Heavy oil cracking in the presence of steam and nanodispersed catalysts based on different metals

Fuel Processing Technology 199 (2020) 106239

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Research article

Heavy oil cracking in the presence of steam and nanodispersed catalysts based on different metals

T

Petr M. Yeletskya, , Olesya O. Zaikinaa,b, Gleb A. Sosnina,b, Roman G. Kukushkina,b, Vadim A. Yakovleva,b ⁎

a b

Federal Research Center, Boreskov Institute of Catalysis SB RAS, Lavrentieva Ave. 5, Novosibirsk 630090, Russian Federation Novosibirsk State University, Pirogova Str. 1, Novosibirsk 630090, Russian Federation

ARTICLE INFO

ABSTRACT

Keywords: Heavy oil upgrading Catalytic steam cracking Dispersed catalysts Heavy oil feedstocks Water-based upgrading

Catalytic steam cracking (CSC) of Tatar heavy oil has been studied using nanodispersed catalysts forming in situ based on K, Fe, Ni, Mo as well as several Mo-based catalysts promoted by Ni-, Co-, and Al-based additives. The upgrading was carried out at 425 °C, catalyst content of 2 wt%, water to heavy oil wt. ratio of 0.3:1, and 1 h of residence time using a batch reactor. The highest upgrading efficacy was found in the case of non-promoted Mobased catalyst: at a low coke yield the upgraded oil had the lowest S content as well as the highest H:С ratio. The participation of water in the case of Mo- and Ni-dispersed catalysts was confirmed by comparison with the waterfree cracking experiments at the same conditions. XRD and TEM characterization have shown that the active components were in a form of oxides and/or sulfides.

1. Introduction Due to the continuous depletion of light and medium crude oil reserves, development of approaches to processing of heavy crudes (heavy and extra-heavy oil, bitumen) as well as heavy residues produced in the petroleum refining industry are essential [1,2]. Currently, in the proven reserves of petroleum crudes, only 20–30% belong to light and medium oil whereas 70–80% is related to unconventional petroleum feedstocks, including oil shales and oil sand bitumen [1,3]. High content of heavy fractions, heteroatoms, metals, resins, and asphaltenes make operations with heavy feedstocks complex at all stages: extraction, transportation and processing. However, in view of the increasing world energy demands and the exhausting of “cheap” petroleum feedstocks (light and medium crude oil), unconventional petroleum feedstocks (UPFs) are considered to be a near future of petroleum industry. At the present time, typical technologies of heavy oil upgrading are based on the two approaches: carbon rejection and hydrogen addition [2,4,5]. Carbon rejection techniques are mainly based on thermal cracking: visbreaking, coking, FCC. In addition, deasphalting is also attributed to carbon rejection. Hydrogen addition is represented mainly

by hydrocracking of various types, which depend predominantly on a form of a used catalyst bed (fixed-bed, moving-bed, ebullated-bed and slurry-bed) [2]. The both approaches have both advantages and shortcomings. Carbon rejection is the cheapest, but the liquid products (upgraded oil) obtained are characterized by a poor quality, in the case of catalytic cracking and coking yields of coke are considerable. From the other hand, hydrocracking produces upgraded oil of high quality, but due a great hydrogen consumption it is a rather expensive process. As a result, in petroleum refining industry, > 77% of commercialized technologies of residue processing are represented by carbon rejection based ones (Fig. 1). Last years, new hydrogen-free techniques on heavy oil upgrading are being emerged, one group of them is based on using of water, which can act differently depending on temperature and a phase state [6,7]. At elevated temperatures, water can be in subcritical (T = 100–374 °C, Psaturation = 1.0–22.1 MPa) and supercritical states (T > 374 °C, P > 22.1 MPa) or in the form of steam (T > 100 °C, P < Psaturation). Upgrading in subcritical water also known as “aquathermolysis” is mainly aimed at viscosity reduction of heavy crudes located directly in oil reservoirs [8–11]. Due to relatively low temperatures (usually ca. 200–370 °C) cracking reactions are very slow and water is not especially

Abbreviations: CSC, catalytic steam cracking; CC, catalytic (water-free) cracking; UPF, unconventional petroleum feedstock; SCW, supercritical water; OSC, oxygen storage and release capacity; CA, citric acid; CSR, coherent scattering region; AHM, ammonium heptamolybdate; XAS, X-ray absorption spectroscopy ⁎ Corresponding author. E-mail address: [email protected] (P.M. Yeletsky). https://doi.org/10.1016/j.fuproc.2019.106239 Received 9 August 2019; Received in revised form 8 October 2019; Accepted 9 October 2019 0378-3820/ © 2019 Elsevier B.V. All rights reserved.

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Fig. 1. Worldwide distribution of commercial residue processing capacity. Adopted from [2].

active relating to interaction with hydrocarbons through partial reforming or other routes. The main action of subcritical water is hydrolysis of S, N and O-containing compounds to produce molecules having lower molecular weight [12]. To improve it, various catalysts [13–17] and/or hydrogen donors like formic acid, tetralin etc. are utilized [12,18,19]. However, in order to increase conversion of heavy fractions and to reach significant yields of the light ones due to both cracking of hydrocarbons and their direct interaction with water, the upgrading should be carried out at higher temperatures (ca. 400–500 °C), when water can be in supercritical state (SCW) or in the form of steam. In several studies, experiments with isotopic water (D2O or H218O) showed, that water acts a source of hydrogen in both cases – SCW [20–22] and steam [23–26] even without utilization of any catalysts. Upgrading of UPFs in SCW is highly effective, and more so in the presence of catalysts [27–30]. Some features of SCW explain this: 1) unlike ordinary liquid water, it is almost nonpolar [31] and is therefore a good solvent for petroleum hydrocarbons; 2) SCW can also emulsify not readily soluble or insoluble high-molecular hydrocarbons resulting in decreased yields of coke and increased yields of liquid products [32,33]. In addition, SCW is used for simultaneous extraction and upgrading of oil sands and oil shales [32]. Nevertheless, high corrosiveness of SCW relating to both catalysts [34,35] and materials of reactors and installation parts [36,37] as well as severe upgrading conditions still hamper an extension of this approach beyond laboratory experiments. Cracking of heavy oil feedstocks in the presence of steam is carried out by two pathways if to consider it from the viewpoint of catalyst using. Unlike sub- and supercritical water, steam cannot act as a solvent. Thence, the use of conventional supported catalysts is conjugated with their rapid deactivation due to blocking of their pore surface and mouths with high-molecular components of the feedstock, which in the further, are transforming into coke deposits. To avoid it, from the literature, two techniques are known to be applied: 1) dilution of UPFs with solvents (typically with benzene or toluene) to obtain a 10% solution of a UPF in a solvent and carrying out the upgrading in a continuous flow reactors with fixed catalyst bed [38–42]; 2) applying of dispersed (unsupported) catalysts in a form of suspension (dispersion)

