Incoloy coking results

An important finding of this investigation was that cokes formed from acetylene on Incoloy 800 surfaces caused what appears to be an ever increasing rate of coke formation, as indicated by the results of Run 15 and especially of Run 18. In other words, this coke resulted in an autoacceleration phenomenon. Yet the coke formed from butadiene (as indicated by the results of Run 14) seemed to deactivate the surface so that a slow and rather steady rate of coke formation occurred. The reason for this difference in the rates of coke formation will be discussed later in this chapter. 

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... 

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. 

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. 

Figure 1 indicates an example of how pretreatment of the Incoloy 800 reactor had a very large effect on the acetylene conversion (or on the kinetics of acetylene decomposition). The coke-coated Incoloy 800 reactor (the coke had been deposited on this reactor when butadiene reacted at 500°-700°C.) used in Run 14 resulted in much lower acetylene conversions in the range of 450° to 550°C compared with the same Incoloy 800 reactor after the coke had been burned off with oxygen and after the reactor had been contacted with hydrogen until nearly all surface oxides were eliminated. Most conversion results for the 11 gas samples collected during Run 15 are shown in Figure 1. Gas Samples 1 through 3 at 350°, 400°, and 450°C, respectively, indicated almost no acetylene conversions. A small amount of carbon dioxide was produced at 450°C, indicating some metal oxides had still been present on the surface after the hydrogen pretreatment. The temperature was then increased to 500°C, and the conversions then increased from 66% to 99% during the first 23 min (Samples 4 and 5). Some carbon oxide production was noted in Sample 4 but none in Sample 5 or in any later samples of the run presumably...

The results for Run 19 (Vycor glass reactor), Run 21 (alonized Incoloy 800 reactor), and Run 14 (coke-covered Incoloy 800 reactor) were similar to both the kinetics and type of products obtained. Although neither oxygen or hydrogen pretreatments were tried in Vycor glass or alonized Incoloy 800 reactors prior to acetylene pyrolyses, it is thought that such pretreatments would have little or no effect on acetylene reactions. This conclusion is based on such pretreatments prior to pyrolysis with other hydrocarbons in these two reactors. It has been concluded that all increases in acetylene conversions above those of Runs 14, 19, and 21 were in some way caused by surface reactions. Based on this assumption, surface reactions were of major importance in Runs 15, 18, and 23. 

Based on the results of this run, the coke formed on the Incoloy 800 surface is quite inactive. Probably even lower conversions would have occurred if the run had continued longer and if more coke had been allowed to form on the surface. Unfortunately, no attempt was made to inspect or analyze the coke formed from benzene. It would be of special interest to determine how much metal was incorporated in the coke formed. 

Considerable information was obtained for ethane pyrolysis relative to coke deposition on and to decoking from the inner walls of a tubular reactor. Both phenomena are affected significantly by the materials of construction (Incoloy 800, stainless steel 304, stainless steel 410, Hastelloy X, or Vycor glass) of the pyrolysis tube and often by their past history. Based on results with a scanning electron microscope, several types of coke were formed. Cokes that formed on metal tubes contained metal particles. The energy of activation for coke formation is about 65 kcal/g mol. 

The effect of the decoking operation on coke formation in subsequent cracking runs was also studied and the results shown pictorially in Figure 10. On an uncoated Incoloy 800 tube, the high coke yield remained approximately constant throughout four successive steam cracking/air decoking cycles. This tube was subsequently coated with silica to produce an immediate xlO... 

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. 

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 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. 

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... 

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. 

Surface reactions including carbon (or coke) deposition (on the reactor surface) varied significantly in the reactors investigated. Table I and Figure 8 show carbon results for several runs in the 304 stainless steel, Incoloy, and Vycor reactors. 

Recent results of Dunkleman and Albright (1) who pyrolyzed ethane have shown that the composition of the product often varies significantly depending on the material of construction of the reactor and on the type of pretreatment of the inner surface of the reactor. Considerably higher yields of ethylene were obtained in a laboratory Vycor reactor as compared to an Incoloy 800 reactor and especially a 304 stainless steel (SS) reactor. Oxidized inner metal surfaces promote the production of coke (or carbon) and carbon oxides, but sulfided surfaces suppress such production. 

In the range of 700 to 900 C, increased temperatures resulted in decreased kinetics for both Reactions 7 and 8. Figure 3 shows the results for runs made in the 304 stainless steel reactor. The inner surface of the reactor was cleaned before each run relatively well of all carbon (or coke) deposits by an oxygen pretreatment, a subsequent reduction with hydrogen, and then a 20 minute treatment with o) gen. In the 700 C run, the carbon dioxide content in the exit stream decreased to 1 or less after about 8 hours in the 800 C run, after 11 hours and in the 900°C run, the content was still 6.6% after 12 hours. The kinetics of reduction also decreased in the Incoloy reactor with increased temperature as shown in Table II. 

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