CN113861159A - Method for preparing lactide by direct polycondensation - Google Patents
Method for preparing lactide by direct polycondensation Download PDFInfo
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- CN113861159A CN113861159A CN202111262295.6A CN202111262295A CN113861159A CN 113861159 A CN113861159 A CN 113861159A CN 202111262295 A CN202111262295 A CN 202111262295A CN 113861159 A CN113861159 A CN 113861159A
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- C07D319/12—1,4-Dioxanes; Hydrogenated 1,4-dioxanes not condensed with other rings
Abstract
The invention provides a method for preparing lactide by direct polycondensation, which comprises the following steps: adding a lactic acid solution, a hydrogen-containing acid catalyst and an entrainer into a mixing tank, stirring and mixing, and then introducing into a reaction tank; heating, catalyzing and condensing to obtain a lactide solution, distilling the lactide solution in a reaction tank under reduced pressure, evaporating an entrainer to carry water vapor into a vacuum condenser, and refluxing the entrainer into the reaction tank after condensing and dividing water; conveying the lactide solution into a purification tank, cooling, crystallizing and filtering to obtain lactide crude crystals, separating the lactide crude crystals from a reaction system, and introducing the filtered lactide mother solution into a reaction tank for circulating participation in reaction; and washing, filtering and drying the crude lactide crystal to obtain the refined lactide. The invention adopts hydrogen-containing acid as catalyst, and utilizes a multilayer rotary disc sieve plate reactor to carry out full stirring reaction, thereby reducing the reaction temperature and simultaneously improving the reaction efficiency, and the final product yield reaches over 86 percent through the dehydration and mother liquor dual-cycle intermittent reaction process.
Description
Technical Field
The invention relates to the field of chemical raw material synthesis, in particular to a method for preparing lactide by direct polycondensation.
Background
With the continuous advance of industrialization, the problem of environmental pollution is widely concerned and regarded by society. White pollution control is also a hot topic of research in industry and academia. Polylactic acid has biodegradability, good mechanical property and processability, is an environment-friendly material with wide application, and is mainly prepared by a lactide ring-opening polymerization method.
In the prior art, the preparation of lactide mainly adopts a two-step method: heating lactic acid at 140-170 ℃ for dehydration to form low molecular weight polylactic acid; then under the high vacuum condition of 210 ℃ and 250 ℃, the oligomer is cracked and cyclized to lactide under the action of a catalyst.
The reaction formula is as follows:
the lactide formed in the process generally contains impurities such as water, lactic acid oligomer and the like, so further dehydration and purification are technical problems for preparing lactide monomers. The operation process often has the problems of low water removal efficiency, more reaction byproducts, low utilization rate of lactic acid and the like. In the polycondensation stage which plays a key role in the process for preparing lactide by direct polycondensation of lactic acid, although water-carrying agents and the like are added in the materials, the generated lactide is not separated from a reaction system, so that a large amount of lactic acid oligomer can be cracked and converted into lactide only at a higher temperature of 230 ℃, and { [1] bud packing, Pengli and Guo water-carrying agents are applied to the lactide synthesis process, chemical engineering and equipment, chemical and environmental engineering academy of Harbin Ringshui university, Heilongjiang Harbin. High-temperature high-vacuum energy consumption is high, the conversion rate is low, and racemization is easy to influence the molecular weight of a polymer in the next step.
