Process for preparing food-grade xylose by extracting hemicellulose from pulp

Abstract

The invention discloses a process for preparing food-grade xylose from hemicellulose extracted from pulp, and particularly relates to the technical field of xylose preparation. Method comprising hemicellulose powder (hemicellulose liquor) 4, milk preparation and size mixing, material heat exchange, continuous dissolution, material heat exchange, cooling, sedimentation suction filtration, tubular membrane filtration, membrane concentration, chromatography desalination and decoloration, MVR plate-type evaporation, granular carbon decoloration, cation exchange and anion exchange, and 5, milk preparation and size mixing, material heat exchange, continuous dissolution, material heat exchange, cooling, sedimentation suction filtration, tubular membrane filtration, membrane concentration, chromatography desalination and decoloration, MVR plate-type evaporation, granular carbon decoloration, cation exchange and anion exchange. The food-grade xylose is obtained by adopting the hemicellulose waste liquid as the raw material and then concentrating, purifying and purifying the hemicellulose waste liquid, and compared with corncob as the raw material, the production cost of the food-grade xylose is lower, and the production process is economic and environment-friendly (mainly physical process); and the device can be recycled.

Description

Technical Field

[0001] The invention relates to the technical field of xylose production, and more specifically to a process for extracting hemicellulose from pulp to produce food-grade xylose. Background Art

[0002] Xylose is a component of pentosan, with the molecular formula C5H10O5. So far, free xylose has not been found in nature. Natural D-xylose exists in plants in the form of polysaccharides, and is more abundant in waste parts of agricultural products such as corn cobs, cottonseed hulls, bagasse, rice husks, and other straws, seed husks, etc. Corn cobs are used in large quantities for xylose production because of their high yield, the highest pentosan content, easy storage, relatively loose structure, and easy extraction. Xylose also exists in animal heparin, chondroitin and glycoproteins. It is the connecting unit of sugar chain and serine (or threonine) in some glycoproteins. The sweetness of xylose is 72% of sucrose, which is close to the sweetness of glucose. The flavor is also similar to glucose. It can improve the flavor and taste of sweets and inhibit odor. After xylose is ingested, it is not digested and absorbed by the human body, so it will not increase the blood sugar level of the human body. It is a non-caloric sweetener; it has good tolerance, and excessive consumption will not cause abdominal distension and diarrhea; it is also not used by oral microorganisms and will not cause dental caries; it has some physiological functions of dietary fiber, which can reduce serum cholesterol and prevent colon cancer. Xylose has good compatibility with food and can be added to food in any proportion as needed. When mixed with amino acids and heated, it is easy to produce Maillard reaction, which has the effect of enhancing flavor. Xylose can activate bifidobacteria in the human intestine and promote their growth. Bifidobacteria are beneficial bacteria. The more bifidobacteria there are, the better for human health. Eating xylose can improve the human microecological environment and enhance the body's immune system. Xylose is a reducing sugar that can be hydrogenated and reduced to xylitol in the presence of a catalyst, so it is widely used as a raw material for xylitol. Glycosides and hydroxy acids can be produced through chemical reactions and can be used as substitutes for glycerol and edible organic acids.

[0003] At present, the main raw material required for domestic xylose production is corn cobs. However, due to the large fluctuations in the purchase price of corn cobs, the large purchase workload, and the fact that the purchase volume is limited by the purchase radius, the production capacity of xylose production companies is also limited. At the same time, the xylose production process using corn cobs as raw materials will produce a large amount of waste residue and wastewater, which has a large impact on the environment and poses certain environmental risks. Summary of the invention

[0004] In order to overcome the above-mentioned defects of the prior art, an embodiment of the present invention provides a process for extracting hemicellulose from pulp to produce food-grade xylose, so as to solve the problems raised in the above-mentioned background technology.

