Lactic acid dehydration device and lactic acid dehydration process

Abstract

The invention provides a lactic acid dehydration device and a lactic acid dehydration process. 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 with the primary condenser fraction tank (4); and the wall of the rectifying tower (2) is made of plastic or alloy. The invention also provides a lactic acid dehydration process, which adopts the lactic acid dehydration device and adopts a bubbling dehydration mode. The lactic acid dehydration device and the lactic acid dehydration process provided by the invention have a good dehydration effect, and the moisture content of lactic acid can be reduced to 1.0% or below from higher.

Description

Technical Field

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

[0002] Biodegradable materials refer to polymer materials that have excellent performance during use and can be rapidly hydrolyzed and degraded by enzymes or microorganisms after use. Among various biodegradable materials, polylactic acid has attracted wide attention from the world due to its good biocompatibility, degradability, and excellent processing performance.

[0003] There are mainly two methods for synthesizing polylactic acid; one is direct polycondensation of lactic acid. The production process of this method is simple, but due to the presence of impurities in the system and the fact that the lactic acid polycondensation reaction is a reversible reaction, the resulting polylactic acid has a relatively low molecular weight, poor strength, and no practical value; the other method for synthesizing polylactic acid is that lactic acid is first oligomerized and depolymerized to obtain cyclic dimer lactide, and then lactide is used as a monomer for ring-opening polymerization reaction to obtain polylactic acid. This method can obtain high molecular weight products of up to millions, and has become the main method for synthesizing polylactic acid.

[0004] Generally, commercially available lactic acid contains 7-20% free water to reduce viscosity and melting point and prevent lactic acid oligomerization, etc. The first step in preparing lactide from lactic acid is to remove the free water in lactic acid. However, simple distillation cannot effectively remove water to a very low level. For this reason, vacuum dehydration is usually used in existing industrial production, and the representative dehydration temperature is 150°C. At this temperature, the vapor pressure of lactic acid is about 8.5 kPa, and there is already a relatively obvious evaporation into the gas phase. Therefore, the mixed vapor of lactic acid and water usually needs to be separated to condense and reflux lactic acid to avoid raw material loss. However, at this temperature, lactic acid has obvious volatilization and enters the gas phase, and lactic acid has strong corrosiveness. Therefore, under the existing technical conditions, the dehydration kettle of the industrial production device must first be equipped with a dehydration rectification tower. Secondly, the dehydration kettle, rectification tower, and rectification tower packing must all use Hastelloy to cope with the corrosion problem during long-term operation, which results in a high investment cost for the device.

[0005] The corrosion ability of acetic acid to metals is similar to that of lactic acid. Examples of anti-corrosion solutions for acetic acid-containing systems in industry are as follows: CN106365977A discloses an acetic acid production device, and the inside of the bubble reaction tower of this device has a 0.5-1 cm zirconia coating, and the reaction temperature is 175-200°C; CN206995905U discloses an acetic acid recovery tower, and the inner wall of this recovery tower uses a titanium-molybdenum alloy. Since acetic acid reaction devices mostly operate under high temperature and high pressure, alloys must be used to withstand high temperature and high pressure.

[0006] In addition to the large investment in equipment, vacuum dehydration of lactic acid requires the use of a vacuum pump and has high requirements for stable vacuum control; in addition, there are high requirements for the air tightness of the device. Since the inside of the device is negative pressure, poor air tightness will cause air to enter the device, causing product oxidation, producing undesirable by-products, and causing the material to turn yellow or black.

[0007] Based on the above content, it can be seen that the existing industrialization plan adopts vacuum and high-temperature dehydration technology, and the key equipment material uses Hastelloy, and there are still many problems that need to be solved urgently. Summary of the invention

[0008] In order to solve the above technical problems, the purpose of the present invention is to provide a lactic acid dehydration device and a lactic acid dehydration process. By adopting a distillation tower made of plastic material, the cost of lactic acid dehydration can be reduced, and by recycling gas bubbling dehydration, the cost of gas use can be reduced and a good dehydration effect can be achieved.

[0009] To achieve the above object, the present invention provides a lactic acid dehydration device, wherein the lactic acid dehydration device comprises a reactor, a distillation tower, a primary condenser, a primary condenser fraction tank, and a gas delivery pump;

[0010] The reactor, the distillation tower, the primary condenser, and the gas delivery pump are sequentially connected together to form a circulation device;

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

[0012] The material of the wall of the reactor is plastic or alloy, preferably reinforced polypropylene or Hastelloy;

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

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

[0015] In the above-mentioned lactic acid dehydration device, preferably, the lactic acid dehydration device further comprises a secondary condenser, and the secondary condenser 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 comprises a secondary condenser distillation tank, and the secondary condenser distillation tank is connected to the secondary condenser.

