A method and system for the production of paraformaldehyde from dimethylformamide

Abstract

The application discloses a method and system for preparing paraformaldehyde from methylal, and relates to the technical field of paraformaldehyde synthesis. The method comprises the steps of methylal refining, catalyst preparation, catalytic oxidation reaction, paraformaldehyde preparation and tail gas treatment: taking crude methylal by-product of glyphosate as raw material, methylal with purity greater than or equal to 98% is obtained through rectification refining; iron-molybdenum catalyst is prepared by iron nitrate and ammonium molybdate; high-concentration formaldehyde is generated by catalytic oxidation under the conditions of 280-390 DEG C and air flow of 0.36-0.47 L / min; paraformaldehyde with purity greater than or equal to 96% is obtained through sodium hydroxide catalytic polymerization and hot air drying, and tail gas realizes recycling and incineration standard emission. The application solves the problems of insufficient resource utilization of methylal by-product of glyphosate, low yield, insufficient purity, poor catalyst stability and poor environmental protection of the existing methylal preparation process of paraformaldehyde. The application realizes the resource utilization of by-products, the formaldehyde yield is more than 70%, the catalyst has long service life, the process is environmentally friendly and efficient, and has significant industrialization potential.

Description

Technical Field

[0001] This invention belongs to the field of paraformaldehyde synthesis technology, specifically to a method and system for preparing paraformaldehyde from methyl acetal. Background Technology

[0002] Currently, methylal is mainly used in low-value-added fields such as paint solvents. However, its economic value can be significantly improved by refining, purifying, and oxidizing it into high-concentration formaldehyde. In existing technologies, the refining of methylal mainly adopts a multi-tower distillation process, including a combination of equipment such as a light-light product removal tower, a methanol removal tower, a pressurized tower, and a methanol reflux tower. High-purity methylal is separated by precisely controlling temperature and pressure.

[0003] Significant progress has been made in the technology of producing high-concentration formaldehyde through the oxidation of methylal. Traditional methanol oxidation methods typically produce formaldehyde with a concentration of 37%, requiring concentration and purification to reach 60-70%, which suffers from high energy consumption, equipment corrosion, and difficulties in recovering diluted formaldehyde. The direct oxidation route of methylal eliminates the need for a concentration unit, significantly reducing energy consumption and directly yielding formaldehyde with a concentration exceeding 70%. Recent research indicates that by adjusting the feed ratio of methylal and methanol, high-concentration formaldehyde of 62-83% can be obtained without introducing external water, simplifying the process.

[0004] However, this technology still faces several challenges: First, the selectivity and stability of the catalyst in the oxidation of methyl acetal need to be improved, and the existing catalytic system is prone to deactivation during long-term operation; second, the heat generated during the reaction is difficult to control, which can easily lead to local overheating and an increase in side reactions; third, the safe storage and transportation of high-concentration formaldehyde is a prominent issue, and its easy polymerization characteristics increase the difficulty of process operation; fourth, the methyl acetal refining process has high energy consumption, and the energy integration scheme needs to be further optimized.

[0005] In terms of technology distribution, my country has several patented technologies for the recovery and utilization of glyphosate byproducts, such as the methylal refining process and the oxidation of methylal to produce concentrated formaldehyde. However, it still lags behind international advanced levels in the development and industrial application of high-efficiency catalysts. Internationally, Japan is in a leading position in the synthesis of methylal by catalytic distillation and has developed reactive distillation technology using solid resin as a catalyst, fundamentally solving the equipment corrosion problem.

[0006] Overall, the technology for producing high-concentration formaldehyde using glyphosate byproduct methylal as a raw material has significant economic and environmental benefits, aligning with the concepts of green chemistry and circular economy development. However, breakthroughs are still needed in catalyst performance, process optimization, and safety control to achieve the technological maturity required for industrial application. Glyphosate, as a highly efficient, low-toxicity, and broad-spectrum non-selective herbicide, has a huge market demand. Currently, the mainstream production process is the dialkyl phosphite method, which generates a large amount of methylal byproduct during production, accounting for approximately 55% of the glyphosate yield by mass. Traditionally, the byproduct methylal is mostly sold as inexpensive fuel or solvent. However, with increasingly stringent national environmental protection requirements, such simple treatment methods can no longer meet environmental standards. The resource utilization of methylal has become crucial for the green production and circular economy of glyphosate.

