Coke Formation Process Research Articles

Insights into the pyrolytic coking process of RP-3 fuel from ReaxFF molecular dynamics

Active cooling technology serves as one of the promising and effective methods for addressing the thermal protection strategy for hypersonic aircraft. While the coke generated through pyrolysis, dehydrogenation and coking reactions of fuel will reduce the heat transfer efficiency, even clog the heat exchange channel. In this work, three-component RP-3 aviation fuel was adopted as model compounds to investigate the cracking coking reaction mechanism and explore the method of inhibiting coking. It is found that the three-component RP-3 molecules generate an intermediate system dominated by C2 products through bond breaking, reforming and bimolecular reactions (mainly hydrogen abstraction reactions) under supercritical state, in which the ·H radicals, as represented radicals, play a crucial role in the opening of benzene rings. The coking process is analyzed in details furtherly, and detailed soot formation mechanisms are revealed. Additionally, a suitable “core-shell structure” model is proposed to describe the coke formation process. Meanwhile, it is found that the addition of a small number of addition agents can effectively inhibit the formation of coke and thus prolong the residence time of fuel molecules in the heat transfer channel for the coking process mainly consisted of addition reaction. The study attempts to provide a theoretical basis for the high-temperature pyrolytic coking mechanism of endothermic hydrocarbon fuel, as well as the significance to understanding the inhibition of coke formation.

A molecular insight into coke formation process of vacuum residue in thermal cracking reaction

Understanding the coking behaviors has been considered to be really essential for developing better vacuum residue processing technologies. A battery of thermal cracking tests of typical vacuum residue at 410 °C with various reaction time were performed to evaluate the coke formation process. The total yields of ideal components including naphtha, atmospheric gas oil (AGO) and vacuum gas oil (VGO) of thermal cracking reactions increased from 10.89% to 40.81%, and the conversion ratios increased from 8.05% to 43.33% with increasing the reaction time from 10 to 70 min. The asphaltene content increased from 12.14% to a maximum of 22.39% and then decreased, and this maximum of asphaltene content occurred at the end of the coking induction period. The asphaltenes during the coking induction period, at the end and after coking induction period of those tested thermal cracking reactions were characterized to disclose the structure changing rules for coke formation process, and the coke formation pathways were discussed to reveal the coke formation process at molecular level.

Characteristic analysis and kinetics description for biochar-based catalysts coking during the biomass catalytic pyrolysis

Coke formation is one of the major obstacles in the development of catalytic pyrolysis of biomass. The experiments were carried out on poplar wood chips using the biochar-based catalysts (poplar biochar: ZSM-5 = 1:2) to study the coke formation characteristics of biochar-based catalysts and to develop the kinetic models of coke formation. It is shown that the high temperature and long reaction time promoted the conversion of carbon from amorphous coke to graphitized coke. The higher the degree of graphitization of the coke, the higher the combustion temperature and the richer the cyclized structure. Activated coke and neutral coke were gradually converted to inert coke per unit mass of catalyst. Based on the experimental data, combined AIC evaluation system, it is found that the MMCGM (monolayer-multilayer coke growth model) can only describe the coke formation at low temperature well. Therefore, the kinetic model of carbon particle stacking model was established. This model can well describe the coke formation process on biochar-based catalysts. This study is expected to provide a guide for understanding the characteristics of catalyst coke formation and optimizing pyrolysis conditions and reactors.

Теоретичний аналіз процесів формування електро-провідності коксу.

