Process for production of a fibrous pulp web

Abstract

A method for producing a fibrous pulp web, in which a fibrous pulp suspension is dewatered in a Twin Wire Former to form a fibrous pulp web which is dried with a TAD dryer. A pre-dewatering device with a hood and suction roll is provided after the Twin Wire Former and before the TAD dryer. The fibrous pulp web is guided through the pre-dewatering device between exactly two machine cloths. The machine cloth between suction roll and the web is a porous felt, and the machine cloth between web and hood is a structured wire. A volume flux of hot drying fluid greater than 100 m.sup.3/(m.sup.2.Math.min), preferably greater than 250 m.sup.3/(m.sup.2.Math.min,) is drawn through the fibrous pulp web into the suction roll.

Claims

1. The method for producing a fibrous pulp web, comprising: dewatering a fibrous pulp suspension between forming wires in a Twin Wire Former to form a fibrous pulp web; conveying the fibrous pulp web from the Twin Wire Former to a pre-dewatering device including a hood and suction roll where the fibrous pulp web is carried through the pre-dewatering device between the hood and the suction roll via guiding only between a felt on the suction roll and a wire between the fibrous web and the hood; in the pre-dewatering device, drawing a volume flux of hot drying fluid greater than 100 m.sup.3/(m.sup.2.Math.min) through the fibrous pulp web into the suction roll; and following the pre-dewatering device, drying the fibrous pulp web in a hot-air dryer on a structured wire and through air dryer (TAD) drum.

2. The method according to claim 1, wherein the fibrous pulp web is guided through the pre-dewatering device with a structured wire.

3. The method according to claim 2, wherein the drying fluid includes hot air at a temperature greater than 150 C.; a differential pressure in the pre-dewatering device between the hood and the suction roll is greater than 0.25 bar; and the flux of drying fluid through the fibrous pulp web is greater than 100 m.sup.3/(m.sup.2.Math.min).

4. The method according to claim 2, wherein the fibrous pulp web passes through first and second zones in succession in the pre-dewatering device, where the drying fluid is at a temperature greater than 200 C. and includes steam and hot air flowing through the fibrous pulp web in the first zone and only hot air flowing through the fibrous pulp web in the second zone; and a differential pressure in the pre-dewatering device between the hood and the suction roll is greater than 0.45 bar.

5. The method according to claim 1 wherein the fibrous pulp web formed on a forming wire of the Twin Wire Former is guided through the pre-dewatering device on said forming wire.

6. The method according to claim 5, wherein said forming wire that guides the fibrous pulp web is unstructured.

7. The method according to claim 1 wherein the drying fluid is hot air at a temperature greater than 150 C.

8. The method according to claim 7, wherein the drying fluid is moist hot air, with a moisture content greater than 150 g.sub.H2O/kg.sub.air.

9. The method according to claim 1, wherein the drying fluid is hot steam.

10. The method according to claim 1, wherein the fibrous pulp web passes through first and second zones in succession in the pre-dewatering device, where steam and hot air flow through the fibrous pulp web in the first zone and hot air flows through the fibrous pulp web in the second zone.

11. The method according to claim 1, wherein a differential pressure in the pre-dewatering device between the hood and the suction roll is greater than 0.25 bar.

12. The method according to claim 1, wherein the felt is a fine-pored material having through-pores between opposite exterior surfaces, with an average pore size of the felt surface facing the fibrous pulp web smaller than the average pore size of the felt surface facing the suction roll.

13. The method according to claim 1, wherein the fibrous pulp web is guided through the pre-dewatering device on the structured wire of the TAD drum.

14. The method according to claim 1, wherein the fibrous pulp web is fed to a Yankee after the TAD drum.

15. The method according to claim 1, wherein a contact roll confronts the suction roll to press the fibrous pulp web against the felt before the fibrous pulp web is guided between said felt and said wire.

16. The method according to claim 1, wherein the flux of drying fluid is greater than 200 m.sup.3/(m.sup.2.Math.min).

17. The method according to claim 1, wherein the drying fluid is at a temperature greater than 200 C.

18. The method according to claim 1, wherein the drying fluid includes moist hot air, with a moisture content greater than 150 g.sub.H2O/kg.sub.air.

19. The method according to claim 18, wherein the moist hot air has a moisture content greater than 300 g.sub.H2O/kg.sub.air.

20. The method according to claim 1, wherein a differential pressure in the pre-dewatering device between the hood and the suction roll is greater than 0.45 bar.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) Two representative embodiments are described with reference to the Drawing, in which:

(2) FIG. 1 shows a schematic (side) view of a tissue machine that is suitable for implementing the method according to the invention; and

(3) FIG. 2 shows schematic (side) view of another tissue machine according to the invention;

(4) Identical reference numerals in the two figures refer to the same components in each case.

