Preparation process of high-purity dipropylene glycol

Abstract

The invention belongs to the technical field of purification of chemical raw materials, and provides a preparation process of high-purity dipropylene glycol, which comprises the following steps: feeding a crude product into a first reduced pressure distillation tower, and extracting an intermediate product from the tower bottom after reduced pressure distillation; feeding the intermediate product into a second reduced pressure distillation tower, and extracting a distillation product from the tower top after reduced pressure distillation; enabling the distillation product to pass through an adsorption column filled with a composite adsorbent, and adsorbing to obtain an adsorption product; carrying out three-stage gradient cooling crystallization treatment on the adsorption product to obtain a crystallized product; and carrying out centrifugal separation on the crystallized product to obtain the high-purity dipropylene glycol. A progressive purification route of coarse screening, fine filtration and final extraction is formed through two-stage gradient reduced pressure distillation, dynamic adsorption of the composite adsorbent and three-stage gradient cooling crystallization, the product quality, the production efficiency and the cost control are balanced, and industrial production and popularization are facilitated.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of chemical raw material purification, and relates to a preparation process of high-purity dipropylene glycol. BACKGROUND

[0002] As an important chemical raw material, dipropylene glycol is widely used in polyester resin, medicine, cosmetics and high-performance lubricant fields, and the purity of the dipropylene glycol directly affects the performance of downstream products. Traditional preparation processes usually adopt normal pressure distillation combined with adsorption or crystallization purification, but in actual production, there are many challenges: normal pressure distillation needs a higher temperature (the normal pressure boiling point is about 230 DEG C or more), which easily leads to partial decomposition of heat-sensitive dipropylene glycol, generates by-products and reduces the yield; the separation efficiency of a single distillation column is limited, and it is difficult to effectively remove light and heavy component impurities, and the product purity can usually only reach 95% to 98%. In addition, the selective adsorption capacity of conventional adsorbents (such as activated carbon or single molecular sieve) for trace polar impurities (such as residual moisture and small molecule alcohols) is insufficient, and the traditional crystallization process often causes the crystal to wrap the impurities or the crystal grain to be too fine due to improper control of the cooling rate, and subsequent centrifugal separation is difficult. In the prior art, although some individual researches try to improve the purity by multi-stage vacuum distillation or combined purification means, there are problems such as complex process flow, high energy consumption or large equipment investment, which are difficult to meet the needs of continuous production of high-purity (≥99.5%) and low energy consumption. Therefore, it is of great significance to develop an efficient, stable and economical high-purity dipropylene glycol preparation process. SUMMARY

[0003] In view of the deficiencies in the prior art, the purpose of the present application is to provide a preparation process of high-purity dipropylene glycol, which forms a progressive purification and purification route of 'coarse screening-precision filtering-final extraction' through two-stage gradient vacuum distillation, dynamic adsorption of composite adsorbent and three-stage gradient cooling crystallization, so that the preparation process provided by the present application achieves a balance between product quality, production efficiency and cost control, and is beneficial to industrial production and popularization.

[0004] To achieve this purpose, the technical scheme adopted by the present application is as follows:

[0005] The present application provides a preparation process of high-purity dipropylene glycol, which comprises:

[0006] (I) feeding the crude product produced by the dipropylene glycol production process into a first vacuum distillation column, and collecting an intermediate product from the bottom after vacuum distillation; feeding the intermediate product into a second vacuum distillation column, and collecting a distillation product from the top after vacuum distillation;

[0007] (II) passing the distillation product obtained in step (I) through an adsorption column filled with a composite adsorbent composed of nano-zirconium oxide loaded with active alumina and amino-modified mesoporous molecular sieve to obtain an adsorption product after adsorption;

[0008] (III) performing three-stage gradient cooling crystallization treatment on the adsorption product obtained in step (II) to obtain a crystallization product; and then performing centrifugal separation on the crystallization product to obtain high-purity dipropylene glycol.

[0009] The present application provides a high-efficiency preparation method of high-purity dipropylene glycol, two-stage vacuum distillation first completes physical separation of most impurities, thereby reducing the burden of subsequent adsorption purification; the composite adsorbent focuses on treating specific impurities that are difficult to remove by distillation; and gradient crystallization utilizes the difference in solubility to complete the final purification. The present application forms a progressive purification and purification route of "coarse screening-precision filtering-final refining" through two-stage gradient vacuum distillation, dynamic adsorption of the composite adsorbent and three-stage gradient cooling crystallization, so that the preparation process provided by the present application achieves a balance between product quality, production efficiency and cost control, and is conducive to industrialized production and popularization.

[0010] The present application is aimed at purifying the crude product produced by the dipropylene glycol production process, which contains unreacted monomer propylene glycol, homologous dipropylene glycol, byproduct acrylic ester compounds, water, ether impurities (such as dipropylene glycol monomethyl ether), residual amine (such as triethylamine) and metal ions (Fe 3+ , Al 3+ ).

[0011] In the two-stage gradient vacuum distillation stage, the first vacuum distillation column is used to remove light component impurities such as unreacted monomer propylene glycol, trace water and low molecular weight ether impurities from the crude product. The remaining components are heavy components, which are enriched in the column bottom of the second vacuum distillation column. The heavy component impurities in the intermediate product are intercepted in the column bottom of the second vacuum distillation column. The target product dipropylene glycol is collected from the top of the second vacuum distillation column. The present application realizes step-by-step interception of light and heavy impurities through the design of double-column series connection. The first vacuum distillation column is used to remove light component impurities from the crude product, and the second vacuum distillation column is used to intercept heavy component impurities in the crude product, thereby providing a high-purity material basis for subsequent deep purification.

[0012] In the dynamic adsorption stage of the composite adsorbent, the distillation product rich in target product taken from the top of the second vacuum distillation column enters the adsorption column for dynamic adsorption. The composite adsorbent filled in the adsorption column is composed of nano-zirconium oxide loaded active alumina and amino-modified mesoporous molecular sieve. The nano-zirconium oxide loaded active alumina is mainly used to remove small polar molecules and metal ions in the distillation product, such as unreacted monomer propylene glycol and ether impurities (such as dipropylene glycol monomethyl ether). The amino-modified mesoporous molecular sieve is mainly used to remove basic substances (such as triethylamine) and moisture in the distillation product. In the dynamic adsorption process, the nano-zirconium oxide modified active alumina can strongly adsorb the residual small polar molecules such as ethylene glycol and short-chain alcohols in the distillation product due to its porous structure and high surface activity. The introduction of zirconium oxide particles also enhances the fixation ability of oxygen-containing compounds through chemical bonding. The mesoporous molecular sieve modified by amino group preferentially captures amine impurities and locks in trace moisture through hydrogen bonding due to its specific pore size (3.5-4.5 nm) and surface amino chemical properties. The nano-zirconium oxide loaded active alumina and the amino-modified mesoporous molecular sieve in the composite adsorbent produce a synergistic effect, forming a three-dimensional purification network. The nano-zirconium oxide loaded active alumina acts as an impurity adsorption carrier, mainly providing a large amount of adsorption space. The amino-modified mesoporous molecular sieve plays a selective filtering function. After the combination of the two, a hierarchical pore network structure with a pore size gradient is formed, allowing different types of impurities to be layered and trapped in the adsorption column, thereby systematically removing multiple impurities such as alcohols, ethers, amines, and moisture, and making the adsorption product meet the purity standards suitable for crystallization.

[0013] In the three-stage gradient cooling crystallization stage, the final removal of impurities is achieved through a staged temperature control scheme. In the rapid pre-cooling stage, the system is rapidly cooled to 30-40°C at a cooling rate of 2.5-3.5°C / min, which prevents random generation of impurity crystals by establishing a moderate supersaturation environment, and also prevents micro-bubbles from causing crystal defects by expelling dissolved gas. In the seed induction stage, high-purity seeds are added twice at temperatures of 18-20°C and 15-18°C, which helps to control the nucleation density. In combination with a stirring speed of 30-40 rpm, the crystal nuclei are uniformly distributed, and the homologous substance tripropylene glycol (which has a solubility 3-5 times higher than that of the target product) is removed from the adsorption product by taking advantage of the solubility difference between impurities and seed crystals. In the low-temperature crystallization stage, the system enters the stable crystallization zone at a slow cooling rate of 0.3-0.5°C / min, which guides the orderly arrangement of dipropylene glycol by reducing molecular thermal motion, while isomers and long-chain ether impurities that are difficult to integrate into the crystal lattice are expelled into the mother liquor. Through the combination of temperature change, stirring intensity, and seed addition method, the three cooling stages form a complete process route from crystal nucleus germination to crystal growth, ultimately obtaining a product with low impurity content and regular crystal form.

[0014] As a preferred technical solution of the present application, in step (I), the operating pressure of the first reduced-pressure distillation column is 10 kPa.

[0015] In some optional examples, the column bottom temperature of the first reduced-pressure distillation column is 130-140°C, for example, can be 130°C, 131°C, 132°C, 133°C, 134°C, 135°C, 136°C, 137°C, 138°C, 139°C or 140°C, but not only limited to the listed values, other values not listed in the range are also applicable.

[0016] In some optional examples, the column top temperature of the first reduced-pressure distillation column is 115-120°C, for example, can be 115°C, 116°C, 117°C, 118°C, 119°C or 120°C, but not only limited to the listed values, other values not listed in the range are also applicable.

[0017] In some optional examples, the reflux ratio of the first reduced-pressure distillation column is (8-9):1, for example, can be 8.0:1, 8.1:1, 8.2:1, 8.3:1, 8.4:1, 8.5:1, 8.6:1, 8.7:1, 8.8:1, 8.9:1 or 9.0:1, but not only limited to the listed values, other values not listed in the range are also applicable.

[0018] As a preferred technical solution of the present application, in step (I), the operating pressure of the second reduced-pressure distillation column is 1 kPa.

[0019] In some optional examples, the column bottom temperature of the second reduced-pressure distillation column is 135-145°C, for example, can be 135°C, 136°C, 137°C, 138°C, 139°C, 140°C, 141°C, 142°C, 143°C, 144°C or 145°C, but not only limited to the listed values, other values not listed in the range are also applicable.

[0020] In some optional examples, the column top temperature of the second reduced-pressure distillation column is 120-125°C, for example, can be 120°C, 121°C, 122°C, 123°C, 124°C or 125°C, but not only limited to the listed values, other values not listed in the range are also applicable.

[0021] In some optional examples, the reflux ratio of the second reduced-pressure distillation column is (5-6):1, for example, can be 5.0:1, 5.1:1, 5.2:1, 5.3:1, 5.4:1, 5.5:1, 5.6:1, 5.7:1, 5.8:1, 5.9:1 or 6.0:1, but not only limited to the listed values, other values not listed in the range are also applicable.

[0022] The present application particularly limits the reflux ratio of the second vacuum distillation column to (5-6):1. Within this reflux ratio range, sufficient gas-liquid contact efficiency can be maintained in the column, which ensures effective separation of target products and impurities and avoids excessive reflux energy waste.

[0023] When the reflux ratio of the second vacuum distillation column is less than 5:1, on the one hand, insufficient liquid reflux in the column causes incomplete coverage of the liquid film on the tray, and the "dry plate" phenomenon occurs in some areas, which reduces the theoretical plate number, causes light components to fail to rise to the top of the column, heavy components to fail to completely settle at the bottom of the column, and some impurities with a boiling point slightly lower than the target product to mix into the column overhead, and part of the heavy components to be retained in the middle section to form an azeotrope. On the other hand, insufficient liquid reflux in the column will cause the gas-liquid mass transfer efficiency to decrease, and more tray numbers are needed to achieve the same separation effect, which means incomplete separation for the actual distillation device with fixed column height. In this case, the purity of the distillation product collected at the top of the column is affected, and more recoverable target components are also left in the heavy components at the bottom of the column.

[0024] When the reflux ratio of the second vacuum distillation column exceeds 6:1, although the separation precision will continue to improve in theory, excessive reflux liquid will cause the liquid holdup in the column to increase, and under the condition of vacuum operation, liquid flooding will occur, the rising vapor cannot effectively penetrate through the thick liquid layer, causing pressure fluctuations in the column, and disrupting the gas-liquid balance. In addition, high reflux ratio will significantly increase the heat load of the reboiler, causing the steam consumption to rise, and the marginal benefit of the purity improvement brought by this part of additional energy consumption gradually decreases. More importantly, the strong reflux effect will "entrain" part of the heavy components at the bottom of the column to the top of the column, and these components that should have settled under normal reflux ratio are re-introduced into the rising vapor due to the excessive liquid reflux, resulting in a decrease in the purity of the distillation product collected at the top of the column.

