SYNTHESIS GAS UTILIZATION FOR TRANSPORTATION FUEL PRODUCTION - Biomass as a Sustainable Energy Source for the Future - page 532

operated in a continuous mode and at steady state. The syngas conversion in a single
pass through a reactor that can be achieved in practice is limited for various reasons.
Typical limitations are heat removal capacity, avoiding condensation of reaction
water, mechanical restrictions to the maximum size of a reactor, and pressure drop
considerations. The bulk of the unconverted syngas is recycled with significant cost
of recompression. Key considerations in developing FTS reactors are heat removal,
pressure drop, and avoidance of diffusion limitations. Often, a distinction is made
between high-temperature FTS (580 − 620 K) and low-temperature FTS (480 − 530 K).

In high-temperature FTS — using iron as a catalyst — the main products are in the

gasoline range (C 4 − C 12 ); this fraction is gaseous under reaction conditions. Low-

temperature FTS mainly produces

longer-chain hydrocarbons and is better suited for diesel production. The product
mix mostly has a liquid nature at operating conditions; typically, a cobalt catalyst
is used for this process.
FTS is a strongly exothermal process (

— which is receiving more attention these days —

mol −1 CO), which means
that proper heat removal is an important aspect of the reactor design. Failing to remove
all heat results in increasing temperatures, which increases the reaction rate but
decreases the selectivity toward the desired long-chain hydrocarbons. Even worse,
it could lead to hot spots and a reactor runaway: an uncontrolled increase in temper-
ature resulting in an explosion. On the other hand, it is crucial to make good use of the
heat released to come to an energy-efficient process: the higher the temperature at
which heat is released, the more valuable it is.
For high-temperature FTS, different kinds of fluidized bed reactors have been used
(see Steynberg et al. (1999) for an overview of these reactors). Because of the larger
importance of low-temperature FTS, we will focus on the two reactors mainly used for
this: multitubular packed beds and slurry bubble columns (Guettel and Turek, 2009;
Hooshyar et al., 2012) (see Figure 17.4).

Δ r H

≈ −

170 kJ

(a) (b)

Liquid Gas Liquid

Gas

Coolant

Coolant

Gas Liquid
FIGURE 17.4 Schematic of (a) a slurry bubble column reactor and (b) a multitubular packed
bed reactor.

Gas Liquid