Method for producing lactide

Abstract

The present invention relates to the field of biodegradable materials, particularly to a lactide production method, which comprises: (1) carrying out a condensation polymerization reaction on lactic acid in a condensation polymerization reaction kettle in the presence of rare earth steel to obtain a lactic acid oligomer with a polymerization degree of less than 10; and (2) carrying out a depolymerization reaction on the lactic acid oligomer in a depolymerization reaction kettle in the presence of rare earth steel to obtain a product containing lactide. According to the method provided by the invention, the yield of the high-gloss pure lactide can be improved, the chemical purity of the product can be guaranteed, and the purification difficulty is reduced.

Description

Technical Field

[0001] The invention relates to the field of biodegradable materials, and in particular to a method for producing lactide. Background Art

[0002] The construction of a green global village requires the vigorous implementation of plastic bans and restrictions. Among them, biodegradable polylactic acid materials are gaining more and more attention, and their consumption is increasing every year. Statistics from the European Bioplastics Association show that the production capacity of biodegradable materials is increasing at a rate of nearly 20% per year, while its demand is increasing at a rate of 30%. The product has been in short supply for a long time.

[0003] Polylactic acid is the most common biodegradable plastic. The ring-opening polymerization of lactide into polylactic acid is the current mainstream production process, which has the characteristics of high product purity and high degree of polymerization. In the lactide ring-opening polymerization method, the synthesis and purification of the intermediate lactide is the core technology and difficulty in the industry. Few foreign companies have achieved technological breakthroughs, and there is a technological monopoly situation. Domestic research is mostly at the pilot research and development stage, and a few companies are promoting industrial trial production.

[0004] The catalyst for lactide synthesis is a key factor in its reaction yield. Among them, stannous octoate catalysts containing rare earth elements have been widely studied and applied at home and abroad, but the cost is still high, accounting for 3-5% of the total cost of lactide production, and it is difficult to recycle. In recent years, as the strategic material attributes of rare earth resources have become prominent, the catalytic effect of rare earth elements has been gradually discovered, but the high price of high-purity rare earth elements has also limited their direct application in related fields. Summary of the invention

[0005] The inventors of the present invention have found that when lactide is prepared by the traditional method of adding a catalyst, especially when lactide is prepared on a large scale, the catalyst is difficult to separate from the product, which easily causes product loss, and the catalyst is difficult to recycle and needs to be added continuously, which greatly increases the production cost. In order to solve the problems of high cost in the synthesis of lactide, poor dispersion ability when using a catalyst, and difficulty in recycling in the prior art, the inventors of the present invention have found through research that a method for preparing lactide in the presence of rare earth steel can improve the yield of high optical purity lactide and reduce production costs.

[0006] In order to achieve the above object, the present invention provides a method for producing lactide in a first aspect, wherein the method comprises:

[0007] (1) subjecting lactic acid to a polycondensation reaction in a polycondensation reactor in the presence of rare earth steel to obtain a lactic acid oligomer having a degree of polymerization of less than 10;

[0008] (2) Depolymerizing the lactic acid oligomer in a depolymerization reactor in the presence of rare earth steel to obtain a product containing lactide.

[0009] Through the above technical scheme, the production method provided by the present invention can improve the yield of high-purity lactide, ensure the chemical purity of the product, and reduce the difficulty of purification. In addition, the reactor made of rare earth steel has a higher tolerance to pressure, strong acid and temperature, and is particularly suitable for the strong acid negative pressure environment of lactic acid polymerization; the production method provided by the present invention can also produce lactide without adding additional catalysts, which can significantly reduce production costs.

[0010] In some preferred embodiments, compared with a conventional falling film reactor with a falling film tube, a rare earth steel reactor with a sieve plate can make the reaction system more uniform and the reaction time more controllable; in the polycondensation reaction and depolymerization reaction, there is no need to introduce other rare earth-containing catalysts, and there is no need to remove the catalyst after the reaction, which can simplify the preparation method, save costs, and reduce the removal of catalysts. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 It is a two-stage series vacuum reactor.

