Application of Water in Hydrothermal Conditions for Upgrading Heavy

Review pubs.acs.org/EF

Application of Water in Hydrothermal Conditions for Upgrading Heavy Oils: A Review P. Arcelus-Arrillaga,† J. L. Pinilla,‡ K. Hellgardt,*,† and M. Millan*,† †

Department of Chemical Engineering, Imperial College London, London SW7 2AZ, United Kingdom Instituto de Carboquímica, CSIC, Calle Miguel Luesma Castán 4, Zaragoza 50018, Spain

‡

ABSTRACT: The use of water in hydrothermal and supercritical conditions as a medium to upgrade heavy oil fractions has shown promising results and presents an interesting alternative for heavy oil processing. Water at these conditions improves transport properties increasing the solubility in the medium and reducing the viscosity of the oil, which facilitates the upgrading process. This review focuses on the use of water as a medium to recover and upgrade heavy oils. An analysis of the main reactions occurring, effect of process conditions, and role of water in the reaction mechanism is carried out based on experimental results found in the literature. Studies performed with model compounds that have enabled a proper understanding of the reaction mechanisms, kinetics, and effect of process conditions in the upgrading of heavy oil in near critical or supercritical water are included. An overview of the main challenges of the technology such as corrosion and salt deposition as well as some innovative reactor designs to solve them is provided. Information regarding research carried out in this field has been linked to the growing industrial interest in the technology showing recent developments and registration of patents on reactor designs and processes involving heavy oil upgrading in near and supercritical water.

1. INTRODUCTION World energy consumption has increased significantly during the last few decades and is expected to increase 54% between the years 2010 and 2040.1 Although the share of liquid fuels (mainly petroleum derived) in the energy market is expected to drop, they will continue to supply the majority of the energy consumed worldwide as shown in Figure 1. This fact, combined

as feedstocks with an API gravity below 20° and viscosities over 100 cP at reservoir conditions.5,6 Heavy oil can also be defined as a feedstock with an API gravity below 22° and as an extra heavy oil if it has less than 10° API but a viscosity below 10,000 cP at reservoir conditions.2 Feedstocks with a low API gravity and viscosities over 10,000 cP are considered natural bitumen.2,5 Heavy oils are difficult to process because of their composition, which shows high heteroatom, asphaltene, and metal content as exemplified in Table 1.7−11 These feeds cause common processing problems such as low yields to light fractions and catalyst deactivation, mainly due to metal deposits and coke formation.12 Because of the nature of these oils, the most suitable refining processes seem to be those based on increasing the hydrogen to carbon ratio of the feedstock. This is achieved mainly with technologies based on hydrogen addition and carbon rejection processes. These technologies present advantages and disadvantages based on technical or economic grounds and are determined by the yields of upgraded oil achieved and its quality. Thermal cracking-based technologies can be generally applied to most feedstocks regardless of composition but result in low yields to light fractions and high coke production. On the contrary, hydrogen addition technologies produce high yields of light fractions but require great initial investments and high hydrogen consumption.13,14 Nowadays, traditional oil refining technologies such as coking or hydrogenation are not suitable as a standalone refining method. For greater yields and an increased quality of oils to be obtained, combination and integration of process technologies to combine the main advantages of each technology in a single process is the best alternative.14 Moreover, other technological

Figure 1. World energy consumption and energy supplied from liquid sources outlook 2010−2040. Adapted from ref 1.

with the gradual decline in light oil production, has made heavy oil production and upgrading increasingly important in the global oil market. It is expected that heavy oil production will increase from 9.7 million barrels per day (mbpd) in 2010 to approximately 13 mbpd in 2035,2 having received a boost from rising oil prices between 2005−2014 and the improvement of extraction and upgrading technologies that enable heavy oil processing.3,4 1.1. Heavy Oil. There is not complete agreement on an accurate definition of heavy oil. Some authors define heavy oil © 2017 American Chemical Society

Received: January 27, 2017 Revised: April 11, 2017 Published: April 12, 2017 4571

DOI: 10.1021/acs.energyfuels.7b00291 Energy Fuels 2017, 31, 4571−4587

Review

Energy & Fuels Table 1. Elemental Analysis Data for Different Heavy Oil Feedstocks with Data Adapted from Refs 7−11 country of origin C (%) H (%) N (%) S (%) O (%) Ni + V (ppm) asphalthenes (%) a

Maya7 oil

Cerro Negro8 oila

Morichal8 oila

Athabaska9 bitumen

vacuum10 residue

Paraho11 oil shale

Mexico 83.96 11.80 0.32 3.57 0.35 351.5 11.32

Venezuela 81.20 7.70 2.10 5.50 1.90 nd 11

Venezuela 80.60 8.10 1.80 5.50 4.00 nd 10

Canada 83.20 9.70 0.40 5.30 1.70 340.0 18.6

N/A 85.57 10.49 0.36 4.7 nd 127 7.5

United States 85.3 11.2 2.0 0.5 1.0 nd nd

Elemental analysis of asphaltene fraction.

Figure 2. Properties of water at NCW and SCW conditions with variation of temperature determined using a Peng−Robinson equation of state and a constant pressure of 230 bar.

Figure 3. Number of publications involving research with SCW and oil between 1985 and 2015.

1.2. Near-Critical Water and Supercritical Water. Water near and above its critical point has properties that differ greatly from those at ambient conditions. When reaching a temperature above 374 °C, a pressure of 221 bar or above, and a density of 0.32 g mL−1, water becomes a supercritical fluid. At these conditions, hydrogen bonding, characteristic of ambient water, is almost completely disrupted, turning it into a good solvent for nonionic species rather than ionic species. Significant variations in properties such as density, viscosity,

challenges are the optimization of operating conditions, minimization of catalyst deactivation, and catalyst coking, which need to be further studied and developed.15 The aforementioned issues provide an opportunity to develop new technologies. One such opportunity is the application of hydrothermal processes in near-critical water (NCW) and supercritical water (SCW) as an alternative upgrading technology for the refining of heavy oil feedstocks. 4572


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Figure 4. Hypothetical asphaltene model compound with the exemplification of characteristic model compound structures.

heavy oil model compounds is included as it greatly aids in the understanding of the main chemical reactions, reaction mechanisms, and kinetic parameters involved in the process. This information is linked to the findings of work performed with heavy oils to provide a better understanding of the oil upgrading process in hydrothermal conditions. Special attention is devoted to hydrolysis, partial oxidation, water gas shift reaction (WGSR), as well as processes concerning removal of heteroatoms (sulfur and nitrogen) and metals (nickel and vanadium) from heavy oil or heavy oil model compounds. It is well accepted that reactions at these conditions mainly occur via free radicals. In the case of hydrolysis and oxidation reactions, the reaction rate has been found to follow first order kinetics with respect to the organic compound.24 Carbon monoxide produced in situ by steam reforming or partial oxidation can further react with the excess water in the system to generate hydrogen by the WGSR. This is influenced by process conditions such as temperature and pressure.25 This potential route for the production of in situ hydrogen is of great importance for the upgrading of hydrocarbons in hydrothermal conditions. Finally, relevant studies addressing the main challenges of the deployment of this technology (corrosion and salt deposition) in the oil industry are examined.

thermal conductivity, and heat capacity occur when water changes from liquid to a supercritical fluid. Figure 2 shows the variations in these properties determined using a Peng− Robinson equation of state for an increase in temperature to values near or above the critical temperature at 230 bar. In addition, important changes in the auto dissociation capacity (ion product), viscosity, and transport coefficients, as well as a decrease in the dielectric constant, can also be observed.16 Besides this, NCW and SCW can be considered as green nontoxic substances. Furthermore, in organic chemical reactions, water at these conditions can play the role of solvent, reactant, and catalyst in the same process.17 The reactivity of different organic compounds and main reactions occurring in NCW and SCW have been reviewed elsewhere.18−22 Potential application has also been shown in oil upgrading processes, as NCW and SCW can inhibit the production of coke and lower the viscosity of the feedstocks.23 These properties and the potential application of NCW and SCW for oil processing has increased the interest of industry and the scientific community for doing research in the field resulting in a considerable increase in the number of related publications as shown in Figure 3. The aim of this review is to illustrate why the application of water in hydrothermal conditions near or above the critical point is becoming increasingly important as an alternative heavy oil upgrading medium. Research related to the feasibility of processing and upgrading heavy oil feedstocks in NCW and SCW has resulted in an important number of articles and patents being published in the last two decades. Therefore, this article aims to provide a critical review of relevant studies on the upgrading of heavy oil in this reaction medium with particular emphasis on the main reactions taking place and the effect of process parameters. Relevant work carried out with

