performance at high pressure is poor, due to produc-
tion of volatile carbonyls. This leaves only cobalt and iron as practical catalysts, and this study
will only consider these two. Iron is cheap, but cobalt has the advantage of higher activity and
longer life, though it is on a metal basis 1000 times more expensive than iron catalyst.
For large-scale commercial Fischer-Tropsch reactors heat removal and temperature con-
trol are the most important design features to obtain optimum product selectivity and long
catalyst lifetimes. Over the years, basically four Fischer-Tropsch reactor designs have been
used commercially (Fig. 7.1).
These are the multi-tubular fixed bed reactor, the slurry reactor, or the fluidized bed
reactor (with either fixed or circulating bed).
The fixed bed reactor consists of thousands of small tubes with the catalyst as surface-
active agent in the tubes. Water surrounds the tubes and regulates the temperature by set-
tling the pressure of evaporation. The slurry reactor is widely used and consists of fluid
and solid elements, where the catalyst has no particular position, but flows around as small
pieces of catalyst together with the reaction components. Overheating the catalyst causes
the decrease of its activity and favors the deposition of carbon on the surface of particles.