A method for preparing high-purity xylan by using viscose fiber alkaline waste liquid

Abstract

A method for preparing high-purity xylan using alkaline waste liquid from viscose cellulose fibers involves precipitating the alkaline waste liquid with ethanol to obtain crude hemicellulose. A ternary eutectic solvent containing choline chloride, acetic acid, and water (0-50% water content) is prepared. The crude hemicellulose and the ternary eutectic solvent are mixed at a solid-liquid ratio of 1:(10-30), and extracted at 110-150℃ for 1-3 hours. The extract is then recovered by ethanol precipitation to obtain high-purity xylan. This invention features a simple, environmentally friendly process, high extraction rate, excellent product purity, and is suitable for industrial application.

Description

Technical Field

[0001] This invention relates to the field of biomass refining technology, specifically a method for preparing high-purity xylan using alkaline waste liquid from viscose fibers. Background Technology

[0002] Viscose fiber is a man-made fiber produced from natural cellulose (wood pulp, cotton pulp, etc.) through processes such as alkalization, aging, xanthation, dissolution, and spinning. Statistics show that my country's annual viscose fiber production exceeds 4 million tons. During the alkalization and pressing processes, a large amount of hemicellulose from wood or cotton linters dissolves into the alkaline waste liquid, forming a pressing lye rich in hemicellulose. The pressing lye produced annually contains hundreds of thousands of tons of usable hemicellulose. Currently, the main treatment methods for this waste liquid are alkali recovery or direct discharge, resulting in extremely low resource utilization of hemicellulose. This not only causes serious waste of biomass resources but also increases the wastewater treatment load and environmental burden.

[0003] Xylan, a major component of hemicellulose, is widely used in food additives (xylooligosaccharide precursors), pharmaceutical excipients, functional films, and bio-based chemicals, possessing significant economic value. However, the efficient extraction of high-purity xylan from alkaline wastewater from viscose cellulose has remained a technical challenge in the industry. Existing technologies for recovering hemicellulose from viscose cellulose wastewater mainly include membrane separation, acid precipitation, and ethanol precipitation. Ethanol precipitation has attracted attention due to its simplicity and high recovery rate, but the resulting crude hemicellulose has low purity and requires further purification to obtain high-purity xylan. Traditional purification methods often employ alkaline extraction and acid precipitation or enzymatic hydrolysis, which suffer from drawbacks such as low extraction rates, high chemical consumption, severe environmental pollution, and high costs. Summary of the Invention

[0004] This invention aims to provide a method for preparing high-purity xylan using alkaline waste liquid from viscose fibers. The process is simple, environmentally friendly, has a high extraction rate, produces products with excellent purity, and is suitable for industrial application.

[0005] To solve the above technical problems, the specific solution adopted in this invention is as follows: a method for preparing high-purity xylan using alkaline waste liquid from viscose fiber, wherein crude hemicellulose is obtained by ethanol precipitation of alkaline waste liquid from viscose fiber, a ternary eutectic solvent containing choline chloride, acetic acid and water with a water content of 0-50% is prepared, the crude hemicellulose and the ternary eutectic solvent are mixed at a solid-liquid ratio of 1:(10-30), and extracted at 110-150℃ for 1-3 hours, and the extract is recovered by ethanol precipitation to obtain high-purity xylan.

[0006] Preferably, the molar ratio of choline chloride to acetic acid is 1:(1.8-2.1).

[0007] Preferably, the molar ratio of choline chloride to acetic acid is 1:2.

[0008] Preferably, the ternary eutectic solvent has a water content of 40%, the extraction temperature is 130℃, the extraction time is 1.5h, and the solid-liquid ratio is 1:20.

[0009] Preferably, crude hemicellulose is obtained by precipitation of the alkaline waste liquid of viscose fiber using ethanol with a concentration of 90% or higher.

[0010] Preferably, the ternary eutectic solvent is prepared by first mixing choline chloride and acetic acid, and then adding water and stirring.

