A distillation system and method for purification of crude acrylic acid

Abstract

The application discloses a rectification system and method for purifying crude acrylic acid, and relates to the technical field of crude acrylic acid separation.The rectification system comprises a supergravity pretreatment unit, a rectification unit and a cracker.The supergravity pretreatment unit comprises a high-speed rotating packed bed, a condenser, a reflux tank and a micro-metering pump.A catalyst loading area is arranged in the rectification tower.A heat pump system for recovering heat is arranged between the overhead condenser and the bottom reboiler of the tower.The overhead condenser is connected with the mechanical heat pump evaporator of the heat pump device through a pipeline.The condensing device of the heat pump system is connected with the reboiler through a pipeline.The rectification system of the application can preliminarily concentrate crude acrylic acid through the supergravity pretreatment unit, and can upgrade the low-temperature waste heat at the top of the reaction rectification tower into a usable high-temperature heat source.The upgraded heat is used to supply the reboiler of the tower, thereby forming an internal heat cycle, reducing the demand for external steam, improving energy efficiency, and reducing operation cost and carbon emission.

Description

Technical Field

[0001] This invention relates to the field of crude acrylic acid separation technology, and more specifically to a distillation system and method for purifying crude acrylic acid. Background Technology

[0002] Acrylic acid is an important organic synthetic monomer. It is an organic compound containing carboxyl groups and carbon-carbon double bonds. Because it has both double bonds and carboxyl groups, acrylic acid can undergo polymerization reactions through free radical polymerization, ionic polymerization and other methods to produce polyacrylic acid and its derivatives. Therefore, it is widely used in the production of polymer materials, such as acrylic resin, superabsorbent resin, coatings, adhesives, textile auxiliaries and so on. Industrially, crude acrylic acid is mainly produced by the gas-phase oxidation of propylene. Its typical composition is: 20-40 wt% acrylic acid, 50-70 wt% water, and impurities such as acetic acid, propionaldehyde, propionic acid, and allyl alcohol. Traditional refining processes usually adopt a multi-tower series process, including a light-removal tower, an extractive distillation tower, a dehydration tower and a product tower. It usually has the following problems: (1) high energy consumption, especially the huge steam consumption required for the evaporation of a large amount of water; (2) large equipment, large footprint, and high investment cost; (3) limited mass transfer efficiency, requiring a high reflux ratio to maintain product purity; (4) system adjustment lag, making it difficult to adapt to raw material fluctuations.

[0003] In recent years, some studies have attempted to introduce heat pump distillation to recover low-temperature heat from the top of the column (CN104876789A), but the energy-saving potential has not been fully realized due to the low efficiency of conventional trays. Other literature reports the use of rotating packed beds (RPB) for solvent dehydration (such as ethanol-water systems), but there are no mature cases of its application in highly corrosive and easily polymerizable systems such as acrylic acid.

[0004] Therefore, there is an urgent need to develop a new acrylic acid refining system that is efficient, compact, energy-saving, and has strong anti-interference capabilities. Summary of the Invention

[0005] To address the problems of existing technologies, this invention provides an integrated refining system and its operation method that combines a supergravity pretreatment unit, a reactive distillation column, and a mechanical heat pump energy recovery mechanism. When used in the high-purity purification process of crude acrylic acid obtained from propylene through a two-stage oxidation method, it has the advantages of low energy consumption, fast response, small footprint, and stable operation.

[0006] The objective of this invention is achieved through the following technical solution: A distillation system for purifying crude acrylic acid, the distillation system comprising a high gravity pretreatment unit, a distillation unit, and a pyrolyzer; The hypergravity pretreatment unit includes a high-speed rotating packed bed, a condenser, a reflux tank, and a micro-metering pump; the gas outlet at the top of the high-speed rotating packed bed is connected to the condenser, the condenser is connected to the reflux tank, the liquid outlet of the reflux tank is connected to the crude acrylic liquid inlet of the high-speed rotating packed bed, and the crude acrylic liquid inlet is also connected to the micro-metering pump. The liquid outlet at the bottom of the high-speed rotating packed bed is connected to the feed inlet of the refining column. The refining column integrates a top condenser and a bottom reboiler. The refining column is equipped with a catalyst loading zone, which is filled with a solid acid catalyst. The upper middle part of the refining column is equipped with a side-stream product outlet. The bottom material outlet of the refining column is connected to a pyrolyzer. A heat pump system for heat recovery is provided between the top condenser and the reboiler. The top condenser is connected to the mechanical heat pump evaporator of the heat pump system through a pipe, and the heat pump condenser of the heat pump system is connected to the reboiler through a pipe.

[0007] As an example, the high-speed rotating packed bed is filled with wire mesh packing material, which is made of titanium alloy or duplex stainless steel.

[0008] As an example, the refining tower is a vertical atmospheric pressure operating tower with 35 trays. The upper part uses sieve trays, and the middle and lower parts use floating valve trays. A catalyst loading zone is provided between the 10th and 20th trays in the middle of the refining tower. The feed inlet is located in the middle and lower part of the refining tower, and the side-stream product outlet is located between the 6th and 8th trays of the refining tower. The refining tower is equipped with multiple sets of temperature sensors.

