WO1992007098A1

WO1992007098A1 - Process and plant for preparing pentose and/or hexose concentrated juice from hemicellulose-rich plant substances

Info

Publication number: WO1992007098A1
Application number: PCT/FR1991/000825
Authority: WO
Inventors: Philippe Sanchez; Luc Rigal; Antoine Gaset; Pierre Mailhol
Original assignee: Agrocinq Recherche S.A.
Priority date: 1990-10-23
Filing date: 1991-10-21
Publication date: 1992-04-30
Also published as: FR2668165A1

Abstract

Process for preparing pentose and/or hexose-based concentrated syrup by hydrolysis of hemicellulose-rich raw plant materials such as young sorghum plants, corn cobs and wheat straw. According to the process, primary reactors (R1, R2, R3) are supplied in parallel with solid raw materials (M1, M2, M3) which undergo softening from the upstream reactor to the downstream reactor. Said materials are also subjected to a post-hydrolysis process under more stringent conditions in a post-hydrolysis reactor (Rp) which receives the solid substances from the primary reactors and is supplied with a higher concentrated acid solution than that of the primary reactors. The process of the invention gives an excellent yield and high xylose concentrations in the extracted juice.

Description

PROCESS AND INSTALLATION FOR PREPARING A CONCENTRATED JUICE OF PENTOSES AND / OR HEXOSES FROM VEGETABLE MATERIALS

RICH IN HEMICELLULOSES

The invention relates to a process for the preparation of concentrated syrups of pentoses and / or hexoses, in particular xylose, by acid hydrolysis of plant materials rich in hemicelluloses such as sorghum, corn, wheat (whole plants or residues). It extends to an installation for implementing said method.

Pentoses and hexoses are sugars useful in human nutrition and, in chemistry, among others as a precursor of polyols; in particular xylose is the precursor of xylitol, a sweet sweetener, acariogenic, with very low heat of dissolution (providing a cooling effect in the mouth).

The processes for preparing xylose by acid hydrolysis at low temperature (below 150 ° C.) of plant materials are well known and described in several prior patents. Certain processes work in a concentrated or very concentrated acid medium (US Patents 2,989,569, US 4,226,638), but the necessary recycling of the acid makes them unprofitable on the economic level and difficult to implement.

Other processes work in a dilute acid medium, generally sulfuric. However, either these methods bring into play small liquid / solid ratios in order to obtain concentrated xylose syrups (patents DE 2,413-306, DE 2,324,022, FR 2,356,728, US 4,226,638 ...) , but they then have low yields (ratio of xylose extracted to xylose contained in the raw materials less than 60%); or else they operate with high liquid / solid ratios with better yields (around 90%) (US patents 4,752,579, FR 906,326 ...), but they then lead to diluted xylose syrups, the subsequent concentration of which is expensive. Certain processes of this latter type recycle the juices in the hydrolysis reactor in order to concentrate them (patents DD 222,635, FR 2,327,314, FR 2,327,315,

SHEET OF R MPLAC M NT DE 2.545.110, DE 2.545.111), but this recycling of juice requires prior treatment of the raw material to avoid a concentration of impurities in the final juice; in addition, xylose being a product sensitive to temperature and pH, this recycling of juice conditions a certain degradation of the products which causes the yield to drop and increases the content of secondary products in the syrups obtained.

Furthermore, patent FR 2,047,193, which is placed in an acid medium diluted with high liquid / solid ratios, proposes a specific mode of implementation of the hydrolysis process in order to increase the concentration of the outlet juices, while maintaining good performance; according to this patent, the plant materials and the acid baths are caused to flow against the current in two successive primary reactors and a post-hydrolysis is carried out on the outlet juice to complete the hydrolysis and lead to xylose; hydrolysis in primary reactors is carried out under harsh conditions (high acidity, high temperature), while post-hydrolysis is carried out under milder acidity conditions. However, the hydrolysis conditions in the primary reactors and the post-hydrolysis carried out on the xylose juice from these reactors lead to a non-negligible degradation of the products and a very troublesome reduction in the purity of the syrups obtained. In addition, if the concentration of outlet syrups is improved compared to the processes involving the same liquid / solid ratios, it remains of mediocre value (of the order of 10 g / 1) and requires a subsequent concentration step which whatever the intended application of the product obtained. In addition, this process is accompanied by relatively high consumption of sulfuric acid.

