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COMPACT BLEACHING REDUCES CAPITAL INTENSITY IN THE BLEACH PLANT
Lennart Gustavsson, Sven-Erik Olsson, Martin Ragnar, Jonas Saetheråsen and Vidar Snekkenes, Kvaerner Pulping AB, Karlstad, Sweden
bleaching, bleach plant, bleaching tower design, feeding of pulp washer, building volume, dilution of pulp suspension, filtrate system
The technology relating to the bleaching of pulp relates not only to the chemicals used in the bleaching sequence but also to the washers and how the bleaching system as a whole is designed. In this paper, a novel bleach plant system called Compact Bleaching is presented. This system represents a paradigm shift in the design of a bleach plant, as it addresses a number of hot topics. With a Compact Bleaching bleach plant it is possible to reduce the electrical energy consumption by some 25 %, to handle the pulp fibres more gently and to reduce the building volume by as much as 25–50 %.
With the environmental aspects of bleaching in focus in the public debate in the early 1990's, action had to be taken by the pulp industry. Apart from the obvious, to stop the further use of elemental chlorine and to turn to bleaching concepts denoted as either ECF or TCF, actions were also taken with regard to the washing of the pulp.
The dominating pulp washers in the bleaching had then been the vacuum filter and the atmospheric diffuser. Now the picture has changed completely and the use of press washers is favoured. The rationale here was simple, to reduce the effluent volume from the bleach plant in order to reduce the overall environmental impact of pulp manufacture. A low effluent volume often follows from a low water consumption. Different types of pulp washers have different potentials to give a highly efficient washing and/or a particularly low water consumption.
It is, for example, well established that in order to reach a particularly low water consumption in the bleaching, the use of press washers is a necessity (Stål 1994), since the water consumption is strongly dependent on the consistency of the pulp suspension leaving the washer. In the case of a press washer, the discharge consistency is typically 30–35 % (high consistency) in contrast to a filter washer discharging at a consistency of 10–16 % (medium consistency).
The main difference between a high and a medium discharge consistency is that different amounts of liquor are carried over from one treatment stage to the following. At 30 % consistency, the carried over liquor is 2.1 m³/ADt, whereas at 14 % consistency it amounts to 5.5 m³/ADt or 2.6 times more liquor. In the bleaching, consecutive stages are normally run at different pH's and different temperatures, which means that the alkali or acid requirement for pH adjustment and the steam requirement for heating (or heat-exchangers for cooling) are much higher with medium consistency than with high consistency discharging washers. In addition, the lower the carry-over of dissolved organic substance to a subsequent stage, the lower is the bleaching chemical requirement in this stage. The shift from filter and diffuser washers to press washers is thus well motivated, although this shift leads to one negative consequence.
Reducing electricity demand and improving strength by DiFeed
An atmospheric diffuser is normally placed on the top of the bleaching tower so that the pulp suspension is brought to the washer without any additional pumping action (Figure 1). The pulp suspension is not diluted, but is washed at the same consistency as it had in the tower. Since the washer is placed on the top of the tower, the wash liquor has to be pumped up. After passing the diffuser washer, the pulp suspension is immediately ready for the next tower.
Figure 1. In a bleaching stage with atmospheric diffuser washers, the pulp suspension passes through the diffuser on its way upwards before being pumped to the next bleaching tower
In a filter-washer-based bleach plant, the pulp suspension is diluted at the top of the upflow tower so that the pulp suspension can flow directly from the tower top to the filter washer (Figure 2). Here the 10–12 % consistency pulp suspension in the tower is first diluted to 3–4 % in a launder after the scraper, before it goes to the filter. The launder is controlled so that a volume of some 2–3 m³ of suspension is kept in order to ensure that the pulp suspension flows smoothly to the filter. At the inlet of the filter, further dilution takes place so that the fed suspension has a consistency of only about 1 %.
Figure 2. In a bleaching stage with filter washer, the pulp suspension is at first diluted to a consistency of 3–4 % in the launder at the top of the tower and then directly fed to the filter. At the inlet to the filter a further dilution to about 1 % takes place
Dilution of the pulp suspension at the top of a bleaching tower and feeding a press washer in the same way as in a filter washer was not a priori considered, since a press washer was considered to be more sensitive towards variations in the feeding consistency than a filter washer. One reason for this is the different feeding consistencies of the two washer types, filter washers being fed at a consistency of about 1 % in contrast to press washers which are fed at up to 3–4 % consistency. It was therefore believed to be necessary to have a stirred dilution tank for the pulp suspension before feeding to the press in order to ensure a uniform consistency. This consistency-equalising tank was of course preferably placed on the ground which meant that the pulp suspension had to be pumped from the tank to the press (Figure 3).
