A closed loop corrosion control drying system and method for a methane chloride rectification column

Abstract

This invention provides a closed-loop anti-corrosion drying system and method for a methane chloride distillation column. The distillation column is equipped with a drying loop, which includes a first condenser, an alkaline washing separator, and an azeotropic column connected in sequence. The vapor phase outlet pipe of the distillation column is connected to the first condenser, and the liquid phase outlet pipe of the azeotropic column is connected to the distillation column. The alkaline washing separator is equipped with an alkaline feed pipe and a demineralized water feed pipe. The beneficial effects of this invention are: by utilizing the azeotropic distillation principle, residual water is removed from the top of the column in the form of an azeotrope, and the deeply dried methane chloride obtained from the bottom of the azeotropic column is returned to the distillation column, forming a continuous closed-loop drying circuit of "distillation-condensation-alkaline washing-azeotropic dehydration-reflux". This keeps the water content of the circulating material in the column at a low level for a long time, significantly reducing the corrosion rate of metal equipment and pipeline inner walls, and extending the maintenance cycle and service life of the equipment.

Description

Technical Field

[0001] This invention relates to the field of distillation column technology, and specifically to a closed-loop anti-corrosion drying system and drying method for a methane chloride distillation column. Background Technology

[0002] In the industrial production of methane chloride, distillation is the core unit operation for obtaining high-purity products. Whether using the natural gas thermal chlorination method or the methanol method, the crude chlorinated liquid produced by the reaction must be separated and purified through multi-stage distillation columns. However, corrosion of these distillation columns has long plagued the industry, becoming a key bottleneck restricting the long-term safe operation of the equipment.

[0003] Studies have shown that the root cause of corrosion in methane chloride distillation columns lies in the unavoidable presence of moisture and a small amount of hydrogen chloride in the crude chlorination liquid. In the low-temperature regions, such as the top of the distillation column, hydrogen chloride and water vapor condense together, forming a highly corrosive dilute hydrochloric acid dew point environment. This acidic condensate causes severe electrochemical corrosion and pitting on internal components made of carbon steel and stainless steel, including trays, float valves, grids, packing, and connecting pipes. This not only leads to thinning of equipment walls and failure of internal components, shortening equipment lifespan, but the corrosion products can also clog trays, affecting separation efficiency and even forcing unplanned shutdowns, severely impacting continuous production and economic benefits for the enterprise.

[0004] To address the corrosion problem in distillation columns, existing technologies primarily seek solutions through material upgrades and front-end dehydration. Regarding materials, some solutions employ distillation columns with glass-lined layers to isolate corrosive media from the carbon steel substrate; others use corrosion-resistant alloys to manufacture key equipment. While these measures alleviate corrosion, the equipment manufacturing costs are high, and they cannot fundamentally eliminate the presence of corrosive media. In terms of front-end dehydration, industrial methods such as molecular sieve adsorption drying and concentrated sulfuric acid drying have been used to pretreat the crude chlorinated liquid entering the distillation column. However, molecular sieves suffer from frequent regeneration and high investment costs; concentrated sulfuric acid drying has drawbacks such as sulfuric acid entrainment and difficulties in waste acid treatment. Another solution uses azeotropic distillation to dehydrate methane chloride materials, but existing azeotropic distillation systems are mostly open-loop designs. The dehydrated water and acidic components are often directly discharged or enter the subsequent alkaline washing and neutralization process after condensation, failing to achieve a closed-loop circulation and resource utilization of the water-containing atmosphere within the entire distillation system. Furthermore, the generation of alkaline washing wastewater adds an additional environmental treatment burden. Summary of the Invention

[0005] To address the issue that existing desulfurization towers' demisters are mostly fixed-channel structures, whose flow area is determined during the design phase based on the rated flue gas volume, making them unable to adapt to real-time load changes; This invention provides a closed-loop corrosion-resistant drying system for a methane chloride distillation column, wherein the distillation column is equipped with a drying loop; The drying loop includes a first condenser, an alkali washing separator, and an azeotropic column connected in sequence. The vapor phase outlet pipe of the distillation column is connected to the first condenser, and the liquid phase outlet pipe of the azeotropic column is connected to the distillation column. The alkali washing separator is equipped with an alkali feed pipe and a demineralized water feed pipe.

