System and method for continuous purification and concentration of lactulose from lactulose feed liquid prepared by sodium metaaluminate catalytic process

Abstract

The invention relates to the technical field of separation and purification after lactulose synthesis. In particular, the present invention provides systems and methods for the continuous purification and concentration of lactulose from a lactulose feed liquid prepared by a sodium metaaluminate catalyzed process. In the system and the method provided by the invention, firstly, the curtain type membrane is adopted to realize solid-liquid separation of the lactulose feed liquid, and the solid-liquid separation treatment speed can reach about 3.0 m < 3 > or above of the lactulose feed liquid per hour; secondly, after solid-liquid separation is completed, filtrate can enter the concentration and desalination procedures, and concentration and ion removal of the filtrate are achieved through nanofiltration concentration, nanofiltration desalination and resin adsorption desalination. The concentration of a lactulose product solution obtained by adopting the system and the method disclosed by the invention can reach at least about 2.5 times of the concentration of original lactulose feed liquid, the conductivity can be about 50 [mu] S / cm or below, and the recovery rate of the lactulose can reach about 90% or above and even about 95% or above.

Description

Technical Field

[0001] The present invention relates to the technical field of separation and purification of lactulose after synthesis, and in particular to a system and method for continuously purifying and concentrating lactulose from a lactulose liquid prepared by a sodium metaaluminate catalytic process. Background Art

[0002] Lactulose is suitable for patients with acute, chronic or habitual constipation, and can be taken by the elderly, children, pregnant women and lactating women. It is a laxative suitable for all ages.

[0003] By comprehensively comparing various lactulose synthesis methods, factors such as cost, yield, equipment requirements, and ease of separation and purification operations, the chemical method is generally preferred as the industrial synthesis process route for lactulose. The chemical method for synthesizing lactulose is further divided into the single alkali method, the boric acid method, and the sodium metaaluminate method.

[0004] The reaction mechanism of preparing lactulose by sodium aluminate catalysis includes: in aqueous solution, aluminate (AlO2 - ) does not actually exist. Its real existence is in the form of tetrahydroxyaluminate ([Al(OH)4] - ); The lactulose molecule contains a furan six-membered ring and a pyran five-membered ring. The furan ring also contains a cis-di-ortho-hydroxyl group, which complexes with the tetrahydroxyaluminate in an alkaline environment, thereby increasing the degree of lactulose's forward reaction. The principle of the complexation is as follows:

[0005]

[0006] The traditional purification and concentration process for preparing lactulose using the sodium aluminate catalytic method includes: adding lactose and sodium aluminate to a reactor and reacting for a certain period of time, then adjusting the pH of the reaction solution to precipitate aluminum hydroxide to form a lactulose solution. The solution is then filtered, and the filtrate is desalted using an anion and cation exchange resin, followed by evaporation to obtain a lactulose solution with a concentration of approximately 50% by weight.

[0007] The advantages of the above-mentioned traditional process include: a lactose conversion rate of more than about 85%, and low catalyst and resin costs.

[0008] However, the conventional process also has the following disadvantages: the aluminum hydroxide formed is in a flocculent state and contains a large amount of lactulose, which inevitably causes a large loss of lactulose, resulting in a final lactulose yield (about 60% to about 70%) that is much lower than the lactulose conversion rate (about 85%).

[0009] In the process of preparing lactulose by sodium aluminate catalysis, after the chemical reaction of preparing lactulose is completed, the liquid (solid and liquid phases) contains a large amount of chloride ions (Cl - ), sodium ion (Na+ ) and aluminum ions (Al 3+ ), where Cl - The concentration is at least about 1.1×10 4 wppm;Na + The concentration is at least about 8.5×10 3 wppm;Al 3+ The concentration is at least about 1.2×10 2 wppm. The lactulose content in the slurry is approximately 5% by weight. The suspended solids in the slurry are primarily aluminum hydroxide waste, accounting for approximately 10% by weight of the slurry. The electrical conductivity of the slurry is as high as approximately 30 mS / cm or above.

[0010] Filtration separation methods are already known in the art for treating lactulose slurries, such as plate and frame filter presses, centrifuge separation, and various filter separations. However, these conventional separation methods cannot achieve complete solid-liquid separation of the lactulose slurry, resulting in poor separation efficiency and a high lactulose loss rate. For example, after the slurry obtained after the reaction is passed through a filtration device (such as a plate and frame filter press) to separate lactulose from aluminum hydroxide, it is often found that the aluminum hydroxide precipitate contains a large amount of lactulose, resulting in material loss. Furthermore, the resulting lactulose solution product contains a large amount of salt, and its final conductivity is difficult to be less than the generally required 50 μS / cm. Summary of the Invention

[0011] Purpose of the Invention

[0012] In view of the problems existing in the prior art described in the above background technology section, the object of the present invention is to provide a system for continuously purifying and concentrating lactulose from a lactulose solution prepared by a sodium aluminate catalytic method, and to provide a method for continuously purifying and concentrating lactulose from a lactulose solution prepared by a sodium aluminate catalytic method using the system.

[0013] Technical Solution

[0014] In order to achieve the above-mentioned purpose of the present invention, the present invention provides the following technical solutions:

[0015] Scheme 1: A system for continuously purifying and concentrating lactulose from a lactulose slurry prepared by a sodium aluminate catalytic method, wherein the system includes a solid-liquid separation unit, the solid-liquid separation unit includes a curtain membrane solid-liquid separation device, the curtain membrane solid-liquid separation device includes a curtain membrane, and is used to achieve solid-liquid separation of a feed containing the lactulose slurry, thereby obtaining aluminum hydroxide slurry and curtain membrane water containing lactulose, wherein the curtain membrane has a filtration accuracy of about 0.4 to about 0.5 μm.

[0016] Option 2: The system according to Option 1 above, wherein the curtain film has a thickness of about 14 to about 16 m3 per cubic meter of the feed.2 membrane area.

[0017] Option 3: The system according to Option 1 or 2 above, wherein the curtain membrane is an organic membrane, such as a polytetrafluoroethylene membrane or a polyvinylidene fluoride membrane.

[0018] Option 4: A system according to any one of Options 1 to 3 above, wherein the solid-liquid separation unit further comprises a centrifuge, and the centrifuge is used to further centrifuge the aluminum hydroxide slurry to obtain aluminum hydroxide mud and centrifuge mother liquor.

[0019] Scheme 5: A system according to any one of schemes 1 to 4 above, wherein the system further comprises a concentration and desalination unit for concentrating and desalting the curtain membrane water containing lactulose, wherein the concentration and desalination unit comprises a primary nanofiltration concentration device, a secondary nanofiltration desalination device, and a resin adsorption desalination device arranged in sequence, wherein

[0020] The primary nanofiltration concentration device comprises a primary nanofiltration membrane, which is used to achieve the concentration of the curtain membrane produced water containing lactulose, thereby obtaining primary nanofiltration produced water and primary nanofiltration concentrated water containing lactulose, wherein the primary nanofiltration membrane has a molecular weight cut-off range of about 300 to about 500 Da;

[0021] The secondary nanofiltration desalination device comprises a secondary nanofiltration membrane, which uses deionized water with a conductivity of less than about 30 μS / cm to replace and remove salt contained in the primary nanofiltration concentrate, thereby obtaining a salt-containing secondary nanofiltration product water and a secondary nanofiltration mixed solution comprising secondary nanofiltration concentrate and deionized water with a conductivity of less than about 300 μS / cm, wherein the volume of the secondary nanofiltration mixed solution is no more than about 110% of the volume of the primary nanofiltration concentrate, wherein the secondary nanofiltration membrane has a molecular weight cut-off range of 300 to 500 Da; and

[0022] The resin adsorption desalination device comprises a cation exchange resin and an anion exchange resin, which are used to remove cations and anions in the secondary nanofiltration mixed solution, respectively, thereby obtaining a lactulose product solution with a conductivity lower than about 50 μS / cm.

[0023] Option 6: The system according to Option 5 above, wherein the primary nanofiltration membrane has a water production capacity of about 10 to about 15 m3 per cubic meter of the curtain membrane. 2 membrane area.

[0024] Option 7: The system according to Option 5 or 6, wherein the secondary nanofiltration membrane has a capacity of about 15 to about 20 m³ per cubic meter of the primary nanofiltration concentrated water. 2 membrane area.

