SE535820C2

SE535820C2 - Process for drying pulp using a perforated suction roll

Info

Publication number: SE535820C2
Application number: SE1051037A
Authority: SE
Inventors: Pekka Kylliainen
Original assignee: Stora Enso Oyj
Priority date: 2010-10-05
Filing date: 2010-10-05
Publication date: 2013-01-02
Also published as: SE1051037A1; WO2012046184A1; NZ608170A; RU2013120321A; RU2582464C2; BR112013008132A2; CN103154360A; EP2625332A1; EP2625332A4

Abstract

Abstract The present invention relates to a process for the drying of pulp whichprocess comprises the steps of, providing a pulp comprising cellulosic fibers,conducting the pulp to a wire, dewatering the pulp by the aid of a suction rollwherein the suction roll has a open surface area of above 60%. By increasingthe open surface area of the suction roll the dewatering of the pulp isincreased. The invention further relates to suction roll for the drying of pulp.

Description

PROCESS FOR THE DRYING OF PULP AND A SUCTION ROLLUSED FOR THE DRYING OF PULP.

Field of the invention The present invention relates to a process for the drying of pulp and asuction roll for the drying of pulp.

Background Pulp comprising cellulosic fibers is used for the production of paper orpaperboard. The pulp can be produced in many different ways and manydifferent cellulosic fibers may be used.

The produced pulp is conducted to a wire in order to form a fibrousweb which eventually will form a paper or paperboard. When the pulp and thepaper or paperboard is manufactured at the same location it is easy totransport the pulp, in a wet state, to the paper or paperboard machine.However, the production of the pulp and the production of the paper orpaperboard can also be done at different locations and it is then necessary totransport the pulp to the paper or paperboard manufacturing location. lt isthus desirable to dry the pulp before transportation in order to avoidexpensive transports due to unnecessary transport of water.

There are two major different type of pulp drying machines, i.e.fourdrinier and cylinder machines. The fourdrinier machine is most commonlyused and the pulp is deposited from a headbox to a continuous wire. Thewater in the pulp drains through the wire forming a pulp mat. After the initialfree drainage through the wire, the pulp mat is further dewatered, as it iscarried on the wire, by the application of increased vacuum. The vacuum isapplied to the underside of the pulp mat by vacuum boxes and/or a couch rolllocated prior to the removal of the mat from the wire. The pulp mat is thenconducted to the press section and the drying section in which further water isremoved.

There is a desire to maximize the dewatering of the pulp in order todecrease transportation costs of the pulp. lt is also advantageous to remove as much water as possible in an early stage of the dewatering or dryingprocess of the pulp since is becomes more expensive to remove water in thedrying section since it is a more energy demanding process compared to theuse of free drainage or vacuum, Dewatering can be maximized in many different ways. By optimizing the performance of the vacuum section, press section and dryer section of apulp drying machine, it is possible to increase the dewatering in an as efficientway as possible. it is also advantageous to keep the pulp temperature as highas possible in order to improve water removal since the viscosity of the wateris decreased at higher temperatures. Furthermore, it is preferred to keep thepH of the pulp low, preferably around 4-5, since the fibers are not as swollenat lower pH and thus facilitates drainage of the water.

Another possibility is to add chemicals to the pulp which improvesdrainage. One example is described in WOO218704.

However, there is still a need for an improved process for the drying ofpulp in an efficient way.

Summary of the invention lt is an object of the present invention to provide a process for thedrying of pulp in an improved and energy efficient way, Another object of the invention is to provide a suction roll for improveddrying of pulp.

These objects and other advantages are achieved by the processaccording to claim 1. The present invention relates a process for the drying ofpulp which process comprises the steps of, providing a pulp comprisingoellulosic fibers, conducting the pulp to a wire, dewatering the pulp by the aidof a suction roll wherein the suction roll has a open surface area of above60%. By increasing the open surface area of the suction roll the dewatering ofthe pulp is increased.

The suction roll preferably has a shell thickness of above 50 mm. lt ispreferred to increase the shell thickness of the roll shell since the open surface area is increased. ln this way the strength of the roll is still good and itis possible to still have high negative pressure (vacuum) or Suction in the roll.