[0009] As an example, the heat pump system includes a compressor, a heat pump condenser, a mechanical heat pump evaporator, a filter, and an expansion valve; The mechanical heat pump evaporator, compressor, condenser, filter, and electronic expansion valve form a heat exchange circuit for circulating refrigerant.

[0010] As an example, the shell-side inlet of the mechanical heat pump evaporator is connected to the shell-side outlet of the top condenser, and the shell-side outlet of the mechanical heat pump evaporator is connected to the shell-side inlet of the top condenser. The shell-side outlet of the heat pump condenser is connected to the shell-side inlet of the reboiler, and the shell-side inlet of the heat pump condenser is connected to the shell-side inlet of the reboiler.

[0011] A distillation method for purifying crude acrylic acid involves a crude acrylic acid mixture being fed into a high-speed rotating packed bed for preliminary concentration to remove low-boiling components. The gaseous components are discharged from the high-speed rotating packed bed and then partially refluxed back to the high-speed rotating packed bed via a condenser. The light components are collected from the top of the reflux tank. The rich liquid at the bottom of the high-speed rotating packed bed is transported to the purification column, where it is further purified through the catalyst packing zone. The high-purity acrylic acid solution is collected through the side stream in the upper part of the purification column, and the heavy components discharged from the bottom of the column are sent to the pyrolyzer. The condenser at the top of the refining column recovers the heat from the top of the column through a heat pump system, and uses the recovered heat to heat the reboiler, thus completing the heat recovery and utilization.

[0012] As an example, the residence time of the crude acrylic acid mixture in a high-speed rotating packed bed is less than 5 seconds.

[0013] As an example, polymerization inhibitors are also added to high-speed rotating packed beds; The circulating refrigerant in the heat pump system is R-1234ze.

[0014] As an example, the temperature at the top of the refining column is 102°C, the temperature in the catalyst packing zone is 108°C, the temperature at the bottom of the column is 116°C, the reflux ratio is initially set to 1.8, and the side stream is taken out at the 7th tray.

[0015] The beneficial effects of this invention are as follows: 1. The present invention provides a distillation system and method for purifying crude acrylic acid, which pre-concentrates crude acrylic acid through a high-gravity pretreatment unit, removes low-boiling substances (such as acrolein and unreacted hydrocarbons), reduces the content of water and impurities, suppresses side reactions involving water, increases the concentration of reactants, reduces the adverse effects of water on the catalyst in the subsequent purification tower, and maintains the stability of the catalyst.

[0016] 2. The present invention provides a distillation system and method for purifying crude acrylic acid. In the production of acrylic acid, the raw gas undergoes two-stage oxidation (propylene → acrolein → acrylic acid), but a small amount of incompletely oxidized acrolein remains. Acrolein has high reactivity and is prone to free radical polymerization or Aldol condensation, which leads to product coloring and coking. It can also copolymerize with acrylic acid to form dark-colored polymeric impurities. Therefore, the present invention converts acrolein into a more stable compound through a solid acid catalyst in the catalyst loading zone during the purification stage, which is then easily separated by distillation, without interfering with the quality of the main product, acrylic acid.

[0017] 3. The present invention provides a distillation system and method for the purification of crude acrylic acid. The heat at the top of the column is recovered and used by a heat pump system and then supplied to the reboiler at the bottom of the purification column. The heat pump upgrades the low-temperature waste heat at the top of the reaction purification column into a usable high-temperature heat source at the cost of electricity. The upgraded heat is used to supply the reboiler of this column, forming an internal heat cycle, reducing the demand for external steam, improving energy efficiency, and reducing operating costs and carbon emissions.

[0018] 4. The present invention provides a distillation system and method for purifying crude acrylic acid. The circulating medium R-1234ze of the heat pump system is an environmentally friendly refrigerant with good thermodynamic properties. By using heat transfer oil as a heat transfer medium, the refrigerant is isolated from direct contact with the acrylic acid material, thus achieving safe application. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of a distillation system for purifying crude acrylic acid according to the present invention; Figure 2 This is a schematic diagram of the high-speed rotating packed bed of the present invention; Figure 3 This is a schematic diagram illustrating the principle of heat exchange between the tower top condenser, heat pump system, and reboiler of the present invention. Figure 4 This is a schematic diagram of the heat pump system of the present invention.