[0025] Option 8: The system according to any one of Options 5 to 7 above, wherein the usage of the anion exchange resin and the cation exchange resin is respectively about 20 to about 21 kg per cubic meter of the secondary nanofiltration mixed liquid.

[0026] Option 9: The system according to any one of options 5 to 8 above, wherein the anion exchange resin is D301 anion exchange resin.

[0027] Option 10: The system according to any one of options 5 to 9 above, wherein the cation exchange resin is D001 cation exchange resin.

[0028] Option 11: The system according to any one of options 5 to 10 above, wherein the concentration and desalination unit further comprises an evaporation device for at least partially evaporating and removing the solvent in the lactulose product solution.

[0029] Option 12: A system according to any one of Options 5 to 11 above, wherein the concentration and desalination unit further includes an ultrafiltration device arranged between the primary nanofiltration concentration device and the secondary nanofiltration desalination device, the ultrafiltration device comprising an ultrafiltration membrane for removing aluminum hydroxide precipitates that may precipitate in the primary nanofiltration concentrated water, thereby obtaining ultrafiltration concentrated water containing the aluminum hydroxide precipitates and ultrafiltration product water from which the aluminum hydroxide precipitates have been removed, and the ultrafiltration product water then replaces the primary nanofiltration concentrated water and enters the secondary nanofiltration device to remove ions therefrom, wherein the ultrafiltration membrane has a filtration accuracy of approximately 0.4 to approximately 0.5 μm.

[0030] Solution 13: The system according to solution 12, wherein the ultrafiltration membrane has a capacity of about 5 to about 10 m³ per cubic meter of the primary nanofiltration concentrated water. 2 membrane area.

[0031] Option 14: A system according to any one of Options 1 to 13 above, wherein the system further comprises a reverse osmosis unit, which comprises a reverse osmosis membrane for removing salt contained in the primary nanofiltration product water and / or the secondary nanofiltration product water and recovering residual lactulose therein, thereby obtaining reverse osmosis concentrated water containing salt and lactulose and reverse osmosis product water with a conductivity of less than about 30 μS / cm.

[0032] Solution 15: The system according to Solution 14, wherein the reverse osmosis membrane has a flow rate of about 300 to about 400 m3 per cubic meter of the first-stage nanofiltration water and / or the second-stage nanofiltration water. 2 membrane area.

[0033] Scheme 16: A method for continuously purifying and concentrating lactulose from a lactulose solution prepared by a sodium metaaluminate catalytic process using the system according to any one of Schemes 1 to 15 above, wherein the lactulose content in the lactulose solution is at most about 50 g / L, the solids content is at least about 10% by weight, and the total salt content is at least about 17.5 g / L, and the method comprises the following steps:

[0034] Step 1: a solid-liquid separation step, comprising allowing the feed containing the lactulose liquid to flow through the curtain membrane of the curtain membrane solid-liquid separation device in the solid-liquid separation unit, thereby utilizing the curtain membrane to achieve solid-liquid separation of the feed containing the lactulose liquid, and obtaining aluminum hydroxide slurry and the curtain membrane water containing lactulose.

[0035] Scheme 17: The method according to Scheme 16 above, wherein during the solid-liquid separation process, the flow rate of the lactulose liquid is preferably controlled at about 3.0 m 3 / h or more.

[0036] Option 18: The method according to the above Option 16 or 17, wherein during the solid-liquid separation process, the pressure difference on both sides of the curtain membrane is controlled in the range of about 20 to about 100 kPa.

[0037] Scheme 19: The method according to any one of Schemes 16 to 18 above, wherein the solid-liquid separation is performed at a temperature of about 20 to about 25°C.

[0038] Option 20: A method according to any one of Options 16 to 19 above, wherein step 1 of the method further includes allowing the aluminum hydroxide slurry to flow through the centrifuge, and further centrifuging the aluminum hydroxide slurry to obtain aluminum hydroxide mud and centrifuge mother liquor.

[0039] Option 21: The method according to the above Option 20, wherein the centrifugal mother liquor is used as part of the feed for the solid-liquid separation step.

[0040] Scheme 22: The method according to any one of Schemes 16 to 21 above, wherein the method further comprises the following steps after step 1:

[0041] Step 2: Concentration and desalination step, for concentrating and desalting the curtain membrane water containing lactulose, the concentration and desalination step comprises the following sequential steps: a primary nanofiltration concentration step, a secondary nanofiltration desalination step, and a resin adsorption desalination step, wherein

[0042] The first-stage nanofiltration concentration step includes allowing the curtain membrane water containing lactulose to flow through the first-stage nanofiltration membrane of the first-stage nanofiltration concentration device, thereby concentrating the curtain membrane water containing lactulose to obtain the first-stage nanofiltration water and the first-stage nanofiltration concentrated water containing lactulose, wherein

[0043] The primary nanofiltration membrane has a molecular weight cut-off range of about 300 to about 500 Da;

[0044] The secondary nanofiltration desalination step comprises passing the primary nanofiltration concentrate through a secondary nanofiltration membrane of a secondary nanofiltration desalination device, and removing salt contained in the primary nanofiltration concentrate by displacement with deionized water, thereby obtaining a salty secondary nanofiltration product water and a secondary nanofiltration mixed solution comprising the secondary nanofiltration concentrate and deionized water having a conductivity of less than about 300 μS / cm, wherein the volume of the secondary nanofiltration mixed solution is no more than about 110% of the volume of the primary nanofiltration concentrate, wherein the secondary nanofiltration membrane has a molecular weight cut-off range of 300 to 500 Da;

[0045] The resin adsorption desalination step includes passing the secondary nanofiltration mixed solution through a cation exchange resin and an anion exchange resin, thereby removing cations and anions in the secondary nanofiltration concentrated water, respectively, to obtain a lactulose product solution with a conductivity of less than about 50 μS / cm.

[0046] Option 23: The method according to the above Option 22, wherein the volume of the primary nanofiltration concentrated water is about 25 to about 40% of the volume of the curtain membrane water production.

[0047] Scheme 24: The method according to Scheme 22 or 23 above, wherein the primary nanofiltration membrane has a water production capacity of about 10 to about 15 m3 per cubic meter of the curtain membrane. 2 membrane area.

[0048] Scheme 25: The method according to any one of Schemes 22 to 24 above, wherein during the primary nanofiltration concentration step,

[0049] Controlling the pH value of the curtain membrane water to be within the range of about 5 to about 7; and / or

[0050] The primary nanofiltration concentration step is carried out at a temperature of about 20 to about 25°C; and / or

[0051] The flow rate of the curtain membrane water production is controlled to be at least about 3.0m 3 / h, and / or

[0052] The pressure difference across the primary nanofiltration membrane is controlled within a range of about 1 to about 3 MPa.

[0053] Scheme 26: The method according to any one of Schemes 22 to 25, wherein the secondary nanofiltration membrane has a capacity of about 15 to about 20 m³ per cubic meter of the primary nanofiltration concentrated water. 2 membrane area.

[0054] Scheme 27: The method according to any one of schemes 22 to 26 above, wherein during the secondary nanofiltration desalination step,

[0055] Controlling the pH value of the primary nanofiltration concentrate to be within the range of about 5 to about 7; and / or

[0056] carrying out the secondary nanofiltration desalination step at a temperature of about 20 to about 25°C; and / or

[0057] The flow rate of the primary nanofiltration concentrated water is controlled to be at least about 0.75m 3 / h, and / or

[0058] The pressure difference across the secondary nanofiltration membrane is controlled in the range of about 0.15 to about 0.3 MPa.

[0059] Scheme 28: A method according to any one of the above schemes 22 to 27, wherein the secondary nanofiltration desalination step includes first allowing the primary nanofiltration concentrate to flow through the secondary nanofiltration membrane, thereby concentrating the primary nanofiltration concentrate to obtain secondary nanofiltration product water and secondary nanofiltration concentrate containing lactulose; then mixing the secondary nanofiltration concentrate with the deionized water to obtain the secondary nanofiltration mixed liquor, the volume of the secondary nanofiltration mixed liquor being approximately 90 to approximately 110% of the volume of the primary nanofiltration concentrate; finally, when the conductivity of the secondary nanofiltration mixed liquor is measured to be higher than approximately 300 μS / cm, repeatedly allowing the secondary nanofiltration mixed liquor to flow through the secondary nanofiltration membrane, thereby concentrating the secondary nanofiltration mixed liquor, further obtaining secondary nanofiltration product water and secondary nanofiltration concentrate containing lactulose, and when the conductivity of the secondary nanofiltration mixed liquor is measured to be lower than approximately 300 μS / cm, allowing the secondary nanofiltration mixed liquor to enter the resin adsorption desalination step.

