Preparation method of xylose

Abstract

The preparation method comprises the following steps: S1, adding xylanase into xylose mother liquor for enzymolysis, viscosity reduction and degradation; s2, adding calcium hydroxide into the enzymatic hydrolysate, and introducing CO2 to generate a nano calcium carbonate flocculating constituent to adsorb impurities; s3, performing cross-flow filtration with a ceramic-polyether sulfone composite membrane to improve the purity of the xylose; s4, carrying out multi-stage counter-current chromatographic separation by adopting calcium type strongly acidic cationic resin, and dynamically adjusting the elution gradient through online HPLC feedback; s5, cooling and crystallizing by stages after triple-effect evaporation and concentration, and adding an alpha-type xylose seed crystal to regulate and control the crystal form; according to the method, the viscosity of the mother liquor is reduced by more than or equal to 45% by virtue of enzymolysis-flocculation synergy, and the impurity adsorption efficiency of nano floccules is more than 85%; the dynamic chromatographic separation factor is increased to 1.8, and the xylose recovery rate is greater than or equal to 96.2%; the purity of alpha-type crystals obtained through gradient crystallization is larger than or equal to 99.5%, and comprehensive energy consumption is reduced; and performing enzymolysis conversion on the raffinate to generate xylooligosaccharide and high fructose corn syrup.

Description

Technical Field

[0001] The present invention relates to the technical field of xylose preparation, and particularly to a method for efficiently separating xylose from xylose mother liquor, especially realizing high-purity extraction and resource utilization of xylose through a multi-stage countercurrent chromatography system combined with a dynamic gradient elution technique. Background Art

[0002] In the industrial production of xylose, the traditional process has long faced three major bottlenecks: low pretreatment efficiency, high separation energy consumption, and insufficient utilization rate of by-products.

[0003] The mainstream technologies rely on acid hydrolysis or single enzymatic hydrolysis. For example, the invention patent application with the application number CN102345234A adopts an acid hydrolysis process, which can reduce the viscosity of the mother liquor by 40%, but produces toxic by-products such as hydroxymethylfurfural (HMF > 500 ppm); while the patent JP2018059492A adopts a single enzymatic hydrolysis technology, and due to incomplete enzymatic hydrolysis, the polysaccharide residue rate exceeds 15%, and the xylose purity is only increased to 55% - 60%. Moreover, for the subsequent chromatographic separation, the separation factor of fixed gradient elution for xylose and arabinose is insufficient, the separation factor is less than 1.3, the resin regeneration is frequent, the service life is shortened to 2 years, and the xylose recovery rate is only 80% - 85%. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for preparing xylose. The present invention combines xylanase and nano-calcium carbonate flocculants, flocculates and removes impurities in the liquid phase system of the reaction by reducing viscosity through enzymatic hydrolysis, and the adsorption rate of the generated nano-calcium carbonate flocculants exceeds 85%. After purification by a composite membrane, the xylose purity can reach 65% - 70%.

[0005] To solve this technical problem, the technical solution of the present invention is: a method for preparing xylose, comprising the following steps: S1. Reducing the viscosity of the mother liquor by xylan enzymatic hydrolysis; Adding xylanase to the xylose mother liquor with a refractive index of 60% - 70%, performing enzymatic hydrolysis, and degrading polysaccharides into xylo-oligosaccharides, and the viscosity reduction of the xylose mother liquor is 45% to 62%; S2. Composite flocculation Adding calcium hydroxide to the enzymatic hydrolysate obtained in S1 and introducing CO2 until the pH is 6.5 - 7.0 to generate nano-calcium carbonate flocculants, and the nano-calcium carbonate flocculants adsorb colloidal and pigment impurities; S3. Composite membrane filtration Subjecting the material obtained by composite flocculation in S2 to cross-flow filtration with a ceramic-polyethersulfone composite membrane, and the xylose purity of the filtrate is increased to 65% - 70%; S4. Chromatographic separation, including the following steps: S41. The feed liquid enters the first chromatographic column for separation, and the effluent is collected in real time and detected by on-line HPLC; Pack a chromatographic column with calcium-type strongly acidic cation resin having a particle size of 200 μm to 400 μm; S42. Based on the ratio of the glucose peak area to the arabinose peak area, the PLC dynamically adjusts the eluent ratio, flow rate, and temperature of the primary separation; In the eluent, based on 100 parts by volume, the adjustment range of the aqueous phase and ethanol phase is that the aqueous phase accounts for 70 to 90 parts by volume, and the balance is the ethanol phase; The flow rate is 0.5 mL / min - 1.5 mL / min; The temperature is 60°C - 80°C; S43. The extract produced by the primary separation is xylose with a purity exceeding 80%. The raffinate is concentrated and then enters the secondary chromatographic column for secondary separation; the extract produced by the secondary separation is xylose with a purity of 50% - 60%, and the raffinate is a raffinate with a purity lower than 3%; S5. Thermal energy nesting and crystallization are synergistically regulated, including the following steps: S51. Concentrate the xylose solution with a triple-effect evaporator; S52. Cool the concentrated solution in stages and add α-type xylose seeds; the crystallization product is vacuum-dried to obtain the target product.

