Tail gas treatment process for synthesizing 1, 5-pentamethylene diisocyanate by phosgenation method

Abstract

The invention discloses a tail gas treatment process for synthesizing 1, 5-pentamethylene diisocyanate by a phosgenation method, which comprises the following specific steps of: firstly, finishing absorption and cyclic utilization of a large amount of phosgene through a phosgene dissolving and absorbing system; secondly, through two-stage falling film absorption, a large amount of hydrogen chloride gas is absorbed, and a byproduct is hydrochloric acid; thirdly, carrying out two-stage hydrolysis reaction to complete the destruction treatment of the residual phosgene; finally, hydrogen chloride gas generated by the hydrolysis reaction is absorbed through protective absorption. In addition, according to the process, a set of HCI and COCI2 emergency damage system is further arranged at the rear end of protection absorption, and it can be ensured that a reaction system is safely stopped in an accident state. The method is low in investment cost, long in continuous operation period, simple and easy to control in process, capable of reducing the discharge amount of acid-alkali wastewater and relieving the pressure of'three wastes' treatment, capable of achieving clean production, suitable for industrial production and high in practicability and generalization performance.

Description

Technical Field

[0001] This invention belongs to the field of chemical engineering, specifically relating to a process for treating the tail gas of phosgenation synthesis of 1,5-pentanediisocyanate. Background Technology

[0002] 1,5-Pentanediisocyanate (PDI) is an aliphatic isocyanate that can be formulated into 1,5-pentanediisocyanate biuret or trimer for the production of polyurethane coatings and adhesives. These coatings are characterized by non-yellowing and strong weather resistance. The product outperforms HDI products in all aspects and can completely replace HDI products. It has good application value and market prospects, but industrial production has not yet been achieved in China.

[0003] PDI is produced by reacting a mixture of PDA (1,5-pentanediamine) and solvent CB (chlorobenzene) with excess phosgene, while also producing hydrogen chloride gas as a byproduct. The tail gas produced by this synthesis reaction contains a large amount of phosgene (in excess) and the byproduct hydrogen chloride gas. The traditional isocyanate phosgenation synthesis tail gas is produced by simply destroying it with alkaline solution (NaOH solution), which is an outdated process. The traditional process requires a large amount of NaOH solution and generates a large amount of acid and alkaline wastewater, which increases production costs and causes environmental pressure. Summary of the Invention

[0004] The purpose of this invention is to absorb the tail gas containing a large amount of phosgene and by-product hydrogen chloride gas generated during the phosgenation synthesis of PDI, so as to solve the problem of tail gas treatment in the PDI industry.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is: a method for synthesizing 1,5-pentanediisocyanate by gas-phase phosgenation. The method is characterized by the following specific steps: a process for treating the tail gas from the phosgenation synthesis of 1,5-pentanediisocyanate, characterized by the following specific steps: Step (1), Phosgene absorption: Using the principle of gas-liquid reverse dissolution absorption, chlorobenzene is used as the absorbent. The photochemical reaction tail gas is sequentially introduced into the first-stage phosgene absorption tower and the second-stage phosgene absorption tower for two-stage series absorption. Step (II), hydrogen chloride absorption: The principle of gas-liquid co-current falling film absorption is adopted, and dilute hydrochloric acid is used as the absorbent. The tail gas after step (I) is sequentially introduced into the hydrochloric acid primary absorption tower and the hydrochloric acid secondary absorption tower for two-stage series absorption. Step (3), Phosgene hydrolysis destruction: Using gas-liquid countercurrent operation and dilute hydrochloric acid as absorbent, the tail gas after step (2) is passed through a phosgene primary hydrolysis tower and a phosgene secondary hydrolysis tower in sequence for two-stage series hydrolysis, so that all the remaining small amount of phosgene in the tail gas is hydrolyzed and the destruction treatment is completed. Step (IV), Protective Absorption: Using the principle of gas-liquid reverse absorption, process water is used as the absorbent. The tail gas after step (III) is passed through the protective absorption tower to completely dissolve and absorb the hydrogen chloride generated by the hydrolysis reaction. Step (5), Emergency Destruction Treatment: Using the principle of acid-base neutralization reaction absorption, alkaline solution is used as the absorbent. The tail gas after the step treatment is passed through the emergency destruction tower to destroy and absorb the tail gas containing a large amount of phosgene and hydrogen chloride gas under the accident condition.

