Fluidized beds attrition

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... 

Detailed reviews of the various test procedures are for example given by Bemrose and Bridgwater (1987) and the British Material Handling Board (1987). The present section is therefore confined to the tests that are most relevant to the subject of fluidized bed attrition. 

Fluidized-bed catalytic reactors tend to generate loss of catalyst through attrition of the solid particles, causing fines to be generated. 

The commercialization by Kureha Chemical Co. of Japan of a new, highly attrition-resistant, activated-carbon adsorbent as Beaded Activated Carbon (BAC) allowed development of a process employing fluidized-bed adsorption and moving-bed desorption for removal of volatile organic carbon compounds from air. The process has been marketed as GASTAK in Japan and as PURASIV HR (91) in the United States, and is now marketed as SOLD ACS by Daikin Industries, Ltd. 

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. 

Fluidized-bed reactor systems put other unique stresses on the VPO catalyst system. The mixing action inside the reactor creates an environment that is too harsh for the mechanical strength of a vanadium phosphoms oxide catalyst, and thus requires that the catalyst be attrition resistant (121,140,141). To achieve this goal, vanadium phosphoms oxide is usually spray dried with coUoidal siUca [7631-86-9] or polysiUcic acid [1343-98-2]. Vanadium phosphoms oxide catalysts made with coUoidal sUica are reported to have a loss of selectivity, while no loss in selectivity is reported for catalysts spray dried with polysUicic acid (140). 

Suspended Particle Techniques. In these methods of size enlargement, granular soHds are produced direcdy from a Hquid or semiliquid phase by dispersion in a gas to allow solidification through heat and/or mass transfer. The feed Hquid, which may be a solution, gel, paste, emulsion, slurry, or melt, must be pumpable and dispersible. Equipment used includes spray dryers, prilling towers, spouted and fluidized beds, and pneumatic conveying dryers, all of which are amenable to continuous, automated, large-scale operation. Because attrition and fines carryover are common problems with this technique, provision must be made for recovery and recycling. 

The modeling of fluidized beds remains a difficult problem since the usual assumptions made for the heat and mass transfer processes in coal combustion in stagnant air are no longer vaUd. Furthermore, the prediction of bubble behavior, generation, growth, coalescence, stabiUty, and interaction with heat exchange tubes, as well as attrition and elutriation of particles, are not well understood and much more research needs to be done. Good reviews on various aspects of fluidized-bed combustion appear in References 121 and 122 (Table 2). 

Size Reauction Three major size-reduction mechanisms occur in the fluidized bed. These are attrition, impact, and thermal decrepitation. 

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. 

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... 

Therefore, in this study, activated carbon, activated alumina, molecular sieve 5A, and molecular sieve 13X were used as dry sorbents to control carbon dioxide in a fluidized bed. In addition, the attrition and percentage loss of adsorption capacity of the dry sorbents were investigated. 

The remaining weight of dry sorbents with time is shown in Fig. 2. For every dry sorbent, attrition mainly still occurs in the early stage of fluidization and A1 test on the basis of the weight after 5 hours shows that Als of molecular sieve 5A and molecular sieve 13X presented 2.1 4.0 times higher than those of activated carbon and activated alumina. Therefore, the use of molecular sieve 5A and 13X in a fluidized bed can cause high maintenance cost and problems in the operating the process. 

Gas velocity is an important operating condition in the fluidized bed process and it can highly affect the attrition of dry sorbents. Therefore, the weight remaining in the bed with fluidization time for gas velocity of 20.59 cm/s, 25.74 cm/s, and 30.89 cm/s was measured to estimate the attrition of dry sorbent with gas velocity. As shown in Fig. 4, attrition mainly occurred in the early stage of fluidization. The attrition rate with time decreased and the regression equations fit natural log functions. In addition, Fig. 4 shows that the attrition of dry sorbents is highly affected by gas velocity in the fluidized bed process. 

In the fluidized bed process, attrition caused dry sorbent to be carryover. This mainly occurred in the early stage of fluidization and was highly affected by gas velocity. The amount of attrition of molecular sieve 5 A and molecular sieve 13X were larger than those of activated carbon and activated alumina. In addition, percentage losses of adsorption capacities of molecular sieve 5A and molecular 13X were 14.5% and 13.5%, whereas those of activated carbon and activated alumina were 8.3% and 8.1%, respectively. This is because retention time of molecular sieve 5A and molecular 13X decreased due to elutriation of particle generated from attrition. Also, Ka of activated alumina and activated carbon were the lower than those of Molecular sieve 13X and 5A. Consequently, molecular sieve 5A and molecular 13X could cause high maintenance cost for dry sorbent and problems in the operation of fluidized bed process. 

One disadvantage of fluidized beds, as discussed previously, is that attrition of the catalyst can cause the generation of catalyst fines, which are then carried over from the bed and lost from the system. This carryover of catalyst fines sometimes necessitates cooling the reactor effluent through direct contact heat transfer by mixing with a cold fluid, since the fines tend to foul conventional heat exchangers. 

Designing a model fluidized bed which simulates the hydrodynamics of a commercial bed requires accounting for all of the mechanical forces in the system. In some instances, convective heat transfer can also be scaled but, at present, proper scaling relationships for chemical reactions or hydromechanical effects, such as particle attrition or the rate of tube erosion, have not been established. 

Particle attrition not due to thermal or chemical reaction effects (i.e., mechanical attrition) occurs much more rapidly in the grid region of fluidized beds than in the bulk of the bed. This is due to high-velocity gas... 

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. 

The attrition rate, i.e., the rate of generation of fines, 0-d microns, at the submerged jets in a fluidized bed, tends to fall off asymptotically with time to a steady-state rate as shown in Fig. 9. Initially the attrition rate is high due to the wearing off of angular comers. Typically, it takes long time, hours to days, for the particles to reach steady-state (equilibrium) where the particles tend to be more rounded. For most catalytic fluidized bed processes, the bed operates at equilibrium. That means the most significant part of the attrition rate curve is the steady-state rate. 

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