Production device and production process of low-methanol high-purity polyvinyl alcohol

Abstract

The invention relates to a production device and a production process of low-methanol high-purity polyvinyl alcohol. According to the production process, semi-finished PVA slurry is firstly soaked, then mechanically dewatered, washed, whitened and finally dried, so that the content of methanol in the prepared finished PVA is 0.3-1.3%, the content of methyl acetate is 0.02-0.3%, the content of sodium acetate is 0.45-0.60%, and the content of ash is 0.16-0.25%.

Description

Technical Field

[0001] This invention belongs to the field of polyvinyl alcohol production, specifically relating to a production apparatus and process for low-methanol, high-purity polyvinyl alcohol. Background Technology

[0002] Polyvinyl alcohol (PVA) is a water-soluble polymer widely used in emulsion protective colloids, dispersants, papermaking, film materials, and thickeners. The production process of PVA mainly includes the synthesis, purification, and polymerization of vinyl acetate (VAC), as well as the alcoholysis of polyvinyl acetate (PVAC). After alkali-catalyzed alcoholysis, a solid product is obtained. This product is then pulverized and extruded using an extruder or centrifuged to remove most of the mother liquor. The PVA product after this stage is called clinker. The clinker is then dried in a dryer to obtain the finished PVA product. For example, a methanol-water solution (60% methanol) is sprayed into the front of the dryer to wash away impurities and simultaneously expand the PVA. The methanol evaporates during the subsequent drying process. Because the methanol-water solution is added at the front of the dryer, a shell film has already formed on some of the PVA surface during the initial drying process, making it difficult for the methanol-water solution to penetrate, resulting in an unstable final methanol content. For example, methyl acetate is sprayed into the middle section of the dryer. Methyl acetate forms an azeotrope with methanol, which carries away the methanol in the PVA. However, because methyl acetate cannot be sprayed in large quantities, it is difficult to add a small amount of methyl acetate evenly. As a result, the methanol content of the low-methanol PVA produced by this method is extremely unstable.

[0003] Current methanol removal technologies involve evaporation during the drying process and the injection of methyl acetate into the dryer. The methyl acetate forms an azeotrope with methanol, carrying away the methanol within the PVA. In addition, there are low-capacity (30-40% below normal capacity) methanol removal methods, but these are less economical.

[0004] With increasing environmental awareness, people are demanding higher methanol content in polyvinyl alcohol (PVA) products. CN102786698A discloses a method for preparing a low-methanol-content PVA aqueous solution, comprising the following steps: mixing PVA with deionized water to obtain a PVA aqueous solution; heating the PVA aqueous solution to swell it; then filtering out the water in the system; repeating this process several times to obtain a low-methanol-content PVA aqueous solution. However, this method uses deionized water for purification, resulting in a high cost for the entire purification process. CN110028602A discloses a method for reducing the impurity content in industrial PVA, which involves first heating and stirring PVA clinker in a solvent, then separating the solid and liquid phases, and finally drying the resulting solid to obtain the finished PVA product. The solvent is a mixture of methanol and water. However, the PVA prepared by this method has a methanol content of 0.08% to 6.04%, indicating instability and a large fluctuation range in methanol content.

[0005] Therefore, it is necessary to study new production processes for polyvinyl alcohol with low methanol content. Summary of the Invention

[0006] The first technical problem this invention aims to solve is the high cost of purification using deionized water in existing methods for producing low-methanol-content polyvinyl alcohol. The second technical problem is the instability and wide fluctuation range of methanol content in the polyvinyl alcohol produced by existing methods. Therefore, this invention provides an apparatus and process for producing high-purity polyvinyl alcohol with low methanol content.

[0007] The specific principle of this invention for producing low-methanol, high-purity polyvinyl alcohol (PVA) is as follows: Semi-finished PVA is immersed in a solvent containing methyl acetate, allowing impurities such as methanol in the semi-finished product to dissolve as much as possible in the methyl acetate solvent. Then, most of the solvent and impurities are removed through mechanical dehydration. After mechanical dehydration, a mixture of methyl acetate, methanol, and water (a third component) is added to wash and whiten the PVA. Simultaneously, the added methyl acetate forms an azeotrope with the methanol in the PVA, lowering the boiling point of methanol during the drying process. Afterward, drying (heat treatment) is performed. During the heat treatment, nitrogen gas at 100-150°C (a fourth component) is added to accelerate the heating rate of the drying equipment, ensuring uniform temperature throughout the drying process and ultimately improving drying efficiency. This allows for the complete evaporation of methanol and methyl acetate from the semi-finished PVA during the drying process. This invention combines immersion washing and drying purification techniques to produce a finished PVA product that meets the requirements of low methanol and high purity, reducing the methanol content to 0.3-1.3 wt% and the methyl acetate content to below 0.02-0.3 wt%.

