Tail gas recovery method and polyoxymethylene production method and apparatus

Abstract

This invention provides a method for recovering exhaust gas, a method for preparing polyoxymethylene (POM), and an apparatus. The exhaust gas enters a primary absorption tower and is absorbed by a first absorption medium, which includes trioxymethylene and a first circulating liquid from the primary absorption tower. The first gas phase output from the primary absorption tower enters the lower section of a secondary absorption tower and is absorbed by a second absorption medium, which includes a second circulating liquid from the lower section of the secondary absorption tower and a fourth circulating liquid from the upper section of the secondary absorption tower. The second gas phase output from the lower section of the secondary absorption tower enters the upper section of the secondary absorption tower and is absorbed by a third absorption medium, which includes water and a third circulating liquid from the upper section. This invention can significantly improve the recovery rate of trioxymethylene and formaldehyde in the exhaust gas (up to 98%–99.99%), while reducing water consumption in the entire absorption process, saving energy, reducing wastewater and other waste emissions, and improving the conversion rate of trioxymethylene and other raw materials in the POM preparation process.

Description

Technical Field

[0001] This invention relates to the field of polyoxymethylene (POM) preparation, specifically to a method for exhaust gas recovery and a method and apparatus for preparing POM. Background Technology

[0002] Recovering and treating exhaust gases containing paraformaldehyde and / or formaldehyde can not only purify the exhaust gases but also recover effective components such as paraformaldehyde and / or formaldehyde, improving raw material utilization and holding significant importance for practical industrial applications. However, existing exhaust gas treatment processes are inefficient in recovering and treating such exhaust gases containing paraformaldehyde and / or formaldehyde, resulting in high energy consumption and severe emissions of waste gas, wastewater, and solid waste. Improving the recovery efficiency of target components such as formaldehyde and paraformaldehyde from these exhaust gases, reducing waste emissions, and lowering energy consumption are urgent technical problems to be solved in this field. Summary of the Invention

[0003] This invention provides a tail gas recovery method and a method and apparatus for preparing polyoxymethylene (POM), which are applicable to the recovery and utilization of tail gas generated during the production of POM. It can improve the recovery efficiency of target components such as formaldehyde and trioxymethylene in the tail gas, reduce the emission of waste gas, reduce energy consumption, and effectively solve the technical problems existing in the prior art.

[0004] In one aspect, the present invention provides a method for recovering exhaust gas containing paraformaldehyde and / or formaldehyde, which is recovered using a primary absorption tower and a secondary absorption tower. The secondary absorption tower includes an upper section and a lower section. The exhaust gas recovery method includes: introducing the exhaust gas into the primary absorption tower and contacting it with a first absorption medium entering the primary absorption tower, so that the first absorption medium absorbs the exhaust gas to obtain a first liquid phase and a first gas phase; at least a portion of the first liquid phase is returned to the primary absorption tower as a first circulating liquid; wherein the first absorption medium includes paraformaldehyde and the first circulating liquid; and introducing the first gas phase into the lower section of the secondary absorption tower and contacting it with a second absorption medium entering the lower section of the secondary absorption tower. The second absorbent medium absorbs the first gas phase to obtain a second liquid phase and a second gas phase, respectively. At least a portion of the second liquid phase is returned to the lower section of the secondary absorber as a second circulating liquid. The second gas phase enters the upper section of the secondary absorber and contacts the third absorbent medium entering the upper section of the secondary absorber, so that the third absorbent medium absorbs the second gas phase to obtain a third liquid phase and a third gas phase, respectively. A portion of the third liquid phase is returned to the upper section of the secondary absorber as a third circulating liquid, and a portion is returned to the lower section of the secondary absorber as a fourth circulating liquid. The second absorbent medium includes the second circulating liquid and the fourth circulating liquid. The third absorbent medium includes water and the third circulating liquid.

[0005] According to one embodiment of the present invention, the mass ratio of the first absorbent medium to the tail gas is (45-60):1; and / or, the operating conditions of the primary absorption tower are: pressure of -0.005 to 0 MPaG and temperature of 75 to 110°C; and / or, the process of allowing the tail gas to enter the primary absorption tower and contact it with the first absorbent medium entering the primary absorption tower includes: rinsing the tail gas entering the primary absorption tower with the first absorbent medium; and / or, the first circulating liquid is cooled by a first cooler and then returned to the primary absorption tower, the first cooler including a refrigerant for cooling, the temperature of the refrigerant being 60 to 80°C.

[0006] According to one embodiment of the present invention, the operating conditions of the lower section of the secondary absorption tower are: pressure of -0.01 to 0 MPaG and temperature of 40 to 70°C; and / or, the first gas phase entering the lower section of the secondary absorption tower is in countercurrent contact with the second absorption medium.

[0007] According to one embodiment of the present invention, the operating conditions of the upper section of the secondary absorption tower are: pressure of 0 to 0.01 MPaG and temperature of 30 to 60°C; and / or, the mass ratio of water to tail gas is (1 to 3):1; and / or, the second gas phase entering the upper section of the secondary absorption tower is in countercurrent contact with the third absorption medium; and / or, the second gas phase is mixed with dilution gas before entering the upper section of the secondary absorption tower.

[0008] According to one embodiment of the present invention, the second gas phase is mixed with the dilution gas and then conveyed to the upper section of the secondary absorption tower by a fan; and / or, the mass ratio of the second gas phase to the dilution gas is 1:(0.5-5); and / or, the dilution gas includes nitrogen.

[0009] According to one embodiment of the present invention, the primary absorption tower has a cavity structure, and / or the secondary absorption tower is provided with trays or packing.

[0010] In another aspect, the present invention provides a method for preparing polyoxymethylene, comprising: feeding a raw material including trioxymethylene into a polymerization reactor to perform a polymerization reaction, thereby obtaining polyoxymethylene and tail gas; recovering the tail gas according to the above-described tail gas recovery method, wherein during the recovery process, a portion of the first liquid phase is returned to the primary absorption tower as a first circulating liquid, and a portion is returned to the polymerization reactor as a raw material to perform the polymerization reaction.

[0011] According to one embodiment of the present invention, the conditions for the polymerization reaction are: reaction temperature of 100-150°C, reaction pressure of 0.001-0.02 MPaG, and reaction residence time of 2-60 min.

[0012] In another aspect, the present invention provides a tail gas treatment device, comprising a primary absorption tower and a secondary absorption tower, wherein the secondary absorption tower comprises an upper section connected to each other and a lower section located below the upper section; the primary absorption tower comprises a first gas phase outlet, a first liquid phase outlet, and a first circulating liquid inlet communicating with the first liquid phase outlet; the lower section of the secondary absorption tower comprises a first gas phase inlet, a second gas phase outlet, a second liquid phase outlet, and a second circulating liquid inlet communicating with the second liquid phase outlet, wherein the first gas phase inlet is communicating with the first gas phase outlet of the primary absorption tower; the upper section of the secondary absorption tower comprises a second gas phase inlet, a third liquid phase outlet, and a third circulating liquid inlet communicating with the third liquid phase outlet, wherein the second gas phase inlet is communicating with the second gas phase outlet of the upper section, and the third liquid phase outlet is communicating with the second circulating liquid inlet of the upper section.

[0013] In another aspect, the present invention provides a polyoxymethylene (POM) preparation apparatus, comprising a polymerization reactor and the aforementioned tail gas treatment apparatus, wherein the first liquid phase outlet of the primary absorption tower is connected to the polymerization reactor, and / or the primary absorption tower includes a tail gas inlet, and the polymerization reactor includes a tail gas outlet connected to the tail gas inlet of the primary absorption tower.

