Preparation method of dissolving pulp with uniform cellulose polymerization degree distribution

Abstract

The invention relates to a preparation method of dissolving pulp with uniform cellulose polymerization degree distribution, and belongs to the technical field of dissolving pulp. The preparation method of the dissolving pulp comprises the following steps: (1) raw material pulp pretreatment and grading screening; (2) cold alkali swelling and pulsed electric field cooperative treatment; (3) selective enzymolysis treatment of a compound enzyme system of cleavage enzyme and excision enzyme; and (4) neutralizing and washing the slurry. Compared with the prior art, the preparation method of the dissolving pulp provided by the invention has the advantages that the centralization of a polymerization degree main distribution interval (such as 800-1000) is realized, the standard deviation is obviously reduced, and the cellulose polymerization degree distribution is narrower and more controllable. No organic solvent or strong oxidant is needed in the whole preparation process of the dissolving pulp, the alkali liquor can be recycled, the enzyme preparation can be repeatedly used, and good environment friendliness is achieved.

Description

Technical Field

[0001] This invention relates to the technical field of dissolving pulp, and more specifically to a method for preparing a dissolving pulp with a uniform degree of polymerization of cellulose. Background Technology

[0002] Dissolving pulp is a high-purity cellulose raw material widely used in high-value-added fields such as viscose fiber, Lyocell fiber, cellulose ether esters, food additives, and medical materials. Its quality directly affects the solubility, processing stability, and performance of end products of cellulose, and it has become a key intermediate product in the pulp and paper industry and the bio-based materials industry.

[0003] In actual production, the degree of polymerization (DP) of cellulose is a core parameter for measuring its molecular chain length and processing behavior. An ideal dissolving paste should have a moderate and concentrated DP range, typically controlled between 800 and 1000, to ensure good solubility, reactivity, and rheological stability. However, current industrial dissolving pastes commonly exhibit a wide DP distribution and a high proportion of extreme components at both ends. High-DP components (DP > 1800) are difficult to fully swell and dissolve, leading to excessively high viscosity, decreased reaction rate, and poor film uniformity in the processing system. Low-DP components (DP < 200) are prone to degradation and hydrolysis, generating small molecular fragments that affect system stability and end-product performance. Furthermore, the non-uniformity of the DP distribution significantly exacerbates system volatility, especially under continuous production conditions, leading to decreased downstream process stability and poor product consistency.

[0004] In existing technologies, degree of polymerization (DP) control mainly relies on methods such as alkali swelling, mechanical homogenization, partial oxidative degradation, and physical screening. While these methods can narrow the DP distribution to some extent, each has significant limitations. For example, mechanical homogenization (such as re-beating and shearing) suffers from high energy consumption and severe damage to fiber structure; cold / hot alkali treatment is prone to uneven swelling and poor selectivity; oxidative degradation methods (such as ozone and H2O2) are difficult to control, easily leading to over-degradation and chemical residues; while bio-enzymatic degradation has good selectivity, its reaction rate is slow and difficult to match industrial pace; physical screening (such as fiber classifiers) is limited by fiber morphology and cannot accurately correspond to DP values, resulting in limited classification accuracy.

[0005] More importantly, existing technologies are mostly based on a "linear serial" processing path, that is, performing steps such as grading, swelling, and degradation sequentially, failing to fully consider the coupling mechanism between the reaction selectivity of cellulose chains and the accessibility of their microstructure. The underlying causes of the uneven distribution of the degree of polymerization are generally overlooked in this field—not only due to differences in chain length, but also influenced by microstructural factors such as the degree of dissociation between primary and secondary walls in the fiber cell walls, the isomeric distribution of crystalline and amorphous regions, and the continuity of the swelling-mass transfer interface on the fiber surface. Therefore, traditional methods often suffer from poor reaction selectivity, severe damage to fiber structure, and insufficient precision in degradation control.

