Patent Publication Number: US-2011056637-A1

Title: Prehydrolysis sulfate cooking process

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a national phase entry under 35 U.S.C. §371 of International Application No. PCT/FI2009/050364 filed May 6, 2009, published in English, which claims priority from Finnish Application No. 20085425 filed May 8, 2008, all of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to an improved prehydrolysis sulfate (Kraft) cooking process. An improved method is disclosed for terminating the hydrolysis stage of the prehydrolysis sulfate cooking process. More precisely, the invention relates to termination of the hydrolysis stage operating in the steam phase, wherein such termination is also performed in the steam phase. 
     BACKGROUND OF THE INVENTION 
     The prehydrolysis-sulfate (Kraft) cooking for the production of special pulps having a high content of alpha cellulose was developed in the 1930&#39;s, see e.g. Rydholm, S. E., Pulping Processes, pp. 649 to 672, Interscience Publishers, New York, 1968. The basic idea was to remove as much hemicellulose as possible from cellulose fibers in connection with delignification, so as to obtain a high content of alpha cellulose. This is essential because the various end uses of such pulps, dissolving pulp for instance, do not tolerate short-chained hemicellulose molecules with a randomly grafted molecular structure. 
     A separate prehydrolysis step permits the desired adjustment of the hydrolysis of hemicelluloses by varying the hydrolysis conditions. In the prehydrolysis-kraft cooking process the necessary delignification is not carried out until a separate second cooking step. The prehydrolysis is carried out either as a water or steam phase prehydrolysis, or in the presence of a catalyst. In the former processes, organic acids liberated from wood during the process perform a major part of the hydrolysis, whereas in the latter process, small amounts of mineral acid or sulfur dioxide are added to “assist” the prehydrolysis. In the prehydrolysis stage carried out in steam phase direct steam is introduced to the chip column in the digester. Due to the material and energy balance, very little, if any, liquid hydrolysate phase is generated, as all condensate is trapped in the porosity of the wood material. 
     Traditionally after prehydrolyzing the cellulosic material in a reactor, the hydrolysate and the prehydrolyzed cellulosic material are neutralized in the reactor with alkaline neutralizing liquor so as to produce neutralized hydrolysate and neutralized prehydrolyzed cellulosic material. There is hydrolysate both in the free liquid outside the chips and also trapped and immobilized inside the chips. If desired, part of the hydrolysate can be recovered before the neutralization step. In the neutralization step, the contents of the digester are prepared for later delignification, to be carried out by alkaline kraft cooking. 
     The prehydrolyzed material is conventionally neutralized with alkali-containing liquid, either by displacement or circulation. Neutralization is achieved by selecting an appropriate neutralizing alkali charge, which results in a clear alkaline neutralization end point. Suitable neutralizing agents to be used contain caustic soda, and the preferred agent is alkaline Kraft cooking liquor, i.e., white liquor. The displacement process can be performed, for example, by filling up the digester with white liquor so that the leading edge of the white liquor neutralizes the hydrolysate. In the circulation process alkali-containing liquid is pumped into the digester so as to displace the hydrolysate from outside the chips. Neutralization is completed by circulating the liquid in the digester and thus mixing the contents therewith. 
     In addition to the primary neutralization function, the neutralization step also serves as an alkaline hemicellulose-dissolving step. The strong alkali and the high temperature directly dissolve and, on the other hand, degrade hemicellulose by the so-called end-wise peeling reaction. The pulp is thus further purified, which leads to higher pulp viscosity and higher alpha cellulose content. In other words, the neutralization step also becomes, in part, an alkaline extraction stage prior to the cooking step. Therefore, the liquor-to-wood ratio in this step is preferably relatively low, such as between about 2.5 and 3.5. 
     The neutralized prehydrolyzed cellulosic material is delignified with alkaline cooking liquor (containing sodium sulfide and sodium hydroxide). The delignification step has been a conventional Kraft cooking method, where white liquor has been added to the digester and the cooking has been carried out as a single step. 
