Encyclopedia of Chemical Processing and Design

ICIATE EDITOR . CUNNINGHAM 1iversity of Texas

Austin , Texas

IN HAPPEL tbia University ork, New York

Encyclopedia of Chemical Processing and Design EXECUTIVE EDITOR

ASSOCIATE EDITOR

33

John J. McKetta William A. Cunningham

Organic Liquids, Thermal Conductivity Estimation to

Peat Supply-Demand · Relationships

828833

c:Mitchef! cM.emoziaf .fibzazg

c:Mi,HiJJippi eState 'llniveu.it!f

MARCEL DEKKER, INC. NEW YORK AND BASEL

Library of Congress Cataloging in Publication Data (Revised) Main entry under title:

Encyclopedia of chemical processing and design .

Includes bibliographical references. I. Chemical engineering- Dictionaries. 2. Chemistry,

Technical- Dictionaries. I. McKetta, John J. II . Cunningham, William Aaron. TP9.E66 660.2'8'003 75 - 40646 ISBN: 0-8247-2480-1

COPYRIGHT@ 1990by MARCEL DEKKER, INC. ALL RIGHTS RESERVED.

Neither this book nor any part may be reproduced or transmitted in any form or" by any

means, electronic or mechanical, including photocopying, microfilming, and recording. or by any information storage and retrieval system, without permission in writing from

the publisher.

MARCEL DEKKER, INC. 270 Madison Avenue, New York, New York, 10016

LIBRARY OF CONGRESS CATALOG CARD NUMBER: 75 - 40646

ISBN : 0-8247-2483-6

Current printing (last digit): 10 9 8 7 6 5 4 3 2 I

PRINTED IN THE UNITED STATES OF AMERICA

I nternation

RAY C. ADAM Fonner Chairman of the Boar< N. L. Industries, Inc. New York, New York

M. A. ALLAWALA Managing Director National Refinery Ltd. Karachi, Pakistan

HAMED H. AMER Chairman Agiba Petroleum Co. Cairo, Egypt

R. G. ANTHONY Professor, Department of Cher

Engineering Texas A & M University College Station, Texas

H.J. AROYAN Fonner Vice President Chevron Research Company Richmond, California

F. SID ASKARI President Technolog, Inc. Engineering and Industrial COJ

sultants Tehran, Iran

DONALD l. BAEDER Fonner Executive Vice Preside

Science and Technology Occidental Petroleum Corpora Los Angeles, California

Wm. A. BAILEY, Jr. Fonner Director, MTM Proce!

search and Development Lal Shell Development Company Houston, Texas

TRAVIS W. BAIN Vice President National Sales, Inc. Jackson, Mississippi

GAREN BALEKJIAN C. F. Braun Arcadia, California

CESAR BAPTISTE Vice President Petroleos Mexicanos Mexico City, Mexico

LEON R. MARTINEZ BASS Sales Manager-Northern Mex Zincamex, S. A. Saltillo, Mexico

ROBERT 0. BATHIANY Technical Planner Weyerhauser Company Tacoma, Washington

312 Paper Manufacture

Palladium (see Platinum Group Metals)

Palladium Catalysis (see Precious Metals Catalysis)

Paper Manufacture

Paper manufacture is one of the largest industries in the civilized world, and its usage is still growing rapidly. The major applications are, first, in communications-newspapers, magazines, books, pamphlets, and in computer readouts. The second major use is in packaging-wrapping paper, bags, cartons, corrugated cardboard, and the like. There are also a host of smaller applications such as insulation, lightweight structures, sporting goods, cleaning, and medical items.

The growth in demand could well outstrip the earth's supply of the basic raw material, trees, except for two mitigating tendencies: the use of thinner grades of paper and the substitution of plastics for paper. The latter are usually made from petroleum or natural gas raw materials. Recycling wastepaper and more efficient processing are also helpful.