[0020] Figure label: 1. Air inlet; 2. High-speed rotating packed bed; 3. Motor; 4. Gas outlet; 5. Shaft; 6. Liquid distributor; 7. Wire mesh packing; 8. Turntable; 9. Support frame; 10. Liquid outlet; 11. Platform; 12. Controller; 13. Liquid inlet; 14. Micro metering pump; 15. Reflux tank; 16. Condenser; 17. Refining tower; 18. Tower top condenser; 19. High-temperature flow meter; 20. Mechanical heat pump evaporator; 21. Compressor; 22. Heat pump condenser; 23. Filter; 24. Expansion valve; 25. Heat pump system controller; 26. Reboiler; 27. Single-phase throttling valve; 28. Cracker; 29. ​​Heat replenishment pipeline; 30. Side-stream product extraction pipeline. Detailed Implementation

[0021] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0022] The terms "including" or "comprising" as used in this invention mean that the element preceding the term encompasses the element listed after the term, and do not exclude the possibility of encompassing other elements as well. Terms such as "upper," "lower," "middle," and "top" are only used as examples in the accompanying drawings to indicate relative positional relationships. When the absolute position of the described object changes, the relative positional relationship may also change accordingly.

[0023] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be obvious to those skilled in the art. This application specification and embodiments are merely exemplary.

[0024] Example 1 This embodiment provides a distillation system for the purification of crude acrylic acid. A schematic diagram of the specific distillation system is shown below. Figure 1 As shown. The distillation system includes a supergravity pretreatment unit, a distillation unit, and a pyrolyzer 28.

[0025] The hypergravity pretreatment unit includes a high-speed rotating packed bed 2, a condenser 16, a reflux tank 15, and a micro-metering pump 14. The gas outlet 4 at the top of the high-speed rotating packed bed 2 is connected to the condenser 16. The condenser 16 is connected to the reflux tank 15. Light components are discharged from the top of the reflux tank 15. The liquid outlet of the reflux tank 15 is connected to the crude acrylic acid liquid inlet 13 of the high-speed rotating packed bed 2. The liquid inlet 13 is also connected to the micro-metering pump 14.

[0026] like Figure 2 As shown, the high-speed rotating packed bed 2 includes a shell, a motor 3, a turntable 8, and wire mesh packing 7. The motor 3 is mounted on the shell, and its output end is connected to a rotating shaft 5. The rotating shaft 5 extends into the shell, and the turntable 8 is fixedly mounted on the rotating shaft 5. The wire mesh packing 7 is mounted on the turntable 8, and a liquid distributor 6 is located above the wire mesh packing 7, communicating with a liquid inlet 13. An air inlet 1 is also provided at the top of the high-speed rotating packed bed 2, and a support frame 9 for supporting the shell is provided at the bottom of the high-speed rotating packed bed 2. Several support frames 9 may be provided. Figure 1 The support frames 9 are four in number, symmetrically installed, and the high-speed rotating filling bed 2 is mounted on the platform 11 via the support frames 9. The high-speed rotating filling bed 2 is also electrically connected to the controller 12. The high-speed rotating filling bed 2 is also equipped with temperature and pressure sensors. The controller 12 controls the working mode and temperature and pressure of the high-speed rotating filling bed 2.

[0027] The wire mesh filler 7 is made of titanium alloy or duplex stainless steel (SUS2205), with a specific surface area ≥500 m² / m³ and a porosity >95%. The rotational speed of the rotating shaft 5 is 1200–1800 rpm, and the equivalent centrifugal acceleration is 100–300g.

[0028] The liquid outlet 10 of the high-speed rotating packed bed 2 is connected to the feed inlet of the refining tower 17 via a pipeline, and the top of the refining tower 17 is connected to the top condenser 18. A valve and a high-temperature flow meter 19 are installed on the pipeline connecting the refining tower 17 and the top condenser 18, and a reboiler 26 is installed at the bottom of the refining tower 17.

[0029] The low-pressure steam at the top of the refining column 17 enters the tube side of the top condenser 18, and the condensed acrylic acid liquid flows back to the top of the refining column 17, with a portion being collected. The components at the bottom of the refining column 17 enter the tube side of the reboiler 26 and then flow back to the refining column 17. A single-phase throttling valve 27 is installed on the pipeline from the reboiler 26 back to the refining column 17.

[0030] The refining tower 17 is a vertical atmospheric pressure operating tower with 35 trays. The upper part of the tower uses sieve trays, the middle and lower parts use valve trays, and a catalyst loading zone (between trays 10 and 20) is located in the middle. The catalyst loading zone is filled with a solid acid catalyst selected from heteropoly acid 20% H3PW. 12 O 40 The catalyst, SiO2 (or sulfonic acid functionalized ion exchange resin), is prepared using the sol-gel method, a preparation method that has been extensively reported, such as in Nat Sci Edit, Dec. 2010, Vol. 7 No. 4. The feed inlet of the refining column 17 is located in the lower middle section. The reboiler 26 is connected to the external heat supply pipe 29, thus providing supplemental heating. The side-stream product outlet is located in the upper middle section (between plates 6 and 8) of the refining column 17, where high-purity acrylic acid is collected. The high-purity acrylic acid is discharged through the side-stream product outlet pipe 30.