Disclosure of Invention
In order to solve the technical problems, the invention provides a method for preparing lactide by direct polycondensation, in which lactide is quickly crystallized, separated out of a reaction system, a lactide synthesis method with recyclable reaction mother liquor is adopted, an entrainer is adopted for recycling water, the reaction operation is simple and easy to control, and the product purity is high and the optical activity is controllable. The method comprises the following steps: mixing hydrogen-containing acid, an entrainer and a lactic acid solution, removing water by using the entrainer in a heating and reduced pressure distillation mode, and then carrying out catalytic polycondensation reaction to obtain lactide; cooling and crystallizing the obtained product to separate crude lactide, and circularly supplementing lactic acid mixed liquor into mother liquor to continue reaction; washing the crude lactide with solvent, filtering and drying to obtain refined lactide. The reaction equation for preparing lactide by direct polycondensation of lactic acid is as follows:
the invention provides a method for preparing lactide by direct polycondensation, which comprises the following steps:
(a) adding a lactic acid solution, a hydrogen-containing acid catalyst and an entrainer into a mixing tank, stirring and mixing, and introducing into a reaction tank as a mixed reactant;
(b) heating and catalytically condensing the mixed reactants in a reaction tank to obtain a lactide solution, distilling the lactide solution in the reaction tank under reduced pressure, evaporating an entrainer to carry water vapor into a condenser, and refluxing the entrainer into the reaction tank after condensation and water separation;
(c) conveying the lactide solution into a purification tank, cooling, crystallizing and filtering to obtain crude lactide crystals, conveying the filtered lactide mother solution into a mixing tank by a conveying pump, mixing with the mixed reactant, and introducing into a reaction tank for circulating participation in reaction;
(d) and washing, filtering and drying the crude lactide crystals in the purification tank to obtain refined lactide, and discharging from a product discharge port of the purification tank to obtain the lactide.
Further, the lactic acid solution is a 60 wt% -90 wt% concentration lactic acid aqueous solution.
Further, the hydrogen-containing acid catalyst is a combination of one or more of sulfuric acid, benzenesulfonic acid and p-toluenesulfonic acid; the addition volume of the catalyst accounts for 0.2 to 1.5 percent of the total volume of the mixed reactants.
Further, the entrainer of step (a) is selected from the group consisting of: one or a mixture of more of toluene, xylene, diphenyl ether, dipropylene glycol methyl ether, diethylene glycol monomethyl ether, ethylene glycol butyl ether, ethylene glycol monoethyl ether, n-octane, butyl acetate and propylene glycol monoethyl ether; the addition volume of the entrainer accounts for 20-60% of the total volume of the mixed reactants.
Furthermore, in the step (b), the reaction tank is a multilayer rotary disc sieve plate film reaction tank, the middle shaft in the reaction tank is provided with a plurality of layers of rotary disc sieve plate pulp coaxially driven by a rotary disc motor, the inner wall of the reaction tank is provided with a plurality of layers of fixed sieve plates, the rotary disc sieve plate pulp and the fixed sieve plates are arranged in a staggered and spaced mode, and a plurality of sieve holes are formed in the rotary disc sieve plate pulp and the fixed sieve plates.
Further, in the step (b), the reaction tank is heated to 90-180 ℃ for catalytic polycondensation reaction, and the catalytic polycondensation reaction time is 2-5 h.
Further, in the step (b), the pressure of the reduced pressure distillation is 80 to 95 kPa.
Further, in the step (b), the vacuum distillation further comprises the following steps: the gas guide port is arranged above the reaction tank, the gas guide port is connected with the gas inlet of the vacuum condenser through a pipeline, the gas outlet above the vacuum condenser is connected with the vacuum pump, liquid condensed by the vacuum condenser flows into the bottom liquid collecting tank, one side of the upper part of the liquid collecting tank is provided with an entrainer overflow port, and light liquid components in the liquid collecting tank are guided out by the entrainer overflow port and are conveyed to the lower part of the reaction tank for recycling.
Further, in the step (c), the temperature reduction and crystallization is to reduce the temperature to 40-60 ℃ in a purification tank.
Further, the crude lactide crystals obtained in step (c) were further washed with ethyl acetate to obtain purified lactide crystals.
The purity of the product obtained by the method can reach more than 98.3 percent, and the yield can reach more than 86 percent. The reaction is carried out homogeneously, and the dehydrated hydrogen-containing acid quickly catalyzes the reaction for polycondensation; the used hydrogen-containing acid is cheap and easy to obtain, and the dosage of the catalyst is small; the entrainer is used for removing water and the lactide mother liquor is subjected to double-circulation reaction, so that the operation temperature is low, the energy consumption is low, and the industrial implementation is easy; the lactide is separated from a reaction system by using a turntable sieve plate film reaction tank in combination with a cooling, crystallization and filtering integrated technology, and the dehydration balance of the lactic acid moves towards the direction of generating the lactide, so that the mass transfer efficiency and the reaction rate are improved, and the yield of the lactide is improved by synchronous dehydration and cyclization; and further washing, filtering and drying the crude lactide crystal obtained in the purification tank by using ethyl acetate, wherein the operation temperature below 180 ℃ is far lower than the vacuum rectification temperature of the lactide at 230 ℃, the lactide is difficult to racemize, and the purity of the obtained lactide is high.
