METHOD FOR EXTRACTING CELLUSOSE, HEMICELLULOSE AND LIGNIN FROM LIGNOCELLULOSE FROM PLANT BIOMASS

Abstract

A process for obtaining cellulose, hemicellulose and lignin from lignocellulose can be carried out in an environmentally friendly manner when it comprises the following steps: step a) wherein a lignocellulose from plant biomass is provided, step b) wherein the lignocellulose is brought into contact with a first mixture M1 containing water and an alkaline component, and a first suspension S1 forms, the first suspension S1 comprises a first solid F1 and a first liquid phase P1, wherein the first solid F1 contains a crude cellulose, and the first liquid phase P1 contains hemicellulose and lignin, and step c) wherein the crude-cellulose-containing solid F1 is brought into contact with a second mixture M2 containing formic acid and water and optionally acetic acid, and a second suspension S2 forms, the second suspension S2 comprises a second solid F2 and a second liquid phase P2, wherein the second solid F2 contains a pure cellulose, and the second liquid phase P2 contains hemicellulose and lignin.

Claims

1. A process for obtaining cellulose, hemicellulose and lignin from lignocellulose, wherein the process comprises: a step a) wherein a lignocellulose from plant biomass is provided, a step b) wherein the lignocellulose is brought into contact with a first mixture M1 containing water and an alkaline component, to form a first suspension S1, the first suspension S1 comprises a first solid F1 and a first liquid phase P1, wherein the first solid F1 contains a crude cellulose, and the first liquid phase P1 contains hemicellulose and lignin, and a step c) wherein the crude-cellulose-containing solid F1 is brought into contact with a second mixture M2 containing formic acid and water and optionally acetic acid, to form a second suspension S2, the second suspension S2 comprises a second solid F2 and a second liquid phase P2, wherein the second solid F2 contains a pure cellulose, and the second liquid phase P2 contains hemicellulose and lignin.

2. The process as claimed in claim 1, wherein step b) and/or step c) are carried out at a pressure of from 0.8 bar to 1.5 bar.

3. The process as claimed in claim 1, wherein the first mixture M1 comprises from 1% by weight to 30% by weight sodium hydroxide and from 70% by weight to 99% by weight water, based on the first mixture M1.

4. The process as claimed in claim 1, wherein in step b) a first ratio MV1 (dry mass in g/volume in ml) of lignocellulose used to first mixture M1 used is from 1:3 to 1:15.

5. The process as claimed in claim 1, wherein after step b), in a step b1), the first solid F1 is separated from the first liquid phase P1, a first precipitating agent FM1, is added to the first liquid phase P1, whereby at least portions of the hemicellulose are precipitated, and a first, hemicellulose-containing precipitate is thus formed in a mixture C1.

6. The process as claimed in claim 1, wherein in step c) the second mixture M2 comprises from 0% by weight to 40% by weight acetic acid, from 40% by weight to 100% by weight formic acid, and from 0% by weight to 40% by weight water, based on the second mixture M2.

7. The process as claimed in claim 1, wherein in step c) the second mixture M2 is in the form of an azeotropic mixture.

8. The process as claimed in claim 1, wherein in step c) a second ratio MV2 (dry mass in g/volume in ml) of crude-cellulose-containing solid F1 used to second mixture M2 used is from 1:5 to 1:30.

9. The process as claimed in claim 1, wherein after step c), in a step c1), the second solid F2 is separated from the second liquid phase P2, the second liquid phase P2 is subjected to precipitation, wherein a second precipitating agent FM2, is optionally added to the second liquid phase P2; whereby at least portions of the lignin are precipitated and a second, lignin-containing precipitate is formed in a second mixture G2.

10. The process as claimed in claim 5, wherein after step b1), in a step b2), the first, hemicellulose-containing precipitate is separated from the first mixture G1, the first precipitating agent FM1 is removed at least in part from the first mixture G1, a third precipitating agent FM3, is added to the first mixture G1, which is depleted in respect of the first precipitating agent FM1, and at least a portion of the lignin is precipitated.

11. The process as claimed in claim 9, wherein after step c1), in a step c2), the second, lignin-containing precipitate is separated from the second mixture G2, and the second mixture G2 is dried that hemicellulose in concentrated form is present.

12. The process as claimed in claim 1, wherein step b) is carried out at a first temperature T1 and step c) is carried out at a second temperature T2, wherein the first temperature T1 and/or the second temperature T2 is from 20 C. to 150 C.

13. The process as claimed in claim 1, wherein in step a) the lignocellulose is provided from a comminuted plant biomass of particles having a maximum spatial extent of from 0.5 mm to 20 mm.