The suction roll preferably comprises a doctor blade. lt is preferred thatthe doctor blade is an air knife. The doctor blade is used in order to clean theopen surface area of the suction roll.

The dry content of the pulp is preferably between 15-30% by weightafter passing the suction roll. lt is thus possible to increase the dry content ofthe pulp by the use of the suction roll with increased open surface area.

The present invention further relates to a suction roll for the drying ofpulp wherein the suction roll has an open surface area of above 60%. Byincreasing the open surface area of a suction roll it is possible to increase thedewatering of a pulp.

The shell of the suction roll has preferably a thickness of above 50 mm.lncreased shell thickness will increase the strength of the roll and thus make itpossible to further increase the open surface area of the roll and/or thenegative pressure of the roll, i.e. increased suction.

The suction roll preferably comprises a doctor blade. lt is preferred thatthe doctor blade is an air knife. The doctor blade is used in order to clean andremove the water from the open surface areas of the roll.

Brief description of the drawingsln the following, the invention will be described with reference to theappended drawings, in which Fig. 1 shows a schematic figure of the suction roll,Fig. 2 shows a schematic figure of the open area surfaces of the suction roll.Detailed description of the invention The present invention relates to process for the drying of pulp and asuction roll used for the drying of pulp. lt has been shown that by increasing the open surface area of the roll,i.e. increase the number and/or size of the holes of the shell of the roll, it ispossible to improve the dewatering of the pulp in a very energy efficient way.The amount of water drained from the pulp can in this way be increased butwithout increasing the energy needed, i.e. by using the same negativepressure, i.e. suction, of the roll. The increased open surface area will allowhigher negative pressure and it is thus possible to increase the negativepressure and increase the amount of water being dewatered from the pulp.

The amount of open surface area of the suction roll shell is above 60%of the total area of the roll shell, preferably above 70%. The open surfacearea is preferably in the form of several holes being evenly distributed overthe whole surface of the roll shell. lt is possible that the open surface area isbigger in the outer part of the shell and smaller in the inner part of the shell,i.e. the holes forming the open surface area are in a tapered form. The openarea surface of the suction roll according to the invention shall preferablyconstitute of more than 60% of the total area of the surface roll, seeing both tothe inner side or the outer side of the roll.

By increasing the size of the holes forming the open surface area ofthe roll shell, marking might occur on the pulp mat. These suction roll vacuummarkings may be beneficial in the subsequent drying steps of the pulp mat.The use of air drying is the most commonly used drying technique in pulpdrying machines and it is then beneficial to have as large surface area aspossible and the vacuum roll markings will increase the surface area of thepulp mat and thus improve the drying.

The thickness of the roll shell is preferably increased compared todrying rolls used in prior art. Due to the increased amount of open surfacearea of the roll it might be necessary or at least advantageous to increase thethickness of the roll shell in order to provide the roll with sufficient strength. ltis also possible to increase the negative pressure of the suction roll since thethickness of the roll shell is increased. The thickness of the roll shell ispreferably above 50 mm, even more preferred above 60 mm. There is anoptimal ratio between the amount of open surface area and thickness of theshell. lf the amount of surface area is high, the thickness of the roll shell is increased in order to be able to run the suction roll in the best way, i.e. mostefficient way.

The negative pressure (vacuum) inside the roll is preferably between10-100 kPa.

The suction roll is preferably provided with at least one doctor blade.The main purpose of the doctor blade is to remove the water from the opensurface area, i.e. from the holes. Since the centrifugal force of the roll is low, itis not possible for the drained water to be removed from the roll. The doctorblade will thus remove the water from the holes making sure that theefficiency of the suction roll is not decreased. The doctor blade may alsoremove impurities, such as deposits, from the suction roll, The doctor blade is preferably an air knife. The air knife comprises anair nozzle which directs pressurized air at the surface of the roll. The air knifemay comprise more than one air nozzles. lt is preferred that the width of theair knife covers the entire width of the shell surface. The width of the at leastone nozzle is preferably wide enough to allow the formation of an air knifecapable of removing water from the open surface areas of the roll, but narrowenough to minimize the amount of air required. The at least one air nozzle ispreferably located far enough from the surface of the roll shell in order toavoid deposit of the removed water and/or deposits on the nozzle itself, butclose enough to remove the water and/or deposits from the open surface areaof the roll. The air of the air knife should preferably be pressured to the pointwhere the air knife will be able to remove water and/or deposits.

