Nylon 6 polymerization production method capable of fully recycling extractable oligomer

Abstract

The invention relates to a nylon 6 polymerization production method capable of fully recycling extractable oligomers, and relates to the technical field of nylon 6 polymerization production processes, the nylon 6 polymerization production method comprises a nylon 6 polymerization section, a nylon 6 slice extraction section, a nylon 6 wet slice drying section, an extract evaporation and concentration section and an extractable oligomer cracking and recycling section; in an extractable oligomer cracking and recycling section, by adjusting process conditions such as temperature, pressure and water content of an oligomer extraction concentrated solution, deep cracking of extractable substances is realized, and all cracking products can be recycled into a polymerization section for recycling, so that rapid accumulation of cyclic dimers in polymers is inhibited, no residues are generated in the polymerization process, and the production cost is reduced. Accumulation of cyclic dimers in a system can be effectively slowed down, the service life of a polymerization reactor is prolonged, and the quality of polymer products is improved.

Description

Technical Field

[0001] The present application relates to the technical field of nylon 6 polymerization production technology, and in particular to a nylon 6 polymerization production method capable of extracting oligomers for full recycling. Background Art

[0002] Polyamide 6, commonly known as nylon 6 (PA6), is a crystalline polymer with good comprehensive properties and is one of the most widely used types of engineering plastics. Nylon 6 chips, also known as nylon 6 chips, are the second largest synthetic fiber and play an important role in industrial use.

[0003] At present, caprolactam hydrolysis polymerization is the most widely used nylon 6 polymerization production process. Nylon 6 prepolymer is obtained by heating caprolactam and water for polymerization. Caprolactam hydrolysis polymerization is usually carried out in a VK tube (vertical continuous polymerization tube). Since the reaction is a reversible equilibrium reaction, the final polymerization product of the VK tube contains about 10% by mass of extractables, mainly including caprolactam monomers and oligomers. The oligomers mainly include linear oligomers and cyclic oligomers, of which caprolactam accounts for more than 70%. In order to reduce production costs, the direct polymerization technology of extractable concentrate is generally adopted, that is, the concentrate is directly returned to the polymerization reaction section for batching and then participates in the reaction. However, since the extract concentrate contains a large amount of cyclic dimers and cannot melt at the polymerization temperature, it stays in the reaction pipeline and accumulates continuously, which not only causes the quality of nylon 6 products to decline, but also causes the production system to stop due to easy precipitation and blocking of the pipeline, resulting in huge economic losses. Therefore, in actual production, in order to make full use of the unreacted caprolactam monomers in the extract and to avoid the accumulation of cyclic dimers in the pipeline, only a portion of the extract concentrate is recycled back to the polymerization section, and the full recycling of the extractable oligomers cannot be achieved. Summary of the invention

[0004] The present application provides a nylon 6 polymerization production method with fully recycled extractable oligomers, which realizes the full recycling of extractable oligomers and inhibits the rapid accumulation of cyclic dimers in the polymer, so that the polymerization process no longer produces residues, can effectively slow down the accumulation of cyclic dimers in the system, extend the service life of the polymerization reactor, and improve the quality of the polymer product.

[0005] In a first aspect, the present application provides a nylon 6 polymerization production method for fully recycling extractable oligomers, the nylon 6 polymerization production method comprising a nylon 6 polymerization section, a nylon 6 chip extraction section, a nylon 6 wet chip drying section, an extract evaporation concentration section and an extractable oligomer cracking and recycling section; specifically comprising the following steps:

[0006] The nylon 6 polymerization section: subjecting the reaction raw materials to polymerization reaction, followed by cooling and pelletizing to obtain coarse nylon 6 slices;

[0007] The nylon 6 chip extraction section: extracting the crude nylon 6 chips to obtain nylon 6 wet chips and oligomer extract;

[0008] The nylon 6 wet slice drying section: drying the nylon 6 wet slice to obtain nylon 6 slices;

[0009] The extract evaporation and concentration section: evaporating and concentrating the oligomer extract to obtain an oligomer extract concentrate having a water content of 50 wt% to 90 wt%;

[0010] The extractable oligomer cracking and recycling section: at a temperature of 230° C. to 350° C. and a pressure of 1 MPa to 8 MPa, the oligomer extract concentrate with a water content of 50wt% to 90wt% is catalytically cracked to obtain cracking products; all of the cracking products are returned to the nylon 6 polymerization section for recycling.

[0011] Furthermore, the catalyst for catalytic cracking is phosphoric acid, and the amount of phosphoric acid added is 5wt% to 30wt% of the oligomer extract concentrate.

[0012] Furthermore, when all the pyrolysis products are returned to the nylon 6 polymerization section for recycling, the cumulative equilibrium value of the cyclic dimer in the nylon 6 polymerization section is 24-26 kg / hr.

[0013] Furthermore, when all the pyrolysis products are returned to the nylon 6 polymerization section for recycling, the reaction raw materials are composed of the pyrolysis products and fresh caprolactam monomers in a weight ratio of 1:(5-25).

[0014] Furthermore, the working parameters of the catalytic cracking include: reaction temperature of 260°C to 290°C, reaction pressure of 2MPa to 5MPa, reaction time of 2h to 4h, and the addition amount of phosphoric acid catalyst of 20wt% to 30wt% of the oligomer extract concentrate.

[0015] Furthermore, the step of subjecting the reaction raw materials to polymerization reaction, cooling and pelletizing, and obtaining coarse nylon 6 chips comprises:

[0016] The uniformly mixed reaction raw materials are sequentially conveyed to a first polymerizer and a second polymerizer for continuous hydrolysis and ring-opening polymerization to obtain a nylon 6 polymer melt;

[0017] The nylon 6 polymer melt is cooled and granulated, and then sliced ​​to obtain a crude nylon 6 slice;

[0018] The working parameters of the first polymerizer include: reaction pressure of 0.25MPa to 0.70MPa, reaction time of 1h to 5h; the first polymerizer is provided with a first heating section and a first heat transfer section, the heat medium temperature of the first heating section is 240°C to 285°C, and the heat medium temperature of the first heat transfer section is 230°C to 270°C;

[0019] The working parameters of the second polymerizer include: reaction pressure of 0.02MPa~0.25MPa, reaction time of 3~10h; the second polymerizer is provided with a second heating section and a second heat transfer section, the heat medium temperature of the second heating section is 230℃~270℃, and the heat medium temperature of the second heat transfer section is 245℃~275℃.

[0020] Furthermore, the step of extracting the crude nylon 6 slices to obtain nylon 6 wet slices and oligomer extracts comprises:

[0021] The crude nylon 6 slices are subjected to multi-stage continuous extraction by countercurrent extraction to obtain nylon 6 wet slices and oligomer extracts;

[0022] The working parameters of the liquid-solid two-phase countercurrent in the multi-stage continuous extraction include: a liquid-solid ratio of 0.5 to 5.0, an extraction temperature of 65° C. to 130° C., and a caprolactam aqueous solution concentration of 0 to 50 wt %.

[0023] Furthermore, the step of drying the nylon 6 wet slices to obtain nylon 6 slices comprises:

[0024] Using an inert gas to countercurrent dry the nylon 6 wet slices to obtain nylon 6 slices;

[0025] The working parameters of the countercurrent drying include: the temperature of the inert gas is 100° C. to 160° C., and the drying time is 15 h to 35 h.

