WO1998031871A1

WO1998031871A1 - Method for fractionating cereal straw, in particular wheat and barley

Info

Publication number: WO1998031871A1
Application number: PCT/FR1998/000082
Authority: WO
Inventors: Luc Rigal; Christian Magro; Antoine Gaset
Original assignee: Ardeval Champagne Ardenne
Priority date: 1997-01-16
Filing date: 1998-01-06
Publication date: 1998-07-23
Also published as: FR2758344A1; FR2758344B1

Abstract

The invention concerns a method for fractionating cereal straw, in particular wheat and barley which consists in contacting the straws with an aqueous basic solution, then subjecting the resulting mixture simultaneously to compression and shearing at a temperature ranging between 20 °C and 100 °C while injecting water, then pressing it for eliminating the excess water to obtain a matrix rich in cellulose fibres and a liquid phase rich in hemicellulose. The invention is applicable to the upgrading of straws.

Description

Title: Process for fractionating cereal straws, in particular wheat and barley.

The present invention relates to a process for fractionating cereal straws, in particular wheat and barley.

Numerous processes are already known for fractionating cereal straws in order to allow either better use by animals or industrial applications.

Thus, in order to improve the digestibility of lignocellulosic substances such as cereal straws, it is known to subject them to an alkaline treatment in an extruder in order to simultaneously disintegrate the substrate. Such a process is for example described in document US-A-4,997,488.

The extraction of hemicelluloses contained in these straws has also been the subject of studies. This is, for example, described in documents US-A-1,819,233 and US-A-2,868,778. The use of an extractor to extract hemicelluloses by breaking the ligniπe-hemicellulose-cellulose complex has also been studied.

Likewise, a large number of methods of hydrolysis of hemicelluloses are known after their alkaline extraction, such as those described in documents EP-A-265,111, EP-A-178,777, US-A-4,070,232, FR-A- 2.327.314, FR-A- 2.327.313. Another traditional use of straws is to mix them with clay to obtain a building material. But in order to allow the use of straw with other materials, studies have been carried out, such as that which is the subject of the publication published in Research and Industry, vol. 24, March 1979, pp. 1 -5 and entitled “Physio-Chemical protection of agricultural residue and strength of Portland cement-bouπd wood products” by SM Singh. So we have tested the manufacture of light mixed materials in association with a hydraulic binder of the cement or plaster type But the incompatibility of lignocellulosic materials with the mineral matrix requires a mineralization pretreatment or the addition of binding additives Also one of the aims of the is the present invention to provide a process for fractionating cereal straws, in particular wheat and barley, in order to obtain highly accessible activated cellulose fibers and hemicelluloses

Another object of the present invention is to provide such a treatment process which makes it possible to manufacture from cellulosic fibers thus obtained, materials which can be molded from such materials find multiple applications, in particular in the fields of packaging and packaging. packaging In addition, these materials are recyclable and compostable

A further object of the present invention is to provide such a process which makes it possible to obtain highly accessible cellulose fibers compatible with hydraulic binders such as cements.

Finally, as said previously, another object of the present invention is to provide such a method which makes it possible to obtain hemicelluloses in the form of powders.

These and other objects which will appear subsequently are achieved by a process for fractionating cereal straws, in particular wheat and barley straws, which is characterized, according to the present invention, by the fact that in contact the straws with a basic aqueous solution, then submitting the mixture thus obtained simultaneously to compression and shearing at a temperature between 20 ^Ù C and 100 ° C while injecting water, then which is pressed in order to remove the excess water and to obtain a matrix of cellulosic fibers and a liquid phase rich in hemicelluloses.

Advantageously, the equivalent OH ^' / mass of dry straw material is between 0.75 and 25 moles / kg; the alkaline solution is in particular an aqueous sodium hydroxide solution whose sodium hydroxide concentration is less than or equal to 50%, and preferably between 5 and 20%.

The total amount of water injected, that is to say the amount of water present in the aqueous sodium hydroxide solution and that injected during shearing, is such that the water / dry straw weight ratio is between 1 and 15. The matrix of recovered cellulosic fibers is thermoformed to obtain molded forms without prior incorporation of a binder or mixed with a hydraulic binder and molded to obtain fibrous composite materials which can be molded with or without mechanical or thermal stress.

