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Title:
DEAERATION OF STOCK IN A PULP DRYING MACHINE
Document Type and Number:
WIPO Patent Application WO/2012/056108
Kind Code:
A1
Abstract:
The present invention relates to a method and arrangement for deaerating incoming thick stock or mixture of white water and thick stock in a pulp drying machine comprising a head box 1 for forming a web, a wire section 2 for removing white water from the web, a wire pit 4 for receiving white water from the wire section 2, thick stock feed 5 for forming a mixture of white water and thick stock and one or more deaeration apparatus 6 for removing air. Air in thick stock or in mixture of white water and thick stock is removed actively by feeding thick stock or at least part of the mixture of white water and thick stock trough at least one deaeration apparatus 6 for removing air. The deaeration apparatus 6 can be any apparatus that is used for deaeration. For example the deaeration apparatus 6 is a deaeration pump or a vacuum tank.

Inventors:
HELLE TOPI (FI)
KOIVISTO TIMO (FI)
Application Number:
PCT/FI2011/050936
Publication Date:
May 03, 2012
Filing Date:
October 26, 2011
Export Citation:
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Assignee:
AIKAWA FIBER TECHNOLOGIES OY (FI)
HELLE TOPI (FI)
KOIVISTO TIMO (FI)
International Classes:
D21C9/18; D21D5/26; D21F5/00
Domestic Patent References:
WO2002022946A12002-03-21
WO2008119872A12008-10-09
WO2007107635A12007-09-27
Foreign References:
US3131117A1964-04-28
US6258213B12001-07-10
Attorney, Agent or Firm:
BORENIUS & CO OY AB (Helsinki, FI)
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Claims:
Claims

1. A method for deaerating thick stock in a pulp drying machine for producing dry market pulp comprising a head box (1) for forming a web, a wire section (2) for removing white water from the web, a wire pit (4) for receiving white water from the wire section (2), and thick stock feed (5) for forming a mixture of white water and thick stock, characterized by that air in the thick stock is actively removed in one or more deaeration apparatus (6).

2. A method according to claim 1, characterized in that the active deaeration is arranged by feeding at least part of the thick stock trough at least one deaeration apparatus (6) for removing air.

3. A method according to claim 1 or 2, characterized in that the deaeration apparatus (6) comprises a deaeration pump.

4. A method according to claim 1 or 2, characterized in that the deaeration apparatus (6) comprises a vacuum tank.

5. A method according to any of the claims 1 to 4, c h a r a c t e r i z e d in that white water is mixed with the thick stock after the deaeration.

6. A method according to any of the claims 1 to 5, characterized in that white water is mixed with the thick stock prior to the deaeration.

7. A method according to any of the claims 1 to 6, characterized in that white water used for making a mixture of white water and thick stock is collected between the wire section (2) and the wire pit (4).

8. A method according to any of the claims 1 to 7, characterized in that white water used for making a mixture of white water and thick stock is collected under the surface of the white water.

9. A method according to claim 7 or 8, characterized in that the mixture of white water and thick stock is fed to the head box (1) after the deaeration.

10. An arrangement in a pulp drying machine for producing dry market pulp, said pulp drying machine comprising a head box (1) for forming a web, a wire section (2) for removing white water from the web, a wire pit (4) for receiving white water from the wire section (2), and thick stock feed (5) for forming a mixture of white water and thick stock, characterized by that the arrangement has means for removing air actively from thick stock in one or more deaeration apparatus (6).

11. An arrangement according to claim 10, characterized by that the arrangement has means for feeding at least part of thick stock trough at least one deaeration apparatus (6) for removing air.

12. An arrangement according to claim 10 or 11, characterized by that the arrangement comprises a deaeration pump.

13. An arrangement according to claim 10 or 11, characterized by that the arrangement comprises a vacuum tank.

14. An arrangement according to any of the claims 10 to 13, characterized by that the arrangement has means for mixing white water with the thick stock after the deaeration.

