Lactic acid dehydration device and lactic acid dehydration process

Abstract

This invention provides a lactic acid dehydration device and a lactic acid dehydration process. The lactic acid dehydration device includes a reactor (1), a distillation column (2), a primary condenser (3), a primary condenser fraction tank (4), and a gas transfer pump (7); the reactor (1), distillation column (2), primary condenser (3), and gas transfer pump (7) are sequentially connected to form a circulation device; the primary condenser (3) is connected to the primary condenser fraction tank (4); the wall material of the distillation column (2) is plastic or alloy. This invention also provides a lactic acid dehydration process, which uses the above-mentioned lactic acid dehydration device and employs a bubbling dehydration method. The lactic acid dehydration device and process provided by this invention have good dehydration effects, reducing the moisture content of lactic acid from a high level to below 1.0%.

Description

Technical Field

[0001] This invention relates to a lactic acid dehydration device and a lactic acid dehydration process, belonging to the field of chemical production equipment manufacturing. Background Technology

[0002] Biodegradable materials are polymeric materials that exhibit excellent performance during use and can be rapidly hydrolyzed and degraded by enzymes or microorganisms after use. Among various biodegradable materials, polylactic acid (PLA) is attracting widespread attention due to its uniquely good biocompatibility, degradability, and excellent processing properties.

[0003] There are two main methods for synthesizing polylactic acid (PLA). One method is direct polycondensation of lactic acid. This method has a simple production process, but due to the presence of impurities in the system and the fact that the polycondensation reaction of lactic acid is reversible, the resulting PLA has a relatively small molecular weight, poor strength, and no practical value. The other method for synthesizing PLA is to first obtain cyclic dimer lactide by oligomerization and depolymerization of lactide, and then carry out ring-opening polymerization of lactide as a monomer to obtain PLA. This method can obtain products with a high relative molecular weight of millions and has become the main method for synthesizing PLA.

[0004] Generally, commercially available lactic acid contains 7-20% free water to reduce viscosity and melting point, and to prevent lactic acid oligomerization. The first step in preparing lactide from lactic acid is to remove the free water. However, simple distillation cannot effectively remove water to a very low level. Therefore, current industrial production typically uses vacuum dehydration, with a representative dehydration temperature of 150°C. At this temperature, the vapor pressure of lactic acid is approximately 8.5 kPa, indicating significant evaporation into the gas phase. Therefore, the mixed vapor of lactic acid and water usually needs to be separated, allowing the lactic acid to be condensed and refluxed to avoid raw material loss. However, at this temperature, significant volatilization of lactic acid occurs, entering the gas phase, and lactic acid is highly corrosive. Therefore, under current technological conditions, the dehydration vessel in industrial production equipment must first be equipped with a dehydration distillation column. Secondly, the dehydration vessel, distillation column, and distillation column packing must all use Hastelloy to cope with corrosion problems under long-term operation, resulting in high equipment investment costs.

[0005] Acetic acid has a similar corrosive effect on metals as lactic acid. Examples of industrial corrosion protection solutions for acetic acid-containing systems include: CN106365977A discloses an acetic acid production apparatus with a 0.5-1 cm zirconium oxide coating inside the bubbling reaction tower, and a reaction temperature of 175-200℃; CN206995905U discloses an acetic acid recovery tower with an inner wall made of titanium-molybdenum alloy. Since acetic acid reactions are often carried out under high temperature and high pressure, alloys must be used to withstand these conditions.

[0006] Besides the large investment in equipment, vacuum dehydration of lactic acid requires the use of vacuum pumps and demands high levels of stable vacuum control. Furthermore, it places stringent requirements on the airtightness of the equipment. Because the equipment operates under negative pressure, poor airtightness allows air to enter, leading to product oxidation, the production of unwanted byproducts, and discoloration such as yellowing or blackening of the material.

