EP0078023A1 - Process for treating cellulosic materials with gaseous hydrofluoric acid - Google Patents

Abstract

The continuous process for hydrolyzing cellulose-containing material (substrate) is carried out by sorption of gaseous HF in a sorption reaction (1) and subsequent desorption in n steps, which are carried out in n reactors which are separated from one another in a gas-tight manner. The substrate is introduced via a gas-tight valve into the sorption reactor (1), passes through this and then reaches consecutively, through gas-tight valves, a hold-up reactor (2) and the first (3c), second (3b), . . . nth desporption reactor, from which it is then removed. The desorption is carried out in each case by the action of one of the n inert gas streams on the substrate at different temperatures, the particular inert gas stream being enriched with the HF being liberated during desorption. The gas streams, which are enriched to different extents with HF, are allowed to act on the substrate in the sorption reactor (1) in such a manner that the gas streams of low HF concentration act on a substrate having a zero or low concentration of HF and thereafter the gas streams of higher HF concentration act on substrate having higher HF concentration. The total gas stream (8a) produced from the individual gas streams leaves, after completion of sorption, the sorption reactor (1) largely freed of HF and is either conveyed to the desorption steps after dividing up into individual gas streams or it initially passes through the last desorption step (3a) and is thereafter divided up and passed to the other desorption steps in order, after passing through the latter, to be returned to the sorption reactor (1).

Description

It is known that cellulosic material, e.g. Wood or waste from annual plants that can chemically digest with mineral acids. Here the cellulose contained, which is a macromolecular substance, is broken down into glycosidic bonds into water-soluble, smaller molecules down to the monomer units, the glucose molecules. The sugars obtained in this way can include fermented into alcohol or used as a fermentation raw material for the production of proteins. This is the technical meaning of wood saccharification. As mineral acids suitable for this purpose, dilute sulfuric acid (Scholler process) and concentrated hydrochloric acid (Bergius process) were used on an industrial scale decades ago; see e.g. Ullmann's Encyclopedia of Technical Chemistry, 3rd edition, Munich-Berlin, 1957, Volume 8, pp. 591 ff.

It is also known that hydrogen fluoride can also be used for wood saccharification. The location of its boiling point (19.7 ° C) allows it to be brought into contact with the substrate to be digested without water as a solvent and to be recovered with comparatively little effort after the digestion has been completed. Not only native material is suitable as a digestion substrate: Rather, it has already been proposed to use waste paper or lignocellulose, the residue of a pre-hydrolysis, which contains very little hemicellulose and other wood-related substances and almost only from cellulose and Lignin exists. Not only wood, but also paper or residues of annual plants of all kinds such as straw or bagasse can be subjected to this pre-hydrolysis. According to the prior art, it consists of an action of water or dilute mineral acid (approx. 0.5%) at 130 to 150 ° C (cf. eg manual "Die Hefen", Volume II, Nuremberg, 1962, p. 114 ff.) or saturated water vapor at 160 to 230 ° C (cf. US -PS 4.160.695).

• die Umsetzung mit gasförmigem Fluorwasserstoff unter Atmosphärendruck,
• die Extraktion mit flüssigem Fluorwasserstoff und schließlich
• die Umsetzung mit gasförmigem Fluorwasserstoff im Vakuum.

Three technical process principles are known from the literature for the implementation of hydrogen fluoride with cellulose-containing material:

• reaction with gaseous hydrogen fluoride under atmospheric pressure,
• the extraction with liquid hydrogen fluoride and finally
• the reaction with gaseous hydrogen fluoride in a vacuum.

DE-PS 585 318 describes a method and a device for treating wood with gaseous hydrogen fluoride, in which hydrogen fluoride gas, which can be diluted with an inert gas, is reacted with wood in a first zone of a reaction tube with a screw conveyor, that this zone is cooled from the outside below the boiling point of the hydrogen fluoride. After the digestion, which may take place in an intermediate zone, the hydrogen fluoride is expelled by external heating and / or blowing out with an inert gas stream in order to be brought back into contact with fresh wood in the cooling zone mentioned.

