Introduction: Fluidization refers to those gas-solids and liquid-solids system in which the solid phase is subjected to behave more or less like a fluid by the upwelling current of gas or liquid stream moving through the bed of solid particles. Fluidized bed combustion and catalytic cracking of heavy crude-oil fractions of petroleum are the two good examples of fluidization. Fluidization starts at a point when the bed pressure drop exactly balances the net downward forces (gravity minus buoyancy forces) on the bed packing.
Pressure Drop Behavior of Fluidized beds: As VS increases, e may increase and hold Dp constant (L will also increase but its effect is much less than the effect of change in e. See all the above equations). Thus the experimental result for such a test is shown in figure.
For velocities less than the minimum fluidization velocity Vmf, the bed behaves as a packed bed. However as the velocity is increased past Vmf, not only does the bed expand (L increases), but also the particles move apart, and also increases to keep the Dp constant. As the velocity is further increased, the bed becomes more and more expanded, and the solid content becomes more and more dilute. Finally, the velocity becomes as large as terminal settling velocity Vt of the individual particles, so the particles are blown out of the system. Thus the velocity range for which a fluidized bed can exist is from Vmf to Vt.
Types of fluidization: The equations derived for minimum fluidization velocity apply to liquids as well as gases, but beyond the minimum fluidization velocity Vmf, the appearance of beds with liquids or gases is quite different.
When fluidizing sand with water, the particles move further apart and their motion becomes more vigorous as the velocity is increased, but the bed density at a given velocity is same in all sections of the bed. This is called particulate fluidization and is characterized by a large but uniform expansion of the bed at high velocities. Beds of solids fluidized with air usually exhibit what is called aggregative or bubbling fluidization. At superficial velocities much greater than Vmf most of the gas passes through the bed as bubbles or voids which are almost free of solids, and only a small fraction of the gas flows in the channels between the particles.
Advantages and Disadvantages of fluidization: The chief advantage of fluidization are that the solid is vigorously agitated by the fluid passing through the bed, and the mixing of the solid ensures that there are practically no temperature gradients in the bed even with quite exothermic or endothermic reactions.
- The main disadvantage of gas-solid fluidization is the uneven contacting of gas and solid.
- Erosion of vessel internals
- Attrition of solids. Because of attrition, the size of the solid particles is getting reduced and possibility for entrapment of solid particles with the fluid are more.
Applications: For more details on applications, see Fluidized bed technology, Fluidized bed combustion, and Fluidized bed reactor. In 1920s, the Winkler process was developed to gasify coal in a fluidized bed, using oxygen. It was not commercially successful. The first large scale commercial implementation, in the early 1940s, was the fluid catalytic cracking (FCC) process, which converted heavier petroleum cuts into gasoline. Carbon-rich "coke" deposits on the catalyst particles and deactivates the catalyst in less than 1 second. The fluidized catalyst particles are shuttled between the fluidized bed reactor and a fluidized bed burner where the coke deposits are burned off, generating heat for the endothermic cracking reaction. By the 1950s fluidized bed technology was being applied to mineral and metallurgical processes such as drying, calcining, and sulfide roasting. In the 1960s, several fluidized bed processes dramatically reduced the cost of some important monomers. Examples are the Sohio process for acrylonitrile and the oxychlorination process for vinyl chloride.
In the late 1970s, a fluidized bed process for the synthesis of polyethylene dramatically reduced the cost of this important polymer, making its use economical in many new applications. The polymerization reaction generates heat and the intense mixing associated with fluidization prevents hot spots where the polyethylene particles would melt. A similar process is used for the synthesis of polypropylene.