A method for determining the composition of a surfactant in an alcohol amine solution

Abstract

The application provides a method for determining the composition of surface active substances in an alcohol amine solution. The method comprises the following steps: adjusting the pH value of the alcohol amine solution with an inorganic acid; extracting the alcohol amine solution after the pH value is adjusted with different organic solvents, and volatilizing the organic solvents; separating components by using a silica gel thin layer chromatography technology with different developing agents; and then performing qualitative and / or quantitative analysis by using an infrared spectrometer, a nuclear magnetic resonance spectrometer, a gas chromatograph-mass spectrometer and a liquid chromatograph-mass spectrometer, so as to obtain qualitative and / or quantitative analysis results of anionic surface active substances in the alcohol amine solution, and qualitative and / or quantitative analysis results of cationic surface active substances and nonionic surface active substances. The method for determining the composition of surface active substances in the alcohol amine solution has the advantages of wide determination range, suitability for determination of unknown surface active substances, accurate qualitative and quantitative results and the like.

Description

Technical Field

[0001] This invention relates to the field of chemical analysis, and in particular to a method for determining the composition of surfactants in an alcohol amine solution. Background Technology

[0002] The produced natural gas contains 60% sulfur. This sulfur-containing natural gas must undergo desulfurization and purification treatment at a natural gas purification plant before it can be used. Therefore, the natural gas purification process is one of the important links in natural gas production and supply. The amine method is the main method for natural gas desulfurization and decarbonization. During the natural gas purification process, the amine desulfurization solution (hereinafter referred to as amine solution) is easily contaminated and foamed by residues from upstream natural gas extraction operations. This causes unstable operation of the desulfurization unit, performance degradation, and substandard product gas quality. When foaming is severe, the desulfurization unit cannot maintain operation, and the purification plant has to shut down, which has a significant impact on natural gas production.

[0003] Residues from various chemical agents used in natural gas extraction are partially introduced into the natural gas pipeline system by being lifted into droplets by high-speed gas flow or by evaporation. Larger particles gradually settle and accumulate in the pipeline, while some that don't settle are easily removed by separators or filters. However, extremely small liquids and even vapors, ranging from 1 to 300 nm in size, exceed the filtration precision of various levels of natural gas separators and filters and cannot be removed. To reduce the amount of impurities entering the desulfurization unit, purification plants continuously improve the filtration efficiency of filters and separators, but they cannot remove all impurities. For example, after introducing advanced PALL high-efficiency separators for filtering raw natural gas, the filtration precision of impurities in the natural gas increased from 500 nm to 300 nm. Field performance tests showed that its impurity removal efficiency ranged from a maximum of 99.8% to a minimum of 93.0%, far superior to the original separators. However, 0.2% to 7% of impurities still failed to be removed and entered the desulfurization unit (not including impurities smaller than 300 nm), contaminating the amine solution. Therefore, foaming due to contamination of the amine solution in the natural gas purification plant is unavoidable.

[0004] While some companies have developed technologies for removing corrosive impurities—thermally stable salts—from amine solutions, they have not yet achieved success in removing effusive substances. One of the key limiting factors is the lack of effective methods for detecting effusive substances, making it impossible to clearly identify the types of effusive substances causing foaming and decreased desulfurization performance in amine solutions; in other words, the target precipitate is unclear. Therefore, before developing effusive substance removal technologies, it is essential to overcome the analytical challenges and clarify the types of target precipitates. Determining the types of effusive substances in amine solutions, i.e., the composition of surfactants, is a recognized challenge in the field of gas purification. With the accelerating pace of natural gas production, the impact of amine foaming on production is more pronounced than in the past, severely restricting the stable and increased production of purification plants. Therefore, it is imperative to conduct technical research to address this challenge and develop methods for determining the composition of surfactants in amine solutions.

[0005] In previous studies, the inventors of this invention used gas chromatography-mass spectrometry (GC-MS) to detect approximately 56 foaming impurities in amine solutions from purification plants (see "Research on CT-FSC Alkylamine Desulfurization Solution Reactivation Technology" in *Petroleum and Natural Gas Chemical Industry*, Vol. 47, No. 5, 2018). These 56 foaming impurities are C7 to C6. 18 Carboxylic acids (salts) and their isomers, C6-C 36 The invention relates to hydrocarbons and their isomers, and includes "a method for removing long-chain carboxylates and hydrocarbons from alkanolamine solutions" (CN103961901A). Application results in various purification plants show that this invention can only solve the foaming problem of amine solutions in some plants, or reduce the foaming tendency of amine solutions in others, but not below the foaming control index. This indicates that in addition to long-chain carboxylates (salts) and hydrocarbons, other types of foaming impurities exist in the amine solutions of purification plants. To comprehensively and thoroughly solve the problem of foaming caused by contamination in amine solutions of various purification plants, it is necessary to upgrade the original invention or develop new separation technologies capable of removing other types of foaming substances. Whether upgrading the original invention or developing new technologies, it is essential to first fully understand the types of foaming substances in the amine solutions of purification plants before developing suitable separation technologies based on their physicochemical and molecular structural characteristics.

[0006] Because the chemical agents used in natural gas extraction are numerous, complex in composition, and kept secret as trade secrets, and because chemical changes occur during operation and after entering the amine solution, the types of impurities in the amine solution are extremely complex. Some impurities do not cause the amine solution to foam; it is only necessary to determine the composition of the foaming surfactants in the amine solution.

[0007] Currently, methods for determining surfactants include spectrophotometry, two-phase titration, liquid chromatography, chromatography-mass spectrometry, thin-layer scanning, electrochemical analysis, resonance scattering analysis, capillary electrophoresis, and oscillometric polarography.

[0008] For example, the various determination methods disclosed in the following inventions are as follows: a method for determining the content of anionic surfactants using resonance Rayleigh scattering (CN106680062A), a method for determining the content of anionic-cationic surfactant compound disinfectant (CN104977386A), a method for simultaneous colorimetric determination of phenol and anionic surfactants (CN102288598A), a method for soil extraction and determination of anionic surfactants (CN102183393A), a method for determining the content of nonionic surfactants in degreasing agents (CN103776788A), a method for detecting detergent residues on fabrics by liquid chromatography (CN109541070A), a method for determining the content of sodium linear alkylbenzene sulfonate residues in beer (CN113310915A), a method for determining polysorbate nonionic surfactants (CN101852739A), and a qualitative determination method for nonionic surfactants in compound systems (CN108152243A).

