Apparatus and method for recovering polyformaldehyde waste liquid by extractive distillation

Abstract

An extraction rectification recovery device and method for waste trioxymethylene liquid, comprising a pre-separation rectification tower, a methyl formate removal tower, an extraction rectification tower, a solvent recovery tower, several delivery pumps and material transfer pumps, several storage tanks, several heat exchangers, a mixer, several condensers and reboilers. The pre-separation rectification tower preliminarily separates methyl formate, benzene, methanol and water, and trioxymethylene in the waste liquid; the methyl formate removal tower obtains methyl formate with a purity of ≥99.5% at the top, and a mixture of benzene and methanol at the bottom; the extraction rectification tower removes benzene with a purity of ≥99.5%; the solvent recovery tower obtains methanol with a purity of ≥99.5% at the top, and the bottom material returns to the extraction rectification tower through the mixer as an organic extraction solvent to further remove benzene, and the cycle is repeated. All materials are separately rectified under normal pressure, which is convenient to operate and saves cost; since each tower separately separates each material under different conditions, compared with the high operation precision of single-tower continuous rectification, the continuous four-tower rectification also reduces the operation difficulty and is easy to realize.

Description

Technical Field

[0001] This invention relates to the field of polyoxymethylene (POM) production technology, specifically to an apparatus and method for extractive distillation to recover POM waste liquid. Background Technology

[0002] Polyoxymethylene (POM) is one of the five most widely used engineering plastics. Trioxymethylene (TOX) is an important intermediate in the production of POM. TOX (C3H6O3) is synthesized from formaldehyde (CH2O) in a large vertical container reactor under the catalysis of sulfuric acid. Polymer-grade TOX is obtained through synthetic distillation, extractive distillation, and the removal of light and heavy components. The production of high-quality POM engineering plastics relies heavily on high-purity TOX. TOX production primarily involves the reaction of formaldehyde solution with a sulfuric acid catalyst to produce crude TOX. Side reactions during this process also result in the generation of byproducts such as methyl formate and paraformaldehyde.

[0003] During the TOX purification stage, water and TOX form an azeotropic mixture, making separation difficult. Industrially, benzene extraction is commonly used to separate water and TOX, but impurities such as methyl formate, water, and methanol are also continuously separated during this process. This separation inevitably carries away useful substances like TOX and benzene, which mix and become difficult to separate and utilize, resulting in hazardous waste. Domestic regulations for this type of waste are stringent, but companies lack corresponding recycling technologies, leading to high costs and resource waste in the treatment of polyoxymethylene (POM) waste. Therefore, developing an economical and efficient POM waste recycling technology can not only achieve comprehensive resource utilization but also reduce waste treatment costs and promote the development of my country's POM industry. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and provide an apparatus and method for extractive distillation to recover polyoxymethylene waste liquid. This invention uses three distillation columns and one extractive distillation column for separation and purification. The extractant is ethylene glycol, which is similar to and compatible with methanol. Based on this, a corresponding process scheme is designed to achieve efficient separation of the mixture.

[0005] To achieve the above objectives, the first aspect of the present invention provides an extractive distillation apparatus for recovering polyoxymethylene waste liquid, specifically comprising: a pre-separation distillation column 1, a methyl formate removal column 2, an extractive distillation column 3, a solvent recovery column 4, a waste liquid storage tank 5, a transfer pump I 6, a methyl formate storage tank 7, a heat exchanger I 8, a transfer pump I 9, a benzene storage tank 10, a heat exchanger II 11, a transfer pump II 12, a methanol storage tank 13, a heat exchanger III 14, a transfer pump III 15, an ethylene glycol storage tank 16, a transfer pump II 17, a mixer 18, a condenser I 19, a condenser II 20, a condenser III 21, a condenser IV 22, a reboiler I 23, a reboiler II 24, a reboiler III 25, and a reboiler IV 26.

