A system and process for the production of polylactic acid by biochemical coupling conversion

Abstract

The application provides a system and a process for preparing polylactic acid through biochemical coupling, comprising a lactic acid synthesis unit, a lactic acid purification unit, a lactide synthesis unit and a lactide polymerization unit; the lactic acid synthesis unit is used for mixing oxygen, methane raw gas, nutrients and a buffer, performing aerobic fermentation, performing gas-liquid separation and solid-liquid separation on the fermentation product, and obtaining a crude lactic acid solution, solid biomass and waste gas; the lactic acid purification unit is used for purifying the crude lactic acid solution obtained by the lactic acid synthesis unit, and obtaining a lactic acid solution meeting the purity requirement; the lactide synthesis unit is used for performing a synthesis reaction on the lactic acid solution obtained by the lactic acid purification unit in a nitrogen environment, and obtaining lactide; and the lactide polymerization unit is used for performing a polymerization reaction on the lactide obtained by the lactide synthesis unit in the presence of a catalyst, and obtaining polylactic acid. The application relieves the competition between lactic acid and food, stabilizes high-energy methane, and realizes the preparation of high-quality polylactic acid from low-cost methane.

Description

Technical Field

[0001] This invention relates to the field of biotransformation technology, specifically to a system and process for the biochemical coupling transformation to prepare polylactic acid. Background Technology

[0002] With increasing environmental pollution and the global ban / restriction of plastics, bio-based biodegradable plastics, represented by polylactic acid (PLA), have attracted much attention. As a thermoplastic polymer, PLA possesses excellent properties such as biodegradability, recyclability, high mechanical strength, low toxicity, good barrier properties, and processability, making it widely used in packaging, pharmaceuticals, and other fields, and an effective way to curb "white pollution." However, PLA production capacity is mainly concentrated overseas, while my country's PLA production is still in its initial stage, with lagging technology and small production scale, posing significant challenges to achieving low-cost, green, low-carbon, and sustainable PLA production. Therefore, exploring practical and efficient PLA synthesis processes is of great practical significance for the development of my country's PLA industry.

[0003] Polylactic acid (PLA) can be obtained from lactic acid through direct polycondensation or ring-opening polymerization of lactide. Lactic acid itself is mainly obtained from starchy raw materials such as corn, rice, and sweet potatoes through bio-fermentation. Currently, the most widely used PLA production process is a two-step process: biomass fermentation to produce lactic acid, followed by PLA production using lactic acid as a raw material. Both methods are technologically mature, but the biomass fermentation process for producing lactic acid will inevitably increase competition between food and lactic acid products. Summary of the Invention

[0004] To overcome the shortcomings of existing polylactic acid (PLA) preparation methods, this invention proposes a system and process for the biochemical coupling preparation of PLA, which not only alleviates the competition between lactic acid and food, but also stabilizes high-energy methane, realizing the preparation of high-quality PLA from low-cost methane.

[0005] This invention is achieved through the following technical solution:

[0006] A system for the biochemical coupling conversion to prepare polylactic acid includes: a lactic acid synthesis unit, a lactic acid purification unit, a lactide synthesis unit, and a lactide polymerization unit;

[0007] The lactic acid synthesis unit is used to mix oxygen, methane feed gas, nutrients and buffers for aerobic fermentation. The fermentation products are subjected to gas-liquid separation and solid-liquid separation to obtain crude lactic acid solution, solid biomass and waste gas.

[0008] The lactic acid purification unit is used to purify the crude lactic acid solution obtained from the lactic acid synthesis unit to obtain a lactic acid solution that meets the purity requirements.

[0009] The lactide synthesis unit is used to synthesize lactide by reacting the lactic acid solution obtained from the lactic acid purification unit in a nitrogen environment.

[0010] The lactide polymerization unit is used to polymerize the lactide obtained from the lactide synthesis unit in the presence of a catalyst to obtain polylactic acid.

[0011] Preferably, the system further includes a gas pretreatment unit, which processes the methane-containing gas and air to obtain oxygen, nitrogen, and methane feed gas that meets the purity requirements of methane; the obtained oxygen and methane feed gas are input into the lactic acid synthesis unit, and the nitrogen is input into the lactide synthesis unit.

[0012] Preferably, the system also includes a combined heat and power (CHP) unit, which is used to ferment the solid biomass produced by the lactic acid synthesis unit to produce biogas, and to burn the biogas and the waste gas produced by the lactic acid synthesis unit together. The combustion gas is used to generate electricity, and the generated electrical energy and heat energy supply the system with electricity and heat.

