A method for the preparation of xylose

Abstract

The application discloses a preparation method of xylose, which comprises the following steps: S1, adding xylanase into a xylose mother liquor for enzyme hydrolysis and viscosity reduction; S2, adding calcium hydroxide into the enzyme hydrolysis liquor and passing in CO2 to generate nanometer calcium carbonate floccus to adsorb impurities; S3, performing cross-flow filtration on the ceramic-polyether sulfone composite membrane to improve the purity of the xylose; S4, performing multistage countercurrent chromatography separation on the calcium type strong acid cation resin, and dynamically adjusting an elution gradient through online HPLC feedback; and S5, performing three-effect evaporation concentration, then stage cooling crystallization, adding alpha type xylose crystal seeds to control the crystal form; the application utilizes enzyme hydrolysis-flocculation to cooperatively reduce the viscosity of the mother liquor to greater than or equal to 45%, and the nanometer floccus has an impurity adsorption efficiency greater than or equal to 85%; a dynamic chromatography separation factor is improved to 1.8, and the xylose recovery rate is greater than or equal to 96.2%; alpha type crystal purity greater than or equal to 99.5% is obtained through gradient crystallization, and the comprehensive energy consumption is reduced; and the enzyme hydrolysis conversion of the raffinate generates xylooligosaccharide and high fructose syrup.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of preparing xylose, in particular to a method for efficiently separating xylose from xylose mother liquor, which realizes high-purity extraction and resource utilization of xylose by combining a multi-stage countercurrent chromatography system with a dynamic gradient elution technique. BACKGROUND

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

[0003] The mainstream technology relies on acidolysis or single enzymolysis. For example, the acidolysis process adopted in the patent application with the application number CN102345234A can reduce the viscosity of the mother liquor by 40%, but generates toxic by-products such as hydroxymethylfurfural (HMF>500 ppm); and the single enzymolysis technology adopted in the patent JP2018059492A leads to more than 15% of polysaccharide residual rate due to incomplete enzymolysis, and the purity of xylose is only improved to 55%-60%. Moreover, the separation factor of the 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 recovery rate of xylose is only 80%-85%. SUMMARY

[0004] The purpose of the present application is to provide a preparation method of xylose. The present application combines xylanase and nano calcium carbonate flocculation body, reduces the viscosity by enzymolysis, removes impurities by flocculation in the liquid phase system of the reaction, and uses the nano calcium carbonate flocculation body with an adsorption rate of more than 85% to obtain xylose with a purity of 65%-70% after purification by a composite membrane.

[0005] To solve the technical problem, the technical scheme of the present application is as follows: a preparation method of xylose, comprising the following steps:

[0006] S1, xylanase enzymolysis to reduce the viscosity of the mother liquor;

[0007] Xylanase is added to the xylose mother liquor with a refractive index of 60%-70%, and enzymolysis is performed to degrade polysaccharide into xylo-oligosaccharide, and the viscosity of the xylose mother liquor is reduced by 45% to 62%;

[0008] S2, composite flocculation

[0009] Calcium hydroxide is added to the enzymolysis liquid after S1, and CO2 is introduced until the pH is 6.5-7.0, to generate nano calcium carbonate flocculation body, which adsorbs colloidal and pigment impurities;

[0010] S3, composite membrane filtration

[0011] The material subjected to the composite flocculation of S2 is subjected to cross-flow filtration by a ceramic-polyether sulfone composite membrane, and the purity of the xylose in the filtrate is improved to 65%-70%.

