EP0012960A1 - Process and apparatus for the continuous cooking of fibrous plant material - Google Patents

Abstract

1. A process for the continuous digestion of plant fibre material, wherein the plant fibre material is treated countercurrently with an organic solvent and wherein the plant fibre material is introduced at the top of a reactor by means of an in-feeding device, is conducted by the latter in a downward direction and cellulose is produced at the bottom of the reactor, organic solvent is conducted upwardly in countercurrent to the plant fibre material and is withdrawn at the top of the reactor, and the laden extraction liquor is separated into its essential components of lignin, hemicellulose and solvent, characterized in that the plant fibre material is impregnated in an impregnating device with organic solvent, the impregnated plant fibre material is positively conducted downwardly through the reactor, organic solvent at a temperature of between 130 and 210 degrees C is introduced as an extraction liquor into the centre of the reactor and at the bottom of the reactor water is fed in as a wash liquor, conducted upwardly in countercurrent to the plant fibre material and is withdrawn at the centre of the reactor.

Description

The invention relates to a method and a device for the continuous digestion of plant fiber material with organic solvent.

The economically important processes for the digestion of cellulose-containing fiber raw materials work today according to the sulfate or sulfite process.

Significant disadvantages of these processes are the necessary size of about 800 to 1000 tonnes per day in order to work economically and the enormous environmental impact in the form of waste water and exhaust gases, in particular because they contain sulfur compounds.

The possibility of digesting cellulose-containing plant fiber materials with organic solvents has already been shown in US Pat. No. 1,856,567.

DE-OS 2 637 449 describes a solvent extraction process for the production of cellulose pulp, in which lignin is extracted from finely divided fibrous plant material at elevated temperature and pressure with an aqueous solution of a lower aliphatic alcohol. This method works with a plurality of extractors working in batches, with a series of work stages following one another in each extractor. This known method is not suitable for the continuous, economical production of cellulose from plant fiber material. The investment costs are disproportionately high due to the large number of extractors required. In particular, the heating-up time per batch, if not disproportionately large amounts of solvent are used very long so that hydrolytic degradation reactions of the cellulose and hemicellulose take place before the high temperature required for the dissolution of the lignin is reached. Therefore, it is not possible with the known method to obtain pulp for paper production with sufficient strength.

In US Pat. No. 3,585,104 and DE-AS 2,644,155, continuous digestion and recovery processes for vegetable fiber raw materials for the production of cellulose using organic solvents are described. No pre-impregnation is used in these processes. For this reason, the material flow is easily blocked, especially since a forced transport through the reactor is not provided. In the known methods, the digested material is washed with cold organic solvent after boiling. However, lignin residues are not removed by washing with cold solvent. The organic solvent is then pressed off the digested material in a screw press. The digested material is then washed in a separate device with an aqueous sodium hydroxide solution. This solution is finally pressed out in a second screw press. This procedure is not very effective and very expensive. In particular, the need to drive the organic solvent out of the digested cold material leads to an increased expenditure of energy.

The invention has for its object a method and an apparatus for continuous digestion To create from vegetable fiber material with organic solvent, the essential components of the vegetable fiber material, namely lignin, hemicellulose and cellulose can be separated and obtained in pure form, which works economically on an industrial scale, which is characterized by a simple process, in particular those described above Disadvantages of the known methods are avoided, and that provides a high quality and easily bleachable cellulose.

The invention relates to a process for the continuous digestion of plant fiber material, in which the plant fiber material is treated in countercurrent at temperatures of 130 to 210 ° C with organic solvent, which is characterized in that the plant material is impregnated with organic solvent in an impregnation device impregnated plant fiber material is introduced into the head of a reactor by means of an introduction device and is forcibly guided downwards through it, cellulose is applied to the bottom of the reactor, organic solvent with a temperature of between 130 and 210 ° C. is introduced as the extraction liquid in the middle of the reactor, countercurrent to the plant fiber material upwards guided and drawn off at the top of the reactor, and water fed as washing liquid at the bottom of the reactor, led upward in countercurrent to the plant fiber material and drawn off in the middle of the reactor, and the loaded extraction liquid is separated into the essential components lignin, hemicellulose and solvent.

