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Significant progress has been made over several decades in the development of F-T synthesis reactors; typical examples include fixed-bed reactors, fluidized-bed reactors, and slurry-bed reactors. In recent years, significant research has been invested in slurry-bed F-T synthesis. This technology involves reacting syngas (H2+CO) by bubbling it upward through a liquid inert medium containing fine particle catalysts. Compared to other reactors, slurry-bed reactors have the following advantages: easy control of temperature and pressure, high operational flexibility, and great product versatility ； Good heat and mass transfer performance ； The catalyst load is relatively uniform ； High one-way conversion rate ； C5+ hydrocarbons have high selectivity. In addition, this reactor also has advantages such as resistance to catalyst failure, simple structure, and low investment costs. Slurry-bed F-T synthesis technology originated from laboratory research by Kolbel and others in Germany in 1938, and a semi-industrial pilot plant capable of producing 11.5 tons of liquid products per day was built in the 1950s. In the 1970s, due to the oil crisis, the U.S. energy sector analyzed the technical and economic advantages of slurry-bed F-T synthesis and began research on this process. The company that achieved great success in this regard is Sasol of South Africa. In May 1993, it became the first in the world to put the slurry-bed F-T synthesis technology into industrial use; the reactor had a diameter of 5 meters and a height of 22 meters, producing 2,500 barrels of liquid fuel per day. Currently, the plant plans to increase the reactor’s diameter to 8 meters, thereby enabling an annual production volume of 425,000 tons per reactor. Since the beginning of the 21st century, in China, the Shanxi Institute of Coal Chemistry under the Chinese Academy of Sciences has been primarily responsible for the research and development of co-precipitated Fe-Cu catalysts and slurry-bed reactors. It completed the design for a pilot-scale SUFT system, and in 2002 a pilot plant with an annual oil production capacity of one thousand tons was built. Preparations are currently underway to build an industrial demonstration plant with a capacity of 150,000 t/a. Currently, the slurry-bed F-T synthesis technology is considered to be the most promising approach for the production of liquid fuels, and companies such as Shell, Exxon, and Syntroleum are working on the development of slurry-bed reactors. This paper will discuss the slurry-bed Fischer-Tropsch synthesis technology from aspects such as the characteristics of the slurry-bed synthesis process, the catalysts used in slurry beds, the influence of process conditions, slurry-bed models, and product separation in slurry beds, with the aim of providing a reference for the research and development of slurry-bed technology. 1 Process flow and characteristics of the slurry bed F-T synthesis process. The feed gas consists mainly of CO and H2 in certain proportions, obtained from coal gasification and natural gas reforming; a schematic diagram of the slurry bed reactor is shown in Figure 1. Its inner chamber is a stirrer-type reactor, while the outer part consists of a thermal insulation unit equipped with heating devices; it also includes cooling coils for heat exchange, a gas distributor, and a gas-liquid separator. The reactor contains finely divided catalyst particles prepared by a special method (black dots in the figure); these highly fine catalyst particles are suspended in a liquid medium to form a slurry. Under normal circumstances, the liquid medium is liquid paraffin with a wide boiling range (boiling point > 340°C). In a slurry bed, syngas with an H2/CO ratio of around 2 is typically used for preheating, and then it enters the slurry composed of catalyst and liquid paraffin from the bottom of the reactor via a gas distributor. The rising bubbles come into contact with the catalyst in the slurry and adsorb onto it, thereby carrying out the synthesis reaction. The cooling coil can remove the heat of reaction from inside the reactor. In industry, the synthetic liquid-phase product is separated from the catalyst-containing slurry inside the reactor through a special separation process. At each outlet within the slurry bed, there are filtering elements made of metal sintered wire meshes. When the liquid product flows through the filtering medium in the forward direction, any obstruction in the flow of the liquid phase causes it to flow in the reverse direction. The filter cake composed of solid particles can be easily removed from the filtering medium, while the lighter, more volatile products flow out of the reactor’s product outlet, where they are then cooled to enable the recovery of various products. The F-T synthesis reaction is a highly exothermic reaction, and temperature control during the reaction process is crucial, as it affects the catalyst’s activity, lifetime, and selectivity. As can be seen from the above process, it is relatively easier to remove the heat of reaction in a slurry bed reactor. The slurry phase has a high heat capacity, and the bubbles generated facilitate the transfer of reaction heat to the cooling coil, thereby allowing the slurry bed to operate near isothermal conditions. This results in a higher reaction rate, easier control over product selectivity, as well as the possibility of replacing and replenishing the catalyst online, making the operation very convenient. 