Device and method for preparing concentrated formaldehyde by oxidizing methylal

Abstract

The invention provides a device and a method for preparing concentrated formaldehyde by oxidizing methylal, and relates to the technical field of chemical production. The device comprises a methylal unit 15, a methylal gasifier 1, a fresh air fan 9, a conduction oil heat exchanger 11 and a concentrated aldehyde heater 12, the circulating gas booster fan 7, the circulating gas mixer 8, the reactor gas inlet and outlet heat exchanger 2, the methylal oxidation reactor 3, the reactor gas inlet and outlet heat exchanger 2, the formaldehyde absorption tower 4, the circulating gas dehumidifier 5, the gas-liquid separator 6 and the absorption tail gas treatment device 14 are connected in sequence; the upstream of the circulating gas mixer 8 is connected with the methylal gasifier 1; the upstream of the gas-liquid separator 6 is connected with a fresh air fan 9; the methylal oxidation reactor 3 is connected with the conduction oil heat exchanger 11; and the formaldehyde absorption tower 4 is connected with a concentrated formaldehyde heater 12. The problem of dilute formaldehyde treatment in a formaldehyde-related process is solved by adopting the byproduct dilute formaldehyde and the raw material methanol in the technical process.

Description

Technical Field

[0001] The invention relates to the technical field of chemical production, and in particular to a device and a method for preparing concentrated formaldehyde by oxidizing methylal. Background Art

[0002] The production of materials such as polyformaldehyde, polyoxymethylene dimethyl ether, methyl acrylate, polyoxymethylene, etc. all uses concentrated formaldehyde as the main raw material. At present, there are two main methods for preparing formaldehyde: direct oxidation of methanol and methylal oxidation.

[0003] The methanol direct oxidation method can be divided into the silver method and the iron-molybdenum method according to the catalyst used. For example, Chinese patent CN107501060A discloses a concentrated formaldehyde production process, in which formaldehyde is prepared by the methanol direct oxidation method, and the bottom discharge of the formaldehyde concentrator is a concentrated formaldehyde solution with a formaldehyde mass fraction greater than 60%. However, this method requires huge energy for sewage treatment, is energy-consuming and not environmentally friendly.

[0004] The methylal method is to synthesize methylal from dilute formaldehyde and methanol under the catalysis of solid acid, and obtain high-concentration methylal at a low energy cost. Methylal is further oxidized and absorbed to obtain concentrated formaldehyde.

[0005] Chinese patent CN20852196U discloses a device for producing concentrated formaldehyde, the working process of which is to mix pressurized air with vaporized methylal and enter a catalytic unit for reaction, and the materials after the reaction are subjected to heat recovery and cooling treatment, and finally about 55% of concentrated formaldehyde is extracted through an absorption unit. The concentration of concentrated formaldehyde obtained by the above scheme is below 60%, which cannot meet the market demand for high-concentration formaldehyde.

[0006] The traditional concentrated formaldehyde preparation process generally uses 45wt%-55wt% formaldehyde to obtain 65wt%-70wt% concentrated formaldehyde through vacuum falling film / rising film evaporation concentration, and the concentrated formaldehyde is then converted into corresponding products through a series of process steps. A large amount of dilute formaldehyde will be generated both in the formaldehyde evaporation concentration process and in the subsequent production process. How to deal with dilute formaldehyde has become a difficult problem for manufacturers of polyformaldehyde, polyoxymethylene dimethyl ether, methyl acrylate and polyoxymethylene.

[0007] At present, most of the dilute aldehydes are recovered by dilute aldehyde pressure distillation. However, the pressure distillation process has serious formaldehyde disproportionation, serious equipment corrosion, high equipment material requirements, relatively large investment, high energy consumption, and waste of resources. In addition, only <45wt% of formaldehyde can be obtained, and it is necessary to further concentrate it through vacuum falling film / rising film evaporation to obtain 65-70wt% concentrated formaldehyde. Since the existing technology often has high requirements for the concentration of formaldehyde products when using formaldehyde products, the value of dilute formaldehyde solution as a product is relatively low. Therefore, how to handle dilute formaldehyde, maximize its value with minimal energy consumption, and drive the market is the research focus of this field. It is also a problem that people in this field need to solve urgently.

[0008] Based on this, it is of great significance to develop a production device and method for preparing concentrated formaldehyde by oxidizing methylal: on the one hand, the by-product dilute formaldehyde produced in the process of preparing polyformaldehyde, polymethoxydimethyl ether, methyl acrylate and polyoxymethylene by effectively utilizing concentrated formaldehyde can solve the problem of treating dilute formaldehyde; on the other hand, it can reduce the energy consumption and operating costs of the process. Summary of the invention

[0009] In view of the above problems, the present invention provides a device and method for preparing concentrated formaldehyde by oxidizing methylal, through reasonable design of the device structure, using the dilute aldehyde produced as a by-product in the process as a raw material, reacting with methanol and refining, and accurately controlling the temperature during the reaction process to obtain 95wt%-98.5wt% of methylal. Methylal is oxidized and absorbed to obtain concentrated formaldehyde, and the dilute aldehyde produced in the subsequent process is recovered and continued to be used as a raw material for synthesizing methylal.

