Pressure screening is a key process in pulp and paper production and is used to enhance the quality of a wide range of pulp and paper products. While the usual goal of screening is to remove oversize contaminants from the pulp, screening is finding increased use for fibre fractionation. Screening is applied in virtually all pulping and papermaking operations.
Advancements in screen cylinder design have been supported by a more detailed appreciation of the connection between performance and screening variables. Performance variables include the traditional parameters of capacity, debris removal efficiency, consistency drop, fractionation, minimum reject rate and power consumption. Technical advances in the industry have provided a more sophisticated definition of these parameters. For example, the increased use of image analysis has led efficiency to be evaluated as a function of contaminant size. The availability of optical fibre length analysers has heightened the awareness of how screens can be used to fractionate fibres on the basis of length.
The ability of pulp screens to accomplish this wide range of tasks follows from the breakthroughs in technology that have occurred over the past few decades. The continuing development of the performance components of a pressure screen, i.e. the cylinder and rotor, has enabled screens to increase both the quality of the accept pulp and the screen capacity. In cases such as in brown stock and headbox screening, advanced technology has led to the development of novel screening solutions.
Advances in screen design have greatly increased the options available to the pulp and paper mills. At the same time, mills have become more sophisticated in their expectations of screen systems. Rules-of-thumb and intuition are no longer sufficient to achieve optimal performance from increasing complex screening systems. Computer-based simulation tools provide a response to this challenge. In their earlier form, simulation was based on spreadsheet programs that created mass, contaminant, and flow balances. The limitations of spreadsheet programs lay in their manual approach to optimising systems, the lack of a user-friendly interface, and their inability to model system dynamics. AFT thus developed an advanced screening simulation which goes far beyond what spreadsheet programs can offer. It provides the ability to improve system performance through optimisation and "what-if" scenarios.
Screens remain a critical part of pulp and paper mill operations. Technical innovations in both component design and simulation systems have led to the creation of significant opportunities to enhance screen system performance. These performance enhancements can be achieved without major investments in capital equipment.
INTRODUCTION
Screening is a key process in pulp and paper production, and is used to enhance the quality of pulp destined for a wide range of pulp and paper products. While the usual goal of screening is to remove oversize contaminants from the pulp, it is finding increasing use for fibre fractionation as well. A product quality can be improved by fractionating fibres for targeted processing or for use in specialty paper products /1/.
For recycled fibre, pulp screens removes stickies, plastic specks and debris so that old newspapers, corrugated containers and other waste fibre can be prepared for making high quality board and paper products. First-stage screens (are also called "fractionators") produce high cleanliness accept pulp and separate long fibres with contaminants for cleaning, dispersion and refining. Fractionation may also be used to separate the fiber furnish according to the furnish's papermaking potential, as is done in multi-ply board production.
In mechanical pulping, pulp screens remove shives, chop and bark fragments that would otherwise compromise the appearance, strength and printability of the paper sheet. Screening is also used to fractionate and remove the undesirable, long/coarse fibre so that it may be receive further refining. In chemical pulping applications, screening removes debris such as plastic specks, fibre bundles and bark. By using more efficient screening processes for the elimination of shives and fibre bundles, screening has provided more flexibility in the choice of bleaching sequences. In approach flow applications, headbox screens protect the headbox and paper machine from large, aggressive contaminants such as rocks and glass. These screens also provide a final opportunity to remove oversize contaminants from the pulp before it is formed into paper, board or tissue.
There are more than 250 different types of pressure screens installed in the pulp and paper mills around the world. Many of these models reflect the differing design philosophies of different manufacturers. However this number also reflects the wide range of applications that exist within the pulp and paper industry. The operating conditions and the quality targets of the screening process will vary with each application. Table 1 defines a number of main product categories, the stages of screening for a particular product and the furnishes typically used for each. While these categories are broad, they are useful in establishing some general guidelines.
QUALITY OBJECTIVES OF SCREENING PROCESSES
Pulp type will have a significant effect on the capacity of a pressure screen and the selection of the screen components. The pulp type is also a consideration in assessing the contaminant removal efficiency because the type and properties of the contaminants will vary. An optimal screening design will take into account the properties of contaminants to be removed, the screen system limitations and product quality targets. The quality requirements for high quality printing papers and speciality papers, for example, are much more stringent than for board. In all cases, the screening system must be designed to avoid fiber loss while ensuring that the quality target is met. To increase efficiency, and at the same time minimise fiber loss, typically calls for increased investment in the screening system, such as by adding more screening stages, which results in capital expenditures and increased operating costs.
