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Experimental methods for developing kinetic models for hydrocracking reactions with slurry phase catalyst using batch reactors Alexander Quitian, and Jorge Ancheyta Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b01953 • Publication Date (Web): 29 Mar 2016 Downloaded from http://pubs.acs.org on April 7, 2016
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Experimental methods for developing kinetic models for hydrocracking reactions with slurry phase catalyst using batch reactors Alexander Quitian†,‡, Jorge Ancheyta‡* †Instituto Mexicano del Petróleo, Eje Central Lázaro Cárdenas Norte 152 Col. San Bartolo Atepehuacan, Mexico D.F. C.P 07730, Email:
[email protected] ‡Facultad de Química, Universidad Nacional Autónoma de México, Ciudad Universitaria, Coyoacán, Mexico D.F. C.P. 04510 ABSTRACT Experimental methods for studying the chemical kinetics of hydrocracking reactions with disperse catalysts in batch operation are reviewed. Batch operating conditions and description of modes of operation of the reactors are discussed. The experimental procedures used to produce, analyze and interpret the experimental data required for determining the chemical kinetics of hydrocracking reactions with catalyst in dispersed phase are also reviewed. Two possible batch operation modes used for the study of such reactions are described: isothermal and temperature scanning operation. The typical kinetic models for hydrocracking are discussed in detail and step-by-step procedures to calculate the stoichiometric coefficients, mass transfer coefficients, rate constants and activation energies from experimental data are provided.
Keywords: Slurry-phase catalyst, hydrocracking, reaction mechanism, reaction kinetics, batch reactor. 1. Introduction The study of hydrocracking of hydrocarbons with dispersed catalysts or catalyst in slurry phase has increased lately due to ability of these processes to improve feeds with low API gravity, high asphaltenes content and high impurities content (S, N, Ni and V) which gives them a great advantage over hydrotreating processes in fixed, moving and ebullated beds.1 The pores of the supported catalysts used in bed reactors rapidly are plugged when handling heavy feeds due to the formation and accumulation of heavy metals on its surface. This causes deactivation and hence forcing to the process shutdown to regenerate or replace the catalyst. 2 The dispersed catalysts are very small particles, thus they have a very large surface area and provide to the catalytic hydrotreating reactions with a high mass transfer rate, consequently reaction rates are faster than those of the catalysts used in the fixed, moving or ebullated bed
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reactors.3 Greater ease of mass transfer and energy disfavors excessive hydrocracking and thereby reducing coking and hydrocracking gases production.4 However, development of an economically viable process for the hydrocracking of a hydrocarbon using a catalyst in slurry phase relies on identifying a catalyst with high efficiency and low cost as well as optimizing the operating conditions. Therefore, in this type of processes, it is preferred to use catalysts with high iron content, instead of transition metals such as molybdenum, nickel, etc., and to operate at moderate operating conditions, but often it requires more severe operating pressures to counteract the lower catalyst efficiency.5 This document discusses the most common experimental methods reported in the literature for catalyst evaluation, definition of operating conditions and data analysis required to develop kinetic models of those reactions occurring during hydrocracking process with dispersed catalyst, such as hydrogenolysis (hydrodesulfurization), hydrogenation (saturation of olefins and aromatics) and hydrocracking.