[0007] Paraformaldehyde is an important raw material for glyphosate production. Currently, industrial production mainly uses formaldehyde solution concentration and polymerization, which suffers from high energy consumption and long process time. To address this issue, the industry is gradually exploring technical routes for preparing paraformaldehyde using methylal as a raw material. Existing technologies include patents disclosing processes for the oxidation of methylal to formaldehyde and paraformaldehyde. For example, invention patent CN104292085B discloses an apparatus and method for producing polyoxymethylene dimethyl ether from methanol or methylal. This method, based on an apparatus, oxidizes methanol or methylal to formaldehyde, polymerizes formaldehyde to obtain paraformaldehyde, reacts paraformaldehyde with methylal to produce polyoxymethylene dimethyl ether, and then separates it by distillation to obtain PODE2 and PODE3. 4. Residual formaldehyde from the reaction is recycled back into the raw materials. The tail gas and wastewater generated from each reaction are treated in tail gas and wastewater treatment systems, respectively, for green emission and recycling. Using an iron-molybdenum catalyst and a reaction temperature of 250-350 ℃, the dilute formaldehyde is recycled back to its oxidation step and the tail gas is catalytically incinerated. However, this patent uses industrial-grade methylal as raw material and does not have a dedicated refining process designed for the impurities in glyphosate by-product methylal (such as methanol and high-boiling-point impurities). This results in poor raw material adaptability, making it difficult to directly apply to the treatment of glyphosate by-product methylal. Furthermore, it lacks a complete tail gas recycling and waste treatment system, making it difficult to balance environmental protection and economic efficiency.

[0008] Therefore, there is an urgent need to develop a method and system that can efficiently utilize methyl acetal byproducts, produce high-purity paraformaldehyde with high yield, and meet green and environmental protection requirements, so as to overcome the shortcomings of existing technologies. Summary of the Invention

[0009] To address the shortcomings of the existing technology, the present invention aims to provide a method and system for preparing paraformaldehyde from methylal, thereby realizing the resource utilization of methylal as a byproduct of glyphosate, improving formaldehyde yield and paraformaldehyde purity, and reducing energy consumption and environmental pressure.

[0010] The method for preparing paraformaldehyde from methyl acetal includes the following steps:

[0011] S1. Refining of methylal: Crude methylal, a byproduct of glyphosate production, is fed from the methylal feed tank into a packed tower, washed with water, and then sent to a distillation tower for distillation to obtain refined methylal with a purity of ≥98%. S2, Catalytic oxidation reaction: The refined methylal obtained in step S1 is vaporized, mixed with air, preheated, and then introduced into a fixed-bed reactor to undergo a catalytic oxidation reaction to generate formaldehyde; the fixed-bed reactor is filled with an iron-molybdenum catalyst and ceramic rings. S3. Preparation of paraformaldehyde: The formaldehyde generated in step S2 is introduced into the polymerization reaction chamber, and sodium hydroxide solution is added as a polymerization catalyst. At the same time, hot air is introduced to allow the formaldehyde to undergo a polymerization reaction in the polymerization reaction chamber. The polymerization product enters the collection tank, which is the paraformaldehyde. The unpolymerized formaldehyde is absorbed by the absorption tower to generate a dilute formaldehyde solution. The dilute formaldehyde solution is depressurized and concentrated to a concentration >65%, and then sent back to the polymerization reaction chamber for catalytic polymerization. S4. Exhaust gas treatment: The exhaust gas discharged from the absorption tower is used to heat the preheater and then sent to the incineration system to convert trace organic matter into carbon dioxide and water before being discharged in compliance with standards.

[0012] In the preferred embodiment, the heating temperature of the methyl acetal raw material tank in step S1 is controlled at 50~60 ℃, the height of the distillation column is 1000 mm, the packing material in the distillation column is a ceramic ring with an outer diameter of 2 mm and a length of 10 mm, and under normal pressure conditions, the temperature of the front fraction is controlled at 35~40 ℃, and the positive fraction at 40~42 ℃ is collected.

[0013] In the preferred embodiment, the iron-molybdenum catalyst in step S2 is prepared by the following method: ferric nitrate and ammonium molybdate are mixed, the pH value of the solution is adjusted, and after standing for 20 h to form a gel, it is filtered; the filtered product is mixed with diatomaceous earth and silica powder, a co-catalyst is added, and then the mixture is stretched into strips, and after being air-dried, baked and calcined, rod-shaped iron-molybdenum catalyst is prepared.

[0014] In a further preferred embodiment, the reagent used to adjust the pH value of the solution is anhydrous sodium carbonate.

[0015] In a further preferred embodiment, the co-catalyst is anhydrous sodium sulfite.

[0016] In a further preferred embodiment, the roasting process is carried out in a muffle furnace.

[0017] In the preferred embodiment, the flow rate of methylal in step S2 is 15~20 mL / h.