[0026] Furthermore, the depolymerization rate of cyclic dimers in the extractable oligomer cracking and recycling section is ≥75%, the conversion rate of caprolactam monomer in the reaction raw material is ≥99%, and the mass fraction of extractables in the nylon 6 chips is <0.6%.

[0027] In a second aspect, the present application provides a nylon 6 polymerization production system for realizing the nylon 6 polymerization production method described in any one of the embodiments of the first aspect, wherein the nylon 6 polymerization production system comprises a static mixer, a polymerization reactor, a multi-stage continuous extraction tower, a vertical drying tower, a multi-effect evaporation system and a cracking reactor;

[0028] The polymerization reactor comprises a first polymerizer and a second polymerizer which are sequentially arranged in series, wherein the first polymerizer is connected to the static mixer, and the second polymerizer is connected to the multi-stage continuous extraction tower, and the multi-stage continuous extraction tower is provided with a first extraction medium input end and a second extraction medium input end;

[0029] The multi-effect evaporation system comprises a first-stage evaporation tower and a second-stage evaporation tower which are sequentially arranged in series, wherein the top of the first-stage evaporation tower is connected to the multi-stage continuous extraction tower, a condenser and a reboiler, the condenser is connected to the reboiler and the multi-stage continuous extraction tower, the top of the second-stage evaporation tower is connected to the reboiler, and the bottom of the second-stage evaporation tower is connected to the cracking reactor;

[0030] The outlet end of the cleavage reactor is connected to the inlet end of the static mixer.

[0031] The above technical solution provided by the embodiment of the present application has at least the following advantages compared with the prior art:

[0032] The embodiment of the present application provides a nylon 6 polymerization production method for fully recycling extractable oligomers. The nylon 6 polymerization production method includes a nylon 6 polymerization section, a nylon 6 chip extraction section, a nylon 6 wet chip drying section, an extract evaporation concentration section, and an extractable oligomer cracking and recycling section. In the extractable oligomer cracking and recycling section, by adjusting process conditions such as temperature, pressure, and water content of oligomer extract concentrate, deep cracking of extractables is achieved and all cracking products can be recycled into the polymerization section for reuse, thereby inhibiting the rapid accumulation of cyclic dimers in the polymer, preventing the polymerization process from generating residues, effectively slowing down the accumulation of cyclic dimers in the system, extending the service life of the polymerization reactor, and improving the quality of the polymer product. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the present application and, together with the description, serve to explain the principles of the present application.

[0034] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0035] Figure 1 A schematic diagram of a process for producing nylon 6 by polymerization with extractable oligomers for full recycling provided in an embodiment of the present application;

[0036] Figure 2A schematic diagram of the structure of a device for fully recycling the cracking products of caprolactam for preparing nylon 6 provided in an embodiment of the present application;

[0037] Among them, 100-static mixer; 200-polymerization reactor; 210-first polymerizer; 220-second polymerizer; 300-multi-stage continuous extraction tower; 310-first extraction medium input end; 320-second extraction medium input end; 400-vertical drying tower; 500-multi-effect evaporation system; 510-first stage evaporation tower; 520-second stage evaporation tower; 530-reboiler; 540-condenser; 600-cracking reactor. DETAILED DESCRIPTION

[0038] In order to make the purpose, technical solution and advantages of the embodiments of the present application clearer, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this application.

[0039] Unless otherwise specified, various raw materials, reagents, instruments and equipment used in this application can be purchased from the market or prepared by existing methods.

[0040] In the first aspect, the present application provides a method for producing nylon 6 by polymerization with extractable oligomers for full recycling, such as Figure 1 As shown, the nylon 6 polymerization production method includes a nylon 6 polymerization section, a nylon 6 chip extraction section, a nylon 6 wet chip drying section, an extract evaporation concentration section, and an extractable oligomer cracking and recycling section; specifically includes the following steps:

[0041] The nylon 6 polymerization section: subjecting the reaction raw materials to polymerization reaction, followed by cooling and pelletizing to obtain coarse nylon 6 slices;

[0042] The nylon 6 chip extraction section: extracting the crude nylon 6 chips to obtain nylon 6 wet chips and oligomer extract;

[0043] The nylon 6 wet slice drying section: drying the nylon 6 wet slice to obtain nylon 6 slices;

[0044] The extract evaporation and concentration section: evaporating and concentrating the oligomer extract to obtain an oligomer extract concentrate having a water content of 50 wt% to 90 wt%;

[0045] The extractable oligomer cracking and recycling section: at a temperature of 230° C. to 350° C. and a pressure of 1 MPa to 8 MPa, the oligomer extract concentrate with a water content of 50wt% to 90wt% is catalytically cracked to obtain cracking products; all of the cracking products are returned to the nylon 6 polymerization section for recycling.

[0046] The embodiment of the present application provides a nylon 6 polymerization production method for fully recycling extractable oligomers. The nylon 6 polymerization production method includes a nylon 6 polymerization section, a nylon 6 chip extraction section, a nylon 6 wet chip drying section, an extract evaporation concentration section, and an extractable oligomer cracking and recycling section. In the extractable oligomer cracking and recycling section, by adjusting process conditions such as temperature, pressure, and water content of oligomer extract concentrate, deep cracking of extractables is achieved and all cracking products can be recycled into the polymerization section for reuse, thereby inhibiting the rapid accumulation of cyclic dimers in the polymer, preventing the polymerization process from generating residues, effectively slowing down the accumulation of cyclic dimers in the system, extending the service life of the polymerization reactor, and improving the quality of the polymer product.

[0047] The extractable oligomers in the present application specifically refer to those obtained by extracting nylon 6 chips in the existing nylon 6 polymerization production process (caprolactam hydrolysis polymerization process), which mainly include linear oligomers and cyclic oligomers, among which the content of cyclic oligomers is about 3wt% to 4wt%, and the amount of cyclic dimers accounts for about 70wt% of the total content of cyclic oligomers.

[0048] The present invention optimizes the process on the basis of the existing nylon 6 polymerization production technology, especially by controlling the process conditions such as temperature, pressure, water content of oligomer extraction concentrate in the extractable oligomer cracking and recycling section, the deep cracking of the extractable is achieved and the cracking products can all be recycled into the polymerization section for reuse, which inhibits the rapid accumulation of cyclic dimers in the polymer, so that no residue is generated in the polymerization process, and the accumulation of cyclic dimers in the system can be effectively slowed down, the service life of the polymerization reactor is extended, and the quality of the polymer product is improved.

[0049] In some specific embodiments, the reaction temperature of the extractable oligomer cracking and recycling section of the present application can be 230°C, 240°C, 250°C, 260°C, 270°C, 280°C, 290°C, 300°C, 310°C, 320°C, 330°C, 340°C, 350°C, etc.; preferably 260°C to 290°C.

[0050] In some specific embodiments, the reaction pressure of the extractable oligomer cracking and recycling section of the present application can be 1MPa, 2MPa, 3MPa, 4MPa, 5MPa, 6MPa, 7MPa, 8MPa, etc.; preferably 2MPa to 5MPa. If the reaction pressure of the extractable oligomer cracking and recycling section is too high or too low, the adverse effects are: too high a reaction temperature will lead to the occurrence of side reactions, destroying the structure and performance of the oligomers, and also reducing the reaction efficiency; too low a reaction temperature will slow down the reaction speed, making it difficult to achieve the desired reaction effect, and will also prolong the reaction time and increase energy consumption.