Nearly all paper is now made from wood cellulose pulp. The pulping process is described in a separate section, and it varies somewhat depending on the type of wood being converted. Basically the process accomplishes the separation of the source material into fine fibers with more or less removal of lignin or other nonfibrous adjuncts. Some specialty papers are made from sources of cellulose other than wood, such as rice hulls, hemp, bagasse, etc.; but a pulping process similar to that used for wood chips is still necessary. In the very early days of papermaking, raw materials were used which were already substantially pure cellulose, such as rags. In these cases, pulping as such could be omitted and the source material merely mechanically chopped up, washed, and bleached. Very high grade writing paper is sometimes still made from rag stock. Manufacture of the cheapest newsprint also skips the normal pulping operation, i.e., removal oflignin from cellulose. In this case a high grade of wood is mechanically ground, washed, and bleached to provide newsprint feedstock.

After pulping, the papermaking process may be subdivided into the following operations.

Paper Manufc

Manufacture

zed world, and ;t, in commun-

in computer g paper, bags, host of smaller g goods, clean-

Jly of the basic use of thinner tter are usually vastepaper and

>. The pulping rhat depending ~omplishes the less removal of tre made from >,bagasse, etc.; II necessary. In ~d which were ses, pulping as ,ically chopped ;ometimes still t also skips the e. In this case a :hed to provide

vided into the

Paper Manufacture 313

A. Stock Preparation

Included in this operation are preparation of a thick slurry of pulp in water, "beating," hydropulping or refining to reduce the thickness of fibers (disintegrating thick clumps or bundles to fine individual fibers or fibrils), addition of various chemicals to modify properties of the finished paper, a final cleaning to remove dirt, sand, lumps, etc., and dilution with more water.

B. Sheet Formation

This is a key step in papermaking, and on a commercial scale is carried out at high speed on a moving filter medium such as a wire or cloth on a Fourdrinier machine or on rotating cylinders or moving belts. Multi-ply sheets can be made by repeated deposition of fibers. In this step a very dilute slurry (less than 1% in solids content) is run onto the filter medium, and water is rapidly drained through the support to yield a sheet of somewhat interlocked fibers. The gravity dewatering action is aided by vacuum, pressure from an adjacent sheet, or even centrifugal force.

C. Drying and Calendering

The moist sheet leaving the forming area has little strength until it is dried from an initial moisture of70 to 80% by weight to approximately 4 to 10% by weight.

The wet sheet may be picked up from the forming section on a carrier felt and transferred to a series of heated steel rolls, where moisture is removed by evaporation. The initial rolls may provide for blowing hot air through the wet paper. The steam heated rolls or "drying cans" are staggered so that both sides of the paper are dried. At tne end of the drying train there will usually be one or more pairs, or a stack, of calender rolls which smooth and densify the paper with a pressure of several hundred pounds per inch. The calendered paper is generally received on a large roll holding several tons of paper.

D. Finishing

Finishing of paper is a concept which may include several different steps along the process. It may include the special additives such as dyes and pigments added to the pulper or beater to enhance color and opacity, or strength additives and fillers, also added in the stock preparation area.

It may include resin or latex dipping or spraying steps included part way along the drying train. Or it may include the application of finish coatings at the calendering stage.

In any case, the roll of paper on the take-up reel is first trimmed to exact width with cutting knives at the edges, and then generally taken to a separate station for cutting into smaller widths for specific uses.


It may be subjected to "creping" where the sheet is jammed against a mechanical resistance as it is unwound and rewound. It may be further treated with resins or surface finishing agents.

Finally, the product is "converted" into specific types of end products.

Details of Papermaking Process

A . Stock Preparation

In most paper mills the cellulose pulp is stored in the form of large sheets, either dry for purchased material or moist ("wet lap") for pulp produced on site. It is generally handled on pallets, and when ready to be used, is dumped from pallets into a machine for quickly breaking up the sheets (about 40 in. 2

and 1/s-in. thick) and slurrying the fibers in water. A typical machine for accomplishing this task is the Beloit hydropulper

shown in Fig. 1. A powerful agitator is driven from the bottom. Its curved knife-like blades provide vigorous agitation for the whole contents of the vessel as well as impelling sufficiently fine fibers through the slotted bottom

Mill water To stock tank

Additives

Claflin refiner

Beloit pulper

FIG. 1. Pulper system.

