A method for preparing ultra-high voltage converter transformer oil

Abstract

This invention discloses a method for preparing ultra-high voltage converter transformer oil. The method uses the full fraction of high-temperature and medium-low temperature coal tar as raw material, employing a combined suspended-bed hydrogenation process. Through the same raw material and a single set of industrial equipment, continuous production of transformer oil base oil and aromatic oil is achieved. After the final addition of an antioxidant, the resulting transformer oil exhibits excellent anti-gas evolution properties, and its low-temperature performance and antioxidant properties are significantly superior to commercially available transformer oils. Furthermore, the method demonstrates strong adaptability to raw materials, flexible operation, and online catalyst addition in small quantities, resulting in improved overall performance of the prepared ultra-high voltage converter transformer oil. This invention provides an economical processing method for coal tar, which has relatively low byproduct utilization, and also develops a new raw material for the production of ultra-high voltage converter transformer oil.

Description

Technical Field

[0001] This invention belongs to the field of coal tar processing technology, and specifically relates to a method for preparing ultra-high voltage converter transformer oil, that is, a method for preparing ultra-high voltage converter transformer oil by hydrogenation of coal tar in a suspended bed. Background Technology

[0002] Transformers are crucial transmission and transformation equipment in power systems, and their development is closely linked to the overall development of the power industry. With the rapid development of the national economy, my country's electricity demand has also grown rapidly, leading to unprecedented growth in demand for electrical equipment. To meet the needs of large-capacity, long-distance power transmission, ultra-high voltage (UHV) and extra-high voltage (EHV) DC and AC transmission are the main development directions for power transmission and transformation technology. UHV and EHV DC transmission have advantages such as long transmission distances, good regulation performance, low overvoltage levels, and low line losses. Their main applications include point-to-point long-distance transmission, submarine cables, and large power grid connections and isolation. They have vast development potential in my country.

[0003] A DC converter transformer is a special electrical device that converts alternating current (AC) to direct current (DC) and then back to AC. This necessitates that the transformer oil used in this device possess higher quality and performance than ordinary AC transformer oil. DC converter transformers are crucial for long-distance DC power transmission technology. Due to the unique operating characteristics of DC converter transformers, particularly their valve-side windings, the operating voltage includes AC, DC, and the polarity reversal voltage during system polarity switching. Therefore, the insulation performance requirements are even higher, demanding that the performance indicators of the DC converter transformer oil be superior to those of ordinary transformer oil. Transformer oil used in DC converter transformers requires high insulation strength: it needs better electrical properties, including higher breakdown voltage and lower dielectric loss factor; high cleanliness: it needs low viscosity to facilitate flushing and filtering of impurities in the converter transformer, while also requiring low impurity content; large capacity and relatively high operating temperature: it needs low viscosity to facilitate oil flow, which is more conducive to transformer cooling and heat dissipation; high flash point for safety; high oxidation stability to prevent oxidation and deterioration; and low gas evolution to prevent affecting the insulation performance of the transformer oil and ensure long-term operation of the transformer.

[0004] Currently, transformer oil production methods generally rely on single mineral oil components. Paraffinic transformer oil base oils have high pour points, are easily oxidized to produce large amounts of acidic compounds, and have low aromatic content, resulting in shortcomings in transformer oil lifespan, oxidation resistance, and gas evolution resistance. Naphthenic transformer oil base oils, on the other hand, meet the insulation and cooling requirements of transformer oils while maintaining long-term stable operation, making them the generally recognized best raw material for transformer oil base oil production. Existing technologies for processing transformer oil from naphthenic distillate oils mainly fall into two categories: one is the traditional three-step process of hydrodeacidification-furfural refining-clay refining; the other is a high-pressure all-hydrogen process technology involving high-pressure hydrotreating or cracking-high-pressure hydrodewaxing or isomerization dewaxing-high-pressure refining. The traditional three-stage process allows base oils to retain some aromatics, resulting in excellent anti-gas evolution and solubility properties, but poor oxidation resistance and environmental pollution issues. The hydrotreating process gives base oils significant performance advantages, such as excellent low-temperature performance, oxidation stability, and thermal stability, and no environmental pollution, but it has the disadvantage of relatively poor anti-gas evolution and solubility properties. More importantly, naphthenic crude oil is a valuable resource among all types of crude oil in the world, with reserves accounting for only 2.2% of the total crude oil reserves. Naphthenic crude oil is a scarce petroleum resource both globally and in my country, and is already in short supply.

[0005] Patent CN115404102A discloses a transformer oil base oil and its preparation method. Using naphthenic distillate oil as raw material, the method involves hydrotreating in a fixed-bed reactor, followed by hydrodewaxing, hydrorefining, and additional hydrotreating, then fractionating to obtain the transformer oil base oil. This method can prepare transformer oil base oil with low sulfur, low nitrogen, low pour point, and low aromaticity characteristics. However, it only meets the requirements for T-40℃ transformer oil (general purpose) base oil in GB2536-2011 standard and is not suitable for ultra-high voltage converter transformers. Furthermore, it suffers from the problem of limited naphthenic distillate oil resources.

[0006] Patent CN103113963A discloses an ultra-high voltage transformer oil and its production process, which consists of naphthenic base oil, dodecylbenzene, metal deactivators (benzotriazole and its derivatives or thiadiazole and its derivatives), and light stabilizers, followed by clay refining to obtain ultra-high voltage converter transformer oil. The transformer oil produced by this process exhibits excellent and stable antioxidant properties and moderate gas evolution, effectively suppressing the charging of transformer oil flow, making it suitable for use in ultra-high voltage converter transformers. However, this method uses benzotriazole or thiadiazole additives, which are restricted by the new IEC 60296-2020 standard. Furthermore, the use of clay refining poses certain environmental pollution problems and does not align with the current development direction of transformer oil. More importantly, it suffers from the limited availability of naphthenic distillate oil resources.