[0006] The waste liquid storage tank 5 stores the mixed materials. The inlet of the transfer pump I6 is connected to the waste liquid storage tank 5, and its outlet is connected to the upper part of the pre-separation distillation column 1. The inlet of the condenser I19 is connected to the top of the pre-separation distillation column 1, and the outlet of the condenser I19 is connected to the upper part of the pre-separation distillation column 1 and the upper part of the methyl formate removal column 2. The reboiler I23 is connected to the bottom of the pre-separation distillation column 1. The inlet of the condenser II20 is connected to the top of the methyl formate removal column 2, and the outlet of the condenser II20 is connected to the upper part of the methyl formate removal column 2 and the methyl formate storage tank 7. The inlet of the reboiler II24 is connected to the bottom of the methyl formate removal column 2, and the outlet of the reboiler II24 is connected to the lower part of the methyl formate removal column 2 and the heat exchanger I8. The inlet of the transfer pump I9 is ​​connected to the heat exchanger I8, and its outlet is connected to the extractive distillation column 3. The inlet of the transfer pump II17 is connected to the ethylene glycol storage tank 16. The outlet of the condenser III 21 is connected to the top of the extractive distillation column 3, and its outlet is connected to the upper part of the extractive distillation column 3 and the benzene storage tank 10, respectively. The inlet of the reboiler III 25 is connected to the bottom of the extractive distillation column 3, and its outlet is connected to the lower part of the extractive distillation column 3 and the heat exchanger II 11, respectively. The inlet of the transfer pump II 12 is connected to the heat exchanger II 11, and its outlet is connected to the solvent recovery column 4. The inlet of the condenser IV 22 is connected to the top of the solvent recovery column 4, and its outlet is connected to the upper part of the solvent recovery column 4 and the methanol storage tank 13, respectively. The inlet of the reboiler IV 26 is connected to the bottom of the solvent recovery column 4, and its outlet is connected to the lower part of the solvent recovery column 4 and the heat exchanger III 14, respectively. The inlet of the transfer pump III 15 is connected to the heat exchanger III 14, and its outlet is connected to the mixer 18. The outlet of the mixer 18 is connected to the methanol storage tank 13.

[0007] A second aspect of the present invention provides a method for recovering paraformaldehyde waste liquid using the above-mentioned apparatus, specifically comprising the following steps: Step 1: The reacted mixture is fed into the pre-separation distillation column 1 for preliminary distillation separation. The top of the column yields low-boiling-point methyl formate, benzene, and methanol, while the bottom yields a small amount of water and trioxymethylene mixture. The mixture is partially returned to the pre-separation distillation column 1 and partially returned to the original process system via reboiler I 23. Step 2: In Step 1, part of the material from the top of the pre-separation distillation column 1 enters the methyl formate removal column 2 through condenser I 19, and part is refluxed back to the pre-separation distillation column 1. The top of the methyl formate removal column 2 yields methyl formate with a purity ≥ 99.5%. After the methyl formate is condensed through condenser II 20, part is refluxed back to the methyl formate removal column 2, and part enters the methyl formate storage tank 7. The bottom of the column yields a mixture of benzene and methanol. Step 3: In Step 2, the material from the bottom of the methyl formate tower 2 passes through heat exchanger 8 and is then fed into extractive distillation tower 3 via transfer pump I 9. Ethylene glycol storage tank 16 is fed into extractive distillation tower 3 via transfer pump II 17 and mixer 18. Benzene with a purity ≥ 99.5% is obtained at the top of extractive distillation tower 3. The material from the bottom of the tower passes through heat exchanger II 11 and transfer pump II 12 into solvent recovery tower 4. The material from the top of the tower enters benzene storage tank 10 with a purity ≥ 99.5% and is returned to the original process system. Step four: The material from the top of the solvent recovery tower 4 in step three enters the methanol storage tank 13, and the material from the bottom of the tower enters the mixer 18 through the heat exchanger III 14 and the transfer pump III 15, and then enters the extractive distillation tower 3 through the mixer 18.

[0008] Preferably, the top temperature of the pre-separation distillation column 1 in step one is 40-50°C, more preferably 41-43°C; and the bottom temperature is 90-100°C, more preferably 90-93°C.