[0012] S4, chromatographic separation, comprising the following steps:

[0013] S41, the feed liquid enters a primary chromatographic column for separation, and the effluent is collected in real time and detected by online HPLC;

[0014] The chromatographic column is packed with calcium-type strong acid cation resin with a particle size of 200 μm to 400 μm;

[0015] S42, the PLC dynamically adjusts the eluent proportion, flow rate and temperature of the primary separation based on the ratio of the glucose peak area to the arabinose peak area;

[0016] The water phase accounts for 70 to 90 volume parts, and the balance is the ethanol phase, based on 100 volume parts of the eluent;

[0017] The flow rate is 0.5 mL / min to 1.5 mL / min;

[0018] The temperature is 60°C to 80°C;

[0019] S43, the extract produced by the primary separation is xylose with a purity of more than 80%, and the raffinate is concentrated and then subjected to secondary separation in a secondary chromatographic column; the extract produced by the secondary separation is xylose with a purity of 50% to 60%, and the raffinate is a raffinate with a purity of less than 3%;

[0020] S5, thermal energy nesting and crystallization synergistic regulation, comprising the following steps:

[0021] S51, concentrating the xylose liquid by a three-effect evaporator;

[0022] S52, the concentrated liquid is cooled in stages, and α-type xylose crystal seeds are added; the crystallization product is vacuum dried to obtain the target product.

[0023] The preferred process conditions for the enzymatic hydrolysis in step S1 are as follows: 50°C to 55°C for 2 hours to 4 hours.

[0024] In step S2, the mass of calcium hydroxide added accounts for 0.3% to 0.8% of the total mass of the enzymatic hydrolysis liquid; the calcium hydroxide is added in the form of a 5% to 10% w / w lime suspension.

[0025] In step S2, the flow rate of the CO2 introduced is 0.4 L / min to 0.6 L / min.

[0026] The raffinate with a xylose content of less than 3% obtained in step S5 is adjusted to a pH of 5.8 to 6.2, xylanase and glucose isomerase are added, and the reaction is carried out at a constant temperature of 45°C to 55°C for 4 hours to 8 hours;

[0027] After the reaction is terminated, centrifugal separation is performed, and the supernatant is obtained as xylo-oligosaccharide and fructose-glucose syrup.

[0028] Preferably, the ratio of the amount of xylanase to glucose isomerase is 1: (0.5-2).

[0029] Preferably, 0.1 mM MgCl2 and 0.05 mM CoCl2 are added to the raffinate to activate glucose isomerase.

[0030] Preferably, the specific process parameters in step S51 are as follows:

[0031] 0.3-0.5 MPa in the first effect;

[0032] 0.2 MPa in the second effect;

[0033] 0.1 MPa in the third effect.

[0034] Preferably, the process parameters of the staged cooling in step S52 are as follows:

[0035] The process parameters of the staged cooling in step S52 are as follows:

[0036] cooling from 70℃ to 50℃ at a rate of ≤2℃ / min, and holding for 20 min;

[0037] cooling from 50℃ to 25℃ at a rate of ≥5℃ / min, and holding for 60 min.

[0038] Preferably, the amount of the added α-type xylose seed in step S52 is 0.1% to 0.3%.

[0039] By adopting the above technical solutions, the present application has the following beneficial effects:

[0040] The present application utilizes xylanase (EC 3.2.1.8) to specifically recognize and cut the β-1,4-glycosidic bond of the xylan main chain through the catalytic domain of its active center, so as to degrade high molecular weight polysaccharides with a polymerization degree of more than 10 into oligomeric xylose with a polymerization degree of 2-6. The xylanase molecule is combined with the xylan chain through a carbohydrate binding module (CBM) to form an enzyme-substrate complex; a glutamic acid residue (Glu) in the catalytic domain attacks the oxygen atom of the glycosidic bond as a proton donor to initiate hydrolysis and rupture; the free xylose and oligomeric xylose are dissociated from the enzyme surface, which reduces the viscosity of the mother liquor by up to 60%, and reduces the subsequent separation resistance;

[0041] The calcium hydroxide reacts with CO2 to generate nanometer calcium carbonate (CaCO3) flocculation body in step S2, the nanometer calcium carbonate has positive charges on the surface and the Zeta potential is +15 mV, the charge neutralization occurs under the action of electrostatic adsorption between the nanometer calcium carbonate and the colloidal particles such as pectin and lignin fragments which have negative charges, and aggregation is formed; the nanometer calcium carbonate particles form a hydrogen bond network with the carboxyl (-COOH) on the surface of impurities through the hydroxyl (-OH), and capture pigments such as flavonoids and phenolic substances; the generated CaCO3 crystals embed the small impurity particles in the growth process, and form dense flocculation bodies with a particle size of 50-200 nm through the co-precipitation effect;

