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The effect of accumulation of polycyclic aromatics in FCC decant oil on subsequent pyrolysis behaviors
Abstract
To control the liquid-phase carbonization of the petroleum residue, the transition of the aromatics in earlier stages of the thermal conversion and its effect on subsequent carbonization behavior were analyzed. It was found that more monocyclic and dicyclic aromatics was converted to polycyclic aromatics in a comparatively shorter time between 450 and 500 °C. The higher polycyclic aromatics content of the oil accelerated the formation of the anisotropic phase, thus the excessive carbonization of the system was prevented and the lamellar orientation of the pyrolysis product was modified (the time required to obtain a fully developed carbonaceous mesophase decreased by 2 h and the stacking height (Lc) increased from 18.58 Å to 21.18 Å when the polycyclic aromatics content increased by 14.92 wt%). The modified oil obtained by thermal pretreatment (HCTO-RF-B) was converted to a carbonaceous mesophase with a large domain texture and good lamellar orientation (d002 = 3.4599 Å, Lc = 21.94 Å), as well as a low softening point (273 °C) in a short carbonization time (8 h), which is attributed to the higher polycyclic aromatics content (which accelerated the formation of an anisotropic phase) and fewer long side chains (which controlled an appropriate free radical concentration) of the thermal pretreated oil. The fully developed carbonaceous mesophase from HCTO-RF-B had a higher stabilization reactivity, which is attributed to the alkyl chains preserved in the system (C–H substitution indice (ICHS) = 0.7513, alkyl indice (IA) = 0.7690).
... Such carbon materials find widespread applications in metallurgy, construction, textiles, papermaking and transportation, exhibiting promising prospects [2][3][4]. The ordered nature of the carbonaceous intermediate phase formed during the pyrolysis of heavy tar is a crucial factor influencing the performance of graphitized foam carbon [5]. ...
In order to achieve the high-value utilization of heavy tar for the production of enhanced-performance graphite foam carbon, the carbon mesophase was ready from the heavy component of low-temperature coal tar, and the coal tar was modified by styrene-butadiene-styrene (SBS), polyethylene (PE) and ethylene-vinyl-acetate (EVA) copolymers. The order degree of the carbonite mesophase was analyzed using a polarizing microscope test, Fourier transform infrared spectroscopy and X-ray diffraction to screen out the most suitable copolymer type and addition amount. Furthermore, the mechanism of modification by this copolymer was analyzed. The results showed that adding SBS, PE and EVA to coal tar would affect the order of carbonaceous mesophase; however, at an addition rate of 10.0 wt.%, the linear-structure SBS copolymer with a styrene/butadiene ratio (S/B) of 30/70 exhibited the optimal degree of ordering in the carbonaceous mesophase. Its foam carbon prepared by polymer modification is the only one that forms a graphitized structure, with d002 of 0.3430 nm, and the maximum values of Lc and La are 3.54 nm and 2.22 nm, respectively. This is because, under elevated pressure and high-temperature conditions, SBS underwent chain scission, releasing a more significant number of methyl and other free radicals that interacted with the coal tar constituents. As a result, it reduced the affinity density of heavy coal tar molecules, enhanced fluidity, promoted the stacking of condensed aromatic hydrocarbons and increased the content of soluble carbonaceous mesophase, ultimately leading to a more favorable alignment of the carbonaceous mesophase.
... The effect of the decantoil hydrocarbon composition on the thermolysis process [19][20][21], as well as the study of the hydrocarbon composition of distillate fractions [22,23] and the kinetics of the hydrocarbon components formation combined, allows us to estimate the stages of mesophase development for different types of hydrocarbon raw materials during the production of needle coke [24][25][26]. This work is devoted to determining the kinetics of decantoils conversion into hydrocarbons of various groups in distillates during coking. ...
Carbon-graphite materials based on needle coke are fundamentally important for the development of related industries worldwide. This paper presents the study results of two types of decantoil coking dis-tillates from the FCC plant, obtained in a laboratory simulation to produce petroleum needle coke. During coking, distillates were sampled by cut fractions at formation temperatures of 410-440, 440-470, 470-500, 500-505°C. The hydrocarbon composition of the original decantoils and obtained cut fractions of the coking distillates were analysed by GC-MS. The selection of short-boiling-range products and analysis of their hydrocarbon composition allowed to estimate indirectly the stages of mesophase development and to determine the narrow temperature interval at which its most active development occurs. For the two types of decantoils this temperature interval is different (for D2 À 440-470°C, for D1 À 470-500°C). This phenomenon allows us to effectively determine the regime parameters of the delayed coking process for needle coke production. The formation kinetics (effective activation energy and pre-exponential factor) of different groups hydrocarbons, including paraffin-naphthenic, unsaturated, aromatic (mono-, bi-and polycyclic) for the first time was determined using the model-free Friedman iso-conversion method. The obtained coefficients of determination are close to 0.9-1.
For the preparation of high-performance pitch-based carbon fibers and other carbon materials, mesophase pitch serves as a high-quality precursor. Since FCC (Fluid Catalytic Cracking) oil slurry is abundant in aromatic hydrocarbons and saturated hydrocarbons (about 95% in total), it has become an ideal choice for developing new carbon material products. This paper details the research progress of preparing mesophase asphalt with FCC oil slurry as a raw material from perspectives including the preparation method of synthesizing mesophase asphalt from FCC oil slurry, the impact factors of the formation process of mesophase asphalt and the industrial application of mesophase asphalt.
The composition of isotropic pitch (IP) is the key factor to determine the performance of carbon fibers (CFs). It is necessary to investigate the effect of chemical structure of pitch and compositions change on the performance of CFs. An isotropic pitch was fractionated into seven fractions by solvent fractionation to characterize the molecular structural features by FT-IR, ¹H NMR etc. The oxidative reactivity of pitch fibers, mechanical properties, microstructure and morphology of CFs derived from four IPs with different components were characterized and evaluated. The results revealed that with the removal of light fractions, the synergistic effect between pitches components in melting significantly decreased, leading to the decrease of spinnability of pitch and deteriorating the uniformity of carbon fiber microstructure. The oxidative reactivity of the pitch fibers gradually decreased with the removal of light fractions. The reason was that these light fractions contained more alkyl side chains, which were preferentially attacked by oxygen at the low oxidation temperature, than that of these heavy fractions. The mechanical properties of CFs showed an increase first and then decrease with the removal of these light fractions. Especially, the carbon fiber derived from the isotropic pitch (H80-T20I) exhibited an excellent high tensile strength of 1190 MPa.
Vacuum residue (VR) is the most complex component of crude oil. Due to the special structure of heavy subfractions, physical aggregation and chemical coking reactions easily occur through molecular force, which affects the normal processing. Therefore, in‐depth study and analysis of their composition, structure and association behaviours are particularly important. In view of the shortcomings of the traditional separation method in terms of separation accuracy, mainly including the purification of asphaltenes and the poor separation of resins. In this paper, a reasonably improved separation method is adopted, and the multi‐stage asphaltene extraction and the multi‐stage silica gel coupling separation are carried out innovatively, which achieves a high yield of 99% while ensuring the separation accuracy. The samples were characterized by elemental analysis (EA), gel permeation chromatography (GPC), Fourier transform infrared spectroscopy (FT‐IR), hydrogen nuclear magnetic resonance spectroscopy (1H‐NMR), X‐ray photoelectron spectroscopy (XPS), X‐ray diffraction (XRD), and scanning electron microscope (SEM) to study their structural characteristics and association behaviours. The results show that the main forms of heteroatoms in asphaltene and resin surface are C‐O‐C, C‐OH, pyridine, pyrrole, and thiophene, and the content of these substances is higher in asphaltene. Compared with resins, asphaltenes contain more peri‐condensed aromatic structure and shorter alkyl substituent side chains. By studying the hydrogen bond and acid–base interaction, it is found that asphaltene and resin mainly contain OH‐OH, OH‐π, and OH‐ether O, of which the content of OH‐OH is the highest. Asphaltene and resin have more neutral and basic substances. These hydrogen bonds and acid–base interactions caused by heteroatoms are the main forces for the association of the heavy subfractions of the VR.
