A method for removing impurities in a propylene oxide separation process

Abstract

The application discloses a method for removing impurities in an epoxypropane separation process. The raw material is from an ethylbenzene co-oxidation method for producing epoxypropane, wherein a titanium-silicon heterogeneous catalyst is used in an epoxidation part, and a tower bottom liquid after separating propylene in the epoxidation reaction is obtained. The material is introduced into an epoxypropane separation tower, crude epoxypropane is separated at the top, a silicon-rich impurity stream is taken out through a side line and introduced into an alkali washing tower, and an oil phase stream with a hexamethyldisilazane content of less than 0.1 ppm is obtained. The oil phase stream is introduced into a desiliconization tower again, a waste liquid rich in silicon impurities is discharged through a side line, and a tower bottom stream is returned to the epoxypropane separation tower, so that epoxypropane separation tower bottom liquid with a total silicon content of less than or equal to 0.1 ppm is obtained. The method can effectively remove silicon-containing impurities in the epoxidation reaction liquid.

Description

Technical Field

[0001] This invention belongs to the field of chemical processes, specifically relating to a method for removing impurities during the separation process in the production of propylene oxide. Technical Background

[0002] Propylene oxide is a colorless, transparent, low-boiling-point flammable liquid. It is the third type of propylene derivative, after acrylonitrile and polypropylene, and plays an important role in organic chemical production. Currently, the main methods for producing propylene oxide in China include the chlorohydrin process, indirect oxidation methods (PO / MTBE, PO / SM, POCHP process), and direct oxidation methods (HPPO process). With increasingly stringent requirements for energy conservation, environmental protection, and low-carbon production in my country, the development space of traditional chlorohydrin process enterprises, which suffer from severe pollution, is facing increasing limitations. While the HPPO process offers advantages such as mild reaction conditions and no pollution, the cost of catalysts and hydrogen peroxide is relatively high. The indirect oxidation method balances environmental protection and economic efficiency, making it more suitable for large-scale industrial production.

[0003] In the process of producing propylene oxide by ethylbenzene peroxidation, when the epoxidation catalyst is a titanium-silicon heterogeneous catalyst, and the tail gas treatment equipment is a catalytic combustion furnace or a regenerative catalytic oxidation furnace, the catalyst life is often short and cannot match the overhaul cycle of the main unit. This is not conducive to the long-term stable operation of the unit and affects economic benefits.

[0004] When the epoxidation catalyst is selected from titanium-silicon heterogeneous catalysts, silicon-containing impurities are present in the exhaust gas. When these impurities enter the catalytic combustion furnace or regenerative catalytic oxidation furnace, they form silica on the catalyst surface, covering the active sites and leading to reduced catalyst lifespan and excessive exhaust emissions. When using catalytic combustion furnaces or regenerative catalytic oxidation furnaces to treat waste gas, manufacturers generally require a minimum Si concentration of 0.05 mg / Nm³ in the gas phase. 3 Furthermore, some manufacturers strictly require that the exhaust gas must not contain silicon elements to avoid catalyst deactivation.

[0005] In the ethylene oxide co-oxidation process for producing propylene oxide, there are currently no industrial-scale facilities or literature reports on how to remove silicon-containing impurities to prevent them from entering the tail gas. Based on our company's actual operation, silicon-containing impurities enter the main process flow through the epoxidation catalyst. During the ethylene oxide reaction liquid separation of ethylene oxide, they enter the peroxidation unit with the recycled EB and ultimately end up in the tail gas. Therefore, it is necessary to develop a new impurity removal process during the propylene oxide separation process to remove silicon-containing impurities during the propylene oxide-ethylene oxide separation stage, thus solving the problem of silicon-containing impurities entering the tail gas through recycled ethylene oxide, leading to a short lifespan of the tail gas treatment system and excessive environmental emissions. Summary of the Invention

[0006] The purpose of this invention is to provide a method for removing impurities during the separation process of propylene oxide. This method can remove silicon-containing impurities during the separation stage of propylene oxide and ethylbenzene, preventing silicon-containing impurities from entering the exhaust gas and solving the problems of short lifespan of exhaust gas treatment systems and excessive environmental emissions.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0008] A method for removing impurities during the separation of propylene oxide, wherein the method employs an alkaline washing tower to wash the side stream ① of the propylene oxide separation tower, wherein the upper part of the alkaline washing tower receives a demineralized water stream ②, the middle part of the alkaline washing tower receives a sodium hydroxide aqueous solution stream ③, the bottom of the alkaline washing tower yields a wastewater stream ④, and the top of the alkaline washing tower yields an oil phase stream ⑤ which enters a desilication tower; the side stream of the desilication tower yields a stream ⑥ enriched with silicon-containing impurities, the bottom of the tower yields a stream ⑦, and the top of the tower yields a stream ⑧, which are returned to the propylene oxide separation tower, and the bottom of the propylene oxide separation tower yields a stream ⑨.

