Process for reducing ash content and oligomer content in polypropylene powder

Abstract

The invention discloses a process for reducing ash content and oligomer content in polypropylene powder. The process comprises the following steps: (1) conveying polypropylene powder and a washing solution in a downstream manner, stirring and premixing to form slurry, and heating, stirring and washing; (2) separating and drying the washed slurry to obtain electrical grade polypropylene; the washing solution is a mixture of alkane and alcohol, ester, epoxide or water. The electrical grade polypropylene product can be obtained by using the process.

Description

Technical Field

[0001] The invention relates to a process for effectively reducing the ash and oligomer contents in polypropylene powder, belonging to the technical field of polypropylene. Background Art

[0004] High dielectric strength and low dielectric loss are the basic requirements for power capacitor films. Since polypropylene film has high dielectric strength (>600V / μm), very low dielectric loss, good high-frequency characteristics, good high-temperature resistance and easy processing, it is irreplaceable as the dielectric of film capacitors by other films. According to the allowable operating temperature of the capacitor, polypropylene capacitor film is divided into "ordinary film" and "high-temperature film". High-temperature film allows capacitors to work in more severe environments without causing thermal breakdown of the capacitor film or a sharp attenuation of the capacitor capacitance value, which is related to the low dielectric loss of polypropylene capacitor film. The factors that affect the dielectric loss value of polypropylene capacitor film, in addition to the characteristics of polypropylene itself, are mainly the ash content in the polypropylene raw material; and the main factors that affect the operating temperature of polypropylene capacitor film, in addition to the characteristics of polypropylene itself, are mainly the crystallinity of polypropylene capacitor film. These two indicators are the two most important technical difficulties in producing this type of polypropylene. In addition, the dielectric strength of polypropylene film with low ash content and high crystallinity is also improved accordingly.

[0005] Polypropylene is a long-chain high molecular polymer, which contains three different main structures, namely isotactic polypropylene, syndiotactic polypropylene and atactic polypropylene. The isotacticity of polypropylene is the content of isotactic polypropylene in the whole polymer, expressed as a mass percentage, and isotacticity is also called isotactic index. Isotacticity affects the crystallinity of polypropylene. The higher the isotacticity, the higher the crystallinity. Crystallinity affects many properties of polypropylene products. Within a certain range, the higher the crystallinity, the higher the tensile yield strength and hardness of the resin. The isotacticity of polypropylene will also affect the processing performance of the resin. If the isotacticity is low, the product will be sticky and have poor fluidity. It is easy to form blocks and lumps during packaging and storage, and it is difficult to add materials during processing, or even impossible to process.

[0006] The impurities in polypropylene are mainly expressed by the ash index. High ash content generally indicates that the activity of the catalyst is low, followed by an increase in the yellow index. High ash content has a great impact on the extrusion process, especially on the fiber spinning process. Too high ash content is mainly due to the high impurity content in the raw materials, such as residual main catalysts, co-catalysts, oligomers, salts, reaction products and other residual metal ions present in the resin. Too high ash content is likely to have the following effects: 1. Polypropylene fiber materials are prone to breakage during spinning, and the film is prone to breakage during film production, affecting product quality; 2. Electrical grade polypropylene has high ash content requirements, and high ash content affects its application in electrical grade products; 3. The granulation unit is prone to clogging the filter screen during extrusion granulation.

[0007] Under certain temperature and pressure conditions, propylene monomer is polymerized into polypropylene powder through the action of main catalyst and co-catalyst. After the polypropylene powder comes out of the reactor, in addition to qualified polypropylene products, it also carries metal substances brought by the main catalyst and co-catalyst and low molecular weight polymers produced during the reaction. This low molecular weight polymer is compared with the molecular weight of polypropylene, usually with a molecular weight of several hundred to several thousand. These metal substances and low molecular weight products must be removed in the production of electrical grade polypropylene, otherwise it is easy to cause short circuit of electrical grade products when electrical materials are used, or even fire.

[0008] Removing the ash and oligomers produced by the catalyst in polypropylene is a technical difficulty in producing electrical grade polypropylene products. my country's current imported high-end electrical film polypropylene uses washing and deashing to obtain polypropylene raw materials with low ash content, and the ash content can be controlled below 20ppm. The ash content of domestically produced polypropylene raw materials is mainly reduced by efficient catalysts, optimized process conditions and auxiliary agent formulations. The ash content is usually controlled at 40-60ppm, which cannot meet the requirements of high-end electrical special materials, resulting in long-term domestic dependence on imports. Therefore, the washing process research conducted in China focuses on removing unnecessary ash from polymer powder. The most representative existing polypropylene powder washing processes are several patents such as CN 114014960 B, CN 114011103 B and CN117619286A.