In needle coke industry, the raffinate oil from raw material refining could be used for co-carbonization to improve the structural evolution of aromatic feedstock with high aromaticity and low alkyl side chain content. High-temperature coal tar pitch (HCTP) and raffinate oil obtained from low-temperature coal tar (LCT) were used to prepare needle coke by liquid phase carbonization, and the characteristics of carbonization behavior as well as the structural evolution of blended materials were also investigated. The raffinate oil possessed abundant saturated compositions and the aromatic component distribution mainly concentrated in 1–3 cyclic aromatic hydrocarbons which resulted in a low yield of derived products and the carbonaceous sheets with poor planarity. Raffinate oil provided sufficient small molecular volatiles of alkyl radicals and generated abundant active sites for the polycondensation of HCTP with high aromaticity and low alkyl side chain content during co-carbonization. The addition of raffinate oil in HCTP facilitated the elimination of steric hindrance and healed the structural defects within the planar aromatic lamellae. This paper improved the utilization of raw material refining treatment and optimized the industrial structure of the needle coke preparation process.
To investigate the effect of calcination process on the characteristics of different carbonaceous structures, decant oil (DO) and low-temperature coal tar (LCT) were selected as materials to refine and blend to prepare the green cokes with different microstructures, and the influence of calcination on the structural evolution of green cokes with different orientation and planarity was studied. It is concluded that the nitrogen content of calcined needle coke decreased to a relatively low value at the calcination temperature of 1300℃ and the removal of QI decreased the sulfur content in calcined cokes. Calcination process would further promote the accumulation of aromatic lamellae on the original basis. The abundant QI in LCT led to the formation of mosaic structures in green coke stage and calcination aggravated the poor planarity of aromatic sheets of mosaic texture. The refining process and blend of raw materials significantly enhanced the proportion of key components and calcination improved the extendability and planarity of graphitic aromatic sheets with the increase of Tri-Ar and Tetra-Ar. In addition, calcination further optimized the aromatic lamellae of acicular domain texture with good planarity and structural orientation, and DO-CNC obtained the highest stacking parameters (Lc = 3.55 nm; N = 11.30; d002 = 3.445 Å).
In the present work, a novel resin molecule based on fluid catalytic cracking (FCC) slurry oil residue was designed and prepared with aid of molecular simulation to improve its ageing resistance. The average molecular structure of resin was characterized with elemental analysis, Fourier-transform IR (FT-IR) spectroscopy, NMR spectroscopy and matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) and designed resin was synthesized by attaching an additional propyl group to the as-received resin. The interactions of resins and asphaltene molecules were investigated via molecular simulation, UV-visible (UV-Vis) spectroscopy and atomic force microscopy (AFM). The results revealed that the equilibrium distance of designed resin-asphaltene dimer decreased from 0.42 nm to 0.38 nm while the lowest contact energy increased from 9.3 kcal/mol to 11.2 kcal/mol, indicating an improved stability of designed resin-asphaltene particles. The adsorption isotherm indicated that more modified resin molecules could be adsorbed by asphaltene than the original resin. Compared with original asphalt, less aggregation of asphaltene occurred in asphalt blended with modified slurry oil residue during ageing process, showing its greatly improved ageing resistance.
Needle coke is mainly used in high-power electrode material for electric arc furnace, the economy of material cost and the quality of co-carbonized products are widely concerned. Low temperature coal tar (LCT) and heavy oil (HO) were used to prepare needle coke by liquid phase carbonization, and the co-carbonization behavior as well as structural evolution of blended materials were also studied. It was concluded that the derived needle coke pyrolyzed from the blended HO and LCT showed obvious structural separation interfaces between the anisotropic acicular structure and isotropic matrix. The abundant primary quinoline insolubles (QI) in LCT prevented the penetration of smaller molecules. The removal of primary QI in LCT improved the compatibility of HO and LCT-derived structure and eliminated the obvious structural separation interfaces. The furfural extraction significantly regulates the molecular composition distribution which is conductive to promote interactions within compositions and improve the order of structural evolution. The sample extracted at 60℃ after removing QI (LCTE60) contained 25.7 wt% of alkanes and naphthenes, and the total content of bicyclic aromatics, tricyclic aromatics and tetracyclic aromatics occupied 45.3 wt%. The derived needle coke prepared from blended HO and LCTE60 with the proportion of 50:50 obtained the needle domain content of 60 wt% and the optical texture index (OTI) of 71.66. This study provides theoretical basis for the co-carbonization of needle coke production.
Needle coke has been widely studied due to its highly oriented carbonaceous structure and low thermal expansion for preparing high power electrode. In order to further investigate the effect of compositions and molecular structure of heavy oil (HO) on the pyrolysis performance and structural evolution of needle coke, the heat pretreatment at 360-400℃ for 2-10 h was used to modify the feedstock and the thermal polymerization method was adopted to prepare needle coke. It was concluded that the heat pretreatment process removed part of small molecular volatiles and increased the polycyclic aromatic content. The decrease of the small molecular volatiles content reduced the disordered disturbance of gas flow and transformed the structure from disordered texture to fine flow texture. The pre-treatment of light components avoided the out-plane crosslinking short carbonaceous sheets within the lamellae to eliminate the steric hindrance and relax the structural strains with few defects. Moreover, the heat pretreatment provided good thermal stability during the accumulation of carbonaceous structure.
Mesophase carbon microbeads (MCMBs) were synthesized by thermal polycondensation with fluidized catalytic cracking residue oil (FRO) as the feedstock. The evolution features of functional groups of the residual oils and the residual solid products from thermal polymerization were analyzed, and the effect of olefinic compounds on the coalescence between mesophase spheres was investigated in depth. The results showed that the olefin contents of residual oil, heptane-soluble, and heptane-insoluble/toluene-soluble decreased simultaneously as the reaction proceeded while that of toluene-insoluble increased, indicating that the olefins in the solvent phase could not only provide molecule sources for the development of carbonaceous mesophase but also directly assemble into the insoluble mesogens. More importantly, the involvement of olefins caused a sharp increase of the system viscosity, making it difficult for the mesophase spheres to completely coalesce into regular large ones after cohering, leading to the apparent deformation. In contrast, catalytic hydrotreatment can saturate olefinic species and enhance the hydrogen transfer ability of FRO to further avoid the induction effect of olefins, slow down the mesophase development, reduce the system viscosity, and consequently avoid the deformation of mesophase spheres.
This work presents the results of thermogravimetric analysis of decantoil. The microstructure of the extracted petroleum coke during the thermolysis of decantoil in an inert nitrogen atmosphere at 1000°C was studied by scanning electron microscopy and interpreted as acicular. The model free kinetic approach based on Friedman's isoconversion method was used to calculate the kinetic characteristics of decantoil thermolysis-activation energy and pre-exponential factor. The individual hydrocarbon composition of decantoil was determined by gas chromatography mass spectrometry, which was used to determine the activation energy of evaporation of its fractions. The total energy spent on the chemical reactions of cracking and thermopolycondensation was determined when the degree of decantoil conversion was changed from 0.1 to 0.9.