[0006] As a preferred embodiment, the outlet pipe of the first condenser is equipped with a reflux tank and a second condenser. When the liquid level in the reflux tank reaches 80%, the methane chloride coolant is discharged. The reflux tank is equipped with a reflux pipe connected to the distillation column.

[0007] As a preferred embodiment, the liquid phase outlet pipe of the azeotropic tower is sequentially equipped with a third condenser and a detection tank.

[0008] As a preferred embodiment, the vapor phase outlet pipe of the azeotropic tower is equipped with a fourth condenser and refluxes to the alkaline washing separator.

[0009] As a preferred embodiment, the outlet pipe of the detection tank is refluxed to the distillation column and a finished product reflux pump is installed.

[0010] A drying method for a methane chloride distillation column is also provided, comprising the following steps: Step 1: Open the DN50 drain valve at the top of the distillation column and pour 10 kg of 15% alkali solution into the distillation column through the drain inlet. After the alkali solution is added, connect the demineralized water to the top drain inlet and flush the inside of the column from top to bottom. When water is discharged from the bottom drain, use pH test paper to monitor the drainage and control the pH value of the drainage to be >9 to avoid acid corrosion of the equipment. Continue flushing until the bottom drain water is visually clear, then stop the flow of demineralized water. Step 2: Connect the nitrogen pipeline to the DN50 backflow valve and purge the entire tower with nitrogen from top to bottom for no less than 24 hours; after purging until there is no obvious water flow and the oxygen content of the sampling analysis system is ≤0.5%, disconnect the nitrogen pipeline and close the top DN50 backflow valve; Step 3: Feed the material into the reflux tank to establish a 30% liquid level. Start the reflux pump to transport the material to the top of the distillation column, carrying away any residual water in the column. The water-containing material discharged from the bottom of the column is collected and disposed of in a ton container. Introduce dichloromethane into the testing tank to a 50% liquid level. Start the finished product reflux pump to feed the material into the distillation column to establish a 50% liquid level and a reflux flow rate of 10 m³ / h. Continuously circulate and dry the material in the drying loop. Take samples of the material in the reflux tank for analysis until the moisture content is ≤50 ppm. The azeotropic drying process is then complete.

[0011] As a preferred option, if the pH value drops during demineralized water rinsing, a small amount of alkali solution can be added to maintain the pH value > 9.

[0012] The beneficial effects of this invention are as follows: 1. This invention constructs an independent external circulation drying loop, utilizing the azeotropic properties of dichloromethane, trichloromethane, and water. It employs acid-free finished material circulating with water, which is then dried in the azeotropic tower and reused. This achieves continuous removal of moisture and acidic impurities from the tower, avoiding secondary corrosion problems caused by incomplete replacement in the distillation tower, and suppressing the corrosion risk of hydrogen chloride hydrolysis to the distillation tower and condenser.

[0013] 2. This invention utilizes the principle of azeotropic distillation to remove residual moisture from the top of the column in the form of an azeotrope. The deeply dried methane chloride obtained from the bottom of the azeotropic column is returned to the distillation column, forming a continuous closed drying loop of "distillation-condensation-alkali washing-azeotropic dehydration-reflux". This keeps the water content of the circulating material in the column at a low level for a long time, significantly reducing the corrosion rate of metal equipment and pipeline inner walls, and extending the maintenance cycle and service life of the equipment. Attached Figure Description

[0014] To make the content of this invention easier to understand, the invention will be further described in detail below with reference to specific embodiments and accompanying drawings, wherein... Figure 1 This is a schematic diagram of the structure of the present invention; The following are the labels in the diagram: 1. Distillation column, 2. First condenser, 3. Alkali wash separator, 4. Azeotropic column, 5. Reflux tank, 6. Second condenser, 7. Third condenser, 8. Detection tank, 9. Fourth condenser, 10. Product reflux pump. Detailed Implementation

