Attrition mechanisms

Attrition cannot normally be directly investigated in the large-scale process. It is, for example, impossible to analyze the whole bulk of material and it is nearly impossible or at least very expensive to vary parameters in a running industrial process. For that reason, attrition has to be investigated in small-scale experiments. The results of these experiments require a model or at least an idea of the governing attrition mechanisms to be applied to the large-scale process. In principle, there are two different philosophies of attrition modeling. 

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. 

More recently, Reppenhagen and Werther (1997) have conducted a comprehensive study ofthe attrition mechanism in a cyclone. Their experimental set-up is shown in Fig. 16. The 0.09 m ID cyclone is operated in the suction mode. The solids (spent FCC, surface mean diameter 105 microns) are introduced via a vibrating feeder into the cyclone inlet tube which allows an independent variation of cyclone inlet velocity Ue and solids loading ll... 

As has been demonstrated above, there are several parts of the fluidized bed system with distinctly different attrition mechanisms. They all... 

Identification of material specific attrition mechanisms for polymers in dilute phase pneumatic conveying... 

In the present paper, the approach chosen at our institute to study the phenomenon of attrition is briefly presented. In the results section, the link between bulk solids material properties and attrition results is discussed. The findings document the suitability of our approach for identifying material specific attrition mechanisms. 

The crucial point is to simulate the previously identified basic stress modes impact and friction under well defined stress conditions. This way, material properties can be related to attrition caused by these stress modes and the respective attrition mechanisms in effect. For this purpose it was chosen to perform single particle experiments in simple experimental setups to realize the defined stress conditions. Details on these setups are given in the next section. 

These findings clearly show the significance of the material function, since the material reaction to similar stress conditions in the pipe bend can vary greatly. In contrast to PP, PMMA and PS apparently are attrited by the same mechanisms in effect under normal impact stress conditions. This also clarifies that it is only possible to deduce the stress mode from the process function but that without knowledge of the material function no conclusions concerning the governing attrition mechanisms can be drawn. 

The results presented show that three levels have to be distinguished when investigating attrition processes. The first one is the stress mode as derived from the process function which is essential to know if the attrition process is to be simulated successfully in a simple experimental setup. The second point is the material reaction to this stress mode, i.e. the material function which varies depending on material properties like storage and loss modulus as measured by DMA. Finally, the microscopic attrition mechanisms (see [18] for impact and [19,20] for sliding friction) describing the formation of attrition on a microscopic scale constitute the bottom level. 

Nevertheless, the presented results indicate that if it is carefully distinguished between the process function (stress mode), the material function (material specific attrition mechanisms) and microscopic attrition mechanisms (attrition formation on the microscale) in the analysis of attrition processes, significant progress in the understanding of the complex phenomenon of attrition can be made. 

The three attrition mechanisms are in turn governed by different failure modes brittle, semibrittle, and ductile (Ghadiri, 1997). Brittle failure occurs when internal or surface cracks already exist and is dominant at low elastic deformation at the powder contact surface (Shipway and Hutchings, 1993). Semibrittle failure, at limited plastic deformation, is responsible for flaw initiation and occurs when the impact forces surpass the yield point. In fact, median and radial cracks cause particle fragmentation, and lateral cracks cause chipping. Soft materials are usually ductile and the mechanisms for particulate solids under ductile mode have not yet been elucidated (Ghadiri, 1997). 

Particle size analysis is useful for assessing attrition because both fragmentation and fine formation yield separate particle populations with different sizes. Production of midsize particles by means of shattering will lower the particle population s mean size and increase its size spread, as formation of fines through surface erosion will make the overall size distribution bimodal or multimodal. Barletta et al. (1993b) summarizes the different size distribution patterns in attrition resulting from the predominant attrition mechanisms and reviews the different models that fit these distributions. 

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. 

Considering the influence of the material properties, it must be acknowledged that attrition is a statistical effect, i.e., there are differences in the attrition susceptibility of the individual bed particles, and the attrition stress is acting randomly on them. As a consequence, there is usually not the influence of one particular property evaluated rather there is an evaluation of the materials statistical attritability as a whole. Section 4 summarizes the relevant attrition test procedures. The results of these attrition tests must then be transferred to the actual process by means of a physically sound description of the process conditions (Werther and Reppenhagen, 1999 Boerefijn et al., 2000). This, however, requires a distinction between different regions of the system that apply different types of stress to the solids. For this reason. Sec. 5 first summarizes the attrition mechanisms prevailing in the relevant sources of a fluidized bed system, and Sec. 6 then finally deals with a description of attrition in the entire process. 

The above definition of the attrition rate considers the bed material as a whole and quantifies solely the production of elutriable material without taking all breakage events or the shrinking of the so-called mother particles into account. More insights into the attrition mechanisms can be obtained from the observation of the change in the particle size distribution as demonstrated by Zenz and Kelleher (1980) and by Lin et al. (1980). An example of one of the results obtained by Zenz and Kelleher (1980) is shown in Fig. 7. 

Reppenhagen and Werther (1999a) developed an alternative method of studying the attrition mechanisms inside the cyclones independently of the collection efficiency. The basic idea is also to pass a batch of particles several times through an isolated cyclone... 

Hence it can be concluded that for each operating condition the fines concentration in the material tends to a characteristic value of which the accumulation of fines is balanced by the release of fines. When this characteristic concentration is reached, the loss rate is at steady state, i.e., it is equal to the production rate of fines. Reppenhagen and Werther (1999a) suggested that we take this steady-state value as a characteristic value for both the assessment of a material s attritabil-ity and the study of cyclone attrition mechanisms in dependence on the various influencing parameters. For this purpose, they defined the cyclone attrition rate as... 

A first approach to finding attrition in fluid beds was made by Zenz (1971). He pointed out that there are various regions in a fluidized bed reactor system in which the stress acting on the bed particles and the corresponding attrition mechanisms are quite different. In the subsequent works (Zenz, 1974 Vaux and... 

Even though most of the attrition tests presented in the literature deal with bubble-induced attrition, the respective attrition mechanisms are not quite clear yet. There are various theoretical and empirical approaches that can in accordance with Eq. P) be summarized in the following definition of a bubble-induced steady-state attrition rate ... 

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