Loss due to Attrition

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

In the moving bed, gradients exist from top to bottom of the reactor with respect to both temperature and concentration of coke on the catalyst. The activity and coke concentration vary from particle to particle at a given level in the reactor because of the intermittent addition of fresh catalyst to replace losses due to attrition or intentional discard. However, a steady state exists with regard to time, so that all increments of the feed stock are exposed to the same cracking conditions and a uniform conversion is maintained. 

A suitable sorbent must be available in large quantities and at low cost. Its performance should not deteriorate in service, that is, it should be almost insoluble in sea water and eluants, and highly stable against physical, chemical, and biological degradation in order to permit long-term recycling procedures and to avoid contamination of the sea. Further, since most of the uranium is sorbed only on the surface of the sorbent particles, any loss due to attrition would mean a serious loss of uranium. 

The process is sometimes operated with fluidised catalyst bed to achieve isothermal conditions. However, more energy is required for increased gas flow velocity and loss due to attrition are also more. The crumbled catalyst particles can get blown off and deposit as a fine powder on heat transfer surfaces of the downstream equipments. Cyclone separator is installed to remove the catalyst particles fi om gas stream... 

The fractional burn-off includes the mass loss due to attrition, but this may not alter above conclusions since mass loss due to attrition is very small compared to that due to combustion. Furthermore, attrition is predicted by the shrinking sphere model. 

The DOE/METC report concludes that both zinc ferrite and zinc tilanate sorbents have the capability to reduce the hydrogen sulfide concentration in coal gas to less than 20 ppm in a fluidized bed reactor. However, the zinc ferrite sorbents were found to suffer structural weakening and losses due to attrition and vaporization at temperatures above about 550°C (1,022°F) whereas, the zinc titanate sorbents, prepared by a proprietary granulation technique, showed excellent sulfur capacity, regenerability, attrition resistance, and zinc vaporization characteristics at temperatures up to about 650°C (1,202°F). 

The improvement in the calibration curve was not significant. A significant retention of the hi er particle sizes occurred in the column following this additional treatment. This is clearly shown in Fig.l for a 312 nm sample which was injected before and after the treatment. Also note the shift in peak position probably caused by loss of pore volume due to attrition between particles of the glass packing. This additional treatment was subsequently abandoned. 

For the special case of an isolated cyclone, which is fed with a mass flux mc in of material sufficiently large to be sent into the catch of the cyclone, the measured mass flux in the loss of the cyclone, mc loss is solely due to attrition inside the cyclone. In such a case the cyclone attrition rate Ra c may be defined by... 

The resin beads used in most columnar operations range in size from 0.3 to 0.9 mm in diameter, which is a compromise based on the effect of ion-exchange rates, capacities, and hydraulic characteristics. The especially made resins used in resin-in-pulp operations range in size from 0.8 to 1.6 mm in diameter. The apparent density of a resin is defined as that weight of backwashed and settled wet resin per cubic foot, which for resins used in the uranium industry is about 38-45 Ib/ft . In column operations, the attrition losses due to swelling and contraction of resin, abrasion of resin-resin surfaces, and abrasion of resin-equipment surfaces are negligible. In resin-in-pulp operations, an appreciable amount of attrition loss is encountered. 

In the early days of development of continuous ion exchange processes the benefits to water treatment of high efficiency and low leakage were paralleled by the rapidly advancing designs of counterflow fixed bed plant. Thus the more complex hydrodynamic requirements of CIX, resin losses due to mechanical attrition, and perhaps a conservative attitude towards availability of plant during periods of unscheduled maintenance meant that, in the UK at least, the CIX... 

From a practical standpoint catalyst loss due to carryover with the gas stream from the reactor and regenerator may be an important problem. Attrition of particles decreases their size to a point where they are no longer fluidized, but move with the gas stream. It has been customary to recover most 6f these catalyst fines by cyclone separators and electrical precipitation equipment placed in the effluent lines from reactor and regenerator. 

In most catalytic systems there is a gradual loss of activity due to contamination or attrition of the catalyst, so the catalyst must be replaced at regular... 

Variances in resin performance and capacities can be expected from normal annual attrition rates of ion-exchange resins. Typical attrition losses that can be expected include (1) Strong cation resin 3 percent per year for three years or 1,000,000 gals/ cu.ft (2) Strong anion resin 25 percent per year for two years or 1,000,000 gals/ cu.ft (3) Weak cation/anion 10 percent per year for two years or 750,000 gals/ cu. ft. A steady falloff of resin-exchange capacity is a matter of concern to the operator and is due to several conditions ... 

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. 

Changes in the fresh catalyst s physical properties may contribute to catalyst losses. The losses could be due to the fresh catalyst s being soft. Softness is evidenced by the quality of the catalyst binder and the large amount of 0-40 microns. It will increase the attrition tendency of the catalyst and thus its losses. 

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. 

CO2 adsorption capacities with dry sorbents before and after attrition were shown in Fig.3. We found variation of CO2 adsorption capacity during operation by examining effect of attrition on adsorption capacity. So, adsorption experiments for each sorbent fluidized for 30hours were carried out. As a result, percentage losses of adsorption capacity of molecular sieve 5A and molecular 13X were 14.5% and 13.5%, but 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. 

Determining the value of potential benefits from risk reduction is relatively straightforward for tangible losses such as property damage, business interruption, and increased insurance costs. However, intangibles such as loss of reputation are difficult to estimate and must be considered on a case-by-case basis. In addition to increased staff costs associated with public relations, items such as possible employee attrition due to low morale and possible loss of market share must be considered. 

Cyclones. According to the model presented above, Eq. (24), a minimum loss rate due to cyclone attrition requires to avoid both high inlet velocities Ue and high solids mass fluxes mc m at the cyclone inlet. The latter requirement can be fulfilled by locating the cyclone inlet above the transport disengaging height (TDH) (Kunii and Levenspiel, 1991). In addition, an enlargement of the freeboard section will reduce the amount of particles that are entrained and thus the mass flux, mc in. 

The mechanical erosion of a solid surface such as a pipe wall in a gas—solid flow is characterized by the loss of solid material from the solid surface due to particle impacts. The collisions of the particles either with other particles or with a solid wall may lead to particle breakup, known as particle attrition. Pipe erosion and particle attrition are major concerns in the design of a gas-solid system and during the operation of such a system. The wear of turbine blades or pipe elbows due to the directional impact of dust or granular materials, the wear of mechanical sieves by the random impact of solids, and the wear of immersed pipes in a fluidized bed by both directional and random impacts are examples of the erosion phenomenon in industrial systems. The surface wear associated with the erosion phenomenon of a gas-solid flow has been exploited to provide beneficial industrial applications such as abrasive guns, as well. 

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