Multi-stage coupling heat exchange and gradient cooling energy-saving cooperative treatment method for waste mineral oil

Abstract

The invention discloses a waste mineral oil multistage coupling heat exchange and gradient cooling energy-saving cooperative treatment method, which belongs to the technical field of heat energy engineering and energy conservation, and comprises the steps of pretreatment refining, fractionation cutting, molecular distillation, hydrofining, gradient cooling and intelligent control. By constructing a multi-stage coupling heat exchange network, hydrofining high-temperature waste heat is used for preheating fractionation feed, and fractionating tower top steam waste heat is used for preheating molecular distillation feed; cooling the high-temperature material flow according to the temperature by stages through a three-stage gradient cooling system, and recycling the low-temperature cold energy of the molecular distillation unit for a pretreatment process; and dynamically optimizing heat exchange and cooling operation based on an energy balance model through the central control system. According to the method, the technical problems of energy utilization splitting and serious waste heat waste in a waste mineral oil regeneration process are solved, the yield and the quality of a base oil product are greatly improved while the system energy consumption and the cooling water consumption are remarkably reduced, and an innovative solution is provided for efficient regeneration of the waste mineral oil.

Description

A method for energy-saving synergistic treatment of waste mineral oil through multi-stage coupled heat exchange and gradient cooling Technical Field

[0001] This invention relates to the fields of thermal energy engineering and energy-saving technology, and in particular to a method for the synergistic energy-saving treatment of waste mineral oil through multi-stage coupled heat exchange and gradient cooling. Background Technology

[0002] The recycling of waste mineral oil is a crucial link in resource recycling and environmental protection, and its technological development is facing the dual pressures of increasingly stringent environmental regulations and resource scarcity. Currently, recycling technology has evolved from simple physical separation to a complex system integrating multiple processes such as chemical refining, distillation fractionation, hydrotreating, and molecular distillation, with the core objective of achieving efficient oil quality improvement and low-energy conversion.

[0003] However, existing regeneration processes generally employ linear, isolated unit operation modes, lacking synergistic optimization of material and energy flows. Chemical refining is prone to chlorine pollution, atmospheric and vacuum distillation is energy-intensive, hydrorefining has stringent requirements for feed purity and requires huge investments, while molecular distillation has limited processing capacity. More importantly, when these processes are directly connected in series, systemic defects such as thermodynamic mismatch, pressure surges, and poor material compatibility exist, leading to problems such as flash coking, catalyst poisoning, and energy waste, severely restricting system energy efficiency, product consistency, and long-term stable operation.

[0004] Therefore, there is an urgent need for an energy-saving synergistic treatment method that can achieve the organic integration of multiple processes, the cascade utilization of energy, and the smooth transition of materials, in order to break through the existing technological bottlenecks. Summary of the Invention

[0005] This invention overcomes the shortcomings of the prior art and provides a method for energy-saving synergistic treatment of waste mineral oil through multi-stage coupled heat exchange and gradient cooling.

[0006] To achieve the above objectives, the technical solution adopted by this invention is: a method for the synergistic energy-saving treatment of waste mineral oil through multi-stage coupled heat exchange and gradient cooling, comprising:

[0007] 1. A method for energy-saving synergistic treatment of waste mineral oil through multi-stage coupled heat exchange and gradient cooling, characterized by comprising the following steps:

[0008] S1. A pretreatment and refining process is carried out on waste mineral oil raw materials to obtain pre-purified oil phase materials;

[0009] S2. The oil phase material is fed into the fractionation and cutting unit and cut under normal or reduced pressure to separate light, medium and heavy fractions.

[0010] S3. The medium and heavy fractions are introduced into the molecular distillation unit for short-path distillation to obtain high-purity base oil components and residual residue oil; and the process heat generated by the fractionation and cutting unit is used to preheat the material entering the molecular distillation unit.

[0011] S4. The high-purity base oil components are fed into the hydrorefining unit for catalytic hydrogenation reaction to obtain regenerated lubricating oil base oil; and the high-temperature waste heat of the hydrorefining unit outlet stream is used to preheat the feed to the fractionation and cutting unit.

[0012] S5. The high-temperature process products generated by the fractionation and cutting unit, molecular distillation unit and hydrorefining unit are subjected to gradient cooling, including: the first stage cooling reduces the material temperature to 115-125℃, the second stage cooling reduces it to 55-65℃, and the third stage cooling reduces it to 15-30℃; at the same time, the low-temperature cold energy generated by the molecular distillation unit is recovered and used in the pretreatment refining process.

[0013] S6. The process parameters of each step are acquired in real time through the central control system, and the heat exchange and cooling operations in steps S3, S4 and S5 are dynamically adjusted based on the energy balance model.

[0014] In a preferred embodiment of the present invention, step S1, the pretreatment and purification includes:

[0015] Pickling is performed using a sulfuric acid solution with a mass concentration of 5-10%, with an oleic acid volume ratio of 8-12:1 and a reaction time of 30-60 min.

