A method for purifying sucralose by crystallization

Abstract

This invention discloses a method for purifying sucralose by crystallization. Using sucralose-6-acetate as raw material, isopropanol as the reaction solvent and tetramethylammonium hydroxide as a metal-free catalyst, a deacetylation reaction is completed. The pH is adjusted to obtain a neutralized crude solution. A refined sugar solution is prepared by continuous ultrafiltration and nanofiltration membrane separation. A rising-film low-temperature thin-film evaporation stage is used for concentration, combined with butyl acetate constant-temperature pulping for impurity removal. Then, single-stage controllable aqueous phase crystallization at 8–12℃ is achieved through gradient temperature control and seed induction. Anhydrous sucralose crystals are obtained by centrifugation and vacuum drying. This invention uses ultrafiltration + nanofiltration dual-membrane continuous purification to replace the traditional adsorption and resin impurity removal process, enabling continuous production, high automation, low operating costs, and no filler regeneration waste liquid. Gradient temperature control single-stage aqueous phase crystallization eliminates the need for deep freezing, significantly reducing refrigeration energy consumption. The addition of seed crystals achieves controllable crystallization and results in uniform crystal particle size.

Description

Technical Field

[0001] This invention relates to the field of sucralose production and purification technology, and particularly to a method for sucralose crystallization and purification. Background Technology

[0002] Sucralose is a highly sweetened compound with a strong sweetness, pure taste, and high stability, and is widely used in the food, beverage, and pharmaceutical industries. The mainstream industrial synthesis route for sucralose uses sucralose-6-acetate as a key intermediate, which is obtained through deacetylation, purification, crystallization, and drying.

[0003] Current deacetylation processes commonly use methanol or anhydrous ethanol as reaction solvents. Methanol is highly toxic, and methanol solvent residue is easily detected in the finished product, making it difficult to meet the stringent solvent residue standards for high-end food and pharmaceutical products. Anhydrous ethanol has high procurement costs, significantly increasing production costs. Furthermore, the deacetylation stage of existing processes uses sodium and potassium-containing alkaline catalysts such as sodium hydroxide, potassium hydroxide, sodium methoxide, sodium ethoxide, and carbonates. After neutralization, a large amount of acetate is generated, resulting in a high salt content in the system. This necessitates the use of activated carbon adsorption, intermittent ion exchange resins, or electrodialysis desalination processes. Trace amounts of metal ions can lead to high ignition residue in the finished product. Long-term recycling of the salt-containing mother liquor can easily lead to impurity accumulation, reducing product purity. Additionally, resin regeneration and activated carbon replacement generate large amounts of hazardous waste, placing significant environmental pressure on the industry.

[0004] Existing refining processes often use intermittent vacuum concentration equipment to process sugar solutions. The material stays in the reactor for a long time. Sucralose is a heat-sensitive substance and is prone to degradation when heated at high temperatures for a long time, resulting in a darker syrup color and an increase in by-products. After concentration, crystallization is carried out directly using a single aqueous phase, which often employs a 4°C low-temperature deep freezing and two-stage step-by-step recrystallization process. This process is extremely energy-intensive, requires a large investment in equipment, and produces a large number of ultrafine crystals. The crystals encapsulate impurities in the mother liquor, making separation and washing difficult and limiting the improvement of the purity of the finished product.

[0005] Existing mother liquor recovery methods are simple, only directly returning to the deacetylated crude liquor end for circulation. Colored degradation products and trace amounts of salt continuously accumulate in the mother liquor. After multiple cycles, the color and purity of the product decline significantly, making it impossible to achieve long-term stable closed-loop production.

[0006] Therefore, in view of the shortcomings of the existing technology, it is necessary to design a crystallization and purification method for sucralose to solve the above problems.

[0007] It should be noted that the above introduction to the technical background is only for the purpose of providing a clear and complete explanation of the technical solution of the present invention and for the convenience of those skilled in the art to understand it. It should not be assumed that the above content is known to those skilled in the art simply because it has been described in the background of the present invention. Summary of the Invention

[0008] To overcome the shortcomings of the prior art, the present invention discloses a crystallization and purification method for sucralose that produces no large amounts of metal salts throughout the process, requires no activated carbon, ion exchange, or electrodialysis to assist in impurity removal, minimizes material degradation due to heat, eliminates the need for deep freezing during crystallization, allows for stable closed-loop recycling of the mother liquor, and results in a product with high purity and low residue upon ignition.