Description of the drawings:
fig. 1 is a schematic diagram of a lactide synthesis apparatus.
In the figure: 1-mixing tank; 11-a feed inlet; 12-a first stirrer; 13-an exhaust port; 14-a feed back port; 2-a reaction tank; 21-heating jacket; 22-rotating disc sieve plate pulp; 23-fixing the sieve plate; 24-a gas guide port; 25-a turntable motor; 26-a first bleeder valve; 3-vacuum condenser; 31-a condenser sleeve; 32-an air inlet; 33-vacuum suction; 34-a liquid collecting tank; 35-entrainer overflow; 36-a water discharge port; 4-a purification tank; 41-a second stirrer; 42-product discharge port; 43-heat exchange jacket; 44-a filter screen; 45-a second bleeder valve; 5-a vacuum pump; 6-a first delivery pump; 7-second delivery pump.
Detailed Description
The present invention will be described in further detail with reference to the accompanying drawings and examples.
The invention provides a method for preparing lactide by direct polycondensation, which comprises the following steps:
(a) adding a lactic acid solution, a hydrogen-containing acid catalyst and an entrainer into a mixing tank 1, stirring and mixing, and introducing into a reaction tank 2 as a mixed reactant;
(b) heating and catalytically condensing the mixed reactant in a reaction tank 2 to obtain a lactide solution, distilling the lactide solution in the reaction tank 2 under reduced pressure, evaporating an entrainer to carry water vapor into a vacuum condenser 3, refluxing the entrainer into the reaction tank 2 after condensation and water separation, continuously evaporating and carrying the water vapor into the vacuum condenser 3;
(c) the lactide solution which is stirred and mixed with the reactant and flows downwards to the bottom of the reaction tank 2 is conveyed into a purification tank 4 through a first conveying pump 6, crude lactide crystals are obtained through cooling, crystallization and filtration, and lactide mother liquor obtained through filtration is conveyed into a material mixing tank 1 through a second conveying pump 7 to be mixed with the reactant and then is introduced into the reaction tank 2 to participate in reaction in a circulating manner;
(d) after the reaction is finished for 2-5 hours, the crude lactide crystals in the purification tank are further washed, filtered and dried to obtain refined lactide, and the refined lactide is discharged from a product discharge port 42 of the purification tank 4.
The lactic acid solution is a 60 wt% -90 wt% lactic acid aqueous solution, and in a preferred technical scheme, the concentration of the lactic acid aqueous solution is 75 wt% -85 wt%.
The hydrogen-containing acid catalyst is one or more of sulfuric acid, benzenesulfonic acid and p-toluenesulfonic acid; the adding volume of the catalyst accounts for 0.2-1.5% of the total volume of the mixed reactants, and in a preferred technical scheme, the adding volume of the catalyst containing the hydrogen acid accounts for 0.2-1.5% of the total volume of the mixed reactants.
In the technical scheme of the invention, the entrainer in the step (a) is one or a mixture of a plurality of toluene, xylene, diphenyl ether, dipropylene glycol methyl ether, diethylene glycol monomethyl ether, ethylene glycol butyl ether, ethylene glycol monoethyl ether, n-octane, butyl acetate and propylene glycol monoethyl ether. In the preferred technical scheme, the addition volume of the entrainer accounts for 20-60% of the total volume of the lactic acid solution; further preferably, the entrainer is added in an amount of 20% to 60% of the total volume of the mixed reactants.
In the step (b), the reaction tank 2 is a multilayer rotary disc sieve plate film reaction tank, the inner middle shaft of the reaction tank 2 is provided with a plurality of layers of rotary disc sieve plate slurry 22 coaxially driven by a rotary disc motor 25, the inner wall of the reaction tank 2 is provided with a plurality of layers of fixed sieve plates 23, the rotary disc sieve plate slurry 22 and the fixed sieve plates 23 are arranged in a staggered and spaced mode, a plurality of sieve holes are formed in the rotary disc sieve plate slurry 22 and the fixed sieve plates 23, the diameter of each sieve hole is 0.5-2cm, the diameter of each sieve hole is increased from top to bottom, the viscosity of the solution at the upper part is lower, the viscosity of the solution at the bottom of the reaction tank 2 is gradually increased along with the progress of reaction and the continuous evaporation of water vapor, in order to ensure the balance of stirring resistance, the sieve holes at the lower part of the reaction tank 2 are properly increased, and meanwhile, the smooth ascending channel of the volatile gas at the bottom can be ensured.