THEORETICAL ANALYSIS OF THE PROCESSES OF COKE ELECTRICAL CONDUCTIVITY FORMATION © I.V. Shulga, Ph.D. in Technical Sciences (State Enterprise “Ukrainian State Research Institute for Carbochemistry (UKHIN)”, 61023, 7 Vesnina str., Kharkiv, Ukraine), V.V. Vladimirenko (National Technical University "Kharkiv Polytechnic Institute", 61002, 2, Kyrpychova str., Kharkiv, Ukraine) A theoretical analysis of the processes of coke conductivity formation based on modern ideas about the molecular structure of coal is carried out. It is shown that, according to the zone theory of solids, the conductive properties are determined by the energy of electrons and their ability to perform work against the forces of binding with the nuclei of atoms. It is shown that coal is a dielectric due to the presence in the side chains of the macromolecule of a large number of σ-bonds formed by carbon in the sp3-hybridisation state, characterized by a significant width of the forbidden energy band (up to 6 eV), and therefore the electrons of these bonds cannot practically enter the conduction zone and become carriers of electric current. In industrial coking, macromolecules are almost completely stripped of their side chains as a result of deep cracking. In this process, coke acquires semiconductor properties. The carriers of electrical conductivity in coke are: ordered carbon of graphite-like blocks, whose electrons provide their own conductivity; coke minerals – p-elements with a lower valence than carbon (primarily aluminum) – impurity (so-called "hole") conductivity. The factors affecting the resistivity and electrical conductivity of coke, as well as for any other solid, are determined by the general zone theory of the physical structure of solids. A theoretical analysis has been carried out, according to which both intrinsic and impurity electrical conductivity of semiconductors increases rapidly with temperature, varying according to an exponential law. At the final stages of the coke formation process, this dependence can be accepted as a working hypothesis that requires experimental verification by pilot coking. Keywords: hard coal, coke, electrical resistivity, temperature dependence, solid structure, carbon, impurities, zone theory, coke readiness. Corresponding author I.V. Shulga, e-mail: ko@ukhin.org.ua

Unsteady state simulation of gasoline fraction pyrolysis

Pyrolysis of various hydrocarbon feedstock is one of the key processes for production of monomers for polymer industry. During this process mixture of hydrocarbons is heated at high temperatures and in the absence of oxygen in furnace coil. However, coke settles on the inner walls of the coil due to side reactions of polymerization and consecutive polycondensation. This phenomenon decreases efficiency of process, thus, making it unstable. Therefore, existing stationary models might not be suitable for simulation of the process as they do not usually take into account influence of coke formations on kinetic, hydrodynamic or thermal aspects of pyrolysis. In this paper, development of unsteady-state model is presented as incremental improvement towards the solution of process optimization and forecasting of its performance. Thus, the model provides ability to understand deeper the influence of operating parameters on the coke formation process. The model allows to simulate coke deposition along the coil through intergeneration cycle, considering composition of provided feedstock. Dependencies of target products yields and coke layer growth rate on such operating parameters as inlet pressure, temperature and feedstock mass flow rate were determined. The inlet pressure varied from 0.3 to 0.75 MPa, the temperature varied from 780 to 850 °C and the flow rate – from 3600 to 5000 kg/h.

Modification of the Acidic and Textural Properties of HY Zeolite by AHFS Treatment and Its Coke Formation Performance in the Catalytic Cracking Reaction of N-Butene

Coke formation on n-butene cracking catalyst is the main reason for the reducing of its lifetime. To study the effects of acidity and textural properties on the coke formation process, a series of HY zeolite-type catalysts were prepared by ammonium hexafluorosilicate treatment (AHFS). NH3-TPD and Py-IR-TPD were used to systematically study the change law of zeolite acidity. It was found that with the increase of AHFS concentration, the acid density decreased, whereas the ratio of Brønsted acid to Lewis acid first increased and then decreased. Meanwhile, the percentage of Brønsted acid inside the supper cages increased and the strength of Brønsted acid increased with the degree of dealumination. Combined with in situ IR study on coke formation, the relationship between coking and acid site was revealed. It was found that the rate of coke formation on zeolites was affected by acid density, which is the rate of coke formation decreased with the decline of acid density. When the acid density remains at the same level, it was the acid strength that determined the coke formation rate—the stronger the acid strength, the faster the coke formation rate.