[0009] The propylene oxide content in the bottom stream ⑨ of the propylene oxide separation tower is ≤1000ppm, preferably ≤100ppm. The content can be obtained by controlling the temperature of the bottom stream. Preferably, the temperature of the bottom stream is 170-175℃.

[0010] In this invention, the receiving streams of the alkaline washing tower, arranged from high to low at the inlet, are: demineralized water stream ②, sodium hydroxide stream ③, and side-collected stream ①; the collected streams include: oil phase stream ⑤ after alkaline washing, located at the top of the alkaline washing tower (above the demineralized water stream ②); and wastewater stream ④, located at the bottom of the alkaline washing tower (below the side-collected stream ①).

[0011] In this invention, the side-stream stream ① contains propylene oxide, ethylbenzene, and silicon impurities. After entering the alkaline washing tower, it comes into contact with the alkaline aqueous phase inside the tower. The alkaline substance is sodium hydroxide. The high-boiling-point silicon impurities in stream ①, typically hexamethyldisilazane (125°C), react in the alkaline washing tower to generate low-boiling-point hexamethyldisilazane (101°C) and trimethylsilanol (100°C). After the oil and water phases are separated in the alkaline washing tower, stream ④ is discharged as wastewater to the unit's wastewater treatment system; the oil phase stream ⑤ enters the desilication tower, where silicon-containing waste liquid is enriched and discharged on the side stream. The top stream ⑧ of the desilication tower merges with the top liquid of the propylene oxide separation tower and is sent downstream, while the bottom stream ⑦ returns to the propylene oxide separation tower.

[0012] In this invention, when the reaction in the alkali washing tower is insufficient, high-boiling-point silicon impurities will enter stream ⑦ in the desilication tower and return to the propylene oxide separation tower, then enter the downstream circulating ethylbenzene and be carried into the tail gas of the unit. The operating temperature of the alkali washing tower is not particularly limited, but is preferably 10-60℃, and the operating pressure is 0.1-1 MPa.

[0013] In addition, if the temperature control at the side sampling point of the desilication tower is not reasonable, silicon-containing impurities cannot be effectively enriched and removed in the side stream, and the above-mentioned problems will also occur. The optimal temperature control range is 110-130℃, the tower bottom temperature is 155-160℃, and the pressure is 0.01-0.1MPa.

[0014] To address this, the present invention achieves maximum removal efficiency of silicon-containing impurities through adjustment methods such as pH control of the alkaline washing tower, oil-water mixing ratio control, and control of the composition of the side stream output from the desiliconization tower.

[0015] In this method, the epoxidation process for preparing propylene oxide using ethylbenzene co-oxidation employs a titanium-silicon heterogeneous catalyst, with the epoxidation reaction liquid after propylene separation serving as the raw material. This raw material passes through an ethylene oxide separation tower, where silicon-containing impurities are initially enriched in the side stream. It then enters an alkaline washing tower to convert high-boiling-point silicon impurities into low-boiling-point silicon impurities, which are subsequently enriched and removed in the side stream of a desilication tower.

[0016] In this invention, the propylene oxide separation tower receives the epoxidation reaction liquid after propylene separation, and the epoxidation reaction liquid comes from the ethylene-benzene co-oxidation method for preparing propylene oxide.

[0017] In this invention, the side-stream extraction stream ① contains silicon-based impurities, including one or more of hexamethyldisilazane, hexamethyldisiloxane, propoxytrimethylsilane, allyloxytrimethylsilane, and trimethylsilanol; preferably, the silicon-based impurity content in the side-stream extraction stream ① is 10ppm to 5000ppm, more preferably 100ppm to 2000ppm.

[0018] In this invention, the mass ratio of the demineralized water stream ② to the side-collected water stream ① is 1:2 to 1:5, such as 1:2.5, 1:3, 1:3.5, 1:4, 1:4.5, preferably 1:3 to 1:4;

[0019] In this invention, the desalinated water stream ② is obtained by ultrafiltration + reverse osmosis + mixed bed treatment.