[0010] To achieve the above purpose, the technical solution adopted by the present invention is as follows: On the one hand, the present invention provides a device for preparing concentrated formaldehyde by oxidizing methylal, comprising: a methylal unit, a methylal gasifier, a fresh air blower, a heat transfer oil heat exchanger, a concentrated formaldehyde heater, a circulating gas pressurizing blower, a circulating gas mixer, a reactor inlet and outlet air heat exchanger, a methylal oxidation reactor, a formaldehyde absorption tower, a circulating gas dehumidifier, a gas-liquid separator, and an absorption tail gas treatment device connected in sequence; The circulating gas mixer is connected to a methylal gasifier upstream; The gas-liquid separator is connected upstream with a fresh air blower; The methylal oxidation reactor is connected to a thermal oil heat exchanger; The formaldehyde absorption tower is connected to a concentrated formaldehyde heater.

[0011] Preferably, the methylal gasifier is connected to low-grade steam.

[0012] Preferably, the first stage of the reactor inlet and outlet air heat exchanger is connected to a boiler water high-level tank.

[0013] Preferably, the circulating cooler is connected to the tower side of the formaldehyde absorption tower.

[0014] Preferably, the concentrated aldehyde heater is connected to low-grade steam.

[0015] Preferably, the tail gas from the top of the formaldehyde absorption tower is connected to an absorption tail gas treatment device or an incinerator.

[0016] Preferably, the circulating gas dehumidifier is used to remove water from the circulating gas and reduce the concentrations of formaldehyde and methanol in the circulating gas.

[0017] The circulating gas dehumidifier can reduce the formaldehyde and moisture in the circulating gas, thereby reducing side reactions, extending the use time of the catalyst, and improving the conversion rate. It requires less chilled water than the traditional absorption tower cooling, reducing the demand for chilled water by more than 50%.

[0018] Preferably, the gas-liquid separator is used to separate the dilute aldehyde entrained in the circulating gas from the circulating gas.

[0019] It can significantly reduce the possibility of dilute aldehyde entering the inlet pipeline of the circulating gas booster fan under various working conditions. If dilute aldehyde enters the pipeline, it will produce polymerized formaldehyde.

[0020] Preferably, the methylal oxidation reactor is a tubular catalyst fixed bed.

[0021] Preferably, a thermal oil electric heater is provided in the bypass between the thermal oil heat exchangers to increase the temperature of the thermal oil.

[0022] Preferably, the formaldehyde absorption tower adopts multi-stage segmented circulation spray absorption; On the one hand, it improves the formaldehyde absorption effect, and on the other hand, it controls the temperature distribution in the absorption tower by controlling the temperature of the spray liquid, thereby removing excess heat in the absorption tower.

[0023] Preferably, a bubble cap absorption tray is provided on the top of the formaldehyde absorption tower, and a multi-stage packing bed layer is provided below the bubble cap tray.

[0024] Preferably, the circulating gas dehumidifier uses a chilled water fin heat exchanger to condense the dilute aldehydes, methanol and water in the circulating gas through a cold surface; the dilute aldehydes are separated and sent to a gas-liquid separator.

[0025] On the one hand, it reduces the formaldehyde and moisture in the circulating gas, which can appropriately reduce useless circulation and thus reduce energy consumption. On the other hand, it can reduce the side reactions in the methylal oxidation reactor, improve the reaction selectivity and yield, and extend the service life of the catalyst.

[0026] Preferably, in the gas-liquid separator, the circulating gas and the air enter from both sides of the top of the gas-liquid separator, and a partition is provided between the two gases; and a liquid seal is provided at the bottom of the gas-liquid separator.

[0027] There may be a certain amount of dilute aldehyde entrained in the circulating gas due to various operating conditions. The use of a gas-liquid separator can more thoroughly separate the dilute aldehyde from the circulating gas and prevent the dilute aldehyde from entering the inlet pipeline of the circulating fan.

[0028] The isolation panels can reduce the amount of dilute formaldehyde volatilized into the fresh air, thereby reducing the total amount of formaldehyde in the circulating air.

[0029] The bottom liquid seal can prevent the circulating gas from short-circuiting from the circulating gas dehumidifier. It can also prevent formaldehyde from polymerizing in the gas-liquid separator under special working conditions.

