A method for separating and preparing light liquid paraffin

Abstract

The present invention belongs to the technical field of separation and purification of chemical raw materials, and relates to a method for separating and preparing light liquid paraffin, the method comprising: performing vacuum dehydration treatment on waxy crude oil to obtain a dehydrated product; performing three-stage countercurrent extraction on the dehydrated product using an extraction solvent, collecting the three-stage extraction phase obtained by the last stage of countercurrent extraction; performing two-stage vacuum distillation on the three-stage extraction phase, collecting the top discharge fraction of the last stage of vacuum distillation, which is the target fraction; performing gradient cooling crystallization on the target fraction, obtaining a crystalline product after centrifugal separation, and vacuum drying the crystalline product to obtain light liquid paraffin. The present invention forms a multi-dimensional collaboratively optimized process system through vacuum dehydration, three-stage countercurrent extraction, two-stage vacuum distillation and gradient cooling crystallization, and obtains a crystalline product with a concentrated particle size distribution and a purity of ≥99%. The yield is increased by 15-20% compared with the traditional process, thereby realizing efficient and green preparation of light liquid paraffin.

Description

Technical Field

[0001] The invention belongs to the technical field of separation and purification of chemical raw materials, and relates to a method for separating and preparing light liquid paraffin. Background Art

[0002] Light liquid paraffin, as an important industrial raw material, is widely used in cosmetics, pharmaceuticals, lubricants, fine chemicals and other fields. The efficiency and purity of its separation and purification process directly affect the quality and cost of downstream products. At present, the industry mainly adopts the separation method of solvent extraction combined with distillation, but the existing technology has significant limitations: traditional extraction systems often rely on a single solvent (such as petroleum ether or n-hexane), which has insufficient solubility selectivity for long-chain hydrocarbons and paraffin, resulting in low extraction efficiency and high solvent residues; at the same time, the multi-stage distillation process often causes azeotropic or thermal decomposition of light paraffin and heavy components due to unreasonable design of pressure and temperature parameters, making it difficult to ensure product purity. In addition, some processes introduce surfactants or polymers to assist extraction. Although this can improve emulsification problems, it is easy to introduce impurities or increase the difficulty of subsequent separation, further increasing production costs.

[0003] In recent years, researchers have attempted to improve separation efficiency by optimizing solvent formulations or modifying distillation gradients, but existing solutions still face key challenges. For example, when using complex solvents, imbalanced component ratios can easily lead to phase equilibrium disturbances, affecting extraction selectivity. In multi-stage vacuum distillation, the overhead fraction collection location does not match the pressure gradient, often causing the target product to mix with high-boiling-point residual liquid, requiring additional purification steps. Furthermore, traditional processes require stringent raw material pretreatment, and the dehydration step is time-consuming and prone to volatilization losses of light components, further reducing overall energy efficiency. Summary of the Invention

[0004] In view of the shortcomings of the existing technology, the purpose of the present invention is to provide a method for separating and preparing light liquid paraffin. The light liquid paraffin separation and preparation process proposed in the present invention forms a multi-dimensional collaboratively optimized process system through vacuum dehydration, three-stage countercurrent extraction, two-stage reduced pressure distillation and gradient cooling crystallization, and obtains a crystal product with concentrated particle size distribution and purity ≥99%. The yield is increased by 15~20% compared with the traditional process, realizing the efficient and green preparation of light liquid paraffin.

[0005] To achieve this object, the present invention adopts the following technical solutions:

[0006] The present invention provides a method for separating and preparing light liquid paraffin, which comprises:

[0007] (I) vacuum dehydrating the waxy crude oil to obtain a dehydrated product; subjecting the dehydrated product to three-stage countercurrent extraction using an extraction solvent, and collecting the three-stage extract phase obtained from the last stage of countercurrent extraction;

[0008] (II) subjecting the three-stage extraction phase of step (I) to two-stage vacuum distillation, collecting the top fraction discharged from the last stage of vacuum distillation as the target fraction;

[0009] (III) subjecting the target fraction of step (II) to gradient cooling crystallization, obtaining a crystalline product after centrifugal separation, and vacuum drying the crystalline product to obtain light liquid paraffin.

[0010] The light liquid paraffin separation and preparation process proposed in the present invention forms a multi-dimensional collaborative optimization process system through vacuum dehydration, three-stage countercurrent extraction, two-stage vacuum distillation and gradient cooling crystallization. First, vacuum dehydration technology is used to accurately remove free water from waxy crude oil, avoiding the volatilization loss of light components caused by traditional high-temperature dehydration, and at the same time creating a stable phase interface environment for subsequent extraction. The three-stage countercurrent extraction process innovatively combines gradient temperature, pressure and a multi-component synergistic solvent system to gradually enhance the mass transfer driving force, thereby increasing the concentration of the target component in the three-stage extraction phase by 4 to 6 times, and controlling the solvent residue to below 0.3%. The two-stage vacuum distillation achieves narrow fraction cutting through segmented pressure gradients, accurately removing low-boiling light components (hydrocarbons below C10) and high-boiling heavy components (C15+ hydrocarbons), and avoiding thermal decomposition of the target component. The gradient cooling crystallization process forms a crystal product with concentrated particle size distribution and purity ≥99% through temperature control rate, ultrasound-assisted nucleation and nitrogen protection. The yield is increased by 15~20% compared with the traditional process, realizing the efficient and green preparation of light liquid paraffin.

[0011] Vacuum dehydration not only effectively removes free water from crude oil but, more importantly, creates a stable interphase environment for subsequent extraction. If trace amounts of water in waxy crude oil are not adequately removed, they can form an emulsion with the organic solvent during the extraction process, making phase separation difficult and reducing the yield of the target product. This process utilizes vacuum dehydration, which not only ensures efficient dehydration but also avoids the volatilization of light components in the crude oil due to high temperatures. This ensures that the feedstock entering the extraction system has an appropriate moisture content and thermal stability, laying the foundation for efficient mass transfer in tertiary countercurrent extraction.

[0012] The three-stage countercurrent extraction process achieves efficient enrichment of the target components through progressively enhanced mass transfer driving forces. Compared to traditional single-stage extraction, countercurrent operation enables dynamic contact between the extraction solvent and the feed liquid in the direction of flow, maximizing the solvent's solubility. The process parameters (such as temperature, pressure, and liquid-liquid ratio) of each extraction tower are gradient-optimized: the first stage uses a lower extraction temperature (45-55°C) and atmospheric pressure (0.01-0.02MPa) to maintain the liquid stability of the solvent and reduce the degradation of heat-sensitive components; as the number of extraction stages increases, the temperature and spray volume are gradually increased to enhance mass transfer efficiency while avoiding excessive dilution. At the same time, the extraction solvent used in the present invention also exerts a synergistic effect: the mixed solvent system of petroleum ether and n-hexane achieves selective dissolution of paraffin molecules by adjusting the ratio of polar and non-polar components; modified nano-silica is used as a surfactant, and significantly reduces the oil-water interfacial tension through modification with a silane coupling agent, thereby promoting the dynamic balance of the emulsified interface; PEG-400 controls the separation rate of the extraction phase by adjusting the solvent viscosity. The synergistic effect of multiple components in the extraction solvent increases the concentration of the target product in the tertiary extraction phase by 3 to 5 times compared with the raw material liquid, providing a high-concentration feed basis for the subsequent distillation process.

[0013] The two-stage vacuum distillation process achieves efficient separation of light and heavy components. The first stage operates at a moderate vacuum of -0.06 to -0.08 MPa, with the bottom temperature controlled at 65-75°C. This primarily removes low-boiling light components (such as hydrocarbons below C10) while preventing excessive volatility of light liquid paraffin. The second stage further reduces the pressure to -0.08 to -0.1 MPa, raising the bottom temperature to 85-95°C, focusing on separating high-boiling heavy components (such as C15+ hydrocarbons). The two-stage vacuum distillation design leverages the differences in component volatility at different pressures, achieving narrow fraction separation through staged interception.

[0014] The gradient cooling crystallization process, used as the final purification step, achieves directional crystal growth and efficient impurity removal through the coordinated regulation of temperature, stirring, ultrasound, and inert gas protection. During the pre-cooling stage (25-35°C → 10-20°C), moderate-intensity stirring (200-300 rpm) is combined to promote uniform cooling of the target fraction and the formation of uniform crystal nuclei. During the crystallization stage, an ultrasonic field (200-300W) and gradient cooling (0.8-1.2°C / min → 0.4-0.5°C / min) are used. The cavitation effect of ultrasound breaks down the energy barrier of the supersaturated solution, inducing the uniform nucleation of fine crystal nuclei while avoiding the formation of inclusions caused by the precipitation of oversized crystals. The nitrogen protective atmosphere effectively suppresses oxidation reactions, ensuring the chemical purity of the crystals. During the final cooling stage (-18~-20℃), slow cooling (0.5~1℃ / min) and long-term heat preservation (80~90min) provide sufficient growth space for the crystals, forming a crystal form with concentrated particle size distribution and dense structure. This multi-physical field synergistic crystallization process increases the yield of the final product by 15~20%, the crystal purity is as high as over 99.5%, and the residual impurities in the mother liquor are significantly lower than the industry standard.

[0015] As a preferred technical solution of the present invention, in step (I), the temperature of the vacuum dehydration is 80-90°C, for example, it can be 80°C, 81°C, 82°C, 83°C, 84°C, 85°C, 86°C, 87°C, 88°C, 89°C or 90°C, but is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0016] In some optional examples, the vacuum dehydration time is 3 to 4 hours, for example, it can be 3.0 hours, 3.1 hours, 3.2 hours, 3.3 hours, 3.4 hours, 3.5 hours, 3.6 hours, 3.7 hours, 3.8 hours, 3.9 hours or 4.0 hours, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0017] As a preferred technical solution of the present invention, in step (I), the operation process of the three-stage countercurrent extraction includes:

[0018] (a) preheating the dehydrated product and feeding it from the bottom of a primary extraction tower, spraying the extraction solvent from the top of the primary extraction tower, and extracting the dehydrated product and the extraction solvent in countercurrent contact within the primary extraction tower. After the extraction is completed, collecting the primary extract phase from the top of the tower;

[0019] (b) preheating the primary extraction phase and feeding it from the bottom of a secondary extraction tower, spraying the extraction solvent from the top of the secondary extraction tower, and extracting the primary extraction phase and the extraction solvent in countercurrent contact within the secondary extraction tower. After the extraction is completed, collecting the secondary extraction phase from the top of the tower;

[0020] (c) The secondary extraction phase is preheated and fed into the bottom of a tertiary extraction tower. The extraction solvent is sprayed from the top of the tertiary extraction tower. The secondary extraction phase and the extraction solvent are contacted and extracted in countercurrent in the tertiary extraction tower. After the extraction is completed, the tertiary extraction phase is collected from the top of the tower.

[0021] The present invention realizes efficient enrichment of target components and step-by-step separation of impurities by connecting a primary extraction tower, a secondary extraction tower and a tertiary extraction tower in series. In the first-stage extraction, the waxy crude oil is preheated and then countercurrently contacted with a low-temperature extraction solvent under normal pressure. At this time, the modified nano-silica is fully dispersed in the petroleum ether / n-hexane extraction solvent, and the lipophilic groups modified by the silane coupling agent are adsorbed on the oil-water interface, significantly reducing the interfacial tension and forming a stable emulsified interface. This interfacial activation effect enables the target paraffin molecules C10-C15 to still maintain a high solubility under low temperature conditions, while avoiding the loss of light components due to volatilization. The dehydrated product is in countercurrent contact with the extraction solvent, and the target component is initially enriched in the first extraction phase through sufficient mixing of the liquid-liquid phase.

[0022] Secondary extraction further improves mass transfer efficiency based on primary extraction. The extraction temperature is raised to 55-65°C, enhancing molecular thermal motion while maintaining the stability of the extraction solvent, thereby increasing the diffusion rate of the target component in the extraction solvent. At the same time, the extraction pressure is moderately reduced to -0.04-0.05 MPa to promote vapor-liquid equilibrium between the extraction solvent and the target component. At this point, the volume ratio of the primary extraction phase to the extraction solvent is reduced to 1:(2-3), increasing the concentration of the target component in the secondary extraction phase by 1-2 times through concentration, while reducing the subsequent distillation load. The dynamic equilibrium system formed within the secondary extraction tower not only maintains a high mass transfer rate, but also begins to separate heavy component impurities through the synergistic effect of pressure and temperature, laying the foundation for final purification.

