Proposal-Basic Design of Digester in Pulp Manufacturing Process - Final [PDF]

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“Basic



Design of Pulp Manufacturing process” Project Proposal



Project Supervisor: Dr. Bibek Uprety



Submitted by: Karuna Chhetri (42335) Sushant Mahat (42345)



DEPARTMENT OF CHEMICAL SCIENCE AND ENGINEERING SCHOOL OF ENGINEERING KATHMANDU UNIVERSITY June, 2020



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Contents 1: Introduction............................................................................................................................................... 4 1.1 Background ......................................................................................................................................... 4 1.2 Objectives: .......................................................................................................................................... 5 1.3 Scope:.................................................................................................................................................. 5 2: Design Basis ............................................................................................................................................. 5 2.1 Table: Battery Limit Conditions and Design Basis............................................................................. 5 2.2 Design Codes ...................................................................................................................................... 6 2.3 Regulation for Safety and Environment.............................................................................................. 6 2.4 Location .............................................................................................................................................. 6 2.5 Climate ................................................................................................................................................ 7 2.6 Kinetic expression ............................................................................................................................... 7 3. Process Description ................................................................................................................................... 8 3.1 Impregnation ....................................................................................................................................... 9 3.2 Cooking ............................................................................................................................................... 9 3.3 Recovery process .............................................................................................................................. 10 3.4 Blowing ............................................................................................................................................. 11 3.5 Screening........................................................................................................................................... 11 3.6 Washing ............................................................................................................................................ 11 3.7 Bleaching .......................................................................................................................................... 12 3.8 Process chemicals ............................................................................................................................. 12 3.9 Byproducts and emissions................................................................................................................. 13 3.10 Chemical reactions involved ........................................................................................................... 13 4. Process Flow Diagram(PFD) .................................................................................................................. 15 5. List of Equipments .................................................................................................................................. 16 6. Energy Intensity of key processes ........................................................................................................... 16 7. Gantt Chart .............................................................................................................................................. 18 8. Expected Results ..................................................................................................................................... 19 9.Conclusions .............................................................................................................................................. 19 9. References ............................................................................................................................................... 20



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List of Figures: Fig (1):



Delignification Kraft Pulping Rate for Softwoods………………………………….7



Fig (2):



Overview of Kraft pulping process………………………………………………….9



Fig (3):



Flow sheet for pulp manufacturing process………………………………………….14



Fig (4):



Process flow diagram for pulp manufacturing………………………………………15



Fig (5):



Material and Energy flows in pulp and paper industry……………………………….17



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1: Introduction 1.1 Background Pulp and paper mills are highly complex and integrate many different process areas including wood preparation, pulping, chemical recovery, bleaching, and papermaking to convert wood to the final product. Processing options and the type of wood processed are often determined by the final product. The pulp for papermaking may be produced from virgin fibre by chemical or mechanical means or may be produced by the repulping of paper for recycling. Wood is the main original raw material. Paper for recycling accounts for about 50 % of the fibres used – but in a few cases straw, hemp, grass, cotton and other cellulose-bearing material can be used. Paper production is basically a two-step process in which a fibrous raw material is first converted into pulp, and then the pulp is converted into paper. The harvested wood is first processed so that the fibres are separated from the unusable fraction of the wood, the lignin. Pulp making can be done mechanically or chemically. The pulp is then bleached and further processed, depending on the type and grade of paper that is to be produced. In the paper factory, the pulp is dried and pressed to produce paper sheets. Postuse, an increasing fraction of paper and paper products is recycled. Non recycled paper is either landfilled or incinerated. The pulp and paper industry is one of the most important industries in the world. It supplies paper to over 5 billion people worldwide. Originally, papermaking was a slow and labor-intensive process. Today pulping and papermaking are driven by capital-intensive technical equipment and high-tech and high-speed paper machines that produce rolls of paper at a speed that may reach 2000 m/min. and with a web width that may exceed 8 m. Paper is essentially a sheet of cellulose fibres with a number of added constituents, when necessary, to affect the quality of the sheet and its fitness for the intended end use. The two terms paper and board generally refer to the weight of the product sheet with paper ranging up to about 160 or 220 g/m 2 and a heavier sheet regarded as board. The grammage above which papers are called board however vary slightly between countries. There are several types of pulping processes. The main ones are:  Chemical pulping  Mechanical pulping  Recycled paper re-pulping Pulp and paper mills convert cellulosic fibers, mostly wood, into pulp and various types of paper products. At present, more than half of the globally produced pulp comes from the Kraft or sulfate pulping process. In the Kraft pulping process, the continuous pulp digester is one of the most critical components. The digester is a complex heterogeneous reactor in which white liquor—an aqueous solution of sodium hydroxide and sodium sulfide—reacts with wood chips comprising mainly of cellulose, hemi-cellulose and lignin, to remove lignin and subsequently free wood fibers. Due to the naturally varying feedstock, long and variable residence time, insufficient measurements and complex physio-chemical characteristics, the digester requires sophisticated control strategies to ensure safe and economically viable operations 4



