Process for the preparation of (meth)acrylates

Abstract

The present application relates to the field of chemical industry, and discloses a preparation method of (meth) acrylate. The method comprises the following steps: (1) heating and reacting (meth) acrylic acid and fatty alcohol in a reaction unit, and transporting the reaction material to a thin film evaporation unit to separate out the light component containing water, and extracting (meth) acrylic acid in the light component with hydrophobic fatty alcohol; (2) optionally, post-treating the heavy component containing (meth) acrylate to obtain (meth) acrylate. The method can greatly improve the reaction efficiency without introducing azeotropic third solvent, can reach a high conversion rate in a short time, has a short reaction period, and can reduce the risk of self-polymerization and process energy consumption.

Description

Technical Field

[0001] This invention relates to the field of chemical engineering, and specifically to a method for preparing (meth)acrylates. Background Technology

[0002] (Meth)acrylates, with their reactive double bonds, are important fine chemical products and polymer monomers. Containing a hydrophobic long-chain aliphatic hydrocarbon backbone, they have wide applications in coatings, oil additives, plastics, papermaking, leather, and cosmetics. (Meth)acrylates are typically prepared using transesterification or direct esterification of (meth)acrylate. Compared to transesterification, the direct esterification of (meth)acrylate with fatty alcohols is more efficient.

[0003] The esterification reaction of (meth)acrylic acid and higher fatty alcohols has a low equilibrium conversion rate, requiring continuous removal of the byproduct water to promote the reaction towards the forward direction. A common approach is to add a low-boiling-point solvent that forms an azeotrope with water to the reaction system, thereby increasing the water removal rate. CN1733687A discloses a method for preparing (meth)acrylates, in which benzene, toluene, cyclohexane, carbon tetrachloride, chloroform, n-pentane, and n-hexane are added to the reaction system as azeotropic agents to enhance water removal, achieving a product yield of up to 97.8%. The drawback of this method is the addition of new substances as azeotropic solvents to the reaction mixture. This increases the difficulty of distillation and purification of the reaction system, and the azeotropic solvent must be purified before being recycled upstream for reuse. While solvent azeotropy can efficiently and rapidly improve the conversion rate of the esterification reaction, it suffers from problems such as secondary distillation of the solvent and process losses, resulting in longer overall energy consumption and time, and a poorer product color.

[0004] Process intensification methods can enhance the removal of the byproduct water. CN101959838B, CN102639482A, and US20040019235A1 disclose a method for preparing (meth)acrylates in a reactor equipped with a circulating evaporator. The circulating evaporator can enhance the removal of water and azeotropic solvents, but the addition of the azeotropic solvent increases the process energy consumption. CN102249913B and CN103193640A disclose a method for preparing (meth)acrylates in a reactive distillation column. The reactive distillation column can achieve in-situ removal of water, but (meth)acrylate is highly prone to self-polymerization in the reactive distillation column, posing a high risk of process self-polymerization. Summary of the Invention

[0005] The purpose of this invention is to overcome the above-mentioned problems existing in the prior art and provide a method for preparing (meth)acrylates. This method can significantly improve the reaction efficiency without introducing an azeotropic third solvent, achieve a high conversion rate in a short time, have a short reaction cycle, and reduce the risk of self-polymerization and process energy consumption.

[0006] To achieve the above objectives, the present invention provides a method for preparing (meth)acrylates, the method comprising:

[0007] (1) (Meth)acrylic acid and fatty alcohol are heated and reacted in a reaction unit, and the reactants are transported to a thin film evaporation unit to separate the water-containing light components;

[0008] (2) Optionally, the heavy component containing (meth)acrylate is post-treated to obtain (meth)acrylate.

[0009] Through the above technical solution, the present invention can achieve the following beneficial effects:

[0010] 1. The technical solution of this invention can achieve a high conversion rate in a short time, with a short reaction cycle, high reaction efficiency, low process energy consumption, and low risk of self-polymerization.

[0011] 2. By adopting the technical solution of the present invention, the azeotropic third solvent can be avoided, which reduces the difficulty of purification and also avoids the purification required for recycling the azeotropic third solvent, further reducing energy consumption. Attached Figure Description

[0012] Figure 1 This invention provides an apparatus for preparing (meth)acrylates.

