CA1229254A - Multi-stage pulp washing within a batch digester - Google Patents
Multi-stage pulp washing within a batch digesterInfo
- Publication number
- CA1229254A CA1229254A CA000454546A CA454546A CA1229254A CA 1229254 A CA1229254 A CA 1229254A CA 000454546 A CA000454546 A CA 000454546A CA 454546 A CA454546 A CA 454546A CA 1229254 A CA1229254 A CA 1229254A
- Authority
- CA
- Canada
- Prior art keywords
- digester
- black liquor
- liquor
- cooking
- wash
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
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- Paper (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE
A method for washing pulp in a batch digester modified pulping process wherein black cooking liquor used for cooking the chips is partially displaced by pumping a first stage wash effluent into the bottom of the digester, additional amounts of the cooking liquor are displaced by pumping a second stage wash effluent, and so on, until sufficient increments of wash effluent have been pumped to displace the cooking liquor. A wash filtrate water is then pumped into the digester from a final pulp washing operation to displace the hot black liquor until all of the hot black liquor is sent to an accumulator, and the various wash effluents have been replenished in their respective stages.
A method for washing pulp in a batch digester modified pulping process wherein black cooking liquor used for cooking the chips is partially displaced by pumping a first stage wash effluent into the bottom of the digester, additional amounts of the cooking liquor are displaced by pumping a second stage wash effluent, and so on, until sufficient increments of wash effluent have been pumped to displace the cooking liquor. A wash filtrate water is then pumped into the digester from a final pulp washing operation to displace the hot black liquor until all of the hot black liquor is sent to an accumulator, and the various wash effluents have been replenished in their respective stages.
Description
I
BACKGROUND OF THE INVENTION
Field of the Invention The present invention is directed to a process for washing pulp which has been cooked in a batch digester using a modified raft pulping process. The invention is particularly concerned with the use of multi-stage pulp washing within the digester itself.
Description of the Prior Art There are numerous types of processes for batch digestion of wood chips in the manufacture of paper, usually taking place in a digester specifically built for that purpose. The digester is filled with the wood chips which are usually compacted therein. Hot solutions of sodium hydroxide alone or in admixture with sodium sulfide are then charged into the digester. The temperature of the digester is conventionally controlled through the intro-diction of steam. After the chips are maintained in contact with the cooking liquor for a predetermined period of time, a blow valve in the digester is opened to dump the contents into a blow tank.
In the previously disclosed pulping process, the hot black liquor is displaced from the digester by pumping washer filtrate into the bottom of the digester at the end of the cook. The hot black liquor leaves the digester through an extraction screen located in the top dome of the digester. Most of the displaced hot black liquor goes to I
the pressurized hot black liquor accumulator and the final volume, as the temperature drops, goes to a warm black liquor accumulator. Tests have shown that pulp washing equivalent to a one-stage filter is achieved in the hot black liquor displacement. Additional pulp washing after the pulp is blown from the digester is required to achieve an acceptably low level of black liquor remaining in thy pulp.
byway The process of the present invention employs a multi-stage washing operation within the digester itself so as to accomplish all or a part of the required pulp washing prior to blowing the pulp from the digester. Among the advantages of this new process are:
(1) An improved degree of pulp washing is accomplished within the digester;
BACKGROUND OF THE INVENTION
Field of the Invention The present invention is directed to a process for washing pulp which has been cooked in a batch digester using a modified raft pulping process. The invention is particularly concerned with the use of multi-stage pulp washing within the digester itself.
Description of the Prior Art There are numerous types of processes for batch digestion of wood chips in the manufacture of paper, usually taking place in a digester specifically built for that purpose. The digester is filled with the wood chips which are usually compacted therein. Hot solutions of sodium hydroxide alone or in admixture with sodium sulfide are then charged into the digester. The temperature of the digester is conventionally controlled through the intro-diction of steam. After the chips are maintained in contact with the cooking liquor for a predetermined period of time, a blow valve in the digester is opened to dump the contents into a blow tank.