[0009] Among the above inventions, some determine the total content of surfactants, such as CN106680062A, CN104977386A, CN102288598A, CN102183393A, CN103776788A, and CN109541070A; some determine the content of a specific surfactant, such as CN113310915A and CN101852739A; and some determine the components of surfactant products without interference from other substances, and the method requires low sensitivity due to the high concentration of surfactants, such as CN108152243A.

[0010] The methods described above cannot accurately determine the content of surfactants in amine solutions, nor can they determine the molecular structure of each surfactant in the amine solution. Existing methods are unsuitable for determining the composition of surfactants in amine solutions for three main reasons: 1) The surfactants in amine solutions are unknown; except for chromatography-mass spectrometry and thin-layer chromatography, the methods described above are only applicable to surfactants with known molecular structures; 2) The concentration of alkanolamines in amine solutions is hundreds or even thousands of times higher than the concentration of surfactants, and the methods described above cannot eliminate the masking effect of high-concentration alkanolamines on surfactants; 3) Amine solutions contain many types of impurities that cause severe mutual interference, and the low content of surfactants in the amine solution results in weak signals. These factors prevent the methods from detecting surfactants in amine solutions, let alone accurately determining or quantifying them.

[0011] Therefore, developing a method for determining the composition of surfactants in alkanolamine solutions has become one of the urgent problems to be solved in this field. Summary of the Invention

[0012] To address the aforementioned technical problems, the present invention aims to provide a method for determining the composition of surfactants in an alkanolamine solution. The method of the present invention has advantages such as a wide measurement range, applicability to the determination of unknown surfactants, and accurate qualitative and quantitative analysis.

[0013] To achieve the above objectives, the present invention provides a method for determining the composition of surfactants in an alcohol amine solution, comprising the following steps:

[0014] (1) Add the alkanolamine solution to an aqueous inorganic acid solution and adjust the pH of the alkanolamine solution to the first pH value; extract the alkanolamine solution after pH adjustment with the first organic solvent (extract once or several times), then evaporate part of the first organic solvent at the first temperature, and then raise the temperature to the second temperature to evaporate to constant weight to obtain all extracts; after completely dissolving the obtained extracts with the first organic solvent, spot the liquid (i.e., the first organic solvent containing all extracts) on the bottom of the silica gel thin-layer plate with a capillary tube, then place the silica gel thin-layer plate into a chromatography tank and separate it with the first developing solvent, and elute the separated components (i.e., spots) with the first organic solvent; perform qualitative and / or quantitative analysis on the components eluted with the first organic solvent using one or more of the following: infrared spectroscopy, nuclear magnetic resonance spectroscopy, gas chromatography-mass spectrometry, liquid chromatography-mass spectrometry, etc., to obtain the qualitative and / or quantitative analysis results of anionic surfactants in the alkanolamine solution;

[0015] (2) Add the alkanolamine solution to an aqueous inorganic acid solution to adjust the pH of the alkanolamine solution to a second pH value. Extract the alkanolamine solution after pH adjustment with a second organic solvent at a third temperature (extract once or several times). Then, evaporate part of the second organic solvent at a fourth temperature, and then raise the temperature to a fifth temperature to evaporate to constant weight to obtain all the extract. After completely dissolving the obtained extract with the second organic solvent, spot the liquid (i.e., the second organic solvent containing all the extract) onto the bottom of a silica gel thin-layer plate using a capillary tube. Then, place the silica gel thin-layer plate into a chromatography tank and separate it with a second developing solvent. Elute the separated components (i.e., spots) with the second organic solvent. Perform qualitative and / or quantitative analysis on the components eluted with the second organic solvent using one or more of the following: infrared spectroscopy, nuclear magnetic resonance spectroscopy, gas chromatography-mass spectrometry, and liquid chromatography-mass spectrometry, to obtain the qualitative and / or quantitative analysis results of cationic and nonionic surfactants in the alkanolamine solution.

[0016] In the above-described method for determining the composition of surfactants in an alkanolamine solution, preferably, in steps (1) and (2), the mass concentration of the inorganic acid aqueous solution is 1% to 38%, more preferably 18% to 19%. The alkanolamine concentration in the solution is very high due to its alkalinity, and the inorganic acid aqueous solution concentration is too low, requiring a large amount of inorganic acid aqueous solution to neutralize to the first or second pH value. Furthermore, the low surfactant concentration in the alkanolamine solution necessitates a large sample volume of amine solution to ensure the extracted surfactant content meets the requirements for accurate subsequent qualitative and quantitative analysis. Adding too much inorganic acid aqueous solution can cause difficulties in subsequent extraction operations, leading to decreased analytical accuracy. Conversely, excessively high inorganic acid aqueous solution concentration results in a vigorous acid-base reaction after sample addition, generating significant heat and potentially causing loss of the analyte. The concentration of the inorganic acid aqueous solution specified in this invention avoids these problems.

[0017] In the above method for determining the composition of surface-active substances in an alcohol amine solution, preferably, in steps (1) and (2), the inorganic acid aqueous solution includes an aqueous solution of one or more of hydrochloric acid, sulfuric acid, nitric acid, and phosphoric acid.

[0018] In the above method for determining the composition of surfactants in an alcohol amine solution, preferably, in step (1), the first pH value is 1 to 6, more preferably 4.

[0019] In the above-described method for determining the composition of surfactants in an alkanolamine solution, preferably, in step (1), the first organic solvent includes diethyl ether, n-butanol, or a mixture of diethyl ether and n-butanol, more preferably a mixture of diethyl ether and n-butanol. In step (1) of the present invention, when the pH of the alkanolamine solution is adjusted to 1-6 and extracted with diethyl ether or n-butanol, the co-extraction rate of the alkanolamine is 0, eliminating the masking and interference of the alkanolamine on the determination of anionic surfactants; the solubility of anionic surfactants in the aqueous phase is very low, and the complete separation of diethyl ether, n-butanol, and aqueous phase is beneficial to improving the extraction rate of anionic surfactants. The extraction rate is highest when the pH is adjusted to 4. Different types of anionic surfactants have different solubilities in diethyl ether or n-butanol; some have higher solubility in diethyl ether, while others have higher solubility in n-butanol. A mixture of diethyl ether and n-butanol is preferred, as it can eliminate the differences in extraction rates among different types of anionic surfactants. Preferably, the volume ratio of diethyl ether to n-butanol in the diethyl ether-n-butanol mixed solvent is (9:1) to (1:9), more preferably 5:5.

[0020] In the above method for determining the composition of surfactants in an alkanolamine solution, preferably, in step (1), when extracting the alkanolamine solution after pH adjustment with a first organic solvent, the volume ratio of the first organic solvent to the alkanolamine solution is 1:2 to 4:1. This volume ratio is the same as the volume ratio of the first organic solvent to the alkanolamine solution used during the first extraction.

