Biermann Ch. Handbook of Pulping and Papermaking

Подождите немного. Документ загружается.

684 33. MISCELLANEOUS TOPICS

9.2 to 9.8 is ideal. In low—pressure steam or hot

water heating systems, nitrite or chromate provides

a protective film on metal surfaces.

Filming amines provide a physical barrier to

protect metal from corrosion. They are effective

in the pH range of 5.5 to 8.5. Amines that are

conmionly used include those with long—chain

hydrocarbons that orient away from the metal

toward the water, such as octadecylamine.

Cy clohexylamine, diethy laminoethanol,

dimethylpropanolamine, and morpholine are

sufficiently volatile to travel with the steam. The

amine in the vapor helps keeps the pH sufficiently

high to avoid corrosion.

Scale control

Scale tends to build up in boiler water (called

water side deposits) for two reasons. First,

bicarbonate ions are converted to carbonate ions

and carbonate salts of calcium and some other

cations have low solubility. Second, the solubility

of calcium carbonate and similar compounds de-

creases with increasing temperature. Heat transfer

surfaces, where the temperature change is most

dramatic and fast, tend to be where scale forms.

The buildup of scale is controlled by the use

of dispersants with chelants and/or phosphate.

Chelants like EDTA (Section 14.7) keep cations

like calcium from precipitating by forming strong,

soluble complexes with them. Phosphate tends to

precipitate these materials early on so they do not

later precipitate on heat transfer surfaces. Com-

pare the solubility products of the compounds in

Table 13-5 to those of CaHP04, 1 x 10"^;

Ca3(P04)2, 2 X 10-2^; Ba3(P04)2, 3.4 x

lO'^^;

Mg3(P04)2, « 10-23 to 10-2^; and Zn3(P04)2, 9 x

10"^^

Remember, however, that the phosphate

salts of divalent cations have five species in the

solubility product.

EXAMPLE 1. Compare the concentration of

calcium ion in calcium carbonate and calcium

phosphate at

°C.

SOLUTION:

[Ca2^][C03-] = 4 X 10-^

[Ca2+] = 6.3 X 10-^

[Ca2+]3 [(P04^)]' = 2 X 10-2^

[Ca^-'] = (2

\0-^y^ = 1.8

10-"

This applies only at high pH (11—12) where

carbonate and phosphate ions occur. At

moderate pH (5—7) calcium bicarbonate is

fairly soluble (apparently too soluble to find

^sp values readily in tables), but calcium

hydrogen phosphate is still fairly insoluble.

PROBLEM: What is the solubility of calcium

over calcium sulfate and calcium hydrogen

phosphate? This is applicable at pH values

found in boiler water.

NTA (nitrilo triacetic acid) is a weaker

chelating agent than EDTA but is used for boiler

water treatment.

Excessive boiler water carryover with the

steam will cause corrosion and scale formation on

turbines and other critical equipment.

Demands of

high—temperature

boilers

New boilers for the pulp and paper industry

operate at 900—1500 psig. While water hardness

is always of concern, the formation of metal oxide

scales is equally important. Iron (and possibly

copper) oxide is often the principal contaminant of

scale formation in high—temperature boilers.

Polymeric dispersants have been used for

scale control at lower temperatures since the

1950s. These include negatively charged poly-

mers,

such as polyacrylate, polymethacrylate, and

polyacrylate—acrylamide copolymer, apparently

bind to the positively charged metal oxide

particles, keep them in solution, and prevent them

from growing, i.e., steric stabilization. (One

might hypothesize that maleic acid copolymers

may chelate better than these other polymers.)

Unlike chelants or precipitants, dispersants do not

react stoichiometrically.

In the mid—1980s Betz introduced a polyeth-

ylene polymer (with every other carbon containing

a methyl group and a phosphonate group) for use

at high temperature.

Boiler water

blowdown

As steam is removed from the boiler, salts

and other materials remain behind, allowing these

materials to concentrate in the boiler water. The

concentration of salts and metal oxides are typical-

ly 10—50 times higher in the boiler water than in

the feedwater. Some boiler water is continuously

BOILER FEEDWATER TREATMENT 685

removed from the system to prevent undue con-

centration of these materials that would eventually

precipitate and cause scale.