[0060] Option 29: The method according to the above Option 28, wherein the volume of the secondary nanofiltration concentrate is about 40 to about 60% of the volume of the primary nanofiltration concentrate.

[0061] Option 30: The method according to any one of the above options 22 to 29, wherein the usage of the anion exchange resin and the cation exchange resin is respectively about 20 to about 21 kg per cubic meter of the secondary nanofiltration concentrated water.

[0062] Scheme 31: The method according to any one of Schemes 22 to 30, wherein the anion exchange resin is D301 anion exchange resin.

[0063] Scheme 32: The method according to any one of Schemes 22 to 31 above, wherein the cation exchange resin is D001 cation exchange resin.

[0064] Scheme 33: The method according to any one of the above schemes 22 to 32, wherein during the resin adsorption desalination step,

[0065] The flow rate of the secondary nanofiltration concentrated water is controlled at about 0.75m 3 / h or more; and / or

[0066] The resin adsorption desalting step is carried out at a temperature of about 20 to about 25°C.

[0067] Option 34: The method according to any one of the above options 22 to 33, wherein the concentration and desalination step further includes an evaporation step after the resin adsorption desalination step, and the evaporation step includes using an evaporation device to at least partially evaporate and remove the solvent in the lactulose product solution.

[0068] Scheme 35: A method according to any one of the above schemes 22 to 34, wherein the method further includes an ultrafiltration step between the primary nanofiltration concentration step and the secondary nanofiltration desalination step, comprising allowing the primary nanofiltration concentrate to flow through the ultrafiltration membrane of an ultrafiltration unit, thereby removing aluminum hydroxide precipitates that may precipitate in the primary nanofiltration concentrate, and obtaining ultrafiltration concentrate containing the aluminum hydroxide precipitates and ultrafiltration product water from which the aluminum hydroxide precipitates have been removed, wherein the ultrafiltration membrane has a filtration accuracy of 0.4 to 0.5 μm, and then allowing the ultrafiltration product water to replace the primary nanofiltration concentrate to undergo the secondary nanofiltration desalination step to remove ions therein.

[0069] Option 36: The method according to the above Option 35, wherein the ultrafiltration concentrated water is combined with the aluminum hydroxide slurry.

[0070] Scheme 37: The method according to the above scheme 35 or 36, wherein the ultrafiltration membrane has a capacity of 5 to 10 m³ per cubic meter of the primary nanofiltration concentrated water. 2 membrane area.

[0071] Scheme 38: The method according to any one of schemes 35 to 37 above, wherein in the ultrafiltration step,

[0072] The flow rate of the primary nanofiltration concentrated water is controlled to at least about 0.75m 3 / h or more; and / or

[0073] The pressure difference across the ultrafiltration membrane is controlled in the range of about 20 to about 500 kPa.

[0074] Scheme 39: The method according to any one of Schemes 22 to 38 above, wherein the method further comprises the following steps:

[0075] Step 3: a reverse osmosis step, comprising passing the primary nanofiltration product water and / or the secondary nanofiltration product water through a reverse osmosis membrane, thereby removing the salt contained in the primary nanofiltration product water and / or the secondary nanofiltration product water and recovering the residual lactulose therein, to obtain reverse osmosis concentrated water containing salt and lactulose and reverse osmosis product water with a conductivity of less than about 30 μS / cm.

[0076] Scheme 40: The method according to Scheme 39, wherein the reverse osmosis membrane has a flow rate of about 300 to about 400 m3 of the first-stage nanofiltration water and / or the second-stage nanofiltration water. 2 membrane area.

[0077] Option 41: The method according to the above option 39 or 40, wherein the pressure difference between the two sides of the reverse osmosis membrane is controlled in the range of about 1.5 to about 2.5 MPa.

[0078] Scheme 42: The method according to any one of Schemes 39 to 41, wherein the reverse osmosis concentrate is at least partially combined with the curtain membrane product water after the lactulose content therein is optionally concentrated.

[0079] Option 43: The method according to any one of the above options 39 to 42, wherein the pH value of the primary nanofiltration product water and / or the secondary nanofiltration product water is controlled in the range of about 5 to about 7.

[0080] Scheme 44: A method according to any one of Schemes 39 to 43 above, wherein at least part or all of the reverse osmosis produced water is used as deionized water in the secondary nanofiltration desalination step; and / or at least part or all of the reverse osmosis produced water is used as washing water for the aluminum hydroxide slurry obtained in step 1.

[0081] Technical Effects

[0082] The system and method of the present invention utilizes a curtain membrane solid-liquid separation device to achieve efficient solid-liquid separation of a lactulose solution having a lactulose content of up to about 50 g / L, a solids content of at least about 10% by weight, and a total salt content of at least about 17.5 g / L. In the present invention, the curtain membrane's high throughput ensures the passage of the lactulose solution while retaining aluminum hydroxide precipitate on the membrane surface, enabling a processing rate of approximately 3.0 m / h. 3lactulose feed liquid or more. When the separation is completed, the aluminum hydroxide precipitate can be allowed to enter the centrifuge for centrifugal operation, and the centrifuged clear liquid can be combined with the feed liquid again as feed to enter the curtain membrane solid-liquid separation device for solid-liquid separation. When the solid-liquid separation is completed, the filtrate can enter the concentration and desalination process, including the use of nanofiltration concentration + nanofiltration desalination + resin adsorption desalination to achieve filtrate concentration and ion removal. The concentration of the lactulose product solution obtained by the system and method of the present invention can reach at least about 2.5 times the concentration of the original lactulose feed liquid, the conductivity can be below about 50μS / cm, and the lactulose recovery rate can reach more than about 90%, or even more than about 95%. BRIEF DESCRIPTION OF THE DRAWINGS

[0083] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the specific implementation methods of the present invention. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0084] Figure 1 A preferred embodiment of the system and method of the present invention for continuously purifying and concentrating lactulose from a lactulose solution prepared by a sodium metaaluminate catalytic process is illustrated. DETAILED DESCRIPTION

[0085] In order to make the purpose, technical solutions and advantages of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. It should be understood by those skilled in the art that the embodiments are only for the purpose of helping to understand the present invention and should not be regarded as specific limitations of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without paying creative work fall within the scope of protection of the present invention. The process parameters for which specific conditions are not specified in the following examples are generally in accordance with conventional conditions.

[0086] The endpoints and any values ​​of the ranges disclosed in the present invention are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. The term "about" as used in the present invention means that the numeral it modifies can fluctuate within ±20%, ±15%, ±10%, ±5% or ±2% of the numeral. For numerical ranges, the endpoints of each range, the endpoints of each range and the individual point values ​​therein, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be considered to be specifically disclosed in the present invention.

[0087] According to a first aspect of the present invention, a system is provided for continuously purifying and concentrating lactulose from a lactulose solution prepared by a sodium metaaluminate catalysis process, wherein the lactulose solution has a lactulose content of up to about 50 g / L, a solids content of at least about 10% by weight, and a total salt content of at least about 17.5 g / L.

[0088] The system according to the first aspect of the present invention comprises a solid-liquid separation unit and an optional concentration and desalination unit.

[0089] Solid-liquid separation unit

[0090] The solid-liquid separation unit includes a curtain membrane solid-liquid separation device, which includes a curtain membrane and is used to achieve solid-liquid separation of the feed containing the lactulose liquid, thereby obtaining aluminum hydroxide slurry and curtain membrane water containing lactulose, wherein the curtain membrane has a filtration accuracy of about 0.4 to about 0.5 μm.

[0091] The curtain membrane separation used in the present invention, which has a filtration accuracy of about 0.4 to about 0.5 μm (e.g., about 0.45 μm), can efficiently achieve solid-liquid separation of the lactulose slurry. Here, the filtration accuracy of the curtain membrane should not be too large, for example, should not exceed about 0.5 μm, otherwise excessive aluminum hydroxide may enter the curtain membrane water, resulting in incomplete solid-liquid separation; nor should it be too small, for example, should not be less than about 0.4 μm, otherwise lactulose molecules may be intercepted by the curtain membrane, resulting in lactulose loss.