[0025] As a preferred technical solution of the present application, in step (II), the filling amount of the composite adsorbent is 60-80% of the volume of the adsorption column, for example, it can be 60%, 62%, 64%, 66%, 68%, 70%, 72%, 74%, 76%, 78% or 80%, but is not limited to the listed values, and other values not listed within this range are also applicable.

[0026] In some optional examples, the height-diameter ratio of the adsorption column is (3-4):1, for example, it can be 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, 3.9:1 or 4.0:1, but is not limited to the listed values, and other values not listed within this range are also applicable.

[0027] In some optional examples, the space velocity of the distillation product is 1-2 h-1 for example, can be 1.0 h -1 , 1.1 h -1 , 1.2 h -1 , 1.3 h -1 , 1.4 h -1 , 1.5 h -1 , 1.6 h -1 , 1.7 h -1 , 1.8 h -1 , 1.9 h -1 or 2.0 h -1 , but not limited to the listed values, other unlisted values within the range are also applicable.

[0028] The present application particularly limits the space velocity of the distillation product to 1-2 h -1 When the space velocity is within this range, the material flow rate is neither too fast to cause insufficient adsorption nor too slow to result in low processing capacity.

[0029] When the space velocity of the distillation product is less than 1 h -1 , although theoretically longer contact time is beneficial to impurity adsorption, in actual operation, it will cause two problems: first, the processing efficiency is significantly reduced, and the material processing capacity per unit time is greatly reduced, which causes a bottleneck in continuous production; second, some adsorbed impurities may be desorbed under certain conditions, such as temperature fluctuations or concentration gradient changes, and the captured amine substances are released into the distillation product. In addition, too low a flow rate will weaken the degree of turbulent flow of the distillation product in the column, resulting in the formation of a laminar boundary layer on the surface of the composite adsorbent, which hinders the diffusion of impurity molecules to the adsorption sites.

[0030] When the space velocity of the distillation product exceeds 2 h -1 , although the processing capacity is improved, the adsorption effect will deteriorate rapidly. The high-speed flow of the distillation product forms a short flow phenomenon in the adsorption column, and part of the distillation product directly passes through the channel with less resistance quickly, without being able to fully contact the composite adsorbent. At the same time, the increase in pressure drop brought by high flow rate not only increases the pumping energy consumption, but also squeezes the composite adsorbent particles, which is prone to cause bed collapse in long-term operation, shortening the service life of the adsorption column.

[0031] In some optional examples, the adsorption temperature is 40-50℃, for example, can be 40℃, 41℃, 42℃, 43℃, 44℃, 45℃, 46℃, 47℃, 48℃, 49℃ or 50℃, but not limited to the listed values, other unlisted values within the range are also applicable.

[0032] As a preferred technical solution of the present application, in step (II), the composite adsorbent is prepared by the following method:

[0033] (1) mixing alumina powder and zirconium oxychloride solution to obtain a precursor solution, adjusting pH to initiate precipitation reaction, standing and aging, filtering, drying and calcining to obtain active alumina support;

[0034] (2) drying mesoporous molecular sieve, dispersing in silane coupling agent solution to obtain molecular sieve suspension, then water bath refluxing, filtering, washing and drying to obtain modified mesoporous molecular sieve;

[0035] (3) mixing active alumina support, modified mesoporous molecular sieve and hydroxypropyl methyl cellulose, adding water to prepare composite slurry, extruding and granulating to obtain wet granules, drying to obtain the composite adsorbent.

[0036] As a preferred technical solution of the present application, in step (II), the composite adsorbent is prepared by the following method:

[0037] (1) mixing active alumina powder and zirconium oxychloride solution and heating to obtain a precursor solution, adding ammonia solution dropwise to adjust the pH value to initiate precipitation reaction, standing and aging after reaction, obtaining reaction product, filtering, drying and calcining to obtain active alumina support;

[0038] (2) vacuum drying and dehydrating mesoporous molecular sieve to obtain dehydrated molecular sieve; immersing the dehydrated molecular sieve in silane coupling agent solution, ultrasonic dispersing to obtain molecular sieve suspension, transferring the molecular sieve suspension to a water bath kettle for refluxing, filtering, washing and drying after reaction to obtain modified mesoporous molecular sieve;

[0039] (3) mixing the active alumina support obtained in step (1), the modified mesoporous molecular sieve obtained in step (2) and hydroxypropyl methyl cellulose, adding deionized water to prepare composite slurry; extruding and granulating the composite slurry by a screw extruder to obtain wet granules, drying the wet granules to obtain the composite adsorbent.

[0040] In the process of preparing the composite adsorbent, the present application first introduces nano zirconium oxide on the active alumina substrate, and forms a stable composite structure through solution mixing and precipitation reaction. When the zirconium oxychloride solution is mixed with the active alumina powder, the active sites provided by the surface of the active alumina become the anchoring basis for the zirconium species. The stepwise addition of ammonia water promotes the uniform deposition of zirconium ions in the form of zirconium hydroxide on the surface and pores of the active alumina. This loading method not only retains the high specific surface area characteristics of the active alumina itself, but also enhances the affinity for oxygen-containing polar molecules through the introduction of the surface active sites of nano zirconium oxide. The calcination treatment converts the deposited layer into stable nano zirconium oxide crystals, thereby significantly improving the capture ability of the composite adsorbent for ethylene glycol, short-chain alcohol and other substances.

[0041] The active alumina supported by nano zirconium oxide forms a multi-level composite structure, and its adsorption is derived from the dual coupling of physical and chemical mechanisms. The active alumina itself has a developed mesoporous network and high specific surface area, providing abundant physical adsorption sites for metal ion impurities (such as Fe 3+ , Al 3+ ). The synergistic effect of the surface hydroxyl groups and nano zirconium oxide particles significantly enhances the capture ability of polar molecules in the crude product. The Zr-OH active sites on the surface of zirconium oxide can produce specific binding with impurity molecules containing hydroxyl groups or ether bonds such as propylene glycol and dipropylene glycol monomethyl ether through hydrogen bonding. At the same time, the tetragonal crystal structure of nano zirconium oxide exposes more unsaturated Zr 4+ , which can interact with polar molecules through Lewis acid-base interaction. In this process, the active alumina support not only serves as a support framework, but also its size effect (2-4 nm) can guide the directional migration of impurity molecules to the active sites of nano zirconium oxide, and promote the enrichment and fixation of metal ions through the electrostatic attraction of surface charges.

[0042] The modification treatment of the mesoporous molecular sieve is mainly used to improve the chemical adsorption performance of the composite adsorbent. First, the water in the mesoporous molecular sieve channels is removed by vacuum drying to ensure the effective penetration of the silane coupling agent. Under the assistance of ultrasonic dispersion, the silane coupling agent molecules can fully contact the inner and outer surfaces of the mesoporous molecular sieve, and through reflux reaction, the amino groups are stably grafted to the mesoporous molecular sieve framework through chemical bonds, thereby changing the surface properties of the mesoporous molecular sieve. It not only retains its screening ability for specific size molecules (such as amine compounds with a molecular diameter of 3.5-4.5 nm), but also endows it with hydrophilic adsorption properties based on hydrogen bonding through the introduction of amino groups. Through the surface functionalization of amino modification, the modified mesoporous molecular sieve can capture impurities based on both physical screening and chemical bonding mechanisms, especially showing excellent adsorption effect on trace amounts of amines and water that are difficult to completely remove in the distillation product.

[0043] Mesoporous molecular sieves form a uniform pore size of 3.5-4.5 nm through the topological structure of the silicon-aluminum framework, intercept macromolecules such as tripropylene glycol homologues by molecular size exclusion effect, and the amino functional groups (-NH2) grafted by silane coupling agent form a basic microenvironment on the inner wall of the mesoporous molecular sieve channel, which protonates with amine impurities such as triethylamine to generate quaternary ammonium salt compounds. This chemical bond effect realizes the irreversible adsorption of basic substances. In addition, the hydrophobic modification of mesoporous molecular sieves makes the silicon hydroxyl groups in the framework be partially replaced, reducing the affinity of the surface to water molecules, while retaining a sufficient number of silicon-oxygen tetrahedra as adsorption sites, adsorbing trace amounts of water through van der Waals forces and dipole interactions. At the same time, the capillary condensation effect caused by the curvature of the pore channel of the modified mesoporous molecular sieve significantly improves the adsorption driving force for moderately polar impurities, and its three-dimensional through-pore structure provides a fast diffusion path for impurity molecules.

[0044] The present application adds hydroxypropyl methyl cellulose in the molding process. Hydroxypropyl methyl cellulose not only can be used as a binder to improve the plasticity of the slurry, but also can maintain the stability of the porous structure of the composite particles due to its unique colloidal properties. The mechanical pressure applied during screw extrusion granulation not only ensures that the wet granules have sufficient mechanical strength, but also avoids the collapse of the pores caused by excessive extrusion. The regular particles produced by the molding method of screw extrusion have an open channel structure, which is beneficial to the efficient diffusion of impurities and can maintain a low pressure drop during dynamic adsorption. At the same time, the interwoven structure of the particles formed during the screw extrusion process of the loaded active alumina and modified mesoporous molecular sieve enables the final composite adsorbent to have both the large adsorption capacity of the loaded active alumina and the high selectivity of the modified mesoporous molecular sieve, thereby effectively avoiding the adsorption competition problem of traditional adsorbents when facing a multi-component impurity system, and enabling the simultaneous adsorption and removal of different types of impurities such as alcohols, ethers and amines remaining in the distillation product.

[0045] The present application produces synergistic effect after loading active alumina and modified mesoporous molecular sieve, the active alumina is mainly aimed at small molecule polar substances and metal ions, the rich active sites on the surface of the active alumina constitute the first adsorption barrier, and large-size impurities which may block the molecular sieve channel are removed in advance; the modified mesoporous molecular sieve plays a precise filtering role, and specific impurities such as amine and homologues are removed through size exclusion and chemical bonding. In the dynamic adsorption process, the metal ions captured by the active alumina alleviate the ion exchange pressure of the modified mesoporous molecular sieve, and the strong adsorption of the amino functional groups in the channel of the modified mesoporous molecular sieve to amine substances avoids the competitive adsorption of these basic substances on the surface of the active alumina. In addition, the Zr-OH on the surface of the active alumina loaded with nano zirconium oxide and the Si-O-Si skeleton of the modified mesoporous molecular sieve form a hydrogen bond network, and the interface interaction optimizes the overall charge distribution of the composite adsorbent, so that the polar impurity molecules produce directional migration between the active alumina and the modified mesoporous molecular sieve. At the same time, the pore size of the active alumina loaded with active alumina is about 2-4 nm, and the pore size of the modified mesoporous molecular sieve is about 3.5-4.5 nm, and the pore size of the two forms a gradient distribution, which can guide the impurity molecules to gradually enrich from the large pore size area to the small pore size area, and finally realize the great improvement of the adsorption capacity.

[0046] As a preferred technical solution of the present application, in step (1), the mass fraction of the zirconium oxychloride solution is 20-25wt%, for example, it can be 20wt%, 20.5wt%, 21wt%, 21.5wt%, 22wt%, 22.5wt%, 23wt%, 23.5wt%, 24wt%, 24.5wt% or 25wt%, but is not limited to the listed values, and other values not listed in the value range are also applicable.

[0047] In some optional examples, the mass ratio of the active alumina powder to the zirconium oxychloride in the zirconium oxychloride solution is 1:(0.3-0.4), for example, it can be 1:0.3, 1:0.31, 1:0.32, 1:0.33, 1:0.34, 1:0.35, 1:0.36, 1:0.37, 1:0.38, 1:0.39 or 1:0.4, but is not limited to the listed values, and other values not listed in the value range are also applicable.

[0048] This invention specifically defines the mass ratio of activated alumina powder to zirconium oxychloride in the zirconium oxychloride solution as 1:(0.3~0.4). When the amount of zirconium oxychloride added is within this range, a uniform zirconium species coating layer can be formed on the surface of the activated alumina support. The zirconium oxide obtained after calcination is dispersed on the alumina surface in the form of nanoparticles. Its exposed hydroxyl groups complement the original active sites of alumina. The large specific surface area of ​​alumina provides the basic capacity for physical adsorption, while the surface hydroxyl groups of zirconium oxide enhance the specific adsorption of hydroxyl-containing impurities such as ethylene glycol and short-chain alcohols through hydrogen bonding. The appropriate zirconium loading will not completely cover the highly active surface of alumina, so that the composite adsorbent maintains an open pore structure, ensuring that impurity molecules can quickly diffuse to the alumina-zirconia interface region. At the same time, it can enhance the capture ability of specific impurities through chemical action. When processing distillation components containing a variety of different impurities, the activated alumina support is used to quickly adsorb a large number of free impurities, while the uniformly distributed nano-zirconia sites continuously remove residual stubborn polar molecules.

