针状焦形成机理及炭化条件

介绍了针状焦形成过程中形态结构变化的基本规律和化学反应机理，芳香性重质油或煤焦油在热解过程中经中间相小球的形成，增长，融并和在气体牵伸作用下形成针状焦。探讨了温度、压力、升温速率等对针状焦形成的影响，在此基础上，对粘度、气体的逸出、原料的反应性与最优炭化条件之间的关系进行了讨论。     

Electrical resistivity of pitch during heat treatment

The d.c. electrical resistivity of coal-tar pitch has been measured from ambient temperature to 450 °C. The change in resistivity with temperature up to 200 °C has been found to be consistent with viscosity-controlled diffusion of current carriers. In an isothermal study at 450 °C, the start of mesophase formation was found to be accompanied by a rapid rise in the pitch resistivity, which subsequently levelled off and remained nearly constant until the onset of mesophase coalescence and phase inversion. The resistivity of the resultant mesophase pitch was found to be less temperature susceptible than that of the parent pitch.     

Production of isotropic coke by thermocracking of the anthracene fraction of coal tar

The properties of coke obtained by heat treatment of the anthracene fraction of coal tar under pressure (by thermocracking) are investigated. Pressures up to 5 MPa are used; the temperature is 500 or 550°C. For comparison, pitch coke is obtained from oxidized pitch with softening temperatures of 166.2 and 190.2°C. The coke yield from thermocracking is 70–75%. The following properties of the coke are determined: the actual density, the ash content, the yield of volatiles, the optical microstructure, the elementary composition, the change in volume on heating to 2400°C, the impurity composition, and the X-ray structural characteristics. High temperatures (at least 550°C) and heating rate of the anthracene fraction facilitate the formation of a large quantity of active radicals, which instantaneously form the three-dimensional coke structure, preventing the growth and coalescence of mesophase particles; isotropic coke is formed, with a microstructure score of 2.2. At 500°C, anisotropic coke is formed, with a microstructure score of 4.3. Despite the high softening temperature and the content of the α1 fraction, the high-temperature pitch does not form isotropic coke on carbonization. The macrostructure of the coke obtained by thermocracking is monolithic, with fine pores. The thermocracking conditions (temperature, pressure, presence of H₂) facilitate partial destruction and hydrogenation of the heterocyclic compounds. As a result, the coke has a reduced N, S, and O content. For pitch coke, the nitrogen content is 20–40% higher. The lack of ash in the anthracene fraction of coal tar results in ash- and metal-free coke. The coke obtained by thermocracking also has satisfactory X-ray structural characteristics and undergoes practically no expansion on graphitization, in contrast to pitch coke. In view of the technological convenience (absence of liquid products, high coke yield) and the quality of the coke, the production of isotropic coke by thermocracking may be regarded as a promising means of supplying the raw material used to produce artificial graphite.     

煤沥青制备针状焦的研究

通过溶剂-离心法净化原料沥青,应用延迟焦化工艺制备针状焦,考察了在聚合过程中QI含量、温度、压力以及时间对成焦的影响.结果表明,原料中QI含量的减少有利于针状结构的生成,当QI含量小于0.1％时能形成大面积的针状结构;适当的温度和压力能使体系的黏度降低,促进气流拉焦作用,在470℃、0.35 MPa条件下得到的针状焦较为理想;充足的反应时间保证小球的充分融并,反应24h体系能融并、重排,形成良好的针状结构.     

High-temperature centrifugation: Application to mesophase pitch

A laboratory high-temperature centrifuge capable of operating up to 500°C at centrifugal accelerations of 600 g has been designed and constructed. The apparatus has been particularly useful in separating the isotropic and anisotropic phases in mesophase pitches and quantitatively determining phase contents. The efficiency of phase separation was determined for a petroleum mesophase pitch as a function of temperature and centrifugal acceleration. Effects of gas evolution, sphere coalescence and intraphase precipitation were also observed.     

