A high-efficiency anti-crystallization vehicle urea solution and preparation method thereof

Abstract

The invention discloses a kind of high-efficiency anti-crystallization vehicle urea solution and preparation method thereof, belong to the technical field of vehicle urea solution preparation, the preparation method includes: preparation inclusion compound, hydrolysis crystallization, dissolving, filtering, fine filtration, ultrafiltration;The method for described hydrolysis crystallization is, after industrial urea and tertiary water are mixed, stirring 30 40min at 70 75 DEG C, carrying out the first gradient cooling, the second gradient cooling, adding inclusion compound, carrying out the third gradient cooling, the fourth gradient cooling, then heat preservation, centrifugation, and taking precipitate. When preparing, the present invention uses industrial urea as raw material, and the impurity content of the high-efficiency anti-crystallization vehicle urea solution prepared is low, without precipitation, and urea yield is high, and urea yield is little affected by the impurity content in industrial urea, and when the precipitation in urea solution is removed by the method for ultrafiltration, membrane flux is little affected by the precipitation content in urea solution, and membrane pollution problem can also be avoided.

Description

Technical Field

[0001] The invention relates to the technical field of preparation of vehicle urea solutions, and in particular to a high-efficiency anti-crystallization vehicle urea solution and a preparation method thereof. Background Art

[0002] Commercial vehicles, the mainstay of the logistics and transportation industry, utilize high-power diesel engines, offering advantages such as low fuel consumption under comprehensive operating conditions and high torque at low speeds. This makes it difficult for power batteries to replace these engines for a considerable period of time. However, the exhaust emissions from these engines contain significant amounts of nitrogen oxides and solid particulate matter. Nitrogen oxides can damage the ozone layer, causing photochemical smog and acid rain, while solid particulate matter can cause smog, ultimately posing significant risks to animals and humans.

[0003] With the implementation of the China VI emission standards, the requirements for exhaust emissions from high-power diesel engines are becoming increasingly stringent. To reduce nitrogen oxide emissions from high-power diesel engines, exhaust gas treatment is necessary. Currently, the most commonly used exhaust gas treatment technology is selective catalytic reduction (SCR). The principle of SCR is to store a urea solution in an onboard storage tank and use a pump to spray the urea solution through a nozzle into the hot engine exhaust. At high temperatures, the urea solution vaporizes and decomposes to produce ammonia. The ammonia then reacts with the nitrogen oxides in the exhaust gas under the action of a catalyst to produce water and nitrogen, while also absorbing harmful smoke particles.

[0004] Automotive urea solution is a colorless, transparent, and clear liquid made from a mixture of high-purity urea and ultrapure water. Currently, commonly used automotive urea solution is generally composed of 32.5% by mass of high-purity urea and 67.5% by mass of ultrapure water. As China strengthens its efforts to prevent and control air pollution, automotive urea solution, as an environmentally friendly product with both economical and environmentally friendly advantages, is widely used in diesel engine exhaust treatment and has broad application prospects.

[0005] When using selective catalytic reduction technology to treat exhaust gas, one of the most prominent problems encountered is the crystallization of automotive urea solution. Specifically, the problem of automotive urea solution crystallization to varying degrees occurs in the head of the automotive urea solution nozzle, the inner wall of the exhaust pipe and the mixer. The problem of automotive urea solution crystallization will lead to increased exhaust back pressure, increased fuel consumption and worsening emissions, seriously affecting the operation of the exhaust system of high-power diesel engines and even causing damage to high-power diesel engines.

[0006] Regarding the causes of the crystallization problem of automotive urea solution, in addition to the injection volume of automotive urea solution, exhaust gas temperature, exhaust gas flow rate and hardware factors, there is also the quality of automotive urea solution. Product quality analysis of automotive urea aqueous solution. Teng Jiangbo, Du Bohui, Bai Linzhi, Zou Huiling, Xia Pandeng. Chemical Engineering Management Issue 9. In March 2024, it was disclosed that when the urea content in the automotive urea solution meets the standard, that is, when the mass fraction of urea in the automotive urea solution is 32.5%, the crystallization point of the automotive urea solution is -11.5℃. When the urea content in the automotive urea solution does not meet the standard, it will affect the crystallization point of the automotive urea solution, resulting in the crystallization problem of the automotive urea solution; the impact of automotive urea solution quality on the SCR system. Hao Chunxiao, Chen Weicheng, Xie Qiong, Zhao Ying, Wang Yanjun, Teng Qi, Ji Zhe, Xiao Han, Fu Yunfang. Environmental and Sustainable Development. Sustainability Development, Issue 1, 2016, January 2016, disclosed impurities in automotive urea solutions, primarily aldehydes, biuret, insoluble matter, phosphates, and metal ions. Among these, aldehydes and biuret are prone to polymerization, forming high-molecular substances that clog pipelines and catalyst frames. Insoluble matter can easily clog nozzles, damaging air pumps and affecting the spray efficiency of automotive urea solutions. Excessive levels of phosphates and metal ions can poison the catalyst and reduce its activity. Xu Yaohui, Research on the Process of Producing Automotive Urea Solutions from Urea, China Nitrogen Fertilizer, Issue 6, November 2017, disclosed that metal ions can cause clogging of automotive urea solution pipelines and nozzles. In summary, substandard urea content or excessively high impurity levels in automotive urea solutions can lead to crystallization. Therefore, the national standard GB 29518-2013 has made strict requirements on the urea content and impurity content in automotive urea solution, requiring the mass fraction of urea to be 31.8-33.2%, the content of aldehydes (calculated as HCHO) ≤5 mg / kg, the mass fraction of biuret ≤0.3%, the content of insoluble matter ≤20 mg / kg, the content of phosphate (calculated as PO4) ≤0.5 mg / kg, the content of calcium, iron, aluminum, magnesium, sodium and potassium ≤0.5 mg / kg, and the content of copper, zinc, chromium and nickel ≤0.2 mg / kg.

[0007] Currently, commonly used methods for preparing automotive urea solutions include industrial urea purification, agricultural urea purification, aqueous solution full-circulation, carbon dioxide stripping, and ammonia stripping. Among these, industrial urea purification is the most traditional method for preparing automotive urea solutions. A study on the process for producing automotive urea using a urea production device was published by Li Lifang and Wang Yonghong in Coal Chemical Industry, Volume 47, Issue 6, in December 2019. The method involves first hydrolyzing industrial urea with ultrapure water at 70-75°C to produce an industrial urea solution. The solution is then cooled to the crystallization temperature for crystallization, yielding high-purity crystalline urea. Each hydrolysis and crystallization step increases the purity of the crystalline urea. Generally, after one to two cycles of hydrolysis and crystallization, high-purity urea is obtained. The resulting high-purity urea is then dissolved in ultrapure water to produce an automotive urea solution.