2. REACTIONS OF HEAVY OIL MODEL COMPOUNDS IN NCW AND SCW Because of the complex nature of heavy oils, getting a proper understanding of the reactions taking place at NCW and SCW conditions is not an easy task. Several authors have opted to study the reaction with specific model compounds that represent chemical structures commonly found in heavy oils. Some of the main structures studied are polycyclic aromatic hydrocarbons (PAH), heterocyclic sulfur, heterocyclic nitrogen, 4573


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greatly influenced by the operating temperature. High temperatures resulted in a decreased molecular size of the oxygenated aromatic products, whereas at conditions below the critical point, polymerization reactions increase in relevance.34 The effect of pressure, oxygen to organic ratio, and temperature in the hydrothermal partial oxidation of heavy oil was further studied with phenanthrene as model compound. It was observed that changes in pressure had a minor effect on the overall conversion, whereas an increase in the oxygen to organic ratio or temperature results in an important increase in conversion. Interestingly, yields and selectivity to different products showed great dependence on changes in these three operating parameters.35 The use of a catalyst to promote the decomposition of PAHs in water at hydrothermal conditions has been considered. Steam catalytic cracking of anthracene at subcritical water pressures was studied at temperatures between 400 and 500 °C using Ni/Al2O3 catalysts doped with Na, K, and Ca. It was observed that the presence and nature of the dopant had an important effect on conversion and product selectivity. Catalysts doped with Na and K resulted in lower anthracene conversions but higher selectivity toward liquid products compared with those of the undoped and Ca-doped catalysts. This behavior was attributed to the Ni crystal size in the catalyst.36 Further studies using the Ni/Al2O3 doped with K catalyst at the same reaction conditions and different reaction times were performed to gain greater understanding of the main reaction pathways. The conclusion was that two simultaneous reactions take place depending on the C−C scission position. The first through the oxidation of the central ring followed by ring opening to produce biphenyl-type products and the second through the reduction of peripheral rings to produce naphthenic-type products.37 The catalytic hydrothermal decomposition of anthracene in SCW was further studied by Reina et al. with a series of NiMo/SiO2 catalysts. Overall, high yields and selectivities to organic soluble products were obtained as well as a hydrogen-rich gas as an added value product. Their results showed that both the catalytic activity and product distribution are strongly dependent on the composition of the catalyst. They reported that higher catalytic activity and a wider product distribution were achieved at higher nickel loadings.38 2.2. NCW and SCW Desulfurization Reaction. The application of NCW and SCW for upgrading and desulfurization of heavy oils with high sulfur content has received relatively little attention and is a research field that offers many opportunities. Despite the great potential of the technology to upgrade and desulfurize heavy oil, fundamental concepts such as the role of water in the reaction, reaction pathways, role of catalyst, and phase behavior in the system are still not completely understood. An intensive research program aiming to study these and other aspects of hydrothermal upgrading of heavy oil with special emphasis on the removal of sulfur has been developed at MIT in the past few decades. A comprehensive review of their achievements and future research in the field has been published elsewhere.39 Studies on the reactivity of organosulfur compounds in highpressure steam were performed using heavy oil model compounds such as thiophenes and hydrothiophenes.40,41 Experiments were carried out at 300 °C, 85 bar, and varying reaction times between 1 and 28 days to approach conditions of steam stimulation of heavy oil reservoirs. It was observed that hydrolysis and thermolysis reactions occur to produce mainly

and metal organic porphyrins found in asphaltenic structures such as those shown in Figure 4. Contrasting results have been published related to the reactivity of model compounds in NCW and SCW as well as the role that water plays in the reaction mechanism. Some of the work carried out in this area is reviewed in the following sections. 2.1. NCW and SCW Decomposition of PAH. The degradation of PAHs such as acenaphthene, phenanthrene, anthracene, fluoranthene, pyrene, perylene, and fluorene was studied in NCW at 165 bar and temperatures ranging between 100 and 350 °C by Andersson et al. They observed that the main reaction products were oxygenated compounds, mainly ketones and quinones, which indicate that water could act as an oxidant under these conditions.26 Similar results were reported in a study concerning the potential of NCW to degrade phenanthrene in the presence and absence of oxygen. The reactions were studied in a temperature range between 150 and 350 °C with and without the addition of hydrogen peroxide as oxygen source. It was observed that most reaction products were oxygen-containing aromatics in both conditions. However, the addition of oxygen resulted in a wider product distribution and in a significant increase in phenanthrene conversion from approximately 61 to 100% at the highest temperature studied.27 Reactivity of carbo- and heterocycles in NCW and SCW has been extensively studied by Katrinsky et al. They performed a series of studies on the reactivity of PAHs, aryl carbonyl compounds, and aryl oxides in SCW. In addition, a series of experiments adding formic acid and sodium formate were performed to study if these species increased reactivity in the medium. In the case of PAHs and aryl carbonyl compounds in water, low yields of ring opening and reduction products were observed. They found evidence that hydrolysis is both acid- and base-catalyzed, and they concluded that there is no significant change in the reaction mechanisms between NCW and SCW processes except for a slight increase in the radical reactions occurring in the supercritical region.28,29 Polyaromatic compounds are highly stable in water at nearcritical and supercritical conditions. However, high conversions can be achieved with the addition of oxygen to the reaction medium. Experiments on the oxidation of PAHs (naphthalene, biphenyl, fluorene, phenanthrene, and pyrene) as model compounds in SCW have shown that, with the use of an adequate oxygen source such as hydrogen peroxide, high destruction efficiencies can be achieved at high temperature and pressure.30 The effect of process variables in the reaction mechanisms of the oxidation PAHs in NCW and SCW was analyzed with naphthalene, phenanthrene, and pyrene at different reaction conditions. It was reported that reactivity increases when temperature approaches that of SCW. Moreover, an increase in temperature, reaction time, or hydrogen peroxide concentration led to an increase in the formation of CO, CO2, and oxygenated organic compounds. In addition, a detailed reaction mechanism was proposed.31,32 Reaction mechanisms for aromatic and aliphatic hydrocarbons (biphenyl, fluorene, hexadecane, and eicosane) commonly found in oil fractions have also been proposed. Overall, aromatic hydrocarbons showed greater resistance to oxidation than aliphatic hydrocarbons in hydrothermal conditions.33 Results of studies focused on the partial oxidation of PAHs in NCW and SCW using phenanthrene as model compound suggest that the reaction proceeds through the central ring position and start at relatively mild temperatures around 300 °C. It was also reported that the nature of the products was 4574


