Method for recovering an alcohol precipitant used in a process for hydrolysis of plant fiber feedstocks to sugar

Abstract

The present application relates to the field of chemical separation, in particular to a method for recovering alcohol precipitants used in the process of hydrolyzing plant fiber raw materials to produce sugar. When C5 and above alcohols are used as precipitants for sugar acid separation, there are problems of high energy consumption for recovering the precipitants and low purity of sugar precipitates. The method of the present application comprises: adding C1-C4 water-soluble low-boiling alcohols into sugar precipitates containing C5 and above alcohol precipitants and sulfuric acid, so that the precipitants and sulfuric acid are redissolved in the alcohols; after solid-liquid separation, the obtained filtrate is distilled to recover the alcohols, and the precipitants containing sulfuric acid are separated out; the sugar precipitates from which the precipitants are removed are mixed with water-soluble sugar and distilled to remove the alcohols; the precipitants containing sulfuric acid are back-extracted with water to recover the precipitants and dilute sulfuric acid. The recovery of the precipitants is converted into the distillation recovery of C1-C4 alcohols, which has the advantages of low energy consumption, complete deacidification, and easy industrialization, and the total deacidification rate can reach more than 98%, and the recovered alcohol precipitants can be recycled.

Description

Technical Field

[0001] This invention relates to the field of chemical separation, and more specifically to a method for recovering alcohol precipitants used in the hydrolysis and sugar production process of plant fiber raw materials. Background Technology

[0002] Plant fiber raw materials are renewable biomass raw materials. Using them as raw materials, monosaccharides can be obtained through hydrolysis, and then processed through fermentation or chemical processes to produce feed protein, food, and chemical products, thus reducing carbon emissions. There are various methods for hydrolyzing plant fiber raw materials, with the concentrated sulfuric acid method being a commonly used one. The concentrated sulfuric acid method generally involves two steps: the first step uses concentrated sulfuric acid to hydrolyze the plant fiber raw material into soluble oligosaccharides, yielding a main hydrolysate, as described in Chinese patent ZL200710030294.2, "Method for preparing main hydrolysate by hydrolyzing plant fiber raw materials with concentrated sulfuric acid"; the second step uses dilute sulfuric acid to hydrolyze the oligosaccharides into monosaccharides, a post-hydrolysis process.

[0003] Because a large amount of sulfuric acid is added when plant fiber raw materials are hydrolyzed with concentrated sulfuric acid, the liquid-to-solid ratio is generally 1.5 to 15. If the sulfuric acid is not separated from the sugar and the sulfuric acid is not recycled, the cost of the sugar obtained will be very high and it will lose its industrialization significance. In other words, the separation of sugar from sulfuric acid is the bottleneck of industrializing the production of sugar from plant fiber raw materials by concentrated sulfuric acid hydrolysis.

[0004] Chinese patent ZL200810145571.9, "Method for recovering sulfuric acid from concentrated acid hydrolysate of plant cellulose raw materials," developed a method for separating sugar and sulfuric acid by adding a water-immiscible alcohol precipitant to the concentrated sulfuric acid hydrolysate of plant cellulose raw materials. However, this method still has the following problems in practical industrial applications: Firstly, although C5 and higher alcohols are immiscible with water, they still have a certain degree of solubility in water. For example, the water solubility of several alcohols is shown in Table 1 below. When sugar precipitates are dissolved in water, some of the precipitant will dissolve in the sugar solution according to its solubility, resulting in a loss. In addition, since C5 and higher alcohols exist in the oil phase in water, they are easily adsorbed by insoluble substances in the sugar solution (such as lignin residues), causing additional adsorption losses and further increasing the consumption of precipitant.

[0005] Table 1

[0006] Secondly, as can be seen from the boiling points of various alcohols in Table 2 below, the boiling points of C5 and above alcohols are much higher than those of water. If direct distillation is used to recover the dissolved and adsorbed precipitates from the sugar solution, not only does the sugar solution need to be heated to a temperature much higher than the boiling point of water, but a large amount of water is also evaporated during the distillation process, resulting in extremely high energy consumption and making it uneconomical.

[0007] Table 2

[0008] Thirdly, ZL200810145571.9 discloses the technical effect of using reagent-grade C5 and above alcohol precipitants to achieve sugar-acid separation, but does not address the performance changes of the precipitant after industrial recycling. As shown in Table 3 regarding the solubility of water in alcohol, after water back-extraction and regeneration, the water content of C5 and above organic precipitant solutions containing sulfuric acid easily reaches saturation. However, the inventors have found through research that the sugar-acid separation capacity of C5 and above organic precipitants after water back-extraction saturation is significantly lower than that of anhydrous reagent-grade precipitants.

[0009] Table 3

[0010] In summary, the recovery of C5 and higher alcohol precipitants in existing technologies faces technical challenges such as significant losses, high energy consumption, and performance degradation after recycling, necessitating the development of new technologies. one A method for efficiently and energy-savingly recovering precipitants, and for improving or restoring the separation performance of precipitants during the recovery process. Summary of the Invention

[0011] To facilitate a full understanding of the technical solution of this invention, some technical terms in this document are defined as follows: Glossary Ultra-concentrated acid hydrolysis: refers to the process of adding concentrated sulfuric acid to plant fiber raw materials for hydrolysis, where the mass-weighted average concentration of sulfuric acid added during the entire hydrolysis process is greater than 80wt%, to distinguish it from traditional concentrated sulfuric acid hydrolysis with a sulfuric acid concentration of 70-72wt%.

[0012] Ultra-concentrated acid hydrolysis main hydrolysate: refers to the hydrolysate formed by the hydrolysis of cellulose and hemicellulose in plant fiber raw materials after ultra-concentrated acid hydrolysis, which is mainly composed of oligosaccharides.