[0030] In another aspect, the present invention provides a method for oxidizing methylal to prepare concentrated formaldehyde, comprising the following steps: S1: The air is pressurized by the fresh air blower and enters the gas-liquid separator, and then mixed with the circulating gas and enters the circulating gas pressurizing blower to obtain a circulating mixed gas; the pressurized circulating mixed gas is mixed with the methylal gas from the methylal gasifier in the circulating gas mixer to obtain a reaction circulating gas; S2: The reaction circulating gas obtained in step S1 is passed through the reactor inlet and outlet gas heat exchanger, and is heat exchanged with the reaction mixed gas after heat energy recovery, and the temperature is raised to 115°C-155°C; S3: the reaction circulating gas obtained in step S2 enters each tubular reactor of the methylal oxidation reactor, is heated to 230°C-250°C, enters the molybdenum-chromium-iron catalyst bed, and reacts under the action of the catalyst to obtain a post-reaction mixed gas; The reaction temperature in the methylal oxidation reactor is controlled by the gasification temperature of the thermal oil; S4: the reaction mixed gas obtained in step S3 is cooled in two stages through the reactor inlet and outlet gas heat exchangers, and enters the formaldehyde absorption tower to absorb formaldehyde through circulating spraying; the distribution of formaldehyde and methanol in the absorption tower is controlled by controlling the circulating spraying temperature; S5: the reaction mixed gas obtained in step S4 is absorbed in a formaldehyde absorption tower and then divided into two parts, one part is drawn out from the side of the formaldehyde absorption tower as circulating gas; the other part is further absorbed by the multi-layer bubble cap tray on the top of the formaldehyde absorption tower to obtain absorbed tail gas, which is then sent to an absorbed tail gas treatment device after pressure control; S6: In step S5, the circulating gas dehumidifier condenses and captures the moisture and dilute aldehydes in the circulating gas, passes through the gas-liquid separator, separates the dilute aldehydes, mixes them with air, and enters the circulation through the circulating gas pressure fan.

[0031] Preferably, in step S1, the amount of air added is adjusted according to the oxygen concentration in the circulating mixed gas; Preferably, in step S1, the pressure of the gas-liquid separator is 3 KPaG-40 KPaG; further preferably, the pressure of the gas-liquid separator is 3 KPaG-35 KPaG.

[0032] Preferably, in step S1, the air intake pressure at the inlet of the circulating gas pressurizing fan is 3KPaG-40KPaG; further preferably, in step S1, the air intake pressure at the inlet of the circulating gas pressurizing fan is 3KPaG-35KPaG.

[0033] Preferably, in step S2, the temperature is raised to 120°C-150°C.

[0034] Preferably, in step S3, the shell-and-tube reactor is selected from at least one of DN15, DN20 or DN25; further preferably, in step S3, the shell-and-tube reactor is DN20.

[0035] Preferably, in step S3, the flow rate of the reaction cycle gas in the tubular reactor is 1.0-5.0 Nm 3 / h / branch tube; Further preferably, in step S3, the flow rate of the reaction cycle gas in the tube reactor is 2.6-2.9 Nm 3 / h / branch tube.

[0036] Preferably, in step S3, the catalyst filling height in the shell-and-tube reactor is 700 mm-1300 mm; further preferably, in step S3, the catalyst filling height in the shell-and-tube reactor is 900 mm-1000 mm.

[0037] Preferably, in step S3, the hot spot temperature of the molybdenum-chromium-iron catalyst bed is 310°C-360°C, and the interlocking temperature is 400°C-410°C; further preferably, in step S3, the hot spot temperature of the molybdenum-chromium-iron catalyst bed is 320°C-350°C, and the interlocking temperature is 400°C-405°C.

[0038] Preferably, in step S3, the catalyst molybdenum-chromium-iron catalyst is selected from type A molybdenum-chromium-iron catalyst and / or type B molybdenum-chromium-iron catalyst; Preferably, the A-type molybdenum-chromium-iron catalyst comprises the following components by weight: 45-75 parts of molybdenum oxide, 20-45 parts of chromium oxide, 5-20 parts of iron oxide, and 3.5-12 parts of metal oxide; Further preferably, the A-type molybdenum-chromium-iron catalyst comprises the following components by mass: 50-70 parts of molybdenum oxide, 25-40 parts of chromium oxide, 10-20 parts of iron oxide, and 3.5-10 parts of metal oxide; The methanol content in the concentrated formaldehyde product prepared by using type A molybdenum-chromium-iron catalyst is ≤0.5wt%.

