Coke deposits characteristics

The carbon residue (ASTM D-189 and ASTM D-524) of a crude oil is a property that can be correlated with several other properties (Figure 2-14). The carbon residue presents indications of the volatility or gasoline-forming propensity of the feedstock and, for the most part in this text, the coke-forming propensity of a feedstock. Tests for carbon residue are sometimes used to evaluate the carbonaceous depositing characteristics of fuels used in certain types of oil-burning equipment and internal combustion engines. 

In Figure 6 the relationship between total acidity decrease and total amount of coke deposited in the catalyst is shown. The acidity-coke content relationship is nearly exponential, that is to say, there is initially an almost linear relationship, which is characteristic of coke deposition in a monolayer, while for high coke content values the acidity decrease is less pronounced. This result can be explained on the basis of either that there is an incipient coke deposition in multilayer or that part of the coke has become inert and has been desorbed by vacating acid sites [10]. The small amount of total coke suggests that pore blockage contribution to aridity deterioration should be negligible. 

It seems that there are two types of coke on the catalyst, soft or hard coke. The coke deposition c defined as soft coke in Eq. (18) has equivalent characteristics to that defined in Eq. (14). The rate of coke deposition is simulated by Eq. (2) where coke is defined as hydrogenable . This coke is speculated to be adsorbed polyaromatics rather than coke. Hard coke c defined in Eq. (19), on the contrary, is steadily produced with a deposition rate of Eq. (2) this affects the diffusivity of reactant. 

Results of experiments varying the ratio of uranium to nickel showed that the ratios giving the largest surface area and catalyst volume were in the range 0.45-0.76 (U Ni). These two characteristics were the most important for activity for these reactions. The catalysts were in a reduced state, which could explain the addition of the reduction-oxidation step in the previous patent A further reason for using catalysts in the 0.45-0.74 U Ni range was that the catalyst demonstrated greatest resistance to coke deposition at a raho of 0.71 1. 

The shape of the TPR curves for Ni/mordenite and Ni/USY (Figures 4 and 5) provides evidence that NiO is reducible only at high temperatures, a behavior which is not characteristic of nickel oxide supported on low surface area carriers [7]. The conclusion is that for zeolites the majority of nickel particles is located inside the crysMine structure of the supports. This is also supported by the results of coke deposition as studied by TGA. As seen in Tables 1 to 3, the total amount of coke found for each catalyst sample explains the deactivation curves in Figure 1 only if the stmctural aspect is considered. Nickel supported on USY contains 4.5 times more coke than nickel supported on ZSM5 and 2.6 times more... 

The most characteristic feature of the Fe-treated Y-zeolite catalyst was that catalyst itself exhibits high activity and low coke deposition for toluene... 

The process (Fig. 2.3) uses the mild thermal cracking partial conversion) as a relatively low-cost and low-severity approach to improving the viscosity characteristics of the residue without attempting significant conversion to distillates. Low residence times are required to avoid polymerization and coking reactions, although additives can help to suppress coke deposits on the tubes of the furnace. 

In addition, catalysts must have high thermal stability to resist sintering, particularly where there is a need for periodic regeneration by combustion of coke deposited on the catalyst. A further desirable characteristic is the ability to resist deactivating influence of poisons, notaby sulfur compounds, which are often present in reactant streams. 

With a steel reactor it is shown that preoxidation of the foil and the reactor surface results in much higher coke formation on the foil compared with prereduction of the reactor system (Figures 6 and 7). This is not surprising since Fe and Ni present on the oxidized surface are known to be good catalysts for carbon formation (21). Prereduced foils, which contain mostly Cr and Mn on the surface, produce much less coke. In agreement with this, the coke deposit from the preoxidized foil surface contains Fe (characteristic of catalytic coke formation) whereas the deposit on the prereduced foils contains effectively no Fe (20). 

The trend in oil processing towards more severe operating conditions and a heavier crude diet in order to enhance the upgrading of the barrel is bringing about more extensive coke formation, leading to catalyst deactivation and, ultimately, to reactor blockage. Characterisation of coke deposits may provide us with clues about catalyst characteristics essential in limiting coke formation under these extreme conditions. Furthermore, the development of deactivation models will be more effective when information not only on the quanti ty of coke hut also on the structure of the deposits is available (refs. 1—3). 

Influence of Catalyst Composition. Coke deposition on the catalyst depends on catalyst composition. The individual characteristics of both catalytic functions, metal and acid, are important as well as how they are balanced during the reaction. 

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