General basic knowledge of lubricants

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Basic Knowledge of Lubricants Lubricants are liquid lubricants used in various types of machinery to reduce friction and protect the machinery as well as the components being processed. They serve functions such as lubrication, cooling, rust prevention, cleaning, sealing, and shock absorption. Lubricating oils account for 85% of all lubricant materials, with a wide variety of types and grades; the global annual consumption is currently around 38 million tons. The general requirements for lubricating oils are: (1) to reduce friction and wear, thereby lowering frictional resistance to save energy, reducing wear to extend the lifespan of machinery, and improving economic efficiency ； 　 (2) Cooling: It is necessary to remove the friction heat from the machine at all times ； 　 (3) Sealing: leakage prevention, dust protection, and air leakage prevention are required ； 　 (4) Corrosion and rust resistance, to protect the friction surface from oil degradation or external erosion ； 　 (5) Clean rinsing: it is necessary to remove the dirt accumulated on the friction surface ； 　 (6) Stress dispersion buffering, load distribution, and impact mitigation as well as shock absorption ； 　 (7) Kinetic energy transmission, hydraulic systems, remote-controlled motors, and frictional continuously variable transmissions, etc. 　　II. Composition of lubricating oil Lubricating oil generally consists of two parts: base oil and additives. Base oil is the main component of lubricants and determines their basic properties, while additives can compensate for and improve the shortcomings of the base oil’s performance, endowing it with certain new properties; they are an important part of lubricants. 　　1. Lubricant base oils　　Lubricant base oils are mainly divided into two categories: mineral base oils and synthetic base oils. Mineral base oils are widely used and consumed in large quantities (over 95% of the total), but in certain applications, products formulated with synthetic base oils must be used; this has led to the rapid development of synthetic base oils. 　　Mineral base oil is derived from crude oil. The main production processes for lubricant base oils include: atmospheric and vacuum distillation, solvent deasphalting, solvent refining, solvent dewaxing, and clay or hydrofinishing. In 1995, China’s current standards for lubricant base oils were revised, mainly by changing the classification method, and standards for two types of specialized base oils—those with low freezing points and those that have undergone advanced refining—were added. In the production of mineral-based lubricants, the most important thing is to select the best crude oil. 　　The chemical composition of mineral base oils includes high-boiling-point, high-molecular-weight hydrocarbons and non-hydrocarbon mixtures. Its composition generally includes alkanes (straight-chain, branched, highly branched), cycloalkanes (monocyclic, bicyclic, polycyclic), aromatics (monocyclic aromatics, polycyclic aromatics), cycloaliphatic aromatics, as well as oxygen-containing, nitrogen-containing, and sulfur-containing organic compounds, along with non-hydrocarbon compounds such as resins and asphaltenes. 　　2. Additives Additives are the essence of modern high-performance lubricants. By selecting and adding them appropriately, it is possible to improve their physicochemical properties, endow the lubricants with new special characteristics, or enhance certain existing properties, thereby meeting higher requirements. Careful selection, meticulous balancing, and rational formulation of additives based on the required quality and performance characteristics of the lubricant are crucial to ensuring its quality. Commonly used additives include: viscosity index improvers, pour point depressants, antioxidants, detergents and dispersants, friction modifiers, oiliness agents, extreme pressure agents, antifoam agents, metal passivators, emulsifiers, corrosion inhibitors, rust inhibitors, and demulsifiers. 　　III. Basic Properties of Lubricating Oils Lubricating oils are technology-intensive products that are mixtures of complex hydrocarbons, and their actual performance in use is the result of a combination of complex physical or chemical processes. The basic properties of lubricating oils include general physical and chemical properties, special physical and chemical properties, and bench test simulations. 　　 General physical and chemical properties: Each type of lubricant has its own common physical and chemical properties, which indicate the inherent quality of the product. For lubricating oils, these general physical and chemical properties are as follows: (1) Appearance (colority) – The color of the oil often reflects its degree of refinement and stability. For base oils, generally the higher the degree of refining, the more thoroughly the hydrocarbon oxides and sulfides are removed, and the lighter the color becomes. However, even under the same refining conditions, the base oils produced from crude oils of different sources and chemical classes may differ in color and transparency. For new finished lubricating oils, due to the use of additives, color has lost its original significance as an indicator of the degree of refinement of the base oil. 　 (2) Density Density is the simplest and most commonly used physical property indicator for lubricants. The density of lubricating oil increases as the amounts of carbon, oxygen, and sulfur in its composition increase. Therefore, at the same viscosity or the same relative molecular mass, lubricating oils with a high content of aromatics, as well as those with high levels of gums and asphalts, have the highest density; those with a high content of naphthenes have a medium density, while those with a high content of alkanes have the lowest density. 　 (3) Viscosity Viscosity reflects the internal friction of an oil and is an indicator of its viscosity and fluidity. Without the addition of any functional additives, the higher the viscosity, the greater the strength of the oil film and the poorer its fluidity. 　 (4) Viscosity Index The viscosity index indicates the degree to which the viscosity of a oil changes with temperature. The higher the viscosity index, the less the viscosity of the oil is affected by temperature, indicating better viscosity-temperature properties; conversely, the lower the viscosity index, the worse these properties are. 　 (5) Flash point The flash point is an indicator of the volatility of oils. The lighter the fraction of the oil, the greater its volatility, and the lower its flash point as well. Conversely, the heavier the distillate of the oil, the lower its volatility, and the higher its flash point. At the same time, the flash point is an indicator of the fire hazard of petroleum products. The hazard level of oils is determined based on their flash point: oils with a flash point below 45°C are considered flammable, while those with a flash point above 45°C are considered combustible. It is strictly prohibited to heat oils to their flash point temperature during storage and transportation. At the same viscosity, the higher the flash point, the better. Therefore, when selecting lubricating oil, users should choose based on the operating temperature and the working conditions of the oil. It is generally believed that a flash point 20–30°C higher than the operating temperature ensures safe use. 　 (6) Freezing point and pour point The freezing point is the highest temperature at which the oil stops flowing under specified cooling conditions. The solidification of oils differs significantly from that of pure compounds. Oil does not have a definite freezing temperature; the so-called “freezing” simply means that it loses its fluidity as a whole, and not all of its components turn into solids. The freezing point of lubricating oil is an important quality indicator that reflects its flowability at low temperatures. It is of great significance for production, transportation, and usage. Lubricants with a high freezing point cannot be used at low temperatures. On the contrary, in areas with higher temperatures, there is no need to use lubricants with a low freezing point. Because the lower the freezing point of the lubricating oil, the higher its production cost, resulting in unnecessary waste. Generally, the freezing point of lubricating oil should be 5–7°C lower than the lowest temperature of the operating environment. It is particularly important to note that when selecting lubricants for low temperatures, a comprehensive consideration of the oil’s freezing point, viscosity at low temperatures, and viscosity-temperature characteristics is necessary. Because oils with low freezing points may also have low-temperature viscosity and viscosity-temperature properties that do not meet the requirements. 　　Both the freezing point and the pour point are indicators of the low-temperature flowability of oils; there is no fundamental difference between them, only slight variations in the methods of measurement. The freezing point and pour point of the same oil product are not exactly equal; generally, the pour point is 2–3°C higher than the freezing point, though there are exceptions. 　　(7) Acid value, alkali value, and neutralization value The acid value is an indicator of the acidic substances present in lubricating oil, with the unit being mgKOH/g. The acid value is divided into strong acid value and weak acid value; the sum of the two gives the total acid value (abbreviated as TAN). What we usually refer to as “acid value” actually means “Total Acid Number (TAN)”. 　　The alkalinity value is an indicator of the content of basic substances in lubricating oil, with the unit being mgKOH/g. Alkali value is also divided into strong alkali value and weak alkali value; the sum of the two constitutes the total alkali value (abbreviated as TBN). What we usually refer to as “alkalinity” actually means “Total Base Number (TBN)”. 　　The neutralization value actually includes the total acid value and the total base value. However, unless otherwise specified, the so-called “neutralization value” generally refers only to the “total acid value,” which is also expressed in mgKOH/g. 　 (8) Moisture Moisture refers to the percentage of water contained in the lubricant, usually expressed as a weight percentage. The presence of moisture in lubricating oil disrupts the oil film formed by the lubricant, thereby reducing its lubricating effectiveness. It also accelerates the corrosion of metals by organic acids, leading to equipment rust and the easy formation of sediment in the oil. In short, the less moisture in the lubricant, the better. 　 (9) Mechanical impurities Mechanical impurities refer to precipitates or gelatinous suspensions present in lubricating oil that are insoluble in solvents such as gasoline, ethanol, and benzene. Most of these impurities are sand, gravel, iron filings, and other such substances, as well as some organometallic salts that are insoluble in solvents and result from additives. Generally, the mechanical impurities in lubricant base oils are kept below 0.005% (levels below 0.005% are considered to be absent). 　 (10) Ash and sulfated ash