[0005] To achieve the above object, the present invention provides the following technical solution: a process for extracting hemicellulose from pulp to produce food-grade xylose, comprising the following steps: S1: prepare milk and slurry, Hemicellulose powder (hemicellulose black liquor) is sent to the stirring and milking tank to be mixed with clean water and 98% sulfuric acid, and the concentration and pH value are adjusted to 15% and PH=1.1~1.2.

[0006] S2: heat exchange between materials, The slurry to be fed into the dissolving tank exchanges heat with the solution discharged by the water dissolution to achieve the purpose of saving steam.

[0007] S3: Pot-type continuous dissolution, The preheated liquid enters the dissolving tank, is heated to about 110°C with direct steam, kept at 120°C, and is discharged after dissolution is completed.

[0008] S4: heat exchange between materials, The dissolved liquid exchanges heat with the liquid ready to enter the water dissolving tank to save steam.

[0009] S5: Cooling down, The dissolved liquid is further cooled by the heat exchanger to the temperature required for entering the next process.

[0010] S6: Sedimentation and filtration, There are a lot of suspended solids in the dissolved liquid, which are settled in the sedimentation tank, and the supernatant liquid directly enters the next process. The settled slurry is filtered by vacuum drum, and the clear liquid and the supernatant liquid enter the next process together. The filtered residue is discharged.

[0011] S7: Tubular membrane filtration, The tubular ultrafiltration membrane has the characteristics of high pollution resistance, high operating flux, and replaceable membrane core and component shell. Tubular membrane filtration is used to remove high molecular colloids in the xylose solution to obtain a clean solution.

[0012] S8: High concentration membrane concentration, The concentration membrane can effectively intercept xylose and other ingredients and pass through water to achieve the purpose of concentration. Membrane concentration does not require steam heating, and the energy consumption is generally only one-third of the traditional multi-effect evaporation; the water permeated by the membrane concentration membrane is clear and transparent, and the xylose content is almost zero, which can be directly reused. After the solution is concentrated by the high-concentration membrane, the sugar concentration is increased from about 12.5% ​​to 18%, and a pre-concentrated solution is obtained.

[0013] S9: Chromatographic desalting and decolorization, The feed liquid passes through the chromatographic separation device, and the salt, pigment and xylose solution are separated due to the different resin adsorption and analysis speeds. After the chromatographic separation removes the sulfate, organic acid salt and pigment, the xylose extract with a conductivity of <1000μS / cm and a concentration of about 7.0% is obtained and enters the next process. The chromatographic raffinate enriched with salts and pigments (conductivity 15000℃30000μS / cm) is sent to the sewage treatment station as production wastewater for treatment and discharge after meeting the standards.

[0014] S10: MVR plate evaporation, The water content of the primary desalted and decolorized xylose solution is evaporated and removed by an MVR plate evaporation system, and the concentration is increased from 7.0% to about 30%, thereby obtaining a xylose pre-concentrate.

[0015] S11: Granular carbon decolorization, The granular activated carbon is used to further adsorb the organic pigments in the pre-treatment solution to obtain a refined xylose solution with a light transmittance of more than 80%. When the granular carbon is saturated with the pigment, it is removed for activation and the activated granular carbon is returned for continued use.

[0016] S12: Cooling down, The decolorized solution is further cooled by the heat exchanger to the temperature required for entering the next process.

[0017] S13: Yang Li Jiao, The xylose decolorized liquid is subjected to cation exchange to remove the residual salts and ash cations, thereby obtaining a xylose cation exchange liquid with a concentration of about 35% and a pH of about 4.6.

[0018] S14: Yin-Li, The residual salt and ash in the xylose decolorized liquid are removed by anionic ion exchange to obtain a xylose ion exchange liquid with a conductivity of <50μS / cm, a concentration of about 33%, and a pH of about 5.9.

[0019] S15: Secondary multi-effect evaporation, The refined xylose solution is evaporated and part of the water therein is removed by a multi-effect falling film vacuum evaporation system, and the concentration is increased from 18% to about 60%, thereby obtaining a xylose concentrate.

