CN120700730A - A process for preparing waterless paper pulp from wheat and rice straw based on waste gas resource recycling - Google Patents
A process for preparing waterless paper pulp from wheat and rice straw based on waste gas resource recyclingInfo
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- CN120700730A CN120700730A CN202510756722.8A CN202510756722A CN120700730A CN 120700730 A CN120700730 A CN 120700730A CN 202510756722 A CN202510756722 A CN 202510756722A CN 120700730 A CN120700730 A CN 120700730A
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Abstract
The invention relates to the technical field of pulp preparation, and discloses a preparation process of wheat and rice straw anhydrous pulp based on waste gas resource recycling, which comprises the following steps of S1, crushing wheat and rice straw and drying and pretreatment of waste gas waste heat; S2, separating and purifying carbon dioxide gas in waste gas, S3, preparing a carbon dioxide activated deep eutectic solvent, S4, mixing pretreated wheat and rice straw with the activated deep eutectic solvent and reacting under the driving of waste gas waste heat, S5, separating gas and liquid to recover carbon dioxide and the deep eutectic solvent, separating solid-phase fiber slurry from lignin-containing liquid phase, S6, refining the fiber slurry and forming paper pulp. Through the collaborative design of cascade waste gas waste heat drying and reaction heating and CO 2 activation of a deep eutectic solvent system, the high-yield extraction of straw fibers, the high-selectivity dissolution of lignin and the efficient cyclic utilization of solvents and carbon sources are realized, the energy consumption and byproduct residues are obviously reduced, and the pulp quality and the overall process economy are improved.
Description
Technical Field
The invention relates to the technical field of pulp preparation, in particular to a preparation process of wheat and rice straw anhydrous pulp based on waste gas resource recycling.
Background
As renewable resources, the green and high efficiency of pulping process is key to transformation and upgrading in paper industry. Is an important research object for replacing raw materials of pulp in recent years. In the existing straw pulp preparation process, an acid-base method, a mechanical method or an organic solvent method is mainly used, and fiber separation is realized by removing lignin and impurities. However, the conventional technology path generally depends on high temperature, strong acid or electric heating technology, so that not only is the energy consumption high, but also the fiber structure is easily damaged, and the pulp quality is limited.
In practical application, the straw fiber extraction process often relies on a high-temperature electric heating mode to carry out drying and reaction operations. However, the electric heating has low thermal efficiency and large temperature fluctuation, and especially under the continuous high-temperature condition, the degradation of cellulose chains is easy to be initiated, the integrity of fiber structure is affected, meanwhile, the excessive crosslinking and oxidation of lignin are also aggravated, insoluble components in pulp are increased, and the final pulp performance is reduced. The heat source mode has heavy energy consumption burden, causes non-negligible carbon emission, and obviously has contradiction with the recycling target of resources.
In addition, in the dissociation process of the lignin and cellulose composite structure in the straw, a solvent system (such as strong acid or salt mixed solution) which is conventionally used lacks the synergism on the molecular level, so that the selective dissolution and the reaction safety are difficult to be considered, a large amount of inorganic salt residues and metal ions are often accumulated, and the post-treatment burden is increased. In particular, in terms of metal impurity migration, fiber dispersion control, and lignin selective removal, the prior art has not yet formed a stable and efficient solvent design mechanism. In addition, due to the lack of an effective activation strategy, the effect of the reaction system on lignin is still limited to physical permeation or simple acidification, resulting in unstable lignin dissolution rate, further affecting pulp quality.
On the other hand, in the aspect of recycling the solvent and the reaction medium, a multistage recycling and separating mechanism is generally lacking in the current straw pulp production process. Especially when separating fiber slurry and reaction liquid, a large amount of liquid phase containing active components is directly abandoned, thereby not only causing resource waste, but also improving operation cost and environmental protection pressure. The single centrifugal or sedimentation mode is difficult to separate the residual solvent and CO 2 from the reaction mixed system, so that the residual quantity in the product is higher, and the pulp performance and the subsequent application safety are seriously affected
Therefore, the invention provides a preparation process of wheat and rice straw anhydrous paper pulp based on waste gas resource recycling, which solves the defects in the prior art.
Disclosure of Invention
Aiming at the defects of the prior art, the invention provides a preparation process of wheat and rice straw anhydrous paper pulp based on waste gas resource recycling, which solves the problems of high energy consumption, more solvent residues, low fiber yield and insufficient resource recovery in the existing preparation process of straw paper pulp.
