What is the production and blending of lubricating oil?

The basic raw material for producing lubricating oil from petroleum is mainly heavy oil in crude oil. In order to produce lubricating oils with different viscosities, heavy oil needs to be fractionated into several fractions and residual oil under reduced pressure. The former is called distillate lubricating oil, which is used to make lubricating oil with low viscosity, such as transformer oil and mechanical oil. The latter is called residual lubricating oil, which is used to make lubricating oil with high viscosity such as cylinder oil.

First, the production of lubricating oil

There are two kinds of lubricating oil production processes: one is the traditional "old three sets" production process, that is, solvent refining, solvent dewaxing and clay supplementary refining; The second is the recent development of lube oil hydrogenation. The following mainly introduces the traditional "old three sets" production procedure of lubricating oil.

(1) solvent refining

Lubricating oil refining refers to the process of removing most polycyclic short-side chain aromatics, resins and asphaltenes from lubricating oil to improve the quality of lubricating oil. Commonly used refining methods include acid-base refining, solvent refining, adsorption refining, hydrofining and so on. At present, solvent refining is widely used in China.

Solvent refining is to select a solvent, dissolve and separate the non-ideal components in lubricating oil, keep the ideal components, and then evaporate the solvent for recycling.

1. Selection requirements of solvents

(1) has strong selectivity and solubility. Solvents must have high solubility for non-ideal components in lubricating oil raw materials and low solubility for ideal components. When the solvent is added to the lubricating oil raw material, the non-ideal components in the lubricating oil quickly dissolve in the solvent, and the solution of the dissolved non-ideal components is separated, leaving the ideal components of the lubricating oil. In general, the former is called extract (extractive solution) and the latter is called raffinate (refined solution). Therefore, the refining process is essentially a solvent extraction or extraction process.

(2) High density. The solvent has high density and enters from the upper part of the extraction tower; Raw oil has low density and enters from the lower part of the tower. The solvent and raw oil are in countercurrent contact in the tower. After a period of time, the solvent completely dissolves the non-ideal components in the lubricating oil. Because the density of the extract is higher than that of the raffinate, there is an obvious interface between the two phases. The raffinate (ideal component+a small amount of solvent) is extracted from the top of the tower, and the extract (non-ideal component+a large amount of solution) is extracted from the bottom of the tower.

(3) Low boiling point and easy recovery. The extract contains more than 90% solvent, and the raffinate also contains a small amount of solvent, which must be recovered. Therefore, the boiling point of the solvent is required to be lower than that of the lubricating oil. For example, the boiling point of furfural is 16 1.7℃, which is lower than the boiling point of lubricating oil. When heated to the boiling point of furfural, furfural evaporates rapidly.

(4) It has stable properties, is not easy to be deteriorated by heat, and does not react chemically with raw materials.

(5) Non-toxic, non-corrosive, low price, etc.

At present, furfural is the most widely used refining solvent in domestic refineries.

2. Solvent refining process

Figure 8-7 is the process principle flow chart of furfural refining.

1) furfural extraction

Raw oil enters the extraction tower from the lower part after heat exchange by the heat exchanger, and furfural enters from the upper part of the tower, and both of them are continuously extracted in countercurrent in the tower. The pressure in the extraction tower is maintained at about 0.5MPa, so that raffinate and extract automatically flow into the solvent recovery system, with raffinate at the top and extract at the bottom.

2) solvent recovery

Solvent recovery of (1) raffinate: the raffinate contains less solvent (about 10%) and can be recovered by distillation. The residual liquid is heated to about 220℃ by the heating furnace and enters the residual liquid stripper. Refined oil (raffinate oil) at the bottom of the stripper is pumped out of the device. The water-containing furfural vapor separated at the top of the tower is cooled by the condenser and then enters the furfural-moisture layer tank.

(2) Solvent recovery of the extractive solution: the extractive solution contains a lot of solvents (about 90%). Most of the solvent is evaporated first, and then a small amount of solvent remaining in the oil is stripped by steam, which not only saves fuel, but also completely recovers the solvent. After exchanging heat with furfural vapor from the high-pressure evaporation tower, the extract enters the low-pressure evaporation tower. After heat exchange, 30% ~ 40% furfural can be distilled from the extract. Furfural vapor at the top of the tower enters the furfural dehydration tower after condensation. The remaining extraction liquid is extracted from the bottom of the low-pressure evaporation tower, heated by the heating furnace and then enters the high-pressure evaporation tower; Most of that furfural vapor evaporate from the top of the tower exchange heat with the extract and then enter the furfural dehydration tower; After evaporation, the bottom extract enters the extract stripper. The bottom of the extraction stripper is an extraction oil outlet device for removing furfural, and the top is furfural vapor containing water. Furfural vapor coming out from the top of the low-pressure evaporation tower and the high-pressure evaporation tower is recovered after removing little water through the furfural dehydration tower.