[0015] To illustrate the features of the present invention, the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0016] Example 1: Please see Figure 1 This invention provides a closed-loop corrosion-resistant drying system for a methane chloride distillation column, comprising a distillation column 1 and a drying loop connected to the distillation column 1. The distillation column 1 is a plate column with a diameter of 1.2 meters and a height of 20 meters, and is equipped with 30 floating valve trays for separating dichloromethane.

[0017] The drying loop includes a first condenser 2, an alkaline washing separator 3, and an azeotropic column 4, which are connected sequentially by pipes along the material flow direction. A vapor outlet pipe is installed at the top of the distillation column 1, which connects to the tube-side inlet of the first condenser 2. The first condenser 2 is a shell-and-tube heat exchanger with a heat exchange area of ​​20 square meters. Circulating cooling water (inlet temperature 32 degrees Celsius, outlet temperature 38 degrees Celsius) is introduced into the shell side to cool the vapor phase at the top of the column to 40 degrees Celsius.

[0018] The outlet pipe of the first condenser 2 is connected to the top inlet of the reflux tank 5. The reflux tank 5 is a horizontal storage tank used to temporarily store condensate. The bottom outlet pipe of the reflux tank 5 is connected to the inlet of the second condenser 6. The second condenser 6 is a shell-and-tube heat exchanger with a heat exchange area of ​​10 square meters. It uses 5-degree Celsius chilled water as the refrigerant to further cool the condensate to 15 degrees Celsius. A level gauge is installed on the side of the reflux tank 5. When the liquid level in the tank reaches 80%, the drain valve automatically opens to discharge part of the methane chloride coolant to the second condenser 6.

[0019] The outlet pipe of the second condenser 6 is connected to the central inlet of the alkaline washing separator 3. The alkaline washing separator 3 is a vertical gravity separator with a diameter of 1.0 meter and a height of 2.0 meters, and is internally equipped with a coalescing packing layer. The upper side of the alkaline washing separator 3 has an alkaline inlet pipe and a demineralized water inlet pipe, used to introduce a 15% sodium hydroxide solution and demineralized water, respectively. The alkaline solution and demineralized water come into countercurrent contact with the condensate inside the tank, neutralizing trace amounts of acidic substances such as hydrogen chloride carried in the material.

[0020] The organic phase outlet pipe of the alkaline washing separator 3 is connected to the central feed inlet of the azeotropic tower 4. The azeotropic tower 4 is a packed tower with a diameter of 0.8 meters and a height of 15 meters, filled with stainless steel Pall ring packing. The operating pressure of the azeotropic tower 4 is atmospheric pressure, and the bottom temperature is controlled at 120 degrees Celsius. It utilizes the property of water forming an azeotrope with methane chloride to further remove trace amounts of residual moisture from the organic phase. The liquid phase outlet pipe at the bottom of the azeotropic tower 4 is connected sequentially to the third condenser 7 and the detection tank 8. The third condenser 7 is a shell-and-tube heat exchanger with a heat exchange area of ​​8 square meters, using circulating cooling water to cool the outlet to 40 degrees Celsius. The detection tank 8 is a vertical tank with a volume of 0.5 cubic meters, equipped with an online moisture analyzer (measuring range 0 to 1000 ppm, accuracy ±5 ppm) and a pH meter to detect the moisture content and acidity in the dried methane chloride.

[0021] The outlet pipe of the testing tank 8 is returned to the distillation column 1 via the finished product reflux pump 10, sending the material that meets the drying standards back into the distillation column 1. The finished product reflux pump 10 is a magnetic pump with a flow rate of 2 cubic meters per hour and a head of 25 meters.