[0051] In some specific embodiments, in the extractable oligomer cracking and recycling section, the water content in the oligomer extract concentrate may be 50wt%, 60wt%, 61wt%, 62wt%, 63wt%, 64wt%, 65wt%, 66wt%, 67wt%, 68wt%, 69wt%, 70wt%, 71wt%, 72wt%, 73wt%, 74wt%, 75wt%, 76wt%, 77wt%, 78wt%, 79wt%, 80wt%, 90wt%, etc., measured in weight percentage; preferably 60wt% to 80wt%. If the water content in the oligomer extraction concentrate of the extractable oligomer cracking and recycling section is too high or too low, the adverse effects are: too high a water content will affect the stability and quality of the concentrate, reduce its dehydration efficiency, and increase the difficulty of processing. At the same time, too high a water content is also prone to corrosion, affecting the life and reliability of the equipment; too low a water content will affect the viscosity and fluidity of the concentrate, increase the energy consumption of the process, and thus affect the economic benefits.

[0052] As an implementation method of an example of the present application, the catalyst for catalytic cracking is phosphoric acid, and the amount of phosphoric acid added is 5wt% to 30wt% of the oligomer extract concentrate.

[0053] Compared with other catalytic cracking catalysts, the advantages of phosphoric acid selected in this application are stable catalytic effect, relatively fast catalytic cracking reaction speed and low toxicity; further controlling the amount of phosphoric acid added to 5wt% to 30wt% of the oligomer extract concentrate is to optimize the effect of phosphoric acid catalytic cracking reaction and prevent excessive addition from causing overreaction and negative effects. In some specific embodiments, the amount of phosphoric acid added can be 5wt%, 6wt%, 7wt%, 8wt%, 9wt%, 10wt%, 11wt%, 12wt%, 13wt%, 14wt%, 15wt%, 16wt%, 17wt%, 18wt%, 19wt%, 20wt%, 21wt%, 22wt%, 23wt%, 24wt%, 25wt%, 26wt%, 27wt%, 28wt%, 29wt%, 30wt%, etc. of the oligomer extract concentrate; preferably 20wt% to 30wt%.

[0054] As an implementation method of the present application, when all the cracking products are returned to the nylon 6 polymerization section for recycling, the cumulative equilibrium value of the cyclic dimer in the nylon 6 polymerization section is 24 to 26 kg / hr.

[0055] In the present application, the cumulative equilibrium value of cyclic dimers in the nylon 6 polymerization section specifically refers to the state in which the formation and decomposition of cyclic dimers reach equilibrium during the polymerization reaction, and the concentration of cyclic dimers remains stable at this time; the role of controlling it to 24-26 kg / hr is to reduce the loss caused by cyclic dimers, improve process stability and save costs, so as to ensure that the caprolactam monomer conversion rate in the system is greater than ≥99%. If the cumulative equilibrium value of cyclic dimers is too large or too small, the adverse effect is that the reaction process is unstable, resulting in a decrease in the physical properties of the product.

[0056] As an implementation method of an example of the present application, when all the pyrolysis products are returned to the nylon 6 polymerization section for recycling, the reaction raw materials are composed of the pyrolysis products and fresh caprolactam monomers in a weight ratio of 1:(5-25).

[0057] When all the pyrolysis products are returned to the nylon 6 polymerization section for recycling, the weight ratio of the pyrolysis products and the fresh caprolactam monomer in the present application is controlled to be 1: (5-25) to reduce production costs, and can reasonably balance the recovery of products and the use of new raw materials to achieve the best solution for combining recycling and production benefits; if the weight ratio of the pyrolysis products and the fresh caprolactam monomer is too large or too small, the adverse effect is that the production process becomes unstable, so that the pyrolysis products that can be used cannot be effectively used, and resources and products are wasted. In some specific embodiments, the weight ratio of the pyrolysis products and the fresh caprolactam monomer can be 1: 5, 1: 6, 1: 7, 1: 8, 1: 9, 1: 10, 1: 11, 1: 12, 1: 13, 1: 14, 1: 15, 1: 16, 1: 17, 1: 18, 1: 19, 1: 20, 1: 21, 1: 22, 1: 23, 1: 24, 1: 25, etc.; preferably 1: (8-15).

[0058] As an implementation method of an example of the present application, the working parameters of the catalytic cracking include: a reaction temperature of 260°C to 290°C, a reaction pressure of 2MPa to 5MPa, a reaction time of 2h to 4h, and an amount of phosphoric acid catalyst added of 20wt% to 30wt% of the oligomer extract concentrate.

[0059] When the working parameters of the extractable oligomer cracking and recycling section are controlled by the present application to be the above-mentioned specific parameters, the deep cracking of the extractable oligomer can be further achieved. By adjusting the process conditions of residence time, temperature, pressure, water content of the concentrate, and phosphoric acid content, more than 80% of the cyclic dimers can be efficiently cracked, and all the cracking products are recycled into the polymerization section for reuse, which inhibits the rapid accumulation of cyclic dimers in the polymer, so that the polymerization process no longer produces residues, and the service life of the polymerization reactor is extended.

[0060] As an implementation method of the present application, the step of subjecting the reaction raw materials to polymerization reaction, cooling and pelletizing to obtain coarse nylon 6 slices comprises:

[0061] The uniformly mixed reaction raw materials are sequentially conveyed to a first polymerizer and a second polymerizer for continuous hydrolysis and ring-opening polymerization to obtain a nylon 6 polymer melt;

[0062] The nylon 6 polymer melt is cooled and granulated, and then sliced ​​to obtain a crude nylon 6 slice;

[0063] The working parameters of the first polymerizer include: reaction pressure of 0.25MPa to 0.70MPa, reaction time of 1h to 5h; the first polymerizer is provided with a first heating section and a first heat transfer section, the heat medium temperature of the first heating section is 240°C to 285°C, and the heat medium temperature of the first heat transfer section is 230°C to 270°C;

[0064] The working parameters of the second polymerizer include: reaction pressure of 0.02MPa~0.25MPa, reaction time of 3~10h; the second polymerizer is provided with a second heating section and a second heat transfer section, the heat medium temperature of the second heating section is 230℃~270℃, and the heat medium temperature of the second heat transfer section is 245℃~275℃.

[0065] The present application selects the above process to carry out the nylon 6 polymerization section, so that the polymerization effect of the fully recycled cracking products and fresh caprolactam monomers is good, thereby improving the quality of the polymer products. Specifically, the first polymerizer (or pre-polymerizer) and the second polymerizer (or post-polymerizer) are both provided with a heating section and a heat transfer section, which are used to provide heat and remove heat respectively; the heat medium temperature of the heating section in the first polymerizer is 240 to 285°C, preferably 248 to 272°C; the heat medium temperature of the heat transfer section in the first polymerizer is 230 to 270°C, preferably 240 to 270°C; the heat medium temperature of the heating section in the second polymerizer is 230 to 270°C, preferably, The temperature of the heat medium in the heat transfer section of the second polymerizer is 245-275°C, preferably 250-268°C; the pressure of the first polymerizer is 0.25-0.70MPa, preferably 0.34-0.55MPa; the reaction time is 1-5h, preferably 1.5-3h; the pressure of the second polymerizer is 0.02-0.25MPa, preferably 0.05-0.20MPa; the reaction time is 3-10h, preferably 3.5-6.5h.

