**4. Comparison of fixed bed, circulating fluidized bed and slurry bed reactors for FTS**

A schematic diagram for a common fixed bed reactor is shown in **Figure 6(a)**. A gas sparger at the reactor inlet is used to remove the initial kinetic energy of the gas stream. A certain gas space is left between the catalyst bed and the gas sparger for fluid buffering and uniform mixing. The packing (e.g., ceramic balls) spreads on the upper part of the bed can further make the stream enter the catalyst bed in a more uniform state. The small and uniform particle size of catalyst makes the resistance of each part of catalyst bed the same to enhance the reaction efficiency. After reacting in the catalyst channels, the unreacted syngas and the formed products leave the catalyst bed and reactor and release a large amount of heat. There is 1 ~ 2 thermocouple inserted in the reactor to monitor the temperature parameters of each section of the catalyst bed in time.

Fluidized bed reactors possess high heat transfer efficiency. Generally, the fluidized bed reactors are classified to circulating fluidized bed reactor and fixed fluidized bed reactor, as shown in **Figure 6**(**b** and **c)**. The feed gas is used as the power source to drive the catalyst, which is suitable for the strong exothermic reactions by using the intermediate heat exchange device to remove the reaction heat in time. However, the device structure of circulating fluidized bed reactor is complex, the investment and maintenance cost are high; Moreover, the reactor is difficult to operate and enlarge. In view of the limitations and defects of circulating fluidized bed reactor, a fixed fluidized bed reactor was designed by Sasol Corporation in 1995 and named as Sasol Advanced Synthnol reactor. Since the diameter of the fixed fluidized bed reactor can be much larger than that of circulating fluidized bed reactor, the space for *Review of Slurry Bed Reactor for Carbon One Chemistry DOI: http://dx.doi.org/10.5772/intechopen.109094*

#### **Figure 6.**

*Reactor types used for FTS. (a) Fixed bed reactor. (b) Circulating fluidized bed reactor. (c) Fixed fluidized bed reactor (d) slurry bubble column reactor [2].*

installing the cooling plate is increased by more than 50%, which benefits to improve the CO conversion. However, the entrainment and attrition of the catalyst during the fluidization process is still very serious, which makes the fluidized bed methanation process is not practical for the expensive catalysts.

Since the heat transfer in the slurry bed reactor is more efficient than that in the fixed bed reactor, the temperature control is relatively easy. Therefore, the temperature of the catalyst active site in the slurry bed reactor could be uniform, which could avoid the formation of hot spots in the catalyst bed. In a continuous operation of the slurry bed reactor, the biggest difficulty and obstacle is the separation of fine catalyst, while in a fixed bed there is no such problem. However, the replacement of


**Table 3.**

*Key parameters and performance comparison of different reactors [27].*

the catalysts in a slurry bed reactor is easier than in a fixed bed reaction. If the catalyst needs to be removed and withdrew frequently in a process, the fixed bed reactor may not be appropriate, it is because that the replacement of the catalyst usually requires to stop and remove the reaction.

The key parameters and catalytic methanation performance of different reactors [27] are compared and summarized in **Table 3**. It can be seen that the heat exchange rate of fluidized bed and slurry bed reactor is significantly higher than that of fixed bed reactor, indicating that the fluidized bed or slurry bed reactor is very suitable for the strong exothermic reaction. However, the cost of fluidized bed reactor is high, the catalyst entrainment and wear are serious, and the catalyst cannot be recovered, which limits the application of large-scale production.
