frim centrifugal cleaners

8/12/2019 FRIM Centrifugal Cleaners

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Reliable designs for centrifugal cleaners

Geoff Covey,Chairman, Covey Consulting Pty. Ltd.

1st Floor, 660 High St. Kew VIC 3102, Australia

Abstract

Centrifugal cleaners have been used extensively in the pulp and paper industry for many years. However, there is very little information available on their performance whenremoving contaminants of different sizes and densities.

Devices which are similar to (or in some cases identical to) centrifugal cleaners have longbeen used in the minerals industry for the separation of particles which separate at differentrate. This paper shows how the methods used there can also be employed with paper-making

fibres, and how estimates of performance can, if necessary be made without extensiveexperimental data.

The paper will also discuss some of the factors that should be considered when selecting ordesigning a centrifugal cleaner system

Keywords: Centrifugal cleaners, pulp cleaning, contaminant removal

INTRODUCTION

For many years centrifugal cleaners have been used in the pulp and paper industry as a meansof removing small contaminants. Initially they were used only for removing dense material

(such as sand and dirt particles) but since the introduction of reverse cleaners they have also been used for the removal of low-density contaminants particularly plastic fragments.

Common applications include:

Pulp mills to remove sand and grit.

Bleach plants to remove dirt.

Recycled fibre plants to remove both heavy and light contaminants.

Paper machine stock preparation for final removal of contaminants.

Recently many papers have suggested the use of centrifugal cleaners to fractionate fibre or toremove fillers from recycled fibre, but it is not clear how widely such uses have been adoptedcommercially.

Despite the publications on their performance when fractionating fibres, comparatively littlehas been written on the contaminant separation performance of hydrocyclones. There is agood deal on the design of systems to give good rejection of contaminants with minimal fibreloss, and there is quite a lot of published information on empirical studies of fibre segregation,

but very little on prediction of efficiency of removal of contaminants of various sizes.

This gap in the knowledge base is very important as sometimes contaminants may be presentin quite specific size ranges, and although a given arrangement may be effective in removingsand that is fairly coarse, it may not be effective in removing the same sand after it has beensubjected to attrition.


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inner vortex that rises to the top of the unit and discharges through a cental pipe (the vortexfinder).

Normally, all of the components of interest are denser than the suspending fluid (wat er) andall solid components will tend to be collected from the bottom (rejects) outlet 1. Thisincludes the fibre, and is responsible for the well-known thickening effect whereby theconsistency of fibre in the rejects stream is typically about twice that in the feed stream.

Therefore, for normal operation, all of the components are denser than water and are trying tosettle. If an infinitely long residence time were available, all of the solids would report to therejects outlet and no segregation would occur. In a real system, what is relied on isdifferences in the settling rates of different solid components.

A schematic cross section of the cylindrical section of a centrifugal cleaner is shown in Figure2

Contaminantsdenser than watermove towards wall

Central region,erse

vortex of watere top

e vortex

where a rev

rises to thoutlet (thfinder)

Figure 2 Cross-section of a centrifugal cleaner showing swirling flow and separation ofdense material near to the wall.

The dense particles have to migrate through a finite distance of rotating fluid to the wall thethickness of this rotating fluid approximates the width of the inlet flow channel.

Ideally, the residence time of the hydrocyclone would be selected so that there is time for thefast settling particles (contaminants) to reach the wall and be discharged as rejects, while noneof the slower settling particles (good fibre) have time to reach it.

However, in practice some of the fibres enter the hydro-cyclone close to the wall and arerejected, and some of the contaminants enter the cyclone far from the wall and do not havetime to migrate to it (so they are included with the accepts). Therefore separation is never

perfect.

There are also other factors that also contribute to imperfect separation:

1 Operation of reverse cleaners is an obvious exception to this generalisation. In this case the fibre is denser

than the water, and some at least of the contaminants are less dense than water. In principle this separation could be effected by gravity separation with fibres sinking and contaminants rising (provided one operated at asufficiently low consistency and had adequate time).


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Short-circuiting some material short-circuits from the rotating outer region directlyto the inner core and out through the vortex finder. Little or no separation occurs withthis component.

Turbulence varied factors can cause the formation of large eddies which sweep fluidfrom close to the wall back into the bulk region, and so negates the separation already

performed on this fluid.

Fluid discharge with rejects It is necessary to discharge some of the inlet liquid withthe rejects concentrate just to maintain movement of the rejects and avoid plugging ofthe bottom outlet. This fluid will have a composition similar to that of the acceptsfluid, and so it represents an inevitable loss of separation efficiency.

The first two of these factors result in more contaminants appearing in the accepts than wouldotherwise be the case. The last factor results in the rejection of more good fibre than wouldotherwise occur.

