Hydroprocessing Catalysts Research Articles

Catalytic pyrolysis of Arab Heavy residue and effects on the chemistry of asphaltene

In an attempt to understand the changes in asphaltene structure during pyrolysis, hydrothermal reactions were performed on Arab Heavy atmospheric residue using FCC catalyst, ZSM-5 zeolite, hydrocracking catalyst (HC-1) and hydroprocessing (NiMo) catalyst in a bench-scale fixed bed reactor. The effect of four different catalysts on pyrolysis reactions showed that the cracking of residue is affected by catalyst type. All four catalysts used in this study are good for achieving hydrocracking selectivity and were found to be effective in conversion of large asphaltene molecules into smaller moieties. Detailed analyses of asphaltenes were carried out using IR, NMR and HP-GPC techniques. These analyses indicated that the mean molecular weight distribution of residue and asphaltenes decreased after pyrolysis reactions indicating the breaking up of larger molecules. The NMR showed the increase of aromatics carbon and aromatic proton contents in asphaltene obtained from all reactions except when using ZSM-5 catalyst. IR analysis showed that there were significant changes in the chemistry of asphaltenes after the pyrolysis of residue. The pyrolysis reactions were performed and the following reaction conditions were maintained; reaction time: 1 h, temperature 400 °C, feed amount (Arab Heavy residue): 10 g, catalyst amount: 5% of feed (0.5 g) and hydrogen gas pressure: 900–1300 psi. The compositional data generated by this work will enhance the understanding of asphaltene structure that will be of great value in improved refining practices, geochemical studies and in assessing performance properties of asphalt.

Hydroprocessing catalyst deactivation in commercial practice

Hydroprocessing catalysts are susceptible to deactivation by several mechanisms, e.g. coke formation, metals deposition and active phase sintering, in their life cycle. Typical examples of catalyst deactivation are presented, showing the phenomena that may be encountered in commercial hydroprocessing units. Spent samples of hydroprocessing catalysts were obtained from various commercial units, as well as pilot plants. The cause(s) of deactivation were identified by analyzing spent catalyst samples for coke and metal content, pore size distribution and the distribution of metals (by SEM-EDX and STEM-EDX). Catalysts applied in the hydroprocessing of light feeds (middle distillates) mainly deactivate due to coke deposition and possibly sintering. Poisoning by metal deposition becomes significant during hydroprocessing of heavy feeds (like VGO and residue) as well as upgraded streams (e.g. coker products). Once the possible causes are known, deactivation mitigation strategies can be defined, which comprise optimizing the catalyst properties, catalyst loading and reactor operation.

Hydroprocessing of jatropha oil and its mixtures with gas oil

Hydroprocessing catalysts, sulfided Ni–W/SiO2–Al2O3, Co–Mo/Al2O3 and Ni–Mo/Al2O3 have been developed, and their performances in hydroprocessing of jatropha oil and its mixtures with refinery gas oil compared in terms of, detailed product distribution in order to optimize the catalyst and conditions that can give maximum yield of desired transportation fuel such as diesel or kerosene (jet). C15–C18 hydrocarbon yield (diesel range) is highest (97.9%) over Ni–Mo catalyst, while it is 80.8% over Ni–W catalyst and surprisingly low (49.2%) over Co–Mo catalyst. Jatropha oil with high as well as low free fatty acid (FFA) contents could be hydroprocessed with little observable effect on reactor metallurgy. The isomers to n-paraffins (i/n) ratio is very low and different for the three types of catalysts- nearly 22–36 times higher for the hydrocracking (Ni–W) catalyst than that for the hydrotreating (Ni–Mo) catalyst. The hydrodeoxygenation pathway for oxygen removal from triglyceride is favored over the fresh Ni–Mo and Co–Mo catalysts, while decarboxylation/decarbonylation pathway is favored over the Ni–W catalyst. But, resulfidation of used Ni–Mo catalyst results in decarboxylation/decarbonylation route being slightly more favored. The yield of diesel range (250–380 °C) product during co-processing varied between 88–92% for the Ni–Mo catalyst. Hydrodesulfurization of gas oil is better during co-processing with jatropha oil. The activation energy for overall S-removal is much lower than that for overall O-removal. Densities of the products were also observed to meet the required specification.

