A method and system for recovering CO from DMC synthesis tail gas by membrane separation

Abstract

The invention discloses a method and system for recovering CO in DMC synthesis tail gas through membrane separation. The method comprises the following steps: pre-treating the DMC synthesis tail gas to obtain membrane separation inlet gas; performing selective component separation on the membrane separation inlet gas through a plurality of membrane modules to obtain permeate gas and retentate gas; wherein: the CO2 content in the DMC synthesis tail gas is 3wt%-15wt%, the sum of the methanol and DMC concentrations in the membrane separation inlet gas is ≤50ppm, the pressure of the membrane separation inlet gas is 1.3-2.8MPaG, the water dew point of the membrane separation inlet gas working pressure is ≤20°C, the temperature of the membrane separation inlet gas is 25-40°C, CO is non-permeate gas, and CO2 and H2O steam are both permeate gas; only a small amount of methanol and DMC need to be removed at the raw gas inlet, the CO recovery rate is high, and the pressure drop of the effective CO gas before and after the membrane separation is ≤100kPa, and the CO gas can be recovered and utilized as the raw gas after being pressurized by a supercharger.

Description

Technical Field

[0001] The invention relates to the technical field of industrial tail gas treatment, and in particular to a method for recovering CO in DMC synthesis tail gas through membrane separation. Background Art

[0002] Dimethyl carbonate (DMC) is a chemical raw material with excellent environmental performance and wide application. It is also an important organic synthesis intermediate. Its molecular structure contains functional groups such as carbonyl, methyl and methoxy groups, and has multiple reaction properties. It has the advantages of safe use, convenience, low pollution and easy transportation in production. Due to its low toxicity, dimethyl carbonate is a "green" chemical product with development prospects. It can meet the requirements of current clean processes and conform to the strategic trend of sustainable development. At the same time, it has multiple excellent properties. Therefore, its synthesis process has received more and more attention.

[0003] Liquid phase oxidative carbonylation of methanol with methanol, CO and O 2 The production of DMC using 2-nitropropene as raw material has the advantages of low production cost, mild reaction conditions, favorable thermodynamics and high product selectivity, and the by-product is water, which has little impact on the environment. It is a very promising method for synthesizing DMC.

[0004] Reaction Mechanism of Synthesis of DMC by Liquid Phase Oxidation of Methanol

[0005] Under the action of the catalyst, methanol reacts with carbon monoxide and oxygen under liquid phase conditions to produce DMC product.

[0006] The main reactions of DMC synthesis process are as follows:

[0007] Main reaction: 2CH 4 O+1 / 2O 2 +CO→(CH 3 O) 2 CO+H 2 O

[0008] Main side reaction: 2CO+O 2 →2CO 2

[0009] In order to ensure the safety and controllability of the reaction process, the DMC synthesis process usually contains O 2 The content is controlled at about 10wt%, which results in a large excess of CO. After heat recovery, cooling and separation, most of the synthesis reaction gas is returned to the raw gas system. 2 During the enrichment in the system, part of the synthesis gas needs to be discharged. The CO content in the discharged synthesis gas is generally above 80wt%. The current industrial treatment method is mainly to send the tail gas to the combustion system for combustion, but no effective recovery method is adopted to recover CO, resulting in waste of effective gas CO and increased production costs. Summary of the invention

[0010] The present invention aims to convert the CO in the exhaust syngas of the existing methanol liquid phase oxidative carbonylation process into CO 2 , no effective recovery method is adopted to recover CO, resulting in the problem of waste of effective gas CO; the present invention provides a method for recovering CO in DMC synthesis tail gas by membrane separation.

[0011] The technical solution of the present invention is: a method for recovering CO in DMC synthesis tail gas by membrane separation, comprising the following steps: pre-treating the DMC synthesis tail gas to obtain membrane separation inlet gas;

[0012] The membrane separation inlet gas passes through multiple membrane modules for selective component separation to obtain permeate gas and retentate gas;

[0013] Among them: CO in DMC synthesis tail gas 2 The content is 3wt%-15wt%, the sum of the methanol and DMC concentrations in the membrane separation inlet gas is ≤50ppm, the pressure of the membrane separation inlet gas is 1.3-2.8MPaG, the water dew point of the membrane separation inlet gas working pressure is ≤20℃, the temperature of the membrane separation inlet gas is 25-40℃, CO is non-permeate gas, CO 2 and H 2 O steam is all permeate gas.