[0018] In the preferred embodiment, the air flow rate in step S2 is controlled at 0.36~0.47 L / min.

[0019] In a preferred embodiment, the temperature of the catalytic bed in the fixed-bed reactor described in step S2 is controlled at 280~390℃.

[0020] In the preferred embodiment, the ceramic rings filled in the fixed-bed reactor in step S2 have an outer diameter of 2 mm and a length of 10 mm.

[0021] In the preferred embodiment, the volume ratio of the iron-molybdenum catalyst and the ceramic ring in step S2 is 0.6~2.2:1.

[0022] In a preferred embodiment, the volume ratio of sodium hydroxide solution to formaldehyde gas in step S3 is 1:10~15.

[0023] In the preferred embodiment, the volume ratio of formaldehyde to air in step S3 is 0.000053-0.00083:1.

[0024] In the preferred embodiment, the polymerization reaction temperature in step S3 is 20~55 ℃.

[0025] In the preferred embodiment, the polymerization chamber described in step S3 adopts a multi-zone temperature control system, which divides the polymerization chamber into three temperature control zones; the temperature of the first zone is controlled at 45~55 ℃, the temperature of the second zone is controlled at 40~45 ℃, and the temperature of the third zone is controlled at 35~40 ℃.

[0026] In the preferred embodiment, the temperature of the air in step S3 is 90~110 ℃.

[0027] In the preferred embodiment, the temperature of the collection tank in step S3 is 85~95 ℃.

[0028] The present invention also provides a system for implementing the method for preparing paraformaldehyde from methyl acetal, comprising a methyl acetal purification unit, a catalytic oxidation unit, a paraformaldehyde preparation unit, and a tail gas treatment unit connected in sequence: The methyl acetal refining unit includes a raw material tank, a packed tower, a distillation tower and a product storage tank connected in sequence. The raw material tank is equipped with a heating device. The distillation tower is filled with ceramic rings with an outer diameter of 2 mm and a length of 10 mm. A temperature monitoring device is provided at the top of the tower to realize the washing and distillation purification of crude methyl acetal. The catalytic oxidation unit includes an air purification device, a flow controller, a preheater, and a fixed-bed reactor. The air purification device is connected to the first inlet of the preheater via the flow controller. The product storage tank outlet is connected to the second inlet of the preheater via a gasification device. The preheater outlet is connected to the fixed-bed reactor. The fixed-bed reactor is filled with an iron-molybdenum catalyst and ceramic rings, and a temperature control device is provided outside the fixed-bed reactor to realize the catalytic oxidation of refined methylal to formaldehyde gas. The paraformaldehyde preparation unit includes a polymerization reaction chamber, a catalyst injection device, a hot air generator, a collection tank, and a concentration device. The outlet of the fixed-bed reactor is connected to the polymerization reaction chamber. The catalyst injection device and the hot air generator are both connected to the polymerization reaction chamber. The outlet of the polymerization reaction chamber is connected to the collection tank. A heating device is installed outside the collection tank. The outlet of the absorption tower is connected to the polymerization reaction chamber via the concentration device. This unit is used to realize the polymerization, drying, and recycling of formaldehyde. The exhaust gas treatment unit includes a circulating fan and an incinerator. The top outlet of the absorption tower is connected to the circulating fan, and the circulating fan is connected to the incinerator via a pipeline. The pipeline surrounds the preheater to heat the preheater, thereby realizing the recycling and emission of exhaust gas in compliance with standards.

[0029] In the preferred embodiment, the concentration device is a pressure-reducing evaporator.

[0030] In the preferred embodiment, the fixed-bed reactor is a stainless steel tubular reactor with a length-to-diameter ratio of 2:1.

[0031] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. This invention is specifically designed for the whole-chain process of glyphosate by-product methyl acetal, which transforms industrial by-products into high-value paraformaldehyde, realizes the "by-product-raw material" cycle, reduces raw material costs, and solves the environmental problems of by-product treatment, which is in line with the concept of circular economy.

[0032] 2. By optimizing the catalytic oxidation conditions, the formaldehyde yield of this invention is as high as 70% or more; by using sodium hydroxide catalytic polymerization combined with hot air drying process, the paraformaldehyde content can be stabilized at over 96%, meeting the requirements of industrial production.

[0033] 3. The iron-molybdenum catalyst provided by this invention has shown stable performance and no significant activity decay after a 7-week life test, effectively solving the short life of iron-molybdenum catalysts in the prior art. It can meet the needs of continuous production and its preparation cost is lower than that of vanadium-titanium composite catalysts.

