System and method for treating washing water after liquefied gas sweetening

Abstract

The invention relates to a system and a method for treating washing water after liquefied gas sweetening. The method comprises the following steps: introducing carbon dioxide captured by air direct capturing equipment into an alkali-containing sewage treatment tank of a liquefied gas sweetening device, and enabling the carbon dioxide to be in contact with alkali-containing sewage in the alkali-containing sewage treatment tank for neutralization reaction to obtain neutralized sewage. According to the system, the alkali-containing sewage is neutralized by using CO2 captured by the air direct capturing equipment in air, so that the difficulty in control of a neutralization process and equipment maintenance is reduced, the safety coefficient of the system is improved, and the system is environment-friendly and energy-saving.

Description

Technical Field

[0001] The present disclosure relates to the field of liquefied gas refining in oil refining processes, and in particular to a system and method for treating water wash water after desulfurization of liquefied gas. Background Art

[0002] At present, in the process of refining liquefied gas products in refineries, the removal of mercaptans in liquefied gas usually adopts 1 to 2 levels of alkaline washing. After alkaline washing, the liquefied gas contains a certain amount of alkali solution (0.2-0.8%wt), which is usually removed by water washing. As the device is running, the pH of the washing water gradually rises to become alkaline wastewater (NaOH content is about 0.001~1.0%wt).

[0003] The usual practice of refineries is to discharge alkaline wastewater to sewage treatment plants regularly, and neutralize the alkaline wastewater to a neutral pH value before entering the salty wastewater treatment series. Strong acids, such as hydrochloric acid or sulfuric acid, are generally used to neutralize alkaline wastewater. The water wash water after liquefied gas desulfurization is weakly alkaline (pH value is 11-14). When using this method to neutralize alkaline wastewater, the pH value of the wastewater during the neutralization process fluctuates between acid and alkaline, and the process control requirements for neutralization are relatively high. It is highly corrosive, and generally non-metallic materials are required for the neutralization reaction tank, which is difficult to construct and install, has a short lifespan, and is prone to leakage. In addition, regardless of whether hydrochloric acid, sulfuric acid or other strong acids are used, the neutralization treatment device needs to add a set of strong acid storage and pressurization facilities, which increases the land occupation and the difficulty of equipment maintenance, and also poses a safety hazard. Summary of the invention

[0004] The purpose of the present disclosure is to provide a system and method for treating wash water after liquefied gas desulfurization. The system uses CO2 captured in the air by direct air capture equipment to neutralize alkaline wastewater, which reduces the difficulty of neutralization process control and equipment maintenance, improves the system safety factor, and is environmentally friendly and energy-saving.

[0005] In order to achieve the above-mentioned objectives, the first aspect of the present disclosure provides a method for treating wash water after liquefied gas desulfurization, the method comprising: introducing carbon dioxide captured by an air direct capture device into an alkaline wastewater treatment tank of a liquefied gas desulfurization device, so that the carbon dioxide contacts the alkaline wastewater in the alkaline wastewater treatment tank for a neutralization reaction to obtain neutralized wastewater.

[0006] Optionally, the NaOH content in the alkaline wastewater is 0.001-1.0% by weight, and the pH value of the alkaline wastewater is 11-14; the conditions of the neutralization reaction include: a temperature of 20-50°C, preferably 30-35°C; a pressure of 0.1-3.0MPa, preferably 0.1-0.5MPa; in the neutralization reaction, the flow rate of the introduced carbon dioxide is 10-100Nm 3 / h.

[0007] Optionally, the carbon dioxide capture method includes: S1, using an air adsorption fan to introduce air into the direct air capture equipment, so that the air contacts the adsorbent on the capture plate for adsorption treatment, and obtains the adsorbent adsorbed with carbon dioxide and the remaining air; S2, providing a heat source to the direct air capture equipment to heat and desorb the adsorbent adsorbed with carbon dioxide to obtain desorbed carbon dioxide; S3, using a carbon dioxide desorption fan to introduce at least part of the desorbed carbon dioxide into an alkaline wastewater treatment tank.

