Mid Sweden University

In this work, high-rate fibre analysis has been used for direct feedback control of pulp quality by application of a new control strategy for a two-stage refining process in the Holmen Hallsta mill, Sweden. The application is based on control of pulp freeness, estimated from the continuous fibre analysis results from a BTG Single Point Morphology ana-lyzer. The goal was to create a robust and simple control strategy. The new strategy includes control of plate gap, con-sistency and the hydraulic force difference between the stages. Expressed as standard deviation, the freeness and av-erage fibre length variations were reduced by 50% and 25% respectively. The small size of the pulp chest in this process also benefits stronger feedback control. Long-term operation suggest that high-rate fibre analysis can be used to reduce faster quality variation.

A successful way to increase the strength properties for pulps based on lignin-rich fibres is to compress the fibre structure at high temperature by means of hot-pressing technology. The fundamental knowledge of how the fi-bre morphology influences the mechanical properties when a paper sheet is hot-pressed is still scarce. Paper sheets based on thermomechanical pulp (TMP) produced with single disc and double disc refiners were compared. The effect of degree of refining was studied as well as the effect of fibre shapes by fractionating pulp with hydrocyclones. Additionally, the effect of fines was studied. All pulps were produced at the Holmen Bra-viken Mill, Norrköping, Sweden with Norway Spruce (Picea abies) as raw material. Handsheets (100 g/m2) with 62% ± 3 dryness were hot-pressed at temperatures up to 260°C at a pressure around 8MPa. The hot-press-ing increased both dry and wet strength for all pulps studied. This was true even for pulps with low fines con-tent and low refining energy. Even thick-walled fibres normally giving lower strength showed an increase of 100% when hot-pressed. In summary, hot-pressing technology can make it possible to use different TMPs to produce strong packaging materials for use in dry and wet conditions.

Valmet has developed a new centre plate, for the RGP68DD refiner, which minimise the free volume in the centre of the refiner. The new centre plate has been evaluated in trials with Norway spruce as raw material for production of TMP for printing paper at the Holmen Braviken Mill, Norrköping, Sweden. The new and old centre plate was compared by generating refining curves both by changing gap at constant production rate and by increasing production rate at constant gap. When the production rate was increased from 13 to 19 adt/h in the trial with the new centre plate, the power and gap varia-tions were reduced by around 70%. The new centre plate did not affect tensile index and shives content at given specific energy. However, the fibre length at given ten-sile index was somewhat higher and the light scattering coefficient lower with the new centre plate, which might be an effect of increased blow line consistency and less power variations. The large improvement of refiner sta-bility and the increased fibre length opens up for seg-ment design development aiming at larger specific en-ergy reduction.

This work shows that, for newsprint quality grades, the production processes for mechanical pulp can be simplified, and the specific electrical energy demand can be reduced with around 600 kWh/ton (30%). The purpose of the work is to demonstrate how the production cost for mechanical pulps can be decreased through increased energy efficiency and reduced number of unit operations. The idea was to improve the main line refining conditions so that no additional fibre development or shive reduction is needed and thereby, the normal screening and rejects treatment system could be omitted.

Mechanical pulp is used to produce a variety of products, where the two largest categories are printing papers and paperboard for packaging. The pulp is mainly produced by the breakdown of wood chips between rotating metal discs in machines called refiners with the product and process generally referred to as thermomechanical pulp(ing) (TMP). The refiner process requires high specific electrical energy to separate and develop the fibres to a pulp intended for the production of printing papers. Today, many processes need over 2000 kWh/ton of refining energy plus 200-300 kWh/ton of auxiliary energy (to drive pumps, agitators, screw conveyors, screens, presses, etc.).

During the last two decades of the 20th century, the chemical processing industry underwent a transformation. The process development changed from being unit operation focused to function focused. The result is more compact processes with less equipment, higher yield and lower energy demand. When the development is made in an innovative way with such large effects on process performance, it is referred to as process intensification. My work is inspired by the concepts of process intensification, especially the striving for more compact processes with higher efficiency. 

