Purification method of caprolactam heavy residual liquid

Abstract

The invention relates to the technical field of caprolactam purification, and discloses a purification method of caprolactam heavy residual liquid. The purification method comprises the following steps: (1) distilling the caprolactam heavy residual liquid to obtain a caprolactam product; and (2) carrying out drying treatment on the distillation residual liquid after distillation to obtain crude caprolactam, returning to the step (1), and continuing distillation. The purification method disclosed by the invention can be used for carrying out efficient impurity removal treatment on the heavy raffinate in the caprolactam production process and recovering valuable caprolactam components in the heavy raffinate, and has the advantages of low ton treatment cost, high caprolactam recovery rate, good finished product quality and the like; and through a series of physical and chemical separation operations, the impurity content in the waste liquid is reduced, and the dual purposes of resource recycling and environment-friendly emission are achieved.

Description

Technical Field

[0001] The invention relates to the technical field of caprolactam purification, in particular to a method for purifying caprolactam heavy residue. Background Art

[0002] Caprolactam, a core raw material for the production of nylon-6 synthetic fibers and engineering plastics, is widely used in new materials fields such as textiles, electronics, automotive, and aerospace. Currently, caprolactam production primarily utilizes the cyclohexanone oxime liquid-phase rearrangement process, which produces a distillation residue during the refining process. This residue, known as caprolactam heavy residue (heavy residue for short), is currently processed by returning it directly to the refining system for recycling into production. The continuous accumulation of caprolactam oligomers and other components in the refining system often leads to fluctuations in product quality and frequent equipment maintenance, which in turn increases production costs.

[0003] CN110092754A discloses a method for refining and purifying caprolactam distillation residue, which involves first evaporating the heavy residue to dryness and then using solvent crystallization to recover the fraction. This method suffers from the following problems: caprolactam loss, and if the distillation residue and crystallization mother liquor are returned to the system, the quality of the main product will be affected. It is also impossible to recover oligomers and directly obtain national standard high-quality products.

[0004] CN113716773A discloses a system and method for separating and crystallizing caprolactam residual liquid, which develops a scheme for refining heavy residual liquid by filtration, evaporation, distillation and crystallization. However, the scheme has the following problems: long process flow, high energy consumption, and large caprolactam loss when the crystallization mother liquor enters the wastewater system.

[0005] Because the heavy residual liquid is rich in caprolactam oligomers, unreacted products and impurities, it can easily lead to fluctuations in caprolactam production quality, increased burden of impurity removal in the refining system, and equipment corrosion, which increases maintenance costs. The valuable components in the heavy residual liquid are not effectively recovered, which does not meet the requirements of high-quality development of the green chemical industry. Summary of the Invention

[0006] The purpose of the present invention is to overcome the problems of high energy consumption and impact on the quality of main production line products in the prior art, and to provide a method for purifying heavy caprolactam residue. The purification method has low ton processing cost, high caprolactam recovery rate, good finished product quality and efficient recovery of oligomers, and has good industrial prospects.

[0007] In order to achieve the above object, the present invention provides a method for purifying caprolactam heavy residue, which comprises:

[0008] (1) distilling the caprolactam heavy residue to obtain a caprolactam product;

[0009] (2) drying the distillation residue to obtain crude caprolactam, and returning to step (1) to continue distillation.

[0010] Through the above technical solution, the beneficial technical effects achieved by the present invention are as follows:

[0011] The purification method of the present invention carries out efficient impurity removal treatment on the heavy residual liquid in the caprolactam production process, recovers the valuable caprolactam component therein, and has the advantages of low ton processing cost, high caprolactam recovery rate, and good finished product quality.

[0012] Furthermore, the present invention reduces the impurity content in the waste liquid through a series of physical and chemical separation operations, realizes the efficient recovery of oligomers, and achieves the dual goals of resource recycling and environmentally friendly emissions, and has good industrial prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 1 is a schematic diagram of a process for purifying a heavy caprolactam residue according to one embodiment of the present invention;

[0014] Figure 2 It is a schematic diagram of a process for purifying caprolactam heavy residue provided by another embodiment of the present invention. DETAILED DESCRIPTION

[0015] The endpoints of the ranges and any values ​​disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoints of each range, the endpoints of each range and individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered to be specifically disclosed herein.