[0013] Explanation of reference numerals in the attached figures

[0014] 1-Reactor inlet, 2-Stirred reactor, 3-Molecular distillation apparatus, 4-Extraction equipment, 5-Raffinate outlet, 6-Extractant inlet, 7-Extractant outlet. Detailed Implementation

[0015] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0016] In this invention, "(meth)acrylic acid" refers to acrylic acid and / or methacrylic acid; "(meth)acrylate" refers to acrylate and / or methacrylate.

[0017] In a first aspect, the present invention provides a method for preparing (meth)acrylates, the method comprising:

[0018] (1) (Meth)acrylic acid and fatty alcohol are heated and reacted in a reaction unit, and the reactants are transported to a thin film evaporation unit to separate the water-containing light components;

[0019] (2) Optionally, the heavy component containing (meth)acrylate is post-treated to obtain (meth)acrylate.

[0020] During the reaction of (meth)acrylic acid and fatty alcohols, water is continuously generated in addition to the formation of the target product (meth)acrylate. The inventors of this invention discovered that thin-film evaporation is used in the preparation of (meth)acrylate. Thin-film evaporation has a large evaporation area and high heat transfer efficiency, which can effectively remove the byproduct water, effectively promoting the esterification reaction towards the forward reaction direction, increasing the conversion rate, shortening the reaction cycle, and preventing the self-polymerization of (meth)acrylic acid. Furthermore, the esterification reaction continues in the reactants during thin-film evaporation.

[0021] The heating can be achieved using heating equipment.

[0022] The heavy component containing (meth)acrylate refers to the material in the system that was not evaporated after the reaction.

[0023] According to the present invention, in order to further ensure sufficient removal of water, thereby further promoting the esterification reaction, further improving the conversion rate and shortening the reaction cycle, it is preferable that the reactants are circulated between the reaction unit and the thin-film evaporation unit. The reaction unit can be single or multiple, and the thin-film evaporation unit can also be single or multiple. More preferably, the reactants are circulated between multiple sets of reaction units and thin-film evaporators connected in series (one reaction unit and one thin-film evaporator are directly connected as a group, and then connected to the next group).

[0024] According to the present invention, preferably, the reactants are heated to above 70°C in the reactor before being first fed into the thin-film evaporator unit to separate the water-containing light components. Heating the reactants to above 70°C further ensures that the reaction proceeds at a higher rate. More preferably, the reactants are first fed into the thin-film evaporator when heated to 80-105°C. The heating rate can be 2-4°C / min, which also saves heating time. The above operations can be carried out under atmospheric pressure.

[0025] According to the present invention, in order to further promote the esterification reaction, preferably, the molar ratio of (meth)acrylic acid (calculated as carboxyl group) to fatty alcohol (calculated as hydroxyl group) is 1-5:1, more preferably 2-3:1.

[0026] According to the present invention, preferably, the fatty alcohol is a fatty alcohol with a boiling point above 100°C at normal pressure, and more preferably a hydrophobic fatty alcohol with a boiling point above 100°C at normal pressure.

[0027] According to the present invention, preferably, the fatty alcohol is at least one of a monohydric alcohol having 9-16 carbon atoms. For example, it can be at least one of (n)decanol, (n)undecylol, (n)dodecanol, and (n)tetradecylol.

[0028] According to the present invention, preferably, the reaction is carried out in the presence of a polymerization inhibitor, which is at least one selected from phenolic polymerization inhibitors, quinone polymerization inhibitors, aromatic amine polymerization inhibitors, and inorganic salt polymerization inhibitors. The amount of the polymerization inhibitor is 0.01-5% by weight of the total weight of (meth)acrylic acid and fatty alcohol. The phenolic polymerization inhibitor may be hydroquinone, pyrogallol, and p-tert-butylcatechol, etc. The quinone polymerization inhibitor may be tetrachlorobenzoquinone, 1,4-naphthoquinone, etc. The aromatic amine polymerization inhibitor may be p-phenylenediamine, benzidine, diphenylamine, and p-(methyl)aniline, etc. The inorganic salt polymerization inhibitor may be ferric chloride, cuprous oxide, etc.