Paper ManufactL

11

tl 0

1~

d k n lc le n lc S1 n gi n Sl

A B a< rr

st p;

dl re

th a< T P< di P< In

sh ra tu th gr sli m In

th

ru ca ell "I=

· Manufacture

nmed against a may be further

f end products.

of large sheets, 1lp produced on 1sed, is dumped ts (about 40 in. 2

oit hydropulper tom. Its curved contents of the ~ slotted bottom

To stock tank

FD


into the small chamber below. Agitator power may be as high as 100 hp per thousand gallons of charge capacity. The charge capacity would be of the order of3000 to 6000 gal, and the run time, 15 min to 1 h, part of which time is devoted to circulating the slurry through the refiner, a high-speed rotating device in which internal conically disposed knives move past stationary knives or bars. A tight clearance is maintained by axial pressure against the rotating cone. The concentration of pulp in water, known as consistency, leaving this system is generally of the order of 1 to 2%, depending on fiber length. The longer the fibers, the lower the consistency must be in order to maintain a reasonable flowability. Besides cellulose pulp (or other fibers) loaded into the pulper, many other additives are generally introduced at this stage. These may be strength additives such as urea-formaldehyde resins, melamine-formaldehyde resins, rosin, or various natural gums. Synthetic gums such as polyethylene oxide derivatives and carboxymethyl cellulose may be used. Pigments and dyes are added for control of transparency, smoothness, and color. The cheapest of these is calcium carbonate or clay.

For effective whiteness, titanium dioxide is used in smaller quantities. Alum is used in many cases to promote adhesion of the other additives. Biocides may be added to prevent slime or mold growth. The total quantity of additives may be as much as 30% of the weight of the pulp, but is generally much less, of the order of 3 to 5%.

The thick slurry from the pulper is dumped into a "dump chest" for storage, then transferred to a "machine chest" for continuous feed to the paper machine area. It is diluted with "white water" or recycled liquid which drains through the paper machine screen and suction boxes into a silo or receiving tank. This system of recycling liquid not only conserves the water used in large quantities on the paper machine (slurry concentrations entering the machine are as low as 0.3% or even less) but also reclaims much of the additives which are not trapped on or adsorbed by the fibers on the machine. Thus, the additives put into the pulper are the net amounts taken up by the paper, while a substantial amount may circulate with the "white water." The diluted stock is generally filtered through a "barrier screen," a long rotating perforated cylindrical screen, and also passed through one or two "cleaners" in series (see Fig 2). These are long conical vessels into which the thinned slurry enters tangentially near the top; the slurry spins around rapidly, its radius of gyration decreasing as it moves downward in the cone, finally turning around near the bottom and passing up through the central core of the cone to an outlet at the top center. Meanwhile, any material such as sand, grit, bits of tramp metal, or wood are thrown out to the side of the cone and slide down into a chamber at the bottom, from which these contaminants may be continuously or batchwise discharged. The cleaned slurry is adjusted in consistency and fed to the "head box" of the paper machine, as described in the next section.

In older paper mills or in highly specialized mills where relatively short runs of a wide variety of products are made, the pulping operation may be carried out in "beaters" or "Hollanders" as shown in Fig. 3. These are long elliptical tubs, with a paddle wheel mounted on one long leg of the ellipse. The "paddles" are actually iron or other metal bars rotating against a bed plate

316

Feed-~•-

Clean slurry

Paper Manufacture

t discharge

~ Trash

Upper section, say 6" diameter

Inlet velocity 50 ft/s

(or more)

Cone length 2 to 4 ft. stain I ess steel

~ Lower section, say 3" diameter

Catch chamber (glass)

FIG. 2 . Hydroclone-type cleaner.

with a sharp knife mounted to give a narrow but adjustable clearance from the bars. The assembly may be provided with a spring or counterweight system designed to allow widening of the clearance in the event of a blockage or oversized lump of hard material coming into the clearance area. The beaters are slow compared to the hydropulper, but provide for good visual observation of the fibrillating action and for holdup of batches pending testing or chemical reaction of additives.

