A static plate-type thin film reactor and its application in lactide synthesis

Abstract

The present invention discloses a static plate-type thin film reactor and its application in the synthesis of lactide. The new process for preparing lactide by using the novel plate-type static thin film reactor disclosed in the present invention can enhance the reaction effect, efficiently prepare lactide products with high optical purity at a higher yield, reduce by-products, simplify the process flow, and significantly reduce investment and downstream separation energy consumption and production costs.

Description

Technical Field

[0001] The invention belongs to the field of lactide synthesis, and particularly relates to a static plate-type thin film reactor and application thereof in lactide synthesis. Background Art

[0002] Polylactic acid (PLA), also known as polylactide, is a typical biodegradable polymer material prepared by chemical synthesis using renewable plant resources as raw materials. The global ban on plastics and waste classification have brought huge potential markets for degradable plastics, especially disposable plastic products. At present, the global market for PLA is growing rapidly and is expected to be in short supply for a long time.

[0003] At present, the industrial production of PLA generally adopts the lactide ring-opening polymerization method. Lactide is a key intermediate for synthesizing PLA, and its synthesis process and key equipment are the key research directions in the industry. At present, the industrial synthesis of lactide generally adopts a certain molecular weight of low polymerization degree polylactic acid to carry out cracking and cyclization reaction in the presence of a catalyst to obtain a lactide product, such as the disclosed technologies of US5247058, US5338822, US5521278, etc. The reactor for preparing lactide by depolymerization of low polymerization degree polylactic acid is the core technology for preparing lactide. Researchers have done a lot of exploration, such as US5258488 using a kettle reactor, JP3083461 / 93, EP0893462A2 using a kettle reactor with reactive distillation, and US5023349, WO9318019A1 using a tower reactor. In view of the thermal sensitivity of the reaction system, a reactor with strong heat and mass transfer capabilities and a low residence time is particularly required. Therefore, more public technologies adopt reactors with enhanced heat and mass transfer, especially thin film reactors, such as WO9509879 and KP20140023143 disclosing horizontal and vertical tubular falling film reactors, as well as WO9509879 and CA2113799 disclosing a vertical scraped film reactor with a rotating part, and EP1873185 disclosing a horizontal thin film reactor.

[0004] Although the existing public technology has solved the high requirements for heat and mass transfer in the depolymerization of polylactic acid to prepare lactide to a certain extent and has achieved industrialization, it still has problems such as complex equipment, low processing energy, and high maintenance costs of moving equipment, and there is still room for improvement. Summary of the invention

[0005] The purpose of the present invention is to further optimize the process and reactor for preparing lactide by depolymerization of polylactic acid based on the existing disclosed technology, simplify the production equipment and reduce the production cost.

[0006] In order to achieve the above invention object, the technical solution of the present invention is as follows:

[0007] A static plate-type thin film reactor includes a plurality of corrugated plates, a material channel is formed between every two corrugated plates, a corrugated area A inclined to the right and a corrugated area C inclined to the left are provided on the corrugated plates, a groove B recessed toward the heat exchange medium side is provided between area A and area C, the corrugations of area A and area C are also provided with raised or recessed small corrugations, and adjacent corrugated plates are sealed by sealing strips or welding. The design of groove B interrupts the direct distribution of liquid from area A to area C, in order to better overcome the problem of no transition between area A and area C in the traditional plate heat exchanger design, and the gradual accumulation of liquid distribution deviation and deviation from uniform distribution. The groove B designed to run through the entire heat exchange plate in the horizontal direction between area A and area C can realize the redistribution of the liquid phase in area A before entering area C, avoid the accumulation of deviations, and ensure that the liquid phase remains uniformly distributed during the flow along this flow direction, which is conducive to the depolymerization reaction.

[0008] Furthermore, the static plate-type thin film reactor is fed from the top, and the preferred raw material inlet 1 is arranged on the side. The bottom of the static plate-type thin film reactor is provided with a liquid phase outlet 5, and the upper end is provided with a gas phase product outlet 4. Compared with the conventional plate heat exchanger, which generally has a gas-liquid co-current design, the present invention adopts a completely opposite design, preferably a gas-liquid counter-current mode. This is mainly for the high viscosity of the lactide reaction system, which requires a greater heat transfer and mass transfer enhancement effect. The gas-liquid counter-current design can utilize the disturbance of the gas phase to the liquid phase to a greater extent to enhance heat and mass transfer.

