Preparation method of nylon 66 salt crystal

Abstract

The invention discloses a preparation method of a nylon 66 salt crystal, which is characterized in that in the method, a nylon 66 salt aqueous solution is subjected to secondary crystallization and a rotary pressure filter process to prepare the nylon 66 salt crystal. According to the technology, the problems of low single-stage yield and large sewage discharge amount are solved by adopting secondary crystallization and evaporation condensate recycling, and the processes of centrifugation, drying, cyclone separation and the like are reduced by replacing a centrifugal machine with a rotary pressure filter, so that the energy consumption per ton of products is reduced.

Description

Technical Field

[0001] This invention relates to a method for preparing nylon 66 salt crystals. Background Technology

[0002] Nylon 66 salt is mainly used as a raw material for nylon 66 polymerization and is an important chemical raw material. The mainstream industrial process is a single-stage crystallization process using nylon 66 salt aqueous solution. While this process meets the requirements of downstream nylon 66 polymerization raw materials, it results in large wastewater discharge, high nylon 66 salt content in the wastewater, and high energy consumption during nylon 66 salt crystal production. Patent CN202111465793.0 discloses a production device and process for nylon 66 salt crystals, solving the problem of long-distance transportation of nylon 66 aqueous solution; however, this method has a low single-stage crystallization yield and unstable product indicators. Summary of the Invention

[0003] This invention is made to further improve the production efficiency of nylon 66 salt crystals, reduce product energy consumption, and further reduce wastewater discharge.

[0004] This invention relates to a method for preparing nylon 66 salt crystals, wherein nylon 66 salt aqueous solution is subjected to a two-stage crystallization and a rotary pressure filtration process to obtain nylon 66 salt crystals.

[0005] In a specific embodiment, the secondary crystallization process includes:

[0006] (1): The crystal slurry after crystallization of Nylon 66 salt solution in the primary crystallizer is separated by a rotary pressure filter to obtain crystal crystals and primary crystallization mother liquor;

[0007] (2): Part of the primary crystallization mother liquor is sent to the primary crystallizer, and the rest is sent to the secondary crystallizer. The crystal slurry after secondary crystallization is separated by a rotary pressure filter to obtain secondary crystals and secondary crystallization mother liquor. The secondary crystals are dissolved in water and sent to the primary crystallizer.

[0008] Furthermore, in a specific embodiment, the secondary crystallization process further includes:

[0009] (3): A portion of the secondary crystallization mother liquor is sent to the secondary crystallizer.

[0010] Furthermore, in a specific embodiment, the nylon 66 salt solution is preheated before entering the primary crystallizer, with a preheating temperature of 40-41°C.

[0011] Furthermore, in a specific embodiment, in step (2), 80%-85% of the primary crystallization mother liquor is sent to the primary crystallizer, and the remainder is sent to the secondary crystallizer.

[0012] Furthermore, in a specific embodiment, in step (3), 90%-95% of the secondary crystallization mother liquor is sent to the secondary crystallizer.

[0013] Furthermore, in a specific embodiment, in step (1), the water in the nylon 66 salt solution is evaporated and condensed through a primary crystallizer and then used as recycled water.

[0014] Furthermore, in a specific embodiment, in step (2), the water in the nylon 66 salt solution is evaporated and condensed through a secondary crystallizer and then used as recycled water.

[0015] The present invention also relates to nylon 66 salt crystals prepared using the above method.

[0016] This invention also relates to the application of the above-mentioned nylon 66 salt crystals in the nylon 66 polycondensation and slicing process.

[0017] The mainstream industrial process is a single-stage crystallization process using nylon 66 salt solution. While this process meets the raw material requirements for downstream nylon 66 polymerization, it suffers from high wastewater discharge, high nylon 66 salt content in the wastewater, and high energy consumption per ton of product during nylon 66 salt crystal production. This technology aims to solve the problems of low single-stage yield, high wastewater discharge, and high energy consumption per ton of product. Through two-stage crystallization of the nylon 66 salt solution, followed by solid-liquid separation, drying, and packaging using a rotary pressure filter, the solution is converted into nylon 66 salt crystals. Simultaneously, the water in the nylon 66 salt solution is evaporated and condensed as recycled water, which is then sent to the upstream salt-forming unit. Compared to single-stage crystallization, this method improves the crystallization yield, increases recycled water volume while maintaining product quality, and reduces wastewater discharge. By using a rotary pressure filter instead of a centrifuge, it reduces the need for centrifugation, drying, and cyclone separation processes.