[0008] However, this preparation method has the following problems:

[0009] First, a quality analysis of automotive urea aqueous solution. Teng Jiangbo, Du Bohui, Bai Linzhi, Zou Huiling, and Xia Pandeng. Chemical Engineering Management, Issue 9, March 2024. The main reason for substandard aldehyde content is that during the production of the raw urea, the final step in forming large urea granules requires mixing urea solution with formaldehyde. Formaldehyde slows the urea's moisture absorption, preventing urea granules from hardening and pulverizing, thus extending storage time. However, poorly controlled production process conditions can easily lead to excessive formaldehyde content in the urea, which can then be introduced into the subsequent automotive urea solution. Furthermore, formaldehyde may also be present in the storage tanks and pipelines (materials containing plastics and adhesives) used during the production and storage of the automotive urea solution, which can also be introduced into the automotive urea solution, resulting in substandard aldehyde content in the automotive urea solution. Experimental Study on Reducing Aldehydes in Automotive Urea. Ding Yan. Nitrogen Fertilizer Technology, Vol. 38, No. 6, 2017. It was disclosed in mid-December 2017 that in the current large-granular urea production process, adding formaldehyde to the evaporation system will greatly increase the particle size and strength of the finished urea product, improve the appearance quality of the finished urea product, and extend the storage time of the finished urea product. However, it will directly lead to unqualified aldehyde content in the automotive urea solution prepared with it.

[0010] To address the above issues, an experimental study on reducing aldehydes in automotive urea was conducted. Ding Yan. Nitrogen Fertilizer Technology, Vol. 38, No. 6, 2017. In December 2017, it was disclosed that by subjecting industrial urea to a single hydrolysis and crystallization step, the content of methylene diurea and, indirectly, the formaldehyde content was reduced. Furthermore, an automotive urea solution that met the national standard GB 29518-2013 was obtained. The industrial urea used, when directly prepared into automotive urea solution, has an aldehyde content (calculated as HCHO) of 13.4-14.1 mg / kg. However, no experiments have been conducted with industrial urea containing higher aldehydes. A study on the purification and preparation of urea by recrystallization was published by Meng Jiaoran, Duan Manlei, Xia Wa, Qi Yu, and Ding Min. Chemical Reagents, Vol. 42, No. 2, published in February 2020. The biuret content and urea yield decrease with increasing recrystallization cycles. The urea yield is approximately 80% for a single recrystallization, 65% for a second, and 52% for a third. To achieve a high urea yield and low impurity content, a second recrystallization cycle was chosen. The first automotive urea film-adding process trial in China was published by Pan Hui. Chemical Engineering Management, published in January 2015. It was also reported that industrial urea is generally hydrolyzed and crystallized several times to meet the purity standards for automotive urea. Therefore, for industrial urea with different impurity contents, as the impurity content increases, the number of hydrolysis and crystallization steps required increases, which further leads to a decrease in urea yield.

[0011] Second, a summary of research on impurity issues in automotive urea solutions. Wang Pan, Li Hongming, Chen Pengjiao, and Ren Mengwei (Shandong Chemical Industry, 2020, Vol. 49, September 2020) disclosed that precipitation is prone to occur during the post-preparation process of automotive urea solutions. Furthermore, dust on the packaging of automotive urea granules, the production environment for preparing automotive urea solutions, and the storage temperature of the prepared automotive urea solutions all affect the generation of impurities. The paper also disclosed that research on the precipitation of automotive urea solutions revealed that addressing the precipitation issue at the source of production is essentially impossible.

[0012] In response to the above problems, a summary of research on impurity issues in automotive urea solutions was published. Wang Pan, Li Hongming, Chen Pengjiao, and Ren Mengwei. Shandong Chemical Industry, Vol. 49, 2020. In September 2020, it was also disclosed that by filtering and ultrafiltration of the prepared automotive urea solution, the precipitation in the automotive urea can be effectively removed without affecting the content of other components in the automotive urea solution. However, according to the research on the efficient concentration of urea aqueous solution by multi-effect membrane distillation. Zhang Lili. Master's thesis of Tianjin University. In June 2012, when ultrafiltration is carried out for a period of time, the membrane flux of the component decreases, causing membrane fouling. According to the experimental research on direct ultrafiltration of mine water. Liang Zhen. Master's thesis of Hebei University of Engineering. In December 2023, turbidity will affect the treatment effect. The higher the turbidity, the lower the membrane flux. Although the study on efficient concentration of urea aqueous solution by multi-effect membrane distillation. Zhang Lili. Tianjin University master's thesis. In June 2012, the effect of mine water turbidity on membrane flux was disclosed, the same problem also exists for automotive urea solution if the turbidity is high. That is, when treating automotive urea solution with high turbidity, there is a serious problem of membrane fouling, which leads to a rapid decrease in membrane flux. Summary of the Invention

[0013] In response to the deficiencies in the prior art, the present invention provides a high-efficiency anti-crystallization automotive urea solution and a preparation method thereof. During preparation, industrial urea is used as a raw material. The prepared high-efficiency anti-crystallization automotive urea solution has a low impurity content, is precipitate-free, and has a high urea yield. The urea yield is less affected by the impurity content in the industrial urea. When the precipitate in the urea solution is removed by ultrafiltration, the membrane flux is less affected by the precipitate content in the urea solution, and membrane contamination problems can also be avoided.

[0014] In order to solve the above technical problems, the technical solutions adopted by the present invention are as follows:

[0015] A method for preparing a high-efficiency anti-crystallization vehicle urea solution, comprising: preparing an inclusion compound, hydrolyzing and crystallizing, dissolving, filtering, fine filtering, and ultrafiltration;

[0016] The method for preparing the inclusion compound comprises mixing polyethylene glycol, industrial urea, and methanol, stirring the mixture at 70-75° C. and 100-400 rpm for 30-40 minutes, naturally cooling the mixture to room temperature, performing low-temperature treatment in a sealed environment, filtering the mixture, collecting the filter residue, and vacuum freeze-drying the filter residue to obtain the inclusion compound;

[0017] In the method for preparing the inclusion compound, the mass ratio of polyethylene glycol, industrial urea, and methanol is 40-42:200-230:600-650;

[0018] The number average molecular weight of the polyethylene glycol is 2000;

