A one-effect evaporation process for the reconcentration of caprolactam concentrate

Abstract

The application discloses a one-effect evaporation process for re-concentration of caprolactam concentrated solution, comprising the following steps: heating caprolactam concentrated solution with a mass fraction of 65%-75% to 125-135 DEG C under pressure of 0.4-0.6 MPa, so that the solution is kept in a superheated pure liquid phase and does not boil; then the superheated liquid enters a dehydration tank with an internal vacuum of-0.06 MPa to-0.08 MPa to perform bulk phase flash evaporation, so that the concentration of the concentrated solution is increased to more than 90%; secondary steam generated in the flash evaporation is washed by condensate water and pure melt, and then part of the secondary steam is mixed with live steam to increase pressure by a TVR heat pump ejector, and is sent back to a tube heater as a heat source; the concentrated solution produced by the dehydration tank is injected into pure caprolactam melt, the Reynolds number is controlled in the range of 1800-2400 to physically reduce viscosity, and finally the concentrated solution is sent out through gradient precision filtration.

Description

Technical Field

[0001] This invention mainly relates to the field of nylon 6 polymerization production technology, specifically a single-effect evaporation process for the reconcentration of caprolactam concentrate. Background Technology

[0002] Nylon 6 is a commonly used polymer material, and its applications are quite extensive, including chemical fiber spinning, engineering plastics, food packaging films, and industrial tire cords.

[0003] Caprolactam is the main raw material for the production of nylon 6. In the production and refining process of caprolactam, it is usually necessary to evaporate and concentrate a low-concentration caprolactam aqueous solution to meet the purity and concentration requirements of subsequent polymerization reactions. Existing concentrate extraction processes mostly employ traditional boiling evaporation technology.

[0004] However, traditional evaporation processes face numerous technical bottlenecks in actual operation. First, during the boiling evaporation of caprolactam concentrate within the tubes, severe scaling easily occurs on the heated tube walls due to localized overheating and the collapse of boiling bubbles, leading to a sharp drop in heat transfer coefficient and requiring frequent equipment shutdowns for cleaning. Second, the average thermal residence time of materials in the high-temperature zone is relatively long in traditional processes, easily triggering thermosensitive degradation of caprolactam, resulting in increased product color (APHA value) and quality impairment. Furthermore, traditional single-effect evaporation processes consume large amounts of live steam, have low thermal energy utilization, and as the concentrate concentration increases, the apparent viscosity of the system increases sharply, leading to high resistance in subsequent filtration processes, easy clogging of filter cartridges, and short lifespans.

[0005] Therefore, there is an urgent need for a new concentration process that can prevent scaling, reduce energy consumption, inhibit degradation, and improve filtration performance. Summary of the Invention

[0006] The present invention addresses the problem that existing technical solutions are too simplistic and provides a solution that is significantly different from existing technologies. It mainly provides a one-effect evaporation process for the reconcentration of caprolactam concentrate, thereby solving the technical problems of easy scaling during boiling evaporation and difficulty in filtering high-viscosity concentrates mentioned in the background.