[0023] The tertiary extraction, as the terminal of the tertiary countercurrent extraction, is primarily used to remove trace impurities and control solvent residue. The temperature of the tertiary extraction is further increased to 60-70°C, and the extraction pressure is further reduced to -0.05-0.06 MPa, creating a strong driving force for mass transfer. The modified nanosilica in the extraction solvent continues to play an interfacial regulatory role during this stage, while the viscosity-regulating function of PEG-400 significantly increases the phase separation rate, avoiding backmixing caused by excessive viscosity. Through extreme concentration, the concentration of the target component in the tertiary extraction phase is further increased to 4-6 times, and the residual solvent content is controlled below 0.3%. The high-concentration environment formed within the tertiary extraction tower, combined with the synergistic effects of temperature and pressure, enables the final concentration of the target component to reach over 90%. Vacuum degassing technology effectively removes residual bubbles, ensuring the purity of the tertiary extraction phase.

[0024] The present invention optimizes the components and ratios of the extraction solvent. The mixed solvent system of petroleum ether and n-hexane achieves selective dissolution of paraffin molecules by adjusting the ratio of polar and non-polar components; modified nano-silica promotes the dynamic equilibrium of the emulsification interface by reducing the oil-water interfacial tension; PEG-400 controls the separation rate of the extraction phase by adjusting the solvent viscosity. The synergistic effect of these components in the three-stage countercurrent extraction process increases the extraction rate of the target product to more than 90%, which is about 25% higher than the traditional single-stage extraction, while the solvent residue is controlled below 0.5%.

[0025] As a preferred technical solution of the present invention, in step (a), the preheating temperature of the dehydrated product is 40-50°C, for example, 40°C, 41°C, 42°C, 43°C, 44°C, 45°C, 46°C, 47°C, 48°C, 49°C or 50°C, but is not limited to the listed values, and other values ​​not listed within this numerical range are also applicable.

[0026] In some optional examples, in the primary extraction tower, the volume ratio of the dehydrated product to the extraction solvent is 1:(3~4), for example, it can be 1:3.0, 1:3.1, 1:3.2, 1:3.3, 1:3.4, 1:3.5, 1:3.6, 1:3.7, 1:3.8, 1:3.9 or 1:4.0, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0027] The present invention specifically limits the volume ratio of the dehydrated product to the extraction solvent to 1:(3-4). Within this volume ratio range, the interfacial renewal rate and mass transfer coefficient are optimally balanced during countercurrent contact between the dehydrated product and the extraction solvent. This allows the modified nano-silica to fully utilize its steric hindrance in the petroleum ether / n-hexane solvent system, effectively suppressing emulsification. Furthermore, the solubility of the extraction solvent for heavy components (such as C15+ hydrocarbons) remains low, preventing high-boiling-point impurities from prematurely entering the primary extraction phase and reducing the separation burden of subsequent vacuum distillation.

[0028] When the amount of extraction solvent used is below the lower limit of the range defined in the present invention, the insufficient amount of extraction solvent will cause the saturation of the target components C10-C15 in the solvent phase to quickly reach the upper limit, weakening the driving force for mass transfer. In this case, although the extraction solvent can quickly dissolve some light paraffin wax during the initial contact with the dehydrated product, as the extraction process progresses, the concentration of the target component in the extraction solvent increases, and the concentration difference between the target component and the dehydrated product decreases, resulting in a significant decrease in subsequent mass transfer efficiency. In addition, insufficient extraction solvent will exacerbate the viscosity difference between the two phases, especially in the case of high viscosity waxy crude oil, which can easily form oil-in-water emulsion droplets, increasing the difficulty of phase separation.

[0029] When the amount of extraction solvent used exceeds the upper limit of the range defined herein, the excess solvent dilutes the concentration gradient of the target component in the extraction phase, weakening the driving force for mass transfer and reducing the extraction load per unit volume of extraction solvent. Furthermore, an excess of extraction solvent prolongs the phase separation time, blurring the emulsion interface formed within the tertiary extraction column, leading to entrainment of the target product in the extraction phase and reducing product yield.

[0030] In some optional examples, the spray rate of the extraction solvent in the primary extraction tower is 1~1.5L / min, for example, it can be 1L / min, 1.05L / min, 1.1L / min, 1.15L / min, 1.2L / min, 1.25L / min, 1.3L / min, 1.35L / min, 1.4L / min, 1.45L / min or 1.5L / min, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0031] In some optional examples, the residence time of the dehydrated product in the primary extraction tower is 40 to 50 min, for example, 40 min, 41 min, 42 min, 43 min, 44 min, 45 min, 46 min, 47 min, 48 min, 49 min or 50 min, but is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0032] In some optional examples, the temperature inside the primary extraction tower is 45~55°C, for example, it can be 45°C, 46°C, 47°C, 48°C, 49°C, 50°C, 51°C, 52°C, 53°C, 54°C or 55°C, but is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0033] In some optional examples, the pressure inside the primary extraction tower is 0.01~0.02MPa, for example, it can be 0.01MPa, 0.011MPa, 0.012MPa, 0.013MPa, 0.014MPa, 0.015MPa, 0.016MPa, 0.017MPa, 0.018MPa, 0.019MPa or 0.02MPa, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0034] In some optional examples, in step (b), the preheating temperature of the primary extraction phase is 50-60°C, for example, it can be 50°C, 51°C, 52°C, 53°C, 54°C, 55°C, 56°C, 57°C, 58°C, 59°C or 60°C, but is not limited to the listed values, and other values ​​not listed within this numerical range are also applicable.

[0035] In some optional examples, in the secondary extraction tower, the volume ratio of the primary extraction phase to the extraction solvent is 1:(2~3), for example, it can be 1:2.0, 1:2.1, 1:2.2, 1:2.3, 1:2.4, 1:2.5, 1:2.6, 1:2.7, 1:2.8, 1:2.9 or 1:3.0, but is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0036] In some optional examples, the spray rate of the extraction solvent in the secondary extraction tower is 1~2L / min, for example, it can be 1.0L / min, 1.1L / min, 1.2L / min, 1.3L / min, 1.4L / min, 1.5L / min, 1.6L / min, 1.7L / min, 1.8L / min, 1.9L / min or 2.0L / min, but is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0037] In some optional examples, the residence time of the primary extraction phase in the secondary extraction tower is 70 to 80 min, for example, it can be 70 min, 71 min, 72 min, 73 min, 74 min, 75 min, 76 min, 77 min, 78 min, 79 min or 80 min, but is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0038] In some optional examples, the temperature inside the secondary extraction tower is 55~65°C, for example, it can be 55°C, 56°C, 57°C, 58°C, 59°C, 60°C, 61°C, 62°C, 63°C, 64°C or 65°C, but is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0039] In some optional examples, the pressure inside the secondary extraction tower is -0.04~-0.05MPa, for example, it can be -0.04MPa, -0.041MPa, -0.042MPa, -0.043MPa, -0.044MPa, -0.045MPa, -0.046MPa, -0.047MPa, -0.048MPa, -0.049MPa or -0.05MPa, but is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0040] In some optional examples, in step (c), the preheating temperature of the secondary extraction phase is 55-65°C, for example, it can be 55°C, 56°C, 57°C, 58°C, 59°C, 60°C, 61°C, 62°C, 63°C, 64°C or 65°C, but is not limited to the listed values, and other values ​​not listed within this numerical range are also applicable.

[0041] In some optional examples, in the three-stage extraction tower, the volume ratio of the secondary extraction phase to the extraction solvent is 1:(1.5~2.5), for example, it can be 1:1.5, 1:1.6, 1:1.7, 1:1.8, 1:1.9, 1:2.0, 1:2.1, 1:2.2, 1:2.3, 1:2.4 or 1:2.5, but is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0042] In some optional examples, the spray rate of the extraction solvent in the three-stage extraction tower is 1.5~2.5L / min, for example, it can be 1.5L / min, 1.6L / min, 1.7L / min, 1.8L / min, 1.9L / min, 2.0L / min, 2.1L / min, 2.2L / min, 2.3L / min, 2.4L / min or 2.5L / min, but is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0043] In some optional examples, the residence time of the secondary extraction phase in the tertiary extraction tower is 100~110min, for example, it can be 100min, 101min, 102min, 103min, 104min, 105min, 106min, 107min, 108min, 109min or 110min, but is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0044] In some optional examples, the temperature inside the three-stage extraction tower is 60~70°C, for example, it can be 60°C, 61°C, 62°C, 63°C, 64°C, 65°C, 66°C, 67°C, 68°C, 69°C or 70°C, but is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0045] In some optional examples, the pressure inside the three-stage extraction tower is -0.05~-0.06MPa, for example, it can be -0.05MPa, -0.051MPa, -0.052MPa, -0.053MPa, -0.054MPa, -0.055MPa, -0.056MPa, -0.057MPa, -0.058MPa, -0.059MPa or -0.06MPa, but is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0046] The present invention specifically limits the pressure within the three-stage extraction tower to -0.05 to -0.06 MPa. Within this pressure range, the boiling point of the extraction solvent drops to 20 to 30°C, ensuring efficient dissolution of paraffin molecules while avoiding phase separation difficulties caused by excessive vaporization. Modified nanosilica maintains a well-dispersed state in this negative pressure environment, and its lipophilic groups are firmly anchored to the oil-water interface via a silane coupling agent, forming a stable interfacial film that promotes the directional migration of target components into the solvent phase. Simultaneously, the synergistic effect of pressure (-0.05 to -0.06 MPa) and temperature (60 to 70°C) creates moderate turbulence within the three-stage extraction tower, enhancing the mixing intensity of the liquid-liquid phase without causing severe backmixing.

[0047] When the pressure inside the tertiary extraction tower is lower than -0.06MPa, the boiling point of the extraction solvent will be further reduced. Although this can theoretically increase the solubility of the target component, in actual operation it will cause the vaporization rate of the extraction solvent to increase sharply, and a large number of tiny bubbles will form in the tower. The violent disturbance of the bubbles will destroy the interfacial stability of the liquid-liquid phase, resulting in a decrease in the mass transfer coefficient of the target component in the solvent phase. In addition, excessive negative pressure will significantly increase the energy consumption of the vacuum system, and the vaporization amount of light components (such as C5-C8 hydrocarbons) in the extraction solvent will increase, which will carry some target components into the raffinate phase, resulting in a loss of product yield. At the same time, when the pressure is too low, the polarity balance of the extraction solvent is broken, and its solubility for high-boiling-point heavy components is enhanced, which will cause some C15+ hydrocarbons to enter the extraction phase prematurely, increasing the burden of subsequent vacuum distillation.

[0048] When the pressure inside the tertiary extraction tower is higher than -0.05MPa, although the operational risks caused by the vaporization of the extraction solvent can be reduced, the driving force for mass transfer will be greatly weakened. At this time, the concentration gradient of the target component in the solvent phase decreases, the diffusion rate decreases, and the extraction efficiency drops significantly. The increase in pressure will also increase the distribution coefficient of the heavy component in the extraction solvent, and some high-boiling point impurities (such as colloids and asphaltenes) can easily penetrate the extraction interface and enter the target phase, increasing the complexity of subsequent distillation. In addition, when the pressure is too high, the density difference between the extraction solvent and the raw material liquid decreases, the contact area between the two phases is reduced, and the dynamic efficiency of the countercurrent contact is reduced. The residence time needs to be extended to achieve the same extraction effect, which will increase the volume of the extraction tower and the equipment cost.

[0049] As a preferred technical solution of the present invention, in step (I), the extraction solvent includes petroleum ether, n-hexane, Tween 80, modified nano-silica and PEG-400.

[0050] In some optional examples, the modified nano-silica is obtained by modifying nano-silica with a silane coupling agent.

