Device and method for preparing crude lactide

Abstract

The invention belongs to the field of polymer preparation processes, and relates to a device and a method for preparing crude lactide, which can be industrially implemented, can be continuously operated and can be operated on a large scale. The device comprises a dehydration unit, a polycondensation unit and a depolymerization unit which are sequentially connected, the dehydration unit comprises a dehydration kettle, a dehydration kettle heater, at least one group of dehydration towers and a condenser thereof, the polycondensation unit comprises a polycondensation kettle and an optional pre-polycondensation kettle arranged at the upstream of the polycondensation kettle and / or an optional final polycondensation kettle arranged at the downstream of the polycondensation kettle, and the depolymerization unit comprises a pre-depolymerization kettle, a depolymerization kettle and a crude lactide condenser. The raw material lactic acid is subjected to dehydration separation, optional pre-polycondensation, polycondensation, optional final polycondensation, pre-depolymerization, depolymerization and the like to finally obtain a crude lactide product. Meanwhile, a small amount of non-condensable gas, depolymerization residues, water and other byproducts are produced. The device provided by the invention can realize industrial large-scale continuous production, and improves the yield of the target product through various strengthening means.

Description

Technical Field

[0001] The invention belongs to the field of polymer materials, and in particular, relates to the preparation of lactide, a degradable plastic monomer, and more particularly, to a device and method for preparing crude lactide. Background Art

[0002] White pollution affects human health and life and urgently needs to be prevented and controlled. It has become one of the major challenges facing mankind today. Degradable plastics that comply with the sustainable development strategy are of great significance to improving the ecological environment and enhancing people's quality of life. In 2020, the "Opinions on Further Strengthening Plastic Pollution Control" issued by the National Development and Reform Commission and the Ministry of Ecology and Environment further promoted the public's attention to biodegradable materials, such as PLA, PBAT, PHA, PGA, etc.

[0003] Polylactic acid or polylactide (PLA) is a bio-based degradable plastic, which is made from lactic acid fermented by microorganisms from biomass raw materials (cassava, beets, straw fiber, sugar cane, etc.) through chemical synthesis. After use, polylactic acid products can be quickly degraded into carbon dioxide and water under composting conditions without polluting the environment.

[0004] PLA can be widely used in medical polymer materials due to its good biocompatibility and degradability, such as surgical sutures, controlled drug release, bone fixation and repair, artificial skin and blood vessels, tissue engineering, etc. In addition, PLA is also used to prepare various biodegradable plastic products, such as films, fibers, packaging materials, etc. Due to its good biodegradable plasticity, PLA is predicted to have broad prospects and will be widely used.

[0005] Lactide is an intermediate monomer for producing PLA. In the prior art, there are two methods for preparing PLA. One is to directly dehydrate and polycondense lactic acid to obtain PLA, also known as the one-step method. The PLA obtained by this method has a low relative molecular mass, and is insufficient in mechanical strength, gas barrier properties, durability, etc., and its application is limited. The other is to first polycondense lactic acid to obtain lactic acid oligomers, and then depolymerize it to obtain cyclic lactide, and use lactide to perform ring-opening polymerization to obtain a polymer with a relative molecular mass of 10,000 to 200,000. This method is also called the two-step method. At present, the second method is usually used in the industry to prepare PLA with a higher molecular mass. An example of the two-step reaction is as follows:

[0006]

[0007] The continuous production of lactide is a key link in the PLA process. How to improve the yield of lactide and achieve continuous production is a technical difficulty.

[0008] CN115010696A discloses a method for continuous preparation and purification of lactide. In this method, lactic acid oligomers and polyether polyols are sent to a supergravity device for step-by-step reaction to obtain crude lactide vapor; the obtained crude lactide vapor is transported to a distillation tower for distillation to obtain a distillate, and the distillate is cooled and crystallized to obtain lactide crystals. This technology introduces supergravity equipment to replace the original kettle reaction device, thereby increasing the reaction rate, and then separating and purifying lactide by distillation. This technical process does not involve the preparation of lactic acid oligomers, and large-scale production is limited. In addition, if a solvent is added, the removal of the third monomer will be involved, which will increase the difficulty of the lactide purification process and increase the complexity of the process.

[0009] CN113582965B discloses a method for preparing lactide based on an organic guanidine complex. The invention relates to a method for preparing high-purity lactide based on catalytic cracking of an organic guanidine complex in the field of polymer material production. It includes the preparation of organic guanidine, the polycondensation of lactic acid to obtain oligomers, the depolymerization of the organic guanidine complex as a depolymerization catalyst to generate crude lactide, and the purification of lactide. The technology does not involve a specific process flow, and it is difficult to implement it industrially.

[0010] CN115028615A discloses a patent for L,L-lactide and its preparation method and application. The patent dehydrates lactic acid, and then prepolymerizes it in the presence of a water absorbent and a solvent, removes the water absorbent, and depolymerizes it to obtain L,L-lactide. However, the method disclosed in the patent is only an intermittent process at the laboratory or small test level, and cannot be mass-produced continuously, and its industrial application is limited.

[0011] CN107438602B discloses a method for producing lactide. The method comprises: forming a solution from lactic acid and a volatile solvent, removing the volatile organic solvent and water from the solution through an evaporation step, and forming lactic acid oligomers, and adding a catalyst for depolymerization to obtain lactide. However, this method causes a third solvent to be mixed in the crude lactide product, which increases the difficulty of the lactide purification process and the complexity of the process. Summary of the invention

[0012] The purpose of the present invention is to provide an industrially feasible, continuously operated, and large-scale lactide production process in view of the deficiencies of the prior art.

[0013] In order to achieve the above-mentioned object, the first aspect of the present invention provides a device for preparing crude lactide, which comprises a dehydration unit, a polycondensation unit and a depolymerization unit connected in sequence; the dehydration unit comprises a dehydration kettle, a dehydration kettle heater, at least one set of dehydration towers and a condenser thereof; the polycondensation unit comprises a polycondensation kettle, and optionally a pre-polycondensation kettle disposed upstream of the polycondensation kettle and / or a final polycondensation kettle disposed downstream of the polycondensation kettle; the depolymerization unit comprises a pre-depolymerization kettle, a depolymerization kettle, and a crude lactide condenser; wherein,

[0014] The dehydration kettle heater is connected to the bottom of the dehydration kettle;

[0015] The top of the dehydration kettle is connected to the dehydration tower, and the bottom is connected to the polycondensation unit;

[0016] The top of one of the at least one group of dehydration towers and their condensers is connected to the dehydration tower condenser, and the bottom is connected to the upper part of the dehydration kettle;

[0017] The polycondensation unit is provided with a gas phase outlet and a liquid phase outlet, and the liquid phase outlet of the polycondensation unit is connected to the pre-depolymerization kettle;

[0018] The top of the pre-depolymerization kettle is connected to the crude lactide condenser, and the bottom is connected to the depolymerization kettle;

[0019] The top of the depolymerization kettle is connected to the crude lactide condenser;

[0020] The lactic acid feed pipeline is connected to the dehydration unit; the catalyst feed pipeline is connected to the dehydration unit or the depolymerization unit.

[0021] A second aspect of the present invention provides a method for preparing crude lactide, using the above-mentioned device, comprising the following steps:

[0022] The lactic acid raw material enters the device through a lactic acid feed pipeline, and the catalyst enters the device through a catalyst feed pipeline. The lactic acid raw material is sequentially subjected to dehydration separation, optional pre-polycondensation, polycondensation, optional final polycondensation, pre-depolymerization, and depolymerization to obtain a crude lactide product.

[0023] The beneficial effects of the present invention include:

[0024] 1) The present invention realizes the preparation of crude lactide through a graded dehydration, polycondensation and depolymerization system.

[0025] 2) The present invention forms oligomers through one-stage or multi-stage polymerization such as pre-condensation, polycondensation and final polymerization with gradual temperature increase and pressure reduction. The pre-depolymerization and depolymerization ensure the steady progress of the reaction and reduce the generation of side reactions.

[0026] 3) The present invention improves process intensification by setting up a dehydration system, thereby ensuring the dehydration residence time and heat transfer.

[0027] 4) The present invention strengthens the polymerization reaction through the reactor of the polycondensation kettle, ensuring that the polymerization reaction reaches the required degree of polymerization.

[0028] 5) The present invention ensures that the target product lactide is generated by maximizing depolymerization, improving the product yield and reducing the generation of residues through the reactor and process settings of one-stage, two-stage or three-stage polymerization and two-stage depolymerization.

[0029] Other features and advantages of the present invention will be described in detail in the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The above and other objects, features and advantages of the present invention will become more apparent through a more detailed description of exemplary embodiments of the present invention in conjunction with the accompanying drawings, wherein like reference numerals generally represent like components throughout the exemplary embodiments of the present invention.

[0031] Figure 1 The present invention is a flow chart of the first device and method for preparing crude lactide.

[0032] Figure 2 The present invention is a flow chart of a second device and method for preparing crude lactide.

[0033] Figure 3 The present invention is a flow chart of a third device and method for preparing crude lactide.

[0034] Figure 4 This is a flow chart of the fourth device and method for preparing crude lactide according to the present invention.

