A method for preparing formaldehyde by oxidation of methyl acetal

Abstract

This invention discloses a method for preparing formaldehyde by oxidizing methylal, belonging to the field of chemical technology. The method uses methylal and oxygen as raw materials, wherein the volume fraction of methylal feed is 2%~6%, the volume ratio of methylal to oxygen feed is 1:1.5~1:5.0, the mass ratio of methylal to water feed is 1:0.09~1:0.24, and the mass ratio of methylal to methanol feed is 1:0.02~1:0.13; the oxidation reaction temperature is 280℃~380℃, and the gas hourly space velocity is 6000~11000 h⁻¹. ‑1 This system regulates reaction selectivity through the synergistic effect of water vapor and oxygen; simultaneously, the staged introduction of methanol not only increases the reaction temperature at the top of the reactor and balances the temperature distribution throughout the reactor, but also achieves efficient control of the heat of reaction, thereby reducing methanol residue in the product. This invention effectively solves the problems of low reaction temperature and high methanol content in the product during the oxidation of methyl acetal, and has the advantages of excellent formaldehyde selectivity and high product purity.

Description

Technical Field

[0001] This invention relates to the field of chemical technology, specifically disclosing a method for preparing formaldehyde by the oxidation of dimethyl acetal (DMM). This method utilizes the synergistic regulation of water vapor and oxygen, co-feeding with methanol for temperature compensation, and staged feeding to achieve reaction temperature control, thereby achieving efficient and highly selective formaldehyde preparation. Background Technology

[0002] Formaldehyde is an important basic chemical raw material. In existing formaldehyde industrial production processes, methanol oxidation is dominant, but the concentration of the formaldehyde aqueous solution produced is only 37%–60%. To meet the demand for high-concentration raw materials from downstream industries such as resin synthesis, high-energy-consuming concentration units are usually required, resulting in high overall energy consumption and production costs. The oxidation of methylal (CH3OCH2OCH3, abbreviated as DMM) to prepare concentrated formaldehyde offers a higher formaldehyde yield; theoretically, the molar concentration of formaldehyde in the product can reach 75%, and the mass fraction can reach 83.3%, while the process energy consumption is relatively low. Therefore, it has attracted widespread attention and represents an important development direction in the formaldehyde production field.

[0003] Current technologies primarily focus on optimizing catalyst systems to improve reaction activity and formaldehyde selectivity. However, the DMM catalytic oxidation system has a complex reaction network and is difficult to control, involving multiple competing reactions such as oxidation, hydrolysis, and pyrolysis. Existing processes lack systematic optimization of reaction conditions, resulting in difficulty in precisely controlling reaction temperature, limiting the improvement of formaldehyde selectivity, and leading to high levels of byproducts such as methanol, dimethyl ether (DME), and methyl formate (MF), severely impacting product quality. Furthermore, high concentrations of formaldehyde are prone to self-polymerization, making further separation and purification of byproducts difficult.

[0004]

[0005] For example, JP1982134722U and JP1985251932A disclose technologies for the oxidation of methyl acetal to formaldehyde using Fe-Mo catalysts, but they do not optimize the reaction conditions. JP1991042253B2 (Asahi Kasei) discloses a process for the oxidation of methyl acetal to formaldehyde, which specifies the matching relationship between methyl acetal concentration and oxygen concentration, but there is still room for further improvement in terms of methyl acetal conversion rate and formaldehyde selectivity.

[0006] Therefore, there is an urgent need in this field to develop an integrated process method that can precisely control the reaction pathway, efficiently suppress side reactions, and simultaneously improve the conversion rate of methylal and methanol, enhance the selectivity and yield of formaldehyde. Summary of the Invention

[0007] The purpose of this invention is to provide an improved technology for the oxidation of methyl acetal to produce concentrated formaldehyde, in order to solve the problems of multiple side reactions, difficulty in controlling the reaction temperature, and high methanol content in concentrated formaldehyde during the oxidation of methyl acetal to produce formaldehyde.

[0008] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0009] A method for preparing formaldehyde by oxidation of methyl acetal, wherein the feed volume fraction of methyl acetal is 2% to 6%, the feed volume ratio of methyl acetal to oxygen is 1:1.5 to 1:5.0, and the feed mass ratio of methyl acetal to water is 1:0.09 to 1:0.24.

[0010] As a preferred technical solution, methanol is also introduced into the feed system.

[0011] As a further preferred technical solution, the mass ratio of methylal to methanol in the feed is 1:0.02 to 1:0.13.

[0012] As a further preferred technical solution, the mass ratio of methylal to methanol in the feed is 1:0.04 to 1:0.11.

[0013] As a further preferred technical solution, the mass ratio of methylal to methanol in the feed is 1:0.06 to 1:0.08.

[0014] As a preferred technical solution, methanol can be fed from the same inlet as methyl acetal and / or from the middle of the reactor.

[0015] As a further preferred technical solution, methanol is fed from the same inlet as methyl acetal and from the middle of the reactor, with the amount of methanol added at the same inlet accounting for 65% to 95% of the total methanol feed.

[0016] As a further preferred technical solution, the amount of methanol fed into the same feed location as methyl acetal accounts for 70% to 90% of the total methanol feed.

[0017] As a further preferred technical solution, the amount of methanol fed into the same feed location as methyl acetal accounts for 75% to 85% of the total methanol feed.

[0018] The catalyst used in the oxidation reaction was an Fe-Mo based oxidation catalyst composed of ferric molybdate and molybdenum trioxide. The reaction temperature was controlled at 280℃ to 380℃, and the gas hourly space velocity (GHSV) was 6000 to 11000 h⁻¹. -1This highly efficient oxidation catalyst enables rapid and highly selective oxidative conversion of methylal; a suitable reaction temperature can both accelerate the main reaction and suppress excessive oxidation caused by local overheating; controlling the space velocity within a suitable range can avoid the problems of incomplete methylal conversion and hydrolysis dominance at high space velocities, and also prevent excessive oxidation of methylal on the catalyst surface at low space velocities.

[0019] The core technical concept of this invention, which addresses the shortcomings of existing technologies, is as follows: methanol is introduced by regulating the synergistic effect of oxygen and water vapor, combined with a specific ratio and feeding method (segmented feeding, controllable ratio).

[0020] Although there are existing reports on the addition of water to participate in oxidation reactions (such as CN107624109A), the mechanism of water's action and the amount added are significantly different from those of this invention. Similarly, although there are literature reports on the introduction of methanol for oxidation reactions (such as US4967014A, EP0327343B1, CN114621072A, etc.), the role, amount, and feeding method of methanol are also significantly different from those of this invention.

[0021] In this invention, an appropriate amount of water vapor can synergistically regulate the reaction selectivity with oxygen; by controlling the amount of methanol and using a unique segmented feeding method, precise control of the reactor temperature field can be achieved, thereby solving problems such as low formaldehyde selectivity, insufficient temperature at the top of the reactor, uneven bed temperature distribution, and high product methanol content in the methyl acetal oxidation process. This process has outstanding advantages such as excellent formaldehyde selectivity (>92%), low product methanol residue (<1 wt.%), and uniform reactor temperature distribution.

[0022] Specifically:

[0023] (1) Synergistic regulation of water vapor and oxygen:

[0024] The feed volume ratio of methyl acetal to oxygen is 1:1.5 to 1:3.5, preferably 1:1.8 to 1:3.0, and more preferably 1:2.0 to 1:2.5. A certain amount of water is introduced into the feed, and the feed mass ratio of methyl acetal to water is 1:0.04 to 1:0.20, preferably 1:0.07 to 1:0.17, and more preferably 1:0.10 to 1:0.14. Water vapor and oxygen enhance the reaction selectivity through a synergistic effect.

[0025] The mechanism by which water vapor and oxygen synergistically enhance formaldehyde selectivity is as follows: water molecules interact with acidic sites on the catalyst surface, regulating the acid strength distribution of the catalyst and thus inhibiting side reactions such as methanol dehydration; an appropriate amount of water vapor can promote the hydrolysis of methyl acetal to generate methanol and formaldehyde, significantly enhancing the reaction activity; by controlling the ratio of water vapor to oxygen to achieve a synergistic effect, the adsorption-desorption balance of reactants on the catalyst surface is optimized, further improving formaldehyde selectivity.