[0051] In some optional examples, the mass fraction of petroleum ether in the extraction solvent is 70~80wt%, for example, it can be 70wt%, 71wt%, 72wt%, 73wt%, 74wt%, 75wt%, 76wt%, 77wt%, 78wt%, 79wt% or 80wt%, but is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0052] In some optional examples, the mass fraction of n-hexane in the extraction solvent is 10~20wt%, for example, it can be 10wt%, 11wt%, 12wt%, 13wt%, 14wt%, 15wt%, 16wt%, 17wt%, 18wt%, 19wt% or 20wt%, but is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0053] In some optional examples, the mass fraction of Tween 80 in the extraction solvent is 1~5wt%, for example, it can be 1wt%, 1.5wt%, 2wt%, 2.5wt%, 3wt%, 3.5wt%, 4wt%, 4.5wt% or 5wt%, but is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0054] In some optional examples, the mass fraction of modified nano-silica in the extraction solvent is 1~2wt%, for example, it can be 1.0wt%, 1.1wt%, 1.2wt%, 1.3wt%, 1.4wt%, 1.5wt%, 1.6wt%, 1.7wt%, 1.8wt%, 1.9wt% or 2.0wt%, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0055] In some optional examples, the mass fraction of PEG-400 in the extraction solvent is 1~3wt%, for example, it can be 1.0wt%, 1.2wt%, 1.4wt%, 1.6wt%, 1.8wt%, 2.0wt%, 2.2wt%, 2.4wt%, 2.6wt%, 2.8wt% or 3.0wt%, but is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0056] As a preferred technical solution of the present invention, the extraction solvent is prepared by the following method:

[0057] (1) dispersing nano-silica in a silane coupling agent solution, ultrasonically dispersing, mixing, stirring, and heating, and then filtering, washing, and drying to obtain modified nano-silica;

[0058] (2) petroleum ether and n-hexane are put into a reactor, mixed, stirred and heated to obtain a homogeneous solution; Tween 80 and PEG-400 are added to the homogeneous solution, mixed, stirred and heated to obtain a premixed solution;

[0059] (3) Dispersing the modified nano-silica obtained in step (1) in the premixed solution obtained in step (2), mixing, stirring and heating, and then vacuum degassing to obtain the extraction solvent.

[0060] The present invention adopts a mixed solvent system consisting of petroleum ether and n-hexane to form a hydrophobic main framework. The mass fractions of the two achieve a balance between polarity and non-polarity. The long-chain alkane structure of petroleum ether gives the extraction solvent a high affinity for paraffin molecules, while the introduction of n-hexane moderately reduces the overall viscosity, avoiding the problem of excessive mass transfer resistance caused by the low polarity of traditional single extraction solvents. The combination of petroleum ether and n-hexane ensures strong solubility for the target components while alleviating the corrosiveness of the extraction solvent to the equipment through the dilution effect of n-hexane, thereby extending the service life of the equipment.

[0061] The present invention uses a silane coupling agent to modify the surface of nano-silica, so that its surface lipophilicity is significantly enhanced, and a stable adsorption layer can be formed at the oil-water interface. The nano-scale size gives it a huge specific surface area, which significantly improves the interfacial mass transfer efficiency, and converts the originally difficult-to-separate colloidal system into a stable suspension that is easy to demulsify.

[0062] As a nonionic surfactant, Tween 80's hydrophilic-lipophilic balance (HLB) complements the petroleum ether / n-hexane base solvent system, significantly accelerating the separation of the oil-water phase by reducing interfacial tension. The addition of PEG-400 modulates the viscosity of the extraction solvent, slowing the diffusion rate of the extraction phase while ensuring sufficient fluidity, thus providing the necessary kinetic window for mass transfer. The synergistic effect of Tween 80 and PEG-400 allows the extraction system to maintain a clear interface even under high-speed stirring, avoiding the phase separation difficulties associated with excessively high viscosity in traditional processes.

[0063] During the multi-stage countercurrent extraction process, the low-polarity environment created by petroleum ether and n-hexane preferentially dissolves the paraffin components in the dehydrated product. Modified nano-silica forms a selective permeability barrier through interfacial adsorption, preventing heavy components from entering the extraction phase. Tween 80 accelerates phase separation by reducing interfacial tension, and PEG-400 regulates the viscosity of the extraction solvent to maintain mass transfer dynamics. This multi-dimensional synergistic effect enables the progressive enrichment of the target product during the three-stage countercurrent extraction. Ultimately, the concentration of light liquid paraffin in the three-stage extraction phase increases by 4-6 times compared to the dehydrated product, while the residual solvent content is controlled below 0.3%.

[0064] As a preferred technical solution of the present invention, in step (1), the silane coupling agent solution consists of a silane coupling agent and an ethanol aqueous solution.

[0065] In some optional examples, the mass fraction of the silane coupling agent in the silane coupling agent solution is 3-4 wt%, for example, 3.0 wt%, 3.1 wt%, 3.2 wt%, 3.3 wt%, 3.4 wt%, 3.5 wt%, 3.6 wt%, 3.7 wt%, 3.8 wt%, 3.9 wt% or 4.0 wt%, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0066] In some optional examples, the ratio of the nano-silica to the silane coupling agent is (2-3) g:1L, for example, 2.0 g:1L, 2.1 g:1L, 2.2 g:1L, 2.3 g:1L, 2.4 g:1L, 2.5 g:1L, 2.6 g:1L, 2.7 g:1L, 2.8 g:1L, 2.9 g:1L or 3.0 g:1L, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0067] In some optional embodiments, the temperature for mixing and stirring the nano-silica and the silane coupling agent solution is 30-40°C, for example, 30°C, 31°C, 32°C, 33°C, 34°C, 35°C, 36°C, 37°C, 38°C, 39°C or 40°C, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0068] In some optional examples, the mixing and stirring time of the nano-silica and the silane coupling agent solution is 40 to 50 minutes, for example, it can be 40 minutes, 41 minutes, 42 minutes, 43 minutes, 44 minutes, 45 minutes, 46 minutes, 47 minutes, 48 ​​minutes, 49 minutes or 50 minutes, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0069] In some optional embodiments, in step (2), the temperature of the mixed stirring of petroleum ether and n-hexane is 25-35°C, for example, it can be 25°C, 26°C, 27°C, 28°C, 29°C, 30°C, 31°C, 32°C, 33°C, 34°C or 35°C, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0070] In some optional examples, the mixing time of the petroleum ether and n-hexane is 30 to 40 minutes, for example, it can be 30 minutes, 31 minutes, 32 minutes, 33 minutes, 34 minutes, 35 minutes, 36 minutes, 37 minutes, 38 minutes, 39 minutes or 40 minutes, but is not limited to the listed values, and other values ​​not listed within this numerical range are also applicable.

[0071] In some optional embodiments, the temperature of the mixing and stirring of the homogeneous solution, Tween 80 and PEG-400 is 40-50°C, for example, 40°C, 41°C, 42°C, 43°C, 44°C, 45°C, 46°C, 47°C, 48°C, 49°C or 50°C, but is not limited to the listed values, and other values ​​not listed within this numerical range are also applicable.

[0072] In some optional examples, the mixing and stirring time of the homogeneous solution, Tween 80 and PEG-400 is 50-60 min, for example, it can be 50 min, 51 min, 52 min, 53 min, 54 min, 55 min, 56 min, 57 min, 58 min, 59 min or 60 min, but is not limited to the listed values, and other values ​​not listed within this numerical range are also applicable.

[0073] In some optional embodiments, in step (3), the temperature for mixing and stirring the modified nano-silica and the premixed liquid is 50-60°C, for example, 50°C, 51°C, 52°C, 53°C, 54°C, 55°C, 56°C, 57°C, 58°C, 59°C or 60°C, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0074] In some optional examples, the mixing and stirring time of the modified nano-silica and the premixed liquid is 30 to 40 minutes, for example, it can be 30 minutes, 31 minutes, 32 minutes, 33 minutes, 34 minutes, 35 minutes, 36 minutes, 37 minutes, 38 minutes, 39 minutes or 40 minutes, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0075] As a preferred technical solution of the present invention, in step (II), the operation process of the two-stage vacuum distillation includes:

[0076] The three-stage extraction phase is sent to a primary vacuum distillation tower for primary vacuum distillation. After the vacuum distillation is completed, the residual liquid in the tower bottom is sent to a secondary vacuum distillation tower for secondary vacuum distillation. After the vacuum distillation is completed, the distillate discharged from the top condenser is collected, which is the target fraction.

[0077] The primary vacuum distillation operates under relatively mild conditions (bottom temperature 65-75°C, pressure -0.06-0.08 MPa), primarily targeting the removal of low-boiling-point impurities from the tertiary extraction phase. At this stage, the system pressure is significantly lower than atmospheric pressure, lowering the boiling point of light hydrocarbons (such as components below C10) to below 65°C. This effectively separates volatile components while preventing the volatilization of light liquid paraffin (boiling point approximately 150-200°C). The key to this stage is balancing separation efficiency and energy consumption. Excessively high pressure will result in residual low-boiling-point impurities, while excessively low pressure will require additional heating, increasing energy consumption.

[0078] Secondary vacuum distillation further reduces the operating pressure to -0.08-0.1 MPa, raising the bottom temperature to 85-95°C. The core purpose of this stage is to separate high-boiling-point heavy components (such as C15+ hydrocarbons), which remain in the primary vacuum distillation due to insufficient volatility. Secondary vacuum distillation uses even more extreme vacuum conditions to lower the boiling point of these high-boiling substances to below 95°C, achieving deep purification while avoiding thermal decomposition of the target product. Furthermore, the temperature of the secondary distillation tower's overhead condenser must be precisely controlled at 55-65°C to ensure that the target fraction is condensed and recovered in liquid form, while uncondensed gases are discharged through the vacuum system.

[0079] The synergistic effect of the two-stage vacuum distillation process is reflected in the matching of operating pressure and temperature. The first stage achieves efficient removal of low-boiling-point impurities through moderate pressure reduction, while simultaneously providing a highly concentrated target fraction feed for the second stage. The second stage, on the other hand, completely removes high-boiling-point residues through extreme pressure reduction, ensuring the purity of the final product. This graded vacuum distillation method avoids the surge in energy consumption caused by excessively low pressure, or the incomplete separation caused by excessively high pressure, which is common in traditional single-stage distillation.

[0080] In some optional examples, the bottom temperature of the first-stage vacuum distillation tower is 65-75°C, for example, it can be 65°C, 66°C, 67°C, 68°C, 69°C, 70°C, 71°C, 72°C, 73°C, 74°C or 75°C, but is not limited to the listed values, and other values ​​not listed within this numerical range are also applicable.

[0081] In some optional examples, the top temperature of the first-stage vacuum distillation tower is 35-45°C, for example, 35°C, 36°C, 37°C, 38°C, 39°C, 40°C, 41°C, 42°C, 43°C, 44°C or 45°C, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0082] In some optional examples, the pressure in the first-stage vacuum distillation tower is -0.06~-0.08 MPa, for example, it can be -0.06 MPa, -0.062 MPa, -0.064 MPa, -0.068 MPa, -0.07 MPa, -0.072 MPa, -0.074 MPa, -0.076 MPa, -0.078 MPa or -0.08 MPa, but is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0083] In some optional examples, the residence time of the tertiary extraction phase in the primary vacuum distillation tower is 40 to 50 min, for example, 40 min, 41 min, 42 min, 43 min, 44 min, 45 min, 46 min, 47 min, 48 min, 49 min or 50 min, but is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0084] In some optional examples, the bottom temperature of the secondary vacuum distillation tower is 85-95°C, for example, it can be 85°C, 86°C, 87°C, 88°C, 89°C, 90°C, 91°C, 92°C, 93°C, 94°C or 95°C, but is not limited to the listed values, and other values ​​not listed within this numerical range are also applicable.

[0085] In some optional examples, the top temperature of the secondary vacuum distillation tower is 55-65°C, for example, it can be 55°C, 56°C, 57°C, 58°C, 59°C, 60°C, 61°C, 62°C, 63°C, 64°C or 65°C, but is not limited to the listed values, and other values ​​not listed within this numerical range are also applicable.