[0006] Furthermore, in step (i), the main components of the photochemical reaction tail gas are excess COCl2, byproduct HCl, and a small amount of non-condensable gases, namely carbon dioxide and nitrogen, in the system. The photochemical reaction tail gas enters from the bottom of the primary phosgene absorber and exits from the top of the secondary phosgene absorber, with the operating pressure controlled at -7.5~-1 kPa and the temperature at -5~5℃.

[0007] Furthermore, in step (ii), the exhaust gas after being treated in step (i) enters from the bottom of the primary hydrochloric acid absorption tower and exits from the top of the secondary hydrochloric acid absorption tower, with the operating pressure controlled at -7.5~-1kPa and the temperature at 18~28℃.

[0008] Furthermore, in step (iii), the tail gas treated in step (ii) enters from the bottom of the phosgene primary hydrolysis tower and exits from the top of the phosgene secondary hydrolysis tower, with the operating pressure controlled at -7.5~-1kPa and the temperature at 40~65℃.

[0009] Furthermore, in step (iv), the exhaust gas after being treated in step (iii) enters from the bottom of the protective absorption tower and exits from the top of the tower, with the operating pressure controlled at -7.5~-1kPa and the temperature at 18~28℃.

[0010] Furthermore, in step (v), the exhaust gas after being treated in step (iv) enters from the bottom of the emergency destruction tower and is discharged from the top of the tower by an induced draft fan, with the operating pressure controlled at -7.5~-1kPa and the temperature at 18~35℃.

[0011] Furthermore, the phosgene primary absorption tower, phosgene secondary absorption tower, hydrochloric acid primary absorption tower, hydrochloric acid secondary absorption tower, phosgene primary hydrolysis tower, phosgene secondary hydrolysis tower, protective absorption tower, and emergency destruction tower all have their own unit tower absorbent circulation system. This system includes an absorbent buffer tank. The liquid outlet at the bottom of the absorbent buffer tank is connected to the top of the unit tower via a circulation pump, and the liquid outlet at the bottom of the unit tower returns to the absorbent buffer tank, realizing the self-circulation and continuous absorption of the absorbent.

[0012] Phosgene hydrolysis reaction in step (II): COCl₂ + H₂O → 2HCl + CO₂ Emergency damage handling in step (five): HCl + NaOH → NaCl + H2O COCI2 + 4NaOH → Na2CO3 + NaCl+ H2O

[0013] 1. The phosgene dissolution and absorption, hydrogen chloride falling film absorption, and phosgene hydrolysis destruction processes used in this invention are mature, have low investment costs, long continuous operation cycles, and are simple and easy to control. 2. The process of this invention does not require a large amount of NaOH solution to destroy the tail gas, which saves NaOH solution and reduces the discharge of acid and alkaline wastewater, alleviating the pressure of "three wastes" treatment and enabling clean production; 3. The process of this invention produces hydrochloric acid (concentration 32%) as a byproduct, which can be sold directly, reducing production costs; 4. The process of this invention is suitable for industrial production and has strong practicality and applicability in the same industry. Attached Figure Description

[0014] Figure 1 This is a process flow diagram of the present invention; Figure 2 This is a schematic diagram of the absorbent circulation system of the unit tower of the present invention.

[0015] In the diagram: A1 - Phosgene primary absorption tower, A2 - Phosgene secondary absorption tower, B1 - Hydrochloric acid primary absorption tower, B2 - Hydrochloric acid secondary absorption tower, C1 - Phosgene primary hydrolysis tower, C2 - Phosgene secondary hydrolysis tower, D - Protective absorption tower, E - Emergency destruction tower, F - Exhaust fan, G - Absorbent buffer tank, H - Circulating pump, K - Online monitoring instrument; 001 - Photochemical reaction tail gas, 002 - Phosgene secondary absorbent, 003 - Phosgene primary absorbent, 004 - Process water, 005 - Dilute hydrochloric acid, 006 - Concentrated hydrochloric acid, 007 - Sodium hydroxide solution, 008 - Inorganic wastewater, 009 - Tail gas after compliance. Detailed Implementation

[0016] The principles and features of the present invention are described below with reference to the accompanying drawings. The examples given are only for explaining the present invention and are not intended to limit the scope of the present invention.

