Classifications

• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C3/00—Pulping cellulose-containing materials
  • D21C3/22—Other features of pulping processes
  • D21C3/222—Use of compounds accelerating the pulping processes
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C3/00—Pulping cellulose-containing materials
  • D21C3/20—Pulping cellulose-containing materials with organic solvents or in solvent environment

Definitions

• the present invention relates to a new method of producing chemical pulp.
• the invention is an improvement in solvent cooking methods (e.g. organic solvent diges ⁇ tion), often referred to as "solvent pulping", being generally as described in U.S. Patent No. 3,585,104.
• Wood comprises cellulosic fibres with lignin both inside the fibres and between the fibres, bonding the fibres together.
• the lignin must be removed from between the fibres, the removal usually being accomplished by dissolving the lignin.
• the cooking liquors consist of sodium hydroxide (i.e. soda cook), sodium hydroxide containing sodium sulphide (i.e. sulphate or kraft cook, also called alkaline cook), or, for example, sulphite ions (i.e. sulphite cook, also called acid sulphite cook).
• Lignin can also be removed by some organic solvents (hence the commercial name organosolv-process), the best known of which are methanol and ethanol. Formic acid is another proposed organic solvent.
• Methanol and ethanol can be utilized as solvents in both alkaline and acid cooks.
• the advantage of acid cooks is the simple recovery of the chemicals, as wood contains acids that make the cook acid, when only methanol or ethanol are used as solvents.
• the disadvantage of acid cooks is the poor quality of the produced pulp, because the cellulosic fibres will, to some extent, degrade in acid treatment. In alkaline cooks the quality of the produced pulp will remain good, but the problem in alkaline cooks is the recovery of chemicals. Some alkali, mostly a sodium-based alkali, will firstly have to be added into the cook and then recovered and used again.
• a known alkaline organic solvent process is the "Organo- cell-process", presently being used in Germany, in which process the cook is a soda-methanol cook.
• the pH of the cook can be kept in the desirable range (generally neutral) by removing the acids from the beginning of the cook, i.e. almost immediately after they are formed.
• the pH generally neutral By keeping the pH generally neutral, the pulp quality is enhanced, both for continuous and batch cooking processes. This is accomplished, in general, by countercurrent flow during continuous cooking, and in batch cooking by withdrawing solvent from the batch digester after a predetermined time from the start of the cook (and perhaps periodically thereafter), while the cook continues after the solvent withdrawal, with new solvent added.
• a method of producing cellulose pulp from comminuted cellulosic fibrous material by organic solvent pulping comprising the step of (a) controlling the pH of the material during cooking so that it is .generally neutral by removing acids formed at the start of the cook together with organic solvent used in the cook and by adding alkali to the cellulosic fibrous material in an amount sufficient to maintain the pH at the desired value.
• This method may be practised utilizing a treatment vessel, comprising further the further steps of: (b) Causing the cellulosic fibrous material to flow in a first direction into the treatment vessel, and to be removed from the treatment vessel in the same, first, direction, whereby the needed amount of alkali is added to the material prior to the treatment vessel; and (c) introducing an organic solvent containing liquid stream to dissolve the lignin of the cellulosic fibrous material into the treatment vessel in a second direction, opposite the first direction.
• Step (a) is practised by removing the acids formed during pulping, together with spent organic solvent, from the treatment vessel.
• a method of producing cellulosic pulp from comminuted cellulosic fibrous material by solvent pulping comprising the steps of: causing the comminuted cellulosic fibrous material to flow in a first stream in a first direction; causing a stream of organic solvent to flow into operative contact with the comminuted cellulosic fibrous material countercurrent to the first stream, that is in a second direction, opposite the first direction throughout the area of operative contact between them, which causes organic acids to be produced as the organic solvent dissolves the lignin from the cellulosic fibrous material; and removing organic acids formed during solvent pulping from intimate contact with the cellulosic fibrous material along with the countercurrent flow of organic solvent.
• Figure 1 is a schematic of an exemplary embodiment of a cook/extraction process according to the invention
• Figure 2 is a schematic of another exemplary embodi ⁇ ment of a cook/extraction process according to the invention
• Figure 3 is a schematic of an exemplary embodiment of the washing stage subsequent to the cook/extraction process
• Figure 4 is a schematic of an exemplary embodiment of solvent recovery process according to the invention.
• Figure 5 is a schematic of another exemplary embodi ⁇ ment of a cook/extraction process according to the invention.