[0018] This technology addresses the problems of low single-stage yield and large wastewater discharge by employing two-stage crystallization and evaporation condensate recycling. By using a rotary pressure filter instead of a centrifuge, it reduces processes such as centrifugation, drying, and cyclone separation, thereby lowering energy consumption per ton of product.

[0019] This technology aims to solve the problems of low yield and large wastewater discharge in single-stage crystallization. Through a two-stage crystallization process of nylon 66 salt solution, followed by solid-liquid separation, drying, and packaging using a rotary pressure filter, it is converted into nylon 66 salt crystals. Simultaneously, the water in the nylon 66 salt solution is evaporated and condensed, then sent as recycled water to the upstream salt-forming unit. Compared to single-stage crystallization, this method improves the crystallization yield, increases recycled water volume, and reduces wastewater discharge and energy consumption per ton of product while ensuring product quality. Attached Figure Description

[0020] Figure 1 : A process flowchart for producing nylon 66 salt crystals according to the present invention.

[0021] Nylon salt solution 1 is sent from the salt-forming unit to the primary feed tank a for storage. Nylon salt solution 2 in primary feed tank a is preheated by heat exchanger b and then enters the upper flash evaporator of primary crystallizer c. Water vapor 5 evaporates in the flash evaporator and is sent to the salt-forming unit. Crystal slurry 4 containing crystals is pumped out at the bottom of primary crystallizer c and sent to primary rotary pressure filter d. Primary centrifuge d separates primary mother liquor 6 and primary nylon 66 salt crystals 9. A portion of the centrifuged primary mother liquor 6 enters the secondary crystallizer, while most of it is returned to primary feed tank a. Primary nylon 66 salt crystals 9 are automatically weighed, packaged (g), transported, and then stacked (h).

[0022] Part of the primary centrifugal mother liquor 6 enters the secondary crystallizer i. Water vapor 13 evaporates in the upper flash evaporator of the secondary crystallizer i and is then sent to the salt-forming unit. The crystal slurry 12 containing crystals is pumped out from the lower part of the secondary crystallizer i and sent to the secondary rotary pressure filter j. The secondary rotary pressure filter j separates the secondary mother liquor 15 and secondary nylon 66 salt crystals 14. The secondary nylon 66 salt crystals 14 dissolve and are sent to the primary feed tank a. Part of the secondary mother liquor 15 is sent to the inlet of the secondary crystallizer i, and part is discharged from the unit. Detailed Implementation

[0023] Example 1

[0024] The process of this invention is carried out in a nylon 66 salt crystallization apparatus.

[0025] A 50% nylon salt aqueous solution 1 (flow rate 12162 kg / h) is fed into the primary feed tank a, preheated to 41°C by heat exchanger b, and then enters the flash evaporator at the top of the primary crystallizer c, producing crystal slurry 4 containing crystals and water vapor 5. Water vapor 5 (flow rate 7167 kg / h) is evaporated and condensed in the flash evaporator and used as recycled water. The crystal slurry 4 containing crystals (flow rate 25510 kg / h) is pumped out at the bottom of the primary crystallizer c and sent to the primary rotary pressure filter d. In the primary rotary pressure filter d, primary mother liquor 6 (flow rate 19282 kg / h) and primary nylon 66 salt crystals 9 (flow rate 6228 kg / h) are separated. 18% of the primary mother liquor 6 goes to the secondary crystallizer i, and the remaining 82% is sent to the primary feed tank a to mix with the nylon salt aqueous solution 1. Grade 1 Nylon 66 salt crystals 9 (flow rate 6228 kg / h) were dehydrated, automatically weighed and packaged to obtain product 10 (flow rate 6037 g), which was then transported and stacked for h.

[0026] The primary mother liquor (flow rate 3469 kg / h) entering the secondary crystallizer i undergoes secondary crystallization in the secondary crystallizer i, generating crystal slurry 12 containing crystals and water vapor 13. Water vapor 13 (flow rate 780 kg / h) is evaporated and condensed from the upper flash evaporator of the secondary crystallizer i and used as recycled water. The crystal slurry 12 containing crystals (flow rate 2688 kg / h) is pumped out from the lower part of the secondary crystallizer i and sent to the secondary rotary pressure filter j. In the secondary rotary pressure filter j, secondary mother liquor 15 (flow rate 2062 kg / h) and secondary nylon 66 salt crystals 14 (flow rate 626 kg / h) are separated. The secondary nylon 66 salt crystals 14 are dissolved in water and sent to the primary feed tank a. 93.5% of the secondary mother liquor 15 is sent to the inlet of the secondary crystallizer i, and the remaining 6.5% is discharged from the nylon 66 salt crystallization unit.