Ash refers to the non-combustible residue remaining after incineration under specified conditions. The composition of ash is generally considered to be various metal elements and their salts. Ash content has different meanings for various types of oils; for base oils or oils without additives, ash content can be used to determine the degree of refinement of the oil. For oils with metal salt additives (new oil), ash content becomes a means to quantitatively control the amount of additives added. Abroad, sulfuric acid ash is used in place of ash. The method is to add a small amount of concentrated sulfuric acid before burning the oil sample and carrying out ashing, so as to convert the metal elements of the additive into sulfates. 　 (11) Residue The dark brown residue formed after the oil is heated, evaporated, and burned under specified experimental conditions is called residue. Residue is an important quality indicator for lubricant base oils; it is a parameter specified to determine the properties of the lubricant and the degree of its refining. In lubricant base oils, the amount of residue is determined not only by its chemical composition but also by the degree of refining of the oil. The main substances that cause residue formation in lubricants are gums, asphaltenes, and polycyclic aromatic hydrocarbons present in the oil. Under conditions of insufficient air, these substances undergo decomposition and condensation under high heat to form residue carbon. The deeper the refining of the oil, the lower its residue value. Generally speaking, the lower the carbon residue value of a base oil, the better. 　　Today, many petroleum products contain additives with elements such as metals, sulfur, phosphorus, and nitrogen; their carbon residue values are very high. As a result, the carbon residue value of oils containing additives no longer serves the original purpose for which carbon residue measurement was intended. Mechanical impurities, moisture, ash, and residue are all quality indicators that reflect the purity of oil products, indicating the degree of refinement of the lubricating base oil. 　　Special physical and chemical properties In addition to the aforementioned general physical and chemical properties, each type of lubricant should also possess special physical and chemical characteristics that reflect its specific application properties. The higher the quality requirements, or the greater the specificity of the oil, the more prominent its special physical and chemical properties become. The test methods reflecting these special physical and chemical properties are briefly introduced as follows: (1) Oxidative stability. Oxidative stability indicates the anti-aging performance of lubricating oils. Many industrial lubricating oils with a long service life must meet this requirement; therefore, it is considered a special property required for such types of oils. There are many methods for determining the oxidation stability of oils. Basically, a certain amount of oil is oxidized at a specific temperature for a set period of time in the presence of air (or oxygen) and metal catalysts, after which the acid value, changes in viscosity, and the formation of precipitates in the oil are measured. All lubricants have different tendencies toward autoxidation, depending on their chemical composition and the external conditions in which they find themselves. Oxidation occurs over time as it is used, gradually producing substances such as aldehydes, ketones, acids, as well as gums and asphaltenes. Oxidation stability is the property that prevents the formation of these substances that are detrimental to the use of oil products. 　 (2) Thermal stability Thermal stability refers to a lubricant’s ability to withstand high temperatures; in other words, it is the lubricant’s resistance to thermal decomposition, which is expressed by the thermal decomposition temperature. Requirements for thermal stability have been established for some high-quality anti-wear hydraulic oils, compressor oils, and similar products. The thermal stability of oils depends primarily on the composition of the base oil; many additives with low decomposition temperatures tend to have an adverse effect on the stability of the oils ； Antioxidants also cannot significantly improve the thermal stability of oils. 　 (3) Oiliness and extreme pressure properties Oiliness refers to the ability of the polar substances in lubricants to form a strong physicochemical adsorption film on the metal surfaces at the points of friction, thereby providing resistance to high loads and frictional wear. Extreme pressure properties, on the other hand, involve the polar substances in lubricants decomposing due to frictional chemical reactions under high temperatures and heavy loads at the metal surfaces in question; these substances then undergo frictional chemical reactions with the surface metal, resulting in the formation of a soft (or plastic) extreme pressure film with a low melting point, which in turn provides protection against impacts as well as high loads and high temperatures. 　 (4) Corrosion and rusting 　　Oxidation of oils or the effect of additives often leads to corrosion of steel and other non-ferrous metals. In corrosion tests, copper strips are generally placed in oil and left at 100°C for 3 hours, after which the changes in the copper are observed ； In the corrosion test, rust forms on the steel surface under the action of water and water vapor. To determine the anti-corrosion properties, 30 ml of distilled water or artificial seawater is added to 300 ml of test oil; then a steel rod is placed in it. The mixture is stirred at 54°C for 24 hours, after which it is examined to see whether any rust has formed on the steel rod. The oil should have metal corrosion resistance and anti-rust properties; in industrial lubricant standards, these two parameters are usually mandatory test items. 　 (5) Antifoaming property: During operation, lubricants often form foam due to the presence of air. This is especially true when the oil contains surfactant additives, as foam is more likely to form and it is also difficult for such foam to dissipate. Foam formation during the use of lubricant can damage the oil film, cause sintering of the friction surfaces or increase wear, accelerate the oxidation and deterioration of the lubricant, and also cause air blockages in the lubrication system, affecting the circulation of the lubricant. Therefore, anti-foaming properties are an important quality indicator for lubricating oils, etc. 　 (6) Hydrolytic stability Hydrolytic stability indicates the stability of oils under the action of water and metals (mainly copper). When an oil has a high acid value, or contains additives that decompose into acidic substances when exposed to water, this parameter often fails to meet the required standards. The determination method involves adding the test oil to a certain amount of water, mixing and stirring it with a copper sheet at a specified temperature for a certain period of time, and then measuring the acid value of the water layer and the weight loss of the copper sheet. 　 (7) Emulsion resistance　　During use, industrial lubricating oils inevitably get mixed with some cooling water. If the lubricating oil has poor emulsion resistance, it will form an emulsion with the entrained water. This makes it difficult to drain the water from the bottom of the circulating oil tank, which may consequently lead to inadequate lubrication. Therefore, anti-emulsibility is a very important physicochemical property of industrial lubricants. For ordinary oils, 40 ml of the oil sample is mixed vigorously with 40 ml of distilled water at a certain temperature for a specified period of time, after which the time it takes for the oil layer, water layer, and emulsion layer to separate into 40–37–3 ml is observed ； For industrial gear oil, the test oil is mixed with water, stirred at a certain temperature and 6,000 rpm for 5 minutes, left to stand for 5 hours, and then the volumes of oil, water, and the emulsion layer are measured. 　 (8) Air release value: This requirement is specified in the hydraulic oil standards, as in hydraulic systems, if the air dissolved in the oil is not released in a timely manner, it will affect the accuracy and sensitivity of hydraulic transmission; in severe cases, it may prevent the hydraulic system from meeting its functional requirements. The method for measuring this property is similar to that for antifoaminess, except that it determines the time it takes for the air (mist) dissolved within the oil to be released. 　 (9) Rubber sealing performance In hydraulic systems, rubber is commonly used as a sealing material. In mechanical devices, oils inevitably come into contact with various sealing elements; oils with poor sealing properties can cause the rubber to swell, shrink, harden, or crack, thereby affecting its sealing ability. Therefore, it is necessary for oils to have good compatibility with rubber. Hydraulic oil standards require a rubber sealing index, which is measured by the change that occurs in a rubber ring of a certain size after being immersed in oil for a specified period of time. 　 (10) Shear stability: In oils to which thickeners have been added, mechanical shear during use causes the high-molecular polymers in the oil to be broken down, resulting in a decrease in the oil’s viscosity and thereby affecting proper lubrication. Therefore, shear stability is a specific physical and chemical property that must be measured for such oils. There are many methods for measuring shear stability, including the ultrasonic shear method, the nozzle shear method, the Wicks pump shear method, and the FZG gearbox shear method; all of these methods ultimately aim to determine the rate of viscosity decrease in the oil. 　 (11) Solubility 　Solubility is usually expressed by the aniline point. The solubility limit aniline point of composite additives varies with different grades of oils; the limit value for oils with low ash content is higher than that for highly alkaline oils, and the limit value for single-grade oils is higher than that for multi-grade oils. 　 (12) Volatility The volatility of the base oil is related to fuel consumption, viscosity stability, and oxidation stability. These properties are particularly important for multi-grade oils and energy-saving oils. 　 (13) Rust resistance This refers specifically to the special physical and chemical properties that rust-proof greases should possess. The testing methods include humidity tests, salt spray tests, lamination tests, and water displacement tests; in addition, there are also box test and long-term storage tests. 　 (14) Electrical properties Electrical properties are unique characteristics of insulating oils, including the dielectric loss angle, dielectric constant, breakdown voltage, pulse voltage, etc. The degree of refinement of the base oil, as well as impurities and moisture, all have a significant impact on the electrical properties of the oil. 　 (15) Special physical and chemical properties of greases In addition to their general physical and chemical properties, greases designed for specific purposes possess special physical and chemical characteristics as well. Greases with good water resistance are required to undergo a water test ； For low-temperature grease, low-temperature torque needs to be measured ； Multi-effect greases need to be tested for extreme pressure and wear resistance as well as rust prevention properties ； Long-life greases require bearing life tests and the like. There are also corresponding test methods for measuring these properties. 　 (16) Other special physical and chemical properties Each type of oil should possess its own unique special properties in addition to its general properties. For example, the cooling rate of quenching oil needs to be determined ； The emulsification stability of the emulsified oil needs to be determined ； The anti-creep coefficient of hydraulic guide rail oil needs to be measured ； The dispersion of oil mist from the sprayed lubricant needs to be measured ； The freezing point and flocculation point of refrigeration oil need to be measured ； Low-temperature gear oil needs to be tested for pitting, etc. All these properties require a special chemical composition of the base oil, or the addition of certain special additives to ensure them. 　　IV. Simulation Bench Tests After their specific physical and chemical properties have been evaluated, lubricants generally undergo certain simulation bench tests, including some engine tests, before they can be put into use. 　　Oils with extreme pressure and anti-wear properties must have their extreme pressure and anti-wear performance evaluated. Commonly used testing machines include the Timken ring block tester, FZG gear tester, Fallex tester, and roller fatigue tester; all of these are used to evaluate a lubricant’s ability to withstand extreme pressure loads or its wear resistance. 　　The most commonly used testing machine for evaluating the extreme pressure properties of oils is the four-ball tester, which can determine the oil’s maximum load without seizure, sintering load, long-term wear, and comprehensive wear index. These indicators can reflect the extreme pressure and anti-wear properties of lubricants to a certain extent, but in many cases they do not show a good correlation with the actual performance in use. It is still widely used simply because this method is simple and easy to implement. 　　In the standards for high-quality vehicle gear oils, a series of gear bench tests are required, including tests under conditions of low speed and high torque, as well as high speed and low torque ； Gear testing with impact load ； Rust resistance test for gearboxes and gear tests for the thermal oxidative stability of oils. 　　 There are many single-cylinder bench test methods for evaluating diesel engine oils, such as the Pitot W-1, AV-1, AV-B, and Lebek L-38 single-cylinder tests, as well as the domestic 1105 and 1135 single-cylinder tests, which can be used to assess diesel engine oils of various grades. Currently, the quality classification standards for API diesel engine oils specify that diesel engine oils are evaluated using the Caterpillar, Mack, Cummins, single-cylinder, and GM multi-cylinder criteria ； Gasoline engine oils are subjected to tests such as MS Program II D (corrosion, wear resistance), III E (high-temperature oxidation), and V E (low-temperature sludge). These bench tests require significant investment, with high costs for each test. They impose strict requirements on testing conditions such as environmental control and fuel standards; not every ordinary laboratory has the necessary facilities to conduct such tests. Therefore, only a few designated centers across the country can be used to evaluate these types of fuels. 　　In short, due to the varying properties of different types of oils and the wide range of applications for them, it is necessary to establish specification standards that reflect the inherent quality of each type of oil, based on its actual characteristics. This ensures that every type of oil produced meets the required quality criteria, thereby satisfying the actual needs of the equipment in use. 　　V. Lubricant Management Lubricants are a category of petrochemical products that come in the greatest variety of grades and are used in a wide range of applications. At the same time, lubricants are also a technology-intensive product. Only by proper management of lubricants and their correct use can their technical properties be utilized, which in turn helps to ensure the proper operation of equipment, extend its lifespan, save on lubricant costs, conserve energy, and improve economic and social benefits. 　　 Management of Lubricant Transportation and Storage The main requirements for the transportation and storage of lubricants are as follows: (I) Bulk lubricants 1. The containers used to hold and store lubricants must be clean ； 　　2. The transportation and storage of transformer oil and turbine oil require \"dedicated tanks and dedicated pipelines\"” ； Other oils should have storage and transportation facilities designated according to the three main categories of diesel engine oil, hydraulic oil, and gear oil. 　　3. Special care must be taken during transportation and storage to prevent the inclusion of moisture and impurities. 　　4. The storage period for bulk lubricating oil should generally not exceed six months. 　　5. The density of lubricating oils is approximately between 0.75 and 0.95 g/cm³; they are lighter than water and insoluble in it. The flash point (open cup) of lubricating oils is generally above 150 °C, classifying them as flammable substances. During storage and transportation, care must be taken to prevent leaks that could contaminate the environment or cause fires. 　　6. Indicate the product name, grade, level, quantity, and date of entry into storage, etc. 　　7. In principle, oils of the same type produced by different manufacturers should not be stored together. If storage together is necessary, a \"compatibility test\" must first be conducted to confirm that there are no adverse reactions before such storage is permitted. 　 (II) Barreled oil products 1. Rough handling is strictly prohibited when loading and unloading oil products, and the stacking height of these products should be appropriate to avoid hazards or damage to the products. 　　2. Special care must be taken during transportation and storage to prevent the intrusion of moisture and impurities. 　　3. The storage period for lubricants in barrels can be longer than that for bulk forms, but it should generally not exceed one year. 4. Different oil products should be stored separately, with clear markings indicating the product name, grade, quality level, quantity, and date of entry, to avoid errors during shipment. 　　Management of lubricant usage (I) Selection of lubricants The selection of lubricants is the first step in their use, and it is crucial for ensuring proper lubrication of equipment as well as maximizing the performance of the lubricants. 　　1. When selecting lubricating oil, the following three factors should be taken into consideration comprehensively: (1) The operating conditions of the mechanical equipment in actual use (i.e., the working conditions) ； 　 (2) Designation or recommendation in the manufacturer’s instructions for mechanical equipment ； 　 (3) Regulations or recommendations from lubricant manufacturers. 　　