Reliable designs for centrifugal cleaners

Transcription

1 Reliable designs for centrifugal cleaners Geoff Covey, Chairman, Covey Consulting Pty. Ltd. 1 st 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 when removing contaminants of different sizes and densities. Devices which are similar to (or in some cases identical to) centrifugal cleaners have long been used in the minerals industry for the separation of particles which separate at different rate. 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 extensive experimental data. The paper will also discuss some of the factors that should be considered when selecting or designing 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 means of 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 to remove fillers from recycled fibre, but it is not clear how widely such uses have been adopted commercially. Despite the publications on their performance when fractionating fibres, comparatively little has been written on the contaminant separation performance of hydrocyclones. There is a good deal on the design of systems to give good rejection of contaminants with minimal fibre loss, 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 present in quite specific size ranges, and although a given arrangement may be effective in removing sand that is fairly coarse, it may not be effective in removing the same sand after it has been subjected to attrition.

2 The minerals industry, which also uses hydrocyclones to separate minerals of different densities and/or sizes has also had to tackle this problem In their case the quantification of separation may be even more difficult as both mineral species are likely to have fairly wide size distributions (wide relative to pulp fibres, where good fibres are usually comparatively closely sized). In order to effectively design systems for hydrocyclones, the minerals industry has successfully developed calculation techniques to quantify the separation, and these techniques can also be applied to other systems, such as centrifugal cleaners in the pulp and paper industry. The cross over of the technique from one industry to another is made easier because not only do centrifugal cleaners in the paper industry and hydrocyclones in the minerals industry work in the same way, but they are often made by the same manufacturers and in many cases the designs of units supplied to the two industries are essentially the same. ACTION OF CENTRIFUGAL CLEANERS A centrifugal cleaners (or hydrocyclones) separates components of the feed by centrifugal action. A schematic of a typical centrifugal cleaner is shown in Figure 1. Accepts Vortex finder Tangential inlet Rejects Figure 1 Schematic of a typical centrifugal cleaner However, unlike in a centrifuge, this centrifugal action is not induced by rotating the equipment. Instead it is induced by introducing the feed stream at relatively high velocity, tangentially into a cylindrical body. This creates a vortex that tends to cause high-density components to move to the wall. The lower portion of the cyclone consists of a convergent cone (although this is not theoretically necessary). Material collected at the wall (the high density fraction) is discharged from the bottom of the cone. The bulk of the flow forms an

3 inner vortex that rises to the top of the unit and discharges through a cental pipe (the vortex finder). Normally, all of the components of interest are denser than the suspending fluid (water) and all solid components will tend to be collected from the bottom ( rejects ) outlet 1. This includes the fibre, and is responsible for the well-known thickening effect whereby the consistency 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 to settle. If an infinitely long residence time were available, all of the solids would report to the rejects outlet and no segregation would occur. In a real system, what is relied on is differences in the settling rates of different solid components. A schematic cross section of the cylindrical section of a centrifugal cleaner is shown in Figure 2 Contaminants denser than water move towards wall Central region, where a reverse vortex of water rises to the top outlet (the vortex finder) Figure 2 Cross-section of a centrifugal cleaner showing swirling flow and separation of dense material near to the wall. The dense particles have to migrate through a finite distance of rotating fluid to the wall the thickness 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 the fast settling particles (contaminants) to reach the wall and be discharged as rejects, while none of 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 are rejected, and some of the contaminants enter the cyclone far from the wall and do not have time 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 a sufficiently low consistency and had adequate time).

4 Short-circuiting some material short-circuits from the rotating outer region directly to the inner core and out through the vortex finder. Little or no separation occurs with this component. Turbulence varied factors can cause the formation of large eddies which sweep fluid from 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 with the rejects concentrate just to maintain movement of the rejects and avoid plugging of the bottom outlet. This fluid will have a composition similar to that of the accepts fluid, 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 would otherwise be the case. The last factor results in the rejection of more good fibre than would otherwise occur. DETERMINING PARTICLE SETTLING RATES As already noted, centrifugal cleaners separate on the basis of differences in settling velocities. 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 about 2600 kg/m 3 and black coal as a less dense particle (density about 1350 kg/m 3, which is not very much greater than that of cellulose fibres kg/m 3 ). The settling velocities must be calculated under the centrifugal acceleration in the hydrocyclones. This can be calculated on a theoretical basis, but results are not always 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 centrifugal acceleration 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 are used for spherical particles can be used (and the results presented below are for equivalent spherical particles ). Using the method in Coulson and Richardson 2 for terminal settling velocities gave the results shown in Table 1. Table 1 Terminal settling velocities of various size contaminants under typical centrifugal cleaner conditions (500g, water at 60 C). Particle size mm Terminal settling velocity coal m/s Terminal settling velocity quartz m/s Term settling velocity coal/quartz 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 not change very much. Therefore, the effect of the spinning action is not to increase separation so much as to accelerate the process by which it occurs.

