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I. The meaning of the petrochemical industry http://www.oilchina.cn/syswsc/images/shangwu/arrow27.gif The petrochemical industry, abbreviated as petrochemics, is an important part of the chemical industry and plays a significant role in the development of the national economy; it is one of the key industrial sectors in China. The petrochemical industry refers to the processing industry that uses oil and natural gas as raw materials to produce petroleum products and petrochemical products. Petroleum products, also known as oils, mainly include various fuel oils (gasoline, kerosene, diesel, etc.) and lubricants, as well as liquefied petroleum gas, petroleum coke, paraffin, asphalt, etc. The processing process for producing these products is often referred to as petroleum refining, or simply refining. Petrochemical products are obtained through further chemical processing of the crude oil supplied by the refining process. The first step in producing petrochemical products is the cracking of crude oil and gases (such as propane, gasoline, diesel, etc.) to generate basic chemical raw materials represented by ethylene, propylene, butadiene, benzene, toluene, and xylene. The second step is to produce a variety of organic chemical raw materials (about 200 types) as well as synthetic materials (plastics, synthetic fibers, synthetic rubber) from basic chemical raw materials. The production of these two types of products falls within the scope of petrochemical industry. Further processing of organic chemical raw materials can yield a wider range of chemical products, which *traditionally do not fall within the scope of petrochemicals. In some sources, the synthesis of ammonia and urea using natural gas, light gasoline, and heavy oil, as well as the production of nitric acid, are also included in petrochemical industry. This book only covers urea. II. The Development of Petrochemical Industry http://www.oilchina.cn/syswsc/images/shangwu/arrow27.gif The development of the petrochemical industry is related to the petroleum refining industry, as well as the production of chemical products and the three major synthetic materials using coal as a basic raw material. Petroleum refining originated in the 1820s. In the 1920s, the automobile industry grew rapidly, driving up gasoline production. To increase gasoline production, a thermal cracking process designed for gasoline production was developed. Subsequently, in the 1940s, a catalytic cracking process was invented, and together with the development of other processing techniques, modern petroleum refining processes were established. To make use of the gases derived from petroleum refining by-products, the production of isopropanol from propylene began in 1920; this is considered to be the first petrochemical product. In the 1950s, hydrocarbon steam high-temperature cracking – abbreviated as cracking – technology was developed on the basis of cracking technology, with the main purpose of producing ethylene. The advancement of this cracking process provided a large amount of raw materials for the development of the petrochemical industry. At the same time, some products that were previously manufactured using coal as the primary raw material (through calcium carbide and coal tar) have gradually begun to use petroleum as their main raw material, such as vinyl chloride. In the 1930s, a large number of polymer synthetic materials were introduced. Ranked by industrial production time: in 1931 it was neoprene and polyvinyl chloride; in 1933 it was high-pressure polyethylene; in 1935 it was nitrile rubber and polystyrene; in 1937 it was styrene-butadiene rubber; and in 1939 it was nylon 66. After World War II, petrochemical technology continued to develop rapidly; acrylic fiber was developed in 1950, polyester in 1953, and polypropylene in 1957. The rapid development of the petrochemical industry is due to the availability of large quantities of cheap raw materials (in the 1950s and 1960s, crude oil cost about $15 per ton) ； There are reliable and potentially scalable production technologies ； The product has a wide range of applications, opening up new fields of use. The combination of raw materials, technology, and applications enabled the transition from coal chemistry to petroleum chemistry, marking a leap in the history of the chemical industry. After the 1970s, rising crude oil prices (around $170 per ton in 1996) led to a slowdown in the development of the petrochemical industry. The development of new processes slowed down as well, and the industry shifted towards the use of new technologies, energy savings, optimized production processes, comprehensive utilization of raw materials, and the production of downstream products. Some developing countries **vigorously develop their petrochemical industries, reducing the share of developed countries**. In 1996, the world’s crude oil processing capacity was 3.8 billion tons, with oil used for producing chemical products accounting for about 10% of that total. III. The Role of Petrochemicals in the National Economy http://www.oilchina.cn/syswsc/images/shangwu/arrow27.gif 1. Petrochemicals are a major source of energy supply. Gasoline, kerosene, diesel, heavy oil, and natural gas produced through petroleum refining are the main suppliers of the current primary energy sources. In 1995, our country produced 80 million tons of fuel oil. Currently, oil and gas consumption worldwide accounts for about 60% of total energy consumption ； Due to its high coal consumption, our country’s oil consumption is less than 20%. The energy provided by the petrochemical industry is mainly used as fuel for cars, tractors, airplanes, ships, and boilers, with a small amount being used as fuel for domestic purposes. Energy is a factor that restricts the development of China’s national economy; the petrochemical industry consumes approximately 8.5% of the total energy used, and it is necessary to continuously reduce energy consumption. 