Deactivation by Coke

Usually, the activity of small-pore molecular sieves can be restored by burning the coke with air at high temperature. A permanent loss of activity has been observed in cation-exchanged chabazites and attributed to structural degradation during 

Gubisch and Bandermann studied the behavior of HCl-exchanged zeolite Na-T as a function of the number of regeneratiye cycles. 

Regeneration was carried out between 693 K and 793 K. They found that the catalyst activity decreased slowly from cycle to cycle, independently of the regeneration temperature. After 16 regenerations the conversion of methanol dropped from 100 to 80%. 

Liang et al. carried out a study on SAPO-34, erionite and erionite-offretite zeolites comprising 55 regenerative cycles. The catalysts were regenerated at 530 C in air after reaction at 450 °C and 5 h WHSV. With SAPO-34 the initial conversion of methanol after 55 regenerations was still 100%, while the total hydrocarbon yield and content of Cj-C olefins in the hydrocarbon fraction were also constant. The offretite-erionite type zeolite used in this work suffered an appreciable deterioration after repeated regenerations. 

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The specific rate is expected to have an Arrhenius dependence on temperature. Deactivation by coke deposition in cracking processes apparently has this kind of correlation. 

Figure 9.10. Scheme of an FCC Unit. Cracking ofthe heavy hydrocarbon feed occurs in an entrained bed, in which the catalyst spends only a few seconds and becomes largely deactivated by coke deposition. Coke combustion in the regenerator is an exothermic process that generates heat for the regeneration and for the endothermic cracking process. 

The hydrocarbon catalytic cracking is also a chain reaction. It involves adsorbed carbonium and carbenium ions as active intermediates. Three elementary steps can describe the mechanism initiation, propagation and termination [6]. The catalytic cracking under supercritical conditions is relatively unknown. Nevertheless, Dardas et al. [7] studied the n-heptane cracking with a commercial acid catalyst. They observed a diminution of the catalyst deactivation (by coking) compared to the one obtained under sub-critical conditions. This result is explained by the extraction of the coke precursors by the supercritical hydrocarbon. 

Catalyst redispersion, 5 230-231 Catalyst regeneration, 5 202, 230, 255-322 catalyst deactivated by coke or carbon, 5 304, 309... 

Notably, since high-temperature steam reforming enhances the r-WGSR, which would produce CO, the undesirable poison of fuel cells, low-temperature reforming is preferable. Low temperatures can be achieved over strong acid catalysts, although the strong acid at the same time tends to cause deactivation by coke formation. 

These interrelations are consistent with the above model of high temperature deactivation by coke formation through a reaction of coke growth with methanol. However, this mechanism needs coke seeds provided as "olefin coke" on external acidic centers. Development of ZSM5-catalysts for high temperature application with long life time thus concerns minimizing of acid sites on crystallite surfaces. 

A substantial difficulty in ethanol SR is a too rapid catalyst deactivation due to coking. This can occur by several reactions, such as methane decomposition (19) or the Boudouard reaction (20), but primarily the polymerization of ethylene is thought to cause the problems (21). Unlike the situation for methane SR, it appears that for ethanol SR the deactivation by coke formation is lower at high temperatures. 

Catalysts deactivated by coking can usually be regenerated if treated properly at high temperatures so that the carbon is burned off. This way, the initial activity can be totally, or to an extent, restored. 

Very high selectivities for ethene at high conversions could be achieved in the absence of gaseous oxygen. As mentioned before, under such conditions, the catalysts would be deactivated by coking. If the catalyst is an oxide, deactivation by reduction also occurs. The catalysts must be regenerated by treatment with oxygen in a cyclic operation (23). 

Ceria-containing samples were prepared that showed a lower tendency towards coke formation, which was most pronounced for an Rh/Pt/Ce02 catalyst. It showed no measurable deactivation for all reaction temperatures applied in the test protocol. Hence this catalyst showed the best performance of all the samples discussed above. For this Rh/Pt/Ce02 catalyst, which was identified as the best catalyst regarding activity, selectivity and deactivation by coke formation, the effect of increasing the S/C ratio was determined at temperatures between 650 and 750°. These measure-... 

