WO2001031118A1 - Use of surfactants in press section of paper machine to enhance water removal - Google Patents

Abstract

The present invention provides a method for manufacturing paper and paper board products which involves the use of surfactants in the press section (14) of a paper machine to enhance water removal from the web. According to the method of the present invention, one or more surfactants is applied to the press section felts (24) before said felts contact the web (22). The presence of the surfactants in the press section felts increases the water removal capacity of the felts by reducing the interfacial surface of the water passing through the felts. In addition, a portion of the applied surfactants may be transferred from the press section felts to the web thereby reducing the interfacial surface tension of any remaining free and interstitial water in the web. Once the interstitial surface tension of the water in the web is reduced, it is far easier to remove such water in subsequent pressing operations. The method of the present invention increases the consistency of the web during pressing and thus results in a substantial reduction in energy needed in the dryer section of the paper machine. Thus, on dryer or press limited machines, the invention facilitates an increase in machine speed.

Description

Title : USE OF SURFACTANTS IN PRESS SECTION OF PAPER MACHINE TO ENHANCE WATER REMOVAL

Field of Invention

The present invention relates to a method for manufacturing paper and paper

board products. More particularly, the present invention relates to a method for

manufacturing paper and paper board products which involves the use of surfactants in

the press section of a paper machine to enhance water removal from the web.

Background

All systems for manufacturing paper and paperboard products, hereinafter

collectively referred to as paper, include a series of operations and processes. Typically,

wood is either digested chemically or comminuted mechanically to form pulp. Pulp

produced by chemical digestion must be washed, and pulp produced by both methods is

often bleached. For most applications, the pulp must be refined before it is used to

manufacture paper.

Refining consists of pumping an aqueous pulp slurry through a series of metal

discs moving at a high speed. During refining, the cellulose fibers are cut and macerated

in order to develop fibrillation. Fibrillation increases the number of inter-fiber contacts

that will occur during formation of the paper and bonding during subsequent pressing and

drying operations. A sheet that is formed from unrefined pulp will generally have a low density and be rather soft and weak, whereas if the same pulp is well-refined, the resultant

paper will be much more dense, hard, and strong. After refining, the aqueous pulp slurry is reduced in consistency by the addition

of white water. At this stage in the process, the diluted aqueous pulp slurry is referred

to as furnish. The furnish has a solids concentration, which is referred to as consistency,

typically within the range of from between 0.2 and 1%. In general, the lower the

consistency of the furnish, the better the formation and homogeneity of appearance of the

finished paper. Functional additives, usually in a cationic form, are conventionally added

to the furnish so that they can be attached to and retained by the fibers.

The furnish is usually fed to a headbox apparatus through one or more screens or

other filtering devices to remove impurities. The furnish then enters a flow spreader

which provides a uniform flowing stream along the width of the paper machine. The

flow spreader discharges the slurry into a headbox where fiber agglomeration is prevented

by controlled turbulence. Pressure is provided to cause the slurry to flow at the necessary

velocity through the slice and onto a moving endless band of a woven synthetic fiber or

metal fabric called a wire. On a modern machine, the wire can be moving at a speed of

about 700 to 2000 meters per minute. Continuous sheet forming and drying can be

accomplished using three different types of equipment: the cylinder. Fourdrinier (i.e..

single wire), and twin- wire machines, all of which are well-known in the art.

In the cylinder machine, a wire-covered cylinder is mounted in a vat containing

the refined fiber slurry. As the cylinder revolves, water drains inward through the screen.

thus forming a paper web on the outside of the cylinder. The wet web is removed at the top of the cylinder, passes through press rolls for water removal, and is then dried over

steam-heated, cylindrical drums. The Fourdrinier machine is more complex than the cylinder machine and basically

consists of a long continuous synthetic fiber wire which is supported by various means

to facilitate drainage of water. The fiber slurry, which is introduced at one end of the

machine through a headbox and slice, loses water as it progresses down the wire, thereby

forming the web. The web is then directed to the press and dryer sections as in the

cylinder machine.