[0007] Patent CN107619706B discloses a transformer oil and its preparation method. This method uses naphthenic and paraffinic vacuum distillate oils as raw materials, and employs a hydrocracking-hydroisomerization-hydrogenation supplementary refining process in a fixed-bed reactor, adding a combination of aromatic additives and antioxidants. The transformer oil produced by this process exhibits excellent carbon-form structure, anti-gas evolution properties, low-temperature performance, and oxidation resistance; some properties of the product are superior to existing products on the market. However, it suffers from the problem of limited naphthenic distillate oil resources.

[0008] As can be seen from the above, CN115404102A, CN103113963A, and CN107619706B all use petroleum fractions as feedstocks and employ traditional fixed-bed reactors for hydrogenation. These methods have strict requirements on the residual carbon and metal content of the feedstocks, poor feedstock adaptability, large catalyst consumption, and the inability to add catalysts online. More importantly, they suffer from the problem of limited naphthenic distillate oil resources.

[0009] my country's resource endowment of being short of oil, gas, and rich in coal determines that the contradiction between oil and gas supply and demand will persist for a long time. From a long-term development perspective, the energy consumption structure dominated by coal is difficult to change. Medium and low temperature coal tar is a liquid-phase byproduct produced during coal pyrolysis. It has a high content of aromatic compounds. Through the coupling of various hydrogenation reactions, the product can be made to be sulfur-free, nitrogen-free, and have extremely low metal content. The hydrogenated product contains a large number of cycloalkanes and isoalkanes, which can be used as a high-grade transformer oil with excellent low-temperature performance, excellent electrical performance, and good oxidation stability.

[0010] Patent CN103436289B discloses a method for producing naphthenic transformer oil base oil from coal tar. This method involves hydrogenating low-temperature coal tar (including hydrorefining and catalytic dewaxing) to obtain a hydrogenated product, followed by fractionation to obtain a transformer oil fraction. The transformer oil fraction is then refined using solvent and clay refining to obtain a qualified transformer oil base oil product. The transformer base oil obtained by this process only meets the standard requirements for No. 45 transformer oil in GB2536-1990 "Transformer Oil," and is not suitable for ultra-high voltage converter transformers. Furthermore, it cannot avoid the use of traditional and outdated methods such as subsequent solvent refining and clay refining.

[0011] Patent CN103789019B discloses a method for producing transformer oil base oil by hydrogenation of medium- and low-temperature coal tar. The method uses light fractions obtained from the pretreatment and fractionation of medium- and low-temperature coal tar (cutoff point 480–510℃) as raw materials. The raw materials are produced through a combined hydrogenation process in a fixed-bed reactor, involving hydrogenation treatment, hydrogenation reforming, and supplementary hydrogenation refining. Although this process uses medium- and low-temperature coal tar as raw material, the resulting transformer oil base oil only meets the performance parameters of No. 45 transformer oil in GB2536-1990 "Transformer Oil" and is not suitable for ultra-high voltage converter transformers.

[0012] Patent CN105419864B discloses a system and method for preparing high-octane gasoline, jet fuel, and naphthenic base oil from fully hydrogenated coal tar. The method uses whole-fraction low-temperature coal tar, or fractions of low-temperature coal tar, or high-temperature coal tar fractions cut below 510℃ by vacuum distillation, or anthracene oil as raw materials. The process involves pretreatment, hydrorefining in a fixed-bed reactor, deep refining, isomerization and dewaxing, and post-refining. High-octane gasoline, jet fuel, and transformer oil are then obtained through naphtha dehydrogenation and aromatics extraction. Although this process uses coal tar as a raw material to prepare transformer oil base oil, the properties of the resulting transformer oil base oil only meet the performance parameters of general-purpose type-30 transformer oil in GB2536-2011 "Unused Mineral Insulating Oils for Electrical Fluid Transformers and Switches," and are not suitable for ultra-high voltage converter transformers.

[0013] It is evident that although CN103436289B, CN103789019B, and CN105419864B use coal tar as raw material to prepare transformer oil, in addition to the inability to avoid environmental pollution, insufficient resource utilization, and low product quality, they all use traditional fixed-bed reactors for hydrogenation, which have strict requirements on the residual carbon and metal content of the raw materials, poor raw material adaptability, large catalyst dosage, and cannot achieve online addition.

[0014] Looking at the existing technologies, there is currently no processing technology that uses the full fraction of coal tar as raw material and employs a combination of suspended bed and fixed bed hydrogenation to prepare ultra-high voltage converter transformer oil. Therefore, developing a method for preparing ultra-high voltage converter transformer oil using coal tar as raw material and employing a combination of suspended bed and fixed bed hydrogenation is of great significance. Summary of the Invention

[0015] To address the aforementioned problems, this invention provides a method for preparing ultra-high voltage converter transformer oil using coal tar via a combined suspended-bed and fixed-bed hydrogenation process, thus compensating for the shortage of naphthenic oil resources. The prepared transformer oil exhibits low kinematic viscosity, high flash point, excellent electrical and antioxidant properties, and moderate gas evolution, making it particularly suitable as an oil for ultra-high voltage converter transformers.

