CA1229334A - Biomass fractionation process - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A process for the delignification of cellulosic materials, wherein a high-yield cellulose pulp and high-value by-products, such as reactive lignins of low molecular weight are produced, which comprises degassing lignocellulosic material, rapidly heating the lignocellulosic material in a liquor comprising water, an organic solvent and a buffer thereby maintaining a substantially neutral solvent extraction, such that said lignocellulosic material is submerged in said liquor at a temperature in the range of 175° to 200°C for a substantially instantaneous period to about one hour at said temperature; and rapidly cooling the mixture to a temperature in the range of 20° to 200°C.

Description

I

BAHAMAS FRACTIONATION PROCESS
BACKGROUND OF THE INVENTION
Field of the Invention:
This invention relates to a process for the production of cellulose pulp from wood and the products obtained thereby.
Discretion of the Prior Art:
Numerous commercial processes exist for producing cellulose pulp from wood. In general, these commercial processes entail exposing wood chips to an aqueous, high temperature environment for greater than one hour. Inorganic chemicals are present in the aqueous phase to control pi and to facilitate lignin dissolution. Typically, the reactor (digester) temperature does not exceed 180C and the system is operated under either acidic or basic conditions. Moreover, the lignin and water-soluble organic by-products recovered from such commercial pulping processes have limited value.
The oldest alkaline pulping process is caustic soda pulping which uses a dilute aqueous sodium hydroxide solution as a pulping agent. however, in modern times, the most significant pulping process is sulfate (Raft) pulping which uses sodium sulfate (salt cake) as the alkali source. The Raft pulping liquor contains caustic soda and sodium sulfide and the process entails a cooking or pulping temperature of 160 to 185C. Kirk Other: Encyclopedia of Chemical Technology, Vol. Cellulose ~lg79).
More recently, other types of alkaline pulping processes have been developed. US. patent 3,278,367 describes the use of sodium sulfite and sodium bicarbonate as constituents of a digesting liquor which removes the cementing bases from wood 933~

chips. US. patent 4,016,029, although not directed to a pulping process, discloses a process for delignifying and bleaching cellulose pulp wherein the cellulose pulp is treated with oxygen and alkali in the presence of sodium bicarbonate. It is known that by using an oxygan-alkali bleaching process in the presence of sodium bicarbonate, the selectivity of delignification is improved.
However, the presence of strongly alkaline aqueous solutions of inorganic materials can lower the cellulose yield due to the degradation of carbohydrates such as the hemicellulose fraction and lead to the generation of undesirable side reactions of the water soluble organic by-products. moreover, the use of sulfites and sulfides pose the additional problem of air and water pollution. In Raft pulping, for example, the volatile, malodorous sulfur compounds hydrogen sulfide, methyl mercaptan, dim ethyl sulfide, and dim ethyl disulfide are produced as by-products. Kirk-Othmer: cyclopedia of Chemical Technology, Vol. 19 Pulp (19~2).
There are also commercial processes which utilize a digesting liquor containing organic solvents. These liquors utilize organic solvents and water, but not co-added buffers. In a typical organic solvent pulping process or organosolv pulping process, as such processes are more commonly known, an organic solvent, such as ethanol, is incorporated in the digester charge along with water and wood chips. The residence time in the digester is generally in excess of one hour and reaction temperatures are usually less than 200C. For example, US.
patent 3,585,104 describes the use of a digesting liquor containing an aqueous mixture of lower aliphatic alcohols, such ~2~33~