2. The petrochemical industry is one of the pillars of the materials industry. Metals, inorganic non-metallic materials, and polymer synthetic materials are known as the three major types of materials. The current global production of polymer synthetic materials produced by the petrochemical industry is approximately 145 million tons; in 1996, China’s production already exceeded 8 million tons. In addition to synthetic materials, the petrochemical industry also provides the vast majority of organic chemical raw materials. Within the field of chemistry, aside from the chemical products derived from mineral sources, the raw materials produced by the petrochemical industry play a crucial role in various industries. 3. The petrochemical industry has promoted agricultural development. Agriculture is a fundamental industry in China’s national economy. Nitrogen fertilizers produced by the petrochemical industry account for 80% of the total amount of chemical fertilizers. The widespread use of agricultural plastic films, along with the rational application of pesticides and the various fuels required for large numbers of agricultural machines, constitute the main force through which the petrochemical industry supports agriculture. 4. All industrial sectors rely on petrochemical products. The development of the modern transportation industry is closely tied to fuel supply; it is no exaggeration to say that without fuel, there would be no modern transportation industry. Metal processing, as well as all types of machinery, require various lubricants and other related materials without exception, consuming a large amount of petrochemical products. The global production of lubricants and greases is about 20 million tons, while in China it is around 1.8 million tons. The building materials industry is a new field for petrochemical products; materials such as plastic components, doors and windows, flooring materials, and coatings are known as chemical building materials. The light industry and textile industry are traditional users of petrochemical products, and the development and promotion of new materials, new processes, and new products all involve petrochemical products. Currently, the rapidly developing electronics industry and various high-tech industries have placed new demands on petrochemical products, particularly those fine chemical products manufactured using petrochemicals as raw materials, which serves as a significant boost to the development of the petrochemical industry. 5. The construction and development of the petrochemical industry rely on the support of various industries. Petrochemical companies, both domestically and internationally, concentrate on building a number of production facilities to create large-scale petrochemical industrial zones. Within the area, the refining units serve as the core, providing cracking feedstocks such as light oil and diesel to the petrochemical units, while also producing petrochemical products ； The cracking unit produces basic petrochemical raw materials such as ethylene, propylene, benzene, and xylene ； According to demand, a series of production facilities are built to manufacture synthetic materials and organic raw materials primarily using the aforementioned raw materials, with a certain proportional relationship between their products and raw materials. To produce 300,000 tons of ethylene per year, rough calculations show that around 1.2 million tons of feedstock are required for cracking. This corresponds to a refining capacity of about 2.5 million tons, enabling the production of 800,000 to 900,000 tons of synthetic materials and basic organic raw materials. It is evident that establishing a petrochemical industrial zone requires substantial investment. The site selection for the plants must be appropriate, ensuring not only the convenient transportation of raw materials and products but also adequate supply of electricity, water, and other necessary infrastructure. Each production unit requires a large number of standard, standardized mechanical components, equipment, instruments, pipelines, as well as non-standard specialized equipment. The manufacturing of mechanical equipment involves a wide variety of materials with different requirements; some key pieces of equipment operate at speeds exceeding 50 meters per second, and the weight of each individual unit is several hundred tons ； Some require heat resistance up to 1000°C, while others require cold resistance down to -150°C. Some key equipment needs to be purchased from the international market. All of these require support from industries such as metallurgy, electricity, machinery, instrumentation, construction, and environmental protection. The petrochemical industry is a technology-intensive industry. The determination of production methods and processes, as well as a range of technologies such as the selection and manufacturing of key equipment, all require to be governed by proprietary or unique technical standards; if these technologies are imported from abroad, fees for patents or know-how must be paid. Therefore, only by strengthening research in basic disciplines, especially organic chemistry, polymer chemistry, catalysis, chemical