ENERGY_PERFORMANCE_ASSESSMENT_FOR_EQUIPMENT_AND_UTILITY_SYSTEMS_(CHAPTER-16:ENERGY PERFORMANCE ASSESSMENT OF PULP & PAPER INDUSTRY)

 

ENERGY PERFORMANCE ASSESSMENT FOR EQUIPMENT B AND UTILITY SYSTEMS

(CHAPTER-16:ENERGY PERFORMANCE ASSESSMENT OF PULP & PAPER INDUSTRY)

Introduction

Paper is produced from cellulose pulp fibers, derived from wood, rags, agricultural residues like bagasse and wheat straw or from waste paper after drying on a battery of steam heated dryers. Paper is a versatile material and finds many uses in our daily life. Traditionally the most common use of paper is for writing and printing, but it also finds various applications as a packaging material, and for large number of industrial and construction processes.

Historically the first use of paper is recorded in ancient Egypt civilisation as a thin paper-like material made from the pith of the papyrus plant, a wetland sedge, grown in the Nile Delta of Egypt. The name paper has therefore originated from the word papyrus. However, the development of pulp and paper making process dates back to 105 AD in China. Over the years, process and technology for paper manufacture has progressed steadily and today in the modern world, paper has linked with cultural and industrial developments. The per capita consumption of paper is often considered as a yardstick of development. The per capita consumption of India stands at only 10.8 kg as against 42 kg in China, 22 kg in Indonesia, 25 kg in Malaysia and 312 kg in the US. This is expected to change in future due to increasing literacy and gradual westernised life style adopted by the Indian populace. The global paper production recorded in 2013 was 403 million tonnes (399 million tonnes in 2012) and the distribution of its production among the different regions is shown in Figure 16.1. Asia, accounts for 45 per cent (182 million tonnes) of paper production and is by far the largest paper producer. Europe (106 million tonnes) and North America (85 million tonnes) are also significant producers. The annual production of paper by leading paper producing countries is shown in Figure 16.2. China is the largest producer of paper with 105 million tons, followed by USA at 74 million tons. India with 13.5 million tons production is ranked 8" largest producer.

The history of mechanised production of paper in India is more than 200 years old, with the first reported setting up of paper mill at Sreerampur, West Bengal in 1812. Presently there are 759 paper industries in India which are divided into three major segments viz., wood based, agro based and recycled fibre / waste paper based, depending on their raw material use. The share of wood based, agro residue and recycled fibre based mills in total production is 31%, 22 % and 47 % respectively. The paper mills are distributed throughout India and the production of different varieties of papers from Indian mills, such as writing/printing, packaging & newsprint produced from different segments of Indian paper industry is about 13.5 million tons.

Pulp and Paper Manufacturing Processes

The pulp and paper manufacturing can be divided into two major operations. - 

Pulp mill- It includes raw material preparation (chippers, chip screen, depither etc.), digesters, Brown stock washers, screening and centricleaning, bleach plant and chemical recovery systems for wood and agro based pulp mills. For waste paper based mills the pulp fiber line includes hydrapulper, pulp cleaning systems, deinking and bleaching etc.

 Stock preparation, paper machine and finishing house- including refining, addition of fillers and additives, paper machines for production of paper / paperboard & newsprint and finishing house to produce paper rolls and sheets ready for distribution.The auxiliary operations linked with pulp and paper making include effluent treatment plant, water treatment, power house (boilers and turbines) and other chemical plants like chlorine dioxide and oxygen generation plants etc.

