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Structure and composition of hard coke deposited on industrial fluid catalytic cracking catalysts by solid state 13C nuclear magnetic resonance
Abstract
Carbonaceous deposits (hard coke) were studied by various solid state C-13 nuclear magnetic resonance (NMR) techniques after demineralization of the spent and regenerated fluid catalytic cracking (FCC) catalysts obtained from Indian refineries. A number of structural parameters such as aromaticity, H/C ratio, fraction of protonated (f(a)(p)) and non-protonated (f(a)(NP)) aromatic carbons, number of pericondensed rings per average molecule (N-peri), and aromatic condensation index (gamma(ar)) are derived from the NMR data. Findings of thirty-two and thirty-nine aromatic rings in coke from regenerated catalysts and five and nineteen rings in coke from spent catalysts are rationalized by various parameters such as feed, temperature and process conditions of FCC reactor on the basis of condensation. The coke are found more condensed in regenerated catalysts compared to spent catalysts revealing that the temperature has a marked effect in the evolution of coke structure whereas feeds and other process conditions govern the nature and composition of coke.
... The thickness of the coke deposits varied little along the axial direction, shown in Fig. 1b. A few other studies also reached similar conclusions [9][10][11][12] . Current research on coking in cyclone has focused on chemical analysis of coke blocks, coking processes, coking mechanisms, and coking prevention [10][11][12][13] , and with some focus on the chemical process of coking. ...
... A few other studies also reached similar conclusions [9][10][11][12] . Current research on coking in cyclone has focused on chemical analysis of coke blocks, coking processes, coking mechanisms, and coking prevention [10][11][12][13] , and with some focus on the chemical process of coking. Kim et al [4][5] analyzed and described the morphology of coke species in a commercial catalytic cracking reactor to understand the formation mechanism of the coke. ...
... Kim et al [4][5] analyzed and described the morphology of coke species in a commercial catalytic cracking reactor to understand the formation mechanism of the coke. Behera et al [10] studied the in uence of factors such as the composition of the catalyst, properties of the feedstock, catalytic cracking reactor parameters, and the conditions leading to regeneration of the coking structure, which may all contribute to the development of FCC catalysts that have better performance and less deactivation. Cho et al [14] investigated the ow pattern of catalyst particles in the cyclone, and used the Computational Particle Fluid Dynamics(CPFD) method to evaluate the conversion of the cracking reaction in the cyclone, determined the inhomogeneous particle distribution ow, and studied how thickness of coking affected the cyclone separation e ciency. ...
Carbon deposits in Residue Fluidized Catalytic Cracking (RFCC) separators could be a safety hazard, they reduce efficiency, may also cause unscheduled shutdowns in industrial production. In this paper, the effect of coking on flow fields and separation performances was studied with an industrial cyclone separator. The time-averaged and dynamic flow-fields were affected significantly by the coking, as well as separation performance. The tangential and axial velocities decreased due to increased coking, with a maximum decrease of 102.9% and 60.7%, respectively. And the pressure drops decreased due to the vortex dissipation losses, with a maximum decrease of 33.8%. The vorticity also aggravated the breakage with the increasing coking as the breakage position moved up to the cone section. The instability of the flow-field affected the separation efficiency, and the separation efficiency, in turn, as a function of coking types, with a maximum decrease of 18.9%. With increased coking, both the pressure eccentricity and dynamic pressure standard deviation increased. And the main frequency of pressure fluctuation decreased from 37 Hz to 9 Hz. This study has the potential to further the understanding of the flow-field characteristics and performance for different coking types in cyclone separators.
... For modifying the FCC catalyst, a refinery equilibrium catalyst (ECat), a mechanical mixing method was employed. The required amount of oxygen carrier; CuO, Co3O4, and Mn2O3, was determined by considering the amount of coke on the FCC catalyst [22,[33][34][35][36][37][38] ...
... 0.46-1.74 [34]. However, the hydrogen contents were slightly higher than the literature values [34] which may be attributed to residual on the catalysts. ...
... [34]. However, the hydrogen contents were slightly higher than the literature values [34] which may be attributed to residual on the catalysts. ...
Oil refineries collectively account for about 4–6% of global CO2 emissions and Fluid Catalytic Cracking (FCC) units are responsible for roughly 25% of these. Although post-combustion and oxy-combustion have been suggested to capture CO2 released from the regenerator of FCC units, Chemical Looping Combustion (CLC) is also a potential approach. In this study, the applicability of CLC for FCC units has been explored. A refinery FCC catalyst (equilibrium catalyst-ECat) was mixed mechanically with reduced oxygen carriers; Cu, Cu2O, CoO, and Mn3O4. To identify any detrimental effects of the reduced oxygen carriers on cracking, the catalyst formulations were tested for n-hexadecane cracking using ASTM D3907-13, the standard FCC microactivity test (MAT). To investigate the combustion reactivity of coke with physically mixed oxidised oxygen carriers, CuO, Co3O4 and Mn2O3, TGA tests were conducted on a low volatile semi-anthracite Welsh coal, which has a similar elemental composition to actual FCC coke, with various oxygen carrier to coke ratios over the temperature range 750–900 °C. The results demonstrated that, whereas Cu was detrimental for cracking n-hexadecane with the ECat, Cu2O, CoO, and Mn3O4 have no significant effects on gas, liquid and coke yields, and product selectivity. Complete combustion of the model coke was achieved with CuO, Co3O4 and Mn2O3, once the stoichiometric ratio of oxygen carrier/coke was higher than 1.0 and sufficient time had been provided. These results indicate that the proposed CLC-FCC concept has promise as a new approach to CO2 capture in FCC.
... The last weight loss is derived from the decomposition/oxidation of stable coke compounds. Generally, FCC coke can be divided into soft (chloroform-soluble) coke and hard (chloroform-insoluble) coke, the former is primarily composed of small aliphatic molecules, and the latter is mainly attributed to stable aromatic hydrocarbons 27,28 . According to TGA results, the second weight loss is due to soft coke and the third weight loss is attributed to hard coke. ...
... The first region of gas evolution is derived from the decomposition of less stable coke, which is attributed to soft coke 27 and decomposes at around 600 °C, generating HCN, NO, CH 4 , etc. (Fig. 2). The second region is attributed to the reaction of hard coke-aromatic compounds and coke deposited in the deep pores of the catalyst 28 , generating CO, NO and CO 2 at an elevated temperature around 950 °C (Fig. 2). ...
... Both aromatic and aliphatic carbons are clearly identified in the initial coked catalyst, with signals centered around 130 and 20 ppm, respectively (Fig. 4a). As reported previously, typical FCC coke is dominated by aromatic carbon and a small fraction of aliphatic carbon 25,27 . The aliphatic species in FCC coke are primarily derived from alkyl groups attached to aromatic rings and hydrocarbons entrained in the catalyst pores (like catalyst-to-oil coke which are not removed by stripping) 4,25 . ...
Regeneration of the coked catalyst is an important process of fluid catalytic cracking (FCC) in petroleum refining, however, this process will emit environmentally harmful gases such as nitrogen and carbon oxides. Transformation of N and C containing compounds in industrial FCC coke under thermal decomposition was investigated via TPD and TPO to examine the evolved gaseous species and TGA, NMR and XPS to analyse the residual coke fraction. Two distinct regions of gas evolution are observed during TPD for the first time, and they arise from decomposition of aliphatic carbons and aromatic carbons. Three types of N species, pyrrolic N, pyridinic N and quaternary N are identified in the FCC coke, the former one is unstable and tends to be decomposed into pyridinic and quaternary N. Mechanisms of NO, CO and CO2 evolution during TPD are proposed and lattice oxygen is suggested to be an important oxygen resource. Regeneration process indicates that coke-C tends to preferentially oxidise compared with coke-N. Hence, new technology for promoting nitrogen-containing compounds conversion will benefit the in-situ reduction of NO by CO during FCC regeneration.
... Conversion of n-hexadecane (Equation (1) (overall conversion) and Equation (2) (excluding CLC conversion)), yields (gas, liquid, coke) (Equations (3)-(5)), and selectivities of cracking products such as C [5][6][7][8][9][10][11][12][13][14][15] (gasoline), C 3-4 (LPG), and C 1-2 (dry gas) (Equation (6)) were determined [25,27]. ...
