Method for displacement in batch digesters

Abstract

The method is for producing pulp in batch digesters. More particularly, the method is for ending a treatment phase of the digester content. In order to obtain a more uniform pulp quality from the process is the treatment phase ended by displacing a liquor volume (WLP) through the digester vessel with an imposed increasing temperature in the displacement liquor added to digester.

Claims

1. A method for ending a heated treatment phase in a displacement batch pulping process in a digester vessel, where the treatment phase has been done at a treatment temperature above 130 C., said digester having at least a bottom, a mid-point and a top liquid exchange position, the method initiated after the heated treatment phase and comprising the following steps; adding a first displacement liquor to the bottom liquid exchange position while having a first lower temperature in the first displacement liquor at start of the displacement filling of a part of the digester with a first volume of the first displacement liquor, wherein the first lower temperature is more than 20 C. below the treatment temperature of the treatment phase; continuing to add the same first displacement liquor to the bottom liquid exchange position while having a higher second temperature in the first displacement liquor in a later phase of the displacement filling at least a part of the digester with a total second volume of the first displacement liquor larger than the first volume; and continuing the displacement with a final displacement liquor; and the first lower temperature in the first displacement liquor is in a range 70-110 C.

2. The method according to claim 1 wherein the final displacement liquor is supplied and displaced until the content of the digester vessel is completely submerged under the total volume of the final displacement liquor added and wherein the first displacement liquor has been displaced from the digester via the top liquid exchange position.

3. The method according to claim 2, wherein a circulation of the liquor is initiated after the digester vessel has been filled by the final displacement liquor.

4. The method according to claim 1, wherein the proportion of the first volume of the first displacement liquor in relation to the total volume of the first displacement liquor is in a range 20-50%.

5. The method according to claim 3, wherein the total volume of the first displacement liquor is in a range 50-75% of the free digester volume.

6. The method according to claim 1, wherein the heated treatment phase is a prehydrolysis phase wherein the digester content is hydrolyzed at a temperature above 150 C.

7. A method for ending a heated treatment phase in a displacement batch pulping process in a digester vessel, where the treatment phase has been done at a treatment temperature above 130 C., said digester having at least a bottom, a mid-point and a top liquid exchange position, the method initiated after the heated treatment phase and comprising the following steps; adding a first displacement liquor to the bottom liquid exchange position while having a first lower temperature in the first displacement liquor at start of the displacement filling of a part of the digester with a first volume of the first displacement liquor, wherein the first lower temperature is more than 20 C. below the treatment temperature of the treatment phase; continuing to add the same first displacement liquor to the bottom liquid exchange position while having a higher second temperature in the first displacement liquor in a later phase of the displacement filling at least a part of the digester with a total second volume of the first displacement liquor larger than the first volume; continuing the displacement with a final displacement liquor; and wherein after adding the first displacement liquor with the higher second temperature and before the content of the digester vessel is completely submerged is the displacement continuing by adding the same first displacement liquor to the bottom liquid exchange position while having a third temperature higher than the second temperature in the displacement liquor in a later phase of the displacement filling a part of the digester with a total third volume of first displacement liquor larger than the total second volume.

8. The method according to claim 7, wherein the temperature in the first displacement liquor increase is done in incremental steps in at least 3 and up to 10 steps during the displacement.

9. The method according to claim 7, wherein the temperature increase in the first displacement liquor is done continuously during the displacement.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 is a schematic layout of a batch digester system with the components necessary to implement the invention in a white liquor displacement phase;

(2) FIG. 2a is showing the a principle effect from exothermic reaction during a conventional displacement front through the digester using a hot white liquor pad WLP at a temperature of 150 C. displaced trough the digester;

(3) FIG. 2b is showing a conventional circulation phase after FIG. 2a in order to establish same temperature throughout the digester after the displacement;

(4) FIG. 3a is showing a principle effect heating from digester content during a conventional displacement front through the digester using a first white liquor pad WLP at a temperature of 150 C. displaced by hot black liquor at a temperature of 130 C.;

(5) FIG. 3b is showing a conventional circulation phase after FIG. 3a in order to establish same temperature throughout the digester after the displacement;

(6) FIG. 4 is showing the principle effect of the inventive displacement front through the digester using a hot white liquor pad at successively higher temperature charged at 90 C. in first phase and up to 150 C. in a second phase;

(7) FIG. 5a is showing the principle effect of the inventive displacement front through the digester using a displacement liquor at successively higher temperature in 7 stages during complete filling of the digester; and

(8) FIG. 5b is showing an optional circulation phase that may be implemented after FIG. 4a in order to absorb some of heat value still contained in the digester content.