The present invention proposes to indicate a process for the hydrolysis of plant materials, which operates in an acidic medium diluted at low temperature and is free from the faults of the previous processes in order to be able to use plant materials rich in hemicelluloses under excellent conditions. economic profitability.

The essential objective of the invention is thus to obtain with an excellent yield (at least equal to that

REPLACEMENT SHEET of patent FR 2,047,193) syrups of pentoses and / or hexoses, in particular xylose, the concentrations of which are high (at least 4 times higher than that of the aforementioned patent); in particular the invention aims to provide xylose syrups whose concentrations are sufficient to be then treated directly, in particular by hydrogenation by fermentation (in particular as described in French patent application No. 89.00209). Another objective is to limit the degradation of the products and to produce syrups of high purity.

Another objective is to considerably reduce acid consumption compared to the processes of the prior art.

A secondary objective is to produce, from vegetable raw materials, hydrolysis by-products which can be recovered both at the level of cellulose (paper pulp, glucose) and at the level of the lignin / cellulose complex (carrier of culture, particle board).

To this end, the process according to the invention for preparing a concentrated syrup of pentoses and / or hexoses by hydrolysis at low temperatures of vegetable raw materials rich in hemicelluloses consists in carrying out on several batches of vegetable raw materials M- _| , M-2, M of hydrolysis in acidulated medium in separate primary reactors R- _j , R ₂ , Ro under the following conditions:

(a) the primary reactors R- _j , R, Ro are supplied in parallel with vegetable raw materials M “ _| , M ₂ ,

(b) the hydrolysis baths are caused to flow successively from one primary reactor to another from a first reactor R- _j to a last reactor Ro, (c) the conditions of duration, temperature and acidity of hydrolyses in primary reactors R -. , R ₂ , R3 ^are adjusted in the ranges of 10 to 120 minutes, 100 to 150 ° C and 0.04 to 0.8 mole of H ⁺ ion per liter of solution respectively, the choice of these parameters in these ranges being performed so that the hydrolysis conditions

EM ACEMENT SHEET are becoming softer since the first primary reactor R- _| until the last Ro,

(d) the plant materials hydrolysed in the primary reactors R- _j , R ₂ , R _o are extracted in parallel and placed in a post-hydrolysis reactor R subjected to more severe acid hydrolysis conditions than those of the first primary reactor R- _j ,

(e) the juice from the post-hydrolysis reactor R is used to constitute the bath of the first primary reactor R- _j ,

(f) the juice from the last primary reactor R _o is recovered and neutralized to provide the concentrated pentose and / or hexose syrup. Thus, in the invention, the primary reactors are supplied in parallel with solid raw material, while the juices circulate in series from one reactor to another, the hydrolysis conditions changing from one reactor to another for soften from upstream to downstream (considering the direction of circulation of liquids). In addition, the post-hydrolysis carried out under more severe conditions relates to the solid materials collected at the outlet of the primary reactors (and not to the juices as is the case in the aforementioned prior patent) and makes it possible to prepare a juice which serves as a bath for the first primary reactor. Preferably, the acid (mineral acid such as sulfuric acid) is added to the post-hydrolysis reactor so that the acidity of this bath is the highest (between 0.2 and 0.6 mole of ion H ⁺ per liter of solution), the hydrolyses in the primary reactors being carried out without adding acid so that the acidity of the baths of these reactors is lower than that of the post-hydrolysis bath and decreases from the first reactor R ^ to the last Ro reactor. If necessary, a fraction of the juice from the post-hydrolysis reactor can be mixed between two successive primary reactors in the bath flowing from one reactor to another. Such a process produces concentrated juices (of the order of 40 to 65 g of xylose per liter) with a remarkable yield (85 to 95% depending on the raw material), without appreciable degradation of the products. The "hydrolysis conditions" will be

REPLACEMENT SHEET represented in the present application by the kinetic expression: Cte " ^{E / RT} in which:

C is the average acid concentration of the reactor,

. t is the duration of hydrolysis,

. T is the absolute temperature prevailing in the reactor,

. E is the activation energy of the degradation reaction of xylose (37 kg cal. / Moles, ie 1.55 × 10 ^ J / moles),

R is the constant of ideal gases (1.98 cal. / Moles, or 8.29 J / moles).