[0021] In the above method for determining the composition of surfactants in an alkanolamine solution, preferably, in step (1), the alkanolamine solution after pH adjustment is extracted with a first organic solvent 1-10 times, more preferably 1-3 times. When the number of extractions with the first organic solvent is 2 or more, the amount of the first organic solvent used each time can be the same, that is, the amount of the first organic solvent used in the first extraction can be used for each extraction.

[0022] In the above method for determining the composition of surface-active substances in an alkanolamine solution, preferably, in step (1), the temperature of the alkanolamine solution after pH adjustment by extraction with a first organic solvent can be room temperature.

[0023] Commonly used extractants for extracting anionic surfactants include benzene, toluene, xylene, pentane, hexane, octane, cyclohexane, acetone, cyclohexanone, methylcyclohexanone, chlorobenzene, dichlorobenzene, chloroform, and dichloromethane. These organic solvents are insoluble in water and can completely separate from the aqueous layer during extraction. Alkylamines, in particular, are water-soluble molecules, and it is generally believed that choosing these extractants would facilitate the separation of alkylamines from anionic surfactants. However, in reality, alkylamines have a strong affinity for anionic surfactants, and direct extraction of the amine solution using the aforementioned organic solvents can hardly extract trace amounts of anionic surfactants from the sample. The solution of this invention is to add an inorganic acid before extraction, introducing hydrogen ions to weaken the affinity between the alkanolamine and the anionic surfactant, and to reduce the solubility of the anionic surfactant in water. At the same time, the alkanolamine is protonated, which significantly reduces its solubility in the organic phase. Diethyl ether and n-butanol, which have high polarity matching with the anionic surfactant, are selected as extractants to improve the solubility of the anionic surfactant in the organic phase. The addition of inorganic acid adjusts the pH value to 1-6, which breaks the hydrogen bonds formed between diethyl ether, n-butanol and water, making their separation from the aqueous phase more thorough and further improving the extraction rate.

[0024] In the above method for determining the composition of surfactants in an alcohol amine solution, preferably, in step (1), the first temperature is room temperature to 35°C, more preferably 27 to 30°C.

[0025] In the above method for determining the composition of surfactants in an alcohol amine solution, preferably, in step (1), the time for evaporating part of the first organic solvent at the first temperature is 1 to 24 hours.

[0026] In the above method for determining the composition of surfactants in an alkanolamine solution, preferably, in step (1), the second temperature is 100–113°C, more preferably 110–112°C. At this temperature, the extraction solvent in step (1) can be completely evaporated, ensuring that the analyte is not lost.

[0027] In the above method for determining the composition of surfactants in an alcohol amine solution, preferably, after step (1) is heated to a second temperature and volatilized to constant weight to obtain all the extract, the method further includes: weighing the total amount of extract.

[0028] In the above method for determining the composition of surface active substances in an alkanolamine solution, in step (1), the amount of the first organic solvent used to completely dissolve the obtained extract can be adjusted by those skilled in the art according to the actual situation, as long as all the extract is dissolved. The present invention does not impose any special limitations on this.

[0029] In the above method for determining the composition of surface-active substances in an alcohol amine solution, preferably, in step (1), the first developing agent includes isopropanol.

[0030] In the above method for determining the composition of surfactants in an alcohol amine solution, in step (1), the liquid (i.e., the first organic solvent containing all the extracts) is spotted onto the bottom of the silica gel thin-layer plate using a capillary tube. The specific spotting operation can be the routine operation of silica gel thin-layer chromatography. For example, the spotting is generally a round dot, the diameter of the spotting is generally about 2 mm or less, the distance from the spotting baseline to the bottom can be 0.5-2 cm, and the spacing between the spots can be 0.5-2 cm.

[0031] In the above method for determining the composition of surfactants in an alcohol amine solution, preferably, in step (1), the specific operation of placing the silica gel thin-layer plate into the chromatography tank and separating it with the first developing solvent is as follows: the silica gel thin-layer plate is placed into the first developing solvent of the chromatography tank, and the depth of the silica gel thin-layer plate immersed in the first developing solvent can be 2-5 mm from the origin (i.e., the spotting point). After developing to 14 cm (i.e., the spotting point rises from the bottom to 14 cm), the silica gel thin-layer plate is taken out and dried.

[0032] In the above method for determining the composition of surface-active substances in an alcohol amine solution, in step (1), the amount of the first organic solvent used to wash off the separated components (i.e., spots) can be adjusted by those skilled in the art according to the actual situation, as long as the subsequent qualitative and / or quantitative analysis can be carried out smoothly.

[0033] In the above-described method for determining the composition of surfactants in an alkanolamine solution, preferably, in step (2), the second pH value is 5-6, more preferably 6. In step (2) of the present invention, when the pH value of the alkanolamine solution is adjusted to 5-6, the co-extraction rate of alkanolamine is 0, which can eliminate the masking and interference of alkanolamine on the determination of cationic and nonionic surfactants. When the second pH value is adjusted to 6, the extraction rate is even higher.

[0034] In the above method for determining the composition of surfactants in an amine solution, preferably, in step (2), the second organic solvent includes a mixed solvent of n-butanol, ethyl acetate, and trioctylamine. The optimal pH range for cationic surfactant extraction is ≥8. To eliminate the masking and interference of amines, step (2) of the present invention adjusts the pH of the amine solution to 5-6. At this acidity, the extraction rate of conventional extractants is low. Using ethyl acetate, trioctylamine, and n-butanol to prepare a mixed extractant can significantly improve the extraction rate of cationic surfactants at pH 5-6. More preferably, the volume ratio of n-butanol, ethyl acetate, and trioctylamine in the n-butanol-ethyl acetate-trioctylamine mixed solvent is (88-92):(6-4):(6-4), and particularly preferably 90:5:5.

[0035] In the above method for determining the composition of surfactants in an alkanolamine solution, preferably, in step (2), when the alkanolamine solution after pH adjustment is extracted with a second organic solvent, the volume ratio of the second organic solvent to the alkanolamine solution is 1:2 to 4:1. This volume ratio is the same as the volume ratio of the second organic solvent to the alkanolamine solution used during the first extraction.

[0036] In the above method for determining the composition of surfactants in an alkanolamine solution, preferably, in step (2), the alkanolamine solution after pH adjustment is extracted with a second organic solvent 1-10 times, more preferably 1-3 times. When the number of extractions with the second organic solvent is 2 or more, the amount of the second organic solvent used each time can be the same, that is, the amount of the second organic solvent used in the first extraction can be used for each extraction.

