Molecular Constitution, Carbonization Reactivity, and Mesophase Development from FCC Decant Oil and Its Derivatives
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Carbonaceous mesophase development takes place during commercial delayed coking of FCC decant oil to produce needle coke. The graphitizabiiity of the needle coke product depends strongly on the extent of mesophase development that indicates the microstructural ordering of large polynuclear aromatic species produced by carbonization. Laboratory carbonization of commercial FCC decant oil samples, their distillation fractions, hydrotreated feedstocks, and the charge to the coker (coker feed) showed significant variations in mesophase development under similar reaction conditions.Differences in molecular composition and initial carbonization reactivity of different samples were related to the observed differences in mesophase development. It was shown that rapid semi-coke formation in the gas oil fractions of the decant oil sample produced poor mesophase development. In contrast, hydrotreated gas oil fractions with a lower coking reactivity in the initial stages of carbonization produced semicokes that display a much higher degree of mesophase development. Also the coker feed samples, that consist of the high-boiling fraction of the decant oils with hydrotreated gas oils and the recycle from the delayed coker, produced a higher degree of mesophase development than that obtained from the coresponding decant oil samples carbonized alone. Blending hydrotreated streams into vacuum fractions of decant oils slows down the initial rate of semi-coke formation and improves the mesophase development, but decreases the semicoke yield. In general, the abundance of pyrene and alkylated pyrenes in the feedstocks was observed to promote the desired mesophase development during carbonization. High concentrations of n-alkanes were found to be detrimental to mesophase development because of increased rates of carbonization.Keywords:
Coker unit
A unique feature of the commercial delayed coking process is that the feed is introduced from the bottom of the coke drums and the coke formation begins at the bottom and proceeds to the top of the of the drum over 16-24 hour cycles. It is believed that the volatiles evolution during delayed coking generates channels and pores within the coke produced in the drums (1). The formation of these passages and pores in the delayed coker is believed to play an important role in the alignment and growth of anisotropic microstructures during the final stages of mesophase development. This alignment is very important for the production of needle coke by delayed coking of FCC decant oils (2). Both computer tomography (3) and nuclear magnetic resonance imaging (4) of petroleum cokes has been reported.
Delayed coker
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Coker unit
Delayed coker
Carbon fibers
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An innovative cokemaking process named SCOPE21 has been developed from 1993 to 2003 by member companies of the Japan Iron and Steel Federation as a national project. The concept of this project is higher utilization of non-, slightly coking coals, improvement of productivity and environmental protection.In order to enhance coke productivity, coals are preheated rapidly, fine coals are agglomerated and coke is discharged at medium temperatures. The strength of coke carbonized at medium temperatures will be lower than that of coke carbonized at a temperature over 1000°C so that coke discharged at medium tempera-ture is further heated in an upper part of CDQ pre-chamber to obtain the conventional coke strength.In this study, the quality of cokes discharged at medium temperatures was analyzed and the upgrading effect of them was investigated by three different reheating methods. As a result, it was confirmed that coke discharged at medium temperature could be improved by reheating treatment and upgrading effect was affected by reheating method.
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Very conclusive document for the influence of coal blend characteristics and coke mass temperature on coke quality are available,however,the influence of carbonization time has not been addressed adequately.Experimentation at the stamp charged battery of Tata Steel reveals that increased carbonization time results in closely sized coke with improved mean size.The coke carbon undergoes significant textural change.The isotropic carbon content decreases,the amount of coarse and fine mosaic texture increases and the pore diameter decreases.The high temperature properties,coke reactivity index(CRI) and coke strength after reaction(CSR) are improved with longer carbonization time.
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Significantly, the change in the carbonization conditions of the dry charge has no effect on the strength of the coke. A table gives data characterizing the physico-chemical properties of the coke, showing that the coke from the dry charge is denser and has a lower reaction capacity than the coke from the moist charge. Thus the studies showed that thorough drying of a coal charge (to 1% moisture) before carbonization significantly improves the quality of the coke and reduces the consumption of heat on carbonization, and that the increase in bulk density and the reduction of the duration of the carbonization period increase the productivity of the coke batteries by 40 to 42%.
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The low temperature carbonization of low-molecular compounds, pitches, reduced crudes, synthetic crude and bitumen has been studied by polarized-light microscopy. Transformation to mesophase from the starting materials given varied depending on thermal decomposition, polymerization-behaviours of the components in them. At the early stage of carbonization, thermally reactive components transform to the mesophase, textures of which are of fine mosaic. Thermally stable components give coarse or fibrous textures at the final stage of mesophase transformation.Rearrangements of polyaromatics, which consists of mesophase lamella, are interrupted by the addition of dehydrogenating reagent through carbonization process, presumably by increasing the degree of crosslinking in the decomposing carbonaceous materials. On the other hand, mesophase transformation alters to coarse or fibrous textures during the carbonization under pressure or in the presence of AlCl3. This is the reason why the viscosity of mesophase formed in the carbonization mentioned above is low enough to permit appreciable structural rearrangement by flow during and after coalescence.
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Superstructure
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The purpose of this work was to study the formation of mesophase spherules from a low-temperature coal tar pitch under carbonization conditions. For comparison, the carbonization of a high-temperature coal tar pitch and a petroleum pitch were also considered. Different operating conditions during the carbonization process were used in an attempt to cover different degrees of mesophase formation and development for each pitch. The parent pitches and the semicokes thus obtained were analyzed by elemental analysis, optical microscopy, and Fourier transform infrared spectroscopy (FT-IR). More significantly, the samples were subjected to thermal decomposition under well-controlled operating conditions from room temperature to 850 °C in a thermogravimetric analyzer (TG). The use of a mass spectrometer linked to the TG (TG-MS) provides additional data about the devolatilization process, yielding information about the evolution of different volatile products and about possible chemical reactions occurring during thermal decomposition. Thus an insight into the process of mesophase formation is obtained. The results from FT-IR, elemental analysis, and the TG-MS tests were compared with the different extents of mesophase formation, checked by optical microscopy. According to the results, several stages can be distinguished as temperature increases in the carbonization process of the pitches. In the low-temperature coal tar pitch, the devolatilization of light components, especially phenols, accounts for the most significant weight loss. Moreover, cross-linking contributes greatly to the formation and development of mesophase, resulting in the predominance of bulk mesophase in a relatively short time in the case of the low-temperature coal tar pitch.
Thermogravimetric analysis
Coal tar
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Correct determination of the technical-economic indices of thermal preparation of coals for carbonization requires objective evaluation of the coke yield. There are a number of contradictory views regarding this problem. Some researchers believe that an increase in carbonization rate should promote a substantial reduction in coke yield while others hold that preheating has no significant influence on this parameter. The present investigation was conducted to determine the coke yield from the carbonization of preheated coals and blends in an experimental 200-kg oven. The well-known method devised by the UKhIN, which is widely employed at coke-chemical enterprises, was used for determination of the coke yield. Gas coal and mixtures containing 70 and 50% gas coal were subjected to carbonization after preheating to 150 to 250/sup 0/C. Preheating was carried out in an experimental apparatus with a capacity of 1 t/h. It was concluded that: The increase in coke yield observed for preheated coals and blends can be attributed to a rise in the concentration of degradation products in the liquid, solid, and gaseous phases; the higher the gas--coal content of the preheated blend, the larger is the absolute increase in coke yield; and the maximum increase in coke yield occurs formore » carbonization of coals and blends preheated to 150/sup 0/.« less
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