[0020] S16: Four-effect TVR plate steaming, The refined xylose liquid is evaporated by a four-effect TVR plate evaporation system to remove part of the water therein, so that the refractive index rises to about 70%, thereby obtaining refined xylose syrup.

[0021] S17: Double-effect standard evaporation, The refined xylose syrup is further evaporated by a double-effect standard evaporation system to remove part of the water therein, so that the refractive index rises to about 85%, thereby obtaining refined xylose syrup.

[0022] S18: Cooling and crystallization, The refined xylose syrup is sent to a horizontal crystallizer, and the temperature of the syrup is slowly lowered by adjusting the amount of cooling water. As the solubility decreases with temperature, xylose crystals precipitate to obtain xylose paste.

[0023] S19: Centrifugal separation, The crystallized xylose mass is centrifuged to remove the mother liquor, thus obtaining xylose crystals and xylose mother liquor. The xylose mother liquor is sold as a by-product, and the xylose crystals enter the next process.

[0024] S20: Dry, The xylose crystals after centrifugal separation still contain about 2% moisture. In order to make the product meet the quality standards and be able to be stored for a long time, the moisture content needs to be reduced to below 0.2% by drying.

[0025] S20: Packaging, 6,000 tons / year of finished food-grade crystalline xylose are packaged in ton bags for easy transportation to users.

[0026] Preferably, in step S1, the hemicellulose black liquor is the pulp porridge generated by the reaction between pulp and soaking alkali solution in the viscose preparation process, and the liquid produced in the pressing process.

[0027] Preferably, in step S17, the steam generated in the evaporation stage and the acid mist generated in the acid adjustment stage are subjected to dust treatment, and the generated xylose dust is intended to be treated by a cyclone dust collector (its treatment efficiency is considered to be 96%). After being treated by the cyclone dust collector, about 0.5kg / h of dust is discharged to the atmosphere through a self-built 15m exhaust pipe. The air volume of the cyclone dust collector is 5000m3 / h. The calculated emission concentration of 98mg / m3 meets the maximum allowable emission concentration of 120 mg / m3 in the "Comprehensive Emission Standard of Air Pollutants" (GB16297-1996). The dust has little impact on the atmosphere, surrounding people and crops. Secondly, sulfuric acid with a concentration of 98% is used in the acid adjustment process, and it is stirred mechanically continuously, and sulfuric acid mist will be generated; hydrochloric acid with a concentration of 30% is used in the primary and secondary separation processes, and it is stirred mechanically continuously, and hydrogen chloride will escape, forming hydrochloric acid mist with strong irritation and corrosiveness with humid air.

[0028] The amount of acid mist produced can be calculated as follows: Gz=M×(0.000352+0.000786×U)×P×F—V water×F Where: Gz——acid mist emission rate (kg / h); M——liquid molecular weight, sulfuric acid is 98, hydrochloric acid is 36.5; U——air velocity on the surface of evaporating liquid (m / s), which should be based on actual measured data. If actual measurement is unavailable, 0.2~0.5m / s can be used or calculated by looking up a table; the U value is 0.5m / s.

[0029] P——The saturated vapor partial pressure in the air corresponding to the liquid temperature, (mmHg); 59mmHg for the acid adjustment section and 47mmHg for the degassing section.

[0030] F——evaporation surface area, m2; this project adopts a circular reaction tank with an evaporation surface area of ​​6.6m2.

[0031] V Water - The evaporation rate of water vapor per unit area is 4.2L / m2•h for sulfuric acid when the evaporation surface temperature is 80-90℃, and 1.25L / m2•h for hydrochloric acid when the evaporation surface temperature is 45℃.

[0032] Calculation results: Gz(H2SO4)=98×(0.000352+0.000786×0.5)×59×6.6-6.6×4.2=0.71kg / h. Gz(HCl)=36.5×(0.000352+0.000786×0.5)×47×6.6-6.6×1.25=0.18kg / h.

