Review on the Processing of Crudes with High Acid Numbers

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Abstract: It introduces the distribution pattern of petroic acid in crude oil, the corrosion mechanism of naphthenic acid, and the factors affecting naphthenic acid corrosion. It reviews the processing methods for high-acid crude oils in recent years, as well as the situation of some domestic refineries in processing such crude oils and the corresponding protective measures. It focuses on the refining of high-acidity crude oil at the Maoming branch, analyzes the acid distribution patterns in various fractions of such crude oil, and proposes strategies for refining high-acidity crude oil. Keywords oleic acid, naphthenic acid, high-acid crude oil, acid removal, acid distribution. With the continuous development and utilization of crude oil, the production of heavy crude oil is increasing, and its density, viscosity, and acid value also rise accordingly. In recent years, the global production of high-acid crude oil has been increasing by about 0.3% per year. In 2001, high-acid crude oils with a acid number (TAN) of more than 1 mgKOH/g accounted for 5.0% of the world’s total crude oil production, and this figure was expected to rise to 5.5% by 2006. The variety and quantity of acid-containing crude oil in the country are also on the rise. Since acidic crude oil causes significant corrosion to production equipment during processing, there is an oversupply of high-acidity crude oil in both international and domestic markets, resulting in low prices. With crude oil procurement costs accounting for 90% of the total costs, purchasing crude oil with a low acid value appropriately can make a significant contribution to economic efficiency. Therefore, it is very important to know how to process crude oils with high acid values. 1 Petrolic acid in crude oil and its distribution pattern. The acidity value of crude oil reflects the amount of acidic oxides such as naphthenic acids, fatty acids, and phenols (collectively referred to as petrolic acids) present in it. The acid value of crude oil is the total amount of KOH consumed by various acidic components in 1 g of crude oil, expressed in mgKOH/g. In addition to naphthenic acids, the acidic substances in crude oil include fatty acids, aromatic acids, inorganic acids, thiols, hydrogen sulfide, and phenol. When the acid value of crude oil is greater than 0.5 mgKOH/g, it can cause equipment corrosion; therefore, crude oil with an acid value above 0.5 mgKOH/g is generally referred to as high-acid-value crude oil. Among the acidic substances in crude oil, naphthenic acid is the most important and is present in high concentrations; it accounts for about 90% of the acidic substances in crude oil. Naphthenic acids are very complex mixtures of carboxylic acids containing five- or six-membered rings; their relative molecular weights vary widely, but they are mostly in the range of 300–400. Low-molecular-weight naphthenic acids have a certain solubility in water, whereas high-molecular-weight naphthenic acids are almost insoluble in water. The distribution pattern of naphthenic acids in crude oil is quite unique: the content of naphthenic acids is highest in the middle distillates (250–500°C), while it is relatively low in both the low-boiling and high-boiling fractions. In other words, the naphthenic acid content generally increases gradually starting from the kerosene fraction, reaches almost its peak in the diesel fraction, and then decreases [3]. 2 Corrosion caused by processing high acid number crude oil: In refineries that process high acid number crude oil, equipment corrosion is mainly caused by naphthenic acids. The high-temperature parts of its distillation equipment are severely corroded; this corrosion occurs mainly in the equipment and pipelines located in these high-temperature areas, such as the elbow at the furnace outlet of atmospheric pressure furnaces and vacuum furnaces, the furnace outlet valves, the high-speed and low-speed sections of the oil transfer lines, as well as the feed evaporation sections of atmospheric pressure columns and vacuum columns, the column walls, trays, main beams, and support beams. The corrosion caused by naphthenic acid differs from sulfur corrosion; it is not uniform corrosion but rather localized or pitting corrosion. Moreover, naphthenic acid corrosion is influenced by various factors such as the acid value, temperature, flow rate, medium, and changes in the physical state of the material, which makes it difficult to detect. 