STOCK PREPARATION SYSTEMS DESIGN CONSIDERATIONS

ABSTRACT

Uniformity is the most important criterion for the stock preparation system. The white water system and the broke system are subsystems of the stock preparation system. Because of this interrelationship, careful consideration must be paid to each of these systems. If one system is allowed to be unstable, the uniformity of the others will be affected. This non- uniformity will result in less than optimal performance of the paper machine, and it may lower both productivity and product quality.

I. SCOPE

Stock preparation is the interface between the pulp mill and the paper machine. In an integrated mill, the boundary limits start at the discharge from the high-density storage chests with the dilution of the heavy stock preparation function is complete when the stock is delivered to the headbox of the paper machine.

However, the white water system of a machine is an integral part of the stock prep area. And so, we must consider the excess water from the paper machine and its treatment

Broke, too, is a factor, not only the broke generated at the time of breaks, either at the wet or the dry end of the machine, but also the off-machine broke generated in the finishing room or from cull rolls. All this broke enters into the broke system and becomes part of the stock preparation system; it is really impossible to segregate the white water system from the stock prep and the broke systems, since they are so closely interrelated.

II. FUNCTION

The function of the stock preparation system is to take the required number of pulp and additives, to develop them to their optimal condition, and to deliver a uniform blend of these furnish components to the headbox. The word “uniform” is the most important criterion that we have to work with and satisfy.

III. DEFINITION OF UNIFORMITY

A dictionary definition tell us that if something is uniform, it has always the same form, manner, or degree: it is not varying or variable. The requirement of uniformity covers all of the items that affect the paper machine operation. All must dovetail and include:

• Consistency
• Freeness
• Flow
• Hydration
• Blend
• Water conditions

Ø White water

Ø Fresh water

• Proportioning

Ø Fiber

Ø Additives

IV CONSEQUENCE OF UNIFORMITY

If we have uniformity, then that implies certain characteristics of our system and they are as follows:

• Repeatability
• Fast turnaround on grade change
• Flexibility
• Dampening variations
• Salable quality of product

The reason for the need for uniformity going into the headbox is the paper machine it self which, in simplest terms, is a mechanical dewatering device through to the press section and a heat transfer device in the dryer section. In essence, the paper machine is a fiber/water classifier running at high speed.

V. PRINCIPAL ACTIVITIES

Within stock preparation there are four principal activities operating in a matrix fashion to meet the criterion of uniformity. The four activities are:

• Furnish development
• Process control
• White water system
• Broke system

VI. DEFINITION OF ACTIVITIES

1.Furnish Development

• Accepts fiber furnishes in their raw condition.
• Modifies the furnish component so that it makes optimum contribution to product quality or machine runnability
• Reduces variations in raw material to provide a uniform product to the machine.

2.Process Control

• Involves the controls of mass flows of various streams without changes to the constituents of the furnish components

3. White Water System

Collect control and condition excess white water from points of generation to reintroduction for:

• Reclamation of useful papermaking components of fines and fillers
• Reuse of white water to highest value
• Minimum loss of heat energy from the system
• Minimum use of fresh water for makeup

4. Broke System

Collect, control and condition broke from points of generation to reintroduction for:

• Reclamation of useful papermaking component
• Maximum uniformity of furnish stream.

I. PROCESS STEPS

Four activities can be broken into discrete process steps. With this division activities, we can define the desires result and list factors that should be considered when troubleshooting an existing system or doing the process design for a new system. Of the twelve process steps, the first five relate to furnish development, the sixt four are in the process control area while the next two are functions of the white water system. The last three relate to the broke system. The

The twelve process steps are:

1.Fiber conditioning

• Refining
• Deflacking

- Criteria

· Modify/enchance fiber properties to meet requirement of finished product.

- Concerns/Design factors

· Variation in quality from pulp mill:

o Short-term

o Long-term

· Variations in volume processed due TO Grade changes.

· Uniformity of treatment over life of wearing parts e.g. refiner tackle.

