FI123100B - Procedure for the collection of by-products - Google Patents

Description

METHOD FOR RECOVERY OF BY-PRODUCTS Field of the Invention

The present invention relates to a process for making pulp. More specifically, it relates to 5 displacement batch cooking processes which include a vapor phase pre-hydrolysis step in which by-product recovery is improved.

Background of the Invention

Pre-hydrolysis sulfate (kraft) soup for the production of specialty pulps with high alpha cellulose content was developed in the 1930s, cf. e.g., Rydholm, S.E., Pulping Processes, pages 649-672, Interscience Publishers, New York, 1968. The main idea is to remove as much hemicellulose from cellulose fibers as possible during delignification (ligninization) to achieve high alpha cellulose content. This is essential because different end uses of pulps, such as soluble pulp, do not accept short-chain hemicellulose molecules having a randomly branched molecular structure.

By means of a separate pre-hydrolysis step, it is possible to adjust the hydrolysis of the hemicelluloses as desired by varying the hydrolysis conditions. In the prehydrolysis-kraft cooking process 20, the delignification required is carried out only in a separate second cooking step. The prehydrolysis is carried out either as aqueous or vapor phase prehydrolysis or in the presence of a catalyst. In earlier processes, organic acids liberated from wood during the process carry out the major part of the hydrolysis, whereas in a later process small amounts of mineral acid or sulfur dioxide can be added to "assist in the pre-hydrolysis." In the vapor phase,

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g In the 25 pre-hydrolysis step, direct steam is added to the chips column in the digester.

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cp q Following the pre-hydrolysis of cellulosic material in a reactor, the hydrolyzate and the pre-hydrolyzed cellulose material are neutralized in the reactor with an alkaline

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hydrolyzate and neutralized pre-hydrolyzed cellulosic material. The hydrolyzate is present both in the free liquid outside the chips and in the closed and trapped within the chips. If desired, the hydrolyzate can be recovered as much as possible prior to the neutralization step in the pre-hydrolysis to utilize the carbohydrates liberated. A separate washing step, in which the kettle is first filled with washing liquid and then the carbohydrate-containing liquid is removed from the digester, can be used between the pre-hydrolysis step and the cooking step. However, this is time consuming, disadvantageous for energy balance and produces a very dilute carbohydrate solution.

WO 2007/090925 describes an improved method for treating lignocellulosic material, wherein the digester and its contents are first heated by direct steam to a predetermined hydrolysis temperature and then a certain volume of washing liquid is introduced into the digester from one end to the other end of the digester.

According to the method described in WO 2007/090926, the hydrolyzate is recovered using a trickle-bed type downstream flow of the hydrolyzate. In this method, the first fraction of the spilled hydrolyzate is collected as a product fraction, and the second fraction is removed from the digester to the hot hydrolyzate storage tank for use as the first tricycle flow fluid in the next batch. This recovery of triclayer layer-15 yields concentrated hydrolyzate, but the recovery step is too slow and therefore unfavorable for pulp quality. In addition, the handling is uneven in the contents of the digester; it is evident that during processing, channeling occurs, the liquid goes where it is most easily reached.

After removal of the hydrolyzate, the process can be continued by a neutralization-cooking process known in the art.

The object of the present invention is to optimize the recovery of hydrolyzate in the pre-hydrolysis sulphate cooking process. By the present method, the disadvantages of the previous processes are eliminated or at least significantly reduced. A kettle is provided

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evenly treating the entire contents, obtaining high quality soluble pulp or paper grade selective oil, and recovering the hydrolyzate substantially free of sulfur. Substantially x sulfur-free means that, according to the invention, it is possible for the hydrolyzate recovered

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sulfur-containing liquids are not added thereto, i.e., according to one embodiment, only sulfur derived from wood co-30 is present. Thus, further processing of carbohydrates is easier. Also the process-

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° s energy efficiency is taken into account, o C \ 1 3

Summary of the Invention

The displacement batch cooking process for pulping according to the present invention comprises the following steps: filling with chips, heating the chips, pre-hydrolysing the chips, recovering the hydrolyzate including neutralizing the contents of the digester, preparing the digester contents for kraft cooking by raising the pH, i.e. tonic and boiling to the required H-factor. In the recovery step of the present invention, after reaching the desired P factor in the hydrolysis step, hot washing fluid is introduced from the bottom of the digester to the digester. The wash liquid may be hot water from a hot water collector. If necessary, sodium hydroxide may be added to the hot water before being introduced into the digester. The digester circulation pump is started when the level of hot washer fluid in the digester is above the suction strainers, and the circulation flow to the top and bottom of the digester is started. Pumping of the hot washer fluid is continued until the chips are "under" the washer fluid. This recovery step washes from the chips of the hydrolyzate and steam condensate produced during the steaming step (i.e., the pre-hydrolysis step) to achieve the highest possible concentration of sugars / carbohydrates dissolved in the washing liquid prior to displacing it in the condensate size. At the same time, prehydrolysis is terminated in a controlled manner, thus preventing fiber quality deterioration. The by-product condensate, i.e. the washing liquid, is displaced from the top of the digester with an alkaline liquid, displacing the condensate is the remainder of the recovery step. The pH of the digester contents 20 is finalized after the recovery step and may range from pH 9 to 12.