[0009] Preferably, the mass ratio of the extractant to the feed flow rate of the mixture of paraformaldehyde, methanol, benzene, and water is 0.9–1.1. Preferably, the total pressure of the pre-separation distillation column 1 is 101.325 kPa; its reflux feed ratio is 3.0–5.0, more preferably 2.0–3.5.

[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 top temperature of the methyl deformate column 2 in step two is 30-40°C, more preferably 31-32°C; and the bottom temperature is 50-60°C, more preferably 58-59°C.

[0012] Preferably, the total pressure of the methyl deformate tower 2 is 101.325 kPa; its reflux feed ratio is 4.0-6.0, more preferably 3.0-4.0.

[0013] Preferably, the top temperature of the extractive distillation column 3 in step three is 70-80°C, more preferably 78-80°C; and the bottom temperature is 80-90°C, more preferably 81-84°C.

[0014] Preferably, the pressure of the entire extractive distillation column 3 is 101.325 kPa; its reflux feed ratio is 1.0-3.0, more preferably 1.0-2.0.

[0015] Preferably, the temperature at the top of the solvent recovery tower 4 in step four is 60-70°C, more preferably 64-65°C; and the temperature at the bottom of the tower is 190-200°C, more preferably 196-198°C.

[0016] Preferably, the total pressure of solvent recovery tower 4 is 101.325 kPa; its reflux feed ratio is 3.0-5.0, more preferably 2.0-3.0.

[0017] Compared with the prior art, the present invention has the following beneficial effects: This invention discloses an extractive distillation device and method for recovering polyoxymethylene waste liquid. The device employs a continuous four-tower distillation process. Pre-separation distillation tower 1, operating at 101.325 kPa, initially separates methyl formate, benzene, methanol, water, and trioxymethylene from the waste liquid. Demethyl formate tower 2, operating at 101.325 kPa, yields methyl formate at the top with a purity ≥99.5%, and a mixture of benzene and methanol at the bottom. Extractive distillation tower 3, operating at 101.325 kPa, removes benzene with a purity ≥99.5%. Solvent recovery tower 4, operating at 101.325 kPa, yields methanol at the top with a purity ≥99.5%, and the bottom material is returned to extractive distillation tower 3 via a mixer to serve as an organic extraction solvent for further benzene extraction, in a continuous cycle.

[0018] This invention relates to an extraction distillation device and method for recovering polyoxymethylene waste liquid. All materials are distilled separately under normal pressure, which is convenient to operate, saves costs, greatly reduces residue in the reactor, and has high efficiency and large output. Since each column is under different conditions, each substance is separated individually. Compared with the high operating precision of single-column continuous distillation, continuous four-column distillation also reduces the operation difficulty and is easy to implement. Attached Figure Description

[0019] Figure 1 This is a process flow diagram of an extraction distillation device and method for recovering polyoxymethylene waste liquid according to the present invention. Detailed Implementation

[0020] 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.

[0021] 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.

[0022] The present invention will now be described in further detail with reference to the accompanying drawings. Example 1

[0023] In this embodiment, the waste liquid composition for producing paraformaldehyde is: 33.04% methyl formate, 20.00 wt% benzene, 27.70 wt% methanol, 3.28 wt% water, and 15.98 wt% paraformaldehyde. The pre-separation distillation column 1 has a reflux ratio R=3, an operating pressure of 101.325 kPa, and 35 theoretical plates; the deformate methyl ester removal column 2 has a reflux ratio R=4, an operating pressure of 101.325 kPa, and 30 theoretical plates; the extractive distillation column 3 has a reflux ratio R=2, an operating pressure of 101.325 kPa, and 46 theoretical plates; and the solvent recovery column 4 has a reflux ratio R=3, an operating pressure of 101.325 kPa, and 30 theoretical plates. The purity of the obtained methyl formate product is ≥99.5%, the purity of the benzene product is ≥99.5%, and the purity of the methanol product is ≥99.5%, all of which meet the national standard requirements or the requirements for recycling in the front-end reaction process. The recovery rate of each product is above 99%, which meets the requirements of the separation process. Example 2