[0009] The washing processes of CN 114014960 B and CN 114011103 B are basically the same, both of which involve countercurrent contact washing of polypropylene powder and washing solution in a washing tower to remove ash. The liquid phase material in the washing tower contains ash and oligomers. After entering the gas-liquid separation tank, the liquid phase material is flashed and separated. The ash-containing material is discharged in the form of a liquid phase. The gas phase detergent is cooled, separated and recovered, and returned to the washing tower for recycling. Small molecules that cannot be cooled (such as propane, etc.) are discharged to the flare system. The washed polypropylene powder is centrifuged and dried with nitrogen to obtain the product. The difference between the two patents is that CN 114014960 B uses isobutylene as the washing solution and operates under supercritical conditions; while CN114011103 B uses saturated alkanes such as n-hexane as the washing solution and operates under normal pressure. The disadvantages of these two patents are: this flow mode leads to a high slurry concentration at the bottom of the washing container, which may block the bottom outlet of the washing container during continuous industrial operation, affecting the long-term stable operation of the device. In addition, the slurry concentration is unevenly distributed, affecting the washing effect of polypropylene powder.

[0010] The washing and separation in CN 117619286 A are both completed in a rotary pressure filter. The polymerization adopts a slurry process, using pentane as a washing solvent. Pentane and the slurry of the polymerization reaction enter the washing filter press together, and under the action of pressurized nitrogen, washing and solid-liquid separation are completed. The separated solvent is refined and returned to the polymerization reactor for recycling. The disadvantage of this patent is that this rotary pressure filter is easily blocked by oligomers and polyolefin powder in the polymer, and needs to be cleaned regularly, and water medium is usually used for cleaning. In the polyolefin process, the co-catalyst and the main catalyst are very sensitive to water, which will not only greatly increase the cost of the material refining system, but also seriously shorten the operation cycle of the device, so it is not conducive to the continuous production of polyolefins. Summary of the invention

[0011] The present invention discloses a process for reducing the ash content and oligomer content in polypropylene powder, comprising the following steps:

[0012] (1) The polypropylene powder and the washing solution are transported in a downstream manner, first pre-mixed by stirring to form a slurry, and then heated, stirred and washed;

[0013] (2) The washed slurry is separated and dried to obtain electrical grade polypropylene;

[0014] The washing solution is a mixture of an alkane and an alcohol, an ester, an epoxide or water.

[0015] According to an embodiment of the present invention, the mass ratio of the alkane to the alcohol, ester, epoxide or water is (1-100):1, for example 2:1, 3:1, 5:1, 10:1, 20:1, 30:1, 40:1 or 50:1, exemplified by 3:1.

[0016] According to an embodiment of the present invention, the alkane is selected from one or more of C1-12 straight-chain alkanes and C3-12 cyclic alkanes, such as one or more of isopentane, n-pentane, cyclopentane, n-hexane, cyclohexane, methylcyclohexane and n-heptane, exemplified by n-hexane.

[0017] According to an embodiment of the present invention, the alcohol is selected from one or more of isopropanol, n-butanol, ethylene glycol, propylene glycol, butylene glycol, glycerol, benzyl alcohol and n-octanol, and isopropanol is exemplified.

[0018] According to an embodiment of the present invention, the ester is selected from one or more of dimethyl carbonate, trimethyl phosphite, dimethyl sulfite, dimethyl phosphate, dimethyl methylphosphonate and trimethyl phosphate.

[0019] According to an embodiment of the present invention, the epoxide is a C3-12 alkylene oxide, for example, one or more selected from propylene oxide and butylene oxide.

[0020] According to an embodiment of the present invention, the mass ratio of the polypropylene powder to the washing solution is 1:(2.5-6), such as 1:3, 1:4 or 1:5. Within this mass ratio range, both the washing effect and the stable operation of the washing system can be ensured.

[0021] According to an embodiment of the present invention, the heating temperature is 70-100° C., for example, 80-90° C. This temperature can ensure that the metal substances and oligomers can be completely dissolved in the washing solution, and the solution temperature needs to be increased to achieve a better dissolution temperature.

[0022] According to an embodiment of the present invention, the time of the heating and stirring washing is 2 to 5 hours, such as 3 hours or 4 hours, to ensure that the metal substances and oligomers can be fully dissolved in the washing solution.

[0023] According to an embodiment of the present invention, the oligomer content in the polypropylene powder is ≥0.5%, and the ash content is >20 ppm; for example, the oligomer content is >0.5%, and the ash content is >40 ppm.

[0024] According to an embodiment of the present invention, the content of oligomers in the electrical grade polypropylene is less than 0.5%, the ash content is ≤20 ppm, and the isotacticity of the polypropylene is ≥98%.

[0025] According to an embodiment of the present invention, the agitation washing uses a double-layer agitator.

[0026] According to an embodiment of the present invention, a portion of the slurry formed by the stirring premixing can also be returned to step (1) to continue mixing with the polypropylene powder and the washing solution.

[0027] According to an embodiment of the present invention, the separation is centrifugal filtration, the obtained liquid material is collected, and the solid material is sent to dry.

