Corrosion Evaluation and Material Selection for Supercritical Water Reactor Used for Heavy Oil Upgradation

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Supercritical water is uniquely a green medium for diverse applications because of its changing nature from polar to non-polar. Owing to this property, it is being considered for heavy oil upgradation since it dissolves both organics (oil) and
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  Vol.:(0123456789)Oxidation of Metalshttps://doi.org/10.1007/s11085-019-09890-5  1 3 REVIEW Corrosion Evaluation and Material Selection for Supercritical Water Reactor Used for Heavy Oil Upgradation M. Faizan Khan 1,3  · Akeem Yusuf Adesina 2  · Sikandar Khan 1  · Anwar Ul-Hamid 3  · Luai M. Al-Hems 3 Received: 20 July 2018 © Springer Science+Business Media, LLC, part of Springer Nature 2019 Abstract Supercritical water is uniquely a green medium for diverse applications because of its changing nature from polar to non-polar. Owing to this property, it is being con-sidered for heavy oil upgradation since it dissolves both organics (oil) and hydro-gen while inorganics behave conversely. However, because of the high pressure and temperature (22.1 MPa, 374 °C), corrosive environment (chlorides, sulfides and salt deposition) and stresses involved, there are serious concerns encountered while utilizing supercritical water in reactors. These include change in the component-material microstructure due to hydrogen ingress, sulfide stress corrosion cracking and salt deposition leading to pitting and de-alloying. Various alloys such as fer-ritic–martensitic steels, austenitic stainless steels, Ti-, Ni- and Zr-based alloys have been used, while new alloys and materials are continuously being investigated to considerably help abate these problems and ultimately improve the life of reactors. Despite significant past efforts in material development, reactors still suffer from these problems and challenges. This review assesses materials selection, the current progress in material development as well as their potentials in ameliorating reac-tors resistance to oxidation, pitting, embrittlement, etc. This study aims to improve understanding of material selection for supercritical water reactors based on the cor-rosive environment of the reactor and hence help engineers to make insightful deci-sions in selecting material for the specific corrosive environment.   *  M. Faizan Khan mfaizan693@gmail.com 1  Department of Mechanical Engineering, King Fahd University of Petroleum and Minerals, Dhahran 31261, Kingdom of Saudi Arabia 2  Center of Research Excellence in Corrosion, King Fahd University of Petroleum and Minerals, Dhahran 31261, Kingdom of Saudi Arabia 3  Center for Engineering Research, King Fahd University of Petroleum and Minerals, Dhahran 31261, Kingdom of Saudi Arabia   Oxidation of Metals  1 3 Graphical Abstract Schematic illustration of materials susceptibility in supercritical water reactor Keywords  Supercritical water · Heavy oil upgrading · Corrosion · Oxidation · Reduction · Reactors · Alloys · Materials Introduction Supercritical water reactor (SCWR) has been identified as a potential and promising technology to combat appalling raise in worldwide CO 2  emission from current tech-nologies utilized for energy production. Besides, SCWR is an economical alterna-tive due to its higher efficiency in heat transfer, lack of complications in its design as well as the fact that water at ambient temperature is relatively cheap and non-toxic. These properties have attracted increasing interest in SCWR and its deployment in different applications including supercritical water oxidation, supercritical water gasification and recently for upgrading heavy oils, etc. It is also being considered under the generation IV program for future nuclear power plants to generate low-cost electricity [1]. Despite the benign, economic, sustainability and higher thermal efficiency characteristics of SCWR, the presence of elevated temperature and pres-sure operating environment posed a serious corrosive challenge to components of the reactor. Most SCWR environments are aggressively oxidizing due to increased content of dissolved oxygen and also the presence of wide range of other substances which make the operating environments more prone to oxidation and other forms of corrosion.   1 3 Oxidation of Metals With the future concern, the continuous increasing demand (Fig. 1) of conven-tional oil resources (gasoline and diesel) has urged oil-producing nations to uti-lize unconventional oil as well parallel to conventional oil. These unconventional oil resources are basically termed as “heavy oils” or extra heavy oils and consist of mainly asphaltene and bitumen [1]. Usually the density of the oil relative to the den-sity of water also known as the specific gravity and the American Petroleum Insti-tute (API) gravity which is expressed as the inverse of the specific gravity are used to categorized the oils. Thus, oils with API gravity less than 22° are termed heavy oil. These are thicker and higher in density but have those entire conventional oil characteristics inherent in them [1–4] and that’s why the name “unconventional oil” was recommended to them.To fully yield the advantages of SCWR for heavy oil upgradation, materials with improved corrosion and oxidation resistance, enhanced strength and embrittlement resistance as well as advanced creep resistant materials must be developed. This review article aimed to summarize recent studies which explore the potentials of various materials and their corrosion resistance behavior under supercritical water condition. This will present a summarized guide to engineers, researcher and sci-entist in understanding the corrosion types, mechanisms and appropriate prevention measures with respect to material in supercritical water (SCW) condition and thus help them make informed decision as to appropriate research paths