The invention further relates to a device for the continuous digestion of plant fiber material with organic solvent, which comprises an impregnator, a reactor for disintegrating the plant fiber material, washing devices for the digested plant fiber material and separating devices, which is characterized in that the impregnator in which the plant fiber material is impregnated with organic solvent is designed as a screw conveyor and directly via a Pressure lock with the head of the reactor is connected, that in the reactor integrally a cooking zone and a washing zone are provided, the reactor at the bottom one or more inlets for the washing liquid and an outlet for the digested vegetable fiber material, in the middle area a discharge for the washing liquid and one or more feeds for the extraction liquid and, in the reactor head, an introduction device for the plant fiber material and furthermore a draw-off for the loaded extraction liquid, and separation devices for the separation of lignin, hemicellulose and solvents are provided.

The advantages of the process according to the invention are, in particular, that economical operating sizes can be achieved with only 50 to 100 tonnes per day, that the raw materials contained in the raw material, namely hemicellulose and lignin, are largely obtained in native form, and that the organic solvents used for the extraction are obtained can be recovered and reused for example by simple distillation and subsequent condensation.

The continuous recovery of the solvents used is practically lossless.

The process can be used for all pumpable, pourable and free-flowing cellulose-containing raw materials. _In particular, wood from indigenous and tropical provinces and annual plants such as straw and bagasse are suitable as raw materials.

The cellulose obtained by the process according to the invention is excellently suitable for paper production due to its high quality.

With the method according to the invention, however, apart from cellulose, the other essential constituents of the plant fiber material can be obtained in native, pure form, so that these products are available for further processing in various areas.

In the only pulping processes that have been used on a large scale so far, namely the sulfite and sulfate processes, the hemicelluloses and the lignin were obtained in the form of derivatives which were not suitable as raw materials for further processing. These substances, which are valuable in themselves, have therefore previously been treated or burned as pure waste products.

In contrast, the method according to the invention now allows for the first time to obtain the further essential constituents, namely lignin and hemicelluloses, in the form of pure, valuable raw materials.

The hemicelluloses are suitable for a variety of uses, their use being particularly noteworthy in the following areas: Production of sugars, in particular sorbitol and mannitol: Production of thickeners, adhesives, size and thickeners for the Paper, textile and coatings industry; Further processing to alcohol by fermentation. It should be emphasized that it can be advantageous to reuse the alcohol obtained from the hemicelluloses as a solvent in the process according to the invention.

With the method according to the invention, lignin can be obtained in native and largely pure form. Lignin is an extremely valuable raw material. It is obtained as a polyol in the process and is particularly suitable for the production of synthetic resins. A reaction with aldehydes is particularly suitable. As synthetic resins that can be obtained starting from lignin, particular mention should be made of polyurethanes, acetal resins, epoxy resins and phenolic resins.

Organic solvents are used as the extraction liquid. The organic solvents should preferably be miscible with water in any ratio. Particularly suitable are aliphatic alcohols with 1 to 6 carbon atoms, including polyhydric alcohols, such as glycol and glycerol, aromatic and aliphatic amines, ketones, such as acetone and methyl ethyl ketone, ethers, such as tetrahydrofuran and dioxane, and solvents, such as dimethylformamide and dimethyl sulfoxide. Aliphatic alcohols, in particular ethyl alcohol and isopropyl alcohol, are preferably used as solvents. The solvents can be used in pure form or as mixtures with water. In the case of aqueous mixtures, concentrations of the solvent in the water of 10 to 100%, preferably between 40 and 60%, are suitable.

Based on the plant fiber material to be digested, it is preferred to work with a two to five times the volume of extractant.

The impregnation can be carried out at atmospheric pressure, which is preferred, or at overpressure. It can be carried out at room temperature or at an elevated temperature, a temperature between 40 and 80 ° C., in particular about 60 ° C., being preferred.

In the cooking or extraction zone, extraction is preferably carried out at temperatures between 130 and 210 ° C., in particular between 180 and 200 ° C. The pressure in the reactor is between 1 and 40 bar, preferably between 15 and 25 bar.

The further processing of the disrupted and washed vegetable fiber material emerging from the reactor, i.e. cellulose, takes place in the usual way, so that it does not have to be discussed in more detail.

The extraction liquid leaving the reactor, which is loaded with the substances extracted from the plant fiber material, is broken down into its constituent parts, i.e. essentially lignin, hemicellulose and solvent, separated.

The solvent, preferably the alcohol, can be removed in a simple manner by distillation, stripping or flashing. The remaining aqueous phase, which contains lignin and hemicellulose, can be separated into its constituents, for example, in that the concentration shift by the ent Removal of the alcohol and by lowering the temperature, the lignin is precipitated and this is isolated in a suitable manner, for example by means of a screw press, centrifuge or a heavy phase separator. The remaining hemicelluloses can then turn a eindarstellung _R and processing are supplied.

