Continuous polymerization apparatus and continuous polymerization method for producing polyamide using dibasic acid and diamine

Abstract

To provide a continuous polymerization apparatus and a continuous polymerization method for producing polyamide by using a dibasic acid and a diamine.SOLUTION: The continuous polymerization apparatus includes an evaporator 1, a reaction rectifying column 2, a flash evaporator 3, and a polymerizer 4, which are sequentially connected by piping. In a tray section of the reaction rectifying column, the number of trays is 15 to 30, an interval between adjacent trays is 400 mm to 600 mm, and a height of an overflow weir of each tray is 50 mm to 300 mm. The continuous polymerization apparatus further includes a preheating heat exchanger 6 and a prepolymerization heat exchanger 7, and the prepolymerization heat exchanger and a tower bottom are connected in a circulating manner via piping. Consequently, a raw material can sequentially enter, through the trays, the preheating heat exchanger, the prepolymerization heat exchanger, and the tower bottom, and can circulate and flow between the prepolymerization heat exchanger and the tower bottom.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] <Cross-reference to related patent applications> This application claims priority from a Chinese patent application filed on May 20, 2024, bearing application number 202410623625.7 and entitled "Continuous polymerization apparatus and method for producing polyamide using dibasic acid and diamine," the entire contents of which are incorporated herein by reference.

[0002] The present invention relates to the technical field of polyamides, and more particularly to a continuous polymerization apparatus and method for producing polyamides using dibasic acids and diamines. [Background technology]

[0003] Polyamide, abbreviated as PA and commonly known as nylon, is a general term for polymers containing an amide group (-NH-C=O) in the repeating unit of the polymer main chain. Polyamides are usually produced by the ring-opening polymerization of lactams, such as nylon 6, or by the condensation polymerization of dibasic acids and diamines, such as nylon 66.

[0004] When producing polyamides by condensation polymerization of dibasic acids and diamines, a typical industrial method is to first mix dibasic acids and diamines in a 1:1 molar ratio in a salt pool with water to prepare a salt solution with a concentration of 50 wt%, and then gradually remove the physical water and generated water in the system to complete the condensation polymerization. Summary of the Invention [Problem to be solved by the invention]

[0005] However, diamines are highly volatile and volatilize from the salt solution during the evaporation process, causing an imbalance in the carboxylic acid / amine ratio in the salt solution, which not only affects the degree of polymerization and performance of polyamide products but also increases production costs due to significant losses.At the same time, the volatilized diamine is emitted into the gas phase, causing environmental pollution. [Means for solving the problem]

[0006] To solve the above problems, the present invention provides a continuous polymerization apparatus and a continuous polymerization method for producing polyamide using dibasic acid and diamine. Producing polyamide using a continuous polymerization apparatus not only ensures the quality of polyamide products, but also reduces losses, reduces the generation of three types of waste, and reduces process risks and operating costs.

[0007] A continuous polymerization apparatus for producing polyamide using a dibasic acid and a diamine includes an evaporator, a reactive distillation column, a flash evaporator, and a polymerization vessel, connected in sequence by piping. The reactive distillation column has a column bottom and a tray section located above the column bottom, the tray section including multiple trays, the number of trays in the tray section being 15 to 30, the spacing between adjacent trays being 400 to 600 mm, and the height of the tray overflow weir being 50 to 300 mm. The continuous polymerization apparatus further includes a preheating heat exchanger and a prepolymerization heat exchanger, the tray closest to the column bottom in the tray section, the preheating heat exchanger, and the prepolymerization heat exchanger being connected in sequence by piping, and the prepolymerization heat exchanger is connected to the column bottom in a cyclical manner via piping, so that raw materials pass through the trays and enter the preheating heat exchanger, the prepolymerization heat exchanger, and the column bottom in that order, and can flow cyclically between the prepolymerization heat exchanger and the column bottom.

[0008] In one embodiment, the number of trays is 20 to 25.

[0009] In one embodiment, the distance between adjacent trays is 450 mm to 550 mm.

[0010] In one embodiment, the height of the overflow weir of the tray is 100mm to 200mm.

[0011] In one embodiment, an online infrared detector is installed in the piping of the vapor phase outlet at the top of the reactive distillation column.

[0012] In one embodiment, the continuous polymerization apparatus further includes a diamine online supply pipe for supplying diamine in real time, and the diamine online supply pipe is connected to the prepolymerization heat exchanger.

[0013] In one embodiment, the polymerization vessel includes a polymerization vessel body, an exhaust pipe communicating with the polymerization vessel body, and a multi-stage heat exchanger incorporated in the exhaust pipe.

[0014] In one embodiment, the number of the polymerization vessels is one or more.

