A method for producing xylo-oligosaccharides

Abstract

The present application relates to the field of xylooligosaccharide production, and discloses a production method of xylooligosaccharide, comprising the following steps: raw material pretreatment: the raw material rich in lignocellulose is removed and crushed; spraying acid for sugar leaching: spraying acid solution into fine powder, and mixing and stirring at the same time, heating for sugar leaching reaction; gradient sugar washing: adding deionized water to the slag sugar mixture to carry out multiple rounds of gradient sugar washing, separating and obtaining high-concentration sugar solution; electrodialysis acid recovery: utilizing electrodialysis to deacidify high-concentration sugar solution, and recovering acid solution for reuse; continuous separation and preparation of xylooligosaccharide: regulating sugar solution pH to 4.0-6.5, adopting nanofiltration membrane to separate xylooligosaccharide, simultaneously adding xylanase in nanofiltration membrane concentration tank, making the intercepted xylan continuously degraded into xylooligosaccharide. The present invention combines xylooligosaccharide separation and preparation together, so that the xylooligosaccharide obtained by xylanase hydrolysis can pass through nanofiltration membrane in time, prevents being further hydrolyzed into xylose, and reduces the generation of by-products.

Description

Technical Field

[0001] The present invention relates to the technical field of xylooligosaccharide production, in particular to a method for producing xylooligosaccharide. Background Art

[0002] Xylooligosaccharides are functional sugars composed of 2-7 xylose molecules connected by β-1,4 glycosidic bonds, also known as xylooligosaccharides. They are not easily digested and absorbed by gastrointestinal digestive enzymes after entering the body. After reaching the large intestine, they are utilized by beneficial intestinal bacteria to proliferate beneficial bacteria. They have a particularly selective proliferation effect on bifidobacteria and are a prebiotic that can improve the host's intestinal health.

[0003] The sweetness of xylooligosaccharide is 40-50% of sucrose, and its properties are stable. For example, it has good heat resistance and acid resistance, and can maintain a stable pH value in a wide range (2.5-8.0). Within this pH range, it will not be destroyed when heated for 1 hour, and it will remain stable when heated at 120℃ for 1 hour.

[0004] Due to its stable properties and good physiological effects, oligoxylose has been widely used in food, health products and feed.

[0005] Currently, the main raw material for xylo-oligosaccharide production is lignocellulosic biomass. Xylanase hydrolyzes the xylan in lignocellulose to produce xylo-oligosaccharides. However, since the xylan in lignocellulose is tightly bound to cellulose and lignin, this hinders the action of xylanase on the xylan. Therefore, acid or alkali pretreatment is required to dissolve the xylan and facilitate the action of xylanase. Acid-base pretreatment consumes a large amount of chemical reagents and generates a large amount of wastewater, which not only increases production costs but also increases the wastewater treatment load. Numerous research efforts have focused on this area, but the results have been less than ideal. Furthermore, xylanase acts on xylan to produce xylo-oligosaccharides. These xylo-oligosaccharides remain in the reaction system, and some of them are further hydrolyzed into xylose, reducing the yield of xylo-oligosaccharides and requiring subsequent separation. Furthermore, after a certain degree of enzymatic hydrolysis, enzyme inactivation is required, which also results in wasteful enzyme preparation. Summary of the Invention

[0006] In response to the deficiencies of the prior art, the present invention provides a method for producing xylooligosaccharides, which overcomes the shortcomings of the prior art in that acid pretreatment of lignocellulose consumes a large amount of acid solution, the resulting sugar solution has a low concentration, and a large amount of steam is consumed during subsequent evaporation. At the same time, the method effectively reduces the generation of xylose during enzymatic hydrolysis, improves the yield and purity of xylooligosaccharides, and enables continuous use of xylanase, saving enzyme preparation.