engineering, computers, and automation, and by improving the training of professionals in these fields so that they can master and apply advanced research results, coupled with relevant engineering technologies, can the petrochemical industry continue to develop and reach new heights. Petroleum refining: http://www.oilchina.cn/syswsc/images/shangwu/arrow27.gif 1. Composition and properties of petroleum Petroleum, also known as crude oil, is a brownish-black, flammable, viscous liquid that is extracted from deep underground. Oil is a mixture formed over a long period of evolution by organisms in ancient oceans or lakes, and like coal, it belongs to fossil fuels. The properties of oil vary depending on its source; its density ranges from 0.8 to 1.0 grams per cubic centimeter. Its viscosity varies widely, and its freezing point differs significantly (from 30 to -60°C). The boiling point ranges from room temperature up to over 500°C. Oil is soluble in various organic solvents, but it is insoluble in water; however, it can form emulsions with water. The chemical elements that make up petroleum are primarily carbon (83%–87%) and hydrogen (11%–14%); the rest include sulfur (0.06%–0.8%), nitrogen (0.02%–1.7%), oxygen (0.08%–1.82%), as well as trace metal elements such as nickel, vanadium, and iron. Hydrocarbons, formed by the combination of carbon and hydrogen, constitute the main component of petroleum, accounting for approximately 95% to 99%. Compounds containing sulfur, oxygen, and nitrogen are harmful to petroleum products and should be removed as much as possible during petroleum processing. The structures and proportions of various hydrocarbons in oils from different sources vary greatly, but they mainly belong to three categories: alkanes, cycloalkanes, and aromatics. Oil that is primarily composed of alkanes is called paraffinic oil ； Those composed mainly of naphthenes and aromatics are called cycloalkyl petroleum ； What lies between the two is called intermediate base oil. The main characteristics of crude oil in our country are a high wax content, a high freezing point, a low sulfur content, moderate levels of nickel and nitrogen, and an extremely low vanadium content. Except for a few oil fields, crude oil contains little gasoline fraction, with residue accounting for 1/3. Oils of different categories have varying processing methods and different product properties; it is necessary to make the best use of each type. The main characteristics of Daqing crude oil are a high wax content, a high freezing point, and a low sulfur content; it belongs to the category of low-sulfur paraffin-based crude oils. 2. Development of the oil refining industry The discovery, extraction, and direct use of petroleum have a long history. The processing of petroleum and the gradual development of the oil refining industry began in the 1830s; by the 1940s and 1950s, the modern oil refining industry had taken shape, becoming one of the largest processing industries. Starting in the 1830s, oil distillation plants were established one after another; their main product was kerosene for lamps, while gasoline had no use and was discarded as waste. In the 1870s, a lubricant factory was built, and high-boiling-point oils obtained through distillation began to be used as fuel for boilers. The advent of the internal combustion engine at the end of the 19th century led to a surge in demand for gasoline and diesel. Distilling crude oil alone (i.e., its primary processing) was not sufficient to meet this demand, which gave rise to secondary processing methods for crude oil that made use of all its various components in order to increase the production of gasoline and diesel. For example, thermal cracking was achieved in 1913, coking in 1930, catalytic cracking in 1930, and catalytic reforming in 1940; thereafter, hydrogenation technology also developed rapidly, giving rise to the modern petroleum refining industry. After the 1950s, petroleum refining provided a large amount of raw materials for the development of chemical products, giving rise to the modern petrochemical industry. In 1996, the world’s oil processing capacity was 3.8 billion tons, while that of our country was 140 million tons. The annual processing capacity of large refineries has exceeded 10 million tons. Petroleum products: http://www.oilchina.cn/syswsc/images/shangwu/arrow27.gif Petroleum products can be divided into 6 categories: petroleum fuels, petroleum solvents and chemical raw materials, lubricants, paraffin, petroleum asphalt, and petroleum coke. Among them, the production of various fuels is the highest, accounting for about 90% of the total output ； There are the most varieties of lubricants, accounting for about 5% of the production. Countries have established product standards to meet the needs of production and use. Gasoline is the type with the highest consumption. The boiling point range (also known as the distillation range) of gasoline is 30 to 205°C, and its density is 0.70 to 0.78 grams per cubic centimeter. Commercial gasoline is classified according to its resistance to knocking during combustion in the engine, and is labeled with octane ratings of 70, 80, 90, or higher. It has a large capacity and excellent performance; gasoline is primarily used as fuel for cars, motorcycles, speedboats, helicopters, and aircraft used in agriculture and forestry. Additives (such as the anti-knock agent tetraethyl lead) are added to commercial gasoline to improve its performance in use and storage. Due to environmental regulations, the levels of aromatics and lead will be restricted in the future. Jet fuel is