[0037] In the above method for determining the composition of surfactants in an alkanolamine solution, preferably, in step (2), the third temperature is room temperature to 55°C, more preferably 35 to 40°C. In step (2) of the present invention, controlling the temperature at 35 to 40°C when extracting cationic and nonionic surfactants can further improve the extraction rate.

[0038] In the above method for determining the composition of surface-active substances in an alcohol amine solution, preferably, in step (2), the fourth temperature is 100-113°C, more preferably 110-112°C.

[0039] In the above method for determining the composition of surface-active substances in an alcohol amine solution, preferably, in step (2), the time for evaporating part of the second organic solvent at the fourth temperature is 1 to 24 hours.

[0040] In the above method for determining the composition of surfactants in an alkanolamine solution, preferably, in step (2), the fifth temperature is 140–163°C, more preferably 155–159°C. At this temperature, the extraction solvent in step (2) can be completely evaporated, ensuring that the analyte is not lost.

[0041] In the above method for determining the composition of surfactants in an alcohol amine solution, preferably, step (2) after evaporating to a fifth temperature to obtain all the extracts by constant weight, further includes: weighing the total amount of extracts.

[0042] In the above method for determining the composition of surface active substances in an alcohol amine solution, in step (2), the amount of the second organic solvent used to completely dissolve the obtained extract can be adjusted by those skilled in the art according to the actual situation, as long as all the extract is dissolved. The present invention does not make any special limitation in this regard.

[0043] In the above method for determining the composition of surfactants in an alkanolamine solution, preferably, in step (2), the second developing agent comprises a mixture of n-hexane-propyl ether-isopropanol-acetonitrile. More preferably, the volume ratio of n-hexane, propyl ether, isopropanol, and acetonitrile in the n-hexane-propyl ether-isopropanol-acetonitrile mixture is (45-55):(35-25):(17-13):(3-7), and particularly preferably 50:30:15:5.

[0044] In the above method for determining the composition of surfactants in an alcohol amine solution, in step (2), the liquid (i.e., the second organic solvent containing all the extracts) is spotted onto the bottom of the silica gel thin-layer plate using a capillary tube. The specific spotting operation can be the routine operation of silica gel thin-layer chromatography. For example, the spotting is generally a round dot, the diameter of the spotting is generally about 2 mm or less, the distance from the spotting baseline to the bottom can be 0.5-2 cm, and the spacing between the spots can be 0.5-2 cm.

[0045] In the above method for determining the composition of surface active substances in an alcohol amine solution, preferably, in step (2), the specific operation of placing the silica gel thin-layer plate into the chromatography tank and separating it with the second developing solvent is as follows: the silica gel thin-layer plate is placed into the second developing solvent of the chromatography tank, and the depth of the silica gel thin-layer plate immersed in the second developing solvent can be 2-5 mm from the origin (i.e., the spotting point). After developing to 14 cm (i.e., the spotting point rises from the bottom to 14 cm), the silica gel thin-layer plate is taken out and dried.

[0046] In the above method for determining the composition of surface-active substances in an alcohol amine solution, in step (2), the amount of the second organic solvent used to elute the separated components (i.e., spots) can be adjusted by those skilled in the art according to the actual situation, as long as the subsequent qualitative and / or quantitative analysis can proceed smoothly.

[0047] In the above method for determining the composition of surfactants in an alcoholic amine solution, preferably, the detection conditions of the infrared spectrometer are: wavenumber range 4000–400 cm⁻¹. -1 Resolution 1-4cm -1 The number of scans ranges from 32 to 64.

[0048] In the above method for determining the composition of surfactants in an alkanolamine solution, preferably, the detection conditions of the nuclear magnetic resonance spectrometer are as follows: 13 C-spectrum, using DEPT pulse program, observe the 180° and 90° pulse widths of the channel and the 180° and 90° pulse widths of the irradiation channel. Set the pulse modulation time τ = 1 / 2 J (J is 140 Hz), the repetition time 2s to 5s, and the inverse gated decoupled irradiation mode. Set the irradiation channel. 1 The Hθ pulses were 135°, 90°, and 45°, respectively.

[0049] In the above method for determining the composition of surfactants in an alkanolamine solution, preferably, the detection conditions of the gas chromatography-mass spectrometry (GC-MS) instrument are as follows: the chromatographic column is HP-5 (60m × 0.25mm × 0.25μm), HP-1 (60m × 0.25mm × 0.25μm), or HP-FFAP (30m × 0.25mm × 0.25μm); the column temperature program is: initial temperature 70℃, hold for 5 min, increase to 240℃ at 5℃ / min, hold for 61 min; column flow rate 1.46 mL / min; split ratio 50:1; injection port temperature 280℃; ion source is an EI source, 70 eV; ion source temperature 220℃, interface temperature 240℃; solvent delay time 2 min; scan speed 1250; scan start time 2 min, scan end time 100 min.

[0050] In the above method for determining the composition of surfactants in an alkanolamine solution, preferably, the detection conditions of the liquid chromatography-mass spectrometry (LC-MS) instrument are as follows: the chromatographic column uses a C24-type column. 18Column (2.1 mm × 150 mm × 3.5 μm); column temperature 40 °C; mobile phase: water and acetonitrile containing 6 mmol / L ammonium acetate, gradient elution, initial volume fraction of water containing 6 mmol / L ammonium acetate 70%, final volume fraction of water containing 6 mmol / L ammonium acetate 0%, initial volume fraction of acetonitrile 30%, final volume fraction of acetonitrile 100%, volume fraction of water containing 6 mmol / L ammonium acetate decreased by 5% every 5 min, volume fraction of acetonitrile increased by 5% every 5 min; mobile phase flow rate 0.3 mL / min; ionization method: electrospray ionization (ESI); scanning method: negative ion scanning; detection method: multiple reaction monitoring; ionization voltage: 4.5 kV; ion source temperature: 550 °C; curtain gas pressure: 0.5 MPa; auxiliary gas pressure: 1.0 MPa; nebulizer pressure: 1.0 MPa.

[0051] In the above-described method for determining the composition of surfactants in an alcohol amine solution, the specific analytical methods used in steps (1) and (2) for qualitative and / or quantitative analysis using an infrared spectrometer, nuclear magnetic resonance spectrometer, gas chromatography-mass spectrometry, or liquid chromatography-mass spectrometry, such as how to analyze the composition based on the spectral results and calculate the amount of the composition, are all conventional operations in the field and will not be elaborated upon in this invention.