[0022] The vapor phase outlet pipe at the top of the azeotropic tower 4 is connected to the inlet of the fourth condenser 9. The fourth condenser 9 is a shell-and-tube heat exchanger with a heat exchange area of ​​6 square meters. It uses chilled water at 5 degrees Celsius to condense the azeotropic vapor into a liquid phase, which is then returned to the top inlet of the alkaline washing separator 3 through a pipe, forming a closed loop.

[0023] In this embodiment, the gaseous material drawn from the top of distillation column 1 is condensed, alkali-washed to remove acid, azeotropically dehydrated, and tested through a drying loop. The dried material is then returned to distillation column 1, effectively reducing the water content and acidity of the material inside the distillation column, preventing corrosion of the column body and internal components, and extending the service life of the equipment. During continuous operation, the water content of the material in distillation column 1 can be controlled below 100 ppm, and the acidity below 5 ppm.

[0024] Example 2: This embodiment provides a drying method for a methane chloride distillation column, and the specific implementation process is as follows: Step 1: Alkaline washing and water washing Open the DN50 backflow valve at the top of distillation column 1 and slowly pour 10 kg of 15% sodium hydroxide solution into the column through the backflow port. The solution flows down the inner wall of the column, neutralizing any remaining acidic substances. After the solution is added, connect the demineralized water pipeline to the top backflow port and flush the entire column from top to bottom, maintaining a flushing water flow rate of 100-200 liters per hour. During flushing, continuously monitor the backflow drainage from the bottom of the column. Once drainage begins to flow from the bottom, use pH test strips to monitor the drainage in real time, ensuring the pH value is greater than 9 to guarantee that the acidic substances in the column have been fully neutralized and to prevent acid corrosion of the column walls and internal components. If the pH value drops below 9 during flushing, add a small amount of 15% sodium hydroxide solution to maintain a pH value above 9. Continue flushing with demineralized water until the bottom backflow effluent is visually clear and free of oil droplets and suspended solids, then stop flushing with demineralized water.

[0025] Step 2: Nitrogen purging Connect a nitrogen line to the DN50 backflow valve at the top of distillation column 1, open the nitrogen valve, ensuring the nitrogen purity is not less than 99.9% and the pressure is controlled between 0.2 and 0.3 MPa. Continuously purge the entire column with nitrogen from top to bottom for at least 24 hours. During purging, periodically check for any standing water discharge from the bottom backflow outlet. After purging until no obvious standing water flows out from the bottom backflow outlet, sample the gas phase inside the column using an oxygen analyzer. When the system oxygen content is less than or equal to 0.5%, it is confirmed that the moisture and air inside the column have been sufficiently replaced. After successful purging, disconnect the nitrogen line, close the top DN50 backflow valve, and maintain a sealed state inside the column.

[0026] Step 3: Azeotropic Cyclic Drying Dichloromethane is introduced into the reflux tank 5 of distillation column 1 to establish a 30% liquid level. The reflux pump is started to deliver the dichloromethane at a flow rate of 3 cubic meters per hour to the top of distillation column 1 for spraying, carrying away any residual water in the column. The water-containing material is discharged from the bottom drain outlet and collected in a ton container for centralized disposal.

[0027] After the residual water in the column has been mostly drained, fresh dichloromethane is introduced into the testing tank 8 to a level of 50%. The finished product reflux pump 10 is started, and the dichloromethane in the testing tank 8 is fed into the distillation column 1 at a reflux rate of 10 cubic meters per hour, gradually establishing a 50% liquid level in the column. At this point, the drying loop enters a continuous circulation drying state.

[0028] During the cyclic drying process, a sample is taken from the sampling port of reflux tank 5 every 30 minutes, and the moisture content of the material is analyzed using the Karl Fischer method. After continuous cyclic drying for approximately 4 to 6 hours, the azeotropic drying process is considered complete when two consecutive sampling analyses show a moisture content of less than or equal to 50 ppm. The entire drying process takes approximately 30 to 36 hours. After drying, the moisture content inside distillation column 1 is less than 50 ppm, effectively preventing corrosion of the equipment by hydrogen chloride hydrolysis and extending the operating cycle of the distillation column.

