A treatment process for wastewater from propylene oxide production

Abstract

The present invention relates to a treatment process for wastewater from propylene oxide production. S1: Dehydrating wastewater containing propylene glycol monomethyl ether through a dehydration tower, distilling the azeotropic composition of the propylene glycol monomethyl ether / water system from the top of the tower, and distilling pure water from the bottom of the tower; S2: The azeotropic composition of the propylene glycol monomethyl ether / water system is extracted and purified through an extraction tower, so that part of the water is distilled from the top of the tower, and a material containing water, an extractant and propylene glycol monomethyl ether is obtained in the bottom of the tower; S3: After the material in the bottom of the extraction tower is dehydrated through a water-permeable membrane, the extractant and propylene glycol monomethyl ether are separated by a purification tower to obtain purified propylene glycol monomethyl ether. This method can realize the resource utilization of wastewater containing propylene glycol monomethyl ether generated in the production of propylene oxide, and this treatment process can recover a propylene glycol monomethyl ether product with a purity greater than 99.5 wt.%.

Description

Technical Field

[0001] The invention belongs to the technical field of water treatment, and mainly relates to a treatment process for propylene oxide production wastewater. Background Art

[0002] Propylene oxide (PO), also known as epoxypropylene, is an important organic chemical raw material, widely used in the production of polyether polyols, propylene glycol, unsaturated polyester resins, surfactants and other chemical products. In the production process of propylene oxide, wastewater containing propylene glycol monomethyl ether (MP) is generated, and the treatment of this wastewater is a challenge in the field of industrial environmental protection.

[0003] In the production process of propylene oxide, especially when the chlorohydrin process is used, wastewater containing propylene glycol monomethyl ether will be produced. This wastewater usually contains a high concentration of propylene glycol monomethyl ether, which is easy to form a stable homogeneous azeotrope with water, with an azeotropic point of 97.19 ° C and an azeotropic composition of 54.87 wt.% PM and 45.13 wt.% water. Therefore, the traditional distillation method cannot effectively purify propylene glycol monomethyl ether, and special distillation methods such as azeotropy, extraction, and pressure swing are required. However, propylene glycol monomethyl ether is sensitive to high temperature, and the use of azeotropic and pressure swing distillation can easily cause propylene glycol monomethyl ether to decompose under heat, resulting in problems such as decreased recovery rate. Although the extractive distillation process can be well used in the purification process of propylene glycol monomethyl ether, the extractant used in the extraction process has certain pollution to the environment, which does not conform to the current background of green chemical development.

[0004] Therefore, there is an urgent need for a new green and environmentally friendly process for the treatment of wastewater containing propylene glycol monomethyl ether. Summary of the invention

[0005] The object of the present invention is to provide a method for treating wastewater containing propylene glycol monomethyl ether to solve the problems of high dosage of extractant, high investment cost and poor separation effect in the prior art.

[0006] The technical solution of the invention is as follows:

[0007] A process for treating wastewater from propylene oxide production comprises the following steps:

[0008] S1: Dehydrating the wastewater containing propylene glycol monomethyl ether through a dehydration tower, distilling out the azeotropic composition of the propylene glycol monomethyl ether / water system at the top of the tower, and distilling out pure water at the bottom of the tower;

[0009] S2: The azeotropic composition of the propylene glycol monomethyl ether / water system is extracted and purified by an extraction tower, so that part of the water is distilled out through the top of the tower, and a material containing water, an extractant and propylene glycol monomethyl ether is obtained in the bottom of the tower;

[0010] S3: After the materials in the extraction tower kettle are dehydrated through a water-permeable membrane, the extractant and propylene glycol monomethyl ether are separated by a purification tower to obtain purified propylene glycol monomethyl ether.

[0011] The aqueous propylene glycol monomethyl ether raw material contains 20-80% water and 20-40% propylene glycol monomethyl ether.

[0012] The total number of plates in the dehydration tower is 25-45, the feed position is on the 5th to 15th plates, and the reflux ratio of the dehydration tower is 1.2-2.0.

[0013] The extractant is dichloromethane or chloroform.

[0014] In the step S2, the water content of the material in the tower kettle is not less than 5wt%, and the flow ratio of the amount of extractant added to the extraction tower to the feed is in the range of 2.0-3.0.

[0015] The reflux ratio of the purification tower is 0.3-0.8, the total number of tower plates is 10-30, and the feed height is 4-8 tower plates.

[0016] The purity of propylene glycol monomethyl ether on the membrane retentate side was calculated by the following formula:

[0017]

[0018] y PM is the mass fraction of propylene glycol monomethyl ether on the retentate side of the membrane, x H2O is the water content of the raw material entering the membrane, F is the flow rate of raw materials into the membrane, S is the flow rate of the extractant.