While the principal objective of screens is typically contaminant removal, screens can also be used to enhance other aspects of pulp quality. The quality specifications for newsprint and lightweight coated (LWC) and super-calendered (SC) paper grades are given in Tables 2 and 3.
PERFORMANCE COMPONENTS - CYLINDERS
The two main components of the screen that determine unit performance are the screen cylinder and screen rotor. Each has evolved significantly over the past two decades. In the case of cylinders, the technology has moved from drilled cylinders with hole diameters in excess of 1 mm to modern, slotted cylinders with slot widths as small as 0.1 mm. The use of slotted cylinders started to increase in the early 1970's. At that time, the contaminant removal efficiency for slotted cylinders was superior to that with holes, but low capacity greatly limited their use. The introduction of contoured cylinders in the 1980's greatly improved the capacity of slotted cylinders. Optimisation of the screen contour and developments in screen cylinder construction has since led to further reductions in slot width. The typical slot width decreased from 0.50 mm to 0.15 during the period from 1975 to 1995 /2/. Of course, the appropriate slot size for a particular mill application will vary greatly depending on the fibre length distribution, consistency, rotor type and other variables. The latest improvement, the wedgewire cylinder, has increased the open area of screen cylinders with slots and further increased screen capacity, especially with low consistency furnishes.
Each cylinder type has an optimum operating point, and holed and slotted cylinders have distinct operating characteristics. Slotted cylinders, for example, typically have higher contaminant removal efficiencies, but also require stronger and more frequent rotor pulsations. Small slots offer particularly high screening efficiencies, especially when they present a barrier for large contaminant particles. For smaller debris, which are governed by "probability screening", efficiency depends on the aperture design and contour height, as well as the reject rate, screen operating variables, degree of reject thickening, etc. Slot width has the greatest effect on screen capacity, efficiency and other performance variables. However slot width is not the only important variable. Figure 1 shows the substantial influence of the cylinder type and aperture type on screen operation for cylinders with the same slot width and contour height /3/.
Total surface area and open area are two of the principal parameters for sizing a screen cylinder. Together, the open area and total accept flow define the average passing velocity, which is used for cylinder sizing and is widely referenced in the literature. There is a belief that each rotor/cylinder application has an optimum passing velocity. For a given screen and capacity, the open area of a cylinder can be adjusted to achieve this value. Developments in cylinder technology, such as the wedgewire cylinder, have increased the open area of cylinders. Figure 2 compares cylinders with different open areas. However the optimal open area will depend on the total required capacity and the minimum practical slot width.
Cylinder contours increase capacity and enable using smaller apertures to be used, which increase screening efficiency. Various theories have been put forward to explain the action of contours. It is widely believed that contours introduce turbulence that deflocculates the pulp and clears fibers from the aperture. Contours have also been thought to reduce the hydraulic resistance of the flow through the aperture, and to local flow conditions where fibers could begin to build up /4/. Indeed all of these factors may be important. What is undeniable though is the very powerful effect that contours have to boost capacity relative to smooth slots.
Contour height has a considerable effect on both screen efficiency and screen capacity. For screening stickies, for example, increasing the contour height will significantly decrease screening efficiency /5/. One study conclude that the best result for screening efficiency can be reached by selecting a shallow contour and then adjusting the slot size to meet the capacity requirement /6/.
PERFORMANCE COMPONENTS - ROTORS
A cross-section of a rotor foil and screen cylinder with an illustration of the local flow patterns is shown in Figure 3. It shows pulp flowing outward through the apertures ahead of the foil, a suction pulse and a flow reversal adjacent the foil. The outward flow through the apertures in the screen cylinder resumes in the foil's wake. During the suction pulse phase, the slot is cleared by the flow which returns from the accept to the feed side of the screen cylinder. The strength of the pulsations may be increased by changing the foil design, decreasing the gap between the rotor and cylinder surface, or by increasing the speed of the rotor. While stronger pulses would more effectively eliminate potential blockages, there are physical limitations on the maximum pulse strength. Stronger pulses may also reduce efficiency. Different rotor designs are used for low and high consistency applications. To optimize the action of a rotor, one may change the overall design (closed versus open-type rotors), the shape, number and placing of the pulse-inducing elements, and the speed of the rotor.