[0011] Preferably, the extract is precipitated and recovered, and then dried to obtain high-purity xylan.

[0012] Preferably, the extract is recovered by precipitation with anhydrous ethanol.

[0013] This invention constructs a ternary eutectic system of choline chloride-acetic acid-water by introducing precisely controlled amounts of water into a binary DES system. Compared to binary DES, this ternary system has the following significant advantages: First, the introduction of an appropriate amount of water effectively reduces the viscosity of the system, improves mass transfer efficiency, and makes it easier for the solvent to penetrate into the interior of crude hemicellulose; second, precise control of the water content optimizes the polarity and hydrogen bond network structure of the system, enhances the selective solubility of xylan, and inhibits the co-extraction of impurities. In particular, when the water content reaches 40%, the various physicochemical properties of the ternary DES system reach an optimal equilibrium state. Under the conditions of 130℃ and 1.5h, highly efficient and selective dissolution of xylan from alcohol-precipitated crude hemicellulose is achieved, with the final product having a sugar content as high as 72.16%, which is a significant improvement compared to the crude product obtained by traditional ethanol precipitation (which typically has a sugar content of only 30%-55%), creating favorable conditions for its subsequent processing and utilization. Attached Figure Description

[0014] Figure 1 The figure shows the results of a single-factor experiment on the effect of DES moisture content on xylan sugar content and extraction rate.

[0015] Figure 2 The figure shows the results of a single-factor experiment on the effect of extraction temperature on the sugar content and extraction rate of xylan.

[0016] Figure 3 The figure shows the results of a single-factor experiment on the effect of extraction time on the sugar content and extraction rate of xylan.

[0017] Figure 4 The figure shows the results of a single-factor experiment on the effect of solid-liquid ratio on the sugar content and extraction rate of xylan.

[0018] Figure 5A bar chart comparing the sugar content of products obtained by the method of the present invention and the DES method of choline chloride-formic acid under optimal conditions. Detailed Implementation

[0019] The technical solution of the present invention will be further described in detail below with reference to specific embodiments and accompanying drawings, but the scope of protection of the present invention is not limited thereto.

[0020] The process flow of this invention is as follows: viscose fiber alkaline waste liquid → 95% ethanol precipitation → crude hemicellulose → DES extraction (choline chloride-acetic acid-water ternary system) → anhydrous ethanol precipitation → high-purity xylan. Specific steps are as follows: (1) Ethanol precipitation to obtain crude hemicellulose: Add 95% (volume fraction) ethanol to the alkaline waste liquid generated during viscose fiber production. The volume ratio of ethanol to waste liquid is (2-5):1. After thorough mixing, let it stand for precipitation for 4-24 hours at 4℃ to room temperature. Collect the precipitate by centrifugation or filtration, wash the precipitate with 95% ethanol 1-2 times, and dry it to obtain crude hemicellulose.

[0021] (2) Preparation of eutectic solvent (DES): Choline chloride and acetic acid are mixed at a molar ratio of 1: (1.8-2.1), stirred evenly, and then a certain amount of deionized water is added to prepare a DES-water ternary system with a water content of 0%-50% (w / w) for later use.

[0022] (3) Extraction of xylan by DES: The crude hemicellulose obtained in step (1) is mixed with the DES-water ternary system prepared in step (2) at a solid-liquid ratio of 1: (10-30) (w / v, g / mL), and extracted by stirring at 110℃-150℃ for 1-3 hours.

[0023] (4) Recovery of xylan: After the extraction reaction is completed, cool to room temperature, add anhydrous ethanol for precipitation, centrifuge to collect the precipitate, and dry to obtain high-purity xylan.

[0024] Example 1: Effect of different DES moisture contents on extraction efficiency Crude hemicellulose was obtained from the alkali liquid pressed from viscose fiber by precipitation with 95% ethanol in step (1) for later use.