The twin- wire machine is the latest development and consists essentially of two

opposing wires. Twin-wire formers have replaced the Fourdrinier, particularly for

lightweight sheets, e.g., tissue, towel, and newsprint. Twin-wire formers also are

operated successfully on fine paper, corrugated media, and liner board grades. In

twin-wire formers, the water is drained from the slurry from both the top and bottom of

the sheet. The two wires, with the slurry between, are wrapped around a cylinder or set

of supporting bars or foils. The tension in the outer wire results in a pressure which is

transmitted through the slurry to the supporting structure. Some machines also use

vacuum to produce the required pressure. The pressurized slurry drains through one or

both of the wires. The web is then directed to the press and dryer sections as in the other

papermaking machines.

In most paper machines, the wire is mounted over a breast roll at the intake end

and at a couch roll at the discharge end. Between these two rolls, the wire is supported

for the most part by table rolls, foils and suction boxes. A substantial vacuum is

developed in the downstream nip between the table roll and the wire. This promotes

water drainage from the slurry on the wire. As speeds increase, however, the suction can

- become too violent and deflect the wire, causing the paper web to be thrown into the air.

A more controlled drainage action is accomplished by the use of foils. Foils are

wing-shaped elements which support the wire and induce a vacuum at the downstream nip. Foil geometry can be varied to provide optimum conditions. After passing over the

foils or table rolls, the wire and sheet pass over suction boxes, where more water is removed. Most machines also include a suction couch roll for further water removal.

In its most typical form, the formation of the paper web takes place in the first few

feet on the wire. The stock issuing from the slice is a suspension of fibers in water,

typically containing from 0.2 to 1.0% solids in a layer some 6-18 millimeters deep and

up to several meters wide. It is deposited on and drains through the wire. At very low

speeds, the force of gravity predominates in causing the drainage. At higher speeds, the

action of gravity becomes negligible compared with the pumping action of the drainage

elements (i.e., the table rolls or foils). A visible change occurs in the appearance of the

stock as it proceeds down the wire when its concentration reaches about 2%. At this

level, the surface ceases to appear mobile, loses its liquid sheen, and takes on a matte

appearance. At this point in the process, the drainage elements are no longer effective

for removing water because the web is formed. Next, consolidation begins, assisted by

the action of the suction boxes. Some slight rearrangement of the fibers may still be achieved by the pressure of an overhead roller, called a dandy roll.

The sheet leaving the wet end has a consistency of 18-23%. Conventionally,

additional water is removed from the sheet mechanically without adversely affecting

sheet properties. This is achieved using one or more rotary presses in the press section of the paper machine. Press rolls may be solid or perforated and often suction is also

applied through the interior of the rolls. The sheet is passed through each rotary press on

continuous felts, usually one and sometimes two for each press, which act as conveyors and porous receptors of water. Conventionally, the solids content of the sheet can be

increased by such pressing to a consistency of about 30 to 40% without crushing.

Crushing, which refers to the direct flow of water in the sheet, occurs when too

much pressure is applied to the wet sheet by the presses. Crushing can be minimized by

applying pressure gradually, since less water is initially removed this way and the fibers are not so likely to be pushed apart. The sheet can stand higher and higher pressure as

water is removed and the sheet becomes stronger. Graduated pressure is particularly

important on heavy boards inasmuch as the danger of crushing increases for greater

thicknesses of paper product. Pressing multi-cylinder boards while they are too wet may

also lead to ply separation as well as crushing.

At a consistency of about 40%, it is no longer feasible to remove additional water

from the sheet using conventional felted rotary presses and evaporative drying must be

employed. This is a costly process and often is the production bottleneck of

papermaking. The dryer section usually includes a series of steam-heated cylinders.

Alternate sides of the wet paper are exposed to the hot surface as the sheet passes from

cylinder to cylinder. In most cases, except for heavy board, the sheet is held closely

against the surface of the dryers by a fabric having carefully controlled permeability to

steam and air. Heat is transferred from the hot cylinder to the wet sheet, and water

evaporates. The water vapor is removed by way of elaborate air systems. Most dryer

-3- sections are covered with hoods for collection and handling of the air, and heat recovery

is practiced in cold climates. The final consistency of the dry sheet is usually between

about 92-96 weight percent, depending upon the type of paper product being

manufactured.