In India, paper mills use diverse raw materials such as wood, agro residues and waste paper. The hardwoods used by the mills are eucalyptus popular, casurina, subabul etc., while the agro residues are wheat straw, bagasse and other annual plants or grasses which are suitable for pulp and paper making. The cellulose fibres in these raw materials are released by chemical pulping actions under strong alkaline conditions at high temperature and pressure, by selectively destroying the chemical bonds between the glue-like substance (lignin) that binds the fibres together. After pulping, the pulp is washed to remove spent chemicals and dissolved lignin in the brown stock washers. The spent pulping liquor separated as filtrate from brown stock washers is concentrated in evaporators and incinerated in specially designed recovery boiler to recover the inorganic chemicals and utilise the heat generated from combustion of organic components as steam for power generation. The washed pulp separated in the brown stock washers is processed further for paper making. The impurities in the pulp such as the foreign materials (sand, dirt etc.) and uncooked/partially cooked fibre bundles are removed from the pulp using screening and centrifugal cleaning equipment. The cleaned pulp is bleached to improve its brightness. Almost 95-97% lignin is removed during pulping and further delignification is continued in bleach plant under milder conditions to reduce cellulose fibre degradation by using milder bleaching chemicals. The bleached pulp produced from wood and agro residues is termed as virgin pulp and is stored in storage towers for paper making. Waste paper is also processed to produce the recycled fibre pulp by using hydrapulpers to disperse the fibers in slurry. The pulp is processed to remove the impurities from the recycled pulp by using different types of centrifugal cleaners and screens. In order to improve brightness of the recycled fibre pulp, it is subjected to deinking and bleaching operations. In the stock preparation stage, the fibres are subjected to mechanical treatment in refiners. The fibresare refined to make them more conformable by opening up the fibrils from the surface. Fibrillation is an important process operation which makes the fibres suitable for paper making and to obtain the desired paper properties by increasing surface area and entanglement between fibres. At the paper making stage, dyes, strength building resins, or texture adding filler materials may be mixed in the pulp, depending on its intended end product use. For paper making the pulp mixture is dewatered, leaving the fibrous constituents and pulp additives on a wire or wire-mesh conveyor. Additional additives are applied depending on requirement after the sheet-making step. The fibres bond together as they are consolidated through a series of presses and dryers. The final paper product is usually spooled on large rolls for storage. In the finishing house paper is rolled and cut into desired roll sizes or sheets as per customer requirement. The layout of a typical wood based mill is shown in Figure 16.3 and details of major processes for production of pulp and paper are discussed briefly below.

Pulping Process

The pulping of the raw material is performed to liberate the cellulose fibres from the raw material matrix by mechanical, chemical and semi chemical pulping methods. Chemical pulping processes are predominantly used by Indian mills using wood and agro based raw materials.

Layout of Typical Wood Based Integrated Pulp and Paper Mill

Two types of digesters used for pulping are, batch and continuous digesters. The chips are fed into digesters along with chemicals (NaOH and Na,S) and digested at high pressure and temperature (7-8 kg/cm2 and 160-165°C). During pulping, fibres are separated and lignin and other organic/inorganic materials are dissolved. Figure16.4(a&b) illustrate the schematic diagram of batch and continuous digesters.

The pulp yield and kappa number are two important variables which are controlled and checked at the end of the pulping process. The kappa number achieved by conventional batch and continuous cooking ranges from 20-30 depending on the type of wood, pulping conditions (temperature, chemical charge and time of cooking etc.) and raw material to liquor ratio etc. maintained during pulping. Kappa Number is the measure of the lignin content (or) bleachability of paper pulp. The International Standard ISO 302:2004 specifies a test method to determine the kappa number. During a test, a known

amount of potassium permanganate is added to the sample dispersed in water. Any oxidisable material

within the sample, such as lignin will consume permanganate, thus giving a higher kappa number.

Washing, screening and centrifugal cleaning

Subsequent to pulping, the pulp is washed and pulp fibres are separated from the residual pulping liquor in a carefully controlled process known as Brown Stock Washing (BSW). The most common washing method employs a series of counter current vacuum drum washers to displace liquor with minimum dilution. A number of efficient alternative options such as rotary pressure washer, horizontal belt washer, twin roll press etc. are available now a day and are being used by modern mills for efficient washing of pulp.

The advanced rotary pressure washers and horizontal belt washers offer a compact design and in a single washer 2 to 3 displacement stages can be operated. The solid of the extracted liquor is also higher than the conventional vacuum washers. The twin roll wash presses or extraction presses are

capable of achieving discharge consistencies around 30-40%. They are quite competitive compared to conventional vacuum washer systems and suitable particularly for pulps that are difficult to permeate, i.e., where a displacement-type washer would not be an appropriate choice.