Features of the organic-mineral intumescent paints structure formation for wooden constructions fire protection

The paper presents the results describing the features of the organic-mineral paints structure formation of the intumescent type for wooden structures fire protection. On the research basis a diagram of the coke formation process has been developed. It is shown that the introduction of titanium oxide and aluminum hydroxide in an amount of 5% promotes the ordering of the foam layer structure, an increase in its thermal stability and the achievement of the highest value of the swelling coefficient – up to 47, which is 1.7 times higher than the values of the swelling coefficient of the base composition. It was found that at 700 °C the formation of refractory compounds of the type TiP2O7, Mg3(PO4)2, AlPO4 occurs, which prevent the burnout of the formed coke foam and increase its strength and heat resistance. On the basis of the main provisions of fuzzy mathematics and data from physicochemical studies, a model of the mechanism of structure and coke formation and control of this process is proposed.

Mathematical modeling and optimization of semi-regenerative catalytic reforming of naphtha

Catalytic naphtha reforming is extensively applied in petroleum refineries and petrochemical industries to convert low-octane naphtha into high-octane gasoline. Besides, this process is an important source of hydrogen and aromatics obtained as side products. The bifunctional Pt-catalysts for reforming are deactivated by coke formation during an industrial operation. This results to a reduction in the yield and octane number. In this paper modeling and optimization of a semi-egenerative catalytic reforming of naphtha is carried out considering catalyst deactivation and a complex multicomponent composition of a hydrocarbon mixture. The mathematical model of semi-egenerative catalytic reforming considering coke formation process was proposed. The operating parameters (yield, octane number, activity) for different catalysts were predicted and optimized. It was found that a decrease in the pressure range from 1.5 to 1.2 MPa at the temperature 478–481 °C and feedstock space velocity equal to 1.4–1 h induces an increase in the yield for 1–2 wt.% due to an increase in the aromatization reactions rate and a decrease in the hydrocracking reactions rate depending on the feedstock composition and catalyst type. It is shown that the decrease in pressure is limited by the requirements for the catalyst stability due to the increase in the coke formation rate. The criterion of optimality is the yield, expressed in octanes per tons.

Intensification of the dehydrogenation process of different hydrocarbons in a catalytic membrane reactor

The process of dehydrogenation of hydrocarbons has attracted a great interest due to the growing demand for monomers for main organic synthesis. Among them, ethylene, propylene, and styrene occupy the leading positions. The efficiency of the dehydrogenation process was numerously shown to be significantly improved by the use of catalytic membrane reactors. Hydrogen elimination from the reaction zone shifts the equilibrium towards products, thus increasing their yields. At the same time, the amount of parallel by-side reactions and, consequently, by-side products can be varied substantially. In the present work, the processes of ethane, propane and ethylbenzene dehydrogenation in catalytic membrane reactors are theoretically compared in terms of efficiency and productivity. A two-dimensional non-isothermal stationary mathematical model of the catalytic membrane reactor was applied to perform the study. The reactor comprises inner hydrogen-permeable tubes with a loaded dehydrogenation catalyst and outer tube. The shell compartment is filled with another catalyst for oxidation of flux hydrogen. It is evident that the heavier is a hydrocarbon, the higher amount of by-products is formed. Since the contribution of the coke formation process is being increased along with temperature, diminishing of the reactor temperature by oxidation of flux hydrogen allows enhancing the target products’ yield.

Molecular dynamics simulations of the coke formation progress on the nickel-based anode of solid oxide fuel cells

Coke formation on anode of solid oxide fuel cells (SOFC) can lead to SOFC performance degradation or even failure. This paper investigates transfer mechanism of carbon atoms in coke formation process on nickel-based anode using the reactive force field molecular dynamics simulation (ReaxFF-MD). The simulation results show that the transfer processes of carbon atoms in methane reforming are composed of three stages, and the alkyl makes formation of coke easier on the surface of Ni(100). The driving force of the carbon atoms diffusion into Ni(100) changes from the carbon concentration difference to the chemical potential difference, which leads to the generation of a new phase of carbon and the structural damage of the SOFC anode. The structural changes of Ni(100) during the carbon atom transport and precipitation are simulated and analyzed to reveal the chemical reaction mechanisms of coke formation, and provide theoretical foundation for optimal design in materials and operating parameters of SOFC anode.