一种新型煤基沥青的炭化性能研究

采用压力反应釜探讨了一种新型煤基沥青的炭化性能。结果发现,在试验的所有炭化条件下该原料均能够形成各向异性焦,说明融并性能良好。在炭化温度450～470℃,生焦各向异性组织的方向性随着温度的提高而改善,但生焦的形貌有所变差;在过高的炭化温度500℃下,生焦的显微结构和形貌均变差;在炭化压力0.3～0.7MPa,随着压力下降各向异性组织的方向性有所改善而单位变细,生焦的形貌改变不明显。选取适中的炭化温度450℃,更高的炭化压力1.3MPa,通过控制炭化体系的温度差和炭化后期适当减压增强气体逸出的剪切作用,制备出了显微组织和形貌得到明显改善的生焦。可见,适中的反应速率、较低的体系黏度和有效的气体逸出对于中间相的定向排列十分重要。     

Microstructural analysis of in situ mesophase transformation in the fabrication of carbon-carbon composites

In this work, we examined the microstructures formed during the pyrolysis of naphthalene mixed with AlCl₃ catalyst, in the critical temperature range of 300-500 °C and at varying pressures. In addition, non-rigidized preforms were densified by multiple cycle in situ transformation and compared the process with impregnation using fully transformed AR mesophase pitch under similar conditions. The process of mesophase formation in the bulk phase and within tightly packed fiber bundles was observed to be similar: spherule nucleation from the isotropic phase, coalescence of spherules forming bulk mesophase, and mesophase flow before hardening. The hardened mesophase displays the coarse, fibrous, and lamellar microstructure observed in needle cokes. The molten naphthalene was observed to evenly penetrate in-depth the large void spaces and fiber bundles. After two in situ cycles, the fiber bundles and the inter-fiber bundle regions were well filled with transformed mesophase. The incremental filling of the larger void spaces reduced the calculated filling efficiencies from 47% in the first cycle to below 15% in the third through fifth cycle. An 8% improvement in densification efficiencies was achieved by applying modest pressures during the pyrolysis. The extent of mesophase penetration with AR mesophase was observed to decrease from the outer to the inner regions of the preform. The results suggest impregnation with naphthalene catalyst mixture is efficient in filling tightly packed fiber bundles but not large void spaces. Multiple cycles are required in order to fill the large void spaces.     

Formation of thin mesophase layers at the interface between filler and binder in prebaked anodes. Effect of mixing on mesophase

Prebaked anodes used for aluminum electrolysis are prepared by mixing petroleum coke as a filler with coal-tar pitch as a binder. Pitches which contain too large amounts of mesophase spheres are not suitable for preparing high quality anodes since mesophase microstructure is destroyed by mixing. Both the well extended parallel preferred orientation and the spherical shape disappear. It is observed that disorganized mesophase forms a shell around the coke particles. Such shells reduce the wettability of the coke by the pitch. This transformation was reproduced experimentally by strong grinding. When a disorganized mesophase is heat-treated at 700°C, it recovers a local molecular orientation, the extension of which widely varies from one area to the next one. The smaller the orientation extent, the lower the graphitizability. A mixture of non graphitizing microporous carbon and of partially graphitizing macroporous carbon is thus obtained. Such porous carbons are also found in anodes after baking as shells around the filler.     

