Method for preparing lactulose

Abstract

The invention discloses a method for preparing lactulose, and belongs to the field of lactulose preparation, and the method for preparing lactulose comprises the following steps: S1, adding boric acid into a lactose solution, adjusting the pH value to 10-11 by using alkali, and carrying out an isomerization reaction to obtain an isomerized sugar solution, S2, carrying out decoloration filtration, precision filtration and ultrafiltration treatment on the isomerized sugar solution obtained in the step S1, the method comprises the following steps: S1, carrying out ultrafiltration treatment on a permeate solution obtained in the step S2, S3, carrying out electrodialysis treatment on the permeate solution obtained in the step S2 to obtain an alkali solution and a dealkalized sugar solution, S4, carrying out nanofiltration treatment on the dealkalized sugar solution obtained in the step S3 to obtain a dilute boric acid solution and a crude lactulose solution, and S5, carrying out resin boron removal treatment on the crude lactulose solution obtained in the step S4 to obtain a refined lactulose solution. Introduction of impurity chloride ions is reduced from the process end, the purity of lactulose is improved, sodium hydroxide and boric acid are recycled respectively, no waste salt or waste water is generated, and the whole production process is green and environmentally friendly.

Description

Technical Field

[0001] The invention belongs to the field of lactulose preparation, and in particular relates to a method for preparing lactulose. Background Art

[0002] The main method of industrialized production of lactulose at present is to prepare lactulose by using NaOH and boric acid as the catalytic system of alkali isomerization. Under alkaline conditions, boric acid can make lactose form lactulose acid, borate complex, so that the balance of reaction moves in the direction that is conducive to lactulose generation. The formation of complex and the movement of reaction balance reduce the generation of degradation products to the greatest extent. Hicks et al. use equimolar lactose and boric acid, carry out isomerization in the presence of amines such as tertiary amines, find that the yield of reaction is higher, and then propose to replace tertiary amine with NaOH, to reduce reaction cost. After completing isomerization, hydrochloric acid needs to be added, and the pH of isomerized liquid is adjusted to 2-3 from the initial 10-11, and then sodium chloride and sodium borate therein are removed by methods such as nanofiltration+resin, resulting in subsequent production of a large amount of waste water containing sodium chloride, sodium borate and COD, increasing the pressure of subsequent environmental protection treatment. Summary of the invention

[0003] The object of the present invention is to provide a method for preparing lactulose to overcome at least one of the above-mentioned defects in the prior art.

[0004] To achieve this object, the present invention adopts the following technical solutions:

[0005] The present invention provides a method for preparing lactulose, comprising the following steps: S1: adding boric acid to a lactose solution, adjusting the pH to 10-11 with alkali, performing an isomerization reaction, and obtaining an isomerized sugar solution; S2: performing decolorization filtration, precision filtration, and ultrafiltration treatment on the isomerized sugar solution obtained in step S1; S3: performing electrodialysis treatment on the permeate obtained by the ultrafiltration treatment in step S2 to obtain an alkali solution and a dealkalized sugar solution; S4: performing nanofiltration treatment on the dealkalized sugar solution obtained in step S3 to obtain a dilute boric acid solution and a crude lactulose solution; and S5: performing a resin deboride treatment on the crude lactulose solution obtained in step S4 to obtain a refined lactulose solution.

[0006] Preferably, in step S1: the concentration of the lactose solution is 20-60%, before adding boric acid, the lactose solution is decolorized and treated with ion exchange resin, the molar ratio of boric acid to lactose in the lactose solution is 0.3-1:1, the isomerization reaction temperature is 50-100°C, and the reaction time is 1-5h.

[0007] Preferably, in step S2: decolorization and filtration include adding activated carbon, stirring at 40-60°C for 0.5-1h, and then filtering through a plate and frame, using activated carbon powder or an activated carbon column for decolorization, and the amount of activated carbon added is 0.5-5% of the mass of lactose, and the precision filtration uses microfiltration, PP melt-blown filter element, flat membrane, high-flux filter or bag filter, and the ultrafiltration uses a hollow fiber ultrafiltration membrane, a tubular ultrafiltration membrane or a ceramic ultrafiltration membrane.

