Particle mixing

Particle mixing is important in processes where the ability of fluidized beds to add and remove particles on line is utilized to send spent (deactivated) catalyst to a separate regenerator column and to replace them by fresh and reactivated catalyst particles. The characteristic mixing time (often called turnover time for fluidized beds) is typically of order 20-200 s. If the mean residence time of the catalyst particles, r, is much greater than this, the particles can be considered to be perfectly mixed. The exit age (or residence time) distribution of the particles is then given by 

When there are broad particle size distributions, entrainment (see Section 4.3.5) of fines from the reactor (called elutriation ) may be contribute significantly to broadening of the catalyst particle residence time distribution. In these cases, it may be 

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Fig. 2. Fuel for high temperature gas-cooled reactor. Fissile material is coated with carbon and siHcon carbide, fertile material with carbon. Particles mixed...

For group B and D particles, nearly all the excess gas velocity (U — U,nj) flows as bubbles tnrough the bed. The flow of bubbles controls particle mixing, attrition, and elutriation. Therefore, ehitriation and attrition rates are proportional to excess gas velocity. Readers should refer to Sec. 17 for important information and correlations on Gel-dart s powder classification, minimum fluidization velocity, bubble growth and bed expansion, and elutriation. 

Colloidal suspensions are, per definition, mixtures of mesoscopic particles and atomic liquids. What happens if there are several different species of particles mixed in the solvent One can invent several different sorts of mixtures small and large particles, differently charged ones, short and long rods, spheres and rods, and many more. Let us look into the literature. One important question when dealing with systems with several components is whether the species can be mixed or whether there exists a miscibility gap where the components macroscopically phase-separate. 

To account for the difference in the dispersion characteristics of the classified, stable fluidised bed and the conventional, well-mixed fluidised bed, the term expanded bed has been used by several authors and the leading manufacture of chromatography media and equipment.38 In the work presented here, the term fluidised bed will be used synonymously with expanded bed to refer to adsorbents fluidised under conditions that seek to minimise particle mixing. 

Alternatives to compounding in the melt are solution mixing or powder blending of solid particles. Mixing with the aid of solvents can be performed at lower temperatures with minimal shear. However, difficulties in removal of the solvent results in plasticization of tJie polymer matrix and altered erosion/drug release performance in addition to residual solvent toxicity concerns. Powder blending at room temperature minimizes thermal/shear stresses, but achieving intimate mixtures is difficult. 

So far, some researchers have analyzed particle fluidization behaviors in a RFB, however, they have not well studied yet, since particle fluidization behaviors are very complicated. In this study, fundamental particle fluidization behaviors of Geldart s group B particle in a RFB were numerically analyzed by using a Discrete Element Method (DEM)- Computational Fluid Dynamics (CFD) coupling model [3]. First of all, visualization of particle fluidization behaviors in a RFB was conducted. Relationship between bed pressure drop and gas velocity was also investigated by the numerical simulation. In addition, fluctuations of bed pressure drop and particle mixing behaviors of radial direction were numerically analyzed. 

Fig. 7 shows temporal changes in the degree of particle mixing, M, at various centrifugal accelerations. The degree of particle mixing, M, is defined as the following equation ... 

Characteristics of particle mixing and fluidization in a fluidized bed ash cooler... 

Yang, W. C., Ettehadieh, B., and Haldipur, G. B., Solids Circulation Pattern and Particles Mixing in a Large Jetting Fluidized Bed, AIChE J., 32 1994 (1986)... 

Bokkers, G. A., Van Sint Annaland, M., and Kuipers, J. A. M., Comparison of continuum models using kinetic theory of granular flow with discrete particle models and experiments extent of particle mixing induced by bubbles. Proceedings of Fluidization XI, May 9-14, 2004, 187-194, Naples, Italy (2004). 

In figure 6 are presented some of the available 210Pb profiles in deep sea sediments. The data available to-date yield values ranging between (20-400) cm2/103 year for particle mixing coefficients, K [69]. The data also show that the mixing coefficients do not exhibit any systematic trend either with the sediment type or with sedimentation rate [69]. 

Figure 6. The 2I0Pb excess profiles in two deep-sea sediment cores (data from Refs. 68 and 69). The data yield particle mixing coefficients in the range of 100...

Once these nuclides deposit on the ocean floor they are likely to be subjected to particle mixing processes. In the following we discuss attenuation due to a simple case of mixing, in which the sedimentary particles are mixed to a constant depth, L, from the sediment-water interface [72,73]. For such a case the temporal variation in the standing crop (atoms/cm2) C, of the radionuclide in the mixed layer is given by ... 

Formulating an hypothesis The clogging of the centrifuge is due to iron particles mixed with coconut oil, soaps, and other normal ingredients of crude coconut oil. 

Figure 5.15. Two examples of age-based micromixing models. In the top example, it is assumed that fluid particles remain segregated until the latest possible age. In the bottom example, the fluid particles mix at the earliest possible age. Numerous intermediate mixing schemes are possible, which would result in different predictions for micromixing-sensitive reactions. 

AL-DlBOUNI, M. R. and Garside, J. Trans. Inst. Chem. Eng. 57 (1979) 94. Particle mixing and classification in liquid fluidised beds. 

Nienow, A.W. (1992) The Suspension of Solid Particles. Mixing in the Process Industries, 2nd edn, Butterworth-Heinemann, Oxford, pp. 364-411. 

Although this expression may not accurately predict circulation time, and in any case particles do not follow a simple predetermined circuit around the bed, it serves to illustrate the significance of the excess gas velocity in determining particle mixing rates. The excess gas flow rate, proportional to the excess gas velocity, is essentially the bubble flow rate. A greater bubble flow generates more bubbles and therefore... 

Figure 1.8 Mechanism of particle mixing in a fluidized bed. From Smith, P.G., Introduction to food process engineering, Kluwer, 2003, figure 13.15. With kind permission of Springer Science and Business Media.

It must be emphasised that particle mixing remains an essential consideration in the use of fluidization in other operations such as drying and granulation. The mixing in a fluidized bed of particles of... 

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