Kraft process

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International Paper: Kraft paper mill InternationalPaper6413.jpg
International Paper: Kraft paper mill
Woodchips for paper production Woodchips for paper production.jpg
Woodchips for paper production

The kraft process (also known as kraft pulping or sulfate process) is a process for conversion of wood into wood pulp, which consists of almost pure cellulose fibres, the main component of paper. The kraft process involves treatment of wood chips with a hot mixture of water, sodium hydroxide (NaOH), and sodium sulfide (Na2S), known as white liquor, that breaks the bonds that link lignin, hemicellulose, and cellulose. The technology entails several steps, both mechanical and chemical. It is the dominant method for producing paper. In some situations, the process has been controversial because kraft plants can release odorous products and in some situations produce substantial liquid wastes. [1] [2] [3]

Contents

The process name is derived from the German word Kraft , meaning "strength" in this context, due to the strength of the kraft paper produced using this process. [4]

History

A roll of kraft paper Papelparamolde.jpg
A roll of kraft paper

A precursor of the kraft process was used during the Napoleonic Wars in England. [5] The kraft process was invented by Carl F. Dahl in 1879 in Danzig, Prussia, Germany. U.S. patent 296,935 was issued in 1884, and a pulp mill using this technology began in Sweden in 1890. [6] The invention of the recovery boiler by G. H. Tomlinson in the early 1930s was a milestone in the advancement of the kraft process. [7] It enabled the recovery and reuse of the inorganic pulping chemicals such that a kraft mill is a nearly closed-cycle process with respect to inorganic chemicals, apart from those used in the bleaching process. For this reason, in the 1940s, the kraft process superseded the sulfite process as the dominant method for producing wood pulp. [6]

The process

Impregnation

Common wood chips used in pulp production are 12–25 millimetres (0.47–0.98 in) long and 2–10 millimetres (0.079–0.394 in) thick. The chips normally first enter the presteaming 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 liquor impregnation is trapped within the 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.

Cooking

The wood chips are then cooked in pressurized digesters. Some digesters operate in a batch manner and some in a continuous process. Digesters producing 1,000 tonnes or more of pulp per day are common, with the largest producing more than 3,500 tonnes per day. [8] Typically, delignification requires around two hours [9] at 170 to 176 °C (338 to 349 °F). Under digesting 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), contain lignin fragments, carbohydrates from the breakdown of hemicellulose, sodium carbonate, sodium sulfate and other inorganic salts.

net reaction in depolymerization of lignin by SH (Ar = aryl, R = alkyl groups). KraftEq.png
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 sulfide (S2−) or bisulfide (HS−) ions. [7]

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" [10] ) 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). [11] [12] 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 sulfate reduction 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". The green liquor's eponymous green colour arises from the presence of colloidal iron sulfide. [13] This liquid is then 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

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 recausticizing 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. [14] Additionally, bark and wood residues are often burned in a separate power boiler to generate steam.

Although recovery boilers using G.H. Tomlinson's invention have been in general use since the early 1930s, attempts have been made to find a more efficient process for the recovery of cooking chemicals. Weyerhaeuser has operated a Chemrec first generation black liquor entrained flow gasifier successfully at its New Bern plant in North Carolina, while a second generation plant is run in pilot scale at Smurfit Kappa's plant in Piteå, Sweden. [15]

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.

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 fibres 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%.

Washing

The brownstock 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 countercurrent 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).

Several types of washing equipment are in use:

Bleaching

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. [16]

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.

Process chemicals

Process chemicals are added to improve the production process:

Comparison with other pulping processes

Pulp produced by the kraft process is stronger than that made by other pulping processes and maintains a high effective sulfur ratio (sulfidity), an important determiner of the strength of the paper. Acidic sulfite processes degrade cellulose more than the kraft process, which leads to weaker fibers. Kraft pulping removes most of the lignin present originally in the wood whereas mechanical pulping processes leave most of the lignin in the fibers. The hydrophobic nature of lignin [19] interferes with the formation of the hydrogen bonds between cellulose (and hemicellulose) in the fibers needed for the strength of paper [6] (strength refers to tensile strength and resistance to tearing).

Kraft pulp is darker than other wood pulps, but it can be bleached to make very white pulp. Fully bleached kraft pulp is used to make high-quality paper where strength, whiteness, and resistance to yellowing are important.

The kraft process can use a wider range of fiber sources than most other pulping processes. All types of wood, including very resinous types like southern pine, [20] and non-wood species like bamboo and kenaf can be used in the kraft process.