[0025] Following step (2), a binary DES with a choline chloride-acetic acid molar ratio of 1:2 was prepared. Calculated amounts of deionized water were added to obtain DES-water ternary systems with water contents of 0%, 10%, 20%, 30%, 40%, and 50%. Extraction experiments were conducted under fixed conditions of a solid-liquid ratio of 1:15, a temperature of 120℃, and a time of 1.5 h.

[0026] The results are shown in Table 1: From Table 1 and Figure 1 It can be seen that with the increase of DES moisture content, the sugar content of xylan shows a trend of first increasing and then decreasing, with the highest sugar content (56.64%) in the single-factor experiment at a moisture content of 30%. However, further orthogonal experiments revealed a significant interaction between moisture content and other factors (temperature, time, and solid-liquid ratio). The optimal combination was 40% moisture content, 130℃ temperature, 1.5h time, and a solid-liquid ratio of 1:20, under which the sugar content reached 72.16%, far exceeding any result in the single-factor experiment. This proves that there is a synergistic effect among the parameters of this invention, and the optimal combination of conditions cannot be predicted by simple single-factor experiments.

[0027] Example 2: Effect of different extraction temperatures on extraction efficiency Crude hemicellulose was obtained according to step (1), and a DES-water ternary system with 0% water content was prepared according to step (2). Extraction experiments were carried out at 110℃, 120℃, 130℃, 140℃ and 150℃ under fixed conditions of solid-liquid ratio of 1:15 and time of 1.5h.

[0028] The results are shown in Table 2: From Table 2 and Figure 2 It can be seen that the sugar content reaches its highest single-factor value (35.99%) at 130℃. When the temperature is too low (110℃), the viscosity of the DES system is too high and the mass transfer efficiency is low. When the temperature is too high (>140℃), it may lead to xylan degradation or increased co-extraction of impurities, resulting in a decrease in sugar content.

[0029] Example 3: Effect of different extraction times on extraction efficiency Crude hemicellulose was obtained according to step (1), and a DES-water ternary system with 0% water content was prepared according to step (2). Under the fixed conditions of solid-liquid ratio of 1:15 and temperature of 120℃, extraction was performed for 1h, 1.5h, 2h, 2.5h and 3h respectively.

[0030] The results are shown in Table 3: Table 3 and Figure 3 Data showed that the sugar content reached its single-factor peak (40.83%) at 2 hours, and the extraction rate exceeded 70% at 1.5 hours, 2 hours, and 3 hours. However, subsequent orthogonal experiments, considering the balance between sugar content and extraction rate, determined 1.5 hours to be the optimal reaction time.

[0031] Example 4: Effect of different solid-liquid ratios on extraction efficiency Crude hemicellulose was obtained according to step (1), and a DES-water ternary system with 0% water content was prepared according to step (2). Extraction was carried out at a fixed temperature of 120℃ and a time of 1.5h with solid-liquid ratios of 1:10, 1:15, 1:20, 1:25 and 1:30, respectively.

[0032] The results are shown in Table 4: From Table 4 and Figure 4 It is evident that the sugar content reaches its highest level (36.02%) at a solid-liquid ratio of 1:20. When the solid-liquid ratio is too low, the amount of DES used is insufficient to fully dissolve the xylan, while when the solid-liquid ratio is too high, it increases the difficulty and cost of subsequent alcohol precipitation and recovery, and the sugar content actually decreases.

[0033] Example 5: Optimization and Verification of Orthogonal Experiments Based on the results of the single-factor experiments above, four factors were selected: water content (A), extraction temperature (B), extraction time (C), and solid-liquid ratio (D), each with three levels. An L9(34) orthogonal array was used for optimization experiments. The factor level table is as follows: The range analysis results in Table 5 show that the order of influence of the four factors on xylan yield is D (moisture content) > A (temperature) > C (solid-liquid ratio) > B (time). This indicates that the moisture content of DES has the most significant impact on the extraction effect, followed by extraction temperature, solid-liquid ratio, and extraction time. The optimal combination of factors is A2B1C2D3, which corresponds to an extraction temperature of 130℃, an extraction time of 1.5 h, a solid-liquid ratio of 1:20 (g / mL), and a DES moisture content of 40%. Under these optimal conditions (Experiment No. 4), the xylan yield reached 72.16%, the highest value among all nine experimental groups.