[0014] In this invention, a series-connected primary and secondary absorption towers are used to recover exhaust gas. The secondary absorption tower is designed with two sections (upper and lower). The exhaust gas first enters the primary absorption tower and is absorbed by the first absorption medium. Then, the first gas phase output from the primary absorption tower enters the lower section of the secondary absorption tower and is absorbed by the second absorption medium. The second gas phase output from the lower section of the secondary absorption tower then enters the upper section of the secondary absorption tower and is absorbed by the third absorption medium. Specifically, paraformaldehyde is introduced into the primary absorption tower, along with the first circulating liquid, as the first absorption medium to absorb the exhaust gas without the introduction of water. The lower section of the secondary absorption tower uses the second circulating liquid generated in the lower section and the fourth circulating liquid from the upper section as the second absorption medium. Water is introduced into the upper section of the secondary absorption tower, along with the third circulating liquid, as the third absorption medium.

[0015] In the above-mentioned recovery process, the primary absorption tower can selectively absorb paraformaldehyde and / or formaldehyde in the exhaust gas, and can absorb most of the paraformaldehyde and / or formaldehyde in the exhaust gas, reducing the vapor pressure of paraformaldehyde and / or formaldehyde in the secondary absorption tower. This significantly reduces the total amount of gas that needs to be absorbed by the secondary absorption tower, thus reducing the amount of water required for the absorption process in the secondary absorption tower. At the same time, the secondary absorption tower is designed with upper and lower sections. The exhaust gas absorbed by the primary absorption tower (i.e., the first gas phase) is absorbed sequentially by the lower and upper sections of the secondary absorption tower. The synergistic effect of each absorption process can greatly reduce the total amount of the third gas phase generated after absorption by the secondary absorption tower, significantly improving the recovery rate of paraformaldehyde and formaldehyde (up to 98% to 99.99%). Meanwhile, the water consumption of the entire absorption process is greatly reduced, saving energy and reducing the discharge of wastewater and other wastes, thereby reducing the cost of waste treatment. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the exhaust gas recovery device according to an embodiment of the present invention.

[0017] Figure 2 This is a schematic diagram of a polyoxymethylene preparation apparatus according to an embodiment of the present invention.

[0018] Explanation of reference numerals in the attached diagram: 1: Primary absorption tower; 2: Secondary absorption tower; 21: Lower section; 22: Upper section; 3: Polymerization reactor; 4: Refining unit; 5: Waste gas treatment unit; 100: Fan; 101: First circulation pump; 102: Second circulation pump; 103: Third circulation pump; 111: First cooler; 112: Second cooler; 113: Third cooler; a: Trioxymethylene; b1: First liquid phase; b 11 g: First circulating liquid; g: Exhaust gas; g1: First gas phase; g2: Second gas phase; g3: Third gas phase; b2: Second liquid phase; b 21 b3: Second circulating liquid; b4: Third liquid phase; b 31 : Third circulating fluid; b 32 Fourth circulating fluid. Detailed Implementation

[0019] To enable those skilled in the art to better understand the present invention, the present invention will be further described in detail below. The specific embodiments listed below are merely descriptions of the principles and features of the present invention, and the examples are only for explaining the present invention and are not intended to limit the scope of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] Recycling exhaust gases containing trioxymethylene and / or formaldehyde can not only purify the exhaust gases, but also recover effective components such as trioxymethylene and / or formaldehyde from the exhaust gases, thereby improving the utilization rate of raw materials. This is of great significance for practical industrial applications.

[0021] For example, polyoxymethylene (POM) is a high-performance engineering plastic with excellent mechanical properties, wear resistance, and corrosion resistance. It is widely used in electronics, machinery, instrumentation, daily light industry, automobiles, building materials, agriculture, and other fields. The production process of POM generates exhaust gases containing formaldehyde, trioxymethylene, and other components. The treatment of these exhaust gases is a crucial step in the POM production process.

[0022] Formaldehyde can be polymerized into polyoxymethylene (POM), but its conversion rate is lower than that of trioxymethylene (TOM). Therefore, using TOM for polymerization to produce POM has a relatively better application prospect. In the TOM polymerization process, the reaction usually needs to be carried out at a high temperature. Unreacted TOM raw materials and by-products such as formaldehyde decomposed from TOM raw materials will leave the polymerization reactor in gaseous form. This means that the tail gas generated during POM production contains TOM and formaldehyde, enabling the recovery and reuse of effective components such as TOM and formaldehyde. This is of great significance for improving the single-pass conversion rate of TOM and the yield of the target POM product.

[0023] In the two-stage absorption tail gas recovery process, the first-stage absorption tower uses the circulating liquid from the bottom of the first-stage absorption tower and the liquid phase output from the bottom of the second-stage absorption tower (or liquid output, i.e., the liquid output from the second-stage absorption tower) as the absorption medium. The liquid phase from the bottom of the first-stage absorption tower is returned to the trioxymethylene refining unit for further refining, and the gas phase output from the top of the first-stage absorption tower enters the bottom of the second-stage absorption tower. The second-stage absorption tower uses the circulating liquid from the bottom of the second-stage absorption tower and water as the absorption medium. The liquid phase output from the bottom of the second-stage absorption tower is returned to the first-stage absorption tower as the absorption medium, and the gas phase output from the top of the second-stage absorption tower is pressurized by a fan and sent to the waste gas treatment unit for purification. The process of recovering tail gas containing trioxymethylene and / or formaldehyde (such as tail gas generated in the production process of polyoxymethylene) using this tail gas recovery treatment process usually has the following defects: (1) The single-pass conversion rate of trioxymethylene is low (only 60-80%). Generally, the conversion rate of raw materials is improved by increasing the number of polymerization reactors (for example, by using 2-4 polymerization reactors in series to increase the catalyst contact amount, reaction time and other conditions, thereby improving the conversion rate of raw materials). However, this not only has a limited effect on improving the conversion rate of raw materials, but also leads to high investment costs and process complexity. (1) High production cost; (2) After absorption, the tail gas must be completely distilled and purified in the trioxymethylene refining unit, resulting in high separation energy consumption; (3) The entire recovery process requires a large amount of water (the mass ratio of water used to the tail gas reacting with trioxymethylene can reach 6 to 10:1), and the water used needs to be discharged to the wastewater system after a series of processes, resulting in a large total amount of wastewater and high cost of treating the three wastes; (4) After the tail gas is absorbed in two stages, the recovery rate of trioxymethylene and formaldehyde is low (studies show that the recovery rate of trioxymethylene and formaldehyde is generally only 90 to 95%), resulting in large material loss.

[0024] In view of the above problems, embodiments of the present invention provide a method for recovering exhaust gas, wherein the exhaust gas contains paraformaldehyde and / or formaldehyde, such as... Figure 1 and Figure 2 As shown, a primary absorption tower 1 and a secondary absorption tower 2 are used for recovery. The secondary absorption tower 2 includes an upper section 22 and a lower section 21. The tail gas recovery method includes:

[0025] The tail gas g is introduced into the first-stage absorption tower 1 and comes into contact with the first absorption medium inside the first-stage absorption tower 1, so that the first absorption medium absorbs the tail gas g and obtains the first liquid phase b1 and the first gas phase g1 respectively.

[0026] The first liquid phase b1 is at least partially used as the first circulating liquid b 11 Return to the primary absorption tower 1; wherein, the first absorption medium includes trioxymethylene (a) and the first circulating liquid (b). 11 ;

[0027] The first gas phase g1 is introduced into the lower section 21 of the secondary absorption tower 2 and comes into contact with the second absorption medium entering the lower section 21 of the secondary absorption tower 2, so that the second absorption medium absorbs the first gas phase g1 and obtains the second liquid phase b2 and the second gas phase g2 respectively.

