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<d id="1001001170" t="none" n="1170">
<issn></issn>
<cod>CATAL. REV.—SCI. ENG., 41(3&4),
255–318 (1999)</cod>
<au fn="GERARD" mn="P." sn="VAN DER
LAAN"><af><od>Department of Chemical
Engineering</od><org>University of
Groningen</org><str>Nijenborgh
4</str><cty>Groningen</cty><zip>9747
AG</zip><cny>The Netherlands</cny></af></au>
<cau fn="A." mn="A.C.M." sn="BEENACKERS"></cau>
<au fn="A." mn="A.C.M."
sn="BEENACKERS"><af><od>Department of Chemical
Engineering</od><org>University of
Groningen</org><str>Nijenborgh
4</str><cty>Groningen</cty><zip>9747
AG</zip><cny>The Netherlands</cny></af></au>
<ti>Kinetics and Selectivity of the
Fischer–Tropsch Synthesis: A Literature
Review</ti>
<toc>
<p>I. INTRODUCTION………</p>
<p>II. FT KINETIC
MEASUREMENTS………</p>
<p>III. ADSORPTION ………</p>
<p>A. H<sb>2</sb>
Adsorption………</p>
<p>B. CO Adsorption………</p>
<p>IV. FISCHER–TROPSCH
CATALYSIS………</p>
<p>A. Catalysts………</p>
<p>B. Catalyst
Pretreatment………</p>
<p>C. Fischer–Tropsch
Activity………</p>
<p>D. Water–Gas Shift
Activity………</p>
<p>V. MECHANISM………</p>
<p>A. Fischer–Tropsch
Synthesis………</p>
<p>B. Water–Gas Shift
Reaction………</p>
<p>VI. SELECTIVITY OF THE FISCHER–TROPSCH
SYNTHESIS………</p>
<p>A. Introduction………</p>
<p>B. Influence of Process Conditions on the
Selectivity………</p>
<p>VII. MODELS OF THE PRODUCT
SELECTIVITY………</p>
<p>A. Anderson–Schulz–Flory
Distribution………</p>
<p>B. Deviations from ASF
Distribution………</p>
<p>C. Comprehensive Product Distribution
Models………</p>
<p>VIII. KINETICS………</p>
<p>A. Introduction………</p>
<p>B. Overall Conversion of Synthesis
Gas………</p>
<p>C. Water–Gas Shift
Kinetics………</p>
<p>D. Hydrocarbon Production
Rate………</p>
<p>IX. CONCLUSIONS………</p>
<p>X. NOTATION………</p>
<p>  REFERENCES………</p>
</toc>
<kwg>
<kwd>Fischer–Tropsch synthesis</kwd>
<kwd>Carbon monoxide hydrogenation</kwd>
<kwd>Water–gas shift</kwd>
<kwd>Product distribution</kwd>
<kwd>Kinetic modeling</kwd>
</kwg>
<abs>
<p>A critical review of the kinetics and
selectivity of the Fischer–Tropsch
synthesis (FTS) is given. The focus is on
reaction mechanisms and kinetics of the
water–gas shift and Fischer–Tropsch
(FT) reactions. New developments in the product
selectivity as well as the overall kinetics are
reviewed. It is concluded that the development
of rate equations for the FTS should be based on
realistic mechanistic schemes. Qualitatively,
there is agreement that the product distribution
is affected by the occurrence of secondary
reactions (hydrogenation, isomerization,
reinsertion, and hydrogenolysis). At high CO and
H<sb>2</sb>O pressures, the most important
secondary reaction is readsorption of olefins,
resulting in initiation of chain growth
processes. Secondary hydrogenation of
α-olefins may occur and depends on the
catalytic system and the process conditions. The
rates of the secondary reactions increase
exponentially with chain length. Much
controversy exists about whether these
chain-length dependencies stem from differences
in physisorption, solubility, or diffusivity.