2 F-T catalysts for slurry beds: Slurry bed FTS requires catalysts with high wear resistance, in order to facilitate the online separation of the product wax from the catalyst and ensure the stability of catalyst operation. Wear includes physical wear and chemical wear. The main cause of chemical wear is the phase transformation in the catalyst during reaction processes, while physical wear results primarily from collisions between catalyst particles as well as between those particles and the reactor walls. Currently, the catalysts used in slurry-bed F-T synthesis reactions are mainly Fe-based and Co-based catalysts. Fe-based catalysts are inexpensive, exhibit good catalytic activity for FTS and WGS reactions, and are suitable for the synthesis of feed gases with a low H2/CO ratio in a slurry phase. The phases formed during the reduction and reaction processes mainly include Fe3O4, α-Fe, ε-Fe2C, ε’-Fe2C, x-Fe5C2, O-Fe3C, Fe7C3, etc. The reduction process involves the conversion of Fe3O4 to FeO, and then to α-Fe. Yaming Jin et al. studied the F-T synthesis reaction using slurry-phase precipitated iron-based catalysts, and found that these catalysts exhibited high catalytic activity for the FTS reaction, low selectivity for methane, and high selectivity for heavy hydrocarbons. They also demonstrated good stability during long-term operation in laboratory FTS reactions, indicating promising prospects for industrial application. Hao Qinglan et al. studied the effect of calcination temperature on the wear resistance of slurry-phase precipitated iron-based catalysts. The results showed that increasing the calcination temperature significantly improved their wear resistance, and catalysts with a calcination temperature above 400°C exhibited good wear resistance. Lü Yijun and others were the first to use Raney Fe catalysts in slurry-phase F-T synthesis reactions. Even after high-temperature drying treatment (which could lead to sintering), Raney Fe retained a specific surface area (BET) of over 20 m2/g, and its catalytic activity was on par with that of precipitation iron-based catalysts with the same composition ； The RaneyFe catalyst also possesses the high strength of iron-melting-based catalysts, overcoming the weakness of precipitation-iron-based catalysts being prone to wear, thus ensuring very stable F-T activity ； At the same time, its unique skeletal structure facilitates the optimal distribution of additives (K, Cr, Zr) on the surface of the catalyst. Moreover, it allows a certain amount of active hydrogen to dissolve into the iron catalyst framework during the preparation process, which is conducive to the progress of hydrogenation reactions and thus improves the selectivity of the resulting alkanes. The Co-based catalysts used in slurry beds mainly include Co/SiO2, Co/γ-Al2O3, Co/TiO2, etc., and the active component undergoes a reduction process of Co3O4 → CoO → Co. γ-Al2O3 possesses good wear resistance, and Zhang J studied the F-T reaction of γ-Al2O3-supported Co-based catalysts in a slurry bed. Studies show that this catalyst has good activity and high selectivity for alkanes, but its stability is average. Co/SiO2 and Co/TiO2 have certain limitations in application due to the easy wear of their supports. To improve the performance of Co-based catalysts in the slurry phase, efforts are often made to modify the properties of the support, either by using a dual-support system, applying chemical modifications, or adding various additives, in order to enhance the catalyst’s activity and wear resistance and thus achieve higher efficiency in Fischer-Tropsch synthesis. 3 FTS operating conditions in a slurry bed: The suitable temperature range for a slurry bed is 210°C to 260°C. Within this temperature range, temperature has a significant impact on the conversion rate of the feed gas used in the Fischer-Tropsch synthesis; generally, the higher the temperature, the higher the gas conversion rate. Compared to fixed-bed reactors, slurry-bed reactors have a lower temperature range and slower reaction rates, which is favorable for the formation of long-chain hydrocarbons. The effect of pressure on the reaction conversion rate is not very significant; the typical operating pressure in slurry beds ranges from 1.0 MPa to 2.2 MPa. Generally, an increase in pressure tends to improve both the selectivity for alkanes and the selectivity for C5+ hydrocarbons. As the space velocity increases, the residence time of the syngas in the slurry bed decreases, which leads to a reduction in the conversion rate of the syngas. The selectivity for C5+ hydrocarbons also drops, and this affects the distribution of liquid hydrocarbons, shifting it toward lighter hydrocarbons. Therefore, to achieve optimal reaction results and conditions, operations are usually carried out at low space velocities. The H2/CO ratio primarily affects the relative amounts of heavy and light hydrocarbon products; numerous studies have shown that a H2/CO ratio of 2 in a slurry bed can yield a higher selectivity for C5+ hydrocarbons. 