A system and method for continuously producing polyester polyols

Abstract

This invention provides a system and method for the continuous preparation of polyester polyols, relating to the field of polymer synthesis technology. The invention constructs a continuous system architecture consisting of four stages in series: esterification premixing, esterification reaction, polycondensation premixing, and polycondensation reaction. The esterification premixing unit preheats and homogenizes the raw materials, providing a stable feed for the esterification reaction. The esterification reaction unit independently completes the atmospheric pressure esterification reaction and the first-stage removal of water as a byproduct. The polycondensation premixing unit provides isothermal and pressure-stabilized buffering of the esterification products, precisely adds catalyst, and achieves homogeneous mixing, eliminating the mismatch in operating conditions caused by differences in pressure, temperature, and reaction rate between the esterification and polycondensation stages. This provides a feed material with uniform composition, stable temperature, and precise proportions for the subsequent polycondensation reaction. The polycondensation reaction unit independently completes the polycondensation reaction and the deep removal of water as a byproduct under high vacuum conditions. This achieves continuous and stable production of polyester polyols throughout the entire process.

Description

Technical Field

[0001] This invention relates to the field of polymer synthesis technology, specifically to a continuous synthesis system and method for polyester polyols. Background Technology

[0002] Polyester polyols, with their outstanding advantages such as adjustable molecular structure, controllable macroscopic properties, excellent mechanical properties, good weather resistance and compatibility, are not only key raw material components in the preparation of polyurethane materials, but also widely used in high-value-added fields such as alkyd resins, unsaturated polyesters, bio-based polymer materials, water-based polymer dispersions and high-performance polymer additives. The advancement of their synthesis technology and the stability of product quality are directly related to the overall efficiency and application level of the downstream industrial chain.

[0003] Currently, the industrial production of polyester polyols is still dominated by batch synthesis processes, with typical batch reaction units such as... Figure 3 As shown, the process mainly includes two stages: esterification and polycondensation. While this traditional process has reached a certain level of maturity through long-term industrial practice, its inherent structural defects have increasingly become bottlenecks restricting the industry's development towards higher efficiency and refinement: First, the single-batch production cycle is as long as 9-18 hours, with non-reaction auxiliary operation time accounting for as much as 30%-50%, resulting in significantly low equipment utilization and production efficiency; Second, the esterification and polycondensation stages are completed in the same reactor at different times, making it impossible to independently control the temperature and adjust the material residence time for different reaction stages, leading to poor matching between process conditions and reaction progress; Third, due to the uneven mixing and heat and mass transfer processes within the reactor... The product exhibits several issues: First, the material residence time is widely distributed, resulting in significant batch-to-batch fluctuations in key quality indicators such as acid value, hydroxyl value, viscosity, and particulate density (PDI). The pass rate for PDI ≤ 1.2, required for high-end applications, is generally below 60%, indicating that product stability fails to meet the demands of the high-end market. Second, the production process involves numerous manual interventions, increasing operational complexity and safety risks, and leading to high energy and material consumption per unit of product. Third, the synergy between the reaction process and byproduct removal is insufficient. Esterification-generated water and small-molecule byproducts from polycondensation are difficult to remove from the reaction system promptly and adequately, directly impacting the product's color, purity, and overall quality.

[0004] To overcome some shortcomings of the aforementioned batch processes, existing technologies, such as patent CN121319343A, have improved upon traditional batch processes. This approach couples a multi-stage gas-liquid separation device to the reactor, using segmented heating and stepwise implementation of atmospheric / reduced pressure esterification and vacuum polycondensation to improve byproduct removal efficiency and correspondingly improve product color and purity. However, it should be noted that the optimization of this improved approach focuses on enhancing the gas-liquid separation process within the reactor. Its overall process architecture still maintains the batch operation mode where esterification and reduced pressure polycondensation are completed in the same reactor at different times, failing to achieve spatial decoupling and separation of the two reaction stages. Simultaneously, the reaction temperature and vacuum level remain confined to the same reaction chamber, making independent control based on the optimal reaction kinetics of esterification and polycondensation. Therefore, this technology essentially remains within the scope of batch production systems, offering limited improvement to fundamental issues such as batch-to-batch quality fluctuations, excessively long non-reaction times, and weak coupling between reaction and separation processes, failing to achieve a breakthrough in process structure.

[0005] Furthermore, in the exploration of continuous synthesis, some existing technologies attempt to achieve continuous production by directly connecting esterification reactors and polycondensation reactors in series. However, their improvements focus only on optimizing components such as packing materials and trays within a single reactor, failing to address the fundamental problem of the unstable connection between the esterification (atmospheric pressure, medium temperature) and polycondensation (high vacuum, high temperature) stages due to significant differences in operating conditions. In the direct connection mode, the residual heat and pressure fluctuations of the esterification products directly impact the polycondensation stage, and the catalyst lacks an independent mixing space, resulting in uneven dispersion. This makes it difficult for the polycondensation reaction to operate steadily under optimal conditions, and continuous production has long been unable to achieve true end-to-end stability.

[0006] In summary, existing intermittent polyester polyol synthesis technologies and continuous improvement schemes are no longer adequate to meet the urgent needs of the industry's development towards continuous, intelligent, and high-end processes in terms of reaction efficiency, process control precision, product consistency, and energy and material consumption. To address these technical challenges, there is an urgent need to develop a polyester polyol synthesis system and corresponding methods that can achieve efficient coupling of reaction and separation, precise temperature control during esterification and polycondensation stages, independently adjustable residence times at each stage, and continuous and stable operation throughout the entire process. This would fundamentally solve many common problems faced by existing technologies and promote the upgrading of key production technologies in this field. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention discloses a system and method for the continuous preparation of polyester polyols. By constructing a continuous system architecture consisting of a four-stage series connection of esterification premixing, esterification reaction, polycondensation premixing, and polycondensation reaction, this invention eliminates the technical obstacle of directly connecting the esterification and polycondensation stages due to significant differences in operating conditions, thus achieving continuous and stable production of polyester polyols throughout the entire process.

[0008] To achieve the above technical objectives, on the one hand, the present invention proposes a system for the continuous preparation of polyester polyols, comprising an esterification premixing unit, an esterification reaction unit, a polycondensation premixing unit, and a polycondensation reaction unit connected in sequence. The esterification premixing unit is equipped with a polyacid inlet and a polyol inlet; The liquid phase inlet of the esterification reaction unit is connected to the outlet of the esterification premix unit. A first condenser is provided at the top of the unit. The liquid phase outlet of the first condenser is branched to the reflux inlet and the first discharge pipeline of the esterification reaction unit. A first overflow outlet is provided at the bottom of the esterification reaction unit. The liquid phase inlet of the polycondensation premixing unit is connected to the first overflow outlet and is also provided with a catalyst inlet. The liquid phase inlet of the polycondensation reaction unit is connected to the outlet of the polycondensation premixing unit. A second condenser is provided at the top of the unit. The liquid phase outlet of the second condenser is branched to the reflux inlet and the second discharge pipeline of the polycondensation reaction unit. The gas phase outlet of the second condenser is connected to a vacuum generator. A second overflow outlet is provided at the bottom of the polycondensation reaction unit.