In the method according to the invention, the impregnation of the plant fiber material, which is usually in the form of wood chips, is important. In contrast to conventional, continuous pulp production processes according to the sulfate or sulfite Z process, in which an expensive impregnation is essential to ensure sufficient pulp quality, the impregnation can be carried out in a simple manner in the present process.

In conventional sulfate or sulfite processes, impregnation essentially fulfills the following functions. Non-condensable gases and terpenes are expelled from the wood chips, so that there is no influence on the heat transfer and the diffusion of the cooking chemicals into the wood chips. The wood chips adsorb condensate vapor, increasing the density at the same time. The moisture displaces air from the wood chip surface. These two effects ensure that the wood chips sink relatively quickly into the cooking liquor. The temperature and moisture content of the wood chips are raised and evened out so that the reactor is cooked evenly. Tem Temperature and moisture lead to swelling of the wood chips, so that the impregnation with the inorganic constituents of the cooking liquor is facilitated.

Inadequate impregnation of the wood chips can lead to excessive condensation of the lignin and thus to the notorious black boiling if the wood chips at the core of the wood chips are at boiling temperature.

Usually the pre-impregnation of wood chips. procedurally carried out as follows. The wood chips are introduced into a so-called steaming container via a metering device. Here, the wood chips are treated with saturated steam at pressures of 1 to 3 bar. Steam pretreatment is done for the reasons mentioned above. The steam-heated chips are then transported to an impregnation tank. This is where the actual impregnation with cooking liquor begins. The wood chips saturated with cooking liquor are then hydraulically pumped out of the impregnation tank by means of a high-pressure conveying device into an inclined separation tube at the head of the reactor. The wood chips are transported into the reactor head by means of a screw conveyor. The excess impregnation liquor is returned to the impregnation tank via a cylindrical sieve located in the separation tube.

The process according to the invention, in which solvents or solvent / water mixtures are used instead of inorganic significant procedural simplifications of the impregnation device. The main advantage of solvent / water mixtures is the much higher wetting speed and diffusion into the wood chips compared to conventional cooking liquors based on solutions of inorganic salts.

The organic solvent removes the terpenes and resins that interfere with the penetration of the digestion liquid. For this reason, the use of a damping device operated under positive pressure can be dispensed with entirely. It goes without saying that the method according to the invention can nevertheless be used to dampen the wood chips.

In order to achieve a satisfactory impregnation with the organic solvent, the entire impregnation device can be reduced to an inclined impregnation tube located at the head of the reactor. The wood chips are pushed upwards by means of a screw in the riser pipe filled with extraction liquid. By preheating the extraction liquid, for example with saturated steam, to temperatures of about 60 ° C., a further improvement in the impregnation is achieved. The residence time of the wood chips in the impregnation zone is, for example, between 1 and 15 minutes, preferably 2 to 5 minutes, depending on the throughput. The wood chips are then sufficiently impregnated with extraction liquid to ensure immediate immersion in the cooking zone. The pressure-free impregnation with the organic solvent ensures an even concentration distribution of the solution achieved in the raw material and thus a defined starting point for the extraction stage.

The chips are introduced against the high pressure prevailing in the extraction vessel preferably by means of a pressure lock controlled in time. It is advantageous to apply pressure to the pressure lock, preferably by means of steam, in order to keep the pressure difference between the pressure lock and the reactor low and thereby avoid pressure fluctuations in the reactor. If the pre-impregnation is carried out without pressure and the reactor operates at 18 bar, it is advantageous, for example, to introduce steam at 16 bar into the pressure lock.

The impregnated wood chips can then be easily pressed under the liquid level in the extraction reactor using an insertion device, preferably a screw. The introduction device also causes the wood chips to be forcibly conveyed through the reactor. Due to the pre-impregnation and the forced conveyance, there is certainly no formation of wood chip bridges at the entry point and thus a blockage of the material flow.

In the continuous pulp cooking process based on sulfite or sulfate, the cooker can be divided into four reaction zones. The first zone is an impregnation zone. With conventional sulfite or sulfate digestion, an impregnation zone is necessary despite the pre-impregnation in order to achieve perfect saturation of the wood chips with cooking chemicals the chips are brought to the desired cooking temperature. This is followed by the cooking zone in which the actual lignin degradation takes place. This is followed by a _J washing and cooling zone, in which the dissolved decomposition components are removed by countercurrent extraction with hot water. The washing water is used as the _Q uench- liquid to stop the degradation reaction.

In the process according to the invention using organic solvents as the extraction liquid, the process engineering of the digestion reaction can be considerably simplified. The reactor comprises only two zones, namely a cooking zone and a washing zone.