[0019] A device for treating wastewater from propylene oxide production, comprising:

[0020] A dehydration tower, used for preliminary dehydration of wastewater containing propylene glycol monomethyl ether;

[0021] An extraction tower is used to extract and separate propylene glycol monomethyl ether from the top material of the dehydration tower, and obtain a mixture of extractant, propylene glycol monomethyl ether and water at the bottom of the tower;

[0022] A membrane unit for dehydrating the material obtained at the bottom of the extraction tower;

[0023] The purification tower is used to separate the dehydrated material obtained from the membrane unit, remove the extractant, and obtain purified propylene glycol monomethyl ether.

[0024] The extractant inlet of the extraction tower is located at the top of the tower, and the raw material inlet is located at the bottom of the tower.

[0025] The beneficial effects of the present invention are as follows: (1) The present invention couples the extraction distillation with the membrane separation technology, so that a certain synergistic effect is generated between the two. The amount of the extractant used is greatly reduced, which further reduces the amount of the extractant used and the recovery cost, greatly improves the economic benefits, and at the same time reduces the environmental pollution caused by the extractant, which is more friendly to the environment and conforms to the background of the era of green chemical development.

[0026] (2) The operating temperature in the process of the present invention is low, and the propylene glycol monomethyl ether will not be decomposed due to excessively high temperature, resulting in problems such as low recovery rate, and the recovery rate of propylene glycol monomethyl ether can be effectively increased.

[0027] (3) The present invention adopts membrane separation technology to remove residual water after extraction. It has high separation efficiency, simple operation process, and little environmental pollution, which can effectively reduce the cost of process and environmental pollution treatment.

[0028] (4) The present invention operates under normal pressure conditions, which is helpful for equipment maintenance and reduces equipment investment and operating costs.

[0029] (5) The present invention constructs a method for predicting the purity of propylene glycol monomethyl ether on the retentate side of the membrane, which is constructed based on the relationship between the flow rate of the extractant and the membrane permeability. This method has the advantage of high prediction accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 : Conventional extractive distillation is used to treat wastewater containing propylene glycol monomethyl ether and purify propylene glycol monomethyl ether.

[0031] Figure 2 : Extraction-membrane separation coupling process is used to treat wastewater containing propylene glycol monomethyl ether and purify propylene glycol monomethyl ether.

[0032] Figure 3 : Relationship curve between extractant flow rate and permeate water volume.

[0033] Explanation of the reference numerals: ET1-extraction tower; T1-dehydration tower; T2-purification tower; T3-extraction agent recovery tower; P1-first DC pump; P2-second DC pump; P3-third DC pump; P4-fourth DC pump; P5-fifth DC pump; V1-first waste liquid collecting tank; V2-propylene glycol monomethyl ether collecting tank; V3-third waste liquid collecting tank; L1-liquid separator; H1-heat exchanger; M1-membrane unit (including pervaporation device). DETAILED DESCRIPTION

[0034] In order to make the present invention easier to understand, the present invention will be described in detail below in conjunction with embodiments and drawings. These embodiments are only for illustration and are not intended to limit the application scope of the present invention.

[0035] like Figure 2As shown, a treatment device for propylene oxide production wastewater comprises:

[0036] A dehydration tower, wherein the feed inlet of the dehydration tower is connected to a first DC pump, and the top of the tower is connected to a second DC pump;

[0037] An extraction tower, wherein the kettle of the extraction tower is connected to a dehydration tower via a second DC pump, the top of the extraction tower is connected to a third DC pump, and the bottom of the kettle is connected to a heat exchanger;

[0038] A membrane unit, wherein the feed side of the membrane unit is connected to the extraction tower via a heat exchanger, and the interception side of the membrane unit is connected to the purification tower;

[0039] A purification tower, the top of which is connected to the extraction tower via a third DC pump;

[0040] A collecting unit, wherein the collecting unit comprises a first waste liquid collecting tank, a permeate collecting tank, and a propylene glycol monomethyl ether collecting tank, wherein the first waste liquid collecting tank is connected to a dehydration tower via a fourth DC pump, the permeate side of the membrane unit is connected to the permeate collecting tank, and the propylene glycol monomethyl ether collecting tank is connected to a purification tower via a fifth DC pump.

[0041] The membrane unit is provided with a pervaporation device, which contains a pervaporation membrane. The pervaporation membrane is a water-permeable molecular sieve membrane. One end of the membrane assembly is connected to a vacuum pump to form a pressure difference between the inside and outside of the membrane to promote the removal of water in the feed liquid. In order to reduce the impact of concentration polarization, nitrogen purge can be added below the membrane assembly.

