Incoloy 800, aluminized

Coke formed on solid surfaces during the pyrolyses of acetylene, ethylene, ethane, propylene, and butadiene were examined by using a scanning electron microscope. Seven types of coke have been identified braided filament, uniform diameter filament, needle or spike, ribbon, fluffy or cottonlike fibers, knobby, and amphorous. The first four types contained metal (especially iron) and were magnetic. Magnetic cokes formed sometimes on Incoloy 800, stainless steel 304, stainless steel 410, and Hastelloy X surfaces, but never on Vycor glass or aluminized Incoloy 800 surfaces. Conditions at which each type of coke was formed are discussed. 

Metal coupons were inserted at various positions in the horizontal tubular reactor. The coupons had two types of surfaces an Incoloy 800 surface and an aluminized Incoloy 800 surface. To prepare these coupons, flat pieces of Incoloy 800 were aluminized (or alonized) by the Alon Processing, Inc. of Tarentum, PA. In this process, gaseous aluminum was contacted with the Incoloy 800, and aluminum diffused into the surface. The alonized samples as received from Alon Processing were cut in small coupons so as to expose an Incoloy 800 surface which was cleaned and polished before use. 

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... 

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. 

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. 

In a second recoking run at 800°C, used iDcoloy 800 and aluminized Incoloy 800 surfaces were positioned side by side near the furnace midpoint and subjected to 0.05 atm. acetylene. Two additional coupons were included a second used Incoloy 800 coupon was placed beside a new Incoloy 800 coupon at a position that was 5 cm. upstream of the furnace midpoint. About 27% more coke formed on this second used coupon than on the new coupon beside it as indicated in Table III. Analogously the amount of coke deposited on the aluminized Incoloy 800 coupon in the recoking was 67% greater than the amount deposited in the initial coking run. However the amount of coke formed on the used Incoloy 800 coupon near the midpoint was 17% less than the amount formed in the initial coking run. 

The tubing containing the coupon samples were placed inside a Vycor glass tube (2.2 cm. I. D. and 85 cm.long) that was positioned horizontally in an electrical furnace controlled at either 700 or 900°C. For inorganic gas treatments Incoloy 800 and polished Incoloy 800 coupons were positioned side by side 4. 5 cm. upstream of the reactor midpoint polished and unpolished aluminized Incoloy 800 coupons were likewise positioned side by side except 2.0 cm. upstream... 

Aluminized Incoloy 800 Samples after 4-Hour Exposures... 

Treatment Aluminized Incoloy 800 Polished Aluminized Incoloy 800... 

An appreciable amount of sulfur (probably as metal su1 f ides> was incorporated in the surface of all coupons exposed to hydrogen sulfide at 700°C as shown in Tables I and II. For Incoloy 800 couponsi there was a very large decrease in the chromium content at the surface. On a sulfur—free basis the iron content increased greatly for polished Incoloy 800 but the nickel content increased for the regular Incoloy 800 coupon. For aluminized Incoloy 800 coupons there was little change in the metal content on a sulfur-free basis as a result of hydrogen sulfide treatments as shown in Table II. 

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. 

Another advantage of aluminized Incoloy 800 surfaces is that diffusion of chromium and titanium to the surface is drastically decreased if not basically stopped. It seems clear that diffusion of aluminum in such alloys is however fairly rapid, as indicated by the present results. One would expect in commercial units however that all aluminum reaching the surface would quickly react with steam or oxygen to produce alumina. 

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