[0034] 4. This invention constructs a dual-cycle system of "exhaust gas recirculation + incineration treatment" and "dilute formaldehyde concentration and recycling", which reduces waste gas emissions and raw material waste; water resources are recycled in the process, which reduces energy consumption compared with the traditional formaldehyde concentration and polymerization process and meets environmental protection requirements.

[0035] 5. The method provided by this invention is simple in process, has controllable parameters, a reasonable system structure, and strong synergy among its units. It can achieve pilot-scale and industrial production by expanding the experimental scale, and is suitable for the supporting needs of glyphosate production enterprises. Attached Figure Description

[0036] Figure 1 This is a schematic diagram of the methyl acetal refining process.

[0037] Figure 2 This is a flow chart of the sodium hydroxide polymerization catalytic process. Detailed Implementation

[0038] The technical solution of the present invention will be further described and illustrated below through examples. All raw materials used in the examples are commercially available or prepared using conventional methods.

[0039] Example 1 This embodiment provides a method for preparing paraformaldehyde by refining and catalytic oxidation of methyl acetal, specifically including the following steps: S1. Refining of Methylal: The methylal feedstock is heated to 55 ℃ and then distilled through a 1000 mm high distillation column. The column is filled with ceramic rings (outer diameter 2 mm, length 10 mm, abbreviated as Φ2×10 mm). Under normal pressure, the temperature of the first fraction is controlled at 38 ℃, the temperature of the refined methylal fraction is controlled at 40 ℃, and the working temperature at the top of the column is maintained at 40 ℃. The fraction collected at 40 ℃ is the refined methylal. Gas chromatography analysis of methyl acetal raw material was performed at an injection temperature of 100 ℃, a column temperature of 50 ℃, and a detector temperature of 280 ℃. The results showed that the contents of the components in the raw material at retention times of 2.293 min, 2.390 min, 2.822 min, and 4.410 min were 0.024%, 3.45%, 91.04%, and 5.49%, respectively. After purification, the contents of the components at the corresponding retention times in methyl acetal became 0.016%, 1.454%, 98.04%, and 0.49%, respectively, indicating a significant improvement in purity. S2. Catalyst Preparation: Prepare ferric nitrate, ammonium molybdate, diatomaceous earth, silica powder, anhydrous sodium sulfite, thymolphthalein, anhydrous sodium carbonate, and a desiccant. Mix a certain amount of ferric nitrate with ammonium molybdate, adjust the pH of the solution, and let it stand for 20 hours to form a gel. Remove the gel and filter it for several hours. Then, mix the filtered material with a small amount of diatomaceous earth and silica powder, and add a certain volume of co-catalyst. Mix the mixture into a ball and then perform a strip-forming process. Air-dry the strip material for several hours, and then dry and calcine it to obtain a rod-shaped catalyst. Mix the catalyst with ceramic rings and fill it into a stainless steel tubular reactor (outer diameter 25 mm, wall thickness 1.5 mm, length 50 mm). Fill both ends of the reactor with a certain volume of ceramic rings. S3. Catalytic oxidation: The refined methylal obtained in step S1 is vaporized at a flow rate of 18 mL / h and mixed with purified and flow-regulated air. After preheating in a preheater, the mixture is introduced into a fixed-bed reactor to produce formaldehyde through catalytic oxidation. The two ends of the fixed-bed reactor are filled with ceramic rings, and the middle section is filled with an iron-molybdenum catalyst and Φ2×10 mm ceramic rings. The total volume of the catalyst and ceramic rings is 50 mL, the ratio is 0.68:1, and the length of the catalyst bed is 14 cm. The temperature of the catalyst bed is controlled at 350℃, and the air flow rate is controlled at 0.36 L / min. The catalytic oxidation reaction produces formaldehyde. S4. Preparation of paraformaldehyde: The formaldehyde generated in step S3 is introduced into the polymerization reaction chamber, and sodium hydroxide solution is added as a polymerization catalyst, wherein the volume ratio of sodium hydroxide solution to formaldehyde gas is 1:12. At the same time, hot air at 100 ℃ is introduced, and the volume ratio of formaldehyde to air is 0.00083:1. The formaldehyde is polymerized in the polymerization reaction chamber at 40 ℃. The polymerization product enters the collection tank at 90 ℃, thus obtaining paraformaldehyde with a content ≥96%. The unpolymerized formaldehyde is absorbed by the absorption tower to generate a dilute formaldehyde solution. The dilute formaldehyde solution is concentrated to a concentration >65% by depressurization and then sent back to the polymerization reaction chamber for catalytic polymerization. S5. Tail gas treatment: The tail gas discharged from the absorption tower is recycled into the process gas system to heat the preheater, and then sent to the incineration system to convert trace organic matter into carbon dioxide and water before being discharged in compliance with standards.