14. The process as claimed in claim 1, wherein the lignocellulose is brought into contact prior to step b) with an alcohol, and at least a portion of the alcohol is removed again prior to step b).

15. The process as claimed in claim 1, wherein the pure cellulose obtained is subjected to a bleaching process, wherein the bleaching process comprises in particular sodium chlorite bleaching, bleaching with hydrogen peroxide, bleaching with peracetic acid and/or bleaching with ozone.

16. A lignin from a first liquid phase P1 and/or from a second liquid phase P2 prepared according to the process of claim 1.

Description

[0058] The invention is explained in greater detail by the following FIGURE, the examples and the claims.

[0059] The invention is illustrated with the aid of the FIGURE (FIG. 1), which shows an example of an embodiment of the process according to the invention. Lignocellulose is used and pure cellulose, hemicellulose and lignin are obtained by carrying out steps a), b), b1), b2), c), c1) and c2), wherein pure cellulose is obtained in the form of a solid in step c), hemicellulose is obtained in the form of a solid, or syrup, in steps b1) and c2), and lignin is obtained in the form of a solid in steps b2) and c1).

EXAMPLE

[0060] Step a)

[0061] Wheat straw was finely ground to a particle size of about 2 mm. The ground wheat straw, which had a moisture content corresponding to ambient humidity, was subjected to extraction with ethanol. The extracted wheat straw could be processed without being dried. In order to determine exact yields, the straw was dried at 60 C. after the ethanol extraction and stored at room temperature. 20 g of dry wheat straw, comprising 0.6 g of dry extract, were present. The sum of dry wheat straw and dry extract is regarded as the starting amount in respect of the mass used in the mass balance.

[0062] After the dried wheat straw had been stored under ambient conditions, the swollen wheat straw had a moisture content of 5% by weight, so that 20.4 g of air-dry wheat straw were present.

[0063] Step b)

[0064] The air-dry wheat straw from step a) having a dry mass of 19.4 g was heated for one hour in a 3-necked flask, with KPG stirring, with 194 ml of a solution of 3.5% strength (g of NaOH/ml of water) sodium hydroxide solution as a first suspension S1 having a ratio, which is also referred to as the liquor ratio, of 1:10 (dry mass in g/volume in ml). The temperature of the 3-necked flask was adjusted to 120 C. by means of a preheated oil bath. The temperature of the first suspension S1, after a short heating phase, was 98 C.

[0065] The first suspension S1 was then filtered in vacuo, and the residue was washed with 100 ml of the 3.5% strength sodium hydroxide solution or with 100 ml of water. The filtrate contained dissolved hemicelluloses and lignin.

[0066] Crude cellulose with a dry mass of 10.7 g was obtained, which corresponds to 53.5% by weight of the starting amount of 20 g of straw. The composition is indicated in Table 1.

[0067] Step b1)

[0068] 230 ml of ethanol were added to 230 ml of the filtrate from the filtration in step b), which had a pH of 13.2, in order to precipitate hemicelluloses. The mixture was stirred for 15 minutes and then filtered, the resulting filtrate was a first mixture, which contained lignin. A first precipitate, which contained hemicelluloses, remained as residue. The first precipitate was washed with 50 ml of ethanol and dried in vacuo. The hemicellulose was precipitated in part as the alkali salt, because it is easier to handle in the form of a salt. The yield of alkali salt was 4.1 g, which corresponds to 20.5% by weight relative to the straw used. The composition is indicated in Table 2. A sugar analysis gave a hemicellulose content of 12.5% by weight, based on the straw used.

[0069] Step b2)

[0070] Ethanol was removed from the first mixture from step b1), which contained lignin, at 40 C. by means of a rotary evaporator, and lignin was precipitated at a pH of 2 by addition of 10% by weight hydrochloric acid, and then filtering was carried out.

[0071] The residue, which contained lignin, was washed twice with 100 ml of water having a pH of 2 and dried in vacuo. Lignin with a dry mass of 2.3 g was obtained, which corresponds to 11.5% by weight of the starting amount of 20 g of straw. The composition is indicated in Table 2.

[0072] Step c)

[0073] To the pressed residue from step b), which consisted of 10.7 g of crude cellulose and water, there were added in a 3-necked flask acetic acid, formic acid and water in such an amount that a second suspension S2 having a ratio of 1:19 (dry mass in g/volume in ml) was obtained and the liquid phase contained 20.4% by weight acetic acid, 60.4% by weight formic acid and 19.2% by weight water and was in the form of an azeotropic mixture having a boiling point of 106 C. The resulting second suspension S2 was heated for one hour, with KPG stirring, and adjusted to a temperature of 120 C. with the aid of a preheated oil bath. The temperature in the second suspension S2 was 106 C.