[0006] Preferably, the process conditions for enzymatic hydrolysis in step S1 are as follows: react at 50°C - 55°C for 2 hours - 4 hours.

[0007] Preferably, the mass of calcium hydroxide added in step S2 accounts for 0.3% - 0.8% of the total mass of the enzymatic hydrolysis solution; calcium hydroxide is added in the form of a 5% - 10% w / w lime milk suspension.

[0008] Preferably, the flow rate of CO2 introduced in step S2 is 0.4 L / min - 0.6 L / min.

[0009] Preferably, adjust the pH of the raffinate with a xylose content lower than 3% obtained in step S5 to 5.8 to 6.2, add xylanase and glucose isomerase, and react at a constant temperature of 45°C to 55°C for 4 hours to 8 hours; After the reaction is terminated, centrifugal separation is performed, and the supernatant obtains xylo-oligosaccharide and fructose-glucose syrup.

[0010] Preferably, the dosage ratio of xylanase to glucose isomerase is 1:(0.5 - 2).

[0011] Preferably, add 0.1 mM MgCl2 and 0.05 mM CoCl2 to the raffinate to activate glucose isomerase.

[0012] Preferably, the specific process parameters in step S51 are as follows: Effect I: 0.3 - 0.5 MPa; Effect II: 0.2 MPa; The third effect is 0.1 Mpa.

[0013] Preferably, the process parameters of the stepwise cooling in step S52 are: The process parameters of the stepwise cooling in step S52 are: Cool down from 70 °C to 50 °C, with a cooling rate ≤ 2 °C / min, and keep warm for 20 min; Cool down from 50 °C to 25 °C, with a rate ≥ 5 °C / min, and keep warm for 60 min.

[0014] Preferably, the addition amount of α-xylose crystal seeds in step S52 is 0.1% to 0.3%.

[0015] By adopting the above technical solutions, the beneficial effects of the present invention are: The present invention uses xylanase (EC 3.2.1.8) to specifically recognize and cut the β-1,4-glycosidic bond of the xylan backbone through the catalytic domain of its active center, and degrades high molecular weight polysaccharides with a degree of polymerization exceeding 10 into xylo-oligosaccharides with a degree of polymerization of 2-6. The xylanase molecule binds to the xylan chain through the carbohydrate-binding module (CBM) to form an enzyme-substrate complex; the glutamate residue (Glu) in the catalytic domain acts as a proton donor to attack the oxygen atom of the glycosidic bond, triggering hydrolysis cleavage; free xylose and xylo-oligosaccharides dissociate from the enzyme surface, reducing the viscosity of the mother liquor, and the reduction can reach 60%, while reducing the subsequent separation resistance; In step S2, calcium hydroxide reacts with CO2 to generate nano-calcium carbonate (CaCO3) flocs. The surface of nano-calcium carbonate is positively charged with a Zeta potential of +15 mV, and it undergoes charge neutralization with negatively charged colloidal particles such as pectin and lignin fragments under the action of electrostatic adsorption to form aggregates; the nano-calcium carbonate particles form a hydrogen bond network with the carboxyl group (-COOH) on the surface of impurities through hydroxyl groups (-OH) to capture pigments such as flavonoids and phenolic substances; the generated CaCO3 crystals embed tiny impurity particles during the growth process and form dense flocs with a particle size of 50-200 nm through the co-precipitation effect; Furthermore, the enzymolysis solution purified by nano-calcium carbonate flocs is filtered through a composite membrane. The ceramic-polyethersulfone composite membrane enhances separation through a dual mechanism. On the one hand, it is pore sieving, and macromolecular impurities such as undegraded polysaccharides and floc complex are precisely intercepted by a pore size of 0.05 μm; on the other hand, it uses charge repulsion. The surface of the ceramic-polyethersulfone composite membrane is negatively charged with a Zeta potential of -30 mV, and it prevents negatively charged colloidal particles such as residual pectin from passing through through electrostatic repulsion, improving the purity of xylose to 65-70%; By separating the pretreated solution that has undergone enzymolysis and nano-calcium carbonate floc purification through intelligent multi-stage countercurrent chromatography in step S4, the problems of low separation factor and short resin life in traditional chromatography are solved.

