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Steaming of Wood Chips - Experimental determination of heating times and effect of different parameters
KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH).
2019 (English)Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesisAlternative title
Basning av flis: Experimentell undersökning av uppvärmningstid samt olika parametrars påverkan (Swedish)
Abstract [en]

The presteaming of wood chips is an important step in the chemical pulping industry. It removes the air from within wood chips, allowing the cooking liquor to better impregnate wood chips, which leads to a more uniform cooking process, and lowers the amount of rejects. When steaming at atmospheric pressure, it is important that the temperature of the wood chips reach 100ᴼ C, as otherwise there will be an equilibrium leaving some air left inside. Having poorly steamed chips in a process could cause severe problems when it comes to reaching the targeted kappa number, or having the adequate retention time in the digester. There are a few different ways in which the wood chips are presteamed within the industry, however, there is little experimental data regarding the heating time of wood chips that can be used when designing these systems. Most studies have mainly focused on the air removal, or improvement of the impregnation step, and the few studies that have included the heating of the wood chips were limited to only one type of wood chip, or failed to specify the experimental details.

Therefore, handmade wood chips pine and birch, two tree species commonly found in Sweden, were steamed in an ATEX designed digester with a steam jacket. The wood chips had thermocouples inside them and the temperature and time was recorded, and the effect of different parameters on the heating could thus be studied.The results revealed that there could be more than a minute in average time difference between wood chips of different thicknesses, both for birch and pine, although the difference in heating time was more linearly correlated to thickness for the birch chips. Pine chips of different thickness were also studied when the pressure inside the digester was allowed to build up, which showed that it is mainly thicker chips that have reduced heating time under such circumstances, as the thinner chips stop heating for a while when the steam condensates on colder surroundings. When comparing heartwood and sapwood chips, it was noted that the difference in heating time could be around 1 minute at most for pine, but only a few seconds for birch. This was most likely due to the pine heartwood and sapwood having distinct moisture contents, 25 % and 58 % respectively, while it was 41% and 42 % in birch heartwood and sapwood. Birch and pine chips wee also steamed together, however, the difference in heating time was only a few seconds on average.

When comparing these experimental results with simulation data of the steaming of wood chips, it fit rather well when it came to the general heating time. However, the effect of increased moisture content had a much larger impact in the simulations, which predicted that more moist wood chips would need several minutes more steaming time, while the experiments only showed at difference of, at most, around 1 minute. When comparing with old experimental data, that has been the basis for the design of older steaming processes, it gave very distinct results, where the effect of thickness did not have as big of an impact as in the old data. No further comparison could be made, however, as the experimental conditions for the old experimental data were not known. Based on these results, it was noted that a steaming time of at least 5 minutes would be needed to ensure that even the largest and more moist chips could reach 100ᴼ C in this system.

Finally, the condensate from the handmade birch and pine chips was analyzed. It revealed the presence of low molecular weight compounds like methanol, formic acid and acetic acid. Common metal ions were also present,although the amount of sodium ions clearly surpassed the rest. The pH of the pine condensate was measured and it was very high, which implies that the condensate was contaminated.

Abstract [sv]

Basning av flis är ett viktigt steg inom kemisk massaindustri. Det avlägsnar luft från flisens insida vilket gör att impregneringen av luten blir bättre, vilket i sin tur leder till en jämnare kokning och färre rejekt. När basningen sker vid atmosfärstryck är det viktigt att flisen når en temperatur på 100ᴼ C, annars kommer det finna ett jämviktstillstånd där lite luft blir kvar på insidan. Att ha otillräckligt basad flis i en process skulle kunna orsaka stora problem när det gäller att nå önskade kappatal, eller att ha en önskad retentionstid i kokaren.Basningen görs på ett par olika sätt inom industrin, men det finns väldigt lite experimentell data tillgänglig angående flisens upvärmning, som skulle kunna användas när dessa system designas. De flesta vetenskapliga studier har fokuserat på luftborttagningen eller på förbättringar av impregneringssteget, medan de få studier som inkluderat mätningar av temperaturen ofta varit begränsade till ett slags trä, eller så har de inte inkluderat detaljer kring experimentet.

Därför basades handgjorda flisbitar av björk och tall från Sverige i en ATEX-designad kokare med en ångjacka. Flisen hade termoelement inuti och temperaturen samt tiden kunde avläsas, vilket gjorde det möjligt att studera effekten av olika parametrar. Resultaten visade att det kunde skilja mer än en minut i uppvärmningstid mellan flisbitar av olika tjocklekar, både för tall och björk, även fast skillnaden i uppvärmningstid var mer linjärt relaterad till tjockleken för björkflisen. Tallflisen studerades också när trycket inuti kokar tilläts stiga vilket visade att det de tjockare flisbitarnas uppvärmningstid som kortas ned mest, eftersom de tunnare flisbitarna slutar värmas upp när ångan börjar kondensera på kallare ytor runt omkring. När flis av splintved och kärnved jämfördes visade det sig att skillnaden i uppvärmningstid kunde vara omkring 1 minut för tall, men endast ett par sekunder för björk. Detta beror troligtvis på att kärnveden och splintveden i tall hade stora skillnader i fukthalt, 25 % respektive 58 %, medan det för björk var 41 % och 42%. Björkflis och tallflis basades även tillsammans men det skillde bara ett fåtal sekunder i genomsnitt i uppvärmningstid.

När den experimentella datan jämfördes med data från simuleringar visade det sig att de stämmer väl överens när det gäller uppvärmningstiden i allmänhet. Å andra sidan förutspådde simuleringsdatan att en ökad fukthalt skulle leda till flera minuters skillnad i uppvärmningstid, medan endast 1 minuts skillnad uppmättes. När jämförelser gjordes med gammal experimentell data som använts som grund för tidigare processers design, var resultaten ganska olika eftersom den gamla datan visade en större effekt av ökad tjocklek än den som uppmättes.Tyvärr kunde inte fler jämförelser göras eftersom detaljerna kring experimentet bakom den gamla datan inte var kända.

Slutligen analyserades även kondensatet från de handgjorda flisbitarna av tall och björk. Det visade att det fanns små mängder av små polära organiska ämnen, såsom metanol, myrsyra och ättiksyra. Vanliga metalljoner detekterades också, där mängden natrium var klart större än övriga metalljoner. Tallkondensatets pH mättes och det visade sig vara väldigt högt, något som tyder på att det troligtvis var förorenat.

Place, publisher, year, edition, pages
2019. , p. 36
National Category
Paper, Pulp and Fiber Technology
Identifiers
URN: urn:nbn:se:kth:diva-259926OAI: oai:DiVA.org:kth-259926DiVA, id: diva2:1353778
External cooperation
Valmet
Educational program
Master of Science in Engineering - Engineering Chemistry
Supervisors
Examiners
Available from: 2019-09-24 Created: 2019-09-24 Last updated: 2019-09-26Bibliographically approved

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