[0007] To achieve the above objectives, the present invention is implemented through the following technical solutions: A method for producing xylooligosaccharides, comprising the following steps:

[0008] (1) Raw material pretreatment: remove impurities from the lignocellulose-rich raw materials, crush them, and screen out fine powders smaller than 0.9 mm;

[0009] (2) Spraying acid to soak sugar: spraying acid solution into the fine powder, while mixing and stirring, heating to carry out the soaking reaction, and obtaining a residue sugar mixture;

[0010] Xylan forms hydrogen bonds and covalent bonds with lignin and cellulose in its natural state, affecting its solubility. The main function of acid pretreatment is to destroy the binding force of xylan to lignin and cellulose, so that xylan is dissolved in the sugar solution. In addition, acid can also promote the degradation of some xylan into oligomeric xylose, improving the efficiency of subsequent enzymatic hydrolysis. However, the excessive effect of acid pretreatment causes xylan to further degrade into xylose, reducing the yield of oligomeric xylose. Therefore, in this method, by controlling the acid concentration, temperature and time, xylan is dissolved to the greatest extent while avoiding its excessive degradation.

[0011] Acid spraying is to spray acid into the lignocellulose powder so that the acid is absorbed into the powder without excess acid. The purpose is to save the amount of acid while meeting the hydrolysis requirements.

[0012] (3) Gradient sugar washing: Deionized water is added to the residue-sugar mixture for multiple rounds of gradient sugar washing to separate and obtain a high-concentration sugar solution;

[0013] The purpose of gradient sugar washing is to increase the sugar concentration of the washed sugar solution, reduce the subsequent evaporation energy consumption, and thus reduce production costs.

[0014] (4) Acid recovery by electrodialysis: Deacidification of the high-concentration sugar solution is performed by electrodialysis, and the acid solution is recovered for reuse in step (2);

[0015] During the electrodialysis process, H + The acid and other cations enter the concentration chamber through the cation exchange membrane and anion exchange membrane, respectively, to achieve effective separation of acid and sugar solution. This not only reduces the subsequent ion exchange load and waste discharge, but also allows the recovered acid to be recycled, improving resource utilization.

[0016] (5) Continuous separation and preparation of xylooligosaccharides: The pH of the sugar solution is adjusted to 4.0-6.5, and xylooligosaccharides are separated using a nanofiltration membrane. At the same time, xylanase is added to the nanofiltration membrane concentration tank to continuously degrade the retained xylan into xylooligosaccharides;

[0017] Xylanase specifically hydrolyzes the β-1,4-glycosidic bonds of xylan to produce xylo-oligosaccharides. However, in traditional processes, xylo-oligosaccharides remain in the reaction system and are partially degraded into xylose, resulting in a low xylo-oligosaccharide yield. This method uses nanofiltration membrane separation to allow the xylo-oligosaccharides produced by enzymatic hydrolysis to quickly permeate the membrane, reducing their residence time in the system and the risk of further degradation, thereby increasing the yield of xylo-oligosaccharides. Furthermore, the xylanase is retained, allowing further hydrolysis of the retained high-molecular-weight xylans and polysaccharides, thereby improving enzyme utilization.

[0018] (6) Refining of oligoxylose liquid: Using activated carbon fiber membrane decolorization, ion exchange and evaporation concentration to obtain oligoxylose products;

[0019] Activated carbon fiber membrane decolorization: the temperature of the material to be decolorized is 50-70℃, the number of activated carbon fiber membrane layers is 35-55 layers, and the flow rate is 2-4 times the volume of the activated carbon fiber membrane per hour.

[0020] Preferably, the raw materials rich in lignocellulose include corn cobs, sugarcane bagasse, corn straw, wheat straw and cottonseed hulls, wherein the preferred particle size of corn cobs is 0.5-0.9 mm to ensure the uniformity of subsequent spray acid soaking.

[0021] Preferably, the step of spraying acid to soak the sugar uses sulfuric acid or hydrochloric acid, the acid concentration is 0.5-1.5%, the mass ratio of raw material fine powder to sprayed acid is 1:2-1:3, stirring is maintained during the acid spraying process to ensure uniform distribution of the acid, the heating temperature is controlled at 100-115°C, and the reaction time is 10-40min.

[0022] In its natural state, xylan is wrapped by lignin and cellulose, forming a stable cross-linked structure that makes it difficult to release directly. The core function of acid treatment is to break the hydrogen bonds and ester bonds between xylan and lignin, so that the xylan is dissolved and partially degraded into oligoxylose. However, the acid concentration and reaction conditions must be precisely controlled:

[0023] Too high an acid concentration (1.5%) will lead to excessive degradation, and some xylooligosaccharides will continue to be broken down into xylose, affecting the yield of the final product.