[0008] Optionally, the adsorbent is selected from organic solid amine adsorbent and / or inorganic solid amine adsorbent; the conditions for the adsorption treatment include: a temperature of 10 to 50°C; an air volume ratio of the air adsorption fan to the carbon dioxide desorption fan of 10 to 50:1; the conditions for the heating desorption treatment include: a temperature of 70 to 120°C.

[0009] Optionally, the method further comprises: allowing the liquefied gas after desulfurization to enter a water washing facility to contact and mix with water for water washing treatment, and introducing the obtained alkaline wastewater into the alkaline wastewater treatment tank; the conditions of the water washing treatment include a temperature of 20 to 50°C, preferably 30 to 35°C; a pressure of 0.1 to 3.0 MPa, preferably 0.1 to 0.5 MPa.

[0010] Optionally, the method further includes: detecting the pH value of the sewage in the alkaline sewage treatment tank, and adjusting the flow of desorbed carbon dioxide entering the alkaline sewage treatment tank so that the pH value of the sewage in the tank is above the external delivery threshold; optionally, the method further includes, when the pH value of the sewage in the tank is less than the external delivery threshold, stopping the introduction of desorbed carbon dioxide, so that the neutralized sewage in the alkaline sewage treatment tank is delivered to a sewage treatment plant; wherein the external delivery threshold is 7 to 9.

[0011] A second aspect of the present disclosure provides a system for treating wash water after desulfurization of liquefied gas, the system comprising: an alkaline wastewater treatment unit; the alkaline wastewater treatment unit comprises an alkaline wastewater treatment tank and an air direct capture device, the alkaline wastewater treatment tank comprises an alkaline wastewater inlet, a neutralized wastewater outlet and a carbon dioxide inlet; the air direct capture device comprises an air inlet, a carbon dioxide outlet and a residual air outlet; the carbon dioxide inlet of the alkaline wastewater treatment tank is connected to the carbon dioxide outlet of the air direct capture device through a first pipeline.

[0012] Optionally, the system also includes a water washing unit; the water washing unit includes a water washing device, the water washing device includes an inlet for liquefied gas after sweetening, an outlet for refined sweetening liquefied gas and an outlet for alkaline wastewater; the alkaline wastewater outlet of the water washing device is connected to the alkaline wastewater inlet of the alkaline wastewater treatment tank.

[0013] Optionally, the system also includes a pH detector, a carbon dioxide desorption fan, a flow regulating valve and an external delivery pump; the pH detector is used to detect the pH of the sewage in the alkaline sewage treatment tank; the external delivery pump is arranged on the neutralized sewage outlet pipeline of the alkaline sewage treatment tank; the carbon dioxide desorption fan and the flow regulating valve are arranged on the first pipeline in sequence along the direction of carbon dioxide flow, and the flow regulating valve is respectively connected to the pH detector and the external delivery pump signal to adjust the flow of desorbed carbon dioxide entering the alkaline sewage treatment tank according to the pH signal detected by the pH detector, and to open and close the external delivery pump according to the pH signal detected by the pH detector.

[0014] Optionally, one or more capture plates are provided in the direct air capture device; an adsorbent is provided on the capture plate; the adsorbent is selected from organic solid amine adsorbent and / or inorganic solid amine adsorbent; an air adsorption fan is provided on the air inlet pipeline of the direct air capture device.

[0015] Through the above technical solution, the present disclosure provides a system and method for treating water wash water after liquefied gas desulfurization, and the use of the system and method has the following advantages:

[0016] (1) The system uses CO2 captured in the air by direct air capture equipment to neutralize alkaline wastewater, which reduces the difficulty of neutralization process control and equipment maintenance and improves the system safety factor.

[0017] (2) The system neutralizes the alkaline wastewater directly in the alkaline wastewater treatment tank, thus avoiding the corrosion of the pipeline caused by the long-distance transportation of the alkaline wastewater.

[0018] (3) Directly using carbon dioxide from the air reduces the CO2 content in the atmosphere, thus playing a role in energy conservation and emission reduction.

