Elutriated mass

A lot of attempts have been made to describe the time dependence of the attrition rate in batch fluidized bed processes. Gwyn (1969) studied the degradation of catalysts in a small-scale test apparatus and defined the elutriated particles as the only attrition product. He described the increase of the elutriated mass, Wel, with time, t, based on the initial solid bed mass, Wbed 0, by the now widely known Gwyn equation ... 

In fluidized bed experiments, most authors assume that all attrition products are elutriated. Consequently, they measure either the decrease in bed mass and use Eq. (2) (e.g.,Kono, 1981 Kokkoris etal., 1991, 1995) or the elutriated mass (e.g., Seville et al., 1992, Werther and Xi, 1993). It should be noted that all these authors used a certain particle size as a threshold below which all particles are assigned to be attrition products provided that all initial particles are clearly larger. Breakage events, which lead to particle sizes above the threshold level are, therefore, not considered. The choice of this threshold is very arbitrary and differs between the various research groups. 

Countercurrent Separation and Elutriation. The process known as elutriation in cell separation is a refined method for separation of cells having close mass densities. Cells can be separated by making use of differences in the critical velocity of cells. If the mass densities of two cells are identical, but the sizes are different, then the larger particle has a higher critical velocity than the smaller one. 

The modeling of fluidized beds remains a difficult problem since the usual assumptions made for the heat and mass transfer processes in coal combustion in stagnant air are no longer vaUd. Furthermore, the prediction of bubble behavior, generation, growth, coalescence, stabiUty, and interaction with heat exchange tubes, as well as attrition and elutriation of particles, are not well understood and much more research needs to be done. Good reviews on various aspects of fluidized-bed combustion appear in References 121 and 122 (Table 2). 

Modeling of Jet-Induced Attrition. Werther and Xi (1993) compared the jet attrition of catalysts particles under steady state conditions with a comminution process. They suggested a model which considers the efficiency of such a process by relating the surface energy created by comminution to the kinetic energy that has been spent to produce this surface area. The attrition rate, RaJ, defined as the mass of attrited and elutriated fines per unit time produced by a single jet, is described by... 

A finely ground mixture of galena and limestone in the proportion of 1 to 4 by mass, is subjected to elutriation by a current of water flowing upwards at 5 mm/s. Assuming that the size distribution for each material is the same, and is as follows, estimate the percentage of galena in the material carried away and in the material left behind. The absolute viscosity of water is 1 mN s/m2 and Stokes equation should be used. 

Carson, G. A. Lynch, J. R. "Calibration of the Vertical Elutriator Cotton Dust Sampler." Presented at the Annual Meeting, A.I.H.A., Boston, Mass. 1973. (Also personal communication to K. Q. Robert). 

The plant DN 400 has a cylindrical glass tube of 400 mm diameter. Using a pressure fan (V = 4000 m3 h-1), the main air mass flux from the environment is sucked in and heated up by two 60-kW electrical heaters. In the extension chamber DN 800 the air flow velocity is reduced such that a percentage of the elutriated dust particles fall back into the fluidized bed. Light particles are tangentially transported to the cyclone. The particles separated in the cyclone can be re-fed to the fluidized bed by the use of a fan. Along with a pressure fan, the experimental plant also possesses an exhaust radial ventilator (v = 4400 m3 h-1). 

The bed mass loss is caused by elutriated particles (dust) generated by attrition, breakage and non-deposited spray drops (overspray). The overspray depends on the position of the nozzle. 

A total mass balance is necessary for qualitative evaluation. A solid mass loss of 1.2 % by depositions of dust on apparatus wall and by elutriation is demonstrated in Tab. 16.16. 

Figure 16.48 presents a comparison of the calculation of bed mass under use of pressure loss and of the Sauter diameter. Although both progressions showed the same tendency, the constant reduction of the particles justified their elutriation and the excessive amount of particles discharged by a industrial vacuum cleaner during the experiments. The total mass balance is listed in Tab. 16.20. 

When bubbles burst at the surface of the fluidized bed, solid material carried along in their wake is ejected into the freeboard space above the bed. The solids are classified in the freeboard particles whose settling velocity ut is greater than the gas velocity fall back into the bed, whereas particles with u < u are elutriated by the gas stream. As a result, both the volume concentration of solids cy and the mass flow rate of entrained solids in the freeboard show a characteristic exponential decay... 

Here x, is the mass fraction of particle-size fraction i in the bed material and x( is the elutriation rate constant for this fraction... 

The literature contains a number of empirical correlations for estimating x [2-4] More physically based are the elutriation models of Wen and Chen [43] and of Kunu and Levenspiel [2, 44], which enable not only calculation of the exiting mass flow rate but also estimation of the concentration versus height cv(h) in the freeboard... 

Elutriation. Gas moving through an expanded or fluidised bed of particles tends to preferentially drag small particles upwards because they are fighter and have greater surface area per unit mass for drag forces to lift the particles. Elutriation may also occur if the particles have different densities. 

The above calculation is quite tedious and gets complicated by the fact that the properties which ultimately control the magnitude of these fourteen unknown quantities further depend on the physical and chemical parameters of the system such as reaction rate constants, initial size distribution of the feed, bed temperature, elutriation constants, heat and mass transfer coefficients, particle growth factors for char and limestone particles, flow rates of solid and gaseous reactants. In a complete analysis of a fluidized bed combustor with sulfur absorption by limestone, the influence of all the above parameters must be evaluated to enable us to optimize the system. In the present report we have limited the scope of our calculations by considering only the initial size of the limestone particles and the reaction rate constant for the sulfation reaction. 

Olech, R.M. Pranis, R.A. Cole, M.J. Janiszewski, J.S. Elutri-Zone Chromatography Applications in a Drug Discovery, in Proceedings of the 50th ASMS Conference on Mass Spectrometry and Allied Topics, Orlando, FL, June, 2002, Abstract No. MPF 205. 

Reduction with carbon is accomplished by heating dry W03 with a slight excess of carbon in closed crucibles at a temperature of 1400" (. At the end of the operation the metallic, powder contains excess carhon and some anrc.dnc.eil oxide. To remove these the mass is elutriated with water, and tho material so removed used in the next charge. A purity of OH per cent may Ik realized by this method, but the product always contains carhon as an impurity, which readers the metal ton brittle for mechanical working. It is, however, suitable for use in making alloy steel and is cheaper tlian the metal obtained by reduction in hydrogen. 

Collection of respirable dust is usually accomplished using a two-stage sampler The first stage, which is often a cyclone or an elutriator, removes the high mass nonrespirable particles and the second stage, a high efficiency filter collects all the particles which penetrate the first stage. 

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