[0013] Precipitating agent: refers to an organic solvent that, when added to the main hydrolysate of ultra-concentrated acid hydrolysis, causes the dissolved sugar to precipitate out in solid form while retaining sulfuric acid in the liquid phase, thereby achieving sugar-acid separation.

[0014] Sugar precipitation: refers to the precipitate formed after adding a precipitant to the main hydrolysate of ultra-concentrated acid hydrolysis and stirring.

[0015] Sugar-acid ratio: refers to the mass ratio of soluble sugar to sulfuric acid in the sugar solution after sugar precipitation and dissolution.

[0016] C5: refers to a cell containing 5 carbon atoms.

[0017] This invention addresses the shortcomings of existing technologies for the recovery of C5 and higher alcohol precipitants that are slightly soluble in water and separate into layers with the aqueous phase in the sugar-acid separation application of concentrated sulfuric acid hydrolysate from plant fiber raw materials. These shortcomings include high energy consumption during recovery, significant losses, and a substantial decline in deacidification performance after recycling. The invention provides a method for recovering precipitants. This method should achieve efficient recovery and recycling of the precipitant, significantly reduce recovery energy consumption, effectively remove residual sulfuric acid during the recovery process, improve the sugar-acid separation effect of the recycled precipitant, and reduce the cost of subsequent sugar solution refining.

[0018] To achieve the above objectives, the inventors conducted in-depth research and obtained the following key findings: Finding 1: Based on the solubility of alcohols in water, alcohols with different carbon numbers can be clearly divided into two groups: C1-C4 alcohols (such as methanol, ethanol, isopropanol, isobutanol, etc.) are miscible to soluble in water and belong to water-soluble alcohols; C5 and above alcohols (such as n-pentanol, isopentanol, n-hexanol, n-octanol, etc.) are slightly soluble in water, meaning they separate into layers with the aqueous phase and belong to slightly soluble alcohols. Due to this difference in solubility, C1-C4 alcohols tend to exist uniformly in an aqueous solution in sugar-containing solutions, while C5 and above alcohols mainly exist in an oil phase. The latter are easily adsorbed by insoluble substances in the sugar solution (such as lignin residues), resulting in additional adsorption losses of the precipitant in addition to the solubility loss caused by solubility. More importantly, the precipitants adsorbed on insoluble matter exhibit significantly different distillation behavior compared to precipitants dissolved in sugar solutions. C1-C4 alcohols existing in aqueous solution are easier to separate by distillation, while C5 and above alcohols in the oil phase that are adsorbed are much more difficult to desorb by distillation due to the binding force of adsorption.

[0019] Finding 2: Based on the boiling points of alcohols, alcohols with different numbers of carbon atoms can also be divided into two groups: C1-C4 alcohols have boiling points lower than or close to water (as shown in Table 2 above, isopropanol 82.5℃, n-propanol 97.2℃, isobutanol 108℃, n-butanol 118℃), belonging to the low-boiling-point alcohol category; C5 and above alcohols have boiling points much higher than water (as shown in Table 2 above, n-pentanol 137.5℃, isopentanol 132℃, n-hexanol 157℃, n-octanol 195℃), belonging to the high-boiling-point alcohol category. When the alcohol solution contains sulfuric acid, the distillation temperature directly determines whether and to what extent side reactions (such as dehydration, esterification, and carbonization of alcohols) occur. Clearly, low-boiling-point C1-C4 alcohols can be distilled out at lower temperatures, effectively avoiding or significantly reducing the occurrence of side reactions. However, high-boiling-point C5 and above alcohols require higher distillation temperatures. Under these conditions, side reactions are difficult to avoid, resulting not only in the loss of the target product but also potentially producing byproducts that inhibit subsequent fermentation.

[0020] Finding 3: C5 and higher alcohol precipitants, after water back-extraction regeneration in industrial production, are water-saturated, resulting in a significantly reduced sugar-acid separation capacity compared to anhydrous reagent-grade precipitants. Taking n-octanol as an example, after five stages of cross-flow precipitation using 99.5% reagent-grade n-octanol at a 3:1 mass ratio (n-octanol:main hydrolysate), the sulfuric acid removal rate in the main hydrolysate was 95.7%, and the sugar-acid ratio of the resulting sugar precipitate was 6.93:1. However, when using water-saturated n-octanol after water back-extraction regeneration under the same conditions, the sulfuric acid removal rate dropped to 90.2%, and the sugar-acid ratio was only 3.04:1, meaning the final sulfuric acid concentration in the sugar solution was more than twice that of when using reagent-grade precipitants. This increase in residual sulfuric acid will significantly increase the equipment investment and operating costs of subsequent sugar-acid refining processes (such as ion exchange, electrodialysis, and simulated moving bed chromatography). The inventors unexpectedly discovered that in the process of washing sugar precipitates with C1-C4 alcohols to recover C5 and above alcohol precipitants, C1-C4 alcohols can not only dissolve and carry out the precipitant adsorbed in the sugar precipitate, but also further remove residual sulfuric acid that cannot be effectively separated by water-saturated precipitants, thereby increasing the total deacidification rate from 90.2% to over 98%, and the sugar-acid ratio of the obtained sugar solution reaches over 15:1, which is even better than the deacidification effect of using reagent-grade precipitants.

[0021] Based on the above findings, the inventors propose a novel approach to precipitant recovery: When using C5 or higher alcohols that separate from the aqueous phase as precipitants in the main hydrolysate of ultra-concentrated acid hydrolysis, their slight solubility and high boiling point make direct distillation of the sugar solution to recover the precipitant problematic due to high evaporation temperatures, high energy consumption caused by large-scale water evaporation, and severe side reactions. However, by using C1-C4 alcohols to dissolve the precipitant adsorbed in the sugar precipitate, a C1-C4 alcohol solution containing the precipitant and a sugar precipitate without the precipitant are obtained. This transforms the previously difficult precipitant recovery problem into a distillation recovery problem of C1-C4 alcohols. Because C1-C4 alcohols are water-soluble and have low boiling points, they are easier to separate using conventional distillation, making precipitant recovery simple, efficient, and energy-saving, while simultaneously achieving deep removal of residual sulfuric acid.

