Elutriation of fines

Leva, M., Elutriation of fines from fluidized systems, Chem. Engrg. Prog., 47 39-45(1951)... 

Consider the reactor of Fig. 26.1(constant flow rates of both solids and gas into and out of the reactor. With the assumption of uniform gas concentration and mixed flow of solids, this model represents a fluidized-bed reactor in which there is no elutriation of fine particles. 

Baeyens, J., Geldart, D., and Wu, S. Y. Elutriation of Fines from Gas Fluidized Beds of Geldart A-Type Powders—Effect of Adding Superfines. Powder Technol. 71 (1992) 71-80. 

In addition to Ni catalysts, Lee and Park explored some unconventional catalysts, such as limestone, dolomite, and iron ore, in a fluidized bed reactor to carry out SR of kerosene and bunker oil. H2 yields from SR of bunker oil over various catalysts (temperature = 800°C, bed height = 10 cm, superficial gas velocity = 20 cm/sec, and S/C = 1.6) were sand (20%), iron ore (29%), commercial Ni catalyst (89%), limestone (93%), and dolomite (76%). Limestone as a SR catalyst looked very promising, but H2 yields over a limestone catalyst decreased over time due to elutriation of fines during the reaction. A fluidized-bed reactor was advantageous for reforming of higher hydrocarbons, due to its ability to replace coked catalyst with fresh catalyst during operation. 

With carbon loading determined, one is in a position to evaluate the carbon loss from the bed. The three principal sources are carbon withdrawn from the bed with spent solids, elutriation of carbon that has burned down to an elutriable size range, and the elutriation of fines produced by attrition. 

D 1,000 coarse e.g., wheat can be spouted mix poorly when fluidized appreciable particle attrition rapid elutriation of fines relatively sticky materials can be fluidized bubbles cloudless slow bubbles bubble velocity less than interstitial gas velocity... 

In designing an FBD for solids drying, it is important to take note about the occurrence of entrainment of fine particles, especially if the solids are polydispersed (i.e., has wide particle size distribution). The gas exit should be placed at a height above the TDH to minimize elutriation of fines. 

Drying air temperature and flow rate are normally fixed at a constant value. However, by adjusting the airflow rate and its temperature, it is possible to save energy and reduce attrition. Mechanical assistance such as agitation or vibration is normally applied for processing materials that are difficult to fluidize. Figure 8.6 shows a typical batch FBD with expanded freeboard and built-in internal bag fillers. Expanded freeboard is used to reduce elutriation of fine particles. 

The quartz made fluidized-bed reactor used for catalyst preparation was electrically heated a cyclone was incorporated into the freeboard of the reactor to prevent elutriation of fines. The support, which was fluidized by N2 was first thermally pretreated to remove physisorbed water. Then the vaporized metal acetyl acetonate was adsorbed on the support at 400 K at constant partial pressure in a flow of N2 for a ven period of time. The subsequent decomposition of the adsorbate was carried out by increasing the fluidized-bed temperature with a rate of 4 K/min. Pt was deposited in either N2 or air at 573 K Cr203 and V2O5 were deposited in air at 673 K. Pt catalysts which were decomposed in N2 were additionally treated in air at 573 K (Pt/Si02 catalysts) respectively at 773 K (Pt/Al203 catalysts) [4, 5]. 

Bachovchin DM, Beer JM, Sarofim AF. An investigation into the steady-state elutriation of fines from a fluidized bed. AIChE Symp. Ser. 77 76-85, 1981. 

Baeyens J, Geldart D, Wu SY. Elutriation of fines from gas fluidized bed of Geldart A-type powders—effect of adding superfines. Powder Technology 71 71-80, 1992. 

Santana D, Rodriguez JM, Macias-Machin. A. Modelling fluidized bed elutriation of fine particles. Powder Technology 106 110-118, 1999. 

Tanaka I, Shinohara H, Hirose H, Tanaka Y. Elutriation of fines from fluidized bed. J. Chem. Eng. Japan 5 57-62, 1972. 

Pemberton ST, Davidson JF. Elutriation of fine particles from bubbling fluidized beds. In Fluidization (D Kunii, R Toei, eds.). Engineering Foundation, New York, 1983 pp 275-282. 

Elutriation is the selective removal of fines by entrainment from a bed of particles with a distribution in particle size. Some work has been published on predicting rates of elutriation. The rate of elutriation of fines with particle diameter, dp, is considered to be proportional to the fraction of the bed consisting of fines with diameter d so that... 

It is common practice in the air elutriation of fine powders to add what is known as a dea lomerating j ent or a flow agent to improve the flow characteristics of the powder. A frequendy used flow agent is colloidal silica. The exact mechanisms by which the improvement of flow is achieved by the addition of these very fine powders is not fully under stood [10] In their discussion of the performance of the elutriator shown in Figure 5.4, Leschonski and Rumpf state that the equipment should not be used to fractionate powders smaller than 10 microns [10]. 

Ui. This range has been plotted as a function of particle size by Richardson (1971) for uniform spheres with mf = 0.4 (Eq. 11.5) as shown in Figure 11.4. It is seen that for large particles (> 1 mm), the allowable range of U is only about 10 Umf- For beds of mixed particle sizes, U is often larger than Ut for the smallest particles, so there is continuous elutriation and a cyclone recycle system is used for particle recycle. The rate of elutriation of fines (Kunii and Levenspiel 1969) at unsteady state is often expressed as ... 

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