[0029] According to the present invention, preferably, the reaction is carried out in the presence of a catalyst, which is at least one of sulfonic acid, heteropolyacid, and acidic resin, and the amount of the catalyst is 0.05-10% by weight of the total weight of (meth)acrylic acid and fatty alcohol. The sulfonic acid can be at least one of p-toluenesulfonic acid (which can be p-toluenesulfonic acid monohydrate) and methanesulfonic acid. Heteropolyacids are a class of oxy-containing polyacids composed of heteroatoms and multicoordinate atoms bridged by oxygen atoms; the heteropolyacids can be phosphotungstic acid, phosphotungmolybdic acid, and silicotungstic acid, etc. The acidic resin can release hydrogen ions; the acidic resin can be a macroporous acidic resin, etc.

[0030] According to a particularly preferred embodiment of the present invention, the reaction is carried out without the presence of other azeotropic agents. These other azeotropic agents include benzene, toluene, cyclohexane, carbon tetrachloride, chloroform, n-pentane, and n-hexane, etc.

[0031] According to the present invention, preferably, the reaction unit is at least one of a batch reactor, a tubular reactor, a fixed-bed reactor, and a fluidized-bed reactor. The number of reaction units is not particularly limited and can be one or more, or, if multiple reaction units are used, can be a combination of different reactors. The reaction unit may be equipped with stirring and heating equipment.

[0032] According to the present invention, the specific type of thin-film evaporation unit is not particularly limited, but preferably, the thin-film evaporation unit is selected from at least one of molecular distillation apparatus, wiped-film evaporator, and falling-film evaporator. There may also be one or more thin-film evaporation units, and if there are multiple distillation units, they may be combinations of different types.

[0033] According to the present invention, preferably, the temperatures of the reaction unit and the thin film evaporation unit are each independently controlled at 50-140°C, more preferably 80-130°C.

[0034] According to the present invention, preferably, the evaporation temperature of the thin-film evaporation unit is controlled at 80-115°C. It is understood that in the reaction unit and the thin-film evaporation unit, as the reaction continues and the water-containing light components in the thin-film evaporation unit are continuously evaporated, the temperature of the material will change. Therefore, when controlling the temperature, the temperature will also fluctuate within a small range.

[0035] During the reaction, samples of the reactants can be taken periodically to analyze the concentration of (meth)acrylic acid or fatty alcohols, thus determining the conversion rate. Preferably, the reaction is terminated when the conversion rate exceeds 98%. The inventors of this invention have discovered that, compared to existing methods for preparing (meth)acrylates, the technical solution provided by this invention achieves a higher conversion rate in a shorter reaction time, a shorter reaction cycle, and higher reaction efficiency.

[0036] According to the present invention, preferably, the reactants undergo at least 0.5 thin-film evaporations per hour, more preferably 1-4 times per hour. It is understood that the reactants enter the thin-film evaporation unit and then leave the unit, thus completing one cycle of thin-film evaporation; the number of evaporations can be controlled by controlling the flow rate. Whether the system has a single thin-film evaporator or multiple thin-film evaporators, better results can be obtained as long as the above-mentioned range of evaporations is met.

[0037] According to the present invention, preferably, the vacuum degree of the reaction unit and the thin-film evaporation unit is independently controlled to be 0-98 kPa. In the present invention, "vacuum degree" refers to absolute vacuum degree.

[0038] The inventors of this invention have discovered that the material further reacts in the thin-film evaporation unit, and by controlling the temperature and pressure in the thin-film evaporation unit within the aforementioned range, the reaction cycle can be further shortened.

[0039] It is understood that the aqueous light component may contain at least one of (meth)acrylic acid, fatty alcohol, and (meth)acrylate, in addition to water. Preferably, at least one of the fatty alcohol, (meth)acrylic acid, and (meth)acrylate in the aqueous light component is recovered. This also saves materials and avoids waste.

[0040] According to the present invention, preferably, the recovery method is to use hydrophobic fatty alcohol for extraction, and the same as the fatty alcohol in step (1);

[0041] Preferably, the volume ratio of the hydrophobic fatty alcohol and the aqueous light component used for extraction is 0.3-10:1, more preferably 1-4:1.