The efficiency of the drying sequence is dependent upon such factors as the amount of applied pressure which squeezes the wet web between the felts, the efficiency

with which water condensed within the dryer cylinder is physically removed, and the

ventilation of the pockets between dryers. During the drying sequence, the consistency

of the product is increased from the entry level of generally about 30-40% up to that of

the emerging dry paper product, i.e., 92-96%.

The energy requirements for removal of water depend upon the form of water

which is present in the paper product. The majority of free water, being that which exists

over and above what is required to saturate the fibers, can be removed on the wire by

gravity or suction. Most of the remaining free water and some of the interstitial water can

be removed from the web in the press section using a conventional felted rotary press

operation. The most tenaciously held water (i.e., that within the lumen and pores of the

fiber wall) must generally be accomplished in the dryer section of the paper machine

using thermal drying.

During the early stages of drying, the fibers are free to slide over one another, but

as the free water is driven off. the fibers are drawn closer together and bonding begins to

take place. Surface tension is primarily responsible for drawing together the fibers in this

stage, but later, molecular attraction brings about the final bonding between fibers. No appreciable fiber-to-fiber bonding takes place until the consistency is raised above about

40 percent, but once this critical drying point is reached, shrinkage begins to take place

and bonding begins.

It will be appreciated that the greatest energy use in the entire papermaking

process occurs in the dryer section of the paper machine. Thus, any significant increase

in the consistency of the web which can be accomplished in the press section of the paper

machine will necessarily result in a substantial energy and cost savings.

U.S. Pat. Nos. 4,684,440 and 5,114,539 to Penniman et al. describe a method for

reducing the water content of the web in the press section of a papermaking machine

using non-polar organic solvents (e.g., odorless kerosene) and water-insoluble liquid hydrocarbons (e.g., ISOPAR G, a proprietary aliphatic iso-paraffinic compound

manufactured by Exxon Corporation), respectively. According to this method, non-polar

organic solvents or water-insoluble liquid hydrocarbons are applied directly to the web

or to the felt surfaces of the rotary press for transfer to the web. These compounds, which

are more readily absorbed by the web than water, displace and replace at least a portion

of the interstitial water in the web during pressing. According to this method, the

consistency of the web can be increased by as much as 10 percent during pressing.

Furthermore, since the compounds added to the web are more volatile than water, they

are more readily removed from the web during drying resulting in an energy consumption

savings of as much as 40 percent.

One of the obvious drawbacks of the method described in the Penniman et al.

patents is the use of non-polar organic hydrocarbon solvents in the papermaking process. These compounds are more expensive than water, some can present odor and toxicity

problems, and some can increase the possibility of fire and/or explosion. Moreover, use

of these solvents requires modification of the paper machine itself such that the

volatilized solvents can be separated and recovered from the water vapor leaving the

dryer section of the paper machine. A method is needed whereby the consistency of the

web can be significantly increased in the press section of a paper machine which does not

involve use of volatile organic compounds and which requires no substantial modification

of the paper machine.

Summary Of The Invention

The present invention relates to a method for increasing the rate of water removal

from a web of paper during pressing in the press section of a paper machine comprising

treating the felt or felts used in the press section with one or more surfactants before the

felt or felts contact the web. Virtually any water soluble or dispersible compound that

reduces the surface or interfacial tension of water can be used as a surfactant in the

invention, but surfactants with an HLB value of from about 4 to about 10 are currently

the most preferred surfactants for use in the invention.

The surfactants are not applied to the press section felts to displace or replace or

substitute for water in the web. but rather they are applied to improve the ability of press

section felts to absorb and transfer water from the web by reducing interfacial surface

tension of the water passing through the felt fibers. Moreover, during pressing the press

section felts transfer a portion of the surfactants from the press section felts to the web thereby reducing the surface or interfacial tension of the remaining free and interstitial water in the web. Once the surface or interfacial tension of the remaining free and

interstitial water in the web is reduced, it can be removed at a greater rate during pressing.