[0092] The curtain membranes that can be used in the present invention include inorganic membranes and organic membranes, and these two membranes each have different advantages and disadvantages. For example, although inorganic membranes (such as ceramic membranes) can also achieve the purpose of the present invention, they require frequent alkali washing and acid washing, which may have an adverse effect on product quality. Organic curtain membranes have lower operating costs and lower operating pressures, and the price of a single membrane is cheap. Therefore, in the present invention, it is preferred to use an organic curtain membrane for the solid-liquid separation, for example, a polytetrafluoroethylene (PTFE) membrane or a polyvinylidene fluoride (PVDF) membrane can be used. In the present invention, a PTFE membrane is more preferably used, because the material of the PTFE can be used for a longer time and the membrane filtration effect is better.

[0093] In a preferred embodiment, in order to ensure the process speed requirements in industrial production, the curtain film preferably has a feed rate of about 14 to about 16 m3 per cubic meter of the lactulose-containing liquid. 2 (For example, about 15m 2 ) of the membrane area, so that the processing capacity of the lactulose liquid-solid-liquid separation can reach at least about 3.0m 3 / h (e.g. about 3.3m 3 / h, or about 3.5m 3 / h).

[0094] In another preferred embodiment, the solid-liquid separation unit may further include a centrifuge, which is used to further centrifuge the aluminum hydroxide slurry to obtain aluminum hydroxide mud and centrifuge mother liquor.

[0095] Concentration and desalination unit

[0096] The concentration and desalination unit is used to treat the curtain membrane water containing lactulose to concentrate and desalinate the curtain membrane water. The concentration and desalination unit includes a primary nanofiltration concentration device, an optional ultrafiltration device, a secondary nanofiltration desalination device, a resin adsorption desalination device, and an optional evaporation device arranged in sequence.

[0097] First-stage nanofiltration concentration device

[0098] The primary nanofiltration concentration device uses a primary nanofiltration membrane to concentrate the curtain membrane produced water containing lactulose obtained from the solid-liquid separation unit, thereby obtaining primary nanofiltration produced water and primary nanofiltration concentrated water containing lactulose.

[0099] The primary nanofiltration membrane should have a molecular weight cut-off range of about 300 to about 500 Da. The molecular weight cut-off range should be neither too high nor too low. For example, it should not exceed about 500 Da, as this may allow excessive lactulose molecules to enter the primary nanofiltration water, resulting in yield loss. It should also not be less than about 300 Da, as excessively small membrane pores increase osmotic resistance, leading to increased operating pressure requirements (possibly exceeding the equipment's pressure range), reduced water production rate per unit membrane area, and poor energy efficiency.

[0100] In some preferred embodiments, in order to ensure the process speed requirements in industrial production, the primary nanofiltration membrane preferably has a water production capacity of about 10 to about 15 m3 per cubic meter of the curtain membrane. 2 (For example, about 11m 2 , about 12m 2 , about 13m 2 or about 14m 2 ) of the membrane area, so that the processing capacity of the concentrated curtain membrane water can reach at least about 3.0m 3 / h, for example, about 3.3m 3 / h or about 3.5m 3 / h.

[0101] Two-stage nanofiltration concentration device

[0102] The secondary nanofiltration desalination device includes a secondary nanofiltration membrane, which uses deionized water with a conductivity of less than about 30 μS / cm to replace and remove the salt contained in the primary nanofiltration concentrate, thereby obtaining salt-containing secondary nanofiltration product water and a secondary nanofiltration mixed liquor containing secondary nanofiltration concentrate and deionized water with a conductivity of less than about 300 μS / cm, wherein the volume of the secondary nanofiltration mixed liquor is not greater than about 110% of the volume of the primary nanofiltration concentrate.

[0103] The secondary nanofiltration membrane should have a molecular weight cut-off range of about 300 to about 500 Da. Here, the molecular weight cut-off range of the secondary nanofiltration membrane should not be too high or too low. For example, it should not exceed about 500 Da, otherwise excessive lactulose molecules may enter the secondary nanofiltration water, resulting in yield loss. It should also not be less than about 300 Da. Excessively small membrane pore size increases osmotic resistance, leading to increased operating pressure requirements (possibly exceeding the pressure range of the equipment), and at the same time, reduces the water production rate per unit membrane area, resulting in poor energy efficiency.

[0104] In some preferred embodiments, in order to ensure the process speed requirements in industrial production, the secondary nanofiltration membrane preferably has a flow rate of about 15 to about 20 m3 per cubic meter of the primary nanofiltration concentrated water. 2 (For example, about 16m 2 , about 17m 2 , about 18m 2 or about 19m 2 ) of the membrane area, so that the treatment capacity of removing salt from the primary nanofiltration concentrated water can reach at least about 0.75m 3 / h, for example, about 1.0m 3 / h or about 1.4m 3 / h.

[0105] Optional ultrafiltration device

[0106] The concentration and desalination unit may optionally further comprise an ultrafiltration device between the primary nanofiltration device and the secondary nanofiltration device.

[0107] Since the first-stage nanofiltration device concentrates the curtain membrane water product to produce first-stage nanofiltration concentrate, the aluminum hydroxide dissolved in the curtain membrane water product may partially precipitate, making the first-stage nanofiltration concentrate turbid. Therefore, the ultrafiltration membrane of the ultrafiltration device can be used to remove the aluminum hydroxide precipitate that may precipitate in the first-stage nanofiltration concentrate, thereby obtaining ultrafiltration concentrate containing the aluminum hydroxide precipitate and ultrafiltration water product from which the aluminum hydroxide precipitate has been removed, and the ultrafiltration water product then replaces the first-stage nanofiltration concentrate and enters the second-stage nanofiltration device to remove the ions therein.

[0108] Here, the ultrafiltration membrane may have a filtration accuracy of about 0.4 to about 0.5 μm (for example, about 0.45 μm); here, the filtration accuracy of the ultrafiltration membrane should not be too large, for example, it should not exceed about 0.5 μm, otherwise it may cause aluminum hydroxide to enter the ultrafiltration water and the aluminum hydroxide separation is incomplete; it should not be too small, for example, it should not be less than about 0.4 μm, otherwise it may cause lactulose molecules to be retained by the ultrafiltration membrane, resulting in lactulose loss.

[0109] In some preferred embodiments, in order to ensure the process speed requirements in industrial production, the ultrafiltration membrane can have a flow rate of about 5 to about 10 m3 per cubic meter of the primary nanofiltration concentrated water. 2 (For example, about 6m 2 , about 7m 2 , about 8m 2 or about 9m 2 ) of the membrane area, so that the processing capacity of separating the aluminum hydroxide in the primary nanofiltration concentrated water can reach at least about 0.75m 3 / h, for example, about 1.0m 3 / h or about 1.4m 3 / h.

[0110] Resin adsorption desalination device

[0111] The resin adsorption desalination device comprises a cation exchange resin and an anion exchange resin, which are used to remove cations and anions in the secondary nanofiltration concentrated water, respectively, thereby obtaining a lactulose product solution with a conductivity lower than about 50 μS / cm.

[0112] In some preferred embodiments, the amount of the anion exchange resin and the cation exchange resin is preferably about 20 to about 21 kg per cubic meter of the secondary nanofiltration concentrate, respectively. The amount of the ion exchange resin should not be too large, for example, not exceeding about 21 kg per cubic meter of the secondary nanofiltration concentrate, as this may result in excessive water consumption; nor should it be too small, for example, not less than about 20 kg per cubic meter of the secondary nanofiltration concentrate, as this may result in incomplete ion adsorption.

[0113] In other exemplary embodiments, the anion exchange resin may be D301 anion exchange resin, and / or the cation exchange resin may be D001 cation exchange resin.

[0114] Optional evaporation device

[0115] The optional evaporation device can be used to evaporate at least part of the solvent in the lactulose product solution to obtain a lactulose product solution with a desired specific gravity or to obtain a lactulose solid product.

[0116] Optional reverse osmosis unit

[0117] In some further preferred embodiments, the system according to the first aspect of the present invention may further include a reverse osmosis unit. The reverse osmosis unit comprises a reverse osmosis membrane for removing salt from the primary nanofiltration product water and / or the secondary nanofiltration product water and recovering lactulose that may be contained therein, thereby obtaining reverse osmosis concentrate containing salt and lactulose and reverse osmosis product water having a conductivity of less than about 30 μS / cm.

[0118] In some preferred embodiments, in order to ensure the process speed requirements in industrial production, the reverse osmosis membrane can have a flow rate of about 300 to about 400 m3 per cubic meter of the first nanofiltration water and / or the second nanofiltration water. 2 (For example, about 350m 2 ) of the membrane area.