Two most important design parameters for a traditional continuous digester are the cross sectional load (typical design was 50 cubic feet of chips per square foot and hour) and retention times (typically 40-60 minutes for impregnation, 75-90 minutes for cooking, 150-220 minutes for washing). In most mills the production has been increased to 50-100% above the design and the kappa target and wood species can also be changed.



1.2 Objectives:     



To provides a detailed description of the continuous pulp cooking process To produce pulp with specific quality while maintaining targeted production rate To design continuous digester To carry out the hazard identification and risk assessment of pulp manufacturing process To learn what modifications can be done in digester design.



1.3 Scope: The scope of pulping is to break the recycled paper or wood down into its component elements so that the fibers can be separated, by removing lignin, a non-fibrous constituent of wood, that is primarily responsible for reducing paper quality and permanence.



2: Design Basis 2.1 Table: Battery Limit Conditions and Design Basis For Digester: Operating Inlet Conditions Temperature(C) 140-150 Pressure(atm) 1 Flowrate



In between



outlet



170-180 10-12



65 80



41700 kg/hr or more



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2.2 Design Codes Various design codes are provided by various companies, among which primary design codes were used as follows: American Society of Mechanical Engineers (ASME)  ASME Section II - Material specification  ASME Section VIII- Pressure Vessel code Tubular Exchanger Manufacturer Association (TEMA)



2.3 Regulation for Safety and Environment According to Environment Protection Act, 1997, the concentrations of pollutants that an industry can release on yearly basis are kept under consideration of:  Environmental Protection Act,1997  Occupational Health and Safety Guidelines  National Occupational Safety and Health Policy  Water Resources Act 1992 and Water Resource Refutation 1993  Industrial Policy 1992



2.4 Location Any ideal industrial areas which have flat land and well drained conditions are suitable for the establishment of industry. Pulp mills are almost always located near large bodies of water due to their substantial demand for water. Delignification of chemical pulps releases considerable amounts of organic material into the environment, particularly into rivers or lakes. The wastewater effluent can also be a major source of pollution, containing lignin from the trees, high biological oxygen demand(BOD)and dissolved organic carbon (DOC), along with alcohols, chlorates, heavy metals, and chelating agents. The process effluents can be treated in a biological effluent treatment plant, which can substantially reduce their toxicity.



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2.5 Climate • Approx. maximum temperature- 45C • Approx. minimum temperature- 10C • Pressure- Normal (1 atm)



2.6 Kinetic expression



Fig (1): Delignification Kraft Pulping Rate for Softwoods at 160▫C At constant sulfidity and alkali charge, the delignification rate is regarded as a homogeneous first order reaction with respect to Lignin concentration (wt %) in the wood using the expression (Vroom (1957)): dL/dt = kL where ‘k’ is the rate constant. The temperature dependency of ‘k’ is provided as Lnk= (43.2 – 16.113/T). where T is the temperature of the digester.