[0019] Other features and advantages of the present disclosure will be described in detail in the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The accompanying drawings are used to provide a further understanding of the present disclosure and constitute a part of the specification. Together with the following specific embodiments, they are used to explain the present disclosure but do not constitute a limitation of the present disclosure. In the accompanying drawings:

[0021] Figure 1 A schematic diagram of the structure of a system for treating post-mercaptan removal wash water in one embodiment of the present disclosure.

[0022] Description of Reference Numerals

[0023] 1 Water washing facility 2 Flow control valve 3 Alkaline wastewater treatment tank

[0024] 4 External delivery pump 5 pH detector 6 Direct air capture equipment

[0025] 7 Air adsorption fan 8 Carbon dioxide desorption fan

[0026] 101 Water washing unit 102 Alkaline wastewater treatment unit DETAILED DESCRIPTION

[0027] The specific implementation of the present disclosure is described in detail below in conjunction with the accompanying drawings. It should be understood that the specific implementation described herein is only used to illustrate and explain the present disclosure, and is not used to limit the present disclosure.

[0028] In the present disclosure, unless otherwise specified, directional words such as "inside" or "within" are used relative to the outline of each device.

[0029] The first aspect of the present disclosure provides a method for treating wash water after liquefied gas desulfurization, the method comprising: introducing carbon dioxide captured by an air direct capture device 6 into an alkaline wastewater treatment tank 3 of a liquefied gas desulfurization device, so that the carbon dioxide contacts the alkaline wastewater in the alkaline wastewater treatment tank 3 for a neutralization reaction to obtain neutralized wastewater.

[0030] In the present disclosure, in the process of alkali neutralization, carbon dioxide (CO2) captured in the air by air direct capture equipment is used to replace the strong acid used in the prior art, which not only solves the problem of difficult control of the neutralization process and difficult equipment maintenance, but also improves the system safety factor; in addition, the direct use of CO2 in the air reduces the CO2 content in the atmosphere, avoiding the harm and pollution to the environment by the strong acid salt with strong stability produced by the strong acid neutralization. Moreover, the method can be used to directly neutralize the alkaline sewage in the alkaline sewage treatment tank, avoiding the corrosion of the pipeline caused by the long-distance transportation of the alkaline sewage. According to the actual production needs of liquefied gas, the scale of liquefied gas to be treated is 100,000-2.5 million tons / year, the alkaline sewage generated is about 100-7000 tons / year, and the CO2 required for neutralization is 7.5-30t / year. The CO2 in the air is greatly reduced, with a good environmental protection effect.

[0031] In one embodiment of the present disclosure, the content of NaOH in the alkaline wastewater is 0.001-1.0% by weight, and the pH value of the alkaline wastewater is 11-14.

[0032] In the present disclosure, the temperature and pressure of the neutralization reaction can vary within a wide range. In one embodiment of the present disclosure, the conditions of the neutralization reaction include: a temperature of 20 to 50° C., preferably 30 to 35° C.; a pressure of 0.1 to 3.0 MPa, preferably 0.1 to 0.5 MPa. When the temperature and pressure of the neutralization reaction in the present disclosure are within the range of the above-mentioned embodiment, the neutralization reaction can be further promoted.

[0033] In the present disclosure, the flow rate of the introduced carbon dioxide can vary within a wide range. In one embodiment of the present disclosure, in the neutralization reaction, the flow rate of the introduced carbon dioxide is 10 to 100 m / s. 3 In the present disclosure, the flow rate of the introduced carbon dioxide can be adjusted according to the pH value of the alkaline sewage.

[0034] In one embodiment of the present disclosure, the method for capturing carbon dioxide comprises:

[0035] S1, using the air adsorption fan 7 to introduce air into the air direct capture device 6, so that the air contacts the adsorbent on the capture plate for adsorption treatment, and obtains the adsorbent adsorbed with carbon dioxide and the remaining air;

[0036] S2, providing a heat source to the air direct capture device 6 to heat and desorb the adsorbent adsorbed with carbon dioxide to obtain desorbed carbon dioxide;

[0037] S3, using the carbon dioxide desorption fan 8 to introduce at least part of the desorbed carbon dioxide into the alkaline wastewater treatment tank 3.