of their particles in the feed and conducting the upgrading in a batch or continuous flow reactors of slurry type without dilution [24,25,43–45]. The technique on the dilution of heavy oil feedstocks to reduce their viscosity, which has been firstly applied by Fumoto et al. in 2004 [46], allowed a screening of several catalyst series. Catalysts attracting the most attention are based on transition metal oxides which have a certain oxygen storage and release capacity (OSC) as well as oxidationreduction (redox) property to be able to oxidize heavy oil molecules followed by reoxidation by water with releasing hydrogen species in the upgrading conditions (Fig. 2) [47]. Such oxides are Fe2O3/Fe3O4, CeO2/ Ce2O3 modified by various promotors (ZrO2, Al2O3, etc.) [38–41,46,47]. In addition, Ni-based catalysts which are known to be active in partial steam reforming of hydrocarbons (1) [48–51] have also been investigated [41,42].

Cn Hx

H2O

(1)

Cm Hy + gas(H2 , CO2 ,…),

where CmHy is a hydrocarbon of lower molecular weight than CnHx. Despite a significant progress attained in this direction, this

MeOx H

Heavy oil

O

Catalyst

H2O

O

Light oil СО2

MeOx-α Fig. 2. Phase change of the metal oxide catalyst during conversion of heavy oil to light fractions in steam atmosphere. Reprinted with permission from [47]. Copyright 2012 American Chemical Society. 2

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approach has significant shortcomings. The first one is the necessity of using significant quantity of solvents (to achieve a 10% solution of a feedstock), and the second one – a very high ratio of water to upgrading feedstock (at least 2:1 to the solution, or, correspondingly – 20:1 to a feedstock self). As water is known to possess very high heat capacity, the upgrading using such high quantities of water is looking highly energy-intensive. Thus, solvent-free UPF upgrading with reasonable water consumption would be closer to petroleum refining industry. To avoid the using of solvents in a heavy oil feedstock upgrading, a technique based on the utilization of unsupported catalysts (usually in the form of particles having a size of several tens of nanometers) suspended in UPFs was developed by several research groups. Such catalysts are used in both hydrocracking and steam cracking of UPFs [24,25,44,52,53]. In the field of catalytic steam cracking, data on properties on a composition of dispersed catalysts as well as their effect on yields and composition of upgraded oil are still scarce. Clark et al. reported on using Ru and Fe-containing salts in a form of aqueous solution to upgrade two types of Canadian bitumen in batch mode [43]. They found that catalyst formed from RuCl3 (~1wt% Ru) and FeSO4 (~2wt% Fe) as precursors led to an increase and to decrease of coke yield correspondingly, in a temperature range of 375–415 °C at a ratio of water to bitumen 1:4 by weight. Pereira et al. developed Aquaprocessing technology on CSC of vacuum residues (VR) in the presence of an ultradispersed bicomponent catalytic system Ni + K (at several hundred ppm concentrations) in continuous flow reactors at temperatures ca. 430–445 °C with adding 5 wt% of water [24,25,54]. Due to increasing of stability of the upgraded oil produced in CSC compared to the one obtained in a simple visbreaking, they were able to obtain higher yields of light fractions [24]. However, when processing of deasphalted feedstock, the effect appeared to be tended to zero [25]. There is a significant lack in the literature data of a study the evolution of the above dispersed catalytic systems in CSC. Moreover, in some cases there is controversial data on an effect of water in the process in both presence and absence of a catalyst [23,25,55]. The mechanisms of water interaction are also needed to be highlighted more clearly. The goal of this study is to investigate dispersed catalysts based on metals of different nature (I, VI and VIII groups – Ni, Mo, Fe, K and Mobased catalysts promoted by additives of Ni, Co and Al) in CSC of high sulfuric heavy oil at a reasonable ratio of water to the feedstock (0.3:1 by weight) in a batch reactor. The selected catalyst concentration was 2 wt% (in recalculation to a metal) as in the most cases the significant effect was obtained at this value. Possible mechanisms of interaction of water with the feedstock molecules are also discussed.

Table 1 Properties of the used Tatar heavy oil (Republic Tatarstan, Russia). Kinematic viscosity at 25 °С, cm2s−1 Density at 25 °С, g·cm−3 CCR, wt% CHNS-composition, wt% С Н N S Н:С ratio Fractional composition (ASTM-1160), wt% Gasoline (IBP < 200 °C) Diesel (200–350 °С) Vacuum gas oil (350–500 °С) Vacuum residue (> 500 °С) SARA analysis, wt% Hydrocarbons Saturated Aromatic Resins Benzene Alcohol-benzene Asphaltenes Content of metals, ppm V Ni

2825 0.96 9.7 83.8 12.1 0.7 4.3 1.74 0 21 31 48 25.4 44.7 13.4 10.3 6.4 200 60

and consists of approximately by a half of non-distillable fractions. 2.3. Heavy oil cracking 2.3.1. Preparation of dispersions of a catalytic metal-containing precursor in heavy oil Before the cracking experiments, catalyst precursors containing the corresponding catalytic metals were introduced into the feedstock through preparation of reverse emulsions followed by their decomposition to obtain corresponding dispersions. For this, 150 g of heavy oil was mixed with the appropriate volume of aqueous solutions of catalytic metal containing precursors (nitrates of nickel, cobalt, iron, aluminum, ammonium heptamolybdate, potassium acetate) in order to obtain 2 wt% of a catalytic metal in a final suspension. In the case of bicomponent dispersed catalysts, citric acid (CA) was used to prevent formation of insoluble molybdates with Ni, Co and Al due to the formation of water-soluble complexes containing various metals [56]. According to the technique from literature [56], CA was taken in the following molar ratios to metals. In the case of MoNi and MoCo catalysts, the required amount of CA was calculated based on the ratio of 1.3 mol of CA to 2 mol of Mo + 1 mol of Ni (or Co), in the case of MoAl catalyst – 1.5 mol of CA to 1 mol of Mo + 1 mol of Al. The purpose of this approach is to study the possibility of obtaining phases of the active component, which would simultaneously contain both catalytic metals (for example, phases of mixed Ni(Co)-Mo-S sulfides, or others). Water content used for the aqueous solution preparation was 5 wt% in the emulsions. To increase stability of the reverse emulsions, additional surfactant SPAN 80 was added to heavy oil (11 wt%). The emulsions were produced by stirring at 24000 rpm for 3 min using an IKA T-25 basic ULTRA-TURRAX (Germany) disperser. Thermal decomposition of the reverse emulsions was carried out in a batch 1000 ml reactor (Parker, USA) equipped with a Magnedrive® stirring system (Fig. 3). Temperature was 210 °C (heating rate 6.3 °C·min−1), stirring – 500 rpm, duration – 1 h. After the decomposition is complete (water and decomposition products stopped appearing in the trap at the reactor outlet), the heating was switched off and reactor was cooled down with compressed air.