[0066] In some specific embodiments, the reaction pressure of the first polymerizer may be 0.25 MPa, 0.35 MPa, 0.45 MPa, 0.55 MPa, 0.65 MPa, 0.70 MPa, etc.; the reaction time may be 1 h, 2 h, 3 h, 4 h, 5 h, etc.

[0067] In some specific embodiments, the temperature of the heat medium in the first heating section may be 240°C, 250°C, 260°C, 270°C, 280°C, 285°C, etc.

[0068] In some specific embodiments, the temperature of the heat medium in the first heat transfer section may be 230° C., 240° C., 250° C., 260° C., 270° C., etc.

[0069] In some specific embodiments, the reaction pressure of the second polymerizer may be 0.02MPa, 0.03MPa, 0.04MPa, 0.05MPa, 0.06MPa, 0.07MPa, 0.08MPa, 0.09MPa, 0.10MPa, 0.11MPa, 0.12MPa, 0.13MPa, 0.14MPa, 0.15MPa, 0.16MPa, 0.17MPa, 0.18MPa, 0.19MPa, 0.20MPa, 0.21MPa, 0.22MPa, 0.23MPa, 0.24MPa, 0.25MPa, etc.; the reaction time may be 3h, 4h, 5h, 6h, 7h, 8h, 9h, 10h, etc.

[0070] In some specific embodiments, the heat medium temperature of the second heating section may be 230°C, 240°C, 250°C, 260°C, 260°C, 270°C, etc.; the heat medium temperature of the second heat transfer section may be 245°C, 255°C, 265°C, 275°C, etc.

[0071] In some specific embodiments, the above nylon 6 polymerization stage can be carried out in a polymerization reactor. Specifically:

[0072] The polymerization reactor comprises at least two first polymerizers and second polymerizers which are arranged in series. The reaction raw materials form feed streams, which include fresh caprolactam feed streams and cracking product feed streams of the extract concentrate. The cracking products of the extract concentrate are derived from the products of catalytic cracking of oligomers in the cracking and recycling section, including unreacted caprolactam monomers.

[0073] The feed stream can be mixed and preheated in a static mixer (the preheating temperature can be 200°C) and then passed through a first polymerizer and a second polymerizer in sequence, where continuous hydrolysis and ring-opening polymerization reactions occur in sequence to obtain nylon 6 polymer, which is discharged from the bottom of the second polymerizer in the form of a melt, cooled and granulated to obtain crude nylon 6 chips.

[0074] As an implementation method of the present application, the step of extracting the crude nylon 6 slice to obtain nylon 6 wet slice and oligomer extract includes:

[0075] The crude nylon 6 slices are subjected to multi-stage continuous extraction by countercurrent extraction to obtain nylon 6 wet slices and oligomer extracts;

[0076] The working parameters of the liquid-solid two-phase countercurrent in the multi-stage continuous extraction include: a liquid-solid ratio of 0.5 to 5.0, an extraction temperature of 65° C. to 130° C., and a caprolactam aqueous solution concentration of 0 to 50 wt %.

[0077] The crude nylon 6 slices described in the present application contain small molecules, which mainly include unreacted monomer caprolactam, cyclic dimers, cyclic trimers and linear oligomers; after the small molecules are extracted through the slice extraction section, the oligomer extract is obtained.

[0078] The multi-stage continuous extraction of the present application can adopt two-stage feeding or two towers in series, the extraction medium of the upper section of the multi-stage continuous extraction tower or the first stage extraction tower is a caprolactam solution, and the medium of the lower section of the multi-stage continuous extraction tower or the second stage extraction tower is hot water. In some specific embodiments, the liquid-solid ratio of the liquid-solid two-phase countercurrent in the multi-stage continuous extraction is preferably 1.0 to 3.0, the extraction temperature is 85°C to 115°C, and the concentration of the caprolactam aqueous solution is 10wt% to 30wt%.

[0079] In some specific embodiments, the nylon 6 chips extraction process can be carried out in a multi-stage continuous extraction tower. Specifically:

[0080] The crude nylon 6 chips are extracted by the extraction medium in the multi-stage continuous extraction tower 300 to obtain an extract containing caprolactam monomers and oligomers and nylon 6 wet chips; the extract is discharged from the upper part of the multi-stage continuous extraction tower 300, and the nylon 6 wet chips are discharged from the bottom of the tower.

[0081] The multi-stage continuous extraction tower is provided with multiple layers of internal components to improve the mass transfer efficiency in the tower; the multi-stage continuous extraction tower is a two-stage feed or two towers connected in series. The extraction medium includes a caprolactam solution and hot water. Specifically, the multi-stage continuous extraction tower with two-end feed includes an upper section and a lower section connected up and down, and the bottom of the upper section is provided with a first extraction medium input end 310, and the caprolactam solution is input into the upper section through the first extraction medium. The bottom of the lower section is provided with a second extraction medium input end, and hot water is input into the tower through the second extraction medium input end.

[0082] The multi-stage continuous extraction tower with two towers connected in series includes a first-stage extraction tower and a second-stage extraction tower arranged in series, wherein the extraction medium of the first-stage extraction tower is caprolactam solution, and the medium of the second-stage extraction tower is hot water, and the temperature of the hot water is the same as the extraction temperature.

[0083] As an implementation method of the present application, the step of drying the nylon 6 wet slices to obtain nylon 6 slices includes:

[0084] Using an inert gas to countercurrent dry the nylon 6 wet slices to obtain nylon 6 slices;

[0085] The working parameters of the countercurrent drying include: the temperature of the inert gas is 100° C. to 160° C., and the drying time is 15 h to 35 h.

[0086] In some specific embodiments, the nylon 6 wet chip drying process can be carried out in a vertical drying tower. Specifically:

[0087] The nylon wet chips drying process is carried out in a vertical drying tower. The nylon 6 wet chips enter through the bottom of the vertical drying tower, and the drying medium enters through the bottom of the tower. The nylon 6 wet chips are dried in the tower to remove the moisture in the chips, and the nylon 6 chips are obtained as the final product, which is used for subsequent processing and molding. Among them, the cyclic dimer content of the nylon 6 chips is ≤0.6%.

[0088] The bottom and middle of the vertical drying tower are equipped with multi-stage gas distributors, and the drying medium is an inert gas such as hot nitrogen; the hot nitrogen enters from the bottom of the vertical drying tower, flows from bottom to top after being distributed by the multi-stage gas distributor, and the nylon 6 wet slices flow from top to bottom. The nylon 6 wet slices are in reverse contact with the hot nitrogen, and the moisture inside and outside the nylon 6 wet slices is gradually taken away by the hot nitrogen until it reaches the bottom of the vertical drying tower and is discharged to obtain the dried nylon 6 slices final product. Preferably, the gas velocity of the hot nitrogen is 0.1 to 2 m / s, preferably 0.2 to 1 m / s; the nitrogen temperature is 100 to 160°C, preferably 110 to 135°C; the drying time is 15 to 35 hours, preferably 22 to 30 hours.