[0012] Figure 2 In the figure, (A) is the front view of the sieve plate, and (B) is the top view of the sieve plate.

[0013] Description of Reference Numerals

[0014] 1, 4 parts with sieve plate 2, 5 parts without sieve plate

[0015] 3, 6 screen plates 7 edge enclosure DETAILED DESCRIPTION

[0016] The endpoints and any values ​​of the ranges disclosed in this article are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of each range, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this article.

[0017] In the present invention, unless otherwise stated, the vacuum pressures referred to are all absolute pressures.

[0018] A first aspect of the present invention provides a method for producing lactide, wherein the method comprises:

[0019] (1) subjecting lactic acid to a polycondensation reaction in a polycondensation reactor in the presence of rare earth steel to obtain a lactic acid oligomer having a degree of polymerization of less than 10;

[0020] (2) Depolymerizing the lactic acid oligomer in a depolymerization reactor in the presence of rare earth steel to obtain a product containing lactide.

[0021] In the present invention, the use of the above method can save the production cost of lactide, improve the lactide yield, improve the optical purity of lactide, and also improve the chemical purity and reduce the difficulty of purification and recovery.

[0022] In the present invention, degree of polymerization (DP) refers to the number of repeating monomer structures contained in the polymer macromolecular chain.

[0023] In the present invention, preferably, the chemical elements contained in the rare earth steel include Fe, C and rare earth elements. The rare earth elements include lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), lutetium (Lu) and scandium (Sc) and yttrium (Y) and the like. The types and proportions of the specific elements in the rare earth elements are not particularly limited, for example, lanthanum (La) may be contained. The rare earth steel also inevitably contains other chemical elements conventionally contained in other fields.

[0024] In the present invention, preferably, the rare earth steel contains ≥ 98 mass %, more preferably 98-99.9 mass % of Fe element.

[0025] In the present invention, preferably, the rare earth steel contains ≤1.7 mass %, more preferably 0.1-1.7 mass % of the C element.

[0026] In the present invention, preferably, the mass content of the rare earth element in the rare earth steel is 6-10ppm, for example, 6ppm, 7ppm, 8ppm, 9ppm, 10ppm and any range composed of any of the above values ​​and any value in the range. The rare earth element in the rare earth steel may exist in the form of a single substance or a compound, for example, a compound generated by reacting with oxygen or sulfur in the rare earth steel.

[0027] In the present invention, the method may also add other catalysts in the polycondensation reaction or depolymerization reaction. In order to further reduce the production cost and reduce the difficulty of product purification, preferably, the method does not add other catalysts. The method provided by the present invention does not require the addition of other catalysts and can produce lactide in high yield. In the present invention, the "other catalysts" refer to catalysts commonly used in the polycondensation reaction or depolymerization reaction when preparing lactide in the art, such as zinc or tin salts, zinc or tin oxides, rare earth catalysts, organic guanidine catalysts, etc.; the zinc or tin salts, or zinc or tin oxides may include stannous octoate, stannous chloride and stannous lactate, etc.; the organic guanidine catalyst may include tetramethylguanidine, biomass creatinine and creatinine zinc chloride, etc. In the present invention, the lactic acid used for the polycondensation reaction may be concentrated lactic acid. The concentration method may refer to conventional methods in the art, such as evaporation concentration and membrane separation concentration, etc., which will not be repeated here.

[0028] According to the present invention, preferably, the chemical purity of the lactic acid used for the polycondensation reaction is above 95%, more preferably above 99.5%.

[0029] In the present invention, the degree of polymerization of the lactic acid oligomer may be 5, 6, 7, 8, 9, or 10.