[0008] Preferably, in step S3: an electrodialysis device with selective removal of alkali is used to remove the alkali solution, and 0.1-0.5 times the volume of the permeate obtained by ultrafiltration in step S2 is used as the alkali receiving solution, and the electrodialysis current density is 10-60 mA / cm 2 .

[0009] Preferably, the electrodialysis device comprises an anode plate, an anode membrane, a repeating unit, a cathode membrane, and a cathode plate arranged in sequence from left to right, the repeating unit comprises an anode membrane and a hydroxide selective membrane, and the anode membrane is located on the left side of the hydroxide selective membrane.

[0010] Preferably, in step S4: the molecular weight cutoff of nanofiltration is 150-300Da, the treatment temperature is 25-35°C, and during the nanofiltration treatment, 3-10 times the volume of the de-alkalized sugar solution is added with pure water for dilution and boron removal.

[0011] Preferably, in step S5: the resin is a boron-removing resin, the skeleton is a styrene-based resin, the functional group is a polyhydroxyamine, the exchange capacity is 0.4-0.6 mol / L, and the boron ion content in the obtained refined lactulose solution is less than 5 ppm.

[0012] Preferably, the method further comprises the following steps: recycling a part of the alkali solution obtained in step S3 to step S1, neutralizing the other part of the alkali solution with the dilute boric acid solution obtained in step S4, controlling the pH value to be 8.5-9.5, subjecting the neutralized solution to high-pressure reverse osmosis treatment, recycling the obtained concentrated solution to step S1, and recycling the obtained permeate to steps S3 and S4 respectively.

[0013] Preferably, the method further includes the following steps: performing acid regeneration, alkali regeneration, and flushing on the resin in step S5, performing acid recovery electrodialysis on the acid regeneration liquid, returning the generated acid liquid for continued use in acid regeneration of the resin, performing alkali recovery electrodialysis on the alkali regeneration liquid, returning the generated alkali liquid for continued use in alkali regeneration of the resin, neutralizing the flushing water, the residual liquid obtained by the acid recovery electrodialysis treatment, and the residual liquid obtained by the alkali recovery electrodialysis treatment, controlling the pH to 8.5-9.5, performing low-pressure reverse osmosis treatment on the neutralized water, performing high-pressure reverse osmosis treatment on the obtained concentrated liquid, and returning the obtained produced water for continued use in flushing the resin.

[0014] Preferably, both high-pressure reverse osmosis and low-pressure reverse osmosis treatment use seawater membranes, with a desalination rate of ≥98%, an operating pressure of 1.5-5MPa for high-pressure reverse osmosis, and an operating pressure of 0.5-2MPa for low-pressure reverse osmosis. The acid regeneration liquid is treated with electrodialysis equipment that selectively removes hydrogen ions, and the acid liquid is selectively recovered, with a recovery rate of ≥90%, and a current density of 10-60mA / cm 2 The alkali regeneration solution is treated with electrodialysis equipment that selectively removes hydroxide ions, and the alkali solution is selectively recovered with a recovery rate of ≥90% and a current density of 10-60mA / cm 2 .

[0015] The beneficial effects of the present invention are:

[0016] Reduce the introduction of impurity chloride ions from the process end, avoid the subsequent removal of chloride ions, reduce the difficulty of subsequent lactulose separation, improve the purity of lactulose, reduce the corrosiveness of the solution, and reduce the requirements for equipment materials. Sodium hydroxide and boric acid are recycled and reused separately, no waste salt or wastewater is generated, and the entire production process is green and environmentally friendly. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is a preparation flow chart of the present invention.

[0018] Figure 2 It is a schematic diagram of the structure of the electrodialysis equipment used in step S3 of the present invention.

[0019] The markings in the accompanying drawings are: 1-anode plate, 2-anode membrane, 4-cathode membrane, 5-cathode plate, 31-cation membrane, 32-hydroxyl selective membrane. DETAILED DESCRIPTION

[0020] The present invention will now be further described with reference to the accompanying drawings and specific implementation methods.