Given the intrinsic interaction between the cylinder and rotor, one must consider the interaction between cylinder and rotor in making an appropriate equipment selection. As noted previously, a higher rotor speed is required for a contour-slotted cylinder than for a cylinder with holes. For a smooth cylinder with holes, an increase in rotor speed generally produces an increase in the reject thickening and fiber fractionation. Although this is not always the case, and a decrease in thickening was found for a rotor with blade-shape tips. With contour-slotted cylinders, rotor speed consistently leads to decreased reject thickening and fractionation /7/.
A final point to consider in selecting a cylinder and rotor for a particular application is the wide range of variables that can influence screen performance. These are summarized in Table. 4. However, while considerable challenge exists in determining the optimal cylinder -rotor combination, supplier companies with significant experience and resources in screening have accumulated the experience and knowledge to make these selections with a high degree of confidence.
SIMULATION AND AUDIT TOOL
The presence of numerous screening variables makes optimizing a screen system a very complex, challenging and time consuming task. The correct operating and sizing parameters are have often been found through trial-and-error. With modern mills, that may have high capacities, high-speed paper machines, complex screening systems and strict contaminant limitations, trial-and-error approaches are impractical.
To meet this need, AFT has developed an Advanced Screening Simulation Tool that takes a wide range of screening variables into account, and estimates the effect of screening conditions, operation parameters and equipment variables on overall system performance. The SimAudit™ screen system simulation software integrates the most current mathematical models of screen performance, AFT's screening knowledge database and a statistically -rigorous audit system into a single tool. The software provides an easy-to-use, graphical interface that allows rapid development of mill models and the assessment of "what-if" scenarios. Figure 4 shows a simulation of a 3-stage screening system with a reject refiner in a Scandinavian TMP mill.
The advanced simulation tool includes a database of screen types, rotor types and screen plate types for all major manufacturers, which allows all of the screens in the system to be quickly characterized. These and other features of the simulation are described below.
ADVANCED PULP AND DEBRIS CHARACTERIZATION
The pulp type significantly affects the performance of the screen and the screen system, with fibre length being the main fiber property affecting performance. Figure 5 shows the length distributions for some typical pulp types, demonstrating the large differences in fibre length distributions between pulps.
To account for the effect of different pulp types, the simulation uses the entire fibre length distribution and tracks 25 fibre length classes. The advanced simulation tool is capable of reading the data files produced by the FQA, Kajaani FS-200 and other commercially available fibre length analyzers directly. The benefit of tracking the detailed fibre length distribution is that it accurately calculates the degree of fractionation during each stage and throughout the entire system. Since fractionation changes the fibre length distribution at each stage, it can be accounted for in the performance of following stages depending on the degree of fractionation and screens position in the system.
The advanced simulation tool also characterizes the debris by a size distribution to accurately reflect the changes in debris content in the various stages. The debris size distribution enables the simulation to base its efficiency estimates on a combination of barrier and probability screening, depending on the relative size of the debris and screen characteristics.
MATHEMATICAL MODEL
The simulation uses a combination of mechanistic and empirical models developed in several laboratories worldwide /8,9,10,11,12/ in combination with AFT's screen performance database. Following Gooding and Kerekes' work, the concentration change of pulp and debris across a screen is derived from a mass balance over a cross-sectional element of the screen.
Passage ratio is defined as the ratio of the concentration of pulp (or debris) passing through the screen, cs, to the upstream concentration of pulp (or debris), cu, i.e.
FIBRE FRACTIONATION
Fractionation is an increasingly important objective of the screening system. This is especially true in TMP screening where selective refining of the long and coarse fibres can significantly improve the tensile strength and the surface and printing characteristics of the pulp. Fractionation can also be a detrimental consequence of screening. For example, in kraft pulping it is desirable to retain the high quality, long fibre fraction.