[0009] This invention discloses a method for purifying sucralose by crystallization, comprising the following steps:

[0010] S1. Using sucralose-6-acetate as raw material, isopropanol is used for dissolution, tetramethylammonium hydroxide is added to catalyze the deacetylation reaction, and then an acid regulator is added to adjust the pH of the system to 6.0-7.0 to obtain a sucralose neutralized crude solution.

[0011] S2. The neutralized crude liquid is sequentially passed through an ultrafiltration membrane module and a nanofiltration membrane module for continuous separation. The nanofiltration retentate concentrate is collected as refined sugar solution.

[0012] S3. The refined sugar solution is concentrated to a syrup with a solid content of 40wt% to 60wt% by low-temperature thin-film evaporation using a rising film method. Butyl acetate is added to the syrup and stirred at a constant temperature. The organic phase is separated by filtration, and the aqueous sugar solution is collected.

[0013] S4. The aqueous sugar solution is concentrated into a saturated aqueous solution by low-temperature evaporation using a rising film method. Anhydrous sucralose seed crystals are added to the saturated aqueous solution at 60°C for 1-2 hours to grow crystals. Then, the temperature is lowered to 25-30°C at a rate of 0.3-0.6°C / h and kept at this temperature for 1-3 hours to grow crystals. Then, the temperature is lowered to 8-12°C at a rate of 0.7-1.0°C / h and kept at this temperature for 6-12 hours to crystallize. After crystallization, the crystals are separated by centrifugation and washed with cold pure water.

[0014] S5. Place the washed crystals under vacuum drying at 45-50℃ for 4-6 hours to obtain anhydrous sucralose crystals.

[0015] S6. Collect the remaining crystallization mother liquor from step S4, add hydrogen peroxide for oxidation treatment, and then reflux the oxidized crystallization mother liquor as refined sugar solution to step S3 after nanofiltration.

[0016] Preferred technical solution: In step S1, the deacetylation reaction temperature is 5-45℃ and the reaction time is 4-5h.

[0017] Preferred technical solution: In step S1, the acidity regulator is selected from any one of acetic acid, dilute hydrochloric acid, and acidic cation exchange resin.

[0018] Preferred technical solution: In step S2, the nanofiltration membrane has a molecular weight cutoff of 150-300 Da and an operating pressure of 1.0-1.8 MPa; the ultrafiltration membrane has a molecular weight cutoff of 3k-10kDa and an operating temperature of 30-45℃.

[0019] Preferred technical solution: In step S3, the amount of butyl acetate added is 0.8 to 1.5 times the mass of the syrup, the pulping temperature is 40 to 60°C, and the stirring time is 0.5 to 2 hours; the butyl acetate organic phase obtained by filtration is recycled for step S3 after distillation.

[0020] Preferred technical solution: In step S4, the seed crystal is anhydrous sucralose crystal powder, and the amount added is 0.1% to 0.5% of the total mass of the saturated aqueous solution.

[0021] Preferred technical solution: In step S6, the hydrogen peroxide is a 30wt% hydrogen peroxide aqueous solution, the added volume is 0.5% to 2% of the mother liquor volume, the oxidation temperature is 40 to 50℃, and the pH of the system is maintained at 6.5 to 7.5 during the oxidation process; after the oxidation is completed, the residual hydrogen peroxide is decomposed by heating.

[0022] Due to the application of the above technical solutions, the beneficial effects of this invention compared with the prior art are as follows:

[0023] This invention uses isopropanol as the deacetylation solvent, resulting in no highly toxic methanol residue in the product and lower solvent evaporation losses during production. The catalyst is a metal-free quaternary ammonium base, tetramethylammonium hydroxide, which neutralizes without generating large amounts of salts, and the calcination residue of the finished product can be stably controlled below 0.03%, eliminating the need for activated carbon, ion exchange, or electrodialysis-assisted desalination. Continuous purification using ultrafiltration and nanofiltration replaces traditional adsorption and resin-based impurity removal processes, enabling continuous production with a high degree of automation, low operating costs, and no waste liquid from filler regeneration. Both stages of concentration utilize rising-film low-temperature thin-film evaporation, resulting in shorter material heating time, effectively inhibiting the thermal degradation of sucralose, lower syrup color, and higher product purity. Butyl acetate is used only as a pulping extraction reagent and does not participate in the crystallization process, allowing for the recycling of organic solvents. Gradient-temperature controlled single-phase aqueous crystallization, with a termination temperature of 8–12°C, eliminates the need for deep freezing, significantly reducing refrigeration energy consumption. The addition of seed crystals enables controlled crystallization, resulting in uniform crystal size, low impurity entrainment, and high separation and washing efficiency. The mother liquor is recycled after being oxidized with hydrogen peroxide and purified by nanofiltration, which will not interfere with the main processes of deacetylation and coarse membrane separation at the front end. Attached Figure Description