This work is focused on mechanical pulp, intended for the manufacture of printing paper, produced in refiners with Norway spruce (Picea abies) as raw material. However, this approach could also be applied to mechanical pulp production in integrated paperboard mills and also using other raw materials e.g., pines or hardwoods. The investigated pulps and processes in this work are mainly intended for uncoated paper grades (newsprint, improved newsprint and book paper) printed by the offset printing process. In all studies, the pulps have been produced with full scale mill equipment and evaluated using laboratory measurements. However, in two studies, the produced pulps were evaluated on paper machines and at printing houses.

A large number of process concepts have been evaluated in which different approaches have been used to reduce the specific energy and, in some cases, improve pulp quality. The approaches include:

1.     Impressafiner chip pretreatment 

2.     Primary high consistency (HC) refiner type (DD, RTS, CD, SD)

3.     Addition of low doses of sodium sulphite 

4.     Increased refining temperature (housing pressure)

5.     Refiner segments and centre plate design

6.     Increased production rate

7.     Low consistency (LC) refining in different process positions and in combination with different HC refiner types

The separate effects of all these techniques have not been evaluated systematically neither have potential synergistic effects of all possible combinations been investigated. Even though a large number of combinations of unit operations have been studied, the emphasis has been on trying to do as much fibre development as possible in a single HC refining stage.

The mill trials with spruce as raw material have shown that a low shive content and appropriate fibre development can be attained in a process without separate treatment of long fibres. High intensity primary stage refining (RTS and DD) was necessary to reach a low shive content at a low specific refining energy (SRE), with DD refiners appearing to be the most suitable for simplified processes. DD and RTS refining produced pulps with fibres exhibiting a higher degree of external fibrillation and share of split fibres than SD refining. DD refining produced fibres with lower cell wall thickness and higher light scattering at given fibre length than RTS refining. The lowest specific refining energy was attained for one of the trials using the process, denoted as S:HT:DD-LC-LC, consisting of DD refining at increased production rate, 18 adt/h, increased housing pressure, 6.6 bar(g), and with 5 kg/adt sodium sulphite added to the chips immediately  before the refiner. After DD refining the pulp was refined in two LC refining stages. This process required only 1280 kWh/adt SRE to reach a tensile index of 52 Nm/g (Rapid-Köthen). This is 900 kWh/adt lower than the final pulp for newsprint based on SD HC refining, and over 500 kWh/adt lower than Scandinavian BAT processes (2014). Additionally, the auxiliary energy was around 150 kWh/adt lower for the processes without a conventional rejects treatment system. At 52 Nm/g tensile index, the light scattering coefficient was 2-3 m2/kg higher, and the length-weighted average fibre length was around 0.1 mm lower for this process than for SD TMP final pulp. The fibre bonding, indicated by density, tensile index and Z-strength of fibre fraction handsheets, was similar or higher for the S:HT:DD-LC-LC process than the reference SD TMP process with a rejects treatment system. 

Other interesting process configurations, with somewhat lower efficiencies, included:

1.     Impressafiner pretreatment of the chips with sodium sulphite before DD refining, with or without subsequent LC refining. Chip pretreatment with the Impressafiner enabled operating the DD refiner at higher intensity (feeding segments and increased production rate) without significant loss of quality and LC refining enabled increased production rate which increased the overall efficiency.

2.     RTS-SD refining with sodium sulphite added before the second stage SD refiner referred to as RTS-S:SD. The pulp from the RTS-S:SD process had similar fibre length as the S:HT:DD-LC-LC process but lower light scattering coefficient.

3.     A single-stage DD refiner operating at 15.5 adt/h and 4 bar(g) housing pressure (no sodium sulphite addition), which produced pulp with lower fibre length but higher light scattering coefficient than the S:HT:DD-LC-LC process. 

Two simplified processes were evaluated on paper machines and in printing houses. The first, denoted DD-LC-F, involved a combination of DD primary refining followed by LC refining and fractionation (screening). The screen rejects were mixed with the main line DD pulp before the LC refiner. The second process was the CPT:S-DD-LC process (№1 above). Good runnability was attained both on the paper machines and in the offset printing presses and the paper quality was similar to the reference paper.