After washing, the pulp is screened and cleaned. Screening of the pulp is performed to separate coarse rejects like knots and uncooked chips from the pulp, remove stones and junk in order to avoid wear and damage of machines. Screening also acts as a “guard” for eventual hard cooks as it is used to remove the shives from the pulp in order to achieve a pulp free from bigger shives. The major types of stock screens employed are vibratory, gravity centrifugal, and pressure (centrifugal or centripetal). The centrifugal cleaners are also used for pulp cleaning. Centrifugal cleaners remove unwanted particles from pulp and paper stock by a combination of centrifugal force and fluid shear on the basis of density differences and particle shape. All centrifugal cleaners work on the principle of a free vortex generated by a pressure drop to develop centrifugal action. The pressure for cleaning is generated from the feed pump and the stock enters the cleaner tangentially imparting a rotating motion through the inlet scroll guides. Due to density difference the long fibre useful for paper making are carried inward and upward to the accept stock outlet. The dirt, held in the downward current, is moved toward the tip and removed from the pulp stock. Figure 16.5 shows the flow sheet of a bleached kraft pulp street.

Extended Delignification

To control bleach chemical cost, it has always made good sense in the production of bleachable grade pulps to de-lignify the raw material as much as possible during cooking, and thereby minimize the amount of residual delignification required during bleaching stage. More recently, with attention focused on reducing the discharge of chlorinated organic compounds from bleach plants, the need to de-lignify pulp more completely prior to conventional bleaching has become essential. Today with the use of extended delignification, it is possible to obtain pulps with kappa number as low as 12- 15 compared to the conventional pulps with kappa number 20-30. Low kappa number of the pulp signifies higher de-lignification in the pulp.

Modern batch digesters are upgraded with Rapid Displacement Heat (RDH) batch cooking or Super batch cooking systems to obtain pulp of high quality with improved energy efficiency. The operations in a modified pulping systems are chip filling, filling of warm impregnation liquor, hot liquor filling for heating and cooking by displacement of warm impregnation liquor, heating and cooking by recirculation of hot liquor using small amount of steam in heat exchanger and finally displacement of the hot spent liquor with cool liquor by pumping into the bottom of the digester on achieving the desired delignification. Displacement of hot liquor from digester reduces the temperature of the pulp below 100° C.

Oxygen delignification (ODL) is also one of the methods used for reducing the lignin content of pulp before conventional bleaching. In modern pulp mills, the digesters equipped with extended delignification (i.e., RDH/super batch pulping) are followed by ODL to obtain low kappa pulps and in these mills the bleach chemical demand is significantly reduced. Although technically independent, the oxygen delignification stage is compatible with the kraft recovery process because its caustic effluent can be added to black liquor and processed through the recovery furnace. In India most of the large wood based mills have oxygen delignification stage prior to bleaching stage.

Bleaching

In order to obtain the white bleached pulp, unbleached pulp is treated with bleaching chemicals like chlorine, chlorine dioxide, hydrogen peroxide, oxygen and ozone etc. Modern bleaching involves continuous sequence of process stages utilizing different chemicals and conditions in each stage, usually with washing provided in between stages. Since elemental chlorine is known to be a major contributor of chlorinated organic compounds (also known as “adsorbable organic halides” or AOX) to the environment, therefore as a Corporate Social Responsibility there has been gradual decline in use of elemental chlorine and increase in substitution / partial substitution of chlorine by chlorine dioxide, known as Elemental Chlorine Free (ECF) bleaching.

Bleaching is achieved through a continuous sequence of process stages utilizing different chemicals and conditions in each stage, usually with washing between stages. The conventional symbols for various bleaching stages are given in Table

Symbols for various bleaching sequences

The common sequences employed in Indian pulp and paper mills are CEPHH, CEDED, (C+D)(E,) DED (in this sequence dioxide is added sequentially to the chlorination stage and oxygen utilized in the extraction stage), D/Eo)DED or D(EoP)DED.