[0049] When the amount of zirconium oxychloride added is below the lower limit defined in this invention, the zirconium hydroxide precipitate generated by the hydrolysis of zirconium oxychloride is difficult to form a continuous coating layer. This results in only sparse nano-zirconia particles adhering to the surface of the activated alumina. The incomplete surface modification means that the original highly polar surface of the activated alumina is not effectively passivated. During the adsorption process, its surface hydroxyl groups preferentially combine with water molecules to form a hydrated layer, which reduces the selective adsorption capacity for target impurities (such as propylene glycol). At the same time, the excessively low zirconium loading leads to Zr 4+ Insufficient active site density prevents effective capture of metal ions via Lewis acid interactions, resulting in residual Fe... 3+ Metallic impurities can become heterogeneous nucleation centers during subsequent crystallization stages, disrupting crystal regularity. Furthermore, the mesoporous structure of the unmodified activated alumina support exposes a large number of non-specific adsorption sites, causing large molecular weight ether impurities (such as dipropylene glycol monomethyl ether) to occupy the pores prematurely, resulting in a sharp decrease in the penetration capacity of the adsorption column.

[0050] When the amount of zirconium oxychloride added exceeds the upper limit defined in this invention, the excess zirconium oxychloride will form a dense zirconium oxide deposit during the precipitation process. Overloading will block the pore structure of the activated alumina, severely hindering the diffusion and mass transfer of impurity molecules. Furthermore, an excessively thick zirconium oxide deposit can induce internal stress concentration, leading to microcracks during subsequent calcination and a decrease in the mechanical strength of the composite adsorbent. Simultaneously, excessive Zr... 4+The dense distribution of the active alumina powder changes the surface charge characteristics of the active alumina, and converts the specific adsorption through hydrogen bonding into strong electrostatic interaction. Although the strong binding force improves the initial adsorption capacity of the composite adsorbent, it causes the increase of the impurity desorption energy barrier, and leads to the premature saturation of the bed in the dynamic adsorption process. In addition, the high oxygen vacancy concentration in the nano-zirconium oxide lattice also causes catalytic side reactions, which causes the residual acrylic ester impurities to polymerize on the surface of the nano-zirconium oxide, resulting in the deactivation of the composite adsorbent.

[0051] In some optional examples, the temperature of the mixing and stirring of the active alumina powder and the zirconium oxychloride solution is 70-80°C, for example, it can be 70°C, 71°C, 72°C, 73°C, 74°C, 75°C, 76°C, 77°C, 78°C, 79°C or 80°C, but not limited to the listed values, and other values not listed in this range are also applicable.

[0052] In some optional examples, the mixing and stirring time of the active alumina powder and the zirconium oxychloride solution is 4-5h, for example, it can be 4h, 4.1h, 4.2h, 4.3h, 4.4h, 4.5h, 4.6h, 4.7h, 4.8h, 4.9h or 5h, but not limited to the listed values, and other values not listed in this range are also applicable.

[0053] In some optional examples, the concentration of the ammonia solution is 20-30wt%, for example, it can be 20wt%, 21wt%, 22wt%, 23wt%, 24wt%, 25wt%, 26wt%, 27wt%, 28wt%, 29wt% or 30wt%, but not limited to the listed values, and other values not listed in this range are also applicable.

[0054] In some optional examples, the ammonia solution is added dropwise to the precursor solution to adjust the pH value to 8.5-9, for example, it can be 8.5, 8.55, 8.6, 8.65, 8.7, 8.75, 8.8, 8.85, 8.9, 8.95 or 9, but not limited to the listed values, and other values not listed in this range are also applicable.

[0055] In some optional examples, after the ammonia solution is completely added, the mixing and stirring is continued for 1-2h, for example, it can be 1.0h, 1.1h, 1.2h, 1.3h, 1.4h, 1.5h, 1.6h, 1.7h, 1.8h, 1.9h or 2.0h, but not limited to the listed values, and other values not listed in this range are also applicable.

[0056] In some optional examples, the time for the standing aging is 10-15 h, for example, can be 10 h, 10.5 h, 11 h, 11.5 h, 12 h, 12.5 h, 13 h, 13.5 h, 14 h, 14.5 h or 15 h, but not only limited to the listed values, other values not listed in the range of values are also applicable.

[0057] In some optional examples, the temperature for the drying is 110-120℃, for example, can be 110℃, 111℃, 112℃, 113℃, 114℃, 115℃, 116℃, 117℃, 118℃, 119℃ or 120℃, but not only limited to the listed values, other values not listed in the range of values are also applicable.

[0058] In some optional examples, the time for the drying is 8-10 h, for example, can be 8.0 h, 8.2 h, 8.4 h, 8.6 h, 8.8 h, 9.0 h, 9.2 h, 9.4 h, 9.6 h, 9.8 h or 10.0 h, but not only limited to the listed values, other values not listed in the range of values are also applicable.

[0059] In some optional examples, the heating rate for the calcination is 3-5℃ / min, for example, can be 3.0℃ / min, 3.2℃ / min, 3.4℃ / min, 3.6℃ / min, 3.8℃ / min, 4.0℃ / min, 4.2℃ / min, 4.4℃ / min, 4.6℃ / min, 4.8℃ / min or 5.0℃ / min, but not only limited to the listed values, other values not listed in the range of values are also applicable.

[0060] In some optional examples, the heating temperature for the calcination is 350-400℃, for example, can be 350℃, 355℃, 360℃, 365℃, 370℃, 375℃, 380℃, 385℃, 390℃, 395℃ or 400℃, but not only limited to the listed values, other values not listed in the range of values are also applicable.

[0061] In some optional examples, the holding time for the calcination is 3-5 h, for example, can be 3.0 h, 3.2 h, 3.4 h, 3.6 h, 3.8 h, 4.0 h, 4.2 h, 4.4 h, 4.6 h, 4.8 h or 5.0 h, but not only limited to the listed values, other values not listed in the range of values are also applicable.

[0062] As a preferred technical solution of the present application, in step (2), the pore size of the mesoporous molecular sieve is 3.5-4.5 nm, for example, it can be 3.5 nm, 3.6 nm, 3.7 nm, 3.8 nm, 3.9 nm, 4.0 nm, 4.1 nm, 4.2 nm, 4.3 nm, 4.4 nm or 4.5 nm, but not limited to the listed values, other values not listed in this range are also applicable.

[0063] In some optional examples, the heating temperature for vacuum drying and dehydration of the mesoporous molecular sieve is 100-120℃, for example, it can be 100℃, 102℃, 104℃, 106℃, 108℃, 110℃, 112℃, 114℃, 116℃, 118℃ or 120℃, but not limited to the listed values, other values not listed in this range are also applicable.

[0064] In some optional examples, the heating time for vacuum drying and dehydration of the mesoporous molecular sieve is 2-3h, for example, it can be 2.0h, 2.1h, 2.2h, 2.3h, 2.4h, 2.5h, 2.6h, 2.7h, 2.8h, 2.9h or 3.0h, but not limited to the listed values, other values not listed in this range are also applicable.

[0065] In some optional examples, the silane coupling agent solution is composed of silane coupling agent and aqueous ethanol solution.

[0066] In some optional examples, the mass fraction of silane coupling agent in the silane coupling agent solution is 3-5wt%, for example, it can be 3.0wt%, 3.2wt%, 3.4wt%, 3.6wt%, 3.8wt%, 4.0wt%, 4.2wt%, 4.4wt%, 4.6wt%, 4.8wt% or 5.0wt%, but not limited to the listed values, other values not listed in this range are also applicable.

[0067] The application particularly limits the mass fraction of silane coupling agent in the silane coupling agent solution to 3-5 wt%, when the concentration of the silane coupling agent solution is in this range, the appropriate silane concentration ensures that the amino groups are stably grafted to the molecular sieve skeleton through chemical bonds to form a uniform active site network, these amino groups can not only capture trace amounts of moisture through hydrogen bonding, but also specifically interact with amine impurities. At the same time, within this concentration range, the silane coupling agent will not block the pore structure of the mesoporous molecular formula, and the mesoporous molecular sieve can still intercept impurity molecules of a specific size through pore size screening. Therefore, when the concentration of the silane coupling agent solution is in the range of 3-5 wt%, the modified mesoporous molecular sieve can not only quickly capture polar impurities in the distillation product through chemical action, but also maintain a high mass transfer rate through the open pore structure. Under the action of the modified mesoporous molecular sieve, the amino groups grafted on the modified mesoporous molecular sieve preferentially adsorb amines and moisture in the distillation product, while alcohol, ether and other substances in the distillation product that are not chemically adsorbed can still be enriched inside the pore of the modified mesoporous molecular sieve through physical adsorption.

[0068] When the mass fraction of silane coupling agent in the silane coupling agent solution is less than 3 wt%, the number of silane coupling agent molecules in the silane coupling agent solution is insufficient, making it difficult to form a continuous coverage on the inner and outer surfaces of the mesoporous molecular sieve, resulting in incomplete modification of some areas of the mesoporous molecular sieve, and the sparse distribution of amino groups significantly weakens the selective adsorption capacity of amine impurities and moisture in the distillation product.

[0069] When the mass fraction of silane coupling agent in the silane coupling agent solution exceeds 5 wt%, excess silane coupling agent molecules in the solution are prone to self-polymerization to form large-size micelles. These micelles are difficult to completely dissociate during ultrasonic dispersion, and will accumulate into a dense layer on the surface of the mesoporous molecular sieve during the subsequent reflux reaction, not only shielding the original pore structure of the mesoporous molecular sieve, but also hindering the entry of impurity molecules in the distillation product into the pore inside the mesoporous molecular sieve, resulting in a decrease in the physical adsorption efficiency of the composite adsorbent.

[0070] In some optional examples, the ultrasonic power for ultrasonic dispersion of the dehydrated molecular sieve in the silane coupling agent solution is 400-500 W, for example, it can be 400 W, 410 W, 420 W, 430 W, 440 W, 450 W, 460 W, 470 W, 480 W, 490 W or 500 W, but is not limited to the listed values, and other values not listed in this range are also applicable.

[0071] In some optional examples, the ultrasonic time for ultrasonic dispersion of the dehydrated molecular sieve in the silane coupling agent solution is 30-50 min, for example, can be 30 min, 32 min, 34 min, 36 min, 38 min, 40 min, 42 min, 44 min, 46 min, 48 min or 50 min, but not only limited to the listed values, other values not listed in the range are also applicable.

[0072] In some optional examples, the temperature of the reflux reaction is 60-70℃, for example, can be 60℃, 61℃, 62℃, 63℃, 64℃, 65℃, 66℃, 67℃, 68℃, 69℃ or 70℃, but not only limited to the listed values, other values not listed in the range are also applicable.

[0073] In some optional examples, the time of the reflux reaction is 8-10 h, for example, can be 8.0 h, 8.2 h, 8.4 h, 8.6 h, 8.8 h, 9.0 h, 9.2 h, 9.4 h, 9.6 h, 9.8 h or 10.0 h, but not only limited to the listed values, other values not listed in the range are also applicable.

[0074] As a preferred technical solution of the present application, in step (3), the mass ratio of the loaded active alumina to the modified mesoporous molecular sieve is (1.5-2):1, for example, can be 1.5:1, 1.55:1, 1.6:1, 1.65:1, 1.7:1, 1.75:1, 1.8:1, 1.85:1, 1.9:1, 1.95:1 or 2:1, but not only limited to the listed values, other values not listed in the range are also applicable.

[0075] The present application particularly limits the mass ratio of the loaded active alumina to the modified mesoporous molecular sieve to be (1.5-2):1. Within this ratio range, the physical adsorption capacity of the loaded active alumina and the selective chemical adsorption function of the modified mesoporous molecular sieve form a complement, which can not only meet the rapid removal demand of a large amount of impurities, but also realize deep adsorption removal of specific stubborn impurities. The proportion of the loaded active alumina is slightly higher than that of the modified mesoporous molecular sieve, the high specific surface area and large pore volume characteristics of the loaded active alumina can ensure the adsorption capacity of common polar impurities such as ethylene glycol and short-chain alcohols, and the appropriate amount of modified mesoporous molecular sieve maintains the directional capture ability of amine impurities and moisture, and the synergy of the two makes the composite adsorbent have wide-spectrum adsorption characteristics.