[0086] In some optional examples, the pressure in the secondary vacuum distillation tower is -0.08~-0.1MPa, for example, it can be -0.08MPa, -0.082MPa, -0.084MPa, -0.086MPa, -0.088MPa, -0.09MPa, -0.092MPa, -0.094MPa, -0.096MPa, -0.098MPa or -0.1MPa, but is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0087] The present invention specifically limits the pressure within the secondary vacuum distillation column to -0.08 to -0.1 MPa. When the secondary vacuum distillation pressure exceeds -0.08 MPa, while still negative, the boiling point of the solvent and the target component is insufficiently depressed, significantly reducing the removal efficiency of high-boiling-point heavy components (such as C15+ hydrocarbons). A small amount of high-boiling-point impurities remains in the resulting target fraction. These impurities are difficult to completely remove during the subsequent cooling crystallization process due to their small solubility differences, ultimately resulting in reduced product purity. Furthermore, excessively high pressure can weaken the solvent's ability to dissolve the target component. In particular, if the distillation pressure is not sufficiently reduced when the tertiary extraction phase is already enriched with a high concentration of the target product, the azeotropic effect between the solvent and the target component can be exacerbated, leading to a complex vapor-liquid equilibrium and increasing the difficulty of distillation separation.

[0088] When the pressure of the secondary vacuum distillation is lower than -0.1MPa, although the boiling point of the solvent can be further reduced, on the one hand, the deep negative pressure significantly increases the requirements for tower sealing, pipe materials and vacuum system, and equipment investment and maintenance costs increase significantly. On the other hand, the solvent vaporizes too quickly at extremely low pressure, which can easily cause turbulence or mist entrainment, disrupting the fluid dynamics balance in the tower and even leading to the risk of flooding. On the other hand, excessive negative pressure can also cause the solvent and the target product to form an azeotropic system, increasing the complexity of distillation separation. At the same time, the petroleum ether in the solvent will undergo thermal decomposition under extreme negative pressure, and the resulting by-products will contaminate the target fraction.

[0089] In some optional examples, the residence time of the residual liquid in the secondary vacuum distillation tower is 80 to 90 min, for example, it can be 80 min, 81 min, 82 min, 83 min, 84 min, 85 min, 86 min, 87 min, 88 min, 89 min or 90 min, but is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0090] As a preferred technical solution of the present invention, in step (III), the operation process of gradient cooling crystallization includes:

[0091] (i) naturally cooling the target fraction to an initial temperature and then feeding it into a crystallization kettle, stirring the target fraction, cooling it to a precooling temperature at a first cooling rate under stirring, holding it at the precooling temperature, continuously stirring the target fraction during the holding process, and obtaining a precooled product after the holding is completed;

[0092] (ii) stirring and ultrasonicating the pre-cooled product, cooling it to a first crystallization temperature at a second cooling rate under the stirring and ultrasonicating conditions; then continuing to stir and stopping the ultrasonicating conditions, cooling it to a second crystallization temperature at a third cooling rate, and holding it at the second crystallization temperature, while continuously stirring the pre-cooled product during the holding process, to obtain an intermediate product after the holding period ends;

[0093] (iii) stirring the intermediate product and charging nitrogen into the crystallization kettle. In the nitrogen atmosphere, cooling the intermediate product at a fourth cooling rate to a final cooling temperature. Holding the intermediate product at the final cooling temperature, stirring the intermediate product during the holding process, and centrifuging the intermediate product at the final cooling temperature after the holding is completed to obtain the crystalline product.

[0094] During the pre-cooling stage, the target fraction is naturally cooled to the initial temperature before entering the crystallization kettle, where it is uniformly cooled by low-speed stirring. The purpose of this stage is to avoid a surge in local supersaturation caused by a sudden drop in temperature, thereby preventing the explosive precipitation of fine grains caused by heterogeneous nucleation. The combination of natural cooling and stirring allows the target fraction to form a uniform supercooled state. The mechanical shear force generated by stirring breaks the natural stratification of the target fraction, promotes the uniform distribution of solute molecules, and prevents component segregation caused by standing. This not only retains sufficient supersaturation to drive crystallization, but also avoids heterogeneous nucleation caused by excessive local concentration gradients in a static state. This homogenization treatment significantly reduces the random generation of fine grains, laying the foundation for subsequent controllable crystallization.

[0095] During the crystallization phase, ultrasound is introduced into the pre-cooled product. The cavitation effect instantly generates microjets and shock waves, disrupting the metastable equilibrium of the solution and inducing the formation of a large number of evenly distributed nuclei. The cooling rate is then adjusted to 0.8-1.2°C / min, ensuring the kinetic conditions for nucleation while avoiding excessive competitive growth of nuclei due to excessive cooling. Ultrasonication is then stopped and the cooling rate is switched to a lower rate (0.4-0.5°C / min), allowing the crystals to grow slowly under controlled conditions. This "nucleation-first, growth-later" staged approach effectively separates the kinetic windows for nucleation and crystal growth, effectively avoiding the wide crystal size distribution caused by the simultaneous nucleation and growth processes in traditional processes. Compared to the pre-cooling phase, the stirring speed during the crystallization phase is slightly reduced to maintain directional solute transport to the crystal surface while reducing mechanical impact on the crystal surface and promoting the integrity of the crystal structure.

[0096] During the final cooling stage, nitrogen is introduced to create an inert atmosphere, effectively isolating oxygen and moisture and preventing oxidation or hydrolysis on the crystal surface. Slow cooling (0.5-1°C / min) combined with extended heat preservation (80-90 minutes) allows ample time for residual solvent molecules and impurities within the crystals to diffuse out, further optimizing the crystal structure. The synergistic effect of the low-temperature environment (-18-20°C) and nitrogen protection not only inhibits the solvent's volatilization rate but also reduces the formation of defects during crystal growth by reducing molecular thermal activity. Finally, centrifugation is performed at low temperatures, avoiding damage to the crystals due to heat sensitivity at high temperatures and ensuring high product yield and purity.

[0097] The gradient cooling crystallization process designed in the present invention forms a progressive synergy of "homogenization-nucleation regulation-purification crystallization". The pre-cooling stage provides uniform starting conditions for ultrasonic nucleation, avoiding the difference in nucleation sites caused by uneven initial concentration distribution; the dense crystal nuclei induced by ultrasound reserve sufficient growth cores for subsequent crystal growth, so that the solute molecules can be filled into the crystal lattice in an orderly manner during the subsequent crystal growth process; the slow cooling and long-term heat preservation in the nitrogen protection and final cooling stages consolidate the results of the previous steps, and finally form high-quality crystals with concentrated particle size distribution and purity of more than 99.5%.

[0098] As a preferred technical solution of the present invention, in step (i), the initial temperature is 25-35°C, for example, it can be 25°C, 26°C, 27°C, 28°C, 29°C, 30°C, 31°C, 32°C, 33°C, 34°C or 35°C, but is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0099] In some optional examples, the stirring speed of the target fraction is 200~300 rpm, for example, it can be 200 rpm, 210 rpm, 220 rpm, 230 rpm, 240 rpm, 250 rpm, 260 rpm, 270 rpm, 280 rpm, 290 rpm or 300 rpm, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0100] In some optional examples, the first cooling rate is 1~2℃ / min, for example, it can be 1.0℃ / min, 1.1℃ / min, 1.2℃ / min, 1.3℃ / min, 1.4℃ / min, 1.5℃ / min, 1.6℃ / min, 1.7℃ / min, 1.8℃ / min, 1.9℃ / min or 2.0℃ / min, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0101] In some optional examples, the pre-cooling temperature is 10~20℃, for example, it can be 10℃, 11℃, 12℃, 13℃, 14℃, 15℃, 16℃, 17℃, 18℃, 19℃ or 20℃, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0102] In some optional examples, the holding time at the pre-cooling temperature is 35 to 45 minutes, for example, it can be 35 minutes, 36 minutes, 37 minutes, 38 minutes, 39 minutes, 40 minutes, 41 minutes, 42 minutes, 43 minutes, 44 minutes or 45 minutes, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0103] In some optional examples, in step (ii), the stirring speed of the pre-cooled product is 150~200 rpm, for example, it can be 150rpm, 155rpm, 160rpm, 165rpm, 170rpm, 175rpm, 180rpm, 185rpm, 190rpm, 195rpm or 200rpm, but is not limited to the listed values, and other values ​​not listed within this numerical range are also applicable.

[0104] In some optional examples, the ultrasonic power of the pre-cooled product is 200~300W, for example, it can be 200W, 210W, 220W, 230W, 240W, 250W, 260W, 270W, 280W, 290W or 300W, but is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0105] The present invention specifically limits the ultrasonic power of the pre-cooled product to 200-300W. When the ultrasonic power is lower than 200W, the microjet intensity generated by the cavitation effect is insufficient, making it difficult to effectively break the metastable equilibrium of the pre-cooled product. At this time, the nucleation process will be limited to the local high supersaturation area, resulting in uneven distribution of crystal nuclei, forming a small number of larger particles and a large number of small grains. The large particles restrict the growth of small particles because they occupy too much growth space, and the small grains gradually dissolve due to lack of sufficient solute supply, ultimately leading to a widening of the crystal size distribution and a reduction in yield. In addition, low-power ultrasound cannot fully disperse the concentration gradient formed in the pre-cooling stage. The residual local supersaturation area is prone to induce heterogeneous nucleation, further exacerbating the heterogeneity of crystal morphology.

[0106] When the ultrasonic power exceeds 300W, the cavitation effect can cause mechanical damage to the crystals due to excess energy. High-intensity ultrasound can trigger violent liquid disturbances, causing already formed nuclei or tiny crystals to collide and shatter, forming a large number of amorphous fragments or ultrafine particles. These fine particles easily agglomerate during subsequent stirring and heat preservation, forming hard agglomerates. This not only increases the difficulty of filtration and washing, but also causes the adsorption of more impurities due to the large surface area, reducing product purity. At the same time, excessive ultrasound can disrupt the adsorption equilibrium between solvent molecules and the crystal surface, resulting in solvent residues inside or on the crystal surface, affecting the crystallinity and thermal stability of the final product.

[0107] In some optional examples, the second cooling rate is 0.8~1.2℃ / min, for example, it can be 0.8℃ / min, 0.85℃ / min, 0.9℃ / min, 0.95℃ / min, 1.0℃ / min, 1.05℃ / min, 1.1℃ / min, 1.15℃ / min or 1.2℃ / min, but is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0108] In some optional examples, the first crystallization temperature is 4~6°C, for example, it can be 4.0°C, 4.2°C, 4.4°C, 4.6°C, 4.8°C, 5.0°C, 5.2°C, 5.4°C, 5.6°C, 5.8°C or 6.0°C, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0109] In some optional instances, the third cooling rate is 0.4~0.5℃ / min, for example, it can be 0.4℃ / min, 0.41℃ / min, 0.42℃ / min, 0.43℃ / min, 0.44℃ / min, 0.45℃ / min, 0.46℃ / min, 0.47℃ / min, 0.48℃ / min, 0.49℃ / min or 0.5℃ / min, but is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0110] In some optional examples, the second crystallization temperature is -10~-12°C, for example, it can be -10°C, -10.2°C, -10.4°C, -10.6°C, -10.8°C, -11°C, -11.2°C, -11.4°C, -11.6°C, -11.8°C or -12°C, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0111] In some optional examples, the holding time at the second crystallization temperature is 45 to 55 minutes, for example, it can be 45 minutes, 46 minutes, 47 minutes, 48 ​​minutes, 49 minutes, 50 minutes, 51 minutes, 52 minutes, 53 minutes, 54 minutes or 55 minutes, but is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0112] In some optional embodiments, in step (iii), the stirring speed of the intermediate product is 250-350 rpm, for example, it can be 250 rpm, 260 rpm, 270 rpm, 280 rpm, 290 rpm, 300 rpm, 310 rpm, 320 rpm, 330 rpm, 340 rpm or 350 rpm, but is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0113] In some optional instances, the fourth cooling rate is 0.5~1℃ / min, for example, it can be 0.5℃ / min, 0.55℃ / min, 0.6℃ / min, 0.65℃ / min, 0.7℃ / min, 0.75℃ / min, 0.8℃ / min, 0.85℃ / min, 0.9℃ / min, 0.95℃ / min or 1℃ / min, but is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0114] In some optional examples, the final cooling temperature is -30~-32°C, for example, it can be -30°C, -30.2°C, -30.4°C, -30.6°C, -30.8°C, -31°C, -31.2°C, -31.4°C, -31.6°C, -31.8°C or -32°C, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0115] The present invention specifically limits the final cooling temperature to -30~-32°C. When the final cooling temperature is higher than -30°C, the supercooling of the solution is insufficient and the crystal growth rate is significantly reduced. At this time, the crystal nuclei formed in the previous stage cannot fully develop due to the lack of sufficient low-temperature driving force, resulting in small crystal particle size and uneven distribution. In addition, higher temperatures will intensify the thermal motion of solvent molecules, and some solvents that do not participate in the crystal structure will remain in the lattice gaps or on the surface, forming a solvent inclusion phenomenon. This residual solvent will not only reduce the purity of the crystal, but will also cause crystal breakage due to volatilization during the subsequent drying process, affecting the product yield. More importantly, if the final cooling temperature is too high, it will not be able to completely suppress the dynamic migration of impurities at low temperatures. Some highly soluble impurities will slowly diffuse to the surface or interior of the crystal with the solvent, resulting in a decrease in the purity of the final product.