Effects of support modifiers on the catalytic performance of Ni/Al2O3 catalyst in CO2 reforming of methane

The effect of alkaline earth metals (MgO, CaO and BaO) on activity, stability and coke formation of Ni/Al2O3 catalysts in CO2 reforming of methane (CRM) was investigated. These catalysts were prepared with impregnation method and characterized by XRD, BET, TPR, TPO and SEM techniques and tested in CO2 reforming of methane. The results showed that addition of these support modifiers to the Ni/Al2O3 catalysts decreased the surface area of catalysts (SBET) but improved the activity and stability and reduced the coke formation due to increasing the basicity of the supports. In addition, among these modifiers, MgO had the greatest impact on catalytic activity and coke formation process. Meanwhile, Ni/MgO–Al2O3 catalyst with various Mg contents were prepared, characterized with BET, TPR, TPO and SEM techniques and studied in CRM. It was found that just a moderate amount of Mg could improve catalytic stability and decreased coke formation in CRM.

Core–shell H-ZSM-5/silicalite-1 composites: Brønsted acidity and catalyst deactivation at the individual particle level

A combination of in situ UV-Vis and confocal fluorescence micro-spectroscopy is applied to investigate the influence of an external silicalite-1 shell on the Brønsted acidity and coke formation process of individual H-ZSM-5 zeolite crystals. Three probe reactions were used: oligomerization of styrene, methanol-to-olefin (MTO) conversion and aromatization of light naphtha (LNA) derivatives. Oligomerization of styrene leads to the formation of optically active carbocationic oligomers. Different styrene substitutions indicate the conversion ability of the catalyst acid core, a preferred alignment of the oligomers within the straight zeolite channels and a Brønsted acidity gradient throughout the zeolite crystal. Both the MTO conversion and the LNA process lead to limited carbonaceous deposition within the external silicalite-1 layer. This outer shell furthermore prevents the growth of extended coke species at the zeolite external surface. During MTO, the formation of carbonaceous compounds initiates at the center of the H-ZSM-5 zeolite core and expands towards the zeolite exterior. This coke build-up starts with a 420 nm UV-Vis absorption band, assigned to methyl-substituted aromatic carbocations, and a second band around 550 nm, which is indicative of their growth towards larger conjugated systems. Aromatization of linear and branched C5 paraffins causes negligible darkening of the zeolite crystals though it forms fluorescent coke deposits and their precursors within the H-ZSM-5 catalyst. Olefin homologues on the contrary cause pronounced darkening of the zeolite composite. Methyl-branching of these reactants slows down the coke formation rate and produces carbonaceous species that are more restricted in their molecular size.

Gas flow behavior and residence time distribution in a FCC disengager vessel with different coupling configurations between two-stage separators

To deeply understand the effect of coupled configuration between two-stage separators on the gas flow behavior in fluid catalytic cracking disengager space, the gas flow field and residence time distribution in a bench-scale disengager is numerically studied on the platform of the commercial computational fluid dynamics software package, Fluent 6.1. Two conventionally used coupling configurations in refineries and a newly-designed configuration are investigated. The Reynolds stress model is applied to simulate the gas flow field and the results agree well with the experimental values measured by a smart five-hole probe. Coupled with the Reynolds stress model, a scalar transport equation is used to obtain the gas residence time distribution. It is shown that the coupling configuration between the primary and secondary cyclones has an apparent effect on the gas flow behavior. The newly-designed configuration is better to quickly discharge the gas into the secondary cyclone inlet with a maximum residence time of 0.38 s, compared to 7.72 s and 2.79 s for the two conventional configurations. The simulation results indicated that the appropriate modification of the coupling configuration can help to improve the gas flow. The knowledge of the gas flow field also help to understand the coke formation process in the disengager space.