[0052] This invention provides an essential analytical method for studying the control parameters of surfactants in amine solutions and developing removal technologies. The method of this invention can accurately determine the molecular structure and content of foaming surfactants in amine solutions, which is essential for studying the control parameters of these substances in amine solutions and their removal technologies. It is also a much-needed analytical method for monitoring the degree of amine solution contamination during natural gas purification production, and for providing early warning and prevention of amine foaming that could lead to malfunctions and shutdowns of natural gas purification units. This is of great significance for ensuring the quality and safe and stable production of natural gas purification plants, and for reducing gas purification production costs. Furthermore, there is widespread demand for this method in oil refineries, chemical plants, and other production sites that use the amine method to purify gases.

[0053] The method for determining the composition of surfactants in an alcoholic amine solution provided by this invention has the following advantages:

[0054] (1) It has a wide range of applications and can accurately measure all surfactants in amine solution, while existing methods can only measure one or a few surfactants.

[0055] (2) Applicable to the determination of unknown surfactants, while most existing methods are only applicable to the determination of surfactants with known molecular structures;

[0056] (3) The qualitative and quantitative methods are accurate. The method provided by the present invention has high selectivity for surfactants in amine solution and can eliminate the masking effect of high concentration of alcohol amine on surfactants and the mutual interference between various impurities in amine solution. However, existing methods cannot eliminate the masking effect of alcohol amine and the mutual interference between impurities, and therefore cannot accurately qualitatively and quantitatively determine surfactants in amine solution. Detailed Implementation

[0057] In order to provide a clearer understanding of the technical features, objectives and beneficial effects of the present invention, the technical solution of the present invention will now be described in detail below, but it should not be construed as limiting the scope of implementation of the present invention.

[0058] Test Example 1

[0059] Add 1000 ppm of anionic surfactant mixture to a fresh methyl diethanol solution and mix well to prepare an alcohol amine solution sample.

[0060] Accurately weigh approximately 20g of alkanolamine solution sample into 10 clean beakers, and accurately record the sample volume. Adjust the pH of the samples in the 10 beakers to 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10 respectively using an 18% hydrochloric acid aqueous solution. After transferring the samples to separatory funnels, add 20 mL of a 5:5 volumetric ratio of diethyl ether to n-butanol for extraction. Transfer the supernatant from each funnel to 10 accurately weighed weighing bottles. Then, add another 20 mL of the 5:5 volumetric ratio of diethyl ether to n-butanol for a second extraction, and transfer the supernatant to the same 10 weighing bottles according to the sample number. Dry at 29°C for 2 hours, then increase the temperature to 112°C and dry to constant weight, and weigh each bottle sequentially.

[0061] The extracts in the 10 weighing bottles were analyzed sequentially using liquid chromatography-mass spectrometry (LC-MS). The LC-MS detection conditions were as follows: the chromatographic column used was C10. 18 Column (2.1 mm × 150 mm × 3.5 μm); column temperature 40 °C; mobile phase: water and acetonitrile containing 6 mmol / L ammonium acetate, gradient elution, initial volume fraction of water containing 6 mmol / L ammonium acetate 70%, final volume fraction of water containing 6 mmol / L ammonium acetate 0%, initial volume fraction of acetonitrile 30%, final volume fraction of acetonitrile 100%, volume fraction of water containing 6 mmol / L ammonium acetate decreased by 5% every 5 min, volume fraction of acetonitrile increased by 5% every 5 min; mobile phase flow rate 0.3 mL / min; ionization method: electrospray ionization (ESI); scanning method: negative ion scanning; detection method: multiple reaction monitoring; ionization voltage: 4.5 kV; ion source temperature: 550 °C; curtain gas pressure: 0.5 MPa; auxiliary gas pressure: 1.0 MPa; nebulizer pressure: 1.0 MPa.

[0062] The results showed that no alkanolamine, diethyl ether, or n-butanol were detected in the extracts after pH adjustments to 1, 2, 3, 4, 5, and 6, indicating that the co-extraction rate of alkanolamine was 0, achieving complete separation from the anionic surfactant, and the extractants diethyl ether and n-butanol were also completely removed. However, no diethyl ether or n-butanol was detected in the extracts after pH adjustments to 7, 8, 9, and 10, but alkanolamine was detected, indicating that alkanolamine was not separated and removed.

[0063] The extraction rate of anionic surfactants in the amine solution was calculated based on the actual amount of anionic surfactants in the sample (sample amount of amine solution × 1000 ppm) and the extraction amount. The results of the extraction rate determination are shown in Table 1.

[0064] Table 1

[0065]

[0066]

[0067] The above experimental results show that when the pH value of the amine solution is adjusted to 1-6, the co-extraction rate of amine is 0, which effectively eliminates the masking effect of amine. Furthermore, the extraction rate of anionic surfactants is high, and the extraction rate is highest when the pH value is adjusted to 4.

[0068] Test Example 2

[0069] Add 1000 ppm of anionic surfactant mixture to a fresh methyl diethanol solution and mix well to prepare an alcohol amine solution sample.

[0070] Accurately weigh approximately 20g of the alkanolamine solution sample into each of eight clean beakers, and accurately record the sample volume. Then, adjust the pH of the samples in all eight beakers to 4 by adding 18% hydrochloric acid aqueous solution. Transfer the samples to separatory funnels, and then add 20 mL of the following extraction solvents to each of the eight funnels: benzene, cyclohexane, acetone, chloroform, dichloromethane, a 9:1 (v / v) mixture of diethyl ether and n-butanol, a 1:9 (v / v) mixture of diethyl ether and n-butanol, and a 5:5 (v / v) mixture of diethyl ether and n-butanol. After extraction, transfer the supernatant from each funnel to eight accurately weighed weighing bottles. Then, add 20 mL of each of the eight extraction solvents for a second extraction, transferring the supernatant to the eight weighing bottles according to the sample number. After evaporating the extraction solvent, weigh each bottle sequentially. The extracts in the above 8 weighing bottles were sequentially tested using liquid chromatography-mass spectrometry under the same testing conditions as in test example 1. No alcoholic amines or extractants were detected, indicating that the alcoholic amines were completely separated and all extractants were completely removed.

[0071] The extraction rate of anionic surfactants in the amine solution was calculated based on the actual amount of anionic surfactants in the sample (sample amount of amine solution × 1000 ppm) and the extraction amount. The results of the extraction rate determination are shown in Table 2.

[0072] Table 2

[0073]

[0074]

[0075] The above experimental results show that the extraction rate of anionic surfactants by the ether-n-butanol mixed solvent is higher than that of commonly used extractants, and the extraction rate is the highest when the ratio of ether to n-butanol (volume ratio) is 5:5.

[0076] Test Example 3

[0077] Add 1000 ppm of a mixture of cationic surfactants and 1000 ppm of a mixture of nonionic surfactants to a fresh methyl diethanol solution and mix well to prepare an alcohol amine solution sample.