[0028] The second liquid phase b2 is at least partially used as the second circulating liquid b. 21 Return to the lower section 21 of the secondary absorption tower 2;

[0029] The second gas phase g2 is introduced into the upper section 22 of the secondary absorption tower 2 and comes into contact with the third absorption medium introduced into the upper section 22 of the secondary absorption tower 2, so that the third absorption medium absorbs the second gas phase g2 to obtain the third liquid phase b3 and the third gas phase g3 respectively.

[0030] The third liquid phase b3 portion serves as the third circulating liquid b. 31 The portion returned to the upper section 22 of the secondary absorption tower 2 is used as the fourth circulating liquid b. 32 Return to the lower section 21 of the secondary absorption tower 2;

[0031] The second absorption medium includes the second circulating liquid b. 21 and the fourth circulating fluid b 32 The third absorption medium includes water and the third circulating liquid b. 31 .

[0032] Thus, trioxymethylene (a) is introduced into the primary absorption tower 1 and reacts with the first circulating liquid (b). 11 As the first absorption medium, the tail gas g is absorbed without the introduction of water; the lower section 21 of the secondary absorption tower 2 uses the second circulating liquid b generated in the lower section 21 of the secondary absorption tower 2. 21 and the fourth circulating liquid b from the upper section 22 of the secondary absorption tower 2. 32 As the second absorption medium, water is introduced into the upper section 22 of the secondary absorption tower 2, which is then mixed with the third circulating liquid b. 31As the third absorption medium, the primary absorption tower 1 selectively absorbs paraformaldehyde and / or formaldehyde in the tail gas g, and ensures that most of the paraformaldehyde and / or formaldehyde in the tail gas g is absorbed, reducing the vapor pressure of paraformaldehyde and / or formaldehyde in the secondary absorption tower 2. This significantly reduces the tail gas treatment volume of the secondary absorption tower 2 and the water required for the absorption process. Furthermore, the secondary absorption tower 2 is designed with two sections. The tail gas (i.e., the first gas phase g1) after absorption by the primary absorption tower 1 is sequentially absorbed by the lower section 21 and the upper section 22 of the secondary absorption tower 2. The coordinated absorption processes greatly reduce the total volume of the third gas phase g3 generated after absorption by the secondary absorption tower 2, significantly improving the recovery rate of paraformaldehyde and formaldehyde (up to 98%–99.99%), greatly reducing material loss. Simultaneously, the water consumption of the entire absorption process is greatly reduced, saving energy and reducing the discharge of wastewater and other waste gases, thereby reducing the cost of waste gas treatment.

[0033] In addition, such as Figure 1 and Figure 2 As shown, the first liquid phase b1 output from the primary absorption tower 1 can be partially used as the first circulating liquid b. 11 Returning to the primary absorption tower 1, a portion of the gas is directly used as raw material for the synthesis of polyoxymethylene (POM), entering the polymerization reactor 3 to participate in the polymerization reaction and generate POM. This eliminates the need for distillation and purification in the purification unit 4 (e.g., when the tail gas g contains trioxymethylene, there is no need for the first liquid phase b1 to enter the trioxymethylene purification unit 4 for purification). This saves separation energy and simplifies the entire tail gas treatment process and the POM preparation process. Simultaneously, this also improves the single-pass conversion rate and total conversion rate of raw materials such as trioxymethylene (the single-pass conversion rate can reach 80%–90%). Generally, only one polymerization reactor 3 is needed to obtain a high single-pass conversion rate and total conversion rate, without the need for complex process structures such as multiple polymerization reactors 3. It also has advantages such as a short POM production process, low cost, and small footprint.

[0034] Under normal circumstances, after the first liquid phase b1 is discharged from the bottom of the primary absorption tower 1, it is pressurized by the first circulation pump 101 and then at least part of it is used as the first circulating liquid b. 11 Returning to the primary absorption tower 1, for example, part of it is used as the first circulating liquid b. 11 It returns to the primary absorption tower 1, and part of it enters the polymerization reactor 3 used to prepare polyoxymethylene as raw material.

[0035] Relatively speaking, if the mass ratio of the first absorbent medium to the tail gas g is too small (less than 45:1), the paraformaldehyde and / or formaldehyde in the tail gas g will be difficult to be fully absorbed, which will affect the recovery rate to some extent and result in material loss; while if the mass ratio of the first absorbent medium to the tail gas g is too large (greater than 60:1), the first circulating liquid b11 Larger flow rates increase power consumption (e.g., the flow rate of liquid b in the first circulating liquid of the first-stage absorption tower 1 is higher). 11 The power consumption and energy consumption of the first circulation pump 101 are considered. Taking these factors into account, the preferred mass ratio of the first absorption medium to the tail gas g is generally (45-60):1, that is, the ratio of the sum of the masses of all the first absorption medium to the mass of the tail gas g is (45-60):1. For example, the first absorption medium consists of the above-mentioned trioxymethylene a and the first circulating liquid b. 11 The composition, then the mass ratio of the first absorption medium to the tail gas g is (45-60):1 refers to the ratio of trioxymethylene a to the first circulating liquid b. 11 The ratio of the sum of their masses to the mass of the exhaust gas (g) is (45-60):1.

[0036] For example, the mass ratio of the first absorption medium to the exhaust gas g can be a range of 45:1, 48:1, 50:1, 52:1, 55:1, 58:1, 60:1, or any two of these.

[0037] In addition, the operating conditions of the primary absorption tower 1 can be: pressure of -0.005 to 0 MPaG, for example, a range of -0.005 MPaG, -0.004 MPaG, -0.003 MPaG, -0.002 MPaG, -0.001 MPaG, 0 MPaG or any two thereof; and temperature of 75 to 110℃, for example, a range of 75℃, 80℃, 85℃, 90℃, 95℃, 100℃, 105℃, 110℃ or any two thereof.

[0038] Specifically, when the operating pressure of the primary absorption tower 1 is too low (less than -0.005 MPaG), energy consumption (such as the energy consumption of the fan 100 described below) increases. Simultaneously, when the primary absorption tower 1 is connected to the polymerization reactor 3 used for preparing polyoxymethylene, so that the first liquid phase b1 output from the primary absorption tower 1 is used as a raw material for preparing polyoxymethylene and enters the polymerization reactor 3 (such as... Figure 2 As shown, since the gas phase space of the primary absorption tower 1 is connected to the gas phase space of the polymerization reactor 3 used to prepare polyoxymethylene, air can easily leak into the polymerization reactor 3, thereby affecting the polymerization reaction environment. For example, components such as water and oxygen in the air can affect the polymerization reaction and thus affect the preparation efficiency of polyoxymethylene. Therefore, controlling the operating pressure (gauge pressure) of the primary absorption tower 1 to -0.005 to 0 MPaG is beneficial to reduce energy consumption and prevent air from leaking into the polymerization reactor 3 and affecting the reaction, thereby improving the preparation efficiency of polyoxymethylene.

[0039] Specifically, such as Figure 2As shown, the tail gas outlet of polymerization reactor 3 is connected to the tail gas inlet of primary absorption tower 1, so that the tail gas g generated in polymerization reactor 3 enters primary absorption tower 1 for tail gas treatment. The operating pressure of polymerization reactor 3 is a slight positive pressure. For example, polymerization reactor 3 is connected to primary absorption tower 1 through a pipeline. When the gas phase (tail gas g) in polymerization reactor 3 enters primary absorption tower 1, it needs to overcome the pipeline resistance drop. Therefore, when the operating pressure of primary absorption tower 1 is -0.005 MPa, the pressure of polymerization reactor 3 is approximately 0.001 MPaG.