[0015] The continuous polymerization method for producing polyamide using a dibasic acid and a diamine is carried out using the above-mentioned continuous polymerization apparatus and includes the following steps: The salt solution is transported to an evaporator for evaporation and concentration, then transported to a reactive distillation column for preheating, and then reaches the trays in the tray section. The trays are layer-by-layer transferred to a preheating heat exchanger, a prepolymerization heat exchanger, and the bottom of the column. The resulting mixture is then transported to a flash evaporator and a polymerizer for flash evaporation and polymerization, and the resulting mixture is granulated to obtain a polyamide product. A circulating flow of raw materials is maintained between the prepolymerization heat exchanger and the bottom of the column.

[0016] The degree of polymerization of the raw material transported to the preheating heat exchanger is 2-3.

[0017] In the present invention, the residence time of the salt solution can be extended and the initial prepolymerization reaction can be carried out by adjusting the number and spacing of trays in the reactive distillation column and the height of the overflow weir. On the one hand, the initial prepolymerization reaction reduces the concentration of free amine in the salt solution, further reducing amine loss during the evaporation and concentration process. On the other hand, after the residence time is extended, the gas phase generated by evaporation and concentration at the bottom of the column rises within the reactive distillation column and comes into countercurrent contact with the salt solution, allowing for sufficient contact with the salt solution during mass and heat transfer. As the free amine in the salt solution is reduced by the initial prepolymerization, the diamine entrained in the gas phase can be absorbed through two mechanisms: physical absorption (cooling and condensation) and chemical absorption (reaction with the carboxylic acid in the salt solution to form a carboxylic acid amine salt). This not only ensures the quality of the polyamide product, but also reduces losses and the generation of three types of waste, thereby improving the competitiveness of the process.

[0018] At the same time, the present invention also adds a thermal heat exchanger and a prepolymerization heat exchanger, so that the raw material passes through the last tray and enters the preheating heat exchanger for preheating, and then enters the prepolymerization heat exchanger for evaporation and prepolymerization, thereby reducing the thermal fluid flow rate and temperature of the prepolymerization heat exchanger and avoiding the formation of scale in the prepolymerization heat exchanger caused by the decomposition and carbonization of the raw material due to the high temperature difference, which not only ensures the quality of the polyamide product, but also ensures the heat exchange efficiency and production capacity of the prepolymerization heat exchanger, and reduces process risks and operating costs. [Brief explanation of the drawings]

[0019] In order to more clearly describe the technical solutions in the embodiments of the present invention, the drawings necessary for describing the embodiments are briefly described below. Obviously, the drawings in the following description are only embodiments of the present invention, and those skilled in the art can obtain other drawings based on the disclosed drawings without any creative efforts.

[0020] [Figure 1]FIG. 1 is a diagram showing a continuous polymerization apparatus for producing polyamide using the dibasic acid and diamine of the present invention. [Figure 2] FIG. 1 is a diagram showing the configuration of a polymerization vessel according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0021] To facilitate understanding of the present invention, the present invention will be described in more detail below. However, it should be understood that the present invention can be embodied in many different forms and is not limited to the embodiments and examples described herein. Rather, the purpose of providing these embodiments and examples is to enable a more thorough and comprehensive understanding of the disclosed subject matter of the present invention.

[0022] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art of the present invention. The terms used in the specification of the present invention are intended to describe specific embodiments and examples and are not intended to limit the scope of the present invention. The term "and / or" used herein includes any one of two or more related items, and also includes any and all combinations of the related items. Here, any and all combinations include any two related items, any more related items, or all combinations of the related items.

[0023] As shown in Figure 1, the continuous polymerization apparatus for producing polyamides using dibasic acids and diamines provided by the present invention is primarily used to evaporate and concentrate a salt solution prepared using dibasic acids and diamines, remove physical water and generated water from the system, and complete the condensation polymerization. Here, the dibasic acid in this invention refers to C4-C12 dicarboxylic acids widely used in the industry, such as succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, and lauric acid. The diamine in this invention refers to C4-C12 diamines widely used in the industry, such as 1,4-butanediamine, 1,5-pentanediamine, 1,6-hexanediamine, 1,7-heptanediamine, 1,8-octanediamine, 1,9-nonanediamine, 1,10-decanediamine, and lauryldiamine. The concentration of the salt solution may be any ratio. Preferably, the dibasic acid and diamine are mixed in a salt pool in a molar ratio of 1:1 in water to form a 50 wt% to 65 wt% salt solution.

[0024] Specifically, the continuous polymerization apparatus of the present invention includes an evaporator 1, a reactive rectification column 2, a flash evaporator 3, and a polymerization vessel 4, which are connected in this order by piping.

[0025] Evaporator 1 is used to evaporate and concentrate the prepared salt solution. Specifically, the salt solution prepared using a dibasic acid and a diamine enters evaporator 1 via pipe a, and evaporator 1 is heated with steam, the temperature of which is preferably 100°C to 200°C. During the heating process, the pressure is preferably maintained at 0.1 MPa(g) to 0.25 MPa(g), which achieves evaporation and concentration of the salt solution and reduces the water content of the salt solution.