Figure 8-7 Flowchart of Furfural Refining Process Principle

1- extraction tower; 2,5-heating furnace; 3- raffinate stripper; 4- extraction stripper; 6- high pressure evaporation tower; 7- low pressure evaporation tower; 8- Furfural-Moisture Layer Tank; 9— Furfural dehydration tower; 10- furfural evaporation tower

(3) Recovery of aqueous solvent: Furfural and water can form a boiling substance, which cannot be separated by simple distillation. The water-containing furfural vapor in the stripper is distilled from the top, cooled by the condenser, and the condensate enters the layering tank. At room temperature, it is divided into two layers, the upper layer is water containing a small amount of furfural, and the lower layer is furfural containing a small amount of water (furfural has a high density). The furfural layer is pumped into the furfural dehydration tower for dehydration, and the boiling substance is distilled from the top of the tower. The steam is condensed and returned to the layered tank, and the dehydrated furfural at the bottom of the tower can be recycled. The water layer enters the furfural evaporation tower, the top of which is a * * * boiling substance, and the bottom of the tower discharges sewage, and the * * * boiling substance at the top of the tower returns to the layered tank after condensation.

(2) solvent dewaxing

The purpose of dewaxing is to reduce the freezing point of lubricating oil, and at the same time, paraffin can be produced by-product. At present, there are many dewaxing methods used in industry, such as cold pressure dewaxing, adsorption dewaxing, urea dewaxing, bacterial dewaxing, solvent dewaxing and so on. Among them, solvent dewaxing is the most widely used, and it can treat a variety of distillate and residual oil.

Solvent dewaxing is to dilute the lubricating oil by adding a solvent to reduce the viscosity of the oil, then cool it to a low temperature, precipitate wax crystals, and separate the oil and wax by filtration. There are many solvents for solvent dewaxing. At present, ketone-benzene mixed solvent is widely used in China, which is called ketone-benzene dewaxing process. The following mainly introduces the ketone-benzene dewaxing process.

1. solvent dewaxing principle

Solvent dewaxing is to separate oil from wax, so as to achieve the purpose of wax removal. The separation of wax and oil adopts filtration method, which is similar to filtering suspended matter in water with gauze. For filtration operation, the larger the solid particles in the mixture, the smaller the viscosity of the liquid, and the easier it is to filter and separate. Therefore, the solvent is used to reduce the viscosity of the raw oil in the oil-wax mixture, which is beneficial to the formation of large-particle crystals of wax and the separation of oil and wax. In order to achieve this goal, at the dewaxing temperature, the solvent to be added should meet the following conditions:

(1) has good selectivity. At the dewaxing temperature, the solubility of oil is large, but the solubility of wax is small, otherwise the wax and oil dissolved in the solvent will enter the filtrate together.

(2) Low viscosity at dewaxing temperature.

(3) Low boiling point, which can be recovered by simple distillation.

(4) The freezing point is low, and it will not solidify at dewaxing temperature.

(5) Non-toxic, non-corrosive, non-reactive with oil and wax, low price, etc.

No ideal single solvent can meet all the above conditions, but mixed solvents can basically meet the production requirements. At present, ketone-benzene mixed solvent is mainly used, so it is often called ketone-benzene dewaxing. The mixed solvent of methyl ethyl ketone (or acetone) and toluene (or benzene) in different proportions is the most widely used solvent for ketone-benzene dewaxing.

In the mixed solvent of acetone-benzene-toluene, benzene has good solubility in oil, but it also has great solubility in wax, so acetone with little solubility in wax is added to reduce its solubility in wax. Therefore, benzene acts as a solvent for oil and acetone acts as a precipitant for wax. Because of the high crystallizing point of benzene, benzene crystals will be precipitated in the process of low temperature dewaxing, so it is necessary to add a certain proportion of low freezing point toluene as a supplement to form a good selective solvent. They have strong solubility in oil, low solubility in wax, low viscosity, low boiling point, low toxicity and low freezing point, and are ideal mixed solvents for solvent dewaxing of lubricating oil. But its flash point is low, so special attention should be paid to safety when using it.

In recent 20 years, the trend of methyl ethyl ketone replacing acetone has developed rapidly. The solubility of methyl ethyl ketone in wax is very small, but it is more soluble in oil than acetone. Using methyl ethyl ketone instead of acetone can improve the yield of dewaxed oil and reduce the temperature difference of dewaxing. At low temperature, the selectivity of toluene is higher than benzene, that is, the solubility of oil is stronger than benzene, and the solubility of wax is worse than benzene, so toluene basically replaces benzene as a solvent. Methyl ethyl ketone-toluene mixed solvent has been widely used in industry.

