Production method of xylooligosaccharide

Abstract

The invention relates to the field of xylooligosaccharide production, and discloses a xylooligosaccharide production method, which comprises: raw material pretreatment: removing impurities from a raw material rich in lignocellulose, and crushing; acid liquid is sprayed into the fine powder, mixing and stirring are conducted at the same time, and heating is conducted for a sugar soaking reaction; gradient sugar washing: adding deionized water into the residue sugar mixture to carry out multiple rounds of gradient sugar washing, and separating to obtain a high-concentration sugar solution; electrodialysis acid recovery: carrying out deacidification treatment on the high-concentration sugar solution by using electrodialysis, and recovering the acid solution for reutilization; and continuous separation and preparation of xylooligosaccharide: adjusting the pH value of the sugar solution to 4.0-6.5, separating the xylooligosaccharide by adopting a nanofiltration membrane, and adding xylanase into a nanofiltration membrane concentration tank at the same time, so that the intercepted xylan is continuously degraded into the xylooligosaccharide. According to the method, xylooligosaccharide separation and xylooligosaccharide preparation are combined together, so that xylooligosaccharide obtained by xylanase hydrolysis can penetrate through a nanofiltration membrane in time, xylooligosaccharide is prevented from being continuously hydrolyzed into xylose, and generation of by-products is reduced.

Description

Technical Field

[0001] The 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 used by beneficial intestinal bacteria to proliferate beneficial bacteria, especially bifidobacteria. They have a highly selective proliferation effect and are a prebiotic that can improve the host's intestinal health.

[0003] The sweetness of xylo-oligosaccharide is 40-50% of that 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°C for 1 hour.

[0004] Due to its stable properties and good physiological effects, xylooligosaccharides have been widely used in the fields of food, health products and feed.

[0005] At present, the main raw material for the production of oligoxylose is lignocellulosic biomass. Xylanase hydrolyzes xylan in lignocellulose to produce oligoxylose. However, since xylan in lignocellulose is closely combined with cellulose and lignin, the effect of xylanase on xylan is affected. It is necessary to use acid or alkali for pretreatment to dissolve xylan to facilitate the action of xylanase. Acid and alkali pretreatment consumes a large amount of chemical reagents on the one hand, and produces a large amount of wastewater on the other hand, which not only increases production costs but also increases the load of sewage treatment. At present, many studies are being carried out around this aspect, but the results are not ideal. In addition, xylanase acts on xylan to produce oligoxylose, and the oligoxylose produced continues to remain in the reaction system, and some of it will continue to be hydrolyzed into xylose, which will reduce the yield of oligoxylose, and it is also necessary to separate it later. At the same time, after the enzymolysis reaches a certain level, it is necessary to inactivate the enzyme, which also causes waste of enzyme preparations. Summary of the invention

[0006] In view of the shortcomings of the prior art, the present invention provides a method for producing xylooligosaccharides, which solves the shortcomings of the prior art that acid pretreatment of lignocellulose consumes a large amount of acid solution, the obtained sugar solution has a low concentration, and a large amount of steam is consumed during subsequent evaporation; at the same time, the generation of xylose during enzymatic hydrolysis is effectively reduced, the yield and purity of xylooligosaccharides are improved, and xylanase can be used continuously, saving enzyme preparations.

[0007] To achieve the above objectives, the present invention is implemented by the following technical scheme: a method for producing xylo-oligosaccharides, comprising the following steps: (1) Raw material pretreatment: remove impurities and crush the raw materials rich in lignocellulose, and screen out fine powder less than 0.9 mm; (2) Spraying acid to soak sugar: spraying acid into the fine powder, mixing and stirring at the same time, heating to carry out the soaking reaction, and obtaining a residue-sugar mixture; Xylan forms hydrogen bonds and covalent bonds with lignin and cellulose in its natural state, affecting its solubility. The main effect of acid pretreatment is to destroy the binding force of xylan with lignin and cellulose, so that xylan is dissolved in the sugar solution. In addition, acid can also promote the degradation of part of xylan into oligomeric xylose, improving the efficiency of subsequent enzymolysis. However, the excessive effect of acid pretreatment causes xylan to be further degraded into xylose, reducing the yield of oligomeric xylose. Therefore, in this method, by controlling acid concentration, temperature and time, xylan is dissolved to the greatest extent while avoiding its excessive degradation.

[0008] Acid spraying is to spray acid into the wood cellulose fine powder so that the acid is absorbed into the fine powder without excess acid. The purpose is to save the amount of acid while meeting the hydrolysis requirements.