[0076] When the addition amount of the active alumina support exceeds the upper limit of the range defined in the present application, the proportion of the modified mesoporous molecular sieve in the composite adsorbent is relatively reduced, although the total amount of physical adsorption is increased, but the number of chemical adsorption sites of the modified mesoporous molecular sieve is insufficient to completely remove the trace amount of amines and moisture remaining in the distillation product, and these impurities which are not effectively removed will interfere with the formation of crystal nucleus in the subsequent crystallization stage, resulting in a decrease in the purity of the crystal. At the same time, the accumulation of excess active alumina support will block the mesoporous structure of the modified mesoporous molecular sieve, weaken its interception ability to impurities, and make it easier for medium-sized ether compounds to penetrate the adsorption column.

[0077] When the addition amount of the active alumina support is less than the upper limit of the range defined in the present application, the proportion of the modified mesoporous molecular sieve in the composite adsorbent is relatively high, and the excessive addition of the modified mesoporous molecular sieve will cause the specific surface area of the composite adsorbent to decrease significantly, and the large-capacity adsorption advantage of the active alumina support is difficult to fully play, and when facing high-concentration impurities, the adsorption saturation phenomenon will appear too early, resulting in a shortening of the effective working time of the adsorption column.

[0078] In some optional examples, the addition amount of the hydroxypropyl methyl cellulose is 5-8wt% of the total mass of the active alumina support and the modified mesoporous molecular sieve, for example, it can be 5.0wt%, 5.5wt%, 6.0wt%, 6.5wt%, 7.0wt%, 7.5 or 8.0wt%, but not limited to the listed values, and other values not listed in the range are also applicable.

[0079] In some optional examples, deionized water is added to adjust the solid content of the composite slurry to 30-35wt%, for example, it can be 30wt%, 30.5wt%, 31wt%, 31.5wt%, 32wt%, 32.5wt%, 33wt%, 33.5wt%, 34wt%, 34.5wt% or 35wt%, but not limited to the listed values, and other values not listed in the range are also applicable.

[0080] In some optional examples, the screw rotation speed of the screw extruder is 100-120rpm, for example, it can be 100rpm, 102rpm, 104rpm, 106rpm, 108rpm, 110rpm, 112rpm, 114rpm, 116rpm, 118rpm or 120rpm, but not limited to the listed values, and other values not listed in the range are also applicable.

[0081] In some optional examples, the barrel of the screw extruder is divided into a first zone, a second zone and a third zone along the flow direction of the composite slurry.

[0082] In some alternative embodiments, the temperature of a third zone of the screw extruder is from 55°C to 65°C, such as 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, although embodiments are not limited to the listed values, as other unlisted values within the range are also applicable.

[0083] In some alternative embodiments, the temperature of a second zone of the screw extruder is from 40°C to 50°C, such as can be 40°C, 41°C, 42°C, 43°C, 44°C, 45°C, 46°C, 47°C, 48°C, 49°C, or 50°C, although embodiments are not limited to the listed values, as other unlisted values within the range are also applicable.

[0084] In some alternative embodiments, the temperature of a third zone of the screw extruder is from 55°C to 65°C, such as 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, although embodiments are not limited to the listed values, as other unlisted values within the range are also applicable.

[0085] In some alternative embodiments, the particle diameter of the wet granules is from 2 mm to 3 mm, such as can be 2.0 mm, 2.1 mm, 2.2 mm, 2.3 mm, 2.4 mm, 2.5 mm, 2.6 mm, 2.7 mm, 2.8 mm, 2.9 mm, or 3.0 mm, although embodiments are not limited to the listed values, as other unlisted values within the range are also applicable.

[0086] In some alternative embodiments, the particle length of the wet granules is from 3 mm to 5 mm, such as can be 3.0 mm, 3.2 mm, 3.4 mm, 3.6 mm, 3.8 mm, 4.0 mm, 4.2 mm, 4.4 mm, 4.6 mm, 4.8 mm, or 5.0 mm, although embodiments are not limited to the listed values, as other unlisted values within the range are also applicable.

[0087] In some alternative embodiments, the drying temperature of the wet granules is from 80°C to 100°C, such as can be 80°C, 82°C, 84°C, 86°C, 88°C, 90°C, 92°C, 94°C, 96°C, 98°C, or 100°C, although embodiments are not limited to the listed values, as other unlisted values within the range are also applicable.

[0088] In some alternative embodiments, the drying time of the wet granules is from 6 hours to 8 hours, such as can be 6.0 hours, 6.2 hours, 6.4 hours, 6.6 hours, 6.8 hours, 7.0 hours, 7.2 hours, 7.4 hours, 7.6 hours, 7.8 hours, or 8.0 hours, although embodiments are not limited to the listed values, as other unlisted values within the range are also applicable.

[0089] As a preferred technical solution of the present application, in step (III), the three-stage gradient cooling crystallization treatment comprises a rapid pre-cooling stage, a seed crystal induction stage and a low-temperature crystallization stage performed in sequence.

[0090] In some optional examples, the rapid pre-cooling stage comprises:

[0091] cooling the adsorption product to a pre-cooling temperature at a first cooling rate, stirring the adsorption product at a first stirring rate when the adsorption product reaches the pre-cooling temperature, and incubating the adsorption product at the first stirring rate and the pre-cooling temperature to obtain a pre-cooled product.

[0092] In some optional examples, the seed crystal induction stage comprises:

[0093] The crystal seeds to be added are divided into first crystal seeds and second crystal seeds by mass, the pre-cooled product is continuously cooled from the pre-cooling temperature to an induction temperature at a second cooling rate; during the continuous cooling process, when the temperature of the pre-cooled product reaches an intermediate induction temperature, the first crystal seeds are added to the pre-cooled product, and at the same time, the stirring rate is increased to a second stirring rate, the pre-cooled product is stirred at the second stirring rate, when the temperature of the pre-cooled product reaches the induction temperature, the second crystal seeds are added to the pre-cooled product, and the pre-cooled product is incubated at the second stirring rate and the induction temperature to obtain an induced product.

[0094] In some optional examples, the low-temperature crystallization stage comprises:

[0095] continuously cooling the induced product from the induction temperature to a crystallization temperature at a third cooling rate, reducing the stirring rate to a third stirring rate when the temperature of the induced product reaches an intermediate crystallization temperature, continuing to stir the induced product at the third stirring rate until the temperature of the induced product decreases to the crystallization temperature, and incubating the induced product at the third stirring rate and the crystallization temperature to obtain the crystallization product.

[0096] In the gradient cooling crystallization stage, by controlling the temperature and adjusting the operating conditions in stages, the adsorption product is gradually transformed from a liquid state to a high-purity crystal.

[0097] In the rapid pre-cooling stage, the adsorption product is rapidly reduced from a higher temperature to 30-40°C by a cooling rate of 2.5-3.5°C / min. The main purpose of this stage is to quickly establish the initial conditions suitable for crystallization, but not to allow it to start crystallization too early. If the cooling rate is too slow, the adsorption product may naturally form a large number of fine nuclei at a higher temperature, leading to subsequent crystal growth disorder; if the cooling rate is too fast, although it can inhibit spontaneous nucleation, it will cause uneven temperature distribution inside the adsorption product. In this stage, with slow stirring at 10-20 rpm, the internal temperature of the adsorption product can be maintained uniform, and too many bubbles can be introduced. When the temperature reaches 30-40°C, it is kept for 10-15 min to make the internal temperature of the adsorption product fully balanced, and at the same time, the gas dissolved in the adsorption product slowly escapes, creating a more stable crystallization environment for the subsequent steps.

[0098] In the seed induction stage, the cooling rate is adjusted to 1-1.5°C / min, and the solution supersaturation is gradually increased by a relatively mild cooling process. In this process, the pre-prepared crystal seeds are added twice. The first time is when the temperature drops to 18-20°C, most of the crystal seeds are added, and the stirring speed is increased to 30-40 rpm to ensure that the crystal seeds are quickly and uniformly dispersed, avoiding local crystal seed accumulation leading to uneven crystal growth. When the temperature continues to drop to 15-18°C, the remaining crystal seeds are added. At this time, the supersaturation of the solution has been further increased, and the newly added crystal seeds can supplement the number of crystal nuclei based on the existing crystal growth. Compared with the one-time addition of crystal seeds, the present application uses a batch addition of crystal seeds, which can better control the total amount and distribution density of crystal nuclei, avoid the situation that too many crystal nuclei lead to too small crystal size, and also avoid the situation that too few crystal nuclei lead to excessive crystal growth and produce wrapped impurities.

[0099] In the low-temperature crystallization stage, a super-slow cooling of 0.3-0.5°C / min is used to gradually reduce the material from 15-18°C to 2-4°C. The purpose of this stage is to optimize the crystal quality. When the temperature drops to 5-7°C, the stirring speed is reduced to 20-30 rpm. At this time, most of the crystals have formed a basic structure, and reducing the stirring intensity can reduce the collision and wear between the crystals, while allowing the solution to flow moderately to remove residual impurities. After reaching the final crystallization temperature of 2-4°C, it is continued to maintain for 3-4 h. The long-term holding process helps to optimize the crystal size, so that the small crystals that do not meet the size requirements are re-dissolved, and the qualified crystals continue to grow, thereby improving the size uniformity of the overall crystal.

[0100] As a preferred technical solution of the present application, the first cooling rate is 2.5-3.5℃ / min, for example, it can be 2.5℃ / min, 2.6℃ / min, 2.7℃ / min, 2.8℃ / min, 2.9℃ / min, 3.0℃ / min, 3.1℃ / min, 3.2℃ / min, 3.3℃ / min, 3.4℃ / min or 3.5℃ / min, but not limited to the listed values, other values not listed in the range are also applicable.

[0101] In some optional examples, the pre-cooling temperature is 30-40℃, for example, it can be 30℃, 31℃, 32℃, 33℃, 34℃, 35℃, 36℃, 37℃, 38℃, 39℃ or 40℃, but not limited to the listed values, other values not listed in the range are also applicable.

[0102] In some optional examples, the first stirring rate is 10-20rpm, for example, it can be 10rpm, 11rpm, 12rpm, 13rpm, 14rpm, 15rpm, 16rpm, 17rpm, 18rpm, 19rpm or 20rpm, but not limited to the listed values, other values not listed in the range are also applicable.

[0103] In some optional examples, the adsorption product is incubated at the first stirring rate and pre-cooling temperature for 10-15min, for example, it can be 10min, 10.5min, 11min, 11.5min, 12min, 12.5min, 13min, 13.5min, 14min, 14.5min or 15min, but not limited to the listed values, other values not listed in the range are also applicable.

[0104] In some optional examples, the mass ratio of the first seed crystal and the second seed crystal is (2.5-3):1, for example, it can be 2.5:1, 2.55:1, 2.6:1, 2.65:1, 2.7:1, 2.75:1, 2.8:1, 2.85:1, 2.9:1, 2.95:1 or 3.0:1, but not limited to the listed values, other values not listed in the range are also applicable.

[0105] In some alternative examples, the total mass of the first seed crystal and the second seed crystal is 0.1-0.2 wt% of the pre-cooling product mass, for example, can be 0.1 wt%, 0.11 wt%, 0.12 wt%, 0.13 wt%, 0.14 wt%, 0.15 wt%, 0.16 wt%, 0.17 wt%, 0.18 wt%, 0.19 wt%, or 0.2 wt%, but not limited to the listed values, other unlisted values within the range are also applicable.

[0106] In some alternative examples, the second cooling rate is 1-1.5 °C / min, for example, can be 1 °C / min, 1.05 °C / min, 1.1 °C / min, 1.15 °C / min, 1.2 °C / min, 1.25 °C / min, 1.3 °C / min, 1.35 °C / min, 1.4 °C / min, 1.45 °C / min, or 1.5 °C / min, but not limited to the listed values, other unlisted values within the range are also applicable.

[0107] In some alternative examples, the induction temperature is 15-18 °C, for example, can be 15 °C, 15.2 °C, 15.4 °C, 15.6 °C, 15.8 °C, 16 °C, 16.2 °C, 16.4 °C, 16.6 °C, 16.8 °C, 17 °C, 17.2 °C, 17.4 °C, 17.6 °C, 17.8 °C, or 18 °C, but not limited to the listed values, other unlisted values within the range are also applicable.

[0108] In some alternative examples, the intermediate induction temperature is 18-20 °C, for example, can be 18 °C, 18.2 °C, 18.4 °C, 18.6 °C, 18.8 °C, 19 °C, 19.2 °C, 19.4 °C, 19.6 °C, 19.8 °C, or 20 °C, but not limited to the listed values, other unlisted values within the range are also applicable.