[0007] The technical solution adopted by the present invention to solve the above-mentioned technical problems is as follows: A single-effect evaporation process for the reconcentration of caprolactam concentrate includes the following steps: S1. Pressure-bearing superheated heating control: Caprolactam concentrate with a mass fraction of 65%-75% enters the tube heater. Under the control of the outlet back pressure valve, the operating pressure inside the tube is maintained at a constant 0.4-0.6MPa, so that the liquid is heated to 125-135℃ in the tube without phase change boiling. S2. Negative pressure bulk phase flash evaporation: The superheated liquid enters the dehydration tank through the tangential inlet. The vacuum inside the dehydration tank is maintained at -0.06MPa to -0.08MPa. The superheated enthalpy value generated by the sudden pressure drop induces flash evaporation inside the liquid and bulk phases, and the water vaporizes and separates, so that the concentration of the concentrate is increased to more than 90%. S3. Condensate-assisted washing: The secondary steam generated by flash evaporation enters the packed tower. The washing liquid is the secondary steam condensate produced by the system itself, and 3%-8% pure caprolactam melt is added online. The mass ratio of washing liquid to secondary steam is controlled at 2:1 to 5:1. The rich liquid after washing is returned to the dehydration tank. S4. TVR heat pump ejector steam cascade pressurization: Part of the washed secondary steam is ejected by the TVR heat pump ejector, mixed with 0.6-0.8MPa live steam and pressurized to 0.3-0.45MPa, and then sent back to the shell side of the tube heater as a heat source. S5. In-situ mixing and viscosity reduction and gradient filtration: The highly concentrated liquid produced at the bottom of the dehydration tank is injected with pure caprolactam melt through a static mixer before entering the gradient filter. The injection mass flow rate is 5%-15% of the concentrated liquid flow rate. Physical viscosity reduction is achieved by adjusting the Reynolds number Re in the mixing section to the range of 1800-2400. Then, it is filtered by the gradient filter and sent to the polymerization process.

[0008] Furthermore, an automatic non-condensable gas venting device is provided at the top of the dead zone at the far end of the shell side of the tube heater. The automatic non-condensable gas venting device captures the static pressure parameters of the top region of the dead zone at the far end of the shell side of the tube heater in real time through temperature monitoring and pressure monitoring. The non-condensable gas automatic venting device compares the actual sensed temperature T. act The theoretical temperature T of saturated steam at the corresponding pressure sat When the difference between the two is ΔT≥2.8℃, it is determined that the volume percentage of non-condensable gas exceeds 5%, and the pulse exhaust logic is triggered. The pulsed exhaust logic adjusts the opening degree and duration of the vent valve to maintain the physical environment of the TVR heat pump ejector in the high-efficiency range of ejection ratio 0.5-0.55.

[0009] Furthermore, the liquid level in the dehydration tank is controlled at 1 / 4 to 1 / 3 of the total tank height to ensure that the average thermal residence time of the concentrate in the high-temperature zone of 125-135℃ is controlled within 20 minutes.

[0010] Furthermore, the TVR heat pump ejector is equipped with a real-time power steam regulating valve. The power steam regulating valve automatically adjusts the power steam flow rate based on the fluctuation of the vacuum degree inside the dehydration tank. By dynamically adjusting the power steam flux, the ejection ratio of the TVR heat pump ejector is locked in the high-efficiency range of 0.5-0.55 in real time.

[0011] Furthermore, the static mixer employs an SK-type mixing element with a three-dimensional cross-flow guiding component, and the surface of the SK-type mixing element is coated with a polytetrafluoroethylene drag-reducing and anti-sticking layer to reduce the shear adhesion force of the oligomer long chain. The physical viscosity reduction process in step S5 is achieved by adjusting the flow rate in the mixing section to bring the fluid into a critical transition flow state within the Reynolds number Re1800-2400 range. Within this range, the microscopic shear force generated by the fluid breaks the association between the long-chain molecules of caprolactam oligomers, while the shear viscosity is reduced by the polytetrafluoroethylene coating on the surface of the SK-type mixing element, so that the apparent viscosity of the filter entering the filter is reduced to below 38 mPa·s.

[0012] Furthermore, the gradient filter includes two alternately used filter cartridges, each cartridge comprising a first-stage coarse filter element with a pore size of 30-50 μm and a second-stage fine filter element with a pore size of 5-10 μm; The filter element material is a surface-modified metal fiber sintered felt.

[0013] Furthermore, a Venturi jet structure is embedded in the tangential inlet of the dehydration tank. The local low-pressure auxiliary bubbles generated at the narrowing of the Venturi jet structure are used to precipitate from the interior of the high-viscosity liquid phase, thereby reducing the flash evaporation resistance.