[0016] To achieve the above objectives, the preparation method of ultra-high voltage converter transformer oil used in this invention includes the following steps:

[0017] Step 1: Pretreatment and Suspended Bed Hydrogenation

[0018] Using the full fraction of medium- or low-temperature coal tar or high-temperature coal tar as raw material, the product sequentially undergoes centrifugation, heating, demulsification and dehydration in a dehydration tank, and flash evaporation in a flash dehydration tower. An oil-soluble catalyst precursor and a sulfurizing agent are added, and the mixture is pressurized by a high-pressure feed pump and mixed with hydrogen before entering the first heating furnace. After heating in the first heating furnace, the mixture is fed into a suspended bed hydrogenation reactor. The suspended bed hydrogenation reaction takes place in the reactor. The suspended bed hydrogenation product is separated into gas and liquid phases by a first high-pressure separator. The gas phase enters a first cold low-pressure separator for further separation into hydrogen and naphtha, while the liquid phase enters a first hot low-pressure separator for further separation into naphtha and suspended bed product oil. The oil generated in the slurry bed is sent to the first fractionation tower for atmospheric and vacuum fractionation, which separates it into naphtha, a 180–360°C fraction, a 360–486°C fraction, and a superplasticized bottom oil with a temperature greater than 486°C. The hydrogen separated by the first cold low-pressure separator is recirculated after being pressurized by a circulating hydrogen compressor. The naphtha separated by the first cold low-pressure separator and the first hot low-pressure separator, as well as the naphtha fractionated by the first fractionation tower, are all output as products. The 360–486°C fractionated by the first fractionation tower is output as a deep hydrorefining feedstock for the production of industrial white oil and rubber plasticizers. The superplasticized bottom oil and catalyst are recycled back to the slurry bed hydrotreating reactor to continue the slurry bed hydrotreating reaction.

[0019] Step 2: Fixed-bed deep hydrogenation refining

[0020] The 180–360°C fraction obtained from the first fractionation tower in step 1 is mixed with a sulfiding agent and hydrogen, and then heated in the second heater. It is then fed into a fixed-bed deep hydrorefining reactor for deep hydrorefining. The product from the fixed-bed deep hydrorefining process is separated into gas and liquid phases by a second high-pressure separator. The gas phase enters a second cold low-pressure separator for further separation into hydrogen and naphtha. The liquid phase enters a second hot low-pressure separator for further separation into naphtha and the deep hydrorefined product oil. The deep hydrorefined product oil enters a first stripping tower for stripping hydrogen sulfide and ammonia. Naphtha exits from the upper part of the first stripping tower, and the stripped oil from the lower part is sent to the second fractionation tower. The distillation tower performs atmospheric and vacuum fractionation, separating naphtha, light diesel oil, a 280–330°C fraction, and a 330–360°C fraction. The hydrogen separated by the second cold low-pressure separator is recirculated after being pressurized by a circulating hydrogen compressor. The naphtha separated by the second cold low-pressure separator, the second hot low-pressure separator, the first stripping tower, and the naphtha fractionated by the second fractionation tower are all output as products. The light diesel oil fractionated by the second fractionation tower is also output as a product. Part of the 280–330°C fractionated by the second fractionation tower is used as aromatic oil, and the other part is mixed with the 330–360°C fractionated by the second fractionation tower through pipelines and used as a feedstock for isomerization dewaxing.

[0021] Step 3: Fixed-bed heterogeneous depressurization

[0022] The isomerized dewaxing feedstock mixed via pipeline in step 2 is mixed with hydrogen and then heated in the third heater. It is then fed into a fixed-bed isomerized dewaxing reactor, where the isomerization dewaxing reaction takes place. The isomerized dewaxing products undergo gas-liquid separation in the third high-pressure separator. The gas phase enters the third cold low-pressure separator for further separation into naphtha and hydrogen, while the liquid phase enters the third hot low-pressure separator for further separation into naphtha and isomerized dewaxing product oil. The isomerized dewaxing product oil enters the second stripping tower for stripping hydrogen sulfide and ammonia. Naphtha is produced from the upper part of the second stripping tower, and the stripped oil from the lower part is sent to the third fractionation tower for atmospheric and vacuum fractionation, resulting in naphtha. Oil, 180-280℃ fraction, 280-330℃ fraction, 330-360℃ fraction; hydrogen separated by the third cold low-pressure separator is recirculated after being pressurized by the circulating hydrogen compressor; naphtha separated by the third cold low-pressure separator, the third hot low-pressure separator, the second stripping tower, and the naphtha fractionated by the third fractionating tower are all output as products; the 180-280℃ fractionated by the third fractionating tower is sent to the next stage as light white oil for further fractionation and cutting according to grade; the 280-330℃ fractionated by the third fractionating tower is used as transformer oil base oil; and the 330-360℃ fractionated by the third fractionating tower is output as refrigeration oil base oil.

[0023] Step 4: Preparation of Product Oil

[0024] The composition by mass percentage is: 5%–15% aromatic oil, 0.2%–0.3% antioxidant, and the balance being transformer oil base oil. The outlet of the aromatic oil obtained from the second fractionation tower in step 2 is connected to the outlet of the transformer oil base oil obtained from the third fractionation tower in step 3 via a pipeline connection. The antioxidant is fully dissolved in a reaction vessel at a temperature of 80–120°C and injected into the outlet pipeline of the transformer oil base oil obtained from the third fractionation tower. The mixture is then online mixed in a static pipeline mixer with a working pressure of 0.6 MPa and a working temperature of 80–100°C. After uniform mixing, ultra-high voltage converter transformer oil is formed.