2. Materials and methods 2.1. Reagents As the catalyst precursors the following salts were used: Ni (NO3)2·6H2O, Co(NO3)2·6H2O, Fe(NO3)3·9H2O, Al(NO3)3·9H2O and potassium acetate. They all were of high grade and supplied by Reakhim Ltd. (Russia); (NH4)6Mo7O24·4H2O was of analytical grade (Vekton Company, Russia). Additional reagents: SPAN 80 (Sigma-Aldrich, purity ≥60%), dichloromethane (Baza No. 1 Khimreaktivov JSC, Russia; purity 99.5%), citric acid (Baza No. 1 Khimreaktivov JSC, Russia; GOST (Russian Standard) 3652-69). Deionized water was obtained by beans of laboratory facilities. 2.2. Feed characterization As a feedstock, highly sulfuric Tatar heavy crude oil was used (S content 4.3 wt%, originated from Tatarstan Republic, Russia). Its characteristics are presented in Table 1. As seen from the Table 1, the used heavy oil has no gasoline fraction

2.3.2. Upgrading procedure Heavy oil upgrading was carried out in the same batch reactor. 45 g of deionized water was added to the obtained suspension followed by 3

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Magnedrive® stirring system

Pressure gauge

Pressure reducer Controller Cranes

Flanges Batch reactor Thermocouples in reactor and heater

Heater

Gaseous products to analysis/exhaust Separator (trap)

Fig. 3. A schematic image of the used batch reactor for the experiments.

sealing of the reactor via connection to the flange. Before the start, the reactor system was tested on leakage by pressurizing with Ar at 10.0 MPa. Temperature in the experiments was 425 °С, stirring rate of 1000 rpm. Initial pressure was 1 atm, operating pressure in the experiments was autogenic and varied from 4.0–6.9 MPa (thermal or catalytic water-free cracking (СС)) to 13.5–15.7 MPa in CSC. Duration of the experiments was 1 h. After 1 h, the heating was switched off and reactor was cooled down with compressed air as in the previous step. When reactor has been cooled down to a room temperature, residual pressure of the gaseous products of cracking was fixed and used for the further taking into account in the mass balance. Before releasing the gaseous products, they were analyzed by means of gas chromatography. A list of the used precursors of dispersed catalysts is presented in Table 2. All the experiments were carried out twice.

and residual upgraded oil dissolved in dichloromethane was then separated from the coke via filtration. The solid residue was washed on a glass Schott filter (porosity S2) in vacuum using a Bunsen flask. The filtered mixture of water and the solution of upgraded oil in dichloromethane was separated on a separatory funnel. Dichloromethane was then evaporated from the solution of residual part of upgraded oil on a rotary evaporator. Then, the separated residual part of upgraded oil was combined with the main part obtained by decantation in the previous step. The coke samples washed with dichloromethane were dried at 100 °С up to constant weight. 2.4. Analysis of products Gaseous non-condensed products (C1–C4-hydrocarbons, hydrogen, СО, СО2) were analyzed when the reactor was cooled down. Gas chromatography was performed on a KhROMOS GKh-1000 chromatograph equipped with TCD and FID detectors, using columns packed with silochrom and activated carbon (length, 3 m; inner diameter, 2 mm). Argon was used as the carrier gas. The CHNS composition of the initial heavy crude oil, upgraded oils and coke residues were determined by means of a CHNS-O elemental analyzer VARIO EL CUBE (Elementar Analysensysteme GmbH, Germany). Every analysis was performed trice with averaging of the obtained results.

2.3.3. Post-treatment of the obtained products After the releasing the gaseous products and reduction of the pressure in the system to atmospheric, the reactor was opened and the mixture of upgraded oil, coke, and water was thoroughly extracted from it into a flask. After the complete sedimentation of coke particles, the liquid products were decanted into another flask. The residual part of upgraded oil mixed with coke was extracted with an excess of dichloromethane. The batch reactor was also thoroughly washed with dichloromethane to minimize the product losses. A mixture of water Table 2 Precursors of the dispersed catalysts used in the experiments on heavy oil cracking. Metal – base of the dispersed catalyst

Contenta, wt%

Precursor of a catalyst

Citric acid using

K Fe Ni Mo Mo + Ni Mo + Co Mo + Al

2

Potassium acetate CH3COOK Iron nitrate Fe(NO3)3·9H2O Nickel nitrate Ni(NO3)2·6H2O Ammonium heptamolybdate (NH4)6Mo7O24·4H2O (NH4)6Mo7O24·4H2O + Ni(NO3)2·6H2O (NH4)6Mo7O24·4H2O + Co(NO3)2·6H2O (NH4)6Mo7O24·4H2O + Al(NO3)3·9H2O

−

a

2 + 0.66 2 + 0.61 2 + 0.56

In recalculation to a metal contained in the dispersion obtained via decomposition of a corresponding reverse emulsion. 4

+

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may follow the path of transformation into molybdenum oxides (MoO2 and MoO3 as well as intermediate Mo oxides), which could also participate in the oxidation-reduction (redox) cycles similarly to Fe2O3 or CeO2 [47]. Exploratory tests showed that a significant effect of water and catalysts in the upgrading in the most cases is being achieved at least at 2.0 wt% of a catalyst (in recalculation to a metal). Thus, a series of experiments on a revealing of an effect of the dispersed catalysts based on the above metals has been carried out in the same conditions (T = 425 °C, process duration – 1 h, stirring rate – 1000 rpm, loading of heavy oil and water – 150 and 45 g correspondingly (ratio of water to heavy oil – 0.3:1). The used temperature has been selected due to a rate of cracking processes is not too high, meanwhile a rate of interaction of water with hydrocarbons via partial steam reforming and oxidative cracking is expected to be significant.