33.6 CORROSION

Corrosion

Corrosion, while always an important consid-

eration, has become of particular concern to the

pulp and paper industry since the 1970s, when

mills have been forced to reuse water in various

processes. As pressure continues to increase

water closure in mills, the problems of corrosion

will increase. Indeed, corrosion may be one of

the limiting factor in determining how much

closure can be obtained. Some basic aspects of

corrosion have been considered in Section 13.11

on page 330.

Types of metals for corrosion resistance

Table 13-8 (page 331) shows some metals

with their relative resistance to corrosion in mov-

ing sea water. A few additional aspects of stain-

less steels will be considered here. In addition to

the compositions shown in Table 13-8, most stain-

less steels have 1—2% Mn, although 201 and 202

have 5.5—7.5% and 7.5—10%, respectively. P is

limited to less than 0.06 or 0.045% in most

stainless steels, although type 303 allows up to

0.2%. S is limited to 0.15%, 0.06%, or 0.03% in

many types. Si is limited to 1.0% in many stain-

less steels; however, type 302B specifies

2—3%,

type 314 specifies 1.5—3%, and type 403 limits it

to 0.5%. Stainless steels with an L suffix are low

carbon. For example, types 304 and 316 have

maximums of

0.08%

carbon while 304L and 316L

have maximums of 0.03% carbon.

Type 317 is designed for maximum corrosion

resistance of standard stainless steels; its composi-

tion is 18—20% Cr, 11—15% Ni, 2% Mn, and

3-4% Mo. Type 347 is 17-19% Cr, 9-13%

Ni,

2% Mn, 1% Si, 0.045% P, 0.03% S, and at

least 10 times the amount of carbon as Cb—Ta.

In addition to the stainless steels, which have

matrixes of iron, alloys which are mixtures of

chromium and molybdenum in a nickel matrix are

very important.

Duplex stainless steels have been used in

selected corrosive environments since the 1960s.

New duplex stainless steels that are more easily

welded have been available since the mid—1980s.

These materials offer the potential to replace other

stainless steels in a wide range of applications.

Acid

cleaning

and

corrosion

(Crowe, 1992)

Many components, such as kraft digesters,

are washed with acid to remove scale. CaCOj and

other carbonates are major species of scale; acid

solubilizes this material by converting the COj^'

into CO2 and HjO by the addition of 2H+. When

HCl is used as the source of acid the remaining

CaCl2 is water soluble. Formic (HCOOH),

sulfamic, and nitric acids are also used for acid

washing. The use of additives (corrosion inhibi-

tors) with these acids may decrease the amount of

corrosion caused by acid washing. These inhibi-

tors are often organic sulfides or amines. One

inhibitor is /?-aminophenol, which has trade

names of Activol and Rodinal. Another agent is

diethylhydroxylamine and is commonly used in

steam cycles. Propargyl alcohol (2—propyn—1—

ol) is used in strong acid baths to decrease hydro-

gen embrittlement and steel corrosion. [The

addition of copper sulfate to hot, dilute, sulfuric

acid often limits its corrosion to stainless steel.]

Often paper mill workers allow suppliers to

give proprietary chemicals as corrosion inhibitors.

This gives the mill no way to effectively test it

against other manufacturers, products without

lengthy studies. Even research papers report

results with unknown inhibitors (Crowe, 1992). It

is also difficult to find information on corrosion

inhibitors that may be particularly effective to the

industry. A mill could even do its own corrosion

tests with small metal coupons in various solutions

to determine what might be most suitable (Crowe,

1992).

The coupon test is appropriate since acid

washing is a short—term corrosion.

Silicates are often other major components of

scale. But, as indicated on page 637, silicates are

soluble under highly alkaline conditions; there-

fore,

they are not present in many components of

the kraft process such as digesters, liquor evapora-

tors,

etc. On the other hand, aluminum, calcium,

and silicate together can form a troublesome scale.

The removal of scale by acid washing is a

tradeoff of many factors. Some acid washing

solution circulation is needed to help dissolve the

scale, but too much circulation leads to increased

rates of corrosion. The time of exposure to the

686 33. MISCELLANEOUS TOPICS

acid cleaning solution must be such that most of

the scale is removed, but not so long that the

amount of corrosion is unnecessarily high.

Ferric ion and iron sulfide in HCl increases

corrosion because as the ferric ion is reduced iron

metal is oxidized. Iron oxides are removed by

HCl treatment.