[0020] In this invention, the flow rate of the sodium hydroxide aqueous solution stream ③ is controlled by the pH value of the aqueous phase in the alkaline washing tower; preferably, the pH value of the wastewater in stream ④ is 5-11, such as 6, 7, 8, 9, 10; preferably 6-9. By controlling the pH of the wastewater in stream ④ to 5-11, the content of hexamethyldisilazane in stream ⑤ is made <0.1ppm.

[0021] In this invention, the mass ratio of propylene oxide to ethylbenzene in stream ⑥ is 1:50 to 50:1, such as 1:40, 1:30, 1:20, 1:10, 10:1, 20:1, 30:1, or 40:1; preferably 1:10 to 10:1. By controlling the mass ratio of propylene oxide to ethylbenzene in stream ⑥ to 1:50 to 50:1, the silicon content in both streams ⑦ and ⑧ is <1 ppm.

[0022] Another object of the present invention is to provide propylene oxide prepared by the ethylbenzene co-oxidation method.

[0023] A propylene oxide prepared by ethylbenzene co-oxidation method, wherein the propylene oxide is purified by the aforementioned method for removing impurities.

[0024] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0025] (1) Through the process flow in this invention, the separation of silicon impurities in the epoxidation reaction liquid purification process is achieved, avoiding silicon impurities from entering the tail gas treatment system and improving the stability of the device operation.

[0026] (2) The process flow of the present invention can reduce the total amount of material to be processed and reduce the loss of ethylbenzene emissions. Attached Figure Description

[0027] Figure 1 This is a flowchart of propylene oxide separation and impurity removal. Detailed Implementation

[0028] The technical solution of the present invention will be further illustrated by the embodiments, but the scope of protection of the present invention is not limited to the scope described in the embodiments.

[0029] Raw material information: All streams originate from the ethylene-benzene co-oxidation process for producing propylene oxide at Wanhua Chemical Co., Ltd.'s Yantai Industrial Park. The ethylene-benzene in the materials involved has a purity ≥99.88%, and is a product of the ethylene-benzene unit in Wanhua's PO / SM plant. The sodium hydroxide has a purity of 32%, and is prepared from sodium hydroxide produced by Wanhua Chlor-Alkali.

[0030] Equipment information: The propylene oxide separation tower is a plate tower, the alkali washing tower is a rotary disc extraction tower, and the desilication tower is a packed tower.

[0031] Device Analysis Methods

[0032] Gas chromatography conditions: Agilent HP-5 column was used for determination. The vaporization chamber temperature was 300℃ and the detector temperature was 300℃. The temperature program was as follows: 50℃ for 2 min; 5℃ / min to 100℃ for 1 min; 10℃ / min to 300℃ for 10 min.

[0033] Gas chromatography-mass spectrometry (GC-MS) conditions: Agilent VF-WAX ms column, vaporization chamber temperature 240℃, detector temperature 250℃; temperature program: 40℃ for 4 min; 5℃ / min to 100℃ for 1 min; 10℃ / min to 240℃ for 10 min.

[0034] Example 1

[0035] The flow rate of the crude propylene oxide separation tower side stream ① is 1 t / h, with a total silicon impurity content of 1000 ppm. The injection rate of the demineralized water stream ② is 0.286 t / h (stream ②:stream ① = 1:3.5). The pH of the post-extraction aqueous phase is adjusted to 5 by controlling the injection rate of the sodium hydroxide stream ③. The alkaline washing tower operates at a temperature of 40℃ and a pressure of 0.35 MPaG. The desilication tower operates at a pressure of 0.08 MPaG, and the side stream sampling point temperature is 120℃. In stream ⑥, the propylene oxide:ethylbenzene ratio is 1:1, and the propylene oxide content in the bottom stream ⑨ of the propylene oxide separation tower is 100 ppm.

[0036] Under the above parameters, the content of hexamethyldisilazane in stream ⑤ was 0.05 ppm, the total silicon impurities in stream ⑦ were 0.4 ppm, the total silicon impurities in stream ⑧ were 0.6 ppm, and the total silicon impurities in stream ⑨ were 0.02 ppm.

