On-line detection method for nitration quality in nitration step of nitrocellulose production process

Abstract

The invention discloses an online detection method for nitration quality of a nitration process in a nitrocellulose production process. The method adopts a back-preparation method to obtain an acid-containing cotton modeling sample, and simultaneously obtains high-precision standard values ​​of the sample nitrogen content and acid value, adopts a near-infrared spectrometer to obtain the sample near-infrared spectrum, and adopts a chemometric method to associate the spectra of a series of acid-containing nitrocellulose modeling samples with the standard values ​​of nitrogen content and acid value, respectively, to establish an analysis model. The nitrogen content and acid value of the sample to be tested are obtained by using the analysis model and the near-infrared spectrum of the sample to be tested. The invention can quickly, accurately and non-destructively detect the nitrogen content and acid value of acid-containing cotton in the production process, predict the nitration quality of the nitration process in the production process, and prejudge the nitrogen content of the product in advance; the invention shortens the detection time of the nitration process from the original 3h to 5h to within 2min, greatly improving the production efficiency; the invention has simple equipment, convenient and safe operation, no pollution, can save labor, and improve the safety of the analysis process.

Description

Technical Field

[0001] The invention belongs to the technical field of quality detection and control in the production process of explosives and firearms, and relates to an online detection method in the production process of nitrocellulose, a dual-use chemical material for military and civilian use. Background Art

[0002] Nitrocellulose, also known as nitrocellulose, has a wide range of applications in both military and civilian applications due to its good mechanical properties, compatibility and solubility. According to different application fields, nitrocellulose is divided into military nitrocellulose and civilian nitrocellulose. In the military industry, nitrocellulose is often used in smokeless gunpowder, blasting glue, explosives, single-base and double-base guns, and rocket boosters; in civilian applications, nitrocellulose is mainly used as an adhesive or film-forming agent in the manufacturing process of coatings, inks, and paints.

[0003] The production principle of nitrocellulose is that the hydroxyl groups in the cellulose molecular structure react with the nitric acid in the nitrating agent to undergo an esterification reaction. The manufacturing process of nitrocellulose consists of several continuous steps, including the preparation of cellulose and the preparation of nitric acid and sulfuric acid, the nitration of cellulose and the stabilization of nitrocellulose. The nitration process is the key process in the production of nitrocellulose. The nitration reaction is a reversible reaction process. The nitration process is accompanied by side reactions such as denitrification and oxidation. With the increase of water and sulfuric acid content in the nitration system and the decrease of nitric acid concentration, the side reactions will intensify. Sulfate esters with poor stability will be formed during the reaction, which will reduce the nitrogen content of the nitrocellulose. Therefore, the adjustment and control of the process is based on the nitration quality. The basis for judging the nitration quality is the test results of the nitrogen content and acid value of the acid-containing cotton samples in the nitration process. The degree of nitration is judged by the nitrogen content, the product quality is predicted, and the decision to continue nitration is made. The acid value is used to determine whether the nitration system needs to be adjusted. It can be seen that the rapid online detection of the nitrogen content and acid value of acid-containing nitrocellulose in the nitration process is an important parameter indicator for process quality detection, which is directly related to the quality of the product.

[0004] Usually, in the quality inspection process of acid-containing nitrocellulose, the acid value test is to soak the sample repeatedly in water, collect the solution, and titrate it by acid-base titration; the nitrogen content test is to first wash the sample with water, boil it with alkali, wash it with water, etc., treat it to neutral, and then dry it in an oven to determine the nitrogen content. There are many methods for determining the nitrogen content of nitrocellulose, including the five-tube nitrogen meter method, ferrous iron-ferrous chloride method, Dewald alloy reduction method, interferometer method, etc. The five-tube nitrogen meter method and ferrous iron-ferrous chloride method have been eliminated due to environmental pollution, cumbersome operation, and long analysis time (more than ten hours). At present, the interferometer method and near-infrared spectroscopy method are more commonly used. Since these two methods have short detection time, but the pretreatment process is still very long, the detection time is 3-5h. However, the nitration process is relatively fast, and the lag results of several hours still cannot meet the needs of rapid online detection. In addition, nitrocellulose itself is flammable and explosive, and the stability of acid-containing nitrocellulose is worse, and it is easy to catch fire, so there are certain safety risks in the analysis process of these detection methods.