[0026] The gas phase produced during the evaporation and concentration process of the salt solution contains water and a very small amount of diamine. Optionally, the gas phase is discharged from the evaporator 1 via pipe b, cooled using a cooling tower or other device, collected, and recycled for use in preparing the salt solution, which realizes resource recycling and is environmentally friendly.

[0027] The reactive distillation column 2 has a column bottom 22 and a tray section located above the column bottom 22, and is used to receive the raw material after it has been evaporated and concentrated in the evaporator 1, i.e., the initially concentrated salt solution. Specifically, the raw material enters a preheater located at the top of the reactive distillation column 2 via pipe c, is preheated by the ascending vapor phase in the reactive distillation column 2, and then enters tray 21 at the top of the tray section of the reactive distillation column 2, for example, the second or third tray 21 from the column top to the column bottom, and then flows layer by layer through each tray 21 and comes into countercurrent contact with the ascending vapor phase in the reactive distillation column 2, thereby transferring mass and heat.

[0028] The number of trays 21 in the tray section is typically 8 to 10, the spacing between adjacent trays 21 typically 200 to 300 mm, and the height of the overflow weir typically 5 to 30 mm. However, with this configuration, the contact residence time between the raw material and the gas phase is short, preventing the prepolymerization reaction from starting in the tray. Therefore, the free amine in the raw material is easily volatilized after further preheating, causing an imbalance in the carboxylic acid / amine ratio in the raw material and hindering the rapid progress of the subsequent prepolymerization reaction. While the lost amine can be optimally adjusted during the salt formation stage, this often results in increased unit consumption and increased costs. Furthermore, the diamine volatilized into the gas phase is discharged along with the gas phase, causing environmental pollution.

[0029] In contrast, the present invention adjusts the installation method of trays 21 in the tray section. Specifically, the number of trays 21 in the tray section in the present invention is 15 to 30, preferably 20 to 25, the spacing between two adjacent trays 21 is 400 mm to 600 mm, preferably 450 mm to 550 mm, and the height of the overflow weir of tray 21 is 50 mm to 300 mm, preferably 100 mm to 200 mm. In the present invention, by adjusting the number and spacing of trays 21 in the reactive distillation column 2, the time during which the raw material flows layer by layer into tray 21 closest to the column bottom is lengthened, and by adjusting the height of the overflow weir of tray 21 in the reactive distillation column 2, the residence time of each layer of raw material in tray 21 is lengthened. This lengthens the overall residence time of the raw material in the reactive distillation column, allowing the raw material to undergo an initial prepolymerization reaction at this stage.

[0030] The initial prepolymerization reaction converts free amines into polyamide dimers, thereby reducing the concentration of free amines in the feedstock and further reducing amine losses during the evaporation and concentration process. At the same time, after the residence time is extended, the gas phase generated by evaporation and concentration at the bottom of the column rises through the reactive distillation column and comes into countercurrent contact with the feedstock, allowing for sufficient mass and heat transfer. As the free amines in the feedstock are reduced by the initial prepolymerization, the diamines entrained in the gas phase can be absorbed through two mechanisms: physical absorption (cooling and condensation) and chemical absorption (reaction with the carboxylic acid in the feedstock to form carboxylic acid amine salts). This not only maintains the carboxylic acid / amine ratio in the feedstock and ensures the quality of the polyamide product, but also reduces losses and the generation of three types of waste, improving the competitiveness of the process.

[0031] Continuing to refer to Figure 1, the continuous polymerization apparatus of the present invention further includes a preheating heat exchanger 6 and a prepolymerization heat exchanger 7. Tray 21 near the bottom of the tray section, preheating heat exchanger 6, and prepolymerization heat exchanger 7 are connected in sequence by piping, and prepolymerization heat exchanger 7 is cyclically connected to the bottom 22 via piping. This allows the raw material to pass through tray 21 and enter preheating heat exchanger 6, prepolymerization heat exchanger 7, and bottom 22 in that order, and to flow cyclically between prepolymerization heat exchanger 7 and bottom 22.

[0032] The preheating heat exchanger 6 is used to receive and preheat the raw material flowing out from the tray 21 near the bottom of the tray section, i.e., the salt solution initially prepolymerized in the tray section. Specifically, as the raw material flows through the tray 21 near the bottom of the column, it enters the preheating heat exchanger 6 via the pipe e and is preheated.

[0033] Alternatively, the effluent may be collected by a collector such as a collection plate on a tray 21 near the bottom of the column, and then connected to a circulation pump 6a to be transported to the preheating heat exchanger 6 for preheating. The circulation pump 6a may be a pump of a type commonly used in the industry, such as a centrifugal pump or a diaphragm pump, and in this embodiment, a centrifugal pump is preferred.