[0014] In the present invention, CO 2 It is a fast gas and is enriched on the permeate side during the membrane separation process, while the effective component CO is a slow gas and is enriched on the retentate side during the membrane separation process. The pressure loss of the effective gas CO is small, which is beneficial to the recycling and utilization of CO and saves investment and operating costs.

[0015] Preferably, the pretreatment of the DMC synthesis tail gas specifically comprises the following steps:

[0016] The methanol and DMC in the DMC synthesis tail gas are removed by water washing to obtain a crude gas;

[0017] After separating the water from the crude gas, it is cooled and subjected to gas-liquid separation to obtain low-temperature crude gas, which is then heated to obtain the membrane separation inlet gas. Gas-liquid separation mainly ensures that the working pressure water dew point of the membrane separation inlet gas is ≤20°C, and removes the water therein to prevent it from adhering to the separation membrane and affecting the separation effect.

[0018] Preferably, after separating the water from the crude gas, it is first heat exchanged with the low-temperature crude gas and then cooled to recover the heat of the crude gas and reduce the load of the cooling system. The crude gas after heat exchange (25-40°C) is used as the membrane separation inlet gas.

[0019] Preferably, a plurality of the membrane modules are connected in parallel in a single stage, and the CO recovery rate is greater than 80% and the purity is above 95wt%.

[0020] Preferably, a plurality of the membrane modules are connected in series in multiple stages, and the CO recovery rate is greater than 90%, and the purity can reach more than 97wt%.

[0021] Preferably, the membrane component is one of a hollow fiber membrane, a flat membrane, a rolled membrane and a tubular membrane.

[0022] Preferably, the membrane core of the membrane assembly is made of a polyimide membrane, which has good thermal conductivity for CO and CO 2 Separation selectivity ≥5, CO and CO 2 The separation effect is good.

[0023] The beneficial effects of the present invention include: no need to provide other chemical reagents such as absorbents and adsorbents, no secondary pollution, only a small amount of methanol and DMC need to be removed at the raw gas inlet, low energy consumption for device operation, high CO recovery rate, and a pressure drop of effective CO gas before and after membrane separation of ≤100 kPa, which can be recovered and utilized as raw gas after being pressurized by a supercharger; the membrane assembly in the present invention can adopt a skid-mounted design, has strong scalability, and is suitable for the treatment of different DMC synthesis tail gases.

[0024] The present invention also discloses a system for recovering CO in DMC synthesis tail gas by membrane separation based on the above method, comprising a water washing tower, a separator, an air-to-air heat exchanger and a cold dryer which are connected in sequence, wherein the air outlet of the cold dryer is connected to the low-temperature air inlet of the air-to-air heat exchanger, and the high-temperature air outlet of the air-to-air heat exchanger is connected to a plurality of membrane components.

[0025] Preferably, a plurality of the membrane modules are connected in parallel in a single stage.

[0026] Preferably, a plurality of the membrane modules are connected in series in multiple stages.

[0027] The beneficial effects of the present invention are simple structure, low investment and low energy consumption in device operation. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 is a flow chart of the method for recovering CO from DMC synthesis tail gas by membrane separation in Examples 1, 4, and 6;

[0029] Figure 2 is a flow chart of the method for recovering CO from DMC synthesis tail gas by membrane separation in Examples 2, 3, 5 and 7;

[0030] Among them, 1-water washing tower; 2-separator; 3-gas-to-gas heat exchanger; 4-cold dryer; 5-membrane assembly; 6-circulating gas compressor. DETAILED DESCRIPTION

[0031] Example 1

[0032] The composition and intake conditions of the DMC synthesis tail gas in this embodiment are shown in Table 1.

