WO1997026401A1

WO1997026401A1 - Process for producing pulp from lignocellulosic plants, and pulp obtained

Info

Publication number: WO1997026401A1
Application number: PCT/FR1997/000065
Authority: WO
Inventors: Didier Cassat
Original assignee: E.Mc2 Developpement
Priority date: 1996-01-17
Filing date: 1997-01-15
Publication date: 1997-07-24
Also published as: ATE192798T1; CN1077630C; CN1214091A; CA2242232A1; DE69701960T2; EP0874935A1; ES2146456T3; FR2743579A1; FR2743579B1; EP0874935B1; DE69701960D1

Abstract

The invention relates to a process for producing pulp, from raw lignocellulosic material based on annual or perennial plants, or residues of such plants. The process comprises impregnating said raw material with an aqueous soda solution such that the weight proportion of soda is substantially comprises between 3.5 % and 10 % based on the dry material and that the weight ratio of hydration of the cellulosic material after impregnation is at least equal to 40 %, and subjecting the lignocellulosic material thus impregnated to vapocracking which consists in pressurising said material under saturated vapour conditions and producing a sudden expansion through a direct passage valve. The solid fraction which forms the pulp is then separated from the aqueous phase and the hydrosoluble products. The process yields good quality pulps characterized particularly by a braking length higher than 5500 meters, and having a high lignine ratio (KAPPA index comprises between 60 and 65).

Description

PROCESS FOR PRODUCING PAPER PULP FROM LIGNOCELLULOSIC PLANTS AND PAPER PULP OBTAINED

The invention relates to a process for manufacturing paper pulp from lignocellulosic raw materials based on annual or perennial plants. By "annual plant" is meant hereinafter any plant having a development period of approximately one year (cotton, hemp, flax, cereal, sugar cane, sorghum, ...); "perennial plants" means plants whose development extends over much longer periods (bamboo, reed, sisal, hardwood or coniferous wood, etc.); the lignocellulosic raw materials targeted by the invention may contain the plants in their entirety, or only parts of these plants (stems, leaves, etc.), or by-products of these plants (straw, bagasse ...).

We know that there are currently essentially four types of paper pulp obtained from perennial or annual plants:

- very low quality pasta obtained with yields higher than 90%: mechanical, thermomechanical and chemothermothermal,

- low quality pasta obtained with yields between 80 and 90%: chemical-mechanical or mechanochemical pasta,

- medium quality pasta obtained with yields between 70 and 80%: semi-chemical pasta, - good quality pasta obtained with yields between 45 and 55%: chemical pasta.

(By "yield" is usually understood in the field of paper pulp, the weight proportion of pulp obtained relative to the initial weight of vegetable material).

Manufacturing costs are increasing from very low quality pasta to good quality pasta, due to increasing losses of materials raw materials, increasing expenditure of energy in mechanical or thermal form, and increasing consumption of the chemicals used.

In the case of paper pulps made from annual plants, only chemical pulps obtained with low yields (less than 55%) have high mechanical strength corresponding to break lengths greater than or equal to 5,500 to 6,000 meters ; in the case of pasta made from perennial plants, some semi-chemical pasta can reach these resistance values. We know that the breaking length represents the length of a strip of arbitrary but uniform width, assumed to be suspended by one of its ends, breaking under the effect of its own weight. This break length is calculated by the

106.R _T formula where:

15g.G

- Rrp is the tensile strength expressed in newtons per meter (standard NFQ 03002),

- G is the grammage of the dough (mass per m ² ) and,

- g is the acceleration of gravity (9.8 m / s / s).

There are also methods of treating plants to make feed components from them. One of these processes, known as steam cracking, consists in subjecting the lignocellulosic raw material to high pressure and temperature (of the order of 20 to 30 bars and from 210 ° C to 240 ° C) and then subjecting it to a sudden decompression with a view to destructuring the plant by this explosive expansion and thus improving its digestibility. By its very nature, such a process leads to significant degradation of the lignocellulosic materials, that is to say to a significant break in the fibers due to the sudden relaxation and intense hydrolysis of the hemicellulosic and cellulosic constituents of the plant material.

