Chemical pulping The most common chemical pulping process used in the USA is the “Kraft” process (also called Sulfate process). Kraft belongs to the “alkaline pulping processes”. Alkaline pulping only means pulping pH has to be alkaline: can be Soda process (NaOH) or Kraft process (NaOH+ Na2S) or any other process with high pH. Today the Kraft process is used for around 90 % of all chemical pulp production in the US. The other main chemical process is sulfite pulping, using H2SO3 (original design used acid conditions, but today also used under neutral or alkaline conditions).Main purpose of chemical pulping it to remove enough lignin so that the fibers can be separated from each other.
The Kraft Process The Kraft process was developed 1879 by Dahl. Dahl added the cheaper sodium sulfate (Na2SO4) as make-up chemical to a recovery system for the soda process (he tried to replace sodium carbonate). In this pulping process the aqueous liquor is collected and incinerated after the pulping process ( to recover chemicals and energy). During the incineration in the recovery system sulfate is reduced to sulfide. So the name “sulfate” process is actually misleading since sulfide is the active ingredient, not sulfate.
In the Kraft process, a mixture of sodium sulfide (Na2S) and sodium hydroxide (NaOH) is used to pulp the wood. The sulfide accelerates the delignification: consequently, the chips are exposed to the hot alkali for a shorter time than in the soda process; this makes it possible to produce a stronger pulp. The name “Kraft” is the German and Swedish word for strength and was chosen because these pulps are very strong.
The Kraft process is superior to the soda process (NaOH only) with respect to the rate of pulping, pulp quality, and production cost. It also has several advantages over the sulfite process (H2SO3) can use any wood species can tolerate more bark cooking times are shorter less problems with pitch extremely high pulp strength very effective, well established recovery system can produce valuable by-products, such as tall-oil and turpentine
Kraft pulping: heating up time = 0.5-1h overall cooking time =1-4 hours max. temp =1800C
Acid sulfite: heating up time = 3-4 h overall cooking time = 5-6 hours max. temp =1400C
The Process: Wood chips are screened to assure uniform size distribution. Chips that are to thick might end up producing uncooked centers (rejects). Thin chips might be “overcooked” and result in yield loss. Chips are presteamed and packed into a
1 digester. The presteaming is done to help pack the chips tightly and to saturate them with water so the chemicals can diffuse into the chips easier. The chips have to be packed as tight as possible; since the chips have to be covered with cooking liquor to many voids would mean the liquor has to be diluted to much. Cooking liquor is added to the digester. This liquor is called “white liquor” and contains the cooking chemicals NaOH and Na2S. In most cases some black liquor ( liquor at the end of the cook) is also recirculated into the digester. Black liquor recirculation results in increased solid concentration of the black liquor, which is important since this means less water has to be evaporated in the recovery system before the black liquor can be incinerated (energy savings). Cooking temperatures are most often around 160-170 o C. At the end of the cook pressure is partially released and with the help of some remaining pressure pulp is blown out of the digester. Chips are softened to the point that they don’t need any mechanical disintegration.
Terminology in Kraft pulping In North America, the widely accepted practice is to express all sodium compounds in the cooking liquors on the basis of the equivalent amount of sodium oxide (Na2O). In Scandinavian and other European literature it is more common to select NaOH equivalent as the basis.The selection of sodium oxide as the North American standard is purely arbitrary. Sodium oxide only exists under anhydrous conditions and therefore is not present in the actual Kraft pulping liquor. It may be present in small amounts in the recovery furnace but converts instantly:
Na2O + H2O 2 NaOH
2 This conversion means, 62 parts of Na2O (molecular weight) are equivalent to 80 parts of NaOH (NaOH molecular weight is 40, you get two molecules for each Na2O). Conversion of Na2S is based on the hypothetical equation;
Na2O + H2S Na2S + H2O
62 parts of Na2O are equivalent to 78 parts of Na2S.