Nitric acid is often used instead of HCl for

cleaning stainless steel surfaces of digesters be-

cause HCl causes pitting or stress corrosion.

Nitric acid rapidly attacks carbon steel.

Formic acid vapors cause corrosion since the

inhibitors are not necessarily

volatile.

Formic acid

can decompose to CO and H2O, so it must be used

with ventilation and containers with relief valves.

Sulfamic acid (NH2SO3H) is used below

50°—60°C since higher temperatures (above about

70°C) leads to some formation of sulfate, which

causes scale formation by formation of insoluble

metal sulfates (such as CaS04).

Corrosion in the wash and

bleach

plants

As late as 1970, most brown stock pulp

washers were constructed of mild carbon steel, but

now stainless steel is specified.

Chlorine dioxide is surprisingly harsh on

nickel—based alloys. Even type 317L stainless

steel (3—4% Mo) is now considered inadequate

for many bleach plants. The stainless steels with

high levels (as much as 6%) of molybdenum are

fairly resistant to attack by CIO2 and have been

incorporated into many bleach plant washers.

Titanium apparently reacts very quickly with

dry chlorine so its use must be limited to areas

where exposure to dry chlorine is avoided. How-

ever, it seems to have very good corrosion resis-

tance in most bleach plants.

Corrosion in the recovery boiler

Corrosion in the recovery boiler is the most

difficult challenge to the pulp and paper mill

corrosion engineer. The potential for explosions

makes corrosion monitoring of the recovery boiler

very important. High temperatures, molten salts,

water, oxygen, and condensation ofmaterials all

contribute to the possibility of rapid corrosion

rates.

This area is discussed in detail by Sharp

(1992).

Prevention

corrosion in

boilers during shutdown

When power boilers are shut down water

may condense on the fireside surfaces if the

temperature of these surfaces falls below the dew

point. Sulfiir deposits may react with this water to

form very acidic solutions that can quickly corrode

the tubes. The corrosion is often localized, caus-

ing severe pitting.

This type of damage can

minimized during

shutdowns by taking the following steps (in the

order presented): removing deposits as quickly as

possible, washing the deposit—free surfaces with

water, air drying the surfaces, applying a thin

layer of lightweight oil, and using a desiccant,

such as unslaked lime, which is replaced as neces-

sary, in the ashpits.

To prevent problems in the future, one might

take several samples of deposits, dissolve them

separately in small amounts of water, and measure

the pH of the water. A pH below 4 may signal

potential problems with the sulfur content of the

fuel supply.

Corrosion

the

paper machine

Corrosion of bronze rolls on paper machines

was studied by Magar (1992). Five corrosion

inhibitors were tested that all contained mercapto-

benzothiazole as the active ingredient. The inhibi-

tors were all more effective if added prior to

exposure of the test liquid to the bronze.

Stress corrosion crack detection

The detection of cracks caused by stress

corrosion is often accomplished nondestructively

by the use of liquid penetrant or magnetic particle

tests.

One study (Reid and Reid, 1993) indicates

that some surface metal must be removed by light

surface grinding for these methods to be effective;

sandblasting and power wire brushing are inade-

quate preparation methods. The paper refutes the

idea that such grinding forms cracks or enlarges

existing cracks for commonly used alloys.

33.7 SAFETY PRACTICES FOR

HAZARDOUS CHEMICALS

Table 33-5 summarizes some steps to take

when handling potentially hazardous chemicals.

Material Safety Data Sheets are available from the

manufacturers and suppliers of chemicals.

WATER CONDITIONING 687

Table 33-5. Procedures for handling hazardous

chemicals.

Read the Material Safety Data Sheet

(MSDS).

Follow the manufacturer's directions for han-

dling the material.

Label all vessels and plumping carrying the

material in a clear fashion with indelible ink.

Be sure to use appropriate protective equip-

ment, clothing, gloves, and safety glasses or

face shields. Have plenty of these materials

on hand, near the point of use, so they are

used.

Do not handle these materials or feed them

into the process by hand; instead use high-

quality metering pumps equipped with power

interlocks to prevent their addition during

shutdowns.

6. Locate showers and eye wash stations near

the point of use of toxic chemicals.

Do not smoke or eat when handling these

materials.

8. Do not wipe or scratch your eyes, mouth,

head, or other body parts with your gloved

hands or clothing that has been in contact

with poisonous or corrosive chemicals.