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sulfur from a gas oil sample doped with model compounds. It was reported that in SCW without the aid of an added catalyst at 400 °C, 250 bar, and 60 min, a low degree of desulfurization was achieved. Sulfur content in the sample was reduced from 0.6 to 0.4% in weight from which it can be concluded that sulfur is very stable to react in SCW alone and that just a slight degree of hydrolysis can occur.49 In order to develop processes based on SCW for the desulfurization of heavy oil, the role that SCW plays in the decomposition of sulfides has to be fully understood. A study combining computational methods and experimental results on the decomposition of sulfides in SCW showed that water plays at least three important functions in the reaction. It can act as a source of hydrogen for the hydrodesulfurization reactions, as a catalyst for hydrogen transfer, and as a reactant in hydrolysis reactions.50 Further studies to gain some understanding of the desulfurization reaction in SCW at a molecular level showed that reactivity was greatly dependent on the nature of the sulfide. It was reported that alkyl sulfides and aromatic sulfides had a similar reactivity that was considerably higher than the one of thiophenes. In addition, it was found that the decomposition of these molecules in SCW was consistent with a radical-based reaction mechanism.51 Studies have proven that when a traditional hydrotreating catalyst is used in an SCW-H2 atmosphere, sulfur-containing compounds including aromatic sulfur compounds can be effectively desulfurized. Adschiri et al. studied desulfurization of dibenzothiophene through in situ generation of hydrogen via WGSR. Results were compared to those obtained when molecular hydrogen was fed to the system. It was found that hydrogen formed in the WGSR was more reactive and had higher hydrogenating capacity due to the formation of active species.52 This confirmed the results of a previous work that concluded that hydrogen formed in situ is significantly more reactive than molecular hydrogen supplied externally.53 2.3. NCW and SCW Denitrogenation Reaction. Nitrogen removal from oil is of great importance because of its potential to cause catalyst poisoning and deactivation, generate toxic oxides during combustion processes, and destabilize fuel. Traditionally, denitrogenation is achieved via hydrodenitrogenation on supported metal sulfide catalysts and, more recently, metal carbides and nitrides.54 Upgrading and heteroatom removal in NCW or SCW is a potential alternative to this process. In similar studies to those performed on sulfur-containing organic compounds, Katritzki et al. performed a series of experiments with nitrogen-containing heterocycles such as pyridine, quinoline, acridine, and phenanthridine in NCW. Thermolysis and aquathermolysis reactions at 350 °C and 3 day reaction time were carried out. They observed that denitrogenation achieved was very low with the exception of acridine, which had a conversion of 50%, but no denitrogenation was achieved. They evaluated the addition of 10% formic acid, 10% sulfuric acid, 10% phosphoric acid, and three clay catalysts to increase conversion and nitrogen removal. When 10% formic acid was added to the system, no increase in conversion was observed for monocyclic structures, but an important increment between 25 and 41% in conversion was observed for polycyclic structures. Overall, the addition of 10% phosphoric acid resulted in a 20% increase in conversion or higher with the exception of pyridine and phenanthridine, which had marginal increases of 9.2 and 8.1%, respectively. In 10% sulfuric acid, only phenanthridine and 2-methyl quinoline showed important

H2S, CO2, CH4, short chain hydrocarbons and substituted thiophenes and hydrothiophenes. They observed that, in the absence of water, greater yields to low molecular weight products are produced, which shows that water has a role in suppressing gas formation. They also noted that water participates in the production of CO2, which suggests it may be a product of the WGSR. A reaction mechanism involving water was proposed. It involves the protonation of the organosulfur, followed by structure opening and desulfurization, decarbonilation, and finally a WGSR.40 Moreover, experiments showed that changes in pH have an impact on the reaction rates. The addition of sulfuric acid to reduce pH increased reactivity in the system, which based on the amount of H2S generated had 10% thiophenes and 14% tetrahydrothiophene hydrolyzed in 14 days compared to 0.75% of thiophenes hydrolyzed after 28 days in the absence of the acid.41 Katrinsky et al. performed studies on the reactivity of different sulfur-containing oil model compounds in NCW and SCW. Compounds such as thiols, sulfides, disulfides, thiophenes, thiophenols, sulfonic acids, alkyl sulfides, and aryl sulfides were subjected to thermolysis and aquathermolysis at different process conditions to evaluate reactivity and desulfurization capacity. In some studies carried out at 250 °C and 5 days, they observed that sulfides and disulfides show greater reactivity in thermolysis than in aquathermolysis conditions, which led to the conclusion that the reaction mechanism predominantly proceeds via free radical over ionic mechanisms.42 On the other hand, they found that phenyl alkyl mercaptans are more reactive in aquathermolysis conditions.43 In the case of arenethiols, some degree of oxidative coupling was observed at 250 °C, whereas diaryl sulfides and heterocyclic sulfur were mainly unreactive. This behavior changed when the reaction took place in aqueous sodium bisulfate where arenthiols reacted through oxidative coupling to give diaryl disulfides in which the rate of conversion increased by a factor of 5.44 In further investigations, they studied the tendency of organic sulfur to generate H2S at 250 and 300 °C. They found that thiophenes are completely unreactive at these conditions and that H2S is mainly obtained from thiols, sulfides, and disulfides through pyrolysis and aquathermolysis with ring cleavage.45 Overall, in NCW, sulfur removal from thiophenes below 1% was achieved with the exception of benzothiophene, which reacted slowly. This is in good agreement with results previously discussed where thiophenes were shown to be almost unreactive in water atmospheres. This shows that heterocyclic sulfur is very stable and cannot be removed through simple aquathermolysis.46 Similar observations were made in studies with SCW where thiophenes showed great stability and a low degree of desulfurization even with the addition of an acid. On the other hand, different model compounds such as thiophenols, alkyl sulfides, and aryl sulfides were effectively desulfurized, especially when an acid was added to the system. The role of the acid is to create a reducing media that proves to enhance the cleavage reaction that is acidcatalyzed.47 The feasibility to enhance the removal of organic and inorganic sulfur with water and methanol blends at supercritical conditions has also been considered. Several experiments were performed varying the methanol/water ratio to find optimum conditions. It was found that in SCW alone the total sulfur removal achieved was up to 14%.48 This topic was further addressed in studies about the potential of SCW to remove 4575


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important influence on the reaction mechanism. It was observed that increments in the initial oxygen loading resulted in greater production of hydrogen favoring the kinetics of the process. Temperature showed a similar trend, resulting in greater reaction rates at higher temperature, whereas pressure showed the opposite effect.61 Similar observations were made in a qualitative investigation on the effect of the operating parameters in the SCW partial oxidation of quinoline using a tubular flow reactor. It was observed that variations in pressure have a minor effect on the oxidation rate if compared to the one of temperature and oxygen loading. Moreover, a detailed kinetic study was performed to determine reaction rates, reaction orders, and kinetic parameters considering quinoline, oxygen, and water as reactants and using a power rate law.62,63 The approximation of the kinetic parameters was made by three different approaches, pseudo first order approximation, integral regression method, and least-squares regression method, and the results are shown in Table 2.63 The authors concluded that

increments in conversion of 50.4 and 44%, respectively. It was observed that the addition of acids to the system did not have an important impact on the rate of denitrogenation with the exception of isoquinoline, which had a rate of 27% with the addition of formic and phosphoric acids. The addition of clays did not improve denitrogenation rates compared to that of water alone.55 Furthermore, thermolysis and aquathermolysis with and without phosphoric acid of pyrroles and indoles were studied at 250 and 350 °C at 1 or 5 day reaction times. It was observed that, at 250 °C, low conversions and low denitrogenation rates were obtained with the exception of 2,5-dimethyl pyrrole, which had a 64.6% conversion. However, when 10% phosphoric acid was added, an increment in conversion was observed for all pyrroles and indoles studied mainly due to methyl transfer and ring opening reactions. When the temperature was increased to 350 °C, mainly char and tar formation was observed.56 As part of the same study, a series of experiments with pyridine analogues and benzopyrroles in SCW or cyclohexane were carried out. It was observed that reactivity in both cases was similar, which indicates that water on its own is not capable of reacting to remove nitrogen from the structure. The possibility of using sodium formate and 15% formic acid as additives to enhance the reduction of the aromatic ring was considered. It was concluded that the potential of denitrogenation is low for most compounds, which indicates that, to effectively remove nitrogen from fossil fuels in SCW, the addition of additives is necessary in most of the systems.57,58 Contrasting results were obtained in a series of experiments to determine the reactivity of quinoline and isoquinoline as model compounds with SCW in the presence or absence of an acid catalyst (ZnCl2). In this study, it was concluded that SCW has a direct influence in the nitrogen removal in the process. It was also reported that the addition of an acid catalyst increases conversion without an apparent change in the product distribution. Evidence of important catalytic activity was observed in the experiments with quinoline for 48 h at 400 and 450 °C, where conversion increased from 27 to 53% and from 7 to 67%, respectively, with the addition of 50 mg of ZnCl2. The same behavior was observed when performing the same experiment at 400 °C with isoquinoline, where conversion increased from 18 to 54%.59 This process was further studied by Li and Houser, who developed a kinetic study with quinoline in SCW also using ZnCl2 as an acid catalyst. Experiments varying time, temperature, water concentration, and catalyst loading were performed. It was found that the rate of reaction is first order for quinoline and catalyst and inverse first order for water. This showed to be consistent with a Langmuir−Hinshelwood surface-catalyzed reaction mechanism with competitive absorption of both quinoline and water.60 As previously mentioned, nitrogen-containing organic compounds show low conversions in SCW, which can be increased with additives to enhance the reaction. This behavior shows similarities with those of sulfur-containing compounds in SCW. However, it has been reported that the removal of nitrogen effectively proceeds via SCW partial oxidation followed by a WGSR on a NiMo/Al2O3 catalyst. Results of a series of batch experiments with quinoline as model compound showed that the reaction pathway consists of two main steps: the in situ production of hydrogen followed by the hydrogenation of quinoline. In addition, process parameters (oxygen loading, temperature, and pressure) were showed to exert an