"Milder" hydrolysis conditions will therefore correspond to a temperature, a duration of hydrolysis and an acidity adjusted in correlation to lead to a lower value of the kinetic expression mentioned above.

This change in hydrolysis conditions from one reactor to another can in particular be obtained:

. for primary reactors, by setting the temperature T of these reactors to a substantially identical value, advantageously between 110 ° C and 130 ° C, and by reducing the hydrolysis times from the first reactor to the last, the acid concentration decreasing similarly by virtue of the implementation process described above (juice circulating in series from one reactor to another),

. for the post-hydrolysis reactor, by adjusting the temperature T of this reactor to a temperature higher than that of the primary reactors (difference of the order of 5 ° to 20 °), and by adopting a hydrolysis time t _p of same order as the duration of the last primary hydrolysis and at most equal to this, the acid concentration being higher than that of the primary reactors by virtue of the implementation process described above.

According to a preferred embodiment, the vegetable raw materials M- _| , M ₂ , M are processed in parallel in three primary reactors R- _j , R, R, the hydrolysis baths flowing successively from the first R- _| at the third Ro with recovery and neutralization of the juice leaving the latter. ^ 0 The duration t ^ of the first hydrolysis in the

TAINT REPLACEMENT SHEET reactor R- _j is in particular adjusted to a value between 20 and 110 minutes; the duration t ₂ of the second hydrolysis in the reactor R is less than this by a value * ~ t ₂ between 5 and 15 minutes, however that the duration t of the third hydrolysis in the reactor Ro. is less than t ₂ by a value -Δto between 5 and 15 minutes.

It should be noted that the times indicated are actual durations of hydrolysis and do not include the temperature rise times. In addition, in the ranges of values indicated, the difference in duration between two hydrolyses is chosen as a function of the duration of the hydrolysis serving as a reference (difference the greater the greater the duration of the latter).

Furthermore, the primary reactors R ^, R, Ro can advantageously be supplied so that their liquid / solid ratio -r- is similar and between 4 and 20 kg of liquid per kg of dry matter. The post-hydrolysis reactor R is preferably supplied so that its ratio -r - is less than -r- and between 0.3 r and 0.5 r.

The primary and post-hydrolysis hydrolyses are in particular carried out, in the usual way, in autoclave reactors at a pressure depending on the temperature; it is also possible to provide different pressure conditions.

In addition, in a manner known per se, the vegetable raw materials will be prepared beforehand by grinding them to reduce them to an average particle size of less than 10 mm, which may vary according to the nature of the vegetable material; however that the juice from the last primary reactor will preferably be neutralized with lime and / or ammonia; the lime will be chosen in the general case, but an addition of ammonia can advantageously be carried out when the xylose is intended for hydrogenation by fermentation.

The process of the invention can be carried out continuously both for the flows of solid materials and for the flows of liquids; a start-up phase is then carried out at the start of implementation, consisting of

REPLACEMENT SHEET introduce into the first primary reactor R- _j an acidic aqueous bath having an acidity adjusted between 0.2 and 0.6 mole of H ⁺ ion per liter of solution. The process of the invention can in particular be applied to prepare a concentrated syrup containing essentially xylose from whole sorghum plants; the implementation conditions are then advantageously as follows:. the duration t ^ and the temperature T- _j of the first hydrolysis are respectively in the following ranges: 60 to 100 minutes, 115 ° to 125 ° C,

. the duration t and the temperature T ₂ of the second hyrolysis are respectively in the following ranges: 50 to 90 minutes, 115 ° to 125 ° C,

. the duration t- and the temperature T of the third hydrolysis are respectively in the following ranges: 40 to 75 minutes, 115 ° to 125 ° C,

. the duration t and the temperature T _p of the post-hydrolysis are respectively within the following ranges: 25 to 50 minutes, 125 ° to 135 ° C. the liquid / solid ratio r of each primary hydrolysis is between 8 and 14 kg of liquid per kg of dry matter,. the liquid / solid ratio r of the post-hydrolysis is between 3 and 6 kg of liquid per kg of dry matter.