     As is disclosed above the acidic hydrolysis stage of the prehydrolysis sulfate (Kraft) cooking of the prior art is terminated (i.e. neutralized) e.g. by filling up the digester with white liquor or with some other alkali-containing liquid to effect the pH change and to be able to transfer from the acidic hemicellulose-splitting condition/state to an alkaline lignin-dissolving sulfate cooking state. The neutralization, i.e. termination of the hydrolysis stage, is performed in the same way independently of whether or not the prehydrolysis is operated in a liquid or in a steam phase. Both of the above-mentioned neutralization methods i.e. circulation and displacement are time-consuming, which makes the uniformity of the neutralization poorer. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, many of these deficiencies in the prior art have now been overcome by the invention of a process for the preparation of prehydrolyzed pulp from lignin-containing cellulosic material comprising prehydrolyzing the lignin-containing cellulosic material with steam in a first steam phase at a first pressure to obtain prehydrolyzed cellulosic material, terminating the prehydrolyzing of the cellulosic material by at least one addition of an alkaline liquid at the first pressure followed by at least one pressure reduction, and delignifying the prehydrolyzed cellulosic material in a cooking stage. This process preferably comprises a batch process. 
     In accordance with one embodiment of the process of the present invention, the alkaline liquid comprises a white liquor. 
     In accordance with another embodiment of the process of the present invention, the at least one addition of the alkaline liquid comprises a single addition of the alkaline liquid at the first pressure, and the at least one pressure reduction comprises a single pressure reduction to a second pressure lower than the first pressure. In a preferred embodiment, the first pressure comprises a pressure of between about 6 and 9 bar and the second pressure comprises a pressure of between about 3 and 5 bar. Preferably, the single pressure reduction is carried out for a period of about 1 second to about 10 minutes, preferably from about 30 seconds to about 5 minutes. 
     In accordance with another embodiment of the process of the present invention, the at least one addition of the alkaline liquid comprises a single addition of the alkaline liquid, and the at least one pressure reduction comprises a gradual pressure reduction. Preferably, the first pressure comprises a pressure of between about 6 and 9 bar, and the gradual pressure reduction comprises a gradual pressure reduction from the first pressure to a second pressure, the second pressure comprising a pressure of between about 3 and 5 bar, and the gradual pressure reduction is carried out by reducing the pressure in increments of from about 0.5 to 2 bar. 
     One object of the present invention is to provide an improved process for the preparation of pulp from lignin-containing cellulosic material, in which the prehydrolysis is carried out in a steam phase and the termination (i.e. neutralization of the contents of the digester) of the prehydrolysis stage is also carried out in a steam phase in order to overcome many of the problems associated with the prior art. According to the present invention steam phase termination is carried out by adding alkaline liquid in a prevailing pressure into the digester and then temporarily reducing the pressure of the digester. As a result of the pressure reduction the added alkaline liquid evaporates to the gas space of the digester, thus neutralizing the prehydrolyzed material in the digester. The termination stage according to the present invention and its modifications will be described in more detail below. 
     The use of a momentary decrease of pressure when impregnating wood by overpressure is known from Canadian Patent No. 597,147. The purpose of this method is to get air contained in the wood cells to escape therefrom with great force. There may be one or several decreases of pressure, and heating of the impregnating liquor may be used in connection therewith. 
     The present invention is directed to a process for the preparation of prehydrolyzed pulp in a digester from lignin-containing cellulosic material comprising
     a) prehydrolyzing the cellulosic material in a first steam phase at a first pressure to obtain prehydrolyzed cellulosic material   b) terminating the prehydrolyzing operation in a termination stage   c) delignifying the prehydrolyzed cellulosic material in a cooking stage,
 