DETAILED DESCRIPTION

(5) In FIG. 1, the pulp suspension is fed to a Twin Wire Former 18 through a headbox 1 between two forming wires 3, carried over a forming roll 2, and dewatered to a dry content of approximately 24% with the aid of vacuum boxes (not shown).

(6) Subsequently, the fibrous pulp web 9 is transferred at the transfer box 11 to a structured TAD wire 4. The structured TAD wire 4 can (but does not have to) be moved a little more slowly than the wire 3 so that the fibers can fit well into the indentations in the TAD wire 4 (wet crepe); the suction box 10 for wet structuring sucks the fibers into the structure of the TAD wire 4.

(7) Structured wires are known in this field of technology, and are sometime also called moulding wires. This can be understood as wire having a multiplicity of surface recesses or indentations into which the fibrous web can be locally deformed.

(8) After this, further dewatering takes place in the pre-dewatering device 20. The pre-dewatering device 20 has a hood 17, a felt 5, and a suction roll 16. The fibrous pulp web 9 attached to the TAD wire 4 can be pressed against the felt 5 and the suction roll 16 by the press roll (kiss press roll) 15. This improves contact between the felt 5 and the fibrous pulp web 9. The line force in this press nip is between 5 kN/m and 30 kN/m. At these pressures, only some 20% of the fibers are compacted, while the remaining 80% of the fibers are protected by the indentations in the TAD wire 4 and thus are not compressed.

(9) The fibrous pulp web 9 is carried through the pre-dewatering device 20 between the felt 5 and the TAD wire 4. The felt 5 is on the surface of the suction roller 16 and the web 9 is on the felt 5, trapped beneath the overlying TAD wire 4. The felt 5 has a fine-pored cross section to enhance capillary dewatering. Unlike the structure on a moulding wire, the fine pores of the felt do not deform the web.

(10) In a first zone, steam is blown onto the fibrous pulp web 9 through the hood 17 in an amount of more than 0.3 metric tons of steam per metric ton of fibrous pulp, where even more than 0.5 metric tons of steam per metric ton of fibrous pulp would be better, ideally even more than 1 metric ton of steam per metric ton of fibrous pulp.

(11) In the subsequent, second zone, the moist, hot air at a temperature of more than 150 C., preferably more than 250 C., is blown through the fibrous pulp web. The moisture content of the hot air blown in through the hood 17 should preferably be more than 150 g.sub.H2O/kg.sub.air, especially more than 300 g.sub.H2O/kg.sub.air, preferably even more than 450 g.sub.H2O/kg.sub.air.

(12) As the fibrous pulp web 9 is still very damp in the area of the pre-dewatering device 20, there is very little evaporation here. On the contrary, the heat supply reduces the viscosity of the water in the fibrous pulp web 9, which causes the water to be sucked out of the fibrous pulp web 9 through the suction roll 16. The fine-pored felt 5 enhances the dewatering process through capillary dewatering. The air volume added through the hood 17 is largely equal to the amount removed by suction through the suction roll 16. According to the invention, the air and steam volumes added are more than 100 m.sup.3/(m.sup.2.Math.min), especially more than 200 m.sup.3/(m.sup.2.Math.min), preferably even more than 250 m.sup.3/(m.sup.2.Math.min).

(13) The pressure in the hood 17 is higher than the ambient pressure here so that none of the cold, ambient air is sucked in through the suction roll 16.

(14) After the pre-dewatering device 20, the fibrous pulp web 9 with a dry content of more than 30%, especially more than 35% and preferably more than 40%, is transferred to the hot-air dryer 19. The hot-air dryer 19 is a TAD dryer, consisting of a TAD drum 13 and a TAD hood 14. With a dry content of 35% or more, there is no need for a second TAD drum.

(15) Subsequently, the fibrous pulp web is transferred from the TAD wire 4 to a Yankee cylinder 6 by means of a press roll 12. On the Yankee cylinder 6, the fibrous pulp web 9 is dried further by the hot air applied through the hood 7 and then scraped off. The surface of the Yankee is sprayed with chemicals by a coating device 8 so that the fibrous pulp web can be scraped off the surface of the Yankee more easily.

(16) FIG. 2 illustrates another device for implementing the method according to the invention. In contrast to FIG. 1, the pre-dewatering device 20 is not arranged inside the structured TAD wire 4, but before this inside the wire 3 of the Twin Wire Former 18. The felt 5 is on the surface of the suction roller 16 and the web 9 is on the felt 5, trapped beneath the overlying forming wire 3.

(17) The fibrous pulp web 9 is not transferred to the structured TAD wire 4 until after the pre-dewatering device 20. The wire 3 in the Twin Wire Former 18 can be a structured or a non-structured wire.