[0109] In some alternative examples, the second agitation rate is 30-40 rpm, for example, can be 30 rpm, 31 rpm, 32 rpm, 33 rpm, 34 rpm, 35 rpm, 36 rpm, 37 rpm, 38 rpm, 39 rpm, or 40 rpm, but not limited to the listed values, other unlisted values within the range are also applicable.

[0110] In some optional examples, the pre-cooled product is held at the second agitation rate and the induction temperature for 20-30 minutes, for example, it can be 20 minutes, 21 minutes, 22 minutes, 23 minutes, 24 minutes, 25 minutes, 26 minutes, 27 minutes, 28 minutes, 29 minutes, or 30 minutes, but not only limited to the listed values, other values not listed in the range of values are also applicable.

[0111] In some optional examples, the third cooling rate is 0.3-0.5 °C / min, for example, it can be 0.3 °C / min, 0.32 °C / min, 0.34 °C / min, 0.36 °C / min, 0.38 °C / min, 0.4 °C / min, 0.42 °C / min, 0.44 °C / min, 0.46 °C / min, 0.48 °C / min, or 0.5 °C / min, but not only limited to the listed values, other values not listed in the range of values are also applicable.

[0112] In some optional examples, the crystallization temperature is 2-4 °C, for example, it can be 2.0 °C, 2.2 °C, 2.4 °C, 2.6 °C, 2.8 °C, 3.0 °C, 3.2 °C, 3.4 °C, 3.6 °C, 3.8 °C, or 4.0 °C, but not only limited to the listed values, other values not listed in the range of values are also applicable.

[0113] In some optional examples, the intermediate crystallization temperature is 5-7 °C, for example, it can be 5.0 °C, 5.2 °C, 5.4 °C, 5.6 °C, 5.8 °C, 6.0 °C, 6.2 °C, 6.4 °C, 6.6 °C, 6.8 °C, or 7.0 °C, but not only limited to the listed values, other values not listed in the range of values are also applicable.

[0114] In some optional examples, the third agitation rate is 20-30 rpm, for example, it can be 20 rpm, 21 rpm, 22 rpm, 23 rpm, 24 rpm, 25 rpm, 26 rpm, 27 rpm, 28 rpm, 29 rpm, or 30 rpm, but not only limited to the listed values, other values not listed in the range of values are also applicable.

[0115] In some optional examples, the induced product is held at the third agitation rate and the crystallization temperature for 3-4 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 not only limited to the listed values, other values not listed in the range of values are also applicable.

[0116] In some optional examples, the seed crystal is a dipropylene glycol crystal with a purity of ≥ 99.5 %.

[0117] Compared with the prior art, the present application has the following advantages:

[0118] The present application provides a high-efficiency preparation method of high-purity dipropylene glycol, two-stage vacuum distillation first completes physical separation of most impurities, reducing the burden of adsorption purification; the composite adsorbent focuses on processing specific impurities that are difficult to remove by distillation; gradient crystallization uses the difference in solubility to complete the final purification. The present application forms a progressive purification and purification route of "coarse screening-precision filtering-final extraction" through two-stage gradient vacuum distillation, dynamic adsorption of composite adsorbent and three-stage gradient cooling crystallization, so that the preparation process provided by the present application balances between product quality, production efficiency and cost control, which is conducive to industrial production and popularization. BRIEF DESCRIPTION OF DRAWINGS

[0119] Figure 1 The present application provides a high-purity dipropylene glycol preparation process flowchart for examples 1-15. DETAILED DESCRIPTION

[0120] The technical solutions of the present application will be described in detail below in combination with specific examples and their drawings. The examples described herein are specific specific embodiments of the present application, which are used to illustrate the concept of the present application; these descriptions are all explanatory and exemplary, and should not be understood as limiting the embodiments of the present application and the protection scope of the present application. In addition to the examples described herein, those skilled in the art can also employ other technical solutions that are obvious based on the content disclosed in the claims and the specification of the present application, which include technical solutions that make any obvious substitutions and modifications to the examples described herein.

[0121] Example 1

[0122] The present embodiment provides a preparation process of high-purity dipropylene glycol, as shown in Figure 1 The preparation process of high-purity dipropylene glycol includes:

[0123] (1) The crude product (which includes 4wt% of unreacted monomer propylene glycol, 3wt% of homologous dipropylene glycol, 1wt% of by-product acrylic ester compounds, 0.5wt% of moisture, 1.5wt% of dipropylene glycol monomethyl ether, 0.2wt% of triethylamine and 30ppm of metal ions, and the rest is the target product dipropylene glycol) produced by the dipropylene glycol production process is sent to the first vacuum distillation column, the operating pressure of the first vacuum distillation column is 10kPa, the column bottom temperature is 130℃, the column top temperature is 115℃, and the reflux ratio is 8:1. After vacuum distillation, the intermediate product is collected from the column bottom;

[0124] The intermediate product is fed into a second vacuum distillation column, the operating pressure of the second vacuum distillation column is 1 kPa, the column bottom temperature is 135℃, the column top temperature is 120℃, and the reflux ratio is 5:1, and after vacuum distillation, a distillation product is collected from the column top;

[0125] (2) The distillation product obtained in step (1) is passed through an adsorption column filled with a composite adsorbent, the filling amount of the composite adsorbent is 60% of the volume of the adsorption column, the height-diameter ratio of the adsorption column is 3:1, the space velocity of the distillation product is 2h -1 -1, and after adsorption, an adsorption product is obtained;

[0126] The composite adsorbent is prepared by the following method:

[0127] (2.1) The active alumina powder and a zirconium oxychloride solution with a mass fraction of 20wt% are mixed and stirred at 70℃ for 5h, the mass ratio of the active alumina powder to the zirconium oxychloride in the zirconium oxychloride solution is 1:0.3, to obtain a precursor solution, 20wt% ammonia water solution is added dropwise to the precursor solution to adjust the pH value to 8.5, after the ammonia water solution is completely added, continue to mix and stir for 1h to initiate the precipitation reaction, after the reaction is completed, stand for aging for 10h to obtain a reaction product, the reaction product is filtered, the filter cake obtained by filtration is dried at 110℃ for 10h to obtain a dry product, and the dry product is heated to 350℃ at a heating rate of 3℃ / min for calcination for 5h to obtain the active alumina support;

[0128] (2.2) The mesoporous molecular sieve with a pore size of 3.5nm is heated to 100℃ for vacuum dehydration for 3h to obtain a dehydrated molecular sieve; the dehydrated molecular sieve is immersed in a silane coupling agent solution composed of silane coupling agent KH550 and an ethanol aqueous solution, the mass fraction of the silane coupling agent KH550 in the silane coupling agent solution is 3wt%, and the volume ratio of ethanol to deionized water in the ethanol aqueous solution is 7:3, ultrasonic dispersion is carried out at an ultrasonic power of 400W for 50min to obtain a molecular sieve suspension, the molecular sieve suspension is transferred to a water bath kettle for reflux reaction, the reflux reaction temperature is 60℃, and the reflux reaction time is 10h, after the reaction is completed, the modified mesoporous molecular sieve is obtained after filtration, washing and drying;

[0129] (2.3) The active alumina support obtained in step (2.1), the modified mesoporous molecular sieve obtained in step (2.2), and hydroxypropyl methyl cellulose are mixed to obtain a composite powder, wherein the mass ratio of the active alumina support to the modified mesoporous molecular sieve is 1.5:1, the addition amount of the hydroxypropyl methyl cellulose is 5wt% of the total mass of the active alumina support and the modified mesoporous molecular sieve, and deionized water is added to the composite powder to prepare a composite slurry with a solid content of 30wt%;

[0130] The composite slurry is fed into a screw extruder, the temperature of the first zone of the screw extruder is 30℃, the temperature of the second zone is 40℃, the temperature of the third zone is 55℃, and the rotation speed of the screw is 100rpm, and the wet granules with a diameter of 2mm and a length of 3mm are obtained by extrusion granulation, and the wet granules are vacuum dried at 80℃ for 8h to obtain the composite adsorbent.

[0131] (3) The adsorption product obtained in step (2) is subjected to three-stage gradient cooling crystallization treatment, first, the adsorption product is cooled to 30℃ at a cooling rate of 2.5℃ / min, when the adsorption product reaches 30℃, the adsorption product is stirred at a stirring speed of 10rpm, and the adsorption product is incubated at 10rpm and 30℃ for 10min to obtain a pre-cooled product;

[0132] Subsequently, the crystal seeds (dipropylene glycol crystals with a purity of 99.8%) to be added are divided into first crystal seeds and second crystal seeds according to mass, the mass ratio of the first crystal seeds to the second crystal seeds is 2.5:1, and the total mass of the first crystal seeds and the second crystal seeds is 0.1wt% of the mass of the pre-cooled product; the pre-cooled product is continuously cooled from 30℃ to 15℃ at a cooling rate of 1℃ / min; during the continuous cooling process, when the temperature of the pre-cooled product reaches 18℃, the first crystal seeds are added to the pre-cooled product, and at the same time, the stirring speed is increased to 30rpm, the pre-cooled product is stirred at a stirring speed of 30rpm, when the temperature of the pre-cooled product reaches 15℃, the second crystal seeds are added to the pre-cooled product, and the pre-cooled product is incubated at 30rpm and 15℃ for 20min to obtain an induced product;

[0133] Finally, the induced product is continuously cooled from 15℃ to 2℃ at a cooling rate of 0.3℃ / min, when the temperature of the induced product reaches 5℃, the stirring speed is reduced to 20rpm, the induced product is continuously stirred at a stirring speed of 20rpm until the temperature of the induced product decreases to 2℃, and the induced product is incubated at 20rpm and 2℃ for 3h to obtain a crystallization product, and the crystallization product is subjected to centrifugal separation to obtain high-purity dipropylene glycol.

[0134] Example 2

[0135] This embodiment provides a preparation process of high-purity dipropylene glycol, as shown in Figure 1 the preparation process of high-purity dipropylene glycol comprises:

[0136] (1) The crude product obtained from the dipropylene glycol production process is fed into a first vacuum distillation column, the operating pressure of the first vacuum distillation column is 10kPa, the column bottom temperature is 132℃, the column top temperature is 116℃, and the reflux ratio is 8.2:1, and the intermediate product is collected from the column bottom after vacuum distillation;

[0137] The intermediate product is fed into a second vacuum distillation column, the operating pressure of the second vacuum distillation column is 1 kPa, the column bottom temperature is 138 ℃, the column top temperature is 121 ℃, and the reflux ratio is 5.2:1, and after vacuum distillation, a distillation product is collected from the column top;

[0138] (2) The distillation product obtained in step (1) is passed through an adsorption column filled with a composite adsorbent, the filling amount of the composite adsorbent is 65% of the volume of the adsorption column, the height-diameter ratio of the adsorption column is 3.2:1, the space velocity of the distillation product is 1.8 h -1 , the adsorption temperature is 42 ℃, and after adsorption, an adsorption product is obtained;

[0139] The composite adsorbent is prepared by the following method:

[0140] (2.1) The active alumina powder and a zirconium oxychloride solution with a mass fraction of 21wt% are mixed and stirred at 72 ℃ for 4.8 h, the mass ratio of the active alumina powder to the zirconium oxychloride in the zirconium oxychloride solution is 1:0.32, to obtain a precursor solution, 22wt% ammonia solution is added dropwise to the precursor solution to adjust the pH value to 8.6, after the ammonia solution is completely added, continue to mix and stir for 1.2 h to initiate the precipitation reaction, after the reaction is completed, stand for aging for 11 h to obtain a reaction product, the reaction product is filtered, the filter cake obtained by filtration is dried at 112 ℃ for 9.5 h to obtain a dry product, and the dry product is heated to 360 ℃ at a heating rate of 3.5 ℃ / min for calcination for 4.5 h to obtain the active alumina support;

[0141] (2.2) The mesoporous molecular sieve with a pore size of 3.8 nm is heated to 105 ℃ for vacuum dehydration for 2.8 h to obtain a dehydrated molecular sieve; the dehydrated molecular sieve is immersed in a silane coupling agent solution composed of silane coupling agent KH550 and an ethanol aqueous solution, the mass fraction of the silane coupling agent KH550 in the silane coupling agent solution is 3.5wt%, and the volume ratio of ethanol to deionized water in the ethanol aqueous solution is 7:3, ultrasonic dispersion is carried out at an ultrasonic power of 420 W for 45 min to obtain a molecular sieve suspension, the molecular sieve suspension is transferred to a water bath kettle for reflux reaction, the reflux reaction temperature is 62 ℃, and the reflux reaction time is 9.5 h, after the reaction is completed, filtration, washing and drying are carried out to obtain a modified mesoporous molecular sieve;

[0142] (2.3) The active alumina support obtained in step (2.1), the modified mesoporous molecular sieve obtained in step (2.2), and hydroxypropyl methyl cellulose are mixed to obtain a composite powder, wherein the mass ratio of the active alumina support to the modified mesoporous molecular sieve is 1.6:1, the addition amount of the hydroxypropyl methyl cellulose is 6wt% of the total mass of the active alumina support and the modified mesoporous molecular sieve, and deionized water is added to the composite powder to prepare a composite slurry with a solid content of 31wt%.