[0014] Furthermore, a water balance compensation system based on online refractive index detection is installed on the pipeline from the bottom of the packed tower back to the dehydration tank. The water balance compensation system offsets the moisture fluctuations caused by the introduction of washing liquid by adjusting the amount of self-generated condensate added.

[0015] Furthermore, the gradient filter has an in-situ backwashing function. When the pressure difference of the gradient filter reaches 0.15MPa, it switches to the standby filter cartridge and introduces pure caprolactam melt at 95-100℃ to soak, dissolve and clean the switched-out filter cartridge to restore the filter cartridge throughput.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. This invention employs a process of pressurized superheated heating and negative pressure phase flash separation, which keeps the liquid under constant pressure of 0.4-0.6 MPa in the heating tube, raising the temperature without phase change boiling, thus completely eliminating the scaling problem on the tube wall caused by boiling bubbles; the heat transfer coefficient of the heater does not decrease significantly during continuous operation. 2. This invention utilizes a TVR heat pump ejector to mix and pressurize the secondary steam after washing with live steam to 0.3-0.45 MPa, and then feeds it back to the shell side of the tube heater as a heat source for recycling. Compared with the traditional single-effect process, the steam consumption per ton of water evaporated is significantly reduced by about 32%. 3. By controlling the liquid level in the dehydration tank, this invention strictly controls the average thermal residence time of the concentrate in the 125-135℃ high-temperature zone to within 20 minutes, effectively inhibiting the heat-sensitive degradation of caprolactam and keeping the color APHA value of the final polymerization product at a low level. 4. This invention injects pure caprolactam melt in situ through a static mixer and precisely adjusts the flow rate in the mixing section to keep the fluid in a critical transition flow state. It uses microscopic shear force to break the association of long oligomer chains to achieve physical viscosity reduction, which significantly reduces the apparent viscosity of the filter, effectively reduces the rate of pressure differential growth, and greatly extends the continuous service life of the filter element. 5. This invention uses self-generated condensate to assist in washing secondary steam and is equipped with a water balance compensation system based on refractive index to eliminate moisture fluctuations. At the same time, the gradient filter integrates a pure melt in-situ backwashing function, which can realize online flow recovery when the pressure difference exceeds the limit, avoiding the tediousness of frequent disassembly and washing.

[0017] The present invention will be explained in detail below with reference to the accompanying drawings and specific embodiments. Attached Figure Description

[0018] Figure 1 This is a flow chart of the single-effect evaporation process of the present invention; Figure 2 This is a comparison chart of the growth rate of filtration pressure difference of the caprolactam concentrate of the present invention under different Reynolds numbers (Re). Detailed Implementation

[0019] To facilitate understanding of the present invention, a more comprehensive description of the present invention will be given below with reference to the accompanying drawings, which illustrate several embodiments of the present invention. However, the present invention can be implemented in different forms and is not limited to the embodiments described in the text. Rather, these embodiments are provided to make the disclosure of the present invention more thorough and complete.

[0020] It should be noted that when an element is said to be fixed to another element, it can be directly on the other element or there may be an intermediate element. When an element is said to be connected to another element, it can be directly connected to the other element or there may be an intermediate element at the same time. The terms vertical, horizontal, left, right and similar expressions used in this article are for illustrative purposes only.

