Pressure screens are used for contaminant removal and fibre length fractionation in the production of pulp and paper products. Axial variations and entry effects in the screen are known to occur and these variations have not been adequately quantified. This thesis describes a fundamental study of the axial variations of several factors that occur within an industrial pressure screen; namely, pulp consistency, fibre length distribution, rotor pressure pulse, and feed annulus tangential velocity. Axial variations of pulp consistency in the screen annulus and the accept chamber of the screen were studied using an internal radial sampling method. Localised pulp samples were taken and evaluated and common measures of screen performance such as fibre passage ratio and fractionation efficiency were calculated along the screen. Consistency generally increased along the length of the screen although under certain conditions the consistency toward the front of the screen was lower than the feed consistency. A two passage ratio model that incorporated forward and reverse passage ratio was derived to elucidate the flow of both fibre and fluid through the screen and their effects on overall screen performance. The passage of fibre through the screen decreased with screen length which generally had a positive effect on the fractionation efficiency toward the back of the screen. The passage of individual fibre length fractions was also studied and it was found that long fibre had a much lower passage than short fibre which caused the average fibre length in the annulus to increase. Rotor induced pressure pulse variations along the screen length were also investigated. The magnitude of the pressure pulse was significantly lower (up to 40 %) at the rear of the screen. The variation in pressure caused by the rotor is due to a Venturi effect and the shape of the rotor. The relative velocity of the fluid and the rotor, called the slip factor, also directly affects the size of the pressure pulse in the annulus. The slip factor decreases along the length of the screen due to the increase in tangential velocity of the fluid. Pressure pulse data was also used to estimate the instantaneous aperture velocity and back-flush ratio. The instantaneous aperture velocity was calculated to vary considerably from the superficial aperture velocity by up to 5 m/s in the forward direction and 10 m/s in the reverse direction. Computational Fluid Dynamics (CFD) was used to model tangential velocity changes in simplified screen annuli with axial through flow. For a smooth screen rotor the mean tangential velocity increased over the entire length of the annulus without reaching a maximum value. A step and bump rotor were modelled and the shape of the pressure pulses showed good agreement with experimentally measured pulses. The mean tangential velocity and the entrance length were found to be heavily dependant on the screen rotor used.

Abstract Plant fibres are an important part of modern daily life. The most obvious consumer products manufactured from them are paper, cardboard and the fibreboards used in the construction and furniture industries. Plants producing a woody stem are the most widely used raw materials for these fibre products but fibres originating from other plants, i.e. non-wood fibres, are used too. One of the most promising potential non-wood fibre resource categories is that of grasses, of which the cereals are the most important plants grown worldwide. A huge amount of straw is generated as an agricultural by-product annually, but the amount used as an industrial raw material is low because it contains components that are detrimental either to processability or to product quality. The purpose of pulp fractionation is to divide pulp into fractions with distinct properties. Industrially feasible fractionation methods are pressure screening, hydrocyclone fractionation and flotation. In pressure screening, separation is based on a mechanical barrier and particles are fractionated according to their dimensions, while a hydrocyclone fractionates particles according to their density and specific surface area and flotation fractionates particles according to their surface chemistry. These methods are traditionally used for removing impurities from pulp but numerous reports on pulp fractionation can also be found. Previous fractionation experiments were performed using wood-based pulps, whereas no previous studies are available concerning the fractionation of pulps manufactured using grasses. The aim of the present work was to determine whether it is possible to fractionate wheat straw pulp by methods that are feasible on an industrial scale. The experimental part was concerned with wheat straw pulp fractionation by pressure screening, hydrocyclone fractionation and flotation. The results show that all these fractionation methods were able to divide the wheat straw pulp into fractions with different cell properties and cell types, e.g. distinct cell lengths, cell wall thicknesses or surface chemistries. Likewise, fractionation can be used to remove detrimental components or to optimize pulp properties according to their end use or to optimize pulp processing sequences. Due to the uniform structure of grasses, it is likely that the results can be generalized to other grasses than that employed here. / Tiivistelmä Kasvisoluja sisältävät tuotteet ovat laajalti mukana ihmisten jokapäiväisessä elämässä. Kaikista näkyvimpiä tuotteita ovat paperi- ja kartonkituotteet sekä rakennus- ja huonekaluteollisuuden käyttämät kuitulevyt. Perinteisesti kasvikuidut näihin tuotteisiin on saatu puuvartisista kasveista, mutta raaka-aineena käytetään muitakin kasveja, niin kutsuttuja non-wood kasveja. Yksi potentiaalinen kasviryhmä teollisuuden raaka-aineeksi ovat heinäkasvit. Muun muassa tärkeimmät viljakasvit ovat heinäkasveja, ja maatalouden sivutuotteina syntyy korsimateriaalia maailmanlaajuisesti huomattava määrä. Heinäkasvien teollinen hyödyntäminen on kuitenkin vähäistä sillä ne sisältävät komponentteja, jotka haittaavat materiaalin prosessointia tai ovat lopputuotteen ominaisuuksia heikentäviä. Kuitususpension fraktioinnin tarkoituksena on jakaa suspensio kahteen tai useampaan jakeeseen, joiden kuituominaisuudet tai solukoostumus ovat erilaiset. Teollisesti käytettävissä olevia fraktiointilaitteita ovat painelajitin, hydrosykloni ja flotaatio: painelajittimessa erottuminen perustuu mekaaniseen esteeseen ja partikkelit erottuvat pääasiassa partikkelin dimensioiden perusteella, hydrosyklonissa erottuminen tapahtuu partikkelien tiheyden ja ominaispinta-alan mukaan kun taas flotaatiolla voidaan erottaa pintakemialtaan erilaisia partikkeleita. Näitä laitteita on perinteisesti käytetty epäpuhtauksien poistamiseen kuitususpensiosta, mutta laitteiden käyttöä puusta valmistettujen massojen fraktiointiin on tutkittu laajasti. Heinäkasveista valmistettujen massojen fraktiointiin näitä menetelmiä ei ole sovellettu. Tämän tutkimuksen tarkoituksena oli selvittää vehnämassan sisältämien solujen fraktiointia teollisuuden käyttöön soveltuvilla menetelmillä. Työn kokeellisessa osassa fraktiointiin vehnäsellua painelajittimella, hydrosyklonilla ja flotaatiolla. Työn tulokset osoittavat, että kaikki tutkitut fraktiointimenetelmät pystyvät jakamaan vehnämassan jakeisiin, joilla on erilaiset solujen ominaisuudet sekä solukoostumukset. Fraktioinnin avulla on mahdollista tuottaa jakeita, joiden kuitupituudet, seinämäpaksuudet sekä pintakemia eroavat. Näin ollen massasta voidaan poistaa jokin haitallinen solujae, tai vaihtoehtoisesti fraktiointia voidaan käyttää optimoitaessa massan ominaisuuksia lopputuotteen ominaisuuksien parantamiseksi, tai optimoitaessa massan prosessointia. Koska heinäkasvit ovat rakenteeltaan samanlaisia, ovat tulokset hyvin todennäköisesti yleistettävissä muidenkin heinäkasvien fraktiointiin.

Diss. (sammanfattning) Umeå : Umeå universitet, 2009. / Härtill 4 uppsatser. Även tryckt utgåva.