2. Selection of lubricant performance indicators (1) Viscosity Viscosity is the indicator used for classifying and grading various lubricants, and it is decisive for quality assessment and determination. The viscosity of the lubricating oil for equipment is determined by referring to relevant charts based on design or calculation data. 　 (2) Pour point The pour point is an indicator that indirectly reflects the low-temperature flow properties of lubricants during storage, transportation, and use. Experience has shown that the operating temperature of ordinary lubricants must be 5–10 °C higher than their pour point. 　 (3) Flash point The flash point is primarily an indicator of the safety of storing, transporting, and using lubricants; it also serves as a criterion for controlling the fractions and volatility of lubricants during production. The principle governing the flash point specification for lubricants is to apply a safety factor of 1/2 in accordance with safety regulations, meaning the value is set 1/2 higher than the actual operating temperature. Since the oil temperature in the crankcase of internal combustion engine oils should not exceed 120 ℃, it is specified that the flash point of such oils must be at least 180 ℃. 　 (4) Selection of performance indicators There are numerous performance indicators, and the differences between different varieties are significant; a reasonable decision should be made by taking into account the operating conditions of the equipment, the requirements of the manufacturer, as well as the specifications and descriptions related to the oil. Strive to meet the requirements of lubrication technology while remaining cost-effective. 　 (II) Substitutes for lubricating oil 1. Different types of lubricating oils have their own specific characteristics or differences in terms of performance. Therefore, it is necessary to select lubricants correctly and appropriately, avoid using substitutes, and under no circumstances permit random substitution. 　　2. Principles for substituting lubricating oils (1) Try to use oils of the same type or those with similar properties as substitutes. 　 (2) The viscosity must be comparable; the viscosity of the substitute oil shall not exceed ±15% of that of the original oil. Oils with slightly higher viscosity should be given priority for substitution. 　 (3) Quality shall be higher rather than lower. 　 (4) When selecting alternative oils, attention should also be paid to the equipment’s operating and ambient temperatures. 　 (III) Mixing of lubricants 1. It is advisable to avoid mixing lubricants of different types, grades, from different manufacturers, as well as old and new lubricants. The following oils must never be mixed together. 　 (1) Military specialty oils and specialized fuels must not be mixed with other types of oils. 　 (2) Oils with anti-emulsification requirements must not be mixed with oils that do not have such requirements. 　 (3) Anti-ammonia turbine oil must not be mixed with other turbine oils. 　 (4) Zn-containing anti-wear hydraulic oil cannot be mixed with silver-resistant hydraulic oil. 　 (5) Gear oil must not be mixed with worm gear oil. 　　2. The following situations allow for mixing: (1) Products of the same manufacturer with similar quality and basic characteristics. 　 (2) Products of the same type but different grades from the same manufacturer. 　 (3) For oils of different types, provided it is known that neither component in the mixture contains additives. 　 (4) When oils of different types are mixed together, no abnormal phenomena or significant changes in performance occur. 　　3. There are a large variety and quantity of additives added to diesel engine oils, with varying performances ； Care must be taken when mixing oils whose performance characteristics are not known. To avoid adverse consequences or even equipment lubrication failures. 　 (IV) Control of lubricant contamination Apart from improper selection or use of lubricants, lubrication failures are mainly caused by contamination. 　　1. Substances that contaminate lubricating oil include dust, impurities, and moisture. 　　2. The control of contamination levels is crucial for the wear resistance of hydraulic oil, turbine oil, hydrostatic film bearing oil, and high-speed bearing oil. 　　3. Measures to control pollution: (1) The containers used for storing lubricants must be clean and airtight, and must not come into contact with metals such as copper and tin, which can facilitate the oxidation and deterioration of lubricants. 　 (2) The oil must undergo sedimentation and filtration before being fed into the equipment, to ensure that its clarity reaches level 5 or higher. 　 (3) Fuel containers must not be left exposed to the atmosphere; in particular, containers holding fuel must not be left uncovered. 　 (4) The oil tanks used for storing lubricating oil should be cleaned regularly, and waste oil should be removed promptly. 　 (5) Install air-filtered respirators on oil tanks or containers; use filters with a mesh size of 100 or finer along with dust caps at the filling ports. Ensure proper sealing in all areas, and install filters and drain valves at appropriate locations in the lubrication system. 　　4. Electrical oils such as transformer oil have high requirements regarding moisture content; oil should be changed whenever the weather is dry. Moreover, the oil should be added to the equipment immediately. 　 (5) Monitoring of the condition of lubricating oil – It is an inevitable phenomenon that lubricating oil gradually ages and degrades over time as it is in use. There are two types of aging and deterioration: one is normal aging and deterioration ； Another type is abnormal deterioration caused by abnormal factors such as water pollution. Monitoring the usage condition of lubricating oil allows for timely awareness of its technical condition, helps prevent lubrication-related equipment failures, and extends the service life of the oil. 　　1. Methods of monitoring (1) Randomly inspect whether operators follow the ‘five fixed principles’ for equipment lubrication. 　 (2) Sample and observe the appearance of the oil, checking its color, transparency, odor, etc. 　 (3) Regularly test key physical and chemical parameters that reflect changes in oil quality, such as viscosity, flash point, moisture content, and acid value (or alkalinity value). 　 (4) Those without a laboratory cannot conduct moisture pop tests and spot tests, etc. 　 (5) Analyze using modern instruments. If an infrared spectrometer is used to determine changes in additives in the oil, then an iron spectrometer or ICP emission spectroscopy is used to detect changes in metal particles or elements in the oil. Instrumental analysis is fast and accurate, and it is of great significance for the lubrication management of large-scale critical equipment such as generator sets. 　 (VI) Replacement of lubricating oil After being in use for a period of time (several months, years, or even decades), lubricating oil gradually deteriorates due to oxidation and the influence of external factors during use, resulting in a decline or change in its properties; therefore, it must be replaced at the appropriate time. 　　1. Determination of oil change timing (1) Determine the oil change timing based on the results of inspections and evaluations ； However, the current difficulty is the relative lack of reporting standards for various types of oils. 　 (2) Replace it regularly based on the recommendations of lubricant manufacturers and equipment manufacturers, as well as practical usage experience. 　　2. Precautions for changing oil (1) Make the decision to change oil easily, and try to extend the service life of the oil. 　 (2) Try to change the oil during the maintenance period. 　 (3) When changing the oil, do not discard it outright; if the quality of the oil is still good, it can be slightly treated (such as through sedimentation and filtration to remove water and impurities) before being reused or used in secondary equipment. Used oil should be collected properly to facilitate future reprocessing and prevent environmental pollution. 　　Preventing lubricants from harming health Many petroleum products are harmful to the human body; if they come into contact with the skin and are not cleaned off promptly, they can cause mild conditions such as dermatitis or bumps, or more severe issues like rashes or skin tumors. If it enters the mouth or is inhaled, it can cause gastrointestinal problems or pneumonia in mild cases; in severe cases, it may lead to cancer. Therefore, it is essential to be careful not to let oil get on food, not to allow it to enter the respiratory tract, and to avoid having oil all over one’s body or on the floor. This not only results in waste but also undermines personal hygiene. 　　Animal tests have shown that refined mineral oil lubricants have low toxicity, but the hazard of lubricants with additives increases, so caution is necessary. Even if new oil is non-toxic, it can become harmful due to deterioration and contamination during use; therefore, care must be taken to avoid contact with the skin, and it must especially not be inhaled or ingested. If it comes into contact with the skin accidentally, rinse it thoroughly with clean water immediately. 　　 Treatment of used oil: The lubricating oil that has been replaced is no longer usable and must be treated as waste oil. This waste oil should be disposed of properly to prevent environmental pollution. 　　1. These used oils should be collected for centralized treatment. Buckets or bottles containing lubricants should not be discarded carelessly; they must also be handled properly to prevent adverse effects on the environment. 　　