33.8 TRANSPORTATION SAFETY

Introduction

This section was prepared with the help of

Mankui (David) Chen who collected most of the

references and wrote some of the first draft of the

text. The author grateftiUy acknowledges his help.

The chemical industry occupies an important

place in the U.S. economy; its total output ac-

counts for about 10% of the GNP in manufactur-

ing. Hauling chemicals is a major source of

revenue for railroads and motor carriers. The

share of freight handled by rail has decreased from

about 70% in 1946 to less than 40% in 1990.

Trucking's share increased from 5 to 25% in the

same time. The railroad industry was deregulated

with the Staggers Rail Act in 1980. In 1989 there

was a record year of nearly 1.5 million rail car-

loads of chemicals and related materials.

Intermodal transportation involves shipping a

material with several forms of transportation (a

combination of truck, ship, rail, and even air).

For example, a motor carrier might deliver and

pick up a bulk handling container at a plant. The

container would be transported by rail over a large

distance and then transferred to a motor carrier for

transportation to the final destination. This trend

has not been widely practiced in the past since the

railroad and trucking industries have been major

competitors rather than cooperators. The ability

of a shipper to deal with one carrier is a driving

force for the partnerships that are forming between

the various types of carriers.

Chemicals can be classified into hazardous

materials and nonhazardous materials. Hazardous

materials may be corrosive, explosive, flanmiable,

and/or toxic. Any major accident in their trans-

portation can directly endanger people's lives and

the environment. Hazardous material handling

regulations include substance classification (corro-

sive,

explosive, etc.), packaging, handling, label-

ing and placarding, accident reports, information

hotlines, filing route plans, etc.

Some incidents involving "nonhazardous"

chemicals may not threaten people, but can de-

stroy an entire ecosystem. For example (Elmer—

DeWitt, 1991), a Southern Pacific tanker car

derailed on a treacherous canyon bridge six miles

north of Dunsumuir on July 14, 1991. Its contents

of 19,500 gal. of metam sodium spilled into the

Sacramento River. Metam sodium was classified

as a nonhazardous material at the time of the

accident. It wiped out aquatic plants, nymphs,

caddis flies, mayflies, and at least 100,000 trout

through a 45 mile stretch.

Every year there are about four billion tons

of hazardous materials and radioactive waste

shipped in the U.S. Accidental chemical releases

seem inevitable—every day there are about 20

accidents where hazardous materials are released

in interstate transportation. These accidents result

in hundreds of deaths, thousands of injuries, great

loss of resources, and damage to the environment.

The public grows increasingly concerned about

this environmental issue.

688 33. MISCELLANEOUS TOPICS

Government agencies and Congress react to

this situation by proposing new laws. Chemical

companies are taking many steps to ensure the safe

handling of their products as the liability is shifting

from the carrier to the shipper.

Safety regulations and codes

There are many regulations, rules, and codes

concerning the transportation of hazardous materi-

als issued by the federal government, the states,

associations, and manufacturers. Three very

important and influential regulations from the

federal government, the shipper, and the carrier,

respectively, are considered here as examples.

The Hazardous Materials

Transportation

Act

of 1974 was the first complete federal law about

hazardous materials transportation; it was origi-

nally passed by Congress as the result (in part) of

public concern about the air accident at Logan

Airport in 1974 (Gerhart, 1992). Three crewmen

aboard a Boeing 707 died in a crash at Logan

Airport in Boston, Mass., in 1974. Investigators

found the dead men had been incapacitated by

ftimes from 10 quarts of improperly packaged

nitric acid. Although correctly marked and la-

beled, the acid was stored on its side and leaked.

The regulation covers substance classification,

packaging, handling, incident report, placarding,

and other topics.

In November 1990, Congress approved the

Uniform Safety Amendments for this act (Stitt,

1991).

It introduced a long list of new regulations

covering emergency response information systems,

specific highway routes, training programs, regis-

tration requirements, safe permit system, inspec-

tion and certification, and in place financial re-

sponsibility (insurance).

This bill allows federal preemption of state

and local hazardous materials laws, calls for state

and local participation in setting routes and stan-

dards,

and sets up a petition process for objections

to routing of hazardous materials. It expanded

federal authority to prosecute violators and in-

creased civil and criminal penalties.