[0030] According to the present invention, the conditions of the polycondensation reaction only need to make the lactic acid polycondensate to obtain lactic acid oligomers having a polymerization degree of less than 10, preferably 5-10. However, in order to further improve the purity and yield of lactide, preferably, the conditions of the polycondensation reaction include: a temperature of 140-180°C, for example, 140°C, 142°C, 144°C, 146°C, 148°C, 150°C, 152°C, 154°C, 156°C, 158°C, 160°C, 162°C, 164°C, 166°C, 168°C, 170°C, 172°C, 174°C, 176°C, 178°C, 180°C and any range of the above values ​​and any value in the range, preferably 150-160°C; pressure is 10-30kPa, for example, 10kPa, 12kPa, 14kPa, 16kPa, 18kPa, 20kPa, 22kPa, 24kPa, 26kPa, 28kPa, 30kPa and any range of the above values ​​and any value in the range, preferably 10-20kPa; time is 3-8h, preferably 4.5-5.5h. The temperature of the polycondensation reaction fluctuates within a certain range, and the fluctuation range is preferably 0-15°C.

[0031] In the present invention, in the polycondensation reaction, the feed volume flow rate of lactic acid can be adjusted according to the volume and height of the reactor. Preferably, relative to 10m 3 The volumetric polycondensation reactor has a feed volume flow rate of lactic acid of 1-2 m3 / h, more preferably 1.2-1.8m 3 / h, more preferably 1.4-1.6m 3 / h.

[0032] According to the present invention, the conditions of the depolymerization reaction are not particularly limited, as long as the lactic acid oligomers can be depolymerized into lactide. Preferably, in order to improve the yield and purity of lactide, the conditions of the depolymerization reaction include: the temperature is 5-80°C higher than the temperature of the polycondensation reaction, for example, it can be 5°C, 10°C, 15°C, 20°C, 25°C, 30°C, 35°C, 40°C, 45°C, 50°C, 55°C, 60°C, 65°C, 70°C, 75°C, 80°C, any range consisting of any of the above values, and any value in the range, more preferably 30-50°C higher; the pressure is lower than the pressure of the polycondensation reaction, preferably 3-10kPa, for example, it can be 3kPa, 3.5kPa, 4kPa, 4.5kPa, 5kPa, 5.5kPa, 6kPa, 6.5kPa, 7kPa, 7.5kPa, 8kPa, 8.5kPa, 9kPa, 9.5kPa, 10kPa, any range consisting of any of the above values, and any value in the range, more preferably 5-8kPa.

[0033] More preferably, the temperature of the depolymerization reaction is 5-80°C higher than the temperature of the polycondensation reaction, and the temperature of the depolymerization reaction is 180-220°C, for example, it can be 180°C, 185°C, 190°C, 200°C, 205°C, 210°C, 215°C, 220°C and any range composed of any of the above values ​​and any value in the range, more preferably 190-210°C. The temperature of the depolymerization reaction fluctuates within a certain range, and the fluctuation range is preferably 0-15°C. In the present invention, the polycondensation reaction and the depolymerization reaction can be carried out in a lactide production system comprising a lactic acid polycondensation reaction unit and a lactic acid oligomer depolymerization unit connected in series, wherein the lactic acid polycondensation reaction unit comprises a polycondensation reaction kettle, and the lactic acid oligomer depolymerization unit comprises a depolymerization reaction kettle.

[0034] The lactide production system may further include a lactic acid concentration unit for concentrating lactic acid, a waste liquid treatment unit, etc. Those skilled in the art may configure these according to actual conditions, which will not be described in detail herein.

[0035] According to the present invention, the polycondensation reactor and the depolymerization reactor may be further provided with a stirring device, and a heating device, such as a heating jacket, may be further provided on the periphery thereof.

[0036] In the present invention, preferably, the depolymerization reactor is made of rare earth steel.

[0037] In the present invention, the material of the polycondensation reactor can be a material commonly used in the art, such as carbon steel, stainless steel (such as 316L or s22053), etc. Preferably, the material of the polycondensation reactor is rare earth steel.