4 Progress in the slurry-bed F-T synthesis model. The scaling up of slurry-bed reactors is one of the key challenges in the industrialization of F-T synthesis. In order to design and scale up bubble-slurry reactors properly and to develop the F-T synthesis process effectively, it is essential to establish mathematical models of the reactors and simulate the impact of various experimental parameters on reactor performance. There have been many studies on the simulation of slurry-bed F-T synthesis reactors. Since the slurry bed reacts under intense stirring conditions, being very similar to a completely mixed flow reactor, the mass transfer resistance at the gas-liquid interface can be ignored. Therefore, the following model assumptions are generally made: (1) The gas phase and liquid phase are completely mixed ； (2) Uniform distribution of catalyst particles ； (3) Reactor steady-state operation ； (4) The reaction occurs only in the liquid phase ； (5) Ignore the diffusion of catalyst particles. Some lumped kinetic models for slurry beds are shown in Table 1. Due to the complexity of the F-T synthesis reaction itself, coupled with the immature state of research on the hydrodynamic behavior of three-phase reactors, simulating and designing slurry-bed F-T synthesis reactors presents significant challenges. The main problem in mathematical simulation is the accurate calculation of the physical, chemical, fluid dynamic, and reaction kinetic parameters used in the model. In short, there is a desire to obtain more data under various operating conditions, so that the model studies can better reflect the real-world situation. 5 Solid-liquid separation methods in slurry bed reactors: In slurry bed reactors, high-boiling wax products are generated as a result of the reactions; these products are difficult to vaporize and tend to accumulate within the reactor. Therefore, it is necessary to remove these wax products from the reactor on a regular basis or continuously in order to ensure the proper operation of the reactor. Since fine particle catalysts are mixed in, it is necessary to separate the catalyst in order to avoid its loss and to facilitate the modification of the wax products. The liquid medium commonly used in slurry reactors is a waxy substance with a wide boiling range, while the catalysts used are typically fine-particle catalysts with high mechanical strength. Both the properties of the liquid phase and those of the catalyst particles affect the separation methods and efficiency. The separation of the catalyst from the liquid-phase products can be carried out either within the reactor or externally, with methods such as sedimentation, pressure filtration, magnetic separation, and supercritical separation being the most common. The gravity sedimentation method is a widely used technique; institutions such as the Shanxi Coal Chemistry Research Institute and MOBIL in the United States employ this method, in which the wax products are introduced into a sedimentator where they are separated by gravity ； Centrifugal separation involves pumping the mixed slurry into a centrifuge for solid-liquid separation, and its greatest advantage is high efficiency and time savings ； Filter separation primarily involves using sintered metal wire meshes with pore sizes much smaller than those of the catalyst particles to filter the slurry. Its biggest drawback is that over time the mesh pores get clogged by catalyst particles, making it difficult to perform backwashing ； Magnetic separation technology primarily utilizes the strong magnetic force generated by high-gradient magnetic fields to separate the catalyst from the liquid-phase products ； Supercritical extraction separation actually utilizes supercritical fluids as a solvent to extract F-T synthesis wax, thereby achieving separation from the catalyst. Based on current applications, the catalyst/wax separation process in slurry beds is still not mature; further improvements to this separation process are needed in order to achieve higher separation efficiency. 6 Conclusion: Slurry-bed F-T synthesis has many advantages, and numerous research institutions and companies around the world are engaged in research on this synthesis technology; however, commercialization has not been achieved anywhere except in South Africa. There are still many aspects that need improvement in slurry bed synthesis technology. The optimization of reactor design, the development of better catalysts, and improvements to process conditions will all contribute to the advancement of this technology.