[0016] The present invention uses a calcium-type strongly acidic cation resin (sulfonic acid group functionalized) to separate xylose and arabinose through the following mechanism: The sulfonic acid group (-SO3H) of the resin dissociates into -SO3 in an aqueous solution - , and forms a double hydrogen bond with the hydroxyl group in the xylose molecule, that is, the cis-hydroxyl groups at the C2 / C3 positions of xylose and -SO3 - form a double hydrogen bond, while arabinose has a weaker binding force due to steric hindrance (configuration difference of the hydroxyl group at the C2 position); Ca 2+ forms a stable five-membered ring chelate with the hydroxyl groups at the C3 and C4 positions of xylose (binding constant K = 1.2 - 1.5), which is significantly higher than that of arabinose (K = 0.8 - 1.0); further, the temperature and flow rate are regulated by PLC. The increase in temperature reduces the viscosity of the eluent and improves the diffusion rate of xylose in the resin pores, and the separation factor is improved by optimizing the dynamic mass transfer; Subsequently, the present invention further performs a three-stage closed-loop separation, and the specific process is as follows: First-stage separation: The purity of xylose in the extract is 80 - 85% (refractive index 20 - 25%), and the xylose in the raffinate is ≤15%; Second-stage separation: The first-stage raffinate is concentrated to a refractive index of 50 - 55% and then separated for the second time. The purity of xylose in the extract is 50 - 60%, and the xylose in the raffinate is ≤3%; The resin used in the present invention is a calcium-type strongly acidic cation resin (sulfonic acid group functionalized), and it is automatically switched to citric acid (2% w / w) for backwashing every 24 hours, and the resin life is extended to more than 4 years. The carboxylic acid group of citric acid and Ca 2+ form a soluble complex (such as Ca(C6H5O7)2 3- ), removing the fouling on the resin surface; at the same time, citric acid can also dissolve inorganic deposits such as calcium carbonate, restoring the resin porosity and exchange capacity, and realizing the regeneration of the resin.

[0017] The present invention uses the cooperation of thermal energy nesting and crystallization to regulate the crystal form of xylose. First, the multi-stage thermal energy nesting concentration makes full use of steam grading. The primary steam (0.3 - 0.5 MPa) drives the triple-effect evaporator to concentrate to a refractive index of 80 - 85%; the secondary steam (0.1 - 0.2 MPa) preheats the chromatographic feed liquid and the eluent, and the comprehensive energy consumption is reduced by 50%; The concentrated liquid obtained by the multi-stage thermal energy nesting concentration in the present invention is cooled in stages to achieve gradient crystallization, and then xylose crystal seeds are added. The purity of the crystallization product is ≥99.5%, and the xylose product has a particle size D50 of 120 μm - 180 μm.