[0009] Unlike existing technologies that directly connect esterification and polycondensation reactors in series, focusing only on optimizing the internal components of a single unit, this invention adds an independent polycondensation premixing unit between the esterification and polycondensation reaction units. This transforms the rigid, direct connection and coupled operation of the esterification and polycondensation sections into a flexible, buffered, and decoupled operation. Specifically, the esterification premixing unit preheats and homogenizes the raw materials, providing a stable feed for the esterification reaction; the esterification reaction unit independently completes the atmospheric pressure esterification reaction and the first-stage removal of water as a byproduct; the polycondensation premixing unit provides constant temperature and pressure buffering for the esterification products, precisely adds catalyst, and achieves homogeneous mixing, eliminating the mismatch in operating conditions caused by differences in pressure, temperature, and reaction rate between the esterification and polycondensation sections, providing a uniformly composed, temperature-stable, and precisely proportioned feed for the subsequent polycondensation reaction; the polycondensation reaction unit independently completes the polycondensation reaction and the deep removal of water as a byproduct under high vacuum conditions.

[0010] In a further example of the present invention, the esterification reaction unit and / or the polycondensation reaction unit is a tower reactor. The tower reactor has a rectification section above the liquid inlet and a reaction section below it. The reaction section is divided into at least two sub-reaction sections connected in series in the vertical direction by a head. Adjacent sub-reaction sections are separated by a head. The head is provided with an overflow pipe and a riser pipe. The overflow pipe extends from the upper reaction section to below the liquid surface of the lower reaction section. The riser pipe extends from the lower reaction section to above the liquid surface of the upper reaction section, and the top of the riser pipe is higher than the top of the overflow pipe.

[0011] Traditional tower reactors typically employ a simple multi-stage stacked structure, where the vapor phase, primarily composed of products, flows upwards through the liquid phase, leading to severe backmixing of byproducts and a gradual weakening of the reaction driving force from bottom to top. This invention addresses this by incorporating relatively multi-stage, interconnected independent sub-reaction sections at the bottom of the tower reactor. This allows the liquid phase to flow downwards in a progressively overflowing manner in the vertical direction, while the gas phase (hot nitrogen and vaporization byproducts) rises along the riser pipes of each sub-reaction section to the rectification section. This constructs an independent, counter-current, multi-stage series reaction system, preventing re-contact between vaporization byproducts and liquid materials and overcoming backmixing. The temperature, gas flow rate, and other parameters of each sub-reaction section can be independently controlled, maintaining a high reaction driving force at each stage. Simultaneously, the overflow pipe extends below the liquid surface of the next stage to form a liquid seal, and the top of the riser pipe is higher than the top of the overflow pipe, ensuring that the gas and liquid phases flow independently. The liquid phase flows downwards along the overflow pipe, while the gas phase rises upwards along the riser pipe, preventing gas-liquid cross-contamination and backmixing.

[0012] In a further preferred embodiment, the liquid inlet of the tower reactor extends below the liquid surface of the first reaction section at the bottom, allowing the feed to directly enter the liquid phase and form a liquid seal, preventing the gas phase from escaping through a short circuit from the feed pipeline and ensuring that the gas phase rises strictly along the riser pipe step by step; at the same time, the feed impact and temperature fluctuations are buffered by the liquid phase, which is beneficial to the temperature stability of the reaction section.

[0013] More preferably, the top end of the riser pipe is at least 3 cm higher than the top end of the overflow pipe.

[0014] In a further example of the present invention, each of the reaction sections is provided with a hot nitrogen inlet extending below the liquid surface, so that the hot nitrogen directly bubbles through the liquid phase in each sub-reaction section, which serves to stir and equalize the temperature on the one hand, and carries the by-product water upward to reduce the water content in the liquid phase, increase the driving force of the reaction, and promote the reaction to proceed in the forward direction.

[0015] In a further example of the invention, the rectification section is packed with packing material and / or trays, and the theoretical number of trays in the rectification section is 5 to 20.

[0016] On the other hand, the present invention also provides a method for continuously preparing polyester polyols using the system described in any of the above examples, comprising the following steps: S1: Polyols and polyacids are mixed in the esterification premixing unit, and the mixed material is continuously conveyed to the esterification reaction unit; S2: In the esterification reaction unit, the esterification reaction is carried out under a hot nitrogen atmosphere. The reaction temperature increases step by step along the material flow direction. Water is generated and evaporated from the top. The esterification product is collected from the first overflow port at the bottom. S3: The esterification product and the catalyst are mixed in the polycondensation premixing unit, and the mixed material is continuously conveyed to the polycondensation reaction unit; S4: In the polycondensation reaction unit, the polycondensation reaction is carried out under a hot nitrogen atmosphere and negative pressure. The reaction temperature increases step by step along the material flow direction. Water is generated and evaporated from the top. The polyester polyol product is collected from the second overflow port at the bottom.

[0017] The above method corresponds to the aforementioned system architecture. By continuously connecting four steps—esterification premixing, esterification reaction, polycondensation premixing, and polycondensation reaction—and coordinating the independent homogeneous mixing of the catalyst in the polycondensation premixing step, esterification and polycondensation can operate synchronously and continuously under their respective optimal conditions.

[0018] During esterification or polycondensation reactions, as the reaction proceeds, the concentration of residual reactants in the system gradually decreases, naturally weakening the driving force of the reaction. Simultaneously, the accumulation of water also inhibits reversible equilibrium. If the same temperature is maintained throughout the process, the reaction rate decreases significantly in the later stages, limiting the conversion rate. This invention addresses this by establishing temperature gradients between each stage of the reaction, matching the reaction temperature to the degree of material conversion in each stage. The initial temperature is lower to avoid over-reaction or side reactions; the subsequent temperature increase compensates for the loss of driving force due to the decrease in reactant concentration and promotes the stripping and removal of the byproduct water, shifting the equilibrium towards the positive direction. This step-by-step heating strategy, combined with a step-by-step overflow of materials and a step-by-step upward flow path of the gas phase, allows each stage of the reaction to be independently maintained at its optimal reaction temperature, maximizing the reaction rate and equilibrium conversion rate.

[0019] In a further example of the present invention, the polybasic acid in step S1 is selected from one or more of dibasic acids, dibasic anhydrides, and hydrogenated dimer acids; And / or, the polyol is selected from one or more of diols, triols, tetraols or pentaols; for example, the diol is ethylene glycol, the triol is glycerol or hexanetriol, the tetraol is tripentaerythritol, and the pentaol is pentapentanol.

