A method for recycling propylene oxide tail gas using molten salt solvent

Abstract

The present invention provides a method for recycling propylene oxide tail gas by using a molten salt solvent. The method includes: (1) bringing the tail gas mixture containing propylene oxide to be absorbed into contact with the molten salt solvent in an absorption tower for absorption to obtain the absorbed tail gas, the molten salt solvent and a propylene oxide mixture; (2) separating the molten salt solvent and the propylene oxide mixture to obtain propylene oxide and the molten salt solvent. The present invention uses a molten salt solvent, and in the separation process, only a gas-liquid separation tank can be used to recycle the solvent. Not only is the process simple and the separation efficiency high, but also the loss of the absorbent and the process energy consumption are reduced, and the yield of propylene oxide is increased.

Description

Technical Field

[0001] The present invention relates to a method for recovering propylene oxide tail gas by using a molten salt solvent. Background Art

[0002] Propylene oxide is an important basic organic chemical raw material and is also the second largest propylene derivative after polypropylene. Propylene oxide has active chemical properties and is easy to ring-opening polymerize. It can be used to synthesize polyether polyols, propylene glycol, etc., and can also be used in surfactants, oilfield demulsifiers, pesticide emulsifiers, etc. At the same time, it is also used in industries such as construction, medicine, and food. Currently, the production of propylene oxide mainly includes the chlorohydrin method, the cumene hydroperoxide method, and the hydrogen peroxide oxidation method, etc. In different production processes, the tail gas in the product intermediate tank or storage tank contains a small amount of propylene oxide and nitrogen. However, due to the low boiling point of propylene oxide and the low total process operating pressure, it is difficult to recover propylene oxide. The literature "Process for Absorbing and Purifying Propylene Oxide Tail Gas with Methanol" reports a method and process for absorbing propylene oxide tail gas with methanol, using the methanol solvent in the reaction section of the device itself to absorb propylene oxide in the tail gas and sending the absorption liquid to the separation section for product recovery.

[0003] 201811275691.0 discloses a method for recycling propylene oxide, using methanol in the reaction section of the propylene oxide device as an absorbent to absorb and recover propylene alkane in the tail gas of the recovery device, and sending the absorption liquid to the separation section for product recovery.

[0004] 201910088390.5 discloses a method and device for treating tail gas during the loading and unloading of propylene oxide and dichloropropane. After being pressurized by a Roots blower, the tail gas is sent to a low-temperature cold box for liquefaction to collect VOCs gases such as propylene oxide in the tail gas.

[0005] 201610008220.8 discloses a tail gas treatment device and process for preparing propylene oxide by epoxidizing propylene with hydrogen peroxide. The collected tail gas containing propylene oxide is sent to a deoxidation reactor for reaction to achieve the purpose of tail gas treatment.

[0006] In the above disclosed technical methods, either organic solvents are used for absorption treatment, or the tail gas is sent to a reactor for reaction or cooled in a cold box, etc. All of them have a long process flow or difficult separation and recovery. Summary of the Invention

[0007] The purpose of the present invention is to provide a method for recovering propylene oxide tail gas with a molten salt solvent. By using the molten salt solvent and process of the present invention, propylene oxide in the tail gas can be efficiently recovered under operating conditions.

[0008] In view of the deficiencies of the prior art, the present invention proposes to use a molten salt solvent composed of a combination of anions and cations. This molten salt solvent exists in a liquid form under normal temperature and pressure, has good thermal stability and chemical stability, is not volatile, not flammable, and non-toxic, and is a green solvent. The present invention utilizes the characteristics of the molten salt solvent being non-volatile and having strong absorption capacity to simplify the propylene oxide recovery process and improve the tail gas recovery efficiency.

[0009] Using the molten salt of the present invention, the separation of propylene oxide and the solvent and the recycling of the solvent can be achieved through a two-tower process. By using the method of the present invention, the molten salt solvent feed with different mass ratios can be selected according to the content of propylene oxide in the tail gas.

[0010] To achieve the foregoing objectives, the present invention provides a method for recovering propylene oxide tail gas using a molten salt solvent, which method includes:

[0011] (1) Contacting and absorbing the tail gas mixture containing propylene oxide to be absorbed with the molten salt solvent in an absorption tower to obtain the absorbed tail gas, the molten salt solvent, and a propylene oxide mixture;

[0012] (2) Separating the molten salt solvent and the propylene oxide mixture to obtain propylene oxide and the molten salt solvent.