Figure 3. In a bleaching stage with a press washer, the pulp suspension is diluted to a consistency of about 3–5 % in a consistency-equalising tank before being pumped to the press washer
It is important to keep in mind not only that this solution involves installation costs for the tank and the pump, but also that it involves costs for electricity to the pump. In addition, the more medium consistency machinery, e.g. pumps and mixers, that the pulp passes in the fibreline, the higher is the likelihood for strength losses in the pulp, so that it is essential to minimise the number of pumps pumping the pulp suspension. There are thus many incentives for the development of a smarter way of feeding the press washer – i.e. feeding it in the same way as the filter washer without any pumps.
The first task was to ensure a uniform feeding consistency without the use of a tank, and a new combined discharge and dilution scraper was developed with a geometry and design which allows a very even dilution. However, some press washers utilise a pressure drop at the inlet as a way of ensuring a uniform distribution of the pulp suspension over the width of the drum, and this may be a problem since a higher consistency of the pulp suspension means both a lower flow and less liquor per ton of pulp and thus an increased risk for plugging. The flexibility of the press washer is also hampered if it relies on a pressure drop for pulp distribution, since the orifice has a diameter designed to provide a uniform distribution at a specific feed consistency.
A uniform distribution of the pulp suspension, without relying on a pressure drop, is a necessity for a successful direct feeding of the press. This can, however, be achieved with a special distribution screw (Eriksson 1985) that mechanically distributes the pulp suspension. The use of upflow–downflow towers instead of e.g. upflow towers is favourable for many reasons, particularly in that it provides a large buffer before the washer which significantly stabilises the operation of the plant. In addition, all the machinery for the discharge scraper is placed close to the ground where it is easily accessible. The complete feeding system is shown in Figure 4.
Figure 4. In a bleaching stage with a press washer fed by DiFeed, the pulp suspension is diluted to a consistency of 6–8 % in the special discharge and dilution scraper located in the bottom of the bleaching tower, before being fed directly to the Compact Press press washer
The pulp is fed to the washer under the influence of the static pressure from the pulp suspension in the tower. Dilution from 10–12 % consistency in the tower to a consistency of 6–8 % in the suspension to the press is carried out in the special discharge and dilution scraper. Two separate pipes then lead from the bottom of this reactor to the inlet screws at either side of the press. Since the pulp suspension goes directly from the tower to the press, this kind of feeding system has been called a "direct feed", the trade name being the abbreviated DiFeed. The first DiFeed system of a Compact Press was installed in the Kishu mill in Japan and was put into successful operation in the beginning of August 2004.
When the pulp suspension has passed the press nip of the press washer it has to be transported away from the washer and diluted before entering the next bleaching stage. So far, these two operations have been carried out in two different screws for all conventional press washers (Luthi and Carlsmith 1973) using upward feeding of the dewatered pulp suspension from the press nip. This kind of press washer gives good washing and it has a high capacity since the pulp flow is split into two parts which are washed separately on the two drums before being recombined in the press nip. Since the discharge screw is located on the top of the two drums and rewetting of the pulp suspension must be avoided, the discharge screw is used as a pure shredding screw and dilution takes place in a special dilution screw below or at a distance from the press. If these two screws could be combined , a lot of benefits would result: the mechanical treatment of the pulp would be reduced, which would be beneficial for the pulp strength, and the electrical energy requirement for the operation of the screws would also be reduced. In addition, the floor level of the press washers could be lowered by at least 2 m, and this would reduce the building costs. Also the maintenance costs and the total investment costs would be reduced.
One way of combining the two screws would of course be to put them in sequence on a common axis. This would mean a very long screw with different diameters at the different ends. After the shredding screw, the pulp suspension is however susceptible to dilution without any major energy input. A device to provide an even more cost-effective dilution was therefore developed (International patent pending) in the form of a ring of dilution nozzles at the top of the standpipe. The shredded pulp is here diluted from a high consistency (around 30–35 %) to a medium consistency, typically 10–12 %. A further benefit of this dilution system over that with a combined shredding and dilution screw is that the material required in the dilution area does not have to be considered also for the shredding screw and for the press. Instead, the demands of the pulp suspension to be washed govern the material choice. The dilution system is shown in Figure 5.
Figure 5. After being transported away from the Compact Press in the shredding screw, the pulp suspension is diluted in the DynaDil system at the top of the standpipe by means of action of a ring of dilution nozzles
Dilution by the action of dilution nozzles in the standpipe instead of in a dilution screw has been named "dynamic dilution", the trade name being the abbreviated DynaDil. The first DynaDil system in a Compact Press was installed in the Gruvön mill in Sweden and it was put into successful operation in the beginning of February 2005. Today, DynaDil dilution is a standard part of the Compact Press.
