Yang Fluidization, Solids Handling, and Processing

436 Fluidization, Solids Handling, and Processing

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.

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.

Unfortunately, the basic physical mechanisms that control the attrition process are still poorly understood. As a consequence, particular test methods are used to evaluate the degradation tendency of the materials or to predict the rate of attrition for a given process. There are a lot of procedures using widely different devices and operations. Some of them observe the degradation of only one individual particle, whereas others treat a considerable amount of material. The particles are subjected to stress systems which range from well-defined ones like impact or compression, to those which are similar to the more or less randomized stresses occurring in natural processes. Section 4 attempts to summarize the huge variety of attrition tests in a systematic way.

Test facilities are also used to investigate specific attrition phenomena of individual processes. Up to now only a few investigations have been carried out in full-scale equipment. Most results were obtained from very special devices which makes it difficult to compare the results of various research groups and to draw general conclusions. Despite of these difficulties, the specific attrition phenomena in fluidized beds and pneumatic conveying lines will be summarized in the Secs. 5 and 6, respectively. It will

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be shown that the fluidized bed systems can be divided into several regions which are characterized by completely different attrition characteristics. These regions will be discussed separately with regard to the respective influence of the operating and design parameters. Some models and correlations to predict or at least describe the extent of attrition will also be given. On the other hand, the number of papers relating to attrition in pneumatic conveying lines is so small that the results can be outlined in a qualitative manner only. At the end of both sections some general rules will be presented to reduce the attrition in a given case.

2.0FACTORS AFFECTING ATTRITION

Before discussing the individual factors affecting attrition in detail, the distinction has to be made between two completely different modes of attrition, namely fragmentation and abrasion. The two modes and their different effects on the particle size distribution are sketched in Fig. 1. Abrasion produces a lot of very fine material removed from the surface of the initial particles that retain their identity and become only slightly smaller. Abrasion leads to a bimodal size distribution. On the other hand, fragmentation destroys the particles and produces a number of particles which are all distinctly smaller than the original ones. The resulting size distribution becomes broader and has a smaller mean diameter than the original one.

In industrial processes the two modes usually do not occur separately. They are rather combined in varying proportions. There may be, for example, an initial breakage of mother particles followed by surface abrasion of the resulting fragments. In this case a small number of intermediately sized particles and many fine particles will be produced. In general, the extent of abrasion in relation to fragmentation depends on the various factors discussed below. However, there have been several rules suggested to identify the dominant mode in a given case. They all use the change in size distribution as criterion. For instance, Dessalces et al. (1994) assessed the degradation behavior of various industrial FCC catalysts by the change

in the ratio dp3,90 / dp3,10, i.e., the ratio of the 90% and 10% values of the cumulative mass density distribution of the particle sizes. They got high

values of this ratio (>100) if abrasion was dominant and smaller values (<10) in the case of fragmentation.

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Figure 1. Attrition modes and their effects on the particle size distribution (q3 = mass density distribution).

Often only that part of degradation which is causing problems is defined as attrition. A good example is the catalyst attrition in fluidized bed reactors where not the particle degradation in itself but the resulting material loss is mostly deemed to be the relevant problem. Consequently many researchers concentrate on the elutriated particles and define them as the only attrition product. This is usually rather fine material which results almost exclusively from abrasion. So, regardless of its extent, fragmentation is not taken into account. The fresh make-up catalyst will additionally be subjected to fragmentation upon entering into the fluidized bed.

The large amount of variables affecting attrition can be classified into two major groups, i.e., the various factors related to material properties and factors related to process conditions.

2.1Material Properties

Particle Structure. First of all, the particle structure has a fundamental influence on the degradation propensity. The extent of degradation as well as its mode will strongly depend on whether the particle is a single crystal, has an amorphous structure or is an agglomerate. For example, spray-dried catalysts, which are often used in fluidized bed reactors, are

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rather susceptible to attrition. Arena et al. (1983) and Pis et al. (1991) studied the attrition in a fluidized bed of amorphous materials such as coal or limestone and found that the size distributions of the attrited materials were independent of the initial particle size and of most operating parameters. Ray et al. (1987a) assumed that unlike crystalline materials, amorphous materials may have some kind of “natural grain size” to which the degradation finally leads.

Pretreatment and Preparation History. The unsteady-state attrition rate of a given powder sample will normally depend on the pretreatment of the solids because the higher and the longer the foregoing stress, the more the weakest points of the particles may have already been attrited. The pretreatment can even occur in the processing route of the solids. If we consider, for example, the spray-drying of catalysts, the weakest agglomerates will immediately break down. Ghadiri et al. (1991) observed that the degradation propensity of salt samples depended on their processing route. They suggested that “rough” processing conditions, as they are occurring in fluidized beds and pneumatic conveying lines, may cause microplastic deformation which in turn work-hardens the surfaces, thus increasing their propensity to degradation.

Particle Size, Shape and Surface Structure. The particle size is of primary interest with respect to particle breakage. This is because the breakage probability strongly depends on the presence of microcracks or imperfections. Smaller particles are, therefore, more difficult to break than larger ones, mainly because they tend to contain fewer faults. The mechanisms of breakage will not be further discussed in the present chapter, but a good survey is given by the British Materials Handling Board (1987).The particle shape is another relevant parameter because irregular and angular particles are inclined to have their corners knocked off in collisions and thus become rounder and naturally smaller with time. This certainly tends more to fragmentation because the debris is much bigger than the fines derived by chipping from a macroscopically smooth surface. The microscopic surface structure is also of interest. If surface asperities are broken off, very fine particles are formed which is characteristic of abrasion.

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

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particle size distribution of the material that is produced by attrition will differ with the initial particle size distribution. A shift of the particle size distribution to smaller particle sizes may, for example, lead to an increased production of fines.