[0021] A single-effect evaporation process for the reconcentration of caprolactam concentrate includes the following steps: S1. Pressure-bearing superheated heating control: Caprolactam concentrate with a mass fraction of 65%-75% enters the tube heater. Under the control of the outlet back pressure valve, the operating pressure inside the tube is maintained at a constant 0.4-0.6MPa, so that the liquid is heated to 125-135℃ in the tube without phase change boiling. S2. Negative pressure bulk phase flash evaporation: The superheated liquid enters the dehydration tank through the tangential inlet. The vacuum inside the dehydration tank is maintained at -0.06MPa to -0.08MPa. The superheated enthalpy value generated by the sudden pressure drop induces flash evaporation inside the liquid and bulk phases, and the water vaporizes and separates, so that the concentration of the concentrate is increased to more than 90%. S3. Condensate-assisted washing: The secondary steam generated by flash evaporation enters the packed tower. The washing liquid is the secondary steam condensate produced by the system itself, and 3%-8% pure caprolactam melt is added online. The mass ratio of washing liquid to secondary steam is controlled at 2:1 to 5:1. The rich liquid after washing is returned to the dehydration tank. S4. TVR heat pump ejector steam cascade pressurization: Part of the washed secondary steam is ejected by the TVR heat pump ejector, mixed with 0.6-0.8MPa live steam and pressurized to 0.3-0.45MPa, and then sent back to the shell side of the tube heater as a heat source. S5. In-situ mixing and viscosity reduction and gradient filtration: The highly concentrated liquid produced at the bottom of the dehydration tank is injected with pure caprolactam melt through a static mixer before entering the gradient filter. The injection mass flow rate is 5%-15% of the concentrated liquid flow rate (mass flow rate is usually expressed as qm). Physical viscosity reduction is achieved by adjusting the Reynolds number Re in the mixing section to the range of 1800-2400. Then, it is filtered by the gradient filter and sent to the polymerization process.

[0022] Example 1: Main Process Flow Example like Figure 1 As shown in the process flow diagram, in a typical industrial application of this application, a 70% caprolactam concentrate output from the upstream refining section is continuously fed into the shell-and-tube heater at a flow rate of 5 t / h via a feed pump. To eliminate scaling on the tube walls caused by boiling bubbles in traditional processes, this embodiment uses a back pressure valve installed at the heater outlet to stably maintain the operating pressure inside the tube at a gauge pressure of 0.5 MPa. At this pressure, the saturation temperature of water is approximately 158°C, while the actual heating temperature of the feed liquid is controlled at only 130°C, ensuring that the feed liquid remains in a superheated pure liquid phase state throughout the 6-meter tube section, without any phase change. The shell-side heat source is the mixed steam pressurized by the subsequent TVR heat pump ejector. The DCS system maintains the outlet temperature in the range of 128–132°C in real time by interlocking the valve opening with temperature and flow rates.

[0023] Actual operation monitoring shows that the heat transfer coefficient U-value of the tube heater of this invention remains above 1450 W / (m²·K) after 90 days of continuous operation (initially 1475 W / (m²·K), with a decay rate of only 4.7%), while the U-value of the traditional boiling evaporation process drops sharply from 1480 W / (m²·K) to 720 W / (m²·K) after 5 days of operation, a decrease of more than 51% within 100 hours. Detailed data are shown in Table 1.

[0024] Table 1 Comparison of heat transfer coefficients of tube heaters with different processes as a function of operating time

[0025] The superheated liquid then enters the dehydration tank through the tangential inlet of the dehydration tank with an embedded Venturi jet structure. The local low pressure of approximately -0.02 MPa generated at the narrowing of the Venturi jet structure effectively assists in the precipitation of bubbles inside the high-viscosity liquid phase, significantly reducing flash evaporation resistance. The vacuum degree of the dehydration tank is maintained at -0.07 MPa by the vacuum pump unit. The liquid instantly releases its superheated enthalpy in the bulk phase, resulting in explosive flash evaporation. The water rapidly vaporizes and separates, and the concentration of the bottom concentrate steadily increases to 91.8 ± 0.3%.

[0026] To control thermosensitive degradation, the level gauge and discharge pump are interlocked to strictly control the liquid level in the dehydration tank to no more than 1 / 3 of the total height. The average thermal residence time in the 130℃ high-temperature zone was measured to be 18±1.2 minutes, which is much lower than the 45±3 minutes of the traditional process. The flash evaporation rate is increased by 11.5% compared with the non-Venturi spray structure, see Example 7 for details.