This steam cracking process has been proposed for producing very low or low quality paper pulp essentially from wood; the units experimental studies have made it possible to manufacture this type of pasta with yields greater than 80%. In some units, steam cracking is combined with treatment with sodium sulfite Na2Sθ3 and also sometimes with soda with low soda contents, less than or equal to 2% (proportion by weight of soda relative to lignocellulosic dry matter). It should be emphasized that the steam cracking process, which imposes very high mechanical stresses on the lignocellulosic material, producing a significant break in the fibers, appears a priori incompatible with obtaining good quality doughs with greater or equal break lengths. at 5,500 meters, because obtaining such quality requires that the fibers be preserved. Thus, all the proposals for using steam cracking to make paper pulps are confined to the manufacture of very low or low quality pulps of the purely mechanical or chemico-mechanical type.

It should be noted as an exception to the above observations that two studies have been carried out on the possibilities of obtaining a good quality pulp using the steam cracking process: "BV KOKTA et al., Cellulose Chem. Technol, 26, 107- 123 (1992) "and US-A-4,798,651, on the one hand; "H. MAMERS et al., TAPPI JOURNAL, Vol. 64 no.7, 93-96 (July 1981)", on the other hand.

The first study (KOKTA) seems to show that obtaining good quality pulp by steam cracking is possible but using a high proportion of sodium sulfite (16%), so that this process is subject to the defects of chemical processes: large quantity of chemicals used making recycling them expensive (moreover, we know that sodium sulfite poses recycling problems which are particularly difficult to solve). It should be noted that US Pat. No. 4,798,651 (KOKTA) also mentions the use of swelling agents, but these swelling agents are used only for the subsequent bleaching operation and not during steam cracking as the examples show described in this patent.

The other study (MAMERS) describes a process in which steam cracking is carried out by forcing the material to pass through a bundle of discharge nozzles ("discharge nozzle arrangements") which subjects said material to a very powerful mechanical shear when it is expelled from the reactor by the explosion. Under these conditions, the lignocellulosic fibers are strongly degraded and part of the material is fragmented into fines and is lost: this explains, in part, that the yield of the process described in this article is low (between (33, 6% and 52.5%) until it is lower than the yield of the chemical processes as indicated in point 4 of the conclusions of this article. the function is not specified.

At present, the only processes actually implemented on an industrial level to obtain good quality pasta from annual plants (breaking lengths greater than or equal to 5,500 meters) are chemical, traditional or derived processes, which use very high proportions of chemicals (in particular 14 to 18% of soda for traditional chemical processes) and which present low yields ranging between 45% and 55%. With regard to perennial plants, certain semi-chemical processes allow these values to be reached, but only with certain (softwood) woods, the performance being much lower with other woods or with annual plants.

The present invention proposes to provide a method of manufacturing paper pulp from annual or perennial plants, which leads to good quality pulps (in particular breaking length at least equal to 5500 meters), which uses quantities moderate chemical reagents much lower than those required in chemical processes, and which has a yield of 60 to 70%, much higher than the yield obtaining chemical pulps of similar quality or the yield of the process proposed by MAMERS et al. The invention relates to a process which makes it possible to obtain these performances whatever the nature of the plants used and which is therefore particularly advantageous in the case of annual plants to provide a new outlet for these plants.

To this end, the process for manufacturing paper pulp according to the invention is characterized in that it combines the following stages: - a lignocellulosic raw material based on annual or perennial plants or residues of these plants is impregnated by means of an aqueous soda solution so that the proportion by weight of soda contained in said lignocellulosic material is substantially between 3.5% and 10% relative to the dry matter and that the weight rate of hydration of said cellulosic material after impregnation is at least equal to approximately 40%,

- The lignocellulosic material thus impregnated is then subjected to a steam cracking, ensuring in a closed enclosure a rise in pressure and temperature of the lignocellulosic material impregnated by the introduction of saturated steam, by cooking said materials in the enclosure. temperature and pressure thus reached, then by subjecting the materials to a sudden expansion by opening a direct passage valve, associated with said enclosure and provided with a free passage adapted to allow the exit of the materials in the absence of notable mechanical shears, and the solid fraction constituting the paper pulp is separated from the aqueous phase and the water-soluble products.