[0013] Preferably, the method includes:

[0014] (1) Feeding the molten salt solvent into the upper middle part of the absorption tower, and feeding the tail gas mixture containing propylene oxide to be absorbed into the lower middle part of the absorption tower, and carrying out countercurrent contact absorption in the absorption tower; obtaining the absorbed tail gas, the molten salt solvent, and a propylene oxide mixture;

[0015] (2) Feeding the molten salt solvent and the propylene oxide mixture into a gas-liquid separation tank for separation to obtain propylene oxide and the molten salt solvent.

[0016] Preferably, the operating conditions of the gas-liquid separation tank include: a temperature of 30 to 55 °C, and / or a pressure of 0.005 to 0.05 MPaG.

[0017] Preferably, the operating conditions of the absorption tower include: an operating pressure of 0.1 to 0.5 MPaG, and / or a total number of theoretical plates of 10 to 30. The higher the pressure or the lower the temperature, the more beneficial it is to the absorption of propylene oxide tail gas.

[0018] Preferably, in step (1), the feeding temperature of the molten salt solvent is 5 to 30 °C lower than the operating temperature of the absorption tower.

[0019] Preferably, in step (1), the feeding temperature of the molten salt solvent is 20 to 45 °C.

[0020] Preferably, in step (1), the feeding temperature of the propylene oxide tail gas is 25 to 50 °C.

[0021] Preferably, the molar ratio of the tail gas mixture containing propylene oxide to be absorbed to the molten salt solvent is 1-10.

[0022] Preferably, the molten salt solvent is composed of 3-hydroxymethylimidazolium cations and thiocyanate anions.

[0023] Preferably,

[0024] The molecular formula of the 3-hydroxymethylimidazolium cation is The molecular formula of the thiocyanate anion is , and preferably R is one of a normal alkyl group having 2-6 carbon atoms or an isoalkyl group having 2-6 carbon atoms.

[0025] Preferably, the absorbed tail gas is led out from the top of the absorption tower.

[0026] Preferably, the molten salt solvent and the propylene oxide mixture are drawn out from the bottom of the absorption tower.

[0027] Preferably, the comprehensive mass loss rate of the molten salt solvent is less than 0.01%.

[0028] Preferably, the tail gas mixture containing propylene oxide contains: propylene oxide, aldehyde and nitrogen, and preferably contains 5-70% by volume of propylene oxide, 1-25% by volume of aldehyde and 5-94% by volume of nitrogen.

[0029] The present invention has the following advantages compared with the prior art:

[0030] (1) The molten salt solvent used in the present invention, such as the molten salt solvent composed of 3-hydroxymethylimidazolium cations and thiocyanate anions, is safe, environmentally friendly, non-toxic, reusable, and has a small dosage and good effect;

[0031] (2) The method of the present invention can reduce the heat load and improve the economy of the device by only a two-tower process and by designing different operating conditions and structural parameters;

[0032] (3) The combination of the molten salt solvent and the two-tower process in the present invention not only has a simple process, high absorption and separation efficiency, but also has a small solvent dosage, less loss, saves production costs, and is environmentally friendly;

[0033] (4) The present invention uses a molten salt solvent, and the separation process can recycle the solvent only by using a gas-liquid separation tank. It not only has a simple process and high separation efficiency, but also reduces the loss of the absorbent and the process energy consumption, and improves the yield of propylene oxide.

[0034] (5) In the preferred embodiment of the present invention, the solvent recovery tower is changed to a gas-liquid separation tank, which greatly reduces the energy consumption of the process flow. Description of the Drawings

[0035] Figure 1 It is a process flow diagram according to a preferred embodiment of the present invention. Specific Embodiment

[0036] In the ranges disclosed herein, the endpoints and any values are not limited to the exact ranges or values, and these ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values and individual point values of each range, and between individual point values, they can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein.

[0037] The present invention provides a method for recovering propylene oxide tail gas using a molten salt solvent, and the method includes: (1) contacting and absorbing the tail gas mixture containing propylene oxide to be absorbed with the molten salt solvent in an absorption tower to obtain the absorbed tail gas, molten salt solvent, and propylene oxide mixture;

[0038] (2) Separating the molten salt solvent and the propylene oxide mixture to obtain propylene oxide and the molten salt solvent.

[0039] In the present invention, the separation method in step (2) can adopt common gas-liquid separation methods. For the present invention, it is preferred that the gas-liquid separation in step (2) adopts flash separation.