Mesophase pitch is an important starting material for making a wide spectrum of industrial and advanced carbon products. It is produced by pyrolysis of petroleum residues. In this work, mesophase formation behavior in petroleum residues was studied to prepare environmentally-benign mesophase pitches, and the composition of petroleum residues and its influence on the mesophase formation was investigated. Two petroleum residues, i.e., clarified oil s (CLO-1, CLO-2) obtained from fluid catalytic cracking units of different Indian petroleum refineries, were taken as feed stocks. A third petroleum residue, aromatic extract (AE), was produced by extraction of one of the CLO-1 by using N-methyl pyrrolidone solvent. These petroleum residues were thermally treated at 380°C to examine their mesophase formation behavior. Mesophase pitches produced as a result of thermal treatment were characterized physico-chemically, as well as by instrumental techniques such as Fourier-transform infrared spectroscopy, nuclear magnetic resonance, X-ray diffraction and thermogravimetry/derivative thermogravimetry. Thermal treatment of these petroleum residues led to formation of a liquid-crystalline phase (mesophase). The mesophase formation behavior in the petroleum residues was analyzed by optical microscopy. Mesophase pitch prepared from CLO-2 exhibited the highest mesophase content (53 vol%) as compared to other mesophase pitches prepared from CLO-1 and AE.
The characterization of the asphaltene fractions of a range of petroleum feedstocks by FT-IR, 1H and 13C NMR spectroscopy indicated distinct differences in the molecular structure of the asphaltenes. Some of these differences could be related to the variations in the size of the principal optical texture of the semi-cokes produced by carbonization. The principal optical texture size was observed to increase steadily with the increasing hydrogen aromaticity of the asphaltenes over the whole range of the feedstocks used. There was no consistent correlation, however, between the carbon aromaticity of the asphaltenes and the optical texture size. The correlation between the hydrogen aromaticity and the principal optical texture size was attributed to structural differences among the asphaltenes that are critically important for the mesophase development.
A petroleum pitch was heated at 420 °C for 7 h in nitrogen to prepare a carbon fiber precursor with a softening point of 295 °C. The precursor was successfully melt-spun into fibers through a circular nozzle of a monofilament spinning apparatus, and these were then stabilized at 320 °C in air and finally carbonized at 1000 °C in nitrogen to produce carbon fibers. SEM, TGA, FT-IR, and XRD were performed to characterize the petroleum pitch, the precursor, the as-spun fibers, the stabilized fibers, and the carbon fibers. It is found that the precursor contains 70.5% mass fraction of mesophase that is aligned upon spinning, and aliphatic side chains that are beneficial to spinning. The carbon fibers have a radial core structure with a linear and bent type anisotropic texture. The maximum tensile strength of the carbon fiber is 650 MPa.
The two-stage pyrolysis was proposed to effectively form uniaxially fibrous mesophase during pyrolysis of FCC decant oil. Detailed investigation focuses on main factors of regulating mesophase orientation and corresponding changes of mesogen stacks. The first-stage treatment under low temperature of 440 °C and high pressure of 2 MPa for 8 h ensures formation of bulk mesophase with well-coalesced texture and good molten fluidity; subsequently, fracture of residual alkyl side-chains together with devolatilization of trapped light molecules under high temperature (450–480 °C) and low pressure (0.2–1.0 MPa), mainly contributes to volatile evolution at solidification of two-stage pyrolysis. Thereby, the overlap between sufficient volatile evolution that provides enough shear force on mesophase and appropriate viscosity of bulk mesophase, high enough for maintenance of mesophase deformation but not too high to prevent penetration of gas bubbles, is readily achieved and then significantly promotes the transition from mesophase domain into acicular flow texture. Moreover, mesophase orientation at solidification is followed by transformation of mesogen stacks from columnar short-range order into nematic order, leading to decrease of stacked height and number of layer per stack. And further cracking and dehydroaromatization reactions at solidification of two-stage pyrolysis facilitate elimination of steric hindrance, relaxation of structural strains and heal of defects within the planar aromatic sheets, thus resulting into a more homogenous distribution of stacked mesogens and a smaller degree of disorder within the 2D mesogen lattice.
Slurry-phase hydrocracking employing dispersed catalyst is an efficient technology for the upgrading of residue to clean fuels. The green synthesis of 2D nanosheets hierarchical composites when carrying out some intrinsic functionalities is interesting for both scientific and technological applications. Herein, we present our effort in simultaneously enhancing the residue upgrading and synergistic fabricating 2D MoS2 nanosheets/carbon hierarchical structures based on amorphous molybdenum sulfide nanocatalysts. The amorphous molybdenum sulfide nanocatalysts (MoSx-AM, x ≈ 2.79) are constructed in an inverse microemulsion system, which are then either modified by oleic acid to synthesize the oleophilic MoSx nanocatalysts (MoSx-OL) or annealed to obtain MoS2 nanocatalysts with the typical layered structure (denoted as MoSx-AN). Benefiting from the decomposition into 2D MoS2 nanosheets with a length less than 20 nm during the residue slurry-phase hydrocracking, MoSx-AM and MoSx-OL exhibit superior hydrogenation and anti-coke activity compared with MoSx-AN. X-ray photoelectron spectroscopy and density functional theory (DFT) calculation reveal that the residue macromolecules with polycyclic aromatic structure and heteroatoms show strong interaction with 2D MoS2 nanosheets, which disturbs the formation of layered-structure of MoS2 and results in isolated 2D MoS2 nanosheets. The generated 2D MoS2 nanosheets with abundant active sites lead to the superior performance in residue slurry-phase hydrocracking. These results demonstrate a dispersed nanocatalyst for slurry-phase hydrocracking and also provide a perspective for the anti-coke of petroleum refining. In particular, this process also enabled the dispersed 2D MoS2 nanosheets/carbon hierarchical structures formation synergistically.
The fused aromatic rings (FARs) of coal are considered to be relatively stable during coalification. However, it is speculated that defects may form in FARs during the deformation of coals under tectonic stress. In order to investigate the possibly existed mechanolysis reactions of FARs, we performed molecular dynamics simulations with the reactive force field to study an anthracite coal model under shear stress. The results show that the FAR rupture reactions occur much more efficiently than expected. There are two important ways to initiate FAR rupture: attacked rupture and direct rupture. Among the mechanisms, two types of rearrangement reactions play crucial roles. The first type is intra-ring rearrangement where the FAR forms a three-membered ring and a five-membered ring before rupture. The second type is extra-ring rearrangement where the location of substituent group changes. Both rearrangement reactions determine which bond in the FAR preferentially breaks but in different manners, and then promote the subsequent reactions. Comparing to pyrolysis mechanism, mechanolysis mechanism follows completely different reaction pathways and result in different products. This research will aid in understanding the reaction mechanisms of coal mechanochemistry, and it opposes the traditional standpoint that the FARs of coal are relatively stable and very hard to be destroyed during coalification.
In this work, we report a simple one-step preparation of a novel graphitic carbon nitride/Polyaniline/Palladium nanoparticles (g-C3N4/PANI/PdNPs) based nanohybrid composite modified screen-printed electrode (SPE) for the efficient electro-oxidation reaction of methanol. g-C3N4/PANI/PdNPs nanohybrid structure was achieved by a single-step simultaneous electro-deposition method with an electrolyte solution containing palladium chloride, aniline, and g-C3N4 nanosheets. Structural and morphological characterizations depicted that the needle-shaped PANI and spherical shaped Pd NPs were deposited between g-C3N4 nanomatrix. The electrocatalytic behavior of the as-prepared nanohybrid composite modified SPE was assessed using cyclic voltammograms and chronoamperometric method, and it showed superior electro-oxidation and excellent catalytic performance over methanol oxidation compared to commercial 10% palladium loaded carbon black material and other previously reported electrode materials. These outcomes proved that the newly developed nanohybrid composite could be used as a potential electrocatalyst material for the direct methanol fuel cell applications.
