Lecture_7_Pulp_Manufacture.ppt

* 15 October 2009 The Road to Copenhagen - Biggest Obstacle to Global Climate Deal May Be How to Pay for It Industry Built From Scratch Pulping * The problem Wood (this is what we have) Fibers (this is what we want) Question of the day Your chance to be rich * Suggest two ways to liberate the fibers (in-tact) from the wood so that you can make paper from them. (i.e. develop ?nanomuncher? to eat inter-fiber lignin) The rich part: Approximately 45 million tons of pulps are produced in the US each year. If you could get just $1/ton for your royalty it would make you $45,000,000 ? not a bad salary. Introduction to Pulping: General Pulping Scheme * Ways to liberate fibers * Chemicals Dissolve the lignin but not the fibers Mechanical Soften the lignin 1st then tear the fiber from the wood Biological The holy grail of pulping Find some ?bug? or enzyme that will eat the lignin and not the carbohydrates Problems ? Most biological agents attack carbohydrates 1st, Reactions are slow, Biological agents are expensive Types of Pulping Two main ways to liberate fibers Mechanically Chemically There are also hybrids of these two methods Chemimechanical Semi-chemical * Popularity of Pulping Technologies % US pulp production Mechanical 20% Chemical 70% Hybrids 10% * The rest of the world is pretty similar Mechanical Pulping Uses mechanical energy with little or no chemicals Two main processes Grinding: logs are forced against a revolving abrasive stone Stone groundwood process (SGW) Refining: wood chips are fed between two metal discs ? one of them rotating Thermomechanical pulp (TMP) * Mechanical Pulping Overview Stone Groundwood: Equipment Stone Groundwood uses logs. Not used at all in the Pacific Northwest because we get almost all our wood as leftovers from saw mills as chips. There are a variety of different designs of this equipment. Basically, logs are pressed into a rotating stone. The fibers are washed off the stone with water. This also cools the stone. Running this system under a slight pressure improves the process. * Refiner Pulping This process uses wood chips as its raw material Two main processes: Thermo-mechanical Pulping (TMP) ? chips are presteamed Chemi-thermomechanical Pulping (CTMP) ? chips are given light chemical treatment * Refiner Plates The refiners contain opposing plates that shred the chips. These plate are separated into different sections 1st section (inlet) rips chips into wood slivers. Each subsequent step reduces the size of the bundles. Water carries the fibers through the disk * Mechanical Pulping Most of the constituents of wood retained This results in high yield (85-95%) Weak sheet. Chemical pulp sometimes needed to be mixed in It discolors easily High energy usage * Products from mechanical pulps Newsprint Tissue Coated publication *                                           TMP Process * Waste heat Steam and Heat Electricity Wood Pulp Refiner (56,000 horse power motor) TMP Economics Economics for 1 ton of medium brightness TMP pulp Accuracy of analysis approximately 20% Capital Cost for 500 ton/day mill = $300,000,000 Production Costs $/Ton pulp Chips 100 Electricity 100 Bleaching 50 Labor, Maintenance, etc. 137 Total Cost 387 Selling Price 625 Profits 238 * Annual profit $238/ton*500 tons/day*350days/year = ~$41 million/year for a mill that costs $300 million TMP Energy Consumption Energy consumption for modern TMP mill is approximately 2400 kWh/ton pulp. Average annual household consumption in Seattle for 2004 was 8,852 kWh. How many homes could NORPAC TMP electricity power? * TMP Energy Consumption * Huge TMP mill ~ 1500 tons pulp/day Annual NORPAC energy usage = 1500tons/day*2400kWh/ton*350days/yr = 1,260,000,000 kWh Number of houses = 1,260,000,000 kWh/yr = 142,000 8,852 kWh/house/yr Chemical Pulping Uses chemicals and heat to dissolve lignin Leave cellulose and some hemicellulose Major processes Kraft Soda Sulfite * Chemical Pulping Kraft process - caustic NaOH and Na2S Soda Process - caustic NaOH Sulfite - acid SO2 and Base: e.g. Mg(OH)2, Ca(OH)2, NH4(OH) * Chemical Pulping Overview Equipment: Batch Digester Large pressure reactors 2500 to 12,500 ft3 Typical mill will have banks of 6-8 digesters Heating of system is either through direct heat (steam) or though indirect heating of the cooking liquor with forced circulation * Chemical Pulping Overview Equipment: Continuous Digester Tube shaped reactor in which the chips move through continuously Various zones in the reactor: charging, impregnation, heating , cooking, etc. Typical reactor: Kamyr digester * * Kraft Pulping Kraft pulping means strong in German. Highest strength pulp Invented by accident ? looking for a cheap source of sodium ? Na2SO4 (saltcake) Dominant process in North America Proven, efficient recovery process Can pulp any wood species Yield is low: 40-45% * Products from Kraft Pulp Linerboard Xerographic Food Boards Source of strong fiber *                    Kraft Process * Reactor (350oF) ? Dissolve all lignin ¼ carbohydrates Wood Pulp Steam Washer Evaporate water Furnace NaOH Na2S in water Excess steam (dry paper) Excess electricity Steam Chemical Recovery