Method for rapidly determining sodium content in MTO-grade methanol

Abstract

The invention provides a method for rapidly determining the content of sodium in MTO-grade methanol, which comprises the following steps: adding ultrapure water into a methanol solution to be detected, and uniformly stirring to obtain a mixed solution in which the volume content of methanol is 50-70%; and detecting the sodium content in the mixed solution by adopting a flame atomic emission spectrometer. The determination method disclosed by the invention is high in speed, the matrix effect is reduced, the process of heating and evaporating a methanol sample is avoided, and the analysis time of a single sample is shortened from 1 hour to 3 minutes. And moreover, the loss of the methanol sample in the evaporation process is eliminated, and the detection accuracy is improved.

Description

Technical Field

[0001] The present invention belongs to the field of analytical technologies, and particularly relates to a method for rapidly determining the sodium content in MTO-grade methanol. Background Art

[0002] In the production process of methanol to olefins, in order to prevent sodium from causing catalyst poisoning, it is required to control the sodium content in MTO-grade methanol (water content is about 4% (v / v)). Since sodium is an alkali metal element that is easily ionized, and its melting point and boiling point are not high, common methods such as flame photometry, inductively coupled plasma emission spectrometry, and atomic absorption spectrometry are used for analysis. The detection method is to first transfer the methanol sample to a quartz beaker, slowly heat it to dryness in a water bath until the methanol sample solution is almost evaporated to dryness, take out the beaker and cool it, add 5% (V / V) nitric acid solution, then slowly boil it on a hot plate for 10 minutes, take down the beaker and cool it, and make up the volume to 25 mL with distilled water in a volumetric flask, and then use an instrument to measure after shaking well. The time required to detect a single sample by this method is about 1 hour, and the detection efficiency is low.

[0003] Flame atomic emission spectrometry uses a flame as the excitation light source for the sample, and has the advantages of low detection limit, high precision, small matrix interference effect, and convenient detection, and is suitable for qualitative and quantitative analysis of alkali metals and alkaline earth metals. Sodium belongs to the atoms of elements with low excitation energy levels and relatively simple spectral lines. When the outer electrons of gaseous sodium atoms are excited and transition from a high energy level to a lower energy level, the excess energy released generates light radiation, which can be measured by flame atomic emission method. Summary of the Invention

[0004] In order to solve the above technical problems, the invention object of the present invention is to provide a method for rapidly determining the sodium content in MTO-grade methanol, which greatly shortens the detection time and improves the work efficiency.

[0005] To achieve the above object, the present invention provides the following technical solutions:

[0006] A method for rapidly determining the sodium content in MTO-grade methanol, which comprises the following steps:

[0007] (1) Add ultrapure water to the methanol solution to be detected, stir evenly to obtain a mixed solution, wherein the volume content of methanol in the mixed solution is 50 - 70%;

[0008] (2) Use a flame atomic emission spectrometer to detect the sodium content in the above mixed solution.

[0009] In a further aspect, the sodium content in the methanol solution in step (1) is 0 - 0.4 mg / L.

[0010] In a further aspect, the methanol solution in step (1) is MTO-grade methanol for methanol to olefins.

[0011] Further solution: In step (2), the lower detection limit is 2.8 μg / L, and the relative standard deviation RSD < 3%.

[0012] Further solution: In step (2), the flame atomic emission spectrometer is an atomic absorption spectrometer equipped with a premixed rotatable burner head and an adjustable nebulizer, with a wavelength of 589.0 nm, a slit of 0.2 nm, a burner height of 13.5 mm, an air flow rate of 13.5 L / min, and an acetylene flow rate of 2.0 L / min.

[0013] Further solution: In step (2), the result of the detection is multiplied by 2 to obtain the sodium content in the methanol solution.

[0014] To improve the detection efficiency of the sodium content in coal chemical MTO-grade methanol, the present invention establishes a flame atomic emission spectrometry method based on the atomic emission function of a flame atomic absorption spectrometer. When using this method to determine the sodium content in MTO-grade methanol, the sample pretreatment is simple. It only needs to be diluted with ultrapure water and then shaken well for direct determination. In the range of 0 - 0.40 mg / L of sodium content, the linear relationship is good, the correlation coefficient is above 0.999, the lower detection limit is 2.8 μg / L, the relative standard deviation RSD < 3%, and the precision and accuracy meet the requirements.