In practice, however, this procedure is difficult to carry out. When the hydrogen fluoride condenses on the substrate, it is distributed only unevenly, so that local overheating occurs. This is evident, for example, from DE-PS 606 009, which states: "It has been shown that when the polysaccharides, for example the wood, are merely moistened with hydrofluoric acid or when the wood and the like are loaded with hydrofluoric acid vapors, temperature increases occur can lead to a partial destruction of the conversion products formed. However, this heat is dissipated by cooling as a result of the poor thermal conductivity of the cellulose-containing material is difficult. "As a remedy, this patent describes an extraction with liquid hydrogen fluoride, which, however, requires large amounts of hydrogen fluoride and has the disadvantage that the evaporation of the hydrogen fluoride from the extract and from the extraction residue (lignin) large amounts of heat must be added and removed again in the subsequent condensation.

The AT-PS 147 494 published a few years later deals with the two methods mentioned. As a remedy for the uneven and imperfect degradation of the wood when digested with highly concentrated or anhydrous hydrofluoric acid in a liquid or gaseous state at low temperatures, as well as against the disadvantages of the high excess of hydrofluoric acid in the extraction process, this patent describes a technically complex process in which the wood before the action of the fluorine _'hydrogen is largely evacuated as possible, and also the recovery of the hydrogen fluoride is conducted in vacuum. The process is also described in the journal "Holz Roh- und Werkstoff" 1 (1938) 342-344. The high technical outlay in this process is not only due to the vacuum technology itself, but also due to the fact that the boiling point of hydrogen fluoride falls below the value of -20 ° C at 150 mbar; this means that condensation is no longer possible without the aid of complex coolants or units.

The literature-known prior art of wood digestion with hydrogen fluoride is characterized by the three methods and devices described. Accordingly, none of these methods or devices combines low expenditure and good digestion results in a technically satisfactory manner. The inherently economical way of reacting cellulose-containing material with a hydrogen fluoride / inert gas mixture which originates from hydrogen fluoride desorption, according to DE-PS 585 318 already mentioned, according to DE-PS 606 009 published later, is apparently impaired by the need to cool below the boiling point of the hydrogen fluoride during absorption.

Surprisingly, it has now been found that gaseous hydrogen fluoride in a mixture with an inert carrier gas can be circulated almost without loss, producing a load of the substrate required for good yields, without the technically highly disadvantageous cooling below the boiling point of the hydrogen fluoride being necessary. This is achieved by dividing the desorption process into several stages, in which the desorption takes place in cocurrent or countercurrent of HF gas mixtures and reaction material (substrate). Corresponding to the different HF loading of the substrate when entering the individual desorption stages, HF gas mixtures of different HF concentrations are formed, which act on the substrate at different points in the sorption stage in such a way that low-HF gas mixtures affect unloaded or HF-loaded material, Act gas mixtures with higher HF concentrations on already heavily loaded material.

This measure was not obvious. Rather, data in the literature allow the conclusion that an adequate loading of wood material even with undiluted hydrogen fluoride above its boiling point is not possible. In a work by Fredenhagen and Cadenbach, Angew. Chem. 46 (1933) 113/7 states (p. 115 bottom right to p. 116 top left): "If gaseous HF is allowed to act on wood at room temperature, HF is absorbed and the temperature rises as a result. This causes but that no further HF quantities are absorbed, so that the reaction comes to a standstill and no further temperature increase occurs. " All the more surprising was the finding that the hydrogen fluoride sorption from the heat of the reaction, which was only noticeable up to relatively low loads makes, is largely independent, rather at a given temperature depends only on the HF concentration in the gas mixture acting. that is, even at temperatures above the boiling point of hydrogen fluoride up to the loading heights required for good yields can be carried out by stepwise generation and use of streams of different HF concentrations.

The subject of the invention is thus a continuous process for the digestion of cellulose-containing material (substrate) with gaseous hydrogen fluoride by sorption of the HF and subsequent desorption, which is characterized in that the HF is sorbed by the substrate at a temperature above its boiling point in a sorption stage, and that the substrate is then freed from the sorbed HF by heating in n desorption stages, n being an integer and the stages mentioned running in gas-tight reactors, and the substrate being introduced into the sorption reactor through a gas-tight lock and then successively through gas-tight locks into the first, second ..... nth desorption reactor and discharged from the last (nth) desorption reactor, and the desorption in each case by the action of one of n heated gas streams in countercurrent to or , preferably, direct current with the substrate with enrichment of the respective gas stream with the HF released during the desorption, and the n HF gas streams, which contain an inert carrier gas in addition to the HF, act in countercurrent to the substrate in such a way that gas streams with a low HF concentration unloaded or still little loaded with HF and gas streams with a high HF concentration act on a substrate more heavily loaded with HF, and the total gas stream resulting from the individual gas streams leaves the sorption reactor, largely depleted in HF, after sorption and either after division into individual gas streams is fed to the desorption stages in the circuit or first goes through the last desorption stage and then divided and fed to the other desorption stages and the sorption reactor.
n is an integer. preferably from 2 to 6, in particular from 2 to 4.

