**1. Introduction**

38 Advanced Fluid Dynamics

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> Fluidization refers to the contact between a bed of solids and a flow of fluid. As a result, the solid particles are transformed into a fluid-like behavior that can be used for different purposes. The fluidized bed reactor is one of the most important technologies for gas-solid heterogeneous operations chemical or petrochemical, considering catalytic or non catalytic processes (Kunii and Levenspiel 1991). The most important industrial applications include catalytic cracking, coal combustion and biomass combustion. One of the most relevant type of fluidized bed reactor is the ascendant flow reactor, which is also known as riser. The riser reactors consist of a tubular column in which both solid and gas flow upwards. The first fluidized bed gas generator was developed in Germany by Fritz Winkler in the 1920s. Later in the 1930s, the american petroleum industry started developing the fluidized bed technology for oil feedstock catalytic cracking, becoming the primary technology for such applications (Tavoulareas 1991).

> Inside the riser reactor, solid particles have a wide range of residence time, which is a disadvantage that reduces the overall conversion and the selectivity of the chemical reactions. For that reason it has recently grown the interest in a new type of gas-solid circulating reactor known as downer. In this reactor the gas and the solid flow cocurrently downward, creating hydrodynamic features comparable to a plug flow reactor and allowing a better control over the conversion, the selectivity and the catalyst deactivation. The concept of downer reactor gas-solid appeared in the 1980s, with the first studies on the fluid dynamics of gas-solid suspensions (Kim and Seader 1983) and with the first downer reactors for patents developed by Texaco for the FCC process (Gross Benjamin and Ramage Michael P 1981; Niccum Phillip K and Bunn Jr Dorrance P 1983). In these studies it is observed that in the downer reactor has a uniform distribution of two-phase flow along the reactor, also observed that the contact time is very low, achieving a 20% decrease in the amounts of coke produced during the FCC process. Applications, differences, advantages and disadvantages to these types of fluidized bed reactors can be found in various publications (Ancheyta 2010; Gonzalez, 2008; Yi Cheng et al. 2008; Crowe 2005; Wen-ching Yang 2003; Grace 1997; Gidaspow 1994; Geldart 1986)

### **2. Fluidization regimes and particle classification**

Fluidization occurs when a gas or liquid is forced to flow vertically through a bed of particles at such a rate that the buoyed weight of the particles is completely supported by the drag force imposed by the fluid.

Fluid Dynamics of Gas – Solid Fluidized Beds 41

form stable spouted beds (Geldart 1973). Desirable properties of particles and gas for

Size distribution Neither too narrow or too broad, e.g., 90th to 10th decile ratio 5 to 25 Density Wide range of values possible, but uniform from particle to particle

Table 1. Desirable properties of particles and gases for Gas-Solid fluidization (Jesse Zhu et

For better understanding of these phenomena and to facilitate the solution of mathematical models is necessary to make an analysis of experimental data. This experimental analysis requires specialized measurement techniques are able to explain the flow field must also be

The measurement techniques, to capture the important fluids dynamic behavior of the twophase flow, can be classified as non-intrusive (**NMT**) and intrusive (**IMT**) techniques. The intrusive techniques are generally probes used to study local basic flow phenomena. Some of these are intended only as research instruments. The most common parameters that are measured with such probes are solids mass flows, radial and axial solids concentration,

The particles can be deposited in the measuring device reducing its performance or causing malfunction. Besides this, the flow area reduction makes of the intrusive devices not the best solution. Non-intrusive techniques to characterize the flow within a fluidized bed are more desirable because it does not disturb the flow behavior. In the Table 2 and Table 3

Shape Rounded and with length to thickness ration no larger than ~3

Property Desirable Range

fluidized bed are delineated in Table 1.

Mean diameter 50 μm to 1.6 mm

Surface stickiness Avoid sticky surfaces

Viscosity No restriction

solids velocities, and distribution.

Relative humidity Typically 10 to 90%

**3. Experimental measurement techniques** 

Attrition resistance Usually strong as possible

Hardness Avoid resilience, but also excessive hardness

Density No restriction, but higher value improves properties

automated to minimize human involvement in the process of collecting data.

classification techniques are included and recent successes have been achieved.

Surface roughness Smooth

Particle Properties

Gas Properties

al. 2005)