Bioleaching of Nickel, Vanadium and Molybdenum from Spent Refinery Catalysts

Spent catalysts represent a large amount of refinery solid waste. In particular, hydro-processing catalysts contain base valuable metals, such as nickel, vanadium and molybdenum and, for their toxic component, these wastes have been classified as hazardous by the Environmental Protection Agency in the USA. The development of an innovative eco-sustainable process for the valorisation of such wastes would undoubtedly give significant advantages also taking into account primary resources preservation. This paper deals with bioleaching of metals from hazardous spent hydro-processing catalyst by means of iron/sulphur oxidizing bacteria. The exhaust catalyst was rich in nickel (45 mg/g), vanadium (44 mg/g) and molybdenum (94 mg/g). Before bioleaching, the solid was washed by means of a mixture of Tween 80 and ethyl alcohol, for hydrocarbon removal. The effects of elemental sulphur, ferrous iron and actions contrasting a possible metal toxicity (either the presence of powdered activated charcoal or the simulation of a cross current process by means of filtration stages in series) was investigated. Ferrous iron resulted to be essential for metal extraction and for bacteria adaptation. Nickel and vanadium were successfully bioleached in the presence of iron, reaching extraction yields of 83% and 90%, respectively; on the other hand extractions around 50% for nickel and vanadium were observed both in biological systems in the absence of iron and in the chemical controls with iron. As concerns molybdenum, the highest extraction yields experimentally observed was about 50%, after 26 days bioleaching in the presence of iron, while a maximum extraction of 25 % was observed in the other treatments. In conclusion, a bio-oxidative attack with iron could successfully extract nickel, vanadium and partially molybdenum. Further actions aimed at contrasting a possible metal toxicity resulted not to be effective.

Influence of sulfuric acid baking on leaching of spent Ni–Mo/Al 2O 3 hydro-processing catalyst

A low temperature sulfuric acid baking and mild acid leaching process was explored for maximum dissolution of metal values from a spent Ni–Mo/Al 2O 3 hydro-processing catalyst obtained from a Korean refinery. The spent catalyst was found to be a matrix of γ-Al 2O 3. containing 27.1% Al, 11.6% Mo, 2.5% Ni, 9.7% S, 4.8% C, 1.9% P, 0.4% Si and trace amounts of Co and Fe. Various baking parameters such as baking temperature, sulfuric acid concentration and baking duration were optimized according to 2 3 full factorial method for maximum dissolutions of Mo, Al and Ni during leaching. Under optimum baking conditions (400 °C; 1.28 stoichiometric H 2SO 4; 1 h), > 96% Mo, 99% Al and 95% Ni were readily dissolved from the spent catalyst by 2% v/v H 2SO 4 at 80 °C. Carbon and sulfur analyses of the baked samples and leaching residues indicated only 10–15% of residual hydrocarbons reacted during acid baking, while most of the sulfur (assumed to be metal sulfides) was converted into soluble sulfates/oxy-sulfates. By comparison, direct sulfuric acid leaching of the catalyst resulted in poor dissolution of Mo and Al, even with large excess of acid, thus demonstrating the beneficial effect of sulfuric acid baking.

Unsupported transition metal sulfide catalysts: 100 years of science and application

For more than 100 years the transition metal sulfides (TMS) have been mainstays of hydro-processing fuels and upgrading bitumen and coal. Every refinery in the world uses them everyday to remove sulfur and other pollutants from transportation liquid fuels. As environmental regulations increase the need for improved active and selective TMS will continue to grow. This need can only be met through increased understanding and research on the next generation of TMS catalysts. In this report we outline the growth in fundamental studies of structure/function in the TMS and suggest are where improved understanding is needed. An understanding of the fundamental properties that lead to both the activity of the simple binary sulfides and the mechanism by which two metals (Co+Mo) acted together to enhance activity (promotion) has been developed. Initial efforts focused on supported commercial catalysts with limited success. In the early 1980s the periodic trends of TMS catalysts on unsupported catalysts were discovered and these results formed the foundation for further basic understanding of the key properties that led to catalytic activity. These results have been extended over the years to include supported catalysts and many petroleum based substrates. Progress has been made by combining synthetic, experimental and theoretical techniques. Theoretical studies support the fact that the d electrons in the frontier orbitals of the catalysts were key in determining catalysis at the surface. The triumph of this approach was that it unified the promoted TMS systems with the binary TMS and provided a common rational for the activity of both. Constant progress since then has been achieved through the application of density functional theory (DFT) narrowing the gap between instinct and a formal description of catalyst structure/function made by combining synthetic, experimental and theoretical techniques. Theoretical studies support the fact that the d electrons in the frontier orbitals of the catalysts were key in determining catalysis at the surface. The triumph of this approach was that it unified the promoted TMS systems with the binary TMS and provided a common rational for the activity of both. Constant progress since then has been achieved through the application of DFT narrowing the gap between instinct and a formal description of catalyst structure/function.It is crucial to remember that for real understanding to develop we must study the catalytically stabilized materials and not materials that are changing under catalytic conditions. In the case of the TMS this means that we must study materials like MoS2−xCx and RuS2−xCx. It has been demonstrated that “surface carbides” are the catalytically stabilized state under hydro-treating conditions. The original relation between the d electrons and later DFT calculations all point to the importance of these electrons in the catalytic reaction. However, more work is needed to define the relation between these electrons and the stabilized carbide surfaces before detailed active site structures can be developed with confidence. In addition the presence of Co metal in active hydro-processing catalysts stabilized for four years in a commercial reactor, calls in to question current theories of the structure of promoted catalysts.