The reactors separated from one another by gas-tight locks can be of the same or different types: for example, stirred vessels are suitable. Rotary tubes, flight dryers, slide beds, screw conveyors, vertical countercurrent or fluidized bed reactors. If necessary, they can be provided with a heating or cooling device.

Wood or waste from annual plants (e.g. straw or bagasse) or, preferably, a pre-hydrolyzate of wood or waste from annual plants, or, also preferably, waste paper, can be used as the cellulose-containing material.

As is known, the presence of a certain amount of water is necessary for the digestion of celluloses, which is hydrolytic cleavage. This water can either be introduced by being present in the substrate as residual moisture of 0.5 to 20, preferably 1 to 10, in particular 3 to 7% by weight, or by being contained in the HF inert gas mixture, or in both.

The reaction material (substrate), the cellulose-containing material, is transported from one reactor to another, for example, by free fall, via rotary feeders and / or by screw conveyors.

Air and nitrogen are suitable as inert carrier gases. Carbon dioxide or one of the noble gases, preferably air or nitrogen.

The gas flow takes place according to the invention in such a way that the gas outlet opening of a sorption reactor is connected via a gas line with an intermediate gas pump (blower) and n-1 branches to the gas inlet openings of n desorption reactors, and the gas outlet openings of these n desorption reactors are connected via gas lines to n gas inlet openings of the sorption reactor. A valve and a heat exchanger are interposed in front of the gas inlet openings of the desorption reactors.

Heat exchangers can also be arranged in front of the gas inlet openings of the sorption reactor.

They may have the task of bringing the gas mixture intended for sorption to the optimum temperature for this. They may also have the task of accompanying substances in the feed material, such as water, which may have been released during the desorption. Condense acetic acid, essential oils, but let the hydrogen fluoride pass in gaseous form.

The gas flow leaving the sorption reactor and containing a maximum of 5% by weight HF, preferably almost completely HF-free, is divided by the branches into n partial gas flows, the size of which depends on the respective setting of the. Valves depends. These partial gas streams are heated in the heat exchangers to the temperature required for the desorption and are allowed to act on the substrate in the desorption reactors in countercurrent to, or preferably in cocurrent with, the substrate. The n partial gas flows are again enriched with HF by the HF released during the desorption.

This enrichment with HF is of different sizes in the individual partial gas streams. In the first desorption reactor, a lot of HF is released during the desorption of the substrate, which is introduced here with maximum HF loading. In the following desorption reactors, the desorption takes place with substrate, that in the previous desorption stages HF has already been freed ever further. In the last (nth) desorption reactor, only a little HF is released, since the substrate is already introduced into HF, largely depleted in HF. When leaving this last desorption reactor, the substrate only contains HF traces.

The division into n partial gas streams can also be carried out in such a way that the gas stream leaving the gas outlet opening of the sorption reactor is first fed completely through the pump to the last (nth) desorption reactor - after heating in the upstream heat exchanger. in which it acts on the already largely depleted HF substrate. Only after leaving this last (nth) desorption reactor is the gas stream divided into an (nth) partial gas stream, which is fed directly to the corresponding gas inlet opening of the sorption reactor, and n-1 partial gas flows, which are directed to the penultimate ((n-1) -th) to the first desorption reactor, after heating in the respective upstream heat exchanger.

The HF concentration in the n-th HF carrier gas stream leaving the last (n-th) desorption reactor is relatively low and increases more and more in the penultimate ((n-1) th) and the previous ones and is in the first HF carrier gas stream, which leaves the first desorption reactor, is highest (up to over 95% by weight). .

The HF carrier gas streams of different HF concentrations are fed through gas lines to the n gas inlet openings of the sorption reactor, in such a way that the nth HF gas flow onto only a little HF-loaded substrate and the first HF gas flow onto the HF ( almost) maximum loaded substrate. The remaining HF gas streams are fed to the substrate at gas inlet openings of the sorption reactor located in between.

The maximum HF loading of the cellulose-containing material depends on its type and nature and on the residence time in the sorption stage and is accordingly between 10 and 120%, preferably between 30 and 80%, based on the weight of the material used.