[0035] Description of reference numerals:

[0036] Dehydration kettle 1, dehydration tower 2, dehydration kettle heater 3, dehydration tower condenser 4, pre-polycondensation kettle 5A, polycondensation kettle 5, pre-polycondensation dehydration tower 6A, polycondensation dehydration tower 6, pre-polycondensation dehydration tower condenser 7A, polycondensation dehydration tower condenser 7, pre-polycondensation kettle heater 8A, polycondensation kettle heater 8, final polycondensation kettle 9, pre-depolymerization kettle 10, crude lactide condenser 11, depolymerization kettle 12, catalyst 13, lactic acid raw material 14, dehydrated non-condensable gas 15, water 16, dehydrated non-condensable gas 15A, water 16A, dehydrated non-condensable gas 15B, water 16B, polycondensation non-condensable gas 17, depolymerization residue 18, crude lactide product 19, depolymerization non-condensable gas mixture 20. DETAILED DESCRIPTION

[0037] The preferred embodiments of the present invention will be described in more detail below. Although the preferred embodiments of the present invention are described below, it should be understood that the present invention can be implemented in various forms and should not be limited to the embodiments set forth herein.

[0038] The present invention provides a device for preparing crude lactide, which comprises a dehydration unit, a polycondensation unit and a depolymerization unit connected in sequence; the dehydration unit comprises a dehydration kettle, a dehydration kettle heater, at least one set of dehydration towers and a condenser thereof; the polycondensation unit comprises a polycondensation kettle, and optionally a pre-polycondensation kettle arranged upstream of the polycondensation kettle and / or a final polycondensation kettle arranged downstream of the polycondensation kettle; the depolymerization unit comprises a pre-depolymerization kettle, a depolymerization kettle and a crude lactide condenser; wherein,

[0039] The dehydration kettle heater is connected to the bottom of the dehydration kettle;

[0040] The top of the dehydration kettle is connected to the dehydration tower, and the bottom is connected to the polycondensation unit;

[0041] The top of one of the at least one group of dehydration towers and their condensers is connected to the dehydration tower condenser, and the bottom is connected to the upper part of the dehydration kettle;

[0042] The polycondensation unit is provided with a gas phase outlet and a liquid phase outlet, and the liquid phase outlet of the polycondensation unit is connected to the pre-depolymerization kettle;

[0043] The top of the pre-depolymerization kettle is connected to the crude lactide condenser, and the bottom is connected to the depolymerization kettle;

[0044] The top of the depolymerization kettle is connected to the crude lactide condenser;

[0045] The lactic acid feed pipeline is connected to the dehydration unit; the catalyst feed pipeline is connected to the dehydration unit or the depolymerization unit.

[0046] According to the present invention, the pre-polycondensation kettle and the final polycondensation kettle are both optional, and the optional meaning is that they can be set or not, and either one of them can be set or both can be set. When a pre-polycondensation kettle is set, the pre-polycondensation kettle is set upstream of the polycondensation kettle, and when a final polycondensation kettle is set, the final polycondensation kettle is set downstream of the polycondensation kettle.

[0047] In the present invention, the dehydration tower and the dehydration tower condenser are arranged as a set, which can be one group or multiple groups. When the at least one group of dehydration towers and their condensers are one group, the group of dehydration towers and their condensers are connected to the top of the dehydration kettle and the top of the polycondensation kettle at the same time (hereinafter referred to as the dehydration tower and the dehydration tower condenser at the top of the dehydration kettle). When there are multiple groups, the multiple groups of dehydration towers and their condensers are respectively connected to the top of the dehydration kettle, the top of the polycondensation kettle and the top of the optional pre-polycondensation kettle. When a pre-polycondensation kettle is provided, the pre-polycondensation kettle can be connected to an independent dehydration tower and its condenser, or can share the dehydration tower and its condenser at the top of the dehydration kettle.

[0048] According to the number of kettles used for polycondensation in the polycondensation unit, the polycondensation unit can be arranged in any of the following schemes:

[0049] (i) the polycondensation unit comprises a polycondensation kettle, a polycondensation dehydration tower and a polycondensation dehydration tower condenser; the bottom of the dehydration kettle is connected to the polycondensation kettle, and the bottom of the polycondensation kettle is connected to the inlet of the pre-depolymerization kettle;

[0050] (ii) the polycondensation unit comprises a polycondensation kettle and a final polycondensation kettle; the bottom of the dehydration kettle is connected to the polycondensation kettle, the bottom of the polycondensation kettle is connected to the inlet of the final polycondensation kettle, and the bottom of the final polycondensation kettle is connected to the inlet of the pre-depolymerization kettle; the polycondensation kettle and the dehydration kettle share a set of dehydration towers and their condensers;

[0051] (iii) the polycondensation unit comprises a polycondensation kettle, a polycondensation dehydration tower, a polycondensation dehydration tower condenser, and a final polycondensation kettle; the bottom of the dehydration kettle is connected to the polycondensation kettle, the bottom of the polycondensation kettle is connected to the inlet of the final polycondensation kettle, and the bottom of the final polycondensation kettle is connected to the inlet of the pre-depolymerization kettle;

[0052] (iv) The polycondensation unit comprises a pre-polycondensation kettle, a pre-polycondensation dehydration tower and a pre-polycondensation dehydration tower condenser, a polycondensation kettle, a polycondensation dehydration tower and a polycondensation dehydration tower condenser, and a final polycondensation kettle; the bottom of the dehydration kettle is connected to the pre-polycondensation kettle, the bottom of the pre-polycondensation kettle is connected to the inlet of the polycondensation kettle, the bottom of the polycondensation kettle is connected to the inlet of the final polycondensation kettle, and the bottom of the final polycondensation kettle is connected to the inlet of the pre-depolymerization kettle.

[0053] According to a preferred embodiment of the present invention, the bottom discharge pipeline of the dehydration kettle is divided into two branches, one branch is connected to the polycondensation unit, and the other branch is connected to the inlet of the dehydration kettle heater.

[0054] According to a preferred embodiment of the present invention, the lactic acid feed pipeline is connected to the dehydration unit, which means that it is connected to the dehydration kettle heater and / or the dehydration kettle.

[0055] In the device of the present invention, the catalyst can enter from the dehydration unit or from the depolymerization unit. Accordingly, when entering from the dehydration unit, the catalyst feed pipeline can be connected to the dehydration kettle heater or the dehydration kettle, and when entering from the depolymerization unit, the catalyst feed pipeline is connected to the pre-depolymerization kettle.

[0056] According to a preferred embodiment of the present invention, the dehydration kettle heater is one of a shell and tube heat exchanger, a plate heat exchanger and an electric heater.

[0057] According to a preferred embodiment of the present invention, the internal parts of the dehydration tower are one or more of a sieve plate, a float valve, a filler and a bubble cap, preferably a filler. When multiple dehydration towers are provided, the internal parts of each dehydration tower may be the same or different.

[0058] According to the present invention, preferably, the dehydration tower condenser is provided with a gas phase non-condensable gas discharge pipeline and a liquid phase water discharge pipeline; the water discharge pipeline is divided into two branches, one is connected to the top of the dehydration tower (corresponding to it), and the other is used as an external discharge pipeline.

[0059] According to the present invention, when a precondensation kettle is provided, the precondensation kettle is provided with a gas phase precondensation non-condensable gas discharge pipeline and a liquid phase precondensation product extraction pipeline; the precondensation kettle is connected to a dehydration tower (precondensation dehydration tower) through the precondensation non-condensable gas discharge pipeline, and is connected to the condensation kettle through the precondensation product extraction pipeline.

[0060] According to a preferred embodiment of the present invention, the precondensation reactor is a vertical or horizontal stirred reactor with a heater disposed inside or a heater and / or a jacket disposed outside, preferably a vertical stirred reactor with a heater disposed outside.

[0061] According to the present invention, preferably, the polycondensation kettle is provided with a gas phase polycondensation non-condensable gas discharge pipeline and a liquid phase polycondensation product extraction pipeline; the polycondensation kettle is connected to a dehydration tower (an independently arranged polycondensation dehydration tower or a dehydration tower shared with a dehydration kettle) through the polycondensation non-condensable gas discharge pipeline, and is connected to the final polycondensation kettle (when set) or the pre-depolymerization kettle through the polycondensation product extraction pipeline.

[0062] According to a preferred embodiment of the present invention, the polycondensation reactor is a vertical stirred tank reactor or a horizontal shaft stirred reactor, preferably a single-shaft or multi-shaft kneading reactor, a screw reactor, a surface renewal reactor, a tubular reactor, a tower reactor, more preferably a horizontal single-shaft or multi-shaft kneading reactor; wherein the surface renewal reactor is preferably a disc reactor or a squirrel cage reactor.

[0063] According to the present invention, when a final polycondensation kettle is set up, the final polycondensation kettle is provided with a gas phase final polycondensation non-condensable gas discharge pipeline and a liquid phase final polycondensation product extraction pipeline; the final polycondensation kettle is connected to the pre-depolymerization kettle through the final polycondensation product extraction pipeline.

[0064] According to a preferred embodiment of the present invention, the final polycondensation reactor is a horizontal shafted stirred reactor, preferably a single-shaft or multi-shaft kneading reactor, a screw reactor, a surface renewal reactor, a tubular reactor, a tower reactor, more preferably a horizontal single-shaft or multi-shaft kneading reactor; wherein the surface renewal reactor is preferably a disc reactor or a squirrel cage reactor.

[0065] According to the present invention, preferably, the pre-depolymerization kettle is provided with a gas phase pre-depolymerization non-condensable gas discharge pipeline and a liquid phase pre-depolymerization product extraction pipeline; the pre-depolymerization kettle is connected to the crude lactide condenser through the pre-depolymerization non-condensable gas discharge pipeline, and is connected to the depolymerization kettle through the pre-depolymerization product extraction pipeline.

[0066] According to a preferred embodiment of the present invention, the pre-depolymerization kettle is a horizontal or vertical evaporator with shaft stirring, with a heater provided inside or a heater and / or a jacket provided outside, preferably a thin film evaporator.