And heating the reaction tank 2 to 90-180 ℃ for catalytic polycondensation, wherein the catalytic polycondensation reaction time is 2-5 hours. The reaction tank 2 is heated to carry out the polycondensation reaction, simultaneously the entrainer, the water in the solution and the water generated by the polycondensation reaction are evaporated, the entrainer in the gas phase carries water vapor to be continuously sucked away from the gas guide port 24 at the top of the reaction tank 2 by the vacuum pump 5, the water in the reaction tank 2 is reduced, and the reaction is carried out towards the polymerization direction. The lactic acid after dehydration is rapidly subjected to polycondensation reaction under the catalysis of hydrogen-containing acid to generate low molecular weight polylactic acid such as dimerization and trimerization, lactide and the like, and the generated low molecular weight polylactic acid such as dimerization and trimerization is continuously subjected to catalytic dehydration, cracking and cyclization to form the lactide.
In the step (b), the pressure of the reduced pressure distillation is 80 to 95 kPa. The vacuum distillation is used for accelerating the evaporation rate of the entrainer and water and improving the reaction efficiency. When the system pressure is lower than 80kPa, the energy consumption is increased and the reaction rate is decreased.
In the step (b), the vacuum distillation further comprises the following steps: an air guide port 24 is arranged above the reaction tank 2, the air guide port 24 is connected with an air inlet 32 of the vacuum condenser 3 through a pipeline, an air outlet 33 of the vacuum condenser 3 is connected with the vacuum pump 5, liquid condensed by the vacuum condenser 3 flows into a liquid collecting tank 34 at the bottom of the vacuum condenser 3, an entrainer overflow port 35 is arranged at the upper part of the liquid collecting tank 34, and light liquid components in the liquid collecting tank 34, namely the entrainer, are guided out from the entrainer overflow port 35 and are conveyed to the lower part of the reaction tank 2 to circularly participate in the reaction.
Wherein in the step (c), the temperature reduction and crystallization is to reduce the temperature to 40-60 ℃ in the purification tank (4). And (c) further washing the crude lactide crystal obtained in the step (c) by using ethyl acetate, filtering and drying to obtain a refined lactide crystal.
The invention further discloses a lactide preparation device, which comprises a mixing tank 1, a reaction tank 2, a vacuum condenser 3 and a purification tank 4. Wherein, the top of compounding jar 1 is equipped with feed inlet 11, feed back 14 and gas vent 13, and feed back 14 communicates through the bottom of pipeline with purification jar 4, and the bottom of compounding jar 1 is connected through pipeline and retort 2 top feed inlet. The top of the reaction tank 2 is provided with a gas guide port 24, the gas guide port 24 is connected with a gas inlet 32 of the vacuum condenser 3 through a pipeline, and a vacuum suction port 33 at the top of the vacuum condenser 3 is connected with the vacuum pump 5. The liquid condensed by the vacuum condenser 3 flows to a bottom liquid collecting tank 34, an entrainer overflow port 35 is arranged at the top of the liquid collecting tank 34, light liquid components, namely the entrainer, in the liquid collecting tank 34 are led out from the entrainer overflow port 35 and conveyed to the bottom of the reaction tank 2 to be circularly heated and lifted, and a water outlet 36 is arranged at the bottom of the liquid collecting tank 34 to discharge the taken water.