If necessary, the substrate loaded with HF, after leaving the sorption reactor and before entering the first desorption reactor, can still pass through a dwell reactor which may have a comminution device for coarse reaction material and the temperature of which is expediently kept in a range which is dependent on the temperatures in the last part of the sorption reactor and is trapped in the first desorption reactor.

The optimal dwell time, i.e. the average length of stay of the substrate in the apparatus from the beginning of the sorption to the end of the desorption depends on the type and nature of the material to be digested and must be matched to the respective case. Accordingly, it can range from about 30 minutes to about 5 hours.

Substrate temperatures in the range from 40 to 120.degree. C., preferably from 50 to 90.degree. C., are chosen for the desorption, the temperatures for the individual stages being different, but a temperature in the range from 20 to 50 for the respectively assigned sorption ° C, preferably 30 to 45 ° C.

In contrast to the normal countercurrent principle according to the prior art, the arrangement according to the invention allows the flow rate and temperature of the HF carrier gas mixture to be adapted to the requirements which are different in the individual regions of the sorption stage and in the individual desorption stages and are dependent on the HF loading degree of the substrate.

The invention will be explained in more detail with reference to FIGS. 1 to 3.

• FIG. 1 shows the flow diagram of a reaction sequence according to the invention in one sorption and three desorption reactors.
• Figure 2 shows a section of the overall flow diagram of Figure 1 with a further subdivision of one of the gas circuits with partial recirculation.
• FIG. 3 shows the flow diagram of a further possibility according to the invention for the course of the reaction in one sorption and three desorption reactors.

In these figures:

The sorption reactor 1 is connected to the desorption reactor 3a via the gas line 8a, the pump 4, the valve 9a and the heat exchanger 5a, and this is connected to the sorption reactor 1 via the gas line 7a and the heat exchanger 6a. Furthermore, the sorption reactor 1 is connected to the desorption reactors 3b or 3c via the gas line 8a, the pump 4, the gas lines 8b or 8c, the valves 9b or 9c and the heat exchangers 5b or 5c, and this is via the gas lines 7b or 7c and the heat exchangers 6b and 6c are connected to the sorption reactor 1.

The cellulosic material (substrate) to be digested is introduced into the sorption reactor 1. This process is symbolized by the arrows 12a in FIGS. 1 and 3.

HF-inert gas mixtures, the HF concentration of which is lowest in gas line 7a and highest in gas line 7c, are supplied to sorption reactor 1 through gas lines 7a, 7b and 7c. In the sorption reactor 1, they flow against the substrate and emerge from the reactor 1 as an almost completely HF-free total gas stream.

The substrate loaded with HF is transported from the sorption reactor 1 into the dwell reactor 2 (arrow 12b) and from there successively into the first, second and third desorption reactors 3c, 3b and 3a (arrows 12c, 12d and 12e).

The gas stream leaving the sorption reactor 1 is divided into three partial flows after passing through the gas line 8a and the pump 4, in accordance with the respective setting of the valves 9a, 9b and 9c. After heating in the heat exchangers 5a or 5b or 5c, these partial gas flows enter the desorption reactors 3a or 3b or 3c and flow through them in countercurrent to, or, preferably, in cocurrent with the substrate.

HF is desorbed by the action of the heated gas streams on the HF-loaded substrate. In the first desorption reactor 3c, since the substrate with maximum HF loading is introduced, most HF is released by desorption, a smaller amount is released in the reactor 3b, and in the last desorption reactor 3a, into which the substrate already largely frees from HF, the smallest amount of HF is released. Accordingly, the HF concentrations in the gas streams leaving the desorption reactors are highest in reactor 3c and lowest in reactor 3a. The HF gas stream which emerges from the reactor 3b has an intermediate HF concentration in between.

The HF gas streams of different HF concentrations are fed into the sorption reactor 1 at various inlet points through the gas lines 7a or 7b or 7c after passing through the interposed heat exchangers 6a or 6b or 6c. The HF gas flow from the gas line 7a with the lowest HF concentration strikes substrate which is only very slightly loaded with HF. The HF gas stream from the gas line 7c with the highest HF concentration strikes the substrate which (almost) has the maximum HF loading. The HF gas flow from the gas line 7b is allowed to act on the substrate at an intermediate point of the sorption reactor 1, which substrate already has a relatively high HF load.

After desorption has taken place in the reactor 3a, the substrate leaves it in an open form (arrow 12f). It only contains traces of residual hydrogen fluoride and is processed, which is carried out in a manner known per se.