Chemical Recovery

The filtrate containing spent pulping chemicals, generated from pulp washers is processed in the recovery section to recycle cooking chemicals for pulping and to utilize energy from the incineration of organic residuals. This leads to minimization of air and water pollution from pulp mill. The steps involved in chemical recovery operation, starting with generation of “weak black liquor” from the brown stock washers, are as follows.

¢ Concentration of the spent weak black liquor in multiple effect evaporators to form "strong black liquor" (45-50% solids) and further concentration to form "heavy black liquor" (60-70% solids).

¢ Salt cake addition to make up soda loss.

¢ Incineration of liquor in the recovery furnace.

¢ Dissolution of smelt from the furnace to form green liquor.

¢ Causticiziation of green liquor with lime to form white liquor.

¢ Burning of lime mud in lime kiln to recover lime.

A typical operating sequence of chemical recovery is illustrated in Figure 16.6. The concentration of black liquor is carried out in the Multiple-Effect Evaporators (MEE), which is a series of live steam or vapour heated bodies operated at different pressures to utilize the vapor from one evaporator body as the steam supply to the next body. Different designs of evaporators are used, but in all cases a main objective is to minimize fouling and scaling and achieve higher efficiency. In Indian mills the most common types of evaporator used are the long tube vertical (LTV) and the free flow falling-film (FFFF) design evaporator bodies. FFFF evaporators are more efficient.

Incineration of the heavy black liquor is performed in the recovery boiler furnace. The liquor droplets dry and partially pyrolysis before falling onto the char bed at the bottom of the furnace, where incomplete combustion in air deficient environment causes carbon and carbon monoxide to act as reducing agents and converts sulfate and thiosulfate to sulfide. The heat at the char bed is sufficient to melt the sodium salts, which then flow by gravity through water-cooled spouts to the smelt dissolving tanks where the smelt is converted into green liquor by addition of weak wash.

In the causticization operation the green liquor is reacted with lime (CaO) to form NaOH or white liquor. The white liquor is clarified to remove precipitated “lime mud” (CaCO,) for cooking. Auxiliary operations include washing of both the dregs and lime mud for soda recovery, and the calcining (“reburning’’) of lime mud to regenerate lime. The lime mud is converted to lime in the the lime kiln by calcining (or lime “reburning”’) for its reuse in the causticizing process.

Stock Preparation

Refining is the most important unit operation in stock preparation in which the fibers are subjected to mechanical action to develop their optimal paper making properties with respect to the product being made. During refining the amount of energy absorbed by the pulp is the major factor affecting the change in pulp properties; but the manner in which the work is carried out is also a significant determinant. Refining carried out at low intensity produces greater fibrillation, less cutting, and more satisfactory development of fiber properties. In other words, a gradual, step by step application of mechanical energy to the fibers provides the optimal treatment, in contrast to that produced by a more abrupt and concentrated application of the same amount of energy. Two major types of refiners used for stock preparation are disc refiners and conical refiners. Disc refiners are a more recent development, and are available in a wide variety of designs and disc patterns. Efficient refining requires control of refining conditions, selection of right equipment and its configuration etc., which results in energy efficiency during refining.

A wide variety of mineral and chemical agents are added to the stock, either to impart specific properties to the paper or to facilitate the paper making process. The fibrous and non-fibrous furnish components are blended to form the paper making stock. Various wet end additives are added to achieve desirable brightness, opacity, wet strength, colour and shade.

Paper Machine

In paper machine the refined pulp stock is converted into a wet web from the pulp suspension which is further dried to get the paper or paper board. Today high speed machines are available to produce upto 8 to 10 meter wide web of paper at speed above 1200 m/min. Mills use variety of machines running on Mono Glazed (MG) cylinder, Fourdrinier, twin wire and duo-former technologies. In terms of energy utilization in paper machine, removal of moisture from the paper web by vacuum dewatering and press drying in steam heated dryers are the major areas which require extensive energy input. For higher energy efficiency and energy cost reduction it is desired that maximum possible moisture removal should be achieved in these steps by following best practices and modern technologies. A basic fourdrinier type machine, common in Indian mills is shown in Figure

In the fourdrinier table or wire part, the paper web is formed by removal of water and the paper web with dryness upto 22% can be obtained by proper operation of various drainage elements in the wire part.

