EEN-E3001 Fundamentals of Industrial Energy Engineering Ilkka Hannula ([email protected]) Lecture #3: Pulp mill fibre line Finland's top export products 2017 Paper and paperboard Wood pulp Softwood sawnwood 1. 5. 6. 6,9 bill. EUR 2,0 bill. EUR 1,9 bill. EUR 2. Diesel fuel 3. Stainless steel 4. Motor vehicles for 7 Ships and boats 4,5 bill. EUR 2,7 bill. EUR personal transport 2,4 bill. 1,5 bill. EUR EUR 8. Electric generators 9. Earth movers and excavators 10. Special machinery and motors and other similar machinery 1,1 bill. EUR 1,4 bill. EUR 1,2 bill. EUR 2 14.2.2018 SOURCE: Finnish Customs Value of forest industry exports Billion EUR 14 12 Value of exports, EUR mill. 2017 Pulp, paper, board and converted products 9 100 Wood products 2 800 10 Furniture industry 200 Forest industry total 12 050 Change from prev. year Pulp, paper, board and 8 converted products 3,4 % Wood products 10,7 % Furniture industry 18,9 % Forest industry total 5,3 % 6 4 2 1970 1975 1980 1985 1990 1995 2000 2005 2010 2015 3 26.11.2018 SOURCE: Finnish Customs Utilisation of wood Small wood, like branches and tops to energy use Pulp wood, i.e. round wood not suitable as logs, to pulp making Round wood to sawmill industry and construction. Side products to pulp industry and energy generation Majority of forest residues, like stumps, branches and needles are left in the forest to secure nutrien cycle. Part of forest residues is used to produce bioenergy. The short history of pulp . Using wood for paper making a fairly recent invention . Egyptians made paper from reeds . In the Middle Ages paper made out of cotton and rags (limited resource) . In 1800 Matthias Koops published a book on papermaking made from straw . Based on grinding of wood, pulping not mentioned. Chemical processes followed: . In the 1870s first commercial sulfite pulp mill was built, in Sweden . By 1900, sulfite pulping had become the dominant means of producing wood pulp, surpassing mechanical pulping methods. The competing chemical pulping process, the sulfate, or kraft, process, was developed by Carl F. Dahl in 1879 . The first kraft paper was produced in 1885 at Munksjö mill in Jönköping, Sweden. The invention of the recovery boiler, by G.H. Tomlinson in the early 1930s,[3] allowed kraft mills to recycle almost all of their pulping chemicals. This, along with the ability of the kraft process to accept a wider variety of types of wood and to produce stronger fibres, made the kraft process the dominant pulping process, starting in the 1940s. White liquor The effective chemicals of white liquor are sodium hydroxide (NaOH) and sodium sulfide (Na2S). 19/01/2020 11 Gross chemical composition of wood Component Hardwood, % Softwoods, % Cellulose 42 – 49 41 – 46 Hemicellulose 23 – 34 25 – 32 Lignin 20–26 26–31 Extractives 3 – 8 10 – 25 Ash 0.2 –0.8 0.2–0.4 Source: Gullichsen & Fogelholm (eds.), Chemical pulping 6A in Papermaking Science and Technology (1999). Fabet, Finland. Dimensions of soft and hard wood Birch Eucalyptus Pine Pine Spruce Spruce (Spring) (Summer) (Spring) (Summer) Fibre lenght, mm 1.1 1 2.9 2.9 2.9 2.9 Diameter, μm 22 16 35 20 33 19 Wall thickness, μm 3 3 2.1 5.5 2.3 4.5 Cooking • Purpose of cooking is to use chemicals and heat to remove lignin and to keep cellulose containing fibres as long, unbroken and strong as possible. • Sulfate cooking (Kraft process) the most commonly used pulp production method. • Chemicals should dissolve as much lignin and as little cellulose as possible • Sulfate process uses white liquor, a mixture of sodium hydroxide (NaOH) and sodium sulfide (Na2S). • Temperature in sulfate pulping is normally 150 - 170 °C. Batch digester Batch cooking phases Source: Gullichsen & Fogelholm (eds.), Chemical pulping 6A in Papermaking Science and Technology (1999). Fabet, Finland. • “Displacement batch cooking” is based on the exploitation of the heat from the previous cooks for the heating of a subsequent cook. • Achieved by storing the hot black liquor in high-pressure accumulator tanks. 1.Chips + impregnation liquor. Filtrate from washing (~90C) is used as impregnation liquor. 2.Hot liquor fill, hot black liquor (HBL) pumped to the digester followed by white liquor + HBL mixture pumping. The displaced liquor taken first to the BL tank and finally to the HBL accumulator. 3.Heating and cooking. Some of the white liquor is added during the cooking stage. 4.Displacement, cooking liquor displaced by pumping first warm BL and later displacement liquor. Displaced liquor is pumped mainly to HBL accumulator. 5.Discharge. Washing filtrate is used for diluting the bottom of the digester and the digester is pumped empty. 