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

[0018] In the above lactic acid dehydration device, preferably, a gas distributor is included in the reactor.

[0019] In industrial production, considering that lactic acid has strong corrosiveness, equipment made of Hastelloy is usually required as the reaction kettle and distillation column, resulting in a relatively high equipment investment. If the reaction temperature can be reduced to reduce the corrosiveness of lactic acid, the service life of the equipment can be extended, the average cost of the equipment can be reduced, and even polypropylene can be used as the equipment material to further reduce the cost. However, there is currently no lactic acid dehydration device with the main process feature of circulating inert gas bubbling for water removal. The present invention uses lightweight materials such as plastics and ceramics, combined with advanced dehydration process conditions, to obtain a lighter and lower-investment-cost lactic acid dehydration device. Using this device can efficiently and inexpensively remove free water in lactic acid and maintain good product quality, avoiding side reactions such as oxidation.

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

[0021] An inert gas is input into the reactor through a gas delivery pump, and water vapor and lactic acid vapor are bubbled out in the reactor and enter the distillation column for separation to obtain a high-temperature mixed gas containing water vapor. After being condensed by a primary condenser, a normal-temperature mixed gas containing saturated water vapor and water is obtained. The normal-temperature mixed gas containing saturated water vapor enters the gas delivery pump to start circulating.

[0022] In the above lactic acid dehydration process, preferably, the water condensed by the primary condenser enters the primary condenser fraction tank.

[0023] In the above 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 lactic acid dehydration process, preferably, the normal-temperature mixed gas containing saturated water vapor enters a secondary condenser for further condensation to obtain a low-temperature mixed gas containing saturated water vapor. The low-temperature mixed gas containing saturated water vapor enters the gas delivery pump to start circulating.

[0025] In the above lactic acid dehydration process, preferably, the water condensed by the secondary condenser enters the secondary condenser fraction tank.

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

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

[0028] In the above 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°C, more preferably 0 - 30°C.

[0029] In the above 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°C, more preferably 0 - 10°C.

[0030] It should be noted that although the main purpose of the lactic acid dehydration device and the lactic acid dehydration process of the present invention is to remove free water in lactic acid, since lactic acid has a spontaneous esterification reaction above 100°C and water is generated in the esterification reaction, the lactic acid dehydration device and the lactic acid dehydration process of the present invention will also simultaneously remove the water generated by the reaction of lactic acid 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 the presence of a catalyst, and 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 the reactor pressure. In the prior art, in order to achieve the effect that the water content in lactic acid ≤ 1%, theoretically, it is necessary to control the vapor pressure of water to 4.76 kPa. When the saturated vapor pressure of water is 4.76 kPa, the corresponding temperature is 32°C. Therefore, in the prior art using vacuum dehydration, it is necessary to control the internal temperature of the reactor to reach 150°C, the reactor pressure to reach about 13.3 kPa (absolute pressure), and the outlet temperature of the condenser ≤ 32°C to achieve this, and at this time, a large amount of lactic acid vapor is contained in the gas phase, and sufficient rectification is required.

[0032] Compared with the prior art, the present invention adopts an inert gas bubbling water-carrying technology under normal pressure or slightly positive pressure, and the water content in the product is no longer limited by the air pressure, but only controlled by the temperature and the saturated vapor pressure of water. When the lactic acid dehydration device and the dehydration process of the present invention dehydrate at 100°C, the saturated vapor pressure of water at 100°C is 101 kPa, and at 105°C is 121 kPa; the saturated vapor pressure of water at 10°C is 1.23 kPa, and at 8°C is 1.07 kPa. Therefore, only by introducing a bubbling gas such as nitrogen without water, or cooling the recycled nitrogen or other bubbling gases to below 10°C through the secondary condenser, can the water in lactic acid be removed to 1% at 105°C; or, by cooling the recycled nitrogen through the secondary condenser to below 8°C, the water in lactic acid can be removed to 1% at 100°C.

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

[0034] The dehydration pressure of the present invention can be controlled near atmospheric pressure, which can effectively reduce the pressure resistance requirement of the container. Therefore, the reactor no longer needs to be a Class I pressure vessel and can use a conventional container, greatly reducing the device design, material, and manufacturing costs. The lactic acid dehydration device of the present invention uses a plastic wall material and can operate at atmospheric pressure. It is easy to operate, can save the device investment cost, reduce device corrosion, save the amount of bubbling gas used, reduce oxygen infiltration, improve product quality, and has broad application prospects and huge economic benefits.