DETERMINING PARTICLE SETTLING RATES

As already noted, centrifugal cleaners separate on the basis of differences in settlingvelocities. Therefore, the settling rates of each of the particles of interest must be determined.For the present purposes, fibres of eucalyptus and pinus radiata, and contaminants of sand and

black coal will be considered. Sand is selected as a common dense contaminant (density about2600 kg/m 3 and black coal as a less dense particle (density about 1350 kg/m 3, which is notvery much greater than that of cellulose fibres 1100-1200 kg/m 3).

The settling velocities must be calculated under the centrifugal acceleration in thehydrocyclones. This can be calculated on a theoretical basis, but results are not alway s reliable because of the effect of friction in slowing the liquid flow. According to Gulichsen 1 centrifugal forces may theoretically be about 800 g (i.e. about 800 times the acceleration due

to gravity) but are usually somewhat less in practice. For the present purposes a centrifugalacceleration of 500g has been used i.

Particles of coal and sand both have dimensions that are of similar magnitude in all directions(isotropic). Therefore, the simple methods for calculating terminal settling velocities that areused for spherical particles can be used (and the results presented below are for equivalentspherical particles).

Using the method in Coulson and Richardson 2 for terminal settling velocities gave the resultsshown in Table 1.

Table 1 Terminal settling velocities of various size contaminants under typical

centrifugal cleaner conditions (500g, water at 60C).

Particle size mm 0.05 0.1 0.2 0.5 1

Terminal settling velocity coal m/s 0.22 0.46 0.85 1.60 2.27Terminal settling velocityquartz m/s 0.64 1.20 2.17 3.43 4.85 Term settling velocity coal/quartz 0.34 0.38 0.39 0.47 0.47

i Performing calculations at a variety of centrifugal accelerations (and under normal gravity) shows that although

the absolute settling velocity changes markedly, the ratio of settling velocities of the various species does notchange very much. Therefore, the effect of the spinning action is not to increase separation so much as toaccelerate the process by which it occurs.


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Pulp fibres are greatly elongated (far from isotropic) and the method used for contaminants isinapplicable for fibres

There are a number of methods that have been presented for predicting the drag or terminal

settling velocity for non-spherical particles3

4

5

. For the present purposes, a method based onthe work of Heywood 3 and of Heiss 4 has been used 2.

This method characterises particles as they lie in their most stable position using a diameterequal to that of a circle having the same area as the projected area of the particle. As shown inFig 4 of Heywoods paper, this permits simple modifications to the equations for spherical

particles to permit representation of non-spherical particles. The results of Heiss coveredisotropic, disc and rod shaped particles.

An alternative, more recent approach is that of Haider and Levenspiel. This method was basedon data for isotropic and disc shaped particles only, and therefore should be used withextreme caution for rod-shaped particles. (There also appears to be a typographical error in

the paper, as it is difficult to make it agree with other published methods even for spherical particles).

The density of cellulose is well known, but the density of individual fibres is more problematic as it depends on the size of the lumen (a large lumen will reduce the fibredensity) and the degree of fibrillation (a high degree will reduce the effective fibre density).Therefore, in Table 2, settling velocities are presented for typical pine and eucalyptus fibres atdensities of 1100-1200 kg/m 3.

Table 2 Terminal settling velocities of wood fibres at various fibre densities (sameconditions as Table 1).

Fibre density kg/m 3 1200 1150 1100PineLength mm 3 3 3Width mm 0.044 0.044 0.044Equivalent diameter mm 0.41 0.41 0.41Shape factor 0.084 0.084 0.084Terminal settling velocity m/s 0.68 0.60 0.50

EucalyptusLength mm 1.1 1.1 1.1Width mm 0.02 0.02 0.02Equivalent diameter mm 0.1670.167 0.167Shape factor 0.094 0.094 0.093Terminal settling velocity m/s 0.38 0.34 0.27

Clearly, separation will only occur when there is a difference in settling velocities. Table 3shows the size of particles of coal and of sand that will have the same terminal settlingvelocities as wood fibres of density 1150 kg/m 3. This table shows that particles of sandsmaller than about 45 m cannot be separated from pine fibre under the conditions of thecalculation. Further, particles smaller than this will actually be more concentrated in theaccepts than in the feed (so very fine particles might be removed by means of reversecleaners). For eucalyptus, the limiting size is smaller at around 30 m.


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Table 3 Settling velocities of wood fibres of density 1150 kg/m 3 and sizes of particles thatsettle at the same rate.

Settlingvel m/s

Coalm

Sandm

Pine 0.60 130 45Eucalypt 0.34 75 30

Clearly, very large particles of sand with settling velocities much greater than those of woodfibres can be readily separated, that particles of 30-45 m will not be separated at all. Particlesof intermediate size will be separated to some extent, but this data is insufficient to determinethe degree of separation of these intermediate size particles.