[0020] And / or, the molar ratio of the polyol to the polyacid is (1.1~2):1; And / or, the mixing temperature in the esterification premixing unit is 90~130℃; And / or, the pressure of the esterification premix unit is atmospheric pressure.

[0021] In a further example of the present invention, the top of the esterification reaction unit in step S2 is at atmospheric pressure and the top temperature is 100~105°C; And / or, the temperature of the reaction section of the esterification reaction unit is 180~240℃, and the temperature difference between two adjacent reaction sections is 10~20℃; And / or, the residence time of the material in each reaction stage of the esterification reaction unit is 30~90min.

[0022] In a further example of the present invention, the amount of catalyst added in step S3 is 0.01~0.1 wt.% of the total mass of the polyol and polyacid; And / or, the mixing temperature in the polycondensation premixing unit is 200~240℃; And / or, the pressure of the polycondensation premix unit is atmospheric pressure.

[0023] In a further example of the present invention, the absolute pressure at the top of the polycondensation reaction unit in step S4 is 0.001~0.1MPa, and the top temperature is 35~100℃. And / or, the temperature of the reaction section of the polycondensation reaction unit is 200~260℃, and the temperature difference between two adjacent reaction sections is 10~20℃; And / or, the residence time of the material in each stage of the polycondensation reaction unit is 30~90 min.

[0024] In a further example of the present invention, the temperature of the hot nitrogen gas in step S2 and / or step S4 is 90~130°C.

[0025] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0026] (1) The continuous preparation system for polyester polyols of the present invention constructs a continuous system architecture of four stages in series: esterification premixing-esterification reaction-condensation premixing-condensation reaction, and adds an independent condensation premixing unit between the esterification reaction unit and the condensation reaction unit. This transforms the esterification section and the condensation section from rigid direct connection and coupled operation to flexible buffer and decoupled operation, fundamentally eliminating the problem of mismatch in operation caused by the difference in pressure, temperature and reaction rate between the two reaction stages, and realizing continuous steady-state operation of the entire process.

[0027] (2) The method for continuous preparation of polyester polyols of the present invention carries out esterification and polycondensation reactions through a system for continuous preparation of polyester polyols, and removes the water generated in the reaction quickly, which not only significantly improves production efficiency and product quality stability of polyester polyols, but also reduces the generation of by-products such as polyethers. Attached Figure Description

[0028] The accompanying drawings, which form part of this application, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings: Figure 1 A structural diagram of a continuous polyester polyol preparation system according to the present invention is shown; Figure 2 A structural diagram of another continuous polyester polyol preparation system of the present invention is shown; Figure 3 A structural diagram of a system for the intermittent preparation of polyester polyols is shown.

[0029] The above figures include the following reference numerals: 1: Esterification premix tank; 2: Esterification reaction tower; 3: First condenser; 4: Polycondensation premix tank; 5: Polycondensation reaction tower; 6: Second condenser; 7: Batch reactor; 8: Distillation column section; 9: Batch reaction condenser; 2-R1: First reaction section of esterification reaction tower; 2-R2: Second reaction section of esterification reaction tower; 2-R3: Third reaction section of esterification reaction tower; 5-R1: First reaction section of polycondensation reaction tower; 5-R2: Second reaction section of polycondensation reaction tower; 10: Overflow pipe; 11: Gas riser pipe; 12: Hot nitrogen inlet. Detailed Implementation

[0030] To facilitate understanding of the present invention, a more comprehensive description will be provided below, along with preferred embodiments. However, it should be understood that these embodiments are merely for more detailed explanation and should not be construed as limiting the invention in any way, i.e., not intended to limit the scope of protection of the invention.

[0031] Unless otherwise defined, the technical terms used in the following embodiments have the same meanings as commonly understood by those skilled in the art to which this invention pertains. Unless otherwise specified, the experimental reagents used in the following embodiments are conventional biochemical reagents; and the experimental methods described are conventional methods.

[0032] A system for the continuous preparation of polyester polyols, such as Figure 1 and Figure 2 As shown, it includes an esterification premix unit, an esterification reaction unit, a polycondensation premix unit, and a polycondensation reaction unit connected in sequence.

[0033] The esterification premixing unit is equipped with a polyacid inlet and a polyol inlet, which are used to continuously feed polyacids and polyols in a set ratio and premix them to form a stable mixture.

[0034] The liquid inlet of the esterification reaction unit is connected to the outlet of the esterification premixing unit. A first condenser 3 is located at the top of the esterification reaction unit, and its liquid outlet branch connects to the reflux inlet and the first discharge line of the esterification reaction unit. A first overflow outlet is located at the bottom of the esterification reaction unit. The mixed material from the esterification premixing unit continuously enters the esterification reaction unit, where the esterification reaction takes place. Water generated during the reaction rises from the top of the tower in gaseous form, is condensed by the first condenser 3, and then splits into two streams: one stream refluxes back to the top of the esterification reaction unit, and the other stream is continuously discharged from the system as generated water through the first discharge line. The esterification product is continuously discharged from the first overflow outlet at the bottom of the esterification reaction unit.

[0035] The liquid phase inlet of the polycondensation premixing unit is connected to the first overflow outlet and is also equipped with a catalyst inlet. The esterification product continuously enters the polycondensation premixing unit through the first overflow outlet, where it is thoroughly mixed with the catalyst added through the catalyst inlet.

[0036] The liquid inlet of the polycondensation reaction unit is connected to the outlet of the polycondensation premixing unit. A second condenser is located at the top of the polycondensation reaction unit. The liquid outlet of the second condenser 6 is branched to the reflux inlet and the second discharge line of the polycondensation reaction unit. The gaseous outlet of the second condenser 6 is connected to a vacuum generator. A second overflow outlet is located at the bottom of the polycondensation reaction unit. The mixed material from the polycondensation premixing unit continuously enters the polycondensation reaction unit and undergoes a polycondensation reaction under the negative pressure provided by the vacuum generator. The water generated in the reaction rises from the top of the tower in gaseous form, is condensed by the second condenser 6, and then splits into two paths: one path flows back to the top of the polycondensation reaction unit, and the other path is continuously discharged from the system as generated water through the second discharge line. The vacuum generator maintains the vacuum level of the polycondensation reaction unit through the gaseous outlet of the second condenser 6. After the polycondensation reaction is completed, the polyester polyol product is continuously collected from the second overflow outlet at the bottom of the polycondensation reaction unit.

[0037] In the above system, the esterification premixing unit, esterification reaction unit, polycondensation premixing unit, and polycondensation reaction unit are connected in series to enable the material to flow continuously in a fully enclosed process, realizing the continuous and stable production of polyester polyols from raw material feeding to product extraction.