[0022] This invention provides a method for recovering alcohol precipitants in a plant fiber raw material hydrolysis sugar production process. The plant fiber raw material hydrolysis sugar production process includes the following steps: First, an alcohol precipitant A (C5 or higher, slightly soluble in water and separated from the aqueous phase) is mixed with the main hydrolysate of the plant fiber raw material using ultra-concentrated acid hydrolysis. The organic precipitant A can precipitate and separate sugars from the sulfuric acid solution. Then, solid-liquid separation is performed to obtain a sugar precipitate B containing alcohol precipitant A and sulfuric acid. The alcohol precipitant recovery method includes the following steps: Step 1: Add water-soluble, low-boiling-point C1-C4 alcohols C to sugar precipitate B and stir to mix. The C1-C4 alcohols C are miscible with organic precipitant A, so that the alcohol precipitant A and sulfuric acid contained in sugar precipitate B are redissolved in C1-C4 alcohols C, and a small amount of sugar precipitate B is also dissolved in C1-C4 alcohols C. Step 2: Solid-liquid separation, yielding sugar precipitate D separated from organic precipitant A, and organic solvent E for dissolving organic precipitant A and sulfuric acid; Step 3: Distill the organic solvent E to obtain alcohols C1~C4 in the distillate. The residue is separated by solid-liquid separation to obtain an organic precipitant G containing sulfuric acid and a precipitated sugar precipitate F. The sugar precipitate F is mixed with sugar precipitate B and then processed in step 1. The alcohols C1~C4 obtained in the distillate can be recycled. Step 4: Dissolve the sugar precipitate D obtained in Step 2 in water to obtain a sugar solution H containing C1~C4 alcohols C; Step 5: Distill the sugar solution H to obtain the distillate containing C1~C4 alcohols C, and the residue is the sugar solution K after removing the C1~C4 alcohols C; Step 6: Add water to the sulfuric acid-containing organic precipitant G obtained in Step 3 for back-extraction to obtain dilute sulfuric acid and organic precipitant A. The obtained organic precipitant A can be recycled, and the obtained dilute sulfuric acid can be concentrated and recycled.

[0023] Furthermore, the C1 to C4 alcohol C is one or more of methanol, ethanol, n-propanol, isopropanol, n-butanol, or isobutanol.

[0024] Furthermore, the C1-C4 alcohols C mentioned in step 1 are added to the sugar precipitate B in a multi-stage countercurrent or multi-stage crosscurrent manner, so that the precipitant A adsorbed by the sugar precipitate B is fully dissolved and carried away by the C1-C4 alcohols C.

[0025] Furthermore, the distillation of the organic solvent E in step 3 is carried out by vacuum distillation, wherein the bottom evaporation temperature is reduced by vacuum distillation to reduce the occurrence of side reactions; more preferably, MVR distillation is used.

[0026] Furthermore, in step 4, the sugar concentration of the sugar solution H is greater than 30 wt%, more preferably greater than 40 wt%, and most preferably 50 wt%~60 wt%. The higher the sugar concentration, the more water tends to be adsorbed by the sugar and less likely to evaporate, thereby increasing the proportion of C1~C4 alcohols in the steam, reducing the amount of water evaporation, and achieving a significant energy-saving effect.

[0027] Furthermore, the distillation of the sugar solution H in step 5 is carried out by vacuum distillation to reduce the evaporation temperature at the bottom of the column and reduce the occurrence of side reactions; more preferably, MVR distillation is used.

[0028] Furthermore, in step 6, the recovered alcohol precipitant A can be returned to the front end of the hydrolysis sugar production process. It can be mixed with freshly replenished alcohol precipitant A and used for sugar-acid separation in the main hydrolysate, achieving long-term recycling of the precipitant. That is, after washing, distillation, and back-extraction regeneration with C1-C4 alcohols, the residual sulfuric acid in the obtained alcohol precipitant A is removed. Although its water content is still water-saturated, it still possesses sugar-acid separation capability after mixing with the ultra-concentrated acid hydrolysis main hydrolysate of plant fiber raw materials. Subsequent washing and purification of the obtained sugar precipitate with C1-C4 alcohols, followed by extraction and recovery of alcohol precipitant A, ensures long-term recycling of the precipitant without frequent replacement or large-scale replenishment of fresh precipitant. This significantly reduces the consumption cost of the precipitant and the process operating cost, providing reliable technical support for continuous industrial production and further enhancing the industrial feasibility and economic competitiveness of the overall process.

[0029] The beneficial effects of this invention are: 1. This invention utilizes water-soluble, low-boiling-point C1-C4 alcohols to dissolve the slightly soluble, high-boiling-point C5 and above alcohols adsorbed in sugar precipitates, transforming the precipitate recovery problem into a conventional C1-C4 alcohol distillation recovery problem. Since the boiling points of C1-C4 alcohols are much lower than those of C5 and above alcohols, complete recovery can be achieved without evaporating large amounts of water during the distillation process, resulting in extremely significant energy savings.

[0030] 2. This invention, while recovering the precipitant, utilizes the polar solubility of C1-C4 alcohols to further remove residual sulfuric acid from the recycled water-saturated precipitant, increasing the overall deacidification rate and sugar-acid ratio. The deacidification effect is far superior to that using reagent-grade precipitants. This effect significantly reduces the residual sulfuric acid content in the sugar solution, which can significantly reduce the scale and operating costs of subsequent high-investment sugar-acid refining equipment such as ion exchange, electrodialysis, and simulated moving bed processes, demonstrating outstanding economic value.