[0027] Flash-evaporated secondary steam enters the packed tower for washing. The washing liquid uses the condensate from the system's own secondary steam, and 5% by mass of 95℃ pure caprolactam melt is added online via a metering pump. The mass ratio of washing liquid to steam is precisely controlled at 3:1. The rich liquid at the bottom of the tower is returned to the dehydration tank via a water balance compensation system based on an online refractive index detector. When the refractive index shows a 0.3% downward trend in concentration, the DCS automatically fine-tunes the condensate replenishment to maintain the system's water balance, with actual concentration fluctuations less than 0.8%.

[0028] After washing, the secondary steam is split into two paths: one path serves as the ejector fluid for the TVR heat pump ejector, and the other path is condensed and recovered. The TVR heat pump ejector uses 0.7MPa live steam as power, which is mixed and pressurized to 0.4MPa before being returned to the shell side of the tube heater. The ejection ratio is stable at 0.52, and the measured energy consumption for evaporating 1 ton of water is 0.83t steam / t water, which is 32% lower than the 1.22t steam / t water of the traditional single-effect process (see Table 2 for details).

[0029] Table 2 Comparison of Steam Consumption for Evaporation of 2 Tons of Water

[0030] To eliminate the influence of non-condensable gases, a temperature sensor and a pressure sensor are installed at the top of the dead zone at the far end of the heater shell side. When the actual sensed temperature T... act With the corresponding pressure and saturated steam temperature T sat When the difference ΔT ≥ 2.8℃, the pulse exhaust valve is automatically triggered. The pulse exhaust valve opens to 40% to 60% and opens for 8 to 12 seconds at a time, controlling the non-condensable gas volume percentage below 4% to ensure the ejector operates in the high-efficiency range. See Example 2 for details.

[0031] The 91.8% high-concentration liquid at the bottom of the dehydration tank enters the in-situ mixing and viscosity-reducing unit. 12% (relative to the concentrate flow rate) of 95°C pure caprolactam melt is injected through a static mixer (SK-type three-dimensional cross-flow guiding element with a PTFE coating). The flow rate in the mixing section is adjusted to maintain the Reynolds number Re at 2050-2150 (critical transition flow state). Figure 2 As shown, this application controls Re in the range of 1800 to 2400. Within this range, the apparent viscosity of the liquid drops sharply from 118 mPa·s to 34 mPa·s, a decrease of 71%. The microscopic shear force effectively breaks the association of long oligomer chains, and the polytetrafluoroethylene coating further reduces the shear viscosity.

[0032] The solution then enters a gradient filter, whose filter element is a surface-modified metal fiber sintered felt. The rate of pressure differential increase in the filter element is reduced by 92% compared to traditional single-stage filtration (0.0011 MPa / h in this invention vs. 0.014 MPa / h in the traditional process). The filtered solution is then sent to the polymerization process, and the product's color APHA value is stabilized at 6–8 (18–25 in the traditional process).

[0033] Table 3 Comparison of Product Color APHA Values ​​over Operating Time

[0034] When the pressure difference reaches 0.15MPa, the system automatically switches to the backup filter cartridge and immerses it in 98℃ pure melt for 30 minutes to dissolve and clean it, achieving a throughput recovery rate of 96% without the need for shutdown and disassembly.

[0035] This embodiment operated continuously for 108 days, with a product concentration of 91.5%–92.2% and a 100% pass rate for polymerization grade, fully verifying the comprehensive advantages of this process in terms of energy saving, long cycle time, no scaling, and anti-degradation.