[0078] Accurately weigh approximately 20g of the alkanolamine solution sample into 10 clean beakers, and accurately record the sample volume. Then, add 18% hydrochloric acid aqueous solution to adjust the pH value of the samples in the 10 beakers to 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10, respectively. After heating to 35℃, extract twice with a mixed solvent of n-butanol-ethyl acetate-trioctylamine (volume ratio 90:5:5), using 20 ml of the mixed solvent for each extraction. Transfer the supernatant from each funnel to 10 accurately weighed weighing bottles. Dry at 110℃ for 2 hours, then raise the temperature to 160℃ and dry to constant weight, and weigh each bottle sequentially.

[0079] The extracts from the 10 weighing bottles were analyzed sequentially using liquid chromatography-mass spectrometry (LC-MS) under the same conditions as in Test Example 1. After adjusting the pH to 1, 2, 3, 4, 5, and 6, no alkanolamine, n-butanol, ethyl acetate, or trioctylamine were detected in the extracts, indicating that the co-extraction rate of alkanolamine was 0, achieving complete separation from the surfactants. The extractants n-butanol, ethyl acetate, and trioctylamine were also completely removed. After adjusting the pH to 7, 8, 9, and 10, no n-butanol, ethyl acetate, or trioctylamine were detected in the extracts, but alkanolamine was detected, indicating that the alkanolamine was not completely removed.

[0080] The extraction rate of cationic and nonionic surfactants in the amine solution was calculated based on the actual amount of cationic and nonionic surfactants in the sample (sample amount of amine solution × 2000 ppm) and the extraction amount. The results of the extraction rate determination are shown in Table 3.

[0081] Table 3

[0082]

[0083]

[0084] The above experimental results show that when the pH of the amine solution is adjusted to 6-9, the extraction rates of cationic and nonionic surfactants are very high. However, when the pH is adjusted to 7-10, the amine will be co-extracted, and subsequent separation using silica gel thin-layer plates will not be complete, which will mask and interfere with the determination of surfactants. Therefore, when extracting cationic and nonionic surfactants from amine solutions, the pH should be adjusted to 5-6, preferably 6.

[0085] Example 1

[0086] This embodiment provides a method for determining the composition of surface-active substances in an alkanolamine solution, which includes the following steps:

[0087] (1) Accurately weigh approximately 40g of foaming amine solution sample from a purification plant into 5 clean beakers. Adjust the pH of the sample to 3 by adding 18% hydrochloric acid aqueous solution. Extract twice with a mixed solvent of diethyl ether and n-butanol (volume ratio of 3:7), using 40 mL of the mixed solvent for each extraction. Then dry at 27℃ for 2 hours, and then heat to 110℃ to constant weight. After cooling to room temperature, weigh the total amount of extract. Dissolve the extract completely with a small amount of the mixed solvent of diethyl ether and n-butanol, and then spot the liquid onto the bottom of a silica gel thin-layer plate using a capillary tube. The spotting is generally circular. The diameter of the spot is generally around 2 mm or less, the baseline of the spot can be 0.5-2 cm from the bottom, and the spacing between the spots can be 0.5-2 cm. Then, the silica gel thin-layer plate is placed in the chromatography tank and separated with isopropanol. The depth of the silica gel thin-layer plate immersed in isopropanol can be 2-5 mm from the origin (i.e., the spot). After it develops to 14 cm (i.e., the spot rises from the bottom to 14 cm), the silica gel thin-layer plate is removed, dried, and the separated components (spots) are eluted with a mixed solvent of diethyl ether and n-butanol. Qualitative and quantitative analysis is then performed using liquid chromatography-mass spectrometry.

[0088] The detection conditions for the liquid chromatography-mass spectrometry (LC-MS) system were as follows: the chromatographic column used was C1000 ppm. 18Column (2.1 mm × 150 mm × 3.5 μm); column temperature 40 °C; mobile phase: water and acetonitrile containing 6 mmol / L ammonium acetate, gradient elution, initial volume fraction of water containing 6 mmol / L ammonium acetate 70%, final volume fraction of water containing 6 mmol / L ammonium acetate 0%, initial volume fraction of acetonitrile 30%, final volume fraction of acetonitrile 100%, volume fraction of water containing 6 mmol / L ammonium acetate decreased by 5% every 5 min, volume fraction of acetonitrile increased by 5% every 5 min; mobile phase flow rate 0.3 mL / min; ionization method: electrospray ionization (ESI); scanning method: negative ion scanning; detection method: multiple reaction monitoring; ionization voltage: 4.5 kV; ion source temperature: 550 °C; curtain gas pressure: 0.5 MPa; auxiliary gas pressure: 1.0 MPa; nebulizer pressure: 1.0 MPa;

[0089] (2) Take five more clean beakers and accurately weigh approximately 40g of the foaming amine solution sample from the above-mentioned purification plant. Add 18% hydrochloric acid aqueous solution to adjust the pH of the sample to 6. After heating to 30°C, extract twice with a mixed solvent of n-butanol-ethyl acetate-trioctylamine (volume ratio of 88:6:6), using 40 ml of the mixed solvent for each extraction. Then bake at 112°C for 2 hours, and then heat to 159°C to constant weight. Weigh the total amount of extract. Dissolve the extract completely with a small amount of n-butanol-ethyl acetate-trioctylamine mixed solvent, and then spot the liquid onto a silica gel thin-layer plate using a capillary tube. At the bottom, the spotting method is the same as in step (1) above. Then, the silica gel thin-layer plate is placed in the chromatography tank and separated with a mixture of hexane-propane-isopropanol-acetonitrile (volume ratio of 45:35:17:3). The silica gel thin-layer plate is immersed in the hexane-propane-isopropanol-acetonitrile mixture to a depth of 2-5 mm from the origin (i.e., spotting). After it develops to 14 cm (i.e., spotting rises from the bottom to 14 cm), the silica gel thin-layer plate is removed, dried, and the separated components (spots) are eluted with a mixed solvent of n-butanol-ethyl acetate-trioctylamine. Qualitative and quantitative analysis is performed using liquid chromatography-mass spectrometry.

