A vacuum device for a caprolactam production aminocapronitrile process

Abstract

The application discloses a vacuum device for a caprolactam production aminocapronitrile process, which comprises a gas washing device, a Roots blower and a liquid ring vacuum pump, and the core is the liquid ring vacuum pump, and the working liquid is aminocapronitrile. The gas feed of the vacuum device is the gas extracted after condensation of the evaporated material at the top of a caprolactam recovery column in the process. The application can solve the engineering problem of blockage of the vacuum system caused by easy crystallization of caprolactam under high vacuum, realize long-period stable operation, and is low in energy consumption, free of waste liquid discharge, and suitable for continuous industrial production of aminocapronitrile by a caprolactam method.

Description

Technical Field

[0001] This invention relates to the field of vacuum devices in chemical production equipment, specifically a high-efficiency vacuum device for the caprolactam process to produce aminohexanonitrile. Background Technology

[0002] In the process of producing aminocaprolactam using the caprolactam method, evaporation and distillation units typically need to be carried out under reduced pressure or high vacuum conditions to lower the operating temperature and improve separation efficiency. Since the process gas often carries a certain amount of caprolactam vapor, especially at the top of the caprolactam recovery tower where the caprolactam content is high, the stability and anti-crystallization capabilities of the vacuum equipment are crucial.

[0003] Caprolactam has a high melting point (around 70°C) and readily crystallizes under cooling conditions. When caprolactam vapor enters vacuum equipment, slight fluctuations in temperature control or the presence of localized supercooling areas can cause crystallization, clogging the equipment and easily leading to vacuum system failure. Especially under high vacuum conditions with high gas velocities (typically >20 m / s), entrainment is severe, and it is inevitable that some caprolactam vapor will enter the vacuum pump and come into contact with the working fluid.

[0004] Dry vacuum pumps (such as claw pumps) are sensitive to crystalline materials and are prone to jamming due to caprolactam crystallization, making them difficult to apply. Existing technologies often use multi-stage steam jet pumps. While jet pumps have strong anti-clogging capabilities, they consume large amounts of steam, resulting in high energy consumption and generating large quantities of wastewater containing low concentrations of caprolactam, increasing the burden on subsequent environmental treatment. Water ring vacuum pumps have a stable structure and can dissolve caprolactam relatively well, but the working fluid (water) has a low boiling point, limiting the achievable vacuum level and making it difficult to meet the vacuum conditions required for the recovery of high-boiling-point caprolactam. Furthermore, since the working fluid is water, it forms an aqueous solution containing caprolactam; although the wastewater volume is small, it still requires wastewater treatment.

[0005] Therefore, it is necessary to provide a vacuum device that can achieve high vacuum operation with low energy consumption, avoid caprolactam crystallization blockage, and generate no additional waste liquid discharge. Summary of the Invention

[0006] To address the problems in the existing technology, the applicant discovered during research that selecting a solvent with a high boiling point, good solubility for caprolactam, and good self-flowability as the working fluid can solve these problems. Based on this design concept, the applicant proposes a vacuum system for the production of aminohexanonitrile from caprolactam, using aminohexanonitrile as the working fluid of a liquid ring vacuum pump. Since aminohexanonitrile meets the aforementioned conditions and is a product of the process itself, with the production process including its own distillation separation unit for aminohexanonitrile and caprolactam, using aminohexanonitrile as the working fluid can achieve a high vacuum while also meeting the industrial application requirements of low energy consumption, zero waste liquid discharge, and stable operation. Furthermore, this invention incorporates a pre-absorption trapping tank and a Roots blower before the liquid ring vacuum pump to construct a Roots-liquid ring vacuum pump unit, achieving stable operation with high vacuum, low energy consumption, and zero waste liquid discharge.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: A vacuum apparatus for the production of aminohexanonitrile from caprolactam includes a liquid ring vacuum pump. The working fluid of the liquid ring vacuum pump is aminohexanonitrile. The gas feed of the vacuum apparatus comes from the gas extracted after condensation of the evaporated material at the top of the caprolactam recovery tower in the process. To reduce the concentration of caprolactam vapor entering subsequent equipment and to reduce the risk of caprolactam condensation, deposition, crystallization, and blockage inside the vacuum apparatus, a gas scrubbing device is also installed before the inlet of the liquid ring vacuum pump. The absorbent of the gas scrubbing device is aminohexanonitrile. The absorbent of the gas scrubbing device comes from the process.