The process of the invention is based on the following surprising observation: when steam cracking of lignocellulosic materials is carried out in the presence of a proportion by weight of soda of between 3.5% and 10% and preferably between 5% and 8%, with a sufficient hydration rate (greater than 40%), we avoid degradation of the lignocellulosic fibers and the paper quality of these is preserved despite the high stresses undergone during the explosion, regardless of the type of plants (annual or perennial). Very resistant pulps of quality corresponding to that of chemical pulps are thus obtained, with a yield much higher than that of chemical processes (yield of the order of 60 to 70% in the experiments carried out). The invention thus makes it possible, compared with traditional chemical or derivative processes, on the one hand to obtain a higher weight of dough compared to the same weight of lignocellulosic raw material, on the other hand to use much lower quantities of products chemical (less than 10% soda); these lower amounts of sodium hydroxide used represent an essential economic advantage in practice since they lead to a considerable reduction in the costs of recycling this product. It should be emphasized that the use of sodium hydroxide represents an essential advantage compared to other chemical reagents offered such as sulfite, because sodium hydroxide is, by nature, easy to recycle chemically (transformation into sodium carbonate, incineration and caustification lime).

The mechanisms by which sodium hydroxide, in an appropriate quantity, is combined with steam cracking to achieve defibration while preserving the paper quality of the fibers is difficult to explain; however, the following assumptions can be made:

1) Part of the sodium hydroxide introduced in the targeted proportions reacts with the organic acids released by hydrolysis of the most fragile constituents of the material under the effect of temperature. The soda and the released acids are neutralized and the cooking medium remains slightly basic thanks to the unreacted soda. The phenomenon ¹ autohydrolysis in these basic pH conditions is inhibited, which results in a yield increase in pulp and preservation of the cellulose fibers which are not attacked. 2) The quantity of soda remaining (soda which has not reacted with organic acids) allows a slight solubilization of the hemicelluloses and lignins which constitute the essential binder between the fibers so that the impregnated raw material which is subjected to steam cracking has a much higher elasticity than its original elasticity and better withstands sudden expansion; under these conditions, this expansion is capable of achieving a release of the fibers (the expected hydration rate authorizing this release) while limiting the degrading breaks of the latter. On the contrary, in known steam cracking processes, implemented in the absence of sodium hydroxide or with proportions less than or equal to 2% (generally between 0.5% and 1%), the highly structured raw material benefits from less elasticity and undergoes explosion in conditions very damaging to the fibers.

It should be emphasized that it is essential that the lignocellulosic fibers do not undergo significant mechanical stresses after explosion at the exit of the enclosure, because such stresses would render inoperative the protective role of the sodium hydroxide which was explained above, and would lead to a fragmentation of the cells of the lignocellulosic material, which precisely seeks to avoid the method of the invention; thus, the method proposed by MAMERS et al. research of diametrically opposite effects to those of the invention (as indicated in point 3 of the conclusions of the article in the TAPPI JOURNAL: fragmentation of cells and fibers). The process of the invention can be carried out from plants, parts of plants or mixtures of plants from the following group:

- annual plants (cotton, hemp, flax ...), - cereal straws (wheat, barley, rye, oats, triticale, rice ...),

- perennial plants (bamboo, reed, sisal, leafy, coniferous ...), - agricultural residues (sugar cane bagasse, sugar sorghum bagasse ...).

The process is particularly interesting for annual plants, because it allows an excellent valorization of products considered today as second category, and this, without presenting the defects of the chemical processes.

Preferably, a prior chopping of the lignocellulosic material is carried out so as to reduce it into fragments of length substantially between 0.5 and 15 centimeters. This increases the specific contact surface of the raw materials with sodium hydroxide and saturated steam.

It seems that we obtain maximum pasta quality and optimal yield improvement by adjusting the parameters as follows:

- proportion by weight of soda substantially between 5% and 8%,

- hydration rate appreciably between 60% and 85%.

According to a first embodiment, the impregnation is carried out by immersion of duration between 10 and 30 minutes, in an aqueous sodium hydroxide solution of concentration substantially between 5 and 15 g / 1, with a hydrovolume (weight of solution / dry weight of lignocellulosic material) substantially between 8 and 20. This embodiment leads to simple and inexpensive equipment.

According to another mode of implementation, the impregnation is carried out by spraying on the cellulosic raw material of an aqueous solution of soda of concentration substantially between 10 and 25 g / 1 in an appropriate amount to reach the weight proportion and the rate hydration aforementioned, the lignocellulosic material undergoing kneading during said spraying.

Impregnation can be carried out at room temperature. A slight increase in temperature favors this impregnation and, in practice, it is possible to choose an impregnation temperature between 40 ° C and 60 ° C.