Production of isotropic coke in industrial trials

The influence of the production conditions on the properties of coke is considered. Coke is produced in a pilot plant at the Institute of Technical Chemistry, Ural Branch, Russian Academy of Sciences, in a 200-L reactor equipped with an air-supply valve and heating elements. The first stage in coke production is air-blowing of the batch with constant temperature rise at 10–12°C/h from 290–310°C; the air flow rate is 45–55 L/kg h. At this stage, the final air-blowing temperature and the batch composition are varied. The second stage is coking, with temperature rise at 25°C/h to 550–600°C. The batch consists of industrial air-blown pitch (ABP), modified by pitch tar (PT). Oxidation of the ABP, even with a very high final temperature (434°C), does not permit the production of isotropic coke. An analogous result is obtained on adding small portions of PT to the batch (15%). On adding >50% PT, totally isotropic coke may be produced. To obtain coke of isotropic microstructure, the optimal content of PT is 36–41%, and the final air-blowing temperature should be high (>390°C). The influence of PT on the structural parameters of the coke is associated with the formation of nonmesogenic structures on air-blowing. On coking, these structures suppress the growth of large mesophase. The isotropic coke produced has the following characteristics: limited expansion in the range 1300–2400°C; high structural strength; and optimal density. Graphite based on such coke is considerably superior to graphite based on industrial pitch coke in terms of its compressive strength, density, and electrical resistivity.     

Carbonization properties of partially hydrogenated aromatic compounds—I: Carbonization of partially hydrogenated pyrene prepared by Birch reduction

The carbonization properties of pyrene hydrogenated by Birch reduction to a variable extent were studied at 600°C under atmospheric pressure, in order to examine the influence of partial hydrogenation on the carbonization reactivity. Pyrene, which did not yield any coke, produced coke with flow texture after partial hydrogenation. Carbon yield of partially hydrogenated pyrene sharply depended on the hydrogenation extent. The intermediate stage of the carbonization pursued by hot-stage microscopy followed the steps of nucleation, growth and coalescence of anisotropic sphere. Considerable fluidity was observed at the final stage of mesophase development prior to the complete solidification where the benzene insoluble content exceeded 90%. The carbonization mechanism was discussed from the view of the polymerization of hydrogenated pyrene in relation with its structure.     

On the activity and stability of platinum/alumina catalyst during multiple deactivation and regeneration

Catalyst surface characterizations have been carried out to investigate the role of dispersion on catalyst activity and to probe the occurrence of oscillations in coking levels with cycle number generally observed during multiple deactivation and regeneration schemes. The titrations were done, cycle by cycle, at 430°C after oxidation and at the same temperature (430°C) after reduction at 500°C. Results show the usually observed trend - that the dispersions after oxidation are higher than those after reduction. The average decline in dispersion from oxidation to reduction was calculated to be 39.25%. It was observed that the cycles with high toxic coke removal were characterised by high deactivation times. The deactivation times were still high even for cycles subsequent to those with low dispersion. At high dispersions the catalyst had short deactivation times, that is the small crystallites deactivate faster than large ones. The nature of reducebale coke and the efficiency of its removal is a much more determinant factor of catalyst activity than the level of metal disperision. Thus prolonged toxic coke reduction at the high temperature of 500°C, though resulting in an apparent lowering of dispersion, does not affect the quality of the catalyst. The dispersion before reduction could be retained on oxidation. Hence reduction at 500°C did not introduce sintering.     

炭化条件对针状焦结构的影响

以除去喹啉不溶物的中温煤沥青为原料,分别在不同的反应温度和保温时间下制备了中间相炭微球(MCMB);在磁场条件下制备了有序结构针状焦;通过扫描电镜(SEM)考察了不同反应条件下中间相炭微球和针状焦的形貌,讨论了中间相形成影响因素对针状焦结构的影响.结果表明,中间相形成阶段的反应温度、保温时间和体系黏度对针状焦的结构和性能具有重要影响,磁场对针状焦的流线型结构有促进作用.     

Development of optical anisotropy in vitrains during carbonization

The purpose of this work was to examine the possible significance in the formation of metallurgical coke of the anisotropic spherical mesophase exemplified by that found during the carbonization of pitch-like materials, and to ascertain if the various types of optical anisotropy found in coke could form a basis for the characterization of cokes produced from different coals. Vitrains from a wide range of coals were carbonized at temperatures from 370 to 1000 °C and the types and amounts of optical anisotropy in the resulting semi-cokes and cokes were determined from microscopic examination, the anisotropic components being classified according to grain size of the granular mosaics and appearance. The anisotropy developed directly from the isotropic phase, appearing initially as a fine-grained mosaic. With increasing carbonization temperature, this fine-grained mosaic was transformed into progressively coarser-grained anisotropy, the extent of this transformation depending on the rank of the vitrain. It is therefore concluded that the formation, growth and coalescence of anisotropic spherical bodies, such as occurs during the carbonization of pitch, is not a necessary precursor of the mosaic anisotropy in coke. The type and amount of anisotropy developed provide a quantitative means of characterising different cokes.     