[0031] like Figure 3 The diagram illustrates the principle of heat exchange between the top condenser 18, the heat pump system, and the reboiler 26. A heat pump system for heat recovery is installed between the top condenser 18 and the reboiler 26. Heat is recovered and utilized between the top condenser 18 and the reboiler 26 through the heat pump system. The top condenser 18 is connected to the mechanical heat pump evaporator 20 of the heat pump system via a pipe. The condenser 22 of the heat pump system is connected to the reboiler 26 via a pipe. Figure 3 The dashed line represents the circulation path of the heat transfer oil used for heat exchange between the top condenser 18, the heat pump system, and the reboiler 26.

[0032] like Figure 4As shown, the heat pump system includes a compressor 21, a heat pump condenser 22, a mechanical heat pump evaporator 20, a filter 23, and an expansion valve 24. The mechanical heat pump evaporator 20, compressor 21, heat pump condenser 22, filter 23, and expansion valve 24 form a heat exchange loop for circulating refrigerant. The compressor 21 is centrifugal and equipped with a variable frequency drive. The circulating refrigerant in the heat exchange loop uses an environmentally friendly HFO-type refrigerant (R-1234ze(E)) with low global warming potential (GWP<1) and high chemical stability. The shell-side inlet of the mechanical heat pump evaporator 20 is connected to the shell-side outlet of the top condenser 18, and the shell-side outlet of the mechanical heat pump evaporator 20 is connected to the shell-side inlet of the top condenser 18. The shell-side outlet of the heat pump condenser 22 is connected to the shell-side inlet of the reboiler 26, and the shell-side inlet of the heat pump condenser 22 is connected to the shell-side inlet of the reboiler 26. The heat pump system is electrically connected to a heat pump system controller 25.

[0033] Example 2 According to the specific distillation method of the distillation system for crude acrylic acid purification in Example 1 above, the crude acrylic acid mixture enters the high-speed rotating packed bed 2 through liquid inlet 13 and then enters the liquid distributor 6. The crude acrylic acid mixture contains 32% acrylic acid, 61.5% water, 2.0% acetic acid, 1.8% propionaldehyde, 0.5% allyl alcohol, and 1.2% other impurities (including acrolein). The material temperature is 45°C, and the feed flow rate is 2,000 kg / h (approximately 24 tons / day). Simultaneously, the micro-metering pump 14 injects the polymerization inhibitor hydroquinone methyl ether (MEHQ) into the feed line, which then enters the high-speed rotating packed bed 2 through liquid inlet 13 along with the crude acrylic acid mixture. The injection rate of MEHQ is 50 ppm (i.e., 0.1 kg / h). The crude acrylic acid mixture is distributed through the liquid distributor 6 into the high-speed rotating wire mesh packing 7, which is a titanium alloy wire mesh structured packing (specific surface area 550 m²). 2 / m 3 (Porosity 96%), initial rotation speed 1500 rpm, equivalent centrifugal acceleration 225g, simultaneously the stripping medium (high-purity nitrogen) is introduced into the high-speed rotating packed bed 2 through inlet 1, ensuring full contact with the crude acrylic acid mixture, high-purity nitrogen flow rate 16 Nm³. 3 / h, the residence time of the material in the high-speed rotating packed bed 2 is <5s, the temperature in the high-speed rotating packed bed 2 is dynamically controlled between 65-70℃, and the pressure is about 30kPa.

[0034] The gas phase component is discharged from the exhaust port 4 of the high-speed rotating packed bed 2, and the gas phase component extraction rate is controlled at 250 kg / h. The gas phase component enters the condenser 16 and is condensed into liquid, and then enters the reflux tank 15, with part of it being returned to the high-speed rotating packed bed 2. The light component is extracted from the top of the reflux tank 15. The crude acrylic acid mixture after preliminary concentration in the high-speed rotating packed bed 2 is discharged from the liquid outlet 10 of the high-speed rotating packed bed 2. The acrolein removal rate in the crude acrylic acid mixture is 90%, and the acrolein content in the liquid phase at the liquid outlet 10 is ≤0.1 wt%. After concentration, the acrylic acid concentration increases to 58.5 wt%, and the water content decreases to ~37 wt%.

[0035] The high-speed rotating packed bed 2, where the wire mesh packing 7, driven by the motor 3, generates a centrifugal force field hundreds of times stronger than gravity (i.e., "hypergravity"). In the high-speed rotating packed bed, the liquid is sheared or dispersed into extremely thin liquid films, filaments, or droplets, and the gas phase flow is also violently disturbed. The preheated crude acrylic acid solution enters from the center of the rotating bed and is thrown to the outer edge at high speed. During this process, the solution comes into countercurrent contact with the inert gas. Lighter components such as acrolein (boiling point approximately 53°C), whose boiling points are much lower than those of acrylic acid, rapidly diffuse from the liquid phase to the gas phase under the strong mass transfer driving force and are removed from the gas outlet 4 at the top of the column. The removal of water requires the controller 12 to control the hypergravity device under reduced pressure, at which point the relative volatility of acrylic acid and water increases, which is beneficial for separation. The water is enriched in the gas phase and discharged from the gas outlet 4 at the top of the column into the condenser 16. Its footprint and energy consumption are lower than those of stripping towers; the residence time of materials in the high-speed rotating packed bed 2 is short, only a few seconds, which can effectively avoid the thermal polymerization of acrylic acid caused by long-term exposure at high temperature; the high-speed rotating packed bed 2 is operated by controller 12, and the operating conditions are easy to control precisely.