[0038] The inventors of the present invention have found that a reactor obtained by rolling and welding rare earth steel is more suitable for the strong acid negative pressure environment of lactic acid polymerization, is safer when used for depolymerization reaction, can effectively alleviate the corrosion problem caused by the operation of traditional reactors, and can also improve the yield and purity of lactide; in the condensation reaction and depolymerization reaction, there is no need to introduce other catalysts containing rare earths, and there is no need to remove the catalyst after the reaction, which can simplify the preparation method, save costs, and reduce the loss of lactide caused by the catalyst removal process.

[0039] In the present invention, the polycondensation reactor and the depolymerization reactor are both continuously fed to react. When the polycondensation reactor and the depolymerization reactor are both made of rare earth steel, continuous production can be carried out for a long time without the need to add other catalysts, which can greatly reduce the use of other catalysts and reduce the difficulty of purifying lactide, simplify the preparation steps, and reduce production costs.

[0040] In the present invention, when a reactor made of a material not containing rare earth is used, rare earth steel can be added in the depolymerization reaction and the polycondensation reaction, respectively. Preferably, the amount of the rare earth steel per ton of lactic acid in the polycondensation reaction is 0.5-10 kg, for example, it can be 0.5 kg, 1 kg, 2 kg, 3 kg, 4 kg, 5 kg, 6 kg, 7 kg, 8 kg, 9 kg, 10 kg and any range composed of any of the above values ​​and any value in the range. Considering the cost and the yield of lactide, the amount of the rare earth steel is more preferably 4-10 kg.

[0041] In the present invention, in order to make the reaction system more uniform, improve the yield and purity of lactide, and reduce the generation of impurities, the polycondensation reactor and / or the depolymerization reactor is further provided with a sieve plate with sieve holes that is staggered and distributed in layers, and the material of the sieve plate is preferably rare earth steel. Figure 1 As shown, the method of the present invention can be implemented in a two-stage series vacuum reactor (a polycondensation reactor and a depolymerization reactor connected in series). The polycondensation reactor includes a portion 1 provided with a sieve plate 3 and a portion 2 without a sieve plate. Similarly, the depolymerization reactor includes a portion 4 provided with a sieve plate 6 and a portion 5 without a sieve plate. The inventors of the present invention have found that compared with a falling film evaporator with a falling film tube as a reactor, the use of a reactor provided with a sieve plate provided by the present invention can make the reaction system more uniform, the reaction time controllable, and can effectively solve the problems of difficult control and low yield of lactide polymerization. Moreover, the reactor provided with a sieve plate can be combined with the method for producing lactide to further improve the yield and purity of lactide.

[0042] Preferably, the sieve plate is parallel to the cross section of the cavity of the reactor. In the reactor, a sieve plate can be provided on each layer, and the volume of the part of the reactor where the sieve plate is provided can account for more than 1 / 5 of the total volume of the reactor. Each layer of sieve plates can be evenly distributed inside the reactor body. The lowest sieve plate can be provided at a position of 1 / 5 or more of the total height of the reactor body.

[0043] Preferably, the shape of the sieve plate is a missing circle, and the central angle of the missing circle is preferably 30 degrees to 90 degrees. The missing edge of the missing circle is preferably a straight edge. In the present invention, preferably, when the sieve plate is arranged in the reactor, its straight edge forms a gap with the inner wall of the reactor. The shape of the gap is not limited, and can be an arch, a polygon, a triangle, etc., and an arch is more preferred, for example Figure 2 As shown, the inventors of the present invention have found that when the notch is set in an arch shape, the sieve plate area can be more fully utilized, which is more conducive to the contact between the sieve plate and the oligomeric lactic acid solution. Without adding a catalyst, the yield of lactide can be further improved.

[0044] In the present invention, preferably, the sieve plate is further provided with an edge enclosure 7, for example Figure 2 As shown, the height of the enclosure is preferably consistent with the thickness of the sieve plate. The edge enclosure 7 can allow the liquid in the reactor to be fully distributed on the sieve plate and then overflow and flow down.

[0045] In the present invention, the number N of the sieve plates can be calculated according to the following formula: N = 5.54 × (t R / W 1 / 2 ) 2 , t R is the residence time of the material (min), W 1 / 2 is the half peak width (min). After setting a certain number of sieve plates in the reactor according to the calculation formula, it is more conducive to the occurrence of polycondensation reaction and / or depolymerization reaction, and the yield of lactide is improved.