[0119] According to a second aspect of the present invention, the present invention provides a method for continuously purifying lactulose from a lactulose solution prepared by a sodium aluminate catalytic method using a system according to the first aspect of the present invention, wherein the lactulose content in the lactulose solution is up to about 50 g / L, the solids content is at least about 10% by weight, and the total salt content is at least about 17.5 g / L.

[0120] All features (including preferred features) described above in relation to the system of the first aspect of the invention are equally applicable to the second aspect of the invention.

[0121] The method according to the second aspect of the present invention comprises the following steps 1, 2 and 3 performed in sequence.

[0122] The step 1 is a solid-liquid separation step, comprising allowing the feed containing the lactulose liquid to flow through the curtain membrane of the curtain membrane solid-liquid separation device in the solid-liquid separation unit, thereby utilizing the curtain membrane to achieve solid-liquid separation of the feed containing the lactulose liquid, and obtaining aluminum hydroxide slurry and curtain membrane water containing lactulose. Here, the conductivity of the curtain membrane water is generally at least about 30 mS / cm, for example, can reach about 33 or about 35 mS / cm.

[0123] In some preferred embodiments, in order to ensure the process speed requirements in industrial production, the flow rate of the lactulose liquid can be controlled at about 3.0 m / s during the solid-liquid separation process. 3 / h or more (e.g. about 3.1m 3 / h, about 3.2m 3 / h, about 3.3m 3 / h, about 3.4m 3 / h or about 3.5m 3 / h), and / or the pressure difference across the curtain membrane can be controlled in the range of about 20 to about 100 kPa (eg, about 40 kPa, about 60 kPa, or about 80 kPa).

[0124] In other preferred embodiments, the solid-liquid separation can be performed at room temperature (e.g., at a temperature of about 20 to about 25° C.). At room temperature, the flux of the curtain membrane is optimal. Higher temperatures may cause more aluminum hydroxide to dissolve in the feed solution, while too low a temperature may significantly reduce the membrane flux, resulting in insufficient solid-liquid separation.

[0125] In some further preferred embodiments, step 1 further comprises passing the aluminum hydroxide slurry through the centrifuge for further centrifugal separation, thereby obtaining aluminum hydroxide slurry and a centrifuge mother liquor. Preferably, the centrifuge mother liquor is used as part of the feed for the solid-liquid separation step. Furthermore, during the centrifugation process, a portion of the reverse osmosis product water obtained below can be optionally used as a washing liquid for the aluminum hydroxide slurry.

[0126] The optional step 2 is a concentration and desalination step for concentrating and desalting the curtain membrane water containing lactulose, comprising the following steps, which are performed in sequence: a primary nanofiltration concentration step, an optional ultrafiltration step, a secondary nanofiltration desalination step, a resin adsorption desalination step, and an optional evaporation step.

[0127] The purpose of the primary nanofiltration concentration step is to enable the curtain membrane water containing lactulose to be concentrated first in the subsequent desalination step to effectively overcome the disadvantages of ion adsorption by anion and cation exchange resins (the disadvantages of ion adsorption by anion and cation exchange resins are that they consume a huge amount of water, require a large amount of resin, and require frequent analysis and regeneration).

[0128] The primary nanofiltration concentration step includes passing the lactulose-containing curtain membrane product water through a primary nanofiltration membrane of a primary nanofiltration concentration device. The primary nanofiltration membrane described in detail above is used to concentrate the lactulose-containing curtain membrane product water, thereby obtaining primary nanofiltration product water and a primary nanofiltration concentrate containing lactulose. The salt concentration in the primary nanofiltration concentrate remains substantially unchanged, and thus its conductivity generally remains above approximately 30 mS / cm, for example, approximately 33 or approximately 35 mS / cm.

[0129] In some preferred embodiments, after the primary nanofiltration concentration step, the volume of the primary nanofiltration concentrate obtained can be about 25 to about 40% of the volume of the curtain membrane water production, for example, about 30% or about 35%. Here, the concentration ratio of the curtain membrane water production should not be too large, for example, not less than about 25% of its original volume, otherwise it may increase operating costs; nor should it be too small, for example, not more than about 40% of its original volume, otherwise the pressure will be low, the water volume will increase in the later stage, and the resin and membrane area will need to be increased.

[0130] In some preferred embodiments, in order to ensure the process speed requirements in industrial production, the flow rate of the curtain membrane water production can be preferably controlled at about 3.0m 3 / h or more (e.g. about 3.1m 3 / h, about 3.2m 3 / h, about 3.3m 3 / h, about 3.4m 3 / h or about 3.5m 3 / h), and / or controlling the pressure difference across the first-stage nanofiltration membrane to be above about 1 MPa (e.g., about 2 MPa or about 3 MPa).

[0131] In other preferred embodiments, the primary nanofiltration concentration step can be performed at room temperature (e.g., at a temperature of about 20 to about 25° C.). At room temperature, the flux of the primary nanofiltration membrane is optimal. Higher temperatures may cause lactulose to produce impurities, while too low a temperature may limit the membrane flux.

[0132] In some preferred embodiments, the pH value of the curtain membrane water is preferably controlled within a range of about 5 to about 7 (e.g., about 6). Here, too high or too low a pH value of the curtain membrane water during the primary nanofiltration concentration process may cause damage to the membrane structure.

[0133] The purpose of the secondary nanofiltration concentration step is to preliminarily remove the salt in the primary nanofiltration concentrated water.

[0134] The secondary nanofiltration concentration step includes allowing the primary nanofiltration concentrate to flow through a secondary nanofiltration membrane of a secondary nanofiltration desalination device, and using the secondary nanofiltration membrane and deionized water described in detail above to displace and remove the salt contained in the primary nanofiltration concentrate, thereby obtaining salt-containing secondary nanofiltration product water and a secondary nanofiltration mixed liquor containing secondary nanofiltration concentrate and deionized water with a conductivity of less than about 300 μS / cm, wherein the volume of the secondary nanofiltration mixed liquor is not greater than about 110% of the volume of the primary nanofiltration concentrate.

[0135] In some preferred embodiments, in order to ensure the process speed requirements in industrial production, during the secondary nanofiltration concentration step, the flow rate of the primary nanofiltration concentrated water can be controlled to at least about 0.75m3 / h (e.g. about 0.8m 3 / h, about 0.9m 3 / h, about 1.0m 3 / h, about 1.1m 3 / h, about 1.2m 3 / h, about 1.3m 3 / h or about 1.4m 3 / h), and / or controlling the pressure difference across the two sides of the secondary nanofiltration membrane in the range of about 0.15 to about 0.3 MPa (e.g., about 0.2 MPa or about 0.25 MPa).

[0136] In other preferred embodiments, the secondary nanofiltration desalination step can be performed at room temperature (e.g., at a temperature of about 20 to about 25° C.). At room temperature, the flux of the secondary nanofiltration membrane is optimal. Higher temperatures may cause lactulose to produce impurities, while too low a temperature may limit the membrane flux.

[0137] In some preferred embodiments, the pH value of the primary nanofiltration concentrate is preferably controlled within a range of about 5 to about 7. Here, excessively high or low pH of the primary nanofiltration concentrate during the secondary nanofiltration desalination process may result in damage to the membrane structure.

[0138] In some particularly preferred embodiments, the specific steps of the secondary nanofiltration desalination may include first allowing the primary nanofiltration concentrated water to flow through the secondary nanofiltration membrane, thereby concentrating the primary nanofiltration concentrated water to obtain secondary nanofiltration product water and secondary nanofiltration concentrated water containing lactulose, wherein the volume of the secondary nanofiltration concentrated water is preferably about 40 to about 60% (e.g., about 50%) of the volume of the primary nanofiltration concentrated water; and then mixing the secondary nanofiltration concentrated water with the deionized water to obtain the secondary nanofiltration mixed solution, wherein the volume of the secondary nanofiltration mixed solution is about 100% to about 200% (e.g., about 50%) of the volume of the primary nanofiltration concentrated water. It is about 90 to about 110% of the volume of the primary nanofiltration concentrated water, for example, about 100%; finally, when the conductivity of the secondary nanofiltration mixed liquid is measured to be higher than about 300 μS / cm, the secondary nanofiltration mixed liquid is repeatedly passed through the secondary nanofiltration membrane, thereby concentrating the secondary nanofiltration mixed liquid, further obtaining secondary nanofiltration product water and secondary nanofiltration concentrated water containing lactulose, and when the conductivity of the secondary nanofiltration mixed liquid is measured to be lower than about 300 μS / cm, the secondary nanofiltration mixed liquid enters the resin adsorption desalination step.