[0027] In this embodiment, the yield of nylon 66 salt crystals was 99.28%.

[0028] Comparative Example 1

[0029] A 50% nylon salt aqueous solution 1 (flow rate 12162 kg / h) is fed into the primary feed tank a, preheated to 40°C by heat exchanger b, and then enters the flash evaporator at the top of the primary crystallizer c, producing crystal slurry 4 containing crystals and water vapor 5. Water vapor 5 (flow rate 6809 kg / h) is evaporated and condensed in the flash evaporator and used as recycled water. The crystal slurry 4 containing crystals (flow rate 25868 kg / h) is pumped out at the bottom of the primary crystallizer c and sent to the primary rotary pressure filter d. In the primary rotary pressure filter d, primary mother liquor 6 (flow rate 19956 kg / h) and primary nylon 66 salt crystals 9 (flow rate 5912 kg / h) are separated. 17.74% of the primary mother liquor 6 goes to the secondary crystallizer i, and the remaining 82.26% is sent to the primary feed tank a to mix with the nylon salt aqueous solution 1. Grade 1 Nylon 66 salt crystals 9 (flow rate 6103 kg / h) were dehydrated, automatically weighed and packaged to obtain product 10 (flow rate 5920 g), which was then transported and stacked for h.

[0030] The primary mother liquor (flow rate 3540 kg / h) entering the secondary crystallizer i undergoes secondary crystallization in the secondary crystallizer i, generating crystal slurry 12 containing crystals and water vapor 13. Water vapor 13 (flow rate 805 kg / h) is evaporated and condensed from the upper flash evaporator of the secondary crystallizer i and used as recycled water. The crystal slurry 12 containing crystals (flow rate 2735 kg / h) is pumped out from the lower part of the secondary crystallizer i and sent to the secondary rotary pressure filter j. In the secondary rotary pressure filter j, secondary mother liquor 15 (flow rate 2127 kg / h) and secondary nylon 66 salt crystals 14 (flow rate 610 kg / h) are separated. The secondary nylon 66 salt crystals 14 are dissolved in water and sent to the primary feed tank a. 93.7% of the secondary mother liquor 15 is sent to the inlet of the secondary crystallizer i, and the remaining 6.3% is discharged from the nylon 66 salt crystallization unit.

[0031] In this embodiment, the yield of nylon 66 salt crystals was 97.35%.

[0032] Comparative Example 2

[0033] A 50% nylon salt aqueous solution 1 (flow rate 12162 kg / h) was sent to the primary feed tank a and preheated to 39°C by heat exchanger b. The nylon salt crystals spontaneously condensed and blocked the pipeline, indicating that the temperature was unsuitable.

[0034] Comparative Example 3

[0035] A 50% nylon salt aqueous solution 1 (flow rate 12162 kg / h) is fed into the primary feed tank a, preheated to 42°C by heat exchanger b, and then enters the flash evaporator at the top of the primary crystallizer c, producing crystal slurry 4 containing crystals and water vapor 5. Water vapor 5 (flow rate 7125 kg / h) is evaporated and condensed in the flash evaporator and used as recycled water. The crystal slurry 4 containing crystals (flow rate 25360 kg / h) is pumped out at the bottom of the primary crystallizer c and sent to the primary rotary pressure filter d. In the primary rotary pressure filter d, primary mother liquor 6 (flow rate 20122 kg / h) and primary nylon 66 salt crystals 9 (flow rate 5238 kg / h) are separated. 19.6% of the primary mother liquor 6 goes to the secondary crystallizer i, and the remaining 80.4% is sent to the primary feed tank a to mix with the nylon salt aqueous solution 1. Grade 1 Nylon 66 salt crystals 9 (flow rate 5238 kg / h) were dehydrated, automatically weighed and packaged to obtain product 10 (flow rate 5080 g), which was then transported and stacked for h.