· Energy costs

· Quality of treatment required

· Blended of separate treatment

· How to maintain uniform treatment with a varying flow rate

· Ease of maintenance

2.Screening

- Criteria

· Remove material by size separation this is deleterious to either the mill product or the processing equipment.

· Disperse fibers, remove foreign materials which would damage wire or machine components.

- Concerns/design factors

· Minimal rejection of acceptable material

· Protection of screening surface from plugging/blinding

· Recycling/processing or reject stream

· Power requirement

· Type of contaminants

· Contaminant disposal

· Ease of maintenance

· Frequency of plugging/stapling

· Ease of unplugging/stapling

3.Cleaning

- Criteria

· Remove material by specific gravity/surface area differential that is deleterious to the mill product or the processing equipment.

- Concern/design factors

· Minimal rejection of acceptable material

· Change in efficiency over life of equipment

· Protection from plugging/wear

· Effectiveness of cleaning

· Number of stages required

· Efficiency required

· Type of contaminants

· Contaminant disposal

4.Deaeration

- Criteria

· Remove excess air from machine furnish so as to assist in stabilizing/improving machine operations, reducing/eliminating pinholes due to air bubbles

- Concerns/design factors

· Effectiveness of equipment

- Configuration

- “Flying wing”

- Tower

- Expandibility

- Power requirements

5.Chemical conditioning

- Criteria

· Maintain a uniform chemical balance of both the fiber constituents, filler additives and water streams for steady state conditions

- Concerns/design factors

· Monitoring of stream from pulp mill for variations in dissolved solids/chemicals

· Control of fresh water makeup/leaks to system

· Uniformity of makeup chemical streams

· Reliability of monitors/sensors

· What streams involved-and why.

- Alum

- pH control

- Sizing

- Color/Tinting

- Brightening

- Zeta potential measurement

6.Consistency control

- Criteria

· Maintain a uniform consistency of the controlled streams under all operating modes

- Concerns/design factors

· Range of operating volume due to differing production rates and/or grade mix

· Reliability of dilution source quality, quantity and constant pressure

· Repeatibility on settings of controls

· Accuracy requirements

· Range of consistencies

· Primary sensing element

· Close control essential for uniform sheet weight

· What can be expected from high density tank?

· Degree of leveling of fluctuations for each consecutive control plant

· How many controllers recommended

· Between high density and headbox?

7.Proportioning

- Criteria

· Maintain uniform material flow to main stream over operating range

- Concerns/design factors

· Sensitivity to consistency variations

· Easy of adjustment to different operating conditions

· Repeatibility

· Are mechanical means variable any more?

· What means of measuring/accuracy?

· Point of addition

· Flow meters are volumetric-need close control of consistency

· Easy of calibrating

8.Blending

- Agitation

- Criteria

· Produce a homogeneous product from various furnish components over the range and mix of the mill production

- Concerns/design factors

· Sensitivity to changes in consistency of components

· Time required to meet criteria

· Shape of chest/volume

· Ability on grade change to reach equilibrium quickly

· Type of blender

- Agitator

- In line

· Materials of construction

9. Surge control

Surge chest

Trim chest

- Criteria

· Sized to dampen flow variations between processing during normal run conditions

· Sized to permit short shutdown of portion of stock prep for equipment inspection/emergency maintenance without affecting productivity

· Sized to permit orderly shut down or star-up of equipment items

- Concerns/design factors

· What is time required to satisfy different criteria?

· Shape

- Round, rectangular, square

- Physical restraints

- Relative power requirements

- Clean out

- Pump suction

- Minimum operating depth

· Consistency control

· Operating range

· Recirculation

· Materials of Construction

10.Dewatering

- Criteria

· Removal of excess water to allow stock storage at a reasonable consistency

· Separation of water/fiber mixture to allow introduction of stock into furnish system without diluting the furnish system appreciably

· Separation of water/fiber mixture to permit use of water for dilution/ showering/makeup

- Concerns/design factors

· Efficiency of separation of components

· Variations in volume or solids may be above maximum operating conditions of dewatering equipment