The method of the invention can also be applied to prepare a syrup concentrated in xylose from corn cobs; the implementation conditions are then advantageously as follows:

. t- _d between 45 and 75 minutes and T- _d between 115 ° C and 125 ° C,

. t ₂ between 35 and 60 minutes and T ₂ between 115 ° C and 125 ° C,

. t between 20 and 40 minutes and Tq between 115 ° C and 125 ° C,

. t _p between 25 and 40 minutes and T _p between 125 ° C and 135 ° C,. r ratio of primary hydrolyses included

REPLACEMENT SHEET between 6 and 10 kg of liquid per kg of dry matter,

. rp ratio of post-hydrolysis between 2.5 and 5 kg of liquid per kg of dry matter. The process of the invention can also be applied to prepare a concentrated xylose syrup from wheat straw; the implementation conditions are then advantageously as follows:

. t-, between 45 and 80 minutes and T- _j between 115 ° and 125 ° C,

. t ₂ between 35 and 65 minutes and T ₂ between 115 ° and 125 ° C, t _q between 30 and 45 minutes and T3 between 115 'and 125 ° C, t „between 25 and 40 minutes and T _n between 125' and 135 ° C,

. ratio r of primary hydrolyses between 7 and 13 kg of liquid per kg of dry matter,

. rp ratio of post-hydrolysis between 3 and 6 kg of liquid per kg of dry matter.

The invention extends to a plant material treatment installation allowing the implementation of the process defined above; this installation is characterized in that it comprises the combination of the following means: at least three primary hydrolysis reactors R - _j , R ₂ , R q, means for supplying said reactors with vegetable matter in parallel, an inlet for liquid to one of the reactors called first primary reactor R < _| , means for circulating the juices in series from one reactor to another from the first reactor - _j to a last primary reactor Rq, an outlet for juice from the last primary reactor R q, a unit for neutralizing the juice from from said last primary reactor Rq, a post-hydrolysis reactor R _p , means for transferring the solid materials from the primary reactors R .., R ₂ , R _q to said post-hydrolysis reactor R, means for extracting residues solids from the post-hydrolysis reactor, an inlet for liquid to said post-hydrolysis reactor R, means for circulating the j us extracted from the post-hydrolysis reactor R _D to the inlet for liquid from the first primary reactor R - _j , of tem pe ratur e re gu l at ion sm oye ns

REPLACEMENT SHEET making it possible to adjust the temperature of the primary reactors and post-hydrolysis reactor, and an acidification unit connected in particular to the liquid inlet of the post-hydrolysis reactor, and, where appropriate, bypass conduits a fraction of the juice from the post-hydrolysis reactor R to the primary reactors R, Rq other than the first.

The description which follows with reference to the single figure of the appended drawing presents an embodiment of such an installation and provides several examples of implementation of the method by means of said installation.

The installation, shown by way of example in the single figure, comprises a mechanical knife grinder 1 in which the plant materials are arranged to reduce them to an average particle size of less than 10 mm, of the order of 5 mm in the examples.

The crushed materials are introduced through a storage hopper 2 to then be distributed in hydrolysis reactors R- _j , R ₂ , Rq. These reactors are identical, they have a volume of 17 1 and are each constituted by an autoclave whose temperature (and pressure) are controlled by regulating means (symbolized by TIC in the figure); the quantities of material M i, M ₂ , Mo introduced into these reactors are identical. At the outlet of the reactors, the solid materials and the liquid phase are separated in separators 3, 4, 5. The solid materials are combined to feed a post-hydrolysis reactor R _p operating in a similar fashion to the previous ones. This reactor has sufficient capacity to allow the introduction of the three batches of material (23 1). On leaving the reactors R- _j and R ₂ (and their respective separator), the liquid phase is sent to the next primary reactor, respectively R ₂ , R ₃ . At the outlet of the separator 5, the liquid phase is sent to a storage tank 6.