[0005] In order to solve the above problems, domestic and foreign literatures have also reported a variety of methods for determining the nitrogen content of nitrocellulose, such as chromatography, calorimetry, salicylic acid-titanium method, ferrous sulfate method, elemental instrument method, etc. However, these methods have not been promoted due to defects such as cumbersome operation process or large errors. The method of measuring the nitrogen content of non-water-containing nitrocellulose by near-infrared spectroscopy reported in the patent "Method for online measurement of nitrogen content of nitrocellulose using near-infrared diffuse reflectance spectroscopy" has greatly improved the analysis speed compared with other methods, but the object of analysis is dry nitrocellulose, and the pre-treatment method of sample analysis is still the oven drying method, which is time-consuming. The patent "Rapid non-destructive detection method of nitrogen content and moisture in the fine washing and mixing process of nitrocellulose production process" uses near-infrared spectroscopy to directly determine the moisture and nitrogen content of water-containing nitrocellulose, which solves the problem of rapid detection of all water-containing nitrocellulose in the production process and realizes online detection of fine washing and mixing processes. Therefore, the detection of nitrogen content and acid value of acid cotton in the nitration process of nitrocellulose production still has long detection time and potential safety hazards, and cannot meet the requirements of rapid online detection of process quality inspection, resulting in low production efficiency and unstable product quality.

[0006] After searching domestic and foreign literature, no literature report on the online detection method of nitration quality in the nitration step of nitrocellulose production process was found. Summary of the invention

[0007] In view of the deficiencies in the prior art, the purpose of the present invention is to provide a method for online detection of nitration quality in the nitration step of a nitrocellulose production process, so as to solve the problems that the quality detection technology of acid-containing nitrocellulose in the nitration step of the nitrocellulose production process cannot meet the requirements of rapid online detection of process quality, resulting in low production efficiency and unstable product quality.

[0008] In order to solve the above technical problems, the present invention adopts the following technical solutions to achieve the above problems:

[0009] An online detection method for nitration quality in a nitration process of a nitrocellulose production process, the method using a near infrared spectrometer can simultaneously and rapidly non-destructively determine the nitrogen content and acid value of the acid-containing nitrocellulose in the nitration process, the method comprising the following steps:

[0010] Step 1, prepare a series of acid-containing nitrocellulose modeling samples:

[0011] Select a series of water-containing nitrocellulose samples with different nitrogen contents, and keep the water content greater than 20%; soak each water-containing nitrocellulose sample in different nitric acid and sulfuric acid mixed solutions, stir, soak, filter, and centrifuge to obtain a series of acid-containing nitrocellulose modeling samples;

[0012] Step 2, spectrum collection of acid-containing nitrocellulose modeling sample:

[0013] The obtained acid-containing nitrocellulose modeling sample is immediately placed in a sample cup with a low-hydroxyl quartz bottom, compacted with a wooden pestle, and a near-infrared spectrometer is used to perform a spectrum scan from the bottom of the sample cup to obtain a near-infrared spectrum of the acid-containing nitrocellulose modeling sample;

[0014] Step 3, determination of the standard value of the acid-containing nitrocellulose modeling sample:

[0015] The standard value of nitrogen content in acid-containing nitrocellulose modeling samples adopts the nitrogen content of its corresponding raw material, water-containing nitrocellulose; the standard value of acid value in acid-containing nitrocellulose modeling samples adopts acid-base titration method and is calculated based on nitric acid content;

[0016] Step 4: Establishment of analysis model:

[0017] The spectra of a series of acid-containing nitrocellulose modeling samples were correlated with the standard values ​​of nitrogen content and acid value by using chemometric methods, and a near-infrared analysis model of nitrogen content and acid value of acid-containing nitrocellulose was established. Based on this model, the nitrogen content and acid value of the produced samples to be tested were tested.