Understanding the deposition process of pyrolytic carbon is essential in controlling its microstructure and consequent macroscale performance. Herein, using molecule dynamics (MD) simulations with the ReaxFF potential, we revealed the initial formation process of pyrolytic carbon near the simplified surface of carbon fiber. Three stages were identified in the deposition process, where polycyclic aromatic hydrocarbons (PAHs) formed from precursor gas at first, followed by the deposition and assembly process of PAHs and subsequently the continuous chemisorption of light carbon species. The impacts of temperature, gas density and surface defects were investigated separately. Our results suggest that PAHs would be the main source of the deposition under high gas density and surface defects would promote the deposition prominently. Comparing formed pyrolytic carbon structures under different conditions, we also found that PAHs would produce more defects and interlayer connections while the growth from light carbon species would achieve a more intact in-plane architecture. Moreover, the potential relations between our simulated structures and previous experimental observations were discussed to explore the initial formation mechanism of pyrolytic carbon in actual deposition process. The MD simulation data and insights obtained here might be significant for understanding the deposition process of pyrolytic carbon at atomistic scale.
To concentrate pre-mesogens (molecules can convert to a mesophase quickly) from petroleum pitch and prevent system from suffering from excessive carbonization, the parent pitch was extracted by various solvents with different solubility parameters and the corresponding pitch residues were carbonized. It was found that the dispersive solubility parameter has a major effect on the FTIR structural indices (i.e., the indices calculated from Fourier transform infrared spectroscopy) of the pitch residue; thus, four pitch residues enriched in molecules characterized by different condensation degrees and numbers of alkyl chains were successfully obtained and their carbonization behaviors were identified to determine the carbonization behavior of pre-mesogens.
The pitch residue obtained by hexamethylene extraction (A1HEX) formed a large domain texture with good lamellar orientation (d002 = 3.4656 Å, Lc = 24.45 Å) and low softening point (SP = 257 °C) in a short carbonization time (6 h), which is attributed to the removal of molecules with a high alkyl carbon atom percentage (which accelerates the formation of an anisotropic phase) and the enrichment of specific molecules. The concentration of molecules with a low condensation degree (condensation index HAU/CA = 0.5491), fewer alkyl chains (C–H substitution indice, ICHS = 0.1511), and a higher condensation reactivity in A1HEX accelerated the formation of the anisotropic phase and provided alkyl free radicals during thermal treatment, which hindered crosslinking between larger free radicals and contributed to the large domain texture with good lamellar orientation and a low softening point.
In addition, because of the lower reactivity of the self-assembled polycyclic aromatic hydrocarbons (PAHs) contained in the anisotropic phase (compared to that of the isolated PAHs in the isotropic matrix), the rapid formation of a mesophase during carbonization prevented excessive condensation to some extent.
TiO 2 -modified activated carbon fiber (ACF) was prepared by a dip-coating method, and supported with CeO 2 nanoparticles. The catalysts were applied in the low temperature selective catalytic reduction (SCR) of NO for the first time and exhibited significantly higher NO conversion than the unmodified catalyst in the entire temperature range. Different characterizations were carried out to show that the redox and acidic properties were closely related to the superior catalytic performance. Furthermore, the promotional mechanism was elucidated from the changes in the structure and surface properties of catalysts. It was verified that the introduction of TiO 2 not only enhanced the concentration of surface chemisorbed oxygen and surface Ce ³⁺ but also improved the adsorption and activation of NH 3 and NO, thus more -NH 2 and nitrate species were generated to participate in SCR reaction, which was attributed to a synergistic interaction between TiO 2 and CeO 2 .
The structural properties and composition of pitches are particularly important for the optimization of pitch processing to high-value products and practical application. Because of their complexity, pitch analysis requires both chemical and spectroscopic tools, as those applied in the present work to coal and petroleum-derived pitch samples. In particular, a large variety of methods, including chromatography, elemental analysis, thermo-gravimetry, mass spectrometry, UV-Visible and FTIR spectroscopy, was applied for evidencing the different characteristics of commercial solid pitch derived from petroleum and coal. The interrelation among some pitch properties was investigated by analyzing petroleum and coal-derived pitches having different softening points and volatility. It was observed that properties as the softening point and the coking yield are mainly related to the pitch volatility rather than on the pitch source (coal or petroleum), whereas some other features as the solubility and the oxidation reactivity of coke produced appeared correlated with the source characteristics.
The purpose of this investigation was to explore the influence of the oxidized diesel oil on the low-rank coal flotation response and flotation kinetics by plasma oxidation method. FTIR analysis showed that the polar groups of the diesel oil after oxidation, such as OH, C[dbnd]O, COOH and C–O–C, showed an obvious increase; and the polar groups generated could bond with the polar functional groups on the low-rank coal surface. Moreover, compared to the diesel oil before oxidation, a stronger interaction between the low-rank coal and the diesel oil after oxidation was found according to the reaction heat analysis. And, the contact angle of the low-rank coal wetted by the diesel oil before and after oxidation is 34.5° and 23.7°, respectively. Therefore, the diesel oil after oxidation showed better positive effect on enhancing the contact angle of the low-rank coal and restraining the negativity of the wetting heat of the low-rank coal wetted by water compared to the raw diesel oil. The FTIR analysis illustrates that the polar oxygen functional groups of the low-rank coal surface are decreased with the oil addition, especially after the oxidized diesel oil addition, indicating that the oxidized diesel oil can better improve the surface hydrophobicity of coal sample compared with the raw diesel oil. Furthermore, five kinetics models were chosen to assess the effect of the diesel oil after oxidation on the kinetics performance of the low-rank coal flotation, and the five test kinetics flotation models could give the excellent fit for the low-rank coal flotation data obtained by the diesel oil before and after oxidation. And, the kinetics fitted results illustrated that the diesel oil after oxidation strongly influenced the flotation kinetics of the low-rank coal and could give a better kinetics effect for the low-rank coal flotation than diesel oil before oxidation.
By adding naphthalene-based mesophase pitch (NBMP) into refined coal tar pitch (RCTP), pyrolysis products were obtained by dry-blending and co-carbonization method. The properties of pyrolysis products were characterized by polarizing microscope, thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FT-IR), X-ray diffractometer (XRD) and Raman spectroscopy. The result shows the NBMP can obviously change the mesophase development of RCTP because it can promote the formation of mesophase spheres during the pyrolysis process. The carbon yield and aromaticity of pyrolysis products were increased with the increase of NBMP content. The anisotropic texture was wide-area structure when the NBMP content was 5 wt%. However, when the content of the additive is increased to 10 wt%, pyrolysis products will appear coking. The main role of this additive is to accelerate the reaction rate as the “nucleating agent” of the thermal condensation reaction.
Molecular dynamic (MD) simulations based on the reactive force field (ReaxFF) were successfully used for the study of coal tar pitch (CTP) oxidation process under medium temperature. Three CTP components were divided into three fractions: pitch acetone solubles (PAS), pitch pyridine soluble-acetone insolubles (PPS), and pitch pyridine insolubles (PPI). A series of ReaxFF MD simulations were conducted to study the influence of temperature and oxygen content on CTP oxidation mechanism. The results demonstrate that the oxidation process is initiated by generation of C–O bonds and O–O bonds following intramolecular hydrogen transfer. The phenoxyl free radical which is initially formed by the cleavage of hydroxyl and the oxidization of benzene ring is an important intermediate product, it plays a key role in the formulation of H2O, CO2 and glyoxal, and it also promotes the combination of two small molecules. The cleavage of C–H bonds is more likely to happen when temperature is larger than 650 K. The PPI fraction is more stable compared with PAS and PPS components. The major reaction pathways are analyzed, and it's observed that the reaction pathways are able to reproduce and explain the experimental findings in the CTP oxidation process.