2. General aspects of slurry phase systems At thermal hydrocracking processes (no catalyst) or coking (thermal decomposition in the absence of hydrogen and catalyst), producing lighter hydrocarbons is associated with increased production of heavy compounds such as coke. The production of these heavy compounds is given by reactions of addition/condensation of the heavier fractions of hydrocarbons (mainly asphaltenes and resins), which are completely or partially inhibited in catalytic hydrocracking processes which increase the yield of liquid distillable products.6 In slurry phase hydrocracking processes, dispersed catalysts can be introduced to the feed as finely divided powders, soluble salts in water or in oil. The presence of a highly dispersed catalyst promotes rapid uptake of hydrogen which prevent the formation of coke. The high activity of the dispersed catalyst increases the conversion of the feed to lighter products. The amounts of coke and asphaltenes are lower in slurry phase hydrocracking processes than hydrocracking processes with supported catalysts.1,2,7 Catalytic hydrocracking processes in disperse phase have been studied by various authors and they are differentiated mainly by the type of catalyst employed.2,7,8 Although many studies have been conducted, it has not been yet developed plants at commercial scale mainly due to high catalyst cost, the difficulty of its recovering from the liquid product and high
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operating conditions necessary to achieve high conversion of the heaviest fractions of the feed. 2.2. Operating conditions Typical operating conditions for hydrocracking with dispersed catalysts at laboratory and pilot scales are similar to those used at industrial scale with supported catalysts, as shown in Table 1. Operating conditions are selected depending on the type of reaction to be performed and the feed used.9,10 In the case of hydrocracking reactions in dispersed phase, according to the operating conditions these can be classified into:11,12 Moderate: If conditions of pressure and temperature are lower than 80-100 kg/cm2 and 410°C respectively. When heavy crude oils or residua are hydrocracked, a vacuum residue conversion of lower than 50% is reached. Severe: When conditions of pressure and temperature are greater than 80-100 kg/cm2 and 410°C respectively. Vacuum residue conversion is greater than 50% when heavy oils or residua are hydrocracked. 2.3. Dispersed catalysts The hydrogenation activity of the metals used as catalysts in catalytic hydrocracking processes decreases in the following order: noble metals, sulfided transition metals, oxides of transition metal and sulfided noble metals. Wherein the noble metals are typically Pt, Pd, Ir and Os, and the most used transition metals are Mo, Fe, Co and Ni.13,14 Dispersed catalysts with noble metals as active sites have not been investigated in the slurryphase hydrocracking due to their high cost. In addition, catalysts of noble metal ostensibly reduce their catalytic activity during the hydrocracking of heavy oil, extra-heavy oil and bitumen. For these reasons, dispersed catalysts are generally oxides, inorganic or organic salts of transition metals, mainly of Mo, Fe, Co and Ni.15,16 Some authors consider metal sulfides as the catalyst, while the added metal compounds together with the feedstock in slurry phase are called catalyst precursors.12,17,18 This is because during hydrocracking reactions of hydrocarbons that have high content of sulfur, catalyst precursors are converted to metal sulfides (e.g. MoS2) and the resulting catalysts are chemically more stable. Catalyst precursors are also catalytically active.19 Dispersed catalysts used in hydrocracking process can be classified according to Figure 1. However, regardless of the dispersed catalyst, their main function is promoting hydrogenation differently to supported catalysts that possess metallic and acid sites. In the supported
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catalysts, the acid sites of the support (generally alumina or silica with high porosity) favor the degradation reactions and isomerization.13,14,20 Slurry catalysts are more efficient than supported catalysts, however they have some disadvantages. The mixing and homogeneity of the catalyst with the hydrocarbon to be treated is difficult because it tends to settle especially when the catalyst concentration is high and the average particle size is greater than 10 microns.21 The main issue is to scale-up these processes to industrial level. Patterns of solid-liquid-gas flow are difficult to predict and the consideration of a homogeneous flow of the solid-liquid phases can produce significant differences between the theoretical and real values of the energy and momentum balances.22 There is a substantial amount of literature citing the broad advantages of soluble catalysts, as they are those having high conversion of heavy fractions, good capacity of hydrogenation and reduction of sulfur content as compared with pulverized catalysts. This is because molybdenum, iron, nickel and other salts can be dissolved in water (water-soluble catalysts) or in a light hydrocarbon (oil-soluble catalysts) and have a microscopic particle size (less than 5 µm) and thus higher surface area. This allows for using low amount of catalyst between 100-5000 ppm.4,23,24 Another important feature of the slurry hydrocracking is that the use of two or more different catalyst precursors does not allow a significant upgrading of the properties or yields of the hydrotreated hydrocarbons.5,25,26 a. Finely powdered catalysts The finely powdered ore catalysts have been widely studied and employed in dispersed phase hydrocracking due to their low cost and high availability. The most used and reported in numerous patents ores are: hematite, laterite, magnetite, limonite, molybdenite, ferrite and others. The particle size and the amount of mineral catalyst used lie between 5-100 microns and 0.5-2 wt.% respectively.27–32 The finely powdered catalysts of technical or analytic grade are oxides, sulfides or salts of iron, molybdenum, titanium, cobalt, nickel, rhodium and chromium, among others and mixtures of these in various proportions. Similar to mineral catalysts, a number of patents and articles reported good results with technical or analytical grade catalyst. The particle size range and concentrations used are similar to those of ores. Finely-power catalysts are prepared by grinding, milling, sieving and drying of pieces of minerals or materials of analytical grade or technical.1,33,34