[0018] The mechanism of the multi-stage thermal energy nesting concentration in the present invention lies in the multi-stage recovery of steam latent heat and the directional regulation of gradient crystallization. [[ID=Q29]]

[0019] The specific process of the multi-stage recovery of steam latent heat is as follows: Primary steam (0.3 - 0.5 MPa): It is depressurized step by step (Effect I → Effect III) in a triple-effect evaporator, and the latent heat of steam (about 2100 kJ / kg) is used to evaporate water, and the refractive index of the concentrated liquid is increased to 80 - 85%; Secondary steam (0.1 - 0.2 MPa): The waste heat is recovered through a plate heat exchanger (NTU ≥ 50) to preheat the chromatographic feed liquid (ΔT = 40°C → 60°C), and the thermal efficiency is increased to 85%; The specific process of gradient crystallization and directional regulation is as follows: Seed induction: Add α-type xylose seeds (0.1 - 0.3% w / w) to provide heterogeneous nucleation sites and inhibit the spontaneous formation of β-type crystals; Stepwise cooling: 70°C → 50°C: Slowly cool (rate ≤ 2°C / min) to control the growth of crystal nuclei and form α-type primary crystals; 50°C → 25°C: Rapidly cool (rate ≥ 5°C / min) to inhibit the secondary nucleation of β-type crystals. The proportion of α-type crystals in the final product is ≥ 99.5%, and the particle size distribution is uniform, i.e., D50 is 120μm - 180μm.

[0020] The raffinate of the extract after crystallization (xylose ≤ 3%) is added with xylanase and glucose isomerase to react to generate xylo-oligosaccharides and fructose-glucose syrup. The mass percentage of xylo-oligosaccharides in the total sugar is 68% - 72%, and the DE value of the fructose-glucose syrup is ≥ 90; The by-products obtained in the present invention are used as functional food additives, and the comprehensive income is increased by 80. The residual xylan (degree of polymerization > 6) in the raffinate is subjected to endo-action and exo-action under the action of xylanase. The endo-action randomly cuts the main chain of xylan to generate xylo-oligosaccharides (degree of polymerization 2 - 4); The exo-action releases xylobiose (DP = 2) from the chain end, and the proportion of xylo-oligosaccharides in the product exceeds 70%. Glucose isomerase (EC 5.3.1.5) catalyzes the isomerization of D-glucose → D-fructose with the assistance of metal ions (Mg 2+ / Co 2+ ) to increase the sweetness. Description of the Drawings

[0021] Figure 1 It is the infrared spectrum of xylose obtained in Example 2 of the present invention; Figure 2 It is the HPLC spectrum of the products obtained in Example 2 and Comparative Example 3 of the present invention. Detailed Embodiments

[0022] In order to further explain the technical solution of the present invention, the present invention will be elaborated in detail through specific examples below.

[0023] Example 1

[0024] This embodiment discloses a method for preparing xylose, which includes the following steps: S1. Lower the viscosity of the mother liquor by xylan enzymolysis; Add xylanase to the xylose mother liquor with a refractive index of 60 - 70%, and conduct enzymolysis. The detailed enzymolysis process conditions are shown in Table 1; degrade polysaccharides into xylo-oligosaccharides, and the viscosity reduction of the xylose mother liquor is shown in Table 1; S2. Composite flocculation Add calcium hydroxide to the enzymolysis solution after S1, stir at 100 rpm for 10 minutes, and introduce CO2 at a flow rate of 0.5 L / min until the pH reaches 7.0 to generate nano-calcium carbonate flocs. Let it stand for 30 min. The nano-calcium carbonate flocs adsorb colloidal and pigment impurities. The amount of calcium hydroxide added in step S2 is shown in Table 1; S3. Composite membrane filtration Cross-flow filter the material after S2 composite flocculation with a ceramic-polyethersulfone composite membrane with a pore size of 0.05 μm. The xylose purity of the filtrate is shown in Table 1; The composition of the ceramic-polyethersulfone composite membrane in this embodiment includes: a ceramic layer and a polyethersulfone coating; Among them, the ceramic layer is an α-Al2O3 substrate with a pore size of 0.05 μm, which is sintered at high temperature; The polyethersulfone coating is a 10% polyethersulfone (PES) DMF solution scraped on the ceramic substrate and cured at 60°C to form a composite membrane. The Zeta potential of the obtained composite membrane is -30 mV.