The bottom of the reaction tank 2 is connected with a feed inlet at the upper part of the purification tank 4 through a pipeline, a filter screen 44 is arranged at the lower part of the purification tank 4, a filtrate discharge outlet is arranged at the bottom of the purification tank 4, and the filtered filtrate is conveyed to a feed back port 14 at the top of the mixing tank 1 through a pipeline. The lactide solution obtained by the reaction is conveyed to a purification tank 4 through a pipeline and a first conveying pump 6 from a reaction tank 2, after cooling, crystallization and filtration, the filtered mother liquor is recovered and is re-injected into a material mixing tank 1 for utilization, and the mother liquor is conveyed into the material mixing tank 2 through a second conveying pump 7 to be mixed with unreacted reactants and then is introduced into the reaction tank 2 to participate in the reaction in a circulating way. The purification tank 4 has a rotary blade therein driven by a second stirrer 41. A product discharge port 42 is arranged on the side wall of the purification tank above the filter screen 44, and after the multiple circulation reactions are finished, the crude lactide crystals accumulated in the purification tank 4 are washed, filtered and dried to obtain refined lactide crystals.
Example 1
240L of entrainer (xylene), 600L of lactic acid solution with the concentration of 80 wt% and the density of about 1.15g/mL, and 5L of hydrogen-containing acid catalyst (sulfuric acid) are added into the mixing tank 1. And starting the first stirrer for stirring, uniformly stirring, introducing into a reaction tank 2 preheated to 90 ℃, reducing the pressure of a vacuum pump 5 to 80kPa, conveying the lactide solution obtained by reaction to a purification tank 4, cooling to 50 ℃, crystallizing and separating out the lactide, filtering and retaining on a filter screen 44, conveying the filtered lactide mother liquor back to a mixing tank 1, and circularly adding the lactide mother liquor into the reaction tank 2 through the mixing tank 1 for continuous reaction. The entrainer gas generated by heating in the reaction tank 2 entrains water vapor into the vacuum condenser 3 from the gas guide port 24, the water vapor is condensed by the vacuum condenser and then collected into the liquid collection tank, and the light entrainer at the upper part in the liquid collection tank overflows and flows back to the reaction tank 2. The reaction was stopped at 3.5h for a double cycle reaction to a solution volume of only the first 30%. The lactide crystallized and precipitated in the purification tank was washed with ethyl acetate 2 times and dried under vacuum to obtain 424kg of purified lactide. In this example, the product yield was 96% and the purity was 99%.
Example 2
290L of entrainer (toluene, xylene and diphenyl ether), 600L of lactic acid solution with the concentration of 60 wt% and the density of about 1.08g/mL and 6L of hydrogenous acid catalyst (p-toluenesulfonic acid) are added into a mixing tank 1, and 10L of entrainer is added into a liquid collecting tank 34 at the lower part of a vacuum condenser 3 to an overflow port of the entrainer. And starting the first stirrer for stirring, uniformly stirring, introducing into a reaction tank 2 preheated to 100 ℃, decompressing to 92kPa by using a vacuum pump 5, conveying the lactide solution obtained by reaction to a purification tank 4, cooling to 60 ℃, crystallizing and separating out the lactide, filtering and retaining on a filter screen 44, conveying the filtered lactide mother liquor back to a mixing tank 1, and circularly adding the lactide mother liquor into the reaction tank 2 through the mixing tank 1 for continuous reaction. The entrainer gas generated by heating in the reaction tank 2 entrains water vapor into the vacuum condenser 3 from the gas guide port 24, the water vapor is condensed by the vacuum condenser and then collected into the liquid collection tank, and the light entrainer at the upper part in the liquid collection tank overflows and flows back to the reaction tank 2. The reaction was stopped by the time of the double circulation reaction for 2h until the solution volume in the reaction tank was only at the beginning 1/3. Lactide crystallized and precipitated in the purification tank was washed with ethyl acetate 2 times, and dried under vacuum to obtain 280kg of purified lactide. In this example, the product yield was 90% and the purity 98%.