[0139] In another particularly preferred embodiment, an ultrafiltration step may be optionally included between the primary nanofiltration concentration step and the secondary nanofiltration desalination step, comprising allowing the primary nanofiltration concentrate to flow through an ultrafiltration membrane of an ultrafiltration unit, thereby utilizing the ultrafiltration membrane described above to remove aluminum hydroxide precipitates that may precipitate in the primary nanofiltration concentrate, thereby obtaining ultrafiltration concentrate containing the aluminum hydroxide precipitates and ultrafiltration product water from which the aluminum hydroxide precipitates have been removed, and then allowing the ultrafiltration product water to replace the primary nanofiltration concentrate to undergo the secondary nanofiltration desalination step to remove ions therein.

[0140] After the ultrafiltration step, the ultrafiltration concentrate is preferably combined with the aluminum hydroxide slurry obtained in step 1.

[0141] In a preferred embodiment of the ultrafiltration step, in order to ensure the process speed requirements in industrial production, the flow rate of the primary nanofiltration concentrated water can be controlled to at least about 0.75m 3 / h or more (e.g. about 0.8m 3 / h, about 0.9m 3 / h, about 1.0m 3 / h, about 1.1m 3 / h, about 1.2m 3 / h, about 1.3m 3 / h or about 1.4m 3 / h), and / or the pressure difference across the ultrafiltration membrane is preferably controlled in the range of about 20 to about 500 kPa (e.g., about 100 kPa, about 200 kPa, about 300 kPa or about 400 kPa).

[0142] The purpose of the resin desalination step is to treat the secondary nanofiltration concentrated water to fully desalinate it.

[0143] The resin desalination step includes passing the secondary nanofiltration concentrate through a cation exchange resin and an anion exchange resin, and utilizing the cation exchange resin and anion exchange resin as described in detail above to fully remove cations and anions from the secondary nanofiltration concentrate, respectively, thereby obtaining a lactulose product solution having a conductivity of less than about 50 μS / cm.

[0144] In some preferred embodiments of the resin desalination step, in order to ensure the process speed requirements in industrial production, the flow rate of the secondary nanofiltration concentrated water can be controlled to at least about 0.75m 3 / h (e.g. about 0.8m 3 / h, about 0.9m 3 / h, about 1.0m 3 / h, about 1.1m 3 / h, about 1.2m 3 / h, about 1.3m 3 / h or about 1.4m3 / h).

[0145] In other preferred embodiments, the resin desalting step can be performed at room temperature (eg, about 20 to about 25° C.). Higher temperatures may cause lactulose to produce impurities, while too low a temperature may not achieve the desired desalting results.

[0146] In some preferred embodiments of the method described in the second aspect of the present invention, the method further comprises an optional evaporation step, comprising using the evaporation apparatus as described above to at least partially evaporate and remove the solvent in the obtained lactulose product solution, thereby obtaining a lactulose product solution having a required specific gravity or obtaining a lactulose solid product.

[0147] In some other preferred embodiments of the method according to the second aspect of the present invention, the method further comprises an optional step 3: a reverse osmosis step.

[0148] The purpose of the reverse osmosis step is to remove the salt contained in the primary nanofiltration product water and / or the secondary nanofiltration product water and to recover the lactulose that may remain therein.

[0149] The reverse osmosis step includes passing the primary nanofiltration product water and / or the secondary nanofiltration product water through a reverse osmosis membrane, and removing salt contained in the primary nanofiltration product water and / or the secondary nanofiltration product water using the reverse osmosis membrane, thereby obtaining reverse osmosis concentrated water containing salt and lactulose and reverse osmosis product water with a conductivity of less than about 30 μS / cm. The reverse osmosis product water may contain residual lactulose.

[0150] In some preferred embodiments, in order to ensure the process speed requirements in industrial production, the reverse osmosis membrane can have a flow rate of at least about 300 m3 per cubic meter of the first nanofiltration water and / or the second nanofiltration water. 2 (For example, about 350m 2 or about 400m 2 ) of the membrane area, and / or controlling the pressure difference across the reverse osmosis membrane to be within a range of about 1.5 to about 2.5 MPa (e.g., about 2.0 MPa).

[0151] In other preferred embodiments, after measuring the lactulose content in the reverse osmosis concentrate, the lactulose content in the reverse osmosis concentrate can be optionally concentrated based on cost considerations, and at least a portion of the concentrated reverse osmosis concentrate can be combined with the curtain membrane product water to recover the lactulose therein. The reverse osmosis concentrate with insufficient lactulose content can be treated separately as saline wastewater.

[0152] In some further preferred embodiments, at least part or all of the reverse osmosis produced water can be used as deionized water in the secondary nanofiltration desalination step, and / or at least part of the reverse osmosis produced water can be used as washing water for the aluminum hydroxide slurry obtained in step 1.

[0153] In some further preferred embodiments, the pH value of the primary nanofiltration product water and / or the secondary nanofiltration product water is preferably controlled within a range of about 5 to about 7. Here, excessively high or low pH of the primary nanofiltration product water and / or the secondary nanofiltration product water in the reverse osmosis step may cause damage to the membrane structure.

[0154] The present invention will be further described in detail below with reference to specific examples and comparative examples.

[0155] Example

[0156] First, a lactulose solution is prepared by a sodium aluminate catalysis method. The lactulose solution has a lactulose content of about 50 g / L, a solid content of about 10% by weight, and a total salt content of about 17.5 g / L.

[0157] Then according to the attached Figure 1 The system shown in FIG. 1 is used to process the lactulose liquid, specifically comprising the following steps:

[0158] Step 1: Place about 20m 3 The feed containing the lactulose liquid enters the PTFE curtain membrane solid-liquid separation device, and the curtain membrane has a filtration accuracy of about 0.45 μm. The temperature of the feed is controlled to be about 23°C and the pH value is about 6. The membrane area of ​​the curtain membrane in the curtain membrane solid-liquid separation device is about 300m 2 , control the pressure difference on both sides of the curtain membrane to be within the range of about 20 to about 100 kPa, and control the flow rate of the lactulose liquid to be within the range of about 3.0 to about 3.5 m 3 / h range.

[0159] The aluminum hydroxide slurry obtained in step 1 (i.e., curtain membrane concentrated water) is fed into a centrifuge for centrifugal separation, and a portion of the reverse osmosis product water obtained subsequently is used to wash the aluminum hydroxide mud. The clear liquid obtained by centrifugation is incorporated into the feed and returned to the curtain membrane solid-liquid separation device for solid-liquid separation.

[0160] The total volume of water produced by the curtain membrane is still about 20m 3 , the conductivity is above about 30mS / cm.

[0161] Step 2: The pH value of the curtain membrane water obtained in step 1 is adjusted to about 6, and then enters the first-stage nanofiltration concentration device to undergo a first-stage nanofiltration concentration step, wherein the first-stage nanofiltration membrane has a molecular weight cut-off range of about 300 to about 500 Da, with a molecular weight cut-off range of about 375 m2 The membrane area is controlled, the pressure on both sides of the first-stage nanofiltration membrane is controlled within the range of about 1 to about 3 MPa, and the temperature of the curtain membrane water production is controlled not to exceed about 25 ° C. The concentration is completed, and the volume of the first-stage nanofiltration water production is about 13m 3 The volume of the primary nanofiltration concentrated water is about 7m 3 , the conductivity is still above about 30mS / cm.

[0162] The pH value of the first-stage nanofiltration concentrated water is adjusted to about 6, and then enters the ultrafiltration device to undergo ultrafiltration steps to remove trace amounts of aluminum hydroxide in the first-stage nanofiltration concentrated water. The filtration accuracy of the ultrafiltration membrane is about 0.45 μm and the area is about 55 m 2 The ultrafiltration water is then sent to the secondary nanofiltration desalination device to undergo the secondary nanofiltration desalination step, and the ultrafiltration concentrated water is then sent to the centrifuge for centrifugal separation. The volume of the ultrafiltration concentrated water is about 7m 3 .