[0067] According to a preferred embodiment of the present invention, the depolymerization reactor is a vertical or horizontal tank reactor with shaft stirring, a screw reactor or a kneading reactor, preferably a single-shaft or multi-shaft kneading reactor.

[0068] According to the present invention, preferably, the depolymerization kettle is provided with a gas phase depolymerization non-condensable gas discharge pipeline and a solid-liquid phase depolymerization residue discharge pipeline, and the depolymerization kettle is connected to the crude lactide condenser via the depolymerization non-condensable gas discharge pipeline.

[0069] According to the present invention, preferably, the crude lactide condenser is provided with a crude lactide product extraction pipeline and a depolymerized non-condensable gas mixture discharge pipeline.

[0070] The present invention also provides a method for preparing crude lactide, using the above-mentioned device, comprising the following steps:

[0071] The lactic acid raw material enters the device through the lactic acid feed pipeline, and the catalyst enters the device through the catalyst feed pipeline. The lactic acid raw material is sequentially subjected to dehydration separation, optional pre-polycondensation, polycondensation, optional final polycondensation, pre-depolymerization, and depolymerization to obtain a crude lactide product. At the same time, non-condensable gas, water alcohol and other by-products are obtained at the top of the dehydration tower and the top of the (pre- and final) polycondensation kettle, and depolymerization residue by-products containing impurities such as coke and polymers are obtained at the bottom of the depolymerization kettle.

[0072] As mentioned above, the optional pre-polycondensation means that the pre-polycondensation may or may not be carried out, and the optional final polycondensation means that the final polycondensation may or may not be carried out, all depending on the device settings.

[0073] According to a preferred embodiment of the present invention, the method comprises the following steps:

[0074] (a) The water-containing lactic acid raw material enters the dehydration kettle heater inlet and / or the dehydration kettle inlet through the lactic acid feed pipeline, and the catalyst enters the dehydration kettle heater inlet or the dehydration kettle inlet or the pre-depolymerization kettle inlet through the catalyst feed pipeline; the dehydration kettle is heated at normal pressure or reduced pressure to evaporate free water and polymerization water, and the evaporated components enter the dehydration tower for reflux separation, the dehydration tower top stream enters the dehydration tower condenser to obtain liquid water and dehydrated non-condensable gas, and the tower bottom stream returns to the dehydration kettle; the dehydration kettle bottom material is divided into two streams, one stream enters the dehydration kettle heater, and the other stream is extracted and enters the downstream polycondensation unit;

[0075] (b) the polycondensation unit receives the material at the bottom of the dehydration kettle, undergoes a polycondensation reaction of lactic acid, and produces lactic acid oligomers; the step (b) is selected from one of the following forms:

[0076] (b-1) The polycondensation unit comprises a polycondensation kettle, which receives the material at the bottom of the dehydration kettle and undergoes a dehydration polycondensation reaction under high temperature and high vacuum operation to obtain lactic acid oligomers, a small amount of polycondensation non-condensable gas and water. The generated water is discharged through a separate dehydration tower or combined into a dehydration tower at the top of the dehydration kettle for discharge;

[0077] (b-2) The polycondensation unit includes a polycondensation kettle and a final polycondensation kettle. The polycondensation kettle receives the material at the bottom of the dehydration kettle and undergoes a dehydration polycondensation reaction under high temperature and low pressure operation. The generated water is discharged through a separate dehydration tower or is combined and discharged into a dehydration tower at the top of the dehydration kettle;

[0078] The final polycondensation kettle receives the material at the bottom of the polycondensation kettle, and continues the polycondensation reaction through high temperature and high vacuum operation to obtain lactic acid oligomers, a small amount of polycondensation non-condensable gas and water;

[0079] (b-3) The polycondensation unit includes a pre-polycondensation kettle, a polycondensation kettle and a final polycondensation kettle. The pre-polycondensation kettle receives the material at the bottom of the dehydration kettle and undergoes a dehydration pre-polycondensation reaction under high temperature and low pressure operation. The generated water is discharged through a separate dehydration tower or is combined and discharged into a dehydration tower at the top of the dehydration kettle;

[0080] The polycondensation kettle receives the material at the bottom of the pre-polycondensation kettle, and further undergoes the polycondensation reaction through high temperature and high vacuum operation. The generated water and non-condensable gas are discharged through the dehydration tower, and the lactic acid oligomers with increased molecular weight are further obtained and enter the final polycondensation kettle;

[0081] The final polycondensation kettle receives the material at the bottom of the polycondensation kettle, and continues the polycondensation reaction at a higher temperature and high vacuum than the polycondensation kettle to obtain lactic acid oligomers with further increased molecular weight, polycondensation non-condensable gas and water;

[0082] (c) the liquid phase at the bottom of the polycondensation unit enters the pre-depolymerization kettle, and undergoes high temperature and high vacuum operation to cause a depolymerization and cyclization reaction of the lactic acid oligomers to generate a lactide-containing stream, which enters the crude lactide condenser through the top of the pre-depolymerization kettle, and the unreacted lactic acid oligomers in the pre-depolymerization kettle enter the depolymerization kettle from the bottom of the pre-depolymerization kettle;

[0083] (d) the depolymerization kettle is operated at a higher temperature and a higher vacuum than the pre-depolymerization kettle, and the depolymerization and cyclization reaction of the lactic acid oligomers continues to occur, and the unreacted lactic acid oligomers in step (c) are converted into lactide, which is discharged from the gas phase pipeline at the top of the depolymerization kettle and then enters the crude lactide condenser, and at the same time, a depolymerization residue by-product containing coke and polymer is produced and discharged from the bottom of the depolymerization kettle;

[0084] (e) The crude lactide condenser condenses the streams from the top of the pre-depolymerization reactor and the top of the depolymerization reactor to obtain a depolymerization non-condensable gas mixture and a crude lactide product.

[0085] According to the method of the present invention, the catalyst can be at least one of tin halides, tin organic acid compounds, antimony oxides, antimony halides, antimony organic acid compounds, zinc oxides, zinc halides and zinc organic acid compounds; preferably, tin organic acid compounds.

[0086] The amount of the catalyst used can be 0.001 to 5% by weight of the amount of lactic acid used.

[0087] According to the method of the present invention, preferably, the number of theoretical plates of the dehydration tower is 2 to 20, the operating pressure at the top of the tower is 50 to 130 kPaA, and the operating temperature at the top of the tower is 30 to 100°C.

[0088] According to the method of the present invention, preferably, the operating temperature of the dehydration kettle is 70-140° C., and the operating pressure is 50-150 kPaA.

[0089] According to the method of the present invention, preferably, the operating temperature of the pre-polycondensation reactor is 120-150° C., and the operating pressure is 10-50 kPaA.

[0090] According to the method of the present invention, preferably, the theoretical plate number of the pre-condensation dehydration tower is 2 to 20, the tower top operating pressure is 10 to 150 kPaA, and the tower top operating temperature is 30 to 80°C.

[0091] According to the method of the present invention, preferably, the operating temperature of the polycondensation reactor is 140-180° C., and the operating pressure is 5-50 kPaA.

[0092] According to the method of the present invention, preferably, the theoretical plate number of the polycondensation dehydration tower is 2 to 20, the tower top operating pressure is 5 to 50 kPaA, and the tower top operating temperature is 30 to 80°C.

[0093] According to the method of the present invention, preferably, the operating temperature of the final polycondensation reactor is 150-190°C, and the operating pressure is 1-20 kPaA

[0094] According to the method of the present invention, preferably, the operating temperature of the pre-depolymerization reactor is 160-210° C., and the operating pressure is 100 PaA-10 kPaA.

[0095] According to the method of the present invention, preferably, the operating temperature of the depolymerization reactor is 180-250° C., and the operating pressure is 100 PaA-10 kPaA.

[0096] According to the method of the present invention, the dehydration kettle mainly removes most of the free water in the lactic acid raw material, and the lactic acid in the dehydration kettle also undergoes a trace amount of prepolymerization to generate lactic acid dimers, trimers, etc., and a trace amount of prepolymer enters the downstream polycondensation unit with the lactic acid raw material.

[0097] In the pre-condensation kettle, the polycondensation reaction continues to take place to remove a small amount of free water and water in the lactic acid. The free water and water produced by the polycondensation are discharged through a separate dehydration tower or combined into a dehydration tower at the top of the dehydration kettle for discharge.

[0098] In the polycondensation kettle, a polycondensation reaction is carried out to polymerize the lactic acid (when a pre-polycondensation kettle is not provided), or a polycondensation reaction is continued to polymerize the remaining lactic acid (when a pre-polycondensation kettle is provided), and a polymerization reaction is carried out to increase the molecular weight, that is, to increase the molecular weight of the lactic acid polycondensate to obtain a lactic acid oligomer.

[0099] In the final polycondensation reactor, the lactic acid oligomers are further condensed to obtain lactic acid oligomers with further increased molecular weight.

[0100] The pre-depolymerization reactor and the depolymerization reactor are used to carry out the depolymerization reaction of lactic acid oligomers to obtain crude lactide and produce coke, polymers and the like as by-products.

[0101] In the present invention, the term "lactic acid" refers to lactic acid prepared by biological method, that is, fermented lactic acid obtained from biological fermentation, preferably L-lactic acid, D-lactic acid, (L / D)-lactic acid or a mixture thereof in any proportion.

[0102] The present invention is further described below by way of examples.