[0007] As can be seen from the above, the existing industrialization solutions use vacuum and high-temperature dehydration technology, and the key equipment is made of Hastelloy. However, there are still many problems that need to be solved. Summary of the Invention

[0008] To solve the above-mentioned technical problems, the present invention aims to provide a lactic acid dehydration device and a lactic acid dehydration process. By using a distillation column made of plastic material, the cost of lactic acid dehydration can be reduced. Furthermore, by using gas bubbling dehydration through recycling, the cost of gas usage can be reduced, and a good dehydration effect can be achieved.

[0009] To achieve the above objectives, the present invention provides a lactic acid dehydration apparatus, wherein the lactic acid dehydration apparatus includes a reactor, a distillation column, a primary condenser, a primary condenser fraction tank, and a gas transfer pump;

[0010] The reactor, distillation column, primary condenser, and gas transfer pump are connected in sequence to form a circulation device;

[0011] The primary condenser is connected to the primary condenser distillation tank;

[0012] The reactor wall is made of plastic or alloy, preferably reinforced polypropylene or Hastelloy.

[0013] The wall material of the distillation column is plastic or alloy, preferably reinforced polypropylene or Hastelloy.

[0014] In the above-mentioned lactic acid dehydration device, preferably, the distillation column is a packed column, and the packing material of the packed column is plastic or ceramic, more preferably ceramic rings.

[0015] In the above-mentioned lactic acid dehydration device, preferably, the lactic acid dehydration device further includes a secondary condenser, which is connected between the primary condenser and the gas delivery pump.

[0016] In the above-mentioned lactic acid dehydration device, preferably, the lactic acid dehydration device further includes a secondary condenser distillation tank, which is connected to the secondary condenser.

[0017] In the above-mentioned lactic acid dehydration device, preferably, the lactic acid dehydration device further includes a buffer tank and a back pressure valve, wherein the buffer tank and the back pressure valve are connected in sequence between the gas delivery pump and the reactor.

[0018] In the above-mentioned lactic acid dehydration apparatus, preferably, the reactor includes a gas distributor.

[0019] In industrial production, given the strong corrosiveness of lactic acid, Hastelloy equipment is typically required for reaction vessels and distillation columns, resulting in high equipment investment. Lowering the reaction temperature and reducing the corrosiveness of lactic acid can extend the equipment's lifespan, reduce average equipment costs, and even allow for the use of polypropylene as the equipment material, further reducing costs. However, currently, there is no lactic acid dehydration equipment that primarily utilizes circulating inert gas bubbling for water removal. This invention employs lightweight materials such as plastics and ceramics, combined with advanced dehydration process conditions, to obtain a lighter and lower-cost lactic acid dehydration device. This device can efficiently and cost-effectively remove free water from lactic acid while maintaining good product quality and avoiding side reactions such as oxidation.

[0020] The present invention also provides a lactic acid dehydration process, which is carried out using the above-mentioned lactic acid dehydration apparatus, and the lactic acid dehydration process includes the following steps:

[0021] An inert gas is fed into the reactor via a gas delivery pump. Inside the reactor, water vapor and lactic acid vapor are bubbled out and then separated in a distillation column to obtain a high-temperature mixed gas containing water vapor. After being condensed in a primary condenser, a room-temperature mixed gas containing saturated water vapor and water are obtained. The room-temperature mixed gas containing saturated water vapor then enters the gas delivery pump to begin circulation.

[0022] In the above-mentioned lactic acid dehydration process, preferably, the water obtained by condensation in the primary condenser enters the primary condenser fraction tank.

[0023] In the above-mentioned lactic acid dehydration process, preferably, the inert gas is one or a combination of two or more of nitrogen, helium, and carbon dioxide.

[0024] In the above-mentioned lactic acid dehydration process, preferably, the ambient temperature mixed gas containing saturated water vapor enters the secondary condenser for further condensation to obtain a low temperature mixed gas containing saturated water vapor, and the low temperature mixed gas containing saturated water vapor enters the gas delivery pump to start circulation.