Preferential physisorption of longer
hydrocarbons and increase of the solubility with
chain length influences the product distribution
and results in a decreasing olefin-to-paraffin
ratio with increasing chain length. Process
development and reactor design should be based
on reliable kinetic expressions and detailed
selectivity models.</p></abs>
<h1><ti>I. INTRODUCTION</ti>
<p>Increasing crude oil prices may cause a shift
to coal and natural gas as the feedstock of the
chemical industry. These can be converted into
CO and H<sb>2</sb> by partial oxidation or
steam-reforming processes. The conversion of the
synthesis gas thus obtained to aliphatic
hydrocarbons was discovered by Fischer and
Tropsch [<lk id="1"></lk>]. The overall
reactions of the Fischer–Tropsch synthesis
(FTS) are summarized in Table <lk id="t1"></lk>.
The FT synthesis product spectrum consists of a
complex multicomponent mixture of linear and
branched hydrocarbons and oxygenated products.
Main products are linear paraffins and
α-olefins. Fuels produced with the FT
synthesis are of a high quality due to a very
low aromaticity and zero sulfur content. The
middle distillate fraction has a high cetane
number, resulting in superior combustion
properties and reduced emissions. New and
stringent regulations may promote replacement or
blending of conventional fuels by sulfur and
aromatic-free FT products [<lk id="2|3"></lk>].
The hydrocarbon synthesis is catalyzed by metals
such as cobalt, iron, and ruthenium. Both iron
and cobalt are used commercially these days at a
temperature of 200–300°C and at
10–60 bars pressure.</p>
<tbl><n>1</n>
<ti>Major Overall Reactions in the
Fischer–Tropsch Synthesis</ti>
<grp c="2">
<tb>
<tr><tey>Main reactions</tey><tey></tey></tr>
<tr><tey> 1.
Paraffins</tey><tey>(2<tex>$n$</tex> +
1)H<sb>2</sb> + <tex>$n$</tex>CO →
C<sb><i>n</i></sb>H<sb>2<i>n</i>+2</sb>+
<tex>$n$</tex>H<sb>2</sb>O</tey></tr>
<tr><tey> 2.
Olefins</tey><tey>2<tex>$n$</tex>H<sb>2</sb>+
<tex>$n$</tex>CO →
C<sb><i>n</i></sb>H<sb>2<i>n</i></sb>+
<tex>$n$</tex>H<sb>2</sb>O</tey></tr>
<tr><tey> 3. WGS reaction</tey><tey>CO
+ H<sb>2</sb>O &lrarr2; CO<sb>2</sb> +
H<sb>2</sb></tey></tr>
<tr><tey>Side reactions</tey><tey></tey></tr>
<tr><tey> 4.
Alcohols</tey><tey>2<tex>$n$</tex>H<sb>2</sb>
+ <tex>$n$</tex>CO →
C<sb><i>n</i></sb>H<sb>2<i>n</i>+2</sb>
+ (<tex>$n$</tex> −
1)H<sb>2</sb>O</tey></tr>
<tr><tey> 5. Catalyst
oxidation/reduction</tey><tey>(a)
M<sb><i>x</i></sb>O<sb><i>y</i></sb> +
<tex>$y$</tex>H<sb>2</sb> &rlarr2;
<tex>$y$</tex>H<sb>2</sb>O +
<tex>$x$</tex>M</tey></tr>
<tr><tey></tey><tey>(b)
M<sb><i>x</i></sb>O<sb><i>y</i></sb> +
<tex>$y$</tex>CO &lrarr2;
<tex>$y$</tex>CO<sb>2</sb> +
<tex>$x$</tex>M</tey></tr>
<tr><tey> 6. Bulk carbide
formation</tey><tey><tex>$y$</tex>C +
<tex>$x$</tex>M &lrarr2;
M<sb><i>x</i></sb>C<sb><i>y</i></sb></tey></tr>
<tr><tey> 7. Boudouard
reaction</tey><tey>2CO → C +
CO<sb>2</sb></tey></tr>
</tb>
</grp>
</tbl>
<p>Political reasons and large resources of
cheap coal resulted in a commercial
Fischer–Tropsch plant in Sasolburg, South
Africa in 1955 [<lk id="4"></lk>]. In 1993,
Sasol realized a commercial-size slurry reactor
of 5 m diameter and 22 m height using an
iron-based catalyst [<lk id="5"></lk>]. The
design capacity of this slurry reactor is about
2500 barrels (bbl)/day. Recently, Sasol,
Qatar General Petroleum Corp., and Phillips
