A method for preparing a dimer acid having low color, low iron content, and low phosphorus content

Abstract

A preparation method of a dimer acid with low color, low iron content and low phosphorus content, comprising polymerization, acidification, coordination, water washing, distillation and temperature reduction storage steps. The preparation process uses organic carboxylic acid as an acidifying agent in the acidification step to avoid the use of phosphoric acid; at the same time, a coordination process is added to remove iron ions and other metal ions through the coordination reaction of the coordination agent with metal ions. Each step uses a continuous closed process, and the connecting pipeline between each step uses 316L stainless steel, finally obtaining a dimer acid with low color, low iron content and low phosphorus content. The dimer acid is measured by iron-cobalt colorimetry, and the color is 2-5, the color storage stability is good; the phosphorus content is ≤1 ppm; the iron content is ≤2 ppm, which can meet the market demand for high-quality dimer acid.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of oil and fat chemical industry, and particularly relates to a preparation technology of dimer acid. BACKGROUND

[0002] Dimer acid, also known as dimer fatty acid, is a viscous and transparent liquid due to containing two carboxylic acid groups in the molecule. The chemical reactivity of dimer acid is active, and dimer acid has the properties of non-toxicity, good high-heat stability and good fluidity, and the structural characteristics of dimer acid itself, so that dimer acid becomes a very important chemical intermediate material, which can be used to prepare polyamide resin, paint, lubricant, fuel additive and other important fine chemical products.

[0003] At present, the preparation process of dimer acid widely used in industry is clay catalysis. The unsaturated fatty acid with wide sources is used as raw material, and clay and lithium carbonate are used as catalysts to obtain crude dimer acid through polymerization reaction. The crude dimer acid is subjected to acidification and filtration to obtain dimer acid with higher purity. Since clay is used as catalyst, metal ions in the clay are inevitably brought into the dimer acid, and the metal ions form soaps under high temperature conditions, which affects the quality of the dimer acid. In order to remove the metal ions and reduce the soaps into dimer acid, the traditional method is to add phosphoric acid into the crude dimer acid, and through the acidification process, the phosphoric acid and the metal ions form phosphate salt precipitate, and the precipitate is removed through pressure filtration. The introduction of phosphoric acid inevitably brings trace phosphate into the dimer acid product. Then, the impurities are removed through water washing and centrifugation, so as to obtain the dimer acid. In industrial practice, the dimer acid prepared by the above method has low purity, heavy color, and high content of metal ions and phosphate.

[0004] In recent years, the requirement for environmental protection is higher and higher, and green and recyclable chemical products gradually become market demand. Phosphate can destroy the ecological balance, cause water quality deterioration, and indirectly affect human health, so many countries and regions are taking measures to reduce the emission of phosphate. Moreover, with the expansion of the application field of dimer acid, dimer acid is applied in the fields of heavy-duty anticorrosive coatings of ships, automobiles and containers, and the dimer acid is required to have higher purity, lighter color and lower metal ion content. For example, the trace metal ions such as iron in the dimer acid for heavy-duty anticorrosive application of ships are less than 2 ppm, and the trace phosphorus is less than 3 ppm. Color is also an important indicator for evaluating the quality of dimer acid. In the use process, dimer acid is often used as the initial raw material, and the color of dimer acid can affect the color and color stability of the final product. At present, the color of the domestic dimer acid product is generally too high, which cannot meet the market demand for high-quality dimer acid, and the high-quality dimer acid has to be imported for a long time. In summary, the current situation of domestic dimer acid seriously restricts the development and application of dimer acid.