The post-hydrolysis reactor R _e is supplied with acidulated water coming from a tank 7 of water and acid mixture. The latter is stored concentrated in a storage tank 8.

REPLACEMENT SHEET At the outlet of the post-hydrolysis reactor R _p , the solids and the liquid phase are separated in a separator 9. The solids are sent to a press 10 in which a strong pressing is carried out to reduce the moisture content of the materials solids and produce cellulosic residues of 80% dry matter.

The liquid phase from the separator 9 and that from the press 10 are combined and sent to the primary reactor R- _j . It should be noted that a bypass 11 allows, during the start-up phase of the installation, to send the acid solution from the mixing tank 7 to the primary reactor R- _j , while conduits 12 make it possible to deliver a fraction juice from the post-hydrolysis reactor R _p to the primary reactors R ₂ and Rq.

Furthermore, at the outlet of the storage tank 6, the hydrolysates are neutralized with lime in a neutralization tank 13 and filtered in a plate filter 14 to separate the calcium sulphate from the xylose syrup.

Example 1: raw materials constituted by sorghum

In this example, the xylose is prepared from a variety of "Sorghum bicolor" fodder sorghum (variety 877 F). Sorghum is mowed and dried in the open air in the field. Its final dry matter is 90%. The whole plant is ground in the grinder 1 provided with a 10 mm grid in order to regulate the particle size.

First, 3 complete operating cycles are carried out corresponding to the start-up phase, without obtaining the desired yields or concentrations.

Then, a sample of 3.0 kg of material (approximately 2.7 kg of dry material) contained in the storage hopper 2 is divided into three to be treated in batches of the same weight in the three primary reactors R-ι, R ₂ , Rq (lots M _1? M ₂ , Mo of 1 kg each).

Is introduced into the primary reactor R ^ the batch Mι and a sulfuric acid bath (juice d) (14.0 1) of concentration equal to about 0.150 mole per liter, entirely from the post-hydrolysis step by l 'intermediate

REPLACEMENT SHEET filter 9 and press 10 (juice d). The amount of dry matter is 0.064 kg / liter of bath (ie a liquid / "raw" matter ratio equal to 14). The residence time in the primary reactor R- _| is 85 minutes while the mixture is brought to a temperature of 120 ° C.

After solid / liquid separation in separator 3 (100 micron filtration mesh), the hydrolyzate (juice a) from this primary reactor R- _j , corresponding to 13 liters / kg of material (13.0 1), is supplemented at the rate of 1 liter / kg (1.0 1) with juice (d) from the post-hydrolysis reactor R, so as to obtain a volume sufficient to hydrolyze the same quantity of material M ₂ (1.0 kg ) in reactor R ₂ . This second batch of plant material M ₂ is brought to a temperature of 120 ° C. in the reactor R ₂ , for the same liquid / solid ratio, but with a residence time of 75 minutes. The overall acidity of the juice is lower than that of the juice (d). After solid / liquid separation in separator 4 (100 micron filtration mesh), the hydrolyzate (juice b) from this second primary reactor R ₂ , corresponding to 13 liters / kg of material (13.0 1), is again topped up at the rate of 1 liter / kg (1.0 1) with juice (d) from post-hydrolysis, so as to obtain a volume sufficient to hydrolyze the same amount of material (M) (1.0 kg) in the reactor Rq.

This third batch of plant material M is brought to a temperature of 120 ° C. in the reactor Rq for the same liquid / solid ratio, but with a residence time of 65 minutes. The overall acidity of the juice is lower than that of the juice (d).

After solid / liquid separation in separator 5 (100 micron filtration mesh), the hydrolyzate (juice c) from this third primary hydrolysis reactor Rq (13 liters / kg of material) is sent to the storage tank 6. The very low pH (around 1) allows good storage at room temperature for a long time. The hydrolysis residues from

REPLACEMENT SHEET primary reactors R- _j , R ₂ , R- after passing through the separators 3, 4 and 5, are combined and sent to the reactor R (23 1) at the same time as a fresh acidulated aqueous bath coming from the mixing tank 7 , with a sulfuric acid concentration of 0.184 mol / liter and prepared from demineralized water. In this hydrolysis step, the quantity of liquid is calculated so as to obtain sufficient juice to carry out the hydrolysis in the reactor R (juice d) and the complement of the juices (a) and (b), i.e. a volume of 16 liters for 3 kg of material. The mixture is then heated at a temperature of 130 ° C for 50 minutes.