[0016] The present invention provides a method for purifying heavy residual liquid of caprolactam, such as Figure 1 As shown, the purification method comprises:

[0017] (1) distilling the caprolactam heavy residue to obtain a caprolactam product;

[0018] (2) drying the distillation residue to obtain crude caprolactam, and returning to step (1) to continue distillation.

[0019] According to the present invention, the caprolactam heavy residue refers to the still residue produced in the last distillation step of the caprolactam refining process. The caprolactam heavy residue has a caprolactam content of about 95-99% by mass and also contains a small amount of caprolactam oligomers (such as dimers, trimers, etc.), sodium hydroxide and other small molecular impurities.

[0020] According to the present invention, oligomer 1 is a water-insoluble polymer, mainly a cyclic dimer of caprolactam, which can be cracked and converted into caprolactam for recycling.

[0021] The purification method of the present invention can efficiently remove impurities from heavy residual liquid in the caprolactam production process and recover valuable caprolactam components therein, and has the advantages of low ton processing cost, high caprolactam recovery rate, and good finished product quality.

[0022] According to the present invention, the heavy residual liquid generated during the production of caprolactam is transported to a distillation device for distillation. The distillation device can be a conventional distillation device in the art. The distillation methods include but are not limited to: ordinary distillation, rectification, molecular distillation, etc.

[0023] According to some embodiments of the present invention, in step (1), the distillation is carried out in a distillation tower with an inner temperature of 120-135°C and a pressure of -98 kPa to -101 kPa; and a tower top temperature of 110-120°C.

[0024] According to the present invention, unless otherwise specified, all pressures herein refer to gauge pressure.

[0025] According to some embodiments of the present invention, the reflux ratio of the distillation is 1:1-1:4.

[0026] According to the present invention, the reflux ratio refers to the ratio of the reflux liquid flow rate to the top product (distillate) flow rate, for example, 1:1, 1:2, 1:3 or 1:4.

[0027] Reflux liquid: refers to the part of the condensate returning to the tower from the top condenser. Its function is to provide mass transfer driving force and cold source in the tower and maintain the gas-liquid balance in the tower.

[0028] Top product: refers to the distillate extracted from the top of the tower as the target product.

[0029] According to the present invention, increasing the reflux ratio can improve the separation effect, but it will increase the energy consumption and equipment load of the condenser and reboiler; if the reflux ratio is too small, the separation requirements may not be met.

[0030] According to the present invention, the heavy residual liquid is pumped into a distillation tower, the temperature in the tower is set at 120-135°C and the pressure is set at -98 kPa to -101 kPa. A defoaming net is installed in the tower to control the temperature at the top of the distillation tower at 110-120°C. The reflux ratio is maintained at about 1:4. The caprolactam component is vaporized and rises by heating. The caprolactam vapor generated during the distillation process is cooled in a condenser and converted into liquid, ensuring efficient distillation and recovery of the caprolactam. Impurities remain at the bottom of the tower to form distillation residual liquid.

[0031] According to some embodiments of the present invention, in step (1), the recovery rate of the caprolactam product is 60-95%.

[0032] According to the present invention, in step (1), the quality of the caprolactam product meets the relevant standards for superior products in GBT13254-2017 "Caprolactam for Industrial Use".

[0033] According to some embodiments of the present invention, in step (2), the drying treatment is performed by steam heating to perform complete evaporation to dryness.

[0034] According to the present invention, the drying process adopts 0.6MPa steam heating.

[0035] According to some embodiments of the present invention, the vacuum degree of the drying process is -101 kPa to -99 kPa.

[0036] According to the present invention, after the distillation residue is discharged from the bottom of the distillation tower, it enters a drying device, which includes but is not limited to a vacuum belt drying device, a vacuum tray drying device, a vacuum rake drying device, and a vertical vacuum stirring drying device.

[0037] According to the present invention, the crude caprolactam obtained after the drying treatment is returned to the distillation system, and the obtained solid residue (solid waste) is subjected to subsequent treatment.

[0038] According to some embodiments of the present invention, in step (2), the crude caprolactam has a chromaticity of <2, an extinction value of <0.2, a PAN value of <8, a basicity of <0.6 mmol / kg, and a volatile base of <0.8 mmol / kg.

[0039] According to some embodiments of the present invention, Figure 2 As shown, the purification method further comprises the following steps:

[0040] (3) washing and separating the solid residue after drying to obtain oligomer 1;

[0041] (4) The waste liquid after washing and separation is subjected to ion exchange, and the clear liquid after ion exchange is dehydrated, and the obtained concentrated liquid is filtered to obtain oligomer 2.