B. Sheet Formation

The basic equipment used for sheet formation is still the Fourdrinier moving wire. Originally the feed end of the wire was mounted on an oscillating track, and the whole feed end was shuttled from side to side a distance of several inches. With the advent of high speed, wide machines, with screens traveling as much as 3000 ft/min, the shaking mechanism became too troublesome to

Paper Manufactur

~ Lapped

loading · pulp [

ben feed mac desiJ box as st maiJ butit hydl

per Manufacture

'·say 6" diameter

velocitY l ft/s morel

ft.

tV 3" diameter

able clearance from g or counterweight : event of a blockage ;learance area. The vide for good visual of batches pending

Fourdrinier moving an oscillating track,

1 distance of several ·ith screens traveling ! too troublesome to


(.___~_____.... ~ _. Path of stock

Additives Motor drive i ~;';.,

~! ,. .... , ~ ''""" . ' ------------- -:.-f. :, Rotor with bars (paddles)

' ~ ' / .. • A' ·~\ pulp 1- _,.-·'·I ~

1

• •

loadtng .; • •I

'····~ ........,..~.r:-

.... .. --.,.'"'- .. ....

Bed plate/knife

FIG. 3. Beater (Hollander).

Leve I of stock

• ( . . -.

Drain

be maintained; it has been abandoned in favor of more accurate control of the feeding of the thin slurry onto the moving wire. The slurry feeding the machine enters through a head box which is, in turn, fed through a carefully designed manifold of piping to insure uniform flow and pressure. The head box discharges through a "slice" or chute onto the surface of the moving wire, as shown in Fig. 4. The head box is contoured and baffled so that the slurry is maintained in moderately turbulent flow so as to maintain a random distribution of fibers in uniform consistency with a combination of static and hydraulic head uniform with respect to machine width and time. The "slice"

318

Pulp slurry

Inlet manifold

From mix boxes

and cleaners

Paper Manufacture

Sheet to

drier

FIG. 4. Fourdrinier sheet former.

is so designed that the slurry leaves the lip at a velocity close to that of the moving screen. A slight excess of slurry velocity is known as "rush" and a slight deficiency is known as "drag." The amount of rush or drag will affect the character of sheet formation. To provide adequate hydraulic impetus, sometimes head boxes are pressurized in the vapor space above the liquid level.

The slurry as it is laid down on the wire should be uniform in depth, consistency, free of waves or ripples, and free ofbubbles. Shortly after leaving the slice, the screen passes over a series of"suction boxes," or chambers with perforated tops connected to a knock-out pot and a uniform vacuum system. The boxes may be 3 to 8 in. wide, of length equal to the full width of the screen, and carefully gasketed so as to draw in water but not air. The water in the slurry should be drawn off uniformly across the width of the machine. Generally there are 3 to 6 wet suction boxes arranged close to each other in series along the wire, and they suffice to draw the free water from the surface of the newly formed sheet. Each box has its vacuum individually controlled so as to achieve uniform proportional draw-off of water from the sheet. The water from the boxes discharges into the "silo" or white water receiver tank, which is generally maintained under some vacuum. The water from the silo is returned to the slurry mixing boxes ahead of the head box via large centrifugal pump ("fan" pump) to maintain constant slurry feed and approximately constant level in the silo.

After the free water is drawn off the surface of the newly formed sheet, air will start to pass through as additional water is drained through. The screen therefore passes over one or more "dry" suction boxes, i.e., suction boxes designed to handle appreciable quantities of air along with water. Such boxes drain into a separator, the vapor space of which is piped to a high capacity wet vacuum pump such as a Nash pump. The water from the dry box separator will still drain into the silo.