[0034] The yield obtained under these optimal conditions (72.16%) is significantly better than the best result under any single-factor experiment (maximum 56.64%), and is 1.27 times that of the single-factor optimal result; it is also far superior to the choline chloride-formic acid DES control system (yield 36%), and is 2.0 times that of the latter. This result fully demonstrates that the parameter combination obtained by orthogonal experimental optimization in this invention produces an unexpected synergistic effect, and there is a significant nonlinear interaction effect among the process parameters, the optimal combination of conditions cannot be predicted by simple single-factor experiments.

[0035] Comparative Example 1: Experimental Study of Different DES Systems Under the same operating conditions as in Example 5 (40% water content, 130°C, 1.5 h, solid-liquid ratio 1:20), the choline chloride-acetic acid DES system of the present invention was replaced with a choline chloride-formic acid (molar ratio 1:2, water content 40%) DES system for comparison. After extraction, anhydrous ethanol was used for precipitation recovery in both cases.

[0036] The results are as follows Figure 5 As shown, the sugar content of the product obtained from the choline chloride-formic acid system is only 36%, which is far lower than the 72.16% of the choline chloride-acetic acid system of this invention. This indicates that not all acidic DES systems can achieve the technical effects of this invention. The choline chloride-acetic acid (1:2) system selected by this invention and the specific water content are unique technical solutions obtained based on creative labor, and their superior effects cannot be reasonably expected.

Claims

1. A method for preparing high-purity xylan using alkaline waste liquid from viscose fibers, characterized in that: Crude hemicellulose was obtained by precipitating alkaline waste liquid from viscose fibers with ethanol. A ternary eutectic solvent containing choline chloride, acetic acid, and water with a water content of 0-50% was prepared. The crude hemicellulose and the ternary eutectic solvent were mixed at a solid-liquid ratio of 1:(10-30) and extracted at 110-150℃ for 1-3 hours. The extract was then recovered by ethanol precipitation to obtain high-purity xylan.

2. The method for preparing high-purity xylan using alkaline waste liquid from viscose fibers as described in claim 1, characterized in that: The molar ratio of choline chloride to acetic acid is 1:(1.8-2.1).

3. The method for preparing high-purity xylan using alkaline waste liquid from viscose fibers as described in claim 1, characterized in that: The molar ratio of choline chloride to acetic acid is 1:

2.

4. The method for preparing high-purity xylan using alkaline waste liquid from viscose fibers as described in claim 3, characterized in that: The ternary eutectic solvent has a water content of 40%, an extraction temperature of 130℃, an extraction time of 1.5h, and a solid-liquid ratio of 1:

20.

5. The method for preparing high-purity xylan using alkaline waste liquid from viscose fibers as described in claim 1, characterized in that: Crude hemicellulose is obtained by precipitating the alkaline waste liquid of viscose fiber with ethanol at a concentration of over 90%.

6. The method for preparing high-purity xylan using alkaline waste liquid from viscose fibers as described in claim 1, characterized in that: The preparation method of the ternary eutectic solvent is to first mix choline chloride and acetic acid, and then add water and stir.

7. The method for preparing high-purity xylan using alkaline waste liquid from viscose fibers as described in claim 1, characterized in that: The extract was precipitated and recovered, and then dried to obtain high-purity xylan.

8. The method for preparing high-purity xylan using alkaline waste liquid from viscose fibers as described in claim 1, characterized in that: The extract was recovered by precipitation with anhydrous ethanol.

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