A smarter filtrate system
In the development of the bleaching processes, the temperature in the bleaching stages has gradually increased. A hot chlorine dioxide stage or a hot acid treatment stage requires a temperature of 85 °C or above in order to be efficient. A high temperature is also clearly an advantage for both extraction (Ragnar 2003) and chelation. Today, temperatures of 80 or 85 °C in a final D-stage are also common, and when hydrogen peroxide is used for brightness increasing bleaching in a pressurised peroxide stage, temperatures of 100 °C or above is the standard. In a filter-washer-based bleach plant process, temperatures above 75–80 °C were problematic since the vacuum in the drop leg became insufficient and the filter-washer thus ceased to function. With the development of temperature-insensitive pulp washers, a high temperature in the washing has become an advantage rather than an obstacle since the lower viscosity of water at a higher temperature increases the washing efficiency. However, washing at a high temperature also means that the temperature of the filtrate is high. This needs to be handled in the design of the filtrate system. It has been found possible to design the filtrate system in a way that makes it feasible to decrease the level at which the press washers are placed.
Reducing building volume
The larger the volume of the building hosting the pulp washers, the higher are the building costs. In the pulp industry, the high capital intensity has been identified as the biggest threat towards an expanding future (Johnson 2000). Every step towards lowering the investment costs should therefore be taken with gratitude. The higher the floor level necessary for the pulp washers, the greater is the building volume. Historically, pulp washers were placed at a floor level of about 15–20 m, the governing factor being the need to ensure a sufficient vacuum in the drop leg of the vacuum filters, but when this factor disappeared following the shift towards other kinds of filter washers, and particularly press washers, the washers have still been placed at a floor level of 10–15 m. The decisive factor has been the size of the filtrate tanks, and the need for separate shredding and dilution screws from the press washers. With the smarter filtrate system and replacing the dilution screws by the dynamic dilution type DynaDil, the whole building volume of the bleach plant and the building costs can be significantly reduced, particularly since the height can be reduced, so that the washers can be placed several meters lower than before.
In order to facilitate a simple reference to all the new aspects of this cost-effective bleach plant, a special trade name has been given to the whole concept, Compact Bleaching. A new bleach plant according to the Compact Bleaching concept is shown in Figure 6, for comparison with Figure 7 which shows the conventional design.
Figure 6. A bleach plant for the 3-stage D*(OP)D sequence built according to the Compact Bleaching concept including upflow-downflow bleaching towers, DiFeed feeding of the Compact Press type press washers, and DynaDil dilution of the pulp suspension after the press washer
Figure 7. A modern bleach plant of the conventional type for a 3-stage D(EO)D sequence including upflow bleaching towers, standpipes and feeding by pumps to press washers
The benefits of the Compact Bleaching concept can be summarised as;
A paradigm shift in the building of bleach plants has been presented. The new system addresses a number of major issues and thus significantly reduces the electrical energy requirement, reduces the investment costs for a new bleach plant and reduces both maintenance and operation costs. In addition, the quality of the pulp is improved. Many of the features of the Compact Bleaching system have been patented.
Gun-Brith Nilsson is gratefully acknowledged for drawing the flowsheets, Hans Furhem for valuable criticism of the manuscript and Dr. Anthony Bristow for the linguistic revision of the manuscript.
Eriksson, E. S. (1985 (publ.)): Apparatus for Discharging Material, USA patent nr. US 4559104
Johnson, A. P. (2000): Fiberline Technology in the New Millenium, International Pulp Bleaching Conference (IPBC), Halifax, Canada, Vol. Oral Presentations: 13-25.
Luthi, O. and Carlsmith, L. A. (1973 (publ.)): Apparatus and Method for Thickening and Washing Suspensions Containing Fibrous Material, USA patent nr. US 3772144
Ragnar, M. (2003): Alkaline Extraction and a Control Strategy for the Chlorine dioxide Charge to the Final Stage in DED Bleaching, Nordic Pulp Paper Res. J. 18: 2, 162-167.
Stål, C. M. (1994): Sunds Defibrator on the Road to the Closed Bleach Plant, IPPTA I-VIII.
ADDRESS OF THE AUTHORS:
Lennart Gustavsson, Sven-Erik Olsson, Martin Ragnar (firstname.lastname@example.org ), Jonas Saetheråsen and Vidar Snekkenes, Kvaerner Pulping AB, R&D, Box 1033, SE-651 15 KARLSTAD, Sweden