Method for recovering 1, 2-dichloroethane in ethylene chlorination process PVC (polyvinyl chloride) production

Abstract

The invention relates to a method for recovering 1, 2-dichloroethane in polyvinyl chloride (PVC) production by an ethylene chlorination process, which comprises the following steps of: treating a high-boiling residue after the PVC production by the ethylene chlorination process by a first-stage heating kettle, and distilling out a high-value component rich in the 1, 2-dichloroethane; distilled materials of the first-stage steam outlet and the second-stage steam outlet are used as feeding materials of the rectifying tower, 1, 2-dichloroethane is extracted from the top of the rectifying tower, part of tower kettle products are extracted from a tower kettle of the rectifying tower, and the other part of tower kettle liquid is returned to a high-boiling-point substance raw material inlet of the first-stage heating kettle; when the residual liquid amount of the first-stage heating kettle is lower than 20% wt, a valve between tower bottoms and a high-boiling-point material inlet of the first-stage heating kettle is opened, the tower bottoms flow back to the first-stage heating kettle, when the materials in the first-stage heating kettle can be freely stirred, the valve is closed to stop the tower bottoms from flowing back to the first-stage heating kettle, a conveying valve is opened, and the materials in the first-stage heating kettle are conveyed to the second-stage heating kettle; the materials are conveyed into a second-stage heating tank; jelly-shaped waste is discharged from the solid discharging port. The defect that in the prior art, a small amount of a high-boiling-point substance mixture is recycled or directly treated as dangerous waste is overcome.

Description

Technical Field

[0001] This invention relates to the field of 1,2-dichloroethane recovery technology in the ethylene chlorination process for PVC production, specifically to a system and method for recovering 1,2-dichloroethane from ethylene chlorination PVC production. The method involves pretreating a mixture of high-boiling-point substances generated during ethylene chlorination PVC production. The pretreated solid waste is then dried and solidified for recycling, while the liquid portion is subjected to distillation to recover 1,2-dichloroethane with a purity of not less than 80%. Background Technology

[0002] The ethylene chlorination process for PVC production generates a large amount of high-boiling-point substances. This mixture contains a significant amount of valuable 1,2-dichloroethane (boiling point 83.5℃) and other substances. The high-boiling-point substances generated in the ethylene chlorination process differ from those generated in the acetylene process. The high-boiling-point substances from the ethylene chlorination process are primarily composed of 1,2-dichloroethane, while those from the calcium carbide process are mainly 1,1-dichloroethane, which is quite viscous. Previous treatment methods have been unable to address the recovery and utilization of 1,2-dichloroethane, forcing it to be treated as hazardous waste. This results in substantial annual hazardous waste disposal costs for companies.

[0003] The traditional method involves directly feeding high-boiling-point substances (HBSs) generated in the PVC production process of ethylene chlorination into a distillation column. The top portion distills 1,2-dichloroethane, while the bottom portion discharges a liquid-solid mixture. However, in practice, these HBSs become viscous after even slight distillation, making the viscous mixture extremely difficult to handle. They easily clog the distillation column and pipelines, and adhere to the reboiler and heating equipment, making cleaning very difficult. The traditional view is that the high viscosity of HBSs makes distillation uneconomical due to problems like column clogging, coking, and reboiler adhesion, leading all companies to treat them as hazardous waste. Even simple distillation is problematic because it cannot operate continuously for extended periods, maintenance is cumbersome, and the bottom portion remains a mixture being discharged. This doesn't truly utilize the HBSs effectively; it only slightly reduces their quantity, resulting in low recovery efficiency and failing to address the issue of HBSs as hazardous waste. Summary of the Invention

[0004] The purpose of this invention is to address the limitations of current technology by providing a method for recovering 1,2-dichloroethane from ethylene chlorination PVC production.

[0005] To achieve the above objectives, the technical solution of the present invention is as follows:

[0006] A method for recovering 1,2-dichloroethane from ethylene chlorination PVC production, the method using a high-boiling-point pretreatment device and a distillation column, wherein the high-boiling-point pretreatment device performs two-stage heating via a primary heating kettle and a secondary heating tank, and is equipped with a primary steam outlet, a secondary steam outlet, a high-boiling-point feed inlet, and a solid discharge outlet; the primary heating kettle and the secondary heating tank are connected by a conveying pipeline equipped with a conveying valve; a valve is installed between the bottom liquid of the distillation column and the high-boiling-point feed inlet of the primary heating kettle; the method includes the following steps:

[0007] After the high-boiling point of PVC production by ethylene chlorination is treated in a primary heating reactor, the high-value component rich in 1,2-dichloroethane is distilled off from the primary steam outlet.

[0008] The distillate from the primary steam outlet and the secondary steam outlet is used as the feed to the distillation column. 1,2-dichloroethane with a purity of not less than 80% is collected from the top of the distillation column. Part of the bottom product is collected from the bottom of the distillation column, and the other part of the bottom liquid is returned to the high-boiling feed inlet of the primary heating vessel.

[0009] When the remaining liquid volume in the primary heating vessel is less than 20%wt, open the valve between the bottom liquid and the high-boiling-point feed inlet of the primary heating vessel, and the bottom liquid flows back to the primary heating vessel. When the material in the primary heating vessel can be freely stirred, close the valve to stop the bottom liquid from flowing back to the primary heating vessel. Then open the conveying valve to convey the material to the secondary heating tank for further heating treatment.

[0010] When (amount of material entering the secondary heating tank - amount of secondary steam output) / amount of material entering the secondary heating tank is not higher than 10%, jelly-like waste is discharged from the solid discharge port 14.

[0011] Furthermore, the temperature of the distillation column is 50-170℃, the pressure is controlled at 10kPa to 300kPa absolute pressure, and the pressure difference between the top and bottom of the column is 5-15kPa.

[0012] Furthermore, the top temperature of the distillation column is 50-120℃, the bottom temperature is 70-180℃, and the bottom product is a mixture mainly composed of trichloroethylene.