[0008] Furthermore, the vacuum device also includes a Roots blower located before the liquid ring vacuum pump. The Roots blower is used to improve the pumping capacity and expand the effective vacuum range of the vacuum device, and it is resistant to clogging, enabling it to meet the high vacuum operating conditions required by the distillation process.

[0009] Furthermore, the working fluid feed of the liquid ring vacuum pump comes from the process described above.

[0010] The air scrubbing device can be installed before or after the Roots blower.

[0011] Furthermore, the liquid outlet of the gas washing device or the working liquid outlet of the liquid ring vacuum pump is connected to the material inlet of the aminohexanonitrile separation device in the process. The rich liquid of the absorption device and / or the rich liquid of the liquid ring working pump are mixed with the material of the previous stage of the reaction process (the feed liquid after deammoniation and dehydration of the product in the reaction stage, mainly aminohexanonitrile, caprolactam and heavy components) and then enter the product aminohexanonitrile purification stage.

[0012] Furthermore, the gas scrubbing device can be a sealed-can absorber or a tower scrubbing device. When a high vacuum level is required, a low-pressure-drop tower scrubbing device can be used to reduce system pressure drop and improve gas-liquid contact efficiency.

[0013] Furthermore, the tower washing device is a packed tower or a plate tower.

[0014] This invention uses aminocapronitrile as the working fluid of a liquid ring vacuum pump. While further pumping gas, it reduces the risk of caprolactam condensation and crystallization clogging within the vacuum system by utilizing the high solubility of caprolactam in aminocapronitrile. To quantitatively demonstrate the solubility of caprolactam in aminocapronitrile, the applicant measured the solubility of caprolactam in aminocapronitrile under different temperature conditions. The results are shown in Table 1 and... Figure 1 As shown in the figure, aminocapronitrile exhibits extremely high solubility for caprolactam, with the solubility increasing rapidly with increasing temperature. Particularly in the 30–60°C range, the solubility shows an exponential increase, enabling substantial dissolution at common vacuum system operating temperatures (30–50°C). Due to its high solubility and large concentration gradient, aminocapronitrile can rapidly absorb caprolactam, exhibiting extremely fast dissolution and mass transfer rates, completing the absorption process in a short time. Furthermore, even when localized cold spots occur during vacuum system operation, caprolactam preferentially dissolves in aminocapronitrile, preventing precipitation and solid formation, thus effectively preventing equipment blockage. Therefore, aminocapronitrile not only possesses excellent dissolving ability but also exhibits rapid dissolution kinetics, making it highly suitable as an absorbent in vacuum systems and a working fluid for liquid ring vacuum pumps.

[0015]

[0016] *Solubility refers to the mass of caprolactam that can dissolve in 100g of aminocaproic acid at a certain temperature.

[0017] The caprolactam-containing mixture obtained at the working fluid outlet of the liquid ring vacuum pump can be transported to the material inlet of the aminohexanonitrile product distillation column in the production process. After mixing with the deammoniation and dehydration product feed (mainly aminohexanonitrile, caprolactam, and heavy components) from the reaction section, it enters the aminohexanonitrile purification stage for separation. This allows caprolactam and aminohexanonitrile to be separated and recovered within the existing separation unit. A small portion of the separated aminohexanonitrile is recycled as absorbent and working fluid for the liquid ring vacuum pump, thus forming a closed-loop material circulation system. The amount of recycled aminohexanonitrile is less than 1% of the total aminohexanonitrile product, therefore it has virtually no impact on the original unit's operating load or energy consumption.