[0025] Record the yield of xylo-oligosaccharides, viscosity reduction, colloidal adsorption rate, pigment removal rate, and xylose purity, which are specifically shown in Table 1.

[0026] S4. Chromatographic separation, which includes the following steps: Load a chromatographic column with calcium-type strongly acidic cation resin with a particle size of 200 μm to 400 μm, the column length is 50 cm, and the diameter is 5 cm; S41. The feed liquid enters the first-stage chromatographic column for separation, and the effluent is collected in real time and detected by on-line HPLC; Load a chromatographic column with calcium-type strongly acidic cation resin with a particle size of 200 μm to 400 μm; S42. PLC dynamically adjusts the eluent ratio, flow rate, and temperature of the first-stage separation based on the ratio of the glucose peak area to the arabinose peak area; In the eluent, the aqueous phase and the ethanol phase are calculated according to 100 volume parts, and the adjustment range is that the aqueous phase accounts for 70 parts to 90 parts by volume, and the balance is the ethanol phase; The flow rate is 0.5 mL / min - 1.5 mL / min; The temperature is 60°C - 80°C; S43. The extract produced by the primary separation is xylose with a purity exceeding 80%. The raffinate is concentrated and then enters the secondary chromatographic column for secondary separation. The extract produced by the secondary separation is xylose with a purity of 50%-60%, and the raffinate has a purity lower than 3%. The evaluation of molecular factors, xylose purity, and resin life in step S4 is shown in Table 2. S5. Thermal energy nesting and crystallization co-regulation, including the following steps: S51. Concentrate the xylose solution in a triple-effect evaporator. The specific process parameters in step S51 are as follows: Effect I: 0.3 - 0.5 MPa; Effect II: 0.2 MPa; Effect III: 0.1 MPa. S52. Cool the concentrated solution in stages. The process parameters for cooling in stages in step S52 are: 70°C → 50°C → 25°C.

[0027] The specific dosage data of adding α-type xylose seeds are shown in Table 3. The α-type xylose seeds have a particle size of 100 μm. The crystalline product is vacuum-dried to obtain the target product. The calculation of thermal efficiency, crystal purity, and particle size distribution are shown in Table 3. The detection of crystal morphology and particle size distribution after vacuum drying of the crystalline product is shown in Table 3.

[0028] Adjust the pH of the raffinate with a xylose content lower than 3% obtained in step S5 to 6.0, add xylanase and glucose isomerase, and react at a constant temperature of 50°C. The reaction time is shown in Table 4. The dosage ratio of xylanase and glucose isomerase is shown in Table 4.

[0029] Add 0.1 mM MgCl2 and 0.05 mM CoCl2 to the raffinate to activate glucose isomerase.

[0030] After the reaction is terminated, centrifuge for separation. The supernatant yields xylooligosaccharide and high fructose syrup. The degree of polymerization of xylooligosaccharide, the DE value of high fructose syrup, and the economic evaluation are shown in Table 4.

[0031] Example 2 The main differences between this example and Example 1 are shown in Tables 1 to 4.

[0032] Example 3 The main differences between this example and Example 1 are shown in Tables 1 to 4.

[0033] Table 1 Process parameters and response analysis in S1 to S3 of Examples 1 to 3

[0034] The specific detection methods in Table 1 are as follows: Yield of xylooligosaccharides: detected by HPLC (chromatographic column: Aminex HPX-87H, mobile phase: 5 mM H2SO4); Viscosity measurement: rotational viscometer (Brookfield DV2T, 25 °C); Particle size of nano-CaCO3: laser particle size analyzer (Malvern Mastersizer 3000).

[0035] As can be seen from Table 1, in terms of optimizing the enzymatic hydrolysis efficiency, when the xylanase addition amount in Example 2 was 0.3% and the reaction time was 3 h, the highest yield of xylooligosaccharides reached 65%, the viscosity of the mother liquor decreased from 70% to 28%, and the decrease was 62%; in terms of the flocculation synergistic effect, when the dosage of calcium hydroxide in Example 2 was 0.5%, the optimal particle size of nano-CaCO3 was 150 nm, and through electrostatic adsorption and hydrogen bond bridging, the colloid adsorption rate was 88% and the pigment removal rate was 75%; Moreover, the xylose purity of the filtrate obtained in Example 2 reached 68%, and the impurity interception rate was ≥90%, providing a high-purity raw material for subsequent chromatographic separation.