[0013] Furthermore, the cogeneration unit includes an anaerobic digester, a second flash tank, a second compressor, a combustion chamber, a gas turbine, a second cooler, and a third flash tank. Solid biomass produced by the lactic acid synthesis unit enters the anaerobic digester. The outlet of the anaerobic digester is connected to the inlet of the second flash tank. The biogas from the second flash tank mixes with the waste gas from the lactic acid synthesis unit and enters the combustion chamber, where it is mixed with compressed air from the second compressor and combusted. The combustion gas outlet of the combustion chamber is connected to the inlet of the gas turbine. The outlet of the gas turbine is connected to the inlet of the second cooler. The outlet of the second cooler is connected to the inlet of the third flash tank. The gas outlet of the third flash tank is connected to the inlet of a carbon dioxide adsorption device, which outputs carbon dioxide.

[0014] Preferably, it also includes a solvent recovery unit; the lactic acid purification unit performs ion exchange on the crude lactic acid solution obtained from the lactic acid synthesis unit, and the eluent generated by the ion exchange is recycled by the solvent recovery unit to recover the buffer, and the recovered buffer is returned to the lactic acid synthesis unit for reuse.

[0015] Furthermore, the buffer is sodium hydroxide or potassium hydroxide; the solvent recovery unit includes a digester, a causticizer, a second centrifuge, a first drying chamber, a calcination chamber, a third cooler, a gas-solid separation tank, a carbon dioxide adsorption device, and a fourth mixer; the eluent generated by ion exchange enters the causticizer, the digester receives water and calcium oxide, the outlet of the digester is connected to the inlet of the causticizer, the material from the causticizer undergoes solid-liquid separation, and the separated solution enters the lactic acid synthesis unit as a buffer; the separated solid enters the first drying chamber, the outlet of the first drying chamber is connected to the inlet of the calcination chamber, the outlet of the calcination chamber is connected to the inlet of the third cooler, the outlet of the third cooler is connected to the inlet of the gas-solid separation tank, the solid outlet of the gas-solid separation tank is connected to the inlet of the digester, and the gas outlet of the gas-solid separation tank outputs carbon dioxide.

[0016] Preferably, the lactic acid purification unit includes an ion exchange device, a quadruple-effect evaporator, and a molecular distillation column. The crude lactic acid solution undergoes ion exchange through the ion exchange device, and the lactic acid solution after ion exchange enters the quadruple-effect evaporator for evaporation and concentration. The concentrated liquid outlet of the quadruple-effect evaporator is connected to the liquid inlet of the molecular distillation column, and the lactic acid solution from the molecular distillation column enters the lactide synthesis unit.

[0017] A process for preparing polylactic acid via biochemical coupling conversion, comprising:

[0018] Oxygen, methane feed gas, nutrients and buffer are mixed and aerobic fermentation is carried out. The fermentation products are subjected to gas-liquid separation and solid-liquid separation to obtain crude lactic acid solution, solid biomass and waste gas.

[0019] The crude lactic acid solution was purified to obtain a lactic acid solution.

[0020] Lactic acid solution was reacted in a nitrogen atmosphere to obtain lactide;

[0021] Polylactic acid is obtained by polymerizing lactide in the presence of a catalyst.

[0022] Preferably, the process for preparing polylactic acid by biochemical coupling conversion further includes: solid biomass undergoes anaerobic fermentation to produce biogas, which is mixed with waste gas for combustion. The combustion gas is used to generate electricity and heat through a gas turbine. The high-temperature gas from the gas turbine is dehydrated and then adsorbed or absorbed to obtain carbon dioxide.

[0023] Preferably, in the process of preparing polylactic acid by biochemical coupling conversion, the crude lactic acid solution is subjected to ion exchange, the eluent generated by ion exchange is reacted with calcium hydroxide, the reaction solution is separated into solid and liquid, the buffer obtained by separation is recycled, the calcium carbonate obtained by separation is calcined, the calcination product is subjected to gas-solid separation to obtain carbon dioxide and calcium oxide, and the calcium oxide reacts with water to generate calcium hydroxide for recycling.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] This invention discloses a system for preparing polylactic acid (PLA). Using methane as a raw material, lactic acid is produced through bio-fermentation. Lactic acid is then used to synthesize lactide, which is polymerized to obtain PLA. This process, through biochemical coupling, yields PLA. This invention, using methane as a raw material to prepare lactic acid, not only alleviates competition between lactic acid and food products but also helps reduce greenhouse gas emissions, stabilizes high-energy methane, and achieves successful low-cost methane-to-high-quality PLA preparation. It provides a necessary reference for the development and large-scale application of PLA production technology in China.