2. Used lubricating oil, especially those containing a high amount of additives, is difficult to recycle. But it can generally be burned as fuel oil. Isomeric Fundamentals Knowledge, Section 1: Principles of Reaction. The low-temperature fluidity of oils is related to the molecular structure of their components; among the isomers of alkanes with the same number of carbon atoms, those that are n-alkanes have the highest freezing point. Isomers exhibit a significant effect on the freezing point and viscosity index depending on the degree of branching and the location of the branches; isomeric alkanes with branches located in the middle of the chain have lower freezing points than those with branches at the ends. For example, the freezing point of n-C20 is 37°C, that of 2M-C19 is 18°C, and that of 5M-C19 is -7°C. 11n-P-C26 has a viscosity index of 20 and a freezing point of 19°C, while 11i-P-C26 has a viscosity index of 125 and a freezing point of -40°C. From the perspective of lowering the pour point, it is undesirable to form highly branched isomers; the isomerization of wax molecules should result in isomers with fewer branches, as having a branch in the middle of the alkane chain leads to a lower pour point. Therefore, the ideal situation is that the tertiary carbocation can rapidly desorb from the acidic site and be hydrogenated to yield less-branched isoparaffins. The greater the number of branches in heteroalkanes, the higher their dynamic viscosity, while their viscosity index is lower. The viscosity of branched alkanes with the same carbon count is greater than that of branched aromatics. I. Principle of the hydrogenation isomerization dewaxing reaction: The wax components in white oil fractions are mainly composed of long-chain alkanes with carbon numbers ranging from C20 to C40, which are either straight-chain or slightly branched at the terminal ends. It is generally believed that the isomerization reaction follows a normal carbocation mechanism; that is, typical dual-functional alkanes isomerization and cracking reactions occur on a bifunctional catalyst that possesses both hydrogenation and acidic functions, consisting of a metal center and an acidic center. In the reaction, long-chain normal alkanes are first dehydrogenated via the strong metal functionality of the catalyst to form long-chain normal alkenes. These long-chain normal alkenes diffuse to acidic sites where they are protonated to form carbocations, which then undergo rapid skeleton isomerization reactions in the pores of the zeolite. Its degree of branching and the length of the branches are entirely controlled by the geometry of the zeolite pores. The isomerized primary carbocation releases a proton and returns to the zeolite, where it undergoes β-scission to form low-molecular-weight products or transforms into another olefin. The olefin generated from the primary carbocation is then hydrogenated through the action of strong metal functions, similar to that in dehydrogenation reactions. With a deeper understanding of macromolecular isomerization reactions, a new theory called the Key-Lock mechanism has been proposed. This mechanism suggests that long-chain normal alkanes can enter two or more pores simultaneously, and the isomerization reaction occurs at the pore openings of the molecular sieve, rather than inside the pores as previously thought; this makes it easier to explain the formation of polybranched isomers. Furthermore, heteroatomic dewaxing catalysts must possess strong hydrogenation-dehydrogenation active centers, which enable the dehydrogenation of alkanes and the hydrogenation of heteroolefin intermediates to occur at lower temperatures. This prevents the occurrence of deep isomerization and hydrocracking reactions, thereby increasing the selectivity of the dewaxing reaction. The yield of light products is much lower when such catalysts are used compared to that obtained with strongly acidic catalysts (catalytic dewaxing). Due to the strong functionality of metals, many aromatic hydrocarbons are also hydrogenated and saturated. 1. Alkane isomerization reaction: During hydroisomerization, both alkanes and alkenes undergo isomerization reactions, thereby increasing the ratio of isomeric hydrocarbons to n-paraffins in the hydrogenated products. The isomerization reactions of alkanes and alkenes also take place on bifunctional catalysts. Figure 2.1 shows the isomerization reaction pathway of n-heptane. As can be seen from Figure 2.1, although this reaction takes place under hydrogen pressure, hydrogen is not included in the stoichiometry of the reaction, and no hydrogen is consumed after the reaction is complete. Therefore, this process is also known as hydroisomerization. The isomerization of the reaction products is related to the hydrogenation activity and acidity of the catalyst. When the acidic activity of the catalyst is relatively high, the degree of isomerization of the product is also relatively high ； And when the hydrogenation activity of the catalyst is relatively high, the degree of isomerization of the product is lower. The figure below shows the hydrogenation isomerization pathway of n-heptane. Alkane isomerization reaction equation: 2. Alkane hydrocracking reaction Under hydroisomerization conditions, alkanes also undergo hydrocracking reactions, resulting in alkanes with lower molecular weights. The general formula for this process is: C2H2n+2 + H2 → CmH2m+2 + Cn-mH2(n-m)+2. Alkane hydrocracking also proceeds through a carbocation reaction mechanism. Alkane macromolecules first undergo dehydrogenation at the metal centers of the hydroisomerization catalyst to form olefin molecules; these olefin molecules then acquire hydrogen protons at the acidic centers of the catalyst to form carbocations. The carbocations undergo C-C bond cleavage at the b-position, resulting in smaller carbocations and olefins. The resulting small molecular olefins can continue to undergo the aforementioned cracking reactions to form even smaller molecules, or they can be hydrogenated at the metal center to become alkane molecules ； The resulting carbocation can either undergo a chain-scission reaction or lose a hydrogen proton at an acidic center to form an alkene, which can then be converted into an alkane through hydrogenation. Therefore, in hydrocracking reactions, the hydrogenation activity and acidity activity of the hydroisomerization catalyst must work well together. If the hydrogenation activity is too strong, it will overly suppress the secondary cracking reaction ； When the acidic activity is too strong, it will cause the secondary cracking reaction to become excessive, resulting in an increase in smaller molecules and unsaturated hydrocarbons in the reaction products; in severe cases, this can even lead to the formation of coke. II. Principle of hydrogenation-based refining reaction: The aromatics in white oil are not converted after passing through the hydrogenation isomerization reactor, and the presence of these aromatics ultimately affects the photostability and thermal oxidative stability of the base oil. To improve the photostability and thermal oxidative stability of the oil, as well as to enhance its color, it is necessary to carry out additional hydrogenation refining of the hydroisomerized product at lower temperatures, thereby promoting further conversion of the aromatics. The main purpose of hydrogenation-based refining is to saturate aromatic compounds; the reactions involved in this process are primarily the hydrogenation and saturation of olefins and aromatics. At the same time, other types of hydrogenation reactions also occur, such as the hydrogenolysis of non-hydrocarbon compounds including those containing sulfur, nitrogen, and oxygen. Since the content of non-hydrocarbon compounds such as organic sulfides, nitrides, and oxides in the crude oil is low, such reactions are not the main chemical processes involved. Therefore, this specification does not go into further detail regarding the principles of these types of reactions. There are fewer olefins in white oil. Olefins hydrogenate rapidly and do not require a high degree of reaction. For olefin hydrogenation, the lower the molecular weight, the easier it is to carry out hydrogenation; n-olefins are more easily hydrogenated than isomeric olefins. The hydrogenation of olefins proceeds as follows: CnH2n + H2 → CnH2n+2. The hydrogenation of aromatics is a reversible reaction; due to the constraints of thermodynamic equilibrium, aromatics cannot be completely converted under typical hydrogenation conditions. The amount of aromatics in the product decreases as the reaction temperature rises, but when the temperature increases further, the concentration of aromatics in the product reaches a maximum value. High pressure is beneficial for maintaining a high conversion rate. The hydrogenation saturation of aromatic rings is a highly exothermic reaction. As a result, the hydrogenation of aromatic hydrocarbons becomes thermodynamically unfavorable. The equilibrium constants for the hydrogenation of aromatics and polycyclic aromatics follow this pattern: (a) the equilibrium constant for the hydrogenation of aromatics decreases as the temperature increases ； (b) In the range of 600–700 K (i.e., 316–371°C), the equilibrium constant Kp for the complete hydrogenation and saturation of the aromatic ring decreases as the number of aromatic rings in the aromatic hydrocarbon molecule increases ； (c) For polycyclic aromatics, the equilibrium constant Kp for hydrogenation of the first ring is relatively large; the equilibrium constant Kp for hydrogenation of the second ring is somewhat smaller, while the equilibrium constant Kp for hydrogenation of all aromatic rings is the smallest. (d) At temperatures above 600 K, the equilibrium constants for the hydrogenation and saturation of aromatics are all low, so higher pressures are required to increase the equilibrium conversion rate ； (e) The hydrogenation saturation of the first aromatic ring in polycyclic aromatics is relatively easy, and its reaction rate constant is an order of magnitude larger than that of benzene ； The hydrogenation saturation of the last remaining aromatic ring is rather difficult, and its reaction rate is similar to that of benzene. Generally, high pressure and low temperature are favorable for the saturation of aromatics. The equilibrium constant for the hydrogenation saturation of aromatic rings with alkyl side chains is lower than that of those without alkyl side chains, and the reaction rate is also slower. 