[0008] The raw material semi-finished product of this invention can be derived from PVA slurry after alcoholysis and pulverization in the PVA production process, with a methanol content of 70-85 wt%. In one specific embodiment, the PVA slurry contains 70-85 wt% methanol, more preferably 71-76 wt%, 20-30 wt% methyl acetate, more preferably 22-27 wt%, 0.5-3 wt% water, more preferably 1.0-2.0 wt%, 0.1-0.8 wt% sodium acetate, more preferably 0.2-0.6 wt%, and 0.02 ± 0.01 wt% aldehydes.

[0009] This invention is achieved through the following technical solution: In a first aspect, the present invention provides a production apparatus for low-methanol, high-purity polyvinyl alcohol, comprising: a PVA slurry soaking device, a slurry conveying device, a mechanical dehydration device, a first-stage drying device, and a second-stage drying device. The inlet of the PVA slurry soaking device is connected to a PVA slurry feed pipeline and a methyl acetate feed pipeline. The outlet of the PVA slurry soaking device is connected to the slurry conveying device. The slurry conveying device is connected to the mechanical dehydration device. The solid discharge pipeline of the mechanical dehydration device is sequentially connected to the first-stage drying device and the second-stage drying device. A third component addition pipeline is provided at the solid outlet of the mechanical dehydration device, or a third component addition pipeline is connected to the pipeline between the mechanical dehydration device and the first-stage drying device. A fourth component addition pipeline is provided in the second-stage drying device. The outlet of the second-stage drying device is connected to the finished polyvinyl alcohol (PVA) discharge pipeline.

[0010] Furthermore, in the aforementioned production apparatus for low-methanol, high-purity polyvinyl alcohol, the slurry conveying device includes a conveying pipeline and a conveying pump, the mechanical desliming device is a centrifuge, and the first-stage drying device and the second-stage drying device are tubular dryers. Preferably, the drying temperature of the first-stage drying device is 50~70℃, and the drying temperature of the second-stage drying device is 70~125℃, more preferably 75~120℃.

[0011] The low-methanol, high-purity polyvinyl alcohol production apparatus of the present invention may further include an alcoholysis unit and a pulverizing unit. The outlet of the alcoholysis unit is connected to the inlet of the pulverizing unit, and the outlet of the pulverizing unit is connected to a PVA slurry soaking unit via a PVA slurry feed pipeline. The PVA semi-finished product produced by the alcoholysis unit is pulverized by the pulverizing unit, washed with a rinsing solution, and then the PVA slurry is sent to the PVA slurry soaking unit.

[0012] Furthermore, in the aforementioned low-methanol high-purity polyvinyl alcohol production device, the third component inlet pipeline is a mixture inlet pipeline for methyl acetate, methanol, and water; and the fourth component inlet pipeline is a nitrogen inlet pipeline.

[0013] Furthermore, in the aforementioned production apparatus for low-methanol high-purity polyvinyl alcohol, the liquid outlet of the mechanical dehydration device is connected to the solvent recovery pipeline.