[0039] Preferably, the metal oxide is selected from at least one of the oxides of aluminum, titanium, strontium, cobalt, bismuth and vanadium; further preferably, the metal oxide is selected from at least one of the oxides of titanium, strontium and cobalt; Preferably, the B-type molybdenum-chromium-iron catalyst comprises the following components by mass: 45-75 parts of molybdenum oxide, 5-30 parts of chromium oxide, 5-20 parts of iron oxide, and 3.5-12 parts of metal oxide; Further preferably, the B-type molybdenum-chromium-iron catalyst comprises the following components in parts by mass: 50-70 parts of molybdenum oxide, 10-30 parts of chromium oxide, 10-20 parts of iron oxide, and 3.5-10 parts of metal oxide; The B-type molybdenum-chromium-iron catalyst is used to prepare the concentrated formaldehyde product, in which 0.5wt%≤methanol≤6wt%.

[0040] Preferably, the metal oxide is selected from at least one of the oxides of aluminum, titanium, strontium, cobalt, bismuth and vanadium; further preferably, the metal oxide is selected from at least one of the oxides of titanium, strontium and cobalt; Preferably, the shape of the catalyst is selected from cylindrical, annular, spherical, bar, honeycomb, gear or hollow cylinder; further preferably, the shape of the catalyst is selected from cylindrical and annular.

[0041] More preferably, the catalyst is in the shape of a Raschig ring.

[0042] Preferably, in step S3, the concentration of methylal at the inlet of the methylal oxidation reactor is 2.5 vol%-4.5 vol%; Preferably, in step S3, the gasification temperature of the thermal oil is controlled by controlling the gas phase pressure of the thermal oil through a thermal oil heat exchanger, thereby controlling the temperature of the methylal oxidation reactor 3.

[0043] Preferably, in step S3, the heat transfer oil is selected from at least one of biphenyl-biphenyl ether heat transfer oil and methylnaphthalene.

[0044] Preferably, in step S4, the two-stage cooling is specifically as follows: in the first stage, boiler water is used to exchange heat with the reaction circulating gas obtained in step S3 through the reactor inlet and outlet gas heat exchanger; after the temperature of the reaction circulating gas obtained in step S3 is reduced, heat exchange is performed with the reaction circulating gas from the circulating gas mixer in the second stage of the reactor inlet and outlet gas heat exchanger.

[0045] Preferably, in step S4, the cyclic spraying is a multi-stage filler cyclic spraying. The multi-stage filler is a PE step ring.

[0046] Preferably, in step S4, the circulating spray temperature is controlled by heating the circulating liquid in the kettle of the formaldehyde absorption tower through a concentrated aldehyde heater in the kettle.

[0047] Preferably, in step S4, the concentration of absorbed formaldehyde is 60wt%-72wt%.

[0048] Preferably, in step S5, softened water is added to the top of the formaldehyde absorption tower to further absorb formaldehyde gas.

[0049] Preferably, in step S5, the absorption tail gas contains carbon monoxide, methyl formate and dimethyl ether, which are oxidized by an absorption tail gas treatment device or an incineration system to meet emission standards.

[0050] Compared with the prior art, the present invention has the following beneficial effects: 1. The present invention realizes precise reaction temperature control of methylal through reasonable design of the device structure, control of the reactor inlet temperature, removal of reaction heat in the methylal oxidation reactor, two-stage cooling of the reactor inlet and outlet air heat exchanger, and multi-stage circulation spray temperature control; the conversion rate of methylal catalytic oxidation reaches more than 99.5%, and the formaldehyde concentration in concentrated formaldehyde is controlled at 60wt%-72wt% as needed; the concentration of methanol in concentrated formaldehyde is 0.5wt%-6.0wt%.

[0051] 2. The present invention achieves precise control of the methanol concentration by precisely controlling the temperature of the formaldehyde absorption tower. In addition, the spray design of the kettle of the formaldehyde absorption tower perfectly solves the scaling problem of concentrated formaldehyde and realizes continuous and stable production of concentrated formaldehyde.

[0052] 3. The present invention realizes the preparation of high-concentration formaldehyde. During the concentration process, the device of the present invention can reduce the energy consumption of formaldehyde concentration and reduce the production of dilute formaldehyde. There is no need for pressurized distillation of dilute formaldehyde, which reduces energy consumption, investment, and wastewater production, thereby significantly reducing carbon emissions.

[0053] 4. The present invention solves the problem of difficult treatment of dilute aldehyde in formaldehyde-related processes. Dilute aldehyde is converted into high-concentration methylal at a very low cost and used as a raw material for generating formaldehyde. The process flow is simple and efficient. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] Figure 1 The present invention is a schematic diagram of a device for oxidizing methylal to produce concentrated formaldehyde.