primarily used for jet aircraft. The boiling point range is 60~280°C or 150~315°C (commonly known as aviation gasoline). To meet the requirements of flight at high altitudes, low temperatures, and high speeds, such oils are required to generate a large amount of heat and not form solid crystals at -50°C. Kerosene has a boiling point range of 180 ~ 310°C and is mainly used for lighting and household cooking. The flame is required to be steady and bright, without producing black smoke. The current production volume is not high. Diesel: The boiling point ranges fall into two categories: 180–370°C and 350–410°C. For petroleum and its refined products, it is customary to refer to those with a low boiling point or boiling point range as light, while the opposite are called heavy. Therefore, the former mentioned above is called light diesel, while the latter is called heavy diesel. Commercial diesel is classified by freezing point, such as 10, -20, etc., which indicate low operating temperatures; diesel is widely used in large vehicles and ships. Since high-speed diesel engines (for use in vehicles) are more fuel-efficient than gasoline engines, the demand for diesel is increasing at a faster rate than that for gasoline, and some small cars are also switching to diesel. The requirements for diesel quality are good combustion performance and fluidity. The combustion performance is indicated by the cetane number, with higher values being better; the diesel fuel produced from Daqing crude oil can have a cetane number of up to 68. The cetane number of light diesel used in high-speed diesel engines ranges from 42 to 55, while that of low-speed engines is below 35. Fuel oil Used as fuel for boilers, ships, and industrial furnaces. Commercial fuel oils are classified into different grades based on their viscosity. Petroleum solvents are used as solvents in the fragrance, oil, reagent, and rubber processing industries, as well as in the paint industry; they are also employed for cleaning instruments, meters, and mechanical parts. Lubricants derived from petroleum account for over 95% of the total production of lubricants. In addition to its lubricating properties, it also serves functions such as cooling, sealing, corrosion prevention, insulation, cleaning, and energy transmission. The largest volume is produced for internal combustion engine oils (accounting for 40%), with the remainder being gear oils, hydraulic oils, turbine oils, electrical insulating oils, and compressor oils, which together account for 40%. Commercial lubricants are classified by viscosity; machines with high loads and low speeds require oils with high viscosity, while those with lower loads and speeds use oils with low viscosity. Refining units produce base oils that are created through various refining processes, to which various additives are added; as a result, they possess specialized functions and generate high additional value. Grease, commonly known as butter, is a solid or semi-fluid substance made from lubricants and thickeners, and it is used in bearings and gears where lubricants cannot be used. Paraffin oil includes paraffin (accounting for 10% of total consumption), ceresin, petrolium wax, etc. Paraffin is mainly used as a packaging material, a raw material for cosmetics, and in the production of wax products; it can also be used as a chemical raw material to produce fatty acids (used in soap manufacturing). Petroleum asphalt is mainly used for roads and construction. Petroleum coke is used as electrodes in the metallurgy (steel, aluminum) and chemical (calcium carbide) industries. In addition to the aforementioned petroleum products, various refining units also produce some gases at room temperature, collectively known as refinery gas. This gas can be used directly as fuel, or it can be pressurized and liquefied to produce liquefied petroleum gas, which can then be used as a raw material or in chemical manufacturing. Refineries supply a wide variety of chemical raw materials and serve as a base for organic chemical products; various oils and refinery gases can be selected according to different production objectives and processes. Gaseous feedstocks at atmospheric pressure are mainly used to produce ethylene, propylene, synthetic ammonia, hydrogen, acetylene, and carbon black. Liquid feedstocks (liquefied petroleum gas, light gasoline, light diesel, and heavy diesel) can be cracked to produce most of the basic raw materials required for the development of the petrochemical industry (with the exception of acetylene), serving as the foundation for its growth. At present, crude oil cannot be directly used to produce basic organic materials due to severe coking caused by high temperatures. Refineries are also sources of important aromatic compounds such as benzene, toluene, and xylene. Finally, it should be noted that gasoline, aviation kerosene, and diesel contain additives, to varying degrees, to improve their performance in use and storage. The products produced by various refining units must have additives added to them in accordance with commercial standards, and oils from different units must be blended together before they can be used as commercial products. Petroleum additives require small amounts but offer significant effects; they are chemically synthesized fine chemical products that are essential for the development of high-quality products, and their production should be vigorously promoted. Introduction to major oil refining processes: http://www.oilchina.cn/syswsc/images/shangwu/arrow27.gif Atmospheric distillation and vacuum distillation Atmospheric distillation and