[0009] Furthermore, a reaction raw material liquid distributor 2 is arranged above the thin film reactor near the feed inlet.

[0010] Furthermore, the thickness of the corrugated plate is 0.1 to 10 mm, preferably 0.2 to 2 mm.

[0011] Furthermore, the corrugations in the A zone and / or the C zone are at an angle of 20-70° to the horizontal direction, preferably 30-60°.

[0012] Furthermore, the groove B runs through the corrugated plate in the horizontal direction, and the bottom of the groove B is flush with or lower than the trough of the corrugation in the A zone and / or the C zone, and preferably the bottom of the groove is 1 to 20 mm lower than the trough of the corrugation in the A zone and / or the C zone. The design of the groove B can enhance the redistribution of the liquid phase, and ensuring good distribution is conducive to enhancing heat and mass transfer.

[0013] Furthermore, the plate in the A zone or the C zone has raised or sunken small corrugations, and the height of the crests and troughs of the small corrugations is 1% to 30% of the height of the crests and troughs of the A zone and / or the C zone, preferably 5% to 20%.

[0014] Furthermore, the A area, groove B and C area are distributed alternately.

[0015] The present invention also provides a method for preparing lactide, comprising the following steps:

[0016] 1) lactic acid oligomers undergo depolymerization reaction in a depolymerization reactor to produce lactide, wherein the depolymerization reactor is a static plate film reactor of the present invention, the static plate film reactor is connected to a reaction distillation tower, or the static plate film reactor is coupled with the reaction distillation tower to form a coupled falling film depolymerization reaction distillation tower, wherein the upper end is the reaction distillation tower and the lower end is the depolymerization reactor;

[0017] 2) The gaseous product produced in the depolymerization reaction section enters the reaction distillation tower from the bottom of the reaction distillation tower.

[0018] 3) A lactide-rich stream is taken out from the side line of the reactive distillation tower as a crude lactide stream for further separation and purification.

[0019] Since the depolymerization reaction of lactic acid oligomers proceeds slowly and the system viscosity is large, in addition to requiring the reactor to have a high heat and mass transfer capacity, if a traditional, simple plate-type falling film reactor is used for the reaction, the depolymerization reaction of lactic acid oligomers cannot occur at a high yield due to the short residence time and poor mass and heat transfer effects. As described above, the present invention adopts a specific structural design to enhance mass and heat transfer; and in the present application, the gas phase outlet is set at the upper end of the plate-type reactor, so that the lactide-rich gas phase produced by the reaction is discharged in countercurrent, which can not only enhance the disturbance of the gas phase to the liquid phase to enhance heat and mass transfer, but also use this disturbance to increase the hysteresis effect of the gas phase on the liquid phase flow, increase the residence time, overcome the problem that the residence time of the traditional downstream heat exchanger is too short, and the reaction can only be enhanced by increasing the temperature, reducing the pressure and increasing the catalyst concentration, resulting in an increase in side reactions and poor reaction effects, and improve the single-pass conversion rate.

[0020] Furthermore, the liquid phase stream of the distillation tower kettle is returned to the depolymerization reaction section for recycling, and an intermediate reactor is provided between the distillation tower kettle and the depolymerization reactor of the coupled falling film depolymerization reaction distillation tower, which can be in the distillation tower or on the pipeline between the distillation tower and the depolymerization reactor;

[0021] Furthermore, the liquid phase stream of the distillation tower bottom stays in the intermediate reactor for a period of time before entering the depolymerization reaction section. The residence time is to promote the reaction of lactic acid in the liquid phase to polylactic acid oligomers, and to avoid low molecular weight lactic acid and oligomers being evaporated to the liquid phase too quickly in the depolymerization reaction section and re-entering the distillation tower instead of participating in the depolymerization reaction, thereby increasing energy consumption in vain. The residence time is generally 0.2 to 2 hours, preferably 1 hour to 2 hours; the reaction temperature of the intermediate reactor is 120-200°C. In the present invention, free lactic acid and the like in the liquid phase of the distillation tower bottom can react in the intermediate reactor to obtain some oligomers, which is beneficial to the reaction.

