WO2009136000A1

WO2009136000A1 - Prehydrolysis sulfate cooking process

Info

Publication number: WO2009136000A1
Application number: PCT/FI2009/050364
Authority: WO
Inventors: Olavi Katajisto; Lari Lammi; Antti Viro
Original assignee: Metso Paper Oy
Current assignee: Valmet Technologies Oy
Priority date: 2008-05-08
Filing date: 2009-05-06
Publication date: 2009-11-12
Anticipated expiration: 2010-11-08
Also published as: FI20085425L; FI20085425A7; US20110056637A1; CN102016162A; FI20085425A0; EP2286022A1

Abstract

A process for the preparation of prehydrolyzed pulp in a digester from lignin-containing cellulosic material is disclosed. According to the present invention the cellulosic material is prehydrolyzed in a first steam phase at a first pressure to obtain prehydrolyzed cellulosic material. The termination of the prehydrolyzing operation i.e. the neutralization of the prehydrolyzed material is performed by adding an alkaline liquid at the first pressure into the digester followed by at least one pressure reduction.

Description

PREHYDROLYSIS SULFATE COOKING PROCESS

Field of the invention

The present invention relates to an improved prehydrolysis sulfate (Kraft) cooking process. It is disclosed an improved method for terminating the hydrolysis stage of the prehydrolysis sulfate cooking process. More precisely the invention relates to the termination of the hydrolysis stage operating in the steam phase, wherein the termination of said stage 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's, see e.g. Rydholm, S. E., Pulping Processes, pp. 649 to 672, Interscience Publishers, New York, 1968. The basic idea is 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 clearly 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. The 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 liquour 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 independent whether the prehydrolysis is operated in liquid or in 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

The main object of the present invention is to provide an improved process for the preparation of pulp from lignin-containing cellulosic material, wherein the prehydrolysis is carried out in a steam phase and the termination (i.e. the neutralization of the contents of the digester) of the prehydrolysis stage is also carried out in a steam phase in order to overcome the problems associated with the prior art. According to the invention the 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 invention and its modifications will be described in more detail below.

The use of momentary decrease of pressure when impregnating wood by overpressure is known from CA 597147. The purpose of this method is to get air contained in the wood cells with great force to escape from them. There may be one or several decreases of pressure and in connection with them heating of impregnating liquor may be used. 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 said 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 of the invention

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 about 150-180⁰C, preferably at a temperature of 160-170⁰C. The steaming is normally started with a low-pressure steam (LP; appr. 3,5 bar, 150⁰C) and then continued with a medium pressure steam (MP; appr. 10,5 bar, 200⁰C) until the desired temperature in the venting line of the digester is obtained, i.e. the temperature of the digester contents is about 150- 180⁰C. 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 4,8 bar and 10,0 bar, preferably it is in the range of 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 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 as a neutralization stage, as the acidic prehydrolyzed material is neutralized during this step. The amount of alkali charged is determined so that all 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 is pH is 7 or more.

According to one embodiment of the present invention, in the termination stage first about 2-15 % (as NaOH on wood) of 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 being the pressure of the end of the prehydrolysis stage i.e. between 4,8 bar and 10,0 bar, preferably being in the range of about 6 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 about 2-7 bar, preferably about 3-5 bar. The pressure is reduced for a period of about 1 second to 10 minutes, preferably for a period of 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 2-15 % (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 0.5-2 bar at a time, to obtain the end pressure, which is about 2-7 bar, preferably 3-5 bar. After completing the termination stage, the digester is filled up with the cooking chemicals and thus the preparation of the pulp is continued by an alkaline delignifying step.

According to a still further embodiment of the invention the total amount of the needed (2-15 % 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. 0,5-1 bar, preferably 0,5-0,7 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 30 seconds to 5 minutes. After the momentary pressure reduction, 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 0,5-1 bar, preferably 0,5-0,7 bar from the previous pressure.

Person 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 ways are also within the scope of the invention.

After the termination stage the prehydrolyzed cellulosic material is delignified by a conventional sulfate cooking method. 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 via indirect heating of the recirculating pulping liquor in a heat exchanger. A suitable cooking temperature is from about 150 ⁰C to 180⁰C, preferably from about 150⁰C to 165 ⁰C for hardwoods, and from about 155 ⁰C to 170⁰C 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 the prehydrolysis the contents of the digester were 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 and 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 in the end of the neutralizing step. In the process according to the invention, the termination or the neutralization of the prehydrolysis stage is carried out very quickly compared to the prior art processes. This leads to several advantages, namely to the time saving in cooking cycle resulting in increased production per digester m³, to improved homogeneity in hemicelluloses content of cooked pulp, to improved uniformity of lignin content of cooked pulp, to increased viscosity of cooked pulp and also to better unbleached pulp brightness.

Claims

CLAIMS:

1. A process for the preparation of prehydrolyzed pulp in a digester from lignin- containing cellulosic material comprising a) prehydrolyzing said 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, characterized in that the termination stage comprises at least one addition of an alkaline liquid at the first pressure followed by at least one pressure reduction.

2. The process according to claim 1, wherein the process is a batch process.

3. The process according to claim 1 or 2, wherein the alkaline liquid is white liquor.

4. The process according to any of the preceding claims, wherein the termination stage comprises one addition of alkaline liquid at the first pressure followed by one pressure reduction to a second pressure.

5. The process according to claim 4, wherein the first pressure is about 6-9 bar and the second pressure is about 3-5 bar.

6. The process according to claim 5, wherein the one pressure reduction takes a period of 1 second to 10 minutes, preferably of 30 seconds to 5 minutes.

7. The process according to claim 1 or 2, wherein the termination stage comprises one addition of alkaline liquid followed by a gradual pressure reduction. The process according to claim 7 wherein the gradual pressure reduction from the first pressure, which is about 6-9 bar, to a second pressure is performed by reducing the pressure 0,5-2 bar at a time, to obtain the second pressure of about 3-5 bar.