Attrition of catalyst

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. 

The particles to be removed may range in size from large molecules, measuring a few hundredths of a micrometre, to the coarse dusts arising from the attrition of catalysts or the fly ash from the combustion of pulverised fuels. 

Modeling of Jet-Induced Attrition. Werther and Xi (1993) compared the jet attrition of catalysts particles under steady state conditions with a comminution process. They suggested a model which considers the efficiency of such a process by relating the surface energy created by comminution to the kinetic energy that has been spent to produce this surface area. The attrition rate, RaJ, defined as the mass of attrited and elutriated fines per unit time produced by a single jet, is described by... 

In general, the high operating gas velocities for lean phase fluidization yield a short contact time between the gas and solid phases. Fast fluidized beds and co-current pneumatic transport are thus suitable for rapid reactions, but attrition of catalyst may be serious. 

Werther J, Xi W. Jet attrition of catalyst particles in gas fluidized beds. Powder Technol 76(l) 39-46, 1993. 

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

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

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

A fluidi2ed-bed catalytic reactor system developed by C. E. Lummus (323) offers several advantages over fixed-bed systems ia temperature control, heat and mass transfer, and continuity of operation. Higher catalyst activity levels and higher ethylene yields (99% compared to 94—96% with fixed-bed systems) are accompHshed by continuous circulation of catalyst between reactor and regenerator for carbon bum-off and continuous replacement of catalyst through attrition. 

Use of the peUetted converter, developed and used by General Motors starting in 1975, has declined since 1980. The advantage of the peUetted converter, which consists of a packed bed of small spherical beads about 3 mm in diameter, is that the pellets were less cosdy to manufacture than the monolithic honeycomb. Disadvantages were the peUetted converter had 2 to 3 times more weight and volume, took longer to heat up, and was more susceptible to attrition and loss of catalyst in use. The monolithic honeycomb can be mounted in any orientation, whereas the peUetted converter had to be downflow. AdditionaUy, the pressure drop of the monolithic honeycomb is one-half to one-quarter that of a similar function peUetted converter. 

ActivatedL yer Loss. Loss of the catalytic layer is the third method of deactivation. Attrition, erosion, or loss of adhesion and exfoHation of the active catalytic layer aU. result in loss of catalyst performance. The monolithic honeycomb catalyst is designed to be resistant to aU. of these mechanisms. There is some erosion of the inlet edge of the cells at the entrance to the monolithic honeycomb, but this loss is minor. The peUetted catalyst is more susceptible to attrition losses because the pellets in the catalytic bed mb against each other. Improvements in the design of the peUetted converter, the surface hardness of the peUets, and the depth of the active layer of the peUets also minimise loss of catalyst performance from attrition in that converter. 

Although mote expensive to fabricate than the pelleted catalyst, and usually more difficult to replace or regenerate, the honeycomb catalyst is more widely used because it affords lower pressure losses from gas flow it is less likely to collect particulates (fixed-bed) or has no losses of catalyst through attrition, compared to fiuidized-bed and it allows a mote versatile catalyst bed design (18), having a weU-defined flow pattern (no channeling) and a reactor that can be oriented in any direction. 

Fig. 6. Catalyst inhibition mechanisms where ( ) are active catalyst sites the catalyst carrier and the catalytic support (a) masking of catalyst (b) poisoning of catalyst (c) thermal aging of catalyst and (d) attrition of ceramic oxide metal substrate monolith system, which causes the loss of active catalytic material resulting in less catalyst in the reactor unit and eventual loss in performance.

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... 

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... 

Catalyst attrition due to the collision of catalyst particles with the vessel internals and other catalyst particles... 

Depending on the design of a cat cracker, the circulating inventory can contain 30-1,200 tons of catalyst. Fresh catalyst is added to the unit continually to replace the catalyst lost by attrition and to maintain... 

Avoid erosion of the riser wall and attrition of the catalyst Perform without plugging or erosion... 

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