[0019] The low temperature treatment is to place the sample in an environment of 1-3°C for 2-2.5 hours, or in an environment of -25°C to -20°C for 2-2.5 hours;

[0020] The vacuum freeze drying process is performed at a vacuum degree of 5-30 Pa, a temperature of -45°C to -35°C, and a time of 30-40 hours;

[0021] The inclusion compound needs to be stored at 2-5°C;

[0022] The hydrolysis crystallization method comprises the following steps: mixing industrial urea and tertiary water, stirring at 70-75° C. and 280-300 rpm for 30-40 minutes, maintaining the stirring speed constant, sequentially performing a first gradient cooling, a second gradient cooling, adding the inclusion compound, sequentially performing a third gradient cooling, a fourth gradient cooling, then keeping the temperature for 18-20 minutes, centrifuging, and collecting a precipitate as the purified urea;

[0023] In the hydrolysis and crystallization method, the mass ratio of industrial urea, tertiary water, and inclusion compound is 365-380:135-140:5-5.2;

[0024] The first gradient cooling is to cool the temperature at a cooling rate of 0.05°C / min for 100-120min;

[0025] The second gradient cooling is to cool the temperature at a cooling rate of 0.1°C / min for 30-35 minutes;

[0026] The third gradient cooling is to cool the temperature at a cooling rate of 0.1°C / min for 30-35 minutes;

[0027] The fourth gradient cooling is to cool the temperature at a cooling rate of 0.2°C / min for 100-110 minutes;

[0028] The centrifugal speed is 3500-4000 rpm and the time is 5-6 min;

[0029] The dissolution method comprises mixing the purified urea with tertiary water, and stirring the mixture at 40-45° C. and 200-400 rpm for 20-30 minutes to obtain a urea solution;

[0030] In the dissolution method, the mass ratio of purified urea to tertiary water is 32.5:67.5;

[0031] The filtering method comprises filtering the urea solution using a bag filter device to obtain a filtered urea solution;

[0032] In the filtering method, the filtering accuracy of the bag filter device is 1 μm, and the filter screen used in the bag filter device is made of polyethylene;

[0033] The fine filtration method comprises: using a fine filtration device to finely filter the filtered urea solution to obtain a finely filtered urea solution;

[0034] In the fine filtration method, the filtration accuracy of the fine filtration equipment is 0.25 μm, and the filter element used in the fine filtration equipment is made of polypropylene;

[0035] The ultrafiltration method comprises the following steps: using an ultrafiltration device to ultrafilter the finely filtered urea solution to obtain a vehicle-use urea solution;

[0036] In the ultrafiltration method, the filtration accuracy of the ultrafiltration equipment is 0.01 μm, the filter membrane used in the ultrafiltration equipment is made of polysulfone, and the stable pure water flux at 25°C and 0.1 MPa is 100 L / (m 2 •h);

[0037] The pressure during the ultrafiltration is 0.24-0.26 MPa.

[0038] The tertiary water used in the hydrolysis crystallization and the dissolution is tertiary water that complies with the national standard GB / T 6682-2008;

[0039] The conductivity of the tertiary water used in the hydrolysis crystallization and the dissolution at 25° C. was 0.31 mS / m.

[0040] A vehicle urea solution prepared by the above preparation method.

[0041] Compared with the prior art, the present invention has the following beneficial effects:

[0042] (1) Study on the process of preparing high-purity automotive urea by cooling crystallization. Li Pan. Master's thesis of Dalian University of Technology. June 2017. Crystals are solid substances with internal structural units arranged in a three-dimensional orderly manner. Under given thermodynamic conditions, crystals crystallized from solution have a specific crystal morphology, which is called crystal habit. Crystal morphology and size distribution have a significant impact on the quality of downstream processes and final products. Small crystals are difficult to wash and filter, and will also carry a large amount of mother liquor containing more impurities, affecting the final crystal quality. Crystals with a particularly wide size distribution are also not conducive to filtration. Small crystals will be inserted into the gaps left by large crystals, making it difficult for the filtrate to pass through the crystal bed. For crystals with a crystal habit of tetrahedral prisms, its aspect ratio also has a significant impact on the downstream process. Needle-shaped crystals with a large aspect ratio are easy to break or stack, which is not conducive to washing and drying, while rod-shaped crystals or cubic crystals with a small aspect ratio have better washing or drying effects. Crystal morphology also includes the roughness of the crystal surface. For crystals with defects on the crystal surface, mother liquor is easy to adhere to and contain, affecting the quality of the crystal. If the crystals adhere excessively to the mother liquor, the impurity content in the crystals will increase due to impurities in the mother liquor. Therefore, minimizing the aspect ratio of the crystals, reducing the surface roughness, and cleaning the crystals can reduce the impurity content in the urea crystals. "Study on the Process of Preparing High-Purity Automotive Urea by Cooling Crystallization," by Li Pan, a master's thesis from Dalian University of Technology, published in June 2017, also revealed that as the crystallization process progresses, the crystals transform from needle-shaped to rod-shaped. A slow cooling rate (0.05°C / min) can inhibit nucleation and reduce the aspect ratio from 8.9 to 6.5, but the final product still has a large aspect ratio. If the cooling rate is slow initially and then gradually increased, the initial slow cooling can inhibit nucleation, while the later rapid cooling can ensure radial growth, resulting in crystals with larger radial dimensions and smaller aspect ratios. However, the later rapid nucleation rate results in the presence of fine crystals, which can easily affect filtration. Fine crystals are also easily lost during filtration, resulting in quality loss. In the crystallization process of the present invention, an inclusion compound is added while gradually increasing the cooling temperature. The inclusion compound consists of urea and polyethylene glycol. The crystallization of urea is generally a tetragonal crystal. After polyethylene glycol is added to the urea solution, urea molecules include the polyethylene glycol to form hexagonal inclusion compound crystals for precipitation. During freeze-drying and low-temperature storage, the molecular chains of the polyethylene glycol are folded. After the inclusion compound is added, the molecular chains of the polyethylene glycol straighten as the temperature rises, thereby inhibiting the generation of crystal nuclei on the urea surface and increasing the nucleation barrier. The inclusion compound can also be combined with urea to grow on the surface to obtain urea crystals with smooth surfaces, reducing the adhesion and inclusion of the mother liquor, ensuring that the prepared high-efficiency anti-crystallization vehicle urea solution has a low impurity content, and the urea yield is less affected by the impurity content in the industrial urea. Therefore, whether the inclusion compound is added and the addition time can affect the quality of the vehicle urea solution, thereby improving the anti-crystallization ability of the vehicle urea.Furthermore, compared to directly adding urea and polyethylene glycol, the urea in the inclusion complex can form a complex with the polyethylene glycol, resulting in a stronger binding force between the urea and polyethylene glycol, ensuring that more polyethylene glycol adheres to the urea surface and controls crystal morphology. Furthermore, based on the impact of urea solution crystallization on SCR system injection components and anti-crystallization strategies (Hu Jiannan, Zhang Qingtang, and Qu Zhaoxue, Petrochemical Technology, 2025, Issue 5, May 2025), polyethylene glycol can serve as an anti-crystallization agent for automotive urea. Therefore, when the polyethylene glycol in the inclusion complex adheres to the surface of purified urea and enters the automotive urea solution, it can also improve the anti-crystallization properties of the automotive urea solution.