[0005] In the prior art, some enterprises have begun to realize the above-mentioned technical problems and try to make various technical improvements. For example, CN102010322A discloses a preparation method of light color, low iron and low phosphorus dimer acid, which adjusts through special equipment and parameter control, such as adjusting the gear pump steady flow delivery, the steady flow delivery of distilled water, the static mixing of dimer acid and process water, the separation of butterfly centrifuge, the vacuum degree, temperature and evaporation area control in the scraped film distillation, the vacuum degree and temperature control in the short path molecular distillation, etc. A light color, low iron and low phosphorus dimer acid is obtained. The color of the dimer acid (Gardner) is 5-7, the trace iron is controlled in 0.5-2 ppm, and the trace phosphorus is controlled in 0.5-7 ppm. However, the preparation method has high requirements for equipment and process parameters, the investment cost of enterprises for equipment is increased, and the dimer acid obtained by the preparation method still has high color. CN101838193A discloses a light color and low phosphorus dimer acid production process, which adopts multi-stage continuous combined molecular distillation technology and new process of pressurization, temperature reduction and high-purity water washing, so that the purity content of the distilled dimer acid is increased to more than 86%, the phosphorus content is reduced to less than 10 ppm, and the product color is 7# (iron-cobalt colorimetry). The purity, phosphorus content and product color of the dimer acid obtained by the process still need to be further improved. CN107141214A discloses a production method of low phosphorus dimer acid, which removes trace phosphates and other impurities by adding metal cation reagents such as aluminum sulfate and calcium carbonate to react with phosphate to generate precipitates and filtering, so as to obtain dimer acid with phosphorus content less than 2 ppm and purity about 80%, but this process inevitably brings aluminum ions and calcium ions into the dimer acid, thereby increasing the content of metal ions in the dimer acid.

[0006] Therefore, it is still urgent to improve the preparation process of dimer acid to solve the problems of low purity, high residual phosphates and iron ions and heavy color. SUMMARY

[0007] The purpose of the present application is to overcome the technical defects of the above-mentioned process and provide a preparation method of dimer acid with low color, low iron content and low phosphorus content. The low color is measured by iron-cobalt colorimetry and is 2-5 color; the low phosphorus content is ≤1 ppm; and the low iron content is ≤2 ppm.

[0008] In order to achieve the above-mentioned purpose, the present application provides a preparation method of dimer acid with low color, low iron content and low phosphorus content, which comprises the following steps:

[0009] 1) Polymerization:

[0010] In the polymerization reactor, add vegetable oil acid, white clay and lithium carbonate, mix well, and carry out polymerization reaction.

[0011] Further, the vegetable oil acid is selected from one or more than two of soybean oil acid, tallow oil acid or cottonseed oil acid.

[0012] Further, the white clay is added in an amount of 6-12% by weight of the vegetable oil acid, preferably 8-10%, more preferably 8%;

[0013] Further, the lithium carbonate is added in an amount of 0.2-0.4% by weight of the vegetable oil acid, preferably 0.3%;

[0014] Further, the above-mentioned materials are heated to 180-230℃ under 1-5 atm for 2-4 h of polymerization reaction; more preferably, the above-mentioned materials are heated to 200-230℃ under 2-3 atm for 2-3 h of polymerization reaction.

[0015] 2) Acidification:

[0016] The polymerized material is flowed into the acidification reactor through a pipeline, and an organic carboxylic acid is added for reaction. The reacted material is filtered or pressure-filtered, and the filtrate is collected to obtain crude dimer acid.

[0017] Further, the acidification reactor has an acidification reaction temperature of 80-180℃, preferably 130-160℃;

[0018] Further, the acidification reaction time is 0.5-5 h, preferably 1-3 h;

[0019] Further, the organic carboxylic acid is selected from one or more of the following: acetic acid, oxalic acid; preferably oxalic acid.

[0020] Further, the organic carboxylic acid is added in an amount of 0.1-3% by weight of the vegetable oil acid, which can be selected from 0.2%, 0.5%, 0.8%, 1%, 1.5%, 2%, 2.5%, 3%; preferably 1-2%, more preferably 1-1.5%.

[0021] 3) Coordination:

[0022] The crude dimer acid is flowed into the coordination reactor through a pipeline, and a coordination agent is added for reaction.

[0023] Further, the reactor temperature is set to 70-120℃, preferably 80-100℃, more preferably 90-95℃;

[0024] Further, the stirring reaction time in the reactor is 2-8 h, preferably 4-6 h.