In the previously disclosed pulping process, the hot black liquor is displaced from the digester by pumping washer filtrate into the bottom of the digester at the end of the cook. The hot black liquor leaves the digester through an extraction screen located in the top dome of the digester. Most of the displaced hot black liquor goes to I
the pressurized hot black liquor accumulator and the final volume, as the temperature drops, goes to a warm black liquor accumulator. Tests have shown that pulp washing equivalent to a one-stage filter is achieved in the hot black liquor displacement. Additional pulp washing after the pulp is blown from the digester is required to achieve an acceptably low level of black liquor remaining in thy pulp.
byway The process of the present invention employs a multi-stage washing operation within the digester itself so as to accomplish all or a part of the required pulp washing prior to blowing the pulp from the digester. Among the advantages of this new process are:
(1) An improved degree of pulp washing is accomplished within the digester;
(2) The process equipment requirements for pulp washing are reduced;
(3) The washing can greatly reduce or eliminate any odor causing sulfur gases in the exhaust gas from the blowing of the pulp from the digester, and (~) The overall displacement efficiency of the hot black liquor from the digester is improved, thus enhancing the energy savings.
In a typical embodiment of the present invention, the washing takes place in three stages. After the chips have been cooked with a cooking liquor to produce pulp and I
a hot black liquor, a portion of the hot black liquor is displaced in the digester with h first wash effluent. An additional portion of the hot black liquor in the digester is displaced with a second wash effluent having a lower concentration of black liquor than the first wash effluent.
Finally, another portion of the hot black liquor is displaced with a third wash effluent which has a still lower concentra-lion of black liquor than either of the first two effluents.
The remaining contents of the digester are displaced with a washer filtrate water and the supplies of first, second and third wash effluents are sequentially replenished from the liquid displaced from the digester.
The pulp washing process of the present invention finds particular use with the batch digester modified raft pulping process, but it also has application in other batch digester pulping systems such as a sulfite pulping system.
BRIEF DESCRIPTION OF THE DRAWINGS
The single Figure of the drawings consists of a schematic flow diagram illustrating a multi-stage pulp washing system of the present invention, when employing three washing stages.
DESCRIPTION THE OR F RUED EMBODIMENTS
The multistage washing process of the present invention can be successfully applied to washing in a digester because of the discovery that when liquor is displaced prom Do a digester by pumping liquor or wash water in the bottom of the digester and removing the displaced liquor from the top, a very efficient displacement can be accomplished in a short time, usually less than 30 minutes. The reason for this was found to be that the pulp in the digester at the end of the cook remains in the form of chips and sinks to the bottom of the digester. Since the pulp sinks in the digester there is no pulp at the top of the digester to plug the outlet screens or restrict the displaced liquor flow from the digester. The observed pulp sinking was found to be true of all types of wood tested which range from low density spruce to high density hardwoods.
Turning now Jo the drawing, reference numeral 10 has been applied generally to a conventional digester of the batch type including a removable lid 11. Steam is introduced into a circulating line 12 through a valve 13 into -the bottom of the digester and circulates through a valve 14 to bring the wood chips which are packed into the digester 10 up to a cooking temperature.
At the completion of the cooking operation when the digester is substantially filled with a mixture of pulp and hot black liquor, a partial displacement of hot black liquor from the digester commences by pumping an effluent from a first stage wash effluent accumulator 15 by means of a pump 16 through a valve 17 and into the base of the digester 10 through an inlet valve 18. Hot black liquor is displaced from the digester 10 through a line 19 and a valve 20 into a hot black liquor accumulator 21.
I
The volume of first stage wash effluent pumped into the digester will depend on system requirements such as the number of wash stages employed, the required pulp cleanliness after washing, the time available, and the allowable dilution of the black liquor with wash water. The volume of wash effluent piped into the digester will normally be in the range of 10 to 30~ of the digester volume.
After pumping the initial violin of first stage wash effluent into the digester, the displacement of hot black liquor from the digester 10 to the accumulator 21 is continued by pumping effluent from a second stage wash effluent accumulator 22 by means of a pump 23 through a valve 24 into the base of the digester through the inlet valve 18.
This second wash effluent has a lower concentration of black liquor than the first wash effluent.
After pumping an additional amount of second stage wash effluent into the digester to thereby displace additional amounts of hot black liquor into the accumulator 21, the flow from the accumulator 22 is slopped and the effluent from a third stage wash effluent accl~nulator 25 is pumped by means of a pump 26 through a valve 27, through inlet valve 18, into the bottom of the accumulator 10 Additional amounts of hot black liquor are thus displaced into the hot black liquor accumulator 21.
