Coke contents

Coke on the catalyst is often referred to as delta coke (AC), the coke content of the spent catalyst minus the coke content of the regenerated catalyst. Delta coke directly influences the regenerator temperature and controls the catalyst circulation rate in the FCCU, thereby controlling the ratio of catalyst hydrocarbon feed (cat-to-od ratio, or C/O). The coke yield as a fraction of feed Cpis related to delta coke through the C/O ratio as ... 

The impact that variations in coke content and burning conditions can have on the overall heat of coke combustion is shown in Table 2. Because the heat balance dictates the amount of heat that is required from burning coke, the heat of combustion then determines the amount of coke that must be burned. 

Delta Coke is the difference between the coke content of the spent catalyst and the coke content of the regenerated catalyst. Numerical value of delta coke is calculated from ... 

Coke formation on these catalysts occurs mainly via methane decomposition. Deactivation as a function of coke content (see Fig. 3 for Pt/ y-AljO,) seems to involve two processes, i e, a slow initial one caused by coke formed from methane on Pt that is non reactive towards CO2 (see Table 3) In parallel, carbon also accumulates on the support and given the ratio between the support surface and metal surface area at a certain level begins to physically block Pt deactivating the catalyst rapidly. The coke deposited on the support very close to the Pt- support interface could be playing an important role in this process. 

Fig. 3.3.2 Relaxation dispersion T7(v) for (a, b) n-heptane and (c, d) water at room temperature in catalyst pellets at various stages of coking and regeneration. Numbers indicate weight-percentages of coke (a, c) and residual coke content during regeneration (b, d).

Contrary to the findings for water, the relative change of the absolute values of Ti and of its frequency dependence is much more pronounced for n-heptane, which possesses a high affinity for the surface coke. One of the steps in the dispersion function can be interpreted as being related to the onset of full coverage of the surface. Because the maximum coke content corresponds to an average layer... 

In Figure 3.3.2, the strong dependence of the 3H relaxation time of n-heptane on coke content was shown for low magnetic field strengths although less pronounced, this 7 dependence still holds for high fields [2]. For large catalyst pellets... 

Fig. 3.3.11 Partially regenerated, coked Al203 catalyst samples with the same residual coke content of 7.65% left, regenerated at 550 °C right, regenerated at 400 °C. (a) Optical photographs of cut samples (b) NMR images with...

The material balance is consistent with the results obtained by OSA (S2+S4 in g/100 g). For oil A, the coke zone is very narrow and the coke content is very low (Table III). On the contrary, for all the other oils, the coke content reaches higher values such as 4.3 g/ 100 g (oil B), 2.3 g/ioo g (oil C), 2.5 g/ioo g (oil D), 2.4/100 g (oil E). These organic residues have been studied by infrared spectroscopy and elemental analysis to compare their compositions. The areas of the bands characteristic of C-H bands (3000-2720 cm-1), C=C bands (1820-1500 cm j have been measured. Examples of results are given in Fig. 4 and 5 for oils A and B. An increase of the temperature in the porous medium induces a decrease in the atomic H/C ratio, which is always lower than 1.1, whatever the oil (Table III). Similar values have been obtained in pyrolysis studies (4) Simultaneously to the H/C ratio decrease, the bands characteristics of CH and CH- groups progressively disappear. The absorbance of the aromatic C-n bands also decreases. This reflects the transformation by pyrolysis of the heavy residue into an aromatic product which becomes more and more condensed. Depending on the oxygen consumption at the combustion front, the atomic 0/C ratio may be comprised between 0.1 and 0.3 ... 

Chen and co-workers have studied the role of coke deposition in the conversion of methanol to olefins over SAPO-34 [111]. They found that the coke formed from oxygenates promoted olefin formation while the coke formed from olefins had only a deactivating effect The yield of olefins during the MTO reaction was found to go through a maximum as a function of both time and amount of coke. Coke was found to reduce the DME dilfusivity, which enhances the formation of olefins, particularly ethylene. The ethylene to propylene ratio increased with intracrystal-line coke content, regardless of the nature of the coke. 