[0006] In summary, there is currently a lack of an effective strategy that can balance the precision of DP distribution control, the greenness of the process, and the adaptability to industrial continuous production without relying on complex new equipment. Especially in the face of the practical needs of high-throughput, continuous production, there is an urgent need to develop a novel, highly integrated, and efficient method for optimizing the degree of polymerization distribution to improve the processability of dissolving pulp and the consistency of end materials, thereby promoting its large-scale application in high-performance regenerated cellulose and its derivatives. Summary of the Invention

[0007] To address the problems of uneven cellulose polymerization degree distribution, large fluctuations in processing performance, and poor environmental friendliness or weak industrial adaptability of existing dissolving pulp preparation methods, this invention provides a method for preparing dissolving pulp with a uniform cellulose polymerization degree distribution to solve these problems. This invention proposes a novel method for controlling the uniformity of the polymerization degree distribution in dissolving pulp. This method can effectively centralize and narrow the polymerization degree distribution of the dissolving pulp, significantly improving its processing adaptability and end-product performance, and fully meeting the needs of large-scale industrial applications. To achieve the above objectives, this invention can cleverly and precisely control the polymerization degree distribution without changing the main structure of the existing production process, achieving significant narrowing and effective centering of the polymerization degree distribution, thereby greatly improving the dissolution performance and processing consistency of the pulp, possessing both good industrial adaptability and broad application value.

[0008] The technical solution of this invention is as follows: A method for preparing a dissolving pulp with a uniform degree of polymerization of cellulose includes the following steps: (1) Pretreatment and grading of dissolving slurry; (2) Synergistic treatment of cold alkali swelling and pulsed electric field; (3) Selective enzymatic hydrolysis of end-cleavage enzyme and exonuclease complex enzyme system; (4) Neutralization and washing of slurry.

[0009] Further, step (1) is as follows: the initially prepared dissolving slurry is input into a mechanical classification device and a two-stage screening process is carried out; the first stage uses a coarse screener to accurately remove low-polymerization fiber segments (DP<200), and the second stage uses a fine separation module to accurately identify and remove ultra-high polymerization fiber segments (DP>1800), and finally retains the fibers in the main distribution range (DP is 800~1200) as the target objects for subsequent processing.

[0010] Furthermore, the mechanical grading device is a centrifugal gas-solid coupling fiber grading device; this device has an automatic screening function and can implement feedback adjustment according to the screening situation to ensure the accuracy and stability of grading.

[0011] Further, step (2) involves: transporting the screened slurry to a cold alkali swelling system, using a 5%~8% sodium hydroxide solution, and swelling at a low temperature of 10℃~30℃ for 30~60 minutes; simultaneously applying an intermittent pulsed electric field with a frequency of 80~120 Hz and a voltage intensity of 1~1.8 kV / cm. This synergistic treatment method can effectively disrupt the hydrogen bond network between the crystalline and amorphous regions in the fiber wall, improve swelling uniformity and reaction permeability, and provide an ideal reaction environment for subsequent enzymatic hydrolysis.

[0012] Furthermore, the pulsed electric field adopts an intermittent on-off pulse mode with a precise on-off cycle of 8 to 15 seconds, and is closely coordinated with and implemented synchronously with the cold alkali swelling process, so as to fully release the synergistic effect of the pulsed electric field on the cold alkali swelling.

[0013] Further, step (3) involves transferring the slurry treated in step (2) to a reaction tank equipped with a mixing device, and adding an immobilized enzyme capsule system composed of end-cleavage enzymes (such as Endo-1,4-β-glucanase) and exonucleases (such as Exo-1,4-β-glucanase) in a mass ratio of 2:1. The enzyme capsules are prepared in microcapsule form using chitosan or sodium alginate as carriers, with an enzyme loading of 25-40 U / g based on the dry weight of the slurry. The reaction conditions are: temperature 35-50 ℃, pH=5.5, and time 10-20 minutes. This selective enzymatic hydrolysis system achieves directional degradation of high-DP cellulose segments and avoids non-selective destruction of segments in the medium- and low-DP regions, ensuring a concentrated degree of polymerization distribution in the dissolved slurry product.

[0014] Furthermore, the immobilized enzyme capsule system uses chitosan or sodium alginate as a carrier and is constructed in the form of microcapsules. The resulting enzyme capsules have excellent recyclability and can be stably recycled 3 to 5 times, significantly reducing enzyme usage costs and improving process economy.

[0015] Furthermore, step (4) involves neutralizing the enzymatically hydrolyzed pulp with dilute acid to stabilize its pH value at a neutral level (around pH 7.0). Subsequently, residual enzymes, sodium ions, and short-chain fragments are thoroughly removed using a conventional pulp washing process, ultimately yielding a dissolving pulp product with a concentrated degree of polymerization, uniform performance, and superior quality, meeting the needs of various high-end applications.