Responsible Care (McConville, 1992;

Ainsworth, 1993) is considered the standard for

the chemical makers' total approach to carrying

out business. It was set up in 1988 by the Chemi-

cal Manufacturers Association (CM

A),

launched in

1989,

and parts of it continue to be ratified. It

provides a framework for good safety practice to

guide manufacturers, distributors, and carriers. It

consists of six operating codes covering every

facet of the industry:

•Community awareness and responsible care

•Pollution prevention

•Process safety

•Distribution

•Employee health and safety

•Product stewardship

Product stewardship is a catch—all for mak-

ing "health, safety, and environmental protection

an integral part of designing, manufacturing,

marketing, distributing, using, recycling, and

disposing of products."

The most stringent code for logistics is the

Distribution Code of Management Practice, which

includes the following five elements:

A. Risk management

Compliance review and training

C. Carrier safety

Handling and storage

E. Emergency procedures

The purpose of this code is to reduce the risk

of chemical exposure to the public, carriers,

distributors, chemical industry employees, and the

environment. To meet the requirements of this

code,

CMA's 178 member companies had to

make drastic changes to their policies and proce-

dures and document their compliance.

Many of the goals of the program are bor-

rowed from TQM such as "constantly upgrading

manufacturing procedures and practices."

The Key Train and Key Route Program is a

program from chemical carriers designed to

improve safety and assure compliance with regula-

tions.

One of the most advanced cross—industry

efforts has brought together representatives of the

railroad, chemical, and rail equipment industries to

develop standards to improve safety. The group,

the Inter—Industry Task Force on the Safe Trans-

portation of Hazardous Materials by Rail, called

for defining trains and routes of highest concern as

"key trains " and "key routes" for limiting train

speeds (50 mph) and for enhancing track mainte-

nance and derailment safeguards. The regulation

TRANSPORTATION SAFETY 689

detailed practices for yard operations, storage, and

employee training (Anon., 1990).

In addition to the regulations listed above,

there is a new suggestion that carriers obtain ISO

9000 certification to compete in the international

market. TRANSCARE (The Transportation

Community Awareness and Emergency program)

calls for the combination of community education

and emergency response training with public

relation efforts.

Carriers

According to the Interstate Commerce Act,

54,

Statute 889, carriers are classified as public or

private. Private carriers are carriers that are (an

incidental) part of the shipping company through

ownership and/or leasing; they are not subject to

government price regulations.

Public carriers can be common, contract, or

exempt. Common carriers agree to serve all who

wish to do business, and their prices are regulated

by the government. Contract carriers are bound

by a contract between the carrier and the shipper,

are limited in the number of firms they can serve,

and are not subject to government price control.

Exempt carriers are not subject to government

price control but are limited to certain products,

geographical regions, and organizations formed.

All carriers are subject to government safety

regulations.

To comply with government regulations and

meet public expectations, logistics departments of

chemical companies are putting a great deal of

effort in the selection of carriers, establishment of

partnerships with carriers, and improvement of

communication with carriers and communities.

Partnerships

Partnership relations between shippers and

carriers are now becoming standard for logistics

management (Rotman and Gibson, 1989; Morris,

1990).

Companies like Du Pont, ICI, and

Monsanto are giving this much concern. They

believe that if service can be improved and if

safety regulations can be completely carried out,

a carrier should be a partner from the beginning.

By reducing their carrier base and by formalizing

that relationship with contract carriers, many

shippers achieve high standards of safety and

performance.

Chemical shippers are reducing their carrier

bases to a few trusted companies; at the same

time,

they are attempting to establish more durable

relationships and monitor more closely the safety

and service of

the

selected carriers. For example,

Monsanto cut its carrier base from 1500 to 100

within three years; PMC's chemical products

group slashed its base of bulk carriers from 40 to

12 and packaged carriers from 500 to 60 in 1992.

With fewer carriers, the existing carriers obtain

more business and gain incentive to improve

service and safety.

One approach used by some companies to

ensure safety and service standards by truck

carriers involves the use of contract rather than

common carriers. For example, Du Pont uses

about 95% of its bulk truckers on a long—term

contract basis. As a major step in beginning its

comprehensive safety program. Ethyl Co. decided

that all shipments of hazardous materials would be

by contract carriers (Cooke, 1992) using Ethyl

owned or leased trailer and tank equipment. Dow

Chemical Co. contracts most of its truck transpor-

tation with motor carriers to ensure that they

adhere to its standards for safe handling. These

companies got away from the traditional, pre-

regulation relationship with carriers and built up

close relationships with contract carriers, sharing

information and even objectives, and in some case

contract carriers could paint their trucks to match

the companies' logos.