as methanol, ethanol, propanol or aqueous mixtures of the lower aliphatic kittens, such as acetone, as appropriate digesting or pulping agents. Additionally, US. patent 3,585,104 discloses a pulping temperature of between 150C and 200C.
It is also possible to effect delignification by steam pulping. Steam pulping is characterized by short residence times, less than 15 minutes, and high operating temperatures of above 200C. In steam pulping very little lignin is solubilized in the digester. Moreover, this lignin must be extracted afterwards by a suitable solvent.
However, each of the available commercial pulping processes has a particular disadvantage. The aqueous pulping liquors containing inorganic material such as sodium hydroxide often cause nonselective degradation of the liberated cellulose pulp as well as of the lignin. At elevated temperatures, several types of alkaline degradative reactions of polysaccharides are known. Kirk-Othmer: Encyclopedia of Chemical Technology, Vol.
19 Pulp (1982). The use of inorganic materials such as sodium sulfite or sulfide, while perhaps more selective, can, nevertheless, cause severe air and water pollution problems.
Moreover, the lignin and water-soluble organic by-products from pulping processes involving these pulping liquors have limited value. Raft lignin and other by-products are usually burned in a recovery boiler. Sulfite lignin is often used as a low cost drilling mud.
While a more valuable lignin by-product is provided by steam or organGsolv pulping, solvent recovery must be incorporated in organosolv pulping. In steam pulping above 200C, the liberated cellulose is rapidly degraded, reducing its market value.

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Moreover, in steam pulping lignin must be extracted in a separate operation in order to obtain a pulp high in cellulose content.
Hence, such processes tend to be very energy intensive.
Therefore, a need continues to exist for a method of Bahamas fractionation wherein both high yield cellulose pulp and high-value by-products are obtained in a cost-effective manner. In order to be cost effective, the method must be non-polluting and not energy intensive.
SUMMERY OF THE INVENTION
Accordingly, the present invention seeks to provide a process for the delignification of cellulosic materials wherein a high yield of cellulose pulp is produced.
This invention also seeks to provide a process for the delignification of cellulosic materials wherein the cellulose in the pulp has a high molecular weight.
Further, this invention seeks to provide a process for the delignification of cellulosic materials wherein a reactive Lenin is produced.
Moreover, this invention seeks to provide a process for the delignification of cellulosic materials wherein no by-product furfural is formed.
Still further, the present invention seeks to provide a pulping liquor composition for effecting the above-mentioned process.
It is also an aspect of the present invention to produce a cellulose pulp having a high molecular weight cellulose having less degradation of the hemicellulose fraction of the pulp than is caused by existing commercial processes.
Further, this invention seeks to produce a low molecular I.

weight, reactive lignin having high market value.
More particularly the invention in one aspect pertains to a process for the delignification of lignocellulosic materials to extract lignin therefrom in low molecular weight, reactive form to yield a high percentage of pulp containing high molecular weight cellulose and a low content of lignin. The process comprises the steps of degas sing lignocellulosic material, rapidly heating the lignocellulosic material submerged in a liquor comprising water, an organic solvent and a buffer in sufficient amount to maintain a substantially neutral solvent extraction yielding a final pi of between 6 - 8, to a temperature in the range of approximately 175 to 280C, and maintaining such temperature for a substantially instantaneous period to about an hour, and rapidly cooling the mixture to a temperature in the range of 20 to 200C.
The invention in another aspect comprehends a cellulose pulp having a high molecular weight cellulose produced by the process of degas sing lignocellulosic material, rapidly heating a mixture of the lignocellulosic material in a liquor comprising water, an organic solvent and a buffer, in sufficient amount to maintain a substantially neutral solvent extraction, to a temperature in the range of approximately 175C to 280C for a substantially instantaneous period to about one hour at the temperature, rapidly cooling the mixture to a temperature in the range of 20 to 200C.
A still further aspect of the invention comprehends a low molecular weight and reactive lignin produced by the process of degas sing lignocellulosic material, rapidly heating a mixture of the lignocellulosic material in a liquor comprising water, an organic solvent and a buffer in sufficient amount to maintain a substantially neutral solvent extraction, to a temperature in the range of 175 to 280C for a substantially instantaneous period to about one hour at the temperature, and rapidly cooling the mixture to a temperature in the range of 20 to 200C.
A still further aspect of the invention comprehends the process of pulping a lignocellulosic material to extract lignin therefrom in low molecular weight, reactive form to yield a high percentage of pulp containing high molecular weight cellulose and a low content of lignin. The process includes the steps of (a) providing a liquor comprising water, an organic solvent and a buffer, (b) rapidly heating a mixture of the liquor with the lignocellulosic material immersed therein to a predetermined temperature, (c) maintaining the predetermined temperature for a predetermined residence time, (d) rapidly cooling the mixture from the predetermined temperature, and (e) controlling the extraction to be one principally achieved by high temperature and short residence time under substantially neutral conditions by selecting the temperature of step (b) within the range of 175C
to 280C such that the residence time of step (c) is in the range of instantaneous to less than 50 minutes, and controlling the amount of buffer in the liquor to provide a substantially neutral pi for the mixture during steps (c) and (d).
An unbleached cellulose pulp having a high molecular weight comprising lignin in an amount less than 5% by weight and hemicellulose in an amount more than 20% by weight is obtained by the process and further low molecular weight lignin may be extracted in accordance with the process.