[0026] (2) Effective control of reaction temperature by methanol feed:

[0027] As mentioned above, the raw materials also contain methanol, and the mass ratio of methylal to methanol in the feed is 1:0.02 to 1:0.13, preferably 1:0.04 to 1:0.11, and more preferably 1:0.06 to 1:0.08.

[0028] The methanol feed point can be the same as that of methylal or it can be fed from the middle of the reactor. The amount of methanol fed from the same feed point as methylal accounts for 65%–95% of the total methanol feed, preferably 70%–90%, and more preferably 75%–85%.

[0029] The reason why methanol feed can effectively control the reaction temperature is that the heat released by methanol oxidation is higher than the net heat released by methylal oxidation. Methanol fed in the same reactor as methylal can compensate for the temperature loss caused by the endothermic hydrolysis of methylal, increasing the temperature at the top of the reactor and thus improving the conversion rate of methylal selective oxidation. Methanol introduced from the middle of the reactor can optimize the temperature distribution within the reactor, avoiding local overheating or undercooling, further improving the reaction conversion rate and formaldehyde selectivity. At the same time, limiting the methanol feed rate within a suitable range can make the reactor bed temperature distribution more uniform, and the reaction heat is easier to control. This not only effectively increases the final formaldehyde concentration but also significantly reduces the amount of residual methanol in the concentrated formaldehyde solution.

[0030] Compared with existing technologies, this invention has the following advantages: the synergistic effect of oxygen and water vapor can effectively suppress side reactions and significantly improve formaldehyde selectivity; by co-feeding methylal and methanol, the temperature at the top of the reactor can be increased, promoting the efficient conversion of methylal and methanol, while introducing methanol in the middle of the reactor can further optimize the bed temperature distribution, avoid local thermal imbalance, and improve the overall reaction performance; the optimized reaction conditions can produce concentrated formaldehyde products with low methanol content, reducing the energy consumption of subsequent separation processes; at the same time, this process can also slow down the catalyst deactivation rate and extend the catalyst life. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of the process flow and apparatus for producing concentrated formaldehyde from methyl acetal according to Embodiment 1 of the present invention.

[0032] Figure 2 A schematic diagram illustrating the mechanism by which water vapor and oxygen synergistically enhance the selectivity of dimethyl acetal (DMM);

[0033] Figure 3 This is a schematic diagram of the packing structure of a tubular reactor;

[0034] Figure 4 This is a schematic diagram of the temperature distribution in a tubular reactor.

[0035] Figure 1 The components are as follows: 1. Methyl acetal storage tank; 2. Methyl acetal pump; 3. Methyl acetal vaporizer; 4. Gas preheater; 5. Methanol storage tank; 6. Methanol pump; 7. Methanol vaporizer; 8. Oxidation reactor; 9. Heat transfer oil system I; 10. Heat transfer oil system II; 11. Concentrated formaldehyde absorption tower; 12. Concentrated formaldehyde storage tank; 13. Concentrated formaldehyde pump; 14. Tail gas treatment device. Detailed Implementation

[0036] To illustrate the technical content, objectives, and application effects of this invention in detail, the invention will be further described below with reference to specific embodiments. However, the scope of protection of this invention is not limited to the following embodiments.

[0037] The following specific embodiments and detailed data further illustrate the technical solution, implementation steps, and excellent effects of the present invention.

[0038] The reaction apparatus structure used in the following embodiments is as follows: Figure 1 As shown, the reaction apparatus includes: a methylal storage tank 1, a methylal pump 2, a methylal vaporizer 3, a gas preheater 4, a methanol storage tank 5, a methanol pump 6, a methanol vaporizer 7, an oxidation reactor 8, a heat transfer oil system I 9, a heat transfer oil system II 10, a concentrated formaldehyde absorption tower 11, a concentrated formaldehyde storage tank 12, a concentrated formaldehyde pump 13, and a tail gas treatment device 14.

[0039] In the following embodiments, methylal in the methylal storage tank 1 is pumped to the methylal vaporizer 3 via the methylal pump 2 to complete the vaporization process; air and nitrogen are introduced into the gas preheater 4 in a set ratio, and after preheating, they are combined with the vaporization products of methylal and enter the oxidation reactor 8 together. Methanol in the methanol storage tank 5 is pumped to the methanol vaporizer 7 via the methanol pump 6, and after vaporization, it is introduced into the middle inlet of the oxidation reactor 8; wherein, the methanol feed at the same feed position as methylal accounts for the majority of the total methanol feed, ensuring that the temperature at the top of the reactor reaches the standard.

[0040] The heat transfer oil system I9 is ​​connected to the upper part of the oxidation reactor 8 to regulate the temperature at the top of the reactor; the heat transfer oil system II10 is connected to the lower part of the oxidation reactor 8 to stabilize the temperature at the bottom of the reactor, achieving a uniform temperature distribution throughout the reaction system. The outlet material of the oxidation reactor 8 is fed into the concentrated formaldehyde absorption tower 11. After absorption and separation, the concentrated formaldehyde enters the concentrated formaldehyde storage tank 12 and is then output through the concentrated formaldehyde pump 13. A portion of the concentrated formaldehyde is returned to the top of the concentrated formaldehyde absorption tower 11 for recycling, while the other portion is discharged as a product (formaldehyde mass fraction > 65%). The tail gas outlet at the top of the concentrated formaldehyde absorption tower 11 is connected to the tail gas treatment device 14, where the treated gas is discharged in compliance with standards.

[0041] Oxidation reactor 8 is a tubular reactor, which can adopt a single or multiple parallel reaction tube structure. Each reaction tube is filled with a reaction bed containing catalyst particles. The reactor is equipped with a heat exchange system to effectively remove the heat of reaction generated during the reaction. Each reactor has a top feed inlet and a side feed inlet, which can realize the feeding of raw materials from different positions. Multiple reactors can be used in series and parallel combinations. In this embodiment, a single-tube reactor is used, with an inner diameter of 21 mm and a length of 1500 mm.

[0042] Oxidation reactor 8 is packed with an oxidation catalyst, which is a cyclic iron-molybdenum catalyst prepared by co-precipitation method, with dimensions of 5.0 × 2.5 × 5.0 mm and a specific surface area of ​​4–6 m². 2 g -1 ;

[0043] The oxidation catalyst packed in the oxidation reactor 8 can be mixed with inert magnetic rings of the same size, with a mixing ratio (catalyst ratio) of 40% to 70%, forming a mixed bed. This mixed bed is uniformly packed in the upper part of a single reaction tube, with a packing height of 300 to 500 mm.

[0044] The pure oxidation catalyst packed in oxidation reactor 8 is uniformly filled in the lower part of a single reaction tube, so that the total filling height of the single reaction tube reaches 1100 mm.

[0045] The concentrated formaldehyde absorption tower 11 is filled with high-efficiency packing material, and a heater is installed on its circulation pipeline to control the temperature of the absorption tower at 85~95℃, which can not only meet the high-efficiency absorption of concentrated formaldehyde, but also prevent concentrated formaldehyde from undergoing self-polymerization.

[0046] The formaldehyde solution (formaldehyde mass fraction > 65%) at the outlet of the concentrated formaldehyde pump 13 was used to determine the formaldehyde content by sodium sulfite titration, and the methanol and methyl acetal contents in the solution were determined by gas chromatography. The gaseous products at the top exhaust outlet of the concentrated formaldehyde absorption tower 11 were analyzed and detected by a gas chromatograph equipped with TCD and FID detectors, GDX-403 and Plot-Q chromatographic columns.

[0047] Example 1

[0048] The oxidation reactor was loaded from bottom to top as follows: a 30 mm high inert magnetic ring (lower inert layer), a 450 mm high pure cyclic iron-molybdenum catalyst (Fe:Mo = 1:2.5, atomic molar ratio, pure catalyst layer), a 600 mm high mixed layer consisting of a uniform mixture of 50 wt.% cyclic iron-molybdenum catalyst and 50 wt.% inert magnetic rings, and a 250 mm high inert magnetic ring (upper inert layer). The total catalyst loading was 209 g.

[0049] Set the oil temperature of heat transfer oil system I to 267℃, turn on the heat transfer oil circulation pump, and perform heat exchange between the upper inert layer and the mixed layer of the oxidation reactor; set the heat transfer oil system II to 262℃, turn on the heat transfer oil circulation pump, and perform heat exchange between the pure catalyst layer and the lower inert layer of the oxidation reactor.