2.1 Corrosion mechanism of naphthenic acids: During petroleum refining, naphthenic acids are heated and distilled along with crude oil; they condense together with oils that have the same boiling point and dissolve in them, thereby causing corrosion of the equipment materials exposed to these fractions. Currently, the reaction mechanism for naphthenic acid corrosion is generally considered to be as follows: 2RCOOH + Fe → Fe(RCOO)2 + H2. Field experience shows that naphthenic acid corrosion often occurs in process media with a acid value greater than 0.5 mgKOH/g, at temperatures between 270 and 400°C, and under high flow rates. It reacts directly with the metal surface or iron sulfide film to produce iron naphthenate. Ferrocyclohexanecarboxylate is oil-soluble, and together with the flow of the medium, it causes the metal surface to be continuously exposed and corroded. Therefore, the metal surface corroded by naphthenic acid is clean, smooth, and free of impurities. In the areas of high temperature and high flow velocity of the material, naphthenic acid corrosion results in streamline grooves with sharp edges that form in the direction of flow ； In the low-flow region, it appears as a pit with sharp edges. Naphthenic acid corrosion occurs in areas such as trays, tower walls, and oil transfer lines. Furthermore, due to their surfactant properties, naphthenates cause severe emulsification of crude oil, leading to fluctuations in plant operation and corrosion at the top of the towers. 2.2 Factors Affecting Naphthenic Acid Corrosion 2.2.1 Influence of Temperature The corrosion caused by naphthenic acid is greatly affected by temperature. Naphthenic acid is non-corrosive to metals at room temperature, but at high temperatures it can react with iron to form iron naphthenate, causing severe corrosion. Naphthenic acid begins to cause corrosion at 220°C, and the corrosion intensifies as the temperature rises. Corrosion is already severe at 270–280°C; it decreases as the temperature rises, but increases sharply at 350–400°C. 2.2.2 Effect of flow rate Flow rate and flow pattern are very important factors affecting naphthenic acid corrosion. Turbulence generated in the elbows, tees, and pumps of refining equipment accelerates equipment corrosion. Jets with a gas volume of more than 60% and a steam flow velocity of more than 60 m/s suffer the most severe corrosion. Under such conditions, the corrosion rate of certain equipment, such as furnace tubes, elbows, and pipelines, can increase by two orders of magnitude. Under high temperature and high flow rate conditions, with an acid value at a very low level (0.3 mgKOH/g), the corrosion rate of carbon steel remains high. 2.2.3 Effect of crude oil sulfur content: The corrosion reaction products of naphthenic acids with metals are oil-soluble Fe(RCOO)2, which can be carried away by the oil flow; as a result, it is difficult for a protective film to form on the surface of metal equipment. These corrosion products tend to dissociate from the metal surface, allowing corrosion to progress deeper into the material. When metal is in prolonged contact with naphthenic acids and the crude oil flow rate is high, a characteristic grooved corrosion can be observed on the metal surface. This is the marker that distinguishes naphthenic acid corrosion from other types of corrosion. If the crude oil contains active sulfides, at high temperatures these active sulfides begin to decompose, producing hydrogen sulfide. The reaction of hydrogen sulfide with metals is: H2S + Fe → FeS + H2. The corrosion product, the FeS film, provides a certain degree of protection under certain conditions. In naphthenic acids, the FeS film is dissolved; the reaction is as follows: FeS + 2RCOOH → Fe(RCOO)2 + H2S. The hydrogen sulfide produced then causes corrosion of the equipment downstream, and this cycle of corrosion exacerbates the degradation of the metal. 3 Anti-corrosion measures for the processing of high-acid crude oil 3.1 Alkali neutralization 3.1.1 Deacidification using NaOH or KOH aqueous solutions Alkali washing is a classical deacidification process. A neutralization reaction is carried out using sodium hydroxide solution with naphthenic acid; the resulting sodium naphthenate salt is separated from the oil, and free naphthenic acid is obtained through acidification. The reaction process can be completed in an instant, but emulsification tends to occur during the reaction. A large amount of research has been conducted on how to prevent the formation of emulsions during processing and on how to break down those that have already formed. Studies have shown that the emulsification phenomenon can be partially improved by adjusting operating parameters (the mixing intensity of oil and alkali solution, alkali concentration, operating temperature, and electric field strength in electrorefining), or by using non-dispersive contact techniques and adding additives. The advantages of this process are: low investment, low operating costs, and simple operation. However, there are also some disadvantages: a large amount of alkali solution is lost along with the processed products, the sodium hydroxide reagent cannot be regenerated, and the sodium sulfate produced during the separation of naphthenic acid causes severe pollution ； The incineration equipment installed to prevent waste pollution, as well as the recycling equipment designed to avoid secondary pollution, increase the investment required for the system ； Moreover, if the alkali in the oil is not fully removed, the small amount of naphthenic acid soaps remaining in the oil will have an adverse effect on the performance of fuel oils and lubricants. 