A system and method for recovering polyformaldehyde waste liquid by extractive rectification coupling process

Abstract

A system and method for recovering polyformaldehyde waste liquid by extractive distillation coupling process, comprising a rectifying column I, an extraction column, a rectifying column II, a rectifying column III, a rectifying column IV, a mixer, a plurality of overhead tanks, a plurality of reboilers and a plurality of condensers, using the characteristics of the boiling point difference of components and the solubility of each component of the benzene-methanol azeotrope system in the extractant, through the operation of the five columns of the rectifying column I, the extraction column, the rectifying column II, the rectifying column III and the rectifying column IV, the efficient separation and recovery of the polyformaldehyde waste liquid mixture are realized. The mass fraction of separated methyl formate, benzene and methanol is greater than 99.50%, and the content of trioxymethylene and water meets the recovery requirements. The present application solves the problems of separation of polyformaldehyde waste liquid multi-component mixture and ordinary distillation of azeotrope system, and is flexible, economical and environmentally friendly, providing a reference scheme for the separation and recovery of polyformaldehyde waste liquid.

Description

Technical Field

[0001] This invention relates to the field of chemical separation and purification, specifically to a system and method for recovering polyoxymethylene waste liquid using an extraction and distillation coupled process. Background Technology

[0002] Polyoxymethylene (POM) is a thermoplastic resin material with excellent comprehensive performance. It possesses high elastic modulus, hardness, rigidity, and mechanical properties over a wide temperature range, making it a viable alternative to metals such as steel, copper, zinc, and aluminum in many applications. Its superior performance has earned it the title of "metal among plastics," and it is widely used in machinery, light industry, electronics, automotive, and agricultural machinery. Among the world's three major general-purpose plastics, POM is the highest-performing engineering plastic. Currently, global POM engineering plastic production capacity ranks third, after polyamide (PA) and polycarbonate (PC). POM products are divided into two main categories: copolymers and homopolymers. Copolymers using trioxymethylene (TOX) and dioxane (dioxane) as monomers account for over 80% of total production capacity. The production of high-quality POM engineering plastics relies heavily on high-purity TOX. TOX production primarily involves reacting formaldehyde solution with a sulfuric acid catalyst to produce crude TOX. This process also involves side reactions that produce byproducts such as methyl formate and paraformaldehyde. In the refining process of crude TOX, water and TOX can form an azeotrope. Therefore, traditional processes typically employ a combination of distillation and benzene extraction to purify crude TOX to polymer-grade TOX. During this process, impurities such as methyl formate, water, and methanol are continuously separated. However, the separation of these impurities inevitably carries away useful substances like TOX and benzene. These components mix and are difficult to separate and utilize, resulting in hazardous waste liquid. Domestic regulations for this type of waste liquid are stringent, but companies lack corresponding recycling technologies. This leads to high costs and significant resource waste in the treatment of polyoxymethylene (POM) waste liquid. Therefore, developing an economical and efficient POM waste liquid recycling technology can not only achieve comprehensive resource utilization but also reduce waste liquid treatment costs and promote the development of my country's POM industry.

[0003] The production of polyoxymethylene (POM) generates a mixed waste liquid containing trioxymethylene, methyl formate, benzene, methanol, and water. Benzene and methanol, as well as trioxymethylene and water, form an azeotrope with azeotropic points of 57.97℃ (61.28 wt% benzene, 38.72 wt% methanol) and 91.72℃ (27.85 wt% water, 72.15 wt% trioxymethylene), respectively. Because benzene and methanol form an azeotrope, simple distillation is insufficient for effective separation; therefore, extraction is necessary. The trioxymethylene-water azeotrope, separated by distillation, can be mixed with the original azeotropic trioxymethylene-water mixture for further separation. This allows for the separation and recovery of methyl formate, methanol, benzene, trioxymethylene, and water, reducing production costs and further minimizing emissions. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and provide a system and method for recovering polyoxymethylene waste liquid through an extraction-distillation coupling process. It utilizes the differences in boiling points of the components and the different solubilities of each component in the extractant in the benzene-methanol azeotropic system to separate them using a distillation-coupling extraction method. The proposed process can achieve efficient separation of a mixture of methyl formate, methanol, benzene, trioxymethylene and water.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: a system for recovering polyoxymethylene waste liquid using an extraction and distillation coupled process, the system mainly comprising distillation column I1, extraction column 2, distillation column II3, distillation column III4, distillation column IV5, mixer 6, top storage tank I7, top storage tank II8, top storage tank III9, top storage tank IV10, reboiler I11, reboiler II12, reboiler III13, reboiler IV14, condenser I15, condenser II16, condenser III17, condenser IV18, condenser V19, condenser VI20, and condenser VII21.