Carbonization of 3.5-dimethyl phenol-formaldehyde resin and its chemical structure at mesophase appearance

The evolving gases during heat treatment of 3.5-dimethyl phenol-formaldehyde resin up to 1000°C were continuously analysed by means of gaschromatography, and the resin's structural change was investigated by using IR spectra. At 430°C, a mosaic type mesophase suddenly appeared over the whole sample. Most of the sample carbonized at 400°C could be dissolved in pyridine, its chemical structure having been analysed by using NMR. Only 5.5 mol/MW of H₂O and 1.5 mol/MW of CH₄ were evolved in the range of carbonization temperature up to 400°C, and the chemical structure at 400°C was nearly the same as that of original resin except for the formation of some methin and ether linkages with naphthene and heterocyclic rings; large aromatic condensed polycyclic structures have not developed yet at this point. At temperatures from 400 to 430°C, only 0.52 mol/MW of H₂O, 0.55 mol/MW of CH₄ and 0.02 mol/MW of H₂ evolved, showing that no severe structural change takes place. Thus, the mesophase formation in this resin does not necessitate the pre-existence of large aromatic polycyclic structures, but is formed rather from planar large molecules.     

Optical properties of (acetoxypropyl)cellulose mesophases: factors influencing the cholesteric pitch

(Acetoxypropyl)cellulose (APC) forms a thermotropic cholesteric liquid crystalline phase, and with dibutyl phthalate (DBP) it also forms a lyotropic cholesteric phase. The reflection bands for the mesophases occur in the visible region, at wavelengths which depend on concentration and temperatures. The pitch of the cholesteric helicoidal structure is derived from measurements of the mean refractive indices and of the reflection band wavelengths for mesophase samples containing from 0 to 30% diluent at temperatures from ambient to 170°C. The pitch of the thermotropic mesophase increases with increasing temperature and with decreasing molar mass. The pitch of the lyotropic mesophase increases with increasing temperature and diluent content. Pitch values approach infinity at temperatures close to the clearing temperature of the mesophase, and no reversal in the sense of the pitch with temperature or diluent content was detected. The experimentally observed changes in pitch with composition and temperature are in reasonable agreement with the predictions of a recent theory for cholesteric mesophases composed of helical rod-like species. The average distance between chains in the mesophase is estimated from X-ray diffraction measurements, and hence the average angle of twist between neighbouring APC molecules may be found. The angle decreased from 2.2° for pure APC to 0.9° for a volume fraction of 0.73 APC in DBP.     

Influence of pyrolysis and melt infiltration temperatures on the mechanical properties of SiCf/SiC composites

In order to improve the degree of matrix densification of SiC_f/SiC composites based on liquid silicon infiltration (LSI) process, the microstructure and mechanical properties of composites according to various pyrolysis temperatures and melt infiltration temperatures were investigated.Comparing the microstructures of SiC_f/C carbon preform by a one-step pyrolysis process at 600 °C and two-step pyrolysis process at 600 and 1600 °C, the width of the crack and microcrack formation between the fibers and matrix in the fiber bundle increased during the two-step pyrolysis process. For each pyrolysis process, the density, porosity, and flexural strength of the SiC_f/SiC composites manufactured by the LSI process at 1450–1550 °C were measured to evaluate the degree of matrix densification and mechanical properties. As a result, the SiC_f/SiC composite that was fabricated by the two-step pyrolysis process and LSI process showed an 18% increase in density, 16%p decrease in porosity, and 150% increase in flexural strength on average compared to the composite fabricated by the one-step pyrolysis process.In addition, among the SiC_f/SiC specimens fabricated by the LSI process after the same two-step pyrolysis process, the specimen that underwent the LSI process at 1500 °C showed 30% higher flexural strength on average than those at 1450 or 1550 °C. Furthermore, under the same pyrolysis temperature, the mechanical strength of SiC_f/SiC specimens in which the LSI process was performed at 1500 °C was higher than that of the 1550 °C although both porosity and density were almost similar. This is because the mechanical properties of the Tyranno-S grade SiC fibers degraded rapidly with increasing LSI process temperature.     

