Coke distributions

Table 1. FCCU Coke Distributions, % of Total Coke...

The use of magnetic resonance imaging (MRI) to study flow patterns in reactors as well as to perform spatially resolved spectroscopy is reviewed by Lynn Gladden, Michael Mantle, and Andrew Sederman (University of Cambridge). This method allows even unsteady-state processes to be studied because of the rapid data acquisition pulse sequence methods that can now be used. In addition, MRI can be used to study systems with short nuclear spin relaxation times—e.g., to study coke distribution in catalytic reactors. 

Fig. 4 Mapping coke distribution with the SPRITE technique (21). (a) Photograph of the sample used for the study. Two layers of coked HZSM-5 (areas a) were separated by a layer of fresh HZSM-5 (area b). Each layer was about 3.3 cm in length, (b) One dimensional SPRITE profile for the sample shown in (a).

NMR spectrometry of Xenon-129 adsorbed in coked samples of the totally protonated H-ZSM-5 zeolite and the modified Na, H-ZSM-5 showed variations attributable to differences in coke distribution. 129Xe NMR spectrometry is extremely useful for probing microporous materials. Ito et al.(2) demonstrated, for example, that NMR spectrometry of adsorbed xenon in coke-fouled H-Y zeolite could probe the deposits after coking and the nature of the internal surfaces after decoking. The NMR results in this study are consistent with a distribution of coke restricted by size selectivity of the acidifying medium. 

Figure 3. Schematic depiction of coke distributions in zeolite ZSM-5. (a) Lightly coked Na,H-ZSM-5 (b) Heavily coked Na,H-ZSM-5 (c) Lightly coked H-ZSM-5 (d) Heavily coked H-ZSM-5. (Reproduced with permission from ref. 16. Copyright 1991 Academic Press Inc.)...

Before regeneration the catalyst carries 2% wt. coke distributed uniformly throughout the pellets, and the whole bed is at the inlet gas temperature. 

The results show that the specificities of catalyst deactivation and it s kinetic description are in closed connection with reaction kinetics of main process and they form a common kinetic model. The kinetic nature of promotor action in platinum catalysts side by side with other physicochemical research follows from this studies as well. It is concern the increase of slow step rate, the decrease of side processes (including coke formation) rate and the acceleration of coke transformation into methane owing to the increase of hydrogen contents in coke. The obtained data can be united by common kinetic model.lt is desirable to solve some problems in describing the catalyst deactivation such as the consideration of coke distribution between surfaces of metal, promoter and the carrier in the course of reactions, diffusion effects etc,. 

The following examples reflect the main advantages of SIMS analysis the microanalysis of elements at trace level in the case of catalytic cracking catalysts (FCC). the sensitivity to light elements for the study of coke distribution and the possibility of providing composition profiles on zeolites modified by chemisorption of metals. 

I. Temperature-programmed oxidation TPO- and coke distribution on catalyst functions... 

Fig. 2 Coke distribution in a fixed bed in relationship with microactivity ...

The reactions were carried out in a TEOM reactor where the weight of the catalyst bed is continuously recorded. The setup is similar to that described previously [8]. The methanol flow was controlled by a liquid flow controller while DME and propene were fed using gas flow controller. The MTO and DTO reactions were carried out at 425°C, WHSV=417h" and a methanol or DME partial pressure of 8 kPa, with helium as diluent. One DTO experiment was also performed at WHSV=600 h" to keep the residence time identical to those from the MTC) experiments. Such high space velocity and low partial pressure were used to avoid non-uniform coke distribution through the catalyst bed, and to keep the conversion well below 100% to minimize secondary reactions of olefins. 

Figure 3a. Coke distribution in the first bed Figure 3b. Metal distribution in the first bed...

As part of this investigation, kerogen pyrolysis models different from the one proposed here were considered. One such model of theoretical appeal is similar in structure to the one given in Figure 9 but with a pure diffusion process for the heavy oil production. However, this alternative model is incompatible with some experimental findings It predicts lower coke concentrations on the surface of the particle than in the interior, whereas microprobe results indicate a uniform coke distribution. Further, this diffusion model predicts zero coke yield for infinitely small particles, whereas the limited amount of data available for small particle sizes suggest a leveling-off of the coke yield below a particle size of 0.4 mm. 

In addition to these kinetic investigations the catalyst was characterized with respect to the following parameters internal surface area, porosity, pore diameter, radial coke distribution within the particle und the tortuosity. Thereby also the change of these parameters for different carbon loads during deactivation and regeneration were determined. 

Fig. 3 Coke distribution in the catalyst pellet after coking...

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