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3. Process Description Generally, wood chips, saw dusts, barks and other small pieces are ingredients for paper and green electricity. Wood chips and saw dusts can be used to manufacture pulps from which paper is made; barks and other small pieces known as hog fuels can be burnt for generating steams that powers paper mills and runs turbine to generate green electricity. Basically wood chips contain 50% water, 25% cellulose fiber (which is main ingredient required to make pulp) and 25% liquid called Lignin which acts as natural glue. To get fiber, we must remove lignin. Digesters are like large pressure cookers where wood chips are places in it along with liquid cooking chemicals called white liquor. Here, steam is introduced. As, heat and pressure increases the lignin begins to dissolve and wood chips begin to break down. White liquor acts as soap separating wood fibers and washing out glue i.e. lignin. The cooking liquid now containing broken down lignin is called Black Liquor. When it leaves the digester, it is very dilute containing a lot of water. Water is evaporated out concentrating remaining lignin and cooking chemicals. This mixture is now burned in especially designed boiler called Recovery Boiler which generates steam electricity for mill. After burning all the remaining of black liquor we get called Green Liquor. In final process, lime is added restoring and recycling original chemical composition. At this stage, mixture is again known as white liquor and is ready to begin cycle again in the digester. This cycle of White to Black to Green and back again to White cycles again and again making process not only environmentally sound but also very cost effective. Back at digesters, the remaining cellulose fiber is not yet totally pure. In order to get rid of remaining lignin, the fiber is transferred to bleaching stations. Here pulp is washed with bleach and water in series of steps assuring all remaining lignin has been removed. In each steps, the fiber becomes whiter and whiter. From here, cellulose fiber i.e. pulp is either sent to the tissue mill, paperboard mill or to pulp dryer where it’s dried, stored for future use. Water is key in paper mill. Millions of gallons per day may be required for cooling and cleaning purposes. It is required for removing product throughout mill and inside boilers to produce steam and cool machinery. Thus water is continuously recycled throughout the mills. In the end, water is returned to river but only after being filtered, cleaned and purified through very complex system.



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Fig(2): overview of Kraft pulping process



3.1 Impregnation Common wood chips used in pulp production are 12-25 millimeters long and 2-10 millimeters thick. The chips normally first enter the pre-steaming where they are wetted and preheated with steam. Cavities inside fresh wood chips are partly filled with liquid and partly with air. The steam treatment causes the air to expand and about 25% of the air to be expelled from the chips. The next step is to saturate the chips with black and white liquor. Air remaining in chips at the beginning of the liquor impregnation is trapped within chips. The impregnation can be done before or after the chips enter the digester and is normally done below 100 °C (212 °F). The cooking liquors consist of a mixture of white liquor, water in chips, condensed steam and weak black liquor. In the impregnation, cooking liquor penetrates into the capillary structure of the chips and low temperature chemical reactions with the wood begin. A good impregnation is important to get a homogeneous cook and low rejects. About 40–60% of all alkali consumption, in the continuous process, occurs in the impregnation zone.



3.2 Cooking The wood chips are then cooked in pressurized vessels called digesters. Some digesters operate in a batch manner and some in a continuous process. There are several variations of the cooking processes both for the batch and the continuous digesters. Digesters producing 1,000 tones’ or more of pulp per day are common, with the largest producing more than 3,500 tones’ per day. In a continuous digester, the materials are fed at a rate that allows the pulping reaction to being completed by the time the materials exit the reactor. Typically, delignification requires several hours (1.5 hours) at 170 to 176 °C (338 to 349 °F). Under these conditions lignin and hemicellulose degrade to give fragments that are soluble in the strongly basic liquid. The solid pulp (about 50% by weight of the dry wood chips) is collected and washed. At this point the pulp is known as brown stock because of its color. The combined liquids, known as black liquor (because of its color), 9



contain lignin fragments, carbohydrates from the breakdown of hemicellulose, sodium carbonate, sodium sulphate and other inorganic salts.



net reaction in depolymerization of lignin by SH− (Ar = aryl, R = alkyl groups). One of the main chemical reactions that underpin the kraft process is the scission of ether bonds by the nucleophilic sulphide (S2−) or bisulphide (HS−) ions.