[0025] Further, the complexing agent is selected from one or more of the following: monoethanolamine, diethanolamine, triethanolamine, diethylenetriamine pentaacetic acid, nitrilotriacetic acid;

[0026] Further, the complexing agent is preferably monoethanolamine, diethanolamine and triethanolamine;

[0027] Further, the complexing agent is more preferably a mixture of triethanolamine and nitrilotriacetic acid, and the weight ratio of the two is preferably 1-3:1, more preferably 1:1;

[0028] Further, the amount of the complexing agent added is 0.01-0.3% by weight of the plant oil acid, which can be selected from 0.02%, 0.025%, 0.05%, 0.08%, 0.1%, 0.15%, 0.2%, 0.25%, 0.3%; preferably 0.05-0.2%, most preferably 0.1%.

[0029] 4) Water washing:

[0030] The crude dimer acid after complexing is flowed into a water washing kettle through a pipeline, and then water washing is performed, followed by centrifugal removal of water.

[0031] Further, the temperature in the water washing kettle is 80-100°C.

[0032] 5) Distillation:

[0033] The water and low-boiling material are removed from the water-washed material, and then multi-stage distillation is performed, to finally obtain a dimer acid with low color, low phosphorus and low iron content.

[0034] Further, the water and low-boiling material are removed by using a wiped-film evaporator. The specific process conditions for the wiped-film evaporation are as follows: heating to 100-140°C, under 0.8-1.2 atm, using a wiped-film evaporator to perform continuous degassing, dewatering and low-boiling material removal.

[0035] Further, the multi-stage distillation is preferably three stages, and a three-stage short-path molecular distillation apparatus is used, and the specific process steps are as follows:

[0036] The dimer acid after removal of water and low-boiling material is sent into a first-stage short-path molecular distillation apparatus, and distillation purification is performed under a vacuum degree of 10-50 Pa and a temperature of 200-240°C, to obtain a heavy-phase material ordinary dimer acid and a light-phase material monomer acid.

[0037] The material after the first-stage short-path molecular distillation is introduced into a second-stage short-path molecular distillation apparatus, and distillation purification is performed under a vacuum degree of 10-30 Pa and a temperature of 210-250°C, to further remove monomer acid and intermediates.

[0038] The material after the second short path molecular distillation enters a third short path molecular distillation device, and is distilled and purified under the conditions of a vacuum degree of 1-5 Pa and a temperature of 230-270 DEG C, so that the light phase is high-purity dimer acid with a content of 98.5% or more, and the heavy phase is polybasic acid.

[0039] 6) cooling storage:

[0040] The dimer acid obtained by the above process is cooled to room temperature, and flows into a storage tank through a pipeline, and can be stored for 24-36 months without discoloration, and the color is stable.

[0041] Further, the polymerization reactor, the acidification kettle, the coordination kettle, the water washing kettle and the storage tank in each step are subjected to nitrogen pre-blowing treatment in advance to remove oxygen therefrom.

[0042] Further, a continuous closed process is used in each step, and the pipeline in each step is made of 316L stainless steel.

[0043] It is detected that the color of the obtained dimer acid is 2-5 in iron-cobalt colorimetry, the phosphorus content is less than or equal to 1 ppm, and the iron content is less than or equal to 2 ppm.

[0044] Compared with the prior art, the present application has the following beneficial effects:

[0045] In the present application, an organic carboxylic acid is used as an acidifying agent instead of the traditional phosphoric acid, so that the addition of phosphorus is avoided, thereby reducing the phosphorus content in the dimer acid. In the present application, a coordination process is added, the coordination reaction between the coordination agent and the metal ions is carried out, and the metal ion coordination body is removed through the water washing step, thereby reducing the content of iron ions and other metal ions. In the prior art of the preparation process of dimer acid, the pipelines in each step are mostly made of 316 type pipe materials. The inventor finds that although the 316 type pipe material has good corrosion resistance and heat resistance, it affects the color, the color of the prepared dimer acid increases after being placed for 24 months, the storage stability of the dimer acid is poor, and it is not conducive to market sales. The inventor finds that the use of 316L type stainless steel can keep the color of the dimer acid unchanged after being stored in the storage tank for 24-36 months.