Simulated coal gas (SCG) was used as pyrolysis atmosphere to improve tar yield and quality of coal pyrolysis, and the effect of SCG compositions including H2, CH4, CO, CO2 on pyrolysis products was also investigated. The results show that tar yield of coal pyrolysis is obviously enhanced under SCG atmosphere compared with that under N2. The components in SCG apparently influence the distribution of pyrolysis products and play a different role in coal pyrolysis. The highest tar yield (16.3 wt%) is obtained under SCG atmosphere at 650 °C. H2 reduces the char yield by suppressing secondary reaction; coexistence of H2 and CO2 in pyrolysis atmosphere increases water yield significantly via reverse water gas shift reaction, but the addition of CO inhibits this reaction. The analysis of tar by simulated distillation and GC/MS showed that the addition of CH4 increases the contents of benzenes and phenols; the introduction of H2 mainly increases naphthalenes; while the introduction of CO inhibits the formation of phenols. Lower aromaticity of tar under SCG than that under N2 indicates that small molecular radicals (i.e. [rad]H, [rad]CHx radicals) generated from SCG participate in the stabilization of free radicals from coal pyrolysis, resulting in high tar yield and quality.
Mesocarbon microbeads (MCMBs) are a valuable carbon material, and the production of this material highly depends on the reaction conditions and feedstock properties. In the present study, supercritical fluid extraction fractionation (SFEF) technology was used to separate coal tar pitch (CTP) into a series of fractions to enhance MCMB preparation. After SFEF separation, most of the toluene-insoluble (TI) components were concentrated into the unextracted residue. First, optimal carbonization conditions were obtained for CTP through systematic experimental evaluations. Then, the SFEF fractions were heat-treated under the optimal conditions to produce MCMBs, and the relationship between chemical composition and product quality was revealed. Compared with CTP products, MCMBs produced from extracted SFEF fractions showed better morphology, and only a bulk mesophase with poor texture was generated from the unextractable residue. Moreover, the TI fraction had a significant negative impact on MCMB production in carbonaceous mesophase development. The results indicate that supercritical fluid extraction can be used to improve the quality of produced MCMBs by removing undesirable components.
The utilization of fluid catalytic cracking (FCC) slurry oil is widely concerned in modern refineries due to its poor processability. The present study used FCC slurry oil (SLO) as feedstock to produce mesocarbon microbeads (MCMBs) according to its rich aromatic content. Supercritical fluid extraction technique was adopted to remove the undesirable components and thus improve the mesophase development performance. Three different alkanes were used as the supercritical fluids and the chemical composition of derived fractions (SFEO) was analyzed. Four stages of mesophase development were observed in the carbonization process. The results showed that heavier feedstock reacted faster and reached the coalescence stage earlier, forming a bulk mesophase. MCMBs with a smooth surface and good morphology were prepared from iso-butane derived SFEO at a reaction time of 3 h. The quality of bulk mesophase produced from SFEOs was significantly improved comparing to DQ SLO.
Partial upgrading is an approach to processing Canadian oil sands bitumen with the objective to produce a synthetic crude oil that meets specifications for pipeline transportation (API Gravity of 19° and viscosity of 350 cSt at 7°C). Most partial upgrading technologies under development rely predominantly on thermal conversion processes combined with additional steps such as solvent deasphalting. Owing to the chemistry of thermal conversion, the liquid product resulting from this route tends to be unstable and susceptible to potential operational issues in pipeline transportation and subsequent refining. Hydroprocessing represents another pathway for bitumen partial upgrading, with the advantage of being able to minimize product instability concerns and achieve the target product quality. The objective of this study was to evaluate the feasibility of using mild hydroprocessing in a fixed-bed reactor for the partial upgrading of Canadian bitumen. Experiments were conducted in a pilot plant that underwent various modifications to handle raw bitumen. Systematic tests at different operating conditions were completed in a time period of 1394 catalyst hours without experiencing any plugging issues. It was demonstrated that stable and pipeline-ready product can be produced at residue conversions of 52.4 wt% or above and with a hydrogen consumption level of about 1005 scf/bbl. Future research must be directed at optimizing the process to reduce hydrogen consumption and investigating catalyst deactivation patterns.
In this work, thermal oxidation reaction of dimethoxymethane (DMM) at low temperature was investigated with a custom-designed mini closed pressure vessel test (MCPVT) employed. The initial auto-oxidation, thermal decomposition and deep radical oxidation processes were revealed by analyzing the behavior of oxidation temperature (T) and pressure (P) in the process of MCPVT monitoring. Peroxides generated from DMM auto-oxidation were measured by iodimetry and thin-layer chromatography (TLC). The dominant peroxide was separated from oxidation products via column chromatography, with structure characterized by mass spectrometry (MS), ¹H and ¹³C nuclear magnetic resonance (NMR). The thermal decomposition characteristics were determined by differential scanning calorimeter (DSC). Results show that the oxidation was mainly initiated by hydrogen abstraction on the methylenedioxy (O-CH2-O) of DMM when temperature was above about 310 K. The primary product of hydroperoxide, hydroperoxy(methoxymethoxy)methane was found in DMM oxidation for the first time, with a high level. Its exothermic onset temperature (T0) and decomposition heat (QDSC) are 372.87 K and 2431.37 J·g⁻¹, respectively. The thermal decomposition could lead to thermal runaway of oxidation, and further transform into an explosion. In most cases, the release of radical pool after peroxides decomposition would induce a rapidly radical oxidation of DMM through an endothermic reaction at about 410 K.
The variations in softening points, sub-fraction contents and carbon residue of air-blown pitches prepared from air blowing treatment of refined hydrocracking tail oil at the different conditions were monitored to select the suitable oxidative condition to prepare the feedstock of mesophase pitch. Furthermore, molecular structures of as-prepared pitches with different oxidative degree were investigated and the influence of cross-linked structural types on the carbonized performance of air-blown pitches was also discussed in terms of carbonized reactivity and planarity of fused-ring aromatic compounds formed at the initial stage of carbonization. During the early stage of oxidative treatment, the dealkylation, dehydro-aromatization of naphthenic ring and the following formation of methylene-bridged polycyclic aromatic compounds are mainly responsible for promoting the softening point of feedstock to 55 °C of PP23 pitch and 91 °C of PP38 pitch; those structural changes contribute to smoothly generating fused-ring aromatic compounds with good planarity configuration, which significantly improves the mesophase development during subsequent carbonization process: i) mesophase content of the carbonized parent pitch (MP-H) is 46% while for the carbonized oxidized pitches MP-PP23 and MP-PP38 it is 84% and 80% respectively; ii) lamellar orientation degree Og increases from 0.957 (MP-H) to 0.970 (MP-PP23 or MP-PP38) and iii) stacking height Lc increase from 2.07 nm (MP-H) to 2.94 nm for MP-PP23 and 2.98 nm for MP-PP38. With the oxidative degree deepening, the competitive cross-linked reaction featured with the formation of the polycyclic aromatics bridged by oxygen-containing functional groups such as CdbndO and OsbndCsbndO becomes dominant, whose cross-linked structure not only accelerates the carbonized reactivity but also kinetically contributes to conversing into five-member-ring bridged fused-ring aromatic compounds with poor molecular planarity, thereby leading to reducing mesophase content of MP-PP48 (prepared from PP48 pitch) and MP-PP52 (derived from PP52 pitch) to 75% and 54%, accompanying by increasing interlayer space d002 to 0.344 nm and 0.350 nm as well as decreasing the stacking height Lc to 2.56 nm and 1.53 nm, respectively.
