Attrition minimization

Figure 20. Schematic drawing of the attrition-minimizing distributor design suggested by Parker and Gwyn (1976, 1977).

Good gas distribution is necessary for the bed to operate properly, and this requites that the pressure drop over the distributor be sufficient to prevent maldistribution arising from pressure fluctuations in the bed. Because gas issues from the distributor at a high velocity, care must also be taken to minimize particle attrition. Many distributor designs are used in fluidized beds. The most common ones are perforated plates, plates with caps, and pipe distributors. 

This shroud length allows the jet issuiag from the orifice to expand and fill the shroud. The gas velocity leaving the shroud should not exceed 70 m/s, to minimize attrition. 

Some SCU products are produced without sealant. These are produced under carefully controlled process conditions that have been optimized to minimize the formation of stress fractures ki the sulfur coating (11). However, such products are more prone to attrition damage when handled than SCU products with a sealant coating. 

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. 

The direction of gas flow through the pellet bed could be important. A pulsating high speed flow of exhaust gases can cause rapid attrition of catalysts, especially if the converter has empty spaces due to catalyst loss or shrinkage, which would promote the internal circulation of catalysts in the converter. The design of a sideflow or an upflow bed must include provisions to avoid empty spaces. A downflow design would minimize these attrition losses. 

A well-defined bed of particles does not exist in the fast-fluidization regime. Instead, the particles are distributed more or less uniformly throughout the reactor. The two-phase model does not apply. Typically, the cracking reactor is described with a pseudohomogeneous, axial dispersion model. The maximum contact time in such a reactor is quite limited because of the low catalyst densities and high gas velocities that prevail in a fast-fluidized or transport-line reactor. Thus, the reaction must be fast, or low conversions must be acceptable. Also, the catalyst must be quite robust to minimize particle attrition. 

Properties which Minimize Attrition Effects of Operating and Formulating Variables... 

In general there are two ways to minimize attrition. First of all the solid particles should be chosen, treated or produced in such a way that they are as attrition-resistant as possible. On the other hand, the fluidized bed system should be designed in such a way that the effects of the various attrition sources are kept as small as possible. 

An analysis of more than 130 preclinical candidates that had attrited during further development showed the failure of the chemotype approach (i.e. that a compound of the same/similar chemotype will have similar risks of attrition and that a structurally diverse chemotype will offer the best approach to minimize attrition risk) and 2D structure-based methods to be able to effectively differentiate compounds [29]. Thus, the risk of failing or succeeding in development is not related to being of the same chemotype , and differentiation by this method may not be the most effective way dangers are both that a valuable series/chemotype could be discarded because of one bad result and that a structurally different compound may actually have similar off-target effects (e.g. due to the decoration versus the scaffold). 

To generate a sufficient pressure drop for good gas distribution, a high velocity through the grid openings may be required. It is best to limit this velocity to less than 60 m/s to minimize attrition of the bed material. The maximum hole velocity allowable may be even lower for very soft materials that attrite easily. The pressure drop and the gas velocity through the hole in the gas distributor are related by the equation... 

Due to the pressure drop requirements across the gas distributor for good gas distribution, the velocity through the grid hole may be higher than desired in order to minimize or limit particle attrition. Therefore, it is common industrial practice to place a length of pipe (called a... 

Powder Formation. Metallic powders can be formed by any number of techniques, including the reduction of corresponding oxides and salts, the thermal dissociation of metal compounds, electrolysis, atomization, gas-phase synthesis or decomposition, or mechanical attrition. The atomization method is the one most commonly used, because it can produce powders from alloys as well as from pure metals. In the atomization process, a molten metal is forced through an orifice and the stream is broken up with a jet of water or gas. The molten metal forms droplets to minimize the surface area, which solidify very rapidly. Currently, iron-nickel-molybdenum alloys, stainless steels, tool steels, nickel alloys, titanium alloys, and aluminum alloys, as well as many pure metals, are manufactured by atomization processes. 

When scaling-up the fluid-bed process, a major requirement is to produce fluidization behavior on the larger machines equivalent to that used on the scale that provided the basis for process development. To achieve this goal, and minimize attritional effects, the same air velocities for each scale of equipment are required. Thus, the overall increase in air volume required during scale-up will be related to the increase in area of the perforated base plate, and, in the case of the Wurster process, the open area of the partition plate immediately beneath each of the inner partitions. Such calculations are simplified when scaling-up from an 18" pilot scale machine to, say, a 32" machine, since the latter represents a three-multiple of the former, and thus would require a threefold increase in airflow. 

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