[0089] As an implementation method of the present application, the depolymerization rate of cyclic dimers in the extractable oligomer cracking and recycling section is ≥75%, the conversion rate of caprolactam monomer in the reaction raw material is ≥99%, and the mass fraction of extractables in the nylon 6 chips is <0.6%.

[0090] In the present application, the extractable oligomers are concentrated and cracked to obtain a recycling polymerization process, the depolymerization rate of the cyclic dimer is ≥75%, and the deep cracking and full recovery process of the oligomer cracking products is realized; at the same time, the nylon 6 polymerization section, the nylon 6 chip extraction section, the nylon 6 wet chip drying section, the extract evaporation concentration section and the extractable oligomer cracking and recycling section work together to achieve the utilization rate of caprolactam (caprolactam monomer conversion rate ≥99%), and improve the quality of the polymer product (the mass fraction of extractables in nylon 6 chips is <0.6%). It is a green and efficient new polymerization process with advanced technology and is in line with the current trend of high-quality development of the new chemical materials industry.

[0091] It should be noted that the steps and specific operating parameters involved in the nylon 6 polymerization production method for fully recycled extractable oligomers provided in this application, if there are no special limitations and specific requirements, can adopt the conventional existing steps in the field, and the specific operating parameters can be selected according to the existing process and actual needs, and will not be described one by one here.

[0092] In a second aspect, based on a general inventive concept, the present application provides a nylon 6 polymerization production system for implementing the nylon 6 polymerization production method described in any embodiment of the first aspect, such as Figure 2 As shown, the nylon 6 polymerization production system includes a static mixer, a polymerization reactor, a multi-stage continuous extraction tower, a vertical drying tower, a multi-effect evaporation system and a cracking reactor;

[0093] The polymerization reactor comprises a first polymerizer and a second polymerizer which are sequentially arranged in series, wherein the first polymerizer is connected to the static mixer, and the second polymerizer is connected to the multi-stage continuous extraction tower, and the multi-stage continuous extraction tower is provided with a first extraction medium input end and a second extraction medium input end;

[0094] The multi-effect evaporation system comprises a first-stage evaporation tower and a second-stage evaporation tower which are sequentially arranged in series, wherein the top of the first-stage evaporation tower is connected to the multi-stage continuous extraction tower, a condenser and a reboiler, the condenser is connected to the reboiler and the multi-stage continuous extraction tower, the top of the second-stage evaporation tower is connected to the reboiler, and the bottom of the second-stage evaporation tower is connected to the cracking reactor;

[0095] The outlet end of the cleavage reactor is connected to the inlet end of the static mixer.

[0096] The present application provides a nylon 6 polymerization production system used in conjunction with the nylon 6 polymerization production method described in the first aspect, including a polymerization reactor 200, a multi-stage continuous extraction tower 300, a vertical drying tower 400, a multi-effect evaporation system 500 and a cracking reactor 600, which are used to sequentially carry out the nylon 6 polymerization production method, including a nylon 6 polymerization section, a nylon 6 slice extraction section, a nylon 6 wet slice drying section, an extract evaporation concentration section and an extractable oligomer cracking and recycling section. The full cycle recycling of the extractable oligomer is achieved, and the rapid accumulation of cyclic dimers in the polymer is suppressed, so that the polymerization process no longer produces residues, which can effectively slow down the accumulation of cyclic dimers in the system, extend the service life of the polymerization reactor, and improve the quality of the polymer product.

[0097] It should be noted that the equipment and equipment connection methods involved in the nylon 6 polymerization production system provided in this application, if there are no special limitations and requirements, can adopt conventional existing equipment in the field and be connected according to existing process design methods, and will not be described one by one here.

[0098] The present application will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are intended only to illustrate the present application and are not intended to limit the scope of the present application. The experimental methods for which specific conditions are not specified in the following examples are usually measured according to national standards. If there is no corresponding national standard, then the conditions recommended by the manufacturer are followed.

[0099] Embodiments 1 to 15 provide a nylon 6 polymerization production method with fully recycled extractable oligomers, including a nylon 6 polymerization section, a nylon 6 chip extraction section, a nylon 6 wet chip drying section, an extract evaporation concentration section and an extractable oligomer cracking and recycling section, in which cyclic dimers are efficiently and deeply cracked, and finally the obtained cracking products containing caprolactam monomers are all recycled into the polymerization section for reuse, the extractable content in nylon 6 can be reduced to below 0.6%, and the caprolactam monomer conversion rate can reach ≥99%.

[0100] like Figure 1 and Figure 2 As shown, the nylon 6 polymerization production method for fully recycling the extractable oligomers in each of the above examples is carried out using the nylon 6 polymerization production system, and the specific process includes:

[0101] First, the nylon 6 polymerization section is carried out in the polymerization reactor 200. The polymerization reactor 100 includes a first polymerization reactor 210 and a second polymerization reactor 220. The reaction raw materials form a feed stream, which includes a fresh caprolactam feed stream and a cracked product feed stream of an extract concentrate. The cracked product of the extract concentrate is derived from the product of catalytic cracking of oligomers in the cracking and recycling section, including unreacted caprolactam monomers.

[0102] The feed stream is mixed and preheated in the static mixer 100 and then passes through the first polymerizer 210 to the second polymerizer 220 in sequence, where continuous hydrolysis and ring-opening polymerization reactions occur in sequence to obtain nylon 6 polymer, which is discharged from the bottom of the second polymerizer 220 in the form of a melt, cooled and granulated to obtain crude nylon 6 chips.

[0103] The reaction in the second polymerizer 210 is carried out under high temperature and high pressure conditions. The reaction in the second polymerizer 220 is carried out under high temperature and low pressure or high temperature and negative pressure conditions. The degree of polymerization gradually increases with the residence time until a high molecular weight nylon 6 polymer melt is obtained at the bottom of the post-polymerizer 210; the nylon 6 polymer melt is discharged from the bottom outlet of the post-polymerizer; the mass ratio of the pyrolysis product of the extract concentrate to the fresh caprolactam is 1:5 to 1:25, preferably 1:8 to 1:15.

[0104] The first polymerizer 210 is provided with a first heating section (or front heating section) and a second heat transfer section (or front heat transfer section) for providing heat and removing heat. The heat medium temperature of the first heating section is 240-285°C, preferably 248-272°C. The heat medium temperature of the second heat transfer section is 230-270°C, preferably 240-270°C. The pressure is 0.25-0.70MPa, preferably 0.34-0.55MPa; the reaction time is 1-5h, preferably 1.5-3h.

[0105] The second polymerizer 220 is provided with a second heating section (or called post-heating section) and a second heat transfer section (or called post-heat transfer section) for providing heat and removing heat. The heat medium temperature of the second heating section is 230-270°C, preferably 240-258°C; the heat medium temperature of the second heat transfer section is 245-275°C, preferably 250-268°C; the pressure is 0.02-0.25MPa, preferably 0.05-0.20MPa; the reaction time is 3-10h, preferably 3.5-6.5h.