[0033] Table 1

[0034]

[0035]

[0036] This embodiment also discloses a method for recovering CO in DMC synthesis tail gas by membrane separation. In this embodiment, the membrane assembly adopts a single-stage multi-membrane group parallel connection mode, including the following steps:

[0037] S1, washing the DMC synthesis tail gas in Table 1 with water through a water washing tower to remove methanol and DMC therein to obtain a crude gas;

[0038] S2, after the coarse gas is separated from the water by the separator, it enters the gas-to-gas heat exchanger to exchange gas-to-gas heat with the low-temperature coarse gas at the outlet of the cold dryer, thereby reducing the temperature of the coarse gas entering the cold dryer;

[0039] S3, the crude gas after gas-to-gas heat exchange is cooled to 10°C by a cold dryer and then separated into gas and liquid to obtain low-temperature crude gas;

[0040] S4, the low-temperature crude gas enters the gas-to-gas heat exchanger for reheating to 30°C and a pressure of 1.78MPaG to obtain the membrane separation inlet gas;

[0041] S5, membrane separation: the inlet gas enters multiple membrane modules connected in parallel in a single stage for component separation to obtain the retentate gas rich in CO and the 2 The permeate gas is sent out of the boundary area as the product gas and tail gas of membrane separation.

[0042] The membrane assembly of this embodiment is equipped with 24 4" membrane cores in total.

[0043] The product CO gas prepared by the above method was analyzed for composition by gas chromatography and measured online by a flow meter, wherein the purity of CO was 98wt%, and the effective recovery rate of CO in the product gas was 82%.

[0044] Example 2

[0045] The composition and intake conditions of the DMC synthesis tail gas in this embodiment are shown in Table 2.

[0046] Table 2

[0047] Element Concentration (wt%) CO 91 <![CDATA[CO 2 ]]> 6.5 <![CDATA[CH 3 OH]]> 1.6 DMC 0.85 <![CDATA[H 2 The]]> 0.05 Inlet pressure(MPaG) 1.8 Inlet air temperature (℃) 50 <![CDATA[Intake air volume (Nm 3 / h)]]> 1000

[0048] This embodiment also discloses a method for recovering CO in DMC synthesis tail gas through membrane separation. In this embodiment, the membrane assembly adopts a two-stage multi-membrane group series connection, including the following steps:

[0049] S1, washing the DMC synthesis tail gas in Table 2 with water through a water washing tower to remove methanol and DMC therein to obtain a crude gas;

[0050] S2, after the coarse gas is separated from the water by the separator, it enters the gas-to-gas heat exchanger to exchange gas-to-gas heat with the low-temperature coarse gas at the outlet of the cold dryer, thereby reducing the temperature of the coarse gas entering the cold dryer;

[0051] S3, the crude gas after gas-to-gas heat exchange is cooled to 10°C by a cold dryer and then separated into gas and liquid to obtain low-temperature crude gas;

[0052] S4, the low-temperature crude gas enters the gas-to-gas heat exchanger for reheating to 30°C and a pressure of 1.78MPaG to obtain the membrane separation inlet gas;

[0053] S5, membrane separation inlet gas enters the first-stage membrane assembly for component separation, and obtains the first-stage retentate gas rich in CO and the second-stage retentate gas rich in CO 2 The first-stage permeate gas is sent out of the boundary area as membrane separation tail gas.

[0054] S6. The primary retentate gas enters the secondary membrane component as the feed gas, the retentate gas of the secondary membrane component is used as the CO product gas obtained by membrane separation, and the secondary permeate gas is pressurized by the circulating gas compressor and returned to the inlet of the primary membrane component for recycling.

[0055] The secondary membrane assembly of this embodiment is equipped with 36 4" membrane cores in total.

[0056] The CO product gas prepared by the above method was analyzed for components by gas chromatography and measured online by a flow meter, wherein the purity of CO was 99.1wt%, and the effective recovery rate of CO in the product gas was 91%.

[0057] Example 3

[0058] The composition and intake conditions of the DMC synthesis tail gas in this embodiment are shown in Table 3.