[0016] Alkaline washing is performed using a sodium hydroxide solution with a mass concentration of 3-6%, and the reaction temperature is 60-80℃;

[0017] Sedimentation and separation are carried out in a cone-bottom settling tank, with a settling time of 4-8 hours and a bottom slag discharge cycle of once every 24 hours.

[0018] In a preferred embodiment of the present invention, in step S2, the fractionation and cutting unit includes a primary distillation column, an atmospheric distillation column, and a vacuum distillation column;

[0019] The pre-purified oil phase material is sequentially passed through a primary distillation column, an atmospheric distillation column, and a vacuum distillation column for three-stage series fractionation.

[0020] The primary distillation column operates at atmospheric pressure, and the top temperature is controlled at 170-190℃.

[0021] The bottom temperature of the atmospheric distillation column is 340-360℃, and the middle fraction is collected from the side stream.

[0022] The reduced pressure distillation column operates at an absolute pressure of 4-6 kPa and a bottom temperature of 370-390℃, and is used to cut heavy fractions.

[0023] In a preferred embodiment of the present invention, in step S3, the short-path distillation is carried out at an evaporation surface temperature of 250-320°C, and the material remains on the evaporation surface for 10-30 seconds.

[0024] In a preferred embodiment of the present invention, in step S4, the reaction temperature of the catalytic hydrogenation is 300-400°C, the hydrogen pressure is 5-15 MPa, and the catalyst used is a nickel-molybdenum or cobalt-molybdenum catalyst supported on γ-alumina.

[0025] In a preferred embodiment of the present invention, in steps S3 and S4, the preheating operation is achieved through a multi-stage coupled heat exchange network;

[0026] The network includes: a high-temperature plate-and-shell heat exchanger for step S4, a medium-temperature spiral plate heat exchanger for step S3, and a low-temperature shell-and-tube heat exchanger for recovering the low-temperature cold energy.

[0027] In a preferred embodiment of the present invention, in step S5, the first-stage, second-stage, and third-stage cooling media correspond to circulating hot water, circulating cooling water, and chilled water, respectively.

[0028] In a preferred embodiment of the present invention, in step S5, the temperature of the circulating hot water is 100-110°C, the temperature of the circulating cooling water is controlled below 35°C, and the temperature of the chilled water is 6-8°C.

[0029] In a preferred embodiment of the present invention, in step S5, the low-temperature cooling energy used to cool the pretreatment refining process is specifically used to reduce the temperature of the washing water in the alkaline washing process.

[0030] In a preferred embodiment of the present invention, the regenerated lubricating oil base oil obtained by the treatment method has a viscosity index of 95-135 and a sulfur content of 1-10 mg / kg.

[0031] This invention addresses the shortcomings of the prior art and has the following beneficial effects:

[0032] (1) This invention constructs a three-stage coupled heat exchange network and a three-stage gradient cooling system for high-temperature waste heat, medium-temperature process, and low-temperature cooling capacity. Following the principle of energy grade matching, the high-temperature waste heat from the hydrorefining unit is used for preheating the feed to the fractionation cutting unit, the steam heat from the top of the fractionation tower is used for heating the feed to the molecular distillation unit, and the circulating hot water recovered in the first stage of gradient cooling is used for heating the plant area. At the same time, the low-temperature cooling capacity of the molecular distillation cold trap is recovered to optimize the pretreatment alkaline washing conditions. Compared with the existing technology where each process unit operates in isolation and high-temperature energy is directly emitted, this invention improves the heat recovery rate, reduces cooling water consumption, and significantly reduces external energy input and water consumption. Furthermore, it realizes closed-loop utilization of energy throughout the entire process, meets the requirements of green and low-carbon development, and significantly reduces the operating cost of industrial production.

[0033] (2) This invention constructs a synergistic system of acid washing for preliminary desulfurization and hydrorefining for deep desulfurization. A smart control system maintains stable conditions for the hydrorefining reaction, and gradient cooling prevents excessive reaction of oil components, ensuring the catalyst functions efficiently. Compared to existing technologies where impurities cause catalyst poisoning and unstable product quality, the recycled lubricating oil base oil produced in this invention has a high viscosity index and low sulfur content. This further broadens the application scenarios of recycled base oil, allowing it to be used in high-end lubricating oil blending, breaking the limitations of low product quality in traditional processes, and enhancing product added value and market competitiveness.

[0034] (3) This invention collects parameters such as temperature, pressure, and flow rate of each unit in real time through a central control system, and dynamically adjusts the opening of heat exchange valves and the flow rate of cooling medium based on the energy balance model and the material conservation equation to achieve coordinated operation of each process unit. Compared with the existing technology where manual adjustment leads to large parameter fluctuations and is prone to coking or shutdown, this invention effectively mitigates the impact of external fluctuations such as raw material composition and ambient temperature, extends the equipment operating cycle, and significantly reduces the number of unplanned shutdowns; it further enhances the large-scale adaptability of the process, can stably process waste mineral oil from different sources and with different impurity contents, and provides reliable technical support for large-scale industrial promotion. Attached Figure Description

[0035] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0036] Figure 1 is a process flow diagram of the present invention. Detailed Implementation

[0037] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0038] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein. Therefore, the scope of protection of the invention is not limited to the specific embodiments disclosed below.

