Homogeneous fluidization

Heterogeneous fluidization Homogeneous fluidization Incipient fluidization Lean phase Minimum fluidization... 

Particulate and Aggregative Fluidization. When the fluid phase is liquid, the difference in the densities of fluid and solid is not very large, and the particle size is small, the bed is fluidized homogeneously with an apparent uniform bed structure as the fluid velocity exceeds the minimum fluidization velocity. The fluid passes through the interstitial spaces between the fluidizing particles without forming solids-free bubbles or voids. This state of fluidization characterizes particulate fluidization. In particulate fluidization, the bed voidage can be related to the superficial fluid velocity by the Richardson-Zaki equation. The particulate fluidization occurs when the Froude number at the minimum fluidization is less than 0.13 [9],... 

Liquid with catalytic solid Particulate fluidization homogeneous with no bubbles. For highly exothermic reactions, consider external recircnilation through a heat exchanger. 

The far left-hand curve of Figure 9.2 shows S to be always positive, so that there is no solution for the stability limit, S = 0 this represents a system that always fluidizes homogeneously. Increasing the particle diameter tends to shift the curve downwards the particle diameter dp which causes the curve to just touch the e-axis (second curve from the left) fixes a point on the homogeneous-transitional boundary of the stability map, as shown in Figure 9.1 the full boundary is obtained by finding this critical particle diameter for a range of values of particle density. 

As is the case for emb> the parameter Aa may be readily evaluated for any defined system. Because it reflects the amplitude growth-rate gradient of perturbations at the critical void fraction that separates stable from unstable fluidization, it provides a direct measure both of the extent of instability for unstable systems and the robustness of the stability manifested by those stable systems for which a minimum bubbling point exists. Systems that fluidize homogeneously for all void fractions cannot be... 

The flow required to maintain a complete homogeneous bed of sohds in which coarse or heavy particles will not segregate from the fluidized... 

Homogeneous reactions. Homogeneous noncatalytic reactions are normally carried out in a fluidized bed to achieve mixing of the gases and temperature control. The sohds of the bed act as a heat sink or source and facihtate heat transfer from or to the gas or from or to heat-exchange surfaces. Reaclious of this type include chlorination of hydrocarbons or oxidation of gaseous fuels. 

The profiles of temperature and composition shown in Fig. 23-3 are not of homogeneous liqmd reactions, but are perhaps representative of all lands of reactions. Only in stirred tanks and some fluidized beds are nearly isothermal conditions practically attainable. 

Fluidization may be described as incipient buoyancy because the particles are still so close as to have essentially no mobility, whereas the usual desire in fluidization is to create bed homogeneity. Such homogeneity can be achieved only by violent mixing. This is brought about by increasing the fluid velocity to the point of blowing "bubbles" or voids into the bed, which mix the bed as they rise. The increased fluid velocity at which bubbles form first is referred to as the incipient (or minimum) bubbling velocity. 

Applicability/Limitations Fluidized beds require frequent attention for maintenance and cleaning purposes. This treatment is ideal for slurries and sludges but not for bulky or viscous wastes. The waste particles should be of a certain size and be homogeneous. Wastes must have a low sodium content and a low heavy metal content. Some refractory wastes may not be fully destroyed since these units operate at low combustion temperatures (750 to 1000°C). 

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). 

Metallocenes are homogeneous catalysts that are often soluble in organic solvents. Therefore, polymerization can occur via a solution process with a non-polar diluent dissolving the propylene gas, the catalyst, and the co-catalyst system. They can also be adsorbed onto an inert substrate which acts as part of the fluidized bed for gas phase polymerization processes. 

The physical situation in a fluidized bed reactor is obviously too complicated to be modeled by an ideal plug flow reactor or an ideal stirred tank reactor although, under certain conditions, either of these ideal models may provide a fair representation of the behavior of a fluidized bed reactor. In other cases, the behavior of the system can be characterized as plug flow modified by longitudinal dispersion, and the unidimensional pseudo homogeneous model (Section 12.7.2.1) can be employed to describe the fluidized bed reactor. As an alternative, a cascade of CSTR s (Section 11.1.3.2) may be used to model the fluidized bed reactor. Unfortunately, none of these models provides an adequate representation of reaction behavior in fluidized beds, particularly when there is appreciable bubble formation within the bed. This situation arises mainly because a knowledge of the residence time distribution of the gas in the bed is insuf-... 

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