Selection of

carriers

and drivers

Carefully scrutinizing transportationproviders

is the first and critical step for hazardous material

shippers to achieve safe transportation. The

screening processes used by some companies gives

us a good model. Auditors from the Department

of Transportation (DOT) are responsible for

checking every aspect of a carrier's safety tech-

niques, from drivers' logs to equipment mainte-

nance records. If a DOT auditor finds violations,

he or she will rate the carrier accordingly with

three options: satisfactory, conditional, and unsat-

isfactory. Most companies require that the hazard-

ous material carriers have the satisfactory rating.

Some shippers also carry out on—site surveys of

all their contract carriers. Du Pont and Rohm and

Haas perform thorough evaluations and on—site

surveys. They go into their multiple facilities and

690 33. MISCELLANEOUS TOPICS

review maintenance programs, logs and training.

They want to see how safe the carriers are with

their products (Gordon, 1990).

Besides examining the DOT audit and

on—site survey, some shipping companies think it

is necessary to train the drivers themselves; Du

Pont trains carrier employees extensively. Its

hazard material training program called RHYTHM

(Remember How You Treat Hazardous Materials)

has become an industry standard (Gordon, 1990).

Ethyl Co. (Cooke, 1992) has developed its own

instructional and certification program for the

drivers of the contract motor carriers who have

excellent safety records at the time of their selec-

tion. Ethyl wants to be sure that the carriers'

drivers are fully knowledgeable about the chemi-

cals they would be hauling and to be sure that the

drivers are taught appropriate emergency—re-

sponse procedures including proper use of person-

al protective equipment. The principal benefit of

this program is the improved safety in transporting

their hazardous materials.

Chemical distribution

growing increasingly

sophisticated and requires improved communica-

tion.

Traditional paper documentation and the

hotline system called CEMTRCE (an emergency

response communication standard) seems unsuit-

able.

Electronic data interchange (EDI) into a

nationwide computer system

has

to be established.

The system would provide fire fighters with the

exact details on all U.S. shipments of hazardous

materials and provide emergency personnel with

dial—up access to a mainframe database of elec-

tronic manifests listing details regarding the con-

tent of every hazardous materials' shipment.

Since the present EDI technology is not advanced

enough to process this vital information, the

proposal to the Congress to mandate the use of a

national computer tracking system has not been

passed yet (Hamilton, 1993).

33.9 OPERATIONS MANAGEMENT

Operations are used to convert resources

(inputs include people, capital, raw materials) into

products (output). Products are usually either

manufactured goods (tangible products like a new

car) or services (intangible goods such as banking

services). The output should have more value

than the input to "create wealth."

Operations management is the decision-

making process whereby all aspects of the process

are selected based on the criteria selected for the

process. Processes must be flexible to respond to

changing market conditions and customer needs,

efficient in order to capture market share from

other competitors,

and

produce high—quality prod-

ucts.

Requirements for an operation include

research and development to design a product;

engineering to scale the process to industrial size;

capital for equipment, facilities, inventory, and

raw materials; marketing; accounting; human

resources; and information systems.

Other criteria include product safety and

limited environmental impact. Consumers and

society have placed higher values on these criteria

in recent years.

A system consists of a group of related

activities, items, and events. A change in one

item will cause a change in other part(s) of the

system. Any system is a part of a larger system

and is usually made up of subsystems.

Flow charts are used to show overall

processes. Process chart symbols include

^^.^^ an "o" for operation, arrow for

' 1^ transportation, triangle for storage, D for

delay, and square for inspection. Some-

times symbols are combined if operations

are combined; for example, a triangle

inside a square indicates storage and

inspection.

Flow charts are often used in con-

junction with time and motion studies to reduce

bottlenecks and improve the overall process.

33.10 PROCESS SIMULATION

Introduction

Process simulation in pulp and paper usually

implies the mathematical solution of mass and

energy balances facilitated by the use of comput-

ers.