Table 3 Yields of upgraded oil, coke and gaseous products as well as H:C ratio in upgraded oil obtained via heavy oil cracking at 425 °С (1 h) in the presence of 2 wt % of dispersed catalysts based on K and Fe (150 g of the heavy oil + 45 g H2O). Yield, wt%

Light fractions (Тb < 350 °C) Synthetic oil (Тbp < 500 °C) Upgraded oil Coke Gas Mass-balance Н:С ratio in upgraded oil a b c

HOa

21 52 100 – – 100 1.74

TCb

47 65 83 8 2 93 1.61

SCc

50 64 82 7 2 91 1.64

Metal – base of dispersed catalyst K

Fe

49 63 81 8 3 92 1.63

47 55 72 16 3 91 1.66

Initial heavy oil. Thermal cracking. Non-catalytic steam cracking.

3.1.1. Activity of K- and Fe-based dispersed catalysts K- and Fe-based dispersed catalysts could be of the highest interest for the water-based upgrading of heavy oil feedstocks as they, to their cheapness are the most available among the tested ones. Table 3 summarizes yields of liquid products (upgraded oil), coke, gas, H:С ratio of the upgraded oil as well as mass-balance of the upgrading. For the comparison, result of blank experiments without catalyst and/or water is also presented. As can be seen from the Table 3, addition of water, compared to thermal cracking, led to some increased yield of light fractions (50 and 47 wt%), slightly elevated H:С ratio (1.64 and 1.61, correspondingly) and decreased content of sulfur (3.5 and 3.3 wt%, Table 4) in upgraded oil, which indicates on some effect of water. The use of K-based dispersed catalyst affected not significantly on yield of either upgraded oil or light fractions as well as other process indicators. However, elevated CO2 content in gas products (Table 5) could be attributed to the both participation of water in the upgrading and products of decomposition of K-containing catalyst precursor (CH3COOK). Decomposable K-based compounds are expected to transform into metallic potassium via reduction of K2O formed in hydrocarbon media through the reaction (2) (T = 427 °C) [67]:

The fractional compositions of the initial heavy oil and obtained upgraded oils were determined by fractionation using a semiautomatic instrument intended for the fractionation of heavy and residual petroleum products (B/R Instrument Corp., United States). The analysis was performed in accordance with the ASTM-1160 standard. The ash content in the coke was determined as described in literature [57]. According to the technique, a catalyst-containing coke sample was combusted in a muffle furnace at 550 °C in air atmosphere for 3 h. 2.5. Characterization of spent catalysts XRD analysis of the solid residues containing particles of dispersed catalysts was conducted by means of a D8 Advance diffractometer (Bruker, Germany) equipped with a Lynxeye (1D) linear detector using monochromatized CuKα radiation (λ = 1.5418 Å). XRD patterns were recorded in a 2θ angle range of 10°–80° in increments of 0.05° at an acquisition time per point of 2 s. The average coherent scattering regions (CSRs) were estimated by the Selyakov–Scherrer formula from the half-width of the diffraction lines [58]. TEM After the tests, the catalyst-containing coke samples were studied by high-resolution transmission electron microscopy on a JEM2010 transmission electron microscope (JEOL, Japan) at an accelerating voltage of 200 kV and a resolution of 0.14 nm. Sample particles were deposited by dispersing a sample suspension in alcohol on an aluminum substrate using an ultrasonic disperser.

4 K0 + K2 CO3 + C6 H6 (g) + H2 (g), G

3K2O + C7 H8 (g) =

(2)

42 kcal/mol 0

When interacting with water, K could give K2O and additional hydrogen, thus closing the loop (3):

2K0 + H2 O

(3)

K2 O + H2

However, participation of K in the cycle (2)–(3) would result in the fact that conversion of most of potassium into K2CO3, reactivity of which is unknown in this process. Cabrales-Navarro et al. suggested another mechanism of participation of K2O in CSC using 18O-labelled water (Fig. 4) [25]. According to it, K2O interacts with oxygen of water to form potassium peroxide. This

3. Results and discussion 3.1. Heavy oil upgrading using 2 wt% monometallic-based dispersed catalysts (K, Fe, Ni, Mo) The four metals – basis of the tested dispersed catalysts have been selected – K, Fe, Ni and Mo. The first three, in composition of catalysts are known to be active in upgrading of heavy oil feedstocks in the presence of water in the form of both steam and SCW. K and Ni are known as components of ultradispersed bicomponent catalyst firstly described by Pereira et al. [24,59]. Also, Ni-based catalysts are widely utilized in steam reforming of hydrocarbons [60–63] as well as in partial steam reforming [48,51,64]. Iron oxide-based catalytic systems are also known to be effective in steam cracking of heavy oil feedstocks [46,47] as well as oxidative cracking in SCW medium [22,65]. Mo-based dispersed catalysts are usually applied in hydrocracking of vacuum residues in reactors of slurry type [4,52,66] where the active Mo-based component of the dispersed catalyst is in MoS2 form. In addition, decomposition of the Mo-based precursor (ammonium heptamolybdate – AHM) without an additional sulfidation in steam medium

Table 4 Sulfur content in upgraded oils and cokes obtained via heavy oil cracking at 425 °С (1 h) in the presence of 2 wt% of dispersed catalysts based on K and Fe (150 g of the heavy oil + 45 g H2O). S content, wt%

Upgraded oil Coked a b c d

5

HOa

4.3 –

TCb

3.5 6.4

Initial heavy oil. Thermal cracking. Non-catalytic steam cracking. Containing dispersed catalyst.

SCc

3.3 6.3

Metal – base of dispersed catalyst K

Fe

3.2 6.5

3.1 8.9

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Table 5 Composition of gaseous products obtained via heavy oil cracking at 425 °С (1 h) in the presence of 2 wt% of dispersed catalysts based on K and Fe (150 g of the heavy oil + 45 g H2O). Gas, vol%

H2 CO CO2 C1–C4 Others a b

TCa

5 0 0 58 37

SCb

3 0.6 0 53 44

C

Metal – base of dispersed catalyst K

Fe

7 0.3 5 55 32

9 1.1 13 49 28

K, CSC 10

20

30

40

50

60

70

2 , deg. Fig. 5. XRD pattern of coke containing particles of 2 wt% K-based dispersed catalyst obtained after CSC of heavy oil.