[0015] The determination method of this application is fast, avoiding the process of heating and evaporating the methanol sample. The analysis time of a single sample is shortened from 1 hour to 3 minutes, reducing the matrix effect. And it eliminates the loss of the methanol sample during the evaporation process, improving the detection accuracy. Specific embodiments

[0016] The following further illustrates the technical solutions of the present invention in conjunction with specific embodiments.

[0017] The reagents and materials used in each embodiment are all commercially available products. The required pure water meets the regulations of secondary water in GB / T 6682. The reagents are all of analytical reagent grade, and the sodium content in water and reagents should be low enough to be negligible.

[0018] Example 1:

[0019] (1) Sampling: Take the MTO-grade methanol solution A to be detected according to the technical requirements specified in GB / T 6680, and then store it in a polyethylene plastic bottle;

[0020] (2) Add ultrapure water to the methanol solution A to be detected and stir evenly to obtain a mixed solution, where the volume content of methanol in the mixed solution is 50%;

[0021] (3) The sodium content in the above-mentioned mixed solution was detected using a flame atomic emission spectrometer, and the detection result was multiplied by 2 to obtain the sodium content in the methanol solution; the flame atomic emission spectrometer was an atomic absorption spectrometer equipped with a premixed rotatable burner head and an adjustable nebulizer, with a wavelength of 589.0 nm, a slit of 0.2 nm, a burner height of 13.5 mm, an air flow rate of 13.5 L / min, and an acetylene flow rate of 2.0 L / min.

[0022] Comparative Example 1:

[0023] (1) Sampling: The MTO-grade methanol solution A to be detected was taken according to the technical requirements specified in GB / T 6680, and then stored in a polyethylene plastic bottle.

[0024] (2) The MTO-grade methanol solution A was transferred to a quartz beaker and slowly evaporated to dryness in a water bath until the methanol sample solution was almost evaporated to dryness. After taking out the beaker and cooling it;

[0025] (3) The substance after evaporation to dryness was added to 5% (V / V) nitric acid solution, and then slowly boiled on a hot plate for 10 minutes. After taking down the beaker and cooling it, it was made up to the mark with distilled water in a 25 mL volumetric flask.

[0026] (4) After shaking well, the sodium content was determined using an atomic absorption spectrometer or an inductively coupled plasma mass spectrometer.

[0027] Example 2:

[0028] (1) Sampling: The MTO-grade methanol solution B to be detected was taken according to the technical requirements specified in GB / T 6680, and then stored in a polyethylene plastic bottle.

[0029] (2) Ultra-pure water was added to the methanol solution B to be detected and stirred evenly to obtain a mixed solution, where the volume content of methanol in the mixed solution was 60%.

[0030] (3) The sodium content in the above-mentioned mixed solution was detected using a flame atomic emission spectrometer, and the detection result was multiplied by 2 to obtain the sodium content in the methanol solution; the flame atomic emission spectrometer was an atomic absorption spectrometer equipped with a premixed rotatable burner head and an adjustable nebulizer, with a wavelength of 589.0 nm, a slit of 0.2 nm, a burner height of 13.5 mm, an air flow rate of 13.5 L / min, and an acetylene flow rate of 2.0 L / min.

[0031] Comparative Example 2:

[0032] Same as Comparative Example 1, the only difference being that the MTO-grade methanol solution to be detected was the MTO-grade methanol solution B in Example 2.

[0033] Example 3:

[0034] (1) Sampling: Take the MTO-grade methanol solution C to be detected according to the technical requirements specified in GB / T 6680, and then store it in a polyethylene plastic bottle.

[0035] (2) Add ultrapure water to the methanol solution C to be detected and stir evenly to obtain a mixed solution, where the volume content of methanol in the mixed solution is 70%.