[0031] 3. C1-C4 alcohols have low boiling points, and the distillation recovery temperature is low, much lower than that of C5 and above alcohols. Low-temperature distillation effectively avoids side reactions such as caramelization and carbonization of sugars under acidic conditions, as well as dehydration and esterification of alcohols, ensuring the sugar recovery rate and quality.

[0032] 4. In the method of the present invention, C1~C4 alcohols C are completely recovered and recycled after distillation in steps 3 and 5, precipitant A is recycled after back-extraction and regeneration in step 6, and dilute sulfuric acid is recycled for the hydrolysis section after concentration. The whole process has no additional waste discharge, high material utilization rate, good industrial application prospects and economic feasibility. Attached Figure Description

[0033] Figure 1This is a flowchart of a method for recovering alcohol precipitants used in a plant fiber raw material hydrolysis sugar production process. Detailed Implementation

[0034] To facilitate understanding by those skilled in the art, the present invention will be further described below with reference to embodiments and accompanying drawings. The content mentioned in the embodiments is not intended to limit the present invention.

[0035] In some embodiments, a method for recovering alcohol precipitants in a plant fiber raw material hydrolysis sugar production process includes the following steps: First, an alcohol precipitant A, which has C5 or higher content, is slightly soluble in water, and separates into layers with the aqueous phase, is mixed with the main hydrolysate of the plant fiber raw material using ultra-concentrated acid hydrolysis. The organic precipitant A can precipitate and separate sugars from the sulfuric acid solution. Then, solid-liquid separation is performed to obtain a sugar precipitate B containing alcohol precipitant A and sulfuric acid. The alcohol precipitant recovery method includes the following steps: Step 1: Add water-soluble, low-boiling-point C1-C4 alcohols C to sugar precipitate B and stir to mix. The C1-C4 alcohols C are miscible with organic precipitant A, so that the alcohol precipitant A and sulfuric acid contained in sugar precipitate B are redissolved in C1-C4 alcohols C, and a small amount of sugar precipitate B is also dissolved in C1-C4 alcohols C. Step 2: Solid-liquid separation to obtain sugar precipitate D, which separates organic precipitant A, and organic solvent E, which dissolves organic precipitant A and sulfuric acid; Step 3: Distill the organic solvent E to obtain alcohols C1~C4 in the distillate. The residue is separated by solid-liquid separation to obtain an organic precipitant G containing sulfuric acid and a precipitated sugar precipitate F. The sugar precipitate F is mixed with sugar precipitate B and then processed in step 1. The alcohols C1~C4 obtained in the distillate can be recycled. Step 4: Dissolve the sugar precipitate D obtained in Step 2 in water to obtain a sugar solution H containing C1~C4 alcohols C; Step 5: Distill the sugar solution H to obtain the distillate containing C1~C4 alcohols C, and the residue is the sugar solution K after removing the C1~C4 alcohols C; Step 6: Add water to the sulfuric acid-containing organic precipitant G obtained in Step 3 for back-extraction to obtain dilute sulfuric acid and organic precipitant A. The obtained organic precipitant A can be recycled, and the obtained dilute sulfuric acid can be concentrated and recycled.

[0036] In some embodiments, the C1 to C4 alcohol C is one or more of methanol, ethanol, n-propanol, isopropanol, n-butanol, or isobutanol.

[0037] In some embodiments, the C1-C4 alcohols C mentioned in step 1 are added to the sugar precipitate B using a multi-stage countercurrent or multi-stage crosscurrent method.

[0038] In some embodiments, the distillation of the organic solvent E in step 3 is carried out by vacuum distillation; more preferably, MVR distillation is used.

[0039] In some embodiments, the sugar concentration of the sugar solution H in step 4 is greater than 30 wt%, more preferably greater than 40 wt%, and most preferably 50 wt% to 60 wt%.

[0040] In some embodiments, the distillation of the sugar solution H in step 5 is performed by vacuum distillation; more preferably, MVR distillation is used.

[0041] Example 1 This embodiment provides a process for producing sugar from plant fiber raw materials by hydrolysis, including the following steps: S1. Preparation of ultra-concentrated acid hydrolysis main hydrolysate of plant fiber raw materials The method described in Example 2 of ZL200810145571.9 for preparing the main hydrolysate using corn stalks includes the following steps: S1-1. The corn stalks are pretreated and crushed into straw powder of about 80 mesh. The moisture content of the straw powder used is about 13%. S1-2. Preparation of initial hydrolysis solution: Prepare the initial hydrolysis solution according to 19% of the total acid amount of 16.5 grams (based on 98 wt% sulfuric acid), that is, weigh 3.1 grams of 98 wt% sulfuric acid, add 1.1 grams of water to prepare a sulfuric acid solution with a concentration of 72 wt%, and use it as the initial hydrolysis solution; and cool the initial hydrolysis solution to room temperature; S1-3. Weigh 10.6 grams of straw powder. At room temperature, add about 1 gram of straw powder to the initial hydrolysate and stir with a glass rod until dissolved to obtain a dark brown hydrolysate. Then add about 1 gram of 98% wt sulfuric acid and stir evenly. Next, add another 1 gram of straw powder and stir with a glass rod until dissolved. Repeat the above process of adding acid, adding straw powder and stirring until all the measured 16.5 grams of 98% wt sulfuric acid and 10.6 grams of straw powder have been added. Finally, stir with a glass rod to ensure the system is fully mixed. S1-4. Place the above mixture in a water bath at 48~50℃ for 8 minutes to obtain a dark brown, highly concentrated acid hydrolysate containing water-soluble oligosaccharides.