[0028] According to an embodiment of the present invention, the collected liquid material is separated into gas and liquid, the obtained gaseous material is cooled and returned to the premixing process, and the liquid material is sent to the flash evaporation process. Further, the gas phase obtained by flash evaporation is condensed and returned to the premixing process, and the remaining material (including metal complexes and oligomers) is discharged as waste.

[0029] According to an embodiment of the present invention, the drying is fluidized bed drying. Further, the solid material is dried in a fluidized bed, the polypropylene powder is recovered or enters a downstream process, the gas-liquid stream containing the washing solution and the drying medium is cooled, the gas and liquid are separated, the obtained liquid is returned to the premixing process of step (1), and the gas is returned to the drying process of step (2) as a drying medium.

[0030] According to an embodiment of the present invention, the process for reducing the ash and oligomer content in the polypropylene powder comprises the following steps:

[0031] (1) The polypropylene powder and the washing solution are transported in a downstream manner, first pre-mixed by stirring to form a slurry, then heated to 70-100° C., and stirred and washed for 2-5 hours;

[0032] The washing solution is a mixture of alkane and alcohol, ester, epoxide or water, and the mass ratio of the alkane to the alcohol, ester, epoxide or water is (1-100):1;

[0033] The mass ratio of the polypropylene powder to the washing solution is 1:(2.5-6);

[0034] (2) The washed slurry is centrifuged, the liquid material is collected, and the solid material is sent to a fluidized bed for drying;

[0035] (3) The collected liquid material is subjected to gas-liquid separation, and the obtained gaseous material is returned to the premixing process of step (1) after cooling, and the liquid material is sent to the flash evaporation process;

[0036] Furthermore, the gas phase obtained in the flash evaporation process is condensed and returned to the premixing process in step (1), and the remaining material is discharged as waste;

[0037] (4) The polypropylene powder obtained by fluidized bed drying is recovered or enters the downstream process, the gas-liquid stream containing the washing solution and the drying medium is cooled and the gas-liquid is separated, the obtained liquid is returned to the premixing process of step (1), and the gas is returned to the fluidized bed drying process of step (2) as the drying medium;

[0038] After the above treatment, electrical grade polypropylene is obtained.

[0039] According to an embodiment of the present invention, the gas-liquid separation in step (3) and step (4) is the same or different, preferably different; for example, the gas-liquid separation in step (3) is carried out in a tower (such as a distillation tower), and the gas-liquid separation in step (4) is carried out in a gas-liquid separator.

[0040] In one embodiment, the process has the following Figure 1 The process shown.

[0041] According to some embodiments of the present invention, the method is performed in the following device, which comprises:

[0042] Slurry premix tanks, washing vessels, centrifugal filters, solvent recovery towers, reflux tanks, fluidized bed dryers and gas-liquid separators;

[0043] The top of the slurry premixing tank is provided with a polypropylene powder inlet and a washing solvent inlet, an agitator is provided inside, and the bottom is connected to the slurry inlet of the washing container through a slurry circulation discharge pump;

[0044] The bottom of the washing container is connected to the centrifugal filter via a discharge pump;

[0045] The centrifugal filter comprises a solid phase outlet and a liquid phase outlet, the solid phase outlet is connected to a fluidized bed dryer, and the liquid phase outlet is connected to a solvent recovery tower;

[0046] The reflux tank is arranged at the top of the solvent recovery tower, and the bottom of the reflux tank is connected to the washing solvent inlet;

[0047] The fluidized bed dryer comprises a gas phase outlet end, and the gas phase outlet end is connected to the gas-liquid separator;

[0048] The gas-liquid separator comprises a liquid outlet end, and the liquid outlet end is connected to the slurry premixing tank.

[0049] The main catalysts used in polypropylene are mostly ionic catalysts containing magnesium, titanium, etc. With the continuous development of catalysts, the activity of catalysts is getting higher and higher, and the amount added is getting smaller and smaller. Therefore, the ash produced by the catalyst residue in polypropylene is only a small part of the ash. The co-catalysts are mostly aluminum-containing substances, and the amount of such co-catalysts added is relatively high. The ash in polypropylene products mainly comes from aluminum-based co-catalysts. In addition, some low molecular weight polymers, usually called oligomers or oligomers, will inevitably be produced during the polymerization process. If these substances are not removed in time, when polypropylene is used in electrical-grade products, it will easily cause current short circuits or breakdown of film materials, thereby causing product performance degradation and even possible fire. Therefore, the key to the production of electrical-grade polypropylene is to ensure lower metal substances and oligomers in polypropylene.

[0050] There are three very important indicators for electrical grade polypropylene products: first, the ash content of the product must be less than 20ppm; second, the isotacticity of the product must reach ≥98%; and third, the product must contain almost no oligomers.