Example 3
280L of entrainer (diethylene glycol monomethyl ether), 420L of lactic acid solution with the concentration of 90 wt% and the density of about 1.18g/mL and 2L of hydrogen-containing acid catalyst (benzenesulfonic acid) are added into the mixing tank 1. And starting the first stirrer for stirring, uniformly stirring, introducing into a reaction tank 2 preheated to 188 ℃, reducing the pressure of a vacuum pump 5 to 92kPa, conveying the lactide solution obtained by reaction to a purification tank 4, cooling to 60 ℃, crystallizing and separating out the lactide, filtering and retaining on a filter screen 44, conveying the filtered lactide mother liquor back to a mixing tank 1, and circularly adding the lactide mother liquor into the reaction tank 2 through the mixing tank 1 for continuous reaction. The entrainer gas generated by heating in the reaction tank 2 entrains water vapor into the vacuum condenser 3 from the gas guide port 24, the water vapor is condensed by the vacuum condenser and then collected into the liquid collection tank, and the light entrainer at the upper part in the liquid collection tank overflows and flows back to the reaction tank 2. The double-circulation reaction is stopped when the solution volume is only 40 percent at the beginning after 4.5 h. Lactide crystallized and precipitated in the purification tank was washed with ethyl acetate 2 times, and dried under vacuum to obtain 335kg of purified lactide. In this example, the product yield was 92% and the purity was 98%.
Example 4
100L of entrainer (butyl acetate and propylene glycol monoethyl ether), 300L of lactic acid solution with the concentration of 85 wt% and the density of about 1.16 g/mL, and 2.4L of hydrogen-containing acid catalyst (sulfuric acid) are added into a mixing tank 1. And starting the first stirrer for stirring, uniformly stirring, introducing into a reaction tank 2 preheated to 115 ℃, reducing the pressure of a vacuum pump 5 to 95kPa, conveying the lactide solution obtained by reaction to a purification tank 4, cooling to 55 ℃, crystallizing and separating out the lactide, filtering and retaining on a filter screen 44, conveying the filtered lactide mother liquor back to a mixing tank 1, and circularly adding the lactide mother liquor into the reaction tank 2 through the mixing tank 1 for continuous reaction. The entrainer gas generated by heating in the reaction tank 2 entrains water vapor into the vacuum condenser 3 from the gas guide port 24, the water vapor is condensed by the vacuum condenser and then collected into the liquid collection tank, and the light entrainer at the upper part in the liquid collection tank overflows and flows back to the reaction tank 2. The reaction was stopped by 4 hours of double cycling to 1/4 where the solution volume was only at the beginning. Lactide crystallized and precipitated in the purification tank was washed with ethyl acetate 2 times, and dried under vacuum to obtain 206kg of purified lactide. In this example, the product yield was 86% and the purity was 99%.
Example 5
210L of entrainer (n-octane), 600L of lactic acid solution with a concentration of 75 wt% and a density of about 1.12g/mL, and 4.8L of hydrogen-containing acid catalyst (benzenesulfonic acid and p-toluenesulfonic acid) are added into the mixing tank 1. And starting the first stirrer for stirring, uniformly stirring, introducing into a reaction tank 2 preheated to 125 ℃, decompressing to 88kPa by using a vacuum pump 5, conveying the lactide solution obtained by reaction to a purification tank 4, cooling to 58 ℃, crystallizing and separating out the lactide, filtering and retaining on a filter screen 44, conveying the filtered lactide mother liquor back to a mixing tank 1, and circularly adding the lactide mother liquor into the reaction tank 2 through the mixing tank 1 for continuous reaction. The entrainer gas generated by heating in the reaction tank 2 entrains water vapor into the vacuum condenser 3 from the gas guide port 24, the water vapor is condensed by the vacuum condenser and then collected into the liquid collection tank, and the light entrainer at the upper part in the liquid collection tank overflows to the reaction tank 2. The reaction was stopped by 5h of double cycling until the solution volume was only at the start 1/4. Lactide crystallized and precipitated in the purification tank was washed with ethyl acetate 2 times, and dried under vacuum to obtain 363kg of purified lactide. In this example, the product yield was 90% and the purity 98%.
Example 6
480L of entrainer (n-octane), 308L of lactic acid solution with the concentration of 75 wt% and the density of about 1.12g/mL and 12L of hydrogen-containing acid catalyst (benzene sulfonic acid and p-toluenesulfonic acid) are added into the mixing tank 1. And starting the first stirrer for stirring, uniformly stirring, introducing into a reaction tank 2 preheated to 125 ℃, decompressing to 88kPa by using a vacuum pump 5, conveying the lactide solution obtained by reaction to a purification tank 4, cooling to 58 ℃, crystallizing and separating out the lactide, filtering and retaining on a filter screen 44, conveying the filtered lactide mother liquor back to a mixing tank 1, and circularly adding the lactide mother liquor into the reaction tank 2 through the mixing tank 1 for continuous reaction. The entrainer gas generated by heating in the reaction tank 2 entrains water vapor into the vacuum condenser 3 from the gas guide port 24, the water vapor is condensed by the vacuum condenser and then collected into the liquid collection tank, and the light entrainer at the upper part in the liquid collection tank overflows to the reaction tank 2. And (5) carrying out double-circulation reaction for 2h, and stopping the circulation reaction. The lactide crystallized and precipitated in the purification tank was washed with ethyl acetate 2 times and dried under vacuum to obtain 190kg of purified lactide. In this example, the product yield was 90% and the purity 98%.