[0036] Example 2: Enhanced Example of Automatic Non-condensable Gas Evacuation Device In this embodiment, the actual efficiency of the automatic non-condensable gas venting device is verified. A high-precision Pt100 temperature sensor (accuracy ±0.1℃) and a pressure transmitter (accuracy 0.1%FS) are installed at the top of the dead zone at the far end of the shell side of the tube heater. The DCS collects data every 30 seconds and calculates ΔT. When ΔT is ≥2.8℃ twice consecutively (corresponding to non-condensable gas volume percentage >5%), the pulse venting logic is triggered: the venting valve first pre-vents at 30% opening for 3 seconds, then steps to 55% opening and maintains it for 8 seconds, and then closes. The function of the venting valve is to eliminate the gas resistance and thermal resistance of the shell side, that is, to prevent the ejector ratio from deteriorating due to external interference by improving the heat exchange environment. In actual operation, after the shell-side gas film thermal resistance is eliminated, the ejector ratio of the TVR heat pump quickly recovers from the fluctuating 0.42±0.03 and stabilizes at 0.53±0.01, the heater outlet steam temperature increases to 143℃, and the overall thermal efficiency increases by 14%. Compared with the control test without an automatic venting device, energy consumption was reduced by 11%, which fully demonstrates the decisive role of the control logic in maintaining the high-efficiency range of the TVR heat pump ejector.

[0037] Example 3: Precise Control of Liquid Level and Residence Time in Dehydration Tank For heat-sensitive quality control, this embodiment strictly locks the liquid level in the dehydration tank at 28% of the total height, slightly below the 1 / 3 upper limit. Through radar level gauge and PID interlocking with the variable frequency discharge pump, the average residence time in the 130℃ high-temperature zone was measured to be only 16.5 minutes. As shown in Table 3, the online colorimeter shows that the product's APHA value is stable at 6-7, a 35% reduction compared to the control group where the liquid level was controlled at 1 / 2 height. The content of the thermal degradation byproduct cyclic dimer decreased from 0.18% to 0.07%, effectively ensuring the monomer purity in the downstream polymerization process.

[0038] Example 4: Real-time regulating valve for power steam of TVR heat pump ejector In this embodiment, an electrically operated real-time regulating valve for power steam is added to the power steam line of the TVR heat pump ejector, forming a closed-loop feedback with the vacuum transmitter of the dehydration tank. The real-time regulating valve directly responds to fluctuations in the vacuum of the dehydration tank by changing the flow rate of power steam, serving as the primary and continuous adjustment method for maintaining the ejection ratio. When the vacuum fluctuates to -0.065 MPa, the real-time regulating valve automatically opens 12% within 2.5 seconds, restoring the vacuum to -0.07 MPa and maintaining an ejection ratio of 0.51 ± 0.01. A continuous 72-hour dynamic disturbance test simulating micro-air leakage shows that the vacuum fluctuation amplitude is ≤0.003 MPa, far superior to the 0.015 MPa of manual adjustment, significantly enhancing the system's robustness.

[0039] Example 5: Static Mixer Physical Viscosity Reduction Example The SK-type mixing element with a PTFE coating was used, with a mixing section length of 1.2 m. The flow rate after mixing was adjusted to 1.8 m / s, and the Reynolds number was precisely controlled within the range of 1980–2150. Rheometer measurements showed that the apparent viscosity entering the cascade filter stabilized at 35.2 ± 1.5 mPa·s, a 71% reduction compared to the initial viscosity. The pressure differential increase rate of the 30 μm filter element decreased from 0.012 ± 0.001 MPa / h to 0.0011 ± 0.0002 MPa / h, extending the continuous filtration cycle by more than four times.

[0040] Example 6: Gradient Filter and In-situ Backwashing Example The gradient filter employs a dual-cartridge design with series filter elements. The first-stage coarse filter element has a pore size of 42μm, while the second-stage fine filter element has a pore size of 7μm. When the total pressure difference reaches 0.15MPa, the PLC automatically switches to the standby filter element and simultaneously initiates a 98℃ pure melt immersion dissolution program at a flow rate of 0.8m³ / h for 28 minutes. The measured filter element flux recovery rate is 95.8±1.2%, and the cumulative service life of a single filter element set reaches 180 days, which is 2.5 times longer than the traditional physical backflushing method.