Due to the easy impregnability of the wood chips with the organic solvent, no further impregnation zone is required.

The wood chips are introduced into the cooking liquid at the top of the reactor, working at extraction liquid temperatures between ₁₃₀ and 210 ° C. Cooking takes place in countercurrent, the delignification of the chips starting practically immediately upon contact with the hot extraction liquid. To achieve sufficient delignification rates, it is necessary to reach a sufficiently high temperature of the wood chips as quickly as possible. Lignin degradation by means of organic solvents or solvent / water mixtures is essentially a thermal radical degradation and only achieves a rate that is sufficient for practical use at higher temperatures. However, since the lignin condensation and thus insolubilization begins at low temperatures, a heating-up period that is too long means that the lignin is only incompletely extracted - can be. The necessary rapid heating of the wood chips takes place in the method according to the invention due to the guidance of the impregnated wood chips in countercurrent to the hot extraction liquid.

In the process, the fresh extraction liquid is preferably fed in from the periphery of the reactor through a ring line. Instead of or preferably in addition to the feed from the periphery, extraction liquid can be fed in the middle of the reactor, for example via a hollow shaft. The extraction liquid, which rises in countercurrent to the wood chips, is drawn off at the top of the reactor. The organic solvent is recovered by distillation and condensation and recycled.

At the bottom of the reactor, water, which can be cold or warm, is fed in as washing liquid, which heats up in contact with the wood chips migrating downward. The feed is preferably carried out via a peripheral ring line and / or a hollow shaft arranged in the reactor axis. Due to the high rate of diffusion of organic solvents, the solvent is practically completely removed from the largely delignified material during countercurrent washing of the wood chips. In the middle of the reactor, the washing liquid loaded with solvent and degradation products is generally drawn off together with part of the hot cooking liquid.

The removed washing liquid is preferably solvent, for example by means of a metering pump added in such an amount that the desired composition of the extraction liquid is reached again: The extraction liquid is then brought to the required temperature, for example via a heat exchanger, and fed in via a peripheral ring line and / or a hollow shaft in the middle of the reactor.

It is essential in the process according to the invention that the washing liquid is not used for quenching the reaction, but rather is a component of the extraction liquid fed into the reactor.

Fluctuations in the process and the raw material, which cause changes in the composition of the extraction liquid, can be easily compensated for in the method according to the invention.

It is also advantageous that the chips are washed in countercurrent with pure water at a relatively high temperature. Because of the low concentration of wash water in organic solvent, the wash is much more effective than a wash using pure extractant as the washing liquid.

The digested material is usually discharged through a blow tank, washed again if necessary, and then sorted and processed in the usual way,

The invention is based on the drawing naner

The single figure shows a flow diagram of the impregnation, extraction, washing and separation stage of the process according to the invention.

The plant fiber material, for example wood chips, is introduced into the impregnator 1, which operates at atmospheric pressure and at a temperature of 60 ° C., via the insertion device 16. An ethanol-water mixture is fed to the impregnator via line 17. The impregnated wood chips are conveyed upwards by means of the screw conveyor 18 through the inclined impregnator and fed to a pressure lock 2 via a discharge device 19. The pressurized wood chips are then introduced into the reactor 3 above.

In the reactor, the chips are pressed below the liquid level by the vertically arranged screw 20 and passed down through the reactor. On their way through the reactor, the chips are first flowed through in countercurrent by the Tisser extraction liquid, the dissolving lignin and other cellulose companions being drawn out of the chips by diffusion and being carried away by the upstream extraction liquid.

The extraction liquid enriched with extract leaves the reactor via line 10. The extraction liquid is then fed via a cooler 23, to a waste-gas relaxation vessel 24. The solvent vapors released are condensed in a vapor condenser 25 and the solvent is recirculated via line 33 to the process.

The remaining aqueous solution of hemicelluloses and lignin is fed to a screw press 28 by a pump 26 via a further cooler 27. A centrifuge or a heavy phase separator can replace the screw press. At low temperatures, the lignin precipitates in the screw press and is separated from the hemicelluloses. The hemicelluloses and the lignin, which are drawn off via lines 29 and 30, can then be sent separately for further processing.

At the end of cooking zone 4, the dissolution of the cellulose companions is complete. The cooking liquid is washed out of the digested material in the subsequent washing zone 5 by rising water and drawn off with it in the middle of the reactor via discharge 8, line 11 and pump 21. Fresh solvent is metered in via line 15 and, after heating in the heat exchanger 14, it is fed in as an extraction liquid via lines 13, 13 'through the hollow shaft 22 reaching into the top of the reactor through feed 9 in the middle of the reactor and via ring line 31.