[0018] Step 5, detection of nitrogen content and acidity of the sample to be tested:

[0019] The acid-containing nitrocellulose sample taken from the nitration process of the production line is immediately placed in a sample cup with a low-hydroxyl quartz bottom, compacted with a wooden pestle, and spectrally scanned from the bottom of the sample cup with a near-infrared spectrometer to obtain a near-infrared spectrum of the acid-containing nitrocellulose; the near-infrared spectrometer and spectrally scanning conditions used are the same as those for collecting the spectrum of the modeling sample; based on the spectrum of the acid-containing nitrocellulose sample to be tested and the established near-infrared analysis model, the nitrogen content and acidity of the nitrocellulose sample to be tested are obtained.

[0020] The present invention also includes the following technical features:

[0021] Specifically, the difference between the maximum and minimum nitrogen contents of the series of acid-containing nitrocellulose modeling samples is greater than 1.2%, including the nitrogen content distribution range of the samples to be tested, and the samples are evenly distributed within the nitrogen content range.

[0022] Specifically, in step 1, the moisture content is 20-30%.

[0023] Specifically, the content of nitric acid in the nitric-sulfur mixed acid solution is 80% to 85%, and the solution temperature is 15 to 25°C.

[0024] Specifically, the stirring and soaking time is 5-8 minutes.

[0025] Specifically, the near-infrared spectrometer is a Fourier transform type, a grating type, a CCD type or a filter type, and the spectral wavelength range includes 6050cm -1 ~5500cm -1(1653nm~1818nm), 4650cm -1 ~4200cm -1 One or two of (2150nm~2381nm).

[0026] Compared with the prior art, the present invention has the following technical effects:

[0027] The present invention breaks through conventional thinking, adopts a reverse preparation method to prepare acid-containing cotton, directly uses the nitrogen content of water-containing cotton as the standard value of the nitrogen content of the corresponding acid-containing cotton, establishes an analysis model, and achieves good results.

[0028] The invention is simple to operate, safe, and pollution-free, and has the advantages of fast, non-destructive, and accurate detection. It can simultaneously detect the nitrogen content and acid value of acid-containing cotton in the production process, shortening the detection time from the original 4h to 6h to within 2min, and greatly improving the production efficiency. DETAILED DESCRIPTION

[0029] At present, the samples in the nitrocellulose production process are directly sampled from the nitration process of the production line, and are centrifuged and sent for inspection as acid-containing nitrocellulose samples to be tested. The samples to be tested adsorb a large amount of nitric acid, sulfuric acid and water. During the analysis and detection, water washing, alkali washing, and water washing are repeatedly performed to remove residual acids such as nitric acid and sulfuric acid, and the samples are treated to be neutral, and then dried to remove moisture. The nitrogen content in the dry nitrocellulose is determined by the interferometer method. This method takes about several hours to detect a sample, and the detection error range is about 0.3% to 1.0%, while the nitrogen content detection error of nitrocellulose products is required to be no more than 0.05%. Therefore, this quality detection method has a long cycle and large errors, and cannot meet the technical requirements of high-precision and rapid online detection in the production process. Therefore, the current test results can only be used as a reference for quality control of the production process, and cannot be used as a timely and accurate guide for the judgment of the quality reliability of the production process. It can be seen from this that the conventional modeling method of collecting samples online as modeling samples and testing them as standard values ​​according to the original method is not feasible.

[0030] Based on near-infrared spectroscopy technology, rapid, safe, and online detection can be achieved. The design idea of ​​the present invention is to use near-infrared spectroscopy technology to achieve a method for rapid online detection of nitrogen content and acid value of acid-containing nitrocellulose in the nitration process of nitrocellulose production. The core of this technology is to establish a near-infrared spectroscopy analysis model. The establishment of the analysis model is to process the spectrum of the modeling sample using a variety of algorithms, use metrological methods to associate it with the corresponding standard value, and construct a mathematical relationship between the sample spectrum and the standard value; then, through the established analysis model and the spectrum of the sample to be tested, the nitrogen content and acid value of the acid-containing nitrocellulose sample to be tested can be directly calculated, so as to achieve rapid online detection of the quality of the nitration process and provide a basis for quality control of the nitration process.