[0036] After pretreatment by the high-speed rotating packed bed 2, the acrylic acid mixture, after the removal of some water and most of the acrolein, collects at the outer edge of the rotating bed and is guided to the purification column 17 via liquid outlet 10 for the next purification step. The pretreatment by high-gravity distillation reduces carbon buildup and poisoning on the catalyst surface in the catalyst packing zone of purification column 17. This is mainly because acrolein and its polymers easily deposit on the surface of solid acid catalysts, clogging pores or covering active sites. High-gravity pretreatment makes the catalyst surface cleaner, enabling it to maintain high activity and long-term operation. It also suppresses acidic catalytic side reactions. The acidic groups on the catalyst itself may catalyze the condensation of aldehydes (Aldol), generating large molecular coke, thereby reducing product concentration. The efficient removal by pretreatment effectively breaks this side reaction chain, extending the catalyst regeneration cycle. It is expected that the catalyst lifespan can be extended by 30–50%, and the frequency of shutdowns for regeneration can be reduced, improving the stability of the unit operation.

[0037] After pretreatment in the high-speed rotating packed bed 2, the coloring precursors in the crude acrylic acid mixture are controlled at the source. Removing acrolein is equivalent to removing the most important color-producing precursor, fundamentally preventing color formation in subsequent processing. Simultaneously, it reduces the risk of acrolein polymerization in the refining tower 17. Reactive distillation towers typically operate at high temperatures (80–120℃), which is precisely the temperature-sensitive range for acrolein polymerization. A low concentration of acrolein in the feed significantly reduces the polymerization probability within the tower, resulting in a significant improvement in product color (APHA / Pt-Co value). Furthermore, it reduces the amount of polymerization inhibitor used. Traditional processes require large amounts of inhibitor to suppress polymerization, but excessive inhibitors can affect product purity or color. Pretreatment in the high-speed rotating packed bed 2 reduces the amount of inhibitor used, indirectly improving product appearance quality. Ultimately, the color of the acrylic product is expected to decrease from >100 APHA to <20 APHA.

[0038] The material discharged from the liquid outlet 10 of the high-speed rotating packed bed 2 enters the refining tower 17. The feed inlet is located in the lower middle part of the refining tower 17. The refining tower 17 is a vertical sieve plate / floating valve composite tower with 35 plates. The catalyst loading area is located between the 10th and 20th plates, using modular catalyst baskets for easy inspection and replacement. The catalyst used is 20% H3PW. 12 O 40 The SiO2 catalyst, with a loading of approximately 60L, is used to promote the reaction of residual acrolein with water to generate more stable 3-hydroxypropanal, achieving "separation while reacting." Multiple temperature monitoring points (temperature sensors) are set on purification column 17. The top temperature is approximately 102℃, the intermediate temperature section (plate 15) in the catalyst loading zone is 108℃, and the bottom temperature is 116℃. The initial reflux ratio is set to 1.8, and the APC dynamic adjustment range is 1.6–2.2. The side-stream product outlet 30 is located on the 7th plate, where high-purity acrylic acid (≥99.0 wt%) is collected. Heavy components (polymer precursors, etc.) are discharged from the bottom of the column and sent to cracker 28. APC dynamic adjustment refers to the automated optimization and adjustment of process parameters by the Advanced Process Control (APC) system. The system will simultaneously coordinate and adjust the reflux flow rate and the heating amount of the reboiler to dynamically control the reflux ratio. The system predicts future operating conditions in real time based on multiple variables such as feed composition, temperature distribution in the tower, and product purity requirements, and calculates the optimal operating instructions in advance so that the reflux ratio will automatically adjust towards the set value.

[0039] To better remove acrolein from the material and improve the purity of acrylic acid, it needs to be converted into a more stable compound through selective hydration during the refining stage. Simultaneously, the hydration product of acrolein (3-hydroxypropanal) is more polar and has a higher boiling point, allowing for easy separation via subsequent distillation without interfering with the quality of the main product. The specific reaction equation for the conversion of acrolein to 3-hydroxypropanal is as follows: CH2=CH–CHO+H2O→HOCH2CH2CHO (3-Hydroxypropanal) The above-mentioned reaction to generate 3-hydroxypropanal requires mild acidic catalysis and must not initiate the self-polymerization of acrylic acid. This necessitates a catalyst with sufficiently strong acidity to catalyze hydration, high selectivity to avoid excessive dehydration or cracking, and long-term stability under humid, hot, and strong organic acid environments. Heteropolyacid catalysts exhibit good tolerance to organic acids, and their framework structure demonstrates excellent resistance to corrosion from weak organic acids such as acrylic acid, making structural disintegration less likely. Acrylic acid itself is a weak acid, but in its concentrated state, it exhibits strong ionization and corrosiveness. Furthermore, the system contains other trace acids, which further lower the pH of the reaction system. This 20% H3PW catalyst... 12 O 40 The SiO2 catalyst itself is a strong protic acid with strong Brønsted acid sites, which facilitates the nucleophilic attack of water molecules on the β-carbon of acrolein (Michael addition hydration). Its pore structure can moderately confine reactants, inhibiting macromolecular polymerization. Electronic structure modulation makes it more active for α,β-unsaturated aldehydes, achieving a selectivity of >85% for acrolein hydration under suitable conditions. It maintains a stable structure under acidic conditions, and its activity is unaffected by fluctuations in acid concentration, preventing deactivation due to acid poisoning. Compared to sulfonic acid resins, this catalyst exhibits stronger hydrothermal stability and resistance to acrylic acid corrosion, and can be regenerated through calcination. In contrast, sulfonic acid resins are lower in cost, more flexible in packing, and more suitable for small to medium-sized plants.