... Additionally, the percentage of carbon and hydrogen in coke, and normalised composition are presented in Table 2. The carbon and hydrogen composition in the cokes (model and produced by the cracking of n-hexadecane and VGO) demonstrate similar elemental composition with the FCC coke [9]. However, the soft coke and hard coke distribution in the coke show differences based on the coke type, which may be attributed to the type of cracking feed. ...
Oil refineries are responsible for ∼5% of total global CO2 emissions and approximately 25–35% of these emissions are released from a single unit called Fluid Catalytic Cracking (FCC). Chemical Looping Combustion (CLC) has been recently proposed as a novel CO2 capture method from the regenerator of FCC units as an integrated process of CLC-FCC. In this study, for the first time, the combustion behaviour of three types of cokes, a model FCC coke (which is a low volatile semi-anthracite coal), and cokes deposited on commercial FCC catalysts by n-hexadecane cracking and Vacuum Gas Oil, were comprehensively investigated with oxygen carriers (Co3O4, CuO, and Mn2O3) in a fixed-bed reactor at 700–850 °C. The results demonstrate that a high coke combustion efficiency was achieved with CuO (98 vol. %), Co3O4 (91 vol. %), and Mn2O3 (91 vol. %) at 800 °C for 30 min. CuO was the most effective oxygen carrier, at temperatures greater than 750 °C for 45 min of residence time. These are the regeneration conditions used in the conventional FCC regenerators.
... The first corresponds to polyglycols, resulting from intermolecular glycerol dehydration, displaying slight signals around 63 and 72 ppm. [25,46] The second refers to aromatics, which exhibits a signal around 130 ppm. [47,48] The signal around 20 refers to methyl groups bonded to the aromatic rings. ...
... [47,48] The signal around 20 refers to methyl groups bonded to the aromatic rings. [48,49] In contrast, those around 31 and 37 ppm refer to aliphatic carbon atoms in hydroaromatic compounds, [46,50] i. e., aromatic precursors that did not have time to complete the aromatization. Subsequently, the peak in 140 ppm was attributed to bridgehead atoms between an aromatic and a non-aromatic ring, [51] such as in fluorenic and indenic compounds, which are intermediates in the formation of ultimate coke compounds. ...
Glycerol conversion into value‐added chemicals has received much attention due to the prospect of increasing the biodiesel industry‘s profitability. The dehydration of glycerol is one of the most explored reactions in the valorization of glycerol, and the zeolite H‐ZSM‐5 is one of the furthermost used structures. In this study, several characterization techniques were used to investigate the detailed aging of coke in spent H‐ZSM‐5 zeolites after their use in the gas‐phase glycerol dehydration to produce acrolein. The carbonaceous deposits formed almost totally in the first minutes of the reaction. They rapidly got trapped inside the pores, hampering the accessibility to active sites and the zeolite‘s full catalytic action. Two types of coke were formed during the reaction: polyglycols, on the external surface, and aromatics, inside the pores, representing the majority (∼98 wt.%) of the carbonaceous compounds. Besides, aromatic compounds age over time, becoming bulkier and more deficient in hydrogen. We also found a correlation between the amount of coke, its size, accessibility to the acid active sites, and changes in crystallographic parameters of the zeolite with the presence of coke.
... The TG/DTA profiles reported in Figure 4 clearly indicate the absence of coke on the investigated samples, as no weight loss is observed above 300 °C [47,48]. The low water content (about 2 wt%), calculated as weight loss in the range of RT-200 °C is typical of hydrophobic materials. ...
... The TG/DTA profiles reported in Figure 4 clearly indicate the absence of coke on the investigated samples, as no weight loss is observed above 300 • C [47,48]. The low water content (about 2 wt%), calculated as weight loss in the range of RT-200 • C is typical of hydrophobic materials. ...
Rare earth elements (REEs) are strategic materials widely used in different applications from Information and Communication Technologies (ICT) to catalysis, which are expected to grow more in the future. In order to reduce the impact of market price and reduce the environmental effect from soil extraction, recovery/purification strategies should be exploited. This paper presents a combined acid-leaching/oxalate precipitation process to recover lanthanum from spent FCC catalyst using nitric acid. Preferred to hydrochloric and sulphuric acid (preliminary assessed), HNO3 showed a good capability to completely leach lanthanum. The combination with an oxalate precipitation step allowed demonstrating that a highly pure (>98% w/w) lanthanum solid can be recovered, with a neglectable amount of poisoning metals (Ni, V) contained into the spent catalyst. This could open a reliable industrial perspective to recover and purify REE in the view of a sustainable recycling strategy.
... The first corresponds to polyglycols, resulting from intermolecular glycerol dehydration, displaying slight signals around 63 and 72 ppm. [25,46] The second refers to aromatics, which exhibits a signal around 130 ppm. [47,48] The signal around 20 refers to methyl groups bonded to the aromatic rings. ...
... [47,48] The signal around 20 refers to methyl groups bonded to the aromatic rings. [48,49] In contrast, those around 31 and 37 ppm refer to aliphatic carbon atoms in hydroaromatic compounds, [46,50] i. e., aromatic precursors that did not have time to complete the aromatization. Subsequently, the peak in 140 ppm was attributed to bridgehead atoms between an aromatic and a non-aromatic ring, [51] such as in fluorenic and indenic compounds, which are intermediates in the formation of ultimate coke compounds. ...
Catalyst deactivation and mechanism of coke formation during the gas phase dehydration of 1,3-butanediol were studied on solid acids of different nature: Al2O3, SiO2/Al2O3, HZSM5 and tungstophosphoric acid (TPA) supported on SiO2. All the catalysts deactivated on stream with a significant coke content ranged between 9% and 14% C. Fresh and spent catalysts as well as carbonaceous deposits were thoroughly characterized using different techniques such as N2 physisorption, FTIR spectroscopy, FTIR of adsorbed pyridine, DSC-TGA, GC-MS and MALDI-TOF MS (matrix-assisted laser desorption/ionization time of flight mass spectrometry). The IR spectra of spent and washed (after direct extraction with CH2Cl2) samples and the superficial soluble coke characterized by MALDI-TOF MS showed the different nature of coke formed in each catalyst with carbonaceous products of higher molecular weights on HZSM5 and TPA/SiO2 than on Al2O3 and SiO2/Al2O3. After catalyst dissolution using hydrofluoric acid and extraction with CH2Cl2 both total soluble and insoluble coke were analyzed. Insoluble coke was only detected on TPA/SiO2 and it was attributed to the strength of its Brønsted acid sites. GC-MS chromatograms of total soluble coke showed the presence of aromatic and polyromantic species, that were very alkylated in the case of HZSM5. Particularly, the well differentiated nature of coke on HZSM5 including alkylnaftalenes and alkylantracenes suggested the coke formation by a shape selectivity mechanism.
... For example, after Mn- based reduced oxygen carrier modification, a dramatic increase in the soft coke was observed (Table 1), which also arise the total coke ratio on the catalyst. However, the coke calculated for the reduced oxygen carriers impregnated ECat was in the range reported in the literature where the FCC catalysts contain 0.46-1.74% of insoluble coke [39] having a H/C ratio of 0.27-0.54. The expected coke was also about 0.8-1.0 ...
... wt% according to the figure drawn for the cracking time versus carbon deposition by Alexis Voorhies [40]. As for the hydrogen content, it was slightly higher due to the moisture content of the catalysts of 2-4 wt% [39]. ...
Fluid Catalytic Cracking (FCC) units are responsible for roughly 25 % of CO2 emissions from oil refineries, which themselves account for 4-6 % of total global CO2 emissions. Although post- and oxy-combustion technologies have been proposed for CO2 capture in FCC, Chemical Looping Combustion (CLC) may also be a potential approach that has lower energy consumption. An equilibrium catalyst (ECat) was first modified with oxidised oxygen carriers (CuO, Co3O4, Mn2O3) using wet-impregnation, and their reduced states (Cu, CoO, Mn3O4, MnO) were generated by hydrogen reduction. To demonstrate that the impregnated reduced oxygen carriers had no significant negative effects on cracking, the prepared catalysts were used to crack n-hexadecane using the standard FCC microactivity test (ASTM D3907-13). The CLC behaviour of coke deposited on the reduced oxygen carrier impregnated ECats, was investigated with the stoichiometrically required amount of oxidised oxygen carrier impregnated ECat in lab scale fixed-bed and fluidised-bed reactors equipped with an online mass spectrometer to monitor CO2 release. Although the conversion and liquid to gas ratio were largely unaffected, coke selectivity did increase with the impregnation of reduced oxygen carriers. However, this increase is mostly attributed to solvent extractable coke. It is possible to reach about 90 vol. % combustion efficiency of the coke deposited on ECat using mechanically mixed with CuO and Mn2O3, but the regeneration temperature required, 800 °C, is considerably higher than that under typical regenerator conditions of 650-750 °C for 30-60 min. However, relatively high combustion efficiencies of greater than 94 vol. % of the coke deposited on reduced Cu and Mn3O4 impregnated ECat were achieved with the stoichiometrically required amount of CuO and Mn2O3 impregnated ECat at 750 °C for 45 min., close to conventional FCC regenerator conditions.