[0024] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0025] Figure 1 This is a flowchart of a crystallization and purification method for sucralose according to the present invention. Detailed Implementation

[0026] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification.

[0027] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this application are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be used interchangeably where appropriate for the description of embodiments of this application herein. Furthermore, the terms "comprising" and "having," and their synonyms, are intended to cover non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0028] In this application, the terms "upper," "lower," "left," "right," "front," "rear," "top," "bottom," "inner," "outer," "middle," "vertical," "horizontal," "lateral," and "longitudinal" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. These terms are primarily for the purpose of better describing the invention and its embodiments, and are not intended to limit the indicated device, element, or component to having a specific orientation, or to be constructed and operated in a specific orientation.

[0029] Furthermore, in addition to indicating direction or positional relationship, some of the aforementioned terms may also have other meanings. For example, the term "above" may also be used in certain situations to indicate a dependency or connection. Those skilled in the art can understand the specific meaning of these terms in this invention based on the specific circumstances.

[0030] Furthermore, the terms "installation," "setting," "equipped with," "connection," "linking," "fitting," and "fitting" should be interpreted broadly. For example, "connection" can be a fixed connection, a detachable connection, or an integral structure; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, or it can be an internal connection between two devices, components, or parts. Similarly, "fitting" can mean completely or partially fitted. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0031] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. This application will now be described in detail with reference to the accompanying drawings and embodiments.

[0032] Example 1:

[0033] like Figure 1 As shown, this invention discloses a method for purifying sucralose by crystallization, comprising the following steps:

[0034] S1. Take 100 kg of sucralose-6-acetic acid ester, add isopropanol to dissolve it completely, add tetramethylammonium hydroxide dropwise to adjust the alkalinity of the system, and keep the reaction at 30℃ for 4.5 h; after the reaction is completed, add acetic acid to adjust the pH to 6.5 to obtain sucralose neutralized crude solution.

[0035] S2. The neutralized crude liquid is sequentially passed through an ultrafiltration membrane (molecular weight cutoff 3 kDa, operating temperature 35℃) and a nanofiltration membrane (molecular weight cutoff 200 Da, operating pressure 1.4 MPa) for continuous separation. The nanofiltration concentrate is collected, which is the refined sugar solution.

[0036] S3. The refined sugar solution is concentrated to a solid content of 50wt% by low-temperature thin-film evaporation using a rising film method. Butyl acetate is added to the syrup at 1.2 times its weight, and the mixture is stirred at 50℃ for 1 hour. After standing and separating by filtration, the organic phase is separated, and the lower aqueous phase sugar solution is collected. Butyl acetate is recovered by distillation of the organic phase and recycled back to this step.

[0037] S4. The aqueous sugar solution is further concentrated to a saturated aqueous solution by low-temperature evaporation using a rising film method; the temperature is raised to 60°C, and 0.3% (by mass) of anhydrous sucralose crystal powder seed is added to the saturated aqueous solution, and the solution is kept at this temperature for 1.5 hours; the temperature is lowered to 28°C at a rate of 0.4°C / h, and the solution is kept at this temperature for 2 hours; the temperature is then lowered to 10°C at a rate of 0.8°C / h, and the solution is kept at this temperature for 10 hours; the solution is then centrifuged, and the crystals are washed with cold pure water.

[0038] S5. After washing, the crystals were vacuum dried at 48°C for 5 hours to obtain anhydrous sucralose product with a purity of 99.91% and a residue of 0.021% on ignition.