[0040] The purity of the paraformaldehyde prepared in this example was determined to be 97.51% by sulfuric acid titration. The calculated yield of paraformaldehyde in this example was 71.62%.

[0041] Example 2 This embodiment is basically the same as that of Example 1, except that in step S3, the total volume of catalyst and ceramic ring is 40 mL, the ratio is 0.68:1, the length of the catalyst bed is 18 cm, the temperature of the catalyst bed is controlled at 370 ℃, and the air flow rate is controlled at 0.47 L / min.

[0042] The purity of the paraformaldehyde prepared in this example was determined to be 72.62% by sulfuric acid titration. The calculated yield of paraformaldehyde in this example was 66.97%.

[0043] Example 3 This embodiment is basically the same as that of Example 1, except that in step S3, the total volume of catalyst and ceramic ring is 60 mL, the ratio is 1:1, the length of the catalyst bed is 18 cm, the temperature of the catalyst bed is controlled at 370 ℃, and the air flow rate is controlled at 0.47 L / min.

[0044] The purity of the paraformaldehyde prepared in this example was determined to be 73.14% by sulfuric acid titration. The calculated yield of paraformaldehyde in this example was 67.45%.

[0045] Example 4 This embodiment is basically the same as that of Example 1, except that in step S3, the total volume of catalyst and ceramic ring is 62 mL, the ratio is 1:2, the length of the catalyst bed is 16.6 cm, the temperature of the catalyst bed is controlled at 370 ℃, and the air flow rate is controlled at 0.54 L / min.

[0046] The purity of the paraformaldehyde prepared in this example was determined to be 70.45% by sulfuric acid titration. The calculated yield of paraformaldehyde in this example was 64.97%.

[0047] The formaldehyde yields in Examples 1-4 were 77.65%, 72.62%, 73.14%, and 70.45%, respectively, as determined by sulfuric acid standard titration. Experimental results show that, with a constant methylal feed rate of 18 mL / h, the yield is relatively high when the catalyst-to-ceramic ring ratio is 0.68:1. At temperatures ranging from 350 to 370 °C and air flow rates from 0.36 to 0.47 L / min, the formaldehyde yield can reach over 70%. After a seven-week catalyst lifespan test, the catalyst's performance and physical state remained normal, and the paraformaldehyde content could be increased to over 96%, meeting quality requirements.

[0048] Example 5 This embodiment is basically the same as embodiment 1, except that in step S4, the volume ratio of formaldehyde to air is 0.000053:1.

[0049] The purity of the paraformaldehyde prepared in this example was determined to be 63.21% by sulfuric acid titration. The calculated yield of paraformaldehyde in this example was 73.76%.

[0050] Example 6 This embodiment is basically the same as embodiment 1, except that in step S4, the temperature of the polymerization reaction chamber is 20 ℃.

[0051] The purity of the paraformaldehyde prepared in this example was determined to be 60.35% by sulfuric acid titration. The calculated yield of paraformaldehyde in this example was 46.56%.

[0052] Example 7 This embodiment is basically the same as embodiment 1, except that in step S4, the polymerization chamber adopts a multi-zone temperature control system, which divides the polymerization chamber into three temperature control zones; the temperature of the first zone is controlled at 50 ℃, the temperature of the second zone is controlled at 45 ℃, and the temperature of the third zone is controlled at 40 ℃, forming a temperature gradient.

[0053] The purity of the paraformaldehyde prepared in this example was determined to be 96.7% by sulfuric acid titration. The calculated yield of paraformaldehyde in this example was 89.25%.

[0054] Example 8 The present invention also provides a system for implementing the method for preparing paraformaldehyde from methyl acetal, comprising a methyl acetal purification unit, a catalytic oxidation unit, a paraformaldehyde preparation unit, and a tail gas treatment unit connected in sequence: The methyl acetal refining unit includes a raw material tank, a packed tower, a distillation tower and a product storage tank connected in sequence. The raw material tank is equipped with a heating device, the distillation tower is filled with Φ2×10 mm ceramic rings, and a temperature monitoring device is installed at the top of the tower to realize the washing and distillation purification of crude methyl acetal. The catalytic oxidation unit includes an air purification device, a flow controller, a preheater, and a fixed-bed reactor. The air purification device is connected to the first inlet of the preheater via the flow controller. The product storage tank outlet is connected to the second inlet of the preheater via a gasification device. The preheater outlet is connected to the fixed-bed reactor (outer diameter 25 mm, wall thickness 1.5 mm, length 50 mm). The fixed-bed reactor is filled with iron-molybdenum catalyst and ceramic rings, and a temperature control device is provided outside the fixed-bed reactor to realize the catalytic oxidation of refined methylal to formaldehyde gas. The paraformaldehyde preparation unit includes a polymerization reaction chamber, a catalyst injection device, a hot air generator, a collection tank, and a concentration device. The outlet of the fixed-bed reactor is connected to the polymerization reaction chamber. The catalyst injection device and the hot air generator are both connected to the polymerization reaction chamber. The outlet of the polymerization reaction chamber is connected to the collection tank. A heating device is installed outside the collection tank. The outlet of the absorption tower is connected to the polymerization reaction chamber via a pressure-reducing evaporator. This unit is used to realize the polymerization, drying, and recycling of formaldehyde. The exhaust gas treatment unit includes a circulating fan and an incinerator. The top outlet of the absorption tower is connected to the circulating fan, and the circulating fan is connected to the incinerator via a pipeline, so as to realize the recycling and emission of exhaust gas in compliance with standards.