Preferably, the liquid phase rich in hemicelluloses is subjected to a concentration in order to obtain in particular a concentrate rich in hemiceiluloses. These hemicelluloses are precipitated by adjusting the pH to a value of about 5 and adding alcohol. The precipitate thus obtained or hemicellulose powder can be easily dried.

The process for fractionating cereal straws, in particular wheat and barley straws, according to the present invention comprises the following steps: a) The straw is brought into intimate contact with an aqueous alkaline solution such as an aqueous sodium hydroxide solution. The equivalent OH ^' / dry matter of straw ratio is between 0.75 and 25 moles / kg; in the case of an aqueous sodium hydroxide solution, the sodium hydroxide concentration of this solution is less than or equal to 50% and, preferably, between 5 and 20%. In this case, the report is based on rai soda / dry matter of straw is between 0.03 and 1. At the end of this step, a heterogeneous mixture is obtained. b) This heterogeneous mixture is subjected to compression and simultaneously to shearing, while bringing it to a temperature between 20 ° C. and 1,00 ° C. and by injecting water so that the water / matter weight ratio dry straw is between 1 and 15: this has in particular the aim of making the cellulosic fibers contained in the straw accessible and consequently of activating chemical reactions and physical transformations. c) The product from step b) is separated, for example by pressing, in order to obtain, on the one hand, a liquid phase rich in hemicelluloses and, on the other hand, a matrix of cellulosic fibers.

For the implementation of the above steps, it is preferable to use a device with a cylindrical wall, and with an elongated axis, and comprising two corotating screws and axes parallel to each other and to the axis of the device. . This device is divided into four zones from upstream to downstream according to the direction of advancement of the straws as the steps progress.

The first zone, or upstream zone, has a large opening in the wall through which the cereal straws are introduced. The screw, or set of screws, located in this zone is said to be direct pitch and has the function of conveying the straws into the second zone further downstream.

At the entrance to this second zone is located in the wall a radial injector of the alkaline aqueous solution, in this case soda. In this zone, the cylindrical wall includes heating means making it possible to bring the mixture to a temperature between 20 ° C and 100 ° C. The screw has a suitable profile in this area to impose the following effects on the mixture: - a shearing effect which reduces the size of the straw particles which corresponds to grinding and defibering,

a kneading effect which allows the straws constituting a solid phase to be brought into intimate contact with the aqueous sodium hydroxide solution injected constituting a liquid phase,

- a compression effect which promotes the diffusion of the liquid phase within the lignocellulosic matrix with a view to allowing the hemicelluloses to be dissolved,

a retention effect of the mixture intended to increase the residence time in the apparatus to allow the advancement of the hemicellulose solubilization reaction,

- and an effect of conveying the mixture to the third zone

A screw profile to obtain such a combination of effects is, for example, a series of mixing elements of a shape suitable for shearing and mixing bilobes, trilobes or toothed wheels.

- screw elements, single or double thread, with direct decreasing pitch to allow the transport of the material, with shear stress (co-penetrating screws) and axial compression (reduction of the screw pitch), - elements screw, single or double thread, reverse pitch, whose role is to form a plug of material which ensures the compression of the mixture In this element of screw, the shearing and mixing action is also very intense The mixture crosses the strip thanks to the space between the screws and the wall, and / or to slots made in the screw In the third zone, the intimate mixture of straw - aqueous soda solution which has crossed the strip, is taken up by direct pitch screws to be conveyed to the water injection point. The mixture is thus relaxed to promote exchanges with water in order to ensure the extraction of hemicelluloses solubilized by the chemical action of soda, by dilution (or washing) effect. The liquid-solid mixture is then subjected to a mechanical effect of mixing and compression thanks to the combination of a sequence of screws with decreasing direct pitch and reverse pitch. The mixing effect promotes the contacting of the phases and the dilution of the solubilized hemicelluloses; the compression effect ensures the pressing (spinning) of the mixture in order to evacuate the liquid through filter grids arranged in the cylindrical wall of the device. The collected filtrate contains the solubilized hemicelluloses.

In the downstream zone or fourth zone, the product drained, which has crossed the strip, is taken up by direct-pitch screws to be sent to the outlet of the device. The activated and highly accessible fiber may, in this downstream area, undergo a drying and shaping treatment, for example, by extrusion through a die. For drying, the cylindrical wall will include heating means and means for removing the water vapor formed.