[0013] The temperature inside the primary heating vessel is 50–120℃, and the pressure is 30–70 kPa absolute pressure; the temperature inside the secondary heating tank is 70–150℃, and the pressure is 10 kPa–90 kPa absolute pressure.

[0014] Furthermore, the trichloroethylene content in the mixture extracted from the bottom of the tower is 30%-50% by mass; the 1,2-dichloroethane content in the high-boiling feedstock is above 20%, preferably 25%-55% by mass.

[0015] Furthermore, the distillation column yields 1,2-dichloroethane with a purity of not less than 95%, preferably 95% to 99.9%; the yield of 1,2-dichloroethane is not less than 95%; and the liquid content of the jelly-like waste is 5% to 10% wt.

[0016] Furthermore, the reflux ratio from the distillation column to the primary heating vessel is 2-10.

[0017] Furthermore, the distillation column is provided with multiple feed inlets from top to bottom, and the outlet of the condenser is connected to the multiple feed inlets through corresponding valves. The space above the feed inlets is the rectification section, and the space below the feed inlets is the stripping section. The purity of the product at the top of the column is controlled by changing the position of the feed inlets.

[0018] Furthermore, the stirring speed inside the primary heating vessel is 50-120 r / min.

[0019] This invention also protects a system for recovering 1,2-dichloroethane from ethylene chlorination PVC production. The system includes a high-boiling-point pretreatment device and a distillation column. The pretreatment device includes a primary heating vessel 3, a secondary heating tank 15, and a conveying pipe 8 connecting the primary heating vessel and the secondary heating tank. A primary heating jacket 2 is provided outside the primary heating vessel 3. A high-boiling-point raw material inlet 1 and a primary steam outlet 6 are respectively provided on both sides of the upper part of the primary heating vessel 3. The top of the primary heating vessel 3 is sealed, and a stirring shaft motor 5 is provided on the top of it. A stirring shaft 4 is connected below the stirring shaft motor 5 and extends into the interior of the primary heating vessel 3.

[0020] The conveying pipe 8 is inclined downwards and installed at the lower part of the primary heating vessel 3, and is connected to the secondary heating tank 15. The conveying pipe 8 is equipped with a conveying valve on the side close to the primary heating vessel, and a stirring cage motor 10 is installed on the side connected to the secondary heating tank. A stirring cage conveying shaft 9 is installed inside the conveying pipe 8 and is connected to the stirring cage motor.

[0021] The secondary heating tank 15 is provided with a secondary heating jacket 11 on the outside and a crushing push shaft 13 is provided inside the secondary heating tank 15. The crushing push shaft 13 is connected to the crushing push motor 12. A solid discharge port 14 is provided at the bottom of the secondary heating tank on the side away from the crushing push motor. A secondary steam outlet 16 is provided at the top of the secondary heating tank on the side away from the crushing push motor.

[0022] The primary steam outlet and the secondary steam outlet are connected to the feed inlet of the distillation column. A distillation column condenser is installed at the top of the distillation column. The distillation column condenser is connected to a vacuum device and a product tank at the top of the column. The bottom of the distillation column is connected to a reboiler, and the bottom liquid tank is connected to the product tank at the bottom of the column.

[0023] Furthermore, part of the reboiler of the distillation column is connected to the bottom liquid tank of the distillation column; the bottom liquid tank of the distillation column is connected to the bottom product tank on one side and to the high-boiling-point feed inlet on the other side.

[0024] Furthermore, metering instruments are installed at the secondary steam outlet, primary steam outlet, and conveying pipeline to collect data on the secondary steam output, primary steam output, and material entering the secondary heating tank, respectively; a metering instrument is also installed at the high-boiling-point raw material inlet; the conveying valve is normally closed, and is opened when the primary steam outlet detects that the steam output reaches the set output threshold.

[0025] The amount of liquid returned from the distillation column bottom tank to the primary heating vessel meets the requirement that the amount of liquid remaining after distillation in the primary heating vessel plus the amount of returned liquid can ensure that the stirring shaft rotates normally at the set stirring shaft speed, which is 50-120 r / min.

[0026] The water content in the system is controlled below 100 ppm, preferably 30-50 ppm;

[0027] When (amount of material entering the secondary heating tank - amount of secondary steam output) / amount of material entering the secondary heating tank is not higher than 10%, jelly-like waste is discharged from the solid discharge port 14.

[0028] Furthermore, the liquid content of the jelly-like waste is 5% to 10% wt.

[0029] Furthermore, the temperature inside the primary heating vessel is 50–120°C, and the pressure is 1 kPa absolute to atmospheric pressure; the temperature inside the secondary heating tank is 70–150°C, and the pressure is 1 kPa absolute to atmospheric pressure.

[0030] Furthermore, the pressure inside the primary heating vessel is 30–70 kPa absolute pressure; the temperature inside the secondary heating tank is 70–150 °C, and the pressure is 10–90 kPa absolute pressure.

[0031] Furthermore, the distillation column is provided with multiple feed inlets from top to bottom, and the outlet of the condenser is connected to the multiple feed inlets through corresponding valves. The space above the feed inlets is the rectification section, and the space below the feed inlets is the stripping section. The purity of the product at the top of the column is controlled by changing the position of the feed inlets.

[0032] Furthermore, the product tank at the top of the tower yields 1,2-dichloroethane with a purity of not less than 80%, preferably 95% to 99.9%; the yield of 1,2-dichloroethane is not less than 95%; and the mass content of 1,2-dichloroethane in the high-boiling feedstock is above 20%, preferably 25% to 55%.

[0033] Furthermore, the height of the trays or packing in the rectification section of the distillation column is 3 to 50 layers or 1 to 15 meters, and the height of the trays or packing in the stripping section is 3 to 50 layers or 1 to 15 meters.

[0034] The height of the overflow weir on the tray of the distillation column is 20–400 mm, preferably 100–400 mm.