[0046] The dimer acid obtained by the preparation process has the following beneficial effects: the color is 2-5 in iron-cobalt colorimetry, the color storage stability is good, the phosphorus content is less than or equal to 1 ppm, and the iron content is less than or equal to 2 ppm, which can meet the market demand for high-quality dimer acid. BRIEF DESCRIPTION OF DRAWINGS

[0047] Figure 1 The present application is a dimer acid production process flow diagram. DETAILED DESCRIPTION

[0048] The following examples are intended to illustrate the present application but not to limit the scope of the present application. The specific techniques or conditions not mentioned in the examples are in accordance with the techniques or conditions described in the literature in the art. The reagents or instruments not mentioned by the manufacturer are all conventional products that can be purchased through regular channels. Unless otherwise specified, the test or test method is a conventional method in the art.

[0049] Example 1

[0050] In a polymerization reactor, 100 kg of cottonseed oil acid, 8 kg of clay and 0.3 kg of lithium carbonate were added and stirred well. The polymerization reaction was carried out at 2 atm and 200°C for 3 hours. The polymerized material was flowed into an acidification reactor through a pipeline, 1 kg of oxalic acid was added, and the reaction was stirred at 130°C for 2 hours. The reacted material was filtered under pressure, and the filtrate was collected to obtain crude dimer acid.

[0051] The crude dimer acid was flowed into a complex reactor through a pipeline, the temperature in the reactor was set to 95°C, and 0.1 kg of monoethanolamine was added while stirring to carry out complexation reaction for 4 hours.

[0052] The complexed crude dimer acid was flowed into a water washing reactor through a pipeline, and then centrifuged to remove water. The water washing temperature was 90°C.

[0053] The dehydrated material was flowed into a wiped film evaporator through a pipeline, and the water and low boiling point substances were continuously removed at 120°C and 0.8 atm. Then the material was flowed into the first stage short path evaporator for purification and refinement, and the heavy phase material dimer acid and the light phase monomer acid were obtained at a vacuum degree of less than 30 Pa and a temperature of 220°C. Then the material was flowed into the second stage short path evaporator for purification and refinement, and the heavy phase material dimer acid and the light phase monomer acid and intermediate were further removed at a vacuum degree of less than 10 Pa and a temperature of 230°C. Then the material was flowed into the third stage short path evaporator for purification and refinement, and the light phase dimer acid and the heavy phase polydimer acid were obtained at a vacuum degree of less than 5 Pa and a temperature of 250°C.

[0054] The dimer acid obtained by the above process was cooled to room temperature and stored in a storage tank through a pipeline.

[0055] Each reaction container in the above preparation process was pre-treated with nitrogen blowing. The above preparation process uses a continuous closed process, and the connecting pipeline between each step uses 316L stainless steel.

[0056] Example 2

[0057] In a polymerization reactor, 100 kg of soybean oil acid, 8 kg of clay and 0.3 kg of lithium carbonate were added, and stirred thoroughly. Polymerization was carried out at 2 atm and 210°C for 3 hours. The polymerized material was flowed into an acidification reactor through a pipeline, 1 kg of oxalic acid was added, and stirred at 130°C for 2 hours. The reacted material was filtered under pressure, and the filtrate was collected to obtain crude dimer acid.

[0058] The crude dimer acid was flowed into a complex reactor through a pipeline, the temperature in the reactor was set to 90°C, and 0.15 kg of nitrilotriacetic acid was added while stirring to carry out complexation reaction for 5 hours.

[0059] The complexed crude dimer acid was flowed into a water washing reactor through a pipeline, and washed with water, and then centrifuged to remove water. The water washing temperature was 90°C.