[0103] Example 1

[0104] Use Figure 1 The device for preparing crude lactide shown in the figure comprises a dehydration unit, a polycondensation unit and a depolymerization unit connected in sequence; the dehydration unit comprises a dehydration kettle 1, a dehydration kettle heater 3, a dehydration tower 2, and a dehydration tower condenser 4; the polycondensation unit comprises a polycondensation kettle 5 and a final polycondensation kettle 9 arranged downstream thereof; the depolymerization unit comprises a pre-depolymerization kettle 10, a depolymerization kettle 12, and a crude lactide condenser 11; wherein,

[0105] The dehydration kettle heater 3 is a shell and tube heat exchanger connected to the bottom of the dehydration kettle 1;

[0106] The top of the dehydration kettle 1 is connected to the dehydration tower 2, and the bottom discharge pipeline is divided into two branches, one is connected to the polycondensation kettle 5, and the other is connected to the inlet of the dehydration kettle heater 3;

[0107] The top of the dehydration tower 2 is connected to the dehydration tower condenser 4, and the bottom is connected to the upper part of the dehydration kettle 1; the internal parts of the dehydration tower 2 are fillers;

[0108] The dehydration tower condenser 4 is provided with a gas phase non-condensable gas discharge pipeline and a liquid phase water discharge pipeline; the water discharge pipeline is divided into two branches, one is connected to the top of the dehydration tower 2, and the other is used as an external discharge pipeline;

[0109] The polycondensation kettle 5 is a vertical stirring kettle with a polycondensation kettle heater 8 provided outside, and is provided with a gas phase polycondensation non-condensable gas discharge pipeline and a liquid phase polycondensation product extraction pipeline; the polycondensation kettle 5 is connected to the dehydration tower 2 through the polycondensation non-condensable gas discharge pipeline, and is connected to the inlet of the final polycondensation kettle 9 through the polycondensation product extraction pipeline;

[0110] The final polycondensation kettle 9 is a horizontal single-shaft kneading reactor, which is provided with a gas phase final polycondensation non-condensable gas discharge pipeline and a liquid phase final polycondensation product extraction pipeline; the final polycondensation kettle 9 is connected to the pre-depolymerization kettle 10 through the final polycondensation product extraction pipeline;

[0111] The pre-depolymerization kettle 10 is a thin film evaporator, provided with a gas phase pre-depolymerization non-condensable gas discharge pipeline and a liquid phase pre-depolymerization product extraction pipeline; the pre-depolymerization kettle 10 is connected to the crude lactide condenser 11 through the pre-depolymerization non-condensable gas discharge pipeline, and is connected to the depolymerization kettle 12 through the pre-depolymerization product extraction pipeline;

[0112] The depolymerization kettle 12 is a uniaxial kneading reactor, provided with a gas phase depolymerization non-condensable gas discharge pipeline and a solid-liquid phase depolymerization residue discharge pipeline, and the depolymerization kettle 12 is connected to the crude lactide condenser 11 through the depolymerization non-condensable gas discharge pipeline;

[0113] The crude lactide condenser 11 is provided with a crude lactide product extraction pipeline and a depolymerized non-condensable gas mixture discharge pipeline;

[0114] The lactic acid feed pipeline and the catalyst feed pipeline are both connected to the dehydration kettle heater 3 .

[0115] The method for preparing crude lactide using the above-mentioned device comprises:

[0116] (a) The water-containing lactic acid raw material 14 enters the inlet of the dehydration kettle heater 3 through the lactic acid feed pipeline, and the catalyst 13 enters the inlet of the dehydration kettle heater 3 through the catalyst feed pipeline; the dehydration kettle 1 is heated at normal pressure or reduced pressure to evaporate free water and polymerization water, and the evaporated components enter the dehydration tower 2 for reflux separation, and the top stream of the dehydration tower 2 enters the dehydration tower condenser 4 to obtain liquid water 16 and dehydrated non-condensable gas 15, and the bottom stream returns to the dehydration kettle 1; the bottom material of the dehydration kettle 1 is divided into two streams, one stream enters the dehydration kettle heater 3, and the other stream is extracted and enters the polycondensation kettle 5;

[0117] (b) The polycondensation reactor 5 receives the material at the bottom of the dehydration reactor 1, and undergoes a dehydration polycondensation reaction through high temperature and low pressure operation. The generated water enters the dehydration tower 2 at the top of the dehydration reactor 1 and is discharged;

[0118] The final polycondensation reactor 9 receives the material at the bottom of the polycondensation reactor 5, and continues the polycondensation reaction through high temperature and high vacuum operation to obtain a small amount of polycondensation non-condensable gas, water and lactic acid oligomers, and the polycondensation non-condensable gas is discharged through the polycondensation non-condensable gas discharge pipeline;

[0119] (c) the liquid phase at the bottom of the final polycondensation reactor 9 containing lactic acid oligomers enters the pre-depolymerization reactor 10, and undergoes high temperature and high vacuum operation to cause a depolymerization and cyclization reaction of the lactic acid oligomers to generate a lactide-containing stream, which enters a crude lactide condenser 11 through the top of the pre-depolymerization reactor 10, and the unreacted lactic acid oligomers in the pre-depolymerization reactor 10 enter a depolymerization reactor 12 from the bottom of the pre-depolymerization reactor 10;

[0120] (d) the depolymerization reactor 12 is operated at a higher temperature and a higher vacuum than the pre-depolymerization reactor 10, and the depolymerization and cyclization reaction of the lactic acid oligomers continues to occur, and the unreacted lactic acid oligomers in step (c) are converted into lactide, which is discharged from the gas phase pipeline at the top of the depolymerization reactor and then enters the crude lactide condenser 11, and at the same time, a depolymerization residue 18 containing coke and polymers is produced and discharged from the bottom of the depolymerization reactor 12;

[0121] (e) The crude lactide condenser 11 condenses the streams from the top of the pre-depolymerization reactor 10 and the top of the depolymerization reactor 12 to obtain a depolymerized non-condensable gas mixture 20 and a crude lactide product 19.

[0122] The lactic acid raw material 14 is L-lactic acid, which comes from outside the world.

[0123] The catalyst 13 is an organic acid compound of tin, and the amount used is 0.1%wt of the lactic acid.

[0124] The dehydration tower 2 has a theoretical plate number of 10, a tower top operating pressure of 100 kPaA, and a tower top operating temperature of 100°C.

[0125] The operating temperature of the dehydration kettle 1 is 120° C. and the operating pressure is 110 kPaA.

[0126] The operating temperature of the polycondensation reactor 5 is 160° C., and the operating pressure is 50 kPaA.

[0127] The polycondensation dehydration tower 6 has a theoretical plate number of 10, a tower top operating pressure of 8 kPaA, and a tower top operating temperature of 60°C.

[0128] The operating temperature of the final polycondensation reactor 9 is 190° C., and the operating pressure is 20 kPaA.

[0129] The operating temperature of the pre-depolymerization reactor 10 is 210° C., and the operating pressure is 5 kPaA.

[0130] The operating temperature of the depolymerization reactor 12 is 230° C. and the operating pressure is 1 kPaA.

[0131] Through the above device and method, the utilization rate of lactic acid is 95% and the yield of lactide is 85%.

[0132] The calculation formula for the utilization rate of lactic acid is: (lactic acid feed molar flow rate-residual lactic acid molar flow rate) / lactic acid feed molar flow rate.

[0133] The calculation formula of the lactide yield is: lactide molar flow rate / (lactic acid feed molar flow rate-residual lactic acid molar flow rate).

[0134] Example 2

[0135] use Figure 2 The device for preparing crude lactide shown in the figure comprises a dehydration unit, a polycondensation unit and a depolymerization unit connected in sequence; the dehydration unit comprises a dehydration kettle 1, a dehydration kettle heater 3, a dehydration tower 2, a dehydration tower condenser 4, a polycondensation dehydration tower 6, and a polycondensation dehydration tower condenser 7; the polycondensation unit comprises a polycondensation kettle 5 and a final polycondensation kettle 9 arranged downstream thereof; the depolymerization unit comprises a pre-depolymerization kettle 10, a depolymerization kettle 12, and a crude lactide condenser 11; wherein,

[0136] The dehydration kettle heater 3 is a shell and tube heat exchanger connected to the bottom of the dehydration kettle 1;

[0137] The top of the dehydration kettle 1 is connected to the dehydration tower 2, and the bottom discharge pipeline is divided into two branches, one is connected to the polycondensation kettle 5, and the other is connected to the inlet of the dehydration kettle heater 3;

[0138] The top of the dehydration tower 2 is connected to the dehydration tower condenser 4, and the bottom is connected to the upper part of the dehydration kettle 1; the internal parts of the dehydration tower 2 are fillers;

[0139] The dehydration tower condenser 4 is provided with a gas phase non-condensable gas discharge pipeline and a liquid phase water discharge pipeline; the water discharge pipeline is divided into two branches, one is connected to the top of the dehydration tower 2, and the other is used as an external discharge pipeline;

[0140] The polycondensation kettle 5 is a vertical stirring kettle with a polycondensation kettle heater 8 disposed outside, and is provided with a gas phase polycondensation non-condensable gas discharge pipeline and a liquid phase polycondensation product extraction pipeline; the polycondensation kettle 5 is connected to the polycondensation kettle dehydration tower 6 through the polycondensation non-condensable gas discharge pipeline, and is connected to the inlet of the final polycondensation kettle 9 through the polycondensation product extraction pipeline;

[0141] The top of the polycondensation kettle dehydration tower 6 is connected to the polycondensation kettle dehydration tower condenser 7, and the bottom is connected to the upper part of the polycondensation kettle dehydration tower 6; the internal parts of the polycondensation kettle dehydration tower 2 are fillers;

[0142] The polycondensation kettle dehydration tower condenser 7 is provided with a gas phase non-condensable gas discharge pipeline and a liquid phase water discharge pipeline; the water discharge pipeline is divided into two branches, one is connected to the top of the polycondensation kettle dehydration tower 6, and the other is used as an external discharge pipeline;