[0035] Furthermore, stirring blades are provided on the stirring shaft. The stirring blades are arranged in layers along the height direction of the primary heating vessel. During the rotation of the stirring blades, they do not touch the wall of the primary heating vessel, but can stir all the liquid in the primary heating vessel, thus avoiding the formation of dead zones in the liquid stirring.

[0036] Alternatively, a spiral stirring blade is provided on the stirring shaft along the height direction. The stirring blade's range of action is to stir all the liquid in the primary heating vessel without hitting the wall, thus avoiding dead zones in the liquid stirring.

[0037] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0038] The pretreatment device in this invention consists of two parts: primary heating and secondary heating. After treatment in the primary heating vessel, a large amount of liquid can be rapidly vaporized and extracted from the primary steam outlet. Then, while maintaining the fluidity of the primary heating vessel itself, the material after the large amount of liquid has been removed from the primary heating vessel is sent to the secondary heating tank. The steam generated during the heating process in the secondary heating tank is extracted through the secondary steam outlet, or the steam can be condensed using condenser 109 before the liquid is sent to distillation column 102. When the liquid content of the material in the secondary heating tank is less than 10%, a jelly-like solid is formed and discharged from the solid outlet. This jelly-like solid has advantages such as elasticity, agglomeration, easy breakage, and non-sticking to the vessel, ensuring the long-term operation of the device and system.

[0039] The pretreatment device of this invention adopts a horizontal two-stage heating tank with the crushing and pushing shaft positioned lower to prevent coking at the bottom of the heating tank. Simultaneously, the up-and-down rotation of the crushing and pushing shaft ensures uniform coking of the material, ultimately forming a jelly-like solid that is easier to push and avoids wall adhesion. When the liquid content meets the pushing requirements at approximately 5-10% wt, and the liquid-to-material ratio in the secondary heating tank is approximately 90-95%, the solid waste is promptly pushed out. The larger heating area facilitates solidification in a shorter time, whereas vertical designs require longer heating times, do not contact the material at the top, and have reduced heating efficiency. Higher heating efficiency results in a smaller area of ​​ineffective heating.

[0040] In this invention, under suitable temperature and pressure control, the required flowability and evaporation rate in the pretreatment device can be guaranteed. The initial evaporation temperature of the primary heating kettle is approximately 50°C. Operation is carried out under normal or negative pressure. During negative pressure operation, it is crucial to prevent system leaks. If a leak occurs, moisture from the air can easily enter the system, causing significant corrosion of the equipment and pipelines. Therefore, temperature and pressure adjustments are used to control the water content within the system to below 100 ppm. If the temperature inside the device is too high, decomposition into HCl can easily occur, leading to material loss and reduced yield. Furthermore, the generated gaseous impurities can corrode the pipelines. Therefore, it is necessary to control the device's operation within a suitable temperature range, preferably 50-120°C.

[0041] This invention provides a system and method that solves the problem of frequent blockages and shutdowns caused by directly feeding the high-boiling-point mixture generated in the ethylene chlorination PVC production process into a distillation column. The system can recover 1,2-dichloroethane from the high-boiling-point mixture into 1,2-dichloroethane with a purity of not less than 99.5%, which can be used directly as a raw material, with a yield of 95%. The yield is calculated as: (mass of recovered 1,2-dichloroethane) / (mass of 1,2-dichloroethane in the high-boiling-point mixture).

[0042] This invention features low equipment investment, low cost, simple operation, high operational flexibility, and can increase the throughput by at least 40% for the same volume diameter. At the same time, it solves the problems of serious material entrainment, frequent blockage of trays or packing in traditional processes, inability to operate continuously for long periods of time, and difficulty in maintenance.

[0043] Traditional processes require shutdowns and maintenance every 1-2 months. This is because high-boiling-point mixtures inevitably enter the distillation column, impacting the gas-liquid phase channels of the trays and packing, causing blockages. Each cleaning is extremely complex, requiring pipe unclogging, dismantling of the column structure, cleaning, unclogging again, and finally restoration – a very cumbersome process. This invention introduces a pretreatment device that makes the solids discharged from the solid outlet easily moldable, non-sticky to fabric and the vessel, fragile, and easy to handle, extending the maintenance cycle to 1-2 years. This significantly solves a major pain point for enterprises.

[0044] This invention transforms hazardous waste into qualified, reusable products. The recovered high-purity 1,2-dichloroethane can be directly used as a raw material in the chlorination production of PVC. The solid portion from the solid discharge port can be reused as carbon powder, improving both environmental and economic benefits for enterprises. Based on an annual production scale of 10,000 tons of high-boiling-point substances, this method can save enterprises at least 20 million yuan annually in hazardous waste treatment costs, and transform these hazardous wastes into products worth 10-11 million yuan. It overcomes the shortcomings of existing technologies that only recover small amounts of high-boiling-point mixtures or treat them directly as hazardous waste. Attached Figure Description

[0045] Figure 1 This is a schematic flowchart of one embodiment of the method for recovering 1,2-dichloroethane in PVC production using the ethylene chlorination process of the present invention.

[0046] Figure 2 This is a schematic diagram of a high-boiling-point pretreatment device according to an embodiment of the present invention.

[0047] Figure 3 This is a schematic diagram of one embodiment of the system for recovering 1,2-dichloroethane in the ethylene chlorination process of PVC production according to the present invention.

[0048] Figure 4 This is a schematic diagram of one embodiment of the system for recovering 1,2-dichloroethane in the ethylene chlorination process of PVC production according to the present invention.

[0049] In the diagram, 101 is the high-boiling-point pretreatment unit, 102 is the distillation column, 103 is the distillation column reboiler, 104 is the distillation column condenser, 105 is the feed tank, 106 is the top product tank, 107 is the bottom liquid tank, 108 is the vacuum equipment, 109 is the condenser, and 110 is the bottom product tank. Detailed Implementation

[0050] The present invention will be further explained below with reference to embodiments and accompanying drawings, but this is not intended to limit the scope of protection of this application. It should be noted that, unless otherwise specified, the embodiments and features in the embodiments of the present invention can be combined with each other.