15. An arrangement according to any of the claims 10 to 14, characterized by that the arrangement has means for mixing white water with the thick stock prior to deaeration.

16. An arrangement according to any of the claims 10 to 15, characterized by that the arrangement has means for collecting white water used for making a mixture of white water and thick stock between the wire section (2) and the wire pit (4).

17. An arrangement according to any of the claims 10 to 16, characterized by that the arrangement has means for collecting white water used for making a mixture of white water and thick stock under the surface of the white water.

18. An arrangement according to claim 16 or 17, characterized by that the arrangement has means for feeding the mixture of white water and thick stock to the head box (1 ) after the deaeration.

19. A method for manufacturing dry market pulp, characterized by that the method comprises a receiving step (11) for receiving thick stock from a pulp source (7), a deaerating step (8) for actively deaerating stock com- prising the thick stock for producing deaerated pulp, a drying step (9) for drying deaerated pulp, and a baling step (10) for baling dry pulp.

20. A method according to claim 19, characterized by that the pulp source (7) is a pulp mill or a deinking plant.

21. A method according to any of the claims 19 to 20, characterized by that the stock which is to be deaerated further comprises white water from a wire section (2).

22. A method according to any of the claims 19 to 21, characterized by that the deaerating of stock (8) is performed by a vacuum tank or a deaeration pump.

23. A method according to any of the claims 19 to 22, characterized by that drying of deaerated pulp (9) is performed in a pulp drying machine.

24. An arrangement for manufacturing dry market pulp, characterized by that the arrangement comprises receiving means (11 ) for receiving thick stock from a pulp source (7), one or more deaeration apparatus (8) for deaerating stock comprising the thick stock for producing deaerated pulp, drying means (9) for drying deaerated pulp, and baling means (10) for baling dry pulp.

25. An arrangement according to claim 24, characterized by that the pulp source (7) is a pulp mill or a deinking plant.

26. An arrangement according to claim 24 or 25, c h a r a c t e r i z e d by that the arrangement further comprises means for mixing white water from a wire section (2) with the thick stock.

27. An arrangement according to any of the claims 24 to 26, c h a r a c t e r - i z e d by that the means for deaerating stock (8) comprises a vacuum tank or a deaeration pump.

28. An arrangement according to any of the claims 24 to 27, c h a r a c t e r - i z e d by that the means for drying deaerated pulp (9) is a pulp drying machine.

Description:
DEAERATION OF STOCK IN A PULP DRYING MACHINE

FIELD OF THE INVENTION

The invention relates to a method and an arrangement of deaerating incoming thick stock and short circulation in a pulp drying machine, and particularly to a method and an arrangement of deaerating incoming thick stock or the mixture of white water and thick stock in a pulp drying machine comprising head box, wire section, wire pit, thick stock feed and deaeration apparatus. The invention relates also to a method and an arrangement for manufacturing dry market pulp. BACKGROUND OF THE INVENTION

The purpose of pulp drying machine is to manufacture bales of dry pulp, so called market pulp, of pulp stock which is a mixture of cellulosic fibers and water, by removing water from the said stock. The pulp stock has been manufactured at a pulp mill of wood chips by dissolving the lignin holding the single fi- bers together. Alternatively the pulp stock has been manufactured at a deink- ing plant by removing printing ink from recycled paper.

The running speed of a pulp drying machine is to a great extent limited by its dewatering capacity. The dewatering of pulp drying machine is performed by similar manners as in paper machine: by wire section, by press section and by evaporation. Most of the dewatering is occurring on the wire section of the machine, where approximately 80 % of all dewatering is performed. The solids content of the web must be sufficiently high when entering the press section in order to avoid web breaks and to be able using higher nip pressure in the first press. Thus, the dewatering capacity of the wire section plays a key role in dewatering in a pulp drying machine.