[0042] Further, the enzyme solution after impurity removal by the nanometer calcium carbonate flocculation body is filtered through a composite membrane, and the ceramic-polyether sulfone composite membrane strengthens separation through a double mechanism, one is pore size screening, and the other is charge repulsion; the ceramic-polyether sulfone composite membrane surface is negatively charged, and the Zeta potential is -30 mV, and the negatively charged colloidal particles such as residual pectin are prevented from penetrating through electrostatic repulsion, so that the purity of xylose is improved to 65-70%;

[0043] The pretreated solution after enzyme hydrolysis and nanometer calcium carbonate flocculation body impurity removal is separated through intelligent multi-stage countercurrent chromatography in step S4, so that the problems of low factor and short resin life in traditional chromatographic separation are solved.

[0044] The application separates xylose and arabinose from the calcium type strong acid cation resin (sulfonic acid functional group) through the following mechanism:

[0045] The sulfonic acid group (-SO3H) of the resin is dissociated into -SO3 - in the aqueous solution, and forms a double hydrogen bond with the cis-hydroxyl group at the C2 / C3 position of the xylose molecule and -SO3 - , while the arabinose has a weaker binding force due to steric hindrance (difference in configuration of the hydroxyl group at the C2 position); Ca 2+ forms a stable five-membered ring chelate with the C3 and C4 hydroxyl groups of the 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 controlled through PLC, the viscosity of the eluent is reduced by increasing the temperature, the diffusion rate of the xylose in the pores of the resin is improved, and the separation factor is improved through optimization of dynamic mass transfer;

[0046] Then, the application further separates through three closed loop separation, and the specific process is as follows:

[0047] First separation: the purity of xylose in the extract is 80-85% (refractive index 20-25%), and the purity of xylose in the raffinate is ≤15%;

[0048] Secondary separation: the first raffinate is concentrated to a refractive index of 50-55% and then subjected to secondary separation, the purity of the extract is 50-60%, and the purity of the raffinate is less than or equal to 3%;

[0049] The resin used in the application is a calcium type strong acid cation resin (sulfonic acid group functionalization), which is automatically switched to citric acid (2% w / w) backwashing every 24 hours, and the service life of the resin is extended to more than 4 years. The carboxyl group of citric acid and Ca 2+ form a soluble complex (such as Ca(C6H5O7)2 3- ), remove the scale on the surface of the resin; at the same time, citric acid can also dissolve inorganic deposits such as calcium carbonate, restore the porosity and exchange capacity of the resin, and realize the regeneration of the resin.

[0050] The application utilizes thermal energy nesting and crystallization to cooperatively regulate the crystal form of xylose. Firstly, multi-stage thermal energy nesting is used to fully utilize steam classification. Primary steam (0.3-0.5 MPa) drives a triple-effect evaporator to concentrate to a refractive index of 80-85%. Secondary steam (0.1-0.2 MPa) preheats the chromatography feed liquid and eluent, and the comprehensive energy consumption is reduced by 50%.

[0051] The concentrated liquid obtained by multi-stage thermal energy nesting is cooled in stages to realize gradient crystallization, and then xylose crystal seeds are added. The purity of the crystalline product is greater than or equal to 99.5%, and the particle size D50 of the xylose product is 120-180 μm.

[0052] The multi-stage thermal energy nesting mechanism in the application is multi-stage recovery of steam latent heat and gradient crystallization directional regulation.

