Globular coke

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. 

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. 

Figure 7 shows the coke formed on the various unpolished Incoloy 800 coupons. In all cases mixtures of filament and globular coke resulted on the Incoloy 800 surfaces certain areas exhibited relatively large clusters of globular coke from which filament coke protruded. 

For polished Incoloy 800 coupons (see Figure 8)< a mixture of filament and globular coke formed on the oxygen-treated surfaces. Filament coke was however not detected on either untreated or steam treated coupons. The coupon treated with steam at 00°C was covered rather uniformly with globular coke. The coupon treated with steam at 900°C exhibited a rippled layer of coalesced globular coke. 

The results of this investigation help clarify why machining is beneficial relative to coke formation rough areas on the surface act as excellent collection sites for coke or tar droplets formed in the gas phase. Such tar droplets eventually become globular coke. Smoother surfaces hence minimize such collection or surface deposits. In addition. smoothing or polishing of both Incoloy 800 and aluminized... 

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 foamed coke formed at the composition testing and not containing modifying additives and fillers has a coarse amorphous structure (Fig. 18.5) there are foamed globular formations of 10-100 pm in the coke volume grouping to associates. 

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