[0014] Secondly, the present invention provides a process for producing low-methanol, high-purity polyvinyl alcohol using the above-mentioned production apparatus, comprising the following steps: (1) Raw material semi-finished product soaking: Methyl acetate and PVA slurry from the alcoholysis and pulverization process in the PVA production process are added to the PVA slurry soaking device, i.e., raw material semi-finished product with methanol content of 70~85wt%, preferably forming a solution with methyl acetate of 30~50wt%, methanol of 40~60wt%, and PVA of 10~20wt%, and soaking at 40~53℃, preferably 43~50℃, and further at 45~48℃ (for example, more than 0.5h, and further 0.5~1h) to obtain a soaked semi-finished product mixture. The soaked semi-finished product mixture is transported to a mechanical desliming device through a slurry conveying device; the methanol in the PVA is replaced into the slurry solution, and other impurities in the PVA are replaced into the slurry solution through soaking; (2) Mechanical dehydration: The semi-finished mixture after soaking is mechanically dehydrated by a mechanical dehydration device to remove the content of solvents methyl acetate and methanol to below 40~60wt%, or even below 30wt% or below 20wt%, to obtain the dehydrated semi-finished mixture; most of the methyl acetate and methanol in PVA are removed by mechanical dehydration; (3) Washing and whitening: Add a third component to the semi-finished product mixture after dehydration, for example, at the solid outlet of the mechanical dehydration device or on the pipeline between the mechanical dehydration device and a drying device, to wash and whiten it, and obtain a washed and whitened semi-finished product mixture. The washed and whitened semi-finished product mixture is transported to a drying device through a pipeline. The third component, namely a mixture of methyl acetate, methanol and water, is sprayed into the PVA after mechanical dehydration. This is mainly to take advantage of the water absorption of PVA. Water is added before drying, and the PVA absorbs water before drying. This is to prevent the formation of a hard shell on the outer surface of the PVA particles during the drying process. A large amount of methyl acetate is added to the PVA slurry and after dehydration, mainly to reduce the boiling point of methanol, promote the evaporation of solvent, and remove impurities such as aldehydes to achieve the whitening effect, so that the purity of PVA is higher during the drying process. (4) Drying: After drying in a first-stage drying device at 50~70℃ for 2~3 hours, it enters a second-stage drying device. The fourth component is added to the second-stage drying device, and it is dried at 70~125℃ for 2~3 hours to obtain the finished polyvinyl alcohol (PVA). The addition of the fourth component to the drying device can accelerate the heating rate of the drying equipment, make the temperature of the entire drying equipment uniform, and ultimately improve the drying efficiency.

[0015] Furthermore, in the above-mentioned process for producing low-methanol high-purity polyvinyl alcohol, in step (1), the amount of methyl acetate added is 10-25 wt% of the raw material semi-finished product, preferably 12-22 wt%, and even more preferably 15-20 wt%, for example 16, 17, 18, 19 and 20 wt%.

[0016] Furthermore, in the above process for producing high-purity polyvinyl alcohol with low methanol, in step (2), the mechanical dehydration is performed by mechanical centrifugation.

[0017] Furthermore, in the above-mentioned process for producing high-purity polyvinyl alcohol with low methanol content, in step (3), the third component is a mixture of methyl acetate, methanol, and water. The volume ratio (or mass ratio) of methyl acetate, methanol, and water in the mixture can be 3.0~3.5:1.0~1.5:1.5~2.0, preferably 3.0~3.2:1.0~1.2:1.5~1.7. The amount of the third component added is 5~15wt% of the semi-finished product mixture after dehydration, preferably 6~13wt%, and even more preferably 9~12wt%, for example 10wt% or 11wt%.

[0018] Furthermore, in the above-mentioned process for producing high-purity polyvinyl alcohol with low methanol content, in step (4), the fourth component is nitrogen gas (100~150℃, preferably 110-120℃), and the amount of the fourth component added is 1.0~5.0wt% of the semi-finished product mixture after dehydration, preferably 2.0~4.0wt%, and the fourth component is added by atomization spraying. The second stage of drying is generally achieved by introducing hot nitrogen gas.

[0019] Furthermore, in the above-mentioned process for producing low-methanol, high-purity polyvinyl alcohol, the finished polyvinyl alcohol (PVA) contains 0.3-1.3 wt% methanol, more specifically 0.4-0.6 wt%, 0.02-0.3 wt% methyl acetate, more specifically 0.02-0.1 wt%, and has a polyvinyl alcohol purity of 98.0 wt% or higher, especially 98.5 wt% or higher, or even 99.0 wt%, 99.5 wt% or higher, or 99.6 wt% or higher.

[0020] The technical solution of the present invention has the following beneficial effects: (1) The production process of low methanol high-purity polyvinyl alcohol of the present invention involves soaking, mechanical dehydration, washing and whitening, and finally drying, so that the methanol content in the prepared polyvinyl alcohol PVA can be reduced to 0.3~1.3%, the methyl acetate content can be reduced to 0.02~0.3%, the sodium acetate content can be reduced to 0.45~0.60%, and the ash content can be reduced to 0.16~0.25%, which solves the problem that the methanol content in the polyvinyl alcohol prepared by the existing low methanol content production method is unstable and fluctuates greatly. (2) The production process of low methanol high purity polyvinyl alcohol of the present invention involves soaking, mechanical dehydration, washing and whitening, and finally drying. Methyl acetate is added in the soaking step, a mixture of methyl acetate, water and methanol is added at the outlet of the mechanical dehydration step, and hot nitrogen is added in the drying step. The production cost is low and solves the problem that the existing production method of low methanol content polyvinyl alcohol uses deionized water for purification, which has a high cost for the entire purification process. Attached Figure Description

