Regeneration of cracking catalysts

SOx Indices of Cracking Catalysts and Additive R at a Regenerator Temperature of 1350°F ... 

Extensive coke deposition takes place during catalytic cracking, resulting in loss of activity. Typically, the catalyst loses 90% of its activity within one second. An elegant solution has been found for this problem. The clue to this solution is a combination of a reactor in which cracking takes place with a reactor used for regeneration of the catalyst by burning the deposited coke. In this set-up coke is... 

Micro activity test (MAT). This test was developed and standardized by ASTM (ASTM-D-3907). In the MAT test, a sample of cracking catalyst is contacted with gas oil in a fixed-bed reactor. Gas chromatographic analysis on gas and liquid products is used to determine the yield structure. Recently, the MAT conditions have been adapted to simulate commercial units more accurately in terms of contact times. A higher catalyst activity results in improved conversion and higher regenerator temperature. 

There are at present two main systems for carrying out the cracking process, both of them incorporating the essential features of catalyst-oil contacting and air regeneration of the catalyst. The principal difference between the two processes, is catalyst particle size. The original Houdry... 

The major part of these catalytic processes is carried out in fixed bed reactors. Some of the main fixed bed catalytic processes are listed in Table 11.1-1. Except for the catalytic cracking of gas oil, which is carried out in a fluidized bed to enable the continuous regeneration of the catalyst, the main solid catalyzed processes of today s chemical and petroleum refining industry appear in Table 11.1-1. However, there are also fluidized bed alternatives for phthalic anhydride— and ethylene dichloride synthesis. Furthermore, Table 11.1-1 is limited to fixed bed processes with only one fluid phase trickle bed process (e.g., encountered in the hydrodesulfurization of heavier petroleum fractions) are not included in the present discussion. Finally, important processes like ammonia oxidation for nitric acid production or hydrogen cyanide synthesis, in which the catalyst is used in the form of a few layers of gauze are also omitted from Table 11.1-1. 

Thus, it has been established that, although the lower members of the paraffin series are highly resistant to the action of cracking catalysts, the continuous formation of a carbon-catalyst linkage and the regeneration of the hydrocarbon from the carbonium ion occur quite readily. 

Coking of catalysts can be reduced by increasing the hydrogen partial pressure or by partial neutralization of the acid sites with promoters, as we have already seen. Coke that has already formed is removed by periodic regeneration of the catalyst. The deactivated catalyst is purified by controlled combustion of the carbon layer. In flui-dized-bed crackers the catalyst circulates continuously between the reactor and the regenerator, in which combustion takes place. The heat of combustion is used to maintain the catalyst at the temperature of the slightly endothermic cracking reaction. 

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