[0042] When the above range is met, a higher extraction rate can be obtained.

[0043] According to the present invention, preferably, the post-treatment includes sequential steam stripping, centrifugation, alkaline washing, water washing and drying.

[0044] The present invention will be described in detail below through embodiments. The following embodiments are... Figure 1 The reaction is carried out in the apparatus shown, which includes a batch reactor 2, a molecular distillation apparatus 3, and an extraction device 4. The batch reactor 2 is equipped with a reactor inlet 1 to add the raw materials required for the reaction. The batch reactor 2 and the molecular distillation apparatus 3 are connected by a pipeline to feed the material from the batch reactor 2 into the molecular distillation apparatus 3. The molecular distillation apparatus 3 and the batch reactor 2 are connected by another pipeline to feed the material after molecular distillation back into the batch reactor 2. The molecular distillation apparatus 3 is connected to the extraction device 4 to feed the aqueous light components into the extraction device 4 for the recovery of fatty alcohols, (meth)acrylic acid, and (meth)acrylates. The extraction device 4 is equipped with a raffinate outlet 5 to remove the raffinate; the extraction device 4 is also equipped with an extractant inlet 6 and an extractant outlet 7 to feed and remove the extractant, respectively.

[0045] The alcohol conversion rate is calculated as follows: the amount of alcohol converted is determined by gas chromatography to obtain the residual alcohol content in the reaction solution, and the ratio of this amount to the initial amount of alcohol input is the conversion rate. In the synthesis of methacrylates, the selectivity is generally high; therefore, the alcohols reacted are usually converted to methacrylates.

[0046] In the following examples, the post-treatment includes steam stripping, centrifugation, alkaline washing, and decolorization, each under the following conditions:

[0047] The conditions for steam stripping include: removing unreacted (meth)acrylic acid from the reaction solution by steam stripping at a temperature not exceeding 150°C;

[0048] Centrifugation conditions include: controlling the centrifugation temperature not to exceed 80℃;

[0049] The conditions for alkaline washing include: alkaline washing with a 5% sodium hydroxide aqueous solution;

[0050] Drying conditions include decolorization by adsorbing the substance with white clay or activated carbon.

[0051] In the following examples, during the reaction, a portion of the reactants was taken as a sample every 0.5 hours to analyze the concentration of fatty alcohols and obtain the conversion rate. The reaction was stopped when the conversion rate was greater than 98%.

[0052] The cycle time of the reactants mentioned in the following examples is started from the time the reactants are heated to the target temperature.

[0053] Example 1

[0054] 426.87 g of methacrylic acid, 400 g of 1-tetradecyl alcohol, 4 g of p-toluenesulfonic acid monohydrate, and 0.4 g of hydroquinone were added to a stirred and heated batch reactor. When the reactants reached 102 °C (heating rate 3.6 °C / min), they were first fed into a molecular distillation apparatus at a rate of 2.36 kg / h under atmospheric pressure for further reaction. The separated aqueous light fraction was then sent to an extraction device. The reactants circulated between the molecular distillation apparatus and the batch reactor, maintaining the temperature between 102 and 112 °C and the pressure at atmospheric pressure. Thin-film evaporation was performed 2.84 times per hour. After 5.5 hours of circulation, the reaction was terminated. The resulting material underwent post-processing to obtain tetradecyl methacrylate. Gas chromatography analysis showed that the tetradecyl alcohol conversion rate at the end of the reaction was 98.74%.

[0055] Example 2

[0056] 294.78 g of methacrylic acid, 266.6 g of 1-tetradecyl alcohol, 2.67 g of p-toluenesulfonic acid monohydrate, and 0.266 g of hydroquinone were added to a stirred and heated batch reactor. After the reactants reached 100°C (heating rate 4°C / min), they were first fed into a molecular distillation apparatus at a rate of 1.56 kg / h under atmospheric pressure for further reaction. The separated aqueous light fraction was then sent to an extraction device. The reactants circulated between the molecular distillation apparatus and the glass batch reactor, maintaining the temperature between 100 and 114°C and the pressure at atmospheric pressure. The reaction solution underwent 2.76 thin-film evaporations per hour. After 4.5 hours of circulation, the reaction was stopped, and the reaction was terminated. The resulting material was post-processed to obtain tetradecyl methacrylate. Gas chromatography analysis showed that the tetradecyl alcohol conversion rate at the end of the reaction was 98.75%.