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DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
According to the present invention, a high yield of cellulose pulp and a reactive lignin is obtained by digesting lignocellulosic material, such as wood chips, in a liquor using short residence times and high operating temperatures. Wood chips or other lignocellulosic material are degassed to remove gases which cause oxidation of the wood chips and to promote uniform penetration of the pulping liquor. The wood chips are rapidly heated in a liquor containing water, an organic solvent and a buffer such that the wood chips are submerged in the liquor, at a temperature of 175 or greater for a substantially instantaneous period of up to an hour at the elevated temperatures. Then, the mixture is rapidly cooled.
While the wood chips may be rapidly heated to a temperature of 175 or greater, it is preferred that the wood chips be rapidly heated to a temperature in the range of approximately 175C to 280C over a period of less than So minutes. Temperatures in excess of about 280C may result in degradation of the liberated cellulose. However, it is especially preferred to rapidly heat the submerged wood chips to a temperature in the range of 200C to 280C over a period of less than 10 minutes.
The digestion of the wood chips in the liquor composition of the present invention is performed for a substantially instantaneous period up to an hour at I 33~
the elevated temperatures. However, it is preferred to perform the digestion of the wood chips in less than 45 minutes.
Neutral conditions are maintained to minimize acid/ba~e hydrolysis of the cellulose and lignin condensation reactions.
After digestion, the mixture is rapidly cooled. It is preferred to rapidly cool the mixture to a temperature in the range of 20~C to 200C over a period of less than 30 minutes. However, it is especially preferred to rapidly cool the mixture to a temperature in the range of 20 to 90C over a period of less than 5 minutes.
A wide variety of organic solvents may be used.
However, preferred as the organic solvents are the straight-chain or branched-chain alcohols having up to about 8 carbons. For example, alcohols such as isopropyl alcohol or ethyl alcohol can be used.
However, ethanol and n-propyl alcohol are the preferred alcohols. A solvent to wood ratio in the range of 3 to 30 ml of solvent to 1 gram of wood may be use However, it is preferred to use a solvent to wood ratio in the range of 4 to 15 ml per 1 gram of wood.
It is preferred that the digesting process of the present invention be carried out as a neutral-process.
More particularly, the neutral solvent extraction of lignin from cellulosic materials reduces the formation of undesired by-products such as furfural and condensed lignins.

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g A neutral solvent extraction may be carried out by the addition of a buffer or by the addition of an acid or a base to adjust phi As a buffer, the use of sodium bicarbonate or sodium bicarbonate is preferred. The controlled addition of a base such as sodium hydroxide may also be employed to maintain a neutral state.
It is desirable to use about 0.5 to 10 grams of buffer material per 15 grams of dry wood. However, it is preferred to use 1 to 5 grams of buffer material lo per 15 grams of dry wood.
While the initial pi of the reaction mixture may be as high as about 10.0, it is important -that the pi ox the product mixture drop and be maintained at a range of about 6.0 to 8.0 during operation, as measured by a conventional pi meter at ambient temperature. It is preferred that this pi be in the range of about 6.8 to 7.5.
Additionally, an organic stabilizer and/or inhibit ion may be added to the liquor to help increase pulp yield while reducing the yield of undesired by-products. For example, the use of methylan~hraqu;none (MA) as an additive at elevated temperatures protects carbohydrate peeling and reduces lignin condensation. Another additive found to work well is knothole. As a general rule, quinines containing election donating groups and fused aromatic ring compounds capable of undergoing a single electrophylic substitution would serve as additives in the present invention.