[0050] A reaction was carried out to produce concentrated formaldehyde by oxidizing a mixture of methylal, methanol, and water (methylal to methanol mass ratio 1:0.07, methylal to water mass ratio 1:0.12). A methylal solution containing a portion of methanol and water (80% of the total methanol feed was from the same feed point as the methylal) was pumped into a methylal vaporizer at a flow rate of 429 g / h to obtain superheated water-containing methylal gas at 220°C. Air at a flow rate of 1180 L / h and nitrogen at a flow rate of 1328 L / h were mixed and heated to 220°C in a gas preheater, then mixed evenly with the water-containing methylal gas. The volume ratio of methylal to oxygen was 1:2.3. This mixture entered the reactor from the top for oxidation. A methanol solution from a methanol storage tank was pumped into the reactor at a flow rate of 5.2 g / h. A flow rate of g / h is pumped into the methanol vaporizer to obtain superheated methanol gas at 220°C, which then enters from the feed inlet in the middle of oxidation reactor 8. The reaction space velocity is 9000 h⁻¹. -1 The volume fraction of methyl acetal was 4%, and the bed reaction temperature distribution was 270~335℃. The reactor temperature distribution is shown in [reference needed]. Figure 4 The reaction results showed that the conversion rate of methyl acetal was 100%, the selectivity of formaldehyde was 94.42%, the selectivity of methanol was 0.5%, the selectivity of dimethyl ether was 1.56%, the selectivity of carbon monoxide was 2.59%, and the selectivity of carbon dioxide was 0.93%. The formaldehyde gas obtained from the reaction outlet entered the concentrated formaldehyde absorption tower, and after absorption, a 75.2% concentrated formaldehyde solution with a capacity of 572 g / h was obtained, in which the methanol mass content was 0.49% and the formic acid content was 231 ppm.

[0051] Example 2

[0052] The filling scheme of the oxidation reactor in this embodiment is the same as that in Embodiment 1, and the setup of heat transfer oil system I and heat transfer oil system II is the same as that in Embodiment 1.

[0053] A reaction was carried out to produce concentrated formaldehyde by oxidizing a mixture of methylal, methanol, and water (methylal to methanol mass ratio 1:0.07, methylal to water mass ratio 1:0.12). A methylal solution containing a portion of methanol and water (80% of the total methanol feed was from the same feed point as the methylal) was pumped into a methylal vaporizer at a flow rate of 482 g / h to obtain superheated aqueous methylal gas at 220°C. Air at a flow rate of 1311 L / h and nitrogen at a flow rate of 877 L / h were mixed and heated to 220°C in a gas preheater, then mixed evenly with the aqueous methylal gas. The volume ratio of methylal to oxygen was 1:2.3. This mixture entered the oxidation reactor from the top for oxidation. A methanol solution from a methanol storage tank was pumped into the reactor at a flow rate of 5.8 g / h. A flow rate of g / h is pumped into the methanol vaporizer to obtain superheated methanol gas at 220°C, which then enters from the feed inlet in the middle of oxidation reactor 8. The reaction space velocity is 8000 h⁻¹. -1 The volume fraction of methyl acetal was 5%, and the bed reaction temperature distribution was 270~350℃. The reaction results showed that the conversion rate of methyl acetal was 100%, the selectivity of formaldehyde was 90.10%, the selectivity of methanol was 1.41%, the selectivity of dimethyl ether was 1.25%, the selectivity of carbon monoxide was 4.04%, and the selectivity of carbon dioxide was 3.44%. The formaldehyde gas obtained from the reaction outlet entered the concentrated formaldehyde absorption tower, and after absorption, a 71.7% concentrated formaldehyde solution with a capacity of 635 g / h was obtained, in which the methanol mass content was 1.19% and the formic acid content was 347 ppm.

[0054] Compared to Example 1, this example reduces the reaction space velocity and increases the concentration of the feedstock methylal, resulting in increased feedstock consumption, particularly a decrease in the concentration of concentrated formaldehyde solution and an increase in methanol content. Increasing the residence time and the feed concentrations of methylal and methanol, while increasing the space-time yield, also increases the reaction temperature, leading to an increase in side reactions producing carbon monoxide and carbon dioxide. Furthermore, the increased methanol concentration reduces the conversion rate of methanol to formaldehyde.

[0055] Example 3

[0056] The filling scheme of the oxidation reactor in this embodiment is the same as that in Embodiment 1, and the setup of the heat transfer oil system I and the heat transfer oil system II is the same as that in Embodiment 1.

[0057] The oxidation of a mixture of methylal, methanol, and water to produce concentrated formaldehyde (methylal to methanol mass ratio 1:0.07, methylal to water mass ratio 1:0.12) is carried out. A methylal solution containing a portion of methanol and water (80% of the total methanol feed is from the same feed point as the methylal) is pumped into a methylal vaporizer at a flow rate of 357 g / h to obtain superheated aqueous methylal gas at 220°C. 983 L / h of air and 1853 L / h of nitrogen are mixed and heated to 220°C in a gas preheater, then mixed evenly with the aqueous methylal gas. The volume ratio of methylal to oxygen is 1:2.3. This mixture enters the oxidation reactor from the top for oxidation. The methanol solution from the methanol storage tank is pumped into the reactor at a flow rate of 4.3 g / h. A methanol vaporizer is pumped at a flow rate of g / h to heat methanol gas to 220°C, which then enters from the feed inlet in the middle of oxidation reactor 8. The reaction space velocity is 10000 h⁻¹. -1 The volume concentration of methyl acetal was 3%, and the bed reaction temperature distribution was 270~345℃. The reaction results showed that the conversion rate of methyl acetal was 100%, the selectivity of formaldehyde was 90.10%, the selectivity of methanol was 1.47%, the selectivity of dimethyl ether was 3.15%, the selectivity of carbon monoxide was 3.09%, and the selectivity of carbon dioxide was 2.60%. The formaldehyde gas obtained from the reaction outlet entered the concentrated formaldehyde absorption tower, and after absorption, a 72.4% concentrated formaldehyde solution with a capacity of 471 g / h was obtained, in which the methanol mass content was 1.26% and the formic acid content was 293 ppm.

[0058] Compared to Example 1, this example increases the reaction space velocity and decreases the concentration of the raw material methylal, resulting in increased raw material consumption and decreased space-time yield, particularly a decrease in the concentration of concentrated formaldehyde solution and an increase in methanol content. Reducing the methylal concentration, residence time, and reaction bed temperature lowers the conversion rate of methanol obtained from methylal hydrolysis to formaldehyde through further oxidation.

[0059] Example 4

[0060] The filling scheme in the oxidation reactor is the same as in Example 1, and the setup of heat transfer oil system I and heat transfer oil system II is the same as in Example 1.

[0061] The oxidation of a mixture of methylal, methanol, and water to produce concentrated formaldehyde (methylal to methanol mass ratio 1:0.08, methylal to water mass ratio 1:0.14) was carried out. A methanol- and aqueous methylal solution (with methanol feed at the same feed point as methylal accounting for 100% of the total methanol feed) from a methylal storage tank was pumped into a methylal vaporizer at a flow rate of 437 g / h to obtain superheated aqueous methylal gas at 220°C. Air at a flow rate of 1231 L / h and nitrogen at a flow rate of 1274 L / h were mixed and heated to 220°C in a gas preheater, then mixed thoroughly with the aqueous methylal gas. The volume ratio of methylal to oxygen was 1:2.4. This mixture entered the reactor from the top for the oxidation reaction. The reaction space velocity was 9000 h⁻¹. -1 The volume concentration of methyl acetal was 4%, and the bed reaction temperature distribution was 270–350℃. The reactor temperature distribution is shown in [reference needed]. Figure 4 The reaction results showed that the conversion rate of methyl acetal was 100%, the selectivity of formaldehyde was 92.0%, the selectivity of methanol was 1.05%, the selectivity of dimethyl ether was 1.31%, the selectivity of carbon monoxide was 4.05%, and the selectivity of carbon dioxide was 1.86%. The formaldehyde gas obtained from the reaction outlet entered the concentrated formaldehyde absorption tower, and after absorption, a 72.6% concentrated formaldehyde solution with a capacity of 567 g / h was obtained, in which the methanol mass content was 0.89% and the formic acid content was 342 ppm.