[0053] The specific process of multi-stage recovery of steam latent heat is as follows:

[0054] Primary steam (0.3-0.5 MPa): stepwise pressure reduction (Ⅰ effect→Ⅲ effect) in a triple-effect evaporator, using steam latent heat (about 2100 kJ / kg) to evaporate water, and the refractive index of the concentrated liquid is increased to 80-85%;

[0055] Secondary steam (0.1-0.2 MPa): through a plate heat exchanger (NTU≥50) to recover waste heat, preheat the chromatography feed liquid (ΔT=40℃→60℃), and the thermal efficiency is increased to 85%;

[0056] The specific process of gradient crystallization directional regulation is as follows:

[0057] Seed induction: adding α-type xylose crystal seeds (0.1-0.3% w / w) to provide heterogeneous nucleation sites and inhibit the spontaneous formation of β-type crystals;

[0058] Stage cooling:

[0059] 70℃→50℃: Slowly cool (rate ≤2℃ / min) to control the growth of crystal nucleus, forming the alpha type primary crystal;

[0060] 50℃→25℃: Rapid cooling (rate ≥5℃ / min) to inhibit the secondary nucleation of beta type crystal, the alpha type crystal accounts for ≥99.5% in the final product, and the particle size distribution is uniform, i.e. D50 is 120μm-180μm.

[0061] The extracted raffinate (xylose ≤3%) after crystallization is added with xylanase and glucose isomerase to generate xylo-oligosaccharide and high fructose syrup, wherein the xylo-oligosaccharide accounts for 68%-72% of the total sugar mass percentage, and the DE value of the high fructose syrup is ≥90; the by-product obtained in the application is used for functional food additives, and the comprehensive income is increased by 80. The residual xylan (polymerization degree >6) in the raffinate is subjected to endo- and exo-acting under the action of xylanase, the endo-acting is random cutting of the xylan main chain to generate xylo-oligosaccharide (polymerization degree 2-4); the exo-acting is to release xylobiose (DP=2) from the chain end, and the xylo-oligosaccharide accounts for more than 70% in the product. Glucose isomerase (EC 5.3.1.5) catalyzes the isomerization of D-glucose→D-fructose with the aid of metal ions (Mg 2+ / Co 2+ ) to improve the sweetness. BRIEF DESCRIPTION OF DRAWINGS

[0062] Figure 1 It is the infrared spectrum of xylose obtained in Example 2 of the application;

[0063] Figure 2 It is the HPLC spectrum of the product obtained in Example 2 and Comparative Example 3 of the application. DETAILED DESCRIPTION

[0064] In order to further explain the technical scheme of the application, the application will be described in detail below through specific examples.

[0065] Example 1

[0066] The embodiment discloses a preparation method of xylose, comprising the following steps:

[0067] S1, xylanase hydrolysis to reduce the viscosity of the mother liquor;

[0068] Xylanase is added to the xylose mother liquor with a refractive index of 60-70%, and the enzyme is hydrolyzed, and the enzyme hydrolysis process conditions are shown in Table 1; the polysaccharide is degraded into xylo-oligosaccharide, and the viscosity reduction of the xylose mother liquor is shown in Table 1;

[0069] S2, composite flocculation

[0070] Calcium hydroxide was added to the enzymatic hydrolysate from S1, stirred at 100 rpm for 10 minutes, and CO2 was passed at a flow rate of 0.5 L / min to pH 7.0 to generate nanometer calcium carbonate flocculation, and the nanometer calcium carbonate flocculation was adsorbed for 30 min, and the colloidal and pigment impurities were adsorbed. The amount of calcium hydroxide added in step S2 is shown in Table 1.

[0071] S3, composite membrane filtration

[0072] The material subjected to composite flocculation in S2 was subjected to cross-flow filtration using a ceramic-polyether sulfone composite membrane with a pore size of 0.05 μm. The purity of the filtrate xylose is shown in Table 1.

[0073] The composition of the ceramic-polyether sulfone composite membrane in this embodiment includes a ceramic layer and a polyether sulfone coating layer.

[0074] The ceramic layer is an α-Al2O3 substrate with a pore size of 0.05 μm, sintered at high temperature.

[0075] The polyether sulfone coating layer is a 10% polyether sulfone (PES) DMF solution scraped onto the ceramic substrate, and the composite membrane is formed after solidification at 60°C. The Zeta potential of the obtained composite membrane is -30 mV.

[0076] The oligomeric xylose yield, viscosity reduction, colloidal adsorption rate, pigment removal rate, and xylose purity were recorded, as shown in Table 1.

