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Solvent dewaxing: To ensure that lubricants maintain good fluidity at low temperatures, it is necessary to remove the waxes that tend to solidify within them; this process is known as dewaxing. The dewaxing process can not only lower the freezing point of lubricating oil but also yield wax. Wax refers to those hydrocarbon compounds that become solid at room temperature (15°C); the main components are n-alkanes and cyclic hydrocarbons with long side chains. N-alkanes with 16 carbon atoms or more are all solid at room temperature. There are many methods for dewaxing, and the commonly used ones at present are cold pressing dewaxing, solvent dewaxing, and urea dewaxing. Propane deasphalting: This method involves using propane to extract the hydrocarbons from residual oil. It takes advantage of the fact that liquid propane has very low solubility for asphalt at temperatures near its critical temperature, but high solubility for oils (alkanes, naphthenes, and minor aromatics), thereby allowing the separation of oils from asphalt. The critical temperature of propane is 96.81°C, and its critical pressure is 4.2 MPa. The critical temperature is the temperature at which, when a liquid is heated above this value, no external pressure, no matter how high it is, can prevent the liquid from boiling and turning into vapor. The external pressure corresponding to the critical temperature is known as the critical pressure. In the region near the critical temperature but below it for propane, the solvating capacity of liquid propane for oils and asphalts decreases as the temperature rises. However, the solvating capacity for asphalt decreases rapidly, while that for oil decreases slowly. Therefore, at a certain temperature within this temperature range, the solubility of oil in propane is much greater than that of asphalt. The deasphalted oil obtained through propane treatment, like other distillate oils, needs to be refined and dewaxed. Refining of white clay: The oil product, after being refined using solvents and dewaxed, already meets the required quality standards to a large extent. However, it usually still contains small amounts of solvents that have not been removed, as well as water. Additionally, there are certain high-molecular-weight condensates, gums, and unstable compounds that form during the heating process of recovering solvents. Mechanical impurities such as iron particles may also be present, coming from the processing equipment. To remove these impurities, further improve the color of the lubricating oil, enhance its stability, and reduce residue carbon, additional refining is required. A commonly used method for refining supplements is clay treatment. Activated clay purification makes use of the adsorption capacity of activated clay to attach various impurities to it, after which the clay is filtered out to remove all the impurities. The method involves adding a small amount (usually a few percent) of pre-dried activated clay to the oil, heating it while stirring to ensure thorough mixing of the oil and the clay; this causes the impurities to be completely adsorbed by the clay. Afterwards, fine filter paper (or cloth) is used for filtration to remove the clay and mechanical impurities, thereby obtaining the refined base oil. Hydrorefining (1) Hydrorefining for further purification: Oil products that have undergone hydrorefining for further purification exhibit improved color, stability, and odor; their sensitivity to antioxidants increases significantly, while changes in viscosity and viscosity-temperature properties are minimal. Additionally, the levels of non-hydrocarbon elements such as sulfur, nitrogen, and oxygen in the oil products decrease. The color and stability of oils depend mainly on the small amounts of polycyclic compounds and polyunsaturated compounds present in them. During hydrogenation, some of the aromatic rings in such compounds are converted into cycloalkanes or open rings, while the unsaturated compounds become saturated compounds. This helps to lighten the color of the oil and improve its stability. Lubricants containing non-hydrocarbon elements such as sulfur, nitrogen, and oxygen generate corrosive acids during use. During hydrogenation, these elements react with hydrogen to produce gases such as hydrogen sulfide, amines, and water, which are separated from the oil; this process improves the quality of the product. The yield of the product obtained through hydrogenation-based refining is higher than that achieved by clay refining; it avoids issues such as the supply of clay and the disposal of waste clay, making it a good alternative to clay refining. (2) Hydroprocessing (or hydrocracking): The hydroprocessing process not only improves the color, stability, and odor of petroleum products, but also enhances their viscosity-temperature properties; it can replace clay treatment and solvent refining, serving two purposes at once. Under conditions that are slightly more stringent than those of hydrorefining, in addition to the various reactions involved in hydrorefining, there are also multiple hydrocracking reactions that convert most or all of the non-ideal components into naphthenes or alkanes through hydrogenation, thereby turning them into ideal components. For example, polycyclic hydrocarbons undergo hydrogenation and ring-opening to form hydrocarbons with fewer rings and longer side chains; as a result, the oil produced by hydrogenation treatment has good viscosity-temperature properties. (3) Hydrodesulfurization: The operating conditions for the hydrodesulfurization process are more stringent than those for hydrogenation. Under the action of a catalyst, the lubricant raw materials undergo hydrogenation isomerization and hydrogenation cracking reactions; this enables the hydrogenation process to not only refine the substances but also isomerize waxes, thereby converting