[0062] Compared to Example 1, in this example, methanol is fed only from the top of the reactor, with no material at the middle feed inlet. This increases raw material consumption per unit area and reduces space-time yield, particularly decreasing the concentration of concentrated formaldehyde solution and increasing the methanol content. The large amount of methanol oxidation at the top of the reactor easily leads to heat concentration and increased hotspot temperatures, increasing side reactions such as carbon monoxide and carbon dioxide production. The lack of methanol replenishment in the middle of the reactor lowers the reaction temperature in the lower bed, reducing the conversion rate of methanol obtained from the hydrolysis of methyl acetal to formaldehyde.

[0063] Example 5

[0064] The filling scheme in the oxidation reactor is the same as in Example 1, and the setup of heat transfer oil system I and heat transfer oil system II is the same as in Example 1.

[0065] The oxidation of a mixture of methylal and water to produce concentrated formaldehyde (methylal to water mass ratio 1:0.12) was carried out. The aqueous methylal solution in the methylal storage tank was pumped into a methylal vaporizer at a flow rate of 418 g / h to obtain superheated aqueous methylal gas at 220°C. Air at a flow rate of 1262 L / h and nitrogen at a flow rate of 1260 L / h were mixed and heated to 220°C in a gas preheater, then mixed thoroughly with the aqueous methylal gas. The volume ratio of methylal to oxygen was 1:2.4. This mixture entered the reactor from the top for the oxidation reaction. The reaction space velocity was 9000 h⁻¹. -1 The volume concentration of methyl acetal was 4.1%, and the bed reaction temperature distribution was 270~350℃. The reactor temperature distribution is shown in [reference needed]. Figure 4 The reaction results showed that the conversion rate of methyl acetal was 100%, the selectivity of formaldehyde was 91.0%, the selectivity of methanol was 2.00%, the selectivity of dimethyl ether was 1.80%, the selectivity of carbon monoxide was 2.70%, and the selectivity of carbon dioxide was 2.47%. The formaldehyde gas obtained from the reaction outlet entered the concentrated formaldehyde absorption tower, and after absorption, a 70.3% concentrated formaldehyde solution with a capacity of 573 g / h was obtained, in which the methanol mass content was 1.65% and the formic acid content was 320 ppm.

[0066] Example 6

[0067] The filling scheme in the oxidation reactor is the same as in Example 1, and the setup of heat transfer oil system I and heat transfer oil system II is the same as in Example 1.

[0068] The oxidation of a mixture of methylal and water to produce concentrated formaldehyde (methylal to water mass ratio 1:0.09) was carried out. The aqueous methylal solution in the methylal storage tank was pumped into a methylal vaporizer at a flow rate of 409 g / h to obtain superheated aqueous methylal gas at 220°C. Air at a flow rate of 1367 L / h and nitrogen at a flow rate of 1165 L / h were mixed and heated to 220°C in a gas preheater, then mixed thoroughly with the aqueous methylal gas. The volume ratio of methylal to oxygen was 1:2.6. This mixture entered the reactor from the top for the oxidation reaction. The reaction space velocity was 9000 h⁻¹. -1 The volume concentration of methyl acetal was 4.1%, and the bed reaction temperature distribution was 270~335℃. The reaction results showed that the conversion rate of methyl acetal was 98%, the selectivity of formaldehyde was 91.0%, the selectivity of methanol was 3.00%, the selectivity of dimethyl ether was 3.80%, the selectivity of carbon monoxide was 1.70%, and the selectivity of carbon dioxide was 0.4%. The formaldehyde gas obtained from the reaction outlet entered the concentrated formaldehyde absorption tower, and after absorption, a 68.3% concentrated formaldehyde solution with a capacity of 572 g / h was obtained, in which the methanol mass content was 2.40% and the formic acid content was 345 ppm.

[0069] Compared to Example 1, Examples 5 and 6, in the process of preparing concentrated formaldehyde by methylal oxidation, lacked methanol in the raw materials, resulting in increased raw material consumption and decreased space-time yield. In particular, the content of concentrated formaldehyde solution was further reduced, while the methanol content was further increased. The lack of methanol in the raw materials led to a lower reactor bed temperature and incomplete reaction, thus reducing the conversion rate of methanol obtained from methylal hydrolysis to formaldehyde through further oxidation.

[0070] Example 7

[0071] The filling scheme in the oxidation reactor is the same as in Example 1, and the setup of heat transfer oil system I and heat transfer oil system II is the same as in Example 1.

[0072] The oxidation of methyl acetal to produce concentrated formaldehyde involves pumping a methyl acetal solution from a storage tank into a methyl acetal vaporizer at a flow rate of 425 g / h to obtain superheated aqueous methyl acetal gas at 220°C. Air at a flow rate of 923 L / h and nitrogen at a flow rate of 1580 L / h are mixed and heated to 220°C in a gas preheater, then thoroughly mixed with the aqueous methyl acetal gas. The volume ratio of methyl acetal to oxygen is 1:1.8. This mixture enters the reactor from the top for the oxidation reaction. The reaction space velocity is 9000 h⁻¹. -1 The volume concentration of methyl acetal was 4%, and the bed reaction temperature distribution was 270~335℃. The reaction results showed that the conversion rate of methyl acetal was 97%, the selectivity of formaldehyde was 91.0%, the selectivity of methanol was 3.50%, the selectivity of dimethyl ether was 1.2%, the selectivity of carbon monoxide was 2.70%, and the selectivity of carbon dioxide was 1.56%. The formaldehyde gas obtained from the reaction outlet entered the concentrated formaldehyde absorption tower, and after absorption, a 67.2% concentrated formaldehyde solution with a capacity of 567 g / h was obtained, in which the methanol mass content was 2.76% and the formic acid content was 453 ppm.

[0073] Compared to Example 1, this example, in the process of preparing concentrated formaldehyde by methylal oxidation, lacks both methanol and water in the raw materials, resulting in increased raw material consumption and decreased space-time yield. In particular, the content of concentrated formaldehyde solution is further reduced, while the methanol content is further increased. The lack of methanol in the raw materials leads to a lower reactor bed temperature and incomplete reaction, reducing the conversion rate of methanol obtained from methylal hydrolysis to formaldehyde through further oxidation. The lack of water in the raw materials reduces the synergistic effect of the water vapor and oxygen ratio, increasing side reactions.

[0074] Example 8 (Influence of water vapor quantity)

[0075] The filling scheme in the oxidation reactor is the same as in Example 1, and the setup of heat transfer oil system I and heat transfer oil system II is the same as in Example 1.

[0076] The oxidation of a mixture of methylal, methanol, and water to produce concentrated formaldehyde (methylal to methanol mass ratio 1:0.07, methylal to water mass ratio 1:0.10) was carried out. A methylal solution containing a portion of methanol and water (80% of the total methanol feed was from the same feed point as the methylal) was pumped into a methylal vaporizer at a flow rate of 422 g / h to obtain superheated aqueous methylal gas at 220°C. Air at a flow rate of 1180 L / h and nitrogen at a flow rate of 1336 L / h were mixed and heated to 220°C in a gas preheater, then mixed thoroughly with the aqueous methylal gas. The volume ratio of methylal to oxygen was 1:2.3, and the mixture entered the reactor from the top for oxidation. The methanol solution from the methanol storage tank was pumped into the reactor at a flow rate of 5.2 g / h. A flow rate of g / h is pumped into the methanol vaporizer to obtain superheated methanol gas at 220°C, which then enters from the feed inlet in the middle of oxidation reactor 8. The reaction space velocity is 9000 h⁻¹. -1 The volume concentration of methyl acetal was 4%, and the bed reaction temperature distribution was 270~345℃. The reaction results showed that the conversion rate of methyl acetal was 100%, the selectivity of formaldehyde was 91.05%, the selectivity of methanol was 2.95%, the selectivity of dimethyl ether was 3.68%, the selectivity of carbon monoxide was 1.75%, and the selectivity of carbon dioxide was 0.65%. The formaldehyde gas obtained from the reaction outlet entered the concentrated formaldehyde absorption tower, and after absorption, a 70.5% concentrated formaldehyde solution with a capacity of 588 g / h was obtained, in which the methanol mass content was 2.44% and the formic acid content was 337 ppm.

[0077] Example 9 (Influence of water vapor quantity)

[0078] The filling scheme in the oxidation reactor is the same as in Example 1, and the setup of heat transfer oil system I and heat transfer oil system II is the same as in Example 1.