[0036] The primary mother liquor (flow rate 3949 kg / h) entering the secondary crystallizer i undergoes secondary crystallization in the secondary crystallizer i, generating crystal slurry 12 containing crystals and water vapor 13. Water vapor 13 (flow rate 889 kg / h) is evaporated and condensed from the upper flash evaporator of the secondary crystallizer i and used as recycled water. Crystal slurry 12 containing crystals (flow rate 3061 kg / h) is pumped out from the lower part of the secondary crystallizer i and sent to the secondary rotary pressure filter j. In the secondary rotary pressure filter j, secondary mother liquor 15 (flow rate 2348 kg / h) and secondary nylon 66 salt crystals 14 (flow rate 713 kg / h) are separated. Secondary nylon 66 salt crystals 14 are dissolved in water and sent to the primary feed tank a. 91% of the secondary mother liquor 15 is sent to the inlet of the secondary crystallizer i, and the remaining 9% is discharged from the nylon 66 salt crystallization unit.

[0037] In this embodiment, the yield of nylon 66 salt crystals was 83.55%.

[0038] Comparative Example 4

[0039] A 50% nylon salt aqueous solution 1 (flow rate 12162 kg / h) is fed into the primary feed tank a, preheated to 41°C by heat exchanger b, and then enters the flash evaporator at the top of the primary crystallizer c, producing crystal slurry 4 containing crystals and water vapor 5. Water vapor 5 (flow rate 7125 kg / h) is evaporated and condensed in the flash evaporator and used as recycled water. The crystal slurry 4 containing crystals (flow rate 25360 kg / h) is pumped out at the bottom of the primary crystallizer c and sent to the primary rotary pressure filter d. In the primary rotary pressure filter d, primary mother liquor 6 (flow rate 19168 kg / h) and primary nylon 66 salt crystals 9 (flow rate 6192 kg / h) are separated. 15% of the primary mother liquor 6 goes to the secondary crystallizer i, and the remaining 85% is sent to the primary feed tank a to mix with the nylon salt aqueous solution 1. Grade 1 Nylon 66 salt crystals 9 (flow rate 6192 kg / h) were dehydrated, automatically weighed and packaged to obtain product 10 (flow rate 6006 g), which was then transported and stacked for h.

[0040] The primary mother liquor (flow rate 2892 kg / h) entering the secondary crystallizer i undergoes secondary crystallization in the secondary crystallizer i, generating crystal slurry 12 containing crystals and water vapor 13. Water vapor 13 (flow rate 651 kg / h) is evaporated and condensed from the upper flash evaporator of the secondary crystallizer i and used as recycled water. The crystal slurry 12 containing crystals (flow rate 2241 kg / h) is pumped out from the lower part of the secondary crystallizer i and sent to the secondary rotary pressure filter j. In the secondary rotary pressure filter j, secondary mother liquor 15 (flow rate 1719 kg / h) and secondary nylon 66 salt crystals 14 (flow rate 522 kg / h) are separated. The secondary nylon 66 salt crystals 14 are dissolved in water and sent to the primary feed tank a. 88.8% of the secondary mother liquor 15 is sent to the inlet of the secondary crystallizer i, and the remaining 11.2% is discharged from the nylon 66 salt crystallization unit.

[0041] In this embodiment, the yield of nylon 66 salt crystals was 98.77%.

[0042] Comparative Example 5

[0043] A 50% nylon salt aqueous solution 1 (flow rate 12162 kg / h) is fed into the primary feed tank a, preheated to 41°C by heat exchanger b, and then enters the flash evaporator at the top of the primary crystallizer c, producing crystal slurry 4 containing crystals and water vapor 5. Water vapor 5 (flow rate 7051 kg / h) is evaporated and condensed in the flash evaporator and used as recycled water. The crystal slurry 4 containing crystals (flow rate 25095 kg / h) is pumped out at the bottom of the primary crystallizer c and sent to the primary rotary pressure filter d. In the primary rotary pressure filter d, primary mother liquor 6 (flow rate 18968 kg / h) and primary nylon 66 salt crystals 9 (flow rate 6127 kg / h) are separated. 10% of the primary mother liquor 6 goes to the secondary crystallizer i, and the remaining 90% is sent to the primary feed tank a to mix with the nylon salt aqueous solution 1. Grade 1 Nylon 66 salt crystals 9 (flow rate 6127 kg / h) were dehydrated, automatically weighed and packaged to obtain product 10 (flow rate 5943.2 g), which was then transported and stacked for h.