[0026] Furthermore, this invention ferments the solid biomass byproducts of the lactic acid synthesis unit to produce biogas, and then burns the biogas and unreacted methane waste gas for power generation. The generated electrical and thermal energy provides power and heat to the system of this invention, thereby achieving efficient energy recovery and utilization.

[0027] Furthermore, by flash evaporation and pressure swing adsorption of the combustion gas, carbon dioxide gas is obtained, achieving low-cost carbon dioxide capture and high-efficiency carbon fixation, which helps reduce greenhouse gas emissions.

[0028] Furthermore, this invention recovers the buffer from the eluent generated by ion exchange via a solvent recovery unit, enabling the buffer to be recycled, improving material utilization, and reducing costs.

[0029] Furthermore, the eluent of this invention reacts with calcium hydroxide to obtain a buffer that can be recycled. At the same time, the calcium carbonate generated is calcined to produce calcium oxide and carbon dioxide. The calcium oxide is recycled, thus achieving low-cost carbon dioxide capture and high-efficiency carbon fixation. Attached Figure Description

[0030] Figure 1 The design flow of the biochemical coupling conversion process for preparing polylactic acid in this invention;

[0031] Figure 2 The present invention provides a process flow for the biochemical coupling conversion preparation of polylactic acid;

[0032] Figure 3 A schematic diagram of the system for preparing polylactic acid by biochemical coupling conversion according to the present invention.

[0033] In the diagram: 1. Methane Pressure Swing Adsorption Unit; 2. Air Separation Unit; 3. First Mixer; 4. First Compressor; 5. Second Mixer; 6. First Pump; 7. Seed Fermentation Tank; 8. Product Fermentation Tank; 9. Pressure Relief Valve; 10. First Flash Evaporator; 11. First Centrifuge; 12. Anaerobic Fermentation Tank; 13. Second Flash Evaporator; 14. Third Mixer; 15. Second Compressor; 16. First Separator; 17. Combustion Chamber; 18. First Cooler; 19. Gas Turbine; 20. Second Cooler; 21. Third Flash Evaporator; 22. Ion Exchange Unit; 32. Quadruple-Effect Evaporator; 33. Molecular Distillation Column; 23. Digester. Causticizer 24, Second centrifuge 25, First drying chamber 26, Calcination chamber 27, Third cooler 28, Gas-solid separator 29, Carbon dioxide adsorption device 30, Fourth mixer 31, Second separator 34, Second pump 35, Fifth mixer 36, First heater 37, Lactide synthesis reactor 38, Fourth cooler 39, Fourth flash tank 40, Fifth cooler 41, Fifth flash tank 42, Third separator 43, Vacuum distillation column 44, Third pump 45, Second heater 46, Polylactic acid reactor 47, Dissolver 48, Second drying chamber 49. Detailed Implementation

[0034] To further understand the present invention, the present invention is described below with reference to examples. These descriptions are only for further explaining the features and advantages of the present invention and are not intended to limit the claims of the present invention.

[0035] Please see Figure 1 and Figure 2 The process for preparing polylactic acid by biochemical coupling conversion according to the present invention includes:

[0036] (1) Gas pretreatment: The methane-containing gas is treated to obtain methane feed gas with a methane content greater than 90%; the air is separated to obtain oxygen;

[0037] (2) Lactic acid synthesis: Methane feed gas, oxygen, nutrients and buffer are mixed and aerobic fermentation is carried out to produce fermentation broth containing lactic acid;

[0038] (3) Lactic acid purification: The fermentation broth containing lactic acid is subjected to gas-liquid separation, centrifugation, ion exchange, evaporation and distillation to obtain a lactic acid solution that meets the requirements for lactide synthesis;

[0039] (4) Synthesis of lactide: The lactic acid solution is reacted after pressure swing and preheating treatment to synthesize lactide. After purification treatment, a lactide product with a certain purity is obtained.

[0040] (5) Polymerization of lactide: The purified lactide is polymerized to produce polylactic acid, which is then purified to obtain the product polylactic acid solid.