[0043] (2) Research on the efficient concentration of urea aqueous solution by multi-effect membrane distillation. Zhang Lili. Master's thesis of Tianjin University. In June 2012, the paper disclosed the following solutions to the membrane fouling and concentration polarization problems in ultrafiltration: In the low-pressure zone: select a membrane material with strong hydrophilicity or strong hydrophobicity according to the properties of the feed liquid, and the membrane surface charge is consistent with the solute charge in the solution. The pore size of the membrane should not be too large, preferably one order of magnitude smaller than the diameter of the macromolecular retention particle; adjust the pH appropriately; increase the temperature and reduce the viscosity according to the properties of the solution. In the medium-pressure zone: increase the feed liquid flow rate; increase the feed liquid temperature; adopt a suitable ultrafiltration membrane module structure. In the high-pressure zone: remove the gel layer in time. However, for automotive urea solutions with high turbidity, how to pre-treat the automotive urea solution is not disclosed. In the present invention, since polyethylene glycol is combined in the formation of urea crystals, polyethylene glycol can be used as a precipitant dispersant in ultrafiltration to prevent the precipitate from agglomerating and adhering to the ultrafiltration membrane surface, thereby alleviating the serious problem of membrane fouling.

[0044] (3) The present invention uses industrial urea as a raw material during preparation. The prepared high-efficiency anti-crystallization automotive urea solution has a low impurity content, no precipitation, and a high urea yield. The urea yield is less affected by the impurity content in the industrial urea.

[0045] (4) When the present invention removes the precipitate in the urea solution by ultrafiltration, the membrane flux is less affected by the precipitate content in the urea solution, and the membrane fouling problem can also be avoided. DETAILED DESCRIPTION

[0046] In order to have a clearer understanding of the technical features, purposes and effects of the present invention, the specific embodiments of the present invention are now described.

[0047] The tertiary water used in the examples and comparative examples all complies with the national standard GB / T 6682-2008, and the conductivity of the tertiary water used at 25° C. is 0.31 mS / m.

[0048] In the Examples and Comparative Examples, when testing the contents of aldehydes, biuret, insoluble matter, phosphate, calcium, iron, aluminum, magnesium, potassium, iron, aluminum, magnesium, sodium, potassium, copper, zinc, chromium, and nickel in the urea solution, the detection methods disclosed in the national standard GB 29518-2013 were followed.

[0049] Example 1

[0050] The industrial urea used in this embodiment was directly mixed with tertiary water to prepare a urea solution with a mass fraction of 32.5%, in which the content of aldehydes (calculated as HCHO) was 14.5 mg / kg, the mass fraction of biuret was 0.31%, the content of insoluble matter was 19 mg / kg, the content of phosphate (calculated as PO4) was 0.42 mg / kg, the content of calcium was 0.18 mg / kg, the content of iron was 0.34 mg / kg, the content of aluminum was 0.37 mg / kg, the content of magnesium was 0.26 mg / kg, the content of sodium was 0.40 mg / kg, the content of potassium was 0.32 mg / kg, the content of copper was 0.09 mg / kg, the content of zinc was 0.11 mg / kg, the content of chromium was 0.07 mg / kg, and the content of nickel was 0.08 mg / kg.

[0051] A method for preparing a high-efficiency anti-crystallization vehicle urea solution, specifically comprising:

[0052] 1. Preparation of inclusion compound: 40 g polyethylene glycol, 200 g industrial urea, and 600 g methanol were added to a reaction device, the temperature in the reaction device was adjusted to 70°C, the stirring speed was adjusted to 100 rpm, and the mixture was stirred for 30 min. After natural cooling to room temperature, the mixture was transferred to a sealed device, and the sealed device was allowed to stand for 2 h in an environment of 1°C and then for 2 h in an environment of -25°C. The mixture was filtered and the filter residue was taken. The filter residue was transferred to a vacuum freeze-drying device, the vacuum degree of the vacuum freeze-drying device was adjusted to 5 Pa, the temperature was adjusted to -45°C, and the mixture was vacuum freeze-dried for 30 h to obtain an inclusion compound. The inclusion compound was transferred to a low-temperature storage device, the temperature of the low-temperature storage device was adjusted to 4°C, and the mixture was stored.

[0053] The number average molecular weight of the polyethylene glycol is 2000;

[0054] 2. Hydrolysis and crystallization: 365 g of industrial urea and 135 g of tertiary water were added to the reaction equipment, the temperature in the reaction equipment was adjusted to 70 ° C, the stirring speed was adjusted to 280 rpm, and stirred for 30 min. The stirring speed was kept constant, and the temperature was lowered at a cooling rate of 0.05 ° C / min for 100 min, and then at a cooling rate of 0.1 ° C / min for 30 min. 5 g of inclusion compound was added, and the temperature was further lowered at a cooling rate of 0.1 ° C / min for 30 min, and then at a cooling rate of 0.2 ° C / min for 100 min. Then, the temperature was kept warm for 18 min, and the reaction mixture was transferred to a centrifugal device, the centrifugal speed of the centrifugal device was adjusted to 3500 rpm, and centrifuged for 5 min. The precipitate was taken as the purified urea;

[0055] 3. Dissolution: Add all the purified urea obtained in step 2 and tertiary water in a mass ratio of 32.5:67.5 into the reaction equipment, adjust the temperature in the reaction equipment to 40°C, adjust the stirring speed to 200 rpm, and stir for 20 minutes to obtain a urea solution;

[0056] 4. Filtration: Use bag filter equipment to filter all the urea solution obtained in step 3 to obtain filtered urea solution;

[0057] The filtration accuracy of the bag filter equipment is 1 μm, and the filter mesh used in the bag filter equipment is made of polyethylene;

[0058] 5. Fine filtration: Use fine filtration equipment to finely filter the entire filtered urea solution obtained in step 4 to obtain a finely filtered urea solution;

[0059] The filtration accuracy of the fine filtration equipment is 0.25 μm, and the filter element used in the fine filtration equipment is made of polypropylene;

[0060] 6. Ultrafiltration: Ultrafiltration equipment is used to ultrafilter the entire finely filtered urea solution obtained in step 5 to obtain a vehicle urea solution;

[0061] The filtration accuracy of the ultrafiltration equipment is 0.01 μm. The filter membrane used in the ultrafiltration equipment is made of polysulfone. The stable pure water flux at 25°C and 0.1 MPa is 100 L / (m 2 •h);

[0062] The pressure during the ultrafiltration was 0.25 MPa.