[0033] The acid adjustment, primary and secondary separation processes are all carried out in a closed system. The acid mist will not overflow and all of it will be recycled without being discharged.

[0034] Technical effects and advantages of the present invention: The present invention uses hemicellulose waste liquid as raw material, and then concentrates, purifies and refines it to obtain food-grade xylose. Compared with using corn cobs as raw materials, the production cost is lower, the production process is economical and environmentally friendly (mainly a physical process), and can be recycled. DETAILED DESCRIPTION

[0035] The following will be combined with the embodiments 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 making creative work are within the scope of protection of the present invention.

[0036] A process for extracting hemicellulose from pulp to produce food-grade xylose according to an embodiment of the present invention comprises the following steps: S1: prepare milk and slurry, Hemicellulose powder (hemicellulose black liquor) is sent to the stirring and milking tank to be mixed with clean water and 98% sulfuric acid, and the concentration and pH value are adjusted to 15% and PH=1.1~1.2.

[0037] S2: heat exchange between materials, The slurry ready to enter the dissolving tank exchanges heat with the solution discharged by water dissolution, thus achieving the purpose of saving steam.

[0038] S3: Pot-type continuous dissolution, The preheated liquid enters the dissolving tank, is heated to about 110°C with direct steam, kept at 120°C, and is discharged after dissolution is completed.

[0039] S4: heat exchange between materials, The dissolved liquid exchanges heat with the liquid ready to enter the water dissolution tank, thus achieving the purpose of saving steam.

[0040] S5: Cooling down, The dissolved liquid is further cooled by the heat exchanger to the temperature required for entering the next process.

[0041] S6: Sedimentation and filtration, There are a lot of suspended solids in the dissolved liquid, which are settled in the sedimentation tank, and the supernatant liquid directly enters the next process. The settled slurry is filtered by vacuum drum, and the clear liquid and the supernatant liquid enter the next process together. The filtered residue is discharged.

[0042] S7: Tubular membrane filtration, The tubular ultrafiltration membrane has the characteristics of high pollution resistance, high operating flux, and replaceable membrane core and component shell. Tubular membrane filtration is used to remove high molecular colloids in the xylose solution to obtain a clean solution.

[0043] S8: High concentration membrane concentration, The concentration membrane can effectively intercept xylose and other ingredients and pass through water to achieve the purpose of concentration. Membrane concentration does not require steam heating, and the energy consumption is generally only one-third of the traditional multi-effect evaporation; the water permeated by the membrane concentration membrane is clear and transparent, and the xylose content is almost zero, which can be directly reused. After the solution is concentrated by the high-concentration membrane, the sugar concentration is increased from about 12.5% ​​to 18%, and a pre-concentrated solution is obtained.

[0044] S9: Chromatographic desalting and decolorization, The feed liquid passes through the chromatographic separation device, and the salt, pigment and xylose solution are separated due to the different resin adsorption and analysis speeds. After the chromatographic separation removes the sulfate, organic acid salt and pigment, the xylose extract with a conductivity of <1000μS / cm and a concentration of about 7.0% is obtained and enters the next process. The chromatographic raffinate enriched with salts and pigments (conductivity 15000℃30000μS / cm) is sent to the sewage treatment station as production wastewater for treatment and discharge after meeting the standards.

[0045] S10: MVR plate evaporation, The water content of the primary desalted and decolorized xylose solution is evaporated and removed by an MVR plate evaporation system, and the concentration is increased from 7.0% to about 30%, thereby obtaining a xylose pre-concentrate.

[0046] S11: Granular carbon decolorization, The granular activated carbon is used to further adsorb the organic pigments in the pre-treatment solution to obtain a refined xylose solution with a light transmittance of more than 80%. When the granular carbon is saturated with the pigment, it is removed for activation and the activated granular carbon is returned for continued use.

