Coke formation, pyrolysis

Reaction Conditions. Typical iadustrial practice of this reaction involves mixing vapor-phase propylene and vapor-phase chlorine in a static mixer, foEowed immediately by passing the admixed reactants into a reactor vessel that operates at 69—240 kPa (10—35 psig) and permits virtual complete chlorine conversion, which requires 1—4 s residence time. The overaE reactions are aE highly exothermic and as the reaction proceeds, usuaEy adiabaticaEy, the temperature rises. OptimaEy, the reaction temperature should not exceed 510°C since, above this temperature, pyrolysis of the chlorinated hydrocarbons results in decreased yield and excessive coke formation (27). 

Deoxygenation reactions are catalyzed by acids and the most studied are solid acids such as zeolites and days. Atutxa et al. [61] used a conical spouted bed reactor containing HZSM-5 and Lapas et al. [62] used ZSM-5 and USY zeolites in a circulating fluid bed to study catalytic pyrolysis (400-500 °C). They both observed excessive coke formation on the catalyst, and, compared with non-catalytic pyrolysis, a substantial increase in gaseous products (mainly C02 and CO) and water and a corresponding decrease in the organic liquid and char yield. The obtained liquid product was less corrosive and more stable than pyrolysis oil. 

Carbon formation is also different for diesel and gasoline. The long chain hydrocarbons present in diesel or kerosene fuel are more difficult to reform than the shorter chain hydrocarbons present in gasoline, while aromatics in gasoline hinder the overall reaction rate. An example is found in the results of Ming et who showed that SR of n-Ci required a higher steam/ carbon ratio to avoid coke formation than i-Cg. The cetane number of the feed had little effect on carbon formation. Carbon formation can often be attributed to fuel pyrolysis that takes place when the diesel fuel is vaporized. This is considerably decreased when the steam content in feed increases. 

The major industrial source of ethylene and propylene is the pyrolysis (thermal cracking) of hydrocarbons.137-139 Since there is an increase in the number of moles during cracking, low partial pressure favors alkene formation. Pyrolysis, therefore, is carried out in the presence of steam (steam cracking), which also reduces coke formation. Cracking temperature and residence time are used to control product distribution. 

Figure 7.13 presents a simplified flowsheet, which concentrates the essential features the balanced VCM technology, as conceptually developed in the previous sections, but this time with the three plants and recycles in place chlorination of ethylene (Rl), thermal cracking of EDC (R2) and oxyclorinahon of ethylene (R3). As mentioned in Section 7.3, from plantwide control three impurities are of particular interest (I]) chloroprene (nbp 332.5 K), (12) trichloroethylene (nbp 359.9K), and (13) tetrachloromethane (nbp 349.8). I, and 12 are bad , since the first can polymerize and plug the equipment, while the second favors the coke formation by EDC pyrolysis. On the contrary, I3 has a catalytic effect on the VCM formation, in some patents being introduced deliberately. 

Coke Formation during the Hydrotreating of Biomass Pyrolysis Oils Influence of Guaiacol Type Compounds... 

Y. H. Lin, P. N. Sharratt, A. A. Garforth, and J. Dwyer, Deactivation of US-Y Zeolite by Coke Formation dnring the Catalytic Pyrolysis of High Density Polyethylene, Thermochim. Acta, 294, 45-50 (1997). 

On the other hand, activated carbon may be considered as a catalyst in the cracking of waste plastics. This is because it is a neutral catalyst with a high surface area and, therefore, it might be more resistant to impurities and coke formation. It has been reported that Pt-, Fe- and Mo-supported activated carbon catalysts were effective for the pyrolysis ofPEandPP [11,14,15]. Use of metal-supported activated carbon catalysts has enhanced the formation of aromatics via dehydrocychzation of straight- or branched-chain radicalic intermediates. 

Chapters 10-12 cover important aspects of coke formation in metal tubular reactors during pyrolysis of hydrocarbons. Chapters 13 and 14 are concerned with coal and lignite pyrolysis. Chapters 15 and 16 deal with pitch formation from, respectively, heavy petroleum fraction and tar sand bitumen. Chapters 17 and 18 cover studies on the mechanisms of thermal alkylation and hydropyrolysis. Chapters 19 and 20 on oil shale deal with the properties of oil shale and shale oil as developed by techniques of microwave heating and thermal analysis. 

It should be noted that Equations 1, 2, 3, 4, and 5 imply a homogeneous kinetic system. Coking in tubular reactors results from a combination of homogeneous and heterogeneous processes. As the kinetics of these processes are not well understood and as the quantitative yield of coke is several orders of magnitude smaller than other pyrolysis products, it is more convenient to model coke formation separately based on commercial operating data. 

Touring the pyrolysis of hydrocarbons, coke is unfortunately always formed in addition to ethylene, propylene, diolefins, aromatics, and other valuable hydrocarbons. Information available on coke formation up to 1965 has been reviewed by Pallmer and Cullis (1). Some (but not extensive) results obtained using an electron microscope have been reported. 

How does the composition of the metal surface change during pyrolysis Surface reactions that have been identified on Incoloy 800 surfaces include oxidation, reduction, sulfidation, desulfidation, and coke formation (14). Do increased concentrations of nickel and chromium ever occur in the surface in view of the fact that iron is incorporated into the coke Tsai and Albright (14) found increased iron concentrations on inner surfaces of tubes used for pyrolyses. 

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. 

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. 

A lthough coke formation is always of importance during pyrolysis processes that are used for production of ethylene and other valuable olefins, diolefins, aromatics, etc., relatively little is known about the factors affecting such coke formation. It has been found that operating conditions, feedstock, pyrolysis equipment, and materials of construction and pretreatments of the inner walls of the pyrolysis tubes all affect the production of coke. General rules that have been devised empirically at one plant for minimizing coke formation are sometimes different than those for another plant. It can be concluded that there is relatively little understanding of, or at least little application of, fundamentals to commercial units. 

Table I presents results for six comparable pyrolysis runs made by using five laboratory reactors all runs were made with approximately 50% steam as diluent in the ethane feed. Conversions at the exit end of the reactor varied from 59% to 65%. Also, results reported for a commercial unit (11) are shown. Ethylene yields varied from about 78% to 89% in all cases except for run D44 made in the stainless steel 304 reactor. In that run, the ethylene yields were very low but production of CO, GOo, and net coke were much higher. Ethylene yields were highest in the run made in the Vycor glass reactor. In this run, coke formation was least of all runs, and no CO or C02 was detected in the product stream.

Thus, before the rate of coke formation can build up to a steady level, the coke precursors must reach some suitable concentration. At this point, the most logical candidates for coke precursors are the / -resins. Plotting the amount of coke formed as a simple function of the quantity of / -resins produces the curve shown in Figure 5. The / -resins/coke data obtained at 800°, 825°, and 850°F (430°, 440°, and 450°C, respectively) lie approximately on the same curve, while the data obtained at 980°F (530°C) follow a separate curve. At relatively low levels of / -resin formation the coke concentration increases only slowly, but as the /3-resin concentration approaches 14-16%, the amount of coke formed rises rapidly and the two curves converge. In the pyrolysis runs where the reactions were terminated before the / -resin concentration had risen much above 10-12%, a substantial portion of the / -resin producing reaction follows first-order kinetics. However, if the pyrolysis reaction is allowed to continue, the concentration of / -resins levels off at about 16-17%, regardless of any further reaction, and the first-order relationship no longer... 

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