[0077] S4, chromatographic separation, including the following steps:

[0078] A calcium-type strong acid cation resin with a particle size of 200 μm to 400 μm was used to pack the chromatographic column, with a column length of 50 cm and a diameter of 5 cm.

[0079] S41, the feed liquid enters the primary chromatographic column for separation, and the effluent is collected in real time for online HPLC detection.

[0080] A calcium-type strong acid cation resin with a particle size of 200 μm to 400 μm was used to pack the chromatographic column.

[0081] S42, the PLC dynamically adjusts the eluent proportion, flow rate, and temperature of the primary separation based on the ratio of glucose peak area to arabinose peak area.

[0082] The water phase and ethanol phase in the eluent are adjusted to a range of 70 to 90 parts by volume of water phase and the balance of ethanol phase, based on 100 parts by volume.

[0083] The flow rate is 0.5 mL / min to 1.5 mL / min.

[0084] The temperature is 60°C to 80°C.

[0085] S43, the extract liquid generated by the first separation is xylose with purity more than 80%, and the raffinate is concentrated and then enters the second chromatographic column for secondary separation; the extract liquid generated by the second separation is xylose with purity of 50%-60%, and the raffinate is the raffinate with purity less than 3%;

[0086] The molecular factor, xylose purity and resin life evaluation after step S4 are shown in Table 2;

[0087] S5, heat energy nesting and crystallization synergistic regulation, including the following steps:

[0088] S51, concentrating the xylose liquid by a three-effect evaporator;

[0089] The specific process parameters in step S51 are as follows:

[0090] 0.3-0.5 MPa for the first effect, 0.2 MPa for the second effect, and 0.1 MPa for the third effect;

[0091] S52, the concentrated liquid is cooled in stages, and the process parameters for the staged cooling in step S52 are as follows:

[0092] 70℃→50℃→25℃.

[0093] The specific data of the amount of added α-type xylose seed are shown in Table 3, and the α-type xylose seed has a particle size of 100 μm. The target product is obtained after vacuum drying of the crystallization product, and the heat efficiency calculation, crystal purity and particle size distribution are shown in Table 3. The crystal morphology and particle size distribution of the crystallization product after vacuum drying are shown in Table 3.

[0094] The pH of the raffinate with xylose content less than 3% obtained in step S5 is adjusted to 6.0, xylanase and glucose isomerase are added, and the reaction is carried out at 50℃, and the reaction time is shown in Table 4;

[0095] The amount of xylanase and glucose isomerase is shown in Table 4.

[0096] 0.1 mM MgCl2 and 0.05 mM CoCl2 are added to the raffinate to activate glucose isomerase.

[0097] After the reaction is terminated, centrifugal separation is carried out, and the supernatant is obtained as xylo-oligosaccharide and high fructose syrup, and the polymerization degree of xylo-oligosaccharide, the DE value of high fructose syrup and the economic evaluation are shown in Table 4.

[0098] Example 2

[0099] The main differences between this example and Example 1 are shown in Tables 1-4.

[0100] Example 3

[0101] The main differences between this example and Example 1 are shown in Tables 1-4.

[0102] Table 1 Process parameters and response analysis in S1-S3 in Examples 1-3

[0103]

[0104] The specific detection methods in Table 1 are as follows:

[0105] Xylo-oligosaccharide yield: HPLC detection (chromatographic column: Aminex HPX-87H, mobile phase: 5 mM H2SO4);

[0106] Viscosity determination: rotary viscometer (Brookfield DV2T, 25°C);

[0107] Nano-CaCO3 particle size: laser particle size analyzer (Malvern Mastersizer 3000).

[0108] As can be seen from Table 1, in terms of enzyme hydrolysis efficiency optimization, in Example 2, when the xylanase addition amount is 0.3% and the reaction time is 3h, the xylo-oligosaccharide yield is as high as 65%, the mother liquor viscosity is reduced from 70% to 28%, and the reduction is 62%; in terms of flocculation synergistic effect, in Example 2, when the calcium hydroxide dosage is 0.5%, the nano-CaCO3 particle size is optimal at 150 nm, and through electrostatic adsorption and hydrogen bond bridging effect, the colloidal adsorption rate is 88% and the pigment removal rate is 75%;

[0109] And the xylose purity of the filtrate obtained in Example 2 is 68%, the impurity interception rate is ≥90%, which provides high-purity raw materials for subsequent chromatographic separation.