[0047] S12: Cooling down, The decolorized solution is further cooled by the heat exchanger to the temperature required for entering the next process.

[0048] S13: Yang Li Jiao, The xylose decolorized liquid is subjected to cation exchange to remove the residual salts and ash cations, thereby obtaining a xylose cation exchange liquid with a concentration of about 35% and a pH of about 4.6.

[0049] S14: Yin-Li, The residual salt and ash in the xylose decolorized liquid are removed by anionic ion exchange to obtain a xylose ion exchange liquid with a conductivity of <50μS / cm, a concentration of about 33%, and a pH of about 5.9.

[0050] S15: Secondary multi-effect evaporation, The refined xylose solution is evaporated and part of the water therein is removed by a multi-effect falling film vacuum evaporation system, and the concentration is increased from 18% to about 60%, thereby obtaining a xylose concentrate.

[0051] S16: Four-effect TVR plate steaming, The refined xylose liquid is evaporated by a four-effect TVR plate evaporation system to remove part of the water therein, so that the refractive index rises to about 70%, thereby obtaining refined xylose syrup.

[0052] S17: Double-effect standard evaporation, The refined xylose syrup is further evaporated by a double-effect standard evaporation system to remove part of the water therein, so that the refractive index rises to about 85%, thereby obtaining refined xylose syrup.

[0053] S18: Cooling and crystallization, The refined xylose syrup is sent to a horizontal crystallizer, and the temperature of the syrup is slowly lowered by adjusting the amount of cooling water. As the solubility decreases with temperature, xylose crystals precipitate to obtain xylose paste.

[0054] S19: Centrifugal separation, The crystallized xylose mass is centrifuged to remove the mother liquor, thus obtaining xylose crystals and xylose mother liquor. The xylose mother liquor is sold as a by-product, and the xylose crystals enter the next process.

[0055] S20: Dry, The xylose crystals after centrifugal separation still contain about 2% moisture. In order to make the product meet the quality standards and be able to be stored for a long time, the moisture content needs to be reduced to below 0.2% by drying.

[0056] S20: Packaging, 6,000 tons / year of finished food-grade crystalline xylose are packaged in ton bags for easy transportation to users.

[0057] Preferably, in step S1, the hemicellulose black liquor is the pulp porridge generated by the reaction between pulp and soaking alkali solution in the viscose preparation process, and the liquid produced in the pressing process.

[0058] Preferably, in step S17, the steam generated in the evaporation stage and the acid mist generated in the acid adjustment stage are subjected to dust treatment, and the generated xylose dust is intended to be treated by a cyclone dust collector (its treatment efficiency is considered to be 96%). After being treated by the cyclone dust collector, about 0.5kg / h of dust is discharged to the atmosphere through a self-built 15m exhaust pipe. The air volume of the cyclone dust collector is 5000m3 / h. The calculated emission concentration of 98mg / m3 meets the maximum allowable emission concentration of 120 mg / m3 in the "Comprehensive Emission Standard of Air Pollutants" (GB16297-1996). The dust has little impact on the atmosphere, surrounding people and crops. Secondly, sulfuric acid with a concentration of 98% is used in the acid adjustment process, and it is stirred mechanically continuously, and sulfuric acid mist will be generated; hydrochloric acid with a concentration of 30% is used in the primary and secondary separation processes, and it is stirred mechanically continuously, and hydrogen chloride will escape, forming hydrochloric acid mist with strong irritation and corrosiveness with humid air.

[0059] The amount of acid mist produced can be calculated as follows: Gz=M×(0.000352+0.000786×U)×P×F—V water×F Where: Gz——acid mist emission rate (kg / h); M——liquid molecular weight, sulfuric acid is 98, hydrochloric acid is 36.5; U——air velocity on the surface of evaporating liquid (m / s), which should be based on actual measured data. If actual measurement is unavailable, 0.2~0.5m / s can be used or calculated by looking up a table; the U value is 0.5m / s.