... The acidity of the solid catalysts has a great influence on carbonaceous deposits or coking during glycerol dehydration. [114,122] The Brønsted acid site is the active site of coke. As the acidity of Brønsted acid site increases, so does the length of the carbon chain of the oligomeric compounds. ...
... [58] The pore structure of solid catalysts also has an impact on carbonaceous deposits or coking during glycerol oxydehydration. [114,122] Pore size of the catalysts in the glycerol dehydration process significantly influences the diffusion and the adsorption-desorption of species in the system of catalytic glycerol dehydration. [45,125] Long channels of a porous catalyst lead to longer residence of glycerol, acrolein, and by-products such as phenol, dihydrofuran, cyclopentenone, methyl cyclopentenone, and cyclohexenone. ...
This article provides a comprehensive and critical review of the latest studies on catalytic glycerol dehydration-oxidation to acrylic acid. The two-bed catalytic system in one or two reactors involves glycerol dehydration to acrolein and subsequent oxidation of acrolein to acrylic acid. Zeolites, metal oxides, heteropoly acids, and phosphates are effective in the dehydration of glycerol to acrolein. Mo–V–O catalysts appear active in the acrolein oxidation to acrylic acid. The glycerol can be completely converted to acrolein with 98% selectivity. In such a two-step process, the step of catalytic dehydration is thought to be critical. A few recent studies reveal that the conversion of glycerol to acrylic acid in two reactors can be also achieved via allyl alcohol as intermediate. For the one-bed catalytic glycerol oxydehydration to acrylic acid, a single catalyst must possess both active acid sites and active redox sites. Mo–V–O, W–V–O, Mo–V–W–O, W–V–Nb–O oxide catalysts, and heteropoly acid catalysts are particularly promising. Currently, a 60% yield of acrylic acid can be achieved over H0.1Cs2.5(VO)0.2(PMo12O40)0.25(PW12O40)0.75 at 340°C. However, all the catalysts rapidly deactivate due to coking. Coking usually occurs during the glycerol oxydehydration step. Optimizing reaction conditions such as increasing water and oxygen feeding, lowering reaction temperature, tuning the catalysts by finely doping, adjusting the surface acidity and enlarging pores of the solid catalysts can inhibit coking to some extent by slowing the deactivation of catalyst. Yet coking over catalysts is a major obstacle when conducting glycerol oxydehydration on a large scale. We suggest that future work should place an emphasis on revealing the essence of coking, further designing coking-resisting catalysts, and developing an efficient reaction and separation system.
... Por isso que apenas em amostras regeneradas de catalisadores de HDT a detecção por DRX dessas estruturas é possível, até porque nessa situação elas não estão mais em meio àquelas menos organizadas (Torres-Mancera, et al., 2015), o que melhora a qualidade da análise devido a eliminação do ruído de fundo causado pelo carbono amorfo ou não aromático ora presente (Machado, et al., 2013;Lu, et al., 2001). Behera, et al., (2013) Os efeitos dos venenos devem ser incorporados à constante cinética da reação (kA), por essa considerar o total de sítios ativos disponíveis (Furimsky, et al., 1999). ...
... Detalhes sobre o método e condições de execução da análise são descritos na Seção 3.3.11. Como referência, outros autores citam essa técnica para quantificação do teor de coque depositados sobre catalisadores (Behera, et al., 2013;Barman, et al., 1997). Espécies precursoras de coque usualmente apresentam valores da relação H/C (molar) de 1,2 a 1,4 e o coque valores ainda bem menores que estes (Rana, et al., 2007). ...
The main objective of this work is to study the residual catalyst activity and the mechanisms of deactivation of catalyst from a lube-oil hydroprocessing industrial unit.
In order to accomplish this, pilot plant tests were carried out followed by spent catalysts characterization. The residual catalytic activity was determined by HDA, HDS and HDN conversions and adjusted by a power law apparent kinetic model.
Pilot plant tests revealed different levels of residual activity for spent catalyst samples. Catalyst samples taken from the first and last of the five catalytic beds showed higher deactivation than others.
Catalyst characterization results pointed out two mechanisms as the main reason for the catalytic deactivation through the industrial reactor: poisoning by metal deposition (mainly Si and As) and coke deposition. Poisoning was the main deactivation mechanism for the first bed spent catalyst sample, while coke deposition was predominant at the last catalytic bed sample. Reactor temperature was identified as the most important operational parameter considering coke aging.
... Table 8 shows that the H/C ratio of the three catalysts is 0.85, 0.30, and 0.19, respectively, which is mostly consistent with the literature (Cerqueira et al. 2008) that the coke component on the catalyst usually has an H/C ratio of 0.3~1.0. The H/C ratio can represent its coking degree because a substance with low H/C ratio may have more condensed rings than that with high H/C ratio (Cerqueira et al. 2008;Behera et al. 2013). ...
Fluid catalytic cracking (FCC) unit is one of the means to lighten heavy oil in refineries, and its regenerated flue gas is also the main source of air pollutants from refinery. However, it is not clear about the type and amount of pollutants discharged from FCC units in China. The emissions of regenerated pollutants in the stack flue gases of three typical FCC units in China were investigated in this study, including a partial regeneration unit without a CO boiler (U1), a partial regeneration unit with a CO boiler (U2), and a full regeneration unit (U3). Different monitoring methods were used to analyze the concentration of sulfur dioxide (SO2) and nitrogen oxides (NOx), and the results showed that Fourier transform infrared spectroscopy (FTIR) monitoring results of SO2 and NOx are approximately 10 times and 5 times larger than those of the continuous emission monitoring system (CEMS) data, respectively. Also, the contents of characteristic pollutants such as NH3, C6H6, HCN, C8H8, C2H4, CH4, and CO were also monitored by FTIR, and the emission factors based on coke burn-off rate and throughput were investigated. The pollutants in U1 exhibited relatively higher contents with the NH3, HCN, and C6H6 of 116.99, 71.94, and 56.41 mg/Nm³ in flue gas, respectively. The emission of regenerated pollutants in U2 and U3 are significantly different from U1. Regeneration processes (including coke properties, operating modes, and presence or absence of CO boilers) affected pollutants’ emission factors in varying degrees. At last, reasonable emission factors based on the different FCC regeneration processes contribute to the prediction, assessment, and control for the pollutant emission.
... However, due to the extremely low content of coke on the FCC catalyst, and the low sensitivity and even technical limit of these bench-scale facilities, the accuracy of the results is still rendered unreliable by a very low signal-to-noise ratio or the destructive pre-treatment step. For instance, for the characterisation of coke by 13 C NMR, the catalyst needs to be demineralised first to concentrate the coke content, which in turn leads to the concern on destruction of the coke structure by demineralisation [19][20][21]. Likewise, Raman spectroscopy can only group coke into aromatic and aliphatic carbon, whereas the solid-state 13 C NMR is unable to detect non-protonated aromatic carbon, due to the absence of magnetisation transfer from 1 H to 13 C[22] and the signal can even be lost in the presence of ferromagnetic minerals in a coke [23]. ...