19/01/2020 21 • Cooking processes divided into batch method & continuous method. • Batch cooking = phase by phase , minimum 4 digesters for uniform production. • Continuous cooking = chips and chemicals are continuously fed from the top and removed from the bottom of the digester. Divided to zones, in which different phases take place. 19/01/2020 25 • The most important factors affecting the washing result is the amount of wash water used in addition to the washing equipment. • The amount of wash water used is expressed by the dilution factor. • Increasing the amount of wash water improves the washing result, but economically as small an amount of wash water as possible should be used. • The more wash water is used, the lower the concentration of the filtrate (liquor) and the more evaporation capacity is needed later. Source: Gullichsen & Fogelholm (eds.), Chemical pulping 6A in Papermaking Science and Technology (1999). Fabet, Finland. In a displacement wash press washing can be divided into three parts:: 1.The pulp is dewatered 2.The remaining water is displaced with cleaner washing liquor and finally 3.The pulp is pressed to a relatively high consistency. 19/01/2020 30 • Lignin amount left in fibers is expressed with a kappa number.. • Lignin causes pulp to turn brown during cooking. --> Because bleaching chemicals are much more expensive than cooking chemicals, as much as possible of the lignin is tried to remove during the cooking process. --> Too extensive lignin removal increases cellulose degradation decreasing pulp strength and yield.. • Typical kappa numbers for pre- bleached pulp is • 14 - 20 for hardwood and • 25 - 30 for softwood pulp. Oxygen delignification Bleaching • In oxygen delignification part of the • The aim of bleaching pulp is to residual lignin in pulp is removed continue delignification and using using oxygen and alkali. bleaching chemicals to remove any • Oxygen delignification is a more lignin known as residual lignin. selective and gentler process for • Chemicals used for delignification in ligning removal than cooking. bleaching are yet more selective • It fragments and oxidizes lignin into (i.e. minimal impact on pulp yield a form which is dissolvable in alkali, and strength) destroys the color compounds in the lignin and removes impurities (resin) from the pulp. • Oxygen delignification process may have one or two stages Drying and finishing Drying and finishing . Unintegrated pulp mill: pulp dried for baling, storage and transport . Integrated pulp mill: pulp undried and transported to paper mill either by pumping or by conveyer . Drying weakens the strength properties of the pulp. The drier the pulp and the higher the drying temperature, the more the pulp loses its tensile strength. Therefore, the lowest possible temperatures are used in pulp drying. After drying, the warmer and wetter the bale is, the faster the pulp turns yellow. The web is often cooled to approx. 40 °C before baling because bale cooling takes a long time. Wet end Wire section . The goal of the wire section is to achieve the highest possible dryness for the web . On the wire section about 80-95 % of water is removed. The water is first removed by its own weight, and as the consistency increases, dewatering is intensified by suction. 19/01/2020 38 Press section . The function of the press section is to remove as much water as possible from the web and to compress and strengthen the web. Much cheaper to remove water from the web by pressing than by evaporating Press section . The dryness after the press depends greatly on the compression impulse, which is the product of compression pressure and retention time. The retention time depends directly on the nip length. Lengthening the nip increases the compression impulse, which increases the dryness after the press and saves drying energy. This also improves the web strength and runnability. The compression impulse is 5 - 10 times higher in the shoe press than in the roller press. Evaporation . The dryness of the web coming from the press section is approx. 50-55 %. The dryness is increased to approx. 90 % in the dryer by evaporation. It is important to dry the web to a high degree of dryness for two reasons: wet pulp does not keep well for very long and it is not sensible to transport wet pulp long distances. The web can be dried in an airborne dryer by convection drying or in a cylinder dryer by contact drying. In convection drying, the web is heated by the surrounding drying air. In contact drying, the web is heated with the hot metal surfaces in contact with the web. Cylinder dryer . There are several cylinder machines in use, but they are not being built for drying purposes any more due to high investment costs. Heat required for evaporation of water from the web is obtained from steam condensed in the cylinder. Heat released from steam condensation moves to the web through the cylinder wall.