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The major disadvantages of the finely divided catalysts are their low activity compared with soluble catalysts and difficulty to keep them in suspension on the hydrocarbon. For these reasons, in hydrocracking processes using finely divided dispersed phase catalysts more severe operating conditions are required and greater mixing or turbulence to avoid the sedimentation of the catalyst. b. Oil soluble catalyst They are organometallic compounds which are soluble in liquid hydrocarbons and are generally organic acids (mainly octanoic, oxalic, naphthenic), amine and carbonyls of transition metals Panarati et al.5 were the first to study in more detail the effect of the type and amount of soluble catalyst used in the hydrocracking processes in disperse phase. For this, they conducted experiments of slurry hydrocracking in batch operation with vacuum residue of Belayin crude oil using various types of oil-soluble catalysts such as: molybdenum naphthenate (MoNaph), molybdenum acetyl acetonate (MoAA), phosphomolybdic acid (PMA), iron naphthenate (FeNaph), Nickel naphthanate (NiNaph), cobal resinate (CoRe), vanadium resinate (VRe) and rothenium acetyl acetonate (RuAA). According to yields and qualities of the liquid product obtained with each of the catalysts, their results allow for establishing the following order of activity for the metals tested: Mo, Ni≈Ru, Co, V y Fe. Also, the results showed that the performance of catalysts used is practically independent of the functional group bonded to the metal. Others authors have more recently confirmed these results with different types of feed and operating conditions, which show that the catalytically active phases are metal sulfides and their organometallic complexes only facilitate the degree of dispersion of the catalyst in the hydrocarbon.32,35,36 37 During the hydrocracking in dispersed phase, oil soluble precursors form crystalline microstructures of metal sulfide that are grouped irregularly. These crystalline structures have average diameters below 2 microns and consist of single layers stacked of crystal sizes less than 40 Å.5,38 Due to its small molecular size, metals sulfide from complex soluble organometallic oil are catalytically more active and exhibit higher yields and quality of liquid products compared with the catalyst precursors of finely powder or water soluble in the slurry hydrocracking processes of heavy crudes, extra heavy crudes, bitumens and vacuum distillation residues.5,26,39 c. Water soluble catalysts
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Various types of water soluble precursors have been studied in the hydrocracking in dispersed phase, including: thiomolybdate, phosphomolybdate, nickel and cobalt nitrates or acetates, but the most used is ammonium molybdate due to its relatively low cost.37,40 The water soluble precursors are less studied than oil soluble because of adding water makes it difficult the refining of obtained products. Further the added water can dissolve salts present in the hydrocarbon and this can decrease dispersion of the catalyst precursor. At the operating conditions of hydrocracking, agglomeration of these sales can promote their deposition on the metal sulfide with which reduces its activity.41 While, the metal sulfides of water soluble precursors have particle size similar to those observed in oil soluble precursors but they do not exhibit similar catalytic activities. The dispersion grade of water-soluble catalysts is close to that of finely powdered precursors because at the temperature of slurry hydrocracking, the water separation occurs. The dispersion of water-soluble catalysts is improved by reducing the surface tension between the aqueous solution and the precursor feed and this is achieved by vigorous stirring and the use of surfactants. However, despite these improvements, the liquid yields and catalytic activity obtained with water soluble precursors still remains lower than that achieved with oil-soluble precursors.41,42 The simplest and efficient method for preparing catalysts is to produce nano-particles of metal sulfides by their synthesis via water-oil micro-emulsions (MEs). This method is superior to simple stirring to produce nano-dispersed metal sulfides from metals, metal oxides and metallic inorganic salts. The MEs are isotopic multi-component and optically transparent that are thermodynamically stable. They are composed of at least three compounds: one polar (aqueous precursor solution), one nonpolar (hydrocarbon) that form the liquid phase and a surfactant.43 The stability of MEs catalyst precursor depends on the temperature, the amount of water used, the degree of agitation to prepare the emulsion, type and amount of surfactant. The more stable MEs are prepared at room temperature and with low amounts of water and surfactant, conditions which can obtain smaller size micelles. Among the surfactants that have been studied are: dodecyl benzene sulfonate (SDBS), dodecyl trimethyl ammonium bromide (DTAB), oleic acid (OA) and coconut amine (CA). Some studies have shown that the use of surfactants favors catalytic hydrocracking reactions and reduces coke formation in particular those with acidic functional group (as SDBS) and suitable straight alkane chain length.43,44 d. Catalyst recovery
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One characteristic of the finely dispersed catalyst is its use at low concentration, this together with its low cost make impractical its recovery. In addition, the slurry catalysts are deactivated with a single use by coke and metal deposition on the surface of active metal, hindering recovery and reuse.45–48 As soluble catalysts are not easy to recover and due to its low concentration of active metal (
