Agglomerates particle size

Both aggregation inefficiency (Adler, 1981) and particle disruption (Hartel and Randolph, 1986) increase with particle size. These dispersive processes can counteract the positive effect of aggregation thereby imposing agglomerate particle size limitations and may give rise to apparent size-independence. 

The particle tenninal velocity Utp) can be estimated based on agglomerate particle size (da) and assuming a spherical shape. U,p in the range 2.0 10 to 6.0 lO cm min can calculated for d in the range 0.5 to 2.5 /rm. These values of t/,, represent about 1/300 of the average fluid velocity in the reactor for a flow rate of 1 L min. In conclusion, the flow direction effect on the catalyst distribution is not an important factor under the operating conditions used. 

In addition to these three parameters, the average agglomerate particle size - for the sake of simplicity also known as particle size - and also the particle-size distribution are important. 

Some methods that are useful in measuring agglomerate particle size or changes in agglomeration with time and process history, and tiKthods tiiat can be used for concoitrated dispersions are discussed in Sectitms 1Z4 and 1Z6. 

The choice of chlorinating agent effect on agglomerate particle size distribution can be explained qualitatively rather easily if we consider the reactions taking place during the precipitation quench ... 

One way to do this is to go through the dispersion milling procedure and then analyze the particles in the dispersed state. Another way to accomplish the same end is to use an ultrasonic dispersion technique such as the one described by Shanefield and then determine the de-agglomerated particle size distribution. As described by Shanefield, the particle size thus determined is actually an aggregate size, since... 

Figure 28.9 summarizes the capabilities of the various gas-phase synthesis methods that have been described. It is evident that all materials can be prepared by means of gas-phase synthesis in a nanocrystalline microstructure. For each case, it is necessary to determine whieh teehnique is most appropriate in terms of cleanliness of the powder surfaces, degree of agglomeration, particle size and distribution, phases, and quantities. In addition to single-phase materials, some of the techniques are also capable of synthesizing metal/metal, metal/ceramic, and ceramic/ceramic composites, as well as coated nanoparticles, potentially leading to interesting applications in the near future. 

Agglomerated fine particles such as catalysts and precipitates may exhibit internal surface areas orders of magnitude greater than that available on the exterior surface. Again, if agglomerate particle size is used as the characteristic dimension, the calculated area is that of the external surface. If the internal surface area is to be calculated then additional knowledge of the constituent crystal size is required, e.g. via microscopy or adsorption. 

---