vacuum distillation are commonly referred to together as atmospheric-vacuum distillation; this process is essentially a physical one. In the distillation tower, crude oil is divided into oils with different boiling points based on their evaporation capacity (referred to as fractions). Some of these oils are exported as finished products after being blended and treated with additives, while a large portion serves as raw material for further processing units. Therefore, atmospheric and vacuum distillation is also known as the primary processing of crude oil. It includes three processes: desalination and dewatering of crude oil ； atmospheric distillation ； Vacuum distillation. Desalination and dewatering of crude oil, also known as pretreatment. Crude oil sent from oil fields to refineries often contains salts (mainly chlorides) and water (either dissolved in the oil or in an emulsified state), which can cause equipment corrosion, lead to scaling on the inner walls of the equipment, and affect the composition of the final petroleum products; therefore, these impurities need to be removed before processing. The common method is to add demulsifiers and water, which causes the water in the oil to aggregate and separate from it; the salts dissolve in the water. A high-voltage electric field is then used to help remove the larger water droplets that are formed. Catalytic cracking Catalytic cracking was developed from the thermal cracking process. It is the most important process operation for increasing the degree of crude oil processing and producing high-quality gasoline and diesel. The feedstock is mainly heavy oil from the 350–540°C fraction of crude oil distillation or other refining units. The catalytic cracking process consists of three parts: catalytic cracking of the feedstock oil, catalyst regeneration, and product separation. The products obtained from catalytic cracking can be separated by distillation to yield gas, gasoline, diesel, and heavy distillates. Some of the oil is returned to the reactor for further processing, which is known as recycled oil. Changes in the operating conditions of catalytic cracking or fluctuations in feedstock can cause variations in the product composition. Catalytic reforming Catalytic reforming (abbreviated as reforming) is a process in which light gasoline obtained through atmospheric distillation is converted into reformate gasoline with a higher content of aromatics, in the presence of a catalyst and hydrogen. If fractions at 80–180°C are used as raw materials, the product is high-octane gasoline ； If distillates at 60–165°C are used as the feed oil, the products are mainly aromatics such as benzene, toluene, and xylene; hydrogen is produced as a by-product of the reforming process, and this hydrogen can be used as a source for hydrogenation operations in refineries. The reaction conditions for reformation are: a reaction temperature of 490–525°C, and a reaction pressure of 1–2 MPa. The reforming process can be divided into raw material pretreatment and reforming. Hydrocracking is carried out under high pressure in the presence of hydrogen; a catalyst is required to convert heavy feedstocks into gasoline, kerosene, diesel, and lubricating oils. Due to the presence of hydrogen in hydrocracking, less coke is formed during the conversion of raw materials, and harmful compounds containing sulfur, nitrogen, and oxygen can be removed. The process is flexible and can be adjusted according to the requirements of the desired products. The product yield is high, and the quality is good. Delayed coking: It involves the deep cracking of feedstock over a prolonged reaction time, with the main objective of producing solid petroleum coke, while also generating gaseous and liquid products. The raw material used in delayed coking is mainly high-boiling-point residue oil. The main operating conditions for delayed coking are: the temperature of the feedstock after heating is around 500°C, and the coke tower operates under a slight positive pressure. Changing the raw materials and operating conditions can adjust the proportions of gasoline, diesel, cracking feedstock oil, and coke. Refinery gas processing: All the production units involved in the primary and secondary processing of crude oil generate gases, which are collectively referred to as refinery gas. In terms of their composition, these gases mainly include hydrogen, methane, ethane and ethylene composed of 2 carbon atoms, propane and propylene composed of 3 carbon atoms, and butane and butenes composed of 4 carbon atoms. Their main uses are as raw materials for producing gasoline and petrochemicals, as well as for producing hydrogen and ammonia. The prerequisite for developing refinery gas processing is to first separate and then utilize the refinery gas. The proportion of refinery gas that is separated for use as chemical raw materials is increasing; for example, purer ethylene can be obtained for use in the production of ethylbenzene ； The purer propylene can be separated out for use in polypropylene and other products. Petroleum product refining: The oils produced by the aforementioned units generally cannot be used directly as commercial products. To meet the requirements for commercial use, in addition to blending and adding additives, further refining is often necessary to remove impurities and improve the properties of these oils so that they can satisfy practical needs. Common impurities include compounds containing sulfur, nitrogen, and oxygen, as well as undesirable components such as waxes and gums mixed in with the oil. They can give oils an unpleasant odor, a