[0042] According to the present invention, the concentrate contains caprolactam and oligomer 2.

[0043] According to the present invention, the chromaticity of the clear solution after ion exchange is reduced to below 20, and the pH is reduced to below 10.

[0044] According to some embodiments of the present invention, in step (3), the water washing separation includes: grinding the solid residue after the drying treatment, adding desalted water, and then performing solid-liquid centrifugal separation.

[0045] According to the present invention, grinding is performed using a grinding method commonly used in the art, for example, a ball mill used in industry.

[0046] According to some embodiments of the present invention, the mass of the desalted water is 5-80 times that of the solid residue, and the desalted water has a conductivity of ≤0.2 μs / cm and a silicate content of ≤20 μg / L.

[0047] According to the present invention, the total hardness of the desalted water is about 0 mg / L.

[0048] According to the present invention, the dried solid residue is ground, desalted water is added, and stirred for approximately 2 hours to thoroughly mix the residue. The residue is then placed in a centrifuge for solid-liquid centrifugal separation. At a centrifuge speed of 4000 r / min, the solid impurities are fully separated from the liquid components, yielding oligomer 1 and a preliminarily separated waste liquid. The preliminarily separated waste liquid is then transferred to a waste liquid collection tank, awaiting subsequent deep purification treatment.

[0049] According to some embodiments of the present invention, in step (4), the ion exchange adopts an anion-cation mixed bed, wherein the cationic adsorption resin accounts for 10-50% of the total volume of the resin.

[0050] According to some embodiments of the present invention, the ion exchange temperature is 25-45°C.

[0051] According to some embodiments of the present invention, the flow rate of the waste liquid is 3-10VB.

[0052] According to the present invention, VB refers to the resin volume multiple.

[0053] According to the present invention, the waste liquid after water washing and separation is subjected to ion exchange for impurity removal, and the waste liquid enters the ion exchange equipment for selective adsorption and exchange of sodium hydroxide and colored impurities, thereby reducing the impurity content and pH of the waste liquid.

[0054] According to the present invention, the ion exchange equipment is a mixed anion and cation bed, wherein the cationic adsorption resin accounts for 10% to 50% of the total resin volume. The ion exchange temperature is 25-45°C, and the flow rate is 3-10VB. After the ion exchange column is saturated with adsorption, the resin is eluted using a specific eluent.

[0055] According to the present invention, the ion exchange device uses a specific eluent to elute the resin after the ion exchange column is saturated with adsorption.

[0056] According to some embodiments of the present invention, in step (4), the dehydration treatment adopts distillation dehydration, and the temperature of the distillation dehydration is 40-100°C; and the pressure is -0.08MPa to 0.05MPa.

[0057] According to the present invention, the clear liquid from the ion exchange process is pumped to a preheater via a feed pump for preheating, then enters the heating chamber of the evaporator. Under the action of the heat, it boils and evaporates, and the resulting secondary steam and concentrated liquid are separated in a separator. The separated secondary steam enters a compressor, where it is compressed to increase its pressure and temperature, and then returns to the evaporator heating chamber as a heating medium, releasing heat and condensing into water for recycling. The feed liquid circulation flow rate is 3-10 times the feed rate, thereby efficiently achieving distillation and dehydration of the ion exchanged material, effectively reducing energy consumption and equipment operation risks. The water obtained by MVR evaporation is returned to the water washing and separation step, and the oligomer 2 obtained by filtering the concentrated slurry is sent to a cracking device for cracking. The filtrate enters the water treatment system for further processing.

[0058] According to some embodiments of the present invention, in step (3), the obtained oligomer 1 is subjected to a cracking treatment.

[0059] According to some embodiments of the present invention, in step (4), the oligomer 2 is subjected to a cracking treatment.

[0060] According to some embodiments of the present invention, in step (4), the water content of the concentrated solution is 2-15 wt %.

[0061] According to the present invention, the concentrated liquid produced after dehydration treatment contains caprolactam and oligomer 2, and has a water content of 2-15 wt%.