[0038] It should be noted that the specific equipment forms of the esterification premixing unit, esterification reaction unit, polycondensation premixing unit, and polycondensation reaction unit are not restrictive. Those skilled in the art can make reasonable selections based on actual process conditions, production scale, and equipment selection requirements. For example, the esterification premixing unit, esterification reaction unit, polycondensation premixing unit, and polycondensation reaction unit are respectively exemplified as esterification premixing tank 1, esterification reaction tower 2, polycondensation premixing tank 4, and polycondensation reaction tower 5, wherein esterification premixing tank 1 and polycondensation premixing tank 4 are both equipped with conventional vertical stirrers.

[0039] As a further option, the esterification reaction unit and / or polycondensation reaction unit are structurally constructed as follows: The esterification and / or polycondensation reaction units employ tower reactors. The section above the liquid inlet is the rectification section, and the section below is the reaction section. The rectification section is packed with packing material and / or trays, with a theoretical plate number of 5-20. This is used to separate and recover organic components (such as unreacted polyols and low-boiling-point feedstocks) entrained in the rising gas phase from the reaction section, condensing and refluxing them back to the reaction section. Simultaneously, byproduct water vapor is allowed to pass through the rectification section and exit from the top of the tower, thus achieving effective separation of the reaction-generated water and organic matter. This ensures smooth water removal while minimizing feedstock loss.

[0040] The reaction section is divided into at least two vertically connected sub-reaction sections by a head, with gas-liquid phase separation between adjacent sections achieved by the head. Each head is equipped with an overflow pipe 10 and a riser pipe 11. The overflow pipe 10 extends from the upper sub-reaction section to below the liquid surface of the lower sub-reaction section, forming a liquid seal. The riser pipe 11 extends from the lower sub-reaction section to above the liquid surface of the upper sub-reaction section, with the top of the riser pipe 11 higher than the top of the overflow pipe 10. This ensures that the gas phase and hot nitrogen generated in the lower reaction can only rise to the upper sub-reaction section through the riser pipe 11, without gas-liquid cross-contamination or short-circuiting via the overflow pipe 10. The transfer of liquid phase materials between the upper and lower reaction sections is achieved by gravity overflow generated by the liquid level difference. That is, after the liquid phase in the upper sub-reaction section reaches a certain level, it continuously flows into the lower sub-reaction section through the overflow pipe 10; the gas phase in each sub-reaction section rises step by step through the riser pipe 11 and finally enters the rectification section. Each sub-reaction section is equipped with a hot nitrogen inlet 12 extending below the liquid surface. After being introduced through each inlet, the hot nitrogen bubbles and rises within the sub-reaction section, serving two purposes: firstly, to stir the gas, promoting gas-liquid mass transfer and temperature homogenization; and secondly, to carry the water vapor generated as a byproduct upwards, reducing the partial pressure of water in the gas phase and driving the esterification / polymerization reaction towards the forward reaction direction. Temperature control for each sub-reaction section can be achieved using independent heating jackets or built-in coils, allowing for different reaction temperatures to be set according to the degree of conversion of the material at each stage.

[0041] It should be noted that both the esterification reaction unit and the polycondensation reaction unit are of the above-mentioned tower reactor structure. They can use the same or different theoretical plate numbers, reaction stages and operating parameters. Specifically, they can be independently optimized according to the separation accuracy and reaction depth required by the esterification stage and the polycondensation stage respectively.

[0042] Device example 1

[0043] A system for the continuous preparation of polyester polyols, such as Figure 1 As shown, it includes an esterification premix tank 1, an esterification reaction tower 2, a first condenser 3, a polycondensation premix tank 4, a polycondensation reaction tower 5, and a second condenser 6.

[0044] The esterification premixing tank 1 is equipped with a polyacid inlet and a polyol inlet. The polyacid inlet is connected to the polyacid inlet pipeline, and the polyol inlet is connected to the polyol inlet pipeline. A vertical stirrer is installed inside the esterification premixing tank 1. The liquid phase outlet of the esterification premixing tank 1 is connected to the liquid phase inlet of the esterification reaction tower 2 through a pipeline.

[0045] Esterification column 2 is divided into upper and lower sections by a liquid inlet. The section above the liquid inlet is the rectification section, and the section below is the reaction section. The rectification section is packed with structured corrugated metal mesh packing with a theoretical plate number of 10. The reaction section consists of a first reaction section 2-R1 and a second reaction section 2-R2, separated by end caps. An overflow pipe 10 is installed on the bottom end cap of the first reaction section 2-R1, extending downwards to below the liquid surface of the second reaction section 2-R2. A riser pipe 11 is installed in the center of the bottom end cap of the first reaction section 2-R1, extending upwards to above the liquid surface of the first reaction section 2-R1, with the top of the riser pipe 11 3 cm higher than the top of the overflow pipe 10. The first reaction section 2-R1 and the second reaction section 2-R2 of the esterification reaction tower are each equipped with a hot nitrogen inlet 12 on their sides. Each hot nitrogen inlet 12 extends inward to below the liquid surface of its respective reaction section and is connected to a hot nitrogen inlet pipeline. The gas phase outlet at the top of the esterification reaction tower 2 is connected to the gas phase inlet of the first condenser 3 via a pipeline. One liquid phase outlet of the first condenser 3 is connected to the liquid phase reflux outlet at the top of the esterification reaction tower 2 via a pipeline, and the other liquid phase outlet of the first condenser 3 is connected to the product water outlet pipeline. A first overflow outlet is provided on the side of the bottom of the esterification reaction tower 2.

[0046] The liquid phase inlet of the polycondensation premixing tank 4 is connected to the first overflow outlet via a pipeline. The polycondensation premixing tank 4 is equipped with a catalyst inlet, which is connected to a catalyst feed pipeline; a vertical stirrer is installed inside the polycondensation premixing tank 4. The liquid phase outlet of the polycondensation premixing tank 4 is connected to the liquid phase inlet of the polycondensation reaction tower 5 via a pipeline.

