Coking activity profile

This model is significant because 1) a variable diffusivity as a function of coke content is incorporated, 2) coke content profiles both within a pellet and the reactor bed are predicted with time and space, 3) catalyst activity is related to coke content, thus with time and space also, and 4) the model is supported by experimental data. 

Comparing eq. (8.183) and (8.117) it can be concluded that the activity profiles are very similar in the cases of coking and poisoning. 

The calculation of the activity profile in the regeneration stage is carried out by using the activity-coke content (uq-Cc) relationship in this stage, eq. (6) [9] ... 

Once the activity profile in the reactor is known, the coke profile in the reaction stage is calculated by means of the activity-coke content relationship in this stage. 

Table 10.6 summarizes data from the TPO profiles for the three samples with and without metals. This table clearly shows how the signal C increase as the concentration of nickel and vanadium increases and supports the hypothesis that this peak corresponds to contaminant coke. It is possible to support the theory that a higher content of vanadium in the catalyst results in a loss of activity because the peak area B, previously attributed to catalytic coke, decreases strongly with vanadium levels. 

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. 

The coke profiles in the reactor bed can be predicted excellently by the model as shown by the solid lines in Figure 1. Figure 2 shows good consistency is also obtained for the average coke content over the reactor bed versus time on stream. Note that within the time period of reactor startup plus one hour of operation, the average coke content of the reactor bed is already at about 5 wt%. The model cannot be applied to this startup and initial period with the rapid transients of temperature, activity "spike" and concentration. However, compensation for this interval can be made by a time translation of the model a model time of 36 hours is fixed at an experimental time of zero. A temperature difference of more than 20C between the center of the bed and outer wall of the reactor in the startup stage has been observed in our laboratory for some experiments. About three-fourths of this difference is across the catalyst bed itself. Startup of the reactor at reasonably lower temperatures in order to control the coke formation and to better maintain the catalyst activity is important, if not critical. 

Figure L Profiles relevant to simple parallel mechanism of coke formation a - rate of blocking the active sites versus normalized distance from the pellet centre b - fraction of blocked sites versus normalized distance from the pellet centre ...

Platinum remains more active than rhenimn even in the presence of CPE and CPD, which confirms that the metals play a dual role in the formation of coke dehydrogenation giving coke precursors (non operating here since CPE and CPD are already present in the reactant) and consolidation of the coke deposited on the support by continuous elimination of hydrogen via a reverse spillover phenomenon. It is clear that Pt remains more effective than Re in coke consolidation. TPO profiles on Re (Fig.4) show a small... 

This coke profile can be used to explain why the maximum reaction rate for toluene formation shifted from the Inlet to the outlet. With the fresh catalyst, the greater the concentration of n-heptane, the greater the rate of toluene formation. This is represented by the curve at time t=0 In Figure 6. When the catalyst is deactivated by coke, the first section loses more activity than the others. Therefore, one point downstream of the first section has greater dehydrocyctization activity. Its toluene formation rate is the highest due to less deactivation compared to its upstream section, and to higher n-heptane concentration compared to its downstream section. 

After panial oxidation of a coked catalyst, the first peak of the TPO profile disappears, and the activity for n-buiane dehydrogenation is completely recovered. A signiricaiu amount of graphitic carbon is detected by TEM and SAED examination of the residual carbon on Pt-Sn catalysts after partial oxidation. This implies that the first peak of the 1 PO profile corresponds to carbon deposits located mainly on the metal surface, wbile the second one derives from the more graphitC Iikc carbon located on the support. 

Figure I displays the profiles of O2 uptake and CO2 formation during the TPO of deactivated Pt/Nb20s, Pt/Al203 and Pt-Sn/Nb205 catalysts. A physical mixture of the deactivated Pt/Al203 catalysts (25 mg) and Nb20s (25 mg) was also analyzed by TPO searching for the activity of niobium oxide on coke oxidation.

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