[0025] In the above-mentioned lactic acid dehydration process, preferably, the water obtained by condensation in the secondary condenser enters the secondary condenser fractionation tank.

[0026] In the above-mentioned lactic acid dehydration process, preferably, the reaction temperature in the reactor is 80-150℃, more preferably 100-125℃, and even more preferably 99-105℃.

[0027] In the above-mentioned lactic acid dehydration process, preferably, the working pressure of the reactor is 80-120 kPa absolute pressure, more preferably 95-105 kPa absolute pressure.

[0028] In the above-mentioned lactic acid dehydration process, preferably, the cooling medium of the primary condenser is cooling water, and the temperature of the cooling water is 0-40℃, more preferably 0-30℃.

[0029] In the above-mentioned lactic acid dehydration process, preferably, the cooling medium of the secondary condenser is chilled water, and the temperature of the chilled water is 0-15℃, more preferably 0-10℃.

[0030] It should be noted that although the main purpose of the lactic acid dehydration device and lactic acid dehydration process of the present invention is to remove free water from lactic acid, since lactic acid undergoes spontaneous esterification reaction at temperatures above 100°C, which generates water, the lactic acid dehydration device and lactic acid dehydration process of the present invention will also simultaneously remove the water generated by the lactic acid reaction under the process conditions. The specific amount of water removed and the degree of reaction are affected by conditions such as temperature, reaction time, and whether there is a catalyst, which will not be elaborated here.

[0031] For lactic acid dehydration, if a vacuum dehydration process is used, the water content in the product is limited by the dehydration temperature and reactor pressure. To achieve a water content of ≤1% in lactic acid using existing technology, the theoretical requirement is to control the water vapor partial pressure to 4.76 kPa. The corresponding temperature for water at a saturated vapor pressure of 4.76 kPa is 32°C. Therefore, existing vacuum dehydration technology requires controlling the reactor internal temperature to 150°C, the reactor pressure to approximately 13.3 kPa (absolute pressure), and the condenser outlet temperature to ≤32°C. Furthermore, at this point, the vapor phase contains a large amount of lactic acid vapor, necessitating thorough distillation.

[0032] Compared with existing technologies, this invention employs an inert gas bubbling and water-removing technology under normal or slightly positive pressure. The water content in the product is no longer limited by gas pressure, but only controlled by temperature and the saturated vapor pressure of water. When the lactic acid dehydration device and process of this invention dehydrate at 100°C, the saturated vapor pressure of water is 101 kPa at 100°C and 121 kPa at 105°C; the saturated vapor pressure of water is 1.23 kPa at 10°C and 1.07 kPa at 8°C. Therefore, by simply introducing anhydrous nitrogen or other bubbling gases, or by cooling recycled nitrogen or other bubbling gases to below 10°C through a secondary condenser, it is possible to remove 1% of the water from lactic acid at 105°C; or by cooling recycled nitrogen to below 8°C through a secondary condenser, it is possible to remove 1% of the water from lactic acid at 100°C.

[0033] Therefore, the technical solution of the present invention can greatly reduce the dehydration temperature and reduce energy consumption.

[0034] The dehydration pressure of this invention can be controlled near atmospheric pressure, effectively reducing the pressure resistance requirements of the container. Therefore, the reactor no longer needs to be a Class I pressure vessel; conventional containers can be used, significantly reducing the design, material, and manufacturing costs. The lactic acid dehydration device of this invention uses plastic walls, can operate at atmospheric pressure, is easy to operate, saves on equipment investment costs, reduces equipment corrosion, saves on bubbling gas consumption, reduces oxygen infiltration, and improves product quality. It has broad application prospects and significant economic benefits.

