INTRODUCTION TO CONTROL IN THE REFINING PROCESS

Author

Colin Baker

Company and address

IPT (Pty) Ltd, Germiston, R.S.A.

email

colin.baker9@btinternet.com

Keywords

review, refining, control, energy

ABSTRACT

Until comparatively recently the process of fibre treatment was predominately by beating, as a batch process. The control of this process was by manual means and control parameters were beating time and applied load (usually measured as weight). The process of beating was considered to be a skill or an art and was heavily dependent on operator experience. An experienced beater man could determine the degree of beating by feel and in many cases measured the wetness or freeness as a last resort.

As the use of refiners on continuous lines became more common the ability to control by touch became obsolete and load was adjusted by mechanical and electrical means.  The control of fibre treatment was however, still in the hands of the beater man and not in the hands of the wet end machine man.  So, in many mills control of refining to achieve required product properties was carried out by indirect involvement. However, as the process of refining became more completely understood and remote control systems became available, control passed into the hands of the machine man.  Control was still manual but more quantitative in terms of power applied to the fibre.

The advent of quantitative terms to describe the refining process has enabled the development of refiner control systems that are capable of continuous measurement and control of the refining process. This paper discusses the relative merits of the available types of refiner control and their relevance to the most important parameters of refining.

1  BACKGROUND

Until comparatively recently the process of fibre treatment was predominately by beating, as a batch process. The control this process was by manual means and control parameters were beating time and applied load (usually measured as weight). The process of beating was considered to be a skill or an art and was heavily dependent on operator experience.

An experienced beater man could determine the degree of beating by feel and in many cases measured the wetness or freeness as a last resort. Control of beating time depended on shift practice and the load on the beater could often be determined by the size of the beater man's boot.

As the use of refiners on continuous lines became more common the ability to control by touch became obsolete and load was adjusted by mechanical and electrical means.  The control of fibre treatment was however, still in the hands of the beater man and not in the hands of the wet end machine man.  So, in many mills control of refining to achieve required product properties was carried out by indirect involvement.

However, as the process of refining became more completely understood and remote control systems became available, control passed into the hands of the machine man.  Control was still manual but more quantitative in terms of power applied to the fibre.

2  REFINER AND STOCK PARAMETERS

The following refining parameters are of most importance to the development of fibre properties:

  • refiner power*
  • refiner energy*
  • refiner tackle
  • rotation speed*
  • no load power
  • residence time (energy)
  • pressure differential*.

Also of importance to the refining process are stock parameters such as:

  • temperature*
  • consistency* (energy)
  • flow*
  • pH
  • chemical additives
  • freeness or wetness*.

* These parameters can be used to control refining

3  CONTROL SYSTEMS

3.1  Control of energy
Expressed as kWhr/t or HP.D/T the term specific energy should be considered as the second equation of the specific edge load theory. The energy equation includes consistency and flow as factors thereby allowing effects of changes in these parameters to be quantified.

In an analysis of a refiner system using the specific edge load theory, it may be concluded that the greatest effect on properties is that of energy with specific edge load playing an important, but lesser role.

The terms energy in this context describes the number of impacts received by the fibre.  The effect of energy differs with the type of fibre. For example, tear index decreases with increasing energy input for every fibre except eucalyptus. On the other hand, strength properties such as burst and tensile increase with energy whatever pulp is being treated.  For this reason, for most pulps, tear and tensile cannot be optimised and a compromise is necessary.

Specific energy is probably the most widely used method of refiner control.  While relatively complex compared to control of power the system provides reliable refiner control as three process signals (consistency, power and flow) are considered, (Figure 1).

Figure 1

Figure 1 Specific energy control (source 1998 Tappi stock preparation short course, Atlanta, 25-27 April)

When consistency of flow vary the refiner set point is maintained and the energy input will vary which in time will cause the stock to be over or under refined.

However, for refiners receiving constant flow, with minimal consistency changes, power control will perform well. On a system with recirculation control, as shown in Figure 2, power control is adequate as flow through the refiner is maintained at constant level.

Figure 2

Figure 2 Recirculation system

3.2  Control of power
Refiner power controls specific edge load or the severity of impact received by the fibre. Power applied is considered in gross or net terms. Many systems no take the refiner no load power into account and control as net power. The higher the power or specific edge load the greater the tendency to cut.

Constant power control is the simplest system, maintaining refiner power at a given point. The system is inexpensive to install and responds quickly. However, the system is not responsive to process or furnish changes.  A system is shown in Figure 3.