Moreover, the fines content can have a strong influence on the attrition propensity of the material. Forsythe et al. (1949) already observed a reduction of the degradation of FCC-catalysts in jet attrition tests due to presence of fines. They supposed some kind of cushioning effect which limits the force of collision impact leading to fragmentation of the coarse particles. The effect of fine particles is of strong interest; fines are produced by attrition, so attrition inhibits itself if the fine particles remain in the system. This is particularly valid for fluidized bed systems where the particles are for long periods subjected to the degradation forces.

Arena et al. (1983) investigated the coal attrition in a mixture with sand under hot but inert conditions. As they increased the sand particle size while keeping its mass in the bed constant, they observed an increase in the coal attrition rate. They interpreted their results by assuming that the abrasion energy is shared out on the entire material surface. On the same basis Ray et al. (1987a) developed their “attrition rate distribution model” for abrasion in a fluidized bed.

2.2Process Conditions

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.

The stress may be a mechanical one due to compression, impact or shear, a thermal one owing to evaporation of moisture or temperature shock, or a chemical one by molecular volume change or partial conversion of the solid into the gas phase. In most cases a combination of these types will occur. The effect of mechanical stress on the particle degradation was intensely investigated by several research groups, e.g., Yuregir et al. (1986, 1987), Paramanathan and Bridgwater (1983), Bridgwater (1987), Neil and Bridgwater et al. (1994), Shipway and Hutchings (1993). The respective investigations were carried out in very special test devices, some of which are described in Sec. 5.

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Gas and Solid Velocity. The gas velocity is usually directly related to the particle velocity which is the most important factor affecting attrition if the particle degradation occurs as a result of mechanical stress due to interparticle collisions or collisions between the particles and a solid wall. The forces involved in the degradation process may be generated by highspeed collisions resulting preferably in breakage. Alternatively, the energy may be transmitted through a matrix of comparatively slow moving particles resulting mostly in abrasion. Particularly in the distributor region of a fluidized bed, where grid jets are issuing into the bed of particles, the impact velocities between particles can be extremely high resulting in significant particle breakage.

Solids Residence Time. The relationship between the solids residence time and the amount of material that is produced by attrition is generally non-linear. As an example, Fig. 2 shows the typical time dependence of the attrition of a fresh catalyst that is subjected to attrition in one of the test devices described in Sec. 4. The elutriated mass is defined to be the attrition product and consequently the attrition rate is defined as elutriated mass per unit time. It is clearly seen that the rate of attrition is decreasing with time. The reason is that, at the beginning, the fresh catalyst particles are very irregular and contain a lot of faults. This results in a high rate of initial particle degradation during which the particles break and their edges and asperities are knocked off. With progressing time the particles become smaller, rounder and smoother and the number of their weak points decreases. The elutriation rate, therefore, decreases continuously with time and tends to a more or less constant value which can be interpreted as some kind of a steady-state level where only abrasion takes place. The shape of the curve in Fig. 2 is typical for all particle degradation processes. However, there are some peculiarities belonging to the particular processes. With respect to pneumatic conveying, the time axis can be substituted by the pipeline length, because the particles are going only once through. With respect to fluidized beds, one has to distinguish between batch and continuous processes. In batch processes, the whole bed material is always at the same state of attrition, which changes during the operating time according to Fig. 2. On the other hand, there is a particle residence time distribution in continuous processes. The attrition rate of the entire material is, therefore, constant although the state of attrition of each particle again changes according to Fig. 2.

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empirical correlations for the time dependence of the mass ratio Wel /Wbed,0 have been suggested by, for example, Pis et al. (1991) and Dessalces et al.

(1994).

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.

The heating-up of fresh cold particles fed into a hot process can cause various phenomena that may lead to particle degradation. These are thermal stress, decrepitation, evaporation of moisture, hydrate decomposition and impurity transformation. On the other hand, particle properties such as strength, hardness and elasticity may be effected by the temperature, too. With respect to the resistance to degradation there is an optimum temperature range for any specific type of material. At lower temperatures particles become brittle and easy to break, while at higher temperatures they may soften, agglomerate or melt and lose discrete particulate properties. Consequently, it is important that the particle friability, which is the major factor for the attrition propensity of a given material, is assessed under conditions that are similar to those found in the respective process where the attrition is occurring. Moreover, the temperature can have a strong effect on the gas density, which affects the fluidization state and with it the particle motion and the stress to which the particles are subjected.

Pressure. The absolute pressure is unlikely to have a direct effect on attrition unless it affects the amount of adsorbed surface layers. But there is again an effect on the gas density that is similar to the effect of temperature mentioned above. Moreover, the rate of pressure change may have more influence.

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.

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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).

3.0ASSESSMENT 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 for this assessment. The British Materials Handling Board (1987) and Bemrose and Bridgwater (1987) have given a lot of examples for the different methods.

3.1Breakage and Selection Functions

A description of the attrition process which is based on the individual particle can be obtained by dividing the bulk material in several size ranges to which both a respective probability of degradation and an individual size distribution of attrition products are assigned. This can be realized by a concept of two separate functions suggested by Broadbent and Calcott (1956). The first one is called selection-for-breakage function or probability function S(dp). It describes the degradation probability of a specific particle of size dp in a specific time-interval. The second one is the so-called breakage function B(dpi,dpj) which describes the mass fraction of breakage products of size dpj originating from a particle of size dpi. The functions S(dp) and B(dpi,dpj) can be used as vectors and matrices, respectively, where the elements are applied to discrete particle size ranges. In detail, the vector elements si describe the rate of material loss of a particular size fraction of mean diameter dpi and the various matrix elements bi,j describe the distribution of attrition products from the fraction i into the smaller fraction j. With a vector of the feed size distribution, they can be combined in a matrix