Coking Behavior Research Articles

The Coking of a Solid Catalyst Rationalized with Combined Raman and Fluorescence Lifetime Microscopy.

The formation of carbon deposits is a major deactivation pathway for solid catalysts. Studying coking on industrially relevant catalysts is, however, often challenging due to the sample heterogeneity. That is especially true for zeolite-containing catalysts where fluorescence often hampers their characterization with Raman spectroscopy. We turned this disadvantage into an advantage and combined Raman and fluorescence (lifetime) microscopy to study the coking behavior of an equilibrium catalyst material used for fluid catalytic cracking of hydrocarbons. The results presented illustrate that this approach can yield new insights in the physicochemical processes occurring within zeolite-containing catalyst particles during their coking process. Ex situ analyses of single catalyst particles revealed considerable intra-sample heterogeneities. The sample-averaged Raman spectra showed a higher degree of graphitization when the sample was exposed to more hexane, while the sample-averaged fluorescence lifetime showed no significant trend. Simultaneous in situ Raman and fluorescence (lifetime) microscopy, used to follow the coking and the regeneration of single particles, gave more insights in the changing fluorescence dynamics. During the coking, the rise and decline of the average fluorescence lifetime suggested the prolonged presence of smaller coke species that are quenched more and more by adjacent larger polyaromatics acting as Förster-resonance-energy-transfer acceptors.

A comprehensive investigation of thermal coke formation during rapid non-catalytic pyrolysis of rubber seed oil

Due to coking deposits, catalyst deactivation severely affects the upgrading of triglyceride biomass. Understanding the chemical nature and development of coke species is essential for mitigating the degree of coking and for the regeneration process of the catalysts. In this study, SEM, XRD, FT-IR, Raman and XPS are employed to gain insights into the coking behavior of RSO during the rapid pyrolysis process. The results show that pyrolysis of RSO produces high quality syngas with 98.72 % H2 and CO. The unstable O-containing functional groups in the cokes continue to be lost, which indirectly promotes the condensation reaction of aromatic hydrocarbons. The hydroxyl group is the main functional group that affects the reactivity of cokes, and it is one of its more active groups. Although the crystal size of the cokes is growing, its degree of graphitization is decreasing with the temperature increases. The size of the aromatic ring system in the cokes gradually increases with increasing temperature. The cross-linking structure in the coke is constantly being destroyed. Resulting in some O-containing functional groups entering the cokes. Based on this work, the coke evolutionary route of rapid pyrolysis of RSO is proposed.

Coking and deactivation behavior study of Ce-modified Cs-Nb/Al2O3 in aldol condensation of methyl propionate with formaldehyde

Coke deposition during the aldol condensation of methyl propionate (MP) and formaldehyde (FA) to produce methyl methacrylate (MMA) is a critical factor limiting the long-term stability of catalysts. In this work, Ce-modified Cs-Nb/Al2O3 catalysts with superior catalytic activity and robust coke resistance were achieved during the reaction by enhancing the oxygen vacancy (OV) ratio. With optimal Ce loading of 1 %, the MP conversion of 35.4 % and MMA selectivity of 90.3 % can be achieved. Meanwhile, the coke content of the modified catalyst significantly reduced from the original 10.49 wt% to 5.44 wt%. It was discovered that higher water content in the reaction feedstock promotes the hydrolysis reaction, resulting in a decrease in MMA selectivity. Furthermore, the presence of water can promote the conversion of Lewis acid sites to Brønsted acid sites, while an excess of Brønsted acid sites can accelerate the coke formation. Various characterization techniques confirmed that the coke species on the used catalysts consisted basically of polycyclic aromatic hydrocarbons and aliphatic compounds. Ultimately, the kinetics of coke deposition on Cs-Nb/Al2O3 and Cs-Nb-1Ce/Al2O3 were established using the modified Voorhies equation, offering a reliable method to predict coke accumulation under specific conditions. The higher apparent activation energy of coke formation observed on Cs-Nb-1Ce/Al2O3 is responsible for its robust coke resistance.

