Attrition regeneration

Most adsorption systems use stationary-bed adsorbers. However, efforts have been made over the years to develop moving-bed adsorption processes in which the adsorbent is moved from an adsorption chamber to another chamber for regeneration, with countercurrent contacting of gases with the adsorbents in each chamber. Union Oil s Hypersorption Process (90) is an example. However, this process proved uneconomical, primarily because of excessive losses resulting from adsorbent attrition. 

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. 

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. 

The lift pipe design was tapered to a larger diameter at the top. This minimized the effects of erosion and catalyst attrition, and also prevented the instantaneous total collapse of circulations when the saltation concentration, or velocity, of solids is experienced (i.e. the slump veloeity-that velocity helow which particles drop out of the flowing gas stream). In a typical operation, 2 % to 4 % eoke can he deposited on the catalyst in the reactor and burned in the regenerator. Catalyst circulation is generally not sufficient to remove all the heat of eombustion. This facilitated the need for steam or pressurized water coils to be located in the regeneration zone to remove exeess heat. 

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... 

An addition of a large amount of steam to the regenerator, particularly to the torch oil nozzles, again causing catalyst attrition... 

In a continuous reformer, some particulate and dust matter can be generated as the catalyst moves from reactor to reactor and is subject to attrition. However, due to catalyst design little attrition occurs, and the only outlet to the atmosphere is the regeneration vent, which is most often scrubbed with a caustic to prevent emission of hydrochloric acid (this also removes particulate matter). Emissions of carbon monoxide and hydrogen sulfide may occur during regeneration of catalyst. 

Of the various mechanical properties of a formed catalyst containing zeolite, attrition resistance is probably the most critical. This is particularly the case for FCC catalysts because of the impact on the addihon rate of fresh catalyst, particulate emissions of fines and overall catalyst flow in the reactor and regenerator. Most attrition methods are a relative determination by means of air jet attrition with samples in the 10 to 180 xm size range. For example the ASTM D5757 method attrites a humidified sample of powder with three high velocity jets of humidified air. The fines are continuously removed from the attrition zone by elucidation into a fines collection assembly. The relative attrition index is calculated from the elutriated fines removed at a specific time interval. 

For some applications, regeneration is not possible, and the material must be discarded. Additional problems include the fact that the charcoal sorbs based on molecular size pollutants with molecular sizes greater than the pores of the charcoal are unaffected. Flow problems and attrition of the carbon particles are other difficulties. Activated charcoal columns are usually pressure vessels due to the large and dynamic pressure drops across the carbon bed. 

Continuous fluidized bed equipment has been utilized for gas adsorption, but usually attrition losses of comparatively expensive adsorbents have been prohibitive and the loss of efficiency because of axial mixing has been a serious handicap. Drying equipment such as those of Figure 9.13 presumably can be operated in reverse to recover valuable substances from a vapor phase, and the forward mode applied for regeneration in associated equipment. Other possibly suitable fluidized bed configurations are those of the reactors of Figures 17.32(a), (c), and (d). 

Advantages of fluidized beds are temperature uniformity, good heat transfer, and the ability to continuously remove catalyst for regeneration. Disadvantages are solids backmixing, catalyst attrition, and recovery of fines. Baffles have been used often to reduce backmixing. 

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