[0038] In the present disclosure, the air direct capture device 6 uses a fixed bed process to capture air, and one or more capture plates may be provided in the air direct capture device 6, for example, 1, 2, 3, 4, 5 or 6; adsorbents are evenly provided on the capture plates; the adsorbents are selected from organic solid amine adsorbents and / or inorganic solid amine adsorbents. When air enters the air direct capture device, the adsorbent therein can adsorb CO2 in the air and discharge the remaining air, and the conditions for the adsorption treatment include: a temperature of 10 to 50°C; the CO2 on the adsorbent can be desorbed from the adsorbent under the heating of a heat source. In the present disclosure, the conditions for the adsorption treatment may also include pressure, which may be normal pressure, negative pressure or slightly positive pressure. The heat source of the present disclosure may be a heat source or electric heating provided by an external shared engineering system, for example, steam. The amount of CO2 treated by the air direct capture device in the present disclosure is adjusted according to the amount of alkaline sewage and the pH of the sewage, which will not be repeated here.

[0039] In one embodiment of the present disclosure, the air volume ratio of the air adsorption fan 7 to the carbon dioxide desorption fan 8 is 10 to 50: 1. In the present disclosure, when the air adsorption fan flow is fixed, the larger the desorption fan flow, the larger the CO2 flow produced by regeneration.

[0040] In the present disclosure, the temperature of the heating desorption treatment can vary within a wide range. In one embodiment of the present disclosure, the conditions of the heating desorption treatment include a temperature of 70 to 120° C. When the temperature of the heating desorption treatment of the present disclosure is within the range of the above-mentioned embodiment, it can further promote the desorption of CO2 from the adsorbent.

[0041] In one embodiment of the present disclosure, the method further comprises: allowing the liquefied gas after desulfurization to enter the water washing facility 1 to contact and mix with water for water washing treatment, and introducing the obtained alkaline wastewater into the alkaline wastewater treatment tank 3.

[0042] In the present disclosure, alkaline wastewater can be intermittently introduced into the alkaline wastewater treatment tank for storage. When the liquid level of the alkaline wastewater in the alkaline wastewater treatment tank reaches the set liquid level of the alkaline wastewater treatment tank, the CO2 desorption fan is activated to inject CO2 into the alkaline wastewater treatment tank; the set liquid level of the alkaline wastewater treatment tank is 20-70% of the height of the alkaline wastewater treatment tank. "The height of the alkaline wastewater treatment tank" refers to the distance between the surface where the alkaline wastewater inlet of the alkaline wastewater treatment tank is located and the surface where the neutralized wastewater outlet is located. The water washing facilities of the present disclosure can adopt the conventional water washing facilities of the present disclosure, for example, one or more of a normal temperature nitrogen stripping device, a high temperature stripping device and a normal temperature fuel gas stripping device can be adopted, which will not be repeated here.

[0043] In the present disclosure, the temperature and pressure of the water washing treatment can vary within a wide range. In one embodiment of the present disclosure, the conditions of the water washing treatment include a temperature of 20 to 50°C, preferably 30 to 35°C; and a pressure of 0.1 to 3.0 MPa, preferably 0.1 to 0.5 MPa.

[0044] In one embodiment of the present disclosure, the method further includes: detecting the pH value of the sewage in the alkaline sewage treatment tank 3, and adjusting the flow of desorbed carbon dioxide entering the alkaline sewage treatment tank 3 so that the pH value of the sewage in the tank is above the external delivery threshold; optionally, the method further includes, when the pH value of the sewage in the tank is less than the external delivery threshold, stopping the introduction of desorbed carbon dioxide, so that the neutralized sewage in the alkaline sewage treatment tank 3 is delivered to the sewage treatment plant; wherein the external delivery threshold is 7 to 9.