[0034] Alternatively, the heat source of the preheating heat exchanger 6 may be a liquid-phase thermal oil at 200°C to 250°C, such as T66 (hydrogenated terphenyl mixture) or similar thermal oils used in other industries, or the heat source may be steam at the same temperature, which preheats the feedstock to 200°C to 220°C. Considering that the feedstock undergoes initial prepolymerization in the tray section of the reaction rectification column 2, in order to avoid clogging of the preheating heat exchanger 6 or a decrease in the heat exchange efficiency due to the high viscosity feedstock, it is preferred that the preheating heat exchanger 6 of the present invention be a high-flow coil-wound heat exchanger, and the heat source is preferably medium-pressure steam, which can be used for heating, thereby utilizing its latent heat to further enhance the heat exchange efficiency and achieve the goal of quickly preheating the feedstock.

[0035] The prepolymerization heat exchanger 7 receives the raw material preheated by the preheating heat exchanger 6 and performs evaporation and prepolymerization to achieve a desired concentration and degree of prepolymerization. Specifically, the raw material preheated by the preheating heat exchanger 6 enters the bottom of the prepolymerization heat exchanger 7 through piping f, and the forced transport of the raw material is achieved in the prepolymerization heat exchanger 7 by the front-end pump and the density difference formed in the prepolymerization heat exchanger 7, where evaporation and prepolymerization are performed.

[0036] After preheating by the preheating heat exchanger 6, the prepolymerization heat exchanger 7 only needs to preheat the feedstock to 220°C to 2250°C. However, if the present invention only adds the prepolymerization heat exchanger 7, the prepolymerization heat exchanger 7 must be heated using thermal oil at 280°C to 300°C, resulting in significant heat loss and making it uneconomical and impractical. The surface temperature of the prepolymerization heat exchanger 7 increases, causing the feedstock to carbonize and form scale after prolonged operation. This significantly reduces the heat exchange efficiency and feedstock fluidity of the scale-formed prepolymerization heat exchanger 7, necessitating an emergency shutdown and cleaning, which increases process risks and operating costs. Furthermore, if the scale breaks off and gets into the feedstock, it may affect the color and performance of the polyamide product.

[0037] In the present invention, the preheating heat exchanger 6 and the prepolymerization heat exchanger 7 are installed at the same time, so that the raw material passes through the last tray 21 into the preheating heat exchanger 6 to be preheated, and then enters the prepolymerization heat exchanger 7 for evaporation and prepolymerization. This reduces the heat fluid flow rate and temperature in the prepolymerization heat exchanger 7 and prevents the formation of scale in the prepolymerization heat exchanger 7, which is caused by the decomposition and carbonization of the raw material due to the high temperature difference. This not only ensures the quality of the polyamide product, but also ensures the heat exchange efficiency and production capacity of the prepolymerization heat exchanger 7, and reduces process risks and operating costs.

[0038] The bottom 22 of the reactive distillation column 2 is used to receive a feedstock containing a gas phase and a liquid phase that has been subjected to evaporation and concentration and prepolymerization in the prepolymerization heat exchanger 7. Specifically, the feedstock obtained in the prepolymerization heat exchanger 7 enters the bottom 22 of the reactive distillation column 2 via pipe g, where gas-liquid separation is carried out. The gas phase undergoes sufficient heat exchange and absorption with the feedstock located on tray 21 in the tray section, and is then discharged from the reactive distillation column 2 via gas-phase outlet pipe d at the top of the column. A portion of the liquid phase enters the flash evaporator 3 via pipe h, where it is further subjected to flash evaporation and polymerization, and the other portion circulates through pipe n to the prepolymerization heat exchanger 7.

[0039] Optionally, an online infrared detector 2a is installed in pipe d at the vapor phase outlet at the top of the reactive distillation column 2 for real-time monitoring of the content of multicomponent low-molecular-weight compounds, particularly the diamine content, in the vapor phase. At the same time, in the present invention, a diamine online supply pipe m may be installed to supply diamine to the prepolymerization heat exchanger 7 in real time in accordance with changes in diamine loss obtained through monitoring, in order to maintain the carboxylic acid / amine ratio in the prepolymerization heat exchanger 7 within an appropriate range. The diamine online supply pipe m may be connected directly to the prepolymerization heat exchanger 7 or may be connected to the prepolymerization heat exchanger 7 via pipe f.

[0040] The flash evaporator 3 uses a piping coil with a variable diameter to utilize the piping resistance to gradually flash evaporate the water in the liquid-phase feedstock, achieving a gas-liquid two-phase state. At the same time, the initially prepolymerized polyamide in the feedstock is further polymerized in the flash evaporator 3. The flash evaporator 3 can heat the feedstock to any temperature between 250°C and 300°C, and the water content in the gas-liquid two-phase feedstock can be any ratio between 0.1 wt% and 2 wt%.