Thermal cracking. Non-catalytic steam cracking.

o +

pathway, nevertheless, is rather discussible as K2O, thus, should be oxidized with water to form a very reactive compound – K2O2, which is, most probably unstable in the CSC conditions in hydrocarbon medium. However, to this date no other mechanisms of participation of K-based compounds in CSC are presented in the literature. Investigation of coke with the spent K-based dispersed catalyst by XRD (Fig. 5) and TEM did not reveal any K-based compounds. This means that K-based components could be in an intercalated form as potassium is known to be tended to the intercalation [68]. Thus, the participation of potassium-based catalysts in CSC needs a deeper investigation, which is out of the focus of this work. The use of Fe-based dispersed catalyst resulted in a more pronounced effect. At the first, yields of upgraded oil decreased due to the elevated coke yield, which increased approximately twice (16 wt%). At the same time, yield of gas products did not change. It should be noted that in this case highest concentrations of H2 and CO2 were observed in gaseous products (9 and 13 wt% correspondingly, Table 5). It should also be noticed that sulfur content in the upgraded oil declined slightly (to 3.1 wt%) with simultaneous increasing in the coke. H:C ratio in liquid products remained practically unchanged. All the above observations in a significant degree are connected with transformations of the Fe-containing precursor – iron (III) nitrate in the CSC conditions. Investigation of spent catalyst accumulated in coke by XRD (Fig. 6) revealed two Fe-containing phases: Fe3O4 (magnetite) [PDF 19629] and Fe0.91S [PDF 20-535]. Average CSR sizes estimated on the most intensive reflexes of the corresponding phase were 66.0 nm for Fe3O4 and 43.0 nm for Fe0.91S. TEM study (Fig. 7) showed only Fe3O4 phase, whose particles have a size of 5–50 nm and are located on the surface of carbon matrix. Fe1-хS phase (detected by XRD) was impossible to be detected by TEM.

C

o

+

+

+ Fe3O4 o

10

20

30

40

50

o Fe0.91S +

+

60

o

Fe, CSC

70

2 , deg. Fig. 6. XRD pattern of coke containing particles of 2 wt% Fe-based dispersed catalyst obtained after CSC of heavy oil.

Similar transformations of a Fe2O3-based dispersed catalyst were observed by Sharypov et al. in CSC of coal-derived liquids [69]. In Fig. 8, a scheme of plausible transformations of iron nitrate in the CSC conditions is presented. At the initial stage – decomposition of the reverse emulsion of aqueous solution of Fe(NO3)3 occurs, iron nitrate decomposes to form of Fe2O3 (hematite) suspension in heavy oil [70] followed by partial reduction to magnetite and sulfidation with heavy oil molecules and/or H2S to form Fe0.91S phase. A relatively high yield of coke can be connected with increased ability of both Fe3O4 and iron sulfides to cracking [71,72]. No Fe2O3 phase was found in the spent catalyst, which could probably indicate on absence of Fe3O4 and/or iron sulfides reoxidation by water. This result is thought to be explained by the encapsulation of the catalyst in coke deposits thus becoming inaccessible or insufficiently accessible for water molecules. The increased S-content in the coke as well as the lower one in the upgraded oil seem to be connected with accumulation of S in coke via iron sulfide formation. Thus, the study of K- and Fe-based dispersed catalysts in the selected conditions of CSC of the Tatar heavy oil showed that a deeper investigation of their applicability is necessary, via a seeking of

Fig. 4. Mechanism of H2O splitting by K+. Reprinted with permission from [25]. Copyright 2017 American Chemical Society. 6

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Fig. 7. Typical TEM images of coke obtained after CSC of heavy oil in the presence of 2 wt% Fe-based dispersed catalyst at 425 °С, 1 h.

CSC at 425 °C

Fe(NO3)3

Decomposition of the reverse emulsion at 210 °C -NO2 -O2

Fe2O3

Partial reduction by hydrocarbons

Sulfidation by Scontaining compounds/ reduction Fe0.91S Fe3O4

conditions that are more appropriate.

Fig. 8. Scheme of plausible transformations of the Fe-based dispersed catalyst precursor – iron nitrate in the conditions of CSC.

crystallites) were almost the same in the both cases – CSC and CC. The scheme of the possible transformation mechanism is presented on Fig. 11. The XRD pattern of cokes, containing Ni-based dispersed catalysts include a number of reflexes located from 15.59 to 55.54° by 2θ corresponding to Ni9S8 [PDF #22–1193] (Fig. 9). A wide halo at 22.0–29.0° by 2θ indicates on the presence of amorphous carbon in the samples. Mean CSRs of the Ni9S8 phase calculated from the most intensive reflex (222, at 31.3°) appeared to be 24.5 and 24.0 nm for CSC and CC correspondingly. TEM investigation of the samples (Fig. 10) showed that the active component is mostly in the form of the highly crystalline Ni9S8 phase represented by parallelepipeds of sizes ca. 15 × 60 nm and cubes with mean sides ca. 40–80 nm. The increased yield of CO2 in gaseous products, a certain suppression of coke formation and the elevated H:C ratio in the upgraded oil in the case of CSC are thought to be explained by the activity of Ni9S8 in complete or partial steam reforming and hydrogenating reactions. Despite for the steam reforming metallic nickel-based catalysts are conventionally applied, nickel sulfides are also known to have a certain activity in partial steam reforming of hydrocarbons [48]. Sulfur content in both upgraded oils and cokes after CSC and CC

3.1.2. Activity of Ni- and Mo-based dispersed catalysts The second group of dispersed catalysts appeared to be more effective in the upgrading includes Ni- and Mo-based ones. Yield of the main products, their composition and some their properties are presented in Tables 6–8. One can see from Table 6 that despite the elevated coke yield, comparing to the blank experiments (13 wt%), the use of Nibased catalyst led to obtaining of upgraded oil having a notably higher H:С ratio (1.69) compared to the CSC using K- and Fe-based catalysts. To distinguish an effect of water and the catalyst, similar water-free experiment with 2 wt% of Ni has been carried out. It can be seen, that using water leads to slight suppression of coke formation, and increased H:С ratio. Sulfur content in upgraded oil became lower (2.9 wt%) than in the case of K- and Fe-based catalysts. Analysis of gaseous products showed that in the case of CSC CO2 concentration of gaseous products compared to water-free experiment was by about two times higher (11 and 6% correspondingly) at equal H2 concentration (12%). Study the cokes containing dispersed catalysts by XRD and TEM (Figs. 9 and 10) revealed that nickel nitrate has transformed to Ni9S8 phase, whose properties (phase composition, size and a form of

Table 6 Yields of upgraded oil, coke and gaseous products as well as H:C ratio in upgraded oil obtained via heavy oil cracking at 425 °С (1 h) in the presence of 2 wt% of dispersed catalysts based on Ni and Mo (150 g of the heavy oil + 45 g H2O). Yield, wt%