DETAILED DESCRIPTION OF THE INVENTION

(9) The description will be made using the schematic layout shown in FIG. 1 which only discloses the essential components for the system used necessary to implement the present invention in a white liquor displacement phase. In a full commercial plant is also additional equipment added for performing the kraft cook and heat recovery after cook, for example using warm and hot black liquor accumulators.

(10) Only one digester is shown but typically a number of digesters are used that operate in sequence and thus in different phases of the cook. If for example 5 digesters are operated the first digester is started and then the remaining digesters are started at some time interval which time interval may correspond to of the total cooking cycle time for one digester. Cooked pulp may then be blow to a blow tank at regular intervals, and the process liquids stored in accumulators and atmospheric tanks may be used in another digester minimizing inactive dwell time for the liquids used. The piping system is simplified showing only one liquid addition point for WL, Wash filtrate, LP_ and MP-steam but in a real system are individual piping connected to the inlet point of the digester.

(11) During the white liquor displacement phase is white liquor used that typically is obtained from the caustization. This white liquor conventionally has a temperature of about 90 C. as it has been stored in atmospheric tanks. The white liquor is heated before supply to the hot white liquor accumulator in at least one heat exchanger HE.sub.2P, using hotter process liquors or steam as heating medium. The content of the hot white liquor accumulator is also heated in a circulation containing an additional heat exchanger HE.sub.2C, using hotter process liquors or steam as heating medium. The heating is performed until the entire accumulator content has reached the intended temperature which in the figure may lie at some 150 C. lif the total digester volume is about 300 m.sup.3, the total free volume inside a digester filled with comminuted chips is about 200 m.sup.3, so the accumulator needs a size of 200 m.sup.3 to store this volume for a full displacement phase.

(12) In FIG. 2a is shown the principle heating effect from exothermic reactions during a conventional displacement front through the digester using a hot white liquor pad at a temperature of 150 C. In this example a drained digester is shown with a digester content of comminuted chips after a heated treatment in form of a prehydrolysis conducted at 170 C. In a first stage, first left hand figure, of the displacement phase is the inlet cone part filled with a volume of white liquor holding 150 C. when added. In a second stage, second figure from left, of the displacement phase is the inlet cone part filled with a volume of hot black liquor holding 130 C. when added, pushing the hot white liquor pad WLP upwardly. During this displacement will exothermic heat be released as alkali is consumed when the upper level of the white liquor pad neutralizes the acidic digester content but now heated to a higher temperature due to exothermic reactions. The heating effect due to exothermic reactions is in the range 0.138-0.206 GJ/odt of wood.

(13) This continues in stages until the entire digester is filled with displacement liquor, and as a result of the heating from the exothermic reaction is a temperature profile established over the height of the digester, with a temperature of the free liquor close to the hydrolysis temperature, i.e. close to 170 C. in top but close to 130 C. in bottom. Now, the digester content in bottom has been flushed by 130 C. wash liquor during the entire displacement and is very close to 130 C. But the digester content in top has only been drenched by heated white liquor with most of the alkali content consumed during neutralization. As a result the hydrolysis is ended much sooner in bottom of digester, as the temperature has been lowered to 130 C. at an early stage and alkali has been present, while the digester content in the top is subjected to extended hydrolysis, as the criteria's for ending the hydrolysis, lowering of temperature and change to alkaline conditions has not been fulfilled.

(14) This temperature profile may be even out by a circulation as shown in FIG. 2b, which starts from a condition corresponding to the rightmost hand figure in FIG. 2a, where a pump starts to withdraw liquor from mid-point liquid exchange position and return this liquor to both top and bottom. This will result in some heating in bottom and cooling in top and ideally the entire content of the digester assumes a temperature lying in-between the hot black liquor temperature and the hydrolysis temperature at end of circulation, as disclosed in the right hand figure.