[0143] The composite slurry is fed into a screw extruder, the temperature of the first zone of the screw extruder is 31℃, the temperature of the second zone is 42℃, the temperature of the third zone is 58℃, and the rotation speed of the screw is 105rpm, and the wet granules with a diameter of 2.2mm and a length of 3.5mm are obtained by extrusion granulation, and the wet granules are vacuum dried at 85℃ for 7.5h to obtain the composite adsorbent.

[0144] (3) The adsorption product obtained in step (2) is subjected to three-stage gradient cooling crystallization treatment, first, the adsorption product is cooled to 32℃ at a cooling rate of 2.8℃ / min, when the adsorption product reaches 32℃, the adsorption product is stirred at a stirring speed of 12rpm, and the adsorption product is incubated at 12rpm and 32℃ for 11min to obtain a pre-cooled product;

[0145] Subsequently, the crystal seeds (dipropylene glycol crystals with a purity of 99.7%) to be added are divided into first crystal seeds and second crystal seeds according to mass, the mass ratio of the first crystal seeds to the second crystal seeds is 2.6:1, and the total mass of the first crystal seeds and the second crystal seeds is 0.12wt% of the mass of the pre-cooled product; the pre-cooled product is continuously cooled from 32℃ to 16℃ at a cooling rate of 1.1℃ / min; during the continuous cooling process, when the temperature of the pre-cooled product reaches 18℃, the first crystal seeds are added to the pre-cooled product, at the same time, the stirring speed is increased to 32pm, the pre-cooled product is stirred at a stirring speed of 32rpm, when the temperature of the pre-cooled product reaches 16℃, the second crystal seeds are added to the pre-cooled product, and the pre-cooled product is incubated at 32rpm and 16℃ for 22min to obtain an induced product;

[0146] Finally, the induced product is continuously cooled from 16℃ to 2℃ at a cooling rate of 0.35℃ / min, when the temperature of the induced product reaches 5℃, the stirring speed is reduced to 22rpm, the induced product is continuously stirred at a stirring speed of 22rpm until the temperature of the induced product decreases to 2℃, and the induced product is incubated at 22rpm and 2℃ for 3.2h to obtain a crystallization product, and the crystallization product is subjected to centrifugal separation to obtain high-purity dipropylene glycol.

[0147] Example 3

[0148] The present embodiment provides a preparation process of high-purity dipropylene glycol, as shown in Figure 1 The preparation process of high-purity dipropylene glycol comprises:

[0149] (1) The crude product obtained from the dipropylene glycol production process is fed into a first vacuum distillation column, the operating pressure of the first vacuum distillation column is 10kPa, the column bottom temperature is 135℃, the column top temperature is 117℃, and the reflux ratio is 8.5:1, and the intermediate product is collected from the column bottom after vacuum distillation;

[0150] The intermediate product is fed into a second vacuum distillation column, the operating pressure of the second vacuum distillation column is 1 kPa, the column bottom temperature is 140℃, the column top temperature is 122℃, and the reflux ratio is 5.5:1, and after vacuum distillation, a distillation product is collected from the column top;

[0151] (2) The distillation product obtained in step (1) is passed through an adsorption column filled with a composite adsorbent, the filling amount of the composite adsorbent is 70% of the volume of the adsorption column, the height-diameter ratio of the adsorption column is 3.5:1, the space velocity of the distillation product is 1.5h -1 , the adsorption temperature is 45℃, and after adsorption, an adsorption product is obtained;

[0152] The composite adsorbent is prepared by the following method:

[0153] (2.1) The active alumina powder and a zirconium oxychloride solution with a mass fraction of 22wt% are mixed and stirred at 75℃ for 4.5h, the mass ratio of the active alumina powder to the zirconium oxychloride in the zirconium oxychloride solution is 1:0.35, to obtain a precursor solution, 25wt% ammonia water solution is added dropwise to the precursor solution to adjust the pH value to 8.7, after the ammonia water solution is completely added, continue to mix and stir for 1.5h to initiate the precipitation reaction, after the reaction is completed, stand for aging for 12h to obtain a reaction product, the reaction product is filtered, the filter cake obtained by filtration is dried at 115℃ for 9h to obtain a dry product, the dry product is heated to 370℃ at a heating rate of 4℃ / min for calcination for 4h to obtain the active alumina supported product;

[0154] (2.2) The mesoporous molecular sieve with a pore size of 4nm is heated to 110℃ for vacuum dehydration for 2.5h to obtain a dehydrated molecular sieve; the dehydrated molecular sieve is immersed in a silane coupling agent solution composed of silane coupling agent KH550 and an ethanol aqueous solution, the mass fraction of the silane coupling agent KH550 in the silane coupling agent solution is 4wt%, the volume ratio of ethanol to deionized water in the ethanol aqueous solution is 7:3, ultrasonic dispersion is carried out at an ultrasonic power of 450W for 40min to obtain a molecular sieve suspension, the molecular sieve suspension is transferred to a water bath kettle for reflux reaction, the reflux reaction temperature is 65℃, the reflux reaction time is 9h, after the reaction is completed, the modified mesoporous molecular sieve is obtained after filtration, washing and drying;

[0155] (2.3) The active alumina supported product obtained in step (2.1), the modified mesoporous molecular sieve obtained in step (2.2) and hydroxypropyl methyl cellulose are mixed to obtain a composite powder, wherein the mass ratio of the active alumina supported product to the modified mesoporous molecular sieve is 1.7:1, the addition amount of the hydroxypropyl methyl cellulose is 7wt% of the total mass of the active alumina supported product and the modified mesoporous molecular sieve, deionized water is added to the composite powder to prepare a composite slurry with a solid content of 32wt%.

[0156] The composite slurry is fed into a screw extruder, the temperature of the first zone of the screw extruder is 32℃, the temperature of the second zone is 45℃, the temperature of the third zone is 60℃, and the rotation speed of the screw is 110rpm, and the wet granules with a diameter of 2.5mm and a length of 4mm are obtained by extrusion granulation, and the wet granules are vacuum dried at 90℃ for 7h to obtain the composite adsorbent.

[0157] (3) The adsorption product obtained in step (2) is subjected to three-stage gradient cooling crystallization treatment, first, the adsorption product is cooled to 35℃ at a cooling rate of 3℃ / min, when the adsorption product reaches 35℃, the adsorption product is stirred at a stirring speed of 15rpm, and the adsorption product is incubated at 15rpm and 35℃ for 12min to obtain a pre-cooled product;

[0158] Subsequently, the crystal seeds (dipropylene glycol crystals with a purity of 99.6%) to be added are divided into first crystal seeds and second crystal seeds according to mass, the mass ratio of the first crystal seeds to the second crystal seeds is 2.7:1, and the total mass of the first crystal seeds and the second crystal seeds is 0.15wt% of the mass of the pre-cooled product; the pre-cooled product is continuously cooled from 35℃ to 17℃ at a cooling rate of 1.2℃ / min; during the continuous cooling process, when the temperature of the pre-cooled product reaches 19℃, the first crystal seeds are added to the pre-cooled product, at the same time, the stirring speed is increased to 35rpm, and the pre-cooled product is stirred at a stirring speed of 35rpm, when the temperature of the pre-cooled product reaches 17℃, the second crystal seeds are added to the pre-cooled product, and the pre-cooled product is incubated at 35rpm and 17℃ for 25min to obtain an induced product;

[0159] Finally, the induced product is continuously cooled from 17℃ to 3℃ at a cooling rate of 0.4℃ / min, when the temperature of the induced product reaches 6℃, the stirring speed is reduced to 25rpm, the induced product is continuously stirred at a stirring speed of 25rpm until the temperature of the induced product decreases to 3℃, and the induced product is incubated at 25rpm and 3℃ for 3.5h to obtain a crystallization product, and the crystallization product is subjected to centrifugal separation to obtain high-purity dipropylene glycol.

[0160] Example 4

[0161] The present embodiment provides a preparation process of high-purity dipropylene glycol, as shown in the following scheme, the preparation process of high-purity dipropylene glycol comprises: Figure 1

[0162] (1) The crude product obtained from the dipropylene glycol production process is fed into a first vacuum distillation column, the operating pressure of the first vacuum distillation column is 10kPa, the column bottom temperature is 138℃, the column top temperature is 118℃, and the reflux ratio is 8.8:1, and the intermediate product is collected from the column bottom after vacuum distillation; ​

[0163] The intermediate product is sent to a second vacuum distillation column, the operating pressure of the second vacuum distillation column is 1 kPa, the column bottom temperature is 142℃, the column top temperature is 123℃, and the reflux ratio is 5.8:1, and after vacuum distillation, a distillation product is collected from the column top;

[0164] (2) The distillation product obtained in step (1) is passed through an adsorption column filled with a composite adsorbent, the filling amount of the composite adsorbent is 75% of the volume of the adsorption column, the height-diameter ratio of the adsorption column is 3.8:1, the space velocity of the distillation product is 1.2h -1 , the adsorption temperature is 48℃, and after adsorption, an adsorption product is obtained;

[0165] The composite adsorbent is prepared by the following method:

[0166] (2.1) The active alumina powder and a zirconium oxychloride solution with a mass fraction of 23wt% are mixed and stirred at 78℃ for 4.2h, the mass ratio of the active alumina powder to the zirconium oxychloride in the zirconium oxychloride solution is 1:0.38, a precursor solution is obtained, 28wt% ammonia solution is added dropwise to the precursor solution to adjust the pH value to 8.8, after the ammonia solution is completely added, the mixture is continuously stirred for 1.8h to initiate the precipitation reaction, after the reaction is completed, it is aged for 13h, and a reaction product is obtained; the reaction product is filtered, the filter cake obtained by filtration is dried at 118℃ for 8.5h, a dry product is obtained, the dry product is heated to 380℃ at a heating rate of 4.5℃ / min, and calcined for 3.5h to obtain a supported active alumina;

[0167] (2.2) The mesoporous molecular sieve with a pore size of 4.2nm is heated to 115℃ for vacuum dehydration for 2.2h to obtain a dehydrated molecular sieve; the dehydrated molecular sieve is immersed in a silane coupling agent solution composed of silane coupling agent KH550 and an ethanol aqueous solution, the mass fraction of silane coupling agent KH550 in the silane coupling agent solution is 4.5wt%, and the volume ratio of ethanol to deionized water in the ethanol aqueous solution is 7:3, ultrasonic dispersion is carried out at an ultrasonic power of 480W for 35min to obtain a molecular sieve suspension, the molecular sieve suspension is transferred to a water bath kettle for reflux reaction, the reflux reaction temperature is 68℃, the reflux reaction time is 8.5h, after the reaction is completed, filtration, washing and drying are carried out to obtain a modified mesoporous molecular sieve;

[0168] (2.3) mixing the loaded active alumina obtained in step (2.1), the modified mesoporous molecular sieve obtained in step (2.2) and hydroxypropyl methyl cellulose to obtain a composite powder, wherein the mass ratio of the loaded active alumina to the modified mesoporous molecular sieve is 1.8:1, the amount of the hydroxypropyl methyl cellulose added is 7wt% of the total mass of the loaded active alumina and the modified mesoporous molecular sieve, and deionized water is added to the composite powder to prepare a composite slurry with a solid content of 33wt%;

[0169] The composite slurry is fed into a screw extruder, the temperature of the first zone of the screw extruder is 33℃, the temperature of the second zone is 48℃, the temperature of the third zone is 62℃, and the rotation speed of the screw is 115rpm, and wet granules with a diameter of 2.8mm and a length of 4.5mm are obtained by extrusion granulation, and the wet granules are vacuum dried at 95℃ for 6.5h to obtain the composite adsorbent.