[0041] This invention can also be used for combined heat and power: byproducts from lactic acid synthesis are anaerobic fermented to produce biogas, which is then mixed with unreacted methane waste gas for combustion. The combustion gas is used to generate electricity and heat through a gas turbine to supply electricity and heat to other processes.

[0042] The present invention can also recover solvents: the fermentation broth after centrifugation to remove solid biomass is recycled by ion exchange to produce an inorganic salt solution, thereby realizing the recycling of the buffer. At the same time, the high-temperature waste heat generated in this process can be used for subsequent purification processes such as lactic acid purification.

[0043] Please see Figure 3 The system for preparing polylactic acid by biochemical coupling conversion according to the present invention includes: a gas pretreatment unit, a lactic acid synthesis unit, a lactic acid purification unit, a lactide synthesis unit, and a lactide polymerization unit.

[0044] The gas pretreatment unit is used to process methane-containing gas and air to obtain oxygen, nitrogen, and methane feed gas that meets the purity requirements of methane.

[0045] The lactic acid synthesis unit is used to mix the oxygen and methane feed gas obtained from the gas pretreatment unit with nutrients and buffers for aerobic fermentation. The fermentation products are subjected to gas-liquid separation and solid-liquid separation to obtain crude lactic acid solution, solid biomass and waste gas.

[0046] The lactic acid purification unit is used to perform ion exchange, evaporation and distillation on the crude lactic acid solution obtained from the lactic acid synthesis unit to obtain a lactic acid solution that meets the purity requirements.

[0047] The lactide synthesis unit is used to react the lactic acid solution obtained from the lactic acid purification unit in the nitrogen gas obtained from the gas pretreatment unit to synthesize lactide.

[0048] The lactide polymerization unit is used to polymerize the lactide obtained from the lactide synthesis unit in the presence of a catalyst to obtain polylactic acid.

[0049] The system of the present invention may also optionally include a combined heat and power unit and / or a solvent recovery unit.

[0050] The combined heat and power (CHP) unit is used to anaerobically ferment the solid biomass produced by the lactic acid synthesis unit to produce biogas. The biogas and the waste gas produced by the lactic acid synthesis unit are burned together, and the combustion gas is used to generate electricity. The generated electricity and heat energy are used to supply electricity and heat to other units.

[0051] The solvent recovery unit processes the eluent generated by ion exchange in the lactic acid purification unit, recovers the buffer, and returns it to the lactic acid synthesis unit for reuse.

[0052] Example 1

[0053] The system for preparing polylactic acid via biochemical coupling conversion described in this embodiment includes: a gas pretreatment unit, a lactic acid synthesis unit, a cogeneration unit, a lactic acid purification unit, a solvent recovery unit, a lactide synthesis unit, and a lactide polymerization unit.

[0054] The gas pretreatment unit includes a methane pressure swing adsorption unit 1, an air separation unit 2, a first mixer 3, and a first compressor 4; the lactic acid synthesis unit includes a second mixer 5, a first pump 6, a seed fermentation tank 7, a product fermentation tank 8, a pressure relief valve 9, a first flash evaporator 10, and a first centrifuge 11; the cogeneration unit includes an anaerobic fermenter 12, a second flash evaporator 13, a third mixer 14, a second compressor 15, a first separator 16, a combustion chamber 17, a first cooler 18, a gas turbine 19, a second cooler 20, and a third flash evaporator 21; the lactic acid purification unit includes an ion exchange unit 22, a quadruple-effect evaporator 32, and a molecular distillation column 33; the solvent... The agent recovery unit includes a digester 23, a causticizer 24, a second centrifuge 25, a first drying chamber 26, a calcination chamber 27, a third cooler 28, a gas-solid separation tank 29, a carbon dioxide adsorption device 30, and a fourth mixer 31; the lactide synthesis unit includes a second separator 34, a second pump 35, a fifth mixer 36, a first heater 37, a lactide synthesis reactor 38, a fourth cooler 39, a fourth flash tank 40, a fifth cooler 41, a fifth flash tank 42, a third separator 43, and a vacuum distillation column 44; the lactide polymerization unit includes a third pump 45, a second heater 46, a polylactic acid reactor 47, a dissolver 48, and a second drying chamber 49.