[0143] The final polycondensation kettle 9 is a horizontal single-shaft kneading reactor, which is provided with a gas phase final polycondensation non-condensable gas discharge pipeline and a liquid phase final polycondensation product extraction pipeline; the final polycondensation kettle 9 is connected to the pre-depolymerization kettle 10 through the final polycondensation product extraction pipeline;

[0144] The pre-depolymerization kettle 10 is a thin film evaporator, provided with a gas phase pre-depolymerization non-condensable gas discharge pipeline and a liquid phase pre-depolymerization product extraction pipeline; the pre-depolymerization kettle 10 is connected to the crude lactide condenser 11 through the pre-depolymerization non-condensable gas discharge pipeline, and is connected to the depolymerization kettle 12 through the pre-depolymerization product extraction pipeline;

[0145] The depolymerization kettle 12 is a uniaxial kneading reactor, provided with a gas phase depolymerization non-condensable gas discharge pipeline and a solid-liquid phase depolymerization residue discharge pipeline, and the depolymerization kettle 12 is connected to the crude lactide condenser 11 through the depolymerization non-condensable gas discharge pipeline;

[0146] The crude lactide condenser 11 is provided with a crude lactide product extraction pipeline and a depolymerized non-condensable gas mixture discharge pipeline;

[0147] The lactic acid feed line is connected to the inlet of the dehydration kettle heater 3; the catalyst feed line is connected to the inlet of the pre-depolymerization kettle 10 (which can be merged with the polycondensation product extraction line).

[0148] The method for preparing crude lactide using the above-mentioned device comprises:

[0149] (a) A water-containing lactic acid raw material 14 enters the inlet of a dehydration kettle heater 3 through a lactic acid feed pipeline, and a catalyst 13 enters the inlet of a pre-depolymerization kettle 10 through a catalyst feed pipeline; the dehydration kettle 1 is heated at normal pressure or reduced pressure to evaporate free water and polymerization water, and the evaporated components enter a dehydration tower 2 for reflux separation, and the top stream of the dehydration tower 2 enters a dehydration tower condenser 4 to obtain liquid water 16 and dehydrated non-condensable gas 15, and the bottom stream returns to the dehydration kettle 1; the bottom material of the dehydration kettle 1 is divided into two streams, one of which enters the dehydration kettle heater 3, and the other is extracted and enters the polycondensation kettle 5;

[0150] (b) The polycondensation kettle 5 receives the bottom material of the dehydration kettle 1, and undergoes a dehydration polycondensation reaction through high temperature and low pressure operation. The generated water enters the polycondensation dehydration tower 6 for reflux separation. The top stream of the polycondensation dehydration tower 6 enters the polycondensation dehydration tower condenser 7 to obtain liquid water 16B and dehydrated non-condensable gas 15B, and the bottom stream returns to the polycondensation kettle 5;

[0151] The final polycondensation reactor 9 receives the material at the bottom of the polycondensation reactor 5, and continues the polycondensation reaction through high temperature and high vacuum operation to obtain a small amount of polycondensation non-condensable gas, water and lactic acid oligomers, and the polycondensation non-condensable gas is discharged through the polycondensation non-condensable gas discharge pipeline;

[0152] (c) the liquid phase at the bottom of the final polycondensation reactor 9 containing lactic acid oligomers enters the pre-depolymerization reactor 10, and undergoes high temperature and high vacuum operation to cause a depolymerization and cyclization reaction of the lactic acid oligomers to generate a lactide-containing stream, which enters a crude lactide condenser 11 through the top of the pre-depolymerization reactor 10, and the unreacted lactic acid oligomers in the pre-depolymerization reactor 10 enter a depolymerization reactor 12 from the bottom of the pre-depolymerization reactor 10;

[0153] (d) the depolymerization reactor 12 is operated at a higher temperature and a higher vacuum than the pre-depolymerization reactor 10, and the depolymerization and cyclization reaction of the lactic acid oligomers continues to occur, and the unreacted lactic acid oligomers in step (c) are converted into lactide, which is discharged from the gas phase pipeline at the top of the depolymerization reactor and then enters the crude lactide condenser 11, and at the same time, a depolymerization residue 18 containing coke and polymers is produced and discharged from the bottom of the depolymerization reactor 12;

[0154] (e) The crude lactide condenser 11 condenses the streams from the top of the pre-depolymerization reactor 10 and the top of the depolymerization reactor 12 to obtain a depolymerized non-condensable gas mixture 20 and a crude lactide product 19.

[0155] The lactic acid raw material 14 is methyl lactate, which comes from outside the world.

[0156] The catalyst 13 is an organic acid compound of tin, and the amount used is 0.2%wt of methyl lactate.

[0157] The dehydration tower 2 has a theoretical plate number of 5, a tower top operating pressure of 125 kPaA, and a tower top operating temperature of 70°C.

[0158] The polycondensation dehydration tower 6 has a theoretical plate number of 5, a tower top operating pressure of 50 kPaA, and a tower top operating temperature of 50°C.

[0159] The operating temperature of the dehydration kettle 1 is 110° C. and the operating pressure is 100 kPaA.

[0160] The operating temperature of the polycondensation reactor 5 is 150° C. and the operating pressure is 50 kPaA.

[0161] The operating temperature of the final polycondensation reactor 9 is 180° C. and the operating pressure is 5 kPaA.

[0162] The operating temperature of the pre-depolymerization reactor 10 is 200° C. and the operating pressure is 5 kPaA.

[0163] The operating temperature of the depolymerization reactor 12 is 250° C. and the operating pressure is 1 kPaA.

[0164] Through the above device and method, the utilization rate of methyl lactate is 95%, and the yield of lactide is 85%.

[0165] The calculation formula for the utilization rate of methyl lactate is: (methyl lactate feed molar flow rate-remaining methyl lactate molar flow rate) / methyl lactate feed molar flow rate.

[0166] The calculation formula for the yield of lactide is: lactide molar flow rate / (methyl lactate feed molar flow rate-remaining methyl lactate molar flow rate).

[0167] Example 3

[0168] use Figure 3 The device for preparing crude lactide shown in the figure comprises a dehydration unit, a polycondensation unit and a depolymerization unit connected in sequence; the dehydration unit comprises a dehydration kettle 1, a dehydration kettle heater 3, a pre-polycondensation dehydration tower 6A, a polycondensation dehydration tower condenser 7A, a polycondensation dehydration tower 6, a polycondensation dehydration tower condenser 7, the polycondensation unit comprises a pre-polycondensation kettle 5A, a polycondensation kettle 5 and a final polycondensation kettle 9 arranged in sequence, the depolymerization unit comprises a pre-depolymerization kettle 10, a depolymerization kettle 12, and a crude lactide condenser 11; wherein,

[0169] The dehydration kettle heater 3 is a shell and tube heat exchanger connected to the bottom of the dehydration kettle 1;

[0170] The top of the dehydration kettle 1 is connected to the dehydration tower 2, and the bottom discharge pipeline is divided into two branches, one is connected to the polycondensation kettle 5, and the other is connected to the inlet of the dehydration kettle heater 3;

[0171] The top of the dehydration tower 2 is connected to the dehydration tower condenser 4, and the bottom is connected to the upper part of the dehydration kettle 1; the internal parts of the dehydration tower 2 are fillers;

[0172] The dehydration tower condenser 4 is provided with a gas phase non-condensable gas discharge pipeline and a liquid phase water discharge pipeline; the water discharge pipeline is divided into two branches, one is connected to the top of the dehydration tower 2, and the other is used as an external discharge pipeline;

[0173] The precondensation reactor 5A is a vertical stirring reactor with a precondensation reactor heater 8 provided outside, and is provided with a gas phase precondensation non-condensable gas discharge pipeline and a liquid phase precondensation product extraction pipeline; the precondensation reactor 5A is connected to the precondensation dehydration tower 6A through the precondensation non-condensable gas discharge pipeline, and is connected to the inlet of the polycondensation reactor 5 through the precondensation product extraction pipeline;

[0174] The top of the pre-condensation dehydration tower 6A is connected to the condensation dehydration tower condenser 7A, and the bottom is connected to the upper part of the pre-condensation kettle 5A; the internal parts of the pre-condensation dehydration tower 6A are fillers; the condensation dehydration tower condenser 7A is provided with a gas phase non-condensable gas discharge pipeline and a liquid phase water discharge pipeline; the water discharge pipeline is divided into two branches, one is connected to the top of the pre-condensation dehydration tower 6A, and the other is used as an external discharge pipeline;

[0175] The polycondensation kettle 5 is a vertical stirring kettle with a polycondensation kettle heater 8 disposed outside, and is provided with a gas phase polycondensation non-condensable gas discharge pipeline and a liquid phase polycondensation product extraction pipeline; the polycondensation kettle 5 is connected to the polycondensation kettle dehydration tower 6 through the polycondensation non-condensable gas discharge pipeline, and is connected to the inlet of the final polycondensation kettle 9 through the polycondensation product extraction pipeline;

[0176] The top of the polycondensation dehydration tower 6 is connected to the polycondensation dehydration tower condenser 7, and the bottom is connected to the upper part of the polycondensation kettle 5; the internal parts of the polycondensation dehydration tower 6 are fillers; the polycondensation dehydration tower condenser 7 is provided with a gas phase non-condensable gas discharge pipeline and a liquid phase water discharge pipeline; the water discharge pipeline is divided into two branches, one is connected to the top of the polycondensation dehydration tower 6, and the other is used as an external discharge pipeline;

[0177] The final polycondensation kettle 9 is a horizontal single-shaft kneading reactor, which is provided with a gas phase final polycondensation non-condensable gas discharge pipeline and a liquid phase final polycondensation product extraction pipeline; the final polycondensation kettle 9 is connected to the pre-depolymerization kettle 10 through the final polycondensation product extraction pipeline;

[0178] The pre-depolymerization kettle 10 is a thin film evaporator, provided with a gas phase pre-depolymerization non-condensable gas discharge pipeline and a liquid phase pre-depolymerization product extraction pipeline; the pre-depolymerization kettle 10 is connected to the crude lactide condenser 11 through the pre-depolymerization non-condensable gas discharge pipeline, and is connected to the depolymerization kettle 12 through the pre-depolymerization product extraction pipeline;

[0179] The depolymerization kettle 12 is a uniaxial kneading reactor, provided with a gas phase depolymerization non-condensable gas discharge pipeline and a solid-liquid phase depolymerization residue discharge pipeline, and the depolymerization kettle 12 is connected to the crude lactide condenser 11 through the depolymerization non-condensable gas discharge pipeline;

[0180] The crude lactide condenser 11 is provided with a crude lactide product extraction pipeline and a depolymerized non-condensable gas mixture discharge pipeline;

[0181] The lactic acid feed line is connected to the inlet of the dehydration kettle heater 3; the catalyst feed line is connected to the inlet of the pre-depolymerization kettle 10 (which can be merged with the polycondensation product extraction line).