Table 2. Kinetic Parameters for the SCW Oxidation of Quinoline with Data Adapted from Ref 63 reaction order Ea (J mol‑1)

model pseudo first order integral regression least squares a

Units in M

quinoline

oxygen

water

3

2.1 × 10

1

0

0

224 × 103

3.68 × 10

1

0.36

0

0.8

0.3

0

234 × 10

226 × 103 −0.36

A (s−1) 14 14a

2.7 × 1013b

−1 b

s . Units in M−0.1 s−1.

SCW oxidation of quinoline is of the first order, that the kinetics also depend on oxygen loading, and that water does not have a great impact on the kinetics of reaction. 2.4. SCW Demetallization Reaction. Recent work on the potential application of SCW for the removal of vanadium and nickel from heavy oil has been developed by Mandal et al. Vanadyl and nickel porphyrin molecules (nickel-5,10,15,20tetraphenylporphine, nickel etioporphyrin, and vanadium etioporphyrin) as model compounds were treated in SCW to remove the metal from the organic compound structure. It was observed that SCW was capable of removing metals from the porphyrin chemical structure with high conversions and percentage of metal removal as seen in Table 3. They Table 3. Metal Removal from Metal Porphyrins in SCW at T = 490°C and 250 bar with Data Adapted from Refs 64−66 metal porphyrin nickel-5,10,15,20tetraphenylporphirine nickel etioporphyrin (Ni-EP) vanadyl etioporphyrin (VOEP)

reaction time (min)

conversion (%)

metal removal (%)

90

90

66

180 180

95 90

91 80

concluded that it is possible to achieve metal removal as a function of temperature and reaction time. From their experimental results, it can be suggested that an increase in reaction time from 90 to 180 min had no impact on the overall conversion; however, it does impact the amount of metal removal in the system. It seems that vanadium is more stable and difficult to remove than nickel from porphyrin structures. They observed that the kinetics of the process were consistent 4576


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Energy & Fuels with first order reactions and that the reaction mechanism proceeded via two-step hydrogenation and hydrogenolysis reactions.64−66 2.5. Reactions with Mixtures of Model Compounds in SCW. Olobunmi et al. studied the removal of heteroatoms through ring cleavage of sulfur and nitrogen model compounds (benzothiophene, thianthrene, thiocroman-4-ol and 2(methylthio) benzothiazole, quinoline, and isoquinoline) alone and in mixtures with SCW and inert nitrogen atmospheres. Slow reactions were observed, which were attributed to the great thermal stability of the reactants. They found that interactions with SCW can promote ring cleavage of heterocyclic sulfur and nitrogen, yielding a wider range of products than in the case of neat pyrolysis in nitrogen.67 Experiments with sulfur- and metal-containing model compounds (benzo and dibenzothiophene, diphenyl sulfide, octadecanethiol, nickel, and vanadyl-tetraphenyl-porphyrin) dissolved in gas oil with SCW were performed to compare the ability of SCW to remove sulfur or metal compounds alone or in a mixture. Moreover, the addition of hydrogen and a presulfided catalyst was considered for comparison. In SCW alone, a degree of desulfurization around 15% was achieved. However, in the presence of a conventional catalyst and H2, conversions increased to values around 80%. In a similar trend, metal content reduction around 15% was achieved in SCW alone, whereas with the addition of hydrogen and a catalyst, metals were completely removed, proving that removal of these compounds is not possible in SCW alone.49

16−20%.83 This has been confirmed in other studies in which extract yields between 70 and 90% of oil were achieved in NCW and SCW.84 Changes in process conditions (temperature and pressure) have been shown to have an impact on the oil extraction yield and product composition. With an increase in temperature, extract yields increase until they reach a maximum and then decrease with any further increase in temperature. This behavior is the result of two opposing effects: an increase in the extract yield due to the increase in solubility and decomposition of heavy organic compounds, and the reduction of extract yield due to the decrease in water density with an increase in temperature. An increase in the system pressure leads to an increase in conversion and extract yield but also impacts the extract composition.85,86 Meng et al. found a maximum extract yield at a temperature of 390 °C. It was also observed that the extract rate for a particular fraction maintained the same trend but had a maximum at 390 °C for saturates and aromatics and at 412 °C for resins. This is of great importance as it shows that the yields to certain fractions can be controlled by the temperature of the process. The effect of pressure variation on the extraction yield was also studied. Extraction yield increased from 73 to 81% with an increase in pressure from 200 to 300 bar. It was also shown that changes in pressure affect the extract composition. Extracts obtained at 200 bar had a higher percentage of saturates and lower percentage of aromatics than extracts obtained at 300 bar.85 It was found that water near or above its critical point plays an important role in the dispersion, dehydrogenation of heavier components, and prevention of recombination reactions. Water molecules surround the activated complex, stabilizing it and slowing free radical reactions in the so-called cage effect.87,88 This, results in higher conversions, lower coke yields, and higher lighter product yields in comparison with results obtained in a nitrogen atmosphere as seen in Table 4. Moreover, the residue from the upgrading in SCW has lower molecular weight compounds than the products obtained in a nitrogen atmosphere.89

3. NCW AND SCW UPGRADING OF HEAVY OIL REAL FEEDSTOCKS Recovering and processing heavy oil is of vital importance for securing the supply of energy in the years to come. Research studies on the implementation of water as a reaction medium for the upgrading of heavy oil have shown that the technology has great potential of implementation in up- and downstream applications. Technologies such as steam-assisted gravity drainage have been implemented commercially have been proven to enhance oil recovery and in situ oil transformation.68−71 Some concerns regarding the use of water in heavy oil recovery and upgrading processes are the high energy requirements and process efficiencies. For the energy and oil extraction efficiencies to be improved, the inclusion of additives in the water has proven to be effective. Mixtures of steam with solvents,72−74 gases such as nitrogen or oxygen,75−77 or surfactants78,79 have been tested with heavy oil feedstocks with positive results, increasing the quality of the heavy oil recovered and its yield. 3.1. Extraction and Upgrading of Oil Sands. Oil sands are extra heavy oil sources with an API gravity below 10° and viscosity over 10,000 cP and are immobile at reservoir conditions.2 These unconsolidated sand deposits consist of high molecular weight viscous petroleum and bitumen, which need to be treated to reduce their viscosities before being transported for refining.80 The use of hot water for the extraction of bitumen from oil sands has been studied for decades, and successful industrial processes have been subsequently developed.81,82 Steam is injected into a reservoir, and the condensate with dispersed bitumen is recovered as an emulsion. Application of NCW or SCW has shown to further enhance bitumen extraction and provide some in situ upgrading with extraction yields of up to 24% in SCW and sulfur removal from the extracted bitumen of

Table 4. Results of the Upgrading of Bitumen in SCW or N2 at 450 °C and Reaction Times of 60 and 120 min with Data Adapted from Ref 89 SCW conversion coke yield middle distillate yield gas yield