[0055] Example 9 The catalyst lifetime evaluation method provided by this invention includes the following steps: S1: Prepare the experimental setup and catalyst samples. Prepare the catalyst lifetime evaluation apparatus, including the reactor, feed system, gas supply system, temperature control system, and product collection and analysis system. Load the catalyst to be evaluated into the reactor, ensuring uniform loading.

[0056] S2: Set the experimental conditions. The reaction temperature was set to 330℃, the feed rate was controlled at 15 ml / h, and the air flow rate was set to 0.36 L / min. These parameters were determined through previous optimization to be the optimal reaction conditions, which can effectively reflect the actual working performance of the catalyst.

[0057] S3: Start the reaction system. First, check the system for airtightness to ensure there are no leaks. Then, heat the system to the preset temperature of 330℃. After the temperature stabilizes, start feeding and aeration according to the set feed rate of 15ml / h and air flow rate of 0.36L / min to start the reaction.

[0058] S4: Timed Sampling and Analysis. After the reaction begins, the product is sampled and analyzed at predetermined time intervals. Sampling time points are set at 1 h, 3 h, 5 h, 7 h, 9 h, 11 h, 13 h, 15 h, 17 h, 19 h, and 21 h of reaction progress. Analysis is performed immediately after each sampling to ensure data accuracy.

[0059] S5: Product Analysis and Determination. The formaldehyde content in the product was determined by sulfuric acid titration. Each sample was analyzed in triplicate, and the volumes of sulfuric acid consumed in each titration (V1, V2, and V3) were recorded. The average titration volume V was calculated. The accuracy and reliability of the analytical method can be evaluated based on the results of the parallel determinations.

[0060] S6: Catalyst lifetime evaluation data processing. Calculate the corresponding formaldehyde yield based on the average volume V of the sulfuric acid titration. Formaldehyde yield (%) is a key indicator for evaluating catalyst activity; the trend of yield change over time can determine the catalyst's lifetime characteristics.

[0061] The reagents for the sulfuric acid titration method include a 1 mol / L sodium hydroxide standard titration solution (C(NaOH)), freshly prepared 3% hydrogen peroxide solution, bromothymol blue indicator (10 g / L), and a 0.24 mol / L sulfuric acid standard titration solution (C(H₂SO₄)). Weigh the sample and proceed with the reaction. Weigh 0.6 g of the sample (accurate to 0.0001 g) into a 250 mL Erlenmeyer flask. Accurately add 50.0 mL of the 1 mol / L sodium hydroxide standard titration solution (C(NaOH)), then slowly add 50 mL of freshly prepared 3% hydrogen peroxide solution, gently shaking the flask occasionally until the reaction is complete. The hydrogen peroxide solution should be prepared fresh each time to ensure its activity. After the reaction is complete, cool the flask and let it stand for 10 min, then add 6 drops of bromothymol blue indicator (10 g / L). Finally, the solution was titrated with a standard sulfuric acid solution C(H2SO4) = 0.24 mol / L until it turned blue-green. At the same time, a blank test was performed, and the volume V of the standard sulfuric acid solution consumed in titrating the sample and the volume V0 of the standard sulfuric acid solution consumed in titrating the blank were recorded.

[0062] After titration, the paraformaldehyde content (calculated as formaldehyde) is calculated using the following formula: X = (V0-V)×C×3.002×100% / m Where: X is the paraformaldehyde content (calculated as formaldehyde); C is the concentration of the sulfuric acid standard titration solution, mol / L; V is the volume of sulfuric acid standard titration solution consumed in titrating the sample, mL; V0 is the volume of sulfuric acid standard titration solution consumed in titrating the blank, mL; m is the mass of the sample, g; 3.002 is the molar mass conversion factor.