[0051] The main sources of this type of ash or oligomers are as follows:

[0052] 1) Metal substances contained in the catalyst itself;

[0053] 2) Metal substances contained in the promoter itself;

[0054] 3) Impurities in the raw materials prevent the catalyst from fully releasing its activity during the polymerization process, and consume part of the co-catalyst, resulting in an increase in the amount of metal substances added and an increase in ash content;

[0055] 4) Oligomers inevitably produced during the polymerization process;

[0056] 5) Large production fluctuations during the polymerization process lead to unstable catalyst addition, increased oligomer production, and increased ash content;

[0057] 6) The above phenomenon may also occur when the polypropylene device is started, stopped or switched brands.

[0058] Definitions and explanations of terms

[0059] Unless otherwise specified, the definitions of terms recorded in the specification and claims of this application, including their definitions as examples, exemplary definitions, preferred definitions, definitions recorded in tables, definitions of specific compounds in examples, etc., may be arbitrarily combined and coupled with each other. Such combinations and couplings shall fall within the scope of the description of this application.

[0060] Unless otherwise specified, the numerical ranges described in this specification and claims are equivalent to at least describing each specific integer value therein. For example, the numerical range "1-12" is equivalent to describing each integer value in the numerical range "1-12", namely 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 or 12.

[0061] The term "C1-12 straight chain alkane" is understood to mean a straight chain alkane having 1 to 12 carbon atoms, preferably a "C5-10 straight chain alkane", for example isopentane, n-pentane, n-hexane, n-heptane, octane, nonane, undecane, dodecane.

[0062] The term "C3-12 cyclic alkane" is understood to mean a saturated monovalent monocyclic, bicyclic (such as condensed ring, bridged ring, spirocyclic) hydrocarbon ring or tricyclic unsubstituted alkane having 3 to 12 (3, 4, 5, 6, 7, 8, 9, 10, 11 or 12) carbon atoms, or a C3-12 cyclic alkane substituted by a C1-12 alkyl group. For example, it may be cyclopentane, cyclohexane, methylcyclohexane, cycloheptane, cyclooctane.

[0063] The term "C1-12 alkyl" is understood to mean straight-chain and branched alkyl groups having 1 to 12 carbon atoms, preferably "C1-6 alkyl". The alkyl group is, for example, methyl, ethyl, propyl, butyl, pentyl, hexyl, isopropyl, isobutyl, sec-butyl, tert-butyl, isopentyl, 2-methylbutyl, 1-methylbutyl, 1-ethylpropyl, 1,2-dimethylpropyl, neopentyl, 1,1-dimethylpropyl, 4-methylpentyl, 3-methylpentyl, 2-methylpentyl, 1-methylpentyl, 2-ethylbutyl, 1-ethylbutyl, 3,3-dimethylbutyl, 2,2-dimethylbutyl, 1,1-dimethylbutyl, 2,3-dimethylbutyl, 1,3-dimethylbutyl or 1,2-dimethylbutyl or the like or isomers thereof.

[0064] The term "C3-12 alkylene oxide" refers to -O-(cycloalkane), wherein cycloalkane is as defined above.

[0065] Beneficial effects of the present invention:

[0066] The present invention focuses on the washing process. First, a washing solution and polypropylene powder with a specific ratio are transported in a downstream manner for premixing. Then, the washing solvent, temperature, time, etc. are controlled to fully dissolve the oligomers. At the same time, the washing solution can fully form a complex with the metal substance to separate it from the polypropylene powder, thereby obtaining an electrical grade polypropylene product.

[0067] In addition, the washing solution is recycled reasonably and effectively, avoiding the accumulation of metal complexes and oligomers dissolved in the washing solution, and ensuring that the washed polymer is not contaminated again. The washing solution entrained in the washed solid material is completely dried and separated.

[0068] This process can produce electrical grade polypropylene:

[0069] 1) The isotacticity of polypropylene is ≥98%;

[0070] 2) Ash content in polypropylene products ≤ 20ppm;

[0071] 3) Oligomers in polypropylene products <0.5%.

[0072] The electrical grade polypropylene obtained by the present invention has relevant indicators that meet the application requirements, and will provide new technologies for the localization of electrical grade polypropylene film materials in terms of key technologies and raw materials, and realize the full-chain localization of dry DC capacitors. At the same time, it can enhance my country's independent innovation capabilities in the research and development and production of electrical grade ultra-clean polypropylene materials, provide a basic guarantee for the construction of my country's new power system, and promote technological breakthroughs and industrial development of domestic manufacturing enterprises in key materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0073] Figure 1 The figure is a schematic diagram of the process flow of the method for reducing the ash content and oligomer content in polypropylene powder of the present invention;

[0074] Figure 2 A schematic diagram of the structure of a device used in the process of reducing the ash and oligomer content in polypropylene powder according to the present invention;

[0075] Description of the serial numbers in the figure:

[0076] 1. Slurry premixing tank; 2. Agitator; 3. First cooler; 4. Slurry circulation discharge pump; 5. First heater; 6. Double-layer agitator; 7. Washing container; 8. Washing container discharge pump; 9. Second cooler; 10. Centrifugal filter; 11. Conveyor; 12. Mother liquor collection tank; 13. Solvent recovery tower feed pump; 14. Preheater; 15. Solvent recovery tower; 16. First condenser; 17. Reflux tank; 18. Solvent recovery tower top reflux pump; 1 9. Discharge pump of the kettle of the solvent recovery tower; 20. Second heater; 21. Flash tank; 22. Second condenser; 23. Discharge pump of the flash vapor phase condenser; 24. Flash vapor phase condenser; 25. Circulation pump of the kettle of the solvent recovery tower; 26. Reboiler of the solvent recovery tower; 27. Fluidized bed dryer; 28. Cyclone separator; 29. ​​Third heater; 30. Circulating nitrogen blower; 31. Gas-liquid separator; 32. Third cooler; 33. Discharge pump of the gas-liquid separator;

[0077] a-polypropylene powder, b-washing solvent. DETAILED DESCRIPTION

[0078] The technical scheme of the present invention will be further described in detail below in conjunction with specific embodiments. It should be understood that the following embodiments are only exemplary descriptions and explanations of the present invention and should not be construed as limiting the scope of protection of the present invention. All technologies implemented based on the above content of the present invention are included in the scope that the present invention is intended to protect.

[0079] The technical solution of the present invention is described in detail below in conjunction with embodiments and comparative examples, but the protection scope of the present invention is not limited to the following embodiments.

[0080] Example 1

[0081] 62.5 kg / h of polypropylene powder (with an oligomer content of 1% and an ash content of 63 ppm) and 187.5 kg / h of washing solution were added downstream to a slurry premixing tank with an agitator, and fully mixed for about half an hour until the mixture was uniformly mixed under the action of the agitator and the premixing tank circulating discharge pump. The mixture was then pumped to a washing container with a double-layer agitator through the premixing tank circulating discharge pump. The washing temperature was set at 80°C and the washing process lasted for 3 hours.

[0082] In this embodiment, the washing solution used has n-hexane:isopropanol=3:1 (by mass ratio). The slurry after leaving the washing container is separated, the obtained liquid material is collected, and the solid material is sent to dry to obtain the product polypropylene.

[0083] The product polypropylene was sampled and analyzed, and the results are listed in Table 1. The ash content of the product polypropylene was 17ppm, the oligomer was 13ppm, and the isotacticity was 98.6%, which fully met the technical indicators of the national requirements for electrical grade polypropylene superior products.

[0084] Table 1

[0085]

[0086] Example 2

[0087] This embodiment is operated according to the operating steps of Example 1, except that 62.5 kg / h of polypropylene powder (wherein the oligomer content is 1% and the ash content is 63 ppm) and 250 kg / h of washing solution are used for washing, that is, the mass ratio of polypropylene powder to washing solution is 1:4.

[0088] The product polypropylene was sampled and analyzed, and the results are listed in Table 2. The ash content of the product polypropylene was 16ppm, the oligomer was 10ppm, and the isotacticity was 98.2%, which fully met the technical indicators of the national requirements for electrical grade polypropylene superior products.

[0089] Table 2

[0090]

[0091] Example 3

[0092] This embodiment is operated according to the operating steps of embodiment 1, except that the washing temperature is increased to 90°C.

[0093] The product polypropylene was sampled and analyzed, and the results are listed in Table 3. The ash content of the product polypropylene was 15ppm, the oligomer was 9ppm, and the isotacticity was 98.4%, which fully met the technical indicators of the national requirements for electrical grade polypropylene superior products.

[0094] Table 3

[0095]

[0096] Example 4

[0097] This embodiment is operated according to the operating steps of embodiment 1, except that the washing time is increased to 4 hours.

[0098] The product polypropylene was sampled and analyzed, and the results are listed in Table 4. The ash content of the product polypropylene was 15ppm, the oligomer was 12ppm, and the isotacticity was 98.2%, which fully met the technical indicators of the state-required superior electrical grade polypropylene.

[0099] Table 4

[0100]

[0101] Comparative Example 1

[0102] This comparative example was operated according to the operating steps of Example 1, except that 62.5 kg / h of polypropylene powder (with an oligomer content of 1% and an ash content of 63 ppm) and 125 kg / h of washing solution were used for washing, that is, the mass ratio of polypropylene powder to washing solution was 1:2.

[0103] The product polypropylene was sampled and analyzed, and the results are listed in Table 5. The ash content of the product polypropylene was 35ppm, the oligomer was 80ppm, and the isotacticity was 97.9%. Comparing this comparative example with the polypropylene prepared in Example 1, it can be seen that the various properties of the prepared polypropylene are significantly lower than those of the polypropylene prepared in Example 1, and cannot meet the technical indicators of the electrical grade polypropylene premium product required by the state.

[0104] Table 5

[0105]

[0106] Comparative Example 2

[0107] This comparative example was operated according to the operating steps of Example 1, except that the washing temperature was lowered to 60°C.