1. The reaction in which polycyclic aromatic hydrocarbons are hydrogenated to form polycyclic cycloalkanes: Viscosity index –60; Viscosity index 20; Pour point > +50°C; Pour point > +20°C. 2. The reaction in which polycyclic cycloalkanes undergo partial hydrogenation and ring opening to form monocyclic cycloalkanes or monocyclic aromatic hydrocarbons with long side chains: Viscosity index 20; Viscosity index 110–140. In addition to these reactions, there is also a possibility of secondary reactions occurring among the decomposition products, leading to an increase in carbon deposition on the catalyst. In most cases, the intermediate products of the condensation reaction are polycyclic aromatics. At a constant temperature, using a higher hydrogen partial pressure will reduce the concentration of such intermediate products, thereby decreasing the formation of coke on the catalyst. An increase in temperature facilitates the formation of intermediate products and increases carbon deposition on the catalyst surface. The higher the concentration of polycyclic molecules in the crude oil, the more coke is produced. The degree and rate of these reactions are related not only to the chemical composition of the raw materials but also closely to the performance of the catalyst and the reaction conditions. Section 2: Catalysts I. Basic Knowledge of Catalyst Theory 1. Catalysts and Catalytic Action: A thermodynamically feasible reaction whose rate is altered due to the action of a certain substance, with no consumption of this substance by the end of the reaction; such a substance is called a catalyst, and the effect it exerts on the reaction is known as catalytic action. Catalysts can change the reaction rate because they alter the reaction pathway in their presence, thereby changing the activation energy of the reaction. Catalysts cannot change the chemical equilibrium; they can only alter the rate at which the equilibrium is reached. 2. Steps of heterogeneous catalytic reactions: When the catalyst and reactants are not in the same phase, it is referred to as heterogeneous catalysis. In most cases of heterogeneous catalysis, the catalyst is solid while the reactants are liquid or gas. In hydroisomerization and hydrorefining processes, the catalyst is solid while the reactants are in gas or liquid phase. The steps of a heterogeneous catalytic reaction are as follows: 1. The reactants diffuse across the membrane on the outer surface of the catalyst particles to reach the outer surface of the catalyst. 2. The reactants diffuse from the outer surface to the inner surface of the catalyst. 3. The reactants adsorb onto the inner surface of the catalyst. 4. The reactants react on the inner surface of the catalyst to form products. 5. The products desorb from the inner surface of the catalyst. 6. The products diffuse from the inner surface to the outer surface of the catalyst. 7. The products diffuse outside through the membrane. These seven steps can be summarized into four phases: external diffusion, internal diffusion, adsorption, and reaction. If one of the stages proceeds at a slower rate than the others, then the overall reaction rate is determined by the rate of that stage; this stage becomes the rate-determining step. 3. Characteristics and Characterization of Hydrogenation Catalysts 3.1 Characteristics Hydrogenation catalysts are generally bifunctional catalysts composed of two basic components: a support and a metal; some also contain other components such as additives. The carrier primarily provides acidity, on which reactions such as cleavage, isomerization, and disproportionation occur. There are two main types of carriers for hydrogenation catalysts: one type is amorphous, such as alumina, silicoalumina, and silicomagnesium. Another category is crystalline zeolite molecular sieves. They can be used separately or together. Metallic components are the main source of hydrogenation activity and can be divided into two categories: non-precious metals and precious metals. The non-precious metals mainly include those from groups VIB and VIII; W, Mo, Co, and Ni are common examples, and they are often used in the form of sulfides. Precious metals, on the other hand, are primarily metals from group VIII such as Pt and Pd, and they are usually used in their metallic form. 3.2 The catalytic performance of a catalyst is determined not only by its chemical composition but also closely related to many of its physical and chemical properties. ①Pore structure: Since the reaction takes place on the solid surface of the catalyst, and mainly on its inner surface, the pore structure of the catalyst is a very important factor. These include: l surface area. Since catalytic reactions take place on the surface of the catalyst, the surface area plays an important role in dispersing the active components of the catalyst. It is closely related to the catalyst’s activity; however, it is not directly proportional to that activity, as only a portion of the surface is active, while another part of the surface cannot function due to either too small pore sizes or mass transfer issues. The pore volume is the sum of the volumes of all pores per unit mass of catalyst; it is one of the parameters describing the pore structure of a catalyst. Certain reactions require that the catalyst have a specific range of pore volumes. The pore diameter is used to indicate the average size of the catalyst’s pores. It is calculated using a formula, based on the assumption that the catalyst has n pores, all of which are cylindrical in shape with uniform sizes from the surface to the interior. The pore volume distribution relative to pore size (referred to as pore distribution) – It is not sufficient to know only the average pore size of a support or catalyst. In general, it is impossible for a catalyst or support to have pores of all the same size; ideally, the pore sizes should be concentrated within a certain range. Therefore, it is necessary to know what proportion of the total pore volume of the catalyst is occupied by pores of different sizes. The pore size of catalysts can be classified into: macropores (>20 nm), mesopores (2–20 nm), and micropores (<2 nm). ②Acidity: Acidity is an important property of hydrogenation isomerization catalysts; it is related to the catalyst’s isomerization activity and serves as a key factor in determining the reaction temperature for the catalyst ； It also affects product distribution. The “acidity” referred to here encompasses three aspects: type of acid (Brønsted acid or Lewis acid), acid strength, and acid concentration (simply referred to as acidity). All three factors influence the properties of catalysts; for instance, a certain reaction can only occur on acid sites with an acidity within a specific range of strength. On a solid acidic catalyst, B-acids and L-acids often coexist, as do acidic centers and basic centers. Moreover, acidity exists as a distribution with respect to strength, and the acid strength and acidity change depending on the treatment conditions of the catalyst. All these factors contribute to the heterogeneity on the surface of solid acid catalysts, which makes the laws governing acid catalysis much more complex than those in liquid-phase reactions. ③Metal dispersion and active phase structure: The hydrogenation activity of hydrogenation catalysts increases as the area of the hydrogenation metal increases. To achieve higher activity with a smaller amount of metal, it is necessary to ensure that the metal components on the catalyst are well dispersed, thereby promoting the formation of more hydrogenation-active phases. ④Other characteristics: such as chemical composition, impurity content, etc. 4. Preparation of general solid hydrogenation catalysts: The preparation of solid catalysts can generally be divided into two stages. First, a matrix with a specific chemical composition and physical structure is prepared, and it is processed into a geometry and particle size that meet the requirements regarding mechanical strength and hydrodynamic properties. Active metal components are added, and after drying and calcination, it becomes a catalyst; prior to use, it is subjected to sulfurization and reduction treatment to enhance its activity. Precious metal catalysts used for aromatic hydrodesulfurization have a low metal loading and are mostly prepared by impregnation. Its unit operations generally include: (1) precipitation (also known as neutralization or gelation), which is usually the first step in catalyst preparation – acidic and alkaline solutions are mixed under specific conditions to form a hydrogel precipitate with a specific chemical composition, also known as the matrix. (2) Filtration, which includes the separation of precipitates from the mother liquor, as well as the separation from the washing water during washing. (3) Washing involves using deionized water to remove the mother liquor from within the pores of the deposits or between the particles, as well as to eliminate impurities adsorbed on the surface of the matrix; this is generally done through pulping and flushing methods. (4) Drying involves removing the water physically adsorbed in the pores and on the surface. Drying is a very important process, as different drying conditions result in carriers with different structures. (5) Shaping: The shape and particle size of the catalyst determine the pressure drop, mass transfer, heat transfer, as well as the pressure resistance and wear resistance of the reactor; therefore, shaping technology is an important step in the catalyst preparation process. Early shaping techniques mainly involved compression, molding with templates, and powder rolling; nowadays, cylindrical shapes, specially shaped extruded strips, or oil-ammonia columns and oil columns are more commonly used. (6) Calcination: The purpose of calcination is to impart a fixed-phase structure to the carrier or catalyst, as well as an appropriate pore volume, surface area, and pore structure, along with good strength. A series of chemical reactions also occur during the calcination process. (7) Impregnation: This is the process of dispersing active metal components on a pre-prepared carrier, and it includes several steps such as preparing the impregnation solution, impregnating, drying, and calcining. In terms of the method of operation, it can be divided into impregnation with an excess solution and saturated (spray) impregnation. The above are the more typical unit operations; variations may exist for a specific catalyst. For example, some undergo an aging stage after gelation, while the addition of active metals may involve methods such as kneading or pulping instead of impregnation. In the co-precipitation method, the carrier and the hydrogenation metal components are precipitated simultaneously, etc. 5. The carriers of hydrogenation catalysts: Among the catalysts used in industry, few are composed of a single component; in particular, catalysts used in the petroleum refining industry consist of both a carrier and an active component. In the early days, the role of the carrier was considered to be merely that of a substrate or support for the active components, with any catalytic activity it might have being very slight. Research in recent decades has shown that this is far from the case; especially for hydrogenation isomerization dewaxing catalysts, the support is an important component of their activity. Generally, the functions of a carrier include: providing mechanical strength, helping to dissipate heat and prevent sintering, increasing the surface area of the active components, and maintaining the separation of the fine grains of these active components in order to reduce sintering and decrease sensitivity to poisons. In addition to the aforementioned common characteristics, the carriers of hydrogenation isomerization dewaxing catalysts also have their own specific features; the most important function of these catalyst carriers is to provide isomerization activity. 5.1 The functions of the hydrogenation catalyst carrier are as follows: l Provide acidic sites. l Improve the thermal stability of the catalyst. l Provide an appropriate pore structure and increase the effective surface area. l Reacts with the active component to form a new compound. 6. Role of additives Adding small amounts of additives or promoters, such as P and B, aims to modify the properties of the carrier, as well as the structure, properties, dispersion, and type of the metal components and active phase, thereby improving the catalyst’s activity, selectivity, hydrogen consumption, and lifespan (reducing coke formation and metal deposition), etc. Section 3: Influencing Factors 1. Reaction Temperature The reaction temperature is the most important process parameter in hydrogenation units, and it must be strictly controlled. In the isomerization dewaxing process using different molecular sieves as catalysts, there is a common pattern regarding the effect of reaction temperature: before the conversion rate reaches a certain value, the isomerization selectivity increases as the conversion rate rises; however, once the conversion rate exceeds that value, an increase in conversion rate leads to a rapid decline in isomerization selectivity. The ratio among various isomers in the isomerization products changes with increasing conversion; the selectivity for monomethyl isomers gradually decreases, while the amount of multi-branched isomers increases, eventually reaching an equilibrium value. To fully utilize the catalyst’s efficiency and appropriately increase the reaction rate, a certain reaction temperature must be maintained; this temperature is determined by the catalyst’s properties, the characteristics of the raw materials, and the desired properties of the product. The presence of sulfur and nitrogen compounds in the feedstock reduces the acidity and activity of the catalyst. To maintain the desired reaction depth, it is also necessary to increase the reaction temperature. Usually, at the beginning of operation, the catalyst has high activity, so the reaction temperature can be set a bit lower. In the later stages of operation, as carbon deposition on the catalyst surface increases, the catalyst’s activity declines; to maintain a certain degree of cracking, the reaction temperature must be increased gradually. Hydroisomerization and aromatic saturation are exothermic reaction processes. As the reaction temperature increases, the reaction rate speeds up, and the heat released during the reaction also increases accordingly. Therefore, it is necessary to control the temperature of the catalyst bed by injecting cold hydrogen into each bed layer in order to protect the catalyst. 2. Reaction pressure: Reaction pressure is one of the main factors affecting hydrogenation isomerization and aromatic saturation reactions; the actual factor influencing the reaction is the hydrogen partial pressure. For hydroisomerization reactions, studies have shown that as the reaction pressure increases, the conversion rate of alkane isomerization on the catalyst decreases, the isomerization selectivity drops, and the rate of the isomerization reaction follows a hyperbolic relationship with the hydrocarbon partial pressure. According to reaction kinetics analysis, for both light alkanes with fewer than 10 carbon atoms and heavy alkanes with more than 11 carbon atoms, the isomerization rate is inversely proportional to the hydrogen partial pressure and directly proportional to the hydrocarbon partial pressure; therefore, reducing the hydrogen-to-oil ratio and the total reaction pressure during isomerization facilitates an increase in conversion rate and isomer selectivity. An increase in reaction pressure can accelerate the hydrogenation-based saturation of refined aromatics. As a result, the aromatic content as well as the levels of sulfur- and nitrogen-containing compounds in the product decrease, while coking is reduced, thereby maintaining the catalyst’s activity and stability. The hydrogen partial pressure in the reactor equals the product of the total pressure of the oil vapor and recycled hydrogen and the fraction of hydrogen molecules in the total number of gas molecules. The hydrogen purity supplied by this device is 99.9%. 3. There are two types of hydrogen-to-oil volume ratios: one is the hydrogen-to-oil ratio at the reactor inlet, and the other is the total cooled hydrogen-to-oil ratio. The hydrogen-to-oil ratio at the reactor inlet is the ratio of the volume of hydrogen passing through the reactor per hour (recycled hydrogen + fresh hydrogen) to the volume of feed oil passing through per hour. (Unit: Nm3/m3). The total cold hydrogen-to-oil volume ratio is the ratio of the total volume of cold hydrogen gas fed into the reactor per hour to the volume of feed oil passing through per hour. Only a portion of the hydrogen reacts in the hydrogenation reactor. Most hydrogen still exists in a free state. Using a high hydrogen-to-oil ratio can increase the partial pressure of hydrogen, which is beneficial for mass transfer and the progress of hydrogenation reactions; it also prevents coking of the oil on the catalyst surface within a certain range. However, the hydrogen-to-oil ratio should not be too high, as this would increase investment and operating costs. Under normal production conditions, slight fluctuations in the hydrogen-to-oil ratio have little noticeable effect on product quality and yield. But in order to protect the catalyst and maintain the design at its minimum value. The designed hydrogen-to-oil ratio at the inlet of unit R-101 is 800:1 (Nm3/m3). If the designed hydrogen partial pressure cannot be maintained, the feed rate must be reduced in order to prevent increased catalyst coking. 4. Space velocity: Volume space velocity is the ratio of the volume of feed passing through the reactor per hour to the volume of catalyst, expressed in m3/h per m3, or h-1. The higher the allowable air velocity, the greater the processing capacity of the device. In fact, the air velocity cannot be increased indefinitely; it is determined by the properties of the feed oil, the activity of the catalyst, and the degree of processing required for the product. Once the type of crude oil and catalyst is determined, under the same operating conditions, if the space velocity is too high (i.e., the feed rate is too high), the contact time between the crude oil and the catalyst is too short; as a result, the degree of processing is insufficient and the conversion rate is low, failing to meet the quality requirements. If the space velocity is too low (i.e., the feed rate is too low), the contact time between the crude oil and the catalyst is longer, allowing for a greater degree of processing. This leads to an increase in unwanted gases, as well as increased coking of the catalyst. It’s economically unreasonable. Therefore, the space velocity is determined through pilot-scale experiments based on the amount of catalyst, the feed flow rate, and the desired extent of the reaction. It is determined based on the most reasonable economic effects. To prevent catalyst coking, the operating procedure requires increasing the feed rate before raising the temperature, and decreasing the temperature before reducing the feed rate. Space velocity is one of the most important factors in the hydroisomerization dewaxing reaction. The heterogeneous dewaxing reaction is highly sensitive to changes in space velocity; therefore, during the reaction process, appropriately adjusting the space velocity can achieve good dewaxing results. Generally, a lower air velocity is beneficial for the dewaxing reaction; however, the cracking reaction also increases accordingly. In actual production, adjusting the space velocity must take into account both the relationship between pour point reduction and yield, so as to achieve maximum yield while ensuring that the product’s pour point meets specifications. 