[0044] The primary mother liquor (flow rate 1897 kg / h) entering the secondary crystallizer i undergoes secondary crystallization in the secondary crystallizer i, generating crystal slurry 12 containing crystals and water vapor 13. Water vapor 13 (flow rate 427 kg / h) is evaporated and condensed from the upper flash evaporator of the secondary crystallizer i and used as recycled water. The crystal slurry 12 containing crystals (flow rate 1470 kg / h) is pumped out from the lower part of the secondary crystallizer i and sent to the secondary rotary pressure filter j. In the secondary rotary pressure filter j, secondary mother liquor 15 (flow rate 1128 kg / h) and secondary nylon 66 salt crystals 14 (flow rate 342 kg / h) are separated. The secondary nylon 66 salt crystals 14 are dissolved in water and sent to the primary feed tank a. 74.3% of the secondary mother liquor 15 is sent to the inlet of the secondary crystallizer i, and the remaining 25.7% is discharged from the nylon 66 salt crystallization unit.

[0045] In this embodiment, the yield of nylon 66 salt crystals was 97.73%.

[0046] Comparative Example 6

[0047] A 50% nylon salt aqueous solution 1 (flow rate 12162 kg / h) is fed into the primary feed tank a, preheated to 41°C by heat exchanger b, and then enters the flash evaporator at the top of the primary crystallizer c, producing crystal slurry 4 containing crystals and water vapor 5. Water vapor 5 (flow rate 7169 kg / h) is evaporated and condensed from the flash evaporator and used as recycled water. The crystal slurry 4 containing crystals (flow rate 25517 kg / h) is pumped out at the bottom of the primary crystallizer c and sent to the primary rotary pressure filter d. In the primary rotary pressure filter d, primary mother liquor 6 (flow rate 19287 kg / h) and primary nylon 66 salt crystals 9 (flow rate 6230 kg / h) are separated. 20% of the primary mother liquor 6 goes to the secondary crystallizer i, and the remaining 80% is sent to the primary feed tank a to mix with the nylon salt aqueous solution 1. Grade 1 Nylon 66 salt crystals 9 (flow rate 6230 kg / h) were dehydrated, automatically weighed and packaged to obtain product 10 (flow rate 6043 g), which was then transported and stacked for h.

[0048] The primary mother liquor (flow rate 3857 kg / h) entering the secondary crystallizer i undergoes secondary crystallization in the secondary crystallizer i, generating crystal slurry 12 containing crystals and water vapor 13. Water vapor 13 (flow rate 868 kg / h) is evaporated and condensed from the upper flash evaporator of the secondary crystallizer i and used as recycled water. The crystal slurry 12 containing crystals (flow rate 2989 kg / h) is pumped out from the lower part of the secondary crystallizer i and sent to the secondary rotary pressure filter j. In the secondary rotary pressure filter j, secondary mother liquor 15 (flow rate 2293 kg / h) and secondary nylon 66 salt crystals 14 (flow rate 696 kg / h) are separated. The secondary nylon 66 salt crystals 14 are dissolved in water and sent to the primary feed tank a. 93.8% of the secondary mother liquor 15 is sent to the inlet of the secondary crystallizer i, and the remaining 6.2% is discharged from the nylon 66 salt crystallization unit.

[0049] In this embodiment, the yield of nylon 66 salt crystals was 99.37%.

[0050] Comparative Example 7

[0051] A 50% nylon salt aqueous solution 1 (flow rate 12162 kg / h) is fed into the primary feed tank a, preheated to 41°C by heat exchanger b, and then enters the flash evaporator at the top of the primary crystallizer c, producing crystal slurry 4 containing crystals and water vapor 5. Water vapor 5 (flow rate 7042 kg / h) is evaporated and condensed in the flash evaporator and used as recycled water. The crystal slurry 4 containing crystals (flow rate 25065 kg / h) is pumped out at the bottom of the primary crystallizer c and sent to the primary rotary pressure filter d. In the primary rotary pressure filter d, primary mother liquor 6 (flow rate 18945 kg / h) and primary nylon 66 salt crystals 9 (flow rate 6120 kg / h) are separated. 25% of the primary mother liquor 6 goes to the secondary crystallizer i, and the remaining 75% is sent to the primary feed tank a to mix with the nylon salt aqueous solution 1. Grade 1 Nylon 66 salt crystals 9 (flow rate 6120 kg / h) were dehydrated, automatically weighed and packaged to obtain product 10 (flow rate 5936 g), which was then transported and stacked for h.