Needle coke formation derived from co-carbonization of ethylene tar pitch and polystyrene

Ethylene tar pitch was co-carbonized with waste polystyrene to prepare needle coke. The modified properties of mesophase, which were greatly improved due to increasing naphthenic and other alkyl content, availed the formation of needle coke with high quality. The coefficient of thermal expansion value was decreased from 3.2 × 10⁻⁶/°C to 0.3 × 10⁻⁶/°C and the optical texture of the coke was changed from coarse mosaic texture to flow domain of high uniaxial orientation after adding waste polystyrene into ethylene tar pitch. The low viscosity of the mesophase pitches favored the development of mesophase and highly uniaxial arrangement. The increase in alkyl group content greatly improved characteristics of the needle coke.     

Heat treatment of anthracene oil under pressure

The heat treatment of the anthracene fraction of coal tar under pressure is investigated. In the production of synthetic pitch, the temperature is varied in the range 400–480°C. The duration of the process is 5 h, and the pressure is 3–5 MPa. After treatment at temperatures below 480°C, the pitch produced lacks the quinoline-insoluble fraction. Chemical analysis shows that increasing the treatment temperature results in consumption of cata-condensed aromatic hydrocarbons, with simultaneous accumulation of peri-condensed aromatic hydrocarbons. NMR and IR spectroscopy indicates simultaneous increase in the aromatic index and the degree of condensation of samples with increase in the treatment temperature, as the result of dealkylation and condensation. Optical analysis shows clearly expressed mesogenic properties of the synthetic pitch. Pitch produced by low-temperature synthesis forms needle coke, with a greater anisotropy and greater real density than coke obtained from pitch produced by high-temperature synthesis. With increase in pressure in the course of coking, the anisotropy and the real density decline for low-temperature pitch, but increase for high-temperature pitch.     

Thermal transformations of polyvinyl alcohol as a source for the preparation of carbon materials

The main volatile products of the pyrolysis of polyvinyl alcohol (PVA)—water, carbon oxides, acetaldehyde, methane, and hydrogen—were determined by gas chromatography with the use of a pyrolytic attachment to a chromatograph (so-called stepwise pyrolysis gas chromatography). The correlations of product ratios over various temperature ranges were found. It was demonstrated that the preliminary thermal oxidation of PVA in air at 200°C, which was responsible for the partial dehydration of PVA and the formation of thermally labile oxygen-containing groups in it, facilitated the occurrence of the subsequent carbonization phase. This manifested itself in much more intense dehydrogenation and methane release from the coke residue of thermally oxidized PVA in the region 700–900°C, when the elimination of oxygen-containing compounds was mainly complete, and in an increase in the yield of coke residue and the C: H ratio in this residue.     

Formation of mesophase in coal tar pitches. Influence of pyrolytic carbon particles

Hitherto it has been considered that the pyrolytic particles present in pitch as a result of high temperature cracking accelerate the formation of mesophase by the nucleation process. It has been demonstrated by optical and hot stage microscopy that the contrary is the case. The pyrolytic particles in pitch retard the development of mesophase so that mesophase starts to be detectable only at higher temperatures of pyrolysis and the coalescence is suppressed or even prevented by pyrolytic particles surrounding the mesophase spheres originally formed.