and/or materials selection and qualification for SCWR applications. Supercritical Water’s Distinct Properties Supercritical water (SCW), a state of water achieved under extreme temperature and pressure (374 °C, 22.1 MPa), is a distinctive source that is being used for diverse applications through a number of years [6–10]. It is one-component water under a temperature and pressure condition exceeding its critical temperature of 374 °C and Fig. 1 Global market demand in millions barrels per day (MM BPD) for refining products. [5]   Oxidation of Metals  1 3 pressure of 22.7 MPa [10]. SCW exhibits properties such as density and viscosity which are considered intermediate in between those of vapor and liquid states. In contrast with other supercritical mediums, supercritical water has a single and non-condensable state. Its density is close to that of its liquid phase, while transporting properties matches with that of the gases. Also, water acts differently from other flu-ids because its solvation nature changes dramatically from subcritical to supercriti-cal condition with the heat phenomenon. With the increase in temperature, SCW losses the hydrogen bonding with the increase in density and hence it behaves as non-polar rather than the polar solvent at ambient conditions [11].Other factors which distinguish the behavior of water, from subcritical to super-critical nature, are dissociation and dielectric constant at very higher pressure (25 MPa). For instance, the dielectric constant was found to be dropped from 80 to 10 when the condition was altered from ambient (25 °C, 0.101325 MPa,) to super-critical (374 °C, 22.1 MPa), respectively. Further increasing in the temperature to 425 °C results in decreasing the dielectric constant to 2 [12]. Similarly, at the same pressure (25 MPa), the dissociation constant fell from -11 to -12 (mol/kg) 2  at 200–300 °C temperature range and abruptly decreases from -11 to -18 (mol/kg) 2  with the increase in temperature above 500 °C [12]. The variation of density, dielec-tric constant and dissociation constant with the increase in temperature is depicted in Fig. 2. Heavy Oil Composition and Characteristics Crude products having American Petroleum Institute (API) gravity less than 22° are classified under the category of heavy oil [1]. Heavy oil for a longer period of time was not considered for refining purpose because they contain mainly Fig. 2 Water properties variation with temperature and pressure [12]   1 3 Oxidation of Metals high molecular weight hydrocarbons with the addition of higher level of hetero-compounds like sulfur, nitrogen, oxygen and metallic elements [13]. However, increasing energy demand on daily basis forced the engineers to consider heavy oil as well for refining purpose in order to meet domestic and industrial needs. Mostly the molecules which form these hydrocarbons contain more than 15 car-bon atoms, and this results in complex and expensive refining processing [4]. These high molecular weight compounds ultimately produce a little amount of desired high-octane hydrocarbons and diesels in the refineries with the addition of secondary byproducts and mixture [14]. Such byproducts include sulfur-con-taining compounds such as sulfides, thiophenes and thiols compounds which are considered more harmful in the refining units. Nitrogen-containing compounds consist of amines, diamines and pyridines, which are among common products. In non-basic form, nitrogen compounds can be in the form of carbazoles, indoles and their derivatives. Typical constituent of porphyrins can also be found in the non-basic nitrogenous compound. Additionally, oil-emulsified water contains the dissolved organic salts which are considered the main source of metals. Fur-thermore, heavy oil usually contains nickel and vanadium, which after reacting with porphyrins, form chelates [13]. These metallic compounds can result in corrosion problem and affect the catalyst function. The oxygenated compounds also appear in the form of carboxylic groups with the addition of ketones, ethers and anhydrides, and the content of these compounds determines the acidity of the heavy oil. This is a principal factor in terms of processing for determining the final cost and market price. So, all these compounds and elements create the complexity in dealing with heavy crude oils.Although the complete elemental characterization of heavy oil product is not possible because of its complex nature, the composition characterization can be achieved to a higher extent by fractioning the process on the basis of polarity and solubility [15, 16]. Non-polar hydrocarbon (both branched and liner chains) and aliphatic cyclic paraffin form saturates. Also, fractions of aromatic rings (one or more rings) attached with aliphatic chains correspond with aromatics [13]. High molecular weight hetero-compounds from the crude oil form the asphaltene and resins, while higher hydrocarbons, like pentane and all those whose density is at least equal to 1 g cm −3  and molecular weight ranges from 500 to 2000 g/mol, can dissolve these resins but they are insoluble in lower hydrocarbons like pro-pane. On contrary, asphaltenes are commonly defined as “the crude oil fraction insoluble in n-alkenes of low molecular mass, they are however soluble in tolu-ene or benzene [13, 17] and are in a class of petroleum macromolecules com- posed of poly condensed aromatic rings and lateral aliphatic chains, presenting a smaller proportion of acidic and basic functional groups.” Table 1 depicts the elemental composition of the samples obtained from Venezuelan heavy oil res-ervoir [18], while elemental composition of asphaltene, after precipitation with n-pentane, obtained from different countries crude oil is given in Table 2 [13]. Aromaticity, which is the ratio of carbon to hydrogen, has been noted for asphal-tene collections with wide range, i.e., from 1 to 1.56.
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