Coke Burn-off Research Articles

Dual-Bed Plasma/Catalytic Synergy for Methane Transformation into Aromatics

Direct conversion of methane to aromatics (benzene, toluene, and xylene, briefly BTX) remains a formidable challenge. Here, we demonstrate that CH4 can be efficiently transformed into BTX through the coupling of a dielectric barrier discharge (DBD) plasma reactor with a thermocatalytic aromatization reactor (at temperatures ≤ 550 °C). With the assistance of DBD plasma, methane can be activated by the energetic electrons to form various radicals, which are recombined to produce a mixture of C2–C6 light alkanes and alkenes. With a plasma power deposition of 12 W, the conversion of CH4 is up to 50% dependent upon the partial pressure. The energy efficiency is between 0.3 to 1.5 mol/(kW h) dependent upon the power deposition. The C2–C6 light hydrocarbons from the plasma reactor can be transformed into BTX over various catalysts at temperatures ≤ 550 °C. With the Ga/HZSM-5 catalyst, the selectivity of total BTX is up to 70% dependent upon the reaction conditions (temperature, partial pressure, and catalyst loading). With such a dual-bed plasma/catalytic tandem system, CH4 conversion and BTX yield only slightly deactivated with time-on-stream up to 1000 min and can be 100% regenerated through coke burnoff. We suggest that the dual-bed plasma/catalytic reaction system could be interesting for small-scale onsite flare gas or biogas conversion.

Non-Destructive characterisation of coke deposit on FCC catalyst and its transient evolution upon Air-Firing and Oxy-Fuel regeneration

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 exothermic C-O2 reactions. As a result, the overall coke including other two species, graphite and aliphatic hydrocarbons were consumed quickly in the O2/CO2 case.

Zeolites as sustainable catalysts for the selective synthesis of renewable bisphenols from lignin-derived monomers.

Alternative biobased bisphenols from lignocellulosic biomass are not only favorable to reduce the environmental impact of current petroleum-derived plastics, but they can be simultaneously beneficial for health issues related to bisphenol A (BPA). Additionally, conventional BPA synthesis entails a large excess of unrecoverable homogeneous acid catalyst (e.g., HCl) or unrecyclable thermolabile sulfonated resins. In this report, zeolites are proposed as recoverable and thermally stable solid acids for the Brønsted-acid-catalyzed condensation between 4-methylguaiacol and formaldehyde to selectively produce renewable bisphenols. It is found that the Brønsted-acid-site density plays a pivotal role for catalyst performance. In particular, the cheap and environmentally friendly FAU 40 exhibits outstanding activity (turnover frequency of 496 h-1 ) and selectivity (>95 %), outperforming even the best benchmark catalyst. Additionally, the zeolite can be easily recycled without activity loss after regeneration by coke burn-off. The catalytic zeolite system also seems very promising for other lignin-derived alkylphenols, alkylguaiacols, and alkylsyringols.

Metals modification of the paraffin dehydrogenation catalyst regenerated by coke combustion

The industrially deactivated Pt0.5-Sn1.5-K0.5-Mg1/γ-Al2O3 catalyst for dehydrogenation of long chain paraffins (n-C10°–C13°) was regenerated by coke burn off and metals (Pt, Sn and K) modification. The catalysts were characterized by TG, BET, CO-chemisorption, H2-TPR, XRD and mechanical crushing strength test. Paraffin dehydrogenation was performed on a bench reactor at the industrial site. Results showed that temperature increase during coke combustion can be effectively controlled with O2 content lower than 1%. The combustion temperature should be higher than 500°C to burn all the coke and lower than 550°C to avoid Pt sintering. Under optimal conditions, coke combustion results only in 92% recovery of catalyst activity. Subsequent metals modification with additional 0.01wt.% Pt addition can recover catalytic performance completely, paraffin conversion and olefin selectivity to the levels of fresh catalyst. The modified catalyst has longer life-time of 77 days compared to fresh catalyst of 69 days. Results are explained in terms of the characteristics of the catalysts.