[0063] This embodiment also provides a vehicle urea solution prepared by the above preparation method.

[0064] In the automotive urea solution obtained in this embodiment, the content of aldehydes (calculated as HCHO) is 1.4 mg / kg, the mass fraction of biuret is 0.09%, the content of insoluble matter is 3.4 mg / kg, the content of phosphate (calculated as PO4) is 0.21 mg / kg, the content of calcium is 0.10 mg / kg, the content of iron is 0.05 mg / kg, the content of aluminum is 0.06 mg / kg, the content of magnesium is 0.06 mg / kg, the content of sodium is 0.09 mg / kg, the content of potassium is 0.11 mg / kg, the content of copper is 0.04 mg / kg, the content of zinc is 0.05 mg / kg, the content of chromium is 0.02 mg / kg, and the content of nickel is 0.03 mg / kg.

[0065] Comparative Example 1

[0066] A method for preparing a high-efficiency anti-crystallization automotive urea solution is specifically as follows: the same preparation method as in Example 1 is adopted, except that the first step of preparing the inclusion compound is omitted, and the addition of the inclusion compound is omitted in the second step of hydrolysis and crystallization.

[0067] This comparative example also provides a vehicle urea solution prepared by the above preparation method.

[0068] In the automotive urea solution obtained in this comparative example, the content of aldehydes (calculated as HCHO) is 3.0 mg / kg, the mass fraction of biuret is 0.17%, the content of insoluble matter is 3.9 mg / kg, the content of phosphate (calculated as PO4) is 0.28 mg / kg, the content of calcium is 0.13 mg / kg, the content of iron is 0.10 mg / kg, the content of aluminum is 0.12 mg / kg, the content of magnesium is 0.17 mg / kg, the content of sodium is 0.15 mg / kg, the content of potassium is 0.17 mg / kg, the content of copper is 0.07 mg / kg, the content of zinc is 0.08 mg / kg, the content of chromium is 0.04 mg / kg, and the content of nickel is 0.05 mg / kg.

[0069] Comparative Example 2

[0070] A method for preparing a high-efficiency anti-crystallization automotive urea solution is specifically as follows: the same preparation method as in Example 1 is used, except that the order of adding the inclusion compound in the second hydrolysis and crystallization step is changed to adding the inclusion compound before cooling at a cooling rate of 0.05°C / min for 100 minutes. Specifically, the second hydrolysis and crystallization step is changed to:

[0071] 365 g of industrial urea and 135 g of tertiary water were added to the reaction equipment, the temperature in the reaction equipment was adjusted to 70°C, the stirring speed was adjusted to 280 rpm, and the mixture was stirred for 30 min. The stirring speed was kept constant, 4 g of the inclusion compound was added, and the mixture was cooled at a cooling rate of 0.05°C / min for 100 min, cooled at a cooling rate of 0.1°C / min for 60 min, and cooled at a cooling rate of 0.2°C / min for 100 min. The mixture was then kept warm for 18 min, transferred to a centrifugal device, and the centrifugal speed of the centrifugal device was adjusted to 3500 rpm. The mixture was centrifuged for 5 min, and the precipitate was taken as the purified urea.

[0072] This comparative example also provides a vehicle urea solution prepared by the above preparation method.

[0073] In the automotive urea solution obtained in this comparative example, the content of aldehydes (calculated as HCHO) is 3.2 mg / kg, the mass fraction of biuret is 0.16%, the content of insoluble matter is 3.7 mg / kg, the content of phosphate (calculated as PO4) is 0.27 mg / kg, the content of calcium is 0.14 mg / kg, the content of iron is 0.11 mg / kg, the content of aluminum is 0.14 mg / kg, the content of magnesium is 0.11 mg / kg, the content of sodium is 0.20 mg / kg, the content of potassium is 0.19 mg / kg, the content of copper is 0.07 mg / kg, the content of zinc is 0.07 mg / kg, the content of chromium is 0.03 mg / kg, and the content of nickel is 0.05 mg / kg.

[0074] Comparative Example 3

[0075] A method for preparing a high-efficiency, crystallization-resistant automotive urea solution is described, specifically: using the same preparation method as Example 1, except that the first step of preparing the inclusion compound is omitted, the addition of the inclusion compound is omitted in the second step of hydrolysis and crystallization, and industrial urea stored at 4°C and polyethylene glycol stored at 4°C are additionally added. Specifically, the second step of hydrolysis and crystallization is changed to:

[0076] 365 g of industrial urea and 135 g of tertiary water were added to the reaction equipment, the temperature in the reaction equipment was adjusted to 70° C., the stirring speed was adjusted to 280 rpm, and the mixture was stirred for 30 min. The stirring speed was kept constant, and the mixture was cooled at a cooling rate of 0.05° C. / min for 100 min and at a cooling rate of 0.1° C. / min for 30 min. 4.2 g of industrial urea stored at a low temperature of 4° C. and 0.8 g of polyethylene glycol stored at a low temperature of 4° C. were added, and the mixture was further cooled at a cooling rate of 0.1° C. / min for 30 min and at a cooling rate of 0.2° C. / min for 100 min. The mixture was then kept warm for 18 min, and the mixture was transferred to a centrifugal device, the centrifugal speed of the centrifugal device was adjusted to 3500 rpm, and the mixture was centrifuged for 5 min. The precipitate was taken as the purified urea;

[0077] The number average molecular weight of the polyethylene glycol in the polyethylene glycol stored at a low temperature of 4° C. is 2000.

[0078] This comparative example also provides a vehicle urea solution prepared by the above preparation method.

[0079] In the automotive urea solution obtained in this comparative example, the content of aldehydes (calculated as HCHO) is 4.1 mg / kg, the mass fraction of biuret is 0.20%, the content of insoluble matter is 3.8 mg / kg, the content of phosphate (calculated as PO4) is 0.31 mg / kg, the content of calcium is 0.19 mg / kg, the content of iron is 0.13 mg / kg, the content of aluminum is 0.17 mg / kg, the content of magnesium is 0.11 mg / kg, the content of sodium is 0.21 mg / kg, the content of potassium is 0.18 mg / kg, the content of copper is 0.08 mg / kg, the content of zinc is 0.09 mg / kg, the content of chromium is 0.02 mg / kg, and the content of nickel is 0.05 mg / kg.