The invention aims at realizing the technical scheme that the preparation process of the wheat and rice straw anhydrous paper pulp based on waste gas resource recycling comprises the following steps:
s1, crushing wheat and rice straw and drying and preprocessing waste heat of waste gas;
s2, separating and purifying carbon dioxide gas in the waste gas;
S3, preparing a carbon dioxide activated deep eutectic solvent;
s4, mixing the pretreated wheat and rice straws with an activated deep eutectic solvent, and reacting under the driving of waste gas waste heat;
s5, separating and recycling carbon dioxide and a deep eutectic solvent by gas-liquid separation, and separating solid-phase fiber slurry from a lignin-containing liquid phase;
s6, refining the fiber slurry and forming paper pulp.
Preferably, the step of crushing wheat and rice straw and drying and preprocessing waste heat of waste gas comprises the following steps:
crushing wheat and rice straw to a grain size of 1-2mm, and removing impurities through a vibrating screen;
and (5) drying the crushed straw by waste heat of waste gas.
Preferably, the waste heat drying of the waste gas comprises two stages of treatment:
the drying temperature of the primary treatment is 100-120 ℃ and the drying time is 10-15 minutes;
the softening temperature of the secondary treatment is 80-100 ℃ and the softening time is 20-30 minutes.
Preferably, the step of separating and purifying the carbon dioxide gas in the exhaust gas includes:
introducing the waste gas into a sodium hydroxide solution absorption tower with the mass concentration of 5-10% for desulfurization treatment, so that the sulfur dioxide content is reduced to 0.005-0.01%;
pressurizing the desulfurized waste gas to 0.3-0.5MPa by a compression separation device, separating out carbon dioxide gas and storing, wherein the concentration of the carbon dioxide gas is 10-15%;
The temperature of the separation process is controlled to be 150-200 ℃ by using waste heat of the waste gas.
Preferably, the step of preparing a carbon dioxide activated deep eutectic solvent comprises:
mixing quaternary ammonium salt compound with alpha-hydroxycarboxylic acid organic acid, and stirring at 50-70 ℃ for 1-2 hours to form a homogeneous deep eutectic solvent;
introducing the separated and purified carbon dioxide gas into deep eutectic solvent at the pressure of 0.3-0.5MPa and the speed of 10-20L/min, and activating for 20-40 min, wherein the pH value of the activated solvent is 5.0-6.0.
Preferably, the quaternary ammonium salt compound comprises 30-40 parts by weight, the alpha-hydroxycarboxylic acid organic acid comprises 60-70 parts by weight, the quaternary ammonium salt compound is choline chloride or betaine, and the alpha-hydroxycarboxylic acid organic acid is lactic acid or malic acid.
Preferably, the step of mixing the pretreated wheat and rice straw with the activated deep eutectic solvent and reacting under the driving of waste gas waste heat comprises the following steps:
mixing the pretreated wheat and rice straw and the activated deep eutectic solvent according to the solid-liquid mass ratio of 1:6-1:10, and carrying out sectional temperature control reaction at 80-100 ℃;
The first stage is carried out for 1.5-2.5 hours at 80-90 ℃, and the second stage is carried out for 0.5-1.5 hours at 90-100 ℃;
maintaining the temperature through an exhaust gas waste heat exchanger in the reaction process, and inputting the exhaust gas waste heat into the reactor at 150-200 ℃;
the stirring speed is controlled to be 200-300 r/min, and the pressure of the reaction system is normal pressure.
Preferably, the step of separating and recovering carbon dioxide and deep eutectic solvent from the gas-liquid and separating the fiber slurry from lignin comprises the following steps:
decompressing the mixed system of the wheat and rice straw and the activated deep eutectic solvent in the step S4 to normal pressure, and recovering carbon dioxide gas through a flash evaporation device to obtain fiber slurry with a solid-to-liquid ratio of 1:4-1:6;
Inputting the fiber slurry into a gas-liquid centrifuge, and centrifugally separating for 10-20 minutes at 3000-5000 rpm to obtain the following components:
A solid phase fiber slurry;
And concentrating the liquid phase containing the deep eutectic solvent and the lignin dissolved by a vacuum thin film evaporator at 60-80 ℃ and minus 0.08-0.10 MPa, and recovering the deep eutectic solvent.
Preferably, the step of refining the fiber and forming the pulp comprises the steps of:
Refining the separated solid-phase fiber slurry in a disc grinder, adjusting the disc grinding gap to 0.05-0.10 mm, controlling the beating degree to 30-40 DEG SR, and refining for 20-40 minutes;
Delivering the refined fiber slurry to a forming machine, and pressing and forming under the pressure of 0.1-0.3MPa, wherein the quantitative pulp is 60-80 g/square meter;
And drying the molded paper pulp by a hot air drying box at 105-120 ℃ until the water content is 5-8%.
The invention provides a preparation process of wheat and rice straw anhydrous paper pulp based on waste gas resource recycling. The beneficial effects are as follows:
1. The invention adopts the cascade recovery strategy of waste gas waste heat grading drying combined reaction heat supply, thereby achieving the effects of reducing energy consumption and stabilizing reaction temperature. Compared with the scheme of directly drying the straw by adopting an electric heating single mode in the prior art, the problems of high energy consumption and easily damaged fiber structure are solved, and the fiber yield and the mechanical property of the slurry are obviously improved.