3.1.2 Deacidification using an ammonia/alcohol solution: Ammonia water is used in place of caustic alkalis to extract petroic acid from the oil phase; essentially, this process converts petroic acid into ammonium salts, which are then separated from the oil. When these ammonium salts are heated, ammonia gas is released, and the petroic acid can be recovered. The ammonia gas can be recycled as well. The addition of alcohols is mainly used to improve emulsification. Studies have shown that when using ammonia-water-methanol solutions, ammonia-water-ethanol solutions, ammonia-water-isopropanol solutions, and other such composite ammonia-water-ethanol solvent systems for acid removal experiments, the highest acid removal rate can reach 99%. Additionally, amines, alkoxy amines, or quaternary ammonium salts can also be used as neutralizing agents in the deacidification process to achieve good deacidification results, but they are more expensive. Ammonia/alcohol deacidification allows for the selection of operating conditions over a wide range; it provides good deacidification results with minimal environmental pollution. However, it requires high energy for solvent recovery, and the color of the deacidified oil is affected to some extent. 3.1.3 Deacidification using metal oxides and sodium (or potassium) salts of organic acids: Calcium oxide, calcium sulfonate or other metal oxides, as well as alkali metal salts of small molecular organic acids, can also be used for deacidification; however, this approach presents the problem of generating large amounts of alkaline slag and saline wastewater. With the increasingly comprehensive and stringent environmental regulations, the problems associated with the treatment of wastewater and alkali slag are becoming more serious, and the disadvantages of the alkali neutralization method are becoming more apparent. 3.2 Extraction and separation: Selective solvents can be used to extract naphthenic acids from petroleum fractions. Studies indicate that formyltrimethylamine, ethylene cyanide, and industrial triethylene glycol can all be used as extraction solvents to effectively remove petroic acid from oils. Using formotrimethylamine as a selective solvent and employing four consecutive extractions, when the acid value of the crude oil was 2.65 mgKOH/g, the acid value decreased by 1.82, 0.40, 0.22, and 0.08 mgKOH/g respectively ； Industrial triethylene glycol was used as a selective solvent to separate naphthenic acids from diesel fractions; when the solvent ratio was 1:1.5, the separation efficiency of naphthenic acids reached 99.9%. Other selective solvents that can be used in addition to the aforementioned solvents include diglycol, polypropylene glycol, dimethyl sulfoxide, N-methylpyrrolidone and ethylene glycol, polypropylene glycol ethers, triethylene glycol aldehyde, etc. However, most of the available solvents also exhibit considerable solubility for aromatic compounds in oils, especially polycyclic aromatic hydrocarbons. Particularly in the case of heavy distillate oils, the differences in properties between large molecular weight petroleum acids and hydrocarbons become smaller, posing significant difficulties in selecting an appropriate extractant. Therefore, finding solvents (or mixed solvents) with good selectivity, low cost, and a low boiling point is key to the practicality of this technique. 3.3 Removal of naphthenic acids from crude oil by hydrogenation: Whether it is the hydrogenation desulfurization and denitration of fractionated oils or the hydrogenation treatment of residue oils, naphthenic acids can be effectively removed during the hydrogenation process. However, due to their strong corrosive nature, they should be removed as early as possible in the refining process. Hydrodeacidification treatment using conventional hydrogenation catalysts under very mild conditions (low reaction temperature and low hydrogen pressure) yields significant deacidification results. In recent years, various oil-soluble and water-soluble dispersive catalysts such as molybdenum naphthenate, cobalt naphthenate, and ammonium phosphomolybdate have been developed for the hydrodeacidification of naphthenic acids, achieving good acid removal results as well. Among oil-soluble catalysts, molybdenum dialkyldithiocarbamate and molybdenum dialkyldithiophosphate exhibit high catalytic activity, while among water-soluble catalysts, ammonium molybdate, ammonium phosphomolybdate, and ammonium thiomolybdate also show high hydrogenation activity and the ability to suppress coking. The hydrogenation method for removing naphthenic acids from crude oil can be considered the most thorough method of acid removal, and it avoids problems such as alkali residue contamination; however, the reaction requires a large amount of hydrogen, resulting in high costs for process installation and operation. 