[0022] Furthermore, the weight average molecular weight Mw of the lactic acid oligomer is 500-5000, preferably 800-2500;

[0023] Furthermore, the reaction temperature of the depolymerization reaction section is 170-240° C., preferably 180-220° C., and the pressure is 0.1-5 kPa, preferably 0.5-2 kPa.

[0024] Furthermore, a stream rich in lactic acid is produced from the top of the reaction distillation section, and a stream rich in lactide is produced from the side line; the pressure of the reaction distillation section is the same as that of the depolymerization reaction section, and the temperature of the reaction distillation section is 100-150°C.

[0025] In the present invention, the lactic acid content in the crude lactide stream withdrawn from the side line of the reactive distillation section is less than 1%, preferably <0.5%, and the lactic acid oligomer content is less than 1%, preferably <0.2%, and more preferably <0.1%.

[0026] Furthermore, the crude lactide stream is purified by distillation in a distillation tower, light components such as lactic acid are removed at the top of the tower, heavy components such as lactic acid oligomers are removed at the bottom of the tower, and a lactide-rich stream is taken out from the side line of the tower. This stream enters the crystallization process below, and a high-purity lactide product is obtained by crystallization.

[0027] In the present invention, by referring to the attached Figure 1 , the "left" in the present invention is attached Figure 1 When the corrugated plate is placed vertically, the direction of the corrugation from bottom to top is the left side, and the "right" mentioned in the present invention means that when the corrugated plate in the accompanying drawings is placed vertically, the direction of the corrugation from bottom to top is the right side. The "left" and "right" in the present invention are relative directions, which are only for the convenience of description and should not limit the present invention.

[0028] The outstanding features and advantages of the present invention are:

[0029] 1. The depolymerization reactor and the distillation tower are coupled, and the gas phase product produced in the depolymerization reaction section enters the reaction distillation section from the bottom of the reaction distillation section, and the crude lactide product is obtained from the side line, which can significantly reduce the lactic acid, water and lactic acid oligomers in the crude lactide, and significantly reduce the technical difficulty and manufacturing cost of the downstream separation process.

[0030] 2. The liquid phase stream passing through the distillation tower is kept in the intermediate reactor for at least 1 hour to convert the remaining lactic acid and lactic acid oligomers into polylactic acid with a higher molecular weight, and then decomposed into lactide again when circulating back to the depolymerization reactor. This avoids the large amount of energy consumption and catalysis of a large number of side reactions caused by the internal circulation of lactic acid oligomers in the gas phase and liquid phase between the distillation tower and the depolymerization reactor, thereby improving the yield and reducing the production cost.

[0031] 3. The novel plate-type thin film depolymerization reactor disclosed in the present invention improves the liquid phase distribution and residence time on the basis of enhancing the heat and mass transfer capabilities, reduces backmixing and retention, and provides a basis for high-yield depolymerization reactions.

[0032] 4. Integrating the high-temperature liquid phase reaction vessel into the distillation tower increases the integration, simplifies the process, and maximizes the flow effect of plug flow, reduces back mixing and side reactions, and ultimately improves the reaction yield and reduces production costs.

[0033] In summary, compared with the known technologies in the current mainstream industry, the method and device disclosed in the present invention significantly reduce equipment investment and production costs. It has outstanding economic advantages.

[0034] Figure 1 A schematic diagram of the corrugations of the corrugated plate of the static plate-type thin film reactor of the present invention;

[0035] Figure 2 It is a schematic diagram of the structure of the plate-type membrane reactor and the material inlet and outlet;

[0036] Figure 3 It is a schematic diagram of the process flow of non-coupled falling film depolymerization reaction distillation depolymerization;

[0037] Figure 4 Schematic diagram of the process flow of the coupled falling film depolymerization reaction distillation tower.