[0024] In this embodiment, the waste liquid composition for producing trioxymethylene is: 36.04% methyl formate, 22.00 wt% benzene, 24.40 wt% methanol, 5.28 wt% water, and 12.28 wt% trioxymethylene. The pre-separation distillation column 1 has a reflux ratio R=2, an operating pressure of 101.325 kPa, and 25 theoretical plates; the demethyl formate column 2 has a reflux ratio R=3.5, an operating pressure of 101.325 kPa, and 30 theoretical plates; the extractive distillation column 3 has a reflux ratio R=1.5, an operating pressure of 101.325 kPa, and 46 theoretical plates; and the solvent recovery column 4 has a reflux ratio R=2.5, an operating pressure of 101.325 kPa, and 35 theoretical plates. The purity of the obtained methyl formate product is ≥99.5%, the purity of the benzene product is ≥99.5%, and the purity of the methanol product is ≥99.5%, all of which meet the national standard requirements or the requirements for recycling in the front-end reaction process. The recovery rate of each product is above 99%, which meets the requirements of the separation process.

[0025] 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.

[0026] 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 extractive distillation to recover polyoxymethylene waste liquid, comprising the following steps: Step 1: The reacted mixture is fed into the pre-separation distillation column (1) for preliminary distillation separation. The top of the column yields low-boiling-point methyl formate, benzene and methanol, while the bottom yields a small amount of water and trioxymethylene mixture. The mixture is then passed through reboiler I (23), with part of it returned to the pre-separation distillation column (1) and part returned to the original process system. Step 2: In Step 1, part of the material at the top of the pre-separation distillation column (1) enters the methyl formate removal column (2) through condenser I (19), and part is refluxed back to the pre-separation distillation column (1). Methyl formate with a purity ≥99.5% is obtained at the top of the methyl formate removal column (2). After the methyl formate is condensed through condenser II (20), part is refluxed back to the methyl formate removal column (2), and part enters the methyl formate storage tank (7). A mixture of benzene and methanol is obtained at the bottom of the column. Step 3: The material from the bottom of the methyl methacrylate tower (2) in step 2 passes through heat exchanger I (8) and enters the extractive distillation tower (3) through transfer pump I (9). The ethylene glycol storage tank (16) adds ethylene glycol to the extractive distillation tower (3) through a mixer (18) via transfer pump II (17). Benzene with a purity ≥99.5% is obtained at the top of the extractive distillation tower (3). The material from the bottom of the tower passes through heat exchanger II (11) and transfer pump II (12) and enters the solvent recovery tower (4). The material from the top of the tower enters the benzene storage tank (10) with a purity ≥99.5% and is returned to the original process system. Step 4: The material at the top of the solvent recovery tower (4) in step 3 enters the methanol storage tank (13), and the material at the bottom of the tower enters the mixer (18) through the heat exchanger III (14) and the transfer pump III (15), and then enters the extractive distillation tower (3) through the mixer (18).