[0040] Regarding the selection of catalyst loading positions in refining column 17: the top of refining column 17 (plates 1-5 from top to bottom) has a relatively low temperature and contains light components, including water, unreacted alcohols, acrolein, formic acid, etc.; the catalyst loading zone (i.e., the reaction zone, plates 10-20) is a medium-temperature zone (~110°C) with a high concentration of acrylic acid and a moderate water content; the bottom of refining column 17 (plates 25-35) has a high temperature (>110°C) and the material is a high-boiling-point substance (acrylic acid with a high polymerization tendency, heavy components, and catalyst residue).

[0041] The catalyst loading zone has a suitable reaction temperature window, balancing activity and stability. The optimal activity temperature range for the solid acid catalyst used in the reactive distillation of acrylic acid is 90–110℃. Too low a temperature leads to a slow reaction rate and low conversion; too high a temperature (e.g., >115℃ at the bottom of the column) intensifies the thermal polymerization and coking of acrylic acid and aldehydes, resulting in catalyst deactivation. Plates 10–20 in the catalyst loading zone are located in the intermediate temperature range, satisfying reaction kinetics requirements while avoiding high-temperature degradation. In the middle section of the column (plates 10–20), acrylic acid from the bottom continuously rises and concentrates. The high liquid concentration of acrylic acid in this region facilitates product removal, disrupting chemical equilibrium. The core advantage of reactive distillation is "separation at the reaction edge," driving the reversible reaction towards the formation of acrylic acid. By placing the reaction zone (catalyst loading zone) in the middle section, the product can be distilled upwards as soon as it is formed, continuously reducing the product concentration, disrupting equilibrium, and increasing the limiting conversion rate (up to over 95%, far exceeding conventional conditions). Simultaneously, extreme environmental zones are avoided to protect the catalyst. While the top region (plates 1-5) has a low temperature, it contains a large amount of water and light aldehydes (such as acrolein and formaldehyde), and may form a two-phase mixture (organic / aqueous phase), leading to catalyst swelling, loss, or poisoning. The bottom region (plates >25) has a high temperature and is enriched with heavy components, resulting in a long residence time for acrylic acid, making it highly susceptible to thermal polymerization, clogging catalyst channels, or forming a coke layer. Plates 10-20 are located in the "clean reaction window," with a moderate water content (beneficial for maintaining the H+ of the protonic acid catalyst). + The polymer and heavy impurities in this section of the column are not yet enriched, resulting in good fluid flow and sufficient mass transfer.

[0042] The catalyst is fixed in a catalyst basket in the catalyst loading zone, employing a structured catalyst design that facilitates installation, replacement, and cleaning, ensuring coordinated reaction and separation. A side-stream outlet is located between plates 6 and 8 to avoid the water-rich zone at the top of the column. Since the catalyst loading zone is located between plates 10 and 20, outlets below plate 10 would be affected by incompletely reacted materials. Furthermore, the peak concentration of acrylic acid typically occurs in the upper-middle part of the column, primarily due to the moderate volatility of acrylic acid, reaching its maximum concentration in the upper-middle section.

[0043] The control strategy for acrylic acid from the side stream involves installing multiple temperature sensors near plates 5-10, using the side stream product outlet as the primary control variable to establish a temperature-component relationship curve. If the temperature is too low (below 110℃), it indicates a high content of light components, posing a risk of entering the side stream; in this case, the reflux ratio of the condensate can be appropriately reduced, or the heat supply to the reboiler increased. If the temperature is too high (above 125℃), it indicates that heavy components may be shifting upwards, or the load is too high; in this case, the feed rate should be reduced. Temperature sensors are also installed on the walls of refining column 17 to prevent localized overheating that could trigger polymerization. Furthermore, the side stream product should be monitored and analyzed. If the water content in the side stream product increases, the reflux ratio should be increased to push the light components back to the bottom of the column. If the acrolein content increases or the color changes, catalyst activity should be considered, or the feed rate adjusted. The side stream is equipped with a GC chromatograph to detect the collected product and control the side stream collection flow rate accordingly, forming a feedback control loop. When the gas chromatograph detects excessive impurities, it is necessary to reduce the opening of the side stream collection regulating valve or even suspend collection.