[0106] Secondly, the extraction of the slice extraction section is carried out in a multi-stage continuous extraction tower. The crude nylon 6 slice contains small molecules, which mainly include unreacted monomer caprolactam, cyclic dimers, cyclic trimers and linear oligomers; the slice extraction section allows the small molecules to enter the extraction liquid through the extraction medium. The granular crude nylon 6 slice moves from top to bottom in the multi-stage continuous extraction tower, and the extraction medium flows from bottom to top, forming a countercurrent extraction; the liquid-solid two-phase countercurrent operation condition in the multi-stage continuous extraction tower is a liquid-solid ratio of 0.5 to 5.0; preferably 1.0 to 3.0.

[0107] The crude nylon 6 chips are extracted by the extraction medium in the multi-stage continuous extraction tower 300 to obtain an extract containing caprolactam monomers and oligomers and nylon 6 wet chips. The extract is discharged from the upper part of the multi-stage continuous extraction tower 300, and the nylon 6 wet chips are discharged from the bottom of the tower.

[0108] The multi-stage continuous extraction tower 300 is provided with multiple layers of internal components to improve the mass transfer efficiency in the tower; the multi-stage continuous extraction tower is a two-stage feed or two towers connected in series. The extraction medium includes a caprolactam solution and hot water. Specifically, the multi-stage continuous extraction tower with two-end feed includes an upper section and a lower section connected up and down, and the bottom of the upper section is provided with a first extraction medium input end 310, and the caprolactam solution is input into the upper section through the first extraction medium. The bottom of the lower section is provided with a second extraction medium input end 320, and hot water is input into the tower through the second extraction medium input end 320.

[0109] The multi-stage continuous extraction tower with two towers connected in series includes a first-stage extraction tower and a second-stage extraction tower arranged in series, wherein the extraction medium of the first-stage extraction tower is a caprolactam solution, and the medium of the second-stage extraction tower is hot water, and the temperature of the hot water is the same as the extraction temperature. The extraction temperature is 65-130°C, preferably 85-115°C; the concentration of the caprolactam aqueous solution is 0-50wt%, preferably 10-30wt%.

[0110] Thirdly, the extract obtained through the slice extraction section and the nylon 6 wet slices enter different processes, the former entering the concentration evaporation section and the latter entering the wet slice drying section.

[0111] On the one hand, the wet chip drying process is carried out in the vertical drying tower 400, the nylon 6 wet chips enter through the bottom of the vertical drying tower 400, the drying medium enters through the bottom of the tower, and is countercurrent dried in the tower to dry the nylon 6 wet chips to remove moisture from the chips, and obtain nylon 6 chips as the final product, which is used for subsequent processing and molding. Among them, the cyclic dimer content of the nylon 6 chips is ≤0.6%.

[0112] Specifically, the bottom and middle of the vertical drying tower 400 are provided with multi-stage gas distributors, and the drying medium is hot nitrogen; the hot nitrogen enters from the bottom of the vertical drying tower, flows from bottom to top after being distributed by the multi-stage gas distributor, and the nylon 6 wet chips flow from top to bottom, and the nylon 6 wet chips are in reverse contact with the hot nitrogen, and the moisture inside and outside the nylon 6 wet chips is gradually taken away by the hot nitrogen until it reaches the bottom of the vertical drying tower and is discharged to obtain the dried nylon 6 chips final product. Preferably, the gas velocity of the hot nitrogen is 0.1 to 2 m / s, preferably 0.2 to 1 m / s; the nitrogen temperature is 100 to 160°C, preferably 110 to 135°C; the drying time is 15 to 35 hours, preferably 22 to 30 hours.

[0113] On the other hand, the concentration and evaporation process is carried out in the multiple-effect evaporation system 500, and the extract is concentrated by evaporation to form an extract concentrate, and the concentration of the extract concentrate is 50-90%, preferably 60-80%.

[0114] The multi-effect evaporation system 500 includes at least two evaporation towers arranged in series; this embodiment takes a two-stage series evaporation tower as an example. They are a first-stage evaporation tower 510 and a second-stage evaporation tower 520. The heat source of the reboiler 530 of the first-stage evaporation tower 510 is provided by the secondary steam of the second-stage evaporation tower 520. The water vapor evaporated from the top of the first-stage evaporation tower 510 is condensed and cooled by the condenser 540, part of which is used as the top reflux of the first-stage evaporation tower 510 and / or the second-stage evaporation tower 520, and part of which is returned to the multi-stage continuous extraction tower 300 in the extraction process as extraction water, realizing a closed-loop circulation of water.

[0115] Finally, the cracking and recycling section is carried out in the cracking reactor 600. The linear oligomers, cyclic dimers and trimers contained in the extract concentrate are catalytically cracked in the cracking reactor to obtain cracking products; all of the cracking products are returned to the polymerization reactor in the polymerization section for recycling.

[0116] The cracking catalyst is phosphoric acid, and the addition amount is 5-30wt% of the extract concentrate, preferably 20-30wt%.

[0117] The catalytic cracking of the extract concentrate is carried out under high temperature and high pressure conditions, the temperature is 230-350° C., preferably 260-290° C.; the pressure is 1-8 MPa, preferably 2-5 MPa.

[0118] The extract concentrate is subjected to catalytic cracking under sufficient mixing to reduce the content of cyclic dimers, with a depolymerization rate of ≥75%.

[0119] 100% of the cracking products obtained by catalytic cracking are recycled to the inlet of the polymerization reactor, and the cumulative equilibrium value of the cyclic dimer in the nylon 6 chips in the polymerization section is controlled to be 24-26 kg / hr.

[0120] Through the hydrolysis polymerization of caprolactam, it enters the polymerization section after preheating, and the polymer product contains about 10% extractables. The extractables are mainly unreacted caprolactam, cyclic oligomers and a small amount of linear oligomers. In the extraction section, the extractables are extracted by hot water to form an extract, and the nylon 6 slices enter the drying section. The extract forms an extract concentrate with a concentration of 50-90% in the concentration and evaporation section. The extract concentrate is deeply cracked by the oligomers through catalytic cracking reaction to obtain cracking products. In addition to the small molecules soluble in water produced by the cracking of oligomers in the cracking products, a large amount of unreacted caprolactam is also included. All the cracking products are mixed with fresh caprolactam monomers in a mass ratio of 1:5 to 1:25 and enter the polymerization section to re-join the polymerization reaction, thereby fully utilizing the caprolactam monomer, and the utilization rate of caprolactam can reach ≥99%.

[0121] The specific reaction parameters of each section in Example 1 are:

[0122] Nylon 6 polymerization section: the reaction pressure of the first polymerizer is 0.25MPa, and the reaction time is 1h; the heat medium temperature of the first heating section is 240°C, and the heat medium temperature of the first heat transfer section is 230°C; the reaction pressure of the second polymerizer is 0.02MPa, and the reaction time is 3h; the heat medium temperature of the second heating section is 230°C, and the heat medium temperature of the second heat transfer section is 245°C; when all the pyrolysis products are returned to the nylon 6 polymerization section for recycling, the cumulative equilibrium value of cyclic dimers in the nylon 6 polymerization section is 24kg / hr, and the reaction raw materials are composed of the pyrolysis products and fresh caprolactam monomers in a weight ratio of 1:5;

[0123] Nylon 6 chip extraction section: the liquid-solid ratio of the liquid-solid two-phase countercurrent in the multi-stage continuous extraction is 1, the extraction temperature is 85°C, and the concentration of the caprolactam aqueous solution is 10wt%;

[0124] Nylon 6 wet chip drying section: the hot nitrogen gas velocity is 1m / s, the nitrogen temperature is 120℃, and the drying time is 25h.