[0031] The establishment of an analytical model is the core and difficulty of near-infrared spectroscopy technology. The quality of the model is determined by the representativeness of the modeling samples, the accuracy of the standard values, and the scientific nature of data processing. Among them, the design and preparation of modeling samples and the acquisition of standard values ​​are the key technologies for establishing an acid-containing cotton model.

[0032] Specific embodiments of the present invention are given below. It should be noted that the present invention is not limited to the following specific embodiments, and all equivalent changes made on the basis of the technical solution of this application fall within the protection scope of the present invention.

[0033] Embodiment 1:

[0034] This embodiment provides a method for online detection of nitration quality in the nitration process of nitrocellulose production. The method uses a near-infrared spectrometer to simultaneously and rapidly non-destructively determine the nitrogen content and acid value of the acid-containing nitrocellulose in the nitration process, and comprises the following steps:

[0035] 1. Prepare a series of acid-containing nitrocellulose modeling samples

[0036] 40 wet nitrocellulose products with different nitrogen contents were selected. The moisture content ranged from 20% to 30%. The nitrogen content range was wide, with the difference between the maximum and minimum values ​​being 1.27%, and was evenly distributed within the nitrogen content range.

[0037] Take 20g of each sample, soak it in nitric acid and sulfuric acid mixed solution of different concentrations, stir and soak for 5-8min, filter and centrifuge to obtain a series of acid-containing cotton standard samples. The nitric acid content in the nitric acid and sulfuric acid mixed solution is 80-85%, and the solution temperature is maintained at 15-25°C.

[0038] 2. Spectral collection of modeling samples

[0039] Select a Fourier transform near-infrared spectrometer and set the instrument resolution to 8 cm -1 , spectral range 4000cm -1 ~12500cm -1 . Take 5g of acid-containing cotton modeling, immediately put it into a sample cup with a diameter of 5cm and a low-hydroxyl quartz bottom, press it tightly with a wooden pestle, and the thickness is greater than 2cm. Collect the spectrum in diffuse reflectance mode. Each sample is measured 3 times, and each spectral line is scanned 64 times.

[0040] 3. Determination of standard value of modeling samples

[0041] The standard value of nitrogen content of the acid-containing cotton standard sample adopts the nitrogen content of its corresponding raw material, the water-containing nitrocellulose, and the detection method is based on GJB 337-1987 "Determination of nitrogen content of nitrocellulose (interferometer method)"; the standard value of acid value of the acid-containing cotton standard sample is determined by acid-base titration and calculated by nitric acid content. The standard value range of nitrogen content of the prepared modeling sample is 11.91-13.18%, and the standard value range of acid value is 15-38%.

[0042] 4. Establishment of analysis model

[0043] A total of 80 modeling samples were prepared according to the above method, and spectra and standard values ​​were obtained. According to the nitrogen content and acid value, 60 samples that met the requirements of both wide distribution and uniformity were selected, and their spectra and standard values ​​were used as the modeling sample database.

[0044] First, the spectrum of the modeling sample is spectrally preprocessed, and the quantitative range is 6050cm -1 ~5500cm -1 and 4650cm -1 ~4200cm -1 Then, the pre-processed spectral data were associated with the standard values ​​of nitrogen content and acid value using partial least squares (PLS) respectively. By optimizing the spectral pre-processing method and other conditions, the near-infrared analysis model of nitrogen content and acid value of acid-containing cotton was established. Optimization results: The combination of the best spectral pre-processing method is data centering + smoothing + first-order derivative; the determination coefficient (R 2 ) is 0.978(R 2 The closer it is to 1, the better the linear relationship between the predicted value and the standard value of the model is). The error between the predicted value and the standard value is very small. It can be seen that the constructed model has good linearity and high accuracy.

[0045] 5. Validation of near infrared spectroscopy analysis method

[0046] Eight batches of acid-containing cotton samples were drawn from the production line, and the nitrogen content and acid value were determined using this model. The original method was also used for determination. Since the parallel error of the original method was large, in order to reduce the error, the average value of 6 determination results was taken for each sample. The determination results of the two methods are shown in Table 1.