Example 7
120L of entrainer (diethylene glycol monomethyl ether), 478.8L of lactic acid solution with the concentration of 90 wt% and the density of about 1.18g/mL and 1.2L of hydrogen-containing acid catalyst (benzenesulfonic acid) are added into the mixing tank 1. And starting the first stirrer for stirring, uniformly stirring, introducing into a reaction tank 2 preheated to 188 ℃, reducing the pressure of a vacuum pump 5 to 92kPa, conveying the lactide solution obtained by reaction to a purification tank 4, cooling to 60 ℃, crystallizing and separating out the lactide, filtering and retaining on a filter screen 44, conveying the filtered lactide mother liquor back to a mixing tank 1, and circularly adding the lactide mother liquor into the reaction tank 2 through the mixing tank 1 for continuous reaction. The entrainer gas generated by heating in the reaction tank 2 entrains water vapor into the vacuum condenser 3 from the gas guide port 24, the water vapor is condensed by the vacuum condenser and then collected into the liquid collection tank, and the light entrainer at the upper part in the liquid collection tank overflows and flows back to the reaction tank 2. And (5) carrying out double-circulation reaction for 5 hours, and stopping the circulation reaction. Lactide crystallized and precipitated in the purification tank was washed with ethyl acetate 2 times, and vacuum-dried to obtain 382kg of purified lactide. In this example, the product yield was 92% and the purity was 98%.
The invention adopts the hydrogen-containing acid as the catalyst, and utilizes the multilayer rotary disc sieve plate reactor to carry out full stirring reaction, thereby reducing the reaction temperature and simultaneously improving the reaction efficiency, and through the cyclic batch reaction process, the yield of the final product is greatly and obviously improved, and simultaneously the productivity consumption is obviously reduced. The preparation reaction of the lactide is carried out in a homogeneous phase state, lactic acid solution is directly condensed under the catalysis of hydrogen-containing acid, dehydrated lactic acid rapidly carries out condensation polymerization under the catalysis of the hydrogen-containing acid, and oligomers in the circulating lactide mother liquor are continuously condensed, cracked and cyclized into the lactide; the used hydrogen-containing acid is cheap and easy to obtain, and the dosage of the catalyst is small; the entrainer is used for removing water and the lactide mother liquor is used for double-circulation reaction, the operation temperature is low, the energy consumption is low, and the industrial implementation is easy.
The embodiments of the present invention are merely illustrative and not restrictive, and those skilled in the art can modify the embodiments without inventive contribution as required after reading the present specification, but the present invention is protected by patent law within the scope of the appended claims.
Claims (10)
1. A method for preparing lactide by direct polycondensation is characterized by comprising the following steps:
(a) adding a lactic acid solution, a hydrogen-containing acid catalyst and an entrainer into a mixing tank (1), stirring and mixing, and introducing into a reaction tank (2) as a mixed reactant;
(b) heating and catalytically condensing the mixed reactants in a reaction tank to obtain a lactide solution, distilling the lactide solution in the reaction tank under reduced pressure, evaporating entrainer to carry water vapor into a vacuum condenser (3), and refluxing the entrainer into the reaction tank after condensation and water separation;
(c) conveying the lactide solution into a purification tank (4), cooling, crystallizing and filtering to obtain crude lactide crystals, conveying lactide mother liquor obtained by filtering into a mixing tank by a conveying pump, mixing with the mixed reactant, and introducing into a reaction tank for circulating reaction;
(d) and washing, filtering and drying the crude lactide crystals in the purification tank to obtain refined lactide, and discharging from a product discharge port of the purification tank to obtain the lactide.