[0018] A second objective of this invention is to provide a vacuum treatment method for the production of aminocapronitrile from caprolactam, comprising: introducing caprolactam-containing gas generated by the process into the aforementioned vacuum device; and dissolving the caprolactam into the working fluid in the liquid ring vacuum pump. Under the premise of achieving high vacuum and anti-clogging, the working fluid and absorbent self-circulate without generating additional waste liquid. The beneficial effects of this invention are: In this invention, the liquid ring vacuum pump uses aminocapronitrile as the working fluid. Under high vacuum, caprolactam-containing gas passes through the condenser and vacuum device at high speed, inevitably carrying caprolactam into the last stage vacuum device, namely the liquid ring pump, where it comes into contact with the working fluid. Since aminocapronitrile has excellent solubility and fluidity for caprolactam, it avoids the formation of solid deposits of caprolactam inside the pump cavity, thus improving the adaptability and stability of the system.

[0019] This invention places a Roots blower with a certain anti-clogging capability before the liquid ring vacuum pump, which expands the effective vacuum range of the system and ensures that the device has a large pumping capacity to achieve a high vacuum level while ensuring anti-clogging performance.

[0020] The present invention also includes a gas scrubbing device at the gas inlet of the Roots blower, which allows most of the caprolactam in the gas to be dissolved and enter the liquid phase at the front end, thereby significantly reducing the concentration of caprolactam vapor entering the subsequent equipment and directly reducing the risk of caprolactam condensing, depositing, crystallizing and clogging inside the vacuum device.

[0021] In this invention, the solution containing caprolactam and aminocaproic acid generated by the gas scrubbing device and / or liquid ring vacuum pump is mixed with the feed liquid (mainly aminocaproic acid, caprolactam, and heavy components) after deammoniation and dehydration of the reaction product, and then enters the aminocaproic acid purification stage. Since the working fluid of the liquid ring vacuum pump is aminocaproic acid, which is an existing material in the system, the introduction of water or steam as an external absorption medium can be effectively avoided, thus preventing the discharge of caprolactam-containing wastewater and improving the system's environmental friendliness. Because no additional external medium or new auxiliary treatment unit is introduced, and the amount of aminocaproic acid recycled is particularly small, less than 1% of the product volume, there is essentially no additional investment or energy consumption.

[0022] This invention has good process compatibility with existing production equipment, is easy to carry out technical transformation and upgrading in existing equipment, and has mature engineering implementation conditions and high feasibility.

[0023] In summary, this invention achieves a high vacuum level under low energy consumption conditions through the synergistic effect of internal coupling circulation of process materials and multi-stage vacuum structure, while also taking into account the comprehensive technical effect of anti-crystallization and blockage capabilities and zero waste liquid discharge characteristics. Attached Figure Description

[0024] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments: Figure 1 The curve shows the solubility of aminocapronitrile for caprolactam as a function of temperature.

[0025] Figure 2 This is a schematic diagram of the process flow for Example 1.

[0026] Figure 3 This is a schematic diagram of the process flow for Example 2.

[0027] Figure 4 This is a schematic diagram of the process flow for Example 3.

[0028] Figure 5 This is a schematic diagram of the process flow for Example 4.

[0029] Figure 6 This is a schematic diagram of the process flow for Example 5.

[0030] Among them, 1 is an aminohexanonitrile distillation column, 2 is a caprolactam recovery distillation column, 3 is a Roots blower, 4 is a liquid ring vacuum pump, 5 is a packed tower washing device, 6 is a plate tower washing device, and 7 is a sealed tank absorption device. Detailed Implementation

[0031] Example 1 like Figure 2 The vacuum apparatus shown is for the production of aminohexanonitrile from caprolactam, comprising a liquid ring vacuum pump. The working fluid of the liquid ring vacuum pump is the aminohexanonitrile product from the upstream process. The gas feed of the vacuum apparatus comes from the gas extracted after condensation at the top of the distillation column in the process. A Roots blower is installed before the liquid ring vacuum pump. The working fluid outlet of the liquid ring vacuum pump mixes with the dehydrated and de-lightened material of the reaction solution to form a confluence stream before entering the aminohexanonitrile distillation column.