In this paper, the regeneration and associated transient evolution of a commercial FCC spent catalyst in both air-firing and oxy-fuel combustion modes (i.e. 0-10% O2 in N2 or CO2, 510-1000°C, and ∼ 4 sec) were studied in a lab-scale drop-tube furnace. A variety of non-destructive techniques including synchrotron NEXAFS, XPS and ATR-FTIR for both non-grounded and grounded spent catalyst and its combustion residues were employed to reveal the speciation, spatial distribution of coke within the catalyst matrix, as well as its evolution upon rapid heart-up and combustion. As have been confirmed, the coke species is highly broad in speciation, covering aliphatic and aromatic compounds as well as graphite that are both physically deposited and chemically adsorbed on acid sites on the catalyst’s exterior surface. The majority of aliphatic hydrocarbons were also embedded deeply within the catalyst matrix, which were presumably caused by the penetration of precursor molecules that are small in size. Irrespective of the combustion mode, upon the initial rapid heat-up, the embedded light hydrocarbons were rapidly volatilised, and diffused outward toward the exterior surface, where it either underwent C-O2/C-CO2 reactions, or simply further recondensed into graphite in the case that oxygen is lean in the air-firing mode. In contrast, in the oxy-firing mode, the coke was confirmed highly reactive for reverse Boudouard reaction and methane drying reaction which were even triggered during the initial particle heat-up stage. Consequently, the initial graphitisation extent of light vapor was mitigated, and the micro-pores of catalysts were opened up to enable a continuous elution of the hydrocarbon vapors to boost the overall coke burn-off. Regarding the competition of the overall C-O2 and C-CO2 reactions on catalyst exterior surface, the former reaction is superior in the case where oxygen is lean (e.g. 3%). In contrast, when the oxygen content is higher (e.g. 10%), the latter one was intensified due to the strong heat feedback from the endothermic C-CO2 reactions. As a result, the overall coke including other two species, graphite and aliphatic hydrocarbons were consumed quickly in the O2/CO2 case.
... shows that the H/C ratio of the three catalysts is 0.85, 0.30, and 0.19, respectively, which is mostly consistent with the literature(Cerqueira et al., 2008) that the coke component on the catalyst usually has an H/C ratio of 0.3 ~ 1.0. The H/C ratio can represent its coking degree because a substance with low H/C ratio may have more condensed rings than that with high H/C ratio( Cerqueira et al., 2008;Behera et al., 2013). So it believes that Cat2 and Cat3 may have more condensed rings. ...
Fluid Catalytic cracking (FCC) unit is one of the means to lighten heavy oil in refineries, and its regenerated flue gas is also the main source of air pollutants from refinery. However, it is not clear about the type and amount of pollutants discharged from FCC units. The emissions of regenerated pollutants in the stack flue gases of three typical FCC units in China were investigated in this study, including a partial regeneration unit without a CO boiler (U1), a partial regeneration unit with a CO boiler (U2) and a full regeneration unit (U3). Different monitoring methods were used to analyze the concentration of sulfur dioxide (SO 2 ) and nitrogen oxides (NO x ), and the results showed that Fourier Transform Infrared Spectroscopy (FTIR) monitoring results of SO 2 and NO x are approximately 10 times and 5 times larger than that of the Continuous Emission Monitoring System (CEMS) data, respectively. Also, the contents of characteristic pollutants such as NH 3 , C 6 H 6 , HCN, C 8 H 8 , C 2 H 4 , CH 4 and CO were also monitored by FTIR, and the emission factors based on coke burn-off rate and throughput were investigated. The pollutants in U1 exhibited relatively higher contents with the NH 3 , HCN and C 6 H 6 of 116.99, 71.94 and 56.41 mg/Nm ³ in flue gas, respectively. The emission of regenerated pollutants in U2 and U3 are significantly different from U1. Regeneration processes (including coke properties, operating modes and presence or absence of CO boilers) affected pollutants emission factors in varying degree. At last, reasonable emission factors based on the different FCC regeneration processes contributes to the prediction, assessment and control for the pollutants emission.
... Fig. 4 shows the NMR 13 C for CatRef before reaction (a), CatRef (b), CatAceMo (c) and CatAXAceMo (d) catalysts. Regarding the pretreatment, all catalysts showed peaks about 0-60 ppm, 60-100 ppm 100-165 ppm and > 165 ppm corresponding to aliphatic, quaternary, aromatic and carbonyl carbons, respectively [37]. In this sense, Table 5 exhibits the semi-quantitative area of aromatic and aliphatic superficial carbon after the HDS reaction of straight run gas oil. ...
Washing with xylene (X), 2,6 bis 1 hydroxy 1,1 diphenyl methyl pyridine (A) and in situ reactivation with molybdenum acetylacetonate (AceMo) on the NiMoP/Al 2 O 3 spent surface catalyst has been investigated in the hydrodesulfurization (HDS) of straight-run gas oil. The spent catalyst was washed and dried at 120 °C before in situ reactivation. The sulfided catalysts were characterized by temperature programmed reduction (TPR), nuclear magnetic resonance (NMR), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The HDS reactions were carried out at 5.5 MPa of H 2 and the reaction temperatures at 340 °C, 360 °C and 380 °C. The CatAXAceMo displayed the highest HDS activity. The TPR results advice that CatAX presented more “rigid” MoS 2 edges than CatRef catalyst. NMR results suggest that CatAXAceMo showed lower aromatic or polyaromatic hydrocarbons surface concentration deposited after HDS reaction than CatAceMo and CatRef. Raman spectroscopy revealed the formation of coke crystallites with shorter size for CatAXAceMo than others catalysts. XPS spectroscopy results exhibited that CatAXAceMo presented smaller superficial carbon and a larger concentration of MoS 2 active phase. A relation between the coke crystallites size and MoS 2 superficial concentration was found. A lower aromatic concentration favors the shorter coke crystallites sizes and a major availability of Mo species for the formation of MoS 2 to HDS reaction.
... The formation of polyglycols involves the activation of glycerol molecules on adjacent acid sites of the catalysts, followed by the successive coupling of hydroxyl groups of glycerol. 71,81 Polyglycols are mainly deposited on the outer surface of the zeolite catalysts. By contrast, polyaromatics were more abundant inside the pores of zeolites. ...
Catalytic glycerol dehydration provides a sustainable route to produce acrolein as glycerol is a bio-available platform chemical. However, in this process catalysts are rapidly deactivated due to coking. This paper examines and discusses recent insights into coking in catalytic glycerol dehydration. The nature and location of coke and the rate of coking depend on feedstock, operating conditions, and the acidity and the pore structure of the solid catalysts. Several methods have been suggested for inhibiting the coking and slowing the deactivation of catalyst, including: (1) co-feeding of oxygen; (2) tuning of the pore size of the solid acid catalysts; (3) doping noble metals (Ru, Pt, Pd) into the solid acid catalysts ; and (4) designing new reactors. The present methods for inhibiting coking are still unsatisfactory. The deactivated catalysts can be regenerated by removing coke. Nevertheless, the rapid deactivation of the regenerated catalyst remains problematic. The literature survey indicates that the exact compositions of the coke in glycerol dehydration remain elusive. The thermodynamics, the kinetics and the formation mechanism of coking need probing so as to advance the development of a catalyst with high activity, selectivity and resistance to coking an to put the catalytic glycerol dehydration into practice.
... No bands in all range of chemical shift can be observed for silica gel after reaction, indicating the catalyst has excellent capacity in anti-coking performance. For other catalysts, two bands are distinguished: (i) aliphatic carbon nuclei in the 0-40 ppm range; and (ii) aromatic carbon nuclei at 130 ppm (Aguayo et al., 2011;Behera et al., 2013;Marcilla et al., 2009). It was reported by Ray et al. that the aromatic peak centred on 130 ppm is mainly due to the quaternary aromatic carbons (Marcilla et al., 2009). ...
The process for dehydration of 3-hydroxypropionic acid to produce acrylic acid was investigated over solid acid catalysts containing HY, ZSM-5, Beta, MCM-41 and silica gel. These catalysts were comprehensively characterized by techniques including XRD, nitrogen adsorption–desorption, NH3-TPD, Pyridine-IR, TGA and ¹³C CP MAS-NMR. Silica gel was found to show the highest acrylic acid selectivity of >99.0% at a complete 3-hydroxypropionic acid conversion. Results from NH3-TPD and Pyridine-IR analyses reveal that the acidity has a significant effect on catalytic performance. The selective conversion of 3-hydroxypropionic acid to acrylic acid over silica gel can be ascribed to the weak and small amount of Lewis acid sites as well as the absence of Brønsted acid sites. Increasing Brønsted acid sites for HY, ZSM-5, Beta and MCM-41 resulted in low acrylic acid selectivities due to increasing of acetic acid formation. TG and ¹³C CP MAS-NMR results further indicate that the presence of Brønsted acid sites favors the formation of coke. And the coke deposited on these catalysts has a primarily aromatic nature. Stability test indicates the silica gel catalyst exhibits an excellent stability for 200 h.