[0035] Because the dehydration temperature of this invention is reduced to around 100°C, the saturated vapor pressure of lactic acid is reduced to about 0.5 kPa, which is only 6% of the vapor pressure (8.5 kPa) at 150°C. Therefore, the lactic acid content in the gas phase is greatly reduced, the difficulty of separating lactic acid in the distillation column is reduced, the height of the distillation column is greatly shortened, and the investment in the equipment is reduced.

[0036] Because the temperature is lower, the corrosiveness of lactic acid is significantly reduced compared to 150°C. Therefore, corrosion-resistant Hastelloy special materials are no longer needed; reinforced polypropylene and other plastics can be used as materials for reactors and distillation columns, greatly reducing the investment cost of the equipment. Furthermore, since reinforced polypropylene has a significantly lower density than stainless steel and Hastelloy, the weight of the entire equipment is reduced, with some equipment even reduced to less than 1 / 5 of its original weight. This correspondingly reduces the load-bearing requirements of the entire equipment and the investment cost of steel structures.

[0037] Even with existing equipment, dehydration using circulating inert gas still offers advantages over vacuum dehydration, for example: 1) In vacuum dehydration, the internal pressure of the reactor is lower than the external pressure, inevitably leading to a small amount of air seeping into the reactor in industrial settings, causing product oxidation and quality degradation; while inert gas dehydration can operate under conditions where the internal pressure is higher than the external pressure, preventing air infiltration; 2) Vacuum dehydration requires continuous vacuum maintenance, resulting in high operating and maintenance costs for vacuum pumps, while circulating inert gas dehydration uses a gas circulation pump, which has lower equipment and operating / maintenance costs than vacuum pumps. Therefore, the circulating inert gas dehydration process of this invention can also be used in existing equipment.

[0038] In summary, compared with the prior art, the present invention has the following advantages:

[0039] 1. The lactic acid dehydration device provided by the present invention uses a distillation column with plastic wall material, which can greatly reduce the use of expensive metal materials such as Hastelloy, reduce the weight of the device, and save on device investment.

[0040] 2. The lactic acid dehydration process of the present invention reduces gas usage costs by using nitrogen and other gases for bubbling dehydration; and reduces heating and cooling costs by selecting appropriate temperature and pressure; and reduces oxygen infiltration and accumulation by operating under near-normal or slightly higher pressure conditions, thereby improving product quality, reducing or eliminating the need for vacuum-resistant container design and manufacturing, and saving equipment design and manufacturing costs.

[0041] 3. The lactic acid dehydration process of the present invention has the advantages of simple operation and low equipment investment, and has great economic benefits. Attached Figure Description

[0042] Figure 1 This is a schematic diagram of a lactic acid dehydration device.

[0043] Explanation of main icon numbers:

[0044] 1. Reactor; 2. Distillation column; 3. Primary condenser; 4. Primary condenser fraction tank; 5. Secondary condenser; 6. Secondary condenser fraction tank; 7. Gas transfer pump; 8. Buffer tank; 9. Back pressure valve; 10. Gas distributor. Detailed Implementation

[0045] In order to provide a clearer understanding of the technical features, objectives and beneficial effects of the present invention, the technical solution of the present invention will now be described in detail below, but it should not be construed as limiting the scope of implementation of the present invention.

[0046] Example 1

[0047] This embodiment provides a lactic acid dehydration device, the structure of which is as follows: Figure 1 As shown, the components within the dashed boxes are optional additions.

[0048] The lactic acid dehydration device includes a reactor 1, a distillation column 2, a primary condenser 3, a primary condenser fraction tank 4, and a gas transfer pump 7;

[0049] The top gas phase outlet of reactor 1 is connected to the bottom inlet of distillation column 2, the top gas phase outlet of distillation column 2 is connected to the inlet of primary condenser 3, the outlet of primary condenser 3 is connected to the inlet of gas transfer pump 7 through a gas phase pipeline, and the outlet of gas transfer pump 7 is connected to the gas inlet of reactor 1; primary condenser 3 is also connected to primary condenser fractionation tank 4.

