Process for Pretreating Biomasses in Advance of Their Enzymatic Treatment

1. Field of the Invention

This invention relates to a process for pretreating biomasses in advance of their enzymatic treatment. More particularly it relates to a process for pretreating biomasses in advance of their enzymatic treatment which comprises treating a mixture of a biomass with an aqueous solution of alkaline compounds by means of a biaxial extruder.

2. Description of the Prior Art

Since the oil crisis in the year 1974 i.e. the so-called primary oil shock, review of various substitute resources and technical development for utilizing the resources have been advanced in order to avoid excessive dependency on fossil resources. Among the resources, biomasses have been particularly noted as reproducible resources, and techniques for utilizing them have been studied. Cellulose as a main component of biomasses is enzymatically or chemically decomposed into glucose which is fermentatively converted into ethanol which is, in turn, used as liquid fuels or chemical raw materials. Enzymolysis process of cellulose has such advantages that the process can be carried out under milder conditions than those in chemical decomposition process, complicated apparatus is unnecessary, etc. but the process also has the following drawbacks: (1) raw material biomasses, as they are, are difficult to be subject to the action of cellulase enzyme; (2) the reaction is slow; (3) a considerable cost is needed for enzyme preparation and its recovery and reuse are difficult; etc. As for the pretreatment process of biomasses in advance of their enzymolysis, fine-grinding treatment, alkali-cooking treatment, etc. have so far been known, but these treatments are not practical so much since any of them needs a considerable cost and treating time. Among them, Japanese patent applications Sho 56-80,998/1981 and Sho 56-80,999/1981 (Japanese patent application laid-open Nos. Sho 57-198094/1982 and Sho 57-198095/1982) (hereinafter referred to as prior inventions) are directed to substantially a combination of fine-grinding treatment with alkali-cooking treatment, and the treating time is as short as e.g. 0.25 to 4.0 hours and the percentage enzymolysis of the pretreated materials is as superior as e.g. 86.1 to 96.4 %. However, the pretreated materials of the prior inventions have been still insufficient in that even if the percentage enzymolysis is high, the treating time is long. Thus, the present inventors have made extensive research on the pretreatment process for decomposing biomasses by means of cellulase enzyme, and have succeeded in a continuous pretreatment of biomasses by means of a biaxial extruder.

The present invention resides in the folloiwng process (1) as a main aspect:

(1) A process for pretreating biomasses in advance of their enzymatic treatment, which comprises subjecting a biomass together with an aqueous solution of an alkaline compound to kneading and reaction in a biaxial extruder.

The above process (1) further has as a first aspect the following constitutions (2).about.(7):

(2) A process for pretreating biomasses in advance of their enzymatic treatment, which comprises mixing a biomass with an aqueous solution of an alkaline compound and thereafter introducing the mixture into a biaxial extruder to subject it to kneading and reaction.

(3) A process according to the above process (2) wherein said biomass is a material obtained by cutting and grinding a biomass and having an average particle diameter of 15 mm or less.

(4) A process according to the above process (2) wherein the amount of said aqueous solution of an alkaline compound is in the range of 0.6 to 2 times by weight the amount of said biomass, and the proportion by weight of said compound to said biomass is in the range of 10 to 24%.

(5) A process according to the above process (2) wherein the treating temperature of said mixture in said biaxial extruder is in the range of 60.degree. to 230.degree. C.

(6) A process according to the above process (2) wherein said biomass is at least one member selected from the group consisting of bagasse, Siebold's beech, silver magnolia, Yezo spruce (Picea jezoensis CARR), white birch, corn rachis and used papers.

(7) A process according to the above process (2) wherein said alkaline compound is NaOH, Na.sub.2 S or mixture thereof.

The above process (1) further has as a second aspect the following constitutions (8).about.(13):

(8) A process for continuously pretreating biomasses in advance of their enzymatic treatment, which comprises introducing a biomass into a biaxial extruder and thereafter subjecting in to contact, kneading and reaction with an aqueous solution of an alkaline compound in the extruder.

(9) A process according to the above process (8) wherein said biomass is a material obtained by cutting and grinding a biomass and having an average particle diameter of 30 mm or less.

(10) A process according to the above process (8) wherein said aqueous solution of an alkaline compound in an amount of 3 to 15 times by weight the amount of said biomass is injected into said biaxial extruder, and the proportion by weight of said compound to said biomass is in the range of 3 to 30%.

(11) A process according to the above process (8) wherein the treating temperature in said biaxial extruder is in the range of 60.degree. to 230.degree. C.

(12) A process according to the above process (8) wherein said biomass is at least one member selected from the group consisting of bagasse, Siebold's beech, silver magnolia, Yezo spruce (Picea jezoensis CARR), white birch, corn rachis and used papers.