[0046] Preferably, the sieve holes of the sieve plate are evenly distributed.

[0047] Preferably, the shape of the sieve hole can be a regular polygon, such as a square, rectangle, rhombus, trapezoid; an irregular polygon, circle, ellipse, etc. It can be one shape or a combination of multiple shapes, preferably a circle.

[0048] Preferably, the mesh holes on the same mesh are of the same size.

[0049] In the present invention, taking a reactor with an annual output of 10,000 tons as an example, the volume of the polycondensation reactor can be 5-10 cubic meters, and can be prepared using 2-4 tons of rare earth steel; the volume of the depolymerization reactor can be 20-50 cubic meters, and can be prepared using 5-20 tons of rare earth steel.

[0050] In the present invention, under the conditions of the above-mentioned preferred method for preparing lactide, the polycondensation reactor and the depolymerization reactor with specific structures and materials can be used to further improve the yield of lactide and the chemical purity of the obtained lactide, reduce production costs, and shorten production time.

[0051] The present invention will be described in detail below through examples. The materials, reagents, etc. used in the examples of the present invention, unless otherwise specified, can be obtained from commercial sources. The experimental methods without specific conditions in the examples are usually carried out under conventional conditions or under conditions recommended by the manufacturer.

[0052] Stannous octoate was purchased from Sinopharm Chemical Reagent Co., Ltd.

[0053] In the following examples, ppm refers to mass concentration.

[0054] In the following embodiments, unless otherwise specified, the amount of L-lactic acid used in the polycondensation reaction is the same.

[0055] The method for detecting the purity of lactic acid shall refer to the enterprise standard Q / JZL03-2021 "Polymer-grade Lactic Acid" of Jilin COFCO Biochemical Co., Ltd. The quality inspection of rare earth steel shall refer to the industry standard "Determination of Lanthanum and Cerium Content in Rare Earth Steel by Inductively Coupled Plasma Mass Spectrometry" YB / T 6125-2023 of the National Technical Committee for Steel Standardization.

[0056] Example 1

[0057] L-lactic acid with 95% chemical purity is continuously introduced into the polycondensation reactor as raw material for polycondensation reaction to obtain lactic acid oligomers, and the lactic acid oligomers are continuously introduced into the depolymerization reactor for depolymerization reaction. 3 ) and depolymerization reactor (volume 25m 3 ) is made of rare earth steel, which is purchased from China North Industries Group Corporation Inner Mongolia North Heavy Industry Group. The rare earth steel contains 98.2% by mass of Fe, 1.6% by mass of C and 10ppm of lanthanide. The number of staggered sieve plates in the polycondensation reactor used is 3, one sieve plate is set on each layer, and each sieve plate is evenly distributed. The bottom sieve plate is set at 1 / 3 of the total height inside the kettle; the number of sieve plates in the depolymerization reactor is 5, one sieve plate is set on each layer, and each sieve plate is evenly distributed. The bottom sieve plate is set at 1 / 5 of the total height inside the kettle. No other catalyst is added to the reactor.

[0058] The rare earth steel polycondensation reactor completes the polycondensation of lactic acid at a reaction temperature of 150±5°C, a vacuum degree of 15kPa, and a time of 240min to obtain lactic acid oligomers (degree of polymerization of 8); the discharge from the polycondensation reactor is fed into a rare earth steel depolymerization reactor for depolymerization at a reaction temperature of 187.5±2.5°C, a pressure of 8kPa, and a time of 180min; wherein, the volume flow rate of lactic acid feed in the polycondensation reactor is 1.3m 3 / h, the feed volume flow rate of lactic acid oligomers in the depolymerization reactor is 2.2m 3 / h.