[0060] The dehydrated material was flowed into a wiped film evaporator through a pipeline, and continuously removed water and low boiling point substances at 130°C and 0.9 atm. Then the material was flowed into a first-stage short-path evaporator for purification and refinement at a vacuum degree of less than 25 Pa and a temperature of 230°C to obtain a heavy phase material of dimer acid and a light phase of monomer acid. Then the material was flowed into a second-stage short-path evaporator for purification and refinement at a vacuum degree of less than 15 Pa and a temperature of 230°C to further remove monomer acid and intermediates, and obtain a heavy phase material of dimer acid. Then the material was flowed into a third-stage short-path evaporator for purification and refinement at a vacuum degree of less than 3 Pa and a temperature of 250°C to obtain a light phase of dimer acid and a heavy phase of polydimer acid.

[0061] The dimer acid obtained by the above process was cooled to room temperature, and stored in a storage tank through a pipeline.

[0062] Each of the reaction vessels in the above preparation process was pre-treated with nitrogen blowing. The above preparation process used a continuous closed process, and the connecting pipelines between the steps used 316L stainless steel.

[0063] Example 3

[0064] In a polymerization reactor, 100 kg of soybean oil acid, 8 kg of clay and 0.3 kg of lithium carbonate were added, and stirred thoroughly. Polymerization was carried out at 2 atm and 210°C for 3 hours. The polymerized material was flowed into an acidification reactor through a pipeline, 1 kg of oxalic acid was added, and stirred at 130°C for 2 hours. The reacted material was filtered under pressure, and the filtrate was collected to obtain crude dimer acid.

[0065] The crude dimer acid was flowed into a complex reactor through a pipeline, the temperature in the reactor was set to 90°C, and 0.15 kg of nitrilotriacetic acid was added while stirring to carry out complexation reaction for 5 hours.

[0066] The coordinated crude dimeric acid is flowed into a water washing kettle through a pipeline, washed with water, and then centrifuged to remove water. The water washing temperature is 100°C.

[0067] The dehydrated material is flowed into a wiped-film evaporator through a pipeline, continuously removed of water and low-boiling substances at 130°C and 1 atm, and then flowed into a first-stage short-path evaporator for purification and refinement, to obtain a heavy-phase material dimeric acid and a light-phase monomer acid at a vacuum degree of less than 40 Pa and a temperature of 240°C. The material is then flowed into a second-stage short-path evaporator for purification and refinement, to further remove monomer acid and intermediates, to obtain a heavy-phase material dimeric acid at a vacuum degree of less than 10 Pa and a temperature of 230°C. The material is then flowed into a third-stage short-path evaporator for purification and refinement, to obtain a light-phase dimeric acid and a heavy-phase polydimeric acid at a vacuum degree of less than 2 Pa and a temperature of 250°C.

[0068] The dimeric acid obtained by the above process is cooled to room temperature and stored in a storage tank through a pipeline.

[0069] Each reaction container in the above preparation process is pre-treated by nitrogen blowing. The above preparation process adopts a continuous closed process, and the connecting pipelines between the steps are made of 316L stainless steel.

[0070] Example 4

[0071] The material is basically the same as in Example 1, except that the complexing agent is a mixture of triethanolamine and nitrilotriacetic acid at a weight ratio of 1:1, 0.1 kg.

[0072] Example 5

[0073] The material is basically the same as in Example 1, except that the complexing agent is a mixture of triethanolamine and nitrilotriacetic acid at a weight ratio of 3:1, 0.1 kg.

[0074] Example 6

[0075] The material is basically the same as in Example 1, except that the complexing agent is 0.12 kg of diethylenetriamine pentaacetic acid.

[0076] Example 7

[0077] The material is basically the same as in Example 1, except that the complexing agent is 0.2 kg of triethanolamine.

[0078] Example 8

[0079] The material is basically the same as in Example 1, except that the connecting pipelines are made of 316 stainless steel.

[0080] Example 9

[0081] The material is basically the same as in Example 4, except that the connecting pipelines are made of 316 stainless steel.