[0024] If the acid concentration is too low (<0.5%), the structure of lignocellulose cannot be fully destroyed, resulting in a decrease in the dissolution rate of xylan.

[0025] Heating temperature of 100-115℃ can effectively promote the dissolution of xylan and increase the formation rate of oligoxylose, but too high temperature will lead to the caramelization of sugar or the increase of by-products.

[0026] In addition, uniform spraying and stirring of the acid solution are also crucial to ensure that all particles are fully exposed to the acid, improve the efficiency of sugar soaking, and prevent unnecessary degradation reactions caused by excessive local acid concentration.

[0027] Preferably, the gradient sugar washing step adopts the following method:

[0028] One round of sugar washing: Mix the residue-sugar mixture with deionized water in a ratio of 1:2-1:3, control the stirring time to 5-10 minutes, and filter at a pressure of 2-3 MPa to obtain sugar solution 1-1, and measure its concentration. Mix the filtered sugar residue with deionized water in the same ratio, filter to obtain sugar solution 1-2, and measure its concentration. Repeat the above process to obtain sugar solutions 1-3 and 1-4, until the concentration of the extruded sugar solution is less than 1%. Stop mixing the sugar residue with deionized water, and the sugar solution obtained in the last step is 1-n.

[0029] Second round of sugar washing: Mix the fresh sugar residue mixture with sugar solution 1-2, filter the sugar solution, record it as 2-1, and measure its concentration. Mix the filtered sugar residue with sugar solution 1-3, filter the sugar solution, record it as 2-2, and measure its concentration. Repeat the above process, mix the filtered sugar residue with sugar solution 1-n, filter the sugar solution, record it as 2-(n-1), and finally mix the filtered sugar residue with fresh deionized water, filter the sugar solution, and record it as 2-n.

[0030] The purpose of using the lower concentration sugar solution from the previous round to continue washing the sugar is to increase the concentration of the sugar solution. The purpose of using deionized water to wash the sugar for the last time is to wash out as much sugar as possible from the residue and reduce sugar loss.

[0031] Three rounds of sugar washing: Use the sugar solution from the previous round to wash the sugar to increase the sugar solution concentration;

[0032] Until the concentration of the sugar solution obtained from the first filtration of two adjacent rounds is consistent, that is, the final concentration of the sugar solution reaches 17-20%, which is the high-concentration sugar solution obtained by gradient sugar washing and enters the subsequent process.

[0033] Preferably, the key parameters of the electrodialysis acid recovery step are as follows:

[0034] The electrodialysis device consists of an anode, a cathode, and an anion and cation exchange membrane;

[0035] Desalination chamber: high concentration sugar solution, initial conductivity 15-25mS / cm, dropped to 2-4mS / cm at the end;

[0036] Concentration chamber: pure water, final conductivity 60-80mS / cm;

[0037] Extreme solution: 2% sodium sulfate;

[0038] The average membrane stack voltage is 1.0-1.2V, and the H+ concentration reaches 0.28-0.35mol / L at the end. The recovered acid is returned to step (2) for reuse.

[0039] The core function of electrodialysis is to use the selective permeability of ion exchange membrane to make H + and SO4 2- (or Cl - ) The acid radical ions migrate directionally and enter the concentration chamber, thereby reducing the acidity of the sugar solution. Since the sugar molecules, electrically neutral organic acids and other macromolecules in the sugar solution cannot pass through the ion exchange membrane, the sugar components will not be lost, and only the acid will be effectively removed. By controlling the average membrane stack voltage at 1.0-1.2V, the electrodialysis process is ensured to be stable, and the hydrolysis side reaction caused by excessive electric field strength is avoided, while ensuring the effective recovery of acid. Finally, the H+ concentration in the acid concentration chamber reaches 0.28-0.35mol / L, and the recovered acid can be directly returned to the spray acid leaching step (2) for continued use, reducing acid consumption and improving the economy and environmental protection of the process.