[0055] The inlet of the methane pressure swing adsorption unit 1 receives methane-containing gas, and its outlet is connected to the methane feed gas inlet of the first mixer 3; the inlet of the air separation unit 2 receives air, and its oxygen outlet is connected to the oxygen inlet of the first mixer 3; the outlet of the first mixer 3 is connected to the inlet of the first compressor 4, and the outlet of the first compressor 4 is connected to the inlet of the seed fermentation tank 7; the second mixer 5 is used to mix buffer and nutrients, and its outlet is connected to the liquid inlet of the seed fermentation tank 7 via the first pump 6; the bottom outlet of the seed fermentation tank 7 is connected to the inlet of the product fermentation tank 8, and the outlet of the product fermentation tank 8 is connected to the liquid inlet of the first flash tank 10 via the pressure relief valve 9; the bottom outlet of the first flash tank 10 is connected to the liquid inlet of the first centrifuge 11, and the liquid after centrifugation by the first centrifuge 11 enters the ion exchange unit 22, while the solid biomass after centrifugation by the first centrifuge 11 enters the anaerobic fermentation tank 12.

[0056] The lactic acid solution after ion exchange in ion exchange device 22 enters quadruple-effect evaporator 32 for evaporation and concentration. The concentrated liquid outlet of quadruple-effect evaporator 32 is connected to the liquid inlet of molecular distillation column 33. The lactic acid solution outlet at the bottom of molecular distillation column 33 is connected to the lactic acid solution inlet of fifth mixer 36 via second pump 35. The nitrogen outlet of air separation unit 2 is connected to the gas inlet of second separator 34. The nitrogen outlet of second separator 34 is connected to the gas inlet of fifth mixer 36. The liquid outlet of fifth mixer 36 is connected to the liquid inlet of first heater 37. The liquid outlet of first heater 37 is connected to the liquid inlet of lactide synthesis reactor 38. The liquid outlet of lactide synthesis reactor 38 is connected to fourth cooler 39. The liquid outlet of the fourth cooler 39 is connected to the liquid outlet of the fourth flash tank 40. The bottom liquid outlet of the fourth flash tank 40 is connected to the liquid outlet of the vacuum distillation column 44. The bottom liquid outlet of the vacuum distillation column 44 is connected to the liquid outlet of the second heater 46 via the third pump 45. The liquid outlet of the second heater 46 is connected to the liquid outlet of the polylactic acid reactor 47. The polylactic acid reactor 47 is provided with a catalyst inlet for receiving catalyst. The discharge port of the polylactic acid reactor 47 is connected to the discharge port of the solvent 48. The solvent 48 is provided with a solvent inlet for receiving solvent. The liquid outlet of the solvent 48 is connected to the liquid outlet of the second drying chamber 49. The outlet of the second drying chamber 49 outputs polylactic acid. The gas outlet of the fourth flash tank 40 is connected to the inlet of the fifth cooler 41. The outlet of the fifth cooler 41 is connected to the inlet of the fifth flash tank 42. The outlet of the fifth flash tank 42 is connected to the gas inlet of the fifth mixer 36.

[0057] The anaerobic fermenter 12 has its liquid outlet connected to the liquid inlet of the second flash tank 13. The gas outlets of the second flash tank 13 and the first flash tank 10 are both connected to the gas inlet of the third mixer 14. The gas outlet of the third mixer 14 is connected to the gas inlet of the combustion chamber 17. The second compressor 15 inputs air, and its gas outlet is connected to the gas inlet of the first separator 16. The first outlet of the first separator 16 is connected to the air inlet of the combustion chamber 17. The second outlet of the first separator 16 is connected to the inlet of the first cooler 18. The outlet of the first cooler 18 and the combustion gas outlet of the combustion chamber 17 are both connected to the inlet of the gas turbine 19. The outlet of the gas turbine 19 is connected to the inlet of the second cooler 20. The outlet of the second cooler 20 is connected to the inlet of the third flash tank 21. The gas outlet of the third flash tank 21 is connected to the gas inlet of the carbon dioxide adsorption device 30.

[0058] The eluent from the ion exchange device 22 enters the causticizer 24. The digester 23 receives water and calcium oxide. The outlet of the digester 23 is connected to the inlet of the causticizer 24. The outlet of the causticizer 24 is connected to the second centrifuge 25. The liquid obtained by centrifugation by the second centrifuge 25 enters the first drying chamber 26. The outlet of the first drying chamber 26 is connected to the inlet of the calcination chamber 27. The outlet of the calcination chamber 27 is connected to the inlet of the third cooler 28. The outlet of the third cooler 28 is connected to the inlet of the gas-solid separator 29. The solid outlet of the gas-solid separator 29 is connected to the inlet of the digester 23. The gas outlet of the gas-solid separator 29 is connected to the inlet of the fourth mixer 31. The carbon dioxide outlet of the carbon dioxide adsorption device 30 is connected to the inlet of the fourth mixer 31. The fourth mixer 31 outputs carbon dioxide.