[0052] The primary mother liquor (flow rate 4736 kg / h) entering the secondary crystallizer i undergoes secondary crystallization in the secondary crystallizer i, generating crystal slurry 12 containing crystals and water vapor 13. Water vapor 13 (flow rate 1066 kg / h) is evaporated and condensed in the upper flash evaporator of the secondary crystallizer i and used as recycled water. The crystal slurry 12 containing crystals (flow rate 3671 kg / h) is pumped out at the lower part of the secondary crystallizer i and sent to the secondary rotary pressure filter j. In the secondary rotary pressure filter j, secondary mother liquor 15 (flow rate 3045 kg / h) and secondary nylon 66 salt crystals 14 (flow rate 626 kg / h) are separated. The secondary nylon 66 salt crystals 14 are dissolved in water and sent to the primary feed tank a. 93.8% of the secondary mother liquor 15 is sent to the inlet of the secondary crystallizer i, and the remaining 6.2% is discharged from the nylon 66 salt crystallization unit.

[0053] In this embodiment, the yield of nylon 66 salt crystals was 97.61%.

[0054] Comparative Example 8

[0055] A 50% nylon salt aqueous solution 1 (flow rate 12162 kg / h) is fed into the primary feed tank a, preheated to 41°C by heat exchanger b, and then enters the flash evaporator at the top of the primary crystallizer c, producing crystal slurry 4 containing crystals and water vapor 5. Water vapor 5 (flow rate 7047 kg / h) is evaporated and condensed in the flash evaporator and used as recycled water. The crystal slurry 4 containing crystals (flow rate 25081 kg / h) is pumped out at the bottom of the primary crystallizer c and sent to the primary rotary pressure filter d. In the primary rotary pressure filter d, primary mother liquor 6 (flow rate 18958 kg / h) and primary nylon 66 salt crystals 9 (flow rate 6124 kg / h) are separated. 18.8% of the primary mother liquor 6 goes to the secondary crystallizer i, and the remaining 81.2% is sent to the primary feed tank a to mix with the nylon salt aqueous solution 1. Grade 1 Nylon 66 salt crystals 9 (flow rate 6124 kg / h) were dehydrated, automatically weighed and packaged to obtain product 10 (flow rate 5940 g), which was then transported and stacked for h.

[0056] The primary mother liquor (flow rate 3563 kg / h) entering the secondary crystallizer i undergoes secondary crystallization in the secondary crystallizer i, generating crystal slurry 12 containing crystals and water vapor 13. Water vapor 13 (flow rate 802 kg / h) is evaporated and condensed from the upper flash evaporator of the secondary crystallizer i and used as recycled water. Crystal slurry 12 containing crystals (flow rate 2761 kg / h) is pumped out from the lower part of the secondary crystallizer i and sent to the secondary rotary pressure filter j. In the secondary rotary pressure filter j, secondary mother liquor 15 (flow rate 2118 kg / h) and secondary nylon 66 salt crystals 14 (flow rate 643 kg / h) are separated. Secondary nylon 66 salt crystals 14 are dissolved in water and sent to the primary feed tank a. 85% of the secondary mother liquor 15 is sent to the inlet of the secondary crystallizer i, and the remaining 15% is discharged from the nylon 66 salt crystallization unit.

[0057] In this embodiment, the yield of nylon 66 salt crystals was 97.68%.

[0058] Comparative Example 9

[0059] A 50% nylon salt aqueous solution 1 (flow rate 12162 kg / h) is fed into the primary feed tank a, preheated to 41°C by heat exchanger b, and then enters the flash evaporator at the top of the primary crystallizer c, producing crystal slurry 4 containing crystals and water vapor 5. Water vapor 5 (flow rate 7094 kg / h) is evaporated and condensed in the flash evaporator and used as recycled water. The crystal slurry 4 containing crystals (flow rate 25249 kg / h) is pumped out at the bottom of the primary crystallizer c and sent to the primary rotary pressure filter d. In the primary rotary pressure filter d, primary mother liquor 6 (flow rate 19084 kg / h) and primary nylon 66 salt crystals 9 (flow rate 6164 kg / h) are separated. 19% of the primary mother liquor 6 goes to the secondary crystallizer i, and the remaining 81% is sent to the primary feed tank a to mix with the nylon salt aqueous solution 1. Grade 1 Nylon 66 salt crystals 9 (flow rate 6164 kg / h) were dehydrated, automatically weighed and packaged to obtain product 10 (flow rate 5979 g), which was then transported and stacked for h.

