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An important method for refining solvent dewaxed petroleum products, which involves diluting the lubricant raw material with a solvent and cooling it to cause the waxes within it to crystallize and precipitate, thereby reducing the freezing point of the lubricant. Industrially, the lubricating oil fractions obtained from crude oil distillation of wax-containing crude oil can be processed through solvent refining, solvent dewaxing, and hydrorefining (or clay refining) to produce lubricating oils (base oils) and paraffin ； Lubricating oils (base oils) and paraffin waxes (see petroleum waxes) can be produced by using vacuum residue from crude oil distillation as raw material, through processes such as solvent deasphalting, solvent refining, solvent dewaxing, and hydrorefining (or clay refining). Process principle: Lubricant raw materials obtained by distilling paraffin-based and intermediate-base crude oils (see crude oil evaluation) all contain waxes. The presence of these waxes affects the low-temperature flow properties of the lubricating oil. Since the boiling point of wax is similar to that of lubricant fractions, distillation cannot be used for separation. However, wax has a high freezing point; by gradually lowering the temperature, wax crystallizes out of the lubricant, allowing the wax and oil to be separated through filtration or centrifugation. At low temperatures, the viscosity of lubricating oil is very high, and the wax crystals formed are small, making filtration or centrifugal separation difficult. Therefore, it is necessary to add some solvents that have a high solubility for oil at low temperatures but a very low solubility for wax in order to dilute it. Benzene-based solvents can dissolve lubricating oils well, but they also have a high solubility for waxes. Similar solvents have very low solubility for waxes. Therefore, some propyl or ** groups are often added to benzene-based solvents to reduce the solubility of these solvents for waxes. Process flow: The first propylene-benzene dewaxing unit was built in 1927; thereafter, other solvents such as propylene-toluene, propane, methyl n-propyl ether, and hydrocarbon chlorides were also used. The process flow for the solvent dewaxing process is generally similar; taking toluene dewaxing as an example (see figure), it includes steps such as crystallization, filtration, solvent recovery, and freezing. The raw materials and solvent are first cooled together with the filtrate in a sleeve crystallizer equipped with a scraper; some solvent is then added, followed by cooling with ammonia and dilution with solvent before filtration. The filtered filtrate and wax solution are evaporated and stripped separately to recover the solvent. The composition of the mixed solvent added and the solvent ratio vary depending on the properties of the raw material (boiling range, wax content, viscosity, etc.) as well as the degree of dewaxing; generally, toluene makes up 40%–60% of the **same-toluene solvent**, with a solvent ratio of 1–4:1. Adding the diluent solvent in several stages helps to form good wax crystals, reduces the dewaxing temperature difference (i.e., the difference between the freezing point of the dewaxed oil and the dewaxing temperature), and increases the yield of the dewaxed oil. The cooling rate of the raw material in the sleeve crystallizer should not be too fast, to avoid the formation of excessive fine wax crystals, which hinders filtration. Filtering is carried out in a drum vacuum filter; depending on the wax content of the raw material, one or two stages of filtration are used. Solvent recovery from both the filtrate and the wax phase is achieved through multi-effect evaporation and stripping to reduce energy consumption. In addition, an inert gas protection system is also installed to reduce solvent loss and prevent explosions. Trends: Solvent dewaxing of lubricating oils is an expensive petroleum refining process, with high costs both in terms of investment and operation. Therefore, countries are striving to find suitable solvents, develop new crystallization equipment, improve filtration equipment, and enhance solvent recovery processes and operating conditions in order to raise the technical level of solvent dewaxing. In addition, research on hydrodesulfurization is underway.