[0051] Example 1

[0052] This embodiment describes a method for recovering 1,2-dichloroethane from ethylene chlorination PVC production. It utilizes a high-boiling-point pretreatment unit and a distillation column. The high-boiling-point mixture from the ethylene chlorination PVC production is processed in the pretreatment unit and separated into liquid and solid components. The liquid component is then processed in the distillation column, yielding 1,2-dichloroethane at the top and a mixture primarily composed of trichloroethylene at the bottom. The solid products are carbon powder or chlorinated pitch, etc.

[0053] The operating conditions of the high-boiling-point pretreatment device 101 are: pressure 1 kPa absolute to 300 kPa absolute, temperature 50-140℃.

[0054] The overall temperature of the distillation column can be controlled within the range of 50-170℃, preferably 80-140℃, and the pressure is 10kPa absolute to 300kPa absolute.

[0055] Example 2

[0056] The system for recovering 1,2-dichloroethane from ethylene chlorination PVC production in this embodiment includes a high-boiling-point pretreatment unit 101 and a distillation column 102.

[0057] Among them, the high-boiling-point pretreatment device is as follows: Figure 2 As shown, it includes a high-boiling-point raw material inlet 1; a primary heating jacket 2; a primary heating vessel 3; a stirring shaft 4; a stirring shaft motor 5; a primary steam outlet 6; a conveying valve 7; a conveying pipeline 8; a stirring cage conveying shaft 9; a stirring cage motor 10; a secondary heating jacket 11 with steam or heat transfer oil inside; a crushing and pushing motor 12; a crushing and pushing shaft 13; a solid discharge port 14; a secondary heating tank 15; and a secondary steam outlet 16.

[0058] The positional relationships are as follows: a primary heating jacket 2 is installed outside the primary heating vessel 3; the high-boiling-point feedstock inlet 1 and the primary steam outlet 6 are both located on the upper side of the primary heating vessel 3, but in different orientations. A stirring shaft motor 5 is installed at the top of the primary heating vessel 3, and a stirring shaft 4 is connected below the stirring shaft motor 5, extending into the interior of the primary heating vessel 3. The conveying pipe 8 is located at the lower part of the primary heating vessel 3, and the conveying valve 7 is located in the upper middle part of the conveying pipe 8. The stirring cage conveying shaft 9 is located inside the conveying pipe 8 and below the conveying valve 7, with the stirring cage motor 10 connected behind the stirring cage conveying shaft 9. The primary steam outlet 6 delivers steam to the distillation column 102.

[0059] The secondary heating tank 15 is equipped with a secondary heating jacket 11 on the outside and a crushing and pushing shaft 13 inside. The bottom is a solid discharge port 14. When the material is heated in the secondary heating tank 15, the crushing and pushing shaft 13 rotates to crush larger solids into smaller pieces and form a jelly-like solid. After heating, the crushing and pushing shaft 13 sends the jelly-like solid out of the equipment through the solid discharge port 14. The steam generated during the heating process is extracted through the secondary steam outlet 16.

[0060] Steam or heat transfer oil is circulated inside the primary heating jacket 2 and the secondary heating jacket 11.

[0061] The primary steam outlet and the secondary steam outlet can be directly or indirectly connected to the feed inlet of the distillation column. A distillation column condenser 104 is installed at the top of the distillation column. The distillation column condenser 104 is connected to the vacuum equipment 108 and the top product tank 106. The bottom of the distillation column is connected to the distillation column reboiler 103. Part of the product from the distillation column reboiler is connected to the bottom liquid tank 107. The bottom liquid tank is connected to the bottom product tank 110 on one side and to the high-boiling-point feed inlet 1 on the other side.

[0062] The product tank 106 at the top of the tower yields 1,2-dichloroethane with a purity of not less than 95%.

[0063] In the primary heating vessel 3, due to the large amount of liquid and the low viscosity of the solid-liquid mixture, a large amount of liquid can be rapidly vaporized and sent to the distillation column. While maintaining its fluidity, the material after removing a large amount of liquid is sent to the secondary heating tank 15. Ensuring fluidity and evaporating the liquid phase requires the primary heating vessel to operate at a specific temperature and pressure for a certain period of time. The temperature in the primary heating vessel 3 is 50-120℃, the pressure is 1 kPa absolute to atmospheric pressure, and the heating time is 2-72 hours after the material enters the vessel.

[0064] Then, in the secondary heating tank 15, the material, being relatively viscous, solidifies under the rotation of the crushing and pushing shaft 13, transforming into smaller solids suitable for subsequent processing. These solids have a jelly-like consistency. The secondary heating tank 15 is located below the primary heating vessel 3, connected in the middle by a conveying pipe 8, with the conveying pipe 8 forming an angle of 10° to 90° with the horizontal plane. The material flows into the secondary heating tank 15 under the agitation of the stirring conveyor shaft 9, and the steam generated during the heating process is sent to the distillation column 102 via the secondary steam outlet 16. The entire secondary heating process requires specific operating conditions to ensure the formation of a jelly-like solid that can be crushed, pushed, and does not stick to the walls. The temperature in the secondary heating tank 15 is 70-150℃, the pressure is 1 kPa absolute to atmospheric pressure, and the heating time is 12-108 hours after entering the vessel.

[0065] In this invention, the temperature of the secondary heating tank can be slightly higher than the temperature of the primary heating vessel, and the two can also remain equal within the above temperature range.

[0066] During the process of conveying materials from the primary heating vessel to the secondary heating tank, the material cannot be too dry, otherwise the flowability cannot be guaranteed. A large amount of liquid is needed to maintain the fluidity of the entire material. However, this contradicts the goal of distilling a large amount of 1,2-dichloroethane from the primary heating vessel. Therefore, in this embodiment, a portion of the liquid from the bottom of the distillation column is introduced to ensure fluidity. The bottom liquid of the distillation column contains a small amount of 1,2-dichloroethane, which provides an opportunity for secondary distillation of this portion of 1,2-dichloroethane. This not only provides fluidity but also increases the yield of 1,2-dichloroethane.

