Aluminized Incoloy coking results

Thermal reactions of acetylene, butadiene, and benzene result in the production of coke, liquid products, and various gaseous products at temperatures varying from 4500 to 800°C. The relative ratios of these products and the conversions of the feed hydrocarbon were significantly affected in many cases by the materials of construction and by the past history of the tubular reactor used. Higher conversions of acetylene and benzene occurred in the Incoloy 800 reactor than in either the aluminized Incoloy 800 or the Vycor glass reactor. Butadiene conversions were similar in all reactors. The coke that formed on Incoloy 800 from acetylene catalyzed additional coke formation. Methods are suggested for decreasing the rates of coke production in commercial pyrolysis furnaces. 

Coking Results Temperature, precursor type, precursor partial pressure, and run duration all significantly affected the amount of coke for Incoloy 800 and aluminized Incoloy 800 coupons and Vycor glass. The... 

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 results of this investigation greatly clarify several factors that affect coking in pyrolysis coils and transfer line exchangers. In particular, the role of the surface is better defined since significant differences in results were obtained when directly comparing Incoloy 800 versus aluminized Incoloy 800 and polished versus unpolished surfaces. 

Incoloy 800 surfaces acts to produce surfaces with more desirable compositions these new surfaces are more resistant to oxidizing. sulfiding. or surface coking reactions. Of interest. Qregg and Leach (11) had found that less coke was formed on electropolished nickel surfaces as compared to unpolished nickel surfaces. In the case of the aluminized Incoloy BOO. care must be taken not to remove too much of the surface since the aluminum has penetrated only short distances into the surface. The results of the present investigation imply that the method or degree of polishing is important. It is not clear why polished coupons in some cases collected considerable coke near the corners of the coupons. 

Results were also obtained at various axial furnace positions. Considerable more coke formed at 800 C from 0.05 atm. acetylene on layered and paired Incoloy 800 coupons at the midstream and downstream positions than at the upstream position. In a run employing ethylene Incoloy 800 and aluminized coupons were positioned side by side in separate Vycor tubes both 4 cm. upstream and 4 cm. downstream of the furnace midpoint. For the Incoloy 800 coupons about twice as much coke was formed at the dowstream position (see Table 1). But. the reverse was true for the aluminized coupons. 

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... 

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