Light fractions (Тb < 350 °C) Synthetic oil (Тbp < 500 °C) Upgraded oil Coke Gas Mass-balance Н:С ratio in upgraded oil a b c d

HOa

21 52 100 – – 100 1.74

TCb

47 65 83 8 2 93 1.61

SCc

50 64 82 7 2 91 1.64

Metal – base of dispersed catalyst Ni + H2O (CSC)

Ni without water (CCd)

Mo + H2O (CSC)

Mo without water (CC)

46 59 76 13 4 93 1.69

48 58 74 15 3 92 1.64

51 66 82 8 3 93 1.70

48 61 82 8 3 93 1.62

Initial heavy oil. Thermal cracking. Non-catalytic steam cracking. Catalytic cracking. 7

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Table 7 Sulfur content in upgraded oils and cokes obtained via heavy oil cracking at 425 °С (1 h) in the presence of 2 wt% of dispersed catalysts based on Ni and Mo (150 g of the heavy oil + 45 g H2O). HOa

S content, wt%

Upgraded oil Cokee a b c d e

TCb

4.3 –

SCc

3.5 6.4

Metal – base of dispersed catalyst

3.3 6.3

Ni + H2O (CSC)

Ni without water (CCd)

Mo + H2O (CSC)

Mo without water (CC)

2.9 10.8

3.0 10.1

2.8 6.6

2.8 12.0

Initial heavy oil. Thermal cracking. Non-catalytic steam cracking. Catalytic cracking. Containing dispersed catalyst.

upgraded oil showed that it has dropped down to 1.62 indicating on the intensified hydrogen saturation in the CSC. It should also be noted that in the case of CSC yields of synthetic oil as well as light fractions were notably higher (66 and 51 wt%, compared to 61 and 48 wt% in waterfree cracking, correspondingly) indicating on a deeper conversion of heavy fractions in the presence of water. A study of S-content in liquid products and coke (Table 7) showed that it has declined in the both upgraded oils in the same degree (to 2.8 wt%) meanwhile in the case of the CC experiment, the accumulation of sulfur in the coke was significant (12 wt%, unlike 6.6 wt% in the case of the CSC). This result means that water acts as a desulfurizing agent: the catalyst without water absorbs sulfur to form molybdenum sulfide, meanwhile the presence of water facilitates the S transfer into gaseous products. Analysis of composition of gaseous products (Table 8) showed that in the case of CSC, CO2 concentration is also higher by more than twice (7 and 3%) at an equal H2 concentration (9% in both cases). This result also indicates on the intensification of interaction of water with the feedstock to form CO2 as a product of steam reforming or oxidative cracking. The cokes containing Mo-based catalyst after CSC and CC were studied by XRD (Fig. 9) and TEM (Fig. 12A and B, correspondingly). XRD pattern of the coke containing Mo-based catalyst after CSC exhibited a number of reflexes located from 26.0 to 72.9° by 2θ corresponding to MoO2 phase [PDF # 32–671]. Average CSR of this phase estimated by the most pronounced (111) reflex appeared to be equal 34.0–36.0 nm. Besides, several small broadened reflexes observed at 33.5 and 58.5° by 2θ are related to MoS2 [PDF 37-1492]. This result indicates on the fact that the CSC conditions lead to formation of poorly crystallized MoS2 phase. XRD pattern of coke containing Mo-based catalyst after CC includes several wide peaks located at 13.2, 25.6, 33.5, 36.9, 53.4 and 59.3° by 2θ corresponding to MoO2 and MoS2 phases. TEM studies of the coke after CSC (Fig. 12A) showed that Mo-containing phases are represented by agglomerates and particles of MoO2, with a size of 20–100 nm, and 1–2-layered linear particles of MoS2 having a length of 10–15 nm. It should be noted that MoS2 is both in direct contact with molybdenum oxide particles and at some distance from them. As can be seen from the comparison of Fig. 12A and B, in the case of water-free catalytic cracking, the proportion of the MoS2 phase increases, thus confirming the CHNS analysis data. At the same time, 1–3-layered particles with the length of 7–10 nm are observed. Mo-containing phases are like core@shell structure, when the shell is MoS2 formed from MoO2 (pseudomorph) and the core is unconverted poorly crystallized MoO2. According to XAS study, after CSC the Mocontaining phase in the spent catalyst consists of 74% of MoO2, 16% of MoO3 and 10% MoS2, whereas after CC it is represented by 63% of MoS2 and 37% of MoO2 [73]. Thus, the XRD and TEM study of the cokes containing dispersed catalyst showed that in the case of Mo-based dispersed catalyst, its composition is strongly affected by the presence of water in the upgrading. Moreover, in spite of equal coke yield in the case of both CSC

Table 8 Composition of gaseous products obtained via heavy oil cracking at 425 °С (1 h) in the presence of 2 wt% of dispersed catalysts based on Ni and Mo (150 g of the heavy oil + 45 g H2O). Gas, vol%

H2 CO CO2 C1–C4 Others a b c

TCa

SCb

5 0 0 58 37

Metal – base of dispersed catalyst

3 0.6 0 53 44

Ni + H2O (CSC)

Ni without water (CCc)

Mo + H2O (CSC)

Mo without water (CC)

12 1.0 11 55 21

12 1.0 6 52 29

9 0.2 7 51 33

9 1.0 3 61 26

Thermal cracking. Non-catalytic steam cracking. Catalytic cracking. v

v - MoO2 * - MoS2

v v *

v v

+ - Ni9S8 v

v

v

v

* * +

+

++

+

+

+ ++

10

20

30

+ ++ ++

40

50

Mo, CSC Mo, CC

+ +

Ni, CC Ni, CSC

60

70

2 , deg. Fig. 9. XRD patterns of cokes containing particles of catalysts obtained after heavy oil upgrading in the presence of 2 wt% Ni- and Mo-based dispersed catalysts in the presence (CSC) and absence (CC – catalytic cracking) of water.

were rather close, the accumulation of sulfur in the cokes seems due to absorption by Ni-containing phase. The most effective in the CSC of heavy oil appeared to be the Mobased dispersed catalyst. At a high yield of upgraded oil (82 wt%), it had the lowest S-content (2.8 wt%) as well as the highest H:C ratio (1.70). In addition, the yield of coke was rather low (8 wt%). As in the case of Ni-based catalyst, the additional water-free experiment on heavy oil cracking (CC) in the presence of 2 wt% Mo-based catalyst was carried out. Determination of H:C ratio in the obtained 8

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Ni9S8

C

C Ni9S8 Fig. 10. Typical TEM images of coke recovered after both CSC of heavy oil in the presence of 2 wt% Ni-based dispersed catalyst at 425 °С, 1 h.