3.4 Mixing Mixing is an effective and low-cost method for preventing naphthenic acid corrosion; it requires no additional expenses or investments, only proper planning and operation. By purposefully adjusting the acid value of the mixed crude oil and ensuring a reasonable and balanced distribution, when the acid value of the mixed crude oil fed into the plant is below 0.5 mgKOH/g, naphthenic acid corrosion can be effectively controlled. 3.5 Use of high-temperature corrosion inhibitors: Employing high-temperature corrosion inhibitor technology is also an economical, simple, and effective way to address high-temperature naphthenic acid corrosion. Currently, the main method used in China to address corrosion in the high-temperature areas of refining units is to replace the materials used, especially for crude oils with low sulfur content and high acid values ； Abroad, while paying attention to material selection, high-temperature corrosion inhibitors are also often used. High-temperature corrosion inhibitors are used to suppress the corrosion caused by organic acids (mainly naphthenic acids); they require small amounts, do not affect the quality of the oil, nor do they impact subsequent processing processes. They overcome the disadvantages of adding alkali to crude oil and can serve as a supplement to changing the material used. High-temperature corrosion inhibitors have seen rapid development in recent years, and many refineries that process high-acid, high-sulfur crude oils are actively using them; their anti-corrosion effect is quite noticeable. However, their performance in areas subject to high temperatures and high speeds is relatively poor. 3.6 Comprehensive improvement of the material grade of equipment and apparatus 3.6.1 Upgrading the materials used in equipment and apparatus Selecting corrosion-resistant materials is one of the most effective measures to prevent naphthenic acid corrosion. In the absence of ideal process-based corrosion prevention methods, improving the corrosion resistance rating of the equipment material is the most effective way to mitigate naphthenic acid corrosion. The essence of improving the corrosion resistance of materials lies in using alloy steels to which certain amounts of alloying elements such as Cr, Ni, and Ti are added, so as to form an oxide protective film that effectively acts as a barrier to the diffusion of metal ions, thereby protecting the metal from further corrosion. In environments subject to organic acid corrosion, when selecting materials for equipment, factors such as the temperature of the medium, flow velocity, and the presence of vortices, turbulence, and phase changes must be taken into full consideration. Materials of higher quality should be used in areas where the flow velocity exceeds 30 m/s, or where vortices, turbulence, and phase changes occur. According to available information, different materials exhibit varying corrosion resistance to naphthenic acid. The corrosion resistance of the specific materials is as follows: carbon steel < low-alloy steel (chromium-molybdenum steel: Cr5Mo < Cr9Mo = < 1Cr13 < 18-8 stainless steel < molybdenum-containing austenitic stainless steel. Therefore, the use of molybdenum-containing austenitic stainless steel can effectively control the corrosion caused by naphthenic acid ； For typical molybdenum-containing austenitic stainless steels 316 and 315L, the molybdenum content should be above 2.3%; a value below this will reduce their corrosion resistance. 3.6.2 Surface modification of materials: Aluminum infiltration technology, as a method for surface modification of materials, can significantly improve the corrosion resistance of metal surfaces. Especially in oil refining for corrosion prevention, aluminum-impregnated materials exhibit excellent corrosion resistance in various media, such as high-temperature sulfur, naphthenic acid, carbon dioxide, low-temperature hydrogen sulfide, and high-temperature oxidation. Due to the increase in the surface microhardness of the metal material after aluminizing, it also possesses excellent wear resistance. Experimental and application results show that aluminum infiltration technology, as a method for modifying metal surfaces, can effectively reduce equipment investment and extend its service life. Currently, the cost of aluminum-doped carbon steel materials is only 40% to 60% of that of 18-8 steel, giving them excellent application prospects. Composite carburizing of chromium, aluminum, and molybdenum on the surfaces of carbon steel and low-alloy steel can meet the corrosion resistance requirements for processing crude oils with high acid values, and the cost of the treated materials increases by only 20% to 50%. 