[0059] Table 3

[0060] Element Concentration (wt%) CO 91 <![CDATA[CO 2 ]]> 6.5 <![CDATA[CH 3 OH]]> 1.6 DMC 0.85 <![CDATA[H 2 The]]> 0.05 Inlet pressure(MPaG) 2.4 Inlet air temperature (℃) 50 <![CDATA[Intake air volume (Nm 3 / h)]]> 1000

[0061] This embodiment also discloses a method for recovering CO in DMC synthesis tail gas through membrane separation. In this embodiment, the membrane assembly adopts a two-stage multi-membrane group series connection, including the following steps:

[0062] S1. Wash the DMC synthesis tail gas in Table 3 with water through a water washing tower to remove methanol and DMC therein to obtain a crude gas;

[0063] S2, after the coarse gas is separated from the water by the separator, it enters the gas-to-gas heat exchanger to exchange gas-to-gas heat with the low-temperature coarse gas at the outlet of the cold dryer, thereby reducing the temperature of the coarse gas entering the cold dryer;

[0064] S3, the crude gas after gas-to-gas heat exchange is cooled to 20°C by a cold dryer for gas-liquid separation to obtain low-temperature crude gas;

[0065] S4, the low-temperature crude gas enters the gas-to-gas heat exchanger for reheating and heating to 35°C and a pressure of 2.38MPaG to obtain the membrane separation inlet gas;

[0066] S5, membrane separation inlet gas enters the first-stage membrane assembly for component separation, and obtains the first-stage retentate gas rich in CO and the second-stage retentate gas rich in CO 2 The first-stage permeate gas is sent out of the boundary area as membrane separation tail gas.

[0067] S6. The primary retentate gas enters the secondary membrane assembly as the feed gas, and the retentate gas of the secondary membrane assembly is used as the CO product gas obtained by membrane separation. The secondary permeate gas is pressurized by the circulating gas compressor and then returns to the inlet of the primary membrane assembly for recycling.

[0068] The secondary membrane assembly of this embodiment is equipped with 24 4" membrane cores in total.

[0069] The CO product gas prepared by the above method was analyzed for components by gas chromatography and measured online by a flow meter, wherein the purity of CO was 99.3wt%, and the CO effective recovery rate of the CO product gas was 90%.

[0070] Example 4

[0071] The components and intake conditions of the DMC synthesis tail gas in this embodiment are shown in Table 4.

[0072] Table 4

[0073]

[0074]

[0075] This embodiment discloses a method for recovering CO in DMC synthesis tail gas by membrane separation. The membrane separation adopts a single-stage multi-membrane group parallel mode, and the specific implementation steps are the same as those in Example 1.

[0076] In this embodiment, a single-stage membrane assembly is equipped with 20 4" membrane cores.

[0077] The product CO gas prepared by the above method was analyzed for composition by gas chromatography and measured online by a flow meter, wherein the purity of CO was 98.3wt%, and the effective recovery rate of CO in the product gas was 84%.

[0078] Example 5

[0079] The intake conditions of the DMC synthetic tail gas in this embodiment are shown in Table 5.

[0080] Table 5

[0081]

[0082]

[0083] This embodiment discloses a method for recovering CO in DMC synthesis tail gas by membrane separation. The membrane separation adopts a two-stage multi-membrane group series mode. The specific implementation steps are the same as those in Example 3.

[0084] The secondary membrane assembly of this embodiment is equipped with 32 4" membrane cores in total.

[0085] The product CO gas prepared by the above method was analyzed for composition by gas chromatography and measured online by a flow meter, wherein the purity of CO was 97.5wt%, and the effective recovery rate of CO in the product gas was 87%.

