Attrition in cyclone

There can also be substantial particle attrition in cyclones in fluidized-bed systems because particles are accelerated at the inlet of the cyclone and impacted against the cyclone wall. Although there is little information on particle attrition in cyclones in the literature, it has been reported (Sishtla) that increasing system pressure decreases the attrition rate in cyclones operating with coal char. The mechanism by which this occurred was not determined. 

Optimum size distribution is important for a fluid bed reactor (Bergoug-nou). Models based on bubbles are not yet capable of predicting the wall effect (Wen). Vertical baflles are most effective in breaking up large bubbles (Volk). The height of the bottom ends of vertical tube bundles above the grid will set the attainable bubble size at the bottom of the bundle. The bundles then essentially maintain the bubble size (Zenz). Horizontal perforated baflle plates reduce the mean residence time of elutriable fine particles in a fluidized bed (Buckham). Observations on attrition in cyclones indicate that it is an exponential function of velocity (Tenney). 

The necessity of distinguishing different modes of attrition is illustrated by the work of Reppenhagen and Werther (1999a), for example, dealing with catalyst attrition in cyclones. Focusing on the mode of pure... 

Reppenhagen J, Werther J. Catalyst attrition in cyclones. Powder Technol 113 55-69, 1999a. 

Attrition in fluidized bed systems leads primarily to a loss of bed material since the cyclones, which are mostly used for the collection of entrained material, are not able to keep the attrition-produced debris inside the fluidized bed system. The material loss through the cyclone is, therefore, usually taken as the attrition rate. This means that among the attrition modes discussed in Sec. 2, namely fragmentation and abrasion, it is abrasion which is the attrition mode of interest for fluidized bed systems. 

Figure 15. Changes in the size distribution of FCC catalyst due to attrition in a 5" cyclone, (inlet velocity 35ft/s). (Zenz andKelleher, 1980.)...

The cyclone inlet velocity is thus shown to be the most significant factor influencing the solids loss due to attrition in the cyclone. 

There is clearly a need to investigate the mechanism of attrition to relate it to the fracture properties of the solids, and to develop a realistic attrition index , similar to that used for abrasion in cyclones. Such an index would indicate the relative importance of operating conditions and design variables such as inlet velocity, feed solids concentration or cyclone diameter. This could then be used in scale-up to predict (or minimize) the effect of the shape, the particle size distribution or the hardness and strength of the feed solids, if known, may allow such predictions without any experimental tests. Generally, better understanding of attrition and its relation to abrasion may lead to better equipment design and operation. 

Other than altering the catalyst level, no operational changes pinpoint a particular type of cyclone failure. However, the troubleshooter can differentiate between catalyst attrition and cyclone damage as a cause of excessive catalyst losses. By collecting a sample of catalyst entrained in the regenerator flue gas, the amount of catalyst fines can be measured. This value is then compared to the percentage of fines in a sample of regenerated catalyst. 

As mentioned earlier, one can distinguish three pure and well-defined mechanical stresses on bulk solids material, namely compression, impact, and shear. There are numerous tests that are based on compression and shear, e.g., Paramanathan and Bridgwater (1983), Neil and Bridgwater et al. (1994), Shipway and Hutchings (1993), but they are not further discussed in this chapter because these stresses are usually not relevant to fluidized beds. On the other hand, impact stress occurs whenever particles hit walls or other particles. Attrition caused by impacts can thus be observed, e.g., in grid jets, in the wake of bubbles, in cyclones, or due to free fall. Consequently, there is a great variety of impact tests that try to simulate these particular stresses. 

For the reasons explained above there is only a limited amount of work published in the open literature on cyclone attrition. In fact there are results from only... 

Figure 23 shows a comparison of the experimental data depicted in Fig. 22 with the calculation from the model equation (27). The required attrition rate constants Cj, K, and Q that describe the materials susceptibility to attrit in the respective regions have been determined by the corresponding attrition tests as described in Sec. 4.3. Cj has been determined from exactly that Gwyn-type test facility that is shown in Fig. 14 and was set to zero in the case of the porous plate distributor has been measured in a 200 mm ID Gwyn-type test apparatus, and Q has been determined from exactly that cyclone attrition-test procedure that is described in Sec. 4.3.4 using the equipment sketched in Fig. 11. The parameters me,in and dpc were measured in the apparatus sketehed in Fig. 21 under the assumption that mc n may be...

The attrition of catalyst particles leads to catalyst loss in cyclones. 

Horizontal sieve-tray baffles reduce the requirement for catalyst hold-up in the unit. This can lead to a significant saving if the catalyst is expensive, i.e. around 25/kg. However, the catalyst must be more attrition resistant. Because of the good contacting, coarse catalyst, say around 200 ym in particle size) can be used and catalyst losses in cyclones are decreased. 

Testing of the mechanical stability of the prepared soibents is carried out by measuring the amount of elutriated material in a fluidized bed with an inner diameter of 4 cm. Superficial gas velocity in the bed is 1 m/s, gas consists of pure nitrogea A special bottom plate is designed in order to increase the amount of attrition in the bed. Batches of 25 g are used. Elutriate is collected by a cyclone, and captured on electrostaticly charged quartz wool kept in a flask. Tests are done at 20°C (cold- elutriation) and 850°C (hot-elutriation). For comparison purposes several natural limestones are also tested in this experimental set-up. Results are given in table III. 

A salient feature of the fluidized bed reactor is that it operates at nearly constant temperature and is, therefore, easy to control. Also, there is no opportunity for hot spots (a condition where a small increase in the wall temperature causes the temperature in a certain region of the reactor to increase rapidly, resulting in uncontrollable reactions) to develop as in the case of the fixed bed reactor. However, the fluidized bed is not as flexible as the fixed bed in adding or removing heat. The loss of catalyst due to carryover with the gas stream from the reactor and regenerator may cause problems. In this case, particle attrition reduces their size to such an extent where they are no longer fluidized, but instead flow with the gas stream. If this occurs, cyclone separators placed in the effluent lines from the reactor and the regenerator can recover the fine particles. These cyclones remove the majority of the entrained equilibrium size catalyst particles and smaller fines. The catalyst fines are attrition products caused by... 

PSD is an important indicator of the fluidization characteristics of the catalyst, cyclone performance, and the attrition resistance of the catalyst. A drop in fines content indicates the loss of cyclone efficiency. This can be confirmed by the particle size of fines collected downstream of the cyclones. An increase in fines content of the E-cat indicates increased catalyst attrition. This can be due to changes in fresh catalyst binder quality, steam leaks, and/or internal mechanical problems, such as those involving the air distributor or slide vah es. 

Matsen (1985) pointed out a number of additional problem areas in scale-up such as consideration of particle size balances which change over time due to reaction, attrition and agglomeration. Erosion of cyclones, slide valves and other components due to abrasive particles are important design considerations for commercial units which may not be resolved in pilot plants. 

It should be noted here that the above definition of the attrition rate considers the bed material as a whole. More insights into the influence of elements of the fluidized bed apparatus, e.g., of the cyclone or of the gas distributor may be obtained from the observation of the change in the particle size distribution as has been demonstrated by Zenz and Kelleher (1980) and Lin et al. (1980). 

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