After the last suction box, the sheet is ready to be removed from the wire. This is usually accomplished by transferring it to a felt, a moving band of high

Paper Manufactu1

qt su

a th m cc "c en se se clt sn

wl th la: fo he lS

di

pa t~

fe, th SH

SH

st1 be

ca se de to at m cy va

pc

C.

Tl th

>er Manufacture

Sheet to

drier

er on felt

Jch pan

lose to that of the n as "rush" and a or drag will affect tydraulic impetus, ~ above the liquid

miform in depth, wrtly after leaving or chambers with

n vacuum system. : full width of the >t air. The water in h of the machine. ;e to each other in :r from the surface ually controlled so >m the sheet. The ater receiver tank, Lter from the silo is ia large centrifugal nd approximately

r formed sheet, air rough. The screen .. e., suction boxes water. Such boxes L high capacity wet dry box separator

ved from the wire. )Ving band of high


quality fabric which acts as a blotter to remove more moisture and as a supporting carrier, moving the sheet to the drying section of the machine.

Pick-up from the screen to the felt is accomplished by the felt passing over a "pick-up slot" vacuum source, which draws air through a narrow slot, through the felt, and through the wet sheet, lifting it off the wire and onto the moving felt. The felt will be moving at the same speed as the wire. The wire continues around over a "couch roll" where adhering water is removed into a "couch pan" for recycle. The wire then returns to the "breast roll" at the feed end, just under the head box. Between the couch roll and the head roll are several rolls which adjust the tension and alignment of the wire, and which seal off water sprayed onto the wire by high pressure jets to keep the wire clean. Under the wire just ahead of, and after, the suction boxes are several small supporting rolls called "table rolls."

For special heavy duty materials, modern paper machines are available which can lay down not only one but two or more layers of wet sheet, one over the other. Often a top layer of thinner but more special quality stock can be laid down over a bottom or base layer of heavier stock. In such cases the forming section of the Fourdrinier is made slightly longer and a second head box and slice are added to lay down the top layer before the bottom layer is fully drained. Care must be taken that the entering top layer slurry does not disrupt the partially formed bottom layer.

For heavy sheet made at high speeds, a considerably different design of paper machine can be used. The wet sheet is formed under pressure between two wires, one on one side, the other on the other side of the sheet, with the feed injected between the two. The clearance between the wires decreases as the sheet moves along, the water being squeezed out into chambers on each side of the wire. To avoid the complication of a multitude of table rolls, one side of the pair of wires may run on a large cylindrical roll, and the other side stretched over it. The action can be repeated with any desired number of head boxes and forming cylindrical rolls.

Returning to the basic simple Fourdrinier operation, the sheet being carried on the felt is moved a relatively short distance (5 to 15ft) to the first of several drying devices, either of squeeze roll, through drier, or "can" type, as described further on. The sheet is removed from the felt at the point of entry to this drying device by "blowing off." The "blow-off" is a slotted pipe placed at the appropriate point behind the felt, through which air is blown at a modest pressure to remove the wet paper. The felt continues on around its cycling loop, generally past a water wash station and squeeze rolls, and a vacuum box to dry the felt.

Felts used on paper machines must be of a high degree of uniformity in porosity, mechanical strength, stretch qualities, and thickness .

C. Drying and Calendering

The wet sheet as transferred from the carrier felt to the drying system is more than 70% by weight water, and a good deal of thermal energy would be required if all the moisture were to be removed via the conventional steam-


heated "can" driers. In many mills a rubber-coated pair of squeeze rolls is used to mechanically remove as much water as possible. One of the rolls may be porous to facilitate taking the water away from the surface of the paper. A high degree of water removal by squeezing through rolls is limited by the poor mechanical strength of the sheet at this point in the process, and also by the disadvantage of excessive compression of the wet sheet, making it less amenable to evaporative drying.