[0041] The raw material (gas-liquid two-phase) obtained in flash evaporation 3 enters polymerizer 4 via pipe k. Polymerizer 4 separates the incoming gas-liquid two-phase mixture, further removes moisture from the system under a vacuum system, and promotes the reaction until final polymerization, thereby obtaining a polyamide product. The moisture content of the polyamide product may be any percentage between 0.01 wt% and 0.1 wt%. Finally, the product obtained in polymerizer 4 is pelletized in pelletizer 5 to obtain a polyamide product.

[0042] In order for the raw materials entering the polymerizer 4 to reach the degree of polymerization and molecular weight distribution required for the final polyamide product, the polymerizer 4 must be able to efficiently and stably remove moisture from the system and ensure that the raw materials remain in the polymerizer 4 for a sufficient period of time to reach the final state of the polyamide product.

[0043] Alternatively, the number of the polymerization vessels 4 may be one or more, preferably two. The two polymerization vessels 4 are each independently connected to a flash evaporator 3. This allows switching between them according to the actual situation.

[0044] When the gas-liquid two-phase raw materials enter the polymerization reactor 4 and are separated, fluctuations in the liquid level can cause uneven heating of some of the product, which can lead to a crosslinking reaction and gelation. Gels affect the quality of the product, and if heated further, they turn into black spots, further degrading the quality of the product.

[0045] Fluctuations in the liquid level are usually caused by fluctuations in the vacuum level. Currently, a water ring vacuum pump or other similar device is typically used to directly extract a large amount of polymerizable process vapor from the polymerizer 4, which is then further cooled and discharged using a cooling method such as spray. However, directly extracting vapor-phase moisture from the polymerizer 4 causes fluctuations in the flow rate of the generated vapor-phase moisture. To balance the fluctuations caused by changes in the amount of gas extracted, the water ring vacuum pump for the polymerizer 4 requires regular adjustment. This inevitably affects the vacuum level of the polymerizer 4. An unstable vacuum significantly impacts liquid level control in the polymerizer 4, causing undesirable crosslinking side reactions and the formation of gels and black spots, necessitating frequent changeover and cleaning. Furthermore, cooling the large amount of process vapor extracted by the vacuum pump requires circulating spray water or other similar means, which not only wastes the heat of the process vapor but also requires additional energy for cooling, resulting in increased production costs.

[0046] 2, a preferred polymerizer 4 of the present invention includes a polymerizer body 41, an exhaust pipe 42 communicating with the polymerizer body 41, and a heat exchanger 43 incorporated in the exhaust pipe 42, and the number of heat exchangers 43 is two or more, including 2, 3, 4, etc. This allows heat exchange with steam using a multi-stage heat exchanger to produce low-pressure steam and hot water.

[0047] Specifically, in this invention, a water ring vacuum pump is used to establish vacuum in the polymerizer 4, and a multi-stage heat exchanger 43 is installed in the exhaust pipe 42 of the polymerizer 4 to exchange heat with the process steam, utilizing the heat from the process steam to produce a certain amount of low-pressure steam and hot water. At the same time, the gas phase cooled by the multi-stage heat exchanger 43 is mainly non-condensable gases and a very small amount of process steam, which ensures the temperature operation of the vacuum system and avoids unnecessary frequent switching and cleaning due to vacuum fluctuations. At the same time, the heat from the process steam can be further utilized and the use of cooling media can be reduced. This reduces operating and production costs, improves the quality stability of polyamide products, and reduces gels and black spots.

[0048] The multi-stage heat exchanger 43 of the present invention is a conventional heat exchanger such as a shell-and-tube heat exchanger or a coiled tube heat exchanger. By installing the multi-stage heat exchanger 43, low-pressure steam and hot water are simultaneously generated in stages, and a second stage of heat exchange is performed on the process steam, making maximum use of the heat of the process steam. After sufficient heat exchange, the process steam consists mainly of non-condensable gases (nitrogen, amines, etc.) and a small amount of water. This is quickly extracted by a water ring vacuum pump to a spray tower or other similar cooling means, where it is finally cooled and discharged.

[0049] Continuing to refer to FIG. 1, the present invention also provides a continuous polymerization method for producing polyamide using the above-mentioned continuous polymerization apparatus, which includes the following steps:

[0050] The salt solution is transported to evaporator 1 for evaporation and concentration, then transported to reactive distillation column 2 where it is preheated and reaches tray 21 in the tray section. After entering tray 21 near the column bottom layer by layer, it is transported in turn to preheating heat exchanger 6, prepolymerization heat exchanger 7 and column bottom 22, and then transported in turn to flash evaporator 3 and polymerizer 4 for flash evaporation and polymerization, and the product is granulated to obtain a polyamide product. There is a circulating flow of raw materials between prepolymerization heat exchanger 7 and column bottom 22.