N2

60 min

120 min

60 min

120 min

0.65 0.12 0.37 0.07

0.71 0.14 0.33 N/A

0.61 0.16 0.32 0.06

0.60 0.23 0.26 N/A

In thermal processes, coke is mainly formed as a result of cracking and recondensation reactions of asphaltenes. Free radicals formed during cracking reactions further react through condensation and polymerization reactions to form coke.90 Experiments on the pyrolysis of asphaltenes showed that considerable thermal decomposition can be achieved at high temperatures starting from 300 °C and ending at 680 °C with a maximum thermal decomposition at 478 °C.91 Coke yield is greatly influenced by interactions between asphaltenes and the solvent as well as by the capacity of asphaltenes to accept hydrogen and of the solvent to donate it.92 It is believed that water in hydrothermal upgrading processes plays an important role in reducing coke yields by donating hydrogen and extracting aromatic species that are coke 4577


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Figure 5. Mechanism of coke suppression in hydrothermal reactions in NCW or SCW. Adapted from ref 93.

water/oil ratio had almost no effect on asphaltene conversions or coke yields. This is because in these processes the effect of temperature or oxygen loading on both asphaltene conversion and coke yield is considerably larger than the effect that water/ oil ratio. On the other hand, an increase in the water/oil ratio seems to decrease the total amount of gas (mainly CO and CO2) produced. This could be explained as greater water/oil ratios will lead to higher pressures, which according to Le Chatelier’s principle will lower the amount of gas produced.96 Sato et al. studied the effects of adding hydrogen, carbon monoxide, and formic acid to SCW as a direct or indirect source of hydrogen for the upgrading of bitumen in a batch reactor at 450 °C and 30 min reaction time. The yield of asphaltenes obtained was in the order SCW > SCW + CO > SCW + H2 > SCW + HCOOH and the yield to coke in the order SCW + H2 > SCW > SCW + CO > SCW + HCOOH. In their experiments, they observed that adding these components enhances the reactivity of asphaltenes and reduces coke formation. In the case of SCW + H2, it gave a higher yield of coke than in the other cases. It seems that the reactivity of hydrogen gas was lower than in the case of hydrogen produced in situ as it was not completely incorporated into the system. On the other hand, when adding carbon monoxide, hydrogen is effectively incorporated into the water through the WGSR (CO + H2O ↔ CO2 + H2O), increasing its reactivity. Overall, it was found that the addition of formic acid showed the best upgrading capacity, reducing yields to coke and increasing asphaltene conversion.95 It has been shown that the WGSR

precursors from the oil phase to the water phase preventing polymerization reactions from occuring. A schematic of the reaction mechanism of coke formation and suppression in low or high extraction medium is proposed in Figure 5.93 Cracking products result from the thermal decomposition of asphaltenes. If the medium has a high extraction capacity, maltenes and cracking products with high hydrogen accepting capacity are mainly solubilized in the water-rich phase preventing coke formation. The cracking products solubilized in the water-rich phase further react to form maltenes. On the other hand, at low extraction conditions, cracking products remain at an important amount in the oil-rich phase. They can react to form coke or maltenes depending on their low or high hydrogen accepting capability. The small fraction extracted from the oil-rich phase will react with water to form maltenes. However, they can also react between them to form asphaltenes that will later precipitate into the oil-rich phase. As NCW or SCW are good solvents for organic species and are considered a high extraction medium, coke yields are expected to be lower than in other processes. Studies on the effect of water density on coke formation during the upgrading of bitumen showed that an increase in density enhances the rate of coke formation and yields a coke with a porous shape.94 Similar behavior was observed in experiments with SCW at 450 °C, where asphaltene conversion and coke formation are enhanced with an increase in the water/ oil ratio.95 However, in NCW or SCW oxidation or partial oxidation where oxygen is present in the system, changes in the 4578


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Table 5. Oil Extract Composition Obtained from Timahdit Oil Shale through Pyrolysis and SCW Extraction with Data Adapted from Ref 107 % weight

pyrolysis

SCW380°C,2.4h,23MPa



asphaltenes maltenes aromatics paraffins polars

8.7 90.2 59.3 20.7 8.1

45.8 53.2 19.4 6.6 25.0

48.4 51.0 17.1 8.6 24.0

32.7 66.2 34.1 12.2 18.5

fluid extraction of oil from Maoming oil shales in water and toluene and compared the results with the ones obtained from neat pyrolysis in argon and Soxhlet extraction with tetrahydrofuran. Similar hydrogen distributions were obtained in all cases, and oil recovery yields were higher in supercritical fluids than in tetrahydrofuran. When comparing the results obtained with supercritical fluids, it was observed that polar compounds are easier to decompose in water than in toluene and that the yield of nonpolar compounds was almost not affected, which indicates that strongly polar or large molecular weight constituents can be decomposed in an easier way into less polar or lower molecular weight compounds in the presence of SCW.106 Similar results were obtained by Harfi et al., who also observed higher oil yields in SCW than in neat pyrolysis conditions. The two oil extract compositions were markedly different as can be observed in Table 5.107 3.3. Upgrading of Oil Residue Fractions. Heavy oils tend to have high asphaltene content and yield a large proportion of heavy fractions such as atmospheric and vacuumdistillation residues, which are of relatively low commercial value and have high heteroatom, metal, and asphaltene content that represents a great challenge for further processing stages.10 Zhao et al. studied the feasibility of upgrading vacuum residue (VR) in SCW at different temperatures, pressures, and H2O/ VR ratios. Optimum experimental conditions were 420 °C, 0.13 g mL−1 water density, water/oil ratio of 2, and 1 h reaction time. At these conditions, yield to coke was low (8.19%), and there was a 53.9% reduction in sulfur content, 83% in nickel, and 86% in vanadium. In addition, the upgraded oil had a 72% reduction in the average molecular weight and a reduction in viscosity of 99.5% in relation to the original feedstock.108 The role of SCW in the upgrading reaction of these fractions was found to be that of a solvent acting as a dispersant of high molecular weight asphaltenes that cannot be completely dissolved. SCW and the asphaltenes constitute a well-dispersed microemulsion that favors the production of lighter oil.109 This is in good agreement with studies by Liu et al., who analyzed the upgrading reaction at both NCW and SCW conditions. It was observed that at these conditions an important suppression of coke and an increase in the yield of liquid products is obtained. In addition, it was found that the reaction mechanism is mainly free radical based and that the influence of the hydrolysis reaction is extremely limited.110 Similar observations were made in hydroconversion experiments of Tahe residue oil when SCW was combined with hydrogen. It was observed that the addition of SCW restrained hydrogenation reactions by quenching free radicals that could potentially lead to further cracking and condensation of high molecular weight compounds, reducing the amount of coke produced and increasing that of liquid products.111 In addition, it has been observed that the presence or addition of light olefins to the feedstock can play an important role as initiator, promoter, and reactant during the cracking of heavy hydrocarbons in SCW.112