[0055] Markings in the figure are as follows: 1: methylal gasifier; 2: reactor inlet and outlet air heat exchanger; 3: methylal oxidation reactor; 4: formaldehyde absorption tower; 5: circulating gas dehumidifier; 6: gas-liquid separator; 7: circulating gas pressure fan; 8: circulating gas mixer; 9: fresh air fan; 10: boiler water high-level tank; 11: thermal oil heat exchanger; 12: concentrated aldehyde heater; 13: circulating cooler; 14: absorption tail treatment device; 15: methylal unit. DETAILED DESCRIPTION

[0056] In order to make the technical means, creative features, purpose and effect of the present invention easy to understand, the present invention is further explained below in conjunction with specific embodiments, but the following embodiments are only preferred embodiments of the present invention, not all. Based on the embodiments in the implementation mode, other embodiments obtained by those skilled in the art without making creative work all belong to the protection scope of the present invention. It is worth noting that the raw materials used in the present invention are all common commercial products, and their sources are not specifically limited. The technology and scientific terms used in the embodiments have the meanings commonly understood by those of ordinary skill in the art to which the present invention belongs.

[0057] Embodiment device: A device for preparing concentrated formaldehyde by methylal oxidation method, comprising a methylal gasifier 1; a reactor inlet and outlet air heat exchanger 2; a methylal oxidation reactor 3, a formaldehyde absorption tower 4; a circulating gas dehumidifier 5; a gas-liquid separator 6; a circulating gas pressure fan 7; a circulating gas mixer 8; a fresh air fan 9; a boiler water high-level tank 10; a heat transfer oil heat exchanger 11; a concentrated formaldehyde heater 12; a circulating cooler 13; an absorption tail treatment device 14; a methylal unit 15; Figure 1 Connections shown.

[0058] Among them, in the methylal gasifier 1, methylal comes from the methylal unit 15, is supplied by a pump, and is accurately metered and sent to the methylal gasifier 1. The -50~50kPa steam is used to enter the condensate from the shell side of the methylal gasifier 1, and the non-condensable gas is extracted from the lower part of the shell side. The raw material methylal is completely converted into methylal gas in the methylal gasifier 1.

[0059] Among them, the reactor inlet and outlet gas heat exchanger 2 is a combined heat exchanger of a steam generator and a gas heat exchanger. The reaction circulating gas first exchanges heat with boiler water in the first section to produce low-pressure steam as a by-product. After completing the first step of cooling, it enters the second section of the gas heat exchanger, exchanges heat with the unreacted reaction circulating gas, and is further cooled before being sent to the formaldehyde absorption tower 4, while the unreacted reaction circulating gas is heated and enters the reactor inlet. The second section of heat exchange is relatively gentle and safe, and will not cause the gas temperature at the reactor inlet to be too high, nor will it cause the temperature of the gas after the reaction entering the absorption tower to be too low. Due to the reasonable design of the heat exchange temperature, the formaldehyde in the circulating mixed gas after the reaction will not be likely to polymerize in the heat exchanger.

[0060] Among them, in the circulating gas mixer 8, the circulating gas and methylal gas with accurate flow rates are fully and evenly mixed in the circulating gas mixer 8. The circulating gas mixer 8 is well grounded for static electricity.

[0061] Among them, the pipeline data of methylal oxidation reactor 3: 20,000 DN20 tubes, length 1500mm. The shell side uses biphenyl-biphenyl ether heat transfer oil as the heat exchange medium.

[0062] The circulating air dehumidifier 5 is a chilled water fin heat exchanger, which uses chilled water as a refrigerant to capture moisture in the circulating air on the fins of the dehumidifier, and also captures most of the formaldehyde and methanol at the same time. The condensed dilute formaldehyde solution flows into the gas-liquid separator 6 under the action of gravity.

[0063] Among them, the circulating gas enters the gas-liquid separator 6 from the top, a small amount of condensate flows in, and the fresh air (air) from the fresh air blower 9 enters from the other side of the top of the gas-liquid separator 6. The fresh air flow rate is fine-tuned according to the oxygen concentration at the outlet of the circulating gas pressurized blower.

[0064] The formaldehyde absorption tower 4 is provided with 5 layers of random packing corresponding to 5 sections of spraying circulation and 13 layers of bubble cap trays on the top. The concentration of concentrated formaldehyde in the bottom of the tower is adjusted by adding the amount of desalted water on the top of the tower.

[0065] Among them, the heat transfer oil gas phase from the shell side of the methylal oxidation reactor 3 enters the heat transfer oil heat exchanger 11 from the lower head of the heat transfer oil heat exchanger 11, is condensed by the boiler water on the shell side in the tubes, and enters from the bottom of the shell side of the methylal oxidation reactor 3 to form a cycle. The boiler water on the shell side of the heat transfer oil heat exchanger 13 is vaporized to take away the reaction heat and produce medium-pressure steam as a by-product.

[0066] Example 1 Set the parameters as follows: The catalyst of the methylal oxidation reactor 3 is a type A molybdenum-chromium-iron catalyst; The catalyst formula is: 50-70 parts of molybdenum oxide, 25-40 parts of chromium oxide, 10-20 parts of iron oxide, and 3.5-10 parts of metal oxide; The metal oxide is a mixture of titanium dioxide, strontium oxide and cobalt oxide, and the mass ratio of titanium dioxide, strontium oxide and cobalt oxide is 1:0.15:0.5; The catalyst filling height is 1000mm; Raschig ring catalyst (outer diameter φ5±0.2mm, inner diameter φ2±0.2mm, height 4±0.2mm); The hot spot temperature of the catalyst bed is 315-360℃, and the interlocking temperature is 405℃.