Coke behavior with H2O in a hydrogen-enriched blast furnace: A review

Coke behavior with H2O in a hydrogen-enriched blast furnace: A review

Numerical simulation on coking process of coke oven with external flue gas recirculation

The coking behaviour and NOx emission characteristics of the coke oven are of great significance for coke production. Based on the realisation k-epsilon turbulence model, the P-1 radiation model, the species transport model with eddy dissipation, and the thermal NOx model. A comprehensive model for traditional regenerative coke oven was established with reversing mode and external flue gas recirculation model considering temperature-dependent thermophysical properties. The convective term of model equations was the discretised using the second-order upwind scheme, while the others were applied by the central difference scheme. The coking behaviour of the coke oven with external flue gas recirculation was numerically solved through the SIMPLE algorithm, and the effects of reversing period and external flue gas recirculation ratio on the coking behaviour were explored. When increasing the reversing period, the mean temperature of the vertical flue increases, and the coking time decreases, but the temperature uniformity of the carbonisation chamber becomes worse. As the external flue gas recirculation ratio increases, the average temperature of the vertical flue decreases, and the NOx concentration at the outlet decreases, yet the coking time increases. These results would provide useful information for guiding the improvement of coke oven operation and reducing NOx emissions.

Kinetic implications of (hierarchical) zeolite fouling during liquid-phase aromatics upgrading

The kinetics of fouling in large pore zeolites (BEA, MOR), including those with hierarchical pore systems, were probed during the Brønsted acid catalyzed reaction of benzyl alcohol (BA) with 1,3,5-trimethylbenzene (TMB) by varying reactant driving force (i.e., [TMB]0/[BA]0; 11–119). In BEA, initial deactivation rate constants (kD,0) decreased exponentially with [TMB]0/[BA]0, highlighting the significance of oxygenates as deactivation precursors. Further, seeding pores with oxygenates completely suppressed measured rates in parent MOR and BEA, while seeding with TMB had no effect. Comparisons of mass accumulations of different organics (low molecular weight reaction species and coke) as functions of ln([TMB]0/[BA]0) revealed that coke (derived from oxygenate-seeded polyalkylation of TMB) disproportionately controlled deactivation rates by damping apparent (diffusion-controlled) and intrinsic (diffusion-corrected) rate constants through proton losses and/or altered molecule confinement within shrinking pores. These kinetic consequences were delayed upon introduction of mesopores, demonstrating how zeolite porosity impacts coke proliferation and behavior in liquid-phase reactions.

Carbon structural evolution and interfacial reaction mechanism investigation of the green anode in kneading and baking via ReaxFF-MD and force-biased Monte Carlo

ABSTRACT Baking is the most costly and critical process in anode preparation, and the study from the microscopic scale is conducive to the optimisation of anode baking process. In this research, a hybrid simulation method of reactive force field molecular dynamics (ReaxFF-MD) and force-biased Monte Carlo (fbMC) was proposed to understand the interaction mechanism between coal tar pitch (CTP) and calcined petroleum coke (CPC) during kneading and baking processes, and to explore the optimal ratio of CTP from the molecular scale. Results showed that the CTP content of 14 wt.% was beneficial for anode production. The mechanism of anode coking behaviour at high temperatures was also analyzed. The carbonisation of CTP during high-temperature baking broke the carbon rings and formed straight chains, exposing active carbon atoms that can bond with the remaining layers. As the high-temperature simulation progresses, the carbon chains at the bonding sites became increasingly cyclized and aromatised, which made the layered structure of the baked block similar to that of graphite. The generation of various volatile hydrocarbons during the baking process was observed, namely CH4, C2H4, C2H2, C2H6, and C3H6. This work provides theoretical basis for process optimisation.

The structure of deposited coke on hydrocracking and reforming catalysts: Coke deactivation and kinetics

AbstractThis comprehensive study investigates the structural deposition of coke, particularly its association with sulphides, in spent hydrocracking and reforming catalysts, leveraging advanced kinetic modelling and characterization techniques. Its objective is to elucidate coke removal kinetics, offering insights into optimizing catalyst regeneration. Utilizing thermogravimetric analysis, spectroscopic methods, and modelling, the research outlines the characteristics and kinetic behaviour of coke during regeneration. The novelty aspect of this study is the use of a reaction‐based model for accurate kinetic analysis and identifying optimal conditions for coke removal. The study recognized both hydrogen‐deficient and hydrogen‐rich amorphous coke, alongside graphitic coke spent catalysts. Thermogravimetric analysis revealed that coke constitutes ~23.8% and 4.6% of the total weight in hydrocracking and reforming catalysts, respectively. The traditional model‐free method for deactivation was found inadequate in predicting coke in the spent reforming catalyst, likely due to the presence of low‐temperature hydrocarbons of soft coke within the catalyst pores. In contrast, a reaction model‐based deactivation approach yielded more consistent evaluations for soft and hard coke of spent catalyst. For the spent hydrocracking catalyst, the estimated activation energies for coke decomposition were 41.56 and 128.97 kJ/mol for soft and hard coke, respectively, during regeneration. Although the coke in the spent reforming catalyst displayed similar temperature trends for decomposition, the average activation energies were 52.61 and 128.62 kJ/mol for soft and hard coke, respectively. The theoretical results from the multi‐reaction model for coke decomposition are consistent with the experimental results for both catalysts.