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-pa-Different additives appear to inhibit different side reactions and, therefore, it may be preferable to use a combination of organic additives. When used, it is preferred to use an additive or combination of additives in an amount up to 15~ of the lignocellulosic material on a dry weight basis.
The obtained lignin has a high market potential as it has a low molecular weight and is more reactive than that obtained by present commercial pulping processes.
The molecular weight range of the lignin is 200 to 10,000. For example, it can be readily converted to monoaromatic compounds in high yield. One approach for 33~

such conversion is catalytic hydrogenolysis a proposed by Hydrocarbon Research, Inc. Gentler, Hebrews and Park Hurst, Wood and Agricultural Residues: Research on Use for Feed, Rules, and Chemicals, E. Solves, Ed., Academic Press, pp. 391-400 (1983). Moreover, the lignln of the present invention can be directly substituted for phenol in numerous polymer applications.
The lignin obtained by the present process has a low molecular weight due to the reduced degree of lignin condensation occurring in the present extraction liquor which maintains a substantially neutral pi during delignification. Thus, both the cellulose pulp and lignin obtained from this invention have high market value, whereas in commercial pulping processes only the cellulose pulp is of value.
The cellulose pulp it produced in high yield by the present process. Moreover, the cellulose in the liberated pulp has a high molecular weight. Also, by using the process of the present invention, it is possible to obtain a cellulose pulp having less degradation of the hemicellulose fraction. As the hemicelluloses are the most abundant of the polymeric cementing materials which hold the cellulose fibers together, their preservation results in the isolation of I

a superior fiber. By maintaining the fibrous structure of the cellulose, the process of the present invention is useful for producing cellulose derivatives as well as non woven cellulosic structures. When the present process is performed under its preferred conditions, it will typically yield an unbleached cellulose pulp leaving less than 5% lignin and more than 20%
hemicellulose by weight.
It is important to degas the wood chips prior to rapidly heating the submerged wood chips in the digesting liquor. By degas sing the wood chips, the rate of oxidative degradation of the liberated cellulose is reduced, so as to ensure the attainment of a cellulose of high molecular weight in the cellulose pulp. Also degas sing facilitates liquor penetrative into the chips, providing more uniform delignificatlon. In degas sing the wood chips, an ordinary aspirator apparatus may be used to produce a vacuum. These devices are well-known, and their operation is understood by those skillet in the Laboratory sciences. Alternatively, hot soaking by means such as hot liquor reflex at atmospheric pressure or steaming may be employed to accomplish degas sing and uniform pulping liquor penetration.
Digestion of the wood chips in a liquor of water, an organic solvent and A buffer allows the lignin to syllables, reduces the degradation of carbohydrate materials and reduces lignin condensation.