[0040] According to a preferred embodiment of the present invention, preferably, the method includes:

[0041] (1) Feeding the molten salt solvent into the upper-middle part of the absorption tower, and feeding the tail gas mixture containing propylene oxide to be absorbed into the lower-middle part of the absorption tower, and carrying out countercurrent contact absorption in the absorption tower; obtaining the absorbed tail gas, molten salt solvent, and propylene oxide mixture;

[0042] (2) Feeding the molten salt solvent and the propylene oxide mixture into a gas-liquid separation tank for separation to obtain propylene oxide and the molten salt solvent. By using the molten salt solvent of the present invention, due to the strong interaction between propylene oxide and the molten salt solvent, strong absorption is achieved, and high-purity propylene oxide is obtained through the gas-liquid separation tank, while realizing the recycling of the molten salt solvent.

[0043] According to the present invention, preferably, the operating conditions of the gas-liquid separation tank include: the temperature is 30 - 55 °C.

[0044] According to the present invention, preferably, the operating conditions of the gas-liquid separation tank include: the pressure is 0.005 - 0.050 MPaG.

[0045] According to the present invention, preferably, the operating conditions of the absorption tower include: the operating pressure is 0.1 - 0.5 MPaG.

[0046] According to the present invention, preferably, the operating conditions of the absorption tower include: the total number of theoretical plates is 10 to 30.

[0047] According to a preferred embodiment of the present invention, preferably, in step (1), the feed temperature of the molten salt solvent is 5 to 30 °C lower than the operating temperature of the absorption tower.

[0048] According to a preferred embodiment of the present invention, preferably, in step (1), the feed temperature of the molten salt solvent is 20 to 45 °C.

[0049] According to a preferred embodiment of the present invention, preferably, in step (1), the feed temperature of the propylene oxide tail gas is 25 to 50 °C.

[0050] The method of the present invention can reduce the heat load and improve the economy of the device through only a two-tower process by designing different operating conditions and structural parameters.

[0051] According to the present invention, preferably, the molar ratio of the tail gas mixture containing propylene oxide to be absorbed to the molten salt solvent is 1-10.

[0052] Through simulation design of long-term experiments, the present invention obtains the optimal molten salt structure for the influence of different molten salts on the relative volatility of the propylene oxide mixture. Preferably, the molten salt solvent is composed of 3-hydroxymethylimidazolium cations and thiocyanate anions.

[0053] According to the present invention, preferably,

[0054] The molecular formula of the 3-hydroxymethylimidazolium cation is The molecular formula of the thiocyanate anion is , and preferably R is one of a normal alkyl group of C2-C6 or an isoalkyl group of C2-C6.

[0055] According to a preferred embodiment of the present invention, preferably, the absorbed tail gas is led out from the top of the absorption tower.

[0056] According to a preferred embodiment of the present invention, preferably, the molten salt solvent and the propylene oxide mixture are drawn out from the bottom of the absorption tower.

[0057] According to a preferred embodiment of the present invention, preferably, the comprehensive mass loss rate of the molten salt solvent is less than 0.01%.

[0058] In the present invention, various components of the tail gas mixture containing propylene oxide are applicable to the present invention. According to a preferred embodiment of the present invention, preferably, the tail gas mixture containing propylene oxide contains: propylene oxide, aldehyde and nitrogen, preferably containing 5-70% by volume of propylene oxide, 1-25% by volume of aldehyde and 5-94% by volume of nitrogen.

[0059] Figure 1 It is a process flow chart according to a preferred embodiment of the present invention. The tail gas from the propylene oxide plant is introduced into the bottom of Tower 1, and countercurrently contacts with the molten salt absorbent entering from the top of the tower to achieve the absorption operation; the absorbed tail gas is discharged from the top of Tower 1 to the next treatment section or directly emptied; the bottom stream of Tower 1 is a mixture of molten salt solvent and propylene oxide, which is pumped to the gas-liquid separation tank of Tower 2; after flashing, propylene oxide product is obtained from the top of Tower 2, and the molten salt solvent at the bottom of Tower 2 is recycled to Tower 1. Figure 1 The most basic process is given, without involving valves, pumps, heaters, condensers, etc., which are well-known to those skilled in the art.

[0060] According to a preferred embodiment of the present invention, the double-tower process includes the absorption tower of Tower 1 and the simple-stage gas-liquid separation tank of Tower 2.