[0047] The polycondensation reaction tower 5 is divided into upper and lower sections by a liquid inlet. The section above the liquid inlet is the rectification section, and the section below is the reaction section. The rectification section is filled with structured corrugated metal mesh packing with a theoretical plate number of 10. The reaction section consists of a first reaction section 5-R1 and a second reaction section 5-R2, separated by a head. An overflow pipe 10 is installed on the bottom head of the first reaction section 5-R1, extending downwards to below the liquid surface of the second reaction section 5-R2. A riser pipe 11 is installed in the center of the bottom head of the first reaction section 5-R1, extending upwards to above the liquid surface of the first reaction section 5-R1, with the top of the riser pipe 11 3 cm higher than the top of the overflow pipe 10. The first reaction section 5-R1 and the second reaction section 5-R2 of the polycondensation reaction tower are each equipped with a hot nitrogen inlet 12 on their sides. Each hot nitrogen inlet 12 extends inward to below the liquid surface of its respective reaction section and is connected to a hot nitrogen feed pipeline. The gas phase outlet at the top of the polycondensation reaction tower 5 is connected to the gas phase inlet of the second condenser 6 via a pipeline. One liquid phase outlet of the second condenser 6 is connected to the liquid phase reflux outlet at the top of the polycondensation reaction tower 5 via a pipeline, and the other liquid phase outlet of the second condenser 6 is connected to the product water outlet pipeline; the tail gas outlet of the second condenser 6 is connected to the vacuum system via a pipeline. A second overflow outlet is provided on the side of the bottom of the polycondensation reaction tower 5, and the second overflow outlet is connected to the polyester polyol outlet pipeline.

[0048] Device example 2

[0049] A system for the continuous preparation of polyester polyols, such as Figure 2 As shown, the difference from apparatus example 1 is that the reaction section of esterification tower 2 is provided from top to bottom as three sub-reaction sections connected in series: esterification tower first reaction section 2-R1, esterification tower second reaction section 2-R2, and esterification tower third reaction section 2-R3.

[0050] The first reaction section 2-R1 and the second reaction section 2-R2 of the esterification reaction tower, and the second reaction section 2-R2 and the third reaction section 2-R3 of the esterification reaction tower, are separated by end caps. Each end cap is equipped with an overflow pipe 10 and a riser pipe 11. The overflow pipe 10 extends downward below the liquid surface of the lower reaction section, and the riser pipe 11 extends upward above the liquid surface of the upper reaction section, with the top of the riser pipe 11 being 3 cm higher than the top of the overflow pipe 10. Hot nitrogen inlets 12 are respectively provided on the sides of the first reaction section 2-R1, the second reaction section 2-R2, and the third reaction section 2-R3 of the esterification reaction tower. Each hot nitrogen inlet 12 extends inward below the liquid surface of its respective reaction section and is connected to a hot nitrogen feed pipeline.

[0051] Comparative Example 1

[0052] A system for the batch preparation of polyester polyols, such as Figure 3 As shown, it includes a batch reactor 7, a distillation column section 8, and a batch reaction condenser 9.

[0053] The batch reactor 7 is equipped with a polybasic acid inlet, a polyol inlet, and a catalyst inlet, which are connected to the polybasic acid inlet pipeline, the polyol inlet pipeline, and the catalyst inlet pipeline, respectively. A vertical stirrer is installed inside the batch reactor 7. The vapor outlet at the top of the batch reactor 7 is connected to the vapor inlet at the bottom of the distillation column section 8, and the vapor outlet at the top of the distillation column section 8 is connected to the vapor inlet of the batch reaction condenser 9. One liquid outlet of the batch reaction condenser 9 is connected to the liquid reflux port at the top of the distillation column section 8 to partially reflux the condensate back to the top of the distillation column section 8; the other liquid outlet of the batch reaction condenser 9 is connected to the product water outlet pipeline to discharge the reaction product water from the system; the tail gas outlet of the batch reaction condenser 9 is connected to the vacuum system for evacuating the system during the later stages of the reaction. The liquid outlet at the bottom of distillation column section 8 is connected to the liquid reflux port at the top of batch reactor 7, used to return the condensate collected in the distillation section to the reactor. The liquid outlet at the bottom of batch reactor 7 is connected to the discharge pipeline, used to collect the polyester polyol product after the reaction is completed.

[0054] Comparative Example 2

[0055] This apparatus provides a system for the direct and continuous preparation of polyester polyols, which differs from apparatus example 1 in that: there is no polycondensation premix tank 4, and the first overflow outlet on the side of the bottom of the esterification reaction tower 2 is directly connected to the liquid phase feed port of the polycondensation reaction tower 5 through a pipeline. The pipeline is equipped with a catalyst injection branch pipe, which is used to introduce the catalyst into the esterification product and then directly enter the polycondensation reaction tower 5.

[0056] Example 1

[0057] This embodiment provides a method for the continuous preparation of polyester polyols, using the continuous polyester polyol preparation system of apparatus example 1 (e.g., ...). Figure 1 (As shown) to carry out continuous production, including the following steps:

[0058] S1: Heat the esterification premix tank 1 to 100℃ and maintain it. Add polyol (ethylene glycol) continuously to the esterification premix tank 1 at a rate of 102 kg / h and polyacid (adipic acid) continuously to the esterification premix tank 1 at a rate of 200 kg / h. The molar ratio of ethylene glycol to adipic acid is 1.2:1. Turn on the stirrer of the esterification premix tank 1 to mix. When the liquid level of the esterification premix tank 1 reaches 50%, continuously transport the mixture to the esterification reaction tower 2 through the liquid phase outlet of the esterification premix tank 1 at a rate of 302 kg / h.

[0059] S2: The top pressure of esterification reaction tower 2 is controlled at atmospheric pressure, and the top temperature is controlled at 100~101℃ by the reflux ratio, which is set to 1. The first reaction section 2-R1 of the esterification reaction tower is heated to 180℃ and maintained by external heating, and the second reaction section 2-R2 of the esterification reaction tower is heated to 200℃ and maintained by external heating. The reaction temperature increases stepwise along the material flow direction. Hot nitrogen gas at 100℃ is introduced into the first reaction section 2-R1 and the second reaction section 2-R2 of the esterification reaction tower through the hot nitrogen gas inlet 12, with a hot nitrogen gas feed rate of 0.2 Nm³ for each reaction section. 3 / h. The liquid material in the first reaction section 2-R1 of the esterification reaction tower enters the second reaction section 2-R2 of the esterification reaction tower through the overflow pipe 10. The height of the overflow pipe 10 ensures that the residence time of the material in both the first reaction section 2-R1 and the second reaction section 2-R2 of the esterification reaction tower is 60 minutes. The liquid material in the second reaction section 2-R2 of the esterification reaction tower is continuously collected through the first overflow outlet on the side of the bottom of the esterification reaction tower 2, which is the esterification product. The water generated by the esterification reaction is distilled off from the top of the esterification reaction tower 2, condensed by the first condenser 3, and partially refluxed and partially discharged as generated water.

[0060] S3: The esterification product collected in S2 is continuously fed into the polycondensation premix tank 4 through the first overflow outlet. The polycondensation premix tank 4 is heated to 200℃ and maintained, and the stirrer of the polycondensation premix tank 4 is turned on. When the liquid level in the polycondensation premix tank 4 reaches 50%, tetrabutyl titanate is continuously added to the polycondensation premix tank 4 as a catalyst at a rate of 0.15 kg / h (the amount of catalyst added is 0.05 wt.% of the total mass of ethylene glycol and adipic acid), and mixed with the esterification product in the polycondensation premix tank 4; the mixed material is continuously conveyed to the polycondensation reaction tower 5 through the liquid phase outlet of the polycondensation premix tank 4.