4. Situation of refining high-acidity crude oil by domestic refineries and protective measures: Based on an analysis of the crude oil produced by various oil fields in China as well as the sources of crude oil for different refineries, refineries in China’s northwest region, such as Lanzhou Refinery, Wuhan Refinery, Duzhou Refinery, and Karamay Refinery, as well as refineries in Jinzhou and Jinxian in Liaoning province, as well as those in North China and Shandong province, have all encountered severe corrosion issues caused by high-acidity crude oil during the crude oil processing process ； In coastal, riverine, and southern refineries, corrosion is mainly caused by the processing of high-sulfur crude oil; in some refineries, it is caused by the processing of crude oil with a high acid value. The acid number and sulfur content of several major acidic crude oils in our country are shown in Table 1. Table 1: Properties of acid-containing crude oil products from various oil fields in China. Crude oil name, Acid value/mgKOH•g-1, Sulfur content, %: Beijiang 4.51 0.13; Bohai Sea 3.61 0.34; Suizhong 3.33 0.39; Liaohe 2.55 0.75; Jinzhou 2.12 0.23; Gudao 1.45–1.83 2.0–2.3; Shengli 1.27 1.03; Wenchang 0.68 0.103; Dagang 0.64 0.21; Jidong 0.56 0.08. As can be seen from Table 1, except for the crude oils from Shengli, Gudao, and Liaohe, which have relatively high sulfur contents, the sulfur content in the crude oils from the other oil fields is not high. The main factor causing severe corrosion of equipment is the organic acids present in the crude oil, namely naphthenic acids. The severe corrosion caused by naphthenic acid in these refineries occurs mainly in the atmospheric and vacuum distillation units as well as on the oil transfer lines, while corrosion caused by naphthenic acid during secondary processing is less common. At present, these refineries have replaced the areas prone to corrosion with stainless steels of different grades, including high-quality grades such as 316L and 317L. After the “upgrade” of the materials, no significant corrosion issues have occurred, enabling the refinery to meet the basic requirement of a repair every 2 years. 4.1 Situation regarding the processing of crude oils with high acid values and protective measures: Before May 1984, the distillation units in the refinery mainly processed Daqing, Renqiu, and Shengli crude oils, and corrosion was not a serious issue. Starting in the second half of 1984, large amounts of Liaohe crude oil and mixed crude oils began to be processed, as shown in Table 2. The acid value of the Shengli crude oil processed at that time also increased, which led to severe corrosion of some carbon steel components in the distillation units. Corrosion mainly occurs in equipment and pipelines in high-temperature areas, such as the elbow at the furnace outlet of atmospheric pressure furnaces and vacuum furnaces, the furnace outlet valves, the high-speed and low-speed sections of the oil transfer lines, the feed evaporation sections of atmospheric pressure columns and vacuum columns, as well as the column walls, trays, main beams, and support beams. Table 2: Acid value and sulfur content of major acidic crude oils used in refineries. Crude oil name, Acid value/mgKOH•g-1, Sulfur content, %. Liaohe: 2.55, 0.75; Shengli: 1.27, 1.03. The main protective measures taken by refineries when processing crude oils with high acid values are as follows: 1) Blending – Purposeful adjustment of the acid value of the crude oils, so as to ensure a reasonable and balanced distribution of acid values among the crude oils fed into the refinery units, thereby effectively controlling naphthenic acid corrosion. 2) Neutralization of naphthenic acids: Sodium hydroxide is added before the crude oil enters the facility to neutralize the naphthenic acids in it, reducing the acid value of the crude oil to below 0.5 mgKOH/g. Although alkali addition can neutralize the acidic substances in crude oil, thereby reducing its acid value and minimizing corrosion, it increases production costs. Moreover, it causes the sodium content in the residue to exceed acceptable levels, rendering the residue unsuitable as a feedstock for catalytic cracking, hydrocracking, and coking. Moreover, alkali injection cannot completely control corrosion in high-temperature areas. 3) Improving the corrosion resistance of equipment materials: The vacuum distillation unit was upgraded in May 1988, with corrosion-resistant materials such as 1Cr18Ni9Ti and 316L being used in areas prone to high-temperature corrosion, and alkali injection was completely stopped in May 1990. After operating for one and a half years, inspections during major overhauls revealed no significant corrosion issues in the high-temperature areas of the distillation unit, thereby ensuring safe production when processing crude oil with high acid values. 4) Modify the unreasonable structure of the equipment to reduce brush corrosion. The equipment structure should be rational, with efforts made to minimize gaps at the joints between components as well as dead zones and blind areas in the flow path of fluids. Reduce pipeline vibration by applying supplementary thickening or overlay welding to the corroded and thinned areas. Reinforcement plates must be added to the outer wall of the thermocouple sleeve. Try to keep the pipeline alignment straight and minimize sharp bends. 4.2 Processing of High Acid Number Crudes by Zhenhai Refining & Chemical Company In recent years, the refining & chemical company’s atmospheric and vacuum distillation units have processed more than 10 different types of high acid number crudes, mainly including Shengli, Liuhua, Liaohe, Kuitu, Roccella, Duri, and Trol. The data regarding their acid number and sulfur content are shown in Table 3. Among them, the Rokelia crude oil had the highest acid value, at 3.40 mgKOH/g. The acid number, processing volume, and proportion of high-acid-number crude oils are all increasing rapidly year by year. Table 3: Acid value and sulfur content of major acidic crude oils processed by Zhenhai Refining & Chemical Company