[0021] To make the content of this invention easier to understand, the invention will be further described in detail below with reference to specific embodiments and accompanying drawings, wherein: Figure 1 This is a schematic diagram of the production apparatus for low-methanol, high-purity polyvinyl alcohol according to the present invention. Figure 2 This is a schematic diagram of the production device for removing methanol from polyvinyl alcohol in the prior art, namely Comparative Example 1. The components are: 1. PVA slurry soaking device, 2. Slurry conveying device, 3. Mechanical desliming device, 4. First-stage drying device, 5. Second-stage drying device, 6. Alcohololysis device, 7. Crushing device, M is the driving device, and 8. Drying device. Detailed Implementation

[0022] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] In the description of the invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installed," "equipped with," "connected," etc., should be interpreted broadly. For example, "connected" can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection, a direct connection, or an indirect connection through an intermediate medium; it can be a connection within two components. The terms "upper," "middle," "outer," "inner," "lower," "around," "left," "right," "front," "rear," "top," "bottom," etc., indicating orientation or positional relationships, are only for the convenience of describing the invention and simplifying the description, and do not indicate or imply that the components or elements referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the invention. Those skilled in the art can understand the specific meaning of the above terms in the invention based on the specific circumstances.

[0024] In the following embodiments and comparative examples of the present invention, the methods for determining the impurity content in semi-finished PVA and finished PVA are as follows: Ash content determination: Determination by ignition method GB / T 12010-2010; Determination of sodium acetate content: GB / T 12010-2010 National Standard Method (Chemical Titration Method); Determination of methanol and methyl acetate content: dissolution method.

[0025] like Figure 1 As shown, the production apparatus for low-methanol high-purity polyvinyl alcohol of the present invention includes: a PVA slurry soaking device 1, a slurry conveying device 2, a mechanical dehydration device 3, a first-stage drying device 4, and a second-stage drying device 5. The inlet of the PVA slurry soaking device 1 is connected to a PVA slurry feed pipeline and a methyl acetate feed pipeline. The outlet of the PVA slurry soaking device 1 is connected to the slurry conveying device 2. The slurry conveying device 2 is connected to the mechanical dehydration device 3. The solid discharge pipeline of the mechanical dehydration device 3 is sequentially connected to the first-stage drying device 4 and the second-stage drying device 5. A third component addition pipeline is provided at the solid outlet of the mechanical dehydration device 3, or a third component addition pipeline is connected to the pipeline between the mechanical dehydration device 3 and the drying device 4. A fourth component addition pipeline is provided in the second-stage drying device 5. The outlet of the second-stage drying device 5 is connected to the finished polyvinyl alcohol (PVA) discharge pipeline.

[0026] In a preferred embodiment, the slurry conveying device 2 includes a conveying pipeline and a conveying pump, the mechanical desliming device 3 is a centrifuge, and the first-stage drying device 4 and the second-stage drying device 5 are tubular dryers. The drying temperature of the first-stage drying device is 50~70℃, and the drying temperature of the second-stage drying device is 70~125℃, preferably 75~120℃.

[0027] The low-methanol, high-purity polyvinyl alcohol production apparatus of the present invention may further include an alcoholysis unit 6 and a pulverizing unit 7. The outlet of the alcoholysis unit is connected to the inlet of the pulverizing unit, and the outlet of the pulverizing unit is connected to the PVA slurry soaking unit 1 through a PVA slurry feed pipeline. The PVA semi-finished product produced by the alcoholysis unit is pulverized by the pulverizing unit, washed with a rinsing solution, and then the PVA slurry is sent to the PVA slurry soaking unit 1.

[0028] In another preferred embodiment, the third component inlet line is a mixture inlet line for methyl acetate, methanol and water; the fourth component inlet line is a nitrogen inlet line.

[0029] In yet another preferred embodiment, the liquid outlet of the mechanical dehydration device 3 is connected to a solvent recovery pipeline.