[0045] In the above embodiment, when the pH value of the neutralized wastewater reaches the external delivery threshold of 7 to 9, the neutralized wastewater can be discharged to a sewage treatment plant.

[0046] The second aspect of the present disclosure provides a system for treating water wash water after desulfurization of liquefied gas, such as Figure 1 As shown, the system includes: an alkaline wastewater treatment unit 102; the alkaline wastewater treatment unit 102 includes an alkaline wastewater treatment tank 3 and an air direct capture device 6, the alkaline wastewater treatment tank 3 includes an alkaline wastewater inlet, a neutralized wastewater outlet and a carbon dioxide inlet; the air direct capture device 6 includes an air inlet, a carbon dioxide outlet and a residual air outlet;

[0047] The carbon dioxide inlet of the alkaline wastewater treatment tank 3 is connected to the carbon dioxide outlet of the air direct capture device 6 through a first pipeline.

[0048] In the present disclosure, the carbon dioxide inlet of the alkaline sewage treatment tank 3 can be provided with a carbon dioxide conduit extending into the alkaline sewage treatment tank 3 along the direction of carbon dioxide introduction. Preferably, the distance between the outlet of the conduit and the bottom of the alkaline sewage treatment tank is 15 to 20 cm. The carbon dioxide conduit can be directly introduced into the alkaline sewage, which is conducive to the neutralization reaction.

[0049] In one embodiment of the present disclosure, the system also includes a water washing unit 101; the water washing unit 101 includes a water washing device 1, and the water washing device 1 includes an inlet for liquefied gas after sweetening, an outlet for refined sweetening liquefied gas and an outlet for alkaline wastewater; the alkaline wastewater outlet of the water washing device 1 is connected to the alkaline wastewater inlet of the alkaline wastewater treatment tank 3.

[0050] In one embodiment of the present disclosure, the system further comprises a pH detector 5, a carbon dioxide desorption blower 8, a flow regulating valve 2 and an external delivery pump 4;

[0051] The pH detector 5 is used to detect the pH of the sewage in the alkaline sewage treatment tank 3; the external delivery pump 4 is arranged on the neutralized sewage outlet pipeline of the alkaline sewage treatment tank 3;

[0052] The carbon dioxide desorption fan 8 and the flow regulating valve 2 are sequentially arranged on the first pipeline along the direction of carbon dioxide flow. The flow regulating valve 2 is respectively connected to the pH detector 5 and the external delivery pump 4 signals to adjust the flow of desorbed carbon dioxide entering the alkaline wastewater treatment tank 3 according to the pH signal detected by the pH detector 5, and to open and close the external delivery pump according to the pH signal detected by the pH detector 5.

[0053] One or more collecting plates are arranged in the air direct capture device 6; an adsorbent is arranged on the collecting plate; the adsorbent is selected from organic solid amine adsorbent and / or inorganic solid amine adsorbent; an air adsorption fan 7 is arranged on the air inlet pipeline of the air direct capture device 6.

[0054] The present invention is further illustrated by the following examples, but the present invention is not limited thereto.

[0055] Unless otherwise specified, the materials used in the examples are all commercially available products.

[0056] The volume content of CO2 in air is 0.03%v. At 20°C and standard atmospheric pressure, the density of air is 1.25kg / m 3 , molecular weight is 29kg / kmol.

[0057] Example 1

[0058] Use Figure 1 The system described treats alkaline wastewater by neutralizing the alkaline wastewater produced after liquefied gas washing after desulfurization in a refinery; the scale of refined liquefied gas in the refinery is 100×10 4 / a, the alkaline wastewater discharge is 4000t / a, the NaOH content is 0.5% by weight, the alkaline wastewater tank is a horizontal tank with a size of 4000×12000mm. During normal operation, the alkaline wastewater content in the tank is about 120t.

[0059] The specific process is:

[0060] (1) The liquefied gas after desulfurization in a refinery enters a water washing facility 1, which uses a nitrogen stripping device; under the conditions of a temperature of 30° C. and a pressure of 1.8 MPa, it is contacted and mixed with water for water washing treatment to obtain alkaline wastewater with a NaOH content of 0.5% and a pH value of 13.7, and the obtained alkaline wastewater is introduced into the alkaline wastewater treatment tank 3.