NiMo supported acidic catalysts for heavy oil hydroprocessing

Large pore diameter Al 2O 3-SiO 2 acidic mixed oxide supports were prepared by using homogeneous co-precipitation method. The acidic properties of supports (oxide) and supported catalysts (sulfided) were chemically probed with cumene cracking (HCR) into propane and benzene at atmospheric pressure and 400 °C. NiMo supported hydroprocessing catalysts activities were also evaluated with heavy Maya crude oil. The use of acidic support is to optimize yield of high octane gasoline components, i.e. selective cracking of complex crude oil molecule. The supported fresh and spent catalysts were characterized by means of N 2 adsorption–desorption and SEM–EDX spectroscopic techniques. The results point out that deactivation takes place mainly at the entrance of pore due to coke deposition, while the depositions of vanadium and nickel sulfides mainly depend on the diameter of the pores. The results also indicated that the deactivation due to carbon deposition is carried out at initial hours of the crude oil processing. The large pore diameter catalyst contains higher amounts of deposited species, which are distributed proportionally along the extrudate radius, while for smaller pore diameter catalyst the deposition mainly occurs on the extrudate surface.

Metal Leaching from Spent Petroleum Catalyst by Acidophilic Bacteria in Presence of Pyrite

This paper describes studies on the recovery of metals from spent hydro-processing catalyst using mixed acidophilic culture in presence of pyrite. This culture was initially grown in the 9K- medium (absence of 9 g/L Fe(II)) where ferrous sulphate (FeSO4) was replaced by pyrite, and then applied in this bioleaching study. Bacterial action on pyrite catalysed the formation of ferric ion (Fe+3), proton (H+) and sulphate ions (SO4-2) in the solution which leached metals (Ni, Mo and V) from the spent catalyst. Experiments were conducted by varying the reaction time, amount of spent catalyst and pyrite, and temperature. After 7 days with 30 g/L of spent catalyst and 50 g/L of pyrite, the leaching of Ni, V and Mo into the solution was 85, 92 and 26%, respectively. With increasing spent catalyst loading, the extent of metal dissolution was decreased, probably due to the precipitation of Fe+3 as a residue. Under all conditions tested, Mo showed recovery due to its precipitation with leach residues as MoO3 observed by applying EDAX and XRD techniques to the leach residues.

Life cycle of hydroprocessing catalysts and total catalyst management

Hydroprocessing catalysts based on Ni, Co, Mo and W are used in various refinery processing applications where several deactivation mechanisms become of importance (coke formation, active phase sintering, metals deposition, poisoning) in the catalyst's life cycle. The life cycle of commercial hydroprocessing catalysts is very complex and includes the catalyst production, sulfidation, use, oxidative regeneration followed by re-sulfidation and reuse or, if reuse is not possible, recycling or disposal. To understand the changes in catalyst properties taking place during a life cycle, the catalyst quality in the different stages can be best monitored by using advanced analytical techniques. The catalyst's life cycle is further complicated by numerous technical, environmental and organizational issues involved. In principle, different companies can be involved in each of the life cycle steps. Leading catalyst manufacturers, together with specialized firms, offer refineries a total catalyst management concept, starting with the purchase of the fresh catalyst and ending with its final recycling or disposal. Total catalyst management includes a broad range of services, ensuring optimal timing during the change-out process, reliable, smooth and safe operations, minimal downtime and maximum catalyst and unit performance.

