title: Pressure screening studies with wood pulp

authors: Weeds, Zuben

subjects: Wood-pulp Refining Separation

abstract: Pressure screening is used in paper making to separate components of a wood pulp slurry (water, wood fibre and solid contaminant material) and to concentrate the wood fibre. The complex solid-liquid and solid-solid mechanisms involved in pressure screening are still poorly understood. This thesis reviews the literature on pressure screening mechanisms and highlights the problems in the models used to describe pressure screening. 
Two models, based on well-mixed tanks in series, are developed to describe reject thickening behaviour of long and short screens. The first model uses a single one row of tanks to model steady flow from feed to accept and from feed to reject streams of a screen. A back-flow model, which had two rows of well-mixed tanks, one for the feedside and another for the accept-side of the screen, was also developed. The passage ratio for back-flow of fibre PB was estimated numerically by deducing the passage ratio for forward-flow of fibre PF. The model showed that PB can decrease more quickly than PF when feed consistency increases, even when overall passage ratio Pp remains constant. 
A small industrial pressure screen (diameter 203 mm, length 220 mm) was used to investigate the effect of screen length Ls and feed consistency Cf on pressure screening performance. The effects of pulp type ( eucalypt for short fibre and pine kraft pulp for short fibre), feed consistency (from 0.005% to 2% compared to approximately 1 % in industrial plants), screen length and placement along the length of the screen, rotor type (bump or step), and screen hole shape (round holes or slots) were investigated. Measurements of pulp consistency in the feed, reject and accept streams under different processing conditions were used to assess the screen models developed. Screening behaviour of screens less than 220 mm was studied extensively for the first time. 
The tanks-in-series model showed that the effect of screen length on reject thickening could be described as well-mixed tanks in series with plug flow behaviour when the screen was modelled as 100 tanks. The reject thickening factor T decreased semilogarithmically when number of tanks was decreased but increased slightly when the number of tanks was increased. . Reject thickening factor decreases when a screen with 1-mm hole is shortened from 220 mm to 27 mm. This reduction can be attributed mainly to the flow regime changing from plug to mixed flow.
The maximum volumetric accept flow rate Qmax decreased by approximately 30 % when a 55-mm screen section was moved from the feed end to the reject end. 1bis decrease in Qmax was consistent with an axial decrease in relative speed ( difference between rotor tip and pulp suspension in the feed annulus), along with an estimated 33% reduction in the negative pressure pulse PNP along the screen. Overall fibre passage ratio PP also decreased when the screen section was moved to the reject end, or when pulp was accelerated in a feed chamber upstream of the feed annulus. 
The three distinct regions and two critical feed consistencies observed when Cr increased from 0.005% to 2% are postulated to indicate when fibre accumulation on the feed and accept sides of the screen produces screen plugging. The critical Cr for the accept side is about ten times greater than that for the feed side due mainly to the fact that the conical holes have a larger diameter on the accept side so more fibre must accumulate before backflow is hindered. In Region 1 ( crowding number < 1 ), reject thickening is independent of feed consistency and remains constant regardless of rotor type (bump or step) and screen type (smooth holes or contoured slots). In Region 2 (1< crowding number< 60), reject thickening decreases with feed consistency. In Region 3 (crowding number > 60), reject thickening either . decreases with feed consistency or remains constant depending on the ratio of forward to back-flushing flow exhibited by the rotor and as predicted by the two-tanks in series model. For the step rotor with a flow ratio of 2:1 reject thickening is constant with feed consistency regardless of screen type. For the bump rotor with a flow ratio of 5: 1 reject thickening decreases with feed consistency regardless of the screen type.
Fibre-length fractionation index <I> increased with increasing Cr to a maximum at about Ct=0.7% when using a 1-mm smooth-holed screen with a bump rotor or at Ct=2% for a 2.4-mm smooth-holed screen. Fractionation index then decreased with increasing Cf. 
It is recommended that the back-flow model be further developed so that the volumetric flow ratio (ratio of forward flow to back flow) can be calculated from a residence time distribution. It is also recommended that fibre-length fractionation be attempted at various positions along the screen for medium consistency pulp (Cr of 8 to 12%) using unconventionally large holes (3-5 mm).