[0025] In step 1 above, the preferred process conditions for suspended bed hydrogenation are: reaction temperature of 360–450℃, reaction pressure of 12–18 MPa, and volume hourly space velocity of 0.5–1.3 h⁻¹. -1 The hydrogen-to-oil volume ratio is 800–1500.

[0026] In step 1 above, the dehydration tank uses ZH-002 or ZH-004 dehydration and desalination aids for demulsification and dehydration, and the injection amount of the dehydration and desalination aids is 200-800 mg / kg.

[0027] In step 1 above, the oil-soluble catalyst precursor is selected as oil-soluble molybdenum isooctanoate, and its addition amount is 100-1000 mg / kg.

[0028] In steps 1 and 2 above, the vulcanizing agent is any one of carbon disulfide, dimethyl disulfide, or liquid sulfur, and the amount added is 200-800 mg / kg.

[0029] In step 2 above, the preferred process conditions for the deep hydrorefining reaction are: reaction temperature of 300–400℃, reaction pressure of 10–18 MPa, hydrogen-to-oil ratio of 1000–1500:1, and volume hourly space velocity of 0.2–0.7 h⁻¹. -1 The catalyst for the deep hydrogenation refining reaction uses modified alumina with a bimodal pore size distribution as a support, loaded with 2%–5% NiO, 5%–10% MoO3, and 15%–30% WO3.

[0030] In step 2 above, the obtained aromatic oil has a CA content greater than 40%, a sulfur content less than 2 ppm, and a nitrogen content less than 5 ppm.

[0031] In step 3 above, the preferred process conditions for the isomerization dewaxing reaction are: reaction temperature 300–400℃, pressure 10–18 MPa, and space velocity 0.6–1.5 h⁻¹. -1 The hydrogen-to-oil ratio is 500–1500:1. The catalyst for the isomerization dewaxing reaction is a mixture of modified ZSM-5 and SAPO-11 in a mass ratio of 1:2–6, supported on 1%–8% NiO.

[0032] In step 3 above, the resulting transformer oil base oil has a CN content of 55%–65% and a kinematic viscosity of 6–8 mm at 40°C. 2 / s, kinematic viscosity less than 800 mm³ at -30℃ 2 / s, flash point above 135℃, pour point less than -50℃.

[0033] In step 4 above, the antioxidant is 2,6-di-tert-butyl-p-cresol or 2,6-di-tert-butylphenol.

[0034] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0035] 1. This invention uses the full fraction of high-temperature and medium-low-temperature coal tar as raw material and employs a combined suspended-bed hydrogenation process. It achieves continuous production of transformer oil base oil and aromatic oil using the same raw material and a single set of industrial equipment. After the addition of antioxidants, the resulting transformer oil exhibits excellent anti-gas evolution properties, and its low-temperature performance and antioxidant properties are significantly superior to commercially available transformer oils. This avoids the environmental pollution, insufficient resource utilization, and low product quality problems associated with traditional processes. Furthermore, it is highly adaptable to raw materials, offers flexible operation, allows for online catalyst addition in small quantities, and improves the overall performance of the prepared ultra-high voltage converter transformer oil.

[0036] 2. The method of this invention provides an economical processing method for coal tar, which has relatively low byproduct utilization, and also develops a new raw material for the production of ultra-high voltage converter transformer oil. my country is a country rich in coal resources, with a large amount of coal tar resources as a byproduct. The method of this invention can replace scarce naphthenic crude oil resources to produce ultra-high voltage converter transformer oil urgently needed by my country's power industry. This creates a new product market for the existing coal tar deep processing market, effectively promotes the high-value utilization of coal tar resources, and drives the high-end, diversified, and low-carbon development of the coal chemical industry. Attached Figure Description

[0037] Figure 1 This is a process flow diagram of the present invention.

[0038] In the diagram: 1-Centrifuge, 2-Heater, 3-Dehydration tank, 4-Flash dehydration tower, 5-First heating furnace, 6-Suspended bed hydrogenation reactor, 7-First high-pressure separator, 8-First cold low-pressure separator, 9-First hot low-pressure separator, 10-First fractionation tower, 11-Second heating furnace, 12-Fixed bed deep hydrogenation refining reactor, 13-Second high-pressure separator, 14-Second cold low-pressure separator, 15-Second hot low-pressure separator, 16-First stripping tower, 17-Second fractionation tower, 18-Third heating furnace, 19-Fixed bed heterogeneous decondensation reactor, 20-Third high-pressure separator, 21-Third cold low-pressure separator, 22-Third hot low-pressure separator, 23-Second stripping tower, 24-Third fractionation tower. Detailed Implementation

[0039] The present invention will now be described in detail with reference to the accompanying drawings and embodiments, but the scope of protection of the present invention is not limited to these embodiments.

[0040] According to the present invention Figure 1 The process flow shown is used to prepare ultra-high voltage converter transformer oil. The specific steps are as follows:

[0041] Step 1: Pretreatment and Suspended Bed Hydrogenation

[0042] Using the full fraction of medium- and low-temperature coal tar or high-temperature coal tar as raw material, most of the solid or insoluble impurities and water in the raw material are removed by centrifuge 1. The raw material is then heated to 100-150°C in heater 2 and then enters dehydration tank 3. In dehydration tank 3, desalting and dehydration aids are added to complete the demulsification and dehydration of the raw material. The demulsified and dehydrated raw material enters flash dehydration tower 4 for deep dehydration to obtain pretreated coal tar. Oil-soluble catalyst precursor and sulfiding agent are added to the pretreated coal tar. After being pressurized by high-pressure feed pump, it is mixed with hydrogen and sent to the first heating furnace 5. After being heated to 360-450°C in the first heating furnace 5, it enters the lower part of the suspended bed hydrogenation reactor 6. In the suspended bed hydrogenation reactor 6, suspended bed hydrogenation reaction is carried out to remove metals and residual carbon, and the mild hydrogenation refining function is completed. The dehydration and desalination aid is ZH-002 or ZH-004 type from Shaanxi Zhongheng New Materials Co., Ltd., with an injection amount of 200–800 mg / kg. The oil-soluble catalyst precursor is oil-soluble molybdenum isooctanoate, with an addition amount of 100–1000 mg / kg. The oil-soluble molybdenum isooctanoate decomposes and sulfides in situ in a hydrogen-rich environment to form nano-sized molybdenum sulfide catalyst, which is highly dispersed in the oil phase. The sulfiding agent is any one of carbon disulfide, dimethyl disulfide, or liquid sulfur, with an addition amount of 200–800 mg / kg. The process conditions for the suspended bed hydrogenation reaction are: reaction temperature 360–450℃, reaction pressure 12–18 MPa, and volume hourly space velocity 0.5–1.3 h⁻¹. -1The hydrogen-to-oil volume ratio is 800–1500. The suspended bed hydrogenation product is separated into gas and liquid phases by the first high-pressure separator 7. The gas phase enters the first cold low-pressure separator 8 for further separation into hydrogen and naphtha. The liquid phase enters the first hot low-pressure separator 9 for further separation into naphtha and suspended bed product oil. The suspended bed product oil is sent to the first fractionation tower 10 for atmospheric and vacuum fractionation, which yields naphtha, a 180–360°C fraction, a 360–486°C fraction, and a bottom oil (a fraction oil with a temperature greater than 486°C). The hydrogen separated by the first cold low-pressure separator 8 is recirculated after being pressurized by the circulating hydrogen compressor. The naphtha separated by the first cold low-pressure separator 8 and the first hot low-pressure separator 9, as well as the naphtha fractionated by the first fractionating tower 10, are all output as products. The 360-486℃ fractionated by the first fractionating tower 10 is output as a deep hydrorefining feedstock for the production of industrial white oil, rubber plasticizers, etc. The bottom oil and catalyst (nano-scale molybdenum sulfide catalyst) are recycled back to the slurry bed reactor to continue the slurry bed hydrogenation reaction.

[0043] Step 2: Fixed-bed deep hydrogenation refining

[0044] The 180–360°C fraction from the first fractionating tower 10 in step 1 is mixed with a sulfiding agent and hydrogen, and then heated in the second heating furnace 11 to 300–400°C. After heating, it is fed into a fixed-bed deep hydrorefining reactor 12 for deep hydrorefining to remove sulfur, nitrogen, aromatics, and other components. The sulfiding agent is the same as in step 1. The process conditions for the deep hydrorefining reaction are: reaction temperature 300–400°C, reaction pressure 10–18 MPa, hydrogen-to-oil ratio 1000–1500:1, and volume hourly space velocity 0.2–0.7 h⁻¹. -1The catalyst for the deep hydrorefining reaction uses modified alumina with a bimodal pore size distribution as a support, loaded with 2%–5% NiO, 5%–10% MoO3, and 15%–30% WO3, where the loading is 100% by mass of the catalyst. The fixed-bed deep hydrorefining product is separated into gas and liquid phases by the second high-pressure separator 13. The gas phase enters the second cold low-pressure separator 14 for further separation into hydrogen and naphtha, while the liquid phase enters the second hot low-pressure separator 15 for further separation into naphtha and deep hydrorefining product oil. The deep hydrorefining product oil enters the first stripping tower 16 for stripping of hydrogen sulfide and ammonia. Naphtha is discharged from the upper part of the first stripping tower 16, and the stripped oil is sent to the second fractionation tower 17 for atmospheric and vacuum fractionation, which yields naphtha, light diesel oil, a 280–330℃ fraction, and a 330–360℃ fraction. The hydrogen separated by the second cold low-pressure separator 14 is pressurized by a circulating hydrogen compressor and then recycled. The naphtha separated by the second cold low-pressure separator 14, the second hot low-pressure separator 15, and the first stripping tower 16, as well as the naphtha fractionated by the second fractionating tower 17, are all output as products. The light diesel oil fractionated by the second fractionating tower 17 is also output as a product. Part of the 280-330℃ fractionated by the second fractionating tower 17 is used as aromatic oil, and the other part is mixed with the 330-360℃ fractionated by the second fractionating tower 17 through pipelines and used as isomerization dewaxing feedstock.