2. The invention realizes the dual functions of high-efficiency lignin dissolution and metal impurity removal by adopting a deep eutectic solvent system activated in situ by CO 2. Compared with the traditional method using a non-synergistic solvent system (such as sodium chloride and oxalic acid), the method solves the problems of poor selectivity and insufficient fiber dissociation, improves the fiber dispersity and reduces the slurry residue.
3. The invention realizes effective regulation and control of reaction paths and byproduct generation by introducing a sectional temperature control reaction mode (low-temperature bond breaking-high-temperature dissolution). Compared with the existing single-temperature-zone constant-temperature treatment technology, the problems of fiber embrittlement and byproduct accumulation caused by heat accumulation are solved, and the flexibility and reaction safety of paper pulp are remarkably improved.
4. The invention realizes the balance of high recovery rate and low residue by adopting the flash evaporation-centrifugation-evaporation three-stage solvent and CO 2 recovery process. Compared with the technical path for separating the slurry only through single centrifugation, the method solves the difficulties of solvent coating, resource loss and high byproduct residue, and realizes synchronous improvement of slurry uniformity and fiber yield.
Drawings
FIG. 1 is a flow chart of the preparation process of the invention.
Detailed Description
The following description of the embodiments of the present invention will be made clearly and fully with reference to the accompanying drawings, in which it is evident that the embodiments described are only some, but not all embodiments of the invention. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.
Choline chloride (Choline Chloride) suppliers in the following examples, comparative examples and experiments were set forth in Nanjing Han Biotechnology Co., ltd;
the lactic acid is food grade lactic acid (LACTIC ACID), the model is WTL-085, the supplier is Shandong Dongming amino acid Co., ltd;
the Betaine is BETA-98, and is purchased from Jiangsu Huaxing biochemical engineering Co., ltd;
The malic acid is food-grade malic acid (MALIC ACID), the model is MAL-001, and the manufacturer is Shandong Kefeng Biotechnology Co., ltd;
The quaternary ammonium salt compound and the alpha-hydroxycarboxylic acid organic acid are calculated according to parts by mass.
The foregoing description will be described in some detail with reference to specific embodiments for the purpose of providing a better understanding of the invention.
Referring to fig. 1, the invention provides a preparation process of wheat and rice straw anhydrous paper pulp based on waste gas resource recycling, which comprises the following steps:
s1, crushing wheat and rice straw and drying and preprocessing waste heat of waste gas;
s2, separating and purifying carbon dioxide gas in the waste gas;
S3, preparing a carbon dioxide activated deep eutectic solvent;
s4, mixing the pretreated wheat and rice straws with an activated deep eutectic solvent, and reacting under the driving of waste gas waste heat;
s5, separating and recycling carbon dioxide and a deep eutectic solvent by gas-liquid separation, and separating solid-phase fiber slurry from a lignin-containing liquid phase;
s6, refining the fiber slurry and forming paper pulp.
The steps of the method of the present invention will be described in detail below.
For the step S1, the wheat and rice straw is crushed to the grain size of 1-2mm, and two-stage drying (primary 100-120 ℃ dehydration and secondary 80-100 ℃ softening) is carried out through waste heat of waste gas. The specific surface area of the crushed straw is increased by 50-80%, the subsequent solvent permeation efficiency is obviously improved, the waste heat of waste gas replaces the traditional electric heating, and the energy consumption is reduced by more than 30%. The secondary softening stage avoids excessive crystallization of the fiber through temperature control, so that the lignin-cellulose composite structure is loose, and active sites are provided for subsequent solvent reaction.
The cascade utilization of waste gas and the cooperative regulation and control of straw structure can achieve the effects of energy conservation and raw material activation.
And for the step S2, the industrial waste gas is desulfurized by sodium hydroxide (the desulfurization rate is more than or equal to 95 percent), and then is compressed to 0.3-0.5MPa to separate the carbon dioxide gas with the concentration of 10-15 percent. The desulfurization process eliminates the poisoning effect of sulfide on the solvent through a neutralization reaction (SO 2+2NaOH→Na2SO3+H2 O), and the compression separation utilizes the characteristic that the critical temperature (31.1 ℃) of CO 2 is higher than N 2 (-147 ℃) to realize high-efficiency enrichment under the temperature control of 150-200 ℃ waste heat.
CO 2 is directionally captured from the low-concentration waste gas, a low-cost carbon source is provided for solvent activation, and a 'waste treatment with waste' circulation mode is formed.