[0016] Furthermore, after step (4) is completed, the process also includes an online monitoring and feedback control step for the degree of polymerization of the dissolving slurry, using an online degree of polymerization analysis module.

[0017] Furthermore, in the post-washing stage, an optional online degree of polymerization (DP) analysis module can be installed to monitor and control the DP distribution of the resulting dissolved slurry in real time. This analysis module employs small-angle light scattering (SALS), capillary viscosity analysis, or electrophoresis to acquire DP distribution data. It is then linked with the central control system to adjust sieving accuracy, electric field parameters, and enzyme dosage in real time, achieving dynamic closed-loop control and ensuring that the final product's DP remains stable within the target range.

[0018] The beneficial effects of this invention are as follows: This invention provides a method for preparing a dissolving pulp with a uniform degree of polymerization of cellulose. Compared with existing technologies, this method centralizes the main distribution range of the degree of polymerization (e.g., 800-1000), significantly reduces the standard deviation, and results in a narrower and more controllable distribution of the degree of polymerization of cellulose. The entire dissolving pulp preparation process requires no organic solvents or strong oxidants; the alkali solution can be recycled; and the enzyme preparation can be reused, demonstrating good environmental friendliness. Key modules can be directly integrated with existing pulping processes (such as washing, alkali treatment, and pulp preparation) without large-scale modifications. Furthermore, the preparation method of this invention has a short processing time, mild reaction conditions, and low energy consumption, making it suitable for matching the cycle time of modern dissolving pulp production lines. In summary, the method described in this invention provides an innovative and feasible technical path for improving the quality and application performance of dissolving pulp, and has good prospects for application and promotion. Detailed Implementation

[0019] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions in the embodiments of this invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of this invention.

[0020] The end-cleavage enzyme used in this embodiment of the invention is Endo-1,4-β-glucanase, trade name Celluclast® 1.5 L, purchased from Novozymes, website: https: / / biosolutions.novozymes.com; The exonuclease used was Exo-1,4-β-glucanase, trade name Cellobiase, purchased from Sigma-Aldrich, website: https: / / www.sigmaaldrich.com.

[0021] Example 1 A method for preparing a dissolving pulp with a uniform degree of polymerization of cellulose, employing synergistic regulation under standard conditions, includes the following steps: (1) Pretreatment and grading of raw material slurry Bleached dissolving hardwood pulp, treated with acidic pre-hydrolysis and alkaline cooking, was selected as the raw material. A two-stage centrifugal air-coupled fiber classification device was used. First, a coarse separation module removed low-polymerization degree fibers (below 200), and then a fine separation module removed high-polymerization degree fibers (above 1500). The DP distribution of the obtained cellulose after classification was mainly concentrated in the range of 600-1200.

[0022] (2) Synergistic treatment of cold alkali swelling and pulsed electric field The screened slurry was prepared to a concentration of 30 wt.% and dispersed in a 6 wt.% sodium hydroxide solution at a slurry-to-liquid mass ratio of 1:10. Cold alkali swelling treatment was performed at 25 °C for 45 minutes, while an intermittent pulsed electric field was simultaneously applied with the following parameters: frequency 100 Hz, field strength 1.2 kV / cm, and on / off cycle 10 seconds. The treatment time was consistent with the swelling process (total 45 minutes). This synergistic treatment significantly disrupted the hydrogen bond network in the cellulose microfiber structure, improving the swelling uniformity of cellulose segments and the permeability of subsequent enzymatic hydrolysis reactions.

[0023] (3) Selective degradation by a complex enzyme system of telase and exonuclease Adjust the pH to 5.5, and add an immobilized complex enzyme system consisting of endo-1,4-β-glucanase and exo-1,4-β-glucanase in a 2:1 mass ratio, using 30 U / g of dry pulp. The enzymatic hydrolysis reaction was carried out at 40 °C for 20 minutes, with stirring to keep the pulp in suspension during the reaction.

[0024] (4) Neutralization and washing of slurry After the enzymatic hydrolysis reaction is completed, a neutralization process is performed using an appropriate amount of sodium hydroxide solution (0.1 mol / L) to adjust the pH of the pulp to approximately 7.0. Then, residual enzymes, sodium ions, short-chain fragments, and other impurities are thoroughly removed using a conventional pulping process to finally obtain dissolved pulp.