[0030] The present invention provides a process for producing low-methanol, high-purity polyvinyl alcohol using the aforementioned production apparatus, comprising the following steps: (1) Raw material semi-finished product soaking: Methyl acetate and PVA slurry from the alcoholysis and pulverization process in the PVA production process and raw material semi-finished product with methanol content of 70~85wt% are added to the PVA slurry soaking device 1. Preferably, a solution of methyl acetate of 30~50wt%, methanol of 40~60wt%, and PVA of 10~20wt% is formed. Soaking is carried out at 40~53℃, preferably 42~52℃, and even more preferably 44~50℃, for example 45℃, 46℃, 48℃ for 0.5~1h to obtain the soaked semi-finished product mixture. The soaked semi-finished product mixture is transported to the mechanical desliming device 3 through the slurry conveying device 2. (2) Mechanical dehydration: The soaked semi-finished mixture is mechanically dehydrated by mechanical dehydration device 3 to remove the content of solvents methyl acetate and methanol to below 40~60wt%, for example, to below 55wt%, or below 50wt%, or below 45wt%, to obtain the dehydrated semi-finished mixture; (3) Washing and whitening: Add a third component to the semi-finished product mixture after dehydration, for example at the solid outlet of the mechanical dehydration device 3 or on the pipeline between the mechanical dehydration device 3 and the drying device 4, and wash and whiten it to obtain the semi-finished product mixture after washing and whitening. The semi-finished product mixture after washing and whitening is transported to the drying device 4 through the pipeline. (4) Drying: After drying in a first-stage drying device 4 at 50~70℃ for 2~3 hours, the product enters a second-stage drying device 5. The fourth component is added to the second-stage drying device 5 and dried at 70~125℃ for 2~3 hours to obtain the finished polyvinyl alcohol PVA.

[0031] In a preferred embodiment, in step (1), the amount of methyl acetate added is 10-25 wt% of the raw material semi-finished product.

[0032] In another preferred embodiment, in step (2), the mechanical dehydration is mechanical centrifugation.

[0033] In another preferred embodiment, in step (3), the third component is a mixture of methyl acetate, methanol and water, wherein the volume ratio of methyl acetate, methanol and water in the mixture is 3.0~3.5:1.0~1.5:1.5~2.0, and the amount of the third component added is 5~15wt% of the semi-finished mixture after dehydration.

[0034] In another preferred embodiment, in step (4), the fourth component is nitrogen gas at 100~150°C, the amount of the fourth component added is 1.0~5.0 wt% of the semi-finished product mixture after dehydration, and the fourth component is added by atomization spraying.

[0035] In another preferred embodiment, the finished polyvinyl alcohol (PVA) contains 0.3-1.3 wt% methanol and 0.02-0.3 wt% methyl acetate. Example 1

[0036] The production process of low-methanol, high-purity polyvinyl alcohol in this embodiment includes the following steps: (1) Raw material semi-finished product soaking: The PVA semi-finished product produced by the alcoholysis unit is crushed by the crushing device and washed by the washing liquid to obtain PVA slurry. The PVA slurry (from the PVA slurry after alcoholysis and crushing in the PVA production process, the contents of each impurity other than PVA are methanol 73%, methyl acetate 25%, water 1.58%, sodium acetate 0.4%, and aldehydes about 0.02%) is sent to the PVA slurry soaking device 1. Methyl acetate (purity 99.95%) is added to the PVA slurry soaking device 1 and soaked for 1 hour at a temperature of about 45°C. The amount of methyl acetate added is 16% of the semi-finished PVA. The soaking step is to replace the methanol in the semi-finished PVA into the slurry solution. At the same time, the soaking replaces other impurities in the semi-finished PVA into the slurry solution. After soaking, it is transported to the mechanical desliming device 3 through the slurry conveying device 2. (2) Mechanical dehydration: Mechanical dehydration is carried out in mechanical dehydration device 3. Specifically, mechanical dehydration is centrifugal dehydration, which removes methyl acetate and methanol from the semi-finished PVA to below 40wt%. Most of the methyl acetate and methanol in the semi-finished PVA are removed by mechanical dehydration. (3) Washing and whitening: A third component is added to the outlet of the mechanical dehydration device 3. The third component is a mixture of methyl acetate, methanol and water (volume ratio of the three is 3.0:1.0:1.7). The amount added is 11 wt% of the semi-finished PVA after dehydration, in order to wash and whiten the PVA after dehydration. A mixture of methyl acetate, methanol and water is sprayed into the semi-finished PVA after mechanical dehydration. This is mainly to take advantage of the water absorption of PVA. After the PVA absorbs water, it is dried to prevent the formation of a hard shell on the outer surface of the PVA particles during the drying process. This promotes the evaporation of solvent and carries away impurities such as aldehydes, thus playing a whitening role. At the same time, the added methyl acetate forms a methanol-methyl acetate azeotrope with the methanol in the PVA. This reduces the boiling point of methanol during the drying process and increases the amount of methanol volatilization, making the purity of PVA higher during the drying process. After washing and whitening, the PVA is transported to the first-stage drying device 4 through a pipeline. (4) Drying: After drying at 60°C for 2 hours in the first-stage drying device 4, the product enters the second-stage drying device 5. The fourth component is hot nitrogen (temperature is about 122°C) added to the second-stage drying device 5 (the heat treatment stage of the drying process). The amount added is 3.0 wt% of the mass of the semi-finished PVA after liquid removal. It is added by atomization to accelerate the heating efficiency of the drying equipment, make the temperature of the entire drying equipment uniform, and ultimately improve the drying efficiency. The second-stage drying time is 3 hours. The finished PVA is obtained from the outlet of the second-stage drying device 5.