[0039] Unless otherwise specified in the following examples, all raw materials are commercially available or prepared by conventional methods in the art.

[0040] As shown in Figure 1, a method for energy-saving synergistic treatment of waste mineral oil using multi-stage coupled heat exchange and gradient cooling includes:

[0041] S1. A pretreatment and refining process is carried out on waste mineral oil raw materials to obtain pre-purified oil phase materials;

[0042] S2. The oil phase material is fed into the fractionation and cutting unit, and the material is cut under normal or reduced pressure to separate light, medium and heavy fractions.

[0043] S3. The medium and heavy fractions are introduced into the molecular distillation unit for short-path distillation to obtain high-purity base oil components and residual residue oil; and the process heat generated by the fractionation cutting unit is used to preheat the material entering the molecular distillation unit.

[0044] S4. The high-purity base oil components are fed into the hydrorefining unit for catalytic hydrogenation reaction to obtain regenerated lubricating oil base oil; and the high-temperature waste heat of the hydrorefining unit outlet stream is used to preheat the feed to the fractionation and cutting unit.

[0045] S5. The high-temperature process products generated by the fractionation and cutting unit, the molecular distillation unit and the hydrorefining unit are subjected to gradient cooling, including: the first stage cooling reduces the material temperature to 115-125℃, the second stage cooling reduces it to 55-65℃, and the third stage cooling reduces it to 15-30℃; at the same time, the low-temperature cold energy generated by the molecular distillation unit is recovered and used in the pretreatment refining process.

[0046] S6. The process parameters of each step are acquired in real time through the central control system, and the heat exchange and cooling operations in steps S3, S4 and S5 are dynamically adjusted based on the energy balance model.

[0047] Each step will be explained in detail below.

[0048] Specifically, step S1 is a pretreatment refining step, which aims to remove metallic impurities, organic acids, colloids and solid particles from waste mineral oil through a combination of acid washing, alkali washing and sedimentation separation, so as to provide pre-purified oil phase materials for subsequent fractionation, distillation and hydrogenation processes, and avoid harmful impurities from affecting the operation of subsequent equipment and product quality.

[0049] Specifically, the pretreatment and refining unit consists of an acid washing reactor, an alkali washing reactor, and a cone-bottom settling tank, with each piece of equipment connected by acid and alkali resistant pipelines.

[0050] Specifically, waste mineral oil raw materials are pumped into an acid washing reactor. At a temperature of room temperature to 50°C, a sulfuric acid solution with a mass concentration of 5-10% is added as an acid washing reagent. The volume ratio of oleic acid is controlled at 8-12:1. The reaction is stirred for 30-60 minutes at a temperature of room temperature to 50°C. The acidity of sulfuric acid reacts chemically with metal impurities and some colloids to form separable precipitates or water-soluble saponifications.

[0051] Further, the mixture flows into the alkaline washing reactor, and a sodium hydroxide solution with a mass concentration of 3-6% is added to the reactor. Alkaline washing is carried out at a stirring rate of 100-300 rpm, and the reaction temperature is controlled at 60-80℃. The organic acids and residual sulfuric acid in the raw materials are removed through acid-base neutralization reaction, while water-soluble saponification is generated, which facilitates subsequent separation.

[0052] Further, the mixture after alkali washing is transported to a cone-bottom settling tank and allowed to settle at room temperature for 4-8 hours to allow solid particles, metal salts, saponifications and water to settle to the bottom of the tank. The slag discharge cycle at the bottom of the tank is set to once every 24 hours. The discharged slag phase is sent to the hazardous waste treatment system, and the clear oil phase on the upper layer is the preliminarily purified oil phase material, which enters the subsequent process.

[0053] Furthermore, step S2 is a fractionation and cutting unit step, which aims to achieve precise cutting of light, medium and heavy fractions through three-stage series fractionation based on the differences in boiling range of each component, and to provide targeted feed for subsequent units, while generating process heat for preheating subsequent materials.

[0054] Specifically, the fractionation and cutting unit adopts a three-stage series structure consisting of a primary distillation column, an atmospheric distillation column, and a vacuum distillation column. The oil phase material obtained in step S1 is pumped into the primary distillation column, which operates at atmospheric pressure and has a top temperature controlled at 170-190℃. This column is used to remove moisture, light hydrocarbons, and other low-boiling-point substances from the feedstock. The bottom material of the primary distillation column is pumped into the atmospheric distillation column, which has multiple side outlets. The bottom temperature of this column is controlled at 340-360℃. Under atmospheric pressure, the side outlets located in the middle of the column are used to collect the medium-quality fraction with a boiling range of 280-400℃. The heavy oil from the bottom of the atmospheric distillation column is pumped into the vacuum distillation column, which operates under a vacuum condition of 4-6 kPa and has a bottom temperature controlled at 370-390℃. This column is used to cut out the heavy fraction with a boiling range higher than 400℃.