Fresh solvent is fed to the process via line 34.

The washed-out pulp is withdrawn from the reactor at the end of the washing zone 5 via outlet 7. Washing liquid in the form of water is fed to the reactor on the reactor floor via the peripheral ring line 32 and / or the hollow shaft 6.

It goes without saying that the individual material flows are controlled by means of suitable measuring and regulating devices.

With the process according to the invention, cellulose of excellent quality and bleachability are obtained, which are ideally suited as a raw material for papermaking. Hemicelluloses and lignin are obtained in pure form and can be used in many different ways as valuable raw materials.

Claims (16)

1. A process for the continuous digestion of plant fiber material, in which the plant fiber material is treated in countercurrent at temperatures of 130 to 210 ° C with organic solvent, characterized in that the plant fiber material is impregnated with organic solvent in an impregnation device, the impregnated plant fiber material at the head of a Reactor introduced by means of an introduction device and is forcibly guided through this down from the cellulose on the reactor bottom is brought in the middle of the reactor organic solvent at a temperature between 130 and 210 ° C as an extraction liquid, countercurrent to the plant fiber material and drawn up at the top of the reactor, and water is fed to the bottom of the reactor as washing liquid, in countercurrent to the Vegetable fiber material is led upwards and drawn off in the middle of the reactor, and the loaded extraction liquid is separated into the essential constituents lignin, hemicellulose and solvent. 2. The method according to claim 1, characterized in that the organic solvent in the form of a mixture of organic solvent and water, with a concentration of 10 to 100%, in particular between 40 and 60%, solvent is used. 3. The method according to claim 1 or 2, characterized in that ethyl alcohol or isopropyl alcohol are used as the organic solvent. 4. The method according to claim 1, characterized in that the impregnation takes place at atmospheric pressure. 5. The method according to claim 1, characterized in that the impregnation is carried out at elevated temperature, in particular between 40 and 80% C. 6. The method according to claim 1, characterized in that the impregnation is carried out in a screw conveyor. 7. The method according to claim 1, characterized in that the extractant introduced in the middle of the reactor has a temperature between 180 and 200 ° C. 8. The method according to claim 1, characterized in that the drawn off in the middle of the reactor washing liquid organic solvent is added and this mixture is fed back as an extraction liquid to the reactor. 9. The method according to claim 1, characterized in that the extraction liquid is fed via a hollow shaft into the center of the reactor and / or a ring line from the periphery of the reactor. 10. The method according to claim 1, characterized in that based on the plant fiber extraction liquid is used in two to five times the volume. 11. Device for the continuous digestion of plant fiber material with organic solvent, which comprises an impregnator, a reactor for digestion of plant fiber material, washing devices for the digested plant fiber material and separating devices, characterized in that the impregnator (1) in which the plant fiber material is impregnated with organic solvent , is designed as a screw conveyor and directly via a pressure lock (2) with the head of the reactor (3) in Ver binding is that in the reactor (3) integrally a cooking zone (4) and a washing zone (5) are provided, the reactor (3) having one or more inlets (6, 32) for the washing liquid and an outlet (7 ) for the digested plant fiber material, in the middle area a discharge (8) for the washing liquid and one or more feeds (9, 31) for the extraction liquid and in the reactor head an introduction device (20) for the plant fiber material and also a discharge (10) for the loaded one Extraction liquid comprises, and that separation devices (24, 25, 28) are provided for the separation of lignin, hemicellulose and solvent. 12. The apparatus according to claim 11, characterized in that the discharge (8) via lines (11, 12, 13, 13 ') and heat exchanger (14) with the feeds (9, 31) is connected, in one of the lines (11, 12, 13, 13 ') opens a feed (15) for metering organic solvent. 13. The apparatus according to claim 11 or 12, characterized in that a hollow shaft (22) extending from above to approximately in the middle thereof is arranged in the reactor for supplying further extraction liquid via a supply (9). 14. The apparatus according to claim 11, characterized in that a further inlet (6) is provided in the form of a hollow shaft for the supply of the washing liquid in the middle of the reactor bottom. 15. Apparatus according to claim 11, characterized in that the _E is formed inbringvorrichtung (20) in the form of a screw conveyor. 16. The apparatus according to claim 11, characterized in that for the separation of lignin, hemicellulose and solvent, an exhaust liquor ( ₂₄ ), a vapor condenser (25) and a screw press (28) are provided.

Images