Catalyst rejuvenation

If the avoidance of deactivation by poisoning is difficult, could a partially deactivated catalyst be regenerated by chemical or physical means In the oil and chemical industry, catalyst reactivation is a standard procedure, done sometimes in situ or after removal from the reactor. Logistically, it does not seem to be feasible to remove an automotive 

There are two main routes for the removal of poisons one is based on chemical, the other on thermal treatment. One of the first patents for the regeneration of automotive catalysts was filed by Houdry and Calvert 

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In-situ ultrasonic catalyst rejuvenation in three-phase hydrogenation of xylose. Chem. Eng. Sci., 54, 1583-1588. 

Continuous regeneration employs a moving bed of catalyst particles that is gradually withdrawn from the reactor system and passed through a regenerator vessel, where the carbon is removed and the catalyst rejuvenated for reintroduction to the reactor system. 

After a given amount of coke is deposited on the catalyst, it is necessary to burn it off in order to recover the initial catal3d ic performance. The regeneration process consists of two well different steps (1) elimination of coke by burning and (2) catalyst rejuvenation. The first step is the most time consuming operation of the regeneration. 

Catalyst Rejuvenation. After all the coke is bum, it is necessary to redisperse, reduce and sulfide the metal, and adjust the acidity by chlorine injection. 

At low (>450° C) temperatures, the presence of these materials, particularly the oxides, leads to simple masking or fouling. In some cases, a catalyst that shows reduced activity beheved to be from poisoning may simply be masked, and activity can be rejuvenated by cleaning with aqueous leaching solutions (21). 

Yoo, J.S. (1998) Metal recovery and rejuvenation of metal-loaded spent catalysts. Catalysis Today, 44 (1—4), 27 16. 

The Rh-based dimerization catalyst described above cannot maintain its activity indefinitely. After prolonged reaction (several hours) or after removal of the products and monomers from the reaction mixture, it becomes deactivated or decays to an inactive species. Because Rh metal is very expensive, it becomes desirable to seek a way to rejuvenate the dead catalyst. The generally recognized mode of decay is the degeneration of the active Rhra to an inactive Rh1 complex (10-12), e.g., R— Rh,uCl RC1 + Rh1. 

The spent-and-stripped cracking catalyst and the rejuvenated SOx catalyst then move from the stripper to the FCCU regenerator for the start of another cycle. 

A method of rejuvenating epoxidation catalysts is described by Grey et al. (3). 

Fig. 25. Deactivation comparison of rejuvenated and fresh catalysts. [From Rothschild (111).] (Reprinted with permission of the Society of Automotive Engineers.)...

Both poisoned catalysts were rejuvenated with 4 hours of O2 followed by 4 hours of H2. Figure 7b shows that the 250 ppm H2S/H2 catalysts have reached comparable activity to that treated with H2 (fig- 7a). The 3000 ppm H2S/H2 treated catalysts have exhibited similar initial activity, as shown in figure 7c, but slowly deactivated to around 75% after 4 hours of reaction. 

As we have pointed out in relation to stability, it is only in theory that the catalyst is found intact at the end of the reaction. All catalysts age and when their activities or their selectivities have become insufficient, they must be regenerated through a treatment that will return part or all of their catalytic properties. The most common treatment is burning off of carbon, but scrubbing with suitable gases is also frequently done to desorb certain reversible poisons hydrogcnolysis of hydrocarbon compounds may be done when the catalyst permits it, as well as an injection of chemical compounds. When the treatment docs not include burning off carbon deposits, it is often called rejuvenation. 

The shorter the cycle of operating time between two regenerations, the more important the regeneration. It becomes apparent that it is not enough for the catalyst to recover its activity and selectivity, it must also preserve its mechanical strength during successive regenerations or rejuvenations. 

It is for the above reasons that rejuvenation of supported metal catalysts is typically performed in oxidizing atmospheres. However, redispersion mechanisms may be much more complex than indicated since fragmentation of particles may also occur during the thermal treatments in oxidizing atmospheres (see subsection C). 

IR spectroscopy is the most widely used technique to characterize catalysts and molecules adsorbed on them. It has been successfully applied to dispersed catalysts as well as to planar model catalyst. Comprehensive reviews by Sheppard and De la Cruz (18,19), Hoffmann (17), Chabal (161), and others (162) describe the basics, technical aspects, and applications of the technique to a variety of catalysts (considering, for example, catalyst preparation, activation and rejuvenation, and the state of the catalyst during the course of a catalytic reaction). The reader is referred to these reviews for details here, we focus on recent developments and high-pressure applications. 

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