[0006] Fresh feed is connected to distillation column I1 via a pipeline; replenished fresh extractant is mixed with the circulating extractant in mixer 6 and then connected to extraction column 2 via a pipeline; distillation column I1 is connected to extraction column 2 via condenser II16 via a pipeline; extraction column 2 is connected to distillation column II3 via a pipeline, and the top of distillation column II3 is connected to distillation column III4 via a pipeline; the bottom of distillation column II3 is connected to distillation column IV5 via condenser V19 via a pipeline; distillation column IV5 is connected to condenser V19 via a pipeline. VII21 is connected to mixer 6; condenser I15 is connected to distillation column I1 and top storage tank I7, condenser III17 is connected to distillation column II3 and top storage tank II8, condenser IV18 is connected to distillation column III4 and top storage tank III9, condenser VI20 is connected to distillation column IV5 and top storage tank IV10; reboiler I11, reboiler II12, reboiler III13, and reboiler IV14 are respectively connected to distillation column I1, distillation column II3, distillation column III4, and distillation column IV5.

[0007] A method for recovering polyoxymethylene waste liquid using an extractive distillation coupled process includes the following steps: (1) A mixture of trioxymethylene, methyl formate, benzene, methanol and water enters from distillation column I1. Methyl formate is separated in distillation column I1. A portion of the material at the bottom of distillation column I1 flows back to distillation column I1 after heat exchange in reboiler I11. A portion is fed into the bottom of extraction column 2 through condenser II16. Methyl formate is condensed by condenser I15 and enters the top storage tank I7. A portion of methyl formate is returned to distillation column I1 and a portion is taken out from the top storage tank I7. (2) The extractant enters from the top of the extraction tower 2. After effective contact extraction and separation, benzene flows out from the top of the extraction tower 2, and the mixture of trioxymethylene, methanol and water flows out from the bottom of the extraction tower 2 and enters the distillation tower II 3. (3) The separation of trioxymethylene and methanol was achieved in distillation column II3. Part of the bottom material of distillation column II3 was returned to distillation column II3 after heat exchange in reboiler II12, and part of it was fed into distillation column IV5 through condenser V19. A large amount of methanol and a small amount of benzene were condensed by condenser III17 and entered the top storage tank II8. Part of it was refluxed from the top storage tank II8 to distillation column II3, and the other part entered distillation column III4 through the top storage tank II8. (4) Methanol is separated in distillation column III4. Part of the methanol at the bottom of distillation column III4 is returned to distillation column III4 after heat exchange in reboiler III13, and part is taken out from the bottom of the column. A small amount of benzene and methanol azeotropically enters the top storage tank III9 after condensation in condenser IV18. Part of it is refluxed from the top storage tank III9 to distillation column III4, and the other part is taken out from the top storage tank III9. (5) After distillation separation in distillation column IV5, part of the water in the bottom material of distillation column IV5 is returned to distillation column IV5 after heat exchange in reboiler IV14, and part is refluxed to mixer 6 through condenser VII21; the trioxymethylene and water azeotrope is condensed in condenser VI20 and enters the top storage tank IV10, part of which is refluxed to distillation column IV5, and the other part is taken out from the top storage tank IV10.

[0008] Preferably, the operating pressure of distillation column I1 is 1 atm, and the number of theoretical plates is 30-45; the operating pressure of extraction column 2 is 1 atm, and the number of theoretical plates is 12-20; the operating pressure of distillation column II3 is 1 atm, and the number of theoretical plates is 30-50; the operating pressure of distillation column III4 is 1 atm, and the number of theoretical plates is 30-40; the operating pressure of distillation column IV5 is 1 atm, and the number of theoretical plates is 14-30; and the feed temperature of the extractant is 25°C.