The wet web is pressed to remove moisture from the paper sheet by mechanical action between the press nips. Pressing operation is considered an extension of the water-removal process that starts on the wire. It is far more economical to remove water by mechanical means than by evaporation, so the papermaker always looks for means to improve pressing efficiency and reduce the evaporative load in the subsequent dryer section. Modern machines are equipped with shoe press which facilitates extended nip dwell time due to its special design. This ensures high performance even at a high machine speed. The machine rebuilds with shoe press in a large number of machines have reported significant energy saving due to high moisture removal leading to steam saving in the dryers.

After pressing, the sheet is passed through the dryer section where the residual water is removed by evaporation on a series of large diameter, rotating, steam-filled cylinders. The dryer section is most costly to operate because of high energy consumption in drying.

The steam economy of the paper dryers is mostly affected due to the steam/condensate and air handling systems. The dryer fabrics also play vital role in energy efficiency. In modern paper machines synthetic fabrics with permeable construction are used to provide ventilation by freely carrying air into and out of the pockets between dryers.

Air has important role in paper drying process, therefore the dryer hood arrangement is also important for water evaporation. In general 3.5 to 10 kg air is utilized for each kg of water evaporated in the dryer section depending on the type of hood arrangement. All modern machines are equipped with hoods having heat recovery system to recover heat from the hot, humid exhaust air into the fresh, ambient supply air.

Proper control and optimization of the paper making operations such as approach flow systems, head box, drainage elements on the wire, forming fabric, type and layout of presses, dryer condensate removal, pocket ventilation and hoods etc, contribute significantly towards product quality and energy savings.

Energy Consumption Pattern

Energy is an essential input and a major cost driver in paper manufacturing. Energy constitutes 20- 30% of the cost of manufacturing and it is the only cost variable which can be controlled to make paper industry cost competitive. Figure 16.8 illustrates the % cost break-up in a wood based mill producing writing printing paper.

Various forms of primary and secondary energy sources are used in pulp and paper making operations.

Among the various energy sources, such as fossil fuels, agro residues and biomass generated within the industry, coal is the largest used fuel. Other fuels such as bagasse pith, rice husk, coconut shell, groundnut shell etc. are also used depending on their availability by the mill. Some internally generated combustible wastes, such as black liquor, pith, chipper dust, sludge etc are also used as fuel. These are generated from raw material preparation, pulping and paper making operations. Black liquor, the spent pulping liquor, accounts for more than 80% of the biomass-based fuel used in the paper industry. Steam and power are the other major secondary forms of energy used in pulping and paper manufacturing processes. Details of the energy consumption in wood, agro and RCF based mills are presented below.

Energy Consumption in Integrated Wood and Agro Based Mills Using Conventional and Nonconventional Chemical Recovery Systems

Unit operations in Integrated wood and agro based mill are generally similar, but technological status

of agro based integrated mill varies based on the size of the mill. In large agro based mills (above 300

TPD) the processes used are based on State-of-Art technologies, however, in smaller sized integrated

agro based mills, with capacity up to 150 TPD, technological level is relatively obsolete. The section

wise breakup of specific steam and power required for unit production of paper from a modern

integrated wood /agro based mills using conventional recovery system integrated with steam turbines

in power house is presented in Table16.2.

The agro based mills below the capacity of 100 TPD utilize non-conventional chemical recovery systems on account of its economic viability. Table 16.2 also presents the section wise breakup of steam and power in an integrated agro based paper mill using Non-conventional chemical recovery system with power house.