The general conditions for implementing steam cracking are known in themselves: this is advantageously carried out by ensuring, in a closed enclosure, a rise in pressure and in temperature of the impregnated lignocellulosic material, by introduction of saturated vapor at a pressure substantially between 12 and 22 bars (12.10 ⁵ and 22.10 ⁵ pascals), and at a temperature substantially between 140 ° C and 230 ° C, then after a cooking time, by decompressing the medium to bring it back to atmospheric pressure by less than 4 seconds, this being carried out, as already indicated, through a direct passage valve of a nature to limit the stresses undergone by the lignocellulosic fibers at the outlet of the enclosure.

The steam cracking time (rise in temperature and pressure, cooking, relaxation) is preferably adjusted between approximately 4 and 8 minutes, which allows good defibration without appreciable degradation of the fibers.

According to a preferred embodiment, prior to steam cracking, the impregnated material is preheated to bring it to a temperature between 60 ° and 100 ° C, said preheating being combined with a mechanical homogenization operation. The steam cracking is then carried out under optimal conditions (better productivity). The invention extends to paper pulps made from annual plants by implementing the method defined above; these paper pulps can be characterized by:

- a length of rupture substantially between 5500 m and 9000 m,

- a KAPPA index substantially between 60 and 65,

- a CMT index (Concora Medium Test) substantially between 1.5 and 2.5 Nm ² / g,

- and the following weight distribution of cellulose, hemicelluloses and lignins: cellulose between 55% and 80%, hemicelluloses between 12% and 25%, and lignins between 8% and 12%.

The breaking length is measured according to standard NFQ 03002, the KAPPA index according to standard NFT 12-018 and the CMT index according to standard NFQ 03044.

Such pulps have never been obtained until now: they are characterized by very good mechanical properties, associated with a very high level of lignin. In known pasta, good mechanical properties are always associated with a low level of lignin (KAPPA index of the order of 30), which translates into a low yield when obtaining the pasta (loss of lignin).

The process of the invention is illustrated by the examples which follow, for which the steam cracking was implemented in an installation of the type of that represented in schematic section in FIGS. 1 and 2 of the drawings; on these drawings:

FIG. 1 is a schematic view of the installation,

- Figure 2 is a section of the outlet of the reactor provided with the outlet valve with direct passage.

The protocol used in the examples is as follows:

The lignocellulosic raw material is firstly rid of undesirable foreign elements (dust, sand, earth, leaves ...) by traditional methods. It is then chopped into strands of 0.5 to

15 cm using a 12-knife forage harvester.

The impregnation step is carried out by total immersion of the raw material in an aqueous sodium hydroxide solution at a temperature of 40 ° C. The fixed sodium hydroxide rate and the hydration rate are adjusted by adjusting the following parameters: concentration of soda solution, soaking time, stirring mode and of drainage (the drainage being carried out by presses with

The impregnated raw material is then introduced into a mixer / preheater as symbolized in 1 in the figure of the drawings. In this device, the material is heated to a temperature of around 90 ° C by admission of steam at 3 bars; the duration of stay of the material is 10 minutes.

The impregnated, homogenized and preheated material is then introduced into a steam cracking reactor 2.

The first phase of steam cracking consists of pressurizing saturated steam in the reactor and raising the temperature of the materials. This phase is short-lived (around 1 to 2 minutes) thanks to the use of a boiler with a high steam flow rate.

When the desired pressure is reached, the next phase of steam cracking is carried out; it consists of isothermal and isobaric cooking of the materials. Maintaining pressure and temperature is achieved through a control loop 3 which supplies the reactor with steam as the reactor vapor condenses.

The last phase is the sudden expansion phase and is operated by abruptly opening a purge valve 4 with direct passage located at the foot of the reactor. In the example, this valve is a knife gate valve which, when open, completely releases the outlet of the reactor and ensures continuity of passage of the material, without any obstacle. Under the effect of the sudden expansion, the pressurized water condensed in the plant turns into vapor and releases the energy necessary for the defibration of the plant material. The material is expelled through the direct passage valve without significant shear stresses.

After the steam cracking operation, a cyclone 5 makes it possible to separate and recover the aqueous and solid phase (outlet 6) from the gaseous phase generated during the explosion (exit 7).

5 examples were used with cereal straws, respectively wheat (example 1), triticale (example 2), rye (example 3), oats (example 4), spring barley (example 5).

The mechanical paper characteristics obtained were measured after shaping the pulps according to the following standards:

- Weight: NF Q 03 019 - Thickness: NF Q 03016

- Tensile strength and elongation: NF Q 03 002

- Tear resistance: NF Q 03 011

- Bursting resistance: NF Q 03 053 - Resistance to flat compression or

CMT: NF Q 03 044

- Ring Crush Test or RCT: TAPPI T822 om 87 The tables below summarize the operating conditions and the results obtained.