[0044] The low-pressure steam (approximately 102°C) at the top of the refining column 17 enters the tube side of the overhead condenser 18 via a valve and a high-temperature gas flow controller 19. Part of the steam is returned to the refining column 17, and part is discharged. The bottom portion of the material from the refining column 17 enters the tube side of the reboiler 26, absorbs heat, and then returns to the refining column 17. The overhead condenser 18 of the refining column 17 recovers the heat from the top of the column through a heat pump system, and uses the recovered heat to heat the material in the reboiler 26, thus completing the heat recovery and utilization.

[0045] The heat pump system uses a single-stage centrifugal compressor 21 (with a gearbox) and a variable frequency motor with a power of 90kW. The circulating working fluid of the heat pump system is R-1234ze(E), with a mass charge of approximately 80 kg. The heat exchange method is an indirect heat exchange system, using an intermediate heat transfer medium (biphenyl-diphenyl ether mixture heat transfer oil). The circulation rate is 12 m³ / h, and the supply and return temperature of the heat transfer oil is 118-122℃.

[0046] Specific indirect heat exchange methods (such as...) Figure 3The process is as follows: Acrylic acid vapor from the top of the refining tower 17 enters the tube side of the top condenser 18, and low-temperature heat transfer oil exchanges heat with the acrylic acid vapor through the shell side of the top condenser 18. The condensed acrylic acid liquid returns to the top of the tower and is partially collected. The heat transfer oil after heat exchange enters the shell side of the mechanical heat pump evaporator 20 to heat the refrigerant of the heat pump cycle. After cooling, it flows back to the shell side inlet of the top condenser 18 for continued recycling. After the circulating refrigerant absorbs heat from the heat transfer oil in the mechanical heat pump evaporator 20 and is heated and pressurized by the compressor 21, it enters the heat pump condenser 22. After the circulating refrigerant releases heat and cools in the tube side of the heat pump condenser 22, it passes through the filter 23 and the expansion valve 24 and returns to the tube side of the low-temperature circulating refrigerant mechanical heat pump evaporator 20. The high-temperature heat transfer oil enters the shell side of the heat pump condenser 22 and absorbs the heat released by the circulating refrigerant. It then circulates to the shell side of the reboiler 26 at the bottom of the tower to provide heat for the acrylic acid solution at the bottom of the tower in the tube side of the reboiler 26. The acrylic acid in the tube side of the reboiler 26 returns to the tower bottom in the form of vapor. The remaining heavy components are discharged from the bottom of the tower for further cracking. The cooled heat transfer oil returns to the shell side inlet of the heat pump condenser 22 for continued recycling. The temperature of the heat pump system is controlled by the heat pump system controller 25.

[0047] R-1234ze is an environmentally friendly refrigerant (GWP<1, ODP=0) suitable for medium- and low-temperature heat pump systems. It possesses excellent thermodynamic properties, suitable for raising 102℃ vapor to 120℃, aligning with the trend of green chemical development. While R-1234ze is stable under dry and clean conditions, it may hydrolyze or corrode metals in wet, acidic environments. To address this issue, the aforementioned indirect heat exchange method avoids direct contact between R-1234ze(E) and acrylic acid vapor, ensuring safe application. This structure guarantees that R-1234ze(E) operates in a closed, clean loop, without contact with acrylic acid. Simultaneously, the intermediate heat exchanger is constructed using corrosion-resistant materials (316L stainless steel). Heat transfer oil serves as the heat transfer medium, isolating the process from the refrigerant, achieving both heat recovery and system safety.

[0048] The operating parameters of the heat pump system are as follows: the inlet temperature of acrylic acid vapor in condenser 18 is 102℃ (flow rate of about 680 kg / h). After heat exchange with heat transfer oil and further heating by compressor 21, the temperature reaches 120℃, with a temperature rise of ΔT = 18℃. At this time, the COP (coefficient of performance) is ≈3.1. The heat load of reboiler 26 is about 1,150 MJ / h (320 kW), of which 72% comes from the heat pump system (828 MJ / h) and 28% is supplemented by high-temperature heat transfer oil (the high-temperature heat transfer oil itself also has heat, and electric heating is used for auxiliary heating during startup or fluctuation). The external supplementary steam consumption is 0 during normal operation, and the annual steam saving (equivalent to standard coal) is about 2,200 tons / year.

[0049] The intelligent control system (APC) employed a sampling period of 45 seconds, ensuring product purity ≥99.5 wt% and color <20 APHA. Upon detecting a 0.3 wt% increase in acrolein in the feed, the system completed feedforward adjustments within 6 minutes: the high-speed rotating packed bed speed increased to 1650 rpm → the reflux ratio increased to 2.1 → the heat pump system load increased by 15% → the MEHQ dosage was automatically adjusted to 65 ppm → product quality remained stable with no exceedances. The operational results are shown in Table 1 below.

