Fluidized-bed operation

Circulating fluidized beds (CFBs) are high velocity fluidized beds operating well above the terminal velocity of all the particles or clusters of particles. A very large cyclone and seal leg return system are needed to recycle sohds in order to maintain a bed inventory. There is a gradual transition from turbulent fluidization to a truly circulating, or fast-fluidized bed, as the gas velocity is increased (Fig. 6), and the exact transition point is rather arbitrary. The sohds are returned to the bed through a conduit called a standpipe. The return of the sohds can be controUed by either a mechanical or a nonmechanical valve. 

The adsorption process is normally performed in a column. The column is run as either a packed- or fluidized-bed operation. The adsorbent, after it has reached the end of its useful life, can either be discarded or regenerated. For further information, the reader is directed to the literature. ... 

The primary advantage of fluidized bed reactors, however, is that they permit continuous, automatically controlled operations using reactant-catalyst systems that require catalyst regeneration at very frequent intervals. Fluidized bed operation permits one to easily add or remove the catalyst from the reactor or the regenerator. Regeneration can be accomplished by any convenient procedure, but the... 

Still another advantage of fluidized bed operation is that it leads to more efficient contacting of gas and solid than many competitive reactor designs. Because the catalyst particles employed in fluidized beds have very small dimensions, one is much less likely to encounter mass transfer limitations on reaction rates in these systems than in fixed bed systems. 

In 2004, a 50 kg coal per day continuous run facility was built to confirm reactor (fluidized bed) operating condition, feeding method, product gas composition, sorbent recycle, etc.18... 

Coalcon A coal gasification process using a fluidized bed operated with hydrogen. Developed by Union Carbide Corporation and the Chemical Construction Company, based on work on liquid-phase hydrogenation completed by Union Carbide in the 1950s. A 20-ton per day pilot plant was operated in the 1960s, but a planned larger demonstration plant was abandoned because of cost. 

Combifining A petroleum refining process which removes asphaltenes, sulfur, and metals from residues, before further treatment. The catalyst is an activated petroleum coke in a fluidized bed, operated under hydrogen pressure at 380 to 420°C. 

Fior Also called Esso Fior. A direct reduction ironmaking process, using natural gas as the reductant, in a fluidized bed. Operated in Venezuela since 1976. Licensed by Davy Corporation. See DR. 

SNAM (2) An ammoxidation process for converting propylene to acrylonitrile. The catalyst is based on molybdenum/vanadium or bismuth, operated in a fluidized bed. Operated in Europe in 1968. 

The fluidized-bed process for this reaction has several advantages over a fixed-bed process. First, the process is highly exothermic, and the selectivity to C3H3N is temperature dependent. The improved temperature control of the fluidized-bed operation enhances the selectivity to acrylonitrile, and substantially extends the life of the catalyst, which readily sinters at temperatures in excess of 800 K. Furthermore, since both the reactants and products are flammable in air, the use of a fluidized bed enables the moving particles to act to quench flames, preventing combustion and ensuring safe operation. 

Circulating Beds These fluidized beds operate at higher velocities, and virtually all the solids are elutriated from the furnace. The majority of the elutriated sohds, still at combustion temperature, are captured by reverse-flow cyclone(s) and recirculated to the foot of the combustor. The foot of the combustor is a potentially very erosive region, as it contains large particles not elutriated from the bed, and they are being fluidized at high velocity. Consequently the lower reaches of the combustor do not contain heat-transfer tubes and the water walls are protected with refractory. Some combustors have... 

In any fluidized bed operating with a gas velocity above M /Some particles will be transported into the gas stream above the bed surface this is called entrainment. Some of these particles will be transported sufficiently so as to leave the fluidized bed column entirely and this mass of solids is then referred to as carry-over. However, the phenomenon is a little more complex. Fractionation or preferential separation of the bed particles occurs and this changes with height above the bed... 

Consider a fluidized bed operated at an elevated temperature, e.g. 800°C, and under atmospheric pressure with ah. The scale model is to be operated with air at ambient temperature and pressure. The fluid density and viscosity will be significantly different for these two conditions, e.g. the gas density of the cold bed is 3.5 times the density of the hot bed. In order to maintain a constant ratio of particle-to-fluid density, the density of the solid particles in the cold bed must be 3.5 times that in the hot bed. As long as the solid density is set, the Archimedes number and the Froude number are used to determine the particle diameter and the superficial velocity of the model, respectively. It is important to note at this point that the rale of similarity requires the two beds to be geometrically similar in construction with identical normalized size distributions and sphericity. It is easy to prove that the length scales (Z, D) of the ambient temperature model are much lower than those in the hot bed. Thus, an ambient bed of modest size can simulate a rather large hot bed under atmospheric pressure. 

The BASF process1026 was developed to use o-xylene exclusively. Most processes, however, such as the Wacker-von Hey den process1027 1028 are capable of operating on both feedstocks or may use a mixture of the two compounds. Fluidized-bed operations were developed in the 1960s (Badger process1029 1030) but have been replaced by improved, more economical fixed-bed processes. The Alusuisse low-air-ratio (LAR) process, for instance, allows the use of a 9.5 1 air o-xylene mixture and achieves an increased catalyst productivity.1031 1032... 

Globulation is the formation of droplets of solution, slurry, or melt followed by solidification by prilling, spray drying, or fluidized bed operation. Control of particle size is best achieved in fluidized beds. 

The overall removal process would consist of two stages the absorption stage, in which the sorbent is converted to the sulfide form by its reaction with the H2S in the feed gas, and the regeneration stage, wherein the sulfide is converted to an oxide. It thus represents a cyclic process. Fixed-bed, moving-bed and fluidized-bed operations have been considered for the purpose. Kinetic data are necessary for choosing a proper reactor type and for the design... 

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