[0050] The walls of reactor 1, the walls of distillation column 2, and the packing of distillation column 2 are made of Hastelloy alloy. The effective volume of reactor 1 is about 75L, and a gas distributor 10 is installed inside.

[0051] The lactic acid dehydration process provided in this embodiment includes:

[0052] Nitrogen gas is pumped into reactor 1 via gas delivery pump 7. In reactor 1, bubbling carries out water vapor and lactic acid vapor. After separation by distillation column 2, a high-temperature mixed gas containing water vapor is obtained. After condensation by primary condenser 3, room-temperature nitrogen gas containing saturated water vapor is obtained and re-enters gas delivery pump 7 to start the cycle. The water obtained from condensation and separation in primary condenser 3 enters primary condenser fraction tank 4.

[0053] The process parameters for this process are as follows:

[0054] The operating temperature of reactor 1 is 140-150℃, and the operating pressure is 110-120kPa absolute pressure.

[0055] The cooling medium for primary condenser 3 is cooling water, and the medium temperature is 22-30℃;

[0056] The feed weight of lactic acid is 50 kg, with a purity of 92% (theoretically containing 4 kg of free water, approximately 8%, and the theoretical maximum water production from the esterification reaction is no more than 9.2 kg). The dehydration time (starting from when the reactor temperature reaches 140℃) is 1.5 hours, and the dehydrated fraction is 8.6 kg. The lactic acid content in the fraction is 0.7% (GC-FID), and the water content is 99.3% (calculated after deducting lactic acid). The moisture content of the lactic acid and oligomers in the reactor after dehydration is 1.0%, and the color number is 90APHA (140℃).

[0057] Example 2

[0058] This embodiment provides a lactic acid dehydration device, the structure of which is as follows: Figure 1 As shown.

[0059] The lactic acid dehydration device is based on Example 1 and includes a secondary condenser 5, a secondary condenser fractionation tank 6, a buffer tank 8, and a back pressure valve 9.

[0060] The top gas phase outlet of reactor 1 is connected to the bottom inlet of distillation column 2. The top gas phase outlet of distillation column 2 is connected to the inlet of primary condenser 3. The outlet of primary condenser 3 is connected to the inlet of secondary condenser 5 via a gas phase pipeline. The outlet of secondary condenser 5 is connected to the inlet of gas transfer pump 7. The outlet of gas transfer pump 7 is connected to the inlet of buffer tank 8. The outlet of buffer tank 8 is connected to the gas inlet of reactor 1 via back pressure valve 9. Primary condenser 3 is connected to primary condenser fractionation tank 4, and secondary condenser 5 is connected to secondary condenser fractionation tank 6.

[0061] The walls of reactor 1 and distillation column 2 are made of reinforced polypropylene, the packing of distillation column 2 is ceramic rings, and the effective volume of reactor 1 is about 25L.

[0062] The process parameters for this process are as follows:

[0063] The operating temperature of reactor 1 is 99-105℃, and the operating pressure is 95-105kPa absolute pressure.

[0064] The cooling medium for primary condenser 3 is cooling water, and the medium temperature is 5-40℃;

[0065] The cooling medium for the secondary condenser 5 is chilled water, and the medium temperature is 3-10℃.

[0066] The feed weight of lactic acid is 15 kg, with a purity of 90% (theoretically containing 1.5 kg of free water, approximately 10%, and the theoretical maximum water production from the esterification reaction is no more than 2.7 kg). The dehydration time (starting from when the reactor temperature reaches 99°C) is 1.5 hours, and the dehydrated fraction is 2.04 kg. The lactic acid content in the fraction is 0.2%, and the water content is 99.8% (calculated by deducting lactic acid). The water content of the lactic acid and oligomers in the reactor after dehydration is 0.89%, and the color number is 25APHA (100°C).

[0067] The results of Examples 1 and 2 show that the lactic acid dehydration device and lactic acid dehydration process provided by the present invention have good dehydration effect and can reduce the lactic acid moisture content from a high level to below 1.0%.