Advanced simulation tool predicts fibre fractionation by calculating the fibre length distribution changes throughout the screen system. This enables the design of screen systems that can tailor the fractionation requirements of an application. The ability to predict fractionation for a wide range of design and operational variables has been determined previously in a series of pilot fractionation trials. In this study, the predictive ability of the advanced simulation is examined in the context of two mill trials: A Scandinavian TMP mill and a North American recycle ONP mill.
The Scandinavian TMP mill used wedge wire screen cylinders with a medium/low profile height. The mean fibre length of the feed, accept and reject streams of the primary and secondary screen was measured and compared to the predicted values in Table 5. Table 5 shows not only the ability of advanced simulation to accurately predict the fractionation across a single screen but the ability to calculate the fractionation in the entire system.
MULTI-LAYER CARDBOARD MACHINE
This mill has a multi-layer cardboard machine that produces approximately 240 tons of box board per day. A detailed audit of the screening system revealed that the screens processing the pulp used in the bottom layer had poor runnability and high fiber loss.
The system balance was not easy to establish, as this particular mill had only reject flow meters. Despite this difficulty, a comprehensive audit was conducted using both simulation and data reconciliation to establish an accurate estimate of efficiency and fibre loss. In addition, a variable reject rate trial was conducted to establish the sensitivity of contaminant removal to reject rate on these screens.
The simulation was used to identify which new screen cylinder would provide the required efficiency while maintaining good runnability. The audit clearly showed that despite the high mass reject rate in the secondary stage, the performance of this screen was unacceptable. In this case, a high contour screen cylinder with different profile type was proposed. Furthermore, the advanced simulation toolwas used to show how to adjust the reject rates of the system to achieve the highest efficiency possible with the new hardware.
As a result of installing the new cylinder and adjusting the reject rates, the fiber loss to the system was reduced by 27% while the efficiency was maintained. In addition, fibre fractionation was reduced so that more of the high quality, long fibre pulp was retained in the system accepts, as shown in Tables 7 and 8.
IMPROVED CLEANLINESS AND CAPACITY IN BROWN STOCK SCREENING BY GLADIATORTM ROTOR AND AFT MACROFLOWTM SCREEN CYLINDER
The mill in this study is an integrated pulp and paper mill producing about 1000 tons of bleached sulphate pulp per day. Pulp is further processed in to various paper grades. The original design for the screenroom using M800 and M400 Centrisorters was for bump rotors, wedge wire cylinders and hole cylinders. Today all stages are running with AFT GladiatorTM rotors and AFT MacroFlowTM cylinders with 0.25 mm and 0.20mm slots in the primary stage and 0.30 mm slot in the secondary and 0.35mm slot in the tertiary stage.
The mill runs both the hardwood and the softwood kraft pulp. The AFT GladiatorTM rotor and AFT MacroFlowTM screen cylinder are working smoothly in all stages. The level of cleanliness is same in the primary stage as before change, but at the secondary stage the level of cleanliness increased by 10% compared to the previous rotor and cylinder combination. Energy consumption at each stage decreased by 20-35%. Total power consumption was reduced also because at the secondary stage the whole capacity runs now with one M400 screen by AFT GladiatorTM rotor and AFT MacroFlowTM screen cylinder combination. Results show also reduced long fibre fractionation.
CONCLUSIONS
The advancements in pulp screening have been matched by increasing demands on the screening process to increase capacity and efficiency. Continuing improvements in pulp screening technology have come from a range of sources. Improved product performance has been obtained from cylinders and rotors. Likewise, the improved design of the screen system has come from the development of a simulation software that is specialized to pulp screening. In particular, a predictive, screen performance model and statistical audit methodology has been integrated into a graphical, easy to use software that enables screening systems to be quickly and accurately modeled and optimized. The simulation uses a detailed characterization of the pulp and contaminants to predict the fibre length distribution changes, debris distribution changes and consistency changes caused by each screen in the system. The predicted values of removal efficiency, fractionation efficiency and consistency changes have been shown to correspond well with experimental results in a series of pilot plant and mill scale screening trials. In addition, it has been demonstrated how the combined simulation and audit features can improve mill screening system performance both by lowering the system reject rate and reducing fibre loss, as well as reducing fractionation. The combination of improved screening hardware, and improved process optimization software provides powerful tools for significant enhancements in pulp screen system performance.
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