In other words, the process of recovering by-products of the present invention in a displacement batch cooking process in a digester comprising a base, a top and a suction screen, the method comprising prehydrolysing the chips in the vapor phase to produce hydrolyzate and vaporconj bring hot washer fluid cp q - add rinsing liquid through suction strainers to top of digester and x to bottom of digester continue to circulate until prehydrolyzed chips are submerged in washing liquid with lye.

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Brief Description of the Drawings

Fig. 1 is a flow chart of a process according to an embodiment of the present invention using hot water from a HOT WATER collector as washing liquid.

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DETAILED DESCRIPTION OF THE INVENTION

The displacement batch cooking process according to the invention is started by filling the digester with lignocellulosic material, i.e. chips. Applicants arrive at the boiler plant with a harness strap and from there to the cooker (s) with the help of the hacker conveyors on the top of the digester (s). Mata-10 blade (LP) steam is used to ensure a good sealing of the chips across the entire cross-section of the digester. During chipping, air is removed through suction strainers. The chip filling is stopped after the cooker level control switch has started and the capping valve is then closed.

The heating of the chips is started using low pressure (LP) steam from the top of the digester and the bottom of the digester, and is continued with medium pressure (MP) steam until the required temperature is reached, i.e. 150-170 ° C. The digester is maintained at this temperature and pressure until the pre-hydrolysis step is completed, i.e. the required factor P is reached. Factor P is a defined factor for controlling the pre-hydrolysis step, taking into account temperature and time (such as, for example, Factor H); Herbert Sixta, Handbook of Pulp, Part 1,

Wiley-VCH Verlag, 2006, pages 343 - 345.

According to the invention, the pre-hydrolysis step is carried out in a vapor phase in which the acids of the wood are hydrolyzed by steam and acidic conditions are created in the digester. Steam-esihydrolyysi-

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g The final pH of the 25 steps varies depending on the wood species and the pre-hydrolysis conditions themselves. pH

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qjj is typically measured from the effluent during hot washer fluid refill and ranges from cp <2.5 to 4.0.

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After the P factor has been reached, the recovery step is initiated by introducing into the digester ^30 hot wash liquid. The temperature of the hot washer fluid is between 100 and 170 ° C, preferably

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5 to 140-160 ° C and is pumped into the digester from the bottom. According to one embodiment, hot water from the HOT WATER collector is used as the washing liquid, as shown in Figure 1. According to another embodiment, sodium hydroxide is added to the hot water prior to being introduced into the digester if the pH of the recovery step needs to be adjusted to enhance completion of the pre-hydrolysis. When the level of the washing liquid reaches the suction strainers, the digester circulation pump is started and circulation to the top of the digester and to the bottom of the digester is started. The suction strainers are normally located on the underside of the digester, typically about one-third the height of the bottom of the digester. Pumping of the washing liquid is continued when the level of the washing liquid reaches the level of the chips, i.e. the whole chips are under the washing liquid. Typically this means that the washing liquid is added in an amount of about 40 to 80% of the free volume of the digester. During the recovery step, the hydrolyzate and steam condensate formed during the pre-hydrolysis step are rinsed from the chips to provide the highest concentration of carbohydrates dissolved in the wash liquid before being displaced into the condensate collector. In addition, the pre-hydrolysis step is terminated during the recovery step in a controlled manner to prevent degradation of the fiber quality (e.g., viscosity or kappa number). The contents of the digester are thus neutralized. If hot water is used as the washing liquid without the addition of sodium hydroxide, the pH of the digester contents will vary between pH 5-7 before displacing the condensate. On the other hand, if the washing liquid contains alkali, the pH of the digester contents prior to displacement is typically pH 7 or higher.

The final neutralization of the contents of the digester, if any, is achieved during condensation displacement. The condensate displacement is started by stopping the digester recycling pump and starting pumping the alkali through the digestion fluid heater to the bottom of the digester.

The condensate containing the dissolved carbohydrates (i.e. by-products) is displaced from the top of the digester to the condensate collector while the alkali pumping is continued. The amount of alkali needed to neutralize depends on the wood material. Any suitable alkali may be used to displace the condensate and / or to neutralize the contents of the digester. According to a preferred embodiment, when substantially sulfur-free condensate is needed, the condensate is displaced by sodium hydroxide. It is also possible to obtain a substantially sulfur-free condensate using white liquor as the alkali source, but in that case cp q is important to ensure that white liquor (WL) does not reach the condensate tank. If the sulfur content of the condensate is not a limiting factor, displacement can be done with white liquor, cc, and is not dangerous, even if a little white liquor can enter the condensate tank. Typically, in the displacement operation of? 30, alkaline liquid is first introduced into the digester from its bottom in an amount necessary to neutralize the contents of the digester and is less than the free volume of the digester. After that, the displacement is continued with hot black liquor and this step is actually the first cooking step. The outgoing hydrolyzate is led to a condensate collector, and when the first displacement alkali begins to discharge, the outgoing liquid is led into a hot black liquor (HBL) collector. After displacement of the condensate, i.e. at the end of the recovery step, the pH of the contents of the digester typically varies between pH 9 and 12.