[0042] The membrane unit may be provided with a plurality of pervaporation devices, the pervaporation devices are connected in series, and the permeation side water outlet is connected to the pervaporation devices connected in series.

[0043] A process for treating wastewater from propylene oxide production comprises the following steps:

[0044] S1: The water-containing propylene glycol monomethyl ether raw material enters a dehydration tower for dehydration, and the azeotropic composition of the propylene glycol monomethyl ether / water system is distilled out from the top of the tower, and pure water is distilled out from the bottom of the tower. The operating pressure of the dehydration tower is normal pressure, and the reflux ratio of the dehydration tower is 1.2-2.0;

[0045] S2: After the azeotropic composition of the propylene glycol monomethyl ether / water system is distilled from the top of the dehydration tower, it is transported to the extraction tower for extraction and purification. The first liquid phase after extraction is distilled from the bottom of the extraction tower, and the second liquid phase is distilled from the top of the tower;

[0046] The azeotropic composition of the propylene glycol monomethyl ether / water system is fed from the last plate of the extraction tower, the extractant is fed from the first plate, the extractant is chloroform or dichloromethane, the first liquid phase is a mixture of propylene glycol monomethyl ether, water and the extractant, and the second liquid phase is a mixture of water and the extractant.

[0047] S3: The distillate from the bottom of the extraction tower passes through a heat exchanger for heat exchange and then enters a membrane unit for dehydration. The operating temperature of the heat exchanger is preferably 95-110°C, the permeate side pressure of the pervaporation membrane is negative pressure, and the operating pressure range is preferably below 2 kPa;

[0048] S4: The aqueous phase flowing out from the permeate side of the membrane unit is collected by condensation and enters the permeate collection tank, and the effluent from the retentate side enters the purification tower for purification. The purified propylene glycol monomethyl ether is distilled from the bottom of the tower, and the extractant is distilled from the top of the tower and circulated to the extraction tower for further extraction. The operating pressure of the purification tower is normal pressure, wherein the effluent from the retentate side enters the 8th plate of the purification tower (total number of plates is 17), and the reflux ratio of the purification tower is 0.5.

[0049] Comparative Example 1

[0050] according to Figure 1 As shown, this comparative example is used to illustrate the process flow of conventional extraction distillation method for treating propylene oxide production wastewater and the energy consumption analysis in the process. Take 1 ton of feed per hour and the feed composition of 30 wt.% (PM) and 70 wt.% (H2O) as an example. The process flow is as follows Figure 1 As shown, the raw material is transported to the last plate of the extraction tower by the first DC pump, and the extractant (chloroform) is circulated and transported to the first plate of the extraction tower by the second DC pump (the mass flow ratio of the extractant to the feed is controlled at 1.5), and most of the water in the raw material is removed by the entrainment effect of the extractant. The top of the extraction tower is distilled as the second liquid phase, and the bottom of the tower is distilled as the first liquid phase, wherein the second liquid phase is mostly water phase (water content is about 88.55wt.%), and the first liquid phase is a mixture of propylene glycol monomethyl ether and the extractant. After that, the second liquid phase enters the 14th plate of the recovery tower (total number of plates: 17), and the reflux ratio of the recovery tower is 0.7, and the extractant and part of propylene glycol monomethyl ether are recovered. The first liquid phase enters the 12th plate of the purification tower (total number of plates: 25), and the top temperature is 60.1°C, the bottom temperature is 99.9°C, and the reflux ratio of the purification tower is 2, which separates propylene glycol monomethyl ether from the extractant, and obtains a propylene glycol monomethyl ether solution with a purity greater than 99.5 wt.% from the bottom of the tower. Finally, the distillate from the top of the recovery tower and the purification tower enters the separator for extractant recovery, and the recovered extractant is circulated to the extraction tower. The extraction tower requires 1.55 tons of chloroform for every ton of propylene glycol monomethyl ether raw material processed. Example 1

[0051] according to Figure 2 As shown, this embodiment is used to illustrate the process flow of the extraction-membrane separation coupling method for treating propylene oxide production wastewater and the energy consumption analysis in the process. Take 1 ton of feed per hour and the feed composition of 30 wt.% (PM) and 70 wt.% (H2O) as an example. The process flow is as follows Figure 2 As shown, the raw material is transported by the first DC pump to the 7th plate of the dehydration tower (total number of plates: 35), the reflux ratio of the dehydration tower is 1.6, the azeotropic composition of propylene glycol monomethyl ether and water is distilled from the top of the tower, and pure water is distilled from the bottom of the tower. The raw material purified by the dehydration tower is transported to the extraction tower by the second DC pump, and the extractant (chloroform) is transported by the third DC pump to the first plate of the extraction tower (the mass flow ratio of the extractant to the feed is controlled at 0.22), the water-containing second liquid phase is distilled from the top of the tower, and the first liquid phase is distilled from the bottom of the tower.