For step S3, choline chloride (or betaine, 30-40 parts by mass) is mixed with lactic acid (or malic acid, 60-70 parts by mass) at 50-70 ℃ to form a deep eutectic solvent. The formation of the deep eutectic solvent is due to the interaction between quaternary ammonium compounds (e.g., choline chloride or betaine) and alpha-hydroxycarboxylic acid organic acids (e.g., lactic acid and malic acid), thereby lowering the melting point and enhancing the solvency.
Then, carbon dioxide gas (0.3-0.5 MPa, for 20-40 min) was introduced to dissolve in the deep eutectic solvent. The CO 2 is dissolved to form bicarbonate ions, the pH value of the solvent is obviously reduced, and the pH value is stabilized between 5.0 and 6.0. The process enhances the hydrogen bond acting force and protonation dissolution capacity of the solvent on the lignin, and promotes the dissolution rate of the lignin to be improved by 15-20%.
The carbon dioxide in-situ acidification is adopted, so that the acid reagent can be replaced, the introduction of ion pollution is avoided, the solvent cost can be reduced by more than 40%, and the economic efficiency is improved. By optimizing the solvent formula and the activation mode, the invention obviously improves the dissolution efficiency of lignin and promotes the high-value utilization of straw resources.
For the step S4, mixing the straw and the activating solvent according to the solid-liquid ratio of 1:6-1:10, and carrying out two-stage reaction (80-90 ℃ C./1.5-2.5 hours- & gt 90-100 ℃ C. & gt/0.5-1.5 hours) under the driving of waste gas waste heat. The first stage of low temperature breaking ether bond and ester bond in LCC to release cellulose, and the second stage of high temperature promoting lignin beta-O-4 bond to break and dissolve. The waste heat of the waste gas is directly supplied by a heat exchanger, the temperature fluctuation is less than or equal to +/-2 ℃, and the energy is saved by 40% compared with the traditional heating.
The lignin-cellulose dissociation kinetics are matched by stage temperature, so that high fiber yield (85-92%) and high lignin purity (precipitation rate 80-90%) are realized.
For the step S5, CO 2 (circulation rate 90-95%) is recovered by flash evaporation of the mixed system after the reaction, and then solid-phase fiber slurry (yield 85-92%) and lignin-containing liquid phase are separated by gas-liquid centrifugation (3000-5000 rpm, 10-20 minutes). The flash evaporation utilizes pressure dip to promote CO 2 to be gasified, so that collapse of fiber structure is avoided, and the fiber and lignin-solvent mixed solution is separated by centrifugation based on density difference. The liquid phase is evaporated by vacuum film (60-80 ℃ C., -0.08 to-0.10 MPa) to recover the solvent (recovery rate is 95-98%).
The flash evaporation-centrifugal-evaporation three-stage separation technology has the solvent loss rate less than or equal to 2 percent and the closed cycle utilization rate of CO 2 and the solvent more than or equal to 90 percent.
For step S6, the fiber slurry is refined by a disc mill (gap 0.05-0.10mm, freeness 30-40 DEG SR), then pressed to form (0.1-0.3 MPa), and dried at 105-120 ℃ to a water content of 5-8%. The disc mill exposes fiber hydroxyl groups through shearing force, enhances the hydrogen bond force (the tensile strength of paper pulp is improved by 20-25%), and realizes heat energy multiplexing by matching the drying temperature with waste heat of waste gas.
The mechanical-physical co-molding process without chemical bleaching agent has the pulp ration of 60-80g/m < 2 >, which accords with the paper standard for culture (GB/T8939-2018).
Example 1:
Raw materials:
the wheat and rice straw used is collected from a local farmland, and the moisture content is 20%.
The steps are as follows:
S1, pretreatment:
Crushing the collected wheat and rice straw to a grain size of 1.0mm, and ensuring uniformity. And then removing impurities through a vibrating screen to realize clean raw materials. Then, drying treatment is carried out under the action of waste heat of industrial waste gas, the primary drying temperature is set to 120 ℃ and the duration time is set to 10 minutes, and the secondary drying temperature is set to 100 ℃ and the duration time is set to 30 minutes, so that moisture is removed and lignin is softened.
S2, CO 2 separation:
The desulfurized waste gas enters an absorption tower of sodium hydroxide solution with the mass concentration of 5% for gas treatment. At this time, sulfur dioxide (SO 2) is effectively removed through a neutralization reaction, and the content of the treated waste gas is ensured to be less than or equal to 0.005 percent. And then, the desulfurized waste gas is lifted to 0.3MPa, CO 2 is pressurized and separated, the target concentration is 10%, and the temperature of 150 ℃ is kept by controlling the temperature, so that the subsequent steps are facilitated.
S3, activating a solvent:
Choline chloride (30 parts by mass) and lactic acid (70 parts by mass) were mixed at 50 ℃ and stirred for 1 hour, ensuring the formation of a uniform deep eutectic solvent. Subsequently, the separated and enriched CO 2 was introduced into the solvent at a rate of 10L/min at a pressure of 0.3MPa, the activation process was continued for 20 minutes, and the pH was ensured to be stabilized at 5.0, enhancing the dissolution effect on lignin.