[0108] The product polypropylene was sampled and analyzed, and the results are listed in Table 6. The ash content of the product polypropylene was 43ppm, the oligomer was 150ppm, and the isotacticity was 97.7%. Comparing this comparative example with the polypropylene prepared in Example 1, it can be seen that the various properties of the prepared polypropylene are significantly lower than those of the polypropylene prepared in Example 1, and cannot meet the technical indicators of the electrical grade polypropylene premium product required by the state.

[0109] Table 6

[0110]

[0111] Comparative Example 3

[0112] This comparative example was operated according to the operating steps of Example 1, except that the washing time was reduced to 1 hour.

[0113] The product polypropylene was sampled and analyzed, and the results are listed in Table 7. The ash content of the product polypropylene was 38ppm, the oligomer was 116ppm, and the isotacticity was 97.5%. Comparing this comparative example with the polypropylene prepared in Example 1, it can be seen that the various properties of the prepared polypropylene are significantly lower than those of the polypropylene prepared in Example 1, and cannot meet the technical indicators of the electrical grade polypropylene premium product required by the state.

[0114] Table 7

[0115]

[0116] Comparative Example 4

[0117] This comparative example was operated according to the operating steps of Example 1, except that the washing liquid was changed to pure n-hexane.

[0118] The product polypropylene was sampled and analyzed, and the results are listed in Table 8. The ash content of the product polypropylene was 38ppm, the oligomer was 116ppm, and the isotacticity was 97.5%. Comparing this comparative example with the polypropylene prepared in Example 1, it can be seen that the various properties of the prepared polypropylene are significantly lower than those of the polypropylene prepared in Example 1, and cannot meet the technical indicators of the electrical grade polypropylene premium product required by the state.

[0119] Table 8

[0120]

[0121] It can be seen from the above examples and comparative examples that the present invention obtains the electrical grade polypropylene superior product that meets national requirements by controlling the process and selecting appropriate washing solvent, washing temperature, time, etc.

[0122] Example 5

[0123] A device for reducing the ash and oligomer content in polypropylene powder, comprising: a slurry premixing tank, a washing container, a centrifugal filter, a solvent recovery tower, a reflux tank, a fluidized bed dryer and a gas-liquid separator;

[0124] The top of the slurry premixing tank is provided with a polypropylene powder inlet and a washing solvent inlet, an agitator is provided inside, and the bottom is connected to the slurry inlet of the washing container through a slurry circulation discharge pump;

[0125] The bottom of the washing container is connected to the centrifugal filter via a discharge pump;

[0126] The centrifugal filter comprises a solid phase outlet and a liquid phase outlet, the solid phase outlet is connected to a fluidized bed dryer, and the liquid phase outlet is connected to a solvent recovery tower;

[0127] The reflux tank is arranged at the top of the solvent recovery tower, and the bottom of the reflux tank is connected to the washing solvent inlet;

[0128] The fluidized bed dryer comprises a gas phase outlet end, and the gas phase outlet end is connected to the gas-liquid separator;

[0129] The gas-liquid separator comprises a liquid outlet end, and the liquid outlet end is connected to the slurry premixing tank.

[0130] Furthermore, the slurry circulation discharge pump is also connected to the top of the slurry premixing tank, that is, the slurry is circulated to allow the polypropylene powder and the washing solution to be fully contacted and mixed; preferably, a first cooler is arranged on the connecting pipeline between the two.

[0131] Furthermore, a first heater is provided on the connecting pipeline between the bottom of the slurry premixing tank and the slurry inlet of the washing container, for heating the slurry so that the temperature of the washing container reaches the set washing temperature to ensure the washing effect. The slurry inlet of the washing container is provided at the top of the washing container.

[0132] Furthermore, a double-layer agitator is arranged inside the washing container to fully and evenly mix the slurry and the washing solvent.

[0133] Furthermore, a second cooler is provided on the connecting pipeline between the discharge pump and the centrifugal filter for cooling the feed.

[0134] Furthermore, the device also includes a conveyor, which is arranged at the solid phase outlet end of the centrifugal filter and is used to convey the solid phase material to the fluidized bed dryer.

[0135] Furthermore, the device also includes a mother liquor collection tank, one end of which is connected to the liquid phase outlet end of the centrifugal filter, and the other end of which is connected to the middle part of the solvent recovery tower.

[0136] Furthermore, the device also includes a first condenser, one end of which is connected to the top of the solvent recovery tower, and the other end of which is connected to the reflux tank.

[0137] Furthermore, the device further comprises a flash tank, the bottom of the solvent recovery tower is connected to the flash tank, and a second heater is preferably arranged on the connecting pipeline between the two.

[0138] Furthermore, the device also includes a second condenser and a flash gas phase condenser; one end of the second condenser is connected to the gas phase outlet of the flash tank, and the other end is connected to the material inlet of the flash gas phase condenser; the liquid phase outlet of the flash gas phase condenser is connected to the solvent recovery tower.