mechanism proceeds in two steps: the formation of formic acid (CO + H2O → HCOOH) followed by the decomposition reaction (HCOOH → CO2 + H2) with the former being the rate-limiting step.97 In the SCW + HCOOH system, lower asphaltene and coke yields are attributed to the presence of reactive species derived from the decomposition of formic acid in early reaction stages but also to the later formation of reactive species from WGSR and reverse WGSR.95 3.2. Extraction and Upgrading of Oil Shale. Oil shales are defined as fine-grained rocks containing refractory organic material that can be upgraded into fossil fuel. A small percentage of this material is made of soluble bitumen fractions, whereas the remainder exists as insoluble kerogene.98 Some authors have studied the potential of NCW and SCW to extract and upgrade shale oil. The process of obtaining hydrocarbons similar to natural crude oil from oil shales in water at high temperature and pressure is commonly known as hydrous pyrolysis.99 The effect of temperature on hydrous pyrolysis of oil shales was studied by Barth et al. They studied a range of T from 200 to 350 °C and identified bitumen as their main product with maximum production at a temperature between 310 and 320 °C. The authors proposed a first order kinetic model that predicts an activation energy for bitumen of 71.48 kJ mol−1.100 Experiments on the extraction of fuel from samples of oil shales and Turkish lignites showed that NCW and SCW are effective extraction media and that SCW is especially good for extracting asphaltenes and kerogene-rich fuels.101 It was also observed that an increase in temperature, pressure, or reaction time led to the decomposition of heavy hydrocarbons to lighter hydrocarbons, polycyclic aromatic hydrocarbons, and heterocyclic compounds. The size of the shale lump has an impact on the yield of oil obtained, especially over short extraction times. At early stages, the extract yield in large size shales is much lower than in smaller samples due to higher mass resistance. An increase in reaction time (over 20 h) showed that extract yields level off for the different sizes tested due to fractures that occur in the oil shale while in contact with water at high temperature and pressure.102,103 Paspek and Klein implemented an SCW/HCl system to upgrade oil shale and studied the effect of process conditions (temperature, water density, HCl loading, reaction time, and HCl/shale oil ratio) in the system. Optimal conditions were found with total nitrogen removal of 89% and product yield of 75% at 425 °C, a water density of 0.2 g mL−1, and an HCl/shale oil ratio of 0.05.11 Similarly, Hu et al. studied the pyrolysis of NCW and SCW upgrading reactions and concluded that the addition of water has a positive impact in the yield and final composition of shale oil, finding a maximum yield at 400 °C.104 This is in good agreement with the results obtained by Yanik et al., who observed an increase in oil yield in NCW and SCW105 and with studies with different oil sources as mentioned earlier.85 Moreover, Funazukuri et al. studied the supercritical 4579


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formation of insoluble products at 400 °C and above. On the contrary, ruthenium-based catalyst promoted C−S bond cleavage, which increased the amount of insoluble products due to coupling of the remaining reactive fragments. In both cases, bitumen viscosity was greatly decreased due to reduction of asphaltene molecular weight.127 Similar results were obtained when heavy oil was upgraded via in situ hydrogen formation with different oil or water-soluble metallic catalyst precursors in a water-syngas atmosphere. The addition of the catalysts improved the product distribution and properties, reducing the amount of asphaltenes and avoiding formation of insoluble products compared to the results obtained without a catalyst. It was reported that the best overall performance was obtained with the addition of phosphomolybdic acid hydrate as catalyst. Its performance was compared with the performance of the system without a catalyst; a marginal increase in conversion from 54.9 to 57.71% was achieved with the use of the catalyst at 420 °C, 70 bar, and 90 min reaction time. However, yields to coke were reduced from 5.48 to 2.2% and a considerable reduction in the content of asphaltenes and resins from 9.28 to 3.23% was achieved. In addition, with the use of catalyst, higher yields to cracking fractions between 300 and 500 °C were obtained. This suggests that the addition of catalyst suppresses cracking and results in more liquid products.128 Further experiments were performed to evaluate different hydrogen sources with phosphomolybdic acid hydrate as catalyst. It was observed that the best performance was obtained when syngas was added to the water to form hydrogen via WGSR. This reduced the yields to toluene insolubles and resulted in the highest yields to cracking products between 300 and 500 °C.129 Following these results, a kinetic study of the hydrocracking reaction through WGSR with phosphomolybdic acid in a syngas-SCW atmosphere was performed. It was concluded that the kinetics of the reaction followed a second order of reaction for carbon monoxide and zero order for water as it is present in great excess.130 The use of iron(III) oxide catalysts supported over zirconium or Zr−Al2O3 to upgrade heavy oils in the presence of steam has been extensively studied by Fumoto et al. In this process, active oxygen species generated from the interaction between water and the catalyst react with heavy oil, achieving yields to light oil above 60% and avoiding coke formation. Catalysts were tested at temperatures ranging from 450 to 550 °C at 1.4 h reaction time and a steam/feedstock ratio of 3.2. It was found that the highest yields to liquid products were obtained at 500 °C without any coke formation. In addition, a kinetic model to describe the process was developed considering four product lumps (heavy oil, gas oil, gasoline + kerosene, and gas). It was found that the reaction pathway was composed of a combination of first and second order reactions. The activation energies obtained were found to be lower than the ones commonly found in hydrocracking processes, which suggests that the catalyst aids in the generation of reactive oxygen species from water that facilitates the cracking process.131−135 A different study incorporating zinc or aluminum shavings showed that their addition to the system increased conversion and hydrogenation of bitumen in SCW. It is believed that these occur due to the formation of hydrogen during the oxidation reaction between the metals and SCW. Overall, the addition of any of the metals produced an increase in hydrogen to carbon ratio, increase in bitumen hydrogenation, and changes in the product composition compared to the results in SCW alone. In addition, yields to volatiles increased 15.3% with the addition of

3.4. Upgrading of Asphaltene Fractions. Asphaltenes are the heaviest and most complex constituents of crude oil. They consist of condensed polycyclic aromatic hydrocarbons that contain heteroatoms (sulfur, nitrogen, or oxygen) and metals such as nickel or vanadium.113 Studies on the molecular structure and hypothetical asphaltene models similar to the one shown in Figure 4, have been developed and can be found elsewhere.114−117 Asphaltenes have no definite melting point and, when exposed to a heat source, decompose leaving carbon residues.118 However, as mentioned previously, NCW and SCW inhibit coke production and are suitable media for carrying out cracking reactions. These properties make water a very attractive medium to process highly asphaltenic feedstocks. A process to precipitate and thermally treat asphaltenes in superheated water has been successfully developed yielding liquid hydrocarbons with low fixed carbon, low sulfur content, and lower molecular weight.119 Asphaltenes undergo cracking in SCW mainly through dealkylation of substituents and aromatization reactions to produce gas, hexane soluble liquids, and carbonized solid residues.120 Similar conclusions were obtained in a study of the aquathermolysis of asphaltenes and resins using oil or watersoluble catalysts. In that study, it was observed that oil soluble catalysts have great activity.121 Moreover, it has been observed that an increase in the temperature or water density results in greater conversions of asphalt in both SCW and SCW oxidation.122 Overall, lower asphaltene conversion but greater desulfurization capacity was observed during partial oxidation. Interestingly, the asphaltene fraction obtained during partial oxidation was heavier and had less sulfur than the one obtained in the absence of oxygen. It is thought that this was due to the polymerization of sulfur-containing reactive radicals that were removed in the insoluble fraction.122 3.5. Catalytic Upgrading of Heavy Oil with NCW or SCW. The use of catalysts in NCW and SCW has been considered mainly in the fields of chemical synthesis, waste destruction, and biomass conversion to fuels.123 It is desirable to find a catalyst capable of enhancing the three main reactions, hydrolysis, partial oxidation, and WGSR, taking place in NCW and SCW. The main challenge is to find a material that is catalytically active and sufficiently stable under these conditions.124 Moreover, it has been mentioned that the successful implementation of hydrothermal processes for the upgrading of heavy oil and highly oxygenated feedstocks is strongly dependent on the development of catalysts that can maintain an adequate performance in the presence of high water loadings. Catalysts based on noble metals on acid supports have been used in batch systems. However, further testing needs to be done before their application in commercial processes at larger scale.125 Another important catalyst design variable to consider when upgrading high molecular weight hydrocarbons is catalyst pore size. Good catalytic performance has been observed in experiments on steam upgrading of vacuum residue with a hierarchical macro-mesoporous Al2O3-supported NiK catalyst. The large pores in the catalyst reduced diffusion times for reactants and enabled access to Ni active sites, resulting in high conversions and liquid yields.126 Studies on the use of transitional metal catalysts for the upgrading of heavy oils in water have been performed. Clark and Kirk compared upgrading of bitumen in different reaction media. The conditions studied were just bitumen in water, iron, or ruthenium water solutions at different temperatures. It was observed that the addition of iron(II) as catalyst minimized the 4580