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.

Unraveling the Coking and Deactivation Behavior of Al–Cs/SiO2 Catalyst in the Aldol Condensation of Methyl Propionate with Formaldehyde

Unraveling the Coking and Deactivation Behavior of Al–Cs/SiO₂ Catalyst in the Aldol Condensation of Methyl Propionate with Formaldehyde

Steam reforming of diethyl ether, ethanol and n-butanol with varied functionality and carbon chain impacts evolution of reaction intermediates and coking behaviors

Molecular structures of organics play key roles in determining the tendency of coking and property of coke through governing reaction intermediates formed. Diethyl ether (DEE) can be produced from etherification of ethanol and has same carbon number with n-butanol (butanol), while they may show distinct coking behaviors in steam reforming (SR). Herein, SR of DEE, ethanol and butanol over Ni/SBA-15 catalyst were performed, aiming to probe impacts of ether functionality and length of carbon chain of the alcohols on coke formation. The results indicated that cracking of DEE formed CxHy* species and unsaturated ketone intermediates, serving as precursors of coke. Dissociative adsorption of butanol formed abundant CxHy* species even from 100 °C, due to its longer carbon chain and lower oxygen content. In comparison, abundant oxygen-containing intermediates were generated from degradation of ethanol and reactions with steam. Abundant CxHy* species from DEE and butanol prompted formation of more coke than that from ethanol (60.6 % of coke in the catalyst for DEE, 66.8 % for butanol and 32.9 % for ethanol). Moreover, the coke from SR of ethanol was thermally less stable than that from DEE or butanol, due to the involvement of more oxygen-containing intermediates, which resulted in more aliphatic coke of amorphous form in ethanol reforming. Additionally, the coke from DEE reforming was more defective with carbon nanotube form of thick walls and little internal cavity. The coke produced from butanol reforming was more graphitic and mainly in carbon nanotube form of smooth surface. Varied abundance of CxHy* and ketone species in reforming of DEE and butanol created such a difference.

A novel ray method for the coking of n-decane in circular tubes with flexible cross-section area

Coking of endothermic hydrocarbon fuels significantly deteriorates the efficiency of regenerative cooling for hypersonic airbreathing vehicles. A novel ray method based on dynamic mesh techniques is proposed to describe the coking behavior in a circular tube with a flexible cross-section area. This method can be used to identify metal and deposit coke efficiently, which promotes the applicability of the dynamic mesh method in flexible structures. The cooling performance of n-decane before and after coking is numerically simulated and compared. The deposition brings higher thermal risks by elevating the solid temperature. However, the deposition reduces the channel’s cross-sectional area and increases the flow speed, leading to decreased flow residence time. Because the suppressing effect from lower flow residence time outweighs the promotion effect from increased temperature, the n-decane conversion decreased after coking.

Elucidating metal composition–coking relationships and coke formation pathways during gas-phase oxydehydration of glycerol over clay mineral-supported Mo-V-O catalysts

Catalyst deactivation by carbon deposition (coking) remains a major obstacle in many important industrial processes, such as catalytic gas-phase oxydehydration of glycerol. Understanding the chemical nature and development of coke species during the time-on-stream (TOS) of the catalytic solids is therefore crucial for mitigating the extent of coking and in the design of regeneration process of the coked catalysts. In this study, multiple analytical techniques, including SEM, XRD, FTIR, Raman, GC-MS, XPS, and 13C solid-state MAS NMR were employed to gain insights into the coking behavior of acid-activated montmorillonite (HMMT) supported Mo-V-O catalysts with modulated metal ratios, along with soluble and insoluble coke compositions and their evolution over time. Insoluble coke was comprised of polycyclic aromatic compounds with 5 or more fused-benzene rings containing different types of oxygen-bonded groups, such as −OCH3, −COOH, −CHO, and −OH. The major chemical constituents of soluble coke were mononuclear aromatic derivatives (i.e., xylenes, 2,4-di-tert-butylphenol, and 1-hydroxycyclohexyl phenyl ketone) and paraffinic hydrocarbons with 12–18 carbon atoms. The latter coke species is thought to be formed via the deoxygenation of linear oligoglycerols. The insights presented in this study may aid in understanding the carbon deposition process during gas-phase oxydehydration of glycerol for improved design of supported Mo-V-O catalyst materials with high coking resistance.