[0067] Example 3

[0068] This embodiment describes a system for recovering 1,2-dichloroethane from PVC production via the ethylene chlorination process. The connections are the same as in Embodiment 2, except that this embodiment includes a condenser 109 and a feed tank 105. The high-boiling-point pretreatment device 101 and the distillation column are both connected to a vacuum system (see [link to documentation]). Figure 3Both the primary and secondary steam outlets are connected to the hot material inlet of condenser 109 via corresponding pipelines. The material after heat exchange in condenser 109 is conveyed to the feed inlet of the distillation column. The top of the condenser is connected to a vacuum device 108. The high-boiling-point feedstock inlet 1 is connected to a feed tank 105 via a pipeline. The high-boiling-point mixture generated in the previous process (waste from ethylene chlorination PVC production) is sent to the feed tank for storage. The outlet of the feed tank is connected to the high-boiling-point feedstock inlet 1 of the high-boiling-point pretreatment device. The distillation column condenser is also connected to the vacuum device 108, thereby maintaining the entire system under vacuum negative pressure.

[0069] The system's working process is as follows:

[0070] 1) The high-boiling-point mixture material is pumped from the feed tank 105 into the high-boiling-point pretreatment unit 101. After the liquid part of the material is vaporized in the high-boiling-point pretreatment unit 101, it is condensed by the condenser and enters the distillation column 102. The distillation column is heated by the distillation column reboiler 103 and cooled by the distillation column condenser 104. The top part of the product is refluxed and drawn into the top product tank 106. The bottom product is pumped into the distillation column bottom liquid tank 107. Part of the liquid in the distillation column bottom liquid tank 107 is returned to the high-boiling-point pretreatment unit 101 as reflux liquid, and part of it is entered into the bottom product tank 110 as bottom product. The vacuum equipment 108 provides a negative pressure environment for the system.

[0071] 2) The high-boiling-point pretreatment device 101 operates under negative or normal pressure and is equipped with auxiliary components such as a stirring shaft and a crushing and pushing shaft. The high-boiling-point mixture enters the pretreatment device through the high-boiling-point raw material inlet 1. In the primary heating vessel 3, a large amount of liquid evaporates under the heating of the primary heating jacket 2 and enters the distillation column through the primary steam outlet 6. When the requirements are met, the conveying valve 7 opens, and the more viscous material enters the secondary heating tank 15 through the conveying pipe 8 with the help of the stirring cage conveying shaft 9. Under the heating of the secondary heating vessel jacket 11, the material becomes dehydrated and shaped. The liquid evaporated from the secondary steam outlet 16 also enters the distillation column. In the secondary heating tank 15, through the control of pressure, temperature, and time, the solid product is controlled at a certain humidity level, thus obtaining a solid with good properties, avoiding the defects of traditional processes. With the help of the crushing and pushing shaft 13, the solid product enters the solid discharge port 14 to obtain the solid product.

[0072] In this embodiment, the reflux ratio in distillation column 102 is 0.3 to 10:1. The height of the trays or packing in the rectifying section of the distillation column is 3 to 50 layers or 1 to 15 meters, and the height of the trays or packing in the stripping section of the distillation column is 3 to 50 layers or 1 to 15 meters.

[0073] The operating conditions of the high-boiling-point pretreatment device 101 are: pressure 10 kPa absolute to atmospheric pressure, temperature 50-120℃.

[0074] The operating conditions for distillation column 102 are: top temperature 50-120℃, bottom temperature 70-180℃, the lower the temperature the better, the entire column is operated under negative pressure, and the pressure difference between the top and bottom of the column can be maintained at 5-15 kPa.

[0075] The bottom product of the tower is a mixture mainly composed of trichloroethylene, wherein the mass content of trichloroethylene in the mixture is 30%-50%. The mass content of 1,2-dichloroethane in the high-boiling feedstock is above 20%, preferably 25%-55%.

[0076] This embodiment incorporates a condenser 109, allowing the entire system to be used in a segmented manner. This separates the pretreatment process from the distillation column's processing. If corrosion occurs, the entire production line does not need to be shut down; only partial repairs are possible, reducing subsequent maintenance costs and facilitating industrial production. Without a condenser, operating the high-boiling-point pretreatment unit necessitates operating the distillation column, requiring the use of the distillation column's top condenser. Considering the system's requirements for sealing, waterproofing, and acid corrosion prevention, a separate condenser 109 is a more rational approach.

[0077] Example 4

[0078] This embodiment describes a system for recovering 1,2-dichloroethane from PVC production via the ethylene chlorination process. In the high-boiling-point pretreatment unit 101, stirring blades are installed on the stirring shaft 4. These blades are arranged in layers along the height of the primary heating vessel. During rotation, the blades neither touch the wall of the primary heating vessel nor disrupt the flow of liquid, thus avoiding dead zones. Alternatively, spiral stirring blades can be installed along the height of the stirring shaft. These blades, when in use, stir the liquid in the primary heating vessel without hitting the walls, also preventing dead zones.

[0079] The primary and secondary heating jackets are heated by circulating steam or heat transfer oil. The upper end of the primary heating jacket is located below the primary steam outlet, and its height is not less than 1 / 4 of the height of the primary heating vessel body. The upper end of the secondary heating jacket is at the same height as the upper end of the secondary heating tank. The crushing and pushing shaft is located at the lower part of the secondary heating tank 15, and does not contact the wall of the secondary heating tank. The crushing and pushing shaft is arranged horizontally, and the secondary heating tank is a horizontal tank.