The next stage consists in pumping a wash filtrate water from a reservoir 28 by means of a pump 29 through a valve 30 into the inlet valve 18 at -the base of the digester.
The filtrate water is normally almost devoid of any black 32~i~
liquor concentration. After the proper volume of hot black liquor has been accumulated in the accumulator I the valve 20 is closed and a valve 31 is opened so that the various effluent fractions are returned to their accumulators using a pump 32 if necessary. The displacement with wash filtrate from the reservoir 28 delivers first stage wash effluent into the accumulator 15 through a valve 33 to replenish the amount of first stage effluent used initially in displacing hot black liquor. When that amount of effluent has been recovered, the valve 33 is closed and a valve 3 is opened to direct flow into the second stage effluent accumulator 22. Upon replenishment of the effluent used in the original displacement, valve 34 is closed and a valve 35 is opened to direct the less concentrated wash effluent back into the third stage wash effluent accumulator 25.
This completes the pulp washing operation in the digester.
The pulp and the remaining liquid are then blown from the digester 10 by means of compressed air entering through a compressed air line 36 and the contents of the digester 10 are directed to a blow tank through a valve 37.
Since most of the black liquor has been removed from the pulp by the washing process, there should be substantially no odorous sulfur gases released with the air from the blow tank. In trials using the modified system of the present invention where the black liquor was displaced from the digester, it was found that there is more than a 98% reduction in odorous sulfur gases as compared to the release from a conventional raft digester blow. The air I
from this blow tank can be vented to the atmosphere with no environmental problem when the new pulp washing system of the present invention is employed.
As a specific example of a system according to the present invention, the relative concentration of black liquor, expressed as percent of dissolved dry black liquor solids in the washing fractions may be as follows:
Black Liquor Wash Liquid Hot Black Liquor I
First Effluent 14%
Second Effluent I
Third Effluent 2%
Wash Filtrate%
The flows for a typical 6000 cu. ft. raft digester could be as follows:
FLOW INTO DIGESTER
SouxceCubic Feet Gallons First Effluent 1000 7,480 Second Effluent 1000 7,480 Third Effluent 1000 7,480 Filtrate 6000 44,880 TOTAL 9000 67,320 FLOW OUT OF DIGESTER
To Cubic Feet Gallons Hot Liquor Accumulator 6000 44,880 First Effluent 1000 7,480 Second Effluent 1000 7,480 Third Effluent 1000 7?480 9000 67,320 It should be evident that various modifications can be made to the described embodiments without departing from the scope of the present invention.
In a typical embodiment of the present invention, the washing takes place in three stages. After the chips have been cooked with a cooking liquor to produce pulp and I
a hot black liquor, a portion of the hot black liquor is displaced in the digester with h first wash effluent. An additional portion of the hot black liquor in the digester is displaced with a second wash effluent having a lower concentration of black liquor than the first wash effluent.
Finally, another portion of the hot black liquor is displaced with a third wash effluent which has a still lower concentra-lion of black liquor than either of the first two effluents.
The remaining contents of the digester are displaced with a washer filtrate water and the supplies of first, second and third wash effluents are sequentially replenished from the liquid displaced from the digester.
The pulp washing process of the present invention finds particular use with the batch digester modified raft pulping process, but it also has application in other batch digester pulping systems such as a sulfite pulping system.
BRIEF DESCRIPTION OF THE DRAWINGS
The single Figure of the drawings consists of a schematic flow diagram illustrating a multi-stage pulp washing system of the present invention, when employing three washing stages.
DESCRIPTION THE OR F RUED EMBODIMENTS
The multistage washing process of the present invention can be successfully applied to washing in a digester because of the discovery that when liquor is displaced prom Do a digester by pumping liquor or wash water in the bottom of the digester and removing the displaced liquor from the top, a very efficient displacement can be accomplished in a short time, usually less than 30 minutes. The reason for this was found to be that the pulp in the digester at the end of the cook remains in the form of chips and sinks to the bottom of the digester. Since the pulp sinks in the digester there is no pulp at the top of the digester to plug the outlet screens or restrict the displaced liquor flow from the digester. The observed pulp sinking was found to be true of all types of wood tested which range from low density spruce to high density hardwoods.
