Particle size distribution changes

Consider how the particle-size distribution changes with time for source-limited kinetics. Suppose that at t = ten the particle-size distribution is given by f(R,t — to) = AR(Rmax - A), where Aroax is the maximum radius of any particle. Calculate the rate at which /(A, t = to) is increasing at R = Amax. At what particle size is the value of /(r, t = to) constant ... 

Besides the growth in the number concentration, also the particle size distribution changes during the smoke episodes. In Kuopio 2010, the diameter of the aerosol particles was twice as large during the smoke event as it was in the background conditions [33], In normal conditions the peak of the size distribution was at 55 nm but during the smoke the peak shifted to 186 nm. The increase in the... 

Other important physical measurements are bulk densities used to estimate hopper contents and circulation factors, and particle size analysis. The correct distribution of fine particles (30 - 180 microns) is essential to proper fluidization and transfer within the FCC unit. Generally, particles less than 30 microns are lost to the atmosphere or fines recovery system and are destined for a landfill. If the catalyst is too coarse, it may not circulate through the unit, necessitating a shutdown. Both problems are costly to the refiner and must be avoided. In addition, observation of particle size distribution changes at various points within the unit can pinpoint equipment malfunctions that might otherwise go undetected. 

Collection efficiency unreliable if gas property or particle size distribution changes. 

Particle Size Distribution Changes. The number frequency distribution of particles for the base and end latexes are plotted in Figure 3. No particles in the end latex formed by agglomeration had a diameter larger than 9600 A. Furthermore, particles of 1300-9600 A constituted only about 1.2% of the total number of particles. The small particle size (200-400 A) that had constituted more than 5% in the base latex disappeared completely. The peak of the distribution curve shifted from about 380 A to 700 A. 

FIGURE 3.8. Particle size distribution change for a Hombikat UV-100 After sonication (—) and after one hour mixing in the reactor (—). (Reprinted with permission of M. SaUces, PhD Dissertation, Univereity of Western Ontario 2002). 

Silver particles deposited on a-Al203 (unlike those on Si02) are not stable at elevated temperatures. Thus, at the calcination step, the particle size distribution changes and becomes polydisperse (a typical range is 50 -1000 A). 

Only one additional stipulation needs to be made before adapting the results that follow from Eq. (5.24) to addition polymers. The mode of termination must be specified to occur by disproportionation to use the results of Sec. 5.4 in this chapter, since termination by combination obviously changes the particle size distribution. We shall return to the case of termination by combination presently. 

Fluidized-bed reaction systems are not normally shut down for changing catalyst. Fresh catalyst is periodically added to manage catalyst activity and particle size distribution. The ALMA process includes faciUties for adding back both catalyst fines and fresh catalyst to the reactor. 

Another technique is to change the particle size distribution. There are, however, disadvantages. If segregation is occurring by the sifting mechanism, the particles must be almost identical in size before sifting is prevented. Alternatively, the mean particle size can be reduced below 100 p.m, but this size reduction (qv) increases the probabiUty of segregation by the too fine powder mechanisms. 

Zinc dust is smaller in particle size and spherical in shape, whereas zinc powder is coarser in size and irregular in shape. The particle size of zinc dust, important in some appHcations, is controUed by adjusting the rate of condensation. Rapid cooling produces fine dust, slower condensation coarse dust. In the case of zinc powder, changes in the atomization parameters can be employed to change particle size to some degree. The particle size distributions for commercial zinc powders range from 44 to 841 p.m (325—20 mesh). The purity of zinc powders is 98—99.6%. 

Aerosol Dynamics. Inclusion of a description of aerosol dynamics within air quaUty models is of primary importance because of the health effects associated with fine particles in the atmosphere, visibiUty deterioration, and the acid deposition problem. Aerosol dynamics differ markedly from gaseous pollutant dynamics in that particles come in a continuous distribution of sizes and can coagulate, evaporate, grow in size by condensation, be formed by nucleation, or be deposited by sedimentation. Furthermore, the species mass concentration alone does not fliUy characterize the aerosol. The particle size distribution, which changes as a function of time, and size-dependent composition determine the fate of particulate air pollutants and their... 

Changes in particle-size distribution affect the pore distribution of the powder. Large pores between particles enhance the rate of binder penetration, whereas they decrease the final extent. In addition, the particle-size distribution affects the ability of the particles to pack within the drop as well as the final degree of saturation [Waldie, Chem. Engin. ScL, 46,2781 (1991)]. 

Figure 6.8 The change in particle size distribution which is brought about by Ostwald ripening of an initial Gaussian distribution of particle size...

Once the unit is running well, it is often assumed that the aeration system is sized properly, but changes in the catalyst physical properties and/or catalyst circulation rate may require a different purge rate. It should be noted that aeration rate is directly proportional to catalyst circulation rate. Trends of the E-cat properties can indicate changes in the particle size distribution, which may require changes in the aeration rate. Restriction orifices could be oversized, undersized, or plugged with catalyst, resulting in over-aeration, under-aeration, or no aeration. All these phenomena cause low pressure buildup and low slide valve differential. 

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