Pyrolysis coke formation during

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

Y. H Lin, P. N. Sharratf A. A. Garforth, and J. Dwyer, Deactivation of US-Y zeolite by coke formation during the catalytic pyrolysis of high density polyethylene, Thermochimica Acta, 294, 45 (1997). 

The present studies were initiated in order to investigate the effect of the reactor surface on the product distribution and on the tendency for coke formation during the steam cracking of propane in a tubular reactor. Attention has been focused on correlating various effects which can arise in the system. Previous studies of the pyrolysis of propane has been reviewed recently (17, 18), and the findings of the present work are related to these studies later in this paper. 

Albright, L.F. Yu, Y.C. Neither, K. "Coke Formation During Pyrolysis Operation". 85th National AIChE Meeting. June 4-8, Philadelphia. Paper no 15E. 

To study the coke formation during ultrapyrolysis of heavy oils, a Model 240 Elemental Analyzer has been employed to analyze the carbon deposited onto the ferromagnetic wire and/or the glass microreactor walls. The influence of temperature and total reaction times on the amount of carbon formed have been studied. The experimental results confirmed that the coke yield increases sharply with temperature and reaction time. However, at the upper boundary of the ultrapyrolytic regime, a maximum coke yield of 17 wt% has been observed at 1000°C with a total reaction time of 1 second. For the same total reaction time, coke yields of 10 wt% and 6.5 wt% have been measured at 900 and 800°C, respectively, sharply decreasing from those values with a decrease in reaction time. Compared to the above values, industrial pyrolysis processes produce very high yields of undesirable coke. 

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. 

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. 

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. 

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. 

Small differences in the kinetics of pyrolysis may have occurred in different reactors see, for example, the results shown in Figures 1 and 3. Such differences could have been caused by one or more factors. One factor that likely is of some importance is small differences in the gas temperature heat transfer obviously depends to some extent on the materials of construction. Small differences in residence times at reaction conditions could also have occurred in the various reactors that did not have exactly the same internal volumes. In addition, surface reactions may affect the kinetics of the reactions. Some initiation or termination of free radicals may have occurred at the reactor surfaces. Probably at least some hydrogen free radicals were formed during coke formation. 

Several gaseous components present during most commercial pyrolysis runs react with or at the surface. For example, hydrogen reduces the surface oxides (6), desulfurizes coke (7), and reacts with the coke Itself to produce methane (8). Cleaning coke from the surface may act to promote more coke fonnatlon, but reduction of surface oxides presumably often decreases the rate of coke formation. Carbon monoxide also Is a reducing agent for metal oxides and Is sometimes employed during the manufacture of steel. 

The present results clearly confirm the Importance and complexity of surface reactions during pyrolysis reactions. Obviously, the composition of the Inner surface of the reactor Is of Importance relative to the level and types of surface reactions. In addition, valuable new Information has been obtained concerning the role of coke In affecting more coke formation. Although the deposition of coke on the walls of a metal reactor decreases the activity of the reactor. It Is of Interest that the surface activities of coke-covered metal reactors always remained higher than those for the Vycor reactor. Lobo and Trimm (11) have Indicated that carbon without contaminants Is Inactive. Based on this finding, metal contaminants were presumably present In the coke formed. Other Investigators (10, 11) have found both nickel and Iron contamination of various cokes. Furthermore, coke Is sometimes reported to be autocatalytic In nature. The evidence that olefins and other hydrocarbons adsorbed on the surface and... 

It is evident, therefore, that the aromatic carbon alone yields coke, and hydroaromatic carbon yields tar. Since neither appears to contribute substantially to the formation of gases (during the low temperature pyrolysis), it seems certain that the gases of low temperature pyrolysis owe their origin largely to the aliphatic structure in coal. At least it is now certain that methane formation is quite independent of the aromatic and hydroaromatic structures in coal. 

The formation of coke is not a problem for UMR since any coke that is formed is burnt off during the air regeneration step. This allows the use of UMR with diesel/logistics fuel and possibly with biomass pyrolysis liquids, though the latter has not yet been demonstrated. 

Miyazawa et al. (92) related rates of decrease of aliphatic hydrogen protons during pyrolysis of ethylene tar pitch to formation of mesophase. Yokono et al, (93) used the model compound anthracene to monitor the availability of transferable hydrogen. Co-carboniza-tions of pitches with anthracene suggested that extents of formation of 9,10-dihydroanthracene could be correlated with size of optical texture. The method was then applied to the carbonization behaviour of hydrogenated ethylene tar pitch (94). This pitch, hydrogenated at 573 K, had a pronounced proton donor ability and produced, on carbonization, a coke of flow-type anisotropy compared with the coarse-grained mosaics (<10 ym dia) of coke from untreated pitch. 

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