After the recovery step, the pH of the digester contents is raised to more than 10, typically between 11 and 512, to continue the alkaline kraft cooking step. Once the required volume of alkali (depending on the species of wood) has been pumped in, the process is continued by pumping in hot black liquor (HBL) from the HBL collector through a cooking liquor heater. The displaced fluid is transferred to the HBL collector while the HBL displacement is continued.

10 Alkaline kraft soup is done with strongly alkaline hot black liquor (HBL) or using hot white liquor after the HBL phase. Typically, the need for additional heating is minimal because the kettle is filled with hot liquor. If heating is required, direct steam nozzles are used in the recycling line for this purpose. Continue cooking until the required factor H is reached.

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After reaching the desired factor H, the cooking is stopped by introducing cool displacement liquor from the bottom of the boiler. This process step can be called the final displacement. The displacement liquor (DPL) is the filtrate from the first brown pulp washing step. DPL displaces used cooking liquor from the digester to the HBL collector. This way, energy and chemicals can be stored / transferred from one cooking to another. This is also an effective way to stop cooking.

The temperature of the contents of the digester is below the boiling point at the end of the displacement, and emptying of the digester is effected by means of a pump. Dilute broth is added to the digester to purify g 25. The diluted wing is taken from the displacement liquor tank. The temperature is low and no strong gases are thus formed. An atmospheric discharge tank cp q (emptying tank) is used and the exhaust gases are introduced into a gas treatment system. After the end of the emptying step, i.e. when the machine is empty, a new cooking cycle can be started.

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In order to carry out the process of the invention, the plant tank area (see Figure 1) typically comprises three manifolds (hot water, condensate and hot black liquor, i.e. HBL-00 manifold) and four atmospheric vessels (displacement, i.e. DPL). tank, discharge tank, kettle and

NaOH / white liquor (WL) tank).

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In the process of Figure 1, the HBL accumulator is used to store the alkaline kraft liquids used in hot liquor fillings and acts as a receptacle for spent cooking liquor from final displacement. The excess liquor from this tank is transferred to evaporation through a heat exchanger and a liquor filter.

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In the embodiment shown in Figure 1, the incoming water is first heated with hot black liquor from the HBL collector and then with condensate from the condensate collector before being fed to a hot water (HW) collector, the heat exchangers are shown in Figure 1. The fibers are separated by a lye filter. The HBL manifold has a recycling line 10 where the HBL temperature is controlled by MP steam when no hot fill is in progress.

The condensate collector is used as a receiving tank for the pre-hydrolysis step condensate. A recycling line is used to control the pH of the collector. If necessary, addition of NaOH is possible. The condensate is cooled by means of a heat exchanger before being further processed.

The hot water collector is a storage tank for hot water from heat exchangers. The hot water collector recycling line is equipped with the ability to heat the water with MP steam 20 to the required temperature. Hot water is used in the recovery step as described above.

The displacement liquor tank is used to store the wash filtrate for final displacement and drain dilution. c \ j δ 25 c \ j £ As shown in Fig. 1, storage tanks for NaOH and / or white liquor are also provided. NaOH can be used to neutralize the condensate in the condensate size recycle line. NaOH is needed in the digester after the pre-hydrolysis step, cc

NaOH / white liquor can also be used to control the alkalinity of HBL during the cooking phase.

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Claims (5)

8 A process for recovering by-products in a displacement batch cooking process in a kitchen comprising a base, a top and a suction screen, said method comprising prehydrolysing the chips in a vapor phase to produce a hydrolyzate and a steam condensate, characterized in that circulate through suction strainers to the top of the digester and continue to circulate at the bottom of the digester until the pre-hydrolyzed chips under the washing liquid are recovered from the digester and the contents of the digester are neutralized by displacing the washing liquid with alkaline lye. The process for recovering by-products according to claim 1, wherein the washing liquid is hot water. The process for recovering by-products according to claim 2, wherein sodium hydroxide is added to the hot water before being introduced into the digester. 20 A process for recovering by-products according to any one of the preceding claims, wherein the temperature of the hot wash liquid is 100-170 ° C, preferably 140-160 ° C. ^ 25 The process for recovering by-products according to any one of the preceding claims, wherein the alkaline solution is NaOH. i oo o o X cc CL CO C \ l m o δ CM 9