[0079] A reaction was carried out to produce concentrated formaldehyde by oxidizing a mixture of methylal, methanol, and water (methylal to methanol mass ratio 1:0.07, methylal to water mass ratio 1:0.17). A methylal solution containing a portion of methanol and water (80% of the total methanol feed was from the same feed point as the methylal) was pumped into a methylal vaporizer at a flow rate of 448 g / h to obtain superheated water-containing methylal gas at 220°C. Air at a flow rate of 1180 L / h and nitrogen at a flow rate of 1304 L / h were mixed and heated to 220°C in a gas preheater, then mixed evenly with the water-containing methylal gas. The volume ratio of methylal to oxygen was 1:2.3. This mixture entered the oxidation reactor from the top for oxidation. A methanol solution from a methanol storage tank was pumped into the reactor at a flow rate of 5.2 g / h. A flow rate of g / h is pumped into the methanol vaporizer to obtain superheated methanol gas at 220°C, which then enters from the feed inlet in the middle of oxidation reactor 8. The reaction space velocity is 9000 h⁻¹. -1The volume concentration of methyl acetal was 4%, and the bed reaction temperature distribution was 270~345℃. The reaction results showed that the conversion rate of methyl acetal was 100%, the selectivity of formaldehyde was 91.05%, the selectivity of methanol was 3.42%, the selectivity of dimethyl ether was 1.22%, the selectivity of carbon monoxide was 2.70%, and the selectivity of carbon dioxide was 1.69%. The formaldehyde gas obtained from the reaction outlet entered the concentrated formaldehyde absorption tower, and after absorption, a 71.2% concentrated formaldehyde solution with a capacity of 583 g / h was obtained, in which the methanol mass content was 2.86% and the formic acid content was 341 ppm.

[0080] Example 10 (Effect of Methanol Content)

[0081] The filling scheme in the oxidation reactor is the same as in Example 1, and the setup of heat transfer oil system I and heat transfer oil system II is the same as in Example 1.

[0082] The oxidation of a mixture of methylal, methanol, and water to produce concentrated formaldehyde (methylal to methanol mass ratio 1:0.04, methylal to water mass ratio 1:0.12) is carried out. A methylal solution containing a portion of methanol and water (80% of the total methanol feed is from the same feed point as the methylal) is pumped into a methylal vaporizer at a flow rate of 421 g / h to obtain superheated aqueous methylal gas at 220°C. Air at a flow rate of 1180 L / h and nitrogen at a flow rate of 1336 L / h are mixed and heated to 220°C in a gas preheater, then mixed evenly with the aqueous methylal gas. The volume ratio of methylal to oxygen is 1:2.3. This mixture enters the reactor from the top for oxidation. The methanol solution from the methanol storage tank is pumped into the reactor at a flow rate of 1.51 g / h. A flow rate of g / h is pumped into the methanol vaporizer to obtain superheated methanol gas at 220°C, which then enters from the feed inlet in the middle of oxidation reactor 8. The reaction space velocity is 9000 h⁻¹. -1 The volume concentration of methyl acetal was 4%, and the bed reaction temperature distribution was 270~340℃. The reaction results showed that the conversion rate of methyl acetal was 100%, the selectivity of formaldehyde was 91.03%, the selectivity of methanol was 2.49%, the selectivity of dimethyl ether was 1.79%, the selectivity of carbon monoxide was 2.70%, and the selectivity of carbon dioxide was 2.03%. The formaldehyde gas obtained from the reaction outlet entered the concentrated formaldehyde absorption tower, and after absorption, a 69.3% concentrated formaldehyde solution with a capacity of 585 g / h was obtained, in which the methanol mass content was 2.02% and the formic acid content was 339 ppm.

[0083] Example 11 (Effect of Methanol Content)

[0084] The filling scheme in the oxidation reactor is the same as in Example 1, and the setup of heat transfer oil system I and heat transfer oil system II is the same as in Example 1.

[0085] The oxidation of a mixture of methylal, methanol, and water to produce concentrated formaldehyde (methylal to methanol mass ratio 1:0.11, methylal to water mass ratio 1:0.12) is carried out. A methylal solution containing a portion of methanol and water (80% of the total methanol feed is from the same feed point as the methylal) is pumped into a methylal vaporizer at a flow rate of 440 g / h to obtain superheated aqueous methylal gas at 220°C. Air at a flow rate of 1180 L / h and nitrogen at a flow rate of 1317 L / h are mixed and heated to 220°C in a gas preheater, then mixed evenly with the aqueous methylal gas. The volume ratio of methylal to oxygen is 1:2.3, and the mixture enters the reactor from the top for oxidation. The methanol solution from the methanol storage tank is pumped into the reactor at a flow rate of 8.0 g / h. A flow rate of g / h is pumped into the methanol vaporizer to obtain superheated methanol gas at 220°C, which then enters from the feed inlet in the middle of oxidation reactor 8. The reaction space velocity is 9000 h⁻¹. -1 The volume concentration of methyl acetal was 4%, and the bed reaction temperature distribution was 270~350℃. The reaction results showed that the conversion rate of methyl acetal was 100%, the selectivity of formaldehyde was 90.16%, the selectivity of methanol was 2.51%, the selectivity of dimethyl ether was 1.33%, the selectivity of carbon monoxide was 4.85%, and the selectivity of carbon dioxide was 1.27%. The formaldehyde gas obtained from the reaction outlet entered the concentrated formaldehyde absorption tower, and after absorption, a 70.3% concentrated formaldehyde solution with a capacity of 613 g / h was obtained, in which the methanol mass content was 2.04% and the formic acid content was 336 ppm.

[0086] Example 12 (The effect of oxygen content)

[0087] The filling scheme in the oxidation reactor is the same as in Example 1, and the setup of heat transfer oil system I and heat transfer oil system II is the same as in Example 1.

[0088] The oxidation of a mixture of methylal, methanol, and water to produce concentrated formaldehyde (methylal to methanol mass ratio 1:0.07, methylal to water mass ratio 1:0.12) was carried out. A methylal solution containing a portion of methanol and water (80% of the total methanol feed was from the same feed point as the methylal) was pumped into a methylal vaporizer at a flow rate of 429 g / h to obtain superheated methylal gas at 220°C. Air at a flow rate of 770 L / h and nitrogen at a flow rate of 1738 L / h were mixed and heated to 220°C in a gas preheater, then mixed thoroughly with the methylal gas at a volume ratio of methylal to oxygen of 1:1.5. This mixture entered the reactor from the top for oxidation. The methanol solution from the methanol storage tank was pumped into the reactor at a flow rate of 5.2 g / h. A flow rate of g / h is pumped into the methanol vaporizer to obtain superheated methanol gas at 220°C, which then enters from the feed inlet in the middle of oxidation reactor 8. The reaction space velocity is 9000 h⁻¹. -1The volume concentration of methyl acetal was 4%, and the bed reaction temperature distribution was 270~350℃. The reaction results showed that the conversion rate of methyl acetal was 96%, the selectivity of formaldehyde was 90.15%, the selectivity of methanol was 3.76%, the selectivity of dimethyl ether was 3.57%, the selectivity of carbon monoxide was 1.80%, and the selectivity of carbon dioxide was 1.09%. The formaldehyde gas obtained from the reaction outlet entered the concentrated formaldehyde absorption tower, and after absorption, a 69.10% concentrated formaldehyde solution with a capacity of 570 g / h was obtained, in which the methanol mass content was 3.07% and the formic acid content was 334 ppm.

[0089] Example 13 (The effect of oxygen content)

[0090] The filling scheme in the oxidation reactor is the same as in Example 1, and the setup of heat transfer oil system I and heat transfer oil system II is the same as in Example 1.