[0060] The primary mother liquor (flow rate 3655 kg / h) entering the secondary crystallizer i undergoes secondary crystallization in the secondary crystallizer i, generating crystal slurry 12 containing crystals and water vapor 13. Water vapor 13 (flow rate 822 kg / h) is evaporated and condensed from the upper flash evaporator of the secondary crystallizer i and used as recycled water. Crystal slurry 12 containing crystals (flow rate 2833 kg / h) is pumped out from the lower part of the secondary crystallizer i and sent to the secondary rotary pressure filter j. In the secondary rotary pressure filter j, secondary mother liquor 15 (flow rate 2173 kg / h) and secondary nylon 66 salt crystals 14 (flow rate 660 kg / h) are separated. Secondary nylon 66 salt crystals 14 are dissolved in water and sent to the primary feed tank a. 90% of the secondary mother liquor 15 is sent to the inlet of the secondary crystallizer i, and the remaining 10% is discharged from the nylon 66 salt crystallization unit.

[0061] In this embodiment, the yield of nylon 66 salt crystals was 98.33%.

[0062] Comparative Example 10

[0063] A 50% nylon salt aqueous solution 1 (flow rate 12162 kg / h) is fed into the primary feed tank a, preheated to 41°C by heat exchanger b, and then enters the flash evaporator at the top of the primary crystallizer c, producing crystal slurry 4 containing crystals and water vapor 5. Water vapor 5 (flow rate 7125 kg / h) is evaporated and condensed in the flash evaporator and used as recycled water. The crystal slurry 4 containing crystals (flow rate 25359 kg / h) is pumped out at the bottom of the primary crystallizer c and sent to the primary rotary pressure filter d. In the primary rotary pressure filter d, primary mother liquor 6 (flow rate 19168 kg / h) and primary nylon 66 salt crystals 9 (flow rate 6191 kg / h) are separated. 20% of the primary mother liquor 6 goes to the secondary crystallizer i, and the remaining 80% is sent to the primary feed tank a to mix with the nylon salt aqueous solution 1. Grade 1 Nylon 66 salt crystals 9 (flow rate 6191 kg / h) were dehydrated, automatically weighed and packaged to obtain product 10 (flow rate 6006 g), which was then transported and stacked for h.

[0064] The primary mother liquor (flow rate 3840 kg / h) entering the secondary crystallizer i undergoes secondary crystallization in the secondary crystallizer i, generating crystal slurry 12 containing crystals and water vapor 13. Water vapor 13 (flow rate 864 kg / h) is evaporated and condensed in the upper flash evaporator of the secondary crystallizer i and used as recycled water. Crystal slurry 12 containing crystals (flow rate 2976 kg / h) is pumped out at the lower part of the secondary crystallizer i and sent to the secondary rotary pressure filter j. In the secondary rotary pressure filter j, secondary mother liquor 15 (flow rate 2283 kg / h) and secondary nylon 66 salt crystals 14 (flow rate 693 kg / h) are separated. Secondary nylon 66 salt crystals 14 are dissolved in water and sent to the primary feed tank a. 95% of the secondary mother liquor 15 is sent to the inlet of the secondary crystallizer i, and the remaining 5% is discharged from the nylon 66 salt crystallization unit.

[0065] In this embodiment, the yield of nylon 66 salt crystals was 98.76%.

[0066] Comparative Example 11

[0067] A 50% nylon salt aqueous solution 1 (flow rate 12162 kg / h) is fed into the primary feed tank a, preheated to 41°C by heat exchanger b, and then enters the flash evaporator at the top of the primary crystallizer c, producing crystal slurry 4 containing crystals and water vapor 5. Water vapor 5 (flow rate 7125 kg / h) is evaporated and condensed in the flash evaporator and used as recycled water. The crystal slurry 4 containing crystals (flow rate 25359 kg / h) is pumped out at the bottom of the primary crystallizer c and sent to the primary rotary pressure filter d. In the primary rotary pressure filter d, primary mother liquor 6 (flow rate 19168 kg / h) and primary nylon 66 salt crystals 9 (flow rate 6191 kg / h) are separated. 20% of the primary mother liquor 6 goes to the secondary crystallizer i, and the remaining 80% is sent to the primary feed tank a to mix with the nylon salt aqueous solution 1. Grade 1 Nylon 66 salt crystals 9 (flow rate 6191 kg / h) were dehydrated, automatically weighed and packaged to obtain product 10 (flow rate 6006 g), which was then transported and stacked for h.