[0110] Table 2 Process parameters of step S4 in Examples 1-3 and response changes

[0111]

[0112] The detection methods in Table 2 are as follows:

[0113] Xylose purity: HPLC detection (chromatographic column: Rezex RPM-Monosaccharide);

[0114] Separation factor (a):

[0115] a = (xylose retention time / arabinose retention time) x (xylose peak area / arabinose peak area);

[0116] Resin life evaluation: exchange capacity decay rate was determined after citric acid backflushing every 24 hours.

[0117] In combination with the process parameters in Table 2 and the changes in response, it can be seen that in Example 2, the viscosity of the eluent at 70°C is reduced by 30%, the diffusion rate of xylose is increased, and the separation factor reaches 1.8 through the synergistic effect of temperature-flow rate, which is 30% higher than that in Example 1; when the reverse flow feedback ratio is optimized to 20%, the xylose recovery rate is increased to 96.2%, while avoiding system overload; compared with Example 3, although the viscosity is further reduced at 80°C, the resin life is shortened by 15%, and the process parameters of S4 are optimized in Example 2 to ensure that the xylose recovery rate is more than 95% and the resin life can exceed 4 years.

[0118] Table 3 Process parameters and response changes in step S5 of Examples 1-3

[0119]

[0120] The detection methods in Table 3 are as follows:

[0121] Thermal efficiency calculation: steam consumption (kg / kg of water evaporated);

[0122] Crystal purity: X-ray diffraction (XRD) analysis of the proportion of α-type;

[0123] Particle size distribution: laser diffraction method (Malvern Mastersizer 3000).

[0124] As can be seen from Table 3, in Example 2, under a steam pressure of 0.4 MPa, the thermal efficiency of three-effect evaporation reaches 82%, and the refractive index is increased from 65% to 83%, achieving steam grading and efficiency enhancement; in Example 2, through gradient crystallization regulation, 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 food-grade xylose standard of purity greater than 99.5%.

[0125] Table 4 Parameters of byproduct conversion enzyme system in step S5 and response

[0126]

[0127] The detection methods in Table 4 are as follows:

[0128] Degree of polymerization of xylooligosaccharides: MALDI-TOF mass spectrometry;

[0129] DE value of fructose-glucose syrup: DNS method for determining reducing sugar content;

[0130] Economic evaluation: ratio of raw material cost to product value (cost / benefit).

[0131] In combination 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 xylooligosaccharide is 68%, the DE value of fructose-glucose syrup is 90%, and the reaction time is 6h; from the aspect of economic balance, the comprehensive income of the 1:1 enzyme ratio scheme is increased by 85%, and the cost is increased by only 8% (cost / revenue ratio 0.18), which is better than Example 1 and Example 3; the present application realizes the high value of by-products, and xylooligosaccharide (polymerization degree 2-4) and high DE value fructose-glucose syrup can replace imported functional sweeteners, and the added value is increased by 80%.

[0132] Comparative Example 1

[0133] The main difference between the present comparative example and Example 2 is that S1 uses a traditional acid hydrolysis method,

[0134] 1.0% w / w sulfuric acid is added to the xylitol mother liquor with a refractive index of 60-70%, and the reaction is carried out at 90°C for 1 hour. The remaining steps S2 to S5 are consistent with Example 2.

[0135] The viscosity reduction of the mother liquor by the traditional acid hydrolysis method in the present comparative example is only 38%, while that in Example 2 is 62%. The content of hydroxymethylfurfural (HMF) generated is >600 ppm, while that in Example 2 is not detected. The purity of xylitol in the present comparative example is ≤98.0%, which is lower than the purity of xylitol 99.6% obtained in Example 2. As can be seen from the comparison between Example 2 and Comparative Example 1, acid hydrolysis leads to the accumulation of by-products, which hinders the subsequent chromatographic separation efficiency.