3.3 Recovery process The excess black liquor contains about 15% solids and is concentrated in a multiple effect evaporator. After the first step the black liquor has about 20–30% solids. At this concentration the rosin soap rises to the surface and is skimmed off. The collected soap is further processed to tall oil. Removal of the soap improves the evaporation operation of the later effects. The weak black liquor is further evaporated to 65% or even 80% solids ("heavy black liquor") and burned in the recovery boiler to recover the inorganic chemicals for reuse in the pulping process. Higher solids in the concentrated black liquor increases the energy and chemical efficiency of the recovery cycle, but also gives higher viscosity and precipitation of solids (plugging and fouling of equipment) During combustion, sodium sulfate is reduced to sodium sulfide by the organic carbon in the mixture: 1. Na2SO4 + 2 C → Na2S + 2 CO2 This reaction is similar to thermochemical sulphate reaction in geochemistry. The molten salts ("smelt") from the recovery boiler are dissolved in a process water known as "weak wash". This process water, also known as "weak white liquor" is composed of all liquors used to wash lime mud and green liquor precipitates. The resulting solution of sodium carbonate and sodium sulfide is known as "green liquor", although it is not known exactly what causes the liquor to be green. This liquid is mixed with calcium oxide, which becomes calcium hydroxide in solution, to regenerate the white liquor used in the pulping process through an equilibrium reaction (Na2S is shown since it is part of the green liquor, but does not participate in the reaction): 2. Na2CO3 + Ca(OH)2 ←→ 2 NaOH + CaCO3 Calcium carbonate precipitates from the white liquor and is recovered and heated in a lime kiln where it is converted to calcium oxide (lime). 3. CaCO3 → CaO + CO2 Calcium oxide (lime) is reacted with water to regenerate the calcium hydroxide used in Reaction 2: 4. CaO + H2O → Ca(OH)2 10



The combination of reactions 1 through 4 form a closed cycle with respect to sodium, sulfur and calcium and is the main concept of the so-called re-causticizing process where sodium carbonate is reacted to regenerate sodium hydroxide. The recovery boiler also generates high pressure steam which is fed to turbogenerators, reducing the steam pressure for the mill use and generating electricity. A modern kraft pulp mill is more than self-sufficient in its electrical generation and normally will provide a net flow of energy which can be used by an associated paper mill or sold to neighboring industries or communities through to the local electrical grid. Additionally, bark and wood residues are often burned in a separate power boiler to generate steam.



3.4 Blowing The finished cooked wood chips are blown to a collection tank called a blow tank that operates at atmospheric pressure. This releases a lot of steam and volatiles. The volatiles are condensed and collected; in the case of northern softwoods this consists mainly of raw turpentine.



3.5 Screening Screening of the pulp after pulping is a process whereby the pulp is separated from large shives, knots, dirt and other debris. The accept is the pulp. The material separated from the pulp is called reject. The screening section consists of different types of sieves (screens) and centrifugal cleaning. The sieves are normally set up in a multistage cascade operation because considerable amounts of good fibers can go to the reject stream when trying to achieve maximum purity in the accept flow. The fiber containing shives and knots are separated from the rest of the reject and reprocessed either in a refiner or is sent back to the digester. The content of knots is typically 0.5–3.0% of the digester output, while the shives content is about 0.1–1.0%.



3.6 Washing The brown stock from the blowing goes to the washing stages where the used cooking liquors are separated from the cellulose fibers. Normally a pulp mill has 3-5 washing stages in series. Washing stages are also placed after oxygen delignification and between the bleaching stages as well. Pulp washers use counter current flow between the stages such that the pulp moves in the opposite direction to the flow of washing waters. Several processes are involved: thickening / dilution, displacement and diffusion. The dilution factor is the measure of the amount of water used in washing compared with the theoretical amount required to displace the liquor from the thickened pulp. Lower dilution factor reduces energy consumption, while higher dilution factor normally gives cleaner pulp. Thorough washing of the pulp reduces the chemical oxygen demand (COD).