[0080] Example 2

[0081] The industrial urea used in this embodiment was directly mixed with tertiary water to prepare a urea solution with a mass fraction of 32.5%, in which the content of aldehydes (calculated as HCHO) was 14.5 mg / kg, the mass fraction of biuret was 0.21%, the content of insoluble matter was 19 mg / kg, the content of phosphate (calculated as PO4) was 0.42 mg / kg, the content of calcium was 0.18 mg / kg, the content of iron was 0.34 mg / kg, the content of aluminum was 0.37 mg / kg, the content of magnesium was 0.26 mg / kg, the content of sodium was 0.40 mg / kg, the content of potassium was 0.32 mg / kg, the content of copper was 0.09 mg / kg, the content of zinc was 0.11 mg / kg, the content of chromium was 0.05 mg / kg, and the content of nickel was 0.08 mg / kg.

[0082] A method for preparing a high-efficiency anti-crystallization vehicle urea solution, specifically comprising:

[0083] 1. Preparation of inclusion compound: 42 g polyethylene glycol, 230 g industrial urea, and 650 g methanol were added to a reaction device, the temperature in the reaction device was adjusted to 75°C, the stirring speed was adjusted to 400 rpm, and the mixture was stirred for 40 min. After natural cooling to room temperature, the mixture was transferred to a sealed device, and the sealed device was allowed to stand for 2.5 h in an environment of 3°C and for 2.5 h in an environment of -20°C. The residue was filtered and transferred to a vacuum freeze-drying device, the vacuum degree of the vacuum freeze-drying device was adjusted to 30 Pa, the temperature was adjusted to -35°C, and the mixture was vacuum freeze-dried for 40 h to obtain an inclusion compound. The inclusion compound was transferred to a low-temperature storage device, the temperature of the low-temperature storage device was adjusted to 4°C, and the mixture was stored.

[0084] The number average molecular weight of the polyethylene glycol is 2000;

[0085] 2. Hydrolysis and crystallization: 380 g of industrial urea and 140 g of tertiary water were added to the reaction equipment, the temperature in the reaction equipment was adjusted to 75 ° C, the stirring speed was adjusted to 300 rpm, and stirred for 40 min. The stirring speed was kept constant, and the temperature was lowered at a cooling rate of 0.05 ° C / min for 120 min, and then at a cooling rate of 0.1 ° C / min for 35 min. 5.2 g of the inclusion compound was added, and the temperature was further lowered at a cooling rate of 0.1 ° C / min for 35 min, and then at a cooling rate of 0.2 ° C / min for 110 min. Then, the mixture was kept warm for 20 min, transferred to a centrifugal device, and the centrifugal speed of the centrifugal device was adjusted to 4000 rpm. Centrifugation was carried out for 6 min, and the precipitate was taken as the purified urea;

[0086] 3. Dissolution: Add all the purified urea obtained in step 2 and tertiary water in a mass ratio of 32.5:67.5 into the reaction equipment, adjust the temperature in the reaction equipment to 45°C, adjust the stirring speed to 400 rpm, and stir for 30 minutes to obtain a urea solution;

[0087] 4. Filtration: Use bag filter equipment to filter all the urea solution obtained in step 3 to obtain filtered urea solution;

[0088] The filtration accuracy of the bag filter equipment is 1 μm, and the filter mesh used in the bag filter equipment is made of polyethylene;

[0089] 5. Fine filtration: Use fine filtration equipment to finely filter the entire filtered urea solution obtained in step 4 to obtain a finely filtered urea solution;

[0090] The filtration accuracy of the fine filtration equipment is 0.25 μm, and the filter element used in the fine filtration equipment is made of polypropylene;

[0091] 6. Ultrafiltration: Ultrafiltration equipment is used to ultrafilter the entire finely filtered urea solution obtained in step 5 to obtain a vehicle urea solution;

[0092] The filtration accuracy of the ultrafiltration equipment is 0.01 μm. The filter membrane used in the ultrafiltration equipment is made of polysulfone. The stable pure water flux at 25°C and 0.1 MPa is 100 L / (m 2 •h);

[0093] The pressure during the ultrafiltration was 0.25 MPa.

[0094] This embodiment also provides a vehicle urea solution prepared by the above preparation method.

[0095] In the automotive urea solution obtained in this embodiment, the content of aldehydes (calculated as HCHO) is 1.1 mg / kg, the mass fraction of biuret is 0.08%, the content of insoluble matter is 3.1 mg / kg, the content of phosphate (calculated as PO4) is 0.20 mg / kg, the content of calcium is 0.08 mg / kg, the content of iron is 0.05 mg / kg, the content of aluminum is 0.06 mg / kg, the content of magnesium is 0.05 mg / kg, the content of sodium is 0.08 mg / kg, the content of potassium is 0.08 mg / kg, the content of copper is 0.03 mg / kg, the content of zinc is 0.04 mg / kg, the content of chromium is 0.02 mg / kg, and the content of nickel is 0.02 mg / kg.

[0096] Example 3

[0097] A method for preparing a high-efficiency anti-crystallization automotive urea solution, specifically comprising: adopting the same preparation method as Example 1, except that different batches of industrial urea are used.

[0098] The industrial urea used in this embodiment was directly mixed with tertiary water to prepare a urea solution with a mass fraction of 32.5%, in which the content of aldehydes (calculated as HCHO) was 31.8 mg / kg, the mass fraction of biuret was 0.43%, the content of insoluble matter was 29 mg / kg, the content of phosphate (calculated as PO4) was 0.62 mg / kg, the content of calcium was 0.31 mg / kg, the content of iron was 0.40 mg / kg, the content of aluminum was 0.65 mg / kg, the content of magnesium was 0.37 mg / kg, the content of sodium was 0.53 mg / kg, the content of potassium was 0.46 mg / kg, the content of copper was 0.12 mg / kg, the content of zinc was 0.15 mg / kg, the content of chromium was 0.20 mg / kg, and the content of nickel was 0.29 mg / kg.

[0099] This embodiment also provides a vehicle urea solution prepared by the above preparation method.

