Method for reducing COD (Chemical Oxygen Demand) content of saponification wastewater

Abstract

The invention discloses a method for reducing the COD (Chemical Oxygen Demand) content of saponification wastewater. According to the method, after a gas phase extracted from a saponification reactor is subjected to n-stage condensation, part of n-stage condensate flows back to the saponification reactor, and the other part of n-stage condensate is extracted; n > = 1 and is an integer. According to the method, a condensate partial extraction process is added after primary saponification condensation, so that the liquid phase load of a saponification reactor can be remarkably reduced, the hydrolysis problem caused by too long retention time of epoxypropane in a saponification tower is solved, and the COD content of saponification wastewater can be reduced by about 200mg / L.

Description

Technical Field

[0001] This invention belongs to the field of propylene oxide saponification, specifically relating to a method for reducing the COD content of saponification wastewater. Background Technology

[0002] Propylene oxide (PO) is a major derivative of propylene and an important basic organic chemical raw material. Propylene oxide is mainly used in the production of polyether polyols for polyurethane, propylene glycol, isopropanolamine, allyl alcohol, and non-polyether polyols, and is widely used in chemical, light industry, pharmaceutical, pesticide, and food industries.

[0003] The chlorohydrin process is a common process for producing propylene oxide. The chlorohydrin process mainly uses propylene and chlorine as raw materials. The chlorohydrin is produced through a chlorohydrin reaction, and then reacted with an alkali in a saponification reactor. The saponification reactor operates under reduced pressure, and crude PO is distilled off from the top of the column under reduced pressure. The crude PO is then separated and purified to obtain propylene oxide. The saponification reaction residue is discharged from the bottom of the column.

[0004] COD (Chemical Oxygen Demand) reflects the degree of pollution in water by reducing substances, which mainly include organic matter, nitrites, ferrous salts, sulfides, and other oxidizable substances. A higher COD indicates a greater amount of organic pollutants in the water. The COD level in the saponification system wastewater is a crucial control point in propylene oxide production. The COD level not only directly characterizes the reaction process in the saponification system but also significantly impacts subsequent wastewater treatment systems.

[0005] How to reduce the COD value of wastewater from saponification systems is a technical problem that those skilled in the art are dedicated to solving. Summary of the Invention

[0006] To improve the above-mentioned technical problems, the present invention provides a method for reducing the COD content of saponification wastewater from the chlorohydrin process for producing propylene oxide, comprising: after the gas phase collected from the saponification reactor is condensed in n stages, a portion of the nth stage condensate is returned to the saponification reactor, and another portion of the nth stage condensate is collected. n≥1 and is an integer, for example n=1~3, such as n=2 (secondary condenser) and 3 (tertiary condenser).

[0007] According to an embodiment of the present invention, the flow rate of the nth stage reflux condensate is denoted as a1, and the flow rate of the extracted condensate is denoted as a2, where a1 / a2 = 15~25, for example, a1 / a2 = 15, 16, 17, 18, 19, 20, 21, 22, 23, 24 or 25.

[0008] According to an embodiment of the present invention, when n=2, i.e., two-stage condensation, the gas phase collected from the saponification reactor is condensed in the first stage to obtain a first-stage condensate and a first-stage gas phase. The first-stage condensate is returned to the saponification reactor, and the first-stage gas phase is condensed in the second stage to obtain a second-stage gas phase and a second-stage condensate. Part of the second-stage condensate a1 is returned to the saponification reactor, and another part of the second-stage condensate a2 is collected. The second-stage gas phase is sent for fractional condensation.

[0009] According to an embodiment of the present invention, the outlet temperature of the nth stage gas phase (e.g., the second stage gas phase) is 20~35°C, preferably 25~30°C. Further, the nth stage gas phase (e.g., the second stage gas phase) is sent to a condenser.

[0010] According to an embodiment of the present invention, the n-stage condensation is carried out in a condenser.

[0011] According to an embodiment of the present invention, the nth stage condensate (e.g., the second stage condensate a2) enters the crude propylene oxide cooling separator.

[0012] According to an embodiment of the present invention, the purity of PO in the crude propylene oxide cooling separator is controlled to be ≥70%, for example, the purity is 70%~90%, and exemplary values ​​are 72%, 74%, 75%, 77%, 78%, 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, or 90%.

[0013] According to an embodiment of the present invention, the saponification reactor is controlled at a pressure of 40-70 kPa (A), a pressure of 50-70 kPa (A), a top temperature of 60-80°C, and a bottom temperature of 80-100°C. For example, the saponification reactor is controlled at a pressure of 45-55 kPa (A), a pressure of 55-70 kPa (A), a top temperature of 65-75°C, and a bottom temperature of 82-90°C.

[0014] According to an embodiment of the present invention, the outlet temperature of the first stage gas phase is ≤40°C, for example, 30~35°C.