[0040] The polymerization reactor 3 operates under slightly positive pressure to prevent air from leaking into it through gaps in connections such as flanges. Simultaneously, to facilitate the polymerization reaction within the reactor 3, the pressure inside the reactor 3 cannot be too high. Therefore, the operating pressure of the primary absorption tower 1, which is connected to the polymerization reactor 3, needs to be coordinated to ensure it is not too low (e.g., not lower than -0.005 MPaG). Furthermore, to facilitate the discharge of tail gas g from the polymerization reactor 3 into the primary absorption tower 1, the pressure inside the primary absorption tower 1 also needs to be controlled to prevent it from becoming too high (e.g., not higher than 0 MPaG).

[0041] Therefore, the pressure of the control-stage absorption tower 1 within the above range (-0.005 to 0 MPaG) is not only beneficial to the purification efficiency of the tail gas, but also facilitates its cooperation with the polymerization reactor 3, thereby improving the polymerization reaction efficiency within the polymerization reactor 3, and thus improving the raw material conversion rate and utilization rate of the entire system as well as the polyoxymethylene preparation efficiency.

[0042] The operating temperature of the primary absorption tower 1 is controlled at 75–110℃ to prevent the absorption medium from solidifying due to excessively low temperatures. This also facilitates the absorption of trioxymethylene and / or formaldehyde gas from the gaseous tail gas g by the liquid-phase primary absorption medium, further improving the recovery efficiency of trioxymethylene and / or formaldehyde. In specific implementation, this can be achieved by cooling the first circulating liquid b. 11 The operating temperature of the primary absorption tower 1 is controlled by methods such as [list of methods].

[0043] Specifically, the first circulating liquid b 11After being cooled by the first cooler 111, the gas returns to the first-stage absorption tower 1. The first cooler 111 includes a refrigerant for cooling, and the temperature of the refrigerant can be 60-80℃, for example, 60℃, 62℃, 65℃, 68℃, 70℃, 73℃, 75℃, 78℃, 80℃ or any combination thereof. This facilitates the heat balance of the entire absorption system and avoids phenomena such as solidification of trioxymethylene and condensation of formaldehyde caused by excessively low operating temperature in the first-stage absorption tower 1. This also avoids blockage of pipes, trays, packing, etc. in the system caused by these phenomena, thereby maintaining the operational stability of the entire system and improving the absorption efficiency of target components such as trioxymethylene and / or formaldehyde in the exhaust gas g.

[0044] Specifically, the tail gas g entering the primary absorption tower 1 can come into countercurrent contact with the first absorption medium. For example, the tail gas g enters the primary absorption tower 1 from the lower part or bottom, and the first absorption medium enters the primary absorption tower 1 from the upper part (top side) or top, so that the two come into countercurrent contact (or reverse contact) in the primary absorption tower 1. This facilitates the first absorption medium to more fully absorb the target components such as trioxymethylene and formaldehyde in the tail gas g.

[0045] In some embodiments, the process of allowing the exhaust gas g to enter the primary absorption tower 1 and come into contact with the first absorption medium entering the primary absorption tower 1 may include: allowing the first absorption medium to scrub the exhaust gas g entering the primary absorption tower 1.

[0046] For example, the first absorption medium is sprayed into the first-stage absorption tower 1 through a nozzle located on the top or upper part of the first-stage absorption tower 1. That is, the first absorption medium is sprayed downward through the nozzle to wash the tail gas g entering the first-stage absorption tower 1 from the lower part or bottom of the first-stage absorption tower 1.

[0047] Specifically, the primary absorption tower 1 may not be equipped with trays or packing.

[0048] Inside the primary absorption tower 1, after the tail gas g is absorbed by the first absorption medium, the resulting first liquid phase b1 can be output from the bottom of the primary absorption tower 1, and then at least part of it is used as the first circulating liquid b. 11 The first absorption medium returns to the first-stage absorption tower 1, and the generated first gas phase g1 can be output from the top of the first-stage absorption tower 1 and then enter the lower section 21 of the second-stage absorption tower 2.

[0049] Generally, a primary absorption tower 1 includes a first absorption section and a first reboiler connected to each other. The direction from the first absorption section to the first reboiler, the length direction of the primary absorption tower 1, and the axial direction of the primary absorption tower 1 are basically parallel. The first absorption section is located above the first reboiler. As mentioned above, the upper part or top of the primary absorption tower 1 is usually also the upper part or top of the first absorption section. The bottom of the primary absorption tower 1 is also the bottom of the first reboiler. The lower part of the primary absorption tower 1 can specifically be the lower part of the first absorption section, or the lower, middle, or upper part of the first reboiler.

[0050] Specifically, the tail gas g can enter the primary absorption tower 1 from the lower part of the first absorption section. After being absorbed by the first absorption medium, the resulting first liquid phase b1 enters the first tower bottom and is output from the bottom of the first tower bottom (i.e., the bottom of the primary absorption tower 1). Then, at least a portion of it is used as the first circulating liquid b. 11 Return to the first absorption section of primary absorption tower 1.

[0051] Specifically, the primary absorption tower 1 can be a cavity structure, meaning that the primary absorption tower 1 does not need to be equipped with tower plates or packing materials. This facilitates the absorption of the tail gas g by the first absorption medium and avoids adverse phenomena such as blockage.

[0052] Furthermore, the first gas phase g1 generated by the primary absorption tower 1 enters the lower section 21 of the secondary absorption tower 2, for example, based on pressure difference.

[0053] The first gas phase g1 entering the lower section 21 of the secondary absorption tower 2 can come into countercurrent contact with the second absorption medium. For example, the first gas phase g1 enters the lower section 21 of the secondary absorption tower 2 from the lower part or bottom, and the second absorption medium enters the lower section 21 of the secondary absorption tower 2 from the upper part, so that the first gas phase g1 can be absorbed by the second absorption medium, and further absorb the residual trioxymethylene and / or formaldehyde therein.

[0054] Specifically, the operating conditions of the lower section 21 of the secondary absorption tower 2 can be as follows: pressure (gauge pressure) of -0.01 to 0 MPaG, for example, -0.01 MPaG, -0.009 MPaG, -0.008 MPaG, -0.007 MPaG, -0.006 MPaG, -0.005 MPaG, -0.004 MPaG, -0.003 MPaG, -0.002 MPaG, -0.001 MPaG, 0 MPaG, or any two of these ranges; and temperature of 40 to 70°C, for example, 40°C, 45°C, 50°C, 55°C, 60°C, 65°C, 70°C, or any two of these ranges.

[0055] After absorption by the lower section 21 of the secondary absorption tower 2, the second liquid phase b2 output from the lower section 21 of the secondary absorption tower 2 is pressurized by the second circulation pump 102 and then at least part of it is used as the second circulating liquid b. 21 Returning to the lower section 21 of the secondary absorption tower 2, the second circulating liquid b 21 Specifically, it can be cooled by the second cooler 112 and then returned to the lower section 21 of the secondary absorption tower 2.

[0056] In specific implementation, the second liquid phase b2 output from the lower section 21 of the secondary absorption tower 2 can be partially used as the second circulating liquid b. 21 Returning to the lower section 21 of the secondary absorption tower 2, a portion enters the purification unit 4 for purification of components such as paraformaldehyde and formaldehyde in the second liquid phase b2, such as through distillation / rectification. Paraformaldehyde and formaldehyde can be separated and recovered as needed. Since the primary absorption tower 1 absorbs most of the target components such as paraformaldehyde and formaldehyde, the amount of the second liquid phase b2 entering the purification unit 4 (e.g., the paraformaldehyde purification unit 4) for purification is greatly reduced, significantly lowering the energy consumption required for purification separation in the purification unit 4. For example, through... Figure 2 The process shown uses trioxymethylene as a raw material to prepare polyoxymethylene. The second liquid phase b2 produced is mainly trioxymethylene. The mass of the second liquid phase b2 that needs to be returned to the purification unit 4 for purification can be as low as 10%-20% of the total mass of trioxymethylene that enters the polymerization reactor 3 as raw material. This is far lower than the amount of trioxymethylene tail gas that needs to be purified produced by conventional polyoxymethylene production processes, which greatly saves energy consumption.