[0100] In the automotive urea solution obtained in this embodiment, the content of aldehydes (calculated as HCHO) is 1.5 mg / kg, the mass fraction of biuret is 0.11%, the content of insoluble matter is 4.3 mg / kg, the content of phosphate (calculated as PO4) is 0.23 mg / kg, the content of calcium is 0.12 mg / kg, the content of iron is 0.07 mg / kg, the content of aluminum is 0.07 mg / kg, the content of magnesium is 0.08 mg / kg, the content of sodium is 0.08 mg / kg, the content of potassium is 0.10 mg / kg, the content of copper is 0.05 mg / kg, the content of zinc is 0.06 mg / kg, the content of chromium is 0.04 mg / kg, and the content of nickel is 0.05 mg / kg.

[0101] Comparative Example 4

[0102] A method for preparing a high-efficiency anti-crystallization automotive urea solution is specifically as follows: the same preparation method as Example 3 is adopted, except that the first step of preparing the inclusion compound is omitted, and the addition of the inclusion compound is omitted in the second step of hydrolysis and crystallization.

[0103] This comparative example also provides a vehicle urea solution prepared by the above preparation method.

[0104] In the automotive urea solution obtained in this comparative example, the content of aldehydes (calculated as HCHO) is 5.5 mg / kg, the mass fraction of biuret is 0.25%, the content of insoluble matter is 4.5 mg / kg, the content of phosphate (calculated as PO4) is 0.37 mg / kg, the content of calcium is 0.15 mg / kg, the content of iron is 0.19 mg / kg, the content of aluminum is 0.34 mg / kg, the content of magnesium is 0.23 mg / kg, the content of sodium is 0.22 mg / kg, the content of potassium is 0.17 mg / kg, the content of copper is 0.07 mg / kg, the content of zinc is 0.08 mg / kg, the content of chromium is 0.11 mg / kg, and the content of nickel is 0.13 mg / kg.

[0105] Comparative Example 5

[0106] A method for preparing a high-efficiency anti-crystallization automotive urea solution is specifically as follows: the same preparation method as in Example 3 is used, except that the order of adding the inclusion compound in the second hydrolysis and crystallization step is changed to adding the inclusion compound before cooling at a cooling rate of 0.05°C / min for 100 minutes. Specifically, the second hydrolysis and crystallization step is changed to:

[0107] 365 g of industrial urea and 135 g of tertiary water were added to the reaction equipment, the temperature in the reaction equipment was adjusted to 70°C, the stirring speed was adjusted to 280 rpm, and the mixture was stirred for 30 min. The stirring speed was kept constant, 4 g of the inclusion compound was added, and the mixture was cooled at a cooling rate of 0.05°C / min for 100 min, cooled at a cooling rate of 0.1°C / min for 60 min, and cooled at a cooling rate of 0.2°C / min for 100 min. The mixture was then kept warm for 18 min, transferred to a centrifugal device, and the centrifugal speed of the centrifugal device was adjusted to 3500 rpm. The mixture was centrifuged for 5 min, and the precipitate was taken as the purified urea.

[0108] This comparative example also provides a vehicle urea solution prepared by the above preparation method.

[0109] In the automotive urea solution obtained in this comparative example, the content of aldehydes (calculated as HCHO) is 6.1 mg / kg, the mass fraction of biuret is 0.27%, the content of insoluble matter is 5.1 mg / kg, the content of phosphate (calculated as PO4) is 0.39 mg / kg, the content of calcium is 0.18 mg / kg, the content of iron is 0.21 mg / kg, the content of aluminum is 0.33 mg / kg, the content of magnesium is 0.21 mg / kg, the content of sodium is 0.25 mg / kg, the content of potassium is 0.14 mg / kg, the content of copper is 0.08 mg / kg, the content of zinc is 0.10 mg / kg, the content of chromium is 0.11 mg / kg, and the content of nickel is 0.12 mg / kg.

[0110] Comparative Example 6

[0111] A method for preparing a high-efficiency, crystallization-resistant automotive urea solution is described, specifically: using the same preparation method as Example 3, except that the first step of preparing the inclusion compound is omitted, the addition of the inclusion compound is omitted in the second step of hydrolysis and crystallization, and industrial urea stored at 4°C and polyethylene glycol stored at 4°C are additionally added. Specifically, the second step of hydrolysis and crystallization is changed to:

[0112] 365 g of industrial urea and 135 g of tertiary water were added to the reaction equipment, the temperature in the reaction equipment was adjusted to 70° C., the stirring speed was adjusted to 280 rpm, and the mixture was stirred for 30 min. The stirring speed was kept constant, and the mixture was cooled at a cooling rate of 0.05° C. / min for 100 min and at a cooling rate of 0.1° C. / min for 30 min. 4.2 g of industrial urea stored at a low temperature of 4° C. and 0.8 g of polyethylene glycol stored at a low temperature of 4° C. were added, and the mixture was further cooled at a cooling rate of 0.1° C. / min for 30 min and at a cooling rate of 0.2° C. / min for 100 min. The mixture was then kept warm for 18 min, and the mixture was transferred to a centrifugal device, the centrifugal speed of the centrifugal device was adjusted to 3500 rpm, and the mixture was centrifuged for 5 min. The precipitate was taken as the purified urea;

[0113] The number average molecular weight of the polyethylene glycol in the polyethylene glycol stored at a low temperature of 4° C. is 2000.

[0114] This comparative example also provides a vehicle urea solution prepared by the above preparation method.

[0115] In the automotive urea solution obtained in this comparative example, the content of aldehydes (calculated as HCHO) is 8.1 mg / kg, the mass fraction of biuret is 0.33%, the content of insoluble matter is 5.7 mg / kg, the content of phosphate (calculated as PO4) is 0.43 mg / kg, the content of calcium is 0.22 mg / kg, the content of iron is 0.24 mg / kg, the content of aluminum is 0.37 mg / kg, the content of magnesium is 0.20 mg / kg, the content of sodium is 0.24 mg / kg, the content of potassium is 0.15 mg / kg, the content of copper is 0.07 mg / kg, the content of zinc is 0.11 mg / kg, the content of chromium is 0.12 mg / kg, and the content of nickel is 0.14 mg / kg.

[0116] Test Example 1

[0117] The automotive urea solutions of Examples 1-3 and Comparative Examples 1-6 were observed for precipitation. The results were as follows:

[0118]

[0119] It can be seen from the above results that by performing filtration, fine filtration and ultrafiltration, it can be ensured that there is no precipitation in the obtained automotive urea solution.