Selective, one-pot catalytic conversion of levulinic acid to pentanoic acid over Ru/H-ZSM5

The direct conversion of levulinic acid (LA) to pentanoic acid (PA) has been studied with six 1 wt% Ru/H-ZSM5 catalysts at 40bar H2 and 473K in dioxane. The influence of ZSM5 cation form, Si/Al ratio and ruthenium precursor on metal dispersion and acidity has been assessed. A highly active bifunctional 1 wt% Ru/H-ZSM5 catalyst was developed to give a PA yield of 91.3% after 10h. The PA productivity of 1.157molPAg−1Ruh−1 is the highest reported to date. The simple preparation method allows for a significant fraction of ruthenium to be located inside the zeolite pores, providing the desired proximity between the hydrogenation function and the strong acid sites, which is key to the conversion of LA into PA. Coke buildup during reaction causes some deactivation, but activity can be almost fully restored after catalyst regeneration by simple coke burn-off.

High-Throughput Investigation of Catalysts for JP-8 Fuel Cracking to Liquefied Petroleum Gas

Portable power technologies for military applications necessitate the production of fuels similar to LPG from existing feedstocks. Catalytic cracking of military jet fuel to form a mixture of C₂-C₄ hydrocarbons was investigated using high-throughput experimentation. Cracking experiments were performed in a gas-phase, 16-sample high-throughput reactor. Zeolite ZSM-5 catalysts with low Si/Al ratios (≤25) demonstrated the highest production of C₂-C₄ hydrocarbons at moderate reaction temperatures (623-823 K). ZSM-5 catalysts were optimized for JP-8 cracking activity to LPG through varying reaction temperature and framework Si/Al ratio. The reducing atmosphere required during catalytic cracking resulted in coking of the catalyst and a commensurate decrease in conversion rate. Rare earth metal promoters for ZSM-5 catalysts were screened to reduce coking deactivation rates, while noble metal promoters reduced onset temperatures for coke burnoff regeneration.

Role of steam, hydrogen and pretreatment in chrysotile gas–solid carbonation: Opportunities for pre-combustion CO2 capture

Enhancing gas–solid carbonation of serpentines could prefigure the emergence of new dry processes that may compete with aqueous carbonation. The hypothesis of pre-combustion CO2 capture to decarbonize shifted syngas in integrated gasification combined cycle plants was examined in this work from a chemistry standpoint. A systematic gas–solid carbonation study of chrysotile was carried out in a basket reactor using model (H2O/H2/CO2) shifted syngas where CO2 uptakes were measured after 1h carbonations between 100°C and 220°C at 3.2MPa total pressures. Partial dehydroxylation and steam mediation substantially enhanced meta-chrysotile carbonation achieving uptakes as high as 0.7 CO2 moles per Mg mole at 130°C. Also, combined X-ray powder diffraction, X-ray photoelectron spectra and CO2 uptake studies indicated that the presence of H2 did not prevent carbonation of meta-chrysotile nor derailed Mg availability towards hydrides or metallic Mg. Chrysotile pre-dehydroxylation in the conditions of gasification was also emulated using catalytic steam reforming of a model-tar compound. However, post-carbonation of spent chrysotile, before and after coke burn-off, achieved at best ca. 0.02 CO2 uptakes suggesting that using the same magnesium silicate batch in gasification then in carbonation may be impractical.

Direct detection of coking and regeneration of single particles and fixed bed reactors by electrical sensors

The activity of solid catalysts is often reduced by the formation of coke. Thus, regeneration by coke burn-off is needed from time to time. A new method to monitor in situ the coke load during coking and decoking by electrical sensors is presented, which could be used as a controlling instrument of high value. Single particles of an Al 2O 3 catalyst were electrically contacted and characterised by impedance spectroscopy. A clear relationship between coke load and the impedance is observed. The sensor was tested by regeneration experiments both with single particles and in a coked fixed bed reactor. The results show that the coke burn-off within a single coked catalyst particle can be monitored and that it is possible to distinguish how strong the decoking of a single particle is influenced by pore diffusion. For a fixed bed, the axial coke profile can be monitored by means of axially distributed sensors, and the velocity of the reaction front and the length of the reaction zone are directly deduced by the local change of the coke profile with time.