[0125] Extractable oligomer cracking and recycling section: the water content in the oligomer extraction concentrate is 50wt%; the amount of phosphoric acid catalyst added is 5wt% of the oligomer extraction concentrate; the reaction temperature of the extractable oligomer catalytic cracking is 230°C, the reaction pressure is 1MPa, and the reaction time is 2 hours.

[0126] In Example 1, the depolymerization rate of cyclic dimers in the extractable oligomer cracking and recycling section was 81%, the conversion rate of caprolactam monomer in the reaction raw material was 76%, and the mass fraction of extractables in the nylon 6 chips was 2.7%.

[0127] Examples 2-8 investigate the influence of the working parameters of the extractable oligomer cracking and recycling section.

[0128] On the basis of Example 1, Examples 2-8 only adjusted the working parameters of the extractable oligomer cracking and recycling section (the remaining steps and parameters were the same as those in Example 1). The working parameters of the extractable oligomer cracking and recycling section in Examples 2-8 are shown in Table 1, and the depolymerization rate of the cyclic dimer, the caprolactam monomer conversion rate and the mass fraction of the extractables in the obtained nylon 6 chips are shown in Table 2; wherein, the amount of phosphoric acid catalyst added is measured as "percentage by weight of the oligomer extract concentrate".

[0129] Table 1 Working parameters of the extractable oligomer cracking and recycling section of Examples 2-8

[0130]

[0131] Table 2

[0132]

[0133]

[0134] Examples 9-15 investigate the influence of the working parameters of the nylon 6 polymerization section.

[0135] On the basis of Example 1, Examples 9-15 only adjusted the working parameters of the nylon 6 polymerization section (the remaining steps and parameters were the same as in Example 1). The working parameters of the extractable oligomer cracking and recycling section in Examples 9-15 are shown in Table 3, and the depolymerization rate of the cyclic dimer, the conversion rate of the caprolactam monomer and the mass fraction of the extractables in the obtained nylon 6 chips are shown in Table 4.

[0136] Table 3 Working parameters of nylon 6 polymerization section in Examples 9-15

[0137]

[0138]

[0139] Table 4

[0140]

[0141]

[0142] Comparative Example 1

[0143] Based on Example 1, the difference between this comparative example 1 and Example 1 is that the cumulative equilibrium value of the cyclic dimer in the nylon 6 polymerization section is 20 kg / hr (that is, the cumulative equilibrium value of the cyclic dimer in the nylon 6 polymerization section is too small), and the other steps and parameters are the same.

[0144] In Comparative Example 1, the depolymerization rate of cyclic dimers in the extractable oligomer cracking and recycling section was 79%, the conversion rate of caprolactam monomer in the reaction raw material was 72%, and the mass fraction of extractables in the nylon 6 chips was 3.2%.

[0145] Comparative Example 2

[0146] Based on Example 1, the difference between this comparative example 1 and Example 1 is that the cumulative equilibrium value of the cyclic dimer in the nylon 6 polymerization section is 30 kg / hr (that is, the cumulative equilibrium value of the cyclic dimer in the nylon 6 polymerization section is too large), and the other steps and parameters are the same.

[0147] In Comparative Example 2, the depolymerization rate of cyclic dimers in the extractable oligomer cracking and recycling section was 78%, the conversion rate of caprolactam monomer in the reaction raw material was 71%, and the mass fraction of extractables in the nylon 6 chips was 3.5%.

[0148] Comparative Example 3

[0149] Based on Example 1, the difference between this comparative example 1 and Example 1 is that the reaction raw materials are composed of the pyrolysis product and fresh caprolactam monomer in a weight ratio of 1:2 (i.e., the amount of pyrolysis product added in the nylon 6 polymerization section is too much), and the other steps and parameters are the same.

[0150] In Comparative Example 3, the depolymerization rate of cyclic dimers in the extractable oligomer cracking and recycling section was 72%, the conversion rate of caprolactam monomer in the reaction raw material was 68%, and the mass fraction of extractables in the nylon 6 chips was 4.0%.

[0151] Comparative Example 4

[0152] Based on Example 1, the difference between this comparative example 1 and Example 1 is that the reaction raw materials are composed of the pyrolysis product and fresh caprolactam monomer in a weight ratio of 1:30 (that is, the amount of pyrolysis product added in the nylon 6 polymerization section is too small), and the other steps and parameters are the same.

[0153] In Comparative Example 4, the depolymerization rate of cyclic dimers in the extractable oligomer cracking and recycling section was 70%, the conversion rate of caprolactam monomer in the reaction raw material was 65%, and the mass fraction of extractables in the nylon 6 chips was 4.5%.

[0154] Comparative Example 5

[0155] Based on Example 1, the difference between this comparative example 1 and Example 1 is only that the catalytic cracking reaction temperature in the extractable oligomer cracking and recycling section is adjusted to 200°C (that is, the catalytic cracking reaction temperature in the extractable oligomer cracking and recycling section is too low), and the other steps and parameters are the same.

[0156] In Comparative Example 5, the depolymerization rate of cyclic dimers in the extractable oligomer cracking and recycling section was 65%, the conversion rate of caprolactam monomer in the reaction raw material was 55%, and the mass fraction of extractables in the nylon 6 chips was 5.6%.

[0157] Comparative Example 6

[0158] Based on Example 1, the difference between this comparative example 1 and Example 1 is only that the catalytic cracking reaction temperature in the extractable oligomer cracking and recycling section is adjusted to 400° C. (that is, the catalytic cracking reaction temperature in the extractable oligomer cracking and recycling section is too high), and the remaining steps and parameters are the same.

[0159] In Comparative Example 6, the depolymerization rate of cyclic dimers in the extractable oligomer cracking and recycling section was 66%, the conversion rate of caprolactam monomer in the reaction raw material was 57%, and the mass fraction of extractables in the nylon 6 chips was 4.7%.

[0160] In summary, the nylon 6 polymerization production method includes a nylon 6 polymerization section, a nylon 6 chip extraction section, a nylon 6 wet chip drying section, an extract evaporation concentration section and an extractable oligomer cracking and recycling section. In the extractable oligomer cracking and recycling section, by adjusting the process conditions such as temperature, pressure, and water content of the oligomer extract concentrate, the deep cracking of the extractable is achieved and the cracking products can be fully recycled into the polymerization section for reuse, which inhibits the rapid accumulation of cyclic dimers in the polymer, so that no residue is generated in the polymerization process, can effectively slow down the accumulation of cyclic dimers in the system, extend the service life of the polymerization reactor, and improve the quality of the polymer product.

[0161] Various embodiments of the present application may be presented in the form of a range; it should be understood that the description in the form of a range is only for convenience and brevity, and should not be understood as a rigid limitation on the scope of the present application; therefore, the range description should be considered to have specifically disclosed all possible sub-ranges and single numerical values ​​within the range. For example, the range description from 1 to 6 should be considered to have specifically disclosed sub-ranges, such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., as well as single numbers within the range, such as 1, 2, 3, 4, 5 and 6, which apply regardless of the range. In addition, whenever a numerical range is indicated herein, it is meant to include any cited number (fractional or integer) within the indicated range.