[0090] The detection conditions for the liquid chromatography-mass spectrometry (LC-MS) system were as follows: the chromatographic column used was C1000 ppm. 18Column (2.1 mm × 150 mm × 3.5 μm); column temperature 40 °C; mobile phase: water and acetonitrile containing 6 mmol / L ammonium acetate, gradient elution, initial volume fraction of water containing 6 mmol / L ammonium acetate 70%, final volume fraction of water containing 6 mmol / L ammonium acetate 0%, initial volume fraction of acetonitrile 30%, final volume fraction of acetonitrile 100%, volume fraction of water containing 6 mmol / L ammonium acetate decreased by 5% every 5 min, volume fraction of acetonitrile increased by 5% every 5 min; mobile phase flow rate 0.3 mL / min; ionization method: electrospray ionization (ESI); scanning method: negative ion scanning; detection method: multiple reaction monitoring; ionization voltage: 4.5 kV; ion source temperature: 550 °C; curtain gas pressure: 0.5 MPa; auxiliary gas pressure: 1.0 MPa; nebulizer pressure: 1.0 MPa.

[0091] The qualitative and quantitative measurement results of this embodiment are shown in Table 4.

[0092] Table 4

[0093]

[0094]

[0095] Example 2

[0096] This embodiment provides a method for determining the composition of surface-active substances in an alkanolamine solution, which includes the following steps:

[0097] (1) Accurately weigh approximately 50g of foaming amine solution sample from a purification plant into 5 clean beakers. Add 18% sulfuric acid aqueous solution to adjust the pH of the sample to 4. Extract twice with a mixed solvent of ether-n-butanol (volume ratio of 8:2), using 50 ml of the mixed solvent each time. Then bake at 30°C for 2 hours, and then heat to 113°C to constant weight. After cooling to room temperature, weigh the total amount of extract. Dissolve the extract completely with a small amount of mixed solvent of ether-n-butanol. Spot the liquid onto the bottom of a silica gel thin-layer plate with a capillary tube. Then place the silica gel thin-layer plate into a chromatography tank and separate with isopropanol. The specific operation of spotting and development is the same as in Example 1. Elute the separated components (spots) with a mixed solvent of ether-n-butanol and perform qualitative and quantitative analysis using liquid chromatography-mass spectrometry. The detection conditions of liquid chromatography-mass spectrometry are the same as in step (1) of Example 1.

[0098] (2) Accurately weigh approximately 50g of the foaming amine solution sample from the above-mentioned purification plant into five clean beakers. Adjust the pH of the sample to 5 by adding 18% sulfuric acid aqueous solution. After heating to 35°C, extract twice with a mixed solvent of n-butanol-ethyl acetate-trioctylamine (volume ratio 92:4:4), using 50 mL of the mixed solvent for each extraction. Then dry at 111°C for 2 hours, and then heat to 155°C to constant weight. Weigh the total amount of extract. Use a small amount of n-butanol-ethyl acetate-trioctylamine mixed solvent to completely remove the extract. After partial dissolution, the liquid was spotted onto the bottom of the silica gel thin-layer plate using a capillary tube. Then, the silica gel thin-layer plate was placed into a chromatography tank and separated using a mixture of hexane-propyl ether-isopropanol-acetonitrile (volume ratio 55:25:13:7). The specific operations for spotting and development were the same as in Example 1. The separated components (spots) were eluted with a mixed solvent of n-butanol-ethyl acetate-trioctylamine and subjected to qualitative and quantitative analysis using liquid chromatography-mass spectrometry (LC-MS). The detection conditions of LC-MS were the same as in step (2) of Example 1.

[0099] The qualitative and quantitative measurement results of this embodiment are shown in Table 5.

[0100] Table 5

[0101]

[0102]

[0103] The analytical results shown in Examples 1 and 2 demonstrate that the method of the present invention can determine the molecular structure of unknown surfactants in alkanolamine solutions; and can determine trace amounts of surfactants (mass fraction ≥ 100 × 10⁻⁶) in alkanolamine solutions. -6 The relative standard deviation can reach <3%, and it is suitable for determining trace amounts of surfactants (mass fraction <100×10⁻⁶) in amine solutions. -6 The relative standard deviation can reach <10%, indicating high precision.

[0104] Example 3

[0105] In Example 1, surfactant standards were added to the foaming alkanolamine solution of the purification plant to prepare alkanolamine solution samples: samples 1, 2, and 3 were a mixture of dodecyltrimethylammonium bromide and nonylphenol polyoxyethylene ether (n=4).

[0106] In Example 2, surfactant standards were added to the foamed amine solution of the purification plant to prepare amine solution samples: samples 4, 5, and 6 were prepared by adding a mixture of oleic acid and lauryl alcohol polyoxyethylene ether.

[0107] This embodiment provides a method for determining the composition of surface-active substances in an alkanolamine solution, which includes the following steps:

[0108] Accurately weigh approximately 25g of each of the 1, 2, and 3 alkanolamine solutions into three clean beakers. Adjust the pH of the samples to 4 by adding 18% hydrochloric acid aqueous solution. Extract twice with a mixed solvent of diethyl ether and n-butanol (volume ratio of 5:5), using 25 ml of the mixed solvent each time. Then bake at 28°C for 2 hours, and then bake at 110°C until constant weight. After cooling to room temperature, weigh the total amount of extract. Dissolve the extract completely with a small amount of the mixed solvent of diethyl ether and n-butanol. Spot the liquid onto the bottom of a silica gel thin-layer plate using a capillary tube. Then place the silica gel thin-layer plate into a chromatography tank and separate with isopropanol. The specific operation of spotting and development is the same as in Example 1. Elute the separated components (spots) with the mixed solvent of diethyl ether and n-butanol and perform qualitative and quantitative analysis using liquid chromatography-mass spectrometry. The detection conditions of the liquid chromatography-mass spectrometry are the same as in step (1) of Example 1.

[0109] Accurately weigh approximately 25g of each of the 4, 5, and 6 alkanolamine solutions into three clean beakers. Adjust the pH of the samples to 6 with 18% hydrochloric acid aqueous solution. After heating to 40°C, extract twice with a mixed solvent of n-butanol, ethyl acetate, and trioctylamine (volume ratio 90:5:5), using 25 mL of the mixed solvent for each extraction. Then, dry at 112°C for 2 hours, followed by heating to 159°C and drying to constant weight. Weigh the total amount of extract. Dissolve the extract completely in a small amount of the n-butanol-ethyl acetate-trioctylamine mixed solvent. Then, the liquid was spotted onto the bottom of the silica gel thin-layer plate using a capillary tube. The silica gel thin-layer plate was then placed into a chromatography tank and separated using a mixture of hexane-propyl ether-isopropanol-acetonitrile (volume ratio 50:30:15:5). The specific operations for spotting and development were the same as in Example 1. The separated components (spots) were eluted with a mixed solvent of n-butanol-ethyl acetate-trioctylamine and qualitatively and quantitatively analyzed using liquid chromatography-mass spectrometry (LC-MS). The detection conditions of LC-MS were the same as in step (2) of Example 1.