[0032] During operation, the gas extracted from the top of the caprolactam recovery distillation column in the caprolactam-aminocaproic acid process after condensation is introduced into a Roots blower. The operating pressure of the distillation column is controlled at 1 kPa, and the temperature of the gas introduced into the vacuum system after condensation of the top material is 90–100°C. Sampling and testing revealed that the mass fraction of caprolactam in the gas was 2.0–3.0%, the mass fraction of aminocaproic acid was 0.5–1.5%, and the remainder consisted of small amounts of light components and non-condensable gases.

[0033] The condensate from the top of the column is pressurized by a Roots blower and then pumped into a liquid ring vacuum pump. The inlet pressure of the Roots blower is maintained at 3.5–4 kPa, and the outlet pressure is controlled at 10–15 kPa. The operating pressure of the liquid ring vacuum pump is 10–15 kPa. The working fluid of the liquid ring vacuum pump is aminohexanonitrile liquid, and the working fluid temperature is controlled at 30–45°C. After three-stage vacuum treatment, the tail gas is discharged at atmospheric pressure, and the working fluid is returned to the distillation process for recycling.

[0034] The system operated continuously and stably for 1000 hours without any crystallization blockage, equipment jamming, or emulsification of the liquid ring vacuum pump, with vacuum fluctuations less than ±0.5 kPa. Compared to traditional liquid ring vacuum systems that use water as the working fluid, this embodiment achieves a caprolactam removal rate of over 99% under the same operating conditions, while significantly reducing the organic component content in the exhaust gas by more than 99%. Furthermore, the working fluid of the liquid ring vacuum pump originates from the existing caprolactam-aminohexanonitrile process system, eliminating the need for external absorption media and avoiding any new wastewater treatment steps. This prevents wastewater generation at the source, enabling the recycling of materials within the system and the long-term stable and coordinated operation of multiple vacuum units.

[0035] To maintain a high vacuum, this embodiment omits a front-end gas scrubbing device. A simple combination of a Roots blower and a liquid ring vacuum pump effectively suppresses the condensation and crystallization of caprolactam in the vacuum system, ensuring stable system operation. This embodiment features a simple structure, low equipment investment, and significant industrial application value and engineering promotion potential.

[0036] Example 2 like Figure 3 The vacuum apparatus shown is for the production of aminohexanonitrile from caprolactam. It includes a liquid ring vacuum pump, whose working fluid is the aminohexanonitrile product from the preceding process. The gas feed to the vacuum apparatus comes from the gas extracted after condensation at the top of the distillation column in the process. A Roots blower is installed before the liquid ring vacuum pump. The working fluid outlet of the liquid ring vacuum pump mixes with the dehydrated and de-lightened material from the reaction liquid to form a confluence. A gas scrubbing device is also installed before the Roots blower. The absorbent of the gas scrubbing device is the aminohexanonitrile product from the preceding process, and the gas scrubbing device is a packed tower absorber. The liquid outlet of the gas scrubbing device also returns to the preceding process, mixing with the dehydrated and de-lightened material from the reaction liquid to form a confluence before entering the aminohexanonitrile distillation column.

[0037] During operation, the gas extracted from the top of the caprolactam recovery distillation column in the caprolactam-aminocaproic acid process after condensation is introduced into the packed column absorption unit. The operating pressure of the distillation column is 3 kPa, and the temperature of the condensate at the top of the column is 115–125℃. Sampling and analysis revealed that the gas contained 2.5–6.0% caprolactam by mass, 0.8–3.5% aminocaproic acid by mass, and the remainder consisted of small amounts of light components and non-condensable gases.

