Fluidization fine particles

Fluidized-bed catalytic reactors. In fluidized-bed reactors, solid material in the form of fine particles is held in suspension by the upward flow of the reacting fluid. The effect of the rapid motion of the particles is good heat transfer and temperature uniformity. This prevents the formation of the hot spots that can occur with fixed-bed reactors. 

Stratification of the particles making up the bed, caused by the fluidization (fines on top), is not desirable. The soflds holding capacity of the bed is best utilized if the filtration flow encounters progressively finer sand particles. This is achieved in upflow filters where the fluidization due to backwash produces the correct stratification in the bed. Unfortunately, the filtration flow and the backwash take place in the same direction the disadvantage is that the washwater goes to the clean side of the filter. 

Transport Disengaging Height. When the drag and buoyancy forces exerted by the gas on a particle exceed the gravitational and interparticle forces at the surface of the bed, particles ate thrown into the freeboard. The ejected particles can be coarser and more numerous than the saturation carrying capacity of the gas, and some coarse particles and clusters of fines particles fall back into the bed. Some particles also coUect near the wall and fall back into the fluidized bed. 

PressurizedFIuidized-Bed Combustors. By 1983 the pressurized fluidized-bed combustor (PFBC) had been demonstrated to have capacities up to 80 MWt (49). PFBCs operate at pressures of up to 1500 kPa (220 psi) and fluidization velocities of 1—2 m/s. Compared to an AFBC of the same capacity, a PFBC is smaller, exhibits higher combustion efficiencies with less elutfiation of fine particles, and utilizes dolomite, CaCO MgCO, rather than limestone to capture SO2. 

Classification The separation of fine particles from coarse can be effected by use of a fluidized bed (see Drying ). However, for economic reasons (i.e., initial cost, power requirements for compression of fluidizing gas, etc.), it is doubtful except in special cases if a fluidized-bed classifier would be built for this purpose alone. 

Measurements in large fluidized beds of fine particles indicate that bubble coalescence often ceases within a short distance above the gas distributor plate. Indications from density measurements or single bubble velocities are that bubble velocity Ug and diameter often reach maximum stable values, which are invariant with height or fluidizing gas velocity. 

A salient feature of the fluidized bed reactor is that it operates at nearly constant temperature and is, therefore, easy to control. Also, there is no opportunity for hot spots (a condition where a small increase in the wall temperature causes the temperature in a certain region of the reactor to increase rapidly, resulting in uncontrollable reactions) to develop as in the case of the fixed bed reactor. However, the fluidized bed is not as flexible as the fixed bed in adding or removing heat. The loss of catalyst due to carryover with the gas stream from the reactor and regenerator may cause problems. In this case, particle attrition reduces their size to such an extent where they are no longer fluidized, but instead flow with the gas stream. If this occurs, cyclone separators placed in the effluent lines from the reactor and the regenerator can recover the fine particles. These cyclones remove the majority of the entrained equilibrium size catalyst particles and smaller fines. The catalyst fines are attrition products caused by... 

Powder coating is a solventless coating system that is not dependent upon a sacrificial medium such as a solvent, but is based on the performance constituents of solid TP or TS plastics. It can be a homogeneous blend of the plastic with fillers and additives in the form of a dry, fine-particle-size compound similar to flour. The three basic methods are the fluidized bed, electrostatic spray, and electrostatic fluidized bed processes (9). 

Practical operations are conducted at two or more multiples of the minimum fluidizing velocity. In reactors, the entrained material is recovered with cyclones and returned to process. In dryers, the fine particles dry most quickly so the entrained material need not be recycled. 

A schematic diagram of the experimental apparatus is shown in Fig. 1. A rotating fluidized bed composes of a plenum chamber and a porous cylindrical air distributor (ID400xD100mm) made of stainless sintered mesh with 20(xm openings [2-3]. The horizontal cylinder (air distributor) rotates around its axis of symmetry inside the plenum chamber. There is a stationary cylindrical filter (ID140xD100mm, 20(o.m openings) inside the air distributor to retain elutriated fine particle. A binary spray nozzle moimted on the metal filter sprays binder mist into the particle bed. A pulse air-jet nozzle is also placed inside the filter, which cleans up the filter surface in order to prevent clogging. 

