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

The multi-tubular fixed bed reactor is a Low Temperature Fischer-Tropsch (LTFT)
reactor ranging from 220-250 ºC. LTFT reactors are better suited for the creation of heavy
hydrocarbons in the form of liquid wax. Later in the process, these long chain hydrocarbons are
cracked to formed FTD [4, 71, 72, 74].
The multi-tubular fixed bed reactor can use either iron or cobalt catalysts. Catalyst
poisoning can occur when coal or biomass is used in the creation of syngas. An advantage of the
TFBR is that only the upper portion of the catalyst may be poisoned, allowing the remaining
catalyst to be operational. However, to replace the catalyst in the TFBR, the reactor must be
taken off line, which causes expensive downtime [72].
The slurry-phase bubbling-bed reactor (SPR) is a three phase reactor with solid, gas and
liquid phases. It is also a LTFT reactor that can utilize both iron and cobalt catalyst. In the SPR
small partials of catalyst are suspend in liquid while syngas bubbles propagate through the slurry.
The reactor temperature is precisely controlled by steam-filled coils that snake through the
slurry. The product hydrocarbons are drawn from the top of the slurry. Light hydrocarbons and
un-reacted syngas exit through the top of the reactor.
The SPR reactors have only recently become a viable reactor. The SPR was tested in the
1950’s and 1960’s; however a viable method of separating the catalyst from the wax product was
not invented until 1990 by Sasol [72]. The SPR is also a LTFT reactor which can utilize both
iron and cobalt catalysts.
If a catalyst poison enters the SPR, all of the catalyst will be deactivated. Nonetheless, the
key advantage of the SPR is that its catalyst can be replaced without taking the reactor offline
[72].
2.4.2. Fischer-Tropsch Catalyst
Iron (Fe), Cobalt (Co), Nickel (Ni) and Ruthenium (Ru) -based catalysts are active
enough to be used in FTS. However, iron and cobalt are the only catalysts utilized in Fischer-
Tropsch synthesis [72] due to the various drawbacks of nickel and ruthenium. Nickel is a strong
hydrogenating catalyst which forms much more methane then iron or cobalt. Nickel also
converts to carbonyls, thus requiring more catalyst to be added. Ruthenium is the most active but
is rare and expensive [71]. Table 5 gives the cost comparison of the four catalysts.
Table 5: Cost scale of Fischer-Tropsch catalyst in 2001 [73].
Catalyst Cost scale
Iron (Fe) 1
Cobalt (Co) 1000
Nickel (Ni) 250
Ruthenium (Ru) 50000
When comparing iron catalysts to cobalt catalysts, iron requires a lower H2/Co ratio from
.8 to 1.7 because in the reactor, it performs a water gas shift to increase the required H2/Co ratio
[75]. Cobalt requires a water gas shift to be preformed on the syngas before it enters the reactor,
to reach its H2/Co ratio of ~2. Cobalt, however has a longer life and is more active [73]. The
higher activity of cobalt is visible by looking at an example given by Dry al. et, in which the
percent conversion is compared to the bed length for iron and cobalt catalyst. Dry uses the
established kinetic equation for cobalt catalysts (2.4.1) [76] and for iron catalysts (2.4.2) [77] to
generate Figure 8.
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