S4, reaction:
Mixing the pretreated wheat and rice straw with a deep eutectic solvent according to a solid-to-liquid ratio of 1:6, and placing the mixture into a reactor. The reaction was first allowed to continue at 80 ℃ for 2.5 hours with the support of a temperature control system, then lifted to 90 ℃ for 1.5 hours with stirring at 200 rpm. Meanwhile, waste heat of waste gas is input into the reactor, and the input temperature is controlled at 150 ℃ so as to improve the reaction efficiency.
S5, separating:
After the reaction is completed, the mixed system is subjected to reduced pressure flash evaporation, CO 2 is recovered, and the recovery rate reaches 90%. Then, the mixture is treated by a centrifuge, the centrifugation speed of 3000 r/min is adopted, the centrifugation time is 15 min, the solid-phase fiber slurry is separated, and the fiber yield reaches 85%. The lignin and deep eutectic solvent in the liquid phase are then recovered by a vacuum thin film evaporator, wherein the recovery of solvent is 95%.
S6, forming:
And (3) inputting the separated solid-phase fiber slurry into a disc mill for fine processing, adjusting the disc mill gap to 0.05mm, controlling the beating degree to be 30 DEG SR, and keeping the refining time at 20 minutes. Finally, the refined fiber slurry is sent into a forming machine to be pressed and formed under the pressure of 0.1MPa, the quantitative amount of paper pulp is 60g/m2, and the formed paper pulp is dried under 105 ℃ by a hot air drying box, and the water content is controlled to be 5%.
Example 2:
Raw materials:
the wheat and rice straw used was the same as in the first example with a moisture content of 20%.
The steps are as follows:
S1, pretreatment:
the straw is crushed to be 1.5mm thick, and impurities are cleaned by a vibrating screen, so that the quality of the raw materials is ensured. And then drying by using the waste heat of industrial waste gas, wherein the primary drying temperature is set to be 110 ℃, the duration is 12 minutes, the secondary drying temperature is set to be 90 ℃ and the duration is 25 minutes, so that the water content is reduced, and the subsequent reaction is facilitated.
S2, CO 2 separation:
And (3) introducing 8% sodium hydroxide solution into the wet waste gas to carry out desulfurization treatment, SO that the content of SO 2 is less than or equal to 0.008%. The desulfurized exhaust gas was pressurized to 0.4MPa to separate CO 2 having a concentration of 12%. The temperature is controlled at 180 ℃ to ensure high-efficiency separation.
S3, activating a solvent:
Betaine (35 parts) and malic acid (65 parts) were stirred at 60 ℃ for 1.5 hours, ensuring that a homogeneous mixed deep eutectic solvent was formed. And then introducing the separated CO 2 into the mixed solvent at a flow rate of 15L/min under a pressure of 0.4MPa, activating for 30 minutes, and regulating the pH value to 5.5.
S4, reaction:
adding the treated wheat and rice straw and the activated deep eutectic solvent into a reactor according to a solid-to-liquid ratio of 1:8. The initial reaction stage was continued at 85 ℃ for 2 hours, raised to 95 ℃ for 1 hour, and the stirring rate was maintained at 250 rpm while maintaining the input temperature at 180 ℃ by waste heat of the exhaust gas.
S5, separating:
After the reaction is finished, the enriched CO 2 is recovered by flash evaporation, and the recovery utilization rate reaches 92 percent. The solid phase fiber slurry was then separated at a fiber yield of 89% using a centrifuge at 4000 rpm for 15 minutes. The separation liquid was subjected to vacuum thin film evaporation, and the solvent recovery rate was 96%.
S6, forming:
The solid phase fiber slurry was fed into a disc mill for fine processing, the disc mill gap was adjusted to 0.08mm, the freeness was controlled at 35 ° SR, and the time was set to 30 minutes. Finally, the slurry is pressed and molded by a molding machine under the pressure of 0.2MPa, the quantitative amount of the pulp reaches 70g/m < 2 >, and the water content of the pulp is 6% after drying treatment.
Example 3:
Raw materials:
the wheat and rice straw used was the same as in the first example with a moisture content of 20%.
The steps are as follows:
S1, pretreatment:
Crushing wheat and rice straw to a grain size of 2.0mm, and removing impurities through a vibrating screen to ensure the cleanliness of the treated raw materials. Drying treatment is carried out with the aid of waste heat of industrial waste gas, wherein the temperature is set to be 100 ℃ in the first stage for 15 minutes, and the temperature is set to be 80 ℃ in the second stage for 20 minutes, so that the required moisture standard is achieved.