[0037] Example 2

[0038] The flow rate of the crude propylene oxide separation tower side stream ① is 1 t / h, with a total silicon impurity content of 5000 ppm. The injection rate of demineralized water stream ② is 0.5 t / h (stream ②:stream ① = 1:2). The pH of the post-extraction aqueous phase is adjusted to 11 by controlling the injection rate of sodium hydroxide stream ③. The alkaline washing tower operates at a temperature of 40℃ and a pressure of 0.35 MPaG. The desilication tower operates at a pressure of 0.08 MPaG, and the side stream sampling point temperature is 130℃. In stream ⑥, the propylene oxide:ethylbenzene ratio is 1:50, and the propylene oxide content in the bottom stream ⑨ of the propylene oxide separation tower is 10 ppm.

[0039] Under the above parameters, the content of hexamethyldisilazane in stream ⑤ was 0.08 ppm, the total silicon impurities in stream ⑦ were 0.8 ppm, the total silicon impurities in stream ⑧ were 0.8 ppm, which met the process target, and the total silicon impurity content in stream ⑨ was 0.01 ppm.

[0040] Example 3

[0041] The flow rate of the crude propylene oxide separation tower side stream ① is 1 t / h, with a total silicon impurity content of 10 ppm. The injection rate of demineralized water stream ② is 0.2 t / h (stream ②:stream ① = 1:5). The injection rate of sodium hydroxide stream ③ is controlled to adjust the pH of the post-extraction aqueous phase to 7. The alkaline washing tower operates at a temperature of 40℃ and a pressure of 0.35 MPaG. The desilication tower operates at a pressure of 0.08 MPaG, and the side stream sampling point temperature is 110℃. Stream ⑥ contains propylene oxide:ethylbenzene in a ratio of 50:1, and the propylene oxide content in the bottom stream ⑨ of the propylene oxide separation tower is 1000 ppm.

[0042] Under the above parameters, the content of hexamethyldisilazane in stream ⑤ was 0.02 ppm, the total silicon impurities in stream ⑦ were 0.1 ppm, the total silicon impurities in stream ⑧ were 0.1 ppm, and the total silicon impurities in stream ⑨ were 0.08 ppm.

[0043] Example 4

[0044] The flow rate of the crude propylene oxide separation tower side stream ① is 1 t / h, with a total silicon impurity content of 100 ppm. The injection rate of demineralized water stream ② is 0.33 t / h (stream ②:stream ① = 1:3). The pH of the post-extraction aqueous phase is adjusted to 8.5 by controlling the injection rate of sodium hydroxide stream ③. The alkaline washing tower operates at a temperature of 40℃ and a pressure of 0.35 MPaG. The desilication tower operates at a pressure of 0.08 MPaG, and the side stream sampling point temperature is 115℃. In stream ⑥, the propylene oxide:ethylbenzene ratio is 10:1, and the propylene oxide content in the bottom stream ⑨ of the propylene oxide separation tower is 500 ppm.

[0045] Under the above parameters, the content of hexamethyldisilazane in stream ⑤ was 0.03 ppm, the total silicon impurity content in stream ⑦ was 0.2 ppm, the total silicon impurity content in stream ⑧ was 0.1 ppm, and the total silicon impurity content in stream ⑨ was 0.05 ppm.

[0046] Example 5

[0047] The side stream effluent ① from the crude propylene oxide separation tower has a flow rate of 1 t / h, with a total silicon impurity content of 2000 ppm. The demineralized water stream ② is injected at a rate of 0.25 t / h (stream ②:stream ① = 1:4). The pH of the post-extraction aqueous phase is adjusted to 10 by controlling the injection rate of sodium hydroxide stream ③. The alkaline washing tower operates at a temperature of 40℃ and a pressure of 0.35 MPaG. The desilication tower operates at a pressure of 0.08 MPaG, and the side stream effluent temperature is 125℃. Stream ⑥ contains propylene oxide:ethylbenzene at a ratio of 1:10, and the propylene oxide content in the bottom stream ⑨ of the propylene oxide separation tower is 200 ppm.

[0048] Under the above parameters, the content of hexamethyldisilazane in stream ⑤ was 0.06 ppm, the total silicon impurities in stream ⑦ were 0.4 ppm, the total silicon impurities in stream ⑧ were 0.6 ppm, and the total silicon impurities in stream ⑨ were 0.03 ppm.