The kinetics of the catalytic polymerization of pyrene monomer to pyrene pitch in the presence of aluminum trichloride (AlCl3) were studied at temperatures between 330 and 370 °C in a 1-L batch reactor. Both reactants and reaction products were isolated from the bulk pitch using supercritical extraction (SCE), so that they could be quantified. Concentrations of monomer, dimer, and trimer were measured at discrete reaction times, and each was found to exhibit first-order reaction behavior. Analysis of the oligomerization reaction behavior at differing temperatures enabled the calculation of apparent activation energies and pre-exponential factors for each reaction. Relatively low activation energies that decreased with increasing oligomer size were obtained, suggesting that the reactions are mass-transfer-limited. The microkinetic model that was developed can be used to estimate the reaction time for which the formation of liquid crystalline, mesophase-forming pyrene trimer is maximized. Other groups have developed models for bulk mesophase formation, but this is the first time that reaction kinetics have been measured for individual oligomers in a carbonaceous pitch formed from pure polycyclic aromatic hydrocarbons (PAHs) or mixtures thereof.
Naphthenic-based petroleum pitch was divided into three components, including heptane-soluble (HS), heptane-insoluble/toluene-soluble (HITS) and toluene-insoluble (TI), by sequential extraction with heptane and toluene. Mesophase pitches were prepared by direct condensation of the extracted components and their mixtures. Effects of different components on formation and development of mesophase structures were investigated. Results showed that HITS was the optimal component for preparation of mesophase pitch with large domain structure, low softening point, high carbon residue and ordered microcrystal structure. Interactive effect indexes were cited to monitor the interaction of these components on mesophase development. The component HS with rich alkyl chains accelerated the carbonization via chain breaking at the initial stage of reaction, and HITS with abundant naphthenic structures alleviated the carbonization through hydrogen-transfer reaction during the aggregation process of aromatic molecules, and TI as initial nucleus could trigger the generation of mesophase spheres. However, the mixture of HS and TI possessed a weaker interactive effect on mesophase development compared to the other two mixtures. This was mainly due to the differences in molecular weight distribution and molecular structure between HS and TI. Furthermore, naphthenic structures had a better improving effect than alkyl chains on the generation of high-quality mesophase pitches.
Three-stage thermal reactions were conducted to investigate empirical trends of petroleum-based pitch production and the chemical characteristics of pitch produced by controlling the temperature and pressure operating parameters. The softening point of pitch increased with increasing reaction temperature under atmospheric pressure, whereas the pitch yield increased with decreasing reaction temperature under a pressurized condition. Elemental analysis supported the result of decreased hydrogen content. In addition, thermogravimetric analysis revealed that higher reaction temperatures could induce the polymerization and condensation of polyaromatic hydrocarbons (PAHs) with carbon yield at 900. °C. The molecular weight distribution (MWD) of pitch was analyzed by MALDI-TOF analysis according to the operating parameters during the thermal reaction. The MALDI-TOF spectrum was normalized to observe the variation in the MWD by Anthracene (178. Da) as a pseudo-component. Range 2-3 and range 6-8 in the MWD coincided with the trends of the pitch yield and the softening point, respectively. The results suggested that pitch yield is related to the PAHs with 2-6 aromatic rings and that the softening point is related to highly condensed PAHs with more than 6 aromatic rings. The anisotropy/isotropy of the pitch was analyzed by polarized light microscopy. The pitch produced under the pressurized condition was isotropic. However, the pitch produced under atmospheric pressure exhibited a well-oriented domain texture. We concluded that volatile contents disturb the polymerization and condensation of PAHs because of side reactions involving the aliphatic contents. © 2018 The Korean Society of Industrial and Engineering Chemistry.
Catalytic thermal cracking of the residue is less common, due to high susceptibility of catalysts to deactivation. In this work, cracking of Athabasca vacuum residue in an autoclave at 400 °C was investigated in presence of in situ prepared and commercial alumina nanoparticles as well as drill cuttings. A high liquid yield of ∼90 wt% was obtained. The nanoparticles and drill cuttings (at 30 wt%) contributed to relatively high coke and gas yields. Nevertheless, the catalytic runs had higher quality maltene product. Reducing drill cuttings concentration to 10 wt% gave the highest liquid yield with high quality maltene. Coke inhibition in presence of optimum drill cuttings concentration is attributed to a balance between the rate of coke precursor formation and adsorption onto the cuttings. Subsequently, at a higher temperature, 420 °C, the higher rate of precursor formation led to similar results between the 10 wt% drill cuttings and the control sample. Both toluene insolubles and produced asphaltenes displayed a lower H/C ratio than asphaltenes in the feed, in line with the thermal cracking activity. Thermogravimetry profiles for the produced asphaltenes was attained at higher temperatures than the toluene insolubles, and was initial-mass sensitive. Accordingly, thermogravimetry profiles should not be indefinitely used to identify the different oil fractions.
Pyrolysis fuel oil (PFO) has been reformed to increase the quinoline-insoluble (QI) content by adding carbon black for use as a binder pitch because petroleum-based pitch generally has a low QI content. The prepared pitch was analyzed for its aromaticity (fa), QI content, molecular weight distribution, coking value (CV), softening point (SP), and carbonization yield. The QI content of the reformed pitches increased from 0.4% to 14.1%, whereas the average molecular weight of only quinoline-soluble content decreased by increasing the carbon black content. These results indicate that carbon black adsorbs low-molecular-weight compounds and serves as nuclei for the growth of QI particles. For these reasons, the SP and carbonization yield showed a decreasing tendency, but increased when QI sharply increased.
Co-pyrolysis of lignite with direct coal liquefaction residue (DCLR) is a promising method in the field of clean coal technology. To investigate the influence of DCLR fraction on the physico-chemical structure and combustion reactivity of the co-pyrolysis chars, char samples with different mass fraction of lignite and DCLR were prepared in a quartz tube fixed-bed reactor at 550 °C. Then the physico-chemical structure and combustion properties of co-pyrolysis chars were characterized by Microscope, Scaning electron microscope, Raman Spectrometer, Fourier Transform Infrared Spectrometer, automatic gas sorption analyzer and thermogravimetric analyzer. Results showed that there was interaction of the co-pyrolysis of lignite and DCLR, both on char yield and on char's structure and combustion reactivity. And such interaction became obvious with the increase of DCLR fraction. Compared with lignite char, the addition of DCLR resulted in high aromaticity and ordered structure of co-pyrolysis chars, while the pore structure of chars from co-pyrolysis became less developed. The combustion reactivity of chars decreased with the increase of DCLR fraction. The chars produced during co-pyrolysis were of special structure, which were different from lignite and DCLR chars pyrolyzed individually, and that was the reason for their different combustion behavior and reactivity. With the increase of DCLR dosage, the combustion reactivity of components corresponding to lignite char in co-pyrolysis chars reduced while components to DCLR char were enhanced. In addition, the combustion reactivity of pyrolysis chars showed good correlations with Raman/FTIR parameters.
A cost-effective method for producing an isotropic pitch-based carbon fiber using the 1-methylnaphthalene (MN) soluble fraction of Hyper-coal (HPC) was newly developed. The extent of MN fractionation was controlled by altering the fractionation temperatures. The MN soluble fraction obtained from HPC at 60 °C had excellent spinnability after the removal of volatile matter by thin layer evaporation. Additionally, the fiber that was subsequently derived from the MN soluble fraction had satisfactory mechanical properties with an averaged tensile strength of 1150 MPa after heating to 800 °C for 5 min.