A special embodiment is shown schematically in Figure 2. A three-way valve (10) is interposed in the gas line 7a, which allows a (more or less large) part of the HF gas stream emerging from the desorption reactor 3a to be passed into a gas line (11) special circuit again and to be introduced between the valve 9a and an intermediate pump (4a) into the gas line 8a via a branch. The three-way valve 10 can also be a control valve. The part of the HF inert gas mixture which is returned in this special cycle is about 10 to about 90%, preferably about 50 to about 90%, of the total mixture which leaves the desorption reactor 3a. Of course, you can replace the three-way valve 10 with a T-piece and install a (control) valve in the gas line 11.

This particular arrangement, which analogously also allows partial recirculation of the HF inert gas mixtures leaving the desorption reactors 3c or 3b, allows the gas velocities of the HF inert gas mixtures passing through to be optimized.

FIG. 3 shows a further special embodiment of the method according to the invention. A three-way valve (10a) is interposed in the gas line 7a, which allows the gas stream leaving the sorption reactor 1 to be divided into partial gas streams only after it has passed the desorption reactor 3a. While a partial flow only passes through the reactor 3a and is fed directly to the sorption reactor 1, the other two partial flows are still passed through a second desorption reactor (3c or 3b) before they are fed to the reactor 1 through the gas lines 7c or 7b.

This particular embodiment allows the largest possible amount of gas, ie the total amount of carrier gas, to act on the substrate in the last desorption stage, as a result of which the desorption is accelerated.

It is advantageous to use any HF still present in the gas stream leaving the sorption reactor for sorption by passing this gas stream through the substrate storage silo before it is fed to the pump 4 via the gas line 8a.

The digested material produced by the process according to the invention is a mixture of lignin and oligomeric saccharides. It can be worked up in a manner known per se by extraction with water, advantageously in the heat or boiling heat, and by simultaneous or subsequent neutralization with lime. Filtration provides lignin, e.g. can be used as fuel, as well as a small amount of calcium fluoride, which results from the residual hydrogen fluoride contained in the reaction material. The filtrate, a clear, slightly yellowish sugar solution, can be added to the alcoholic fermentation or fermentation either immediately or after setting an appropriate concentration. The dissolved, oligomeric sugars can also be treated by brief post-treatment, e.g. with highly diluted mineral acid at temperatures above 100 ° C, almost quantitatively converted into glucose.

example 1

Example 1 was carried out in an apparatus arrangement which is shown schematically in FIG. 1. It consisted of a sorption reactor (1), a residence reactor (2) and three desorption reactors (3a, 3b, 3c), which were connected to each other by pipelines and cellular wheel locks. A vertical tube made of stainless steel with a clear width of 5 cm and a length of 80 cm was used as the sorption reactor, which carried a gas-tight rotary valve with a filling funnel at the upper end and was also provided with a gas-tight rotary valve at the lower end. In the longitudinal axis of the tube was a slowly rotating shaft with narrow wings. At 3 points, which were distributed over the lower two thirds of the pipe length, there were inlets for HF-containing gases. The gas outlet opening was located just below the top rotary valve. The indwelling reactor was a cylindrical vessel of approx. 2 1 contents made of semi-transparent polyethylene. The desorption reactors were made of stainless steel and were heatable rotary tube reactors trained, which could be flowed through in the same direction by the substrate and by the flowing gases. The usable volume of the desorption reactors was approximately 3 l each.

• Erster Desorptionsreaktor (3c): Eingetragen wurde aus dem Verweilreaktor (2) mittels gasdichter Zellenradschleuse im Gewichtsverhältnis 60 : 100 mit HF-beladene Lignocellulose; ausgetragen wurde ein Substrat mit einer Beladung von ca. 35 : 100 (Gew.-Verhältnis HF zu Lignocellulose); die Desorptionstemperatur lag bei 60 - 70°C; das austretende Gasgemisch enthielt ca. 65 Gew.-% HF.