[0080] When the secondary heating tank heats the material, the crushing and pushing shaft 13 rotates and flips the material up and down. When pushing the material, the crushing and pushing shaft pushes the material out. This pushed material is in the form of jelly and is treated as waste.

[0081] Example 5

[0082] In this embodiment, the high-boiling-point pretreatment device 101 is equipped with metering instruments at the secondary steam outlet, primary steam outlet, and conveying pipeline to collect the secondary steam output, primary steam evaporation output, and material quantity entering the secondary heating tank, respectively. A metering instrument is also installed at the high-boiling-point raw material inlet. The conveying valve is normally closed. When the primary steam outlet detects that the steam evaporation output reaches the set evaporation threshold (in this embodiment, the evaporation threshold is set to 80% of the total added liquid volume), the conveying valve is opened.

[0083] The reflux liquid flow from the distillation column reboiler to the primary heating vessel must meet the following requirements: the remaining liquid volume after distillation in the primary heating vessel plus the reflux liquid volume must be sufficient to ensure that the stirring shaft rotates normally at the current stirring shaft speed of 50-120 r / min. When this requirement is met, the valve on the pipeline connecting the distillation column reboiler to the high-boiling-point feed inlet can be closed. If this requirement is not met, this valve will be opened to provide reflux liquid to the primary heating vessel. This valve is usually normally closed and will be opened when the set requirements are met.

[0084] Once the return liquid volume meets the requirements, close the valve on the pipeline connecting the distillation column bottom liquid tank and the high-boiling-point raw material inlet, and open the conveying valve 7 to supply material to the secondary heating tank.

[0085] The water content in the system is controlled below 100 ppm, preferably 30-50 ppm;

[0086] When (amount of material entering the secondary heating tank - amount of secondary steam output) / amount of material entering the secondary heating tank is not higher than 10%, jelly-like waste is discharged from the solid discharge port 14.

[0087] Example 6

[0088] This embodiment describes a method for recovering 1,2-dichloroethane from PVC production via the ethylene chlorination process. The method involves pretreatment, solidification, crushing, and distillation of a high-boiling-point mixture containing a large amount of high-boiling-point substances to recover 1,2-dichloroethane and solid products. A high-boiling-point pretreatment unit and a distillation column are used. The high-boiling-point pretreatment unit employs two-stage heating via a primary heating kettle and a secondary heating tank. It is equipped with a primary steam outlet, a secondary steam outlet, a high-boiling-point raw material inlet, and a solid discharge outlet. The primary heating kettle and the secondary heating tank are connected by a conveying pipeline equipped with a conveying valve. Specifically:

[0089] After the high-boiling point of PVC production by ethylene chlorination is treated in a primary heating reactor, the high-value component rich in 1,2-dichloroethane is distilled off from the primary steam outlet.

[0090] The distillate from the primary steam outlet and the secondary steam outlet is used as the feed to the distillation column. 1,2-dichloroethane with a purity of not less than 95% is collected from the top of the distillation column. Part of the bottom product is collected from the bottom of the distillation column, and the other part of the bottom liquid is returned to the high-boiling feed inlet of the primary heating vessel.

[0091] When the remaining liquid volume in the primary heating vessel is less than 20%wt, open the valve between the bottom liquid and the high-boiling-point feed inlet of the primary heating vessel, and the bottom liquid flows back to the primary heating vessel. When the material in the primary heating vessel can be freely stirred at the current stirring speed, close the valve to stop the bottom liquid from flowing back to the primary heating vessel. Then open the conveying valve to convey the material to the secondary heating tank for further heating treatment.

[0092] When (amount of material entering the secondary heating tank - amount of secondary steam output) / amount of material entering the secondary heating tank is not higher than 10% wt, jelly-like waste is discharged from solid outlet 14;

[0093] The water content in the entire system is controlled to be below 100 ppm by operating under negative pressure.

[0094] Example 7

[0095] The system for recovering 1,2-dichloroethane from ethylene chlorination PVC production in this embodiment mainly includes a high-boiling-point pretreatment unit 101 and a distillation column 102.

[0096] The high-boiling-point pretreatment device 101 includes a primary heating vessel 3, a secondary heating tank 15, and a conveying pipeline connecting the primary heating vessel and the secondary heating tank; heating jackets are provided outside the primary heating vessel 3 and the secondary heating tank, and steam or heat transfer oil is circulated inside the two heating jackets.

[0097] A stirring shaft driven by a motor is installed inside the primary heating vessel. A high-boiling-point raw material inlet is installed at the top of the primary heating vessel, which is connected to the high-boiling-point mixture produced by the ethylene chlorination process PVC production in the upper stage. At the same time, a primary steam outlet 6 is installed at the top of the primary heating vessel. A conveying valve 7 is installed on the conveying pipe 8 near the primary heating vessel. The side of the conveying pipe near the secondary heating tank is connected to the secondary heating tank, and a stirring conveying shaft driven by a stirring cage motor is installed inside the conveying pipe.

[0098] A crushing push shaft 13 driven by a crushing push motor 12 is installed in the secondary heating tank. A secondary steam outlet 16 is installed at the upper part of the secondary heating tank on the side away from the conveying pipeline, and a solid discharge port 14 is installed at the lower part.

[0099] The primary steam outlet and the secondary steam outlet are connected to the feed inlet of the distillation column via condensers. A distillation column condenser 104 is installed at the top of the distillation column. The distillation column condenser 104 is connected to a vacuum device 108 and a top product tank 106. The bottom of the distillation column is connected to a reboiler 103. Part of the product from the reboiler is connected to a bottom liquid tank 107. The bottom liquid tank is connected to the bottom product tank 110 on one side and to the high-boiling-point feed inlet 1 on the other side.

[0100] The distillation column 102 includes a rectification section and a stripping section; it can be a plate distillation column, a packed distillation column, or a combined distillation column, etc. The top of the distillation column 102 produces recyclable 1,2-dichloroethane, and the bottom of the column produces products such as trichloroethylene.