Chemical Molecular Equation for Conversion of Conversion of weight conversion to chemical to Na2O to Na2O Na2O chemical NaOH 40 62/80 0.775 1.290 Na2S 78 62/78 0.795 1.258 Na2S X 9 H2O 240 62/240 0.258 3.871 Na2CO3 106 62/106 0.585 1.710
Standard Kraft Pulping terms:
1. Total Alkali: All sodium salts : NaOH + Na2S + Na2CO3 + Na2SO3 + Na2SO4 + Na2S2O3 etc. expressed as Na2O
2. Total Titratable alkali (TTA): NaOH + Na2S + Na2CO3
3. Active Alkali: NaOH + Na2S, expressed as Na2O
4. Effective Alkali: NaOH + ½ Na2S, expressed as Na2O
5. Activity: the percentage ratio of active alkali to total alkali, both expressed as Na2O
6. Causticizing efficiency: in white liquor the percentage ratio of NaOH to the sum of NaOH + Na2CO3, (both chemicals expressed as Na2O), and corrected for the NaOH content of the original green liquor in order to represent only the NaOH produced.
7. Causticity:The percentage ratio of NaOH, expressed as Na2O, to active alkali
8. Sulfidity: in white liquor, the percentage ratio of Na2S to Active Alkali, both expressed as Na2O. In green liquor, the percentage ratio of Na2S to total alkali both expressed as Na2O.
9. Reduction: in green liquor, the percentage ratio of Na2S to the sum of Na2SO4 + Na2S (+ any other soda-sulfur compounds if present), all expressed as Na2O.
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10. Unreduced salt cake: Na2SO4 in the green liquor expressed as Na2SO4
11. Make up chemical consumption: The pounds of Na2SO4 , or other sodium compounds all expressed as Na2 SO4, added as new chemicals per ton of air- dry pulp production.
12. Chemical recovery efficiency: the percentage ratio – total chemical fed to the digester minus total chemical in the new Na2 SO4 ( make-up chemical) divided by total chemical fed to the digester.
13. Chemical loss: Total loss: the percentage ratio – total chemical in the new Na2 SO4 divided by the total chemical fed to the digester.
Loss in cooking and pulp washing: the percentage ratio – total chemical fed to the digester minus total chemical fed to the evaporator divided by total chemical fed to the digester. Loss in evaporator and furnaces: the percentage ratio – total chemical fed to the evaporator minus total chemical in the green liquor divided by total chemical fed to digester
Loss in recausticizing and mud washing: the percentage ratio- total chemical in the green liquor minus the total chemical in the white liquor divided by the total chemical fed to the digesters.
Kraft Liquors White liquor: This is the cooking liquor added to the chips at the beginning, it contains NaOH and Na2S. pH is around 13.5-14. White liquors in mill can have several impurities (inefficiencies in recovery and causticizing). Impurities can be: 1. Sodium sulfate, sodium carbonate, sodium thiosulfate 2. Sodium chloride, Potassium salts 3. Iron, Manganese, Calcium
These “ineffective” chemicals are called “dead load”. 2nd group (chloride) can cause corrosion problems. “Dead load” chemicals have no direct effect on pulping. But they help determine ionic strength of the liquor, therefore they can result in decreased lignin solubility.
Black Liquor Liquor exiting the digester with the cooked chips. In addition to the inorganic material that entered with the white liquor, black liquor contains both organic and inorganic material removed from the wood during the cook. Significant portions of the sulfur compounds have been oxidized to sulfate and thiosulfate, also hydroxide concentration was reduced (lower pH). Trace metals have increased.
4 Concentration of Calcium and Silicate ion concentration is very important; it can cause scaling problems later on in the recovery system.
Major difference between white liquor and black liquor is presence of organic material. Organic material is present mainly as organic acids. Also, significant amounts of dissolved alcohols such as methanol are present. Some organic material may be recovered in the chemical recovery system as the mixture of resin and fatty acids known as tall oil. Most of the organics carry through to the recovery furnace, where they are burned, and generating heat.