[0015] According to an embodiment of the invention, the chloropropanol solution and lime slurry react in a saponification reactor, for example, the flow ratio of the chloropropanol solution to the lime slurry is (4~10):1, preferably (5~7):1. The chloropropanol solution is the reaction liquid after the reaction product of propylene with chlorine and water undergoes a chlorohydrin reaction.

[0016] According to some embodiments of the present invention, the flow rate of the chloropropanol solution is 200~300 m³ / h, for example 230~280 m³ / h, exemplarily 220 m³ / h, 240 m³ / h, 250 m³ / h, 260 m³ / h, 270 m³ / h or 280 m³ / h.

[0017] According to some embodiments of the present invention, the concentration of the chloropropanol solution is 2-6%, for example 3%, 4%, 5% or 6%.

[0018] According to some embodiments of the present invention, the concentration of the lime slurry is 13-20%, for example 13%, 14%, 15%, 16%, 17%, 18%, 19% or 20%.

[0019] According to a preferred embodiment of the present invention, the method for reducing the COD content of the saponification wastewater from the chloropropanol process for producing propylene oxide includes: a chloropropanol solution and lime slurry react in a saponification reactor; the gas phase collected from the saponification reactor is condensed in a first stage to obtain a first-stage condensate and a first-stage gas phase; the first-stage condensate is returned to the saponification reactor; the first-stage gas phase is condensed in a second stage to obtain a second-stage gas phase and a second-stage condensate; a portion of the second-stage condensate a1 is returned to the saponification reactor; another portion of the second-stage condensate a2 is collected; and the outlet temperature of the second-stage gas phase is controlled at 20~35℃ (preferably 25~30℃) and sent for further condensation. a1 / a2 = 15~25, preferably 18~23, more preferably 19.

[0020] According to an embodiment of the present invention, the COD content of the saponification wastewater in the method is <1400mg / L, preferably 1200~1350mg / L.

[0021] The present invention also provides a saponification reaction apparatus, comprising: a saponification reactor and an n-stage condenser; The material inlet of the first-stage condenser is connected to the gas phase outlet of the saponification reactor; The condensate outlet of the nth stage condenser is provided with two branches. The first branch is connected to the saponification reactor, and the second branch collects the condensate. When n>1, the gas phase outlet of the first-stage condenser is connected to the material inlet of the nth-stage condenser, and the condensate outlet of the first-stage condenser is connected to the saponification reactor.

[0022] According to an embodiment of the present invention, the saponification reaction apparatus further includes a saponification condenser connected to the gas phase outlet of the nth stage condenser.

[0023] According to an embodiment of the present invention, the saponification reaction apparatus further includes a crude propylene oxide cooling separator connected to a second branch of the nth stage condenser.

[0024] According to an embodiment of the present invention, the saponification reaction apparatus further includes a flash tank connected to the bottom of the saponification reactor.

[0025] According to a preferred embodiment of the present invention, the saponification reaction apparatus includes: a saponification reactor, a first-stage condenser, a second-stage condenser, a crude propylene oxide cooling separator, and a saponification condenser; The material inlet of the first-stage condenser is connected to the gas phase outlet of the saponification reactor, the gas phase outlet of the first-stage condenser is connected to the material inlet of the second-stage condenser, and the condensate outlet of the first-stage condenser is connected to the saponification reactor. The condensate outlet of the second-stage condenser is provided with two branches. The first branch is connected to the saponification reactor, and the second branch is connected to the crude propylene oxide cooling separator. The gas phase outlet of the second-stage condenser is connected to the saponification condenser.

[0026] The present invention also provides the application of the above-mentioned saponification reaction device in reducing the COD content of saponification wastewater from the chlorohydrin process for producing propylene oxide.

[0027] Beneficial effects This invention significantly reduces the liquid load in the saponification reactor by adding a partial condensate extraction process after the primary saponification condensation, thereby mitigating the hydrolysis problem caused by excessive residence time of propylene oxide in the saponification tower. The COD content of the saponification wastewater can be reduced by approximately 200 mg / L. It achieves selective separation of PO, reduces hydrolysis reactions, and improves product yield. It also lowers environmental treatment costs and wastewater treatment expenses. Attached Figure Description

[0028] Figure 1 A schematic diagram of a method for reducing COD content in saponification wastewater; Attached reference numerals: 1-Saponification reactor, 2-First-stage condenser, 3-Second-stage condenser, 4-Saponification fractionator, 5-Crude propylene oxide cooling separator, 6-Pump, 7-Flash tank; a1 - Second-stage condensate returned to the saponification reactor, a2 - Second-stage condensate entering the crude propylene oxide cooling separator. Detailed Implementation

[0029] The technical solution of the present invention will be further described in detail below with reference to specific embodiments. It should be understood that the following embodiments are merely illustrative and explanatory of the present invention, and should not be construed as limiting the scope of protection of the present invention. All technologies implemented based on the above content of the present invention are covered within the scope of protection intended by the present invention.