[0041] Example 7: Venturi jet structure assisted flash evaporation example The tangential inlet Venturi jet structure has a diameter reduction ratio of 1:0.65, and the flow velocity in the local low-pressure zone reaches 4.2 m / s. The flash evaporation resistance coefficient is reduced by 18% compared with the structure without Venturi jet, the flash evaporation rate is increased by 11.5% under the same vacuum, and the concentration fluctuation of the concentrate is reduced from ±1.2% to ±0.4%.

[0042] Example 8: Water Balance Compensation System Example The packed tower's reflux pipeline integrates an online refractive index detector with an accuracy of ±0.1%. When a concentration drop of 0.4% is detected, the DCS automatically increases the condensate replenishment by 1.8% while simultaneously fine-tuning the melt addition ratio. The system concentration fluctuation over 24 hours is measured to be ≤0.7%, completely eliminating moisture disturbances introduced by the washing liquid.

[0043] Example 9: Comprehensive Long-Term Validation Example After integrating all the above optimization measures, the device operated continuously for 108 days. The tube heaters required no cleaning, the ejector ratio fluctuation of the TVR heat pump ejector was <0.02, and the filter was switched 8 times, all online. All product indicators fully met the requirements for polymerization grade. The comprehensive energy consumption for evaporation per ton of water was 0.82t steam / t water, a 31% reduction compared to existing technologies, fully demonstrating the reliability and economy of this invention in industrial applications.

[0044] The above embodiments are merely preferred embodiments of the present invention. The technical solutions of the present invention are not limited to the above embodiments. Without departing from the spirit and scope of the present invention, those skilled in the art can reasonably adjust or equivalently replace the parameters according to actual needs, and all such adjustments should be considered to fall within the protection scope of the present invention.

[0045] Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make possible changes and modifications to the technical solutions of the present invention by utilizing the methods and techniques disclosed above without departing from the spirit and scope of the present invention. Therefore, any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solutions of the present invention shall fall within the protection scope of the technical solutions of the present invention.

Claims

1. A single-effect evaporation process for the reconcentration of caprolactam concentrate, characterized in that, Includes the following steps: S1. Pressure-bearing superheated heating control: Caprolactam concentrate with a mass fraction of 65%-75% enters the tube heater. Under the control of the outlet back pressure valve, the operating pressure inside the tube is maintained at a constant 0.4-0.6MPa, so that the liquid is heated to 125-135℃ in the tube without phase change boiling. S2. Negative pressure bulk phase flash evaporation: The superheated liquid enters the dehydration tank through the tangential inlet. The vacuum inside the dehydration tank is maintained at -0.06MPa to -0.08MPa. The superheated enthalpy value generated by the sudden pressure drop induces flash evaporation inside the liquid and bulk phases, and the water vaporizes and separates, so that the concentration of the concentrate is increased to more than 90%. S3. Condensate-assisted washing: The secondary steam generated by flash evaporation enters the packed tower. The washing liquid is the secondary steam condensate produced by the system itself, and 3%-8% pure caprolactam melt is added online. The mass ratio of washing liquid to secondary steam is controlled at 2:1 to 5:

1. The rich liquid after washing is returned to the dehydration tank. S4. TVR heat pump ejector steam cascade pressurization: Part of the washed secondary steam is ejected by the TVR heat pump ejector, mixed with 0.6-0.8MPa live steam and pressurized to 0.3-0.45MPa, and then sent back to the shell side of the tube heater as a heat source. S5. In-situ mixing and viscosity reduction and gradient filtration: The highly concentrated liquid produced at the bottom of the dehydration tank is injected with pure caprolactam melt through a static mixer before entering the gradient filter. The injection mass flow rate is 5%-15% of the concentrated liquid flow rate. Physical viscosity reduction is achieved by adjusting the Reynolds number Re in the mixing section to the range of 1800-2400. Then, it is filtered by the gradient filter and sent to the polymerization process.