Green liquor The liquor that results when the inorganic smelt from the recovery furnace is dissolved in water is called green liquor. The name comes from small amounts of iron sulfide, giving the liquor a greenish color. Green liquor has very high carbonate levels and a low hydroxide concentration. This combination does not provide the high pH necessary for the Kraft process -- carbonate has to be converted to hydroxide in the causticizing process before it can be used.
Chemical Reactions of Wood Constituents
Properties of wood which are of interest in chemical pulping are:
1. Porous structure, permitting penetration of water and chemical reagents
2. Chemical heterogeneity, allowing selective chemical reaction of its constituents ( you are trying to remove preferentially lignin)
3. The fibrous form of wood cells, which allows them, when separated, to be reorganized into the random network of fibers and fiber debris.
In chemical pulping the fiber separation occurs in the middle lamella, leaving the primary wall intact. In order to achieve this separation, the components (lignin) of the middle lamella must be chemically removed, at least to the point that fiber separation is possible. In addition, chemical attack on the components of the cell wall, mainly the S2 layer must be controlled so that the resulting pulp has the necessary physical and chemical characteristics.
The major component in the middle lamella is lignin, therefore lignin reactions are the key to successful chemical pulping. Unfortunately not all of the lignin in a fiber is in the middle lamella, majority by weight is in the S2 layer and reacts before or at the at the same time than the lignin from the CML (compound middle lamella = primary wall of one cell + Middle Lamella + primary wall of next cell). This greatly increases the amount of material that is removed before enough CML lignin is degraded to permit fiber separation. It also means that major structural changes occur in the S2 layer when lignin is removed during chemical pulping.
5 Both cellulose and hemicelluloses are attacked during pulping (yield loss and additional chemical consumption). Polysaccharide degree of polymerization (DP) is reduced and pulp viscosity and strength decrease
Reactions of lignin Reaction of lignin during alkaline pulping are complex. Since the pH is so high in - Kraft cooking the chemical NaOH and Na2S actually exists as OH and HS. Sulfide accelerates lignin dissolution without increasing cellulose degradation. Sulfide reacts chemically with the lignin and becomes organically bound to it, the result are thio-lignins. But later during the cooking stages most of the sulfur containing lignin products are decomposed, resulting in formation of elemental sulfur. This sulfur reacts with hydrosulfide ions (HS-) to form polysulfides (H-S-S- S-S-S-….). Kraft lignin contains around 2-3 % bound lignin (this means around 25-30 % of the sulfur added to the cook stays with the lignin). Addition of sulfides leads to pulps with a lower lignin content for a given yield (or higher yield for a given lignin content). Reactions of lignin are related to both, the hydroxyl (HO-) and the hydosulfide (HS-) concentrations-. In both, Soda and Kraft liquors, the lignin is degraded by the cooking chemicals, and the fragments dissolved in the liquor. Fiber separation in this process happens in the middle lamella.
Kinetics – Lignin The dissolution of lignin can be divided into three phases: Initial phase: temperatures below 1400C (controlled by diffusion) 0 Bulk delignification: above 140 C ( controlled by chemical reactions) Accelerates steadily with increasing temperature Until about 90 % lignin removal.
-dL = kL L = lignin content dt t =time k= reaction constant
Residual delignification: slow phase
6 30
Intitial delignification 25
20
15 Bulk delignification
Lignin (% of wood) 10
Residual 5 delignification
0 0 200 400 600 800 1000 1200 1400 1600 1800 H-factor
H-factor , G-factor
Kraft pulping is influenced by a wide variety of factors, for example:
1. Transport of the chemical ions from liquor to the exterior surface of the chips 2. Diffusion of the ions to the inside of chips 3. Chemical reaction between ions and wood components 4. Diffusion of the reaction products to the chip exterior 5. Transport of reaction products into bulk liquor
The overall reaction rate will be determined by the slowest of these factors. While 1 and 5 are not considered important 2 and 4 are. Even when transport phenomena are eliminated, the development of a rate expression is not easy. Reasons for this are the chemical complexity of the reacting substances and the fact that the liquor composition changes continuously during the cook.