[0057] Generally, the second gas phase g2 is mixed with the dilution gas c before entering the upper section 22 of the secondary absorption tower 2. By introducing the dilution gas c, components such as paraformaldehyde and formaldehyde in the tail gas g can be diluted, making it easier for these components to be captured and recovered by the third absorption medium in the upper section 22 of the secondary absorption tower 2. The dilution gas c may include an inert gas, such as nitrogen.

[0058] Furthermore, the mass ratio of the second gas phase g2 entering the upper section 22 of the secondary absorption tower 2 to the dilution gas c can be 1:(0.5 to 5), for example, 1:0.5, 1:1, 1:1.5, 1:2, 1:2.5, 1:3, 1:3.5, 1:4, 1:4.5, 1:5 or any combination thereof.

[0059] Studies show that, relatively speaking, if the mass ratio of the second gas phase g2 to the dilution gas c is higher than 1:0.5, it will affect the recovery rate of target components such as paraformaldehyde and formaldehyde to a certain extent. If the mass ratio of the second gas phase g2 to the dilution gas is lower than 1:5, there will be too much dilution gas c, resulting in a large system gas volume and high energy consumption (for example, the second gas phase g2 and dilution gas are transported to the upper section 22 of the secondary absorption tower 2 by the fan 100, which increases the power consumption due to the large gas volume transported by the fan 100). Therefore, controlling the mass ratio of the second gas phase g2 to the dilution gas c to 1:(0.5~5) can further improve the recovery rate of target components such as paraformaldehyde and formaldehyde in the tail gas g. Specifically, the recovery rate of target components such as paraformaldehyde and formaldehyde can reach more than 98%, such as 98%~99.99%, while also reducing energy consumption, which is more conducive to industrial application.

[0060] In practice, the second gas phase g2 and the dilution gas c are mixed and then conveyed to the upper section 22 of the secondary absorption tower 2 by the blower 100. The blower 100 can also be used to extract gas and regulate the operating pressure within the structures of the primary absorption tower 1 and the secondary absorption tower 2. The blower 100 can be located in the lower section 21 of the secondary absorption tower 2, specifically at the outlet of the second gas phase of the lower section 21, to convey the dilution gas c and the second gas phase g2 output from the lower section 21 of the secondary absorption tower 2 to the upper section 22 of the secondary absorption tower 2. This facilitates pressure regulation within the structures of the primary absorption tower 1, the lower section 21 of the secondary absorption tower 2, the upper section 22 of the secondary absorption tower 2, and the polymerization reactor 3 connected to the primary absorption tower 1. It also facilitates the assembly of various components, offering advantages such as low equipment investment and low operating costs.

[0061] For example, as described above, when the polymerization reactor 3 used to prepare polyoxymethylene is connected to the primary absorption tower 1, the first liquid phase b1 output from the primary absorption tower 1 enters the polyoxymethylene reactor as a raw material for polyoxymethylene synthesis. If the blower 100 is located at the first gas phase outlet of the primary absorption tower 1, which outputs the first gas phase g1, the blower 100 will have a large air volume and high power consumption. If the blower 100 is located at the third gas phase outlet of the upper section 22 of the secondary absorption tower 2, which outputs the third gas phase g3, it will increase the difficulty of adjusting the pressure of the primary absorption tower 1 to some extent and affect the timely and effective discharge of the tail gas g generated by the polymerization reactor 3, thus affecting the polymerization reaction effect and the recovery effect of the tail gas g. However, by placing the blower 100 at the second gas phase outlet of the lower section 21 of the secondary absorption tower 2, the above problems can be avoided, and the exhaust pressure of the polymerization reactor 3 can be controlled more precisely.

[0062] Specifically, the second gas phase g2 entering the upper section 22 of the secondary absorption tower 2 can come into countercurrent contact with the third absorption medium. For example, the second gas phase g2 enters the upper section 22 of the secondary absorption tower 2 from the lower part of the upper section 22, and the third absorption medium enters the upper section 22 of the secondary absorption tower 2 from the upper part or top of the upper section 22, so that the two come into countercurrent contact in the upper section 22 of the secondary absorption tower 2.

[0063] Specifically, the operating conditions of the upper section 22 of the secondary absorption tower 2 can be: pressure (gauge pressure) of 0 to 0.01 MPaG and temperature of 30 to 60°C, such as 30°C, 35°C, 40°C, 45°C, 50°C, 55°C, 60°C or any combination thereof.

[0064] The exhaust gas recovery process of this invention can significantly reduce water consumption. In some specific embodiments, the mass ratio of the water used (i.e., the water entering the upper section 22 of the secondary absorption tower 2 as the third absorption medium) to the exhaust gas g can be (1-3):1, for example, 1:1, 1.2:1, 1.5:1, 1.8:1, 2:1, 2.2:1, 2.5:1, 2.8:1, 3:1 or any combination thereof. This can improve the recovery efficiency of target components such as paraformaldehyde and formaldehyde in the exhaust gas g, and can reduce water consumption, energy consumption and emissions of waste gas, wastewater, and solid waste.

[0065] After absorption by the upper section 22 of the secondary absorption tower 2, the third liquid phase b3 output from the upper section 22 of the secondary absorption tower 2 is pressurized by the third circulation pump 103 and then partially used as the third circulating liquid b. 31 Part of the liquid returned to the upper section 22 of the secondary absorption tower 2 is used as the fourth circulating liquid b. 32 Returning to the lower section 21 of the secondary absorption tower 2, where the third circulating liquid b 31 It can be cooled by the third cooler 113 and then returned to the upper section 22 of the secondary absorption tower 2, the fourth circulating liquid b 32 After being cooled by the second cooler 112, it returns to the lower section 21 of the secondary absorption tower 2.

[0066] In addition, the third gas phase g3 output from the upper section 22 of the secondary absorption tower 2 can at least partially enter the waste gas treatment unit 5 for further harmless treatment. That is, it can all enter the waste gas treatment unit 5, or partially enter the waste gas treatment unit 5 and partially return to the upper section 22 of the secondary absorption tower 2 (such as being transported into the upper section 22 of the secondary absorption tower 2 by the fan 100) to form a cycle.

[0067] Specifically, the secondary absorption tower 2 may be equipped with trays or packing. The upper section 22 and the lower section 21 of the secondary absorption tower 2 may be equipped with trays or packing respectively. The trays or packing may be conventional structures and materials in the art. For example, the packing may include random materials, such as plastic materials, but is not limited to these.

[0068] The exhaust gas treatment device provided in this embodiment of the invention is used to implement the above-mentioned exhaust gas recovery process, such as... Figure 1 and Figure 2 As shown, the exhaust gas treatment device includes a primary absorption tower 1 and a secondary absorption tower 2. The secondary absorption tower 2 includes an upper section 22 connected to each other and a lower section 21 located below the upper section 22. The primary absorption tower 1 includes a first gas phase outlet, a first liquid phase outlet, and a first circulating liquid inlet connected to the first liquid phase outlet. The lower section 21 of the secondary absorption tower 2 includes a first gas phase inlet, a second gas phase outlet, a second liquid phase outlet, and a second circulating liquid inlet connected to the second liquid phase outlet. The first gas phase inlet is connected to the first gas phase outlet of the primary absorption tower 1. The upper section 22 of the secondary absorption tower 2 includes a second gas phase inlet, a third liquid phase outlet, and a third circulating liquid inlet connected to the third liquid phase outlet. The second gas phase inlet is connected to the second gas phase outlet of the lower section 21, and the third liquid phase outlet is connected to the second circulating liquid inlet of the upper section 22.