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Energy & Fuels Table 6. Main Corrosion Types, Cause, and Mechanisms Adapted from Refs 144 and 145 type of corrosion

cause

mechanism

pitting corrosion

caused by halogenated anions (Cl, Br, etc.) that penetrate the protective oxide layer; mainly affects stainless steel and Ni-based alloys occurs mainly by a general attack to the oxide film affecting the whole metal surface chemical differences and formation of new phases (carbides and nitrides) at grain boundaries and neighboring areas combination of chemical corrosion and mechanical stress

typically penetrate into grain boundaries and inclusions to form pits; pit size increases by dissolving Ni and Cr components in the metal

general corrosion intercrystalline corrosion stress corrosion cracking

starts as pitting and spreads to the whole material surface causing great mass loss; linear and predictive corrosion rates segregation and enrichment of trace elements at the boundaries weakens the structure making it susceptible of corrosion normally occurs around or through grain boundaries; high stress leads to cracking even with low corrosion potential and vice versa

play an important role in the dissociation of corrosive species and removal of the metal oxide protective layer from the equipment.142 Different factors such as high temperature and pressure, the species present in the medium, and oxygen concentration in the system are relevant for the long-term operation regardless of the material of construction.143 The most common corrosion mechanisms normally encountered in processes operating with NCW and SCW are briefly described in Table 6.144,145 It has been stated that areas in the reactor at hot temperatures but in the subcritical region such as the feed inlets are particularly vulnerable to corrosion.145 For this reason, reactor design and operation become relevant in the long term performance of the reactor. An overview of the mechanism of corrosion and construction materials including different kinds of steel, ceramics, and Ni-, Ti-, and Zr-based alloys capable of withstanding SCW corrosion in different reactive environments have been reported in the literature.143 Analysis of corrosion and oxidation rates of ferritic-martensitic steels in SCW processes showed that dissolved oxygen is responsible for the high corrosion rates and that the effect is increased with temperature.146,147 In studies concerning the corrosion mechanism of stainless steel 316 (SS-316) in SCW oxidation processes, it was shown that duplex layers of oxides were formed and some degree of dealloying was observed.148,149 Moreover, high corrosion rates were observed on Ni-based alloys and SS-316 when used under SCW oxidation conditions.150 Watanabe et al. found that weight loss in SS-316 was less pronounced than in Ni alloys and showed good resistance in SCW.151 However, contrasting results were obtained by Saito et al., who found a lower weight loss in Ni alloys, which was attributed to the higher chromium content in its structure. In the same study, the resistances of alloys with different metal compositions were evaluated, and it was concluded that the most suitable construction materials based on resistance and cost were Ti, Ta, and Ti−Pd alloys.152 The use of alternative materials such as ceramics has been considered to deal with corrosion issues. It was observed that most ceramic materials with the exception of monolithic alumina and partially stabilized zirconia showed low resistance to corrosion and severe weight loss.153 Corrosion in SCW, materials of construction, corrosion mechanisms, and influences of process parameters and conditions are reviewed in great detail elsewhere.143,144,154 As mentioned before, several factors promote corrosion in SCW reactors. This makes it impossible to find a single solution to control it; thus, a combination of control measurements can be the best alternative. Traditionally, two main approaches have been followed to protect equipment from corrosion in SCW. The first focused on the resistance and physical protection of the equipment against the corrosive environment, and the second focused on the preconditioning of the process feed as summarized in Figure 6. A detailed review

zinc and 38.2% with aluminum. However, it was observed that, when zinc was added, a maximum increment in product yield up to 62.3%, a high yield of resins up to 33.5%, and a high oxygen content in the products were achieved. On the contrary, the addition of aluminum reduced the yields to resins to 7.5% and resulted in the absence of oxygen in the liquid products.136 Desulfurization of a vacuum residue sample has been performed by partial oxidation and in situ hydrogen generation in SCW with the addition of a CoMo/Al2O3 catalyst. It was shown that pyrolysis and desulfurization in SCW can be carried out simultaneously with high reduction of sulfur content.137 Ates et al. assessed the use of ZnO, MoO3, and MoS2 as economic catalysts for the upgrading of heavy oil in SCW. Overall, the addition of the catalyst had a positive effect, increasing the desulfurization capacity of the system. Analysis of the spent catalysts showed that MoS2 was resistant to the reaction medium, whereas ZnO and MoO3 showed important structural changes.138

4. MAIN TECHNOLOGICAL CHALLENGES Because of the nature of heavy oil feedstocks and the conditions of the process, several technological challenges have to be overcome to enable the application of SCW in upgrading processes. It has been identified that the main problems affecting industrial scale up and commercialization of the process are corrosion, salt deposition, and high cost.139 Although most petroleum refining processes include a desalting stage, desalting efficiency is not 100% and often leaves a considerable amount of salt in the oil stream. This represents a great challenge to refineries as salt deposits can cause plugging in different units, which may result in the shutdown of certain operations or the entire refining complex. Salt solutions and deposits can, depending on their nature, present a high risk of localized corrosion in process units.140 Corrosion in hydrothermal processes is accentuated by the presence of water, which at these conditions is highly corrosive itself. Moreover, salt solubility in SCW is very low, aggravating fouling and corrosion due to salt deposition. These technological challenges are further magnified in the case of a process involving oxidation or partial oxidation reactions, which lead to problems in the choice of materials of construction and design. An overview of the different technical aspects and challenges of the partial oxidation process in SCW and a review of related literature has been presented elsewhere.141 4.1. Corrosion. NCW and SCW can become a potentially corrosive environment, especially if there is dissolved oxygen due to the high gas miscibility. The low solubility of ionic compounds promotes the precipitation of salts that can be highly corrosive in nature. The extent of corrosion in the system is greatly dependent on the physical properties of water, especially density, ion product, and dielectric constant, which 4581


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Figure 6. Corrosion control methods for supercritical water processes adapted from ref 145.

operates at lower temperature up to 400 °C and is designed to withstand high pressure. The pressurized feed stream flows between the two concentric tubes and cools the reaction medium as can be observed in Figure 7a. The main advantages of the reactor is the high temperature, high conversion, and corrosion control that can be achieved.163,164 The design has been tested in a pilot plant scale in the University of Valladolid achieving total organic carbon (TOC) removal efficiencies above 99%.163,165 The reactor has been further scaled up to a demonstration plant scale for wastewater treatment.166 This reactor design has proven to be very effective for dealing with the problem of corrosion but has the limitation of operating with influents with low salt concentration to avoid plugging in the system. A novel reactor designed to overcome both corrosion and salt deposition is the transpiring wall reactor. This reactor design also consists of two vessels: an external vessel made of SS-316, which can withstand the high pressure and temperature of the process, and an internal vessel where the reaction takes place made of a porous material that is highly resistant to corrosion. Water and oxidant enter the reactor vessel in radial flow through the porous barrier and the organic feed at one of the ends of the reactor as seen in Figure 7b. This design is employed to dilute corrosive species in the wall vicinity, to cool the reaction medium where the exothermic reaction is taking place, and to prevent deposition of solids in the reactor wall. The reactor design has been modeled and tested with TOC removal efficiencies over 99%.167,168 Bermejo et al. applied the same principle with some operation modifications to build a pilot plant in the University of Valladolid to process 20 L h1− of wastewater at supercritical conditions with high conversions.169 A study on the effect of operating conditions was performed in a pilot plant with a maximum capacity of 25 k gh−1 built in Shandong University in China. It was found that parameters such as feed concentration and residence time are important factors affecting feed degradation, whereas transpiration intensity and feed temperature have a minor impact on the overall TOC removal efficiency.170 An additional improvement to the concept of a transpiring wall is the implementation of a hydrothermal flame as heat source as shown in Figure 7c. This allows the reactants to reach reaction conditions and achieve high conversions without the need to be in contact with the reactor wall. This concept was