5 Pulp fibres are greatly elongated (far from isotropic) and the method used for contaminants is inapplicable for fibres There are a number of methods that have been presented for predicting the drag or terminal settling velocity for non-spherical particles For the present purposes, a method based on the 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 diameter equal to that of a circle having the same area as the projected area of the particle. As shown in Fig 4 of Heywood s paper, this permits simple modifications to the equations for spherical particles to permit representation of non-spherical particles. The results of Heiss covered isotropic, disc and rod shaped particles. An alternative, more recent approach is that of Haider and Levenspiel. This method was based on data for isotropic and disc shaped particles only, and therefore should be used with extreme 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 fibre density) 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 at densities of kg/m 3. Table 2 Terminal settling velocities of wood fibres at various fibre densities (same conditions as Table 1). Fibre density kg/m Pine Length mm Width mm Equivalent diameter mm Shape factor Terminal settling velocity m/s Eucalyptus Length mm Width mm Equivalent diameter mm Shape factor Terminal settling velocity m/s Clearly, separation will only occur when there is a difference in settling velocities. Table 3 shows the size of particles of coal and of sand that will have the same terminal settling velocities as wood fibres of density 1150 kg/m 3. This table shows that particles of sand smaller than about 45 µm cannot be separated from pine fibre under the conditions of the calculation. Further, particles smaller than this will actually be more concentrated in the accepts than in the feed (so very fine particles might be removed by means of reverse cleaners). For eucalyptus, the limiting size is smaller at around 30 µm.

6 Table 3 Settling velocities of wood fibres of density 1150 kg/m 3 and sizes of particles that settle at the same rate. Settling vel m/s Coal µm Sand µm Pine Eucalypt Clearly, very large particles of sand with settling velocities much greater than those of wood fibres can be readily separated, that particles of µm will not be separated at all. Particles of intermediate size will be separated to some extent, but this data is insufficient to determine the degree of separation of these intermediate size particles. USE OF REDUCED-RECOVERY CURVES The minerals industry commonly uses cascades of hydrocyclones to separate minerals initially 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 a method which is commonly used in the mineral processing industry and which is described in SME Mineral Processing Handbook (p 3D-22 et seq.) 6. As reported in the SME Handbook, workers have found that the proportion of a component rejected varies according to the equation: exp( αx) 1 Y = (1) exp( αx) + exp( α) 2 Where: Y is the fraction of a particular size passing to the rejects stream α 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 size which 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 of the particles must be substantially the same, and the shapes must also be similar to the extent that the effect of shape on settling velocity is approximately uniform. The parameter α is determined by fitting available data on rejects vs. accepts split at different sizes for a particular installation. According to the SME Handbook, the value of α is usually in the 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 and obtaining such data requires experimental equipment that is not readily available. However results by Kadant 7 give sufficient information to calculate a value for α and to calculate the rejection efficiencies of coal and sand particles of various sizes. From this data it is found that with a conventional cleaner (as used in most pulp mills) there is about 92-93% rejection of 100μm sand at typical operating consistency of % of hardwood pulp. This can be combined with information on sizes of coal and sand particles having the same settling