[0017] Example 1, see Figure 1 A process for treating the tail gas from the synthesis of 1,5-pentanediisocyanate via phosgenation is disclosed. The tail gas treatment process is achieved through a treatment system, which includes a phosgene primary absorption tower A1, a phosgene secondary absorption tower A2, a hydrochloric acid primary absorption tower B1, a hydrochloric acid secondary absorption tower B2, a phosgene primary hydrolysis tower C1, a phosgene secondary hydrolysis tower C2, a protective absorption tower D, and an emergency destruction tower E, all connected in sequence.

[0018] The specific steps of the processing technology are as follows: Step (1), Phosgene absorption: Using the principle of gas-liquid reverse dissolution absorption, chlorobenzene is used as the absorbent. The photochemical reaction tail gas 001 is sequentially introduced into the phosgene primary absorption tower A1 and the phosgene secondary absorption tower A2 for two-stage series absorption. Step (II), hydrogen chloride absorption: Using the gas-liquid co-current falling film absorption principle, dilute hydrochloric acid is used as the absorbent. The tail gas after step (I) is sequentially introduced into the hydrochloric acid primary absorption tower B1 and the hydrochloric acid secondary absorption tower B2 for two-stage series absorption. Step (3), Phosgene hydrolysis destruction: Using gas-liquid countercurrent operation and dilute hydrochloric acid as absorbent, the tail gas after step (2) is sequentially passed through the phosgene primary hydrolysis tower C1 and the phosgene secondary hydrolysis tower C2 for two-stage series hydrolysis, so that all the remaining small amount of phosgene in the tail gas is hydrolyzed and the destruction treatment is completed. Step (IV), Protective Absorption: Using the principle of gas-liquid reverse absorption, process water is used as the absorbent. The tail gas after step (III) is passed through protective absorption tower D to completely dissolve and absorb the hydrogen chloride generated by the hydrolysis reaction. Step (5), Emergency Destruction Treatment: Using the principle of acid-base neutralization reaction absorption, alkaline solution is used as the absorbent. The tail gas treated in step (4) is passed through emergency destruction tower E to destroy and absorb the tail gas containing a large amount of phosgene and hydrogen chloride gas under the accident condition.

[0019] In step (i), the main components of the photochemical reaction tail gas 001 are excess COCl2, byproduct HCl, and a small amount of non-condensable gases, namely carbon dioxide and nitrogen, within the system; The photochemical reaction tail gas 001 enters from the bottom of the phosgene primary absorption tower A1 and exits from the top of the phosgene secondary absorption tower A2. The operating pressure is controlled at -7.5~-1kPa and the temperature is -5~5℃.

[0020] In step (ii), the exhaust gas after being treated in step (i) enters from the bottom of the primary hydrochloric acid absorption tower B1 and exits from the top of the secondary hydrochloric acid absorption tower B2. The operating pressure is controlled at -7.5~-1 kPa and the temperature is controlled at 18~28℃.

[0021] In step (iii), the tail gas after being treated in step (ii) enters from the bottom of the C1 column of the phosgene primary hydrolysis tower and exits from the top of the C2 column of the phosgene secondary hydrolysis tower, with the operating pressure controlled at -7.5~-1kPa and the temperature at 40~65℃.

[0022] In step (four), the exhaust gas after being treated in step (three) enters from the bottom of the protective absorption tower D and exits from the top of the tower, with the operating pressure controlled at -7.5~-1kPa and the temperature at 18~28℃.

[0023] In step (5), the exhaust gas after being treated in step (4) enters from the bottom of the emergency destruction tower E and is discharged from the top of the tower by the induced draft fan F, with the operating pressure controlled at -7.5~-1kPa and the temperature at 18~35℃.

