Particles degradation

Any reduction in the normai value of the impact force on the particles will help and so long radius bends are to be recommended. Reducing the magnitude of the deceleration force on the particles as a result of impact will also help. The use of rubber hose for pipeline 

Rubber is a resilient material and so is capable of absorbing the energy of impact. As a consequence both the damage to the particles and the damage to the rubber itself will be reduced. If the energy of impact, however, is above the threshold for the rubber, due to particle velocity, size or density, rubber will wear rapidly if the material is abrasive. A particular advantage with rubber is that the material is flexible and can be rotated when used for bends and so the life can be prolonged. 

Agarwal, V.K. (2004) Handbook of Pneumatic Conveying Engineering. Marcel Dekker, New York. (Note that this book is in US Engineering Units) 

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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. 

Both, the mechanism and the extent of particle degradation depend not only on the process type but also on properties of the solid material, and to a large extent on the process conditions. Clift (1996) has stated that attrition is a triple-level problem, i.e., one is dealing with phenomena on three different length and time scales the processing equipment, the individual particles, and the sub-particle phenomenon such as fracture which leads to the formation of fines. The appearance of attrition can, therefore, differ very much between the various applications. For that reason, the following section deals with the various modes of attrition and the factors affecting them. 

Particle Size Distribution. The particle size distribution is a significant factor with respect to attrition. Coarser particles tend more to fragmentation while smaller particles have a stronger inclination to abrasion because of their large specific surface. Since the particle degradation is composed of fragmentation as well as abrasion, both the amount and the... 

The process conditions will influence the particle degradation by generating the stress on the individual particles on the one hand and by affecting the material properties and consequently the particle friability on the other. 

Temperature. There are three conceivable temperature effects that may influence the particle degradation in an either direct or indirect way, i.e., thermal shock, changes in particle properties and changes in the gas density. 

Humidity. There are two conceivable effects of humidity. Particularly with respect to agricultural products the moisture content can influence the hardness and elasticity. For example, Segler (1951) observed that dry peas are more sensitive to breakage than wet ones. Moreover, Wyszynski and Bridgwater (1993) have reported that a lubricating layer of moisture on the particle surface can reduce the particle degradation. 

Wall-Hardness. One can assume that the particle degradation increases with the hardness of the vessel wall. This effect will increase with increasing ratio of particle-to-tube diameter and will thus in practice be relevant in pneumatic conveying lines only. 

Chemical Reaction. Chemical reaction of particulate material generates stresses within the particle that can lead to fracture. In the case of gas-solid reactions, the particle degradation is also desired because it accelerates the reaction by extending the reactive surface. A relevant commercial example is the particle degradation of solid fuels in combustion processes. This latter topic has been studied by Massimilla and coworkers extensively. The reader is referred for further details to a review given by Chirone et al. (1991). 

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. 

Despite these potential problems, remarkably little work has been published on particle degradation in pneumatic conveying systems. That may be explained by experimental problems that are even much more serious than with fluidized beds. According to the different problems mentioned above, there are more individual measurement techniques and assessment procedures required than with fluidized bed attrition. Usually, the assessment is restricted to the comparison of the particle size distribution before and after conveying. Moreover, there is no steady-state attrition that could be measured. It is only possible to measure an integrated value, which... 

Both Reed and Bradley (1991) and Wypych and Arnold (1993) have given surveys of techniques to minimize the particle degradation. 

Mills, D., Particle Degradation in Pneumatic Conveying, 7th Int. Symp. on Freight Pipelines, Wollongong, NSW, Australia (1992)... 

Reed, A. R., and Bradley, M. S., Techniques for Minimising Particle Degradation... 

As discussed in Chapter 15, the size distribution of particles in an agglomeration process is essentially determined by a population balance that depends on the kinetics of the various processes taking place simultaneously, some of which result in particle growth and some in particle degradation. In a batch process, an equilibrium condition will eventually be established with the net rates of formation and destruction of particles of each size reaching an equilibrium condition. In a continuous process, there is the additional complication that the residence time distribution of particles of each size has an important influence. 

Soil. Diafenthiuron and its main metabolites show a strong sorptivity to soil particles. Degradation in soils proceeds rapidly DTJ0 <1 hr to 1.4 days... 

Attrition Breaking down or wearing away by friction. Usually as particle to particle degradation in a diatomite slurry. 

Product quality deterioration due to thermal effects or particle degradation... 

Besides liposomes, polymeric nanoparticles may be used as effective drug carrier systems using cytotic pathways. Particle size and polymeric composition help control particle degradation and drug release. Recently, it was shown in a rat study that polybutylcyanoacrylate nanoparticles, which had been surface coated with polysorbate 80, exhibited a 20-fold higher uptake into brain capillary endothelial cells compared to noncoated nanoparticles [17]. It is assumed that association of lipoproteins at the surface triggers the endocytotic uptake of the nanoparticles. 

Second, during regeneration these metals oxidize and act as oxidation catalysts, leading to excessive combustion rates and sintering. Especially bad is V,Oi, not only because it is a strong oxidation agent but also because it melts and forms a flux to accelerate particle degradation. 

Stability is directly related to the lifetime of the resin. This, in turn, directly affects the cost of the process. Physical stresses can occur through swelling and shrinking cycles due to osmotic pressure changes. Mechanical forces, such as static pressure load, and abrasion can cause breakage. Operation outside the normal temperature range will also add to particle degradation. 

The accelerating effect of the phospholipid on release from lipid microparticles might be even more pronounced in vivo, where it is reported that lipid particles degrade faster in the presence of surfactant, which enables the contact with lipases [3],... 

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