ASSESSMENT OF ATTRITION

The effects of attrition on a given particulate material may be assessed in many different ways. One may base this assessment on the observation of an individual particle. Alternatively, the fate of a group of particles may be examined, or the effect of attrition on the bulk properties of a powder may be taken forthis assessment. The British Materials Handling Board (1987) and Bemrose and Bridgwater (1987) have given a lot of examples for the different methods. 

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The assessment of attrition is mainly based on the examination of the behavior of a group of particles. In fluidization technology the total or overall attrition rate Ratot is commonly defined by the relative change of bed weight, Wbed, with time,... 

This definition, however, includes effects of initial breakage and of initial fines content and will therefore normally not give an unambiguous assessment of attrition. 

In order to evaluate the extent of attrition and its impact on the particle size distribution, there is a need of a qualitative and quantitative characterization. This, however, is not as simple as it may seem at first. There are many different properties, parameters and effects that manifest themselves and could be measured. In addition, as will be shown, the choice of the assessment procedure is strongly connected with the definition of attrition which, on its part, depends on the degradation mechanism that is considered to be relevant to the process. Hence there are a lot of procedures and indices to characterize the process of particle attrition. Section 3 deals with those which are relevant to fluidized beds and pneumatic conveying lines. 

Knight, P. C., and Bridgwater, J., Comparison of Methods for Assessing Powder Attrition, Powder Tech., 44 99 (1985)... 

We would also like to stress the importance of additional requirements related to the physical properties such as the mechanical strength of potential CO2 sorbents. The calcium looping process will most likely be operated in a circulating fluidized bed reactor set-up. This requires the development of attrition-resistant materials. However, the mechanical strength or attrition resistance of new developed CO2 sorbents is rarely assessed and, thus, requires more attention if the materials are to be relevant for practical applications. 

Naturally, the primary interest in the field of attrition is to reduce its extent to a minimum. An obvious approach is to use an optimum attrition resistant bed material. Hence the majority of work deals with the particles degradation in standardized bulk attrition tests to determine the relative tendency of materials to attrit (Davuluri and Knowlton, 1998). These so-called friability tests are quite useful in the quick and first assessment of a candidate bed material. But they solely focus on one factor affecting attrition, namely the materials properties, whereas the prevailing stress mechanisms may differ considerably from those in the process. For that reason, the pure tests cannot predict the quantitative extent of attrition that will occur with the tested materials in a technical fluidized bed process. They cannot even guarantee that the ratio of the attrition rates of two materials will be the same in the test and in the process (Werther and Reppenhagen, 1999a Boerefijn et al., 2000). 

The chosen type of assessment procedure basically depends on whether the attrition propensity of a given material, i.e., its material strength, or attrition in a given process is to be considered. Furthermore, the choice of the assessment procedure depends on the attrition effect that is of interest. When the entire process is considered, it is usually either the assessment of the attrition-induced material loss or the assessment of the attrition-induced changes in the particle size distribution of the bed material. 

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