[0038] The condensed gas from the top of the column first enters the packed tower absorption unit. The packing material is stainless steel random-packed Pall rings. A certain amount of aminohexanonitrile liquid is sprayed into the top as the absorbent. The gas from the top of the column enters the column from the bottom through the gas distribution pipe and comes into full contact with the absorbent liquid on the surface of the packing, achieving gas-liquid mass transfer absorption. The operating temperature of the packed tower is controlled at 45-50℃, and the pressure is maintained at 2-2.5 kPa, which is basically matched with the pressure at the top of the distillation column to ensure stable gas entry and avoid pressure fluctuations. The residence time of the gas in the liquid phase is 3-5 seconds. After absorption treatment, the caprolactam content in the gas phase is reduced from the original 2.5-6.0% to below 0.1%, and the aminohexanonitrile content is reduced to below 0.02%, with an absorption efficiency greater than 98%. A portion of the rich liquid obtained from absorption is recycled back to the top of the packed tower to ensure the spray density of the absorbent liquid, while another portion is discharged from the bottom and directly returned to the distillation unit. After being mixed with the dehydrated and light-weight material of the reaction liquid, it enters the feed section in the middle of the aminohexanonitrile distillation tower to participate in material separation and circulation, forming an internal closed-loop coupling. The mass ratio of circulation volume to discharge volume is 5-20:1.

[0039] Unabsorbed gas is pumped and pressurized by a Roots blower before entering a liquid ring vacuum pump for suction. The inlet pressure of the Roots blower is maintained at 2.5–3 kPa, and the outlet pressure is controlled at 10–15 kPa. The operating pressure of the liquid ring vacuum pump is 10–15 kPa. The working fluid of the liquid ring vacuum pump is also aminohexanonitrile liquid, and the working fluid temperature is controlled at 30–45°C. After three-stage vacuum treatment, the exhaust gas is discharged at atmospheric pressure, and the working fluid is returned to the distillation process for recycling.

[0040] The system operated continuously and stably for 480 hours without any crystallization blockage or emulsification of the liquid ring vacuum pump. Compared with traditional processes using external condensation and water ring vacuum pumps, this system reduced the concentration of organic matter in the exhaust gas by more than 99%, significantly improved vacuum stability, and reduced overall operating energy consumption by approximately 10-15%. Since both the absorbent and the liquid ring working fluid are derived from the existing caprolactam-aminohexanonitrile process system, no external solvents were introduced or additional waste liquid treatment units were added. This achieved closed-loop circulation of materials internally and synergistic operation of the multi-stage vacuum structure, effectively reducing material loss and equipment blockage risks while ensuring the stability of negative pressure distillation.

[0041] This example achieves efficient recovery of caprolactam from exhaust gas, stable operation of the vacuum system, and reduced system energy consumption without changing the main production process, demonstrating good industrial applicability and engineering feasibility.

[0042]

[0043] Example 3 like Figure 4The vacuum apparatus shown is for the production of aminohexanonitrile from caprolactam. It includes a liquid ring vacuum pump, whose working fluid is the aminohexanonitrile product from the preceding process. The gas feed to the vacuum apparatus comes from the gas extracted after condensation at the top of the distillation column in the process. A Roots blower is installed before the liquid ring vacuum pump. The working fluid outlet of the liquid ring vacuum pump mixes with the dehydrated and delighted material from the reaction solution to form a confluence. A gas scrubbing device is also installed before the Roots blower. The absorbent of the gas scrubbing device is the aminohexanonitrile product from the preceding process, and the gas scrubbing device is a plate column scrubbing device. The liquid outlet of the gas scrubbing device also returns to the preceding process, mixes with the dehydrated and delighted material from the reaction solution to form a confluence, and then enters the aminohexanonitrile distillation column.