On the Use of Acid-Base-Supported Catalysts for Hydroprocessing of Heavy Petroleum

The environmental regulation pressure is being increased to reduce precursors of pollution contaminants (sulfur, nitrogen, and aromatics) in fuels to lower levels. There are various processes to upgrade heavy and extra-heavy crude oils, such as solvent deasphalting, thermal conversion, catalytic conversion, distillation, and hydroprocessing. Hydroprocessing not only upgrades the crude oil but also produces synthetic crude oil that has a lower impurities content and a higher liquid yield of products. The fuels upgrading currently is achieved in refinery hydroprocessing units using different new-generation catalysts, along with several modifications of process conditions such as multistage reactors, type of catalyst loading, and reactor internals. However, it has been widely recognized that, for deep removal of these contaminants by hydroprocessing, research must be more oriented to the catalyst developments, rather than the process conditions. Actual commercial catalysts are based on well-known and studied...

Bioleaching of spent hydro-processing catalyst using acidophilic bacteria and its kinetics aspect

Bioleaching of metals from hazardous spent hydro-processing catalysts was attempted in the second stage after growing the bacteria with sulfur in the first stage. The first stage involved transformation of elemental sulfur particles to sulfuric acid through an oxidation process by acidophilic bacteria. In the second stage, the acidic medium was utilized for the leaching process. Nickel, vanadium and molybdenum contained within spent catalyst were leached from the solid materials to liquid medium by the action of sulfuric acid that was produced by acidophilic leaching bacteria. Experiments were conducted varying the reaction time, amount of spent catalysts, amount of elemental sulfur and temperature. At 50 g/L spent catalyst concentration and 20 g/L elemental sulfur, 88.3% Ni, 46.3% Mo, and 94.8% V were recovered after 7 days. Chemical leaching with commercial sulfuric acid of the similar amount that produced by bacteria was compared. Thermodynamic parameters were calculated and the nature of reaction was found to be exothermic. Leaching kinetics of the metals was represented by different reaction kinetic equations, however, only diffusion controlled model showed the best correlation here. During the whole process Mo showed low dissolution because of substantiate precipitation with leach residues as MoO 3. Bioleach residues were characterized by EDX and XRD.

Periodic Trends Transition Metal Sulfide Catalysis: Intuition and Theory

The use of Transition Metal Sulfide (TMS) catalysts came about as the need to hydrogenate coal to liquid fuels became urgent in the 1920's. Sabatier won the Nobel Prize in 1912 for describing the phenomenon of hydrogenation by transition metals. However, when the transition metals were used to hydrogenate coal, the TMS emerged as the stable catalytic states due to high sulfur content in the coal liquids. From 1920–1930 I.G. Farbenindustrie A.G. tested over 6000 catalysts. It was from these origins that modern Co (Ni)/Mo/Al2 O3 arose. Since that time scientists have developed an understanding of the fundamental properties that lead toboth the activity of the simple binary sulfides and the mechanism by which two metals (Co+Mo ) acted together to enhance activity (promotion). Initial efforts focused on supported commercial catalysts with limited success. In the early 1980's the periodic trends of TMS catalysts on unsupported catalysts were discovered and these results formed the foundation for further basic understanding of the key properties that led to catalytic activity. These results have been extended over the years to include supported catalysts and many petroleum based substrates. Early results related catalytic activity to the heats of formation of the sulfides and Pauling % d-character of the transition metals. These correlations both pointed to the importance of the 4d an 5d electrons and also to the well established catalytic principle that the metal sulfur bonds needed to be “just right”, not too strong nor too weak thus allowing facile turnover of the adsorbing and desorbing molecules. Averaging of the heat of formation of Co9 S8 and MoS2 suggested that promotion operated through sulfur vacancies formed by atoms that shared both a Co atom and a Mo atom on the surface. Theoretical studies also began in this period that further supported the idea that d-electrons in the frontier orbitals of the catalysts were key in determining catalysis at the surface. The triumph of this approach was that it unified the promoted TMS systems with the binary TMS and provided a common rational for the activity of both. Constant progress since then has been achieved through the application of density functional theory (DFT) narrowing the gap between instinct and a formal description of catalyst structure/function. It is crucial to remember that for real understanding to develop we must study the catalytically stabilized materials and not materials that are changing under catalytic conditions. In the case of the TMS this means that we must study materials like MoSCx and RuSCx . It has been demonstrated that “surface carbides” are the catalytically stabilized state under hydrotreating conditions. The original relation between the d-electrons and later DFT calculations all point to the importance of these electrons in the catalytic reaction. However, more work is needed to define the relation between these electrons and the stabilized carbide surfaces before detailed “active site” structures can be developed with confidence. In addition the presence of Co metal in active hydroprocessing catalysts stabilized for four years in a commercial reactor, calls in to question current theories of the structure of promoted catalysts.