[0061] According to a preferred embodiment of the present invention, the number of theoretical plates of Tower 1 is 10 to 30.

[0062] According to a preferred embodiment of the present invention, Tower 2 is a simple-stage gas-liquid separation tank.

[0063] According to a preferred embodiment of the present invention, the recycling of the molten salt solvent means introducing the molten salt solvent stream at the bottom of Tower 2 to the feed position at the top of Tower 1.

[0064] According to a preferred embodiment of the present invention, it is carried out according to the Figure 1 shown process, specifically including:

[0065] (1) Introduce the molten salt solvent into the middle upper part of the absorption tower Tower 1, for example, from the top, and at the same time introduce the propylene oxide-containing tail gas mixture to be absorbed into the middle lower part of Tower 1, for example, from the bottom. Preferably, the operating pressure of Tower 1 is 0.1 to 0.5 MPaG, and the total number of theoretical plates of Tower 1 is 10 to 30. The absorbed tail gas is led out from the top of Tower 1; preferably, the feed temperature of the molten salt solvent is 5 to 10 °C lower than the operating temperature of Tower 1;

[0066] (2) Pump out the mixture of molten salt solvent and propylene oxide in Tower 1 from the bottom and send it to Tower 2 (gas-liquid separation tank). Preferably, the operating pressure of the gas-liquid separation tank is 0.005 to 0.050 MPaG. High-purity propylene oxide product is obtained from the top of the gas-liquid separation tank of Tower 2, and the molten salt solvent is led out from the bottom of the gas-liquid separation tank of Tower 2 and returned to Tower 1 for recycling.

[0067] According to a preferred embodiment of the present invention, in step (1), the feed temperature of the molten salt solvent is 20 to 45 °C; the feed temperature of the propylene oxide tail gas is 25 to 50 °C.

[0068] According to a preferred embodiment of the present invention, the operating temperature of the gas-liquid separation tank is

[0069] 30 to 55 °C, and the operating pressure is 0.005 to 0.050 MPaG.

[0070] According to a preferred embodiment of the present invention, by using the method of the present invention, the comprehensive mass loss rate of the molten salt solvent (the mass ratio of the solvent make-up flow rate to the solvent circulation amount) is less than 0.01%.

[0071] According to Figure 1 the process shown, the present invention provides a method for recycling propylene oxide tail gas using a molten salt solvent, and the specific operations include:

[0072] a) Introducing the propylene oxide-containing tail gas into Tower 1 from the bottom of Tower 1;

[0073] Preferably, the temperature of the propylene oxide tail gas entering Tower 1 is 25 to 50 °C. Preferably, the operating pressure range of Tower 1 is 0.1 to 0.5 MPaG. Within this pressure range, it can be operated, and the product can meet the indicators. It can adapt to different operating conditions and has low energy consumption;

[0074] b) At the same time, mixing the molten salt solvent returned from Tower 2 with the fresh molten salt solvent and introducing it into Tower 1 from the top of Tower 1;

[0075] Preferably, when the molten salt solvent is used to absorb propylene oxide, the operating temperature range is 10 to 80 °C, and the operating pressure is 0.01 to 0.3 MPaG;

[0076] The temperature of the molten salt solvent entering Tower 1 is 20 to 45 °C, and the feed temperature is 5 to 15 °C lower than the operating temperature at the top of Tower 1;

[0077] c) After absorption, the absorbed tail gas is led out from the top of Tower 1;

[0078] d) The molten salt solvent and propylene oxide mixture are drawn out from the bottom of Tower 1 and sent to Tower 2;

[0079] e) High-purity propylene oxide product is obtained from the top of Tower 2;

[0080] f) The molten salt solvent drawn out from the bottom of Tower 2 is returned to Tower 1;

[0081] g) The operating pressure of Tower 2 is selected so that the propylene oxide vapor at the top of the tower can be cooled by circulating cooling water.

[0082] According to the method of the present invention, the mass concentration of propylene oxide in the absorbed tail gas led out from the top of Tower 1 is not more than 0.1%, and the mass concentration of the high-purity propylene oxide product obtained from the top of Tower 2 is greater than 99.99%.

[0083] According to the method of the present invention, the operating temperature of Tower 1 is preferably 30-65°C, the operating pressure is 0.1-0.5 MPaG, the operating temperature of Tower 2 is 30-55°C, and the operating pressure is 0.005-0.050 MPaG.

[0084] The comprehensive mass loss rate of the molten salt solvent is less than 0.01%.