to -ha-The present process also utilizes short residence times and high operating temperatures. It has been found that by utilizing these conditions, it is possible to effect the separation of the cellulose from the lignin with the least damage to the liberated cellulose.
The use of high temperatures ensures rapid delignification. The use of short residence times ensures minimal product degradation. These conditions allow high production rates per unit volume thus allowing low processing costs.
Generally, it is preferable to raise the temperature of the pulping mixture as quickly as possible. Similarly it is preferable to reduce the pulping temperature of the mixture after treatment as quickly as possible in order to reduce the amount of time during which the wood chips are at high temperature.
Systems which may be utilized to heat and cool the pulping liquor mixture of the present invention are known to those skilled in the pulping art. For example, rapid heating can be achieved by vapor condensation.
Similarly, rapid cooling can be achieved by vaporization at reduced pressure.
In addition to using the preferred lower alcohols having up to 8 carbons, any of the other conventional organic solvents used in the delignification of cellulosic materials may be used in the present process. For example, aqueous solutions of lower aliphatic kittens may also be used in the present invention. Aqueous solutions of other organic solvents such as ethylene glycol, or Dixon may also be used.
Other buffers may be used instead of sodium bicarbonate, such as sodium carbonate, sodium phosphate, sodium citrate, and calcium phosphate. Any buffer may be used which functions to maintain a substantially neutral solvent extraction, thereby reducing the unwanted formation of by-products. The selection of such a buffer may be easily made by anyone skilled in the chemical arts.
Moreover, batch digesters may be used as well as continuous divesters. A varying temperature profile may be used as well as a sequential addition of solvent and buffer. Compared to organosolv pulping, the present invention provides a higher yield of cellulose pulp and higher production rates per unit of reactor volume.
Also, practically any hard or soft wood or other lignocellulosic material may be treated according to the present invention. Of course, by treating wood chips it it possible to expose a large surface area of the wood to the pulping liquor, thereby producing better results.
The present invention will be further illustrated by certain examples and references, tabulated below, which are provided for purposes of illustration only and are not intended to limit the present invention ~2C~33~ -lo in C to 'l' cud 8 -- on Al or o CDC~ I Us I
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In the control and examples 1-22 presented in Table I ground tulip poplar is subjected to a vacuum using an aspirator. Solvent, buffer and any additive are added before the vacuum is broken. The mixture of wood and liquor, which comprises the solvent, buffer and any additive is rapidly heated in a pressure reactor to the temperature indicated in Table I, held at that temperature for the specified residence time, and rapidly cooled to ambient temperature. The pi of the liquor after pulping is reported as the final pi in Table I. The pulp is then washed, filtered and dried to obtain the pulp yield The lignin content of the pulp is determined by the Klason lignin test (TAIPEI Standard T222).
The control presented in Table I does not have a buffer, organic co-solvent or any additive prevent. A
low yield pulp with a high residual lignin content is obtained. Also the final pulping liquor is acidic reflecting the acids formed during pulping.

In examples 23 and 24 presented in Table II, below, ground tulip poplar is subjected to a vacuum using an aspirator. Solvent, buffer and additive are added before the vacuum is broken. The mixture of wood and liquor is rapidly heated in a pressure reactor to the temperature indicated in Table II, held at that temperature for the specified residence time, and rapidly cooled to ambient temperature. The pulp is then washed, filtered and dried to obtain the pulp yield.
I- The lignin content ox the pulp is again determined by my t Jo r r,'", or I " I, ,, 933~

the Klason lignin test TAIPEI Standard T222). The carbohydrates in the pulp were analyzed by gas chromatography by converting the carbohydrate to alditol acetates, according to TAIPEI Standard Procedure TP249 as modified by Borchardt and Hasty, Taipei, 65(~),127(1982) The hemicellulose content of the pulp reported in Table II is the sum of the Arabian, galactan, manna and Dylan present.
Examples 23 and 24 illustrate that the pulps obtained retain a substantial portion of hemicellulose after a majority of the lignin present in the wood has been removed.

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Having now fully described this invention, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made thereto without departing from the spirit or scope of the invention as set forth herein.

Claims (24)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for the delignification of lignocellulosic materials to extract lignin therefrom in low molecular weight, reactive form to yield a high percentage of pulp containing high molecular weight cellulose and a low content of lignin, comprising the steps of:
(a) degassing lignocellulosic material;
(b) rapidly heating the lignocellulosic material submerged in a liquor comprising water, an organic solvent and a buffer in sufficient amount to maintain a substantially neutral solvent extraction yielding a final pH of between 6 - 8, to a temperature in the range of approximately 175° to 280°C, and maintaining such temperature for a substantially instantaneous period to about an hour; and (c) rapidly cooling said mixture to a temperature in the range of 20° to 200°C.

2. The process as claimed in Claim 1, wherein said organic solvent comprises a straight-chain or branched-chain alcohol having up to about 8 carbon atoms.

3. The process as claimed in Claim 2, wherein said alcohol is selected from the group consisting of ethanol and n-propyl alcohol.

4. The process as claimed in Claim 1, 7 or 3 wherein said buffer is selected from the group consisting of sodium bicarbonate, sodium carbonate, sodium phosphate, sodium citrate and calcium phosphate.