Table  Section-wise Energy Consumption in an Integrated Wood/Agro Based Mill Using Conventional Recovery System and the Integrated Agro Based Mill using Non-Conventional Recovery System. (Source: CPPRI)

Energy Consumption in Recycled Fiber (RCF)/Waste Paper Based Mills

The RCF based mills can be grouped into two categories depending on the manufacturing process and the final product. These are (1). Mills producing white varieties using De-inking and bleaching systems and (2). Mills without De-inking systems.

The overall energy consumption in the major unit operations of RCF based mill producing white  varieties using De-inking systems as well as the mill producing white and unbleached varieties without De-inking system is presented in Table

Table  Section-wise Energy Consumption of RCF Based Mill Producing White Grade of Paper Using Deinking and Oxidative Bleaching Systems as well as the Mill Producing White and Unbleached Varieties Without De-inking System. (Source: CPPRI)

Material and Energy Balance Calculations in Pulping and Paper Making Pulping

(i) Chemistry of Pulping

The two commonly used processes by Indian pulp & paper mills for pulping are soda and kraft processes. The soda process is used by medium sized agro based mills based on non-conventional recovery and the kraft process by large wood and agro based mills using conventional chemical recovery systems. In soda process sodium hydroxide is the major chemical used for cooking whereas in kraft process, cooking liquor or white liquor, an aqueous solution of sodium hydroxide (NaOH) and 

sodium sulphide (Na2S) is used for pulping. The hydroxyl ion (OH) and the hydrosulfide ion (SH ) are the active components in the cooking liquor. They originate from NaOH and Na2S as follows.

The concentration and total charge of the ions are the key elements in all kraft pulping reactions.

(ii) Terms Used in Pulping

Liquor to raw material ratio

Liquor to raw material ratio is a term that describes the amount of total liquor per unit weight of dry raw material in the digester. The moisture content of the raw material is also included in total liquor. The liquor to raw material ratio varies for each type of raw material and typically varies between three to five.

Alkali Charge

Two commonly used terms to indicate the chemicals charged in digester are total and active alkali.  Total alkali indicates concentration of sodium salts in white liquor on basis of equivalent amount of sodium oxide (Na,O) or as sodium hydroxide (NaOH) and it includes NaOH, Na2S, Na2CO3,1/2 Na2SO3 present in white liquor. The active alkali includes only NaOH and Na2S, which are the active chemicals for pulping. Both total and active alkali are expressed as percentage of dry lingo-cellulosic raw material weight are then referred to as total or active chemical / alkali charge.

Cooking Cycle

Various stages of cooking cycle are pre-steaming, raw material impregnation, cooking and blowing. Total cooking cycle is 3-4 hrs for a batch digester. The cooking however, takes place at a constant temperature of approximately 160 to 165° C for one to two hours. Most of the delignification takes place in the cooking stage.

Kappa Number

Kappa Number is a measure of residual lignin in the pulp. It is helpful to determine the degree of delignification achieved during cooking and to find out the bleach chemical demand during bleaching.

Example 16.1: Pulp cooking and steam requirements for digestion In a digester with 42.5 MT digester shell, 100 MT of chips with 50% moisture are cooked at 170°C by maintaining 1:4 bath ratio. The digester is filled with 112.5 MT of white liquor at 90°C and 37.5MT of black liquor at 85°C. Find out the total steam requirement per cook and steam per tonne of the chips and pulp.

- Sp. heat of chips at 50% moisture is 0.33, kcal/kg°C at 35°C

- Sp. heat of white liquor =0.91 kcal/kg °C at 90°C

- Sp. heat of Black liquor =0.94 kcal/kg° C at 85°C

- Sp. heat of digester shell =0.117 kcal/kg°C at 90°C.

- Temperature after blow = 105°C

- Radiation losses from digester = 8%

- Heat of reaction = -10% (exothermic)

- Sp heat of cooking liquor after cooking = 0.92 kcal/kg°C

- Pulp Yield= 40%

- Allowance for vent steam and heat blow = 5%

Solution:

The heat requirement for each component in digester is calculated as

Paper Machine

The paper making process is essentially a very large dewatering operation where a diluted solution of pulp suspension with less than 0.5 % fibre solids is formed into a web. Regardless of the nature of the pulp, whether it is chemical, mechanical or recycled from softwood, hardwood, agricultural residue, the basics of paper making are similar.