[0029] The above embodiments and accompanying drawings are only used to illustrate the technical solutions of the present invention and are not intended to limit the present invention. The present invention has been described in detail with reference to preferred embodiments. Those skilled in the art should understand that any changes, modifications, additions, or substitutions made by those skilled in the art within the scope of the present invention do not depart from the spirit of the present invention and should also fall within the protection scope of the claims of the present invention. Other related technical structures not disclosed in detail in the present invention are existing technologies in the art.

Claims

1. A closed-loop corrosion-resistant drying system for a methane chloride distillation column, characterized in that, Distillation column (1) is equipped with a drying loop; The drying loop includes a first condenser (2), an alkali washing separator (3), and an azeotropic tower (4) connected in sequence. The vapor phase outlet pipe of the distillation tower (1) is connected to the first condenser (2), and the liquid phase outlet pipe of the azeotropic tower (4) is connected to the distillation tower (1). The alkali washing separator (3) is provided with an alkali feed pipe and a demineralized water feed pipe.

2. The closed-loop corrosion-resistant drying system for a methane chloride distillation column according to claim 1, characterized in that: The outlet pipe of the first condenser (2) is equipped with a reflux tank (5) and a second condenser (6). When the liquid level in the reflux tank (5) reaches 80%, the methane chloride coolant is discharged. The reflux tank (5) is equipped with a reflux pipe connected to the distillation column (1).

3. The closed-loop corrosion-resistant drying system for a methane chloride distillation column according to claim 1, characterized in that: The liquid phase outlet pipe of the azeotropic tower (4) is equipped with a third condenser (7) and a detection tank (8) in sequence.

4. The closed-loop corrosion-resistant drying system for a methane chloride distillation column according to claim 1, characterized in that: The vapor phase outlet pipe of the azeotropic tower (4) is equipped with a fourth condenser (9) and refluxes to the alkaline washing separator (3).

5. The closed-loop corrosion-resistant drying system for a methane chloride distillation column according to claim 3, characterized in that: The outlet pipe of the detection tank (8) is returned to the distillation column (1) and a finished product reflux pump (10) is installed.

6. A drying method for a methane chloride distillation column, characterized in that, Includes the following steps: Step 1: Open the DN50 drain valve at the top of the distillation column (1) and pour 10 kg of 15% alkali solution into the distillation column (1) through the drain port; after the alkali solution is added, connect the deionized water to the top drain port and rinse the inside of the column from top to bottom; when water is discharged from the bottom drain, use pH test paper to monitor the drainage and control the drainage pH value to be >9 to avoid acid corrosion of the equipment. Continue rinsing until the bottom drain water is visually clear, and then stop the flow of deionized water. Step 2: Connect the nitrogen pipeline to the DN50 backflow valve and purge the entire tower with nitrogen from top to bottom for no less than 24 hours; after purging until there is no obvious water flow and the oxygen content of the sampling analysis system is ≤0.5%, disconnect the nitrogen pipeline and close the top DN50 backflow valve; Step 3: Feed the material into the reflux tank (5) to establish a 30% liquid level. Start the reflux pump to transport the material to the top of the distillation column (1), carrying away the residual water in the column. The water-containing material discharged from the bottom of the column is recycled into the ton container for collection and disposal. Introduce dichloromethane into the detection tank (8) to a 50% liquid level. Start the finished product reflux pump (10) to feed the material into the distillation column (1) to establish a 50% liquid level and a reflux flow rate of 10 m³ / h in the column. Continuously circulate and dry in the drying loop. Take samples of the material in the reflux tank (5) for analysis until the moisture content is ≤50 ppm. The azeotropic drying process is then completed.

7. The drying method for a methane chloride distillation column according to claim 6, characterized in that, If the pH value drops during the desalination rinse, a small amount of alkali solution can be added to maintain the pH value > 9.

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