[0136] Comparative Example 2

[0137] The main difference between the present comparative example and Example 2 is that sodium carbonate flocculation is used instead of in-situ generated nano-CaCO3, and the specific technical scheme is as follows:

[0138] 1.5% w / w sodium carbonate solution is directly added. The remaining process parameters are the same as those in Example 2.

[0139] In the present comparative example, the average particle size of the generated flocculation body is greater than 500 nm, the colloidal adsorption rate is 68%, and the purity of xylitol after composite membrane filtration is only 60%. However, in Example 2, the particle size of the calcium carbonate flocculation body is 150 nm, the colloidal adsorption rate is 88%, and the purity of xylitol after composite membrane filtration is 68%. As can be seen from Comparative Example 2, exogenous addition of sodium carbonate cannot form nano-sized flocculation body, and the impurity removal efficiency is insufficient.

[0140] Comparative Example 3

[0141] The main difference between the present comparative example and Example 2 is that the present comparative example uses fixed gradient chromatographic separation instead of dynamic control in Example 2, and the specific technical scheme is as follows:

[0142] Cancel the dynamic gradient adjustment based on online HPLC feedback in step S4, and use a fixed water-ethanol volume ratio of 85:15 (corresponding to an ethanol proportion of 15%). The remaining conditions are the same as in Example 2.

[0143] In the present comparative example, the xylose-arabinose separation factor α = 1.25, the xylose recovery rate ≤ 83%, and the resin life attenuation rate reaches 0.15% / cycle. As can be seen from Comparative Example 3, the fixed gradient cannot adapt to fluctuations in raw material components, resulting in a decrease in separation efficiency and resin stability.

[0144] Comparative Example 4

[0145] The main difference between the present comparative example and Example 2 is that single-stage crystallization is used instead of gradient cooling. The specific technical solution is as follows:

[0146] Cancel the stepwise cooling and seed addition in step S52, and directly cool the concentrated solution from 70℃ to 25℃ at a constant rate of 3℃ / min. The remaining steps are the same as in Example 2.

[0147] In the product obtained in Comparative Example 4,

[0148] The proportion of α-type crystals is 85%,

[0149] The coefficient of variation of crystal particle size distribution is > 30%,

[0150] The content of β-type xylose impurities in the product reaches 12%, and as can be seen from Comparative Example 4, single-stage cooling induces spontaneous nucleation of β-type crystals, destroying the crystal form uniformity.

[0151] The infrared spectrum of the xylose obtained in Example 2 of the present application is shown in Figure 1 The strong and wide peak of O-H stretching vibration at 3420 cm -1 , the intermolecular / intramolecular hydrogen bond network of xylose is formed, and the peak width increases due to the coupling of multimer hydrogen bonds; the weak peak of aliphatic C-H symmetric stretching vibration at 2920 cm -1 , the intensity is weak because xylose has no long-chain alkyl group, which is consistent with the structure of pentose; the strong sharp peak at 1070 cm -1 is the asymmetric stretching vibration of α-D-xylopyranose ring C-O-C and the deformation vibration of C1-H, and the α-type anomeric form (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 (δC1-H) of the C1-H of the α-anomeric form, which is a marker for α-type pyranose, while the β-type appears at 840-850 cm -1 , and Figure 1 the medium peak at 845 cm -1No absorption peak can confirm that the content of β-type impurities is less than 0.5%. The products obtained from Example 2 and Example 3 are respectively tested by HPLC, as shown in Figure 2 , the retention time of xylose peak obtained from Example 2 is 3.2 min, and the peak shape is sharp and symmetrical; the retention time of arabinose peak is 4.2 min, which is completely separated from the xylose peak; there is no significant impurity peak in the interval of 2.0-3.0 min, indicating that the gelatin and HMF are effectively removed. The retention time of xylose peak obtained from Comparative Example 3 is 3.6 min, and the peak is obviously broadened; the retention time of arabinose peak is 4.0 min, which is partially overlapped with the xylose peak, and the separation degree Rs is less than 0.8; there is an obvious shoulder peak at 2.5 min, which is confirmed by standard sample as hydroxymethylfurfural HMF. Therefore, it can be known from Figure 2 that the intelligent dynamic elution of Example 2 increases the separation degree of xylose-arabinose by 87.5% (Rs is increased from 0.8 to 1.5), and eliminates the interference of HMF impurities; and the fixed gradient of Comparative Example 3 leads to co-elution, and the recovery rate of xylose is reduced to 83%, which verifies the necessity of online HPLC feedback control.