Ni(NO3)2

Decomposition of the reverse emulsion at 210 °C

CSC/CC at 425 °C

-NO2 -O2

Sulfidation by S-containing compounds

NiO

MoO2 + MoS2

Ni9S8 (NH4)6Mo7O24

-NH3 -H2O

Fig. 11. Scheme of plausible transformations of the Ni-based dispersed catalyst precursor – nickel nitrate in the conditions of CSC or CC.

MoO2@MoS2

Fig. 13. Scheme of plausible transformations of the Mo-based dispersed catalyst precursor – ammonium heptamolybdate in the heavy oil upgrading conditions.

Ni- and Co-containing additives were taken so as the atomic ratio of Mo to Ni and Co was like in conventional hydrocracking catalysts – 2:1. Aluminum-containing additive was taken as a cracking component with the atomic ratio to Mo 1:1. To prevent deposition of molybdates of the above metals, which are known to be water-insoluble, citric acid was used as an agent forming corresponding mixed complex compounds (please, see Part 2.3.2 in the Experimental section). In Tables 9–11 yields and properties of the upgrading products obtained in CSC of heavy oil in the presence of Mo-based dispersed catalyst modified by Ni, Co and Al are presented. Also, for comparison, the same is presented for non-modified Mo-based catalyst. From Table 9 it can be seen that the using Ni for the promotion has not affected significantly yields of the main products as well H:C in the upgraded oil. Moreover, yield of synthetic oil slightly declined (from 66 to 63 wt%) due to increased coke yield (10 wt%). Sulfur content in liquid products

3.2. Heavy oil CSC using Mo-based dispersed catalyst promoted with Ni, Co, and Al As the Mo-based catalyst has exhibited the highest efficiency in steam cracking of heavy oil, it has been selected for promotion by the Ni-, Co- and Al-based additives to improve its catalytic properties. The

C

MoO3 similar to core@shell structure

and CC, when using water for the upgrading, MoO2 (or MoOx) phase is thought to participate in oxidative cracking (according to the mechanism presented in Fig. 2), meanwhile MoS2 phase catalyzes hydrogenation thus increasing H:С ratio in the upgraded oil. Schemes of transformations of Mo-containing phases in the CSC and CC are presented in Fig. 13. In addition, as sulfur content decreased in the both upgraded oil and coke, MoO2 phase participates in desulfurization instead of the sulfur uptake. In order to increase the performance of the Mo-based catalyst in CSC, attempts to its promotion by Ni, Co and Al additives were further carried out.

A

T > 300 °C

MoO2

C MoO2 covered with MoS2 (pseudomorph)

B

C C Fig. 12. Typical TEM images of coke obtained after CSC (A, up) and CC (B, below) of heavy oil in the presence of 2 wt% Mo-based dispersed catalyst at 425 °С, 1 h. 9

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Table 9 Yields of upgraded oil, coke and gaseous products as well as H:C ratio in upgraded oil obtained via heavy oil CSC at 425 °С (1 h) in the presence of 2 wt% of Mo-based dispersed catalysts promoted with additives of Ni, Co and Al (atomic ratios: Mo:Ni = Mo:Co = 2:1, Mo:Al = 1:1) (150 g of the heavy oil + 45 g H2O). Yield, wt%

Light fractions (Тb < 350 °C) Synthetic oil (Тb < 500 °C) Upgraded oil Coke Gas Mass-balance Н:С ratio in upgraded oil

Table 11 Composition of gaseous products obtained via heavy oil cracking at 425 °С (1 h) in the presence of 2 wt% of Mo-based dispersed catalysts promoted with additives of Ni, Co and Al (atomic ratios: Mo:Ni = Mo:Co = 2:1, Mo:Al = 1:1) (150 g of the heavy oil + 45 g H2O). Gas, vol%

Metal – base of dispersed catalyst 2% Mo

2% Mo + 0.66% Ni

2% Mo + 0.61% Co

2% Mo + 0.56% Al

51

50

39

42

66

63

55

55

82 8 3 93 1.70

81 10 3 94 1.70

78 10 3 91 1.67

69 17 4 90 1.65

TCa

SCb

Metal – base of dispersed catalyst 2% Mo

H2 CO CO2 C1–C4 Others a b

5 0 0 58 37

3 9 0.6 0.2 0 7 53 51 44 33

2% Mo + 0.66% Ni

2% Mo + 0.61% Co

2% Mo + 0.56% Al

10 0.2 8 55 27

10 0 10 52 28

9 0.2 13 49 29

Thermal cracking. Non-catalytic steam cracking. v

and coke increased significantly (up to 3.1 and 10.5 wt% correspondingly) indicating on the impaired desulfurizing properties of the Mobased active component in steam atmosphere. The use of Co as well as Al for the modification resulted in poorer results: decreased yield of upgraded oil having lower H:С ratio, elevated coke yields (especially in the case of Al – 17 wt%). The obtained results could be explained by altered properties of the Mo-based active component as well as an effect of the additives. The spent catalysts in coke were studied by XRD (Fig. 14) and TEM (Figs. 15–17). XRD investigation showed that no any mutual phases containing Mo and any other metal were found. XRD pattern of coke with the Mo-Ni-based spent catalyst was found to include reflexes corresponding to MoO2, MoS2 and Ni9S8 phases. XRD studies of coke residues with the Mo-Co- and Mo-Al-based dispersed catalyst have not reevealed the MoO2 phase in the samples. This result could be explained by the action of citric acid, the use of which led to formation of Mo-based complex compounds with Co and Al followed by their decomposition in the CSC conditions and formation of MoO2 phase having too high dispersity. The both samples were revealed to contain MoS2, and Co-containing catalyst – Co9S8 phase (reflexes at 15.45, 29.85, 31.20, 39.55, 47.67 and 52.20° by 2θ, [PDF # 19-364]). No other reflexes, except the ones of MoS2 phase was detected in Mo-Al-based spent catalyst, a nature of peaks at 2θ = 22.5 and 26.9° is difficult to be explained. TEM studies of the coke deposits containing Mo-Ni-, Mo-Co- and Mo-Al-based catalysts (Figs. 15–17) showed that all the samples contain MoS2 phase located on the carbon surface in the form of 1–2-layered linear chains of length of 5–30 nm. Also, all the samples include MoO2 phase, morphology of which appeared to be slightly different: in the case of Mo-Ni-containing catalyst it is in a form of agglomerates (20–100 nm), meanwhile, in the case of Co- and Al-containing samples

v MoO2 * MoS2 + Ni9S8 Co9S8

v v

v * v

* +

+ * +

C

20

* * *

30

+

v

v

v

v

v

Mo

v +

+ +

v

++

Mo-Ni *

*

Mo-Co Mo-Al

*

40

50

60

70

2 , deg. Fig. 14. XRD patterns of cokes containing particles of catalysts obtained after CSC heavy oil in the presence of Mo-based dispersed catalyst – bare and promoted with additives of Co, Ni and Al.