USE OF REDUCED-RECOVERY CURVES

The minerals industry commonly uses cascades of hydrocyclones to separate mineralsinitially present at similar concentrations, and sometimes to produce two saleable products.Therefore it regularly needs to determine the efficiency of removal of intermediate size

particles.

The extent to which impurity particles of various sizes can be removed may be estimated by amethod which is commonly used in the mineral proce ssing industry and which is described inSME Mineral Processing Handbook (p 3D-22 et seq.) 6 .

As reported in the SME Handbook, workers have found that the proportion of a componentrejected varies according to the equation:

2)exp()exp(

1)exp(

+

=

x

xY (1)

Where:

Y is the fraction of a particular size passing to the rejects stream

is a characteristic of the particle-fluid combination and of the cleaner configuration.

x is the ratio of the diameter of the particle of interest to the diameter of the particle sizewhich passes equally to the accepts and the rejects (usually designated d 50 ).

The curve that can be fitted by the equation is known as the reduced-recovery curve.

Obviously, this equation is only applicable for particles that only differ in size. The density ofthe particles must be substantially the same, and the shapes must also be similar to the extentthat the effect of shape on settling velocity is approximately uniform.

The parameter is determined by fitting available data on rejects vs. accepts split at differentsizes for a particular installation. According to the SME Handbook, the value of is usually inthe range 2.5 to 6, and is most commonly 3 to 4.

Unfortunately, very data is available on the size of sand removed by pulp cleaners andobtaining such data requires experimental equipment that is not readily available. Howeverresults by Kadant 7 give sufficient information to calculate a value for and to calculate therejection efficiencies of coal and sand particles of various sizes. From this data it is found thatwith a conventional cleaner (as used in most pulp mills) there is about 92-93% rejection of100 m sand at typical operating consistency of 0.6-0.9% of hardwood pulp. This can becombined with information on sizes of coal and sand particles having the same settling


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velocity as pulp fibres and typical cleaner operating parameters to calculate the performanceof cleaners in removing coal particles of various sizes.

The parameter can be estimated by the following steps:

1. Part of the reduced recovery curve can be deduced from the thickening effect with pulp in

a known centrifugal cleaner.Data for two commercial cyclones are given in Table 4

Cleaner A Cleaner BRejects % 10 8Thickening factor 2 2.8

Pulp to rejects % 20 22.4Pulp to accepts % 80 77.6Corrected pulp to rejects % 10 14.4Corrected pulp to accepts % 88.89 84.35Y 0.111 0.157

Table 4 Performance of two commercial cyclones.As before, Y is the fraction of a particular size passing to the rejects stream.

The corrected pulp to rejects is allowing for the fact that some of the fibre in the rejectsstream is entrained with the accompanying water, rather than there as a result ofclassification so an amount is subtracted from th e rejects pulp equivalent to the amountof pulp that would be in the same volume of accepts #.

2. The value of is then determined by assuming that it will be the same for pulp andcontaminants in a given cyclone. This assumption is not strictly correct, but it provides areasonable approximation in the absence of better experimental data.I is taken that the contaminant size having the same settling velocity as the pulp fibres willalso be rejected at the same rate as the pulp.

The value of is found by fitting the reduced recovery curve (equation (1)) to the two points corresponding to the equivalent of the pulp rejects rate and the rejection of 92-3%of 100m sand (or same rejection rate of another contaminant having the same settlingvelocity as 100m sand). The highly non-linear nature of equation (1) makes it convenientto substitute one of the points into the equation and then solve the remainder numerically.

3. For the data used here, it was found that the best fit for Cleaner A was with = 3.0, andfor Cleaner B with = 3.6; both of these values lie within the most common range quoted

by SME as 3-4.

Figure 3 presents a graph shows that the data from the two types of cyclone give quitesimilar results. This is not surprising as they are of similar geometric proportions, andmuch of the difference relates to set-up for operation.

# In mineral processing applications, there can be some uncertainty as to whether the concentration applied here

should be that of the feed or the accepts, or some intermediate value. However in normal pulp cleaner operation,the consistency of the accepts is not much lower than that of the feed, and it makes little difference which ofthese values is used.


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Reject vs size for sand

0

20

40

60

80

100

0.000 0.050 0.100 0.150 0.200

Particle size mm

% R

e p o r t

i n g t o r e

j e c t s

Cleaner BCleaner A

Average

Figure 3 Reduced recovery curves for sand using rejects data from two commercialcyclones and = 3.6.

Figure 4 shows the relative performance of a centrifugal cleaner in removing contaminants ofsand and coal of various sizes.