[0182] The method for preparing crude lactide using the above-mentioned device comprises:

[0183] (a) The water-containing lactic acid raw material 14 enters the inlet of the dehydration kettle heater 3 through the lactic acid feed pipeline, and the catalyst 13 enters the inlet of the pre-depolymerization kettle 10 through the catalyst feed pipeline; the dehydration kettle 1 is heated at normal pressure or reduced pressure to evaporate free water and polymerization water, and the evaporated components enter the dehydration tower 2 for reflux separation, and the top stream of the dehydration tower 2 enters the dehydration tower condenser 4 to obtain liquid water 16 and dehydrated non-condensable gas 15, and the bottom stream returns to the dehydration kettle 1; the bottom material of the dehydration kettle 1 is divided into two streams, one stream enters the dehydration kettle heater 3, and the other stream is produced and enters the pre-condensation kettle 5A;

[0184] (b) The precondensation reactor 5A receives the material at the bottom of the dehydration reactor 1, and undergoes a dehydration precondensation reaction through high temperature and low pressure operation. The generated water enters the precondensation dehydration tower 6A for reflux separation. The top stream of the precondensation dehydration tower 6A enters the precondensation dehydration tower condenser 7A to obtain liquid water 16A and dehydrated non-condensable gas 15A, and the bottom stream returns to the precondensation reactor 5A;

[0185] The polycondensation kettle 5 receives the bottom material of the pre-polycondensation kettle 5A, and after further high-temperature and high-vacuum operation, the polycondensation reaction continues, and the generated water and non-condensable gas enter the polycondensation dehydration tower 6 for reflux separation, and the top stream of the polycondensation dehydration tower 6 enters the polycondensation dehydration tower condenser 7 to obtain liquid water 16B and dehydrated non-condensable gas 15B, and the bottom stream returns to the polycondensation kettle 5;

[0186] The final polycondensation reactor 9 receives the material at the bottom of the polycondensation reactor 5, and continues the polycondensation reaction at a higher temperature and high vacuum than the polycondensation reactor to obtain polycondensation non-condensable gas, lactic acid oligomers with further increased molecular weight, and water. The polycondensation non-condensable gas is discharged through the polycondensation non-condensable gas discharge pipeline;

[0187] (c) the liquid phase at the bottom of the final polycondensation reactor 9 containing lactic acid oligomers enters the pre-depolymerization reactor 10, and undergoes high temperature and high vacuum operation to cause a depolymerization and cyclization reaction of the lactic acid oligomers to generate a lactide-containing stream, which enters a crude lactide condenser 11 through the top of the pre-depolymerization reactor 10, and the unreacted lactic acid oligomers in the pre-depolymerization reactor 10 enter a depolymerization reactor 12 from the bottom of the pre-depolymerization reactor 10;

[0188] (d) the depolymerization reactor 12 is operated at a higher temperature and a higher vacuum than the pre-depolymerization reactor 10, and the depolymerization and cyclization reaction of the lactic acid oligomers continues to occur, and the unreacted lactic acid oligomers in step (c) are converted into lactide, which is discharged from the gas phase pipeline at the top of the depolymerization reactor and then enters the crude lactide condenser 11, and at the same time, the depolymerization residue 18 containing coke and polymers is produced and discharged from the bottom of the depolymerization reactor 12;

[0189] (e) The crude lactide condenser 11 condenses the streams from the top of the pre-depolymerization reactor 10 and the top of the depolymerization reactor 12 to obtain a depolymerized non-condensable gas mixture 20 and a crude lactide product 19.

[0190] The lactic acid raw material 14 is methyl lactate, which comes from outside the world.

[0191] The catalyst 13 is an organic acid compound of tin, and the amount used is 0.2%wt of methyl lactate.

[0192] The dehydration tower 2 has a theoretical plate number of 5, a tower top operating pressure of 125 kPaA, and a tower top operating temperature of 70°C.

[0193] The theoretical plate number of the pre-condensation dehydration tower 6A is 5, the tower top operating pressure is 125 kPaA, and the tower top operating temperature is 70°C.

[0194] The polycondensation dehydration tower 6 has a theoretical plate number of 5, a tower top operating pressure of 50 kPaA, and a tower top operating temperature of 50°C.

[0195] The operating temperature of the dehydration kettle 1 is 110° C. and the operating pressure is 100 kPaA.

[0196] The operating temperature of the precondensation reactor 5A is 150° C. and the operating pressure is 50 kPaA.

[0197] The operating temperature of the polycondensation reactor 5 is 180° C. and the operating pressure is 5 kPaA.

[0198] The operating temperature of the final polycondensation reactor 9 is 180° C., and the operating pressure is 2 kPaA.

[0199] The operating temperature of the pre-depolymerization reactor 10 is 200° C. and the operating pressure is 5 kPaA.

[0200] The operating temperature of the depolymerization reactor 12 is 240° C., and the operating pressure is 1 kPaA.

[0201] Through the above device and method, the utilization rate of methyl lactate is 98% and the yield of lactide is 88%.

[0202] The calculation formula for the utilization rate of methyl lactate is: (methyl lactate feed molar flow rate-remaining methyl lactate molar flow rate) / methyl lactate feed molar flow rate.

[0203] The calculation formula for the yield of lactide is: lactide molar flow rate / (methyl lactate feed molar flow rate-remaining methyl lactate molar flow rate).

[0204] Example 4

[0205] Use Figure 4The device for preparing crude lactide shown in the figure comprises a dehydration unit, a polycondensation unit and a depolymerization unit connected in sequence; the dehydration unit comprises a dehydration kettle 1, a dehydration kettle heater 3, a dehydration tower 2, a dehydration tower condenser 4, a polycondensation kettle dehydration tower 6, and a polycondensation kettle dehydration tower condenser 7; the polycondensation unit comprises a polycondensation kettle 5; the depolymerization unit comprises a pre-depolymerization kettle 10, a depolymerization kettle 12, and a crude lactide condenser 11; wherein,

[0206] The dehydration kettle heater 3 is a shell and tube heat exchanger connected to the bottom of the dehydration kettle 1;

[0207] The top of the dehydration kettle 1 is connected to the dehydration tower 2, and the bottom discharge pipeline is divided into two branches, one is connected to the polycondensation kettle 5, and the other is connected to the inlet of the dehydration kettle heater 3;

[0208] The top of the dehydration tower 2 is connected to the dehydration tower condenser 4, and the bottom is connected to the upper part of the dehydration kettle 1; the internal parts of the dehydration tower 2 are fillers;

[0209] The dehydration tower condenser 4 is provided with a gas phase non-condensable gas discharge pipeline and a liquid phase water discharge pipeline; the water discharge pipeline is divided into two branches, one is connected to the top of the dehydration tower 2, and the other is used as an external discharge pipeline;

[0210] The polycondensation kettle 5 is a vertical stirring kettle with a polycondensation kettle heater 8 provided outside, and is provided with a gas phase polycondensation non-condensable gas discharge pipeline and a liquid phase polycondensation product extraction pipeline; the polycondensation kettle 5 is connected to the polycondensation kettle dehydration tower 6 through the polycondensation non-condensable gas discharge pipeline, and is connected to the inlet of the pre-depolymerization kettle 10 through the polycondensation product extraction pipeline;

[0211] The top of the polycondensation dehydration tower 6 is connected to the polycondensation dehydration tower condenser 7, and the bottom is connected to the upper part of the polycondensation kettle 5; the internal parts of the polycondensation dehydration tower 6 are fillers; the polycondensation dehydration tower condenser 7 is provided with a gas phase non-condensable gas discharge pipeline and a liquid phase water discharge pipeline; the water discharge pipeline is divided into two branches, one is connected to the top of the polycondensation dehydration tower 6, and the other is used as an external discharge pipeline;

[0212] The pre-depolymerization kettle 10 is a thin film evaporator, provided with a gas phase pre-depolymerization non-condensable gas discharge pipeline and a liquid phase pre-depolymerization product extraction pipeline; the pre-depolymerization kettle 10 is connected to the crude lactide condenser 11 through the pre-depolymerization non-condensable gas discharge pipeline, and is connected to the depolymerization kettle 12 through the pre-depolymerization product extraction pipeline;

[0213] The depolymerization kettle 12 is a uniaxial kneading reactor, provided with a gas phase depolymerization non-condensable gas discharge pipeline and a solid-liquid phase depolymerization residue discharge pipeline, and the depolymerization kettle 12 is connected to the crude lactide condenser 11 through the depolymerization non-condensable gas discharge pipeline;

[0214] The crude lactide condenser 11 is provided with a crude lactide product extraction pipeline and a depolymerized non-condensable gas mixture discharge pipeline;

[0215] The lactic acid feed line is connected to the inlet of the dehydration kettle heater 3; the catalyst feed line is connected to the inlet of the pre-depolymerization kettle 10 (which can be merged with the polycondensation product extraction line).