A DFT‐Based Investigation of Hydrogen Abstraction Reactions from Methylated Polycyclic Aromatic Hydrocarbons

The growth of polycyclic aromatic hydrocarbons (PAHs) is in many areas of combustion and pyrolysis of hydrocarbons an inconvenient side effect that warrants an extensive investigation of the underlying reaction mechanism, which is known to be a cascade of radical reactions. Herein, the focus lies on one of the key reaction classes within the coke formation process: hydrogen abstraction reactions induced by a methyl radical from methylated benzenoid species. It has been shown previously that hydrogen abstractions determine the global PAH formation rate. In particular, the influence of the polyaromatic environment on the thermodynamic and kinetic properties is the subject of a thorough exploration. Reaction enthalpies at 298 K, reaction barriers at 0 K, rate constants, and kinetic parameters (within the temperature interval 700-1100 K) are calculated by using B3LYP/6-31+G(d,p) geometries and BMK/6-311+G(3df,2p) single-point energies. This level of theory has been validated with available experimental data for the abstraction at toluene. The enhanced stability of the product benzylic radicals and its influence on the reaction enthalpies is highlighted. Corrections for tunneling effects and hindered (or free) rotations of the methyl group are taken into account. The largest spreading in thermochemical and kinetic data is observed in the series of linear acenes, and a normal reactivity-enthalpy relationship is obtained. The abstraction of a methyl hydrogen atom at one of the center rings of large methylated acenes is largely preferred. Geometrical and electronic aspects lie at the basis of this striking feature. Comparison with hydrogen abstractions leading to arylic radicals is also made.

CFD Flow and Heat Transfer Simulation for Empty and Packed Fixed Bed Reactor in Catalytic Cracking of Naphtha

This work aims to test the application of computational fluid dynamics (CFD) modeling to fixed bed catalytic cracking reactors. Studies of CFD with a fixed bed design commonly use a regular packing with N=2 to define bed geometry. CFD allows us to obtain a more accurate view of the fluid flow and heat transfer mechanisms present in fixed bed equipment. Naphtha was used as feedstock and the reactor length was 80cm. It is divided in three sections that catalyst bed packed in the middle section of the reactor. The reaction scheme was involved one primary reaction and 24 secondary reactions. Because of high CPU times in these simulations, parallel processing have been used. In this study the coke formation process in fixed bed and empty tube reactor was simulated and coke in these reactors are compared. In addition, the effect of steam ratio and feed flow rate on coke formation was investigated. Keywords—Coke Formation, CFD Simulation, Fixed Bed, Catalyitic Cracking.

Synthetic fuel for imitation of municipal solid waste in experimental studies of waste incineration

Supercritical fluid extraction, narrow-cut characterization of bitumen pitch, hydrocracker and once-through coker vacuum resids revealed the unexpected process chemistry of bitumen hydrocracking. Hydrocracking appears to prolong the coke formation process of the end-cut material of bitumen, by converting it to coke precursors. The conversion (or removal) mechanism for most key species (microcarbon residue (MCR), metals, N) in hydrocracking is by partitioning, which is similar to coking. Sulfur species that convert in hydrocracking are from the front-end of bitumen pitch; there is only a small sulfur reduction in the end-cut.Bitumen pitch and resid products have similar MCR distribution which is dependent on the “depth” of resid, not the conversion processes. Coke yield did not correlate with the MCR content of narrow-cut feed; coke yield was insignificant at the front-fractions and high at the end-cut. The sub-fractions of bitumen derived resid products have lower H/C ratios than those of bitumen pitch, but have the same MCR content. This can be explained by the pendant-core model, in which bitumen pitch and resid products have the same amount of aromatic cores (coke precursors). The inverse correlation of key species with H/C ratio is process dependent. The reaction products of end-cut (mainly asphaltenes) remained in resid product slate (524°C+ materials) in once-through coking and hydrocracking operations. The implications for the current commercial bitumen upgrading flowsheet are also discussed.