[0055] Furthermore, step S3 is a molecular distillation unit step, which aims to use short-path distillation technology to deeply purify medium and heavy fractions to obtain high-purity base oil components, while recovering the process heat of the fractionation cutting unit to achieve feed preheating and improve energy utilization efficiency.

[0056] Specifically, the processing unit adopts a scraped film molecular distillation unit, and the medium and heavy fractions are preheated through a spiral plate heat exchanger in the medium temperature section. The heat source of the heat exchanger comes from the top vapor of the fractionation cutting unit in step S2 and the high-temperature material at the bottom of the column. The preheated material enters the molecular distillation unit, and the evaporation surface temperature is controlled at 250-320℃. The system maintains a high vacuum environment, and the scraped film rotor rotates at a speed of 200-500 rpm to ensure that the material is heated evenly. The residence time of the material on the evaporation surface is strictly controlled at 10-30s to achieve short-path distillation.

[0057] Specifically, short-path distillation utilizes the difference in the molecular free path of different substances to deeply purify medium and heavy fractions. High-purity base oil components have a longer molecular free path, enabling them to quickly reach the condensing surface under high vacuum conditions for condensation and separation, forming high-purity base oil components. In contrast, the heavy components in residual residue oil have a shorter molecular free path, which prevents them from reaching the condensing surface and causes them to remain at the bottom of the evaporation surface and be discharged.

[0058] Furthermore, step S4 is a hydrorefining unit step, in which impurities such as sulfur, nitrogen, and oxygen are deeply removed from the base oil components through catalytic hydrogenation reaction, and unsaturated hydrocarbons are saturated, improving the product color, oxidation stability and viscosity index, and producing recycled lubricating oil base oil that meets high-end standards; at the same time, the high-temperature waste heat after the catalytic hydrogenation reaction is recovered for preheating of the feed to the fractionation and cutting unit, realizing energy cascade utilization.

[0059] Specifically, step S4 is carried out in a fixed-bed hydrotreating reactor within the hydrorefining unit. The high-purity base oil component obtained in step S3 is mixed with high-purity hydrogen in a specific ratio. The mixed gas enters the fixed-bed hydrotreating reactor, which is filled with a nickel-molybdenum or cobalt-molybdenum bimetallic catalyst supported on γ-alumina. The reaction is carried out at a temperature of 300-400℃, a hydrogen pressure of 5-15 MPa, and a volume hourly space velocity of 0.5-2 h⁻¹. -1 Under certain conditions, selective hydrogenation saturation and desulfurization and denitrification reactions are carried out, so that the sulfides and nitrides in the high-purity base oil components react with hydrogen to generate hydrogen sulfide and ammonia, respectively, which are discharged with the tail gas for further treatment, while the unsaturated hydrocarbons are hydrogenated and saturated.

[0060] Furthermore, the high-temperature stream after the hydrogenation reaction first enters a high-temperature plate-and-shell heat exchanger. The cold side of this heat exchanger is the feed oil from the fractionation and cutting unit in step S2. The feed oil is preheated using the principle of heat transfer, recovering a large amount of high-temperature waste heat and reducing the heating energy consumption of the fractionation unit. The hydrogenation product after heat exchange is separated and cooled to obtain regenerated lubricating oil base oil.

[0061] Furthermore, step S5 is a gradient cooling and cold energy recovery step, the main purpose of which is to cool down the high-temperature process materials generated by the fractionation, molecular distillation and hydrogenation refining units in stages to ensure that the materials meet the requirements for subsequent storage or processing, while recovering the low-temperature cold energy for the pretreatment refining process, so as to achieve efficient utilization of cold energy.

[0062] Specifically, the cooling system adopts a three-stage gradient cooling design. The first stage of cooling targets the high-temperature materials discharged from each unit, using circulating hot water at a temperature of 100-110℃ as the cooling medium to reduce the material temperature from the operating temperature to 115-125℃. This stage of cooling is based on the principle of high-temperature waste heat recovery. After absorbing heat, the circulating hot water can be used for factory heating, domestic hot water supply, and other scenarios, realizing high-grade waste heat reuse. The second stage of cooling uses circulating cooling water with a temperature controlled below 35℃ as the medium to further cool the material from 115-125℃ to 55-65℃. It utilizes the low cost and high heat dissipation efficiency of circulating cooling water to achieve rapid cooling in the medium temperature range. The third stage of cooling uses chilled water at a temperature of 6-8℃ as the medium to cool the material to 15-30℃, meeting the temperature requirements for material storage or subsequent processing.

[0063] Furthermore, the low-temperature cold energy discharged from the cold trap of the molecular distillation unit is recovered and transferred to the alkaline washing process of the pretreatment step S1 through a shell-and-tube heat exchanger in the low-temperature section. This is used to reduce the temperature of the alkaline washing water, optimize the alkaline washing reaction conditions, avoid oil oxidation or side reactions caused by high temperature, and form an energy closed loop of high-temperature waste heat recovery - medium-temperature rapid cooling - low-temperature deep cooling - cold energy recycling.

[0064] Specifically, the preheating operation is achieved through a multi-stage coupled heat exchange network, including a high-temperature plate-and-shell heat exchanger for step S4, a medium-temperature spiral plate heat exchanger for step S3, and a low-temperature shell-and-tube heat exchanger for recovering low-temperature cold energy.