[0170] (3) the adsorption product obtained in step (2) is subjected to three-stage gradient cooling crystallization treatment, first, the adsorption product is cooled to 38℃ at a cooling rate of 3.2℃ / min, when the adsorption product reaches 38℃, the adsorption product is stirred at a stirring speed of 18rpm, and the adsorption product is incubated at 18rpm and 38℃ for 13min to obtain a pre-cooled product;

[0171] Subsequently, the crystal seeds (dipropylene glycol crystals with a purity of 99.5%) to be added are divided into first crystal seeds and second crystal seeds according to mass, the mass ratio of the first crystal seeds to the second crystal seeds is 2.8:1, and the total mass of the first crystal seeds and the second crystal seeds is 0.18wt% of the mass of the pre-cooled product; the pre-cooled product is continuously cooled from 38℃ to 17℃ at a cooling rate of 1.3℃ / min; during the continuous cooling process, when the temperature of the pre-cooled product reaches 19℃, the first crystal seeds are added to the pre-cooled product, and at the same time, the stirring speed is increased to 38rpm, the pre-cooled product is stirred at a stirring speed of 38rpm, when the temperature of the pre-cooled product reaches 17℃, the second crystal seeds are added to the pre-cooled product, and the pre-cooled product is incubated at 38rpm and 17℃ for 28min to obtain an induced product;

[0172] Finally, the induced product is continuously cooled from 17℃ to 3℃ at a cooling rate of 0.45℃ / min, when the temperature of the induced product reaches 6℃, the stirring speed is reduced to 28rpm, the induced product is continuously stirred at a stirring speed of 28rpm until the temperature of the induced product decreases to 3℃, and the induced product is incubated at 28rpm and 3℃ for 3.8h to obtain a crystallization product, and the crystallization product is subjected to centrifugal separation to obtain high-purity dipropylene glycol.

[0173] Example 5

[0174] The present embodiment provides a preparation process of high-purity dipropylene glycol, as shown in Figure 1As shown, the preparation process of the high-purity dipropylene glycol comprises:

[0175] (1) The crude product produced by the dipropylene glycol production process is sent into a first vacuum distillation column, the operating pressure of the first vacuum distillation column is 10 kPa, the column bottom temperature is 140 ℃, the column top temperature is 120 ℃, and the reflux ratio is 9:1. After vacuum distillation, the intermediate product is collected from the column bottom;

[0176] The intermediate product is sent into a second vacuum distillation column, the operating pressure of the second vacuum distillation column is 1 kPa, the column bottom temperature is 145 ℃, the column top temperature is 125 ℃, and the reflux ratio is 6:1. After vacuum distillation, the distillation product is collected from the column top;

[0177] (2) The distillation product obtained in step (1) is passed through an adsorption column filled with a composite adsorbent, the filling amount of the composite adsorbent is 80% of the volume of the adsorption column, the height-diameter ratio of the adsorption column is 4:1, the space velocity of the distillation product is 1 h -1 -1, and the adsorption temperature is 50 ℃. After adsorption, the adsorption product is obtained;

[0178] The composite adsorbent is prepared by the following method:

[0179] (2.1) The active alumina powder is mixed with a zirconium oxychloride solution with a mass fraction of 25wt% at 80 ℃ for 4h, the mass ratio of the active alumina powder to the zirconium oxychloride in the zirconium oxychloride solution is 1:0.4, to obtain a precursor solution. A 30wt% ammonia solution is added dropwise to the precursor solution to adjust the pH value to 9. After the ammonia solution is completely added, continue to mix and stir for 2h to initiate the precipitation reaction. After the reaction is completed, stand for aging for 15h to obtain a reaction product. The reaction product is filtered, and the filter cake obtained by filtration is dried at 120 ℃ for 8h to obtain a dry product. The dry product is heated to 400 ℃ at a heating rate of 5 ℃ / min for calcination for 3h to obtain the active alumina support;

[0180] (2.2) The mesoporous molecular sieve with a pore size of 4.5 nm is heated to 120 ℃ for vacuum dehydration for 2h to obtain a dehydrated molecular sieve. The dehydrated molecular sieve is immersed in a silane coupling agent solution composed of a silane coupling agent KH550 and an ethanol aqueous solution, the mass fraction of the silane coupling agent KH550 in the silane coupling agent solution is 5wt%, and the volume ratio of ethanol to deionized water in the ethanol aqueous solution is 7:3. Ultrasonic dispersion is carried out at an ultrasonic power of 500W for 30min to obtain a molecular sieve suspension. The molecular sieve suspension is transferred to a water bath kettle for reflux reaction, the reflux reaction temperature is 70 ℃, and the reflux reaction time is 8h. After the reaction is completed, filtration, washing and drying are carried out to obtain a modified mesoporous molecular sieve;

[0181] (2.3) mixing the loaded active alumina obtained in step (2.1), the modified mesoporous molecular sieve obtained in step (2.2) and hydroxypropyl methyl cellulose to obtain a composite powder, wherein the mass ratio of the loaded active alumina to the modified mesoporous molecular sieve is 2:1, the amount of the hydroxypropyl methyl cellulose added is 8wt% of the total mass of the loaded active alumina and the modified mesoporous molecular sieve, and deionized water is added to the composite powder to prepare a composite slurry with a solid content of 35wt%;

[0182] The composite slurry is fed into a screw extruder, the temperature of the first zone of the screw extruder is 35℃, the temperature of the second zone is 50℃, the temperature of the third zone is 65℃, and the rotation speed of the screw is 120rpm, and wet granules with a diameter of 3mm and a length of 5mm are obtained by extrusion granulation, and the wet granules are vacuum dried at 100℃ for 6h to obtain the composite adsorbent.

[0183] (3) the adsorption product obtained in step (2) is subjected to three-stage gradient cooling crystallization treatment, first, the adsorption product is cooled to 40℃ at a cooling rate of 3.5℃ / min, when the adsorption product reaches 40℃, the adsorption product is stirred at a stirring speed of 20rpm, and the adsorption product is incubated at 20rpm and 40℃ for 15min to obtain a pre-cooled product;

[0184] Subsequently, the seed crystals (dipropylene glycol crystals with a purity of 99.8%) to be added are divided into first seed crystals and second seed crystals according to mass, the mass ratio of the first seed crystals to the second seed crystals is 3:1, and the total mass of the first seed crystals and the second seed crystals is 0.2wt% of the mass of the pre-cooled product; the pre-cooled product is continuously cooled from 40℃ to 18℃ at a cooling rate of 1.5℃ / min; during the continuous cooling process, when the temperature of the pre-cooled product reaches 20℃, the first seed crystals are added to the pre-cooled product, and at the same time, the stirring speed is increased to 40rpm, and the pre-cooled product is stirred at a stirring speed of 40rpm, when the temperature of the pre-cooled product reaches 18℃, the second seed crystals are added to the pre-cooled product, and the pre-cooled product is incubated at 40rpm and 18℃ for 30min to obtain an induced product;

[0185] Finally, the induced product is continuously cooled from 18℃ to 4℃ at a cooling rate of 0.5℃ / min, when the temperature of the induced product reaches 7℃, the stirring speed is reduced to 30rpm, and the induced product is continuously stirred at a stirring speed of 30rpm until the temperature of the induced product decreases to 4℃, and the induced product is incubated at 30rpm and 4℃ for 4h to obtain a crystallization product, and the crystallization product is subjected to centrifugal separation to obtain high-purity dipropylene glycol.

[0186] Example 6

[0187] The embodiment provides a preparation process of high-purity dipropylene glycol, which is different from the embodiment 1 in that in step (1), the reflux ratio of the second vacuum distillation tower is adjusted to 4:1, and other operation steps and process parameters are completely same as those of the embodiment 1.

[0188] Embodiment 7

[0189] The embodiment provides a preparation process of high-purity dipropylene glycol, which is different from the embodiment 1 in that in step (1), the reflux ratio of the second vacuum distillation tower is adjusted to 7:1, and other operation steps and process parameters are completely same as those of the embodiment 1.

[0190] Embodiment 8

[0191] The embodiment provides a preparation process of high-purity dipropylene glycol, which is different from the embodiment 1 in that in step (2), the space velocity of the distillation product is adjusted to 0.5h -1 -1, and other operation steps and process parameters are completely same as those of the embodiment 1.

[0192] Embodiment 9

[0193] The embodiment provides a preparation process of high-purity dipropylene glycol, which is different from the embodiment 1 in that in step (2), the space velocity of the distillation product is adjusted to 3h -1 -1, and other operation steps and process parameters are completely same as those of the embodiment 1.

[0194] Embodiment 10

[0195] The embodiment provides a preparation process of high-purity dipropylene glycol, which is different from the embodiment 1 in that in step (2.1), the mass ratio of the active alumina powder to zirconium oxychloride in the zirconium oxychloride solution is adjusted to 1:0.2, and other operation steps and process parameters are completely same as those of the embodiment 1.

[0196] Embodiment 11

[0197] The embodiment provides a preparation process of high-purity dipropylene glycol, which is different from the embodiment 1 in that in step (2.1), the mass ratio of the active alumina powder to zirconium oxychloride in the zirconium oxychloride solution is adjusted to 1:0.5, and other operation steps and process parameters are completely same as those of the embodiment 1.

[0198] Embodiment 12

[0199] The embodiment provides a preparation process of high-purity dipropylene glycol, which is different from the embodiment 1 in that in step (2.2), the mass fraction of the silane coupling agent KH550 in the silane coupling agent solution is adjusted to 1wt%, and other operation steps and process parameters are completely same as those of the embodiment 1.

[0200] Embodiment 13

[0201] The embodiment provides a preparation process of high-purity dipropylene glycol, and the difference from the embodiment 1 is that in step (2.2), the mass fraction of the silane coupling agent KH550 in the silane coupling agent solution is adjusted to 7 wt%, and other operation steps and process parameters are completely same as those of the embodiment 1.

[0202] Embodiment 14

[0203] The embodiment provides a preparation process of high-purity dipropylene glycol, and the difference from the embodiment 1 is that in step (2.3), the mass ratio of the loaded active alumina to the modified mesoporous molecular sieve is adjusted to 1:1, and other operation steps and process parameters are completely same as those of the embodiment 1.

[0204] Embodiment 15

[0205] The embodiment provides a preparation process of high-purity dipropylene glycol, and the difference from the embodiment 1 is that in step (2.3), the mass ratio of the loaded active alumina to the modified mesoporous molecular sieve is adjusted to 2.5:1, and other operation steps and process parameters are completely same as those of the embodiment 1.

[0206] The product yield and product purity of the high-purity dipropylene glycol prepared in the embodiments 1-15 are tested, and the specific test steps are as follows:

[0207] (1) Dipropylene glycol product yield

[0208] According to the initial content of the dipropylene glycol in the crude product (determined by GC pre-analysis), the maximum theoretical content without loss is assumed, and is denoted as m n . The mass of the crude product is weighed, and is denoted as m0. The mass of the wet crystal after centrifugation of the crystallization product is weighed, and is denoted as m1.

[0209] The product yield is calculated by using the following formula:

[0210]

[0211] (2) Dipropylene glycol product purity

[0212] The test was performed by using a gas chromatograph (GC) equipped with a hydrogen flame ionization detector (FID). The chromatographic column was a DB-5 capillary column (30 m x 0.32 mm x 0.25 μm). The carrier gas was high-purity nitrogen (flow rate 1.5 mL / min). The injection port temperature was set to 250°C. The detector temperature was set to 300°C. The column temperature program was set as follows: initial 80°C, holding for 2 min, increasing to 280°C at a rate of 10°C / min, holding for 10 min. The injection volume was set to 1 μL. A calibration curve was established using a standard dipropylene glycol solution with a purity of ≥99.99%. Concentration gradient standard solutions (0.1%, 0.5%, 1.0%, 5.0%, 10.0%, 50.0%, 100%) were prepared. The sample was injected for analysis. The relationship between the main peak area and the concentration was recorded.

[0213] High-purity dipropylene glycol (100 mg) was diluted with anhydrous ethanol to 10 mL. After ultrasonic dissolution, the solution was filtered through a 0.22 μm filter membrane. Three samples were prepared in parallel to avoid light and moisture absorption. After the sample was injected, the main peak position was confirmed by retention time. The main component peak area ratio was calculated according to the calibration curve. After deducting the impurity peaks (such as ethylene glycol, propylene glycol, moisture, etc.), the purity of the dipropylene glycol product (%) was obtained.

[0214] The calculation results are shown in Table 1.