Complete process of water removal on the paper machine 1s carried out by four principal operations: 1) By “free drainage” on the fourdrinier, 2) By “suction” on the wire part, 3) By “pressing” on the press part and 4) by “evaporation” on the dryer part. The majority of the water is removed in the wire part to form the wet web of the paper. The sheet dryness after couch is 20-22%. The wet paper sheet after wire part is pressed and consolidated into a dense sheet with removal of moisture by pressing. After mechanical limits of water removal in the press is reached, the wet sheet enters the dryer part, where the dryness content of the sheet is increased by evaporation. The dryer section of a paper machine removes between 1.1 to 1.3 kg of water per kg of paper compared to 200 kg and 2.6 kg in the forming and press section respectively. It is significantly more expensive to remove water in the dryers than the forming and press section. The relative cost of dewatering are, forming section 10%, press Section 12% and dryer section 78 %. The dryer section is by far the largest consumer of thermal energy of the paper machine.

In this section some examples of the paper de-watering and steam requirements for paper drying are discussed.

Example 16.2:

A multicylinder fourdrinier paper machine is producing 100.0 tonnes cream woven paper with 6% moisture content without break per day. The consistency of the fibre suspension inside the head box is 0.5% and the dry content leaving the couch and the last press is 20% and 35% respectively. Determine

percentage of water removed,

By free drainage and suction on the wire part

By pressing on the press part

By evaporation from the dryers

Total quantity of water evaporated from the dryers per day

ao Tf

Neglect fibre losses, if any, with the back water.

Solution:

Basis: 100 tonnes /day production of cream woven paper

Hence, quantity of dry content carried by the paper per day = 0.94 x 100 = 94 tonnes

Ans:

a) Water removed on the wire — part = 97.98%

b) Water removed by the presses = 1.07%

c) Water evaporated by the dryers = 0.90%

d) Total quantity of water evaporated by the dryers perday = 168.57 tonnes

Example 16.3:

The average rpm of the dryer of a paper machine, producing 70 GSM Maplitho paper of 70% moisture, is 65. The deckle of the paper on the reel is 3.3 meters. The diameter of the drier is 1.5 meters. The steam pressure inside the drying cylinder is kept constant at 2.5 kg/cm’. If the sheet dryness after presses is reduced from 40% to 35%, Calculate:

a. Amount of additional water evaporated per day for paper at 93% dryness.

b. Amount of additional steam required per day.

Given that latent heat of evaporation of water at 100°C from the paper at zero pressure is 540 kcal/kg

and that of the steam in the drier at 2.5 kg/cm? at 126°C is 522 kcal/kg.

a) Evaporation when dryness after presses is 40 %

Moisture in finished paper will remain same

Therefore evaporation per kg of dry fibre = 1.857 — 0.075 = 1.782

Hence Evaporation per kg of paper produced = 1.782 x 0.93 = 1.657

Additional water evaporated = 1.657 — 1.325 = 0.332

Thus, additional water evaporated per day = 97.67 x 0.332

(kg of water evaporated / kg of paper produced) = 32.46 Tons

b. Since the steam pressure inside the drying cylinders is kept constant at 2.5 kg/cm2, therefore additional steam requirement shall be only due to the change in the dryness after presses.At 40% dryness, evaporation = 1.325 kg / kg of paper produced

Conclusions

The technological complexity and variation in process layouts in different pulp and paper mills based on wood, agro and RCF, leads to wide difference in energy consumption for production of similar products. A large number of energy management, best practices, advanced process control and State-of-Art technologies have been adopted by the mills, however still there is ample scope for improvement and energy efficiency enhancement in pulp and paper manufacturing. The energy auditors with sound understanding of pulp and paper mill processes can contribute significantly towards energy conservation, as the process optimization often leads to large savings.

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Chapter 15

Chapter 17