[0065] Furthermore, step S6 is the central intelligent control step, which ensures optimal coordination between heat exchange and cooling operations throughout the entire process through real-time monitoring and dynamic adjustment, thereby maximizing system energy utilization efficiency and maintaining stable product quality.

[0066] Specifically, the central control system consists of a data acquisition module, a model calculation module, and an execution control module. The data acquisition module acquires key process parameters of steps S1-S5 in real time through distributed temperature sensors, pressure transmitters, mass flow meters, and online near-infrared spectrometers. These parameters include the inlet and outlet temperatures, pressures, and flow rates of each unit, the temperature and flow rate of the cooling medium, and the material composition. The sampling frequency is no less than once per second.

[0067] Specifically, the model calculation module has a built-in energy balance model based on the first and second laws of thermodynamics, and the calculation formula is as follows:

[0068] ;

[0069] Among them, E x Let Q be the available energy (kW), Q be the heat flow (kW), T0 be the ambient temperature (K), and T be the material temperature (K). This model is used to evaluate the available energy loss rate at each heat exchange node and identify energy efficiency bottlenecks. Another core algorithm is the material-energy coupling balance equation:

[0070] ;

[0071] Where, m i m j The mass flow rates (kg / s) and h of the input and output materials are respectively. i h o The specific enthalpy (kJ / kg) of the input and output materials, respectively, Q in For input thermal power (kW), W loss The irreversible power loss is expressed in kW.

[0072] The execution control module outputs a 4-20mA standard signal through a programmable logic controller to dynamically adjust the opening of the bypass valves of the spiral plate heat exchanger in step S3, the plate and shell heat exchanger in step S4, and the cooling medium circulation flow rate of each stage of the cooler in step S5, so as to ensure that the heat exchange efficiency and cooling effect match the process requirements.

[0073] Furthermore, when external conditions such as raw material composition and ambient temperature fluctuate, the system can respond quickly and maintain the stable operation of each unit by adjusting operating parameters, so that the entire system is always in the optimal state of energy utilization, improving the overall heat recovery efficiency, reducing cooling water consumption, and ensuring that the viscosity index of the regenerated lubricating oil base oil is stable at 95-135 and the sulfur content is controlled at 1-10 mg / kg.

[0074] Example 1:

[0075] Waste engine oil generated by auto repair shops was selected as the raw material for treatment. Its viscosity was 120 cSt / 40℃, sulfur content was 1200 mg / kg, metal impurity content was 0.8%, and the treatment capacity was 5 t / h.

[0076] Step S1: Pump 5t / h of waste engine oil raw material into the pickling reactor, control the reactor temperature at 40℃, add 8% sulfuric acid solution (oil-to-acid volume ratio 10:1), stir at 150rpm for 45min, utilize the sulfuric acid to react with Fe, Cu and other metallic impurities and colloids in the raw material to generate soluble sulfate precipitates; the pickling mixture flows by gravity into the alkaline washing reactor, add 5% sodium hydroxide solution, stir at 200rpm, control the reactor temperature at 70℃, and perform alkaline washing for 30min to neutralize residual sulfuric acid and convert organic acids into water-soluble saponifications; after alkaline washing, the mixture is transported to a cone-bottom settling tank, allowed to settle at room temperature for 6h, solid particles, metal salts and other slag phases settle to the bottom of the tank, the slag phase is discharged once every 24h and sent to the hazardous waste disposal center, the upper clear oil phase is the preliminarily purified oil phase material.

[0077] Step S2: Pump the oil phase material from Step S1 into the primary distillation column, maintain atmospheric pressure operation, and control the top temperature of the column at 180℃ to remove moisture and light hydrocarbons; send the bottom material of the primary distillation column into the atmospheric distillation column, control the bottom temperature of the column at 350℃, and collect the medium fraction through the side stream in the middle of the column; send the heavy oil at the bottom of the atmospheric distillation column into the vacuum distillation column via a vacuum pump, maintain the absolute pressure at 5kPa and the bottom temperature at 380℃, cut off the heavy fraction, and send the remaining residue for external processing.

[0078] Step S3: After mixing the medium and heavy fractions from Step S2, the mixture is introduced into a medium-temperature spiral plate heat exchanger. The process heat of the 180°C steam at the top of the fractionation tower in Step S2 is used to preheat the material to 200°C. The preheated material enters a scraped film molecular distillation unit. The evaporation surface temperature is controlled at 280°C and the system vacuum is 5Pa. The scraped film rotor rotates at 300 rpm to make the material form a uniform film. The material stays on the evaporation surface for 20 seconds. Based on the difference in molecular free path, the high-purity base oil components are collected after being enriched on the condensation surface, and the residual residue oil is sent out as fuel.