[0059] The biochemical coupling conversion process for preparing polylactic acid described in this embodiment specifically includes:

[0060] (1) Gas pretreatment: Natural gas containing methane is passed through a methane pressure swing adsorption unit 1 (or a CO2 absorption unit or other equipment that can remove CO2) to remove some carbon dioxide and obtain methane feed gas with a methane content of 90% (mass fraction); air is passed through an air separation unit to remove nitrogen and obtain oxygen with a purity greater than 95%. After the oxygen and methane feed gas are mixed in the first mixer 3, they are pressurized by the first compressor 4 to 1.17 atm and then enter the seed fermentation tank 7 in the lactic acid synthesis unit.

[0061] (2) Lactic acid synthesis: Sodium hydroxide solution acts as a buffer to adjust the pH value in the seed fermenter, maintaining the fermentation conditions at a weakly acidic pH of approximately 6.0-6.5. The sodium hydroxide solution is mixed with ammonia in the second mixer 5 and pressurized to 1.17 atm by the first pump 6. This mixture, along with the gas from the gas pretreatment unit and nutrients, is then introduced into the seed fermenter 7 to obtain a culture medium and inoculum that meet the conditions for lactic acid fermentation. Subsequently, it enters the product fermenter 8, where aerobic fermentation takes place at 30°C and 1.17 atm. With the participation of methanogens, methane is converted into lactic acid and biomass (C4H8O2N). After passing through the pressure relief valve 9, the pressure drops to 1 atm, reducing the solubility of unreacted methane and oxygen. The mixture is then introduced into the first flash tank 10 for gas-liquid separation. The fermentation broth is then introduced into the first centrifuge 11 for solid-liquid separation. The removed solid biomass enters the anaerobic fermenter 12 in the cogeneration unit, while the remaining fermentation broth enters the ion exchange device 22.

[0062] (3) Lactic acid purification: The remaining liquid after the ion exchange device consists of water and lactic acid, with a water content of 97.4%. In order to obtain a lactic acid feedstock solution that meets the requirements for lactide production, a quadruple-effect evaporator 32 is used to remove excess water. This device can achieve efficient utilization of heat energy, and the lactic acid content reaches 73.1%. Subsequently, it is passed into a molecular distillation column 33 to remove excess water, and a crude lactic acid solution with a mass fraction of 85% is obtained.

[0063] (4) Lactide Synthesis: The crude lactic acid solution enters the lactide synthesis unit and is pressurized to 1 bar by the second pump 35. To reduce the formation of oligomers, the pressurized crude lactic acid solution is mixed with a small amount of nitrogen generated sequentially by the air separation unit 2 and the second separator 34 in the fifth mixer 36 and then preheated to 160°C by the first heater 37. After that, it enters the lactide synthesis reactor 38 and reacts at 240°C and 1 atm. The crude lactide solution discharged from the lactide synthesis reactor 38 is rapidly cooled to 110°C by the fourth cooler 39 (to reduce the formation of isomers). At the same time, water and nitrogen can be separated in the fourth flash tank 40. The mixture of water and nitrogen is further condensed to 28°C by the fifth cooler 41. Nitrogen and water are separated in the fifth flash tank 42. The nitrogen is separated by the third separator 43 and can be recycled in the fifth mixer 36. The crude lactide separated from the fourth flash evaporator 40 was purified by vacuum distillation column 44, and the lactide purity reached 97.4%.

[0064] (5) Polymerization of lactide: The purified lactide is heated to about 150°C and pressure is 0.02 atm. It is then pressurized to 94 kPa by the third pump 45 and heated to 160°C by the second heater 46. It is then introduced into the polylactic acid reactor 47 and polylactic acid is synthesized under the action of a catalyst. Ethyl acetate and other solvents are added to the polylactic acid containing impurities to dissolve the solid polylactic acid. Then it is introduced into the drying chamber for drying until all the solvent evaporates, and polylactic acid is obtained.