[0085] The pressures described in the present invention are all absolute pressures.

[0086] Example 1

[0087] Using 1-butyl-3-hydroxymethylimidazole thiocyanate as the absorption solvent, the absorption and separation operation of the propylene oxide-containing tail gas is carried out according to the Figure 1 process as follows:

[0088] a) A tail gas mixture with a propylene oxide molar content of 35%, an aldehyde molar content of 20%, and a nitrogen molar content of 45% is introduced into the bottom of Tower 1 at a temperature of 30°C and a pressure of 0.20 MPaG.

[0089] b) At the same time, the molten salt solvent returned from Tower 2 is mixed with the fresh solvent and then pumped to the top of Tower 1. The temperature of the circulating molten salt solvent entering Tower 1 is 25°C, the pressure is 0.20 MPaG, and the ratio of the total amount of the circulating molten salt solvent and the fresh solvent to the total amount of the tail gas is 1:1 (molar ratio).

[0090] c) After absorption, the absorbed tail gas is led out from the top of Tower 1.

[0091] d) The molten salt solvent and propylene oxide mixture are drawn out from the bottom of Tower 1 and sent to Tower 2.

[0092] e) High-purity propylene oxide product is obtained from the top of Tower 2.

[0093] f) The molten salt solvent drawn out from the bottom of Tower 2 is returned to Tower 1.

[0094] The above operating conditions, raw material composition, solvent ratio, circulating solvent concentration, propylene oxide content in the absorbed tail gas, and propylene oxide product indicators are listed in Table 1.

[0095] Example 2

[0096] This example is basically the same as Example 1, except that:

[0097] The propylene oxide molar content in the tail gas feed is 50%, the aldehyde molar content is 10%, and the nitrogen is 40%. Since the propylene oxide content in the raw material to be treated increases, while the molar ratio of the molten salt solvent to the raw material remains 1:1. The operating conditions and various product indicators of this example are listed in Table 1.

[0098] Example 3

[0099] This embodiment is basically the same as Embodiment 1, except that:

[0100] The solvent ratio in the absorption process is 1.5:1. The operating conditions and various product indexes of this embodiment are listed in Table 1.

[0101] Embodiment 4

[0102] This embodiment is basically the same as Embodiment 1, except that:

[0103] 1-(2,2-dimethylbutane)yl-3-hydroxymethylimidazole thiocyanate is used as the absorbent. The operating conditions and various product indexes of this embodiment are listed in Table 1.

[0104] Comparative Example 1

[0105] This comparative example is basically the same as Embodiment 1, except that:

[0106] 1-propyl-3-methylimidazole methyl sulfate is used as the absorbent. The operating conditions and various product indexes of this embodiment are listed in Table 1.

[0107] Table 1 Comparison of Embodiment Results

[0108] Item Example 1 Example 2 Example 3 Example 4 Comparative Example 1 Theoretical number of plates of Tower 1 15 15 15 15 15 Temperature of Tower 1 50 52 45 55 60 Operating pressure of Tower 1, MPaG 0.42 0.30 0.50 0.16 0.40 Operating pressure of Tower 2, MPaG 0.02 0.03 0.01 0.05 0.02 Temperature of Tower 2 40 40 40 40 40 Solvent ratio, mol / mol 1.35:1 1.20:1 1.5:1 1.05:1 1.25:1 Propylene oxide content in the feed tail gas, mol% 35％ 50％ 35％ 35％ 35％ Propylene oxide content in the tail gas after absorption, mol% 0.03％ 0.06％ 0.01％ 0.08％ 5.50％ Propylene oxide product concentration, mol% 99.90％ 99.92％ 99.9％ 99.90％ 99.5％ Molten salt content in the circulating solvent, mol% 99.99％ 99.95％ 99.98％ 99.99％ 99.89％ Propylene oxide recovery rate, mol% 99.91％ 99.88％ 99.97％ 99.77％ 84.29％ Comprehensive mass loss rate of the molten salt solvent 0.01％ 0.01％ 0.01％ 0.01％ 0.01％

[0109] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited thereto. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solutions of the present invention, including any other suitable combination of each technical feature. These simple modifications and combinations should also be regarded as the content disclosed by the present invention and fall within the protection scope of the present invention.