Turning now Jo the drawing, reference numeral 10 has been applied generally to a conventional digester of the batch type including a removable lid 11. Steam is introduced into a circulating line 12 through a valve 13 into -the bottom of the digester and circulates through a valve 14 to bring the wood chips which are packed into the digester 10 up to a cooking temperature.
At the completion of the cooking operation when the digester is substantially filled with a mixture of pulp and hot black liquor, a partial displacement of hot black liquor from the digester commences by pumping an effluent from a first stage wash effluent accumulator 15 by means of a pump 16 through a valve 17 and into the base of the digester 10 through an inlet valve 18. Hot black liquor is displaced from the digester 10 through a line 19 and a valve 20 into a hot black liquor accumulator 21.
I
The volume of first stage wash effluent pumped into the digester will depend on system requirements such as the number of wash stages employed, the required pulp cleanliness after washing, the time available, and the allowable dilution of the black liquor with wash water. The volume of wash effluent piped into the digester will normally be in the range of 10 to 30~ of the digester volume.
After pumping the initial violin of first stage wash effluent into the digester, the displacement of hot black liquor from the digester 10 to the accumulator 21 is continued by pumping effluent from a second stage wash effluent accumulator 22 by means of a pump 23 through a valve 24 into the base of the digester through the inlet valve 18.
This second wash effluent has a lower concentration of black liquor than the first wash effluent.
After pumping an additional amount of second stage wash effluent into the digester to thereby displace additional amounts of hot black liquor into the accumulator 21, the flow from the accumulator 22 is slopped and the effluent from a third stage wash effluent accl~nulator 25 is pumped by means of a pump 26 through a valve 27, through inlet valve 18, into the bottom of the accumulator 10 Additional amounts of hot black liquor are thus displaced into the hot black liquor accumulator 21.
The next stage consists in pumping a wash filtrate water from a reservoir 28 by means of a pump 29 through a valve 30 into the inlet valve 18 at -the base of the digester.
The filtrate water is normally almost devoid of any black 32~i~
liquor concentration. After the proper volume of hot black liquor has been accumulated in the accumulator I the valve 20 is closed and a valve 31 is opened so that the various effluent fractions are returned to their accumulators using a pump 32 if necessary. The displacement with wash filtrate from the reservoir 28 delivers first stage wash effluent into the accumulator 15 through a valve 33 to replenish the amount of first stage effluent used initially in displacing hot black liquor. When that amount of effluent has been recovered, the valve 33 is closed and a valve 3 is opened to direct flow into the second stage effluent accumulator 22. Upon replenishment of the effluent used in the original displacement, valve 34 is closed and a valve 35 is opened to direct the less concentrated wash effluent back into the third stage wash effluent accumulator 25.
This completes the pulp washing operation in the digester.
The pulp and the remaining liquid are then blown from the digester 10 by means of compressed air entering through a compressed air line 36 and the contents of the digester 10 are directed to a blow tank through a valve 37.
Since most of the black liquor has been removed from the pulp by the washing process, there should be substantially no odorous sulfur gases released with the air from the blow tank. In trials using the modified system of the present invention where the black liquor was displaced from the digester, it was found that there is more than a 98% reduction in odorous sulfur gases as compared to the release from a conventional raft digester blow. The air I
from this blow tank can be vented to the atmosphere with no environmental problem when the new pulp washing system of the present invention is employed.
As a specific example of a system according to the present invention, the relative concentration of black liquor, expressed as percent of dissolved dry black liquor solids in the washing fractions may be as follows:
Black Liquor Wash Liquid Hot Black Liquor I
First Effluent 14%
Second Effluent I
Third Effluent 2%
Wash Filtrate%
The flows for a typical 6000 cu. ft. raft digester could be as follows:
FLOW INTO DIGESTER
SouxceCubic Feet Gallons First Effluent 1000 7,480 Second Effluent 1000 7,480 Third Effluent 1000 7,480 Filtrate 6000 44,880 TOTAL 9000 67,320 FLOW OUT OF DIGESTER
To Cubic Feet Gallons Hot Liquor Accumulator 6000 44,880 First Effluent 1000 7,480 Second Effluent 1000 7,480 Third Effluent 1000 7?480 9000 67,320 It should be evident that various modifications can be made to the described embodiments without departing from the scope of the present invention.