After solid / liquid separation in the separator 9, then mechanical pressing in the press 10, part of the juice (14 liters / kg of material out of 16 collected) from this post-hydrolysis is used to feed the primary reactor R ^ of the first step. The other part will be used to complete the juices (a) and (b) through the conduits 12.

The hydrolyzate collected after storage in tank 6 is brought back to a pH of 4.0 by adding lime. The whole is filtered on the plate filter 14 (porosity of 40 to 50 microns) so as to remove the precipitated calcium sulphate salts (CaSO 4).

The results obtained in this example are shown in the table below:

Reactor 1 Reactor 2 Reactor 3 Reactor R,

Extraction rate = (quantity of xylose extracted / initial dry matter) = 16.2%

Yield = (amount of xylose extracted / amount of potential xylose) = 84%

REPLACEMENT SHEET Example 2: raw materials constituted by wheat straw In this example, xylose is prepared from wheat straw collected after harvesting the grain. The straws are harvested at harvest time and baled for storage (92% dry matter). The hydrolysis protocol is the same as that followed in Example No. 1. The amount of material treated is also 3.0 kg.

The results obtained in this example are shown in the table below:

Reactor 1 Reactor 2 Reactor 3 Reactor R,

Extraction rate = (quantity of xylose extracted / initial dry matter) = 18.4%

Yield = (amount of xylose extracted / amount of potential xylose) = 87.6%.

Example 3: raw materials constituted by corn cobs In this example, the xylose is prepared from corn cobs collected after harvesting the grain. The final dry matter is 90%. The hydrolysis protocol is the same as that followed in Example 1, except that the size of the grate of the grinder is reduced to 5 mm. The quantity of material treated is also 3.0 kg but the liquid / solid ratio goes to 12 in the reactors R- _j , R ₂ , Rq and to 14 liters for 3 kg of material in the post-hydrolysis reactor R.

The results obtained in this example are shown in the table below:

REPLACEMENT SHEET Reactor 1

Reactor 2 Reactor 3

Reactor R

Extraction rate = (amount of xylose extracted / initial dry matter) = 23.5% Yield = (amount of xylose extracted / amount of potential xylose) = 92%.

REPLACEMENT SHEET

Claims

CLAIMS 1 / - Process for preparing a concentrated syrup of pentoses and / or hexoses by hydrolysis of vegetable raw materials rich in hemicelluloses, characterized in that it consists in producing on several batches of vegetable raw materials (M, M ₂ ,) hydrolysed in acidulated medium in separate primary reactors (R- _j , R ₂ , Rq) under the following conditions: (a) the primary reactors (R- _j , R ₂ , Rq) are supplied in parallel with vegetable raw materials (M - _j , M ₂ , M ₃ ),

(b) the hydrolysis baths are caused to flow successively from one primary reactor to another from a first reactor (^) to a last reactor

⁽ R ₃ ⁾ ,

(c) the conditions of duration, temperature and acidity of the hydrolyses in the primary reactors (R- _j , R ₂ , Rq) are adjusted within the ranges of 10 to 120 minutes, 100 to 150 ° C and 0.04 to 0.8 mole of H ⁺ ion per liter of solution, the choice of these parameters in these ranges being made so that the hydrolysis conditions are more and more mild from the first primary reactor (R- _j ) until the last (Rq), (d) the plant materials hydrolysed in the primary reactors (R ^, R ₂ , Rq) are extracted in parallel and placed in a post-hydrolysis reactor (R _D ) subjected to more severe acid hydrolysis conditions than those of the first primary reactor (R- _j ), (e) the juice from the post-hydrolysis reactor

(R _p ) is used to constitute the bath of the first primary reactor (R- _j ),

(f) the juice from the last primary reactor (Rq) is recovered and neutralized to provide the concentrated pentose and / or hexose syrup.