Improvement of technology for preparation of coal suspensions in water

Supercritical fluid extraction, narrow-cut characterization of bitumen pitch, hydrocracker and once-through coker vacuum resids revealed the unexpected process chemistry of bitumen hydrocracking. Hydrocracking appears to prolong the coke formation process of the end-cut material of bitumen, by converting it to coke precursors. The conversion (or removal) mechanism for most key species (microcarbon residue (MCR), metals, N) in hydrocracking is by partitioning, which is similar to coking. Sulfur species that convert in hydrocracking are from the front-end of bitumen pitch; there is only a small sulfur reduction in the end-cut.Bitumen pitch and resid products have similar MCR distribution which is dependent on the “depth” of resid, not the conversion processes. Coke yield did not correlate with the MCR content of narrow-cut feed; coke yield was insignificant at the front-fractions and high at the end-cut. The sub-fractions of bitumen derived resid products have lower H/C ratios than those of bitumen pitch, but have the same MCR content. This can be explained by the pendant-core model, in which bitumen pitch and resid products have the same amount of aromatic cores (coke precursors). The inverse correlation of key species with H/C ratio is process dependent. The reaction products of end-cut (mainly asphaltenes) remained in resid product slate (524°C+ materials) in once-through coking and hydrocracking operations. The implications for the current commercial bitumen upgrading flowsheet are also discussed.

Narrow-cut characterization reveals resid process chemistry

Supercritical fluid extraction, narrow-cut characterization of bitumen pitch, hydrocracker and once-through coker vacuum resids revealed the unexpected process chemistry of bitumen hydrocracking. Hydrocracking appears to prolong the coke formation process of the end-cut material of bitumen, by converting it to coke precursors. The conversion (or removal) mechanism for most key species (microcarbon residue (MCR), metals, N) in hydrocracking is by partitioning, which is similar to coking. Sulfur species that convert in hydrocracking are from the front-end of bitumen pitch; there is only a small sulfur reduction in the end-cut. Bitumen pitch and resid products have similar MCR distribution which is dependent on the “depth” of resid, not the conversion processes. Coke yield did not correlate with the MCR content of narrow-cut feed; coke yield was insignificant at the front-fractions and high at the end-cut. The sub-fractions of bitumen derived resid products have lower H/C ratios than those of bitumen pitch, but have the same MCR content. This can be explained by the pendant-core model, in which bitumen pitch and resid products have the same amount of aromatic cores (coke precursors). The inverse correlation of key species with H/C ratio is process dependent. The reaction products of end-cut (mainly asphaltenes) remained in resid product slate (524°C+ materials) in once-through coking and hydrocracking operations. The implications for the current commercial bitumen upgrading flowsheet are also discussed.

Narrow pore distribution formed in steam and CO 2 gasified cokes of different origin

A new high resolution sorption (HRS) technique has been employed to characterize steam and/or CO 2 gasified petroleum cokes, coal chars, and carbons derived from peat and bone char. A lignite gasified by mild oxidation in steam was also studied. We have found that all of these coke samples show a very narrow pore feature centered at 3.9 nm diameter. High pressure mercury porosimetry of a petroleum coke gasified in a fluidized bed at temperatures >900°C confirm the HRS analysis of 3.9 nm pores formed on gasification. We propose that a primary structural unit, analogous to a micelle or a spherulitic polymer micro domain, forms during the coke-formation process, and this unit is 4 nm in diameter. We believe that gasification attacks these structural units generating a very narrow pore structure within the coke. In other work, gasification of carbon blacks by mild oxidation has been shown to attack the center of the particle and proceed to the exterior of the spherulitic structure. Such a process would account for the generation of narrow pores within a coke following gasification in steam and/or CO 2.

Some features of the structure of metallurgical cokes and their effects on strength

Five theoretical models relating strength to total pore volume are fitted to the experimental data for seven cokes and the goodness of fit is compared. Estimates of zero-porosity strength are obtained by extrapolation and are compared with alternative estimates obtained by fracturing small ( approximately 0.1 mm) coke particles containing only 5% of pores. Predictions of the surface area of the microfissures formed when some constituents of coke contract around non-contracting particle constituents during the coke formation process are compared with experimental results obtained by microscopy.