[0039] S6. Collect the crystallization mother liquor, add 30wt% hydrogen peroxide (1.2% of the mother liquor volume), oxidize at 45℃, and maintain the pH of the system at 7.0. After oxidation is complete, heat to decompose the residual hydrogen peroxide. The oxidized mother liquor is desalted by nanofiltration to obtain purified sugar solution, which is then sent to step S3 for recycling.

[0040] Example 2:

[0041] S1. Take 100 kg of sucralose-6-acetate, dissolve it in isopropanol, catalyze with tetramethylammonium hydroxide, and keep the reaction at 20°C for 5 h; adjust the pH to 6.2 with dilute hydrochloric acid to obtain a neutralized crude solution.

[0042] S2, Ultrafiltration membrane with a molecular weight cutoff of 3kDa and a temperature of 32℃; Nanofiltration membrane with a molecular weight cutoff of 180Da and an operating pressure of 1.2MPa, collects the refined sugar solution filtered by nanofiltration.

[0043] S3. Evaporate and concentrate the syrup to a solid content of 45wt% using a rising film method. Add 0.9 times the mass of butyl acetate, stir at 42℃ for 0.8h, filter in layers, and the aqueous sugar solution enters the next process. The butyl acetate is then recovered by distillation.

[0044] S4. Concentrate the aqueous sugar solution into a saturated aqueous solution, heat to 60℃, add 0.15% anhydrous sucralose seed crystals (by mass of the saturated aqueous solution) to grow crystals for 1 hour; cool to 26℃ at 0.3℃ / h to grow crystals for 1.2 hours; cool to 9℃ at 0.7℃ / h to precipitate crystals for 8 hours, centrifuge, and wash with cold water.

[0045] S5. After washing, the crystals were vacuum dried at 46°C for 6 hours. The purity of the anhydrous sucralose product was 99.87%, and the residue on ignition was 0.024%.

[0046] S6. Add 0.8 times the volume of 30wt% hydrogen peroxide to the mother liquor, oxidize at 42℃, and maintain pH at 6.8; after decomposing the hydrogen peroxide, purify by nanofiltration and return to S3 for reuse.

[0047] Example 3:

[0048] S1. Dissolve 100 kg of sucralose-6-acetate in isopropanol, catalyze with tetramethylammonium hydroxide, and react at 40°C for 4 h; adjust the pH to 6.8 with acidic cation exchange resin to obtain a neutralized crude solution.

[0049] S2, ultrafiltration membrane with a molecular weight cutoff of 3 kDa and an operating temperature of 42℃; nanofiltration membrane with a molecular weight cutoff of 280 Da and an operating pressure of 1.7 MPa, for collecting refined sugar solution.

[0050] S3. Evaporate and concentrate the syrup to a solid content of 58wt% using a rising film evaporator. Add 1.4 times the mass of butyl acetate, stir at 58℃ for 1.8h, filter in layers, and the aqueous sugar solution enters the next process. The butyl acetate is then recovered by distillation.

[0051] S4. Concentrate the aqueous sugar solution into a saturated aqueous solution, heat to 60℃, add 0.45% (by mass) of seed crystals from the saturated aqueous solution and grow crystals for 2 hours; cool to 29℃ at 0.6℃ / h and grow crystals for 2.8 hours; cool to 11℃ at 1.0℃ / h and crystallize for 12 hours, then centrifuge and wash with water.

[0052] S5. After washing, the crystals are vacuum dried at 50°C for 4 hours to obtain anhydrous sucralose product with a purity of 99.90% and a residue of 0.021% on ignition.

[0053] S6. Add 1.8% hydrogen peroxide to the mother liquor, oxidize at 48℃, pH 7.2, decompose the hydrogen peroxide, then nanofilter and reflux into S3 circulation.

[0054] Comparative Example 1:

[0055] Methanol was used to dissolve sucralose-6-acetate, followed by sodium hydroxide-catalyzed deacetylation. Acetic acid neutralized the solution to produce a large amount of sodium acetate. The crude solution was then desalted via activated carbon adsorption and ion exchange resin, concentrated under reduced pressure, and then recrystallized in two stages at 4°C. The mother liquor was directly refluxed back to the deacetylated crude solution. The finished product had a methanol residue of 0.018 mg / kg, a residue on ignition of 0.065%, and a purity of 99.51%. The refrigeration energy consumption was 1.8 times that of this invention. After five batches of mother liquor recycling, the color of the mother liquor significantly deepened.