[0035] Since the dehydration temperature of the present 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 extremely shortened, and the device investment is reduced.

[0036] Due to the temperature reduction, the corrosiveness of lactic acid is significantly reduced compared to that at 150°C. Therefore, there is no longer a need for special corrosion-resistant Hastelloy materials, and plastics such as reinforced polypropylene can be used as the materials for the reactor and distillation column, greatly reducing the device investment cost. Moreover, since the density of reinforced polypropylene is significantly lower than that of stainless steel and Hastelloy, the weight of the entire device is reduced, and the corresponding equipment can even be reduced to less than 1 / 5 of the original weight, correspondingly reducing the load-bearing requirement of the entire device and the steel structure investment cost.

[0037] Even when using an existing device, dehydrating with recycled inert gas still has advantages compared to vacuum dehydration. For example: 1) In the case of vacuum dehydration, the internal pressure of the reactor is lower than the external pressure, and inevitably a small amount of air infiltrates into the reactor in an industrial device, causing product oxidation and quality degradation; while inert gas dehydration can operate under the condition that the internal pressure is higher than the external pressure, avoiding air infiltration; 2) The vacuum dehydration device needs to continuously maintain a vacuum, and the operation and maintenance costs of the vacuum pump are relatively high, while the recycled inert gas dehydration uses a gas circulation pump, and the equipment cost and operation and maintenance costs are lower than those of the vacuum pump. Therefore, the existing device can also use the recycled inert gas dehydration process of the present invention.

[0038] Generally speaking, 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 a plastic wall material, which can greatly reduce the use of expensive metal materials such as Hastelloy, reduce the weight of the device, and save the device investment.

[0040] 2. The lactic acid dehydration process of the present invention can reduce the gas usage cost by bubbling and dehydrating with recycled gases such as nitrogen; and by selecting appropriate temperature and pressure, the heating and cooling costs can be reduced; by operating under conditions close to or slightly above atmospheric pressure, the infiltration and accumulation of oxygen can be reduced, the product quality can be improved, and the design and manufacturing requirements for the container's vacuum resistance can be reduced or eliminated, saving the equipment design and manufacturing costs.

[0041] 3. The lactic acid dehydration process of the present invention has the advantages of simple operation and small equipment investment, and contains huge economic benefits. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0043] Description of the main reference numerals:

[0044] 1. Reactor, 2. Rectifying 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 DESCRIPTION OF THE EMBODIMENTS

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

[0046] Example 1

[0047] This example provides a lactic acid dehydration device, the structure of which is as Figure 1 shown, where the components within the dashed box are optional additional components.

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

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

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

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

[0052] Nitrogen is pumped into the reactor 1 by the gas transfer pump 7, and water vapor and lactic acid vapor are bubbled out in the reactor 1. After the nitrogen, water vapor and lactic acid vapor are separated by the rectification column 2, a high-temperature mixed gas containing water vapor is obtained. After being condensed by the primary condenser 3, normal-temperature nitrogen containing saturated water vapor is obtained and enters the gas transfer pump 7 again to start cycling; the water separated and condensed by the primary condenser 3 enters the primary condenser fraction tank 4;

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

[0054] The working temperature of the reactor 1 is 140 - 150 °C, and the working pressure is an absolute pressure of 110 - 120 kPa.

[0055] The cooling medium of the primary condenser 3 is cooling water, and the medium temperature is 22 - 30 °C;

[0056] The mass of the fed lactic acid is 50 kg, and the purity is 92% (theoretically containing 4 kg of free water, about 8%, and the theoretically generated water by the esterification reaction does not exceed 9.2 kg at most). The dehydration time (starting from when the reactor temperature reaches 140 °C) is 1.5 hours, the distilled fraction is 8.6 kg, the lactic acid content in the fraction is 0.7% (GC - FID), and the water content is 99.3% (deduced by back - calculating lactic acid); the water content in the lactic acid and oligomers in the reactor after dehydration is 1.0%, and the color number is 90 APHA (140 °C).

[0057] Example 2

[0058] This example provides a lactic acid dehydration device, and its structure is as Figure 1 shown.