on corrosion control methods for processes involving SCW conditions can be found elsewhere in the literature.145 4.2. Salt Deposition. When water approaches its critical point, salt solubility is greatly decreased. This potentially leads to plugging and fouling in the reactors, which damage the equipment and cause elevated maintenance and operational costs. Salts can be present in heavy oil feedstocks, especially in the remaining brine, and are normally removed in a desalting unit.155 If oil is not pretreated or salt is not completely removed, potential problems can be encountered in further processes, especially if SCW is involved. Salt deposition models of sodium and potassium sulfates commonly found in SCW processes have been developed taking into account transport parameters and solubility of inorganic salts.156,157 Salt precipitation and scale build up has hindered the spreading of SCW processes in the industrial sector due to the operational challenges and high cleaning costs. One possible solution is the continuous removal of salts from feed mixtures at SCW conditions. Schubert et al. studied the possibility of installing a salt separator vessel to precipitate and separate salts continuously in a laboratory scale hydrothermal gasification plant. In their studies, they assessed the performance of the separator with feeds of water with different types of salt and mixtures of them. They observed separation efficiencies ranging from 80 to 97% and concluded that salt separation greatly depends on the type of salt and the phase behavior of the mixture.158−160 Some fundamental principles, research, and commercial approaches to control salt precipitation and solid build up have been reviewed elsewhere.161,162 4.3. Innovative Reactor Designs. Corrosion and fouling are major obstacles to the commercialization and positioning of SCW-based technologies. Different innovative reactors as the cool wall reactor, transpiring wall reactor, hydrothermal flame reactor, and floating-type reactor have been developed in an attempt to reduce exposure of the pressure-bearing section to the highly corrosive process fluid. These reactors have been successfully tested in SCW processes aiming to remove pollutants from water effluents. The cool wall reactor has been designed to separate the temperature and pressure effects into two different chambers. In the inner chamber, made of corrosion-resistant material, main reactions occur in a highly corrosive medium at high temperature, whereas the outer chamber made of SS-316 4582


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Figure 7. Innovative reactor designed to overcome corrosion and salt deposition in SCWO processes: (a) cool wall reactor, (b) transpiring wall reactor, (c) hydrothermal flame, and (d) SUWOX reactor. Adapted from refs 141,167,171, and 176.

incorporated to a transpiring wall reactor achieving conversions over 99% at feed temperatures as low as 125 °C.171,172 The conditions for spontaneous ignition of hydrothermal flames were studied with an organic decomposition rate up to 99.9%. It was found that a 2% v/v of fuel, temperatures above 470 °C, and air to carbon ratios over 1.8 are necessary to achieve spontaneous flame ignition using 2-propanol as flame fuel.173 The use of methanol−water and methane−water have also been analyzed to determine minimum ignition conditions. It was found that with both fuels concentrations as low as 6% mol and temperatures of 450 °C and greater lead to the spontaneous ignition of the flame.174 Further experiments to analyze the possibility of scaling up transpiring wall reactors with a hydrothermal flame were performed by Bermejo et al.175 Depending on the nature of the feed, the content of heteroatoms can vary and cause severe corrosion problems in the reactor. This problem gets worse when halogenated species are present. A very effective design to overcome this problem is

the SUWOX reactor. This reactor is especially designed to resist acidic species at SCW oxidation conditions. The fundamental aspect of the design is similar to the reactors previously explained with a vertical double wall reactor with an outer tube designed to resist high pressure and a ceramic nonporous inner tube designed to resist corrosion. Water is supplied at the bottom, and a neutralizing agent is fed at the top of the reactor, leaving the supercritical zone floating in the vertical double wall reactor. The space between the inner and outer wall is filled with water, which plays the role of coupling agent with respect to temperature and pressure in the system. At the outlet of the reaction chamber, the product stream with the acidic species is neutralized by the neutralizing agent at subcritical water conditions before leaving the reactor as seen in Figure 7d. A complete conversion of model compounds has been achieved with the system without any corrosion or fouling detected.176 This reactor has proven to be very effective when treating feedstocks with halogenated or acidic compounds.177 4583


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Energy & Fuels Table 7. Oil Upgrading with SCW Relevant Patents Registered between 2008 and Present179−190 patent number US US US US US US US US US US US US

20140334985 20130140214 20120181217 20110315600 20110203973 20110147266 20100314583 20100189610 20090166262 20090166261 20080099378 20080099377

A1 A1 A1 A1 A1 A1 A1 A1 A1 A1 A1 A1

title

year

assignee

process for upgrading heavy and highly waxy crude oil without supply of hydrogen supercritical water process to upgrade petroleum petroleum upgrading and desulfurizing process removal of sulfur compounds from petroleum stream process for upgrading hydrocarbons and device for use therein petroleum upgrading process supercritical water processing of extra heavy crude in a slurry-phase up-flow reactor system apparatus for upgrading heavy hydrocarbons using supercritical water simultaneous metal, sulfur, and nitrogen removal using supercritical water upgrading heavy hydrocarbon oils process and reactor for upgrading heavy hydrocarbon oils process for upgrading heavy hydrocarbon oils

2014 2013 2012 2011 2011 2011 2010 2010 2009 2009 2008 2008

Saudi Arabian Oil Company Saudi Arabian Oil Company Saudi Arabian Oil Company Saudi Arabian Oil Company Chevron USA Inc. Saudi Arabian Oil Company Conoco Phillips Air Products and Chemicals Inc. Chevron Corporation Chevron Corporation Chevron Corporation Chevron Corporation

cracking, heteroatom removal, and demetallization reactions can be successfully achieved in NCW and SCW. Moreover, studies with model compounds enable the proper understanding and determination of reaction mechanisms, kinetics, and effect of process conditions in the reaction system. The main technological challenges identified are corrosion and salt deposition, which increase operating and maintenance costs, preventing the widespread application of the technology. Reactors built of different materials and with different configurations have been designed to address the main technological challenges in the process. The promising results have developed an interest in companies to develop research in the area to evaluate potential reduction of operating costs and make the process technically viable. This has resulted in a number of patents being registered on reactor and process design concerning SCW upgrading of heavy oil in the past decade.

The concept of the SUWOX reactor was further studied at the lab scale and in a 400 kg h−1 pilot plant for the treatment of industrial wastewater with TOC removal percentages over 99%.178

5. APPLICATIONS AND PROCESSES FOR THE OIL INDUSTRY Cracking of high molecular weight hydrocarbons is an industrial process yielding liquid and gaseous products. It is of great interest to increase selectivity of the products toward valuable liquid products. It has been observed that when cracking processes are carried out in supercritical fluids, yields to liquid products are increased and, consequently, gas production decreases. The environmentally friendly nature of water and the characteristic properties of SCW as solvent and reactant have developed interest in the oil industry. Oil companies such as Chevron, Conoco Philips, and Saudi Arabian Oil have considered the use of water at hydrothermal conditions for the upgrading of heavy and extra heavy oil into lighter fractions suitable for transportation fuel. This has resulted in a number of patents on process designs, reactor designs, and prototypes filed by these companies between the years 2008 and 2016. Some relevant patents published between the years 2008 and 2014 can be found in Table 7.179−190

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

M. Millan: 0000-0003-2019-6525 Notes

6. CONCLUSIONS In this work, relevant studies published on the application of water near or above its critical point to upgrade heavy oil feedstocks and related model compounds have been reviewed. The particular properties of water at these conditions make it a favorable medium for the upgrading of heavy oil. This is mainly because of the capacity of NCW and SCW to play the role of solvent to organic compounds and dispersant of very heavy compounds, which cannot be dissolved completely but form an emulsion favorable for upgrading reactions. In addition, some experimental results show that NCW or SCW participates as reactant in the system. It can be concluded that reaction mechanisms proceed mainly via free radicals (more pronounced in SCW conditions than in NCW) and that the presence of water reduces the formation of coke in the reaction. Reaction rates and conversions in SCW alone are relatively low; implying that upgrading in SCW alone is not feasible. However, when oxygen, acidic additives, light olefins, or catalysts are added to the system, conversions increase. Reactions such as hydrolysis, partial oxidation, and WGSR take place in NCW and SCW and play a major role in the reaction mechanisms for the upgrading of model compounds and heavy oil. Overall,

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS P.A.-A. wants to thank CONACYT Mexico for financial support. J.L.P. would like to thank the Spanish Economy and Competitiveness Ministry (MINECO) for his Ramon y Cajal research contract (RYC-2013-12494).

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