[0216] The method for preparing crude lactide using the above-mentioned device comprises:

[0217] (a) A water-containing lactic acid raw material 14 enters the inlet of a dehydration kettle heater 3 through a lactic acid feed pipeline, and a catalyst 13 enters the inlet of a pre-depolymerization kettle 10 through a catalyst feed pipeline; the dehydration kettle 1 is heated at normal pressure or reduced pressure to evaporate free water and polymerization water, and the evaporated components enter a dehydration tower 2 for reflux separation, and the top stream of the dehydration tower 2 enters a dehydration tower condenser 4 to obtain liquid water 16 and dehydrated non-condensable gas 15, and the bottom stream returns to the dehydration kettle 1; the bottom material of the dehydration kettle 1 is divided into two streams, one of which enters the dehydration kettle heater 3, and the other is extracted and enters the polycondensation kettle 5;

[0218] (b) The polycondensation kettle 5 receives the bottom material of the dehydration kettle 1, and undergoes a dehydration polycondensation reaction through high temperature and low pressure operation. The generated water enters the polycondensation dehydration tower 6 for reflux separation. The top stream of the polycondensation dehydration tower 6 enters the polycondensation dehydration tower condenser 7 to obtain liquid water 16B and dehydrated non-condensable gas 15B, and the bottom stream returns to the polycondensation kettle 5;

[0219] (c) the liquid phase at the bottom of the polycondensation reactor 5 containing lactic acid oligomers enters the pre-depolymerization reactor 10, and undergoes high temperature and high vacuum operation to cause a depolymerization and cyclization reaction of the lactic acid oligomers to generate a lactide-containing stream, which enters a crude lactide condenser 11 through the top of the pre-depolymerization reactor 10, and unreacted lactic acid oligomers in the pre-depolymerization reactor 10 enter a depolymerization reactor 12 from the bottom of the pre-depolymerization reactor 10;

[0220] (d) the depolymerization reactor 12 is operated at a higher temperature and a higher vacuum than the pre-depolymerization reactor 10, and the depolymerization and cyclization reaction of the lactic acid oligomers continues to occur, and the unreacted lactic acid oligomers in step (c) are converted into lactide, which is discharged from the gas phase pipeline at the top of the depolymerization reactor and then enters the crude lactide condenser 11, and at the same time, a depolymerization residue 18 containing coke and polymers is produced and discharged from the bottom of the depolymerization reactor 12;

[0221] (e) The crude lactide condenser 11 condenses the streams from the top of the pre-depolymerization reactor 10 and the top of the depolymerization reactor 12 to obtain a depolymerized non-condensable gas mixture 20 and a crude lactide product 19.

[0222] The lactic acid raw material 14 is L-lactic acid, which comes from outside the world.

[0223] The catalyst is an organic acid compound of tin, and the amount used is 0.05%wt of the lactic acid.

[0224] The dehydration tower 2 has a theoretical plate number of 18, a tower top operating pressure of 70 kPaA, and a tower top operating temperature of 90°C.

[0225] The operating temperature of the dehydration kettle 1 is 95° C. and the operating pressure is 75 kPaA.

[0226] The polycondensation dehydration tower 6 has a theoretical plate number of 5, a tower top operating pressure of 50 kPaA, and a tower top operating temperature of 50°C.

[0227] The operating temperature of the polycondensation reactor 5 is 130° C., and the operating pressure is 40 kPaA.

[0228] The operating temperature of the pre-depolymerization reactor 10 is 200° C. and the operating pressure is 10 kPaA.

[0229] The operating temperature of the depolymerization reactor 12 is 230° C., and the operating pressure is 5 kPaA.

[0230] Through the above device and method, the utilization rate of lactic acid is 90% and the yield of lactide is 82%.

[0231] The calculation formula for the utilization rate of lactic acid is: (lactic acid feed molar flow rate-residual lactic acid molar flow rate) / lactic acid feed molar flow rate.

[0232] The calculation formula of the lactide yield is: lactide molar flow rate / (lactic acid feed molar flow rate-residual lactic acid molar flow rate).

[0233] The above embodiments illustrate that the device of the present invention can realize large-scale and continuous production, and improve the yield of the target product through various strengthening means.

[0234] The embodiments of the present invention have been described above, and the above description is exemplary, not exhaustive, and is not limited to the disclosed embodiments. Many modifications and changes will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments.

Claims

1. A device for preparing crude lactide, characterized in that: The device comprises a dehydration unit, a polycondensation unit and a depolymerization unit connected in sequence; the dehydration unit comprises a dehydration kettle, a dehydration kettle heater, at least one set of dehydration towers and a condenser thereof; the polycondensation unit comprises a polycondensation kettle, and optionally a pre-polycondensation kettle arranged upstream of the polycondensation kettle and / or a final polycondensation kettle arranged downstream of the polycondensation kettle; the depolymerization unit comprises a pre-depolymerization kettle, a depolymerization kettle and a crude lactide condenser; wherein, The dehydration kettle heater is connected to the bottom of the dehydration kettle; The top of the dehydration kettle is connected to the dehydration tower, and the bottom is connected to the polycondensation unit; The top of one of the at least one group of dehydration towers and their condensers is connected to the dehydration tower condenser, and the bottom is connected to the upper part of the dehydration kettle; The polycondensation unit is provided with a gas phase outlet and a liquid phase outlet, and the liquid phase outlet of the polycondensation unit is connected to the pre-depolymerization kettle; The top of the pre-depolymerization kettle is connected to the crude lactide condenser, and the bottom is connected to the depolymerization kettle; The top of the depolymerization kettle is connected to the crude lactide condenser; The lactic acid feed pipeline is connected to the dehydration unit; the catalyst feed pipeline is connected to the dehydration unit or the depolymerization unit.

2. The device for preparing crude lactide according to claim 1, characterized in that: When the at least one group of dehydration towers and their condensers are a group, the group of dehydration towers and their condensers are connected to the top of the dehydration kettle and the top of the polycondensation kettle at the same time; when there are multiple groups, the multiple groups of dehydration towers and their condensers are respectively connected to the top of the dehydration kettle, the top of the polycondensation kettle and the top of the optional pre-polycondensation kettle.

3. The device for preparing crude lactide according to claim 2, characterized in that: The polycondensation unit is provided in any of the following ways: (i) the polycondensation unit comprises a polycondensation kettle, a polycondensation dehydration tower and a polycondensation dehydration tower condenser; the bottom of the dehydration kettle is connected to the polycondensation kettle, and the bottom of the polycondensation kettle is connected to the inlet of the pre-depolymerization kettle; (ii) the polycondensation unit comprises a polycondensation kettle and a final polycondensation kettle; the bottom of the dehydration kettle is connected to the polycondensation kettle, the bottom of the polycondensation kettle is connected to the inlet of the final polycondensation kettle, and the bottom of the final polycondensation kettle is connected to the inlet of the pre-depolymerization kettle; the polycondensation kettle and the dehydration kettle share a set of dehydration towers and their condensers; (iii) the polycondensation unit comprises a polycondensation kettle, a polycondensation dehydration tower, a polycondensation dehydration tower condenser, and a final polycondensation kettle; the bottom of the dehydration kettle is connected to the polycondensation kettle, the bottom of the polycondensation kettle is connected to the inlet of the final polycondensation kettle, and the bottom of the final polycondensation kettle is connected to the inlet of the pre-depolymerization kettle; (iv) The polycondensation unit comprises a pre-polycondensation kettle, a pre-polycondensation dehydration tower and a pre-polycondensation dehydration tower condenser, a polycondensation kettle, a polycondensation dehydration tower and a polycondensation dehydration tower condenser, and a final polycondensation kettle; the bottom of the dehydration kettle is connected to the pre-polycondensation kettle, the bottom of the pre-polycondensation kettle is connected to the inlet of the polycondensation kettle, the bottom of the polycondensation kettle is connected to the inlet of the final polycondensation kettle, and the bottom of the final polycondensation kettle is connected to the inlet of the pre-depolymerization kettle.

4. The device for preparing crude lactide according to claim 1, characterized in that: The bottom discharge pipeline of the dehydration kettle is divided into two branches, one is connected to the polycondensation unit, and the other is connected to the inlet of the dehydration kettle heater; The lactic acid feed pipeline is connected to the dehydration kettle heater and / or the dehydration kettle; The catalyst feeding pipeline is connected to the dehydration kettle heater or the dehydration kettle, or is connected to the pre-depolymerization kettle.

5. The device for preparing crude lactide according to any one of claims 1 to 4, characterized in that: The dehydration kettle heater is one of a shell and tube heat exchanger, a plate heat exchanger and an electric heater; The internal parts of the dehydration tower are one or more of a sieve plate, a float valve, a filler and a bubble cap, preferably a filler; The dehydration tower condenser is provided with a gas phase non-condensable gas discharge pipeline and a liquid phase water discharge pipeline; the water discharge pipeline is divided into two branches, one is connected to the top of the dehydration tower, and the other is used as an external discharge pipeline.

