3. 2 Justification for Proposed Work 3. 2. 1 Theory Fluidization is a process where solid-state particles are moved into a fluid-like state by means of suspension from either a gas or a liquid. This is accomplished by passing an upward flow of either a gas or liquid up through a fixed bed of small solid particles. Fluidized beds are used in industry for a number of reasons.
First, they are an easily controlled process due to smooth, liquid-like flow of the particles. Also, compared to other methods, the rates of mass and heat transfer are high, providing good contact. Furthermore, since the solids are mixed so quickly, the fluidized bed represents near isothermal conditions, which leads to a very reliable process. Despite these advantages, there are some problems associated with fluidization. One of which is that solids may have nonuniform residence times due to the rapid mixing, which may lead to nonuniform products. Another disadvantage is that small particles may be entrained in the passing gas, or they may agglomerate forming larger particles.
Overall, fluidization of solid beds is an important process used in industry, and it is important to review the theory behind the mode of fluidization to fully understand the details of this proposal. Fluidization begins with a fixed bed of small solid particles. A flow of gas is then passed upwards, through a distributor plate to avoid channeling, through the bed at a known velocity. The bed exhibits a pressure drop due to the friction between the gas and the solid particles. As the velocity of the gas increases, the solid particles expand and rearrange themselves to a configuration to offer less resistance to flow. The pressure drop across the bed continues to increase as the gas velocity increases until it reaches the minimum fluidization velocity.
The minimum fluidization velocity, Umf, is the velocity at which the pressure drop across the bed becomes great enough to support the weight of the bed, or. (1) All symbols are defined in the table of nomenclature located at the end of the text. When a bed reaches the point of minimum, or incipient, fluidization, the particles are in their loosest configuration, but the bed is still fixed. This is the point at which the particles are no longer touching and begin to exhibit behavior like a fluid. At the point of minimum fluidization, the pressure drop remains constant even as the gas velocity increases. Figure 1 shows a curve of pressure drop versus gas velocity for ideal fluidization behavior..