wherein the termination stage comprises at least one addition of an alkaline liquid at the first pressure followed by at least one pressure reduction.
   

    
    
     DETAILED DESCRIPTION 
     According to the present invention the prehydrolysis stage is carried out in a steam phase, by introducing direct steam to the chip column in the digester at a temperature of from about 150 to 180° C., preferably at a temperature of 160 to 170° C. The steaming is normally initiated with a low-pressure steam (LP; appr. 3.5 bar, 150° C.) and then continued with a medium pressure steam (MP; appr. 10.5 bar, 200° C.) until the desired temperature in the venting line of the digester is obtained, i.e. the temperature of the digester contents is about 150 to 180° C. The temperature inside the digester is measured from the temperature of the venting gas in the venting line. During the prehydrolysis stage the pressure inside the digester can vary between about 4.8 bar and 10.0 bar, preferably it is in the range of from about 6 to about 9 bar. 
     A suitable prehydrolyzing time is from about 10 to 200 minutes, preferably from about 20 to 120 minutes. 
     According to the present invention after completing the prehydrolysis stage, the contents of the digester are neutralized, i.e. the prehydrolysis stage is terminated by a termination stage, which stage comprises at least one addition of an alkaline liquid at a prevailing pressure (i.e. at a first pressure) followed by at least one pressure reduction to a second pressure. Thus, according to the present invention the first pressure is always higher than the second pressure. The pressure difference between the first pressure and the second pressure, i.e. the magnitude of the pressure drop, depends on several process variables, e.g. strength of the alkali, degree of packing and pressure of the digester. The pressure of the digester is reduced so that the alkali charged/introduced evaporates and thus neutralizes the prehydrolyzed material in the digester. This termination stage can also be called a neutralization stage, as the acidic prehydrolyzed material is neutralized during this step. The amount of alkali thus charged is determined so that all of the liquid (free liquid outside the chips and liquid trapped and immobilized inside the chips) in the digester is neutralized, and alkaline conditions are obtained that are at a pH of 7 or greater. 
     According to one embodiment of the present invention, in the termination stage first about 2 to 15% (as NaOH on wood) of a suitable alkaline liquid, for example alkalized black liquor, preferably white liquor, is added into the digester at the prevailing pressure i.e. at the first pressure. The first pressure is the pressure of the end of the prehydrolysis stage, i.e. between 4.8 bar and 10.0 bar, preferably in the range of from about 6 bar to about 9 bar. The pH of the digester contents cannot be directly measured, thus, normally, the alkaline conditions are ensured by feeding some extra amount of alkaline liquid into the digester. Secondly, after the addition of the alkaline liquid, the pressure of the digester is momentarily reduced from the first pressure to a second pressure, which is from about 2 to 7 bar, preferably from about 3 to 5 bar. The pressure is reduced for a period of about 1 second to 10 minutes, preferably for a period of about 30 seconds to 5 minutes. After the momentary pressure reduction the process is continued by an alkaline delignifying step. In the delignifying step the digester is filled up with the cooking chemicals, the temperature is increased to the cooking temperature, which is maintained until the desired degree of delignification is obtained. 
     According to a further embodiment of the present invention the pressure reduction of the termination stage is performed gradually. In that case first about 2 to 15% (as NaOH on wood) of suitable alkaline liquid, for example alkalized black liquor, preferably white liquor, is charged into the digester at the prevailing pressure. After the addition of the alkaline liquid the pressure is reduced gradually step-by-step, for example, at increments of about 0.5 to 2 bar at a time, to obtain the end pressure, which is from about 2 to 7 bar, preferably from about 3 to 5 bar. After completing the termination stage, the digester is filled up with the cooking chemicals and thus preparation of the pulp is continued by an alkaline delignifying step. 
     According to a still further embodiment of the present invention, the total amount of the needed (2to 15% as NaOH on wood) suitable alkaline liquid, for example alkalized black liquor, preferably white liquor, is gradually added into the digester and after each addition the pressure is reduced from the previous pressure. For example, first, half of the needed alkaline liquid is added into the digester and immediately after that the pressure is reduced e.g. by an increment of from about 0.5 to 1 bar, preferably from about 0.5 to 0.7 bar. The pressure is reduced for a suitable period of time, i.e. for a period of about 1 second to 10 minutes, preferably for a period of about 30 seconds to 5 minutes. After the momentary pressure reduction, the rest of the alkaline liquid is added into the digester followed by a second pressure drop. The amount of alkaline liquid needed can also be added in more than two batches. Each addition of alkaline liquid is followed by a pressure reduction. The pressure can be reduced e.g. gradually so that after the addition of alkaline liquid the reduction of the pressure is in increments of from about 0.5 to 1 bar, preferably from about 0.5 to 0.7 bar from the previous pressure. 
     Persons skilled in the art can determine the optimal termination conditions and procedural steps based on the process variables in question. 
     The neutralizing liquid is preferably introduced into the digester through the bottom of the digester, but other possible methods are also within the scope of the present invention. 
     After the termination stage the prehydrolyzed cellulosic material is delignified by conventional sulfate cooking. In the beginning of this conventional Kraft cooking, the cooking chemicals, e.g. white liquor and spent black liquor, are fed into the digester. The digester contents are then heated by direct steam or by means of indirect heating of the recirculating pulping liquor in a heat exchanger. A suitable cooking temperature is from about 150° C. to 180° C., preferably from about 150° C. to 165° C. for hardwoods, and from about 155° C. to 170° C. for softwoods. After the desired degree of delignification is reached, the digester contents are blown to a blow tank. 
     In the prior art after completing prehydrolysis the contents of the digester are neutralized, in other words the prehydrolysis stage was completed by a liquid phase neutralization process, i.e. circulation or displacement, which are very time-consuming. Therefore, the process conditions in the upper part of the digester are totally different compared to the process conditions in the lower part of the digester at the end of the neutralizing step. In the process according to the invention, termination or neutralization of the prehydrolysis stage is carried out very quickly compared to the prior art processes. This leads to several advantages, namely to time savings in the cooking cycle, resulting in increased production per m 3  of the digester, to improved homogeneity in the hemicelluloses content of the cooked pulp, to improved uniformity of the lignin content of the cooked pulp, to increased viscosity of the cooked pulp and also to better unbleached pulp brightness. 
     Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.