[0057] Example 3

[0058] 457.7 g of methacrylic acid, 488 g of 1-tetradecyl alcohol, 4.86 g of p-toluenesulfonic acid monohydrate, and 0.486 g of hydroquinone were added to a stirred and heated batch reactor. After the reactants reached 100°C (heating rate 3°C / min), they were first fed into a molecular distillation apparatus at a rate of 1.56 kg / h under atmospheric pressure for further reaction. The separated aqueous light fraction was then sent to an extraction device. The reactants circulated between the molecular distillation apparatus and the batch reactor, maintaining the temperature between 100 and 115°C and the pressure at atmospheric pressure. Thin-film evaporation was performed 1.64 times per hour. After 4.5 hours of circulation, the reaction was terminated. The resulting material underwent post-processing to obtain tetradecyl methacrylate. Gas chromatography analysis showed that the tetradecyl alcohol conversion rate at the end of the reaction was 98.30%.

[0059] Example 4

[0060] 240.93 g of methacrylic acid, 400 g of 1-tetradecyl alcohol, 4 g of p-toluenesulfonic acid monohydrate, and 0.4 g of hydroquinone were added to a stirred and heated batch reactor. After the reactants reached 97°C (heating rate 2°C / min), they were first fed into a molecular distillation apparatus at a rate of 2.55 kg / h under atmospheric pressure for further reaction. The separated aqueous light fraction was then sent to an extraction device. The reactants circulated between the molecular distillation apparatus and the batch reactor, maintaining the temperature between 90 and 100°C and the pressure at atmospheric pressure. Thin-film evaporation was performed 3.95 times per hour. After 7 hours of circulation, the reaction was terminated. The resulting material underwent post-processing to obtain tetradecyl methacrylate. Gas chromatography analysis showed that the tetradecyl alcohol conversion rate at the end of the reaction was 91.65%.

[0061] Comparative Example 1

[0062] 120.47 g of methacrylic acid, 200 g of 1-tetradecyl alcohol, 2 g of p-toluenesulfonic acid monohydrate, and 0.2 g of hydroquinone were added to a four-necked flask. N2 was introduced into the reaction solution, and the temperature was raised at a rate of 2 °C / min. The esterification reaction was started at 100 °C, and the reaction temperature was controlled between 100 °C and 130 °C. Under normal pressure, water generated during the esterification reaction was continuously distilled off using a water separator. After 5 hours of reaction, a sample of the reaction solution was taken for analysis, revealing a tetradecyl alcohol conversion rate of 89.52%. After 8 hours of reaction, unreacted methacrylic acid was distilled off under reduced pressure, and further post-processing yielded tetradecyl methacrylate. Gas chromatography analysis showed a tetradecyl alcohol conversion rate of 98.52% after 8 hours.

[0063] As can be seen from the above embodiments and comparative examples, the method provided by this invention can significantly improve reaction efficiency and achieve a higher conversion rate without introducing an azeotropic third solvent; within the same reaction cycle, the method of this invention can achieve a higher conversion rate. Furthermore, no precipitate formation was observed in the reactants during the reaction process of the above embodiments, indicating that methacrylic acid did not undergo self-polymerization. The method of this invention shortens the reaction time and reduces the risk of self-polymerization. Moreover, in Comparative Example 1, the tetradecanol conversion rate was only 89.52% after 5 hours of reaction, while according to Examples 2-3 of this application, the tetradecanol conversion rate reached over 98.30% after 4.5 hours of reaction using the technical solution of this application, indicating that this reaction is more efficient and has a shorter cycle. Furthermore, the inventors of this invention have also discovered that the effect is even better when the number of times the reactants undergo thin-film evaporation per hour is within a preferred range.

[0064] The following examples illustrate the steps for recovering methacrylic acid from an aqueous light component containing methacrylic acid. The extraction recovery rate is calculated as follows: the amount of extracted methacrylic acid is obtained based on the residual methacrylic acid content in the aqueous phase after extraction, and the ratio of this amount to the initial amount of methacrylic acid in the aqueous solution containing methacrylic acid is the extraction rate.