[0024] The phosgene primary absorption tower A1, phosgene secondary absorption tower A2, hydrochloric acid primary absorption tower B1, hydrochloric acid secondary absorption tower B2, phosgene primary hydrolysis tower C1, phosgene secondary hydrolysis tower C2, protective absorption tower D, and emergency destruction tower E each have their own unit tower absorbent circulation system. This system includes an absorbent buffer tank G. The liquid outlet at the bottom of the absorbent buffer tank G is connected to the top of the unit tower via a circulation pump H. The liquid outlet at the bottom of the unit tower returns to the absorbent buffer tank G, realizing the self-circulation and continuous absorption of the absorbent.

[0025] Work process: (1) The photochemical reaction tail gas 001 passes through the phosgene primary absorption tower A1 and the phosgene secondary absorption tower A2 in sequence to complete the dissolution and absorption of a large amount of phosgene in the tail gas. The phosgene primary absorption liquid 003 is output from the bottom of the phosgene primary absorption tower A1 to the recycling system, and the phosgene secondary absorption liquid 002 is output from the bottom of the phosgene secondary absorption tower A2 to the recycling system. (2) The tail gas from the top of the phosgene secondary absorption tower A2 passes through the hydrochloric acid primary absorption tower B1 and the hydrochloric acid secondary absorption tower B2 in sequence to complete the falling film dissolution and absorption of hydrogen chloride in the tail gas. 32% concentrated hydrochloric acid 006 is output from the bottom of the hydrochloric acid primary absorption tower B1. The primary absorbent is dilute hydrochloric acid 005 from the bottom of the hydrochloric acid secondary absorption tower B2, and the secondary absorbent is dilute hydrochloric acid 005 from the bottom of the phosgene primary hydrolysis tower C1. (3) The tail gas from the top of the hydrochloric acid secondary absorption tower B2 passes through the phosgene primary hydrolysis tower C1 and the phosgene secondary hydrolysis tower C2 in sequence to complete the hydrolysis and destruction absorption of all remaining phosgene. The primary hydrolysant comes from the dilute hydrochloric acid 005 in the phosgene secondary hydrolysis tower C2, and the secondary hydrolysant comes from the dilute hydrochloric acid 005 in the bottom of the protective absorption tower D. (4) The tail gas from the top of the phosgene secondary hydrolysis tower C2 passes through the protective absorption tower D to complete the solvent absorption of all hydrogen chloride. The absorbent comes from the process water 004 of the utility. (5) The exhaust gas from the top of the protective absorption tower D is tested and qualified by the online detector K, and is directly discharged into the air through the emergency breaking tower E (empty tower) and the induced draft fan F. The inorganic wastewater 008 is discharged after passing the test. (6) In the event of an accident, the tail gas from the top of the protective absorption tower D is found to be unqualified by the online detector K (containing a large amount of phosgene and hydrogen chloride gas). The emergency destruction tower E is activated by interlock. After emergency destruction absorption by the emergency destruction tower E, all phosgene and hydrogen chloride are destroyed and treated. The gas is then directly discharged into the air by the induced draft fan F. The emergency destruction absorption liquid is a 19% sodium hydroxide solution from the public works.