[0163] The ultrafiltration product water obtained is adjusted to a pH of about 6 and then subjected to a secondary nanofiltration desalination step at room temperature. The secondary nanofiltration membrane has a molecular weight cut-off range of about 300 to about 500 Da, and about 125 m 2 The pressure on both sides of the secondary nanofiltration membrane is controlled in the range of about 0.15 to about 0.3 MPa. When the secondary nanofiltration water production reaches about 3.5m 3 When the secondary nanofiltration concentrated water is mixed with about 3.5m 3 The reverse osmosis product water was mixed as described below, and the mixture was subjected to secondary nanofiltration desalination again. The dialysis was repeated 10 times. The conductivity of the obtained secondary nanofiltration mixture was detected to be below about 280μS / cm. At this time, the secondary nanofiltration concentrate was subjected to the resin adsorption desalination step.

[0164] The pH value of the secondary nanofiltration mixed solution from the secondary nanofiltration desalination step was adjusted to about 6, and then subjected to treatment with D301 anion exchange resin and D001 cation exchange resin, respectively, with the resin dosage being about 143.5 kg. The flow rate of the secondary nanofiltration concentrated water was controlled to be at least about 0.75 m 3 / h(about 7m 3 After the filtration, the conductivity of the obtained lactulose product solution was detected to be below about 45 μS / cm, and the lactulose content was about 136.7 g per liter of solution.

[0165] The lactulose product solution is sent to a double-effect evaporator to evaporate and remove at least part of the solvent in the solution.

[0166] Step 3: The first-stage nanofiltration product water and the second-stage nanofiltration product water obtained above are mixed and the pH value is adjusted to about 6, and then subjected to a reverse osmosis treatment step, wherein the reverse osmosis membrane has a pH of about 350m3 per cubic meter of the first-stage nanofiltration product water and / or the second-stage nanofiltration product water. 2 The membrane area is controlled to maintain a pressure differential across the reverse osmosis membrane within a range of 1.5 to 2.5 MPa, producing a reverse osmosis concentrate containing salt and lactulose and a reverse osmosis product with a conductivity of less than approximately 30 μS / cm. A portion of this product is fed to the secondary nanofiltration desalination step as deionized water makeup, while the remainder is fed to solid-liquid separation for washing aluminum hydroxide sludge. The lactulose content of the reverse osmosis concentrate obtained in the reverse osmosis treatment step is analyzed and, after concentration, returned to the curtain membrane product tank for lactulose recovery.

[0167] After stable operation, based on the volume and lactulose content of the lactulose liquid as the raw material, and the volume and lactulose content of the obtained lactulose product solution, the lactulose yield of the embodiment of the present invention can be calculated to be above about 95.7%.

[0168] Comparative Example

[0169] About 20m 3 The lactulose liquid (with the same composition as that in the embodiment) enters the plate-frame filter press solid-liquid separation device, and the liquid temperature is controlled at about 23°C and the pH value is about 6. The volume of the clear liquid obtained after the plate-frame filter press solid-liquid separation is about 16m 3 A large amount of lactulose remains on the filter cake. The filter cake is stirred with water and filtered for the second time using a plate and frame filter to obtain a clear liquid of about 3m 3 The combined volume of the clear liquid is about 19m 3 Lactulose serum.

[0170] The resulting lactulose solution was filtered through anion and cation resins using approximately 1000 kg of D001 cationic resin and approximately 1100 kg of D301 anionic resin. After filtration, the remaining lactulose solution contained within the resins was replaced with deionized water. After the entire solution was filtered, the conductivity of the lactulose product solution was tested to be approximately 340 μS / cm, indicating that the product was unqualified. Based on the volume and lactulose content of the raw lactulose solution, as well as the volume and lactulose content of the resulting lactulose product solution, the lactulose yield of the comparative example was calculated to be only approximately 69%.

[0171] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions claimed to be protected by the present invention.

Claims

1. A system for continuously purifying and concentrating lactulose from a lactulose solution prepared by a sodium metaaluminate catalytic process, characterized in that: The system includes a solid-liquid separation unit, which includes a curtain membrane solid-liquid separation device and an optional centrifuge. The curtain membrane solid-liquid separation device comprises a curtain membrane for achieving solid-liquid separation of the feed containing the lactulose liquid, thereby obtaining aluminum hydroxide slurry and curtain membrane water containing lactulose, wherein the curtain membrane has a filtration accuracy of 0.4 to 0.5 μm, and the curtain membrane preferably has a filtration accuracy of 14 to 16 m3 per cubic meter of the feed. 2 The membrane area, and / or the curtain membrane is preferably an organic membrane; and The centrifuge is used to further centrifuge the aluminum hydroxide slurry to obtain aluminum hydroxide mud and centrifuge mother liquor.

2. The system according to claim 1, wherein: The system further comprises a concentration and desalination unit for concentrating and desalting the curtain membrane water containing lactulose, wherein the concentration and desalination unit comprises a primary nanofiltration concentration device, a secondary nanofiltration desalination device, a resin adsorption desalination device and an optional evaporation device arranged in sequence, wherein The primary nanofiltration concentration device comprises a primary nanofiltration membrane for concentrating the curtain membrane water containing lactulose, thereby obtaining primary nanofiltration water and primary nanofiltration concentrated water containing lactulose, wherein the primary nanofiltration membrane has a molecular weight cut-off range of 300 to 500 Da, and the primary nanofiltration membrane preferably has a molecular weight cut-off range of 10 to 15 m3 per cubic meter of curtain membrane water. 2 Membrane area; The secondary nanofiltration desalination device comprises a secondary nanofiltration membrane, which uses deionized water with a conductivity of less than 30 μS / cm to replace and remove the salt contained in the primary nanofiltration concentrated water, thereby obtaining a salt-containing secondary nanofiltration product water and a secondary nanofiltration mixed solution comprising secondary nanofiltration concentrated water and deionized water with a conductivity of less than 300 μS / cm, wherein the volume of the secondary nanofiltration mixed solution is not greater than 110% of the volume of the primary nanofiltration concentrated water, wherein the secondary nanofiltration membrane has a molecular weight cut-off range of 300 to 500 Da, and the secondary nanofiltration membrane preferably has a molecular weight cut-off range of 15 to 20 m3 per cubic meter of the primary nanofiltration concentrated water. 2 Membrane area; The resin adsorption desalination device comprises a cation exchange resin and an anion exchange resin, which are respectively used to remove cations and anions in the secondary nanofiltration mixed solution, thereby obtaining a lactulose product solution with a conductivity of less than 50 μS / cm, wherein the amount of the anion exchange resin and the cation exchange resin is preferably 20 to 21 kg per cubic meter of the secondary nanofiltration mixed solution; and / or the anion exchange resin is preferably D301 anion exchange resin; and / or the cation exchange resin is preferably D001 cation exchange resin; and The optional evaporation device is used to at least partially evaporate the solvent in the lactulose product solution.

3. The system according to claim 2, characterized in that: The concentration and desalination unit further includes an ultrafiltration device arranged between the primary nanofiltration concentration device and the secondary nanofiltration desalination device, the ultrafiltration device including an ultrafiltration membrane for removing aluminum hydroxide precipitates that may be precipitated in the primary nanofiltration concentrated water, thereby obtaining ultrafiltration concentrated water containing the aluminum hydroxide precipitates and ultrafiltration product water from which the aluminum hydroxide precipitates have been removed, and the ultrafiltration product water then replaces the primary nanofiltration concentrated water and enters the secondary nanofiltration device to remove ions therein, wherein the ultrafiltration membrane has a filtration accuracy of 0.4 to 0.5 μm; and the ultrafiltration membrane preferably has a filtration accuracy of 5 to 10 m3 per cubic meter of the primary nanofiltration concentrated water. 2 membrane area.

4. The system according to any one of claims 2 to 3, characterized in that: The system further comprises a reverse osmosis unit, which comprises a reverse osmosis membrane for removing salt contained in the primary nanofiltration product water and / or the secondary nanofiltration product water and recovering residual lactulose therein, thereby obtaining reverse osmosis concentrated water containing salt and lactulose and reverse osmosis product water with a conductivity lower than 30 μS / cm, wherein the reverse osmosis membrane preferably has a conductivity of 300 to 400 m3 per cubic meter of the primary nanofiltration product water and / or the secondary nanofiltration product water. 2 membrane area.