S2, CO 2 separation:
10% sodium hydroxide solution is adopted to carry out desulfurization treatment on the waste gas, so that the sulfur dioxide content is ensured to be less than 0.01%. After desulfurization is completed, the waste gas is pressurized to 0.5MPa to separate CO 2 with the concentration of 15%, and the temperature is controlled under the condition of 200 ℃, so that the separation efficiency is improved.
S3, activating a solvent:
Choline chloride (40 parts) was mixed with malic acid (60 parts) and stirred at 70 ℃ for 2 hours to form the desired deep eutectic solvent. With the aid of the CO 2 separated above, the solvent was introduced at a pressure of 0.5MPa and at a rate of 20L/min, and the activation process was continued for 40 minutes, ensuring that the pH was maintained at 6.0.
S4, reaction:
In the reactor, the pretreated wheat and rice straw and the activating solvent are mixed according to a solid-to-liquid ratio of 1:10. The reaction was started at 90 ℃ for 1.5 hours, then the temperature was raised to 100 ℃ for 0.5 hours, and the stirring scheme was set to 300 rpm, and the waste heat of the waste gas was input at 200 ℃ to ensure the stability of the reaction temperature.
S5, separating:
After the reaction was completed, CO 2 was recovered by flash evaporation, with a recovery of 95%. Then, the solid-phase fiber slurry was separated by a centrifuge at a speed of 5000 rpm for 10 minutes, and the yield reached 92%. Thereafter, the liquid phase was concentrated by a vacuum thin film evaporator, and the solvent recovery rate was as high as 98%.
S6, forming:
the solid phase fiber slurry was fed into a disc mill, the gap was adjusted to 0.10mm, the freeness was controlled at 40 ° SR, and the refining time was 30 minutes. Finally, the pulp is pressed and molded by a molding machine, the quantitative content of the pulp reaches 80g/m < 2 >, and the water content after drying is 8%.
Example 4:
Raw materials:
the wheat and rice straw used was the same as in the first example with a moisture content of 20%.
The steps are as follows:
S1, pretreatment:
Crushing wheat and rice straw to a grain size of 1.2mm, filtering impurities by a vibrating screen, and performing two-stage drying by using waste heat of waste gas. The drying temperature in the first stage was set at 105℃for 13 minutes and in the second stage at 85℃for 22 minutes until the desired moisture was reached.
S2, CO 2 separation:
And 6% sodium hydroxide solution is used for desulfurization treatment, SO that the content of SO 2 is less than or equal to 0.006%. The treated exhaust gas was then pressurized to 0.35MPa to separate 11% CO 2 and optimize the separation at a controlled temperature of 160 ℃.
S3, activating a solvent:
Betaine (32 parts) and lactic acid (68 parts) were stirred at 55 ℃ for 1.2 hours to form a homogeneous solution. Then, the separated CO 2 was introduced into the mixed solvent at a pressure of 0.35MPa and a speed of 12L/min, and activated for 25 minutes, to ensure that the pH was stabilized at 5.2.
S4, reaction:
In the reactor, the pretreated wheat and rice straws and the activated deep eutectic solvent are mixed according to a solid-to-liquid ratio of 1:7. The reaction was first maintained at 82 ℃ for 2.2 hours, then raised to 88 ℃ for 1.2 hours, and the stirring speed was set at 220 rpm while maintaining the input temperature at 160 ℃ by using the waste heat of the exhaust gas.
S5, separating:
CO 2 was recovered by flash distillation technique with 91% recovery. Then, the solid phase fiber slurry was treated with a 3500 rpm centrifuge for 12 minutes, and the yield of the separated solid phase fiber slurry reached 87%. The liquid phase was concentrated by vacuum thin film evaporation with a solvent recovery of 96.5%.
S6, forming:
The solid phase fiber slurry was fed into a disc mill for fine processing with a disc mill gap set at 0.06mm and a freeness controlled at 32 ° SR for refining time of 25 minutes. Finally, the slurry is pressed and molded by a molding machine under the pressure of 0.15MPa, the quantitative amount of the pulp is 65g/m < 2 >, and the water content after drying is 5.5%.
Comparative example 1:
In comparison with example 1, the waste heat drying was omitted, and the electric heating drying was employed, and the classification drying (120 ℃ C. Continuous drying for 40 minutes) was not performed. The rest steps and parameters are the same.
Comparative example 2:
Compared with example 2, the method is characterized in that betaine is replaced by sodium chloride (non-quaternary ammonium salt compound), malic acid is replaced by oxalic acid (non-alpha-hydroxycarboxylic acid organic acid), and the mixing ratio of sodium chloride and oxalic acid is kept at 35:65. The rest steps and parameters are the same.
Comparative example 3:
compared with the embodiment 3, the method is different in that two-stage temperature control is canceled in the step S4, and the whole process adopts a 90 ℃ single temperature zone for reaction for 4 hours.