This will have a positive effect upon the operation during subsequent drying steps.

The method of the present invention has several distinct advantages when

compared to prior art methods. The surfactants used in the present invention are

generally more cost effective than the compounds used in prior art methods, and

substantially lesser amounts of surfactant must be added to the press felts in order to

obtain comparable results. Furthermore, the surfactants used in the invention are non-

toxic, water-dispersible compounds that do not present fire or explosion hazards.

Furthermore, practice of the invention does not require modification of the dryer section

of the paper machine.

The foregoing and other features of the invention are hereinafter more fully

described and particularly pointed out in the claims, the following description setting

forth in detail certain illustrative embodiments of the invention, these being indicative,

however, of but a few of the various ways in which the principles of the present invention

may be employed.

Brief Description of the Drawings In the annexed drawings:

FIG. 1 is a schematic illustration of a typical Fourdrinier paper-making machine;

and,

FIG. 2 is a schematic illustration showing a surfactant according to the method

of the present invention being applied to one embodiment of a felted press roll assembly. Detailed Description

The present invention generally relates to the application of a water-soluble or

dispersible surfactant to a press felt before the press felt contacts the web. After a

sufficient amount of a surfactant has been applied to a press felt, the web is pressed

against the press felt as the web and press felt pass through a press nip. During pressing,

water is pressed out of the web in order to increase its consistency. The water passes from the web into or through the press felt where it will later be removed by suction.

When exiting the nip, the pressed web will try to reabsorb or be rewet by water, since all

water cannot be removed by the press nip. According to the invention, however, the

water-dispersible surfactant reduces the interfacial surface tension of any water it comes

into contact with, both in the web and in the press felt, increasing the water removal

capacity of the press felt. The economic effect of the improvement in water removal rate

is significant because less energy must be used to further dry the web in the dryer section

of the papermaking machine. Use of the present invention may also inhibit the deposition

of undesirable organics in the felt and thus it may help to reduce the use of cleaning

chemicals.

Referring to the drawings, and initially to Fig. 1 , there is schematically illustrated

a typical Fourdrinier paper machine 10. Machine 10 includes a flow spreader 1 1, a

headbox 12, a Fourdrinier table 13, a press section 14, a dryer section 15. a calender stack

16 and a reel 17. As best seen in Fig. 2, the surfactant of the present invention is used in the press section 14. Press section 14 includes a pair of rolls 20 that press paper 22.

Contacting paper 22 is a continuous loop of felt 24. Felt 24 is conditioned with a detergent shower 26 and a high pressure shower 28. Of course, care must be taken to

prevent the felt washes or detergent from stripping the beneficial surfactant. Press section

14 includes a flooding shower 30 and lubricating shower 32. The vacuum box or felt

suction pipe 34 is located after the lubricating shower 32. In accordance with the present

invention, a surfactant is applied to the felt 24 at shower 38. Other locations of course,

such as 39 or 40 may be utilized to apply the surfactant.

Any one or a combination of water-soluble or dispersible non-ionic surfactants,

being agents that reduce surface interfacial tension when dissolved in or mixed with

water, can be utilized in the invention. Preferably, the surfactant has an HLB number of

from about 4 to about 10. The HLB number (hydrophilic-lipophilic balance) of an

emulsifier is calculated according to the following formula: HLB=(100-L)/15 in which

L denotes the percentage by weight of the lipophilic group relative to the weight of the

entire molecule.

The use of a surfactant or combination of surfactants with an HLB value that is

too high will result in undesirable foaming. The use of a surfactant or combination of

surfactants with too low of an HLB value will result in plugging or ineffective bridging

between the felt and the water. When selecting the proper surfactant, care must be taken

to utilize a surfactant that is compatible with the type of roll covers being used in the

press (i.e. the surfactant must not damage or attack the roll covers). Also, the type of felt

being used in the machine should be considered when selecting the proper surfactant.