5. The process as claimed in Claim 1, 2 or 3 wherein said submerged lignocellulosic material is rapidly heated to a temperature of between about 200°-280°C, and digested at said temperature for less than 45 minutes.

6. The process as claimed in Claim 1, 2 or 3 wherein said heated, digesting lignocellulosic material is rapidly cooled to temperature in the range of 20° to 90°C.

7. The process as claimed in Claim 1, 2 or 3 wherein the solvent to wood ratio is in the range of 3 to 30 ml of solvent to 1 gm of wood.

8. A process as claimed in Claim 1, 2 or 3 wherein said lignocellulosic material is wood chips.

9. A process as claimed in Claim 1, 2 or 3 wherein the degassing is carried out in a vacuum.

10. A process as claimed in Claim 1, 2 or 3 wherein the degassing is carried out by liquor reflux at atmospheric pressure.

11. A process as claimed in Claim 1, 2 or 3 wherein the degassing is carried out by steaming.

12. A process as claimed in Claim 1, wherein said liquor includes an additive selected from the group consisting of anthraquinones, containing electron donating groups and fused aromatic ring compounds capable of undergoing a single electrophylic substitution.

13. A process as claimed in Claim 12, wherein said additive group is selected from the group consisting of methylanthraquinone and 2-napthol.

14. A process as claimed in Claim 12, wherein said additives are present in an amount up to 15% of dry lignocellulosic material by weight.

15. A cellulose pulp having a high molecular weight cellulose produced by the process of:
(a) degassing lignocellulosic material;
(b) rapidly heating a mixture of the lignocellulosic material in a liquor comprising water, an organic solvent and a buffer, in sufficient amount to maintain a substantially neutral solvent extraction, to a temperature in the range of approximately 175°C to 280°C for a substantially instantaneous period to about one hour at said temperature;
(c) rapidly cooling said mixture to a temperature in the range of 20° to 200°C.

16. A low molecular weight and reactive lignin produced by the process of:
(a) degassing lignocellulosic material;
(b) rapidly heating a mixture of the lignocellulosic material in a liquor comprising water, an organic solvent and a buffer in sufficient amount to maintain a substantially neutral solvent extraction, to a temperature in the range of 175°C to 280°C for a substantially instantaneous period to about one hour at said temperature;
(c) rapidly cooling said mixture to a temperature in the range of 20° to 200°C.

17. The process of pulping a lignocellulosic material to extract lignin therefrom in low molecular weight, reactive form to yield a high percentage of pulp containing high molecular weight cellulose and a low content of lignin, which includes the steps of:
(a) providing a liquor comprising water, an organic solvent and a buffer;
(b) rapidly heating a mixture of said liquor with the lignocellulosic material immersed therein to a predetermined temperature;
(c) maintaining said predetermined temperature for a predetermined residence time;
(d) rapidly cooling the mixture from said predetermined temperature; and (e) controlling the extraction to be one principally achieved by high temperature and short residence time under substantially neutral conditions by:
(1) selecting the temperature of step (b) within the range of 175°C to 280°C such that the residence time of step (c) is in the range of instantaneous to less than 50 minutes, and (2) controlling the amount of buffer in the liquor to provide a substantially neutral pH for the mixture during steps (c) and (d).

18. The process as defined in Claim 17 wherein the temperature of step (b) is in the range of 200 - 280°C.

19. The process as defined in Claim 18 wherein the residence time of step (c) is less than 45 minutes.

20. The process as defined in Claim 18 wherein the residence time of step (c) is less than 10 minutes.

21. The process as defined in Claim 17 including the step of degassing said lignocellulosic material prior to step (b).

22. An unbleached cellulose pulp having a high molecular weight comprising lignin in an amount less than 5% by weight and hemicellulose in an amount more than 20% by weight obtained by the process of Claim 21.

23. An unbleached cellulose pulp as defined in Claim 22, further comprising xylan in an amount more than 10%.

24. Low molecular weight lignin extracted in accordance with the process of Claim 21.