[0063] The following results were obtained by analyzing the collected data: After 1 hour of reaction, the average volume of sulfuric acid titration was 11.79 mL, corresponding to a paraformaldehyde yield of 48.28%. After 3 hours of reaction, the average volume of sulfuric acid titration was 21.85 mL, corresponding to a paraformaldehyde yield of 67.12%. After 5 hours of reaction, the average volume of sulfuric acid titration was 26.27 mL, corresponding to a paraformaldehyde yield of 80.58%. After 7 h of reaction, the average volume of sulfuric acid titration was 24.86 mL, corresponding to a paraformaldehyde yield of 76.36%. After 9 hours of reaction, the average volume of sulfuric acid titration was 24.25 mL, corresponding to a paraformaldehyde yield of 74.5%. After 11 h of reaction, the average volume of sulfuric acid titration was 25.14 mL, corresponding to a paraformaldehyde yield of 77.23%. After 13 h of reaction, the average volume of sulfuric acid titration was 25.86 mL, corresponding to a paraformaldehyde yield of 79.45%. After 15 h of reaction, the average volume of sulfuric acid titration was 23.25 mL, corresponding to a paraformaldehyde yield of 71.42%. After 17 h of reaction, the average volume of sulfuric acid titration was 26.05 mL, corresponding to a paraformaldehyde yield of 80.04%. After 19 h of reaction, the average volume of sulfuric acid titration was 24.36 mL, corresponding to a paraformaldehyde yield of 74.83%. After 21 h of reaction, the average volume of sulfuric acid titration was 29.38 mL, and the yield data of paraformaldehyde was not fully recorded.

[0064] Analysis of the paraformaldehyde yield over time revealed that the catalyst activity gradually increased in the initial stage of the reaction (1-5 hours), reaching its first activity peak at 5 hours with a yield of 80.58%. Subsequently, the catalyst activity fluctuated slightly but remained at a relatively high level, peaking again at 17 hours with a yield of 80.04%. This indicates that the catalyst possesses good stability and a long lifespan, with no significant deactivation observed during the continuous 21-hour reaction process.

[0065] In a preferred embodiment, the effect of temperature on catalyst lifetime can be investigated by adjusting the reaction temperature within the range of 310~350℃. Experiments show that when the temperature is below 330℃, the catalyst activity is lower but the stability is better; when the temperature is above 330℃, the initial activity increases but the deactivation rate accelerates.

[0066] In another preferred embodiment, the effect of oxygen concentration on catalyst lifetime can be studied by adjusting the air flow rate within the range of 0.30~0.42 L / min. The results show that appropriately increasing the air flow rate can slow down carbon deposition on the catalyst surface and extend catalyst lifetime.

[0067] This catalyst lifetime evaluation method is characterized by its simple operation, reliable data, and comprehensive evaluation. It can effectively predict the service life of catalysts in actual production and provide an important basis for the industrial application of catalysts.

[0068] It should be understood that the above embodiments are for illustrative purposes only and are not intended to limit the scope of protection of the present invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

Claims

1. A method for preparing paraformaldehyde from methyl acetal, characterized in that, Includes the following steps: S1. Refining of methylal: Crude methylal, a byproduct of glyphosate production, is fed from the methylal feed tank into a packed tower, washed with water, and then sent to a distillation tower for distillation to obtain refined methylal with a purity of ≥98%. S2, Catalytic oxidation reaction: The refined methylal obtained in step S1 is vaporized, mixed with air, preheated, and then introduced into a fixed-bed reactor to undergo a catalytic oxidation reaction to generate formaldehyde; the fixed-bed reactor is filled with an iron-molybdenum catalyst and ceramic rings. S3. Preparation of paraformaldehyde: The formaldehyde generated in step S2 is introduced into the polymerization reaction chamber, and sodium hydroxide solution is added as a polymerization catalyst. At the same time, hot air is introduced to allow the formaldehyde to undergo a polymerization reaction in the polymerization reaction chamber. The polymerization product enters the collection tank, which is the paraformaldehyde. The unpolymerized formaldehyde is absorbed by the absorption tower to generate a dilute formaldehyde solution. The dilute formaldehyde solution is depressurized and concentrated to a concentration >65%, and then sent back to the polymerization reaction chamber for catalytic polymerization. S4. Exhaust gas treatment: The exhaust gas discharged from the absorption tower is used to heat the preheater and then sent to the incineration system to convert trace organic matter into carbon dioxide and water before being discharged in compliance with standards.