text: Pressure screening studies with wood pulp Pressure screening is used in paper making to separate components of a wood pulp slurry (water, wood fibre and solid contaminant material) and to concentrate the wood fibre. The complex solid-liquid and solid-solid mechanisms involved in pressure screening are still poorly understood. This thesis reviews the literature on pressure screening mechanisms and highlights the problems in the models used to describe pressure screening. 
Two models, based on well-mixed tanks in series, are developed to describe reject thickening behaviour of long and short screens. The first model uses a single one row of tanks to model steady flow from feed to accept and from feed to reject streams of a screen. A back-flow model, which had two rows of well-mixed tanks, one for the feedside and another for the accept-side of the screen, was also developed. The passage ratio for back-flow of fibre PB was estimated numerically by deducing the passage ratio for forward-flow of fibre PF. The model showed that PB can decrease more quickly than PF when feed consistency increases, even when overall passage ratio Pp remains constant. 
A small industrial pressure screen (diameter 203 mm, length 220 mm) was used to investigate the effect of screen length Ls and feed consistency Cf on pressure screening performance. The effects of pulp type ( eucalypt for short fibre and pine kraft pulp for short fibre), feed consistency (from 0.005% to 2% compared to approximately 1 % in industrial plants), screen length and placement along the length of the screen, rotor type (bump or step), and screen hole shape (round holes or slots) were investigated. Measurements of pulp consistency in the feed, reject and accept streams under different processing conditions were used to assess the screen models developed. Screening behaviour of screens less than 220 mm was studied extensively for the first time. 
The tanks-in-series model showed that the effect of screen length on reject thickening could be described as well-mixed tanks in series with plug flow behaviour when the screen was modelled as 100 tanks. The reject thickening factor T decreased semilogarithmically when number of tanks was decreased but increased slightly when the number of tanks was increased. . Reject thickening factor decreases when a screen with 1-mm hole is shortened from 220 mm to 27 mm. This reduction can be attributed mainly to the flow regime changing from plug to mixed flow.
The maximum volumetric accept flow rate Qmax decreased by approximately 30 % when a 55-mm screen section was moved from the feed end to the reject end. 1bis decrease in Qmax was consistent with an axial decrease in relative speed ( difference between rotor tip and pulp suspension in the feed annulus), along with an estimated 33% reduction in the negative pressure pulse PNP along the screen. Overall fibre passage ratio PP also decreased when the screen section was moved to the reject end, or when pulp was accelerated in a feed chamber upstream of the feed annulus. 
The three distinct regions and two critical feed consistencies observed when Cr increased from 0.005% to 2% are postulated to indicate when fibre accumulation on the feed and accept sides of the screen produces screen plugging. The critical Cr for the accept side is about ten times greater than that for the feed side due mainly to the fact that the conical holes have a larger diameter on the accept side so more fibre must accumulate before backflow is hindered. In Region 1 ( crowding number < 1 ), reject thickening is independent of feed consistency and remains constant regardless of rotor type (bump or step) and screen type (smooth holes or contoured slots). In Region 2 (1< crowding number< 60), reject thickening decreases with feed consistency. In Region 3 (crowding number > 60), reject thickening either . decreases with feed consistency or remains constant depending on the ratio of forward to back-flushing flow exhibited by the rotor and as predicted by the two-tanks in series model. For the step rotor with a flow ratio of 2:1 reject thickening is constant with feed consistency regardless of screen type. For the bump rotor with a flow ratio of 5: 1 reject thickening decreases with feed consistency regardless of the screen type.
Fibre-length fractionation index <I> increased with increasing Cr to a maximum at about Ct=0.7% when using a 1-mm smooth-holed screen with a bump rotor or at Ct=2% for a 2.4-mm smooth-holed screen. Fractionation index then decreased with increasing Cf. 
It is recommended that the back-flow model be further developed so that the volumetric flow ratio (ratio of forward flow to back flow) can be calculated from a residence time distribution. It is also recommended that fibre-length fractionation be attempted at various positions along the screen for medium consistency pulp (Cr of 8 to 12%) using unconventionally large holes (3-5 mm).

year: 2006

title: Why the trip home is shorter, but not faster

authors: Charlton, Samuel G.