[0067] Target reaction tube single tube flow rate: 2.0Nm 3 / h / branch tube, 2.4 Nm 3 / h / branch tube, 2.8 Nm 3 / h / branch tube, 3.0 Nm 3 / h / branch tube.

[0068] Methylal concentration at the reactor inlet: 2.5 vol%, 2.9 vol%, 3.3 vol%, 3.7 vol%, 4.1 vol%, 4.5 vol%.

[0069] Absorption tower target formaldehyde concentration: 60±0.5wt%. Methanol concentration: ≤0.5wt%.

[0070] Methylal raw material: methylal ≥ 98.0wt%.

[0071] Here are the steps: S1: air is pressurized by the fresh air blower 9 and enters the gas-liquid separator 6, and then enters the circulating gas pressurizing blower 7 after mixing with the circulating gas to obtain a circulating mixed gas; the pressurized circulating mixed gas is mixed with the methylal gas from the methylal gasifier 1 in the circulating gas mixer 8 to obtain a reaction circulating gas; the pressure of the gas-liquid separator 6 is 3KPaG-35KPaG; the inlet pressure of the circulating gas pressurizing blower 7 is 3KPaG-35KPaG; S2: The reaction circulating gas obtained in step S1 passes through the reactor inlet and outlet gas heat exchanger 2, exchanges heat with the post-reaction mixed gas after heat energy recovery, and is heated to 120°C-150°C; S3: The reaction circulating gas obtained in step S2 enters each tubular reactor (DN20) of the methylal oxidation reactor 3, is heated to 235°C-245°C, enters the molybdenum-chromium-iron catalyst bed, and reacts under the action of the catalyst to obtain a reaction mixed gas; The reaction temperature in the methylal oxidation reactor 3 is controlled by the gasification temperature of the thermal oil; the gasification temperature of the thermal oil is controlled by the thermal oil heat exchanger 11 to control the gas phase pressure of the thermal oil. Then the temperature of the methylal oxidation reactor 3 is controlled; The heat transfer oil is selected from at least one of biphenyl-biphenyl ether heat transfer oil and methyl naphthalene; S4: the reaction mixed gas obtained in step S3 is cooled in two stages through the reactor inlet and outlet gas heat exchangers 2, and enters the formaldehyde absorption tower 4 to absorb formaldehyde through multi-stage filler circulation spraying; the multi-stage filler is PE step ring; the formaldehyde and methanol concentration distribution in the absorption tower is controlled by controlling the circulation spraying temperature; the circulation spraying temperature is controlled by heating the circulating liquid in the tower bottom of the formaldehyde absorption tower 4 through the concentrated formaldehyde heater 12 in the tower bottom; The two-stage cooling is specifically as follows: in the first stage, boiler water is used to exchange heat with the reaction circulating gas obtained in step S3 through the reactor inlet and outlet gas heat exchanger 2. After the temperature of the reaction circulating gas obtained in step S3 is reduced, heat exchange is performed with the reaction circulating gas from the circulating gas mixer 8 in the second stage of the reactor inlet and outlet gas heat exchanger 2; S5: A part of the post-reaction mixed gas obtained in step S4 is introduced into the circulating gas dehumidifier 5 from the side of the formaldehyde absorption tower 4 for absorption to obtain circulating gas; the other part is further absorbed by the multi-layer bubble tower plate on the top of the formaldehyde absorption tower to obtain absorbed tail gas, which is then sent to the absorption tail gas treatment device 14 after pressure control; softened water is added to the top of the formaldehyde absorption tower 4 to further absorb formaldehyde gas, and the absorbed tail gas contains carbon monoxide, methyl formate, and dimethyl ether, which are oxidized by the ECS reactor or the incineration system and meet the emission standards.

[0072] S6: In step S5, the circulating gas dehumidifier 5 condenses and captures moisture and formaldehyde in the circulating gas to obtain dilute formaldehyde, which is separated by the gas-liquid separator 6 and then mixed with air and enters the circulation through the circulating gas pressurizing fan 7.

[0073] Table 1. Collection operation data 1

[0074] Table 2. Collection operation data 2

[0075] Table 3. Collection operation data 3

[0076] Note: “ / ” means not running.