• Figure 6 and 7 illustrate the change of pH as observed when examining the cook/extraction process of Figure 1;
• Figure 8 is a schematic diagram illustrating ap ⁇ paratus for practising a method according to the present invention.
• Figure 9 a schematic of an exemplary embodiment of solvent and alkali recovery process according to the invention.
• Figure 10 is a possible process alternative of an application of the invention.
• Figure 11 is a possible process alternative of an application of the invention on an industrial scale.
• Figure 1 illustrates a schematic representation of the principle of a countercurrent cook/extraction treat ⁇ ment vessel/stage 20 according to the invention.
• the number 10 refers to comminuted cellulosic fibrous material (e.g. wood) being fed into the process and the number 11 refers to cellulosic fibres of the wood, produced by dissolving and extracting lignin from the wood.
• the number 12 refers to the organic solvent being fed to the process and the number 13 refers to the spent organic solvent being exhausted from the process, with dissolved lignin also present.
• the process of Fig. 1 was examined in laboratory conditions with the surprising result shown in Fig. 6.
• the pulp quality can be further improved by adding alkali to the countercurrent cook at the beginning of the cook. This case is shown in Fig. 7 by curve 102.
• the alkali added in the beginning increases the pH and acids formed or dissolved neutralize the cook and the pH decreases to an almost neutral value during the counter ⁇ current cook.
• a countercurrent extraction according to Fig. 2 ensures that the pH does not decrease at the initial stages of the cook/extraction 20.
• the amount of acid forming at the initial stages of the cook is 8-20 kg/ton of pulp, and an amount of alkali corresponding to the amount of acid is added, if the pH is to be kept out of the acid range (e.g. generally neutral, for example a pH of about 6.5 - 7.5). Due to the essentially completely countercurrent nature of the cook there is some alkali present at the initial stages of the cook 20, but it is then washed away and removed with the dissolved lignin in stream 13.
• the residual solvent in the stream of fibres sub ⁇ sequent to the extraction 20 must be washed away as shown in Fig. 3.
• the stream 11 of fibres is a mixture of organic solvents and wood fibres. Because of the countercurrent nature of the cook/extraction, it can be presumed that the fibres are suspended in nearly pure solvent.
• the solvent is washed away with washing liquid 16, mostly water.
• the result is a wood fibre-water suspension 15 and a solution 17 of water and solvent, mostly a mixture of water and ethanol or methanol.
• the solvent is recovered from stream 17 by a process according to, for example, Fig. 4.
• the solution 17 of water and solvent is directed to distillation apparatus
• the solvent 12 is directed to cook 20. As is shown in Fig. 4, it is possible to effect a wash in 21 with water and still maintain a high ethanol and/or methanol content in the actual cook 20.
• the pulp, if any, still in the form of chips prior to the wash, is fiberized prior to the wash to optimize the wash.
• the organic solvent 12 preferably has a substantially greater than 30% methanol and/or ethanol content.
• Fig. 5 illustrates a process more developed than heretofore described.
• the wood 30 introduced into the process is heated at the beginning of the cook by feeding steam 31 in the wood material in presteamer 23.
• Alkali 32 is at the same time introduced into the wood material to increase the pH at the initial stage of the cook.
• a catalyst 33 can be introduced during either the pretreat ⁇ ment 23 or at the beginning of the cook 20 to enhance the dissolving of lignin.
• a commonly used catalyst in acid process is CaCl 2 .
• the Ca 2+ ions are advantageous to the process, but on the other hand the amount of acid is detrimentally increased in the reaction 2C1- + 2H+ -> 2HC1.
• Organic bases, such as amine can be used as catalysts in acid, neutral and alkaline processes.
• Ethanol or methanol 34 is added to make up for the losses of solvent in the process.
• the process has further been complemented by treat ⁇ ment of the lignin-containing outlet liquid 36.
• the outlet liquids 13 of the previous embodiments can also be treated accordingly.
• the outlet stream 36 of solvent from the cook 20 mostly contains solvent, polysaccharides, lignin and water.
• the mixture 36 is directed to distilla ⁇ tion/evaporation stage 24, where the solvent, mostly ethanol and/or methanol 37, is separated from the water 38 and lignin and polysaccharide material 39.
• the solvent 37 can be returned to the cook 20 together with the solvent 12 from the distillation stage 22, while the water and lignin material are directed to combustion apparatus 25 or other treatment device.
• One possibility is to separate the lignin and to produce, for example, vanillin from the separated lignin.