[0036] (3) Use a flame atomic emission spectrometer to detect the sodium content in the above mixed solution, and multiply the detection result by 2 to obtain the sodium content in the methanol solution; among them, the flame atomic emission spectrometer is an atomic absorption spectrometer equipped with a premixed rotatable burner head and an adjustable nebulizer, with a wavelength of 589.0 nm, a slit of 0.2 nm, a burner height of 13.5 mm, an air flow rate of 13.5 L / min, and an acetylene flow rate of 2.0 L / min.

[0037] Comparative Example 3:

[0038] Same as Comparative Example 1, the only difference is that the MTO-grade methanol solution to be detected is the MTO-grade methanol solution C in Example 2.

[0039] Examples 4-5 are the same as Example 1, the only difference is that the MTO-grade methanol solutions to be detected are TO-grade methanol solutions D, E, and F.

[0040] Comparative Examples 4-5 are the same as Comparative Example 1, the only difference is that the MTO-grade methanol solutions to be detected are the MTO-grade methanol solutions D, E, and F in Examples 4-6.

[0041] The detection results of the above Examples 1-6 and Comparative Examples 1-6 are as follows:

[0042]

[0043]

[0044] As can be seen from the above table, there is no significant difference in the detection results between Example 1 and Comparative Example 1, but the detection time is greatly shortened.

[0045] Verification Example 1:

[0046] The preparation steps of the sodium standard solution are specifically as follows:

[0047] Accurately pipette 1.0 mL of a sodium stock solution with a content of 1000 mg / L into a 100 mL volumetric flask, and make up the volume with pure water (meeting the requirements of secondary water in GB / T 6682) to obtain an intermediate solution with a sodium content of 10 mg / L. Similarly, use a pipette to separately transfer 0, 1.0, 2.0, 3.0, and 4.0 mL of the sodium intermediate solution into 100 mL volumetric flasks and make up the volume with pure water to prepare sodium working standard solutions with concentrations of 0.0, 0.1, 0.2, 0.3, and 0.4 mg / L respectively.

[0048] Take the above-prepared sodium standard solutions (0.0, 0.1, 0.2, 0.3, 0.4 mg / L), make up the volume with ultrapure water and then measure them on a spectrometer using the same flame atomic emission spectrometer as in Example 1; among them, the flame atomic emission spectrometer is an atomic absorption spectrometer equipped with a premixed rotatable burner head and an adjustable nebulizer, with a wavelength of 589.0 nm, a slit of 0.2 nm, a burner height of 13.5 mm, an air flow rate of 13.5 L / min, and an acetylene flow rate of 2.0 L / min.

[0049] After detection, the linear regression equation of sodium is Y = 2.406X - 0.02043, and its linear correlation coefficient r = 0.9997. That is, the linear range of sodium content in MTO-grade methanol solution is 0 - 0.4 mg / L.

[0050] Verification Example 2:

[0051] Select three identical MTO-grade methanol samples according to the method of Comparative Example 1, and successively prepare samples with water contents of 50%, 30%, 15%, and 4% (v / v) with ultrapure water. Take three samples for each, totaling 12 experimental samples; then heat and evaporate them to dryness, dissolve them with 5% (V / V) nitric acid solution, boil for 10 minutes and then make up the volume; finally, measure the initial value of their sodium content;

[0052] Take 100 mL of each of the above experimental samples, and add 1.0, 1.5, and 2.0 mL (spiking amount) of the sodium standard solution (sodium concentration is 10 mg / L) prepared in Verification Example 1 to them respectively, resulting in a total of 36 spiked samples.

[0053] Then the theoretical calculated value (mg / L) of the sodium concentration after spiking is calculated according to the formula u = (C1×V1 + C0×V0) / (V1 + V0)), and the spiking recovery rate (%) is calculated according to the formula P = ((C2×V2 - C1×V1) / (C0×V0))×100%; where C1 is the initial value of the sodium concentration before spiking, V1 is the volume of the solution before spiking; C2 is the measured value of the spiked sodium concentration, V2 is the volume of the solution after spiking; C0 is the sodium concentration of the added standard sample, V0 is the volume of the added standard sample. Continuously measure 5 times under the same conditions and take the average value to statistically analyze the recovery rate of sodium. The measurement results are shown in the following table:

[0054]

[0055]

[0056] As can be seen from the above table, when the water content in MTO-grade methanol is 50% and 30% (V / V), that is, when the methanol content in the methanol aqueous solution is 50 - 70% (V / V), the recovery rate is between 98.0% and 104.8%, indicating that the matrix effect is not obvious and meets the requirements of quantitative analysis;

[0057] When the water content is 15% and 4% (V / V), the recovery rate is between 130.2% and 158.9%, far greater than 120%, and the matrix effect is obvious, so it cannot be directly analyzed.