5. A method for continuously purifying and concentrating lactulose from a lactulose solution prepared by a sodium metaaluminate catalytic process using the system according to any one of claims 1 to 4, wherein the lactulose solution has a lactulose content of at most 50 g / L, a solids content of at least 10% by weight, and a total salt content of at least 17.5 g / L, characterized in that: The method comprises the following steps: Step 1: solid-liquid separation step, comprising allowing the feed containing the lactulose liquid to flow through the curtain membrane of the curtain membrane solid-liquid separation device in the solid-liquid separation unit, thereby utilizing the curtain membrane to achieve solid-liquid separation of the feed containing the lactulose liquid, and obtaining the aluminum hydroxide slurry and the curtain membrane water containing lactulose, wherein during the solid-liquid separation process, the flow rate of the lactulose liquid is preferably controlled at 3.0 m / s. 3 / h or more, and / or the pressure difference across the curtain membrane is preferably controlled within a range of 20 to 100 kPa; and / or the solid-liquid separation is preferably performed at a temperature of 20 to 25°C.

6. The method according to claim 5, characterized in that Step 1 of the method further includes passing the aluminum hydroxide slurry through the centrifuge, and further centrifuging the aluminum hydroxide slurry to obtain aluminum hydroxide mud and centrifuge mother liquor, wherein the centrifuge mother liquor is preferably used as part of the feed of the solid-liquid separation step.

7. The method according to claim 5, characterized in that The method further comprises the following steps after step 1: Step 2: a concentration and desalination step for concentrating and desalting the curtain membrane water containing lactulose, wherein the concentration and desalination step comprises the following steps: a primary nanofiltration concentration step, a secondary nanofiltration desalination step, a resin adsorption desalination step, and an optional evaporation step, which are performed in sequence. The first-level nanofiltration concentration step comprises allowing the curtain membrane water containing lactulose to flow through the first-level nanofiltration membrane of the first-level nanofiltration concentration device, thereby concentrating the curtain membrane water containing lactulose, obtaining first-level nanofiltration water and first-level nanofiltration concentrated water containing lactulose, wherein the first-level nanofiltration membrane has a molecular weight cut-off range of 300 to 500 Da, and the volume of the obtained first-level nanofiltration concentrated water is preferably 25 to 40% of the volume of the curtain membrane water; and the first-level nanofiltration membrane preferably has a molecular weight cut-off range of 10 to 15 m3 per cubic meter of the curtain membrane water. 2 and / or during the primary nanofiltration concentration step, the pH value of the curtain membrane water is preferably controlled within the range of 5 to 7; and / or the primary nanofiltration concentration step is preferably performed at a temperature of 20 to 25° C.; and / or the flow rate of the curtain membrane water is preferably controlled to be at least 3.0 m 3 / h; and / or the pressure difference on both sides of the primary nanofiltration membrane is preferably controlled within the range of 1 to 3 MPa; The secondary nanofiltration desalination step includes passing the primary nanofiltration concentrate through a secondary nanofiltration membrane of a secondary nanofiltration desalination device, and using deionized water to replace and remove the salt contained in the primary nanofiltration concentrate, thereby obtaining a salt-containing secondary nanofiltration product water and a secondary nanofiltration mixed solution containing secondary nanofiltration concentrate and deionized water with a conductivity lower than 300 μS / cm, wherein the volume of the secondary nanofiltration mixed solution is not greater than 110% of the volume of the primary nanofiltration concentrate, wherein the secondary nanofiltration membrane has a molecular weight cut-off range of 300 to 500 Da, and the secondary nanofiltration membrane preferably has a molecular weight cut-off range of 15 to 20 m3 per cubic meter of the primary nanofiltration concentrate. 2 and / or during the secondary nanofiltration desalination step, the pH value of the primary nanofiltration concentrate is preferably controlled within the range of 5 to 7; and / or the secondary nanofiltration desalination step is preferably performed at a temperature of 20 to 25° C.; and / or the flow rate of the primary nanofiltration concentrate is preferably controlled to be at least 0.75 m 3 / h; and / or the pressure difference on both sides of the secondary nanofiltration membrane is preferably controlled in the range of 0.15 to 0.3 MPa; The resin adsorption desalination step comprises passing the secondary nanofiltration concentrated water through a cation exchange resin and an anion exchange resin, thereby removing cations and anions in the secondary nanofiltration concentrated water, respectively, to obtain a lactulose product solution with a conductivity of less than 50 μS / cm, wherein the amount of the anion exchange resin and the cation exchange resin is preferably 20 to 21 kg per cubic meter of the secondary nanofiltration concentrated water; and / or the anion exchange resin is preferably D301 anion exchange resin, and / or the cation exchange resin is preferably D001 cation exchange resin; and / or during the resin adsorption desalination step, the flow rate of the secondary nanofiltration concentrated water is preferably controlled at 0.75 m 3 / h or more; and / or the resin adsorption desalination step is preferably carried out at a temperature of 20 to 25 ° C; and The optional evaporation step comprises at least partially evaporating the solvent from the lactulose product solution using an evaporation device.

8. The method according to claim 7, characterized in that The secondary nanofiltration desalination step includes first allowing the primary nanofiltration concentrate to flow through the secondary nanofiltration membrane, thereby concentrating the primary nanofiltration concentrate to obtain secondary nanofiltration product water and secondary nanofiltration concentrate containing lactulose, and the volume of the secondary nanofiltration concentrate is preferably 40 to 60% of the volume of the primary nanofiltration concentrate; then mixing the secondary nanofiltration concentrate with the deionized water to obtain the secondary nanofiltration mixed liquid, and the volume of the secondary nanofiltration mixed liquid is 90 to 110% of the volume of the primary nanofiltration concentrate; finally, when the conductivity of the secondary nanofiltration mixed liquid is measured to be higher than 300 μS / cm, repeatedly allowing the secondary nanofiltration mixed liquid to flow through the secondary nanofiltration membrane, thereby concentrating the secondary nanofiltration mixed liquid, further obtaining secondary nanofiltration product water and secondary nanofiltration concentrate containing lactulose, and when the conductivity of the secondary nanofiltration mixed liquid is measured to be lower than 300 μS / cm, allowing the secondary nanofiltration mixed liquid to enter the resin adsorption desalination step.

9. The method according to claim 7 or 8, characterized in that The method further includes an ultrafiltration step between the primary nanofiltration concentration step and the secondary nanofiltration desalination step, comprising allowing the primary nanofiltration concentrated water to flow through an ultrafiltration membrane of an ultrafiltration unit, thereby removing aluminum hydroxide precipitates precipitated in the primary nanofiltration concentrated water, obtaining ultrafiltration concentrated water containing the aluminum hydroxide precipitates and ultrafiltration product water from which the aluminum hydroxide precipitates have been removed, wherein the ultrafiltration membrane has a filtration accuracy of 0.4 to 0.5 μm, and then allowing the ultrafiltration product water to replace the primary nanofiltration concentrated water and enter the secondary nanofiltration desalination step to remove ions therein, and preferably merging the ultrafiltration concentrated water with the aluminum hydroxide slurry; and the ultrafiltration membrane preferably has a filtration accuracy of 5 to 10 m3 per cubic meter of the primary nanofiltration concentrated water. 2 and / or in the ultrafiltration step, the flow rate of the primary nanofiltration concentrated water is preferably controlled at least 0.75m 3 / h or more; and / or the pressure difference on both sides of the ultrafiltration membrane is preferably controlled in the range of 20 to 500 kPa.

10. The method according to claim 7 or 8, characterized in that The method further comprises the following steps: Step 3: Reverse osmosis step, comprising passing the primary nanofiltration product water and / or the secondary nanofiltration product water through a reverse osmosis membrane, thereby removing the salt contained in the primary nanofiltration product water and / or the secondary nanofiltration product water and recovering the residual lactulose therein, obtaining reverse osmosis concentrated water containing salt and lactulose and reverse osmosis product water with a conductivity lower than 30 μS / cm, wherein the reverse osmosis membrane preferably has a conductivity of 300 to 400 m3 per cubic meter of the primary nanofiltration product water and / or the secondary nanofiltration product water. 2 membrane area; and / or the pressure difference across the reverse osmosis membrane is preferably controlled within the range of 1.5 to 2.5 MPa; and / or the reverse osmosis concentrated water is at least partially combined with the curtain membrane produced water after the lactulose content therein is optionally concentrated; and / or the pH value of the primary nanofiltration produced water and / or the secondary nanofiltration produced water is preferably controlled within the range of 5 to 7; and / or the reverse osmosis produced water is preferably at least partially or entirely used as deionized water in the secondary nanofiltration desalination step; and / or the reverse osmosis produced water is preferably at least partially or entirely used as washing water for the aluminum hydroxide slurry obtained in step 1.

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