[0021] like Figure 1-2 As shown, a method for preparing lactulose provided in this embodiment comprises the following steps:

[0022] S1: Prepare 40% lactose solution, decolorize the lactose solution and treat it with ion exchange resin, add boric acid to the lactose solution, the molar ratio of boric acid to lactose in the lactose solution is 0.37:1, use sodium hydroxide to adjust the pH to 11, heat to 85°C, perform isomerization reaction for 2h, cool to 30°C, and obtain isomerized sugar solution.

[0023] S2: Add activated carbon powder with a mass percentage of 2% of lactose to the isomerized sugar solution obtained in step S1, stir at 50°C for 1 hour, and then filter through a plate and frame. The filtrate obtained by plate and frame filtration is subjected to microfiltration with a filtration accuracy of 1um to remove larger particles, colloids, insoluble impurities, etc. The filtrate obtained by microfiltration is subjected to ultrafiltration treatment using a ceramic membrane with a filtration accuracy of 0.05μm to filter and remove suspended matter and activated carbon impurities, and remove suspended matter with a content of less than 1ppm.

[0024] S3: The permeate obtained by ultrafiltration in step S2 is subjected to electrodialysis. The permeate obtained by ultrafiltration is added to one side of the electrodialysis, and pure water (the volume is 0.5 times the volume of the permeate obtained by ultrafiltration) is added to the other side. Add 50mA / cm 2 The current density is used for recovery, and the pH value of the sugar solution after de-alkali recovery is reduced to 2-3, and the alkali concentration is recovered to 1 mol / L.

[0025] The electrodialysis device includes an anode plate 1, an anode membrane 2, a repeating unit, a cathode membrane 4, and a cathode plate 5, which are arranged from left to right. The repeating unit includes a cation membrane 31 and a hydroxide selective membrane 32. The cation membrane 31 is located on the left side of the hydroxide selective membrane 32. Hydroxyl ions are well separated and recovered by the hydroxide selective membrane 32 to achieve efficient recovery of sodium hydroxide. The hydroxide selective membrane 32 can be a quaternary ammonium anion exchange membrane.

[0026] S4: The de-alkali sugar solution obtained in step S3 is subjected to nanofiltration treatment, with a molecular weight cut-off of 150Da and a treatment temperature of 35°C, to remove boron and other small molecular fructose. The boron in the de-alkali sugar solution can be reduced to 300ppm by adding 8 times the volume of pure water to dilute the de-alkali sugar solution, thereby obtaining a crude lactulose solution after deboronization and desalination, and a dilute boric acid solution.

[0027] S5: The crude lactulose solution obtained in step S4 is subjected to resin deboronization treatment to perform deep deboronization, and the boron ion content is reduced to below 5 ppm to obtain a refined lactulose solution. The resin is a boron removal resin with high selectivity for boron, the skeleton is a styrene-based resin, the functional group is a polyhydroxyamine, specifically a styrene-based polyhydroxyamine resin, the exchange capacity is 0.7 mol / L, and the boron ion content in the obtained refined lactulose solution is less than 5 ppm.

[0028] The following steps are also included:

[0029] A portion of the sodium hydroxide obtained in step S3 is recycled to step S1 for isomerization reaction treatment, and the other portion of the sodium hydroxide is neutralized with the dilute boric acid solution obtained in step S4, and the pH is controlled to be 8.5. The neutralized solution is subjected to high-pressure reverse osmosis treatment, specifically using a desalination membrane for concentration, with an operating pressure of 4MPa, and the obtained concentrated solution (sodium borate solution) is recycled to step S1, and the obtained permeate is recycled to steps S3 and S4 respectively. The boron ion content in the concentrated solution is 12000mg / L after concentration.

[0030] The following steps are also included:

[0031] The resin in step S5 is acid-regenerated using hydrochloric acid, the resin in step S5 is alkali-regenerated using sodium hydroxide solution, and the resin in step S5 is rinsed using pure water.