Analysis of the constituents of the dough obtained in Example 1 gave the following results (weight distribution):

- cellulose: 58.4%

- hemicelluloses: 17.3%

- lignins: 10.5%

The KAPPA index of the examples varied between 62 and 65, the average index being 64.

Claims

CLAIMS 1 / - Process for manufacturing paper pulp, characterized in that it combines the following steps: - a lignocellulosic raw material based on annual or perennial plants or residues of these plants is impregnated by means of a solution aqueous soda so that the weight proportion of soda contained in said lignocellulosic material is substantially between 3.5% and 10% based on the dry matter and the weight rate of hydration of said cellulosic material after impregnation is at least equal at substantially 40%, - the lignocellulosic material thus impregnated is then subjected to steam cracking, ensuring in a closed enclosure an increase in pressure and temperature of the lignocellulosic material impregnated by introduction of saturated steam, by cooking said materials in the enclosure at the temperature and pressure thus reached, then subjecting the materials to sudden expansion by opening a direct passage valve, associated with said enclosure and provided with a free passage adapted to allow the materials to exit in the absence of significant mechanical shearing, - and the solid fraction constituting the paper pulp is separated from the aqueous phase and the water-soluble products. 2/ - Method according to claim 1, in which a preliminary chopping of the lignocellulosic material is carried out so as to reduce it into fragments of length substantially between 0.5 and 15 centimeters. 3/ - Method according to one of claims 1 or 2, in which the impregnation is carried out so that the weight proportion of soda in the lignocellulosic material is substantially between 5% and 8% and its hydration rate substantially included between 60% and 85%. 4/ - Method according to one of claims 1, 2 or 3, characterized in that the impregnation is carried out by immersion for a duration of between 10 and 30 minutes, using an aqueous solution of soda with a concentration substantially between 5 and 15 g/1, with a hydrovolume (weight of solution/dry weight of lignocellulosic material) substantially between 8 and 20. 5/ - Process according to one of claims 1, 2 or 3, characterized in that the impregnation is carried out by spraying the cellulosic raw material with an aqueous sodium hydroxide solution with a concentration substantially between 10 and 25 g/l in an appropriate quantity to achieve the aforementioned weight proportion and hydration rate, the lignocellulosic material undergoing mixing during of said spraying. 6/ - Method according to one of claims 1, 2, 3, 4, 5, characterized in that the impregnation is carried out using an aqueous soda solution at a temperature between 40° C and 60° C. 7/ - Method according to one of claims 1, 2, 3, 4, 5 or 6, in which the steam cracking is carried out by ensuring in a closed enclosure a rise in pressure and temperature of the lignocellulosic material impregnated by introduction of 'a saturated steam at a pressure substantially between 12 and 22 bars (12.10 ^ and 22.105 pascals), and at a temperature substantially between 140 ° C and 230 ° C, then after the cooking time, by decompressing the medium to reduce substantially to atmospheric pressure in less than 4 seconds through the direct-flow valve. 8/ - Method according to claim 7, characterized in that the steam cracking of the impregnated lignocellulosic material (rise in temperature and pressure, cooking, relaxation) is carried out in a time substantially between 4 and 8 minutes. 9/ - Method according to one of claims 7 or 8, characterized in that, prior to steam cracking, preheating of the impregnated material is carried out to bring it to a temperature between 60° and 100° C, said preheating being combined with a mechanical homogenization operation. 10/ - Process according to one of claims 1 to 9, in which plants, parts of plants or mixtures of plants from the following group are used as lignocellulosic raw material: - annual plants (cotton, hemp, linen, etc.), - cereal straw (wheat, barley, rye, oats, triticale, rice, etc.), - perennial plants (bamboo, reed, sisal, hardwood, softwood, etc.), - agricultural residues (sugar cane bagasse, bagasse of sorghum sugar bowl...). 11/ - Paper pulp manufactured from annual plants by implementing the process according to one of claims 1 to 10 having a breaking length substantially between 5,500 m and 9,000 m, characterized by:

(1) a KAPPA index substantially between 60 and 65,

(2) a CMT index (Concora Medium Test) substantially between 1.5 and 2.5 N.m^/g, (3) the following weight distribution: cellulose between 55% and 80%, hemicelluloses between 12% and 25%, and lignins between 8% and 12%.