In the sorption reactor (1), granular lignocellulose, which had been obtained as a residue from a pre-hydrolysis of spruce wood chips and had a water content of about 3% by weight, was continuously conveyed from top to bottom by its own weight. HF-nitrogen mixtures of various concentrations originating from the desorption were introduced through the three gas inlets, specifically at the lowest inlet point with the highest, at the uppermost with the lowest HF concentration. With the aid of samples taken from the lower cellular wheel, the conveying speed was adjusted so that the reaction mixture emerging from the reactor contained about 60 g HF per 100 g lignocellulose used. From the lower cellular wheel sluice, the substrate reached the indwelling reactor (2) by free fall and remained there for an average of 30 minutes. A temperature of 50 ° C. was maintained inside by blowing the vessel with warm air. The almost HF-free nitrogen leaving the sorption reactor (1) above was distributed to the three desorption reactors (3a, 3b, 3c) through a gas line (8a) via a gas pump (4) and the branching gas lines (8b, 8c). With the help of the throttle valves (9a, 9b, 9c) and the gas heaters (5a, 5b, 5c), the nitrogen introduced into the respective desorption reactor was regulated in such a way that the following gas mixtures and degrees of desorption were obtained with the help of the heating provided on the reactor itself:

• First desorption reactor (3c): The dwelling reactor (2) was introduced by means of a gas-tight rotary valve in a weight ratio of 60: 100 with HF-loaded lignocellulose; a substrate with a loading of approx. 35: 100 (weight ratio HF to lignocellulose) was discharged; the desorption temperature was 60 - 70 ° C; the exiting gas mixture contained approx. 65% by weight HF.

Second desorption reactor (3b): The product loaded with HF was introduced from the first desorption reactor (3c) by means of a gas-tight rotary valve; a substrate with a loading of approx. 10: 100 was discharged; the desorption temperature was 70 - 80 ° C; the exiting gas mixture contained about 25% by weight of HF.

Third desorption reactor (3a): The product loaded with HF was introduced from the second desorption reactor (3b) by means of a gas-tight rotary valve; a substrate with approximately 0.5% by weight HF was discharged; the desorption temperature was approx. 90 ° C; the exiting gas mixture contained about 5% by weight of HF.

The three gas mixtures generated in the desorption reactors were fed through the pipelines (7a, 7b, 7c) and the heat exchangers (6a, 6b, 6c), where they were cooled to 25-30 ° C., into the sorption reactor ( 1) passed so that circuits of carrier gas (nitrogen) and HF came about with continuous conveyance of substrate through the apparatus.

The digested substrate, largely freed from HF, was extracted in the usual way with hot water

neutralized solution thus obtained with calcium hydroxide, filtered and evaporated. Wood sugar of light color was obtained in a yield of 90%, based on the cellulose originally present.

Example 2

In the apparatus described in Example 1 and according to the process described in detail there, raw spruce wood shavings were broken down to a residual moisture content of about 5% by weight. During the desorption in the reactors 3c to 3a, wood accompanying substances such as acetic acid were driven off and condensed and separated in the heat exchangers 6c to 6a. After the usual work-up, as described in Example 1, wood sugar was obtained in a yield of about 70% by weight, based on the carbohydrates contained in the material used.

Claims (5)

1. Continuous process for the digestion of cellulose-containing material (substrate) with gaseous hydrogen fluoride by sorption of the HF and subsequent desorption, which is characterized in that the HF is sorbed by the substrate at a temperature above its boiling point in a sorption stage, and after that the substrate is freed of the sorbed HF by heating in n desorption stages, where n is an integer and wherein the stages mentioned take place in reactors which are each separated from one another in a gastight manner, and the substrate is introduced into the sorption reactor through a gas-tight lock, passes through it and then successively through gas-tight locks into the first, second ..... nth desorption reactor and discharged from the last (nth) desorption reactor, and the desorption in each case by the action of one of n heated gas streams in countercurrent to or, preferably, direct current with the substrate under excitation The respective gas stream is secured with the HF released during the desorption, and the n HF gas streams, which contain an inert carrier gas in addition to the HF, act in countercurrent to the substrate in such a way that gas streams with a low HF concentration have no or little charge substrate loaded with HF and gas streams with a high HF concentration act on substrate more strongly loaded with HF, and the total gas stream resulting from the individual gas streams leaves the sorption reactor, largely depleted in HF, after sorption and is either fed into the desorption stages after being divided into individual gas streams in the circuit is or first goes through the last desorption stage and then divided and fed to the other desorption stages and the sorption reactor. 2. The method according to claim 1, characterized in that n is an integer from 2 to 6, in particular from 2 to 4. 3. The method according to claim 1 or 2, characterized in that a pre-hydrolyzate of wood or waste from annual plants or waste paper are used as the substrate. 4. The method according to claim 1 to 3, characterized in that air or nitrogen are used as the inert carrier gas. 5. The method according to claim 1 to 4, characterized in that an HF gas flow or several HF gas flows after leaving the (the) desorption reactor (desorption reactors) is (are) and a part to the entrance of the (the) desorption reactor (desorption reactors) is returned directly. -

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