For printing paper applications, the proportion of fibre development in LC refining should preferably be relatively low, since it was shown that LC refiners have limited capacity to reduce fibre wall thickness and thereby develop light scattering and fibre fraction Z-strength.

Explicit effects on the number of unit operations and production cost have not been evaluated in this work, but clearly both investment and variable costs as well as fixed costs can be reduced with a simplified process.

Det här arbetet visar att det är möjligt att kraftigt förenkla tillverknings-processen för mekanisk massa och samtidigt minska totala elenergibehovet med omkring 600 kWh/ton (30 %) jämfört med dagens bästa teknik. Syftet med detta arbete är att visa hur produktionskostnaden för mekanisk massa kan sänkas genom ökad energieffektivitet och minskat antal enhets-operationer. Utgångspunkten var att det borde vara möjligt att förbättra betingelserna vid högkoncentrations(HC)raffinering så att det endast krävs ett HC steg och bara mindre efterföljande fiberutveckling. Därmed kan den specifika elenergin minskas avsevärt och det normala rejektbehandlings-systemet uteslutas.

Mekanisk massa används för att producera en mängd olika produkter, där de två största kategorierna är tryckpapper och kartong för förpackningar. Massan framställs huvudsakligen genom att träflis mals mellan roterande metallskivor i maskiner som kallas raffinörer. Separationen och bearbet-ningen av fibrerna till en massa avsedd för produktion av tryckpapper kräver mycket elenergi. Idag använder många processer över 2000 kWh/ton elenergi för raffinering plus 200-300 kWh/ton elenergi för att driva övrig utrustning, t.ex. pumpar, omrörare, silar, skruvtransportörer, pressar, mm.

Under slutet av 1900-talet genomgick den kemiska processindustrin en genomgripande omvandling. Processutvecklingen gick från att vara fokuserad på enhetsprocesser till att bli funktionsfokuserad. Resultatet är mer kompakta processer med mindre utrustning, högre utbyte och lägre energibehov. När utvecklingen görs på ett innovativt sätt med stor effekt på processprestanda kallas det processintensifiering. Mitt arbete är inspirerat av metodiken inom processintensifiering, särskilt strävan efter mer kompakta processer med högre effektivitet. Arbetet är inriktat på mekanisk massa avsedd för tillverkning av tryckpapper, som produceras i raffinörer med gran (Picea abies) som råvara. Metodiken kan dock med stor sannolikhet tillämpas för produktion av mekanisk massa i integrerade kartongbruk och även för andra råvaror, till exempel tall eller lövträ.

De undersökta massaprocesserna i detta arbete är främst avsedda för obestrukna papperskvaliteter (t.ex. tidningspapper, bokpapper och förbättrat tidningspapper) tryckta i offset. I alla studier har massan producerats med stora raffinörer i pappersbruk och utvärderats med hjälp av laboratorie-mätningar. I två studier utvärderades dessutom den producerade massan på pappersmaskin och i tryckeri.

Ett stort antal processkoncept har utvärderats där olika metoder har använts för att minska den specifika energin och förbättra massakvaliteten:

1.     Flisförbehandling med Impressafiner 

2.     Typ av HC primärraffinör (DD, RTS, CD, SD)

3.     Tillsats av natriumsulfit 

4.     Ökad raffineringstemperatur (malhustryck)

5.     Raffinörsegmentdesign

6.     Ökad produktionstakt

7.     Lågkoncentrations(LC)raffinering i olika positioner och tillsammans med olika primärraffinörer.

De separata och eventuellt synergistiska effekterna av dessa tekniker har inte utvärderats systematiskt. Även om ett stort antal konfigurationer har studerats, har fokus legat på att försöka göra så mycket fiberutveckling som möjligt i ett enda HC-raffineringssteg.