Crude oil name | Acid value/mgKOH•g-1 | Sulfur content, %
--- | --- | ---
Luokelia | 3.40 | 0.47
Liaohé | 2.55 | 0.75
Huìtǔ | 1.90 | 0.69
Shènglì | 1.27 | 1.03
Dùlǐ | 1.26 | 0.47
Liúhuā | 1.12 | 0.24
Tèluòěr | 0.88 | 0.28

Impact of processing high-acid-value crude oils on production operations: 1) The low-quality crude oils with high acid values processed by this company are almost all heavy crude oils, containing little light components and many heavy components. This results in a shift of the heat release point to later stages; as a consequence, there is less heat in the lower temperature areas during production, leading to lower temperatures in the electrodialysis units and thus reducing the efficiency of desalination by these units. 2) The amount of wax oil is high, causing the cooling equipment of the unit to become overloaded; as a result, the temperature of the wax oil after cooling exceeds the specified limit. 3) The heavy residue volume and high viscosity make it difficult to discharge. 4) High-acid-value low-quality crude oil has an uneven distribution of its components, resulting in a low amount that can be extracted under atmospheric pressure and a large amount of residue. This increases the load on the heating furnaces, makes it difficult to control product quality, and hinders an increase in the processing capacity of the plant. Zhenhai Refining & Chemical Company’s measures to process high-acidity crude oil are as follows: 1) Strengthening system upgrades. To address the issues of insufficient processing capacity in subsequent units, low alkali washing capacity, and limited hydrogenation capacity when dealing with large amounts of high-acidity crude oil, Zhenhai Refining & Chemical Company has made significant efforts to upgrade its systems. It has built a new diesel hydrogenation unit with a capacity of 3.0 Mt/a and a wax oil hydrogenation unit with a capacity of 1.8 Mt/a, thereby overcoming the shortage in hydrogenation capacity and improving the ability to process high-acidity crude oil. 2) Comprehensively improve the material grade of equipment and apparatus. In the absence of ideal process-based corrosion prevention methods, raising the corrosion resistance of the equipment materials is the most effective way to mitigate naphthenic acid corrosion. The essence of improving the corrosion resistance of materials lies in using alloy steels to which certain amounts of alloying elements such as Cr, Ni, and Ti are added, so as to form an oxide protective film that effectively acts as a barrier to the diffusion of metal ions, thereby protecting the metal from further corrosion. 3) Adopt blending processing technology: To ensure the long-term operation of the facility, when processing low-quality crude oils with high acid values, blending and mixing should be the main methods used, and it is necessary to keep the acid value of the blended crude oil below 0.5 mgKOH/g. 4) Pay attention to desalination operations. 5) Use new types of organic amines and high-temperature corrosion inhibitors. Situation and countermeasures for refining high acid number crude oil at the 5th branch company: The branch company has 4 distillation units with a crude oil processing capacity of 13.5 Mt/a. A distillation unit was put into operation in March 1963, and after several modifications, its oil refining capacity has now reached 3.0 Mt/a. Due to the variable properties of crude oil and the complexity of the plant’s operations, especially since 1987, the acid value of the crude oil supplied to the plant has been on the rise, which has accelerated corrosion of the equipment and pipelines in the high-temperature heavy oil processing section of the plant. Given that, as the acid value of crude oil increases, the carbon steel and Cr5Mo steel used in the areas prone to corrosion at high temperatures can no longer meet the requirements of production, a material upgrade was implemented starting from the end of 1988. At present, the internal components and pipelines in the high-temperature heavy oil section of this unit are primarily made of austenitic stainless steel, which meets the requirements for processing crude oil with high acid values. The two-distillation unit was put into operation in September 1974. It was originally designed as a lubricant-producing facility focused on processing Daqing crude oil, with a processing capacity of 2.5 Mt/a. After two upgrades in 1978 and 1980, its processing capacity has reached 3.0 Mt/a. The types of crude oil processed have also evolved from the original single Daqing crude to more than 10 kinds of crude oils with high acid values and high sulfur content, sourced both domestically and internationally. The third and fourth distillation units are primarily used for processing imported crude oil with high sulfur content, and the materials used in their equipment are chosen to be resistant to sulfur corrosion. 5.1 Types of high acid number crude oils refined by the branch company The acid number and sulfur content of more than 10 main acid-containing crude oils refined by the branch company are shown in Table 4. Table 4: Acid value and sulfur content of the main acidic crude oils processed by the subsidiaries