[0037] After testing, the finished PVA prepared in Example 1 contained 0.83% methanol, 0.03% methyl acetate, 0.56% sodium acetate, 0.21% ash, and had a purity of approximately 98.5 wt%. Example 2

[0038] The production process of low-methanol, high-purity polyvinyl alcohol in this embodiment includes the following steps: (1) Raw material semi-finished product soaking: Methyl acetate and raw material semi-finished PVA (from the PVA slurry after alcoholysis and pulverization in the PVA production process, except for PVA, the contents of each impurity are methanol 73%, methyl acetate 25%, water 1.58%, sodium acetate 0.4%, aldehydes about 0.02%) are added to the PVA slurry soaking device 1. The soaking time is 1h, the temperature is 42℃, and the amount of methyl acetate added is 17wt% of the semi-finished PVA. The soaking step is to replace the methanol in the semi-finished PVA into the slurry solution. At the same time, the soaking replaces other impurities in the semi-finished PVA into the slurry solution. After soaking, the slurry is conveyed to the mechanical desliming device 3 through the slurry conveying device 2. (2) Mechanical dehydration: Mechanical dehydration is carried out in mechanical dehydration device 3. Specifically, mechanical dehydration is centrifugal dehydration, which removes methyl acetate and methanol from the semi-finished PVA to below 50wt%. Most of the methyl acetate and methanol in the semi-finished PVA are removed by mechanical dehydration. (3) Washing and whitening: A third component is added to the outlet of the mechanical dehydration device 3. The third component is a mixture of methyl acetate, methanol and water (volume ratio of the three is 3.2:1.2:1.7). The amount added is 12wt% of the semi-finished PVA after dehydration, in order to wash and whiten the PVA after dehydration. A mixture of methyl acetate, methanol and water is sprayed into the semi-finished PVA after mechanical dehydration. The main purpose is to take advantage of the water absorption of PVA. Water is added before drying, and the PVA absorbs water before drying. This is to prevent the formation of a hard shell on the outer surface of the PVA particles during the drying process, promote the increase of solvent evaporation, and remove impurities such as aldehydes, thus playing a whitening role. At the same time, the added methyl acetate forms a methanol-methyl acetate azeotrope with the methanol in the PVA. During the drying process, the boiling point of methanol is reduced and the amount of methanol volatilization is increased, so that the purity of PVA is higher during the drying process. After washing and whitening, it is transported to the first-stage drying device 4 through the pipeline. (4) Drying: After drying at 58°C for 3 hours in the first-stage drying device 4, the product enters the second-stage drying device. The fourth component is hot nitrogen (temperature 119°C) added to the second-stage drying device 5 (heat treatment stage of the drying process). The amount added is 4wt% of the mass of the semi-finished PVA after liquid removal. It is added by atomization spraying to accelerate the heating rate of the drying equipment, make the temperature of the entire drying equipment uniform, and ultimately improve the drying efficiency. The second-stage drying time is 2 hours. The finished PVA is obtained from the outlet of the second-stage drying device 5.

[0039] After testing, the finished PVA prepared in Example 2 contained 0.73% methanol, 0.04% methyl acetate, 0.59% sodium acetate, 0.22% ash, and had a purity of approximately 98.5 wt%. Example 3