normal alkanes with high freezing points into isoparaffins or lower-molecular-weight alkanes with lower freezing points, achieving the goal of reducing the freezing point. Lubricant product blending Blending is the final and crucial step in the lubricant production process. According to the formula specified for the lubricant, the base oil components and additives are added to the blending container in the correct proportions and sequence. Mechanical stirring (or stirring using compressed air), pumping for circulation, and static mixing through pipelines are used to ensure uniform mixing. Once sampling and analysis according to product standards show that the mixture meets the requirements, it becomes a finished product. Typically, after refinement in refineries, only several base oils with different viscosities are obtained, such as normal third fraction, second vacuum fraction, third vacuum fraction, fourth vacuum fraction, and bright oil (i.e., a high-viscosity oil obtained by deasphalting and refining vacuum residue). Many grades of lubricant products are often formulated by mixing two or more base oil components with different viscosities in a certain ratio (which is commonly referred to as the blending ratio). The blending of these base oils forms the basis for the formulation of lubricant products. (1) Calculation of the viscosity of the blended oil and the mixing ratio: When oils with different viscosities are mixed together, their viscosity does not follow an additive relationship; rather, it should be calculated using the following formula: lgV = N1·BlgV1 + N2·lgV2. Here, V, V1, and V2 represent the dynamic viscosities of component 1 and component 2 of the blended oil, in mm2/s; N1 and N2 represent the mixing ratios of component 1 and component 2, expressed as percentages (as decimals in calculations, with N1 = 1 – N2). (2) Changes in the properties of the blended oil: When two or more component oils are blended to obtain an oil with the desired viscosity, not only is the viscosity not equal to the arithmetic average, but other property values also do not follow an arithmetic average; rather, the properties tend to be those of the component oil with the lower properties. For example: 1. The flash point of an oil mixture composed of components with different flash points generally tends to be that of the component with the lower flash point; in other words, there is a decrease in the flash point. 2. The freezing point of an oil mixture composed of components with different freezing points generally tends to be that of the component with the higher freezing point; in other words, there is an increase in the freezing point. 3. The viscosity index of the oil obtained by mixing component oils with different viscosity indices generally tends to be that of the oil from the group with the higher viscosity index. Within a certain range, it also exhibits a certain degree of additivity, that is, an increase in the viscosity index. 4. For oils formed by mixing components with different oiliness levels, their overall oiliness generally follows a linear relationship with the arithmetic mean. 5. Other indicators of blended oil, such as acid value, ash content, impurities, and residue, are additive indicators. Atmospheric and vacuum distillation: The most commonly used lubricants today are produced from petroleum distillates, and are generally referred to as mineral oil-based lubricants. The raw materials used to produce such lubricants are abundant, inexpensive, and of good quality; moreover, appropriate additives can be added to improve their performance, which is why they are widely used. The crude oil used for producing lubricating oils has been selected; taking advantage of the difference in boiling points among its various components, various petroleum fractions can be separated from the crude oil using atmospheric and vacuum distillation units. A atmospheric and vacuum distillation unit can be divided into a primary distillation section, an atmospheric pressure section, and a vacuum section. At atmospheric pressure distillation, fractions with a boiling point below 400°C are distilled off. Atmospheric pressure distillation can only produce lubricants with low viscosity, because when crude oil is heated to 400°C, some of the hydrocarbons undergo cracking and coking occurs in the heating furnace, which affects the quality of the lubricants. Based on the principle that as external pressure decreases, the boiling point of a liquid also decreases, vacuum distillation is used to fractionate high-boiling-point (350–500°C), high-viscosity fractions. However, there are still some heavy lubricating oils that cannot be vaporized in the vacuum distillation tower and remain as residue; these oils need to have their gums and asphalts removed before they can be further processed. Solvent refining: Solvent refining involves using solvents to extract certain undesirable components from oil, thereby altering the properties of that oil. Lubricating oils that have undergone solvent refining exhibit significantly improved properties such as viscosity-temperature characteristics and oxidation resistance. Solvents used in industry include acids and bases, phenols, furfural, and cresol, etc. The basic principle of solvent refining is to take advantage of the fact that solvents have a high solubility for the non-ideal components in oils but a low solubility for the ideal components. The solvent is added to the lubricating oil, where the non-ideal components dissolve rapidly in the solvent. The solution containing these non-ideal components is separated out, leaving behind the ideal components of the lubricating oil. The former is usually referred to as extracted oil, while the latter is called raffinate oil or refined oil. Solvent refining essentially involves removing the non-ideal components from the lubricating oil; hence this process is also known as solvent extraction.