[0045] Step 3: Fixed-bed heterogeneous depressurization

[0046] The isomerization decondensation feedstock mixed via pipeline in step 2 is then mixed with hydrogen and fed into the third heating furnace 18 for heating to 300–400°C. After heating, it is fed into a fixed-bed isomerization decondensation reactor 19, where selective ring-opening and isomerization reactions occur. The process conditions for the isomerization decondensation reaction are: reaction temperature 300–400°C, pressure 10–18 MPa, and space velocity 0.6–1.5 h⁻¹. -1The hydrogen-to-oil ratio is 500–1500:1. The catalyst for the isomerization dewaxing reaction is a mixture of modified ZSM-5 and SAPO-11 in a mass ratio of 1:2–6, supported on 1%–8% NiO. The isomerization dewaxing products undergo gas-liquid separation in the third high-pressure separator 20. The gas phase enters the third cold low-pressure separator 21 for further separation into naphtha and hydrogen. The liquid phase enters the third hot low-pressure separator 22 for further separation into naphtha and isomerization dewaxing product oil. The isomerization dewaxing product oil enters the second stripping tower 23 for stripping hydrogen sulfide and ammonia. Naphtha is discharged from the upper part of the second stripping tower 23, and the stripped oil is sent to the third fractionation tower 24 for atmospheric and vacuum fractionation, which yields naphtha, a 180–280℃ fraction, a 280–330℃ fraction, and a 330–360℃ fraction. The hydrogen separated by the third cold low-pressure separator 21 is recirculated after being pressurized by a circulating hydrogen compressor. The naphtha separated by the third cold low-pressure separator 21, the third hot low-pressure separator 22, and the second stripping tower 23, as well as the naphtha fractionated by the third fractionating tower 24, are all output as products. The 180–280°C fraction from the third fractionating tower 24 is sent to the next stage for graded fractionation as light white oil. The 280–330°C fraction from the third fractionating tower 24 is used as transformer base oil, and the 330–360°C fraction is output as refrigeration oil base oil. The aromatic oil has a CA content greater than 40%, a sulfur content less than 2 ppm, and a nitrogen content less than 5 ppm. The transformer oil base oil has a CN content of 55%–65% and a kinematic viscosity of 6–8 mm at 40°C. 2 / s, kinematic viscosity less than 800 mm³ at -30℃ 2 / s, flash point above 135℃, pour point less than -50℃.

[0047] Step 4: Preparation of Product Oil

[0048] The composition by mass percentage is: 5%–15% aromatic oil, 0.2%–0.3% antioxidant, and the balance being transformer oil base oil. The outlet of the aromatic oil obtained from fractionation in the second fractionation tower 17 in step 2 is connected to the outlet of the transformer oil base oil obtained from fractionation in the third fractionation tower 24 in step 3 via a pipeline connection. The mass of the added aromatic oil is controlled by a flow meter. The antioxidant, 2,6-di-tert-butyl-p-cresol or 2,6-di-tert-butylphenol, is fully dissolved in a reaction vessel at a temperature of 80–120°C. The mass flow rate of the antioxidant is adjusted by a metering pump and injected into the outlet pipeline of the transformer oil base oil obtained from fractionation in the third fractionation tower 24. The mixture is then online mixed by a static mixer with a working pressure of 0.6 MPa and a working temperature of 80–100°C. After uniform mixing, ultra-high voltage converter transformer oil is formed.

[0049] Example

[0050] Using medium- and low-temperature coal tar and high-temperature coal tar with the physicochemical properties shown in Table 1 as raw materials, and in accordance with the suspension bed hydrogenation reaction conditions, fixed bed deep hydrogenation refining reaction conditions, fixed bed isomerization dewaxing reaction conditions and mass percentage composition in Table 2, ultra-high voltage converter transformer oils of Examples 1 to 4 were prepared respectively. The physicochemical properties of the prepared ultra-high voltage converter transformer oils are shown in Table 3.

[0051] Comparative Example 1

[0052] Using the same raw materials as in Example 1, the process conditions for suspended bed hydrogenation and fixed bed deep hydrogenation refining were the same as in Example 1. The 280-330℃ fraction obtained by fractionation in the second fractionation tower 17 after fixed bed deep hydrogenation refining was used as the product base oil and antioxidants were directly added as the final product. The process conditions and product properties of the hydrogenation process are shown in Tables 2 and 3, respectively.

[0053] Comparative Example 2

[0054] Using the same raw materials as in Example 1, the process conditions for suspended bed hydrogenation, fixed bed deep hydrogenation refining, and fixed bed isomerization dewaxing were also the same as in Example 1. The difference was that the transformer oil base oil fractionated in the third fractionation tower 24 was only added with antioxidants and not with aromatic oil components. As the final product, the process conditions and product properties of the hydrogenation process are shown in Tables 2 and 3.

[0055] Comparative Example 3

[0056] Using the same raw materials as in Example 1, the fixed-bed deep hydrorefining and fixed-bed isomerization dewaxing process conditions were also the same as in Example 1. The difference was that the suspended-bed hydrorefining reactor 6 was replaced with a fixed-bed hydrorefining reactor. The fixed-bed hydrorefining reactor was sequentially loaded with TK-10 hydroprotectant (Topsoe), RN-5030 hydrodemetallizer (Criterion), ICR-137 hydrodesulfurizer (Chevron), and TK-555 hydromodification catalyst (Topsoe). The inlet temperature of TK-10 hydroprotectant was 230°C, the reaction temperature of RN-5030 hydrodemetallizer was 240°C, the reaction temperature of ICR-137 hydrodesulfurizer was 330°C, and the reaction temperature of TK-555 hydromodification catalyst was 370°C. The pressure was 13.5 MPa, and the liquid hourly space velocity was 0.3 h⁻¹. -1 The hydrogen-to-oil volume ratio is 1500:1. The 280–330°C fraction from the final third fractionation tower is used as the product base oil. After adding aromatic oil and antioxidants, it becomes the final product.

[0057] Table 1. Physicochemical properties of coal tar used in the examples.

[0058]

[0059]

[0060] Table 2. Basic conditions for hydrogenation in examples and comparative examples.

[0061]

[0062]

[0063] Table 3. Physical and chemical properties of oil in ultra-high voltage converter transformers.