[0065] (6) Cogeneration: Solid biomass is fed into anaerobic digester 12, where it undergoes anaerobic fermentation at 42°C and 1 atm to produce biogas. After passing through the second flash tank 13, the biogas is separated and mixed with unreacted methane waste gas from the first flash tank 10 in the third mixer 1 before entering the combustion chamber 17. Air passes through the second compressor 15, where the pressure and temperature are raised to 17.1 atm and 427.1°C, respectively. 83% of the compressed air enters the combustion chamber 17 and burns with methane waste gas at 1400°C to generate a large amount of high-temperature waste heat, which is used as the heat fluid for the heat exchange equipment to supply heat to the system. The remaining 17% of the air is cooled to ambient temperature and pressure of 14.87 atm by the first cooler 18 and is fed into the gas turbine 19 together with the combustion gas from the combustion chamber 17 to generate electricity while obtaining high-temperature waste heat, which is supplied to the gas pretreatment, lactic acid purification unit and lactide synthesis unit through the heat exchange equipment. The high-temperature gas from the gas turbine 19 is cooled by the second cooler 20 to obtain cooling gas, and then passes through the third flash tank 21 to remove excess moisture. The gas obtained from flash evaporation has a high carbon dioxide content, and then passes through the carbon dioxide adsorption device 30 to obtain carbon dioxide gas.

[0066] (7) Solvent Recovery: The fermentation broth after removing solid biomass mainly consists of inorganic salts such as sodium hydroxide and sodium carbonate, water, lactic acid, and small amounts of carbon dioxide and methane gas. The fermentation broth passes through ion exchange device 22, and the inorganic salt solution obtained by elution enters the solvent recovery unit. To achieve the recycling of sodium hydroxide, water at 50°C and calcium oxide are fed into digester 23, where they react at 84°C and 1 atm to produce calcium hydroxide. Subsequently, it enters causticizer 24 together with the inorganic salt solution, where sodium hydroxide and calcium carbonate solids are generated at 70°C and 1 atm. After separation by second centrifuge 25, the alkaline solution rich in sodium hydroxide is recycled into the second mixer 5 in the lactic acid synthesis unit. After drying in the first drying chamber 26, calcium carbonate is fed into calcination chamber 27, where it is calcined at 825°C to produce calcium oxide and carbon dioxide. After cooling in the third cooler 28, it enters gas-solid separator 29. Calcium oxide enters digester 23 to participate in the recycling. Carbon dioxide is mixed with carbon dioxide emitted from the cogeneration unit in the fourth mixer 31 and enters carbon dioxide storage device.

[0067] This invention, through process simulation, realizes the design of a biochemical coupling conversion process for the preparation of polylactic acid (PLA). By analyzing energy and mass flows, and the interconnection and coupling of each component unit, the corresponding key operating parameters were determined, providing a technical route for the preparation of lactide through biochemical coupling. This process enables the production of high-quality PLA, low-cost carbon dioxide capture, and high-efficiency carbon fixation, providing necessary reference for the development and large-scale application of PLA production technology in China.

Claims

1. A system for the biochemical coupling conversion to prepare polylactic acid, characterized in that, include: The unit includes a lactic acid synthesis unit, a lactic acid purification unit, a lactide synthesis unit, a lactide polymerization unit, a cogeneration unit, and a solvent recovery unit. The lactic acid synthesis unit is used to mix oxygen, methane feed gas, nutrients and buffers for aerobic fermentation. The fermentation products are subjected to gas-liquid separation and solid-liquid separation to obtain crude lactic acid solution, solid biomass and waste gas. The lactic acid purification unit is used to perform ion exchange, evaporation and distillation on the crude lactic acid solution obtained from the lactic acid synthesis unit to obtain a lactic acid solution that meets the purity requirements; the eluent generated by ion exchange is recycled to the buffer in the solvent recovery unit, and the recycled buffer is returned to the lactic acid synthesis unit for reuse; The lactide synthesis unit is used to synthesize lactide by reacting the lactic acid solution obtained from the lactic acid purification unit in a nitrogen environment. The lactide polymerization unit is used to polymerize the lactide obtained from the lactide synthesis unit in the presence of a catalyst to obtain polylactic acid. The cogeneration unit is used to ferment the solid biomass produced by the lactic acid synthesis unit to produce biogas, and to burn the biogas and the waste gas produced by the lactic acid synthesis unit together. The combustion gas is used to generate electricity, and the generated electrical energy and heat energy supply the system with electricity and heat.