[0032] The acid regeneration liquid is treated with an electrodialysis device that selectively removes hydrogen ions to recover the hydrochloric acid selectively. Specifically, the acid regeneration liquid is added to one side of the electrodialysis and pure water is added to the other side. Add 50mA / cm 2 The acid recovery rate is 95% and the recovery concentration is 5%. The acid solution produced is returned to be used for acid regeneration of the resin. The cationic membrane of the electrodialysis device that selectively removes hydrogen ions can be a sulfonic acid cationic membrane, a carboxylic acid cationic membrane or a phosphate cationic membrane.

[0033] The alkali regeneration liquid is treated with an electrodialysis device that selectively removes hydroxide ions to selectively recover sodium hydroxide. Specifically, the alkali regeneration liquid is added to one side of the electrodialysis and pure water is added to the other side. Add 50mA / cm 2 The alkali recovery rate is 95% and the recovery concentration is 5%. The generated alkali solution is returned to continue to be used for alkali regeneration of the resin. The anion membrane of the electrodialysis device that selectively removes hydroxide ions can be a quaternary ammonium anion exchange membrane.

[0034] The rinse water, the residual liquid obtained by the acid recovery electrodialysis treatment, and the residual liquid obtained by the alkali recovery electrodialysis treatment are neutralized and the pH is controlled to 8.5. The neutralized water is subjected to low-pressure reverse osmosis treatment, and the obtained concentrated liquid is subjected to high-pressure reverse osmosis treatment, and the boron ion content in the concentrated liquid is concentrated to 3000mg / L, and the operating pressure is 1.5Mpa. The obtained produced water is returned to continue to be used for flushing the resin. Among them, both high-pressure reverse osmosis and low-pressure reverse osmosis treatment use anti-pollution seawater desalination membranes.

[0035] The invention adopts electrodialysis to replace the hydrochloric acid acidification process, directly recovers and concentrates the sodium hydroxide therein, and the concentrated sodium hydroxide is reused as a front-end raw material. At the same time, since the hydroxide selective membrane therein has the selective permeability of hydroxide ions, it can be separated from the borate therein, and then the lactulose solution and the boric acid are separated by nanofiltration, the separated boric acid is neutralized with the recovered sodium hydroxide, the neutralized sodium borate is concentrated and returned to the front end for reuse, the produced water is used for sodium filtration and the diluent and receiving liquid of the electrodialysis, most of the sugar solution after the boron is removed enters the resin system for deep boron removal, and the refined lactulose solution is met after the boron is removed, the acid and alkali of the resin regeneration liquid are respectively recovered and reused, the regenerated boric acid and the recovered sodium hydroxide are neutralized, the low-pressure reverse osmosis is concentrated, the concentrated liquid enters the upper high-pressure reverse osmosis for concentration, the produced water is used as resin flushing water, and the whole cycle resource recycling is realized, and the generation of wastewater is avoided.

[0036] The present invention reduces the introduction of impurity chloride ions from the process end, avoids the subsequent removal of chloride ions, reduces the difficulty of subsequent lactulose separation, improves the purity of lactulose, reduces the corrosiveness of the solution, and reduces the material requirements of the equipment. Sodium hydroxide and boric acid are recycled and reused separately, no waste salt or wastewater is generated, and the entire production process is green and environmentally friendly.

[0037] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, it should be understood by those skilled in the art that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features thereof may be replaced by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing lactulose, characterized in that: The following steps are involved: S1: Add boric acid to the lactose solution, adjust the pH to 10-11 with alkali, perform isomerization reaction, and obtain an isomerized sugar solution; S2: Decolorizing, filtering, fine filtering, and ultrafiltration the isomerized sugar solution obtained in step S1; S3: subjecting the permeate obtained by ultrafiltration in step S2 to electrodialysis to obtain an alkali solution and a de-alkalized sugar solution; S4: subjecting the de-alkalized sugar solution obtained in step S3 to nanofiltration to obtain a dilute boric acid solution and a crude lactulose solution; S5: The crude lactulose solution obtained in step S4 is subjected to resin deboronization treatment to obtain a refined lactulose solution.