[There are many other types of simulation as

well, such as simulating the steps in an industrial

process or a grocery store. In the latter, the

computer would "create" customers that enter the

store at times based on a probability distribution.

These customers would spend various lengths of

time in the store and purchase various items. The

average length of

time

(and variability) they would

be required to spend in a checkout line (queue)

PROCESS SIMULATION 691

and other parameters could be determined. These

simulations allow one to predict where bottlenecks

will occur and how scheduling of employees might

change the situation.]

Steady—state

simulators work on the assump-

tion that variables are constant with time or reach

a constant level. The heart of simulation is the

selection of mathematical equations that represent

the relationships between

variables.

These mathe-

matical equations are called models. In turn,

many of the mathematical equations are based on

certain assumptions.

The results of simulation are as good as the

assumptions of the models, the models, and the

degree of precision of the actual values of a

variable. The sensitivity of a variable is the

amount that a small change in the value of that

variable causes the final result to change. If a

small change in the value of a variable causes a

large change in the result, it is important to know

the value of that variable with a high level of

precision. The term garbage—in—garbage—out is

used to describe simulation processes where there

is a high level of uncertainty in assumptions,

models, and the actual values of variables, so the

results are essentially meaningless.

Simulation does not have to be elaborate to

be useful and effective. Some of the most useful

simulations are quite simple. The degree of cook

in the kraft process is modeled by the H—factor

which combines the variables time and tempera-

ture.

The mathematical relationship is sound and

the variables of time and temperature can both be

measured with a high degree of precision. The

effects of time and temperature of the pulping

process on the yield can be predicted accurately

for an infinite number of combinations ahead of

time.

That is why this has enjoyed widespread

use.

The model does not describe the influence of

pulping chemicals on the pulp yield but other

models with these refinements have been described

(page 371). Section 17.6 describes the simulation

of fiber cleaning systems.

Process simulation, however, usually implies

the use of a specially designed program to keep

track of large numbers of variables. The first

process simulation programs of large scope were

developed in the late 1960s. Several popular ones

(roughly in order of decreasing popularity) for

pulp and paper include GEMS, which was devel-

oped by L.L. Edwards at the University of Idaho,

MAPPS,

which was developed by the Institute of

Paper Chemistry and is a modification of GEMCS,

MASSBAL, developed by SACDA at the Univer-

sity of Western Ontario, and FlowCalc, which is

written in Basic and is designed specifically for

microcomputer use; all of

these

have versions that

run on IBM/PC compatible computers. Many

people are performing sophisticated simulations

using spreadsheet programs such as QuattroPro®

or Lotus

1—2—3®.

Models should

checked for agreement with

reality by

validation.

The model should be tested

under several sets of conditions (where the re-

al—world results are known) that were not used to

develop the model. If

the

model consistently gives

results that are in agreement with real—world re-

sults,

then it is probably very useful. However,

one should be careful to not used the model under

conditions where its underlying assumptions are

not valid or it has not been validated.

Once a process has been adequately modelled

and validated, variables can be changed in the

computer and the result will be determined. This

allows a large number of conditions to be studied

without having to perform numerous, expensive

mill trials. Processes can be made more efficient

or economically favorable. It may also be a useful

tool for operator training.

Simulation a particular process

As always, one must clearly define the

objectives of the simulation project. The simula-

tion of a process requires much background

information on the process. What variables are

involved? What are the relationships between the

variables? What equipment will be used in the

process? What are the typical operating condi-

tions?

This information is used to formulate

mathematical models. These models are then

converted into a form that the process simulation

program can use. The model is then tested and

validated. The model can then be used under to

predict real—world behavior.

Processes may be dynamic or steady—state.

The grocery store is an example of dynamic

simulation. Sometimes there may be many people

in the store, and other times there may be very

692 33. MISCELLANEOUS TOPICS

few. The values of variables of dynamic process-

es change with time. Most process simulation is

approximated by assuming the process reaches a

steady state where the values of the variables do

not change with time after steady state is reached;

in this case the system can be modeled as a series

of algebraic equations. The two principal solution

algorithms of steady state simulations are sequen-

tial and

simultaneous.

In sequential simulation the

variables are solved sequentially in a defined

order, solved again as better values for each

variables are determined, and resolved until the

convergence (change in the value of the variables

from a solution to the next solution) is within

acceptable limits. With simultaneous solutions

each equation is put into a linear form and all of

the equations are solved simultaneously.