Here, Cc is the concentration of the active sites covered by coke. The authors presented empirical relations to connect (pA with the coke content of the catalyst Cc ... 

There is a complex and little understood relationship between coke content, catalyst activity, and the chemical nature of the coke. For instance, the H/C ratio of coke depends on how the coke was formed its exact value will vary from system to system (Cumming and Wojciechowski, 1996). And it seems that catalyst decay is not related in any simple way to the hydrogen-to-carbon atomic ratio of the coke, or to the total coke content of the catalyst, or any simple measure of coke properties. Moreover, despite many and varied attempts, there is currently no consensus as to the detailed chemistry of coke formation. There is, however,... 

A. Net Coke Content and Coke Selectivity at Constant Conversion... 

The coke content of the zeolite samples was measured on a Perkin-Elmer 240B CHN instrument which uses a combustion method to convert the sample elements to simple gases (C02, H20 and N2). The sample is first oxidized in a pure oxygen environment the resulting gases are then controlled to exact conditions of pressure, temperature and volume. Finally, the product gases are separated. Then, under steady-state conditions, the gases are measured as a function of thermal conductivity. The results are accurate to + 0.5%, absolute. 

Table I. Treatment and Coke Contents of Protonated H-ZSM-5 and Modified Na, H-ZSM-5 Samples...

Figures 1A and 1B show the adsorption isotherms of xenon on the Na, H-ZSM-5 and H-ZSM-5 zeolites, respectively. From the comparison, one sees that xenon uptake decreases slightly (about 10%) with coke content in the Na, H-ZSM-5 with a low (1%) coke content, on zeolite H-ZSM-5, and decreases only slightly more with heavy coking (12%).

Figure 1. (a) Xenon adsorption isotherms (at 297 K) of the size-selectively modified Na,H-ZSM-5 zeolites having different coke contents --uncoked A--1 wt % coke B--12 wt % coke, (b) Xenon adsorption isotherms (at 297 K) of fully protonated H-ZSM-5 zeolites having different coke contents -uncoked A—1 wt%coke 12wt%coke. (Reproduced with permission from ref. 16. Copyright 1991 Academic Press Inc. 

The deactivation of a lanthanum exchanged zeolite Y catalyst for isopropyl benzene (cumene) cracking was studied using a thermobalance. The kinetics of the main reaction and the coking reaction were determined. The effects of catalyst coke content and poisoning by nitrogen compounds, quinoline, pyridine, and aniline, were evaluated. The Froment-Bischoff approach to modeling catalyst deactivation was used. 

The influence of the coke on the kinetics of the main reaction can be accounted empirically by multiplying the kinetic coefficient of eq. (4) by a deactivation function [Pg.251]

Figure 4. The linear relationship of poisoning might be due to uniform poisoning, i.e., sites of equal activity were deactivated at zero coke content. Figure 4 shows that pyridine and quinoline are more poisonous than aniline. It shows that the higher basicity compounds have greater effectiveness as poisons. Quinoline which has a higher molecular weight and lower basicity than pyridine showed a slightly lower effectiveness than pyridine.

A trickle-bed reactor was used to study catalyst deactivation during hydrotreatment of a mixture of 30 wt% SRC and process solvent. The catalyst was Shell 324, NiMo/Al having monodispersed, medium pore diameters. The catalyst zones of the reactors were separated into five sections, and analyzed for pore sizes and coke content. A parallel fouling model is developed to represent the experimental observations. Both model predictions and experimental results consistently show that 1) the coking reactions are parallel to the main reactions, 2) hydrogenation and hydrodenitrogenation activities can be related to catalyst coke content with both time and space, and 3) the coke severely reduces the pore size and restricts the catalyst efficiency. The model is significant because it incorporates a variable diffusi-vity as a function of coke deposition, both time and space profiles for coke are predicted within pellet and reactor, activity is related to coke content, and the model is supported by experimental data. 

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