[0009] (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; 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.

[0010] (4) Acid recovery by electrodialysis: Deacidification of the high-concentration sugar solution is carried out by electrodialysis, and the acid solution is recovered for repeated use in step (2); During the electrodialysis process, H + The acid and sugar solution are separated by cation exchange membrane and anion exchange membrane respectively, which can reduce the subsequent ion exchange load and waste liquid discharge, and the recovered acid can be recycled to improve resource utilization.

[0011] (5) Continuous separation and preparation of xylooligosaccharides: The pH of the sugar solution is adjusted to 4.0-6.5, and xylooligosaccharides are separated by nanofiltration membrane. At the same time, xylanase is added to the nanofiltration membrane concentration tank to continuously degrade the retained xylan into xylooligosaccharides; Xylanase can specifically hydrolyze the β-1,4-glycosidic bonds of xylan to generate xylo-oligosaccharides. However, in traditional processes, xylo-oligosaccharides will continue to remain in the reaction system and be partially degraded into xylose, resulting in a reduced yield of xylo-oligosaccharides. This method uses nanofiltration membrane separation to allow the xylo-oligosaccharides produced by enzymatic hydrolysis to quickly pass through the membrane, reducing their residence time in the system, thereby reducing the risk of further degradation and increasing the yield of xylo-oligosaccharides. At the same time, xylanase is retained to continue hydrolyzing the retained high molecular weight xylans and polysaccharides, thereby improving the utilization of the enzyme.

[0012] (6) Refining of oligoxylose liquid: using activated carbon fiber membrane decolorization, ion exchange and evaporation concentration to obtain oligoxylose products; Activated carbon fiber membrane decolorization: the temperature of the material to be decolorized is 50-70°C, 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.

[0013] Preferably, the raw materials rich in lignocellulose include corn cobs, 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.

[0014] 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 the 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.

[0015] In its natural state, xylan is wrapped by lignin and cellulose to form a stable cross-linked structure, making it difficult to release directly. The core function of acid treatment is to destroy the hydrogen bonds and ester bonds between xylan and lignin, so that xylan is dissolved and partially degraded into oligoxylose. However, the acid concentration and reaction conditions must be precisely controlled: Too high 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.

[0016] 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.

[0017] Heating temperature of 100-115°C can effectively promote the dissolution of xylan and increase the formation rate of xylo-oligosaccharides, but too high temperature will lead to the caramelization of sugars or the increase of by-products.

[0018] 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.

[0019] Preferably, the gradient sugar washing step adopts the following method: One round of sugar washing: the mixture of sugar residue and deionized water is mixed and stirred in a ratio of 1:2-1:3, the stirring time is controlled to be 5-10min, the filtering pressure is 2-3MPa, and the sugar solution 1-1 is obtained, and its concentration is measured; the filtered sugar residue is mixed and stirred with deionized water in the same ratio as above, and the sugar solution is filtered and recorded as 1-2, and its concentration is measured; the above process is repeated to obtain sugar solutions 1-3 and 1-4, until the concentration of the squeezed sugar solution is lower than 1%, and the mixing of the sugar residue and deionized water is stopped, and the sugar solution obtained in the last time 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); finally, mix the filtered sugar residue with fresh deionized water, filter the sugar solution, record it as 2-n.

[0020] 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 slag and reduce sugar loss. Three rounds of sugar washing: Use the sugar solution from the previous round to wash the sugar to increase the concentration of the sugar solution; 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.

[0021] Preferably, 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 liquid: 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 repeated use.

[0022] The core function of electrodialysis is to utilize the selective permeability of ion exchange membrane to make H + and SO4 2- (or Cl -) The acid radical ions migrate in a directional manner 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 stability of the electrodialysis process is ensured, and the hydrolysis side reaction caused by excessive electric field strength is avoided, while ensuring the effective recovery of the 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 acid spraying sugar soaking step (2) for continued use, reducing acid consumption and improving the economy and environmental protection of the process.

[0023] Preferably, the step of continuous separation and preparation of xylo-oligosaccharides uses a nanofiltration membrane, 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 material temperature in the concentration tank is maintained at 50-60°C; 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 xylooligosaccharides.

[0024] The core function of nanofiltration membrane is based on molecular sieve selectivity and electrostatic repulsion effect, which can effectively separate large molecular weight xylan and small molecular weight xylo-oligosaccharides. At the same time, controlling the appropriate operating pressure (0.3-0.6MPa) can not only ensure efficient permeation rate, but also prevent excessive pressure from causing damage to the membrane structure or structural degradation of xylo-oligosaccharides.