[0040] Preferably, the continuous separation and preparation of xylo-oligosaccharides uses a nanofiltration membrane, wherein:

[0041] The molecular weight cut-off of the nanofiltration membrane is 1000Da, and the operating pressure is controlled at 0.3-0.6MPa;

[0042] Add xylanase at a dosage of 1-3‰ of the dry basis of the material;

[0043] The material temperature in the concentration tank is maintained at 50-60℃;

[0044] During operation, the flow rate of the material entering the nanofiltration membrane concentration tank is controlled to be consistent with the flow rate of the permeate;

[0045] Through the method of degradation and permeation, xylan is continuously degraded into small molecular weight oligoxylose.

[0046] The core function of nanofiltration membranes is to effectively separate large-molecule xylans from small-molecule xylooligosaccharides based on molecular sieve selectivity and electrostatic repulsion. At the same time, controlling the operating pressure (0.3-0.6MPa) ensures a high permeation rate while preventing excessive pressure from damaging the membrane structure or degrading the xylooligosaccharides.

[0047] During this process, xylanase is added at a level of 1-3‰ based on the dry basis of the material, and the enzymatic reaction is maintained during the nanofiltration membrane separation process. Xylanase can specifically hydrolyze β-1,4-glycosidic bonds, degrading large molecular weight xylans into oligosaccharides.

[0048] The degradation and permeation method is adopted, that is, during the enzymatic hydrolysis process, the generated oligoxylose can quickly pass through the nanofiltration membrane and be separated, preventing the oligoxylose from being further degraded into xylose in the system, thereby increasing the yield of oligoxylose.

[0049] Preferably, the step of refining the xylooligosaccharide solution comprises:

[0050] Activated carbon fiber membrane decolorization: the temperature of the material to be decolorized is 50-70℃, the number of activated carbon fiber membrane layers is 35-55 layers, and the flow rate is 2-4 times the volume of the activated carbon fiber membrane per hour.

[0051] Preferably, in the process of recovering the acid solution, after each round of electrodialysis operation, the acid concentration of the recovered acid solution needs to be tested and adjusted to 0.5-1.5%. When the acidity is insufficient, new acid is added to the set concentration and then returned to the acid spraying and sugar soaking step for repeated use.

[0052] During the electrodialysis process, the acid solution is mainly composed of H + Ions and acid radicals (such as SO4 2- or Cl - ) migrates to the concentrating chamber, forming a recyclable acid solution. However, because the acid migration rate is affected by the initial acidity of the sugar solution, membrane conductivity, and operating voltage, the concentration of the recovered acid fluctuates after each electrodialysis cycle. If the acidity is too low, returning directly to the spray acid soaking step will reduce the efficiency of xylan release, thereby affecting the yield of xylo-oligosaccharides. Therefore, the acidity must be monitored after each cycle, and fresh acid must be added as appropriate to ensure the stability and reusability of the acid solution.

[0053] In the traditional sugar washing process, the sugar solution obtained by washing has a low sugar concentration, and subsequent evaporation consumes a large amount of steam, resulting in excessively high production costs. The present invention uses a gradient sugar washing method to replace deionized water with a low-concentration sugar solution to increase the sugar concentration in the sugar washing solution.

[0054] During the electrodialysis process, acid ions (H + and SO4 2- / Cl - ) is driven by an electric field and migrates through the ion exchange membrane to the concentration chamber, thereby reducing the acidity of the sugar solution and achieving acid recovery. However, if the electrodialysis process is not properly controlled, the acid loss will be large, resulting in too low a concentration of recovered acid, reducing its reuse value. The present invention optimizes the membrane stack voltage (1.0-1.2V) and the operating parameters of acid recovery to ensure the efficiency of acid migration, improve the acid recovery rate, and reduce the acid residue in the sugar solution, thereby reducing the burden on subsequent processes. Through an efficient acid recovery system, the need for external acid supplementation is reduced, resource utilization is improved, and the entire process is more environmentally friendly and economical.

[0055] Preferably, the method is applicable to lignocellulosic raw materials from different sources, including agricultural waste and forestry waste.

[0056] The present invention provides a method for producing xylooligosaccharides. It has the following beneficial effects:

[0057] 1. The present invention adopts the method of spraying acid to soak sugar, which greatly reduces the amount of acid used, and adopts a gradient sugar washing method to wash out the leached sugar, thereby increasing the sugar concentration of the washed sugar solution and reducing subsequent steam consumption.