Key features of nano NixMg1-xAl2O4 spinel for biogas bi-reforming: CO2 activation and coke elimination

Using green and sustainable biogas as raw material to produce syngas (H2/CO≈2) suitable for direct synthesis of green methanol has attracted wide attention, but developing efficient catalysts with high activity/stability remains a big challenge. A series of MgAl2O4 spinel catalysts (denoted as NixMg1-xAl2O4) is designed and applied in biogas bi-reforming. The nano NixMg1-xAl2O4 catalysts were comprehensively characterized by Raman, SEM, HR-TEM, H2-TPR, CO2-TPD, etc. The CO2 activation was verified by in situ CH4-TPSR/CO2-TPO, and the key role of NixMg1-xAl2O4 in coke removal behavior mechanism was further confirmed. Due to the efficient CO2 adsorption/activation and strong metal–support interaction, nano NixMg1-xAl2O4 shows excellent catalytic stability to inhibit sintering and carbon deposition. At reforming conditions of 750 °C, a weight hourly space velocity (WHSV) of 32,400 mL gCat−1 h−1, the Ni0.4Mg0.6Al2O4 catalyst exhibits excellent CH4/CO2 conversion of 91/51 % are achieved, followed by slight decrease after 40 h, and remain basically stable for the next 40 h.

Effects of CaO addition on the reactivity and reaction kinetics of coke

ABSTRACTIn recent years, researches have shown that highly reactive coke for blast furnace ironmaking not only reduces fuel ratio but also reduces carbon emissions. Therefore, the research on developing high reactivity coke has received unprecedented attention. In this work, CaO was added into coals with different degrees of metamorphism to caking coke to study the effect of CaO amount on the non-isothermal gasification reaction behavior of coke. Results showed that the reactivity is significantly improved as the mass of added calcium oxide accounting for less than 0.4% of the dry coal mass. However, CaO exerted little effect on the micro-strength and structural strength of coke. The analysis of Mercury intrusion porosimetry of coke samples demonstrates that the addition of CaO increased the porosity of coke, resulting in the contact area with CO2, leading further to an increase in coke reactivity. At different heating rates, as the amount of CaO added increased, both the initial reaction temperature and the corresponding temperature of the maximum reaction rate of the gasification from CO2 decreased little by little. The addition of CaO was increased from 0% to 0.6%, the activation energy of the reaction was decreased by 16.41 kJ/mol. The kinetic model of the gasification of coke by CO2 conforms to the unreacted shrinking core model. The reaction mechanism function is converted from f(α) = 3(1-α)2/3 to f(α) = 2(1-α)1/2 (α means carbon conversion ratio), indicating that the addition of CaO accelerates the dissolution rate of coke by CO2.

Steam reforming of n-butanol and tetrahydrofuran: Influence of aliphatic carbon chain and furan ring on coking behaviors

Almost all small organics, in theory, could be potential feedstock for production of hydrogen via steam reforming (SR). Nevertheless, not all organics contain a linear carbon chain like ethanol or acetone. How do ring structure of an organic substrate impacted coking behaviors in SR were explored herein with n-butanol and tetrahydrofuran (THF) as the research objects over Ni/SiO2 catalyst, due to the same carbon number but distinct carbon chains in these two molecules. The results indicated that furan ring in THF lowered reactivity towards SR, as its adsorption on metallic sites was more difficult than n-butanol. The in-situ IR analysis for measuring reaction intermediates suggested that n-butanol and THF involved generation of intermediates with CO formed via dehydrogenation, CxHy formed via cracking and CC species that can be further gasified or aggregate to coke. However, more oxygenated intermediates generated in SR of n-butanol than in THF, rendering n-butanol more reactive towards SR. This, however, also formed substantially more coke of aliphatic nature in amorphous form, which led to the coke of thermally unstable and had higher tendency being oxidized. In converse, coke formed from SR of THF was more aromatic, showing less impact on catalytic activity, higher thermal stability, and larger crystal size with more carbon nanotube form of coke. This was closely connected with the ring structures that could dehydrogenated to form the unsaturated ring with capability to involve in Diels-Alder reactions, forming coke of aromatic nature.

Non-isothermal kinetics of coke and iron ore melting reduction with variable activation energy model

Due to the significant consumption of coke in blast furnace ironmaking, a large amount of carbon emissions is generated. Thus, addressing the issues of energy conservation and carbon emission reduction in blast furnace ironmaking is imperative. The melting reduction reaction constitutes the predominant portion of carbon consumption during the ironmaking process. However, its kinetic behavior remains unclear to date, impeding energy-saving and carbon reduction efforts in blast furnaces. Consequently, this study conducted melting reduction experiments on two types of sinters and coke under simulated blast furnace conditions. A variable activation energy model (VAEM) was established for kinetic analysis of melting reduction. The conclusion is as follows: the variable activation energies of two types of sinters were determined to vary between 110 and 50 kJ/mol and 115–60 kJ/mol, the pre-exponential factors were 1.36 and 2.57. The deviation analysis showed that the predicted values using VAEM deviated from the experimental values by 1.17 % and 0.95 %. The reliability of VAEM was demonstrated. In turn, the kinetic behavior of coke and iron ore melting reduction was declared. It offers a theoretical basis for promoting the rational use of coke resources in blast furnace ironmaking and reducing carbon emissions.