Fabriksförsöken har visat att låg spethalt och tillräcklig fiberutveckling kan uppnås i en process utan rejektraffineringssystem. HC-raffinering med hög intensitet (RTS och DD) var nödvändigt för att uppnå en låg spethalt vid låg specifik energi, där DD-raffinörer visade sig vara de mest lämpliga. DD och RTS raffinörerna producerade massa vars fibrer hade högre grad av extern fibrillering och mer sprickor i fiberväggen. DD raffinering resulterade i fibrer med tunnare cellväggar och högre ljusspridning vid viss fiberlängd jämfört med RTS raffinering.

Lägst specifik raffineringsenergi erhölls i ett av försöken med en process bestående av DD raffinering vid hög temperatur och med tillsatts av 5 kg/ton natriumsulfit precis före raffinören. Raffinören kördes med hög produktions-takt (18 adt/h) och högt hustryck (6,6 bar(g)). Därefter LC-raffinerades massan i två steg. Denna process, benämnd S:HT:DD-LC-LC, krävde endast 1280 kWh/adt i specifik raffineringsenergi till dragindex 52 Nm/g (Rapid-Köthen), vilket är 900 kWh/adt lägre än en process med SD-raffinering i två steg samt ett normalt rejektsystem och cirka 500 kWh/adt lägre än de bästa processerna i Skandinavien (2014). Dessutom var behovet av övrig elenergi (pumpar, silar, pressar, mm) cirka 150 kWh/adt lägre för en process utan rejektbearbetnings-system. Vid 52 Nm/g i dragindex var ljusspridningskoefficienten 2-3 m2/kg högre och längdviktade medelfiberlängden cirka 0.1 mm lägre för den förenklade S:HT:DD-LC-LC processen jämfört med färdigmassa från SD TMP. Massan från den förenklade processen hade bättre eller lika bra fiberbindning, bedömd utifrån densitet, dragindex och Z-styrka på fiber-fraktionsark, som en SD TMP process med sileri och rejektbearbetning.

Ytterligare intressanta processer som dock hade något lägre energi-effektivitet var:

1.     Flisförbehandling med Impressafiner och natriumsulfit följt av DD raffinering, med eller utan efterföljande LC raffinering. Flisför-behandlingen med Impressafiner möjliggjorde högre raffinerings-intensitet (Matande segment och hög produktion) och därmed högre energieffektivitet utan att massakvaliteten blev sämre. LC raffinering i huvudlinjen möjliggjorde ökad produktionstakt vilket sammantaget ökade energieffektiviteten.

2.     Tvåstegsraffinering med RTS i första steget och SD raffinering i andra steget där 5 kg/adt natriumsulfit tillsattes till massan före andrasteget. Denna process producerade massa med samma fiberlängd som S:HT:DD-LC-LC processen, men med lägre ljusspridning.

3.     DD raffinering utan sulfit, men vid relativt hög produktion, 15.5 adt/h, och vid normalt hustryck, 4 bar. Detta är en mycket enkel process som dock resulterade i en massa med lägre fiberlängd men högre ljusspridning än S:HT:DD-LC-LC processen. 

Två förenklade processer utan sileri och rejektbearbetning utvärderades på pappersmaskiner och i tryckeri. Den första bestod av DD raffinering i första steget följt av LC raffinering och silning. Silrejektet blandades med DD massan före LC raffinören. Den andra processen inleddes med flisför-behandling med Impressafiner och natriumsulfit följt av DD raffinering och ett LC raffineringssteg. Massorna från de två förenklade processerna uppvisade bra körbarhet på pappersmaskinerna och i tryckerierna och gav liknande pappers-kvalitet som den normala massan, med undantag av något lägre rivstyrka.

Vid tillverkning av massa för tryckpapper är det fördelaktigt att kombinera LC raffinering med flisraffinering som ger hög ljusspridning, till exempel DD-raffinörer. Dessutom bör andelen av den totala bearbetningen vara relativt låg i LC raffineringen eftersom den har begränsad förmåga att minska fiberväggs-tjockleken och därigenom utveckla ljusspridning och fiberfraktionens bindningsförmåga (mätt som Z-styrka).

Effekten på antal enhetsoperationer och produktionskostnaden har inte utvärderats explicit i detta arbete, men det är uppenbart att både investerings-kostnad samt rörlig och fast kostnad kan minskas med en förenklad process. 