[0059] On the basis of Example 1, this lactic acid dehydration device is increased with a secondary condenser 5, a secondary condenser fraction tank 6, a buffer tank 8, and a back - pressure valve 9;

[0060] Among them, the top gas phase outlet of the reactor 1 is connected to the bottom inlet of the rectification column 2, the top gas phase outlet of the rectification column 2 is connected to the inlet of the primary condenser 3, the outlet of the primary condenser 3 is connected to the inlet of the secondary condenser 5 through a gas pipeline, the outlet of the secondary condenser 5 is connected to the inlet of the gas transfer pump 7, the outlet of the gas transfer pump 7 is connected to the inlet of the buffer tank 8, and the outlet of the buffer tank 8 is connected to the gas inlet of the reactor 1 through the back - pressure valve 9; the primary condenser 3 is connected to the primary condenser fraction tank 4, and the secondary condenser 5 is connected to the secondary condenser fraction tank 6;

[0061] The wall material of the reactor 1 and the tower wall of the rectification column 2 is reinforced polypropylene, the packing of the rectification column 2 is porcelain rings, and the effective volume of the reactor 1 is about 25 L.

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

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

[0064] The cooling medium of the primary condenser 3 is cooling water, and the medium temperature is 5 - 40 °C;

[0065] The cooling medium of the secondary condenser 5 is chilled water, and the medium temperature is 3 - 10 °C;

[0066] The mass of the charged lactic acid is 15 kg, and the purity is 90% (theoretically containing 1.5 kg of free water, about 10%, and the highest amount of water generated by the esterification reaction does not exceed 2.7 kg). The dehydration time (starting from when the reactor temperature reaches 99 °C) is 1.5 hours, the removed distillate is 2.04 kg, the lactic acid content in the distillate is 0.2%, and the water content is 99.8% (deduced by back-calculating the lactic acid). The water content in the lactic acid and oligomers in the reactor after dehydration is 0.89%, and the color number is 25 APHA (100 °C).

[0067] It can be seen from the results of Examples 1 and 2 that: the lactic acid dehydration device and lactic acid dehydration process provided by the present invention have good dehydration effects, and can reduce the lactic acid from a relatively high water content to below 1.0%.

[0068] The present invention is not limited to the above embodiments. Those skilled in the art, based on the disclosure of the present invention, the improvements and modifications made without departing from the scope of the present invention should be within the protection scope of the present invention.

Claims

1. A lactic acid dehydration device, wherein, The lactic acid dehydration device includes a reactor (1), a rectifying column (2), a primary condenser (3), a primary condenser fraction tank (4), and a gas transfer pump (7); The reactor (1), the rectifying column (2), the primary condenser (3), and the gas transfer pump (7) are connected in sequence to form a circulation device; The primary condenser (3) is connected to the primary condenser fraction tank (4); The material of the reactor wall of the reactor (1) is plastic or alloy, preferably reinforced polypropylene or Hastelloy; The material of the rectifying column wall of the rectifying column (2) is plastic or alloy, preferably reinforced polypropylene or Hastelloy.

2. The lactic acid dehydration device according to claim 1, wherein, The rectifying column (2) is a packed column, and the packing material of the packed column is plastic or ceramic, preferably porcelain rings.

3. The dehydration device according to claim 1 or 2, wherein, A gas distributor is included inside the reactor (1).

4. The lactic acid dehydration device according to claim 1, wherein, The lactic acid dehydration device further includes a buffer tank (8) and a back pressure valve (9), and the buffer tank (8) and the back pressure valve (9) are connected in sequence between the gas transfer pump (7) and the reactor (1).

5. A lactic acid dehydration process, which is carried out by using the lactic acid dehydration device according to any one of claims 1-4. The lactic acid dehydration process includes the following steps: An inert gas is input into the reactor (1) through the gas transfer pump (7), bubbles out water vapor and lactic acid vapor in the reactor (1), and enters the rectifying column (2) for separation to obtain a high-temperature mixed gas containing water vapor. After being condensed by the primary condenser (3), a normal-temperature mixed gas containing saturated water vapor and water is obtained. The normal-temperature mixed gas containing saturated water vapor enters the gas transfer pump (7) to start circulation.

6. The lactic acid dehydration process according to claim 5, wherein, The inert gas is one or a combination of two or more of nitrogen, helium, and carbon dioxide.

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

8. The lactic acid dehydration process according to claim 5, wherein, The reaction temperature in the reactor (1) is 80 - 150 °C, preferably 100 - 125 °C, and more preferably 99 - 105 °C.

9. The lactic acid dehydration process according to claim 5, wherein, The working pressure of the reactor (1) is an absolute pressure of 80 - 120 kPa, preferably an absolute pressure of 95 - 105 kPa.

10. The lactic acid dehydration process according to claim 5, wherein, The cooling medium of the primary condenser (3) is cooling water, and the temperature of the cooling water is 0 - 40 °C, preferably 0 - 30 °C.

11. The lactic acid dehydration process according to claim 7, wherein, The cooling medium of the secondary condenser (5) is chilled water, and the temperature of the chilled water is 0 - 15 °C, preferably 0 - 10 °C.

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