[0042] S2. Prepare sugar precipitate using reagent-grade n-octanol with a mass fraction of 99.5% (for comparison). The method for preparing sugar precipitate using Example 4 in ZL200810145571.9 includes the following steps: S2-1. Take 26.4 g of the ultra-concentrated acid hydrolysis main hydrolysate obtained in step S1 above into a beaker. Under stirring, slowly add it to 79.2 g of 99.5% n-octanol organic precipitant (the mass ratio of 99.5% reagent-grade n-octanol to the main hydrolysate is 3:1) to form a fine granular precipitate. Let it stand for 15 minutes to allow it to fully precipitate. Then filter it with filter paper to obtain sugar precipitate and a light yellow filtrate containing acidic n-octanol organic precipitant. S2-2. Use 79.2 g of 99.5% n-octanol to wash the sugar precipitate on the filter paper into a beaker, stir, and then filter with filter paper to obtain the sugar precipitate and a pale yellow filtrate containing acidic n-octanol organic precipitant. S2-3. Repeat step S2-2 three times, that is, perform a total of 5 cross-flow sedimentation. S2-4. The final sugar precipitate was determined to have a sugar-acid ratio of 6.93:1, and a sulfuric acid removal rate of 95.7%. S2-5. Comparison of sample labels: The sugar precipitate prepared in this study is for comparative purposes only and will not be used as raw material for subsequent examples and comparative cases.

[0043] It should be noted that in step B3, the five cross-flow precipitations are as follows: First: in step S2-1, n-octanol is added to the main hydrolysate, followed by precipitation and filtration; Second: in step S2-2, the sugar precipitate is washed with n-octanol and filtered; Third to fifth: in step S2-3, the operation of step S2-2 is repeated three times, for a total of five times.

[0044] S3. Back-extraction to prepare water-saturated n-octanol Take 50 g of the acid-containing n-octanol filtrate obtained in step S2-1 above, place it in a separatory funnel, add 16 g of pure water, and back-extract at room temperature. After standing and separating the layers, separate the aqueous phase. Repeat the above back-extraction operation 6 times. It was determined that a total of 99.3% of the sulfuric acid was removed by back-extraction, and the obtained n-octanol was a water-saturated precipitant, which was recycled for sugar precipitation preparation.

[0045] S4. Prepare sugar precipitate using water-saturated n-octanol (raw materials from Examples 2-6 and Comparative Example 1). The method for preparing sugar precipitate using Example 4 in ZL200810145571.9 includes the following steps: S4-1. Take 26.4 g of the ultra-concentrated acid hydrolysis main hydrolysate obtained in step S1 above into a beaker. Under stirring, slowly add it to the water-saturated n-octanol organic precipitant obtained in step S3 above (the mass ratio of water-saturated n-octanol to main hydrolysate after back-extraction is 3:1) to form a fine particulate precipitate. Let it stand for 15 minutes to allow it to fully precipitate. Then filter it with filter paper to obtain sugar precipitate and a light yellow filtrate containing acidic n-octanol organic precipitant. S4-2. Use 79.2 g of the water-saturated n-octanol organic precipitant prepared in step S3 above to wash the sugar precipitate on the filter paper into a beaker, stir, and then filter with filter paper to obtain the sugar precipitate and a pale yellow filtrate containing acidic n-octanol organic precipitant. S4-3. Repeat step S4-2 three times, that is, perform a total of 5 cross-flow sedimentation processes. S4-4, The final sugar precipitate was determined to have a sugar-acid ratio of 3.04:1, and the sulfuric acid removal rate was 90.2%. S4-5, Raw material labeling: The sugar precipitate prepared in this study will be used as the raw material for subsequent Examples 2-6 and Comparative Example 1.

[0046] As shown in Example 1, the sugar precipitate prepared using reagent-grade n-octanol with a mass fraction of 99.5% had a sugar-acid ratio of 6.93:1 and a sulfuric acid removal rate of 95.7%. However, the sugar precipitate prepared using water-saturated n-octanol had a sugar-acid ratio reduced to 3.04:1 and a sulfuric acid removal rate reduced to 90.2%. This demonstrates that the precipitant, after water back-extraction regeneration, is in a water-saturated state, and its sugar-acid separation ability is significantly reduced compared to the reagent-grade precipitant. The final sulfuric acid concentration in the sugar solution is more than twice that when using the reagent-grade precipitant.

[0047] Example 2 In this embodiment, methanol is selected as the C1-C4 alcohol.

[0048] This embodiment provides a method for recovering alcohol precipitants in a plant fiber raw material hydrolysis sugar production process, including the following steps: Step 1: Take 100g of the sugar precipitate B obtained in step S4 of Example 1, add 200g of 99.5% methanol, stir and mix, so that the n-octanol and sulfuric acid contained in the sugar precipitate B are redissolved in the methanol, and a small amount of sugar precipitate B is dissolved in the methanol at the same time. Step 2: Filter the mixture from Step 1 using filter paper to obtain filtrate E and sugar precipitate B. Repeat Step 1 six times to completely dissolve n-octanol and sulfuric acid in methanol. Separate the solid and liquid phases to obtain sugar precipitate D after removing n-octanol and some sulfuric acid. Perform sugar solubility analysis on sugar precipitate D. The sugar-acid ratio is 15.87:1. The total deacidification rate of sulfuric acid added during ultra-concentrated acid hydrolysis is 98.1%. This total deacidification rate is the cumulative value achieved after further removal of residual acid by subsequent methanol washing, based on the deacidification rate of 90.2% in the water-saturated n-octanol precipitation step. Step 3: Perform vacuum distillation on the first filtrate E obtained in Step 2. The evaporation temperature is 60℃, and the distillation ends when the vacuum pressure drops to 80KPa. The condensate obtained from the distillation is methanol. The residue is separated into solid and liquid components to obtain n-octanol containing sulfuric acid and sugar precipitate F. The sugar precipitate F obtained is combined with the sugar precipitate B obtained in Step 1 and returned to Step 1 for processing. The methanol obtained can be recycled. Step 4: Add the sugar precipitate D obtained in Step 2 (after removing n-octanol) to 30g of water, stir to dissolve, and obtain a sugar solution H with a sugar concentration of approximately 50%. Step 5: Perform vacuum distillation on the sugar solution H obtained in Step 4. The bottom temperature of the column is 60℃. Methanol is obtained at the top of the column, and methanol-free sugar solution K is obtained at the bottom of the column. The obtained methanol can be recycled. Step 6: Add the sulfuric acid-containing n-octanol obtained in Step 3 to 1 / 3 of its mass of water for back-extraction, allow it to stand and separate into layers, and separate the aqueous phase; repeat the above back-extraction operation on the oil phase 6 times to obtain n-octanol and dilute sulfuric acid that are basically free of sulfuric acid; the obtained n-octanol is recycled, and the obtained dilute sulfuric acid is concentrated and then recycled.