Direct detection of coke deposits on fixed bed catalysts by electrical sensors

The activity and the rate of deactivation of heterogeneous fixed bed catalysts are substantially influenced by the formation of coke deposits. In the chemical industry, especially in refineries and in petrochemical processes, the continuous direct and in situ monitoring of the level of the coke load in a catalytic reactor would be an additional controlling instrument of high value. In order to contribute to the development of such sensors, a representative pellet of an alumina-based chromium-oxide-coated catalyst was used in this work as a model system. The pellet was contacted and electrically measured by impedance spectroscopy. By combination with a gravimetrical method, a clear relationship between coke loading and the respective electrical signal could be observed, both during coke formation and during the regeneration by coke burn-off.

Regeneration of coked catalysts—modelling and verification of coke burn-off in single particles and fixed bed reactors

The regeneration of a coked naphtha reforming catalyst (Pt/Re– Al 2 O 3 ) was studied by kinetic investigations on the effective rate of coke burn-off. For temperatures of industrial relevance for the catalyst, i.e., below 550 ∘ C (deactivation), the coke burn-off within the cylindrical particles ( d P ≈ 2 mm ) is determined by the interplay of chemical reaction and pore diffusion; limitation by external mass transfer can be excluded for T < 750 ∘ C . Based on the parameters of the intrinsic kinetics and of the structure of the catalyst (porosity, tortuosity), the regeneration process is modelled and discussed both on the level of a single particle and in a technical fixed bed reactor. The results of modelling are compared with data from lab-scale investigations (coke profiles within the particles) and the performance data of the regeneration in an industrial fixed bed reactor (moving reaction zone); the agreement of calculation and measurement is in both cases complete.

Influence of chemical reaction rate, diffusion and pore structure on the regeneration of a coked Al 2O 3-catalyst

As a contribution to a better basic understanding and an accurate modelling of the regeneration of coked catalyst particles, the following investigations were done with pure Al 2O 3 as a model catalyst: (1) determination of the intrinsic and effective kinetics of coke burn-off; (2) characterisation of the catalyst’s morphology with respect to the influence of the carbon load on the surface area, porosity, pore diameter, and tortuosity; (3) measurement of radial coke profiles within partly regenerated particles; (4) numerical simulation of the regeneration within coked particles and comparison with experimental data of radial coke profiles and the time needed for a certain degree of burn-off. For the modelling of coke burn-off within single particles, the chemical reaction rate, pore diffusion, radial gradients of the O 2- and the carbon-concentration, and the influence of the carbon load on the porosity and tortuosity have to be considered. Only the resistances of external heat and mass transfer and of intraparticle heat conduction can be neglected, at least for particle sizes and temperatures of technical relevance for fixed beds (<5 mm, <700 °C). The measured and numerically simulated data according to this model presented in detail are in good agreement. The results show that temperatures above about 400 °C are needed to achieve regeneration within an acceptable timeframe. On the other hand, a temperature of more than about 500 °C will not anymore accelerate the burn-off, at least in case of fixed bed reactors with particles in the region of mm. This effect can be attributed to the increasing strength of pore diffusion resistance, and eventually the complete resistance is confined to the outer carbon-free shell.

Coke burnoff in a typical FCC particle analyzed by an SEM mapped 2-D network pore structure

The pore morphology of a selected ( ≅70 μm ) fluid catalytic cracking (FCC) catalyst particle, as viewed on a SEM image of sectioned surface of low melting point alloy (LMPA) impregnated FCC catalyst sample, was mapped onto an approximately ‘equivalent’ 2-D 30×30 pseudo-random pore network to investigate the consequences for burnoff characteristics in the particle. The mapped 2-D structure was constructed from a set of pores having a psd close to that given by BJH adsorption results by preferentially assigning larger macropores to corresponding apparent locations in the SEM image while the rest of the meso- and micro-pores were then randomly assigned across the rest of the network. Major parameters such as pore structural configuration, residual coke, and spatial oxygen profile were tracked as a function of time. The model results were compared to experimental coke burnoff data, carried out in a micro-scale reaction unit. The model prediction fits fairly well within the experimental fluctuations. The results also showed that over 90% coke is burnt off within the first 50% of reaction time. For comparative purposes, the model predictions were also applied to a ‘shuffled’ structure, produced by random permutation of the same radii of the mapped structure as well as an ‘optimally sparsed’ structure. Burnoff in the shuffled structure exhibited the worst performance. This was attributed to the decrease in the proportion of larger pores on the network skin in contrast to the mapped and optimally sparsed structures. Since visual inspections of SEM images of most particles tend to reveal a randomly oriented pore configuration (whereas the mapped structure was for a selected particle), the predicted aggregate performance of the FCC catalyst sample studied would be expected to lie somewhere in between the curves of the shuffled and mapped structures. The support pore architectural design of those particles could, therefore, still be improved for better performance in terms of accessibility, reactivity and selectivity.