The matrix of cellulosic fibers, collected at the outlet of the apparatus, as described above, can be molded by thermoforming to obtain molded forms sufficiently resistant to be substituted for parts obtained from plastics or composites in their applications. Such shapes cannot be obtained by direct thermoforming of straws, even after grinding. Furthermore, this matrix of cellulosic fibers can be incorporated into a hydraulic binder such as a cement to produce a lightweight composite material, the mechanical strength of which allows its use in the construction field, in particular for insulation, facing and rehabilitation. As for the liquid phase rich in hemicelluloses, this can be subjected to settling and concentration by ultrafiltration on the membrane to obtain, on the one hand, a permeate which can be recycled or evacuated and, on the other hand , a concentrate rich in hemicelluloses.

By adjusting the pH of the concentrate to a value of about 5 and adding alcohol, the hemicelluloses are precipitated and collected by filtration: the hemicellulose powder thus obtained can be easily dried. The high purity of the hemicelluloses contained in this powder is demonstrated by its high xylose content obtained by their acid hydrolysis: these yields are of the order of 86%. The method according to the present invention was compared with that described by

RW DETROY et al. with regard to the production of xylose from straws In Table I below, the elements of this comparison have been grouped:

BOARD

Hydrolysis of this hemicellulose powder is carried out, for example in an autoclave. In Table II below, the results obtained have been grouped according to different operating conditions.

TABLE II

* mo = organic matter As for the matrix of cellulosic fibers which crosses the fourth zone of the reactor, it is thermoformable either directly, or after washing with acidulated water. The formed products thus obtained have a smooth and homogeneous appearance. They can be given during thermoforming of complex shapes, for example conical, rounded, tee ... These products are machinable, nailable, sawable and printable in particular.

Thus, 50 g of dry matter of cellulose fibers were introduced after acidic washing and drying, in a cylindrical mold having a diameter of 100 mm and placed in a hydraulic press with a heating plate.

In Table III, the results of three trials are combined.

TABLE III

It is also possible to incorporate the matrix of cellulosic fibers washed with water in a paste of Portland cement (water cement: about 0.5) under mixing. The mixture thus obtained is shaped by molding. The composite material thus produced has densities and mechanical strengths which are a function of the rate of incorporation of cellulosic fibers, as shown by the results grouped in Table IV below.

TABLE IV

In order to further understand the present invention, there will be described examples of embodiments of the present invention. In these examples, the same apparatus with a cylindrical wall and an elongated axis is used. This apparatus or reactor is, for example, of the twin-screw type such as that sold by the company CLEXTRAL. Example 1 In this apparatus or reactor, straw is introduced. at a flow rate of 4.3 kg / h is related to the dry matter 4 kg / h The basic aqueous solution is a sodium hydroxide solution at 15% sodium hydroxide, and is injected with a flow rate of 10.7 kg of sodium hydroxide solution / hour The weight ratio of soda / straw dry matter is 0.4. Water is injected at a rate of 27.6 kg / h and the temperature is

About 50 ° C

The reactor screws have a rotational speed of the order of 170 revolutions / minute

The total water / dry matter weight ratio of straw is approximately 9.25

At the end of the reactor, therefore from step b above, a flow rate of cellulose fibers of 2.7 kg of dry matter / hour is obtained, ie a yield of approximately 67.5%. As for the purity of cellulose, it is approximately 72% relative to the organic matter of cellulosic fiber introduced. The filtrate is collected in the third zone of the reactor with a flow rate of the order of 2.9 kg of dry matter / hour. hemicelluloses is approximately 24.5 g / kg, i.e. an efficiency of hemicellulose extraction, that is to say ratio of solubilized hemicelluloses to initial hemicelluloses, of approximately 75% The yield of separate hemicelluloses present in the filtrate is about 58%

Solid particles of size less than 1 μm (10 ^"s m) suspended in this filtrate are eliminated for example by centπfugation The dry matter thus settled is about 10% of the dry matter of straw introduced This dry matter is advantageously reincorporated in the cellulosic fiber the total fiber yield is therefore around 70% compared to the dry matter of treated straw