In a batch cooking, the liquor is typically introduced into the digester at a temperature of 70 to 80o C and the contents of the digester are then heated to a predetermined maximum cooking temperature. The rise to temperature is generally carried out as rapidly as possible within the limitations of the equipment and the available steam pressure -- but can take 1.5 - 2 hours. In continues cooking, the rise to temperature can be faster but, time is needed for impregnation of chips by the pulping liquor therefore heating up time still is a substantial part of the total cooking cycle.
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The removal of lignin is slow at the beginning of the cook regardless of alkali concentration. This indicates, that at this point lignin removal is not chemically controlled but diffusion limited Initial phase.
As the temperature increases, delignification accelerates and alkali concentration influences rate. By the time maximum temperature is reached generally more than 40 % of lignin is removed and the reaction becomes chemically controlled at these intermediate cooking times Bulk delignification. Later in the cook, the lignin content levels off Residual delignification. At this point the most reactive lignin units have reacted, some condensations reactions have happened and the remaining lignin is difficult to remove.
Reactions of hemicelluloses depend on type of hemicelluloses. Glucomannan is removed relative rapidly at the beginning of the cook. This is believed to be caused by dissolution of soluble fractions. At 100 o C rate of glucomannan removal increases further. When about 70% are removed rate drops sharply.
Xylan removal looks similar to lignin removal. The initial phase is slow, is accelerated at maximum temperature and levels off. The main loss of xylan is caused by dissolution. Cellulose removal starts at around 120 - 130 o C and levels off when maximum temperature is reached. About 10 % of the cellulose (around 5 % of the wood) is lost. Cellulose can not be dissolved (like some of the hemicelluloses), the yield loss is caused by peeling reactions.
During the initial stages of the cook lignin is removed mainly from the secondary wall. When delignification is about 50 % complete rate of dissolution of the middle lamella becomes dominant. The residual lignin at the end of the cook is mainly located in the cell wall. Delignification slows down considerable at the end of the cook. Since cellulose reactions have to be considered too, delignification should be stopped before “residual delignification” begins which generally is at a pulp lignin content of around 2.5-3.0 %. Some mills stop around 4-5%.
Besides chemical concentration it is well know that the rate of Kraft pulping is very sensitive to temperature. Bulk delignification is very slow at temperatures below 100o C but accelerates markedly as the temperature rises above 160 o C . To achieve a given degree of delignification, various combinations of heat-up time, maximum temperature, and time at maximum temperature can be used.
The selection of these variables was made simpler by the introduction of the so called H-factor. It provides a method of expressing cooking times and cooking temperatures as a single variable so that the times and temperatures of any cycle can be expressed as a single numerical value. This value (the H-factor) , represents the area under the curve in which relative reaction rate is plotted against time. If all other factors (such as wood, alkalinity etc.) are kept the same a given H-factor should always result in the same lignin content at the end of the
8 cook, no matter at which temperature it was reached. (lower temperature means time has to be longer)
Relative reaction rates were determined using several assumption - for example it was assumed that during bulk delignification variations in hydroxyl and hydrosulfide ion concentrations are moderate and that the bulk delignification follows pseudo-first –order kinetics.
-dL = kL L = lignin content dt t =time k= reaction constant
K is dependant on temperature. Vroom determined k at varying temperatures, which allowed it to calculate the activation energy Ea using the Arrhenius equation.
-Ea/RT k = A e or taking the natural log ln k = ln A- (Ea/RT)
An activation energy of around 32 kcal/mol or 134 KJ/mol was determined. A simplified system uses the rate constant at 100o C as the standard.