An alternative method of removing the large initial content of water in the sheet is to use a "through drier," as shown in Fig. 5. The through drier is a large diameter perforated metal drum which rotates, carrying the sheet around under a large, closely fitting metal hood. Hot combustion gases pass from the hood through the moist sheet, vaporizing water. The vapors pass through the paper sheet and the perforations of the drum into the interior of the drum which is maintained under a moderate vacuum (say 4-8 in. water). The connection to the source of vacuum is a stationary shoe inside the rotating drum, so placed that the sheet is under vacuum from the moment it reaches the drum until shortly before it reaches the point of discharge (see Fig. 5). At the discharge a short stationary shoe provides a modest blast of compressed air to ensure removal of the sheet from the perforated drum.

The moist air coming from the sheet passes into the suction of a large fan, which circulates the air past a chamber where natural gas or LPG is burned to quickly heat the air to a controlled temperature well above the boiling point of water (say 300-SOOoF). The upper limit of temperature depends upon the residual moisture in the sheet leaving the drier-the greater the moisture content, the higher the allowable temperature. Moisture balance in the circulating air system is controlled by bleeding moist air out after the blower, before the combustion chamber. The rejected moist air may be passed through an economizer to recover its heat, transferring it to cold make-up air required to keep the circulation volume in balance. Operation of a throughdrier system is usually controlled by a microcomputer.

Final drying of the paper is usually accomplished by use of the timehonored "can" driers which are simply cylindrical drums over which the paper passes, with alternating sides facing the drum surface. The cans are heated internally by steam at a moderate pressure (say 10 to 50 lb/in.2),

admitted through rotary "Johnson joints." The condensate is similarly removed, being picked up at the bottom of the rotating drum by a stationary pickup siphon or manifold. The cans are built with as thin a shell as possible to withstand pressure and mechanical requirements, to facilitate heat transfer. They are highly polished and accurately aligned. A typical can-drier arrangement is shown in Fig. 6.

After the paper has been dried to about 3 to 5% moisture it is ready for the final steps in the ordinary papermaking process. The paper passes through one or more calendering rolls which are highly polished, precision squeeze rolls which provide for densification and a smooth finish on the paper. The pressure on the rolls is hydraulically controlled to give a total load of many tons across the width of the moving sheet. To ensure against differences in pressure across the width of the calender, elaborate internal compensating devices are built into the rolls.

Paper ManufactUI

From

drier

Wet frorr

ti' m a{:

of sp to m

1aper Manufacture

tir of squeeze rolls is . One of the rolls may 1rface of the paper. A is limited by the poor )Cess, and also by the heet, making it less

ontent of water in the 'he through drier is a :, carrying the sheet Jmbustion gases pass tter. The vapors pass m into the interior of n (say 4-8 in. water). nary shoe inside the 1 from the moment it )int of discharge (see ies a modest blast of .he perforated drum. suction of a large fan , s or LPG is burned to ve the boiling point of re depends upon the greater the moisture iture balance in the rout after the blower, : air may be passed it to cold make-up air >eration of a through-

by use of the time·ums over which the urface. The cans are ;ay 10 to 50 lb/in.2),

1densate is similarly drum by a stationary hin a shell as possible , facilitate heat trans. A typical can-drier

ture it is ready for the paper passes through ~d, precision squeeze ish on the paper. The a total load of many

against differences in tternal compensating


From

drier

Wet sheet from felt

Hot air hood

Rotating cylinder

Combustion chamber

Circulating fan

FIG . 5 . Through drier.

Make-up air

Moist air discharge

As the mechanical and chemical properties of paper are extremely sensitive to final moisture content, it is sometimes desirable to use sophisticated means to control it. One such means is the use of a "moisturizer," which applies a controlled amount of a fine mistlike spray of water onto the surface of the paper after the calender. A high voltage may be utilized to direct the spray specifically to the paper surface. The can drier system is operated so as to overdry the paper, and the final moisture is established by use of the moisturizer.

exits via Johnson joint

FIG. 6. "Can" driers.