The impact of various catalysts on pyrolysis bio-oil characteristics and catalyst coking behavior of corn stover

ABSTRACT Biomass has immense potential to address energy and environmental issues. Catalytic pyrolysis is an effective technology for utilizing biomass. Six catalysts (CaO, Al2O3, Fe2O3, MCM-41, HZSM-5, and red mud(RM)) were used to pyrolyze corn stover in a fixed-bed reactor. Six catalysts (CaO, Al2O3, Fe2O3, MCM-41, HZSM-5, and red mud (RM)) were used in a fixed-bed reactor. The feedstock-to-catalyst (FCR) ratio was adjusted to optimize the catalytic effect. The comprehensive evaluation of the catalysts was carried out by analyzing the deoxygenation effect, liquid yield, and catalyst coke accumulation behavior through TG, GC-MS, SEM/EDS, and XRD techniques. The study revealed that superior deoxygenation performance among the six investigated were HZSM-5, CaO, and RM. At the highest FCR (2:1), the liquid yield and deoxygenation effect of CaO are both at their optimal state simultaneously. The bio-oil yield reached 51.98%, but it has a coke accumulation of 27.07% by TG. MCM-41 and HZSM-5 exhibited more severe coking with coke contents of 8.14% and 8.04%, respectively, along with higher maximum heat loss temperature, upon dichloromethane extraction. The relative peak areas of alkanes in the coke accumulation were more than 50% except for RM, which the main component of the latter was polycyclic aromatic hydrocarbons (78.43%). By comparing the bio-oil characteristics and carbon accumulation behavior of different catalysts, it provides a reference for the study of carbon accumulation in pyrolysis catalysts.

Analysis of the differences in coke thermal properties testing

Coke thermal properties affects the operation of blast furnaces, but currently there are certain issues with the accuracy of the results(CRI&CSR), which affects the determination of the quality of coke. Using the coke of enterprise as the experimental sample and base on the Chinese standard as the experimental conditions, comparative-experiment is carried out under two different temperature modes: constant center temperature of 1100 °C and constant heating temperature of 1100 °C, next, by simulating the ore-coke experimental under blast furnace conditions, the reliability of CRI&CSR for comparative-experiment is demonstrated. The research conclusion is as follows: there are significant differences of CRI&CSR of different coke in comparison-experiment, according to thermodynamic analysis, the reason is that thermal compensation determines the degree of absorption heat of coke solution loss affects the experimental results, which leads to the deviation. The difference in deviation is due to the varying degree to which the solution loss reaction rate of different coke is affected by temperature, the ore-coke experimental shows that the reactivity of coke, CSL (Coke Solution Loss), and the strength after reaction, CSRCSL, is a consistent trend with constant center temperature method, therefore the constant center temperature method can simulate the solution and degradation behavior of coke in blast furnaces more accurately, and can evaluate the metallurgical performance of coke more accurately.

Behaviors of coking and kinetics of volatiles’ reaction during coal pyrolysis in a two-stage reactor

The reaction of volatiles, especially the coking of volatiles, is the core reaction in the thermal conversion of low-rank coal. In this work, a two-stage reactor is designed and used to investigate the coking behavior of volatiles by adjusting the temperature and residence time of volatiles in second-stage reactor. The variation of tar properties and coke structure is analyzed by Fourier transform infrared spectroscopy (FTIR), electron spin resonance (ESR), and Gel permeation chromatography (GPC). The results show that the temperature of tar condensation (540 °C) is 60 °C lower than that of coke formation (600 °C). The oxygen-containing functional groups in tar can inhibit the coking of volatiles, although oxygen content is higher in the initial coking. The coking behavior of volatiles can be expressed by first-order + autocatalytic kinetics with activation energy lower than 40 kJ/mol when the high temperature volatiles produced by the first-stage reactor directly enter the second-stage reactor to generate coke. The coke is categorized into two types, one suspended in the tar (coke-tar) and another deposited on the wall of reactor (coke-wall). The conversion of coke-tar to coke-wall lead to a big error in the first order + autocatalytic kinetics fitting.