[0120] Test Example 2

[0121] According to the preparation methods of the automotive urea solutions of Examples 1-3 and Comparative Examples 1-6, automotive urea was prepared, and the masses of the prepared automotive urea solutions were statistically analyzed. The statistical results are as follows:

[0122]

[0123] It can be seen from the above results that the mass of the automotive urea solutions prepared in Comparative Examples 1-3 is less than the mass of the automotive urea solution prepared in Example 1; the mass of the automotive urea solutions prepared in Comparative Examples 4-6 is less than the mass of the automotive urea solution prepared in Example 3; this indicates that whether the inclusion compound is added and the order of adding the inclusion compounds will affect the mass of the prepared automotive urea solutions, that is, the yield of urea.

[0124] Test Example 3

[0125] 100 batches of vehicle urea solutions were prepared repeatedly according to the preparation methods of Examples 1-3 and Comparative Examples 1-6, respectively. For Examples 1-3 and Comparative Examples 2 and 5, the inclusion complex required for 100 batches of vehicle urea solutions was first prepared, and then purified urea was prepared according to the amount used in the hydrolysis and crystallization. Then, one batch of vehicle urea solution was prepared according to the method of dissolution, filtration, fine filtration, and ultrafiltration. The process of hydrolysis and crystallization, dissolution, filtration, fine filtration, and ultrafiltration was repeated to prepare the remaining 99 batches of vehicle urea solutions. For Comparative Examples 1, 3, 4, and 6, purified urea was directly prepared according to the amount used in the hydrolysis and crystallization. Then, one batch of vehicle urea solution was prepared according to the method of dissolution, filtration, fine filtration, and ultrafiltration. The process of hydrolysis and crystallization, dissolution, filtration, fine filtration, and ultrafiltration was repeated.

[0126] After preparing 100 batches of automotive urea solutions, the ultrafiltration equipment used in the ultrafiltration was tested for its stable pure water flux at 25°C and 0.1 MPa. The test results are as follows:

[0127]

[0128] It can be seen from the above results that the pure water flux of the ultrafiltration membranes used in Comparative Examples 1 and 3 is lower than that of the ultrafiltration membrane used in Example 1. The pure water flux of the ultrafiltration membranes used in Comparative Examples 4 and 6 is lower than that of the ultrafiltration membrane used in Example 3.

[0129] Test Example 4

[0130] The crystallization points of the automotive urea solutions of Examples 1-3 and Comparative Examples 1-6 were tested, and the test results are as follows:

[0131]

[0132] It can be seen from the above results that the crystallization point of the automotive urea solution of Example 1 is lower than those of Comparative Examples 1-3, and the crystallization point of the automotive urea solution of Example 3 is lower than those of Comparative Examples 4-6.

Claims

1. A method for preparing a high-efficiency anti-crystallization vehicle urea solution, characterized in that: include: Preparation of inclusion compounds, hydrolysis and crystallization, dissolution, filtration, fine filtration, and ultrafiltration; The method for preparing the inclusion compound comprises mixing polyethylene glycol, industrial urea and methanol, stirring at 70-75° C., cooling, performing low-temperature treatment in a sealed environment, filtering, collecting the filter residue, and vacuum freeze-drying to obtain the inclusion compound; In the method for preparing the inclusion compound, the number average molecular weight of the polyethylene glycol is 2000; The hydrolysis crystallization method comprises mixing industrial urea and tertiary water, stirring at 70-75° C. and 280-300 rpm, sequentially performing a first gradient cooling, a second gradient cooling, adding the inclusion compound, sequentially performing a third gradient cooling, a fourth gradient cooling, then keeping the temperature, centrifuging, and taking a precipitate as the purified urea; The dissolution method is to mix the purified urea with tertiary water and stir at 40-45° C. to obtain a urea solution; In the dissolution method, the mass ratio of purified urea to tertiary water is 32.5:67.

5.

2. The method for preparing a high-efficiency anti-crystallization vehicle urea solution according to claim 1, characterized in that: In the method for preparing the inclusion compound, the mass ratio of polyethylene glycol, industrial urea and methanol is 40-42:200-230:600-650.

3. The method for preparing a high-efficiency anti-crystallization vehicle urea solution according to claim 1, characterized in that: In the method for preparing the inclusion compound, the low-temperature treatment is to stand still for 2-2.5 hours in an environment of 1-3°C or to stand still for 2-2.5 hours in an environment of -25°C to -20°C.

4. The method for preparing a high-efficiency anti-crystallization vehicle urea solution according to claim 1, characterized in that: In the method for preparing the inclusion compound, the vacuum freeze drying is performed at a vacuum degree of 5-30 Pa, a temperature of -45°C to -35°C, and a time of 30-40 hours; The inclusion compound needs to be stored at 2-5°C.

5. The method for preparing a high-efficiency anti-crystallization vehicle urea solution according to claim 1, characterized in that: In the hydrolysis and crystallization method, the mass ratio of industrial urea, tertiary water, and inclusion compound is 365-380:135-140:5-5.

2.

6. The method for preparing a high-efficiency anti-crystallization vehicle urea solution according to claim 1, characterized in that: In the hydrolysis crystallization method, the first gradient cooling is to cool the temperature at a cooling rate of 0.05°C / min for 100-120min; The second gradient cooling is to cool the temperature at a cooling rate of 0.1°C / min for 30-35 minutes; The third gradient cooling is to cool the temperature at a cooling rate of 0.1°C / min for 30-35 minutes; The fourth gradient cooling is to cool the temperature at a cooling rate of 0.2°C / min for 100-110 min.

7. The method for preparing a high-efficiency anti-crystallization vehicle urea solution according to claim 1, characterized in that: The filtering method comprises filtering the urea solution using a bag filter device to obtain a filtered urea solution; In the filtering method, the filtering accuracy of the bag filter device is 1 μm, and the filter screen used in the bag filter device is made of polyethylene; The fine filtration method comprises: using a fine filtration device to finely filter the filtered urea solution to obtain a finely filtered urea solution; In the fine filtration method, the filtration accuracy of the fine filtration equipment is 0.25 μm, and the filter element used in the fine filtration equipment is made of polypropylene.

8. The method for preparing a high-efficiency anti-crystallization vehicle urea solution according to claim 1, characterized in that: The ultrafiltration method comprises the following steps: using an ultrafiltration device to ultrafilter the finely filtered urea solution to obtain a vehicle-use urea solution; In the ultrafiltration method, the filtration accuracy of the ultrafiltration equipment is 0.01 μm, the filter membrane used in the ultrafiltration equipment is made of polysulfone, and the stable pure water flux at 25°C and 0.1 MPa is 100 L / (m 2 •h); The pressure during the ultrafiltration is 0.24-0.26 MPa.

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