· Compatibility of recovered water and point of reuse

- Solid content

- Temperature

- Chemical content

· Means of limiting the variations at the dewaterer from normal to break conditions

· Save-all sweetener

11.High density storage

- Criteria

· Sized to reduce production interruptions and losses due to downtime of either pulp mill or paper mill

· Delivers continuous supply of fiber at a controlled consistency

- Concerns/design factors

· Non uniformity of pulp quality from pulp mill

· H.D. tank not a blender

· Layered structure of fiber

· Plug flow

· Blending effectiveness

- Time

- Throughput rate

- Agitation

- Bottom form

12. Repulping

- Criteria

· Makedown of fiber to a processing or storage consistency from a paper machine operating consistency

- Concerns/design factors

· Variations in tonnage due to different operating conditions

· Variations in volume or solids may be above maximum operating conditions

· Power requirements

· Material of constructions

· Energy costs

· Protections from plugging

· Reliability of dilution source quality, quantity and constant pressure

VII. WHITE WATER SYSTEM

Included in this activity are the following process steps:

- Consistency control (6)

- Proportioning (7)

- Blending (8)

- Surge control (9)

- Dewatering ( 10)

The concerns/design factors that we should be cognizant of include:

- Slime build-up

- Temperature build-up

- Dissolved solid build-up

- Maintain white water in constant state with variations in machine operations

· Normal

· Break

· Stock off wire

- Maintain constant

· pH

· Temperature

· Fines content under various operating modes

- Compatibility (pH, color, etc.) at points of reuse

- Volume vs. operating conditions

· Surplus

· Deficiency

IX. BROKE SYSTEM

In this activity we may find the process steps

v Fiber conditioning

v Screening

v Cleaning

v Consistency control

v Proportioning

v Blending

v Surge control

v Dewatering

v High density storage

v Repulping

In this activity we would be concerned with the following:

Ø Have sufficient storage time available

Ø Be able to provide needed functions at various operating rates

Ø Ability to return to furnish at constant

§ Rate

§ Consistency

§ condition

Ø Direct return to main stream with ability to divert/hold when appropriate

Ø Pulper configuration, agitation, sheet circulation, dilution control of :

§ Couch pit

§ Press pit

§ Dry end

§ Trim/broke

Ø Fiber reconditioning

Ø Point of re-entry

X. SYSTEM ANALYSIS

The purpose of this exercise has been to provide the ability to break down any a stock prep system into various pieces that can be analyzed readily for suitability in meeting the goal. Each stock prep system is unique. This is because of different products, quality need, furnish component and water quality and quantity.

A goal of a stock prep system is not the same as its function (II). The goal is to do the necessary work so that the mill profitability will be maximized

The cleanliness requirements for fine papers are different from bleached bag grades. Therefore, we would expect to see a more sophisticated cleaning and screening system for the fine paper stock prep system. Another example would be that a desirable product quality for facial tissue grades is softness whereas for milk carton grades we want stiffness. The design and operation of the stock prep system either controls or influences the finished product quality. Therefore, we expect to see significant differences in installed equipment for the various grades of paper and board: however, the criteria and concerns for each process step need to be addressed.

Common sense tells us some system will be more complex than others. A system with only a single furnish component, Producing a single grade would be the easiest system to design. For an integrated mill, a sack kraft machine would be an example. Furnish development would be concentrated on fiber conditioning. Cleaning would be for gross contaminant removal. Screening would be for fiber bundle separation and wire protection. Because of high retention of the wire and absence of fillers, the white water system would be simple also, the broke system need not be complex. With proper system design, broke repulping should be done at a consistency compatible with storage consistencies. Our system is simple in three of its four activities and we might consider the fourth- -process control- -to be simple relative to the possible complexity. Because a system is simple, this is no guarantee that the design will meet the design criteria of uniformity. With responsibility for process steps meet the criteria.

Stock pulp system become more as we process a number of fiber stream, add fillers and dyes in order to produce a variety of grades. The process control system matches the complexity as we look at the complete system. The advantage of our exercise of categorizing activities and defining the process steps becomes evident when we analyze a complex system.