[0036] Table 2 Process parameters of step S4 in Examples 1 to 3 in response to changes

[0037] The detection methods in Table 2 are as follows: Xylose purity: detected by HPLC (chromatographic column: Rezex RPM-Monosaccharide); Separation factor (α): α = (xylose retention time / arabinose retention time) × (xylose peak area / arabinose peak area); Resin life evaluation: measure the exchange capacity decay rate after backwashing with citric acid every 24 hours.

[0038] Combined with the process parameters in Table 2 and the changes in responses, it can be seen that in Example 2, when the temperature was 70 °C, the viscosity of the eluent decreased by 30%, the xylose diffusion rate increased, and the separation factor reached 1.8 through the temperature-flow rate synergistic effect, which was 30% higher than that in Example 1; when the optimized countercurrent feedback ratio was 20%, the xylose recovery rate increased to 96.2%, while avoiding system overload; compared with Example 3, although the viscosity was further reduced at 80 °C, the resin life was shortened by 15%. The process parameters of S4 were comprehensively optimized to be Example 2, ensuring that the xylose recovery rate exceeded 95% and the resin life could exceed 4 years.

[0039] Table 3 Process parameters of step S5 in Examples 1 to 3 and response changes

[0040] The detection methods in Table 3 are as follows: Calculation of thermal efficiency: steam consumption (kg / kg of water evaporation); Crystal purity: proportion of α-type analyzed by X-ray diffraction (XRD); Particle size distribution: laser diffraction method (Malvern Mastersizer 3000).

[0041] Combined with Table 3, it can be seen that in Example 2 under a steam pressure of 0.4 MPa, the thermal efficiency of triple-effect evaporation reaches 82%, the refractive index increases from 65% to 83%, achieving steam grading and efficiency improvement; in Example 2, through gradient crystallization control, the proportion of α-type crystals is 99.6%, the particle size D50 is 150 μm, and the coefficient of variation is less than 10%; in Example 2, the comprehensive energy consumption is reduced by 55% compared with traditional single-effect evaporation, and the crystal morphology is uniform, meeting the requirement of the food-grade xylose standard purity greater than 99.5%.

[0042] Table 4 Parameters and responses of the by-product conversion enzyme system in step S5

[0043] The detection methods in Table 4 are as follows: Degree of polymerization of xylo-oligosaccharides: analyzed by MALDI-TOF mass spectrometry; DE value of fructose syrup: determined by DNS method for reducing sugar content; [[ID=I21]] Economic evaluation: ratio of raw material cost to product value (cost / benefit).

[0044] Combined with Table 4, it can be seen that in Example 2 when the ratio of 0.2% xylanase to 0.2% glucose isomerase is 1:1, the yield of xylo-oligosaccharides is 68%, the DE value of fructose syrup is 90%, and the reaction time is 6 h; in terms of economic balance, the comprehensive benefit of the enzyme ratio 1:1 scheme is increased by 85%, and the cost is only increased by 8% (cost / benefit ratio 0.18), which is better than Example 1 and Example 3; the present invention realizes the high-value utilization of by-products, and xylo-oligosaccharides (degree of polymerization 2-4) and high-DE fructose syrup can replace imported functional sweeteners, with the added value increased by 80%.

[0045] Comparative Example 1 The main difference between this comparative example and Example 2 is that in S1, the traditional acid hydrolysis method is adopted. 1.0% w / w sulfuric acid was added to the xylose mother liquor with a refractive index of 60-70%, and the reaction was carried out at 90 °C for 1 hour. The remaining steps S2 to S5 were the same as those in Example 2.

