Attrition of Solids

Paramanathan, B. K. and Bridgwater, J. (1983). Attrition of Solids-II. Chem. Eng. Sci., 38,207. Peddieson, J. (1975). Gas-Particle Flow Past Bodies with Attached Shock Waves. AIAA J., 13,939. Rietema, K. (1991). The Dynamics of Fine Powders. London Elsevier Applied Science. 

The attrition of solid particles is an unavoidable consequence of the intensive solids motion resulting from the presence of bubbles in the fluidized bed. The attrition problem is especially critical in processes where the bed material needs to remain unaltered for the longest possible time, as in fluidized-bed reactors for heterogeneous catalytic gas-phase reactions. Catalyst attrition is important in the economics of such processes and may even become the critical factor. 

Attrition of solids is known to take place during handling or on collection in gas cyclones, for example, but little is known of how it is related to particle properties, which particle sizes are most affected, and how attrition can be related to abrasion. Clearly, large particles are more likely to be affected by attrition finer fractions are generated by knocking off corners or by complete breakage of the larger particles. 

Cyclone inlet velocity not only affects efficiency of separation but also reflects in pressure loss and possible erosion. Gas viscosity has an important effect on particle efficiency, and so it is advisable to check its dependency with temperature and consider those cases in which a gas different from air is involved in the process. Smaller cyclone diameters increase overall efficiency, but also promote erosion. In addition to this, it is sometimes necessary to consider possible attrition of solids in the cyclone, which will result in pr< uction of fines and considerable losses. Erosion occurs primarily where the particles first impact the cyclone wall, but also occur at the bottom of cyclones too short to accommodate the length of the naturally occurring vortex. 

Fluidized-bed catalytic reactors tend to generate loss of catalyst through attrition of the solid particles, causing fines to be generated. 

Fluidized-bed adsorbers have several disadvantages. The continuous handling and transport of solids is expensive from an equipment standpoint fluidized-bed systems must be large to be economical. Solids handling also presents a potential for mechanical problems. Careful control is required to keep the adsorbent fluidized, while minimizing adsorbent loss with the gas-phase attrition of the adsorbent can be high, requiring substantial makeup. 

The lift pipe design was tapered to a larger diameter at the top. This minimized the effects of erosion and catalyst attrition, and also prevented the instantaneous total collapse of circulations when the saltation concentration, or velocity, of solids is experienced (i.e. the slump veloeity-that velocity helow which particles drop out of the flowing gas stream). In a typical operation, 2 % to 4 % eoke can he deposited on the catalyst in the reactor and burned in the regenerator. Catalyst circulation is generally not sufficient to remove all the heat of eombustion. This facilitated the need for steam or pressurized water coils to be located in the regeneration zone to remove exeess heat. 

Theoretical representation of the behaviour of a hydrocyclone requires adequate analysis of three distinct physical phenomenon taking place in these devices, viz. the understanding of fluid flow, its interactions with the dispersed solid phase and the quantification of shear induced attrition of crystals. Simplified analytical solutions to conservation of mass and momentum equations derived from the Navier-Stokes equation can be used to quantify fluid flow in the hydrocyclone. For dilute slurries, once bulk flow has been quantified in terms of spatial components of velocity, crystal motion can then be traced by balancing forces on the crystals themselves to map out their trajectories. The trajectories for different sizes can then be used to develop a separation efficiency curve, which quantifies performance of the vessel (Bloor and Ingham, 1987). In principle, population balances can be included for crystal attrition in the above description for developing a thorough mathematical model. 

Neil, A. U., and Bridgwater, J., Attrition of particulate solids under shear. Powder Technol. 80, 207-209 (1994). 

Attrition of particulate materials occurs wherever solids are handled and processed. In contrast to the term comminution, which describes the intentional particle degradation, the term attrition condenses all phenomena of unwanted particle degradation which may lead to a lot of different problems. The present chapter focuses on two particular process types where attrition is of special relevance, namely fluidized beds and pneumatic conveying lines. The problems caused by attrition can be divided into two broad categories. On the one hand, there is the generation of fines. In the case of fluidized bed catalytic reactors, this will lead to a loss of valuable catalyst material. Moreover, attrition may cause dust problems like explosion hazards or additional burden on the filtration systems. On the other hand, attrition causes changes in physical properties of the material such as particle size distribution or surface area. This can result in a reduction of product quality or in difficulties with operation of the plant. 

The Bed Material. Only catalytic processes are relevant with respect to modifying the attrition resistance of the bed material. In other processes, e g., drying, the bed material is the product and cannot be changed. In the combustion of solid fuels, the particle degradation due to attrition enlarges the reacting surface and thus increases the reactivity of the fuel. On the other hand, the lack of attrition resistance is often a major obstacle that hinders the commercialization of fluidized bed catalytic processes. 

Kokkoris, A., and Turton, R., The Reduction of Attrition in Fluidized Beds by the Addition of Solid Lubricants, AIChE Symp. Ser., 281(87) 20 (1991)... 

Yuregir, K. R., Ghadiri, M., and Clift, R., Observation of Impact Attrition of Granular Solids, Powder Tech., 49 53 (1986)... 

---