Types of cokes formed

Variables Affecting Type of Coke. The material of construction of the solid on which the coke formed, the temperature, and the space time all affected the type of coke formed. Cokes formed on Incoloy 800, SS 304, SS 410, and Hastelloy X were sometimes magnetic. Cokes formed in an alonized surface were always nonmagnetic, and no metals were detected by EDAX except for a trace of aluminum. Cokes formed in Vycor glass reactors were also nonmagnetic. 

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. 

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... 

Figures 4 and 5 indicate that the types of coke formed on Incoloy 800 as ethylene and propylene, respectively, were contacted with an Incoloy 800 surface at various temperatures and at slightly different conversion levels. In these experiments, the Incoloy 800 coupons were positioned at different positions in the horizontal tubular reactor. The residence time of gases in the reactor was about 7, 10, 15, and 25 sec by the time the gases reached the coupon. The temperature of each location was about 460°, 560°, 600°, and 565°C, respectively. The cokes...

How does sulfiding or oxidizing the metal surfaces affect the types of coke formed ... 

Although more information is needed to determine details concerning factors that favor inactive coke formation, relatively high levels of surface sulfides probably promote formation of such coke. On the other hand, metal oxides on the surface likely favor production of active coke. Sulfiding the reactor tube immediately upon completion of the decoking step would form metal sulfides. An aluminized surface, such as provided by the alonized Incoloy 800 reactor, also has been found to be an effective way to prevent the production of active coke. Quite possibly, the initial type of coke formed on the just-cleaned tube would have an important effect on the length of time a reactor tube could be used in a commercial plant before decoking would be required. 

A reactor constructed of stainless steel 410 was used for pyrolysis since it contained no nickel. The coke layer formed during pyrolysis was usually thin and greyish. Less frequently, a piece of black coke was found on the surface. The metal surface (Surface C) was always grey. Figure 5 shows the two types of coke formed at Surface A in the stainless steel 410 reactor. The black (less frequent) coke appeared to be a floe of fine filaments, about 0.05 / m in diameter, with occasional 0.4- m filaments. The predominant deposit seems to be platelets of coke that include metal crystallite inclusions, the lighter area. The metal particles in the coke deposits, as detected by EDAX, were chromium rich compared with the bulk metal, as reported in Table III. Some sulfur also was present in the deposit the sulfur was present, no doubt, because of the prior treatment of the surface with hydrogen sulfide. Surfaces B and C for the stainless steel 410 reactor are also shown in Figure 6. Surface B indicated porous coke platelets. Surface C was covered mostly with coke platelets, and cavities existed on the surface. Metal crystallites rich in iron apparently were pulled from the metal surface and were now rather firmly bound to Surface B. Surface C was richer in chromium than the bulk metal. 

Figure 14. Abundance of various types of coke on zeolite H-Y during reaction with propene. The soluble fractions (A-C, black symbols) represent different aromatic structures which form during the first polymerization steps. The insoluble coke is the final product of polymerization which is affected by the solid state acidity of the zeolite, represented here as the Si A1 ratio.

Coke Formation. An example of the change in the amount of coke formed with respect to time on stream under the aromatization reaction conditions of several types of hydrocarbons on the H-Ga-silicate prototype catalyst is shown in Fig. 4. In this figure, feed rate was set to be uniform at an LHSV of 2h" in terms of the n-hexane equivalent to compare the amount of coke formed. The rhajor components of light paraffin consist of n-butane and n-hexane, while ethylene, cyclohexane and toluene are compounds that contribute largely to the amount of coke formed. 

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. 

Miscellaneous Coke Results. More than one type of coke was noted in many pictures. Figure 9 shows six cokes formed from various hydrocarbons ... 

How do the amounts and types of coke deposited on the various metal surfaces vary as a function of time In the present investigation, the resulting coke was obtained during 120-min runs. In the future, shorter and longer runs are needed to determine the kinetics of coke formation and to determine whether one type of coke is a precursor for another type. Possibly both filament and needle cokes act to some extent as a filter for gas phase coke to form eventually amorphous or knobby coke in which metal-containing coke is eventually covered with metal-free coke. 

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 reversible type of coke can be removed, be converted into a harmless type of coke, or form irreversible coke on a time-scale slow enough to be detected by the GC analysis. The amount of reversible coke is changed by modifications in the reaction conditions. 

The reversible and irreversible types of coke occupy the same number of active sites per carbon atom. The irreversible coke is formed only from the reversible coke. 

As reported by Ho [12], the types of delta coke formed in Resid FCC can be classified based on the length of time needed for their formation. CCR coke will form nearly instantaneously at the inlet of the reactor and is therefore also called entrance coke." The second type of coke is formed by the adsorption of highly aromatic and basic materials on even weakly acidic surfaces this process also occurs quite rapidly. Finally, reaction or catalytic coke will form in what is clearly the slowest coke formation process. 

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