[0062] According to a particularly preferred embodiment of the present invention, a purification method for purifying a heavy caprolactam residue is provided, such as Figure 2 As shown, the specific steps include:

[0063] (1) Distillation of heavy residue: The heavy residue produced during the production of caprolactam is transported to a distillation device, which includes but is not limited to ordinary distillation, rectification, and molecular distillation. The heavy residue is pumped into a distillation tower, and the temperature in the tower is set at 120-135°C and the pressure is -98kPa to -101kPa. The tower is equipped with a defoaming net to control the temperature at the top of the distillation tower at 110-120°C. The reflux ratio is maintained at about 2:8. The caprolactam component is vaporized and raised by heating. The caprolactam vapor produced during the distillation process is cooled by a condenser and converted into liquid, ensuring efficient distillation and recovery of caprolactam. Impurities remain at the bottom of the tower to form distillation residue. Among them, the recovery rate of caprolactam is controlled between 60-95%, and the product quality is guaranteed to meet the relevant standards for superior products in GBT13254-2017 "Industrial Caprolactam".

[0064] (2) Treatment of distillation residue: After the distillation residue is discharged from the bottom of the distillation tower, it enters the drying equipment, which includes but is not limited to vacuum belt drying equipment, vacuum disc drying equipment, vacuum rake drying equipment, and vertical vacuum stirring drying equipment. The drying equipment uses 0.6MPa steam heating, and the vacuum degree is -101kPa to -99kPa. The crude caprolactam obtained by drying is returned to the distillation system. The solid residue (solid waste) obtained is ground and added with 5-80 times the mass of the solid waste desalted water. After stirring for 2 hours, it enters the centrifuge for solid-liquid centrifugal separation. Under the condition of a centrifuge speed of 4000r / min, the solid impurities and liquid components therein are fully separated to obtain oligomer 1 and the waste liquid after preliminary separation. The obtained oligomer 1 is sent to the cracking device for cracking treatment. The waste liquid after preliminary separation flows into the waste liquid collection tank and waits for subsequent deep purification treatment.

[0065] (3) Ion exchange impurity removal: The waste liquid enters the ion exchange device for selective adsorption and exchange of sodium hydroxide and colored impurities, reducing the impurity content and pH of the waste liquid. The ion exchange equipment is a mixed anion and cation bed, in which the cationic adsorption resin accounts for 10%-50% of the total resin volume. The ion exchange temperature is 25-45°C, and the flow rate is 3-10VB. After the ion exchange column is saturated with adsorption, the resin is eluted with a specific eluent.

[0066] (4) Dehydration and recovery of oligomer 2: The ion exchange output is transported to the preheater through the feed pump for preheating treatment, and then enters the evaporator heating chamber, where it boils and evaporates under the action of heating. The secondary steam generated is separated from the concentrated liquid in the separator to achieve gas-liquid separation; the separated secondary steam enters the compressor, is compressed to increase the pressure and temperature, and then returns to the evaporator heating chamber as a heating medium, releasing heat and condensing into water for recycling. The process conditions of this distillation dehydration process are: temperature is controlled at 40-100℃; operating pressure is maintained at a negative pressure state of -0.08MPa to 0.05MPa; the water content of the material output is controlled at 2%-15% (mass fraction); the material liquid circulation flow rate is 3-10 times the feed amount, thereby efficiently achieving distillation and dehydration of the ion exchange output and effectively reducing energy consumption and equipment operation risks. The water obtained by MVR evaporation is returned to the water washing separation step, and the oligomer 2 obtained by filtration of the concentrated slurry is sent to the cracking device for cracking treatment, and the filtrate enters the water treatment system for further treatment.

[0067] According to the present invention, through independent treatment and closed-loop reuse, the caprolactam in the heavy residual liquid is first effectively recovered, thereby maximizing the utilization rate of resources and preventing the heavy residual liquid from flowing back to the refining system, avoiding negative impact on the original production system. At the same time, the oligomers are recovered, which not only ensures that the recycled products generate economic benefits, but also significantly reduces the maintenance costs of the equipment due to corrosion and scaling, thereby extending the service life of the device.

[0068] The present invention is an independent impurity removal and refining system, and all heavy residual liquids are processed in this system without having to be put back into the refining system of the production device.

[0069] According to the present invention, since the heavy residue is not returned to the refining system, the refining system's impurities are reduced, the load is lowered, and the quality and stability of the caprolactam finished product of the device are improved. Furthermore, returning the heavy residue directly to the refining system consumes approximately 2 tons of steam per ton of heavy residue, while the present invention consumes less than 1 ton of steam per ton of heavy residue. For a medium-sized plant with an annual output of 300,000 tons, for example, this saves at least 45,000 tons of steam consumption annually. Due to the reduced load on the refining system, the cyclohexanone production section can increase the amount of cyclohexanone added, achieving a production increase of approximately 15%.