[0101] The tray or packing height of the rectifying section of distillation column 102 is 10–40 layers or 3–12 meters, and the tray or packing height of the stripping section is 10–40 layers or 3–12 meters. Multiple feed inlets are provided on the distillation column from top to bottom. The outlet of the condenser 109 is connected to these multiple feed inlets via corresponding valves. The space above the feed inlets is the rectifying section, and the space below the feed inlets is the stripping section. The purity of the top product is controlled by changing the position of the feed inlets.

[0102] The height of the overflow weir on the tray of the distillation column is 20–400 mm, preferably 100–400 mm.

[0103] Example 8

[0104] This embodiment describes a system for recovering 1,2-dichloroethane from ethylene chlorination PVC production, such as... Figure 4 As shown, in this embodiment, no reflux is set between the high-boiling-point pretreatment device and the distillation column. At this time, when the remaining liquid in the primary heating vessel is about 30%, the conveying valve can be opened to carry out the secondary heating tank treatment.

[0105] In this embodiment, the recovery rate of 1,2-dichloroethane is over 80% (the recovery rate of 1,2-dichloroethane in existing systems is generally only around 40%). In comparison, adding the bottom liquid of the distillation column as reflux liquid to the high-boiling-point pretreatment unit can significantly improve the overall recovery rate of 1,2-dichloroethane, which can reach over 95%.

[0106] Example 9

[0107] This embodiment describes a method for recovering 1,2-dichloroethane from PVC production via the ethylene chlorination process. The distillation column 102 is made of stainless steel with 4 meters of packing. The high-boiling-point mixture of raw materials contains approximately 50% 1,2-dichloroethane by mass. The process is as follows:

[0108] 1) The high-boiling-point mixture enters the high-boiling-point pretreatment unit 101. In the high-boiling-point pretreatment unit, the liquid material vaporizes and enters the distillation column 102 in gaseous or liquid form (liquid form requires condenser 109). The viscous material is solidified, broken, and pulverized. The high-boiling-point pretreatment unit is divided into two stages. The primary function of the primary heating vessel is to collect large quantities of high-value components such as 1,2-dichloroethane after vaporization. The primary function of the secondary heating tank is to solidify the material into a jelly-like solid product with a certain moisture content. The jelly-like material agglomerates into a solid, has a certain elasticity, and can be broken into smaller solids for easier subsequent processing.

[0109] In this embodiment, the temperature inside the primary heating vessel is 60°C and the pressure is 30 kPa absolute; or 80°C and 70 kPa absolute pressure. The temperature, pressure, and processing time of the secondary heating tank are 30 kPa absolute pressure, 110°C, and 20 hours; or 50 kPa absolute pressure, 95°C, and 24 hours. The vacuum levels in the primary heating vessel, secondary heating tank, and distillation column can be controlled independently.

[0110] 2) The liquid vaporized in the high-boiling-point pretreatment unit 101 enters the distillation column 102, and is collected by distillation to separate 1,2-dichloroethane. The top product of the distillation column 102 is 1,2-dichloroethane, and the bottom product of the distillation column 102 is a mixture of trichloroethylene and other substances.

[0111] The distillation column was operated at an absolute pressure of 70 kPa, a top temperature of 73°C, 20 theoretical plates, and a reflux ratio of 5 to obtain 1,2-dichloroethane with a purity of 99.5%.

[0112] Example 10

[0113] In this embodiment, the pressure and temperature of the secondary heating tank are maintained at 30 kPa absolute pressure and 110°C. The mass content of 1,2-dichloroethane in the high-boiling mixture raw material to be processed is about 50%. The top temperature of the distillation column is 57°C, the bottom temperature is 100°C, the top pressure is 60 kPa absolute pressure, and the bottom pressure is 70 kPa absolute pressure. Under the same temperature and pressure control of the secondary and primary heating tanks, and while keeping the temperature and pressure operating conditions in the distillation column constant, the reflux ratio from the distillation column to the primary heating vessel is changed, and the purity of 1,2-dichloroethane at the top of the column is detected.

[0114] When the reflux ratio is 2-4, the purity of 1,2-dichloroethane at the top of distillation column 102 is not less than 95%.

[0115] When the reflux ratio is 8-9, the purity of 1,2-dichloroethane at the top of distillation column 102 is not less than 99%.

[0116] 1,2-Dichloroethane with a purity of 95%-99.9% can be used directly as a raw material, and the recovery rate of 1,2-Dichloroethane is >95%.

[0117] A higher reflux ratio results in higher purity of dichloroethane, but also higher energy consumption. In actual production, the ratio can be adjusted according to the manufacturer's requirements. For example, in some cases, if the manufacturer wants 95% pure dichloroethane, a reflux ratio of 2 can meet the requirements.

[0118] Example 11

[0119] In this embodiment, the reflux ratio from the distillation column to the primary heating vessel is set to 3. The temperature and pressure at the top of the distillation column are controlled. When the pressure inside the column is 70 kPa absolute, the temperature at the bottom of the column is 80°C, and the temperature at the top of the column is 70°C, the temperature difference between the bottom and the top of the column is small. The purity of 1,2-dichloroethane at the top of distillation column 102 is about 80%, but the overall yield is still above 95%.

[0120] When the pressure inside the column is 70 kPa absolute, the temperature at the bottom of the column is 90℃, and the temperature at the top of the column is 72℃, the purity of 1,2-dichloroethane at the top of distillation column 102 is about 99.5%, and the overall yield is still above 95%.

[0121] This invention abandons the traditional distillation route of directly feeding solid-liquid mixtures into the distillation column. The solid output is a jelly-like solid with good breakability. It can maintain the solid form while having a certain degree of elasticity, which can be further crushed and transported, avoiding solid coking and sticking to the vessel. This brings convenience to subsequent processing and completely changes the industry's process route. If the humidity is too high, it will stick to the vessel; if the humidity is too low, it will harden.

