Elutriation, attrition and

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). 

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. 

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... 

Gas velocity Increases attrition and elutriation rates (major effect) Decreases coalescence for inertial growth Has no effect on coalescence for noninertial growth, unless altering bed moisture through drying Increases granule consolidation and density... 

The fluidized bed process performance significantly decreased after 135 hours of uninterrupted operation. This low conversion to hydrogen was most likely due to the catalyst losses by attrition and elutriation from the reactor, though some deactivation effects cannot be excluded at this time. 

Size distributions of solids (coal and limestone) in the feed and in the bed shoiald be considered in the evaluation of attrition and elutriation loss. Standard correlations for elutriation rate constants have been found to be inadequate for the calcijlation of solids elutriation. Data obtained from large pilot-scale FBC show large disagreement from those calciilated based on existing elutriation rate correlations. Recently, correlations for attrition and elutriation of bed particles have been proposed by Merrick and Highley (106). For larger particles, this correlation underestimates the carbon loss. 

Any non-conversed reactant of the suspension and the reaction product are deposited on fluidized-bed material after the drying process, and can be removed from the bed material by attrition. The dust due to the attrition is elutriated with the gas flow from the fluidized bed. 

In the case of fluidized bed gasifiers, fine powders are formed by erosion of the mineral bed inventory due to limitations on its resistance to attrition and ash and char formation in the biomass conversion process they are elutriated by the gas leaving the reactor. 

For application in fluidization and fluid-particle systems, the attrition index is probably the most important particle characteristic. The particle attrition can affect the entrainment and elutriation from a fluidized bed and thus subsequently dictate the design of downstream equipment. The attrition in a pneumatic transport line can change the particle size distribution of the feed material into a fluidized bed reactor and thus alter the reaction kinetics. Davuluri and Knowlton (1998) have developed standardized procedures to evaluate the Attrition Index employing two techniques, solids impaction on a plate and the Davison jet cup. The two test units used are shown in Figs. 6 and 7. They found that these two test techniques are versatile enough to be applicable for a wide range of materials, such as plastic, alumina, and lime-... 

It should be noted here that the quantitative results obtained in a Gwyn-type attrition apparatus will in general depend not only on the cut size of the gravity separator but also on the entrainment and elutriation conditions in the main column. Werther and Xi (1993) compared, for example, attrition test results of the same catalyst obtained from three differently sized Gwyn-type units, one with column A having 50 mm ID and 500 mm height, one with column B having... 

CO2 adsorption capacities with dry sorbents before and after attrition were shown in Fig.3. We found variation of CO2 adsorption capacity during operation by examining effect of attrition on adsorption capacity. So, adsorption experiments for each sorbent fluidized for 30hours were carried out. As a result, percentage losses of adsorption capacity of molecular sieve 5A and molecular 13X were 14.5% and 13.5%, but those of activated carbon and activated alumina were 8.3% and 8.1% respectively. This is because retention time of molecular sieve 5A and molecular 13X decreased due to elutriation of particle generated from attrition. 

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. 

Consequently, it is very difficult to evaluate the cyclone attrition rate from the measured elutriation rate. In order to study the cyclone attrition mechanism in detail it is necessary to study the cyclone in isolation. This can be achieved by feeding a cyclone batch-wise and directly without any additional equipment that could contribute to attrition. 

Moreover, not all attrition products will be directly elutriated from the cyclone. Instead, a part will be collected by strands of the material and will be transported via the solids return line into the fluidized bed process. In subsequent passes through the cyclone, the accumulated attrition-produced fines will be elutriated due to the sifting effect of the cyclone. 

Of the various mechanical properties of a formed catalyst containing zeolite, attrition resistance is probably the most critical. This is particularly the case for FCC catalysts because of the impact on the addihon rate of fresh catalyst, particulate emissions of fines and overall catalyst flow in the reactor and regenerator. Most attrition methods are a relative determination by means of air jet attrition with samples in the 10 to 180 xm size range. For example the ASTM D5757 method attrites a humidified sample of powder with three high velocity jets of humidified air. The fines are continuously removed from the attrition zone by elucidation into a fines collection assembly. The relative attrition index is calculated from the elutriated fines removed at a specific time interval. 

Elutriation is important in most industrial fluidized beds and is generally thought of as a disadvantage. In addition to the small particles which may be present in the initial particle size distribution, fines may be created in the course of operation by the attrition of bed particles. Elutriated particles usually need to be collected and recovered either because they represent the loss of product particles of a given size, because they must be separated from the exhaust gas for environmental reasons, or because of safety concerns there is a considerable risk of a dust explosion with very fine particles and perhaps especially so with many food particulates. Therefore the fluidized bed plant will require ancillary gas cleaning equipment such as a cyclone, filter or electrostatic precipitator to separate the fines from the gas. The loss of a particular size fraction from the bed may change fluidized bed behaviour and it then becomes important to return the fines to the bed continuously. 

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