[0040] The production process of low-methanol, high-purity polyvinyl alcohol in this embodiment includes the following steps: (1) Soaking of raw material semi-finished product: Methyl acetate and raw material semi-finished product PVA (from PVA slurry after alcoholysis and pulverization in the PVA production process, the contents of each impurity other than PVA are methanol 73%, methyl acetate 25%, water 1.58%, sodium acetate 0.4%, aldehydes about 0.02%) are added to PVA slurry soaking device 1. The soaking time is 0.5h, the temperature is 50℃, and the amount of methyl acetate added is 18% of the amount of semi-finished product PVA. The soaking step is to replace the methanol in the semi-finished product PVA into the slurry solution. At the same time, the soaking replaces other impurities in the semi-finished product PVA into the slurry solution. After soaking, the slurry is transported to the mechanical desliming device 3 through slurry conveying device 2. (2) Mechanical dehydration: Mechanical dehydration is carried out in mechanical dehydration device 3. Specifically, mechanical dehydration is centrifugal dehydration, which removes methyl acetate and methanol from the semi-finished PVA to below 45wt%. Most of the methyl acetate and methanol in the semi-finished PVA are removed by mechanical dehydration. (3) Washing and whitening: A third component is added to the outlet of the mechanical dehydration device 3. The third component is a mixture of methyl acetate, methanol and water (volume ratio of the three is 3.0:1.0:1.6). The amount added is 9 wt% of the semi-finished PVA after dehydration, in order to wash and whiten the PVA after dehydration. A mixture of methyl acetate, methanol and water is sprayed into the semi-finished PVA after mechanical dehydration. This is mainly to take advantage of the water absorption of PVA. After the PVA absorbs water, it is dried to prevent the formation of a hard shell on the outer surface of the PVA particles during the drying process. This promotes the evaporation of solvent and carries away impurities such as aldehydes, thus playing a whitening role. At the same time, the added methyl acetate forms a methanol-methyl acetate azeotrope with the methanol in the PVA. This reduces the boiling point of methanol during the drying process and increases the amount of methanol volatilization, making the purity of PVA higher during the drying process. After washing and whitening, the PVA is transported to the first-stage drying device 4 through a pipeline. (4) Drying: After drying at 62°C for 2 hours in the first-stage drying device 4, the product enters the second-stage drying device. The fourth component is hot nitrogen (temperature 120°C) added to the second-stage drying device 5 (heat treatment stage of the drying process). The amount added is 2wt% of the mass of the semi-finished PVA after liquid removal. It is added by atomization spraying to accelerate the heating efficiency of the drying equipment, make the temperature of the entire drying equipment uniform, and ultimately improve the drying efficiency. The drying time is 3 hours. The finished PVA is obtained from the outlet of the second-stage drying device 5.

[0041] After testing, the finished PVA prepared in Example 3 contained 1.16% methanol, 0.04% methyl acetate, 0.54% sodium acetate, 0.20% ash, and had a purity of approximately 98.3 wt%. Comparative Example 1

[0042] like Figure 2 As shown, the PVA slurry after alcoholysis and pulverization (i.e., the raw material semi-finished PVA of Example 1, with the same source and the same content of each impurity) was sprayed with methanol and water (water:methanol mass ratio 2:3) in the front section of the dryer to wash the impurities in the PVA and to make the PVA swell. The amount added was 8 wt% of the mass of the semi-finished PVA after dehydration. The residual solvent was dried and evaporated by the drying device 8. Methyl acetate (concentration of 99.95%) was sprayed in the middle section of the dryer at a spraying amount of 3 wt% to obtain the finished PVA.

[0043] After testing, the finished PVA prepared in Comparative Example 1 contained 0.73% sodium acetate, 2.97% methanol, 0.25% methyl acetate, and 0.28% ash. Comparative Example 2

[0044] The process is basically the same as in Example 1, except that in step (3), the third component is a mixture of water and methanol (volume ratio of 1:1.5), and its addition amount is 10 wt% of the semi-finished PVA after dehydration. After testing, the finished PVA prepared in Comparative Example 2 has a methanol content of 2.57%, a methyl acetate content of 0.30%, a sodium acetate content of 0.77%, and an ash content of 0.29%. Comparative Example 3

[0045] The process is basically the same as in Example 1, except that in step (1), water and raw material semi-finished PVA (from the PVA slurry after alcoholysis and pulverization in the PVA production process, the contents of each impurity other than PVA are methanol 73%, methyl acetate 25%, water 1.58%, sodium acetate 0.4%, and aldehydes about 0.02%) are added to the PVA slurry soaking device 1. The soaking time is 1 hour, the temperature is 42~52℃, and the amount of water added is 10% of the amount of semi-finished PVA. After measurement, the methanol content of the finished PVA prepared in Comparative Example 3 is 2.76%, the methyl acetate content is 0.25%, the sodium acetate content is 0.72%, and the ash content is 0.27%.

