Coking or fouling

Tendency to coke or foul far less than other grids, due to elimination of horizontal planes where liquids or solids can stagnate. 

Deactivation by Coking or Fouling. This mechanism of decay (see Figure 10-21) is common to reactions involving hydrocarbons. It results from a carbonaceous (coke) material being deposited on the surface of a catalyst. 

Coking or fouling loss of activity, normally in reactions involving hydrocarbons, due to reactant or product degradation, producing a carbonaceous residue on the surface which is inactive for catalysis. 

Nearly every cat cracker experiences some degree of coking/fouling. Coke has been found on the reactor walls, dome, cyclones, overhead vapor line, and the slurry bottoms pumparound circuit. Coking and fouling always occur, but they become a problem when they impact throughput or efficiency. 

A common form of pretreatment is to remove the risk of fouling and staining by iron and manganese using an aeration tower with a coke or media catalyst, onto which the aerated water falls, promoting the oxidation and precipitation of insoluble ferric hydroxide and manganese dioxide. 

Catalysts can be regenerated, that is, the performance of deactivated catalysts can be improved by regeneration. That is, when the catalyst activity and/or selectivity is reduced during operation, a particular treatment allows the proper activity and/or selectivity of the catalysts to be restored [7], Deactivation is produced by inhibition, fouling, or sintering, and all of these can be reversed, by the removal of poisons, or fouling agents, like coke, or by the re-dispersion of the active species [8], The reproducibility of a catalyst is related to the consistency of its properties in different sets of production lots [7],... 

The deactivation of catalysts concerns the decrease in concentration of active sites on the catalyst Nj. This should not be confused with the reversible inhibition of the active sites by competitive adsorption, which is treated above. The deactivation can have various causes, such as sintering, irreversible adsorption and fouling (for example coking or metal depositions in petrochemical conversions). It is generally attempted to express the deactivation in a time-dependent expression in order to be able to predict the catalyst s life time. An important reason for deactivation in industry is coking, which may arise from a side path of the main catalytic reaction or from a precursor that adsorbs strongly on the active sites, but which cannot be related to a measurable gas phase concentration. For example for the reaction A B the site balance contains also the concentration of blocked sites C. A deactivation function is now defined by cq 24, which is used in the rate expression. 

It follows that regeneration may consist of either (i) removal of IS sometimes poisons, most often inhibitors or fouling agents, e.g., coke (hydrogenation catalysts, e.g., selective hydrogenation of pyrolysis gasoline) or (ii) redispersion of the active species (platinum catalysts) or (iii) both (hydrodesulfurization or catalytic reforming catalysts). 

The major portion of sait is found in residues as these streams serve as the bases for fuels, or as feeds for asphalt and petroleum coke production, the presence of salt in these products causes fouling of burners, the alteration of asphalt emulsions, and the deterioration of coke quality. Furthermore, calcium and magnesium chlorides begin to hydrolyze at 120°C. This hydrolysis occurs rapidly as the temperature increases (Figure 8.1) according to the reaction i. ... 

Application all services except extremely coking, polymer formation or other high fouling conditions. Use for extremely low flow conditions where tray must remain wet and maintain a vapor seal. 

The quality of the FCC feed also impacts coke buildup in the reactor internals and vapor line and fouling/coking of the main column circuit. The asphaltene or the resid content of the feed, if not converted in the riser, can contribute to this coking. 

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