Byproducts and emissions

Forchem tall oil refinery in Rauma, Finland. Forchem Rauma 2.jpg
Forchem tall oil refinery in Rauma, Finland.

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. [21] 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. [21]

Various byproducts containing hydrogen sulfide, methyl mercaptan, dimethyl sulfide, dimethyl disulfide, and other volatile sulfur compounds are the cause of the malodorous air emissions characteristic for pulp mills utilizing the kraft process. [22] [23] The sulfur dioxide emissions of kraft-pulp mills are much lower than those from sulfite mills. In the ambient air outside a typical modern kraft-pulp mill, the sulfur-dioxide odour is perceivable only during disturbance situations, for example when the mill is shut down for a maintenance break, or when an extended power outage occurs. Control of odours is achieved through the collection and burning of these odorous gases in the recovery boiler alongside the black liquor. In modern mills, where well-dried solids are burned in the recovery boiler, hardly any sulfur dioxide leaves the boiler. At high boiler temperatures, the sodium released from the black liquor droplets reacts with sulfur dioxide, thereby effectively scavenging it by forming odourless sodium sulfate crystals.

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 lignins 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. [24] [25]

See also

Related Research Articles

<span class="mw-page-title-main">Pulp (paper)</span> Fibrous material used notably in papermaking

Pulp is a fibrous lignocellulosic material prepared by chemically, semi-chemically or mechanically producing cellulosic fibers from wood, fiber crops, waste paper, or rags. Mixed with water and other chemicals or plant-based additives, pulp is the major raw material used in papermaking and the industrial production of other paper products.

<span class="mw-page-title-main">Pulpwood</span> Timber intended for processing into wood pulp for paper production

Pulpwood can be defined as timber that is ground and processed into a fibrous pulp. It is a versatile natural resource commonly used for paper-making but also made into low-grade wood and used for chips, energy, pellets, and engineered products.

Sulfur dyes are the most commonly used dyes manufactured for cotton in terms of volume. They are inexpensive, generally have good wash-fastness, and are easy to apply. Sulfur dyes are predominantly black, brown, and dark blue. Red sulfur dyes are unknown, although a pink or lighter scarlet color is available.

<span class="mw-page-title-main">Pulp mill</span> Facility which pulps wood or plant fibre

A pulp mill is a manufacturing facility that converts wood chips or other plant fiber sources into a thick fiber board which can be shipped to a paper mill for further processing. Pulp can be manufactured using mechanical, semi-chemical, or fully chemical methods. The finished product may be either bleached or non-bleached, depending on the customer requirements.

<span class="mw-page-title-main">Black liquor</span> Industrial by-product

In industrial chemistry, black liquor is the by-product from the kraft process when digesting pulpwood into paper pulp removing lignin, hemicelluloses and other extractives from the wood to free the cellulose fibers.

Tall oil, also called liquid rosin or tallol, is a viscous yellow-black odorous liquid obtained as a by-product of the kraft process of wood pulp manufacture when pulping mainly coniferous trees. The name originated as an anglicization of the Swedish tallolja. Tall oil is the third largest chemical by-product in a kraft mill after lignin and hemicellulose; the yield of crude tall oil from the process is in the range of 30–50 kg / ton pulp. It may contribute to 1.0–1.5% of the mill's revenue if not used internally.

Lignosulfonates (LS) are water-soluble anionic polyelectrolyte polymers: they are byproducts from the production of wood pulp using sulfite pulping. Most delignification in sulfite pulping involves acidic cleavage of ether bonds, which connect many of the constituents of lignin. Sulfonated lignin (SL) refers to other forms of lignin by-product, such as those derived from the much more popular Kraft process, that have been processed to add sulfonic acid groups. The two have similar uses and are commonly confused with each other, with SL being much cheaper. LS and SL both appear as free-flowing powders; the former is light brown while the latter is dark brown.

<span class="mw-page-title-main">Kraft paper</span> Paper or paperboard produced from chemical pulp produced in the kraft process

Kraft paper or kraft is paper or paperboard (cardboard) produced from chemical pulp produced in the kraft process.

<span class="mw-page-title-main">Recovery boiler</span> Industrial equipment used in pulping

Recovery boiler is the part of kraft process of pulping where chemicals for white liquor are recovered and reformed from black liquor, which contains lignin from previously processed wood. The black liquor is burned, generating heat, which is usually used in the process of making electricity, much as in a conventional steam power plant. The invention of the recovery boiler by G.H. Tomlinson in the early 1930s was a milestone in the advancement of the kraft process.