[0038] Among them, 1 is the raw material inlet, 2 is the liquid distributor, 3 is the defoamer, 4 is the gas phase product outlet, 5 is the liquid phase outlet, 6 is the heat transfer oil outlet, and 7 is the heat transfer oil inlet;

[0039] R1 is a reaction distillation tower, R2 is a depolymerization reactor, C1 is a distillation tower, and D1 is a crystallizer. DETAILED DESCRIPTION

[0040] The following further describes the embodiments of the present invention in conjunction with the accompanying drawings and examples. However, the present invention is not limited to the listed examples, but also includes any other known changes within the scope of the rights claimed by the present invention.

[0041] Embodiment 1:

[0042] The prepolymerization was carried out in an intermittent stirring reactor, with lactic acid (Jiangxi Musashino Company, chemical purity 88%, optical purity 99.7%) as the raw material, the system pressure was controlled to be 2.5 kPa, the reaction temperature was gradually increased to 150°C, and the temperature was kept for 2 hours. The reaction product, lactic acid oligomer, Mw=800, was used as the raw material for the depolymerization reaction.

[0043] Reference Figure 1-2As shown, the depolymerization reactor adopts the new plate-type film reactor disclosed in the present invention. The heat exchange plate has a single plate width of 0.2m, a height of 0.8m, a single plate thickness of 0.6mm, and is composed of 5 plates, with a projected heat exchange area of ​​0.8m2. The angle between the corrugation in zone A and zone C and the horizontal direction is 45°, the crest spacing of the corrugated plate is b=20mm, the width is a=10mm, the groove width of zone B is 20mm, and the depth is 10mm deeper than the trough of zone A. Small strip-shaped corrugations parallel to the horizontal direction are processed on the large corrugations, the crest spacing of the small corrugations is 3mm, and the trough depth is 1.5mm.

[0044] The hot oil temperature on the heating side of the depolymerization reactor was controlled at 220°C, the feed rate of polylactic acid was 6 kg / h, and the product gas phase was collected and condensed to obtain a crude lactide product. The crude lactide yield was 22.6%, and the optical purity of L-lactide was 98.8%.

[0045] Example 2

[0046] The reactor of Example 1 and the same lactic acid oligomer raw material were used, the hot oil temperature was controlled at 220°C, the feed rate of polylactic acid was 3 kg / h, and the product gas phase was collected and condensed to obtain a crude lactide product. The crude lactide yield was 46.5%, and the optical purity of L-lactide was 98.2%.

[0047] Example 3

[0048] The reactor of Example 1 and the same lactic acid oligomer raw material were used, the hot oil temperature was controlled at 240°C, the feed rate of polylactic acid was 2kg / h, and the product gas phase was collected and condensed to obtain a crude lactide product. The crude lactide yield was 86.5%, and the optical purity of L-lactide was 97.6%.

[0049] Comparative Example 1

[0050] The same process and the same lactic acid oligomer raw materials as in Example 1 were used. The depolymerization reactor was a conventional tubular falling film reactor with a tube diameter of 32*2 mm and a length of 3 m. There were 3 tubes in total and a heat exchange area of ​​0.79 m 2 The reactor is used with a hot oil temperature of 220°C and a feed rate of polylactic acid of 6 kg / h. The product gas phase is collected and condensed to obtain a crude lactide product. The crude lactide yield is 10.5%, and the optical purity of L-lactide is 97.7%.

[0051] Comparative Example 2

[0052] The same process and the same lactic acid oligomer raw material as in Comparative Example 1 were used. The reactor was used with a hot oil temperature of 220°C and a feed polylactic acid flow rate of 3 kg / h. The product gas phase was collected and condensed to obtain a crude lactide product. The crude lactide yield was 14.5%, and the optical purity of L-lactide was 97.4%.

[0053] Comparative Example 3

[0054] The same process and the same lactic acid oligomer raw material as in Comparative Example 1 were used. The hot oil temperature was controlled at 240°C, the feed polylactic acid flow rate was 2kg / h, and the product gas phase was collected and condensed to obtain a crude lactide product. The crude lactide yield was 36.9%, and the optical purity of L-lactide was 96.1%.