Felts generally comprise nylon or some other synthetic fiber. Different surfactants may-

have varied attractions to certain types of felt. Sulfonate-type surfactants are generally not suited for use in the present invention for they tend to lead to excessive foam

formation. The surfactants utilized in the present invention must be no or low foaming.

Also, since the surfactant may come into contact with the paper fibers, FDA food contact issues may need to be taken into consideration when selecting the surfactant. Also, it is

believed that cationic and anionic surfactants are generally not suitable for use in the

present invention.

Presently, the most preferred surfactant is a polyethylene glycol 400, which is

available as FELTMASTER 15-LF from GEO Specialty Chemicals of Charlotte, North

Carolina. Examples of other general types of surfactants suitable for use in the invention

include alcohol ethoxylates; alkylphenol ethoxylates; glycerol esters; polyoxyethylene

esters; anhydrosorbitol esters; ethoxylated anhydrosorbitol esters; natural ethoxylated

fats, oil, and waxes; glycol esters of fatty acids, diethanol amine condensates (amides);

monoalkanol amine condensates (amides) and poly alkylene oxide block polymers.

It will be appreciated that water-dispersible surfactants used in the invention

advantageously have little or no odor. Accordingly, they can be used in various paper

making operations without need for special water treatment equipment and do not raise

occupational health concerns. Moreover, because the surfactants used in the invention

are water-dispersible, they present virtually no risk of fire or explosion (i.e. they have a

high flash point and are substantially non-combustible).

If the surfactant is a liquid, it can be applied in a substantially pure form directly

to the press felt, preferably by spraying. Of course, other techniques such as misting or

showering may be employed. It will be appreciated that if the surfactant is a solid, it must be dissolved in water before being applied to the press felt. Preferably, whether the

surfactant is a liquid or a solid, it is applied to the press felt as a solution with water by

misting, spraying, showering or dripping onto either side of the felt surface in a manner

similar to that used for cleaning the felt. The surfactants may also be advantageously

applied from the inside of a press section roll, and directed close to the nip so that the

force of pressure can also be used to dispense water from the web.

The amount of surfactant utilized is not per se critical to the method of this

invention. From a practical standpoint, a minimum amount of surfactant should be used

in order to minimize cost. The minimum necessary amount of surfactant to be applied

to the felt will depend upon the speed of the press (i.e. how fast the felt moves) and the

amount of water being contacted and can be routinely determined by one skilled in the art. The felt surfaces must be uniformly and partially wetted by the surfactant for

optimum results (preferably by spraying). Complete saturation of the felt by the

surfactant is counterproductive because the felt can no longer absorb water from the web.

Preferably, some of the surfactant is available for transfer to the web to assist in the

reducing the surface interfacial tension of the free and interstitial water in the web. With

these considerations in mind, the lowest practical application limit would be an amount

sufficient to at least partially coat the press felt before the press felt contacts the web.

The maximum practical amount utilized would be influenced by cost and performance

considerations. At present, it is believed that application of surfactants at a rate of from

about 0.25 pounds to about 1 pound per ton of finished paper is optimal. Excessive use could impact upon paper sizing operations.

-ι: The use of surfactants as described in the present invention increases the water

transfer capacity of the press felt. It is believed that the surfactants reduce the surface

interfacial tension of the water within the felt fibers making the water pass through the

felt more easily. In addition, at least a portion of the surfactants are believed to be

transferred to the web thereby reducing the surface interfacial tension of any remaining

free and interstitial water in the web. Once the surface interfacial tension of the water in

the web has been reduced, it can more easily be removed by pressing in the press section

of the papermaking machine. Thus, there is less to be removed during subsequent drying.

When a surfactant is applied to the press felts as disclosed herein, an increase in

consistency of the web exiting the press section is achieved as compared to the

consistency of a web exiting from an untreated press section. The increase in consistency

can range from about 0.1% to about 5%, with increases of at least about 0.5% to about

1.2% being most preferred. Over and above the consistency increase, use of surfactants

as described herein will lead to less of a need to remove water in the dryer section. Relative to paper machines that have limited drying ability, the use of the present

invention facilitates higher machine speeds. Additionally, the use of the present

invention can lower steam consumption since there is less water to remove from the

paper.