[0025] (5) Analysis and performance The degree of polymerization (DOP) of the cellulose in the dissolving pulp product obtained in Example 1 was concentrated in the range of 780–1050, reflecting more focused control over the DOP. The following characterization data further validated the treatment effect: Lyocell fiber breaking strength increased from 3.5 cN / dtex to 3.9 cN / dtex, an increase of approximately 12% compared to the original pulp, indicating that the DOP distribution was more suitable for the spun fiber structure. The time required for complete dissolution in NMMO (N-methylmorpholine oxide) solution was reduced from 28 minutes to 23 minutes, a reduction of approximately 18%, indicating enhanced reactivity of the treated dissolving pulp and faster swelling and molecular chain unfolding. The viscosity fluctuation coefficient (CV) of the dissolving system decreased from 12.6% to 6.4%, indicating a more stable processing process suitable for high-end Lyocell fiber continuous spinning systems.

[0026] Example 2 A method for preparing a dissolving slurry with a uniform degree of polymerization of cellulose, employing a low-temperature, high-electric-field enhancement mode, includes the following steps: (1) Pretreatment and grading of raw material slurry Hardwood pulp was selected as raw material and prebleached dissolving pulp was prepared by acid pre-hydrolysis and alkaline cooking. A two-stage air classifier was used to remove fiber components with a degree of polymerization of less than 150 and greater than 1500, so that the degree of polymerization of cellulose was mainly concentrated in the range of 650~1200.

[0027] (2) Synergistic treatment of cold alkali swelling and pulsed electric field The slurry was dispersed at a concentration of 3 wt.% in an 8 wt.% NaOH solution at a slurry-to-liquid mass ratio of 1:10 and swelled for 60 minutes under cooling conditions at 15 °C. An intermittent pulsed electric field (120 Hz, 1.8 kV / cm, 8-second on / off) was applied for 60 minutes.

[0028] (3) Selective degradation by a complex enzyme system of telase and exonuclease Adjust the pH to 5.5, add an immobilized complex enzyme system consisting of end-cleavage enzyme and exonuclease in a mass ratio of 2:1, with an enzyme amount of 40 U / g dry pulp, react at 35 ℃ for 15 minutes.

[0029] (4) Neutralization and washing of slurry After enzymatic hydrolysis, the pulp is neutralized with dilute acid to control the pH of the hydrolysate to approximately 7.0. Then, a conventional washing process is used to thoroughly remove residual enzymes, sodium ions, short-chain fragments, and other impurities, yielding the dissolved pulp product.

[0030] (5) Analysis and performance The degree of polymerization of cellulose in the dissolving pulp product obtained in Example 2 was more concentrated in the range of 820-980. The dissolution time of the treated pulp in NMMO (N-methylmorpholine oxide) solution was shortened by 50%, and the viscosity fluctuation of the system was reduced by 30%. Specifically, the dissolution time of the control (the original pulp without this process) in NMMO solution was 30 minutes, while the dissolution time of the treated dissolving pulp was shortened to 15 minutes.

[0031] Example 3 A method for preparing a dissolving pulp with a uniform degree of polymerization of cellulose, employing a medium-temperature rapid processing technology, includes the following steps: (1) Pretreatment and grading of raw material slurry Hardwood pulp was selected as raw material and prebleached dissolving pulp was prepared by acid pre-hydrolysis and alkaline cooking. The degree of polymerization of the original cellulose ranged from 150 to 2800. A continuous cyclone classification system was used to remove fiber components with a degree of polymerization of less than 200 mm and greater than 1500 mm, resulting in cellulose components with a degree of polymerization mainly concentrated in the range of 700 to 1200.

[0032] (2) Synergistic treatment of cold alkali swelling and pulsed electric field The slurry was dispersed in a 5 wt.% NaOH solution at a concentration of 30 wt.% with a solid-liquid ratio of 1:10, and swelled for 30 minutes under cooling conditions at 30 °C. An intermittent pulsed electric field (80 Hz, 1.0 kV / cm, 15-second on / off) was applied for 30 minutes.

[0033] (3) Selective degradation by a complex enzyme system of telase and exonuclease Adjust the pH to 5.5, add an immobilized complex enzyme system consisting of end-cleavage enzyme and exonuclease in a mass ratio of 2:1, with an enzyme amount of 25 U / g dry pulp, react at 50 ℃ for 10 minutes.