[0082] Comparative Example 1

[0083] Into a polymerization reactor, 100 kg of cottonseed oil acid, 8 kg of clay and 0.3 kg of lithium carbonate were added and mixed well. Polymerization was carried out at 2 atm and 200°C for 3 hours. The polymerized material was then fed into an acidification reactor, 2 kg of phosphoric acid was added, and the mixture was stirred at 150°C for 1.5 hours. The reacted material was filtered, and the filtrate was collected to obtain crude dimer acid.

[0084] The crude dimer acid was fed into a water washing reactor, washed with water, and then centrifuged to remove water. The water washing temperature was 95°C.

[0085] The dehydrated material was fed into an evaporator, and water and low-boiling substances were removed at 110°C and 1 atm. The material was then fed into a short-path evaporator, and dimer acid was obtained at 230°C and a vacuum degree of 25 Pa.

[0086] The dimer acid obtained by the above process was cooled to room temperature, and stored in a tank through a pipeline.

[0087] Each reactor in the above preparation process was pre-treated with nitrogen blowing. The above preparation process was a continuous and closed process, and the connecting pipelines between the steps were made of 316 stainless steel.

[0088] Comparative Example 2

[0089] Into a polymerization reactor, 100 kg of cottonseed oil acid, 8 kg of clay and 0.3 kg of lithium carbonate were added and mixed well. Polymerization was carried out at 2 atm and 200°C for 3 hours. The polymerized material was then fed into an acidification reactor, 2 kg of phosphoric acid was added, and the mixture was stirred at 150°C for 1.5 hours. The reacted material was filtered, and the filtrate was collected to obtain crude dimer acid.

[0090] The crude dimer acid was fed into a water washing reactor, washed with water, and then centrifuged to remove water. The water washing temperature was 95°C.

[0091] The dehydrated material was fed into a wiped-film evaporator, and water and low-boiling substances were continuously removed at 130°C and 1 atm. The material was then fed into a first-stage short-path evaporator for purification and refining, and dimer acid was obtained as the heavy phase at a vacuum degree of less than 40 Pa and a temperature of 240°C. The light phase was monomer acid. The material was then fed into a second-stage short-path evaporator for purification and refining, and further removal of monomer acid and intermediates was carried out at a vacuum degree of less than 10 Pa and a temperature of 230°C to obtain dimer acid as the heavy phase. The material was then fed into a third-stage short-path evaporator for purification and refining, and dimer acid was obtained as the light phase at a vacuum degree of less than 1 Pa and a temperature of 250°C. The heavy phase was polydimer acid.

[0092] The dimer acid obtained from the above process is cooled to room temperature and stored in a tank through a pipeline.

[0093] Each reaction vessel in the above preparation process is pre-treated with nitrogen blowing. The above preparation process adopts a continuous closed process, and the connecting pipeline between each step is made of 316 stainless steel.

[0094] Comparative Example 3

[0095] The material is basically the same as that in Comparative Example 2, except that the acidifying agent phosphoric acid is replaced by oxalic acid.

[0096] The above Examples 1-9 and Comparative Examples 1-3 are detected by the following specific detection methods.

[0097] Detection method:

[0098] Purity detection: The purity is measured by high performance liquid chromatography.

[0099] Color detection: The color of the dimer acid is measured by an iron-cobalt colorimeter. The sample is compared with a series of iron-cobalt standard color solutions with different color values by visual method to evaluate the color depth, which is represented by numbers from 1 to 18. The lightest is 1 and the darkest is 18. The color number is used to represent. The dimer acid obtained by the above process is represented by "initial color".

[0100] Phosphorus content detection: The sample is ashed by burning, and then dissolved in hydrochloric acid. After adding reagents such as sodium molybdate, phosphomolybdate is generated. The phosphomolybdate is reduced by ascorbic acid to produce colored complexes such as molybdenum blue. The color depth is proportional to the phosphorus content in the sample by colorimetric determination at 650 nm wavelength by spectrophotometer. The phosphorus content in the sample is calculated by using the standard phosphorus content curve. The unit of measurement is ppm.