' 4 • To calender


D. Finishing

The paper leaving the forming and drying section of the mill is normally wound up, under controlled tension, on a "jumbo roll" which covers the full width of the machine and may be wound to 5 ft in diameter or more. This heavy roll is then removed (while a new roll is started) and taken to a cutting/slitting machine, where the jumbo roll is unwound and rapidly run past a series of circular knives set at the proper spacing to cut the sheet into whatever widths may be desired.

For may purposes a highly glossy finish may be preferred, in which case a separate calendering operation can be carried out whereby a layer of resin or plastic material is sheeted through a pair of calender rolls and then immediately laid onto the paper as it passes through another pair of rolls. A pattern may be embossed on the plastic surface during such an operation.

For speCial strength additives to the paper, such as vinyl acetate emulsions, viscose solutions, rubber latex, and a wide variety of other materials, a dipping plus squeeze-roll arrangement may be inserted into the drying train part of the plant. For example, after a half-dozen drying cans, the paper with a moisture content of 5-10% is dipped and squeezed to yield an impregnated paper with 20-30% reinforcing agent and perhaps another 20-30% water. This sheet then passes over another 6-12 drying cans or even a through drier to work the moisture back to 3-5%.

A creping operation has already been described.

Miscellaneous Auxiliary Equipment

In this brief description a large number of auxiliary items have been skipped, but perhaps deserve passing mention.

1. The white water circulation system takes advantage of gravity to a considerable degree. The Fourdrinier wire with its suction boxes is located on a floor of the mill well above the basement, while the white water silo is well below the machine. This provides a natural drainage suction, and vacuum pumps are needed only to trim the pressure in the vapor space of the silo. Of course, the dry vacuum boxes do need Nash vacuum pumps.

2. Because of many small sources of water around the paper machine (wirecleaning sprays or "showers") and felt cleaning showers, wet end paper trimming jets, water seals, etc., a fair amount of excess white water is developed (beyond what can normally be recycled to the machine). This excess white water passes into a "save all" filter, a large rotary vacuum filter which recovers most of the fibers in the excess water, and even some of the pigments (which are present only in small amounts). The save all product is recycled to the pulpers.

Paper Manufact

aper Manufacture

the mill is normally which covers the full meter or more. This :ed) and taken to a und and rapidly run to cut the sheet into

rred, in which case a by a layer of resin or ls and then immedi.ir of rolls. A pattern lperation. yl acetate emulsions, f other materials, a into the drying train cans, the paper with ·ield an impregnated 'ther 20-30% water. even a through drier

shave been skipped,

age of gravity to a ts suction boxes is ent, while the white ; a natural drainage 1 the pressure in the Joxes do need Nash

aper machine (wirewers, wet end paper "cess white water is , the machine). This arge rotary vacuum rater, and even some ounts). The save all


3. When starting up a machine, or when operational upsets occur, a large amount of damaged sheet "broke" is generated. This broke is also usually recycled to the pulpers unless it is contaminated, in which case it may be burned.

4. To facilitate startup with a minimum of scrap or "broke," a usual practice is to cut only a narrow "tail" of sheet leaving the Fourdrinier wire (the rest being recycled wet to the mix boxes). This "tail" is passed under a rope at one side of the machine which threads over and under all the equipment the sheet must pass through on its way through the drying end. If the tail makes its run successfully, the tail is quickly widened to full sheet width and the drying system thus becomes fully threaded. The edge of the sheet that passed under the rope is trimmed away at the wet end, and the rope no longer carries a "tail."

5. Since can driers are bulky, they are arranged close together in staggered fashion. As water vapors emanate from the paper passing over the closely stacked cans, it is advantageous to provide good ventilation between successive cans to provide dry air and to remove hot, moist air.

6. As through driers handle very hot gases and since occasionally some paper may be trapped in the through-drier ductwork, it is desirable to provide adequate fire prevention measures. These include automatic shutoff of the combustion system, shutoff of the fresh air intake, and replacing the air with a steam snuffing system.

