Butadiene coke formed

One of the issues that concern liquid feedstock cracking operations is a higher rate of fouling. This is not only a consequence of heavier coke forming precursors, but also as a consequence of long lived free radicals which act as agents for the formation of a polymer (often referred to as pop-corn polymer) in the primary fractionator and downstream units. For instance, free radicals based on styrene or indene have sufficiently long half-lives to pass from the pyrolysis section into the primary fractionator. These can concentrate in this unit and produce polymer (free radical polymerisation) when sufficient amounts of suitable olefins are present, in particular styrene itself and di-olefins such as cyclo-pentadiene or butadiene. 

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 granules also have been found in cokes formed or deposited on iron, cobalt, and nickel foils in experiments using methane, propane, propylene, and butadiene (7-10). Platelet-type coke, whose properties match those of graphite also was produced in some cases. Lahaye et al. (11) investigated the steam cracking of cyclohexane, toluene, and n-hexane over quartz, electrode graphite, and refractory steel. They report that heavy hydrocarbon species form in the gas phase, condense into liquid droplets which then strike the solid surface, and finally react on the solid surfaces to produce carbonaceous products. The liquid droplets wet and spread out on certain surfaces better than on others. 

Figure 3. Cokes formed by butadiene at 465°C. (Left) Incoloy 800 (right) alonized Incoloy 800.

Figure 6. Coke formed at 600°C on alonized Incoloy 800 with four unsaturated hydrocarbons. (Top left) acetylene (top right) ethylene (bottom left) propylene (bottom right) butadiene.

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. 

The results of this investigation and particularly of those with butadiene strongly suggest that at least portions of the inactive coke formed during pyrolyses involve the following sequence of events (a) production in the gas phase of unsaturated hydrocarbons, (b) chemical condensation or polymerization of unsaturated hydrocarbons to produce rather heavy hydrocarbons, (c) physical condensation of these heavy hydrocarbons as liquids on the reactor walls or in the transfer line exchangers, and (d) decomposition of the liquids to coke (or tars) and hydrogen. This sequence of events is essentially identical to the one proposed by Lahaye et al. (12) for coke production from cyclohexane, toluene, or n-hexane. 

Table 8-6 lists poisons for various catalysts and reactions. The materials that are added to reactant streams to improve the performance of a catalyst are called accelerators. They are the counterparts of poisons. For example, steam added to the butene feed of a dehydrogenation reactor appeared to reduce the amount of coke formed and increase the yield of butadiene. The catalyst in this case was iron. ... 

The dimerization of butadiene to form 4-vinyl cyclohexane-1 The decomposition of butadiene to form hydrogen, methane, ethylene, acetylene, and coke... 

Mechanism 3 has, as a first step, reactions between surface radicals on the coke and the acetylene, butadiene, and gaseous free radicals reactions probably also occur with ethylene and propylene. Reactions with gaseous free radicals were discussed earlier as a termination step in the gas-phase reactions. When acetylene reacts with the surface radicals, aromatic structures are formed on the surface. When the C—H bonds on the surface later break, graphitic coke is formed. The cokes produced by both Mechanisms 1 and 3 tends to be highly graphitic. Microscopic photographs have shown that Mechanism 3 thickens filamentous coke and causes spherical coke particles formed by Mechanism 2 to grow in diameter. 

The process is cydic. The feedstodt and C4 recycle are preheated to 600 and sent to the catalyst bed, forming butadiene, butenes, a number of gaseous by-products and coke. After a reaction period of 5 to 10 min. depending on the number of reactors in the unit, the temperature drops by-15 to 20 C Regeneration is then carried out, lasting 5 to 10 min. The reactor is first purged with steam, and air at 600 C is then introduced to bum the carbon deposits formed. The heat liberated raises the temperature of the... 

Diolefins also occur in relatively small concentrations in olefin feed streams. Their concentration increases as cracking severity increases, such as in fluid coking. As in the case of ethylene, the butadienes do not appear to react with isobutane in the presence of strong sulfuric acid. The dienes are believed to form reaction products, most of which are acid soluble, and if this general premise is accepted the fresh acid make-up rate for a dilution range of 98.5 to 90.0% (wt.) can be calculated to be 2465 pounds of acid per barrel of butadiene. Industry practice is to use rates in the range of 1890 to 4200 pounds per barrel. 

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. 

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. 

Incoloy 800 1,3-butadiene runs to form coke on surface 350-650... 

Ideally this system would consist of three phases only. Two of these phases would be liquid, water and cooled hydrocarbons, and the third would be the cracked gas stream. Unfortunately, solids may accumulate in this vessel. Coke fines and small particles from the furnace may be entrained into this unit, and activated olefins such as butadiene or styrene may polymerize to form insoluble particles. All of this behavior promotes foaming. In this aqueous system, foam formation may be controlled by addition of a polyglycol. 

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