[0058] 2. The present invention uses electrodialysis to recover and reuse the acid in the washed sugar solution, further reducing the amount of acid used and the amount of wastewater discharged.

[0059] 3. The present invention combines the separation and preparation of xylo-oligosaccharides together, so that the xylo-oligosaccharides obtained by xylanase hydrolysis can pass through the nanofiltration membrane in time, preventing them from being further hydrolyzed into xylose, reducing the generation of by-products, improving the yield of xylo-oligosaccharides, and obtaining high-purity xylo-oligosaccharide products; at the same time, the xylanase is retained and the retained high-molecular-weight xylans and polysaccharides are further hydrolyzed, thereby improving the utilization rate of the enzyme; and continuous operation can be achieved, thereby improving production efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0060] Figure 1 It is a schematic diagram of the sugar washing process of the present invention. DETAILED DESCRIPTION

[0061] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the drawings in the present specification. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0062] Example 1:

[0063] The corn cobs are cleaned, crushed and screened, and 10 kg of fine powder with a diameter of less than 0.9 mm is selected. While stirring, 25 kg of 1% sulfuric acid solution is sprayed into the powder. The powder is heated to 105 ° C and maintained for 30 minutes. Then, the sugar is washed according to the gradient process. Figure 1The sugar is washed in this way to obtain a sugar solution with a concentration of 19%. The acid in the sugar solution is recovered by electrodialysis. The electrodialysis desalination chamber is a sugar solution with a concentration of 19%, the concentration chamber is pure water, the polar liquid is 2% sodium sulfate, and the average membrane stack voltage is 1.0-1.2V. At the end, the conductivity of the desalination chamber is reduced from the initial 20mS / cm to 3mS / cm, the conductivity of the concentration chamber reaches 75mS / cm, and the H+ concentration is 0.32mol / L. It can be returned to step (2) for reuse. The material in the desalination chamber is adjusted to a pH value of 4.0-6.5 and then nanofiltered, and 2‰ of xylanase on a dry basis of the material is added to the nanofiltration storage tank. The pH value is maintained at 4.0-6.5 and the temperature is 50-60°C during operation. The permeate is xylooligosaccharide, and the liquid phase detection shows that the xylooligosaccharide content reaches 96.3%, which can reach more than 95% without special separation and purification. The 50-60°C permeate is decolorized by passing through 45 layers of activated carbon fiber membranes at a flow rate of 3 times the volume of the activated carbon fiber membrane per hour, and then a high-content xylooligosaccharide product is obtained by ion exchange and evaporation concentration.

[0064] Example 2:

[0065] The corn cobs are cleaned, crushed and screened, and 10 kg of fine powder with a diameter of less than 0.9 mm is selected. While stirring, 30 kg of sulfuric acid solution with a concentration of 0.5% is sprayed into the powder. The powder is heated to 115 ° C and maintained for 40 minutes. Then, the sugar is washed according to the gradient process. Figure 1 The sugar was washed in this way to obtain a sugar solution with a concentration of 17%. The acid in the sugar solution was recovered by electrodialysis. The electrodialysis desalination chamber was a sugar solution with a concentration of 17%, the concentration chamber was pure water, the polar liquid was 2% sodium sulfate, and the average membrane stack voltage was 1.0-1.2V. At the end, the conductivity of the desalination chamber dropped from the initial 15mS / cm to 2mS / cm, the conductivity of the concentration chamber reached 61mS / cm, and the H+ concentration was 0.29mol / L. It can be returned to step (2) for reuse. The material in the desalination chamber is adjusted to a pH value of 4.0-6.5 and then nanofiltered, and 3‰ of xylanase on a dry basis of the material is added to the nanofiltration tank. The pH value is maintained at 4.0-6.5 and the temperature is 50-60°C during operation. The permeate is xylooligosaccharide, and the liquid phase detection shows that the xylooligosaccharide content reaches 97.4%, which can reach more than 95% without special separation and purification. The 60-70°C permeate is decolorized through 35 layers of activated carbon fiber membranes at a flow rate of 4 times the volume of the activated carbon fiber membrane per hour, and then a high-content xylooligosaccharide product is obtained through ion exchange and evaporation concentration.