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Several types of washing equipment are in use:     



Pressure diffusers Atmospheric diffusers Vacuum drum washers Drum displacers Wash presses



3.7 Bleaching To produce white paper, the pulp is bleached. The chemicals used to bleach pulp must be environment friendly. Bleaching with chlorine produces dioxins and other undesirable products. So, nowadays pulp is bleached with hydrogen peroxide, ozone, chlorine dioxide, oxygen etc. The objective of bleaching is to remove small fractions of lignin that remains after digestion In a modern mill, brownstock (cellulose fibers containing approximately 5% residual lignin) produced by the pulping is first washed to remove some of the dissolved organic material and then further delignified by a variety of bleaching stages. In the case of a plant designed to produce pulp to make brown sack paper or linerboard for boxes and packaging, the pulp does not always need to be bleached to a high brightness. Bleaching decreases, the mass of pulp produced by about 5%, decreases the strength of the fibers and adds to the cost of manufacture.



3.8 Process chemicals Process chemicals are added to improve the production process: Impregnation aids. Surfactants may be used to improve impregnation of the wood chips with the cooking liquors. Anthraquinone is used as a digester additive. It works as a redox catalyst by oxidizing cellulose and reducing lignin. This protects the cellulose from degradation and makes the lignin more water-soluble An emulsion breaker can be added in the soap separation to speed up and improve the separation of soap from the used cooking liquors by flocculation Defoamers remove foam and speed up the production process. Drainage of washing equipment is improved and gives cleaner pulp. Dispersing agents, detackifiers and complexing agents keep the system cleaner and reduce the need for maintenance stops.



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Fixation agents are fixating finely dispersed potential deposits to the fibers and thereby transporting it out of the process.



3.9 Byproducts and emissions The main byproducts of kraft pulping are crude sulfate turpentine and tall oil soap. The availability of these is strongly dependent on wood species, growth conditions, storage time of logs and chips, and the mill's process. Pines are the most extractive rich woods. The raw turpentine is volatile and is distilled off the digester, while the raw soap is separated from the spent black liquor by decantation of the soap layer formed on top of the liquor storage tanks. From pines the average yield of turpentine is 5–10 kg/t pulp and of crude tall oil is 30–50 kg/t pulp.



3.10 Chemical reactions involved (i) Digestion (hydrolysis and solubilization of lignin) R-R’ + NaOHR”COONa + ROH R-R’ + Na2S Mercaptans



(ii) Chemical recovery from black liquor 



Smelting



2NaR + air Na2CO3 + CO2 (lignin) Na2SO4 + 2C  Na2S + 2CO2 (from R) (white liquor) 



Causticizing



Na2CO3 (aq) + Ca(OH)2 (s)  2NaOH (aq) + CaCO3 (s) (green liquor)



(white liquor)



CaCO3CaO + CO2 CaO + H2O Ca(OH)2



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Fig (3): Flow sheet for pulp manufacturing process chemical recovery



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4. Process Flow Diagram(PFD)



Fig (4): process flow diagram for pulp manufacturing



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5. List of Equipments The list of equipments is given below:       



Chipper Bin Digester tower Heat Exchanger Blow down valve Blow tank Screens Filters



6. Energy Intensity of key processes Pulp and paper production is an energy-intensive industry. On a global scale, it is the fourth largest industrial consumer of energy, consuming 5.7% of total industrial energy use. Production of pulp and paper requires energy input in the form of heat and power According to BREF, the main energy input in pulp and paper manufacturing are shown in the Figure below:



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Fig (5): Material and Energy flows in pulp and paper industry