Air trapped in the stock among other things hinders dewatering on the wire section. Thus, machine running speed must be low enough in order to achieve sufficient dewatering. Alternatively the wire section could be made longer so that water has more time to exit the web. This will add to the establishment expenses and is therefore preferred to be avoided.

The air is dissolved in the stock in tanks with high pressure and liquid-air interface. Air is also entrapped when the stock is mixed hard, e.g. in pumps or tanks with mixing equipment. In pulp drying process the pumping and mixing of the stock can not be avoided. Especially fresh pulp from pulp mill or deinking plant comprises a lot of air.

Traditionally the wire pit is used to remove entrapped air in pulp drying machine. The relative stillness of the white water in wire pit allows the air bubbles to rise up to the surface. This can be considered passive deaeration. However, air removal in wire pit as a method is not effective enough in order to achieve sufficient deaeration. If the flow rate is too strong the air bubbles are entrapped with the flow and do not rise up. In addition if the air bubbles get too deep in the wire pit the hydrostatic pressure reduces the buoyancy of the bub- ble and thus hindering the air removal efficiency.

BRIEF DESCRIPTION OF THE INVENTION

An object of the present invention is to provide a method and an arrangement so as to solve the above problems.

The invention is based on the idea of removing air from incoming thick stock and the short circulation of a pulp drying machine using active deaeration. In active deaeration air is removed by feeding thick stock or mixture of white water and thick stock trough an apparatus for removing air.

An advantage of the invention is that air entrapped in the thick stock or mixture of white water and thick stock is removed efficiently. Thus, dewatering on the wire section is improved, which allows using higher machine running speeds and higher nip pressure in the first nip of the press section, thus improving the dewatering on the press section. This reduces dewatering needed on the drying section, where dewatering is performed by evaporation. Thus, the amount of energy needed for evaporation is reduced.

Another advantage of the invention is that the air is removed from the incoming fresh pulp before it is formed as a web at the pulp drying machine.

It is generally thought that machines producing cellulosic fiber webs, e.g. paper machines, board machines or pulp drying machines, do not need special air removal if the machine running speed is less than 1000 m/min. In the present invention however, it has been noticed that the dewatering is improved also in a pulp drying machine running at 100-300 m/min. BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be described in greater detail by means of preferred embodiments with reference to the attached drawing, in which

Figure 1 is a schematic flow sheet of the inventive process according to an embodiment.

Figure 2 is a schematic flow sheet of the inventive process according to an embodiment.

Figure 3 is a schematic flow sheet of the inventive process according to an embodiment. SUMMARY OF THE INVENTION

The wet end of a pulp drying machine comprises feed of thick stock and a head box 1 . The head box 1 spreads the pulp evenly on a rolling wire of the wire section 2. The wire section 2 can also comprise of twin wire former. Pulp retains on the wire and water is filtrated trough the wire. Said water, called white water, is collected to wire pit 4 using white water channels 3 under the wire section 2. White water in the wire pit 4 is used for diluting the thick stock coming from a pulp mill or deinking plant. This circulation of white water is called short circulation.

The present invention relates to a method for deaerating thick stock in a pulp drying machine for producing dry market pulp comprising a head box 1 for forming a web, a wire section 2 for removing white water from the web, a wire pit 4 for receiving white water from the wire section 2, thick stock feed 5 for forming a mixture of white water and thick stock and one or more deaeration apparatus 6 for removing air. Air in the thick stock is removed actively by feeding at least part of the thick stock trough at least one deaeration apparatus 6 for removing air. The deaeration apparatus 6 can be any apparatus that is used for deaeration. For example the deaeration apparatus 6 is a deaeration pump or a vacuum tank. After the deaeration the thick stock is mixed with white water.

Another embodiment of the method is to mix white water with thick stock prior to deaeration. White water used for making the mixture of white water and thick stock is collected between the wire section 2 and the wire pit 4. Preferably white water is collected under the surface of the white water in the wire pit 4 in order to prevent any extra air entering the stream of white water. Alternatively white water is collected under the surface of white water stream in white water channels 3. After mixing thick stock with white water the mixture is lead to the deaeration. After the deaeration the mixture of white water and thick stock is fed to the head box 1 .