Comparative example 4:
In comparison with example 4, the difference is that the flash evaporation and vacuum thin film evaporation steps are eliminated in step S5, the solid phase fiber slurry is separated by only a single centrifugation (3000 rpm, 15 minutes), and the liquid phase (containing lignin and solvent) is discarded. The rest steps and parameters are the same.
Experiment 1:
The experimental steps are as follows:
Sample preparation two sets of slurries were prepared, one set prepared according to the procedure and parameters of example 1 and the other set prepared according to the procedure and parameters of comparative example 1, and the two sets of slurries were equilibrated under the same conditions (temperature 25 ℃, humidity 50%) for 24 hours for subsequent testing.
And (3) testing:
Fiber yield 10g of dried fiber slurry was taken for each group of samples, dried to constant weight at 105 ℃, and weighed to calculate yield (%).
Mechanical properties of pulp Each set of samples produced a sheet with a basis weight of 60g/m2, and tensile strength (kN/m) was measured according to ISO 1924-2.
Solvent residue detection 5g of fiber slurry was taken for each group of samples, extracted with ultrapure water for 24 hours, and the residual amount (ppm) of quaternary ammonium salt in the liquid phase was detected by ion chromatography (ICS-5000).
Recording the fiber yield, tensile strength and quaternary ammonium salt residue.
Experimental data are shown in table 1:
TABLE 1 comparison of waste gas Heat and electric heating Process end product Performance
The sectional temperature control strategy of waste gas waste heat step drying obviously protects the natural integrity of the fiber structure. The fiber yield of example 1 is as high as 91.3%, which is 16.3% higher than that of comparative example 1. The fiber break was reduced and hemicellulose retention was more adequate, directly translating into a jump in tensile strength (2.15 kN/mvs1.47 kN/m). The complete cross-linking of the molecular chains gives the pulp a higher mechanical toughness.
The rough high temperature of single-stage electric heating not only damages the fiber length, but also causes lignin to be excessively oxidized. Residual moisture (implicit in yield differences) synergistically compromises solvent permeation efficiency with hardened lignin. The quaternary ammonium salt residue of comparative example 1 rose violently to 89.4ppm, exposing the complete failure of its recovery system, and the waste of resources and the risk of pollution increased.
Experiment 2:
The experimental steps are as follows:
sample preparation two sets of slurries were prepared, one set according to the procedure and parameters of example 2 and the other set according to the procedure and parameters of comparative example 2, and the two sets were equilibrated under the same conditions (temperature 25 ℃, humidity 50%) for 24 hours for subsequent testing.
And (3) testing:
Metal ion residue 5g of dry fiber slurry was taken per group of samples, ashed (550 ℃ C., 4 hours), acid dissolved (10% HNO 3), and the total content of K +、Ca2+、Fe3+ was measured by ICP-OES.
Fiber dispersion degree Each group of samples was diluted to 0.5% concentration with slurry, magnetically stirred for 30 minutes, and the suspension was measured for D90 particle size with a laser particle sizer (Malvern Mastersizer3000,3000).
Lignin residual ratio 5g of slurry was taken from each group of samples, hydrolyzed with 72% sulfuric acid for 2 hours, and the precipitate was dried and weighed after centrifugation, and the acid-insoluble lignin ratio (%) was calculated.
Slurry viscosity the slurry viscosity (mpa·s) was determined for each set of samples using a rotational viscometer (Brookfield DV 2T), spindle No. 3, at 60rpm,25 ℃.
The total K +、Ca2+、Fe3+ content, fiber D90, lignin residue, slurry viscosity were recorded.
Experimental data are shown in table 2:
TABLE 2 comparison of different solvent systems for fiber dissociation and metal removal
The synergistic effect of the quaternary ammonium salt compound (betaine) and the alpha-hydroxycarboxylic acid organic acid (malic acid) obviously improves the metal removal efficiency. The quaternary ammonium cation of betaine displaces the fiber surface bound K +、Ca2+ by electrostatic action, while the alpha-hydroxyl group of malic acid forms a stable complex with the carboxylic acid group, locking Fe 3+ synchronously (residual only 0.5 ppm). The sodium chloride (not quaternary ammonium salt) and oxalic acid (not alpha-hydroxycarboxylic acid) of comparative example 2 lack a synergistic mechanism, and the metal removal rate is reduced by more than 40%.
Optimization of the fiber dispersion (d90=37.2 μm) and the slurry viscosity (218 mpa·s) results from the low viscosity characteristics of the quaternary ammonium salt-carboxylic acid system, facilitating solvent penetration into the fiber micropores. Whereas the fiber agglomeration of comparative example 2 (d90=124.7 μm) resulted in a 3-fold increase in viscosity and an increase in energy consumption.