Hydroprocessing of heavy petroleum feeds: Tutorial

The world wide trends in crude oil supply indicate a declining availability of conventional crudes, which has been offset by increasing volume of heavy crudes. For the latter, the yield of liquid fractions can be increased by upgrading the distillation residues. A number of thermal processes (e.g., visbreaking, delayed-, fluidand flexi-coking), so-called carbon rejecting processes, have been used on a commercial scale for several decades [1,2]. Heavy feeds can also be upgraded by hydroprocessing, so-called hydrogen addition option [3,4]. This requires the presence of hydrogen and an active catalyst. Compared with thermal processes, hydroprocessing operations are more flexible, giving higher yields of liquid fractions. An optimum of hydroprocessing operation can be achieved by properly matching the type of reactor and catalyst with the properties of heavy feeds. Several types of catalytic reactors, i.e., fixed-, movingand ebullated-bed reactors are available commercially. In spite of some similarities, hydroprocessing of heavy feeds differs markedly from that of light feeds. This results from the presence of high molecular weight asphaltenic molecules and organometallic compounds in the former. Then, the catalyst design has to take this fact into consideration. Special attention has to be paid to the textural properties of catalysts, such as pore diameter and pore volume. These parameters have to be optimized to ensure adequate surface area. The size and shape of catalyst particles are important for the efficient operation as well. An active catalyst for hydroprocessing of heavy feeds has to be resistant to deactivation by coke and metal deposits.

Structural characterization of coke on spent hydroprocessing catalysts used for processing of vacuum gas oils

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-Al 2O 3 and NiMo/γ-Al 2O 3) 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, f a > 0.95).

Preparation, characterization and evaluation of Maya crude hydroprocessing catalysts

Two supports, one prepared in our laboratory and other supplied commercially, were used to prepare CoMo and NiMo catalysts. A commercial catalyst was also used as a reference in our present investigation. These catalysts were characterized by XRD and TPR experiments. HDM, HDS, HDN and HDAsp reactions were carried out in a microreactor using a mixture of Maya crude with naphtha as feed. The deactivation of catalysts was also studied. XRD results revealed that the active metals were well dispersed into the supports. Two broad peaks were observed on promoted catalysts in our TPR experiment. Lower temperature peak was assigned as reduction of multilayer MoO 3 species and higher temperature peak was due to reduction of monolayer MoO 3 species. The reduction peak patterns were different for respective catalysts. The catalyst deactivation was observed with time on the stream and it was faster at initial period of reaction. The coke formation was prime concerned for the deactivation of these catalysts as well as the decrease of specific surface area. The activities were correlated with the pore structure of the catalyst.

A model for the hydrogenation of aromatic compounds during gasoil hydroprocessing

In the present work, on the basis of a detailed experimental analysis, we propose a mathematical model for the interpretation of the hydrogenation of aromatic compounds during gasoil hydroprocessing. The model considers a lumped scheme for gasoil composition and assumes that hydrogenation/dehydrogenation reactions occur according to the Langmuir–Hinshelwood mechanisms, where hydrocarbons and hydrogen adsorption occur on different active sites. Kinetic rate constants were evaluated through the fitting of a set of experimental data obtained on a pilot unit processing with a commercial NiMo/Al 2O 3 catalyst a gasoil mixture in a wide range of operating conditions, spanning the typical industrial ranges of pressure, temperature and LHSV. Further, independent sets of data obtained with the same catalyst but different feed qualities are correctly predicted by the model, confirming its reliability. Also, the model is applicable for the interpretation of hydrogenation behavior with different types of commercial hydroprocessing catalysts.