[0052] At this time, the first liquid phase still contains about 10 wt.% of water, and a membrane unit device is required for dehydration operation (NaA molecular sieve membrane is used in the membrane unit). The permeate side is water, which is collected by condensation, and the retentate side is a mixed solution of extractant and propylene glycol monomethyl ether. The retentate side is finally transported to the 8th plate of the purification tower (total number of plates is 17), with a top temperature of 49.2°C and a bottom temperature of 119.5°C. At normal pressure, the reflux ratio of the purification tower is 0.5, and a propylene glycol monomethyl ether solution with a purity greater than 99.5 wt.% is obtained from the purification tower kettle. The extractant distilled from the top of the tower is transported to the extraction tower through the third DC pump. The extraction tower requires 0.22 tons of chloroform for every ton of propylene glycol monomethyl ether raw material processed by the extraction tower.

[0053] Compared with the original conventional extraction and distillation process, it greatly saves the amount of extraction agent used and reduces the investment in extraction agent and equipment.

[0054] According to the extraction agent flow S (100 kg / h~250 kg / h, increment of 25 kg / h) and the corresponding membrane permeate volume The seven experimental values ​​of the extractant flow rate were fitted. S and membrane permeate The relationship is ; The relationship between the extractant flow rate S and the membrane permeation water volume is as follows Figure 3 shown.

[0055] According to the law of conservation of mass , the relationship between the purity of propylene glycol monomethyl ether on the membrane retentate side and the flow rate of raw materials into the membrane, water content, and the flow rate of the extractant is obtained:

[0056]

[0057] In the formula y PM is the mass fraction of propylene glycol monomethyl ether on the retentate side of the membrane, x H2O is the water content of the raw material entering the membrane, F is the flow rate of raw materials into the membrane, S is the extraction agent flow rate, is the membrane permeation water volume, and the relative errors of the PM purity calculated according to the relationship and the simulated value are 0.089%, 0.034%, 0.051%, 0.093%, 0.072%, 0.004% and 0.106%, respectively.

[0058] Furthermore, the other three groups of experimental data were used to verify the model, and the relative errors between the true value and the experimental value were 0.106%, 0.407%, and 0.765%, respectively.

Claims

1. A process for treating wastewater from propylene oxide production, characterized in that: The steps include: S1: Dehydrating the wastewater containing propylene glycol monomethyl ether through a dehydration tower, distilling out the azeotropic composition of the propylene glycol monomethyl ether / water system at the top of the tower, and distilling out pure water at the bottom of the tower; S2: The azeotropic composition of the propylene glycol monomethyl ether / water system is extracted and purified by an extraction tower, so that part of the water is distilled out through the top of the tower, and a material containing water, an extractant and propylene glycol monomethyl ether is obtained in the bottom of the tower; S3: After the materials in the extraction tower kettle are dehydrated through a water-permeable membrane, the extractant and propylene glycol monomethyl ether are separated by a purification tower to obtain purified propylene glycol monomethyl ether.

2. The process for treating propylene oxide production wastewater according to claim 1, characterized in that: The wastewater containing propylene glycol monomethyl ether contains 20-80% water and 20-40% propylene glycol monomethyl ether.

3. The process for treating propylene oxide production wastewater according to claim 1, characterized in that: The total number of plates in the dehydration tower is 25-45, the feed position is on the 5th to 15th plates, and the reflux ratio of the dehydration tower is 1.2-2.

0.

4. The process for treating propylene oxide production wastewater according to claim 1, characterized in that: The extractant is dichloromethane or chloroform.

5. The process for treating propylene oxide production wastewater according to claim 1, characterized in that: In the step S2, the water content of the material in the tower kettle is not less than 5wt%, and the mass flow rate ratio of the amount of extractant added to the extraction tower to the feed is in the range of 2.0-3.

0.

6. The process for treating propylene oxide production wastewater according to claim 1, characterized in that: The reflux ratio of the purification tower is 0.3-0.8, the total number of tower plates is 10-30, and the feed height is 4-8 tower plates; the tower top temperature is 45-55°C, and the tower bottom temperature is 115-125°C.

7. The process for treating propylene oxide production wastewater according to claim 1, characterized in that: The purity of propylene glycol monomethyl ether on the retentate side of the water permeable membrane was calculated by the following formula: ； y PM is the mass fraction of propylene glycol monomethyl ether on the retentate side of the membrane, x H2O is the water content of the raw material entering the membrane, F is the flow rate of raw materials into the membrane, S is the flow rate of the extractant.

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