Similar dependence of catalyst deactivation on coke or catalyst residence time is suggested by Corella et al. (5-7). The authors give details on possible mechanisms of catalyst deactivation by coke, and also suggest, based on their data, that the deactivation order n may not be a constant. For our analysis, however, we will assume that n is constant and a function of catalyst type. Further theoretical treatment of catalyst decay is given by Wojdechowski (8.9). 

The catalyst and oil are in plug flow and the contact time is short so that secondary reactions are avoided and catalyst deactivation by coke formation is properly simulated. The resulting product selectivity, then, is similar to commercial units. Experimental results from a laboratory scale unit can thus be translated to commercial units. 

The problem associated with zeolites as nitration catalysts will be a reversible deactivation by coke deposition, and an irreversible deactivation by framework A1 removal (acid leaching). Optimization of zeolite activity, selectivity and life will be controlled by density of acid sites, crystalline size and hydrophobic/hydrophilic surface properties. 

Modeling Zeolite Catalyst Deactivation by Coking and Nitrogen Compound Poisoning... 

Catalyst deactivation by coke deposition is a major concern in upgrading coal-derived oils. Coke forms as a results of a sequence of side reactions which may be simplified as follows ... 

A recycle electrobalance reactor for the study of catalyst deactivation by coke formation... 

Since the coking reaction is not deactivated by coke formation, its deactivation function equals one. 

Basically, we could consider the FCC catalyst system as a combination of a shrinking core of sites not yet deactivated by coke and a progressing shell of large hydrocarbon molecules and metal contaminants, penetrating into the catalyst particle. 

G.F.A. Froment "A quantitative Approach of Catalyst Deactivation by Coke Formation", Proceedings International Symposium on Catalyst Deactivation, Elsevier, Amsterdam 1980,... 

In a study of the deactivation by coking of an atmospheric residue HDM catalyst, we have been able to obtain coked catalysts almost free from metal deposits in batch reactor and coked catalysts containing small amounts of metal sulfide deposits in continuous flow reactor using a Safaniya atmospheric residue under similar experimental conditions (30). We report in this paper a study of the deactivating effects of the deposits using toluene hydrogenation, cyclohexane isomerization and thiophene hydrodesulfurization reactions. 

The activity decay of cracking catalysts chemical and structural deactivation by coke... 

Regeneration is a critical step in catalytic reformer operation to regain activity, selectivity and stability of deactivated catalyst. Regeneration procedures and capabilities are dependent on the causes of deactivation. The procedures are proprietary in nature and supplied by catalyst vendors or process licensors The catalyst deactivated by coke can be easily regenerated to restore it s activity, Modified methods are adopted when catalyst had suffered from sulfur or water upset. It is important to emphasize that on line catalyst samplers are good tools to know the state of catalyst, causes of deactivation and help in improving operational and regeneration effkiency[ll]. There are no samplers installed in the reformer under discussion... 

The total acidity deterioration and the acidity strength distribution of a catalyst prepared from a H-ZSM-5 zeolite has been studied in the MTG process carried out in catalytic chamber and in an isothermal fixed bed integral reactor. The acidity deterioration has been related to coke deposition. The evolution of the acidic structure and of coke deposition has been analysed in situ, by diffuse reflectance FTIR in a catalytic chamber. The effect of operating conditions (time on stream and temperature) on acidity deterioration, coke deposition and coke nature has been studied from experiments in a fixed integral reactor. The technique for studying acidity yields a reproducible measurement of total acidity and acidity strength distribution of the catalyst deactivated by coke. The NH3 adsorption-desorption is measured by combination of scanning differential calorimetry and the FTIR analysis of the products desorbed. 

Using fixed dolomite guard beds to lower the input tar concentration can extend Ni catalyst lifetimes. Adding various promoters and support modifiers has been demonstrated to improve catalyst lifetime by reducing catalyst deactivation by coke formation, sulfur and chlorine poisoning, and sintering. Several novel, Ni-based catalyst formulations have been developed that show excellent tar reforming activity, improved mechanical properties for fluidized-bed applications, and enhanced lifetimes. 

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