The difference in lignin residual rate (2.3% vs 8.9%) further shows the solvent selectivity that betaine preferentially dissolves lignin phenolic hydroxyl structures, while oxalic acid strongly breaks cellulose crystallization areas, which in turn causes lignin to encapsulate fibers. This result demonstrates the decisive effect of molecular design of quaternary ammonium salt and alpha-hydroxycarboxylic acid on the fiber separation efficiency.
Experiment 3:
The experimental steps are as follows:
sample preparation two sets of slurries were prepared, one set according to the procedure and parameters of example 3 and the other set according to the procedure and parameters of comparative example 3, and the two sets were equilibrated under the same conditions (temperature 25 ℃, humidity 50%) for 24 hours for subsequent testing.
And (3) testing:
Solvent residue detection, namely taking 5g of dry slurry from each group of samples, separating the residual solvent by a supercritical CO 2 extraction method, and quantitatively analyzing (mg/kg) by gas chromatography (GC-2010).
By-product formation amount the reaction waste liquid was collected for each group of samples, diluted 20 times, and then the formation amount (ppm) of furfural (Fur) was measured by HPLC (Agilent 1260).
Fiber flexibility-60 g/m2 of paper was made from each set of sample slurries, and elongation at break (%) was measured according to ISO 1924-3.
Recording solvent residue (mg/kg), furfural production (ppm), elongation at break (%)
Experimental data are shown in table 3:
TABLE 3 comparison of the comprehensive Properties of the final product of the stage temperature control and Single temperature zone reaction
The sectional temperature control process obviously reduces solvent residues and byproducts. The solvent residue (83 mg/kg) of example 3 is 75.6% lower than that of comparative example 3, and the two-stage temperature control (70 ℃ to 110 ℃) reduces the solvent wrapping through low-temperature infiltration, thereby improving the recovery efficiency. Continuous heating at 90 ℃ in a single temperature zone leads to local carbonization of the solvent, and the residual quantity is increased sharply.
The difference in the formation of furfural (9.2 ppmvs46.8 ppm) revealed that the key of reaction path control is that the low-temperature section protected the hemicellulose structure and reduced degradation, while the uniform temperature accelerated the dehydration of the sugars and the by-product fluctuated 5 times.
Elongation at break (3.1% vs 0.9%) verifies fiber integrity protection. The sectional temperature control avoids high temperature continuous damage, the flexibility of the fiber is improved by 244%, and the durability of paper products is directly improved. The data indicate that the temperature gradient design balances the reaction efficiency with the product quality.
Experiment 4:
The experimental steps are as follows:
Sample preparation two sets of slurries were prepared, one set according to the procedure and parameters of example 4 and the other set according to the procedure and parameters of comparative example 4, and the two sets were equilibrated under the same conditions (temperature 25 ℃, humidity 50%) for 24 hours for subsequent testing.
And (3) testing:
Solvent residue 5g of dry slurry was taken for each group of samples, soxhlet extraction with acetone was performed for 12 hours, and GC-MS quantified for solvent residue (mg/kg).
Fiber yield: each set of samples was weighed after solid phase drying and the yield (%) relative to the initial slurry was calculated.
Slurry uniformity Each set of sample slurries was diluted to 1% concentration and the slurry particle size distribution span ((D90-D10)/D50) was determined by laser scatterometer (Malvern Mastersizer 3000,3000).
Byproduct carryover: dry slurry was ashed (550 ℃) for each set of samples, and the total amount of furfural and HMF (ppm) was measured by HPLC after dissolution of the residue.
Recording solvent residue (mg/kg), fiber yield (%), particle size distribution span, total by-product (ppm).
Experimental data are shown in table 4:
TABLE 4 influence of flash evaporation-evaporation combination and centrifugation Process on fiber Properties
The combined flash evaporation-evaporation process significantly optimizes solvent residue and fiber retention. The solvent residue (42 mg/kg) of example 4 was 85.7% lower than that of comparative example 4, indicating that the gradient recovery effectively peeled off the solvent in the fiber pores, whereas the centrifugation process resulted in solvent encapsulation (residue 293 mg/kg) due to liquid phase discard, and the recovery efficiency was low.
The difference in fiber yield (92.3% vs 84.7%) reflects the loss control of the process on the fiber. Flash evaporation-evaporation reduces fiber loss with waste solvent, whereas single centrifugation results in 7.6% fiber loss, directly pushing up raw material costs.
The particle size distribution span (1.6vs 3.4) shows that the slurry of the combined process is more uniform, the fiber dispersibility is improved by 112%, and the subsequent processing is facilitated. The total amount of byproducts (5.8 ppmvs18.2 ppm) further demonstrates the ability of the combined process to inhibit side reactions, reducing toxic residue. The data show that the combination of flash evaporation and evaporation gives consideration to the quality of the fiber and the process economy.
Although embodiments of the present invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made therein without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents.
Claims (9)
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