[0079] Step S4: Mix the high-purity base oil component from step S3 with high-purity hydrogen at a hydrogen-to-oil ratio of 800:1, and introduce the mixture into a hydrogenation reactor. Control the reaction temperature at 350°C, the hydrogen pressure at 8 MPa, and the volumetric hourly space velocity at 1 h⁻¹ to complete desulfurization, denitrification, and unsaturated hydrocarbon saturation. The 350°C high-temperature stream after the hydrogenation reaction enters a high-temperature plate-and-shell heat exchanger. The cold side is fed with the purified oil phase from step S2, which is preheated from 50°C to 150°C. After heat exchange, the hydrogenation product undergoes gas-liquid separation to obtain regenerated lubricating oil base oil.

[0080] Step S5: The bottom stream from the vacuum distillation column in Step S2, the residual residue from the molecular distillation in Step S3, and the heat-exchanged stream from the hydrogenation product in Step S4 are collected and enter the gradient cooling system. The first stage of the cooling system uses circulating hot water at 110°C to cool the mixture stream from an average temperature of approximately 300°C to 120°C. The second stage uses circulating cooling water at 30°C to cool the stream from 120°C to 60°C. The third stage uses chilled water at 7°C to finally cool the stream to 25°C before it enters the storage tank. At the same time, the cold energy of the 40°C low-temperature stream discharged from the cold trap of the molecular distillation unit in Step S3 is recovered, and the washing water from the alkaline washing process in Step S1 is cooled from 60°C to 50°C through a shell-and-tube heat exchanger in the low-temperature section.

[0081] Step S6: Collect inlet and outlet temperature, pressure, flow rate and composition information of each process unit in real time, and dynamically adjust the opening of bypass valves, cooling medium flow rate and pumping power of each heat exchanger to ensure that the multi-stage coupled heat exchange network and gradient cooling system operate in the optimal energy efficiency state.

[0082] Comparative Example 1:

[0083] This comparative example uses a traditional waste mineral oil treatment process, without multi-stage coupled heat exchange, gradient cooling, or intelligent energy collaborative control. Each process unit operates independently, and the raw materials processed are the same as those in Example 1.

[0084] Step S1: A pretreatment unit consisting of a horizontal settling tank and a bag filter is used. 5t / h of waste oil is directly pumped into the horizontal settling tank and allowed to settle at room temperature for 12 hours to remove solid particles with a diameter >100μm. After settling, the material is filtered through a bag filter to obtain a coarse purified oil phase.

[0085] Step S2: Using an atmospheric distillation column, the crude purified oil phase from step S1 is pumped into the atmospheric distillation column, and the bottom temperature of the column is raised to 380°C by an external steam heater to separate the mixed fraction.

[0086] Step S3: Using a scraped-film molecular distillation apparatus, the mixed fraction from S2 is directly fed into the molecular distillation apparatus. An additional electric heating system needs to be turned on to raise the material from room temperature to 280°C. Finally, the base oil components and residual residue oil are obtained.

[0087] Step S4: The base oil components from step S3 are mixed with hydrogen and then introduced into the reactor under the same reaction conditions as in Example 1; however, the high-temperature stream at 350°C after hydrogenation is directly introduced into the cooling water cooler without any residual heat recovery.

[0088] Step S5: Single-stage cooling water and chilled water are used for cooling. The hydrogenation product is directly cooled to 60°C with 30°C circulating cooling water and then cooled to 25°C with 7°C chilled water. There is no molecular distillation cold energy recovery process, and the cold energy is directly discharged.

[0089] Experimental Example 1:

[0090] The heat recovery rate is obtained by measuring and calculating the percentage of heat recovered and utilized through the multi-stage coupled heat exchange network relative to the total recoverable heat of the system. The base oil recovery rate is calculated by the ratio of the mass of the regenerated lubricating oil base oil to the input waste mineral oil feedstock.

[0091] The total amount of circulating cooling water consumed within 72 hours to maintain the process flow of each unit at the specified temperature is calculated and converted into the cooling water consumption per unit of raw material.

[0092] The viscosity of the base oil of the recycled lubricating oil was determined by GB / T 1995-1998 "Calculation Method of Viscosity Index of Petroleum Products"; the sulfur content of the base oil was determined by GB / T 34100-2017 "Determination of Total Sulfur Content in Light Hydrocarbons, Engine Fuels and Other Oils by Ultraviolet Fluorescence Method".

[0093] Table 1 Test Results

[0094] Indicators Example 1 Comparative Example 1 Heat recovery rate (%) 71.5 33.8 Base oil yield (%) 56.8 50.5 Cooling water consumption (t / t feedstock) 8.2 11.8 Base oil viscosity index 1 16 85 Base oil sulfur content (mg / kg) 5 mg / kg 18 mg / kg surface

[0095] The heat recovery rate of Example 1 reached 71.5%, significantly higher than that of Comparative Example 1 (33.8%). The core difference stems from the multi-stage coupled heat exchange design of the process. Example 1 utilizes the process heat of the top steam of the fractionation tower to preheat the molecular distillation feed and the high-temperature waste heat of the hydrogenation outlet to preheat the fractionation feed. At the same time, the first-stage circulating hot water of the gradient cooling system recovers the heat of the high-temperature feed, forming a closed-loop energy network of heat generation and heat utilization. In contrast, Comparative Example 1 adopts an independent heating / cooling mode for each unit, without a waste heat recovery link. The high-temperature heat from fractionation and hydrogenation is directly discharged through cooling water, resulting in a significant reduction in the utilization rate of recoverable heat. This demonstrates the role of multi-stage coupled heat exchange in improving system energy efficiency.