[0049] As can be seen from Example 2, the above process recovered n-octanol from sugar precipitate B, as well as the added methanol.

[0050] Example 3 In this embodiment, ethanol is selected as the C1-C4 alcohol.

[0051] This embodiment provides a method for recovering alcohol precipitants in a plant fiber raw material hydrolysis sugar production process, including the following steps: Step 1: Take 100g of the sugar precipitate B obtained in step S4 of Example 1, add 200g of 99.5% ethanol, stir and mix, so that the n-octanol and sulfuric acid contained in the sugar precipitate B are redissolved in the ethanol, and a small amount of sugar precipitate B is dissolved in the ethanol at the same time. Step 2: Filter the mixture from Step 1 using filter paper to obtain filtrate E and sugar precipitate B. Repeat Step 1 six times to completely dissolve n-octanol and sulfuric acid in ethanol. Separate the solid and liquid phases to obtain sugar precipitate D after removing n-octanol and some sulfuric acid. Perform sugar solubility analysis on sugar precipitate D. The sugar-acid ratio is 17.57:1. The total deacidification rate of sulfuric acid added during ultra-concentrated acid hydrolysis is 98.3%. This total deacidification rate is the cumulative value achieved after further removal of residual acid by subsequent ethanol washing, based on the deacidification rate of 90.2% in the water-saturated n-octanol precipitation step. Step 3: Perform vacuum distillation on the first filtrate E obtained in Step 2. The evaporation temperature is 65℃, and the distillation ends when the vacuum pressure drops to 55KPa. The condensate obtained from the distillation is ethanol. The residue is separated into solid and liquid components to obtain n-octanol containing sulfuric acid and sugar precipitate F. The sugar precipitate F obtained is combined with the sugar precipitate B from Step 1 and returned to Step 1 for processing. The obtained ethanol can be recycled. Step 4: Add the sugar precipitate D obtained in Step 2 (after removing n-octanol) to 30g of water, stir to dissolve, and obtain a sugar solution H with a sugar concentration of approximately 50%. Step 5: Perform vacuum distillation on the sugar solution H obtained in Step 4. The bottom temperature of the column is 65℃. The distillate ethanol is obtained at the top of the column, and the sugar solution K after removing ethanol is obtained at the bottom of the column. The obtained ethanol can be recycled. Step 6: Add the sulfuric acid-containing n-octanol obtained in Step 3 to 1 / 3 of its mass of water for back-extraction, allow it to stand and separate into layers, and separate the aqueous phase; repeat the above back-extraction operation on the oil phase 6 times to obtain n-octanol and dilute sulfuric acid that are basically free of sulfuric acid; the obtained n-octanol is recycled, and the obtained dilute sulfuric acid is concentrated and then recycled.

[0052] As can be seen from Example 3, the above process recovered the n-octanol precipitant from the sugar precipitate, and also recovered the added ethanol.

[0053] Example 4 In this embodiment, isopropanol is selected as the C1-C4 alcohol.

[0054] This embodiment provides a method for recovering alcohol precipitants in a plant fiber raw material hydrolysis sugar production process, including the following steps: Step 1: Take 100g of the sugar precipitate B obtained in step S4 of Example 1, add 200g of 99.5% isopropanol, stir and mix, so that the n-octanol and sulfuric acid contained in the sugar precipitate B are redissolved in the isopropanol, and at the same time a small amount of sugar precipitate B dissolves in the isopropanol. Step 2: Filter the mixture from Step 1 using filter paper to obtain filtrate E and sugar precipitate B. Repeat Step 1 six times to completely dissolve n-octanol and sulfuric acid in isopropanol. Separate the solid and liquid phases to obtain sugar precipitate D after removing n-octanol and some sulfuric acid. Perform sugar solubility analysis on sugar precipitate D. The sugar-acid ratio is 19.68:1. The total deacidification rate of sulfuric acid added during ultra-concentrated acid hydrolysis is 98.5%. This total deacidification rate is the cumulative value achieved after further removal of residual acid by subsequent isopropanol washing, based on the deacidification rate of 90.2% in the water-saturated n-octanol precipitation step. Step 3: Perform vacuum distillation on the first filtrate E obtained in Step 2. The evaporation temperature is 68℃, and the distillation ends when the vacuum pressure drops to 50KPa. The condensate obtained from the distillation is isopropanol. The residue is separated into solid and liquid components to obtain n-octanol containing sulfuric acid and sugar precipitate F. The sugar precipitate F obtained is combined with the sugar precipitate B obtained in Step 1 and returned to Step 1 for processing. The obtained isopropanol can be recycled. Step 4: Add the sugar precipitate D obtained in Step 2 (after removing n-octanol) to 30g of water, stir to dissolve, and obtain a sugar solution H with a sugar concentration of approximately 50%. Step 5: Perform vacuum distillation on the sugar solution H obtained in Step 4. The bottom temperature of the column is 68°C. Isopropanol is obtained at the top of the column, and sugar solution K with isopropanol removed is obtained at the bottom of the column. The obtained isopropanol can be recycled. Step 6: Add the sulfuric acid-containing n-octanol obtained in Step 3 to 1 / 3 of its mass of water for back-extraction, allow it to stand and separate into layers, and separate the aqueous phase; repeat the above back-extraction operation on the oil phase 6 times to obtain n-octanol and dilute sulfuric acid that are basically free of sulfuric acid; the obtained n-octanol is recycled, and the obtained dilute sulfuric acid is concentrated and then recycled.