[0061] (2) using an air adsorption fan 7 to introduce air into the air direct capture device 6, so that the air contacts the organic solid amine adsorbent on the capture plate for adsorption treatment, thereby obtaining an adsorbent adsorbing carbon dioxide and residual air; providing a heat source to the air direct capture device 6 to heat it to 80° C., performing a heating desorption treatment on the organic solid amine adsorbent adsorbing carbon dioxide, thereby obtaining desorbed carbon dioxide;

[0062] (3) The carbon dioxide captured by the air direct capture device 6 is introduced into the alkaline wastewater treatment tank 3 by using the carbon dioxide desorption fan 8, so that the carbon dioxide contacts the alkaline wastewater in the alkaline wastewater treatment tank 3 at a temperature of 40°C, a pressure of 0.5 MPa, and a carbon dioxide flow rate of 20 Nm 3 / h to carry out the neutralization reaction;

[0063] According to the complete neutralization reaction of CO2 and NaOH without any side reaction, the reaction in the tank is:

[0064] 2NaOH+CO2→Na2CO3+H2O.

[0065] The amount of CO2 required to neutralize the NaOH in the tank is 7.5 kmol, and the pH value of the neutralized wastewater is 8.

[0066] Comparative Example 1

[0067] The alkaline wastewater was treated in the same manner as in Example 1, except that the alkaline wastewater was neutralized with a 98 wt% H2SO4 solution after being sent to the wastewater treatment plant. A total of 900 kg of H2SO4 was consumed.

[0068] Compared with Comparative Example 1, Example 1 saves the step of sending out the alkaline wastewater and avoids the probability of corrosion of pipelines caused by long-distance transportation of alkaline wastewater. In addition, Example 1 uses carbon dioxide in the air instead of sulfuric acid, which reduces the danger of operation and the difficulty of controlling the neutralization reaction process, avoids the generation of sulfates with strong stability, and reduces the harm and pollution to the environment; neutralization treatment of the alkaline wastewater generated after the liquefied gas washing after desulfurization consumes about 5000Nm per year. 3 of CO2, which has a good effect of reducing emissions.

[0069] The preferred embodiments of the present disclosure are described in detail above in conjunction with the accompanying drawings; however, the present disclosure is not limited to the specific details in the above embodiments. Within the technical concept of the present disclosure, a variety of simple modifications can be made to the technical solution of the present disclosure, and these simple modifications all fall within the protection scope of the present disclosure.

[0070] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, the present disclosure will not further describe various possible combinations.

[0071] In addition, various embodiments of the present disclosure may be arbitrarily combined, and as long as they do not violate the concept of the present disclosure, they should also be regarded as the contents disclosed by the present disclosure.

Claims

1. A method for treating water wash water after desulfurization of liquefied gas, characterized in that: The method includes: The carbon dioxide captured by the air direct capture device (6) is introduced into the alkaline wastewater treatment tank (3) of the liquefied gas desulfurization device, so that the carbon dioxide contacts the alkaline wastewater in the alkaline wastewater treatment tank (3) for a neutralization reaction to obtain neutralized wastewater.

2. The method according to claim 1, characterized in that The NaOH content in the alkaline wastewater is 0.001 to 1.0% by weight, and the pH value of the alkaline wastewater is 11 to 14; The conditions of the neutralization reaction include: temperature of 20 to 50°C, preferably 30 to 35°C; pressure of 0.1 to 3.0 MPa, preferably 0.1 to 0.5 MPa; In the neutralization reaction, the flow rate of the introduced carbon dioxide is 10 to 100 Nm 3 / h.

3. The method according to claim 1, characterized in that The carbon dioxide capture method comprises: S1, using an air adsorption fan (7) to introduce air into the air direct capture device (6), so that the air contacts the adsorbent on the capture plate for adsorption treatment, and obtains the adsorbent adsorbed with carbon dioxide and the remaining air; S2, providing a heat source to the air direct capture device (6) to heat and desorb the adsorbent adsorbed with carbon dioxide to obtain desorbed carbon dioxide; S3. Using a carbon dioxide desorption fan (8) to introduce at least part of the desorbed carbon dioxide into the alkaline wastewater treatment tank (3).