Crude oil name, Acid value/mgKOH•g-1, Sulfur content, %, Evaluation year
Brazilian Malin, 1.34, 0.788, 2004
Shengli, 1.27, 0.77, 1997
Indonesian Duri, 1.26, 0.47, 2006
Norwegian Troll, 0.85, 0.30, 2005
Hainan Wenchang, 0.68, 0.103, 2002
Equatorial Guinea Zafiro, 0.67, 0.27, 1997
Nigerian Bonga, 0.59, 0.25, 2006
Angolan Hango, 0.56, 0.62, 2005
Congo Jenou, 0.50, 0.233, 1997
Oman, 0.47, 1.18, 2004
Luzhou, 0.45, 0.10, 2003
Nile, 0.42, 0.05, 2003
Note: The data in the table are taken from the crude oil evaluation reports. As can be seen from Table 4, the crude oil with the highest acid value processed by the branch companies is the Brazilian Marín crude, whose acid value reaches 1.34 mgKOH/g ； Apart from the high-acid, sulfur-rich crude oils from Marlim and Vitória in Brazil and Hango in Angola, the refined high-acid crude oils are generally high-acid but low-sulfur. Crudes such as Oman, Weizhou, and Nile have an acid number of less than 0.5 mgKOH/g, so they are not considered high-acid crudes; however, their acid numbers are still relatively high, and the acid number of vacuum wax oil is also high ； When using these reduced-pressure paraffin oils to produce lubricants, it is difficult to lower the acid value of the base oil, which affects the quality of the product. 5.3 Acid distribution in various fractions of high acid number crude oils The acid distribution in the various fractions of the 3 types of high acid number crude oils processed by the branch company is shown in Table 5. Table 5 Acid distribution in various fractions of three high acid number crude oils
Crude Oil Fraction Duri, Oman Mix Hargo Bunga Acid value/mgKOH•g-1 Acidity/mgKOH•(100mL)-1 Acid value/mgKOH•g-1 Acidity/mgKOH•(100mL)-1 Acid value/mgKOH•g-1 Acidity/mgKOH•(100mL)-1
Crude Oil 1.00 - 0.56 - 0.59 - Light distillate - 0.073 - 0 0.228
Middle distillate 0.212 - 0.203 - 0.559 Heavy middle distillate 10.43 15.8 14.3
Heavy middle distillate 72.3 28.11 - 28.11 Heavy heavy distillate 0.903 - 48.5 45.6
Light residue 1.26 - 0.756 - 0.699 Medium residue 1.44 1.031 - 0.808
Heavy residue 1.46 1.063 - 1.05 Very heavy residue 0.993 - 0.949 - 0.860
Tar 0.312 - 0.209 - 0.381
Note: The heavy middle distillate has had alkali added to it. As can be seen from Table 5, the acidic substances in crude oils with high acid values are mainly present in normal paraffins, normal olefins, reduced paraffins, reduced olefins, reduced terpenes, and reduced tetraenes – that is, in fractions with a boiling range of 250–500°C, which correspond to diesel components and vacuum wax oil components. Among these, reduced terpenes have the highest acid value. Therefore, the acid values of both the diesel fractions and the vacuum wax oil fractions produced from crude oils with high acid values are high, which affects the quality of the products. 5.3 Problems in the Processing of Crudes with High Acid Value The processing of crudes with high acid value at the Maoming branch faces the following main problems: 1) It can cause severe corrosion in the atmospheric and vacuum distillation units. 2) The acid value of the diesel component is on the high side. 3) The acid value of the reduced paraffin oil is high, which in turn causes the neutralization value of the lubricating oil to be high. 5.4 Countermeasures for Processing Crude Oil with High Acid Value The Maoming branch can adopt the following countermeasures for processing crude oil with high acid value: 1) Paying attention to desalination operations. Crude oil desalination is not only a key factor in preventing corrosion in the overhead system of crude oil distillation units, but it is also an important pretreatment process for crude oil that influences subsequent processes such as heavy oil catalytic cracking and hydrogenation. Improving the desalination and dewatering efficiency can, on the one hand, reduce corrosion and scaling in the equipment; on the other hand, after desalination and dewatering using an electrodesalination unit, it effectively reduces metal ions and chloride ions in crude oil, removes impurities, and enhances the quality of the products produced by atmospheric and vacuum distillation units. Developing efficient demulsifiers that are generally suitable for high-acidity, low-quality crude oils, optimizing the desalination conditions for such crude oils (such as desalination temperature, water injection volume, mixing intensity, etc.), and using an appropriate electric field gradient are effective ways to improve the efficiency of electrodesalination. 