[0056] Comparative Example 2:

[0057] Anhydrous ethanol was used as the solvent, sodium methoxide was used as the catalyst, resin was desalted, intermittently concentrated under reduced pressure, and recrystallized twice at 4°C; the mother liquor was directly refluxed to the front end. The finished product had a residue of 0.058% on ignition, and the raw material cost was higher than that of this invention, with severe impurity accumulation after multiple cycles.

[0058] The process of this invention uses isopropanol solvent, tetramethylammonium hydroxide metal-free catalysis, dual-membrane continuous purification, rising membrane low-temperature concentration, medium-temperature single gradient crystallization, and closed-loop circulation of oxidative nanofiltration mother liquor. The product has high purity, low ignition residue, no methanol residue, lower energy consumption, and the mother liquor can be stably reused for a long time.

[0059] Finally, it should be noted that the above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for purifying sucralose by crystallization, characterized in that, Includes the following steps: S1. Using sucralose-6-acetate as raw material, isopropanol is used for dissolution, tetramethylammonium hydroxide is added to catalyze the deacetylation reaction, and then an acid regulator is added to adjust the pH of the system to 6.0-7.0 to obtain a sucralose neutralized crude solution. S2. The neutralized crude liquid is sequentially passed through an ultrafiltration membrane module and a nanofiltration membrane module for continuous separation. The nanofiltration retentate concentrate is collected as refined sugar solution. S3. The refined sugar solution is concentrated to a syrup with a solid content of 40wt% to 60wt% by low-temperature thin-film evaporation using a rising film method. Butyl acetate is added to the syrup and stirred at a constant temperature. The organic phase is separated by filtration, and the aqueous sugar solution is collected. S4. The aqueous sugar solution is concentrated into a saturated aqueous solution by low-temperature evaporation using a rising film method. Anhydrous sucralose seed crystals are added to the saturated aqueous solution at 60°C for 1-2 hours to grow crystals. Then, the temperature is lowered to 25-30°C at a rate of 0.3-0.6°C / h and kept at this temperature for 1-3 hours to grow crystals. Then, the temperature is lowered to 8-12°C at a rate of 0.7-1.0°C / h and kept at this temperature for 6-12 hours to crystallize. After crystallization, the crystals are separated by centrifugation and washed with cold pure water. S5. Place the washed crystals under vacuum drying at 45-50℃ for 4-6 hours to obtain anhydrous sucralose crystals. S6. Collect the remaining crystallization mother liquor from step S4, add hydrogen peroxide for oxidation treatment, and then reflux the oxidized crystallization mother liquor as refined sugar solution to step S3 after nanofiltration.

2. The method for crystallizing and purifying sucralose according to claim 1, characterized in that: In step S1, the deacetylation reaction temperature is 5–45°C, and the reaction time is 4–5 h.

3. The method for crystallizing and purifying sucralose according to claim 1, characterized in that: In step S1, the acidity regulator is selected from any one of acetic acid, dilute hydrochloric acid, and acidic cation exchange resin.

4. The crystallization and purification method for sucralose according to claim 1, characterized in that: In step S2, the nanofiltration membrane has a molecular weight cutoff of 150–300 Da and an operating pressure of 1.0–1.8 MPa; the ultrafiltration membrane has a molecular weight cutoff of 3k–10kDa and an operating temperature of 30–45℃.

5. The method for crystallizing and purifying sucralose according to claim 1, characterized in that: In step S3, the amount of butyl acetate added is 0.8 to 1.5 times the mass of the syrup, the pulping temperature is 40 to 60°C, and the stirring time is 0.5 to 2 hours. The butyl acetate organic phase obtained by filtration is recycled for step S3 after distillation.

6. The method for crystallizing and purifying sucralose according to claim 1, characterized in that: In step S4, the seed crystal is anhydrous sucralose crystal powder, and the amount added is 0.1% to 0.5% of the total mass of the saturated aqueous solution.

7. The method for crystallizing and purifying sucralose according to claim 1, characterized in that: In step S6, the hydrogen peroxide is a 30wt% hydrogen peroxide aqueous solution, and the added volume is 0.5% to 2% of the mother liquor volume. The oxidation temperature is 40 to 50℃, and the pH of the system is maintained at 6.5 to 7.5 during the oxidation process. After the oxidation is completed, the residual hydrogen peroxide is decomposed by heating.

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