7. The paper, as it passes through the drying system, shrinks, and the speed of the rolls handling it must be correspondingly adjusted. This is not a simple task but is accomplished by providing many individually speed controlled motor drives along the length of the machine. The speed is adjusted to maintain constant tension in the moving sheet.

8. Occasionally it will happen that there is uneven shrinkage across the width of the machine, a condition that can cause serious tracking problems and sheet breakage. This problem can be alleviated by judicious placement and adjustment of Mount Hope rolls. These are rubber (or elastomer) covered bowed rolls, the degree and direction of bowing being controllable by simple adjustments which can be made while the machine is running. The internals of these rolls are made up of a multitude of short cylindrical sections, all rotating on a common small diameter flexible axis.

Instrumentation

The instrumentation of a modern paper machine has become highly sophisticated. Generally the machine is subdivided into a number of separate control stations, each of which is monitored and controlled by a microcomputer. These stations then feed information to a master computer and control station which keeps the whole machine under the specified operating conditions.


Sensing instruments include flow to the head box, consistency, flow through each suction box, vacuum, wire speed, wet and dry sheet thickness, moisture content, porosity, presence of holes or contaminants, and many other variables. Many of these are monitored across the width of the sheet, and a degree of uniformity is controlled.

Some Paper Product Properties

Some key properties of typical grades of paper as made today may be listed as follows.

Basis Weight (in grams per square meter)*

Tissue Towel Newsprint Grocery bag Fine papers Kraft linerboard Box board

Caliper (thickness) (in micrometers)

Capacitor tissue Facial tissue Newsprint Offset bond Linerboard Book cover

15-20 50-60 45-50 50-100 60-150 (loaded)

125-440 190-585

7.6 65 85

100 230-600 770-7600

Tensile Strength (in kgjcm width or sometimes in breaking length, i.e., length which is self-supporting)

Newsprint Linerboard Aluminum foil

2 12 20-25

The strength in the machine direction will be 1.5 to 2.5 times that in the cross direction. The strength in thickness direction is even less than in the cross

*In older terminology, lb/3000 ft2 ream; lib/ream= 1.62 g/m2•

Paper and Pulp, E1

din nat 30.

Ot.

De

Me Me Liq Elo Te:: Op Ta~

Ele

Paper and I (see also Energy Cc

Ent lati cos fin1 alte enc Co OPI reg for

pla cas ste< cor cia! use tot

per Manufacture

consistency, flow lry sheet thickness, 1inants, and many width of the sheet,

lay may be listed as

eaking length,

nes that in the cross 5S than in the cross

Paper and Pulp, Energy Management 325

direction. A practically used measure of strength is the Mullen burst pressure, namely the pounds pressure required to burst an unsupported disk of paper 30.5 mm in diameter. (Pressure may run lO to 100 lb.)

Other Properties Measured

Depending on the ultimate use, properties measured may include:

Moisture Moisture vapor permeability Liquid permeability Elongation Tear strength Opacity, brightness, gloss, color stability Taste, odor Electrical resistance, dielectric strength

Paper and Pulp, Energy Management

ERNEST 0 . OHSOL

(see also Energy Conservation, Paper Operations, Fuel Choices, and Energy Demand)

Energy management is a vital concern throughout the paper industry. Regulations requiring decreased consumption per unit of production, increasing costs per Btu, and threats of curtailment have made it imperative that mills find and implement means of increasing energy efficiency and of using alternative sources. The control technology applicable to accomplishing energy efficiency improvements in the pulp and paper industries is discussed. Computer implementation of energy-saving techniques offer outstanding opportunities because of the accuracy with which individual units can be regulated, and the coordination which can be enforced over entire complexes for broad-based optimization.

Early energy management efforts in paper mills and other industrial plants were focused on electrical load control. Savings were achieved in many cases without affecting production. Attention is now being turned toward steam generation and distribution systems, because these relate directly to the consumption of expensive purchased fuels. This can be particularly beneficial in pulp and paper applications because of the large amounts of steam used for direct and indirect heating needs in the processes and for conversion to mechanical shaft work or electricity.