... Specifically, the peaks at ca. 1093 and 1628 cm −1 correspond to ethereal (C−O str ) carbon species and coke having an aromatic skeleton, respectively. 23,50 The existence of methyl/methylene groups is verified by the peak at ca. 1394 cm −1 , while the 730 and 3440 cm −1 bands are ascribed to C−H out of plane in graphite and surface-condensed O−H str vibration of the hydroxyl functional group, respectively. The presence of different carbon species as shown by the infrared (IR) spectroscopy results were further collaborated with 13 C CP/MAS NMR analysis. ...
The deposition of appreciable amounts of metal poisons and carbon poses serious problems to the refiner during fluid catalytic cracking (FCC) unit operation. To check the effect of these contaminants on the catalyst, an in-depth understanding of their locations and existing states becomes necessary. In this work, the location and nature of vanadium, nickel and coke species on FCC catalyst were investigated. Detailed analyses of catalyst samples including industrial equilibrium catalysts (E-cats) were accomplished by using a variety of characterization techniques. It was found that nickel and vanadium concentrated mainly in mesopores and micropores of the FCC catalyst, respectively. On the surface of E-cats, vanadium exists mainly in +4 and +5 oxidation states, while nickel is present as NiO, NiAl2O4 and surface hydroxylated nickel and as NiAl2O4 with traces of nickel hydro-silicates in the bulk. The formation of a large amount of NiAl2O4 on alumina support by nickel indicates its preferential location in the alumina component of the FCC catalyst. When co-existing, a synergic effect between vanadium and nickel is likely. On the hand, coke distributed within these pore spaces, exhibiting different behavior in different catalysts due to the effect of metals and steam treatment. The coke deposits consist of a layer of graphitized carbon with both hydrocarbon and aromatic carbon species. Results obtained in this study provide insights into the nature of contaminants of FCC catalysts and could help in the rational design of catalysts to alleviate the metal poisons during catalytic cracking.
Benzene series are considered as air pollutants in refineries. However, the emissions of benzene series in fluid catalytic cracking (FCC) flue gas are poorly understand. In this work, we conduct stack tests on three typical FCC units. Benzene series, including benzene, toluene, xylene and ethyl benzene, are monitored in the flue gas. It shows that the coking degree of the spent catalysts affect the emissions of benzene series significantly, and there are four kinds of carbon-containing precursors in the spent catalyst. A fixed-bed reactor is used to conduct the regeneration simulation experiments, and the flue gas is monitored by TG-MS and FTIR. The emissions of toluene and ethyl benzene are mainly emitted in the early and middle stage of the reaction (250-650 °C), while the emission of benzene is mainly detected in the middle and late stage of the reaction (450-750 °C). Xylene group is not detected in the stack tests and regeneration experiments. Higher emissions of benzene series are released from the spent catalyst with lower C/H ratio during regeneration process. With the increase of oxygen content, the emissions of benzene series decrease, and the initial emission temperature is advanced. These insights can improve the refinery's awareness and control of benzene series in the future.
Selective hydrogenolysis of glycerol to propanol is a sustainable reaction, which can play a critical role in the future of the propylene market, as one step dehydration of this sustainable propanol provides access to sustainable propylene. In this work, Copper (Cu) – Tungstosilicic acid (HSiW) catalysts were studied for the hydrogenolysis of glycerol. For the first time over a Cu catalyst, this study reports a yield of ∼76 % for direct conversion of glycerol to propanols in continuous flow mode. Several advanced characterization techniques were employed to study the fresh and used catalysts. The presence of intact Keggin units of HSiW solid acid (with medium acidity) in the vicinity of small clusters of copper sites improved the performance and lifetime of these bifunctional catalysts. This study offers insights into the nature of the active sites required for the hydrogenolysis reaction, which was achieved without using expensive PGMs.
Fluid catalytic cracking processes generate various gas pollutants, which are the major concern for environmental protection in refineries. However, the transformation mechanism from deposits to gas pollutants is not well understood. In this work, spent catalysts from different typical FCC units were analyzed to evaluate the effect of pollutant precursors on pollutant emission. The main carbon, nitrogen and sulfur precursors of FCC pollutant emission were identified. Regeneration experiments were conducted to assess the relationship between regeneration temperature and pollutant emission under partial combustion conditions. Results showed that the same precursors from different catalysts result in close temperatures at which characteristic pollutants occurred. VOCs are firstly generated by the coke branches cracking under low temperature. Then, nitrogen-containing compounds convert to NH3 and HCN, and some of them are oxidized into NOx. After that, sulfur-containing compounds convert to C-S group and H-S group, which finally convert to SO2 at high temperature. At last, a full view of transformation mechanism of FCC pollutant emission is presented, which will give a further insight into the prediction and control of pollutants.
Fluid catalytic cracking (FCC) unit emits a large amount of flue gas, which is a major issue of environmental protection supervision. A spent FCC catalyst from a typical FCC unit was characterized by NMR, XPS, EA TGA and TPD-MS methods. XPS results presented that the nitrogen compounds in coke were pyridine nitrogen, pyrrole and quaternary nitrogen. Sulfur compounds in coke are in the form of thiophene and thiols. TGA and TPD-MS results indicated that the soft coke in the spent catalyst may decompose to small molecular such as NOx, SO2 and HCN. NH3 and HCN are mainly emitted due to the incomplete combustion. The flue gas from the FCC unit is monitored by different on-line monitoring instruments. Results showed that the existence of ammonia greatly affected the value of SO2 during the venting process or instruments' inlet piping system, where saturated vapor in flue gas is partially condensed. The concentration of NH3 and HCN is more than 100 ppm, which should be paid more attention to. Taken together, fourier transform infrared method was more applicable for monitoring FCC flue gas than non-dispersed infrared method and ultraviolet fluorescence method.
On purpose of industrial application, deactivation behavior of hydroxyapatite catalyst was studied in dehydration of lactic acid to acrylic acid. The hydroxyapatite catalyst shows stable acrylic acid selectivity with 140 h on stream. Obvious increases in acetaldehyde and lactide selectivities were observed from 140 to 250 h, which resulted in decrease of sharp acrylic acid selectivity. The fresh and used hydroxyapatite catalysts were further characterized by techniques including N2 adsorption–desorption, X-ray diffraction, transmission electron microscopy, 13C cross polarization magic-angle spinning nuclear magnetic resonance, thermal gravity analysis, temperature-programmed desorption of CO2 and NH3. These results indicate that mechanism of coke deactivation consists of two steps. The one is the formation of aliphatic carbon by the deposition of low carbon products with 140 h; the other is the formation of aromatic coke by the condensation of lactide. Also, the best generated time of 140 h was determined.
The influence on gas-phase catalytic glycerol dehydration of crystal size (S: small, or L: large crystals), acidity, and synthesis procedure for isomorphous incorporation of gallium (Ga-S; Ga-L) or aluminum (Al-S; Al-L) in MFI zeolites was studied. The main product observed was acrolein, with the undesirable parallel formation of deactivating coke molecules such as polyglycols and polyaromatics. The Ga-S zeolite showed the best performance in this reaction, as it provided a combination of adequate accessibility to the microporous system and weak Brønsted acid sites. The chemical and structural properties of the fresh MFI zeolites were studied by X-ray diffraction, nitrogen sorption measurements, scanning electron microscopy, temperature-programmed desorption of NH3, X-ray photoelectron spectroscopy, and 27Al and 29Si MAS-NMR. Solid-state 13C MAS-NMR and thermogravimetric analyses of the spent MFI zeolites confirmed the differences in the nature and amounts of the carbonaceous deposits formed. The polyglycols were preferentially formed on the external surface of the zeolite crystals, as expected due to the greater exposed area. On the other hand, the polyaromatic compounds formed were more abundant inside the micropores of the MFI zeolites, especially those composed of larger crystals and with a greater number of strong Brønsted acid sites.