[0066] Example 3:

[0067] The corn cobs are cleaned, crushed and screened, and 10 kg of fine powder with a diameter of less than 0.9 mm is selected. While stirring, a 1.5% hydrochloric acid solution is sprayed in an amount of 20 kg. The mixture is heated to 100 ° C and maintained for 10 minutes. Then, the sugar is washed according to the gradient process. Figure 1The sugar was washed in this way to obtain a sugar solution with a concentration of 19.5%. The acid in the sugar solution was recovered by electrodialysis. The electrodialysis desalination chamber was a sugar solution with a concentration of 19.5%, the concentration chamber was pure water, the polar liquid was 2% sodium sulfate, and the average membrane stack voltage was 1.0-1.2V. At the end, the conductivity of the desalination chamber dropped from the initial 24mS / cm to 3.7mS / cm, and the conductivity of the concentration chamber reached 79mS / cm. The H+ concentration was 0.33mol / L, and it could be returned to step (2) for reuse. The material in the desalination chamber is adjusted to a pH value of 4.0-6.5 and then subjected to nanofiltration, and 3‰ of xylanase on a dry basis of the material is added to the nanofiltration storage tank. The pH value is maintained at 4.0-6.5 and the temperature is 50-60°C during operation. The permeate is xylooligosaccharide, and the liquid phase detection shows that the xylooligosaccharide content reaches 96.7%, which can reach more than 95% without special separation and purification. The 50-60°C permeate is decolorized through 55 layers of activated carbon fiber membranes at a flow rate of 2 times the volume of the activated carbon fiber membrane per hour, and then a high-content xylooligosaccharide product is obtained through ion exchange and evaporation concentration.

[0068] Example 4:

[0069] This embodiment is a traditional process:

[0070] The corn cobs are cleaned, crushed and screened, and 10 kg of fine powder with a diameter of less than 0.9 mm is selected. 80 kg of 1.2% sulfuric acid solution is added and heated to 105°C. After maintaining for 30 minutes, plate and frame filtration is performed to separate the sugar solution and the residue. The obtained sugar solution has a concentration of 5.5%. The pH value is adjusted to 4.0-6.5 with light calcium carbonate and 3‰ xylanase is added for enzymatic hydrolysis. After enzymatic hydrolysis for about 12-16 hours, the enzyme is inactivated by heating. The oligoxylose content of the material after enzyme inactivation is 73%. After activated carbon decolorization, ion exchange and evaporation concentration, chromatographic purification is performed to increase the content to more than 95%.

[0071] Comparison between this research process and traditional process (processing 10 kg of lignocellulose):

[0072]

[0073] Through the above comparison, the present invention has obvious advantages over the traditional process in saving acid dosage, increasing sugar concentration, saving enzyme preparations and increasing the yield of oligoxylose.