Petroleum Co. started a feasibility study for a
20,000-bbl/day natural-gas-based plant in
Qatar [<lk id="6"></lk>].</p>
<p>In the eighties, Shell developed a novel FTS
process starting from natural gas. This
so-called Shell Middle Distillate Synthesis
(SMDS) process [<lk id="7|8"></lk>] produces
heavy paraffins at a design capacity of 20,000
bbl/day in multitubular trickle-bed reactors
in Bintulu, Malaysia. Part of these are sold as
wax specialties; another part is hydrocracked
over a noble metal catalyst into clean diesel
fuels.</p>
<p>Exxon's Natural Gas to Liquids
Conversion Technology was demonstrated at a
scale of 200 bbl/day in a slurry process. A
feasibility study to scale up to commercial
sizes of 50,000–100,000 bbl/day at a
location in Qatar was completed recently [<lk
id="9"></lk>].</p>
<p>The Fischer–Tropsch synthesis in slurry
bubble columns is very attractive relative to
fixed-bed reactors [<lk id="10"></lk>]. The
advantages are the following: (<lk id="1"></lk>)
a low-pressure drop over the reactor; (<lk
id="2"></lk>) excellent heat-transfer
characteristics resulting in stable reactor
temperatures; (<lk id="3"></lk>) no diffusion
limitations; (<lk id="4"></lk>) the possibility
of continuous refreshment of catalyst
particles.</p>
<p>An optimal design with respect to product
yield and selectivity requires a deep
understanding of hydrodynamics, reaction
kinetics, and FT chemistry. The mathematical
modeling of FT slurry bubble columns was
reviewed by Saxena et al. [<lk id="10"></lk>]
and more recently by Saxena [<lk id="11"></lk>].
He showed that none of the available models is
accurate enough for a reliable reactor design.
The bottleneck appears to be the lack of
reliable kinetic equations for all products and
reactants based on realistic reaction
mechanisms.</p>
<p>Literature on the kinetics and selectivity of
the Fischer–Tropsch synthesis can be
divided into two classes. Most studies aim at
catalyst improvement and postulate empirical
power-law kinetics for the carbon monoxide and
hydrogen conversion rates and a simple
polymerization reaction following an
Anderson–Schulz–Flory (ASF)
distribution for the total hydrocarbon product
yield. This distribution describes the entire
product range by a single parameter, α,
the probability of the addition of a carbon
intermediate (monomer) to a chain. Relatively
few studies aim at understanding the reaction
mechanisms. Some authors derived
Langmuir–Hinshelwood–Hougen–Watson
(LHHW) rate expressions for the reactant
consumption and quantitative formulations to
describe the product distribution of linear and
branched paraffins and olefins, and alcohols.
Models which combine the overall consumption of
reactants and the product distribution are very
scarce in the literature, but they are extremely
valuable for understanding and modeling this
process.</p>
<p>Recently, interest in the distribution of the
Fischer–Tropsch products was a result of
improvements of the analysis of all isomers and
products which can- not be described with the
classical ASF distribution. Also, the mechanism
of CO hydrogenation has remained a subject of
immense controversy and uncertainty. A critical
review on the various reaction mechanisms and
kinetic relations proposed is the subject of
this article. Areas which require further
research will be defined.</p></h1>
<h1><ti>II.
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