This article presents macromolecular models of sulfur-containing Qingdao petroleum. The results were investigated with ultimate analysis, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy. The models showed that organic sulfur, nitrogen, and oxygen in the coke were mainly in the forms of thiophene-containing polycyclic aromatic hydrocarbon, polycyclic aromatic hydrocarbons containing pyrrole or pyridine, and C=O or C-O-C, respectively. During calcination, the aromaticity of coke increased, with element contents and functional groups changing substantially. Moreover, the models explained the desulfurization mechanism of coke during calcination.
The deactivation of a catalyst located at the first bed of a bench-scale hydrotreating reactor operated at co-current downflow mode was studied. The deactivation, caused by metal and coke depositions, is analyzed by the characterization of catalyst recovered along the reactor bed. Metals trapped in catalysts remain as metal sulfides in the spent catalysts and their amount decreases from top to bottom of the bed length whereas coke increases toward bed outlet. Deposited foulants are classified according to removability by regeneration at 550 °C. Below this temperature the removable coke by oxidation is labeled as soft coke and hard coke corresponds to carbonaceous compounds that can only be oxidized above this temperature. By 13C CP-MAS NMR it was found that the aromaticity of the deposited coke is constant throughout the catalytic bed. By regeneration of spent catalysts near half of the hydrodesulfurization activity is recovered. However, the hydrocracking activity is the same or higher compared with the fresh catalyst. The extra hydrocracking activity is the result of the creation of Brønsted acid sites after regeneration.
While solid state 13C n.m.r. has made a major contribution to the characterization of coal and other insoluble carbonaceous materials over the past decade, there has been considerable uncertainty concerning the quantitative reliability of the technique. This debate addresses this important topic and comprises six contributions from authors who are recognized experts in n.m.r. characterization of solid fuels. The principal issue discussed is the accuracy of aromaticity measurements on coals by cross-polarization — magic-angle spinning (CP/MAS) 13C n.m.r., together with additional problems posed by high field measurements and spectral editing, and with some discussion of emerging techniques. There is a consensus that significant errors can arise in CP/MAS 13C n.m.r. measurements of aromaticity due to the unfortunate spin-dynamics of coals, which typically result in only ≈50% of the carbon being observed for bituminous coals. There is clear discrimination against aromatic carbon, but differences of opinion exist over the magnitude of the errors (from 2 to 15 mole carbon %) and whether high field (⩾ 50 MHz) measurements are as accurate as those of low field (< 25 MHz) because either sideband suppression or extremely high speed MAS has to be employed to eliminate sidebands. From the evidence presented, it is suggested that a combination of low field, single pulse excitation with long relaxation delays and the use of a suitable reagent to quench paramagnetic centres is the most satisfactory, albeit time consuming, recipe for obtaining reasonably reliable results on unknown samples. An inter-laboratory exercise is being organized by Argonne National Laboratory to check the precision and to further investigate quantitative reliability of 13C n.m.r. measurements on coals from their Premium Coal Sample Bank.
IR and high-resolution solid-statel3C-NMR studies provided evidence for successive steps in coke formation upon reaction of ethene over the zeolite hydrogen mordenite. Adsorbed at room temperature, ethene polymerizes to branched and possibly also linear alkanes. Upon heating up to 500 K, the alkanes isomerize and crack. Meanwhile, the13C-NMR spectra show the formation of carbocations of alkylic, allylic and aromatic nature. Above 500 K, high-temperature coke develops under formation of alkylaromatics and subsequently small polyaromatic and/or polyphenylene molecules. Over dealuminated hydrogen mordenites, ethene also forms paraffinic deposits at low temperatures. Above 500 K, however, a substantial amount of C1C4 alkanes are formed together with unsaturated species.
Carbon aromaticities for a selection of 19 bituminous coals and vitrinite concentrates have been obtained by solid-state 13C n.m.r. using the inherently quantitative single pulse excitation technique. Within experimental error, the aromaticity values in the range 0.75–0.90 correlate extremely well with the ratios (A2 = 0.93); the 11 samples investigated with ratios between 0.74 and 0.77 all have aromaticities in the range 0.75–0.81. Furthermore, the correlation holds for the coals investigated with high inertinite contents.
Carbonaceous deposits (coke) formed on zeolites H-ZSM-5 and Linde L during methanol conversion at 460°C have been studied with transmission electron microscopy (TEM). At ca. 6% (w/w) of coke, deposits could just be detected, and at ca. 10% (w/w) the zeolite was enveloped in coke layers > 5 nm thick. Catalyst performance was not affected by coke deposits which were void of any structure in the transmission electron microscope (amorphous), but catalysts became deactivated when coke deposits contained packets of aromatic sheets of carbon parallel to the catalyst surface.
Catalyst deactivation due to coke formation is an important technological and economic problem in petroleum refining and in the petrochemical industry. Remedies to catalyst deactivation are sought by a variety of strategies involving modification of catalyst surface composition such as the use of polymetallic catalysts and/or by manipulation of the reaction environment which often limits the yield due to thermodynamic constrains (i.e., high hydrogen pressures, etc.). In the limit, when the activity reaches unacceptable limits, regeneration by burning off carbon residues can usually be attained, Regeneration can take place in situ, as in fixed-bed reactors, or in an adjacent reactor to which the catalysts is transported to, such as in moving-bed reactors or in fluidized-bed reactors. In the first case intermittent operation is required, whereas in the second case the operation is continuous, but a second regeneration reactor is required. The choice of the proper process cycle is an economic optimization problem constrained by catalysts cost, operational and regenerational cost, and by the value of the final product [1–5]. Process optimization under catalyst decay is an engineering problem that requires a knowledge of the catalyst deactivation kinetic.
The authors investigated the formation of coke on zeolite catalysts in the disproportionation of n-propylbenzene by means of [sup 129]Xe, [sup 27]Al, and [sup 13]C NMR spectroscopy. According to [sup 13]C CP-MAS (proton cross polarization with magic angle spinning) spectra, the coke formed in USY and ZSM-5 zeolites was classified into aliphatics and aromatics. It was found that aliphatic coke could be removed by a simple evacuation procedure. The location of the coke determined from [sup 129]Xe NMR spectra and xenon adsorption isotherms varies with the zeolite catalysts. For zeolite USY, the coke was formed inside the supercages and is mainly polyaromatic. For zeolite ZSM-5, the coke is alkylaromatic and tended to deposit at the openings of the channels rather than at their intersections. In regard to the mechanism, the reaction on USY followed bimolecular kinetics, whereas on ZSM-5 the reaction was monomolecular. 38 refs., 5 figs., 1 tab.
Over the course of the commercial fluid catalytic cracking (FCC), catalyst deactivation occurs both reversibly, as a result of side reactions that eventually yields coke, and irreversibly, due to contaminants present in the feedstock or to the dealumination of the zeolite catalyst component. Herein, we discuss the deactivation of HY zeolite and FCC catalysts from a fundamental as well as an applied point of view. Aspects related to the various causes of FCC catalysts (and additives) deactivation under industrial conditions are also summarized.
In order to understand the effects of acidity and acid strength distribution on the nature of coke formed over ZSM-5 zeolite in n-heptane aromatization reaction, a series of ZSM-5 samples dealuminated to different degrees are prepared. Microcalorimetric ammonia adsorption studies indicated the presence of very strong acid sites (ΔH>140kJ/mol) in the catalysts steamed at 400 and 500°C. 13C CP/MAS NMR along with dipolar dephasing is used to estimate the fraction of aromatic carbon and variation in aliphatic components with change in acidity of the ZSM-5 zeolites. Maximum selectivity to aromatics in n-heptane conversion is observed in the case of partly dealuminated ZSM-5 catalyst, whereas the aromatic nature of the coke is continuously decreased with steaming temperature, i.e. dealumination.