[0059] Example 2

[0060] L-lactic acid with a chemical purity of 98% is continuously introduced into a polycondensation reactor as a raw material for polycondensation reaction to obtain lactic acid oligomers, and the lactic acid oligomers are continuously introduced into a depolymerization reactor for depolymerization reaction. 3 ) and depolymerization reactor (volume 20m 3 ) is made of rare earth steel, which was purchased from Baotou Iron and Steel (Group) Co., Ltd. Rare earth steel contains 98.1% by mass of Fe; unit mass of rare earth steel contains 1.6% by mass of C and 10ppm of lanthanide. The number of sieve plates in the polycondensation reactor used is 3, one sieve plate is set on each layer, each sieve plate is evenly distributed, and the bottom sieve plate is set at 1 / 7 of the total height inside the reactor body; the number of sieve plates in the depolymerization reactor is 8, one sieve plate is set on each layer, each sieve plate is evenly distributed, and the bottom sieve plate is set at 1 / 4 of the total height inside the reactor body. No other catalysts were added.

[0061] The rare earth steel polycondensation reactor completes the polycondensation of lactic acid at a reaction temperature of 160±5°C, a vacuum degree of 10kPa, and a time of 330min; the discharge from the polycondensation reactor is fed into a secondary rare earth steel depolymerization reactor for depolymerization at a reaction temperature of 192.5±2.5°C, a pressure of 5kPa, and a time of 78min. Among them, the volume flow rate of lactic acid feed in the polycondensation reactor is 1.2m 3 / h, the feed volume flow rate of lactic acid oligomers in the depolymerization reactor is 3.2m 3 / h.

[0062] Example 3

[0063] The method of Example 1 is followed, except that the conditions for the polycondensation reaction are: reaction temperature 165±5°C, pressure 20 kPa, and time 240 min.

[0064] The conditions for the depolymerization reaction are: reaction temperature 205±5°C, pressure 6 kPa, and time 210 min.

[0065] Example 4

[0066] The method of Example 1 is followed, except that 0.5‰ of stannous octoate powder by mass of the lactic acid is added as a catalyst in the polycondensation reaction, 0.5‰ of stannous octoate powder by mass of the lactic acid oligomer is added as a catalyst in the depolymerization reaction, and the catalyst is removed after the reaction is filtered.

[0067] Example 5

[0068] The method of Example 1 is followed, except that no sieve plate is provided and a falling film tube made of rare earth steel is used.

[0069] Example 6

[0070] The method of Example 1 is the same, except that the reaction kettle for the polycondensation reaction and the depolymerization reaction is made of glass lining, and rare earth steel balls with a radius of 1 cm are added as catalysts during the polycondensation reaction and the depolymerization reaction. After the reaction, the rare earth steel balls are removed by filtration.

[0071] In the polycondensation reaction, the amount of rare earth steel used is 6 kg for every ton of lactic acid; in the depolymerization reaction, the amount of rare earth steel used is 9 kg for every ton of lactic acid oligomers.

[0072] Example 7

[0073] The method of Example 5 was followed, except that a two-stage falling film evaporator (made of 22053 stainless steel) was used for the polycondensation reaction and the depolymerization reaction, and rare earth steel balls with a radius of 1 cm were added as catalysts during the polycondensation reaction and the depolymerization reaction. After the reaction, the rare earth steel balls were removed by filtration.

[0074] In the polycondensation reaction, the amount of rare earth steel used is 6 kg for every ton of lactic acid; in the depolymerization reaction, the amount of rare earth steel used is 9 kg for every ton of lactic acid oligomers.

[0075] Comparative Example 1

[0076] The method of Example 1 is followed, except that the polycondensation reaction and depolymerization reaction are carried out in a glass-lined reactor, stannous octoate powder with a mass of 0.5‰ of the mass of lactic acid is added as a catalyst in the polycondensation reaction, and stannous octoate powder with a mass of 0.5‰ of the mass of lactic acid oligomers is added as a catalyst in the depolymerization reaction, and the catalyst is removed after the reaction is filtered.