2. The method for preparing lactulose according to claim 1, characterized in that: In step S1: The concentration of the lactose solution is 20-60%. Before adding boric acid, the lactose solution is decolorized and treated with ion exchange resin. The molar ratio of boric acid to lactose in the lactose solution is 0.3-1:

1. The isomerization reaction temperature is 50-100°C and the reaction time is 1-5h.

3. The method for preparing lactulose according to claim 1, characterized in that: In step S2: Decolorization and filtration include adding activated carbon, stirring at 40-60°C for 0.5-1h, and then filtering through a plate and frame; Use activated carbon powder or activated carbon column for decolorization, the amount of activated carbon added is 0.5-5% of the mass of lactose; Precision filtration uses microfiltration, PP melt-blown filter elements, flat membranes, high-flux filters or bag filters; Ultrafiltration uses hollow fiber ultrafiltration membrane, tubular ultrafiltration membrane or ceramic ultrafiltration membrane.

4. The method for preparing lactulose according to claim 1, characterized in that: In step S3: The alkali solution is removed by using an electrodialysis device that selectively removes alkali, and 0.1-0.5 times the volume of pure water of the permeate obtained by ultrafiltration in step S2 is used as the alkali receiving solution; The electrodialysis current density is 10-60mA / cm 2 .

5. The method for preparing lactulose according to claim 4, characterized in that: The electrodialysis device comprises an anode plate, an anode membrane, a repeating unit, a cathode membrane, and a cathode plate which are arranged in sequence from left to right; The repeating unit includes a cation membrane and a hydroxide selective membrane, wherein the cation membrane is located on the left side of the hydroxide selective membrane.

6. The method for preparing lactulose according to claim 1, characterized in that: In step S4: The molecular weight cut-off of nanofiltration is 150-300Da, and the processing temperature is 25-35°C; During the nanofiltration treatment, pure water 3-10 times the volume of the de-alkali sugar solution is added for dilution and boron removal.

7. The method for preparing lactulose according to claim 1, characterized in that: In step S5: The resin adopts a boron removal resin, the skeleton is a styrene-based resin, the functional group is a polyhydroxyamine, the exchange capacity is 0.4-0.6 mol / L, and the boron ion content in the obtained refined lactulose solution is less than 5ppm.

8. The method for preparing lactulose according to claim 1, characterized in that: The following steps are also included: A portion of the alkali solution obtained in step S3 is recycled to step S1, and the other portion of the alkali solution is neutralized with the dilute boric acid solution obtained in step S4 to control the pH value to 8.5-9.5; The neutralized solution is subjected to high-pressure reverse osmosis treatment, and the obtained concentrated solution is recycled to step S1, and the obtained permeate is recycled to step S3 and step S4 respectively.

9. The method for preparing lactulose according to claim 1, characterized in that: The following steps are also included: The resin in step S5 is subjected to acid regeneration, alkali regeneration, and rinsing; The acid regeneration liquid is subjected to acid recovery electrodialysis treatment, and the generated acid liquid is returned to continue to be used for acid regeneration of the resin; The alkali regeneration liquid is subjected to alkali recovery electrodialysis treatment, and the generated alkali liquid is returned to continue to be used for alkali regeneration of the resin; The rinse water, the residual liquid obtained by the acid recovery electrodialysis treatment, and the residual liquid obtained by the alkali recovery electrodialysis treatment are neutralized to control the pH to 8.5-9.5; The neutralized water is subjected to low-pressure reverse osmosis treatment, the obtained concentrated liquid is subjected to high-pressure reverse osmosis treatment, and the obtained produced water is returned to continue to be used for flushing the resin.

10. The method for preparing lactulose according to claim 9, characterized in that: Both high-pressure reverse osmosis and low-pressure reverse osmosis treatment use seawater desalination membranes, with a desalination rate of ≥98%. The operating pressure of high-pressure reverse osmosis is 1.5-5MPa, and the operating pressure of low-pressure reverse osmosis is 0.5-2MPa; The acid regeneration liquid is treated with electrodialysis equipment that selectively removes hydrogen ions, and the acid liquid is selectively recovered with a recovery rate of ≥90% and a current density of 10-60mA / cm 2 ; The alkali regeneration solution is treated with electrodialysis equipment that selectively removes hydroxide ions, and the alkali solution is selectively recovered with a recovery rate of ≥90% and a current density of 10-60mA / cm 2 .

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