(15) In FIG. 3a is shown the additional principle heating effect from the residual heat content of the digester during a conventional displacement front through the digester using a hot white liquor pad at a temperature of 150 C. In this example is a drained digester shown with a digester content of comminuted chips after a heated treatment in form of a prehydrolysis conducted at 170 C. In a first stage, first left hand figure, of the displacement phase is the inlet cone part filled with a volume of hot white liquor holding 150 C. when added. In a second stage, second figure from left, of the displacement phase is the inlet cone part filled with a volume of hot black liquor holding 130 C. when added, pushing the first volume upwardly but now heated to a higher temperature by the digester content. This continues in stages until the entire digester is filled with displacement liquor, and as a result of the heating from the digester content is a temperature profile established over the height of the digester, with a temperature of the free liquor elevated some 10 C. in top but close to 130 C. in bottom. Now, the digester content in bottom has been flushed by 130 C. hot black liquor during the entire displacement and is very close to 130 C. But the digester content in top has only been drenched for a short time by heated white liquor that now holds a temperature of about 150+10 C., and has thus most of the heat value from hydrolysis left also in the digester content.

(16) This temperature profile may be even out by a circulation as shown in FIG. 3b, which starts from a condition corresponding to the rightmost hand figure in FIG. 3a, where a pump starts to withdraw liquor from mid-point liquid exchange position and return this liquor to both top and bottom. This will result in some heating in bottom and cooling in top and ideally the entire content of the digester assumes a temperature lying in-between the hot black liquor temperature, about 130 C., and the added heating from the digester material release, about 140 C., at end of circulation, as disclosed in the right hand figure.

(17) These two examples in FIGS. 3a and 4a show the two independent heating effects from exothermic reactions and heating from digester material respectively, and that the usage of an isothermal displacement liquor results in a temperature profile in the digester, and hence is the digester content in top and bottom of digester subjected to quite a difference in H- or P-factor exposure resulting a variance in the pulp quality.

(18) According to the invention a deliberate temperature profiling is instead implemented in the displacement liquor used, either as a part of a displacement pad or throughout a complete filling of the digester.

(19) Embodiment in White Liquor Pad

(20) In FIG. 4 is a first embodiment of the inventive displacement through the digester shown using displacement liquor, i.e. the one and same displacement liquor as of chemical content, which in at least 2 incremental steps, at temperatures of 90 C. and finally 150 C. is used. In this example a drained digester is shown with a digester content of comminuted chips after a heated treatment in form of a prehydrolysis conducted at 170 C. In a first stage, first left hand figure, of the displacement phase is the inlet cone part filled with a first volume of white liquor holding 90 C. when added. In a second stage, second figure from left, of the displacement phase is the inlet cone part filled with a second volume of hot white liquor holding 150 C. when added, pushing the first volume upwardly. The first and second volumes establish a white liquor pad (WLP) that is thereafter displaced trough the digester content by adding hot black liquor holding a temperature of about 150 C. With this temperature profiling of the white liquor, using an unheated white liquor in first phase, will this low temperature part of the WLP absorb the exothermic heat release as well has residual heat in the digester content, avoiding the temperature to become excessive. As indicated an upper layer of the first volume will be heated during displacement and this heated layer HL will increase during the displacement. Due to the initial low temperature at some 90 C., the heating from both exothermic reactions and residual heat in digester content will not be able to raise the temperature close to full hydrolysis temperature which will guarantee that the hydrolysis will be ended. This even if the alkali content has been depleted by the consumption during neutralization. The total volume of the White Liquor Pad WLP is 50-70% of the free volume inside digester and the first colder volume of the WLP is 20-50% of the WLP.

(21) Embodiment in Digester Filling Phase

(22) In FIG. 5a is an alternative embodiment of the invention shown during a complete filling of the digester with one and the same liquor, but with a forced temperature profile. This temperature profiling may be implemented after the White Liquor Pad displacement as shown in FIG. 4.