[0116] When the final cooling temperature drops below -32°C, the solution enters an extreme supercooled state. While the crystal growth rate is rapid, it is prone to forming a non-equilibrium structure. At this point, the solvent partially solidifies due to the low temperature, causing a sharp increase in the solution's viscosity, hindering the free diffusion of solute molecules and, in turn, inhibiting normal crystal growth. Furthermore, ultra-low temperatures can cause a hydrogen bond network to form between the solvent and the target product, leading to crystal structure distortion or the formation of eutectics. This structural defect not only reduces the physical stability of the crystal but also increases the risk of phase separation during subsequent storage due to increased solvent residue.

[0117] In some optional embodiments, the holding time at the final cooling temperature is 80 to 90 minutes, for example, 80 minutes, 81 minutes, 82 minutes, 83 minutes, 84 minutes, 85 minutes, 86 minutes, 87 minutes, 88 minutes, 89 minutes, or 90 minutes, but the holding time is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0118] Compared with the prior art, the present invention has the following beneficial effects:

[0119] The light liquid paraffin separation and preparation process proposed in the present invention forms a multi-dimensional collaborative optimization process system through vacuum dehydration, three-stage countercurrent extraction, two-stage vacuum distillation and gradient cooling crystallization. First, vacuum dehydration technology is used to accurately remove free water from waxy crude oil, avoiding the volatilization loss of light components caused by traditional high-temperature dehydration, and at the same time creating a stable phase interface environment for subsequent extraction. The three-stage countercurrent extraction process innovatively combines gradient temperature, pressure and a multi-component synergistic solvent system to gradually enhance the mass transfer driving force, thereby increasing the concentration of the target component in the three-stage extraction phase by 4 to 6 times, and controlling the solvent residue to below 0.3%. The two-stage vacuum distillation achieves narrow fraction cutting through segmented pressure gradients, accurately removing low-boiling light components (hydrocarbons below C10) and high-boiling heavy components (C15+ hydrocarbons), and avoiding thermal decomposition of the target component. The gradient cooling crystallization process forms a crystal product with concentrated particle size distribution and purity ≥99% through temperature control rate, ultrasound-assisted nucleation and nitrogen protection. The yield is increased by 15~20% compared with the traditional process, realizing the efficient and green preparation of light liquid paraffin. BRIEF DESCRIPTION OF THE DRAWINGS

[0120] Figure 1 Flowchart of the method for separating and preparing light liquid paraffin provided in Examples 1-15 of the present invention. DETAILED DESCRIPTION

[0121] The technical solutions of the present invention are described in detail below in conjunction with specific embodiments and their accompanying drawings. The embodiments described herein are specific embodiments of the present invention and are used to illustrate the concept of the present invention; these descriptions are explanatory and exemplary and should not be understood as limiting the embodiments of the present invention and the scope of protection of the present invention. In addition to the embodiments described herein, those skilled in the art can also adopt other obvious technical solutions based on the contents disclosed in the claims of this application and its specification, including technical solutions that adopt any obvious replacements and modifications to the embodiments described herein.

[0122] Example 1

[0123] This embodiment provides a method for separating and preparing light liquid paraffin, such as Figure 1 As shown, the specific steps include:

[0124] (1) Vacuum dehydration of waxy crude oil at 80°C for 4 h to obtain a dehydrated product;

[0125] The dehydrated product was preheated to 40°C and then fed into the bottom of the first-stage extraction tower. The extraction solvent was sprayed from the top of the first-stage extraction tower at a spray rate of 1 L / min. The temperature in the tower was 45°C and the pressure in the tower was 0.01 MPa. The dehydrated product and the extraction solvent were contacted and extracted in a countercurrent manner in the first-stage extraction tower. The volume ratio of the dehydrated product to the extraction solvent was 1:4. The residence time of the dehydrated product in the tower was 50 minutes. After the extraction was completed, the first-stage extract phase was collected from the top of the tower.

[0126] The primary extraction phase was preheated to 50°C and then fed into the bottom of the secondary extraction tower. The extraction solvent was sprayed from the top of the secondary extraction tower at a spray rate of 1 L / min. The temperature in the tower was 55°C and the pressure in the tower was -0.04 MPa. The primary extraction phase and the extraction solvent were contacted and extracted in countercurrent in the secondary extraction tower. The volume ratio of the primary extraction phase to the extraction solvent was 1:3. The residence time of the primary extraction phase in the tower was 80 min. After the extraction was completed, the secondary extraction phase was collected from the top of the tower.

[0127] The secondary extraction phase was preheated to 55°C and then fed into the bottom of the tertiary extraction tower. The extraction solvent was sprayed from the top of the tertiary extraction tower at a spray rate of 1.5 L / min. The temperature in the tower was 60°C, the pressure in the tower was -0.05 MPa, and the secondary extraction phase and the extraction solvent were contacted and extracted in countercurrent in the tertiary extraction tower. The volume ratio of the secondary extraction phase to the extraction solvent was 1:2.5. The residence time of the secondary extraction phase in the tower was 110 min. After the extraction was completed, the tertiary extraction phase was collected from the top of the tower.

[0128] The extraction solvent used in the extraction process includes 70 wt% petroleum ether, 20 wt% n-hexane, 5 wt% Tween 80, 2 wt% modified nano-silica and 3 wt% PEG-400, which is prepared by the following method:

[0129] Nano-silica was dispersed in a silane coupling agent solution consisting of a silane coupling agent KH550 and an ethanol aqueous solution (the volume ratio of ethanol to deionized water was 8:2), the mass fraction of the silane coupling agent KH550 in the silane coupling agent solution was 3 wt%, and the ratio of nano-silica to the silane coupling agent KH550 was 2 g:1 L. After ultrasonic dispersion, the mixture was mixed and stirred at 30° C. for 50 minutes, and then filtered, washed, and dried to obtain modified nano-silica.

[0130] Petroleum ether and n-hexane were added to a reaction kettle, mixed and stirred at 25°C for 40 minutes to obtain a homogeneous solution; Tween 80 and PEG-400 were added to the homogeneous solution, mixed and stirred at 40°C for 60 minutes to obtain a premixed solution;

[0131] The modified nano-silica was dispersed in the premixed solution, mixed and stirred at 50° C. for 40 minutes, and then vacuum degassed to obtain an extraction solvent.

[0132] (2) The tertiary extraction phase is fed into a primary vacuum distillation tower for primary vacuum distillation. The bottom temperature of the primary vacuum distillation tower is 65°C, the top temperature is 35°C, the pressure in the tower is -0.06 MPa, and the residence time of the tertiary extraction phase in the tower is 50 min.

[0133] After the vacuum distillation is completed, the residual liquid in the tower bottom is sent to a secondary vacuum distillation tower for secondary vacuum distillation. The tower bottom temperature of the secondary vacuum distillation tower is 85°C, the tower top temperature is 55°C, the pressure in the tower is -0.08MPa, and the residence time of the residual liquid in the tower is 90min. After the vacuum distillation is completed, the distillate discharged from the tower top condenser is collected, which is the target fraction;

[0134] (3) The target fraction was naturally cooled to 25°C and then sent into the crystallization kettle. The target fraction was stirred at a speed of 200 rpm. Under stirring conditions, the temperature was cooled to 10°C at a cooling rate of 1°C / min and kept warm for 35 minutes. The target fraction was continuously stirred during the insulation process. After the insulation was completed, a pre-cooled product was obtained;

[0135] The pre-cooled product was stirred and ultrasonicated, and cooled to 4°C at a cooling rate of 0.8°C / min under a stirring speed of 150 rpm and an ultrasonic power of 300 W. Then, the stirring was continued and the ultrasonication was stopped, and the temperature was further cooled to -10°C at a cooling rate of 0.4°C / min and kept warm for 55 minutes. During the insulation process, the pre-cooled product was continuously stirred, and an intermediate product was obtained after the insulation was completed.

[0136] The intermediate product was stirred at a stirring speed of 250 rpm, and nitrogen was filled into the crystallization kettle. In the nitrogen atmosphere, the temperature was continued to be lowered to -30°C at a cooling rate of 0.5°C / min and kept warm for 90 minutes. During the insulation process, the intermediate product was continuously stirred. After the insulation was completed, centrifugation was performed at the final cooling temperature to obtain a crystalline product; the crystalline product was vacuum dried at 80°C for 4 hours to obtain light liquid paraffin.

[0137] Example 2

[0138] This embodiment provides a method for separating and preparing light liquid paraffin, such as Figure 1 As shown, the specific steps include:

[0139] (1) Vacuum dehydration of the waxy crude oil at 82°C for 3.8 h to obtain a dehydrated product;

[0140] The dehydrated product was preheated to 42°C and then fed into the bottom of the first-stage extraction tower. The extraction solvent was sprayed from the top of the first-stage extraction tower at a spray rate of 1.2 L / min. The temperature in the tower was 48°C, the pressure in the tower was 0.012 MPa, and the dehydrated product and the extraction solvent were extracted in countercurrent contact in the first-stage extraction tower. The volume ratio of the dehydrated product to the extraction solvent was 1:3.8. The residence time of the dehydrated product in the tower was 48 minutes. After the extraction was completed, the first-stage extract phase was collected from the top of the tower.

[0141] The primary extraction phase was preheated to 52°C and then fed into the bottom of the secondary extraction tower. The extraction solvent was sprayed from the top of the secondary extraction tower at a spray rate of 1.2 L / min. The temperature in the tower was 58°C, the pressure in the tower was -0.042 MPa, and the primary extraction phase and the extraction solvent were subjected to countercurrent contact extraction in the secondary extraction tower. The volume ratio of the primary extraction phase to the extraction solvent was 1:2.8. The residence time of the primary extraction phase in the tower was 78 minutes. After the extraction was completed, the secondary extraction phase was collected from the top of the tower.

[0142] The secondary extraction phase was preheated to 58°C and then fed into the bottom of the tertiary extraction tower. The extraction solvent was sprayed from the top of the tertiary extraction tower at a spray rate of 1.8 L / min. The temperature in the tower was 62°C, the pressure in the tower was -0.052 MPa, and the secondary extraction phase and the extraction solvent were subjected to countercurrent contact extraction in the tertiary extraction tower. The volume ratio of the secondary extraction phase to the extraction solvent was 1:2.2. The residence time of the secondary extraction phase in the tower was 108 min. After the extraction was completed, the tertiary extraction phase was collected from the top of the tower.