[0044] During operation, the gas extracted from the top of the caprolactam recovery distillation column in the caprolactam-aminocaproic acid process after condensation is introduced into the gas scrubbing unit. The operating pressure of the distillation column is controlled at 5 kPa, and the top temperature is 130–140 °C. Under conditions of increased bottom temperature and reflux ratio, the caprolactam content in the gas escaping from the top increases to 6.5–12.0%, the aminocaproic acid content is 1.5–4.0%, and the remainder consists of small amounts of light components and non-condensable gases. Due to the significantly increased caprolactam vapor content, it is more likely to condense and precipitate in the low-temperature part of the vacuum system; at the same time, the vacuum requirement in this case is not very high, and a certain pressure drop is allowed. Therefore, a tower-type scrubbing unit with more sufficient gas-liquid contact is used for treatment.

[0045] The plate scrubbing unit consists of three bubble-cap trays. The absorbent is sprayed down from the top of the column, while the gas from the top enters from the bottom and flows upward through the trays, forming a countercurrent contact with the downward-flowing aminocapronitrile absorbent to enhance gas-liquid mass transfer. The operating pressure inside the column is approximately 1-2 kPa higher than the top pressure, and the operating temperature is 50-60℃. After column scrubbing and absorption, the caprolactam content in the gas phase is reduced to below 0.2%, and the aminocapronitrile content is reduced to below 0.1%, with an overall absorption efficiency greater than 99%. The rich liquid after absorption is discharged from the bottom of the column and directly returned to the feed inlet of the stripping section of the aminocapronitrile distillation column, achieving separation of aminocapronitrile and caprolactam.

[0046] Unabsorbed gas is pumped and pressurized by a Roots blower before entering a liquid ring vacuum pump for further extraction. The Roots blower inlet pressure is maintained at 6–7 kPa, and the outlet pressure at 10–15 kPa. The liquid ring vacuum pump operates at 10–15 kPa, and the working fluid is aminohexanonitrile liquid, with the working fluid temperature controlled at 35–50°C. After three stages of vacuum treatment, the exhaust gas is discharged at atmospheric pressure, and the working fluid is returned to the distillation process for recycling.

[0047] The system operated continuously for 720 hours without any scaling or clogging of the packing material, and the vacuum fluctuation was less than ±0.5 kPa. Compared with Example 2, under the condition of significantly increased caprolactam content at the top of the tower, the plate tower scrubbing device can provide a larger gas-liquid mass transfer area and residence time, resulting in a more thorough scrubbing effect. This ensures that the concentration of organic components is significantly reduced before the tail gas enters the vacuum system, thereby maintaining the stable operation of the multi-stage vacuum system.

[0048] This example further demonstrates that the internal material circulation structure of the present invention has good adaptability to multi-stage vacuum systems. Different types of absorption units can be selected according to the composition of the gas at the top of the tower, achieving efficient recovery and stable vacuum operation without changing the main process system. This reflects the strong process compatibility and engineering implementation flexibility of the present invention.

[0049]

[0050] Example 4 like Figure 5 The vacuum apparatus shown is for the production of aminohexanonitrile from caprolactam. It includes a liquid ring vacuum pump, whose working fluid is the aminohexanonitrile product from the preceding process. The gas feed to the vacuum apparatus comes from the gas extracted after condensation at the top of the distillation column in the process. A Roots blower is installed before the liquid ring vacuum pump. The working fluid outlet of the liquid ring vacuum pump mixes with the dehydrated and delighted material from the reaction solution to form a confluence. A gas scrubbing device is also installed before the Roots blower. The absorbent in the gas scrubbing device is the aminohexanonitrile product from the preceding process, and the gas scrubbing device is a sealed-tank type absorbent. The liquid outlet of the gas scrubbing device also returns to the preceding process, mixing with the dehydrated and delighted material from the reaction solution to form a confluence before entering the aminohexanonitrile distillation column.

