Role of water-gas-shift reaction in Fischer–Tropsch synthesis on iron catalysts: A review
Graphical abstract
Introduction
Fischer–Tropsch Synthesis (FTS) reaction is a key part of technology to convert natural resources, such as coal, natural gas and biomass, into liquid hydrocarbon fuels. FTS is a heterogeneous reaction discovered in the early twentieth century [1]. Reactants, carbon-monoxide and hydrogen (syngas), are converted to an array of hydrocarbons (mainly n-paraffins and olefins), and FTS reaction can be described by the following stoichiometry:
The catalysts of choice for industrial FTS are cobalt and iron. If the raw material is coal, then the preferred catalyst is iron [2], [3]. One of the main features of iron FTS catalysts is their water-gas-shift (WGS) activity. WGS reaction can be presented as:
The WGS reaction provides additional hydrogen for FTS, which is needed in the case of coal-derived syngas. Coal-derived syngas normally has a H2/CO ratio below 2, the latter being an approximate stoichiometric H2/CO ratio (i.e. m/n = 2) needed to produce hydrocarbons according to equations (1).
The main objectives for industrial FTS plants are to achieve efficient utilization, high productivity, long term operation and high selectivity to C5+ hydrocarbons and low methane selectivity. Kinetics of various parallel reactions (FTS, WGS and olefin secondary reactions) determines the overall selectivity of FTS products. Proper selection of process and syngas feed conditions is essential in achieving activity, stability and selectivity targets in commercial FTS reactors. With Fe-based catalysts the effect of WGS is very significant. This reaction affects concentrations (partial pressures) of CO, H2, CO2 and H2O in the system, which in turn has an impact on kinetics of both primary FTS and 1-olefin secondary reactions and thus the product distribution.
In this work we focus on the role of WGS in FTS and review available information on the effect of process conditions on the WGS activity, and the impact of WGS reaction on selection of process conditions used in industrial practice and reactor design considerations.
This paper is dedicated to Professors Mark Dry and Hans Schulz, two giants in the field of FTS catalysis, who have made pioneering contributions to our knowledge about various aspects of industrial applications of FTS and its fundamentals.
Section snippets
Measures of WGS activity and stoichiometric constraints
The extent of WGS reaction, or WGS activity, has been expressed in terms of: (1) magnitude of H2/CO usage ratio, U; (2) CO2 selectivity; (3) partial pressure quotient or ; or (4) as ratio of WGS rate to FTS rate, rWGS/rFTS. First we introduce definitions and provide some useful relations between different variables and conversions of reactants.
Fractional conversion of a reactant (Xi), is defined as:where:
Effect of temperature
Results from experiments in our laboratory with precipitated iron catalyst (Ruhrchemie catalyst with nominal composition 100Fe/5Cu/4.2K/25SiO2 by weight) in a stirred tank slurry reactor are shown in Fig. 3. The usage ratio decreases with an increase in conversion (at constant temperature) or with an increase in temperature (at constant conversion) for F = 0.67 (Fig. 3a). At higher conversions the effect of temperature is less pronounced. Similar trends are observed with F = 2 (Fig. 3b) except that
Catalyst deactivation and WGS reaction
Iron FTS catalysts lose a part of their initial activity with time on stream due to several deactivation mechanisms: poisoning (mainly by sulfur in the feed), carbonaceous deposits, sintering and oxidation (i.e. conversion of iron carbides to magnetite) [10], [32], [33]. The latter two mechanisms are considered to be the most important for precipitated Fe catalysts used in low temperature Fischer–Tropsch synthesis [32], [33] and this is attributed to hydrothermal sintering and oxidation by
Kinetics and mechanism of WGS
Since the WGS reaction has a significant influence on the partial pressure of FTS reactants, understanding its kinetics is very important. Several reviews of WGS kinetics have been published to date [9], [36] and proposed rate expressions are summarized in Table 3. Some of the early kinetic studies focused on using empirical rate equations [37], [38], [39], the most popular one utilized a simple first order in CO [37], [38]. Even though they provided a reasonable fit of the experimental data, a
WGS and industrial practice
Sasol has been operating fixed bed reactors (Arge reactors) for low temperature FTS (LTFT) since mid 1950's. Over the years researchers from Sasol have presented information on various aspects of the LTFT process including product distribution, operating conditions, reaction kinetics and mechanism of FTS, catalyst composition and the overall process including upstream and downstream units in an integrated plant to produce fuels and chemicals from coal. Process conditions for the LTFT have been
Conclusions
WGS reaction plays an important role in CTL processes based on iron FT catalysts. The extent of WGS reaction determines concentrations of reactants and water, and this in turn has significant impact on product distribution and catalyst deactivation. High partial pressure of water is a major cause of deactivation (oxidation of active carbide phase). In addition, water inhibits the rate of FTS. The usage ratio is strongly affected by syngas feed ratio and total syngas conversion. As the syngas
Acknowledgments
This work was supported in part by US DOE grant DE-FG26-02NT41540 and Qatar Foundation.
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