[0051] Optionally, the degree of polymerization of the raw material transported to the preheating heat exchanger 6 is 2 to 3, which can prevent the formation of scale in the prepolymerization heat exchanger 7, which is caused by decomposition or carbonization of the raw material due to the high temperature difference in the prepolymerization heat exchanger 7. This not only ensures the quality of the polyamide product, but also ensures the heat exchange effect of the prepolymerization heat exchanger 7.

[0052] Alternatively, the water content of the raw material obtained by the prepolymerization heat exchanger 7 is controlled to preferably 10 wt % to 12 wt %.

[0053] The above-mentioned continuous polyamide polymerization apparatus and continuous polymerization method will be further described below with reference to specific examples.

[0054] Example 1 Adipic acid and hexamethylenediamine are mixed in a 1:1 molar ratio in water in a salt pool to form a salt solution containing 50 wt % nylon 66.

[0055] The salt solution was poured into the tube a for 0.5 m. 3 The salt solution is transported to evaporator 1 at a rate of 1 / h, and evaporator 1 heats the salt solution to 140°C and maintains the pressure at 0.2 MPa(g) to remove some of the water from the salt solution, obtaining a raw material with a water content of 25 wt%, i.e., an initially concentrated salt solution.

[0056] The raw material is delivered through pipe c by 0.33 m. 3 The feedstock is transported to the reactive distillation column 2 at a rate of 1 / h. The reactive distillation column 2 has 20 trays 21, with a 450 mm spacing between adjacent trays 21 and a 150 mm overflow weir height. After entering the reactive distillation column 2, the feedstock is first preheated by the top preheater and then enters the third tray 21 at the top of the reactive distillation column 2. The feedstock flows through each tray 21 layer by layer, yielding an initially prepolymerized salt solution on the last tray 21 near the bottom. Sampling tests revealed that the degree of polymerization at this stage was DP = 2-3. The feedstock is collected on a collection plate and then transported by a centrifugal pump to preheating heat exchanger 6, where it is preheated to 220°C. It then enters prepolymerization heat exchanger 7 via pipe f, where it is heated to 45°C for further moisture removal and further prepolymerization, yielding a feedstock with a moisture content of 10 wt%. Next, the raw material is transported to the bottom of the reaction rectification column 2 through the pipe g, where gas-liquid separation is carried out. A part of the liquid phase is circulated through the pipe n into the prepolymerization heat exchanger 7, and the other part is circulated through the pipe h to the bottom of the reaction rectification column 2 through the pipe g. 3 / h to the flash evaporator 3, which heats the raw material to 270°C to obtain a raw material with a moisture content of 0.5 wt%.

[0057] The raw material is delivered through the pipe k to 0.26 m 3The mixture is transported to the polymerizer 4 at a rate of 1 / h, where it is further polymerized to obtain a product with a moisture content of 0.1 wt% and a relative viscosity of 2.4. The product is finally transported to the pelletizer 5 and pelletized to obtain a polyamide product. The process steam (0.027 m) generated in the polymerizer 4 is also used. 3 / h) is cooled by heat exchange in a multi-stage heat exchanger 43 incorporated in the exhaust pipe 42 and then discharged.

[0058] Comparative Example 1 The only differences between Comparative Example 1 and Example 1 are that the number of trays 21 in the reactive distillation column 2 is 20, the distance between two adjacent trays 21 is 300 mm, and the height of the overflow weir is 50 mm.

[0059] Sampling analysis showed that the feedstock entering prepolymerization heat exchanger 7 did not undergo prepolymerization, i.e., the degree of polymerization was 0. Furthermore, tracking by an online infrared detector installed in pipe d at the overhead gas phase outlet revealed a significant increase in the hexamethylenediamine peak under comparable evaporation rates. Test analysis after complete condensation showed that the amine content of the overhead cold condensate in Example 1 was 0.12%, while the amine content of the overhead cold condensate in Comparative Example 1 was 0.27%. Testing of the end group content in the final polyamide product showed that the end group content was 51 mol / kg in Example 1 and 45 mol / kg in Comparative Example 1.

[0060] This shows that if the setting method of tray 21 of reactive distillation column 2 is not within the scope of the present invention, the initial prepolymerization reaction will not occur on tray 21, resulting in a significant increase in the loss of diamine, which will result in an increase in the COD in the wastewater, and in particular, ammonia nitrogen compounds, which will make wastewater treatment more difficult. Furthermore, the loss of a large amount of amine will also reduce the quality of the polyamide product.