[0139] Furthermore, the device also includes a cyclone separator, which is arranged above the fluidized bed dryer, the gas phase outlet end of the fluidized bed dryer is connected to the material inlet of the cyclone separator, the gas outlet of the cyclone separator is connected to the gas-liquid separator, and the solid outlet of the cyclone separator is connected to the fluidized bed dryer. Preferably, a third cooler is arranged on the connecting pipeline between the gas outlet of the cyclone separator and the gas-liquid separator.

[0140] Furthermore, the gas outlet of the gas-liquid separator is connected to the fluidized bed dryer, and preferably a circulating nitrogen blower and / or a third heater are arranged on the connecting pipeline between the two.

[0141] Furthermore, those skilled in the art can arrange a pump body on the pipeline as required.

[0142] Example 6

[0143] Different from Example 1, this process is as follows Figure 1 The process is carried out in the device shown in the figure, specifically, the premixing process is carried out in a slurry premixing tank, the washing process is carried out in a washing container, the slurry separation process after washing is carried out in a centrifugal filter, the liquid material obtained by centrifugal separation is collected by a mother liquor collection tank, and gas-liquid separation is carried out in a solvent recovery tower, the obtained gaseous material is returned to the slurry premixing tank after cooling, the liquid material is sent to a flash tank, and the gas phase obtained by flash evaporation is returned to the slurry premixing tank after condensation; the drying process is carried out in a fluidized bed dryer, the polypropylene powder obtained by fluidized bed drying is recovered or enters the downstream process, the gas-liquid flow containing the washing solution and the drying medium passes through the gas-liquid separator, the obtained liquid is returned to the slurry premixing tank, and the gas is returned to the fluidized bed dryer as a drying medium.

[0144] The device includes:

[0145] Slurry premixing tank 1, washing container 7, centrifugal filter 10, conveyor 11, mother liquor collecting tank 12, solvent recovery tower 15, first condenser 16, reflux tank 17, flash tank 21, second condenser 22, fluidized bed dryer 27, cyclone separator 28 and gas-liquid separator 31;

[0146] The top of the slurry premixing tank 1 is provided with a polypropylene powder inlet and a washing solvent inlet, an agitator 2 is provided inside, and the bottom is connected to the slurry inlet of the washing container 7 through a slurry circulation discharge pump 4. A first heater 5 is provided on the connecting pipeline between the slurry circulation discharge pump 4 and the slurry inlet of the washing container 7, which is used to heat the slurry so that the temperature of the washing container reaches the set washing temperature to ensure the washing effect; the slurry circulation discharge pump 4 is also connected to the top of the slurry premixing tank 1, that is, the slurry is circulated so that the polypropylene and the washing solution are fully contacted and mixed; further, a first cooler 3 is provided on the connecting pipeline between the slurry circulation discharge pump 4 and the top of the slurry premixing tank 1;

[0147] A double-layer agitator 6 is arranged inside the washing container 7 to fully mix the slurry and the washing solvent; the bottom of the washing container 7 is connected to the centrifugal filter 10 through the washing container discharge pump 8, and further, a second cooler 9 is arranged on the connecting pipeline between the two to cool the feed;

[0148] The centrifugal filter 10 includes a solid phase outlet and a liquid phase outlet. The solid phase outlet is connected to the fluidized bed dryer 27 through a conveyor 11, and the liquid phase outlet is connected to a mother liquid collection tank, and is connected to the middle of a solvent recovery tower 15 through a solvent recovery tower feed pump 13 and a preheater 14.

[0149] A first condenser 16 is disposed at the top of the solvent recovery tower 15, one end of the first condenser 16 is connected to the top of the solvent recovery tower 15, and the other end is connected to a reflux tank 17, and the bottom of the reflux tank 17 is connected to the washing solvent inlet via a solvent recovery tower top reflux pump 18;

[0150] A solvent recovery tower kettle discharge pump 19 is provided at the bottom of the solvent recovery tower 15, which is connected to a flash tank 21 via a second heater 20;

[0151] The bottom of the solvent recovery tower 15 is also provided with a solvent recovery tower kettle circulation pump 25 and a solvent recovery tower reboiler 26 to form a loop at the bottom of the tower;

[0152] A second condenser 22 is provided at the top of the flash tank 21 (i.e., the gas phase outlet), one end of the second condenser 22 is connected to the flash gas phase condensation tank 24, and the other end is connected to the material inlet of the flash gas phase condensation tank 24;

[0153] The bottom of the flash gas phase condenser 24 (i.e., the liquid phase outlet) is connected to the solvent recovery tower 15 via the flash gas phase condenser discharge pump 23;

[0154] The cyclone separator 28 is arranged above the fluidized bed dryer 27, the gas outlet of the fluidized bed dryer 27 is connected to the material inlet of the cyclone separator 28, the gas outlet of the cyclone separator 28 is connected to the gas-liquid separator 31, and the solid outlet of the cyclone separator 28 is connected to the fluidized bed dryer 27. A third cooler 32 is arranged on the connecting pipeline between the gas outlet of the cyclone separator 28 and the gas-liquid separator 31. The liquid outlet end of the gas-liquid separator 31 is connected to the slurry premixing tank 1 through the gas-liquid separator discharge pump 33, that is, the recycling and reuse of the washing solvent is realized. The gas outlet of the gas-liquid separator 31 is connected to the fluidized bed dryer 27, and a circulating nitrogen blower 30 and a third heater 29 are further arranged on the connecting pipeline between the two.