[0009] Preferably, the mass ratio of the feed flow rate of the extractant to the mixture of trioxymethylene, methanol, benzene and water is 0.9 to 1.1.

[0010] Preferably, in the mixture of paraformaldehyde, methanol, benzene, methyl formate and water, the mass fraction of paraformaldehyde is 10%–20%, the mass fraction of methanol is 20%–30%, the mass fraction of benzene is 20%–30%, the mass fraction of methyl formate is 28%–38%, and the mass fraction of water is 3%–10%.

[0011] Preferably, the purity of the separated methyl formate is greater than 99.50% by mass, the purity of the methanol is greater than 99.50% by mass, the purity of the benzene is greater than 99.50% by mass, and the purity of the extractant recovered from the bottom of the distillation column T5 is greater than 99.90% by mass.

[0012] Compared with the prior art, the present invention has the following beneficial effects: This invention discloses a system and method for recovering polyoxymethylene waste liquid using an extraction-distillation coupled process. The extraction-distillation coupled process achieves efficient separation of a multi-component mixture of trioxymethylene, water, benzene, methanol, and methyl formate.

[0013] The present invention discloses a system and method for recovering polyoxymethylene waste liquid using an extraction and distillation coupled process. The designed process flow enables the economical and clean separation of mixtures.

[0014] This invention provides a system and method for recovering polyoxymethylene waste liquid using an extraction and distillation coupled process. This system and method are clean, efficient, economical, environmentally friendly, simple, and flexible in operation. Attached Figure Description

[0015] Figure 1 This is a process flow diagram of a system and method for recovering polyoxymethylene waste liquid using an extraction and distillation coupled process according to the present invention. Detailed Implementation

[0016] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0017] In the description of this invention, it should be noted that the terms "top," "bottom," "one side," "the other side," "front," "rear," "middle part," "inner," "top," and "bottom," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing the invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the invention. The terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance. Furthermore, unless otherwise explicitly specified and limited, the terms "installed," "connected," and "linked" 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 or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal communication of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0018] The present invention will now be described in further detail with reference to the accompanying drawings. Example

[0019] The feed flow rate was 1000 kg / h, the feed temperature was 30℃, the pressure was 1 atm, and the mass fraction of the feed was 15.98% formate, 27.70% methanol, 20.00% benzene, 33.04% methyl formate, and 3.28% water. The extractant feed flow rate was 700 kg / h. The following parameters are given for distillation column I: operating pressure 1 atm, feed temperature 25℃, pressure 1 atm, 35 trays, feed position 20th tray, top temperature 31.48℃, bottom temperature 59.76℃; extraction column II: operating pressure 1 atm, 12 trays, extractant feed position 1st tray, waste liquid feed position 12th tray, extraction column temperature 35℃; distillation column II: operating pressure 1 atm, 40 trays, feed position 25th tray, top temperature 63.66℃, bottom temperature 94.71℃. Distillation column III4 operates at a pressure of 1 atm, has 30 trays, feeds at the 8th tray, has a top temperature of 58.51℃, and a bottom temperature of 64.18℃; distillation column IV5 operates at a pressure of 1 atm, has 16 trays, feeds at the 10th tray, has a top temperature of 91.19℃, and a bottom temperature of 99.64℃; after separation, the resulting products have a concentration of methyl formate greater than 99.50%, methanol greater than 99.50%, and benzene greater than 99.50%, and the azeotrope of paraformaldehyde and water meets the requirements for return to the original process for separation. Example