The changes in zeolitic structure (Y zeolite) of fresh, equilibrium, spent and regenerated FCC catalysts obtained from an Indian FCC refining unit were investigated by 29Si magic angle spinning (MAS) and cross polarization (CP)-MAS, 27Al MAS and multiple quantum MAS (MQMAS) NMR. Deconvoluted 29Si MAS NMR along with 29Si CP-MAS NMR of these four catalysts provide the detailed local structural changes occurring at all Si(nAl) sites. 27Al MAS NMR provides the relative changes of tetrahedral (Td) and octahedral (Oh) aluminum sites of the FCC catalysts at different stages of the cracker. Second order quadrupolar effect (SOQE) and quadrupole coupling constant (CQ) obtained from both computation of 27Al MQMAS and simulation of 27Al 1D MAS spectra of these catalysts give imposition of structural asymmetries in Td and Oh sites that can be correlated with catalytic activities. Local structural changes are explained by NMR and are supplemented by Si/Al ratio, shrinkage in unit cell size and crystallinity obtained from XRD and BET. Si(3Al) sites of Y zeolite are found to be more deactivated and may be the primary sites of coking. 13C single hard pulse excitation (SHPE)-MAS, CP-MAS, cross polarization-dipolar dephasing (CPDD) NMR and CP dynamics analysis of hard coke extracted from the spent catalyst from the FCC unit reveal essential structural properties and nature of coke.
Coking of a USHY was studied in a fixed-bed reactor at 450°C. Different model compounds were used: an alkane (n-heptane), an alkene (propene), a naphthenic (methylcyclohexane) and aromatics (m-xylene, 1-methyl-naphthalene). Those reactants present very different coking rates in the order: 1-methyl-naphthalene>propene⪢m-xylene>n-heptane>methylcyclohexane. The coke profile along the reactor is a function of the coke precursors. By using a triple-bed reactor, it is shown that the coking process proceeds primarily with olefins and aromatic reactants and secondarily with alkanes and naphthenes. In the first case, coke is preferentially deposited on the first part of the bed, whereas in the second case, the coke deposit is rather concentrated on the last part of the bed (after formation of coke maker molecules). GC-MS analysis has shown that whatever the reactant, coke is mainly constituted by small polyaromatic compounds with four to six aromatic rings located in the zeolite pores. Also, highly polyaromatic compounds located either inside the pores or on the external surface of zeolite crystallite were observed.
Spent alumina-based nickel-molybdenum catalysts used for hydroconversion of heavy oil or residuum upgrading are subjected to thermogravimetric analysis (TGA) to determine oil and coke contents simultaneously. TGA or differential TGA curves clearly indicate that oil and carbonaceous materials are driven off at 100-300 degrees C and 300-700 degrees C ranges, respectively. The amounts of oil, coke, and inorganic residue obtained by the single-step TGA procedure are found to be in excellent agreement with those obtained by conventional methods such as Soxhlet extraction for the oil content followed by calcining of the deoiled catalyst in a muffle furnace for the coke and inorganic residue contents. Since the TGA results at heating rates of 10, 20, and 30 degrees C/min are found to be statistically invariant, the analysis time can be as short as 26 min, which is advantageous for monitoring and control of catalyst performance in an operating unit.
Conventional and dipolar dephasing CP/MAS /sup 13/C NMR experiments are reported on 63 coals and coal macerals from lignite to anthracite ranks (from the US, the United Kingdom, and Australia). While the conventional experiment can yield only f/sub a/ (the fraction of carbon that is sp/sub 2/ hybridized), the dipolar dephasing experiments provide estimates of four other structural parameters. Examination of the dipolar dephasing data reveals an overall increase. While loss of substituents from aromatic rings with little aromatic cross-linking occurs until the anthracite stage is reached, the dipolar dephasing experiments also yield decay constants for different functional groups that are similar to the decay constants obtained with simple organic compounds. Because full characterization of a sample by the dipolar approach has been developed for obtaining less precise f/sub a/sup a,H/, f/sub a/sup H/, f/sub Me/, and H/sub a/ values. The data so obtained are particularly useful for quickly comparing samples. 43 references, 7 tables, 9 figures.
Scientific research is often compared to exploration. Expeditions (research programs) are mounted, discoveries made, and reports presented. As the information from the various expeditions accumulates, the time finally comes when a map may be drawn. Curiously, few maps of catalytic cracking have been attempted. The authors integrate related literature from the range of fields that impact on catalytic cracking, illustrate methodologies for studying complicated reaction networks, and detail important new information on catalyst selectivity and decay.
A combination of analytical techniques, including X-ray photoelectron spectroscopy (XPS), solid state 13C nuclear magnetic resonance (NMR) spectroscopy, and supercritical fluid extraction/mass spectrometry (SFE/MS), were used to characterize the detailed composition and structure of coke formed on catalyst in the fluid catalytic cracking (FCC) process. By characterizing coke samples from a series of designed FCC experiments, the effects of conversion on coke composition were systematically studied. SFE is shown to be an effective technique for removing low molecular weight coke molecules from the catalyst. When combined with mass spectrometry, the technique provided molecular level information of the extracted coke species. The coked catalysts were directly analyzed by XPS and NMR to obtain information relevant to surface and bulk coke structures, respectively. The study revealed the presence of two types of nitrogen-based coke and showed that N distributions were strongly affected by FCC conversion level. The study also suggests that most nitrogen-containing coke is formed in the earlier stages of cracking while hydrocarbons are the primary contributors to coke yield in the later stages of cracking. The aromaticity of coke remains fairly constant at high conversions.
It is now generally accepted that single pulse excitation (SPE) or Bloch decay measurements are essential for obtaining reliable aromaticity values for coals by solid-state C-13 NMR although such measurements are considerably more time consuming than cross-polarization (CP). Just as with normal CP, SPE can be combined with dipolar dephasing (DD) to derive nonprotonated aromatic carbon concentrations. This approach has been applied to four of the Argonne premium coal samples (APCS, N. Dakota lignite, Wyodak, Illinois no. 6, and Pocahontas), two vitrinite concentrates from UK bituminous coals, and an anthracite. The results have been compared with those obtained from CP and, for the APCS, nonprotonated aromatic carbon concentrations derived indirectly from FTIR and H-1 CRAMPS. As with the overall aromaticity, SPE invariably gives higher values than CP for nonprotonated aromatic carbon concentrations with differences of up to 10 mol % carbon being found, the greatest being found for the two low-rank coals. However, in some instances, reasonable agreement has been obtained with CP data when long contact times are employed to minimize the discrimination against aromatic carbons. The aliphatic H/C ratios in the range 2.0-2.5 derived from the SPE-DD measurements are much more consistent with other available structural information on coals than the much lower values generally derived from CP, FTIR, and H-1 CRAMPS.
In view of the results of this study and the well-documented inherent problems with cross-polarization (CP) C-13 NMR concerning quantification for coals, it is now clear that the more time-consuming single-pulse excitation (SPE) or Bloch decay measurements are essential for obtaining aromaticities and other carbon skeletal parameters for coals. SPE C-13 NMR has been carried out on the Argonne Premium Coal Samples at both a low and a high field strength (25 and 75 MHz, respectively), high-speed magic angle spinning (13 kHz) being used to suppress spinning sidebands at the higher field. Aromaticity values measured by SPE at low and high field were generally in excellent agreement and were consistently higher than those from CP, the greatest differences being found for the two low-rank coals in the Argonne suite. The use of tetrakis(trimethylsilyl)silane as an internal standard in the low-field measurements indicated that, in general, over 75% of the carbon in the coals is typically observed by SPE.
We report the structural variations of a number of coals and coal macerals. Through the time constants associated with dipolar-dephasing techniques, CP/MAS spectral data reveal the presence of segmental motion in certain low-rank coal samples. The motion detailed is in the aliphatic region and is thought to be due to CH2 groups associated with hydroaromatic and/or polymethylene structural units.
Coke has been concentrated from two deactivated FCC refinery catalysts via demineralisation to facilitate detailed characterisation by solid state 13C NMR. The catalysts were obtained from runs with a residue feed (5% Conradson carbon) and a hydrotreated vacuum gas oil (HVGO). As for solid fuels, the use of a low-field spectrometer in conjunction with the single pulse excitation (SPE or Bloch decay) technique has enabled quantitative carbon skeletal parameters to be obtained for the cokes. Internal standard measurements demonstrated that most of the carbon was observed by SPE and, therefore, NMR-invisible graphitic layers are not thought to be major structural features of the cokes. SPE gave much higher values for both the carbon aromaticities and the proportions of non-protonated aromatic carbon than the less quantitatively reliable cross-polarisation (CP) technique. Differences in feedstock composition were reflected in the structure of the cokes with the aromatic nuclei being more highly condensed in the residue-derived coke and corresponding to 15–20 pericondensed aromatic rings.