[0061] Furthermore, due to the large amount of amine loss, a longer residence time in the polymerization reactor 4 was required for Comparative Example 1 to achieve the target degree of polymerization / viscosity. The longer the residence time, the higher the liquid level, resulting in greater amine loss and fluctuations in the loss rate, causing the liquid level to constantly change. Frequent changes in the liquid level increase the rate of gelation and black spots in the polyamide product, significantly affecting its quality. Specifically, after long-term operation in Example 1, the number of black spots was less than 10 per kilogram, whereas after long-term operation in Comparative Example 1, the number of black spots exceeded 20 per kilogram.

[0062] Comparative Example 2 The difference between Comparative Example 2 and Example 1 is that the raw material passes through tray 21 near the bottom of the column and then directly enters prepolymerization heat exchanger 7, which uses high-temperature thermal oil at 300°C to rapidly heat the raw material.

[0063] In Comparative Example 2, the preheating heat exchanger 6 is not used, so the temperature and flow rate of the thermal oil in the prepolymerization heat exchanger 7 must be significantly increased. Converted to equivalent production capacity, the total heat value in Example 1 is approximately 430,000 kcal / h, while the heat value of the thermal oil required in Comparative Example 2 (including heat dissipation losses) is 500,000 kcal / h, resulting in a 16% increase in energy consumption. Long-term operation results show that the color of the product in Comparative Example 2 is generally YI = 5-10, while that of the product in Example 1 is YI = 1-3. This is because the temperature of the thermal oil in the prepolymerization heat exchanger 7 in Comparative Example 2 is relatively high, causing some of the prepolymer to polymerize rapidly, rapidly forming scale in the piping of the prepolymerization heat exchanger 7, which then decomposes after long-term operation, resulting in unwanted color contamination in the final product. At the same time, after scale formation in the prepolymerization heat exchanger 7, the heat exchange efficiency is significantly reduced, further reducing the flow rate within the prepolymerization heat exchanger 7, ultimately forcing operation to be shut down, increasing operating costs and the difficulty of cleaning.

[0064] Example 2 The difference between Example 2 and Example 1 is that the generated process steam (0.027 m 3 / h) is directly extracted by a water ring pump, spray cooled and then discharged.

[0065] Comparing the long-term operation data of Example 1 and Example 2, the gelation rate of the product of Example 1 is 0.1‰~0.15‰, the black points are 0.16‰~0.23‰, and the equipment changeover time is 7~8 months, while the gelation rate of the product of Example 2 is 0.23‰~0.32‰, the black points are 0.2~0.29‰, and the equipment changeover time is 3~4 months.

[0066] According to calculations for large-scale production, the polymerization unit 4 can continuously generate 1 to 1.5 t / h of 280°C steam (approximately 500,000 kcal / h), generating approximately 400,000 kcal of heat per year, which is equivalent to approximately 500 to 600 tce / year in standard coal terms. A multi-stage heat exchanger 43 is installed to cool the process steam and simultaneously generate 0.15 t / h of low-pressure steam and 8 t / h of 75°C hot water. Implementing the above process solution reduces the amount of circulating water used and also regenerates a certain amount of low-pressure steam and a large amount of high-temperature water for heating and insulation of other equipment throughout the plant, such as hexanediamine and nylon salt pipes. This not only reduces the unnecessary use of circulating water but also reduces the purchase of external steam, saving approximately 2 to 3 million yuan in energy consumption each year.

[0067] Furthermore, a comparison between Example 1 and Example 2 shows that after installing the multistage heat exchanger 43, the operating time of the equipment is significantly extended, and the number of annual replacements and maintenance of the equipment is reduced by one to two times. Generally, the maintenance and production costs for one equipment are approximately 1 million to 1.5 million yuan. By installing the multistage heat exchanger 43, the energy savings and reduced equipment maintenance costs combined can reduce production costs by approximately 3.5 million to 5 million yuan per year.

[0068] The technical features of the above embodiments can be combined in any combination, and for the sake of brevity, not all combinations of the technical features of the above embodiments are described. However, as long as the combinations of these technical features are not contradictory, they are deemed to be included in the scope described herein.

[0069] The above examples are specifically and in detail described, showing only some embodiments of the present invention, but are not intended to limit the scope of the invention patent. It should be noted that those skilled in the art can make some variations and modifications without departing from the spirit and scope of the present invention, and all of them fall within the scope of protection of the present invention. Therefore, the scope of patent protection of the present invention is defined by the appended claims. [Explanation of symbols]

[0070] 1. Evaporator 2. Reactive rectification column 3. Flash evaporator 4 Polymerization vessel 5 Pelletizer 6 Preheating heat exchanger 7 Prepolymerization heat exchanger 21 Tray 22 Tower bottom 2a Online infrared detector 6a Circulation pump 41 Polymerization vessel body 42 Exhaust pipe 43 Heat exchanger