[0068] This invention is not limited to the above embodiments. Any improvements and modifications made by those skilled in the art based on the disclosure of this invention without departing from the scope of this invention should be within the protection scope of this invention.

Claims

1. A lactic acid dehydration apparatus, wherein, The lactic acid dehydration device comprises a reactor (1), a rectifying tower (2), a primary condenser (3), a primary condenser fraction tank (4), and a gas delivery pump (7); The reactor (1), the rectifying tower (2), the primary condenser (3), and the gas delivery pump (7) are sequentially connected together to form a circulating device. The primary condenser (3) is connected to the primary condenser fraction tank (4). The material of the reactor (1) is plastic or alloy. The material of the rectifying tower (2) is plastic or alloy. The gas delivery pump (7) is used for delivering inert gas and mixed gas containing water vapor.

2. The lactic acid dehydration apparatus according to claim 1, wherein The rectifying tower (2) is a packed tower, and the material of the packing of the packed tower is plastic or ceramic.

3. The lactic acid dehydration apparatus according to claim 1 or 2, wherein The reactor (1) comprises a gas distributor.

4. The lactic acid dehydration apparatus according to claim 1, wherein The lactic acid dehydration device further comprises a buffer tank (8) and a back pressure valve (9), which are sequentially connected between the gas delivery pump (7) and the reactor (1).

5. The lactic acid dehydration apparatus according to claim 1, wherein The material of the reactor (1) is reinforced polypropylene or hastelloy.

6. The lactic acid dehydration apparatus according to claim 1, wherein The material of the rectifying tower (2) is reinforced polypropylene or hastelloy.

7. The lactic acid dehydration apparatus according to claim 2, wherein The material of the packing of the packed tower is porcelain ring.

8. A lactic acid dehydration process, which is performed by using the lactic acid dehydration device according to any one of claims 1-7, and comprises the following steps: Inert gas is input into the reactor (1) through the gas delivery pump (7), water vapor and lactic acid vapor are brought out by bubbling in the reactor (1), and then enter the rectifying tower (2) for separation to obtain high-temperature mixed gas containing water vapor, which is condensed by the primary condenser (3) to obtain normal-temperature mixed gas containing saturated water vapor and water, and the normal-temperature mixed gas containing saturated water vapor enters the gas delivery pump (7) to start circulation.

9. The lactic acid dehydration process of claim 8, wherein, The inert gas is one or a combination of two or more of nitrogen, helium, and carbon dioxide.

10. The lactic acid dehydration process of claim 8 wherein, The normal-temperature mixed gas containing saturated water vapor enters the secondary condenser (5) for further condensation to obtain low-temperature mixed gas containing saturated water vapor, which enters the gas delivery pump (7) to start circulation.

11. The lactic acid dehydration process of claim 8 wherein, The reaction temperature in the reactor (1) is 80-150℃.

12. The lactic acid dehydration process of claim 11, wherein, The reaction temperature in the reactor (1) is 100-125℃.

13. The lactic acid dehydration process of claim 11, wherein, The reaction temperature in the reactor (1) is 99-105℃.

14. The lactic acid dehydration process of claim 8, wherein, The working pressure of the reactor (1) is 80-120 kPa.

15. The lactic acid dehydration process of claim 14, wherein, The working pressure of the reactor (1) is 95-105 kPa.

16. The lactic acid dehydration process of claim 8 wherein, The cooling medium of the primary condenser (3) is cooling water, and the temperature of the cooling water is 0-40℃.

17. The lactic acid dehydration process of claim 16, wherein, The temperature of the cooling water is 0-30℃.

18. The lactic acid dehydration process of claim 10, wherein, The cooling medium of the secondary condenser (5) is chilled water, and the temperature of the chilled water is 0-15℃.

19. The lactic acid dehydration process of claim 18, wherein, The temperature of the chilled water is 0-10℃.

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