[0162] In the present application, in the absence of any contrary description, the directional words used, such as "upper" and "lower", are specifically the directions of the drawings in the accompanying drawings. In addition, in the description of the present specification, the terms "including", "comprising", etc. refer to "including but not limited to". In this article, relational terms such as "first" and "second" are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply that there is any such actual relationship or order between these entities or operations. In this article, "and / or" describes the association relationship of the associated objects, indicating that there may be three relationships, for example, A and / or B, which can represent: A exists alone, A and B exist at the same time, and B exists alone. Wherein A, B can be singular or plural. In this article, "at least one" refers to one or more, and "plural" refers to two or more. "At least one", "at least one of the following" or similar expressions refer to any combination of these items, including any combination of singular or plural items. For example, "at least one of a, b, or c", or "at least one of a, b and c", can both mean: a, b, c, ab (i.e. a and b), ac, bc or abc, where a, b, c can be single or plural, respectively.

[0163] The above description is only a specific implementation of the present application, so that those skilled in the art can understand or implement the present application. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to the embodiments shown herein, but will conform to the widest range consistent with the principles and novel features applied for herein.

Claims

1. A nylon 6 polymerization production method capable of fully recycling extractable oligomers, characterized in that: The nylon 6 polymerization production method comprises a nylon 6 polymerization section, a nylon 6 chip extraction section, a nylon 6 wet chip drying section, an extract evaporation concentration section and an extractable oligomer cracking and recycling section; specifically comprises the following steps: The nylon 6 polymerization section: subjecting the reaction raw materials to polymerization reaction, followed by cooling and pelletizing to obtain coarse nylon 6 slices; The nylon 6 chip extraction section: extracting the crude nylon 6 chips to obtain nylon 6 wet chips and oligomer extract; The nylon 6 wet slice drying section: drying the nylon 6 wet slice to obtain nylon 6 slices; The extract evaporation and concentration section: evaporating and concentrating the oligomer extract to obtain an oligomer extract concentrate having a water content of 50 wt% to 90 wt%; The extractable oligomer cracking and recycling section: at a temperature of 230° C. to 350° C. and a pressure of 1 MPa to 8 MPa, the oligomer extract concentrate with a water content of 50wt% to 90wt% is catalytically cracked to obtain cracking products; all of the cracking products are returned to the nylon 6 polymerization section for recycling.

2. The nylon 6 polymerization production method according to claim 1, characterized in that: The catalyst for catalytic cracking is phosphoric acid, and the amount of phosphoric acid added is 5wt% to 30wt% of the oligomer extract concentrate.

3. The nylon 6 polymerization production method according to claim 1, characterized in that: When all the pyrolysis products are returned to the nylon 6 polymerization section for recycling, the cumulative equilibrium value of the cyclic dimer in the nylon 6 polymerization section is 24-26 kg / hr.

4. The nylon 6 polymerization production method according to claim 1, characterized in that: When all the pyrolysis products are returned to the nylon 6 polymerization section for recycling, the reaction raw materials are composed of the pyrolysis products and fresh caprolactam monomers in a weight ratio of 1:(5-25).

5. The nylon 6 polymerization production method according to claim 1, characterized in that: The working parameters of the catalytic cracking include: reaction temperature of 260° C. to 290° C., reaction pressure of 2 MPa to 5 MPa, reaction time of 2 h to 4 h, and the addition amount of phosphoric acid catalyst of 20 wt % to 30 wt % of the oligomer extract concentrate.

6. The method for producing nylon 6 by polymerization according to any one of claims 1 to 5, characterized in that: The step of subjecting the reaction raw materials to polymerization reaction, cooling and pelletizing to obtain coarse nylon 6 chips comprises: The uniformly mixed reaction raw materials are sequentially conveyed to a first polymerizer and a second polymerizer for continuous hydrolysis and ring-opening polymerization to obtain a nylon 6 polymer melt; The nylon 6 polymer melt is cooled and granulated, and then sliced ​​to obtain a crude nylon 6 slice; The working parameters of the first polymerizer include: reaction pressure of 0.25MPa to 0.70MPa, reaction time of 1h to 5h; the first polymerizer is provided with a first heating section and a first heat transfer section, the heat medium temperature of the first heating section is 240°C to 285°C, and the heat medium temperature of the first heat transfer section is 230°C to 270°C; The working parameters of the second polymerizer include: reaction pressure of 0.02MPa~0.25MPa, reaction time of 3~10h; the second polymerizer is provided with a second heating section and a second heat transfer section, the heat medium temperature of the second heating section is 230℃~270℃, and the heat medium temperature of the second heat transfer section is 245℃~275℃.

7. The method for producing nylon 6 by polymerization according to any one of claims 1 to 5, characterized in that: The step of extracting the crude nylon 6 slices to obtain nylon 6 wet slices and oligomer extracts comprises: The crude nylon 6 slices are subjected to multi-stage continuous extraction by countercurrent extraction to obtain nylon 6 wet slices and oligomer extracts; The working parameters of the liquid-solid two-phase countercurrent in the multi-stage continuous extraction include: a liquid-solid ratio of 0.5 to 5.0, an extraction temperature of 65° C. to 130° C., and a caprolactam aqueous solution concentration of 0 to 50 wt %.

8. The method for producing nylon 6 by polymerization according to any one of claims 1 to 5, characterized in that: The step of drying the nylon 6 wet slices to obtain nylon 6 slices comprises: Using an inert gas to countercurrent dry the nylon 6 wet slices to obtain nylon 6 slices; The working parameters of the countercurrent drying include: the temperature of the inert gas is 100° C. to 160° C., and the drying time is 15 h to 35 h.

9. The method for producing nylon 6 by polymerization according to any one of claims 1 to 5, characterized in that: The depolymerization rate of cyclic dimers in the extractable oligomer cracking and recycling section is ≥75%, the conversion rate of caprolactam monomers in the reaction raw materials is ≥99%, and the mass fraction of extractables in the nylon 6 chips is <0.6%.

10. A nylon 6 polymerization production system used in conjunction with the nylon 6 polymerization production method according to any one of claims 1 to 9, characterized in that: The nylon 6 polymerization production system comprises a static mixer, a polymerization reactor, a multi-stage continuous extraction tower, a vertical drying tower, a multi-effect evaporation system and a cracking reactor; The polymerization reactor comprises a first polymerizer and a second polymerizer which are sequentially arranged in series, wherein the first polymerizer is connected to the static mixer, and the second polymerizer is connected to the multi-stage continuous extraction tower, and the multi-stage continuous extraction tower is provided with a first extraction medium input end and a second extraction medium input end; The multi-effect evaporation system comprises a first-stage evaporation tower and a second-stage evaporation tower which are sequentially arranged in series, wherein the top of the first-stage evaporation tower is connected to the multi-stage continuous extraction tower, a condenser and a reboiler, the condenser is connected to the reboiler and the multi-stage continuous extraction tower, the top of the second-stage evaporation tower is connected to the reboiler, and the bottom of the second-stage evaporation tower is connected to the cracking reactor; The outlet end of the cleavage reactor is connected to the inlet end of the static mixer.

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