[0070] The present invention will be described in detail below through examples. It should be understood that the specific embodiments described herein are only used to illustrate and explain the present invention and are not intended to limit the present invention.

[0071] In the following examples and comparative examples, if no specific conditions are specified, the experiments were carried out under conventional conditions or those recommended by the manufacturer. All reagents and instruments used, if no manufacturer is specified, are commercially available conventional products.

[0072] In the following examples and comparative examples, the components of the heavy residue produced during the caprolactam production process are shown in Table 1.

[0073] Table 1

[0074]

[0075] 1. Parameter determination method:

[0076] Color: GBT 13255.1-2009 Test method for industrial caprolactam Part 1: Determination of color of 50% aqueous solution - Spectrophotometric method.

[0077] Volatile alkali: GBT 13255.4-2009 Test method for industrial caprolactam Part 4: Determination of volatile alkali content - Titration method after distillation.

[0078] Alkalinity: GBT 13255.6-2009 Test methods for industrial caprolactam Part 6: Determination of acidity or alkalinity.

[0079] Extinction value: GBT 13255.5-2009 Test method for industrial caprolactam Part 5: Determination of absorbance at a wavelength of 290 nm.

[0080] PAN: GBT 13255.3-2009 Test method for industrial caprolactam - Part 3: Determination of potassium permanganate absorption - Spectrophotometric method.

[0081] Purity: Determined by gas chromatography. Chromatographic conditions: Vaporizer temperature: 260°C; Detection chamber temperature: 280°C; Column temperature: Programmed temperature: 100°C for 5 minutes, then increase at 10°C / min to 180°C for 8 minutes, then increase at 25°C / min to 230°C for 12 minutes, total run time: 35 minutes; Gradient flow: 1.0 mL / min, then increase to 2.0 mL / min, then maintain for 15 minutes, then increase at 1.0 mL / min to 3.0 mL / min, then maintain for 13 minutes; Hydrogen flow rate: 35 mL / min; Air flow rate: 350 mL / min; Split ratio: 50:1; Injection volume: 0.2 pL.

[0082] 2. Main equipment:

[0083] Distillation equipment: equipped with fillers, defoaming nets, reflux systems and auxiliary heating and condensing systems, used to distill and separate heavy residual liquids to purify them to obtain national standard high-quality caprolactam.

[0084] Drying equipment: including but not limited to all drying equipment including vacuum belt drying, vacuum horizontal drying, vacuum vertical drying, and scraped film evaporator, used to dry the heavy residue distillation residue and recover caprolactam.

[0085] Centrifugal separation equipment: water washing centrifuge, used to wash the dried solid waste and perform solid-liquid separation, obtain oligomers and further process the liquid phase.

[0086] Ion exchange equipment: used to deeply purify the separated waste liquid and remove organic impurities, metal ions and other impurities.

[0087] Dehydration equipment: MVR evaporator is preferred to dehydrate the clear liquid of ion exchange and recover the residual caprolactam and oligomers.

[0088] Example 1

[0089] (1) Distillation of heavy residue:

[0090] The heavy residue produced during caprolactam production is transferred to a conventional distillation unit. This heavy residue is pumped into a distillation tower, set at a temperature of 125°C and a pressure of -100 kPa. A defoaming screen is installed inside the tower, maintaining a top temperature of 115°C and a reflux ratio of 2:8. Heating causes the caprolactam components to vaporize and rise. The caprolactam vapor produced during distillation is cooled in a condenser and converted to liquid, ensuring efficient distillation and recovery of the caprolactam. Impurities remain at the bottom of the tower, forming a distillation residue. The single-pass recovery rate is 85%, and the product quality is shown in Table 2.

[0091] (2) Treatment of distillation residue:

[0092] After the distillation residue is discharged from the bottom of the distillation tower, it enters a drying device, which uses a vacuum belt dryer. The drying device uses 0.6MPa steam heating and a vacuum degree of -101kPa. The crude caprolactam obtained after drying for 10 hours is returned to the distillation system. The resulting solid residue (solid waste) is ground and added with desalted water 10 times the mass of the solid waste. After stirring for 2 hours, it enters a centrifuge for solid-liquid centrifugal separation. At a centrifuge speed of 4000r / min, the solid impurities are fully separated from the liquid components, resulting in oligomer 1 and the waste liquid after preliminary separation. The waste liquid after preliminary separation flows into a waste liquid collection tank, awaiting subsequent deep purification treatment.

