Coke deposit filamentous

A second finding is that the morphology of the coke deposited on aluminized Incoloy 800 and Incoloy 800 surfaces often was quite different. As depicted in Figures 2B, 2D, 2F, 4D, 4F, 5B> 6B, and 8B> the predominant structure of the coke observed on aluminized surfaces tended to be either a film of tar or a globular coke deposit. Filamentous coke was found on numerous Incoloy 800 samples (Figures 2A, 2C> 2E, 3B> 5C-5F, 7A, and 70 whereas filamentous coke was detected at most in only small amounts on the aluminized surfaces of only three runs (Figures 2F, 7B, and 7D). Third, EDAX analyses indicated appreciable metal, generally mainly iron and nickel, in the coke formed on (or brushed off) the Incoloy 800 surfaces analyses of coke deposited on aluminized Incoloy 800 coupons in the same runs indicated trace amounts of aluminum but no detectable iron, nickel, or chromium. 

How do the amounts and types of coke deposited on the various metal surfaces vary as a function of time In the present investigation, the resulting coke was obtained during 120-min runs. In the future, shorter and longer runs are needed to determine the kinetics of coke formation and to determine whether one type of coke is a precursor for another type. Possibly both filament and needle cokes act to some extent as a filter for gas phase coke to form eventually amorphous or knobby coke in which metal-containing coke is eventually covered with metal-free coke. 

A reactor constructed of stainless steel 410 was used for pyrolysis since it contained no nickel. The coke layer formed during pyrolysis was usually thin and greyish. Less frequently, a piece of black coke was found on the surface. The metal surface (Surface C) was always grey. Figure 5 shows the two types of coke formed at Surface A in the stainless steel 410 reactor. The black (less frequent) coke appeared to be a floe of fine filaments, about 0.05 / m in diameter, with occasional 0.4- m filaments. The predominant deposit seems to be platelets of coke that include metal crystallite inclusions, the lighter area. The metal particles in the coke deposits, as detected by EDAX, were chromium rich compared with the bulk metal, as reported in Table III. Some sulfur also was present in the deposit the sulfur was present, no doubt, because of the prior treatment of the surface with hydrogen sulfide. Surfaces B and C for the stainless steel 410 reactor are also shown in Figure 6. Surface B indicated porous coke platelets. Surface C was covered mostly with coke platelets, and cavities existed on the surface. Metal crystallites rich in iron apparently were pulled from the metal surface and were now rather firmly bound to Surface B. Surface C was richer in chromium than the bulk metal. 

The coke deposit obtained in the Hastelloy X reactor was black, thin, and adherent to the metal wall. Hence only Surface A was inspected in the reactor. Rather large diameter (about 0.3 / m) filaments predominate, as shown in Figure 6. Some smaller diameter filaments also were observed. Table III indicates the analysis of the metal in the coke filaments. The signals generated were so low in magnitude that the results should only be considered as approximate. Apparently iron and molybdenum were being extracted preferentially by the coke. 

Figure 15. TEM of filamentous coke deposit (particles arrowed).

End effects were likely important for Incoloy 800 coupons. Relatively coke—free surface metal) enriched in both chromium and titanium> was visible at the upstream and/or downstream ends of several Incoloy 800 coupons subjected to 0.05 atm. acetylene at 800°C. Numerous small craters> or holes> were observed on the metal. A few isolated filaments protruded from the surface> and sparse amounts of globular coke were also detected at the ends. The bulk of the coke deposit) which consisted of filaments intermixed with larger amounts of globular coke (Figure 8A)> occurred near the midsection of the coupon. A matrix of smooth and rather solid carbon was visible at the metal to coke transition regions the smooth matrix lay under the globular coke. 

Observations near the edges of several recoked Incoloy 800 coupons indicated that the structure of the coke deposit varied considerably with depth as the coke metal interface was approached. The innermost layer i. e. the layer in contact with the coupon surface was observed to be a matrix of smooth and rather solid carbon. The outermost layer however consisted of the profuse filament growth which was described earlier from two-dimensional observations. This filamentous growth occurred in the region between the carbon matrix and the reacting gas phase. 

Figure 6.4 shows the result of SEM observations and EDS analysis for coke deposited on Ni. As shown in Fig. 6.4(b), the coke contained some metals. There were fewer imaged bright spots for Ni than there were for Fe. The coke looked filamentous, but the filaments were thinner as eompared with the coke on Fe. At the bright spots imaged in Fig. 6.4(b), EDS deteeted Ni, supporting the theory that it can also act as a catalyst of coke formation (Fig. 6.4(d)). 

As the metal particle size decreases the filament diameter should also decrease. It has been shown that the surface energy of thirmer filaments is larger and hence the filaments are less stable (11,17-18). Also the proportion of the Ni(l 11) planes, which readily cause carbon formation, is lower in smaller Ni particles (19). Therefore, even though the reasons are diverse, in practice the carbon filament formation ceases with catalysts containing smaller Ni particles. Consequently, well dispersed Ni catalysts prepared by deposition precipitation of Ni (average metal particle size below 2-3 nm) were stable for 50 hours on stream and exhibited no filamentous coke [16]. 

Co, Fe, Ni/ Y-AI2O3 H2, H2O, CH4 500-700°C micro-TEM Sintering and/or coking occur more rapidly in gas mixtures (e.g. CH4 + H2O) relative to pure gases. Under simulated steam-reforming conditions deactivation occurs by carbon deposits (films, patches, and filaments), severe sintering, deformation of crystallites, and loss of metal as carbonyls. 2 ... 

Surface heterogeneities described earlier often were important relative to coke formed or deposited on aluminized Incoloy 800 surfaces. Figures SB, 6B, and 10B show globular and cylindrical coke which resulted preferentially in the pitted areas of aluminized surfaces in several runs. Filamentous coke formed at 700 C on an aluminized Incoloy 800 coupon subjected to a 0.05 atm. acetylene feed is depicted in Figure 7D every filament observed was in or near a pitted area but interestingly not every pitted area contained filaments. 

The reactions shown in Eqs. 3a and 3c are endothermic in nature, whereas the water-gas shift (WGS) reaction shown in Eq. 3b is moderately exothermic. If these reactions are carried out externally (external reformaticm), the efficiency and operation of the cell are significantly affected. Hence, internal reforming is preferred however, Ni acts as an excellent coking catalyst. As a consequence, in the presence of carbonaceous fuels (and in the absence of sufficient water vapor), there is always a possibility of deposition of carbon filament on the surface of Ni. The mechanism involves carbon formation on the metal surface followed by dissolution of the carbon into the bulk of the metal and finally precipitation of graphitic carbon at some surface of the metal particles after it becomes supersaturated with carbon [6]. It not only reduces the active sites for reactions mentioned in Eqs. 2c-j and 3a-c but also destroys the whole anode over a period of time. The following three reactions are the most probable catalytic reactions that lead to carbon formation in high-temperature systems ... 

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