[0110] The qualitative and quantitative measurement results of this embodiment are shown in Tables 6 and 7.

[0111] Table 6

[0112]

[0113] Table 7

[0114]

[0115] The alkanolamine concentration in the foaming alkanolamine solution used in this embodiment is 44%, which is several thousand times higher than the concentration of surfactants. In the presence of high concentrations of alkanolamine, the method of the present invention can detect trace and ultra-trace amounts of surfactants in the alkanolamine solution, and the sample spike recovery rate can reach 90% to 110%. This shows that the method of the present invention has strong anti-interference ability and high detection sensitivity and accuracy.

[0116] The above description is only a preferred embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for determining the composition of surfactants in an alcohol amine solution, comprising the following steps: (1) Add the alkanolamine solution to the inorganic acid aqueous solution and adjust the pH of the alkanolamine solution to the first pH value; extract the alkanolamine solution after pH adjustment with the first organic solvent, then evaporate part of the first organic solvent at the first temperature, and then raise the temperature to the second temperature to evaporate to constant weight to obtain all the extract; after completely dissolving the extract with the first organic solvent, spot the liquid onto the bottom of the silica gel thin-layer plate with a capillary tube, then place the silica gel thin-layer plate into the chromatography tank and separate it with the first developing solvent, and elute the separated components with the first organic solvent; use an infrared spectrometer and nuclear magnetic resonance spectrometer. A combination of one or more of the following instruments—resonance spectroscopy, gas chromatography-mass spectrometry, and liquid chromatography-mass spectrometry—is used to perform qualitative and / or quantitative analysis of the components eluted with a first organic solvent, in order to obtain qualitative and / or quantitative analysis results of anionic surfactants in an alcoholic amine solution; wherein the first pH value is 1–6, the first organic solvent includes diethyl ether, n-butanol, or a mixture of diethyl ether and n-butanol, the first temperature is room temperature to 35°C, the second temperature is 100–113°C, and the first developing solvent includes isopropanol; (2) Add the alkanolamine solution to the inorganic acid aqueous solution to adjust the pH of the alkanolamine solution to the second pH value. Extract the alkanolamine solution after pH adjustment with the second organic solvent at the third temperature. Then, evaporate part of the second organic solvent at the fourth temperature and then raise the temperature to the fifth temperature to evaporate to constant weight to obtain all the extract. After completely dissolving the extract with the second organic solvent, spot the liquid onto the bottom of the silica gel thin-layer plate with a capillary tube. Then, place the silica gel thin-layer plate into the chromatography tank and separate it with the second developing solvent. Elute the separated components with the second organic solvent. Use an infrared spectrometer, nuclear magnetic resonance spectrometer, and gas chromatography-mass spectrometry to analyze the results. The components eluted with a second organic solvent are qualitatively and / or quantitatively analyzed using one or more of the following methods: liquid chromatography-mass spectrometry (LC-MS) instruments, to obtain qualitative and / or quantitative analysis results of cationic and nonionic surfactants in the alkanolamine solution; wherein the second pH value is 5-6, the third temperature is room temperature to 55°C, the second organic solvent includes a mixed solvent of n-butanol-ethyl acetate-trioctylamine, the fourth temperature is 100-113°C, the fifth temperature is 140-163°C, and the second developing solvent includes a mixture of n-hexane-propyl ether-isopropanol-acetonitrile.

2. The method for determining the composition of surfactants in an alkanolamine solution according to claim 1, wherein, In steps (1) and (2), the mass concentration of the inorganic acid aqueous solution is 1% to 38%.

3. The method for determining the composition of surfactants in an alkanolamine solution according to claim 2, wherein, In steps (1) and (2), the mass concentration of the inorganic acid aqueous solution is 18-19%.

4. The method for determining the composition of surfactants in an alkanolamine solution according to claim 1, wherein, In steps (1) and (2), the inorganic acid aqueous solution includes an aqueous solution of one or more of hydrochloric acid, sulfuric acid, nitric acid and phosphoric acid.

5. The method for determining the composition of surfactants in an alcoholamine solution according to claim 1, wherein, In step (1), the first pH value is 4.

6. The method for determining the composition of surfactants in an alkanolamine solution according to claim 1, wherein, In step (1), the first organic solvent is a mixture of diethyl ether and n-butanol.

7. The method for determining the composition of surfactants in an alkanolamine solution according to claim 6, wherein, In step (1), the volume ratio of diethyl ether to n-butanol in the diethyl ether-n-butanol mixed solvent is (9:1) to (1:9).

8. The method for determining the composition of surfactants in an alkanolamine solution according to claim 7, wherein, In step (1), the volume ratio of diethyl ether to n-butanol in the diethyl ether-n-butanol mixed solvent is 5:

5.

9. The method for determining the composition of surfactants in an alcoholamine solution according to claim 1, wherein, In step (1), the first temperature is 27 to 30°C.

10. The method for determining the composition of surfactants in an alkanolamine solution according to claim 1, wherein, In step (1), the second temperature is 110 to 112°C.

11. The method for determining the composition of surfactants in an alkanolamine solution according to claim 1, wherein, In step (2), the second pH value is 6.

12. The method for determining the composition of surfactants in an alkanolamine solution according to claim 1, wherein, In step (2), the volume ratio of n-butanol, ethyl acetate and trioctylamine in the n-butanol-ethyl acetate-trioctylamine mixed solvent is (88-92):(6-4):(6-4).

13. The method for determining the composition of surfactants in an alkanolamine solution according to claim 12, wherein, In step (2), the volume ratio of n-butanol, ethyl acetate and trioctylamine in the n-butanol-ethyl acetate-trioctylamine mixed solvent is 90:5:

5.

14. The method for determining the composition of surfactants in an alcoholamine solution according to claim 1, wherein, In step (2), the third temperature is 35 to 40°C.

15. The method for determining the composition of surfactants in an alkanolamine solution according to claim 1, wherein, In step (2), the fourth temperature is 110 to 112°C.

16. The method for determining the composition of surfactants in an alkanolamine solution according to claim 1, wherein, In step (2), the fifth temperature is 155 to 159°C.

17. The method for determining the composition of surfactants in an alkanolamine solution according to claim 1, wherein, In step (2), the volume ratio of hexane, propyl ether, isopropanol and acetonitrile in the hexane-propyl ether-isopropanol-acetonitrile mixture is (45-55):(35-25):(17-13):(3-7).

18. The method for determining the composition of surfactants in an alkanolamine solution according to claim 17, wherein, In step (2), the volume ratio of hexane, propyl ether, isopropanol and acetonitrile in the hexane-propyl ether-isopropanol-acetonitrile mixture is 50:30:15:5.

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