[0061] S4: Activate the vacuum system to control the vacuum level at the top of the polycondensation reaction tower 5 to -0.095 MPa (absolute pressure approximately 0.005 MPa). The top temperature is controlled at 36.2~36.4℃ via a reflux ratio of 1. The first reaction section 5-R1 of the polycondensation reaction tower is heated to 200℃ and maintained by external heating, while the second reaction section 5-R2 is heated to 220℃ and maintained by external heating. The reaction temperature increases progressively along the material flow direction. Hot nitrogen gas at 100℃ is introduced into both the first reaction section 5-R1 and the second reaction section 5-R2 of the polycondensation reaction tower through the hot nitrogen inlet 12, with a feed rate of 0.2 Nm³ for each reaction section. 3 / h. The liquid material in the first reaction section 5-R1 of the polycondensation reactor enters the second reaction section 5-R2 of the polycondensation reactor through the overflow pipe 10. The height of the overflow pipe 10 ensures that the residence time of the material in both the first reaction section 5-R1 and the second reaction section 5-R2 of the polycondensation reactor is 60 minutes. The liquid material in the second reaction section 5-R2 of the polycondensation reactor is continuously collected through the second overflow outlet on the side of the bottom of the polycondensation reactor 5, which is the polyester polyol product. The water generated by the polycondensation reaction is distilled off from the top of the polycondensation reactor 5, condensed by the second condenser 6, and partially refluxed and partially discharged as generated water.

[0062] During the reaction, samples of the esterification product collected from the first overflow outlet and the polyester polyol product collected from the second overflow outlet were taken and analyzed every 30 to 60 minutes to monitor the reaction progress and product quality.

[0063] Example 2

[0064] This embodiment provides a method for the continuous preparation of polyester polyols, using the continuous polyester polyol preparation system of apparatus example 2 (such as...). Figure 2 (As shown) to carry out continuous production, including the following steps:

[0065] S1: Heat the esterification premix tank 1 to 100℃ and maintain it. Add polyol (ethylene glycol) continuously to the esterification premix tank 1 at a rate of 102 kg / h and polyacid (adipic acid) continuously to the esterification premix tank 1 at a rate of 200 kg / h. The molar ratio of ethylene glycol to adipic acid is 1.2:1. Turn on the stirrer of the esterification premix tank 1 to mix. When the liquid level of the esterification premix tank 1 reaches 50%, continuously transport the mixture to the esterification reaction tower 2 through the liquid phase outlet of the esterification premix tank 1 at a rate of 302 kg / h.

[0066] S2: The top pressure of esterification reaction tower 2 is controlled at atmospheric pressure, and the top temperature is controlled at 100~101℃ by the reflux ratio, which is set to 1. The first reaction section 2-R1 of the esterification reaction tower is heated to 180℃ and maintained by external heating; the second reaction section 2-R2 is heated to 200℃ and maintained by external heating; and the third reaction section 2-R3 is heated to 220℃ and maintained by external heating. The reaction temperature increases progressively along the material flow direction. Hot nitrogen gas at 100℃ is introduced into the first reaction section 2-R1, the second reaction section 2-R2, and the third reaction section 2-R3 of the esterification reaction tower through the hot nitrogen inlet 12, with a hot nitrogen feed rate of 0.2 Nm³ for each reaction section. 3 / h. The liquid material in the first reaction section 2-R1 of the esterification reaction tower enters the second reaction section 2-R2 of the esterification reaction tower through the overflow pipe 10. The liquid material in the second reaction section 2-R2 of the esterification reaction tower enters the third reaction section 2-R3 of the esterification reaction tower through the overflow pipe 10. The height of each overflow pipe 10 ensures that the residence time of the material in the first reaction section 2-R1, the second reaction section 2-R2, and the third reaction section 2-R3 of the esterification reaction tower is 60 minutes. The liquid material in the third reaction section 2-R3 of the esterification reaction tower is continuously collected through the first overflow outlet on the side of the bottom of the esterification reaction tower 2, which is the esterification product. The water generated by the esterification reaction is distilled from the top of the esterification reaction tower 2, condensed by the first condenser 3, and part of it is refluxed and part of it is discharged as generated water.

[0067] S3: The esterification product collected in S2 is continuously fed into the polycondensation premix tank 4 through the first overflow outlet. The polycondensation premix tank 4 is heated to 220°C and maintained, and the stirrer of the polycondensation premix tank 4 is turned on. When the liquid level in the polycondensation premix tank 4 reaches 50%, tetrabutyl titanate is continuously added to the polycondensation premix tank 4 as a catalyst at a rate of 0.15 kg / h (the amount of catalyst added is 0.05 wt.% of the total mass of ethylene glycol and adipic acid), and mixed with the esterification product in the polycondensation premix tank 4; the mixed material is continuously conveyed to the polycondensation reaction tower 5 through the liquid phase outlet of the polycondensation premix tank 4.

[0068] S4: Activate the vacuum system to control the vacuum level at the top of the polycondensation reaction tower 5 to -0.095 MPa (absolute pressure approximately 0.005 MPa). The top temperature is controlled at 36.2~36.4℃ via the reflux ratio, which is set to 1. The first reaction section 5-R1 of the polycondensation reaction tower is heated to 220℃ and maintained by external heating, while the second reaction section 5-R2 is heated to 240℃ and maintained by external heating. The reaction temperature increases progressively along the material flow direction. Hot nitrogen gas at 100℃ is introduced into both the first reaction section 5-R1 and the second reaction section 5-R2 of the polycondensation reaction tower through the hot nitrogen inlet 12, with a hot nitrogen feed rate of 0.2 Nm³ for each reaction section. 3 / h. The liquid material in the first reaction section 5-R1 of the polycondensation reactor enters the second reaction section 5-R2 of the polycondensation reactor through the overflow pipe 10. The height of each overflow pipe 10 ensures that the residence time of the material in both the first reaction section 5-R1 and the second reaction section 5-R2 of the polycondensation reactor is 60 minutes. The liquid material in the second reaction section 5-R2 of the polycondensation reactor is continuously collected through the second overflow outlet on the side of the bottom of the polycondensation reactor 5, which is the polyester polyol product. The water generated by the polycondensation reaction is distilled off from the top of the polycondensation reactor 5, condensed by the second condenser 6, and partially refluxed and partially discharged as generated water.

[0069] During the reaction, samples of the esterification product collected from the first overflow outlet and the polyester polyol product collected from the second overflow outlet were taken and analyzed every 30 to 60 minutes to monitor the reaction progress and product quality.