[0091] The oxidation of a mixture of methylal, methanol, and water to produce concentrated formaldehyde (methylal to methanol mass ratio 1:0.07, methylal to water mass ratio 1:0.12) was carried out. A methylal solution containing a portion of methanol and water (80% of the total methanol feed was from the same feed point as the methylal) was pumped into a methylal vaporizer at a flow rate of 429 g / h to obtain superheated aqueous methylal gas at 220°C. Air at a flow rate of 1796 L / h and nitrogen at a flow rate of 711 L / h were mixed and heated to 220°C in a gas preheater, then mixed thoroughly with the aqueous methylal gas. The volume ratio of methylal to oxygen was 1:3.5, and the mixture entered the reactor from the top for oxidation. The methanol solution from the methanol storage tank was pumped into the reactor at a flow rate of 5.2 g / h. A flow rate of g / h is pumped into the methanol vaporizer to obtain superheated methanol gas at 220°C, which then enters from the feed inlet in the middle of oxidation reactor 8. The reaction space velocity is 9000 h⁻¹. -1 The volume concentration of methyl acetal was 4%, and the bed reaction temperature distribution was 270~350℃. The reaction results showed that the conversion rate of methyl acetal was 100%, the selectivity of formaldehyde was 88.20%, the selectivity of methanol was 3.23%, the selectivity of dimethyl ether was 3.68%, the selectivity of carbon monoxide was 2.70%, and the selectivity of carbon dioxide was 2.54%. The formaldehyde gas obtained from the reaction outlet entered the concentrated formaldehyde absorption tower, and after absorption, a 69.5% concentrated formaldehyde solution with a capacity of 577 g / h was obtained, in which the methanol mass content was 2.71% and the formic acid content was 344 ppm.

[0092] Example 14 (Impact of methanol feed rate at the same feed location as methylal on the total methanol feed rate)

[0093] The filling scheme in the oxidation reactor is the same as in Example 1, and the setup of heat transfer oil system I and heat transfer oil system II is the same as in Example 1.

[0094] The oxidation of a mixture of methylal, methanol, and water to produce concentrated formaldehyde (methylal to methanol mass ratio 1:0.07, methylal to water mass ratio 1:0.12) is carried out. A methylal solution containing a portion of methanol and water (70% of the total methanol feed is from the same feed point as the methylal) is pumped into a methylal vaporizer at a flow rate of 429 g / h to obtain superheated aqueous methylal gas at 220°C. Air at a flow rate of 1180 L / h and nitrogen at a flow rate of 1328 L / h are mixed and heated to 220°C in a gas preheater, then mixed thoroughly with the aqueous methylal gas. The volume ratio of methylal to oxygen is 1:2.3. This mixture enters the oxidation reactor from the top for oxidation. The methanol solution from the methanol storage tank is pumped into the reactor at a flow rate of 7.8 g / h. A flow rate of g / h is pumped into the methanol vaporizer to obtain superheated methanol gas at 220°C, which then enters from the feed inlet in the middle of oxidation reactor 8. The reaction space velocity is 9000 h⁻¹. -1 The volume concentration of methyl acetal was 4%, and the bed reaction temperature distribution was 270~345℃. The reactor temperature distribution is shown in [reference needed]. Figure 4 The reaction results showed that the conversion rate of methyl acetal was 100%, the selectivity of formaldehyde was 90.25%, the selectivity of methanol was 2.76%, the selectivity of dimethyl ether was 1.34%, the selectivity of carbon monoxide was 3.80%, and the selectivity of carbon dioxide was 2.20%. The formaldehyde gas obtained from the reaction outlet entered the concentrated formaldehyde absorption tower, and after absorption, a 68.5% concentrated formaldehyde solution with a capacity of 598 g / h was obtained, in which the methanol content was 2.24% and the formic acid content was 332 ppm.

[0095] Example 15

[0096] This embodiment verifies the impact of the methanol feed rate at the same feed location as methyl acetal on the total methanol feed rate.

[0097] The filling scheme in the oxidation reactor is the same as in Example 1, and the setup of heat transfer oil system I and heat transfer oil system II is the same as in Example 1.

[0098] The oxidation of a mixture of methylal, methanol, and water to produce concentrated formaldehyde (methylal to methanol mass ratio 1:0.07, methylal to water mass ratio 1:0.12) is carried out. A methylal solution containing a portion of methanol and water (85% of the total methanol feed is from the same feed point as the methylal) is pumped into a methylal vaporizer at a flow rate of 429 g / h to obtain superheated aqueous methylal gas at 220°C. Air at a flow rate of 1180 L / h and nitrogen at a flow rate of 1328 L / h are mixed and heated to 220°C in a gas preheater, then mixed evenly with the aqueous methylal gas. The volume ratio of methylal to oxygen is 1:2.3. This mixture enters the oxidation reactor from the top for oxidation. The methanol solution from the methanol storage tank is pumped into the reactor at a flow rate of 3.9 g / h. A flow rate of g / h is pumped into the methanol vaporizer to obtain superheated methanol gas at 220°C, which then enters from the feed inlet in the middle of oxidation reactor 8. The reaction space velocity is 9000 h⁻¹. -1 The volume concentration of methyl acetal was 4%, and the bed reaction temperature distribution was 270~355℃. The reactor temperature distribution is shown in [reference needed]. Figure 4 The reaction results showed that the conversion rate of methyl acetal was 100%, the selectivity of formaldehyde was 89.18%, the selectivity of methanol was 3.21%, the selectivity of dimethyl ether was 1.34%, the selectivity of carbon monoxide was 4.18%, and the selectivity of carbon dioxide was 2.44%. The formaldehyde gas obtained from the reaction outlet entered the concentrated formaldehyde absorption tower, and after absorption, a 69.7% concentrated formaldehyde solution with a capacity of 594 g / h was obtained, in which the methanol mass content was 2.65% and the formic acid content was 338 ppm.

[0099] Comparative Example 1

[0100] The filling scheme in the oxidation reactor is the same as in Example 1, and the setup of heat transfer oil system I and heat transfer oil system II is the same as in Example 1.

[0101] Anhydrous methyl acetal solution from a methyl acetal storage tank is pumped into a methyl acetal vaporizer at a flow rate of 371 g / h to obtain superheated anhydrous methyl acetal gas at 220°C. Air at a flow rate of 1334 L / h and nitrogen at a flow rate of 1245 L / h are mixed and heated to 220°C in a gas preheater, then mixed thoroughly with the aqueous methyl acetal gas. The volume ratio of methyl acetal to oxygen is 1:2.6. This mixture enters the oxidation reactor from the top for oxidation. The reaction space velocity is 9000 h⁻¹. -1The volume concentration of methyl acetal was 4%, and the bed reaction temperature distribution was 270~335℃. The reaction results showed that the conversion rate of methyl acetal was 96.20%, the selectivity of formaldehyde was 88.0%, the selectivity of methanol was 3.98%, the selectivity of dimethyl ether was 4.20%, the selectivity of carbon monoxide was 2.70%, and the selectivity of carbon dioxide was 1.08%. The formaldehyde gas obtained from the reaction outlet entered the concentrated formaldehyde absorption tower, and after absorption, a 65.2% concentrated formaldehyde solution with a capacity of 561 g / h was obtained, in which the methanol mass content was 3.15% and the formic acid content was 454 ppm.

[0102] Compared with Example 1, the raw material methyl acetal in this comparative example does not contain water and methanol. Due to the decrease in reaction bed temperature and the synergistic effect of the lack of water on the reaction, the concentration of formaldehyde aqueous solution is reduced, the concentration of methanol is increased, and the side reaction of dimethyl ether formation is increased.

[0103] Comparative Example 2

[0104] The filling scheme in the oxidation reactor is the same as in Example 1, and the setup of the heat transfer oil system I and the heat transfer oil system II is the same as in Example 1.

[0105] Anhydrous methanol solution from a methanol storage tank is pumped into a methanol vaporizer at a flow rate of 403 g / h using a methylal pump to obtain superheated methanol gas at 220°C. Air at a flow rate of 1616 L / h and nitrogen at a flow rate of 789 L / h are mixed and heated to 220°C in a gas preheater, then mixed thoroughly with the aqueous methylal gas. The volume ratio of methanol to oxygen is 1:1.20. This mixture enters the oxidation reactor from the top for oxidation. The reaction space velocity is 9000 h⁻¹. -1 The methanol volume concentration was 10.5%, and the bed reaction temperature distribution was 270~360℃. The reaction results showed a methanol conversion rate of 98.0%, formaldehyde selectivity of 92.0%, dimethyl ether selectivity of 2.60%, carbon monoxide selectivity of 2.50%, and carbon dioxide selectivity of 1.30%. The formaldehyde gas obtained from the reaction outlet entered a concentrated formaldehyde absorption tower, and after absorption, a 55% concentrated formaldehyde solution with a capacity of 619 g / h was obtained, containing 1.30% methanol and 275 ppm formic acid.