4. The method according to claim 3, characterized in that The adsorbent is selected from organic solid amine adsorbent and / or inorganic solid amine adsorbent; the conditions of the adsorption treatment include: the temperature is 10 to 50°C; The air volume ratio of the air adsorption fan (7) to the carbon dioxide desorption fan (8) is 10 to 50:1; The conditions of the heating desorption treatment include: a temperature of 70 to 120°C.

5. The method according to claim 1, characterized in that The method further comprises: allowing the liquefied gas after desulfurization to enter a water washing facility (1) to contact and mix with water for water washing treatment, and introducing the obtained alkaline wastewater into the alkaline wastewater treatment tank (3); The conditions of the water washing treatment include a temperature of 20 to 50° C., preferably 30 to 35° C.; and a pressure of 0.1 to 3.0 MPa, preferably 0.1 to 0.5 MPa.

6. The method according to claim 2, characterized in that The method further comprises: detecting the pH value of the sewage in the alkaline sewage treatment tank (3), and adjusting the flow rate of the desorbed carbon dioxide entering the alkaline sewage treatment tank (3) so that the pH value of the sewage in the tank is above a threshold value for external transmission; Optionally, the method further comprises, when the pH value of the sewage in the tank is less than the external delivery threshold, stopping the introduction of desorbed carbon dioxide, so that the neutralized sewage in the alkaline sewage treatment tank (3) is externally delivered to a sewage treatment plant; wherein the external delivery threshold is 7 to 9.

7. A system for treating water wash water after liquefied gas desulfurization, characterized in that: The system comprises: an alkaline wastewater treatment unit (102); The alkaline wastewater treatment unit (102) comprises an alkaline wastewater treatment tank (3) and an air direct capture device (6), wherein the alkaline wastewater treatment tank (3) comprises an alkaline wastewater inlet, a neutralized wastewater outlet and a carbon dioxide inlet; the air direct capture device (6) comprises an air inlet, a carbon dioxide outlet and a residual air outlet; The carbon dioxide inlet of the alkaline wastewater treatment tank (3) is connected to the carbon dioxide outlet of the direct air capture device (6) through a first pipeline.

8. The system according to claim 7, characterized in that The system further comprises a water washing unit (101); the water washing unit (101) comprises a water washing device (1), the water washing device (1) comprises an inlet for liquefied gas after sweetening, an outlet for refined sweetening liquefied gas and an outlet for alkaline wastewater; the outlet for alkaline wastewater of the water washing device (1) is connected to the inlet for alkaline wastewater of the alkaline wastewater treatment tank (3).

9. The system according to claim 8, characterized in that The system also includes a pH detector (5), a carbon dioxide desorption fan (8), a flow regulating valve (2) and an external delivery pump (4); The pH detector (5) is used to detect the pH of the sewage in the alkaline sewage treatment tank (3); the external delivery pump (4) is arranged on the neutralized sewage outlet pipeline of the alkaline sewage treatment tank (3); The carbon dioxide desorption fan (8) and the flow regulating valve (2) are sequentially arranged on the first pipeline along the direction of carbon dioxide flow. The flow regulating valve (2) is respectively connected to the pH detector (5) and the external delivery pump (4) by signal, so as to adjust the flow of desorbed carbon dioxide entering the alkaline wastewater treatment tank (3) according to the pH signal detected by the pH detector (5), and to open and close the external delivery pump according to the pH signal detected by the pH detector (5).

10. The system according to claim 8, characterized in that The air direct capture device (6) is provided with one or more capture plates; the capture plates are provided with an adsorbent; the adsorbent is selected from an organic solid amine adsorbent and / or an inorganic solid amine adsorbent; An air adsorption fan (7) is provided on the air inlet pipeline of the air direct capture device (6).

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