0 ln k100 = ln A – (Ea/ 373R) (373 is absolute temp. in K at 100 C) subtraction the equation before from this one gives you:
ln (k/k100) = -(Ea/RT )+ (Ea / 373R)
relative rate constant kr
kr = exp ( 43.2 – 16.113/T)
Values of relative rate constants at different temperatures are listed in tables (see following pages in handouts).
-dL = kL integration gives you : f(L) = ∫ k dt dt using our relative rate constants:
∫ kr dt = ∫ exp (43.2 – 16,113/T) dt
The left side of this equation is called the H-factor and expresses the area under the curve when relative rate constants are plotted against time. Cooking to a given H-factor will give you the same amount of lignin removal, no matter if you
9 use a high temperature ( and a short time) or a low temperature and longer reaction times.
The H-factor is can be determined by a numerical method that is equivalent to plotting the relative rate constant vs. cooking time and evaluating the area under the curve.
1000
900
800
700
600
500
400
300
200 Relative reaction rate constant
100
0 0 20 40 60 80 100 120 140 160 Time [min]
Numerical method to determine H-factor. For details, please see Lab section “How to determine H-factor”.
G-factor H –factor is designed to predict lignin content but not cellulose viscosity. Activation energy for cellulose chain cleavage is not the same than for lignin degradation (activation energy for cellulose degradation was determined to be 179 kJ/mol – lignin 134 kJ/mol). A similar derivation than for the H-factor leads to the so-called G-factor, describing cellulose viscosity loss.
The higher activation energy means that cellulose reactions are slower at lower temperatures, but are going to increase much faster with increasing temperatures than lignin reactions. Therefore you can not use excessively high cooking temperatures to shorten your cooking cycle. Generally cooking temperature should be kept below 180 0 C. Above this temperature cellulose degradation becomes severe.
10 Process variables:
Cooking Liquor . Penetration into chips . Sulfidity . Active Alkali . Liquor to Wood ratio Cooking control . H-factor . Temperature cycle . Residual alkali Wood parameters . Species . Size, distribution . Contamination . Moisture content
The objective is to cook to given lignin content at lowest possible yield and cellulose viscosity loss and with minimum of screen rejects. Obviously enough time has to be provided at lower temperatures to achieve good liquor penetration into the chips. Mechanism of penetration is by capillary movement along the Lumina (tracheids and ray cells), pits, resin ducts, and for hardwood, the vessels and into longitudinal fissures. A secondary mechanism is by diffusion through the cell wall. (Diffusion only refers to the movement of dissolved ions through the chips).
Non-swelling liquors (for example acid conditions) penetrate 50-200 times more rapidly in the longitudinal direction (capillary movement) than in the transverse direction (by diffusion). With a swelling agent such as sodium hydroxide, the speed in the longitudinal direction is only around six times faster (the overall speed of liquor penetration is much faster). Insufficient penetration lowers the degree of cooking and increases reject levels (uncooked chip centers). Once initial bulk penetration has taken place and the chip is saturated with liquor, additional chemical can only enter the wood by diffusion.
Air in chips can interfere with penetration-- presteaming helps removing air, also higher temperatures increase liquor transport. Chip size is very important, reduced chip thickness means, chips can be pulped faster and have reduced reject rates. But, smaller length generally means lower yield, weaker pulps, higher chemical consumption (some are overcooked) . Also, a high percentage of fine material can cause poor liquor circulation – pinchips or sawdust are often cooked separately.
Sulfidity Effect of Sodium Sulfide is very apparent up to sulfidity levels of around 15-20%, (depends on wood species) higher levels than that show only very slight
11 improvements. In commercial Kraft mills, sulfidity varies from 15 to 35 % (most common range 20-30 %). The sulfidity level cannot always be readily controlled by the operator since it is dependant on the makeup chemical (see recovery section) used and the sulfur losses in the digestion, pulp washing and liquor recovery system. Because of the increased odor levels at higher sulfidity, many mills prefer to keep sulfidity as low as practical by using sodium hydroxide as part of the makeup chemicals.