[0155] The process of this embodiment can not only obtain electrical grade polypropylene as shown in Example 1, but also achieve full recycling of materials and energy.

[0156] The above is an explanation of the embodiments of the present invention. However, the present invention is not limited to the above embodiments. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A process for reducing the ash and oligomer content in polypropylene powder, characterized in that: The process comprises the following steps: (1) The polypropylene powder and the washing solution are transported in a downstream manner, first pre-mixed by stirring to form a slurry, and then heated, stirred and washed; (2) The washed slurry is separated and dried to obtain electrical grade polypropylene; The washing solution is a mixture of an alkane and an alcohol, an ester, an epoxide or water.

2. The process according to claim 1, characterized in that The mass ratio of the alkane to the alcohol, ester, epoxide or water is (1-100):

1.

3. The process according to claim 1, characterized in that The alkane is selected from one or more of C1-12 straight-chain alkanes and C3-12 cyclic alkanes, such as one or more of isopentane, n-pentane, cyclopentane, n-hexane, cyclohexane, methylcyclohexane and n-heptane; The alcohol is selected from one or more of isopropanol, n-butanol, ethylene glycol, propylene glycol, butylene glycol, glycerol, benzyl alcohol and n-octanol; The ester is selected from one or more of dimethyl carbonate, trimethyl phosphite, dimethyl sulfite, dimethyl phosphate, dimethyl methylphosphonate and trimethyl phosphate; The epoxide is a C3-12 alkylene oxide, for example, one or more selected from propylene oxide and butylene oxide.

4. The process according to any one of claims 1 to 3, characterized in that The mass ratio of the polypropylene powder to the washing solution is 1:(2.5-6); And / or, the temperature to which the heating is performed is 70 to 100° C.; And / or, the heating, stirring and washing time is 2 to 5 hours.

5. The process according to any one of claims 1 to 3, characterized in that The oligomer content in the polypropylene powder is ≥0.5%, and the ash content is >20ppm; The content of oligomers in the electrical grade polypropylene is less than 0.5%, the content of ash is less than or equal to 20 ppm, and the isotacticity of the polypropylene is greater than or equal to 98%.

6. The process according to claim 1, characterized in that The stirring washing uses a double-layer agitator; And / or, a portion of the slurry formed by the stirring premixing can also be returned to step (1) to continue mixing with the polypropylene powder and the washing solution; And / or, the separation is centrifugal filtration, the obtained liquid material is collected, and the solid material is sent to drying; And / or, the drying is fluidized bed drying.

7. The process according to claim 6, characterized in that The collected liquid material is separated into gas and liquid, and the obtained gaseous material is returned to the premixing process after cooling, and the liquid material is sent to the flash evaporation process; further, the gas phase obtained by flash evaporation is condensed and returned to the premixing process, and the remaining material is discharged as waste.

8. The process according to claim 6, characterized in that The solid material is dried in a fluidized bed, the polypropylene powder is recovered or enters a downstream process, the gas-liquid flow containing the washing solution and the drying medium is cooled and the gas-liquid is separated, the obtained liquid is returned to the premixing process of step (1), and the gas is returned to the drying process of step (2) as a drying medium.

9. The process according to any one of claims 1 to 8, characterized in that: The process is carried out in the following device, which comprises: Slurry premix tanks, washing vessels, centrifugal filters, solvent recovery towers, reflux tanks, fluidized bed dryers and gas-liquid separators; The top of the slurry premixing tank is provided with a polypropylene powder inlet and a washing solvent inlet, an agitator is provided inside, and the bottom is connected to the slurry inlet of the washing container through a slurry circulation discharge pump; The bottom of the washing container is connected to the centrifugal filter via a discharge pump; The centrifugal filter comprises a solid phase outlet and a liquid phase outlet, the solid phase outlet is connected to a fluidized bed dryer, and the liquid phase outlet is connected to a solvent recovery tower; The reflux tank is arranged at the top of the solvent recovery tower, and the bottom of the reflux tank is connected to the washing solvent inlet; The fluidized bed dryer comprises a gas phase outlet end, and the gas phase outlet end is connected to the gas-liquid separator; The gas-liquid separator comprises a liquid outlet end, and the liquid outlet end is connected to the slurry premixing tank.

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