[0055] From the data of Examples 1-3 and Comparative Examples 1-3, it can be seen that, compared with the traditional falling film reactor, the new plate-type thin film reactor disclosed in the present invention can achieve higher product yield and product purity, with obvious advantages. The reason is that the new plate-type thin film reactor disclosed in the present invention overcomes the existing bottlenecks of flow and distribution, heat transfer and mass transfer in the traditional falling film reactor, greatly improves the heat transfer and mass transfer efficiency and longer residence time, which is more beneficial for the high viscosity and slow reaction system of lactide reaction.

[0056] Example 4

[0057] Use Figure 3 The process shown is to couple the reaction distillation tower R1 with the plate reactor R2. The structure of the plate reactor is the same as that of Example 1. The reaction distillation tower R1 adopts the reaction distillation tower of POPE Company. The distillation tower has a diameter of 100 mm, a packing height of 1.2 m, and is filled with 2.5 mm*2.5 mm triangular spiral packing. The side line extraction position is the lower 1 / 3 of the packing section, and the side line liquid phase is extracted. 10 sieve plate trays are arranged at the lower part of the packing. The lower end of the distillation tower is connected to the depolymerization reactor, and the depolymerization reactor adopts the same new static plate film reactor as in Example 1.

[0058] The conditions and results of the reactive distillation column 1 are shown below.

[0059] Table 1 Process parameters of each section of the reactive distillation tower

[0060]

[0061] Table 2 Reaction distillation tower each section extraction

[0062]

[0063] Table 3 Composition data of each stream

[0064] Lactic acid (%) Oligomer(%) Lactide(%) Top extraction 55.70 1.52 42.78 Side line extraction 1.18 0.87 97.95

[0065] The distillation tower C1 is a 100mm diameter distillation tower from POPE, USA, with a packing height of 1.2m, filled with 2.5mm*2.5mm triangular spiral packing, the feed position is the upper 1 / 3 of the packing section, the side line withdrawal position is the lower 1 / 3 of the packing section, and the side line gas phase is withdrawn. The top pressure of the distillation tower 1 is 1kPaA, the reflux ratio is 15:1, the top temperature is 120℃, and the bottom temperature is about 150℃. The distillation results are as follows:

[0066] Table 4

[0067]

[0068] After being purified by the distillation tower C1, a high-purity M-lactide product can be obtained through further crystallization for synthesizing PLA or modified products, thereby improving the overall yield and economic benefits of the device.

[0069] The test results of the crystalline product are as follows: purity 99.9%, acid value 3mmol / kg, which can meet the quality requirements for high-purity lactide in the market.

Claims

1. A static plate-type thin film reactor, characterized in that: It comprises a plurality of corrugated plates, a material channel is formed between every two corrugated plates, the corrugated plates are provided with a corrugated area A inclined to the right and a corrugated area C inclined to the left, a groove B recessed toward the heat exchange medium side is provided between the A area and the C area, and the corrugations in the A area and the C area are also provided with raised or recessed small corrugations; the corrugations in the A area and / or the C area are at an angle of 20-70° to the horizontal direction, the groove B passes through the corrugated plates in the horizontal direction, the bottom of the groove B is flush with or lower than the trough of the corrugations in the A area and / or the C area, and adjacent corrugated plates are sealed by sealing strips or welding; The static plate type thin film reactor is fed from the top, a liquid phase outlet is arranged at the bottom of the static plate type thin film reactor, and a gas phase product outlet is arranged at the top.

2. The static plate type thin film reactor according to claim 1, characterized in that: The feed inlet of the static plate-type thin film reactor is arranged on the side.

3. The static plate type thin film reactor according to claim 1, characterized in that: A reaction raw material liquid distributor is arranged above the film reactor near the feed inlet.

4. The static plate type thin film reactor according to claim 1, characterized in that: The thickness of the corrugated plate is 0.1-10 mm.

5. The static plate type thin film reactor according to claim 4, characterized in that: The thickness of the corrugated plate is 0.2-2 mm.