[0025] During this process, xylanase is added at the same time, and its addition amount is controlled at 1-3‰ of 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 and degrade macromolecular xylan into oligoxylose.

[0026] The degradation-while-permeation method is adopted, that is, during the enzymatic hydrolysis process, the generated xylooligosaccharides can be quickly separated through the nanofiltration membrane, preventing the xylooligosaccharides from being further degraded into xylose in the system, thereby increasing the yield of xylooligosaccharides.

[0027] Preferably, the step of refining the xylooligosaccharide solution comprises: Activated carbon fiber membrane decolorization: the temperature of the material to be decolorized is 50-70°C, 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.

[0028] 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%. 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.

[0029] 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 concentration chamber to form a recoverable acid solution. However, since the migration rate of the acid is affected by the initial acidity of the sugar solution, membrane conductivity, and operating voltage, the concentration of the recovered acid will fluctuate after each round of electrodialysis. If the acidity is too low, directly returning to the acid-spraying sugar soaking step will reduce the efficiency of xylan release, thereby affecting the yield of oligoxylose. Therefore, the acidity must be monitored after each round of operation, and new acid must be appropriately added to ensure the stability and reusability of the acid solution.

[0030] In the traditional sugar washing process, the sugar solution obtained by sugar 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.

[0031] During electrodialysis, acid ions (H + and SO4 2- / Cl - ) is driven by an electric field and migrates to the concentration chamber through the ion exchange membrane, thereby reducing the acidity of the sugar solution and realizing 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 ensures the migration efficiency of the acid, improves the acid recovery rate, reduces the acid residue in the sugar solution, and reduces the burden of subsequent processes by optimizing the membrane stack voltage (1.0-1.2V) and the operating parameters of acid recovery. 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.

[0032] Preferably, the method is applicable to lignocellulosic raw materials from different sources, including but not limited to agricultural waste and forestry waste.

[0033] The present invention provides a method for producing xylooligosaccharides. It has the following beneficial effects: 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.

[0034] 2. The present invention adopts electrodialysis to recover and reuse the acid in the washed sugar solution, further reducing the amount of acid used and the amount of sewage discharged.

[0035] 3. The present invention combines the separation and preparation of xylooligosaccharides together, so that the xylooligosaccharides obtained by xylanase hydrolysis can pass through the nanofiltration membrane in time to prevent further hydrolysis into xylose, thereby reducing the generation of by-products, improving the yield of xylooligosaccharides, and obtaining high-purity xylooligosaccharide products; at the same time, the xylanase is retained to continue to hydrolyze the retained high-molecular-weight xylans and polysaccharides, thereby improving the utilization rate of the enzyme; and continuous operation can be achieved, thereby improving production efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 It is a schematic diagram of the sugar washing process of the present invention. DETAILED DESCRIPTION

[0037] The following will be combined with the drawings in the specification of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. 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 creative work are within the scope of protection of the present invention. Example 1

[0038] The corn cobs are cleaned, crushed and screened, and 10 kg of fine powder with a diameter less than 0.9 mm is selected. A 1% sulfuric acid solution is sprayed into the powder while stirring. The spraying amount is 25 kg. The powder is heated to 105°C and maintained for 30 minutes. Then, the sugar is washed according to the gradient process. Figure 1 The sugar is washed in the manner of 19% sugar solution, and 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 drops from the initial 20mS / cm to 3mS / cm, and the conductivity of the concentration chamber reaches 75mS / cm. The H+ concentration is 0.32mol / L, and it can be returned to step (2) for reuse. The material in the desalination chamber is adjusted to pH 4.0-6.5 and then nanofiltered, and 2‰ 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 xylo-oligosaccharide, and the liquid phase detection xylo-oligosaccharide content reaches 96.3%, which can reach more than 95% without special separation and purification. The 50-60°C permeate is decolorized by 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 xylo-oligosaccharide product is obtained by ion exchange and evaporation concentration. Example 2