it is represented by particles of a poorly ordered structure (5–20 nm). In the Ni-containing sample, Ni9S8 phase representing crystallites of ca. 100 nm size was found. In the Co-containing sample Co3O4 phase was detected, Co9S8 phase was impossible to be revealed. In the Alcontaining phase it was discovered, that Al is present in a form of Al2O3, which can be in a contact with MoO2 phase. Thus, the promotion of the Mo-based dispersed catalyst by Ni-, Co-, and Al-containing additives, has not resulted in improving of the target upgrading indicators (yields of upgraded oil, light fractions and coke, H:C ratio etc.) compared to non-modified Mo-based dispersed catalyst. As an interesting effect should be noted, the use of citric acid has given

Table 10 Sulfur content in upgraded oils and cokes obtained via heavy oil cracking at 425 °С (1 h) in the presence of 2% of Mo-based dispersed catalysts promoted with additives of Ni, Co and Al (atomic ratios: Mo:Ni = Mo:Co = 2:1, Mo:Al = 1:1) (150 g of the heavy oil + 45 g H2O). S content, wt%

Upgraded oil Coked a b c d

HOa

4.3 –

TCb

3.5 6.4

SCc

3.3 6.3

Metal – base of dispersed catalyst 2% Mo

2% Mo + 0.66% Ni

2% Mo + 0.61% Co

2% Mo + 0.56% Al

2.8 6.6

3.1 10.5

3.1 12.2

3.3 10.0

Initial heavy oil. Thermal cracking. Non-catalytic steam cracking. Containing dispersed catalyst. 10

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MoO2

MoS2 MoO2 C

C Ni9S8

Fig. 15. Typical TEM images of coke obtained after CSC of heavy oil in the presence of 2 wt% Mo–0.66 wt% Ni dispersed catalyst at 425 °С, 1 h.

rise to increase in the dispersity of the MoO2 phase, mostly in the case of Co- and Al-containing catalysts, and deep sulfurization of its surfaces to form MoS2 shell. This effect, most probably diminished activity of the catalysts in steam cracking. Moreover, introduction of Al led to the highest yield of coke, possibly due to intensified cracking processes.

H:С ratio in upgraded oils diminished from 1.70 and 1.69 to 1.62 and 1.64 in the case of Mo- and Ni-based dispersed catalysts, correspondingly; 3) in the case of Ni-based dispersed catalyst coke yield increased from 13 to 15 wt%. Also, in the case of water-free cracking with Mobased catalyst, S content in the coke containing the catalyst increased from 6.6 to 12.0 wt% indicating on participation of water in desulfurization processes. The use of K-based dispersed catalyst resulted in achieving upgrading indicators practically the same as in the case of non-catalytic steam cracking. Application of Fe-based catalyst gave rise to high coke yield (16.0 wt%) with no other significant improvement of the upgrading indicators. The attempts to the promotion of Mo-based catalyst by Ni-, Co- and Al-containing additives was found to be worsening its catalytic properties: in the case of the Ni-based additive, coke yield slightly increased with no other significant improvements of the upgrading indicators; in the case of Co- and Al-based additives, the upgrading indicators were much worse, especially in the case of the Al-based additive, when coke yield was found to be the highest (17 wt%). Investigation of coke residues containing spent dispersed catalysts by XRD and TEM have shown that except of K-based catalyst, other active components were found to be in the form of oxides and/or sulfides: Fe3O4, Fe0.91S, Ni9S8, MoO2, MoS2, Co9S8, Co3O4 and Al2O3. All the phases were found in the form of particles having a size of several tens of nanometers in the most cases. Nevertheless, application of the Mo-based dispersed catalyst was shown to be enough perspective for heavy oil upgrading in the presence of steam. In the whole, the upgrading in the presence of of steam needs a further search of appropriate process conditions, which can be different depending on a catalyst type.

4. Conclusions Catalytic steam cracking of Tatar heavy high-sulfuric oil has been studied using several types of nanodispersed catalysts based on K, Fe, Ni, Mo as well as several Mo-based catalysts promoted by Ni-, Co-, and Al-based additives. The upgrading was carried out at 425 °C, catalyst content of 2 wt%, water to heavy oil wt. ratio of 0.3:1, and 1 h of residence time using a batch reactor. The dispersed catalysts were introduced into the feedstock through preparation of reverse emulsions of the heavy oil and aqueous solutions of the corresponding salts-precursors followed by their thermal decomposition to obtain corresponding dispersions of nanodispersed particles of the catalysts in the feedstock. The upgrading efficacy was estimated by yields of upgraded oil, light fractions, and coke as well as by reduction of sulfur content and H:С ratio in the produced upgraded oils, compared to thermal cracking or non-catalytic steam cracking blank experiments. It was found that the highest upgrading efficacy was achieved in the case of non-modified Mo-based catalyst: at the comparable yields of upgraded and synthetic oils (82 and 66 wt%) the upgraded oil had the lowest S content (2.8 wt%) and H:С ratio (1.70) at relatively low coke and gaseous products yields (8 and 3 wt% correspondingly). In the case of Ni-based dispersed catalyst, H:С ratio and S-content in the upgraded oil was almost the same (1.69 and 2.9 wt%, correspondingly), but the yield of coke was significantly higher – 13 wt%. The participation of water through steam reforming or oxidative cracking in the case of Moand Ni-dispersed catalysts was confirmed by carrying out water-free experiments at the same conditions: 1) CO2 concentration in gaseous products was ca. by twice lower than the one in CSC experiments; 2)

Declaration of competing interest The authors declare that they have no known competing financial

C

Co3O4

MoS2

C Fig. 16. Typical TEM images of coke obtained after CSC of heavy oil in the presence of 2 wt% Mo–0.61 wt% Co-dispersed catalyst at 425 °С, 1 h. 11

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C MoO2

Al2O3 MoO2

C

Fig. 17. Typical TEM images of coke obtained after CSC of heavy oil in the presence of 2 wt% Mo–0.56% Al-dispersed catalyst at 425 °С, 1 h.

interests or personal relationships that could have appeared to influence the work reported in this paper.

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