Bleaching of wood pulp is the chemical processing of wood pulp to lighten its color and whiten the pulp. The primary product of wood pulp is paper, for which whiteness is an important characteristic. These processes and chemistry are also applicable to the bleaching of non-wood pulps, such as those made from bamboo or kenaf.

The sulfite process produces wood pulp that is almost pure cellulose fibers by treating wood chips with solutions of sulfite and bisulfite ions. These chemicals cleave the bonds between the cellulose and lignin components of the lignocellulose. A variety of sulfite/bisulfite salts are used, including sodium (Na+), calcium (Ca2+), potassium (K+), magnesium (Mg2+), and ammonium (NH4+). The lignin is converted to lignosulfonates, which are soluble and can be separated from the cellulose fibers. For the production of cellulose, the sulfite process competes with the Kraft process which produces stronger fibers and is less environmentally costly.

Soda pulping is a chemical process for making wood pulp with sodium hydroxide as the cooking chemical. In the Soda-AQ process, anthraquinone (AQ) may be used as a pulping additive to decrease the carbohydrate degradation. The soda process gives pulp with lower tear strength than other chemical pulping processes, but has still limited use for easily pulped materials like straw and some hardwoods.

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Knots are unwanted, large, dark aggregates of wood fibres when making chemical pulp.

White liquor is a strongly alkaline solution mainly of sodium hydroxide and sodium sulfide. It is used in the first stage of the Kraft process in which lignin and hemicellulose are separated from cellulose fiber for the production of pulp. The white liquor breaks the bonds between lignin and cellulose. It is called white liquor due to its white opaque colour.

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<span class="mw-page-title-main">Paper chemicals</span> Chemicals used in paper manufacturing

Paper chemicals designate a group of chemicals that are used for paper manufacturing, or modify the properties of paper. These chemicals can be used to alter the paper in many ways, including changing its color and brightness, or by increasing its strength and resistance to water. The chemicals can be defined on basis of their usage in the process.

Wet Processing Engineering is one of the major streams in Textile Engineering or Textile manufacturing which refers to the engineering of textile chemical processes and associated applied science. The other three streams in textile engineering are yarn engineering, fabric engineering, and apparel engineering. The processes of this stream are involved or carried out in an aqueous stage. Hence, it is called a wet process which usually covers pre-treatment, dyeing, printing, and finishing.

Red Scar Works was built in 1939 by Courtaulds and produced rayon. It was located in Ribbleton Preston off Longridge Road. The closure of the works was announced in November 1979 and the issue raised in the UK Parliament House of Commons by the constituency MP. At the time of closure approximately 2,600 people were employed there but there were approximately 4,000 at its peak. It was the largest rayon producing site in Britain. Two main products were manufactured with one being tyre cord by a process known as CSPT – Continuous Spinning Process Tenasco. Two main denier of this product were manufactured. The other product was a general textile called Bright. A range of deniers of this were produced in a range of colours. The trade name for the coloured product was Duracol. At the time of closure, one reason given by management for the closure was the rising popularity of steel belt radial tires thus reducing demand for tyre cord.