[0034] (4) Neutralization and washing of slurry After enzymatic hydrolysis, the pulp is neutralized with alkali to control the pH of the hydrolysate to approximately 7.0. Then, a conventional washing process is used to thoroughly remove residual enzymes, sodium ions, short-chain fragments, and other impurities, yielding the dissolved pulp product.

[0035] (5) Analysis and performance The degree of polymerization of cellulose in the dissolving pulp product obtained in Example 3 was more concentrated in the range of 750-1100. The processing time was reduced by 33% compared to Example 2, making it suitable for matching the pace of continuous industrial production. Verification showed that the obtained pulp exhibited good viscosity control during wet spinning, a breakage rate of less than 0.8 times / km, and significantly reduced batch-to-batch fiber variability.

[0036] Examples 1-3 systematically demonstrate the adaptability and controllability of the dissolving slurry preparation method provided by this invention under different process parameters, further verifying its feasibility and flexibility in industrial applications. A comparison of parameters and results for Examples 1-3 is shown in Table 1 below: Table 1 - Parameters and Results of Examples 1-3

[0037] By comparing the dissolution time and viscosity fluctuations before and after the treatment, the optimization effect of this process on the degree of polymerization distribution of cellulose and its improvement effect on subsequent processing performance were verified.

[0038] Although the present invention has been described in detail by way of preferred embodiments, the present invention is not limited thereto. Various equivalent modifications or substitutions can be made to the embodiments of the present invention by those skilled in the art without departing from the spirit and essence of the invention, and such modifications or substitutions should all be within the scope of the present invention. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should also be covered within the protection scope of the present invention.

Claims

1. A method for preparing a dissolving pulp with a uniform degree of polymerization of cellulose, characterized in that, Includes the following steps: (1) Pretreatment and grading of dissolving slurry; (2) Synergistic treatment of cold alkali swelling and pulsed electric field; (3) Selective degradation of slurry by a complex enzyme system of end-cleavage enzymes and exonucleases; (4) Neutralization and washing of slurry.

2. The preparation method according to claim 1, characterized in that, Step (1) is as follows: The initially prepared dissolving slurry is fed into a mechanical classification device for two-stage screening. The first stage uses a coarse screener to remove cellulose with a degree of polymerization <200. The second stage uses a fine separation module to remove cellulose with a degree of polymerization >1800.

3. The preparation method according to claim 2, characterized in that, The mechanical grading device is a centrifugal gas-solid coupling fiber grading device.

4. The preparation method according to claim 1, characterized in that, In step (2), the cold alkali swelling conditions are as follows: the mass fraction of sodium hydroxide solution is 5%~8%, the swelling temperature is 10℃~30℃, and the treatment time is 30~60 minutes; and a pulsed electric field with a frequency of 80~120 Hz and a voltage intensity of 1~1.8 kV / cm is applied simultaneously.

5. The preparation method according to claim 4, characterized in that, The pulsed electric field adopts an intermittent on-off pulse mode with an on-off period of 8 to 15 seconds, and is carried out synchronously with the cold alkali swelling process.

6. The preparation method according to claim 1, characterized in that, In step (3), the complex enzyme system is formed by combining end-cleavage enzyme and exonuclease in a mass ratio of 2:1 and then immobilizing them to form an immobilized enzyme capsule system; the enzyme loading is 25~40 U / g based on the dry weight of the slurry, the reaction conditions are 35 ℃~50 ℃, pH=5.5, and the enzymatic hydrolysis time is 10~20 minutes.

7. The preparation method according to claim 6, characterized in that, The immobilized enzyme capsule system uses chitosan or sodium alginate as a carrier and is prepared into microcapsule form.

8. The preparation method according to claim 1, characterized in that, In step (4): the neutralization is achieved by adjusting the slurry to pH=7.0 with alkali.

9. The preparation method according to claim 8, characterized in that, After step (4) is completed, the following steps are included: online monitoring and feedback control of the degree of polymerization of the dissolved slurry. The online monitoring adopts the online degree of polymerization analysis module.

10. The preparation method according to claim 9, characterized in that, The online degree of polymerization analysis module uses small-angle light scattering, capillary viscosity or electrophoresis technology to obtain cellulose degree of polymerization distribution data in real time and accurately.

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