[0060] P——The saturated vapor partial pressure in the air corresponding to the liquid temperature, (mmHg); 59mmHg for the acid adjustment section and 47mmHg for the degassing section.

[0061] F——evaporation surface area, m2; this project adopts a circular reaction tank with an evaporation surface area of ​​6.6m2.

[0062] V Water - The evaporation rate of water vapor per unit area is 4.2L / m2•h for sulfuric acid when the evaporation surface temperature is 80-90℃, and 1.25L / m2•h for hydrochloric acid when the evaporation surface temperature is 45℃.

[0063] Calculation results: Gz(H2SO4)=98×(0.000352+0.000786×0.5)×59×6.6-6.6×4.2=0.71kg / h. Gz(HCl)=36.5×(0.000352+0.000786×0.5)×47×6.6-6.6×1.25=0.18kg / h.

[0064] The acid adjustment, primary and secondary separation processes are all carried out in a closed system. The acid mist will not overflow and all of it will be recycled without being discharged. Example

[0065] 1) Hemicellulose alkali solution Hemicellulose refers to the part of plant polysaccharides that coexists with cellulose in plant cell walls and is much easier to dissolve than cellulose in an acidic environment. A plant often contains several types of hemicellulose composed of two or three sugar groups, and their chemical structures are different. The hemicellulose content and composition of tree stems, branches, roots and bark are also different. Therefore, hemicellulose is the name of a class of substances.

[0066] The sugar groups that make up hemicellulose mainly include D-xylose, D-mannose, D-glucose, D-galactose, L-arabinose, 4-O-methyl-D-glucuronic acid, D-galacturonic acid and D-glucuronic acid, as well as a small amount of L-rhamnose, L-fucose, etc. Hemicellulose is mainly divided into three categories, namely polyxylose, polyglucose mannose and polygalactose glucose mannose.

[0067] Hemicellulose black liquor refers to the pulp porridge generated by the reaction between pulp and impregnation alkali solution in the viscose preparation process, and the liquid produced in the pressing process. The raw material of this project is mainly hemicellulose liquid. The raw liquid workshop of Alaertai Textile Technology Co., Ltd. provides the raw material hemicellulose liquid for this project.

[0068] Table 1 Hemicellulose liquid composition Serial number name unit index 1 Sodium hydroxide % 4.5 2 Hemicellulose % 12.0 3 H2O % 83.5 2) Hydrochloric acid Hydrochloric acid is a colorless liquid (industrial hydrochloric acid may appear slightly yellow due to the presence of impurities such as trivalent iron salts), corrosive, and is an aqueous solution of hydrogen chloride. pKa: -7; melting point (℃): -114.8 (pure HCl); boiling point (℃): 108.6 (20% azeotropic solution); relative density (water = 1): 1.20; relative vapor density (air = 1): 1.26; saturated vapor pressure (kPa): 30.66 (21℃); solubility: miscible with water, concentrated hydrochloric acid releases heat when dissolved in water. Soluble in alkali solution and react with alkali solution to neutralize. Miscible with ethanol at will, hydrogen chloride is soluble in benzene.

[0069] The hydrochloric acid used in the present invention is mainly used for resin regeneration in the ion exchange process.

[0070] 3) Sulfuric acid Molecular weight: 98.08, a colorless and odorless oily liquid, a high boiling point, non-volatile strong acid, easily soluble in water, miscible with water in any ratio, and miscible with ethanol. Melting point 10°C, boiling point 338°C. Relative density 1.84 (water = 1), acute toxicity: LC50, 2140mg / kg (rat oral); LC50, 510mg / m3, 2 hours (rat inhalation); 320mg / m3, 2 hours (mouse inhalation).

[0071] The sulfuric acid used in the present invention is mainly used to control the acidity during dissolution.