[0086] Example 6

[0087] This embodiment discloses a system for recovering CO from DMC synthesis tail gas through membrane separation, such as Figure 1 As shown, it includes a water washing tower 1, a separator 2, an air-to-air heat exchanger 3 and a cold dryer 4 which are connected in sequence, the air outlet of the cold dryer 4 is connected to the low-temperature air inlet of the air-to-air heat exchanger 3, and the high-temperature air outlet of the air-to-air heat exchanger 3 is connected to multiple membrane assemblies 5, wherein the multiple membrane assemblies 5 are in a single-stage parallel form;

[0088] The DMC synthesis tail gas passes through a water washing tower 1 to remove methanol and DMC therein, and then enters a separator 2 for gas-liquid separation, where the water therein is separated to obtain a crude gas. The crude gas enters an air-to-air heat exchanger to exchange heat with the low-temperature crude gas, and then enters a cold dryer for cooling to obtain a low-temperature crude gas. The low-temperature crude gas enters an air-to-air heat exchanger to exchange heat with the crude gas, and then enters a membrane component for separation. This system is not only conducive to the recovery and reuse of DMC synthesis tail gas, saves CO resources, and reduces the cost of synthesizing DMC, but also can increase the service life of the membrane component and save purification costs.

[0089] Example 7

[0090] In this embodiment, the membrane assembly is two-stage. This embodiment discloses a system for recovering CO in DMC synthesis tail gas by membrane separation, such as Figure 2As shown, it includes a water washing tower 1, a separator 2, an air-to-air heat exchanger 3 and a cold dryer 4 which are connected in sequence, the air outlet of the cold dryer 4 is connected to the low-temperature air inlet of the air-to-air heat exchanger 3, and the high-temperature air outlet of the air-to-air heat exchanger 3 is connected to multiple membrane components 5, wherein the multiple membrane components are connected in multi-stage series, and the gas to be circulated is circulated by a circulating gas compressor 6.

[0091] The DMC synthesis tail gas passes through a water washing tower 1 to remove methanol and DMC therein, and then enters a separator 2 for gas-liquid separation, in which the water therein is separated to obtain a crude gas. The crude gas enters an air-to-air heat exchanger 3 to exchange heat with the low-temperature crude gas, and then enters a cold dryer 4 for cooling to obtain a low-temperature crude gas. The low-temperature crude gas enters an air-to-air heat exchanger 3 to exchange heat with the crude gas, and then enters a membrane component 5 for separation. This system is not only conducive to the recovery and reuse of DMC synthesis tail gas, saving CO resources and reducing the cost of synthesizing DMC, but also can increase the service life of the membrane component and save purification costs.

[0092] Finally, it should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the protection scope of the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for recovering CO from DMC synthesis tail gas by membrane separation, characterized in that: The following steps are involved: S1, washing the DMC synthesis tail gas with water through a water washing tower to remove methanol and DMC therein to obtain a crude gas; S2, after the coarse gas is separated from the water by the separator, it enters the gas-to-gas heat exchanger to exchange gas-to-gas heat with the low-temperature coarse gas at the outlet of the cold dryer, thereby reducing the temperature of the coarse gas entering the cold dryer; S3, the crude gas after gas-to-gas heat exchange is cooled to 20°C by a cold dryer for gas-liquid separation to obtain low-temperature crude gas; S4, the low-temperature crude gas enters the gas-to-gas heat exchanger for reheating and heating to 35°C and a pressure of 2.38MPaG to obtain the membrane separation inlet gas; S5, the membrane separation inlet gas enters the primary membrane assembly for component separation to obtain a primary retentate gas rich in CO and a primary permeate gas rich in CO2, and the primary permeate gas is sent out of the boundary area as the membrane separation tail gas; S6, the primary retentate gas enters the secondary membrane assembly as the feed gas, the retentate gas of the secondary membrane assembly is used as the CO product gas obtained by membrane separation, and the secondary permeate gas is pressurized by the circulating gas compressor and returned to the inlet of the primary membrane assembly for recycling; The secondary membrane assembly is equipped with 24 4” membrane cores in total; In terms of weight percentage, the main components of the DMC synthesis tail gas include: CO 91%, CO 6.5%, CH OH 1.6%, DMC 0.85%, H 2 O 0.05%; Intake condition parameters include: intake pressure 2.4MPaG, intake temperature 50℃, intake volume 1000Nm 3 / h.

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