2) Use of new organic bases and high-temperature corrosion inhibitors: Given the impact of sodium ions on secondary processing equipment in traditional alkali injection methods, organic bases can be considered as alternatives to caustic soda as neutralizers for highly acidic crude oil, such as monoethanolamine and diethanolamine; however, their cost is relatively high. The acids in reduced paraffin oil can also be neutralized with organic bases. Considering the characteristics of naphthenic acid corrosion, the addition of a new high-temperature corrosion inhibitor can help reduce naphthenic acid corrosion in the high-temperature areas of distillation units without the need to use alkali. New types of high-temperature corrosion inhibitors can form an adsorptive protective film on the surface of metal materials. Meanwhile, some of these inhibitors react directly with naphthenic acid to produce naphthenate esters. These high-molecular-weight naphthenate esters establish an adsorption equilibrium on the metal surface, separating organic acids such as naphthenic acid from the metal surface thereby achieving the purpose of protecting the material. 3) Mixing processing technology: Mixing is an effective and low-cost method for preventing naphthenic acid corrosion; it requires no additional expenses or investments, only proper planning and operation. By purposefully adjusting the acid value of the mixed crude oil to ensure a reasonable and balanced level, with the acid value of the mixed crude oil entering the plant being less than 0.5 mgKOH/g, it is possible to effectively control naphthenic acid corrosion; moreover, the acid values of the diesel fractions and vacuum wax oils can also be reduced. 4) Neutralization of naphthenic acid: The acid value of diesel components can be neutralized by the alkali injection method. However, when the naphthenic acid content in diesel components is high, emulsification tends to occur during alkaline neutralization; therefore, an appropriate demulsifier must be added when neutralizing the naphthenic acid in diesel components. 5) Strengthen corrosion monitoring: Install fixed monitoring points in areas where corrosion is severe, and conduct regular wall thickness inspections. Increase the monitoring frequency when the corrosion rate changes significantly or when the corrosion environment changes greatly (such as when the corrosivity of crude oil is high or the type of oil changes). 6) Monitoring the acid value of materials: The acid values of various crude oils vary; even different batches of the same type of crude oil can have different acid values. It is essential to know the acid values in various parts of the distillation unit at high temperatures in order to monitor corrosion in the equipment and pipelines in those areas. By monitoring the acid value of the materials, it is possible to track the corrosion progression in the high-temperature areas of the equipment. By examining the acid value and sulfur content of these materials, it is feasible to determine the relationship between them as well as their connection to equipment corrosion. This approach helps to understand the corrosion mechanisms associated with crude oils that are high in acid and sulfur content, thereby providing guidance for implementing anti-corrosion measures on the equipment. 5 Conclusions 1) Among the acidic substances in crude oil, naphthenic acid is the most important and is present in high concentrations; it accounts for about 90% of the acidic substances in crude oil. 2) The distribution pattern of naphthenic acids in crude oil is quite special: the content of naphthenic acids is highest in the middle distillates (250–500°C), while it is relatively low in both the low-boiling and high-boiling fractions. 3) The main factors affecting naphthenic acid corrosion are temperature, flow rate, and crude oil sulfur content. 4) In refineries that process crude oil with a high acid value, the high-temperature parts of their distillation units suffer severe corrosion. Corrosion mainly occurs in equipment and pipelines in high-temperature areas, such as the elbow at the furnace outlet of atmospheric furnaces and vacuum furnaces, the furnace outlet valves, the high-speed and low-speed sections of the oil transfer lines, the feed evaporation sections of atmospheric towers and vacuum towers, as well as the tower walls, trays, main beams, and support beams. 5) Processing crude oils with high acid values can lead to higher acid values in the diesel components and vacuum wax oils, thereby resulting in higher acid values in diesel and a higher neutralization value in lubricating oils. 6) There are many effective measures to prevent naphthenic acid corrosion, among which blending is the most effective and low-cost method for preventing corrosion caused by high-temperature naphthenic acids. It requires no additional expenses or investments; proper planning and operation are sufficient. 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