[0047] Table 1 Test results of 8 acid-containing cotton samples

[0048]

[0049]

[0050] Embodiment 2:

[0051] This embodiment provides a method for online detection of nitration quality in a nitration step of a nitrocellulose production process, comprising the following steps:

[0052] 1. Prepare a series of acid-containing nitrocellulose modeling samples

[0053] 45 columns of water-containing nitrocellulose products with different nitrogen contents were selected. The moisture content ranged from 20% to 30%, the nitrogen content range was wide, the difference between the maximum and minimum values ​​was 1.35%, and the nitrogen content was evenly distributed within the range.

[0054] Take 20g of each sample, soak it in different nitric acid and sulfuric acid mixed solutions, stir and soak for 5-8min, filter and centrifuge to obtain a series of acid-containing cotton standard samples. The nitric acid content in the nitric acid and sulfuric acid mixed solution is 80-85%, and the solution temperature is maintained at 15-25°C.

[0055] 2. Spectral collection of modeling samples

[0056] Select a filter-type near-infrared spectrometer and set the instrument resolution to 12.5 nm and the spectral range to 4000 cm -1 ~12500cm -1 . Take 5g of acid-containing cotton modeling, immediately put it into a sample cup with a diameter of 5cm and a low-hydroxyl quartz bottom, press it tightly with a wooden pestle, and the thickness is greater than 2cm. Collect the spectrum in diffuse reflectance mode. Each sample is measured 3 times, and each spectral line is scanned 64 times.

[0057] 3. Determination of standard value of modeling samples

[0058] The standard value of nitrogen content of the acid-containing cotton standard sample adopts the nitrogen content of its corresponding raw material, the water-containing nitrocellulose, and the detection method is based on GJB 337-1987 "Determination of nitrogen content of nitrocellulose (interferometer method)"; the standard value of acid value of the acid-containing cotton standard sample is determined by acid-base titration and calculated by nitric acid content. The standard value range of nitrogen content of the prepared modeling sample is 11.80-13.15%, and the standard value range of acid value is 20-45%.

[0059] 4. Establishment of analysis model

[0060] A total of 60 modeling samples were prepared according to the above method, and spectra and standard values ​​were obtained. According to the requirements of wide distribution and uniformity of nitrogen content and acid value, 51 were selected for model establishment.

[0061] The partial least squares (PLS) method was used to associate the preprocessed spectral data of the modeled samples with the standard values ​​of nitrogen content and acid value, and the near-infrared analysis model of nitrogen content and acid value of acid-containing cotton was established by optimizing the spectral quantitative range of the modeled samples and the spectral preprocessing method. The optimization results showed that the optimal spectral quantitative range was 1653nm~1818nm, and the optimal spectral preprocessing method combination was data centralization + smoothing + first-order derivative; the determination coefficient (R 2 ) is 0.966(R 2 The closer it is to 1, the better the linear relationship between the predicted value and the standard value of the model is). The error between the predicted value and the standard value is very small. It can be seen that the constructed model has good linearity and high accuracy.

[0062] 5. Validation of near infrared spectroscopy analysis method

[0063] Eight batches of acid-containing cotton samples were drawn from the production line, and the nitrogen content and acid value were determined using this model. The original method was also used for determination. Since the original method had a large parallel error, in order to reduce the error, the average of six determination results was taken for each sample. The determination results of the two methods are shown in Table 2.

[0064] Table 2 Test results of 8 acid-containing cotton samples

[0065]

[0066] The present invention obtains an acid-containing nitrocellulose modeling sample by using a method for preparing acid-containing nitrocellulose with water-containing nitrocellulose. Since the concentration of the mixed acid used is consistent with the concentration of the process residual acid, no reaction occurs in a short time, that is, the nitrogen content in the pure nitrocellulose remains unchanged and is consistent with the nitrogen content of the water-containing cotton used. Since the error of the nitrogen content detection method of the water-containing cotton is not greater than 0.05%, and the detection error of the nitrogen content of the acid-containing cotton is greater than 1.0%, when the nitrogen content standard value of the acid-containing cotton adopts the corresponding nitrogen content result of the water-containing cotton, the detection accuracy is significantly improved. In addition, since the nitrogen content distribution of the water-containing cotton is easy to select and adjust, the problem of low nitrogen content detection accuracy and large error is solved by the acid-containing cotton back preparation method, and a modeling sample with a wide and uniform distribution of nitrogen content and acidity is obtained. On this basis, the spectrum of the acid-containing nitrocellulose modeling sample and the standard values ​​of nitrogen content and acid value are obtained, and a stable and reliable near-infrared analysis model is established. Based on the analysis model, one only needs to scan a near-infrared spectrum of the sample to be tested, call in the model calculation, and the nitrogen content and acid value of the sample can be directly obtained.