2 / - Process according to claim 1, characterized in that a mineral acid is added to the post-hydrolysis reactor (R _p ) so that the acidity of the post-hydrolysis bath is between 0.2 and 0 , 6 mole of H ⁺ ion per liter of solution, hydrolysed in reactors

REPLACEMENT SHEET primers being carried out without adding acid so that the acidity of the baths of these reactors is lower than that of the post-hydrolysis bath and decreases from the first reactor (R- _j ) to the last reactor (R).

3 / - Process according to claim 2, characterized in that a fraction of the juice from the post-hydrolysis reactor (R _D ) is mixed between two successive primary reactors in the bath flowing from one reactor to the other.

4 / - Method according to one of claims 2 or 3, characterized in that the vegetable raw materials (M- ,, M ₂ , Mq) are processed in parallel in three primary reactors (R- _j , R, Rq), the hydrolysis baths circulating successively from the first (Ri) to the third (Rq) with recovery and neutralization of the juice leaving the latter.

5 / - Method according to claim 4, characterized in that:

. the hydrolyses in the three primary reactors (R- _j , R ₂ , Rq) are carried out at a substantially identical temperature T of between 110 ° and 130 ° C,

. the duration (t- _d ) of the first hydrolysis in the reactor (^) is adjusted between 20 and 110 minutes,

. the duration (t) of the second hydrolysis in the reactor (R ₂ ) is less than the duration (t- _j ) by a value (Λ t ₂ ) of between 5 and 15 minutes,

. the duration (tq) of the third hydrolysis in the reactor (Rq) is less than the duration (t ₂ ) by a value (Δ tq) between 5 and 15 minutes,. . the post-hydrolysis in the reactor (R) is carried out at a temperature (T) higher than the temperature T by a value (Δ T _p ) between 5 ° and 20 °, for a duration of the same order as the duration ( tq) of the third hydrolysis and at most equal to it. 6 / - Method according to one of claims 1 to 5, characterized in that:

. the primary reactors (R- ,, R ₂ , Rq) are supplied so that the liquid / solid ratio -r- of each primary reactor is between 4 and 20 kg of liquid per kg of dry matter,

REPLACEMENT SHEET the post-hydrolysis reactor (R _p ) is supplied so that the liquid / solid ratio -rp- of said post-hydrolysis reactor is between -0.3 r- and -0.5 r-.

7 / - Method according to one of claims 1 to 6, wherein the hydrolysis and post-hydrolysis are carried out in autoclave reactors (R- _j , R ₂ , R, _p ) at a pressure depending on the temperature. 8 / - Method according to one of claims 1 to 7, characterized in that the solid residue from the post¬ hydrolysis is washed with water, this washing water being used as post-hydrolysis bath after acidification.

9 / - Method according to one of claims 1 to 8, characterized by a continuous implementation for the flow of solid materials and for the flow of liquids, a start-up phase being carried out at the start of the implementation and consisting to introduce into the first primary reactor (R- _j ) an acidic aqueous bath having an acidity adjusted between 0.2 and 0.6 mole of H ⁺ ion per liter of solution.

10 / - Method according to one of claims 1 to 9, wherein the juice from the last primary reactor (R ₃ ) is neutralized with lime and / or ammonia.

11 / - Method according to one of claims 1 to 10, wherein the vegetable raw materials (M- _j , M ₂ , M ₃ ) are previously ground to be reduced to an average particle size less than 10 mm.

12 / - Process according to claim 5 for preparing a concentrated syrup containing essentially xylose, in which the vegetable raw materials consist of whole sorghum plants, characterized in that:

. the duration (t- _j ) and the temperature (T- _j ) of the first hydrolysis are respectively included in the following ranges: 60 to 100 minutes, 115 ° to 125 ° C,

. the duration (t ₂ ) and the temperature (T ₂ ) of the second hyrolysis are included respectively in the following ranges: 50 to 90 minutes, 115 ° to 125 ° C,

. the duration (t-q) and the temperature (Tq) of the third hydrolysis are included respectively in the

REPLACEMENT SHEET following ranges: 40 to 75 minutes, 115 ° to 125 ° C,

. the duration (t _p ) and the temperature (T _p ) of the post-hydrolysis are respectively included in the following ranges: 25 to 50 minutes, 125 ° to 135 ° C

13 / - Process according to claim 12, characterized in that the liquid / solid ratio (r) of each primary hydrolysis is between 8 and 14 kg of liquid per kg of dry matter and the liquid / solid ratio (r) of the post-hydrolysis is between 3 and 6 kg of liquid per kg of dry matter.