Claims

1. A method for recovering propylene oxide tail gas using a molten salt solvent, characterized in that, The method includes: (1) Contacting and absorbing the tail gas mixture containing propylene oxide to be absorbed with a molten salt solvent in an absorption tower to obtain the absorbed tail gas, the molten salt solvent, and a propylene oxide mixture; (2) Separating the molten salt solvent and the propylene oxide mixture to obtain propylene oxide and the molten salt solvent; The molten salt solvent consists of 3 - hydroxymethylimidazole - type cations and thiocyanate anions; The molar ratio of the tail gas mixture containing propylene oxide to be absorbed to the molten salt solvent is 1 - 10; The tail gas mixture containing propylene oxide contains: propylene oxide, aldehyde, and nitrogen.

2. The method according to claim 1, wherein The method includes: (1) Feeding the molten salt solvent into the upper middle part of the absorption tower, and feeding the tail gas mixture containing propylene oxide to be absorbed into the lower middle part of the absorption tower, and carrying out counter - current contact absorption in the absorption tower; obtaining the absorbed tail gas, the molten salt solvent, and a propylene oxide mixture; (2) Feeding the molten salt solvent and the propylene oxide mixture into a gas - liquid separation tank for separation to obtain propylene oxide and the molten salt solvent.

3. The method according to claim 2, wherein The operating conditions of the gas - liquid separation tank include: temperature of 30 - 55 °C, and / or pressure of 0.005 - 0.05 MPaG.

4. The method according to claim 1 or 2, wherein The operating conditions of the absorption tower include: operating pressure of 0.1 - 0.5 MPaG, and / or total number of theoretical plates of 10 - 30; temperature of 40 - 60 °C.

5. The method according to claim 1 or 2, wherein In step (1), the feeding temperature of the molten salt solvent is 5 - 30 °C lower than the operating temperature of the absorption tower.

6. The method according to claim 5, wherein In step (1), the feeding temperature of the molten salt solvent is 20 - 45 °C; the feeding temperature of the propylene oxide tail gas is 25 - 50 °C.

7. According to the method described in claim 1, wherein, the molar ratio of the tail gas mixture containing propylene oxide to be absorbed to the molten salt solvent is 1.05 - 1.6; and / or the tail gas mixture containing propylene oxide contains 5 - 70 vol% of propylene oxide, 1 - 25 vol% of aldehyde, and 5 - 94 vol% of nitrogen.

8. According to the method described in claim 1, wherein, The molecular formula of the 3-hydroxymethylimidazole-based cation is The molecular formula of the thiocyanate anion is 9. The method according to claim 8, wherein R is one of a normal alkyl group of C2 - C6 or an isoalkyl group of C2 - C6.

10. According to the method described in claim 1 or 2, wherein, the absorbed tail gas is led out from the top of the absorption tower; and / or the molten salt solvent and the propylene oxide mixture are drawn out from the bottom of the absorption tower; and / or the comprehensive mass loss rate of the molten salt solvent is less than 0.01%; the method includes: returning the molten salt solvent separated in step (2) to step (1) for recycling.

11. The method according to claim 1 or 2, wherein, The method includes: a) Introducing the propylene oxide - containing tail gas from the bottom of tower 1 into tower 1; b) Mixing the molten salt solvent returned from tower 2 with fresh molten salt solvent and feeding it into tower 1 through the top; c) After absorption, leading out the absorbed tail gas from the top of tower 1; d) Drawing out the molten salt solvent and the propylene oxide mixture from the bottom of tower 1 and feeding it into tower 2; e) Obtaining a high - purity propylene oxide product from the top of tower 2; f) Leading out the molten salt solvent from the bottom of tower 2 and returning it to tower 1; The mass concentration of propylene oxide in the absorbed tail gas led out from the top of tower 1 is not more than 0.1%, and the mass concentration of the propylene oxide product obtained from the top of tower 2 is greater than 99.99%.

12. The method according to claim 11, wherein The method includes: In step a), the temperature of the propylene oxide tail gas entering tower 1 is 25 - 50 °C; In step b), the feeding temperature of the molten salt solvent is 20 to 45 °C, and the feeding temperature of the molten salt solvent is 5 to 15 °C lower than the operating temperature of Tower 1; The operating temperature of Tower 1 is 30 to 65 °C; the operating pressure is 0.1 to 0.5 MPaG, the operating temperature of Tower 2 is 30 to 55 °C, and the operating pressure is 0.005 to 0.050 MPaG; The comprehensive mass loss rate of the molten salt solvent is less than 0.01%.

13. The method according to claim 12, wherein, This method includes: The operating temperature of Tower 1 is 40 - 60 °C.

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