[0093] (3) Ion exchange removal:

[0094] The wastewater enters the ion exchange equipment, where it selectively adsorbs and exchanges sodium hydroxide and colored impurities, reducing the wastewater's impurity content and pH. The ion exchange equipment utilizes a mixed anion and cation bed, with the cationic adsorption resin comprising 33% of the total resin volume. The ion exchange temperature is 35°C, and the flow rate is 6VB. Once the ion exchange column is saturated with adsorption, the resin is eluted using a specific eluent.

[0095] (4) Dehydration to recover oligomer 2:

[0096] The effluent from the deionization process is transported to the preheater via a feed pump for preheating, and then enters the evaporator heating chamber. Under the action of the heat, it boils and evaporates, and the secondary steam generated is separated from the concentrated liquid in the separator. The separated secondary steam enters the compressor, is compressed to increase the pressure and temperature, and then returns to the evaporator heating chamber as a heating medium, releasing heat and condensing into water for recycling. The process conditions of this distillation dehydration process are as follows: the temperature is controlled at 60°C; the operating pressure is maintained at a negative pressure state of -0.07MPa (gauge pressure); the moisture content of the material output is controlled at 5% (mass fraction); the feed liquid circulation flow rate is 4 times the feed amount, thereby efficiently achieving distillation and dehydration of the effluent from the deionization process, and effectively reducing energy consumption and equipment operation risks. The water obtained by MVR evaporation is returned to the water washing separation step, and the oligomer 2 obtained by filtration of the concentrated slurry is sent to the cracking device for cracking treatment, and the filtrate enters the water treatment system for further treatment.

[0097] According to calculation, the recovery rate of caprolactam is 99.8%.

[0098] Example 2

[0099] (1) Heavy residue distillation

[0100] The heavy residue produced during caprolactam production is transported to a distillation unit, which uses a rectification device. This heavy residue is pumped into a distillation tower, which is set at a temperature of 135°C and a pressure of -101 kPa. The tower is packed with packing, and the top temperature is controlled at 120°C. The reflux ratio is 2:8. Heating causes the caprolactam components to vaporize and rise. The caprolactam vapor produced during distillation is cooled in a condenser and converted to liquid, ensuring efficient distillation and recovery of caprolactam. Impurities remain at the bottom of the tower, forming a distillation residue. The single-pass recovery rate is 95%, and the product quality is shown in Table 2.

[0101] The other steps are the same as those in Example 1.

[0102] According to calculation, the recovery rate of caprolactam is 99.7%.

[0103] Example 3

[0104] (1) Distillation of heavy residue:

[0105] The heavy residue produced during caprolactam production is transported to a conventional distillation unit. This heavy residue is pumped into a distillation tower, set at a temperature of 135°C and a pressure of -98 kPa. A defoaming screen is installed inside the tower, maintaining a top temperature of 115°C and a reflux ratio of 1:1. Heating causes the caprolactam components to vaporize and rise. The caprolactam vapor produced during distillation is cooled in a condenser and converted to liquid, ensuring efficient distillation and recovery of the caprolactam. Impurities remain at the bottom of the tower, forming a distillation residue. The single-pass recovery rate is 70%, and the product quality is shown in Table 2.

[0106] (2) Treatment of distillation residue:

[0107] After the distillation residue is discharged from the bottom of the distillation tower, it enters a drying device, which uses a vacuum belt dryer. The drying device uses 0.6MPa steam heating and a vacuum degree of -100kPa. The crude caprolactam obtained after drying for 12 hours is returned to the distillation system. The resulting solid residue (solid waste) is ground and added with desalted water 70 times the mass of the solid waste. After stirring for 1 hour, it enters a centrifuge for solid-liquid centrifugal separation. At a centrifuge speed of 4000r / min, the solid impurities are fully separated from the liquid components, resulting in oligomer 1 and the waste liquid after preliminary separation. The waste liquid after preliminary separation flows into a waste liquid collection tank, awaiting subsequent deep purification treatment.