Heat energy is usually made available in the form of high pressure steam, used to generate electrical power in turbo-generators, with low or medium pressure steam extraction. Steam can be used both in pulping and papermaking. A very large steam consumption is related to the drying section, to evaporate water both from pulp (in pulping plants) or paper (in the continuous papermaking machine). Electricity is used in several parts of the pulp and paper manufacturing, in general to move motors for different equipment (e.g. refiners, pulpers, vacuum pumps, compressors). Moreover, some modern equipment might act in a way that they reduce consumption of heat, but increase the one of electricity, e.g. electric infrared drying or use of modern presses that increase the dryiness of the paper at the press section exit and lower steam consumption in the drying section.



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7. Gantt Chart Month Week Tasks Idea brain storming Literature Review Proposal Preparation Preliminary PFD Intensive study Digester design Bolw Tank design Screen and Filter design Multi effect Evaporator Design P&ID Verification Final Defense



June June July July 3 4 1 2



Legend Completed Remaining



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July 3



July 4



August 1



August 2



8. Expected Results After the completion of project, it is expected that we will be able to design the process flow diagram, piping and instrumentation diagram, equipment design of Pulp manufacturing process and also able to determine its feasibility. We may learn what modifications can be done in digester design particularly as well.



9.Conclusions This is most popularly used process and is an alkaline process; Na2SO4 is added to the cooking liquor. So its common name is sulfate process. The presence of sodium sulfide makes bleaching of pulp easier and the paper produced has better strength. In particular, it is more expensive than mechanical pulping, but the output has higher quality in terms of fiber length, brightness and strength. Chemical pulping foresees the cooking of wood chips with chemicals, in order to remove lignin. The water network has an important impact on heat consumption in pulp and paper mills. Water is the main heat transporter and dissipator. Non- isothermal mixing is an important source of heat degradation. Problems of build-up of contaminants from wood or process chemicals limit the extent of water closure achievable. A modern Kraft pulp mill produces more heat and power than necessary for the process. In existing mills, improved Process Integration typically enables reduction of heat consumption by 15–30%. Integrating bioprocesses producing high-value-added chemicals and materials into pulp and paper mills is presently the object of intensive development in the forestry sector. Energy, mass and infrastructure integration is crucial for the success of such eco-cyclic systems, also called ‘integrated forest bio-refineries’ in the literature. Optimization techniques are also increasingly used for this purpose. Process Integration in the pulp and paper industry classically leads to fossilfuel savings, increase in power production, and reduction of effluents and greenhouse gas emissions. Beyond these results, Process Integration is also a key factor in the success for ecological industrial systems around the mills of the future.



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9. References 1. Dryden C. E. (2008). Outlines of Chemical Technology, East-West Press. 2. Shreve R. N., Austin G. T. (1984), Shreve's Chemical process industries, McGraw – Hill. 3. Hamaguchi, M., Cardoso, M., & Vakkilainen, E. (2012). Alternative technologies for biofuels production in kraft pulp mills—Potential and prospects. Energies, 5(7), 22882309. 4. European Union., (2018). Technical analysis – Pulp and Paper sector (NACE C17), EU coordinated MEthods and procedures based on Real Cases for the effective implementation of policies and measures supporting energy efficiency in the Industry, HORIZON 2020 Project, Nr. 693845 5. Bajpai, P. (2018). Biermann's Handbook of Pulp and Paper: Volume 1: Raw Material and Pulp Making. Elsevier. 6. Klemes, J. J. (Ed.). (2013). Handbook of process integration (PI): minimisation of energy and water use, waste and emissions. Elsevier. 7. Zhang, W. H., Wu, J., Weng, L., Zhang, H., Zhang, J., & Wu, A. (2020). Understanding the role of cellulose fiber on the dewaterability of simulated pulp and paper mill sludge. Science of The Total Environment, 702, 134376. 8. Ariono, D., Aldiyana, G., Adrian, R., & Indarto, A. (2020). Fractionation of Turpentine. MS&E, 742(1), 012029.



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