2. The method for extractive distillation to recover polyoxymethylene waste liquid according to claim 1, further comprising an apparatus for extractive distillation to recover polyoxymethylene waste liquid, specifically including: Pre-separation distillation column (1), methyl formate removal column (2), extractive distillation column (3), solvent recovery column (4), waste liquid storage tank (5), transfer pump I (6), methyl formate storage tank (7), heat exchanger I (8), transfer pump I (9), benzene storage tank (10), heat exchanger II (11), transfer pump II (12), methanol storage tank (13), heat exchanger III (14), transfer pump III (15), ethylene glycol storage tank (16), transfer pump II (17), mixer (18), condenser I (19), condenser II (20), condenser III (21), condenser IV (22), reboiler I (23), reboiler II (24), reboiler III (25), reboiler Device IV (26), characterized in that: the waste liquid storage tank (5) stores the mixed materials, the inlet of the transfer pump I (6) is connected to the waste liquid storage tank (5), and its outlet is connected to the upper part of the pre-separation distillation column (1), the inlet of the condenser I (19) is connected to the top of the pre-separation distillation column (1), and the outlet of the condenser I (19) is connected to the upper part of the pre-separation distillation column (1) and the upper part of the methyl formate column (2) respectively, the reboiler I (23) is connected to the bottom of the pre-separation distillation column (1), the inlet of the condenser II (20) is connected to the top of the methyl formate column (2), and the outlet of the condenser II (20) is connected to the upper part of the methyl formate column (2) and the methyl formate storage tank (7 ... condenser I (20) is connected to the top of the methyl formate column (2), and the outlet of the condenser II (20 The inlet of reboiler II (24) is connected to the bottom of the methyl deformate column (2), and the outlet of reboiler II (24) is connected to the lower part of the methyl deformate column (2) and heat exchanger I (8), respectively. The inlet of the transfer pump I (9) is connected to the heat exchanger I (8), and the outlet is connected to the extractive distillation column (3). The inlet of the transfer pump II (17) is connected to the ethylene glycol storage tank (16), and the outlet is connected to the mixer (18). The inlet of the condenser III (21) is connected to the top of the extractive distillation column (3), and the outlet is connected to the upper part of the extractive distillation column (3) and benzene storage tank (10), respectively. The inlet of reboiler III (25) is connected to the bottom of the extractive distillation column (3), and the outlet of reboiler III (25) is connected to the extractive distillation column (3), respectively. The lower part of the distillation column (3) is connected to the heat exchanger II (11). The inlet of the transfer pump II (12) is connected to the heat exchanger II (11) and the outlet is connected to the solvent recovery column (4). The inlet of the condenser IV (22) is connected to the top of the solvent recovery column (4) and the outlet is connected to the upper part of the solvent recovery column (4) and the methanol storage tank (13) respectively. The inlet of the reboiler IV (26) is connected to the bottom of the solvent recovery column (4) and the outlet is connected to the lower part of the solvent recovery column (4) and the heat exchanger III (14) respectively. The inlet of the transfer pump III (15) is connected to the heat exchanger III (14) and the outlet is connected to the mixer (18). The outlet of the mixer (18) is connected to the methanol storage tank (13).

3. The method for extractive distillation and recovery of polyoxymethylene waste liquid according to claim 1, characterized in that: The top temperature of the pre-separation distillation column (1) in step one is 40-50°C or 41-43°C; the bottom temperature is 90-100°C or 90-93°C.

4. The method for extractive distillation and recovery of polyoxymethylene waste liquid 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, with the mass fractions of trioxymethylene, methanol, benzene and water being 10% to 20%, methanol, benzene, methyl formate, and water being 3% to 10%.

5. The method for extractive distillation and recovery of polyoxymethylene waste liquid according to claim 1, characterized in that: The pressure of the pre-separation distillation column (1) is 101.325 kPa; its reflux feed ratio is 3.0 to 5.0, or 2.0 to 3.

5.

6. The method for extractive distillation and recovery of polyoxymethylene waste liquid according to claim 1, characterized in that: In step two, the top temperature of the methyl methacrylate tower (2) is 30-40°C or 31-32°C; the bottom temperature is 50-60°C or 58-59°C; the total pressure of the methyl methacrylate tower (2) is 101.325 kPa; and the reflux feed ratio is 4.0-6.0 or 3.0-4.

0.

7. The method for extractive distillation and recovery of polyoxymethylene waste liquid according to claim 1, characterized in that: The top temperature of the extractive distillation column (3) in step three is 70-80°C or 78-80°C; the bottom temperature is 80-90°C or 81-84°C; the total pressure of the extractive distillation column (3) is 101.325 kPa; and the reflux feed ratio is 1.0-3.0 or 1.0-2.

0.

8. The method for extractive distillation and recovery of polyoxymethylene waste liquid according to claim 1, characterized in that: The temperature at the top of the solvent recovery tower (4) in step four is 60-70°C or 64-65°C; the temperature at the bottom of the tower is 190-200°C or 196-198°C; the pressure of the entire solvent recovery tower (4) is 101.325 kPa; and the reflux feed ratio is 3.0-5.0 or 2.0-3.0.

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