Figure 3

Figure 3 Power control (source 1998 Tappi stock preparation short course, Atlanta, 25-27 April)

Figure 4 shows results from a refining study carried out at the Finnish Pulp and Paper Institute which shows a clear dependence between gap and specific edge load. This appears to demonstrate that specific edge load characterises the severity of conditions under which fibres absorb the impact.

Figure 4 Bar gap as a function of specific edge load (source Paperi ja Puu - Paper and Timber, vol 72, no 2, 1990)

The load carrying capacity will be dependent on the stock being treated (see pulp/refiner interactions).

There have been proposals to control refining by gap measurement, but as yet no commercial application exists. A major problem would be the effect of heat.

3.3  Rotation speed
Most refiners have fixed speed motors. The rotational speed of a refiner determines no load and cutting edge length and therefore specific edge load (ag).  The higher the rotation speed, the higher the no load. Therefore, although one can lower specific edge load and increase fibrillation, major changes in rotation speed will reduce refiner energy.

In most cases, the rotation speed of a refiner motor is determined by the size of refiner. When variable speed motors are installed, for example the Adapta Set system, changes in speed are low to minimise changes in no load.

Control of refining by power or energy lack one aspect in that changes in power applied will also change the intensity and type of fibre treatment. This is because net power is common to both specific edge load and energy calculations.  Changes in power to cope with changes in throughput will lead to changes in tendency to cut or fibrillate.

A method of overcoming this problem is the Adapta Set method of control. In this system variable speed motors are used and are controlled as part of the system. Changes in power can now be accompanies by speed changes to keep specific edge load to a set point.

This system is a good control method, but is probably only suitable for large new installations where a variety of products are to be made. This control system is no longer commercially available.

3.4 Control of pressure differential

Figure 5

Figure 5 Recirculation/pressure control system (source Tappi Journal, September 1989)

3.5  Stock temperature
There are very few references to the influence of stock temperature. Over the normal range of refining, say up to 200 kWhr/t, temperature rise is low enough not to have too great an effect on the refining process. However, it has been found that higher temperatures (60-80C) slow down the beating rate, whereas intermediate temperatures (40C) gave the highest tensile.

Much of the energy used in refining is released as an increase in the temperature, but over a range of energy inputs up to 300 net kWhr/t temperature still remains below 50C.

3.6  Drainage rate and freeness control

Figure 6

Figure 6 Control of refining by freeness (source 1990 Tappi Stock Preparation Short Course, Atlanta, April 25-27)

The Kajaani PDA is a continuous pulp drainage analyser which has corrections for up to four separate sampling devices on the process pipeline. The following parameters for the sampling device can be selected:

  • number of partial samples
  • dilution water quality
  • sampling interval
  • mixing and rinsing time.

The measurement is closely allied to the measurement of wetness and freeness.

3.7  Couch vacuum control
As with freeness control, couch vacuum controls using a property of the stock. However, this control system can only be used for the last refiner in the sequence. Measurement is taken on the paper machine and wet web breaks can be directly related. However, couch vacuum is also sensitive to other factors, for example basis weight changes, filler addition, retention and machine speed. There is also a delay between pulp refining and the couch roll, since refined pulp is normally blended in a chest before going to the paper machine.

Figure 6a

3.9  Fibroptronic
A new method of control is the Fibrotronic 3000 control system developed by Acieres de Bonpertuis in conjunction with EFPG.  The system has two optic scanners systems measuring 3000 fibres/minutes. One scanner measures refined the other unrefined stock.  Addition measurements are made on two further samples each of 300 fibres. Measurements made and parameters calculated from them are shown below.

Parameters measured by Fibroptronic 3000 system.

For 3000 fibres

Scanner gives:

  • fibre length distribution
  • fibre width distribution
  • fibre curvature distribution

Computer calculates:

  • average fibre length
  • weighted average fibre length
  • ditto according to pulp composition
  • average fibre diameter
  • ratio of long fibres to short fibres
  • weak morphological points and location on fibre length
  • curvature factor of fibres.

For 300 fibres:

Scanner gives:

  • specific surface distribution
  • ratio of lumen to cell wall distribution
  • weak points from cooking process.

Computer calculates:

  • freeness (SR or CSF)
  • water retention value
  • K factor (relates plates geometry, specific energy and pulp characterise).

With the number of real time measured parameters the potential for control is considerable and a futuristic concept for control is to use the Fibroptronic 3000 to modify the disc refining zone by zero motors in order to control refining intensity.

4  SUMMARY

Sterrazza tabulates the relative benefits of refiner control systems as shown in Figure 7.

Figure 7

Figure 7   Relative values for refiner control schemes (source Spectrum, the ABB North American Pulp & Paper Organisation of Asea Brown Boven Inc)

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