[0074] The second suspension S2 was then filtered in vacuo. The second residue, which contained pure cellulose, was washed again, this time with 400 ml of water. Pure cellulose with a dry mass of 8.8 g was obtained, which corresponds to 44.0% by weight of the starting amount of 20 g of straw. The composition is indicated in Table 1.

[0075] Step c1)

[0076] For precipitation of lignin, acetic acid, formic acid and water were removed as far as possible from the filtrate of the filtration in step c) at 40 C. by means of a rotary evaporator, whereby a dark syrup remained, which was taken up in 100 ml of water. A pH of 2.2 thereby resulted. The mixture was adjusted to a temperature of 120 C. for one hour by means of a preheated oil bath. Filtration was then carried out. The resulting filtrate formed a second mixture G2, which contained hemicellulose in water.

[0077] The residue, which contained lignin, was washed with 50 ml of water and dried in vacuo. Lignin with a dry mass of 0.6 g was obtained, which corresponds to 3.0% by weight of the starting amount of 20 g of straw. The composition is indicated in Table 2.

[0078] Step c2)

[0079] Water was removed from the second mixture G2 from step c1), which contains hemicellulose, at 40 C. by means of rotary evaporator. The resulting syrup was dried in vacuo. 1.2 g of hemicellulose were obtained in the form of syrup, which corresponds to 6% by weight of the starting amount of 20 g of straw.

[0080] Bleaching Process

[0081] The pure cellulose from step c) was bleached by a standard bleaching process with three-stage bleaching, with the bleaching sequence CT-E-CT, that is to say with sodium chlorite bleaching (CT), alkaline extraction (E) and a second sodium chlorite bleaching (CT), as is described in greater detail in J. Beringer (Zellstoff aus Weizenstroh: Gewinnung durch Aufschlussverfahren mit Ameisen-und Essigsure sowie Untersuchungen zur Zellstoffstruktur und Eignung als Papier-und Chemiezellstoff, dissertation 2004, University of Stuttgart). Bleached pure cellulose with a dry mass of 8.4 was obtained, which corresponds to 42.0% by weight of the starting amount of 20 g of straw. The composition is indicated in Table 1.

[0082] In total, approximately 75% by weight, based on the straw used, of cellulose, hemicellulose and lignin, in each case in isolated form, were obtained. Taking into account 3% by weight natural wheat straw extract from the extraction with ethanol and approximately 6% by weight mineral substances, especially silicates, this corresponds to a product yield of approximately 84% by weight, based on the straw used.

TABLE-US-00001 TABLE 1 Composition of the different celluloses obtained Bleached pure Crude cellulose Pure cellulose cellulose Analyses based in % by weight on Cellulose Cellulose Cellulose fraction Straw fraction Straw fraction Straw Cellulose 63.1 33.8 76.2 33.4 81.0 34.0 Hemicellulose 18.6 9.9 11.5 5.0 13.1 5.5 Lignin 5.6 3.0 2.6 1.1 0.2 0.1 Ash 2.0 1.1 0.3 0.1 0.1 0.1 Mass balance 89.3 47.8 90.5 39.6 94.4 39.7 Correlation 99.9 53.5 100.6 44.0 99.9 42.0 with amounts isolated

TABLE-US-00002 TABLE 2 Composition of lignin and hemicellulose obtained Hemicellulose Lignin Lignin from step b1) from step b2) from step c1) Analysis based in % by weight on Hemi- cellulose Lignin Lignin fraction Straw fraction Straw fraction Straw Cellulose not determined not determined Hemicellulose 61.5 12.5 Lignin 3.3 0.7 91.3 11.5 90.0 2.7 Ash 34.7* 7.1* 4.0** 0.5** 1.7 0.1 Mass balance 99.5 20.3 95.6 10.9 93.3 2.8 Correlation 100.5 20.5 100.9 11.5 100.0 3.0 with amounts isolated *Alkali salts **Amount varies (0.4-6%), always over 80% silicates

[0083] The bleached pure cellulose produced had the characteristics shown in Table 3.

TABLE-US-00003 TABLE 3 Characterization of the bleached pure cellulose Criterion Result -Cellulose content 98% Degree of whiteness according to Hunter 89 Degree of polymerization DP.sub.EWNN 1670

[0084] Within the EU, a content of -cellulose of at least 92% and an ash content of not more than 0.3% is currently required for use in the food sector, which is achieved by the cellulose produced by the process according to the invention.