[0049] Example 6

[0050] The flow rate of the crude propylene oxide separation tower side stream ① is 1 t / h, with a total silicon impurity content of 5000 ppm. The injection rate of demineralized water stream ② is 0.5 t / h (stream ②:stream ① = 1:2). The pH of the post-extraction aqueous phase is adjusted to 11 by controlling the injection rate of sodium hydroxide stream ③. The alkaline washing tower operates at a temperature of 40℃ and a pressure of 0.35 MPaG. The desilication tower operates at a pressure of 0.08 MPaG, and the side stream sampling point temperature is 130℃. In stream ⑥, the propylene oxide:ethylbenzene ratio is 1:50, and the propylene oxide content in the bottom stream ⑨ of the propylene oxide separation tower is 2000 ppm.

[0051] Under the above parameters, the content of hexamethyldisilazane in stream ⑤ was 0.08 ppm, the total silicon impurities in stream ⑦ were 0.8 ppm, the total silicon impurities in stream ⑧ were 0.8 ppm, which met the process target, and the total silicon impurity content in stream ⑨ was 0.3 ppm.

[0052] Example 7

[0053] The flow rate of the crude propylene oxide separation tower side stream ① is 1 t / h, with a total silicon impurity content of 2000 ppm. The injection rate of demineralized water stream ② is 0.125 t / h (stream ②:stream ① = 1:8). The pH of the post-extraction aqueous phase is adjusted to 10 by controlling the injection rate of sodium hydroxide stream ③. The alkaline washing tower operates at a temperature of 40℃ and a pressure of 0.35 MPaG. The desilication tower operates at a pressure of 0.08 MPaG, and the side stream sampling point temperature is 125℃. Stream ⑥ contains propylene oxide:ethylbenzene in a ratio of 1:10, and the propylene oxide content in the bottom stream ⑨ of the propylene oxide separation tower is 200 ppm.

[0054] Under the above parameters, the content of hexamethyldisilazane in stream ⑤ was 0.5 ppm, the total silicon impurities in stream ⑦ were 1.1 ppm, the total silicon impurities in stream ⑧ were 0.3 ppm, and the total silicon impurities in stream ⑨ were 0.15 ppm.

[0055] Example 8

[0056] The side stream effluent ① from the crude propylene oxide separation tower has a flow rate of 1 t / h, with a total silicon impurity content of 5 ppm. The demineralized water stream ② is injected at a rate of 0.286 t / h (stream ②:stream ① = 1:3.5). The pH of the post-extraction aqueous phase is adjusted to 5 by controlling the injection rate of sodium hydroxide stream ③. The alkaline washing tower operates at a temperature of 40℃ and a pressure of 0.35 MPaG. The desilication tower operates at a pressure of 0.08 MPaG, and the side stream effluent temperature is 120℃. Stream ⑥ contains propylene oxide:ethylbenzene in a 1:1 ratio, and the propylene oxide content in the bottom stream ⑨ of the propylene oxide separation tower is 100 ppm.

[0057] Under the above parameters, the content of hexamethyldisilazane in stream ⑤ was 0.03 ppm, the total silicon impurities in stream ⑦ were 0.3 ppm, the total silicon impurities in stream ⑧ were 0.4 ppm, and the total silicon impurities in stream ⑨ were 0.17 ppm.

[0058] Example 9

[0059] The side stream effluent ① from the crude propylene oxide separation tower has a flow rate of 1 t / h, with a total silicon impurity content of 5000 ppm. The demineralized water stream ② is injected at a rate of 1 t / h (stream ②:stream ① = 1:1). The pH of the post-extraction aqueous phase is adjusted to 11 by controlling the injection rate of sodium hydroxide stream ③. The alkaline washing tower operates at a temperature of 40℃ and a pressure of 0.35 MPaG. The desilication tower operates at a pressure of 0.08 MPaG, and the side stream effluent temperature is 130℃. Stream ⑥ contains propylene oxide:ethylbenzene at a ratio of 1:50, and the propylene oxide content in the bottom stream ⑨ of the propylene oxide separation tower is 10 ppm.

[0060] Under the above parameters, the content of hexamethyldisilazane in stream ⑤ was 0.5 ppm, the total silicon impurities in stream ⑦ were 1.8 ppm, the total silicon impurities in stream ⑧ were 0.3 ppm, which met the process target, and the total silicon impurities in stream ⑨ were 0.14 ppm.