7 velocity as pulp fibres and typical cleaner operating parameters to calculate the performance of cleaners in removing coal particles of various sizes. The parameter α 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 B Rejects % 10 8 Thickening factor Pulp to rejects % Pulp to accepts % Corrected pulp to rejects % Corrected pulp to accepts % Y 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 rejects stream is entrained with the accompanying water, rather than there as a result of classification so an amount is subtracted from the rejects pulp equivalent to the amount of pulp that would be in the same volume of accepts #. 2. The value of α is then determined by assuming that it will be the same for pulp and contaminants in a given cyclone. This assumption is not strictly correct, but it provides a reasonable approximation in the absence of better experimental data. I is taken that the contaminant size having the same settling velocity as the pulp fibres will also be rejected at the same rate as the pulp. The value of α 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 100µm sand (or same rejection rate of another contaminant having the same settling velocity as 100µm sand). The highly non-linear nature of equation (1) makes it convenient to 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 α = 3.0, and for Cleaner B with α = 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 quite similar results. This is not surprising as they are of similar geometric proportions, and much 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 of these values is used.

8 % Reporting to rejects Reject vs size for sand Cleaner B Cleaner A Average Particle size mm Figure 3 Reduced recovery curves for sand using rejects data from two commercial cyclones and α = 3.6. Figure 4 shows the relative performance of a centrifugal cleaner in removing contaminants of sand and coal of various sizes. Reduced recovery curves for sand and coal % Reporting to rejects Sand 30 Coal Particle diameter mm Figure 4 Reduced recovery curves for sand and coal for α = 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 µm sand particles, but for coal only 25% of particles of this size are removed. This shows that contamination with fine, lowdensity particles can present difficulties for centrifugal cleaners. Only the first stage of cleaners in a system removes contaminants, and subsequent stages only work to recover good fibre and so to reduce the losses from the system (in the process they also recover some of the contaminants with the good fibre and so slightly reduce the rejection efficiency of the system). The amount of material that can be rejected by a cleaner system can only be achieved by either:

9 Rejecting more pulp in the first stage so that more contaminants are also rejected. This approach requires the use of additional stages of cleaning if fibre loss is not also to increase; 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 second stage. In theory this likes quite attractive (even if expensive) as cleaners work on a statistical basis, and if single screening will remove (say) 90% of a contaminant, then double cleaning will remove 99% in total. Unfortunately it is found that with this type of arrangement, the second cleaners are less effective than the first. Although the cleaner will theoretically remove 90% of a particular size, the 10% it does not remove often appears more difficult to treat in the next step. The reduction in efficiency per stage when one of these approaches is used is significant, but not necessarily sufficiently so great as to make double cleaning impractical. The problem is that in the paper industry we do not normally have reduced-recovery curves (or gradeefficiency 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 very effective in removing this. It should also be noted that the treatment given above has been based on the requirement to remove 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 when designing 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 to induce cleaning if the inlet velocity falls, cleaning will deteriorate. Too high a velocity does not adversely affect cleaning but it can lead to rapid wear of cleaner elements. Inlet velocity is directly related to pressure drop between the inlet and the accepts, and this pressure drop should always be kept in the range specified by the manufacturer. This has two important consequences: firstly, the cleaner system must be of adequate capacity for the largest anticipated stock flow; and secondly, if flow is less than design, the pressure drop must be maintained by either shutting off some cleaner bottles, or by recycling some of the accepts flow. 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 their removal. Therefore, there is little point in running a cleaner system at a higher consistency than the manufacturer recommends, as it will not give good performance. For long fibre pulps, the maximum cleaning consistency is usually about % and for short fibre pulps a 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 consistency about 0.05% lower than that of the preceding stage.

10 Keeping the cleaners working. A cleaner is only useful if all the inlets and outlets are clear. The most common problem is blocking of the outlet tips of individual cleaners. A blocked cleaner is not removing contaminants, and any trapped grit can rapidly wear a hole in the wall. Therefore it is essential that the outlets be checked regularly and blocked cleaners cleared or taken out of service. This is particularly important with recycled fibre. Canister vs. modular The original way of installing cleaners was from linear, horizontal headers, with each cleaner having separate valves from the feed and accepts headers (the rejects header was usually at atmospheric pressure and no valves were required. This arrangement gives great flexibility, but requires a lot of floor space, a problem that could be reduced by mounting the headers vertically if there was sufficient head-room. Figure 5 Individually mounted cleaners with horizontal headers (above) and vertical headers (right) More recently, cleaners mounted in canisters have become popular. In this arrangement the cleaners are installed in vessels that have partitions to create separate chambers for feed accepts and rejects. There is no requirement (or possibility) for individual valves for each cleaner bottle. The resulting arrangement is very compact as can be seen from Figure 6. As noted, the canister arrangement saves on space and is simpler. It also permits a pressurised rejects chamber, which can allow the use of larger rejects nozzle-tips and hence reduced danger of tip blockage (particularly important with recycled fibre). However, the canister arrangement does have the problem that it is usually impossible to maintain it or alter its configuration without shutting-down the stock line. Blocked cleaners do not remove any material, each blocked cleaner is equivalent to a fraction of the flow bypassing the cleaning system. There is no way to clean a blocked tube in a canister while the system is running.