• the added alkali 32 if any, is directed to combustion apparatus 25 and can thereafter be reused, or, if desired, removed. Removal is typically practised if the amount of the added alkali is small, e.g. 10-50 kg/ton of pulp.
• Ethanol is probably a better solvent than methanol for the practice of the invention, but the deacetylation that occurs in the wood produces some methanol. Thus there is always some methanol present in the process, even if the only fresh solvent 34 is ethanol.
• the invention can be applied to numerous cooks, both batch cooks and continuous cooks.
• the invention can, for example, be utilized in continuous digesters with a pretreatment zone or in continuous digesters with a separate pretreatment vessel.
• the acids produced at the beginning of the cook can be removed by withdrawing solvent from the digester after a predetermined time (e.g. about 5 - 10 minutes, but dependent upon material being cooked, the exact composition of the organic* solvent, etc. ) - the produced acids being removed with the solvent - and introducing at least some new solvent. While normally not necessary since most of the acids are produced within the first five minutes or so, the with- drawal and replacement procedure can be practised periodi ⁇ cally if desirable.
• the pH during batch cooking can alternatively, or in addition, be maintained generally neutral during the initial stages of the cook by adding alkali to the material prior to or simultaneously with introduction into the digester.
• the invention may be further illustrated by the following examples.
• Example A series of laboratory tests on softwood were performed to study the method.
• the or ⁇ ganosolv cooking process was performed as follows:
• the chips were pretreated in a pretreatment liquor consisting of NaOH dissolved in water.
• the pretreatment time was about 30 minutes and the temperature 120 °C.
• the idea of the pretreatment step was to cause the fibre to swell so as to enhance the subsequent ethanol extraction step.
• Another reason for the pretreatment step was to enable the addition of alkali to the chips to keep the pH at a sufficiently high level during the rest of the cook.
• the amount of alkali was varied between 0.25 to 1.50 mol NaOH/l. It was found that when the amount of alkali was below 0.25 mol NaOH/l the delignification was insufficient and the amount of unfiberized wood high. At alkali levels above 1.00 mol NaOH/l the yield loss in the cook became too high. Probably the amount of alkali actually needed depends on type of wood used. For the tested Scandinavian spruce the optimal alkali level was between 0.5 and 1.0 mol NaOH/l.
• the chips were removed from the pretreatment liquor by lifting the chips out. It was found that about 30 - 70 kg of NaOH per ton of pulp was entrained with the chips.
• Alkali had also been consumed in the pretreatment vessel.
• the consumption was 50 - 300 kg NaOH per ton of pulp.
• Step 3 Subsequent to steps 1 and 2 the chips were taken to a countercurrent ethanol extraction step.
• the duration of this solvent extraction step was about 120 minutes and the temperature 185 °C Anthraquinone was added to the solvent to improve delignification.
• the added amount of anthra ⁇ quinone was between 0 and 1.0 mmol.
• the strength of the ethanol solvent was varied between 25% and 100%. The lowest residual lignin contents were achieved when the ethanol content was between 40 and 70% in the liquid phase. When being tested for bleach- ability and pulp properties, the pulp was found to have pulp properties close to those of kraft pulp regarding both bleachability and strength.
• FIG. 8 shows an apparatus with which the process of example 1 can be performed.
• the apparatus comprises a conventional chips bin 201 connected by a conventional low pressure feeder 202 to a conventional horizontal steaming vessel 203, which in turn is connected by a conventional chute 203 to a conventional high pressure feeder 204.
• Wood chips are first steamed and preheated and then taken through the first pressure feeder 204 in line 205 to the pretreatment vessel 206 in which the chips are treated in an alkaline solution.
• the alkaline solution is introduced into the system in line 207.
• a conventional liquid/ material separator system is provided at the top of the vessel, with withdrawn liquid recirculated via line 208 to the inlet high pressure port of the feeder 204.
• the alkali can come from one or several of the following sources:
• the chips are taken to the extraction zone 215 in the second pressure vessel 213 in which the pressure is much higher than in pressure vessel 206.
• the chips material passes via line 211 to the top of the vessel 213 from a high pressure feeder 209 in the bottom of the pretreatment vessel 206.
• a conventional liquid/material separator system is provided at the top of the vessel 213, with withdrawn liquid recirculated via line 212 to the inlet high pressure port of the feeder 209.
• the liquid is passed through a heat exchanger 210.