[0106] Compared with Example 1, the concentration of formaldehyde aqueous solution obtained in this comparative example using methanol as raw material was significantly reduced.

[0107] Comparative Example 3 (Effect of Water Vapor Content)

[0108] The filling scheme in the oxidation reactor is the same as in Example 1, and the setup of heat transfer oil system I and heat transfer oil system II is the same as in Example 1.

[0109] A reaction was carried out to produce concentrated formaldehyde by oxidizing a mixture of methylal, methanol, and water (methylal to methanol mass ratio 1:0.07, methylal to water mass ratio 1:0.03). A methylal solution containing a portion of methanol and water (80% of the total methanol feed was from the same feed point as the methylal) was pumped into a methylal vaporizer at a flow rate of 397 g / h to obtain superheated aqueous methylal gas at 220°C. Air at a flow rate of 1180 L / h and nitrogen at a flow rate of 1367 L / h were mixed and heated to 220°C in a gas preheater, then mixed evenly with the aqueous methylal gas. The volume ratio of methylal to oxygen was 1:2.3, and the mixture entered the oxidation reactor from the top for oxidation. A methanol solution from a methanol storage tank was pumped into the reactor at a flow rate of 5.2 g / h. A flow rate of g / h is pumped into the methanol vaporizer to obtain superheated methanol gas at 220°C, which then enters from the feed inlet in the middle of oxidation reactor 8. The reaction space velocity is 9000 h⁻¹. -1 The volume concentration of methyl acetal was 4%, and the bed reaction temperature distribution was 270~345℃. The reaction results showed that the conversion rate of methyl acetal was 98%, the selectivity of formaldehyde was 88.21%, the selectivity of methanol was 3.88%, the selectivity of dimethyl ether was 4.06%, the selectivity of carbon monoxide was 2.70%, and the selectivity of carbon dioxide was 1.24%. The formaldehyde gas obtained from the reaction outlet entered the concentrated formaldehyde absorption tower, and after absorption, a concentrated formaldehyde solution of 69.30% with a capacity of 569 g / h was obtained, in which the methanol mass content was 3.25% and the formic acid content was 342 ppm.

[0110] Comparative Example 4 (Effect of Water Vapor Content)

[0111] The filling scheme in the oxidation reactor is the same as in Example 1, and the setup of heat transfer oil system I and heat transfer oil system II is the same as in Example 1.

[0112] A reaction was carried out to produce concentrated formaldehyde by oxidizing a mixture of methylal, methanol, and water (methylal to methanol mass ratio 1:0.07, methylal to water mass ratio 1:0.25). A methylal solution containing a portion of methanol and water (80% of the total methanol feed was from the same feed point as the methylal) was pumped into a methylal vaporizer at a flow rate of 422 g / h to obtain superheated aqueous methylal gas at 220°C. Air at a flow rate of 1180 L / h and nitrogen at a flow rate of 1268 L / h were mixed and heated to 220°C in a gas preheater, then mixed evenly with the aqueous methylal gas. The volume ratio of methylal to oxygen was 1:2.3. This mixture entered the oxidation reactor from the top for oxidation. A methanol solution from a methanol storage tank was pumped into the reactor at a flow rate of 5.2 g / h. A flow rate of g / h is pumped into the methanol vaporizer to obtain superheated methanol gas at 220°C, which then enters from the feed inlet in the middle of oxidation reactor 8. The reaction space velocity is 9000 h⁻¹. -1The volume concentration of methyl acetal was 4%, and the bed reaction temperature distribution was 270~345℃. The reaction results showed that the conversion rate of methyl acetal was 96%, the selectivity of formaldehyde was 90.16%, the selectivity of methanol was 4.29%, the selectivity of dimethyl ether was 1.23%, the selectivity of carbon monoxide was 2.70%, and the selectivity of carbon dioxide was 1.70%. The formaldehyde gas obtained from the reaction outlet entered the concentrated formaldehyde absorption tower, and after absorption, a 66.30% concentrated formaldehyde solution with a capacity of 602 g / h was obtained, in which the methanol mass content was 3.37% and the formic acid content was 320 ppm.

[0113] Comparative Example 5 (Effect of Methanol Content)

[0114] The filling scheme in the oxidation reactor is the same as in Example 1, and the setup of heat transfer oil system I and heat transfer oil system II is the same as in Example 1.

[0115] The oxidation of a mixture of methylal, methanol, and water to produce concentrated formaldehyde (methylal to methanol mass ratio 1:0.01, methylal to water mass ratio 1:0.12) is carried out. A methylal solution containing a portion of methanol and water (80% of the total methanol feed is from the same feed point as the methylal) is pumped into a methylal vaporizer at a flow rate of 411 g / h to obtain superheated aqueous methylal gas at 220°C. Air at a flow rate of 1180 L / h and nitrogen at a flow rate of 1343 L / h are mixed and heated to 220°C in a gas preheater, then mixed evenly with the aqueous methylal gas. The volume ratio of methylal to oxygen is 1:2.3. This mixture enters the oxidation reactor from the top for oxidation. The methanol solution from the methanol storage tank is pumped into the reactor at a flow rate of 0.7 g / h. A flow rate of g / h is pumped into the methanol vaporizer to obtain superheated methanol gas at 220°C, which then enters from the feed inlet in the middle of oxidation reactor 8. The reaction space velocity is 9000 h⁻¹. -1 The volume concentration of methyl acetal was 4%, and the bed reaction temperature distribution was 265~345℃. The reaction results showed that the conversion rate of methyl acetal was 97%, the selectivity of formaldehyde was 90.02%, the selectivity of methanol was 4.58%, the selectivity of dimethyl ether was 1.79%, the selectivity of carbon monoxide was 2.70%, and the selectivity of carbon dioxide was 0.95%. The formaldehyde gas obtained from the reaction outlet entered the concentrated formaldehyde absorption tower, and after absorption, a 68.1% concentrated formaldehyde solution with a capacity of 558 g / h was obtained, in which the methanol mass content was 3.86% and the formic acid content was 360 ppm.

[0116] Comparative Example 6 (Effect of Methanol Content)

[0117] The filling scheme in the oxidation reactor is the same as in Example 1, and the setup of heat transfer oil system I and heat transfer oil system II is the same as in Example 1.

[0118] The oxidation of a mixture of methylal, methanol, and water to produce concentrated formaldehyde (methylal to methanol mass ratio 1:0.15, methylal to water mass ratio 1:0.12) is carried out. A methylal solution containing a portion of methanol and water (80% of the total methanol feed is from the same feed point as the methylal) is pumped into a methylal vaporizer at a flow rate of 451 g / h to obtain superheated aqueous methylal gas at 220°C. Air at a flow rate of 1180 L / h and nitrogen at a flow rate of 1308 L / h are mixed and heated to 220°C in a gas preheater, then mixed evenly with the aqueous methylal gas. The volume ratio of methylal to oxygen is 1:2.3, and the mixture enters the reactor from the top for oxidation. The methanol solution from the methanol storage tank is pumped into the reactor at a flow rate of 10.75 g / h. A flow rate of g / h is pumped into the methanol vaporizer to obtain superheated methanol gas at 220°C, which then enters from the feed inlet in the middle of oxidation reactor 8. The reaction space velocity is 9000 h⁻¹. -1 The volume concentration of methyl acetal was 4%, and the bed reaction temperature distribution was 280~365℃. The reaction results showed that the conversion rate of methyl acetal was 100%, the selectivity of formaldehyde was 86.62%, the selectivity of methanol was 4.30%, the selectivity of dimethyl ether was 1.33%, the selectivity of carbon monoxide was 4.80%, and the selectivity of carbon dioxide was 3.13%. The formaldehyde gas obtained from the reaction outlet entered the concentrated formaldehyde absorption tower, and after absorption, a 68.3% concentrated formaldehyde solution with a capacity of 610 g / h was obtained, in which the methanol mass content was 3.72% and the formic acid content was 335 ppm.

[0119] Comparative Example 7 (Impact of methanol feed rate at the same feed location as methyl acetal on the total methanol feed rate)

[0120] The filling scheme in the oxidation reactor is the same as in Example 1, and the setup of heat transfer oil system I and heat transfer oil system II is the same as in Example 1.