[0065] Example 5

[0066] 49.54 g of an aqueous solution containing 1.76% methacrylic acid was taken. 123.98 g of 1-dodecyl alcohol was added to this aqueous solution at 30 °C to extract and recover the methacrylic acid (volume ratio of dodecyl alcohol to aqueous methacrylic acid solution was 3), yielding a heavy component aqueous phase and a light component organic phase; the methacrylic acid content in the aqueous phase was 0.05%, and the methacrylic acid content in the organic phase was 0.46%. The extraction recovery rate was 97.58%.

[0067] As can be seen from the above examples, using hydrophobic fatty alcohols for extraction results in a higher extraction rate, which can further avoid raw material waste and reduce production costs.

[0068] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A method for preparing (meth)acrylate, characterized in that, The method includes: (1) The (meth)acrylic acid and fatty alcohol are heated and reacted in the reaction unit, and the reactants are transported to the thin film evaporation unit to separate the water-containing light component; at least one of the fatty alcohol, (meth)acrylic acid and (meth)acrylate in the water-containing light component is recovered; the recovery method is to extract with a hydrophobic fatty alcohol, and the hydrophobic fatty alcohol used for extraction is at least one of undecyl alcohol, dodecanol and tetradecanol; The reactants are circulated between the reaction unit and the thin-film evaporation unit; the reactants undergo at least 0.5 thin-film evaporations per hour; the molar ratio of (meth)acrylic acid (based on carboxyl groups) to fatty alcohol (based on hydroxyl groups) is 2-3:1; the fatty alcohol is at least one of undecyl alcohol, dodecanol, and tetradecanol; (2) Optionally, the heavy component containing (meth)acrylate is post-treated to obtain (meth)acrylate.

2. The method according to claim 1, wherein, In the reactor, when the reactants are heated to above 70°C, they are first fed into the thin-film evaporation unit to separate the water-containing light components.

3. The method according to claim 1, wherein, The reaction is carried out in the presence of a polymerization inhibitor, which is at least one of phenolic polymerization inhibitors, quinone polymerization inhibitors, aromatic amine polymerization inhibitors, and inorganic salt polymerization inhibitors, and the amount of the polymerization inhibitor is 0.01-5% by weight of the total weight of (meth)acrylic acid and fatty alcohol.

4. The method according to claim 1, wherein, The reaction is carried out in the presence of a catalyst, which is at least one of sulfonic acid, heteropoly acid and acidic resin, and the amount of the catalyst is 0.05-10% by weight of the total weight of (meth)acrylic acid and fatty alcohol.

5. The method according to claim 1, wherein, The reaction unit is at least one of a batch reactor, a tubular reactor, a fixed-bed reactor, and a fluidized-bed reactor; And / or, the thin-film evaporation unit is selected from at least one of molecular distillation apparatus, scraped film evaporator and falling film evaporator.

6. The method according to claim 1, wherein, The temperatures of the reaction unit and the thin-film evaporation unit are independently controlled between 50-140℃.

7. The method according to claim 6, wherein, The temperatures of the reaction unit and the thin-film evaporation unit are independently controlled between 80-130℃.

8. The method according to claim 7, wherein, The evaporation temperature of the thin-film evaporation unit is controlled at 80-115℃.

9. The method according to claim 1, wherein, The reaction is terminated when the conversion rate is greater than 98%.

10. The method according to claim 1 or 2, wherein, The reactants undergo 1-4 thin-film evaporations per hour.

11. The method according to claim 1, wherein, The vacuum levels of the reaction unit and the thin-film evaporation unit are independently controlled from 0 to 98 kPa.

12. The method according to claim 1, wherein, The volume ratio of the hydrophobic fatty alcohol and the aqueous light component used in the extraction is 0.3-10:

1.

13. The method according to claim 12, wherein, The volume ratio of the hydrophobic fatty alcohol and the aqueous light component used in the extraction was 1-4:

1.

14. The method according to claim 1, wherein, The post-treatment includes sequential steam stripping, centrifugation, alkaline washing, and decolorization.

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