6. The static plate type thin film reactor according to claim 1, characterized in that: The corrugations in the A zone and / or the C zone form an angle of 30-60° with the horizontal direction.

7. The static plate type thin film reactor according to claim 1, characterized in that: The bottom of groove B is 1 to 20 mm lower than the trough of the corrugations in area A and / or area C.

8. The static plate type thin film reactor according to claim 1, characterized in that: There are raised or sunken small corrugations on the plate body of the A zone or the C zone, and the height of the crest and trough of the small corrugations is 1% to 30% of the height of the crest and trough of the A zone and / or the C zone.

9. The static plate type thin film reactor according to claim 8, characterized in that: The height of the crest and trough of the small corrugation is 5% to 20% of the height of the crest and trough of the A zone and / or the C zone.

10. The static plate type thin film reactor according to claim 1, characterized in that: The A area, groove B and C area are distributed alternately.

11. A method for preparing lactide, characterized in that: The following steps are involved: 1) lactic acid oligomers undergo depolymerization reaction in a depolymerization reactor to produce lactide, wherein the depolymerization reactor is the static plate film reactor according to any one of claims 1 to 5, the static plate film reactor is connected to a reaction distillation tower, or the static plate film reactor is coupled with the reaction distillation tower to form a coupled falling film depolymerization reaction distillation tower, wherein the upper end is the reaction distillation tower and the lower end is the depolymerization reactor; 2) The gas phase product produced in the depolymerization reaction section enters the reaction distillation tower from the bottom of the reaction distillation tower; 3) A lactide-rich stream is taken out from the side line of the reactive distillation tower as a crude lactide stream for further separation and purification.

12. The preparation method according to claim 11, characterized in that: The liquid phase stream of the distillation tower kettle is returned to the depolymerization reaction section for recycling. An intermediate reactor is arranged between the distillation tower kettle and the depolymerization reactor of the coupled falling film depolymerization reaction distillation tower, in the distillation tower or on the pipeline between the distillation tower and the depolymerization reactor.

13. The preparation method according to claim 11, characterized in that: The liquid phase stream of the distillation tower bottom stays in the intermediate reactor for a period of time before entering the depolymerization reaction section. The residence time is 0.2 to 2 hours, and the reaction temperature of the intermediate reactor is 120-200°C.

14. The preparation method according to claim 13, characterized in that: The residence time is 1 h to 2 h.

15. The preparation method according to claim 11, characterized in that: The weight average molecular weight Mw of the lactic acid oligomer is 500-5000.

16. The preparation method according to claim 15, characterized in that: The weight average molecular weight Mw of the lactic acid oligomer is 800-2500.

17. The preparation method according to claim 11, characterized in that: The reaction temperature of the depolymerization reaction section is 170-240° C., and the pressure is 0.1-5 kPa.

18. The preparation method according to claim 17, characterized in that: The reaction temperature of the depolymerization reaction section is 180-220° C. and the pressure is 0.5-2 kPa.

19. The preparation method according to claim 11, characterized in that: A stream rich in lactic acid is taken out from the top of the reaction distillation section, and a stream rich in lactide is taken out from the side line; the pressure of the reaction distillation section is the same as that of the depolymerization reaction section, and the temperature of the reaction distillation section is 100-150°C.

20. The preparation method according to claim 11, characterized in that: The lactic acid content in the crude lactide stream taken out from the side line of the reactive distillation section is less than 1%, and the lactic acid oligomer content is less than 1%.

21. The preparation method according to claim 20, characterized in that: The lactic acid content in the crude lactide stream taken out from the side line of the reactive distillation section is less than 0.5%, and the lactic acid oligomer content is less than 0.2%.

22. The preparation method according to claim 21, characterized in that: The content of lactic acid oligomers in the crude lactide stream withdrawn from the side line of the reactive distillation section is less than 0.1%.

23. The preparation method according to claim 11, characterized in that: The crude lactide stream is purified by distillation in a distillation tower, light components are removed at the top of the tower, heavy components are removed at the bottom of the tower, and a lactide-rich stream is produced from the side line of the tower; this stream enters the downstream crystallization process, and a high-purity lactide product is obtained by crystallization.

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