Claims

1. A method for preparing xylose, characterized in that: The following steps are involved: S1, enzymatic hydrolysis of xylan to reduce the viscosity of the mother liquor; In step S1, xylanase is added to the xylose mother liquor with a refractive index of 60%-70%, and enzymatic hydrolysis is performed to degrade the polysaccharide into xylooligosaccharides, and the viscosity of the xylose mother liquor is reduced by 45% to 62%; The process conditions of enzymatic hydrolysis in step S1 are as follows: 50°C-55°C for 2 hours-4 hours; S2, composite flocculation Add calcium hydroxide to the enzymatic hydrolyzate after S1 and introduce CO2 until the pH reaches 6.5-7.0 to generate nano-calcium carbonate flocs; In step S2, the amount of calcium hydroxide added is 0.3%-0.8% of the total mass of the enzymatic hydrolyzate; the calcium hydroxide is added in the form of a 5%-10% w / w lime milk suspension; The flow rate of CO2 in step S2 is 0.4 L / min-0.6 L / min; S3, composite membrane filtration; The material after S2 composite flocculation is filtered through a ceramic-polyethersulfone composite membrane cross-flow, and the xylose purity of the filtrate is increased to 65%-70%; S4, chromatographic separation, comprising the following steps: S41, the feed liquid enters the primary chromatographic column for separation, and the effluent is collected in real time and detected by online HPLC; The chromatographic column is packed with calcium-type strong acidic cationic resin with a particle size of 200 μm to 400 μm; S42, PLC dynamically adjusts the eluent ratio, flow rate, and temperature of the primary separation based on the ratio of the glucose peak area to the arabinose peak area; The aqueous phase and the ethanol phase in the eluent are calculated based on 100 parts by volume, and the adjustment range is 70 to 90 parts by volume of the aqueous phase, and the balance is the ethanol phase; Flow rate: 0.5 mL / min-1.5 mL / min; Temperature is 60℃-80℃; S43, the extract produced by the primary separation is xylose with a purity of more than 80%, and 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 of less than 3%; S5, thermal energy nesting and crystallization synergistically regulate the target product; Step S5: Thermal energy nesting and crystallization coordinated regulation includes the following steps: S51, triple-effect evaporator to concentrate xylose liquor; S52, cooling the concentrated solution in stages, adding α-xylose seed crystals; and vacuum drying the crystallized product to obtain the target product; The process parameters for the staged cooling in step S52 are: Cool from 70°C to 50°C at a cooling rate of ≤2°C / min and keep warm for 20 min; Cool from 50°C to 25°C at a rate of ≥5°C / min and keep warm for 60 min; Adjusting the pH of the raffinate obtained in step S5 with a xylose content of less than 3% to 5.8 to 6.2, adding xylanase and glucose isomerase, and reacting at a constant temperature of 45° C. to 55° C. for 4 to 8 hours; After the reaction is terminated, the mixture is centrifuged and the supernatant is used to obtain oligoxylose and fructose syrup.

2. The preparation method according to claim 1, wherein: The dosage ratio of xylanase to glucose isomerase is 1:(0.5-2).

3. The preparation method according to claim 1, wherein: 0.1 mM MgCl2 and 0.05 mM CoCl2 were added to the raffinate to activate glucose isomerase.

4. The preparation method according to claim 1, wherein: The specific process parameters in step S51 are as follows: Ⅰ effect 0.3-0.5 MPa; II effect 0.2 MPa; Effect III: 0.1 MPa.

5. The preparation method according to claim 1, wherein: In step S52 , the amount of α-xylose seed crystals added is 0.1% to 0.3%.

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