[0069] Specifically, the primary absorption tower 1 also includes a tail gas inlet and a trioxymethylene inlet. Tail gas g enters the primary absorption tower 1 through the tail gas inlet, and trioxymethylene a enters the primary absorption tower 1 through the trioxymethylene inlet. The first circulating liquid b... 11 The exhaust gas g enters the primary absorption tower 1 through the first circulating liquid inlet. Inside the primary absorption tower 1, the exhaust gas g reacts with the first absorption medium (trioxymethylene a and the first circulating liquid b). 11 The first liquid phase b1 is absorbed by the first absorbent medium upon contact with the first liquid phase outlet, and at least a portion of it is used as the first circulating liquid b. 11 Returning from the first circulating liquid inlet to the first-stage absorption tower 1, the first gas phase g1 is output from the first gas phase outlet and enters the lower section 21 of the second-stage absorption tower 2 from the first gas phase inlet.

[0070] The first circulating liquid inlet and the trioxymethylene inlet can be the same inlet or different inlets, and there is no particular limitation on this.

[0071] For example, such as Figure 1 and Figure 2 As shown, the trioxymethylene inlet and the first circulating liquid inlet can be located at the top of the primary absorption tower 1, the first gas phase outlet can be located at the top of the primary absorption tower 1 (the top of the primary absorption tower 1), and the first liquid phase outlet can be located at the bottom of the primary absorption tower 1.

[0072] Furthermore, the aforementioned exhaust gas treatment device may also include nozzles respectively located at the paraformaldehyde inlet and the first circulating liquid inlet, so that paraformaldehyde a and the first circulating liquid b... 11 The exhaust gas g inside the primary absorption tower 1 is sprayed into the primary absorption tower 1 through a nozzle.

[0073] Specifically, the primary absorption tower 1 includes a first absorption section and a first tower bottom. In the primary absorption tower 1, the absorption process of the tail gas g by the first absorption medium is mainly carried out in the first absorption section. The first tower bottom is used to hold the first liquid phase b1. The first gas phase outlet, the trioxymethylene inlet, and the first circulating liquid inlet are specifically located in the first absorption section, and the first liquid phase outlet is located in the first tower bottom.

[0074] Furthermore, the aforementioned exhaust gas treatment device may also include a first circulating pump 101 and a first cooler 111. The first liquid phase outlet, the first circulating pump 101, the first cooler 111, and the first circulating liquid inlet of the primary absorption tower 1 are sequentially connected, so that the first liquid phase b1 output from the first liquid phase outlet is pressurized by the first circulating pump 101 and part of it becomes the first circulating liquid b. 11 After being cooled by the first cooler 111, it returns to the first-stage absorption tower 1 through the first circulating liquid inlet.

[0075] After the first gas phase g1 exits from the first gas phase outlet of the primary absorption tower 1, it enters the secondary absorption tower 2 from the first gas phase inlet of the lower section 21. The second liquid phase b2 in the secondary absorption tower 2 exits from the second liquid phase outlet, and at least part of it is used as the second circulating liquid b. 21 The second circulating liquid returns to the upper section 22 of the secondary absorption tower 2 from the second circulating liquid inlet, while the fourth circulating liquid b from the upper section 22 of the secondary absorption tower 2... 32 The circulating liquid enters the lower section 21 of the secondary absorption tower 2 from the second circulating liquid inlet, or the lower section 21 of the secondary absorption tower 2 is also provided with a fourth circulating liquid inlet that is connected to the third liquid phase outlet of the upper section 22 of the secondary absorption tower 2. 32 The second circulating liquid (b) enters the lower section 21 of the secondary absorption tower 2 from the fourth circulating liquid inlet; 21 and the fourth circulating fluid b 32 As the second absorption medium, it absorbs the first gas phase g1. After absorption, the generated second gas phase g2 is output from the second gas phase outlet and enters the upper section 22 of the secondary absorption tower 2 through the second gas phase inlet of the upper section 22.

[0076] The upper section 22 of the secondary absorption tower 2 is also equipped with an aqueous phase inlet and a third gas phase outlet. Water enters the upper section 22 of the secondary absorption tower 2 from the aqueous phase inlet. The third liquid phase b3 generated in the upper section 22 of the secondary absorption tower 2 is output from the third liquid phase outlet, and part of it is used as the third circulating liquid b. 31The liquid returns from the third circulating liquid inlet to the upper section 22 of the secondary absorption tower 2, and part of it becomes the fourth circulating liquid b. 32 It enters the lower section 21 of the secondary absorption tower 2; the third gas phase g3 generated in the upper section 22 of the secondary absorption tower 2 is output from the third gas phase outlet.

[0077] For example, such as Figure 1 and Figure 2 As shown, the first gas phase inlet is located at the lower part of the lower section 21 of the secondary absorption tower 2, the second gas phase outlet and the second circulating liquid inlet (also the fourth circulating liquid inlet) are respectively located at the upper part of the lower section 21 of the primary absorption tower 1, and the second liquid phase outlet is located at the bottom of the lower section 21 of the secondary absorption tower 2.

[0078] Specifically, the lower section 21 of the secondary absorption tower 2 includes a second absorption section and a second tower bottom. In the lower section 21 of the secondary absorption tower 2, the absorption process of the first gas phase g1 by the second absorption medium is mainly carried out in the second absorption section. The second tower bottom is used to hold the second liquid phase b2. The first gas phase inlet, the second gas phase outlet, the second circulating liquid inlet / the fourth circulating liquid inlet are respectively located in the second absorption section, and the second liquid phase outlet is located in the second tower bottom.

[0079] In addition, the above-mentioned exhaust gas treatment device may also include a purification unit 4 connected to the second liquid phase outlet, so that a portion of the second liquid phase b2 output from the second liquid phase outlet enters the purification unit 4, and the second liquid phase b2 is further purified by distillation / rectification or other methods, such as separating trioxymethylene and formaldehyde therein.

[0080] For example, such as Figure 1 and Figure 2 As shown, the second gas phase inlet and the third liquid phase outlet are respectively located at the lower part of the upper section 22 of the secondary absorption tower 2, the water phase inlet and the third circulating liquid inlet are located at the upper part of the secondary absorption tower 2, and the third gas phase outlet is located at the top of the upper section 22 of the secondary absorption tower 2 (the top of the secondary absorption tower 2).

[0081] The aqueous phase inlet and the third circulating liquid inlet can be the same inlet or different inlets, and there are no special restrictions on this.

[0082] The exhaust gas treatment device may further include an exhaust gas treatment unit 5 connected to the third gas phase outlet, wherein the third gas phase g3 output from the third gas phase outlet may at least partially enter the exhaust gas treatment unit 5 for further harmless treatment.

[0083] Optionally, such as Figure 1 and Figure 2 As shown, the third gas phase outlet can also be connected to the second gas phase inlet. The third gas phase g3 output from the third gas phase outlet can be partially returned from the second gas phase inlet to the upper section 22 of the secondary absorption tower 2, forming a cycle.

[0084] In addition, such as Figure 1 and Figure 2 As shown, the above-mentioned exhaust gas treatment device may also include a fan 100 and a dilution gas source for providing dilution gas c. The second gas phase outlet, the fan 100, and the second gas phase inlet are connected in sequence, and the dilution gas source, the fan 100, and the second gas phase inlet are connected in sequence to allow the second gas phase g2 and the dilution gas c to be transported into the upper section 22 of the secondary absorption tower 2 through the fan 100. The fan 100 can also be used to regulate the operating pressure and other conditions of the primary absorption tower 1 and the secondary absorption tower 2.

[0085] In addition, such as Figure 1 and Figure 2 As shown, the exhaust gas treatment device may further include a second circulation pump 102, a second cooler 112, a third circulation pump 103, and a third cooler 113.