[0096] The base oil yield of Example 1 was 56.8%, an increase of 6.3 percentage points compared to 50.5% of Comparative Example 1. This difference is directly related to the refinement of pretreatment and fractionation. Example 1, through a combination of acid washing, alkali washing, and sedimentation, deeply removed ineffective components such as metallic impurities and colloids, avoiding component cracking losses caused by impurities in subsequent processes. At the same time, the three-stage fractionation tower achieved precise cutting of light, medium, and heavy fractions, and the preheated feed design of molecular distillation reduced the excessive heating loss of high-quality components. In contrast, Comparative Example 1 only used simple sedimentation filtration, which did not remove impurities thoroughly. The mixed fraction cutting accuracy of single-tower fractionation was low, and direct heating in molecular distillation easily triggered recombination and decomposition, ultimately leading to a decrease in base oil yield.

[0097] The cooling water consumption in Example 1 was 8.2 t / t of raw material, which was 30.5% lower than the 11.8 t / t of raw material in Comparative Example 1. This is a direct result of the gradient cooling and cold energy recovery design. Example 1 adopted a three-stage gradient cooling system. The first stage used 110℃ circulating hot water to recover the heat of the high-temperature material, reducing the cooling water demand in the subsequent medium and low temperature stages. At the same time, the low-temperature cold energy of the molecular distillation cold trap was recovered for the pretreatment alkaline washing water cooling, further reducing the load on the cooling system. In contrast, Comparative Example 1 adopted a single-stage cooling mode, where the high-temperature material was directly cooled by low-temperature cooling water, and there was no cold energy recovery process, resulting in a significant increase in cooling water consumption. This demonstrates the optimization effect of gradient cooling on resource consumption.

[0098] The base oil viscosity index of Example 1 reached 116, which is much higher than that of Comparative Example 1 (85). This difference stems from the process's retention of high-quality components and deep removal of impurities: the refined pretreatment of Example 1 removed components such as gum and metallic impurities that would lower the viscosity index, and the three-stage fractionation and molecular distillation accurately separated the high viscosity index base oil components. The stable reaction conditions during hydrorefining ensured the full saturation of unsaturated hydrocarbons. In contrast, Comparative Example 1 had more residual impurities after pretreatment, and the mixed fraction from the single-tower fractionation contained more light components with low viscosity index. During the hydrorefining process, the catalyst activity was insufficient due to impurity interference, resulting in a lower base oil viscosity index.

[0099] The base oil in Example 1 had a sulfur content of only 5 mg / kg, far lower than the 18 mg / kg in Comparative Example 1. The core reason is the deep desulfurization synergistic design of the process. The acid washing process in Example 1 has initially removed some sulfur-containing gums. During the hydrorefining process, the sulfides have been fully converted by hydrogenation through stable reaction temperature, pressure and efficient catalyst. In contrast, Comparative Example 1 did not have fine pretreatment. A large amount of sulfur-containing impurities in the raw material directly entered the hydrogenation process, which not only increased the desulfurization load on the catalyst, but also easily led to catalyst poisoning. In the end, the hydrodesulfurization effect was poor, and the sulfur content was much higher than that in Example 1.

[0100] In use, the multi-stage coupled heat exchange and gradient cooling energy-saving synergistic processing system of this invention includes a pretreatment and refining unit, a fractionation and cutting unit, a molecular distillation unit, a hydrorefining unit, a multi-stage coupled heat exchange network, a gradient cooling system, and a central control system. The pretreatment and refining unit consists of an acid washing reactor, an alkali washing reactor, and a cone-bottom settling tank, with each unit connected via acid- and alkali-resistant pipelines. The fractionation and cutting unit includes a primary distillation column, an atmospheric distillation column, and a vacuum distillation column connected in series, with the top steam pipeline connected to the hot-side inlet of the intermediate-temperature section heat exchanger. The molecular distillation unit adopts a scraped-film structure, and its cold trap outlet is connected to the alkali washing water storage tank of the pretreatment and refining unit through the tube side of the low-temperature section heat exchanger. The reactor outlet pipeline of the hydrorefining unit first connects to the hot side of the high-temperature section heat exchanger to preheat the fractionation feed before it passes through the three-stage cooling loop of the gradient cooling system into the product storage tank. The three types of heat exchangers in the multi-stage coupled heat exchange network are strictly zoned according to temperature ranges to avoid cross-contamination and thermal stress concentration. The gradient cooling system consists of three independent circulation loops, each equipped with a dedicated pump set, heat exchanger, and temperature control valve. The central control system communicates with all field instruments and actuators via industrial Ethernet to achieve coordinated optimization of energy and material flow throughout the plant.

[0101] Based on the preferred embodiments of the present invention described above, those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification, but must be determined according to the scope of the claims.