[0051] During operation, the gas extracted from the top of the caprolactam recovery distillation column in the caprolactam-aminocaproic acid process after condensation is introduced into the gas washing unit. The operating pressure of the distillation column is 10 kPa, and the top temperature is 140–145 °C. Analysis shows that the mass fraction of caprolactam in the escaping gas from the top is 10.5–20.0%, the mass fraction of aminocaproic acid is 3.0–5.0%, and the remainder consists of light components and a small amount of non-condensable gases. Due to the low load on the system, the concentration of organic vapors in the top gas is relatively high, resulting in a high risk of condensation and precipitation.

[0052] The gas washing unit employs a simplified sealed tank absorption structure. The sealed tank maintains a constant liquid level of aminohexanonitrile liquid. Gas from the top of the column enters the tank directly through a gas distribution device at the bottom, where it naturally contacts the liquid surface and undergoes partial bubbling absorption. No packing or trays are used. The sealed tank operating temperature is controlled at 50–60℃, and the operating pressure is 10–15 kPa. After absorption, the caprolactam content in the gas is reduced to below 0.05%, and the aminohexanonitrile content is reduced to below 0.02%. The resulting mixture is directly returned to the pre-distillation stage via a pump.

[0053] After primary capture, the gas enters a Roots blower for pressurization, with the inlet pressure maintained at 10–15 kPa and the outlet pressure at 20–30 kPa. The gas then enters a liquid ring vacuum pump for final suction. The working fluid of the liquid ring vacuum pump is aminohexanonitrile liquid, and the working fluid temperature is controlled at 30–50°C. Due to the low upstream absorption load, the concentration of organic vapor entering the liquid ring vacuum pump is low, and no crystallization or emulsification occurs in the system. After gas-liquid separation, the entire liquid ring working fluid is returned to the distillation system for recycling.

[0054] The system operated continuously for 720 hours with vacuum fluctuations less than ±0.4 kPa and no blockages or abnormal vibrations occurred. Compared to high-concentration conditions, this embodiment allows for a simpler absorption structure under low organic vapor loads, further reducing equipment investment and operating energy consumption, while still maintaining the synergistic effect of the multi-stage vacuum structure and the closed-loop circulation characteristics within the material. This example demonstrates that the present invention exhibits good stability and engineering adaptability even under low-concentration, low-load operating conditions.

[0055]

[0056] Example 5 like Figure 6 The vacuum apparatus shown is for the production of aminohexanonitrile from caprolactam. It includes a liquid ring vacuum pump, whose working fluid is the aminohexanonitrile product from the preceding process. The gas feed to the vacuum apparatus comes from the gas extracted after condensation at the top of the distillation column in the process. A Roots blower is installed before the liquid ring vacuum pump. The working fluid outlet of the liquid ring vacuum pump mixes with the dehydrated and de-lightened material from the reaction liquid to form a confluence. A gas scrubbing device is also installed after the Roots blower. The absorbent in the gas scrubbing device is the aminohexanonitrile product from the preceding process, and the gas scrubbing device is a plate column absorber. The liquid outlet of the gas scrubbing device also returns to the preceding process, mixing with the dehydrated and de-lightened material from the reaction liquid to form a confluence before entering the aminohexanonitrile distillation column.

[0057] During operation, the gas extracted from the top of the caprolactam recovery distillation column in the caprolactam-aminocaproic acid process after condensation is introduced into a Roots blower. The operating pressure of the distillation column is controlled at 7 kPa, and the temperature of the gas introduced into the vacuum system after condensation of the top material is 135–140 °C. Sampling and testing revealed that the mass fraction of caprolactam in the gas was 8.5–16.0%, the mass fraction of aminocaproic acid was 2.0–4.5%, and the remainder consisted of small amounts of light components and non-condensable gases.