[0143] The extraction solvent used in the extraction process includes 73 wt% petroleum ether, 19 wt% n-hexane, 3 wt% Tween 80, 2 wt% modified nano-silica and 3 wt% PEG-400, which is prepared by the following method:

[0144] Nano-silica was dispersed in a silane coupling agent solution consisting of a silane coupling agent KH550 and an ethanol aqueous solution (the volume ratio of ethanol to deionized water was 8:2), the mass fraction of the silane coupling agent KH550 in the silane coupling agent solution was 3.2 wt%, and the ratio of nano-silica to the silane coupling agent KH550 was 2.2 g:1 L. After ultrasonic dispersion, the mixture was mixed and stirred at 32° C. for 48 minutes, and then filtered, washed, and dried to obtain modified nano-silica.

[0145] Petroleum ether and n-hexane were added to a reaction kettle, mixed and stirred at 28°C for 38 minutes to obtain a homogeneous solution; Tween 80 and PEG-400 were added to the homogeneous solution, mixed and stirred at 42°C for 58 minutes to obtain a premixed solution;

[0146] The modified nano-silica was dispersed in the premixed solution, mixed and stirred at 52° C. for 38 minutes, and then vacuum degassed to obtain the extraction solvent.

[0147] (2) The tertiary extraction phase is fed into a primary vacuum distillation tower for primary vacuum distillation. The bottom temperature of the primary vacuum distillation tower is 68°C, the top temperature is 38°C, the pressure in the tower is -0.065 MPa, and the residence time of the tertiary extraction phase in the tower is 48 min.

[0148] After the vacuum distillation is completed, the residual liquid in the bottom of the tower is sent to a secondary vacuum distillation tower for secondary vacuum distillation. The bottom temperature of the secondary vacuum distillation tower is 88°C, the top temperature is 58°C, the pressure in the tower is -0.085MPa, and the residence time of the residual liquid in the tower is 88min. After the vacuum distillation is completed, the distillate discharged from the top condenser is collected, which is the target fraction;

[0149] (3) The target fraction was naturally cooled to 28°C and then sent into the crystallization kettle. The target fraction was stirred at a speed of 220 rpm. Under stirring conditions, the temperature was cooled to 12°C at a cooling rate of 1.2°C / min and kept warm for 38 minutes. The target fraction was continuously stirred during the insulation process. After the insulation was completed, a pre-cooled product was obtained.

[0150] The precooled product was stirred and ultrasonicated, and cooled to 4.5°C at a cooling rate of 0.9°C / min under a stirring speed of 160 rpm and an ultrasonic power of 280 W. Then, the stirring was continued and the ultrasonication was stopped, and the temperature was further cooled to -10.5°C at a cooling rate of 0.42°C / min and kept warm for 52 minutes. The precooled product was continuously stirred during the insulation process, and an intermediate product was obtained after the insulation was completed.

[0151] The intermediate product was stirred at a stirring speed of 280 rpm, and nitrogen was filled into the crystallization kettle. In the nitrogen atmosphere, the temperature was continued to be lowered to -30.5°C at a cooling rate of 0.6°C / min and kept warm for 88 minutes. During the insulation process, the intermediate product was continuously stirred. After the insulation was completed, centrifugation was performed at the final cooling temperature to obtain a crystalline product; the crystalline product was vacuum dried at 82°C for 3.8 hours to obtain light liquid paraffin.

[0152] Example 3

[0153] This embodiment provides a method for separating and preparing light liquid paraffin, such as Figure 1 As shown, the specific steps include:

[0154] (1) Waxy crude oil (comprising 30 wt% of light liquid paraffin and 10 wt% of free water, with the remainder being a hydrocarbon mixture) was subjected to vacuum dehydration at 85°C for 3.5 h to obtain a dehydrated product;

[0155] The dehydrated product was preheated to 45°C and then fed into the bottom of the first-stage extraction tower. The extraction solvent was sprayed from the top of the first-stage extraction tower at a spray rate of 1.3 L / min. The temperature in the tower was 50°C, the pressure in the tower was 0.015 MPa, and the dehydrated product and the extraction solvent were contacted and extracted in a countercurrent manner in the first-stage extraction tower. The volume ratio of the dehydrated product to the extraction solvent was 1:3.5, and the residence time of the dehydrated product in the tower was 45 minutes. After the extraction was completed, the first-stage extract phase was collected from the top of the tower.

[0156] The primary extraction phase was preheated to 55°C and then fed into the bottom of the secondary extraction tower. The extraction solvent was sprayed from the top of the secondary extraction tower at a spray rate of 1.5 L / min. The temperature in the tower was 60°C, the pressure in the tower was -0.045 MPa, and the primary extraction phase and the extraction solvent were subjected to countercurrent contact extraction in the secondary extraction tower. The volume ratio of the primary extraction phase to the extraction solvent was 1:2.5. The residence time of the primary extraction phase in the tower was 75 min. After the extraction was completed, the secondary extraction phase was collected from the top of the tower.

[0157] The secondary extraction phase was preheated to 60°C and then fed into the bottom of the tertiary extraction tower. The extraction solvent was sprayed from the top of the tertiary extraction tower at a spray rate of 2 L / min. The temperature in the tower was 65°C and the pressure in the tower was -0.055 MPa. The secondary extraction phase and the extraction solvent were contacted and extracted in countercurrent in the tertiary extraction tower. The volume ratio of the secondary extraction phase to the extraction solvent was 1:2. The residence time of the secondary extraction phase in the tower was 105 min. After the extraction was completed, the tertiary extraction phase was collected from the top of the tower.

[0158] The extraction solvent used in the extraction process includes 75 wt% petroleum ether, 16 wt% n-hexane, 4 wt% Tween 80, 2 wt% modified nano-silica and 3 wt% PEG-400, which is prepared by the following method:

[0159] Nano-silica was dispersed in a silane coupling agent solution consisting of a silane coupling agent KH550 and an ethanol aqueous solution (the volume ratio of ethanol to deionized water was 8:2), the mass fraction of the silane coupling agent KH550 in the silane coupling agent solution was 3.5 wt%, and the ratio of nano-silica to the silane coupling agent KH550 was 2.5 g:1 L. After ultrasonic dispersion, the mixture was mixed and stirred at 35° C. for 45 minutes, and then filtered, washed, and dried to obtain modified nano-silica.

[0160] Petroleum ether and n-hexane were added to a reaction kettle, mixed and stirred at 30°C for 35 minutes to obtain a homogeneous solution; Tween 80 and PEG-400 were added to the homogeneous solution, mixed and stirred at 45°C for 55 minutes to obtain a premixed solution;

[0161] The modified nano-silica was dispersed in the premixed solution, mixed and stirred at 55° C. for 35 minutes, and then vacuum degassed to obtain an extraction solvent.

[0162] (2) The tertiary extraction phase is sent to a primary vacuum distillation tower for primary vacuum distillation. The bottom temperature of the primary vacuum distillation tower is 70°C, the top temperature is 40°C, the pressure in the tower is -0.07 MPa, and the residence time of the tertiary extraction phase in the tower is 45 min.

[0163] After the vacuum distillation is completed, the residual liquid in the bottom of the tower is sent to a secondary vacuum distillation tower for secondary vacuum distillation. The bottom temperature of the secondary vacuum distillation tower is 90°C, the top temperature is 60°C, the pressure in the tower is -0.09MPa, and the residence time of the residual liquid in the tower is 85min. After the vacuum distillation is completed, the distillate discharged from the top condenser is collected, which is the target fraction;

[0164] (3) The target fraction was naturally cooled to 30°C and then sent into the crystallization kettle. The target fraction was stirred at a speed of 250 rpm. Under stirring conditions, the temperature was cooled to 15°C at a cooling rate of 1.5°C / min and kept warm for 40 minutes. The target fraction was continuously stirred during the insulation process. After the insulation was completed, a pre-cooled product was obtained.

[0165] The pre-cooled product was stirred and ultrasonicated, and cooled to 5°C at a cooling rate of 1°C / min under a stirring speed of 170 rpm and an ultrasonic power of 250 W. Subsequently, the stirring was continued and the ultrasonication was stopped, and the temperature was further cooled to -11°C at a cooling rate of 0.45°C / min and kept warm for 50 minutes. During the insulation process, the pre-cooled product was continuously stirred, and an intermediate product was obtained after the insulation was completed.

[0166] The intermediate product was stirred at a stirring speed of 300 rpm, and nitrogen was filled into the crystallization kettle. In the nitrogen atmosphere, the temperature was continued to be lowered to -31°C at a cooling rate of 0.7°C / min and kept warm for 85 minutes. During the insulation process, the intermediate product was continuously stirred. After the insulation was completed, centrifugation was performed at the final cooling temperature to obtain a crystalline product; the crystalline product was vacuum dried at 85°C for 3.5 hours to obtain light liquid paraffin.

[0167] Example 4

[0168] This embodiment provides a method for separating and preparing light liquid paraffin, such as Figure 1 As shown, the specific steps include:

[0169] (1) The waxy crude oil was vacuum dehydrated at 88°C for 3.2 h to obtain a dehydrated product;

[0170] The dehydrated product was preheated to 48°C and then fed into the bottom of the first-stage extraction tower. The extraction solvent was sprayed from the top of the first-stage extraction tower at a spray rate of 1.4 L / min. The temperature in the tower was 52°C, and the pressure in the tower was 0.018 MPa. The dehydrated product and the extraction solvent were contacted and extracted in a countercurrent manner in the first-stage extraction tower. The volume ratio of the dehydrated product to the extraction solvent was 1:3.2. The residence time of the dehydrated product in the tower was 42 minutes. After the extraction was completed, the first-stage extract phase was collected from the top of the tower.

[0171] The primary extraction phase was preheated to 58°C and then fed into the bottom of the secondary extraction tower. The extraction solvent was sprayed from the top of the secondary extraction tower at a spray rate of 1.8 L / min. The temperature in the tower was 62°C, and the pressure in the tower was -0.048 MPa. The primary extraction phase and the extraction solvent were contacted and extracted in countercurrent in the secondary extraction tower. The volume ratio of the primary extraction phase to the extraction solvent was 1:2.2. The residence time of the primary extraction phase in the tower was 72 minutes. After the extraction was completed, the secondary extraction phase was collected from the top of the tower.

[0172] The secondary extraction phase was preheated to 62°C and then fed into the bottom of the tertiary extraction tower. The extraction solvent was sprayed from the top of the tertiary extraction tower at a spray rate of 2.2 L / min. The temperature in the tower was 68°C, the pressure in the tower was -0.058 MPa, and the secondary extraction phase and the extraction solvent were subjected to countercurrent contact extraction in the tertiary extraction tower. The volume ratio of the secondary extraction phase to the extraction solvent was 1:1.8, and the residence time of the secondary extraction phase in the tower was 102 min. After the extraction was completed, the tertiary extraction phase was collected from the top of the tower.

[0173] The extraction solvent used in the extraction process includes 78 wt% petroleum ether, 19 wt% n-hexane, 1 wt% Tween 80, 1 wt% modified nano-silica and 1 wt% PEG-400, which is prepared by the following method:

[0174] Nano-silica was dispersed in a silane coupling agent solution consisting of a silane coupling agent KH550 and an ethanol aqueous solution (the volume ratio of ethanol to deionized water was 8:2), the mass fraction of the silane coupling agent KH550 in the silane coupling agent solution was 3.8 wt%, and the ratio of nano-silica to the silane coupling agent KH550 was 2.8 g:1 L. After ultrasonic dispersion, the mixture was mixed and stirred at 38° C. for 42 minutes, and then filtered, washed, and dried to obtain modified nano-silica.

[0175] Petroleum ether and n-hexane were added to a reaction kettle, mixed and stirred at 32°C for 32 minutes to obtain a homogeneous solution; Tween 80 and PEG-400 were added to the homogeneous solution, mixed and stirred at 48°C for 52 minutes to obtain a premixed solution;

[0176] The modified nano-silica was dispersed in the premixed solution, mixed and stirred at 58° C. for 32 minutes, and then vacuum degassed to obtain the extraction solvent.

[0177] (2) The tertiary extraction phase is fed into a primary vacuum distillation tower for primary vacuum distillation. The bottom temperature of the primary vacuum distillation tower is 72°C, the top temperature is 42°C, the pressure in the tower is -0.067 MPa, and the residence time of the tertiary extraction phase in the tower is 42 min.