For each ton of pulp produced, 3000-5000 gallons of black liquor produced Black liquor contains Organics ? dissolved wood components Inorganics ? cooking chemicals, now spent (inactive) form * Chemical Recovery For economic and environmental reason, the black liquor MUST be recovered ? hence the Chemical Recovery System Goals Burn organics to generate heat Convert inactive cooking chemicals back into active form. 98% of pulping chemicals are regenerated * Chemical Recovery ? process Black liquor is washed out of the pulp in a brownstock washer This weak black liquor is thickened by removing water in the evaporators The resulting strong black liquor is then sprayed into the Recovery Boiler * Simplified Flow Diagram of the Kraft Process * Pulping Chips Wash Water Pulp Evaporators Recovery Boiler Causticization Lime Kiln * * Kraft Pulp Economics Economics for 1 ton of Kraft pulp Accuracy of analysis approximately 20% Capital Cost for 1000 ton/day mill = $1,000,000,000 Production Costs $/Ton pulp Chips 200 Electricity 0 Chemicals 70 Labor, Maintenance, etc. 190 Total Cost 460 Selling Price 850 Profits 390 * Annual profit $390/ton*1000 tons/day*350days/year = $136,500,000/year for a mill that costs $1,000,000,000 Kraft Pulp Production Costs - Brazil Economics for 1 ton of Kraft pulp Accuracy of analysis approximately 20% Capital Cost for 1000 ton/day mill = $1,000,000,000 Production Costs $/Ton pulp Chips 80 Electricity 0 Chemicals 70 Labor, Maintenance, etc. 110 Total Cost 260 Selling Price 850 Profits 590 * Annual profit $590/ton*1000 tons/day*350days/year = $206,500,000/year for a mill that costs $1,000,000,000 Kraft Pulp Economics-Finland Economics for 1 ton of Kraft pulp Capital Cost for 1000 ton/day mill = $1,000,000,000 Expensive electricity ($0.30/kWhr) ? sell excess power (~30MW from 1000 ton/day mill. Production Costs $/Ton pulp Chips 200 Electricity 0 Chemicals 70 Labor, Maintenance, etc. 190 Total Cost 460 Selling Price 850 Electrical power 200 Profits 390 * Annual profit $590/ton*1000 tons/day*350days/year = $206,500,000/year for a mill that costs $1,000,000,000 Kraft Water Consumption Water consumption for kraft mill is approx. 21,000 gal/ton pulp. 21,000,000 gal/day @ 1000 tons pulp/day production 21,000 gal/ton*1,000tons/day = 21,000,000 gal/day Average annual per household consumption of water in Seattle is about 27,000 gal. 27,000 gal/year ÷ 365 days/year ~ 80 gal/day Kraft mill producing 1000 tons pulp per day uses same amount of water 262,500 houses. 21,000,000 gal/day ÷ 80 gal/day/house ~ 262,500 houses * ?????????? Which type of pulp mill is more environmentally friendly and why? TMP Kraft * * Drought in Southeast US 2008 Largest pulp producing region of the country Carbon emissions (global warming) Pulp mill controversy * http://www.youtube.com/watch?v=yOu-beqDh3M Bleaching * Who needs white paper anyway? Who Needs White Paper???? * Little Yellow Pills * What is Bleaching? Bleaching is the chemical process applied to cellulosic materials to increase their brightness Cellulose and hemicellulose are white and do not contribute to color Lignin, dirt, fiber bundles (shives) do contribute to color * Goals of Bleaching Attack and remove remaining lignin Attack and destroy color-causing molecules Attack and remove/decolorize dirt and fiber bundles (shives) Improve pulp brightness and cleanliness * Measurement of Brightness Brightness is measured by shining light on a sheet of paper and measuring how much is reflected. Units: % * Paper Sheet Incident Light Reflected Light Brightness of Paper Type of Paper Copy/Printing paper Newsprint Grocery Bag Brightness, %ISO 80 ? 90 60 ? 70 20 - 30 * How Does Bleaching Occur? Most common bleaching chemicals are very strong oxidizers In most cases, these oxidizers are strong electrophiles ? they steal electrons from lignin and other molecules, causing chemical bonds to break * Bleaching Chemicals * Oxygen Chlorine Dioxide Hydrogen Peroxide Ozone Extraction O D P Z E O2 ClO2 H2O2 O3 NaOH General Principles: Process Bleaching uses a combination of chemicals in series. One chemical alone will not remove residual lignin. Each step reacts with material modified in previous step. * Unbleached Pulp Bleached Pulp O D EO D NaOH O2 ClO2 NaOH O2 ClO2 Remove lignin Brighten pulp Bleach Towers * Environmental Impact of Bleaching * Washing Pulping Chlorine Based Bleaching Wash Water Wash Water To Recovery Boiler Effluent Treatment Process Description * Washing Pulping Chlorine Based Bleaching Wash Water Chlorine To Recovery Boiler Chlorine in Effluent Environmental Impact of Bleaching Effluent from any chlorine-based bleaching cannot go to the recovery boiler ? causes corrosion and explosions Effluent must be treated and released; affects aquatic life Bleaching is the biggest environmental problem in a kraft pulp mill * Overall Kraft Water Usage Source Gal/ton Wood Prep 1000 Pulping 5000 Bleaching 10000 Papermaking 5000 Total 21000 * Making bleaching better for the Environment Switch to sequences using only oxygen-based agents ? oxygen, ozone, peroxide Permits effluents to go to chemical recovery Close up mill ? no fresh water in, no effluent out *