Neural network kinetic prediction of coke burn-off on spent MnO2/CeO2 wet oxidation catalysts

Combustion kinetics of coke laydown on wet oxidation catalysts is studied by means of temperature-programmed oxidation “TPO” and mass spectrometry “MS” in the temperature range from 30 to 600°C. The study is designed to allow a better understanding of the influence of wet oxidation conditions (catalyst-phenol contacting time and temperature) on the combustion kinetics of coke deposited on MnO2/CeO2 and 1wt.% Pt-MnO2/CeO2 catalysts during phenol degradation. In this respect, the experimental procedure involves the continuous monitoring of carbon oxides and O2 fluxes resulting form the combustion of carbonaceous deposits in a 5% O2/He mixture. Based on the experimental data, an artificial neural network-based (ANN) modeling approach is implemented to represent, as accurately as possible, the complex combustion phenomenon so as to provide the opportunity to predict its evolution.In this context, the resulting ANN model is used as a “black-box” to approximate the complex non-linear conversion rate of the wet oxidation coke. The conversion rate is thus, expressed in terms of the TPO ramp temperature, running oxygen concentration, wet oxidation temperature, and phenol oxidation time.The proposed ANN-based modeling approach proved to be an accurate, reliable and effective tool for the quantification of the coke burn-off kinetics. The method has then great potential as a means to compensate for the lack of efficient phenomenological kinetic modeling techniques. It can also be used in the design of regenerative units downstream of catalytic wastewater treatment reactors based on wet oxidation.

Kinetics of carbon oxide evolution in temperature-programmed oxidation of carbonaceous laydown deposited on wet oxidation catalysts

The combustion kinetics of coke laydown on wet oxidation catalysts was studied by means of temperature-programmed oxidation and mass spectrometry within the temperature range (30–600°C). The coke deposits were formed over three different catalysts 1 wt.% Pt/Al 2O 3, MnO 2/CeO 2 and 1 wt.% Pt–MnO 2/CeO 2 during phenol deep oxidation in a three-phase slurry reactor at various reaction conditions (exposure time, temperature, oxygen pressure, catalyst loading). The carbon oxides, oxygen and water fluxes arising from the combustion of the carbonaceous deposits in a 5% O 2/He mixture, were continuously monitored. In all cases, unimodal quasi-Gaussian distributions were obtained for CO 2 while no CO was detected. These evolutions were successfully described by a modified “fractal power-law” grain model. The coke-dependence of the carbon dioxide profiles was related to the fractal dimension of the catalyst surface and to the oxygen partial order during coke burn-off. The corresponding change in O 2 partial order was ascribed to competition between three steps in the combustion mechanism: non-dissociative O 2 chemisorption, interaction of oxygen with undissociated dioxygen bearing surface species, physical desorption of the complex oxide as carbon dioxide.

Support-pore architecture optimization in FCC catalyst particles using designed pore networks

SEM images of FCC catalyst particles reveal the internal pore space to consist of massive networks of randomly interconnected pores of varying sizes and orientation. Until recently, design of catalyst particles has been restricted mainly to composition, particle size and pore size distribution with little attention paid to pore architecture. With recent advances in computing technology and catalyst characterization techniques it is now possible to move from these ‘jumbled’ configurations towards more structured controlled pore architectures that could greatly enhance effective utilization of catalyst pore space. In this paper, an application of 2D stochastic networks to investigate the direct influence of pore assembly on diffusion and reaction in FCC catalyst particles is described. Coke burn-off in heavily coked particles was used. Various pore architectural structures were tested including random, positively spiralled, negatively spiralled, structured and interspersed 2D networks. The interspersed network exhibited fastest burn-off kinetics relative to other structures while the random configuration, which probably characterizes most current catalyst particles, showed results better only than the negatively spiralled network. This can be attributed to enhanced reactant accessibility resulting from absence of transport inefficient micro- and macro-pore clusters and an increase in direct micro–macro pore links characteristic of the designed interspersed network.