[0030] Unless otherwise stated, the raw materials and reagents used in the following examples are commercially available products or can be prepared by known methods.

[0031] Examples 1-13 A 4% chloropropanol solution (250 m³ / h) and a 16% lime slurry (40 m³ / h) react in saponification reactor 1. The saponification reactor is controlled with a top pressure of 50 kPa (A), a bottom pressure of 63 kPa (A), a top temperature of 70 °C, and a bottom temperature of 86 °C. The gas phase after the reaction enters the first-stage condenser 2 located at the top of the tower, with an outlet gas phase temperature of 33-33.5 °C. All the condensed liquid phase is returned to the tower, and the non-condensable gas enters the second-stage condenser 3. The parameters of the gas phase outlet temperature of the second-stage condenser, the portion of the second-stage condensate a1 returned to the saponification reaction, and the collected second-stage condensate a2 are shown in Table 1.

[0032] Table 1

[0033] As shown in Table 1, controlling the gas phase outlet temperature of the second-stage condenser and the a1 / a2 ratio can reduce the COD value of the saponification residue while meeting the requirement that the crude PO purity is ≥70%.

[0034] The embodiments of the present invention have been described above. However, the present invention is not limited to the above embodiments. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for reducing the COD content of wastewater from the saponification of propylene oxide produced by the chlorohydrin process, characterized in that, The method includes: after the gas phase collected from the saponification reactor is condensed in n stages, part of the nth stage condensate is returned to the saponification reactor, and another part of the nth stage condensate is collected; n≥1 and is an integer, for example, n=1~3.

2. The method according to claim 1, characterized in that, The flow rate of the nth stage reflux condensate is denoted as a1, and the flow rate of the extracted condensate is denoted as a2, where a1 / a2 = 15~25.

3. The method according to claim 1, characterized in that, When n=2, which is a two-stage condensation, the gas phase collected from the saponification reactor is condensed in the first stage to obtain the first-stage condensate and the first-stage gas phase. The first-stage condensate is returned to the saponification reactor, and the first-stage gas phase is condensed in the second stage to obtain the second-stage gas phase and the second-stage condensate. Part of the second-stage condensate a1 is returned to the saponification reactor, and the other part of the second-stage condensate a2 is collected. The second-stage gas phase is sent for fractional condensation.

4. The method according to claim 1, characterized in that, The outlet temperature of the nth stage gas phase (e.g., the second stage gas phase) is 20~35°C; furthermore, the nth stage gas phase (e.g., the second stage gas phase) is sent to a condenser.

5. The method according to claim 1, characterized in that, The condensate from the nth stage (e.g., the second-stage condensate a2) enters the crude propylene oxide cooling separator.

6. The method according to claim 1, characterized in that, The saponification reactor is controlled at a pressure of 40~70 kPa (A), a pressure of 50~70 kPa (A), a top temperature of 60~80℃, and a bottom temperature of 80~100℃.

7. The method according to claim 1, characterized in that, The outlet temperature of the first stage gas phase is ≤40℃, for example, 30~35℃.

8. The method according to claim 1, characterized in that, The method for reducing the COD content of propylene oxide saponification wastewater produced by the chlorohydrin process includes: reacting chloropropanol solution and lime milk in a saponification reactor; the gas phase collected from the saponification reactor is condensed in a first stage to obtain a first-stage condensate and a first-stage gas phase; the first-stage condensate is returned to the saponification reactor; the first-stage gas phase is condensed in a second stage to obtain a second-stage gas phase and a second-stage condensate; a portion of the second-stage condensate a1 is returned to the saponification reactor, and another portion of the second-stage condensate a2 is collected; the outlet temperature of the second-stage gas phase is controlled at 20~35℃ (preferably 25~30℃) and sent for fractional condensation. a1 / a2 = 15~25.

9. The saponification reaction apparatus used in the method of any one of claims 1-8, characterized in that, include: Saponification reactor and n-stage condenser; The material inlet of the first-stage condenser is connected to the gas phase outlet of the saponification reactor; The condensate outlet of the nth stage condenser is provided with two branches. The first branch is connected to the saponification reactor, and the second branch collects the condensate. When n>1, the gas phase outlet of the first-stage condenser is connected to the material inlet of the nth-stage condenser, and the condensate outlet of the first-stage condenser is connected to the saponification reactor.

10. The saponification reaction apparatus according to claim 9, characterized in that, The saponification reaction apparatus also includes a saponification condenser, which is connected to the gas phase outlet of the nth stage condenser; And / or, the saponification reaction apparatus further includes a crude propylene oxide cooling separator connected to a second branch of the nth stage condenser.

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