Claims

1. A continuous polymerization apparatus for producing a polyamide using a dibasic acid and a diamine, comprising an evaporator, a reactive distillation column, a flash evaporator, and a polymerization vessel, which are connected in this order by piping, the reactive distillation column having a column bottom and a tray section located above the column bottom, the tray section including a plurality of trays, the number of trays in the tray section being 15 to 30, the interval between adjacent trays being 400 mm to 600 mm, and the height of an overflow weir on each of the trays being 50 mm to 300 mm; The continuous polymerization apparatus for producing a polyamide using a dibasic acid and a diamine further comprises a preheating heat exchanger and a prepolymerization heat exchanger, wherein the tray in the tray section closest to the bottom of the column, the preheating heat exchanger, and the prepolymerization heat exchanger are connected in this order by piping, and the prepolymerization heat exchanger is connected to the bottom of the column via piping in a cyclical manner, whereby raw materials pass through the trays and enter the preheating heat exchanger, the prepolymerization heat exchanger, and the bottom of the column in this order, and can flow cyclically between the prepolymerization heat exchanger and the bottom of the column.

2. 2. The continuous polymerization apparatus for producing polyamide using dibasic acid and diamine according to claim 1, wherein the number of trays is 20 to 25.

3. 2. The continuous polymerization apparatus for producing polyamide using a dibasic acid and a diamine according to claim 1, wherein the interval between adjacent trays is 450 mm to 550 mm.

4. 2. The continuous polymerization apparatus for producing polyamide using dibasic acid and diamine according to claim 1, wherein the height of the overflow weir of the tray is 100 mm to 200 mm.

5. 2. The continuous polymerization apparatus for producing polyamide using a dibasic acid and a diamine according to claim 1, wherein an online infrared detector is installed in a pipe at a vapor phase outlet at the top of the reactive distillation column.

6. 6. The continuous polymerization apparatus for producing polyamide using a dibasic acid and a diamine according to claim 5, wherein the continuous polymerization apparatus further comprises a diamine online supply pipe for supplying diamine in real time, the diamine online supply pipe being connected to the prepolymerization heat exchanger.

7. 2. The continuous polymerization apparatus for producing polyamide using a dibasic acid and a diamine according to claim 1, wherein the polymerization vessel comprises a polymerization vessel body, an exhaust pipe communicating with the polymerization vessel body, and a multi-stage heat exchanger incorporated in the exhaust pipe.

8. 2. The continuous polymerization apparatus for producing polyamide using a dibasic acid and a diamine according to claim 1, wherein the number of the polymerization vessels is one or more.

9. A continuous polymerization method for producing a polyamide using a dibasic acid and a diamine, which is carried out using the continuous polymerization apparatus according to claim 1, and which comprises the following steps: The salt solution is transported to an evaporator for evaporation and concentration, then transported to a reactive distillation column for preheating, and then reaches the trays in the tray section. After entering the trays near the bottom of the column layer by layer, it is transported in turn to a preheating heat exchanger, a prepolymerization heat exchanger, and the bottom of the column, and then transported in turn to a flash evaporator and a polymerizer for flash evaporation and polymerization, and the product is granulated to obtain a polyamide product. There is a circulating flow of raw materials between the prepolymerization heat exchanger and the bottom of the column.

10. 10. The continuous polymerization method for producing polyamide using a dibasic acid and a diamine according to claim 9, wherein the degree of polymerization of the raw material transported to the preheating heat exchanger is 2 to 3.

11. 10. The continuous polymerization method for producing polyamide using dibasic acid and diamine according to claim 9, wherein the number of trays is 20 to 25.

12. 10. The continuous polymerization method for producing polyamide using dibasic acid and diamine according to claim 9, wherein the interval between adjacent trays is 450 mm to 550 mm.

13. 10. The continuous polymerization method for producing polyamide using dibasic acid and diamine according to claim 9, wherein the height of the overflow weir of the tray is 100 mm to 200 mm.

14. 10. The continuous polymerization method for producing polyamide using a dibasic acid and a diamine according to claim 9, wherein an online infrared detector is installed in the piping of the vapor phase outlet at the top of the reactive distillation column.

15. 15. The continuous polymerization method for producing polyamide using a dibasic acid and a diamine according to claim 14, wherein the continuous polymerization apparatus further comprises a diamine online supply pipe for supplying diamine in real time, and the diamine online supply pipe is connected to the prepolymerization heat exchanger.

16. 10. The continuous polymerization method for producing a polyamide using a dibasic acid and a diamine according to claim 9, wherein the polymerization vessel comprises a polymerization vessel body, an exhaust pipe communicating with the polymerization vessel body, and a multi-stage heat exchanger incorporated in the exhaust pipe.

17. 10. The continuous polymerization method for producing polyamide using a dibasic acid and a diamine according to claim 9, wherein the number of the polymerization vessels is one or more.

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