[0068] The primary mother liquor (flow rate 3913 kg / h) entering the secondary crystallizer i undergoes secondary crystallization in the secondary crystallizer i, generating crystal slurry 12 containing crystals and water vapor 13. Water vapor 13 (flow rate 880 kg / h) is evaporated and condensed in the upper flash evaporator of the secondary crystallizer i and used as recycled water. Crystal slurry 12 containing crystals (flow rate 3033 kg / h) is pumped out at the lower part of the secondary crystallizer i and sent to the secondary rotary pressure filter j. In the secondary rotary pressure filter j, secondary mother liquor 15 (flow rate 2326 kg / h) and secondary nylon 66 salt crystals 14 (flow rate 706 kg / h) are separated. Secondary nylon 66 salt crystals 14 are dissolved in water and sent to the primary feed tank a. 96% of the secondary mother liquor 15 is sent to the inlet of the secondary crystallizer i, and the remaining 4% is discharged from the nylon 66 salt crystallization unit.

[0069] In this embodiment, the yield of nylon 66 salt crystals was 98.76%.

[0070] Comparing Example 1 with Comparative Examples 1, 2, and 3, by adjusting different preheating temperatures, the highest yield of nylon 66 salt solution was observed at a preheating temperature of 40-41℃. When the preheating temperature was below 40℃, pre-crystallization of nylon 66 salt occurred, leading to experimental failure. When the preheating temperature was above 41℃, the nylon 66 salt crystals decomposed, resulting in a decrease in yield. Therefore, the optimal preheating temperature was determined to be 40-41℃.

[0071] Comparing Example 1 with Comparative Examples 4, 5, 6, and 7, the yield of nylon 66 salt crystals varied slightly by adjusting the proportion of the primary crystallization mother liquor fed to the primary crystallizer. The highest yield of nylon 66 salt crystals, exceeding 98.7%, was achieved when the proportion of the primary crystallization mother liquor fed to the primary crystallizer was 80-85%. When the proportion was below 80% or above 85%, the yield of nylon 66 salt crystals did not exceed 98%. Therefore, the appropriate proportion of the primary crystallization mother liquor fed to the primary crystallizer is 80-85%.

[0072] Comparing Example 1 with Comparative Examples 8, 9, 10, and 11, the yield of nylon 66 salt crystals varied slightly depending on the proportion of the secondary crystallization mother liquor fed to the secondary crystallizer. The highest yield of nylon 66 salt crystals, exceeding 98.3%, was achieved when the proportion was 90-95%. Below 90%, the yield did not exceed 98%. Above 95%, although the product yield remained the same, the nylon 66 crystals contained excessive impurities, resulting in a product that did not meet requirements. Therefore, the appropriate proportion of the secondary crystallization mother liquor fed to the secondary crystallizer is 90-95%.

Claims

1. A method for preparing nylon 66 salt crystals, characterized in that, In the method, nylon 66 salt aqueous solution is used to prepare nylon 66 salt crystals through a two-stage crystallization and rotary pressure filtration process.

2. The method for preparing nylon 66 salt crystals according to claim 1, characterized in that, The secondary crystals include: (1): The crystal slurry after crystallization of Nylon 66 salt solution in the primary crystallizer is separated by a rotary pressure filter to obtain crystal crystals and primary crystallization mother liquor; (2): Part of the primary crystallization mother liquor is sent to the primary feed tank, and the rest is sent to the secondary crystallizer. The crystal slurry after secondary crystallization is separated by a rotary pressure filter to obtain secondary crystals and secondary crystallization mother liquor. The secondary crystals are dissolved in water and sent to the primary feed tank.

3. The method for preparing nylon 66 salt crystals according to claim 2, characterized in that, The secondary crystallization also includes: (3): A portion of the secondary crystallization mother liquor is sent to the secondary crystallizer.

4. The method for preparing nylon 66 salt crystals according to claim 2, characterized in that, The Nylon 66 salt solution is preheated before entering the primary crystallizer at a temperature of 40-41℃.

5. The method for preparing nylon 66 salt crystals according to claim 2, characterized in that, In step (2), 80%-85% of the primary crystallization mother liquor is sent to the primary feed tank, and the remainder is sent to the secondary crystallizer.

6. The method for preparing nylon 66 salt crystals according to claim 2, characterized in that, In step (3), 90%-95% of the secondary crystallization mother liquor is sent to the secondary crystallizer.

7. The method for preparing nylon 66 salt crystals according to claim 2, characterized in that, In step (1), the water in the nylon 66 salt solution is evaporated and condensed in a primary crystallizer and then used as recycled water.

8. The method for preparing nylon 66 salt crystals according to claim 2, characterized in that, In step (2), the water in the nylon 66 salt solution is evaporated and condensed through a secondary crystallizer and then used as recycled water.

9. Nylon 66 salt crystals prepared by the method of any one of claims 1-6.

10. The application of the nylon 66 salt crystals according to claim 9 in the nylon 66 polycondensation and slicing process.

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