[0046] In this comparative example, the viscosity reduction of the mother liquor by the traditional acid hydrolysis method was only 38%, while that of Example 2 was 62%. The content of hydroxymethylfurfural (HMF) generated was >600 ppm, and it was not detected in Example 2. The purity of the xylose product in this comparative example was ≤98.0%, which was lower than the xylose purity of 99.6% obtained in Example 2. Comparing Example 2 and Comparative Example 1, it can be seen that acid hydrolysis leads to the accumulation of by-products, hindering the subsequent chromatographic separation efficiency.

[0047] Comparative Example 2 The main difference between this comparative example and Example 2 is that sodium carbonate flocculation is used instead of in-situ generation of nano-CaCO3. The specific technical solution is as follows: Directly add 1.5% w / w sodium carbonate solution. The other process parameters are the same as those in Example 2.

[0048] In this comparative example, the average particle size of the flocs generated was greater than 500 nm, the colloid adsorption rate was 68%, and the xylose purity after composite membrane filtration was only 60%. While the size of the calcium carbonate floc particles obtained in Example 2 was 150 nm, the colloid adsorption rate was 88%, and the xylose purity after composite membrane filtration was 68%. It can be seen from Comparative Example 2 that adding sodium carbonate externally cannot form nano-scale flocs, and the impurity removal efficiency is insufficient.

[0049] Comparative Example 3 The main difference between this comparative example and Example 2 is that this comparative example uses fixed-gradient chromatography separation instead of dynamic regulation in Example 2. The specific technical solution is as follows: Cancel the dynamic gradient adjustment based on online HPLC feedback in step S4, and adopt a fixed water-ethanol volume ratio of 85:15 (corresponding to an ethanol ratio of 15%). The other conditions are the same as those in Example 2.

[0050] In this comparative example, the separation factor α of xylose-arabinose was 1.25, the xylose recovery rate was ≤83%, and the resin life decay rate reached 0.15% / cycle. It can be seen from Comparative Example 3 that the fixed gradient cannot adapt to the fluctuations of the raw material components, resulting in a decrease in separation efficiency and resin stability.

[0051] Comparative Example 4 The main difference between this comparative example and Example 2 is that single-stage crystallization is used instead of gradient cooling. The specific technical solution is as follows: Cancel the stepwise cooling and seed addition in step S52, and directly cool the concentrated solution from 70°C to 25°C at a constant speed of 3°C / min. The other steps are the same as those in Example 2.

[0052] In the product obtained in Comparative Example 4 The proportion of α-type crystals was 85%, The coefficient of variation of the crystal particle size distribution was >30%, The content of β-xylose impurities in the product reaches 12%. From Comparative Example 4, it can be seen that single-stage cooling triggers the spontaneous nucleation of β-type crystals, destroying the crystal form homogeneity.

[0053] The infrared spectrum of the xylose obtained in Example 2 of the present invention is as Figure 1 shown. There is a strong and broad peak of O-H stretching vibration at 3420 cm -1 , indicating the formation of an intermolecular / intramolecular hydrogen bond network in xylose molecules. The increase in peak width is due to the coupling of poly-mer hydrogen bonds; there is a weak peak of aliphatic C-H symmetric stretching vibration at 2920 cm -1 . The weak intensity is because xylose has no long-chain alkyl groups, which is in line with the pentose structure; there is a strong sharp peak at 1070 cm -1 , which belongs to the asymmetric stretching vibration of C-O-C and the deformation vibration of C1-H in the α-D-xylopyranose ring. The α-anomer (C1-OH and C5-CH2OH on the same side) produces a characteristic peak at 1070 cm -1 ; the medium-strong peak at 890 cm -1 is the out-of-plane bending vibration of C1-H (δC1-H) of the α-anomer, which is a marker for α-pyranose. The β-type appears at 840–850 cm -1 , while Figure 1 shows no absorption peak at 845 cm -1 , which can confirm that the content of β-type impurities is less than 0.5%. The products obtained in Example 2 and Example 3 were respectively subjected to HPLC tests. As Figure 2 shows, the retention time of the xylose peak obtained in Example 2 is 3.2 min, and the peak shape is sharp and symmetric; the retention time of the arabinose peak is 4.2 min, and it is completely baseline-separated from the xylose peak; there are no significant impurity peaks in the 2.0–3.0 min interval, indicating that the colloid and HMF are effectively removed. In the product obtained in Comparative Example 3, the retention time of the xylose peak is 3.6 min, and the peak is significantly broadened; the retention time of the arabinose peak is 4.0 min, and it partially overlaps with the xylose peak, and the resolution Rs < 0.8; there is an obvious shoulder peak at 2.5 min, which is confirmed to be hydroxymethylfurfural HMF by the standard sample. Therefore, from Figure 2 it can be seen that the intelligent dynamic elution in Example 2 improves the separation degree of xylose-arabinose by 87.5% (Rs increases from 0.8 to 1.5), and eliminates the interference of HMF impurities; while in Comparative Example 3, due to the fixed gradient, co-elution occurs, and the xylose recovery rate is reduced to 83%, verifying the necessity of online HPLC feedback regulation.