[0055] As can be seen from Example 4, the above process recovered the n-octanol precipitant from the sugar precipitate, and also recovered the added isopropanol.

[0056] Example 5 In this embodiment, isobutanol is selected as the C1-C4 alcohol.

[0057] This embodiment provides a method for recovering alcohol precipitants in a plant fiber raw material hydrolysis sugar production process, including the following steps: Step 1: Take 100g of the sugar precipitate B obtained in step S4 of Example 1, add 200g of 99.5% isobutanol, stir and mix, so that the n-octanol and sulfuric acid contained in the sugar precipitate B are redissolved in the isobutanol, and at the same time a small amount of sugar precipitate B dissolves in the isobutanol. Step 2: Filter the mixture from Step 1 using filter paper to obtain filtrate E and sugar precipitate B. Repeat Step 1 six times to completely dissolve n-octanol and sulfuric acid in isobutanol. Separate the solid and liquid phases to obtain sugar precipitate D after removing n-octanol and some sulfuric acid. Perform sugar solubility analysis on sugar precipitate D. The sugar-acid ratio is 18.92:1. The total deacidification rate of sulfuric acid added during ultra-concentrated acid hydrolysis is 98.4%. This total deacidification rate is the cumulative value achieved after further removal of residual acid by subsequent isobutanol washing, based on the deacidification rate of 90.2% in the water-saturated n-octanol precipitation step. Step 3: Perform vacuum distillation on the first filtrate E obtained in Step 2. The evaporation temperature is 70℃, and the distillation ends when the vacuum pressure drops to 17KPa. The condensate obtained from the distillation is isobutanol. The residue is separated into solid and liquid components to obtain n-octanol containing sulfuric acid and sugar precipitate F. The sugar precipitate F obtained is combined with the sugar precipitate B from Step 1 and returned to Step 1 for processing. The obtained isobutanol can be recycled. Step 4: Add the sugar precipitate D obtained in Step 2 (after removing n-octanol) to 30g of water, stir to dissolve, and obtain a sugar solution H with a sugar concentration of approximately 50%. Step 5: Perform vacuum distillation on the sugar solution H obtained in Step 4. The bottom temperature of the column is 70°C. Isobutanol is obtained at the top of the column, and sugar solution K with isobutanol removed is obtained at the bottom of the column. The obtained isobutanol can be recycled. Step 6: Add the sulfuric acid-containing n-octanol obtained in Step 3 to 1 / 3 of its mass of water for back-extraction, allow it to stand and separate into layers, and separate the aqueous phase; repeat the above back-extraction operation on the oil phase 6 times to obtain n-octanol and dilute sulfuric acid that are basically free of sulfuric acid; the obtained n-octanol is recycled, and the obtained dilute sulfuric acid is concentrated and then recycled.

[0058] As can be seen from Example 5, the above process recovered the n-octanol precipitant from the sugar precipitate, and also recovered the added isobutanol.

[0059] In conclusion,

[0060] Examples 2 to 5 show that using methanol, ethanol, isopropanol, and isobutanol—four typical C1-C4 water-soluble low-boiling-point alcohols—as the C1-C4 alcohols can successfully dissolve and remove the n-octanol precipitant and residual sulfuric acid adsorbed in the sugar precipitate. Solvent recovery and recycling are achieved through vacuum distillation. All four C1-C4 water-soluble low-boiling-point alcohols significantly increased the total deacidification rate from 90.2% when water-saturated n-octanol was used alone in Example 1 to 98.1%-98.5%, and the sugar-acid ratio from 3.04:1 to 15.87:1-19.68:1. The deacidification effect is not only far superior to water-saturated n-octanol, but even superior to using 99.5% reagent-grade n-octanol (deacidification rate 95.7%, sugar-acid ratio 6.93:1). Simultaneously, the distillation temperature is only 60-70℃, far lower than the 195℃ required for n-octanol, effectively avoiding sugar caramelization and side reactions.

[0061] Example 6 Unlike Example 5, step 5 in this example has the following specific steps: Step 5-1: Add the sugar solution H obtained in step 4 to a round-bottom flask, and then carry out vacuum distillation at a water bath temperature of 70°C. Stop when the sugar solution evaporates to a viscous state, and about 21 grams of condensed water are obtained. It is determined that the isobutanol has not been completely evaporated. Step 5-2: Add 21 grams of water to the viscous residue obtained in Step 5-1, stir well, and then perform vacuum distillation again at a water bath temperature of 70°C. Stop distillation when the amount of condensed water is about 21 grams. It is determined that the isobutanol has been completely evaporated. Step 5-3: Calculate the total amount of water evaporated: A total of 3 distillations were performed (30 grams of water were added for distillation in step 4, and 21 grams of water were added for distillation in the subsequent 2 distillations). The amount of water evaporated each time was about 21 grams, and the total amount of water evaporated was about 2 times × 21 grams / time = 42 grams.