Alkali Charge Normal alkali charges are: softwood 12-14 % effective alkali and 8-10 % for hardwood (based on dry weight of wood). A slight excess of chemical is needed to provide sufficient driving force and prevent redeposition of dissolved lignin. Alkali concentration continuously decreases during pulping. Alkali charge does influence reaction rate, but it is more common to control reaction rate through temperature. Higher alkali charge leads to lower H-factors (need cooks with shorter times or at lower temperatures). But high alkali charge also reduces hemicelluloses retention at given KAPPA # (KAPPA is measure for remaining lignin content, see lab section) and changes the composition of the hemicelluloses.
Higher alkali charge is used for unbleached board grades -- increased alkali results in easier separation of fibers at slightly higher KAPPA #, this results in better overall yield (cook stopped earlier). This pulp is used as unbleached pulp, so high KAPPA # doesn’t influence subsequent bleaching stages.
Maximum temperature Temperature has effect on reaction rate. If temperature becomes higher than approximately 180 o C loss in yield and strength become significant (this “maximum” temperature is slightly different for different species). Since cellulose degradation reactions have higher activation energy than lignin reactions increasing temperatures increase cellulose reactions faster than lignin reactions. It is important to make sure that alkali has penetrated into chips before temperature rises above 140 o C, since undesirable lignin condensation reactions can occur at high temperature in absence of alkali. Too rapid heating also can lead to non-uniformity of temperature throughout the digester
Effect of Liquor-to-Wood Ration All chip surfaces have to be wetted. Normally about 75 % of the digester volume are filled with liquor at the start of the cook. As the cook proceeds, chip water and lignin enter the liquid phase while chip mass settles. The liquor level rises in relation to the chip level.
Usually some Black liquor (from previous cooks) is re-circulated into the digester (decreased black liquor dilution). The overall liquor to wood ratios vary from 3 to 5. Liquor concentration should be kept as high as possible (higher reaction rate, less dilution). To ensure minimum dilution and maximum productivity (especially
12 for batch digesters) is necessary to use a reliable method of chip packing (less void volume). Steam packing is a common method of making sure chips are packed effectively.
Wood Species Hardwoods delignify faster than softwoods. Softwoods tend to give lower yields but result in pulps with higher strength. Juvenile wood has lower density, shorter fibers and higher levels of moisture, lignin, hemicelluloses and extractives, this results in lower yield and shorter fiber length for juvenile wood (especially low tear strength).
Characteristics of Kraft pulp fibers
Chemical composition of Kraft pulp fibers differs strongly from original wood. Pulp will retain only around 2-5 % lignin, around 25 % of the original glucommannan and around 50 % of the original xylan. Cellulose is the main component of the pulp. Some cellulose degradation occurs during Kraft cook (peeling reactions yield loss and random chain cleavage decrease in cellulose viscosity). The decrease in the DP of cellulose is accompanied by an increase in its carbonyl content (stopping reactions in cellulose create acids groups).
Residual lignin content is usually around 2% for hardwoods and 3% for softwoods. Hardwoods need to have lower lignin level to completely difiber. Lignin content influences physical and optical properties including strength and color. It also influences chemical behavior in the bleaching process. When pulp is to be used unbleached delignification should be stopped at as high a level as possible ( as long as is can be defibered) to preserve good yield while getting good fiber separation without to much rejects. Amount of remaining pitch (extractives) is very low in Kraft pulps.
Kraft fibers have extremely high strength properties, which makes them attractive for use in linerboard, bags and wrapping papers or bleached board. They are very dark in unbleached condition and therefore are not used in the unbleached form for printing grades. Chromophoric groups formed from lignin during pulping (condensation reactions) are responsible for most of the color in Kraft pulps.
It has been estimated that 90 % of the color in Kraft pulps originates from lignin, 9 % from carbohydrates and 1% from extractives. In some special cases extractives can have stronger influence on the color, for example some eucalyptus pulps contain highly colored extractives.
