Erosion and Attrition

The mechanical erosion of a solid surface such as a pipe wall in a gas—solid flow is characterized by the loss of solid material from the solid surface due to particle impacts. The collisions of the particles either with other particles or with a solid wall may lead to particle breakup, known as particle attrition. Pipe erosion and particle attrition are major concerns in the design of a gas-solid system and during the operation of such a system. The wear of turbine blades or pipe elbows due to the directional impact of dust or granular materials, the wear of mechanical sieves by the random impact of solids, and the wear of immersed pipes in a fluidized bed by both directional and random impacts are examples of the erosion phenomenon in industrial systems. The surface wear associated with the erosion phenomenon of a gas-solid flow has been exploited to provide beneficial industrial applications such as abrasive guns, as well. 

The most commonly used pipe materials may be classified into four categories based on the mechanical erosion modes metals such as copper, aluminum, and steel ceramics 

---

In practice, it is prudent to increase Lmin by a factor of 50 to 100%. A shroud length less than Lmin causes significantly more erosion and attrition than no shroud at all. Significant attrition can also occur if the shroud is not centered over the smaller hole. 

Briefly, three points of porous SiC-based catalytic support properties can be emphasized (i) SiC shows very good mechanical properties which gives resistance to erosion and attrition, in addition to a high thermal stability (ii) SiC has a higher thermal conductivity compared with the more conventional supports which could prevent the metal sintering (iii) SiC is particularly inactive with respect to chemical reagents such as acids or bases. Therefore, the active phase can be easily reprocessed after simple acidic or basic treatments. Among refractory materials, the thermal conductivity of silicon carbide, SiC (500 W m-1K-1 for crystalline state, at room temperature) is close to that of metals such as Ag or Cu (400-500 Wm K-1). 

Disadvantages, however, can be erosion and attrition and problems in gas-solids separation. Furthermore, there may be doubt eOsout some of the claims. Fig. 17 shows a fast bed or riser set-up. Next to it a typical pressure drop graph is given schematically. The accelleration zone may extend to 4 or more meters length even in small scale units (78). 

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. 

Avoid erosion of the riser wall and attrition of the catalyst Perform without plugging or erosion... 

Excessive vapor entrainment down the dipleg can increase erosion and possibly catalyst attrition. On the reactor side, excessive entrainment will send more cracked product vapors to the stripper. 

The choice and properties of the aeration gas are important factors for maintaining stable standpipe operation. The condensate source for steam aeration can cause several problems. If the steam is not kept dry, the condensate can lead to stress cracking of the tap piping, plugging of the tap nozzle with mud, erratic aeration rates, orifice erosion, and potentially catalyst attrition. Similar problems can occur with wet fuel gas as an aeration source. When possible, dry air and/or nitrogen are preferred rather than steam as aeration media for standpipes. However, in actual... 

This behavior is illustrated in Figure 39. As a result, when cyclones are used in series, the catalyst recovered in the first stage is coarser than in the second. The particle size ( cut size ) above which recovery efficiency is good depends upon the physical dimensions of the equipment, gas velocity, particle density, and properties of the gas. High inlet velocities result in a greater separating force and a smaller cut size but also cause increased erosion of the equipment and attrition of the catalyst. The inlet-vapor velocity is normally limited to 60 ft./second in the reactor cyclones and 75 ft./second in the regenerator cyclones (97). 

Particle size distribution Weight mean diameter 50 < dp < 70 jum + 80 jum, 5-20 wt. % -40 /tm, 10-30 wt.% Improved fluidity Good fluidization Decreased catalyst loss and attrition Se vere erosion if particles are coarse Difficulty in recovering particles <10 /am Installation of internals Multistage fluid bed with horizontal baffles... 

The term tooth wear is commonly used to describe the loss of tooth hard tissue due to non-carious causes [1], This encompasses a variety of both chemical and mechanical causes of both intrinsic and extrinsic origin. The term tooth wear is preferred over some of the more precise definitions of individual hard tissue loss mechanisms, because it acknowledges the fact that wear is usually a multifactorial process one mechanism may dominate, but the overall wear is commonly due to the interaction between two or more wear mechanisms. In dentistry, the terms erosion, abrasion, attrition and abfraction are widely used to describe particular mechanisms of hard tissue loss. 

The mechanisms of tooth wear fall into two distinct types those of chemical origin (e.g. erosion) and those of physical origin (e.g. abrasion, attrition). In any individual, both chemical and physical insults to the tooth hard tissue will be present in some form or other, so tooth wear is the combined effect of these insults. Despite the clear definition of a number of distinct tooth wear mechanisms, it is uncommon to find a single wear mechanism present in the... 

Broad residence time distributions of solids due to intense mixing, erosion of the bed internals, and attrition of the catalyst particles. 

Erosion/Attrition/Thermal Shock. The high velocities of balls in lift pipes and the turbulent nature of the fluidized beds lead to the possibility of erosion of the equipment and attrition or fracturing of the balls. Erosion can be reduced by using abrasion resistant refractory linings in pipes. Attrition and fracturing of balls can be reduced by proper design to reduce the effect of impaction at elbows and on deflection plates. 

Bends complicate the design of pneumatic dilute phase transport systems and when designing a transport system it is best to use as few bends as possible. Bends increase the pressure drop in a line, and also are the points of most serious erosion and particle attrition. 

---