2. The direct polycondensation lactide preparation method according to claim 1, wherein the lactic acid solution is an aqueous lactic acid solution having a concentration of 60 wt% to 90 wt%.
3. The direct polycondensation preparation of lactide according to claim 1, wherein the hydrogen-containing acid catalyst is a combination of one or more of sulfuric acid, benzenesulfonic acid, and p-toluenesulfonic acid; the addition volume of the catalyst accounts for 0.2 to 1.5 percent of the total volume of the mixed reactants.
4. The process for the preparation of lactide by direct polycondensation according to claim 1, wherein the entrainer of step (a) is selected from the group consisting of: one or a mixture of more of toluene, xylene, diphenyl ether, dipropylene glycol methyl ether, diethylene glycol monomethyl ether, ethylene glycol butyl ether, ethylene glycol monoethyl ether, n-octane, butyl acetate and propylene glycol monoethyl ether; the addition volume of the entrainer is 20-60% of the total volume of the mixed reactants.
5. The direct polycondensation lactide preparation method according to claim 1 or 4, wherein in step (b), the reaction tank is a multilayer rotary disc sieve plate thin film reaction tank, the central shaft in the reaction tank is provided with a plurality of layers of rotary disc sieve plate pulp (22) coaxially driven by a rotary disc motor (25), the inner wall of the reaction tank is provided with a plurality of layers of fixed sieve plates (23), the rotary disc sieve plate pulp and the fixed sieve plates are alternately arranged, and a plurality of sieve holes are arranged on the rotary disc sieve plate pulp and the fixed sieve plates.
6. The direct polycondensation lactide preparation method according to claim 1, wherein in step (b), the reaction tank is heated to 90-180 ℃ for catalytic polycondensation reaction, and the catalytic condensation reaction time is 2-5 h.
7. The direct polycondensation lactide production process according to claim 1, wherein the reduced pressure distillation pressure in step (b) is 80 to 95 kPa.
8. The direct polycondensation lactide production method according to claim 1, wherein the reduced pressure distillation further comprises the steps of, in the step (b): the gas guide port (24) is arranged above the reaction tank, the gas guide port is connected with a gas inlet (32) of the vacuum condenser through a pipeline, a gas outlet (33) above the vacuum condenser is connected with the vacuum pump (5), liquid condensed by the vacuum condenser flows into the bottom liquid collecting tank (34), one side of the upper part of the liquid collecting tank is provided with an entrainer overflow port (35), and light liquid components in the liquid collecting tank are guided out through the entrainer overflow port and are conveyed to the lower part of the reaction tank for recycling.
9. The process for the preparation of lactide by direct polycondensation according to claim 1, wherein the temperature-reduced crystallization in step (c) is carried out by reducing the temperature to 40-60 ℃ in a purification tank.
10. The process for preparing lactide by direct polycondensation according to claim 1, wherein the crude lactide crystals obtained in step (c) are further washed with ethyl acetate to obtain purified lactide crystals.
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| Publication number | Priority date | Publication date | Assignee | Title |
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| CN115232102A (en) * | 2022-09-06 | 2022-10-25 | 中国科学院长春应用化学研究所 | Preparation method and production device of lactide |
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| CN102766132A (en) * | 2012-08-14 | 2012-11-07 | 北京普利玛科技有限责任公司 | Method for preparing lactide continuously |
| CN206508865U (en) * | 2017-02-17 | 2017-09-22 | 广州市健谊生物科技有限公司 | A kind of food additives agitation mixer |
| CN109563067A (en) * | 2016-06-20 | 2019-04-02 | 道达尔研究技术弗吕公司 | Pass through the water catalyst of recycling and the single stage lactide production technique of hydrolyzing oligomer |
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| CN102766132A (en) * | 2012-08-14 | 2012-11-07 | 北京普利玛科技有限责任公司 | Method for preparing lactide continuously |
| CN109563067A (en) * | 2016-06-20 | 2019-04-02 | 道达尔研究技术弗吕公司 | Pass through the water catalyst of recycling and the single stage lactide production technique of hydrolyzing oligomer |
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| CN115232102B (en) * | 2022-09-06 | 2024-02-13 | 中国科学院长春应用化学研究所 | Preparation method and production device of lactide |
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