[0122] This invention employs a process route combining a pretreatment device and a distillation column, and innovatively incorporates a bottom reflux process in the distillation column to further improve the yield and purity of 1,2-dichloroethane, achieving process indicators of a 1,2-dichloroethane yield greater than 95% and a purity greater than 99.5%. Under the set operating conditions, high-boiling-point mixtures can be separated into solid and liquid products, achieving triple benefits in terms of social, economic, and environmental impact.

[0123] This invention, through multiple experiments, utilizes a combination of equipment and control conditions to achieve a discharge of solids that are easy to shape, non-sticky, non-adhesive to the vessel, fragile, and easy to handle. The stirring blades of the stirring shaft do not touch the vessel wall while ensuring thorough mixing, avoiding dead zones in the liquid. The primary heating vessel is heated from top to bottom, and since the heating medium generally enters from the top, the upper part of the vessel has a higher temperature. This stirring shaft configuration ensures thorough mixing along the entire height, maintaining the material's fluidity and preventing coking on the vessel wall. In the secondary heating tank, through controlled operating conditions, the liquid content of the discharged material is maintained between 5% and 10% wt, forming a jelly-like solid. This solid is neither too dry nor too wet, maintaining a certain level of moisture to prevent complete drying and adhesion to the vessel wall. It also possesses a certain degree of moisture and elasticity, making it easily breakable for subsequent processing. Furthermore, this processing method achieves more thorough separation of 1,2-dichloroethane, a higher recovery rate, and a purity that meets the 99.5% raw material requirement for direct reuse.

[0124] The initial evaporation temperature of the primary heating vessel of this invention is 50°C. If the pressure is too low, the entire system will leak, and moisture from the air will enter the system. Therefore, it is necessary to control the system pressure within a suitable range so that the system water content is controlled below 100ppm to avoid pipeline corrosion. It is preferable to operate the system at a lower temperature so that the water content in the system is controlled within the range of 30-50ppm. If the pressure is too high, the temperature will be too high, which will easily cause cracking, loss of materials, and generation of gaseous impurities. The HCl gas generated by cracking will be extremely corrosive in the presence of water.

[0125] The entire system of this invention can control temperature and pressure as a whole, or monitor them individually. Each device can also be equipped with monitoring devices for temperature, pressure, liquid level, etc., according to actual needs. Both inlet and outlet liquids are metered, realizing the integration of the distillation column and the high-boiling-point pretreatment device.

[0126] Inter-agency control.

[0127] Matters not covered in this invention are common knowledge.

Claims

1. A process for the recovery of 1,2-dichloroethane in the production of PVC by the ethylene chlorination process, characterized in that, The method uses a high-boiling pretreatment device and a rectifying column, the high-boiling pretreatment device is heated by a first heating kettle and a second heating tank in two stages, a first steam outlet, a second steam outlet, a high-boiling raw material inlet and a solid discharge port are arranged, the first heating kettle and the second heating tank are connected by a conveying pipeline, a conveying valve is arranged on the conveying pipeline, a valve is arranged between the column liquid of the rectifying column and the high-boiling raw material inlet of the first heating kettle, and the method comprises the following steps: The high-boiling substance after the production of PVC by the ethylene chlorination method is treated by the first heating kettle, and high-value components rich in 1,2-dichloroethane are evaporated from the first steam outlet; The evaporated materials of the first steam outlet and the second steam outlet are used as the feed of the rectifying column, 1,2-dichloroethane with a purity of not less than 80% is collected from the top of the rectifying column, and part of the column liquid is collected from the bottom of the rectifying column, and the other part of the column liquid returns to the high-boiling raw material inlet of the first heating kettle; When the residual liquid amount in the first heating kettle is less than 20% wt, the valve between the column liquid and the high-boiling raw material inlet of the first heating kettle is opened, the column liquid returns to the first heating kettle, the valve is closed to stop the column liquid from returning to the first heating kettle when the materials in the first heating kettle can be freely stirred, then the conveying valve is opened, and the materials are conveyed to the second heating tank for further heating treatment; When the amount of materials entering the second heating tank minus the amount of steam outlet is not more than 10% of the amount of materials entering the second heating tank, the solid discharge port 14 discharges the jelly-like waste.

2. The method of claim 1, wherein, The temperature of the rectifying column is 50-170℃, the pressure is controlled at 10kPa-300kPa absolute pressure, and the pressure difference between the top and the bottom of the column is 5-15kPa.

3. The method of claim 2, wherein, The temperature of the top of the rectifying column is 50-120℃, the temperature of the bottom of the rectifying column is 70-180℃, and the mixture collected from the bottom of the rectifying column mainly contains trichloroethylene; The temperature in the first heating kettle is 50-120℃, and the pressure is 30-70kPa absolute pressure; the temperature in the second heating tank is 70-150℃, and the pressure is 10kPa-90kPa absolute pressure.

4. The method of claim 3, wherein, The mass content of trichloroethylene in the mixture collected from the bottom of the column is 30%-50%; the mass content of 1,2-dichloroethane in the high-boiling raw material is more than 20%, and is preferably 25%-55%.

5. The method of claim 1, wherein, The 1,2-dichloroethane collected from the top of the rectifying column has a purity of not less than 95%, and the purity is preferably 95%-99.9%; the yield of 1,2-dichloroethane is not less than 95%; and the liquid content of the jelly-like waste is 5%-10% wt.

6. The method of claim 1, wherein, The reflux ratio of the column liquid returning to the first heating kettle from the rectifying column is 2-10.

7. The method of claim 1, wherein, A plurality of feed inlets are arranged on the rectifying column from top to bottom, the outlet of the condenser is connected to the plurality of feed inlets through corresponding valves, the space of the rectifying column above the feed inlets is a rectifying section, and the space of the rectifying column below the feed inlets is a stripping section, and the purity of the product at the top of the column is controlled by changing the position of the feed inlets.

8. The method of claim 1, wherein, The stirring speed in the first heating kettle is 50-120r / min.

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