[0086] The second liquid phase outlet, the second circulation pump 102, the second cooler 112, and the second circulation liquid inlet / fourth circulation liquid inlet are connected in sequence so that the second liquid phase b2 output from the second liquid phase outlet is pressurized by the second circulation pump 102 and partially becomes the second circulation liquid b. 21 After being cooled by the second cooler 112, it returns to the lower section 21 of the secondary absorption tower 2 from the second circulating liquid inlet / fourth circulating liquid inlet.

[0087] The third liquid phase outlet, the third circulation pump 103, the second cooler 112, and the second / fourth circulation liquid inlet are connected in sequence so that the third liquid phase b3 output from the third liquid phase outlet is pressurized by the third circulation pump 103 and partially becomes the fourth circulation liquid b. 32 After being cooled by the second cooler 112, it enters the lower section 21 of the secondary absorption tower 2 from the second circulating liquid inlet / fourth circulating liquid inlet.

[0088] The third liquid phase outlet, the third circulation pump 103, the third cooler 113, and the third circulation liquid inlet are connected in sequence so that the third liquid phase b3 output from the third liquid phase outlet is pressurized by the third circulation pump 103 and part of it becomes the third circulation liquid b. 31 After being cooled by the third cooler 113, it returns to the upper section 22 of the secondary absorption tower 2 through the third circulating liquid inlet.

[0089] Specifically, the secondary absorption tower 2 can be divided into an upper section 22 and a lower section 21 by a partition. That is, a partition is installed between the upper section 22 and the lower section 21 of the secondary absorption tower 2, thus dividing the secondary absorption tower 2 into two absorption zones, the upper section 22 and the lower section 21. This not only facilitates energy matching of the entire system and improves the absorption efficiency of target components such as paraformaldehyde and formaldehyde in the exhaust gas g, but also simplifies the overall structure of the device, reduces its footprint, and lowers costs. The upper section 22 and the lower section 21 can be coaxially arranged.

[0090] Under normal circumstances, such as Figure 1 and Figure 2 As shown, the primary absorption tower 1 and the secondary absorption tower 2 are placed vertically. The length / axial direction of the primary absorption tower 1, the length / axial direction of the secondary absorption tower 2, the direction from the lower section 21 to the upper section 22, the direction from the bottom to the top of the primary absorption tower 1, and the direction from the bottom to the top of the secondary absorption tower (or its upper section 22 or lower section 21) are basically parallel to each other.

[0091] like Figure 1 and Figure 2 As shown, the polyoxymethylene preparation method provided in this embodiment of the invention includes: feeding a raw material including trioxymethylene into a polymerization reactor 3 for polymerization reaction to obtain polyoxymethylene and tail gas g; recovering the tail gas g according to the above-described tail gas recovery method, wherein during the recovery process, a portion of the first liquid phase b1 is used as the first circulating liquid b. 11 The contents are returned to the primary absorption tower 1, and part of them are returned as raw materials to the polymerization reactor 3 for polymerization.

[0092] Generally, polyoxymethylene (POM) is produced by polymerization of trioxymethylene as a raw material. The tail gas g generated during the polymerization process contains unreacted trioxymethylene and byproducts such as formaldehyde. This tail gas g can be recycled through the above-mentioned tail gas recovery process to improve the raw material conversion rate and utilization rate of trioxymethylene and improve the preparation effect of POM.

[0093] Specifically, trioxymethylene a can be fed into the primary absorption tower 1. Trioxymethylene a inside the primary absorption tower 1 acts as the first absorption medium to absorb trioxymethylene and formaldehyde and other components in the tail gas g. The resulting first liquid phase b1 is then used as raw material to enter the polymerization reactor 3 for polymerization reaction to produce polyoxymethylene.

[0094] Specifically, the polymerization conditions can be: the reaction temperature can be 100–150°C, for example, 100°C, 110°C, 120°C, 130°C, 140°C, 150°C or any combination thereof; the reaction pressure can be 0.001–0.02 MPaG, for example, 0.001 MPaG, 0.003 MPaG, 0.005 MPaG, 0.008 MPaG, 0.01 MPaG, 0.013 MPaG, 0.015 MPaG, 0.018 MPaG, 0.02 MPaG or any combination thereof; and the reaction residence time can be 2–60 min, for example, 2 min, 5 min, 10 min, 15 min, 20 min, 25 min, 30 min, 35 min, 40 min, 45 min, 50 min, 55 min, 60 min or any combination thereof.

[0095] The polyoxymethylene (POM) preparation apparatus provided in this embodiment of the invention is used to implement the above-described POM preparation method, such as... Figure 1 and Figure 2 As shown, the polyoxymethylene (POM) preparation apparatus includes a polymerization reactor 3 and the aforementioned tail gas treatment device. The first liquid phase outlet of the primary absorption tower 1 is connected to the polymerization reactor 3 so that the first liquid phase b1 portion output from the first liquid phase outlet of the primary absorption tower 1 is used as raw material to enter the polymerization reactor 3 for polymerization reaction to prepare POM.

[0096] In some embodiments, such as Figure 2 As shown, the polymerization reactor 3 is connected to the primary absorption tower 1. That is, the polymerization reactor 3 includes a tail gas outlet connected to the tail gas inlet of the primary absorption tower 1. The tail gas g generated in the polymerization reactor 3 is output from the tail gas outlet and enters the primary absorption tower 1 through the tail gas inlet of the primary absorption tower 1. It is absorbed by the primary absorption tower 1 and the secondary absorption tower 2 to recover components such as trioxymethylene and formaldehyde.

[0097] The present invention will be further described below through specific embodiments.

[0098] In Examples 1 and 2 below, the calculation process for the single-pass conversion rate w1 of paraformaldehyde a, the recovery rate w2 of paraformaldehyde, and the recovery rate w3 of formaldehyde is as follows:

[0099] The single-pass conversion rate of trioxymethylene a is w1 = (m0-m1) / m0, where m0 is the mass of trioxymethylene a entering the first-stage absorption tower 1, and m1 is the mass of trioxymethylene in the first gas phase g1.

[0100] The recovery rate of trioxymethylene is w2 = (m2-m3) / m2, where m2 is the mass of trioxymethylene in the tail gas g, and m3 is the mass of trioxymethylene in the third gas phase g3 output from the third gas phase outlet of the upper section 22 of the secondary absorption tower 2.

[0101] The formaldehyde recovery rate w3 = (m4-m5) / m4, where m4 is the mass of formaldehyde in the tail gas g, and m5 is the mass of formaldehyde in the third gas phase g3 output from the third gas phase outlet of the upper section 22 of the secondary absorption tower 2.

[0102] Example 1

[0103] Using trioxymethylene (A) as raw material, and employing methods such as... Figure 2 The trioxymethylene preparation apparatus shown is used to prepare polyoxymethylene. The apparatus structure and related process flow are as described above. Figure 2 I will not go into further detail here.

[0104] In the exhaust gas treatment process, trioxymethylene (a) and the first circulating liquid (b) are used. 11 The gas, as the first circulating medium, is sprayed into the first-stage absorption tower 1 through nozzles located at the trioxymethylene inlet and the first circulating liquid inlet, respectively, to spray the tail gas g in the first-stage absorption tower 1.

[0105] The first-stage absorption tower 1 is a cavity, meaning it does not have any trays or packing structures.

[0106] Inside the primary absorption tower, the mass ratio of the first absorption medium to the tail gas g is 45:1;

[0107] The operating pressure of the primary absorption tower 1 is -0.005 MPaG, and the operating temperature is 100℃;

[0108] The first liquid phase b1 output from the first liquid phase outlet of the primary absorption tower 1 is pressurized by the first circulation pump 101, and part of it becomes the first circulating liquid b. 11 After being cooled by the first cooler 111, the product returns to the first-stage absorption tower 1. The remaining portion is directly used as raw material in the polymerization reactor 3 to participate in the polymerization reaction without further separation. The temperature of the refrigerant in the first cooler 111 is 80°C.