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References

  1. ↑ Hoffman, E., Lyons, J., Boxall, J., Robertson, C., Lake, C. B., & Walker, T. R. (2017). Spatiotemporal assessment (quarter century) of pulp mill metal (loid) contaminated sediment to inform remediation decisions. Environmental Monitoring and Assessment, 189(6), 257.
  2. ↑ Hoffman, E., Bernier, M., Blotnicky, B., Golden, P. G., Janes, J., Kader, A., ... & Walker, T. R. (2015). Assessment of public perception and environmental compliance at a pulp and paper facility: a Canadian case study. Environmental Monitoring and Assessment, 187(12), 766.
  3. ↑ Rudolf Patt et al. "Paper and Pulp" in Ullmann's Encyclopedia of Industrial Chemistry 2002 Wiley-VCH, Weinheim. doi : 10.1002/14356007.a18_545.pub4
  4. ↑ Both capitalized and lowercase spelling ("Kraft process" and "kraft process") appear in the literature, but "kraft" is most commonly used in the pulp and paper industry.
  5. ↑ Joint Textbook Committee of the Paper Industry (1969). Macdonald, Ronald Gordon; Franklin, John N. (eds.). Pulp and Paper Manufacture, vol. 1 : The Pulping of Wood. Vol. 1 (2nd ed.). McGraw Hill. OCLC   867778880.
  6. 1 2 3 Biermann, Christopher J. (1993). Essentials of Pulping and Papermaking . San Diego: Academic Press, Inc. ISBN   0-12-097360-X.
  7. 1 2 E. Sjöström (1993). Wood Chemistry: Fundamentals and Applications. Academic Press. ISBN   0-12-647480-X.
  8. ↑ Woodman, Jocelyn (1993). "Pollution Prevention Technologies for the Bleached Kraft Segment of the U.S. Pulp and Paper Industry". U.S. Environmental Protection Agency. p. 66. Retrieved 2022-02-06.
  9. ↑ "Basics of Kraft Pulping" (PDF). University of Tennessee Knoxville - Department of Chemical and Biomolecular Engineering. Archived from the original (PDF) on 2021-10-23. Retrieved 2021-10-23.
  10. ↑ "Equipment to handle heavy black liquor". Archived from the original on 2005-04-20. Retrieved 2007-10-09.
  11. ↑ Hsieh, Jeffery S.; Smith, Jason B. "Second Critical Solids Black Liquor Scaling" (PDF). Pulp and Paper Engineering, School of Chemical Engineering, Georgia Institute of Technology. Archived from the original (PDF) on 2011-08-31. Retrieved 2007-10-09.
  12. ↑ USgranted 5527427,Mualla Berksoy&Yaman Boluk,"High solids black liquor of reduced viscosity and viscosity reduction method for high solids black liquor",issued 1996-06-18, assigned to Optima Specialty Chemicals & Technology Inc
  13. ↑ Giddings, J.F.; Roll, D.R.; Cappellino, C.A.; Day, M.; Nardone, R.A. (2008). "12.12.4.". In Karassik, I.; Messina, J.; Cooper, P.; Heald, C. (eds.). Pump Handbook (4th ed.). New York: McGraw Hill. ISBN   9780071460446. Archived from the original on March 4, 2013. Retrieved February 21, 2013.
  14. ↑ Jeffries, Tom (March 27, 1997). "Kraft pulping: Energy consumption and production". University of Wisconsin Biotech Center. Archived from the original on September 28, 2011. Retrieved 2007-10-21.
  15. ↑ "Chemrec web site". Archived from the original on 2012-07-09. Retrieved 2011-01-07.
  16. ↑ "Environmental Comparison of Bleached Kraft Pulp ManufacturingTechnologies" (PDF). Archived from the original (PDF) on 2004-12-18. Retrieved 2007-09-28.
  17. ↑ Goyal, Gopal C. (1997). Anthraquinone Pulping. A TAPPI Press Anthology of Published Papers, 1977-1996. Atlanta: TAPPI Press. ISBN   0-89852-340-0.
  18. ↑ "Tall Oil and Tall Oil Soap Recovery" (PDF). Archived from the original (PDF) on 2012-03-05. Retrieved 2008-12-19.
  19. ↑ Hubbe, Martin a.; Lucian A. Lucia (2007). "The "Love-Hate" Relationship Present in Lignocellulosic Materials". BioResources. 2 (4): 534–535. doi:10.15376/biores.2.4.534-535 . Retrieved 2015-02-03.
  20. ↑ "The Southern Pines" (PDF). US Department of Agriculture. 1985. Archived from the original (PDF) on 2007-07-09. Retrieved 2007-09-13.
  21. 1 2 Stenius, Per, ed. (2000). "2". Forest Products Chemistry. Papermaking Science and Technology. Vol. 3. Helsinki, Finland: Fapet OY. pp. 73–76. ISBN   952-5216-03-9..
  22. ↑ Hansen, G. A. (1962). "Odor and Fallout Control in a Kraft Pulp Mill". Journal of the Air Pollution Control Association. 12 (9): 409–436. doi:10.1080/00022470.1962.10468107. PMID   13904415.
  23. ↑ Hoffman, E., Guernsey, J. R., Walker, T. R., Kim, J. S., Sherren, K., & Andreou, P. (2017). Pilot study investigating ambient air toxics emissions near a Canadian kraft pulp and paper facility in Pictou County, Nova Scotia. Environmental Science and Pollution Research, 24(25), 20685-20698.
  24. ↑ Hoffman, E., Bernier, M., Blotnicky, B., Golden, P. G., Janes, J., Kader, A., ... & Walker, T. R. (2015). Assessment of public perception and environmental compliance at a pulp and paper facility: a Canadian case study. Environmental monitoring and assessment, 187(12), 766.
  25. ↑ Hoffman, E., Lyons, J., Boxall, J., Robertson, C., Lake, C. B., & Walker, T. R. (2017). Spatiotemporal assessment (quarter century) of pulp mill metal (loid) contaminated sediment to inform remediation decisions. Environmental monitoring and assessment, 189(6), 257.

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