(23) The principle effect of the inventive displacement front through the digester shown using a displacement liquor, i.e. the one and same displacement liquor as of chemical content, that in at least 2 or 3 incremental steps, which in FIG. 5a are 7 incremental steps, at temperatures of 90-92.5-95-97.5-100-102.5 and finally 105 C. is used. In this example a drained digester is shown with a digester content of comminuted chips after a heated treatment in form of a prehydrolysis conducted at 170 C. In a first stage, first left hand figure, of the displacement phase is the inlet cone part filled with a volume of wash liquid holding 90 C. when added. In a second stage, second figure from left, of the displacement phase is the inlet cone part filled with a volume of wash liquid holding 92.5 C. when added, pushing the first volume upwardly but now heated to a higher temperature by the digester content to about 92.5 C. This continues in stages until the entire digester is filled with displacement liquor, and as a result of the heating from the digester content is an ISO temperature profile established over the height of the digester, with a temperature of the free liquor close to about 105 C. in the entire digester. The total heating effect is about 10 C. from exothermic reactions and about 10 C. from heat value in digester content. Now, the digester content in bottom has been flushed by liquor during the entire displacement and most of the heat value in the digester content has been absorbed in the liquor, while the digester content in top has only been drenched by heated liquor and has thus most of the heat value from hydrolysis left in the digester content.

(24) This temperature profile may be even out by a circulation as shown in FIG. 5b, which starts from a condition corresponding to the rightmost hand figure in FIG. 4a, with an isothermal temperature profile of the free liquor, where a pump starts to withdraw liquor from mid-point liquid exchange position and return this liquor to both top and bottom. This will result in some heating of the free liquor by the excess residual heat in the digester content in top throughout the digester. Ideally, the entire content of the digester, both the chips and the free liquor, assumes a temperature lying somewhat above the temperature of the free liquor at end of displacement, as disclosed in the right hand figure.

(25) Alternative Embodiments

(26) Alternatively, the forced temperature profiling of the displacement liquor may even be modified so that the temperature of the free liquor in the final phase of displacement is not isothermal throughout the digester, but could still have a slight temperature profile with either colder or warmer temperature in final 7.sup.th displacement phase. Hence, the digester content in bottom that is displaced by largest amount of displacement liquor may have the lowest residual heat value in the digester content, and therefore could the temperature increase be larger in the steps disclosed in FIG. 5a.

(27) As the objective is to expose the digester content of iso-H factor exposure, could the forced temperature profiling be controlled exponentially so that the digester content may be exposed to less total cooling effect in latter stages of displacement, i.e. using less temperature increase in first 1-3 phases and then successively higher temperature increases in last 4-7 phases.

(28) The effect of the temperature profiling could be controlled in the pulp finally blown from the digester, taking a sample of the first blow pulp and then a sample from the last blown pulp from the digester and compare pulp quality between these samples as of viscosity, tear strength or other pulp characteristics that may be effected by the specific displacement process.

(29) If the inventive displacement process is implemented after a prehydrolysis, could for example differences in first and last blow pulp be compared as to residual content of hemicellulose that is supposed to dissolve during the prehydrolysis. If the first blown pulp, i.e. the digester content in bottom during treatment, has a higher content of hemicellulose, one may assume that the hydrolysis has not been obtained to the same extent as the last blown pulp thus suggesting an alteration of the temperature profiling towards a higher temperature in the lower part during the displacement phase.

(30) The temperature profiling during the displacement could easily be implemented in a principal system as that disclosed in FIG. 1 by using only unheated liquor (90 C.) in first phase, i.e. opening valve V.sub.2A while having valve V.sub.2B closed. Then as disclosed in figures could a change be implemented in several stages, gradually opening the valve V.sub.2B in steps as functionally disclosed in figures, or alternatively opening the valve gradually over the entire control process. The total volume of the liquor accumulator could thus be reduced considerably, as the total heated liquor volume is reduced in proportion to usage. The opening of the valve V.sub.2B may be controlled by a temperature sensor located after mixing of the unheated and heated wash liquors, as disclosed in FIG. 1.

(31) While the temperature profiling has been disclosed after a prehydrolysis, the very same temperature profiling may be forced to any displacement liquor added to batch digester to end a preceding heated phase where temperature and time exposure on the digester content of cellulosic material play a role in that treatment phase. Thus, the Hot White Liquor accumulator shown in FIG. 1 may alternatively be a Wash Liquor or Hot Black Liquor accumulator.

(32) While the present invention has been described in accordance with preferred compositions and embodiments, it is to be understood that certain substitutions and alterations may be made thereto without departing from the spirit and scope of the following claims.