[0108] (3) Ion exchange removal:

[0109] The wastewater enters the ion exchange equipment, where it selectively adsorbs and exchanges sodium hydroxide and colored impurities, reducing the wastewater's impurity content and pH. The ion exchange equipment utilizes a mixed anion and cation bed, with the cationic adsorption resin comprising 50% of the total resin volume. The ion exchange temperature is 25°C, and the flow rate is 10VB. Once the ion exchange column is saturated with adsorption, the resin is eluted using a specific eluent.

[0110] The other steps are the same as those in Example 1.

[0111] According to calculation, the recovery rate of caprolactam is 98.0%.

[0112] Comparative Example 1

[0113] (1) Drying of heavy residual liquid

[0114] The heavy residual liquid produced during the caprolactam production process is transported to a drying facility, which utilizes a vacuum belt dryer. The drying facility utilizes 0.6MPa steam heating and a vacuum of -101kPa. The solid caprolactam product is dried for 15 hours; the product quality is shown in Table 2. The resulting solid residue (solid waste) is ground and then added with desalted water (10 times the mass of the solid waste). After stirring for 2 hours, the product is placed in a centrifuge for solid-liquid centrifugation. At a centrifuge speed of 4000 rpm, the solid impurities are fully separated from the liquid components, resulting in oligomer 1 and the initially separated waste liquid. The initially separated waste liquid flows into a waste liquid collection tank, awaiting subsequent deep purification treatment.

[0115] (2) Ion exchange removal:

[0116] The wastewater enters the ion exchange equipment, where it selectively adsorbs and exchanges sodium hydroxide and colored impurities, reducing the wastewater's impurity content and pH. The ion exchange equipment utilizes a mixed anion and cation bed, with the cationic adsorption resin comprising 33% of the total resin volume. The ion exchange temperature is 35°C, and the flow rate is 6VB. Once the ion exchange column is saturated with adsorption, the resin is eluted using a specific eluent.

[0117] (3) Dehydration to recover oligomer 2:

[0118] The effluent from the deionization process is transported to the preheater via a feed pump for preheating, and then enters the evaporator heating chamber. Under the action of the heat, it boils and evaporates, and the secondary steam generated is separated from the concentrated liquid in the separator. The separated secondary steam enters the compressor, is compressed to increase the pressure and temperature, and then returns to the evaporator heating chamber as a heating medium, releasing heat and condensing into water for recycling. The process conditions of this distillation dehydration process are as follows: the temperature is controlled at 60°C; the operating pressure is maintained at a negative pressure state of -0.07MPa (gauge pressure); the moisture content of the material output is controlled at 5% (mass fraction); the feed liquid circulation flow rate is 4 times the feed amount, thereby efficiently achieving distillation and dehydration of the effluent from the deionization process, and effectively reducing energy consumption and equipment operation risks. The water obtained by MVR evaporation is returned to the water washing separation step, and the oligomer 2 obtained by filtration of the concentrated slurry is sent to the cracking device for cracking treatment, and the filtrate enters the water treatment system for further treatment.

[0119] According to calculation, the recovery rate of caprolactam is 99.8%.

[0120] Comparative Example 2

[0121] The heavy residue produced during caprolactam production is transferred to a conventional distillation unit. This heavy residue is pumped into a distillation tower, set at a temperature of 125°C and a pressure of -100 kPa. A defoaming screen is installed inside the tower, maintaining a top temperature of 115°C and a reflux ratio of 2:8. Heating causes the caprolactam components to vaporize and rise. The caprolactam vapor produced during distillation is cooled in a condenser and converted to liquid, ensuring efficient distillation and recovery of the caprolactam. Impurities remain at the bottom of the tower, forming a distillation residue. The single-pass recovery rate is 85%, and the product quality is shown in Table 2.

[0122] The distillation residue of the heavy residue is returned to the refining system, and the recovery rate of caprolactam is 85%.

[0123] Comparative Example 3

[0124] (1) Vacuum distillation

[0125] The caprolactam distillation residue having a caprolactam content of 85% (wt) was subjected to vacuum distillation at a pressure of 0.1 kPa and a temperature of 115° C., gaseous caprolactam was obtained at the top of the distillation tower, and molten caprolactam at a temperature of 93° C. was obtained after condensation, and a kettle residue having a caprolactam content of 0.6% was obtained at the bottom of the distillation tower;

[0126] (2) Cooling crystallization

[0127] Add pure water containing 10% by weight of crude caprolactam to the molten caprolactam at 93° C. obtained in step (1) to obtain a caprolactam solution at a temperature of 40° C., and then cool it down to 3° C. at a cooling rate of 3° C. / h, and then keep it warm for 35 minutes to obtain uniform caprolactam grains;

[0128] (3) Centrifugation and drying

[0129] The caprolactam crystals obtained in step (2) are centrifuged to obtain caprolactam crystals and centrifugal mother liquor; the caprolactam crystals are dried to obtain a solid caprolactam product; the product quality is shown in Table 2.