[0050] Table 1

[0051] As can be seen from Table 1 above, the final acrylic product has a purity of ≥99.5% and a color of <20 APHA. The output product meets the standards for purity and color, while reducing energy consumption.

[0052] The above description is merely a preferred embodiment of the present invention and should not be construed as limiting the scope of the invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications are also considered to be within the scope of protection of the present invention.

Claims

1. A distillation system for purifying crude acrylic acid, characterized in that, The distillation system includes a supergravity pretreatment unit, a distillation unit and a pyrolyzer (28). The hypergravity pretreatment unit includes a high-speed rotating packed bed (2), a condenser (16), a reflux tank (15), and a micro-metering pump (14); the gas outlet (4) at the top of the high-speed rotating packed bed (2) is connected to the condenser (16), the condenser (16) is connected to the reflux tank (15), the liquid outlet of the reflux tank (15) is connected to the crude acrylic liquid inlet (13) of the high-speed rotating packed bed (2), and the crude acrylic liquid inlet (13) is also connected to the micro-metering pump (14); The liquid outlet (10) at the bottom of the high-speed rotating packed bed (2) is connected to the feed inlet of the refining tower (17). The refining tower (17) integrates a top condenser (18) and a bottom reboiler (26). The refining tower (17) is provided with a catalyst loading zone, which is filled with a solid acid catalyst. The upper middle part of the refining tower (17) is provided with a side-stream product outlet. The bottom material outlet of the refining tower (17) is connected to a cracker (28). A heat pump system for recovering heat is provided between the top condenser (18) and the reboiler (26). The top condenser (18) is connected to the mechanical heat pump evaporator (20) of the heat pump system through a pipe. The heat pump condenser (22) of the heat pump system is connected to the reboiler (26) through a pipe.

2. The distillation system for purifying crude acrylic acid according to claim 1, characterized in that, The high-speed rotating filling bed (2) is filled with wire mesh filler (7), which is made of titanium alloy or duplex stainless steel.

3. The distillation system for purifying crude acrylic acid according to claim 1, characterized in that, The refining tower (17) is a vertical atmospheric pressure operating tower with 35 trays. The upper part uses sieve trays, and the middle and lower parts use floating valve trays. A catalyst loading area is provided between the 10th and 20th trays in the middle of the refining tower (17). The feed inlet is located in the middle and lower part of the refining tower, and the side-stream product outlet is located between the 6th and 8th trays of the refining tower (17). The refining tower (17) is equipped with a temperature sensor.

4. The distillation system for purifying crude acrylic acid according to claim 1, characterized in that, The heat pump system includes a compressor (21), a heat pump condenser (22), a mechanical heat pump evaporator (20), a filter (23), and an expansion valve (24). The mechanical heat pump evaporator (20), compressor (21), heat pump condenser (22), filter (23) and expansion valve (24) form a heat exchange circuit for circulating refrigerant.

5. The distillation system for purifying crude acrylic acid according to claim 4, characterized in that, The shell-side inlet of the mechanical heat pump evaporator (20) is connected to the shell-side outlet of the top condenser (18), and the shell-side outlet of the mechanical heat pump evaporator (20) is connected to the shell-side inlet of the top condenser (18). The shell-side outlet of the heat pump condenser (22) is connected to the shell-side inlet of the reboiler (26), and the shell-side inlet of the heat pump condenser (22) is connected to the shell-side inlet of the reboiler (26).

6. The distillation method for the distillation system for purifying crude acrylic acid according to claim 1, characterized in that, The distillation method includes the following steps: the crude acrylic acid mixture enters a high-speed rotating packed bed (2) for preliminary concentration to remove low-boiling substances. The gas phase components are discharged from the high-speed rotating packed bed (2) and then enter the reflux tank (15) through the condenser (16) to return to the high-speed rotating packed bed (2). The light components are taken out from the top of the reflux tank (15). The rich liquid at the bottom of the high-speed rotating packed bed (2) is transported to the purification tower (17) and further purified through the catalyst packing zone in the purification tower (17). The high-purity acrylic acid solution is collected through the side stream at the top of the purification tower (17), and the heavy components discharged from the bottom of the tower are sent to the pyrolyzer (28). The top condenser (18) of the refining column (17) recovers the heat from the top of the column through a heat pump system and uses the recovered heat to heat the reboiler (26), thus completing the heat recovery and utilization.

7. The distillation method according to claim 6, characterized in that, The residence time of the crude acrylic acid mixture in the high-speed rotating packed bed (2) is less than 5 seconds.

8. The distillation method according to claim 6, characterized in that, The high-speed rotating packed bed (2) also contains a polymerization inhibitor.

9. The distillation method according to claim 6, characterized in that, The circulating refrigerant in the heat pump system is R-1234ze.

10. The distillation method according to claim 6, characterized in that, The temperature at the top of the refining tower (17) is 102°C, the temperature in the catalyst loading zone is 108°C, the temperature at the bottom of the tower is 116°C, the initial reflux ratio is set to 1.8, and the side stream is taken out at the 7th tray.

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