[0178] After the vacuum distillation is completed, the residual liquid in the tower bottom is sent to a secondary vacuum distillation tower for secondary vacuum distillation. The tower bottom temperature of the secondary vacuum distillation tower is 92°C, the tower top temperature is 62°C, the pressure in the tower is -0.095MPa, and the residence time of the residual liquid in the tower is 82min. After the vacuum distillation is completed, the distillate discharged from the tower top condenser is collected, which is the target fraction;

[0179] (3) The target fraction was naturally cooled to 32°C and then sent into the crystallization kettle. The target fraction was stirred at a speed of 280 rpm. Under stirring conditions, the temperature was cooled to 18°C ​​at a cooling rate of 1.8°C / min and kept warm for 42 minutes. The target fraction was continuously stirred during the insulation process. After the insulation was completed, a pre-cooled product was obtained.

[0180] The precooled product was stirred and ultrasonicated, and cooled to 5.5°C at a cooling rate of 1.1°C / min under a stirring speed of 180 rpm and an ultrasonic power of 220 W. Then, the stirring was continued and the ultrasonication was stopped, and the temperature was further cooled to -11.5°C at a cooling rate of 0.48°C / min and kept warm for 48 minutes. The precooled product was continuously stirred during the insulation process, and an intermediate product was obtained after the insulation was completed.

[0181] The intermediate product was stirred at a stirring speed of 320 rpm, and nitrogen was filled into the crystallization kettle. In the nitrogen atmosphere, the temperature was continued to be lowered to -31.5°C at a cooling rate of 0.8°C / min and kept warm for 82 minutes. During the insulation process, the intermediate product was continuously stirred. After the insulation was completed, centrifugation was performed at the final cooling temperature to obtain a crystalline product; the crystalline product was vacuum dried at 88°C for 3.2 hours to obtain light liquid paraffin.

[0182] Example 5

[0183] This embodiment provides a method for separating and preparing light liquid paraffin, such as Figure 1 As shown, the specific steps include:

[0184] (1) Vacuum dehydration of waxy crude oil at 90°C for 3 h to obtain a dehydrated product;

[0185] The dehydrated product was preheated to 50°C and then fed into the bottom of the first-stage extraction tower. The extraction solvent was sprayed from the top of the first-stage extraction tower at a spray rate of 1.5 L / min. The temperature in the tower was 55°C and the pressure in the tower was 0.02 MPa. The dehydrated product and the extraction solvent were contacted and extracted in a countercurrent manner in the first-stage extraction tower. The volume ratio of the dehydrated product to the extraction solvent was 1:3. The residence time of the dehydrated product in the tower was 40 minutes. After the extraction was completed, the first-stage extract phase was collected from the top of the tower.

[0186] The primary extraction phase was preheated to 60°C and then fed into the bottom of the secondary extraction tower. The extraction solvent was sprayed from the top of the secondary extraction tower at a spray rate of 2 L / min. The temperature in the tower was 65°C and the pressure in the tower was -0.05 MPa. The primary extraction phase and the extraction solvent were contacted and extracted in countercurrent in the secondary extraction tower. The volume ratio of the primary extraction phase to the extraction solvent was 1:2. The residence time of the primary extraction phase in the tower was 70 min. After the extraction was completed, the secondary extraction phase was collected from the top of the tower.

[0187] The secondary extraction phase was preheated to 65°C and then fed into the bottom of the tertiary extraction tower. The extraction solvent was sprayed from the top of the tertiary extraction tower at a spray rate of 2.5 L / min. The temperature in the tower was 70°C, and the pressure in the tower was -0.06 MPa. The secondary extraction phase and the extraction solvent were contacted and extracted in countercurrent in the tertiary extraction tower. The volume ratio of the secondary extraction phase to the extraction solvent was 1:1.5. The residence time of the secondary extraction phase in the tower was 100 min. After the extraction was completed, the tertiary extraction phase was collected from the top of the tower.

[0188] The extraction solvent used in the extraction process includes 80 wt% petroleum ether, 10 wt% n-hexane, 5 wt% Tween 80, 2 wt% modified nano-silica and 3 wt% PEG-400, which is prepared by the following method:

[0189] Nano-silica was dispersed in a silane coupling agent solution consisting of a silane coupling agent KH550 and an ethanol aqueous solution (the volume ratio of ethanol to deionized water was 8:2), the mass fraction of the silane coupling agent KH550 in the silane coupling agent solution was 4 wt%, and the ratio of nano-silica to the silane coupling agent KH550 was 3 g:1 L. After ultrasonic dispersion, the mixture was mixed and stirred at 40° C. for 40 minutes, and then filtered, washed, and dried to obtain modified nano-silica.

[0190] Petroleum ether and n-hexane were added to a reaction kettle, mixed and stirred at 35°C for 30 minutes to obtain a homogeneous solution; Tween 80 and PEG-400 were added to the homogeneous solution, mixed and stirred at 50°C for 50 minutes to obtain a premixed solution;

[0191] The modified nano-silica was dispersed in the premixed solution, mixed and stirred at 60° C. for 30 minutes, and then vacuum degassed to obtain an extraction solvent.

[0192] (2) The tertiary extraction phase is fed into a primary vacuum distillation tower for primary vacuum distillation. The bottom temperature of the primary vacuum distillation tower is 75°C, the top temperature is 45°C, the pressure in the tower is -0.08 MPa, and the residence time of the tertiary extraction phase in the tower is 40 min.

[0193] After the vacuum distillation is completed, the residual liquid in the tower bottom is sent to a secondary vacuum distillation tower for secondary vacuum distillation. The tower bottom temperature of the secondary vacuum distillation tower is 95°C, the tower top temperature is 65°C, the pressure in the tower is -0.1MPa, and the residence time of the residual liquid in the tower is 80min. After the vacuum distillation is completed, the distillate discharged from the tower top condenser is collected, which is the target fraction;

[0194] (3) The target fraction was naturally cooled to 35°C and then sent into the crystallization kettle. The target fraction was stirred at a speed of 300 rpm. Under stirring conditions, the temperature was cooled to 20°C at a cooling rate of 2°C / min and kept warm for 45 minutes. The target fraction was continuously stirred during the insulation process. After the insulation was completed, a pre-cooled product was obtained;

[0195] The precooled product was stirred and ultrasonicated, and cooled to 6°C at a cooling rate of 1.2°C / min under a stirring speed of 200 rpm and an ultrasonic power of 200 W. Then, the stirring was continued and the ultrasonication was stopped, and the temperature was further cooled to -12°C at a cooling rate of 0.5°C / min and kept warm for 45 minutes. During the insulation process, the precooled product was continuously stirred, and an intermediate product was obtained after the insulation was completed.

[0196] The intermediate product was stirred at a stirring speed of 350 rpm, and nitrogen was filled into the crystallization kettle. In the nitrogen atmosphere, the temperature was continued to be lowered to -32°C at a cooling rate of 1°C / min and kept warm for 80 minutes. During the insulation process, the intermediate product was continuously stirred. After the insulation was completed, centrifugation was performed at the final cooling temperature to obtain a crystalline product; the crystalline product was vacuum dried at 90°C for 3 hours to obtain light liquid paraffin.

[0197] Example 6

[0198] This embodiment provides a method for separating and preparing light liquid paraffin. The difference from Example 1 is that in step (1), in the primary extraction tower, the volume ratio of the dehydrated product to the extraction solvent is adjusted to 1:2.5, and the other process parameters and operating steps are exactly the same as those in Example 1.

[0199] Example 7

[0200] This embodiment provides a method for separating and preparing light liquid paraffin. The difference from Example 1 is that in step (1), in the primary extraction tower, the volume ratio of the dehydrated product to the extraction solvent is adjusted to 1:4.5, and the other process parameters and operating steps are exactly the same as those in Example 1.

[0201] Example 8

[0202] This embodiment provides a method for separating and preparing light liquid paraffin. The difference from Example 1 is that in step (1), the pressure inside the three-stage extraction tower is adjusted to -0.04 MPa, and the other process parameters and operating steps are exactly the same as those in Example 1.

[0203] Example 9

[0204] This embodiment provides a method for separating and preparing light liquid paraffin. The difference from Example 1 is that in step (1), the pressure inside the three-stage extraction tower is adjusted to -0.07 MPa, and the other process parameters and operating steps are exactly the same as those in Example 1.

[0205] Example 10

[0206] This embodiment provides a method for separating and preparing light liquid paraffin. The difference from Example 1 is that in step (2), the pressure inside the secondary vacuum distillation tower is adjusted to -0.07 MPa, and the other process parameters and operating steps are exactly the same as those in Example 1.

[0207] Example 11

[0208] This embodiment provides a method for separating and preparing light liquid paraffin. The difference from Example 1 is that in step (2), the pressure inside the secondary vacuum distillation tower is adjusted to -0.12 MPa, and the other process parameters and operating steps are exactly the same as those in Example 1.

[0209] Example 12

[0210] This embodiment provides a method for separating and preparing light liquid paraffin. The difference from Example 1 is that in step (3), the ultrasonic power of the pre-cooled product is adjusted to 150 W, and the other process parameters and operating steps are exactly the same as those in Example 1.

[0211] Example 13

[0212] This embodiment provides a method for separating and preparing light liquid paraffin. The difference from Example 1 is that in step (3), the ultrasonic power of the pre-cooled product is adjusted to 350 W, and the other process parameters and operating steps are exactly the same as those in Example 1.

[0213] Example 14

[0214] This embodiment provides a method for separating and preparing light liquid paraffin. The difference from Example 1 is that in step (3), the final cooling temperature is adjusted to -15°C, and the other process parameters and operating steps are exactly the same as those in Example 1.

[0215] Example 15

[0216] This embodiment provides a method for separating and preparing light liquid paraffin. The difference from Example 1 is that in step (3), the final cooling temperature is adjusted to -25°C, and the other process parameters and operating steps are exactly the same as those in Example 1.

[0217] The light liquid paraffin product was prepared by the method for separating and preparing light liquid paraffin provided in Examples 1-15, and the product purity and product yield were calculated. The specific test steps are as follows:

[0218] (1) Product purity test:

[0219] The light liquid paraffin prepared in the present invention refers to C10-C15 alkanes, and gas chromatography (GC) is used to determine the content of the target component (C10-C15 alkanes).

[0220] Take a dried light liquid paraffin sample and dilute it with n-hexane to an appropriate concentration.

[0221] Gas chromatography conditions:

[0222] Chromatographic column: non-polar capillary column (DB-1, 30m×0.25mm×0.25μm);

[0223] Carrier gas: nitrogen or helium, flow rate 1.0 mL / min;

[0224] Inlet temperature: 300°C;

[0225] Detector: FID, temperature 320℃;

[0226] Column temperature program: initial temperature 50 °C, hold for 1 min, increase to 300 °C at 10 °C / min, hold for 10 min;

[0227] Injection volume: 1 μL, split ratio 50:1.

[0228] The sample solution was injected into the gas chromatograph, the chromatogram was recorded, and the percentage of the peak area of ​​the target component (C10-C15 alkane) to the total peak area was calculated by the area normalization method, which was the purity.

[0229] (2) Product yield

[0230] The yield refers to the percentage of the mass of the target component C10-C15 alkanes in the light liquid paraffin product actually obtained to the mass of the target component (light liquid paraffin) in the raw waxy crude oil.

[0231] Take a certain mass of waxy crude oil and determine the mass of light liquid paraffin (C10-C15 alkanes) in it through solvent extraction combined with distillation, recorded as M0. Weigh the mass of the final light liquid paraffin product and record it as M1. Calculate the product yield using the following formula:

[0232] Product yield = (M1 / M0) × 100%

[0233] The test results are shown in Table 1.