An experimental fluidized bed reactor has a 2.5 cm in diameter and 230 cm in height, and the distributor has 32 holes and each hole was 2 mm in diameter. 200 mesh net was put on the distributor to prevent particles from falhng down. The cyclone was made by standard proportion to collect fine particles. Air flow rate was controlled by a flow meter, CO2 (99.9%) flow rate was controlled by mass flow controller and then 10% CO2 inlet concentration was maintained by mixing in a mixing chamber. CO2 outlet concentration was also measured by CO2 analyzer (CD 95, Geotechnical instruments, England). 

Certain three-dimensional electrodes, also known as slurry or fluidized-bed electrodes, are sometimes used as well in order to have a strongly enhanced working surface area. Electrodes of this type consist of fine particles of the electrode material (metal, oxide, carbon, or other) kept in suspension in the electrolyte solution by intense mixing or gas bubbling. A certain potential difference is applied to the system between an inert feeder elecnode and an auxiliary electrode that are immersed into the suspension. By charge transfer, the particles of electrode material constantly hitting the feeder electrode acquire its potential (fully or at least in part), so that a desired electrochemical reaction may occur at their surface. In this reaction, the particles lose their charge but reacquire it in subsequent encounters with the feeder electrode. 

Werther, J., and Schoessler, M., Modeling Catalytic Reactions in Bubbling Fluidized Beds of Fine Particles, Heat and Mass Transfer, (W. P. M. Van Swaay, and H. H. Afgan, eds.), Springer, Berlin (1986)... 

Leva, M., Weintrub, M., and Grummer, M., Heat Transmission Though Fluidized Beds of Fine Particles, Chem. Eng. Progr., 45(9) 563-572 (1949)... 

In operating a fluidized bed reactor such as a fluidized bed coal gasifier, fine particles tend to be elutriated from the fluidized bed. The elutriated fines, if not recovered, represent a significant carbon loss and thus a significant loss of reactor efficiency. In actual industrial practice, the fines are recycled back to the fluidized bed for further consumption. The location of the fines reinjection point into the fluidized bed reactor is important in order to maximize the consumption of fines in each pass. Otherwise, the fines will build up in the recycle loop and increase the heat load of the reactor operation. The fines reinjection location is selected to maximize the fines residence time in the bed and to provide an conducive environment for consumption, such as high temperature and an oxidizing atmosphere. 

Geldart, D., Behavior of Fine Particles in a Fluidized Bed of Coarse Solids, EPRI Report CS-2094, Electric Power Research Institute, Palo Alto, CA. (1981)... 

Morooka, S., Kawazuishi, K., and Kato, Y., Holdup and Flow Pattern of Solid Particles in Freeboard of Gas-Solid Fluidized Bed with Fine Particles, Powder Technol., 26 75 (1980)... 

Yang, W. C., and Keaims, D. L., A Study of Fine Particles Residence Time in a Jetting Fluidized Bed, Powder Technol., 53 169 (1987)... 

Tsutsumi, A., Nakmoto, S., Mineo,T., and Yoshida, K., A Novel Fluidized-Bed Coating of Fine Particles by Rapid Expansion of Supercritical Fluid Solutions, Proc. 1st Int. Particle Technol. Forum, pp. 452-455, Denver, CO (1994)... 

Kato, K., Takaradu, T., Matsuo, N., and Suto, T., Residence-Time Distribution of Fine Particles in a Powder-Particle Fluidized Bed, Int. Chem. Eng., 34 605(1994)... 

Nakagawa, N., Li, Z., Bai, D., and Kato, K., Fundamental Study of Fine Particle Residence in a Powder-Particle Fluidized Bed / Effect of Fine Powder Concentration, Preprint Fluidization VIII, 1 287 (1995)... 

Catalyst powders with carefully specified particle size distribution have been known to possess good fluidization characteristics. Generally, addition of fine particles to coarse particles tends to improve the latter s fluidization characteristics. Experiments were thus conducted on binary particle mixtures, each consisting of a fairly close particle size distribution. 

Figure 52 plots the dimensionless subsidence time for the six sets of Group A-A and Group B-A binary mixtures for different fractions of fines, Xp showing that the improvement of fluidizing characteristics by addition of fine particles increases monotonically with increasing percentage of the fines. 

---