Claims

1. A method for preparing xylose, characterized in that: It includes the following steps: S1. Using xylanase to hydrolyze and reduce the viscosity of the mother liquor; S2. Composite flocculation Adding calcium hydroxide to the enzyme hydrolyzate after S1 and introducing CO2 until the pH is 6.5 - 7.0 to generate nano-calcium carbonate flocculants; S3. Composite membrane filtration; S4. Chromatographic separation; The feed liquid after enzymatic hydrolysis and flocculation filtration is successively subjected to primary separation, dynamic control is executed by a programmable logic controller, secondary separation, and the elution gradient of the primary separation is closed-loop regulated; S5. Thermal energy nesting and crystallization are synergistically regulated to obtain the target product.

2. The preparation method according to claim 1, wherein: In step S1, xylanase is added to the xylose mother liquor with a refractive index of 60% - 70%, and enzymatic hydrolysis is carried out to degrade polysaccharides into xylo-oligosaccharides, and the viscosity of the xylose mother liquor is reduced by 45% to 62%; The process conditions for enzymatic hydrolysis in step S1 are as follows: reacting at 50°C - 55°C for 2 hours - 4 hours.

3. The preparation method according to claim 1, characterized in that: The mass of calcium hydroxide added in step S2 accounts for 0.3% - 0.8% of the total mass of the enzyme hydrolyzate; calcium hydroxide is added in the form of a 5% - 10% w / w lime milk suspension.

4. The preparation method according to claim 1, characterized in that: The flow rate of CO2 introduced in step S2 is 0.4 L / min - 0.6 L / min.

5. The preparation method according to claim 1, characterized in that: The synergistic regulation of thermal energy nesting and crystallization in step S5 includes the following steps: S51. Concentrating the xylose solution with a triple-effect evaporator; S52. Cooling the concentrated solution in stages and adding α-type xylose crystal seeds; the crystallization product is vacuum-dried to obtain the target product; Adjust the pH of the raffinate with a xylose content of less than 3% obtained in step S5 to 5.8 to 6.2, add xylanase and glucose isomerase, and react at a constant temperature of 45°C to 55°C for 4 hours to 8 hours; After the reaction is terminated, centrifugal separation is carried out, and the supernatant obtains xylo-oligosaccharides and fructose-glucose syrup.

6. The preparation method according to claim 5, characterized in that: The dosage ratio of xylanase to glucose isomerase is 1:(0.5 - 2).

7. The preparation method according to claim 5, characterized in that: Adding 0.1 mM MgCl2 and 0.05 mM CoCl2 to the raffinate to activate glucose isomerase.

8. The preparation method according to claim 5, characterized in that: The specific process parameters in step S51 are as follows: Effect I: 0.3 - 0.5 MPa; Effect II: 0.2 MPa; Effect III: 0.1 MPa.

9. The preparation method according to claim 5, characterized in that: The process parameters for cooling in stages in step S52 are: Cooling from 70°C to 50°C, the cooling rate ≤ 2°C / min, and keeping warm for 20 min; Cooling from 50°C to 25°C, the rate ≥ 5°C / min, and keeping warm for 60 min.

10. The preparation method according to claim 5, characterized in that: The addition amount of α-type xylose crystal seeds in step S52 is 0.1% to 0.3%.

Images