[0077] Table 4. Collection operation data 4

[0078] Example 2 Set the parameters as follows: The catalyst of the methylal oxidation reactor 3 is a B-type molybdenum-chromium-iron catalyst; The catalyst formula is: 50-70 parts of molybdenum oxide, 10-30 parts of chromium oxide, 10-20 parts of iron oxide, and 3.5-10 parts of metal oxide; The metal oxide is a mixture of titanium dioxide, strontium oxide and cobalt oxide, and the mass ratio of titanium dioxide, strontium oxide and cobalt oxide is 1:0.15:0.5; The catalyst filling height is 1000mm; Raschig ring catalyst (outer diameter φ5±0.2mm, inner diameter φ2±0.2mm, height 4±0.2mm); The hot spot temperature of the catalyst bed is 315-350℃, and the interlocking temperature is 405℃.

[0079] Target reaction tube single tube flow rate 2.8 Nm 3 / h / branch tube.

[0080] Methylal concentration at reactor inlet: 3.7 vol%, 4.1 vol%.

[0081] Absorption tower target formaldehyde concentration: 69±1.0wt%. Methanol concentration: ≤6.0wt%.

[0082] Methylal raw material: methylal ≥ 98.0wt%.

[0083] The operation steps are the same as those in Example 1.

[0084] Table 5. Collection operation data 5

[0085] Note: “ / ” means not running.

[0086] According to the data in Table 1-Table 5, at 2.8±0.05Nm 3 / h flow rate, the best effect is achieved when the methylal concentration is controlled between 4.1%±0.05%; the present invention realizes precise reaction temperature control of methylal through reasonable design of the device structure, reactor inlet temperature, removal control of reaction heat in the methylal oxidation reactor, two-stage cooling of the reactor inlet and outlet air heat exchanger, and multi-stage circulation spray temperature control; the conversion rate of methylal catalytic oxidation reaches more than 99.5%, and the formaldehyde concentration in concentrated formaldehyde is controlled between 60wt% and 72wt% as needed; the concentration of methanol in concentrated formaldehyde is 0.5wt% to 6.0wt%.

[0087] Finally, it should be noted that the above content is only used to illustrate the technical solution of the present invention, rather than to limit the scope of protection of the present invention. Simple modifications or equivalent substitutions of the technical solution of the present invention by ordinary technicians in this field do not deviate from the essence and scope of the technical solution of the present invention.

Claims

1. A device for producing concentrated formaldehyde by oxidizing methylal, characterized in that: The invention comprises: a methylal unit (15), a methylal gasifier (1), a fresh air blower (9), a heat transfer oil heat exchanger (11), a concentrated aldehyde heater (12), a circulating gas pressurizing blower (7), a circulating gas mixer (8), a reactor inlet and outlet air heat exchanger (2), a methylal oxidation reactor (3), a formaldehyde absorption tower (4), a circulating gas dehumidifier (5), a gas-liquid separator (6) and an absorption tail gas treatment device (14) connected in sequence; The circulating gas mixer (8) is connected upstream to the methylal gasifier (1); The gas-liquid separator (6) is connected upstream to a fresh air blower (9); The methylal oxidation reactor (3) is connected to a heat transfer oil heat exchanger (11); The formaldehyde absorption tower (4) is connected to a concentrated formaldehyde heater (12).

2. The device according to claim 1, characterized in that The methylal gasifier (1) is connected to low-grade steam; The reactor inlet and outlet air heat exchanger (2) is connected to the boiler water high level tank (10) in the first stage; The formaldehyde absorption tower (4) is connected to a circulating cooler (13); The concentrated aldehyde heater (12) is connected to low-grade steam; The pipeline bypass of the methylal oxidation reactor (3) is connected to a thermal oil electric heater; The tail gas at the top of the formaldehyde absorption tower (4) is connected to an absorption tail gas treatment device (14) or an incinerator.

3. The device according to claim 1, characterized in that The circulating gas dehumidifier (5) is used to remove water from the circulating gas, thereby reducing the concentrations of formaldehyde and methanol in the circulating gas.

4. The device according to claim 1, characterized in that The gas-liquid separator (6) is used to separate the dilute aldehydes entrained in the circulating gas.

5. The device according to claim 1, characterized in that The circulating gas dehumidifier (5) uses a chilled water fin heat exchanger to condense the dilute aldehydes, methanol and water in the circulating gas through a cold surface; the dilute aldehydes are separated and sent to a gas-liquid separator (6).

6. The device according to claim 1, characterized in that The circulating gas and air enter from both sides of the top of the gas-liquid separator (6), and a partition is provided between the two gases; a liquid seal is provided at the bottom of the gas-liquid separator.