Study on the sulfurization of molybdate catalysts for slurry-bed hydroprocessing of residuum

Sulfurization remarkably affects the physical and chemical properties of dispersed catalysts. In this study, molybdate is taken as catalyst precursor for slurry-bed hydroprocessing of residuum to study the mechanics of sulfurization. Several conditions are used to evaluate the activities of the sulfurized catalysts, and XRD, TEM and BET are employed to characterize the sulfurized catalysts. The results show that molybdate is firstly transferred into some kind of crystallized oxothiomolybdate and then mainly into micro-crystallized MoS 2 in the end. The dispersivity of the MoS 2 granules is greatly enhanced compared to the oxothiomolybdate. XPS analysis shows the portion of Mo IV in molybdenum species on the surface of the catalysts is enhanced by the addition of NH 4Cl during sulfurization. Therefore, the catalytic activity, measured using anthracene as chemical probe in the practical reaction system of hydroprocessing for residue, is also enhanced.

Study on the sulfurization of molybdate catalysts for slurry-bed hydroprocessing of residuum

Sulfurization remarkably affects the physical and chemical properties of dispersed catalysts. In this study, molybdate is taken as catalyst precursor for slurry-bed hydroprocessing of residuum to study the mechanics of sulfurization. Several conditions are used to evaluate the activities of the sulfurized catalysts, and XRD, TEM and BET are employed to characterize the sulfurized catalysts. The results show that molybdate is firstly transferred into some kind of crystallized oxothiomolybdate and then mainly into micro-crystallized MoS 2 in the end. The dispersivity of the MoS 2 granules is greatly enhanced compared to the oxothiomolybdate. XPS analysis shows the portion of Mo IV in molybdenum species on the surface of the catalysts is enhanced by the addition of NH 4 Cl during sulfurization. Therefore, the catalytic activity, measured using anthracene as chemical probe in the practical reaction system of hydroprocessing for residue, is also enhanced.

Catalysts for hydroprocessing of Maya heavy crude

In this work the effect of active metals, promoters, phosphorous and lithium on hydroprocessing reactions of Maya heavy crude was studied. Four reactions (hydrodemetallization (HDM), hydrodesulfurization (HDS), hydrodenitrogenation (HDN), and asphaltene conversion (HDAsp)) were carried out on seven different catalysts. The support of these catalysts was prepared by urea hydrolysis method. Temperature programmed reduction (TPR) results showed that more reducible species were present on cobalt promoted catalyst compared with unpromoted one. We observed that active metal was more dispersed on P containing catalyst. The conversions, determined as Time Weighted Mean Conversion (TWMC), were very high for a PCoMo catalyst, however, it exhibited a rapid fall of activation. A linear correlation was found between the loss of specific surface area (SSA) and coke formation. The catalysts deactivation was mainly due to coke formation over catalyst surface. Cobalt promoted catalysts showed higher initial HDM conversion compared with Ni one. Mo active metal was better for HDM reaction. Even though lithium-doped catalyst showed slightly higher activities it had no effect on catalyst life.

Effect of coke deposition upon pore structure and self-diffusion in deactivated industrial hydroprocessing catalysts

Pulsed field gradient-nuclear magnetic resonance (PFG-NMR) has been applied to probe molecular motion of hydrocarbon probe molecules in the pores of deactivated catalysts. It is demonstrated that the technique works very well for coked catalysts, which were operated in an industrial reactor for up to 4 years. Cross-polarisation-magic angle spinning (CP-MAS) 13 C NMR was used to characterise the chemical nature of the coke deposited. It was found that reductions of up to 16% in average BJH pore diameter, 40% in surface area of 48% in pore volume of the catalyst occurred in the most heavily coked sample compared with the fresh catalyst. The decrease in pore volume could not be explained by the reduction in pore diameter alone, implying that significant pore blockage also occurred. The self-diffusivities of pentane and heptane probe molecules were found to decrease linearly with increasing coke content. The catalyst effectiveness factor was estimated to decrease by 10% in the most heavily coked sample compared with the fresh catalyst. This reduction in effectiveness factor was considered to have less influence on the overall rate of reaction per unit mass of catalyst than the considerable loss of pore volume. The tortuosities experienced by heptane imbibed in pellets were in the range 2.37–3.71, which was higher than those for pentane in the range 1.6–1.91; for both molecules, tortuosity showed a general increase with coke deposition.