Catalyst coke content

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. 

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. 

Figure 4. Most frequent pore diameter vs. catalyst coke content. Key model prediction A, experimental data.

Catalyst coke content is a good measure of activity. Both hydrogenation and hydrodenitrogenation can be related to coke content. 

The deactivation of cracking catalysts by coking with vacuum gas oils (VGO) is studied in relation to the chemical deactivation due to site coverage, and with the increase of diffusional limitations. These two phenomena are taken into account by a simple deactivation function versus catalyst coke content. The parameters of this function arc discussed in relation to feedstock analysis and change of effective diffiisivity with catalyst coke content. 

Three extraction experiments, runs 11-13, were conducted with carbon dioxide. Run 12 was conducted at a reduced pressure of 0.93 and a reduced temperature of 1.05 for 13 h. The catalyst coke content was reduced from 17.5% to 11%, where the coke was primarily removed from pores of 9.6 nm diameter. This represented a 37% removal of coke from the catalyst and resulted in a bimodal pore size distribution with a pore volume of 0.22 nr/g and a surface area of 137 mz/g. The changes in the pore size distribution are shown in Fig. 1. The other two extractions with carbon dioxide... 

Figure 5. Effect of catalyst/feed ratio on catalyst coke content (%) with time on stream. 

Furthermore, the catalyst status at this conversion level can be obtained from Figures 3 to 5 in terms of the catalyst coke content and surface area. Hence the status of the equilibrium catalyst used in the commercial operation should be adjusted by ... 

Coke content Wt.% of the catalyst Coke content Wt% of the catalyst... 

To account for these different time scales, different activity functions are used for the formation of coke and for the formation of the other products the activity for coke formation is described by an exponentially decaying function of the residence time (characteristic time of about 0.02 s) the conversion and formation of the other products are described by one activity function that decreases exponentially as function of the catalyst coke content. This model describes the conversion obtained with the regenerated catalyst according to the curve that was drawn in Figure 2. 

The lower part of Figure 1 also shows that the presence of nickel promoter ions in the catalyst had no influence on the catalyst coke content. This is consistent with Equation 13 which shows that undesirable reactions are not influenced by the number of electron holes (0) in the catalyst. 

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 ... 

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).

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...

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,... 

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 coke content of the catalyst, Cc ... 

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