7. A method for preparing concentrated formaldehyde by oxidizing methylal, characterized in that: Using the device according to any one of claims 1 to 6 comprises the following steps: S1: air is pressurized by the fresh air blower (9) and enters the gas-liquid separator (6), and then is mixed with the circulating gas and enters the circulating gas pressurizing blower (7) to obtain a circulating mixed gas; the pressurized circulating mixed gas is mixed with the methylal gas from the methylal gasifier (1) in the circulating gas mixer (8) to obtain a reaction circulating gas; S2: The reaction circulating gas obtained in step S1 passes through the reactor inlet and outlet gas heat exchanger (2), exchanges heat with the post-reaction mixed gas after heat energy recovery, and is heated to 115°C-155°C; S3: the reaction circulating gas obtained in step S2 enters each tubular reactor of the methylal oxidation reactor (3), is heated to 230°C-250°C, enters the molybdenum-chromium-iron catalyst bed, and reacts under the action of the catalyst to obtain a post-reaction mixed gas; The reaction temperature in the methylal oxidation reactor (3) is controlled by the gasification temperature of the thermal oil; S4: the reaction mixture obtained in step S3 is cooled in two stages by the reactor inlet and outlet heat exchangers (2), and then enters the formaldehyde absorption tower (4) to absorb formaldehyde by circulating spraying; the distribution of formaldehyde and methanol in the absorption tower is controlled by controlling the circulating spraying temperature; S5: the reaction mixed gas obtained in step S4 is absorbed by the formaldehyde absorption tower (4) and then divided into two parts, one part is drawn out from the side of the formaldehyde absorption tower (4) as circulating gas; the other part is further absorbed by the multi-layer bubble cap tray on the top of the formaldehyde absorption tower (4) to obtain absorbed tail gas, which is then sent to the absorbed tail gas treatment device (14) after pressure control; S6: In step S5, the circulating gas dehumidifier (5) condenses and captures the moisture and dilute aldehydes in the circulating gas, passes through the gas-liquid separator (6), separates the dilute aldehydes, mixes with air, and enters the circulation through the circulating gas pressure fan (7).

8. The method according to claim 7, characterized in that In step S1, the pressure of the gas-liquid separator (6) is 3 KPaG-40 KPaG.

9. The method according to claim 7, characterized in that: In step S1, the air intake pressure at the inlet of the circulating air pressurizing fan (7) is 3KPaG-40KPaG.

10. The method according to claim 7, characterized in that In step S3, the flow rate of the reaction cycle gas in the tubular reactor is 1.0-5.0 Nm 3 / h / branch tube.

11. The method according to claim 7, characterized in that In step S3, the molybdenum-chromium-iron catalyst is selected from type A molybdenum-chromium-iron catalyst and / or type B molybdenum-chromium-iron catalyst; The A-type molybdenum-chromium-iron catalyst comprises the following components by weight: 45-75 parts of molybdenum oxide, 20-45 parts of chromium oxide, 5-20 parts of iron oxide, and 3.5-12 parts of metal oxide; The metal oxide is selected from at least one of the oxides of aluminum, titanium, strontium, cobalt, bismuth and vanadium; Using type A molybdenum-chromium-iron catalyst, the methanol content in the prepared concentrated formaldehyde product is ≤0.5wt%; The B-type molybdenum-chromium-iron catalyst comprises the following components by weight: 45-75 parts of molybdenum oxide, 5-30 parts of chromium oxide, 5-20 parts of iron oxide, and 3.5-12 parts of metal oxide; The metal oxide is selected from at least one of the oxides of aluminum, titanium, strontium, cobalt, bismuth and vanadium; The B-type molybdenum-chromium-iron catalyst is used to prepare the concentrated formaldehyde product, in which 0.5wt%≤methanol≤6wt%.

12. The method according to claim 7, characterized in that In step S3, the gasification temperature of the heat transfer oil is controlled by the heat transfer oil heat exchanger (11), and the gas phase pressure of the heat transfer oil is controlled, thereby controlling the temperature of the methylal oxidation reactor (3).

13. The method according to claim 7, characterized in that In step S3, the heat transfer oil is selected from at least one of biphenyl-biphenyl ether heat transfer oil and methylnaphthalene.

14. The method according to claim 7, characterized in that In step S4, the two-stage cooling is specifically as follows: in the first stage, boiler water is used to exchange heat with the reaction circulating gas obtained in step S3 through the reactor inlet and outlet gas heat exchanger (2); after the temperature of the reaction circulating gas obtained in step S3 is reduced, heat exchange is then carried out with the reaction circulating gas from the circulating gas mixer (8) in the second stage of the reactor inlet and outlet gas heat exchanger (2).

15. The method according to claim 7, characterized in that In step S4, the cyclic spraying is a multi-stage filler cyclic spraying; the multi-stage filler is a PE step ring.

16. The method according to claim 7, characterized in that In step S4, the circulating spray temperature is controlled by heating the circulating liquid in the bottom of the formaldehyde absorption tower (4) through the concentrated formaldehyde heater (12) in the bottom of the tower.

17. The method according to claim 7, characterized in that In step S4, the concentration of absorbed formaldehyde is 60wt%-72wt%.

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