Reduced recovery curves for sand and coal

010203040

5060708090

100

0.0 0.1 0.2 0.3 0.4Particle diameter mm

% R

e p o r

t i n g

t o r e

j e c

t s

Sand

Coal

Figure 4 Reduced recovery curves for sand and coal for = 3.4

Although both types of particle are more dense than pulp fibres, the performance in separating

them is very different because of the differences in the densities of the two contaminants.One stage of cleaning will remove more than 90% of 100 m sand particles, but for coal only25% of particles of this size are removed. This shows that contamination with fine, low-density particles can present difficulties for centrifugal cleaners.

Only the first stage of cleaners in a system removes contaminants, and subsequent stages onlywork to recover good fibre and so to reduce the losses from the system (in the process theyalso recover some of the contaminants with the good fibre and so slightly reduce therejection efficiency of the system).

The amount of material that can be rejected by a cleaner system can only be achieved by

either:


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Rejecting more pulp in the first stage so that more contaminants are also rejected. Thisapproach requires the use of additional stages of cleaning if fibre loss is not also toincrease; OR

Use of a double first stage of cleaning whereby two sets of cleaners (of the same size)are operated in series so that the accepts from one set is re-processed in a secondstage. In theory this likes quite attractive (even if expensive) as cleaners work on astatistical basis, and if single screening will remove (say) 90% of a contaminant, thendouble cleaning will remove 99% in total. Unfortunately it is found that with this typeof arrangement, the second cleaners are less effective than the first. Although thecleaner will theoretically remove 90% of a particular size, the 10% it does not removeoften appears more difficult to treat in the next step.

The reduction in efficiency per stage when one of these approaches is used is significant, butnot necessarily sufficiently so great as to make double cleaning impractical. The problem isthat in the paper industry we do not normally have reduced-recovery curves (or grade-efficiency curves) and there is a tendency to just look at the quantity of contaminant

remaining rather than its size distribution. The reality is that the portion of contaminant passing the first step of cleaning is the finest part, and additional passes will not be veryeffective in removing this.

It should also be noted that the treatment given above has been based on the requirement toremove contaminants from uniformly sized fibres. The same approach can also be used to

predict the separation of shives, fines or fibres of different types.

DESIGN AND OPERATION OF CLEANER SYSTEMS

This section of the paper will briefly discuss some matters that should be considered whendesigning a cleaner system, and in their operation, particularly when the duty changes.

Keeping the pressure drop right.

Cleaners rely on an adequate inlet velocity to achieve the necessary centrifugal force toinduce cleaning if the inlet velocity falls, cleaning will deteriorate. Too high a velocity doesnot adversely affect cleaning but it can lead to rapid wear of cleaner elements. Inlet velocity isdirectly related to pressure drop between the inlet and the accepts, and this pressure dropshould always be kept in the range specified by the manufacturer. This has two importantconsequences: firstly, the cleaner system must be of adequate capacity for the largestanticipated stock flow; and secondly, if flow is less than design, the pressure drop must bemaintained by either shutting off some cleaner bottles, or by recycling some of the acceptsflow.

Keeping the consistency right.

As the consistency of a fibre stock rises, so the fibres begin to interact and form a network.This network is very effective in holding contaminant particles and in preventing theirremoval. Therefore, there is little point in running a cleaner system at a higher consistencythan the manufacturer recommends, as it will not give good performance. For long fibre

pulps, the maximum cleaning consistency is usually about 0.75-.08% and for short fibre pulpsa little higher. These figures are for the first stage of cleaning, but subsequent stages must be

operated at a lower consistency (typically, each stage should be operated at a consistencyabout 0.05% lower than that of the preceding stage.


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Figure 6 An example of canistermounted centrifugal cleaners

Modular cleaners can be individually while the rest of the system is running. They also permitthe number of elements in use to be changed to maintain optimum performance when thethroughput of the system changes.

Designing in flexibility.

Accurate prediction of the duty of a cleaning system, particularly one handling recycled fibre,is extremely difficult. Throughput will change depending on the grade made and marketconditions. It is likely that the grade of waste to be processed will change with time, and evenwithin a grade, different shipments from different sources will have different concentrationsand types of fine contaminants.

Therefore it is wise to build a system with a degree of flexibility. This is most readilyachieved with a modular rather than with a canister system.

Ensure that the system has sufficient capacity to run at the maximum anticipatedthroughput while remaining within the recommended consistency and pressure droprange.

Allow for the possible need to run at higher rejects than originally expected. This can be to process fibre with higher than anticipated contaminants, or to avoid blockage problems. In practical terms this means that there should be additional capacityavailable in the second and each subsequent stage. It must be remembered that theneed for additional capacity will grow exponentially.

For example, if it is found necessary to increase the rejects fraction from eachstage by 20%:

o The second stage will need to process 20% more fibre, it will reject anextra 20%, multiplied by an extra 20% for the increased rejectionfactor.

o Thus the third sage will need to have an extra 44% capacity available.


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