[0121] The oxidation of a mixture of methylal, methanol, and water to produce concentrated formaldehyde (methylal to methanol mass ratio 1:0.07, methylal to water mass ratio 1:0.12) is carried out. A methylal solution containing a portion of methanol and water (60% of the total methanol feed is from the same feed point as the methylal) is pumped into a methylal vaporizer at a flow rate of 429 g / h to obtain superheated aqueous methylal gas at 220°C. Air at a flow rate of 1180 L / h and nitrogen at a flow rate of 1328 L / h are mixed and heated to 220°C in a gas preheater, then mixed evenly with the aqueous methylal gas. The volume ratio of methylal to oxygen is 1:2.3. This mixture enters the oxidation reactor from the top for oxidation. The methanol solution from the methanol storage tank is pumped into the reactor at a flow rate of 10.8 g / h. A flow rate of g / h is pumped into the methanol vaporizer to obtain superheated methanol gas at 220°C, which then enters from the feed inlet in the middle of oxidation reactor 8. The reaction space velocity is 9000 h⁻¹.-1 The volume concentration of methyl acetal was 4%, and the bed reaction temperature distribution was 270~350℃. The reactor temperature distribution is shown in [reference needed]. Figure 4 The reaction results showed that the conversion rate of methyl acetal was 100%, the selectivity of formaldehyde was 87.25%, the selectivity of methanol was 5.11%, the selectivity of dimethyl ether was 4.06%, the selectivity of carbon monoxide was 2.70%, and the selectivity of carbon dioxide was 1.23%. The formaldehyde gas obtained from the reaction outlet entered the concentrated formaldehyde absorption tower, and after absorption, a 601 g / h concentrated formaldehyde solution with a concentration of 66.7% was obtained, in which the methanol content was 4.16% and the formic acid content was 334 ppm.

[0122] Comparative Example 8 (Impact of methanol feed rate at the same feed location as methyl acetal on the total methanol feed rate)

[0123] The filling scheme in the oxidation reactor is the same as in Example 1, and the setup of heat transfer oil system I and heat transfer oil system II is the same as in Example 1.

[0124] The oxidation of a mixture of methylal, methanol, and water to produce concentrated formaldehyde (methylal to methanol mass ratio 1:0.07, methylal to water mass ratio 1:0.12) was carried out. A methylal solution containing a portion of methanol and water (100% of the total methanol feed was from the same feed point as the methylal) was pumped into a methylal vaporizer at a flow rate of 429 g / h to obtain superheated aqueous methylal gas at 220°C. Air at a flow rate of 1180 L / h and nitrogen at a flow rate of 1328 L / h were mixed and heated to 220°C in a gas preheater, then mixed thoroughly with the aqueous methylal gas. The volume ratio of methylal to oxygen was 1:2.3. This mixture entered the reactor from the top for the oxidation reaction. The reaction space velocity was 9000 h⁻¹. -1 The volume concentration of methyl acetal was 4%, and the bed reaction temperature distribution was 270~350℃. The reactor temperature distribution is shown in [reference needed]. Figure 4 The reaction results showed that the conversion rate of methyl acetal was 100%, the selectivity of formaldehyde was 88.40%, the selectivity of methanol was 4.06%, the selectivity of dimethyl ether was 3.93%, the selectivity of carbon monoxide was 2.70%, and the selectivity of carbon dioxide was 1.26%. The formaldehyde gas obtained from the reaction outlet entered the concentrated formaldehyde absorption tower, and after absorption, a 67.6% concentrated formaldehyde solution with a capacity of 587 g / h was obtained, in which the methanol mass content was 3.31% and the formic acid content was 334 ppm.

[0125] Table 1 Reaction conditions of Examples 1-15 and Comparative Examples 1-8

[0126]

[0127] In Table 1, "DMM:water" refers to the mass ratio of methyl acetal (DMM) to water during feeding, "DMM:methanol" refers to the mass ratio of methyl acetal (DMM) to methanol during feeding, and "methanol feed percentage" refers to the percentage of methanol feed at the same feed location as methyl acetal to the total methanol feed.

[0128] The preparation results of each embodiment and comparative example are shown in Table 2.

[0129] Table 2. Preparation results data for each example and comparative example.

[0130]

[0131] Examples 8, 9, Comparative Example 3, and Comparative Example 4 illustrate the effect of water vapor content on the oxidation reaction compared to Example 1. A decrease in water content in the raw materials reduces the synergistic effect of the water vapor-oxygen ratio, leading to an increase in side reactions. An increase in water content in the raw materials reduces the adsorption of methylal and methanol on the catalyst, thus decreasing the oxidation conversion rate of methylal and methanol. All of these process changes result in a decrease in the concentration of concentrated formaldehyde and an increase in the methanol content.

[0132] Examples 10, 11, 5, and 6 compare the effects of methanol content on the oxidation reaction with those of Example 1. Decreasing the methanol content in the feedstock reduces the heat released during the oxidation reaction, lowers the bed temperature, and decreases the conversion rates of methylal and methanol. Increasing the methanol content in the feedstock increases the heat released during the oxidation reaction, raises the bed temperature, and increases the number of side reactions. Simultaneously, increasing the amount of methanol fed into the middle of the oxidation reactor reduces the methanol conversion rate in the lower part of the reactor. All of these process changes result in a decrease in the concentration of concentrated formaldehyde and an increase in the methanol content.

[0133] Examples 12 and 13 compare the effects of oxygen content on the oxidation reaction with Example 1. When the oxygen content decreases, the oxidation conversion rate of methyl acetal decreases, and the oxidation conversion rate of hydrolyzed methanol also decreases. When the oxygen content increases, the oxidation reaction increases, over-oxidation is more likely to occur, and the bed temperature is more difficult to control, leading to an increase in side reactions. All of the above process changes result in a decrease in the concentration of concentrated formaldehyde and an increase in the methanol content.

[0134] Examples 14, 15, Comparative Examples 7 and 8, compared with Example 1, investigated the effect of the methanol feed rate at the same feed location as methylal on the total methanol feed rate. When the methanol feed rate at the same feed location as methylal increased, more oxidation reactions occurred at the top of the catalyst, resulting in higher hotspot temperatures, more side reactions, and difficulty in controlling the bed temperature. When the methanol feed rate at the same feed location as methylal decreased, the methanol feed rate in the middle of the oxidation reactor increased, making it difficult to adjust the bed temperature and reducing the methanol conversion rate at the bottom of the reactor. All of the above process changes resulted in a decrease in the concentration of concentrated formaldehyde and an increase in the methanol content.

[0135] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing formaldehyde by oxidation of methyl acetal, characterized in that, The feed volume fraction of methyl acetal is 2% to 6%, the feed volume ratio of methyl acetal to oxygen is 1:1.5 to 1:3.5, and the feed mass ratio of methyl acetal to water is 1:0.09 to 1:0.

24.

2. The method according to claim 1, characterized in that: The feed volume ratio of methyl acetal to oxygen is 1:1.8 to 1:3.0; methanol is also introduced during the feeding process.

3. The method according to claim 2, characterized in that: The feed volume ratio of methyl acetal to oxygen is 1:2.0 to 1:2.5; the feed mass ratio of methyl acetal to methanol is 1:0.02 to 1:0.

13.

4. The method according to claim 3, characterized in that: The mass ratio of methyl acetal to methanol in the feed is 1:0.04 to 1:0.

11.

5. The method according to claim 4, characterized in that: The mass ratio of methylal to methanol in the feed is 1:0.06 to 1:0.

08.

6. The method according to claim 2, characterized in that: The methanol may be fed from the same inlet as the methyl acetal and / or from the middle of the reactor.

7. The method according to claim 6, characterized in that: The methanol is fed from the same inlet as the methyl acetal and from the middle of the reactor, respectively. The amount of methanol fed from the same inlet as the methyl acetal accounts for 65% to 95% of the total methanol feed.

8. The method according to claim 7, characterized in that: The amount of methanol fed into the same feed location as methyl acetal accounts for 70-90% of the total methanol feed.

9. The method according to claim 8, characterized in that: The amount of methanol fed into the same feed location as methyl acetal accounts for 75-85% of the total methanol feed.

10. The method according to claim 1 or 2, characterized in that, The catalyst added during oxidation is an oxidation catalyst composed of a mixture of ferric molybdate and molybdenum trioxide. The oxidation reaction temperature is 280℃~380℃, and the gas hourly space velocity is 6000~11000 h⁻¹. -1 .

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