**2.1 Mechanically agitated slurry reactor**

The mechanically agitated slurry reactors are commonly used in many chemical processes, such as the hydrogenation of unsaturated oils and nitro compounds. It has the advantages of high mass and heat transfer efficiency, which are best suited for laboratory kinetic studies. And it is because the catalyst particles are small in the reactor, the diffusion resistance in the particles is very low, and the utilization rate of the catalyst is very high. The disadvantages are the high power required for stirring, the presence of significant liquid back-mixing, and the difficulty of catalyst separation in continuous operation. The commonly used agitator in the reactor is a turbine-type radial flow agitator, and there is an optimal ratio of paddle to vessel diameter. To keep the catalyst particles suspended evenly, the number of the baffles should be 4 in most cases and the size is 1/10 ~ 1/12 of the container diameter. Moreover, the use of baffles

**Figure 1.** *Three different types of slurry bed reactors [7].*

*Review of Slurry Bed Reactor for Carbon One Chemistry DOI: http://dx.doi.org/10.5772/intechopen.109094*

#### **Figure 2.**

*Slurry bed reactor with multiple paddles [7]. 1. Paddle, 2. baffle, 3. cooling or heating coils, 4. annular pipes for supplying H2.*

could provide a proper gas distribution and avoid the formation of vortex on the surface of the liquid. Generally, the gas inlet should be as close to the paddle as possible, using an annular sparger rather than a single nozzle. The width of the sparger should be about 0.8 times of the paddle diameter. **Figure 2** shows the slurry bed reactor with multiple paddles, which is commonly used for the hydrogenation of unsaturated oils [7]. To reduce the back mixing in continuous operation, the horizontal inlets are provided at various locations in the reactor.

#### **2.2 Bubble column slurry reactor**

In a bubble column slurry reactor, the dispersion of the feed gas takes place through a deep pool of inert liquid in which the catalyst particles are suspended, and the momentum is transferred to the inert liquid by the movement of the bubbles. The operation is usually implemented in the columns with a height-to-diameter ratio of 4 to 10. For the conversion of liquid reactants, the operation can be semi-batch or continuously. The advantages of this type of reactor are that there are no moving parts, no need to seal the stirrer, less floor space and low power consumption compared to stirred reactors. Its main disadvantage is that the liquid phase has considerable backmixing, which needs to overcome a large pressure drop, and when the height-diameter ratio is higher than 10, the gas-liquid interface area will be decreased quite rapidly. The bubble column slurry reactors have been used in many industrial processes, such as FTS to hydrocarbons, and the production of caprolactam, etc. **Figure 3(a)** shows a continuous bubble column slurry reactor with solid-liquid separation [7]. In another type of bubble column slurry reactor, the catalyst and liquid are circulated by using external pumps, and the catalyst can be operated at higher loads, either countercurrent or co-current. The schematic diagram is shown in **Figure 3(b)**.

#### **2.3 Three-phase fluidized bed reactor**

In a three-phase fluidized bed reactor, the catalyst particles are primarily fluidized by the liquid, while the gas stream flows co-currently in the form of intermittent bubbles. An important difference between the three-phase fluidized bed reactor and

#### **Figure 3.**

*(a) Schematic diagram of continuous bubble column slurry reactor, (b) schematic diagram of a bubble column slurry reactor circulating with a pump [7].*

the bubble column slurry reactor is that the former transfers momentum through the movement of liquid, while the latter transfers momentum through the movement of the bubbles. Moreover, the relatively large catalyst particles can be used in threephase fluidized bed reactors, which makes it suitable for continuous operation due to easy separation of catalyst particles from the liquid. The main industrial application of three-phase fluidized reactor is the hydrogenation of petroleum, including the hydrodesulfurization, hydrocracking to produce olefins, and the partial oxidation to produce aromatics. The three-phase fluidized bed reactors are generally operated in co-current flow, but in some cases, they can also be operated in counter-current flow (the liquid flows down while the gas flows up), such as when the density of catalyst particles (ion exchange resin) is lower than that of the liquid. In the industry, the particles are kept fluidized with internal circulation of the liquid, which can be accomplished by using a pump. This makes it possible to fluidize large catalyst particles. If the internal circulation is not used, the single-pass conversion rate will be too low due to the excessive liquid flow rate used.

#### **2.4 Comparison of three types of slurry reactors**


difference between a fluidized bed and a bubbling bed is that the suspension of the particles for the former is the result of liquid flow, thus, the relatively large catalyst particles can be used, which benefits the catalyst separation. Due to the absence of moving parts, the design of bubble columns and three-phase fluidized bed reactors is simpler than that of mechanically stirred reactors.