[0062] Comparative Example 1 This comparative example provides a method for recovering alcohol precipitants in a plant fiber raw material hydrolysis sugar production process, including the following steps: Step 1: Take 100g of the sugar precipitate obtained in step S4 of Example 1, add 30g of water, stir to dissolve, and obtain a sugar solution with a sugar concentration of about 50%. Step 2: Add the sugar solution obtained in Step 1 to a round-bottom flask and perform vacuum distillation at a water bath temperature of 70°C. Stop distillation when the sugar solution evaporates to a viscous state. About 21 grams of condensed water is obtained. It is determined that the n-octanol has not been completely evaporated. Step 3: Add 21 grams of water to the viscous residue obtained in Step 2, stir well, and then perform vacuum distillation again at a water bath temperature of 70°C. When the amount of condensed water is about 21 grams, stop distillation. It is found that the n-octanol has not been completely evaporated. Step 4: Repeat the operation of step 3, adding 21 grams of water each time and distilling until about 21 grams of condensate is obtained, until the 12th time water is added. After the distillation process is completed, it is measured that n-octanol has been completely evaporated and recovered. Step 5: Calculate the total amount of water evaporated: A total of 13 distillations were performed (30 grams of water were added for the first distillation, and 21 grams of water were added for each of the subsequent 12 distillations). The amount of water evaporated each time was about 21 grams, and the total amount of water evaporated was about 13 times × 21 grams / time = 273 grams.

[0063] Comparative Example 1 and Example 6 show that the direct distillation method for recovering n-octanol in Comparative Example 1 is extremely energy-intensive (requiring the evaporation of 273 grams of water), while the isobutanol replacement method in Example 6 only requires the evaporation of about 42 grams of water to completely recover the alcohol, a difference of about 6.5 times. Furthermore, isobutanol has lower volatility among C1-C4 alcohols than methanol, ethanol, and isopropanol in the above examples. This means that the amount of water required to evaporate when evaporating methanol, ethanol, and isopropanol is even lower than that required for isobutanol. Therefore, the methods in Examples 2-5 are more energy-efficient than the direct sugar-dissolving method for recovering n-octanol.

[0064] The above description is only a preferred embodiment of the present invention. For those skilled in the art, there will be changes in the specific implementation and application scope based on the ideas of the present invention. The content of this specification should not be construed as a limitation of the present invention.

Claims

1. A method for recovering alcohol precipitants in a plant fiber raw material hydrolysis sugar production process, wherein the plant fiber raw material hydrolysis sugar production process includes the following steps: firstly, mixing an alcohol precipitant A (C5 or higher, slightly soluble in water and separated into layers with the aqueous phase) with the main hydrolysate of the plant fiber raw material using ultra-concentrated acid hydrolysis, and then performing solid-liquid separation to obtain a sugar precipitate B containing organic precipitant A and sulfuric acid, characterized in that... The method for recovering alcohol precipitants includes the following steps: Step 1: Add water-soluble, low-boiling-point C1-C4 alcohols C to sugar precipitate B and stir to mix. The C1-C4 alcohols C are miscible with organic precipitant A, so that the alcohol precipitant A and sulfuric acid contained in sugar precipitate B are redissolved in C1-C4 alcohols C, and a small amount of sugar precipitate B is also dissolved in C1-C4 alcohols C. Step 2: Solid-liquid separation to obtain sugar precipitate D, which separates organic precipitant A, and organic solvent E, which dissolves organic precipitant A and sulfuric acid; Step 3: Distill the organic solvent E to obtain alcohols C1~C4 in the distillate. The residue is separated by solid-liquid separation to obtain an organic precipitant G containing sulfuric acid and a precipitated sugar precipitate F. The sugar precipitate F is mixed with sugar precipitate B and then processed in step 1. The alcohols C1~C4 obtained in the distillate can be recycled. Step 4: Dissolve the sugar precipitate D obtained in Step 2 in water to obtain a sugar solution H containing C1~C4 alcohols C; Step 5: Distill the sugar solution H to obtain the distillate containing C1~C4 alcohols C, and the residue is the sugar solution K after removing the C1~C4 alcohols C; Step 6: Add water to the sulfuric acid-containing organic precipitant G obtained in Step 3 for back-extraction to obtain dilute sulfuric acid and organic precipitant A. The obtained organic precipitant A can be recycled, and the obtained dilute sulfuric acid can be concentrated and recycled.

2. The method for recovering alcohol precipitants in the hydrolysis and sugar production process of plant fiber raw materials according to claim 1, characterized in that, The C1 to C4 alcohols are one or more of methanol, ethanol, n-propanol, isopropanol, n-butanol, or isobutanol.

3. The method for recovering alcohol precipitants in the hydrolysis and sugar production process of plant fiber raw materials according to claim 1, characterized in that, The C1-C4 alcohols C mentioned in step 1 are added to the sugar precipitate B using a multi-stage countercurrent or multi-stage crosscurrent method.

4. The method for recovering alcohol precipitants in the hydrolysis and sugar production process of plant fiber raw materials according to claim 1, characterized in that, The organic solvent E mentioned in step 3 is distilled using vacuum distillation or MVR distillation.

5. The method for recovering alcohol precipitants in the hydrolysis and sugar production process of plant fiber raw materials according to claim 1, characterized in that, In step 4, the sugar concentration of the sugar solution H is greater than 30%wt.

6. The method for recovering alcohol precipitants in the hydrolysis and sugar production process of plant fiber raw materials according to claim 1, characterized in that, In step 4, the sugar concentration of the sugar solution H is greater than 40%wt.

7. The method for recovering alcohol precipitants in the hydrolysis and sugar production process of plant fiber raw materials according to claim 1, characterized in that, In step 4, the sugar concentration of the sugar solution H is 50%wt~60%wt.

8. The method for recovering alcohol precipitants in the hydrolysis and sugar production process of plant fiber raw materials according to claim 1, characterized in that, The sugar solution H described in step 5 is distilled using vacuum distillation or MVR distillation.

9. The method for recovering alcohol precipitants in the hydrolysis and sugar production process of plant fiber raw materials according to claim 1, characterized in that, The acid-containing precipitant G described in step 6 is subjected to water back-extraction using a multi-stage countercurrent or multi-stage crosscurrent method.

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