The filtered filtrate is concentrated 5 times by ultrafiltration and then acidified to a pH of about 5 with acetic acid and completed volume by volume with ethanol at 95 °. This gives a precipitate of hemicelluloses which is isolated by filtration, for example using a filter press system The powder hemicelluloses thus collected is dried in the open air until its water content becomes less than 10% by weight. The hemicellulose powder yield is approximately 15% relative to the straw dry matter for a production of 0.6 kg of dry matter / hour, corresponding to a yield relative to the hemicelluloses introduced of approximately 50%. The mineral matter of this hemicellulose powder is 6.6% by weight relative to the dry matter, and its sugar content is greater than 89% by weight relative to the dry matter, including 70% by weight of xylose. compared to dry matter. Example 2 In this apparatus or reactor, straw is introduced at a flow rate of

3 kg / h, or related to the dry matter 2.8 kg / h. The basic aqueous solution is a 15% sodium hydroxide solution and is injected with a flow rate of 10.7 kg of sodium hydroxide / hour. The weight ratio soda / dry matter of straw is 0.4.

Water is injected with a flow rate of 23.7 kg / h and the temperature is approximately 50 ° C.

The reactor screws have a rotation speed of the order of 200 revolutions / minute.

The total water / dry matter weight ratio of straw is approximately 12.9. At the end of the reactor, therefore from step b above, a flow rate of cellulose fibers of 1.32 kg of dry matter / hour is obtained, ie a yield of approximately 47%. As for the purity of the cellulose, it is approximately 81% relative to the organic matter of cellulosic fiber introduced.

The filtrate is collected in the third zone of the reactor with a flow rate of the order of 3.65 kg of dry matter / hour. The hemicellulose concentration is approximately 18 g / kg, or an efficiency of hemicellulose extraction, that is to say the ratio of solubilized hemicelluloses to initial hemicelluloses, of approximately 88%. The yield of separate hemicelluloses present in the filtrate is approximately 77%. The hemicellulose powder yield is approximately 16% compared to the dry matter of the straw for a production of 0.4 kg of material per hour.

As for the cellulosic fibers produced in this example, they have the same physical properties as those of Example 1. Example 3 In this apparatus or reactor, straw is introduced at a flow rate of 7.7 kg / h, ie 7 kg / h of dry matter. The basic aqueous solution is a sodium hydroxide solution at 7% sodium hydroxide, and is injected with a flow rate of 9.5 kg of sodium hydroxide / hour. The weight ratio soda / dry matter of straw is 0.095. Water is injected with a flow rate of 23 kg / h and the temperature is

50 ° C approximately.

The reactor screws have a rotational speed of the order of 170 revolutions / minute.

The total water / dry matter weight ratio of straw is approximately 4.5.

At the end of the reactor, therefore from step b above, a flow rate of cellulose fibers of 6.3 kg of dry matter / hour is obtained, ie a yield of approximately 90%. As for the purity of the cellulose, it is approximately 72% relative to the organic matter of cellulose fiber introduced. In this exemplary embodiment, the yield of hemicelluloses extracted being low, the filtered filtrate is directly recycled into the reactor with straw.

On the other hand, the cellulosic fibers obtained have properties similar to those of Example 1.

Claims

1. Process for fractionating cereal straws, in particular wheat and barley straws, characterized in that the straws are brought into contact with a basic aqueous solution, then that the mixture thus obtained is simultaneously subjected to compression and shearing at a temperature between 20 ° C and 100 ° C while injecting water, then pressing to remove excess water to obtain a matrix of cellulosic fibers and a liquid phase rich in hemicelluloses.

2. Method according to claim 1, characterized by the weight ratio soda / straw dry matter is between 0.03 and 1.

3. Method according to claim 1, characterized in that the aqueous sodium hydroxide solution has a concentration at most equal to 50%.

4. Method according to claim 3, characterized in that the concentration of the aqueous solution is between 5 and 20%.

5. Method according to any one of claims 1 to 4, characterized in that the total quantity of water injected is such that the weight ratio water / dry matter of straw is between 1 and 15.

6. Method according to any one of claims 1 to 5, characterized in that the recovered cellulose fiber matrix is thermoformed to obtain molded forms without prior incorporation of a fibrous composite binder or mixed with a hydraulic binder and molded to obtain materials.

7. Method according to claim 1, characterized in that the liquid phase rich in hemicelluloses is subjected to a concentration to obtain a concentrate rich in hemicelluloses.

8. Method according to claim 7, characterized in that hemicelluloses are precipitated by adjusting the pH to a value of approximately 5 and by adding alcohol, the precipitate thus obtained or hemicellulose powder which can be easily dried.