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Fischer–Tropsch (F–T) synthesis produces sulphur-free hydrocarbons from synthesis gas (syngas), which is a mixture of carbon monoxide and hydrogen. Syngas can be obtained from biomass, coal and natural gas by gasification and/or reforming. Therefore, by using the F–T synthesis reaction, biomass, coal and natural gas, rather than crude petroleum, can be used to obtain hydrocarbons (fuel gas, gasoline, diesel and F–T waxes).

Fe, Co and Ru are known to be active metal catalysts for F–T synthesis. Among them, Co is the most attractive active metal catalyst due to its high catalytic performance (compared with Fe) and low price (compared with Ru). Refractory oxides (such as SiO₂ and Al2O3) are typically used as supports, while some transition metals (such as Zr and Mn) are used as promoters for improving the activity and stability of Co catalysts.

Due to the highly exothermic character of F–T synthesis, it is difficult to control the reaction temperature in a fixed-bed reactor, especially in commercial plants. Therefore, the gas-flowed slurry-phase reaction system is very attractive because the isothermal operations can be controlled by changing the reaction heat using a solvent during the reaction. Slurry reactors can be divided into two main types: stirred tank slurry reactors (STSRs) and slurry bubble column reactors (SBCRs).

Materials and Methods
The Mn–Zr–Co/SiO₂ catalyst was prepared using an incipient wet impregnation method. The SiO₂ support had a particle size of 75–150nm, a Brunauer–Emmett–Teller (BET) surface area of 300m²g-1 and an average pore size of 10nm. The loadings of Co, Zr and Mn in the Mn–Zr–Co/SiO₂ catalyst were 20wt%, 2wt% and 2wt%, respectively.

The F–T synthesis was performed in a gas-flowed slurry-phase reaction system. During the reaction, the feed gas was continuously introduced into the slurry (containing solvents and catalysts) from the bottom of the reactor. The formed diesel-range hydrocarbons and F–T waxes remained in the slurry reactor. A cooling trap was set between the reactor exit and back-pressure regulator to collect the gasoline-range hydrocarbons and water. The reaction temperature was 240°C, and the total pressure was 30atm. The fed syngas consisted of 60% H₂, 30% CO and 10% N₂.

Results and Discussion
Slurry Reactor Combining Slurry Bubble Column Reactor with Stirred Tank Slurry Reactor for the Slurry-phase F–T Synthesis
Figure 1 shows a schematic diagram and photograph of the pilot-scale slurry reactor used in this study. The reactor has an inner diameter (ID) of 100mm and an inner height (IH) of 1,600mm. Its volume is approximately 12.5 litres, and the slenderness ratio of IH to ID is 16. Although an SBCR is not usually equipped with a stirrer for economic reasons, the reactor has a stirrer (like an STSR) for investigating the influence of stirring on the reaction. A hollow T-shaped tubular stirring rod was used in the stirrer. There are several small holes on the upper part of the tubular T-rod. When the stirring speed is higher than 600 revolutions per minute (rpm), the gas above the slurry can be aspirated into the rod through the holes and bubble out from the ends of the tubular T-rod owing to centrifugal force (Venturi effect). Hence, the gas is circulated through the route inside the hollow tubular T-rod, and a complete mixing system is accomplished in the reactor. The maximum speed of the stirrer is 2,000rpm. However, the safe stirring zones for the stirrer are from 0 to 700rpm and from 1,400 to 2,000rpm because the dangerous co-vibration zone of the long stirrer rod (1,500mm) ranges from 750 to 1,350rpm.

A cast iron filter (pore size 50nm) was set in the reactor close to the bottom for supporting the catalyst (particle size 75–150nm). Two furnaces were set for heating the reactor. The lower furnace supplied the primary power during the catalyst pre-treatment (H2 reduction). Furthermore, the two furnaces were used co-operatively to control the reaction temperature during the reaction.

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