[0020] The feed flow rate was 1300 kg / h, the feed temperature was 30℃, the pressure was 1 atm, and the mass fraction of the feed was 12.58% formate, 21.70% methanol, 23.40% benzene, 37.24% methyl formate, and 5.08% water. The extractant feed flow rate was 820 kg / h. The following parameters are given: kg / h, feed temperature 25℃, pressure 1 atm; Distillation column I1 operating pressure 1 atm, 32 trays, feed position 22nd tray, top temperature 31.47℃, bottom temperature 57.48℃; Extraction column 2 operating pressure 1 atm, 12 trays, extractant feed position 1 tray, waste liquid feed position 12th tray, extraction column 2 temperature 35℃; Distillation column II3 operating pressure 1 atm, 45 trays, feed position 36th tray, top temperature 63.22℃, bottom temperature 95.15℃. Distillation column III4 operates at a pressure of 1 atm, has 30 trays, feeds at the 8th tray, has a top temperature of 55.25℃, and a bottom temperature of 64.17℃. Distillation column IV5 operates at a pressure of 1 atm, has 16 trays, feeds at the 10th tray, has a top temperature of 91.01℃, and a bottom temperature of 99.63℃. After separation, the product concentrations of methyl formate, methanol, and benzene are greater than 99.50%, and the azeotrope of paraformaldehyde and water meets the requirements for return to the original process for separation. Example

[0021] The feed flow rate was 1600 kg / h, the feed temperature was 30℃, the pressure was 1 atm, and the mass fraction of the feed was 16.08% formate, 24.20% methanol, 27.40% benzene, 28.24% methyl formate, and 4.26% water. The extractant feed flow rate was 1160 kg / h. The following parameters are given: Distillation column I1 operates at 1 atm, has 34 trays, feeds on the 20th tray, has a top temperature of 31.48℃, and a bottom temperature of 58.62℃. Extraction column II operates at 1 atm, has 16 trays, feeds the extractant on the 1st tray and the waste liquid on the 16th tray, and has a temperature of 35℃. Distillation column II operates at 1 atm, has 38 trays, feeds on the 29th tray, has a top temperature of 63.42℃, and a bottom temperature of 93.82℃. The feed flow rate is 1 kg / h, feed temperature 25℃, pressure 1 atm. The operating pressure of distillation column I1 is 1 atm, with 34 trays, feed position 20th tray, top temperature 31.48℃, and bottom temperature 58.62℃. Distillation column III4 operates at a pressure of 1 atm, has 30 trays, feeds at the 8th tray, has a top temperature of 55.90℃, and a bottom temperature of 64.16℃; distillation column IV5 operates at a pressure of 1 atm, has 16 trays, feeds at the 10th tray, has a top temperature of 91.20℃, and a bottom temperature of 99.64℃; after separation, the resulting products have a concentration of methyl formate greater than 99.50%, methanol greater than 99.50%, and benzene greater than 99.50%, and the azeotrope of paraformaldehyde and water meets the requirements for return to the original process for separation.

[0022] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention. No reference numerals in the claims should be construed as limiting the scope of the claims.

[0023] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A method for recovering polyoxymethylene waste liquid using an extractive distillation coupled process, characterized in that, The steps include the following: (1) A mixture of trioxymethylene, methyl formate, benzene, methanol and water enters from distillation column I (1). Methyl formate is separated in distillation column I (1). A portion of the material at the bottom of distillation column I (1) is returned to distillation column I (1) after heat exchange in reboiler I (11). A portion is fed into the bottom of extraction column (2) through condenser II (16). Methyl formate is condensed by condenser I (15) and enters the top storage tank I (7). A portion of methyl formate is returned to distillation column I (1) and a portion is taken out from the top storage tank I (7). (2) The extractant enters from the top of the extraction tower (2). After effective contact extraction and separation, benzene flows out from the top of the extraction tower (2), and the mixture of trioxymethylene, methanol and water flows out from the bottom of the extraction tower (2) and enters the distillation tower II (3). (3) The separation of trioxymethylene and methanol was achieved in distillation column II (3). Part of the bottom material of distillation column II (3) was returned to distillation column II (3) after heat exchange in reboiler II (12), and part of it was fed into distillation column IV (5) through condenser V (19). A large amount of methanol and a small amount of benzene were condensed in condenser III (17) and entered the top storage tank II (8). Part of it was returned to distillation column II (3) from the top storage tank II (8), and the other part entered distillation column III (4) through the top storage tank II (8). (4) Methanol is separated in distillation column III (4). A portion of the methanol at the bottom of distillation column III (4) is returned to distillation column III (4) after heat exchange in reboiler III (13), and a portion is taken out from the bottom of the column. A small amount of benzene and methanol azeotropically enters the top storage tank III (9) after condensation in condenser IV (18). A portion of the methanol is refluxed from the top storage tank III (9) to distillation column III (4), and another portion is taken out from the top storage tank III (9). (5) After distillation separation in distillation column IV (5), part of the water in the bottom material of distillation column IV (5) is returned to distillation column IV (5) after heat exchange in reboiler IV (14), and part of it is refluxed to mixer (6) through condenser VII (21); the trioxymethylene and water azeotrope is condensed in condenser VI (20) and enters the top storage tank IV (10), part of which is refluxed to distillation column IV (5), and the other part is taken out from the top storage tank IV (10).