The molecular structure and composition of carbonaceous deposits commonly known as coke deposited inside the pores and on the surface of aged hydroprocessing (hydrotreating and hydrocracking) catalysts in value upgradation of vacuum gas oils were investigated by various analytical methods, such as NMR, HPLC and TGA. The soft (dichloromethane extractable) and hard (solvent insoluble) coke deposits from two spent hydrotreating catalysts (CoMo/γ-Al2O3 and NiMo/γ-Al2O3) and one spent commercial hydrocracking catalyst with carbon contents ranging from 5 to 10% were characterized. HPLC and high resolution liquid state 1H and 13C NMR methods were used for the characterization of the soft coke whereas hard insoluble coke deposits were characterized by high resolution solid-state 13C NMR and TGA. The results indicate that the composition of soft coke is mostly alkylated mono-, di-aromatics and fewer amounts of polyaromatics. The composition of the hard coke in spent hydrotreating catalysts is relatively high in aliphatic content with high H/C ratio obtained in the present study unlike the highly aromatic coke obtained with residue hydrotreating as reported in the literature. However, the insoluble coke deposits in spent hydrocracking catalysts are highly aromatic in nature (aromaticity, fa > 0.95).
Coke deposition on spent noble metal catalysts used in petroleum/petrochemical industries is of serious concern on account of its impact on the catalyst deactivation and consequent loss in the production yield. In order to counteract the effects of coke deposition, it is vital to know the location as well as the nature and composition coke deposited on the spent catalysts. In the present study, spent Pt-Sn/Al 2 O 3 catalysts used in the industrial reactors for selective dehydrogenation of C 10 –C 13 n-paraffins to mono-olefins at different coke loading (approximately 7–9%, w/w) were characterized. The characterization of coke deposits were analyzed by the combination of analytical techniques including HPLC, solid-state 13 C CP/MAS NMR, and TGA. Average structural information has been obtained from the quantitative analysis of NMR data. The results indicate that the nature of coke present in the soluble coke extracts of spent catalysts is rich in alkylated mono-and diaromatics with low percentage of polyaromatics whereas the nature of insoluble coke is highly polyaromatic (aromaticity, f a > 0.95). In addition, temperature programmed oxidation studies by TGA reveals that the coke is deposited on the dispersed metal as well as on the support.
The location and composition of the coke formed over H-ZSM-5 zeolites during the cracking of hexadecane at 350°C and at 20 atmospheres was studied. A solvent extraction technique using two different solvents located the coke either on the external surface or within the pores of the zeolite and GC-MS analysis characterised the soluble coke. Coke was also examined using 13C MAS NMR and high resolution TEM. Substituted benzenes were identified as the major precursors to coke formation, and the external coke had a graphitic structure. The contact time was shown to affect only the rate of formation but not the composition of the coke.
Chemical functionalities of hydrogen and carbon in a series of cokes obtained from heavy crude oils in a Mobile continuous flow laboratory coker pilot unit are probed with high-resolution solid state NMR. The fractions of aromatic hydrogen and carbon, as determined from 1H combined rotation and multiple-pulse spectroscopy (CRAMPS), and 13C magic angle spinning (MAS) experiment with and without cross polarization (CP), varied only slightly between 0.49 and 0.65 and between 0.88 and 0.92, respectively, for the samples studied. A comparison with the results of direct excitation (13C MAS) NMR showed that CP/MAS NMR spectra taken with a contact time of 1 ms well represented relative carbon intensities. The high-resolution spectra, in combination with previously reported wideline 1H NMR data and the results of elemental analysis, are used to derive several structural parameters, including aromatic and aliphatic hydrogen to carbon ratios and the average formula per 100 carbon atoms. Finally, the “average” structures for studied cokes are proposed and discussed. Most cokes are concluded to consist of molecules having approximately 10 aromatic rings bearing only few substitutions.
A link has been observed between the decline in catalyst activity and coke formation in the cracking of n-nonane. The link involves the chemistry and kinetics of the reaction, as well as the chemical structure of coke as observed by 13C CP/MAS-NMR spectroscopy. We find that the chemical structure of surface species normally analyzed as coke plays an important role in the activity of the catalyst. This we believe reflects the influence of the structure of surface carbenium ions on their reactivity in chain propagation and in olefin-forming, site-releasing desorption reactions. Both these processes are vital to the maintenance of catalyst activity. As the average surface species becomes more dehydrogenated, it also becomes less reactive both in bimolecular chain propagation with gas phase reactant molecules and in olefin formation by desorption. This time-dependent loss of reactivity of the average carbenium ion manifests itself in a decline in total catalyst activity with time on stream.
Fluid catalytic cracking (FCC) feeds from four Indian refineries are structurally characterized by 1H, gated-decoupled 13C, distortionless enhancement by polarization transfer (DEPT) and 2D 1H–13C HETeronuclear CORrelation (HETCOR) and other 2D nuclear magnetic resonance (NMR) methods. Detailed structural analyses are completely supported by a range of NMR information including chemical shifts of 1H and 13C, CHn type distributions and 1H –13C connectivities. The average structural parameters like branching sites, average number of branching per molecule, average length of side chains, percentage of saturates, aromatics and naphthenes are obtained from these NMR data. A novel approach based on “multipoint spline base line correction” is employed for estimation of naphthenes and n-paraffins that gives better quantitative estimation than the conventional methods. In this paper, importance is given to the study of those structural parameters that plays a key role in cracking chemistry as well as coke forming tendency of the feedstock. To the best of our knowledge, this is the first attempt to characterize and quantitatively estimate compositions of the high boiling fractions of petroleum feed by NMR methods and especially the complex structure of vacuum gas oil (VGO) fractions used in Indian refineries. The importance of this paper is to help in optimizing the product slate of Indian refineries through proper feedstock blending using few hundreds of million metric tons (MMT) of crude oil consisting of blends of light crudes with different heavy crudes and bottom of the barrel due to escalating cost of crudes.
The modes of coking and of deactivation of zeolites during n-heptane cracking at 723 K were established on the basis of (i) the composition of the carbonaceous compounds responsible for deactivation (coke), (ii) the deactivating effect of the coke molecules and (iii) the reduction by coke of the volume accessible to nitrogen and to n-hexane (kinetic diameter similar to n-heptane). The zeolites [USHY, H Mordenite (HMOR), HZSM5 and H Erionite (HERI)] were chosen to determine the effect of different parameters of the pore structure: (i) pore size, (ii) existence (USHY, HERI) or non-existence of cavities (HMOR, HZSM5), (iii) the possibility for the reactant to diffuse unidirectionally (HMOR) or tridirectionally. The retention of coke molecules is due to trapping in the cavities (or at channel intersections). Their size is intermediate between that of the apertures and that of the cavities (or channel intersections). The coking rate is all the faster when the space available near the acid sites is large and when the coke precursors desorb slowly. On all the zeolites, coke formation occurs through oligomerization of the olefinic cracking products followed by cyclization of the oligomers, transformation through hydrogen transfer into monoaromatics, alkylation of these monoaromatics, then cyclization and hydrogen transfer to give bi-aromatics, tri-aromatics, etc. There is no site poisoning by coke; deactivation occurs through the three following modes: (i) limitation of the access of n-heptane to the active sites, (ii) blockage of the access to the sites of the cavities (or channel intersections) in which the coke molecules are situated and (iii) blockage of the access to the sites of the pores in which there are no coke molecules.
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After reviewing some recent studies on the characterisation of coke deposits on fluid catalytic cracking (FCC) and hydroprocessing catalysts by solid state 13C-NMR, the quantitative structural information that has been obtained through the use of demineralisation of FCC catalysts to provide coke concentrates for analysis will be described. The deactivated catalysts investigated contain only approx. 1% (w/w) carbon and were obtained both from refinery units operating with heavy feeds and from laboratory fluidised-bed tests with n-hexadecane. As for other carbonaceous materials, the use of a low-field field strength in conjunction with the single pulse excitation (SPE or Bloch decay) technique has enabled most of the carbon to be detected and, therefore, NMR-invisible graphitic layers are not thought to be major structural features of the cokes. Although stripping the catalysts gives rise to highly aromatic cokes (aromaticity>0.95), even for n-hexadecane, differences in feedstock composition are still reflected in the structure of the resultant cokes with those derived from n-hexadecane containing less condensed aromatic nuclei than those from heavy feeds.
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