[0061] Example 10

[0062] The flow rate of the crude propylene oxide separation tower side stream ① is 1 t / h, with a total silicon impurity content of 10 ppm. The injection rate of demineralized water stream ② is 0.2 t / h (stream ②:stream ① = 1:5). The injection rate of sodium hydroxide stream ③ is controlled to adjust the pH of the post-extraction aqueous phase to 7. The alkaline washing tower operates at a temperature of 40℃ and a pressure of 0.35 MPaG. The desilication tower operates at a pressure of 0.08 MPaG, and the side stream sampling point temperature is 85℃. Stream ⑥ contains propylene oxide:ethylbenzene in a ratio of 80:1, and the propylene oxide content in the bottom stream ⑨ of the propylene oxide separation tower is 1000 ppm.

[0063] Under the above parameters, the content of hexamethyldisilazane in stream ⑤ was 0.03 ppm, the total silicon impurities in stream ⑦ were 3.8 ppm, the total silicon impurities in stream ⑧ were 0.05 ppm, and the total silicon impurities in stream ⑨ were 0.26 ppm.

Claims

1. A method for removing impurities in the separation process of propylene oxide, the method using an alkali washing tower to alkali wash a side draw stream ① of a propylene oxide separation tower, wherein a desalted water stream ② is received at the upper part of the alkali washing tower, an alkali water solution stream ③ is received at the middle part of the alkali washing tower, a waste water stream ④ is obtained at the bottom of the alkali washing tower, and an oil phase stream ⑤ is obtained at the top of the alkali washing tower and enters a desiliconization tower; a stream ⑥ rich in silicon-containing impurities is obtained at the side line of the desiliconization tower, a stream ⑦ is obtained at the bottom of the desiliconization tower and is returned to the propylene oxide separation tower, a stream ⑧ is obtained at the top of the desiliconization tower, and a stream ⑨ is obtained at the bottom of the propylene oxide separation tower, the content of silicon-containing impurities in the side draw stream ① being 10 ppm to 5000 ppm.

2. The method of claim 1, wherein, The silicon-containing impurities in the side draw stream ① include one or more of hexamethyldisilazane, hexamethyldisiloxane, propoxytrimethylsilane, allyloxytrimethylsilane, and trimethylsilanol, and the content of the silicon-containing impurities in the side draw stream ① is 100 ppm to 2000 ppm.

3. The method of claim 1, wherein, The mass ratio of the desalted water stream ② to the side draw stream ① is 1:2 to 1:

5.

4. The method of claim 1, wherein, The mass ratio of the desalted water stream ② to the side draw stream ① is 1:3 to 1:

4.

5. The method according to any one of claims 1 to 4, characterized in that, The content of propylene oxide in the stream ⑨ obtained at the bottom of the propylene oxide separation tower is ≤1000 ppm.

6. The method according to any one of claims 1 to 4, wherein The content of propylene oxide in the stream ⑨ obtained at the bottom of the propylene oxide separation tower is ≤100 ppm.

7. The method according to any one of claims 1 to 4, wherein According to the mass ratio, the content of propylene oxide to ethylbenzene in the stream ⑥ is 1:50 to 50:

1.

8. The method of any one of claims 1-4, wherein, According to the mass ratio, the content of propylene oxide to ethylbenzene in the stream ⑥ is 1:10 to 10:

1.

9. The method of any one of claims 1-4, wherein, The pH of the waste water in the stream ④ is 5 to 11, and the content of hexamethyldisilazane in the stream ⑤ is <0.1 ppm.

10. The method of any one of claims 1-4, wherein, The streams received by the alkali washing tower, from high to low at the inlet, are the desalted water stream ②, the alkali water solution stream ③, and the side draw stream ①. The obtained stream includes the oil phase stream ⑤ after alkali washing, which is located at the top of the alkali washing tower. The waste water stream ④ is located at the bottom of the alkali washing tower.

11. The method of any one of claims 1-4, wherein, The side draw stream ① contains propylene oxide, ethylbenzene, and silicon-containing impurities; the waste water stream ④ is discharged to a waste water treatment system of the device; and the oil phase stream ⑤ enters the desiliconization tower and is discharged at the side line of the desiliconization tower. The stream ⑧ at the top of the desiliconization tower is combined with the stream at the top of the propylene oxide separation tower and is sent to the downstream, and the stream ⑦ at the bottom of the desiliconization tower is returned to the propylene oxide separation tower.

12. The method of any one of claims 1-4, wherein, The propylene oxide separation tower receives a reaction liquid after the separation of propylene, the reaction liquid is obtained from a process for preparing propylene oxide by ethylbenzene co-oxidation, and the reaction uses a titanium-silicon heterogeneous catalyst.

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