[0101] Iron content detection: The sample is ashed by burning, and then dissolved in hydrochloric acid to form a dilute hydrochloric acid solution. Iron (III) is reduced to iron (II) by hydroxylamine hydrochloride, and iron (II) forms a stable red complex with 1,10-phenanthroline in the pH range of 3-9. The absorbance is measured at 510 nm wavelength by spectrophotometer. The iron ion content in the sample is calculated by using the standard iron content curve. The unit of measurement is ppm.

[0102] Storage stability detection: The prepared dimer acid is placed in a nitrogen pre-blowing tank and stored in a dark cool place for 24 months. The color is measured by an iron-cobalt colorimeter, and the color number is used to represent. The storage stability is represented by "color after 24 months of storage". Compared with the initial color, the less the color changes, the better the storage stability.

[0103] The detection results are shown in Table 1 Performance Test Table.

[0104] Table 1 Performance test table

[0105]

[0106] From the above test results, it can be seen that the dimer acid obtained by using the preparation process of the present application has purity of more than 99%, phosphorus content of <1 ppm, iron content of <1.2 ppm, and initial color number of <3; after storage for 24 months, the color does not change, and the storage stability is good. The performance indicators are all better than those of the dimer acid prepared by Comparative Examples 1-3.

[0107] Although the embodiments of the present application have been shown and described, it will be understood by those of ordinary skill in the art that various changes, modifications, substitutions and alterations can be made thereto without departing from the principles and spirit of the present application, the scope of which is defined by the following claims and their equivalents.

Claims

1. A method for preparing a dimer acid with low color, low iron content, and low phosphorus content, wherein the low color is measured by an iron-cobalt colorimeter and is color number 2-5; the low phosphorus content is phosphorus content ≤1ppm; and the low iron content is iron content ≤2ppm. It includes the following steps: 1) Polymerization: Add vegetable oleic acid, kaolin and lithium carbonate to the polymerization reactor, stir thoroughly to mix, and carry out the polymerization reaction; 2) Acidification: The polymerized material is piped into an acidification kettle, an organic carboxylic acid is added to react, the reacted material is filtered or pressure filtered, and the filtrate is collected to obtain crude dimer acid; 3) Coordination: Crude dimer acid flows into the coordination vessel through a pipeline, a coordination agent is added, and the mixture is stirred to carry out the reaction; 4) Water washing: The coordinated crude dimer acid flows into the water washing tank through the pipeline for water washing, and then centrifugation is used to remove water; 5) Distillation: After washing, the material is dehydrated and low-boiling substances are removed. Then, it is distilled in multiple stages to finally obtain a dimer acid with low color, low phosphorus, and low iron content. 6) Cooling and storage: The dimer acid obtained by the above process is cooled to room temperature and then piped into a storage tank for storage; The organic carboxylic acid is selected from one or two of the following: acetic acid and oxalic acid; The ligand is selected from one or more of the following: monoethanolamine, diethanolamine, triethanolamine, diethylenetriaminepentaacetic acid, and hypotriacetic acid; Each step employs a continuous closed-loop process, and the pipes in each step are made of 316L stainless steel.

2. The method for preparing dimer acid according to claim 1, characterized in that, The organic carboxylic acid mentioned is selected from oxalic acid.

3. The method for preparing dimer acid according to claim 1, characterized in that, The amount of the organic carboxylic acid added is 0.1-3% of the weight of the vegetable oleic acid.

4. The method for preparing dimer acid according to claim 1, characterized in that, The ligand is selected from one or more of the following: monoethanolamine, diethanolamine, and triethanolamine.

5. The method for preparing dimer acid according to claim 1, characterized in that, The ligand is a mixture of triethanolamine and hypotriacetic acid in a weight ratio of 1-3:

1.

6. The method for preparing dimer acid according to claim 5, characterized in that, The ligand is a mixture of triethanolamine and hypotriacetic acid in a weight ratio of 1:

1.

7. The method for preparing dimer acid according to claim 1, characterized in that, The amount of the ligand added is 0.01-0.3% of the weight of oleic acid.

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