09-04-2011, 11:50 AM
Presented by:
ABHIRAJ H. PATHAK
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FLUIDISATION AND FLUIDISED BED
INTRODUCTION
Fluidization is a process in which solids are caused to behave like a fluid by blowing gas or liquid upwards through the solid-filled reactor. Fluidization is widely used in commercial operations; the applications can be roughly divided into two categories, i.e.
• Physical operations, such as transportation, heating, absorption, mixing of fine powder, etc. and
• Chemical operations, such as reactions of gases on solid catalysts and reactions of solids with gases etc.
The fluidized bed is one of the best known contacting methods used in the processing industry, for instance in oil refinery plants. Among its chief advantages are that the particles are well mixed leading to low temperature gradients, they are suitable for both small and large scale operations and they allow continuous processing. There are many well established operations that utilize this technology, including cracking and reforming of hydrocarbons, coal carbonization and gasification, ore roasting, Fisher-Tropsch synthesis, coking, aluminium production, melamine production, and coating preparations. The application of fluidization is also well recognized in nuclear engineering as a unit operation for example, in uranium extraction, nuclear fuel fabrication, reprocessing of fuel and waste disposal.
The fluidisation principle is straight forward, passing a fluid upwards through a packed bed of solids produces pressure drop due to fluid drag. When fluid drag is equal to the bed weight the particles no longer rest on each other, this is the point of fluidisation.
The fluid velocity is sufficient to suspend the particles, but it is not large enough to carry them out of the vessel. The solid particles swirl around the bed rapidly, creating excellent mixing among them. The material “fluidized” is almost always a solid and the “fluidizing medium” is either a liquid or gas. The characteristics and behavior of a fluidized bed are strongly dependent on both the solid and liquid or gas properties.
BASIS
We are going to use the Kunii-Levenspiel bubbling bed model to describe reactions in fluidized beds. In this model, the reactant gas enters the bottom of the bed and flows up the reactor in the form of bubbles. As the bubbles rise, mass transfer of the reactant gases takes place as they flow (diffuse) in and out of the bubble to contact the solid particles where the reaction product is formed. The product then flows back into a bubble and finally exits the bed when the bubble reaches the top of the bed. The rate at which the reactants and products transfer in and out of the bubble affects the conversion, as does the time it takes for the bubble to pass through the bed.
FLUIDIZATION REGIMES
When the solid particles are fluidized, the fluidized bed behaves differently as velocity, gas and solid properties are varied. It has become evident that there are number of regimes of fluidization, as shown in Figure 1. When the flow of a gas passed through a bed of particles is increased continually, a few vibrate, but still within the same height as the bed at rest. This is called a fixed bed (Figure 1A). With increasing gas velocity, a point is reached where the drag force imparted by the upward moving gas equals the weight of the particles, and the voidage of the bed increases slightly: this is the onset of fluidization and is called minimum fluidization with a corresponding minimum fluidization velocity. Increasing the gas flow further, the formation of fluidization bubbles sets in. At this point, a bubbling fluidized bed occurs as shown in Figure 1C. As the velocity is increased further still, the bubbles in a bubbling fluidized bed will coalesce and grow as they rise. If the ratio of the height to the diameter of the bed is high enough, the size of bubbles may become almost the same as diameter of the bed. This is called slugging (Figure 1D). If the particles are fluidized at a high enough gas flow rate, the velocity exceeds the terminal velocity of the particles. The upper surface of the bed disappears and, instead of bubbles, one observes a turbulent motion of solid clusters and voids of gas of various sizes and shapes. Beds under these conditions are called turbulent beds as shown in Figure 1E. With further increases of gas velocity, eventually the fluidized bed becomes an entrained bed in which we have disperse, dilute or lean phase fluidized bed, which amounts to pneumatic transport of solids.
GELDART’S CLASSIC CLASSIFICATION OF POWDERS
Not every particle can be fluidized. The behavior of solid particles in fluidized beds depends mostly on their size and density. A careful observation by Geldart (1973, 1978) is shown in Figure 2 in which the characteristics of the four different powder types were categorized as follows:
• Group A is designated as ‘aeratable’ particles. These materials have small mean particle size (dp < 30 μm) and/or low particle density (<~1.4 g/cm3). Fluid cracking catalysts typically are in this category. These solids fluidize easily, with smooth fluidization at low gas velocities without the formation of bubbles. At higher gas velocity, a point is eventually reached when bubbles start to form and the minimum bubbling velocity, Umb is always greater than Umf.
• Group B is called ‘sandlike’ particles and some call it bubbly particles. Most particles of this group have size 150 μm to 500 μm and density from 1.4 to 4 g/cm3. For these particles, once the minimum fluidization velocity is exceeded, the excess gas appears in the form of bubbles. Bubbles in a bed of group B particles can grow to a large size. Typically used group B materials are glass beads (ballotini) and coarse sand.
• Group C materials are ‘cohesive’, or very fine powders. Their sizes are usually less than 30 μm, and they are extremely difficult to fluidize because interparticle forces are relatively large, compared to those resulting from the action of gas. In small diameter beds, group C particles easily give rise to channeling. Examples of group C materials are talc, flour and starch.
• Group D is called ‘spoutable’ and the materials are either very large or very dense. They are difficult to fluidize in deep beds. Unlike group B particles, as velocity increases, a jet can be formed in the bed and material may then be blown out with the jet in a spouting motion. If the gas distribution is uneven, spouting behavior and severe channeling can be expected. Roasting coffee beans, lead shot and some roasting metal ores are examples of group D materials.
Geldart’s classification is clear and easy to use as displayed in figure for fluidization at ambient conditions and for U less than about 10•Umf. For any solid of a known density ρs and mean particle size dp this graph shows the type of fluidization to be expected. It also helps predicting other properties such as bubble size, bubble velocity, the existence of slugs.