[0074] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A method for producing xylo-oligosaccharides, characterized in that: The following steps are involved: (1) Raw material pretreatment: remove impurities from the lignocellulose-rich raw materials, crush them, and screen out fine powders smaller than 0.9 mm; (2) Spraying acid to soak sugar: spraying acid solution into the fine powder, while mixing and stirring, heating to carry out the soaking reaction, and obtaining a residue sugar mixture; (3) Gradient sugar washing: Deionized water is added to the residue-sugar mixture for multiple rounds of gradient sugar washing to separate and obtain a high-concentration sugar solution; (4) Acid recovery by electrodialysis: Deacidification of the high-concentration sugar solution is performed by electrodialysis, and the acid solution is recovered for reuse in step (2); (5) Continuous separation and preparation of xylooligosaccharides: The pH of the sugar solution is adjusted to 4.0-6.5, and xylooligosaccharides are separated using a nanofiltration membrane. At the same time, xylanase is added to the nanofiltration membrane concentration tank to continuously degrade the retained xylan into xylooligosaccharides; (6) Refining of oligoxylose liquid: Using activated carbon fiber membrane decolorization, ion exchange and evaporation concentration to obtain oligoxylose products; The gradient sugar washing step adopts the following method: One round of sugar washing: Mix the residue-sugar mixture with deionized water in a ratio of 1:2-1:3, control the stirring time to 5-10 minutes, and filter at a pressure of 2-3 MPa to obtain sugar solution 1-1, and measure its concentration. Mix the filtered sugar residue with deionized water in the same ratio, filter to obtain sugar solution 1-2, and measure its concentration. Repeat the above process to obtain sugar solutions 1-3 and 1-4, until the concentration of the extruded sugar solution is less than 1%. Stop mixing the sugar residue with deionized water, and the sugar solution obtained in the last step is 1-n. Second round of sugar washing: Mix the fresh sugar residue mixture with sugar solution 1-2, filter the sugar solution, record it as 2-1, and measure its concentration. Mix the filtered sugar residue with sugar solution 1-3, filter the sugar solution, record it as 2-2, and measure its concentration. Repeat the above process, mix the filtered sugar residue with sugar solution 1-n, filter the sugar solution, record it as 2-(n-1), and finally mix the filtered sugar residue with fresh deionized water, filter the sugar solution, record it as 2-n. The purpose of using the lower concentration sugar solution from the previous round to continue washing the sugar is to increase the concentration of the sugar solution. The purpose of using deionized water to wash the sugar for the last time is to wash out as much sugar as possible from the residue and reduce sugar loss. Three rounds of sugar washing: Use the sugar solution from the previous round to wash the sugar to increase the sugar solution concentration; Until the concentration of the sugar solution obtained from the first filtration of two adjacent rounds is consistent, that is, the final concentration of the sugar solution reaches 17-20%, which is the high-concentration sugar solution obtained by gradient sugar washing and enters the subsequent process; The continuous separation and preparation steps of xylooligosaccharides use nanofiltration membranes, wherein: The molecular weight cut-off of the nanofiltration membrane is 1000Da, and the operating pressure is controlled at 0.3-0.6MPa; Add xylanase at a dosage of 1-3‰ of the dry basis of the material; The pH value of the material in the concentration tank is maintained at 4.0-6.5 and the temperature is maintained at 50-60℃; During operation, the flow rate of the material entering the nanofiltration membrane concentration tank is controlled to be consistent with the flow rate of the permeate; Through the method of degradation and permeation, xylan is continuously degraded into small molecular weight oligoxylose.

2. The method for producing xylo-oligosaccharides according to claim 1, wherein The raw materials rich in lignocellulose include corn cobs, sugarcane bagasse, corn straw, wheat straw and cottonseed hulls, wherein the particle size of the corn cobs is 0.5-0.9 mm to ensure the uniformity of subsequent spray acid soaking sugar.

3. The method for producing xylo-oligosaccharides according to claim 1, wherein: The sugar-spraying acid soaking step uses sulfuric acid or hydrochloric acid, the acid concentration is 0.5-1.5%, the mass ratio of raw material fine powder to sprayed acid is 1:2-1:3, stirring is maintained during the acid spraying process to ensure uniform distribution of the acid, the heating temperature is controlled at 100-115°C, and the reaction time is 10-40 minutes.

4. The method for producing xylo-oligosaccharides according to claim 1, wherein: The key parameters of the electrodialysis acid recovery step are as follows: The electrodialysis device consists of an anode, a cathode, and an anion and cation exchange membrane; Desalination chamber: high concentration sugar solution, initial conductivity 15-25mS / cm, dropped to 2-4mS / cm at the end; Concentration chamber: pure water, final conductivity 60-80mS / cm; Extreme solution: 2% sodium sulfate; The average membrane stack voltage is 1.0-1.2V, and the H+ concentration reaches 0.28-0.35mol / L at the end. The recovered acid is returned to step (2) for reuse.

5. The method for producing xylo-oligosaccharides according to claim 1, wherein: The oligoxylose liquid refining step comprises: Activated carbon fiber membrane decolorization: the temperature of the material to be decolorized is 50-70℃, the number of activated carbon fiber membrane layers is 35-55 layers, and the flow rate is 2-4 times the volume of the activated carbon fiber membrane per hour.

6. The method for producing xylo-oligosaccharides according to claim 1, wherein: During the acid recovery process, after each round of electrodialysis operation, the acid concentration of the recovered acid solution needs to be tested and adjusted to 0.5-1.5%. If the acidity is insufficient, new acid is added to the set concentration and then returned to the acid spraying and sugar soaking step for repeated use.

7. The method for producing xylo-oligosaccharides according to claim 1, wherein: The method is applicable to lignocellulosic raw materials from different sources, including agricultural waste and forestry waste.

Images