• the second vessel 213 preferably consists of two zones: (1) zone 215 in which the chips are extracted with ethanol/methanol; and (2) zone 216 in which the chips are washed before being discharged from the vessel.
• washing liquid Filtrate from a subsequent washing or bleaching step is used as washing liquid.
• the washing liquid is intro ⁇ quizd into the bottom of the vessel 213 through line 218.
• the pulp at the bottom of the vessel 213 is washed and discharged into line 219.
• the ethanol and/or methanol is added at a point above the washing zone 216 in line 217.
• the ethanol is intro ⁇ **d at such a strength and amount that optimal extrac- tion conditions are achieved in the extraction zone 215.
• water can be separated from the circulation 220 by distillation to control methanol/ethanol strength in the extraction zone 215.
• the extraction liquor is withdrawn from the extrac ⁇ tor/digester 213 into a withdrawal conduit 214.
• FIG. 9 A simplified recovery system for solvent and alkali is shown in Fig. 9.
• the extraction liquor 214 from Fig. 8 is treated in the following steps: (a) Ethanol/methanol separation (250). The solvent is then reused in the extraction vessel 213. (b) Evaporation (252) in which water is separated, (c) Combustion (254) of lignin and polysaccharides.
• melt (255) consisting essential ⁇ ly of Na 2 C0 3 is produced.
• This melt is dissolved in water and used as a Na 2 C0 3 containing liquid in the pretreatment vessel 206 or causticized to NaOH before using in the pretreatment vessel 206. If the Na 2 C0 3 amount is small it does not have to be reused.
• Fig. 10 illustrates a diagram of a process in which the initial bleaching is effected by oxygen and the actual final bleaching by ozone and peroxide.
• the wood chips 300 that are introduced into the process are first heated by feeding steam 301 to the wood material to a pretreatment vessel 323.
• Alkali 302 is at the same time introduced into the wood material so that the chips are treated at a pH of 11 - 12.
• the alkali is obtained from the bleach plant effluents, which contain 40 - 120 kg of NaOH/adt. If the volume of the effluent is too big, the effluent has to be evaporated in order to reduce the volume. Additional alkali is brought if needed in the form of NaOH or Na 2 C0 3 .
• the chips 310 are introduced to the extraction stage 320 where some methanol and/or ethanol 312' may be added in the beginning to control the liquor- to - wood ratio, or to increase the content of methanol/ethanol in the beginning of the extraction process 320.
• the chips 311 are washed with the bleach plant effluents 302', 303, 304, which may be acid or alkali depending on how the bleaching has done. If possible, the acid filtrates 303 and 304 are used in the washing 321 and the alkali filtrate 302 in chips pretreat ⁇ ment 323.
• the washing liquid 317 containing solvent may have to be strengthened by distillation 322 before adding methanol and/or ethanol 312 to the extraction stage 320.
• the pulp 315 from the washing stage is introduced into the bleach plant 330 in which the pulp is bleached in the sequence OZP.
• the waste liquor 306 is recovered, evaporated and combusted. If the alkali amount is low, the Na 2 C0 3 formed in the recovery process is removed from the mill and fresh NaOH is brought in. Some of the Na 2 C0 3 may be used in the pretreatment stage. If the amount of alkali in the effluent stream is large it is probably more practical to causticize the formed Na 2 C0 3 to NaOH and thus generate new NaOH for bleaching and pre ⁇ treatment at the mill.
• Example 1 the wood was pretreated with a mixture of 75 % Na 2 C0 3 and 25 % NaOH in amount that corresponds about 200 kg Na/adt expressed as NaOH. This means that about 50 kg/adt of NaOH was used and the rest was Na 2 C0 3 .
• the amount of NaOH needed in the bleaching sequence OZEP is also about 50 kg/adt.
• all alkali needed in the pretreatment of the chips in the form of NaOH is received from the bleach plant effluent water.
• the rest of the alkali can be used as Na 2 C0 3 which is the form of the Na when burning the residual liquor in a recovery boiler.
• a mill does not need any causticizing plant, but uses Na 2 C0 3 from the combustion of spent liquor and NaOH from bleach plant effluents.
• Filtrates from the bleach plant can be used to dissolve the Na 2 C0 3 from the recovery boiler thus further reducing the water effluent volume.
• Fig. 11 also illustrates a schematic technical diagram of a process in which the initial bleaching is e ected by oxygen and the actual final bleaching by ozone and peroxide.

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• 1993
  • 1993-04-01 DE DE69308831T patent/DE69308831T2/de not_active Expired - Fee Related
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