Claims

1. A method for energy-saving synergistic treatment of waste mineral oil through multi-stage coupled heat exchange and gradient cooling, characterized in that: Includes the following steps: S1. The waste mineral oil feedstock is pretreated and refined to obtain a preliminarily purified oil phase material; S2. The oil phase material is fed into a fractionation and cutting unit and cut under normal or reduced pressure to separate light, medium and heavy fractions; S3. The medium and heavy fractions are introduced into a molecular distillation unit for short-path distillation to obtain high-purity base oil components and residual residue oil; The process heat generated by the fractionation and cutting unit is used to preheat the material entering the molecular distillation unit; S4, the high-purity base oil component is sent to the hydrorefining unit for catalytic hydrogenation reaction to obtain regenerated lubricating oil base oil; and the high-temperature waste heat of the hydrorefining unit outlet stream is used to preheat the feed to the fractionation and cutting unit. S5. The high-temperature process products generated by the fractionation and cutting unit, molecular distillation unit, and hydrorefining unit are subjected to gradient cooling, including: first-stage cooling to reduce the material temperature to 115-125℃, second-stage cooling to 55-65℃, and third-stage cooling to 15-30℃; at the same time, the low-temperature cold energy generated by the molecular distillation unit is recovered and used in the pretreatment refining process; S6. The process parameters of each step are acquired in real time through the central control system, and the heat exchange and cooling operations in steps S3, S4, and S5 are dynamically adjusted based on the energy balance model.

2. The method for energy-saving synergistic treatment of waste mineral oil through multi-stage coupled heat exchange and gradient cooling according to claim 1, characterized in that: In step S1, the pretreatment purification includes: acid washing with a sulfuric acid solution with a mass concentration of 5-10% and an oleic acid volume ratio of 8-12:1 for a reaction time of 30-60 min; alkaline washing with a sodium hydroxide solution with a mass concentration of 3-6% for a reaction temperature of 60-80℃; and sedimentation separation in a cone-bottom settling tank for a settling time of 4-8 h, with bottom slag discharge every 24 h.

3. The method for energy-saving synergistic treatment of waste mineral oil through multi-stage coupled heat exchange and gradient cooling according to claim 1, characterized in that: In step S2, the fractionation and cutting unit includes a primary distillation column, an atmospheric distillation column, and a vacuum distillation column; the preliminarily purified oil phase material is sequentially passed through the primary distillation column, the atmospheric distillation column, and the vacuum distillation column for three-stage series fractionation; the primary distillation column operates at atmospheric pressure, and the top temperature is controlled at 170-190℃; the bottom temperature of the atmospheric distillation column is 340-360℃, and the middle fraction is collected from the side stream; the vacuum distillation column operates at an absolute pressure of 4-6 kPa, and the bottom temperature is 370-390℃, and is used to cut the heavy fraction.

4. The method for energy-saving synergistic treatment of waste mineral oil through multi-stage coupled heat exchange and gradient cooling according to claim 1, characterized in that: In step S3, the short-path distillation is carried out at an evaporation surface temperature of 250-320℃, and the material remains on the evaporation surface for 10-30 seconds.

5. The method for energy-saving synergistic treatment of waste mineral oil through multi-stage coupled heat exchange and gradient cooling according to claim 1, characterized in that: In step S4, the reaction temperature for catalytic hydrogenation is 300-400℃, the hydrogen pressure is 5-15MPa, and the catalyst used is a nickel-molybdenum or cobalt-molybdenum catalyst supported on γ-alumina.

6. The method for energy-saving synergistic treatment of waste mineral oil through multi-stage coupled heat exchange and gradient cooling according to claim 1, characterized in that: In steps S3 and S4, the preheating operation is achieved through a multi-stage coupled heat exchange network; the network includes: a high-temperature plate-and-shell heat exchanger for step S4, a medium-temperature spiral plate heat exchanger for step S3, and a low-temperature shell-and-tube heat exchanger for recovering the low-temperature cold energy.

7. The method for energy-saving synergistic treatment of waste mineral oil through multi-stage coupled heat exchange and gradient cooling according to claim 1, characterized in that: In step S5, the first, second, and third stage cooling media correspond to circulating hot water, circulating cooling water, and chilled water, respectively.

8. The method for energy-saving synergistic treatment of waste mineral oil through multi-stage coupled heat exchange and gradient cooling according to claim 1, characterized in that: In step S5, the temperature of the circulating hot water is 100-110℃, the temperature of the circulating cooling water is controlled below 35℃, and the temperature of the chilled water is 6-8℃.

9. The method for energy-saving synergistic treatment of waste mineral oil through multi-stage coupled heat exchange and gradient cooling according to claim 1, characterized in that: In step S5, the low-temperature cooling energy used to cool the pretreatment refining process is specifically used to reduce the temperature of the washing water in the alkaline washing process.

10. The method for energy-saving synergistic treatment of waste mineral oil through multi-stage coupled heat exchange and gradient cooling according to claim 1, characterized in that: The recycled lubricating oil base oil obtained by the treatment method has a viscosity index of 95-135 and a sulfur content of 1-10 mg / kg.

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