Particle spray fluidized beds

Many reports have been made on experimental and theoretical investigations in the area of liquid sprayed-fluidized beds by Heinrich [30], who dealt comprehensively with the modeling of heat and mass transfers, as well as with particle populations in the fluidized beds. 

If sufficient calcium hydroxide is present in the spraying drop, then the diffusion of sulfur dioxide is small. In liquid-sprayed fluidized beds the drops or films on the particles are under permanent stresses, and particle-particle impacts lead to constant destruction of the film surface. Thus, no converted calcium hydroxide reaches the film surface to perform the reaction for disposal, and consequently a strongly over-stoichiometric process is not necessary. 

The average particle diameter which is growing during granulation and agglomeration has no effect on the conversion of sulfur dioxide. An increase in particle diameter leads to higher Re-numbers, and thus to an enhanced mass transfer of sulfur dioxide. However, the opposite phenomenon is that the increased mass transfer of the evaporation reduces the mass transfer surface. In addition, these experimental studies illustrate that the liquid-sprayed fluidized bed can be used as a reactive absorber. 

Panda, R.C. Zank, J. Martin, H. Modelling the droplet deposition behaviour on a single particle in fluidized bed spray granulation process. Powder Technol. 2001, 115, 51-57. 

The choice of dryer is more difficult when the drying step is also used to shape the product. Typical designs here are the spray dryer and the sprayed fluidized bed. If necessary, drying can be followed by a classification step in which undesired particles, for example, fines, are reyded to the feed stream for the dryer. When the desired product properties, for example, freedom from dust, pourabUity, rate of dissolution, and bulk density, are not attainable in the drying step, additional steps such as compaction and granulation must be used. 

Attrition-resistant microspheroidal catalyst particles for fluidized beds are manufactured by means of spray drying. The catalyst particles, for instance zeolite crystals, are suspended in an aqueous sol or hydrogel of binder particles that, after processing, serve as a mechanically strong and porous matrix. Typical binders are silica-alumina and alumina gels, clays (e.g. kaolin, bentonite), and... 

General Principles of Particle Formulation in Spray Fluidized Beds... 

Despite Tab. 7.2, powders can be the preferred product form for various applications, due, for example, to their high volume-specific surface area and the resulting high reactivity. Even ultra-fine powders can be produced in spray fluidized beds. For this purpose, one follows Fig. 7.1c. However, the coating is created with the sole purpose of its subsequent in situ destruction by particle-particle collisions. The material created by attrition is now the real product of the process, which is carried out of the fluidized bed by the gas and can be collected in a cyclone or in filter bags. The core particles are carriers for the coating and, simultaneously, the promoters of its destruction, so that they have to be heavy and rigid. Metallic carrier particles can be heated by wall contact or by induction, so that the creation of ultra-fine powder can be combined with its thermal treatment. 

The rate of agglomeration in spray fluidized beds is proportional to the frequency of collisions and the probability that colliding particles will stick together rather than... 

The previous discussion pointed out that various material properties have a more or less strong influence on particle formulation in spray fluidized beds. Process conditions, such as the spraying rate, gas temperature, or the mass flow rate of the fluidization gas, can also very significantly affect the properties of the resulting particles. This aspect will be illustrated in the present section by means of selected examples. 

Drying and liquid penetration are also important for the process already discussed in Section 7.3.3, namely spray fluidized bed agglomeration. The reason for this is that agglomeration takes place with the help of droplets sprayed on the particles, so that it slows down when such droplets are lost either by evaporation (drying) or by liquid penetration into the porous substrate. Influences of this kind can be captured very well with the help of respective micro-scale models integrated into discrete simulations, as we will see in Section 7.7. 

Each of the mentioned main constmctive elements of spray fluidized bed equipment can be used to manipulate product properties in the desired direction. For example, droplet size and spray pattern have an influence on the particle wetting and on the local liquid distribution in the fluidized bed - thus also on particle growth kinetics, the type of particle size enlargement (agglomeration in comparison to granulation and coating) and product properties (e.g., particle porosity and density, and surface morphology). 

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