The present invention relates also to an arrangement in a pulp drying machine for producing dry market pulp. Said pulp drying machine comprises a head box 1 for forming a web, a wire section 2 for removing white water from the web, a wire pit 4 for receiving white water from the wire section 2, thick stock feed 5 for forming a mixture of white water and thick stock and one or more deaeration apparatus 6 for removing air. The arrangement has means for removing air from thick stock actively by feeding at least part of thick stock trough at least one deaeration apparatus 6 for removing air. The deaeration apparatus 6 in the arrangement can be any apparatus that is used for removing air. For example the deaeration apparatus 6 is a deaeration pump or a vacuum tank. After the deaeration the thick stock is mixed with white water.

Another embodiment of the arrangement is to mix white water with thick stock prior to deaeration. The arrangement has means for collecting white water for making the mixture of white water and thick stock between the wire section 2 and the wire pit 4. Preferably white water is collected under the surface of the white water in the wire pit 4 in order to prevent any extra air entering the stream of white water. Alternatively white water is collected under the surface of white water stream in white water channels 3.

After mixing thick stock with white water the mixture is lead to the deaeration. The arrangement has also means for feeding the mixture of white water and thick stock to the head box 1 after the deaeration.

The invention relates also to a method for manufacturing dry market pulp, wherein the method comprises a receiving step 1 1 for receiving thick stock from a pulp source 7, a deaerating step 8 for actively deaerating stock com- prising the thick stock for producing deaerated pulp, a drying step 9 for drying deaerated pulp, and a baling 10 step for baling dry pulp. Deaerating thick stock actively means that thick stock is fed trough one or more deaeration apparatus 6 for removing air.

According to an embodiment of the invention the pulp source 7 is a pulp mill or a deinking plant. Advantageously thick stock is fresh pulp from pulp mill or deinked pulp from deinking plant. According to an advantageous embodiment the stock which is to be deaerated further comprises white water from a wire section 2.

According to an embodiment deaerating 8 of stock is performed by a vacuum tank or a deaeration pump. For example the deaeration is performed by a deculator. Advantageously deaeration 8 is performed by an apparatus which removes air from liquid by centrifugal force.

Advantageously drying 9 of deaerated pulp is performed by a pulp drying machine. In the pulp drying machine the pulp is dried as a web of dry pulp. After the pulp drying machine the web of dry pulp is cut and stacked into bales in baling 10.

The invention relates also to an arrangement for manufacturing dry market pulp, wherein the arrangement comprises receiving means 1 1 for receiving thick stock from a pulp source 7, one or more deaeration apparatus 8 for deaerating stock comprising thick stock for producing deaerated pulp, drying means 9 for drying deaerated pulp, and baling means 10 for baling dry pulp.

According to an advantageous embodiment the arrangement further comprises means for mixing white water from a wire section (2) with the thick stock

According to an embodiment of the invention the means for deaerating stock 8 comprises a vacuum tank or a deaeration pump. Advantageously the means for deaerating stock 8 removes air from liquid by centrifugal force.

Advantageously the means for drying deaerated pulp 9 is a pulp drying machine.

According to an embodiment of the invention the thick stock is received from a pulp mill 7 and diluted with white water from a wire section 2 of a pulp drying machine 9. Stock comprising thick stock and wire water is deaerated using centrifugal deaerating apparatus 6 and deaerated pulp is produced. Deaerated pulp is then led to a head box 1 of the pulp drying machine 9 and formed onto a wire at the wire section 2. Water contained in the pulp is drained trough the wire as white water and recirculated in the process. Remaining water in formed pulp web is removed in one or more phases. Usually water removal phases comprise pressing and drying. After drying pulp web is cut and stacked into bales in baling 10.




 
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