[0130] The kettle residue with a caprolactam content of 0.6% obtained in step (1) is incinerated;

[0131] The centrifuged mother liquor obtained in step (3) is fed into the benzene extraction section of the original production process.

[0132] According to calculation, the recovery rate of caprolactam is 65%.

[0133] Table 2

[0134]

[0135] The results in Table 2 indicate that the quality of the caprolactam solid products obtained in Examples 1-3 meets the standards for superior products in GBT 13254-2017, "Caprolactam for Industrial Use," while neither Comparative Examples 1 nor 2 meet these standards. Furthermore, the methods of Examples 1-3 do not return the heavy residue to the refining system, reducing impurities and load in the refining system and enabling full recovery of various high-value components. This demonstrates that the method of the present invention offers significantly superior results.

[0136] The preferred embodiments of the present invention have been described in detail above, but the present invention is not limited thereto. Within the technical concept of the present invention, various simple variations of the technical solution of the present invention may be made, including combining the various technical features in any other appropriate manner. These simple variations and combinations should also be regarded as disclosed in the present invention and fall within the scope of protection of the present invention.

Claims

1. A method for purifying caprolactam heavy residue, characterized in that: The purification method comprises: (1) distilling the caprolactam heavy residue to obtain a caprolactam product; (2) drying the distillation residue to obtain crude caprolactam, and returning to step (1) to continue distillation.

2. The purification method according to claim 1, wherein In step (1), the distillation is carried out in a distillation tower with an inner temperature of 120-135° C. and a pressure of -98 kPa to -101 kPa; and a tower top temperature of 110-120° C.; And / or, the reflux ratio of the distillation is 1:1-4.

3. The purification method according to claim 1 or 2, wherein In step (1), the mass content of caprolactam in the caprolactam residual liquid is 95-99%; And / or, in step (1), the recovery rate of the caprolactam product is 60-95%.

4. The purification method according to any one of claims 1 to 3, wherein In step (2), the drying treatment is performed by steam heating to perform complete evaporation; Preferably, the vacuum degree of the drying process is -101 kPa to -99 kPa.

5. The purification method according to any one of claims 1 to 4, wherein In step (2), the crude caprolactam has a chromaticity of less than 2, an extinction value of less than 0.2, a PAN value of less than 8, a basicity of less than 0.6 mmol / kg, and a volatile base of less than 0.8 mmol / kg.

6. The purification method according to any one of claims 1 to 5, wherein The purification method further comprises the steps of: (3) washing and separating the solid residue after drying to obtain oligomer 1; (4) The waste liquid after washing and separation is subjected to ion exchange, and the clear liquid after ion exchange is dehydrated, and the obtained concentrated liquid is filtered to obtain oligomer 2.

7. The purification method according to claim 6, wherein In step (3), the water washing and separation comprises: grinding the solid residue after the drying treatment, adding desalted water, and then performing solid-liquid centrifugal separation; Preferably, the mass of the desalted water is 5-80 times that of the solid residue, the conductivity of the desalted water is ≤0.2 μs / cm, and the silicate radical is ≤20 μg / L.

8. The purification method according to claim 6 or 7, wherein In step (4), the ion exchange adopts an anion-cation mixed bed, wherein the cationic adsorption resin accounts for 10-50% of the total volume of the resin; and / or, the ion exchange temperature is 25-45° C.; And / or, the flow rate of the waste liquid is 3-10VB.

9. The purification method according to any one of claims 6 to 8, wherein In step (4), the dehydration treatment adopts distillation dehydration, the temperature of the distillation dehydration is 40-100°C, and the pressure is -0.08MPa to 0.05MPa.

10. The purification method according to any one of claims 6 to 9, wherein: In step (3), the obtained oligomer 1 is subjected to a cracking treatment; and / or, in step (4), the oligomer 2 is subjected to a cracking treatment; And / or, in step (4), the water content of the concentrated solution is 2-15 wt%.

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