2. The system for preparing polylactic acid via biochemical coupling conversion according to claim 1, characterized in that, It also includes a gas pretreatment unit, which processes methane-containing gas and air to obtain oxygen, nitrogen, and methane feed gas that meets the purity requirements of methane; the obtained oxygen and methane feed gas are input into the lactic acid synthesis unit, and the nitrogen is input into the lactide synthesis unit.

3. The system for preparing polylactic acid via biochemical coupling conversion according to claim 1, characterized in that, The cogeneration unit includes an anaerobic digester (12), a second flash tank (13), a second compressor (15), a combustion chamber (17), a gas turbine (19), a second cooler (20), and a third flash tank (21). The solid biomass produced by the lactic acid synthesis unit enters the anaerobic digester (12). The outlet of the anaerobic digester (12) is connected to the inlet of the second flash tank (13). The biogas from the second flash tank (13) is mixed with the waste gas produced by the lactic acid synthesis unit and then enters the combustion chamber (17), where it is mixed with compressed air from the second compressor (15) and burned. The combustion gas outlet of the combustion chamber (17) is connected to the inlet of the gas turbine (19). The outlet of the gas turbine (19) is connected to the inlet of the second cooler (20). The outlet of the second cooler (20) is connected to the inlet of the third flash tank (21). The gas outlet of the third flash tank (21) is connected to the inlet of the carbon dioxide adsorption device (30). The carbon dioxide adsorption device (30) outputs carbon dioxide.

4. The system for preparing polylactic acid via biochemical coupling conversion according to claim 1, characterized in that, The buffer is sodium hydroxide or potassium hydroxide; the solvent recovery unit includes a digester (23), a causticizer (24), a second centrifuge (25), a first drying chamber (26), a calcination chamber (27), a third cooler (28), a gas-solid separation tank (29), a carbon dioxide adsorption device (30), and a fourth mixer (31); the eluent generated by ion exchange enters the causticizer (24), the digester (23) receives water and calcium oxide, the outlet of the digester (23) is connected to the inlet of the causticizer (24), and the causticizer (24)... The material is separated into solid and liquid. The separated solution is used as a buffer and enters the lactic acid synthesis unit. The separated solid enters the first drying chamber (26). The outlet of the first drying chamber (26) is connected to the inlet of the calcination chamber (27). The outlet of the calcination chamber (27) is connected to the inlet of the third cooler (28). The outlet of the third cooler (28) is connected to the inlet of the gas-solid separator (29). The solid outlet of the gas-solid separator (29) is connected to the inlet of the digester (23). The gas outlet of the gas-solid separator (29) outputs carbon dioxide.

5. The system for preparing polylactic acid via biochemical coupling conversion according to claim 1, characterized in that, The lactic acid purification unit includes an ion exchange device (22), a quadruple-effect evaporator (32), and a molecular distillation column (33). The crude lactic acid solution undergoes ion exchange through the ion exchange device (22), and the lactic acid solution after ion exchange enters the quadruple-effect evaporator (32) for evaporation and concentration. The concentrated liquid outlet of the quadruple-effect evaporator (32) is connected to the liquid inlet of the molecular distillation column (33), and the lactic acid solution from the molecular distillation column (33) enters the lactide synthesis unit.

6. A process for preparing polylactic acid through biochemical coupling conversion, characterized in that, include: Oxygen, methane feed gas, nutrients and buffer are mixed and aerobic fermentation is carried out. The fermentation products are subjected to gas-liquid separation and solid-liquid separation to obtain crude lactic acid solution, solid biomass and waste gas. The crude lactic acid solution was subjected to ion exchange, evaporation, and distillation to obtain a lactic acid solution. Lactic acid solution was reacted in a nitrogen atmosphere to obtain lactide; Polymerization of lactide in the presence of a catalyst yields polylactic acid; Solid biomass undergoes anaerobic fermentation to produce biogas, which is then mixed with waste gas for combustion. The combustion gas is used to generate electricity and heat through a gas turbine. The high-temperature gas from the gas turbine removes moisture and obtains carbon dioxide through adsorption or absorption. The eluent generated by ion exchange is reacted with calcium hydroxide. The reaction solution is separated into solid and liquid components. The buffer obtained from the separation is recycled. The calcium carbonate obtained from the separation is calcined. The calcination product is subjected to gas-solid separation to obtain carbon dioxide and calcium oxide. The calcium oxide reacts with water to generate calcium hydroxide, which is recycled.

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