[0070] Example 3

[0071] This embodiment provides a method for the continuous preparation of polyester polyols, which differs from Example 2 in that the temperature conditions are different, and specifically includes the following steps:

[0072] S1: The operating conditions for the esterification premixing stage are the same as in Example 2.

[0073] S2: During the esterification reaction stage, the first reaction section 2-R1 of the esterification reaction tower is heated to 180°C and maintained by external heating; the second reaction section 2-R2 of the esterification reaction tower is heated to 190°C and maintained by external heating; and the third reaction section 2-R3 of the esterification reaction tower is heated to 200°C and maintained by external heating. The reaction temperature increases gradually along the material flow direction. Other operating conditions are the same as in Example 2.

[0074] S3: During the polycondensation premixing stage, the polycondensation premixing tank 4 is heated to 200°C and maintained. All other operating conditions are the same as in Example 2.

[0075] S4: During the polycondensation reaction stage, the first reaction section 5-R1 of the polycondensation reaction tower is heated to 200°C and maintained by external heating, while the second reaction section 5-R2 of the polycondensation reaction tower is heated to 220°C and maintained by external heating. The reaction temperature increases gradually along the material flow direction. Other operating conditions are the same as in Example 2.

[0076] Comparative Example 1

[0077] This comparative example provides a method for batch preparation of polyester polyols, using the batch preparation system of Comparative Example 1 for polyester polyol production, including the following steps:

[0078] S1: In the esterification reaction stage, ethylene glycol and adipic acid are added to batch reactor 7 at a molar ratio of 1.2:1. Feeding is stopped when the liquid level reaches 70%, the stirrer is turned on, and the temperature is raised to initiate the esterification reaction. The heating program is as follows: first, the temperature is raised to 160℃ at a rate of 20℃ / h and held for 60 minutes; then, the temperature is raised to 220℃ at a rate of 20℃ / h and held for 60 minutes. During the reaction, the pressure at the top of distillation column section 8 is at atmospheric pressure, and the top temperature is controlled at 100~101℃ by adjusting the reflux ratio. When the acid value of the reactants drops to 45 mg KOH / g, tetrabutyl titanate is added through the catalyst inlet at a rate of 0.05 wt.% of the total mass of ethylene glycol and oxalic acid. The reaction continues until the acid value drops to 30 mg KOH / g, at which point the esterification reaction ends.

[0079] S2: Polycondensation reaction stage. Maintain the temperature of batch reactor 7 constant, turn on the vacuum system, and control the vacuum degree at the top of distillation column section 8 at -0.095MPa (absolute pressure is about 0.005MPa) to carry out decompression polycondensation reaction. After continuous polycondensation for 120 minutes, the production of one batch of batch polyester polyol is completed.

[0080] Comparative Example 2

[0081] This comparative example provides a method for the direct and continuous preparation of polyester polyols, using the same direct and continuous polyester polyol production system as Comparative Example 2. The difference from Example 1 lies in the operating conditions of step S3, which specifically includes the following steps:

[0082] S1: The operating conditions for the esterification premixing stage are the same as in Example 1.

[0083] S2: The operating conditions for the esterification reaction stage are the same as in Example 1.

[0084] S3: The esterification product collected in S2 is continuously fed into polycondensation reaction tower 5 through the first overflow outlet. Tetrabutyl titanate is fed into polycondensation reaction tower 5 continuously at a rate of 0.15 kg / h through a branch on the phase-connected pipeline (the amount of catalyst added is 0.05 wt.% of the total mass of ethylene glycol and adipic acid).

[0085] S4: The operating conditions for the polycondensation reaction stage are the same as in Example 1.

[0086] Comparative Example 3

[0087] This comparative example provides a method for the continuous preparation of polyester polyols, which differs from Example 2 in that the temperature conditions are different, and specifically includes the following steps:

[0088] S1: The operating conditions for the esterification premixing stage are the same as in Example 2.

[0089] S2: During the esterification reaction stage, the first reaction section 2-R1, the second reaction section 2-R2, and the third reaction section 2-R3 of the esterification reaction tower are heated to 180°C by external heating and maintained. Other operating conditions are the same as in Example 2.

[0090] S3: The operating conditions for the polycondensation premixing stage are the same as in Example 2.

[0091] S4: During the polycondensation reaction stage, the first reaction section 5-R1 and the second reaction section 5-R2 of the polycondensation reaction tower are heated to 200°C by external heating and maintained, wherein the operating conditions are the same as in Example 2.

[0092] Polyester polyol products were prepared according to the methods of Examples 1-3 and Comparative Examples 1-3, respectively. The esterification products and polyester polyol product indicators of each example and comparative example are shown in Table 1.

[0093] Table 1. Specifications of Esterification Products and Polyester Polyols from Examples and Comparative Examples

[0094] Note: PDI: Polydispersity Index, used to describe the molecular weight distribution of a polymer.

[0095] As shown in Table 1, the polyester polyol products prepared in each embodiment of the present invention exhibit excellent performance in key indicators. The PDI of the products in each embodiment can be controlled below 1.25. In particular, Examples 2 and 3 have met the stringent requirements of high-end applications for PDI ≤ 1.2. At the same time, the color of the products is below 10 APHA and the polyether content is below 70 ppm. This indicates that the present invention, through a four-stage continuous system architecture of "esterification premixing - esterification reaction - polycondensation premixing - polycondensation reaction", combined with the precise control of gas-liquid countercurrent stepwise flow and gradient temperature in multiple reaction stages, has achieved deep removal of water generated in the reaction and homogeneous mixing of the catalyst. This effectively promotes the equilibrium of esterification and polycondensation reactions to shift in the positive direction, thereby obtaining high-quality polyester polyol products with narrow molecular weight distribution, low color, and high purity.

[0096] In contrast, Comparative Example 1 uses a batch synthesis process, completing esterification and polycondensation reactions in the same reactor in separate time steps. Its product has a PDI of 1.31, which cannot meet the requirements of high-end applications. Furthermore, its color (10 APHA) and polyether content (88 ppm) are significantly inferior to those of the embodiments of this invention. This indicates that the inherent defects of the batch process, such as the simultaneous operation of esterification and polycondensation in the same reactor in separate time steps, the lack of an independent mixing space for the catalyst, and the insufficient removal of by-products, seriously restrict the improvement of product quality. Comparative Example 2 uses a direct continuous process where the esterification product is directly fed into the polycondensation reaction tower. Its PDI is 1.27, and the hydroxyl value (53 mg KOH / g) and acid value (0.28 mg KOH / g) of the product are significantly worse than those of Example 1 under the same operating conditions (PDI is 1.25, hydroxyl value is 48 mg KOH / g, and acid value is 0.25 mg KOH / g). This indicates that the absence of the polycondensation premixing unit leads to the lack of independent mixing space for the catalyst, resulting in uneven dispersion. Fluctuations in feed temperature and composition directly affect the steady-state operation of the polycondensation section, thereby deteriorating the product quality. Comparative Example 3 used the same temperature for esterification and polycondensation reactions in the reaction section (each section of the esterification section was 180°C, and each section of the polycondensation section was 200°C). Its PDI was 1.24, which was higher than that of Example 2 (1.15) under the same equipment conditions. This shows that setting a temperature gradient in the reaction section and gradually increasing the temperature along the material flow direction helps to compensate for the decline of driving force in the later stage of the reaction and promotes the stripping and removal of water by-products. This is a key operating condition for improving product quality.

