Page 230 - Synthetic Fuels Handbook

216 CHAPTER SEVEN

Syngas inlet
Gas
outlet Gas
outlet Gas
outlet
Hopper
Catalyst
tubes Catalyst

Slurry Wax
Steam bed out
Steam
Catalyst
Stanoppe bed
Cooling Cooling
Gas
water water
outlet Syngas
Syngas
inlet
inlet
Syngas
Wax outer
inlet
Multi-tubular Circulating Fixed Fixed
(ARGE) (Synthol) (Sasol advanced Synthol) slurry bed
fixed bed fluidized bed fluidized bed
FIGURE 7.1 Types of Fischer-Tropsch reactors.

In the slurry reactor, the catalyst is suspended in the liquid and the gas is bubbled
through the suspension. Generally this type of reactor gives (a) a more uniform tempera-
ture, (b) there is less catalyst present and consumed per ton of product, and (d) the differ-
ential pressure over bed is lower.
The slurry reactor and the fixed bed reactor are used in LTFT. The fluidized bed reac-
tors are diverse, but characterized by the fluid behavior of the catalyst; the fluidized bed
reactor is used in the HTFT.
Sasol in South Africa uses coal and natural gas as a feedstock, and produces a variety
of synthetic petroleum products. The process was used in South Africa to meet its energy
needs during its isolation under apartheid. This process has received renewed attention in
the quest to produce low-sulfur diesel fuel in order to minimize the environmental impact
from the use of diesel engines.
The Fischer-Tropsch technology as applied at Sasol can be divided into two operating
regimes: (a) high-temperature Fischer Tropsch and (b) low-temperature Fischer-Tropsch.
The high-temperature Fischer Tropsch technology uses a fluidized catalyst at 300 to
330°C. Originally circulating fluidized bed units were used (Synthol reactors). Since 1989, a
commercial scale classical fluidized bed unit has been implemented and improved upon.
The low temperature Fischer Tropsch technology has originally been used in tubular
fixed bed reactors at 200 to 230°C. This produces a more paraffinic and waxy product
spectrum than the high-temperature technology. A new type of reactor (the Sasol slurry
phase distillate reactor) has been developed and is in commercial operation. This reactor
uses a slurry phase system rather than a tubular fixed bed configuration and is currently the
favored technology for the commercial production of synfuels.
The commercial Sasol Fischer-Tropsch reactors all use iron-based catalysts on the basis
of the desired product spectrum and operating costs. Cobalt-based catalysts have also been
known since the early days of this technology and have the advantage of higher conversion