Redeposition of lignin (during the cook) can influence color strongly. Redepositon happens if pH is to low at the end of the cook. At low pH the dissolved lignin precipitates. Especially precipitated thiolignins contain high levels of chromophores and are extremely dark. This means you should avoid low alkalinity and high solid concentrations especially at the end of the cook.
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Strength of pulps depends on parameters such as cooking time, temperature, alkali charge, sulfidity, degree of delignification and chip size. Also, wood parameters such as fiber length, fiber dimensions, cell-wall thickness, ratio springwood/summerwood are very important.
Physical properties also depend on hemicellulose content and degree of polymerization of the cellulose. Tear strength can be related to a large extend to long fiber content of pulps (20 mesh fraction). Hardwoods have a much shorter fiber length and are not used for products requiring extremely high tear strength. The main use for hardwood pulps is in bleached printing grades. The thin smooth hardwood fiber forms a smooth uniform paper surface with good printing properties.
For higher lignin content pulp strength increases with decreasing lignin content in the fiber. Higher lignin content reduces the bonding capabilities and makes fiber stiffer (less collapsed). Strength starts to decrease if delignification is continued to extremely low lignin content (cellulose damage, continued loss of hemicelluloses).
Chemical pulp properties can be changed through beating/ refining after the pulping process. Depending on refining process variables, fibers can be shortened or can develop new fiber surfaces through external and internal fibrillation.
Cooking Equipment
Kraft cooking is done in a pressure vessel (digester). Digesters are designed to operate either in batch or continuous mode. A third method, using conveyer cooking process is used only for small scale operations, for example for saw dust of non-wood fiber processing.
Batch digesters typically are 70-350 m3 (2,500-12,500 ft3) in volume. They are cylindrical vessels with a conical bottom. Typically a mill has six to eight batch digesters: while several are cooking, one is filling, one blowing etc. Heating with steam may be direct (steam added directly to the digester) -- which results in dilution of the cooking liquor, or indirect, where steam is passed through the inside of tubes within the digester (more uniform heating, no dilution, steam condensate might be used for other proposes).Digesters are packed with 180- 200 kg of chips per cubic meter (Softwood) and around 220-240 Kg/m3 for hardwoods. Chip filling operations take 20-30 minutes.
A continuous digester can be a tube shaped digester where chips are moved through a course that may contain elements of presteaming, liquor impregnation, heating, cooking and washing. Chips enter and exit the digester continuously. continuous digesters tend to be more space efficient, easier to control and give
14 increased yield and lower chemical demand. They are less labor intensive and more energy efficient than batch digesters.
Since continuous digesters are always pressurized, special feeders must be used to allow chips at atmospheric pressure to enter. Screw feeders are used for material like sawdust and straw. Rotary valves are used for chips (work like revolving doors).Kamyr digester is the best known continues digester; they are large, vertical digesters, where chips enter the top and exit the bottom. The chips go through various areas of the digester where they are impregnated with liquor, heated to cooking temperature, held at cooking temperature and washed.
The M&D digester ( Messer and Durkee) is a long digester inclined at a 45o angle. This digester is often used for Kraft pulping of sawdust or semi-chemical pulping. It has some use for Kraft pulping of chips. The chips or sawdust enter the top, go down one side of the digester, return back up the other side (conveyer belt) and exit at the top. A plate separates the two sides. Since the size oft the digesters is limited to about 2.4 m diameter, they are limited to relatively small production levels.
Kraft Recovery system
The Kraft recovery system is a very important part of the Kraft process. It makes it possible to : a) Reuse chemicals b) Recover heat c) Reduce discharge to environment
Modern facilities recover around 98 % of the chemicals applied to the digester while using low rates of wash water (2-3 t of water per ton of pulp).