[0039] The corn cobs are cleaned, crushed and screened, and 10 kg of fine powder with a diameter less than 0.9 mm is selected. A 0.5% sulfuric acid solution is sprayed into the corn cobs while stirring. The spraying amount is 30 kg. The corn cobs are heated to 115°C and maintained for 40 minutes. Then, the sugar is washed according to the gradient process. Figure 1 The sugar is washed in the manner of 17% sugar solution, and electrodialysis is used to recover the acid in the sugar solution. The electrodialysis desalination chamber is a sugar solution with a concentration of 17%, 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 drops from the initial 15mS / cm to 2mS / cm, and the conductivity of the concentration chamber reaches 61mS / cm. The H+ concentration is 0.29mol / L, and it can be returned to step (2) for reuse. The material in the desalination chamber is adjusted to pH 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 xylo-oligosaccharide, and the liquid phase detection xylo-oligosaccharide 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 xylo-oligosaccharide product is obtained through ion exchange and evaporation concentration. Example 3

[0040] The corn cobs are cleaned, crushed and screened, and 10 kg of fine powder with a diameter less than 0.9 mm is selected. A 1.5% hydrochloric acid solution is sprayed into the corn cobs while stirring. The spraying amount is 20 kg. The corn cobs are heated to 100°C and maintained for 10 minutes. Then, the sugar is washed according to the gradient process. Figure 1 The sugar is washed in the manner of 19.5% sugar solution, and the acid in the sugar solution is recovered by electrodialysis. The electrodialysis desalination chamber is a sugar solution with a concentration of 19.5%, 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 drops from the initial 24mS / cm to 3.7mS / cm, and the conductivity of the concentration chamber reaches 79mS / cm. The H+ concentration is 0.33mol / L, and it can be returned to step (2) for reuse. The material in the desalination chamber is adjusted to pH 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 xylo-oligosaccharide, and the liquid phase detection xylo-oligosaccharide 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 xylo-oligosaccharide product is obtained through ion exchange and evaporation concentration. Example 4

[0041] This embodiment is a traditional process: 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 then 3‰ xylanase is added for enzymolysis. After about 12-16 hours of enzymolysis, 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%.

[0042] Comparison between this research process and traditional process (processing 10 kg of lignocellulose):

[0043] 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 xylo-oligosaccharides.

[0044] Although 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 the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present 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 and crush the raw materials rich in lignocellulose, and screen out fine powder less than 0.9 mm; (2) Spraying acid to soak sugar: spraying acid into the fine powder, mixing and stirring at the same time, 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 carried out by electrodialysis, and the acid solution is recovered for repeated use 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 by 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: Use activated carbon fiber membrane decolorization, ion exchange and evaporation concentration to obtain oligoxylose products.

2. The method for producing xylo-oligosaccharides according to claim 1, characterized in that: The raw materials rich in lignocellulose include corn cobs, 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.

3. The method for producing xylo-oligosaccharides according to claim 1, characterized in that: 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 the 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.

4. The method for producing xylo-oligosaccharides according to claim 1, characterized in that: The gradient sugar washing step adopts the following method: One round of sugar washing: the mixture of sugar residue and deionized water is mixed and stirred in a ratio of 1:2-1:3, the stirring time is controlled to be 5-10min, the filtering pressure is 2-3MPa, and the sugar solution 1-1 is obtained, and its concentration is measured; the filtered sugar residue is mixed and stirred with deionized water in the same ratio as above, and the sugar solution is filtered and recorded as 1-2, and its concentration is measured; the above process is repeated to obtain sugar solutions 1-3 and 1-4, until the concentration of the squeezed sugar solution is lower than 1%, and the mixing of the sugar residue and deionized water is stopped, and the sugar solution obtained in the last time 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); 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 slag and reduce sugar loss. Three rounds of sugar washing: Use the sugar solution from the previous round to wash the sugar to increase the concentration of the sugar solution; 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.

5. The method for producing xylo-oligosaccharides according to claim 1, characterized in that: 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 liquid: 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 repeated use.

6. The method for producing xylo-oligosaccharides according to claim 1, characterized in that: The continuous separation and preparation steps of xylo-oligosaccharides 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 material in the concentration tank maintains a pH value of 4.0-6.5 and a temperature of 50-60°C; 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 xylooligosaccharides.

7. The method for producing xylo-oligosaccharides according to claim 1, characterized in that: The step of refining the xylooligosaccharide solution comprises: Activated carbon fiber membrane decolorization: the temperature of the material to be decolorized is 50-70°C, 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.

8. The method for producing xylo-oligosaccharides according to claim 1, characterized in that: During the acid solution 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.

9. The method for producing xylo-oligosaccharides according to claim 1, characterized in that: The method is applicable to lignocellulosic raw materials from different sources, including but not limited to agricultural waste and forestry waste.

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