[0097] It should be noted that for polyester polyols, the acid value is caused by the carboxyl groups at the polyester end. As an inhibitor of the polyurethane reaction, a fluctuation of 1 mg KOH / g in the acid value can significantly affect the product's appearance, mechanical properties, and curing speed. The hydroxyl value directly affects the crosslinking density and microstructure of polyurethane products. When the hydroxyl value fluctuates by more than 3 mg KOH / g, the product's hardness, deformation characteristics, curing behavior, brittleness, and color, among other macroscopic properties, will all change significantly. When the product is in a low PDI system (PDI in the range of 1.15~1.25), a fluctuation of 0.1 in PDI can significantly affect the product's viscosity aging performance. Therefore, although the numerical differences in some indicators between the various embodiments and comparative examples of this invention may seem limited, the differences in the effects on product application are quite significant for evaluating the quality of polyester polyols.

[0098] It should be noted that the above description is a further detailed explanation of the present invention in conjunction with specific embodiments, and should not be construed as limiting the specific implementation of the present invention to these descriptions. The specific parameters in this embodiment do not necessarily limit the technical solution, but merely illustrate one specific working condition. For those skilled in the art, various simple improvements and modifications can be made without departing from the concept of the present invention, and all such improvements and modifications should be considered to fall within the scope of protection of the present invention.

Claims

1. A system for the continuous preparation of polyester polyols, characterized in that, It includes an esterification premix unit, an esterification reaction unit, a polycondensation premix unit, and a polycondensation reaction unit connected in sequence. The esterification premixing unit is equipped with a polyacid inlet and a polyol inlet; The liquid phase inlet of the esterification reaction unit is connected to the outlet of the esterification premix unit. A first condenser is provided at the top of the unit. The liquid phase outlet of the first condenser is branched to the reflux inlet and the first discharge pipeline of the esterification reaction unit. A first overflow outlet is provided at the bottom of the esterification reaction unit. The liquid phase inlet of the polycondensation premixing unit is connected to the first overflow outlet, and a catalyst inlet is provided. The liquid phase inlet of the polycondensation reaction unit is connected to the outlet of the polycondensation premixing unit. A second condenser is provided at the top of the unit. The liquid phase outlet of the second condenser is branched to the reflux inlet and the second discharge pipeline of the polycondensation reaction unit. The gas phase outlet of the second condenser is connected to a vacuum generator. A second overflow outlet is provided at the bottom of the polycondensation reaction unit.

2. The system for continuous preparation of polyester polyols according to claim 1, characterized in that, The esterification reaction unit and / or the polycondensation reaction unit are tower reactors. The tower reactor has a rectification section above the liquid inlet and a reaction section below it. The reaction section is divided into at least two sub-reaction sections connected in series vertically by a head. Adjacent sub-reaction sections are separated by a head. The head is provided with an overflow pipe and a riser pipe. The overflow pipe extends from the upper reaction section to below the liquid surface of the lower reaction section. The riser pipe extends from the lower reaction section to above the liquid surface of the upper reaction section, and the top of the riser pipe is higher than the top of the overflow pipe.

3. The system for continuous preparation of polyester polyols according to claim 2, characterized in that, Each of the reaction sections is equipped with a hot nitrogen inlet that extends below the liquid surface.

4. The system for continuous preparation of polyester polyols according to claim 2, characterized in that, The rectification section is filled with packing material and / or trays, and the theoretical number of trays in the rectification section is 5 to 20.

5. A method for continuously preparing polyester polyols using the system described in any one of claims 1-4, characterized in that, Includes the following steps: S1: Polyols and polyacids are mixed in the esterification premixing unit, and the mixed material is continuously conveyed to the esterification reaction unit; S2: In the esterification reaction unit, the esterification reaction is carried out under a hot nitrogen atmosphere. The reaction temperature increases step by step along the material flow direction. Water is generated and evaporated from the top. The esterification product is collected from the first overflow port at the bottom. S3: The esterification product and the catalyst are mixed in the polycondensation premixing unit, and the mixed material is continuously conveyed to the polycondensation reaction unit; S4: In the polycondensation reaction unit, the polycondensation reaction is carried out under a hot nitrogen atmosphere and negative pressure. The reaction temperature increases step by step along the material flow direction. Water is generated and evaporated from the top. The polyester polyol product is collected from the second overflow port at the bottom.

6. The method for continuous preparation of polyester polyols according to claim 5, characterized in that, The polybasic acid mentioned in step S1 is selected from one or more of diabasic acids, diabasic anhydrides, and hydrogenated dimer acids; And / or, the polyol is selected from one or more of diols, triols, tetraols or pentaols; And / or, the molar ratio of the polyol to the polyacid is (1.1~2):1; And / or, the mixing temperature in the esterification premixing unit is 90~130℃; And / or, the pressure of the esterification premix unit is atmospheric pressure.

7. The method for continuous preparation of polyester polyols according to claim 5, characterized in that, The top of the esterification reaction unit in step S2 is at atmospheric pressure and the top temperature is 100~105℃; And / or, the temperature of the reaction section of the esterification reaction unit is 180~240℃, and the temperature difference between two adjacent reaction sections is 10~20℃; And / or, the residence time of the material in each reaction stage of the esterification reaction unit is 30~90min.

8. The method for continuous preparation of polyester polyols according to claim 5, characterized in that, The amount of catalyst added in step S3 is 0.01~0.1 wt.% of the total mass of the polyol and polyacid. And / or, the mixing temperature in the polycondensation premixing unit is 200~240℃; And / or, the pressure of the polycondensation premix unit is atmospheric pressure.

9. The method for continuous preparation of polyester polyols according to claim 5, characterized in that, The absolute pressure at the top of the polycondensation reaction unit in step S4 is 0.001~0.1MPa, and the top temperature is 35~100℃; And / or, the temperature of the reaction section of the polycondensation reaction unit is 200~260℃, and the temperature difference between two adjacent reaction sections is 10~20℃; And / or, the residence time of the material in each stage of the polycondensation reaction unit is 30~90 min.

10. The method for continuous preparation of polyester polyols according to claim 5, characterized in that, The temperature of the hot nitrogen gas in step S2 and / or step S4 is 90~130℃.

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