[0234] Table 1 Test results

[0235]

[0236] It can be seen from the test data of Examples 1, 6, and 7 that, compared with Example 1, Examples 6 and 7 adjusted the volume ratio of the dehydrated product to the extraction solvent in the primary extraction tower. The present invention limits the volume ratio of the dehydrated product to the extraction solvent to 1:(3-4). Example 6 adjusted the volume ratio of the dehydrated product to the extraction solvent to 1:2.5, resulting in reduced extraction efficiency, insufficient solvent, insufficient dissolution of the target component, decreased product purity, and reduced product yield. Example 7 adjusted the volume ratio of the dehydrated product to the extraction solvent to 1:4.5, resulting in dilution of the extract phase, a decrease in the concentration gradient, and a weakening of the mass transfer driving force, which also affected the product purity and product yield.

[0237] The test data from Examples 1, 8, and 9 indicate that, compared to Example 1, Examples 8 and 9 adjusted the internal pressure of the three-stage extraction column. The present invention limits the internal pressure of the three-stage extraction column to -0.05 to -0.06 MPa. The excessively high internal pressure (-0.04 MPa) in the three-stage extraction column in Example 8 resulted in excessive solvent vaporization, difficult phase separation, impurities entering the extract phase, decreased product purity, and reduced product yield due to solvent loss. The excessively low internal pressure (-0.07 MPa) in the three-stage extraction column in Example 9 resulted in insufficient mass transfer driving force, reduced extraction efficiency, and decreased product purity and yield.

[0238] The test data from Examples 1, 10, and 11 indicate that, compared to Example 1, Examples 10 and 11 adjusted the internal pressure of the two-stage vacuum distillation column. The present invention limits the internal pressure of the two-stage vacuum distillation column to -0.08 to -0.1 MPa. The internal pressure of the two-stage vacuum distillation column in Example 10 was too high (-0.07 MPa), failing to effectively remove high-boiling-point heavy components, resulting in residual high-boiling-point heavy components and reduced product purity. The internal pressure of the two-stage vacuum distillation column in Example 11 was too low (-0.12 MPa), leading to volatilization loss of light components. This reduced product purity due to an increase in residual low-boiling-point impurities and a decrease in product yield due to excessive evaporation.

[0239] As can be seen from the test data of Examples 1, 12, and 13, compared with Example 1, Examples 8 and 9 adjusted the ultrasonic power of the pre-cooled product. The present invention limits the ultrasonic power of the pre-cooled product to 200-300W. The ultrasonic power of the pre-cooled product in Example 12 was too low (150W), resulting in uneven nucleation and a wide crystal size distribution, which affected filtration and washing, resulting in a decrease in product purity and a decrease in product yield. The ultrasonic power of the pre-cooled product in Example 13 was too high (350W), which mechanically damaged the crystals, resulting in an increase in fine particles and increased solvent residues, resulting in a decrease in product purity and a decrease in product yield.

[0240] The test data of Examples 1, 14, and 15 show that, compared with Example 1, Examples 14 and 15 adjusted the final cooling temperature. The present invention limits the final cooling temperature to -18 to -20°C. The final cooling temperature in Example 14 was too high (-15°C), resulting in insufficient supercooling, slow crystal growth, and a high amount of residual solvent, leading to decreased product purity and reduced product yield due to smaller crystals. The final cooling temperature in Example 15 was too low (-25°C), resulting in a non-equilibrium structure and solvent entrapment, which reduced product purity and yield.

[0241] The applicant declares that the above is only a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention fall within the scope of protection and disclosure of the present invention.

Claims

1. A method for separating and preparing light liquid paraffin, characterized in that: The method comprises: (I) vacuum dehydrating the waxy crude oil to obtain a dehydrated product; the vacuum dehydration temperature is 80-90° C., and the vacuum dehydration time is 3-4 hours; the dehydrated product is subjected to three-stage countercurrent extraction using an extraction solvent, and a three-stage extraction phase obtained from the last stage of countercurrent extraction is collected; the extraction solvent includes petroleum ether, n-hexane, Tween 80, modified nano-silica, and PEG-400; The modified nano-silica is obtained by modifying nano-silica with a silane coupling agent; The mass fraction of petroleum ether in the extraction solvent is 70-80wt%; The mass fraction of n-hexane in the extraction solvent is 10-20 wt %; The mass fraction of Tween 80 in the extraction solvent is 1-5wt%; The mass fraction of the modified nano-silica in the extraction solvent is 1-2 wt %; The mass fraction of PEG-400 in the extraction solvent is 1-3 wt %; (II) subjecting the three-stage extraction phase of step (I) to two-stage vacuum distillation, collecting the top fraction discharged from the last stage of vacuum distillation as the target fraction; (III) subjecting the target fraction of step (II) to gradient cooling crystallization, obtaining a crystalline product after centrifugal separation, and vacuum drying the crystalline product to obtain light liquid paraffin.

2. The method for separating and preparing light liquid paraffin according to claim 1, wherein In step (I), the operation process of the three-stage countercurrent extraction includes: (a) preheating the dehydrated product and feeding it from the bottom of a primary extraction tower, spraying the extraction solvent from the top of the primary extraction tower, and extracting the dehydrated product and the extraction solvent in countercurrent contact within the primary extraction tower. After the extraction is completed, collecting the primary extract phase from the top of the tower; (b) preheating the primary extraction phase and feeding it from the bottom of a secondary extraction tower, spraying the extraction solvent from the top of the secondary extraction tower, and extracting the primary extraction phase and the extraction solvent in countercurrent contact within the secondary extraction tower. After the extraction is completed, collecting the secondary extraction phase from the top of the tower; (c) The secondary extraction phase is preheated and fed into the bottom of a tertiary extraction tower. The extraction solvent is sprayed from the top of the tertiary extraction tower. The secondary extraction phase and the extraction solvent are contacted and extracted in countercurrent in the tertiary extraction tower. After the extraction is completed, the tertiary extraction phase is collected from the top of the tower.

3. The method for separating and preparing light liquid paraffin according to claim 2, wherein: In step (a), the preheating temperature of the dehydrated product is 40-50°C; In the primary extraction tower, the volume ratio of the dehydrated product to the extraction solvent is 1:(3-4); The spraying rate of the extraction solvent in the primary extraction tower is 1-1.5 L / min; The residence time of the dehydrated product in the primary extraction tower is 40 to 50 minutes; The temperature inside the primary extraction tower is 45-55°C; The pressure inside the first-stage extraction tower is 0.01-0.02 MPa; In step (b), the preheating temperature of the primary extraction phase is 50-60°C; In the secondary extraction tower, the volume ratio of the primary extraction phase to the extraction solvent is 1:(2-3); The spraying rate of the extraction solvent in the secondary extraction tower is 1-2 L / min; The residence time of the primary extraction phase in the secondary extraction tower is 70 to 80 minutes; The temperature inside the secondary extraction tower is 55-65°C; The pressure inside the secondary extraction tower is -0.04 to -0.05 MPa; In step (c), the preheating temperature of the secondary extraction phase is 55-65°C; In the three-stage extraction tower, the volume ratio of the secondary extraction phase to the extraction solvent is 1:(1.5-2.5); The spraying rate of the extraction solvent in the three-stage extraction tower is 1.5~2.5L / min; The residence time of the secondary extraction phase in the tertiary extraction tower is 100-110 min; The temperature inside the three-stage extraction tower is 60-70°C; The pressure inside the three-stage extraction tower is -0.05~-0.06MPa.

4. The method for separating and preparing light liquid paraffin according to claim 1, wherein The extraction solvent is prepared by the following method: (1) dispersing nano-silica in a silane coupling agent solution, ultrasonically dispersing, mixing, stirring, and heating, and then filtering, washing, and drying to obtain modified nano-silica; (2) petroleum ether and n-hexane are put into a reactor, mixed, stirred and heated to obtain a homogeneous solution; Tween 80 and PEG-400 are added to the homogeneous solution, mixed, stirred and heated to obtain a premixed solution; (3) Dispersing the modified nano-silica obtained in step (1) in the premixed solution obtained in step (2), mixing, stirring and heating, and then vacuum degassing to obtain the extraction solvent.

5. The method for separating and preparing light liquid paraffin according to claim 4, characterized in that: In step (1), the silane coupling agent solution consists of a silane coupling agent and an ethanol aqueous solution; The mass fraction of the silane coupling agent in the silane coupling agent solution is 3-4 wt %; The ratio of the nano-silica to the silane coupling agent is (2-3) g:1 L; The temperature of mixing the nano-silica and silane coupling agent solution is 30-40° C. The mixing time of the nano-silica and silane coupling agent solution is 40 to 50 minutes; In step (2), the temperature of the mixing of petroleum ether and n-hexane is 25-35°C; The mixing time of the petroleum ether and n-hexane is 30 to 40 minutes; The temperature of the mixing of the homogeneous solution, Tween 80 and PEG-400 is 40-50°C; The mixing time of the homogeneous solution, Tween 80 and PEG-400 is 50-60 minutes; In step (3), the temperature of mixing and stirring the modified nano-silica and the premixed liquid is 50-60°C; The time for mixing and stirring the modified nano-silica and the premixed liquid is 30 to 40 minutes.

6. The method for separating and preparing light liquid paraffin according to claim 1, characterized in that: In step (II), the operation process of the two-stage vacuum distillation includes: The three-stage extraction phase is sent to a first-stage vacuum distillation tower for first-stage vacuum distillation, and the residual liquid in the tower kettle is sent to a second-stage vacuum distillation tower for second-stage vacuum distillation after the vacuum distillation is completed. After the vacuum distillation is completed, the distillate discharged from the top condenser is collected, which is the target fraction; The bottom temperature of the first-stage vacuum distillation tower is 65-75°C; The top temperature of the first-stage vacuum distillation tower is 35-45°C; The pressure inside the first-stage vacuum distillation tower is -0.06 to -0.08 MPa; The residence time of the tertiary extraction phase in the primary vacuum distillation tower is 40 to 50 minutes; The bottom temperature of the secondary vacuum distillation tower is 85-95°C; The top temperature of the secondary vacuum distillation tower is 55-65°C; The pressure inside the secondary vacuum distillation tower is -0.08 to -0.1 MPa; The residual liquid resides in the secondary vacuum distillation tower for 80 to 90 minutes.

7. The method for separating and preparing light liquid paraffin according to claim 1, wherein: In step (III), the operation process of gradient cooling crystallization includes: (i) naturally cooling the target fraction to an initial temperature and then feeding it into a crystallization kettle, stirring the target fraction, cooling it to a precooling temperature at a first cooling rate under stirring, holding it at the precooling temperature, continuously stirring the target fraction during the holding process, and obtaining a precooled product after the holding is completed; (ii) stirring and ultrasonicating the pre-cooled product, cooling it to a first crystallization temperature at a second cooling rate under the stirring and ultrasonicating conditions; then continuing to stir and stopping the ultrasonicating conditions, cooling it to a second crystallization temperature at a third cooling rate, and holding it at the second crystallization temperature, while continuously stirring the pre-cooled product during the holding process, to obtain an intermediate product after the holding period ends; (iii) stirring the intermediate product and charging nitrogen into the crystallization kettle. In the nitrogen atmosphere, cooling the intermediate product at a fourth cooling rate to a final cooling temperature. Holding the intermediate product at the final cooling temperature, stirring the intermediate product during the holding process, and centrifuging the intermediate product at the final cooling temperature after the holding is completed to obtain the crystalline product.

8. The method for separating and preparing light liquid paraffin according to claim 7, characterized in that: In step (i), The initial temperature is 25-35°C; The stirring speed of the target fraction is 200-300 rpm; The first cooling rate is 1-2°C / min; The pre-cooling temperature is 10-20°C; The holding time at the pre-cooling temperature is 35 to 45 minutes; In step (ii), the stirring speed of the pre-cooled product is 150-200 rpm; The ultrasonic power of the pre-cooled product is 200-300W; The second cooling rate is 0.8-1.2°C / min; The first crystallization temperature is 4-6°C; The third cooling rate is 0.4-0.5°C / min; The second crystallization temperature is -10 to -12°C; The holding time at the second crystallization temperature is 45 to 55 minutes; In step (iii), the stirring speed of the intermediate product is 250-350 rpm; The fourth cooling rate is 0.5-1°C / min; The final cooling temperature is -30~-32°C; The holding time at the final cooling temperature is 80 to 90 minutes.

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