2. The single-effect evaporation process for re-concentrating caprolactam concentrate according to claim 1, characterized in that, An automatic non-condensable gas venting device is provided at the top of the dead zone at the far end of the shell side of the tube heater. The automatic non-condensable gas venting device captures the static pressure parameters of the top region of the dead zone at the far end of the shell side of the tube heater in real time through temperature monitoring and pressure monitoring. The non-condensable gas automatic venting device compares the actual sensed temperature T. act The theoretical temperature T of saturated steam at the corresponding pressure sat When the difference between the two is ΔT≥2.8℃, it is determined that the volume percentage of non-condensable gas exceeds 5%, and the pulse exhaust logic is triggered. The pulsed exhaust logic adjusts the opening degree and duration of the vent valve to maintain the physical environment of the TVR heat pump ejector in the high-efficiency range of ejection ratio 0.5-0.

55.

3. The single-effect evaporation process for re-concentrating caprolactam concentrate according to claim 1, characterized in that, The liquid level in the dehydration tank is controlled at 1 / 4 to 1 / 3 of the total tank height to ensure that the average heat residence time of the concentrate in the high-temperature zone of 125-135℃ is controlled within 20 minutes.

4. The single-effect evaporation process for re-concentrating caprolactam concentrate according to claim 1, characterized in that, The TVR heat pump ejector is equipped with a real-time power steam regulating valve. The power steam regulating valve automatically adjusts the power steam flow rate based on the fluctuation of the vacuum degree inside the dehydration tank. By dynamically adjusting the power steam flux, the ejection ratio of the TVR heat pump ejector is locked in the high-efficiency range of 0.5-0.55 in real time.

5. A single-effect evaporation process for re-concentrating caprolactam concentrate according to claim 1, characterized in that, The static mixer employs an SK-type mixing element with three-dimensional cross-flow guiding components, and the surface of the SK-type mixing element is coated with a polytetrafluoroethylene drag-reducing and anti-sticking layer to reduce the shear adhesion force of the long oligomer chains. The physical viscosity reduction process in step S5 is achieved by adjusting the flow rate in the mixing section to bring the fluid into a critical transition flow state within the Reynolds number Re1800-2400 range. Within this range, the microscopic shear force generated by the fluid breaks the association between the long-chain molecules of caprolactam oligomers, while the shear viscosity is reduced by the polytetrafluoroethylene coating on the surface of the SK-type mixing element, so that the apparent viscosity of the filter entering the filter is reduced to below 38 mPa·s.

6. The single-effect evaporation process for re-concentrating caprolactam concentrate according to claim 1, characterized in that, The gradient filter includes two alternately used filter cartridges, each cartridge comprising a first-stage coarse filter element with a pore size of 30-50 μm and a second-stage fine filter element with a pore size of 5-10 μm. The filter element material is a surface-modified metal fiber sintered felt.

7. A single-effect evaporation process for re-concentrating caprolactam concentrate according to claim 1, characterized in that, The dehydration tank has a venturi jet structure embedded in its tangential inlet. The local low-pressure auxiliary bubbles generated at the narrowing of the venturi jet structure help to precipitate from the high-viscosity liquid phase, thereby reducing flash evaporation resistance.

8. The single-effect evaporation process for re-concentrating caprolactam concentrate according to claim 1, characterized in that, The pipeline from the bottom of the packed tower back to the dehydration tank is equipped with a water balance compensation system based on online refractive index detection. The water balance compensation system offsets the moisture fluctuations caused by the introduction of washing liquid by adjusting the amount of self-generated condensate added.

9. A single-effect evaporation process for re-concentrating caprolactam concentrate according to claim 1, characterized in that, The gradient filter has an in-situ backwashing function. When the pressure difference of the gradient filter reaches 0.15MPa, it switches to the standby filter cartridge and introduces pure caprolactam melt at 95-100℃ to soak, dissolve and clean the switched-out filter cartridge to restore the filter cartridge flow rate.

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