2. The method for preparing paraformaldehyde from methyl acetal according to claim 1, characterized in that, In step S1, the heating temperature of the methyl acetal raw material tank is controlled at 50~60 ℃, the height of the distillation column is 1000 mm, the packing material in the distillation column is a ceramic ring with an outer diameter of 2 mm and a length of 10 mm, under normal pressure conditions, the temperature of the first fraction is controlled at 35~40 ℃, and the positive fraction at 40~42 ℃ is collected.

3. The method for preparing paraformaldehyde from methyl acetal according to claim 1, characterized in that, The iron-molybdenum catalyst mentioned in step S2 is prepared by the following method: ferric nitrate and ammonium molybdate are mixed, the pH value of the solution is adjusted, and after standing for 20 h to form a gel, it is filtered; the filtered product is mixed with diatomaceous earth and silica powder, a co-catalyst is added, and then the mixture is stretched into strips, and after being air-dried, baked and calcined, rod-shaped iron-molybdenum catalyst is prepared.

4. The method for preparing paraformaldehyde from methyl acetal according to claim 3, characterized in that, The reagent used to adjust the pH of the solution is anhydrous sodium carbonate; the co-catalyst is anhydrous sodium sulfite; and the calcination process is carried out in a muffle furnace.

5. The method for preparing paraformaldehyde from methyl acetal according to claim 1, characterized in that, The flow rate of methylal in step S2 is 15~20 mL / h; the air flow rate in step S2 is controlled at 0.36~0.47 L / min; the temperature of the catalyst bed in the fixed-bed reactor in step S2 is controlled at 280~390 ℃; the outer diameter of the ceramic rings filled in the fixed-bed reactor in step S2 is 2 mm and the length is 10 mm.

6. The method for preparing paraformaldehyde from methyl acetal according to claim 1, characterized in that, The volume ratio of the iron-molybdenum catalyst and the ceramic ring in step S2 is 0.6~2.2:

1.

7. The method for preparing paraformaldehyde from methyl acetal according to claim 1, characterized in that, The volume ratio of sodium hydroxide solution to formaldehyde gas in step S3 is 1:10~15; the volume ratio of formaldehyde to air in step S3 is 0.000053-0.00083:1; the polymerization reaction temperature in step S3 is 20~55 ℃; the air temperature in step S3 is 90~110 ℃; and the temperature of the collection tank in step S3 is 85~95 ℃.

8. The method for preparing paraformaldehyde from methyl acetal according to claim 1, characterized in that, The polymerization chamber described in step S3 adopts a multi-zone temperature control system, which divides the polymerization chamber into three temperature control zones: the temperature of the first zone is controlled at 45~55 ℃, the temperature of the second zone is controlled at 40~45 ℃, and the temperature of the third zone is controlled at 35~40 ℃.

9. A system for implementing the method for preparing paraformaldehyde from methyl acetal as described in any one of claims 1 to 8, characterized in that, It includes a methyl acetal refining unit, a catalytic oxidation unit, a paraformaldehyde preparation unit, and a tail gas treatment unit connected in sequence: The methyl acetal refining unit includes a raw material tank, a packed tower, a distillation tower and a product storage tank connected in sequence. The raw material tank is equipped with a heating device, the distillation tower is filled with ceramic rings and a temperature monitoring device is installed at the top of the tower. The catalytic oxidation unit includes an air purification device, a flow controller, a preheater, and a fixed-bed reactor. The air purification device is connected to the first inlet of the preheater through the flow controller. The outlet of the product storage tank is connected to the second inlet of the preheater through a gasification device. The outlet of the preheater is connected to the fixed-bed reactor. The fixed-bed reactor is filled with iron-molybdenum catalyst and ceramic rings, and a temperature control device is provided outside the fixed-bed reactor. The paraformaldehyde preparation unit includes a polymerization reaction chamber, a catalyst injection device, a hot air generator, a collection tank, and a concentration device. The outlet of the fixed bed reactor is connected to the polymerization reaction chamber. The catalyst injection device and the hot air generator are both connected to the polymerization reaction chamber. The outlet of the polymerization reaction chamber is connected to the collection tank. A heating device is provided outside the collection tank. The outlet of the absorption tower is connected to the polymerization reaction chamber via the concentration device. The exhaust gas treatment unit includes a circulating fan and an incinerator. The top outlet of the absorption tower is connected to the circulating fan, and the circulating fan is connected to the incinerator via a pipeline. The pipeline surrounds the preheater.

10. The system for preparing paraformaldehyde from methyl acetal according to claim 9, characterized in that, The concentration device is a pressure-reducing evaporator; the fixed-bed reactor is a stainless steel tubular reactor with a length-to-diameter ratio of 2:1.

Images