5. Properties of the feedstock oil Since the quality of the feedstock has a great influence on the performance of the catalyst, especially the sulfur and nitrogen contents in the feedstock. This is because heterogeneous and post-supplemented refined catalysts require that the sulfur content in the feedstock does not exceed 10–20 μg/g, and the nitrogen content is no higher than 10 μg/g. Therefore, during operation, close attention should be paid to whether the properties of the feedstock meet the requirements, so as to ensure the full performance of both catalysts and prolong their service life. ① Nitrogen content: Nitrogen compounds in the feed can poison the acidic sites of the catalyst. Studies show that as the nitrogen content in the feed increases, the isoparaffin/norparaffin ratio in the pyrolysis products decreases. When the nitrogen content is between 6 and 20 μg/g, after the acidity of the catalyst is somewhat suppressed, the yield of base oil actually increases slightly. The poisoning effect of nitrides is reversible; when the nitrogen content in the feed increases, cracking activity decreases. However, when a low-nitrogen or nitrogen-free oil feed is used, the catalyst’s activity can gradually be restored. The maximum allowable nitrogen content in the feed varies depending on the catalyst. The organic nitrogen content in the feed should preferably not exceed 10 μg/g. During the operation of actual industrial plants, it is required that the nitrogen content in the feed be less than 2 μg/g. ② The essence of sulfur poisoning in metal-type catalysts due to sulfur content is the dissociative adsorption of sulfides on the metal surface; sulfur binds with the metal to form strong chemical bonds, which prevents reactant molecules from adsorbing on the metal surface and results in catalyst deactivation. If the sulfur content in the feed is too high, it has an impact on hydroisomerization and aromatic saturation; the presence of sulfides primarily undermines the hydrogenation capabilities of the active metals, leading to the cracking of isomeric hydrocarbons. Studies show that when the sulfur content in the feed exceeds 200 μg/g, the reaction temperature rises significantly, which **reduces the isomerization selectivity and also decreases the yield of lubricating oil**. ③ The level of olefin content has little effect on the activity of the catalyst for isomerization dewaxing and aromatic saturation. However, as precursors for coking, olefins easily cause coking on the catalyst surface, simultaneously leading to a rapid increase in the pressure drop across the reactor’s catalyst bed and shortening the operating cycle of the plant. Furthermore, the hydrogenation saturation of olefins is a highly exothermic reaction; a high olefin content in the feed oil leads to a higher temperature rise in the catalyst bed and greater consumption of chemical hydrogen. ④ As the dry point of the crude oil increases, the size of the reactant molecules grows and the content of polycyclic aromatics rises, which can lead to steric hindrance in the reaction process. This hinders the saturation of aromatics, ultimately resulting in a decline in the stability and color of the base oil. At the same time, since macromolecules and polycyclic aromatic hydrocarbons tend to coking more easily during hydrogenation, this leads to a decrease in catalyst activity and a rapid increase in the pressure difference across the catalyst bed. Appropriately increasing the reaction temperature can compensate for the deterioration in the quality of the feed oil, but it is not conducive to the long-term operation of the plant. ⑤ An increase in the carbon residue content in the feed oil has little effect on the product yield, and the carbon residue content in the resulting oil also increases only slightly. However, a high residue content in the feed oil can cause the catalyst to coking rapidly. The decrease in catalyst activity must be compensated for by raising the reaction temperature. 6. Catalyst poisons Catalyst poisons refer to substances that can cause catalyst poisoning. After catalyst poisoning, its activity decreases, resulting in a shorter operating cycle for the catalyst. Replacing or regenerating catalysts is costly and delays production; therefore, it is essential to strictly control the quality of the feed oil and fresh hydrogen to prevent catalyst poisoning. The toxins that affect precious metal catalysts generally include asphaltenes, heavy metal contents, sulfur, and nitrogen compounds. The effects of sulfur and nitrogen compounds on catalysts have been discussed above; below, the impacts of other such substances on catalysts will be analyzed separately. ① Asphaltene is a high-boiling-point polycyclic molecule and a major precursor to coking; it readily causes rapid deactivation of catalysts. Even a slight increase in the asphaltene content can significantly accelerate the rate of catalyst deactivation, requiring an increase in reaction temperature and thus shortening the operation cycle. Additionally, asphaltene often exists in combination with other poisons such as metals. Therefore, the asphaltenes content in the crude oil must be strictly controlled, with a requirement of less than 0.01%. Asphaltenes are molecules with high boiling points; the key to controlling their content is to ensure that the final boiling point does not exceed the designated value. A high asphaltene content has a significant impact on the color of the base oil; in severe cases, the product turns black. ② In hydrogenation reactions, heavy metals (nickel, vanadium, and other metals) present in the feedstock, especially nickel (Ni), vanadium (V), copper (Cu), lead (Pb), etc., will deposit in the micropores of the catalyst. This leads to poisoning of the active areas and blockage of the pores, thereby reducing the catalyst’s activity. It is necessary to increase the reaction temperature in order to compensate for this loss of catalyst activity. Metals such as Ni and V can easily cause catalyst poisoning; even trace amounts of such metals can lead to the permanent deactivation of the catalyst and shorten the operational life of the device. At the same time, metals such as Ni and V are permanent poisons that cannot have their activity restored through catalyst regeneration; therefore, after one cycle of operation, even after conventional coking, the hydrogenation activity of the catalyst remains insufficient, and it is necessary to replace the catalyst due to metal deactivation. Controlling the metal content in the feed is one of the important methods to ensure the operating cycle of the catalyst. ③ In iron, magnesium, calcium, and sodium feedstocks, Fe ions readily form sulfides that deposit on the surface of catalyst particles as well as in the spaces between them. These deposits typically appear in the form of a crust at the top of the catalyst bed, causing an increase in the pressure drop across the bed; in severe cases, this can force the plant to shut down prematurely in order to replace the catalyst (or remove the contaminated portion of it). This type of blockage generally occurs in the bed at the top of the first reactor. To minimize the occurrence of this phenomenon, it is necessary to regularly analyze the metal content in the feed to ensure that the control parameters are met. Fe ions are present in the feedstock in two forms: one as suspended particles, and iron in this form can be minimized by installing a feed filter to reduce the amount of iron particles (such as iron oxides) that enter the catalyst bed ； Another form is the formation of oil-soluble substances through reaction with hydrocarbons, which cannot be resolved by filtration and requires an increase in the volume of the protective agent. For this device, the source of Fe is mainly equipment corrosion in upstream units and during storage and transportation processes, which enters the crude oil. It is generally required that the iron content in the feed to the hydrogenation unit be no more than 1 μg/g. Similar to Fe, high metal contents of Ca, Mg, and Na can also lead to metal deposition on the surface of the catalyst bed. However, since ions such as Ca, Mg, and Na are less likely to be generated during subsequent processing, the quality of the feed can basically be ensured. ④ Chlorinated feed oils generally also contain trace amounts of organic chlorides, which generate hydrogen chloride in the hydrogenation reactor. At the same time, if the new hydrogen is hydrogen produced by a reforming unit, it can also easily lead to hydrogen chloride. On the one hand, hydrogen chloride alters the hydrogenation-acidic properties of hydrogenation catalysts, affecting the catalyst’s selectivity. On the other hand, hydrogen chloride reacts with ammonia produced during the hydrogenation process to form ammonium chloride; these substances tend to deposit in the heat exchangers for feed/reactant effluents, reducing heat transfer efficiency and causing pipe blockages and erosion. Therefore, it is necessary to regularly analyze the chlorine content in the feed and fresh hydrogen to ensure that it remains below the specified values.