33.11 SUPERABSORBENCY

Superabsorbent polymers (SAPs) have been

used in hygiene products since the late 1960s

(Masuda, 1993). About 250,000 tons are used

globally each year, most of which are used in

disposable diapers for babies. Most SAPs for hy-

giene products are the sodium salts of moderately

crosslinked polyacrylic acid (not all of the acid

groups are in the salt form) in a dry powder of

300 iim particles. The crosslinking agents are

typically glycol diacrylate or A^,Ar—methylene-

bisacrylamide. Residual monomer (which may be

a skin irritant) levels are now below 100 ppm.

The famous polymer scientist P. J. Flory had

theoretically determined the important parameters

for water absorption by polymers. The important

factors for absorption capacity include a polymer

with a high affinity for water (i.e., a polar poly-

mer),

a low crosslinking density in the polymer

(just enough to keep the polymer insoluble), and a

high ion density on the polymer.

Electrolytes dissolved in the water to be

absorbed decrease the amount of water ultimately

absorbed by the crosslinked polymer, and here the

type of electrolyte is important, with polyvalent

cations decreasing absorption to a higher degree

than monovalent cations (as expected from colloid

chemistry principles); these factors become

important when SAP systems are extended to non-

hygiene product uses such as those pressed be-

tween two papers and used in sheet form.

The rate of absorption is determined by

physical properties, especially the surface area of

the SAP. Finer particles generally absorb faster,

although if the particle size is too small, a gummy

material is obtained that does not absorb water

quickly. Masuda (1993) states that diaper dryness

is correlated to absorbency under load (AUL, r =

0.81) and diaper leakage is inversely correlated to

gel—stability (r = 0.93). The trend has been to

use lower amounts of fluff fiber and higher levels

of SAP in hygiene products; these products may

be as much as 30% SAP.

33.12 BEARINGS

Introduction

This section will provide a brief overview of

the different types of bearings and their use. This

information comes largely from the SKF USA Inc.

(King of Prussia, Pennsylvania) Product Service

Guide (April, 1992). Fig. 33-5 shows the basic

types of bearings that are discussed here.

Bearings range in size from the precision

miniature bearings used in medical and aerospace

applications to deep grove ball bearings with bore

sizes exceeding 55 in.

Single

row

deep groove

ballbearings—Conradtype

This popular type of bearing is a relatively

simple, nonseparable (since elastic deflection of

the rings is used to introduce the balls into the

bearing during the Conrad assembly method)

design. It can carry significant radial loads, and,

due to the uninterrupted raceway grooves and the

high degree of conformity between balls and

raceways, substantial thrust loads in either direc-

tion, even at very high speeds. Single row deep

groove ball bearings are available in open form or

with various types of seals or snap rings.

Single row deep groove ball bearings of the car-

tridge—type

Cartridge—type bearings have the same stan-

dard bore and outside diameter as single row deep

groove ball bearings but are as wide as double row

bearings. They are supplied with two seals or

shields.

Max type

(filling

slot)/single row ball bearings

Max type single row ball bearings (also

called filling slot type) employ a filling slot (rather

-WIDTH-

Self'Aligning

Ball Bearing

BEARINGS 693

Single Row

Deep Groove

Ball Bearing

Angular Contact

Ball Bearing

Double Row

Deep Groove

Ball Bearing

Spherical Roller

Bearing

Cylindrical Roller

Bearing

Spherical Roller

Thrust Bearing

Tapered Roller

Bearing

Inner ring

Inner ring corner

Inner ring land

Outer ring land

Outer ring

6. Ball

Counter bore

8. Thrust plate

10.

11.

12.

13.

14.

15.

16.

Outer raceway

Inner ring raceway

Outer ring corner

Spherical roller

Lubrication feature

Outer ring raceway

Floating ring guide

Inner ring face

17.

Outer ring face

18.

CyUnderical roller

19.

Outer ring rib

20.

Cone front face

21.

Cone front face rib

22.

Cup (outer ring)

23.

Tapered roller

24.

Cone back face rib

Fig. 33-5. Eight basic bearing types and their terminology. Courtesy of SKF USA, Inc

25.

26.

27.

28.

30.

32.

33.

Cone back face

Undercut

Cone (inner ring)

Cage

Face

Shaft washer

Housing washer