14 / - Process according to claim 5 for preparing a concentrated syrup containing essentially xylose, in which the vegetable raw materials consist of corn raffles, characterized in that:

. the duration (t- _j ) and the temperature (T- _j ) of the first hydrolysis are respectively included in the following ranges: 45 to 75 minutes, 115 ° to 125 ° C,

. the duration (t ₂ ) and the temperature (T ₂ ) of the second hydrolysis are respectively included in the following ranges: 35 to 60 minutes, 115 ° to 125 ° C,

. the duration (tq) and the temperature (T ₃ ) of the third hydrolysis are respectively included in the following ranges: 20 to 40 minutes, 115 ° to 125 ° C,. the duration (t _p ) and the temperature (T _p ) of the post-hydrolysis are respectively included in the following ranges: 25 to 40 minutes, 125 ° to 135 ° C

15 / - Process according to claim 14, characterized in that the liquid / solid ratios (r) and (r _p ) of the primary hydrolyses and of the post-hydrolysis are respectively between 6 and 10 or between 2.5 and 5 kg of liquid per kg of dry matter.

16 / - Process according to claim 5 for preparing a concentrated syrup containing essentially xylose, in which the vegetable raw materials consist of wheat straw, characterized in that:

. the duration (t) _e T the temperature (T _j) of the first hydrolysis are respectively within the following ranges: 45 to 80 minutes, 115 ° to 125 ° C. the duration (t2) and the temperature (T2 ^ ^{of the}

REPLACEMENT SHEET second hyrolysis are respectively in the following ranges: 35 to 65 minutes, 115 ° to 125 ° C,

. the duration (tq) and the temperature (Tq) of the third hydrolysis are respectively included in the following ranges: 30 to 45 minutes, 115 ° to 125 ° C,

. the duration (t) and the temperature (T) of the post-hydrolysis are respectively in the following ranges: 25 to 40 minutes, 125 ° to 135 ° C. 17 / - Process according to claim 16, characterized in that the liquid / solid ratios (r) and (r) of the primary hydrolyses and of the post-hydrolysis are respectively between 7 and 13 or between 3 and 6 kg of liquid per kg of dry matter. 18 / - Plant treatment plant for implementing the process according to one of claims 1 to 17, characterized in that it comprises at least three primary hydrolysis reactors (R- _j , R ₂ , Rq), means for supplying said reactors with vegetable matter in parallel, an inlet for liquid to one of the reactors known as the first primary reactor (Ri), means for circulating the juices in series from one reactor to another from the first reactor (R- _| ) to a last primary reactor (Rq), a juice outlet αiu the last primary reactor (Rq), a unit for neutralizing the juice from said last primary reactor (Rq), a post-reaction reactor hydrolysis (R _p ), means for transferring solid matter from the primary reactors (R- _j , R ₂ , Rq) to said post-hydrolysis reactor (R _p ), means for extracting solid residues from the post reactor - hydrolysis, an entry of liquid to said reactor post-hydrolysis ur (R _p ), means for circulating the juice extracted from the post-hydrolysis reactor (R _p ) to the liquid inlet of the first primary reactor (R- _j ), temperature regulation means allowing to adjust the temperature of the primary reactors and post-hydrolysis reactor, and an acidification unit connected in particular to the liquid inlet of the post-hydrolysis reactor.

19 / - Treatment installation according to claim 18, characterized in that it comprises bypass conduits (12) of a fraction of the juice from the

REPLACEMENT SHEET post-hydrolysis reactor (R) to primary reactors

(R ₂ , Rq) other than the first.

REPLACEMENT SHEET