abstract: When we travel by car the trip home often seems shorter than the initial trip away from the place of origin. This perceptual phenomenon (called the Return Trip Effect) has been noted in many contexts and its existence has puzzled drivers and researchers alike since the distance travelled and time spent traveling are usually identical. It has recently been proposed that the reason for this effect may be due to the fact that the outward trip has more unpredictable elements which make the initial trip seem longer. An alternative interpretation is that drivers pay less attention to familiar tasks (like the trip home) and our sense of trip duration is shorter when we are engaged in taskunrelated thoughts. In this paper we provide data from a driving simulator experiment that contrast these two interpretations as well as determine actual travel times as drivers complete return trips on familiar roads. The results indicate that there is no difference in participants' travel times on the two legs of the journey (i.e., the return trip is not actually faster). When elements of the scene are changed to include new elements and change existing elements during the return leg, the participants' actually did travel faster (decreasing their return trip time). Although changing expectations may play a role in the Return Trip Effect for unfamiliar destinations, when traveling on familiar roads these data suggest that lower levels of conscious engagement with driving is the likely reason for the subjective experience of shorter return trips.

text: Why the trip home is shorter, but not faster When we travel by car the trip home often seems shorter than the initial trip away from the place of origin. This perceptual phenomenon (called the Return Trip Effect) has been noted in many contexts and its existence has puzzled drivers and researchers alike since the distance travelled and time spent traveling are usually identical. It has recently been proposed that the reason for this effect may be due to the fact that the outward trip has more unpredictable elements which make the initial trip seem longer. An alternative interpretation is that drivers pay less attention to familiar tasks (like the trip home) and our sense of trip duration is shorter when we are engaged in taskunrelated thoughts. In this paper we provide data from a driving simulator experiment that contrast these two interpretations as well as determine actual travel times as drivers complete return trips on familiar roads. The results indicate that there is no difference in participants' travel times on the two legs of the journey (i.e., the return trip is not actually faster). When elements of the scene are changed to include new elements and change existing elements during the return leg, the participants' actually did travel faster (decreasing their return trip time). Although changing expectations may play a role in the Return Trip Effect for unfamiliar destinations, when traveling on familiar roads these data suggest that lower levels of conscious engagement with driving is the likely reason for the subjective experience of shorter return trips.

year: 2013-08-28

title: Whakapiki Wairua: Co-designing and implementing a Māori mindfulness mental health intervention in a Wharekura.

authors: McDonald, Marama

abstract: This article explores the process of co-designing a mātauranga-Māori-informed mindfulness intervention with rangatahi in a wharekura and examines the effects on wellbeing. Mahitahi co-design methodology underpinned the design, implementation and evaluation of the intervention, and quantitative psychological tests measured improvements in wellbeing and dispositional mindfulness. Findings showed positive indications for a decrease in levels of psychological distress, improvements in Māori quality of life domains, and higher levels of dispositional mindfulness. The effective mātaurangaMāori-informed wellbeing components of the intervention were he āhuru mōwai, mahi a ngā tīpuna, ngā kaitiaki, te taiao, whanaungatanga, and hohou te rongo. Future application of the intervention in mainstream schools and communities is needed to assess the efficacy of the intervention for rangatahi in other environments, the sustainability of mindfulness practice for rangatahi, and the long-term effects on wellbeing.

text: Whakapiki Wairua: Co-designing and implementing a Māori mindfulness mental health intervention in a Wharekura. This article explores the process of co-designing a mātauranga-Māori-informed mindfulness intervention with rangatahi in a wharekura and examines the effects on wellbeing. Mahitahi co-design methodology underpinned the design, implementation and evaluation of the intervention, and quantitative psychological tests measured improvements in wellbeing and dispositional mindfulness. Findings showed positive indications for a decrease in levels of psychological distress, improvements in Māori quality of life domains, and higher levels of dispositional mindfulness. The effective mātaurangaMāori-informed wellbeing components of the intervention were he āhuru mōwai, mahi a ngā tīpuna, ngā kaitiaki, te taiao, whanaungatanga, and hohou te rongo. Future application of the intervention in mainstream schools and communities is needed to assess the efficacy of the intervention for rangatahi in other environments, the sustainability of mindfulness practice for rangatahi, and the long-term effects on wellbeing.

year: 2021