Claims (8)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A method for cooking and washing pulp in a batch digester which comprises:
cooking wood chips in a digester with a cooking liquor to produce pulp and a hot black liquor, when said cooking is completed displacing a first portion of said black liquor substantially less than the volume of said digester from said digester with a first wash effluent, displacing a second portion of the remaining black liquor substantially less than the volume of said digester from said digester with a second wash effluent, directing the displaced hot black liquor portions to an accumulator, directing the first and second wash effluents from said digester to separate storage means, and emptying the pulp and remaining liquid from said digester after said black liquor portions have been displaced.
cooking wood chips in a digester with a cooking liquor to produce pulp and a hot black liquor, when said cooking is completed displacing a first portion of said black liquor substantially less than the volume of said digester from said digester with a first wash effluent, displacing a second portion of the remaining black liquor substantially less than the volume of said digester from said digester with a second wash effluent, directing the displaced hot black liquor portions to an accumulator, directing the first and second wash effluents from said digester to separate storage means, and emptying the pulp and remaining liquid from said digester after said black liquor portions have been displaced.
2. A method according to claim 1 in which said first and second wash effluents are displaced from said digester with a washer filtrate water.
3. A method according to claim 1 which includes the additional step of displacing a third portion of said black liquor with a third wash effluent.
4. A method according to claim 1 in which said cooking liquor is an alkaline kraft liquor.
5. A method according to claim 1 in which said cooking liquor is a sulphite cooking liquor.
6. A method according to claim 1 in which:
said first wash effluent has a higher concentration of black liquor than said second wash effluent.
said first wash effluent has a higher concentration of black liquor than said second wash effluent.
7. A method according to claim 3 in which:
said third wash effluent has a lower concentration of black liquor than said second wash effluent.
said third wash effluent has a lower concentration of black liquor than said second wash effluent.
8. A method for cooking wood chips and washing the resulting pulp in a batch digester which comprises:
cooking said wood chips in a digester with a cooking liquor to produce pulp and a hot black liquor, after the cooking is completed displacing a portion of the hot black liquor in said digester with a first wash effluent, displacing an additional portion of the hot black liquor in said digester with a second wash effluent having a lower concentration of black liquor than said first wash effluent, displacing another portion of the hot black liquor in said digester with a third wash effluent having a lower concentration of black liquor than said second wash effluent, displacing the remaining contents of said digester with a washer filtrate water, the displacement of said hot liquor portions being carried out while the cooked pulp is still in the digester, and sequentially replenishing the supplies of first, second and third wash effluent from the liquid displaced from said digester.
cooking said wood chips in a digester with a cooking liquor to produce pulp and a hot black liquor, after the cooking is completed displacing a portion of the hot black liquor in said digester with a first wash effluent, displacing an additional portion of the hot black liquor in said digester with a second wash effluent having a lower concentration of black liquor than said first wash effluent, displacing another portion of the hot black liquor in said digester with a third wash effluent having a lower concentration of black liquor than said second wash effluent, displacing the remaining contents of said digester with a washer filtrate water, the displacement of said hot liquor portions being carried out while the cooked pulp is still in the digester, and sequentially replenishing the supplies of first, second and third wash effluent from the liquid displaced from said digester.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US51549383A | 1983-07-20 | 1983-07-20 | |
| US515,493 | 1990-04-30 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1229254A true CA1229254A (en) | 1987-11-17 |
Family
ID=24051595
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000454546A Expired CA1229254A (en) | 1983-07-20 | 1984-05-17 | Multi-stage pulp washing within a batch digester |
Country Status (2)
| Country | Link |
|---|---|
| JP (1) | JPS6039494A (en) |
| CA (1) | CA1229254A (en) |
Families Citing this family (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPH02250706A (en) * | 1989-03-17 | 1990-10-08 | O S G Kk | End mill and manufacture thereof |
| CN100451235C (en) * | 2006-05-22 | 2009-01-14 | 李国林 | Paper pulp manufacturing process and equipment |
-
1984
- 1984-05-17 CA CA000454546A patent/CA1229254A/en not_active Expired
- 1984-07-19 JP JP14869484A patent/JPS6039494A/en active Granted
Also Published As
| Publication number | Publication date |
|---|---|
| JPS6039494A (en) | 1985-03-01 |
| JPS627318B2 (en) | 1987-02-17 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKEX | Expiry |