[0046] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A production apparatus for high-purity polyvinyl alcohol with low methanol content, characterized in that, include: The device consists of a PVA slurry soaking device (1), a slurry conveying device (2), a mechanical desliming device (3), a first-stage drying device (4), and a second-stage drying device (5). The inlet of the PVA slurry soaking device (1) is connected to the PVA slurry feed pipeline and the methyl acetate feed pipeline. The outlet of the PVA slurry soaking device (1) is connected to the slurry conveying device (2). The slurry conveying device (2) is connected to the mechanical desliming device (3). The solid discharge pipeline of the mechanical desliming device (3) is connected to the first-stage drying device (4) and the second-stage drying device (5) in sequence. A third component addition pipeline is provided at the solid outlet of the mechanical desliming device (3), or a third component addition pipeline is connected to the pipeline between the mechanical desliming device (3) and the drying device (4). A fourth component addition pipeline is provided in the second-stage drying device (5). The outlet of the drying device (4) is connected to the finished polyvinyl alcohol (PVA) discharge pipeline.

2. The production apparatus for low-methanol, high-purity polyvinyl alcohol according to claim 1, characterized in that, The slurry conveying device (2) includes a conveying pipeline and a conveying pump; the mechanical desliming device (3) is a centrifuge; the first-stage drying device (4) and the second-stage drying device (5) are tubular dryers. Preferably, the drying temperature of the first-stage drying device is 50~70℃, and the drying temperature of the second-stage drying device is 70~125℃, more preferably 75~120℃.

3. The production apparatus for low-methanol, high-purity polyvinyl alcohol according to claim 1 or 2, characterized in that, The third component addition line is a mixture addition line for methyl acetate, methanol, and water; and / or, The fourth component addition pipeline is a nitrogen addition pipeline.

4. The apparatus for producing low-methanol, high-purity polyvinyl alcohol according to any one of claims 1-3, characterized in that, The liquid outlet of the mechanical desolvation device (3) is connected to the solvent recovery pipeline.

5. A process for producing low-methanol, high-purity polyvinyl alcohol using the production apparatus according to any one of claims 1-4, characterized in that, Includes the following steps: (1) Raw material semi-finished product soaking: Methyl acetate and PVA slurry from the alcoholysis and pulverization process in the PVA production process are added to the PVA slurry soaking device (1), that is, raw material semi-finished product with methanol content of 70~85wt%, and soaked at 40~53℃ to obtain the soaked semi-finished product mixture. The soaked semi-finished product mixture is transported to the mechanical desliming device (3) through the slurry conveying device (2); (2) Mechanical dehydration: The semi-finished mixture after soaking is mechanically dehydrated by a mechanical dehydration device (3) to remove the content of solvents methyl acetate and methanol to below 40~60wt%, and obtain the dehydrated semi-finished mixture; (3) Washing and whitening: Add the third component to the semi-finished product mixture after dehydration, and wash and whiten it to obtain the washed and whitened semi-finished product mixture. The washed and whitened semi-finished product mixture is transported to a drying device (4) through a pipeline. (4) Drying: After drying in a first-stage drying device (4) at 50~70℃, it enters a second-stage drying device. The fourth component is added to the second-stage drying device (5) and dried at 70~125℃ to obtain the finished polyvinyl alcohol (PVA).

6. The process for producing low-methanol, high-purity polyvinyl alcohol according to claim 5, characterized in that, In step (1), the amount of methyl acetate added is 10-25 wt% of the raw material semi-finished product, preferably 15-20 wt%.

7. The process for producing low-methanol, high-purity polyvinyl alcohol according to claim 5, characterized in that, In step (2), the mechanical dehydration is mechanical centrifugation.

8. The process for producing low-methanol, high-purity polyvinyl alcohol according to claim 5, characterized in that, In step (3), the third component is a mixture of methyl acetate, methanol and water. The volume ratio of methyl acetate, methanol and water in the mixture is 3.0~3.5:1.0~1.5:1.5~2.0, preferably 3.0~3.2:1.0~1.2:1.5~1.

7. The amount of the third component added is 5~15wt% of the semi-finished mixture after dehydration, preferably 9~12wt%.

9. The process for producing low-methanol, high-purity polyvinyl alcohol according to claim 5, characterized in that, In step (4), the fourth component is nitrogen gas at 100~150℃, the amount of the fourth component added is 1.0~5.0wt% of the semi-finished product mixture after dehydration, and the fourth component is added by atomization spraying.

10. The process for producing low-methanol, high-purity polyvinyl alcohol according to any one of claims 5-9, characterized in that, The finished polyvinyl alcohol (PVA) contains 0.3 to 1.3 wt% methanol.

Images