Gegeneration of Deactivated β-Zeolite by Coke Oxidation

An accurate description of coke burn off is obtained from a catalyst based on β-zeolite and used for benzene alkylation with propylene giving cumene by using the thermogravimetric technique. A simple empirical kinetic model was successfully applied to interpolate the data of thermogravimetric analysis performed on samples after partial burn off. Different temperatures, partial pressures of oxygen and gas flow rates were the variables studied in order to calculate the apparent rates and the activation energy for the coke oxidation reaction and to outline the best conditions for the industrial regeneration procedure of our proprietary catalyst PBE-1 for cumene synthesis. Combining the unusually long lifetime per reaction cycle with the optimized regeneration procedure leads to an outstanding overall catalyst life.

Carbon Dioxide Jet Spray Polishing

The coking and regeneration of a reforming catalyst was studied by physical characterization methods (pore volume, tortuosity, porosity, carbon distribution) as well as by kinetic investigations on the reaction rate of coke burn-off. For temperatures of industrial relevance for the Pt/Re-Al2O3 catalyst, i.e. below 550°C (deactivation), the bum-off rate is determined by the interplay of chemical reaction and pore diffusion; limitation by external mass transfer can be excluded. Based on the kinetic parameters, the process of the regeneration of a technical reactor is discussed.

Regeneration of a Deactivated Hydrotreating Catalyst

A commercial NiMo/Al2O3 catalyst deactivated in a shale oil hydrogenation plant was submitted to several regeneration procedures. The results show that direct coke burnoff with an oxidizing mixture containing low oxygen levels (1.6% v/v O2) was more efficient in recovering the catalytic activity and the textural properties of the spent catalyst than reductive treatments (5% v/v H2) or sequential treatments of solvent extraction with acetone or cyclohexane/regeneration. Diffuse reflectance spectroscopy results suggest that, after the regeneration step, there was some nickel promoter loss due to nickel spinel (NiAl2O4) formation. The spinel formation explains why the catalytic activity of the fresh catalyst was not fully recovered after the regeneration of the spent catalyst, and its formation might be attributed to an unobserved temperature increase in the catalytic bed during the carbon burnoff step.

CATCRAK: A Process Simulator for an Integrated FCC-Regenerator System

A comprehensive integrated simulator has been developed to simulate the riser and the regenerator of a fluid catalytic cracking (FCC) process. The gas-oil feed sent to the reactor of an FCC unit is cracked to yield gasoline and light fractions. The simulator predicts the yield of C-lump (C 1 -C 4 + H 2 ), gasoline, light fuel oil, heavy fuel oil, regenerator temperature, coke burn-off, and the flue gas composition for different feed characteristics and operating conditions (temperature, pressure, catalyst circulation rate). It can also be used for trouble-shooting, optimization, and control of the process, so as to obtain the maximum yield of cracked products. The simulator integrates Weekman's 10-lump kinetic model and plug flow hydrodynamics for the riser reactor with Errazu et. al's grid model for the regenerator. A reactor-regenerator heat balance has been incorporated in the simulator to calculate the required catalyst to oil ratio. On testing the simulator over a wide range of operating conditions the predicted C-lump, gasoline, light fuel oil, and heavy fuel oil yields show similar results when compared with the existing limited information in the literature.

Partial oxidation of methane to formaldehyde by an abrupt-cooling method

Samples of commercial Pt-Re/Al2O3 catalysts coked during plant operation or in laboratory were decoked with ozone-air and oxygen-nitrogen mixtures. The coke burnoff with ozone occurs at lower temperatures than the one with oxygen. As ozone is decomposed at high temperatures, its burning capacity presents and optimum at about 400 K. The coke burning with ozone is nonselective, while the burning with oxygen is selective, being firstly removed the coke deposited on the metallic function. This difference is ascribed to the different mechanisms of the burning process: controlled by the step of pore diffusion in the case of ozone and by the chemical reaction in the case of oxygen.