2. Ketobenzene dewaxing process

The ketone-benzene dewaxing process includes five parts, as shown in Figure 8-8.

Fig. 8-8 schematic diagram of ketone-benzene dewaxing process.

(1) crystallization system: Its function is to cool the solution mixed with raw oil and solvent to a low temperature, so that wax can crystallize out of the solution and fully form large-particle crystals which are beneficial to filtration.

(2) Refrigeration system: used to refrigerate and take out the heat released during crystallization. Liquid ammonia is selected as the refrigerant, and the temperature of raw oil is reduced by evaporation and heat absorption. The evaporated ammonia gas is compressed and cooled to become liquid ammonia for recycling.

(3) filtration system: the function is to separate the crystalline wax from the oil and divide the mixture into two parts, one is filtrate and the other is dewaxed oil containing solvent; The other part is wax paste, which contains a small amount of oil and solvent. Both parts should be sent to the solvent recovery system for solvent recovery and reuse.

(4) Solvent recovery system: the function is to evaporate the solvent in wax paste and filtrate. The method used is still evaporation first and then stripping.

(5) Safe gas system: The ketone and benzene in the solvent volatilize, and their vapor and air will form an explosive mixture. Therefore, when a large amount of solvent is used, a safe gas system must be set up. The safety gas is composed of carbon monoxide, carbon dioxide, H2O, nitric oxide, oxygen and other mixed gases. In the circulation process of safety gas, when the oxygen content increases and exceeds 5%, a part of safety gas is discharged, and fresh safety gas is supplemented from the safety gas generator.

(3) clay refining

After solvent refining, solvent dewaxing and other processes, the obtained oil still contains harmful impurities (such as gum, naphthenic acid, acid residue and a small amount of residual solvent). These impurities must be removed to further improve the color and stability of lubricating oil and reduce carbon residue. Clay refining is to treat lubricating oil with activated clay at a high temperature and adsorb these impurities on the surface of clay to obtain refined oil.

1. Basic principle of clay refining

Clay is a kind of crystalline or amorphous substance with micropore structure, with large surface area and strong adsorption capacity. Clay can be divided into natural and activated types. Natural clay is weathered feldspar. The pores of natural clay often contain some impurities. 8% ~ 15% dilute sulfuric acid is used to remove impurities. The treated clay is called active clay, and its adsorption capacity is greatly enhanced.

Under the condition of clay refining, the adsorption capacity of clay to various components in lubricating oil is different. Clay is easy to adsorb gum, asphaltene and residual solvent in lubricating oil, but its adsorption capacity for lubricating oil is poor. Therefore, using the selective adsorption performance of clay, mixing clay with oil, and then filtering the clay adsorbed with impurities can get refined lubricating oil.

2. White clay refining process

Clay refining process can be divided into three types: fixed bed infiltration, continuous infiltration and contact method. The first two have been eliminated because of low production efficiency. At present, contact clay refining is widely used. Figure 8-9 shows the process flow of contact clay refining.

Figure 8-9 Process Flow of Contact Clay Refining

(1) White clay mixed with oil. The raw oil is preheated to 80 ~ 90℃, sent to a blender, added with white clay as required, and fully stirred in the blender. Because of the high density of clay, it must be fully stirred, otherwise it will not sink. Whether oil and clay can be mixed evenly has a great influence on the whole refining process.

(2) heating. The paste mixture of oil and clay is heated in the heating furnace, which reduces the viscosity of raw oil and is beneficial to the adsorption of clay.

(3) Contact adsorption. After the heated mixture enters the contactor or contact tower for a period of time, the clay is fully adsorbed. There is a vacuum pump device at the top of the tower, which pumps out the light components and evaporation solvent produced by cracking when the heating furnace is heated, and then enters the intermediate tank. A stirrer is installed in the tank to prevent the clay from settling.

(4) filtering. The mixture of oil and clay from the tundish is first filtered in Smith filter to remove most of the clay. This kind of filter is relatively thick, and some fine clay can pass through it, so it can be filtered again through the plate-and-frame filter to ensure that there is no solid clay in the product. The obtained refined oil discharge device is used as base oil and waste clay discharge device.

Second, the lubricating oil blending

The quality of lubricating oil is determined by the quality of lubricating base oil and additives used in lubricating oil. A small amount of effective additives can obviously improve the performance of lubricating oil, greatly improve the quality of lubricating oil and meet the needs of industrial application.

Blending is the last important process in lubricating oil production. Lubricating base oils with different viscosities obtained by solvent refining, dewaxing and clay refining. Mix them in a certain proportion and add appropriate additives. What kind of blending component additives to use depends on the actual situation. For example, blending 20 # engine oil requires low viscosity, and low viscosity lubricating base oil is usually blended with appropriate additives.