Weak black liquor has a dissolved solids content of around 13-17 %. This has to be concentrated to around 70 % (higher if possible) to be fired in the recovery boiler. The objective in the recovery boiler is to complete the combustion of the organic matter in the black liquor and to recover sodium and sulfur in a form that is suitable for regeneration of the pulping chemicals. The overall reaction is:
Black Liquor + O2 Na2 CO3 + Na2S + Heat
Black liquor is sprayed into the furnace through liquor guns located about 15 feet above char bed. Relatively coarse sprays are used to minimize material carried out with flue gas. The actual burning can take place in three different modes. In flight burning, bed burning and burning on the walls. Since burning involves chemical reactions between oxygen and black liquor, the location of burning
15 depends on air injection and spraying pattern. For example, smaller droplets have a larger surface and burn faster (in flight). Generally pyrolysis is followed by char burning. Highly swollen char particles burn faster (higher surface area). As the carbon is burned the particle shrinks and becomes denser (smelt drop). Conditions inside the burning char particle are favorable for reduction, and a substantial amount of sulfide formation may take place during in-flight burning. As the carbon is used up reoxidation will become more important.
The char bed consists of the residual material from in flight burning that arrives at the hearth. It provides a fuel reservoir, which helps to stabilize the combustion process. Stability of the bed depends on rate at which fresh material is deposited to burning processes. If out of balance char bed can grow or burn down to nothing. Char bed form can be influenced by spray pattern and air supply.
Bed burning has two roles: combust or gasify the organic material that was not burnt in flight and allow for smelt reduction, liquefying and drainage. Combustion and gasification on the bed require oxygen from air or carbon dioxide or water vapor. Since the bed is relatively impenetrable to gases bed burning is mostly limited to the surface. The active zone is only a few inches thick. Very important are reduction reactions again. Generally, reactions happening here are the same than for single particle char burning. All reactions that could take place below the active zone (no oxygen) are endothermic and therefore self-limiting. Sharp temperature gradients from bed surface inwards are typical. Char beds can “black out” (stop burning, either locally or all together). Main reasons for this are wet black liquor drops landing on the char bed, air inlets being plugged or excessive bed growth.
TRS gases are produced by pyrolysis of black liquor as part of the burning process. They mainly consist of H2S, methyl mercaptane ( CH3SH), dimethylsulfide (CH3SCH3) and dimethyldisulfide (CH3SSCH3). Discharge limits generally are around 5ppm. TRS is reduced by oxidizing to sulfur dioxide (oxidizing zone in recovery boiler). This can only be achieved by oxidizing all combustibles in the furnace and requires excess air and good mixing. The SO2 generated this way reacts with any alkaline fume (dust), for example Na2CO3 dust. Generally high temperatures favor larger dust levels, resulting in good SO2 control, but large dust levels in precipitator.
Second big advantage of Kraft recovery boiler is generation of heat. Typically 3-4 lbs. of steam are generated per pound of black liquor. Steam pressure and temperatures in a recovery steam boiler are generally limited to around 1500 psi and 480oC (most often below these values). The floor and the walls of the furnace consist of water tubes. In addition there are economizer sections (picking up low heat residues) and superheater section (final high steam temperature).
16 Causticizing
In the causticizing process, lime (CaO) slakes with water (formation of Ca(OH)2) and reacts with sodium carbonate in the green liquor to produce sodium hydroxide (NaOH) and CaCO3
CaO + H2O Ca(OH)2
Na2CO3 + Ca(OH)2 2NaOH + CaCO3
The causticizing reaction is an equilibrium. Both calcium hydroxide and calcium carbonate are insoluble under these conditions. Hydroxide and carbonate ions are exchanged with the liquid phase at the surface of the lime particles. This process is necessary to convert the lower pH Na2CO3 to NaOH. Not all Na2CO3 is converted; white liquor generally still contains small amounts of carbonate.
The remaining operations are the separation of lime mud from liquor (which is now mainly NaOH and Na2S) , washing of the lime (CaCO3) and reburning of lime in the lime kiln.
CaCO3 + heat CaO + CO2
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