11 Figure 6 An example of canister mounted centrifugal cleaners Modular cleaners can be individually while the rest of the system is running. They also permit the number of elements in use to be changed to maintain optimum performance when the throughput 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 market conditions. It is likely that the grade of waste to be processed will change with time, and even within a grade, different shipments from different sources will have different concentrations and types of fine contaminants. Therefore it is wise to build a system with a degree of flexibility. This is most readily achieved with a modular rather than with a canister system. Ensure that the system has sufficient capacity to run at the maximum anticipated throughput while remaining within the recommended consistency and pressure drop range. 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 capacity available in the second and each subsequent stage. It must be remembered that the need for additional capacity will grow exponentially. For example, if it is found necessary to increase the rejects fraction from each stage by 20%: o The second stage will need to process 20% more fibre, it will reject an extra 20%, multiplied by an extra 20% for the increased rejection factor. o Thus the third sage will need to have an extra 44% capacity available.

12 o The fourth stage will need an extra 77%. These numbers look large, but under normal conditions each stage will only have about one-quarter of the number of elements of the preceding stage, s the effect on the total number of elements in the system and its cost is not large, and the additional cost is readily recouped when production is saved. Keep a spare set of polymer cones with small tips. If the characteristics of the feedstock changes it may be necessary to alter the fraction rejected (in severe cases it may not be possible to run the system at anywhere near its capacity until this is done). To do this normally requires replacement of the cone tips with ones of a different opening size. It is expensive to keep full sets of every size tip in store, but polymer cones can usually be cut back (or drilled through) so that a small outlet can be turned into a larger one. Ceramic cones usually offer much better wear resistance than polymer ones, but this increased life does not always justify the extra cost. Particularly in the first stage of the system, the grit concentration should be low and wear should not be excessive and polymer cones will usually give good life. Conversely, in the last stage grit concentrations will be high and abrasion resistant cones are highly desirable. Leave space for future expansion of all stages. It is not particularly difficult or expensive to extend the frames and headers of most types of modular cleaner, and this can provide an in-expensive capacity increase, provided that space is available. CONCLUSIONS Although centrifugal cleaners have been widely used for removing dense contaminants form pulp for many years, there has been little information on their relative removal efficiency of contaminants of different sizes and densities. This paper has explained how a relatively simple technique, which has been used for many years in the minerals industry with the same equipment, can be used to predict the grade efficiency for contaminant removal. Ideally, the parameter α should be determined experimentally for the particular cleaner and contaminant. However in the absence of better information, an estimate of removal efficiency that is adequate for most purposes can be obtained by assuming a value of α of 3-4. REFERENCES 1 Gulichsen, J. and Fogelbohm, C-J Papermaking science and technology Helsinki Coulson, J.M., Richardson, J.F., Blackhurst, J.R. and Harker, J.H. Chemical Engineering Vol 2, 3 rd Edn 1985, Pergamon. 3 Haider, A and Levenspiel, O. Drag coefficient and Terminal Velocity of Spherical and Nonspherical Particles. Powder Tech (1989). 4 Heywood, H The calculation of particle terminal velocities J Imp Coll Chem Eng Soc (1948). 5 Heiss, J.F. and Coull J. The effect of orientation and shape on the settling velocity of non-isometric particles in a viscous medium Chem Eng Prog 48(3) (Mar 1952). 6 Weiss, N.L. (Ed) SME Minerals Processing Handbook Vol 1, 1985, Society of Mining Engineers, New York 7 Kadant Lamort: Kadant Cyclotech General Trade literature on centrifugal cleaners.