(13) A process according to the above process (8) wherein said alkaline compound is NaOH, Na.sub.2 S or mixture thereof.

(i) Biomasses as a raw material of the present invention:

Any of cellulosic resources may be generally used therefor. However, since the present invention is directed to an efficient pretreatment process, the form and moisture of biomasses raise a problem. Whether the resources are arboreus plants or perbaceous plants, it is desirable that they have been cut or ground to an adequate dimension and the moisture contained therein has been adequately removed or adjusted. As to the form and moisture, in order to ease rapid permeation of the aqueous solution of an alkaline compound and washing-off of the treating solution, the length is made 15 mm or less, preferably 2 mm or less and the moisture is made 100% by weight or less, preferably 5 to 50% by weight based on their dry weight, in the case of the above process (2) as the first aspect. On the other hand, in the case of the above process (8) as the second aspect, the average particle diameter is made 30 mm or less, preferably 15 mm or less. As to the kinds of biomasses, any of cellulosic materials may be used such as arboreus materials e.g. Siebold's beech, silver magnolia, Yezo spruce (Picea jezoensis CARR), white birch, etc. or perbaceous materials e.g. bagasse, rice straw, wheat straw, corn rachis, etc. or cellulosic material e.g. used papers. Among them, bagasse which has already been ground and pressed up to a considerable extent when produced, is one of desirable raw materials.

(ii) The aqueous solution of an alkaline compound used in the present invention:

In the above process (2) as the first aspect, the solution is used in an amount of 0.6 to 2 times by weight the amount of the biomass. If the amount of the solution is less than 0.6 times by weight, rapid and uniform agitation and permeation are difficult. On the other hand, even if it exceeds twice by weight, there is no particular improvement in effectiveness. Such an excess amount is rather undesirable since the heating effect is reduced, the quantity of energy required is increased, and the moisture is squeezed out in an excess amount exceeding a certain extent by the screw and cylinder of the extruder so that the amount of the solution collects in the vicinity of the biomass-introducing port of the extruder to hinder a long time operation. In the above process (10) as the second aspect, the aqueous solution of an alkaline compound is used in an amount of 3 to 15 times the amount of biomass. If the amount is less than 3 times by weight, rapid and uniform mixing and permeation are difficult, while even if it exceeds 15 times by weight, there is no particular improvement in effectiveness. As the alkaline compound, NaOH, Na.sub.2 S or mixture thereof is used in an amount of 10 to 24% by weight based on the amount of biomass in the case of the above process (2) as the first aspect. On the other hand, in the case of the above process (10) as the second aspect, it is used in an amount of 3 to 30% by weight. If the proportion is lower than those in the above ranges, the pretreatment effectiveness of the present invention is reduced, while even if it exceeds the above ranges, there is no particular effectiveness. As the compound used, any of NaOH, Na.sub.2 S and mixture thereof is effective, and the proportion of the mixture may be optional, but the proportion by weight of NaOH/Na.sub.2 S is preferred to be in the range of 1 to 5 times.

(iii) Pretreatment conditions:

The biomasses of the above item (i) and the aqueous solution of an alkaline compound, both used in the above process (2) as the first aspect, are in advance mixed with stirring and then introduced into an extruder. As this feeder, a biaxial screw feeder may be used which has usually been used for extruding plastic pellets or powder.

The biomasses of the above item (i) used in the above process (8) as the second aspect are introduced into an extruder, and an aqueous solution of an alkaline compound is injected through a feeding port or a vent port separately provided at a biomass-feeding port, and they are kneaded and reacted. For the introduction, a biaxial screw feeder may be used which has usually been used for plastic pellets or powder.

A biaxial extruder is used for the extruder. Monoaxial extruders are unsuitable for pretreating biomasses since they are insufficient in kneading and extruding performances.

The revolving direction of the screw may be either in the same or different directions. The treating temperature (the cylinder temperature of the extruder) is in the range of 60.degree. to 230.degree. C., preferably 90.degree. to 230.degree. C. The retention time of the biomass mixed with the aqueous solution of an alkaline compound with stirring, within the biaxial extruder, i.e. the pretreating time, is one minute or shorter.

(iv) Characteristic effectiveness of the present invention:

As apparent from the foregoing description, the effectiveness of the present invention is as follows:

It is possible to carry out a continuous treatment in a very short pretreating time. Further, particularly in the case of the above process (2) as the first aspect, the liquid ratio is low; hence it is also possible to reduce the heat quantity required for heating down to a large extent. Further, in the case of the above process (8) as the second aspect, when the biomass is introduced into the extruder, it has not yet been mixed with the aqueous solution of an alkaline compound; hence it is easy to introduce the biomass.