dark color, cause corrosion to mechanical equipment, and make them difficult to store. Common methods for removing impurities include acid-base purification, deodorization, hydrogenation, solvent purification, clay purification, dewaxing, etc. Acid refining involves treating oils with sulfuric acid to remove certain sulfur-containing compounds, nitrogen-containing compounds, and gums. Alkali refining involves treating oils such as gasoline, diesel, and lubricants with an aqueous solution of caustic soda; this process removes oxidized compounds and sulfides, as well as the sulfuric acid that remains from acid refining. Acid refining and alkali refining are often used together, hence it is called acid-alkali refining. Deodorization is applied to gasoline, coal oil, and diesel produced from crude oils with high sulfur content, which develop an unpleasant odor due to thiol compounds. A high thiol content can cause gum formation in the oil, making it difficult to store. A treatment with an alkaline solution followed by oxidation with air can be employed in the presence of a catalyst. Hydrogenation is a process that takes place in the presence of a catalyst at temperatures between 300 and 425°C and under a pressure of 1.5 megapascals; it allows for the removal of compounds containing sulfur, nitrogen, and oxygen, as well as metal impurities. This process improves the storage properties, corrosion resistance, and combustion characteristics of oils, and can be applied to various types of oils. Wax removal is primarily used to refine aviation kerosene, diesel, etc. Oil contains wax, which forms wax crystals at low temperatures, affecting its flow properties and causing pipes to become clogged easily. Wax removal is very important for aviation fuels. Wax removal can be achieved through molecular sieve adsorption. The refining of lubricating oils often involves solvent refining to remove undesirable components, in order to improve its composition and color. Wax removal is sometimes required. Clay purification is generally carried out at the end of the purification process, using clay (composed mainly of silicon dioxide and aluminum oxide) to adsorb harmful substances. Acid refining involves treating oils with sulfuric acid, which can remove certain sulfur-containing compounds, nitrogen-containing compounds, and gums. Alkali refining involves treating oils such as gasoline, diesel, and lubricants with an aqueous solution of caustic soda; this process removes oxygenated compounds and sulfides, as well as the sulfuric acid that remains after acid refining. Acid refining and alkali refining are often used together, hence it is called acid-alkali refining. Deodorization is a process applied to gasoline, coal oil, and diesel produced from crude oil with high sulfur content; these fuels develop an unpleasant odor due to thiols, and high levels of thiols can cause the formation of gums in the oils, making them difficult to store. A treatment with an alkaline solution followed by oxidation with air can be employed in the presence of a catalyst. Hydrogenation involves adding hydrogen in the presence of a catalyst at temperatures between 300 and 425°C and under a pressure of 1.5 megapascals. This process removes compounds containing sulfur, nitrogen, and oxygen, as well as metal impurities, thereby improving the storage properties, corrosion resistance, and combustion characteristics of the oil. It can be applied to various types of oils. Wax removal is mainly used for refining aviation kerosene, diesel, etc. Oil contains wax, which forms wax crystals at low temperatures, affecting its flow properties and causing pipes to become clogged easily. Wax removal is very important for aviation fuels. Wax removal can be achieved through molecular sieve adsorption. The refining of lubricating oils often involves solvent refining to remove undesirable components, in order to improve its composition and color. Wax removal is sometimes required. White clay purification This step is generally carried out at the end of the purification process, using white clay (composed mainly of silicon dioxide and aluminum oxide) to absorb harmful substances. Lubricating oils: Their raw materials are primarily derived from the distillation of crude oil. The most important properties of lubricating oils are viscosity, stability, and lubricity. The basic process of producing lubricating oil essentially involves removing the undesirable components from the crude oil, mainly gums, asphaltenes, compounds containing sulfur, nitrogen, and oxygen, as well as waxes and polycyclic aromatic hydrocarbons. These components primarily affect viscosity, stability, and color. The methods include solvent refining, dewaxing and deasphalting, hydrogenation, and clay refining. Solvent refining involves using the different solubilities of various components in solvents to achieve purification, and it is employed in the production process of the vast majority of lubricants. Common solvents include furfural and phenol. The production process is similar to the aromatic extraction in reforming units. Solvent dewaxing is a process used to remove those components in lubricant raw materials that tend to crystallize at low temperatures, primarily paraffin. This dewaxing is carried out using the cold crystallization method. To overcome the problem of high viscosity at low temperatures, as well as the issue of very small paraffin crystals that make filtration difficult, mixed solvents that do not dissolve paraffin are often added, such as toluene – xylene; hence, this dewaxing method is often referred to as xylene dewaxing.