[0109] The operating pressure of the lower section 21 of the secondary absorption tower 2 is -0.002 MPaG, and the operating temperature is 65℃.

[0110] The mass ratio of the second gas phase g2 to the dilution gas c is 1:1, and the dilution gas c is nitrogen.

[0111] The operating pressure of the upper section 22 of the secondary absorption tower 2 is 0.005 MPaG, and the operating temperature is 50℃.

[0112] The mass ratio of water used in the secondary absorption tower 2 (i.e., water entering the upper section 22 of the secondary absorption tower 2) to tail gas g is 2:1;

[0113] The polymerization reaction conditions in polymerization reactor 3 are as follows: reaction temperature is 130℃, reaction pressure is 0.008MPaG, and reaction residence time is 10min.

[0114] Tests showed that the single-pass conversion rate of trioxymethylene (a) was 85%; after the tail gas (g) was absorbed by the primary absorption tower 1 and the secondary absorption tower 2, the recovery rate of trioxymethylene was 99%, and the recovery rate of formaldehyde was 99.5%.

[0115] Example 2

[0116] The differences from Example 1 are as follows; all other conditions are the same except for the following differences:

[0117] Inside the primary absorption tower, the mass ratio of the first absorption medium to the tail gas g is 52:1;

[0118] The operating pressure of the primary absorption tower 1 is -0.002 MPaG, and the operating temperature is 80℃.

[0119] The refrigerant temperature in the first cooler 111 is 60°C;

[0120] The operating pressure of the lower section 21 of the secondary absorption tower 2 is -0.001 MPaG, and the operating temperature is 50℃.

[0121] The mass ratio of the second gas phase g2 to the dilution gas c is 1:1.5, and the dilution gas c is nitrogen.

[0122] The operating pressure of the upper section 22 of the secondary absorption tower 2 is 0.01 MPaG, and the operating temperature is 40℃.

[0123] The mass ratio of water used in the secondary absorption tower 2 to the tail gas g is 1.5:1;

[0124] The polymerization reaction conditions in polymerization reactor 3 are as follows: reaction temperature is 118℃, reaction pressure is 0.003MPaG, and reaction residence time is 22min.

[0125] Tests showed that the single-pass conversion rate of trioxymethylene (a) was 90%; after the tail gas (g) was absorbed by the primary absorption tower 1 and the secondary absorption tower 2, the recovery rate of trioxymethylene was 99.5% and the recovery rate of formaldehyde was 99.8%.

[0126] In the description of this invention, unless otherwise explicitly specified and limited, the terms "set up," "located in," "connected," and "connected" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection, an electrical connection, or a communication connection (network connection); they can refer to a direct connection, an indirect connection through an intermediate medium, or the internal connection of two components, such as a connection through a pipe. Those skilled in the art will understand the specific meanings of the above within the context of this invention based on the specific circumstances.

[0127] Furthermore, terms such as "first," "second," "third," and "fourth" are used for descriptive purposes only, such as distinguishing components to more clearly illustrate / explain the technical solution, and should not be construed as indicating or implying the number of technical features indicated or their substantial order.

[0128] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for recovering exhaust gas, characterized in that, The exhaust gas contains paraformaldehyde and formaldehyde, which are recovered using a primary absorption tower and a secondary absorption tower. The secondary absorption tower includes an upper section and a lower section. The exhaust gas recovery method includes: The exhaust gas is introduced into the primary absorption tower and comes into contact with the first absorption medium inside the primary absorption tower, so that the first absorption medium absorbs the exhaust gas to obtain a first liquid phase and a first gas phase respectively. The first liquid phase is returned to the first-stage absorption tower in at least a portion as the first circulating liquid, and in part as the raw material for synthesizing polyoxymethylene, entering the polymerization reactor for preparing polyoxymethylene to participate in the polymerization reaction. The tail gas outlet of the polymerization reactor is connected to the tail gas inlet of the first-stage absorption tower so that the tail gas generated in the polymerization reactor enters the first-stage absorption tower for tail gas treatment. The first absorption medium includes trioxymethylene and the first circulating liquid. The first gas phase is introduced into the lower section of the secondary absorption tower and comes into contact with the second absorption medium that also enters the lower section of the secondary absorption tower, so that the second absorption medium absorbs the first gas phase, thereby obtaining the second liquid phase and the second gas phase respectively. The second liquid phase is returned at least partially as a second circulating liquid to the lower section of the secondary absorption tower, and partially enters the purification unit to purify the paraformaldehyde and formaldehyde in the second liquid phase; The second gas phase is introduced into the upper section of the secondary absorption tower and comes into contact with the third absorption medium introduced into the upper section of the secondary absorption tower, so that the third absorption medium absorbs the second gas phase to obtain the third liquid phase and the third gas phase respectively. The third liquid phase is partially returned to the upper section of the secondary absorption tower as the third circulating liquid, and partially returned to the lower section of the secondary absorption tower as the fourth circulating liquid. Wherein, the second absorption medium includes the second circulating liquid and the fourth circulating liquid; the third absorption medium includes water and the third circulating liquid; The second gas phase is mixed with the dilution gas and then conveyed to the upper section of the secondary absorption tower by a fan; The mass ratio of the second gas phase to the dilution gas is 1:(0.5~5); The dilution gas includes nitrogen.

2. The exhaust gas recovery method according to claim 1, characterized in that, The mass ratio of the first absorption medium to the exhaust gas is (45~60):1; And / or, the operating conditions of the primary absorption tower are: pressure of -0.005~0MPaG and temperature of 75~110℃; And / or, the process of allowing the exhaust gas to enter the primary absorption tower and come into contact with the first absorption medium entering the primary absorption tower includes: allowing the first absorption medium to scrub the exhaust gas entering the primary absorption tower. And / or, the first circulating liquid is cooled by a first cooler before being returned to the primary absorption tower, the first cooler comprising a refrigerant for cooling, the temperature of the refrigerant being 60~80°C.

3. The exhaust gas recovery method according to claim 1, characterized in that, The operating conditions for the lower section of the secondary absorption tower are: pressure -0.01~0 MPaG and temperature 40~70℃. And / or, the first gas phase entering the lower section of the secondary absorption tower comes into countercurrent contact with the second absorption medium.

4. The exhaust gas recovery method according to claim 1, characterized in that, The operating conditions for the upper section of the secondary absorption tower are: pressure 0~0.01MPaG and temperature 30~60℃. And / or, the mass ratio of the water to the exhaust gas is (1~3):1; And / or, the second gas phase entering the upper section of the secondary absorption tower comes into countercurrent contact with the third absorption medium.

5. The exhaust gas recovery method according to claim 1, characterized in that, The primary absorption tower has a hollow structure, and / or the secondary absorption tower is equipped with trays or packing.

6. A method for preparing polyoxymethylene, characterized in that, include: The raw materials, including trioxymethylene, are fed into a polymerization reactor to undergo a polymerization reaction, yielding polyoxymethylene and tail gas. The tail gas is recovered according to the tail gas recovery method according to any one of claims 1-5. During the recovery process, part of the first liquid phase is returned to the first-stage absorption tower as the first circulating liquid, and part is returned to the polymerization reactor as raw material for the polymerization reaction.

7. The method for preparing polyoxymethylene according to claim 6, characterized in that, The polymerization reaction conditions are as follows: reaction temperature of 100~150℃, reaction pressure of 0.001~0.02MPaG, and reaction residence time of 2~60min.

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