Whereas, in the past pretreatment processes (excluding the prior inventions), for example, fine-grinding treatment requires times as long as several hours to several tens hours, and for example, alkali treatment requires cooking times as long as several tens minutes to several hours in order to obtain a percentage enzymolysis of 50% or higher. Further, cooking requires the aqueous solution of an alkaline compound in an amount of 5 to 20 times by weight the amount of biomass; hence the heat quantity required for heating water becomes very large.

The present invention will be further described in detail by way of Examples.

Various tests in Examples were carried out as follows:

To a sample (1%) and an enzyme (1%) was added a 0.1M-acetic acid buffer so as to give a volume of 10 ml. Using a L-letter test tube (18 mm.phi., 100 mm high.times.120 mm long), mono-shaking (46 reciprocations/min.) was carried out at 50.degree. C. for 4 hours. As the enzyme, Cellulase Onozuka R-10 (Trichoderma Viride Cellulase made by Yakuruto Biochemicals) was used. After the reaction, the enzymatic reaction liquor was filtered through a glass fiber filter paper (Toyo filter paper GC-50) and the filtration residue was washed with water, followed by drying at 105.degree. C. overnight and weighing. The percentage enzymolysis was calculated from the following equation: ##EQU1## Further, glucose in the filtrate was measured and the percentage enzymatic saccharification was calculated from the following equation: ##EQU2##

This content was sought by a testing method for lignin in pulp materials according to JIS P8008 (1976).

To bagasse ground to an average particle diameter of 15 mm or less was added an aqueous solution containing NaOH in an amount of 20% by weight based on the bagasse, in definite amounts shown in Table 1, followed by mixing with stirring. The mixture of bagasse was introduced into a biaxial extruder and treatment was carried out at a cylinder temperature of the extruder of 200.degree. C. After the treatment, the bagasse was washed with water and then with acetone and air-dried. The lignin content and the percentage enzymolysis were measured according to the above definite testing methods.

The results are shown in Table 1. As seen from this Table, the values of the lignin content and the percentage enzymolysis are both inferior to those of the prior inventions, but the values of the percentage enzymatic saccharification as a finally decisive factor are to the same extent to that of the prior inventions. On the other hand, the treating time of the prior inventions is 15 minutes or longer, whereas that of the present invention is as extremely short as one minute or less.

Examples 1.about.8 were repeated except that the alkaline compound was replaced by a mixture of Na.sub.2 S with NaOH. The results are shown in Table 2.

Example 3 was repeated except that the amount of NaOH was varied. The results are shown in Table 3.

Example 11 was repeated except that the amount of the mixture of Na.sub.2 S.multidot.NaOH was varied. The results are shown in Table 4.

Examples 17.about.20 were repeated except that the reagent was replaced by Na.sub.2 S. The results are shown in Table 5.

Example 26 was repeated except that the treating temperature was replaced by 60.degree..about.210.degree. C. The results are shown in Table 6.

Treatments were carried out wherein the particle diameter of sample was varied. The results are shown in Table 7.

Treatments were carried out wherein samples obtained by grinding and sieving Siebold's beech, silver magnolia, Yezo spruce and corn rachis to 2 mm or less were used. The results are shown in Table 8.

Bagasse ground to a length of 30 mm or less was introduced and a 4% aqueous solution of the mixture of Na.sub.2 S.multidot.NaOH was injected through a vent port, followed by kneading and reaction. Treatment was carried out at a cylinder temperature of extruder of 200.degree. C. After the treatment, the bagasse was filtered, washed with water and further with acetone and air-dried. The lignin content and the percentage enzymolysis of the thus treated bagasse were measured according to the definite testing methods. The results are shown in Table 9.

The present invention is different from the prior inventions in cooking conditions; hence a simple comparison between them is impossible. The former is somewhat inferior to the latter in both the lignin content and the percentage enzymolysis. However, the percentage enzymatic saccharification as a finally decisive factor, of the present invention is to the same extent as those of the prior inventions. On the other hand, the treating time of the present invention is as extremely short as one minute or less, whereas those of the prior inventions are 15 minutes or longer.

Example 3 was repeated except that the concentration of the mixture of Na.sub.2 S.multidot.NaOH was varied. The results are shown in Table 10.

Example 47 was repeated except that the particle diameter of sample was varied. The results are shown in Table 11.

Example 45 was repeated except that the temperature was varied. The results are shown in Table 12.

Treatments were carried out with samples obtained by grinding and sieving Siebold's beech, silver magnolia, Yezo spruce and corn rachis to 15 mm or less. The results are shown in Table 13.