2. The method for recovering polyoxymethylene waste liquid using an extractive distillation coupled process according to claim 1 relates to a system for recovering polyoxymethylene waste liquid using an extractive distillation coupled process, comprising distillation column I (1), extraction column (2), distillation column II (3), distillation column III (4), distillation column IV (5), mixer (6), top storage tank I (7), top storage tank II (8), top storage tank III (9), top storage tank IV (10), reboiler I (11), reboiler II (12), reboiler III (13), reboiler IV (14), condenser I (15), condenser II (16), condenser III (17), condenser IV (18), condenser V (19), condenser VI (20), and condenser VII (21), characterized in that... Fresh feed is connected to distillation column I (1) via a pipeline; fresh extractant and recycled extractant are mixed in mixer (6) and then connected to extraction column (2) via a pipeline; distillation column I (1) is connected to extraction column (2) via condenser II (16) via a pipeline; extraction column (2) is connected to distillation column II (3) via a pipeline, and the top of distillation column II (3) is connected to distillation column III (4) via a pipeline; the bottom of distillation column II (3) is connected to distillation column IV (5) via condenser V (19) via a pipeline; distillation column IV (5) is connected to the mixing column via condenser VII (21) via a pipeline. The condenser (6) is connected to the distillation column I (15) and the top storage tank I (7); the condenser III (17) is connected to the distillation column II (3) and the top storage tank II (8); the condenser IV (18) is connected to the distillation column III (4) and the top storage tank III (9); the condenser VI (20) is connected to the distillation column IV (5) and the top storage tank IV (10); the reboilers I (11), II (12), III (13), and IV (14) are respectively connected to the distillation column I (1), II (3), III (4), and IV (5).

3. The method for recovering polyoxymethylene waste liquid using an extractive distillation coupled process according to claim 1, characterized in that... The distillation column I (1) operates at a pressure of 1 atm and has 30 to 45 theoretical plates; the extraction column (2) operates at a pressure of 1 atm and has 12 to 20 theoretical plates; the distillation column II (3) operates at a pressure of 1 atm and has 30 to 50 theoretical plates; the distillation column III (4) operates at a pressure of 1 atm and has 30 to 40 theoretical plates; the distillation column IV (5) operates at a pressure of 1 atm and has 14 to 30 theoretical plates, and the feed temperature of the extractant is 25°C.

4. The method for recovering polyoxymethylene waste liquid using an extractive distillation coupled process according to claim 1, characterized in that... The mass ratio of the feed flow rate of the extractant to the mixture of trioxymethylene, methanol, benzene and water is 0.9 to 1.

1.

5. The method for recovering polyoxymethylene waste liquid using an extractive distillation coupled process according to claim 1, characterized in that... In the mixture of paraformaldehyde, methanol, benzene, methyl formate and water, the mass fraction of paraformaldehyde is 10%–20%, the mass fraction of methanol is 20%–30%, the mass fraction of benzene is 20%–30%, the mass fraction of methyl formate is 28%–38%, and the mass fraction of water is 3%–10%.

6. The method for recovering polyoxymethylene waste liquid using an extractive distillation coupled process according to claim 1, characterized in that... The purity of the separated methyl formate is greater than 99.50% by mass, the purity of the methanol is greater than 99.50% by mass, the purity of the benzene is greater than 99.50% by mass, and the purity of the extractant recovered from the bottom of distillation column T5 is greater than 99.90% by mass.

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