Acetylene coke formed from

Recently both Lobo and associates (2,3) and Baker and associates (4,5,6) have investigated the mechanism of coke formed from acetylene on nickel surfaces. Such investigations should be most helpful since acetylene is considered to be an important coke precursor in both pyrolysis tubes and in the transfer line exchanger connected to the tubes. 

Figure 1 shows that the type of coke formed from acetylene in the range of about 410°- 460°C depends on the metal surface. The coke on the Incoloy 800 surface appears to be braided or rope-like filaments. In another picture, as shown in Figure 2, two types of filaments were produced—both braided and constant-diameter filaments. Both types of filaments were relatively long compared with their diameters, which were approximately 0.25 /an. Each filament was firmly attached to the metal surface and could not be removed from it easily by mechanical means. 

Figure 1. Coke formed from acetylene at about 410°-460°C. (Left) Incoloy 800 (right) alonized Incoloy 800.

Figure 2. Effect of temperature on coke formed from acetylene on Inco-loy 800. (Top left,) 325°C, (top right,) 560°C, (bottom left) 600°C, (bottom right) 770°C.

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. 

As shown in Figure 2, the types of coke formed on Incoloy 800 surfaces from acetylene varied significantly with temperature in at least the range 325°-770°C. At 325°C both braided and constant-diameter filaments occurred. In a photograph that is not shown here, part of a filament was apparently braided and the remainder had a constant... 

Figure 6 indicates that amorphous coke was formed from acetylene, ethylene, propylene, and butadiene at 600°C on alonized Incoloy 800 surfaces. These cokes were in all cases nonmagnetic in character and contained no detectable iron. They did contain a trace of aluminum, probably as alumina. 

In the steam cracking of hydrocarbons, a small portion of the hydrocarbon feed gases decomposes to produce coke that accumulates on the interior walls of the coils in the radiant zone and on the inner surfaces of the transferline exchanger (TLX). Albright et identified three mechanisms for coke formation. Mechanism 1 involves metal-catalyzed reactions in which metal carbides are intermediate compounds and for which iron and nickel are catalysts. The resulting filamentous coke often contains iron or nickel positioned primarily at the tips of the filaments. This filamenteous coke acts as excellent collection sites for coke formed by mechanisms 2 and 3. Mechanism 2 results in the formation of tar droplets in the gas phase, often from aromatics. These aromatics are often produced by trimerization and other reactions involving acetylene. Some, but not all, of these droplets collect... 

From the sulfuric acid solution, thorium may also be obtained by precipitation with sodium fluosilicate, sodium hypo-phosphate,1 or sodium pyrophosphate.2 An ingenious method for removing the phosphorus has been proposed by Basker-ville3 and used on a large scale. It consists in heating in an electric furnace a mixture of monazite, coke, lime, and feldspar. The phosphorus is distilled out and the mass allowed to cool. When extracted with water, acetylene is evolved from the calcium carbide formed during the heating, and the remainder crumbles to a fine powder. This is dissolved in hydrochloric acid, and the cerium earths removed. 

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. 

An alternative process to VCM is from calcium carbide. This process is used in China [11]. Calcium carbide, CaC2, is made by heating lime with coal-derived coke. After treatment with water, acetylene is formed. The acetylene is then reacted with HCl to produce VCM. 

The oligomerization of propene on zeolite H-Y has been studied [33,37] by variable-temperature MAS NMR. Alkoxy species formed between protonated alkenes and oxygens of the zeolitic framework were found to be important long-lived intermediates in these reactions. Simple secondary or tertiary carbocations are either absent in the zeolite at low temperatures, or are so transient as to be undetectable by NMR even at temperatures as low as 163 K. There was, however, evidence for long-lived alkyl-substituted cyclopentenyl carbocations, which are formed as free ions in the zeolite at room temperature. At 503 K the oligomers crack to form branched butanes, pentanes and other alkanes. The final product was highly aromatic coke. The structure, dynamics and reactivity of an alkoxy intermediate formed from acetylene on zeolite catalysts have been investigated by Lazo et. al. [32]. 

Acetylene was discovered m 1836 by Edmund Davy and characterized by the French chemist P E M Berthelot m 1862 It did not command much attention until its large scale preparation from calcium carbide m the last decade of the nineteenth century stim ulated interest m industrial applications In the first stage of that synthesis limestone and coke a material rich m elemental carbon obtained from coal are heated m an electric furnace to form calcium carbide... 

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. 

Acetylene is most generally produced by the reaction of water on calcium carbide which has been formed in electric furnaces from coke and... 

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