Claims

1. A method for online detection of nitration quality in the nitration process of nitrocellulose production, characterized in that: The method uses a near-infrared spectrometer to simultaneously and rapidly non-destructively determine the nitrogen content and acid value of the acid-containing nitrocellulose in the nitration process. The method comprises the following steps: Step 1, prepare a series of acid-containing nitrocellulose modeling samples: Select a series of water-containing nitrocellulose samples with different nitrogen contents, and keep the moisture content greater than 20%; soak each water-containing nitrocellulose sample in different nitric acid and sulfuric acid mixed solutions, stir, soak, filter, and centrifuge to obtain a series of acid-containing nitrocellulose modeling samples; Step 2, spectrum collection of acid-containing nitrocellulose modeling sample: The obtained acid-containing nitrocellulose modeling sample is immediately placed in a sample cup with a low-hydroxyl quartz bottom, compacted with a wooden pestle, and a near-infrared spectrometer is used to perform a spectrum scan from the bottom of the sample cup to obtain a near-infrared spectrum of the acid-containing nitrocellulose modeling sample; Step 3, determination of the standard value of the acid-containing nitrocellulose modeling sample: The standard value of nitrogen content in acid-containing nitrocellulose modeling samples adopts the nitrogen content of its corresponding raw material, water-containing nitrocellulose; the standard value of acid value in acid-containing nitrocellulose modeling samples adopts acid-base titration method and is calculated based on nitric acid content; Step 4: Establishment of analysis model: The spectra of a series of acid-containing nitrocellulose modeling samples were correlated with the standard values ​​of nitrogen content and acid value by using chemometric methods, and a near-infrared analysis model of nitrogen content and acid value of acid-containing nitrocellulose was established. Based on this model, the nitrogen content and acid value of the produced samples to be tested were tested. Step 5, detection of nitrogen content and acidity of the sample to be tested: The acid-containing nitrocellulose sample taken from the nitrification process of the production line is immediately placed in a sample cup with a low-hydroxyl quartz bottom, compacted with a wooden pestle, and spectrally scanned from the bottom of the sample cup with a near-infrared spectrometer to obtain a near-infrared spectrum of the acid-containing nitrocellulose; the near-infrared spectrometer and spectrally scanning conditions used are the same as those for the spectrum acquisition of the modeling sample; based on the spectrum of the acid-containing nitrocellulose sample to be tested and the established near-infrared analysis model, the nitrogen content and acidity of the nitrocellulose sample to be tested are obtained; The content of nitric acid in the nitric-sulfur mixed acid solution is 80% to 85%, and the solution temperature is 15 to 25°C.

2. The method for online detection of nitration quality in the nitration step of nitrocellulose production process according to claim 1, characterized in that: The difference between the maximum and minimum nitrogen contents of the series of acid-containing nitrocellulose modeling samples is greater than 1.2%, covering the nitrogen content distribution range of the samples to be tested, and the samples are evenly distributed within the nitrogen content range.

3. The method for online detection of nitration quality in the nitration step of nitrocellulose production process according to claim 1, characterized in that: In the step 1, the moisture content is 20-30%.

4. The method for online detection of nitration quality in the nitration step of nitrocellulose production process according to claim 1, characterized in that: The stirring and soaking time is 5-8 minutes.

5. The method for online detection of nitration quality in the nitration step of nitrocellulose production process according to claim 1, characterized in that: The near infrared spectrometer is of Fourier transform type, grating type, CCD type or filter type, and the spectral wavelength range includes 6050cm -1 ~5500 cm -1 、4650 cm -1 ~4200cm -1 One or two of.

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