[0058] The condensate from the top of the column is first pumped and pressurized by a Roots blower before entering the plate scrubbing unit. The inlet pressure of the Roots blower is maintained at 6.5–7.5 kPa, and the outlet pressure is controlled at 12–15 kPa. The plate scrubbing unit consists of three bubble cap trays. The absorbent is sprayed down from the top of the column, while the gas from the top enters from the bottom and flows upward through the trays, forming a countercurrent contact with the downward-flowing aminohexanonitrile absorbent, thus enhancing gas-liquid mass transfer. The operating pressure inside the column is 1–2 kPa higher than the outlet pressure of the Roots blower, and the operating temperature is 60–70°C. After column scrubbing and absorption, the caprolactam content in the gas phase is reduced to below 0.2%, and the aminohexanonitrile content is reduced to below 0.1%, with an overall absorption efficiency greater than 99%. The rich liquid after absorption is discharged from the bottom of the column and directly returned to the feed inlet of the stripping section of the aminohexanonitrile distillation column to participate in material separation and circulation, forming an internal closed-loop coupling.

[0059] Unabsorbed gas is drawn into a liquid ring vacuum pump for extraction. The liquid ring vacuum pump operates at a pressure of 15–17 kPa, and the working fluid is aminohexanonitrile liquid, with the working fluid temperature controlled at 30–45°C. After three-stage vacuum treatment, the exhaust gas is discharged at atmospheric pressure, and the working fluid is returned to the distillation process for recycling.

[0060] The system operated continuously and stably for 600 hours, with vacuum fluctuations less than ±0.4 kPa, and no crystallization blockage, equipment jamming, or emulsification of the liquid ring vacuum pump occurred. Compared with the structures using a pre-washing device in Examples 2 and 3, this embodiment places the absorption unit after the Roots blower, allowing the gas to undergo a pressurization before entering the absorption unit, increasing gas density and enhancing mass transfer driving force, thereby achieving higher absorption efficiency under the same gas-liquid contact conditions. Simultaneously, the gas flow is more stable after pre-treatment by the Roots blower, which helps reduce the risk of local condensation and crystallization of high-concentration caprolactam vapor in the absorption unit. Furthermore, the post-absorption structure can achieve efficient capture of high-concentration organic vapors without significantly increasing the system pressure drop, and reduces the processing load on the liquid ring vacuum pump, making its operation more stable.

[0061] This embodiment demonstrates that by optimizing the position of the gas scrubbing device, the stable operation of the vacuum system and the efficient absorption can be synergistically enhanced without changing the main process structure. This further expands the applicability of the invention under different operating pressures and gas composition conditions, and has good engineering adjustment flexibility and industrial application value.

Claims

1. A vacuum apparatus for the production of aminohexanonitrile from caprolactam, characterized in that, The device includes a liquid ring vacuum pump, the working fluid of which is aminohexanonitrile, and the gas feed of the vacuum device comes from the gas extracted after condensation of the material evaporated at the top of the caprolactam recovery tower in the process.

2. The vacuum device according to claim 1, characterized in that, The working fluid for the liquid ring vacuum pump is fed from the aforementioned process.

3. The vacuum device according to claim 1, characterized in that, The vacuum device also includes a Roots blower located upstream of the liquid ring vacuum pump.

4. The vacuum device according to claim 1, characterized in that, The working fluid outlet of the liquid ring vacuum pump is connected to the material inlet of the aminohexanonitrile separation unit in the process.

5. The vacuum device according to claim 1, characterized in that, A gas washing device is also provided before the inlet of the liquid ring vacuum pump, and the absorbent of the gas washing device is aminohexanonitrile.

6. The vacuum device according to claim 5, characterized in that, The liquid outlet of the gas washing device or the working fluid outlet of the liquid ring vacuum pump is connected to the material inlet of the aminohexanonitrile separation device in the process.

7. The vacuum device according to claim 5, characterized in that, The gas scrubbing device is either a sealed-can absorber or a tower scrubbing device.

8. The vacuum device according to claim 7, characterized in that, The tower washing device is a packed tower or a plate tower.

9. A vacuum treatment method for the production of aminohexanonitrile from caprolactam, characterized in that, This includes introducing the caprolactam-containing gas generated by the process into the vacuum device according to any one of claims 1-8, wherein the caprolactam dissolves into the working fluid in the liquid ring vacuum pump.

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