The water-dispersible surfactants used in the invention preferably do not need to

be recovered from waste water through special treatment. The preferred water-dispersible surfactants are biodegradable and can be discharged into conventional water treatment

facilities without special treatment. The scope of the invention is further described in connection with the following

examples which are set forth for the sole purpose of illustrating the preferred

embodiments of the invention and which are not to be construed as limiting the scope of

the invention in any manner.

Example I

Feltmaster 15-LF surfactant was applied to the felts of a paper machine at a rate

of about 0.2 - 0.3 pounds per ton of finished paper. Use of the surfactant yielded a more

open and drier felt. Use of the surfactant also yielded about a 4,500 pound/hour decrease

in steam usage.

Example II

Feltmaster 15-LF surfactant was applied to the felts of a paper machine at the rate

of about 0.25-0.3 pounds per ton of finished paper. Use of the surfactant resulted in

significantly drier felts in the machine. Moister levels exiting the first press fell from about 69.5% to as low as about 66% over a 5 hour period.

Claims

What is claimed is:

1. A method for improving the rate of water removal from a web of pulp

fiber in a paper-product making machine having a felted press section comprising

applying a water-dispersible non-ionic surfactant to a press section felt before the press

section felt contacts the web and pressing the web against the press section felt to remove a portion of the water from the web.

2. The method of claim 1 wherein the water-dispersible surfactant is applied

to the press section felt in an amount sufficient to increase the consistency of the web

exiting the press section of the paper-product making machine by at least about 0.1% as

compared to the consistency of a web that has been pressed by a press section felt to which no water-dispersible surfactant has been applied.

3. The method of claim 1 wherein the water-dispersible surfactant is applied

continuously to the press section felt at a rate of from about 0.25 pounds to about 1 pound

per ton of finished paper-product produced using the paper-product making machine.

4. The method of claim 1 wherein the press section felt passes through a

plurality of press nips during pressing and the water-dispersible surfactant is applied to

the press section felt prior to entering each press nip.

5. The method of claim 1 wherein the water-dispersible surfactant is applied

to a press section felt before the press section felt and web pass through a final press nip.

6. The method of claim 1 wherein the water-dispersible surfactant comprises

a surfactant having an HLB value of from about 4 to about 10.

7. In a paper-product manufacturing process using a paper-product making

machine which deposits a pulp fiber furnish upon a wire to form a web, removes a

portion of the water from the web by pressing the web against a felt surface in a press

section, and dries the pressed web, the improvement comprising applying a water-

dispersible non-ionic surfactant to a press section felt before it contacts the web to

improve the water removal efficiency of the press section felt during the manufacture of

the paper product.

8. The method of claim 7 wherein the water-dispersible surfactant is applied

to the press section felt in an amount sufficient to increase the consistency of the web

exiting the press section of the paper-product making machine by at least about 0.5% as

compared to the consistency of a web that has been pressed by a press section felt to

which no water-dispersible surfactant has been applied.

9. The method of claim 7 wherein the water-dispersible surfactant is applied

continuously to the press section felt at a rate of from about 0.25 pounds to about 1 pound

per ton of finished paper-product produced using the paper-product making machine.

10. The method of claim 7 wherein the press section felt passes through a

plurality of press nips during pressing and the water-dispersible surfactant is applied to

the press section felt prior to entering each press nip.

11. The method of claim 7 wherein the water-dispersible surfactant is applied

to a press section felt before the press section felt and web pass through a final press nip.

12. The method of claim 7 wherein the water-dispersible surfactant comprises

a surfactant having an HLB value of from about 4 to about 10.

13. The method of claim 12 wherein said surfactant comprises a material

selected from the group of alcohol ethoxylates; alkylphenol ethoxylates; glycerol esters;

polyoxyethylene esters; anhydrosorbitol esters; ethoxylated anhydrosorbitol esters;

natural ethoxylated fats, oil, and waxes; glycol esters of fatty acids, diethanol amine

condensates (amides); monoalkanol amine condensates (amides) and poly alkylene oxide

block polymers or mixtures thereof.