[0064]

[0065]

[0066]

[0067] As shown in Table 3, the transformer oils produced in Examples 1-4 of this invention have a pour point of less than -50°C and a kinematic viscosity of 7-8 mmHg at 40°C. 2 / s, kinematic viscosity less than 800 mm³ at -30℃ 2 / s, aniline point below 80℃, acid value less than or equal to 0.03mgKOH / g after 500h oxidation, sludge content less than 0.01%, medium loss factor at 90℃ less than 0.008, and gas evolution less than 10mm. 3 The oil prepared at Comparative Example 1 not only meets the requirements for I-30 (special type) transformer oil in GB 2536-2011 "Unused Mineral Insulating Oil for Electrical Fluid Transformers and Switches", but its physical, chemical, and electrical properties also meet the quality indicators for DC converter transformer oil. The transformer oil prepared in Comparative Example 1, while having an advantage in gas evolution resistance, fails to meet the requirements for I-30 (special type) transformer oil in GB 2536-2011 and related indicators for ultra-high voltage converter transformer oil in terms of pour point, breakdown voltage, density, dielectric loss factor, acid value, and oxidation stability. Comparative Example 2, except for its gas evolution resistance, has similar indicators to Example 1. Although it meets the requirements for I-30 (special type) transformer oil in GB 2536-2011, its poor gas evolution resistance makes it unsuitable for long-term operation of ultra-high voltage transformers. Comparative Example 3, except for kinematic viscosity (-30℃) and density (20℃), is similar to Example 1 in all other aspects. Although it meets the requirements for transformer oil I-30 (special type) in standard 2536-2011, its kinematic viscosity (-30℃) is greater than 800 mm³. 2The current density is too high, failing to meet the requirements for ultra-high voltage converter transformer oil, and is therefore unsuitable for use in ultra-high voltage converter transformers. Therefore, the transformer base oil provided by this invention has characteristics such as low low-temperature kinematic viscosity, low pour point, excellent gas evolution properties, and excellent oxidation stability. It can simultaneously meet the requirements for transformer oil I-30 (special type) in GB2536-2011 standard, and its physical, chemical, and electrical properties can all meet the quality indicators for DC converter transformer oil, making it suitable for long-term operation of ultra-high voltage transformers.

Claims

1. A method for the production of an ultra-high voltage converter transformer oil, characterized in that comprising the steps of: Step 1: Pretreatment and slurry-bed hydrogenation With the full fraction of low-medium temperature coal tar or high temperature coal tar with metal content ≥ 190 ppm, carbon residue ≥ 6% as raw material, it is sequentially centrifuged by centrifuge (1), heated by heater (2), demulsified and dehydrated by dehydration tank (3), and flashed by flash dehydration tower (4), then oil-soluble catalyst precursor and sulfurizing agent are added, pressurized by high-pressure feed pump, mixed with hydrogen, heated by first heating furnace (5), sent to slurry-bed hydrogenation reactor (6), slurry-bed hydrogenation reaction is carried out in the slurry-bed hydrogenation reactor (6), the slurry-bed hydrogenation product is separated into gas and liquid phases by the first high-pressure separator (7), the gas phase is further separated into hydrogen and naphtha by the first cold low-pressure separator (8), the liquid phase is further separated into naphtha and slurry-bed generated oil by the first hot low-pressure separator (9), the slurry-bed generated oil is sent to the first fractionating column (10) for atmospheric and vacuum distillation, and is separated into naphtha, 180-360℃ fraction, 360-486℃ fraction, and bottom oil greater than 486℃; the hydrogen separated by the first cold low-pressure separator (8) is pressurized by a recycle hydrogen compressor and recycled, the naphtha separated by the first cold low-pressure separator (8) and the first hot low-pressure separator (9) and the naphtha fractionated by the first fractionating column (10) are all output as products, the 360-486℃ fraction fractionated by the first fractionating column (10) is output as a deep hydrogenation refining raw material for producing industrial white oil and rubber plasticizer, and the bottom oil and catalyst are recycled back to the slurry-bed hydrogenation reactor (6) for slurry-bed hydrogenation reaction; Step 2: Fixed-bed deep hydrogenation refining The 180-360℃ fraction of the first fractionating column (10) in step 1 is mixed with a sulfurizing agent and hydrogen, heated in the second heating furnace (11), and then sent to the fixed-bed deep hydrofining reactor (12) for deep hydrofining reaction. The fixed-bed deep hydrofining product is separated into gas and liquid phases in the second high-pressure separator (13), the gas phase is further separated into hydrogen and naphtha in the second cold low-pressure separator (14), and the liquid phase is further separated into naphtha and deep hydrofining product oil in the second hot low-pressure separator (15). The deep hydrofining product oil is sent to the first stripping column (16) for stripping of hydrogen sulfide and ammonia. The naphtha is discharged from the upper part of the first stripping column (16), and the stripped oil is sent to the second fractionating column (17) for atmospheric and vacuum distillation, which is separated into naphtha, light diesel, 280-330℃ fraction, and 330-360℃ fraction. The hydrogen separated in the second cold low-pressure separator (14) is recycled after being pressurized by a recycle hydrogen compressor. The naphtha separated in the second cold low-pressure separator (14), the second hot low-pressure separator (15), the first stripping column (16), and the second fractionating column (17) is output as a product. The light diesel separated in the second fractionating column (17) is output as a product. Part of the 280-330℃ fraction separated in the second fractionating column (17) is used as an aromatic oil, and the other part is mixed with the 330-360℃ fraction separated in the second fractionating column (17) and used as an isomerization and pour point depressing raw material. Step 3: Fixed-bed isomerization and pour point depression The isomerization and pour point depressing raw material mixed in step 2 is mixed with hydrogen, heated in the third heating furnace (18), and then sent to the fixed-bed isomerization and pour point depression reactor (19) for isomerization and pour point depression reaction. The isomerization and pour point depression product is separated into gas and liquid phases in the third high-pressure separator (20), the gas phase is further separated into naphtha and hydrogen in the third cold low-pressure separator (21), and the liquid phase is further separated into naphtha and isomerization and pour point depression product oil in the third hot low-pressure separator (22). The isomerization and pour point depression product oil is sent to the second stripping column (23) for stripping of hydrogen sulfide and ammonia. The naphtha is discharged from the upper part of the second stripping column (23), and the stripped oil is sent to the third fractionating column (24) for atmospheric and vacuum distillation, which is separated into naphtha, 180-280℃ fraction, 280-330℃ fraction, and 330-360℃ fraction. The hydrogen separated in the third cold low-pressure separator (21) is recycled after being pressurized by a recycle hydrogen compressor. The naphtha separated in the third cold low-pressure separator (21), the third hot low-pressure separator (22), the second stripping column (23), and the third fractionating column (24) is output as a product. The 180-280℃ fraction separated in the third fractionating column (24) is sent to the next section for fractionation and cutting according to the grade as light white oil. The 280-330℃ fraction separated in the third fractionating column (24) is used as transformer oil base oil. The 330-360℃ fraction separated in the third fractionating column (24) is output as refrigerator oil base oil. Step 4: Preparation of product oil According to the mass percentage composition: aromatic oil 5%~15%, antioxidant 0.2%~0.3%, the balance is transformer oil base oil, the outlet of the aromatic oil obtained by the second fractionating column (17) in step 2 is connected with the outlet of the transformer oil base oil obtained by the third fractionating column (24) in step 3 by pipeline connection; the antioxidant is dissolved in the charging kettle, the temperature of the antioxidant charging kettle is 80~120℃, and is injected into the outlet pipeline of the transformer oil base oil obtained by the third fractionating column (24), and is mixed on line by the pipeline static mixer with the working pressure of 0.6MPa and the working temperature of 80~100℃, and after uniform mixing, the super-high voltage converter transformer oil is formed; the obtained super-high voltage converter transformer oil meets: the pour point is less than-50℃, the 40℃ kinematic viscosity is 7~8mm 2 / s, the-30℃ kinematic viscosity is less than 800mm 2 / s, the aniline point is lower than 80℃, the total acid value is less than or equal to 0.03mgKOH / g after 500h oxidation at 120℃, the oil sludge is less than 0.01%, the 90℃ medium loss factor is less than 0.008, and the gas evolution is less than 10mm 3 / min.