By the process according to the invention it is possible to increase thedry content of the pulp already at the wet end of the pulp drying machine. lt ispreferred that the dry content of the pulp after passing the suction roll of theinvention is preferably between 15-30 % by weight. By increasing the drycontent of the pulp at an early stage in the pulp drying machine it is possibleto use higher pressure in the subsequent press section.

The suction roll according to the invention can be of any conventionaltype used for the drying of pulp and/or production of paper or paperboard,with the difference that the amount of open surface area is increased. Other components, such as type of shell, vacuum system or water removal system,etc can be of any conventional type.

The suction roll according to the invention can be used in anyconventional pulp drying machine or process. lt is preferred to use it insteadof a couch roll in a fourdrinier machine. lt may also be advantageous to use a lumpbreaker roll in combinationwith the suction roll. The lumpbreaker roll preferably comprises a felt, i.e. afelted lumpbreaker roll. The lumpbreaker roll preferably forms a nip togetherwith the suction roll. The nip pressure between the lumpbreaker roll and thesuction roll is preferably between 25-40 kN/m. lt is advantageous to use thelumpbreaker roll since it can increase the drying of the pulp since the drainedwater tends to go to the open surface area of the suction roll to a greaterextent.

Figure 1 and figure 2 shows a suction roll (1) according to theinvention. A wire (2) is conducting a pulp mat or web. The wire (2) is partlywrapped round the suction ro|| (1). The suction roll (1) comprises a roll shell(3) which rotates around a non rotating central roll core. The roll shell (3)comprises a number of holes forming an open surface area. As can be seenfrom figure 2, the open area surface (9) is in the form of holes that have beenformed by drilling of the roll shell (3). The open surface area (9) constitutes atleast 60% of the total surface of the roll shell (3). The holes makes sure thatthe pulp mat on the wire (2) will be exposed to the lower pressure (vacuum)inside the roll (1). The roll shell (3) is made of metal and the thickness of theroll shell (3) is above 50 mm.

The roll (1) further comprises vacuum seals (4) which make sure that itis possible to create a negative pressure inside the roll. The roll (1) furthercomprises a non-rotating suction quadrant (5). The quadrant (5) creates thenegative pressure or vacuum inside the roll. The water withdrawn from thepulp is removed by a white water removal system (6) which removes waterfrom inside the roll (1). The withdrawn water is also removed by the aid of anair Knife (7). The air knife (7) removes water from the open surface areas (9)of the roll shell (3). The roll (1) also comprises an end plate (8).

The pulp may be of any conventional type. lt can be mechanical,chemimechanical, chemithermomechnical and/or chemical pulp. Thecellulosic fibers of the pulp may be of any conventional kind wood cellulosefibers, such as hardwood fibers, softwood fibers or agricultural raw materialsor waste products. ln view of the above detailed description of the present invention, othermodifications and Variations will become apparent to those skilled in the art.However, it should be apparent that such other modifications and Variationsmay be effected without departing from the spirit and scope of the inventiont

Claims

1. Claims ._Å _ A process for the drying of pulp which process comprises the steps of: - providing a pulp comprising cellulosic flbers, - conductlng the pulp to a wire, - dewatering the pulp by the aid of a suction roll wherein thesuction roll has a open surface area of above 60%. The process according to claim 1 wherein the suction roll has ashell thickness of above 50 mm. The process according to any of the preceding claims wherein thesuction roll comprises a doctor blade The process according to claim 3 wherein the doctor blade is an airknife. The process according to any of the preceding claims wherein thedry content of the pulp is 15-30 % by weight after passing thesuction roll. A suction roll for the drying of pulp wherein the suction roll has anopen surface area of above 60%. The suction roll according to claim 6 wherein the shell thickness ofthe roll has a thickness of above 50 mm: The suction roll according to any of claim 6-7 wherein the suctionroll comprises a doctor blade. The suction roll according to claim 8 wherein the doctor blade is anair knife.