[0072] 4) Caustic soda Caustic soda (NaOH) is a white crystal at room temperature and is highly corrosive. Melting point: 318.4℃, boiling point: 1390℃, it is hygroscopic and can be used as a desiccant, but it cannot dry acidic gases such as sulfur dioxide, carbon dioxide, nitrogen dioxide and hydrogen chloride (it will react with acidic gases), and it is easy to deliquesce and liquefy in the air (the phenomenon of dissolving due to water absorption is a physical change), dissolve in water, and release a lot of heat. Its aqueous solution is strongly alkaline and can turn phenolphthalein red. Sodium hydroxide is a very commonly used alkali.

[0073] The sodium hydroxide used in the present invention is mainly used for resin regeneration in the ion exchange process.

Claims

1. A process for extracting hemicellulose from pulp to produce food-grade xylose, characterized in that: The following steps are involved: S1: prepare milk and slurry, Hemicellulose powder (hemicellulose black liquor) is sent to the mixing tank and mixed with clean water and 98% sulfuric acid, and the concentration and pH value are adjusted to 15% and PH=1.1~1.2; S2: heat exchange between materials, The slurry to be fed into the dissolving tank exchanges heat with the solution discharged by water dissolution to save steam; S3: Pot-type continuous dissolution, The preheated liquid enters the dissolving tank, where it is heated to about 110°C with direct steam, kept at 120°C, and discharged after dissolution is completed; S4: heat exchange between materials, The dissolved liquid exchanges heat with the liquid ready to enter the water dissolving tank to save steam; S5: Cooling down, The dissolved liquid is further cooled by the heat exchanger to the temperature required for entering the next process; S6: Sedimentation and filtration, There are a lot of suspended matter in the dissolved liquid, which is settled in a sedimentation tank, and the supernatant directly enters the next process. The settled slurry is filtered by a vacuum drum, and the clear liquid and the supernatant enter the next process together; the filtered residue is discharged; S7: Tubular membrane filtration, Tubular membrane filtration is used to remove high molecular colloids in the xylose solution to obtain a clean solution. S8: High concentration membrane concentration, The concentration membrane intercepts xylose and other ingredients and passes through water to achieve the purpose of concentration; membrane concentration does not require steam heating, and the energy consumption is generally only one-third of the traditional multi-effect evaporation; the water permeated by the membrane concentration membrane is clear and transparent, and the xylose content is almost zero; after the solution is concentrated by the high-concentration membrane, the sugar concentration is increased from about 12.5% ​​to 18%, and a pre-concentrated solution is obtained; S9: Chromatographic desalting and decolorization, The feed liquid passes through the chromatographic separation device, and due to the different resin adsorption and decomposition speeds, the salt, pigment and xylose solution are separated; After chromatographic separation to remove sulfate, organic acid salt and pigment, the xylose extract with conductivity <1000μS / cm and concentration of about 7.0% is entered into the next process, and the chromatographic residue enriched with salts and pigments with conductivity of 15000℃30000μS / cm is sent to the sewage treatment station as production wastewater for treatment and discharge after meeting the standards; S10: MVR plate evaporation, The water content of the initially desalted and decolorized xylose solution is evaporated and removed by the MVR plate evaporation system, and the concentration is increased from 7.0% to about 30%, thereby obtaining a xylose pre-concentrated solution. S11: Granular carbon decolorization, The granular activated carbon is used to further adsorb the organic pigments in the pre-treated solution to obtain a refined xylose solution with a light transmittance of more than 80%; when the granular carbon is saturated with the adsorption of the pigments, it is removed for activation, and the activated granular carbon is returned for continued use; S12: Cooling down, The decolorized solution is further cooled by the heat exchanger to the temperature required for entering the next process; S13: Yang Li Jiao, The xylose decolorized liquid is subjected to cation exchange to remove the residual salts and ash cations, thereby obtaining a xylose cation exchange liquid with a concentration of about 35% and a pH of about 4.6; S14: Yin-Li, The residual salt and ash in the xylose decolorized liquid are removed by anionic ion exchange to obtain a xylose ion exchange liquid with a conductivity of <50μS / cm, a concentration of about 33%, and a pH of about 5.9; S15: Secondary multi-effect evaporation, The refined xylose solution is evaporated and part of the water is removed by a multi-effect falling film vacuum evaporation system, and the concentration is increased from 18% to about 60%, thereby obtaining a xylose concentrate; S16: Four-effect TVR plate steaming, The refined xylose liquid is evaporated through a four-effect TVR plate evaporation system to remove part of the water therein, so that the refractive index rises to about 70%, thereby obtaining refined xylose syrup; S17: Double-effect standard evaporation, The refined xylose syrup is further evaporated through a double-effect standard evaporation system to remove part of the water therein, so that the refractive index rises to about 85%, thereby obtaining refined xylose syrup; S18: Cooling and crystallization, The refined xylose syrup is sent to a horizontal crystallizer, and the temperature of the syrup is slowly lowered by adjusting the amount of cooling water. Since the solubility decreases with temperature, xylose crystals are precipitated to obtain xylose paste; S19: Centrifugal separation, The crystallized xylose paste is centrifuged to remove the mother liquor, thus obtaining xylose crystals and xylose mother liquor. The xylose mother liquor is sold as a by-product, and the xylose crystals enter the next process. S20: Dry, After centrifugation, the xylose crystals still contain about 2% moisture. In order to make the product meet the quality standards and be able to be stored for a long time, it is necessary to reduce its moisture to below 0.2% by drying; S20: Packaging, 6,000 tons / year of finished food-grade crystalline xylose is packaged in ton bags.