2. The method of producing an ultra-high voltage converter transformer oil according to claim 1, characterized in that, The process conditions of the suspended bed hydrogenation in step 1 are as follows: reaction temperature 360-450℃, reaction pressure 12-18 MPa, volume space velocity 0.5-1.3 h -1 , hydrogen / oil volume ratio 800-1500.

3. The method of producing an ultra-high voltage converter transformer oil according to claim 1, characterized in that, In step 1, the dehydration tank (3) is subjected to demulsification and dehydration by using ZH-002 or ZH-004 dehydration desalination aids, and the injection amount of the dehydration desalination aids is 200-800 mg / kg.

4. The method of producing an ultra-high voltage converter transformer oil according to claim 1, characterized in that, In step 1, the oil-soluble catalyst precursor is selected to be oil-soluble molybdenum isooctoate, and the addition amount is 100-1000 mg / kg.

5. The method of preparing ultra-high voltage converter transformer oil according to claim 1, characterized in that, In steps 1 and 2, the vulcanizing agent is any one of carbon disulfide, dimethyl disulfide and liquid sulfur, and the addition amount is 200-800 mg / kg.

6. The method of producing an ultra-high voltage converter transformer oil according to claim 1, characterized in that, In step 2, the process conditions of the deep hydrofining reaction are as follows: reaction temperature is 300-400℃, reaction pressure is 10-18MPa, hydrogen / oil ratio is 1000-1500:1, volume space velocity is 0.2-0.7h -1 The catalyst for the deep hydrofining reaction uses modified alumina with bimodal pore distribution as carrier, and is loaded with 2%-5% NiO, 5%-10% MoO3 and 15%-30% WO3.

7. The method of producing an ultra-high voltage converter transformer oil according to claim 1, characterized in that, In step 2, the obtained aromatic oil CA has a content of greater than 40% and a sulfur content of less than 2 ppm and a nitrogen content of less than 5 ppm.

8. The method of producing an ultra-high voltage converter transformer oil according to claim 1, characterized in that, The process conditions of the isomerization and dewaxing reaction in step 3 are as follows: reaction temperature 300-400℃, pressure 10-18 MPa, space velocity 0.6-1.5 h -1 , hydrogen / oil ratio 500-1500:1, and the isomerization and dewaxing catalyst is a mixture of modified ZSM-5 and SAPO-11 with a mass ratio of 1:2-6 as a carrier, and 1%-8% NiO is loaded.

9. The method of producing an ultra-high voltage converter transformer oil according to claim 1, characterized in that, In step 3, the obtained transformer oil base oil CN content is 55% to 65%, the 40℃ kinematic viscosity is 6 to 8mm 2 / s, the -30℃ kinematic viscosity is less than 800mm 2 / s, the flash point is higher than 135℃, and the pour point is less than -50℃.

10. The method of producing an ultra-high voltage converter transformer oil according to claim 1, characterized in that, In step 4, the antioxidant is 2,6-di-tert-butyl-p-cresol or 2,6-di-tert-butyl phenol.

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