Comparative studies in carbonization behaviors of petroleum pitch (SS70 pitch) and SS70 pitch-additive mixtures were conducted to develop a better understanding of the modified mechanisms of additives. The carbonization of solvent fractions in SS70 pitch was also investigated to reveal the relationships among solvent fractions and the additives. The characterization of resultant solid products by X-ray diffraction and scanning electron microscope analysis showed that samples obtained from co-carbonization of SS70-aromatic oil (P1) and SS70- aromatics enriched fractions of FCC slurry (P2) mixtures respectively had higher stacking layer numbers of mesogens and flatter morphology than those of specimens produced by carbonization of SS70 pitch and SS70-deasphalt oil (P3) mixtures. Adding the additives P1 and P2 into SS70 pitch lowered not only the rates of carbonization but also the aromaticity and size of aromatic molecules in toluene insolubles formed in the early stage of carbonization, both jointly contributing to the formation of well-developed mesophase. The asphaltenes of SS70 pitch were prone to forming the poorly-developed mesophase due to its high reactivity and also interfered with the carbonized performance of maltenes. The additives P1 and P2 played the “dominant partner effect” on mesophase development by providing physical fluidity of the reaction system and some chemical stability for asphaltenes via their dilution effect and H-transfer reactions.
The kinetics of mesophase formation for three petroleum residues of different aromaticities (R1, R2 and R3) have been followed by optical microscopy (anisotropic content ‘A’) and solvent extraction (insolubility in heptane (HI), toluene (TI) and 1-methyl-2-pyrrolidinone (NMPI)). Kinetics of mesophase formation followed as development of HI, TI, NMPI and ‘A’, approximate to apparent first order kinetics. Deviations from Arrhenius plots are observed. These could be caused by: a) depletion of reactants; b) effect of consecutive reactions, c) effect of reversible reactions, d) submicron mesophase in the isotropic phase and e) mesophase solubility. Activation energies for the three feedstocks are in close agreement. Values are between 160 and 270 kJ mol−1 and are a function of the experimental method (extraction with different solvents and optical microscopy). A consecutive reaction model has also been applied for R1 and R2 pyrolyses. Experimental values adjust reasonably to calculated values applying this method. Both R1 and R2 follow this model, with calculated activation energies of 240±20 kJ mol−1.
Dense-gas/supercritical extraction (DGE/SCE) was used for the fractionation of a representative petroleum pitch, M-50, into its oligomeric constituents using toluene as the extractive solvent. A small, pilot-scale column was operated in the semi-continuous/semi-batch mode under a linear positive temperature gradient of 380–330 °C from the top to the bottom of the column and over a pressure range of 15–75 bar. This DGE column was used to produce high-purity monomer, dimer, and trimer fractions of M-50 suitable for use as molecular standards for petroleum pitches and for polycyclic aromatic hydrocarbon (PAH) oligomers. During the experimental runs, matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS) was used to monitor in real time the progress of the separation (and oligomeric purity) by performing rapid analyses of the molecular-weight distributions (MWDs) of the overhead fractions being collected. These real-time analyses provided us with the ability to fine-tune in situ the operating conditions according to the separation desired. The separation of petroleum pitches and other heavy fossil fuels into narrow molecular-weight fractions by semi-continuous DGE has proven to be an invaluable first step in the isolation and structural characterization of the individual species present in these multi-component, poorly defined systems. Furthermore, these molecular standards are also suitable for quantitative analysis.
The anisotropic content of a pitch is one of the most important parameters for characterizing such materials. Polarized light optical microscopy is the technique most commonly employed (ASTM D 4616 standard procedure) to measure this pitch parameter. However, this standard procedure is limited to pitches with mesophase contents only up to 20%. An alternative technique for determining the anisotropic content of a pitch is high-temperature centrifugation, which can be used without limitation for pitches with up to 100% anisotropic content. In this work, the two techniques have been compared; samples of four pitches with mesophase contents lower than 20% have been analyzed by both techniques and the results have been compared. The high-temperature centrifugation technique showed good repeatability, and the results that it yielded matched those obtained from optical microscopy when the anisotropic content of the pitch was higher than around 5%. The centrifugation technique is always faster, simpler, and possibly more accurate than optical microscopy for pitches with mesophase contents higher than 20%.
Carbonization studies of petroleum vacuum distillation residua have been carried out on a laboratory scale with the objective of developing a better understanding of the causes and mechanisms of the incidence of shot coke formation in delayed coking. The rates of carbonization of the residua are found to inversely relate to the extent of mesophase development in the resulting semicoke. The asphaltenic fractions of the petroleum stocks carbonized most rapidly and produced cokes containing small anisotropic regions (fine mosaics), similar to that of commercially produced shot coke and consistent with arrested mesophase development. The addition of aromatic petroleum materials enhanced mesophase development commensurate with their reducing the rate of carbonization. While the microstructural features of shot coke are reproducibly produced in the laboratory, the characteristic spherical morphology is not and appears to be determined by the conditions obtaining in large-scale operation. It is suggested that this morphology may arise from the segregation of regions of rapidly carbonizing constituents from a less reactive matrix.
Incipient coke formation was studied by heating a heavy hydrocarbon residue in isothermal batch reactor at atmospheric pressure of nitrogen at temperatures 320, 357, and 387°C over times from 0 to 7h. The kinetics of coke formation was followed by solvent extraction (insolubility in hexane (HI), toluene (TI)) and as development of HI and TI approximate to apparent first-order kinetics. Results appeared to follow the Wiehe’s phase separation model for coke formation. The density of the pyrolysis residues was found to be independent of condition and it shows a linear relation to TI content.
Thermal pretreatment of coker feeds at sub-carbonization temperatures can lead to considerable development of the optical texture of semi-cokes produced upon carbonization at 723 K. The extent of the development is dependent upon the nature of the residual feed and pretreatment time-temperature history. Under suitable pretreatment conditions, highly reactive feed components can be induced to react and self-stabilize. During subsequent carbonization, the slower processes required for the development of carbonaceous mesophase can then proceed relatively unhindered. These same reactive species are believed to give rise to the formation of shot coke in delayed coking and cause the production of other cokes with poorly developed mesophase.
Individual dimer, trimer, and tetramer constituents that comprise the higher molecular weight (mol wt), mesophase-forming fraction of M-50 petroleum pitch were characterized in terms of polycyclic aromatic hydrocarbon (PAH) backbone structure, the extent of alkylation, and the nature of the bonds connecting the monomeric “building blocks” comprising the oligomeric species. Isolation of individual oligomeric constituents for subsequent analysis and identification was made possible by applying our two-step, sequential fractionation technique of dense-gas/supercritical extraction (DGE) followed by prep-scale gel permeation chromatography (GPC). MALDI and MALDI-post source decay (PSD) mass spectrometry of the narrow mol wt fractions obtained indicate that the major oligomeric constituents are grouped in terms of well-defined molecular weight distributions, with each composed of base monomer PAH backbone structure(s) (e.g., pyrene, triphenylene, benzopyrenes, chrysene, and benz[a]anthracene) possessing various degrees (typically 1–4/monomer unit) of methylation. UV–Vis analysis indicates that the oligomerization process occurs via a condensation reaction, with the loss of four hydrogens as a nonalternant, 5-membered PAH ring forms to join the reacting monomer units. FT-IR spectroscopy indicates that this reaction results in relatively uncondensed structures over the entire range of oligomers investigated. Analogous results to those given above were obtained for the thermal polymerization of an anthracene pitch.