2. The process for extracting hemicellulose from pulp to produce food-grade xylose according to claim 1, characterized in that: In the step S1, the hemicellulose black liquor is the pulp porridge generated by the reaction between pulp and soaking alkali solution in the viscose preparation process, and the liquid produced in the pressing process.

3. The process for extracting hemicellulose from pulp to produce food-grade xylose according to claim 1, characterized in that: In the step S17, the steam generated in the evaporation stage and the acid mist generated in the acid adjustment stage are subjected to dust treatment. The generated xylose dust is intended to be treated by a cyclone dust collector. After being treated by the cyclone dust collector, about 0.5 kg / h of dust is discharged to the atmosphere through a self-built 15 m exhaust pipe. The air volume of the cyclone dust collector is 5000 m3 / h. The dust has little impact on the atmosphere, the surrounding people and crops. Secondly, sulfuric acid with a concentration of 98% is used in the acid adjustment process, and it is stirred mechanically continuously, which will produce sulfuric acid mist; hydrochloric acid with a concentration of 30% is used in the primary and secondary separation processes, and it is stirred mechanically, and hydrogen chloride will escape, forming irritating and corrosive hydrochloric acid mist with humid air. The amount of acid mist produced is calculated as follows: Gz=M×(0.000352+0.000786×U)×P×F—Vwater×F Where: Gz——acid mist emission rate (kg / h); M——liquid molecular weight, sulfuric acid is 98, hydrochloric acid is 36.5; U——air velocity on the surface of evaporating liquid (m / s), should be based on actual measured data. If actual measurement is unavailable, take 0.2~0.5m / s or calculate by table. U value is 0.5m / s. P——corresponds to the saturated vapor partial pressure in the air at liquid temperature (mmHg); 59mmHg for the acid adjustment section and 47mmHg for the degassing section. F——the area of ​​evaporation surface, m2; this project adopts a circular reaction tank with an evaporation surface area of ​​6.6m2, Vwater——the evaporation rate of water vapor per unit area, which is 4.2L / m2·h when the evaporation surface temperature of sulfuric acid is 80-90℃, and 1.25L / m2·h when the evaporation surface temperature of hydrochloric acid is 45℃.