6. The device for preparing crude lactide according to any one of claims 1 to 4, characterized in that: The pre-condensation reactor is provided with a gas phase pre-condensation non-condensable gas discharge pipeline and a liquid phase pre-condensation product extraction pipeline; the pre-condensation reactor is connected to the dehydration tower through the pre-condensation non-condensable gas discharge pipeline, and is connected to the polycondensation reactor through the pre-condensation product extraction pipeline; The precondensation reactor is a vertical or horizontal stirring reactor, with a heater disposed inside or a heater and / or a jacket disposed outside, preferably a vertical stirring reactor with a heater disposed outside.

7. The device for preparing crude lactide according to any one of claims 1 to 4, characterized in that: The polycondensation kettle is provided with a gas phase polycondensation non-condensable gas discharge pipeline and a liquid phase polycondensation product extraction pipeline; the polycondensation kettle is connected to the dehydration tower through the polycondensation non-condensable gas discharge pipeline, and is connected to the final polycondensation kettle or the pre-depolymerization kettle through the polycondensation product extraction pipeline; The polycondensation reactor is a vertical stirred tank reactor or a horizontal shaft stirred reactor, preferably a single shaft or multi-shaft kneading reactor, a screw reactor, a surface renewal reactor, a tubular reactor, a tower reactor, more preferably a horizontal single shaft or multi-shaft kneading reactor; The surface renewal reactor is preferably a disc reactor or a squirrel cage reactor.

8. The device for preparing crude lactide according to any one of claims 1 to 4, characterized in that: The final polycondensation kettle is provided with a gas phase final polycondensation non-condensable gas discharge pipeline and a liquid phase final polycondensation product extraction pipeline; the final polycondensation kettle is connected to the pre-depolymerization kettle through the final polycondensation product extraction pipeline; The final polycondensation reactor is a horizontal shaft-stirred reactor, preferably a single-shaft or multi-shaft kneading reactor, a screw reactor, a surface renewal reactor, a tubular reactor, or a tower reactor, more preferably a horizontal single-shaft or multi-shaft kneading reactor; The surface renewal reactor is preferably a disc reactor or a squirrel cage reactor.

9. The device for preparing crude lactide according to any one of claims 1 to 4, characterized in that: The pre-depolymerization kettle is provided with a pre-depolymerization non-condensable gas discharge pipeline of the gas phase and a pre-depolymerization product extraction pipeline of the liquid phase; the pre-depolymerization kettle is connected to the crude lactide condenser through the pre-depolymerization non-condensable gas discharge pipeline, and is connected to the depolymerization kettle through the pre-depolymerization product extraction pipeline; The pre-depolymerization kettle is a horizontal or vertical evaporator with shaft stirring, with a heater disposed inside or a heater and / or a jacket disposed outside, and is preferably a thin film evaporator.

10. The device for preparing crude lactide according to any one of claims 1 to 4, characterized in that: The depolymerization reactor is a vertical or horizontal reactor with shaft stirring, a screw reactor or a kneading reactor, preferably a single-shaft or multi-shaft kneading reactor; The depolymerization kettle is provided with a gas phase depolymerization non-condensable gas discharge pipeline and a solid-liquid phase depolymerization residue discharge pipeline, and the depolymerization kettle is connected to the crude lactide condenser through the depolymerization non-condensable gas discharge pipeline; The crude lactide condenser is provided with a crude lactide product extraction pipeline and a depolymerized non-condensable gas mixture discharge pipeline.

11. A method for preparing crude lactide, using the device according to any one of claims 1 to 10, characterized in that: The following steps are involved: The lactic acid raw material enters the device through a lactic acid feed pipeline, and the catalyst enters the device through a catalyst feed pipeline. The lactic acid raw material is sequentially subjected to dehydration separation, optional pre-polycondensation, polycondensation, optional final polycondensation, pre-depolymerization, and depolymerization to obtain a crude lactide product.

12. The method for preparing crude lactide according to claim 11, characterized in that: The method comprises the following steps: (a) The water-containing lactic acid raw material enters the dehydration kettle heater inlet and / or the dehydration kettle inlet through the lactic acid feed pipeline, and the catalyst enters the dehydration kettle heater inlet or the dehydration kettle inlet or the pre-depolymerization kettle inlet through the catalyst feed pipeline; the dehydration kettle is heated at normal pressure or reduced pressure to evaporate free water and polymerization water, and the evaporated components enter the dehydration tower for reflux separation, the dehydration tower top stream enters the dehydration tower condenser to obtain liquid water and dehydrated non-condensable gas, and the tower bottom stream returns to the dehydration kettle; the dehydration kettle bottom material is divided into two streams, one stream enters the dehydration kettle heater, and the other stream is extracted and enters the downstream polycondensation unit; (b) the polycondensation unit receives the material at the bottom of the dehydration kettle, undergoes a polycondensation reaction of lactic acid, and produces lactic acid oligomers; the step (b) is selected from one of the following forms: (b-1) The polycondensation unit comprises a polycondensation kettle, which receives the material at the bottom of the dehydration kettle and undergoes a dehydration polycondensation reaction under high temperature and high vacuum operation to obtain lactic acid oligomers, a small amount of polycondensation non-condensable gas and water. The generated water is discharged through a separate dehydration tower or combined into a dehydration tower at the top of the dehydration kettle for discharge; (b-2) The polycondensation unit includes a polycondensation kettle and a final polycondensation kettle. The polycondensation kettle receives the material at the bottom of the dehydration kettle and undergoes a dehydration polycondensation reaction under high temperature and low pressure operation. The generated water is discharged through a separate dehydration tower or is combined and discharged into a dehydration tower at the top of the dehydration kettle; The final polycondensation kettle receives the material at the bottom of the polycondensation kettle, and continues the polycondensation reaction through high temperature and high vacuum operation to obtain lactic acid oligomers, a small amount of polycondensation non-condensable gas and water; (b-3) The polycondensation unit includes a pre-polycondensation kettle, a polycondensation kettle and a final polycondensation kettle. The pre-polycondensation kettle receives the material at the bottom of the dehydration kettle and undergoes a dehydration pre-polycondensation reaction under high temperature and low pressure operation. The generated water is discharged through a separate dehydration tower or is combined and discharged into a dehydration tower at the top of the dehydration kettle; The polycondensation kettle receives the material at the bottom of the pre-polycondensation kettle, and further undergoes the polycondensation reaction through high temperature and high vacuum operation. The generated water and non-condensable gas are discharged through the dehydration tower, and the lactic acid oligomers with increased molecular weight are further obtained and enter the final polycondensation kettle; The final polycondensation kettle receives the material at the bottom of the polycondensation kettle, and continues the polycondensation reaction at a higher temperature and high vacuum than the polycondensation kettle to obtain lactic acid oligomers with further increased molecular weight, polycondensation non-condensable gas and water; (c) the liquid phase at the bottom of the polycondensation unit enters the pre-depolymerization kettle, and is subjected to high temperature and high vacuum operation to cause a depolymerization and cyclization reaction of the lactic acid oligomers to occur, thereby generating a lactide-containing stream, which enters the crude lactide condenser through the top of the pre-depolymerization kettle, and the unreacted lactic acid oligomers in the pre-depolymerization kettle enter the depolymerization kettle through the bottom of the pre-depolymerization kettle; (d) the depolymerization kettle is operated at a higher temperature and a higher vacuum than the pre-depolymerization kettle, and the depolymerization and cyclization reaction of the lactic acid oligomers continues to occur, and the unreacted lactic acid oligomers in step (c) are converted into lactide, which is discharged from the gas phase pipeline at the top of the depolymerization kettle and then enters the crude lactide condenser, and at the same time, a depolymerization residue by-product containing coke and polymer is produced and discharged from the bottom of the depolymerization kettle; (e) The crude lactide condenser condenses the streams from the top of the pre-depolymerization reactor and the top of the depolymerization reactor to obtain a depolymerization non-condensable gas mixture and a crude lactide product.

13. The method for preparing crude lactide according to claim 11 or 12, characterized in that: The catalyst is at least one of tin halides, tin organic acid compounds, antimony oxides, antimony halides, antimony organic acid compounds, zinc oxides, zinc halides and zinc organic acid compounds; preferably tin organic acid compounds; The amount of the catalyst used is 0.001 to 5% wt of the amount of lactic acid used; The lactic acid is fermented lactic acid obtained from biological fermentation, preferably L-lactic acid, D-lactic acid, (L / D)-lactic acid or a mixture thereof in any proportion; The number of theoretical plates of the dehydration tower is 2 to 20, the operating pressure at the top of the tower is 50 to 130 kPaA, and the operating temperature at the top of the tower is 30 to 100°C; The operating temperature of the dehydration kettle is 70-140°C, and the operating pressure is 50-150kPaA; The operating temperature of the pre-condensation reactor is 120-150°C, and the operating pressure is 10-50kPaA; The theoretical plate number of the pre-condensation dehydration tower is 2 to 20, the top operating pressure is 10 to 150 kPaA, and the top operating temperature is 30 to 80°C; The operating temperature of the polycondensation reactor is 140-180°C, and the operating pressure is 5-50kPaA; The polycondensation dehydration tower has a theoretical plate number of 2 to 20, a tower top operating pressure of 5 to 50 kPaA, and a tower top operating temperature of 30 to 80°C; The operating temperature of the final polycondensation reactor is 150-190°C, and the operating pressure is 1-20 kPaA; The operating temperature of the pre-depolymerization reactor is 160-210°C, and the operating pressure is 100PaA-10kPaA; The operating temperature of the depolymerization reactor is 180-250° C., and the operating pressure is 100 PaA-10 kPaA.

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