Claims

1. A process for treating off-gas from the synthesis of 1,5-pentamethylene diisocyanate by the phosgenation method, characterized in that, The specific steps are as follows: Step (one), phosgene absorption: using the principle of gas-liquid reverse dissolution absorption, chlorobenzene as the absorbent, the phosgene tail gas (001) is sequentially introduced into the first-stage phosgene absorption tower (A1) and the second-stage phosgene absorption tower (A2) for two-stage series absorption; Step (two), hydrogen chloride absorption: using the principle of gas-liquid co-current falling film absorption, dilute hydrochloric acid as the absorbent, the tail gas treated in step (one) is sequentially introduced into the first-stage hydrochloric acid absorption tower (B1) and the second-stage hydrochloric acid absorption tower (B2) for two-stage series absorption; Step (three), phosgene hydrolysis destruction: using the principle of gas-liquid reverse operation, dilute hydrochloric acid as the absorbent, the tail gas treated in step (two) is sequentially introduced into the first-stage phosgene hydrolysis tower (C1) and the second-stage phosgene hydrolysis tower (C2) for two-stage series hydrolysis, so that the remaining small amount of phosgene in the tail gas is completely hydrolyzed to complete the destruction treatment; Step (four), protection absorption: using the principle of gas-liquid reverse absorption, process water as the absorbent, the tail gas treated in step (three) is introduced into the protection absorption tower (D) to completely dissolve and absorb the hydrogen chloride generated in the hydrolysis reaction; Step (five), emergency destruction treatment: using the principle of acid-base neutralization reaction absorption, alkali solution as the absorbent, the tail gas treated in step (four) is introduced into the emergency destruction tower (E) to destroy and absorb the tail gas containing a large amount of phosgene and hydrogen chloride gas in the accident state.

2. The process for treating the exhaust gas of the synthesis of 1,5-pentamethylene diisocyanate by the phosgenation method according to claim 1, characterized in that: In step (one), the main components of the phosgene tail gas (001) are excess COCI2, by-product HCI, and a small amount of non-condensable gas, i.e., carbon dioxide and nitrogen, in the system.

3. The process for treating the exhaust gas of the synthesis of 1,5-pentamethylene diisocyanate by the phosgenation method according to claim 1, characterized by the fact that: In step (one), the phosgene tail gas (001) is introduced from the bottom of the first-stage phosgene absorption tower (A1) and discharged from the top of the second-stage phosgene absorption tower (A2), with the control operation pressure being -7.5~-1kPa and the temperature being -5~5℃.

4. The process for treating the exhaust gas of the synthesis of 1,5-pentamethylene diisocyanate by the phosgenation method according to claim 1, characterized by the fact that: In step (two), the tail gas treated in step (one) is introduced from the bottom of the first-stage hydrochloric acid absorption tower (B1) and discharged from the top of the second-stage hydrochloric acid absorption tower (B2), with the control operation pressure being -7.5~-1kPa and the temperature being 18~28℃.

5. The process for treating the exhaust gas of the synthesis of 1,5-pentamethylene diisocyanate by the phosgenation method according to claim 1, characterized by the fact that: In step (three), the tail gas treated in step (two) is introduced from the bottom of the first-stage phosgene hydrolysis tower (C1) and discharged from the top of the second-stage phosgene hydrolysis tower (C2), with the control operation pressure being -7.5~-1kPa and the temperature being 40~65℃.

6. The process for treating the off-gas from the synthesis of 1,5-pentamethylene diisocyanate by the phosgenation process according to claim 1, characterized by the fact that: In step (four), the tail gas treated in step (three) is introduced from the bottom of the protection absorption tower (D) and discharged from the top, with the control operation pressure being -7.5~-1kPa and the temperature being 18~28℃.

7. The process for treating the off-gas from the synthesis of 1,5-pentamethylene diisocyanate by the phosgenation process according to claim 1, characterized by the fact that: In step (five), the tail gas treated in step (four) is introduced from the bottom of the emergency destruction tower (E) and discharged from the top through the induced draft fan (F), with the control operation pressure being -7.5~-1kPa and the temperature being 18~35℃.

8. The process according to any one of claims 1 to 7, characterized in that: The primary phosgene absorption tower (A1), the secondary phosgene absorption tower (A2), the primary hydrochloric acid absorption tower (B1), the secondary hydrochloric acid absorption tower (B2), the primary phosgene hydrolysis tower (C1), the secondary phosgene hydrolysis tower (C2), the protection absorption tower (D) and the emergency destruction tower (E) all have their own unit tower absorbent circulating system, which comprises an absorbent buffer tank (G), a liquid outlet at the bottom of the absorbent buffer tank (G) is connected to the top of the unit tower through a circulating pump (H), and a liquid outlet at the bottom of the unit tower is connected back to the absorbent buffer tank (G), so as to realize the self-circulation and continuous absorption of the absorbent.

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