[0077] Comparative Example 2

[0078] The method of Example 1 is different in that the catalyst added in the polycondensation reaction and the depolymerization reaction is a rare earth metal complex (self-made in the laboratory), and the catalyst is filtered out after the reaction. The structure of the catalyst is as follows:

[0079] Among them, Me is methyl.

[0080] Comparative Example 3

[0081] The method of Example 5 is followed, except that the falling film tube is made of enamel.

[0082] Test Example 1

[0083] The yield of lactide in each of the above examples and comparative examples was tested.

[0084] The reaction equation is

[0085]

[0086] Calculation of lactide yield:

[0087] Yield = (lactide production / lactic acid input) × 1.25 × 100%

[0088] Note: The unit of lactide output is kilogram, and the unit of lactic acid input is kilogram.

[0089] Table 1

[0090]

[0091]

[0092] As can be seen from Table 1, the reaction production of lactide prepared by the rare earth steel described in the present invention can improve the yield of lactide, reduce the production cost of lactide, and reduce the difficulty of purification. Referring to the "Lactide" enterprise standard Q / SWCL001-2023 of COFCO Biomaterials (Yushu) Co., Ltd., the optical purity of the lactide obtained in each embodiment and comparative example was detected. The mass content of D-lactide was less than 1%. The method described in the present invention can also ensure the chemical purity of the product. Moreover, the reactor prepared by rare earth steel has a higher tolerance to pressure, strong acid and temperature, and is particularly suitable for the strong acid negative pressure environment of lactic acid polymerization. The rare earth steel reactor with sieve plates used in Examples 1-3 and 5 can make the reaction system more uniform and the reaction time more controllable compared with the falling film tube. After using the catalyst in Example 5, due to the difficulty in separation, the final yield of lactide is similar to that without adding the catalyst, which further illustrates that the method described in the present invention can obtain high-purity lactide without adding an additional catalyst, and can further improve the yield of lactide, simplify the preparation method, and further reduce the production cost.

[0093] The preferred embodiments of the present invention are described in detail above, but the present invention is not limited thereto. Within the technical concept of the present invention, the technical solution of the present invention can be subjected to a variety of simple modifications, including the combination of various technical features in any other suitable manner, and these simple modifications and combinations should also be regarded as the contents disclosed by the present invention and belong to the protection scope of the present invention.

Claims

1. A method for producing lactide, characterized in that: The method includes: (1) subjecting lactic acid to a polycondensation reaction in a polycondensation reactor in the presence of rare earth steel to obtain a lactic acid oligomer having a degree of polymerization of less than 10; (2) Depolymerizing the lactic acid oligomer in a depolymerization reactor in the presence of rare earth steel to obtain a product containing lactide.

2. The method according to claim 1, wherein: No other catalysts are added to the process.

3. The method according to claim 1 or 2, wherein: The chemical elements contained in the rare earth steel include Fe, C and rare earth elements.

4. The method according to claim 3, wherein: The rare earth steel contains Fe in an amount of 98 mass % or more; and / or, The rare earth steel contains the element C in an amount of 1.7 mass % or less.

5. The method according to claim 4, wherein: The mass content of rare earth elements in the rare earth steel is 6-10ppm.

6. The method according to claim 1 or 2, wherein: The conditions of the polycondensation reaction include: a temperature of 140-180° C. and a pressure of 10-30 kPa.

7. The method according to claim 1 or 2, wherein: The conditions of the depolymerization reaction include: the temperature is 5-80° C. higher than the temperature of the polycondensation reaction, and the pressure is 3-10 kPa.

8. The method according to claim 1 or 2, wherein: The polycondensation reaction and depolymerization reaction are carried out in a lactide production system comprising a polycondensation reactor and a depolymerization reactor connected in series.

9. The method according to claim 1 or 2, wherein: The depolymerization reactor is made of rare earth steel; and / or, The polycondensation reactor is made of rare earth steel.

10. The method according to claim 1 or 2, wherein: The polycondensation reactor and / or the depolymerization reactor is further provided with staggered sieve plates; and / or, The material of the sieve plate is rare earth steel.

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