The isotropic phase isolated from a thermally treated coal-tar pitch was studied as a possible precursor for carbon fibres. Extraction with different solvents was performed in order to increase its softening point and so enable higher stabilisation temperatures to be used, with a significant reduction in time. The extraction conditions were selected studying the softening temperatures of the residues, the results of their thermogravimetric analysis and reactivity in air studied by means of differential scanning calorimetry. The residue obtained with a mixture of 40% acetone–60% acetonitrile was found to be the most suitable precursor for the fibres. The carbonised fibres showed a homogeneous surface and diameter, and had tensile properties comparable to other isotropic fibres described in the literature.
Simulated delayed coking characteristics of five petroleum residues from various sources in China and four fractions from one of the residues were investigated by thermogravimetry (TG), and interactions among the fractions were thus revealed. Results showed that properties of these petroleum residues and fractions varied over a broad range. The coke yields from residues were closely related to their carbon residues and those from the fractions varied much. Apparent thermal-cracking activity was in the order saturates>aromatics>resins>asphaltenes, while actual overall cracking intensity appeared in reverse order when measured by the heat effect derived from differential thermal analysis (DTA). The apparent cracking reactions of the fractions and their parent residue could be described in first order kinetics in two coking temperature zones with activation energies being 75–120 kJ mol−1 and 130–210 kJ mol−1. Saturates promoted coke formation from other fractions, while aromatics inhibited coke formation from both resins and asphaltenes; the coke yield from a residue was smaller than that by physical summation based on residue's compositional fractions. It is thus possible to inhibit coke formation and enhance liquid distillate production by delayed coking of certain mixed petroleum residues.
The oxidative stabilization process of mesophase pitch fibers was monitored by thermal analyses at several heating rates. Methylnaphthalene-derived mesophase pitch (mNP) fiber showed five exothermic oxidation peaks, three (A, B, C) and two (D, E) of which were ascribed to the formations of oxygen functional groups and oxidation-decompositions of the groups in the stabilization process, respectively. The latter two peaks appeared always at higher temperature, being accompanied by weight loss. The former three peaks reflected the weight gain, with the maximum weight gain just before the peak D. The lower heating rate showed the larger maximum weight gain by oxygen uptake, which is due to the least participation of the oxidation-decomposition into the oxygen-uptaking steps. Slower heating in stabilization endowed higher tensile strength on the resulting graphitized fiber such as 480 kg/mm2 at a heating rate as slow as 0.1°C/min. The reactivity of the mesophase pitch fibers and the optimal temperature for the stabilization completion can be easily monitored by the thermal analyses. Among the mesophase pitches available, the thermal analyses proved easily that mNP possessed the highest oxidation reactivity and the largest maximum weight gain in the stabilization.
Much recent interest has been directed toward understanding the reactivity characteristics of polynuclear aromatic systems. The major emphasis has pertained to simple radical substitution and oxidation-reduction reactions. Relatively little work has been applied to studies of self-condensation sequences, which are generally accomplished thermally and lead to the formation of complex carbonaceous residues from the typical polynuclear aromatic hydrocarbon. We report herein investigations of the thermal reactivity for eighty-four polynuclear aromatic hydrocarbons. Our approach has employed differential thermal analysis (d.t.a.) to categorize and delineate thermal reactivity. These results emphasize the importance of intermolecular thermal hydrogen transfer for the polynuclear aromatics. Thermal condensation reactivity is found to be dependent on molecular structure and correlates with other reactivity criteria but includes the additional parameter of molecular size.
The effect of cooling and depressurization on the formation and growth of carbonaceous mesophase in petroleum vacuum residue was investigated using a novel stirred hot-stage reactor. This apparatus allowed the in situ observation of mesophase formation at 440 °C and 4–6.5 MPa under both hydrogen and nitrogen atmospheres. The use of a magnetic stirrer enabled experiments with milliliter volumes of liquid and the addition of catalyst. The in situ observations showed that cooling at constant pressure did not change the amount and size of the mesophase domains, except to arrest their growth by stopping the chemical reactions. Depressurization to atmospheric pressure, while maintaining reaction temperature, after the onset of mesophase formation increased the amount of observable mesophase material significantly. Depressurization before the onset of mesophase formation also induced the formation of observable mesophase earlier than if the reactor had been maintained at pressure.
The average chemical structure of asphaltenes present in a vacuum resid feed is related to the morphology of the coke that is produced in a delayed coker (shot coke vs sponge coke). A combination of solid-state 13C NMR, X-ray photoelectron spectroscopy (XPS), and elemental abundance was used to characterize the average chemical structure of several n-heptane asphaltenes from shot-coke- and sponge-coke-producing vacuum resid feeds. The chemical structural properties of the asphaltenes are discussed in relation to the coke morphology produced from the parent resid. The average asphaltene aromatic carbon per cluster size is between 14 and 22 carbon atoms, which corresponds to three-to-five-ring average clusters. When the ratio of aromatic carbon to unreactive (i.e., heterocyclic aromatic) nitrogen and sulfur in asphaltenes is <16, the feed tendency is to produce shot coke. Representative chemical structural models of asphaltenes reveal significant differences (1.5 cal/cm3)1/2 in the calculated solubility parameter of the PNA core structure. Feeds with larger-solubility-parameter asphaltene PNA cores tend to produce a shot coke morphology. The differentiation appears despite much smaller differences (<0.3 cal/cm3)1/2 in the solubility parameter for the full non-thermally treated asphaltene average structure. The quantity of asphaltenes and the asphaltene:concarbon ratio are not reliable predictors of coke morphology produced in delayed cokers. Microcarbon residue (MCR) tests on the vacuum resid feeds were performed, followed by cross-polarized light optical microscopy on the resultant cokes. The micrographs show a small (2−10 μm) domain size/anisotropy in the optically anisotropic coke produced from shot-coke-forming feeds. This is comparable to the mosaic size of commercial shot cokes. In contrast, larger flow domains of more highly anisotropic appearance are characteristic of sponge-coke-forming feeds. The approach provides a method of assessing the morphology of the coke that will be produced from a given feed.
Supercritical fluid (SCF) extraction has been used to fractionate an isotropic petroleum pitch and produce a number of pitch fractions, many of which contain 100% mesophase. Toluene was used as the supercritical solvent, and experiments were carried out in a region of liquid-liquid equilibrium that exists for mixtures of pitch and toluene above the critical pressure of toluene. A central composite design was used to investigate the effects of the operating variables of temperature, solubility parameter and solvent-to-pitch () ratio on the yield and softening point of the produced mesophase pitch fractions. Temperatures, pressures and ratios from 310 to 360 °C, 45 to 155 bar and 2.5 to 4.0, respectively, were investigated. Using the developed statistical models, the effect of operating variables on softening point and mesophase yield can be quantitatively predicted. Chemical analyses of the mesophases by elemental analysis and DRIFT indicate that the extraction process can also be used to alter the chemical composition (e.g. the degree of alkyl substitution) of the produced mesophases. The flexibility of SCF extraction for processing pitches is illustrated by the fact that one can change the operating temperature and pressure and still maintain a constant mesophase yield or softening point.
At temperatures up to about 200 ° the viscosity of pitch, of approximately 100 °C Ring and Ball Softening Point, follows a predictable variation with temperature. Above 200 °C, variations occur depending on the source of the pitch, although a minimum viscosity is always achieved between 320 and 400 °C, followed by a rise. On heat treating at a constant temperature of 400 °C the viscosity rises. Although some preliminary work on two low-temperature pitches indicates a different regime, the work on coke-oven pitches shows a clear relation between viscosity rise and the formation of toluene- and quinoline-insolubles by what appears to be consecutive reactions from tar oils, during which the rheological behaviour becomes increasingly pseudoplastic.