Production of mechanical pulps requires high specific electrical energy compared to many other attrition processes. In Scandinavia, the lowest specific refining energy for production of thermomechanical pulp is around 1800 kWh/t for newsprint quality, which is roughly 60 times higher than for crushing of stone to a similar size distribution. The high specific energy demand for refining has naturally motivated large efforts in the search for improved efficiency. It is always practical to be able to quantify improvements in efficiency for comparison of process designs and of different machine types. However, there is no commonly accepted definition of efficiency for mechanical pulping processes. In published work within mechanical pulping, energy efficiency has been presented in different ways. In this paper, we discuss definitions of energy efficiency and aspects that ought to be considered when energy efficiency is presented. Although focus of this work is on energy efficiency for refiner processes, the principles can be applied to other types of mechanical pulping processes such as stone groundwood. 

Mechanical pulp for printing paper can be produced with a process that involve much less equipment and that require much lower specific energy compared to conventional processes. Even though common evaluation methods, e.g. handsheet testing, have shown that the pulp quality is similar for the simplified and the conventional processes, it is not known how fibre properties, at the microscopic level, is developed with the simplified process. In this mill scale study, the fibre properties attained with an "intensified"mechanical pulping process, consisting of single stage high consistency double disc refining followed by two stage low consistency refining and no reject treatment was investigated. The simplified process was compared to a process with a reject system. The simplified process rendered fibres with higher degree of fibrillation, higher share of axial splits, lower fibre wall thickness but slightly lower length than the conventional process. The fibrillar fines size distribution of the two processes was different. The conventional process generated more of small fibrillar fines which probably explains the higher tensile index at given density for that process. The results show that it is possible to simplify the production process for mechanical pulp and reduce the specific energy with over 700 kWh/adt.

In this study, the effect of process- and online analyser configuration on pulp quality control is explored. The following parameters were included: analyser sampling interval, time delay, measurement error magnitude, and latency chest residence time. Using different values of parameters in a process model, a range of configurations were constructed. For each configuration, the achievable control performance was evaluated using an optimization approach. PI controller settings were chosen based on minimization of the integrated absolute error (IAE) in pulp quality after an input step disturbance. The results show that reducing the sampling interval improves performance also when the interval is smaller than the chest residence time or the analyser delay. Moreover, reducing the chest residence time can reduce the IAE by up to 40 %. However, reducing the residence time to lower than 1/3 of the sampling interval does not improve performance. Further improvement is possible if the analyser delay is reduced. The compromise between reducing the IAE and avoiding creating variation by acting on measurement error has a strong influence on the results. In conclusion, pulp quality control performance can be improved significantly by making changes to the studied configuration parameters. 

The aim of this study was to evaluate changes in fibre properties with high (HC)- and low consistency (LC) refining of TMP and determine how these contribute to tensile index. Two process configurations, one with only HC refining and another with HC refining followed by LC refining were evaluated in three TMP mainline processes in two mills using Norway spruce. An increase in tensile index for a given applied specific energy was similar for all LC refiners in the three lines, despite differences in the fibre property profiles of the feed pulps. Compared with only HC refined pulps at a given tensile index, HC+LC refined pulps had greater fibre wall thickness, similar fibre length, strain at break and freeness, but lower light scattering coefficient, fibre curl and external fibrillation. The degree of internal fibrillation, determined by Simons' stain measurements, was similar for both configurations at a given tensile index. The results indicate that the increase in tensile index in LC refining is largely influenced by a decrease in fibre curl and in HC refining by peeling of the fibre walls. Compared at a given tensile index, the shive content (Somerville mass fraction) was similar for both HC+LC and HC refining. 

This paper is a review of the development of the mechanical pulping process with focus on refiner-based processes. The intention is to provide an overview of the trends and the major advances in the development of the mechanical pulping (MP) process. The focus is on the development of the entire MP process, rather than the refiner as such. However, when discussing the MP process development, it is inevitable to consider the development of the refiner unit operation briefly. Processes for printing papers based on softwood is mainly discussed, but board processes are discussed briefly as well.