[0215] Table 1 Product purity and product yield

[0216]

[0217] As can be seen from the test data of Example 1, Example 6 and Example 7, the reflux ratio of the second vacuum distillation column is adjusted in Example 6 and Example 7, resulting in a decrease in the yield and purity of the dipropylene glycol product. This is because the reflux ratio of the second vacuum distillation column in Example 6 is too low, resulting in incomplete separation of light components and an increase in heavy component residues, which reduces the yield and purity of the product. In Example 7, the reflux ratio of the second vacuum distillation column is too high. Although high reflux ratio improves separation accuracy, the risk of thermal decomposition causes the loss of part of the target product, which reduces the yield and purity of the product.

[0218] As can be seen from the test data of Example 1, Example 8 and Example 9, the space velocity of the distillation product is adjusted in Example 8 and Example 9, resulting in a decrease in the yield and purity of the dipropylene glycol product. This is because the space velocity of the distillation product in Example 8 is too low, resulting in local saturation of the composite adsorbent and an increase in the penetration rate of impurities, which reduces the yield and purity of the product. In Example 9, the space velocity of the distillation product is too high, resulting in insufficient adsorption and the presence of a small amount of amine and ether impurities in the adsorption product, which reduces the yield and purity of the product.

[0219] As can be seen from the test data of Example 1, Example 10 and Example 11, Example 10 and Example 11 adjust the mass ratio of active alumina powder and zirconium oxychloride in the zirconium oxychloride solution, resulting in the decrease of the product yield and product purity of dipropylene glycol, which is due to the fact that the amount of zirconium oxychloride added in Example 10 is too low, the adsorption capacity of the polar impurities in the distillation product is weakened, resulting in the decrease of the product yield and product purity. The amount of zirconium oxychloride added in Example 11 is too high, resulting in the excess of zirconium oxide generated, which blocks the pores of the active alumina, and the physical adsorption is invalid, resulting in the decrease of the product yield and product purity.

[0220] As can be seen from the test data of Example 1, Example 12 and Example 13, Example 12 and Example 13 adjust the mass fraction of silane coupling agent in the silane coupling agent solution, resulting in the decrease of the product yield and product purity of dipropylene glycol, which is due to the fact that the concentration of the silane coupling agent solution in Example 12 is too low, the amino modification is insufficient, resulting in the weakening of the selective adsorption capacity of the modified mesoporous molecular sieve, resulting in the decrease of the product yield and product purity. The concentration of the silane coupling agent solution in Example 12 is too high, the silane self-polymerization causes the pore structure of the modified mesoporous molecular sieve to be blocked, the physical adsorption is invalid, resulting in the decrease of the product yield and product purity.

[0221] As can be seen from the test data of Example 1, Example 14 and Example 15, Example 14 and Example 15 adjust the mass ratio of active alumina and modified mesoporous molecular sieve, resulting in the decrease of the product yield and product purity of dipropylene glycol, which is due to the fact that the amount of active alumina added in Example 14 is too low, the proportion of the modified mesoporous molecular sieve is too high, the physical adsorption capacity is insufficient, resulting in the decrease of the product yield and product purity. The amount of active alumina added in Example 15 is too high, the proportion of the modified mesoporous molecular sieve is too low, the active site is insufficient, resulting in the decrease of the product yield and product purity.

[0222] The inventor declares that the above is only a specific embodiment of the present application, but the protection scope of the present application is not limited thereto, and those skilled in the art should understand that any changes or replacements within the technical scope disclosed by the present application can be easily thought of by those skilled in the art, which falls within the protection scope and disclosure scope of the present application.

Claims

1. A process for preparing high-purity dipropylene glycol, characterized in that, The preparation process of the high-purity dipropylene glycol includes: (I) The crude product produced by the dipropylene glycol production process is fed into the first vacuum distillation column. After vacuum distillation, the intermediate product is collected from the bottom of the column. The intermediate product is fed into the second vacuum distillation column. After vacuum distillation, the distilled product is collected from the top of the column. (II) The distillation product is passed through an adsorption column filled with a composite adsorbent to obtain the adsorbed product. The composite adsorbent is composed of nano-zirconia supported active alumina and amino-modified mesoporous molecular sieve. (III) The adsorption product is subjected to a three-stage gradient cooling crystallization process to obtain a crystalline product, which is then separated by centrifugation to obtain high-purity dipropylene glycol.

2. The preparation process of high-purity dipropylene glycol according to claim 1, characterized in that, In step (I), the operating pressure of the first vacuum distillation column is 10 kPa; Alternatively, the bottom temperature of the first vacuum distillation column is 130~140℃; The top temperature of the first vacuum distillation column is 115~120℃; The reflux ratio of the first vacuum distillation column is (8~9):

1.

3. The preparation process of high-purity dipropylene glycol according to claim 1, characterized in that, In step (I), the operating pressure of the second vacuum distillation column is 1 kPa; Alternatively, the bottom temperature of the second vacuum distillation column is 135~145℃; The top temperature of the second vacuum distillation column is 120~125℃; The reflux ratio of the second vacuum distillation column is (5~6):1; Alternatively, in step (II), the amount of the composite adsorbent is 60-80% of the volume of the adsorption column; Alternatively, the height-to-diameter ratio of the adsorption column is (3~4):1; The space velocity of the distillation product is 1-2 h⁻¹. -1 ; The adsorption temperature is 40~50℃.

4. The preparation process of high-purity dipropylene glycol according to claim 1, characterized in that, In step (II), the composite adsorbent is prepared by the following method: (1) Alumina powder was mixed with zirconium oxychloride solution to obtain a precursor solution. The pH was adjusted to initiate a precipitation reaction. The mixture was allowed to stand for aging, filtered, dried and calcined to obtain loaded active alumina. (2) The mesoporous molecular sieve is dried, dispersed in a silane coupling agent solution to obtain a molecular sieve suspension, then refluxed in a water bath, filtered, washed and dried to obtain a modified mesoporous molecular sieve; (3) After mixing the loaded active alumina, modified mesoporous molecular sieve and hydroxypropyl methylcellulose, water is added to form a composite slurry, which is then extruded and granulated to obtain wet granules. After drying, the composite adsorbent is obtained.

5. The preparation process of high-purity dipropylene glycol according to claim 1, characterized in that, In step (II), the composite adsorbent is prepared by the following method: (1) Mix and stir the active alumina powder with zirconium oxychloride solution and heat to obtain a precursor solution. Add ammonia solution dropwise to the precursor solution to adjust its pH value and initiate a precipitation reaction. After the reaction is completed, let it stand and age to obtain the reaction product. Filter, dry and calcine the reaction product to obtain loaded active alumina. (2) The mesoporous molecular sieve is vacuum dried and dehydrated to obtain a dehydrated molecular sieve; the dehydrated molecular sieve is immersed in a silane coupling agent solution, and after ultrasonic dispersion, a molecular sieve suspension is obtained; the molecular sieve suspension is transferred to a water bath for reflux reaction; after the reaction is completed, it is filtered, washed and dried to obtain a modified mesoporous molecular sieve. (3) The loaded active alumina obtained in step (1), the modified mesoporous molecular sieve obtained in step (2) and hydroxypropyl methylcellulose are mixed and deionized water is added to prepare a composite slurry; the composite slurry is extruded and granulated by a screw extruder to obtain wet granules, and the wet granules are dried to obtain the composite adsorbent.

6. The preparation process of high-purity dipropylene glycol according to claim 5, characterized in that, In step (1), the zirconium oxychloride solution has a mass fraction of 20-25 wt%. Alternatively, the mass ratio of the activated alumina powder to the zirconium oxychloride in the zirconium oxychloride solution is 1:(0.3~0.4); The mixing and stirring temperature of the activated alumina powder and zirconium oxychloride solution is 70~80℃; The active alumina powder and zirconium oxychloride solution are mixed and stirred for 4-5 hours. The concentration of the ammonia solution is 20-30 wt%. Add aqueous ammonia solution dropwise to the precursor solution to adjust its pH to 8.5-9; After all the ammonia solution has been added dropwise, continue mixing and stirring for 1-2 hours. The static aging time is 10-15 hours; The drying temperature is 110~120℃; The drying time is 8-10 hours; The heating rate of the calcination is 3~5℃ / min; The calcination heating temperature is 350~400℃; The calcination holding time is 3-5 hours.

7. The preparation process of high-purity dipropylene glycol according to claim 5, characterized in that, In step (2), the pore size of the mesoporous molecular sieve is 3.5~4.5 nm; Alternatively, the heating temperature for vacuum drying and dehydration of the mesoporous molecular sieve is 100~120℃; The heating time for vacuum drying and dehydration of the mesoporous molecular sieve is 2-3 hours; The silane coupling agent solution is composed of a silane coupling agent and an aqueous ethanol solution; The mass fraction of the silane coupling agent in the silane coupling agent solution is 3-5 wt%. The ultrasonic power of the dehydrated molecular sieve in the silane coupling agent solution is 400~500W. The ultrasonic dispersion time of the dehydrated molecular sieve in the silane coupling agent solution is 30-50 min. The reflux reaction temperature is 60~70℃; The reflux reaction time is 8-10 hours.

8. The preparation process of high-purity dipropylene glycol according to claim 5, characterized in that, In step (3), the mass ratio of the supported active alumina to the modified mesoporous molecular sieve is (1.5~2):1; Alternatively, the amount of hydroxypropyl methylcellulose added is 5-8 wt% of the total mass of the supported activated alumina and the modified mesoporous molecular sieve; Deionized water is added to adjust the solid content of the composite slurry to 30-35 wt%. The screw speed of the screw extruder is 100~120 rpm; The screw extruder barrel is divided into three zones along the flow direction of the composite slurry: zone one, zone two, and zone three. The temperature in zone one of the screw extruder is 30~35℃; The temperature in the second zone of the screw extruder is 40~50℃; The temperature of the three zones of the screw extruder is 55~65℃; The wet granules have a particle diameter of 2-3 mm; The wet granules have a particle length of 3-5 mm; The drying temperature of the wet granules is 80~100℃; The drying time for the wet granules is 6-8 hours.

9. The preparation process of high-purity dipropylene glycol according to claim 1, characterized in that, In step (III), the three-stage gradient cooling crystallization process includes a rapid precooling stage, a seed induction stage, and a low-temperature crystallization stage performed sequentially. Alternatively, the rapid precooling stage may include: The adsorbed product is cooled to a pre-cooling temperature at a first cooling rate. When the adsorbed product reaches the pre-cooling temperature, the adsorbed product is stirred at a first stirring rate and kept warm at the first stirring rate and the pre-cooling temperature to obtain a pre-cooled product. The seed induction stage includes: The seed crystals to be added are divided into first seed crystals and second seed crystals according to their mass. The pre-cooled product is continuously cooled from the pre-cooling temperature to the induction temperature at a second cooling rate. During the continuous cooling process, when the temperature of the pre-cooled product reaches the intermediate induction temperature, the first seed crystal is added to the pre-cooled product. At the same time, the stirring rate is increased to a second stirring rate, and the pre-cooled product is stirred at the second stirring rate. When the temperature of the pre-cooled product reaches the induction temperature, the second seed crystal is added to the pre-cooled product. The pre-cooled product is kept at the second stirring rate and induction temperature to obtain the induced product. The low-temperature crystallization stage includes: The induced product is continuously cooled from the induction temperature to the crystallization temperature at a third cooling rate. When the temperature of the induced product reaches the intermediate crystallization temperature, the stirring rate is reduced to a third stirring rate. The induced product is stirred at the third stirring rate until the temperature of the induced product drops to the crystallization temperature. The induced product is then kept at the third stirring rate and crystallization temperature to obtain the crystallized product.

10. The preparation process of high-purity dipropylene glycol according to claim 9, characterized in that, The first cooling rate is 2.5~3.5℃ / min; Alternatively, the precooling temperature is 30~40℃; The first stirring speed is 10~20 rpm; The adsorbed product was kept at the first stirring rate and pre-cooling temperature for 10-15 minutes. The mass ratio of the first seed crystal to the second seed crystal is (2.5~3):1; The total mass of the first seed crystal and the second seed crystal is 0.1~0.2 wt% of the mass of the precooled product; The second cooling rate is 1~1.5℃ / min; The induction temperature is 15~18℃; The intermediate induction temperature is 18~20℃; The second stirring speed is 30~40 rpm; The pre-cooled product was kept at the second stirring rate and induction temperature for 20-30 minutes. The third cooling rate is 0.3~0.5℃ / min; The crystallization temperature is 2~4℃; The intermediate crystallization temperature is 5~7℃; The third stirring rate is 20~30 rpm; The induced product was kept at the third stirring rate and crystallization temperature for 3-4 hours. The seed crystal is a dipropylene glycol crystal with a purity of ≥99.5%.

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