[0058] It can also be seen from the above table that for MTO-grade methanol with a water content of about 4% (V / V), it can be directly analyzed when the methanol content is about 50% after being diluted twice with ultrapure water.

[0059] Randomly select the sample No. 12 after spiking in the above table, divide it into 9 parts, dilute each part with ultrapure water at a ratio of 1:1 and then measure. Take twice the value shown by the instrument as the measured value of the sample, compare it with its theoretical calculated value, and continuously measure these 9 samples. The measurement results are shown in the following table:

[0060]

[0061] According to the t-test method, compare the measured value of the sample with the theoretical calculated value, and calculate according to the formula where is the average value of the measured values of the sample, μ is the theoretical calculated value, s is the standard deviation of the measured values of the sample, and n is the number of measurements. The calculated t 计 value is 0.98, while the critical value t 0.05 is 2.31 (α = 0.05), and t 计 < t 0.05 , indicating that there is no significant difference between the measured average value and the theoretical calculated value, that is, the measurement method of the present invention does not cause systematic errors.

[0062] Verification Example 3:

[0063] Randomly select two samples to be measured, and continuously perform 11 measurements under the steps and the same conditions as in Example 1 to examine its precision. The test results are shown in Table 4 below, where the RSDs are 2.75% and 2.47% respectively, meeting the measurement requirements (<3%).

[0064]

[0065] Verification Example 4:

[0066] The sodium content in pure water was detected 12 times using the flame atomic emission spectrometer in Example 1. The detection limit (LOD) was estimated by three times the standard deviation. The test results are shown in the following table, where the LOD is 2.8 μg / L.

[0067]

[0068] From the above analysis, it can be seen that the present invention directly dilutes and determines the sodium content in MTO-grade methanol by flame atomic emission spectrometry, without the need to evaporate methanol to dryness and then make an aqueous solution for determination. The analysis time for a single sample is shortened from the original 1 hour to 3 minutes. The operation is convenient, the instrument maintenance is simple, the detection limit is low, the accuracy is high, the precision is good, and the consumption of contaminated reagents is small. It is very suitable for quality control analysis in the production process of MTO-grade methanol and has reference value for analyzing the potassium content in methanol using this method.

[0069] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A method for rapidly determining the sodium content in MTO grade methanol, characterized in that: The following steps are involved: (1) adding ultrapure water to the methanol solution to be tested, stirring evenly to obtain a mixed solution, wherein the volume content of methanol in the mixed solution is 50-70%; (2) The sodium content in the mixed solution is detected by flame atomic emission spectrometry.

2. The rapid determination method according to claim 1, characterized in that: The sodium content in the methanol solution in step (1) is 0-0.4 mg / L.

3. The rapid determination method according to claim 1, characterized in that: The methanol solution in step (1) is MTO grade methanol from methanol to olefins.

4. The rapid determination method according to claim 1, characterized in that: The detection limit in step (2) was 2.8 μg / L, and the relative standard deviation RSD was <3%.

5. The rapid determination method according to claim 1, characterized in that: The flame atomic emission spectrometer in step (2) is an atomic absorption spectrometer equipped with a premixing rotatable burner head and an adjustable atomizer, with a wavelength of 589.0 nm, a slit of 0.2 nm, a burner height of 13.5 mm, an air flow rate of 13.5 L / min, and an acetylene flow rate of 2.0 L / min.

6. The rapid determination method according to claim 1, characterized in that: The result detected in step (2) multiplied by 2 is the sodium content in the methanol solution.

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